稀土掺杂的钙钴氧系热电转换材料的研究
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摘要
随着经济的快速发展,能源短缺和环境污染问题日益严重,开发环保新能源已成为全球能源发展趋势。热电材料通过载流子的移动来实现热能与电能的直接相互转化,是一种先进的能量转换材料,由于其在环境保护和能源利用等方面的优势,受到了科学家的高度重视。
本文采用溶胶-凝胶法合成稀土单元素掺杂的Ca3-xAxCo4O9(A=La or Sm)和复合掺杂的Ca3-2xLaxSmxCo4O9材料的前驱粉末,再通过烧结得到热电材料烧结体。研究粉末和材料烧结体制备的工艺条件以及稀土离子对Ca位的替代对材料的结构和性能的影响。用差热分析仪对预处理得到的凝胶粉末进行差热-热重分析,用扫描电子显微镜观察前驱粉末以及材料烧结体的微观形貌,用X-射线衍射仪分析材料的组成,用四探针法测量热电材料的电阻率从而得到电导率,并通过估算得出热电材料的热导率。主要研究结果如下：
1、采用溶胶-凝胶法,并分别通过常规加热法和微波加热法成功地制备了材料的前驱粉末。微波加热法与常规加热法相比可以更快地得到纯相的前驱粉末,且粉末的结晶程度更高,晶形发育更完善。将粉末在20 MPa下压片、900℃下高温烧结5小时并进行微波二次烧结,最终得到热电材料烧结体。
2、XRD测试结果表明,单元素掺杂以及复合掺杂后材料的物相都没有发生改变,没有杂相生成,稀土离子都能成功地取代Ca位,进入到Ca-Co-O体系的晶格结构中,成为单一化合物。
3、电导率的测试结果表明,热电材料的电导率随温度的升高整体呈现出增加的趋势；单元素掺杂以及复合掺杂La和Sm后,材料的电导率都高于不掺杂材料的电导率；材料的电导率随掺杂量的增加而逐渐增加,当电导率达到最大值后继续增大掺杂量,材料的电导率逐渐减小。本实验中,单元素掺杂当La的量为0.7、掺杂Sm的量为0.5、复合掺杂当La和Sm的量都为0.35(即总掺杂量为0.7)时,掺杂材料的电导率分别达到最大值。
相同掺杂量下,单元素掺杂La材料的电导率大于掺杂Sm材料的电导率,复合掺杂热电材料的电导率介于两种单元素掺杂材料的电导率之间,但不是二者的平均值,在温度低时接近于单元素掺杂Sm材料的电导率,在温度高时接近于掺杂La材料的电导率。
4、热电材料的电子热导率估算结果表明,稀土掺杂后Ca3-xAxCo4O9和Ca3-2xLaxSmxCo4O9体系材料的电子热导率都没有降低,且随着温度的升高有所增加,但数值都非常小；而稀土掺杂后引起晶格结构变化,使声子散射作用加强,热导率中起主导作用的声子热导率大幅度地降低。所以综合考虑电子热导率和声子热导率的变化,掺杂材料的总体热导率是减小的。
稀土掺杂后,材料的热电参数得到了一定的改善,稀土元素La、Sm的单元素掺杂以及二者的复合掺杂都有利于优化材料的热电性能。
With the rapid economic development, the problems of energy shortages and environment pollution have become increasingly serious. Developing a new environment-friendly energy source has become a global energy trend. Thermoelectric material is an advanced energy conversion material, which can achieve the energy conversion between the heat and electricity directly through the migration of carriers. It has been attached great importance from scientists because of its environmental protection and energy utilization advantages.
In this paper, the precursor powders of the rare-earth single-element doped Ca3-xAxCo4O9 (A=La or Sm) and complex doped Ca3_2xLaxSmxCo4O9 materials were synthesized by sol-gel method, and the bulk thermoelectric materials were obtained through high sintering. The impact to the material structure and performance, from the preparation process of powders and bulk materials as well as the rare-earth ion substitution for the Ca-bit, has been studied. DTA-TG analysis of the gel powders obtained by pretreatment was made by using differential thermal analysis instrument, the microcosmic appearances of precursor powders and bulk materials were observed by using scanning electron microscope (SEM), and the composition of the materials was analyzed by using X-ray diffraction (XRD). The resistivity of thermoelectric materials was measured through four-probe method, thus obtained the electrical conductivity, and got the thermal conductivity of thermoelectric materials by estimating. The main results are as follows:
1. Through the sol-gel method, the precursor powders of materials are synthesized respectively by conventional heating method and microwave heating method. Compared with the conventional heating method, the microwave heating method can make pure precursor powders get faster, and can obtain powders with a higher crystalline degree and better crystal growth. The powders are pressed under 20 MPa, sintered 5 hours at 900℃, and have the second sintering by microwave. Ultimately the bulk thermoelectric materials are obtained.
2. XRD results show that, after the single-element doping and complex doping, the phase of materials has not changed, and there is no impurity formation. Rare-earth ions can successfully replace Ca-bit, and disperse in the lattice structure of the Ca-Co-O system, composing a single compound.
3. The results of conductivity test show that, the conductivity of thermoelectric materials appears an upward trend with increasing temperature; the conductivity of the materials, after single-element doping and complex doping La and Sm, is higher than the conductivity of the material without doping; the conductivity increases with increasing doped content, and when it reaches a maximum it decreases with increasing doping. In this study, when the amount of single-element doping La and Sm is 0.7 and 0.5 respectively, the complex doped amount of La and Sm are 0.35 (the total doping amount is 0.7), the conductivity of doped materials reaches maximums respectively.
In the same doping amount, the conductivity of the single-element doping La materials is greater than the Sm doped materials, and the conductivity of complex doped thermoelectric material is between the two single-element doped materials, but not the average of them. It is close to single-element doping Sm materials in low temperatures, and close to La doped materials in high temperatures.
4. The estimation results of the electronic thermal conductivity show that the electronic thermal conductivity of the rare-earth doped Ca3-xAxCo4O9 and Ca3-2xLaxSmxCo4O9 system are not decrease, and increase with increasing temperature, but the values are very small; rare-earth doping changes the lattice structure, and strengthens the phonon scattering, then the phonon thermal conductivity substantially lowers, which plays a dominant role in the thermal conductivity. Therefore, comprehensive consideration of the changes of electronic thermal conductivity and phonon thermal conductivity, the overall thermal conductivity of doped materials is reduced.
After rare-earth doping, the thermoelectric parameters have been some improvement. The single-element doping rare-earth elements La, Sm and complex doping of them are conducive to optimizing the thermoelectric property of materials.
本文采用溶胶-凝胶法合成稀土单元素掺杂的Ca3-xAxCo4O9(A=La or Sm)和复合掺杂的Ca3-2xLaxSmxCo4O9材料的前驱粉末,再通过烧结得到热电材料烧结体。研究粉末和材料烧结体制备的工艺条件以及稀土离子对Ca位的替代对材料的结构和性能的影响。用差热分析仪对预处理得到的凝胶粉末进行差热-热重分析,用扫描电子显微镜观察前驱粉末以及材料烧结体的微观形貌,用X-射线衍射仪分析材料的组成,用四探针法测量热电材料的电阻率从而得到电导率,并通过估算得出热电材料的热导率。主要研究结果如下：
1、采用溶胶-凝胶法,并分别通过常规加热法和微波加热法成功地制备了材料的前驱粉末。微波加热法与常规加热法相比可以更快地得到纯相的前驱粉末,且粉末的结晶程度更高,晶形发育更完善。将粉末在20 MPa下压片、900℃下高温烧结5小时并进行微波二次烧结,最终得到热电材料烧结体。
2、XRD测试结果表明,单元素掺杂以及复合掺杂后材料的物相都没有发生改变,没有杂相生成,稀土离子都能成功地取代Ca位,进入到Ca-Co-O体系的晶格结构中,成为单一化合物。
3、电导率的测试结果表明,热电材料的电导率随温度的升高整体呈现出增加的趋势；单元素掺杂以及复合掺杂La和Sm后,材料的电导率都高于不掺杂材料的电导率；材料的电导率随掺杂量的增加而逐渐增加,当电导率达到最大值后继续增大掺杂量,材料的电导率逐渐减小。本实验中,单元素掺杂当La的量为0.7、掺杂Sm的量为0.5、复合掺杂当La和Sm的量都为0.35(即总掺杂量为0.7)时,掺杂材料的电导率分别达到最大值。
相同掺杂量下,单元素掺杂La材料的电导率大于掺杂Sm材料的电导率,复合掺杂热电材料的电导率介于两种单元素掺杂材料的电导率之间,但不是二者的平均值,在温度低时接近于单元素掺杂Sm材料的电导率,在温度高时接近于掺杂La材料的电导率。
4、热电材料的电子热导率估算结果表明,稀土掺杂后Ca3-xAxCo4O9和Ca3-2xLaxSmxCo4O9体系材料的电子热导率都没有降低,且随着温度的升高有所增加,但数值都非常小；而稀土掺杂后引起晶格结构变化,使声子散射作用加强,热导率中起主导作用的声子热导率大幅度地降低。所以综合考虑电子热导率和声子热导率的变化,掺杂材料的总体热导率是减小的。
稀土掺杂后,材料的热电参数得到了一定的改善,稀土元素La、Sm的单元素掺杂以及二者的复合掺杂都有利于优化材料的热电性能。
With the rapid economic development, the problems of energy shortages and environment pollution have become increasingly serious. Developing a new environment-friendly energy source has become a global energy trend. Thermoelectric material is an advanced energy conversion material, which can achieve the energy conversion between the heat and electricity directly through the migration of carriers. It has been attached great importance from scientists because of its environmental protection and energy utilization advantages.
In this paper, the precursor powders of the rare-earth single-element doped Ca3-xAxCo4O9 (A=La or Sm) and complex doped Ca3_2xLaxSmxCo4O9 materials were synthesized by sol-gel method, and the bulk thermoelectric materials were obtained through high sintering. The impact to the material structure and performance, from the preparation process of powders and bulk materials as well as the rare-earth ion substitution for the Ca-bit, has been studied. DTA-TG analysis of the gel powders obtained by pretreatment was made by using differential thermal analysis instrument, the microcosmic appearances of precursor powders and bulk materials were observed by using scanning electron microscope (SEM), and the composition of the materials was analyzed by using X-ray diffraction (XRD). The resistivity of thermoelectric materials was measured through four-probe method, thus obtained the electrical conductivity, and got the thermal conductivity of thermoelectric materials by estimating. The main results are as follows:
1. Through the sol-gel method, the precursor powders of materials are synthesized respectively by conventional heating method and microwave heating method. Compared with the conventional heating method, the microwave heating method can make pure precursor powders get faster, and can obtain powders with a higher crystalline degree and better crystal growth. The powders are pressed under 20 MPa, sintered 5 hours at 900℃, and have the second sintering by microwave. Ultimately the bulk thermoelectric materials are obtained.
2. XRD results show that, after the single-element doping and complex doping, the phase of materials has not changed, and there is no impurity formation. Rare-earth ions can successfully replace Ca-bit, and disperse in the lattice structure of the Ca-Co-O system, composing a single compound.
3. The results of conductivity test show that, the conductivity of thermoelectric materials appears an upward trend with increasing temperature; the conductivity of the materials, after single-element doping and complex doping La and Sm, is higher than the conductivity of the material without doping; the conductivity increases with increasing doped content, and when it reaches a maximum it decreases with increasing doping. In this study, when the amount of single-element doping La and Sm is 0.7 and 0.5 respectively, the complex doped amount of La and Sm are 0.35 (the total doping amount is 0.7), the conductivity of doped materials reaches maximums respectively.
In the same doping amount, the conductivity of the single-element doping La materials is greater than the Sm doped materials, and the conductivity of complex doped thermoelectric material is between the two single-element doped materials, but not the average of them. It is close to single-element doping Sm materials in low temperatures, and close to La doped materials in high temperatures.
4. The estimation results of the electronic thermal conductivity show that the electronic thermal conductivity of the rare-earth doped Ca3-xAxCo4O9 and Ca3-2xLaxSmxCo4O9 system are not decrease, and increase with increasing temperature, but the values are very small; rare-earth doping changes the lattice structure, and strengthens the phonon scattering, then the phonon thermal conductivity substantially lowers, which plays a dominant role in the thermal conductivity. Therefore, comprehensive consideration of the changes of electronic thermal conductivity and phonon thermal conductivity, the overall thermal conductivity of doped materials is reduced.
After rare-earth doping, the thermoelectric parameters have been some improvement. The single-element doping rare-earth elements La, Sm and complex doping of them are conducive to optimizing the thermoelectric property of materials.
引文
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