【作者】 穆武第；

【导师】 陈朝辉；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2009， 博士

【摘要】 热电材料是实现热能和电能直接转换的材料,可用于温差发电和通电制冷。Bi₂Te₃基化合物是室温性能最好的热电材料,PbTe基化合物是中温（300⁹00K）性能较好的热电材料。经过几十年的研究,块体Bi₂Te₃基和PbTe基材料的热电优值一直徘徊在1左右。随着纳米技术的兴起,近年来有关在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或在材料内部添加纳米级第二相粒子并降低材料维数,可以增加对载流子和声子的散射,提高Seebeck系数,降低热导率,提高热电性能。本文从理论模拟计算入手,设计了（Bi₂Te₃/PbTe）n薄膜的结构,采用磁控溅射法制备了不同结构的p型Bi₂Te₃薄膜和Bi₂Te₃与PbTe的复合薄膜,对薄膜结构与性能作了系统研究。本文首先从波尔兹曼方程出发,首次引入了Bi₂Te₃/PbTe粗糙界面效应,限定量子隧道效应,模拟计算（Bi₂Te₃/PbTe）n多层量子阱结构的热电性能,结果表明:当PbTe障碍层的宽度为1nm时,隧道传输系数为0.15;粗糙的（Bi₂Te₃/PbTe）n界面对载流子产生漫反射,使得（Bi₂Te₃/PbTe）n多层量子阱的最大ZT值急剧降低;限定PbTe障碍层的宽度为1nm,PbTe障碍层的存在使得镜面参数p为1时的ZT值比Bi₂Te₃理想超晶格的ZT值低近1倍。当Bi₂Te₃亚层宽度增大时,计算值还表明（Bi₂Te₃/PbTe）n多层量子阱的ZT值急剧下降;当p为0.5,Bi₂Te₃亚层宽度超过2nm时,其ZT值比Bi₂Te₃块体材料的还要低。模拟结果与制备得到的（Bi₂Te₃/PbTe）n多层膜的实验结果中功率因子较低相一致。本文系统的研究了磁控溅射工艺制备得到的Bi₂Te₃薄膜、（Bi₂Te₃/PbTe）n多层膜和（PbTe）np/Bi₂Te₃纳米复合薄膜的结构。研究发现,采用射频磁控溅射,在低功率（25W）溅射条件下,在平整的表面（解理云母片或冷抛石英玻璃）上,Bi₂Te₃以非晶态的结构沉积在基底表面;随着沉积时间的延长,Bi₂Te₃薄膜变厚的同时发生结晶,形成纳米晶薄膜;采用间歇沉积（沉积1min,停止溅射约1min后再溅射）的方式首次得到了非晶态的微米量级厚度的Bi₂Te₃薄膜。采用直流磁控溅射沉积PbTe时,PbTe以颗粒形态沉积在基底表面,首先形成不连续的岛状结构,当连续沉积3s以上时,形成连续的薄膜。控制磁控溅射工艺成功地得到（Bi₂Te₃/PbTe）n纳米多层膜,多层膜中PbTe亚层厚度最小约为6nm。首次采用磁控溅射多层膜的工艺得到了PbTe纳米颗粒弥散分布在Bi₂Te₃基体中的（PbTe）np/Bi₂Te₃纳米复合薄膜,其中PbTe纳米颗粒的尺寸在2⁵nm,并且PbTe纳米颗粒均匀弥散分布在Bi₂Te₃基体中,不存在搭接现象。射频磁控沉积的Bi₂Te₃非晶态薄膜在300℃条件下退火3小时后,薄膜发生结晶,晶粒尺寸不超过20nm。退火前非晶态的Bi₂Te₃中载流子处于定域态中,限制了载流子的迁移,使得非晶态中的电导率比退火后的晶态Bi₂Te₃薄膜的电导率要低。研究Bi过量程度不同的p型Bi₂Te₃晶态薄膜,其电导率在300⁷00S/cm范围内变化,电导率较小,主要原因是受到薄膜内部大量的缺陷和界面的散射造成;Seebeck系数在80¹60μV/K范围内变化;最大的功率因子只有8×10-4WK-2m-1。采用磁控溅射沉积制备得到（Bi₂Te₃/PbTe）n纳米多层膜,PbTe亚层是结晶态,Bi₂Te₃亚层是非晶态的。由理论分析表明Bi₂Te₃/PbTe界面的镜面参数p约为0.3⁰.4时,理论计算得到的电学性能与实验吻合;退火后镜面参数降低。载流子主要在Bi₂Te₃亚层中传输,受到强烈的界面散射使得p型（Bi₂Te₃/PbTe）n多层膜的Seebeck系数在100℃时为250μV/K;但其电导率低,功率因子总体较低。在（PbTe）np/Bi₂Te₃纳米复合薄膜中,载流子主要在Bi₂Te₃基体中进行输运,一方面沉积得到的薄膜中PbTe纳米颗粒是晶态的,Bi₂Te₃在沉积过程中依托PbTe晶粒生长,有利于Bi₂Te₃与PbTe之间形成比较理想的界面,界面散射较弱,电导率相对退火后的（PbTe）np/Bi₂Te₃纳米复合薄膜较高;而退火后的Seebeck系数由于比较强烈的界面散射而有所增大。与Bi₂Te₃/PbTe多层膜相比,（PbTe）np/Bi₂Te₃纳米复合薄膜的功率因子较高。采用3ω法测量了（PbTe）np/Bi₂Te₃纳米复合薄膜的热导率,发现薄膜的声子热导率在300³60K的温度范围内随着温度的升高而增大,说明（PbTe）np/Bi₂Te₃纳米复合薄膜中声子的散射机制主要是界面散射。利用有效介质理论和界面热阻,引入基体晶粒大小的尺寸效应,分析了（PbTe）np/Bi₂Te₃纳米复合薄膜的热导率随PbTe纳米颗粒体积含量增加的变化,得到Bi₂Te₃/PbTe的界面热阻在PbTe体积含量较低时,随PbTe体积含量的增大而变小;当PbTe体积含量较大时,界面热阻又变大,且界面热阻变化范围小,0.94².48×10-9m2K/W。针对军事装备上高温部位的热红外隐身的需求,本文首次提出设计温差发电和通电制冷两种方式来解决这一问题。理论分析和实验表明,采用温差发电方式不能有效地降低“表面”温度;采用通电制冷的方式能迅速有效地降低“表面”温度,且能通过调节外加电流大小来控制“表面”温度,有望实现智能热红外隐身。更多还原

【Abstract】 Thermoelectric materials interconvert heat and electricity directly. They are used in power generation and cooling. Bi₂Te₃ based materials are the best thermoelectric materials at room temperature. And PbTe based materials are the better thermoelectric materials from 300 to 900K. But the figure of merits ZT of Bi₂Te₃ and PbTe are about 1 for decades. In recent years, the research of thermoelectric material has made new progress by nanotechnology. Low dimensional and nanostructured materials have a large amount of boundaries that will strongly scatter the phonons and carriers. Therefore the Seebeck coefficient is improved and the thermal conductivity reduced. In this paper, structures of （Bi₂Te₃/PbTe）n multilayer films were designed according to the theoretical calculation from Boltzmann equation and various nanostructured p type Bi₂Te₃ films and composite films of Bi₂Te₃ and PbTe were fabricated by magnetron sputtering. The structures and properties are systematically investigated.In this paper, rough interface of Bi₂Te₃/PbTe and quantum tunnel effect are induced to analyse the ZT of （Bi₂Te₃/PbTe）n multilayer quantum well from Boltzmann equation for the first time. The result shows that increases rapidly when the thickness of PbTe barrier is under 3 nm. Rough interface of Bi₂Te₃/PbTe scatters the carriers strongly and ZT of （Bi₂Te₃/PbTe）n multilayer quantum well decreases rapidly because of the rough interface. If the thickness of PbTe barrier is restricted to 1 nm （the tunnel transmission coefficient is 0.15）, the ZT of Bi₂Te₃/PbTe multilayer quantum well is much lower than the ZT of ideal superlattice even if the specularity parameter p is 1. The calculational result also shows that the ZT decreases with increasing of the Bi₂Te₃ sub-layer. If the p is 0.5, the ZT of Bi₂Te₃/PbTe multilayer quantum well is lower than the ZT of bulk Bi₂Te₃ when the thickness of Bi₂Te₃ sub-layer is over 2nm. The calculational results are consistent to the experimental results of （Bi₂Te₃/PbTe）n nultilayer films with low power factors.The influences of magnetron sputtering technics on the structures of Bi₂Te₃, （Bi₂Te₃/PbTe）n multilayer and （PbTe）np/Bi₂Te₃ nanocomposite films are systematically studied in this paper. The results show that amorphous Bi₂Te₃ deposit on the glabrous surfaces （mica or cool polishing quartz glass） perfectly with low sputtering power 25W. Amorphous Bi₂Te₃ will become nano-grain with the increasing of continuous sputtering time, and constitute tree structure vertical the substrate. Amorphous Bi₂Te₃ films can be acquired by the method of intermittent depositing. PbTe deposits on the surfaces with nano-particles magnetron sputtering. When the depositing time is over 3s, PbTe nano-particles will be continuous films with tight contact among the particles. Otherwise, PbTe nano-particles will be nano-island structure on the surfaces. So it can be acquired that （Bi₂Te₃/PbTe）n nano-multilayer and （PbTe）np/Bi₂Te₃ nanocomposite films of PbTe nano-particles dispersing in Bi₂Te₃ substrate. In the multilayer, the thicknesses of PbTe sub-layer are over 6 nm. The size of PbTe nano-particles are 2⁵nm in the Bi₂Te₃ substrate without contact.The amorphous Bi₂Te₃ films crystallize completely after annealing under the condition of 300℃and 3 hours and the grain size is under 20nm. The carriers are confined in the confined states of amorphous Bi₂Te₃ films. So the conductivities of amorphous Bi₂Te₃ films are less than the conductivities in crystalline Bi₂Te₃ films. The conductivities are 300⁷00 S/cm in crystalline Bi₂Te₃ films with over Bi. The conductivities are low because the hole is scattered strongly by the plentiful defects and interface. Seebeck coefficients are 80 to 160μV/K in the films. And the best power factor is only 8×10-4WK-2m-1. In the （Bi₂Te₃/PbTe）n nano-multilayer films, the sub-layer of PbTe is crystalline and the sub-layer of Bi₂Te₃ amorphous. According to the model of rough interface, the specularity parameter p of the Bi₂Te₃/PbTe interface is about 0.3⁰.4. And the value is higher than the p of the Bi₂Te₃/PbTe interface after annealing. Because Bi₂Te₃ deposits on the surface of PbTe sub-layer, but the coherent deposition is broken after annealing. The carriers are scattered strongly by the interface when they transmit in the Bi₂Te₃ sub-layer. So the Seebeck coefficients increase and Seebeck coefficient of p type Bi₂Te₃/PbTe multilayer films is 250μV/K at 100℃. But the conductivities are low because of scattering to the carriers and the power factors are low. In the （PbTe）np/Bi₂Te₃ nanocomposite films, Bi₂Te₃ deposits along the nucleuses of nano-grain and that is benefit to form coherent interface between Bi₂Te₃ and PbTe. The carriers are scattered weakly by the interface. But the coherent interfaces are destroyed after annealing as the same of （Bi₂Te₃/PbTe）n multilayer films. So the conductivities of the （PbTe）np/Bi₂Te₃ nanocomposite films as grown are higher than the conductivities after annealing. In verse, the Seebeck coefficients are higher after annealing because of the interface strongly scattering to carriers. The power factors are higher to the （PbTe）np/Bi₂Te₃ nanocomposite films than that to the （Bi₂Te₃/PbTe）n multilayer films.The 3ωthermal conductivity measurement technique was introduced to measure the thermal conductivities of the （PbTe）np/Bi₂Te₃ nanocomposite films. The results show that the lattice thermal conductivities increase with the increasing of temperatures in the temperature zone 300 to 360K. That is, the phonons are scattered by the interfaces when they transmit in the films in the temperature zone. For interpreting the regular of phonon thermal conductivities depending on fraction of PbTe nano-particles, a model is introduced to integrate Effective Medium Theory, interfacial thermal resistance and nano-grain size effect in the substrates. The interfacial thermal resistances of Bi₂Te₃/PbTe are about 0.94 to 2.48×10-9m2K/W in this paper. For the （PbTe）np/Bi₂Te₃ nanocomposite films after annealing, Bi₂Te₃ crystallize along on the PbTe and so the coherent interface is better in the higher fraction of PbTe nano-particles when the fraction is small. But the coherent interface is destroyed when the fraction of PbTe nano-particles is large enough. That exhibits that the interfacial thermal resistances increase with the increasing of PbTe nano-particles at first. But the interfacial thermal resistances decrease with the increasing of PbTe nano-particles at last when the volume of PbTe nano-particles is large enough.For the need to thermal infrared stealth of high temperature surfaces of martial equipments, two models are designed to resolve the difficulty with the effects to giving power and refrigeration of thermoelectric materials for the first time. The results show that the effect to giving power can’t to decrease the external temperature effectively. And the effect to refrigeration can decrease the external temperature effectively and rapidly and control the external temperature by varying the electric current of the thermoelectric module. That is imaginable to achieve intelligent thermal infrared stealth.更多还原