Vaporization Waves in Metals

The vaporization wave hypothesis Is discussed and Its merits and defects are examined. The vaporizing model is visualized on thermodynamic grounds as carrying the liquid metal through a continuous succession of states either on or near the liquidus line in the two-phase region. On this line the adiabatic sound speed for wet vapor will limit the rate of propagation of the vaporization front into the liquid. Experimental data for wire explosions of Al, Ag, Cu, Au, Pb and Hg (frozen) are analyzed for wave speeds. While the influence of thermal expansion of the liquid can be accounted for theoretically, Insufficient thermal data are available for the metals to permit correction of the wave speeds for this effect. The experimentally derived wave speeds are compared with theoretical values of the adiabatic sound speed in the wet vapor obtained from a modified, van der Waals equation of state. At low velocities the agreement is satisfactory but higher values deviate considerably from theory. Possible causes of the deviations are discussed. These include the crudity of the fluid dynamic model, neglect of thermal expansion, lack of information about the relationship between density and electrical conductivity and the approximation imposed by the van der Waals equation. TABLE OF CONTENTS Page ABSTRACT 3 INTRODUCTION 7 THEFMODYNAMICAL MODEL 12 EXPERIMENTAL 20 DISCUSSION 393 INTRODUCTION 7 THEFMODYNAMICAL MODEL 12 EXPERIMENTAL 20 DISCUSSION 39 SIAMARY 1*5 REFERENCES U? DISTRIBUTION LIST 1+9 INTRODUCTION We discuss here some recent experiments to examine further the hypothesis that vaporization waves are responsible for the anomalous resistance rise in exploding wires. While vaporization waves may be expected in all high temperature matter on general thermodynamic 1 5* grounds, * the first instance in which their existence could be 2 recognized appears to be the exploding wire phenomenon. If the vaporization of a superheated metal cylinder is limited by the speed with which the head of the vaporization wave travels from periphery to' the axis, then an upper bound is set on the rate at which the conducting cross section diminishes. Resistance increases above the melting point, larger than the usual linear rise with temperature, can be related to the decreasing cross section of the conductor; thus, wave speeds can be obtained from the electrical pulse data. In our earlier 2 paper we reported wave speeds for Cu wires. Here we present data for Cu, Pb, Al, Au, Ag and Hg (frozen) wires and make comparison with predictions from theory. From a critic's point of view a number of limitations on the applicability of the vaporization wave hypothesis can be stated. The success of the experimental method of analysis depends on the assumption f a linear relation between resistivity and specific energy applied above the melting point up through the two-phase region. This is, at best, a fairly crude approximation but cannot be replaced until better Information is available. Certain metals such as Fe, Ni and W » Supereoript nvmbera denote refeven*«* which may be found on page 47, 1