The properties of composites can be roughly divided into two categories: those that can be predicted from the properties of constituent phases, and those that cannot. In the first category we can include such properties as electrical conductivity, Young modulus, thermal expansion and hardness, and in the second, flexural strength, fracture toughness and others. The maximum toughening effect of the secondary phase in CMC materials is reached with additions of 10 to 20 vol.%, the maximum flexural strength is reached at 25-35 vol.%. At these levels, properties vs. concentrations curves have the appearance of non-monotone lines with unpredictable behavior trends. Disadvantages may result: (i) all investigators were studying materials with step of composition of about 5%. We question the adequacy of this technique; (ii) very often the investigators do not carry out analysis of structure vs. properties correlations, so that there is no possibility to design materials with new properties level; (iii) today no one has a trustworthy model for designing physical properties via microstructure design - all solutions are based on empirical or accidental specific knowledge.

When a multi-component composite is designed with an insulator as a matrix and a conductor (material with different conductivity level) as a secondary phase (filler), then both the mechanical and electrical properties of the composite are governed by a bifurcation behavior due to the formation of a continuous network of filler particles throughout the matrix.

We would like to present a new approach in the development of the CMM's microstructure and improvement in the preparation process of the raw material mixture and optimization of the sintering step. The analysis of the microstructure networks shows two typical patterns: array and random type structures. The morphology, distribution and size of conducting particles, for both structural patterns in a multi component composite, depends on the purity and dispersion of the initial powder and on the temperature and environment of the sintering process. Fluctuation in the sintering behaviour influences the structural pattern evolution. We have shown that particles size and shape and packing density of CMM are changing in a non-monotone manner and at present we have no satisfactory method of monitoring their microstructure.

In addition, the percolation theory approach can be used. This theory was developed with respect to DC conductivity. We have shown that AC conductivity gives a new possibility to understand the microstructure vs. properties correlation. It has been observed that in the multi-component composite with a certain volume fraction of the conductor Xm, there exists a threshold conductor volume fraction, Xc, above which the electrical conduction takes place through the composite. The electrical resistivity of the CMC strongly depends not only on the Xm, but also on the Xc, which in turn strongly relies on the microstructure of the composites. Following this approach, it is possible to determine basic micro structural parameters as particles shape factor (h/D), fractal index t, size factor Rm/Rd, threshold concentration Xc and irregularity of particles contour, due to the agglomeration degree, by a multiplier factor (M).

The goal of this work has been the systematic investigation of the sintering behavior, microstructure, electrical and mechanical properties of multi-component Ceramic Matrix Materials (CMM). The design of both mechanical and electrical properties is possible through the realization of a customized microstructure with controlled size, shape and concentration of additives and phases. We also propose simple methods of monitoring the evolution of microstructure of practicable CMM's and new solutions in the area of designing energy saving heating devices for wider applications.

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