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An experimental investigation has been made of the flow patterns at the initiation of convection in a layer of a high Prandtl number liquid confined between rigid, horizontal surfaces and heated from below. The experiment was designed to overcome the limitations of earlier experiments and to correspond closely to the conditions of the theory. In particular, the upper and lower rigid surfaces which enclosed the layer were made of copper which has a high thermal conductivity. To aid in the analysis of the experimental results some supplementary observations of the flow patterns were made using a glass upper plate. For small fluid depths and large temperature differences between the upper and lower surface the initial flow was in the form of hexagonal cells as predicted theoretically. With increasing Rayleigh number the cellular flow appeared to transform into rolls as predicted. For large fluid depths and small temperature differences only circular plan-form rolls were observed. This is in agreement with the results of other experimenters. It is tentatively proposed that this non-appearance of an initial cellular flow is due to the shape of the test chamber having a dominating influence on the flow pattern when the temperature gradient in the fluid is small. Measurements were also made of the development time for the flow patterns and the critical Rayleigh number.

In this article, we fabricated one-dimensional 0.5 vol% silicon nitride (Si 3 N 4) fibre reinforced Alumix123 matrix composites by the powder metallurgy method. Si 3 N 4 is a ceramic material with high strength, stiffness and low coefficient of thermal expansion. The Si 3 N 4 used in the study is comprised of single crystal fibres with sub-micrometre diameter and length < 25 μm. The size of the fibre is smaller than the metal matrix powder allowing it to coat the metal powder before consolidation rather than forming agglomerations that might lead to pores and poor mechanical properties. We investigated the effects of Si 3 N 4 fibre additions on the densification, hardness, elastic modulus and tensile strength. The experimental results found that the Si 3 N 4 fibre effectively enhanced the mechanical properties of the composites at low additions. The microstructural analysis clearly shows the Si 3 N 4 fibres in the sintered grain boundaries. The interface between the fibre and the Alumix123 matrix showed good wetting and bonding with only minimal reaction. The Si 3 N 4 fibre reinforced Alumix123 composites have potential uses in the automobile and aerospace industries due to their improved specific mechanical properties.