Patent Publication Number: US-2021162502-A1

Title: Additive manufacturing techniques using noble metals and/or copper metal and related methods and compositions

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 62/943,143, filed Dec. 3, 2019, titled “Additive Manufacturing Techniques Using Noble Metals and/or Copper Metal and Related Methods And Compositions,” which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     Methods of additive manufacturing, binder compositions for additive manufacturing, and articles produced by and/or associated with methods of additive manufacturing are generally described. 
     BACKGROUND 
     Additive manufacturing may be employed to form structures, such as three-dimensional structures. Some methods of additive manufacturing involve employing a binder composition to adhere together a metal powder. However, these methods of additive manufacturing typically suffer from a number of drawbacks. Examples of such drawbacks include undesirable chemical interactions between the binder composition and the metal powder, poor mechanical integrity of metal-based composite objects fabricated from the binder composition, limited shelf stability of the binder composition, and/or the binder composition having a chemical composition unsuitable for being deposited by a print head. Accordingly, improved methods of additive manufacturing, binder compositions for additive manufacturing, and articles produced by and/or associated with methods of additive manufacturing are needed. 
     SUMMARY 
     Methods of additive manufacturing using noble metals and/or copper metal, and binder compositions for use during the additive manufacturing methods, are generally described. In some instances, the methods of additive manufacturing include de-binding (and in some cases sintering steps) that afford metal-based composites, de-bound metal structures, and metal objects containing noble metals (e.g., silver, gold, platinum) and/or copper that have improved properties, such as relatively high densities. In certain aspects, combinations of certain metal powders (e.g., noble metal and/or copper powders) with certain binder compositions may result in improved properties of resulting metal objects produced by the additive manufacturing process, such as relatively low surface roughnesses. The binder compositions described may include a low molecular weight polymer (e.g., including an acrylic acid repeat unit) and, in some cases, a cross-linking agent. The subject matter of the present invention involves, in some cases, interrelated products, alternative solutions to a particular problem, and/or a plurality of different uses of one or more systems and/or articles. 
     In one aspect, methods of additive manufacturing metal-based composite structures by binder jet printing are provided. In some embodiments, the method comprises depositing a first layer of metal powder, the metal powder comprising a noble metal powder and/or copper powder; depositing a binder composition on at least a portion of the first layer of metal powder, the binder composition comprising water and a low molecular weight polymer including an acrylic acid repeat unit; and drying and/or cross-linking at least the binder composition deposited on the first layer of the metal powder, thereby forming a metal-based composite structure. 
     In some embodiments, the method comprises depositing a first layer of metal powder; depositing a binder composition on at least a portion of the first layer of metal powder, the binder composition comprising water and a low molecular weight polymer including an acrylic acid repeat unit; drying and/or cross-linking at least the binder composition deposited on the first layer of the metal powder, thereby forming a metal-based composite structure; and heating the metal-based composite structure in an oxidative environment having a temperature of greater than or equal to 220° C. and less than or equal to 450° C. 
     In another aspect, three-dimensional compositions formed by additive manufacturing are provided. In some embodiments, the three-dimensional composition comprises a metal powder comprising a noble metal powder and/or a copper powder; and a binder composition, wherein the binder composition comprises: water; a low molecular weight polymer including an acrylic acid repeat unit; and a cross-linking agent comprising a polyol, a multifunctional amine, and/or a multifunctional thiol. In another aspect, metal-based composite structures formed by additive manufacturing are provided. In some embodiments, the metal-based composite structure comprises a metal powder comprising a noble metal powder and/or copper powder, wherein the wt % of the noble metal powder and/or copper powder in the metal-based composite structure is between 92 wt % and 99.9 wt %; and a binder, wherein the binder comprises a low molecular weight polymer including an acrylic acid repeat unit. 
     In another aspect, methods of producing an additively manufactured metal object through binder jet printing is provided. In some embodiments, the method comprises heating a metal-based composite structure in a first environment in a chamber to form a de-bound metal structure, wherein the first environment is an oxidative environment having a temperature of greater than or equal to 220° C. and less than or equal to 450° C., wherein the metal-based composite structure comprises a metal powder and a binder, and wherein the binder comprises a low molecular weight polymer including an acrylic acid repeat unit; and sintering the de-bound metal structure in a second environment in the chamber, the second environment having a temperature of greater than or equal to 750° C. and less than or equal to 1700° C. to form a metal object. 
     Other advantages and novel features of the present invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and/or inconsistent disclosure, the present specification shall control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In the figures: 
         FIGS. 1A-1B  show non-limiting diagrams of methods of additive manufacturing, in accordance with some embodiments; 
         FIG. 2  shows a non-limiting embodiment of a method of depositing a layer of metal powder, in accordance with some embodiments; 
         FIG. 3A  shows a non-limiting embodiment of a method of depositing a binder composition onto at least a portion of a layer of metal powder, in accordance with some embodiments; 
         FIG. 3B  shows a non-limiting embodiment of a method step of depositing a second powder layer on a first powder layer onto which a binder composition had been deposited, in accordance with some embodiments; 
         FIG. 3C  shows a non-limiting embodiment of a layer onto which a binder composition has been deposited, in accordance with some embodiments; 
         FIG. 4  shows one non-limiting embodiment of a method of drying and/or cross-linking a binder composition, in accordance with some embodiments; 
         FIG. 5  shows one non-limiting embodiment of a method of heating a metal-based composite structure, in accordance with some embodiments; 
         FIG. 6  shows one non-limiting embodiment of a method of heating a de-bound metal structure, in accordance with some embodiments; 
         FIGS. 7A-7B  show two similar versions of an exemplary additive manufacturing system, in accordance with some embodiments; and 
         FIG. 8  shows one non-limiting embodiment of an additive manufacturing plant, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Methods of additive manufacturing using noble metals and/or copper metal, and binder compositions for use during the additive manufacturing methods, are generally described. In some instances, the methods of additive manufacturing include de-binding (and in some cases sintering steps) that afford metal-based composites, de-bound metal structures, and metal objects containing noble metals (e.g., silver, gold, platinum) and/or copper that have improved properties, such as relatively high densities. In certain aspects, combinations of certain metal powders (e.g., noble metal and/or copper powders) with certain binder compositions may result in improved properties of resulting metal objects produced by the additive manufacturing process, such as relatively low surface roughnesses. The binder compositions described may include a low molecular weight polymer (e.g., including an acrylic acid repeat unit) and, in some cases, a cross-linking agent. 
     Certain aspects of embodiments of the present disclosure relate to additive manufacturing processes and/or conditions (e.g., printing, curing, de-binding, and/or sintering) that can produce noble-metal and/or copper-containing products (e.g., metal objects) having beneficial properties. One aspect, for example, relates to particular de-binding processes that have been discovered to be particularly well-suited for additive manufacturing with noble metals and/or noble metal alloys. For example, in some instances, metal-based composite structures comprising noble metal powder and/or copper powder and a binder composition are heated in particular environments (e.g., oxidative gaseous environments) and at particular temperatures (e.g., between 220° C. and 450° C.) that may afford beneficial properties to the resulting articles and/or avoid potentially problematic phenomena with noble metals and/or copper observed using certain existing techniques. Moreover, certain sintering steps may be employed under particular environments (e.g., vacuum, inert, oxidative, reductive, depending on the noble metal) and temperatures that afford noble-metal and/or copper-containing finished metal objects having beneficial properties (e.g., high density, good finish). An overview of steps that may be included in methods of additive manufacturing is provided below. It should be understood that some methods of additive manufacturing may comprise some of the steps described below but lack other of the steps described below, that some methods of additive manufacturing may comprise all of the steps described below, that some methods of additive manufacturing may comprise none of the steps described below, and that some methods of additive manufacturing may comprise further steps not described below. 
     Other aspects relate to binder compositions that may, in some instances, have advantageous properties for additive manufacturing using noble metals (e.g., silver, gold, platinum) and/or copper. For example, the binder composition may include a low molecular weight hydrophilic polymer such as poly(acrylic) acid and a cross-linker, in combination may lead to the formation of relatively strong, well-resolved noble metal-based composites. As another example, the binder composition may have a relatively high surface tension, which may, when used in the additive manufacturing methods of the present disclosure, result in finished metal objects having relatively smooth surfaces. Such smooth surfaces may be desirable in certain applications of additive manufacturing of noble metals (e.g., silver, gold, platinum) and noble metal alloys (e.g., sterling silver), such as the production of jewelry. As another example, in some embodiments, a binder composition as a whole may be configured to interact with a metal powder comprising a noble metal, noble metal alloy, and/or copper such that the metal powder undergoes minimal amounts of deleterious chemical reactions. Binder compositions having this property may desirably allow for the formation of metal-based composite structures and/or metal objects in which the metal component(s) has a chemical composition close to (or identical to) its composition in powder form and/or in which the metal powder has a desirable chemical composition. 
     In some embodiments, a binder composition as a whole is configured to interact with a layer of metal powder such that it penetrates the layer of metal powder and/or spreads within the layer of metal powder in a desirable manner. The binder composition may be configured to readily penetrate through the depth of the layer of metal powder, which may assist with adhering the layer of metal powder together and/or may reduce the amount of undesired pores in a metal-based composite structure formed therefrom prior to sintering. In some embodiments, a binder composition is configured to spread laterally to a relatively low extent within a layer of metal powder. As excessive spreading is believed to cause the formation of metal objects that are oversized and/or rough, this property may assist with the formation of metal objects having fine features and/or that are smooth. 
     As another example, in some embodiments, a binder composition as a whole is configured such that it can be printed by an additive manufacturing system in a desirable manner. For instance, the binder composition as a whole may be configured to allow for the formation of droplets of a desired size and/or uniformity by a print head of the additive manufacturing system. Such properties may allow for additive manufacturing system to be capable of facilely printing the binder composition in a manner that results in the formation of desirable metal-based composite structures without appreciable wear and tear of the on-demand printer. 
     In some embodiments, a binder composition described herein is configured to be stored for an appreciable amount of time without undergoing undesirable transformations. For instance, in some embodiments, a binder composition described herein has a composition that retards and/or prevents the growth of biological contaminants therein. 
     It should be understood that some binder compositions described herein may have all of the above-described advantages, some binder compositions described herein may have a subset of the above-described advantages, and binder compositions described herein may have none of the above-described advantages. Similarly, some binder compositions described herein may have advantages not described above and/or may be desirable for use in a variety of applications for reasons not described above. Particular features of binder compositions that may promote one or more of the above-described advantages are described in further detail below. 
     It has been observed that certain de-binding steps involving heating metal-based composite structures under oxidative conditions (e.g., in air at temperatures up to 700° C.) may lead to rapid removal of binder components via oxidative and/or thermal degradation. However, it has been observed in the context disclosure that such harsh conditions may result in finished metal object parts having low densities. Without wishing to be bound by any particular theory, the harsh conditions may cause deleterious oxidation of metal components to occur. As a purely illustrative example of such a metal oxidation, in embodiments in which a metal powder comprising sterling silver is used for the additive manufacturing, the oxidative heating may cause the copper present in the sterling silver to form copper oxides. These copper oxides may negatively affect the properties of the final sterling silver metal part. Specifically, the presence of these copper oxides may impede densification of the sterling silver powder (e.g., during a downstream sintering step), even when attempts to reduce the oxides are performed (e.g., using reductive conditions during a de-binding and/or a sintering step). Low densities in finished metal objects may result in poor surface finishes, poor mechanical properties of the metal part, or poor conductivity. The inventors have realized that, surprisingly, certain combinations of materials (e.g., certain binder compositions, such as those with low molecular weight polymers with acrylate repeat units), conditions (e.g., environments, temperatures, timing) and/or sequences of steps (e.g., printing, curing, low-temperature oxidative de-binding, and sintering) that avoid deleterious phenomena with noble-metal and/or copper-containing metal powders are possible. Additive manufacturing processes combining one or more of these materials, conditions, and steps may result in highly-dense metal objects with good finishes and low oxygen and carbon content, and are described in more detail below. The formation of highly dense metal objects during the additive manufacturing process may also reduce or eliminate a potential need for further production steps, such as infiltration or high isostatic pressing/pressure. 
     In some embodiments, a method of additive manufacturing comprises forming a metal-based composite structure.  FIG. 1A  shows a non-limiting diagram of additive manufacturing method  50  comprising step  60  in which a metal-based composite structure is formed. Step  60  may comprise one or more sub-steps of depositing a layer of metal powder, depositing a binder composition, and “curing” deposited components via drying and/or cross-linking, according to certain embodiments. For example, step  60  may comprise a step of depositing a layer of metal powder. This step may comprise dispersing a metal powder to form a layer thereof. The metal powder may initially not be in the form of layer (e.g., it may be in the form of a source of metal powder enclosed in a container, in the form of a pile, etc.).  FIG. 2  shows one non-limiting embodiment of a method of depositing a layer of metal powder in which a metal powder  10  is deposited to form a layer of metal powder  20 . In some embodiments, a metal powder is deposited to form the layer thereof by one or more tools, non-limiting examples of which include rollers, doctor blades, and sifters. Depositing a metal powder to form a layer thereof is typically performed such that the resultant layer of metal powder is formed on a substrate. Appropriate examples of substrates include bases on which the article formed by the additive manufacturing method is designed to be formed (e.g., platforms comprising metals and/or ceramics, sheets comprising metals and/or ceramics) and layers disposed on such bases (e.g., one or more layers of metal powder disposed on a base on which the article formed by the additive manufacturing method is designed to be formed, one or more layers formed in an additive manufacturing process, such as one or more of the layers formed by one or more of the processes described below). Layers disposed on such bases may include layer(s) configured to be incorporated into an article formed by additive manufacturing (e.g., in the case of layer(s) themselves formed by additive manufacturing and/or layer(s) not configured to be incorporated into an article formed by additive manufacturing (e.g., in the case of layer(s) of metal powder). 
     In some embodiments, the metal powder deposited during the formation the metal-based composite structure in step  60  comprises a noble metal powder and/or a copper (Cu) powder. Certain method steps and/or binder compositions described herein may help overcome challenges associated with additive manufacturing with noble metal and/or copper powders using existing techniques and binders, such as low density of completed metal objects. Referring again to  FIG. 2 , metal powder  10 , which is deposited to form a layer of metal powder  20 , is or comprises a noble metal powder (e.g., comprising one or more noble metals), according to some embodiments. Alternatively or additionally, metal powder  10  may be or comprise a copper powder (e.g., comprising copper metal or a copper alloy). As used herein, the “noble metals” are ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au). In certain instances in which the metal powder comprises a noble metal powder, the noble metal powder comprises silver metal and/or a silver alloy. In an exemplary embodiment, the noble metal powder comprises sterling silver. Sterling silver generally refers to a silver alloy having a silver content of greater than or equal to 92.5 weight percent (wt %). Sterling silver may contain silver alloyed with copper (e.g., at 7.5 wt %) to improve properties such as hardness and strength, though in some cases other elements such as germanium (Ge) are used). In some embodiments, the noble metal powder comprises platinum and/or a platinum alloy (e.g. platinum alloyed with cobalt (Co), ruthenium, iridium). In some instances, the noble metal powder comprises gold metal and/or a gold alloy (e.g., gold alloyed with zinc (Zn), copper, silver, nickel (Ni), iron (Fe), cadmium (Cd), aluminum (Al), palladium (Pd)). The metal powder may have only a single noble metal present, though in some embodiments the metal powder comprises two or more noble metals. As one non-limiting example, the metal powder may comprise a mixture of silver metal powder and gold metal powder. In some embodiments in which the metal powder comprises a metal alloy (e.g., a noble metal alloy such as sterling silver), the metal powder comprises a plurality of particles of the metal alloy (e.g., a plurality of sterling silver particles). 
     The metal powder may have a relatively high content of the noble metal, noble metal alloy, and/or copper, by weight. Having a relatively high content of noble metal, noble metal alloy, and/or copper in the metal powder may result in the manufacture of a metal-based composite structure and/or a completed metal object having a relatively high noble metal and/or copper content during or following the additive manufacturing process. In some embodiments, the noble metal (e.g., silver, platinum, gold), noble metal alloy (e.g., sterling silver), and/or copper is present in the metal powder in an amount of greater than or equal to 25 wt %, greater than or equal to 30 wt %, greater than or equal to 40 wt %, greater than or equal to 50 wt %, greater than or equal to 60 wt %, greater than or equal to 65 wt %, greater than or equal to greater than or equal to 70 wt %, greater than or equal 75 wt %, greater than or equal to 80 wt %, greater than or equal to 85 wt %, greater than or equal to 90 wt %, greater than or equal to 91 wt %, greater than or equal to 92 wt %, greater than or equal to 92.5 wt %, greater than or equal to 93 wt %, greater than or equal to 93.5 wt %, greater than or equal to 94%, greater than or equal to 94.5 wt %, greater than or equal to 95 wt %, greater than or equal to 95.5 wt %, greater than or equal to 96 wt %, greater than or equal to 96.5 wt %, greater than or equal to 97 wt %, greater than or equal to 97.5 wt %, greater than or equal to 98 wt %, greater than or equal to 98.5 wt %, greater than or equal to 99 wt %, greater than or equal to 99.5 wt %, greater than or equal to 99.7 wt %, greater than or equal to 99.8 wt %, or greater than or equal to 99.9 wt %. The noble metal (e.g., silver, platinum, gold), noble metal alloy (e.g., sterling silver), and/or copper may be present in the metal powder in amount of less than or equal to substantially 100 wt %, less than or equal to 99.9 wt %, less than or equal to 99.8 wt %, less than or equal to 99.7 wt %, less than or equal to 99.5 wt %, less than or equal to 99 wt %, less than or equal to 98.5 wt %, less than or equal to 98 wt %, less than or equal to 97.5 wt %, less than or equal to 97 wt %, less than or equal to 96.5 wt %, less than or equal to 96 wt %, less than or equal to 95.5 wt %, less than or equal to 96 wt %, less than or equal to 95.5 wt %, less than or equal to 95 wt %, less than or equal to 94.5 wt %, less than or equal to 94 wt %, less than or equal to 93.5 wt %, less than or equal to 93 wt %, less than or equal to 92.5 wt %, less than or equal to 92 wt %, less than or equal to 91 wt %, less than or equal to 90 wt %, less than or equal to 85 wt %, less than or equal to 80 wt %, less than or equal to 75 wt %, less than or equal to 70 wt %, less than or equal to 65 wt %, less than or equal to 60 wt %, less than or equal to 50 wt %, less than or equal to 40 wt %, or less. Combinations of these ranges are possible (e.g., greater than or equal to 25 wt % and less than or equal to substantially 100 wt %, greater than or equal to 25 wt % and less than or equal to 99.9 wt %, or greater than or equal to 92.5 wt % and less than or equal to 99.9 wt %). Other ranges are possible. 
     For example, in some embodiments, the metal powder comprises silver metal and/or a silver alloy in certain of the ranges described above. In some embodiments, silver is present in the metal powder in an amount of greater than or equal to 90 wt %, greater than or equal to 91 wt %, greater than or equal to 92 wt %, greater than or equal to 92.5 wt %, greater than or equal to 93 wt %, greater than or equal to 93.5 wt %, greater than or equal to 94%, greater than or equal to 94.5 wt %, greater than or equal to 95 wt %, greater than or equal to 95.5 wt %, greater than or equal to 96 wt %, greater than or equal to 96.5 wt %, greater than or equal to 97 wt %, greater than or equal to 97.5 wt %, greater than or equal to 98 wt %, greater than or equal to 98.5 wt %, greater than or equal to 99 wt %, greater than or equal to 99.5 wt %, greater than or equal to 99.7 wt %, greater than or equal to 99.8 wt %, or greater than or equal to 99.9 wt %. In some embodiments, silver is present in the metal powder in an amount of less than or equal to substantially 100 wt %, less than or equal to 99.9 wt %, less than or equal to 99.8 wt %, less than or equal to 99.7 wt %, less than or equal to 99.5 wt %, less than or equal to 99 wt %, less than or equal to 98.5 wt %, less than or equal to 98 wt %, less than or equal to 97.5 wt %, less than or equal to 97 wt %, less than or equal to 96.5 wt %, less than or equal to 96 wt %, less than or equal to 95.5 wt %, less than or equal to 96 wt %, less than or equal to 95.5 wt %, less than or equal to 95 wt %, less than or equal to 94.5 wt %, less than or equal to 94 wt %, less than or equal to 93.5 wt %, less than or equal to 93 wt %, less than or equal to 92.5 wt %, less than or equal to 92 wt %, less than or equal to 91 wt %, or less. Combinations (e.g., greater than or equal to 90 wt % and less than or equal to 100 wt %, or greater than or equal to 92.5 wt % and less than or equal to 100 wt %) are possible. Other ranges for the amount of silver in the metal powder are also possible. 
     In some embodiments, the metal powder comprises gold metal and/or a gold alloy in certain of the ranges described above. In some embodiments, gold is present in the metal powder in an amount of greater than or equal to 25 wt %, greater than or equal to greater than or equal to 30 wt %, greater than or equal to 40 wt %, greater than or equal to 50 wt %, greater than or equal to 60 wt %, greater than or equal to 65 wt %, greater than or equal to greater than or equal to 70 wt %, greater than or equal 75 wt %, greater than or equal to 80 wt %, greater than or equal to 85 wt %, greater than or equal to 90 wt %, greater than or equal to 91 wt %, greater than or equal to 92 wt %, greater than or equal to 92.5 wt %, greater than or equal to 93 wt %, greater than or equal to 93.5 wt %, greater than or equal to 94%, greater than or equal to 94.5 wt %, greater than or equal to 95 wt %, greater than or equal to 95.5 wt %, greater than or equal to 96 wt %, greater than or equal to 96.5 wt %, greater than or equal to 97 wt %, greater than or equal to 97.5 wt %, greater than or equal to 98 wt %, greater than or equal to 98.5 wt %, greater than or equal to 99 wt %, greater than or equal to 99.5 wt %, greater than or equal to 99.7 wt %, greater than or equal to 99.8 wt %, or greater than or equal to 99.9 wt %. Gold metal may be present in the metal powder in amount of less than or equal to substantially 100 wt %, less than or equal to 99.9 wt %, less than or equal to 99.8 wt %, less than or equal to 99.7 wt %, less than or equal to 99.5 wt %, less than or equal to 99 wt %, less than or equal to 98.5 wt %, less than or equal to 98 wt %, less than or equal to 97.5 wt %, less than or equal to 97 wt %, less than or equal to 96.5 wt %, less than or equal to 96 wt %, less than or equal to 95.5 wt %, less than or equal to 96 wt %, less than or equal to 95.5 wt %, less than or equal to 95 wt %, less than or equal to 94.5 wt %, less than or equal to 94 wt %, less than or equal to 93.5 wt %, less than or equal to 93 wt %, less than or equal to 92.5 wt %, less than or equal to 92 wt %, less than or equal to 91 wt %, less than or equal to 90 wt %, less than or equal to 85 wt %, less than or equal to 80 wt %, less than or equal to 75 wt %, less than or equal to 70 wt %, less than or equal to 65 wt %, less than or equal to 60 wt %, less than or equal to 50 wt %, less than or equal to 40 wt %, or less. Combinations of these ranges are possible (e.g., greater than or equal to 25 wt % and less than or equal to substantially 100 wt %, greater than or equal to 25 wt % and less than or equal to 99.9 wt %, or greater than or equal to 92.5 wt % and less than or equal to 99.9 wt %). Other ranges are possible. It should be understood that the amount of gold metal in a material may also be expressed in terms of karat. An alloy having gold present in an amount of 100 wt % (pure gold) has 24 karats. Therefore, an alloy having gold present in an amount of 75 wt % has 18 karat, an alloy having gold present in an amount of 50 wt % has 12 karats, and so forth. In some embodiments, the metal powder is a gold alloy powder having greater than or equal to 6 karats, greater than or equal to 8 karats, greater than or equal to 12 karats, greater than or equal to 14 karats, greater than or equal to 16 karats, greater than or equal to 18 karats, greater than or equal to 20 karats, greater than or equal to 22 karats, greater than or equal to 23 karats, or more. In some embodiments, the metal powder is a gold alloy powder having less than or equal to 24 karats, less than or equal to 23 karats, less than or equal to 22 karats, less than or equal to 21 karats, less than or equal to 20 karats, less than or equal to 18 karats, less than or equal to 16 karats, less than or equal to 14karats, less than or equal to 12 karats, less than or equal to 10 karats, less than or equal to 8 karats, or less. Combinations (e.g., greater than or equal to 6 karats and less than or equal to 24 karats) are possible. Other ranges are also possible. 
     Once a layer of metal powder (e.g., comprising a noble metal powder and/or copper powder) is obtained, the step of forming the metal-based composite structure (e.g., step  60 ) may further comprise depositing a binder composition onto at least a portion of the layer of metal powder.  FIG. 3A  shows one example of this method step, as it depicts the deposition of a binder composition  100  on a layer  200  of metal powder. The metal powder comprises a plurality  210  of metal particles. In some embodiments, like the embodiment shown in  FIG. 3A , the binder composition are deposited on the metal powder in the form of droplets, such as in the form of a plurality of droplets formed by a print head. By way of example, a method of additive manufacturing described herein may comprise performing a binder jet printing process. 
     Step  60  of additive manufacturing method  50  may comprise performing the steps shown in  FIGS. 2 and 3A  multiple times successively. For instance, a method of additive manufacturing may comprise depositing a first layer of metal powder, then depositing a binder composition on at least a portion of the first layer of metal powder, and then depositing a second layer of metal powder on the first layer of metal powder. As another example, a method of additive manufacturing may comprise depositing a binder composition on at least a portion of a first layer of metal powder, then depositing a second layer of metal powder on the first layer of metal powder, and then depositing a binder composition on at least a portion of the second layer of metal powder. It can be seen that some methods of additive manufacturing may comprise performing these two steps in an alternating manner at least twice, at least three times, at least four times, at least five times, at least ten times, at least a hundred times, or a number of times sufficient to build up a metal-based composite structure. 
     Methods comprising performing successive steps of depositing a layer of metal powder and depositing a binder composition onto at least a portion of the layer of metal powder may be performed in a variety of manners. By way of example,  FIG. 3B  shows a method step of depositing a second powder layer  252  on the first powder layer onto which a binder composition had been deposited. 
     In some embodiments, the sequential steps of depositing a layer of metal powder and depositing a binder composition thereon may be performed in a manner in which the binder deposited on at least a portion of a first layer of metal powder is not dried or cross-linked prior to depositing a second layer of metal powder on the first layer of metal powder (e.g., the second layer of metal powder is deposited on the first layer of metal powder prior to drying or cross-linking the binder composition). The article formed by such successive steps may be referred to elsewhere herein as a “three-dimensional composition”. In some embodiments, the sequential steps of depositing a layer of metal powder and depositing a binder composition thereon may be performed in a manner in which the binder deposited on at least a portion of a first layer of metal powder is dried and/or cross-linked prior to depositing a second layer of metal powder on the first layer of metal powder. In some such embodiments, drying the binder composition may occur passively at least in part; that is, without a step in which a source of heat is actively applied (examples of applied heating steps being radiative heating (e.g., with an infrared lamp) or convective (e.g., via blowing with hot gas). Passive drying may comprise, for instance, drying at room temperature over some period of time. 
     It should be noted that some embodiments may comprise both of the above-referenced sequences of steps (e.g., during step  60 ). For instance, the steps of sequentially depositing a layer of metal powder and then depositing a binder composition onto at least a portion of the layer of metal powder may be repeated a number of times without performing any drying or heating process on the binder composition (e.g., one or more layers of metal powder may be deposited prior to cross-linking or drying the binder composition previously deposited, binder composition may be deposited onto at least a portion of a layer of metal powder deposited prior to the cross-linking or drying of the binder composition previously deposited). These steps may result in the formation of a three-dimensional composition. Then, the binder composition may be dried and/or cross-linked to form a metal-based composite structure from the three-dimensional composition. After which, further steps of sequentially depositing a layer of metal powder and then depositing a binder composition onto at least a portion of the layer of metal powder may be performed thereon. The second three-dimensional composition may also be dried and/or cross-linked. This drying and/or cross-linking may result in the formation of a new metal-based composite structure comprising the prior metal-based composite structure and the dried and/or cross-linked three-dimensional composition formed thereon. 
     In some embodiments, a step of depositing a binder composition on at least a portion of a layer of metal powder like that shown in  FIG. 3A  or  FIG. 3B  comprises depositing a binder composition on a layer of metal powder such that it contacts some portions of the layer of metal powder and does not contact other portions of the layer of metal powder. The binder composition may penetrate into and/or spread into portions of the layer of metal powder that it contacts. This process may result in the formation of a layer having a morphology like that shown in  FIG. 3C . In  FIG. 3C , a layer  304  comprises a portion  354  comprising both a binder composition and a portion of the layer of metal powder and a portion  364  comprising a portion of the layer of metal powder but lacking the binder composition. The portions of the layer of metal powder through which the binder composition has penetrated and/or spread may be adhered together by one or more components of the binder composition (e.g., a polymer) upon deposition thereof and/or during later processing steps. The portions of the layer of metal powder through which the binder composition has not penetrated or spread may remain unadhered to each other. 
     During formation of a three-dimensional composition, deposition of a binder composition on a layer or metal powder may also comprise depositing a portion of the binder composition onto another layer positioned beneath the layer (e.g., binder composition is deposited on a second layer and a first layer beneath the second layer). Advantageously, this may adhere together layers in the three-dimensional composition with the layers to which they are directly adjacent, which may result in the formation of a three-dimensional object, metal-based composite structure, or combination of metal-based composite structures adhered together in all three dimensions and/or having a continuous morphology. 
     As described above, some methods of additive manufacturing comprise forming a metal-based composite structure (e.g., from a layer comprising a binder composition, from a three-dimensional composition) (e.g., in a process comprising drying and/or cross-linking a binder composition). The metal-based structure may comprise a metal powder comprising a noble metal powder and/or a copper metal powder. As also described above, the binder composition may be a binder composition present in a three-dimensional composition and/or may be a binder composition present in a layer disposed on a metal-based composite structure. 
     As mentioned above step  60  of additive manufacturing method  50 , corresponding to forming a metal-based composite structure, may comprise a step of drying and/or cross-linking the binder composition (e.g., to “cure” the metal-based composite structure). Drying the binder composition may comprise exposing the binder composition to a stimulus that causes one or more volatile components therein to evaporate (e.g., free water, organic solvents, volatile pH modifiers). Other, non-volatile and/or less volatile components of the binder composition may not be removed by a drying process (e.g., bound water, a polymer, a cross-linking agent). 
     Cross-linking the binder composition may comprise exposing the binder composition to a stimulus that causes one or more portions thereof to undergo a cross-linking reaction (e.g., a polymer, a cross-linking agent). Non-limiting examples of suitable stimuli include heat and light (e.g., microwave radiation, UV light). As such, in some embodiments, step  60  includes a heating step. Heat transfer during the heating step may include any combination of conduction, convection and/or radiation. Convective heat transfer may include, but is not limited to, forced convection through the powder bed. Heat and/or light stimuli may be suitable both for drying the binder composition and cross-linking the binder composition; other such stimuli may only be suitable for one or the other. In some embodiments, a binder composition may be dried and then cross-linked. The drying step may comprise removing one or more components that would interfere with the cross-linking step. For instance, a drying step may comprise removing water (e.g., water that causes the equilibrium of the cross-linking reaction to favor breaking cross-links instead of forming cross-links) and/or may comprise removing an optional pH modifier, if present (e.g., a pH modifier that would interfere with the cross-linking reaction). 
       FIG. 4  shows one example of a step of drying and/or cross-linking a binder composition, in which a stimulus is applied to a layer  306  to form a metal-based composite structure  406 . In some embodiments, a method like that shown in  FIG. 4  is performed on a single layer comprising a layer of metal particles and a binder (and/or a layer of metal particles on which a binder is disposed). In some embodiments, a method like that shown in 
       FIG. 4  is performed on a series of such layers disposed on each other in a three-dimensional composition simultaneously. 
     In some embodiments, a metal-based composite structure may undergo one or more further steps. By way of example, portion(s) of a layer of metal powder (and/or layers of metal powder forming a metal powder bed) onto which the binder has been deposited may be incorporated into the metal-based composite structure while other portion(s) (e.g., portions onto which the binder composition was not deposited) may not be incorporated into the metal-based composite structure. One or more portion(s) of the layer(s) of metal powder and/or metal powder bed not incorporated into the metal-based composite structure may be removed therefrom (e.g., following step  60  in  FIG. 1A ). This may be accomplished by, for example, removing the metal-based composite structure from a powder bed. Removing the metal powder not incorporated into the metal-based composite structure may include a “depowdering” process. A depowdering process may be accomplished using any of a variety of techniques, including, but not limited to, manual removal (e.g., by brushing excess powder), directed gas stream flow, and collision with solid particles (e.g., particles of solid CO 2 ) to dislodge unincorporated metal powder from the metal-based composite structure. 
     In some embodiments, the additive manufacturing method includes a step of further heating the metal-based composite structure. For example, following step  60  in which the metal-based composite structure is formed, an ensuing step  70  of heating the metal-based composite structure to de-bind the structure may be performed. It should be understood that one or more intervening steps may be performed between step  60  and step  70 , such as the depowdering step described above. While the heating to de-bind of step  70  is optional, it may, in certain cases, be useful for removing some or all of the binder from the metal-based composite structure. Removal of the binder (e.g., in step  70 ) may, in some cases, advantageously contribute to high metal (e.g., noble metal) densities in the finished product of the additive manufacturing method. 
     It has been discovered that in certain instances in which the metal powder comprises a noble metal powder (e.g., silver, platinum, gold) and/or copper powder, certain heating conditions (e.g., temperature, gaseous environment) contribute to a reduction or avoidance in common deleterious phenomena associate with the use of noble metals and copper in additive manufacturing techniques. As a non-limiting example, step  70  may performed using particular temperature ranges and/or particular gaseous environments that can lead to a high amount of carbon removal during a de-binding process while avoiding excessive metal oxidation. For example, and without wishing to be bound by any particular theory, in some embodiments where copper is present, such as in sterling silver, excessive oxidation of copper to form copper oxides may occur, and step  70  may be performed using particular temperature ranges and/or particular gaseous environments that can lead to a high amount of carbon removal during a de-binding process while reducing the extent of copper oxide formation. 
     The heating (e.g., in step  70 ) may comprise positioning the metal-based composite structure in an environment at a temperature that results in the removal of one or more components of the binder composition previously retained in the composite structure. For instance, the heating may remove a polymer from the binder composition retained in the composite structure and/or one or more other components of the binder composition not removed from the composite structure by prior drying and/or cross-linking steps. In some embodiments, heating the composite structure may cause thermal decomposition of these components of the binder composition that are then volatilized or retained as solids (e.g., as char) positioned within the resultant structure. The resultant structure may also be referred to herein as a “de-bound metal structure”.  FIG. 5  shows one example of a heating step, in which heat is applied to a metal-based composite structure  408  to form a de-bound metal structure  608 . During a heating step, the particles present in the metal-based composite structure may adhere together directly as the portion(s) of the binder composition are being removed. 
     During this heating process (e.g., during step  70 ), a temperature is typically selected that promotes volatilization (e.g., evaporation, thermal decomposition and/or degradation) of the binder and/or binder composition. However, in some cases the temperature should not be so high as to cause deleterious processes such as oxide formation when certain metals are present. It has unexpectedly been observed that there exists a range of temperatures suitable for de-binding metal-based composite structures (e.g., comprising noble metals such as silver, platinum, gold, and/or copper) with little to no such deleterious phenomena. Consequently, in some embodiments, an environment in which a metal-based composite structure is positioned is heated to a temperature of greater than or equal to 220° C., greater than or equal to 230° C., greater than or equal to 240° C., greater than or equal to 250° C., greater than or equal to 275° C., greater than or equal to 300° C., greater than or equal to 350° C., greater than or equal to 400° C., or greater than or equal to 450° C. In some embodiments, an environment in which a metal-based composite structure is positioned is heated to a temperature of less than or equal to 450° C., less than or equal to 400° C., less than or equal to 350° C., less than or equal to 300° C., less than or equal to 280° C., less than or equal to 275° C., less than or equal to 250° C., less than or equal to 240° C., or less than or equal to 230° C. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 220° C. and less than or equal to 450° C., or greater than or equal to 240° C. and less than or equal to 280° C.). Other ranges are also possible. The temperature of an environment may be determined by use of a thermocouple positioned in the environment. 
     As an illustrative example, in some embodiments, an environment in which a metal-based composite structure comprising a metal powder comprising silver metal and/or a silver alloy (e.g., sterling silver) is positioned is heated to a temperature of greater than or equal to 220° C., greater than or equal to 230° C., greater than or equal to 240° C., greater than or equal to 250° C., greater than or equal to 275° C., greater than or equal to 300° C., greater than or equal to 350° C., greater than or equal to 400° C., or greater than or equal to 450° C. In some embodiments, an environment in which a comprising a metal powder comprising silver metal and/or a silver alloy (e.g., sterling silver) is positioned is heated to a temperature of less than or equal to 450° C., less than or equal to 400° C., less than or equal to 350° C., less than or equal to 300° C., less than or equal to 280° C., less than or equal to 275° C., less than or equal to 250° C., less than or equal to 240° C., or less than or equal to 230° C. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 220° C. and less than or equal to 450° C., or greater than or equal to 240° C. and less than or equal to 280° C.). 
     It should be understood that some heating steps may comprise heating an environment in which the metal-based composite structure is positioned to two or more temperatures in sequence. For instance, a heating step may comprise heating a metal-based composite structure at one temperature at which one portion of the binder is expected to degrade (e.g., based on a thermogravimetric analysis performed on the binder) and then heating the metal-based composite structure at another temperature at which another portion of the binder is expected to degrade (e.g., based on a thermogravimetric analysis performed on the binder). These temperatures may be successively increasing (e.g., each temperature to which the metal-based composite structure is heated during the heating step may be higher than the previous temperature at which the metal-based composite structure was heated during the heating step). Some such heating steps may comprise heating the metal-based composite structure at three, four, five, or even more temperatures in sequence. Some or all of the temperatures may be within one or more of the above-described ranges. 
     In some embodiments, a heating step (e.g., during step  70 ) is performed in a manner that reduces the tendency of the metal-based composite structure to form cracks. For instance, the heating step may be performed in a manner such that changes between temperatures are accomplished relatively slowly. In some embodiments, a heating step is performed such that the change in temperature of the environment in which the metal-based composite structure is positioned is less than or equal to 2° C./min, less than or equal to 1.5° C./min, less than or equal to 1° C./min, less than or equal to 0.75° C./min, less than or equal to 0.5° C./min, or less than or equal to 0.25° C./min. In some embodiments, a heating step is performed such that that the change in temperature of the environment in which the metal-based composite structure is positioned is greater than or equal to 0.1° C./min, greater than or equal to 0.25° C./min, greater than or equal to 0.5° C./min, greater than or equal to 0.75° C./min, greater than or equal to 1° C./min, or greater than or equal to 1.5° C./min. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 2° C./min and less than or equal to 0.1° C./min). Other ranges are also possible. In some embodiments, the temperature of the environment in which the metal-based composite structure is positioned is either constant or changes at a rate in one or more of the ranges described above throughout the heating step. In some embodiments, the heating step comprises a change in temperature at a rate in one or more of the ranges described above but also comprises further changes in temperature (e.g., at a rate in one or more of the ranges described above, at a rate outside of the ranges described above). 
     In some embodiments, during a heating step (e.g., step  70  in  FIG. 1A ), the pressure of the environment to which the metal-based composite structure is exposed is set to a partial vacuum to contribute to the removal of the decomposition products from the metal-based composite structure. It should be understood that in a partial vacuum, one or more gases may be present, but the environment has a pressure lower than ambient pressure. In some embodiments, the pressure of the environment is greater than or equal to 10 −5  bar, greater than or equal to 10 −4  bar, greater than or equal to 10 −3  bar, greater than or equal to 10 −2  bar, greater than or equal to 10 −1  bar, greater than or equal to 1 bar, greater than or equal to 10 bar, greater than or equal to 20 bar, or greater than or equal to 50 bar. In some embodiments, the pressure is less than or equal to 70 bar, less than or equal to 50 bar, less than or equal to 20 bar, less than or equal to 10 bar, less than or equal to 1 bar, less than or equal to 10 −1  bar, less than or equal to 10 −2  bar, less than or equal to 10 −3  bar, less than or equal to 10 −4  bar, less than or equal to 10 −5  bar, less than or equal to 10 −6  bar, less than or equal to 10 −7  bar, less than or equal to 10 −8  bar, less than or equal to 10 −9  bar, or less than or equal to 10 −10  bar. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 10 −11  bar and less than or equal to 70 bar, greater than or equal to 10 −11  bar and less than or equal to 10 −3  bar, greater than or equal to 10 −3  bar and less than or equal to 70 bar, greater than or equal to 10 −3  bar and less than or equal to 1 bar, or greater than or equal to 10 −3  bar and less than or equal to 10 −1  bar). Other ranges are also possible. The pressure may be determined by a pressure gauge. In some embodiments, the pressure of the environment to which the metal-based composite structure is exposed is cycled between atmospheric pressure and a pressure in one or more of the above-referenced ranges. 
     In some embodiments, an environment to which a metal-based composite structure is exposed during a heating step is a gaseous environment, where the environment comprises one or more gases. For instance, in some embodiments, the relevant environment may comprise one or more species that are reactive (e.g., with one or more components of the binder) at the temperature to which the environment is heated. For example, the relevant environment may be an oxidative environment (e.g., it may comprise air). An oxidative environment generally refers to one comprising oxidizing species, including but not limited to oxygen gas (O 2 ), ozone (O 3 ), gaseous hydrogen peroxide (H 2 O 2 ), other peroxides (e.g., sodium peroxide (Na 2 O 2 ), barium peroxide (BaO 2 ), magnesium peroxide (MgO 2 ), gaseous nitric acid (HNO 3 ), and carbon dioxide (CO 2 ). It has been unexpectedly observed in the context of the present disclosure that the use of an oxidative environment during a thermal de-binding step (i.e., involving a heating step) can lead to effective de-binding of metal-based composites formed from noble metal and/or copper powder while reducing or avoiding of typical deleterious phenomena, such as copper oxide formation in the case of sterling silver. The formation of such oxides may cause a loss of density of the finished metal object of the overall additive manufacturing method. 
     In some embodiments in which the environment in which a metal-based composite structure is heated in an oxidative environment, the environment is a gaseous environment having a suitable oxygen (O 2 ) content. For example, in some embodiments, an oxidative environment in which a metal-based composite structure is heated has an oxygen content of greater than or equal to 10%, greater than or equal to 12%, greater than or equal to 15%, greater than or equal to 18%, greater than or equal to 20% by volume, greater than or equal to 30%, greater than or equal to 50%, greater than or equal to 75%, or greater. In some cases, an oxidative environment in which a metal-based composite structure is heated has an oxygen content of less than or equal to 100%, less than or equal to 90%, less than or equal to 75%, less than or equal to 60%, less than or equal to 50%, less than or equal to 40%, less than or equal to 30%, less than or equal to 25%, less than or equal to 23%, less than or equal to 22%, less than or equal to 21%, less than or equal to 20%, less than or equal to 18%, less than or equal to 15%, or less than or equal to 10% by volume. Combinations of these ranges are possible. For example, in some embodiments, an oxidative environment in which a metal-based composite structure is heated has an oxygen content of greater than or equal to 10% and less than or equal to 100% or greater than or equal to 10% and less than or equal to 23% by volume. In some embodiments, the oxidative environment is a gaseous environment that is or comprises air. 
     In some embodiments in which the environment in which a metal-based composite structure is heated in an oxidative environment, the environment is a gaseous environment having oxygen (O 2 ) at a suitable partial pressure. For example, in some embodiments, an oxidative environment in which a metal-based composite structure is heated has an oxygen partial pressure of greater than or equal to 10 −5  bar, greater than or equal to 10 −4  bar, greater than or equal to 10 −3  bar, greater than or equal to 10 −2  bar, greater than or equal to 10 −1  bar, 2×10 −1  bar, greater than or equal to 1 bar, greater than or equal to 10 bar, greater than or equal to 20 bar, or greater than or equal to 50 bar. In some cases, an oxidative environment in which a metal-based composite structure is heated has an oxygen partial pressure of less than or equal to 70 bar, less than or equal to 50 bar, less than or equal to 20 bar, less than or equal to 10 bar, less than or equal to 1 bar, less than or equal to 2×10 −1  bar, less than or equal to 10 −1  bar, less than or equal to 10 −2  bar, less than or equal to 10 −3  bar, or less than or equal to 10 −4  bar. Combinations of these ranges are possible. For example, in some embodiments, an oxidative environment in which a metal-based composite structure is heated has an oxygen partial pressure of greater than or equal to 10 −5  bar and less than or equal to 70 bar. In some instances, step  70  further comprises a step of exposing the resulting de-bound metal structure to a reductive environment (e.g., following a heating step in which the metal-based composite is exposed to an oxidative environment). The reductive environment may comprise hydrogen (H 2 ). For instance, in some embodiments, a reductive environment in which a de-bound metal structure is heated after the oxidative de-debinding is a gaseous environment having a hydrogen content of greater than or equal to 2 vol %, greater than or equal to 4 vol %, greater than or equal to 6 vol %, greater than or equal to 8 vol %, greater than or equal to 10 vol %, greater than or equal to 20 vol %, greater than or equal to 30 vol %, greater than or equal to 50 vol %, greater than or equal to 75 vol %, greater than or equal to 90 vol %, greater than or equal to 95 vol %, or higher. The reductive gaseous environment in which the de-bound metal structure is heated may have a hydrogen content of less than or equal to 100 vol %, less than or equal to 99 vol %, less than or equal to 95 vol %, less than or equal to 90 vol %, less than or equal to 75 vol %, less than or equal to 50 vol %, less than or equal to 40 vol %, less than or equal to 30 vol %, less than or equal to 20 vol %, less than or equal to 10 vol %, less than or equal to 8 vol %, less than or equal to 6 vol %, or less than or equal to 4 vol % hydrogen. Combinations of the above-referenced ranges are also possible (e.g., a hydrogen content of greater than or equal to 2 vol % and less than or equal to 100 vol %, or a hydrogen content of greater than or equal to 2 vol % and less than or equal to 10 vol %). Other ranges are also possible. 
     In some embodiments, a reductive gaseous environment in which a de-bound metal structure is heated after the oxidative de-debinding has a partial pressure of hydrogen of greater than or equal to 10 −5  bar, greater than or equal to 10 4  bar, greater than or equal to 10 −3  bar, greater than or equal to 10 −2  bar, greater than or equal to 10 −1  bar, greater than or equal to 1 bar, greater than or equal to 10 bar, greater than or equal to 20 bar, or greater than or equal to 50 bar. In some embodiments, a reductive gaseous environment in which a de-bound metal structure is heated after the oxidative de-debinding has a partial pressure of hydrogen of less than or equal to 70 bar, less than or equal to 50 bar, less than or equal to 20 bar, less than or equal to 10 bar, less than or equal to 1 bar, less than or equal to 10 −1  bar, less than or equal to 10 −2  bar, less than or equal to 10 −3  bar, or less than or equal to 10 4  bar. Combinations of these ranges (e.g., greater than or equal to 10 −5  bar hydrogen and less than or equal to 70 bar hydrogen) are possible. 
     As one non-limiting example, a metal-based composite structure may be heated as part of a de-binding step in an oxidative gaseous environment (e.g., air) having a temperature of greater than or equal to 220° C. and less than or equal to 450° C. to form a de-bound metal structure, and then the environment may be changed a second environment that is a reductive gaseous environment (e.g., having a hydrogen content of greater than or equal to 2 vol % and up to 10 vol %, or even up to 100 vol % hydrogen) having a temperature that is also of greater than or equal to 220° C. and less than or equal to 450° C. The de-bound metal structure may be held in the second environment for greater than or equal to 1 hour and less than or equal to 6 hours, for example. 
     A metal-based composite structure may be heated (e.g., in an oxidative environment) for a variety of suitable amounts of time. In some embodiments, an environment in which a metal-based composite material is positioned (e.g., an oxidative environment) is heated for a time period of greater than or equal to 15 minutes, greater than or equal to 0.5 hours, greater than or equal to 1 hour, greater than or equal to 1.5 hours, greater than or equal to 2 hours, greater than or equal to 2.5 hours, greater than or equal to 3 hours, greater than or equal to 4 hours, greater than or equal to 4.5 hours, greater than or equal to 5 hours, greater than or equal to 5.5 hours, greater than or equal to 6 hours, greater than or equal to 6.5 hours, greater than or equal to 7 hours, greater than or equal to 7.5 hours, greater than or equal to 8 hours, greater than or equal to 8.5 hours, greater than or equal to 9 hours, greater than or equal to 9.5 hours, greater than or equal to 10 hours, greater than or equal to 15 hours, greater than or equal to 20 hours, or greater. In some embodiments, an environment in which a metal-based composite material is positioned is heated for a time period of less than or equal to 72 hours, less than or equal to 48 hours, less than or equal to 36 hours, less than or equal to 24 hours, less than or equal to 20 hours, less than or equal to 15 hours, less than or equal to 10 hours, less than or equal to 9.5 hours, less than or equal to 9 hours, less than or equal to 8.5 hours, less than or equal to 8 hours, less than or equal to 7.5 hours, less than or equal to 7 hours, less than or equal to 6.5 hours, less than or equal to 6 hours, less than or equal to 5.5 hours, or less than or equal to 5 hours. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 15 minutes and less than or equal to 1 day, or greater than or equal to 4 hours and less than or equal to 10 hours). Other ranges are also possible. 
     In some embodiments, heating a metal-based composite material comprises heating the environment in which the metal-based composite material is positioned to one temperature in one or more of the above-referenced ranges and holding the temperature of the environment thereat for an amount of time in one of the above-referenced ranges. In some embodiments, heating a metal-based composite material comprises heating an environment in which the metal-based composite is positioned to two or more temperatures in the above-referenced ranges sequentially and holding the temperature of the environment at each of the two or more temperatures. In such embodiments, the relevant environment may be held at each of the relevant temperatures for a period of time in one or more of the above-referenced ranges and/or may be heated such that the total time it is held at all of the relevant temperatures is in one or more of the above-referenced ranges. 
     Non-limiting examples of suitable environments in which a metal-based composite structure may be positioned during heating include suitable chambers such as an oven. 
     In some embodiments where the additive manufacturing method comprises heating a metal-based composite structure in a first, oxidative environment in a chamber (e.g., oven) to de-bind the metal-based composite structure and then comprises holding the resulting de-bound metal structure in a second environment (e.g., a reductive gaseous environment), the holding step can also be performed in the same chamber as the first, oxidative environment. For example, the oxidizing gas of the first environment may be replaced with a reducing gas (e.g., hydrogen) by flowing in a reducing gas, or evacuating the chamber and back-filling with the reducing gas. 
     In some embodiments, the additive manufacturing method includes a step of sintering the de-bound metal structure to form a metal object. For example,  FIG. 1B  shows an exemplary embodiment of additive manufacturing method  50  in which a step  80  of sintering a de-bound metal structure is performed following step  60  and step  70 . The sintering step may be performed by heating the de-bound metal structure. In some embodiments, sintering the de-bound metal structure (e.g., by heating) forms a metal object. The heating process of the sintering step (e.g., step  80 ) may comprise heating an environment in which the de-bound metal structure is positioned to a temperature that allows for diffusion of metal components within the de-bound metal structure but that does not melt the de-bound metal structure to an undesirable extent. For example, this heating step may comprise heating the environment to a temperature that promotes sintering of the de-bound metal structure. Advantageously, diffusion that occurs during sintering may further bond together the resultant metal object and/or may reduce (and/or eliminate) any porosity present in de-bound metal structure. This diffusion may also cause de-bound metal structure to densify, which may enhance its surface finish, mechanical properties, and/or electrical conductivity.  FIG. 6  shows one example of a step of heating a de-bound metal structure, in which heat is applied to a de-bound metal structure  610  to form a metal object  710 . 
     The temperature of the environment in which the sintering step is performed may be chosen based on one or more properties of the metal and/or metal alloy (e.g., copper or a noble metal such as silver, platinum, gold) present in the de-bound metal structure. For example, during this heating process, it may be desirable for the de-bound metal structure to undergo sintering without undergoing appreciable melting, and an appropriate temperature should be chosen accordingly. Heating may be performed using any of a variety of techniques, such as radiative heating, convective heating, conduction, or combinations thereof. For example, in some embodiments, the sintering is performed in a thermal radiation oven. In certain other embodiments, the sintering is performed in an oven configured to reach relatively high temperatures, such as a tube furnace. In some embodiments, an environment in which a de-bound metal structure is positioned is heated to a temperature of greater than or equal to 750° C., greater than or equal to 800° C., greater than or equal to 850° C., greater than or equal to 900° C., greater than or equal to 950° C., greater than or equal to 1000° C., greater than or equal to 1050° C., greater than or equal to 1100° C., greater than or equal to 1150° C., greater than or equal to 1200° C., greater than or equal to 1250° C., greater than or equal to 1300° C., greater than or equal to 1350° C., greater than or equal to 1400° C., greater than or equal to 1500° C., or greater than or equal to 1600° C. In some embodiments, an environment in which a de-bound metal structure is positioned is heated to a temperature of less than or equal to 1700° C., less than or equal to 1600° C., less than or equal to 1500° C., less than or equal to 1400° C., less than or equal to 1350° C., less than or equal to 1300° C., less than or equal to 1250° C., less than or equal to 1200° C., less than or equal to 1150° C., less than or equal to 1100° C., less than or equal to 1050° C., less than or equal to 1000° C., less than or equal to 950° C., less than or equal to 900° C., less than or equal to 850° C., or less than or equal to 800° C. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 750° C. and less than or equal to 1700° C., greater than or equal to 750° C. and less than or equal to 1200° C., greater than or equal to 750° C. and less than or equal to 900° C., greater than or equal to 800° C. and less than or equal to 1000° C., or greater than or equal to 1400° C. and less than or equal to 1700° C.). Other ranges are also possible. The temperature of an environment may be determined by use of a thermocouple positioned in the environment. 
     In some embodiments, during the heating step of a sintering process (e.g., during step  80  in  FIG. 1B ), the pressure of the environment to which the de-bound metal structure is exposed is set to a full vacuum or a partial vacuum to contribute to the removal of, for example, decomposition products from the metal-based composite structure that may still be present in the de-bound metal structure. In some embodiments, the pressure is greater than or equal to 10 −11  bar, greater than or equal to 10 −10  bar, greater than or equal to 10 −9  bar, greater than or equal to 10 −8  bar, greater than or equal to 10 −7  bar, greater than or equal to 10 −6  bar, greater than or equal to 10 −5  bar, greater than or equal to 10 4  bar, greater than or equal to 10 −3  bar, greater than or equal to 10 −2  bar, greater than or equal to 10 −1  bar, greater than or equal to 1 bar, greater than or equal to 10 bar, greater than or equal to 20 bar, or greater than or equal to 50 bar. In some embodiments, the pressure is less than or equal to 70 bar, less than or equal to 50 bar, less than or equal to 20 bar, less than or equal to 10 bar, less than or equal to 1 bar, less than or equal to 10 −1  bar, less than or equal to 10 −2  bar, less than or equal to 10 −3  bar, less than or equal to 10 4  bar, less than or equal to 10 −5  bar, less than or equal to 10 −6  bar, less than or equal to 10 −7  bar, less than or equal to 10 −8  bar, less than or equal to 10 −9  bar, or less than or equal to 10 −10  bar. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 10- 11  bar and less than or equal to 70 bar, greater than or equal to 10 −11  bar and less than or equal to 10 −3  bar, greater than or equal to 10 −3  bar and less than or equal to 70 bar, greater than or equal to 10 −3  bar and less than or equal to 1 bar, or greater than or equal to 10 −3  bar and less than or equal to 10 −1  bar). Other ranges are also possible. The pressure may be determined by a pressure gauge. In some embodiments, the pressure of the environment to which the de-bound metal structure is exposed is cycled between atmospheric pressure and a pressure in one or more of the above-referenced ranges. 
     In some embodiments, an environment to which a de-bound metal structure is exposed during a heating step of a sintering process is a gaseous environment, where the environment comprises one or more gases. For instance, in some embodiments, the relevant environment may comprise one or more species that are reactive at the temperature to which the environment is heated. For example, the relevant environment may be an oxidative environment (e.g., it may comprise air) or a reductive environment. A reductive environment generally refers to one containing reducing species, such as hydrogen (e.g. pure hydrogen, a mixture of hydrogen and argon comprising 1 volume percent (vol %) to 6 vol % hydrogen, a mixture of hydrogen and argon comprising up to 10 vol % hydrogen). In some embodiments, the relevant environment is an inert environment. By way of example, the relevant environment may comprise, consist of, and/or consist essentially of inert gases, such as nitrogen, argon, and/or helium. 
     In some embodiments in which the environment in which a de-bound metal structure is heated in an oxidative environment (e.g., during sintering step  80 ), the environment is a gaseous environment having a suitable oxygen (O 2 ) content. For example, in some embodiments, an oxidative environment in which a de-bound metal structure is heated has an oxygen content of greater than or equal to 10%, greater than or equal to 12%, greater than or equal to 15%, greater than or equal to 18%, greater than or equal to 20% by volume, greater than or equal to 30%, greater than or equal to 50%, greater than or equal to 75%, or greater by volume. In some cases, an oxidative environment in which a de-bound metal structure is heated has an oxygen content of less than or equal to 100%, less than or equal to 90%, less than or equal to 75%, less than or equal to 60%, less than or equal to 50%, less than or equal to 40%, less than or equal to 30%, less than or equal to 25%, less than or equal to 23%, less than or equal to 22%, less than or equal to 21%, less than or equal to 20%, less than or equal to 18%, less than or equal to 15%, or less than or equal to 10% by volume. Combinations of these ranges are possible. For example, in some embodiments, an oxidative environment in which a de-bound metal structure is heated has an oxygen content of greater than or equal to 10% and less than or equal to 100% by volume, or greater than or equal to 10% and less than or equal to 23% by volume. In some embodiments, the oxidative environment is a gaseous environment that is or comprises air. 
     In some embodiments in which the environment in which a de-bound metal-structure is heated in an oxidative environment, the environment is a gaseous environment having oxygen (O 2 ) at a suitable partial pressure. For example, in some embodiments, an oxidative environment in which a de-bound metal structure is heated has an oxygen partial pressure of greater than or equal to 10 −5  bar, greater than or equal to 10 4  bar, greater than or equal to 10 −3  bar, greater than or equal to 10 −2  bar, greater than or equal to 10 −1  bar, 2×10 −1  bar, greater than or equal to 1 bar, greater than or equal to 10 bar, greater than or equal to 20 bar, or greater than or equal to 50 bar. In some cases, an oxidative environment in which a de-bound metal structure is heated has an oxygen partial pressure of less than or equal to 70 bar, less than or equal to 50 bar, less than or equal to 20 bar, less than or equal to 10 bar, less than or equal to 1 bar, less than or equal to 2×10 −1  bar, less than or equal to 10 −1  bar, less than or equal to 10 −2  bar, less than or equal to 10 −3  bar, or less than or equal to 10 4  bar. Combinations of these ranges are possible. For example, in some embodiments, an oxidative environment in which a de-bound structure is heated has an oxygen partial pressure of greater than or equal to 10 −5  bar and less than or equal to 70 bar. 
     In some embodiments in which the environment in which a de-bound metal structure is heated in a reductive environment (e.g., during sintering step  80 ), the environment is a gaseous environment having a suitable hydrogen (H 2 ) content. For instance, in some embodiments, an environment in which a de-bound metal structure is heated during sintering is a gaseous environment having a hydrogen content of greater than or equal to 2 vol %, greater than or equal to 4 vol %, greater than or equal to 6 vol %, greater than or equal to 8 vol %, greater than or equal to 10 vol %, greater than or equal to 20 vol %, greater than or equal to 30 vol %, greater than or equal to 50 vol %, greater than or equal to 75 vol %, greater than or equal to 90 vol %, greater than or equal to 95 vol %, or higher. The reductive gaseous environment in which the de-bound metal structure is heated during sintering may have a hydrogen content of less than or equal to 100 vol %, less than or equal to 99 vol %, less than or equal to 95 vol %, less than or equal to 90 vol %, less than or equal to 75 vol %, less than or equal to 50 vol %, less than or equal to 40 vol %, less than or equal to 30 vol %, less than or equal to 20 vol %, less than or equal to 10 vol %, less than or equal to 8 vol %, less than or equal to 6 vol %, or less than or equal to 4 vol % hydrogen. Combinations of the above-referenced ranges are also possible (e.g., a hydrogen content of greater than or equal to 2 vol % and less than or equal to 100 vol %, or a hydrogen content of greater than or equal to 2 vol % and less than or equal to 10 vol %). Other ranges are also possible. 
     In some embodiments, a reductive gaseous environment in which a de-bound metal structure is heated during sintering has a partial pressure of hydrogen of greater than or equal to 10 −5  bar, greater than or equal to 10 −4  bar, greater than or equal to 10 −3  bar, greater than or equal to 10 −2  bar, greater than or equal to 10 −1  bar, greater than or equal to 1 bar, greater than or equal to 10 bar, greater than or equal to 20 bar, or greater than or equal to 50 bar. In some embodiments, a reductive gaseous environment in which a de-bound metal structure is heated during sintering has a partial pressure of hydrogen of less than or equal to 70 bar, less than or equal to 50 bar, less than or equal to 20 bar, less than or equal to 10 bar, less than or equal to 1 bar, less than or equal to 10 −1  bar, less than or equal to 10 −2  bar, less than or equal to 10 −3  bar, or less than or equal to 10 4  bar. Combinations of these ranges (e.g., greater than or equal to 10 −5  bar hydrogen and less than or equal to 70 bar hydrogen) are possible. 
     While in some embodiments the various heating steps are each performed in a different chamber (e.g., oven), in other embodiments multiple heating steps can be performed in the same chamber. For example, in certain instances, some or all the heating for de-binding (e.g., step  70  in  FIGS. 1A-1B ) occurs in a chamber, and some or all of the sintering (e.g., step  80  in  FIG. 1B ) also occurs in the chamber (e.g., the same oven). Heating for de-binding and sintering may be performed in the same chamber even in instances in which de-binding and sintering are performed in different environments. For example, a single chamber may be configured to heat contents of the chamber within first and second environments having different temperatures and/or comprising different constituent gas(es). Surprisingly, it has been observed that a single chamber may be configured to successfully de-bind and sinter even in instances in which de-binding occurs a first environment having a first type of reactivity (e.g., oxidative) and sintering in a second environment having a second, different type of reactivity (e.g., reductive). For example, the chamber may comprise components that have little to no reactivity with either oxidative gases (e.g., oxygen) or reductive gases (e.g., hydrogen). In at least some instances, performing different heating steps (e.g., de-binding, sintering) in the same chamber (e.g., oven) increases the efficiency and/or speed of the overall additive manufacturing process. For example, because de-bound metal structures produced through de-binding do not need to be removed from one device and supplied to another before sintering, this time may be saved. Additionally, in some instances use of the same chamber for different heating steps reduces complexity and cost by reducing the number of process steps, separate components/apparatuses, and overall system footprint. 
     In some embodiments, an additive manufacturing method comprises heating a metal-based composite structure (e.g., comprising a metal powder such as a noble metal powder and/or copper metal powder) in a first environment in a chamber to form a de-bound metal structure, and sintering the de-bound metal structure in a second environment in the same chamber to form a metal object. In some such embodiments, the first environment is an oxidative environment having a temperature of greater than or equal to 220° C. and less than or equal to 450° C., as described above. The second environment within the chamber during sintering may have an elevated temperature, such as a temperature of greater than or equal to 750° C. and less than or equal to 1700° C., as described above. In some embodiments, the de-bound metal structure formed during the heating in the first environment is not removed from the chamber prior to the sintering step. For example, the metal-based composite structure may be heated in a first environment (e.g., an oxidative environment having a temperature of greater than or equal to 220° C. and less than or equal to 450° C.), and then, without removing the resulting de-bound metal structure, the temperature is increased (e.g., ramped) to a second environment having an elevated temperature for sintering (e.g., at a temperature of greater than or equal to 750° C. and less than or equal to 1700° C.). 
     The change in temperature between the first environment and the second environment may be achieved using any of a variety of techniques. In some embodiments, the chamber is configured to heat contents of the chamber to the temperature of a first environment and to also heat contents of the chamber to the temperature of a second environment by operating heating elements of the same type, or even by operating a single heating element. For example, in some embodiments the chamber is configured to heat contents of the chamber at a first temperature (in a first environment) by operating a heating element configured for convective heating, and the chamber is also configured to heat contents of the chamber at a second temperature (in a second environment) for sintering using the same convective heating element. As another example, the chamber may, in some instances, be configured to heat contents of the chamber in both in the first environment and the second environment using a radiative heating element. In some embodiments, the chamber comprises multiple heating elements, and is configured to heat contents of the chamber at a first temperature (in the first environment) using a first heating element and to heat contents of the chamber at a second temperature (in the second environment) using a second, different heating element. As one non-limiting example, the chamber may be configured to heat contents of the chamber in a first environment (e.g., an oxidative gaseous environment) using a first heating element configured for convective heat transfer, and the chamber may be configured to heat contents of the chamber in the second environment (e.g., a vacuum environment) for sintering using a second heating element configured for radiative heat transfer (e.g., an infrared lamp). 
     In some embodiments where the heating for de-binding and the sintering are performed in the same chamber, the second environment has the same type of gaseous environment as the first environment. For example, in some embodiments, the first environment is an oxidative environment (e.g., a gaseous environment containing oxygen, such as air), and the second environment used for sintering has the same oxidative environment. In some such embodiments, the metal-based composite structure (e.g., comprising platinum metal and/or a platinum alloy) is placed in the chamber at room temperature, and the temperature is first elevated to a de-binding temperature and then elevated to a sintering temperature without a change in the type of gaseous environment (e.g., oxidative) in the chamber. The change in temperature may occur in stages (e.g., with holds), or the change in temperature (e.g., from room temperature to the temperature of the first environment to the temperature of the second environment) may occur via a continuous temperature ramp, such that de-binding occurs during a portion of the temperature ramp. In some embodiments, the second environment used for sintering has a different type of gaseous environment than the first gaseous environment. For example, in some embodiments, the first environment is an oxidative environment (e.g., a gaseous environment containing oxygen, such as air), and the second environment used for sintering is an inert environment, or a vacuum environment, or a reductive environment (e.g., a gaseous environment containing hydrogen). In some instances where the second environment has a different type of gaseous environment than the first environment used de-binding, an intermediate environment is used between the first environment and the second environment. As one non-limiting example, the first environment used for de-binding may be an oxidative environment in a first temperature range (e.g., greater than or equal to 220° C. and less than or equal to 450° C.), and subsequently the chamber is changed to an intermediate environment having a reductive gaseous (or inert gaseous, or vacuum) environment but still in the first temperature range (e.g., greater than or equal to 220° C. and less than or equal to 450° C.). The de-bound metal structure is held at the intermediate environment for a period of time. The environment in the chamber is then changed to the second environment for sintering, the second environment having the same reductive gaseous (or inert gaseous, or vacuum) environment as the intermediate environment, but having an elevated temperature range (e.g., greater than or equal to 750° C. and less than or equal to 1700° C.). 
     The type of gaseous environment of the chamber may be changed using any of a variety of suitable techniques. For example, the chamber may be configured such that one or more gases (or gas mixtures) may be flowed into and/or through the headspace of the chamber. Exemplary configurations include chambers equipped with gas inlets and gas outlets, which may be operated (manually and/or automatically) to direct gas into or out of the chamber. In some embodiments, the gas inlet is fluidically connected to a gas mixture source and, optionally, configured with one or more pumps configured to flow the gas mixture into and/or through the chamber. In some embodiments in which the chamber is configured to heat a metal-based composite structure in a first environment and sinter a de-bound metal structure in a second environment having a different gaseous environment, the chamber is configured to be fluidically connected to two or more different gas mixture sources (e.g., with a gas inlet connected to two different gas tanks and valving for switching flow to the chamber between the different gas tanks, or two or more gas inlets connected to different pressurized gas tanks and valving for switching flow to the chamber between different gas inlets). As a specific example, the first environment may be air, and the second environment may be established by flowing a different gas mixture (e.g., a reductive gas such as forming gas) into and/or through the chamber (e.g., via a gas inlet optionally connected to a pump). In some cases, the chamber may be coupled to a reduced pressure source such as a vacuum pump, which may be operated to evacuate or otherwise remove gas from the interior of the chamber. 
     Another way to change the type of gaseous environment of the chamber may be to configure the chamber such that the gaseous environment can be evacuated via vacuum. For example, the chamber may comprise a headspace fluidically connected to a vacuum source and valving for exposing the headspace to the vacuum source when desired. The chamber may also be configured to flow gas mixtures into and through the headspace (e.g., via one or more gas inlets, gas outlets, valves and pumps connected to a gas mixture source). The chamber may be configured to back-fill the headspace with a gas mixture to establish a gaseous environment after a vacuum evacuation process. In some cases, the first environment is gaseous (e.g., air), the gas is evacuated from the chamber, and the chamber is subsequently back-filled with a different gas mixture (e.g., forming gas) to establish the second environment. 
     Formation of a metal object by sintering may comprise heating an environment in which a de-bound metal structure is positioned in for a variety of suitable amounts of time. In some embodiments, an environment in which a de-bound metal structure is positioned is heated for a time period of greater than or equal to 30 minutes, greater than or equal to 1 hour, greater than or equal to 2 hours, greater than or equal to 3 hours, greater than or equal to 6 hours, greater than or equal to 9 hours, greater than or equal to 12 hours, greater than or equal to 18 hours, greater than or equal to 1 day, or greater than or equal to 1.5 days. In some embodiments, an environment in which a de-bound metal structure is positioned is heated for a time period of less than or equal to 2 days, less than or equal to 1.5 days, less than or equal to 1 day, less than or equal to 18 hours, less than or equal to 12 hours, less than or equal to 9 hours, less than or equal to 6 hours, less than or equal to 3 hours, less than or equal to 2 hours, or less than or equal to 1 hour. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 30 minutes and less than or equal to 2 days). Other ranges are also possible. 
     In some embodiments, heating a de-bound metal structure heating the environment in which the de-bound metal structure is positioned to one temperature in one or more of the above-referenced ranges and holding the temperature of the environment thereat for an amount of time in one of the above-referenced ranges. As a non-limiting example, the entire sintering step may involve holding the temperature of the environment at 830° C. for 6 hours in certain cases. In some embodiments, heating a de-bound metal structure comprises heating an environment in which the de-bound metal is positioned to two or more temperatures in the above-referenced ranges sequentially and holding the temperature of the environment at each of the two or more temperatures. In such embodiments, the relevant environment may be held at each of the relevant temperatures for a period of time in one or more of the above-referenced ranges and/or may be heated such that the total time it is held at all of the relevant temperatures is in one or more of the above-referenced ranges. As a non-limiting example, the sintering step may involve holding the temperature at 700° C. for up to 3 hours, then holding the temperature at 820° C. for up to 3 hours, and then holding the temperature at 840° C. for up to 3 hours. 
     While some embodiments comprise sintering the de-bound metal structure a single time using the conditions described above, other embodiments comprise multiple such sintering steps. For example, an additive manufacturing method may comprise a first sintering step that forms an initial metal object, and then the initial metal object may undergo a second sintering step (e.g., under the same or different conditions as the first sintering step). It has been observed in the context of the present disclosure that a second sintering step (e.g., a second heating step during step  80 ) can lead to higher densities of finished metal products relative to instances where an otherwise identical process is performed but using just a single sintering step. 
     It has been discovered in the context of the present disclosure that certain combinations of environments and temperatures may, in some cases, lead to advantageous results with particular metal powders during de-binding and sintering process (e.g., steps  70  and  80  in  FIG. 1B ). The advantageous results may include, but are not limited to, high levels of densification and low levels of oxygen and carbon in final metal parts. As one example, in some embodiments in which the metal powder comprises a noble metal powder comprising silver metal and/or a silver alloy (e.g., in an amount greater than or equal to 90 wt %, greater than or equal to 95 wt %, greater than or equal to 99 wt %, or higher), the following steps of an additive manufacturing method may be performed. During a de-binding step, the metal-based composite structure comprising silver metal and/or a silver alloy (e.g., sterling silver) may be heated in an oxidative gaseous environment (e.g., having oxygen present in an amount of greater than or equal to 10 vol % and less than or equal to 23 vol %) at a temperature in the range of greater than or equal to 220° C. and less than or equal to 450° C. Then, during a sintering step, the resulting de-bound metal object may be heated in a reductive gaseous environment (e.g., having hydrogen present in an amount of greater than or equal to 2 vol % and less than or equal to 100 vol %) at a temperature in the range of greater than or equal to 750° C. and less than or equal to 900° C., thereby forming a metal object containing the silver metal and/or silver alloy. 
     As another non-limiting example, in some embodiments in which the metal powder comprises a noble metal powder comprising platinum metal and/or a platinum alloy (e.g., in an amount greater than or equal to 50 wt %, greater than or equal to 75 wt %, greater than or equal to 90 wt %, greater than or equal to 95 wt %, greater than or equal to 99 wt %, or higher), the following steps of an additive manufacturing method may be performed. During a de-binding step, the metal-based composite structure comprising platinum metal and/or a platinum alloy may be heated in an oxidative gaseous environment (e.g., having oxygen present in an amount of greater than or equal to 10 vol % and less than or equal to 23 vol %) at a temperature in the range of greater than or equal to 220° C. and less than or equal to 450° C. Then, during a sintering step, the resulting de-bound metal object may be heated in a vacuum environment (e.g., full vacuum or partial vacuum), an inert environment, or an oxidative gaseous environment (e.g., having oxygen present in an amount of greater than or equal to 10 vol % and less than or equal to 23 vol %) at a temperature in the range of greater than or equal to 1400° C. and less than or equal to 1700° C., thereby forming a metal object containing the platinum metal and/or platinum alloy. 
     As yet another non-limiting example, in some embodiments in which the metal powder comprises a noble metal powder comprising gold metal and/or a gold alloy (e.g., in an amount greater than or equal to 25 wt %, greater than or equal to 50 wt %, greater than or equal to 75 wt %, greater than or equal to 90 wt %, greater than or equal to 95 wt %, greater than or equal to 99 wt %, or higher), the following steps of an additive manufacturing method may be performed. During a de-binding step, the metal-based composite structure comprising gold metal and/or a gold alloy may be heated in an oxidative gaseous environment (e.g., having oxygen present in an amount of greater than or equal to 10 vol % and less than or equal to 23 vol %) at a temperature in the range of greater than or equal to 220° C. and less than or equal to 450° C. Then, during a sintering step, the resulting de-bound metal object may be heated in a vacuum environment (e.g., full vacuum or partial vacuum), an inert environment, or a reductive gaseous environment (e.g., having hydrogen present in an amount of greater than or equal to 2 vol % and less than or equal to 100 vol %) at a temperature in the range of greater than or equal to 800° C. and less than or equal to 1000° C., thereby forming a metal object containing the gold metal and/or gold alloy. 
     In some embodiments, one or more of the method steps described above may be performed in an additive manufacturing system.  FIGS. 7A and 7B  show two similar versions of an exemplary additive manufacturing system  1100 . The various components of this additive manufacturing system and its operation are described below. 
     The additive manufacturing system  1100  shown in  FIGS. 7A and 7B  may be used to form an article  1102  from a metal powder  1104 . The article  1102  may be a three-dimensional composition as described elsewhere herein. For instance, it may comprise a binder composition and a metal powder comprising a plurality of metal particles (e.g., as shown in  FIGS. 3A-3C ). As also described elsewhere herein, the three-dimensional composition  1102  can undergo subsequent steps to form a metal object. The additive manufacturing system shown in  FIGS. 7A and 7B  may be suitable for performing a binder jetting process to form a three-dimensional composition (e.g., by selectively joining portions of layers of metal powder with a binder composition in a sequential manner). 
     The additive manufacturing system  1100  shown in  FIGS. 7A and 7B  can include a powder deposition mechanism  1106  (e.g., shown in  FIG. 7B ) and a print head (e.g., shown as print head  1118  in  FIG. 7A  and print head  1108  in  FIG. 7B ), which may be coupled to and moved across the print area by a unit  1107  (e.g., as shown in  FIG. 7B ). The powder deposition mechanism  1106  may be operated to deposit a layer of metal powder by depositing powder  1104  onto the powder bed  1114 . 
     In some embodiments, a powder deposition mechanism comprises a metal powder supply  1112 , a metal powder bed  1114 , and a spreader  1116  (e.g., as shown in  FIG. 7A ). When present, the spreader  1116  can be movable from the metal powder supply  1112  to the metal powder bed  1114  and along the metal powder bed  1114  to deposit a metal powder onto the metal powder bed  1114  and to deposit successive layers of the metal powder across the metal powder bed  1114 . As discussed in more detail below, the additive manufacturing apparatus  1100  and/or the spreader  1116  therein may be configured to deposit layers of metal powder on the powder bed having any suitable geometry (e.g., layers of metal powder having a homogeneous, planar geometry; layers of metal powder having a morphology other than a homogeneous, planar geometry). Depending on the particular embodiment, the spreader  1116  may include, for example, a roller rotatable about an axis perpendicular to an axis of movement of the spreader  1116  across the powder bed  1114 . The roller can be, for example, substantially cylindrical. In use, rotation of the roller about the axis perpendicular to the axis of movement of the spreader  1116  can deposit the metal powder from the metal powder supply  1112  to the metal powder bed  1114  and form a layer of the metal powder along the metal powder bed  1114 . It should be appreciated, therefore, that a plurality of sequential layers of the material  1104  can be formed in the metal powder bed  1114  through repeated movement of the spreader  1116  across the metal powder bed  1114 . 
     The print head  1108  (in  FIG. 7B ) and/or  1118  (in  FIG. 7A ) can be movable (e.g., in coordination with movement of the spreader  1116 ) across the metal powder bed  1114  and/or can be stationary (e.g., in embodiments in which the platform  1105  is movable). In some embodiments, the print head  1108  and/or  1118  includes one or more orifices through which a liquid (e.g., a binder composition) can be delivered from the print head  1118  to each layer of the metal powder along the metal powder bed  1114 . In certain embodiments, the print head  1108  and/or  1118  can include one or more piezoelectric elements, and each piezoelectric element may be associated with a respective orifice and, in use, each piezoelectric element can be selectively actuated such that displacement of the piezoelectric element can expel liquid from the respective orifice. In some embodiments, the print head  1108  and/or  1118  may be arranged to expel a single liquid formulation from the one or more orifices. In other embodiments, the print head  1108  and/or  1118  may be arranged to expel a plurality of liquid formulations from the one or more orifices. For example, the print head  1108  and/or  1118  can expel a plurality of liquids (e.g., a plurality of solvents), a plurality of components of a binder composition, or both from the one or more orifices. Moreover, in some instances, expelling or otherwise delivering a liquid from the print head may include emitting an aerosolized liquid (i.e., an aerosol spray) from a nozzle of the print head. 
     In general, the print head  1108  in  FIG. 7B and/or 1118  in  FIG. 7A  may be controlled to deliver liquid such as a binder composition to the metal powder bed  1114  in predetermined two-dimensional patterns, with each pattern corresponding to a respective layer of the three-dimensional composition  1102 . In this manner, the delivery of the binder composition may be a printing operation in which the metal powder in each respective layer of the three-dimensional composition is selectively joined along the predetermined two-dimensional layers. After each layer of the three-dimensional composition is formed as described above, the platform  1105  may be moved down and a new layer of metal powder deposited, binder composition again applied to the new metal powder, etc. until the object has been formed. 
     In some embodiments, the print head  1108  (in  FIG. 7B ) and/or  1118  (in  FIG. 7A ) can extend axially along substantially an entire dimension of the metal powder bed  1114  in a direction perpendicular to a direction of movement of the print head  1108  and/or  1118  across the metal powder bed  1114 . For example, in such embodiments, the print head  1118  can define a plurality of orifices arranged along the axial extent of the print head  1108  and/or  1118 , and liquid can be selectively jetted from these orifices along the axial extent to form a predetermined two-dimensional pattern of liquid along the metal powder bed  1114  as the print head  1108  and/or  1118  moves across the metal powder bed  1114 . In some embodiments, the print head  1108  and/or  1118  may extend only partially across the metal powder bed  1114 , and the print head  1108  and/or  1118  may be movable in two dimensions relative to a plane defined by the powder bed  1114  to deliver a predetermined two-dimensional pattern of a liquid along the powder bed  1114 . 
     The additive manufacturing system  1100  generally further includes a controller  1120  in electrical communication with one or more other system components. For instance, in  FIG. 7A , a controller  1120  is in electrical communication with the metal powder supply  1112 , the metal powder bed  1114 , the spreader  1116 , and the print head  1118 . In  FIG. 7B , the controller  1120  is in electrical communication with the unit  1107 , the powder deposition mechanism  1106 , and the print head  1108 . Also in  FIG. 7B , the controller  1120  may be configured to control the motion of the unit  1107 , the material deposition mechanism  1106 , and the print head  1108  as described above. 
     A non-transitory, computer readable storage medium  1122  may be in communication with the controller  1120  and have stored thereon a three-dimensional model  1124  and instructions for carrying out any one or more of the methods described herein. Alternatively, the non-transitory, computer readable storage medium may comprise previously prepared instructions. With reference to  FIG. 7B , such instructions, when executed by the controller  1120 , may operate the platform  1105 , the unit  1107 , the material deposition mechanism  1106 , and the print head  1108  to fabricate one or more three-dimensional compositions. For example, one or more processors of the controller  1120  can execute instructions to move the unit  1107  forwards and backwards along an x-axis direction across the surface of the powder bed  1114 . One or more processors of the controller  1120  also may control the material deposition mechanism  1106  to deposit build material onto the metal powder bed  1114 . 
     With reference to  FIG. 7A , one or more processors of the controller  1120  can execute instructions to control movement of one or more of the metal powder supply  1112  and the metal powder bed  1114  relative to one another as the three-dimensional composition  1102  is being formed. For example, one or more processors of the controller  1120  can execute instructions to move the metal powder supply  1112  in a z-axis direction toward the spreader  1116  to direct the metal powder  1104  toward the spreader  1116  as each layer of the three-dimensional composition  102  is formed and to move the metal powder bed  1114  in a z-axis direction away from the spreader  1116  to accept each new layer of the metal powder along the top of the metal powder bed  1114  as the spreader  1116  moves across the metal powder bed  1114 . One or more processors of the controller  1120  also may control movement of the spreader  1116  from the metal powder supply  1112  to the metal powder bed  1114  to move successive layers of the metal powder across the metal powder bed  1114 . 
     In some embodiments, one or more processors of the controller  1120  can control movement of the print head  1108  (in  FIG. 7B ) and/or  1118  (in  FIG. 7A ) to deposit liquid such as a binder composition onto selected regions of the metal powder bed  1114  to deliver a respective predetermined two-dimensional pattern of the liquid to each new layer of the metal powder  1104  along the top of the metal powder bed  1114 . In general, as a plurality of sequential layers of the metal powder  1104  are introduced to the metal powder bed  1114  and the predetermined two-dimensional patterns of the liquid are delivered to each respective layer of the plurality of sequential layers of the metal powder  1104 , the three-dimensional composition  1102  is formed according to the three-dimensional model (e.g., a model stored in a non-transitory, computer readable storage medium coupled to, or otherwise accessible by, the controller  1120 , such as three-dimensional model  1124  stored in the non-transitory, computer readable storage medium  1122 ). In certain embodiments, the controller  1120  may retrieve the three-dimensional model (e.g., three-dimensional model  1124 ) in response to user input, and generate machine-ready instructions for execution by the additive manufacturing system  1100  to fabricate the three-dimensional object  1102 . 
     As described above, it will be appreciated that the illustrative additive manufacturing system  1100  is provided as one example of a suitable additive manufacturing system and is not intended to be limiting with respect to the techniques described herein for controlling the flow behavior of a metal powder. For instance, it will be appreciated that the techniques may be applied within an additive manufacturing apparatus that utilizes only a roller as a material deposition mechanism and does not include material deposition mechanism  1106 . 
     According to some embodiments, the techniques described herein for controlling the flow behavior of a metal powder may be employed to control properties of a metal powder for a binder jet additive manufacturing system. Such a system may comprise additive manufacturing system  1100  in addition to one or more other apparatus for producing a completed part (e.g., a metal object as described herein). Such apparatus may include, for example, an oven for debinding and/or sintering a three-dimensional composition fabricated by the additive manufacturing system  1100  (or for de-binding and/or sintering such a three-dimensional composition subsequent to applying other post-processing steps upon the three-dimensional composition). 
     Techniques described herein may refer to a “metal powder,” although it will be appreciated that the techniques described herein are not necessarily limited to use cases in which the metal material employed to form one or more of the articles described herein comprises or consists of a powder. As such, while the discussion above may focus primarily on depositing a binder composition onto a metal powder (e.g., a noble metal powder), it will be appreciated that any binder deposition process described herein may also apply to deposition of a binder onto any granular material(s). 
     Referring now to  FIG. 8 , an additive manufacturing plant  1300  can include the additive manufacturing system  1100 , a conveyor  1304 , and a post-processing station  1306 . The metal powder bed  1114  containing the three-dimensional composition  1102  can be moved along the conveyor  1304  and into the post-processing station  1306 . The conveyor  1304  can be, for example, a belt conveyor movable in a direction from the additive manufacturing system  1100  toward the post-processing station. Additionally, or alternatively, the conveyor  1304  can include a cart on which the powder bed  1114  is mounted and, in certain instances, the powder bed  1114  can be moved from the additive manufacturing system  1100  to the post-processing station  1306  through movement of the cart (e.g., through the use of actuators to move the cart along rails or by an operator pushing the cart). 
     In the post-processing station  1306  shown in  FIG. 8 , the three-dimensional composition  1102  can be heated in the metal powder bed  1114  to remove at least some of the liquid of the binder composition in the three-dimensional composition and to form a metal-based composite structure (e.g., a self-supporting metal-based composite structure) within the metal powder bed. The metal-based composite structure can be removed from the metal powder bed  1114 . According to some aspects, the binder compositions described herein may aid in attaining a desired mechanical strength characteristic of the metal-based composite structure, thereby allowing for improved ability to handle the metal-based composite structure and improved consistency in metal objects formed from such metal-based composite structures. The metal powder  1104  remaining in the metal powder bed  1114  upon removal of the metal-based composite structure can be, for example, recycled for use in subsequent fabrication of additional parts. Additionally, or alternatively, in the post-processing station  1306 , the metal-based composite structure can be cleaned (e.g., through the use of pressurized air) of excess amounts of the metal powder  1104 . 
     In systems employing a binder jetting process, a metal-based composite structure can undergo one or more de-binding processes (e.g., step  70  of  FIGS. 1A-1B ) in the post-processing station  1306  to remove all or a portion of a polymer of the binder composition from the metal-based composite structure  1102 . One example of a suitable de-binding process is a thermal de-binding process (e.g., heating as described elsewhere herein, particularly in the context of noble metals and/or copper). 
     The post-processing station  1306  shown in  FIG. 8  can include oven  1308 . The metal-based composite structure can undergo de-binding in oven  1308 . In some embodiments, the de-bound metal structure can undergo sintering (e.g., step  80  of  FIG. 1B ) in oven  1308  such that the metal particles of the powder  1106  melt (e.g., to an extent not overall undesirable) and combine with one another to form a metal object. 
     As described above, in some embodiments, a binder composition (and/or one or more components thereof) is configured to form one or more of the articles described herein (e.g., a three-dimensional composition) in combination with a metal powder (e.g., a noble metal powder and/or copper powder) described herein. In some embodiments, one or more of the articles described herein (e.g., a de-bound metal composition or a metal object) may be formed from a binder composition described herein. Such articles may comprise the binder composition, may comprise some components of the binder composition but lack other components of the binder composition (e.g., may comprise a polymer present in the binder composition but lack a solvent present in the binder composition), or may not include any components of the binder composition. In some embodiments, an article described herein comprises a reaction product of a binder composition (e.g., a polymer present in the binder composition that has been cross-linked, such as by a cross-linking agent present in the binder composition; a thermal decomposition product of a component of the binder composition, such as char). 
     As also described above, some binder compositions described herein may have one or more physical properties that enhances their suitability for use in one or more of the methods described herein, such as one or more of the methods for additive manufacturing described herein, and/or in one or more of the articles described herein, such as a three-dimensional object, a metal-based composite structure, a de-bound metal structure, and/or a metal object. Further details regarding some such physical properties is provided below. 
     The binder compositions described herein may have an advantageous surface tension. Without wishing to be bound by any particular theory, it is believed that the surface tension of the binder composition may affect the surface finish of a final product of the additive manufacturing process (e.g., the completed metal object). As one example, a binder composition having a relatively high surface tension may contribute, at least in part, to a final product (e.g., metal object) have a relatively low surface roughness. A low surface roughness may be desirable in certain applications of noble metal products, such as jewelry. In some embodiments, a binder composition has a surface tension of greater than or equal to 18 dynes/cm, greater than or equal to 20 dynes/cm, greater than or equal to 22.5 dynes/cm, greater than or equal to 25 dynes/cm, greater than or equal to 28 dynes/cm, greater than or equal to 30 dynes/cm, greater than or equal to 32.5 dynes/cm, greater than or equal to 35 dynes/cm, greater than or equal to 40 dynes/cm, greater than or equal to 45 dynes/cm, greater than or equal to 50 dynes/cm, greater than or equal to 55 dynes/cm, greater than or equal to 60 dynes/cm, or greater than or equal to 65 dynes/cm at a printing temperature. In some embodiments, a binder composition has a surface tension of less than or equal to 75 dynes/cm, less than or equal to 70 dynes/cm, less than or equal to 65 dynes/cm, less than or equal to 60 dynes/cm, less than or equal to 55 dynes/cm, less than or equal to 50 dynes/cm, less than or equal to 45 dynes/cm, less than or equal to 40 dynes/cm, less than or equal to 35 dynes/cm, less than or equal to 32.5 dynes/cm, less than or equal to 30 dynes/cm, less than or equal to 28 dynes/cm, less than or equal to 25 dynes/cm, less than or equal to 22.5 dynes/cm, or less than or equal to 20 dynes/cm at a printing temperature. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 18 dynes/cm and less than or equal to 75 dynes/cm, or greater than or equal to 30 dynes/cm and less than or equal to 75 dynes/cm). Other ranges are also possible. The surface tension of a binder composition may be measured in accordance with ASTM D1331-14. 
     In some embodiments, a binder composition described herein may have an advantageous viscosity. Without wishing to be bound by any particular theory, it is believed that the viscosity of the binder composition may affect its ability to be printed by a particular print head. For instance, some print heads may be designed to print binder compositions having a certain range of viscosities and may be unable to print compositions having viscosities outside of this range in a manner that is reliable and/or desirable. By way of example, binder compositions having viscosities above the range for which the print head is configured may not flow or may not flow appreciably at the pressures provided by the print head. As another example, binder compositions having viscosities below the range for which the print head is configured may flow in undesirable manners at the pressures provided by the print head (e.g., flow in a manner that produces droplets that are coalesced, take the form a mist, and/or misdirected), resulting in poor control over the deposition of the binder composition from the print head. 
     In some embodiments, a binder composition has a viscosity at a printing temperature of greater than or equal to 0.55 cP, greater than or equal to 1 cP, greater than or equal to 1.5 cP, greater than or equal to 2 cP, greater than or equal to 2.5 cP, greater than or equal to 3 cP, greater than or equal to 3.5 cP, greater than or equal to 4 cP, greater than or equal to 5 cP, greater than or equal to 6 cP, greater than or equal to 7 cP, greater than or equal to 8 cP, greater than or equal to 10 cP, greater than or equal to 12.5 cP, greater than or equal to 15 cP, greater than or equal to 17.5 cP, greater than or equal to 20 cP, greater than or equal to 22.5 cP, greater than or equal to 25 cP, or greater than or equal to 27.5 cP. In some embodiments, a binder composition has a viscosity at a printing temperature of less than or equal to 30 cP, less than or equal to 27.5 cP, less than or equal to 25 cP, less than or equal to 22.5 cP, less than or equal to 20 cP, less than or equal to 17.5 cP, less than or equal to 15 cP, less than or equal to 12.5 cP, less than or equal to 10 cP, less than or equal to 8 cP, less than or equal to 7 cP, less than or equal to 6 cP, less than or equal to 5 cP, less than or equal to 4 cP, less than or equal to 3.5 cP, less than or equal to 3 cP, less than or equal to 2.5 cP, less than or equal to 2 cP, less than or equal to 1.5 cP, or less than or equal to 1 cP. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 0.55 cP and less than or equal to 30 cP, greater than or equal to 1 cP and less than or equal to 10 cP, greater than or equal to 3 cP and less than or equal to 30 cP, greater than or equal to 3.5 cP and less than or equal to 10 cP, greater than or equal to 3.5 cP and less than or equal to 7 cP or greater than or equal to 4 cP and less than or equal to 6 cP). Other ranges are also possible. The viscosity of the binder composition may be determined by use of a cone and plate rheometer operated at a shear rate of 300 s −1 . 
     The viscosities described above may be desirable for use with particular print heads of interest (e.g., piezoelectric print heads, thermal print heads, print heads suitable for ink jet printing). By way of example, in some embodiments, it may be desirable for a binder composition configured to be deposited thermally (e.g., by a thermal print head) to have a viscosity of greater than or equal to 1 cP and less than or equal to 10 cP at the printing temperature. As another example, in some embodiments, it may be desirable for a binder composition configured to be deposited piezoelectrically (e.g., by a piezoelectric print head) to have a viscosity of greater than or equal to 3 cP and less than or equal to 30 cP at the printing temperature. 
     The printing temperature may be a temperature at which the binder composition is ejected from a print head (e.g., by an additive manufacturing process, by a binder jetting process). In some embodiments, the printing temperature is greater than or equal to 20° C., greater than or equal to 25° C., greater than or equal to 30° C., greater than or equal to 35° C., or greater than or equal to 40° C. In some embodiments, the printing temperature is less than or equal to 45° C., less than or equal to 40° C., less than or equal to 35° C., less than or equal to 30° C., or less than or equal to 20° C. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 20° C. and less than or equal to 40° C.). Other ranges are also possible. 
     As described above, some binder compositions have pHs that are non-corrosive to one or more articles with which the binder composition is configured to contact during formation of a metal-based composite structure. As also described above, these components may include portions of a printer, such as a print head, and/or components to be incorporated into a metal-based composite structure, such as a metal powder. 
     In some embodiments, a binder composition has a pH of greater than or equal to 1.5, greater than or equal to 2, greater than or equal to 2.5, greater than or equal to 3, greater than or equal to 3.5, greater than or equal to 4, greater than or equal to 4.5, greater than or equal to 5, greater than or equal to 5.5, greater than or equal to 6, greater than or equal to 6.5, greater than or equal to 7, greater than or equal to 7.5, greater than or equal to 8, greater than or equal to 8.5, greater than or equal to 9 greater than or equal to 9.5, greater than or equal to 10, or greater than or equal to 10.5. In some embodiments, a binder composition has a pH of less than or equal to 11, less than or equal to 10.5, less than or equal to 10, less than or equal to 9.5, less than or equal to 9, less than or equal to 8.5, less than or equal to 8, less than or equal to 7.5, less than or equal to 7, less than or equal to 6.5, less than or equal to 6, less than or equal to 5.5, or less than or equal to 5. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 2 and less than or equal to 11, greater than or equal to 5 and less than or equal to 8, greater than or equal to 7 and less than or equal to 11, or greater than or equal to 7 and less than or equal to 9). Other ranges are also possible. The pH of a binder composition may be measured with a pH meter. 
     As described above, in some embodiments, a binder composition as a whole may comprise a combination of advantageous components. Further details regarding such components are provided below. 
     In some embodiments, a binder composition comprises one or more polymers. One example of a suitable type of polymer that may be included in a binder composition is a polymer including an acrylic acid repeat unit, the chemical structure of which is shown below: 
     
       
         
         
             
             
         
       
     
     Advantageously, the acrylic acid repeat unit may be esterified as part of a cross-linking reaction. The acrylic acid repeat unit may be an acidic repeat unit (e.g., comprising an acidic hydrogen) or may be a repeat unit in a deprotonated state (e.g., it may be deprotonated and negatively charged, it may be associated with a counter ion). Without wishing to be bound by any particular theory, in some embodiments, a polymer including at least some acrylic acid repeat units that are in a deprotonated state (e.g., exclusively acrylic acid repeat units that have been deprotonated, both acrylic acid repeat units that have been deprotonated and those that are acidic) may be beneficially hygroscopic and/or soluble. In at least some instances, this is believed to reduce the latency of the binder formulation and enhance its decap performance. However, in some embodiments, most (e.g., greater than or equal to 90 mol %, greater than or equal to 99 mol %, greater than or equal to 99.9 mol %, or more) or all of the repeat units are acidic repeat units (i.e., in a protonated state). 
     Polymers including acrylic acid repeat units may further comprise other, different repeat units. By way of example, a binder composition may comprise a polymer comprising an acrylic acid repeat unit and comprising further, non-acrylic acid, repeat units (e.g., styrene monomers, maleic anhydride repeat units). The repeat units in a polymer (e.g., acrylic acid repeat units, others) may be distributed within the polymer in a variety of suitable manners (e.g., randomly, statistically, alternatingly, in one or more blocks, etc.). In some embodiments, a polymer may comprise acrylic acid repeat units and may comprise one or more end groups lacking an acrylic acid functional group (e.g., one or more initiators, one or more RAFT agents). It is also possible for the polymer to be a homopolymer comprising acrylic acid repeat units. By way of example, a binder composition may comprise poly(acrylic acid). 
     When a polymer comprising an acrylic acid repeat unit further comprises other repeat units, the acrylic acid repeat unit may make up a variety of suitable amounts of the polymer. 
     In some embodiments, a polymer comprises an acrylic acid repeat unit that makes up greater than or equal to 30 mol %, greater than or equal to 40 mol %, greater than or equal to 50 mol %, greater than or equal to 60 mol %, greater than or equal to 70 mol %, greater than or equal to 80 mol %, or greater than or equal to 90 mol % of the repeat units within the polymer. In some embodiments, a polymer comprises an acrylic acid repeat unit make up less than or equal to 100 mol %, less than or equal to 90 mol %, less than or equal to 80 mol %, less than or equal to 70 mol %, less than or equal to 60 mol %, less than or equal to 50 mol %, or less than or equal to 40 mol % of the repeat units within the polymer. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 30 mol % and less than or equal to 100 mol %). Other ranges are also possible. 
     Further examples of suitable polymers for use in binder compositions include synthetic polymers and natural polymers. Non-limiting examples of suitable synthetic polymers include poly(ethylenimine)s (which may be a suitable cross-linking agent as described elsewhere herein), poly(vinyl alcohol), and poly(vinyl pyrrolidone). Non-limiting examples of suitable natural polymers include chitosan, gelatin, starches, and sugars. While in certain instances these further examples may be used instead of a low molecular weight polymer including an acrylic acid repeat unit, in certain other instances these further examples may be included in the binder composition in addition to the low molecular weight polymer comprising the acrylic acid repeat unit. 
     In some embodiments, a binder composition comprises a low molecular weight polymer (e.g., a low molecular weight polymer including an acrylic acid repeat unit). Advantageously low molecular weight polymers may increase the viscosity of binder compositions to a smaller extent than otherwise equivalent polymers having a higher molecular weight. This may desirably allow for binder compositions to be formulated that both have a beneficial viscosity and include a larger wt % of polymer. Increased amounts of polymer in the binder composition are believed to enhance the transverse flexural strength of articles formed therefrom (e.g., metal-based composite structures). Lower molecular weight polymers are also believed to be beneficial because it is believed that they are more soluble in many binder formulations than otherwise equivalent polymers having a higher molecular weight. This enhanced solubility is believed to result in improved binder jetting performance (e.g., improved latency and/or decap performance). These possible advantages in strength and processability due to the use of low molecular weight polymers may result in good resolution during manufacturing. 
     A binder composition described herein may comprise a polymer (e.g., a polymer including an acrylic acid repeat unit) having a weight average molecular weight of less than or equal to 40 kDa, less than or equal to 35 kDa, less than or equal to 30 kDa, less than or equal to 25 kDa, less than or equal to 20 kDa, less than or equal to 15 kDa, less than or equal to 10 kDa, less than or equal to 7.5 kDa, less than or equal to 6 kDa, less than or equal to 5 kDa, less than or equal to 4 kDa, less than or equal to 3 kDa, less than or equal to 2 kDa, or less than or equal to 1 kDa. In some embodiments, the binder composition comprises a polymer (e.g., a polymer including an acrylic acid repeat unit) having a weight average molecular weight of greater than or equal to 500 Da, greater than or equal to 1 kDa, greater than or equal to 2 kDa, greater than or equal to 3 kDa, greater than or equal to 4 kDa, greater than or equal to 5 kDa, greater than or equal to 6 kDa, greater than or equal to 7.5 kDa, greater than or equal to 10 kDa, greater than or equal to 15 kDa, greater than or equal to 20 kDa, greater than or equal to 30 kDa, or greater than or equal to 35 kDa. In some embodiments, a binder composition comprises a polymer having a weight average molecular weight of Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 40 kDa and greater than or equal to 500 Da, less than or equal to 40 kDa and greater than or equal to 2 kDa, less than or equal to 6 kDa and greater than or equal to 2 kDa, or less than or equal to 5 kDa and greater than or equal to 1.5 kDa). Other ranges are also possible. The weight average molecular weight may be determined by GPC. 
     In some instances, a binder composition has a single type of polymer (e.g., a single type of polymer including an acrylic acid repeat unit), while in other instances a binder composition comprises two or more types of polymer. It should also be understood that a single type of polymer present in a binder composition (e.g., a polymer including an acrylic acid repeat unit, a polymer lacking an acrylic acid repeat unit) may have a weight average molecular weight in one or more of the above-referenced ranges (e.g., in a binder composition that further comprises other, different types of polymers or that lacks other, different polymers) and/or the binder composition may comprise more than one type of polymer and all of the types of polymers together may have a weight average molecular weight in one or more of the above-referenced ranges. 
     In some embodiments, a binder composition comprises both a low molecular weight polymer (e.g., a polymer comprising an acrylic acid repeat unit) and further comprises a non-low molecular weight polymer. The non-low molecular weight polymer may have a molecular weight outside of one or more of the ranges described above (e.g., of greater than 40 kDa). In some embodiments, the non-low molecular weight polymer comprises a polymer lacking an acrylic acid repeat unit (e.g., a binder composition may comprise synthetic polymer and/or a natural polymer as described above that is also a non-low molecular weight polymer). 
     Polymers suitable for use in the binder compositions described herein may be present in the binder compositions in a variety of suitable amounts. In some embodiments, a binder composition comprises a polymer or combination of polymers that make up greater than or equal to 1 wt %, greater than or equal to 2 wt %, greater than or equal to 3 wt %, greater than or equal to 4 wt %, greater than or equal to 5 wt %, greater than or equal to 7.5 wt %, greater than or equal to 10 wt %, greater than or equal to 15 wt %, greater than or equal to 20 wt %, greater than or equal to 25 wt %, greater than or equal to 30 wt %, or greater than or equal to 35 wt % of the binder composition. In some embodiments, a binder composition comprises a polymer or combination of polymers that make up less than or equal to 40 wt %, less than or equal to 35 wt %, less than or equal to 30 wt %, less than or equal to 25 wt %, less than or equal to 20 wt %, less than or equal to 15 wt %, less than or equal to 10 wt %, less than or equal to 7.5 wt %, less than or equal to 5 wt %, less than or equal to 4 wt %, less than or equal to 3 wt %, or less than or equal to 1 wt % of the binder composition. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 1 wt % and less than or equal to 40 wt %, greater than or equal to 1 wt % and less than or equal to 30 wt %, greater than or equal to 1 wt % and less than or equal to 25 wt %, greater than or equal to 2 wt % and less than or equal to 30 wt %, or greater than or equal to 3 wt % and less than or equal to 40 wt %). Other ranges are also possible. It should also be understood that a single polymer may be present in a binder composition in one or more of the above-referenced ranges (e.g., that further comprises other, different polymers or that lacks other, different polymers) and/or the total amount of polymer in a binder composition may be in one or more of the above-referenced ranges. 
     In some embodiments, a binder composition comprises one or more cross-linking agents. The cross-linking agent(s) may be configured to cross-link one or more polymers also present in the binder composition. This may advantageously cause the polymer(s) to form a cross-linked network with enhanced cohesive strength, which may enhance the mechanical properties (e.g., flexural strength) of a metal-based composite structure formed from the binder composition. 
     When present, a cross-linking agent may be configured to cross-link one or more polymers in a variety of suitable manners. For instance, in some embodiments, a cross-linking agent is configured to undergo a reaction with one or more functional groups on a polymer. By way of example, a cross-linking agent may comprise a nucleophilic functional group that is configured to undergo a reaction with an electrophilic functional group included in the polymer (e.g., an acrylic acid repeat unit). Non-limiting examples of suitable nucleophilic functional groups include alcohol-based functional groups (e.g., hydroxyl functional groups), amino functional groups, thiol functional groups, and epoxide functional groups. Non-limiting examples of suitable cross-linking agents comprising such functional groups include poly(ethylenimine)s, polyols (e.g., aliphatic diols such as 1,2-hexanediol, 1,3-hexanediol, 1,2-butanediol, 1,4-cyclohexane diol, ethylene diol, diethylene glycol, and/or propylene glycol; aliphatic triols such as glycerol; aliphatic tetrols, such as erythritol), aliphatic poly(ether)s (e.g., poly(propylene glycol), poly(ethylene glycol)), multifunctional amines (e.g., diethanolamine, triethanolamine, polyetheramines), multifunctional thiols, and metallic cross-linking agents (e.g., zirconium carbonate, titanium isopropoxides, aluminum isopropoxides). In some embodiments, in which the metal powder comprises a noble metal powder, the cross-linking agent comprises a polyol. In some such cases, the polyol comprises glycerol and/or 1,2-hexanediol. In some embodiments, a cross-linking agent in a binder composition may also be able to serve other roles besides facilitating cross-linking. For example, a cross-linking agent may also act as humectant. In certain cases, the cross-linking agent is a co-solvent in the binder composition. One-non-limiting example of a cross-linking agent that can be a humectant and a co-solvent is a polyol such as 1,2-hexanediol. 
     The cross-linking agents described herein typically comprise at least two portions configured to undergo a reaction with a polymer (e.g., at least two nucleophilic functional groups). In other words, the cross-linking agents described herein are typically at least difunctional. In some embodiments, a binder composition comprises a cross-linking agent that is difunctional, trifunctional, tetrafunctional, or having a higher level of functionality. 
     Cross-linking agents suitable for use in the binder compositions described herein may have a variety of suitable molecular weights. In some embodiments, a binder composition comprises a cross-linking agent having a weight average molecular weight of less than or equal to 2 kDa, less than or equal to 1 kDa, less than or equal to 750 Da, less than or equal to 500 Da, less than or equal to 200 Da, less than or equal to 100 Da, or less than or equal to 75 Da. In some embodiments, a binder composition comprises a cross-linking agent having a weight average molecular weight of greater than or equal to 62 Da, greater than or equal to 75 Da, greater than or equal to 100 Da, greater than or equal to 200 Da, greater than or equal to 500 Da, greater than or equal to 750 Da, or greater than or equal to 1 kDa. Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 2 kDa and greater than or equal to 62 Da). Other ranges are also possible. The weight average molecular weight may be determined by GPC. 
     In some instances, a binder composition has a single type of cross-linking agent, while in other instances a binder composition comprises two or more types of cross-linking agent. It should also be understood that a single type of cross-linking agent present in a binder composition may have a weight average molecular weight in one or more of the above-referenced ranges (e.g., in a binder composition that further comprises other, different types of cross-linking agents or that lacks other, different cross-linking agents) and/or the binder composition may comprise more than one type of cross-linking agents and all of the types of cross-linking agents together may have a weight average molecular weight in one or more of the above-referenced ranges. 
     Cross-linking agents suitable for use in the binder compositions described herein may be present in the binder compositions in a variety of suitable amounts. In some embodiments, a binder composition comprises a cross-linking agent or combination of cross-linking agents that make up greater than or equal to 0 wt %, greater than or equal to 0.1 wt %, greater than or equal to 0.2 wt %, greater than or equal to 0.3 wt %, greater than or equal to 0.4 wt %, greater than or equal to 0.5 wt %, greater than or equal to 0.75 wt %, greater than or equal to 1 wt %, greater than or equal to 1.5 wt %, greater than or equal to 2 wt %, greater than or equal to 5 wt %, or greater than or equal to 7.5 wt % of the binder composition. In some embodiments, a binder composition comprises cross-linking agent or combination of cross-linking agents that make up less than or equal to 10 wt %, less than or equal to 7.5 wt %, less than or equal to 5 wt %, less than or equal to 2 wt %, less than or equal to 1.5 wt %, less than or equal to 1 wt %, less than or equal to 0.75 wt %, less than or equal to 0.5 wt %, less than or equal to 0.4 wt %, less than or equal to 0.3 wt %, less than or equal to 0.2 wt %, or less than or equal to 0.1 wt % of the binder composition. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 0 wt % and less than or equal to 10 wt %, greater than or equal to 0.1 wt % and less than or equal to 10 wt %, or greater than or equal to 0.2 wt % and less than or equal to 10 wt %). Other ranges are also possible. It should also be understood that a single cross-linking agent may be present in a binder composition in one or more of the above-referenced ranges (e.g., that further comprises other, different cross-linking agents or that lacks other, different cross-linking agents) and/or the total amount of cross-linking agent in a binder composition may be in one or more of the above-referenced ranges. 
     In some embodiments, a binder composition comprises a solvent. The solvent may solvate the other components therein (e.g., one or more polymers therein, one or more optional pH modifiers therein, one or more surfactants therein, one or more biocides therein, one or more humectants therein, one or more adhesion promoters therein, one or more cross-linking agents therein). In some embodiments, the solvent is a liquid and/or the binder composition is a liquid solution. 
     In some embodiments, a binder composition comprises a solvent comprising water. 
     In other words, a binder composition may comprise an aqueous solvent and/or an aqueous solution. Without wishing to be bound by any particular theory, it is believed that aqueous solvents may be desirable for use in binder compositions because they may be more environmentally friendly and/or less toxic than other types of solvents (e.g., than organic solvents). It is also believed that, when water is a reaction product of a cross-linking reaction that one or more components of the binder composition are configured to undergo, the presence of water in the binder composition may advantageously suppress cross-linking at points in time before it is desired. Then, the water may be removed and cross-linking may be performed. 
     Solvents suitable for use in the binder compositions described herein may be present in the binder compositions in a variety of suitable amounts. In some embodiments, a binder composition comprises a solvent that makes up greater than or equal to 50 wt %, greater than or equal to 55 wt %, greater than or equal to 60 wt %, greater than or equal to 65 wt %, greater than or equal to 70 wt %, greater than or equal to 75 wt %, greater than or equal to 80 wt %, greater than or equal to 85 wt %, greater than or equal to 90 wt %, or greater than or equal to 95 wt % of the binder composition. In some embodiments, a binder composition comprises a solvent that makes up less than or equal to 99 wt %, less than or equal to 95 wt %, less than or equal to 90 wt %, less than or equal to 85 wt %, less than or equal to 80 wt %, less than or equal to 75 wt %, less than or equal to 70 wt %, less than or equal to 65 wt %, less than or equal to 60 wt %, or less than or equal to 55 wt % of the binder composition. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 50 wt % and less than or equal to 99 wt %, greater than or equal to 75% and less than or equal to 85%). Other ranges are also possible. 
     In some cases, water present in the binder composition (e.g., as at least a part of the solvent) makes up greater than or equal to greater than or equal to 50 wt %, greater than or equal to 55 wt %, greater than or equal to 60 wt %, greater than or equal to 65 wt %, greater than or equal to 70 wt %, greater than or equal to 75 wt %, greater than or equal to 80 wt %, greater than or equal to 85 wt %, greater than or equal to 90 wt %, or greater than or equal to 95 wt % of the binder composition. In some embodiments, water present in the binder composition makes up less than or equal to 99 wt %, less than or equal to 95 wt %, less than or equal to 90 wt %, less than or equal to 85 wt %, less than or equal to 80 wt %, less than or equal to 75 wt %, less than or equal to 70 wt %, less than or equal to 65 wt %, less than or equal to 60 wt %, or less than or equal to 55 wt % of the binder composition. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 50 wt % and less than or equal to 99 wt %, greater than or equal to 75% and less than or equal to 85%). Other ranges are also possible. 
     Binder compositions comprising a solvent may also comprise one or more co-solvents. The co-solvent(s) may enhance the solubility of one or more components of the binder composition in the solvent (e.g., it may enhance the solubility of one or more polymers, one or more pH modifiers, one or more surfactants, one or more biocides, one or more humectants, one or more adhesion promoters, and/or one or more cross-linking agents in the binder composition). The co-solvent(s) may be liquid(s). 
     Non-limiting examples of suitable co-solvents include solvents that are miscible with the solvent and readily solubilize one or more components of the binder composition. For instance, when the solvent comprises water and the binder composition comprises one or more organic components, one or more water-soluble organic solvents may be suitable for use as co-solvents. Non-limiting examples of suitable co-solvents include alcohols (e.g., monofunctional alcohols, diols, triols), ketones (e.g., acetone, diacetone, butanone), esters (e.g., ethyl acetate), ethers, lactones (e.g., hydroxybutyrolactone), lactams, pyrrolidones (e.g., N-methyl pyrrolidone, N-phenyl pyrrolidone, 2-pyrrolidone), amides (e.g., dimethyl acetamide), nitriles (e.g., acetonitrile), sulfones (e.g., dimethyl sulfone), and sulfoxides (e.g., dimethyl sulfoxide), and combinations thereof. Further examples of alcohols include methanol, ethanol, isopropanol, 1-butanol, 2-butanol, 1,2-hexanediol, ethylene glycol, propylene glycol, 1-(1-hydroxypropoxy)propan-1-ol, 1-(2-hydroxypropoxy)propan-2-ol, 3,3′-oxybis(propan-1-ol), and dipropylene glycol (a mixture of isomers 1-(1-hydroxypropoxy)propan-1-ol, 1-(2-hydroxypropoxy)propan-2-ol, and 3,3′-oxybis(propan-1-ol)). 
     Co-solvents suitable for use in the binder compositions described herein may be present in the binder compositions in a variety of suitable amounts. In some embodiments, a binder composition comprises a co-solvent or combination of co-solvents that make up greater than or equal to 0 wt %, greater than or equal to 1 wt %, greater than or equal to 2 wt %, greater than or equal to 3 wt %, greater than or equal to 4 wt %, greater than or equal to 5 wt %, greater than or equal to 7.5 wt %, greater than or equal to 10 wt %, greater than or equal to 15 wt %, greater than or equal to 20 wt %, or greater than or equal to 25 wt % of the binder composition. In some embodiments, a binder composition comprises a co-solvent or combination of co-solvents that make up less than or equal to 30 wt %, less than or equal to 25 wt %, less than or equal to 20 wt %, less than or equal to 15 wt %, less than or equal to 10 wt %, less than or equal to 7.5 wt %, less than or equal to 5 wt %, less than or equal to 4 wt %, less than or equal to 3 wt %, less than or equal to 2 wt %, or less than or equal to 1 wt % of the binder composition. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 0 wt % and less than or equal to 30 wt %, or greater than or equal to 0 wt % and less than or equal to 5 wt %). Other ranges are also possible. It should also be understood that a single co-solvent may be present in a binder composition in one or more of the above-referenced ranges (e.g., that further comprises other, different co-solvents or that lacks other, different co-solvents) and/or the total amount of co-solvent in a binder composition may be in one or more of the above-referenced ranges. 
     In some, but not necessarily all embodiments, a binder composition comprises one or more pH modifiers. The optional pH modifier(s) may be configured to interact with the other components of the binder composition to adjust the pH to a desired value. For instance, the pH modifier may be configured to interact with the other components of the binder composition to adjust the pH to a value that will not appreciably corrode one or more articles with which the binder composition is configured to contact during formation of a composite structure. It should be understood that in some instances, the binder composition does not comprise a pH modifier. 
     The binder compositions described herein may comprise pH modifier(s) that are basic. A variety of types of bases may be suitable for use as pH modifiers. For instance, examples of suitable pH modifiers include Arrhenius bases, Lewis bases, and Bronsted-Lowry bases. As another example, a binder composition may comprise an inorganic base and/or an organic base. Non-limiting examples of suitable pH modifiers include hydroxides (e.g., sodium hydroxide, potassium hydroxide, lithium hydroxide), carbonates (e.g., sodium carbonate, potassium carbonate, lithium carbonate), ammonia, tetramethyl ammonium hydroxide, and amine-containing species (e.g., organic amine-containing species that may be aliphatic or aromatic, such as methyl amine, ethyl amine, triethyl amine, ethanolamine, triethanolamine, diaminobenzene), and amine derivatives (e.g., pyridine, imidazole). 
     The pH modifiers described herein may be selected such that the pH of the binder composition is in a desirable range. In some embodiments, a pH modifier is selected such that the pH of the binder composition is greater than or equal to 2, greater than or equal to 2.5, greater than or equal to 3, greater than or equal to 3.5, greater than or equal to 4, greater than or equal to 4.5, greater than or equal to 5, greater than or equal to 5.5, greater than or equal to 6, greater than or equal to 6.5, greater than or equal to 7, greater than or equal to 7.5, greater than or equal to 8, greater than or equal to 8.5, greater than or equal to 9 greater than or equal to 9.5, greater than or equal to 10, or greater than or equal to 10.5. In some embodiments, a binder composition has a pH of less than or equal to 11, less than or equal to 10.5, less than or equal to 10, less than or equal to 9.5, less than or equal to 9, less than or equal to 8.5, less than or equal to 8, less than or equal to 7.5, less than or equal to 7, less than or equal to 6.5, less than or equal to 6, less than or equal to 5.5, less than or equal to 5, less than or equal to 4.5, less than or equal to 4, less than or equal to 3.5, or less than or equal to 3. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 2 and less than or equal to 11, greater than or equal to 5 and less than or equal to 8, greater than or equal to 7 and less than or equal to 11, or greater than or equal to 7 and less than or equal to 9). Other ranges are also possible. 
     The pH modifiers described herein may have a variety of suitable boiling points. In some embodiments, it may be advantageous for a pH modifier to have a relatively low boiling point, as this may facilitate removal of the pH modifier from the binder composition (or from a three dimensional composition and/or metal-based composite structure fabricated from a binder composition) at relatively low temperatures. This property may be advantageous when it is desirable to remove the pH modifier from a three-dimensional composition and/or metal-based composite structure into which it has been incorporated during one of the steps described herein (e.g., a drying and/or heating step) and/or prior to performing some of the steps described herein (e.g., a further heating step). As one specific example, it may be advantageous to remove the pH modifier from binder composition prior to cross-linking, as the pH modifier may interfere with the cross-linking reaction in certain cases. The inclusion of such a pH modifier in the binder composition may desirably suppress cross-linking at points in time before it is desired. In other embodiments, a pH modifier may also function as a cross-linking agent. 
     In some embodiments, a binder composition comprises a pH modifier having a boiling point of less than or equal to 150° C., less than or equal to 125° C., less than or equal to 100° C., less than or equal to 75° C., less than or equal to 50° C., less than or equal to 25° C., less than or equal to 0° C., or less than or equal to −25° C. In some embodiments, a binder composition comprises a pH modifier having a boiling point of greater than or equal to −40° C., greater than or equal to −25° C., greater than or equal to 0° C., greater than or equal to 25° C., greater than or equal to 50° C., greater than or equal to 75° C., greater than or equal to 100° C., or greater than or equal to 125° C. Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 150° C. and greater than or equal to −40° C.). Other ranges are also possible. 
     In some embodiments, a binder composition comprises one or more surfactant(s). The surfactant(s) may increase the penetration of the binder composition into a composite layer and/or may enhance the jetting performance of the binder composition. It is also believed that the surfactants may increase the amount of spreading of the binder composition in a powder layer. In some embodiments, the surfactant(s) may reduce the level of foaming in the binder composition during one or more processes associated with additive manufacturing (e.g., transport to and/or deposition by a print head) and/or may enhance the rate at which the binder composition can refill a print head (e.g., a thermal print head). 
     Some binder compositions may comprise ionic surfactants and some binder compositions may comprise non-ionic surfactants. Non-limiting examples of suitable ionic surfactants include sulfates (e.g., ammonium lauryl sulfate, sodium lauryl sulfate, sodium laureth sulfate, sodium myreth sulfate, perfluorooctanesulfonate, perfluorobutanesulfonate), sulfosuccinates (e.g., dioctyl sodium sulfosuccinate), ethers (e.g., alkyl-aryl ether phosphates, alkyl ether phosphates), sodium stearate, sodium lauroyl sarcosinate, perfluorononanoate, and perfluorooctanoate. Non-limiting examples of suitable non-ionic surfactants include Surfynol 440, Surfynol 2502, Surfynol 604, Thetawet TS 8230, Thetawet FS-8150, polyoxyl 35 castor oil, lauryldimethylamine oxide, Triton X-100, and Dynol 604. 
     In some embodiments, a binder composition comprises a surfactant or combination of surfactants that make up greater than or equal to 0 wt %, greater than or equal to 0.01 wt %, greater than or equal to 0.02 wt %, greater than or equal to 0.05 wt %, greater than or equal to 0.075 wt %, greater than or equal to 0.1 wt %, greater than or equal to 0.2 wt %, greater than or equal to 0.3 wt %, greater than or equal to 0.4 wt %, greater than or equal to 0.5 wt %, greater than or equal to 0.6 wt %, greater than or equal to 0.7 wt %, greater than or equal to 0.8 wt %, or greater than or equal to 0.9 wt % of the binder composition. In some embodiments, a binder composition comprises a surfactant or combination of surfactants that make up less than or equal to 1 wt %, less than or equal to 0.9 wt %, less than or equal to 0.8 wt %, less than or equal to 0.7 wt %, less than or equal to 0.6 wt %, less than or equal to 0.5 wt %, less than or equal to 0.4 wt %, less than or equal to 0.3 wt %, less than or equal to 0.2 wt %, less than or equal to 0.1 wt %, less than or equal to 0.075 wt %, less than or equal to 0.05 wt %, less than or equal to 0.02 wt %, or less than or equal to 0.01 wt % of the binder composition. In some cases, however, no surfactant is present in the binder composition. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 0 wt % and less than or equal to 1 wt %, greater than or equal to 0.01 wt % and less than or equal to 1 wt %, greater than or equal to 0.01 wt % and less than or equal to 0.05 wt %, greater than or equal to 0.1 wt % and less than or equal to 1 wt %, greater than or equal to 0.1 wt % and less than or equal to 0.5 wt %, or greater than or equal to 0.5 wt % and less than or equal to 1 wt %). Other ranges are also possible. It should also be understood that a single surfactant may be present in a binder composition in one or more of the above-referenced ranges (e.g., that further comprises other, different surfactants or that lacks other, different surfactants) and/or the total amount of surfactant in a binder composition may be in one or more of the above-referenced ranges. 
     In some embodiments, a binder composition comprises one or more biocide(s). The biocide(s) may inhibit the growth of biological species (e.g., bacteria, yeast, fungi) in the binder composition during storage and/or inhibit enzymatic degradation of a polymer in the binder composition during storage. Without wishing to be bound by any particular theory, it is believed that biologically-derived polymers, such as chitosan and gelatin, may be particularly susceptible to such degradation. 
     Binder compositions described herein may comprise one or more biocides that are microbicides and/or one or more biocides that are fungicides. In some embodiments, a binder composition comprises a biocide that is an isothiazolinone, such as ProxelGXL, 1,2-benzisothiazolin-3-one, 4,5-dichloro-2-octyl-4-isothiazolin-3-one, and 2-n-octyl-4-isothiazolin-3-one. Further examples of suitable biocides include 3-(3,4-dichlorophenyl)-1,1-dimethylurea, 2-bromo-2-nitropropane-1,3-diol, lauryldimethylamine oxide, benzalkonium chloride, and/or rotenone. 
     In some embodiments, a binder composition comprises a biocide or combination of biocides that make up greater than or equal to 0 wt %, greater than or equal to 0.01 wt %, greater than or equal to 0.02 wt %, greater than or equal to 0.05 wt %, greater than or equal to 0.075 wt %, greater than or equal to 0.1 wt %, greater than or equal to 0.15 wt %, or greater than or equal to 0.2 wt % of the binder composition. In some embodiments, a binder composition comprises a biocide or combination of biocides that make up less than or equal to 0.25 wt %, less than or equal to 0.2 wt %, less than or equal to 0.15 wt %, less than or equal to 0.1 wt %, less than or equal to 0.075 wt %, less than or equal to 0.05 wt %, less than or equal to 0.02 wt %, or less than or equal to 0.01 wt % of the binder composition. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 0 wt % and less than or equal to 0.25 wt %). Other ranges are also possible. It should also be understood that a single biocide may be present in a binder composition in one or more of the above-referenced ranges (e.g., that further comprises other, different biocides or that lacks other, different biocides) and/or the total amount of biocide in a binder composition may be in one or more of the above-referenced ranges. 
     In some embodiments, a binder composition comprises one or more humectants. Non-limiting examples of suitable humectants include alcohols (e.g., mono- or multifunctional alcohols), ethers, lactones, lactams (e.g., substituted lactams, unsubstituted lactams), amides, amines, sulfones, sulfoxides, sulfides, carbonates, and carbamates. Further non-limiting examples of suitable humectants include 1,3-propanediol, 1,4-butanediol, 1,4-cyclohexanedimethanol, 1,5-pentanediol, 1,6-hexanediol, 1,2-hexanediol, 1,8-octanediol, 1,2-propanediol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, poly(ethylene glycol) having a weight average molecular weight of less than 2000 Da, dipropylene glycol, propylene glycol, polypropylene glycol having weight average molecular weight less than 2000, glycerol, 1,2,6-hexanetriol, sorbitol, 2-pyrrolidone, 1-methyl-2-pyrrolidone, 1-methyl-2-piperidone, N-ethylacetamide, N-methlpropionamide, N-acetyl ethanolamine, N-methylacetamide, formamide, 3-amino-1,2-propanediol, 2,2-thiodiethanol, 3,3-thiodipropanol, tetramethylene sulfone, butadiene sulfone, ethylene carbonate, butyrolacetone, tetrahydrofurfuryl alcohol, glycerol, 1,2,4-butenetriol, trimethylpropane, pantothenol, urea, biuret, triethanolamine, and diethanolamine. 
     In some embodiments, a binder composition comprises a humectant or combination of humectants that makes up less than or equal to 30 wt %, less than or equal to 25 wt %, less than or equal to 20 wt %, less than or equal to 17.5 wt %, less than or equal to 15 wt %, less than or equal to 12.5 wt %, less than or equal to 10 wt %, less than or equal to 7.5 wt %, less than or equal to 5 wt %, less than or equal to 2.5 wt %, or less than or equal to 1 wt % of the binder composition. In some embodiments, a binder composition comprises a humectant or combination of humectants that makes up greater than or equal to 0 wt %, greater than or equal to 1 wt %, greater than or equal to 2.5 wt %, greater than or equal to 5 wt %, greater than or equal to 7.5 wt %, greater than or equal to 10 wt %, greater than or equal to 12.5 wt %, greater than or equal to 15 wt %, greater than or equal to 17.5 wt %, greater than or equal to 20 wt %, or greater than or equal to 25 wt % of the binder composition. Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 30 wt % and greater than or equal to 0 wt %). Other ranges are also possible. It should also be understood that a single humectant may be present in a binder composition in one or more of the above-referenced ranges (e.g., that further comprises other, different humectants or that lacks other, different humectants) and/or the total amount of humectant in a binder composition may be in one or more of the above-referenced ranges. 
     It has been observed that certain combinations of components of the binder composition may lead to any of a variety of possible advantages (e.g., good resolution, good processability), including in embodiments in which the metal powder comprises a noble metal powder and/or a copper powder. In some such embodiments, the binder composition comprises an aqueous solution containing between 40-60 wt % (e.g., 50 wt %) of the low molecular weight polymer including an acrylic acid repeat unit (e.g., polyacrylic acid having a weight average molecular weight of between 2-5 kDa, such as 4 kDa), the polymer-containing aqueous solution being present in the binder composition an amount of between 15-25 wt % (e.g., 20 wt %). In some embodiments, the binder composition comprises one or more polyol cross-linking agents (e.g., glycerol, 1,2-hexanediol). The glycerol may be present an amount between 1.8-2.2 wt % (e.g., 2-2.12 wt %). The 1,2-hexanediol may be present in an amount between 1.8-2.3 wt % (e.g., 2.22 wt %). While in some embodiments co-solvents (e.g., 2-propanol in an amount of between 5-15 wt %, such as 10 wt %), surfactants (e.g., Thetawet TS 8230 in an amount of 0.1 wt %), and pH modifiers (e.g., sodium hydroxide in an amount of between 0.1-0.3 wt %) may also be present, in certain cases these components are not present, and the balance of the binder composition is water. In some embodiments, the balance of the binder composition is water, and the balance water is present in an amount of between 60-80 wt % (e.g., 74-76 wt %). It has been observed that some such binder compositions have a relatively high surface tension, such as greater than or equal to 28 dyne/cm, greater than or equal to 30 dyne/cm, greater than or equal to 35 dyne/cm, greater than or equal to 40 dyne/cm, greater than or equal to 43 dyne/cm, and/or up to 45 dyne/cm, or up to 50 dyne/cm. The binder compositions may have a viscosity of less than or equal to 6 cP, less than or equal to 5.5 cP, less than or equal to 4.5 cP, and/or as low as 4 cP at a printing temperature. The binder composition may have a pH of between 2-4. 
     The metal powders described herein may comprise a plurality of metal and/or metal alloy particles having a variety of suitable sizes. In some embodiments, a metal powder may comprise a plurality of particles having a size suitable for the formation of metal objects by additive manufacturing methods (e.g., that have good flow behavior and/or are suitable for sintering). For instance, the plurality of particles may have an advantageous value of D50 (i.e., an advantageous median particle size). In some embodiments, the plurality of particles has a D50 of greater than or equal to 3 microns, greater than or equal to 5 microns, greater than or equal to 6 microns, greater than or equal to 7 microns, greater than or equal to 9 microns, greater than or equal to 10 microns, greater than or equal to 12.5 microns, greater than or equal to 13 microns, greater than or equal to 15 microns, greater than or equal to 20 microns, greater than or equal to 25 microns, greater than or equal to 30 microns, greater than or equal to 35 microns, greater than or equal to 40 microns, or greater than or equal to 45 microns. In some embodiments, the plurality of particles has a D50 of less than or equal to 50 microns, less than or equal to 45 microns, less than or equal to 40 microns, less than or equal to 35 microns, less than or equal to 30 microns, less than or equal to 25 microns, less than or equal to 20 microns, less than or equal to 15 microns, less than or equal to 13 microns, less than or equal to 12.5 microns, less than or equal to 10 microns, less than or equal to 9 microns, or less than or equal to 7 microns. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 3 microns and less than or equal to 50 microns, greater than or equal to 5 microns and less than or equal to 50 microns, or greater than or equal to 7 microns and less than or equal to 20 microns). Other ranges are also possible. The D50 of a plurality of particles may be determined in accordance with ASTM E2651-13. 
     As described above, certain embodiments relate to methods of additive manufacturing by binder jet printing. Further details regarding such embodiments and the articles produced by methods of additive manufacturing are provided below. 
     As also described above, some methods of additive manufacturing comprise depositing a binder composition on a layer of metal powder. The layer of metal powder may be a layer that comprises a plurality of particles that are not adhered together. For instance, the metal particles in a layer of metal powder may be readily separated from each other by the application of minimal amounts of force, such as the application of forces present during typical processes of depositing a layer of metal powder and/or the application of gravity. 
     When present, layers of metal powder may have a variety of suitable thicknesses. In some embodiments, a layer of metal powder has a thickness of greater than or equal to 10 microns, greater than or equal to 15 microns, greater than or equal to 20 microns, greater than or equal to 25 microns, greater than or equal to 30 microns, greater than or equal to 35 microns, greater than or equal to 40 microns, greater than or equal to 45 microns, greater than or equal to 50 microns, greater than or equal to 60 microns, greater than or equal to 70 microns, greater than or equal to 80 microns, or greater than or equal to 90 microns. In some embodiments, a layer of metal powder has a thickness of less than or equal to 200 microns, less than or equal to 150 microns, less than or equal to 125 microns, less than or equal to 100 microns, less than or equal to 90 microns, less than or equal to 80 microns, less than or equal to 70 microns, less than or equal to 60 microns, less than or equal to 50 microns, less than or equal to 45 microns, less than or equal to 40 microns, less than or equal to 35 microns, less than or equal to 30 microns, or less than or equal to 20 microns. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 10 microns and less than or equal to 200 microns, or greater than or equal to 25 microns and less than or equal to 100 microns). Other ranges are also possible. 
     Once a layer of metal powder (e.g., noble metal powder) has been deposited, deposition of a binder composition thereon may occur (e.g., by means of a print head, such as an ink jet print head, that ejects a plurality of droplets) at a variety of suitable velocities. In some embodiments, the binder composition is deposited at a velocity of greater than or equal to 3 m/s, greater than or equal to 4 m/s, greater than or equal to 5 m/s, greater than or equal to 6 m/s, greater than or equal to 7 m/s, greater than or equal to 8 m/s, greater than or equal to 9 m/s, greater than or equal to 10 m/s, or greater than or equal to 11 m/s. In some embodiments, the binder composition is deposited at a velocity of less than or equal to 12 m/s, less than or equal to 11 m/s, less than or equal to 10 m/s, less than or equal to 9 m/s, less than or equal to 8 m/s, less than or equal to 7 m/s, less than or equal to 6 m/s, less than or equal to 5 m/s, or less than or equal to 4 m/s. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 3 m/s and less than or equal to 12 m/s greater than or equal to 6 m/s and less than or equal to 12 m/s). Other ranges are also possible. Binder composition velocity may be measured using a high speed camera, or a stroboscope/camera apparatus, with imaging software as described in the Examples. The velocity may be measured when droplets of the binder composition are 0.5 mm from the orifice of the print head from which they are ejected. 
     In some embodiments, a binder composition is deposited in the form of droplets. For instance, in some embodiments, a step of depositing a binder composition on layer of metal powder comprises producing a droplet of the binder composition and depositing the droplet of the binder composition on the layer of metal powder. When produced, droplets may have a variety of suitable volumes. In some embodiments, a method comprises producing a droplet having a volume of greater than or equal to 0.5 pL, greater than or equal to 0.75 pL, greater than or equal to 1 pL, greater than or equal to 1.5 pL, greater than or equal to 2 pL, greater than or equal to 3 pL, greater than or equal to 5 pL, greater than or equal to 7.5 pL, greater than or equal to 10 pL, greater than or equal to 12 pL, greater than or equal to 15 pL, greater than or equal to 20 pL, greater than or equal to 25 pL, greater than or equal to 30 pL, or greater than or equal to 35 pL. In some embodiments, a method comprises producing a droplet having a volume of less than or equal to 40 pL, less than or equal to 35 pL, less than or equal to 30 pL, less than or equal to 25 pL, 20 pL, less than or equal to 15 pL, less than or equal to 12 pL, less than or equal to 10 pL, less than or equal to 7.5 pL, less than or equal to 5 pL, less than or equal to 3 pL, less than or equal to 2 pL, less than or equal to 1.5 pL, less than or equal to 1 pL, or less than or equal to 0.75 pL. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 0.5 pL and less than or equal to 40 pL, greater than or equal to 0.5 pL and less than or equal to 20 pL, greater than or equal to 0.5 pL and less than or equal to 2 pL, greater than or equal to 2 pL and less than or equal to 20 pL, or greater than or equal to 2 pL and less than or equal to 12 pL). Other ranges are also possible. Droplet volume may be measured using a high speed camera, or a stroboscope/camera apparatus, with imaging software as described in the examples. It should also be understood that some methods may comprise producing a plurality of droplets comprising one or more droplets having a volume in one or more of the above-referenced ranges and that some methods may comprise producing a plurality of droplets having an average volume in one or more of the above-referenced ranges. 
     In some embodiments in which a plurality of droplets of a binder composition are produced, they may be produced in a manner such that they have a relatively uniform volume. Production of droplets with relatively uniform volumes may enhance the precision with which features in a composite layer can be formed, as it may allow more control over the amount and location of the binder composition in the composite layer by reducing the amount of unwanted droplets and/or droplets of unwanted volumes. Volume uniformity may enhance control over the volumetric ratio of the binder composition to the metal powder on which it is deposited, which may promote better control over the properties of the metal object fabricated therefrom. 
     In some embodiments, a plurality of droplets comprises almost exclusively main droplets and very few satellite droplets. In other words, the binder composition may form droplets in a manner that does not substantially form satellite droplets. The satellite droplets may be droplets having a smaller volume than the main droplets. In some embodiments, satellite droplets have a volume of less than or equal to 1.5 pL, less than or equal to 1 pL, or less than or equal to 0.5 pL. For instance, in some embodiments, less than 1% of the droplets within a plurality of droplets are satellite droplets (e.g., less than 1% of the droplets have a volume of less than or equal to 1.5 pL, less than or equal to 1 pL, or less than or equal to 0.5 pL when the main droplets have a volume of greater than or equal to 0.5 pL, greater than or equal to 1 pL, or greater than or equal to 1.5 pL). In some embodiments, a plurality of droplets comprises exclusively main droplets and no satellite droplets and/or a binder formulation forms droplets in a manner that does not form satellite droplets. The presence of satellite droplets, and their amount, may be determined by using the technique described for measuring droplet volume described above. 
     Droplets of a binder composition may be produced in a variety of suitable manners. In some embodiments, one or more droplets of a binder composition are produced by a print head, such as a piezoelectric print head or a thermal print head. Without wishing to be bound by any particular theory, it is believed that piezoelectric print heads may be configured to form larger droplets comprising a binder composition than thermal print heads (e.g., piezoelectric print heads may be configured to form droplets having volumes of greater than or equal to 2 pL and less than or equal to 20 pL, while thermal print heads may be configured to form droplets having volumes of greater than or equal to 0.5 pL and less than or equal to 2 pL). Non-limiting examples of suitable print heads include SAMBA (FujiFilm Co.), SG-1024 (Fujifilm Co.), XAAR 5601 (XAAR, plc), VersaPass (Memjet), Duralink (Memjet), and Duraflex (Memjet). 
     As described above, certain embodiments relate to three-dimensional compositions. The three-dimensional compositions may include a metal powder and a binder composition. The binder composition may comprise one or more of the components described elsewhere herein with respect to binder compositions (e.g., water, a polymer including an acrylic acid repeat unit, a cross-linking agent, and/or an optional surfactant or co-solvent. 
     As described above, certain embodiments relate to the drying and/or cross-linking of a binder composition and certain embodiments relate to metal-based composite structures formed by the drying and/or cross-linking of a binder composition positioned in a three-dimensional composition. Further details regarding such embodiments are provided below. 
     As also described above, the drying and/or cross-linking of a binder composition may be accomplished by exposing the binder composition to a stimulus that is heat. The temperature to which the binder composition is heated may be sufficient to dry and/or cross-link the binder composition without appreciably degrading the portion(s) of the binder composition, if any, configured to remain in the metal-based composite structure during this step (e.g., a polymer). In some embodiments, drying and/or cross-linking a binder composition comprises heating an environment in which a three-dimensional composition is positioned to a temperature of greater than or equal to 90° C., greater than or equal to 100° C., greater than or equal to 110° C., greater than or equal to 120° C., greater than or equal to 130° C., greater than or equal to 140° C., greater than or equal to 150° C., greater than or equal to 160° C., greater than or equal to 170° C., greater than or equal to 180° C., greater than or equal to 190° C., greater than or equal to 200° C., greater than or equal to 210° C., greater than or equal to 220° C., greater than or equal to 230° C., or greater than or equal to 240° C. In some embodiments, drying and/or cross-linking a binder composition comprises heating an environment in which the three-dimensional composition is positioned to a temperature of less than or equal to 250° C., less than or equal to 240° C., less than or equal to 230° C., less than or equal to 220° C., less than or equal to 210° C., less than or equal to 200° C., less than or equal to 190° C., less than or equal to 180° C., less than or equal to 170° C., less than or equal to 160° C., less than or equal to 150° C., less than or equal to 140° C., less than or equal to 130° C., less than or equal to 120° C., or less than or equal to 100° C. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 90° C. and less than or equal to 250° C., greater than or equal to 120° C. and less than or equal to 220° C.). Other temperatures are also possible. The temperature chosen for the environment during the drying and/or cross-linking may depend on the composition of the environment (e.g., to avoid prematurely de-binding). For example, in some embodiments in which the heating environment is an oxidative gaseous environment (e.g., air), the environment may be heated to a temperature of less than or equal to 220° C., while in some embodiments in which the heating environment is an inert environment, the environment may be heated to a temperature of less than or equal to 250° C. The temperature of an environment may be determined by use of a thermocouple positioned in the environment. 
     Non-limiting examples of suitable environments in which a three-dimensional composition may be positioned during drying and/or heating of the binder composition include an oven and a powder bed. The relevant environment may comprise a variety of suitable types of gases. By way of example, the relevant environment may comprise an oxidative environment such as air, may comprise hydrogen, and/or may comprise an inert gas (e.g., nitrogen, argon, helium). Certain gas species (e.g., hydrogen, helium) may contribute to a relatively high thermal conductivity during drying and/or heating. In some embodiments, the relevant environment may lack species that are reactive at the temperature to which the environment is heated. By way of example, the relevant environment may be an inert environment (e.g., it may comprise, consist essentially of, and/or consist of an inert gas such as nitrogen and/or argon). The pressure of the relevant environment may generally be selected as desired. Some relevant environments may be at atmospheric pressure; some may be at a pressure less than atmospheric pressure. 
     Drying and/or heating a three-dimensional composition may be performed for a variety of suitable amounts of time. The time may be selected to provide a desired level of drying and/or cross-linking of the binder composition. By way of example, if a light level of drying and/or cross-linking is desired, a drying and/or cross-linking step may be performed for a relatively short time. Similarly, if a relatively high level of drying and/or cross-linking is desired, a drying and/or cross-linking step may be performed for a relatively long time. In some embodiments, a drying and/or cross-linking step comprises heating an environment in which a three-dimensional composition is positioned for a time period of greater than or equal to 15 minutes, greater than or equal to 30 minutes, greater than or equal to 45 minutes, greater than or equal to 1 hour, greater than or equal to 90 minutes, greater than or equal to 2 hours, greater than or equal to 3 hours, greater than or equal to 4 hours, greater than or equal to 5 hours, greater than or equal to 6 hours, greater than or equal to 8 hours, greater than or equal to 10 hours, greater than or equal to 12 hours, greater than or equal to 14 hours, greater than or equal to 16 hours, greater than or equal to 18 hours, greater than or equal to 20 hours, greater than or equal to 1 day, greater than or equal to 2 days, greater than or equal to 3 days, greater than or equal to 4 days, greater than or equal to 100 hours, greater than or equal to 5 days, or greater than or equal to 6 days. In some embodiments, a drying and/or cross-linking step comprises heating an environment in which a three-dimensional composition is positioned for a time period of less than or equal to 1 week, less than or equal to 6 days, less than or equal to 5 days, less than or equal to 100 hours, less than or equal to 4 days, less than or equal to 3 days, less than or equal to 2 days, less than or equal to 1 day, less than or equal to 20 hours, less than or equal to 18 hours, less than or equal to 16 hours, less than or equal to 14 hours, less than or equal to 12 hours, less than or equal to 10 hours, less than or equal to 8 hours, less than or equal to 6 hours, less than or equal to 5 hours, less than or equal to 4 hours, less than or equal to 3 hours, less than or equal to 140 minutes, less than or equal to 120 minutes, or less than or equal to 100 minutes. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 15 minutes and less than or equal to 6 days, greater than or equal to 15 minutes and less than or equal to 120 minutes, or greater than or equal to 45 minutes and less than or equal to 120 minutes). Other ranges are also possible. 
     In some embodiments, drying and/or heating a three-dimensional composition comprises heating the environment in which the three-dimensional composition is positioned to one temperature in one or more of the above-referenced ranges and holding the temperature of the environment thereat for an amount of time in one of the above-referenced ranges. In some embodiments, drying and/or heating a three-dimensional composition comprises heating an environment in which the three-dimensional composition is positioned to two or more temperatures in the above-referenced ranges sequentially and holding the temperature of the environment at each of the two or more temperatures. In such embodiments, the relevant environment may be held at each of the relevant temperatures for a period of time in one or more of the above-referenced ranges and/or may be heated such that the total time it is held at all of the relevant temperatures is in one or more of the above-referenced ranges. 
     In some embodiments, drying and/or heating a three-dimensional composition is performed in a manner that reduces the tendency of the three-dimensional object to form cracks. For instance, drying and/or heating a three-dimensional composition may be performed in a manner such that changes between temperatures are performed relatively slowly. In some embodiments, drying and/or heating a three-dimensional composition is performed such that the change in temperature of the environment in which the three-dimensional object is positioned is less than or equal to 2° C./min, less than or equal to 1.5° C./min, less than or equal to 1° C./min, at less than or equal to 0.75° C./min, less than or equal to 0.5° C./min, or less than or equal to 0.25° C./min. In some embodiments, drying and/or heating a three-dimensional composition is performed such that that the change in temperature of the environment in which the three-dimensional composition is positioned is greater than or equal to 0.1° C./min, greater than or equal to 0.25° C./min, greater than or equal to 0.5° C./min, greater than or equal to 0.75° C./min, greater than or equal to 1° C./min, greater than or equal to 1.5° C./min. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 2° C./min and less than or equal to 0.1° C./min). Other ranges are also possible. In some embodiments, the temperature of the environment in which the three-dimensional composition is positioned is either constant or changes at a rate in one or more of the ranges described above throughout a drying and/or cross-linking process. In some embodiments, a drying and/or cross-linking process comprises a change in temperature at a rate in one or more of the ranges described above but also comprises further changes in temperature (e.g., at a rate in one or more of the ranges described above, at a rate outside of the ranges described above). 
     As described above, certain embodiments relate to metal-based composite structures. Further details regarding such embodiments are provided below. 
     In some embodiments, a metal-based composite structure is provided. The metal-based composite structure may comprise a metal powder (e.g., a metal powder comprising a noble metal or noble metal alloy), one or more components of a binder composition (e.g., a polymer including an acrylic acid repeat unit, a cross-linking agent), and/or one or more reaction products of one or more components of a binder composition (e.g., a cross-linked polymer formed by a reaction of a polymer present in the binder composition with a cross-linking agent present in the binder composition). The components of the binder composition and the reaction products thereof may together form a binder. In some embodiments, the metal powder comprises a plurality of particles that is embedded in the binder. For instance, the binder may form a matrix in which the plurality of particles is disposed. As another example, the binder may coat the surfaces of the particles within the plurality of particles and/or the particles within the plurality of particles may be in topological contact with each other through the binder. 
     In some embodiments, a metal-based composite structure comprises a binder. For instance, the binder may adhere to particles positioned in the composite layer and may have sufficient cohesive strength to form a self-supporting structure in which the particles are embedded. 
     A metal powder (e.g., noble metal powder) present in a metal-based composite structure may make up any suitable amount thereof. In some embodiments, the metal powder (e.g., noble metal powder) makes up greater than or equal to 92 wt %, greater than or equal to 93 wt %, greater than or equal to 94 wt %, greater than or equal to 95 wt %, greater than or equal to 96 wt %, greater than or equal to 97 wt %, greater than or equal to 98 wt %, greater than or equal to 99 wt %, or greater than or equal to 99.8 wt % of the metal-based composite structure. In some embodiments, the metal powder (e.g., noble metal powder) makes up less than or equal to 99.9 wt %, less than or equal to 99.5 wt %, less than or equal to 99 wt %, less than or equal to 98 wt %, less than or equal to 97 wt %, less than or equal to 96 wt %, less than or equal to 95 wt %, less than or equal to 94 wt %, or less than or equal to 93 wt % of the metal-based composite structure. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 92 wt % and less than or equal to 99.9 wt %, greater than or equal to 96 wt % and less than or equal to 99.9 wt %, or greater than or equal to 97 wt % and less than or equal to 99.8 wt %). Other ranges are also possible. 
     The metal-based composite structures described herein may have advantageously have relatively high transverse flexural strengths. Desirably, high values of transverse flexural strength may reduce the tendency of metal-based composite structures to fail during further additive manufacturing steps. In some embodiments, a metal-based composite structure has a transverse flexural strength of greater than or equal to 1 MPa, greater than or equal to 2 MPa, greater than or equal to 3 MPa, greater than or equal to 5 MPa, greater than or equal to 7.5 MPa, greater than or equal to 10 MPa, greater than or equal to 20 MPa, greater than or equal to 50 MPa, greater than or equal to 75 MPa, greater than or equal to 100 MPa, or greater than or equal to 125 MPa. In some embodiments, a metal-based composite structure has a transverse flexural strength of less than or equal to 150 MPa, less than or equal to 125 MPa, less than or equal to 100 MPa, less than or equal to 75 MPa, less than or equal to 50 MPa, less than or equal to 20 MPa, less than or equal to 10 MPa, less than or equal to 7.5 MPa, less than or equal to 5 MPa, or less than or equal to 2 MPa. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 1 MPa and less than or equal to 150 MPa, greater than or equal to 2 MPa and less than or equal to 150 MPa, or greater than or equal to 3 MPa and less than or equal to 100 MPa). Other ranges are also possible. 
     The transverse flexural strength of a metal-based composite structure may be the transverse flexural strength as determined by the three-point bending test described in ASTM B312-14 and/or may be the transverse flexural strength as determined by the four-point bending test described in ASTM C1161-18. In other words, some metal-based composite structures may have transverse flexural strengths as determined by the three-point bending test described in ASTM B312-14 in one or more of the above-referenced ranges, some metal-based composite structures may have transverse flexural strengths as determined by the four-point bending test described in ASTM C1161-18 in one or more of the above-referenced ranges, and some metal-based composite structures may have transverse flexural strengths as determined by the three-point bending test described in ASTM B312-14 and as determined the four-point bending test described in ASTM C1161-18 in one or more of the above-referenced ranges. 
     As described above, certain embodiments relate to heating metal-based composite structures. Certain embodiments relate to de-bound metal structures formed by heating metal-based composite structures (e.g., during step  70  in  FIGS. 1A-1B ). Further details regarding such embodiments are provided below. 
     De-bound metal structures may advantageously include relatively low levels of certain elements. For instance, in some embodiments, a de-bound metal structure comprises relatively small amounts of carbon and/or oxygen. As described elsewhere herein, such components may react undesirably with the metal in the de-bound metal structure during further additive manufacturing steps (e.g., during a sintering step). By way of example, carbon in a de-bound metal structure may react undesirably with surface oxides also therein. 
     In some embodiments, carbon makes up less than or equal to 0.5 wt %, less than or equal to 0.4 wt %, less than or equal to 0.2 wt %, less than or equal to 0.1 wt %, less than or equal to 0.05 wt %, or less than or equal to 0.02 wt % of the de-bound metal structure. In some embodiments, carbon makes up greater than or equal to 0 wt %, greater than or equal to 0.02 wt %, greater than or equal to 0.05 wt %, greater than or equal to 0.1 wt %, greater than or equal to 0.2 wt %, or greater than or equal to 0.4 wt % of the de-bound metal structure. Combinations of the above-referenced ranges are also possible (e.g., less than or equal to 0.5 wt % and greater than or equal to 0 wt %, or less than or equal to 0.1 wt % and greater than or equal to 0 wt %). Other ranges are also possible. The amount of carbon in the de-bound metal structure may be determined in accordance with ASTM E1019. 
     In some embodiments, oxygen makes up less than or equal to 1.5 wt %, less than or equal to 1.3 wt %, or less than or equal to 1.1 wt % of the de-bound metal structure. The amount of oxygen in the de-bound metal structure may be determined in accordance with ASTM E1019. 
     As described above, certain embodiments relate to the formation of metal objects from de-bound metal structures and/or composite metal structures. Certain embodiments relate to metal objects. Further details regarding such embodiments are provided below. 
     Some metal objects described herein advantageously both comprise a metal and/or metal alloy (e.g., a noble metal and/or noble metal alloy) and have a density that is relatively close to the density of the metal and/or alloy included therein. Metal objects having this property may include a relatively low amount of internal pores (i.e., pores included in the bulk of the metal object and not in fluidic communication with an environment external to the metal object) and/or may include internal pores that make up a relatively small volume fraction of the metal object. Low amounts and/or volume fractions of internal pores may desirably increase the robustness and strength of the metal object. 
     The relationship between the density of a metal object and the density of a metal alloy included therein may be parametrized by a relative density. As used herein, the relative density may be computed by dividing the bulk density of the metal object by the bulk density of the relevant metal alloy and multiplying by 100%. Accordingly, a relative density of 100% would indicate that the metal object has a density identical to the bulk metal alloy included therein while a relative densities of less than 100% would indicate that the metal object has a density less than the metal alloy included therein. The bulk density of a metal object may be computed in accordance with ASTM B962-17. It should be understood that internal pores would contribute to this volume (because they are entirely enclosed by the outer boundary of the metal object) while open pores and other features partially enclosed by a metal object would not contribute to this volume. 
     In some embodiments, a metal object has a relative density of greater than or equal to 90%, greater than or equal to 91%, greater than or equal to 92%, greater than or equal to 93%, greater than or equal to 94%, greater than or equal to 95%, greater than or equal to 96%, greater than or equal to 97%, greater than or equal to 98%, or greater than or equal to 99%. In some embodiments, a metal object has a relative density of less than or equal to 100%, less than or equal to 99.9%, less than or equal to 99%, less than or equal to 98%, less than or equal to 97%, less than or equal to 96%, less than or equal to 95%, less than or equal to 94%, less than or equal to 93%, less than or equal to 92%, or less than or equal to 91%. Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 90% and less than or equal to 100%, or greater than or equal to 93% and less than or equal to 100%). Other ranges are also possible. 
     Metal objects described herein may have a chemical composition similar to the metal powders from which they were formed. For instance, a metal object may comprise a noble metal or noble metal alloy from the noble metal powder, such as sterling silver. Non-limiting examples of suitable metal alloys include argentium silver (e.g., 93.5 wt % or 96 wt % silver alloyed with other metals or metalloids, such as germanium), yellow gold (e.g., 75 wt % gold, 16 wt % silver, 9 wt % copper), rose gold (e.g., 75 wt % gold, 6 wt % silver, 19 wt % copper), platinum (e.g., 95 wt % platinum and 5 wt % ruthenium). In some instances, the weight percent of the noble metal and/or noble metal alloy from the noble metal powder in the metal object is greater than or equal to 90 wt %, greater than or equal to 93 wt %, greater than or equal to 95 wt %, greater than or equal to 97 wt %, greater than or equal to 98 wt %, and/or up 99 wt %, up to 99.5 wt %, up to 99.9 wt %, or more. Combinations of these ranges are possible (e.g., a wt % of between 93 wt % and 99 wt %. 
     For convenience, certain terms employed in the specification, examples, and appended claims are listed here. Definitions of specific functional groups and chemical terms are described in more detail below. For purposes of this invention, the chemical elements are identified in accordance with the Periodic Table of the Elements, CAS version, Handbook of Chemistry and Physics, 75th Ed., inside cover, and specific functional groups are generally defined as described therein. Additionally, general principles of organic chemistry, as well as specific functional moieties and reactivity, are described in Organic Chemistry, Thomas Sorrell, University Science Books, Sausalito: 1999. 
     The term “aliphatic,” as used herein, includes both saturated and unsaturated, nonaromatic, straight chain (i.e., unbranched), branched, acyclic, and cyclic (i.e., carbocyclic) hydrocarbons, which are optionally substituted with one or more functional groups. As will be appreciated by one of ordinary skill in the art, “aliphatic” is intended herein to include, but is not limited to, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, and cycloalkynyl moieties. Thus, as used herein, the term “alkyl” includes straight, branched and cyclic alkyl groups. An analogous convention applies to other generic terms such as “alkenyl”, “alkynyl”, and the like. Furthermore, as used herein, the terms “alkyl”, “alkenyl”, “alkynyl”, and the like encompass both substituted and unsubstituted groups. In certain embodiments, as used herein, “aliphatic” is used to indicate those aliphatic groups (cyclic, acyclic, substituted, unsubstituted, branched or unbranched) having 1-20 carbon atoms. Aliphatic group substituents include, but are not limited to, any of the substituents described herein, that result in the formation of a stable moiety (e.g., aliphatic, alkyl, alkenyl, alkynyl, heteroaliphatic, heterocyclic, aryl, heteroaryl, acyl, oxo, imino, thiooxo, cyano, isocyano, amino, azido, nitro, hydroxyl, thiol, halo, aliphaticamino, heteroaliphaticamino, alkylamino, heteroalkylamino, arylamino, heteroarylamino, alkylaryl, arylalkyl, aliphaticoxy, heteroaliphaticoxy, alkyloxy, heteroalkyloxy, aryloxy, heteroaryloxy, aliphaticthioxy, heteroaliphaticthioxy, alkylthioxy, heteroalkylthioxy, arylthioxy, heteroarylthioxy, acyloxy, and the like, each of which may or may not be further substituted). 
     The term “alkyl” refers to the radical of saturated aliphatic groups, including straight-chain alkyl groups, branched-chain alkyl groups, cycloalkyl (alicyclic) groups, alkyl substituted cycloalkyl groups, and cycloalkyl substituted alkyl groups. The alkyl groups may be optionally substituted, as described more fully below. Examples of alkyl groups include, but are not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, 2-ethylhexyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and the like. “Heteroalkyl” groups are alkyl groups wherein at least one atom is a heteroatom (e.g., oxygen, sulfur, nitrogen, phosphorus, etc.), with the remainder of the atoms being carbon atoms. Examples of heteroalkyl groups include, but are not limited to, alkoxy, poly(ethylene glycol)-, alkyl-substituted amino, tetrahydrofuranyl, piperidinyl, morpholinyl, etc. 
     The terms “alkenyl” and “alkynyl” refer to unsaturated aliphatic groups analogous to the alkyl groups described above, but containing at least one double or triple bond respectively. The “heteroalkenyl” and “heteroalkynyl” refer to alkenyl and alkynyl groups as described herein in which one or more atoms is a heteroatom (e.g., oxygen, nitrogen, sulfur, and the like). 
     The term “aryl” refers to an aromatic carbocyclic group having a single ring (e.g., phenyl), multiple rings (e.g., biphenyl), or multiple fused rings in which at least one is aromatic (e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl), all optionally substituted. “Heteroaryl” groups are aryl groups wherein at least one ring atom in the aromatic ring is a heteroatom, with the remainder of the ring atoms being carbon atoms. Examples of heteroaryl groups include furanyl, thienyl, pyridyl, pyrrolyl, N lower alkyl pyrrolyl, pyridyl N oxide, pyrimidyl, pyrazinyl, imidazolyl, indolyl and the like, all optionally substituted. 
     The terms “amine” and “amino” refer to both unsubstituted and substituted amines, e.g., a moiety that can be represented by the general formula: N(R′)(R″)(R′″) wherein R′, R″, and R′″ each independently represent a group permitted by the rules of valence. The terms “acyl,” “carboxyl group,” or “carbonyl group” are recognized in the art and can include such moieties as can be represented by the general formula: 
     
       
         
         
             
             
         
       
     
     wherein W is H, OH, O-alkyl, O-alkenyl, or a salt thereof. Where W is O-alkyl, the formula represents an “ester.” Where W is OH, the formula represents a “carboxylic acid.” In general, where the oxygen atom of the above formula is replaced by sulfur, the formula represents a “thiolcarbonyl” group. Where W is a S-alkyl, the formula represents a “thiolester.” Where W is SH, the formula represents a “thiolcarboxylic acid.” On the other hand, where W is alkyl, the above formula represents a “ketone” group. Where W is hydrogen, the above formula represents an “aldehyde” group. 
     As used herein, the term “heteroaromatic” or “heteroaryl” means a monocyclic or polycyclic heteroaromatic ring (or radical thereof) comprising carbon atom ring members and one or more heteroatom ring members (such as, for example, oxygen, sulfur or nitrogen). Typically, the heteroaromatic ring has from 5 to about 14 ring members in which at least 1 ring member is a heteroatom selected from oxygen, sulfur, and nitrogen. In another embodiment, the heteroaromatic ring is a 5 or 6 membered ring and may contain from 1 to about 4 heteroatoms. In another embodiment, the heteroaromatic ring system has a 7 to 14 ring members and may contain from 1 to about 7 heteroatoms. Representative heteroaryls include pyridyl, furyl, thienyl, pyrrolyl, oxazolyl, imidazolyl, indolizinyl, thiazolyl, isoxazolyl, pyrazolyl, isothiazolyl, pyridazinyl, pyrimidinyl, pyrazinyl, triazinyl, triazolyl, pyridinyl, thiadiazolyl, pyrazinyl, quinolyl, isoquinolyl, indazolyl, benzoxazolyl, benzofuryl, benzothiazolyl, indolizinyl, imidazopyridinyl, isothiazolyl, tetrazolyl, benzimidazolyl, benzoxazolyl, benzothiazolyl, benzothiadiazolyl, benzoxadiazolyl, carbazolyl, indolyl, tetrahydroindolyl, azaindolyl, imidazopyridyl, qunizaolinyl, purinyl, pyrrolo[2,3]pyrimidyl, pyrazolo[3,4]pyrimidyl, benzo(b)thienyl, and the like. These heteroaryl groups may be optionally substituted with one or more substituents. 
     The term “substituted” is contemplated to include all permissible substituents of organic compounds, “permissible” being in the context of the chemical rules of valence known to those of ordinary skill in the art. In some cases, “substituted” may generally refer to replacement of a hydrogen with a substituent as described herein. However, “substituted,” as used herein, does not encompass replacement and/or alteration of a key functional group by which a molecule is identified, e.g., such that the “substituted” functional group becomes, through substitution, a different functional group. For example, a “substituted phenyl” must still comprise the phenyl moiety and cannot be modified by substitution, in this definition, to become, e.g., a heteroaryl group such as pyridine. In a broad aspect, the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and nonaromatic substituents of organic compounds. Illustrative substituents include, for example, those described herein. The permissible substituents can be one or more and the same or different for appropriate organic compounds. For purposes of this invention, the heteroatoms such as nitrogen may have hydrogen substituents and/or any permissible substituents of organic compounds described herein which satisfy the valencies of the heteroatoms. This invention is not intended to be limited in any manner by the permissible substituents of organic compounds. 
     Examples of substituents include, but are not limited to, alkyl, aryl, aralkyl, cyclic alkyl, heterocycloalkyl, hydroxy, alkoxy, aryloxy, perhaloalkoxy, aralkoxy, heteroaryl, heteroaryloxy, heteroarylalkyl, heteroaralkoxy, azido, amino, halogen, alkylthio, oxo, acyl, acylalkyl, carboxy esters, carboxyl, carboxamido, nitro, acyloxy, aminoalkyl, alkylaminoaryl, alkylaryl, alkylaminoalkyl, alkoxyaryl, arylamino, aralkylamino, alkylsulfonyl, carboxamidoalkylaryl, carboxamidoaryl, hydroxyalkyl, haloalkyl, alkylaminoalkylcarboxy, aminocarboxamidoalkyl, alkoxyalkyl, perhaloalkyl, arylalkyloxyalkyl, and the like. The following examples are intended to illustrate certain embodiments of the present invention, but do not exemplify the full scope of the invention. 
     EXAMPLE 1 
     This Example describes the preparation of two binder compositions and the formation of sterling silver metal-based composite structures therefrom. 
     Two binder compositions having the components shown in Table 1 were prepared by first charging the components into a flask at the indicated percentages. Then, the components in the flask were stirred at room temperature using magnetic stirrer until a homogeneous solution was formed. The homogeneous solutions were then filtered through 5 micron glass filter. Sokolan CP10S (50 wt % poly(acrylic acid) aqueous solution, M w =4,000 Da) was purchased from BASF Inc. The 63 wt % poly(acrylic) acid aqueous solution (M w =2,000 Da) was purchased from Polyscience Inc. Glycerol, 1,2-hexanediol, and 2-propanol were purchased from Sigma-Aldrich, Inc. Viscosity measurements were performed using a Brookfield DV3T rheometer at 25° C., and surface tension measurements were performed using a SITA Dynotester at 20° C. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Binder Composition Formulations 
               
            
           
           
               
               
            
               
                   
                 Binder Composition No. 
               
            
           
           
               
               
               
            
               
                   
                 1 
                 2 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Weight percent of  
                 20 
                   
               
               
                 63 wt % aqueous 
                   
                   
               
               
                 solution of 
                   
                   
               
               
                 poly(acrylic) acid 
                   
                   
               
               
                 with M w  = 2000 Da; 
                   
                   
               
               
                 purchased from 
                   
                   
               
               
                 Polyscience Inc. 
                   
                   
               
               
                 Weight percent of 
                   
                 20 
               
               
                 Sokolan CP10S 
                   
                   
               
               
                 Weight percent of 
                 2 
                 2.12 
               
               
                 glycerol 
                   
                   
               
               
                 Weight percent of 
                   
                 2.22 
               
               
                 1,2-hexanediol 
                   
                   
               
               
                 Weight percent of  
                 10 
                   
               
               
                 2-propanol 
                   
                   
               
               
                 Weight percent of 
                 0.1 
                   
               
               
                 Thetawet TS 8230 
                   
                   
               
               
                 Weight percent of 
                 67.9 
                 75.66 
               
               
                 water 
                   
                   
               
               
                 Viscosity (cP) 
                 5.76 
                 4.34 
               
               
                 Surface tension 
                 28 
                 43.2 
               
               
                 (dyne/cm) 
                   
                   
               
               
                 pH 
                 3 
                 2.2 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 2 
     This Example describes the measurement of the transverse rupture strength (TRS) of a sanded sterling silver composite bar prepared according to Example 1. Sterling silver composite bars for TRS measurement were made using binder compositions 1 and 2 above. The composite bars were prepared by charging about 250 g of sterling silver powder supplied by Cimini &amp; Associates, Inc., of Westerly, RI (having a D50 of approximately 15 microns) and about 40 g of binder composition into a mixing cup of Flack Tech Speed Mixer. The cup was spun at about 800 RPM under a vacuum environment (20 mmHg pressure) for 4 minutes to produce even, bubble-free suspensions. The suspensions were poured into four 5.08 cm×1.91 cm×1.59 cm silicon molds. The filled molds were allowed to sit for 2 hours to allow the sterling silver powder to settle, and then supernatant liquid was removed from the settled sterling silver powder mixture by pipette. Then, the molds were allowed to dry in air for 14 hrs. Then, the molds were placed in a re-circulating air oven and subjected to the following temperature profile for curing: (1) heating to 60° C. and holding for 45 minutes, (2) heating to 75° C. and holding for 45 minutes, and (3) heating to 90° C. and holding for 1 hour, and (4) heating to 180° C. and holding for 2 hours. Then, the heating element of the oven was turned off and the oven was allowed to cool to room temperature. The resulting sterling silver composite bars were de-molded and sanded with 220 grit sandpaper to produce smooth, even surfaces and corners without sharp edges. 
     Sterling silver composite bar 1, which was prepared using binder composition 1 in Example 1, was measured in accordance with ASTM B528-16. The results are shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Mechanical properties of sterling silver composite bar 
               
            
           
           
               
               
               
               
            
               
                 Composite Bar 
                 Binder Composition 
                 Cure Temperature 
                 TRS  
               
               
                 No. 
                 No. 
                 (° C.) 
                 (MPa) 
               
               
                   
               
               
                 1 
                 1 
                 180 
                 19.6 
               
               
                   
               
            
           
         
       
     
     The results shown in Table 2 demonstrates that the binder compositions of the present disclosure are capable of forming metal-based composite structures that are strong enough to produce self-supporting “brown” parts during some additive manufacturing processes. 
     EXAMPLE 3 
     This Example demonstrates the suitability of binder compositions from Example 1 for printing using ink-jet printhead. Each binder composition shown in Table 1 was printed with droplets produced using a Samba G3L ink-jet print head equipped with Megnajet CIMS-2HFR (available from Megnajet, Northamptonshire, England) recirculation system. Droplet volume and velocity were determined using a stroboscope, a camera, and imaging software, all available from JetXpert, Nashua. N.H., USA. For each binder formulation, the droplet formation was adjusted by changing ink-jet pulse voltage, pulse time, and pulse sequence to achieve individual droplet volumes in the range of from 9 pL to 12 pL while keeping velocities in the range of from 8 m/s to 12 m/s (measured at a distance 0.5 mm from the printhead nozzle at a 24 kHz pulse frequency) and minimizing satellite droplet formation. The resulting pulse sequence is referred to as “the large drop waveform.” After droplet formation parameters were configured, each binder formulation was tested for droplet production latency as follows: a printhead was turned on and stable droplet formation was established, and then the print head was turned off for 5 seconds and turned back on again. The number of droplets required to re-establish stable droplet formation was recorded. Results are shown in Table 3. 
                                             TABLE 3                                       Latency                           (average                           number of                           droplets                           ejected from                           the print                           head prior to           Binder   Drop           resumption           composition   Volume    Drop       of normal           No.   (pL)   velocity   Satellites   printing)                          1   14   10   0   200           2   15   10   0   110                        
The results in Table 3 show that binder compositions 1 and 2 are suitable for printing using ink-jet printheads.
 
     EXAMPLE 4 
     This Example describes exemplary de-binding processes using sanded sterling silver composite bars prepared according to Example 2. The results in this Example demonstrate the unexpected advantages of using temperatures between 220° C. and 450° C. for de-binding of metal composite structures containing noble metals. For the samples heated at a de-binding temperature of 330° C. or 240° C., a sterling silver composite bar prepared as in Example 2 was heated by ramping the temperature of the tube furnace to the desired de-binding temperature in air at a rate of 2° C./minute and held at the de-binding temperature for 2 hours. A third sample was heated at two different temperatures, with a sterling silver composite bar prepared as in Example 2 being heating by ramping the temperature of the tube furnace to 330° C. in air at a rate of 2° C./minute, holding at 330° C. for 1 hour, ramping the temperature of the tube furnace to a temperature of 550° C. in air at a rate of 1° C./minute, and holding at 550° C. for 1 hour. At the conclusion of the de-binding heating in air for all three samples, the tube furnace was cooled to room temperature and then heated again to the sintering temperature at a rate of 2° C./minute in a flow of a gas mixture consisting of 3% hydrogen and 97% argon. After a 3 hour hold at the sintering temperature, the tube furnace was allowed to cool to room temperature in the same gaseous environment. The density of sample was measured according to ASTM B962-17, and the results are summarized in the Table 4. 
                             TABLE 4               De-binding temperature    Sintering temperature    Sintered        (° C.)   (° C.)   density                  330 and 550   800   73%       330   800   90%       240   830   94%                    
The results in Table 4 demonstrate that while sintered sterling silver metal objects with high densities were achieved when the de-binding process occurred at 240° C. and 330° C., de-binding at a temperature that exceed 330° C. resulting in a much less dense sterling silver sintered metal object.
 
     EXAMPLE 5 
     This Example describes exemplary de-binding processes using sanded sterling silver composite bars prepared according to Example 2. The sanded sterling silver composite bars were placed into a forced-air convention Binder model 56 oven. Samples were heated to pre-determined de-bind temperatures at a rate of 2° C./minute and held at the de-bind temperatures for 4 hours. At the conclusion of that 4 hours, the heating element in the oven was turned off and the oven was allowed to cool to room temperature. The carbon and oxygen content of the samples following the heating process of this Example was determined at Dirats Laboratories in Westfield, Mass. 
     Then, the sterling silver de-bound samples were first heated in a tube furnace in a forming gas (3% hydrogen, 97% argon) environment having a temperature starting at room temperature and ramped up 400° C. at a rate of 1° C./minute. The temperature of the tube furnace environment was then ramped to 800° C. at a rate of at 2° C./minute, held at 800° C. for 2 hours, and allowed to cool to room temperature. All steps in the tube furnace were performed in a forming gas environment (3% hydrogen, 97% argon). The density of the sintered samples was measured according to ASTM B962-17, and the results are summarized in Table 5. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                 Binder 
                 De-bind 
                   
                   
                   
                 Archimedes 
                 Relative 
               
               
                 Sample 
                 Composition 
                 temperature 
                   
                   
                   
                 density 
                 Density 
               
               
                 No. 
                 No. 
                 (° C.) 
                 % C 
                 % O 
                 Comments 
                 (g/cm 3 ) 
                 (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 1 
                 1 
                 240 
                 0.023 
                 0.43 
                 Bubbling, 
                 NA 
                 NA 
               
               
                   
                   
                   
                   
                   
                 indicating 
                   
                   
               
               
                   
                   
                   
                   
                   
                 open 
                   
                   
               
               
                   
                   
                   
                   
                   
                 porosity 
                   
                   
               
               
                 2 
                 1 
                 260 
                   
                   
                 No bubbles 
                 9.74 
                 94% 
               
               
                 3 
                 1 
                 270 
                 0.009 
                 1.15 
                 No bubbles 
                 9.74 
                 94% 
               
               
                 4 
                 1 
                 280 
                 0.005 
                 1.3 
                 No bubbles 
                 8.5 
                 82% 
               
               
                 5 
                 2 
                 240 
                 0.03 
                 0.42 
                 Bubbling, 
                 NA 
                 NA 
               
               
                   
                   
                   
                   
                   
                 indicating 
                   
                   
               
               
                   
                   
                   
                   
                   
                 open 
                   
                   
               
               
                   
                   
                   
                   
                   
                 porosity 
                   
                   
               
               
                 6 
                 2 
                 270 
                 0.004 
                 1.05 
                 No bubbles 
                 9.59 
                 92% 
               
               
                   
               
            
           
         
       
     
     The data from Table 5 show that heating in air at temperatures between 240 and 280° C. removes between 97% and 99.7% of carbon. The results in this Example further indicate that 
     Samples 1 and 5 had an open porosity after sintering and therefore showed relatively poor sintering, likely due to the high carbon content after de-binding. This Example further indicated that Samples 2 and 3 were fully sintered with good sintering density, likely due to the low carbon and oxygen content after de-binding. Sample 4 was observed to have low density, likely due to the high oxygen content after de-binding. Overall, this Example shows that sterling silver objects having high relative densities are achievable using the binder compositions and de-binding and sintering methods described herein. 
     EXAMPLE 6 
     This Example describes evaluation of surface quality of printed sterling silver composite bars using binder compositions having different surface tensions. Rectangular composite bars were printed with sterling silver powder and binder compositions 1 and 2 from Example 1 using a process similar to the one described in U.S. Patent Publication No. 2019/0210294, published on Jul. 11, 2019, filed as U.S. patent application Ser. No. 15/971,541, and entitled “Method for Reducing Layer Shifting and Smearing During 3D Printing,” which is incorporated herein by reference in its entirety for all purposes. The composite bars were cured according to the process described in Example 2. Results are summarized in the Table 6. 
                                     TABLE 6                       Binder Composition    Surface tension    Surface quality of printed           No.   (dynes/cm)   bars                          1   28   Poor (uneven and rough)           2   43   Good (smooth and flat)                        
The results in Table 6 demonstrate that high surface tensions of binder compositions improves the surface finish of printed metal-based composite parts.
 
     EXAMPLE 7 
     This Example describes an exemplary de-binding and sintering oven cycle for processing sterling silver composites with binder composition no. 2. Cubes having dimensions of 10 mm×10×10 mm 3  and small gears were printed with sterling silver powder and binder composition no. 2 from Example 1 using a process similar to the one described in in U.S. Patent Publication No. 2019/0210294, and cured according to the process described in Example 2. The cubes were heated with a temperature ramp of 2° C./minute in air at ambient pressure in a tube furnace at a flow rate of 0.5 slpm and held for 60 minutes at 245° C. to de-bind the part. The cubes were then held at 245° C. for 90 minutes in forming gas (3% hydrogen, 97% argon) at a flow rate of 3 standard litres per minute (slpm). Finally, the de-bound parts were heated with a temperature ramp of 2° C./minute in 3 slpm forming gas until the environment reached 700° C., held for 30 minutes, at which point the environment was heated to 820° C., held for 120 minutes, and finally the environment was heated to 840° C. and held for 30 minutes. The samples were then cooled at a rate of 5° C./minute in the tube furnace. The de-binding and sintering conditions in this Example showed that high sintered densities (determined according to ASTM B962-17) are achievable for sterling silver parts using the binder compositions described herein. 
     
       
         
           
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                   
                 Archimedes 
                 % of  
               
               
                 Sample 
                 Density (g/cm 3 ) 
                 theoretical 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Cube 
                 9.96 
                 96 
               
               
                 Gear 
                 10.00 
                 96.5 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 8 
     This Example describes an exemplary debinding and sintering oven cycle for processing 18 karat yellow and rose gold composites with binder composition no. 2 from Example 1. Powders of 18 karat yellow gold (75 wt % gold (Au, 16 wt % Ag, 9 wt % Cu) and 18 karat rose gold (75 wt % Au, 6 wt % Ag, 19 wt % Cu) were procured from Cimini &amp; Associates with the following powder size distribution: d10=6.0 μm, d50=13 μm, d90=22 μm. Composite bars of these two powders were prepared with binder composition no. 2 from Example 1 and using steps adapted from Example 2. Samples were heated in a 4 inch outer diameter tube furnace at ambient pressure, flowing 3.0 slpm of clean dry air, with the tube furnace reaching 300° C. at a rate of 1° C./minute. After a 1 hour hold at 300° C., the gas flow was changed from 3.0 slpm of clean dry air to 3.0 slpm of 3% H2, 97% Ar forming gas. The bars were held at 300° C. for an additional 2 hours, and then the tube furnace was ramped to 710° C. at a rate of 2° C./minute. After holding at 710° C. for 1 hour, the tube furnace was ramped to 875° C. at a rate of 2° C./minute. After holding at 875° C. for 3 hours, the tube furnace was allowed to cool. 
     The densities of the sintered samples were measured by ASTM B962-17 and are summarized in Table 8. 
     
       
         
           
               
               
               
             
               
                 TABLE 8 
               
               
                   
               
               
                   
                 Archimedes 
                 % of 
               
               
                 Powder Type 
                 density (g/cm 3 ) 
                 theoretical 
               
               
                   
               
             
            
               
                 Rose gold 
                 14.51 
                 96 
               
               
                 Yellow gold 
                 14.90 
                 96 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 9 
     This Example describes an exemplary debinding and sintering oven cycle for processing platinum composites with binder composition no. 2. Platinum powder (95 wt % Pt, 5 wt % Ru) was procured from Cimini &amp; Associates with the following powder size distribution: d10=6.0 μm, d50=13 μm, d90=22 μm. Composite bars of the powder was prepared with binder composition no. 2 according to Example 1 and the methods in Example 2. Samples were heated in air, with the tube furnace reaching 1650° C. at a rate of 1° C./minute. After holding at 1650° C. in air for 2 hours, the tube furnace was allowed to cool. 
     The density of sintered samples were measured by ASTM B962-17 and are summarized in Table 9. 
     
       
         
           
               
               
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 Powder  
                 Archimedes 
                 % of  
               
               
                 Type 
                 density (g/cm 3 ) 
                 theoretical 
               
               
                   
               
             
            
               
                 Platinum 
                 20.43 
                 99 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 10 
     This Example describes an exemplary de-binding and sintering oven cycle for processing copper composites with binder composition no. 2. Copper powder (&gt;99.8 wt % Cu) was procured from Carpenter with the following powder size distribution: d10=5.0 μm, d50=11 μm, d90 =16 μm. Composite bars of this powder were prepared with binder composition no. 2 from Example 1 and using steps adapted from Example 2. Samples were heated in a 4 inch outer diameter tube furnace at ambient pressure, flowing 3.0 slpm of clean dry air, with the tube furnace reaching 300° C. at a rate of 1° C./minute. After a 1 hour hold at 300° C., the gas flow changed from 3.0 slpm of clean dry air to 3.0 slpm of 3% H 2 , 97% Ar forming gas. The copper composite bars were held at 300° C. for an additional 2 hours, then the tube furnace was ramped to 890° C. at a rate of 2° C./minute. After holding at 890° C. for 1hour, the tube furnace was ramped to 1065° C. at a rate of 2° C./minute. After holding at 1065° C. for 3 hours, the tube furnace was allowed to cool. 
     The density of sintered samples were measured by ASTM B962-17 and are summarized in Table 10. 
     
       
         
           
               
               
               
             
               
                 TABLE 10 
               
               
                   
               
               
                 Powder  
                 Archimedes 
                 % of 
               
               
                 Type 
                 density (g/cm 3 ) 
                 theoretical 
               
               
                   
               
             
            
               
                 Copper 
                 8.67 
                 97 
               
               
                   
               
            
           
         
       
     
     While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, and/or methods, if such features, systems, articles, materials, and/or methods are not mutually inconsistent, is included within the scope of the present invention. 
     The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” 
     The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to “A and/or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. 
     As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law. 
     As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. 
     In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.