基于石墨烯复合和液相烧结法协同调控的N型碲化铋基热电材料研究

Bismuth telluride based thermoelectric materials as the best room-temperature thermoelectric materials are promising for large-scale applications in thermoelectric power generation and solid-state refrigeration. Although the figure-of-merit ZT value of P-type bismuth telluride based material has been greatly improved in recent years, the ZT value of the relevant N-type bismuth telluride material has been increasing very slowly, which has become a bottleneck problem in the large-scale application of bismuth telluride based thermoelectric devices. What’s more, reducing the preparation cost and enhancing the mechanical strength are also the challenge problem in the research field of thermoelectric materials. In order to solve these challenging scientific problems, this project will adopt the chemical solution synthesis method and introduce the synergetic strategy of graphene compositing and liquid-phase sintering to prepare N-type bismuth telluride based thermoelectric materials with enhanced thermoelectric and mechanical performance. In this project, we will make full use of the advantages of chemical solution-processed methods in the preparation of nanomaterials, the single-atomic-layer and high-mechanical-strength characteristics of graphene as well as the liquid-solid-coexistence characteristics of the liquid-phase sintering process to synergistically optimize the interfacial structure of the N-type bismuth telluride based nanocomposites, which will be benefit for reducing the lattice thermal conductivity, achieving high power factor and enhancing mechanical strength in the N-type bismuth telluride based nanocomposite. The project for N-type bismuth telluride based nanocomposites includes the following specific research contents: the control of graphene compositing and liquid-phase sintering process; the control of thermoelectric and mechanical properties of N-type bismuth telluride based nanocomposites by controlling the synergetic strategy of graphene compositing and liquid-phase sintering; interfacial structures and thermoelectric transport mechanism.

碲化铋基热电材料作为目前最好的室温热电材料，在发电和固态制冷领域都具有广阔的应用前景。虽然近年来P型碲化铋基材料的热电优值ZT有了大幅度提升，但是相关N型材料的ZT值的增长却很缓慢，这已成为碲化铋基热电器件大规模应用的瓶颈问题。另外，降低制备成本和提高机械强度也是热电领域的关键问题。针对这些挑战，本项目拟在化学溶液合成法的基础上，引入石墨烯复合和液相烧结协同优化的方法来制备热电和机械性能增强的N型碲化铋基热电材料。本项目拟充分利用化学溶液法在纳米材料制备上的优势、石墨烯的单原子层和机械强度高的特点以及液相烧结过程中液相和固相共存的特性，通过协同优化来调控材料的界面结构，以降低晶格热导率、维持较高的功率因子和增强机械强度。本项目针对N型碲化铋基热电材料的具体研究内容包括：石墨烯复合过程和液相烧结过程的控制；石墨烯复合和液相烧结过程的协同调控对热电和机械性能的影响；界面结构和电热输运机理研究。

为提升N型碲化铋合金的热电性能和机械性能，本项目发展了化学溶液法来合成具有各类纳米结构的碲化铋基纳米材料（如二维纳米片、一维碲化铋-碲异质结），并通过石墨烯复合和液相烧结法来协同调控纳米复合材料的界面结构。基于“盐辅助球磨法”和化学溶液法，实现了N型碲化铋合金与石墨烯氧化物或氮掺杂石墨烯的复合材料，也研究了一维碲化铋-碲异质结与一维碳纳米管（可看作石墨烯卷曲而成）或一维Ag纳米线的复合材料。对N型碲化铋-石墨烯基纳米复合热电材料的研究表明：适量的石墨烯氧化物可以增强声子在界面的散射，降低晶格热导率；石墨烯氧化物在界面处对碲化铋合金中的“类施主效应”有一定抑制作用，从而能优化材料的载流子浓度；过量碲诱发的液相烧结过程可以优化界面连接，从而维持材料较高的功率因子；这些综合效应使得N型碲化铋-石墨烯基纳米复合热电材料能获得极低的晶格热导率（约为0.21 Wm-1K-1），在473K左右的热电优值（ZT）能达到1.0以上。通过比较石墨烯氧化物和氮掺杂石墨烯对N型碲化铋-石墨烯复合热电材料的影响规律，我们发现缺陷多的氧化石墨烯对优化材料的界面结构更有利，从而改善载流子、声子的输运形式。. 在溶液法制备、石墨烯复合和液相烧结法工艺的基础上，我们开发了一系列适合柔性薄膜热电器件的制备工艺，从而提升本项目研究内容的应用前景。相比于溶液法制备的碲化铋基二维纳米片，我们通过“两步法”获得的一维碲化铋-碲异质结对制备柔性更强的薄膜热电材料更具有优势。制备的N型碲化铋基薄膜热电器件具有领先水平的功率密度，并展现出较好的柔性。为了抑制N型碲化铋基合金中的“类施主效应”并提升其机械性能，我们发展了硼或硼化物（硼化镁）介导的界面工程。这类界面调控方法对碲化锗材料也适用，从而能同时提升材料的热电性能和机械性能，为下一步的热电器件开发奠定良好的基础。以上成果对发展更高效率的致冷器件和中低温废热发电技术都具有积极意义。.