Silicon carbide matrix

Phosphoric acid fuel cells (PAFC) use liquid phosphoric acid as an electrolyte - the acid is contained in a Teflon-bonded silicon carbide matrix - and porous carbon electrodes containing a platinum catalyst. The PAFC is considered the "first generation" of modern fuel cells. It is one of the most mature cell types, the first to be used commercially, and features the most proven track record in terms of commercial applications with over 200 units currently in use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses. 

A silicon carbide-bonded graphite material in which graphite particles are distributed through the silicon carbide matrix has high thermal shock resistance and is suitable for applications including rocket nose cones and nozzles and other severe thermal shock environments (155) (see Ablative materials). 

Recent research has explored a wide variety of filler-matrix combinations for ceramic composites. For example, scientists at the Japan Atomic Energy Research Institute have been studying a composite made of silicon carbide fibers embedded in a silicon carbide matrix for use in high-temperature applications, such as spacecraft components and nuclear fusion facilities. Other composites that have been tested include silicon nitride reinforcements embedded in silicon carbide matrix, carbon fibers in boron nitride matrix, silicon nitride in boron nitride, and silicon nitride in titanium nitride. Researchers are also testing other, less common filler and matrix materials in the development of new composites. These include titanium carbide (TiC), titanium boride (TiB2), chromium boride (CrB), zirconium oxide (Zr02), and lanthanum phosphate (LaP04). 

Phosphoric-acid fuel cell (PAFC) — In PAFCs the -> electrolyte consists of concentrated phosphoric acid (85-100%) retained in a silicon carbide matrix while the -> porous electrodes contain a mixture of Pt electrocatalyst (or its alloys) (-> electrocatalysis) supported on -> carbon black and a polymeric binder forming an integral structure. A porous carbon paper substrate serves as a structural support for the electrocatalyst layer and as the current collector. The operating temperature is maintained between 150 to 220 °C. At lower temperatures, phosphoric acid tends to be a poor ionic conductor and poisoning of the electrocatalyst at the anode by CO becomes severe. 

H. Kodama, H. Sakamoto, and T. Miyoshi, Silicon Carbide Monofilament-Reinforced Silicon Nitride or Silicon Carbide Matrix Composites, J. Am. Ceram. Soc., 72[4], 551-558 (1989). 

Pt, which is not trivial over the projected lifetime for a PAFC. Migration of the platinum from the cathode towards the anode is due to the platinum being deposited, not on the anode catalyst, but rather on the silicon carbide matrix, adjacent to the anode, as a consequence of the small solubility of hydrogen in solution at the electrode/matrix interface. 

VAPOR SILICON INFILTRATION FOR FIBER REINFORCED SILICON CARBIDE MATRIX COMPOSITES... 

Vapor Silicon Infiltration for Fiber Reinforced Silicon Carbide Matrix Composites... 

Owing to their outstanding properties, silicon carbide fiber reinforced silicon carbide matrix (SiC/SiC) composites have been specified in several applications especially in recent fusion power plant design studies and have been considered internationally in several power plant studies. These characteristics include high-temperature properties and stability, corrosion resistance, as well as low induced radioactivity, quick decay of activity, low afterheat, low atomic number, good fracture resistance and more. 

A. Sayano, F. Sutoh, S. Suyama, Y. Itoh, and S. Nakagawa, Development of a Reaction-sintered Silicon Carbide Matrix Composite, J. Nucl, Mater., 271-272, 467-71 (1999). 

The coated preform is densified with a silicon carbide matrix via CVl. The gaseous precursor is methyl chi orosilane (MTS). When the CVl process is carried out isothermally (I-CVl), surface pores tend to close, restricting the gas flow to the interior of the preform. This phenomenon requires an intermediate operation of surface machining to obtain an adequate density. The CVl parameters (i.e. total pressure, temperature and gas flow rate) have to be selected according to the preform geometry defined by the pore spectmm and the thickness, the number of preforms present in the chamber and the size of the chamber. Finally several coating systems can be applied to these composites via CVD to provide environmental and oxidation protection. 

Ansorge F, Characterisation of carbon fibre/silicon carbide matrix composites, Naslain R, Lamon J, Doumeingts D eds.. High Temperature Ceramic Matrix Composites, Proceedings of 6th European Conference on Composite Materials, European Association for Composite Materials, American Ceramic Soc Inc, Ceramic Society of Japan, Bordeaux, 491-498, 20-24 Sep 1993. 

Xu Y, Cheng L, Zhang L, Yan D, Mechanical properties and microstructural characteristics of carbon fibre reinforced silicon carbide matrix composites by chemical vapour infiltration, Niihara K, Nakano K, Sekino T, Yasuda E eds.. Ceramic Society of Japan, High Temperature Ceramic Matrix Composites III, Proc 3rd Int Conf, Osaka, Sep 6-9 1998, 73-16, Key Eng Mater, Vol 164-165. 

The carbon-fiber composites show substantially higher strength than the silicon carbide materials but degrade more rapidly in air at 1100°C. The fracture modes of a carbon fiber/silicon-carbide matrix composite are shown in Fig. 9.9. 

Figure 9.9. Fracture of a carbon fiber/silicon-carbide matrix composite. (Photograph courtesy Rockwell International, Canoga Park, CA.)...

Classical phosphoric add fuel cells use phosphoric add as the electrolyte, which is immobilized in a Teflon bonded silicon carbide matrix. Phosphoric acid fuel cells usually work at temperatures around 200 °C and are able to tolerate carbon monoxide levels of up to 2 vol.% [1]. Platinum/ruthenium as the anode catalyst may improve the performance in presence of carbon monoxide, similar to PEM fuel cells [33]. 

Polymers are nsed in fnel cells. Those of particular interest are the polymer electrolyte membrane (PEM) and the phosphoric acid fuel cell (PAFC) designs. The latter design contains the liquid phosphoric acid in a Teflon bonded silicon carbide matrix. In March 2005 Ticona reported that it had bnilt the first fnel cell prototype made solely with engineering thermoplastics. They claimed that this approach rednced the cost of the fuel by at least 50% when compared with fuel cells fabricated from other materials. The 17-cell unit contains injection moulded bipolar plates of Vectra liquid crystal polymer and end plates of Fortron polyphenylene sulfide (PPS). These two materials remain dimensionally stable at temperatures up to 200 "C. The Vectra LCP bipolar plates contain 85% powdered carbon and are made in a cycle time of 30 seconds. 

From the point of view of polymer usage the types of particular interest are the PEM and the phosphoric acid PAFC designs. The latter design contains the liquid phosphoric acid in a Teflon bonded silicon carbide matrix. 

The electrolyte is concentrated liqnid phosphoric acid (H3PO4), immobilized in a porous silicon carbide matrix. It exhibits good proton conductivity at medium temperature (180°C-250°C). However, it solidifies at around 42°C, so this type of fuel cell caimot begin operating at ambient temperature, so it is necessary to maintain the temperature of the cell, even when it is not operating. Platinum is needed to catalyze the reactions. 

Coatings were applied to bend bars made from various commercial types of silicon nitride or silicon carbide, fiber-reinforced, silicon carbide matrix composites. In some cases the silicon nitride bend bars had as-processed surfaces, but in most cases the surfaces were machined and ground. The surfaces of the SiCf/SiCn, bend bars were cleaned prior to coating, but no other treatment was applied to them. 

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