Automated non-contact ultrasonic measurements of single crystals

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.

The work contained within this proposal is focussed on development of novel techniques for ultrasonic measurements of the elastic properties of single crystals, but is necessarily interdisciplinary, drawing on expertise from condensed matter physics, non-destructive testing and digital signal processing. This cross-disciplinary approach to solving a problem will bring significant benefits to the scientific community. Currently there is limited research in the UK into ultrasonic measurements of single crystals, with few facilities and very few advances made in recent years. Such a facility is essential, putting the UK at the forefront of ultrasonic research, and this proposal aims to develop a state of the art system for these measurements, focussing on the benefits and improvements that non-contact techniques will bring. The use of EMATs brings the benefits of an inexpensive, adaptable system, in particular when compared with other elastic constant measurement systems, such as neutron scattering. The benefits of an inexpensive test, identifying the materials which are best suited to the more expensive and time consuming measurements, will bring great benefits to materials researchers and physicists at a time when funding is an issue. Whilst commercialisation of the new instrument developed within this work is not feasible, this work aims to create an open facility which will be made available to other UK researchers, and collaborations will be pursued throughout the programme. The main beneficiaries of this research will be academia, and in particular researchers in ultrasonic measurements. On the academic side the work has the potential to impact the following groups; * The condensed matter / materials science / engineering research communities, through the new capability for studying elastic constants and phase changes in materials, and in particular those which have previously caused difficulties * Research groups at the University of Warwick, through providing a new facility for collaboration within the department for materials characterisation * Researchers within the ultrasound group, who will be able to test new materials for piezoelectric generation of ultrasound, develop new thickness gauging techniques, and build collaborations with other researchers in materials characterisation. Through this research collaborations will be built, in order to apply the techniques to a wide range of materials and investigate interesting physics. There is a focus on development of skills within this project. The named technician involved in the project will gain experience in working with cryogenics and in building electronic circuits, allowing him to progress in his job and gain valuable knowledge. The PI will gain the benefits of time to work on this new area and develop new expertise, and will be able to build up new collaborations through dissemination of the research. A further benefit comes from the potential application of the velocity measurement improvements to thickness gauging in non-destructive testing, which is of interest to many industrial members of the EPSRC funded Research Centre in Non-Destructive Evaluation (RCNDE). The new digital signal processing and analysis developed within this proposal will potentially give a higher accuracy to thickness gauging measurements, and will allow new techniques which require a narrowband signal to be developed for applications such as corrosion and wall thinning measurements in pipework, and testing can production for the food and drink industry. The RCNDE industrial members will be informed about the work and the potential for new thickness gauging techniques, and any potential new avenues will be pursued initially as part of this work. Dissemination is essential, and will involve publication of the results and presentation at conferences and seminars for interested parties.