电脉冲调节的碲化锗纳米线热电性能

发布时间：2018-06-07 13:12

  本文选题：热输运 + 热电　； 参考：《南京大学》2017年硕士论文

【摘要】：纳米结构的引入对于热学输运及热电调控方面具有重要的应用价值。由于德拜模型,非金属态材料的热导率主要受到热容和各种传输机制的影响,如点缺陷,杂质,界面,倒逆散射等。另一方面,这些因素的引入往往也会对材料的热电性能产生影响,是调控热电系数的一个方法;此外,为了测试低维纳米结构的热学和电学输运性能,我们介绍了一种热桥方法。这种热桥方法可以同时测量纳米线的几个热电参数。尽管如此,纳米结构引入的步骤通常较为复杂,且需要较极端的高温合成条件;在低维纳米材料中对热电系数的调控尤其困难,且由于热电各系数的相关性,我们很难同时定向调控并优化材料的各个热学和电学参数。另一方面,相变材料作为一种已经较为深入研究的材料,可通过电学或激光脉冲改变材料的局部结构,完全改变材料的电学输运性质;但关于相变材料热学性能的研究还比较少,且对脉冲前后热学输运性质随温度的变化了解不多。在这篇论文里,我们通过化学气相沉积方法,合成了两种相变材料纳米线。并通过扫描电镜,透射电镜,X射线衍射谱等方法确定了对合成的纳米线进行了表征。我们分别在二氧化硅基底和热桥芯片上进行了电脉冲测试,并第一次实现了碲化锗纳米线在悬浮基底上的电脉冲相变。在将碲化锗样品转移到热桥芯片上后,我们对样品的热电参数进行了随温度变化的测试。首先,通过电脉冲控制纳米线不同相的形成,我们探究了相变碲化锗纳米线随温度变化的电导率的不同趋势,进一步确定了纳米线不同的相及不同的输运机制。其次,我们测试了碲化锗非晶相纳米线和晶相纳米线的赛贝克系数,发现尽管两相之间电导率有几个数量级的差距,塞贝克系数的变化和大小却比较类似。我们推测是因为非晶相仅占纳米线的一小部分所致。最后,我们还测试了碲化锗纳米线通过电脉冲变为脏金属态纳米线前后的热导率,因为纳米线尺寸上远大于其声子平均自由程,其热导率数值与体块晶态纳米线大小类似;我们第一次发现热导率在脉冲前后的变化趋势与电导率在脉冲前后的变化趋势变化相反,这一点为对各相碲化锗纳米线的热学输运机制的知识得到了提升,并为在各相纳米线之中的电脉冲调控提供了很大的参考价值。通过综合几个测试数据,我们得到了第一次碲化锗非晶态纳米线的热电优值。且发现在不同纳米线相之间的调控或许可对热电参数进行同时的优化。并实现了在测试热桥芯片上对纳米线性质的原位调控。同时,在之后的工作中,我们希望能实现同一个样品各相在芯片上的脉冲总体调控,并对样品进行透射电镜,表面电子散失谱等进行进一步的表征。并对热学和电学的参数变化进行理论上的模拟和解释。
[Abstract]:The introduction of nanostructures plays an important role in thermal transport and thermoelectric regulation. Due to Debye model, the thermal conductivity of nonmetallic materials is mainly affected by heat capacity and various transport mechanisms, such as point defects, impurities, interface, inverse scattering and so on. On the other hand, the introduction of these factors often affects the thermoelectric properties of the materials and is a method of regulating thermoelectric coefficients. In addition, in order to test the thermal and electrical transport properties of low-dimensional nanostructures, We introduce a hot bridge method. This method can simultaneously measure several thermoelectric parameters of nanowires. Nevertheless, the steps introduced into nanostructures are usually more complex and require more extreme high temperature synthesis conditions; in low-dimensional nanomaterials, it is particularly difficult to regulate thermoelectric coefficients, and because of the correlation of thermoelectric coefficients, It is difficult to control and optimize the thermal and electrical parameters of materials simultaneously. On the other hand, as a kind of material which has been studied deeply, the phase change material can change the local structure of the material by electricity or laser pulse, and completely change the electrical transport property of the material. However, there are few studies on the thermal properties of phase change materials, and little is known about the change of thermal transport properties with temperature before and after pulse. In this paper, we synthesized two phase change nanowires by chemical vapor deposition. The synthesized nanowires were characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). We have carried out the electric pulse test on silicon dioxide substrate and thermal bridge chip respectively, and have realized the electric pulse phase transition of germanium telluride nanowires on the suspension substrate for the first time. After the germanium telluride sample was transferred to the thermal bridge chip, the thermoelectric parameters of the sample were measured with the change of temperature. Firstly, by controlling the formation of different phases of nanowires by electric pulse, we investigate the different trends of conductivity of phase change germanium telluride nanowires with temperature, and further determine the different phases and transport mechanisms of nanowires. Secondly, we have measured the Seebeck coefficient of amorphous and crystalline germanium telluride nanowires. It is found that the variation and magnitude of Seebeck coefficient are similar even though there are several orders of magnitude difference in conductivity between the two phases. We speculate that the amorphous phase accounts for only a small part of the nanowires. Finally, we also measured the thermal conductivity of germanium telluride nanowires before and after they were transformed into dirty metal nanowires by electric pulse, because the size of nanowires was much larger than the average free path of phonon, and the thermal conductivity of nanowires was similar to that of bulk nanowires. For the first time, we found that the change trend of thermal conductivity before and after pulse is opposite to that of electrical conductivity before and after pulse, which is an improvement in the knowledge of thermal transport mechanism of germanium telluride nanowires. It also provides a great reference value for the control of electric pulse in various phase nanowires. The thermoelectric excellent values of the first amorphous germanium telluride nanowires have been obtained by synthesizing several test data. It is found that the thermoelectric parameters can be optimized simultaneously by the regulation of different nanowires. The in-situ control of the properties of nanowires is realized on the test hot bridge chip. At the same time, in the later work, we hope to realize the pulse control of each phase of the same sample on the chip, and to further characterize the sample by transmission electron microscope, surface electron dissipation spectrum and so on. The variation of thermal and electrical parameters is simulated and explained theoretically.
【学位授予单位】：南京大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O614.431;TB383.1
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本文编号：1991293