Cationic heats of transport in potassium chloride and sodium chloride single crystals

Abstract

Flows of heat through a hot solid entrain flows of material. These flow rates of matter and heat in a solid are related by a term called 'heat of transport'. In the experiments described, independent measurements of heats of transport and thermoelectric powers have been made in two strongly ionic solids - potassium chloride and sodium chloride single crystals in Pt/salt/Pt cells. Measurements of electrical resistance in these materials have been made both at equilibrium and non-equilibrium conditions. The changes in resistance due to application of thermal and electric field forces have been used as indicators of deviations of equilibrium vacancy concentrations. The heat of transport which is related to the flow of vacancies can then be calculated from resistance change. The values of cationic heats of transport for pure potassium chloride and -4 sodium chloride crystals are found to be (0.37+4.74x10-4xT]) eV and (0.52+2.87x10-4xT) eV respectively. The measurement of heat of transport from considerations of vacancy concentrations is quite distinct. from the traditional approach which attempts to evaluate it from the thermoelectric power. Measured heat of transport can then be used to evaluate homogeneous thermoelectric power of the solid. Comparison of this component of thermoelectric power with the total thermoelectric power using Platinum electrodes offers a value of heterogeneous component. In addition to these measurements, the present approach also gives values of vacancy life time and vacancy mean free path. An estimate of effectiveness of vacancy traps can be obtained from considerations of mean free path. These parameters together elucidate the actual process involved in vacancy migration under driving forces.