CaMnO₃基热电材料的制备与热电性能研究

【作者】 李翠芹；

【导师】 陈前林；

【作者基本信息】 贵州大学 ， 材料化学工程， 2019， 博士

【摘要】 热电材料是可以实现将热能和电能相互转化的能源材料,广泛用于发电和制冷,以热电材料为核心的热电器件具有可靠性高、无传动部件、使用寿命长等众多优点,因而近年来成为新能源材料领域的研究热点。本文选取CaMnO₃热电材料体系作为研究对象,对样品进行XRD物相分析及Rietveld精修、形貌表征、XPS分析、热电性能表征及第一性原理计算,深入探讨Ca位掺杂改性及形成固溶体对CaMnO₃基热电材料的精细结构、电子结构及热电性能的影响机理。主要包括以下研究内容:一、对共沉淀法制备CaMnO₃热电材料工艺条件做了研究,确定了最佳制备工艺。分别控制共沉淀反应的p H值为6.5、7.5及8.5制备CaMnO₃前驱粉体,通过对比样品的电输运性能发现,p H=8.5对降低样品的电阻率最为有效,所以确定共沉淀法制备CaMnO₃的最佳p H=8.5。采用1150℃-1300℃温度范围内对CaMnO₃热电材料进行烧结,通过对比材料的微观形貌、致密度及电输运性能,得到最佳烧结温度为1250℃。然后对CaMnO₃的Ca位做单元素掺杂改性,研究单掺杂改性对热电性能影响规律。发现:以高价稀土离子Dy掺杂Ca_1-xDy_xMnO₃（x=0.03,0.05,0.10,0.15）热电材料作为研究对象进行了不同掺杂浓度实验,通过对样品物相结构、致密度及电输运性能的表征与对比,Ca_0.9Dy_0.1MnO₃取得了高功率因子3.01×10-4 Wm-1K-2,为未掺杂CaMnO₃的1.5倍。二、在单掺杂改性的基础上,对CaMnO₃基热电材料进行了Ca位Pr和Yb双掺杂改性,以提高其电输运性能和降低其热导率。通过XRD分析及Rietveld精修,微观形貌分析、XPS分析、第一性原理计算及热电性能表征,探讨Ca位不同浓度Pr和Yb双掺杂对Ca_1-2xPr_xYb_xMnO₃（x=0.01,0.02,0.03,0.04,0.05）微观结构、电子结构和热电性能的影响规律。得到以下研究结果:1)通过XRD分析及Rietveld精修得到得出不同Pr和Yb掺杂量样品的准确的晶胞参数、Mn-O键长、Mn-O键键角等信息。从材料的结构决定性能方面分析了Pr和Yb双掺杂引起掺杂CaMnO₃晶胞结构中Mn O6八面体畸变,从而降低双掺杂体系的电阻率。2)第一性原理关于电子结构（态密度）的计算结果从理论上阐述了Pr和Yb双掺杂使CaMnO₃的电阻率下降的原因。3)共沉淀法制备的双掺杂样品小的晶粒尺寸增加了晶界的散射,导致双掺杂样品的热导率降低。此外,Mn O6八面体的结构畸变以及双掺杂离子与钙离子质量的巨大差异是热导率降低的另一个主要原因。经计算得Ca_0.92Pr_0.04Yb_0.04MnO₃的功率因子值最高为3.49×10-4 Wm-1K-2,说明Ca双掺杂热电材料的电输运性能优于单掺杂。ZT值最大为0.24,是未掺杂CaMnO₃的3倍。而这一结果,同样优于大多数文献报道的CaMnO₃基材料的ZT值。三、在Ca位双掺杂的最佳浓度x=0.04的条件下,选取La和Sm、Dy、Yb对CaMnO₃体系Ca位进行相同量不同稀土元素双掺杂改性,研究Ca位双掺杂不同稀土元素对CaMnO₃基热电材料的精细相结构、微观形貌、电子结构及热电性能的影响规律。得到以下主要结果:1)通过XRD分析及Rietveld精修得到得出不同稀土元素双掺杂的准确晶体结构信息。揭示双掺杂导致的电输运性能的改变有强烈的结构依赖性。2)第一性原理关于电子结构（态密度）的计算结果从理论上阐述了不同元素双掺杂改变CaMnO₃热电材料电输运性能的原因。3)Ca位稀土元素双掺杂有效的抑制的晶粒的生长,增加了晶界的散射的同时提高样品致密度。分析结果发现,Ca_0.92Pr_0.04Yb_0.04MnO₃的ZT值最高为0.24,说明摩尔质量大的元素在Ca位双掺杂中的作用更显著。四、采用固相反应法制备CaMnO₃前驱粉体,向前驱粉体中添加Bi₂O₃粉末做矿化剂,在较低烧结温度下得到了致密的CaMnO₃热电材料,探讨添加Bi₂O₃对CaMnO₃精细结构、微观形貌和热电性能的影响规律。得到如下结果:X射线衍射表明Bi₂O₃与CaMnO₃在高温烧结的过程中形成固溶体,Rietveld精修结果表明CaMnO₃体系的晶胞参数和晶胞体积随着Bi₂O₃的添加量的增加而逐渐增大,其结果是Mn O6八面体的Jahn-Teller畸变程度随着Bi₂O₃添加量的增加而变大。同时CaMnO₃热电材料的晶粒尺寸随着Bi₂O₃添加量的增加而减小,密度随Bi₂O₃添加的增加而增大,增大到一定程度后又有一定程度的下降。所以随着Bi₂O₃添加量的增加,样品的电阻率减小,泽贝克系数的绝对值减小,功率因子的值变大,而热导率的值变小。在整个测温区间内,化合物CaMnO₃-5Bi₂O₃具有最小的热导率,0.63@973 K,并获得最大ZT值0.21@973 K。是未添加Bi₂O₃的CaMnO₃样品的约3倍。本文采用优化制备工艺、单元素掺杂、双元素掺杂及形成固溶体等方式,系统的研究了CaMnO₃基热电材料的热电性能。发现共沉淀法Ca位双掺杂可以有效的提升CaMnO₃的热电性能。向CaMnO₃前驱粉体中加入Bi₂O₃可得到低热导率的较高热电性能CaMnO₃热电材料。本论文的研究方法对类似的热电材料体系提供了借鉴和参考。更多还原

【Abstract】 Thermoelectric materials are energy materials that can realize the conversion of heat energy to electricity,and vice versa.They are widely used in power generation and refrigeration.There are many advantages such as high reliability,no transmission parts and long service life for using thermoelectric devices cored with thermoelectric materials.Therefore,they have become a hotspot of research in the field of new energy materials in recent years.In this paper CaMnO₃ thermoelectric material system was selected as the research object,by means of microstructural optimization and doping modification to improve the thermoelectric properties of the materials.The following research contents included:1.The process conditions for preparing CaMnO₃ thermoelectric materials by coprecipitation were studied and the optimum process was determined.CaMnO₃ precursor powders were prepared by controlling the co-precipitation with pH value at 6.5,7.5 and 8.5respectively.By comparing the electrical transport performance of the samples,it was found that pH=8.5 was the effective to reduce the resistivity of the samples.Therefore,the optimal pH value for the preparation of CaMnO₃ by co-precipitation was determined to be 8.5.After sintering the CaMnO₃ thermoelectric materials at temperature within the range of 1150℃to1300℃,the material microstructure,density and electrical transport properties were compared to determine the optimum sintering temperature,which was determined as1250℃.Then the Ca site of CaMnO₃ was modified by single element doping to study the influence of single doping modification on thermoelectric properties.The high-valent rare earth ion Dy doped Ca_1-xDy_xMnO₃（x=0.03,0.05,0.10,0.15）thermoelectric material was used as the research object to conduct experiments with different doping concentrations.Through the characterization and comparison of the phase structure,density and electrical transport performance of the sample,it was found that the Ca_0.9Dy_0.1MnO₃ sample obtained a high power factor of 3.01×10^-44 Wm^-1K^-2,which is 1.5 times that of the undoped CaMnO₃.2.On the basis of single-doping modification,CaMnO₃ thermoelectric materials were modified by Ca site Pr and Yb double-doping to improve their electrical transport properties and reduce their thermal conductivity.Through XRD analysis and Rietveld refinement,SEM,XPS,first-principles calculation and thermoelectric properties characterization,the effects of different Pr and Yb double doping concentration at Ca site on the microstructures,electronic structures and thermoelectric properties of Ca_1-2xPr_xYb_xMnO₃（x=0,0.01,0.02,0.03,0.04,0.05）were investigated,with the results obtained as follows:1)The cell parameters,Mn-O bond length and Mn-O bond angle of the samples doped with different Pr and Yb were obtained by XRD analysis and Rietveld refinement.The octahedron distortion of MnO₆ in doped CaMnO₃ cell structure caused by double doping of Pr and Yb was analyzed from the aspect of structure-determining properties of materials,which reduced the resistivity of double doped system.2)The first-principles calculation results of the electronic structure（density of states）explain theoretically the reason why the resistivity of CaMnO₃ decreases due to Pr and Yb doping.3)The small grain size of the double-doped sample prepared by coprecipitation increases the scattering of grain boundaries,resulting in the decrease of thermal conductivity of the double-doped sample.In addition,the structural distortion of MnO₆ octahedron and the huge difference between the mass of double-doped ions and calcium ions are another main cause of the decrease of thermal conductivity.The highest power factor of Ca_0.92Pr_0.04Yb_0.04MnO₃ as per calculation is 3.49×10^-44 Wm^-1K^-2,which implies that the electrical transport performance of Ca-doped thermoelectric materials is better than that of the single-doped.The maximum ZT value is 0.24,which is three times much as that of undoped CaMnO₃,and is more optimal than the ZT value of CaMnO₃materials reported in most documents.3.La and Sm,Dy and Yb were selected under the condition of optimum concentration x=0.04,to modify Ca sites of CaMnO₃ system by double doping with the same amount of different rare earth elements,for the study of the effects of Ca sites double doping with different rare earth elements on the fine phase structure,the micro-morphology,the electronic structure and the thermoelectric properties of CaMnO₃ thermoelectric materials.The main results concluded are as follows:1)The accurate crystal structure information of double doping of different rare earth elements were obtained through XRD analysis and Rietveld refinement,which revealed that the change of electrical transport properties caused by double doping is strongly structure-dependent.2)The results of first-principles calculation regarding the electronic structure（density of states）explain theoretically the cause of the change brought by the double doping with different elements to the electrical transport properties of CaMnO₃ thermoelectric materials.3)Double doping of rare earth elements at Ca site can effectively inhibit the growth of grains,and increase the scattering of grain boundaries as well as the density of samples.The results show that the optimal ZT value of Ca_0.92La_0.04Yb_0.04MnO₃ is 0.24,which indicates that elements with high molar mass play a more significant role in double doping of Ca sites.4.CaMnO₃ precursor powders were prepared by solid-state reaction method,then added Bi₂O₃ powders to the precursor powders as mineralizers to obtain Dense Bi₂O₃ ceramics at lower sintering temperatures.The impact of Bi₂O₃ addition on the fine structure,micro-morphology and thermoelectric properties of CaMnO₃ has also been discussed in this paper.The conclusions are as follows:X-ray diffraction shows that formation of solid solution takes place during sintering of Bi₂O₃ and CaMnO₃ at high temperature;Rietveld refinement results show that the cell parameters and cell volume of the CaMnO₃ system increase with the augment of Bi₂O₃ content,resulting in the increased Jahn-Teller distortion of MnO₆ octahedron following the adding of Bi₂O₃ content.At the same time,the grain size of CaMnO₃ ceramics declines when the addition of Bi₂O₃ is increased,while its density becomes greater and greater.It drops with a certain amount after it increases to certain extent.Therefore,with the increase of Bi₂O₃ content,the resistivity of the sample decreases,the absolute value of Seebeck coefficient decreases,the value of power factor increases,and the value of thermal conductivity decreases.In the whole temperature range,the compound CaMnO₃-5Bi₂O₃ possess the lowest thermal conductivity,0.63@973 K,and the maximum ZT value is 0.21@973 K,which is about three times as much as that of CaMnO₃ without Bi₂O₃.In this paper,the thermoelectric properties of CaMnO₃ thermoelectric materials were systematically studied by optimizing the preparation process,element doping,double element doping and solid solution formation.It is concluded that co-precipitation method with double doping of Ca sites can effectively improve the thermoelectric properties of CaMnO₃,while the adding of Bi₂O₃ into the precursor powder of CaMnO₃ will result in CaMnO₃thermoelectric materials with low thermal conductivity but high thermoelectric properties.The research methods in this paper will serve as a constructive reference for other similar thermoelectric materials systems.更多还原