  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.
