Freezing of molecular hydrogen and its isotopes in porous vycor glass

Abstract

We have made a detailed ultrasonic study of freezing and melting of molecular H₂, HD and D₂ in the pores of Vycor glass. The behavior was similar to that seen in previous measurements with argon and helium. The hydrogen liquids under cooled about 2.5 K below their bulk triple points before freezing began and there was substantial hysteresis between freezing and melting. The velocity and attenuation began to increase suddenly at the onset of freezing. The velocity continued to increase to the lowest temperatures (2 K) and the attenuation had a broad peak at about two thirds of the freezing temperature. We attribute these effects to stress relaxation via thermally activated vacancy motion in the solid hydrogen, an interpretation confirmed by looking at the frequency dependence of the velocity and attenuation. The magnitude of the velocity and attenuation changes increased in going from H₂ to HD to D₂, as expected based on their increasing densities and elastic constants. However, there were no qualitative differences between the boson (H₂ and D₂) and fermion (HD) cases nor, for that matter, between hydrogen and argon. We believe that essentially all the hydrogen was frozen a few tenths of a kelvin belowT _F, at the point where the melting/freezing hysteresis began. If even a few per cent of the hydrogen had remained liquid and become superfluid at some lower temperature, it would have been seen as a further increase in the velocity and a critical attenuation peak. The sensitivity of our ultrasonic measurements allowed us to make accurate measurements of the freezing and melting temperatures of the different liquids in Vycor. We found that the fractional undercooling, (T _B -T _F )/T _B , increased as the molecular mass decreased which may indicate the importance of quantum effects on the liquid-solid interfacial energy σ _ls .