THERMODYNAMICS OF PHASE-TRANSITIONS IN CURRENT-CARRYING CONDUCTORS

This work studies the thermodynamics of phase transitions of the first kind in current-carrying conductors when these transitions are accomp anied by a sharp change of the electrical conductivity. It is shown th at the critical current in the normal conductor, i.e., the current tha t generates the critical pressure, may be considerably lower than is g enerally believed. The reason for the lower value of the critical curr ent is the shift of the whole curve of phase equilibrium in the presen ce of a strong electric current. This shift arises due to the addition al work performed against ponderomotive forces, which prevents the for mation of the nucleus of a phase with the lower value of electric cond uctivity. In case of the van der Waals model of the critical state the value of the critical current calculated taking into account the shif t of the phase equilibrium curve is 2-3 times less than the critical c urrent determined when this shift is neglected. It is shown that under these conditions there occurs a splitting of the phase-equilibrium cu rve into two separate curves for direct and inverse phase transitions. Depending upon the mutual location of both curves two opposite situat ions may occur. The first case is that of regular hysteresis when ther e exists a domain of stability of both phases and the realization of a particular phase is determined by the initial conditions and the dire ction of the process. In the second case there exists a region where b oth phases are unstable. This region is considered as a domain of the fragmentation of material into small particles. This work determines v arious thermodynamic parameters: latent heat of the phase transition, shift of the phase-equilibrium curve, and the size of the critical nuc leus. It is shown that the value of the shift of the phase-equilibrium curve under the current densities employed in the experiments with ex ploding wires is of order 1. A mechanism for the formation of small pa rticles is suggested and theoretical results are compared with experim ental data.