Thermoelectric Properties of n -Type Bi 2Te 3/PbSe 0.5Te 0.5Segmented Thermoelectric MaterialSEJIN YOON,1JUN-YOUNG CHO,1HYUN KOO,1SUNG-HWAN BAE,1SEUNGHYUN AHN,1GWI RANG KIM,1JIN-SANG KIM,2and CHAN PARK 1,3,41.—Department of Materials Science and Engineering,Seoul National University,Daehak-dong,Gwanak-gu,Seoul,Republic of Korea.2.—Electronic Materials Research Center,Korea Institute of Science and Technology,Wolgok 2-dong,Seongbuk-gu,Seoul,Republic of Korea.3.—Research Institute of Advanced Materials,Seoul National University,Daehak-dong,Gwanak-gu,Seoul,Republic of Korea.4.—e-mail:pchan@snu.ac.krTo investigate the effects of segmentation of thermoelectric materials on performance levels,n -type segmented Bi 2Te 3/PbSe 0.5Te 0.5thermoelectric material was fabricated,and its output power was measured and compared with those of Bi 2Te 3and PbSe 0.5Te 0.5.The two materials were bonded by diffusion bonding with a diffusion layer that was $18l m thick.The electrical conductivity,Seebeck coefficient,and power factor of the segmented Bi 2Te 3/PbSe 0.5Te 0.5sample were close to the average of the values for Bi 2Te 3and PbSe 0.5Te 0.5.The output power of Bi 2Te 3was higher than those of PbSe 0.5Te 0.5and the segmented sample for small D T (300K to 400K and 300K to 500K),but that of the segmented sample was higher than those of Bi 2Te 3and PbSe 0.5Te 0.5when D T exceeded 300K (300K to 600K and 300K to 700K).The output power of the segmented sample was about 15%and 73%higher than those of the Bi 2Te 3and PbSe 0.5Te 0.5samples,respectively,when D T was 400K (300K to 700K).The efficiency of thermoelectric materials for large temperature differences can be enhanced by segmenting materials with high performance in different temperature ranges.Key words:Thermoelectric material,segmented thermoelectric material,Bi 2Te 3,Pb(Se,Te),output powerINTRODUCTIONMany studies have focused on improving the energy conversion efficiency of thermoelectric materials for their potential use in power-generation and cooling systems.The efficiency of a thermoelectric material is characterized by its dimensionless figure of merit [zT =(S 2r T )/j ,where S is the Seebeck coefficient,r is the electrical conductivity,and j denotes the thermalconductivity].1The upper limit of the efficiency (g ¼g c ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þzT p À1 =ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þzT p þ1Àg c h i ,whereg c =D T /T hot is the Carnot efficiency,and zT is the average zT over the temperature range)is determined by the temperature difference D T ,2,3which means that better energy conversion can be expected with the use of a larger D T .Most thermoelectric materials,how-ever,have high zT over a very limited temperature range,indicating that most thermoelectric materials cannot effectively utilize such a large D T .High ther-moelectric energy conversion efficiency can be obtained when a large D T can be used,which can only be realized by using a thermoelectric material with high zT value across a broad temperature range.Materials with large average zT values over a wide temperature range have to be used to convert large amounts of energy from a large D T .Connection of materials with high zT values in different tempera-ture ranges,forming a segmented thermoelectric(Received May 10,2013;accepted October 14,2013;published online November 27,2013)Journal of ELECTRONIC MATERIALS,Vol.43,No.2,2014DOI:10.1007/s11664-013-2869-4Ó2013TMS414material,can lead to improvement of the efficiency in a wide temperature range.Most research on thermoelectric materials has focused on improving the zT value of a single material by optimizing the carrier concentration with dopants and/or by using low-dimensional structures such as quantum dots and superlattic-es.4–11The zT values of such thermoelectric mate-rials with low-dimensional structures have been significantly improved compared with those of bulk thermoelectric materials.8,11Thermoelectric mate-rials with a low-dimensional structure,however, have mainly been fabricated in the form of thin films or nanowires,for instance,which are not appropriate for use in large-scale energy conversion systems with a large temperature difference. Given that the thermoelectric properties(S,r,j)of materials vary with temperature,Snyder et al.3,12 suggested that different thermoelectric materials could be segmented or cascaded together to improve their efficiency for a large temperature difference. Recently,many research groups have reported calculations and experimental data for segmented thermoelectric materials with improved perfor-mance.Kuznetsov et al.2used various amount of Te and SbI3as dopants to control the carrier concen-tration of Bi2Te3,connecting these materials to improve efficiency from8.8%to10%at D T=200K. Cui reported results for segmented p-type FeSi2/ Bi2Te3,showing1.4times more output power than FeSi2at D T=485K.13Zeng et al.14,15reported that the output power of a thermoelectric module of 16916segmented legs consisting of Bi2Te3/ ErAs:(InGaAs)1Àx(InAlAs)x was6.3W when the heat source was set to610K and the cooling water was kept at285K.Oh fabricated p-type(Bi,Sb)2Te3/ (Pb,Sn)Te by hot-pressing,showing,at a D T value of 320K,output power of$72mW,while(Bi,Sb)2Te3 and(Pb,Sn)Te showed63.9mW and26mW,respec-tively.16Recent studies of segmented thermoelectric materials have shown improved efficiency of seg-mented thermoelectric materials compared with nonsegmented ones,as mentioned above.Detailed understanding of the effects of different fabrication methods and interface properties of segmented materials on the thermoelectric properties will help to improve the efficiency further.In this work,to investigate the effects of segmen-tation of thermoelectric materials on their perfor-mance,an n-type segmented thermoelectric material consisting of Bi2Te3(highest zT at$400K)and PbSe0.5Te0.5(highest zT at$700K)was fabricated by spark plasma sintering(SPS)of Bi2Te3and PbSe0.5Te0.5powders prepared by mechanical alloy-ing.Quantitative elemental analyses were performed by means of wavelength-dispersive spectrometer (WDS)measurements at the interface to investigate the diffusion of the constituent elements.Output powers for various D T conditions(100K,200K, 300K,and400K)and the thermoelectric properties of the segmented Bi2Te3/PbSe0.5Te0.5sample were measured and compared with those of Bi2Te3and PbSe0.5Te0.5made by SPS.EXPERIMENTAL PROCEDURES Sample PreparationElemental Te,Bi,Pb,and Se(>99.999%)powders were weighed according to the stoichiometric com-positions of Bi2Te3and PbSe0.5Te0.5,and charged into a stainless-steel(SS)vial with SS balls under Ar atmosphere.SS balls with diameters of2mm and8mm were used with ball-to-powder weight ratio of10:1.Mechanical alloying of the powders was carried out using high-energy ball milling at 250rpm for20h.Contamination of the powders during the high-energy ball milling process was checked by analyzing the elements present in the powder using an inductively coupled plasma mass spectrometer.The prepared powders were then sintered by SPS. Approximately7g of each powder was packed into a carbon die with diameter of10mm and sintered under vacuum(<200mTorr)at400°C for10min under uniaxial pressure of50MPa.To fabricate the segmented sample,about3.5g Bi2Te3powder was charged into the carbon die and then pressed under 10MPa of uniaxial pressure,followed by charging and pressing of PbSe0.5Te0.5powder under the same conditions as for Bi2Te3.The green body consisting of Bi2Te3and PbSe0.5Te0.5was then spark plasma sintered(SPSed)using the same pressure,temper-ature,and sintering time as the nonsegmented samples.Figure1a and b show an image of the fab-ricated sample and a schematic of the fabrication method of the segmented sample,respectively.Each sample prepared by SPS was machined into bar-type (3mm,3mm,and10mm)and disk-type(1mm and 10mm)samples for further measurements. Characterization of the SamplesTo identify the phases present at each processing step,h–2h x-ray diffraction(XRD,D-8Advance; Bruker)with Cu K a radiation was performed for all powders and sintered samples.WDS quantitative elemental analysis was carried out to investigate the diffusion of elements across the interface of the segmented sample.The electrical conductivity and Seebeck coefficient were measured using a ZEM-2 instrument(Ulvac).Measurements were performed at intervals of50°C from room temperature to 400°C.The temperature difference was kept at 10°C,20°C,and30°C to measure the Seebeck coef-ficient.The relative density was measured by Archimedes’method.The output power of each sample was measured in a vacuum chamber under various D T conditions(D T=100K,200K,300K, and400K)by heating one side of the sample while the other side was in direct contact with a coolingThermoelectric Properties of n-Type Bi2Te3/PbSe0.5Te0.5Segmented Thermoelectric Material415block (300K).During the measurement of the seg-mented sample,Bi 2Te 3and PbSe 0.5Te 0.5were in contact with the cooling block and the heater,respectively.RESULTS AND DISCUSSIONFigure 2shows the XRD patterns of the powders after mechanical alloying and the SPSed samples of Bi 2Te 3and PbSe 0.5Te 0.5.The diffraction peaks were compared with those in the published powder dif-fraction files:PDF 01-082-0358(ICDD,1993)and PDF 03-065-8019(ICDD,1969).The peak positions of the powders and the SPSed samples were iden-tical to those of the PDF data of Bi 2Te 3and PbSe 0.5Te 0.5,indicating that no second phase was formed in the sample during the mechanical alloy-ing or SPS process.Broad peaks can be observed in Fig.2b,e,as the mechanical alloying is accompa-nied by a decrease in the powder size and the occurrence of strain.Grain growth and strain release during the sintering process can lead to a large decrease in the full-width at half-maximum of the diffraction peaks of SPSed samples (Fig.2c,f)compared with those of the powders (Fig.2b,e).To analyze the diffusion of atoms through the interface of the segmented Bi 2Te 3/PbSe 0.5Te 0.5sample,quantitative elemental analysis was per-formed by WDS.The atomic percentages of Bi,Pb,Se,and Te were measured every 2l m across the segmented interface;these results are shown in Fig.3.Pb and Se diffuse into Bi 2Te 3and Bi diffuses into PbSe 0.5Te 0.5to form a diffusion layer at the interface.This is similar to diffusion bonding of bulk materials at high temperature and pressure.17,18The length of the diffusion layer across the interface was about 18l m,much smaller than that reported by Oh,which was 350l m.16These different lengths of the diffusion layer could result from the different sintering conditions applied,which were 10min by SPS at 400°C in this work and 1h by hot pressing at 400°C for the result reported by Oh.The composi-tion of the diffusion layer is different from those of the materials at each side,allowing the diffusion layer to have different thermoelectric properties from the two end materials,which could affect the total efficiency of the sample.Investigation of the thermoelectric properties of the diffusion layer and its effect on the performance of the segmented sample could lead to further optimization of the thermoelectric properties of such segmented ther-moelectricmaterials.Fig.1.(a)Fabricated segmented Bi 2Te 3/PbSe 0.5Te 0.5sample and (b)schematic of the fabrication method of the segmented Bi 2Te 3/PbSe 0.5Te 0.5sample by spark plasmasintering.Fig.2.PDF data of (a)Bi 2Te 3and (d)PbSe 0.5Te 0.5,and XRD pat-terns of (b)the Bi 2Te 3powder,(c)the Bi 2Te 3spark-plasma-sintered sample,(e)the PbSe 0.5Te 0.5powder,and (f)the PbSe 0.5Te 0.5spark-plasma-sinteredsample.Fig.3.WDS data of the segmented Bi 2Te 3/PbSe 0.5Te 0.5sample.Yoon,Cho,Koo,Bae,Ahn,G.R.Kim,J.-S.Kim,and Park416The electrical conductivity and Seebeck coefficient were measured for all three sintered samples at various temperatures in air.The temperature dependences of the electrical conductivity and See-beck coefficient are presented in Fig.4a and b,respectively.The resulting electrical conductivity values for the Bi 2Te 3and PbSe 0.5Te 0.5samples werelower than those reported by Zhao et al.19and Li et al.,20whose samples were fabricated by methods similar to that used in this work.The low electrical conductivity of the samples could result from the low relative densities of the Bi 2Te 3(95.2%)and PbSe 0.5Te 0.5(94.8%)samples compared with the reported relative density of Bi 2Te 3of 98.5%.19This difference in the electrical conductivity can be explained in terms of the effective medium theory (EMT)which provides the relationship between the relative density and the electrical conductivity as r =[(3f À1)r 0]/2,where f is the fraction of the material except the pores and r 0is the electrical conductivity of the material without any pores.21The Seebeck coefficients of Bi 2Te 3and PbSe 0.5Te 0.5were similar to the reported values.19,20The power factor of each sample was calculated from the elec-trical conductivity and Seebeck coefficient,as shown in Fig.4c.The Bi 2Te 3and PbSe 0.5Te 0.5samples showed their highest power factor at 425K and 625K,respectively.The electrical conductivity,Seebeck coefficient,and power factor of the seg-mented Bi 2Te 3/PbSe 0.5Te 0.5sample were close to the average of the values for the nonsegmented samples.Figure 5shows the maximum output power of the Bi 2Te 3,PbSe 0.5Te 0.5,and segmented Bi 2Te 3/PbSe 0.5Te 0.5samples for various D T conditions.The temperature of the cold side was kept at 300K by cooling water,and the hot side of the samplewasFig.4.Temperature dependence of (a)electrical conductivity,(b)Seebeck coefficient,and (c)powerfactor.Fig.5.Output power of the Bi 2Te 3,PbSe 0.5Te 0.5,and segmented Bi 2Te 3/PbSe 0.5Te 0.5samples with temperature differences across the samples from 100K to 400K.Thermoelectric Properties of n -Type Bi 2Te 3/PbSe 0.5Te 0.5Segmented Thermoelectric Material 417heated by a local heater.The output power of the Bi2Te3sample was higher than that of the PbSe0.5Te0.5 and segmented Bi2Te3/PbSe0.5Te0.5samples when the D T value was lower than200K,but that of the seg-mented Bi2Te3/PbSe0.5Te0.5sample was higher than the other samples when D T exceeded300K.The out-put power of the Bi2Te3,PbSe0.5Te0.5,and segmented Bi2Te3/PbSe0.5Te0.5samples was38.8mW,26.0mW, and45.3mW,respectively,when D T was400K.The output power of the segmented sample was about15% and73%higher than that of the Bi2Te3and PbSe0.5Te0.5sample,respectively,at a D T value of 400K.The output power of the segmented sample was improved compared with those of the nonsegmented samples for large D T.Optimizing the length of the segmented materials and controlling the diffusion at the interface could lead to further improvement of the performance of segmented thermoelectric materials at large D T.CONCLUSIONSTo investigate the effects of segmentation of thermoelectric materials on performance levels,a segmented thermoelectric material was fabricated, and its output power was measured and compared with those of the nonsegmented materials.In this study,n-type Bi2Te3and PbSe0.5Te0.5,having their highest zT values at about400K and700K, respectively,were used to make the segmented thermoelectric material.The n-type segmented Bi2Te3/PbSe0.5Te0.5thermoelectric material was fabricated by SPS of a green body consisting of a stacked structure of Bi2Te3and PbSe0.5Te0.5.The two materials were bonded by diffusion bonding with a diffusion layer of$18l m.The electrical conductivity,Seebeck coefficient,and power factor of the segmented Bi2Te3/PbSe0.5Te0.5sample were near the average of the values of the two nonseg-mented materials.The output power of Bi2Te3was higher than those of the PbSe0.5Te0.5and segmented samples for small D T(300K to400K and300K to 500K),but the output power of the segmented sample was higher compared with those of Bi2Te3 and PbSe0.5Te0.5when D T exceeded300K(300K to 600K and300K to700K).The output power of the segmented sample was approximately15%and73% higher than those of the Bi2Te3and PbSe0.5Te0.5 samples,respectively,when D T was400K(300K to700K).The results of this work highlight that seg-mented thermoelectric materials can offer improved efficiency over a wide temperature range,and that optimization of the length of each segment and proper control of the diffusion layer could further increase the efficiency of the thermoelectric mate-rial for large D T.ACKNOWLEDGEMENTSThis research was supported by the Converging Research Center Program through the Ministry of Sci-ence,ICT and Future Planning,Korea(2013K000170) and by the Power Generation&Electricity Delivery of the Korea Institute of Energy Technology Evaluation and Planning(KETEP)grant funded by the Korea government Ministry of Knowledge Economy(No. 20111020400090).REFERENCES1. 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