Transport properties of helium near the liquid-vapor critical point. II. Thermal conductivity of a³He-⁴He mixture

Abstract

Measurements of the thermal conductivity are reported for an 80%³He-20%⁴He mixture above the critical point along several isotherms and near-critical isochores, using the same techniques and apparatus described for a study of³He. Using again the assumption that the observed conductivity λ can be decomposed into a sum of a regular and a singular contribution λ_reg and λ_sing, it is shown that along two near-critical isochores, λ_sing diverges. In particular, along the isochore showing the largest λ at the phase transition, the divergence is nearly the same as for³He and can be roughly characterized by a simple power law (T-T_c)^−ψ with Ψ≈0.58. This observation is contrary to predictions that foresee λ_sing→0 asT _c is approached. The relaxation times characterizing the attainment of steady state conditions after switching the heat flux on and off show a similar behaviour as a function of reduced temperature as do those for pure³He. This result might indicate a substantial coupling between concentration and entropy diffusion. In the Appendix, the correlation length for³He nearT _c is calculated from heat conductivity, viscosity, and specific heat data.