Patent Description:
The present invention relates to the additive manufacture of complex objects using refractory materials for a variety of energy-related applications.

The majority of worldwide energy systems today are designed to convert heat to electricity. The heat may be generated from fossil fuels, solar thermal methods, or nuclear fission or fusion, for example. The second law of thermodynamics dictates that in order to extract the maximum efficiency from heat engines, high operating temperatures are necessary. A majority of the pathways that allow for conversion of heat to electricity, such as Rankine or Brayton cycles, require a thermal fluid. The ability to achieve high efficiencies requires high fluid temperatures. This in turn requires components that are made from materials that can withstand these high temperatures. These materials may form the combustion vessel or reactor core in a fossil or fission energy system, respectively, as well as piping, heat exchangers, and power conversion components. Refractory materials that can withstand high temperatures are therefore ideal in these applications.

While the production of simple geometries (e.g., piping) from refractory metals or ceramics is possible today, components of higher complexity, for example heat exchangers, flanges, and turbines, are not readily produced. The ability to use refractory metals and ceramics for the manufacture of complex components would greatly improve the thermal efficiency of these energy systems, well beyond what is possible with conventional means, for example high temperature Ni-based superalloys. Accordingly, there remains a continued need for methods for the manufacture of components from refractory materials, including for example ceramics and metals, the components having complex three-dimensional geometries for use in energy systems and other applications. Reference may be made to <CIT> which relates to a member for semiconductor equipment. Reference may be made to <CIT> which relates to a method for preparing silicon carbide ceramic matrix composite through combination of spray granulation, 3DP (three-dimensional printing) and CVI (chemical vapor infiltration).

The present invention is defined by the appended independent claims, to which reference should now be made. Specific embodiments are defined in the dependent claims.

The resulting object includes a substantially pure microstructure with excellent resistance to high temperatures. Example objects include heat exchangers, turbines, flanges, to name only a few.

As set forth herein, the present method is readily adapted for fabricating objects having complex geometries in applications where high heat resistance is desired. Additive manufacturing of a green body is generally performed at room temperature, and the CVI furnace operates at temperatures far below sintering temperatures. In embodiments having nuclear fuel contained therein, the packing fraction of nuclear fuel particles was found to be greater than <NUM>%, outpacing the packing density found in pressed and sintered nuclear fuels, resulting in smaller nuclear fuel assemblies.

These and other features of the invention will be more fully understood and appreciated by reference to the description of the embodiments and the drawings.

Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited to the details of operation or to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention may be implemented in various other embodiments and of being practiced or being carried out in alternative ways not expressly disclosed herein, but within the scope of the appended claims. In addition, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of "including" and "comprising" and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. Further, enumeration may be used in the description of various embodiments. Nor should the use of enumeration be construed as excluding from the scope of the invention any additional steps or components that might be combined with or into the enumerated steps or components.

As discussed herein, the current embodiments generally relate to a method for the manufacture of a wide-variety of object using a refractory matrix material. The method includes the additive manufacture of a green body from a powder-based refractory matrix material followed by densification via CVI. The method of manufacture is generally discussed in Part I below, followed by a description of an integral nuclear fuel element formed according to this method in Part II below. Though described in connection with a nuclear fuel element, the present method is applicable in effectively any application in which a complex three-dimensional object requires high heat resistance, including heat exchangers, flanges, and turbines blades for example. Similarly, just as nuclear fuel may be embedded inside the refractory matrix, other constitutes and devices may be incorporated into the matrix.

A method according to one embodiment includes the manufacture of a three-dimensional object using a refractory matrix material. With reference to <FIG>, the method generally includes (a) the selection of a refractory feedstock, (b) the additive manufacture of a green body using the refractory feedstock, (c) the introduction of a refractory gaseous precursor for CVI, (d) CVI of the green body for densification and removal of the binder, and (e) the completion of the final part having a complex three-dimensional geometry. Each such operation is separately discussed below.

The selection of a refractory feedstock at step <NUM> includes the selection of a refractory ceramic powder feedstock or a refractory metal powder feedstock. A suitable refractory ceramic can include, for example, SiC, C, or ZrC, and a suitable refractory metal can include, for example, Mo or W. At step <NUM>, the green body is formed according to an additive manufacturing process to produce a three-dimensional object. In the current embodiment, the green body is formed according to a binder-jet printing process. In the binder-jet printing process, a powder bed of the refractory material is printed at ambient temperatures with a binder pattern layer-by-layer, as optionally set forth in <CIT> and <CIT>.

More particularly, the powder feedstock is deposited in sequential layers, one on top of the other. Following the deposit of each layer of powder feedstock, a liquid binder material, for example a polymeric binder, is selectively supplied to the layer of powder feedstock in accordance with a computer model (e.g., CAD model) of the three-dimensional object being formed.

Once the three-dimensional object is completed, the unbound powder is removed, yielding a near net-shaped green body held together by the removable polymeric binder. The green body can have a binder content on the order of a few wt%, for example <NUM>-<NUM>%, with a density of about <NUM>-<NUM>% of their theoretical limit. For example, the green body is a dimensionally stable object of greater than <NUM>% by weight of SiC (or other refractory material) in one embodiment, further optionally greater than <NUM>% by weight SiC (or other refractory material) in other embodiments. At step <NUM>, a gaseous refractory precursor for CVI is selected, such that the finished object can include a highly pure and uniform matrix. For example, the gaseous refractory precursor for CVI of a SiC green body can include MethylTrichloroSilane (MTS) that gives SiC by the MTS decomposing. Further by example, the gaseous refractory precursor for a ZrC green body can include zirconium tetrachloride (ZrCl<NUM>) gas, the gaseous refractory precursor for a graphite (C) green body can include methane (CH<NUM>) gas, the gaseous refractory precursor for CVI of a W green body can include tungsten hexafluoride (WF<NUM>) gas, and the gaseous refractory precursor for CVI of a Mo green body can include molybdenum hexafluoride (MoF<NUM>) or molybdenum pentachloride (MoCl<NUM>) gas. In other embodiments, however, a composite matrix may be realized by printing one material in powder form and depositing another material around the powder with CVI.

At step <NUM>, the green body is placed in a CVI furnace (reactor vessel) into which the gaseous precursor and carrier gas that could be inert (e.g. Ar) or otherwise (e.g. H<NUM>) is admitted. The pressure and temperature within the furnace and the composition, partial pressure and flow rate of the gaseous precursor are selected to allow the gaseous precursor to diffuse within the pores of the green body. More specifically, CVI involves the temperature decomposition of the gaseous precursor (e.g., MTS or WF<NUM>) and the infiltration and then absorption of the decomposed precursor within the pores of the matrix material (e.g., SiC or W). The CVI process for SiC involves a process temperature of between <NUM> and <NUM>, <NUM> and <NUM>, optionally <NUM>, which is far below the temperatures required for sintering in existing methods (~<NUM>). Of note, as the temperatures increase within the CVI furnace, the binder dissociates and is removed prior to the start of the CVI process. The CVI process initially uniformly densifies the green body, and as the pores inside the green body become closed, the CVI selectively deposits a fully dense coating on all internal and external surfaces of the three-dimensional object. The densified green body can include a density of greater than <NUM>% by weight of SiC in some embodiments, and greater than <NUM>% by weight of SiC in other embodiments. This phenomenon is further illustrated in <FIG>, which includes a cross-section of a binder-jet printed SiC specimen having a complex geometry. The cross-section includes a dense and hermetic outer layer with a thickness on the order of <NUM> to <NUM>, further optionally <NUM> to <NUM>. As also shown in the inset of <FIG>, a continuous CVI SiC matrix is deposited around the SiC powders, such that the microstructure includes both the original 3D printed SiC powder and a continuous CVI SiC matrix.

Completion of the final part is depicted at step <NUM>. Owing to the formation of the green body by additive manufacturing, the finished article can possess almost any geometry, including overhangs, undercuts, and internal volumes. As shown in <FIG> for example, green body can include a heat exchanger <NUM> having a multi-channel primary loop <NUM> and a helical secondary loop <NUM> (or vice versa), each defining a complex internal volume not readily formed according to conventional methods. As alternatively shown in <FIG> for example, the present method can be used to form a turbine blade <NUM> or other objects whose manufacture is difficult or not possible according to conventional methods.

An integral nuclear fuel element and its method of manufacture will now be described. As set forth below, the integral nuclear fuel element generally includes a CVI-densified fuel envelope formed of a 3D printed refractory matrix material and containing uniformly dispersed fuel particles therein, for example TRISO nuclear fuel particles, the fuel envelope optionally being shaped as a prismatic fuel block.

As shown in <FIG>, an integral nuclear fuel element is formed by binder-jet printing a rigid envelope <NUM> from a refractory powder feedstock. The envelope <NUM> can include any construction having at least one internal volume (or cavity) <NUM> for a nuclear fuel and optionally at least one cooling channel <NUM>. In the illustrated embodiments, the internal volume <NUM> is defined between an outer sidewall <NUM>, an inner sidewall <NUM>, a base <NUM>, and a cap <NUM>, the internal volume <NUM> being accessible through one or more openings <NUM> in the cap <NUM>. The envelope <NUM> includes a hexagonal construction in the present embodiment, but can include other constructions in other embodiments, including for example cylindrical or any other construction including those that are axial and radially asymmetric. In the embodiment shown in <FIG>, a single cooling channel <NUM> extends vertically through the center of the envelope <NUM>, interconnecting the base <NUM> with the cap <NUM>, such that the internal volume <NUM> concentrically surrounds the cooling channel <NUM>. In the embodiments of <FIG> and <FIG>, multiple cooling channels <NUM> extend vertically through the envelope <NUM>, but differ from the cooling channel of <FIG> in that the cooling channels of <FIG> and <FIG> are non-linear or curvilinear, diverging and/or converging, and optionally port the cooling gas from the envelope at a non-zero angle relative to vertical. Because the binder-jet printing process can accommodate overhangs, undercuts, and internal volumes, the internal volume and the cooling channel can achieve effectively any geometry, with the geometries of <FIG> being depicted for illustrative purposes. <FIG> shows the general manufacturing steps for the nuclear fuel envelope of <FIG> and <FIG> with the leftmost illustration showing the CAD model for the fuel envelope, the center illustration showing the 3D printed envelope and the rightmost illustration showing the CVI-densified fuel envelope (without the fuel particles for the sake of disclosure).

The envelope is generally formed of a non-fuel refractory powder feedstock. Examples include SiC, C, ZrC, Mo, and W. <FIG> shows an SiC powder morphology and size distribution suitable for use in manufacturing the fuel envelope. Refractory powders in a range of alternative morphologies and alternative size distributions may be used in alternative applications. In this embodiment, the SiC powder is α-SiC (hexagonal phase) feedstock from Sigma Aldrich with a purity ><NUM>%. The powder feedstock is deposited in successive layers according to a binder-jet printing process, with a liquid binder being selectively supplied to each layer of powder feedstock in accordance with the CAD model of the envelope. In the illustrated embodiment, the envelope is 3D printed using an Innovent binderjet system from ExOne Company (North Huntingdon, PA). The envelope may, however, be formed using a wide variety of alternative binder-jet printers. After printing, the powder bed may undergo a binder curing step that drives off the majority of the aqueous or organic-based solvent. For example, the powder bed may be heated at about <NUM> for approximately <NUM> hours in air.

Once the envelope is fully printed, the unbound powder is removed, yielding for example the near net-shaped green body shown at left in <FIG>. Subsequent to the formation of the green body, and prior to CVI, fuel particles <NUM> are added to the internal fuel cavity. The fuel particles can include uranium or other fissile elements, and can be bare fuel kernels or coated particles, for example tri-structural isotropic (TRISO) particles, bi-structural isotropic (BISO) particles, and bare uranium-bearing (e.g., UO<NUM>, UC, UN, or there combinations) spheres (fuel kernels) containing fissile uranium. Further optionally, the fuel particles can include a combination of bare fuel kernels and coated particles. The fuel particles are added to the internal cavity according to any desired technique, for example from a hopper, until substantially full. Additional matrix powder feedstock is then added to the internal cavity, being at least an order of magnitude smaller than the fuel particles, to occupy the voids between adjacent fuel particles, while also coating the exposed fuel particles at the openings <NUM>. Further optionally, the additional matrix powder feedstock can be vibro-packed to ensure maximum densification prior to chemical vapor infiltration.

Subsequent to filling and vibro-packing the envelope with fuel particles and optional additional matrix material, the envelope is inserted within a CVI furnace and elevated to a temperature that is ideal for the specific CVI process. For instances, a temperature of between <NUM> and <NUM>, <NUM> and <NUM>, optionally <NUM>, is ideal for SiC deposition with MTS, while temperatures < <NUM> are ideal for W deposition using WF<NUM>. As the temperature is elevated in the CVI furnace, the polymeric binder dissociates starting at ~<NUM> with dissociation complete at <NUM>. During dissociation, the continuous inert gas flow in the CVI furnace vessel purges binder dissociation products. Once at the target CVI temperature, a gaseous precursor is introduced within the CVI furnace to allow additional matrix material deposition within the pores of the envelope. As the pores inside the envelope become closed, the CVI process selectively deposits a fully dense coating on all internal and external surfaces of the envelope. The resulting microstructure of the envelope includes high purity and an optionally uniform matrix, while sealing the nuclear fuel particles therein. As shown at right in <FIG>, a cross-section of the resulting integral nuclear fuel element include a dense fuel compact with uranium fuel particles embedded in a hermetically sealed refractory envelope. The content of the matrix material in the densified envelope is greater than <NUM>% in some embodiments, further optionally <NUM>% in other embodiments, and still further optionally greater than <NUM>% in other embodiments. Further, the matrix material (e.g., SiC) can comprise less than the <NUM>% by volume of the integral nuclear fuel element <NUM>. <FIG> shows the weight evolution in a green part produced with an aqueous binder after the optional curing step and upon heating in Ar to high temperature.

The integral nuclear fuel element includes a generally stackable construction. When arranged and stacked, the cooling channel(s) of each nuclear fuel element are in fluid communication with the cooling channel(s) of a vertically adjacent nuclear fuel element. The densified and highly pure refractory envelope can withstand normal operating temperatures within a reactor core, for example the reactor core of a high temperature gas-cooled reactor (HTGR) having a Brayton closed-cycle gas turbine or other power conversion means. In addition, the nuclear fuel within the nuclear fuel element includes an increased packing fraction over conventional fuels. For example, existing methods (pressing and sintering) provide packing fractions of up to <NUM>%. By contrast, the packing fraction of the nuclear fuel particles within the fuel envelope of the present invention can be greater than <NUM>%. As a result, nuclear fuel assemblies including the integral nuclear fuel elements of present invention can be made more compact. Further, the cooling channels can be manufactured with optimized geometries and surface features to improve cooling of the nuclear fuel compact therein, as the thermal energy is optimally transmitted to the cooling gas.

Claim 1:
A method for manufacturing an article comprising:
providing (<NUM>) a powder feedstock of silicon carbide;
selectively (<NUM>) depositing a binder onto successive layers of the powder feedstock to produce a dimensionally stable object of greater than <NUM>% by weight of silicon carbide;
positioning (<NUM>) the object within a chemical vapor infiltration, CVI, reactor and elevating the temperature therein, thereby debinding the object; and
introducing within the CVI reactor a precursor gas including silicon and a hydrocarbon while at an elevated temperature, such that a breakdown of the precursor gas at the elevated temperature causes silicon carbide to infiltrate the object and seal the object with a densified outer layer, the object including a substantially pure silicon carbide microstructure and high heat resistance with a density of greater than <NUM>% by weight of silicon carbide.