| 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. |
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