Acoustic Shear Impedance Research Articles

A−Btransition of superfluidHe3in aerogel and the effect of anisotropic scattering

We report the results of high-frequency acoustic shear impedance measurements on superfluid $^{3}\mathrm{He}$ confined in 98% porosity silica aerogel. Using $8.69\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ continuous wave excitation, we measured the acoustic shear impedance as a function of temperature for the sample pressures of 28.4 and $33.5\phantom{\rule{0.3em}{0ex}}\text{bar}$. We observed the $A\text{\ensuremath{-}}B$ transition on warming in zero magnetic field. Our observations show that the $A$ and $B$ phases in aerogel coexist in a temperature range of about $100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in width. We propose that differences in the relative stability of the $A$ and $B$ phases arising from anisotropic scattering can account for our observations.

Monitoring the setting behavior of cementitious materials using one-sided ultrasonic measurements

Cementitious materials are transformed from a fluid to a solid state due to a chemical reaction known as hydration. These cementitious materials exhibit a continuous change in the mechanical properties with time; there is a steady increase in the stiffness after setting. An ultrasonic test setup and the data analysis procedure, which provide for continuous monitoring of the hydrating cementitious materials from a very early age, have recently been developed. The test procedure for obtaining the ultrasonic test data from cementitious material at different stages of hydration and the theoretical analysis, which allows interpreting the ultrasonic response in terms of the changes in the acoustic shear impedance of the hydrating cementitious material, are presented in this paper. Experimental test results obtained from mortar mixtures of known composition are presented. It is shown that the initial and final setting times correspond approximately with the occurrence of distinctive features in the ultrasonic response.

Measuring Newtonian viscosity from the phase of reflected ultrasonic shear wave

In this paper, an acoustic shear impedance model is employed to obtain a relation between the viscosity of a Newtonian fluid and phase characteristics of ultrasonic shear wave reflection from a solid–fluid interface. The phase and magnitude of the reflection coefficient can be decoupled in this model. The decoupling allows an independent relation between the acoustic shear impedance (viscosity–density product) and phase of the reflection coefficient. The model was experimentally verified for different fluid–solid combinations. Comparison of the results with the commonly used absolute reflection coefficient method demonstrates that phase measurement provides improved measurements.

Measurements of the acoustic shear impedance of superfluid3He

Measurements of the complex acoustic shear impedance of3He-B at 19 bar and 36 and 60 MHz, and of3He-A at 23 bar and 12, 36, and 60 MHz, have been made by observing the time constant and the frequency shift of the decay of a transiently excited AC-cut quartz transducer.