An Ultrasonic Interferometer for High-Pressure Research

A new method in which ultrasonic interferometry is used to measure the pressure dependence of elastic constants and the density of solids has been applied to polycrystalline I{C1 to pressures of 36 kilobars. Simultaneous measurement of longitudinal and transverse wave velocities in a specimen of initial thickness of approximately 0.2 mm, compressed between two tungsten carbide anvils, yields the adiabatic pressure derivative of density, which is numerically integrated to give the pressure-density relation, permitting direct comparison with Bridgrnan's data. Densities obtained by the present method are within 0.7 per cent of Bridgman's throughout the pressure range studied. The 19.7-kb phase transition of KC1 is marked by a 6 and 12 per cent increase in bulk and shear moduli, the former in good agreement with Bridgman. Extension of the present method to higher pressures and high temperatures and to a variety of materials appears feasible. Introduction. The pressure dependence of elastic constants of selected solids has been measured by ultrasonic pulse travel time [Lazarus, 1949; Hughes, 1960] and by ultrasonic interference methods [McSkimin, 1958] to pressures of 10 kb. The pressure-volume measurements of Bridgman give densities and compressibilitics to pressures of 100 kb under hydrostatic conditions. In the present experiment he ultrasonic pulse interference methods developed by McSkimin [1950] are used in a pressure cell of the type first developed by Bridgman [1952] and later modified by Griggs and Kennedy [1956] to determine the variation of the elastic constants and the density of a polycrystalline material, KC1 with pressure. This material was chosen because the dependence of the elastic constants and of the density of the low-pressure phase on pressure had been well established [Lazarus, 1949], and because KC1 is known to have a polymorphic phase transition [Bridgman, 1940] at a pressure readily attainable in our present pressure cell. The present experimental technique, while similar to that of Anderson [1960], should permit extension of pressure-dependence studies of elastic constants and density of solids to the limit.s of the Bridgman-type anvil apparatus. Harris, Vaisnys, Stromberg, and Jura, x In partial fulfillment of the requirements for the degree of Doctor of Philosophy. •' Contribution 10. [1960] report achieving pressures of 400 kb in such an apparatus. Straightforward modifications of the present equipment should permit the introduction of temperature as a variable in the measurements. Description a•d theory o/measurement. McSkimin [1950] developed a method for obtaining elastic constants of small specimens by measuring the carrier frequency of an ultrasonic RF pulse reflected from opposite sides of the specimen. If the pulse duration is long in comparison with the transit time, the carrier frequency may be varied until a condition of dest -ructive interference between pulses reflected at opposite sides of the specimen is produced. From specimen thickness and carrier frequency it is then possible, after some corrections, to determine elastic wave velocities to high precision. A buffer ro(l is often useful in separati•g the quart• transducer from the specimen, especially when the conditions of stress or temperature imposed upon the specimen are unfavorable for the operation of the quartz transducer [McSkimin, 1953]. In the present experiment (Figs. I and 2) an axial force is applied through two tungsten carbide inserts to a thin specimen, of the order of 0.2 mm thick and contained laterally by a pipestone washer. At the stress-free sides of the inserts are two quartz transducers, x-cut and ycut. An RF pulse is applied to either transducer. The resulting ultrasonic pulse is internally re-