Isotropic thermal expansion in anisotropic thermal management composites filled with carbon fibres and graphite

Light materials with high thermal conductivity and low thermal expansion have a wide application potential for the thermal management of high-performance electronics, in particular in mobile and aerospace applications. We present here metal matrix composites with a mixture of graphite flakes and pitch-based carbon fibres as filler. The production by spark plasma sintering orients the filler particles on to a plane perpendicular to the pressing axis. The obtained materials have lower density than aluminium combined with a thermal conductivity significantly outperforming the used metal matrix. Depending on the ratio of the filler components, a low thermal expansion along the pressing direction (high graphite flakes content) or across the pressing direction (high carbon fibre content) is achieved. For a 1:3 ratio of carbon fibres to graphite, we measured an isotropic reduction of the thermal expansion of the matrix by up to 55%. We present a detailed characterisation of composites with two aluminium alloys as matrix and an overview of the properties for six different metal matrices including magnesium and copper. With the goal of a technical application, we show that the described properties are intrinsic to the material compositions and are achieved with a wide spectrum of production methods

Temperature dependence of the thermal boundary conductance in Ag–3Si/diamond composites

The effect of oxygen termination of diamond surfaces on the interface thermal conductance between Ag–3Si and diamonds is investigated in dependence of temperature by measuring thermal conductivity in the temperature range from 4 K up to ambient. Composites for O-terminated diamonds exhibit a marked maximum of 960 W m− 1 K− 1 at roughly 150 K. Calculation of the interface conductance by the differential effective medium (DEM) scheme obtained marked difference between H- and O-terminated diamond surfaces in composites and is supported by AFM force distance measurements on native, H-terminated, and O-terminated diamond single crystal (100) surfaces. O-termination can be easily induced by immersing diamonds in hot aqua regia solution