热电材料是一种能将热能和电能相互转换的新型功能材料，可以实现废热发电和无氟制冷，有利于缓解能源及环境问题，得到了广泛的关注。近年来，热电材料发展迅速，得益于性能优化策略的成功应用，许多热电材料性能取得了巨大的提升。但受限于较低的转换效率及较高的应用成本，热电材料仍然无法进行大规模商业应用。因此，继续提高材料的热电性能以及开发新型低成本热电材料具有重要的意义。 CuFeX₂(X=O，S，Se)系列材料，组成元素在地壳含量丰度高，具有低成本的应用优势。其中CuFeO₂是一种具有层状结构的铜铁矿氧化物，CuFeS₂是具有黄铜矿结构的合金，它们的热电性能已经得到了一定的研究。CuFeSe₂合金是比较新的热电材料，研究相对较少，热电传输特性尚未被揭示。本论文对此体系材料的制备及相结构进行了详细的研究，明确了样品相结构与制备工艺的关系。在此基础上，通过对材料热电性能的研究，分析了其中的热电传输机制，论文的主要研究内容如下： (一)使用固相反应法制备了化学组成为(1-x)CuFeO₂+xCuAlO₂的固溶体陶瓷样品。通过X射线衍射发现，(1-x)CuFeO₂+xCuAlO₂固溶体样品中，随着x从0.0增加1.0，样品主相存在相转变CuFeO₂-Cu(Fe，Al)₂O₄-CuAlO₂。在相转换临界区域，CuFeO₂样品中的空穴增多，显著降低了材料的电阻率，最终使得样品功率因子和zT值得到了提高。样品x=0.2的热电优值zT在温度为907K下为0.04，这个数值是未固溶的CuFeO₂的2.5倍以上。研究还发现固相反应法制备CuFeO₂的过程中，样品中极易存在杂相。本工作详细研究了烧结温度对样品相结构的影响，发现在1000℃-1010℃下可以成功制备杂相较少的CuFeO₂样品。此外，本实验还制备了CuFeO₂-CuMnO₂固溶体，并发现Mn替代Fe后会造成a／b轴晶格常数减小，这是由Mn原子发生姜-泰勒畸变导致的。 (二)为了得到纯相的CuFeO₂样品，本工作进行了以微波水热法快速制备纯相CuFeO₂纳米前驱体的研究。通过改变反应温度(130-210℃)得到了一系列形貌不同的样品，研究了晶粒尺寸并分析了晶粒生长机制。通过X射线衍射可以发现，合成样品的衍射峰峰强随着温度的升高而增强，低温下制备的样品峰强较弱，表明结晶较差。样品平均晶粒尺寸结果表明，在180℃之前数值随温度变化不大，在180℃之后则迅速增大。通过研究不同温度下样品晶粒尺寸的分布，发现高温下得到的样品中小晶粒数量减少，大晶粒数量增多。结合透射电镜照片与晶粒尺寸结果分析发现，微波水热法制备CuFeO₂的晶粒生长过程符合奥斯瓦尔德熟化机制，即小晶粒不断溶解到溶液中，并再次沉积到大晶粒上。随着反应温度升高，奥斯瓦尔德熟化进程加快。研究还表明，将微波水热法合成的粉体制备成块体材料时，由于粉体颗粒度较小，表面能较大，使得样品化学反应活性高，高温烧结时容易发生氧化或分解反应。 (三)通过淬火-退火-热压的过程制备了化学组成为Cu₂FeSe₂(x=0-0.05)的样品。通过X射线衍射测试发现当x>0.01时，样品中会出现Cu₂Se的杂峰，随x增大，第二相含量增多。由于Cu₂Se的生成会夺取CuFeSe₂中的Se原子，由此导致主相晶格中存在Se空位，这里称之为竞争Se空位。此外，通过X射线荧光光谱测试发现样品中Cu与Se的含量与理论化学计量比相比有所降低，这表明样品制备的高温过程特别是淬火过程中存在元素挥发导致元素的缺失。这种由样品制备导致的Se空位在所有样品中是相同的，这里称之为本征Se空位。样品扫描电镜照片可以看出，样品的微观形貌表现出层状结构，样品中无孔隙，致密度较好。对x=0.05样品的元素能谱测试发现，样品中Cu₂Se第二相分布不均，部分富集成较大的晶粒。样品的电学性能测试结果发现，所有样品的Seebeck系数在323K时为正值，继续升温变为负值。即随着温度的升高样品导电性出现p-n转换的现象，这在CuFeSe₂样品中是首次发现。进一步分析表明，产生p-n转变的原因是样品中存在的大量Se空位在高温下产生了额外的电子。电子数量与竞争Se空位成正相关关系，因此随x增大，电子数量增多，样品的电阻率显著降低，最终导致了功率因子的提高。此外，铜过量加强了样品中的点缺陷散射，同时引入了第二相散射，极大的降低了样品的晶格热导率。功率因子和晶格热导率的优化，最终使得样品的zT值得到极大的优化。 (四)以熔融法制备了CuFeS₂样品，并采用不同的降温处理方式来得到不同的样品，研究了由此带来的相结构及热电性能的变化。通过X射线衍射图谱发现，自然降温(Natural cooling，简写为NC)和淬火(Quenching cooling，简写为QC)得到的CuFeS₂样品具有不同的相结构，NC-CuFeS₂为单一的黄铜矿相，而QC-CuFeS₂为黄铜矿和波流铁铜矿的混合相。通过对样品的电学性能测试发现，QC-CuFeS₂和NC-CuFeS₂都是n型导电性，但电学性能的数值相差很大，且随温度的变化规律也不同。QC-样品具有较低的电导率但同时其Seebeck系数要比NC-样品低很多，最终其功率因子数值远不如NC-样品。此外，Ca掺杂后的QC-CuFeS₂样品相结构由混合相变为单一的波流铁铜矿相。通过对其电学性能的研究发现，样品的电导率和Seebeck系数经过Ca掺杂后变化规律与原样品有较大不同。掺杂后的样品，Seebeck系数绝对值在全温度区间得到了显著的提高，同时低温下的电阻率也有明显降低，最终使得功率因子在全温度区间均得到了较大的提高。本文还尝试采用Cu位Mg掺杂来优化NC-CuFeS₂样品的电学性能。研究发现，Mg掺杂样品的电阻率和Seebeck系数随掺杂量降低，这说明Mg的掺杂提高了样品的载流子浓度。但由于Seebeck系数降低幅度较大，样品最终的功率因子降低。 本论文通过对上述内容的研究，系统的阐明了CuFeX₂(X=O，S，Se)系列热电材料的制备及性能特点。本论文中对CuFeO₂的固相及微波水热合成法的制备过程进行了系统的探索，明确了不同方法的问题及优势，对材料的应用有一定的指导作用。对于CuFeSe₂材料，本论文首次完整的研究了以熔融法制备的材料的热电性能，并研究了其热电传输特性。研究还首次发现了样品中存在的随温度变化而产生的p-n导电性的转变，并分析了产生此转变的内在机制，这给此材料未来的研究和应用提供了借鉴。而对于CuFeS₂则是通过不同的降温处理方式得到了不同相结构的样品，发现了淬火样品的混合相存在形式。 关键词：低成本热电材料；样品制备；物相表征；热电传输特性

 Thermoelectric material is a new kind of functional material which can convert heat into electricity. It can recycle waste heat or realize the fluorine-free refrigeration, and has received wide attention. In recent years, the thermoelectric material is in a rapid development period. With the successful applying of the performance optimization strategies, the properties of many excellent materials have been enhanced tremendously. However, thermoelectric materials are still difficult to be commercially used on a large scale due to the low efficiency and high cost. So it is significant to further improve the performance of thermoelectric materials or develop new materials. CuFeX₂(X=0, S, Se) materials, containing abundant elements, have the advantage of low cost in application. CuFeO₂ is a kind of delafossite oxide with layered structure. CuFeS₂ is a kind of alloy with chalcopyrite structure. Both CuFeO₂ and CuFeS₂ have been studied in thermoelectric performance. CuFeSe₂ is a new thermoelectric and the thermoelectric transport properties are not revealed. In this thesis, the synthesis and phase structure of this material system are studied in detail, and the relationship between them is established. Basing on that, the thermoelectric performance is studied and the thermoelectric transport is analyzed. The main content is as follows: (1)Solid solutions of (1-x) CuFeO₂+CuAlO₂ ceramics are fabricated by the solid state reaction. From the X-ray diffraction (XRD) pattern, a phase transition of CuFeO₂Cu(Fe,Al)₂O₄-CuAlO₂ has been found in the samples with x increasing. The electrical resistivity of delafossite samples can be decreased obviously near the phase change areas, which is due to the increased carriers. At last, the power factor and zT values are enhanced due to the improved electrical resistivity. The highest zT is 0.04 for sample of x=0.2 at 907K, which is 2.5 times of the pristine CuFeO₂. Besides, it is found that the impurity phases are easy to exist in the samples. The relationship between the synthesis temperature and the sample phase structure has been studied in detail. The result shows that the temperature should be in the range of 1000℃-1010℃ to achieve the sample with less impure phases. The CuFeO₂-CuMnO₂ solid solutions have also been prepared in this thesis. The lattice parameters of a/b are found decreasing with Mn replacing Fe. This is due to the Jahn-Teller distortion induced by Mn. (2)To get the pure phase of CuFeO₂, the microwave hydrothermal method has been employed to prepare the pure CuFeO₂ nano-percursors rapidly. Samples with different micro-morphology have been got by changing reaction temperatures between 130-210℃, and the grain growth mechanism has been analyzed by studying the grain size. From the XRD patterns, the peaks of CuFeO₂ are found increasing with the increase of temperature, indicating the poor crystallinity at low temperatures. The average grain sizes are almost the same below 180℃ and increase rapidly after the temperature is over 180℃. The grain size distributions are also studied for samples at different temperatures. It is found that samples synthesized at high temperature have more large grains and less small grains. These results are consistent with the Oswald ripening mechanism which describes the process that small grains dissolve and deposit on larger grains. With the temperature increasing, the ripening progress is accelerated. Besides, it is found the CuFeO₂ nano-precursors will be oxidized or decomposed when they are used to fabricate the thermoelectric bulk material. This is due to the high chemical reactivity which is relevant with the high surface energy of nano-materials. (3)Samples of Cu〓FeSe₂ (x=0-0.05) are fabricated by the quenching, annealing, and hot-pressing progress. The secondary phase of Cu₂Se is detected when x>0.01 from the XRD patterns. The peaks of Cu₂Se increase with the increase of x, indicating the increase of the secondary phase. As the excessive Cu reacting with Se to form Cu₂Se, the Se in the CuFeSe₂ lattice will be deficient. This Se vacancies is called the competitive Se vacancy here. Besides, the X-ray fluorescence (XRF) shows the decreased ratios of Cu and Se in samples which is due to the elements volatilizations during the high-temperature progress, especially the quenching procedure. This kind of Se vacancies are defined as the Se intrinsic vacancy. The microstructure is seen laminar and no holes are detected, indicating the good density of the bulks. The energy dispersive spectrum of x=0.05 show the enriched area of Cu₂Se in samples. The Seebeck coefficients of all samples show p-n transformation with the temperature increasing. This transformation is first discovered in this material. This transformation is due to the Se vacancies which contribute more electrons to the conduction band. As the number of the electrons is positive correlation with the amount of Cu₂Se, the electrical resistivity is decreased with the x increasing, which lead to the increase of power factor. Moreover, the excessive Cu strengthen the point defect scattering and induce secondary phase scattering which decreased the thermal conductivity obviously. At last, zT is improved tremendously by the excess Cu. (4) CuFeS₂ samples are synthesized by the melting method, and different cooling ways are used to get samples. From the XRD patterns, samples with natural cooling (NC) and quenching cooling (QC) have different phase structures. The phase of QC-CuFeS₂ is a mixed phase of chalcopyrite and putoranite, while that of NC-CuFeS₂ is only chalcopyrite. The electrical properties measurement shows that these two samples are all n-type conductive behavior. But the values of two samples are much different as well as the change trend depend on temperature. The QC- sample has low electrical resistivity and Seebeck coefficient, and finally the power factor is much low than NC-sample. Besides, it is found that the phase structures of Ca doped samples change from the mixed phase to single putoranite phase. The electrical resistivity and Seebeck coefficient show completely different change tendency due to the phase change. The Ca-doped samples have the larger Seebeck coefficients which lead to the higher power factor. Moreover, the Mg doped NC-CuFeS₂ samples have also been fabricated to study the electrical properties. The electrical resistivity and Seebeck coefficient decrease with the increase of doping amount. This result proves the increase of carriers and is consistent with the n-type doping. However, the power factor is decreased due to the much decreased Seebeck coefficient. In this thesis, the synthesis and performance characteristics of CuFeX₂ (X=0, S, Se) series thermoelectric materials are systematically expounded through the study of the above contents. The preparation methods of CuFeO₂ by solid phase and microwave hydrothermal synthesis have been systematically explored, and the problems and advantages of different methods have been clarified. These results would be a reference for this material. For CuFeSe₂ materials synthesized by the melting method, our work is the first complete study for the thermoelectric performance optimization. In addition, the p-n conductivity transformation with temperature change in the sample is first discovered, and the mechanism of this transformation is analyzed, which will provide a reference for future research and application of this material. For CuFeS₂, samples with different phase structures have been obtained through different cooling treatments, and the presence of mixed phase of quenching samples has been found. Keywords: Low-cost thermoelectric materials; Sample preparation; Phase charactization; Thermoelectric transport