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

现有的商用高温、高电荷存储电介质无法在200℃以上成功运行，但对于可再生能源、航空航天和汽车工业中重要的新兴电力和恶劣环境电子产品，电容器材料需要在300℃及更高的温度下具有稳定、强大的介电性能。在此背景下，我们提出了一项局部晶体结构的基础研究，以发现非常规类型的极性氧化物陶瓷背后的科学原理，这可能为高温电容器技术提供突破。这种材料来源于弛豫铁电体，之所以叫弛豫铁电体是因为它们的介电性质有宽频率弛豫。其动机是允许英国电容器制造业创造新产品，并带来电力和恶劣环境电子产品的进步。弛豫铁电体，如基于铌酸铅镁的铁电体，与普通铁电体不同，因为它们在仅几纳米（与普通铁电体的微米相反）的长度尺度上表现出极性顺序。相对介电常数-温度响应的一个强峰值是由于极性长度尺度的增加和极性耦合对冷却的动力学变化的相互作用。传统的弛豫器显示出很大的温度依赖性，使它们不适合用于电容器。通过经验成分工程，已经证明，相对介电常数峰值可以在宽温度范围内被抑制，温度稳定的电荷存储诱导，上限温度为300℃。这些新的温度稳定的高温弛豫器有望创造下一代高温电容器，但现有材料无法满足工业需求：(a)稳定的相对介电常数不能扩展到工业标准的较低温度-55℃；(b)相对介电常数小于商用低于200℃电容器的50%；(c)介电损耗过高，特别是在极端温度下。对于弛豫铁电的极性纳米结构是如何通过增加晶格取代水平来改变以创造温度稳定性能的，缺乏任何科学的理解是器件标准突破的主要障碍。我们将消除这一障碍，通过发现将正常弛豫变为温度稳定弛豫的纳米结构和纳米化学因素，从而促进创新高温电介质的设计。目前，没有人知道为什么某些化学修饰会使介电响应变平。我们将通过研究现有最佳的温度稳定弛豫固溶体体系之一：Ca改性BaTiO3-Bi(Mg0.5Ti0.5)O3来揭示基础的科学原理。随着配方中Bi和Mg取代Ba/Ca和Ti的水平的增加，从铁电变为常规弛豫或铁电变为温度稳定的弛豫。结构将使用先进的纳米级分析技术进行研究：原子图像分辨率扫描电子显微镜，用于10 -100纳米的纳米结构的直接成像；利用x射线吸收技术进行短距离分析，以获得平均局部协调环境、键长和电子结构的细节；并使用原子计算机模型来支持数据解释。结合电性能测量，这种多学科的方法将阐明结构性能标准。目的是将新知识应用于设计从-55℃到300℃的高温介电材料，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.