Liquid metal infiltration into ceramic particle preforms with bimodal size distributions

The work carried out during the last years on pressure infiltration of liquid metals into ceramic compacts made of bimodal mixtures of particles is reviewed. The use of bimodal mixtures of particles largely different in size, allows obtaining preforms, and thus, composites, with particle volume fractions Vp high enough to be used as supports in electronic packaging. The experimental results for the particle volume fraction are rationalized using a simple model that assumes that small particles can be easily accommodated in the free space left by large particles. Threshold pressure for infiltration P0 seems to be mainly controlled by the local compactness of fine particles. The experimental results for P0 can be fitted using the particle surface area per unit volume given by the linear rule of mixtures. The intrinsic permeability of the preforms, evaluated by means of pressureless infiltration of an organic liquid, can be rationalized in terms of models used in soil science. Thermal properties of the composites are also briefly discussed. The coefficient of thermal expansion calculated over a wide temperature range decreases linearly with the particle volume fraction, indicating that the key parameter is Vp, while other characteristics of the composite are irrelevant or play a minor role. The experimental results for the thermal conductivity can be accounted for by Hasselman-Johnson formula, using, as above, the particle surface area per unit volume given by the linear mixture rule.