层状热电氧化物晶体的生长与输运性质研究
发布时间：2018-08-14 15:10
【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。热电性能较好的半导体合金材料由于具有在高温下易分解、易氧化、制造成本高、元素储量少和含有对人体有害的元素等缺点,并不是理想的高温热电材料。氧化物热电材料具有高温稳定性、安全无毒和元素储量丰富等优点受到热电研究者的广泛关注。层状氧化物体系具有自然的超晶格结构,导电层与绝热层分开,可以实现电输运和热输运的独立调控。被认为是声子玻璃-电子晶体热电材料的一类候选材料,成为近年来热电研究领域的热点之一。目前,层状热电材料的研究主要集中在多晶样品的热电性能和低温输运性质的研究,而对这类材料体系的本征物理机制缺少系统的研究,比如：在层状结构热电氧化物中能否在同一个方向上对热导和电导同时进行调控?层状氧化物中热导率和电导率的各向异性有多大?同价替换对电导率和热导率有何影响?为此,我们选择具有层状结构的(AE代表碱土金属元素Ca, Sr, Ba)和BiCuSeO体系为研究对象。采用光学浮区法和助熔剂法制备高质量的晶体,对其微结构、电输运和热输运进行系统的研究,并对以上问题展开深入地研究。主要研究工作如下：1、Bi2AE2Co2Oy(AE=Ca, Sr, Ba)单晶样品的微结构与热输运性质研究(1)微结构表征的结果表明：Bi2Sr2Co2Oy晶体质量最好,而Bi2Ba2Co2Oy晶体沿c轴方向存在织构,与前两者不同,Bi2Ca2Co2Oy晶体沿b方向具有复杂的非公度调制结构。(2)高温热导率的各向异性结果表明：当AE从Ca到Ba变化时,Bi2AE2Co2Oy系列单晶样品ab面的热导率相差不大,而B12Sr2CoO2Oy单晶c方向的热导率是Bi2Ca2Co2Oy和Bi2Ba2Co2Oy晶体c方向热导率的两倍。各向异性弹性常数的测量表明：AE从Ca变化到Ba时,声速的各向异性不大,因而不能解释c方向的热导率的差异。该结果表明同价替换在该材料中主要调控c方向的热导率,对ab面热导率调控作用不大。(3)基于体系中的织构结构与非公度调制结构会导致原子间的弹性常数具有一定的涨落,我们提出了具有弹性常数涨落的一维原子链模型来模拟微结构差异对热扩散常数的影响。该模型能定性地解释Bi2AE2Co2Oy (AE=Ca, Sr, Ba)系列单晶沿c方向热导率的差异。这揭示了热导率与微结构的紧密联系。a, Sr, Ba)及掺杂单晶的电输运性质研究低温电输运测试结果显示当AE从Ca到Ba变化时,Bi2Ca2CoO8+δ和Bi2Sr2-xBaxCo2O8+8(x=0,0.5,1.0)系列单晶表现出半导体行为,而Bi2Sr0.5Ba1.5Co2O8+δ和Bi2Ba2Co2O8+δ单晶却表现出金属行为。电阻-温度曲线、低温下磁阻-磁场关系和磁化强度-温度关系分析表明：Bi2Ca2Co2O8+δ显示出半导体行为来源于安德森局域化和电子-电子关联作用,而Bi2Sr2-xBaxCo2O8+δ(x=0,0.5,1.0)晶体的半导体行为是由安德森局域化引起的。这表明同价替换对于该体系ab面的低温电输运有显著的影响。因此,同价替换改善氧化物热电材料的电导率是一个有效的手段。3. Bi2AE2Co2Oy(AE=Ca, Sr, Ba)的热电性能Bi2AE2Co2Oy(AE=Ca, Sr, Ba)系列单晶ab面的Seebeck系数随温度的增加而增加,且AE从Ba到Ca变化时,该系列单晶的Seebeck系数增加。Bi2AE2Co2Oy(AE=Ca, Sr, Ba)系列单晶中Bi2Ca2Co2Oy的ZT值最大,ZT值为0.24(873 K)。4、BiCuSeO体系的输运性质BiCuSeO体系具有低本征热导率,如何提高导电性是该体系的重要研究内容。我们采用助熔剂法在不同温度下生长了BiCuSeO晶体。通过对电输运的研究发现：改变生长条件可以实现从半导体行为到金属性的转变。金属性的电输运行为的演变归因于高温下生长的BiCuSeO单晶的Bi元素缺失,导致体系的空穴载流子浓度的增加。室温下的化学计量比和非化学计量比的BiCuSeO单晶的电阻率分别为0.77 Ω·cm和0.092 Ω·cm。这表明通过优化生长条件可以调控晶体的电输运性能,从而提高热电性能。通过上述研究,我们对层状结构的Bi2AE2Co2Oy (AE=Ca, Sr, Ba)和BiCuSeO体系的微结构、电输运和热输运有了更深入的理解。在层状氧化物Bi2AE2Co2Oy系统中,同价替换可以实现电输运和热输运的独立调控,但是很难实现在同一个方向上的同时调控。同价替换对Bi2AE2Co2Oy体系的ab面热导率调控作用不大。因此,在这类层状氧化物材料中很难通过降低热导率来提高热电性能。那么,应该选择本征低热导率的氧化物材料作为研究体系,集中提高功率因子。其次,同价替换对低温电阻影响非常大,ab面电输运实现从半导体(Bi2Ca2Co2Oy)到金属的转变(Bi2Ba2Co2Oy)。因此,同价替换是一个很好的提高电导率的方法。这些结果对于进一步改善氧化物热电材料的热电性能具有重要意义。
[Abstract]:Thermoelectric material is a kind of functional material which can transform electric energy and heat energy directly.Semiconductor alloy materials with good thermoelectric properties are not ideal high temperature thermoelectric materials because they are easy to decompose at high temperature, easy to oxidize, high cost of manufacture, less element reserves and contain harmful elements. Layered oxide systems with natural superlattice structure and separate conductive layer from insulating layer can realize independent regulation of electrical and thermal transport. They are considered as a kind of candidate materials for phonon glass-electron crystal thermoelectric materials. Material has become one of the hot topics in thermoelectric research in recent years. At present, the research of layered thermoelectric materials mainly focuses on the thermoelectric properties and low-temperature transport properties of polycrystalline samples, but there is no systematic study on the intrinsic physical mechanism of such materials, such as whether the layered thermoelectric oxides can be carried out in the same direction. Thermal conductivity and conductivity are controlled simultaneously? How much is the anisotropy of thermal conductivity and conductivity in layered oxides? What is the effect of Isovalent substitution on thermal conductivity and conductivity? For this reason, we choose layered structure (AE represents alkaline earth metal elements Ca, Sr, Ba) and BiCuSeO system as the research object. Optical floating zone method and flux method are used to prepare the layered oxides. The main research work is as follows: 1. Study on the microstructure and thermal transport properties of Bi2AE2Co2Oy (AE = Ca, Sr, Ba) single crystal samples (1) The results of microstructure characterization show that Bi2Sr2Co2Oy crystal has the best quality, while Bi2Ba2Co2 crystal has the best quality. Bi2Ca2Co2Oy crystals have complex incommensurate modulation structures along the B direction. (2) The anisotropy of high temperature thermal conductivity shows that the thermal conductivity of the ab plane of Bi2AE2Co2Oy crystals varies little with the change of AE from Ca to Ba, while the thermal conductivity of B12Sr2CoO2 Oy crystals varies little with the change of AE from Ca to Ba. Anisotropic elastic constants measurements show that the anisotropy of the velocity of sound in AE from Ca to Ba is small and therefore the difference of thermal conductivity in C direction can not be explained. (3) Based on the fact that the texture structure and the incommensurate modulation structure in the system lead to the fluctuation of the elastic constants between atoms, we propose a one-dimensional atomic chain model with the fluctuation of the elastic constants to simulate the effect of microstructure differences on the thermal diffusion constants. The model can qualitatively explain the Bi2AE2Co2Oy (AE = Ca, Sr, Ba) series single crystals along C. Differentiation of thermal conductivity in direction reveals a close relationship between thermal conductivity and microstructure. Studies on electrical transport properties of a, Sr, Ba and doped single crystals at low temperatures show that Bi2Ca2CoO8 + delta and Bi2Sr2-xBaxCo2O8 + 8 (x = 0, 0.5, 1.0) crystals exhibit semiconductor behavior when AE changes from Ca to Ba, while Bi2Sr0.5Ba 1.5Co2O8 + delta and Bi2Ba2C show semiconductor behavior. The results of resistance-temperature curve, magnetoresistance-magnetic field relationship and magnetization-temperature relationship show that Bi2Ca2Co2O8+delta shows that the semiconductor behavior originates from Anderson localization and electron-electron correlation, while the semiconductor behavior of Bi2Sr2-xBaxCo2O8+delta (x=0,0.5,1.0) crystals is attributed to Anderson. This indicates that the Isovalent substitution has a significant effect on the low-temperature electrical transport of AB planes in the system. Therefore, the Isovalent substitution is an effective means to improve the conductivity of oxide thermoelectric materials. 3. The thermoelectric properties of Bi2AE2Co2Oy (AE = Ca, Sr, Ba) Bi2AE2Co2Oy (AE = Ca, Sr, Ba) Single Crystal AB planes have Seebeck coefficients increasing with temperature. The ZT value of Bi2Ca2Co2Oy is the highest in Bi2AE2Co2Oy (AE = Ca, Sr, Ba) single crystals, and the ZT value is 0.24 (873 K). The transport property of BiCuSeO system is low intrinsic thermal conductivity. How to improve the conductivity is an important research content of the system. BiCuSeO crystals were grown at different temperatures by flux method. It is found that the transition from semiconductor behavior to metallicity can be achieved by changing the growth conditions. The evolution of metallicity is attributed to the absence of Bi element in BiCuSeO single crystals grown at high temperatures, resulting in the increase of hole carrier concentration. The resistivity of BiCuSeO single crystals with stoichiometric and non-stoichiometric ratios at room temperature is 0.77 and 0.092_ cm, respectively. This indicates that the electrical transport properties of the crystals can be controlled by optimizing the growth conditions, thus improving the thermoelectric properties. In the layered oxide Bi2AE2Co2Oy system, Isovalent substitution can achieve independent regulation of electrical transport and thermal transport, but it is difficult to achieve simultaneous regulation in the same direction. It is difficult to improve the thermoelectric properties of oxide materials by reducing the thermal conductivity. Therefore, the oxide materials with intrinsic low thermal conductivity should be selected as the research system to concentrate on increasing the power factor. Secondly, the effect of the substitution of the same valence on the low temperature resistance is very great. Ab surface electrical transport realizes the transition from semiconductor (Bi2Ca2Co2Oy) to metal (Bi2Ba2Co2Oy). Covalent substitution is a good method to improve the conductivity. These results are of great significance for further improving the thermoelectric properties of oxide thermoelectric materials.
【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O78;TB34
本文编号：2183255
[Abstract]:Thermoelectric material is a kind of functional material which can transform electric energy and heat energy directly.Semiconductor alloy materials with good thermoelectric properties are not ideal high temperature thermoelectric materials because they are easy to decompose at high temperature, easy to oxidize, high cost of manufacture, less element reserves and contain harmful elements. Layered oxide systems with natural superlattice structure and separate conductive layer from insulating layer can realize independent regulation of electrical and thermal transport. They are considered as a kind of candidate materials for phonon glass-electron crystal thermoelectric materials. Material has become one of the hot topics in thermoelectric research in recent years. At present, the research of layered thermoelectric materials mainly focuses on the thermoelectric properties and low-temperature transport properties of polycrystalline samples, but there is no systematic study on the intrinsic physical mechanism of such materials, such as whether the layered thermoelectric oxides can be carried out in the same direction. Thermal conductivity and conductivity are controlled simultaneously? How much is the anisotropy of thermal conductivity and conductivity in layered oxides? What is the effect of Isovalent substitution on thermal conductivity and conductivity? For this reason, we choose layered structure (AE represents alkaline earth metal elements Ca, Sr, Ba) and BiCuSeO system as the research object. Optical floating zone method and flux method are used to prepare the layered oxides. The main research work is as follows: 1. Study on the microstructure and thermal transport properties of Bi2AE2Co2Oy (AE = Ca, Sr, Ba) single crystal samples (1) The results of microstructure characterization show that Bi2Sr2Co2Oy crystal has the best quality, while Bi2Ba2Co2 crystal has the best quality. Bi2Ca2Co2Oy crystals have complex incommensurate modulation structures along the B direction. (2) The anisotropy of high temperature thermal conductivity shows that the thermal conductivity of the ab plane of Bi2AE2Co2Oy crystals varies little with the change of AE from Ca to Ba, while the thermal conductivity of B12Sr2CoO2 Oy crystals varies little with the change of AE from Ca to Ba. Anisotropic elastic constants measurements show that the anisotropy of the velocity of sound in AE from Ca to Ba is small and therefore the difference of thermal conductivity in C direction can not be explained. (3) Based on the fact that the texture structure and the incommensurate modulation structure in the system lead to the fluctuation of the elastic constants between atoms, we propose a one-dimensional atomic chain model with the fluctuation of the elastic constants to simulate the effect of microstructure differences on the thermal diffusion constants. The model can qualitatively explain the Bi2AE2Co2Oy (AE = Ca, Sr, Ba) series single crystals along C. Differentiation of thermal conductivity in direction reveals a close relationship between thermal conductivity and microstructure. Studies on electrical transport properties of a, Sr, Ba and doped single crystals at low temperatures show that Bi2Ca2CoO8 + delta and Bi2Sr2-xBaxCo2O8 + 8 (x = 0, 0.5, 1.0) crystals exhibit semiconductor behavior when AE changes from Ca to Ba, while Bi2Sr0.5Ba 1.5Co2O8 + delta and Bi2Ba2C show semiconductor behavior. The results of resistance-temperature curve, magnetoresistance-magnetic field relationship and magnetization-temperature relationship show that Bi2Ca2Co2O8+delta shows that the semiconductor behavior originates from Anderson localization and electron-electron correlation, while the semiconductor behavior of Bi2Sr2-xBaxCo2O8+delta (x=0,0.5,1.0) crystals is attributed to Anderson. This indicates that the Isovalent substitution has a significant effect on the low-temperature electrical transport of AB planes in the system. Therefore, the Isovalent substitution is an effective means to improve the conductivity of oxide thermoelectric materials. 3. The thermoelectric properties of Bi2AE2Co2Oy (AE = Ca, Sr, Ba) Bi2AE2Co2Oy (AE = Ca, Sr, Ba) Single Crystal AB planes have Seebeck coefficients increasing with temperature. The ZT value of Bi2Ca2Co2Oy is the highest in Bi2AE2Co2Oy (AE = Ca, Sr, Ba) single crystals, and the ZT value is 0.24 (873 K). The transport property of BiCuSeO system is low intrinsic thermal conductivity. How to improve the conductivity is an important research content of the system. BiCuSeO crystals were grown at different temperatures by flux method. It is found that the transition from semiconductor behavior to metallicity can be achieved by changing the growth conditions. The evolution of metallicity is attributed to the absence of Bi element in BiCuSeO single crystals grown at high temperatures, resulting in the increase of hole carrier concentration. The resistivity of BiCuSeO single crystals with stoichiometric and non-stoichiometric ratios at room temperature is 0.77 and 0.092_ cm, respectively. This indicates that the electrical transport properties of the crystals can be controlled by optimizing the growth conditions, thus improving the thermoelectric properties. In the layered oxide Bi2AE2Co2Oy system, Isovalent substitution can achieve independent regulation of electrical transport and thermal transport, but it is difficult to achieve simultaneous regulation in the same direction. It is difficult to improve the thermoelectric properties of oxide materials by reducing the thermal conductivity. Therefore, the oxide materials with intrinsic low thermal conductivity should be selected as the research system to concentrate on increasing the power factor. Secondly, the effect of the substitution of the same valence on the low temperature resistance is very great. Ab surface electrical transport realizes the transition from semiconductor (Bi2Ca2Co2Oy) to metal (Bi2Ba2Co2Oy). Covalent substitution is a good method to improve the conductivity. These results are of great significance for further improving the thermoelectric properties of oxide thermoelectric materials.
【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O78;TB34
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相关期刊论文 前3条
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