Effects of solidification rate on pore shape in solid

Abstract

The effects of solidification rate on the shape of a pore, resulting from a bubble entrapped by a solidification front, are investigated in this study. Solidification rate plays an important role in the pore shape due to its strong influence on solute concentration, nucleation rate, solute transport across the bubble cap, morphology of the solidification front, etc. Understanding and controlling porosity in solid is important for manufacturing porosity-free materials, and functional materials such as lotus-type porous materials in nanotechnologies and scaffolds in tissue engineering, etc. In addition to self-consistent determination of the cap shape at which balance of pressures and physico-chemical equilibrium are satisfied, this model accounts for solute transport across the cap in three different cases. Case 1 is subject to solute transport from the pore across an emerged cap in the concentration boundary layer along the solidification front in the early stage. Cases 2a and 2b indicate opposite directions of solute transport across a submerged cap in the concentration boundary layer. In contrast to Case 2a, expansion of pore volume in Case 2b exhibits a stronger effect on solute gas pressure in the pore than solute transport in the late stage. The results show that an increase in solidification rate in Case 1 decreases and increases pore radius in the early and late stage, respectively. The bubble is entrapped earlier by increasing solidification rate. The cap cannot be entrapped, because solute concentration increases and decreases rapidly in the late stage in Cases 2a and 2b, respectively. Predicted and measured pore shapes in solid are in good agreement. The pore shape can be controlled via selecting solidification rate to change directions and magnitudes of solute transport.