Patent Application
20180071950
This disclosure relates to generally heat exchangers and other fluid guides and more particularly to methods of manufacturing such articles.
Heat exchangers are devices built for transferring heat from a heat source to a heat sink. This transfer of heat often utilizes fluids (e.g., air, water, or organic compound or composition), either as a heat transfer medium or as the principal heat source or heat sink. The fluids may be separated by a solid barrier or other divider that keeps them from mixing. Heat exchangers are commonly used in refrigeration, air conditioning, space heating, electricity generation, and chemical processing. Heat exchangers are typically designed to provide a high surface area of the barrier between the two fluids, while minimizing resistance to fluid flow through the heat exchanger. The performance of the heat exchanger may also be affected by the addition of fins, corrugations or other forms of protuberances in one or both flow directions which increase surface area, channel fluid flow, and induce turbulence.
The manufacture of heat exchangers typically involves complex assembly and multiple steps of attachment, brazing, soldering, or welding operations of various metal heat exchanger components, including but not limited to the core, the exterior, fins, and manifolds. Additionally, many heat exchanger designs and methods for manufacture necessitate the use of metal parts, which may not be suited for certain environments such as corrosive or high temperature environments.
According to some embodiments of this disclosure, a method of making a heat exchanger comprises identifying a space for a heat exchanger fluid flow path. A template comprising carbon is formed in the shape of the flow path space, with void space in the shape of a fluid guide that forms the flow path space. A fluid composition comprising ceramic or a ceramic precursor is deposited to the template void space, and a solid ceramic is formed from the fluid composition. The template is removed by oxidizing the carbon.
According to some embodiments, a method of making a fluid guide comprises identifying a space for a fluid flow path. A template comprising carbon is formed in the shape of the flow path space, with void space in the shape of the fluid guide. A fluid composition comprising ceramic or a ceramic precursor is deposited to the template void space, and a solid ceramic is formed from the fluid composition. The template is removed by oxidizing the carbon.
Subject matter of this disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
With reference now to the Figures,
As mentioned above, the template comprises carbon. Various types of carbon-containing materials can be used, including but not limited to graphite or amorphous carbon. In some embodiments, the template material can be prepared by mixing carbon powder with a resin binder (e.g., 30 to 80 weight percent (based on total weight of carbon powder and binder) of a binder such as coal tar pitch, petroleum pitch, synthetic polymer resins) to form an extrudable or moldable material, and extruding or molding the material into the shape of the desired template structures. The molded material can optionally be cured with heat (e.g., 100-250° C.) to graphitize the carbon. Other carbon-containing template materials can be used as well. For example, wood materials or organic resins also contain carbon atoms, and can be used with lower temperature deposition processes such as sol-gel processes, discussed in greater detail below. In some embodiments, a larger piece of graphite could be machined into a more complex geometry if extruding it would be difficult. The template material can include additives or surface treatments that may promote deposition of the ceramic. For example, the template can include atomic content matching portions of the ceramic (e.g., silicon content), such as ceramic or ceramic precursor material (e.g., particles or layer) on or near the surface to promote formation of an interphase layer. In some embodiments, the template can be coated with a thin ceramic layer using chemical vapor deposition (CVD), or other thin film technique to help protect the carbon template from the harsher processing methods.
With reference now to
The ceramic material can be deposited or infiltrated into the template and densified by various techniques. Various ceramic deposition or infiltration processes and methodologies can be used. These include, but are not limited to, chemical vapor infiltration (CVI), polymer impregnation and pyrolysis (PIP), melt infiltration (MI), or sol-gel processing. Chemical vapor infiltration involves utilizing vapor deposition to apply a ceramic precursor to the template that is chemically transformed during the deposition process into a ceramic material. Examples of CVI ceramic precursors for ceramics include but are not limited to methyltrichlorosilane with hydrogen gas (for deposition of silicon carbide), boron trichloride and methane with hydrogen gas for deposition of boron carbide or titanium tetrachloride, methane with hydrogen gas for titanium carbide, silicon tetrachloride and ammonia gas for silicon nitride, aluminum isopropoxide for the deposition of aluminum oxide, or tetraethylorthosilicate for the deposition of silicon dioxide. CVI can be performed by introducing a carrier gas comprising the precursor composition into a heated space (e.g., an oven). This isothermal/isobaric process is also known as the “hot wall” technique. CVI can also be performed under non-isothermal conditions where the template is subjected to a temperature gradient and is infiltrated with a forced gas flow. CVI can be performed under reduced pressure or a vacuum, or at atmospheric pressure. Process variations commonly employed for chemical vapor deposition (CVD) can also be utilized for CVI, including but not limited to aerosol assist for the infiltrating gas flow, plasma assist for the infiltrating gas flow, or direct liquid injection (where liquid precursors are injected to a vaporization chamber prior to infiltration into the template).
Polymer impregnation and pyrolysis involves the introduction of a liquid composition comprising a pre-ceramic polymer into the template void space, followed by pyrolysis to convert the pre-ceramic polymer to a solid ceramic. Several subsequent polymer infiltration and pyrolysis cycles (6 to 11) are typically required to bring the ceramic to full density. Examples of pre-ceramic polymers include but are not limited to polycarbosilane (for deposition of silicon carbide), polysilazanes (for deposition of silicon nitride), and polysiloxanes (for deposition of silicon dioxide). The liquid composition can be introduced to the template void spaces using various known liquid molding, injection, infiltration, or other liquid processing techniques. Examples of methods for introducing the liquid composition to the template void spaces include immersion of the template into the liquid composition, injection of liquid into the template void spaces, vacuum transfer where a vacuum is utilized to draw the liquid into the template void spaces by evacuating air from the template void spaces in the presence of the liquid pressure-driven infiltration, pressure transfer where a pressure differential drives higher pressure liquid metal into the spaces in the pre-form. Combinations of these methods can be used as well. For example, the liquid can be injected under pressure into the template void spaces from one side of the template (e.g., the side 12c, 14c (
Melt infiltration involves the introduction of a liquid composition comprising a carbonaceous precursor into the template void space, followed by pyrolysis to convert the carbonaceous material to a solid carbon. Molten silicon metal is then introduced to the material, reacting with the carbonaceous material to form silicon carbide. For example, a liquid phenolic resin can be injected and then cured to surround the carbon fluid flow path template. The composite material would then be packed with a silicon metal and phenolic resin cover mixture, placed in a vacuum furnace, and heated above the melting point of silicon (1414° C.). The silicon melts, diffusing in to the composite and reacting with the cured phenolic resin to form the silicon carbide ceramic. For another example, the carbon fluid flow path template could be partially densified with carbon using CVI. The resulting partial composite structure can then be covered with a silicon metal and phenolic resin cover mix and heated under vacuum like the prior example. In some embodiments, the liquid composition can comprise carbon (e.g., a slurry of carbon powder, a carbonaceous resin, or a gaseous carbon species in the case of CVI). The liquid composition can be introduced to the template void spaces using various known liquid molding, injection, infiltration, or other liquid processing techniques. Examples of methods for introducing the liquid composition to the template void spaces include immersion of the template into the liquid composition, injection of liquid into the template void spaces, vacuum transfer where a vacuum is utilized to draw the liquid into the template void spaces by evacuating air from the template void spaces in the presence of the liquid pressure-driven infiltration, pressure transfer where a pressure differential drives higher pressure liquid metal into the spaces in the pre-form. Gaseous deposition methods such as CVI can also be used to deposit a carbon layer. Since the molten silicon will react with any carbon materials, this processing method can include applying a thin ceramic coating later to the carbon fluid flow path template. This layer can comprise a material resistant to attack by molten silicon, such as silicon carbide, titanium carbide, or boron nitride.
Sol-gel processing can also be used to introduce the ceramic into the template void spaces. Sol-gel formation of ceramics utilizes a ceramic precursor dissolved or in stable suspension in a liquid carrier (i.e., the “sol”). A chemical change is induced to this liquid carrier system that causes the ceramic precursor to drop out of the solution or suspension (the “gel”). The gelled coating can then be further dried and pyrolyzed to form a ceramic material. As applied herein, the liquid sol can be introduced to the template void spaces, followed by introduction of the chemical change to form the solid ceramic and subsequent drying to remove residual liquid. For example, a ceramic precursor such as an alkoxide or a metal alkoxide can undergo hydrolysis in an aqueous liquid comprising a miscible organic solvent to form a sol comprising a metal alkoxide or metal hydroxide. A chemical change (e.g., dilution of the miscible solvent with additional water or addition of an acid or base to cause a pH change) is used to initiate a polycondensation reaction of the metal alkoxide or metal hydroxide, resulting in the formation of a metal oxide or metal oxide amorphous structure. The condensed metal alkoxide is then dried and heated to from the crystalline ceramic material. Additional ceramic content can be incorporated into a sol-gel ceramic by inclusion of a source of a ceramic powder in the sol mixture (e.g., aluminum oxide or silicon dioxide powder) that is incorporated into domains (e.g., nanoscopic domains) dispersed in the gel phase such as a silica gel phase, followed by condensation and heating to form the ceramic monolith. Sol-gel processing lends itself to the formation of oxide ceramic materials such as aluminum oxide, silicon dioxide, or titanium dioxide.
Deposition and densification of the solid ceramic into the template void spaces produces the composite structure 20 comprising the carbon template and the deposited ceramic. The template is then removed to form the plate fin heat exchanger structure 30 in
The above description is of a particular example embodiment for a particular heat exchanger design, and can be subject to various modifications and changes. For example, the example embodiment in
While the present disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present disclosure is not limited to such disclosed embodiments. Rather, the present disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the present disclosure. Additionally, while various embodiments of the present disclosure have been described, it is to be understood that aspects of the present disclosure may include only some of the described embodiments. Accordingly, the present disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
