Properties Optimization Of GeTe Alloy Based Thermoelectric Material

Thermoelectric materials are attractive for electrical power generation andrefrigeration as a global sustainable energy solution. Improvement forthermoelectric properties requires a better understanding of how the optimalparameters can be achieved in the given system. As a p-type narrow bandsemiconductors, the alloy of GeTe shows excellent electrical transmissionperformance, whose conductivity is determined by vacancies on the Ge sites.These vacancies on the Ge sites affect not only the electric properties viageneration of two holes per vacancy, but also contribute to phonon scatteringwith a reduction in lattice thermal conductivity. This makes GeTe a uniquematrix where doping with various elements can significantly affect multiplemechanisms responsible for the thermoelectric properties.The samples with a given stoichiometric ratio were sealed in the evacuatedquartz tube and then heated at1237K, and subsequently quenched in liquidnitrogen. The quenched alloys were powdered in agate mortar and then ballmilled in a planetary ball mill using a hard stainless steel vial and zirconia balls.The milled powders were consolidated by spark plasma sintering （SPS） at823Kfor5minutes under and axial pressure of48MPa with a peak impulse value of675A to obtain the high density specimen. The density of6.2g/cm3could beattained for the samples in this work.Doping of GeTe with Ag produces a system of Ag8xGeTe1+5x, written as（GeTe）1-x（A_g8GeTe₆）x. The （GeTe）1-x（A_g8GeTe₆）xcomposites with x=0.02,0.05,0.08and0.11were prepared by melting, ball milling and spark plasma sinteringtechniques. The thermoelectric properties of the composites were investigated.The experimental results show that all samples are composites consisting of theprimary solidification phase GeTe and eutectic microstructures with GeTe andA_g8GeTe₆phases. The composites are p-type semiconductors. As compared tothe pure compound GeTe, the existence of A_g8GeTe₆compound in the GeTephase increases its electrical resistivity at low temperatures while decreases itselectrical resistivity at high temperatures. It also decreases its Seebeckcoefficient slightly. However, the thermal conductivities of the composites arereduced significantly. As a result, the figure of merit ZT for the materials can beenhanced. The maximum figure of merit ZT as1.15was obtained in the sample（GeTe）_0.89（A_g8GeTe₆）_0.11at673K.The thermoelectric parameters of electrical resistivity ρ, Seebeck coefficient S, thermal conductivity κ and figure of merit ZT for the Ag_xBi_xGe_1-xTe_1+xcomposites with x=0.05,0.10and0.15are measured in the temperature range of300K to750K. The Seebeck coefficient achieved the value of2.00×10^-4V/Kwhich is two times of the pure GeTe alloy. The maximum value of ZT at thetemperature of723K is obtained as1.23for the sample with x=0.1.