Electric charge on dislocation arrays in sodium chloride

The electric charge on dislocation arrays was studied as a function of temperature by observing the displacement of pure tilt low angle grain boundaries in sodium chloride bicrystals maintained in an electric field. The sign of the charge was determined unambiguously by direct observation of the boundary displacement relative to the polarity of the electric field; the existence of an isoelectric point was confirmed. No measurable boundary displacement is found below a certain (threshold) value of electric field which depends on temperature. For larger fields boundary migration is observed to occur with no delay in time and at a uniform rate which increases monotonically with increasing field. The method used permits the determination of the driving force on the boundary and of the migration velocity as two independent quantities ; from the temperature dependence of the boundary mobility thus determined an activation energy for the process was obtained.