【作者】 周爱军；

【导师】 赵新兵；

【作者基本信息】 浙江大学 ， 材料学， 2010， 博士

【摘要】 热电材料是一种将电能与热能直接互相转换的功能材料,它提供了一种安全可靠、全固态的发电和制冷方式,具有较大的应用潜力。但大多性能较好的热电材料都是由价格昂贵且有毒的元素如Te、Sb、Co、Pb等组成,它们所带来的环境问题和高经济成本成为热电材料应用的瓶颈之一。高锰硅因具有价格低廉、无毒环保、热化学稳定等优良特性成为当今倍受关注的环保型热电硅化物之一。目前世界上研究高锰硅热电材料的主要国家是俄罗斯和日本,所报导的最高热电优值ZT在0.7附近,且利用高锰硅作为P型单元的热电模块的转换效率可达到8%-12%。而国内对高锰硅热电材料和器件的的研究正处于起步阶段。本文以环保型硅化物和氧化物为研究方向,选择了高锰硅和钙钛矿氧化物La_1-xSr_xCoO₃作为研究对象,从材料热电性能的调控、结构的纳米化以及器件的模拟计算几个方面进行了详细研究。以高锰硅热电性能的提高为目的,通过原位熔炼、机械研磨和热压烧结等方法制备了含第二组元的高锰硅复合材料CeSi₂/HMS、PbTe（Ag₂Te）/HMS和SiGe（Ge）/HMS,对各种高锰硅复合材料相结构、微观形貌和热电性能进行了研究。实验表明,由于第二相的物理特性和在基体中的非弥散分布,CeSi₂和PbTe（Ag₂Te）的引入使高锰硅的热电性能下降。在SiGe（Ge）/HMS复合材料中,虽然也形成了非弥散分布的SiGe相,但由于部分Ge原子对高锰硅中Si原子的取代效应,复合材料的功率因子增加,同时其晶格热导率降低,ZT值得到了25%-50%的提到。SiGe（Ge）第二相对高锰硅的影响还体现在对其显微结构的影响。SiGe（Ge）加入后,高锰硅中的MnSi条纹的数量和形态发生了明显变化,很可能对高锰硅的热学或电学输运性能产生了重要影响。以实现低热导的高锰硅纳米结构材料为目的,通过高能机械球磨法制备了不同晶粒尺寸和相组成的高锰硅粉末。本文首次发现了高锰硅的机械力化学分解现象,并研究了高锰硅的相分解率与各球磨参数和晶粒尺寸之间的关系。将单相的高锰硅纳米粉末进行热压后发现,单相高锰硅在高温下发生了相变,产生了MnSi相。通过同步辐射X射线衍射进行原位分析后发现,纳米尺寸对高锰硅的相变有重要影响。和微米粉末相比,纳米高锰硅粉末的相变温度较低,且相变速率和相变程度较大。压力对纳米高锰硅的相变在不同温度下也会产生抑制或加强。本文分析认为,纳米高锰硅高温下的相变主要是高锰硅非公度相之间的自转变。这种自转变相变发生的条件和程度与高锰硅体系的自由能和热动力学特性相关。高锰硅自身结构的特殊性,即强烈的空间各向异性和由此导致的纳米尺度下的结构不稳定性是高锰硅机械力化学分解和自转变的本征原因。尽管机械球磨纳米化被证明能够提高很多其它材料的热电性能,但本文的实验表明,纳米高锰硅的相变产生MnSi金属相,这种金属相会使材料的热电性能大幅度降低。因此,通过球磨法制备的纳米高锰硅材料无法应用于热电器件。分别通过固相反应法和溶胶凝胶法与热压烧结的结合,制备了不同Sr掺杂的La_1-xSr_xCoO₃固体氧化物材料,并研究了温度和Sr掺杂对氧化物材料热电性能的影响。结果表明,未掺杂的LaCoO₃在室温下具有高达到555μV/K的Seebeck系数,但随着温度的升高而迅速降低;随着Sr掺杂量的增加,试样的Seebeck系数发生明显下降。在溶胶凝胶法制备的试样中,过多的Sr掺杂提高了氧化物的热导率,降低了ZT值。溶胶凝胶法制备的La_0.9Sr_0.1CoO₃氧化物在室温下具有最大的ZT值仅有0.046。温度和Sr掺杂对La_1-xSrxCoO₃电学输运性能的影响与载流子浓度的变化以及Co³⁺离子在不同温区和掺杂量下的的自旋态转变有关。热电器件的制作需要优化各种器件参数以获得最佳的工作性能。本文以热电传输学基本理论为指导,通过Mathematica数学软件和程序,对由P型高锰硅和N型Mg₂（Si,Sn）构成的单对热电模块的尺寸优化参数A_p/A_n、热流输入密度Q、电能输出功率密度P_max和最大转换效率η_max进行了计算,并模拟了接触条件对热电模块工作性能的影响。计算表明,当冷热端温度分别固定为298K和898K时,该热电模块在理想接触状态下可产生8.29%的最大转换效率。同样的条件下,获得最大输出功率P_max和最大转换效率η_max的尺寸优化参数A_p/A_n分别为1.97和1.89。模拟结果表明,当接触层电导率和热导率的数值和P型或N型单元的数值接近或更大时,模块的工作性能和理想接触状态时接近;而当两者同时以数量级的程度减小时,模块的转换效率和输出功率都会大幅度降低。更多还原

【Abstract】 Thermoelectric（TE） materials are functional materials that can convert heat energy to electricity directly and visas verse.They show great potential in practical applications providing a safe,reliable and full-solid-state way of power generation and refrigeration.Most of the state-of-the-art TE materials are however alloys composed of high cost and toxic elements,such as Te,Sb,Co,Pb,etc.,which constrains the development of TE applications due to the environmental and economic issues with them.Higher manganese silicides（HMS）, one of the most promising TE silicides,have attracted increasing interest for their low cost of raw materials,environmental friendliness and thermal and chemical stabilities.Most of the research on HMS have being carried out in Russia and Japan,where a maximum dimensionless figure of merit of 0.7 was reported and a conversion efficiency of 8%-12%of a HMS-based silicide TE module was achieved.However,China is just on the starting line for the research of HMS TE materials and devices.Based on the concept of developing environmental-friendly TE materials,detailed investigation was performed on HMS and oxide materials.Focuses have been made in several aspects from property tuning,nanostructuring to module simulation.Aiming at improving the TE performance,HMS composites with secondary phases were prepared through in situ melting or ex situ mechanical mixing combined with hot-pressing densification process.Composites of CeSi₂/HMS,PbTe（Ag₂Te）/HMS and SiGe（Ge）/HMS were synthesized and their phase structures,microstructure and TE properties were investigated.It was found that the TE performance of CeSi₂/HMS and PbTe（Ag₂Te）/HMS was degraded due to the physical transport characteristics of the secondary phases and their inhomogeneous distribution in the matrix.In the composite SiGe（Ge）/HMS,although inhomogeneity of SiGe precipitations was observed,the figure of merit was improved due to increased power factor and reduced lattice thermal conductivity by partial substitution of Ge for Si.The ZT value was improved by 25%-50%in SiGe（Ge）/HMS composites.Moreover, the microstructure of the inherent MnSi layers in SiGe（Ge）/HMS was significantly influenced by addition of Ge,which may also affect the transport properties of the material.Nanostructuring is an effective way to improve the TE performance by quantum effect in many material systems.HMS powders with different grain sizes and phase constitution were obtained by high-energy ball milling.The mechanochemical decomposition of HMS was for the first time discovered in this work,and the effects of ball milling parameters on the decomposition rate of HMS and their grain sizes were studied in detail.It was found that by hot-pressing the single phase HMS nanopowders,the MnSi phase was obtained in the pellet. In situ energy dispersive x-ray diffraction revealed some phase transformations under high temperature,on which the grain size effect was important.Compared to the micropowders, the HMS nanopowders showed much lower temperature and faster rate of phase transformation.High pressure could either depress or accelerate the phase transformation of nanopowders in different temperature ranges.It was understood that the phase transformation of HMS should be attributed to the inter-evolution of several incommensurate phases,which is closely related to the total internal energy of the system and the thermodynamic features of Mn-Si system.Intrinsically,the highly anisotropic crystal structures of HMS and their decreased stability under nanoscale dimension were supposed to introduce mechanochemical decomposition and inter-evolution of HMS.The presence of metallic MnSi phase during the phase transformation neglected the possibility of using ball-milled HMS nanopowders as TE materials.Perovskite-type La_1-xSr_xCoO₃ oxides were synthesized by solid state reaction and sol-gel method combined with hot-pressing,and the effects of temperature and Sr-doping on the TE properties were explored.It was found that the undoped LaCoO₃ showed an extremely high Seebeck coefficient of 555μV/K at 300K,which however decreased quickly with increasing temperature.As Sr-doping was introduced,the Seebeck coefficient showed also sudden decrease,and a good TE performance was not expected due to reduced power factors.The maximum figure of merit was obtained in sol-gel prepared La_0.9Sr_0.1CoO₃ with a ZT value of only 0.046 at 300K,which is even not considerable for TE use.The increase of carrier concentration by Sr-doping and the spin state transition of Co³⁺ at different temperatures were attributed to the change of electrical transport properties of La_1-xSr_xCoO₃.Optimization of size parameters is usually needed before building TE devices to obtain best performances.Using the well-known TE theories and Mathematica programming software,the optimized size parameter A_p/A_n,heat flow Q,maximum power density P_max and efficiencyη_max were simulated for a simple TE module based on P-type HMS and N-type Mg₂（Si,Sn）.The results show that the module is able to produce a maximum efficiency of 8.29%in ideal contact condition when the temperatures of the cold and hot side maintain 298K and 898K,respectively.The size parameter A_p/A_n for obtaining P_max andη_max should be 1.97 and 1.89,respectively.Contact simulation revealed that when the electrical and thermal conductivity of the contact layer is comparable or better than the P- and N-type materials,the performance of the TE module shows no significant decline,while it is the opposite case when the electrical and thermal conductivity of the contact layer decrease simultaneously by the magnitude of 10¹-10².更多还原