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Development of colloidal processing for critical components

An aqueous colloidal processing route was developed for the manufacture of ceramics, namely yttria stabilized zirconia ceramics, with improved microstructure, strength, reliability and thermal shock resistance after firing. The potential properties of the zirconia were demonstrated in a fuel cell and in other ceramic parts (needles and fibres). Zirconia powders were synthesized and characterized to confirm purity, grain size and surface structure. These powders were colloidally processed. This involved the comminution of the raw zirconia dispersed in a surfactant solution whereby the particles were reduced to less than 1 micron in size after milling. To this stable dispersion, a polymeric solution containing binder and plasticiser was added. Stability of this dispersion was maintained for a reasonable length of time and this slurry passed through a 1 um filter. After removal of excess water. the plastic paste was extruded to form thin-walled tubes which were demonstrated in a 1000 tube fuel cell stack. These were then compared with conventional extruded tubes. Test showed that the colloidal process gave substantially higher strengths of over 500 MPa and good reliability.

A zirconia fuel cell system was designed and the system construction was studied. Individual cells were designed to fit within this system and cells were made by the colloidal process route, dried, fired and studied. These cells were incorporated into a 200 tube reactor and a 1000 tube reactor. The entire system was controlled by a computer. Both reactors survived rapid warm-up times of 60 s without catastrophic failure, in contrast to the 5 h needed for the Westinghouse tubular design to reach the operating temperature of 800 C. Electricity and heat were successfully produced using natural gas as the fuel and air as the oxidant. This device could have enormous benefit for cogeneration applications.

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