All-ceramic composites

Within the field of materials science, the term composite is appUed to a material that consists of a combination of phases. Typically, there are two phases, a continuous one, known as the matrix, and a discontinuous one, dispersed within the matrix phase [1]. The majority of composites consist of an inorganic filler, either particles or fibres, dispersed in a matrix of organic polymer. However, other technically useful composites exist, including all-ceramic composites [2] and metal matrix composites [3]. 

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. 

While necessary, the property measurements alone do not provide all the necessary information about the functionality of the seal material. However, material screening and evaluation using stack tests are not practical. In this section experimental techniques to evaluate the seal material in addition to property measurements are discussed. While discussions focus on glass or glass-ceramic composite seal materials, many of the techniques apply to other types of seal materials. 

As for enviromnental resistance, there exists a design chart that is somewhat useful for this case study, but, more importantly, may be of use in other designs. The compatibility of various materials in six common environments is shown in Figure 8.16. The suitability of a material for each of the six environments improves as you move from the center of the chart outward. In this case, resistance to organic solvents is of primary importance. We see that all ceramics and glasses, all alloys, and some polymers such as poly(tetrafluoroethylene), PTFE, will provide excellent resistance. Composites will provide good resistance, which may be satisfactory for our application. 

By making use of the Internet I have searched all over the world for photographs of ceramic composite structures. In the end I received some from Mr K. Yoshida of the Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Japan. These are photographs... 

During the last few years, ceramic- and zeolite-based membranes have begun to be used for a few commercial separations. These membranes are all multilayer composite structures formed by coating a thin selective ceramic or zeolite layer onto a microporous ceramic support. Ceramic membranes are prepared by the sol-gel technique described in Chapter 3 zeolite membranes are prepared by direct crystallization, in which the thin zeolite layer is crystallized at high pressure and temperature directly onto the microporous support [24,25],... 

The consolidation mechanism of many of these ceramic composites involves a thin glassy phase linking the matrix grains, the reinforcement and the crystalline portion of the sintering additive.14 The thermal and mechanical properties of all these phases are quite different, which will therefore influence the behaviour of the composite. Although the coefficient of thermal expansion... 

The application of ceramics has infiltrated almost all fields in the last 20 years, because of their advantages over metals due to their strong ionic or covalent bonding. But it is just this bonding nature of ceramics that directly results in their inherent brittleness and difficulty in machining. In other words, ceramics show hardly any macroscopic plasticity at room temperature or at low temperatures like metals. Hence, superplasticity at room temperature is a research objective for structural ceramics. In recent years, many researches have been carried out to investigate nanophase ceramic composites. 

As can be inferred from the equations outlined above, none of the different models can adjust the creep parameters for all the different ceramics, especially in the case of YTZP,7 explaining why there is still controversy over the accommodation process controlling superplasticity. The same conclusions can be outlined for ceramic composites, although more experimental work should be done.20,31... 

In this section we review experimental observations on the creep of ceramic matrix composites. Observations that apply to all ceramic matrix composites are discussed. Creep curves obtained on ceramic matrix composites are compared with curves obtained on metals and metallic alloys. The role of a second phase in increasing the creep resistance of composites is emphasized. Finally, a discussion of creep asymmetry is presented, wherein creep occurs more easily in tension than in compression. 

Some ceramic materials are not found widely or at all in nature, and thus are synthesized for use. To prepare more complex ceramic compositions such as perovskites of general structural formula ABO3, and ferrites, of formula MFc204, the individual oxides or salts of the cations A, B, and M are often combined as powders and then reacted at high temperature by a solid-state diffusion mechanism. Silicon nitride (Si3N4) can be manufactured from either the nitridation of silicon metal or from the reaction of silicon tetrachloride with ammonia. Silicon carbide (SiC) is obtained from the reduction of silica with a carbon containing source. 

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