Freezing and melting of fluids in porous glasses

We have studied the freezing and melting of a number of cryogenic fluids (hydrogen, neon, oxygen, and argon) confined in porous glasses (Vycor and a silica xerogel). ac heat-capacity measurements show broadened latent-heat peaks associated with both freezing and melting at temperatures substantially below the bulk melting temperatures. Thermal cycling shows pronounced hysteresis, with melting occurring at a higher temperature than freezing. Also, the latent heat of freezing appears to be much smaller than that of melting. The hysteresis in the argon-Vycor system was studied in detail using high-resolution ultrasonic techniques which directly probe the shear modulus of the material in the pores. We found that the onset of freezing is extremely sharp, despite the random pore geometry, and that freezing continues over a range of temperatures. The freezing process is extremely irreversible, in the sense 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 small fraction of the fluid is initially frozen. The melting branch of the hysteresis loop is more nearly reversible. In order to correctly measure the latent heat of freezing in the presence of such hysteresis, a technique should be used in which cooling is performed monotonically, for example, thermal relaxation or differential scanning calorimetry.