Fabrication And Thermoelectric Properties Of AgPb_mSbTe_2+m Compounds

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%.