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Low Temperature Measurements

Ultrasound also has uses in measurements of the fundamental properties of single crystals, such as identifying phase changes and measuring the elastic constants. For these measurements, ultrasonic echoes are measured and the velocity calculated from the time spacing between echoes.

Ultrasound echoes in Gd64Sc36

The calculated velocity can then be used to calculate elastic constants. In the above sample, Gd64Sc36, for longitudinal waves generated along the crystal c-axis, C33= ρv2. Measuring C33 over a wide range of temperatures and magnetic fields will show phase transitions. The figure below was plotted with C33 shown by the colour scale.

Phase diagram

Standard measurements use contact transducers, such as quartz, to generate and detect ultrasound. We are developing EMAT techniques to remove the need for couplant. In NDT, the required magnetic field is provided by a permanent magnet; however, here we have a superconducting magnet which is used to generate a magnetic field to change the sample properties.

Image of EMATs for NDT and low temperature measurements