The application of radioactivity to studies of diffusion in solids

Abstract

The aim of the investigation was the study of the influence of crystal fine structure on conductivity and cation self diffusion in single crystals of sodium chloride. Crystals were grown by the Kyropoulos technique and subjected to thermal treatment likely to alter the crystal fine structure. Both annealing and chilling of single crystals produced changes in the crystals conductivity and in sodium-22 self-diffusion rate at temperatures below 520ºC. In the investigation of conductivity a study was made of electrode materials and for the diffusion measurements a sensitive absorption method was used. The changes in diffusion rates and conductivity might have been the result of changes in internal surfaces and grain boundaries. The possibility of ion movement in grain boundaries was investigated by autoradiography of sections of crystal into which sodium-22 had penetrated. The rte of movement of sodium-22 on the surface of sodium chloride was also investigate and a limit set for the surface diffusion coefficient. It was concluded that cation movement in sodium chloride single crystals does not take place more rapidly in the grain boundaries or on internal surfaces than it does through the bulk of the crystal. An explanation of the changes in diffusion rates and conductivity on thermal treatment of the crystal is offered in terms of vacancies, rain boundaries and impurity ions.