【作者】 鄢永高；

【导师】 唐新峰；

【作者基本信息】 武汉理工大学 ， 材料学， 2007， 博士

【摘要】 理论计算和实验研究都表明，结构低维化和纳米化可以大幅度地提高材料的热电性能，AgPb_mSbTe_2+m族化合物代表了一类包含纳米第二相的复合材料，具有潜在的高热电性能而在热电材料研究领域受到了极大的关注。已有的研究结果表明，这类化合物的高热电性能可能与富含AgSb的纳米第二相有关，这类化合物的热电性能对制备工艺和组成的微小变化十分敏感。因此，研究这类化合物组成、制备工艺对微观结构和热电传输性能的影响规律显得尤为重要。本论文主要讨论了不同制备工艺以及偏离化学计量比对材料相组成和热电性能的影响规律，同时探索了采用Ga和In取代Sb制备具有类似结构高性能热电材料的可行性。主要工作和结果如下：采用熔融法制备了AgPb_mSbTe_2+m（m=4，6，10，12，23，48，99，199）化合物，研究了AgSb掺杂量对化合物结构和热电传输性能的影响规律。结果表明，AgSb的掺杂显著影响了PbTe化合物的微观结构和电热输运特性，一定量的AgSb掺杂有助于提高PbTe材料的热电性能。AgSb掺杂后，样品中出现了明显的成分偏析，形成了富Pb相和富AgSb相调幅变化的结构。随着AgSb含量的逐渐增加，n型化合物电导率和功率因子均呈现下降的趋势，Seebeck系数绝对值先增加后降低，化合物的热导率和晶格热导率大幅降低，m值在10～23之间的样品具有最佳的热电性能。在所有样品中，AgPb₂₃SbTe₂₅化合物常温下具有最大的ZT值。研究了不同的熔体冷却速率对AgPb₁₈SbTe₂₀化合物结构和热电性能的影响规律，并在此基础上探索了粉末冶金工艺制备单相高性能AgPb_mSbTe_2+m化合物的可行性。研究表明，冷却速率越大，化合物成分偏析越严重，淬火样品中，作为第二相而存在的AgSb使化合物热电性能降低。采用熔融-淬火-退火-SPS烧结工艺制备了AgPb_mSbTe_2+m（m=18，10，6）样品，m为10和18时，淬火样品经过450℃退火40h及SPS烧结后可得到基本为单相的化合物，两者电导率和Seebeck系数相近，且几乎不随温度而改变，由于后者具有更低的热导率，在800K时具有最大ZT值0.90。研究了Ag、Pb、Sb偏离化学计量比对AgPb₁₈SbTe₂₀样品热电传输特性的影响规律。采用熔融-淬火-SPS烧结工艺制备了Ag含量偏离化学计量比的Ag_1-xPb₁₈SbTe₂₀（x=0～0.75）样品，研究表明，当Ag含量下降时，部分Sb以第二相Sb₂Te₃存在，样品的载流子浓度增加，当Ag含量低于0.5后，样品载流子浓度增加到5×10¹⁸cm^-3左右后不再增加，样品热导率和晶格热导率随Ag含量的降低而增加，Ag_0.25Pb₁₈SbTe₂₀样品在520K时具有最大的ZT值0.5；采用熔融缓冷法制备了Pb、Sb过量的AgPb_18+xSbTe₂₀和AgPb₁₈Sb_1+xTe₂₀样品，结果表明，Pb含量增加时，Te优先和Pb结合而使PbTe基体中固溶的AgSb含量降低，多余Ag和Sb以单质或二元化合物形式存在。Pb含量超过化学计量比1％时有助于提高材料电性能，而Sb高于化学计量比使样品的电性能下降。采用熔融缓冷法制备了（AgIn/Ga）_xPb_1-2xTe化合物，探索了AgIn和AgGa共同掺杂时对PbTe化合物相组成和热电性能的影响规律。结果表明，AgInTe₂在PbTe中的最大固溶度为4％，而AgInGa₂在PbTe中的固溶浓度低于1％。AgIn含量增大时，样品载流子浓度基本保持不变，而迁移率和电导率降低，样品均为n型传导，Seebeck系数的绝对值随AgIn含量的增加而增大，样品热导率随AgIn含量的增加而显著降低。AgIn和AgGa的共同掺杂有效的提高了PbTe材料的热电性能，Ag_0.01In_0.01Pb_0.98Te样品在800K具有最大的ZT值1.1；而Ag_0.01Ga_0.01Pb_0.98Te样品在700K时样品具有最大的ZT值1.12，ZT值比传统的n型PbTe材料提高了近40％。更多还原

【Abstract】 Both theoretical calculation and experimental results have showed that great enhancement in thermoelectric properties can be obtained for thermoelectric materials charactered with low-dimensional and nano structure. As a kind of composite consisting of nanoscaled second phase, AgPb_mSbTe_2+m compounds have potential high thermoelectric figure of merits and attract much attention recently. The reported researches and results show that high ZT value of this series of compound possibly relates to nanoscaled second phase rich in AgSb, and the electrical and thermal transport properties are sensitive to preparation conditions and chemical composition. In this thesis, the effect of the fabrication process and nonstoichimetric composition on the microstructure and thermoelectric properties of the compounds were investigated. Also it is explored to synthesize material with similar structure and excellent thermoelectric properties by substituting Sb with Ga or In. The following results have been obtained in this thesis.AgPb_mSbTe_2+m （m=4, 6, 10, 12, 23, 48, 99, 199） samples were prepared from melts and the effect of AgSb concentration on the microstructure and thermoelectric properties of AgPb_mSbTe_2+m compounds have been investigated. Results show that Ag-Sb-doping significantly affects the microstructure and electrical and thermal transport properties, and a proper AgSb concentration can enhance the thermoelectric properties of PbTe compound. Obvious phase segregation occurs during cooling process of the ingots, and samples contain compositional modulation of Pb-rich phase and Ag-Sb-rich phase. As the AgSb concentration increases, the electrical conductivity, and power factor of the n-type samples show a decreasing trend, and the absolute Seebeck coefficient increase firstly and then decrease; also the thermal conductivity and lattice thermal conductivity of the samples decrease dramatically. The samples have the highest thermoelectric properties for m in the range from 10 to 23, and AgPb₂₃SbTe₂₅ samples have the maximum ZT value at room temperature among all samples.Effects of cooling rates on the microstructure and thermoelectric properties of AgPb₁₈SbTe₂₀ compounds have been investigated, and the feasibility to prepare single-phase high performance AgPb_mSbTe_2+m compounds by powder metallurgy technology have also been explored. Results show that phase segregation happens more seriously in samples synthesized with faster cooling rate. Quenched samples have poor thermoelectric performance with a large amount of AgSb existing as a second phase in the samples. AgPb_mSbTe_2+m（m=18, 10, 6） samples were prepared by melt-quench-anneal-spark plasma sintering （SPS） process. Results show that the second phase produced in quenching process can be eliminated by annealing at 450℃for 40h and SPS when m equals 18 and 10 for AgPb_mSbTe_2+m samples. The two samples have similar electrical conductivity and Seebeck coefficient, both nearly independent on temperature. Owing to its lower thermal conductivity, samples with m=10 have higher ZT value during the whole measuring temperature range,, and the maximum ZT value reach 0.90 at 800K.Effects of nonstoichimetric AgPb₁₈SbTe₂₀ compounds in content of Ag, Pb and Sb on the microstructure and thermoelectric properties of this compound have been discussed. In this part, Ag_1-xPb₁₈SbTe₂₀ （x=0-0.75） samples were prepared by melt-quench-SPS process. Results show that, as the Ag content decreases, part of Sb exists as a second phase Sb₂Te₃; the carrier concentration increases and reaches to a maximum value of about 5×10¹⁸cm^-3, when x equals 0.5. The thermal conductivity and lattice thermal conductivity increase as Ag content decreases. Ag_0.25Pb₁₈SbTe₂₀ sample has a maximum ZT value of 0.5 at 520K. N-type AgPb_18+xSbTe₂₀ and AgPb₁₈Sb_1+xTe₂₀ compounds were prepared by slow cooling from melts. Results indicate that with the increasing of the Pb concentration, Te combines firstly with excessive Pb leaving corresponding Ag and Sb existing as elements or their binary compounds in the material. The electrical properties can be enhanced for samples with 1% excessive Pb content whereas excessive Sb makes the compounds thermoelectric properties decrease.（AgIn/Ga）_xPb_1-2xTe compounds were prepared by melting and slow cooling method, and the effects of codoping of AgIn and AgGa on the phase consititution and thermoelectric properties have been investigated. Results show that AgInTe2 can solve in PbTe with a maximum solid solubility of 4%, whereas the solubility of AgInGa₂ in PbTe is less than 1%. As Ag-In content increases, the carrier concentration remains unchanged while the carrier mobility and electrical conductivity decrease. As Ag-In content increase, the absolute Seebeck coefficient increase and the thermal conductivity decrease dramatically. The co-doping of AgIn and AgGa can effectively enhance the thermoelectric properties of the PbTe compound. Ag_0.01In_0.01Pb_0.98Te and Ag_0.01Ga_0.01Pb_0.98Te samples have maximum ZT value of 1.1 and 1.12 at 800K and 700K, respectively. Compared with the traditional n-type PbTe materials, these values are increased by 40%.更多还原