HPHT Synthesis Investigations Of High Performance Thermoelectric Materials

Thermoelectric material is one kind of function materials that can directlyconvert electricity and heat. The thermoelectric materials could be used asgenerators and refrigeration. The thermoelectric generators can convertterrestrial heat and the waste heat of industry to electricity directly;Thermoelectric refrigeration devices use electricity to pump heat from cold tohot, both without any moving parts or bulk fluids. They are lightweight, small,portable, inexpensive, quiet performance and the ability for localized'spot'cooling. Thus thermoelectric materials are wildly used.Presently, the use of TE devices is limited by their low efficiencies. Up todate, thermoelectric materials were mostly used in the field of national defenceand high-tech. The efficiency of a TE device depends on the TE material. Theefficiency of a TE material can be defined by the dimensionless thermoelectricfigure of merit, ZT, where T is the absolute temperature and Z =σκS2(S is thethermopower,σis the electrical conductivity andκis the total thermalconductivity). In the function,σS2 or S2/ρ(ρis the electrical resistivity) is thepower factor which characters the electric properties.There are now several good reasons to renew the quest for superior TE materials, with worsening environment and energey source. Several approacheshave been adopted in the research for the improved thermoelectric materials,ranging from the synthesis of new bulk materials with complex structure andquantum-well structures that may exhibit improved ZT to combinatorialsynthesis techniques that rapidly screen materials for desirable thermoelectricproperties. Pressure tuning may offer a new mean.The group of J. V. Badding in Pennsylvania State University and Sergey VOvsyannikov in Russian Academy of Sciences found that the power factor and figure ofmerit for many materials could be improved largely. For example, the power factor ofPbTe based materials could be improved 107 times. Unfortunately, these highthermoelectric performance can not be kept after unload the pressure. While the hightemperature combined with high pressure could resolve this defect for high pressuresolely.About high pressure and high temperature synthesized TE materials, DoctorZhu first prepared TE materials used cubic multi-anvil high pressure apparatus.PbTe samples that have NaCl construction were synthesized under pressureranging 3.0-5.0GPa and temperature ranging 900-1000℃and time ranging 10-30 minuter. His results showed that the electrical properties of PbTe sampleswere modulated HPHT and the thermoelectric performance was improvedlargely. The in-stiu measurement of PbTe under HPHT shows that the highperformance of PbTe under high pressure was kept by quenching successfully. Inaddition, the PbTe doped with Bi, Sb and I were studied and the TE propertieswere improved farther.Up to date, there is slightly report on thermoelectric materials except PbTebased materials synthesized by HPHT. As mention above, the best bulk TEmaterials are the compound with complex structure. For example: LAST(AgPb18SbTe₂0) and TAGS (GeTe-AgSbTe₂) which contain all four elements. In addition, they could be regarded as the alloys of AgSbTe₂. The ternarycompound AgSbTe₂ has the same crystal structure (Fm3m) with PbTe. AgSbTe₂is one kind of thermoelectric material with high performance which was foundin 1960s. But there was found that this material has many drawbackssynthesized with normal methods. For example: it is difficult to obtain singlephase AgSbTe₂; the TE properties is not uniform; it is difficult to doping and theelectrical resistivity is so high that harms the TE performance. According to thestudies about PbTe early, the HPHT method many advantages, such as restrainingthe disorder, phase separation and other complicating factors during the preparation formaterials.First, we prepared ternary TE materials AgSbTe₂ by vacuum melting andHPHT methods. The measurement of XRD shows that the sample prepared byvacuum melting contains many other phases such as Ag2Te and Sb2Te3. Whilethe HPHT synthesized samples are all near single phase except a small mount ofSb2Te3. The SEM-EDS show that the sample prepared at HPHT are moreuniformly than that of the same sample prepared by vacuum melting.The electrical resistivity and Seebeck coefficient of AgSbTe₂ synthesized byHPHT were measured at room temperature. The measured results show that theelectrical resistivity and Seebeck coefficient decrease with an increase ofpressure. The power factor of AgSbTe₂ increases with an increase of syntheticpressure. In order to study the mechanism of AgSbTe₂ under high pressure, thein-stiu measurement of electrical resistivity was used. The result show that theresistivity of the AgSbTe₂ sample prepared by vacuum melting decreases with anincrease of pressure, however, regretfully, low electrical resistivity under highpressure returns again to the primary state once unloading the pressure. The in-stiumeasurement results and the HPHT synthesis experimental results indicate thatAgSbTe₂ sample has low electrical resistivity under high pressure, which is partially kept by HPHT quenching.Nonstoichiometric AgSbTe₂ with excessive Ag2Te (with the expression as(AgSbTe₂)1-x(Ag2Te)x) were prepared by high pressure and high temperature (HPHT)method. The samples are near single phase AgSbTe₂ with a small quantity of impuritiesincluding Sb2Te3 and Ag2Te. The measurements of electrical properties at roomtemperature show that the Seebeck coefficient and the electrical resistivity for(AgSbTe₂)1-x(Ag2Te)x decrease with an increase of the synthetic pressure and x, whichindicate that high pressure combining with alloying with Ag2Te could modulate theelectrical properties of AgSbTe₂ effectively.Nonstoichiometric AgSbTe₂ with excessive Sb2Te3 (AgSbTe₂)1-x(Sb2Te3 )x) werealso studied under HPHT. The results show that the resistivity of AgSbTe₂ wasdepressed by the effect of high pressure and the doping of Sb2Te3. In addition, theSeebeck coefficient was improved when doped with a small quantity of Sb2Te3. So thepower factor was improved effectively by alloying with a little of Sb2Te3. Thetemperature dependence of thermoelectric performance was studied for theAg0.9Sb2.1Te₂.2 prepared at 2.0 GPa which has the largest power factor at roomtemperature. The temperature dependent electrical properties indicate that the sample isdegenerate semiconductor. The thermal conductivity is much lower than that of PbTe andBi2Te3 and similar to the AgSbTe₂ sample prepared at normal pressure. The largestfigure of merit ZT=1.03 was obtained at 500K, which is match to the state of artthermoelectric materials.The alloy of AgSbTe₂ and PbTe (Ag0.8Pb18SbTe₂0) were synthesized by HPHTquenching. The samples were single phase with the structure of NaCl. The pressuredependence of thermoelectric properties were studied at room temperature, which hasthe similar effect to that of AgSbTe₂. Especially, the Seebeck coefficient and electricalresistivity dropped suddenly at the pressure of 4.0 GPa. This pressure may be thechanging pressure of semiconductor to semimetal. Thermoelectric materials Ag0.8Pb18SbTe₂0-xSex were studied under high pressure. Themeasurement results show that pressure and alloying with Se have the similar effect onthe electrical properties for AgPb18SbTe₂0. The reason should be contribute to thedifferent ionic radii of Te and Se which is 0.142 and 0.122nm respectively. The chemicalinternal stress should be improved when the Te atom was substituted by Se, which isequal to the effect of exterior pressure. Large power factor and lower thermalconductivity was obtained for the sample of Ag0.8Pb18SbTe10Se10. The high figure ofmerit, ZT reach to 0.43 at room temperature which is nearly to the result of Hus report atScience (0.44).Skutterudites compound CoSb3 were studied by HPHT. Single phase CoSb3 could besynthesized within just 20 minutes, which indicates that HPHT is effective method tofabricate this kind of compound. The measurement of transport properties show that theSeebeck coefficient and resistivity increase with an increase of synthetic pressure. Thecalculation of the band structure studies show that the band gap increases with anincrease of pressure which may be source of decreasing of carrier concentration.New kind of TE materials Zn4Sb3 were fabricated by HPHT successfully. Theelectrical properties measurements show that the TE performance of Zn4Sb3 was nearlynot impacted by high pressure.PbSe with NaCl structure were prepared by HPHT. The electrical resistivity decreaseswith increase of pressure up to 4.5 GPa and then increase largely. The largest powerfactor at room temperature reach to 29μWcm-1K-2 for the sample prepared at 4 GPawhich is much higher that of the same sample prepared by the method at ambientpressure. The temperature- dependent TE properties study show that the PbSe sampleprepared at 4 GPa is a degenerate semiconductor. The maximum figure of merit,ZT=0.88 was obtained at 530K which matches to that of the state of art TE materialPbTe.The transport properties and thermoelectric performance of PbTe doped with Bi2Te3 and Sb2Te3 at HPHT were studied. The carrier concentrations increase with the dopedcontent. The Seebeck coefficient and resistivity were sensitive to the dopant, whichdecrease largely when doped with trice of Bi2Te3 and Sb2Te3. The phonon thermalconductivity decreases with an increase of the content of dopant. High figure of merit0.28 and 0.26 were obtained at room temperature.In conclusion, the results obtained in our work indicate that HPHT is an effectivemethod for synthesizing the TE materials. The TE performance for the materials with thestructure of NaCl could be improved by high pressure according to the results by now.