Ceramic grains

Fig. 16.7. Microstructural features of a crystalline ceramic grains, grain boundaries, pores, microcracks and second phases.

A cermet is a particulate composite similar to a dispersion-hardened alloy, but consists of larger ceramic grains (cer-) held in a metal matrix (-met) (see Figure 1.78). The refractory particulates can be from the oxide category, such as alumina (AI2O3),... 

E., (2000), Adsorption and wetting mechanisms at ceramic grain boundaries , Ceramic Transactions, 118, 427 -44. 

Eqs (2.88) and (2.89) cannot be applied to all solids but they are valid for the many high symmetry ionic structures that are non-polar and non-conductive. Also it has to be borne in mind that in the case of ceramics, grain boundaries can give rise to anomalies in the applied field distribution, and occluded layers of water can contribute to increased permittivities. R.D. Shannon [9] and others have calculated polarizabilities using Eq. (2.89) with the established values of molecular volume (Fm= 1 /N) and permittivity. They find that each constituent ion can be assigned a unique polarizability which is the same whatever other ions they are associated with. Table 2.5 gives the polarizabilities of a wide selection of ions and using these it is possible to calculate the permittivity of any combination... 

Ceramics based on SiC and ZnO are two materials in everyday use that have the characteristic shown in Fig. 4.7. In both cases it has been established that the resistance is controlled by the region in which the ceramic grains contact one another. 

When an ordered array of monosized particles is sintered, the vacancies and grain boundaries create pores, cracks, and other flaws which are difficult to remove dming sintering. Pores are flaws about the size of the original ceramic particles in the final sintered ceramic. Grain boundaries are flaws that lead to cracks of a size equivalent to the size of the ordered domain. Both these flaws lead to the failure of the final ceramic piece according to Griffiths s analysis ... 

The synthesis of nanophase ceramics is one of these concepts, it allows micro-porous ceramic materials with ceramic grains in the nanometer range to be obtained. Research in the field of nanophase materials is very active. A number of results on the control of microstructure and temperature stability of metal oxide ceramics can be applied to membrane preparation. Works carried out on non-oxide ceramics such as silicon carbide, silicon oxinitride or aluminum nitride should be regarded in order to extend the domain of available membrane materials. 

Kingery, W. D. (1974). Plausible concepts necessary and sufficient for interpretation of ceramic grain-boundary phenomena I, Grain boundary characteristics, structure and electrostatic potential II, Solute segregation, grain-boundary diffusion and general discussion. Journal of the American Ceramic Society, 57, 1-8 and 74-83. 

Previous conventional and depth sensing Vickers hardness measurements performed on sapphire and with sintered traditional AI2O3 ceramics (grain sizes 2-10 pm. 

The depth of modulation is determined by the ratio of the thickness of the laser material to the grain size of the ceramics [287, 289-291]. Analytical expressions for eigen polarizations and phase delays in grains of thermally loaded Nd YAG ceramic rods have been established [287]. It was suggested that the depolarization of radiation in polycrystalline ceramics led to beam modulation with a characteristic size of the order of the ceramic grain size. Therefore, increasing the ratio of rod... 

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