Influence of powder morphology on thermoelectric anisotropy

Influence of powder morphology on thermoelectric anisotropy of spark-plasma-sintered Bi–Te-based thermoelectric materialsDong Hwan Kim a ,Cham Kim a ,b ,Seong Hyeon Heo a ,c ,Hoyoung Kim a ,⇑aDaegu Gyeongbuk Institute of Science and Technology (DGIST),711-623Hosan-dong,Dalseo-gu,Daegu 704-230,Republic of KoreabDepartment of Chemical Engineering,Pohang University of Science and Technology (POSTECH),San 31Hyoja-dong,Pohang 790-784,Republic of KoreacSchool of Materials Science and Engineering,Pusan National University,Geumjeong-gu,Busan 600-735,Republic of KoreaReceived 28June 2010;accepted 26September 2010Available online 20October 2010AbstractThe influence of the starting powders’morphology on the thermoelectric anisotropy of Bi–Te-based thermoelectric materials that are fabricated by spark plasma sintering has been investigated.Starting powders with three types of morphologies are prepared through a chemical reaction method (nano-sized particles with a spherical shape),a conventional pulverization method (flake-like shape)and a gas atomizing method (spherical shape).The thermoelectric anisotropy of each sintered body is determined by measuring the electrical resis-tivity and thermal conductivity in both the parallel and perpendicular directions to the spark plasma sintering pressing direction.The p-type Bi 0.5Sb 1.5Te 3.0-sintered body that is composed of the spherically shaped powder has isotropic thermoelectric properties,while the same material composed of the flake-like powder shows anisotropic behavior.The n-type Bi 2Te 3sintered body has a relatively small anisotropy when it is composed from the spherically shaped powder.Ó2010Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.Keywords:Thermoelectric;Anisotropy;Morphology;Spark plasma sintering1.IntroductionBismuth–telluride-based alloys are well known as the most efficient thermoelectric (TE)materials that operate near room temperature [1,2].Bi 2Te 3has a rhombohedral structure with the space group R3m.To visualize the struc-ture more clearly,the layered structure is frequently stud-ied.The hexagonal cell is composed of atomic layers that are stacked in the following order along the c -axis:–Te–Bi–Te–Bi–Te–.It has been shown that the Te and Bi layers are held together by strong ionic–covalent bonds,but no bonding electrons remain to connect the adjacent Te layers.The Te–Te layers are bonded by a weak van der Waals force,and the crystal has distinct cleavage planes that are perpendicular to the c -axis [3].It is also well known that the structural anisotropy of the lamellar structured alloys leads to corresponding aniso-tropies in many of their other physical properties.The comprehensive performance of thermoelectric materials is evaluated via the dimensionless figure of merit ZT ,which is a function of the resistivity,Seebeck coefficient and ther-mal conductivity [4,5].Because these physical properties are related to the transport of the charge carriers,Bi–Te-based alloys have TE anisotropy due to their structural anisotropy.The anisotropy of the electrical resistivity of single-crystal Bi 2Te 3along the crystalline axis was first reported by Delves and co-workers [6].They found that the electrical resistivity varied by a factor of approximately 3.2when measured parallel to the c -axis of the crystals in comparison with that measured perpendicular to the c -axis.Stordeur et al.[7]modeled the transport properties of Bi–Te-based thermoelectric materials and indicated that the resistivity in the direction perpendicular to the cleavage planes was greater than that measured in the parallel direction.These results should be considered in explana-tion of the anisotropic resistivity.However,there have not been any reports relating the Seebeck coefficient to1359-6454/$36.00Ó2010Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.doi:10.1016/j.actamat.2010.09.054⇑Corresponding author.Tel.:+82547702936;fax:+82547702940.E-mail address:hoykim@dgist.ac.kr (H.Kim)./locate/actamatActa Materialia 59(2011)405–411the structural anisotropy.Many researchers have studied the anisotropy of Bi–Te-based TE materials[8–13]. According to these reports,the anisotropies of the electrical resistivity and the thermal conductivity were clearly observed along the crystalline axis while the Seebeck coef-ficient was nearly isotopic regardless of the crystalline axis and was only dependent on the carrier concentration. These results are consistent with experiments suggesting that the anisotropy ratios ranged from4.3to6.7for the resistivity and from2.1to2.5for the thermal conductivity. Based on the above results,we can predict that ZT will be enhanced in crystalline-oriented TE materials due to the decrease in the resistivity.On the other hand,the structural anisotropy of the sin-gle-crystal Bi–Te-based alloys presents the disadvantage of poor mechanical integrity.These crystals are easily frac-tured along the cleavage planes during cutting or during operation of the modules,which causes problems in the production yield and the reliability of the modules[14–16].Thus,investigations have been focused on polycrystal-line Bi–Te-based alloys that are fabricated by powder met-allurgy methods,such as pressure-less sintering[17–19], mechanical alloying[20–23],hot extrusion[24,25]and hot pressing[26–29].Powder metallurgy techniques are broadly defined as the processes whereby powders are compacted and then sintered at elevated temperatures to form a dense body with a well-defined,coherent grain structure.These techniques are used to fabricate a variety of common ther-moelectric materials.It has been reported that the thermo-electricfigure of merit of hot-pressed materials is comparable to that of single crystals[30].Recently,spark plasma sintering(SPS)was used to fabricate thermoelectric materials.SPS is a newly developed rapid sintering process that makes it possible to sinter high-quality materials.A particular advantage of SPS is its sintering speed,which allows the formation offine crystalline materials with less grain growth.SPS reflects a recent research trend that tries to fabricate nanostructural thermoelectric materials with phonon-grain boundary scattering[31,32].The purpose of this research was to study the influence of powder morphology on the thermoelectric anisotropy of the sintered body.SPS was applied to fabricate the ther-moelectric materials.The morphologies of the starting powders were controlled by various synthesis methods:a chemical reaction method,a conventional pulverization method and a gas atomizing method.The thermoelectric properties of sintered bodies were evaluated parallel and perpendicular to the pressing direction.2.Experimental details2.1.Specimen preparationTo investigate the anisotropy of Bi–Te thermoelectric materials,three powder samples were prepared.Thefirst one,which was named BTn,was synthesized by a chemical reaction method.Bismuth(III)nitrate and elemental tellu-rium powder were employed as precursors.Ascorbic acid and ethylenediaminetetraacetic acid were used to dissolve the bismuth source and to stabilize it in deionized water, respectively.Sodium borohydride was chosen to reduce tel-lurium.Bi2Te3powder was obtained as a precipitate through the chemical reaction.The precipitate wasfiltered and rinsed with both dry ethanol and deionized water,then dried under vacuum at60°C overnight,whichfinally resulted in Bi2Te3powder.The second powder sample,which was named BSTi,was prepared by the conventional pulverization method.The metal powders(Bi,Sb and Te),which had over99.99% purity,were weighed to a composition of Bi0.5Sb1.5Te3.0 and mixed in a quartz tube.The tube was evacuated below 10À5torr and sealed.The tube was heated to1000°C in a rocking furnace to prevent any phase segregation.After 24h of heating,the tube was cooled to room temperature in a furnace.The obtained Bi0.5Sb1.5Te3.0ingot was crushed and sieved with a36l m mesh.The last powder sample,which was named BSTs,was prepared by a gas atomization method.The Bi,Te and Sb powders,with over99.99%purity were weighed to a composition of Bi0.5Sb1.5Te3.0.After melting in a high-fre-quency induction furnace under an argon atmosphere in a graphite crucible,the Bi0.5Sb1.5Te3.0powder was atomized from a nozzle with compressed nitrogen gas and sieved with a36l m mesh.The thermoelectric powders were sintered into cylinders that were12mm in height and15mm in diameter with a SPS machine(DR.Sinter,SPS-3,20MK-IV)in the temper-ature range of350–520°C and under a pressure of50MPa in an argon atmosphere.The heating rate was80K minÀ1, and the holding time at the sintering temperature was 2min.The sintering conditions were chosen such that the sintered body had a density larger than95%and avoided liquid phase sintering.2.2.CharacterizationThe size and morphology of the powders were charac-terized byfield emission scanning electron microscopy (Hitachi,S-4800FE-SEM).The microstructure of the sin-tered body that was composed of the powder derived via the chemical reaction was observed with electron backscat-ter diffraction scanning microscopy(Jeol,JSM-7000F with TSL OIM EBSD),while the microstructures of the other two bodies were observed with optical microscopy(Hit-achi,S-4800FE-SEM).The Hall coefficient(R H)was mea-sured by the van der Pauw method at room temperature by using a Hall measurement system(Ecopia,HMS-3000). Both the carrier concentration(n)and Hall mobility(l H) were calculated by assuming the single carrier conduction model with the following relations:n¼À1H;l H¼1, where e is the electronic charge and q is the electrical resistivity.The electrical resistivity(q)and the Seebeck coefficient (a)were measured with commercial equipment(Ulvac-406 D.H.Kim et al./Acta Materialia59(2011)405–411Rico,ZEM3).ByLFA447),thermalbetween300and(j)was evaluatedj¼k C P d,where dheat,respectively.specimens adoptedelectric materialsof merit waswhich consists ofZT¼a2T.3.Results andThe morphologytering was observedmicroscope,asmorphology and thethe preparationthesized by theof sphericalthemselves werepowder,which wasmethod,showed aflake-like shapes.Itingot has van deralong the c-axis,theflake shapes.Thecal shape of gasparticles about30lFig.2shows thefrom the three typese)and perpendicularThe sintered bodiespowders because theature was not long.1l m and did notobserved directions.of grain sizespicture with respectdifference was thatthe pressing5and30l m,andtive of the observedin the BTn body.Fig.3showsimens as Fig.2.Theybodies gave diffraction patterns that did not change withthe observed surface,whereas the BSTi body gave different patterns for each observed surface.In the BSTi body,the number of grains in which the c-axis aligned with the press-ing direction increased,which is in agreement with the microstructures shown in Fig.2.Fig.4shows the electron/Hall mobility vs.the carrier concentration of the sintered bodies that were measured at room temperature,where_33denotes the value mea-sured in the pressing direction and_11is that measured perpendicular to the_33direction.The carrier concentra-tions did not show any dependence on the measured direc-tion,whereas the mobility showed a considerable dependence on the measured direction(particularly for the BSTi body).The mobility of the BSTs body was mea-sured as250cm2VÀ1sÀ1irrespective of the measured direction,whereas the BSTi and BTn bodies gave different values along the different measuring directions.The BSTi body gave250cm2VÀ1sÀ1along the pressing direction(_33)and 300cm 2V À1s À1along the direction perpendicu-lar to pressing (_11).In the case of the Bi–Te single crystals,it is known that the mobility at room temperature has different values along the crystalline direction such that l H c =l H ab $1/3for Bi 2Te 3and Bi 0.5Sb 1.5Te 3.0[7].Thus,when a polycrystal has a preferred crystal orientation,the mobility can vary according to the measured direction.As an extension of this concept,the BSTi results could be qualitatively explained by considering the anisotropy of the microstructure and the X-ray diffraction pattern.In this case,the mobility in the pressing direction should be less35 m 35 m 35 m35 m(a)1m1m (b)(c)(d)(e)(f)408D.H.Kim et al./Acta Materialia 59(2011)405–411because the number of the grains in which the c-axis is aligned to the pressing direction was greater than that of the other surfaces.The BTn body gave130cm2VÀ1sÀ1 along the pressing direction and150cm2VÀ1sÀ1in the perpendicular direction.Thisfinding could also be qualita-tively explained in a way similar to the BSTi body;how-ever,in this case,the mobility difference in the measured direction was relatively small because the anisotropy in the microstructure was not as pronounced as that in the BSTi body.Fig.5shows the directional electrical resistivity of the sintered bodies;the BSTs body showed isotropic behavior, the BTn was somewhat anisotropic and the BSTi was strongly anisotropic in the measured temperature range. The BSTi_33was1.4times higher than the BSTi_11.It is well known that(n l H)values of$8Â1021for BSTs, $5.4Â1021for BSTi_11,$4.5Â1021for BSTi_33and $7Â1021for BTn(Fig.4)qualitatively reflect the electri-cal resistivity at room temperature when we consider the relationship between the electrical resistivity and the mobil-ity:q¼1H .Fig.6shows the directional Seebeck coefficient vs.the temperature of the sintered bodies.The Seebeck coefficient was observed to be an isotropic property.These results could be explained by considering that the Seebeck coeffi-cient is known to be dependent on the carrier concentration and independent of the mobility,as expressed in the follow-ing equation:a¼Æk Beðrþ2Þþln2ð2p mÃk B TÞ3=2h n!ð1Þwhere k B is Boltzmann’s constant,r is the scattering factor, m*is the effective mass and h is the Plank constant.Fig.7shows the directional thermal conductivity vs.the temperature of the sintered bodies;the BSTs body showed isotropic behavior,the BTn had somewhat anisotropic behavior and the BSTi had strong anisotropic behavior that was similar to the electrical resistivity.These results could be explained by considering the carrier concentration and the mobility,which is shown in Fig.4.The thermal conductivity is known to be composed of two components, the lattice thermal conductivity(j L)and the electronic thermal conductivity(j e),such that j=j L+j e.The elec-tronic thermal conductivity is known to be affected by both the carrier concentration and the carrier mobility in terms of the following equation:j e¼k Be2LTq¼k Be2LT e n l Hð2Þwhere L is the Lorentz number.However,the lattice thermal conductivity is not known to be significantlyD.H.Kim et al./Acta Materialia59(2011)405–411409dependent on the carrier concentration.The thermal con-ductivity at room temperature of the BSTs,BSTi_11and BST_33seemed to show the effect of the(n l H)values as ex-pressed in the above equation,which is similar to the elec-trical resistivity.It is worthwhile considering the BTn result more closely because the nanosized powder may play a role.Although its(n l H)value was almost the same as that of the BSTs(i.e.it had the same values for j e),the thermal conductivity of BTn had a considerably lower value in the measured temperature range.Thus,this result is caused by a decrease in the lattice thermal conductivity(j L)in the BTn body,which is the claim of nanostructured thermo-electric materials in terms of the phonon-grain boundary scattering mechanism[34,35].Fig.8shows the directionalfigure of merit vs.the tem-perature of the sintered bodies.The ZT values did not reveal significant differences along the evaluated directions in the measured temperature range because the anisotro-pies in the electrical and thermal conductivities cancel each other when evaluating thefigure of merit:ZT=a2T/(jq).4.ConclusionsIn this study,we investigated the influence of the pow-der’s morphology for sintering on the thermoelectric prop-erties of the sintered bodies.The sintered body,whose powder was composed of spherical particles,had a micro-structure that was independent of the observed direction, whereas the sintered body that was derived fromflake-shaped powders had different microstructures in the press-ing direction and perpendicular to the pressing direction. The directional difference in the microstructure was mainly responsible for the directional difference in the electron/ Hall mobility,which was supported by measurements of the electrical resistivity,Seebeck coefficient and thermal conductivity.In particular,the sintered body that used the nanosized powder had a thermal conductivity less than the body that was composed of the microsized powder;this was attributed to so-called phonon scattering.This study experimentally suggests that aligning the crystal orienta-tion properly may enhance the performance of Bi–Te ther-moelectric materials.AcknowledgementsThis work was supported by the Energy Efficiency&Re-sources of the Korea Institute of Energy Technology Eval-uation and Planning(KETEP)grant funded by the Ministry of Knowledge Economy,Republic of Korea. 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