ELECTRICAL-PROPERTIES OF LINEAR ALIPHATIC POLYESTER METAL COMPOSITE-MATERIALS .2. GRAIN-SIZE AND MICROSTRUCTURAL EFFECTS ON THE DC ELECTRICAL-CONDUCTIVITY OF METAL POWDER-POLY(ETHYLENE-ADIPATE) COMPOSITES

The dc electrical conductivity of composites prepared by dispersion of metal powders of various grain size in molten poly(ethylene adipate) (PEA) in the complete absence of air was measured as a function of temperature, applied voltage and low metal content. Moreover, the microstructure of the composites was investigated by SEM. The dc electrical conductivity was found to depend chiefly on the grain size rather than the nature of the metal powders. The formation of conducting networks was observed only for the higher powder grain size, an effect which is quite the opposite of that found in segregated composites obtained from sinterization of metal-polymer powders mixtures. On the contrary, a continuous decrease of the electrical conductivity was found as a function of the metal content with the smallest grain size. Microstructural investigations showed that detachments of polymer matrix from metal grains occurred at the metal-matrix interface or microcraks formed near these interfaces for metal grains with complex surface morphology. These effects are clearly enhanced in the case of decreasing powder grain size, i.e., increasing powder surface, which thus introduce several discontinuities in the bulk of the composites. This arises during material cooling from the preparation to room temperature, owing to the difference in the thermal expansion coefficients of the phases, and results in decreasing electrical conductivity as a function of metal content. © 1979.