R. Tavangar (Sp), M. Vetterli, L. Weber, Swiss Federal Institute of Technology, Lausanne (Switzerland)
Materials with tailored coefficient of thermal expansion (CTE) are of interest wherever assemblies of different
materials are subjected to thermal cycling, e.g. in electronic devices. The composite approach is the most
promising to access a large range of CTEs. While long-fiber reinforced composites have the capacity to adapt the
CTE in one direction or in a plane, particle reinforced composites have the advantage of offering isotropic
properties. However, to access very low values of CTE, e.g. that of silicon (2.6 ppm/K at room temperature), very
high loadings of very low expansion phase in the matrix are necessary. It is commonly perceived that, for a fixed
volume fraction of inclusion phase, a lower CTE of the low expansion phase should lower the CTE of the
composite. We will show in this contribution, that there are two cases where this may not be true: the first is the,
somewhat trivial, case of insufficient bonding strength between the reinforcement and the matrix, leading
essentially to debonding of matrix and inclusion and free expansion of the matrix. The second, more subtle, case
is when the low expansion inclusion phase also has a lower bulk modulus than the matrix: the deviation from the
rule of mixtures in the elastic models, e.g. by Shapery and Kerner, is then towards higher CTEs while in the
general case of the low expansion phase having also the higher bulk modulus, the deviation from rule of mixture
is towards lower CTE values. We exemplify these considerations by presenting data on Cu-diamond, Cu-Bdiamond
and Cu-fused silica composites: the copper diamond system debonds readily while the boron addition to
copper confers good adhesion to the diamond copper interface. The Cu-B-diamond and the Cu-fused silica
composites on the other hand represent two extreme cases of elastic property mismatch between matrix and
inclusion phase at similar CTE of the inclusion phase.Klicken Sie hier, um Text einzufügen...