【作者】 王阳；

【导师】 周建十； 苏文辉； 隋郁；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2010， 博士

【摘要】 热电材料是一种能够将电能与热能直接相互转化的功能材料,可以广泛的用于发电和制冷,并且具有固态运行、可精确控制、无辐射无污染、不需维护、使用寿命长等众多优点,因而这些年来得到了广泛的研究。目前技术上较为成熟、性能较好的热电材料多为金属半导体合金,但这些热电合金在高温下不稳定,容易氧化,造价高,并大多含有对人体有害的重金属,并不是理想的热电材料。相比之下,氧化物具有优良的结构稳定性和化学稳定性,安全无毒,不易氧化,成本低,易于大规模生产,被认为是一种潜在的热电材料。然而早期所研究的一些氧化物因其热电性能太低以至于无法应用,因而氧化物热电材料的研究一直没有得到足够的重视。但是,随着近年来在层状Co氧化物、钙钛矿型Co氧化物、电子掺杂的Mn氧化物,以及电子掺杂的Ti氧化物中陆续发现反常大的热电响应后,热电氧化物重新吸引了广泛的研究兴趣。在本论文中,我们详细地研究了Ca3Co4O9体系,CaMnO3体系,与LaCoO3体系这三种典型的高性能热电氧化物体系的结构、形貌、电输运、磁输运、热输运、磁性质、及热电性质等各方面的物理性质,对这三个体系的晶体结构和热电响应及电、磁、热多方面物理性质的关联有了深入的理解。通过离子掺杂、复合、改进制备手段等方法成功地提高了体系的热电性能。对于Ca3Co4O9体系,研究发现通过对Ca位Ag、Y等离子的掺杂,Co位Fe、Mn、Cu等离子的掺杂,体系的热电性能得到很大的改进。掺杂的离子改变了体系的载流子浓度,引入了化学压力,影响了迁移率与电子关联,并能够有效地抑制热导。这些掺杂离子对体系的输运机制、比热、磁性质等也有重要影响。同时研究了Ag复合的Ca3Co4O9的输运与热电性质,发现Ag复合有效的降低了晶界散射,从而提高了热电性能。此外研究表明,冷高压处理可以显著的提高这种层状氧化物陶瓷材料的织构,增大材料密度,降低孔隙、减少晶界并使晶粒长大,对热电性能起到了进一步的促进作用。结合离子掺杂与冷压技术改进,得到的Ca3Co4O9体系的最大ZT在1000K下可超过0.5,这在陶瓷氧化物中是一个非常高的值。对于电子掺杂的CaMnO3体系,研究揭示了决定该体系热电响应的主导因素。进行掺杂优化后,得到最高的ZT值在1000K下为0.2。尽管这个值在n型氧化物中很高,但与应用标准还是相距很远。深入地研究证明在电子掺杂的CaMnO3中难于实现超过1的ZT。由此探索了高性能热电氧化物应具有的特点,提出了寻找新的高性能热电氧化物的一系列切实可行的途径与策略。随后深入详细地研究了体系的电磁输运、热、磁等方面的性质,包括晶体结构与物性的关联、电磁输运机制、金属-绝缘体转变、磁性相分离、渗渝输运行为、磁电阻效应、电荷有序现象、热输运性质、反常的热导率行为、点缺陷散射机制、自旋/轨道简并及电子组态与热电势的关系、反常的磁致热电势效应、临界现象、相变与涨落等丰富的物理现象。对于LaCoO3体系,研究表明空穴掺杂的La1-xCaxCoO3与La1-xSrxCoO3具有不同的整体畸变和局域畸变,由此造成了两个系列表现出不同的电输运、磁输运、热输运性质与磁性质。空穴掺杂有效地改进了体系的热电性能。随后详细研究了体系的电磁输运行为与磁性演变,发现了玻璃铁磁性与再入自旋玻璃现象,并利用Arrott图做出了磁性相图。研究了体系的磁电阻效应,发现了磁阻的标度行为。同时分析了结构与物性的关联。此外,还研究了电子掺杂的La1-xCexCoO3,成功地得到了一种新的高性能的n型热电氧化物,发现其反常大的室温热电响应来源于Co3+离子的自旋态转变,由此分析了自旋组态与热电性质之间的关联,并发现了电子、空穴掺杂的不对称性。通过这些研究,对层状的Ca3Co4O9体系,钙钛矿CaMnO3体系,与钙钛矿LaCoO3体系的结构、物性、热电等方面有了全面深入的理解;这些研究对于进一步深入探索氧化物热电材料本征的物理机制与寻找新的高性能热电氧化物具有重要的意义。更多还原

【Abstract】 Thermoelectric materials and thermoelectric effects, which are responsible for the direct conversion of heat into electrical energy and vice versa, have attracted much attention recently driven by their application as clean energy sources and device cooler. The many advantages of thermoelectric devices include solid-state operation, vast scalability, zero-emissions, no maintenance, and long operating lifetime.To date only a few intermetallic compound semiconductors exhibit good thermoelectric performance, and these thermoelectric alloys remain the state-of-the-art high ZT materials even today. However, these thermoelectric alloys are not stable in high temperature, easy oxidation, high-cost, and including heavy metal, so they are not ideal thermoelectric materials. Compared with the conventional thermoelectric alloys, metal oxides are more suitable for high temperature applications because of their structural and chemical stabilities, oxidation resistance, easy manufacture and low-cost. But the thermoelectric performance of earlier oxides is far away from application so that they did not receive enough attention. However, since the discovery large thermoelectric response in some layered cobaltites, perovskite cobaltites, electron-doped manganites, and electron-doped titanates recent years, thermoelectric oxides have received a renewed interesting.In this thesis, we detailedly investigate the structure, morphology, electric transport, magnetic transport, thermal transport, magnetic properties, and thermoelectric properties etc of three typical high-performance thermoelectric oxide systems: Ca3Co4O9 system, CaMnO3 system, and LaCoO3 system. From the study, we have an in-depth understanding of crystalline structure, thermoelectric response, electrical, magnetic, and thermal properties of these three systems, and successfully improve their thermoelectric performance by combining doping, composing, and developing preparation method.In Ca3Co4O9 system, the substitutions of Ag, Y for Ca and Fe, Mn, Cu for Co efficiently improve the thermoelectric properties. Ions doping alters the carrier concentration, induces chemical pressure, influences mobility and electronic correlation, and suppress thermal conductivity of the system. The dopings also have important effects on the transport mechanism, specific heat, and magnetic properties. We also investigate the transport and thermoelectric properties of Ag-added Ca3Co4O9, and find the addition of Ag can efficiently reduce the boundary scattering and thus enhance thermoelectric performance. Moreover, the study indicates that cold high-pressure can obviously improve the texture of such layered materials, increase density, decrease porosity and grain boundary, and thus further facilitate the thermoelectric response. Combining ion doping and cold high-pressure, the largest ZT of Ca3Co4O9 system reaches 0.5 at 1000K, which is a quite high value among ceramic oxides.In electron-doped CaMnO3 system, we study the dominant factors determining the thermoelectric response of this system. After optimizing doping, we get the highest ZT is ~0.2 at 1000K. Although this ZT is large among n-type oxides, it is still far away from application criterion. We demonstrate that a ZT value larger than one in electron-doped CaMnO3 systems seems rather unlikely. Then we analyze the characteristic of high-performance thermoelectric oxides; some strategies for searching new thermoelectric materials with high performance in transition metal oxides are proposed. Subsequently, we systematically investigate the electric, magnetic, thermal, and transport properties etc, and detailedly discuss many rich physical phenomena in this system, such as the correlation between crystalline structure and physical properties, electric and magnetic transport mechanism, metal-insulator transition, magnetic phase separation, percolative transport behavior, magnetoresistance effect, charger ordering phenomenon, thermal transport properties, anomalous thermal conductivity behavior, point-defect scattering mechanism, relationship between spin/orbital degeneracy and electronic configuration and thermopower, unusual magneto-thermopower effect, critical phenomenon, phase transition and fluctuation.In LaCoO3 system, we find hole-doped La1-xCaxCoO3 and La1-xSrxCoO3 have different global distortion and local distortion, which results in their different electric transport, magnetic transport, thermal transport, and magnetic properties. Hole-doping efficiently improve the thermoelectric properties of the system. Then we study the electric and magnetic transport behaviors and magnetic evolution. The glassy ferromagnetism and reentrant spin glass phenomenon are found. Using Arrott plot, we show the magnetic phase diagram of the system. We also investigate the magnetoresistance effect, and find the scaling behavior of magnetoresistance in this system. Similar with manganites, we analyze the correlation between crystalline structure and physical properties. Furthermore, we investigate electron-doped La1-xCexCoO3, and successfully obtain a new promising n-type thermoelectric oxide. The unusual large room-temperature thermoelectric response results from the spin state transition of Co3+. We discuss the relationship between spin blockade effect and thermoelectric properties in this system, and find the asymmetry of electron-doping and hole-doping.On the basis of these studies, we widely and deeply understand the structure, properties, and thermoelectrics of Ca3Co4O9, CaMnO3, and LaCoO3 systems. These results are highly significant for the further study of intrinsic physical mechanism of thermoelectric oxides and the search of new high-performance thermoelectric oxides.更多还原