【作者】 余健；

【导师】 张清杰； 赵文俞；

【作者基本信息】 武汉理工大学 ， 材料加工工程， 2014， 博士

【摘要】 热电材料是一种利用Seebeck效应或Peltier效应实现热能和电能直接转换的功能材料，在工业余热发电和热电制冷等领域具有重要和广泛应用。填充方钴矿热电材料在中温区具有优异的热电性能，被认为是最具应用潜力的热电材料。针对填充方钴矿材料目前存在的制备周期过长和难以进一步降低晶格热导率的两大瓶颈问题，本文发展了一种熔体淬火（MQ）结合放电等离子体烧结（SPS）快速制备填充方钴矿热电材料的高效方法（MQ-SPS），制备周期由传统方法的9~10天缩短至30小时以内；发展了一种制备微米孔结构填充方钴矿热电材料的新方法；并以快速工艺制备的铁基填充方钴矿热电材料为研究对象，研究了该材料的热稳定性。得到如下主要结论：采用MQ-SPS快速工艺制备了名义组成为BaxFe4Sb1（2x=1.0,1.2,1.4,1.5,1.6）的Ba单原子填充铁基方钴矿热电材料，研究了Ba填充对材料的物相组成、显微结构和热电性能的影响规律。结果发现，在放电等离子体活化作用下，淬火材料可以快速转变为仅含有少量Sb和FeSb2杂质相的填充方钴矿，制备周期缩短为30h以内；优化x值，可以有效降低SPS材料中杂质相的含量，x≥1.4样品由单相填充方钴矿组成，x>1.4样品因BaSb3液相量增大而出现晶粒异常长大现象。输运性能测量表明，材料电导率随x值增大先逐渐增大，x>1.4样品虽含有可降低电导率的气孔，但电导率未出现明显降低，电导率先增大与杂质相FeSb2含量减少有关，电导率保持不变源于异常长大晶粒导致迁移率显著提高；Seebeck系数随x增大先增大后减小，Seebeck系数先增大与杂质相含量减少有关，Seebeck系数减小源于气孔增多；晶格热导率随x增大先降低后增大，晶格热导率降低与Ba填充量增大导致声子共振散射作用增强有关，晶格热导率增大源于晶粒异常长大导致晶界对声子的散射作用减弱。x=1.4样品具有最大的功率因子和最低晶格热导率，800K时分别为2.9mW·K-2·m-1和0.63W·m-1·K-1，x=1.4样品800K时最大的ZT值达到0.62，与x=1.0样品相比提高了34%。以名义组成为BaIn0.5Fe3.7Co0.3Sb12的(Ba,In)双原子填充铁基方钴矿材料为研究对象，研究SPS烧结温度对快速制备(Ba,In)双原子填充铁基方钴矿材料物相组成和热电性能的影响规律，揭示MQ-SPS工艺快速制备填充方钴矿的结晶反应机制。结果发现，在SPS条件下，淬火过程中包晶反应形成的FeSb首先与Sb反应生成FeSb2，随后FeSb2与其他淬火物相(BaSb3,InSb,Sb)直接反应形成BaIn0.5Fe3.7Co0.3Sb12。烧结温度为723K时，FeSb2和FeSb可全部转变成单相填充方钴矿，高于723K后由于Sb挥发导致产物中重新出现FeSb2。输运性能测量表明，烧结温度为723K时材料具有最高电导率，烧结温度为848K时材料电导率最低；升高烧结温度可小幅度提高Seebeck系数。烧结温度为798K时材料具有最大功率因子，800K时达到2.8mW·K-2·m-1。热导率和晶格热导率随烧结温度升高先降低后增大，烧结温度为773K时材料具有最低晶格热导率，800K时仅为0.57W·m-1·K-1。材料ZT值随烧结温度升高先增大后减少，烧结温度为798K时材料ZT值最大,800K时达到0.61。利用ZnSb在高温下易分解挥发特性，发展了熔体淬火-扩散退火-SPS-高温退火工艺制备具有微米孔结构的钴基(Ba,In)双原子填充方钴矿热电材料的新方法，系统研究了微米孔结构对物相组成、显微结构和热电性能的影响规律。结果表明，钴基(Ba,In)双原子填充方钴矿热电材料中ZnSb第二相在高温退火过程中被分解、挥发并形成约1~4μm的微米孔结构，同时发现微米孔边界表面富集大量由Ba和Sb构成的尺寸约为20~80nm的纳米结构。输运性能测量表明，这种微米孔材料的Seebeck系数显著提高，但电导率并未因微米孔出现而明显降低；功率因子明显增大，800K时最大功率因子达到4.96mW·K-2·m-1，与同成分的致密材料相比，增大了6.7%。这种微米孔结构还导致晶格热导率大幅度降低，800K时最低晶格热导率仅为0.9W·m-1·K-1，与同成分的致密材料相比，降低了26%。故微米孔材料的ZT值大幅度提高，800K时最大ZT值达到1.36，与同成分的致密材料相比，提高了22.5%。微米孔材料的电导率变化不明显源于电输运的逾渗效应，Seebeck系数显著增大源于孔表面的纳米结构对载流子的能量过滤效应，晶格热导率大幅度降低是微米孔边界对长波声子强烈散射和声子弹道输运的综合体现。以MQ-SPS快速工艺制备的名义组成为Ba1.4Fe4Sb12的铁基填充方钴矿材料为研究对象，研究了该材料在773K下长时间真空退火过程中其物相组成、显微结构和热电性能的热稳定性。结果发现，材料内部组成和显微结构具有良好的热稳定性，退火30d后内部仍为单相方钴矿，但因Sb挥发其表面分解层的厚度达49μm，通过外推法预测退火1年后，分解层厚度约为130μm。输运性能测量表明，MQ-SPS工艺制备的材料在长时间真空退火过程中表现出良好的热电性能稳定性，退火处理前材料ZT值在800K时为0.61；退火30d后，材料ZT值为0.62，基本无变化。更多还原

【Abstract】 Thermoelectric (TE) material is a kind of functional material that can directlyconvert heat to electric energy and vice versa through the Seebeck and Peltier effects.The TE materials have important and widespread applications in the fields of powergeneration of industrial waste heat and TE cooling. Filled skutterudites exhibit superiorTE properties in the moderate temperature range, which are considered as one of themost promising TE materials for practical applications. However, there are somebottleneck problems in the field of filled skutterudite, such as the too long preparationtime and the difficulty to further decrease the lattice thermal conductivity. In thisdissertation, an efficient method has been developed to prepare filled skutteruditematerials by a combination of melting-quenching (MQ) and spark plasma sintering(SPS). The preparation period of MQ-SPS method is shortened to less than30hoursfrom9-10days for the traditional preparation method. A new technology wasdeveloped to prepare the filled skutterudite TE materials with micropores. Fe-basedfilled skutterudite material prepared by MQ-SPS method was chosen as a researchobject to investigate the thermal stability of the material was studied. The main resultsare as following:Ba single-filled Fe-based skutterudite bulk materials with nominal compositionBaxFe4Sb12(x=1.0,1.2,1.4,1.5,1.6) were prepared by MQ-SPS method. The effects ofBa filling on composition, microstructure, and TE properties of BaxFe4Sb12wereinvestigated. The results indicated that the quenched materials were rapidly transformedinto the bulk BaxFe4Sb12materials with trace FeSb2and Sb impurities on the conditionof the spark plasma activation. The whole processing time is shortened to less than30hours. The contents of impurities in bulk materials can be significantly reduced byoptimizing the x. Single-phase filled skutterudites were obtained for the samples with x≥1.4. Abnormal grain growth was observed in the samples with x>1.4, which wasinduced by the increase in the content of liquid phase of BaSb3. The transport propertiesmeasurements indicated that the electrical conductivity increased first with increasingthe x, and remained almost unchanged when x>1.4, although the samples with x>1.4contain pores that should decrease the electrical conductivity. The enhanced electrical conductivities are attributed to the reduction in the content of impurity phase of FeSb2,and the unchanged electrical conductivities are due to the enhancement in carriermobility induced by the abnormal grain growth. With increasing the x, the Seebcekcoefficient increased first and then decreased. The increase in Seebeck coefficient isclosely related to the reduction in contents of impurity phases, and the decrease inSeebeck coefficient is due to the increase in the amount of pores. The lattice thermalconductivity first decreased and then increased with increasing the x. The decrease inlattice thermal conductivity is due to the enhanced resonance phonon scattering inducedby the increasing Ba filling fraction, and the increase originates from the weakenedgrain boundary scattering of phonons induced by the abnormal grain growth. Themaximum power factor and minimum lattice thermal conductivity are2.9mW·K-2·m-1and0.63W·m-1·K-1at800K for the sample with x=1.4, respectively. The maximum ZTvalue reached0.62at800K for the sample with x=1.4, increased by34%as comparedwith that of the sample with x=1.0.(Ba,In) double-filled Fe-based skutterudites with nominal compositionBaIn0.5Fe3.7Co0.3Sb12prepared by MQ-SPS method were chosen as research objects. Theeffects of the SPS temperature on composition and TE properties of the (Ba,In)double-filled Fe-based skutterudites were investigated to reveal the crystallizationmechanism of the filled skutterudites during the SPS process. The results indicated thatFeSb was formed in the quenching process because of the incompletely peritecticreaction. In the SPS process, FeSb first reacted with Sb to form FeSb2, and then FeSb2reacted with other quenched phases (BaSb3, InSb, Sb) to form BaIn0.5Fe3.7Co0.3Sb12.FeSb and FeSb2were totally transformed into single-phase filled skutterudites when thesintering temperature was723K. However, FeSb2appeared again due to the Sbsublimation when the sintering temperature was higher than723K. The transportproperties measurements indicated that the material obtained at723K had the highestelectrical conductivity. The material obtained at848K showed the lowest electricalconductivity. The Seebeck coefficient was slightly enhanced with increasing thesintering temperature. The power factor of the material obtained at798K was thehighest, and a maximum power factor of2.8mW·K-2·m-1was obtained at800K. Thethermal conductivity and lattice thermal conductivity first decreased and then increasedwith increasing the sintering temperature. A minimum lattice thermal conductivity of 0.57mW·K-2·m-1was obtained at800K for the material obtained at798K. The ZTvalues first increased and then decreased with increasing the sintering temperature. Themaximum ZT value reached0.61at800K for the material obtained at798K.Taking advantage of the decomposition and volatilization natures of ZnSb at hightemperatures, a new technology was developed to prepare (Ba,In) double-filledCo-based skutterudite materials with microporous structures by a combination ofmelting-quenching, diffusion annealing, SPS and high-temperature annealing. Theeffects of microporous structures on phase composition, microstructure and TEproperties were systematically investigated. The results indicated that the secondaryphase ZnSb in (Ba,In) double-filled Co-based skutterudite materials was decomposedand sublimated completely during the high-temperature annealing process, and formedsome randomly arranged micropores about1-4μm in diameter. Furthermore, a greatamount of nanostructures about20-80nm in size were found in the internal surfaces ofthose micropores. The chemical compositions of the nanostructure were consisted of Baand Sb. The transport properties measurements indicated that the Seebeck coefficients ofthe microporous materials were significantly increased while the electricalconductivities were not obviously deteriorated. The power factors of the microporousmaterials were significantly enhanced. A maximum power factor of4.96mW·K-2·m-1was obtained at800K, increased by6.7%as compared with that of the dense materialwith the same chemical composition. The lattice thermal conductivities of themicroporous materials were dramatically suppressed. A minimum lattice thermalconductivity of0.9W·m-1·K-1was obtained at800K, reduced by26%as comparedwith that of the dense material with the same chemical composition. As a result, the ZTvalues of the microporous materials were substantially enhanced. The maximum ZTvalue of1.36was achieved, increased by22.5%as compared with that of the densematerial with the same chemical composition. The electrical conductivity of themicroporous materials was almost invariable, which was related to the percolation effectof conducting network of filled skutterudites. The increase in Seebeck coefficient isattributed to the electron energy filtering induced by nanostructures in the internalsurfaces of micropores. The dramatic decrease in lattice thermal conductivity originatesfrom the pore-edge boundary scattering of the long-wavelength phonons and theballistic transport nature of phonons. Fe-based filled skutterudite materials with the nominal compositionBaIn0.5Fe3.7Co0.3Sb12prepared by MQ-SPS method were chosen as the research object.The thermal stabilities of phase composition, microstructure and TE properties of thefilled skutterudites materials were investigated during annealing process in vacuumunder773K. The results indicated that the phase composition and microstructure of theinterior part of materials had excellent thermal stability. After having been annealed for30days, the interior part was still single-phase filled skutterudite. However, the surfaceof the material was decomposed due to the the sublimation of Sb. The thickness ofdecomposed layer reached about49μm after30days, which might reach about130μmafter1year. The transport properties measurements indicated that the filled skutteruditematerials prepared by MQ-SPS method had excellent performance stability during thelong-time vacuum annealing process. The ZT value of the filled skutterudite was0.61at800K for as prepared materials, which remained0.62after having been annealed for30days. The variation of the ZT values is very small.更多还原