Automated non-contact ultrasonic measurements of single crystals

超声波检测因其在医学物理学中的应用而广为人知，但在工程中也有应用，用于确定材料和结构的结构完整性，以及新材料的基本物理测量。目前，英国很少进行基于物理的超声波研究，尽管这些测量提供了有关材料弹性常数和相变的信息，并且与中子散射等测量相比，可以提供相对快速和廉价的测试。目前单晶的标准超声波测量使用接触式超声波换能器，这需要粘合到样品上，并且需要复杂的程序来测量声波的速度。这项提议寻求通过开发使用电磁声换能器(EMAT)的非接触式测量和实时数据分析来创建新的实验技术，并在这样做的过程中创建一种可供其他研究人员使用的设施。非接触式测量带来了几个优点；由于不需要物理接触，传感器不会给系统带来明显的负载。不再需要偶联剂(胶水)意味着样品不会受到污染，而且可以在广泛的温度范围内进行实验。最后，由于产生机制取决于材料的磁性状态，EMAT的效率可以清楚地显示任何磁性相变。然而，电磁技术产生超声波的效率比标准接触技术低得多，而且对电磁噪声也很敏感。我们将研究如何通过使用滤波、脉冲编码和开发新的电磁换能器设计来克服这一问题，在相同的环境中利用一系列线圈，从而减少电气噪声。另一个要求是一种新的数据处理。个人计算机现在能够记录和分析数据，我们将开发新的分析例程，特别是通过研究基于频率的分析技术，如小波和傅立叶变换。这也将有助于对回波可能重叠的薄样品的调查。使用非接触技术还可以进行进一步的改进，包括使用宽带超声波脉冲而不是窄带，并通过设计新的实验探头，允许样品在实验期间在磁场中旋转。通过这项工作开发的新设备将允许对许多材料进行测量，识别使用非接触技术工作良好的样品。将被测量的样品包括具有磁热效应的Gd基磁性材料，以及在结构相变附近表现出声发射和磁噪声的FEPD和Ni2MnGa。像BaTiO3这样的铁电材料在很宽的温度范围内都有相变，而无偶联剂的系统在这里将非常有用。最后，非接触技术将非常适合于易碎材料，如基于BEDT-TTF分子的有机超导体。进一步的好处来自于数据分析改进在厚度测量中的潜在应用，如管道工程中的腐蚀和壁厚测量。

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.