The thermophysical behaviour of dispersion-filled polymer composites can be tailored over a wide range by varying the type, the loading and, importantly, the dispersity of the filler introduced into the matrix. The classical relations most often used to estimate the effective thermal conductivity of such two-phase systems are formulated only in terms of the conductivities of the constituent phases and their fractions, and contain no explicit dependence on the size of the filler particles; consequently, no single classical relation can reproduce the experimental conductivity curves obtained for the same composition but for fillers of different particle diameter. In the present work, a single particle-size-dependent calculation formula is proposed for the effective thermal conductivity λ of a two-phase composite and is validated experimentally. Composites were prepared from ED-20 epoxy-diane resin, cured with a polyethylene-polyamine hardener, and filled with M-type barium hexaferrite (BaO？6Fe2O3) powder of two dispersities, with effective particle diameters of 2 μm and 20 μm, at filling degrees of 0% - 60% by mass; the thermal conductivity was measured with an IT-λ-400 instrument. The proposed formula, which supplements the linear rule of mixtures with a correction term proportional to the square root of the effective particle diameter, reproduces the measured curves for both the 2 μm and the 20 μm fillers, whereas the classical size-independent relations cannot describe both data sets simultaneously.
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