【作者】 张胜楠；

【导师】 赵新兵； 朱铁军；

【作者基本信息】 浙江大学 ， 材料学， 2010， 博士

【摘要】 热电材料是指可以将热能与电能进行直接转换的半导体功能材料。基于不同的能量转换过程,热电材料可以用于热电发电和热电制冷两大领域。一般而言,热电器件具有体积小、重量轻、无污染、无噪音、维护成本低、安全可靠等优点,目前已经在航空航天、军事、医学、工业废热发电以及精确控温等领域具有了广泛的应用。但是,热电材料较低的能量转换效率仍然是限制其应用的瓶颈,使其只能应用在小功率条件下。热电材料的能量转换效率可以使用热电优值衡量,热电优值的定义为ZT=α2σT/K,与电导率σ和Seebeck系数α的平方(PF=α2σ也被定义为功率因子)成正比,而与热导率κ反比。目前大部分热电材料的性能优化方法都是针对高功率因子体系,通过纳米化和固溶等手段降低体系的晶格热导率,以提高材料的热电优值。但是热导率的降低具有其自身的局限性,非晶结构时能够获得的热导率为材料的最小热导率。因此,本文从相反的方向考虑,选择几种典型的低热导率碲化物热电材料作为研究对象,通过固溶及原位析出第二相、界面净化及掺杂等方法,在保持或进一步强化材料低热导率的同时,强化其电输运性能,达到了优化热电性能的目的。获得的结论如下：1.使用晶化动力学理论,分析了不同成分的SiTe基非晶材料的晶化过程。在SiTe基非晶材料的晶化过程中,晶核是在退火期间形成的,而且其晶核形成机制为二维和三维混合生长模型。计算得到Si15Te85和Si20Te80的晶化激活能分别为198-204kJ/mol和236-244 kJ/mol.与Si15Te85相比,Si20Te80非晶具有更高的热稳定性和玻璃形成能力,但晶化能力较弱。2.另一方面,结合实际测量的低温热输运性能和低温声子输运理论,对Se掺杂的GeTe基非晶材料的低温热输运性能进行分析,对非晶体系中以爱因斯坦局域化振动模式为特征的低温声子输运性能进行了分析,并且由于声子平均自由程被限制在原子间距内,因此,非晶热导率仅为～0.1 Wm-1K-1左右,达到了这种成分可以获得的最小热导率。这些研究为后续对低热导率热电碲化物的热性能分析提供理论依据。3.使用固溶并结合原位析出纳米结构的方法,对三元低热导率碲化物AgSbTe2的电输运性能进行优化。通过调整AgSbTe2 ((Ag2Te)x(Sb2Te3)1-x)基固溶体中Ag2Te和Sb2Te3的比例,分别获得了具有Sb2Te3和Ag2Te原位析出相的复合材料。由于析出相在基体中分布特征的不同其对复合体系热电性能的影响完全不同,实验证明Ag2Te呈纳米点和纳米层状结构的析出能够有效地提高材料的热电性能。整个体系中最大热电优值1.53出现在673 K时,Ag2Te析出的复合材料Ag0.90Sb1.10Te210中,与单相材料相比热电性能提高了40%左右。结合一定的理论计算,证明了原位析出的微结构不仅通过能量过滤效应提高了Seebeck系数,并且对电导率的提高也有积极作用,从而获得了热电性能的较大提高。此外,掺杂等工艺对AgSbTe2基热电材料的电输运性能也有优化作用。4.熔炼法制备了(GeTe)85(AgSbTe2)15(简称TAGS-85)基热电材料,通过改变球磨过程中的保护气氛(空气、氩气和液氮),研究了球磨气氛对材料热电性能的影响。研究表明,体系的最大热电优值ZTmax为氩气中球磨后烧结材料的1.8,比空气中球磨后烧结材料的1.4有明显的提高,说明清洁的界面和细小晶粒可以有效地降低热导率,提高材料的Seebeck系数,从而优化材料的热电性能。5.首先通过熔炼热压的方法制备得到了单相的Ag8GeTe6,并且通过Ag位自掺杂成功地优化了材料的热电性能,这一系列的最大热电优值在成分为Ag7.99GeTe6时,在623K达到了0.85,比化学计量比材料的热电性能提高了30%。另一方面,通过与Ag8SiTe6基化合物固溶,进一步的降低材料的热导率,并获得电导率的提高。更多还原

【Abstract】 Thermoelectric (TE) materials can be used for the direct conversion between electricity and thermal energy. There are mainly two effects during the energy conversion process, namely Seebeck effect and Peltier effect. Based on these, TE materials can be used as power generator, cooler and also precise temperature controller. Due to the advantages TE devices exhibiting, such as small and lightweight, maintenance-free, no moving parts, acoustically silent and electrically quiet etc., they have been widely utilized in many different fields, for example, the power generator in spaceflights, infrared-seeking missiles, laser diode coolers, low noise amplifiers and also wine cabinets. However, the low energy conversion efficiency is still the neck, which limited the application of TE devices. The optimization of TE materials with higher TE performance is very essential for the further development of TE industry.Based on the definition of thermoelectric figure of merit, ZT=α2CσT/κ. The TE performance is proportional to the electrical conductivity, a and Seebeck coefficient, a and inverse proportional to the thermal conductivity,κ. Most of the researches today focus on the reduction of thermal conductivity in the TE materials with high electrical transport properties. However, there is a minimum limitation for the thermal conductivity, which is called the amorphous limitation. So in this study, we focus our attention on the TE materials with intrinsically low thermal conductivity. In-situ precipitates, solid solution, boundary clearance and doping were performed on different low-κTE systems to enhance the electrical transport properties, while maintain or further decrease the thermal conductivity to final improvement of the TE properties. The conclusions are listed below:1. The crystalline kinetics theory has been used to analyze the crystallization process of SiTe-based amorphous. The activation energies of crystallizationΔEa have been calculated to be 198-204 kJ/mol and 236-244 kJ/mol for Si15Te85 and Si20Te80, respectively. By the analysis of related data, Si2oTego amorphous has been found with lower crystallization ability but higher thermal stability comparing with Si15Te85 And the crystallization processes for these two amorphous both indicated a mixed mechanism of two and three-dimensional growth.2. Low temperature transport properties measurements and related theories were combined together to analyze the phonon transport properties in Se doped GeTe-based amorphous. The low temperature specific heat measurements identified some localized low-frequency oscillation modes (Einstein modes) in conjunction with a Debye-like behavior. The thermal conductivity of all Ge20Te80-xSex exhibited typical amorphous heat conduction behavior, which has been discussed in connection with the small phonon mean free path. All these studies provided the theoretical basis for the afterwards study on the low-κTE materials.3. In-situ nanocomposites with different precipitations (Ag2Te or Sb2Te3) have been obtained in AgSbTe2 system by tuning the ratio of Ag2Te to Sb2Te3. It proved that these in-situ precipitates can contribute to the improvements of Seebeck coefficient and electrical conductivity at the same time based on the energy filtering effect. On the other hand, the precipitates of Ag2Te nanodots and nano-lamellae structures are much more effective for the optimization of thermoelectric properties comparing with Sb2Te3 precipitates. The maximum ZT value of 1.53 has been obtained in AgiTe precipitated samples at 673 K with the enhancement of～40% comparing with the single phased samples.4. The boundary clearance process has also proved to be very effective to (GeTe)85(AgSbTe2)15 (TAGS-85). By changing the ball-mill atmosphere from air to Argon, clean boundaries and smaller grains have been obtained. The ZT value has been improved from 1.4 to 1.8, which was sintered by SPS after the ball-mill process in air and Argon, respectively.5. Non-stoichiometric Ag8GeTe6 based alloys have been successfully synthesized. The Ag self-doping has proved to be very effective for the enhancement of the thermoelectric properties due to the improvement in Seebeck coefficient, while maintaining the intrinsically low thermal conductivity. The maximum ZT of 0.86 has been obtained in Ag7.99GeTe6 at 623 K which is more than 30% improvement comparing with the stoichiometric AggGeTe6.更多还原