Nanostructured Thermoelectric Oxides for Energy Generation: A Combined Experimental and Modelling Investigation

塞贝克效应是一种热电效应，通过这种效应，材料上的温度梯度被转换成电压，这种电压可以用于发电。对化石燃料和碳排放的日益关注导致对能源生产的各个方面和减少消耗的路线进行了详细的审查。热电（TE）技术，利用废热直接转化为电能，已经成为一个重要的竞争者，特别是在汽车和发动机相关应用中。热电模块采用多对n型和p型TE材料。传统的金属TE材料（如Bi2Te3和PbTe），可用50年，不太适合高温应用，因为它们容易汽化，表面氧化和分解。此外，许多是有毒的。硅锗合金也很成熟，在高达1200K的温度下具有良好的TE性能，但每瓦成本可高达传统材料的10倍。在过去的十年中，氧化物热电材料已经成为有前途的TE候选者，特别是钙钛矿（如n型CaMnO3）和层状钴矿（如p型Ca3Co4O9），因为它们具有灵活的结构，高温稳定性和令人鼓舞的ZT值，但它们尚未具有商业可行性。因此，本研究旨在了解和改善基于CaMnO3和ZnO的氧化物材料的热电性能。此外，它们不仅代表了非常有前途的n型材料，而且通过使用它们作为具有不同和特征良好的结构的模型材料，我们的目标是使用它们定量地确定不同因素如何控制热电性能。热电材料的转换效率用性能曲线ZT表示（其中T为温度）；ZT应该尽可能高。为了使Z值最大化，需要高塞贝克系数(S)，再加上小导热系数和高导电性。原则上，导电性可以通过改变正离子/阴离子的组成来调节。更大的挑战是同时降低热导率。然而在氧化物陶瓷中，晶格电导率主导着热传递，因为声子是热的主要载体。这为一系列减少热传导的策略提供了基础；本质上是纳米级的微结构工程，以增加声子散射。纳米结构的方法将是：(i)将外来离子引入晶格，（ii）发展超晶格结构，（iii）通过引入纹理或纳米尺寸特征进行纳米复合，（iv）发展不同尺寸和结构的可控孔隙，所有这些都提供了额外的散射中心。另外，TE增强也可以通过替代掺杂剂来调节电导率来实现。通过系统地研究纳米结构对CaMnO3和ZnO陶瓷的影响，以及自组装纳米结构的发展，我们将能够定义这些因素的相对重要性，并了解控制热电氧化物中热传导和电子传递的机制。这项工作的一个关键特点是，我们将采用综合方法，结合先进的实验和建模技术来研究纳米结构特征对重要热电氧化物性能的影响。建模研究既可以指导实验人员，也可以为原子水平到颗粒水平上发生的控制机制和过程提供定量的见解，而实验将为不同热电性质的计算提供严格的测试。我们将评估优化的n型和p型材料的机械性能，然后构建热电模块，将在汽车测试环境中进行评估。

The Seebeck effect is a thermoelectric effect whereby a temperature gradient across a material is converted to a voltage, which can be exploited for power generation. The growing concern over fossil fuels and carbon emissions has led to detailed reviews of all aspects of energy generation and routes to reduce consumption. Thermoelectric (TE) technology, utilising the direct conversion of waste heat into electric power, has emerged as a serious contender, particular for automotive and engine related applications. Thermoelectric power modules employ multiple pairs of n-type and p-type TE materials. Traditional metallic TE materials (such as Bi2Te3 and PbTe), available for 50 years, are not well suited to high temperature applications since they are prone to vaporization, surface oxidation, and decomposition. In addition many are toxic. Si-Ge alloys are also well established, with good TE performance at temperatures up to 1200K but the cost per watt can be up to 10x that of conventional materials. In the last decade oxide thermoelectrics have emerged as promising TE candidates, particularly perovskites (such as n-type CaMnO3) and layered cobaltites (e.g. p-type Ca3Co4O9) because of their flexible structure, high temperature stability and encouraging ZT values, but they are not yet commercially viable. Thus this investigation is concerned with understanding and improving the thermoelectric properties of oxide materials based on CaMnO3 and ZnO. Furthermore, not only do they represent very promising n-type materials in their own right but by using them as model materials with different and well-characterised structures we aim to use them to identify quantitatively how different factors control thermoelectric properties.The conversion efficiency of thermoelectric materials is characterised by the figure of merit ZT (where T is temperature); ZT should be as high as possible. To maximise the Z value requires a high Seebeck coefficient (S), coupled with small thermal conductivity and high electrical conductivity. In principle electrical conductivity can be adjusted by changes in cation/anion composition. The greater challenge is to concurrently reduce thermal conductivity. However in oxide ceramics the lattice conductivity dominates thermal transport since phonons are the main carriers of heat. This affords the basis for a range of strategies for reducing heat conduction; essentially microstructural engineering at the nanoscale to increase phonon scattering. The nanostructuring approaches will be: (i) introduction of foreign ions into the lattice, (ii) development of superlattice structures, (iii) nanocompositing by introducing texture or nm size features (iv) development of controlled porosity of different size and architecture, all providing additional scattering centres. Independently, TE enhancement can also be achieved by substitution of dopants to adjust the electrical conductivity. By systematically investigating the effect of nanostructuring in CaMnO3 and ZnO ceramics, plus the development of self-assembly nanostructures we will be able to define the relative importance of the factors and understand the mechanisms controlling thermal and electron transport in thermoelectric oxides.A key feature of the work is that we will adopt an integrated approach, combining advanced experimental and modelling techniques to investigate the effect of nanostructured features on the properties of important thermoelectric oxide. The modelling studies will both guide the experimentalists and provide quantitative insight into the controlling mechanisms and processes occurring at the atom level to the grain level, while the experiments will provide a rigorous test of the calculation of the different thermoelectric properties. We will assess the mechanical performance of optimised n-type and p-type materials, and then construct thermoelectric modules which will be evaluated in automobile test environments.