Thermal conductance of randomly oriented composites of thin layers

This work studied the thermal conductance of thin layers with randomly orie nted composites by the percolation theory, and developed an effective-mediu m approximation (EMA) model for triple bond percolation systems. The result s showed that the thin layer is anisotropic in conductivities when its thic kness is lower than the correlation length. The conductivity in normal dire ction increases with decreasing thickness while the in-plane conductivity d eclines. The significance of this thickness effect is a function of the con centration of good conductor and the thermal conductivity ratio of the phas es in the composites. A threshold concentration exists for the conductivity of bulk composites, beyond which the effective thermal conductivity increa ses significantly from that of the poor conductor with increasing concentra tion. The developed EMA model agrees quite well with the numerical simulation and is expected being applicable to predict thermal conductivities of composit es using the coordination number as a fitting parameter (C) 2000 Elsevier S cience Ltd. All rights reserved.