【作者】 曹一琦；

【导师】 赵新兵；

【作者基本信息】 浙江大学 ， 材料学， 2009， 博士

【摘要】 热电材料是一种能够实现电能与热能之间直接转换的功能材料,其提供了一种安全可靠、全固态的发电和制冷方式,具有广泛的应用前景。Te基热电材料中,Bi₂Te₃基化合物是室温下性能最好的热电材料之一,经过几十年的研究,块体Bi₂Te₃基材料的最高热电优值ZT徘徊在1左右。而PbTe基化合物是目前中温附近应用的最好的热电材料之一,据报道其最高热电优值ZT可高达2.2。随着纳米技术的兴起,近年来有关低维材料中取得高热电优值的报道不断出现。将材料的晶粒细化到纳米级别,可以增加对载流子和声子的散射,从而提到Seebeck系数,降低热导率,最后提高材料的热电性能。本文分别对Bi₂Te₃基和PbTe基热电材料作了广泛的研究。分别采用溶剂热/水热法、低温湿化学法以及循环环流法制备Te基纳米热电材料,对合成产物的形貌进行观察,并讨论了各自的反应机理;同时利用块体材料的纳米效应,实现对电声输运的有效调控,从而提高材料的热电优值。取得的主要成果如下:低温湿化学法合成Bi₂Te₃纳米颗粒过程中,系统中加入过多的络合剂不利于Bi₂Te₃的合成;分步加入还原剂NaBH₄有利于合成出纯度较高的Bi₂Te₃;反应物浓度低,则Bi₂Te₃形核率也相应较低,在低浓度下合成出较纯的Bi₂Te₃纳米颗粒需要较长的时间。分别以纯Te粉和亚碲酸钠（Na₂TeO₃）为反应Te源,制备出Bi₂Te₃颗粒的形貌不同,前者形貌为纳米管,其直径在100 nm左右,管长为500 nm至1μm不等,壁厚则为30 nm左右,后者的形貌为一些聚集在一起的颗粒尺寸在30 nm左右的石榴状结构。低温湿化学法所合成出Bi₂Te₃纳米颗粒的尺寸比循环环流法制备出的要小,这可能和两种方法的反应温度不同有关系。对反应过程进行非原位观察和分析得出,化学合成Bi₂Te₃晶体的反应基本上可以分为原子反应过程以及离子反应过程这两种反应机制。三元合金对短波长声子的散射较强,能够降低热导率,材料的能带结构得到优化,因此具有更高的热电性能。用水热法在180℃的温度下制备出纯度较高的三元（Bi,Sb）₂Te₃纳米粉末,该粉末具有中空的结构,包含很多弯曲的薄片,这些薄片的尺寸在100-150 nm左右,壁厚则大概为20 nm。其块体的晶粒尺寸在几十至几百纳米之间,与未烧结前粉末的尺寸相差不大,这可能和粉末中空弯曲的形貌有关。在边界对声子的散射作用下,其热导率在室温下低达0.9 Wm^-1K^-1,最大ZT值在室温下达到1.28。分别用低温湿化学法和水热法在70℃和180℃的温度下合成纳米的Bi₂Te₃和Sb₂Te₃颗粒。放电等离子体烧结不同比例复合的Bi₂Te₃/Sb₂Te₃块体材料,在500 K的温度下,Bi₂Te₃和Sb₂Te₃以1:1复合热电材料的热导率低达0.7 W/（m·K）。真空热压烧结不同比例复合的Bi₂Te₃/Sb₂Te₃块体材料,Bi₂Te₃和Sb₂Te₃按1:1复合的材料具有相对较高的Seebeck系数和电导率,其ZT值在450 K的时候达到1.47左右。通过Sn掺杂有效提高了材料的电导率,2%Sn掺杂的试样在500 K左右的时候,ZT值达到最大1.4。微米Sb₂Te₃块体材料的热导率在室温下高达2.0 W/（m·K）,复合了40%Bi₂Te₃纳米粉末之后,热导率在温室下就低达1.1 W/（m·K）。在综合了纳米效应对Seebeck系数、电导率和热导率的综合影响之后,复合了30%Bi₂Te₃纳米粉末的试样在很大的测量温度范围内具有相对较高的ZT值,在450 K的时候,达到最大值0.9。以醋酸铅（Pb（CH₃COO）₂）和Te粉为反应原料,在100℃的温度下用循环环流法反应48小时后制备出颗粒尺寸在100 nm左右的PbTe粉末。24小时取样得到的是球状PbTe的形貌,颗粒尺寸在50至75 nm左右,这可能是PbTe在形核和长大过程中的中间形貌。本文认为,化学合成PbTe纳米粉末的反应基本上可以分为原子反应过程以及离子反应过程这两种机制。通过不同的热压工艺把上述合成的PbTe纳米粉末热压成型,得到晶粒尺寸不同的四个纳米块体材料。经过孔隙率修正之后,在边界效应的作用下,材料仍然具有较低的热导率。由于具有较高的载流子浓度,在孔隙率和边界效应的双重影响下,块体纳米材料仍然具有较高的电导率。400℃热压制备的块体试样的ZT值在580 K温度的时候达到最高为0.8,这个值和之前所报道的二元PbTe合金的最好热电性能相接近。更多还原

【Abstract】 As one of the options tor the energy conversion between thermal and electrical energy,thermoelectric（TE） materials are widely used as clean,reliable,and sustainable energy sources.Among all the tellurium based TE materials,Bi₂Te₃-based alloys are the state-of-art room temperature TE materials.After decades of research,the dimensionless figure of merit ZT of bulk Bi₂Te₃-based alloys is kept at about 1.On the other hand,for medium temperature range application,PbTe-based alloys are considered to be the most convenient choice,and the best ZT value reported is 2.2.With the development of nano-technique,more and more low-dimensional TE materials are studied and great improvements are made.With the crystal size decreasing from micron-to nano-meter,the carrier and phonon scattering increased,which lead to the increasing of Seebeck coefficient and decreasing of thermal conductivity,and resultantly the improvement of the thermoelectric properties.In the present work,Bi₂Te₃-and PbTe-based TE materials are systemically studied. These telluride-based materials are prepared using solvo-/hydro-thermal method, low-temperature aqueous chemical route and reflux method.The structures and morphologies of these nanopowders were investigated by XRD,SEM,TEM,and HRTEM, and the reaction mechanism were also discussed in detail here.The electrical transport properties can be tuned due to the nano effect in these bulk samples,and the ZT values were improved a lot.Some important results of the present work are listed as follows:During the low-temperature aqueous chemical route of Bi₂Te₃ process,too much complexing agent is not good for the synthesis.The reductant NaBH₄ should be added by several steps,which is good for the synthesis of purer Bi₂Te₃.The lower the concentration of reactions is,the lower the nucleation ratio of Bi₂Te₃ will be,and longer time will be needed.With different Te source,Te powders and Na₂TeO₃ namely,different morphologies of Bi₂Te₃ powders are obtained.The samples prepared with Te powders are mainly about 100nm diameters,0.5～1μm long nanotubes,with the walls about 30nm thick.The powders obtained from the later one are megranate like structures,which were formed by 30nm sized powders gathered together.The Bi₂Te₃ nanopowders synthesized by low-temperature aqueous chemical route have smaller sized than those by reflux method.This can be explained by the different reaction temperature.By ex-situ investigation and analyzing,the mechanism of chemical synthesis of Bi₂Te₃ was assumed to be a mixture of two mechanisms,namely,atom reaction and ion reaction.Ternary alloys have strong scattering effect to short-wave phonon,which can decrease the thermal conductivity and optimize the electron band structure,and lead to a higher thermoelectric properties.By hydrothermal method at 180℃,high purity ternary （Bi,Sb）₂Te₃ nanopowders were prepared successfully.The morphology of these powders was hollow,curve sheets.The size of these sheets was about 100-150nm with a thickness of 20nm.The crystal size in the bulk samples is about 10s to 100s nanometers.The powder sizes had no obvious change before and after sintering,which was maybe due to the hollow,curve morphology.Because of the boundary scattering effect,the thermal conductivity dropped to as low as 0.9Wm^-1K^-1,and the highest ZT reached 1.28 at room temperature.The nanopowders Bi₂Te₃ and Sb₂Te₃ were prepared by low-temperature chemical route at 70℃and solvothermal at 180℃,respectively.With different Bi₂Te₃ to Sb₂Te₃ ratio,these nanopowders are consolidated by Spark Plasma Sintering Method（SPS）.At 500K,the thermal conductivity of the sample with a Bi₂Te₃/Sb₂Te₃=1:1 is 0.7Wm^-1K^-1. on the other hand,these powders are also consolidated by hot pressing in vacuum.The sample with Bi₂Te₃/Sb₂Te₃=1:1 performed higher Seebeck coefficient and electrical conductivity,the figure of merit ZT value reached 1.47 at 450K.The electrical conductivity can be effectively improved by Sn doping.The value 1.4 of ZT at 500K was obtained for the sample of 2%doping.The thermal conductivity at room temperature of micron-sized Sb₂Te₃ bulk sample was about 2.0W/（m-K）.While the thermal conductivity of the nano-micro composite with a nano/micro=4:6 dropped to 1.1W/（m-K）. Considering all the nano effect to Seebeck coefficient,electrical conductivity and thermal conductivity,the optimized ZT of 0.9 was obtained at 450K for the sample with nano/micro=3:7.Using Pb（CH₃COO）₂ and Te powders as precursor,the PbTe powders with 100nm size were prepared by reflux method at 100℃for 48h.The morphology of PbTe powders sampling at 24h is 50-75nm sized sphere.This could be the medium step during the nuclear and growth process.Here,the chemical synthesis of PbTe nanopowders was assumed to be a mixture of two mechanisms,namely atom reaction and ion reaction.Four PbTe bulk samples were consolidated by hot pressing with different parameters.Due to the boundary effect,even after Porosity modification,these samples still showed very low thermal conductivities.And even with the porosity and boundary double effect,the electrical conductivities remained at very high level in virtue of the high carrier concentration.The ZT value of 0.8 was obtained at 580K for the sample hot pressed at 400℃,which is comparable with the optimized binary PbTe alloy in previous reports.更多还原