Influence of particle size on ice nucleation and growth during the freeze-casting process

The solidification behaviour of suspensions of alumina particles during directional solidification is investigated here by in situ observations using X-ray radiography and tomography. The objective of this study is to assess the influence of particle size on the solidification behaviour of the suspensions during the early stages of solidification. Four powders with particle size in the range 0.2-3.4{\mu}m (median size) were investigated. Solidification is obtained by cooling at a constant rate, starting from room temperature. Attention is specifically paid to the nucleation and growth behaviour of the ice crystals in these suspensions. We propose that the nucleation of ice crystals is controlled by the particle size, the surface of the particles acting as nucleation sites. Smaller particle size leads to a lower degree of supercooling since nucleation and growth can proceed at higher temperature than with larger particles. The initial interface velocity is dependent on the degree of supercooling, and controls the extent of the initial structural gradient in the resulting porous materials.