GRAPHITE-FIBER CARBIDE-MATRIX COMPOSITES .2. STRUCTURE AND ELECTRICAL-PROPERTIES

The measurement of electrical properties of ceramic-matrix composites supplies data which can be used directly with information about the st ructure of the composites. The structures of graphite-fibre/carbide-ma trix composites may be varied within large intervals of appropriate pa rameters, as shown in a previous publication. A corresponding variety of behaviour patterns of such materials in the electrical field had be en expected and was actually observed. The matrices of the composites were boron, niobium, and tantalum carbides, and the fibres were Kulon and VMN-4 The electrical conductivity of graphite-fibre/carbide-matrix composites has been determined, including that at cryogenic temperatu res. A preliminary series of experiments gives the characteristics of piezo-resistance of the C/NbC composites. The experimental data yield a set of characteristics of the fibre, matrix, and composite structure . The conductivity characteristics of the graph ite fibres and carbide matrices obtained in this way correspond to the expected ones. That a lso includes a type of conductivity behaviour revealed by the temperat ure dependence of the conductivity. Graphite fibre behaviour is of the metallic type, while boron carbide is a semiconductor-type material. The conductivity of pure matrices differs essentially from that of the matrices obtained as an extrapolation of the conductivity versus fibr e volume fraction dependence to zero fibre content. Active diffusion o f carbon from the fibre/matrix interface makes the stoichiometry of a carbide better and its conductivity higher. But a distinctive dependen ce of the piezo-resistance of the C/NbC composites on the fibre volume fraction may lead to an assumption about the possibility of the exist ence of another mode of influence of the carbon fibres on the conducti ve properties of carbides. Calculation of the ineffective length of a fibre (from the viewpoint of conductivity) yields an estimate of the c onductivity of the interface. It is clearly connected to the structure of the interface.