多元化纳米复合热电薄膜微结构演化的强磁场调控及热电性能研究

ZnO thermoelectric film is an ideal material for low-grade waste heat recovery because of its fast conversion speed, high energy density, low cost, non-toxicity and good thermal stability. At present, the property (ZT value) of ZnO film is much lower than that of theoretical value, and it does not meet the needs in application. It is the key and difficulty in improving the ZT value to break the trade-off between Seebeck coefficient, electrical conductivity and thermal conductivity. In this project, the multi-nanocomposite ZnO:Al films are deposited by high magnetic field assisted radio frequency atomic source in vacuum thermal evaporation system, and the electron-phonon transport of the ZnO:Al films are controlled by nanocomposite of Au, CNT and nanopore. The aim of this project is to improve the ZT value by breaking the trade-off between thermoelectric parameters. The effects of Au-layer thickness, CNT content on microstructure, Seebeck coefficient, electrical conductivity and thermal conductivity are studied; the effects of nanoporous size on microstructure, electrical conductivity and thermal conductivity are studied; the effects of Au, CNT and nanopore on microstructure and electron-phonon transport in multi-nanocomposite films are analyzed, and the trade-off between them to thermoelectric parameters is analyzed; the mechanisms of electron-phonon transport controlled by high magnetic field are discussed through microstructure evolution in nanocomposite film; it is to reveal the method and mechanism of breaking the trade-off between thermoelectric parameters; and it is to propose the technical model of improving the ZT value by breaking the trade-off between thermoelectric parameters. This project provides a theoretical basis of thermoelectric transport mechanism and properties control in nanostructure of low-dimension materials.

ZnO薄膜的热电转换速度快、能量密度高、成本低、无毒、热稳定性高，是低品位余热回收发电的理想材料。目前，ZnO薄膜的性能(ZT值)同理论值差距较大，未达到使用要求。热导率、电导率和Seebeck系数的去耦合调控是提高ZT值的关键和难点。本项目采用强磁场辅助射频原子源真空热蒸发方法，通过Au、CNT、纳米孔多元化纳米复合调控ZnO:Al薄膜的电声输运，实现去耦合调控热电参数，提高ZT值。研究Au层厚度、CNT含量对微结构及Seebeck系数、电导率、热导率的影响；研究纳米孔尺寸对微结构及电导率、热导率的影响；分析Au、CNT、纳米孔多元化纳米复合对微结构与电声输运的影响及三者之间对热电参数的竞争与配合关系；研究强磁场通过微结构演化调控纳米复合薄膜电声输运的机制；揭示去耦合调控热电参数的途径和机制；提出去耦合提高ZT值的技术原型。为低维材料纳米结构的热电传输机理和性能控制提供理论基础。

热电材料是基于Seebeck效应将热能直接转化为电能的材料，可用于回收低品位余热，提高能源利用率。在大规模商业化余热回收中，热电材料须具备转换效率高、成本低、热稳定性高、绿色环保等特点。ZnO薄膜的热电转换速度快、能量密度高、成本低、无毒、热稳定性高，是低品位余热回收发电的理想材料。目前，ZnO薄膜的性能(ZT值)同理论值差距较大，未达到使用要求。主要原因是与载流子有关的Seebeck系数和电导率以及与声子散射有关的热导率之间的独立调控机制不明确、且缺乏相关的控制途径。热导率、电导率和Seebeck系数的去耦合调控是提高ZT值的关键和难点。本项目采用强磁场辅助射频原子源真空热蒸发方法，通过Au、CNT、纳米孔多元化纳米复合调控ZnO:Al薄膜的电声输运，实现去耦合调控热电参数，提高ZT值。研究了Au层厚度、CNT含量对微结构及Seebeck系数、电导率、热导率的影响；研究了纳米孔尺寸对微结构及电导率、热导率的影响；分析了Au、CNT、纳米孔多元化纳米复合对微结构与电声输运的影响及三者之间对热电参数的竞争与配合关系；研究了强磁场通过微结构演化调控纳米复合薄膜电声输运的机制；揭示了去耦合调控热电参数的途径和机制。研究结果表明，RF原子源辅助真空热蒸发方法可以在AAO模板上原位生长出透明的有序纳米孔结构的ZnO:Al薄膜。纳米孔结构会使薄膜电阻率增加，热传导降低，有利于保持温度梯度。在Au和CNT两种复合方式中，Au复合可以提高薄膜载流子浓度，CNT复合可以提高薄膜载流子迁移率。高迁移率低载流子浓度的300℃-CNT-ZnO:Al薄膜的ZT值在523 K时为0.081。纳米孔和CNT的多元化复合可以保留原有ZnO:Al薄膜的透光率，同时使薄膜的电阻率和热导率在低温阶段(<450 K)降低50%以上，从而实现了热导率和功率因子的单独调控。纳米孔结构使这种多元化的复合ZnO:Al薄膜具有较高的输出功率，使室温ZT值提高70%。因此，提出了CNT和纳米孔多元化复合的方法，可以实现ZnO:Al薄膜热电参数去耦合化提高ZT值的技术原型。为低维材料纳米结构的热电传输机理和性能控制提供理论基础。