Reliable, Scalable and Affordable Thermoelectrics: Spin Seebeck Based Devices for Energy Harvesting

作为《能源效率指令》的一部分，英国承诺到2020年将能源效率提高20%，温室气体排放量至少减少20%，并增加可再生能源的份额（与1990年的水平相比）。为了应对这些挑战，需要开发一种稳定和多样化的能源，毫不奇怪，这一直是国际上激烈研究的焦点。相关的研究挑战可以大致分为可再生能源（太阳能、风能、潮汐能）、可持续能源（如碳捕获、聚变）和微型发电（如从热、光、声或振动源收集能量）。这种热源的一个例子是利用热电发电机（teg）收集废热，这种技术具有可靠性（无活动部件）的优势，但受到高成本（使用Te等关键元素）和低效率（200K温差<10%）的限制。考虑到日常生活中大量的废热来源（锅炉、发动机、计算机、区域热网），开发可以轻松应用于各种表面的低成本teg可能是一个重要的变革载体。例如，在英国，收集汽车发动机作为废热损失的能量的5%，每年将节省相当于10万吨当量的石油（或2014年英国总能源使用量的1%）。传统的teg通常基于塞贝克效应：当材料两端存在温差时产生电流的物理过程。提高这些器件效率的瓶颈之一是两个关键材料性能的相互依赖性：导热性和导电性。虽然通过纳米工程已经取得了一些进展，但要实现广泛的商业化仍有一段路要走。然而，这可以通过基于自旋塞贝克效应的teg来克服，其中额外的自由度-电子的自旋-导致器件结构与表面积成比例（与传统热电器件不同），使热导率和导电性分离，从而提高器件的效率，并拥有可从丰富来源（如铁或铜，而不是碲化铋）采购的活性材料。本奖学金的目的是研究自旋塞贝克效应及其作为TEG的应用。需要解决的关键挑战有5个：(1)精确确定基于自旋塞贝克的TEGs的效率；(2)发现新材料（来源丰富）；(3) teg原型机的研制；(4)确定与整个装置效率有关的控制因素；(5)了解这种效应的基本物理原理。例如，利用由自旋塞贝克效应产生的最大自旋极化电流通常需要使用昂贵的铂触点。因此，要使这种技术在经济上可行，就需要发现更便宜的替代品，例如将要研究的掺杂金属。此外，自旋塞贝克效应的精确表征受到仪器的限制，这些仪器通常只监测温差（而不是热流），因此将开发仪器来监测这两个参数，以便确定功率转换。到目前为止，还没有一个综合系数可以用来比较不同的材料系统（如传统热电材料的塞贝克系数），也没有一个经过严格测试的优点数字。一旦确定了这一点，就可以对整个设备的不同材料和工程进行全面比较。

As part of the Energy Efficiency Directive, the UK has committed to a 20% increase in energy efficiency, a reduction of greenhouse gas emissions by at least 20% and an increased share of renewable energy sources (compared to 1990 levels) by 2020. To address these challenges a stable and diverse range of energy sources will need to be developed and, unsurprisingly, this has been the focus of an intense international research effort. The associated research challenges can be loosely categorised into renewable sources (solar, wind, tidal), sustainable sources (e.g. carbon capture, fusion), and micro generation (e.g. energy harvesting from thermal, light, sound, or vibrational sources). One example of such sources is the harvesting of waste heat with thermoelectric generators (TEGs), a technology that has the advantage of reliability (no moving parts), but is limited by high costs (use of critical elements such as Te) and low efficiencies (<10% for a 200K temperature difference). Given the abundant sources of waste heat in everyday life (boilers, engines, computers, district heat networks), development of low-cost TEGs that could easily be applied to various surfaces could present a significant vector for change. For example, harvesting just 5% of the energy lost as waste heat by car engines in the UK would save the equivalent of 1 hundred thousand equivalent tonnes of oil per year (or ~1% of the UK's total energy usage in 2014).Conventional TEGs are typically based on the Seebeck effect: a physical process that results in the generation of an electric current when a temperature difference exists between two ends of a material. One of the bottlenecks for improvement of the efficiency of these devices is the co-dependence of two key material properties: the thermal and electric conductivity. Whilst some progress has been made to circumvent this by nano-engineering, there is still some way to go before widespread commercialisation becomes viable. This could, however, be overcome with TEGs based on the spin Seebeck effect, where an additional degree of freedom - the spin of the electrons - results in a device architecture that scales with surface area (unlike conventional thermoelectrics), enables separation of the thermal and electric conductivities that drive the efficiency of the device and boasts active materials that could be sourced from abundant sources (such as iron or copper, rather than bismuth telluride). The aim of this Fellowship is to investigate the spin Seebeck effect with regards to its application as a TEG. There are 5 key challenges that will be addressed: (1) precise determination of the efficiency of such spin Seebeck based TEGs; (2) discovery of new materials (from abundant sources); (3) development of prototype TEGs; (4) identifying the controlling factors with regards to the efficiency of the overall device; and(5) understanding the underlying physics of this effect. For example, harnessing the maximum spin polarised current generated by the spin Seebeck effect typically requires the use of expensive platinum contacts. For such technology to become economically viable would therefore require discovery of cheaper alternatives, such as the doped metals that will be investigated. In addition, precise characterisation of the spin Seebeck effect is limited by instrumentation that typically only monitors the temperature difference (rather than heat flow), hence instrumentation will be developed to monitor both these parameters so that the power conversion can be determined. There is also, as of yet, no comprehensive coefficient that can be used to compare different material systems (such as the Seebeck coefficient for conventional thermoelectrics), nor a rigorously tested figure of merit. Once this has been established, a comprehensive comparison of different materials and engineering of the overall device can be made.