Transport properties of helium near the liquid-vapor critical point. III. Thermal diffusion ratio, conductivity, and relaxation times of3He-4He mixtures

Abstract

Measurements of the density and temperature gradient induced by a heat flux in two3He-4He mixtures along their critical isochore are reported. From these measurements, the thermal diffusion ratiokT and the thermal conductivity κ are derived. In addition, similar measurements were made for3He. It is found that for the mixtureskT and κ diverge with respective exponents of φ=1.23 and ψ≈0.6, which differs from the asymptotic predictions for binary mixtures. It therefore appears that the crossover to the expected “mixture” behavior has not been reached within the investigated range 10−1>(T-Tc)/Tc>7×10−4, where the limit close toTc is set by the very long observed relaxation times. The characteristic times for the establishment of steady state conditions in the density and temperature gradients τ(Δρ) and τ(ΔT) are reported. Within experimental uncertainty they are found to be the same and nearly independent of isotopic composition. They are compared with the predictions from the hydrodynamic diffusion equations that lead to the characteristic times of the diffusion modes. The slowest calculated times are found to be in fair agreement with the observed ones.