Ceramic bodies processing

Ultimately, the surface energy is used to produce a cohesive body during sintering. As such, surface energy, which is also referred to as surface tension, y, is obviously very important in ceramic powder processing. Surface tension causes liquids to fonn spherical drops, and allows solids to preferentially adsorb atoms to lower tire free energy of tire system. Also, surface tension creates pressure differences and chemical potential differences across curved surfaces tlrat cause matter to move. 

Deteriora.tlon. Ceramic objects are fragile, and mechanical damages through breakage and abrasions are the most likely source of destmction. Low fired ceramics can suffer through the rehydration of the body material this process results ia a complete loss of mechanical streagth. The preseace of soluble salts ia porous ceramic bodies has the same disastrous results as ia stoae (136). 

Plastic Forming. A plastic ceramic body deforms iaelastically without mpture under a compressive load that produces a shear stress ia excess of the shear strength of the body. Plastic forming processes (38,40—42,54—57) iavolve elastic—plastic behavior, whereby measurable elastic respoase occurs before and after plastic yielding. At pressures above the shear strength, the body deforms plastically by shear flow. 

The processes employed in manufacturing a ceramic are defined and controlled to produce a product with properties suited to a specific appHcation. Processing—microstmcture—property relationships are deterrnined by characterizing the ceramic raw materials, mixes, and the formed ceramic body intermittently during processing and after final thermal consoHdation. It is possible to modify and optimize processes to optimize properties and to identify and correct processing deficiencies when less than optimal properties are obtained. Examples of some process—microstmcture—property relations in advanced ceramics are outlined in Figure 4. 

Using 2eohte catalysts, the NO reduction takes place inside a molecular sieve ceramic body rather than on the surface of a metallic catalyst (see Molecularsieves). This difference is reported to reduce the effect of particulates, soot, SO2/SO2 conversions, heavy metals, etc, which poison, plug, and mask metal catalysts. ZeoHtes have been in use in Europe since the mid-1980s and there are approximately 100 installations on stream. Process applications range from use of natural gas to coal as fuel. Typically, nitrogen oxide levels are reduced 80 to 90% (37). 

Applications. CVD ceramic powders such as SiC and Si3N4 are used to produce ceramic bodies for a wide variety of applications, either experimentally or in production. These include structural applications in high temperature or corrosive environments where metals are not suitable, in such areas as reciprocating engines, gas turbines, turbochargers, bearings, machinery, and process equipment. 

Novak, S., Kosmac, T., Kmel, K. et al.. Principles of the hydrolysis assisted solidification (HAS) process for forming ceramic bodies from aqueous suspension, J. Eur. Ceram. Soc., 22, 289, 2002. 

ITRI has developed and patented processes for preparing and utilizing stable aqueous colloidal sols of tin(IV) oxide and related materials. These products, which contain nanometer-scale particulate tin oxide, are useful precursors for the synthesis of ceramic bodies, powders, and coatings, and may And application in encapsulated pigments, electroconductive materials, catalysts, and transparent tin oxide Aims on glass and other substrates. 

Rosenholm, J.B. et al., Colloidal processing related to the properties of the ceramic pai ticles and the sintered ceramic body, in Proceedings of the International Workshop on Ceramic Intetfaces, Properties and Applications III, Smart, R.St.C and Nowotny, J. eds.. Institute of Materials Press, London, 1998, p. 433. 

R.E. Gould and J. Lux, Some experiences with the control of plastic bodies for automatic jiggering, in Ceramic Fabrication Processes, W.D. Kingery (ed.), John Wiley, New York, 1958, pp. 98-107. 

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