【作者】 冯斌；

【导师】 谢健；

【作者基本信息】 浙江大学 ， 材料科学与工程， 2015， 博士

【摘要】 石墨烯是一种由碳原子组成的二维周期蜂窝状点阵结构材料,具有高的电导率和迁移率、大的比表面积、良好的机械强度及热稳定性等特点,因此是理想的复合材料组元。由于石墨烯优异的物理化学性能,石墨烯复合材料在能源转换和存储方面的应用也被认为具有广阔前景。尤其在热电材料和锂离子电池领域,石墨烯复合材料被广泛关注。本文设计和制备了石墨烯-合金复合材料,研究了它们的热电性能或电化学吸放锂性能。具体研究了石墨烯-CoSb3(CoSb3/G)体系、石墨烯-PbTe(PbTe/G)体系的热电性能以及石墨烯-锡基合金复合物(CoSn2/G和FeSn2/G)的电化学吸放锂性能。主要成果可以归结如下：提出纳米化结合复合化的策略提高p型CoSb3体系的热电性能。采用溶剂热法制备了CoSb3/G纳米粉末,其中CoSb3颗粒尺寸约为5-10 nm。由于石墨烯的存在,经过600℃热压烧结2 h后的复合材料中CoSb3晶粒尺寸仍保持在100nm左右,远小于纯CoSb3试样的颗粒尺寸。热电性能研究发现,与含量为1.5 wt%的石墨烯原位复合后,材料的室温电导率从约11000提高到了51000 Sm-1,室温晶格热导率从1.05降低到了0.9W m-1K-1。CoSb3/G试样在800 K时zT值达到0.61,而纯CoSb3仅为0.26。体系电导率的提高是由于石墨烯对体系载流子浓度和载流子迁移率的共同提高。体系晶格热导率的降低是由于均匀分散的石墨烯造成更多界面散射。采用共沉淀法制备了CoSb3/G的前驱体,将前驱体在不同气氛(氢气和氢氮混合气)中烧结得到CoSb3/G复合材料。热电测试发现,石墨烯的引入提高了体系电导率并降低了热导率,这与溶剂热结合热压法制备的CoSb3/G试样类似。说明在不同合成工艺或合成条件下,石墨烯的引入都能在一定程度上提高电导率并降低热导率。采用溶剂热法制备了PbTe/G复合物,尺寸为100-200 nm的PbTe立方体颗粒被包裹在石墨烯片层中。由于石墨烯的存在,经过600℃热压烧结2h后,PbTe的纳米结构仍得到保留。热电性能研究发现,相比于纯PbTe,复合材料的室温电导率从约1300提高至22000 S m-1,室温晶格热导率从1.15降低到了0.85 W m-1K-1,这与溶剂热结合热压法制备的CoSb3/G复合材料结果类似。说明对于不同材料体系,石墨烯的引入都能提高体系电导率并降低晶格热导率。通过模型化处理发现石墨烯复合热电材料的电导率遵循渗流理论。在渗流阈值以下,随着石墨烯含量增加,体系的电导率也随之增大。用有效介质理论分析复合材料热性能变化,发现石墨烯对体系热导率的影响依赖于石墨烯在基体中的分散状态。采用一步溶剂热法制备了CoSn2/G和FeSn2/G复合物。两种复合物均呈现夹层结构,细小的纳米颗粒均匀分布在石墨烯片层上及片层间,其中CoSn2/G中CoSn2颗粒尺寸仅为2-4 nm, FeSn2/G中FeSn2颗粒尺寸为10-30 nm。对复合材料及其相应合金的电化学性能对比,可以发现石墨烯的引入显著提高了材料的电化学循环稳定性及倍率性能。电化学性能的改善可以归功于石墨烯防止了纳米颗粒的团聚、阻碍了合金颗粒的长大以及在脱嵌锂过程中抑制了体积膨胀,同时石墨烯也充当合金颗粒的导电网络。更多还原

【Abstract】 Graphene, a two-dimensional sheet composed of single-layer carbon atoms with the honeycomb lattice structure, has high electrical conductivity, high mobility, large specific surface area, good mechanical strength and thermal stability. Therefore, graphene is an ideal building block of composites. Because of the unique physical and chemical properties of graphene, graphene-based composites are considered promising for energy conversion and storage. Extensive attention has been paid to graphene-based composites for thermoelectric materials and Li ion batteries. This work focused on the design, synthesis and thermoelectric properties or Li-storage properties of graphene-alloy composites. Thermoelectric properties of graphene-CoSb3(CoSb3/G) and graphene-PbTe (PbTe/G) composites and Li-storage properties of graphene-Sn based alloy(CoSn2/G and FeSn2/G) composites were studied. The role of graphene in the composites is also discussed. The main results are summarized as follows:A strategy which combines nanostructuring and second phase incorporation for improving the thermoelectric performance of p-type CoSb3 is proposed. CoSb3/G, in which the crystal size of CoSb3 is around 5-10 nm, was prepared via solvothermal route. Due to the existence of graphene, grain size of CoSb3 maintained around 100 nm after hot pressing at 600℃ for 2 h, much smaller than that of graphene-free CoSb3,and the electrical conductivity at room temperature increased from 11000 to 51000 S m-1. The thermal conductivity at room temperature decreased from 1.05 to 0.9 W m-1 K-1. A dimensionless figure of merit zT= 0.61 at 800 K has been obtained for the CoSb3/G. while it is 0.26 for graphene-free CoSb3. The introducing of graphene benefits the composites in both carrier concentration and mobility. Therefore the electrical conductivity increases. In addition, the well dispersed graphene in the matrix also contributes to the low lattice thermal conductivity due to the increased interface scattering.The precursors of CoSb3/G were fabricated via the co-precipitation method. CoSb3/G composites were synthesized by sintering the precursors in different sintering atmosphere(H2 and H2/N2). Similar to the solvothermal route/hot-pressed CoSb3/G, the room temperature electrical conductivity of CoSb3/G prepared this way increased and thermal conductivity decreased. This means introducing graphene to the system may help increasing the electrical conductivity as well as decreasing the thermal conductivity, which has nothing to do with the synthetic process. PbTe/G powders have been prepared by solvothermal route. The cubic shaped PbTe particles with 100-200 nm are wrapped by graphene nanosheets. Due to the presence of graphene, the nanostructure of PbTe was maintained after hot pressing at 600℃ for 2 h. Similar to the solvothermal route/hot-pressed CoSb3/G, the electrical conductivity and thermal conductivity decreased. The electrical conductivity of PbTe/G at room temperature increased from 1300 to 22000 S m-1, and the thermal conductivity at room temperature decreased from 1.15 to 0.85 W m-1 K-1. This means introducing graphene to the system may help increasing the electrical conductivity as well as decreasing the thermal conductivity, which has nothing to do with the matrixs. Importantly, it was found that the electrical conductivity of graphene-based composites followed percolation theory. Below the percolation threshold, the more graphene was added, the electrical conductivity of graphene-based composites are better. And according to the model analysis, the thermal conductivity of the system depends on how the graphene dispersed in the matrix.CoSn2/G and FeSn2/G were synthesized by an in situ solvothermal route. CoSn2 (or FeSn2) nanoparticles with a size of 2-4 nm (10-30 nm) are uniformly dispersed and confined by graphene, forming a sandwich structure. The composites exhibit improved electrochemical cycle stability and rate capability compared to the bare alloy. The enhancement in the electrochemical properties could be attributed to the introduction of graphene that not only constructs two-dimensional conductive networks but also disperses and confines the nanoparticles, in addition to the buffering effect for the large volume changes.更多还原