DYNAMIC MEASUREMENTS OF THERMAL TRANSPORT-COEFFICIENTS AND BOUNDARY RESISTANCE .1. NORMAL HE-4

The thermal conductivity, kappa, and the thermal diffusivity, D-T, of normal liquid He-4 have been obtained from the temperature response, D elta T(omega), across a fluid layer of thickness, d, to an ac heat flu x, Q(t) = Q(0) exp(i omega t). Previous transient heat flux experiment s measured the thermal relaxation of the fluid towards equilibrium and assumed the dominance of a single slowest mode. The present ac techni que allows measurements tinder steady-state conditions while driving t he system at a single frequency, omega. The response cur ve for Delta T(omega)/Q(0) yields data for kappa, D-T and the boundary resistance, R(b). Boundary effects appear at frequencies higher than tau(-1) = D-T /d(2) where the fluid is unresponsive to bulk heat transport. We use t his fact to obtain R(b) with high accuracy in the normal phase from th e high frequency response. In addition, the apparatus permits the flui d thickness, d, to be varied continuously and in situ from zero to 3 m m, allowing for further consistency in the fluid measurements. This wo rk also includes data for the onset of convection where Q(0) > Q(c), a nd Q(c) corresponds to the heat amplitude at convective onset.