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

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.

The project will add to UK research strengths in Advanced Functional Materials by pioneering the study of local structure using an integrated approach for the purpose of designing high-temperature capacitor materials with a step-change in performance for use in power and harsh environment electronics.

The anticipated end-result is to break through the 200 C barrier facing high temperature capacitors. Successfully applying scientific discoveries to design high relative permittivity, high temperature dielectric ceramics will have economic impact in the UK. New products are required to continue the advances in high temperature electronics to benefit the rapidly expanding power electronics, and harsh environment electronics sectors. Understanding the origin of a near-temperature invariant dielectric performance over wide temperature ranges with upper operating temperatures of 300 C will permit tailored design to achieve innovative, device-quality capacitor materials. Downstream this will lead to new capacitor products and strengthen the UK capacitor manufacturing base for bulk and multilayer capacitors. Capacitor companies will be able to implement product development programmes based on Demonstrator Materials emerging from the project.

The demand for > 200 C capability passive components is driven by recent advances in high-temperature semiconductor and packaging technology. Lack of > 200 C Class II capacitors is limiting growth in high temperature electronics in numerous applications where cooling is not practical or improved system reliability or better efficiency would result from using high temperature components. Power conversion, bypassing, filtering and signal coupling rely on capacitor technology. New capacitor materials would be employed where high power dissipation creates self-heating, as in power conditioning electronics for sustainable energy generation and grid integration: within the automotive sector, superior inverters are required for electric and hybrid vehicles. Harsh environment applications in avionics and aerospace include distributed control schemes, where engine controls are located close to the engine, reducing the complexity of the interconnections, saving hundreds of kg of aircraft weight, saving fuel and increasing the reliability of the system. Other examples include military and deep-well drilling.

Encouraging young people to study science and engineering will be part of the project's remit, implemented with the assistance of the Leeds Festival of Science, by schools outreach and interactions with University Technical Colleges, including project display booths at Immersion Weeks. The Team for Public Engagement with Research at University of Leeds will assist in reaching the wider public to illustrate the importance of fundamental scientific research to the UK's prosperity. Career opportunities of researchers will be improved by expanding skill-sets and working co-operatively in a multidisciplinary team. Transferrable skills will be improved by attending formal training courses at the Universities, and practicing communication skills within the project, for example by presentations to industrialists at steering meetings and at the project workshop.

Our new scientific discoveries in the field of relaxor dielectrics, both in terms of experimental and theoretical research are expected to stimulate other research groups to apply a similar integrated analytical approach in other areas.

The project will impact on advanced electron microscopy researchers, X-ray absorption specialists and the modelling community working on relaxor and ferroelectrics materials. We will communicate the findings of our multi-disciplinary research with leading international researchers via personal links, high impact journal publications and at international and national conferences.