SOLIDIFICATION AND MELTING OF SUCCINONITRILE WITHIN THE POROUS NETWORK OF AN AEROGEL

As a model for a three-dimensional nanostructured material, molten suc cinonitrile (SCN) is infiltrated into aerogels, directionally solidifi ed and remelted with various temperature gradients and solidification front velocities. Static light scattering was used to study the phase transformation. We observe that solidification occurs over a wide temp erature range beginning from the equilibrium freezing temperature to u ndercoolings of up to 20 K below it. The wide freezing range is explai ned by pure capillary undercooling due to the spectrum of pore sizes r anging from a few nanometers to a few mu m. A model is developed to ca lculate the capillary undercooling in a cylindrical pore. It is shown by repeated solidification and remelting that the fragile aerogels are not destroyed by the infiltration process, since the light scattering pattern is unaltered. The scattered light intensity profile is descri bed with the Debye model of light scattering in a random inhomogeneous medium, assuming a certain pore size distribution in the aerogel. Dur ing remelting a hysteresis of several degrees in the melting temperatu re is observed, whose origin is explained by dynamic wetting.