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

现有的商用高温、高电荷存储介质无法在200℃以上成功运行，但对于新兴电源和恶劣环境的电子产品(在可再生能源、航空航天和汽车行业中非常重要)，电容器材料需要在300℃或更高的温度下具有稳定、坚固的介电性能。在此背景下，我们提出了对局部晶体结构的基础性研究，以揭示非常规类型的极性氧化物陶瓷背后的科学原理，从而为高温电容器技术提供突破。这种材料来源于弛豫铁电体，之所以被称为弛豫铁电体，是因为它们的介电性能具有很宽的频率驰豫。其动机是允许英国电容器制造行业创造新产品，并带来电力和恶劣环境电子产品的进步。弛豫铁电体，如那些基于Nb镁铅的，不同于普通的铁电体，因为它们在仅几纳米的长度尺度上表现出极有序(与正常的铁电体的微米相反)。相对介电常数-温度响应中的一个强峰值是由于极长尺度的增加和冷却时极耦合动力学的变化所致。传统的弛豫剂表现出很大的温度依赖性，因此不适合用于电容器。通过经验组成工程已经证明，相对介电常数峰值可以被抑制，并且可以在很宽的温度范围内诱导温度稳定的电荷存储，最高温度为&gt；300℃。这些新型的温度稳定的高温驰豫材料有望制造出新一代高温电容器，但现有材料无法满足工业需求：(A)稳定的相对介电常数没有扩展到-55℃的行业标准较低温度；(B)相对介电常数低于商用<200℃电容器的50%；(C)介质损耗太高，尤其是在极端温度下。对于弛豫铁电体的极性纳米结构是如何通过增加晶格替代水平来产生温度稳定性能而改变的，缺乏任何科学了解，这是器件标准突破的主要障碍。我们将消除这一障碍，从而通过发现负责将正常温度转变为温度稳定驰豫的纳米结构和纳米化学因素来促进创新高温介质的设计。目前，没有人知道为什么某些化学修饰会使介电响应变得平坦。我们将通过研究现有最好的温度稳定的弛豫固溶体系统之一：CA改性的BaTiO_3-Bi(Mg_(0.5)Ti0.5)O_3来揭示其支持的科学原理。这从铁电到传统的弛豫铁电体转变为温度稳定的弛豫体，随着在配方中用铋和镁取代BaCa和Ti的水平的增加。将使用先进的纳米分析技术来研究结构：原子图像分辨率扫描电子显微镜，用于对10‘S-100’S纳米范围内的纳米结构直接成像；使用X射线吸收技术进行较短距离的分析，以获得平均局部配位环境、键长和电子结构的详细信息；以及使用原子计算机模拟来支持数据解释。结合电学特性测量，这种多学科方法将阐明结构性能标准。其目的是将新知识应用于设计-55至300摄氏度的高温介质材料，这将彻底改变高温电容器技术，为英国带来经济和环境效益。

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.