摘要

Based on a brief review of the history of heat theory the concept of thermal mass is proposed. The relation between thermal energy and thermal mass is established according to the special relativity theory of Einstein. The physical quantity of thermal mass here developed in this work is different from the concept of caloric developed in nineteen century. It is the difference between of the relativistic mass and the rest mass corresponding to the total thermal energy of all the atoms moving randomly in an object. The relativistic kinetic mass of the thermal energy of solid is further related to that of phonon gas and the phonon gas state equation for temperature far beyond the Debye temperature is derived based on the Debye equation of free energy. The driving force and resistance for the phonon motion are clarified and the conservation equations of mass, momentum and energy for phonon gas are established based on the induced quantities of the thermal mass density, speed of phonon gas etc. The concepts of thermal mass density, speed of phonon gas are introduced. The relationship between the Fourier conduction law and the mass motion of phonon gas is found. The result shows that the Fourier conduction law is the motion equation of phonon gas under the condition of neglecting the kinetic energy of phonon gas. In extreme conditions, such very low temperature and ultra fast heating, the Fourier's law does not hold and the kinetic energy of phonon gas must be considered. The concept of dissipation of phonon gas energy is introduced and can be used as a target function in the optimization of heat transfer processes.