層狀熱電氧化物晶體的生長與輸運(yùn)性質(zhì)研究
發(fā)布時(shí)間：2018-08-14 15:10
【摘要】：熱電材料是一種能夠?qū)㈦娔芘c熱能直接相互轉(zhuǎn)化的功能材料。熱電性能較好的半導(dǎo)體合金材料由于具有在高溫下易分解、易氧化、制造成本高、元素儲量少和含有對人體有害的元素等缺點(diǎn),并不是理想的高溫?zé)犭姴牧�。氧化物熱電材料具有高溫穩(wěn)定性、安全無毒和元素儲量豐富等優(yōu)點(diǎn)受到熱電研究者的廣泛關(guān)注。層狀氧化物體系具有自然的超晶格結(jié)構(gòu),導(dǎo)電層與絕熱層分開,可以實(shí)現(xiàn)電輸運(yùn)和熱輸運(yùn)的獨(dú)立調(diào)控。被認(rèn)為是聲子玻璃-電子晶體熱電材料的一類候選材料,成為近年來熱電研究領(lǐng)域的熱點(diǎn)之一。目前,層狀熱電材料的研究主要集中在多晶樣品的熱電性能和低溫輸運(yùn)性質(zhì)的研究,而對這類材料體系的本征物理機(jī)制缺少系統(tǒng)的研究,比如：在層狀結(jié)構(gòu)熱電氧化物中能否在同一個(gè)方向上對熱導(dǎo)和電導(dǎo)同時(shí)進(jìn)行調(diào)控?層狀氧化物中熱導(dǎo)率和電導(dǎo)率的各向異性有多大?同價(jià)替換對電導(dǎo)率和熱導(dǎo)率有何影響?為此,我們選擇具有層狀結(jié)構(gòu)的(AE代表堿土金屬元素Ca, Sr, Ba)和BiCuSeO體系為研究對象。采用光學(xué)浮區(qū)法和助熔劑法制備高質(zhì)量的晶體,對其微結(jié)構(gòu)、電輸運(yùn)和熱輸運(yùn)進(jìn)行系統(tǒng)的研究,并對以上問題展開深入地研究。主要研究工作如下：1、Bi2AE2Co2Oy(AE=Ca, Sr, Ba)單晶樣品的微結(jié)構(gòu)與熱輸運(yùn)性質(zhì)研究(1)微結(jié)構(gòu)表征的結(jié)果表明：Bi2Sr2Co2Oy晶體質(zhì)量最好,而Bi2Ba2Co2Oy晶體沿c軸方向存在織構(gòu),與前兩者不同,Bi2Ca2Co2Oy晶體沿b方向具有復(fù)雜的非公度調(diào)制結(jié)構(gòu)。(2)高溫?zé)釋?dǎo)率的各向異性結(jié)果表明：當(dāng)AE從Ca到Ba變化時(shí),Bi2AE2Co2Oy系列單晶樣品ab面的熱導(dǎo)率相差不大,而B12Sr2CoO2Oy單晶c方向的熱導(dǎo)率是Bi2Ca2Co2Oy和Bi2Ba2Co2Oy晶體c方向熱導(dǎo)率的兩倍。各向異性彈性常數(shù)的測量表明：AE從Ca變化到Ba時(shí),聲速的各向異性不大,因而不能解釋c方向的熱導(dǎo)率的差異。該結(jié)果表明同價(jià)替換在該材料中主要調(diào)控c方向的熱導(dǎo)率,對ab面熱導(dǎo)率調(diào)控作用不大。(3)基于體系中的織構(gòu)結(jié)構(gòu)與非公度調(diào)制結(jié)構(gòu)會導(dǎo)致原子間的彈性常數(shù)具有一定的漲落,我們提出了具有彈性常數(shù)漲落的一維原子鏈模型來模擬微結(jié)構(gòu)差異對熱擴(kuò)散常數(shù)的影響。該模型能定性地解釋Bi2AE2Co2Oy (AE=Ca, Sr, Ba)系列單晶沿c方向熱導(dǎo)率的差異。這揭示了熱導(dǎo)率與微結(jié)構(gòu)的緊密聯(lián)系。a, Sr, Ba)及摻雜單晶的電輸運(yùn)性質(zhì)研究低溫電輸運(yùn)測試結(jié)果顯示當(dāng)AE從Ca到Ba變化時(shí),Bi2Ca2CoO8+δ和Bi2Sr2-xBaxCo2O8+8(x=0,0.5,1.0)系列單晶表現(xiàn)出半導(dǎo)體行為,而Bi2Sr0.5Ba1.5Co2O8+δ和Bi2Ba2Co2O8+δ單晶卻表現(xiàn)出金屬行為。電阻-溫度曲線、低溫下磁阻-磁場關(guān)系和磁化強(qiáng)度-溫度關(guān)系分析表明：Bi2Ca2Co2O8+δ顯示出半導(dǎo)體行為來源于安德森局域化和電子-電子關(guān)聯(lián)作用,而Bi2Sr2-xBaxCo2O8+δ(x=0,0.5,1.0)晶體的半導(dǎo)體行為是由安德森局域化引起的。這表明同價(jià)替換對于該體系ab面的低溫電輸運(yùn)有顯著的影響。因此,同價(jià)替換改善氧化物熱電材料的電導(dǎo)率是一個(gè)有效的手段。3. Bi2AE2Co2Oy(AE=Ca, Sr, Ba)的熱電性能Bi2AE2Co2Oy(AE=Ca, Sr, Ba)系列單晶ab面的Seebeck系數(shù)隨溫度的增加而增加,且AE從Ba到Ca變化時(shí),該系列單晶的Seebeck系數(shù)增加。Bi2AE2Co2Oy(AE=Ca, Sr, Ba)系列單晶中Bi2Ca2Co2Oy的ZT值最大,ZT值為0.24(873 K)。4、BiCuSeO體系的輸運(yùn)性質(zhì)BiCuSeO體系具有低本征熱導(dǎo)率,如何提高導(dǎo)電性是該體系的重要研究內(nèi)容。我們采用助熔劑法在不同溫度下生長了BiCuSeO晶體。通過對電輸運(yùn)的研究發(fā)現(xiàn)：改變生長條件可以實(shí)現(xiàn)從半導(dǎo)體行為到金屬性的轉(zhuǎn)變。金屬性的電輸運(yùn)行為的演變歸因于高溫下生長的BiCuSeO單晶的Bi元素缺失,導(dǎo)致體系的空穴載流子濃度的增加。室溫下的化學(xué)計(jì)量比和非化學(xué)計(jì)量比的BiCuSeO單晶的電阻率分別為0.77 Ω·cm和0.092 Ω·cm。這表明通過優(yōu)化生長條件可以調(diào)控晶體的電輸運(yùn)性能,從而提高熱電性能。通過上述研究,我們對層狀結(jié)構(gòu)的Bi2AE2Co2Oy (AE=Ca, Sr, Ba)和BiCuSeO體系的微結(jié)構(gòu)、電輸運(yùn)和熱輸運(yùn)有了更深入的理解。在層狀氧化物Bi2AE2Co2Oy系統(tǒng)中,同價(jià)替換可以實(shí)現(xiàn)電輸運(yùn)和熱輸運(yùn)的獨(dú)立調(diào)控,但是很難實(shí)現(xiàn)在同一個(gè)方向上的同時(shí)調(diào)控。同價(jià)替換對Bi2AE2Co2Oy體系的ab面熱導(dǎo)率調(diào)控作用不大。因此,在這類層狀氧化物材料中很難通過降低熱導(dǎo)率來提高熱電性能。那么,應(yīng)該選擇本征低熱導(dǎo)率的氧化物材料作為研究體系,集中提高功率因子。其次,同價(jià)替換對低溫電阻影響非常大,ab面電輸運(yùn)實(shí)現(xiàn)從半導(dǎo)體(Bi2Ca2Co2Oy)到金屬的轉(zhuǎn)變(Bi2Ba2Co2Oy)。因此,同價(jià)替換是一個(gè)很好的提高電導(dǎo)率的方法。這些結(jié)果對于進(jìn)一步改善氧化物熱電材料的熱電性能具有重要意義。
[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.
【學(xué)位授予單位】：南京大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：O78;TB34
本文編號：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.
【學(xué)位授予單位】：南京大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：O78;TB34
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相關(guān)期刊論文 前3條
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2 李洪濤;朱志秀;吳益文;吳曉紅;周輝;華沂;孫明星;宿太超;季誠昌;;熱電材料的應(yīng)用和研究進(jìn)展[J];材料導(dǎo)報(bào);2012年15期
3 ;Thermoelectric Characterization of (Na_(1-y)M_y)_(1.6)Co_2O_4(M=K, Ca, Sr)[J];Journal of Materials Science & Technology;2006年04期
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