FREEZING AND MELTING OF HELIUM IN DIFFERENT POROUS-MEDIA

We have studied the freezing and melting of He-4 confined in several p orous media with very different structures. These include Vycor glass, which has very small pores with a rather narrow, size distribution, a silica aerogel with a much more open geometry and a wider range of po re sizes, and art alumina membrane with larger but non-intersecting po res. We measured the velocity and attenuation of ultrasonic waves, whi ch allowed us to detect the decoupling of any superfluid in the pores as well as the modulus changes associated with freezing and melting. W e find that the shift of the melting curve due to confinement is small er in the aerogel than in Vycor, but the hysteresis between freezing a nd melting is similar. The effects of confinement are very small for t he alumina membrane and hysteresis is only observed at the lowest pres sures.