Abstract
High thermal conductivity, low thermal expansion and low density are three important features in novel materials for high performance electronics, mobile applications and aerospace. Spark plasma sintering was used to produce light metal–graphite composites with an excellent combination of these three properties. By adding up to 50 vol.% of macroscopic graphite flakes, the thermal expansion coefficient of magnesium and aluminum alloys was tuned down to zero or negative values, while the specific thermal conductivity was over four times higher than in copper. No degradation of the samples was observed after thermal stress tests and thermal cycling. Tensile strength and hardness measurements proved sufficient mechanical stability for most thermal management applications. For the production of the alloys, both prealloyed powders and elemental mixtures were used; the addition of trace elements to cope with the oxidation of the powders was studied.
Highlights
The low thermal expansion of typical semiconducting materials found in electronics induces mechanical stress at the interface with conventional metallic heat sinks [1,2]
Aluminum alloys and mixtures were provided by Ecka Granules (Velden, Germany), magnesium by laborladen.de (Hufingen, Germany), AZ31 by MSE-Clausthal (Clausthal, Germany), Mg-0.9Ca was provided by the HelmholtzZentrum Geesthacht (Geesthacht, Germany) and for further experiments produced by mechanical alloying of Mg and Mg-9.7Ca (20 min, 200 rpm ball milling with cyclohexane in Ar atmosphere), AZ61 was mixed in our lab from its components
Thermal conductivity was used as the primary parameter for the optimization of the sintering process, since it indirectly gives information about the porosity of the material and the presence of interfaces, e.g. because of
Summary
The low thermal expansion of typical semiconducting materials found in electronics induces mechanical stress at the interface with conventional metallic heat sinks [1,2]. In graphene and carbon based materials, in contrast, thermal transport is dominated by phonons This results in a large interface thermal resistance (Kapitza resistance) between filler and metal matrix and strongly decreases the TC of the composite [6]. The current flowing through the sample was shown to have a positive effect on mass transport [14] It allows the sintering of alloys, starting from pure metals instead of prealloyed powders. We report new graphite-reinforced composites using light aluminum alloys and magnesium alloys as matrix with density below 2000 kg m–3 Though these metals have much lower TC than copper, in the obtained metal–graphite composites values up to 380 W m–1 K–1 were measured. Most interestingly for mobile and aerospace applications, the low density of these composites led to a specific thermal conductivity over four times lower than in copper
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