Automated non-contact ultrasonic measurements of single crystals

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Ultrasonic testing is well known for its use in medical physics, but also has uses in engineering, for determining the structural integrity of materials and structures, and in fundamental physics measurements of new materials. There is currently very little physics-based ultrasound research being performed in the UK, despite the fact that these measurements give information about the elastic constants and phase changes in the materials, and can give a relatively quick and inexpensive test when compared to measurements such as neutron scattering.The current standard ultrasonic measurements of single crystals use contacting ultrasonic transducers which require gluing to the sample, and complicated procedures to measure the velocity of the sound waves. This proposal seeks to create new experimental techniques, through developing non-contact measurements using electromagnetic acoustic transducers (EMATs) and real-time data analysis, and in doing so create a facility which can be used by other researchers. Non-contact measurements bring several advantages; as there is no need for physical contact, the transducers don't significantly load the system. Removing the need for the couplant (glue) means samples are not contaminated, and experiments over a wide range of temperatures is possible. Finally, as the generation mechanism depends on the magnetic state of a material, the efficiency of the EMAT shows clearly any magnetic phase changes.However, electromagnetic techniques of ultrasound generation have a much lower efficiency than standard, contact techniques, and are also sensitive to electromagnetic noise. We will investigate how to overcome this through using filtering, pulse-encoding, and through development of new designs of electromagnetic transducer, utilising a series of coils in the same environment so that electrical noise can be subtracted. Another requirement is a new kind of data processing. PCs are now able to both record and analyse data, and we will develop new analysis routines, in particular through looking at frequency-based analysis techniques such as wavelets and Fourier transforms. This will also help with investigations of thin samples, where echoes are likely to overlap. Using non-contact techniques also allows further improvements to be made, including using broadband ultrasonic pulses rather than narrowband, and through designing new experimental probes which will allow samples to be rotated in a magnetic field during the experiment.The new equipment developed through this work will allow measurements to be performed on a number of materials, identifying samples which work well with non-contact techniques. Samples which will be measured include magnetic materials such as Gd-based materials, which can exhibit magnetocaloric effects, and FePd and Ni2MnGa which show acoustic emission and magnetic noise around structural phase changes. Ferroelectric materials such as BaTiO3 have phase changes over a wide range of temperatures, and a couplant-free system will be very useful here. Finally, non-contact techniques will be well-suited to fragile materials, such as the organic superconductors based on the BEDT-TTF molecule.A further benefit comes from the potential application of the data analysis improvements to thickness gauging for applications such as corrosion and wall thinning measurements in pipework.