高压热电材料

Journal of Alloys and Compounds460(2008)8–12Effects of high-pressure high-temperature treatment on thethermoelectric properties of PbTeMichael A.McGuire a,1,Abds-Sami Malik b,Francis J.DiSalvo c,∗a Department of Physics,Clark Hall,Cornell University,Ithaca,NY14853,USAb Diamond Innovations,Worthington,OH43085,USAc Department of Chemistry and Chemical Biology,Baker Laboratory,Cornell University,Ithaca,NY14853,USAReceived21January2007;accepted22May2007Available online25May2007AbstractChanges in the thermoelectric properties of nominally undoped stoichiometric PbTe caused by high-pressure high-temperature(HPHT)treatment near6.5GPa and900◦C are parison of the electrical resistivity,thermopower,and thermal conductivity suggest that the carrier concentration and lattice defect density are decreased by HPHT treatment.Annealing the treated samples at intermediate temperatures(500–600◦C) reversed the changes.These observations are consistent with Pb vacancies being“squeezed out”by the applied pressure,and subsequently reintroduced by annealing.©2007Elsevier B.V.All rights reserved.Keywords:Semiconductors;Point defects;Electrical transport;Heat conduction1.IntroductionGood thermoelectric(TE)performance requires a specific combination of transport properties[1].In particular,the effi-ciency of a material for TE conversion increases monotonically with the parameter Z=S2/ρκ,where S is the Seebeck coefficient or thermopower,ρthe electrical resistivity,andκis the thermal conductivity.Often Z is quoted as the dimensionless product ZT,where T is the absolute temperature,and ZT values near1 are typical for good TE materials near their optimal operating temperature[2].Lead Telluride(PbTe),which adopts the rock salt crystal structure,is a good TE material when optimally doped with ZT values close to1near400◦C.Although this material has been extensively studied for many years,researchers are still working toward improving the TE performance of PbTe.Recent reviews of the TE properties of PbTe[3]and its use in TE power generation devices[4]have been published.As with other TE materials,considerable work has been devoted to ∗Corresponding author.Tel.:+16072557238;fax:+16072554137.E-mail address:fjd3@(F.J.DiSalvo).1Present address:Department of Chemistry,Frick Laboratory,Princeton Uni-versity,Princeton,NJ08644,USA.understanding the effects of synthetic techniques and materials processing on the properties of PbTe,including single crystals [5],thin-films[6],andfine grained materials[7].Since many TE devices are made from sintered polycrystalline materials, the effects of the hot-pressing process are of particular rele-vance.Recently,large improvements in the TE properties of PbTe synthesized under high-pressure high-temperature(HPHT)con-ditions have been reported[8].In contrast to hot-pressing,in which powdered PbTe is sintered under applied pressure at ele-vated temperatures(but below the melting point of PbTe which is near910◦C),the researchers loaded Pb and Te powders into a pressure cell and reacted them to form PbTe under pressure at high temperature.The reported result is a significant improve-ment in ZT at room temperature and pressure,which changed from about0.04for a cast sample to0.87for a sample made at the highest pressure(5.2GPa).Here we report the effects of post-synthetic HPHT treatment on the TE properties of nominally undoped,stoichiometric PbTe. Significant changes in the measured properties are observed after HPHT treatment,which can be partially reversed by annealing the HPHT treated samples.We propose that the observed effects are related to vacancies in the PbTe struc-ture,which may prove to be important in controlling the carrier0925-8388/$–see front matter©2007Elsevier B.V.All rights reserved. doi:10.1016/j.jallcom.2007.05.072M.A.McGuire et al./Journal of Alloys and Compounds460(2008)8–129concentration in doped PbTe TE materials prepared by hot-pressing.2.Experimental detailsInside an argonfilled glove box,pieces were broken away from a high purity Te ingot(99.9999%,Johnson Matthey)and cut from a high purity Pb rod (99.999%,ASARCO),and loaded into silica tubes in a1:1molar ratio(total sample mass of10g per tube).A conservative estimate of the uncertainty in the loaded compositions,assuming an error of±0.5mg in weighing the starting materials,is about±0.01%.After removal from the glove box the tubes were quickly attached to a vacuum line for sealing,minimizing exposure of the reac-tants to air.The sealed tubes were then heated in an electrical box furnace.The best PbTe ingots,which included the fewest and smallest voids,were made by heating at930◦C for1h and then cooling to850◦C over6h.Powder X-ray diffraction and electron microprobe analysis showed the product to be single phase PbTe.A rectangular parallelepiped sample was cut from the ingot using a dia-mond wire saw,and subsequently used for TE property measurements.HPHT treatment of a coarse powder of this same material was performed at Diamond Innovations using a hydraulic press at∼6.5GPa and∼900◦C.Powder X-ray diffraction under ambient conditions after HPHT treatment confirmed that the sample was still single phase with the NaCl structure type.The sample was cut into several rectangular bars using wire electrical discharge machining(EDM) at Diamond Innovations.The surfaces of the bars were cleaned with SiC paper prior to characterization.Inspection of polished surfaces of the samples by elec-tron microprobe in composition mode showed no significant difference between the cast sample and the HPHT treated material(Fig.1).Annealing of the HPHT treated samples was carried out in evacuated,sealed silica tubes in an electrical box furnace.Before annealing,the surfaces of the pellets were cleaned with a file and SiC paper and rinsed with hexanes to remove any contamination from the application of the electrical and thermal contacts.Preliminary attempts to synthesize PbTe from Pb and Te powders under HPHT conditions following Zhu et al.[8]were not position maps and wavelength dispersive electron microprobe analysis showed the prod-ucts of these reactions to be multiphase,made up of a PbTe matrix incorporating Te rich regions near the composition“PbTe3”and Pb rich regions containing Pb,Te and O(Fig.1).This is likely due to the presence of an oxide layer on the surface of thefine Pb powder starting material.Measurements of TE properties were performed over the temperature range of100–300K using the home built apparatus described elsewhere[9].Copper was plated onto the ends of the samples so that good thermal and electrical contacts could be made using indium metal for the measurement of thermal conductivityκ,thermopower S,and electrical resistivityρ.The voltage contacts for the resistivity measurements were made usingfine gauge copper wire and silver paste(Dupont Conductor Composition4922N).Based on measurements of reference materials with similar values of S andκ,and on errors associated with geometrical measurements,conservative estimates of the uncertainties on these measurements are given as±3%for S,±5%forρ,and±15%forκ. 3.Results and discussionThe TE properties of two bars cut from the HPHT treated material are shown in Fig.2,along with the data from the cast sample before HPHT treatment for comparison.We willfirst address the properties of the cast sample,and then discuss the effects of the HPHT treatment.The positive values of S and the low values ofρwhich increase with temperature indicate that the cast material may be best described as a heavily p-doped semiconductor.Stoichio-metric PbTe is a small band gap semiconductor and is expected to be n-type since the mobility of electrons in the Pb-based conduc-tion band is greater than that of holes in the Te-based valence band.The observed p-type behavior of the cast samplemay Fig.1.Electron microprobe(JEOL8900,15kV,20nA)composition maps for conventionally synthesized PbTe before(a)and after(b)HPHT treatment,and for HPHT synthesized PbTe(c).The light grey regions in all three maps are PbTe. The medium grey regions in(c)are Te rich with compositions near“PbTe3”. The dark grey regions in(c)contain Pb,Te,and O.indicate the presence of Pb vacancies.The congruently melt-ing composition in the Pb–Te phase diagram has been reported as PbTe1.003[5],equivalent to about5×1019Pb vacancies per cubic centimeter.If each Pb vacancy acts as a p-type dopant, degenerate behavior is expected,as observed in the cast sample.10M.A.McGuire et al./Journal of Alloys and Compounds460(2008)8–12Fig.2.The measured electrical resistivityρ,thermopower S,and thermal con-ductivityκof conventionally synthesized PbTe and two samples of the same material after HPHT treatment.Also shown is ZT=S2T/ρκcalculated from the measured data.Fig.2shows that HPHT processing had little effect on themeasured thermal conductivity,but dramatically changed S and ρ.The resistivity increased by over two orders of magnitude, and the temperature dependence changed to display more of anactivated type of behavior.HPHT treatment changed the sign ofthe thermopower,indicating that the conduction is dominatedby electrons in the HPHT processed sample.Due to the largeincrease in resistivity,ZT of the HPHT treated material is signif-icantly diminished,from about0.2for the cast sample to about0.005for the HPHT treated sample at300K.In situ measurements of Pb chalcogenides under pressures up to∼10GPa have revealed changes inρand S of similar magnitude to those shown in Fig.2[10,11].In those investi-gations,samples were subjected to pressure,without heating, and it was found that the changes were reversible upon release of pressure.The observed changes inρand S with increas-ing pressure were attributed to a narrowing of the band gap and subsequent metallization,followed by a structural phase transition near6GPa.At this pressure the cubic lattice of PbTe distorts to orthorhombic symmetry.Much of the earlier literature ascribes the GeS structure-type to this phase;how-ever,recent synchrotron powder diffraction experiments have shown that it is a different orthorhombic modification[12].It is proposed that this structural phase transition is electronically driven,resulting in the opening of a band gap and a transi-tion back into a semiconducting state with a high thermopower [10,11].We believe that the changes we observe after HPHT treatment(Fig.2)arise from a fundamentally different mecha-nism than those observed at high pressure in previous investi-gations.The n-type conduction,larger thermopower,higher resistiv-ity,and activated behavior of the material after HPHT treatment suggest that these samples are closer to intrinsic PbTe than the cast material(Fig.2).We propose that this may be attributed to the“squeezing out”of Pb vacancies from the cast material.To test this hypothesis,a HPHT treated sample was annealed for 12h at500◦C,and then again for12h at600◦C.The TE proper-ties of the sample were measured after each anneal.It is expected that annealing should allow vacancies to diffuse back into the bulk.The samples were not visibly changed by the annealing processes.No sign of grain growth was seen on the sample sur-faces,and no vapor transported material was observed in the silica tube.The results of the properties measurements after annealing are shown in Fig.3.Annealing the samples is seen to have decreased the resistivity to values and behavior intermediate between the cast material and HPHT treated samples.After annealing,the thermopower was again positive,but greater than that in the cast sample.These observations indicate that the density of p-type dopants increased during annealing,but not up to the level present in the cast sample.This is consis-tent with Pb vacancies beginning to diffuse back into the bulk of the material from the sample surface or the grain bounda-ries.The thermal conductivity of the annealed samples is lower than that of the cast sample and the HPHT treated samples (Fig.3).It is important to note that the measured values shown in Fig.3are the total thermal conductivities,which include contributions from the lattice as well as the electronic charge carriers.To understand the behavior of the thermal con-ductivity in this series of measurements,it is instructive to consider separately the lattice thermal conductivityκL,by sub-tracting the electronic contributionκe,which is related to the electrical resistivity by the Wiedemann–Franz lawκe=L0T/ρ, L0=2.45×10−8W /K2[13].The results are shown in Fig.4. HPHT treatment clearly increased the lattice thermal conductiv-ity.This is consistent with phonon scattering defects(vacancies)M.A.McGuire et al./Journal of Alloys and Compounds460(2008)8–1211Fig.3.The measured electrical resistivityρ,thermopower S,and thermal con-ductivityκafter annealing of HPHT treated,conventionally synthesized PbTe. Also shown is ZT=S2T/ρκcalculated from the measured data.being removed by HPHT processing.Fig.3also shows that annealing the HPHT treated samples decreased the lattice ther-mal conductivity,consistent with vacancies diffusing back into the material.A decrease in lattice thermal conductivity is expected in hot-pressedfinegrained materials due to enhanced grain boundary scattering of heat carrying phonons and has been observed exper-imentally[14–16].This phenomenon does not seem to play a significant role in the behavior of the thermal conductivity in the present study.This is consistent with SEM observations,which Fig. 4.The lattice thermal conductivity of PbTe samples as cast,after HPHT treatment,and after subsequent annealing.The electronic contribu-tion was subtracted from the measured total thermal conductivity using the Wiedemann–Franz law[13].did not show afine grained sintered compact,but rather a smooth appearance with no obvious grain structure.It is not surprising that the high pressures(6.5GPa)and high temperatures(900◦C, near the melting point of PbTe)used here did not produce the fine grained structure observed in many hot-pressed refractory materials.Longer annealing times were also briefly explored.After5 days at500◦C small PbTe crystals(approximately0.5mm)had formed on the surface of the pellet and on the tube walls.This likely indicates the presence of some impurities in the sample or tube which acted as vapor transport agents for the crystals. After the5-day anneal,this sample had a negative thermopower and low resistivity(close to that of the cast sample).This is not consistent with the explanation given above for the effects of the shorter anneals,and suggests an increase in the concentra-tion of some n-type dopant species.This is likely due to some unidentified impurity unintentionally introduced into the sample or tube,or to a change in composition of the bulk sample due to the transport of material away from the pellet.Further work is required to understand how these small crystals are transported and how they grow.4.ConclusionsHigh-pressure high-temperature tuning of the TE proper-ties of conventionally synthesized,nominally undoped PbTe was demonstrated.The effects of HPHT processing presented above are satisfactorily explained by the removal of lattice defects,which act like p-type dopants(possibly Pb vacancies) by HPHT treatment,and their reintroduction through annealing. This may prove to be a technologically important observation, since PbTe TE materials are sometimes prepared by hot-pressing,and since it is expected that vacancy concentration may influence the effectiveness of intentionally introduced dopants. These results suggest that further work should be carried out to examine the use of pressure to control the defect density, and therefore the carrier concentration,in optimally doped PbTe.12M.A.McGuire et al./Journal of Alloys and Compounds460(2008)8–12AcknowledgementsThis work was funded by Cornell University.We thank John W.Lucek at Diamond Innovations for helpful discussions regarding the HPHT work,and John Hunt for assistance with the electron microprobe facility,which is part of the Cornell Center for Materials Research(MRSEC Grant DMR-0520404). References[1]R.Heikes,R.Ure Jr.,Thermoelectricity:Science and Engineering,Firsted.,Interscience Publishers Inc.,1961.[2]G.Mahan,Solid State Phys.51(1997)81–157.[3]Z.H.Dughaish,Physica B322(2002)205.[4]J.Yang,T.Caillat,MRS Bull.31(2006)224.[5]I.K.Avetisov,A.Y.Mel’kov,A.Y.Zinov’ev,E.V.Zharikov,Crystallogr.Rep.50(2005)S124.[6]K.Kishimoto,M.Tsukamoto,T.Koyanagi,J.Appl.Phys.92(2002)5331.[7]K.Kishimoto,T.Koyanagi,J.Appl.Phys.92(2002)2544.[8]P.Zhu,X.Jia,H.Chen,L.Chen,W.Guo,D.Mei,B.Liu,H.Ma,G.Ren,G.Zou,Chem.Phys.Lett.359(2002)89.[9]T.Reynolds,M.McGuire,F.DiSalvo,J.Solid State Chem.177(2004)2998.[10]V.Shchennikov,S.Ovsyannikov,A.Derevskov,Phys.Solid State44(2002)1845.[11]S.Ovsyannikov,V.Shchennikov,Phys.Status Solidi B241(2004)3231.[12]G.Rousse,S.Klotz,A.Saitta,J.Rodriguez-Carvajal,M.McMahon,B.Couzinet,M.Mezouar,Phys.Rev.B71(2005)224116.[13]N.W.Ashcroft,N.D.Mermin,Solid State Physics,First ed.,SaundersCollege,1976.[14]J.Parrott,J.Phys.C:Solid State Phys.2(1969)147.[15]N.Savvidest,H.Goldsmid,J.Phys.C:Solid State Phys.13(1980)4671.[16]N.Savvidest,H.Goldsmid,J.Phys.C:Solid State Phys.13(1980)4657.。