The Effect of Polydispersivity on the Thermal Conductivity of Particulate Thermal Interface Materials

Abstract

A critical need in developing thermal interface materials (TIMs) is an understanding of the effect of particle/matrix conductivities, volume loading of the particles, the size distribution, and the random arrangement of the particles in the matrix on the homogenized thermal conductivity. Commonly, TIM systems contain random spatial distributions of particles of a polydisperse (usually bimodal) nature. A detailed analysis of the microstructural characteristics that influence the effective thermal conductivity of TIMs is the goal of this paper. Random microstructural arrangements consisting of lognormal size-distributions of alumina particles in silicone matrix were generated using a drop-fall-shake algorithm. The generated microstructures were statistically characterized using the matrix-exclusion probability function. The filler particle volume loading was varied over a range of 40-55 %. For a given filler volume loading, the effect of polydispersivity in the microstructures was captured by varying the standard deviation(s) of the filler particle size distribution function. For each particle arrangement, the effective thermal conductivity of the microstructures was evaluated through numerical simulations using a network model previously developed by the authors. Counter to expectation, increased polydispersivity was observed to increase the effective conductivity up to a volume loading of 50%. However, at a volume loading of 55%, beyond a limiting standard deviation of 0.9, the effective thermal conductivity decreased with increased standard deviation suggesting that the observed effects are a trade-off between resistance to transport through the particles versus transport through the inter-particle matrix gap in a percolation chain.

Stochasticity in materials structure, properties, and processing—A review

“…The thermal conductivity of composites containing inclusions of same properties and various dimensions (filler size polydispersity) exhibits an opposite trend. 75 As the volume fraction of inclusions, the mean inclusion size, the thermal conductivity of inclusions, and of the matrix are kept constant, but the inclusion size is sampled from a distribution function of increasing variance, the thermal conductivity of the composite increases.…”

Topological design of channels for squeeze flow optimization of thermal interface materials

“…The discretization in finite element approaches may be thought of as resulting in a conductance network between the nodes. Conductance networks have been constructed to study near-percolation thermal transport behavior of TIMs [18,6]. Flow resistances are commonly used when modeling flow through pipes in fluid systems, as well.…”

“…Alumina and other ceramic materials, in addition to metallic particles such as silver, are common choices for the particles, while silicone and other similar materials are common choices for the grease matrix. The particles are typically present in the fluid at loading volume fractions of 30-60% [6], enabling a considerable improvement of the bulk thermal conductivity of the TIM-often an order of magnitude improvement or more.…”