The effect of particle size on the thermal conductivity of ZnS/diamond composites

We have observed that the thernal conductivity of zincsulphide is increased by adding large particles of highly conducting diamond, but lowered by the addition of sub-micron size particles of diamond. This effect is explained in terms of the interfacial thermal resistance which becomes increasingly dominant as the particles becomes smaller (because that increases their surface to volume ratio). A phenomonological model in which the interface resistance is expressed as an effective Kapitza radius, ak, is presented. The conductivity of the composite is analyzed for different values of α, which is defined to be equal to the Kapitza radius divided by the particle radius. If α = 1, that is, the actual particle radius is equal to ak then the effective thermal conductivity of the particles is equal to that of the matrix. If α > 1, that is the particles are very small, then the contribution of the particles to the thermal conductivity of the composite is dominated by interfaces; if α < 1 then the bulk property of the particles is important. Our measurements yield ak ≈ 1.5 μm for the ZnS-diamond interface.