Investigation On Fabrication And Transportation Property For GeTe Based Thermoelectric Materials

Thermoelectric materials can directly realize the conversion between heat and electric energy for thermoelectric power generation or refrigeration,showing a great prospect in the clean and efficient utilization of energy and semiconductor refrigeration.The power generation or refrigeration efficiency of thermoelectric materials is affected by the Seebeck coefficient,electrical conductivity and thermal conductivity of the materials themselves.The Seebeck coefficient,electrical conductivity and thermal conductivity are coupled with each other,resulting in generally low and difficult to improve the efficiency of thermoelectric power generation.The IV-VI compound semiconductor GeTe with narrow band gap is a kind of intermediate temperature thermoelectric material with excellent performance.Due to a large number of Ge vacancies are generated in the synthesis of GeTe,one Ge vacancy produces two holes,so the high hole carrier concentration of GeTe affects the thermoelectric performance of GeTe.In order to improve the thermoelectric application performance of GeTe,this experiment explored ways to improve the thermoelectric performance of GeTe by different sintering methods and element doping according to the characteristics of GeTe,and carried out the following research:(1)GeTe has a high carrier concentration due to the existence of Ge vacancies,and the Ge/Te atomic ratio has an important effect on the concentration of Ge vacancies.Meanwhile,the thermoelectric properties of GeTe are also greatly affected by the preparation process.In order to reflect the effect of Ge/Te atomic ratio and preparation process on the thermoelectric properties of GeTe,two methods of plasma sintering(SPS)and high pressure sintering(HPS)were used to prepare different atomic ratio of Gextey(x:y=1:1.08,1:1.06,1:04,1:1,1.05:1,1.075:1,1.1:1,respectively).The thermoelectric performance test of the samples showed that the conductivity of the samples with excess Te was high and the Seebeck coefficient was low,while that of the samples with excess Ge was on the contrary.The hole carrier concentration decreased with the increase of Ge/Te atomic ratio.However,the lattice thermal conductivity increases due to the presence of pure Ge in the samples with excess Ge.GeTe obtained by HPS sintering has lower lattice thermal conductivity due to the presence of nanocrystals.Among the samples obtained by the two preparation methods,Ge1Te1 has the highest power factor due to its high Seebeck coefficient and electrical conductivity.The ZTmax value of HPS sintered Ge1Te1 is the highest,reaching 1.37 at 723 K.At the same time,because HPS process has the advantages of low cost,high efficiency and suitable for mass industrial production,this experiment found a preparation process of GeTe based thermoelectric materials with low cost,high thermoelectric performance,stable nanocrystalline polycrystalline.(2)The main problems which GeTe is facing are high carrier concentration and stress caused by crystal transformation during heating process,limiting the application of GeTe.Element doping is an effective means to improve the thermoelectric properties of GeTe.In order to study the specific influence of element doping on GeTe,this experiment studied the thermoelectric properties of GeTe by single element doping/compound of Bi2Te3,Sb2Te3,Mn and In.Among them,Bi2Te3 or Sb2Te3 doped samples were sintered by HPS method,and Mn or In doped samples were sintered by SPS method.It is found that the lattice constants of Bi2Te3/Sb2Te3 doped GeTe after HPS sintering are very different from those of pure GeTe.The space group of Bi2Te3/Sb2Te3 doped GeTe sample is R3m(166),which is different from the rhombohedral phase(R3m(160))and cubic phase(Fm 3 m)of pure GeTe.The Bi2Te3/Sb2Te3 doped GeTe sintered by HPS has nanocrystalline properties.After heat treatment,the thermoelectric properties of the samples are improved.The carrier scattering mechanism of the samples is the phonon scattering mechanism,and the conduction mechanism is the small polaron transition mechanism.For the samples sintered by SPS,15 mol%Mn doping can result in the appearance of cubic phase for GeTe at room temperature,avoiding the stress caused by crystal transformation during heating.However,the carrier mobility of the sample doped with Mn is low,and MnTe will be precipitated when above 670 K,which ultimately leads to that the thermoelectric optimal value above 600 K is lower than that of the undoped GeTe.The conduction mechanism of Mndoped GeTe sample is small polaron transition mechanism,and the carrier scattering mechanism is phonon scattering mechanism.The increases of effective mass,density of states and Seebeck coefficient for In-doped GeTe finally improved the thermoelectric optimal value.The crystal transformation of In-doped GeTe sample occurs at 550 K,and the conduction mechanism is the small polaron transition mechanism.The carrier scattering mechanism is the phonon scattering mechanism before the phase transition,while it becomes the alloy scattering mechanism after the phase transition.When the doping amount of In is 4 mol%,the thermoelectric performance is the highest,and the ZTmax is 1.14.(3)The improvement of thermoelectric properties of GeTe by single element doping is limited,while the multi-element doping can take advantage of different elements to produce a synergistic regulation effect on the thermoelectric performance of GeTe.In this experiment,three co-doping methods of Mn,Se;In,Mn;and In,Sb,Se were used to prepare Ge0.85Mn0.15Te1-xSex(x=0,0.01,0.03,0.06),Ge(1-0.04-x)In0.04MnxTe(x=0,0.05,0.1,0.15)and Ge0.84In0.01Sb0.1Te1-xSex(x=0,0.01,0.03,0.06).The results show that Mn and Se co-doping make the cubic phase GeTe gradually transform into rhombohedral phase,and the carrier concentration increases with the increase of Se content.The XRD peaks and SEM images of the sample show that MnSe is generated.The conduction mechanism of GeTe with the Mn and Se co-doping is the small polaron transition mechanism,and the carrier scattering mechanism is mainly phonon scattering mechanism.Due to the presence of MnSe,the thermoelectric performance does not improve.When In and Mn are co-doped,In doping can reduce the amount of Mn doped into room temperature cubic phase of GeTe caused by Mn doping.When x=0.1,the sample shows the characteristics of cubic phase;when x=0.05,the crystal transformation of the sample occurs at 550 K.The conduction mechanism of GeTe with In and Mn co-doping is the small polaron transition mechanism,and the carrier scattering mechanism is the alloy scattering mechanism.With the increase of Mn doping,the thermoelectric properties of the samples decrease.Finally,the sample with the highest ZTmax value is Ge0.96In0.04Te.The crystal transition temperature of In,Sb and Se co-doped samples is 630 K.XRD results at room temperature show that the samples tend to form cubic or pseudocubic structures.SEM results show that the samples have lamellar morphology.The carrier concentration decreases with the increase of Se content.Ge0.84In0.01Sb0.1Te0.94Se0.06 have the highest Seebeck coefficient,the lowest conductivity and lattice thermal conductivity,the highest ZTmax value.Its ZTmax of 1.61 is achieved at 723 K.In,Sb,Se co-doping is an effective way to improve the thermoelectric properties of GeTe.