Multiphase ceramics

Silicon nitride has been obtained by the pyrolysis (in a stream of NHj) of the perhydropolysilazane prepared by the ammonialysis of the H SiClj-pyridine adduct [23, 24]. The gas stream employed during the pyrolysis of the preceramic polymer plays a crucial role [25]. Pyrolysis of [B, H,j diamine] polymers in an NHj stream gives BN [26]. TiN is similarly obtained by the pyrolysis of an amine precursor [27]. TiN has been prepared from titanazane [28]. Pyrolysis of Nicalon in NHj is reported to give SijN [24]. Besides single-phase ceramics, multiphase ceramics (e.g. composites of SiC and TiC, BiN and SijN ) have been prepared from precursors [29, 30]. Group 13 metal nitrides (GaN, AIN, hiN) have been prepared by the decomposition of urea complexes [31]. This method has been extended for the synthesis of BN, TiN and NbN [32]. 

Hollandite does not contain long-lived ACTs and, therefore, it is undergoing p-y-irradiation from fission and corrosion products, but in multiphase ceramics it can also be a-irradiated from neighbouring ACT-bearing phases. Irradiation by a-particles from external 238PuOz sources and heavy ions results in a volume expansion of 2-2.5% and transformation of tetragonal to monoclinic symmetry. 

Other multiphase ceramics. Numerous multiphase ceramic formulations for conditioning of various wastes have been designed (Harker 1988). These so-called tailored ceramics were developed for immobilization of complex defence wastes at the Savannah River Plant and Rockwell Hanford Operation (Harker 1988). Tailored ceramics include ACT and REE hosts (fluorite-structure solid solutions, zirconolite. 

Let us concentrate a little longer on ceramics. Here micro-analysis only slowly won ground and the application of solid state physics lagged behind. Very slowly the relationship between the properties of a material and its microstructure was being discovered. Metallurgy had already been characterized by a theoretical approach for some time and consequently metals were about 15 times as important as ceramic materials in 1960 (see Ashby s graph). This was of course influenced by the fact that metals have relatively simple structures which, in their turn, simplify theoretical comtemplations. Ceramic structures are very often complex and are characterized by multiphase systems. However, at present ceramic materials are approached much differently than for instance in 1900. 

Silicon nitride ceramics are not merely only one material but several classes of materials. All of them are multiphased, i.e., they exhibit a heterogeneous microstructure which has formed during sintering (Sect. 6). Therefore in all classes a large variety of properties is predominant and as a consequence also a large variety of potential applications (Sect. 10). Often little variations in the powders and the processing parameters cause remarkable changes in the microstructure which have a pronounced effect on properties (Sects. 6 and 7). 

Thompson DP (1986) Phase relationships in Y-Si-Al-O-N ceramics. In Tressler RE, Messing GL, Pantano CG, Newnham RE (eds) Mat Sci Res Proc 21st Univ Conf in Ceramic Science Tailoring Multiphase and Composite Ceramics 20. Plenum Press, New York, p 79... 

In practice ceramics are usually multiphase, consisting of crystalline phases, glasses and porosity. The overall behaviour depends on the distribution as well as the properties of these constituents. A minor phase that forms a layer round each crystallite of the major phases, and therefore results in a 3-0 connectivity system (see Section 2.7.4), can have a major effect. If the minor phase is conductive it can greatly reduce the resistivity of the composite or, if insulating, it can reduce its conductivity. Also, an abrupt change in the mode of conduction at the main phase-intercrystalline phase boundary may introduce barriers to conduction that dominate the overall electrical behaviour. In contrast, minor phases present as small discrete particles, or porosity present as empty cavities, can only modify properties to a minor extent as indicated by one of the mixture relations such as Lichtenecker s rule (see Section 2.7.4). 

In contrast to dielectric losses permittivity is not, in general, sensitive to small amounts of impurities and for homogeneous dielectrics values can be calculated as described in Section 2.7.1, and the various mixture rules allow good estimates to be made for multiphase dielectrics. For Ba- and Sr-based dielectrics having the perovskite structure the variation of permittivity with temperature, which determines rf (see Eq. (5.37)), can be correlated with the tolerance factor t (see Section 2.7.3) [13] providing guidance for tailoring ceramics to have xf = 0. 

The power of 31P INADEQUATE experiment in the investigations of multiphase phosphate ceramics, especially those composed of a mixture of crystalline and amorphous components, was demonstrated by O Dell et al.,177 who investigated Ti-, Sr- and Zn-containing sodium calcium phosphates. From conducted... 

In spatially evolving multiphase media (e.g., during dissolution of a porous medium, or phase separation in a polymer blend), the mean curvature of the interface between two phases is of interest. Curvature is a sensitive indicator of morphological transitions such as the transition from spherical to rod-like micelles in an emulsion, or the degree of sintering in a porous ceramic material. Furthermore, important physicochemical parameters such as capillary pressure (from the Young-Laplace equation) are curvature-dependent. The local value of the mean curvature K — (1 /R + 1 /Ri) of an interface of phase i with principal radii of curvature Rx and R2 can be calculated as the divergence of the interface normal vector ,... 

R. Hayami, K. Ueno, I. Kondou, N. Tamari, and Y. Toibana, Si3N4-SiC Whisker Composite Material, in Tailoring Multiphase and Composite Ceramics, eds. R. E. Tressler, G. L. Messing, C. G. Pontano, and R. E. Newnham, Materials Science Research Series, Plenum Press, New York, NY, 1986, pp. 663-674. 

N. Claussen and G. Petzow, Whisker-Reinforced Zirconia-Toughened Ceramics , in Tailoring Multiphase and Composite Ceramics, eds. R. E. 

Consider a continuous fiber-reinforced ceramic as a multiphase system where the individual phases are parallel to one another and to the uniaxial loading direction. The fibers (or fiber bundles), matrix, and interface zone are treated as individual phases. In general, each phase undergoes elastic-plastic (creep) deformation. In the present analysis, the creep rate of each phase, e is assumed to obey a general creep law of the following form... 

In the metallurgical sense, alloys are mixtures of metals with each other or with certain nonmetals, resulting in substances that may be single phase or multiphase. It should be noted that the term alloy, as referring to a macroscopically homogeneous mixture, is also used for nonmetals such as ceramics and semiconductors (e.g. Si-Ge alloys) and polymers (e.g. block copolymers) these alloys, however, are outside the scope of the present article. 

The examples reported in this chapter show that the new processing routes using liquid organometalfic ceramic precursors allow the tailoring of multiphase and composite materials at the micro- and nanoscale, and hence the development of functionally graded materials. The stringent advantages are a better control of... 

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