Analysis of Polar Nanostructures in High Temperature Relaxor Dielectrics: a Framework for Materials Discovery

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Existing commercial high temperature, high charge storage dielectrics fail to operate successfully above 200 C - but for emerging power and harsh environment electronics which are important in renewable energy, aerospace and automotive industries, capacitor materials are required with stable, robust dielectric performance to temperatures of 300 C and higher. Against this background, we propose a fundamental study of local crystal structure to discover the scientific principles behind a non-conventional type of polar oxide ceramic which could offer a breakthrough in high temperature capacitor technology. The materials are derived from relaxor ferroelectrics, so called because of a wide frequency relaxation in their dielectric properties. The motivation is to permit the UK capacitor manufacturing industry to create new products and to bring about advances in power and harsh environment electronics. Relaxor ferroelectrics, such as those based on lead magnesium niobate, differ from normal ferroelectrics as they exhibit polar order over length scales of only a few nanometers (as opposed to microns in a normal ferroelectric). A strong peak in the relative permittivity-temperature response is due to the interplay of increased polar length scales and changes to the dynamics of polar coupling on cooling. Conventional relaxors show a large temperature dependence, making them unsuitable for use in capacitors. By empirical compositional engineering, it has been shown that the relative permittivity peak can be supressed and temperature-stable charge storage induced over wide temperature ranges, with ceiling temperatures > 300 C. These new temperature-stable, high temperature relaxors show promise for creating next-generation high-temperature capacitors but existing materials fail to meet industry needs: (a) stable relative permittivity does not extend to industry standard lower temperatures of -55 C; (b) relative permittivity is less than 50% of commercial sub-200 C capacitors; (c) dielectric losses are too high, especially at the extremes of temperature. A lack of any scientific understanding of how the polar nanostructure of a relaxor ferroelectric is changed by increasing levels of crystal lattice substitution to create temperature stable performance is the major obstacle to device-standard breakthroughs. We will remove this barrier, and so facilitate the design of innovative high-temperature dielectrics by discovering the nanostructural and nanochemical factors responsible for converting a normal to a temperature-stable relaxor. Currently, no one knows why certain chemical modifications flatten the dielectric response. We shall reveal the underpinning scientific principles by studying one of the best existing temperature-stable relaxor solid solution systems: Ca modified BaTiO3-Bi(Mg0.5Ti0.5)O3. This changes from a ferroelectric to a conventional relaxor ferroelectric to a temperature-stable relaxor with increasing levels of substitution of Bi and Mg for Ba/Ca and Ti in the formulation. Structures will be studied using advanced nanoscale analysis techniques: atomic image resolution scanning electron microscopy for direct imaging of nanostructure over 10's-100's of nm; shorter range analysis to yield details of average local co-ordination environments, bond lengths and electronic structure using X-ray absorption techniques; and with atomistic computer modelling to support data interpretation. In conjunction with electrical property measurements, this multi-disciplinary approach will elucidate structure-performance criteria. The aim is to apply the new knowledge to design high temperature dielectric materials specified from -55 to 300 C that will revolutionise high-temperature capacitor technology, bringing economic and environmental benefits to the UK.