导电有机物-石墨烯纳米结热电输运及转换机制的研究

The thermoelectric (TE) material, which directly converts heat to electricity, and vice versa, is attracting great interests for the advantages of small volume, light weight, no transmission unit, no noise, accuracy and reliability. Recently, much attention has been paid to the thermoelectric transport properties of low-dimensional organic-inorganic hybrid structures since the thermal conductance of organic-inorganic interface is small and conducting polymers are low cost and can be flexibly synthesized. We focus on the TE transport and transformation properties of polymer-graphene nanoribbon junction, where a graphene nanoribbon is trapped between two polymer leads. This structure has unique advantage to achieve better thermoelectric performance: 1. In polymer leads, the small polaron transport due to strong electron-phonon interaction should be considered. Narrow small polaron band provides much sharper density of states (DOS) than that in metal or semiconductor materals, which gives larger Seebeck coefficient. The band mismatch at the interface induces the DOS distortion, which also enhance the Seebekc coefficient; 2. The thermal conductance could be strongly reduced due to the mode mismatch and interface scattering of the phonon at the polymer and graphene nanoribbon interfaces; 3. The electrical conductance in the polymer-graphene nanoribbon junction can be enhanced by tuning the electron chemical potential of the polymer leads, which can be realized by changing the doping concentration of the polymer material. We will study the electron transport based on the Holstein small polaron model by using the Green’s function method and the Landauer formula to obtain the elecltrical conductance, electronic thermal conductance, and Seebeck coefficient. We will investigate the phonon transport by molecular dynamics method to obtain the thermal conductance. The basic mechanism of thermoelectric transport will be indicated. The largest figure of merit (ZT) of this structure will be predicted. This work will provide necessary theoretical guide for the thermoelectric application of the conducting polymer-graphene nanoribbon junction.

热电材料可以实现热能和电能的直接转化，具有体积小、重量轻、无噪声运行、精确可靠等优点。由于低维材料中量子效应可以有效增大Seebeck系数且有机无机材料界面声子散射降低热导，近年来有机无机纳米复合结构的热电性质备受关注。本项目旨在研究导电有机物-石墨烯纳米结中的热电输运及转换机制。此结构有以下优点可以得到高的热电优值：有机物中小极化子的输运导致尖锐的载流子态密度，界面上能带不匹配导致能带弯曲，都有利于得到大Seebeck系数；有机物石墨烯界面上声子模的不匹配和声子界面散射会显著降低声子热导；通过调节掺杂浓度改变化学势可以提高电导。本项目将基于Holstein小极化子模型采用格林函数方法和朗道公式研究其中的电子输运，利用分子动力学方法研究声子输运，得到本体系中热电输运的基本物理机制，探讨各种因素对热电输运的影响，预测最大热电优值，为有机物石墨烯纳米结在热电领域的应用提供理论基础。

热电材料可以实现热能和电能的直接转化，具有体积小、重量轻、无噪声运行、精确可靠等优点。由于低维材料中量子效应可以有效增大Seebeck系数且有机无机材料界面声子散射降低热导，近年来有机无机纳米复合结构的热电性质备受关注。本项目研究了有机物-无机物复合纳米结中的热电输运及转换机制。此结构有以下优点可以得到高的热电优值：有机物中小极化子的输运导致尖锐的载流子态密度，界面上能带不匹配导致能带弯曲，都有利于得到大Seebeck系数；界面上声子模的不匹配和声子界面散射会显著降低声子热导；通过调节掺杂浓度改变化学势可以提高电导。我们基于Holstein小极化子模型采用格林函数方法和朗道公式研究其中的电子输运，利用分子动力学方法研究声子输运，得到本体系中热电输运的基本物理机制，探讨了各种因素对热电输运的影响，预测了最大的热电优值，为有机无机纳米结在热电领域的应用提供理论基础。事实上，此结构还有优秀的自旋热电性质。我们随后研究了金属-非金属界面的传热问题，发现能量可以通过三个通道实现从金属向非金属的传输：1)界面的声子-声子耦合；2)界面的电子-声子耦合；3)金属内的电子-声子耦合以及随之存在的界面的声子-声子耦合。正是以上三种能量输运通道的竞争决定了不同的金属-非金属材料的界面热导性质的不同。接着我们研究了不同的纳米复合材料体系的热电性能，我们合成了Cu掺杂的CZTSe 纳米晶体以及Ni掺杂的Cu3SbS4纳米晶体，发现由于掺杂，材料的电导率大幅度提高、热导率降低，同时Seebeck系数缺并没有显著降低，因此材料的热电性质有了大幅度提高。前者在450摄氏度时热电优值可以达到0.7。在纳米复合热电材料性能研究的基础上，我们接着研究了热电制冷和热电冷却器件的性能。我们采用纳米流体作为冷却液强化热电器件冷端的散热，发现纳米流体能有效帮助提高热电体系的发电效率。还引入梯度热电材料，使得热电制冷器件可以得到更大的温度差。