Fabrication And Thermoelectric Properties Of P-type PbTe-Based Composite Thermoelectric Materials

PbTe based thermoelectric material is one of the most attractive thermoelectric materials possessing excellent properties in the middle temperature zone.Due to the relationship between thermoelectric properties with the band and microstructure,we can improve the thermoelectric properties through doping and alloying.On this basis,we fabricate nano-composites by introducing nano phase to the matrix for the purpose of regulating the microstructure of the materials in multi-scale range,tuning the thermoelectric parameters.In addition,the fabrication process is simple and highly productive,which is benefit to fabricate the high performance thermoelectric materials in batch production and low cost,and has profound meanings to broaden the application field of thermoelectric materials.In this research,the melt-quenching-annealing technology was adopted to prepare quaternary alloy of x% Na doping PbTe-y%SrTe.The optimum chemical composition and sintering process were also explored.Based on the optimum chemical composition of Pb_0.98Na_0.02Te-4% SrTe,combined with mechanical milling technology,a lot of nano-composite second phase such as SiC nanoparticles and multi walled carbon nanotubes?MWCNT?were used to composite with the matrix material for controlling the microstructure and the properties.Finally,the composite powders were sintered by spark plasma sintering?SPS?,the microstructure and thermoelectric properties of the composite thermoelectric materials were studied systematically.?1?The x% Na doped PbTe-y% SrTe alloy were obtained through the melt-quenching-annealing technology,whose shapes are irregular and size distribution range is wide.Maintaining the condition of chemical composition and adjusting the sintering temperature and sintering pressure,we successfully obtain the optimum sintering temperature and sintering pressure,which is 550? and 60 MPa respectively,with a series of thermoelectric performance characterization.Under the proper condition,we further explored the effects of Na and SrTe content on the thermoelectric properties of PbTe alloy.Ultimately,we determine the optimal proportion for Pb_0.98Na_0.02Te-4% SrTe,and the matrix maximum ZT can reach 1.53 at 750 K.?2?Based on the above work,the SiC/Pb_0.98Na_0.02Te-4% SrTe composites with different contents of SiC were fabricated by mechanical ball milling method.The characterization methods such as XRD,SEM and TEM were used to characterize the phase composition and microstructure of the composites.Results show that the density decreases after adding a certain amount of SiC to the matrix and the thermal conductivity of matrix can significantly reduce.When the SiC content is 0.5 vol.%,the composites achieve the lowest thermal conductivity value,which is reduced by 28% compared with the matrix.Thus,the ZT values of composite materials are improved compare to Pb_0.98Na_0.02Te-4% SrTe matrix,and when SiC content is 0.25 vol.%,the composites achieved the maximum ZT value of 1.73 at 750 K,which is 11% higher than that of Pb_0.98Na_0.02Te-4% SrTe matrix.?3?The MWCNT/Pb_0.98Na_0.02Te-4% SrTe composites with different contents of MWCNT were fabricated by mechanical ball milling method.We studied the effects of MWCNT contents on Pb_0.98Na_0.02Te-4% SrTe phase composition,microstructure and thermoelectric performance.Results indicate that MWCNT has great impact on the thermal conductivity of the composite.The thermal conductivity of MWCNT composites are lower than that of the matrix,and with a increase of MWCNT content,the thermal conductivity shows a decreasing trend.When the content of MWCNT is 2.0 vol.%,the thermal conductivity of the composites can reach 0.85W/?m·K?at 700 K,which is lowest and 15% lower than the matrix.