Phase behavior of helium in porous media
A comprehensive study of liquid helium in nanoporous structures using quartz crystal microbalances (QCMs) is described. The principal goal was to study the interaction between superfluid onset and capillary condensation. Three types of porous materials were used: porous gold, calcium fluoride, and anodized aluminum oxide. Forward and reverse vapor pressure isotherms were measured in the temperature range of 1 K to 3 K. Hysteresis loops associated with capillary condensation in these isotherms give information on the geometry of the porous materials while sudden changes in apparent mass loading and dissipation locate the superfluid transition.
The isotherms were used to construct the first phase diagrams in the μ--T plane showing hysteresis closure points and superfluid onsets in porous media. Superfluid onset in porous substrates was found to be significantly different than the superfluid transition in either bulk 3D fluid or in 2D thin films. The characteristics of superfluid onset in the porous samples depended crucially on whether the transition occurred at low coverage and low temperature in the single phase regime, or at higher temperatures and coverage in the hysteretic capillary condensation phase regime. In the single phase regime, superfluid onset showed both the abrupt frequency shift and dissipation peak characteristic of a conventional Kosterlitz-Thouless transition, corrected for the increased area and tortuosity of the substrate. At higher temperatures and coverages, a dissipation peak is observed, but the nearly discontinuous frequency shift is replaced by a smooth and gradual decoupling of the superfluid component as the coverage is increased. The gradual onset of superflow can be approximately described by a percolation model which yields a power law with an exponent near 1.5.
Most porous materials are random, inhomogeneous structures. In contrast, nanoporous alumina can be made to have a highly uniform array of cylindrical pores. We have measured the first isotherms on a highly ordered porous substrate grown on a QCM. The fact that the pores are cylindrical in shape allows one to theoretical model adsorption in the structure and to quantitatively analyze isotherms near the critical point. Because the axis of the pores is perpendicular to the mode of oscillation, the superfluid fraction is hydrodynamically locked to the structure. This phenomenon is the basis for a novel low temperature pressure gauge.
0794: Materials science