No abstract

Lattice dynamics and elasticity of silver thiogallate ( AgGaS 2 ) from ab initio calculations Ab initio calculations of the basic properties of solids have advanced significantly, and it is now possible to simply access a crystal structure database, and from the given constituent atoms and their positions, calculate reasonably accurate values for elastic constants and thermomechanical properties that may be derived from them. However, progress has been impeded by a unique discrepancy involving the sign of one of the elastic constants of an important material: Al 2 O 3 . In this letter, this longstanding discrepancy is resolved with experimental measurements.

Measurements of elastic constants of strained 200 and 400 nm thin films, as well as unstrained samples, of the colossal magnetoresistance (CMR) material La0.67Ca0.33MnO3 are presented. Since the peak resistance temperature of a strained CMR film decreases as the film thickness decreases, it is of interest to see if features in the elastic constants, reflecting structural or magnetic changes, follow the peak resistance temperature. It is observed that features in the elastic constants appear not only at the peak resistance temperatures of the CMR samples, but also at a temperature about 17 K higher. A new technique, thin-film resonant ultrasound spectroscopy, was used to make the measurements.

Various oscillating-wave thermodynamic devices, including orifice and feedback pulse tube refrigerators, thermoacoustic-Stirling hybrid engines, cascaded thermoacoustic engines, and traditional Stirling engines and refrigerators, utilize regenerators to amplify acoustic power ͑engines͒ or to pump heat acoustically up a temperature gradient ͑refrigerators͒. As such a regenerator is scaled to higher power or operated at lower temperatures, the thermal and hydrodynamic communication transverse to the acoustic axis decreases, allowing for the possibility of an internal acoustic streaming instability with regions of counterflowing streaming that carry significant heat leak down the temperature gradient. The instability is driven by the nonlinear flow resistance of the regenerator, which results in different hydrodynamic flow resistances encountered by the oscillating flow and the streaming flow. The instability is inhibited by several other mechanisms, including acoustically transported enthalpy flux and axial and transverse thermal conduction in the regenerator solid matrix. A calculation of the stability limit caused by these effects reveals that engines are immune to a streaming instability while, under some conditions, refrigerators can exhibit an instability. The calculation is compared to experimental data obtained with a specially built orifice pulse tube refrigerator whose regenerator contains many thermocouples to detect a departure from transverse temperature uniformity.

The ultrasonic absorption and velocity at 21 MHz and also the shear viscosity were measured for poly (vinyl alcohol) and water solutions. The concentrations of poly (vinyl alcohol) by weight that were used were 1%, 3%, 5%, 9%. Measurements were made at 25 ø, 30 ø, 35 ø, 40 ø, and 45 øC. It was found that a/f 2 decreased with increasing temperature and increased with increasing concentration.The wave velocity increased with both temperature and concentration. The shear viscosity was observed to decrease with temperature and increase with concentration.