Patent Application
20060183018
The present invention relates to a method of fabricating a component of an electrochemical converter.
Electrochemical converters generally comprise a series of electrolyte units onto which electrodes are applied, and a series of interconnector units, disposed between the electrolyte units, to provide serial electrical connections. Each electrolyte unit is typically an ionic conductor having low ionic resistance, thereby allowing the transport of an ionic species from one electrode-electrolyte interface to the opposite electrode-electrolyte interface under the particular operating conditions of the converter.
Various electrolytes can be used in such electrochemical converters. For example, zirconia stabilized with such compounds as magnesia, calcia or yttria can satisfy these requirements when operating at an elevated temperature, e.g., about 1000° C. These electrolyte materials utilize oxygen ions to carry electrical current. Generally, the electrolyte does not conduct electrons which can cause a short-circuit of the converter. The interconnector unit, on the other hand, is typically a good electronic conductor. In operation, the interaction of the input reacting gas, electrode and electrolyte occur at the electrode-electrolyte interface, which requires that the electrodes be sufficiently porous to admit the reacting gas species to, and to permit exit of product gas species from the electrolyte surfaces.
The approach of forming electrochemical converters from electrolyte and interconnector components, and the bulk integration thereof were disclosed by the present inventor in U.S. Pat. No. 5,833,822, U.S. Pat. No. 5,747,185, U.S. Pat. No. 5,338,622, U.S. Pat. No. 4,496,445, U.S. Pat. No. 4,629,537, and U.S. Pat. No. 4,721,556, all of which are herein incorporated by reference. In particular, U.S. Pat. No. 5,833,822 describes an electrochemical converter assembly having one or more converter elements having a peripheral edge. The converter element includes a series of electrolyte plates having an oxidizer electrode material on one side and a fuel electrode material on the opposing side, and a series of interconnector plates, alternately stacked with the electrolyte plates, that provide electrical contact with the electrolyte plates. The interconnector plate or the electrolyte plate may have a textured pattern that forms reactant-flow passageways. These passageways selectively distribute the fuel and oxidizer reactants introduced to the columnar converter element. For example, the passageways distribute the fuel reactant over the fuel electrode side of the electrolyte plate, and the oxidant reactant over the oxidizer electrode side of the electrolyte plate. Alternatively, a spacer plate can be interposed between the electrolyte and interconnector plates to provide passageways through which the reactants can flow. The spacer plate can be either a corrugated or a perforated plate.
The present invention provides an improved method for fabricating a component of an electrochemical converter. The process of the present invention includes forming a free standing thin plate by using a tape casting method to form thin green sheets, and then applying a hot press method to densify the sheet to a near zero porosity state: A plurality of tapes may be laminated together prior to hot pressing to provide a thicker structure or a composite structure comprising layers of different materials. The resulting component is ultra dense, thin, has high oxidation and corrosion resistance, high electrical and thermal conductivity, hydrogen reduction stability, and low thermal expansion to match with ceramic components also used in the electrochemical converter. The process of the present invention can be applied to silicon carbide SiC, high chromium alloys, chromium iron alloys (e.g., Cr-5 wt % Fe-1 wt % Y2O3), chromium magnesium alloys (e.g., Cr-5 wt % Ni-1 wt % MgO) and mixtures thereof.
The present invention provides an improved method of fabricating a component of an electrochemical converter. The present invention will be described below relative to an illustrative embodiment. Those skilled in the art will appreciate that the present invention may be implemented in a number of different applications and embodiments and is not specifically limited in its application to the particular embodiments depicted herein.
Gas seals and electrical contact between plates are obtained in the assembly by spring loading the interconnector plates against the surface of the electrolyte plates with or without seal materials. For example, the plates of the electrochemical converter 10 are held in compression by a spring loaded tie-rod assembly 12. The tie-rod assembly 12 includes a tie-rod member 14 seated within a central oxidizer manifold that includes an assembly nut 14A. A pair of endplates 16 mounted at either end of the electrochemical converter element 10, protects the interconnector and electrolyte plates from damage caused by these rigid structural components during compression of the plates. Compressing the interconnector plates 30 and the electrolyte plates 20 together maintains the electrical contact between the plates and provides gas sealing at appropriate places within the assembly.
High temperature electrochemical converters generally employ components that satisfy a number of demanding requirements, including the use of a thermally conductive metal of 10 btu/F-ft-hr and an electrically conductive metal of 104 mho/cm, low thermal expansion ceramics of 5×10−6 in/in-F, a lightweight thin plate, for example, about 0.02 inch thickness, and a gas tight structure with no gas permeation. Moreover, electrochemical converters generally have an oxidation resistance up to about 1000° C.
In one aspect, zirconia, a refractory ceramic material, can be used as the electrolyte in the electrochemical converter due to its ability to conduct oxygen ions but not electrons at an elevated temperature of about 800-1000° C. The zirconia electrochemical converter offer higher efficiency than most other conventional or advanced energy conversion systems.
A zirconia electrochemical converter is assembled with thin plates of zirconia electrolyte alternately arranged with interconnector plates. The electrolyte plates are made of thin zirconia plates with electrode coatings. The interconnector plates are fabricated from corrosion-resistant, electrically conductive materials. To fabricate a zirconia electrochemical converter a freestanding electrolyte plate 20 with electrode coatings and a freestanding interconnector plate 30 are first fabricated. The plates are assembled by compression, with or without sealing material. Then, internal holes or manifolds and reactant-flow passageways can be formed in the plates to facilitate the passage of reactants and exhaust species.
The illustrated interconnector plates 30 provide multiple functions in the electrochemical converter. For example, the interconnector plates 30 provide low-loss electrical connections from cell-to-cell through a stack by contact with adjacent electrodes. The interconnector plates 30 also provide gas partitions to allow a repetitive series voltage connection of cells. The interconnector plates 30 also form effective thermal paths to conduct heat from the electrode surfaces to the outer edge of the plates, i.e., surfaces of the cell stack. The interconnector plates 30 may also form gaskets to prevent leakage of the reactants by the controlled yield of the material at operating temperature. Finally, the interconnector plates 30 also form stable structural members in the composite assembly encompassing the ceramic electrolyte plates and electrical conducting interconnector plates.
A wide variety of conductive materials can be used for the thin interconnector plates of this invention. Such materials should meet the following requirements: (1) high strength, as well as electrical and thermal conductivity; (2) good oxidation resistance up to the working temperature; (3) chemical compatibility and stability with the input reactants; and (4) manufacturing economy when formed into the textured plate configuration exemplified by reactant-flow passageways. The material also optionally and preferably exhibits a coefficient of thermal expansion that correlates closely with the ceramic electrolyte plates, including Zirconia electrolyte plates.
Traditional metallic plates are good electrical conductors in ambient environments. However, when subjected to high temperature and/or moist environment, the metallic conductors experience accelerated oxidation and corrosion degradations. One such circumstance exists in high temperature electrochemically converters. The fictional stacks of high temperature electrochemical converters require electrically conductive plates to act as serial electrical conductors and gas separation barriers for individual cells within the stack Such high temperature electrochemical converters today utilize super alloys or ceramic conductive plates to perform the described functions. Problems with the ceramic interconnectors include high cost, fragility, and low electrical conductivity. Drawbacks of the super alloy approach are multiple, including oxidation and corrosion problems that diminish its electrical conductivity and mechanical strength, and coefficient of thermal expansion (CTE) mismatch with the cell material that affects adversely the mechanical integrity of the stack.
Suitable materials for interconnector plate fabrication include silicon carbide (SiC), high chromium alloys, such as a chromium oxide mixture, chromium iron alloys (Cr-5 wt % Fe-1 wt % Y2O3) and chromium magnesium alloys (Cr-5 wt % Ni-1 wt % MgO). Chromium alloys are typically suitable for high temperature applications that employ ambient air of an oxidation environment One skilled in the art will recognize that the invention is not limited to these materials and that any suitable material may be used.
To facilitate radial heat transfer in the electrochemical converter stacks, electrolyte and interconnector plates of between about 5 centimeters and about 15 centimeters in diameter are appropriate. However, depending upon the application and design parameters, other sizes obvious to the ordinary skilled artisan will be obvious. Stacks of a relatively smaller diameter are suitable for systems requiring quick transient response, while stacks of a relatively larger diameter are appropriate for base-load power systems. A zirconia converter of modular design can be conveniently packaged as a small kW-level generator and also in a 10-25 kW module as a building block in scaled-up MW-level general applications. The distinctive features that make this electrochemical converter stack suitable for practical power applications are its high power density, ease of heat removal, structural ruggedness and low stress assembly.
The present invention provides an improved fabrication method for producing a component of an electrochemical converter.
As shown in
After trimming, the laminate structure is hot pressed in step 70 by applying heat and pressure to form a high density, near zero porosity sintered structure. As used herein, the term “high density” refers to a material having a specific density of about 96% or greater, i.e., the material oecupiesat least 96% of the volume of the component, such that the total pore volume is less than about 4% of the total volume of the component. According to an illustrative embodiment, the step of hot pressing comprises sintering the laminate structure into a dense structure using a pressure-assisted furnace or kiln in an inert or reducing atmosphere. Suitable temperatures and pressure for performing hot-pressing are obvious to those of ordinary skill in the art, and are generally in the range of about 1300° C. and 1000 psi, respectively. After pressure sintering, the sintered structure may be mechanically, chemically, or thermally machined or trimmed to a desirable configuration in step 80. The trimmed sintered structure may be coated with other compounds, such as protective coatings, in step 90. According to one aspect, the coating may be applied using plasma spray, chemical vapor deposition, or physical vapor deposition equipment, though one skilled in the art will recognize that the invention is not limited to these coating techniques.
According to an alternate embodiment of the invention, the fabrication method includes a step of subjecting the laminated structure to a furnace sintering process before the hot press process in step 70.
According to an alternate embodiment, a raw material comprising a fine powder i.e., having a particle size on the nanometer scale, is used to form the component. In this embodiment, step 70 may alternatively comprise a pressure-free sintering step, which uses heat only to sinter the structure.
According to an illustrative embodiment, the component is an electrically conductive interconnector plate, though one skilled in the art will recognize that the illustrated fabrication method may be used to fabricate any suitable component, including that of an electrochemical converter, or any other plate. According to another aspect, the fabrication method of an illustrative embodiment of the invention produces a composite plate consisting of a high chromium core and a lanthanum chromite surface protection layer.
For example, according to an illustrative embodiment, the illustrated method produces an interconnector plate for an electrochemical converter that comprises a high chromium composite core and a lanthanum chromite surface protective sheet, though one skilled in the art will recognize that any suitable material may be used in the fabrication method of the present invention and any suitable component maybe produced. For example, to produce a chromium-lanthanum chromite composite plate, a sheet having a high chromium content, formed of a powder material that is preferably more than 95% chromium, is produced by tape casting, following steps 10-30 in
The interconnector plate formed by the illustrative fabrication method may have textured surfaces, which oppose flat surfaces of adjacent electrolyte plates in the stacked electrochemical converter. Alternatively, the interconnector plate may have flat surfaces, which oppose textured surfaces of adjacent electrolyte plates in the stacked electrochemical converter.
The fabrication method of an illustrative embodiment of the present invention provides an ultra dense.(i.e., having a specific density of at least 96%), thin and cheaper component for an electrochemical converter. The fabrication method produces a component having high oxidation and corrosion resistance, high electrical and thermal conductivity, hydrogen reduction stability of up to 1000° C., and low thermal expansion to match with ceramic components.
It will thus be seen that the invention contains improvements over the prior art. Since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense.
It is also to be understood that the following claims are to cover all generic and specific features of the invention described herein, and all statements of the scope of the invention which as a matter of language, might be said to fall therebetween. For example, the fabrication method for dense thin plates of the present invention call also employ in any applications including the electrochemical devices such as molten carbonate, phosphoric acid, and proton exchange membrane converters.
The present invention claims priority to U.S. Provisional Patent Application Ser. No. 60/403,218, filed Aug. 13, 2002, the contents of which are herein incorporated by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US03/25517 | 8/13/2003 | WO | 4/5/2005 |
| Number | Date | Country | |
|---|---|---|---|
| 60403218 | Aug 2002 | US |
