Optimizing Thermoelectric Performance Of GeTe Alloys Through Doping

Based on the Seebeck effect,the thermoelectric device enables direct conversion between thermal and electrical energy with the advantages of no pollution,high reliability,and long life,leading to prominsing potential application in future.However,the thermoelectric conversion efficiency between thermal and electrical energy is much lower than the traditional energy conversion method,making it imminent to optimize the performance of thermoelectric materials.As a narrow band gap semiconductor,GeTe alloys are typical moderate temperature thermoelectric materials and have shown excellent thermoelectric properties without the toxicity.Due to the large number of Ge vacancies,the hole concentration of GeTe always exceeds 1021 cm-3,which is much higher than the typical materials,resulting in a low ZT value about 1.2.To further enhance the thermoelectric properties of GeTe alloys,we attempted to reduce the carrier concentration or regulate the band structure through doping,and simultaneously examine its impact on the lattice thermal conductivity.In order to reduce the carrier concentration,we doped Zn,Pb,Cd respectively into the GeTe matrix,hoping that these atoms would occupy the Ge vacancies for lower hole density and then optimize the three parameters of Seebeck coefficient,electrical conductivity and thermal conductivityfor higher ZT values.Then,the ZnxGe1-xTe(x=0.01,0.02,0.03,0.04),Zn0.03PbyGe097-yTe(y=0.01,0.03,0.05,0.07,0.09)and CdzGeuzTe(z=0.001,0.02,0.03,0.04)bulk thermoelectric materials are prepared by high energy ball milling and hot pressing technology,respectively.The results showed that the power factor changed little,but the thermoelectric ZTs have been optimized to some extent,yvhich mainly came from the reducing of the thermal conductivity.At 773 K,the Zn0.02Ge0.98Te,Zn0.03Pb0.07Ge.90Te and Cd0.04Ge0.9%Te sample has peak ZT of 1.4,1.63 and 1.53,respectively,showing about 15?30%improvement than the pure GeTe sample.For the purpose of ZT improving by increasing the hole concentration,Al was introduced into GeTe in this work.Previous reports showed that A1 doping in ?-? compounds(PbSe,SnTe)are capable of forming a resonant level in conduction or valence band,which significantly optimizes the thermoelectric properties of materials.However,in the same family GeTe alloy,the effects of A1 doping on the thermoelectric properties have not been studied yet The AlrGe1-rTe(r=0.01,0.02,0.03,0.04,0.05)samples were fabricated by ball milling and hot pressing,and the influence of A1 on thermoelectric properties were explored.The study showed that Al doping in GeTe brought reduced electrical conductivity and increased Seebeck coefficient,resulting in optimized power factor.In addition,the lattice was distorted after doping Al atoms into the GeTe crystal lattice,which led to an evident reducing of the thermal conductivity and large improvement of thermoelectric performance.When r=0.04,thermoelectric figure of merit of AlrGe1-rTe samples reached the maximumof 1.47 at a temperature of 773 K.This study shows that doping GeTe with Zn,Pb,Cd elements would reduce the thermal conductivity of the material.At the same time,doping with Al can not only enhance the power factor,but also reduce the thermal conductivity.Through all these doping,the thermoelectric properties of GeTe were improved 15?30%.