Influence of particle diameter on coefficient of thermal expansion of SiO₂/epoxy particulate composites

We summarize the results of recent experiments and finite element analysis (FEA) micromechanics simulations aimed at exploring the effect of SiO ₂ particle diameter on the coefficient of thermal expansion (CTE) of SiO₂/epoxy composites. Of particular interest are reductions of particle diameter from the micrometer range into the nanometer range, since a group of nanoparticles having the same volume as a single microparticle will have a total surface area up to 1000 times that of the single microparticle. In a composite, the large increase in total particle/matrix interfacial surface area resulting from the use of low-CTE, high stiffness nanoparticles can be expected to increase the degree of constraint of thermal expansion of the high-CTE, low stiffness polymer matrix, leading to reductions in the composite CTE. Our experiments show that reducing the particle diameter at constant particle volume fraction leads to small but significant reductions in the composite CTE. 3D FEA models accurately predicted the composite CTE for large diameter particles, but they could not be used to study the effect of using larger numbers of smaller particles because the required number of FEA nodes became too large. Less accurate 2D, 1D and axisymmetric FEA models made it possible to extend the range of particle diameters, but predicted CTEs did not consistently follow the experimentally observed trend of reduced CTE with decreasing particle size.