【作者】 周书才；

【导师】 白晨光；

【作者基本信息】 重庆大学 ， 冶金工程， 2012， 博士

【摘要】 热电材料是将热能和电能实现直接相互转换的一种功能材料，可用热电材料制成热电器件实现温差发电，从而充分利用工业上的废气和废热；也可用热电材料制成制冷机在一些特殊环境中替代传统的制冷设备。热电器件具有污染小、噪声低、重量轻、体积小、携带方便、可靠性高等优点，因此关于热电材料的研究与应用具有广阔前景。Mg₂Si是一种应用于500K-800K温度区间的性能优良的热电材料，其组成元素具有原料丰富、价格低廉、环境友好等特点，但镁化学活性高，容易氧化，导致Mg₂Si金属化合物的制备比较困难，因此Mg₂Si基热电材料的制备已成为研究重点之一。本论文采用微波加热合成技术，实现了Mg₂Si基热电材料的快速合成与产品致密化。微波加热合成法是利用微波与物质之间的相互作用产生的介质损耗、电导损耗和磁损耗，在物质内部产生热量，使材料被整体加热至反应温度从而实现相关的化学反应的一种新型材料合成方法，该方法具有体积加热、选择性加热、非热效应等特点。论文从理论和实验方面探索微波加热金属粉体的加热特性及发热机理，并以Mg₂Si微波合成实验为基础，研究了微波固相合成Mg₂Si反应的动力学及机制，同时研究了微波合成Mg₂Si和Mg₂Si1-xSn热电材料的工艺、热电性能、物相结构的特点。通过实验研究工作，本论文得到如下主要研究结论：（1）Mg₂Si基半导体材料在微波加热过程中，加热机理低温时主要以介电损耗为主，高温时主要以电导损耗加热为主。（2）Mg₂Si压坯在微波场中的加热升温特性与Mg₂Si压坯密度、元素组成和微波输出功率有关：压坯密度越小，加热速率越快，最终所能加热的温度也越高，但Mg₂Si压坯在微波场中最终所能达到的最高温度与Mg₂Si压坯初始密度无关；微波输出功率越大，Mg₂Si压坯加热速度越快；随Sn含量增加，Mg₂Si_1-xSn_x压坯加热速度降低，Mg₂Si压坯的加热速度较Mg₂Sn压坯快。（3）采用微波辅助加热，制备了Sn掺杂的Mg₂Si_1-xSn_x（x=0、0.2、0.4、0.6、0.8）热电材料，并应用X-射线分析仪（XRD）、电子显微镜和扫描电镜（带能谱分析）分析了试样的物相组成、结构形貌。分析结果表明：采用合适的微波加热工艺制度可以有效抑制Mg氧化；合适的Mg过量可以补偿Mg的挥发；当Mg过量8at%、微波加热功率在2.5Kw（853K）条件下保温30min时，可以得到纯度较高的Mg₂Si热电材料。同时XRD分析表明：在微波辐射下Mg₂Sil_1-xSn_x形成了良好的固溶体并原位形成Mg₂（Si，Sn）结构的复合材料，该方法具有反应温度低，合成速度快等优点。（4）通过对Mg₂Si基热电材料进行热电性能测试，结果表明：所有的Mg₂Si基热电材料试样的电导率均随温度的升高而升高，这表明Mg₂Si基热电材料呈现半导体传输特性，Seebeck系数为负值，材料为n型半导体。而且随着Sn含量的增加，Mg₂Si_1-xSn_x合金的Seebeck系数的绝对值和电导率增加，热导率降低60%。在500K时，Mg₂Si_0.4Sn_0.6固溶体的ZTmax为0.26，为未掺杂试样的2倍，可见Sn能极大地改善Mg₂Si的热电性能，相比较其它重金属元素， Sn可获得更高的热电性能，且资源更为丰富。因此，采用微波辅助加热合成并掺杂Sn元素形成固溶体是一种提高Mg₂Si热电材料性能的新途径。（5）通过对微波加热合成Mg₂Si反应动力学的研究，建立了转化率与时间关系曲线，从而计算出Mg₂Si生成反应的活化能，研究结果表明：微波电磁场对扩散过程的影响主要表现在对指前因子及扩散活化能的影响，微波加热降低了Mg₂Si反应活化能，其活化能为102KJ/mol，而常规合成Mg₂Si活化能为120¹90kJ/mol；合成时间较常规合成时间降低85⁹0%。Mg₂Si合成过程大致可分为四个阶段，其反应的动力学模型符合金斯特林格扩散反应壳—核心模型。（6）微波电磁场作用下对Mg-Si反应体系的影响不仅表现出“热效应”，而且也存在着“非热效应”，微波对反应的促进作用主要体现在微波加热体系热点的形成，电场的取向作用及PMF效应。更多还原

【Abstract】 Thermoelectric material is a kind of functional materials which can convert heatinto electricity directly. The thermoelectric materials could be used as generators andrefrigeration. The thermoelectric generators can convert terrestrial heat and the wasteheat of industry to electricity directly. The thermoelectric devices own some advantagesas no pollution, no voice, and lightweight, small, portable and safety, which are ofpromising application value for thermoelectric generation and thermoelectric cooling,mainly used in the fields of low-grade heat power generation, aerospace andmicroelectronics. Magnesium silicide is a kind of middle temperature thermoelectricmaterials, owing to some advantages as the abundance of raw materials, low cost andenvironment friendly. However, the phase purity and microstructure of the productMg₂Si are difficult to control by conventional technique because of the easyvolatilization and oxidation of Mg. In this paper, the method of microwave-assistedsynthesis was used to realize the rapid reaction and densification of Mg₂Si basedthermoelectric materials. Microwave synthesizing is a method through using the heatingproduced by the couple of microwace with the microstucture of materials and whichhave these features on bulk heating, selective heating and non-therm effect. This methodis a very promising preparation method for fabricating many materials because it is fast,clean energy efficient and does not suffer from the disadvantages of the classicalpreparation technique. In this paper, the behaviour and mechanism of microwaveheating metal powders was investigated, and the kinetics and mechanism of microwavesynthesizing Mg₂Si was researched. The process, thermoelectric properties andmicrosture of microwave synthesizing Mg₂Si and Mg₂Si_1-xSn_xwere studies. On the baseof study, the magjor results of this paper are shown as following.（1） In the microwave heating of Mg₂Si-based thermoelectric materials, dielectriclosses was the main mode at lower temperature, while conductive losses would get therun upon at higher temperature.（2） The heating behavior of Mg, Si and Sn fixed powder was investigated undermicrowave irradiation. The microwave heating therm profiles of Mg₂Si–based compactsare dependent on composition, porosity and power. The faster of heating rate and higherof final temperature are. The larger of porosity is, the faster of heating rate is, And thehigher of output power is, the faster of heating rate is. The heating rate increases as the green density decreases （porosity increases）. Higher is the porosity, higher is the heatingrate for a particular choice of experimental variables, the temperature rise is restricted toa certain level, but the final temperature doesn’t depend on the porosity. The heatingrate Mg₂Si_1-xSn_xcompacts decreases at a particular power setting with the increase of x.（3） Mg₂Si_1-xSn_x（x=0、0.2、0.4、0.6、0.8） solid solutions were prepared bymicrowave irradiation and the morphologies, microstures and phase compositions of thesample were mainly studied using scanning electron microscope（SEM） with energydisperse spectroscopy analyzer（EDS） and X-ray diffraction（XRD）. The oxidation of Mgcan be rest rained by changing microwave heating programs, the volatility of Mg can bemade up by controlling excessive content of Mg. When the excessive content of Mg isabout8at%, the samples is kept under853K for30min, high purity Mg₂Si powders canbe obtained under this experimental conditions. XRD patterns show that Mg₂Si_1-xSn_xsolid solutions have been well formed under microwave irradiation and microstructurecharacterization revealed a continuous network of Sn particulates and fabricate self-assembled Mg₂（Si, Sn） composites. This method is a very promising preparationmethod for many materials because it is fast, clean energy efficient and does not sufferfrom the disadvantages of the classical preparation technique.（4）Test of thermoelectric properties demonstrate that the electrical conductivity ofMg₂Si-based thermoelectric materials increases monotonically with temperature,indicating semiconducting behavior and a negative Seebeck coefficient suggests that theobtained materials exhibit n-type conductivity. What is more, increase of Sn ispositively related to the absolute value of Seebeck coefficient and electricalconductivity for Mg₂Si_1-xSn_xalloy.The thermal conductivity of Mg₂Si sample dopedwith Sn element had been reduced by60%. A maximum figure of merit, ZT=0.26, wasobtained for Mg₂Si0.4Sn0.6at about500K, the figure of merit ZT achieved2times ofpure Mg₂Si at600K, and its ZTmaxwas0.13. Sn element is better than the heavy ones inthermoelectric properties of of Mg₂Si owing to some advantages as the abundance of itsraw materials and low cost. It is really a renewed approach to enhance thethermoelectric performances of Mg₂Si by doping Sn elements and using microwaveIrradiation technology.（5）Experiments of the kinetic analysis on Mg₂Si synthesizing were carried outby microwave irradiation and the curve of rate of conversion as the function of reactiontime is obtained based on former experiment. The kinetic analysis showed that theactivation energy Eain Mg-Si system decreased under microwave irradiation. The activation energy was102KJ/mol by microwave heating and was120¹90kJ/mol byconventional heating. The synthesizing time of Mg₂Si sample by microwave heatinghad been reduced by85⁹0%. The microwave could increase the activation energy andpre-exponential factor. The kinetic model of the reaction process has been establishedand the reaction mechanism of Mg-Si system belonged to the Kingsburg diffusionreaction shell-core model. The results show that the reaction process of Mg-Sithermoelectric materials could be expressed as:（i） Mg is melted and the surface of Siparticle is covered by Mg liquid;（ii） formation of Mg₂Si;（iii） Mg penetrate Mg₂Siphase and move to interface of Si particle;（iv） The following reaction occurs at Mg₂Si/Si side:2Mg+Si=Mg₂Si.（6）In comparison with the conventional heating processing, the results ofmicrowave direct synthesis of Mg₂Si-based thermoelectric materials show that thekinetic induced by microwave irradiation owing to microwave thermal effect and non-thermal effect. The results that it is possible by microwave heating to induce localizedsuperheating （eventually thermal runaway） which lead to localized reaction rateenhancements, PMF effect and microwave E-field has orientation and, which canaccelerate the diffusion rate of particles. So the reaction rate of Mg-Si by microwavequickly than that conventional heating.更多还原