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

Lead Research Organisation: University of Glasgow
Department Name: School of Physics and Astronomy

Abstract

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.

Publications

10 25 50
 
Description It is becoming apparent that local chemical ordering on the nanoscale is present in the ceramics with the high temperature - wide temperature range stable dielectric constant. This seems to correlate with some local polarisation ordering. Data is currently being analysed prior to publication.
Exploitation Route We need to finish the analysis of the data taken in the programme. Also, the samples are still present and will be analysed once the current national crisis is over in the aim of publishing the results in the next year.
Sectors Aerospace, Defence and Marine,Electronics

 
Description Presentation at MMC conference by Mr Thomas MacGregor 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Poster presented at MMC2019 on some of the preparation techniques used in the work. It was well received as it won a presentation prize.
Year(s) Of Engagement Activity 2019
URL https://www.mmc-series.org.uk/general-information/previous-congresses.html
 
Description Publication of an article in the "Microscopy and Analysis" trade magazine. 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited submission to "Microscopy and Analysis" (a widely circulated trade publication in the microscopy community) as a result of Tom MacGregor winning a presentation prize at MMC2019 describing some of the preparation techniques used in making samples for high temperature, high field experiments.
Year(s) Of Engagement Activity 2019
URL https://www.microscopyebooks.com/Europe/Supplements/2019/November/#p=5