Estimation of heat loss in thermal wave experiments

Abstract

The theoretical description of thermal wave experiments assumes harmonic heat introduction (sinusoidally modulated heating and cooling) and an adiabatic (thermally insulated from the environment) sample. In most experimental setups, however, only pulsed heat is introduced, and the mean sample temperature stabilizes at a value above ambient determined by the strength of the heat loss to the surroundings. The question arises as to what conditions have to be satisfied such that the assumption of an adiabatic sample is applicable to the experimental situation. In this contribution we present a treatment of heat-loss mechanisms (radiation, conduction/convection to air, and conduction to the sample holder) together with their implications for signal shape and temperature control of the sample. The treatment leads to a general mathematical criterion to decide whether heat conduction inside the sample can be considered to be adiabatic or not, depending on excitation frequency and spatial frequency content of the heating power distribution. The theoretical considerations are applied experimentally to typical situations of thermally thin samples (where heat loss to the sample holder is critical) and thermally thick samples (where temperature control can be a problem).