Pyrolysis Sintering method

Sihcon carbide is also a prime candidate material for high temperature fibers (qv). These fibers are produced by three main approaches polymer pyrolysis, chemical vapor deposition (CVD), and sintering. Whereas fiber from the former two approaches are already available as commercial products, the sintered SiC fiber is still under development. Because of its relatively simple process, the sintered a-SiC fiber approach offers the potential of high performance and extreme temperature stabiUty at a relatively low cost. A comparison of the manufacturing methods and properties of various SiC fibers is presented in Table 4 (121,122). 

The slurry process has been enhanced with vacuum to fabricate planar SOFCs [78], This method is of low cost and thus has been widely used to develop low-cost SOFCs. However, together with other liquid precursor methods such as sol-gel and spray pyrolysis, it is time, labor, and energy intensive because the coating-drying-sintering has to be repeated in order to avoid cracking formation. 

The conventional industrial method for the synthesis of a-silicon carbide is to heat silica (sand) with coke in an electric furnace at 2,000-2,500 °C. However, because of the high melting point of the product, it is difficult to fabricate by sintering or melt techniques. Thus, the discovery of a lower temperature fabrication and synthesis route to silicon carbide by Yajima and coworkers in 197526,27 proved to be an important technological breakthrough. This is a preceramic polymer pyrolysis route that has been developed commercially for the production of ceramic fibers. 

La0 2Sro8Feo55Tio 4503 8 (LSFTO) powder, synthesized by using the spray pyrolysis method, was obtained from Praxair Specialty Ceramics (Woodinville, WA). The powder was preheated at 1250°C for 10 h in air with a 3°C/min ramp rate. The preheated powder was ground with ethanol in a mortar for more than 30 min. After drying to remove the ethanol, the powder was pressed into a cylindrical shape at 5000 psi with a 1 in. diam die. Dense ceramic disks were obtained by sintering in air at 1300°C for 20 h and 1400°C for 10 h. The sintered disks had a density of 90.3% of the theoretical den-... 

Dry methods and postcalcination methods The industrial micron sized R2O3 powder is commonly made by thermal pyrolysis of rare earth carbonates or oxalates at a temperature of 600-1000 °C. The dry methods usually result in fine powders with a relatively wide size distribution. After the sintering, the surface OH and other solvent related species are generally removed, therefore, the powder may exhibit better luminescence efficiency and longer decay time. Nano-sized rare earth oxide products could be obtained from finely selected precursors like hydroxides gels, premade nanostructures, through heat treatment, spray pyrolysis, combustion, and sol-gel processes. 

The design of the interstices filling in colloidal crystals with appropriate media and subsequently fluid-solid transformation is central to the whole synthesis. Fluid precursors in the voids of crystal arrays can solidify by polymerization and sol-gel hydrolysis. More recently, many methods have been developed including salt precipitation and chemical conversion, chemical vapor deposition (CVD), spraying techniques (spray pyrolysis, ion spraying, and laser spraying), nanocrystal deposition and sintering, oxide and salt reduction, electrodeposition, and electroless deposition. 

Ultra-flne-grained highly reactive yttria powders, suitable especially for the preparation of transparent ceramics, are prepared by various methods including combustion synthesis [293], precipitation [294, 295], hydrothermal synthesis [296], electrospray pyrolysis [297], and sol-gel [298]. In order to improve the dispersion and sinterability of yttria powders, seed crystals are often added [296]. A significant refinement of yttria powders prepared by precipitation from solution may be achieved by the addition of sulfate ions to the reaction mixture [299] (Figure 1.25). 

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