Patent Publication Number: US-2021163766-A1

Title: Chalcogenide glass based inks obtained by dissolution or nanoparticles milling

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to, and the benefit of, U.S. Provisional Patent Application No. 62/943,031, filed on Dec. 3, 2019, and entitled “Materials Characterization of Thin Films Printed with Ge20Se80 Ink,” and U.S. Provisional Patent Application No. 62/943,044, filed on Dec. 3, 2019, and entitled “Studies and Analysis of GexSe100-x Based Spin Coated Chalcogenide Thin Films,” the contents of each of which are incorporated by reference herein in their entirety. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     This invention was made with Government support under contract number FPK809-SB-001 awarded by NASA. The Government has certain rights in the invention. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure is generally related to the field of additive manufacturing and, in particular, to chalcogenide glass nanoparticle-based ink for additive manufacturing and chalcogenide glass ink obtained by dissolution. 
     BACKGROUND 
     Chalcogenide glasses and electrical systems made with chalcogenide glasses may be beneficial for several reasons. For example, chalcogenide glasses may include up to 5% impurities without affecting their electrical performance. They may also be radiation hard. They can further have various optical properties for use in sensing and communication systems. Based on these properties, chalcogenide glasses offer a wide range of applications including phase change memory, temperature sensing, infrared laser power delivery, high-speed communications, and ultra-fast switching. 
     Chalcogenide glass layers may typically be formed through conventional deposition methods, such as evaporative deposition techniques. These techniques may be costly and may not be suitable from some applications. Additive manufacturing technologies may help reduce the cost of manufacturing in general, however, typical additive manufacturing techniques rely on inks that include the desired material. Traditional additive manufacturing inks may not be compatible with the properties of chalcogenide glasses. Other disadvantages may exist. 
     SUMMARY 
     In an embodiment, an additive manufacturing ink composition may include a fluid medium and a chalcogenide glass suspended within the fluid medium to form a chalcogenide glass mixture solution. It should be noted that, as used herein, the term “solution” may refer to a solute dissolved in a solvent, or a suspension of nanoparticles in a fluid medium, which may not be dissolved. The additive manufacturing ink composition may further include a surfactant. 
     In some embodiments, the fluid medium includes terpineol or cyclohexanone. In some embodiments, the chalcogenide glass includes a variety of glass compositions, such as Ge—Se, Ge—S, Ge—Sn—Se. Ge—Sn—S Ge—Sb—Te, Ge—Pb—S, and other chalcogenide glass systems. In some embodiments, the surfactant includes ethyl cellulose. In some embodiments, the chalcogenide glass mixture includes between 0.15 grams of chalcogenide glass per milliliter of the fluid medium and 0.3 grams of chalcogenide glass per milliliter of the fluid medium. In some embodiments, the chalcogenide glass mixture includes 0.12 grams of the surfactant per milliliter of the fluid medium. 
     In an embodiment, a method for forming an additive manufacturing ink includes wet milling a chalcogenide glass in a fluid medium and a surfactant to produce a chalcogenide glass mixture. The method further includes, after wet milling the chalcogenide glass, processing the chalcogenide glass mixture to reduce an average particle size of the chalcogenide glass. 
     In some embodiments, the method includes synthesizing the chalcogenide glass from bulk materials having 5N purity using a melt quenching process. In some embodiments, the fluid medium is free of amines. In some embodiments, the fluid medium includes cyclohexanone. In some embodiments, the surfactant includes ethyl cellulose. In some embodiments, processing the chalcogenide glass mixture includes ultrasonicating the chalcogenide glass mixture and centrifuging the chalcogenide glass mixture. In some embodiments, the method includes adjusting a viscosity of the chalcogenide glass mixture by adding additional fluid medium to the chalcogenide glass mixture. In some embodiments, after adjusting the viscosity of the chalcogenide glass mixture, the chalcogenide glass mixture includes between 0.15 grams of chalcogenide glass per milliliter of the fluid medium and 0.3 grams of chalcogenide glass per milliliter of the fluid medium. In some embodiments, after adjusting the viscosity of the chalcogenide glass mixture, the chalcogenide glass mixture includes 0.12 grams of the surfactant per milliliter of the fluid medium. In an embodiment, during the processing of the chalcogenide glass mixture, the average particle size of the chalcogenide glass is reduced to less than or equal to 100 nm. 
     In an embodiment, a method for forming a dissolution based chalcogenide glass ink includes dissolving a chalcogenide glass into an amine-based solvent to form a chalcogenide glass solution. The method further includes filtering the chalcogenide glass solution. 
     In some embodiments, the solvent includes ethylenediamine or propylamine. In some embodiments, dissolving the chalcogenide glass into the amine-based solvent includes stirring the chalcogenide glass and the solvent using a magnetic stirrer at the rate of 700 rpm for at least 72 hours. In some embodiments, the method includes synthesizing the chalcogenide glass from bulk materials having 5N purity using a melt quenching process. In some embodiments, the method includes filtering the chalcogenide glass solution through a filter. In some embodiments, the method includes adding terpineol to the chalcogenide glass solution to increase its viscosity. In some embodiments, particles of chalcogenide glass from the chalcogenide glass solution are configured to agglomerate into a solid film upon application of a two-part sintering process. In some embodiments, the two-part sintering process includes heating the chalcogenide glass solution in a vacuum furnace to 100° C. for at least 24 hours and heating the chalcogenide glass solution to 130° C. for at least 24 hours. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an embodiment of an additive manufacturing ink obtained by dissolution or milling to nanoparticle size with added solvent. 
         FIG. 2  depicts an embodiment of a method for forming an additive manufacturing ink. 
         FIG. 3  depicts a transformation of bulk elements to bulk chalcogenide glass particles. 
         FIG. 4  depicts an embodiment of a chalcogenide glass paste. 
         FIG. 5  depicts the formation of a chalcogenide glass nanoparticle ink with a predetermined viscosity. 
         FIG. 6  depicts an embodiment of a method for making a dissolution based chalcogenide glass ink. 
         FIG. 7  depicts a chalcogenide glass ink including glass particles dissolved in a solvent. 
         FIG. 8  depicts a chalcogenide glass ink after evaporation has occurred. 
         FIG. 9  depicts a dissolution based chalcogenide ink after a second sintering phase has occurred. 
     
    
    
     While the disclosure is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the disclosure is not intended to be limited to the particular forms disclosed. Rather, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the disclosure. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an embodiment of an additive manufacturing ink is depicted. In the case of  FIG. 1 , the additive manufacturing ink may be a chalcogenide glass nanoparticle ink  100 . The depictions of  FIG. 1  are for clarity and may accurately depict the shape and proportions of the components of the chalcogenide glass nanoparticle ink  100 . The chalcogenide glass nanoparticle ink  100  may include a fluid medium  102 , a chalcogenide glass  104 , and a surfactant  106 . 
     The fluid medium  102  may include cyclohexanone (C6H100), which may have a boiling point of 156° C. and a viscosity 2.02 cP at 25° C. This high boiling point may prevent the fluid medium  102  from evaporating while flowing through a nozzle of a printer or of other piece of additive manufacturing equipment. Further, cyclohexanone may not react with the chalcogenide glass  104 . Other compositions may also be used as the fluid medium  102 . In order to reduce the risk of reaction with printer parts and nozzles, the fluid medium  102  may be free of amines. Amines may be more reactive with types of plastic that may be used in additive manufacturing. 
     The surfactant  106  may include ethyl cellulose (n-CH2CH3). As the surfactant  106 , the ethyl cellulose may include long chain polymers that can connect themselves to the chalcogenide glass  104  and prevent agglomeration. Thus, a texture and viscosity of the chalcogenide glass nanoparticle ink  100  may be maintained. 
     The chalcogenide glass  104  may include multiple compositions, such as a germanium-sulfide (Ge—S), a germanium-selenide (Ge—Se), a germanium-tin-sulfide (Ge—Sn—S), a germanium-tin-selenide (Ge—Sn—Se), a germanium-antimony-telleride (Ge—Sb—Te), or a germanium-lead-sulfide (Ge—Pb—S). Other possibilities may also exist. The chalcogenide glass  104  may be mixed with the fluid medium  102  at a concentration of between 0.15 grams of chalcogenide glass per milliliter of the fluid medium  102  and 0.3 grams of chalcogenide glass per milliliter of the fluid medium  102 . The surfactant  106  may be mixed with the fluid medium  102  at a concentration of 0.12 grams of the surfactant  106  per milliliter of the fluid medium  102 . An average particle size of the chalcogenide glass  104  may be less than or equal to 100 nm. Hence, the chalcogenide glass  104  may be referred to as nanoparticles. 
     A benefit of the chalcogenide glass nanoparticle ink  100  is that it may be used with printing processes and other additive manufacturing processing to form chalcogenide glass layers. Using printing processes may be less expensive than typical vapor deposition processes. Further, the chalcogenide glass nanoparticle ink  100  may include components that do not harm typical printers. Other benefits may exist. 
     Referring to  FIG. 2 , an embodiment of a method  200  for forming an additive manufacturing ink is depicted. The method  200  may include synthesizing a chalcogenide glass, at  202 . Some of the options for the chalcogenide glass include germanium-sulfide (Ge—S), germanium-selenide (Ge—Se), germanium-tin-sulfide (Ge—Sn—S), germanium-tin-selenide (Ge—Sn—Se), germanium-antimony-telluride (Ge—Sb—Te), and germanium-lead-sulfide (Ge—Pb—S) because they are thermally stable, have wide glass forming regions, and may be less toxic as compared to other chalcogenides, for example those containing Amines. Further, ternaries such as Pb2Ge8S15 and Sn2Ge8S15 may belong to a family of chalcogenide glasses where the members have more than one crystallization temperature. These chalcogenide glasses could be beneficial for increasing temperature sensing resolutions and could also make sensing devices more compact. 
     Ge20Se80, Ge30Se70, Ge33Se67, Ge40Se60, Sn2Ge8S15, Ge2Sb2Te5, and Pb2Ge8S15 bulk glasses may be synthesized through melt quenching technique. Pure 5N elements may be weighed and loaded into quartz ampules, which are sealed under a vacuum at about 10 −4  mbar. The glass synthesis may be carried out in a programmable tube furnace for 168 hours with a peak temperature of 750° C. The furnace may be programmed at different rates, depending on the composition, to reach 750° C. within the first 24 hours of synthesis with some holdings at characteristic temperatures. The quartz ampules may be kept at the highest temperature for 168 hours (about one week). This may result in a good homogenization of the melt. The resulting glass may be quenched in water from a temperature of 20° C. above the melting temperature for the synthesized composition as can be derived from phase diagrams. 
     The method  200  may further include crushing the chalcogenide glass into a chalcogenide glass powder, at  204 . For example, after synthesis, a chalcogenide glass may include an uneven distribution of chunks and shards. In order to further process it, it may be beneficial to crush the chalcogenide glass into substantially more uniform particles. 
     The method  200  may also include wet milling the chalcogenide glass powder, at  206 . For milling 2 mm stainless steel milling balls may be used. The starting particle size of the chalcogenide glass may be smaller than the milling balls. An agate mortar and pestle may be used to ensure that particles within the chalcogenide glass powder less than about 2 mm. In general, ball milling can be done either dry or wet. In this case, wet milling may help prevent particle agglomeration. Further, the use of a surfactant during milling may produce finer particles and also reduce wastage of material by preventing particle adhesion to a milling jar and to the milling balls. The chalcogenide glass (e.g., germanium-selenide), the surfactant (e.g., ethyl cellulose) and the fluid medium (e.g., cyclohexanone) may be mixed and introduced into a milling jar with the milling balls. 
     Ethyl cellulose may takes some time to dissolve in cyclohexanone. However, un-dissolved ethyl cellulose may not have a significant impact the milling and may be dissolved during the process instead of before the process begins. The fluid medium may be selected to prevent chemical reaction with either the chalcogenide glass particles or the surfactant. 
     In some cases, a ball mill may not be designed for continuous production. In these cases, it may be set to mill for 30 minutes, pause for 30 minutes, and then repeat the process until the milling time has been completed. During milling, the temperature may be kept below 50° C. to prevent undesired crystallization of the chalcogenide glass particles. Milling may be continued until a particle size of the chalcogenide glass becomes saturated. 
     The method  200  may include determining whether a particle size of the chalcogenide glass is about 250 nm, at  208 . If the target particle size has not been reached, the method  200  may include continuing to wet mill the chalcogenide glass power, at  206 . A dynamic Light Scattering (DLS) system may be used to measure the particle size every 24 hours. DLS utilizes light scattering to measure particle size. Pure cyclohexanone may be poured in a vendor recommended glass cuvette. A drop of ink may be mixed with the cyclohexanone. Individual particle sizes may then be determined using DLS. 
     If the target particle size has been reached, then the method  200  may include mixing the resulting paste with additional fluid medium, at  210 . Milling alone may not result in a desired viscosity. After ball milling the chalcogenide glass mixture may come out as a paste. A viscosity between 8 cP and 24 cP may be desirable for an additive manufacturing process. For final adjustment of the ink viscosity, cyclohexanone and ethyl cellulose may be added. For example, another 50 ml of cyclohexanone may be added to the paste to prepare a less viscous solution. In an example preparation, a good concentration was found to be between 0.15 g/ml and 0.3 g/ml of chalcogenide glass in cyclohexanone and about 0.12 g/ml of ethyl cellulose in cyclohexanone. Further, a particle size between 100 nm and 270 nm showed good results in terms of processing and device performance. In some embodiments, this mixing of the paste with additional fluid medium may be performed after the following steps of ultrasonicating and centrifuging. 
     The method  200  may further include ultrasonicating the mixture, at  212 . For example, an ultrasonicator may utilize a probe to transfer vibrational energy to the mixture. The diameter of the probe may be selected based on the volume of the mixture. For example, a 2 mm diameter probe may be used for a 30 ml mixture. Other possibilities may exist based on experimentation. To reduce the size of the particles, the mixture may be sonicated for about 2½ hours. A diameter of the particles after ultrasonication may be as low as 145 nm. However, the average size may be much higher due to a wide variability in particle size. Further ultrasonication can be used to both disperse and reduce particle size. For example, in 10 to 12 hours ultrasonication can reduce the average particle size from its bulk size to less than 250 nm. 
     The method  200  may also include centrifuging the mixture, at  214 . Centrifuging the mixture at 4500 rpm for about 1.5 hours may help create a uniform particle size within the mixture. The resulting mixture may have chalcogenide glass nanoparticles having a diameter of less than 100 nm, which may be sufficient for sintering or melting the particles at a temperature below 427° C., which may be lower than the lowest crystallization temperature of the materials used in the mixture. 
     The method  200  may include determining whether a particle size of the chalcogenide glass less than 100 nm, at  216 . If the target particle size has not been reached, the method  200  may include repeatedly ultrasonicating the mixture, at  212 , and centrifuging the mixture, at  214 . If the target particle size has been reached, then the resulting ink may be ready for printing, at  218 . 
     A benefit of the method  200  is that it may be used to form a chalcogenide glass nanoparticle ink for printing in conventional additive printers. Using printers may be less expensive than typical vapor deposition processes for chalcogenide glass layers. Further, the chalcogenide glass nanoparticle ink may include components that do not harm typical printers. Other benefits may exist. 
     Referring to  FIG. 3 , a transformation  300  of bulk elements to bulk chalcogenide glass particles  306  is depicted. For example, bulk germanium  302  and bulk selenium  304  may be synthesized into chalcogenide glass and then crushed into a powder that includes the chalcogenide glass particles  306 . The chalcogenide glass particles  306  may correspond to a state after the step  204  of the method  200 . It should be noted that the germanium-selenide composition depicted in  FIG. 3  is only for example, purposes. Other compositions may exist as described herein. The bulk chalcogenide glass powder may have an average diameter  308  that may be less than about 2 mm after the transformation  300 . 
     Referring to  FIG. 4 , an embodiment of a chalcogenide glass paste  400  is depicted. The paste  400  may correspond to a state after the step  208  of the method  200 . For example, the paste  400  may be the result of wet milling as described herein. The paste  400  may include a fluid medium  402 , chalcogenide glass particles  404 , and a surfactant  406 . The chalcogenide glass particles  404  may have a diameter  408  of about 250 nm. 
     Referring to  FIG. 5 , the formation of a chalcogenide glass nanoparticle ink  510  with a desired viscosity is depicted. For example, the paste  400  may be mixed with additional fluid medium  502  to reduce a concentration of the chalcogenide glass particles  404  and the surfactant  406  within the combined fluid medium  512 . In some cases, the addition of the additional fluid medium  502  may be performed after the step  208  of method  200 , as shown in  FIG. 2 . In other cases, the additional fluid medium  502  may be added after the ultrasonication step  212  and the centrifuge step  214 . 
     Amines, as described herein, may be used to make dissolution-based chalcogenide glass inks. In some cases, the amines described herein may be reactive with various parts of printers. For example, the chalcogenide glass mixtures used may include corrosive fluid mediums or solvents, such as amine-based mediums. These mixtures may not be suitable for many printing applications because they can damage printers and printer components if made from plastic polymers. However, these inks may be appropriate for other additive manufacturing processes, such as screen printing, nScrypt printer, and spin coating. 
     Referring to  FIG. 6 , an embodiment of a method  600  for making a dissolution based chalcogenide glass ink is depicted. The method  600  may include synthesizing a chalcogenide glass, at  602 . The process of synthesizing the chalcogenide glass may be the same process as described with reference to the method  200 . In particular, Ge20Se80, Ge30Se70, Ge33Se67, Ge40Se60, Sn2Ge8S15, Ge2Sb2Te5, and Pb2Ge8S15 bulk glasses may be synthesized through melt quenching technique using pure 5N elements. 
     The method  600  may further include crushing the chalcogenide glass into a chalcogenide glass powder, at  604 . The crushing may be performed using an agate mortar and pestle. Other techniques are possible. 
     The method  600  may also include dissolving the chalcogenide glass powder into a solvent, at  606 . Amine solvents may be capable of dissolving the chalcogenide glass powder. Examples of amines that may be used are ethylenediamine (EDA) and propylamine (PA). Other solvents are possible. A volumetric flask may be used to perform the mixing. The dissolution rate of Ge—Se based chalcogenide glass, for example, into an amine-based solvent may be about 0.08 g per 20 ml. To speed up the dissolution process, the solution may be stirred by using a magnetic stirrer at a rate of 700 rpm for 72 hours. During the process, a lid of the volumetric flask may be kept closed by para-film to prevent evaporation. 
     The method  600  may include increasing a viscosity of the solution, at  608 . For example, terpineol may be added to the solution to increase its viscosity. The terpineol and the increased viscosity may improve a resolution of a resulting printed film. In some applications 5% to 10% of terpineol is sufficient to achieve a good resolution for screen-printed patterns. 
     The method  600  may further include performing vacuum filtration of the solution, at  610 . For example, the solution may be filtered through a 0.025 μm nylon filter using a vacuum filtration technique. The filtration may remove larger particles of chalcogenide glass that have not been dissolved in the solvent. In some embodiments, the step  608  of increasing the viscosity of the solution may be performed after the step  610  of filtering the solution. 
     The resulting ink may be ready for printing, at  612 . Before the printing process, the dissolution based ink may be characterized using a tensiometer. A contact angle of the ink when applied to a substrate may provide information about how well the ink will spread over the substrate. It may be an important indicator of proper ink viscosity and the quality of the ink formulation process. 
     The tensiometer may be used to measure the contact angle by applying a droplet of ink to a stage and then analyzing image data of the droplet. First, the stage may be flattened to ensure that the droplet of ink does not move during deposition. After the droplet is deposited on the stage, an image may be recorded by a camera. The image may be analyzed using software to determine the contact angle. 
     The prepared ink can be applied for printing on a substrate using a screen printer or a 3D printer or by spin coating the ink on the substrate. In the case of screen printing, a monofilament polyester fabric may be stretched extremely tightly on a metal frame. A photosensitive emulsion may be coated over the fabric to form a stencil. The combination of the stencil and the metal frame may form a screen. After drying the photosensitive emulsion, the desired printing design may be transferred to the screen. In some screen printing processes, a desired print design transparent sheet may be laid onto an emulsion coated screen. The screen may be exposed to UV light and the exposed area may be hardened. The unexposed area may be washed away using water. 
     The screen may be placed on a printing press. The substrate may be placed on a flat printing board underneath the screen and the screen may be lowered onto the printing board. The chalcogenide glass dissolution based ink may be added to a top of the screen and a squeegee may be used to pull the ink along the full length of the screen. The ink may then transfer through the open areas of the stencil design. As a result, the desired design may be imprinted on the substrate. A thickness of the printed film can be varied by changing an emulsion thickness of the screen. 
     After the printing process, a sintering process may be applied to the printed ink. The printed and dried film patterns may be highly resistive due to the inconsistencies in the films, which may not be sufficient for some applications. For example, referring to  FIG. 7 , a chalcogenide glass ink  700  may include chalcogenide glass particles  702  dissolved in a solvent  704 . The solvent  704  may obstruct electrical conduction within the chalcogenide glass ink  700 . Sintering may combine particles of chalcogenide glass to create a solid film. This may form a conductive printed pattern. The sintering process may depend on factors, such as the composition of the solution, the particle size of the chalcogenide glass, a heat rate, a sintering temperature, a sintering time, liquid phase formation properties, and so on. The sintering process may also help the printed pattern or film to have better adhesion to the substrate. The basic sintering process may expose the printed film to heat, intense light, microwave radiation, plasma, or an electric field, which may trigger the formation of continuous films. The main challenge may be to remove the solvent from the surface of the printed pattern and to break down polymer backbones of the solution. Since the chalcogenide glass may be a photosensitive material, a thermal sintering process may be more appropriate. 
     Referring to  FIG. 8 , a chalcogenide ink  800 , which may correspond to the chalcogenide ink  700 , is depicted after evaporation has occurred. As mentioned before, FDA or PA may be used as the solvent  704 . The boiling point of EDA may be 116° C., and the boiling point of PA may be 51° C. The printed film may be placed into a vacuum furnace at 100° C. for 24 hours. In this phase, the solvent  704  of the dissolution based ink may be evaporated from a surface of the printed film, placing the chalcogenide glass particles  704  in close contact. 
     Referring to  FIG. 9 , a dissolution based chalcogenide ink  900 , which may correspond to the chalcogenide ink  700  and to the chalcogenide glass ink  800 , is depicted after a second sintering phase has occurred. The second sintering phase may include heating the chalcogenide glass ink  900  to 130° C. for 24 hours. In this phase, the particles  702  of the printed film may agglomerate and create a solid film. A vacuum furnace window may be covered with Aluminum to avoid light and assist with the occurrence of photoinduced processes in the sintering films, due to the photosensitivity properties of the chalcogenide glass. The temperature may be increased in increments of 20° C. with a hold of 15 minutes between them starting at room temperature and reaching 100° C. slowly during the first phase of the sintering process to avoid cracks formation on the printed films. 
     In an example, an EDA based Ge30Se70 ink printed into a film composition had about 1% variance from the bulk materials used to formulate the ink. As another example, a PA-based Ge30Se70 printed film may vary by about 5% to 6% as compared to the bulk material. 
     Although various embodiments have been shown and described, the present disclosure is not so limited and will be understood to include all such modifications and variations as would be apparent to one skilled in the art.