JOURNAL OF MATERIALS SCIENCE35 (2000 )1549– 1554

Thermoelectric properties of n-type SbI3-doped

Bi2Te2.85Se0.15 compound fabricated by hot

pressing and hot extrusion

J. SEODepartment of Metallurgical Engineering, Inha University, Inchon 402-751, Korea

C. LEEJointly Appointed at the Center for the Advanced Aerospace Materials,Pohang University of Science and Technology, Pohang 790-784, Korea

K. PARKDepartment of Materials Engineering, Chung-ju National University, Chungbuk 380-702,Korea

n-type 0.1 wt% SbI3-doped Bi2Te2.85Se0.15 compounds were fabricated by hot pressing andhot extrusion. The hot pressed compounds were densified up to 99.2% of theoreticaldensity. The grains were preferentially oriented and contained many dislocations due to thehot pressing. The figure of merit of the compounds hot pressed at 420 ◦C was 2.35× 10−3/K.On the other hand, the grains of the extruded compounds were small, equiaxed, (∼1.0 µm)and contained many dislocations due to the dynamic recrystallization during the extrusion.The fine grains significantly improved the bending strength and figure of merit. The grainswere also preferentially oriented through the extrusion. The bending strength and figure ofmerit were increased with increasing extrusion temperature. The figure of merit of thecompounds hot extruded at 440 ◦C was 2.62× 10−3/K. C© 2000 Kluwer Academic Publishers

1. IntroductionBismuth telluride (Bi2Te3) and its related solid solu-tions have been used as thermoelectric cooling and heat-ing materials, since they have a high figure of merit atroom temperature. The crystal structure of Bi2Te3 atroom temperature is rhombohedral (a = 0.438 nm andc = 3.049 nm) [1]. This crystal structure is composedof atomic layers in the order of Te/Bi/Te/Te/Bi/Te/Bi/Te/Te/. . .along thec-axis. The Te/Te layers are consid-ered to be weakly bound with van der Waal forces [2].The solid solutions are distinguished by the high de-gree of anisotropy in most of physical properties. Theelectrical and mechanical properties along the direc-tion parallel to the cleavage plane are better than thosealong the direction perpendicular to the plane, i.e., thec-axis [3, 4]. As an example, the electrical conductivityof Bi2Te3 along the direction parallel to the cleavageplane is three to four times as large as that of Bi2Te3along the direction perpendicular to the plane.

Owing to the cleavage features, the crystal haslow mechanical properties and poor ability in micro-processing for fabricating the miniature thermoelectricmodules. It is thus inappropriate for mass production ofthe thermoelectric modules. Many workers attempted tofabricate the thermoelectric materials without cleavagefeatures. Sintering technique is not effective becausethe figure of merit of sintered compounds is lower thanthat of single crystals. It has previously been reported

that good n-type materials are the Bi2Te3-rich solid so-lutions [5]. The solid solution alloying leads to an in-crease in the figure of merit (Z) owing to the scatteringof phonons, which is caused by the disorder in the lat-tice due to alloying. In this work, we fabricated then-type SbI3-doped Bi2Te2.85Se0.15 compounds by twodifferent processings, hot pressing and hot extrusion,and then investigated the microstructure and thermo-electric properties of the compounds.

2. ExperimentalTo fabricate the hot-pressed and hot-extruded n-typeBi2Te2.85Se0.15 compounds doped with 0.1 wt% SbI3,the powders of Bi, Te, Se, and SbI3 with >99.99%purity were mixed. The mixture was placed into SiO2tube with 25 mm diameter and 330 mm length. Then,the tube was evacuated below 10−4 torr and sealed. Themixture was heated at 700◦C to make a melt. The meltin the tube was stirred under a frequency of 5 times/minat 700◦C for 6 hours using a rocking furnace to make ahomogeneous melt without segregation. The tube con-taining the melt was cooled to room temperature in fur-nace. The solidified ingot was crushed into fine flakesusing Al2O3 bowl. The resulting flakes were ball milledfor 12 hours and then sieved to prepare powders with45–74µm size. To remove the oxygen developed dur-ing the crushing and ball milling, the resulting powders

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were reduced in hydrogen atmosphere at 360◦C for4 hours. The powders were compacted by the hot press-ing in the temperature range 380 to 420◦C at steps of20◦ under 200 MPa in Ar, producing the billets with30 mm diameter and 6 mm length. Subsequently, thebillets hot pressed at 420◦ and 200 MPa were hot ex-truded in the temperature range 300–510◦C at steps of70◦C under an extrusion ratio of 20 : 1 and a ram speedof 50 mm/min.

The density of the hot-pressed and hot-extruded com-pounds was measured by pycnometer (MicrometricCo.). The mechanical properties were measured at roomtemperature under a crosshead speed of 0.5 mm/minby three-point bending in accordance with ASTMD790 [6] using a universal testing machine. Detailedmicrostructural information on the compounds was ob-tained from transmission electron microscopy (TEM)and X-ray diffraction (XRD). TEM specimens weresectioned from the perpendicular and parallel sectionsto the processing direction with a low-speed diamondsaw. TEM specimens were sectioned from the perpen-dicular and parallel sections to the processing directionwith a low-speed diamond saw. TEM specimens wereprepared by mechanical grinding, dimpling, and ionmilling. Ion milling was conducted under 1 mA cur-rent and 3 kV Ar+ at an incident angle of 12◦ withliquid nitrogen to minimize ion-induced damage. Themicrostructure of the specimens was investigated usinga Philips CM20 transmission electron microscope.

The thermoelectric properties of the hot-pressedand hot-extruded compounds were measured at roomtemperature along the directions perpendicular to thepressing direction and parallel to the extrusion direc-tion, respectively. The specimens with dimensions of2 × 2 × 15 mm and of 4× 4 × 4 mm were cut outof the compounds for the measurements of the See-beck coefficientα and thermal conductivityκ and ofthe electrical resistivityρ, respectively. Then, their sur-faces were polished with a series of SiC polishing pa-per of up to #2000 and further polished on a polishingcloth impregnated with Al2O3 powders of 0.3µm size.To measure the Seebeck coefficientα, heat was appliedto the specimen which was placed between the two Cudiscs. The thermoelectric electromotive forceE wasmeasured upon applying small temperature difference(1T < 2 ◦C) between the both ends of the specimen.The Seebeck coefficientα was determined from theE/1T . The thermal conductivityκ was measured bythe static comparative method using a transparent SiO2(κ = 1.36 W/Km at room temperature) as a standardsample in 5×10−5 torr. The electrical resistivityρ wasmeasured by the four-probe technique. The repeat mea-surement was made rapidly with a duration smaller thanone second to prevent errors due to the Peltier effect.

3. Results and discussion3.1. Hot-pressed Bi2Te2.85Se0.15 compoundsIt was found that the n-type 0.1 wt% SbI3-dopedBi2Te2.85Se0.15 compounds were relatively dense. Thepowders of the Bi2Te2.85Se0.15 compounds are flaky sothat the flat faces of the powders are contacted by hot

pressing. The bonding between the powders becamestrong with increasing the pressing temperature, result-ing in an increase in the density. The densities of thecompounds hot pressed at 380, 400, and 420◦C were97.9, 98.5, and 99.2% of theoretical density, respec-tively.

Fig. 1 shows the TEM bright field image from theperpendicular section to the hot pressing direction forthe compounds hot pressed at 420◦C. The compoundscontain many dislocations within grains. The grain sizewas found to be∼30µm. No crystalline defects suchas stacking faults and microtwins were observed. Thecompounds hot pressed at 420◦C showed the bendingstrength of 51 MPa. The fractograph of the compoundshot pressed at 420◦C is shown in Fig. 2. It is apparentthat the fractograph represents transgranular cleavagefeatures. The fracture path follows transgranular cleav-age planes. The orientation change from grain to grainis also found.

Fig. 3a and b show the XRD patterns obtained fromthe perpendicular and parallel sections, respectively,for the compounds hot pressed at 420◦C. The inten-sity of (0 0 6), (0 0 15), and (0 0 18) planes, which areperpendicular to thec-axis, is only observed at the

Figure 1 TEM bright field image from the perpendicular section to thehot pressing direction for the compounds hot pressed at 420◦C.

Figure 2 Fractograph of the compounds hot pressed at 420◦C.

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Figure 3 XRD patterns obtained from the (a) perpendicular and (b) par-allel sections to the hot pressing direction for the compounds hot pressedat 420◦C.

Figure 4 Carrier concentrationnc and mobilityµ as a function of thehot pressing temperature.

perpendicular section. This indicates that the grains arepreferentially oriented through the hot pressing. It haspreviously been reported that the preferred orientationof grains is observed in unidirectionally solidified ma-terials, in which the growing direction is perpendicularto thec-axis [7]. This preferred orientation leads to anincrease in the thermoelectric properties.

The carrier concentrationnc and mobilityµas a func-tion of pressing temperature are shown in Fig. 4. Withincreasing the pressing temperature, the carrier con-centration is slightly decreased, whereas the mobilityis increased due to the porosity decrease. At present,the reason for the decrease in carrier concentration isnot identified.

The variation of Seebeck coefficientα and electricalresistivityρ with the pressing temperature is shown in

Figure 5 Variation of the Seebeck coefficientα and electrical resistivityρ with hot pressing temperature.

Fig. 5. With increasing the pressing temperature, theabsolute value of Seebeck coefficient|α| is increasedbecause of the decrease in carrier concentration. Therelationship between the|α| andnc can be expressed asfollows:

|α| ≈ r − ln nc (1)

where,r is the scattering factor [8]. Also, the electricalresistivity is slightly decreased with the pressing tem-perature. The electrical resistivity can be expressed asthe following relationship:

ρ = 1

nceµ(2)

Consequently, two competing factors, carrier concen-tration and mobility, determine the electrical resistiv-ity. Therefore, the slight decrease in electrical resis-tivity would result from the increase in mobility andthe slight decrease in carrier concentration. In addition,the thermal conductivityκ increased with the press-ing temperature probably because of the density in-crease. The thermal conductivities of the compoundshot pressed at 380, 400, and 420◦C were 1.358, 1.389,and 1.423 W/Km, respectively.

The figure of meritZ was calculated using the fol-lowing equation:

Z = α2

ρκ(3)

Fig. 6 shows the figure of meritZ of the compoundshot pressed at various temperatures. The figure of meritis increased with the pressing temperature. This is be-cause with increasing the pressing temperature, the ab-solute value of Seebeck coefficient is increased andthe electrical resistivity is slightly decreased, althoughthe thermal conductivity is increased. The compoundshot pressed at 420◦C show the figure of merit of2.35× 10−3/K.

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Figure 6 Figure of meritZ of the compounds hot pressed at varioustemperatures.

3.2. Hot-extruded Bi2Te2.85Se0.15compounds

The bonding between the powders became strong withincreasing the extrusion temperature, giving rise to anincrease in the density. The density was achieved upto 99.5% of theoretical density, which was obtained at440◦C. Also, high-quality extruded bars without anydefects such as tearing, orange peel, and blister were ob-tained in the hot extrusion temperature of 300–440◦C.However, hot cracks were developed during the hot ex-trusion at 510◦C. This would result from the local melt-ing due to the heat generated by the friction betweenthe billet and die.

Fig. 7a and b show the TEM bright field images fromthe perpendicular and parallel sections to the extrusiondirection, respectively, for the compounds extruded at440◦C. The crystalline defects such as stacking faultsand microtwins were not observed. It is evident that thegrains are small equiaxed (∼1.0µm) and contain manydislocations because of the dynamic recrystallization(DRX) during the extrusion. The DRX is important forincreasing the ductility during the extrusion and for re-fining the grains. The fine grains significantly improvethe product mechanical strength and toughness. Thecompounds hot extruded at 440◦C showed the bendingstrength of 95 MPa. The fractograph of the compoundshot extruded at 440◦C is shown in Fig. 8. This figurerepresents the transgranular cleavage features.

Fig. 9a and b show the XRD patterns obtained fromthe perpendicular and parallel sections to the extrusiondirection, respectively, for the compounds extruded at440◦C. The intensity of (0 0 6), (0 0 15), and (0 0 18)planes is only observed at the parallel section, stronglyindicating that the hot extrusion gave rise to the pre-ferred orientation of grains.

As shown in Fig. 10, with increasing the extrusiontemperature, the carrier concentrationnc is decreased

(a)

(b)

Figure 7 TEM bright field images from the (a) perpendicular and (b)parallel sections to the hot extrusion direction for the compounds hotextruded at 440◦C.

Figure 8 Fractograph of the compounds hot extruded at 440◦C.

and the mobilityµ is slightly increased due to the poros-ity decrease. At this time, we do not know the originof the decrease in carrier concentration. Fig. 11 showsthe variation of Seebeck coefficientα and electrical re-sistivity ρ with the extrusion temperature. This figurerepresents that the absolute value of Seebeck coefficient

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Figure 9 XRD patterns obtained from the (a) perpendicular and (b) par-allel sections to the hot extrusion direction for the compounds hot ex-truded at 440◦C.

Figure 10 Carrier concentrationnc and carrier mobilityµ as a functionof hot extrusion temperature.

|α| increased with the extrusion temperature probablybecause of the decrease in carrier concentration. Thevalues of Seebeck coefficient for the compounds hot ex-truded at 300, 370, and 440◦C are−138.2,−143.4, and−161.5 µV/K, respectively. Also, the electrical resis-tivity is increased with the extrusion temperature proba-bly because of the slight increase in mobility and the de-crease in carrier concentration. The electrical resistivityof the compounds extruded at 440◦C is 0.798× 10−5

Ä m.The thermal conductivityκ is significantly decreased

with increasing the temperature, as shown in Fig. 12.The thermal conductivity (1.249 W/Km) of the com-pounds hot extruded at 440◦C is much smaller than that

Figure 11 Variation of the Seebeck coefficientα and electrical resistivityρ with hot extrusion temperature.

Figure 12 Relationship between the thermal conductivityκ and hot ex-trusion temperature.

of the hot-pressed compounds because of the increasein phonon-grain boundary scattering, which originatesfrom the grain refinement. It has been reported that thephonon-grain boundary scattering has a significant ef-fect on reducing the lattice thermal conductivity of SiGealloys [9, 10]. The figure of meritZ was calculated us-ing Equation 3. As shown in Fig. 13, the figure of meritincreased with the extrusion temperature. This is due tothe fact that with increasing the extrusion temperature,the absolute value of Seebeck coefficient is increasedand the thermal conductivity is significantly decreased,although the electrical resistivity is increased. The fig-ure of merit of compounds hot extruded at 440◦C isequal to 2.62× 10−3/K.

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Figure 13 Figure of meritZ of the compounds hot extruded at varioustemperatures.

4. ConclusionsThe n-type 0.1 wt% SbI3-doped Bi2Te2.85Se0.15 com-pounds fabricated by hot pressing were found to bedensified up to 99.2% of theoretical density. The hotpressing gave rise to the preferred orientation of grains.It was also found that the bending strength and the figureof merit increased with increasing the pressing temper-ature. The bending strength and figure of merit of thecompounds hot pressed at 420◦C were 51 MPa and2.35× 10−3/K, respectively.

On the other hand, the grains of hot-extruded n-type0.1 wt% SbI3-doped Bi2Te2.85Se0.15 compounds werefine equiaxed (∼1.0µm) and contained many disloca-tions owing to the dynamic recrystallization during theextrusion. The fine grains contribute to an increase in

the bending strength and figure of merit. The bendingstrength of the compounds hot extruded at 440◦C was97 MPa. Also, the hot extrusion gave rise to the pre-ferred orientation of grains. The highest figure of merit(2.62× 10−3/K) was obtained at 440◦C. We believethat the extruded compounds could increase the bond-ing strength between the thermoelectric materials andmetal electrode during soldering for the fabrication ofthermoelectric modules. The hot extrusion techniqueis an effective to fabricate the thermoelectric materialswith high thermoelectric performance.

AcknowledgementWe would like to thank the support from the RA schol-arship and post doctor program in Inha University.

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Received 16 June 1997and accepted 30 September 1999

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