摘要
Bi_2Te_3系化合物是室温下热电性能最好的热电材料之一,已在微电子,光电子器件的冷却和恒温、生物芯片、医疗器材及国防特种制冷等多种领域有所应用。块体Bi_2Te_3系化合物的制备通常采用单晶的制备方法,尽管单晶材料的一致取向性使它在某个方向上的性能优于多晶材料,但单晶材料容易沿解理面发生断裂,机械加工性能差,为后续的模块化加工带来了困难,造成了大量不必要的浪费。因此,具有各向同性和微细结构的多晶Bi_2Te_3系材料受到了研究者的青睐,制备多晶材料的粉末冶金方法受到了重视,在保证多晶材料机械性能的同时最大限度的提高其热电性能,成为Bi_2Te_3系热电材料研究的重点。
本文采用机械合金化(MA)结合不同的成形工艺,包括热压(HP)、等离子体活化烧结(PAS)或等通道转角挤压(ECAE)制备了p型和n型Bi_2Te_3系热电材料,通过X射线衍射(XRD),场发射扫描电子显微分析(FE-SEM),X射线能谱(EDS),差热分析(DTA),透射电子显微分析(TEM),正电子湮灭寿命分析(PAT),红外光谱分析(FTIR),Seebeck系数,电阻率,载流子浓度,迁移率及热导率测试等多种分析检测手段,研究了成形工艺条件,掺杂种类及掺杂量对Bi_2Te_3系合金的微观结构和热电性能的影响。
通过分析Bi_2Te_3系热电材料的性能优化设计方案及国内外的研究现状、发展趋势,提出了本文的研究目的、意义及研究内容。考察了MA工艺对Bi_2Te_3系化合物合金化过程的影响,优化了合成Bi_2Te_3系热电材料的MA工艺条件。研究了HP和PAS工艺中烧结时间、烧结温度对p型Bi_(0.5)Sb_(1.5)Te_3和n型Bi_2Te_(2.85)Se_(0.15)合金微观结构和热电性能的影响,发现烧结体内晶粒的基本面(0 0 l)沿垂直于压力方向出现择优取向。室温下,p型Bi_(0.5)Sb_(1.5)Te_3和n型Bi_2Te_(2.85)Se_(0.15)的PAS烧结样品的最大热电优值分别为2.85×10-3K-1和1.8×10-3K-1,p型Bi_(0.5)Sb_(1.5)Te_3和n型Bi_2Te_(2.85)Se_(0.15)合金的HP烧结样品最大热电优值分别为2.84×10~(-3)K~(-1)和1.4×10~(-3)K~(-1)。
考察了HP和PAS工艺中Sb_2Te_3名义成分变化及Te掺杂对p型合金(Bi_2Te_3)1-x(Sb_2Te_3) x微观结构和热电性能的影响,首次在Bi_2Te_3系合金中发现了孪晶亚结构和调幅组织,增强了缺陷对声子的散射效应,降低了合金的晶格热导率。首次采用正电子湮灭谱学分析了成分为Bi_(0.4)Sb_(1.6)Te_3的单晶试样、HP和PAS烧结样品中晶格缺陷浓度的高低,单晶试样的空位浓度要明显低于HP烧结样,PAS烧结样介于两者之间。FTIR分析表明,随Sb_2Te_3含量增加,(Bi_2Te_3)1-x(Sb_2Te_3)x合金的能隙从0.26eV降低到了0.23eV。PAS工艺中,当名义成分为(Bi_2Te_3)0.2(Sb_2Te_3)0.8时得到了最大热电优值,Z=5.26×10~(-3) K~(-1),这是目前所有文献报导的块体Bi_2Te_3系合金中热电性能的最高值。过量Te的掺杂削弱了合金Bi_(0.4)Sb_(1.6)Te_3的热电性能,当掺杂Te量增加到8wt.%时,热电优值Z从未掺杂样的5.26×10-3K-1降低到了4.44×10-3K-1。
考察了Se、Sb含量变化,AgI、CuBr2掺杂对n型Bi2-xSbxTe2.85Se0.15合金热电性能的影响。当采用0.2wt.%的AgI掺杂时,得到了最大热电优值Z=1.86×10~(-3)K~(-1)。采用ECAE工艺,使p型Bi_(0.4)Sb_(1.6)Te_3和n型Bi_2Te_(2.85)Se_(0.15)合金的基本面(0 0 l)在平行于挤压方向出现了择优取向,n型和p型样品的最高取向因子分别达到了0.28和0.36。择优取向的形成使平行于挤压方向的热电性能要远远高于垂直于挤压方向,343K时p型Bi_(0.4)Sb_(1.6)Te_3和n型Bi_2Te_(2.85)Se_(0.15)合金在平行于挤压方向得到了最大无量纲热电优值ZT,分别为0.979和0.66。
在微波辅助湿化学法合成Bi_2Te_3纳米片和电弧等离子体沉积合成椭球形Bi_2Te_3纳米粉末基础上,分别考察了Bi_2Te_3纳米片和椭球形Bi_2Te_3纳米粉末复合对块体Bi_2Te_3合金热电性能的影响,当纳米粉末的掺杂量提高到15wt.%时,相比无掺杂试样,合金的晶格热导率降低了近20%。
As one of the most excellent thermoelectric materials near room temperature, bismuth telluride based compounds are extensively used as Peltier cooler for microelectronic devices, photoelectronic devices, biologic slug, medical appliance, military equipment and etc. Currently, they are usually prepared by unidirectional crystal growth methods. Although the resulting single crystal materials present excellent thermoelectric properties along the growth direction, the Te(1)–Te(1) layers bonded with weak Van der Waals force,which aligns orientationally in the single crystal materials, makes them more fragile and difficult for machining. Therefore, the isotropic mechanically strong materials with good thermoelectric properties are more preferable. Powder metallurgical methods, which produce randomly oriented polycrystalline and fine microstructure, and thus good mechanical properties for bismuth telluride based materials, are extensively studied.
p-type and n-type Bi_2Te_3 based thermoelectric materials were prepared by mechanical alloying (MA) and different sintering processes, including hot pressing (HP), plasma activated sintering (PAS) and equal channel angular extrusion (ECAE) in the present work. The X-ray diffraction (XRD), differential thermal analysis (DTA), field emission scanning electron microscope (FE-SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscope (TEM), infrared spectroscopy (FTIR), positron annihilation technology (PAT) , electrical resistivity, Seebeck coefficient, carrier concentration, mobility and thermal conductivity measurement were applied to analysis the microstructure and thermoelectric properties. Effects of sintering process parameters, doping on the microstructure and thermoelectric properties of Bi_2Te_3 based alloys were investigated.
Based on a detailed review on the research and development status for Bi_2Te_3 based thermoelectric materials, the research goal, task and content of this work were then put forward. The effect of MA process parameters on the alloying process of Bi_2Te_3 based alloys was investigated and the MA process parameters were optimized. The effects of sintering time, sintering temperature on the microstructure and thermoelectric properties of p-type Bi_(0.5)Sb_(1.5)Te_3 and n-type Bi_2Te_(2.85)Se_(0.15) alloys were researched. A preferentially orientated microstructure with the basal planes (0 0 Z) perpendicular to the pressing direction was formed in the sintering samples. The maximum figures of merit (Z) of the as-PASed p-type Bi_(0.5)Sb_(1.5)Te_3 and n-type Bi_2Te_(2.85)Se_(0.15) alloys at room temperature were 2.85×10-3K-1 and 1.8×10-3K-1, and the maximum figures of merit (Z) of the as-HPed p-type Bi_(0.5)Sb_(1.5)Te_3 and n-type Bi_2Te_(2.85)Se_(0.15) alloys at room temperature were 2.84×10~(-3)K~(-1) and 1.4×10~(-3)K~(-1), respectively.
The effects of nominal Sb_2Te_3 concentration and Te doping on the microstructure and thermoelectric properties of p-type (Bi_2Te_3)1-x(Sb_2Te_3)x alloys were investigated. For the first time, both twin crystals and spinodal decomposition were observed in the HPed and PASed Bi_2Te_3 based alloys, which enhanced the phonon scattering contribution of crystal defects and thus reduced the crystal thermal conductivity. The positron annihilation spectroscopy was firstly applied to analyze the concentration of deffect of the HPed, PASed polycrystal materials and the single crystal Bi_(0.4)Sb_(1.6)Te_3. The vacancy concentration of the as-PASed alloy was lower than that of the as-HPed alloy but higher than that of the single crystal. FTIR analysis showed that the band gap decreased from 0.26eV to 0.23eV with increasing the nominal concentration of Sb_2Te_3. The maximum figure of merit (Z) of the as-PASed Bi_(0.4)Sb_(1.6)Te_3 alloys reached 5.26×10-3K-1 at 300 K. To our knowledge, this was also the highest figure of merit that has ever been reported for p-type Bi_2Te_3 based bulk materials. Te doping deteriorated the thermoelectric properties of Bi_(0.4)Sb_(1.6)Te_3, and the figure of merit (Z) decreased from 5.26×10-3K-1 of the undoping alloys to 4.44×10-3K-1 with 8wt.% Te dopant.
The effects of the concentration of Se, Sb and AgI, CuBr2 dopants on the thermoelectric properties of n-type Bi_2Te_3 based alloys were also investigated. A maximum figure of merit (Z) of the n-type alloys Bi_2Te_(2.85)Se_(0.15) with 0.2wt.% AgI dopant was obtained as 1.86×10-3K-1.
By the ECAE process, an obvious preferentially orientated microstructure with the basal planes (0 0 l) in parallel to the extrusion direction was formed in the p-type Bi_(0.4)Sb_(1.6)Te_3 and n-type Bi_2Te_(2.85)Se_(0.15) alloys. The maximum orientation factor of the p-type and n-type alloys reached 0.36 and 0.28. The formation of preferentially orientated microstructure resulted in the thermoelectric properties along the parallel direction to the extrusion was more optimized than that of the perpendicular direction to the extrusion. The maximum dimensionless figures of merit (ZT) of p-type Bi_(0.4)Sb_(1.6)Te_3 and n-type Bi_2Te_(2.85)Se_(0.15) alloys reached 0.979 and 0.66 at 343K, respectively.
The nano-sheet and ellipsoid shaped nano-powders Bi_2Te_3 were prepared by microwave assistant wet-chemical technique and arc-plasma deposition technique respectively and then they were added into the as-MAed powders for consolidation. The effects of the concentration of nano-sheet and ellipsoid shaped nano-powders Bi_2Te_3 on the thermoelectric properties of bulk Bi_2Te_3 were investigated. When the doping concentration was 15wt.%, the crystal thermal conductivity reduced 20% than the undoping alloys.
引文
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