【作者】 王伟；

【导师】 隋郁；

【作者基本信息】 哈尔滨工业大学 ， 凝聚态物理， 2009， 硕士

【摘要】 进入21世纪以来,随着全球环境污染和能源危机的日益严重,以及对人类可持续发展的广泛关注,开发新型环保的替代能源已经越来越受到世界各国的重视。热电材料是一种将热能和电能相互转换的功能材料。其中,氧化物热电材料作为一种新型的能源转化材料受到越来越广泛的重视,其最大优点是可以在氧化气氛的高温下长期工作,大多数无毒性、无污染而且制备简单。因此,研究氧化物热电材料具有十分重要的意义。本文以探索提高热电材料的热电性能为目的,对LaCoO₃热电材料掺杂Ca,Sr元素,深入分析了掺杂元素和掺杂量对LaCoO₃材料的输运以及热电性能的影响。本论文使用传统的固相反应法制备了掺杂的系列样品La_1-xCa_xCoO₃ （x=0.05,0.1,0.2,0.3）和La_1-xSr_xCoO₃ （x=0,0.1,0.2,0.25,0.3）。通过XRD分析显示,合成的样品均为单相。对样品磁学性能的分析发现,掺杂后的LaCoO₃样品具有自旋玻璃性质。通过对样品高温电阻率的研究发现,相对于未掺杂的LaCoO₃,掺杂后能够显著降低样品的电阻率。电阻率的下降主要是因为掺杂改变了样品中钴离子的平均价态,使部分的Co3+离子变为Co4+离子,进而引入了大量的空穴载流子,致使电阻率下降。对于样品的高温导电机制进行了分析,分析发现在部分温区符合绝热小极化子跳跃导电模型。系统研究了La_1-xCa_xCoO₃ （x=0.05,0.1,0.2,0.3）和La_1-xSr_xCoO₃ （x=0,0.1,0.2,0.25）系列样品的低温热电性能。研究发现LaCoO₃经过掺杂以后,能够明显降低样品的电阻率,而且同时能够保证样品的热电势绝对值保持在较大的数值。此外,在LaCoO₃的La位掺杂后,由于晶格畸变影响,增加了声子散射,使得热导率降低,。之后随着掺杂量的增加,由于电阻率显著降低,使得热导率略微增加。基于以上的分析表明通过在La位进行掺杂能够有效的提高LaCoO₃材料的热电性能。计算了系列样品的热电优值,其中ZT值最高的为其中La_0.8Sr_0.2CoO₃,在157K左右达到最大值0.11。更多还原

【Abstract】 Since entering the 21st century, with the global environmental pollution and the increasingly serious energy crisis, as well as a wide range of sustainable human development concerns, the development of new environmentally-friendly alternative sources of energy had been receiving increasing attention around the world. Thermoelectric materials is a thermal and electrical energy can be converted each other functional materials. As potential candidates for a new thermoelectric material several electronic conductive oxides have attracted more and more research interests. These oxides have many advanced properties, which can be used at high temperature for long time, nontoxic, without pollutions and can be maked easily. Therefore , it is getting more and more important to study the thermoelectric oxide materials. The thesis was originally intended to improve the thermoelectric performance for the thermoelectric materials in the use of Ca,Sr as the dopants in the p type LaCoO₃ material. This paper analyzed the impact of the doped elements and the doped density for the transport and thermoelectrical properties.In this thesis, polycrystalline samples La_1-xCa_xCoO₃（x=0.05,0.1,0.2,0.3） and La_1-xSr_xCoO₃（x=0,0.1,0.2,0.25） were prepared by the traditional solid-state reaction method. From the X-ray diffraction （XRD） data, the all prepared samples exhibited single-phase structure. From the magnetic properties,the La1-xRxCoO₃ are the spin-glass simple. From the investigation of the resistivity in the high temperature, it was seen that the resistivity of the doped La1-xRxCoO₃ samples significantly decreased compared the undoped semiconductor-like LaCoO₃. The reduction of the resistivity as doping at first should be attributed to the change of the valence of Co ions by Ca and Sr elements substitution. The substitution will induce Co4+ in the matrix of Co3+, and create a large number of hole carriers. Analyzing the conduction mechanism, the resistivity is consist with the adiabatic small polaron hopping conduction model in somepart of the high temperature. Systematic investigation the low temperature thermoelectric properties of the La_1-xCa_xCoO₃ （x=0.05,0.1,0.2,0.3） and La_1-xSr_xCoO₃（x=0,0.1,0.2,0.25,0.3） samples. After use Ca and Sr elements as a dopant in LaCoO₃ material, the resistivity can significantly decrease. Meanwhile, the absolute value of the thermal power can maintain at a good value. Besides, doping at La site with Ca and Sr ions in LaCoO₃ will enhance the distortion of lattice, which may be enhance the scattering of the phonon in the result of lowering the thermal conductivity,and then the resistivity can significantly decrease,which can be enhanced the result of lowering the thermal conductivity. Based on the analysis, doping at La site with rare Ca and Sr elements may improve the thermoelectric performance of LaCoO₃ material. We calculated the figure of merit of the samples, obtaining the figure of merit 0.11 at about 157K in La_0.8Sr_0.2CoO₃ sample.更多还原