New directions in high temperature dielectrics: unlocking performance of doped tungsten bronze oxides through mechanistic understanding

需要新型的高温、高压多层陶瓷电容器(MLCC)来推动电力电子技术的发展，电力电子技术是到2050年实现二氧化碳净零排放的能源过渡的重要技术。用于电力电子设备的宽带隙半导体技术已经提供了可以在250-300℃下工作的有源元件，从而减少了散热器的尺寸和设备的重量。然而，由于宽带隙器件的高开关速度，无源和有源元件必须非常接近，这对(无源)电容器的高温运行提出了更高的要求。除了在可再生能源分配中的应用外，在运输电气化中还需要更高温度的电容器，其中电子设备需要在较高的环境温度下运行。不幸的是，现有的II类电容器都是基于钙钛矿晶体结构的，只能在125-175℃下工作。过去十年来，全球对新型高温电容器材料的研究未能生产出任何适合大众市场MLCC的介质材料，现在使用廉价的镍金属内电极制造。障碍一直是陶瓷配方中存在的铋或铅氧化物。这将导致电介质材料和电极在高温、化学还原气氛中降解，这些气氛用于制造现代MLCC。在研究方向的转变中，我们最近获得了原理证明，基于钨青铜晶体结构的新型介质在所需的-55至300℃温度范围内具有均匀的高介电常数(&gt；1300+/-15%)。该材料是以仅掺有1-2.5at.%的钙、Y和锆的铌酸锶钠(SNN)为基础的。尽管前景看好，但其介电性能仍达不到下一代电容器材料所需的卓越性能水平。例如，介质损耗(目前为4%)超过了工业规范(2.5%)。由于缺乏对支撑机制的了解，新的钨青铜方法的真正潜力被严重阻碍。例如，为什么低水平的掺杂剂会产生与温度有关的弥散的孪生介电异常。在前期的工作中，我们已经证明了现有的基于钛钙钛矿的高温稳定介质的组成-结构-性能关系不适用于这种新型的高温介质。我们建议通过应用新的科学理解来释放钨青铜的真正潜力，以克服现有的限制。我们将结合使用电子、中子和同步加速器衍射、原子柱分辨电子显微镜和电化学阻抗谱，通过研究特定掺杂系统如何改变结构(晶体、纳米、微米、缺陷)，发现如何在具有挑战性的温度范围-55至300℃范围内提高掺杂SNN陶瓷的介电常数和降低介电损耗。第一性原理模拟还将帮助我们解释实验结果和开发结构-性能模型。从这个理解框架中，我们将设计出新的作文。最终的材料选择标准将包括一系列其他介电参数，包括介电击穿强度和储能密度。我们的电容器行业合作伙伴KEMET将帮助评估材料并进行加速寿命测试。最好的材料将在项目中展示在带有集成高压直流链接MLCC的宽带隙开关单元中。除了直接参与已建立的公司合作外，还将通过与业界开发新的活动来最大限度地提高更广泛的利益。这将在一定程度上利用利兹大学研究和创新服务以及新的创新和企业中心Nexus的资源来实现。

New higher temperature, high-voltage multilayer ceramic capacitors (MLCCs) are required to advance power electronics - an important technology in the energy transition to net zero CO2 emissions by 2050. Wide-bandgap semiconductor technologies for power electronic equipment already provide active components that can operate at 250-300C, allowing reductions in heatsink size and equipment weight. However due to the high switching speeds of wide-bandgap devices, passive and active components must be in close proximity, demanding high temperature operation of the (passive) capacitors. In addition to applications in renewable energy distribution, there are demands for higher temperature capacitors in transport electrification where electronic equipment needs to operate at high ambient temperatures. Unfortunately existing Class II capacitors, which are all based on the perovskite crystal structure, can only operate to 125-175 C. Global research into new higher temperature capacitor materials over the past decade has failed to produce any dielectric material suitable for mass market MLCCs, now manufactured using inexpensive nickel metal internal electrodes. The obstacle has been the presence of bismuth or lead oxide in the ceramic formulation. This would cause the dielectric materials and electrodes to degrade in the high temperature, chemically reducing atmospheres used to manufacture modern MLCCs. In a shift of research direction, we have recently obtained proof-of-principle that a new type of dielectric based on the tungsten bronze crystal structure offers uniformly high permittivity (>1300 +/- 15%) over the requisite -55 to 300 C temperature range. The material is based on strontium sodium niobate (SNN) co-doped with only 1-2.5 at.% calcium, yttrium and zirconium. Although promising, the dielectric properties fall short of the exceptional performance levels required of a next generation capacitor material. For example, dielectric losses (currently 4%) exceed industrial specifications (2.5%). Unlocking the true potential of the new tungsten bronze approach is severely hindered by a lack of knowledge as to underpinning mechanisms. For example, why low levels of dopants create extremely diffuse twin temperature-dependent dielectric anomalies. In preliminary work we have demonstrated that composition-structure-property relationships for existing temperature stable dielectrics based on titanate perovskites do not apply to this new type of high-temperature dielectric. We propose to unlock the true potential of tungsten bronzes by application of new scientific understanding to overcome existing limitations. We will discover how to raise permittivity and reduce dielectric losses in doped SNN ceramics across the challenging temperature range -55 to 300 C by studying how structure (crystal, nano, micro, defect) is modified by specific dopant systems, using a combination of: electron, neutron and synchrotron diffraction; atom column resolution electron microscopy; electrochemical impedance spectroscopy. First principles simulations will also assist us in interpreting experimental findings and developing structure-property models. From this framework of understanding, new compositions will be designed. Final materials selection criteria will include a range of other dielectric parameters, including dielectric breakdown strength and energy storage density. Our capacitor industry partner KEMET will help evaluate materials and conduct highly accelerated lifetime testing. The best material will be demonstrated within the project in a wide-bandgap switching cell with integrated high-voltage DC-link MLCCs. Alongside direct engagement with established company collaborations, wider benefits will be maximized by developing new activities with industry. This will be achieved in part using the resources of the University of Leeds's Research and Innovation Service, and the new Innovation and Enterprise Centre, Nexus.