Properties Characterization Of Doped Ag_0.8 Pb_m SbTe_m+2 Thermoelectric Materials Via Rapid Hot-pressing

With the NaCl crystal structure, Ag_0.8Pb_mSbTe_m+2 (LAST alloy) thermoelectric materials possess the thermal and electrical transfer properties similar to that of low-dimensional structure materials and marked thermoelectric performance index, making them the target of widespread investigations as medium temperature (500⁸00 K) thermoelectric materials in recent years. The thermoelectric properties can be enhanced to a large extent by optimization of the material composition and preparation process, combined with grain refinement and increased density. Therefore, it is of great importance to investigate the influence of material composition and preparation process on the microstructure and the resultant thermoelectric performance of the materials.In the present work, n-type Ag_0.8Pb_mSbTe_m+2 (m=12, 14, 16, 18) and Ag_0.8Pb₁₈In_xSb_1-xTe₂₀ (x=0.25, 0.5, 0.75, 1) alloy powders were fabricated via vacuum sealed melting technique, followed by high energy ball milling. Rapid hot-pressing sintering was employed to prepare bulk materials and the thermoelectric properties were optimized by doping methods. The products prepared by hot-pressing sintering were characterized by using X-ray diffraction and scanning electron microcopy and the influence of process parameters of hot-pressing sintering on the resulting microstructure, electrical conductivity, Seebeck coefficient and power factor were systematically investigated. The work herein suggests the following results:1. The results suggest that Ag_0.8Pb_mSbTe_m+2 (m=12, 14, 16, 18) alloy powders posess the face centered cubic structure and the relevant diffraction peaks shift towards large-angle side with the decreasing of the value of m compared to PbTe, indicating that both Ag and Sb have been successfully doped into the PbTe matrix. After high energy ball milling, the resulting powders achieved micron size, providing extra sintering impetus and thus favoring the sintering process, resulting in a high relative density of 94%.2. The rapid hot-pressing sintering process was optimized by a systematical investigation of the influence of the relevant process parameters on the material density and grain growth behavior, including sintering temperature, time, pressure, etc. The corresponding optimum process conditions were confirmed to be: a sintering temperature of 673 K, a sintering period of 30 min, and sintering pressure of 20 MPa with a concomitant heating rate of 70 K/min. The as-obtained products possessed high density and fine grains, most of which have a particle size less than 3μm.3. For Ag_0.8Pb_mSbTe_m+2 thermoelectric materials, with the increasing content of Ag and Sb, the electrical conductivity decreased whereas Seebeck coefficients increased first and subsequently declined. The results suggested that the resulting materials had high Seebeck coefficients with a maximum of -634μV/K and a minimum of -407μV/K. The maximum power factor of 7.5×10^-4 Wm^-1K^-2 was obtained at 548 K with m=18.4. The results show that Ag_0.8Pb₁₈In_xSb_1-xTe₂₀ (x=0.25, 0.5, 0.75, 1) alloy powders the face centered cubic structure and in comparison with PbTe. All diffraction peaks showed a slight shift towards small-angle side with the increasing value of x, confirming the successful doping of In into Ag_0.8Pb₁₈SbTe₂₀. The relevant optimum process conditions were determined to be: a sintering temperature of 693 K, a sintering period of 30 min, and sintering pressure of 15 MPa combined with a heating rate of 70 K/min, yielding products with fine grains and high density exceeding 96%.5. The thermoelectric performance of Ag_0.8Pb₁₈SbTe₂₀ based materials was improved by doping of In. Together with the increasing In, the decreased content of Sb improved the electrical conductivity and also decreased the Seebeck coefficient which achieved a maximum of 3.1×10^-3 Wm^-1K^-2 at 623 K for an x value of 0.5.In summary, good thermoelectric performance of Ag_0.8Pb_mSbTe_m+2 was obtained by using rapid hot-pressing methods and the thermoelectric performance of Ag_0.8Pb₁₈SbTe₂₀ based materials was improved by the doping of In. The work may provide a new avenue for the promotion of thermoelectric properties of such materials.