DISLOCATION MOBILITY AND PINNING IN HARD MATE RIALS THROUGH INTERNAL FRICTION STUDIES.

The internal friction due to dislocation motion in silicon crystals which have undergone con trolled degrees of plastic deformation near 900 C is measured in the 3 kc to 80 kc frequency re gion. The observation reported previously, of a thermally activated rise between 600 C and 900 C, is confirmed, the activation energy usually being 1.6 eV, although some less reliable measurements gave values below 1.5 eV. The results in this region are consistent with a modification of Brailsfords built-in kink theory of damping. Attempts to produce a peak in internal friction by suitable treatment of specimens were unsuc cessful, and so confirmation of the theory which the peak could have given is still required. At temperatures above 900 C the increase of internal friction with temperature becomes less, corre sponding to an activation energy of about 1 eV. Annealing above 1000 C produces a considerable reduction in the internal friction, while main taining the same activation energy for the proc ess. The degree of anneal appears to depend on anneal temperature, the temperature of initial deformation, and oxygen content of the crystals. Further anneal at lower temperature can produce partial recovery of the internal friction. Author