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Fabrication, phase transformation studies and characterization of SiC-AlN-Al sub 2 OC ceramics

Description: SiC and AlN are two of the important high temperature structural ceramics. AlN and the 2H polytype of SiC are isostructural. Prior work has shown that they form an extension solid solution at temperatures {ge} 2000{degrees}C. At lower temperatures, the solid solution can undergo phase separation. Additionally, Al{sub 2}OC is also isostructural and can form extensive solid solutions with SiC and AlN. The formation of solid solutions in such refractory materials as well as the tendency to undergo diffusional phase transformations suggests that a potential exists to improve properties through alloying. The principal objective of the proposed work is to examine phase relations, phase transformations, the resulting precipitate morphologies and their influence on mechanical properties of SiC-AlN-Al{sub 2}OC ceramics. Formation of modulated structures have been documented in SiC-AlN ceramics in our work. It has been shown that modulations occur along directions other than the (0001) direction and this results in the formation of a tweed type of a microstructure. In the AlN-Al{sub 2}OC system, the occurrence of cellular precipitates as well as coherent, disc-shaped precipitates has been observed. During the past year, work has progressed in the following areas: (1) Phase separation in SiC-AlN system: The effect of coherency strain energy on the precipitate morphology. (2) High temperature creep of SiC-AlN ceramics containing modulated structures and SiC-Al{sub 2}OC ceramics. (3) Fabrication and characterization of damage-resistant SiC-AlN ceramics. Three manuscripts have been submitted for publication.
Date: January 12, 1992
Creator: Virkar, A.V.
Partner: UNT Libraries Government Documents Department

Fabrication, phase transformation studies, and characterization of SiC-AlN-Al{sub 2}OC ceramics. Final report

Description: Principal focus was on phase transformation, microstructure development, and elevated temperature creep, with some effort on room- temperature mechanical properties of selected materials. Fabrication was largely hot pressing, although many of the compositions can be densified by pressureless sintering; hot pressing was to ensure full attainment of density with fine microstructure. Most of the work was on SiC-AlN and AlN-Al{sub 2}OC pseudobinaries.
Date: February 28, 1994
Creator: Virkar, A. V.
Partner: UNT Libraries Government Documents Department

The effect of pressure on solid oxide fuel cell performance

Description: Current work in solid oxide fuel cells (SOFCs) is on cathode-supported, anode-supported, or electrolyte-supported cells. In electrode-supported cells, a thin film (5 to 30 microns) of an electrolyte (YSZ) is deposited on a relatively thick, porous electrode. In electrolyte-supported cells, the electrolyte thickness is typically greater than or equal 150 microns upon which thin electrodes are screen printed. Both types of SOFCs are being explored for hybrid applications, that is, in combination with a gas turbine, for which the exit gases from an SOFC generator must be at a high pressure (3 to 15 atm) for input into a gas turbine. It is necessary to examine the expected performance of an SOFC under a high pressure. Work at Westinghouse Ontario Hydro has shown that the performance improvement at high pressures is greater than that can be expected based on an increased Nernst potential alone. This increased performance can in part be attributed to a lower concentration polarization. The objective of this work was to conduct a preliminary analysis of the effect of pressure on the performance of both cathode-supported and electrolyte-supported cells. Flux equations for transport through porous electrodes are formulated and are solved in combination with those for electrochemical operation of an SOFC for cathode-supported and electrolyte-supported cells. The analysis shows that the overall cell performance increases significantly with increasing pressure in the case of cathode-supported cells due to a lowering of concentration polarization at high pressures. Similar effects (not presented here) are also observed on anode-supported cells. By contrast, only a modest improvement is observed in the case of electrolyte-supported cells, commensurate with the fact that in the latter, the ohmic contribution of the electrolyte is the most dominant one, which is not altered by pressure.
Date: December 1, 1997
Creator: Virkar, A.V.; Fung, K.Z. & Singhal, S.C.
Partner: UNT Libraries Government Documents Department

Fabrication, phase transformation studies and characterization of SiC-AlN-Al{sub 2}OC ceramics. [Annual report, February 1, 1993--July 30, 1993]

Description: During the first year of the above grant, research has been conducted in three years: (1) Research underway on the effect of coherency strains on phase separation in the AlN-Al{sub 2}OC was written into a manuscript. (2) A study of interdiffusion in the SiC-AlN system was initiated. (3) Work on theoretical analysis for determination of symmetry of morphology of phase separation in a hexagonal system was initiated. A brief description of the progress during the past six months is given, followed by a summary of work planned for next year.
Date: December 31, 1993
Creator: Virkar, A. V.; Tian, Qiang & Chen, Jong
Partner: UNT Libraries Government Documents Department

Salt splitting of sodium-dominated radioactive waste using ceramic membranes

Description: The potential for salt splitting of sodium dominated radioactive wastes by use of a ceramic membrane is reviewed. The technical basis for considering this processing technology is derived from the technology developed for battery and chlor-alkali chemical industry. Specific comparisons are made with the commercial organic membranes which are the standard in nonradioactive salt splitting. Two features of ceramic membranes are expected to be especially attractive: high tolerance to gamma irradiation and high selectivity between sodium and other ions. The objective of the salt splitting process is to separate nonradioactive sodium from contaminated sodium salts prior to other pretreatment processes in order to: (1) concentrate the waste in order to reduce the volume of subsequent additives and capacity of equipment, (2) decrease the pH of the waste in preparation for further processing, and (3) provide sodium with very low radioactivity levels for caustic washing of sludge or low level and mixed waste vitrification.
Date: August 1, 1994
Creator: Hollenberg, G. W.; Carlson, C. D.; Virkar, A. & Joshi, A.
Partner: UNT Libraries Government Documents Department

DEGRADATION ISSUES IN SOLID OXIDE CELLS DURING HIGH TEMPERATURE ELECTROLYSIS

Description: Idaho National Laboratory (INL) is performing high-temperature electrolysis research to generate hydrogen using solid oxide electrolysis cells (SOECs). The project goals are to address the technical and degradation issues associated with the SOECs. This paper provides a summary of various ongoing INL and INL sponsored activities aimed at addressing SOEC degradation. These activities include stack testing, post-test examination, degradation modeling, and a list of issues that need to be addressed in future. Major degradation issues relating to solid oxide fuel cells (SOFC) are relatively better understood than those for SOECs. Some of the degradation mechanisms in SOFCs include contact problems between adjacent cell components, microstructural deterioration (coarsening) of the porous electrodes, and blocking of the reaction sites within the electrodes. Contact problems include delamination of an electrode from the electrolyte, growth of a poorly (electronically) conducting oxide layer between the metallic interconnect plates and the electrodes, and lack of contact between the interconnect and the electrode. INL’s test results on high temperature electrolysis (HTE) using solid oxide cells do not provide a clear evidence whether different events lead to similar or drastically different electrochemical degradation mechanisms. Post-test examination of the solid oxide electrolysis cells showed that the hydrogen electrode and interconnect get partially oxidized and become non-conductive. This is most likely caused by the hydrogen stream composition and flow rate during cool down. The oxygen electrode side of the stacks seemed to be responsible for the observed degradation due to large areas of electrode delamination. Based on the oxygen electrode appearance, the degradation of these stacks was largely controlled by the oxygen electrode delamination rate. University of Utah (Virkar) has developed a SOEC model based on concepts in local thermodynamic equilibrium in systems otherwise in global thermodynamic non-equilibrium. This model is under continued development. It shows that electronic conduction through ...
Date: June 1, 2010
Creator: O'Brien, J. E.; Stoots, C. M.; Sharma, V. I.; Yildiz, B. & Virkar, A. V.
Partner: UNT Libraries Government Documents Department