Theoretical considerations of the effects of rapid heating of solids on their apparent thermal properties

The conditions are investigated for thermal properties to change from their normal values when solids are heated very rapidly. The properties considered are specific heat, thermal expansion, thermal conductivity, and thermal diffusivity. Over times which may be as long as a microsecond, the heated solid is unable to expand: the appropriate values of specific heat and thermal conductivity are then those at constant volume rather than constant pressure. In those alloys where thermal equilibrium requires diffusion, its establishment is delayed, and if solids do not have time to expand, the diffusion coefficient is reduced. For heating times below nanoseconds, the electrons and the lattice may be at different temperatures, particularly if the energy is initially imparted to the electrons. The temperature of the electron gas of metals may then approach the degenerary temperature. The apparent specific heat of a decoupled system departs from the steady-state value in a manner which depends on how temperature is measured. In such a decoupled system the concepts of thermal conductivity and thermal diffusivity must be used with care.