Numerical and experimental analysis of the thermal conductivity of metallic hollow sphere structures
Introduction
Porous metals are characterised by high specific stiffness, the ability to absorb high amounts of energy and potential for noise control, mechanical damping and thermal insulation [1]. However, classical cellular metals such as aluminium metallic foams often exhibit inconstant material parameters [2] due to their stochastic geometry. Local density inhomogeneities [3], [4] yield a scattering of macroscopic properties. This problem can be decreased by MHSS which are assembled by spheres with defined geometry and possess a more homogenous structure which drives to optimized properties. Therefore, MHSS open a wide field of potential multifunctional applications, e.g. in automotive or aerospace industry [5]. The exact knowledge about the thermal conductivity enables the consideration of these novel materials in the design of improved heat shields, thermal insulators or heat exchangers. MHSS are assembled by spherical metallic shells which can be joined by different technologies, such as sintering, soldering or adhesive bonding [6], in order to obtain a coherent structure. The metallic hollow spheres are manufactured by a sintering process as described in [7], [8]. Preliminary numerical investigations [9] have shown that adhesively bonded structures exhibit minimum thermal conductivity due to the insulating effect of the adhesive within the structure. Baumeister and colleagues [10] investigated the thermal properties of syntactic hollow sphere composites. Corundum based hollow spheres were embedded in an epoxy matrix and the thermal expansion coefficient was determined. It was found that the thermal behaviour of these composites was mainly governed by the used epoxy resin. Such materials with low thermal conductivities can be used as thermal insulators. Two different morphologies can be distinguished in the case of hollow sphere structures. In the case of partial MHSS (density ρ ≈ 0.6 kg / dm3) the adhesive is concentrated at the contact points of neighbouring spheres. In contrast, the metallic spheres with a syntactic structure (ρ ≈ 1.2 kg / dm3) are completely embedded within the adhesive matrix (cf. Fig. 1).
Section snippets
Methodology
Three different approaches to obtain the thermal conductivities of MHSS, namely a numerical, analytical and experimental one, have been performed.
Results
Fig. 4 visualises the temperature distribution inside the MHSS for steady state conditions (finite element method). Due to the high thermal conductivity of the steel, the temperature within the metallic shell is approximately constant. In contrast, high temperature gradients, visible by short distances between isotherms (borders between two different grey areas), occur inside the adhesive. Due to these high temperature gradients, it can be concluded that the adhesive acts as a thermal insulator
Conclusions
In the scope of this article the thermal overall conductivities of adhesively bonded MHSS are determined for partial and syntactic morphology. Three different methodologies are applied and good agreement between the experimental, numerical and analytical results is found. The low thermal conductivity of the composite enables its application as a thermal insulator, in particular if other potentials of this lightweight composite material can be exploited.
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