Related Ceramic Processing Routes

In RBSN, a silicon metal powder preform reacts with nitrogen gas to form silicon nitride. The reaction occurs by percolation of the gas phase into the open porosity of the silicon preform, while the large specific volume of the silicon nitride product substantially reduces the volume fraction of residual porosity (down to a lower limit of approximately 9%), The mechanical properties of RBSN are inferior to those of sintered silicon nitride powder products, but the reduced costs of machining the near-net-shape product is a very attractive advantage. 

RBAO involves a similar production route, but in this case the aluminum metal powder is a minor component in a powder mix that contains a substantial fraction of alumina. Oxidation of the aluminum metal by gas percolation yields a metastable oxide phase, which is replaced at higher temperatures by the stable corundum phase. Again, the large specific volume of the oxide product reduces the volume fraction of porosity, but in this case the thermal cycle actually sinters the product, and for the optimized powder formulation, expansion during oxidation of the metal at a moderate temperature is followed by the sintering contraction at a higher temperature to yield a dense alumina product whose final shape and volume is still very close to that of the green powder preform. 

A similar reaction route has been developed for reaction bonded mullite (RBM, aluminum silicate). In this case the powder formulation also includes a source of silicon (silicon metal, silicon carbide, or silica), in addition to aluminum metal and alumina, and the final product contains mullite as the major phase. The RBM process is frequently modified by additions of zircon (ZrSi04), which provide an additional source of silicon and result in a dispersion of fine zirconia particles in the product which restricts grain growth to ensure a fine uniform microstructure. It is again possible to reduce residual porosity while retaining the near-net-shape characteristics, but at the time of writing this process is still under development and some way from commercial exploitation. 

A rather different approach has been developed for reaction sintered silicon carbide (RSSC), first developed in the former Soviet Union. In this process a powder preform of mixed graphite and silicon carbide is immersed in a liquid bath of molten silicon. The silicon wets and infiltrates the preform, reacting with the finely-divided graphitic component (carbon black). In the best case, all graphite is reacted and the residual sihcon content is no more than a few percent. The product of the reaction, silicon carbide, firmly bonds the silicon carbide preform powder 

The combination of infiltration and reaction that characterizes DMO has been exploited to make a number of composites. As long ago as 1953, it was shown that silica containing refractories were reduced by molten aluminum to form alumina and silicon [4], Subsequently [27], the displacement reaction was extended to the formation of composites of alumina with residual Al-Si. More recently, the Al-Si02 displacement reaction has been used in the infiltration of dense preforms of silica [28] and mullite [29,30] by molten aluminum. Extension of the reactive infiltration process to porous silica-containing preforms [31,32] has resulted in the fabrication of metal-matrix composites in which the silica was replaced by a mixture of about 65% alumina and 35% metal, while the pores were infiltrated by molten alloy. In contrast to DMO, the displacement reaction appears to proceed at a critical temperature of 1100-1200°C and without the need for a volatile solute element or oxygen. Borosilicate glass has also been used as an initiator to enable the infiltration of Al-Si alloys into alumina preforms [33]. 

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It will be clear to any ceramic engineer that the support structure requirements for a 20 pm thick microfiltration membrane with 200 nm pores and for a support structure for a 100 nm thick hyperfiltration membrane with 0.7 nm pores are very different. These differences relate to the final properties of the support structure needed as well as the processing route to prepare them. A discussion of these differences is included in this chapter. 

Abstract Refractory oxides encompass a broad range of unary, binary, and ternary ceramic compounds that can be used in structural, insulating, and other applications. The chemical bonds that provide cohesive energy to the crystalline solids also influence properties such as thermal expansion coefficient, thermal conductivity, elastic modulus, and heat capacity. This chapter provides a historical perspective on the use of refractory oxide materials, reviews applications for refractory oxides, overviews fundamental structure-property relations, describes typical processing routes, and summarizes the properties of these materials. 

The CRC-Elsevier materials selector , 2nd edition, N.A. Waterman, and M.E Ashby CRC Press (1996) ISBN 0412615509. (Now, also available on CD-ROM). Basic reference work. Three-volume compilation of data for all materials includes selection and design guide. The Materials Selector is the most comprehensive and up-to-date comparative information system on engineering materials and related methods of component manufacture. It contains information on the properties, performance and processability of metals, plastics, ceramics, composites, surface treatments and the characteristics and comparative economics of the manufacturing routes which convert these materials into engineering components and products. 

Domier has developed a production route for continuous fiber-reinforced ceramics based on the impregnation and pyrolysis of Si-polymers. This process is related to the manufacturing of fiber-reinforced plastics and allows the cost-effective production of large and complex CMC-structures. 

Convenient sol-gel processing, compaction, and firing allow the synthesis of optically clear ceramics. Two examples are presented in Figure 12.6, a ferroelectric PLZT 65/35 ceramic prepared by hybrid sol-gel route and a mulhte matrix prepared using aluminum-sihcon ester and silicon-methoxide. Note the opaque skin around the transparent PLZT ceramics is related to PbO loss. When a partial pressure higher than the equihbrium pressure required to avoid any PbO excess... 

As it can be seen from Table 24.3, processing of inorganic material using SAS and related processes is dominantly applied to obtain organic compounds which serve as precursors for the synthesis of inorganic solids. An interesting route to synthesize ceramic material was proposed... 

Polymer pyrolysis refers to the pyrolytic decomposition of metal-organic polymeric compounds to produce ceramics. The polymers used in this way are commonly referred to as preceramic polymers in that they form the precursors to ceramics. Unlike conventional organic polymers (e.g., polyethylene), which contain a chain of carbon atoms, the chain backbone in preceramic polymers contains elements other than carbon (e.g., Si, B, and N ) or in addition to carbon. The pyrolysis of the polymer produces a ceramic containing some of the elements present in the chain. Polymer pyrolysis is an extension of the well-known route for the production of carbon materials (e.g., fibers from pitch or polyacrylonitrile) by the pyrolysis of carbon-based polymers (54). It is also related to the solution sol-gel process described in the previous section where a metal-organic polymeric gel is synthesized and converted to an oxide. 

The objective of this chapter is to adopt partial/pressureless sintering and freeze-casting routes for the preparation of high-strength porous titanium diboride (TiB2) and porous Zr02 ceramics, with particular emphasis on the processing-microstructure-property relations inherent to each process. 

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