FREEZING AND MELTING OF FLUIDS IN POROUS GLASSES

We have studied the freezing and melting of a number of cryogenic flui ds (hydrogen, neon, oxygen, and argon) confined in porous glasses (Vyc or and a silica xerogel). ac heat-capacity measurements show broadened latent-heat peaks associated with both freezing and melting at temper atures substantially below the bulk melting temperatures. Thermal cycl ing shows pronounced hysteresis, with melting occurring at a higher te mperature than freezing. Also, the latent heat of freezing appears to be much smaller than that of melting. The hysteresis in the argon-Vyco r system was studied in detail using high-resolution ultrasonic techni ques which directly probe the shear modulus of the material in the por es. We found that the onset of freezing is extremely sharp, despite th e random pore geometry, and that freezing continues over a range of te mperatures. The freezing process is extremely irreversible, in the sen se that, once the solid appears, subsequent warming does not cause it to melt until a much higher temperature. This is true even if only a s mall fraction of the fluid is initially frozen. The melting branch of the hysteresis loop is more nearly reversible. In order to correctly m easure the latent heat of freezing in the presence of such hysteresis, a technique should be used in which cooling is performed monotonicall y, for example, thermal relaxation or differential scanning calorimetr y.