L.A. Thomas

Bio: L.A. Thomas is an academic researcher from General Electric. The author has contributed to research in topics: Quartz & Crystal twinning. The author has an hindex of 1, co-authored 1 publications receiving 37 citations.

The control and elimination of electrical (Dauphiné) twinning in quartz

Abstract: Quartz for telecommunication purposes has to be of the highest crystalline quality. A considerable proportion of raw material is rejected on account of electrical twinning. The war-time need for economy of quartz led to an investigation of means of removing this defect. The paper surveys the experimental methods used in exploring the possibilities of untwinning quartz. The basis of these methods is a heat treatment of the quartz plate while subjected to a system of stresses applied to the plate by temperature gradients, pure bending, longitudinal compression or torque. The effect of crystallographic orientation is discussed, and the new physical phenomenon of “piezocrescence” is defined.The paper also describes the practical application of the untwinning techniques to quartz-crystal plates, with particular reference to the requirements of telecommunication. It is shown that the method employing heat treatment with an applied torque may be successfully applied to many important types of quartz crystals including the BT and AT cuts. The proportion of success and the occurrence of quartz showing anomalous behaviour are discussed.

Plasticity and hydrolytic weakening of quartz single crystals

Abstract: The historical record and experimental data on the plasticity of quartz crystals are reviewed, including the discovery of the important phenomenon of hydrolytic weakening. Quartz deforms by slip on numerous planes, generally with Burgers vectors 1/3 and [0001]; slip may be activated when resolved shear stresses on a and c slip systems are low. Dry natural crystals deformed in an anhydrous environment are very strong, with strengths approaching the intrinsic “theoretical” values, at temperatures up to 1000°C. Synthetic crystals with high OH concentrations and dry crystals heat-treated in a hydrous environment are anomalously weak above a critical temperature (Tc) that varies inversely with OH content. There is a transition in the preferred slip system in crystals equally stressed for a and c slip that coincides with the hydrolytic weakening temperature (Tc). Yielding and flow in all crystals below their respective critical weakening temperatures appear to be controlled by a lattice resistance mechanism, characterized by a low activation energy and very small activation area. Above the critical temperatures, it appears that crystal flow is facilitated by diffusion of “water,” or some related hydroxy defect, to the dislocations, causing hydrolysis of the strong Si-O-Si bonds and aiding bond exchange. This process may assist glide or climb of dislocations–or both–and appears to be rate-limiting. There is evidence that hydrolytic weakening is strongly influenced by confining pressure, possibly through the solubility or diffusivity of H2O or its components in quartz. The change in slip systems at the weakening temperature is attributable to anisotropy of the diffusivity. Similarly, a discontinuity in the creep rates of synthetic crystals, with a concomitant change of activation energy, at the α-β transition is consistent with observed changes in diffusivity at the transition temperature. The experimental data are not yet complete enough to construct an adequate microdynamical model of the flow mechanism.