FABRICATION AND THERMOELECTRIC PROPERTIES OF n-TYPESbI3-DOPED Bi2Te2.85Se0.15 COMPOUNDS BY HOT EXTRUSION

J. Seo1, K. Park2, and C. Lee1*†1Department of Metallurgical Engineering, Inha University,

Inchon 402–751, Republic of Korea2Department of Materials Engineering, Chung-ju National University,

Chungbuk 380–702, Republic of Korea

(Refereed)Received June 9, 1997; Accepted October 10, 1997

ABSTRACTThe n-type 0.1 wt% SbI3-doped Bi2Te2.85Se0.15 compounds were fabricatedby hot extrusion in the temperature range 300–510°C under an extrusion ratioof 20:1. The extruded compounds were highly dense. The grains were small,equiaxed (;1.0 mm), and contained many dislocations due to the dynamicrecrystallization during the extrusion. The grains were also preferentiallyoriented through the extrusion. The bending strength and the figure of merit ofthe compounds, hot-extruded at 440°C, were 97 MPa and 2.623 1023/K,respectively. © 1998 Elsevier Science Ltd

KEYWORDS: A. intermetallic compounds, C. electron microscopy, D. elec-trical properties, D. microstructure, D. thermal conductivity

INTRODUCTION

Bismuth telluride (Bi2Te3) based compounds have been used as thermoelectric cooling andheating materials, because they have the high figure of merit at room temperature. Thecompounds have a rhombohedral structure (a 5 0.438 nm andc 5 3.049 nm) and belong tospace groupR3#m [1]. This crystal structure is composed of atomic layers in the order of

*To whom correspondence should be addressed.†Jointly appointed at the Center for Advanced Aerospace Materials, Pohang University of Science andTechnology, Pohang 790–784, Republic of Korea.

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Te/Bi/Te/Te/Bi/Te/Bi/Te/Te/ . . . along thec axis. The Te/Te layers are considered to beweakly bound with van der Waal forces [2]. The electrical and mechanical properties alongthe direction parallel to the cleavage plane are better than those along the direction perpen-dicular to the plane, i.e., thec axis [3,4]. Because the compounds have cleavage features,there is difficulty in the mass production of miniature thermoelectric modules. It has beenreported that the phonons for SiGe alloys are scattered at the grain boundaries. The scatteringgives rise to a decrease in thermal conductivity, resulting in an increase in thermoelectricperformance [5,6]. One way of increasing the scattering is by refining the grain size. Thegrain refinement can be achieved by hot extrusion. In the present study, we fabricated then-type 0.1 wt% SbI3-doped Bi2Te2.85Se0.15 compounds with fine grains by means of hotextrusion, and thus, we improved the thermoelectric properties.

EXPERIMENTAL

To fabricate the n-type 0.1 wt% SbI3-doped Bi2Te2.85Se0.15 compounds, the powders of Bi,Te, Se, and SbI3 with .99.99% purity were mixed. The mixture was placed into an SiO2 tubethat was 25 mm in diameter and 330 mm in length. The tube was then evacuated below 1024

torr and sealed. The mixture was heated at 700°C to make a melt. The melt in the tube wasstirred at a frequency of 5 times/min at 700°C for 6 h, using a rocking furnace to make ahomogeneous melt without segregation. The tube containing the melt was cooled to roomtemperature in the furnace. The solidified ingot was crushed into fine flakes using an Al2O3

bowl. The resulting flakes were ball-milled using Al2O3 balls for 12 h in air then sieved toprepared powders 45–74mm in size. To remove the oxygen developed during the crushingand ball milling, the resulting powders were reduced in hydrogen atmosphere at 360°C for4 h. The powders were compacted by hot pressing at 420°C under 200 MPa in Ar to producebillets 30 mm in diameter and 6 mm in length. Subsequently, the compacted billets werehot-extruded in a temperature range of 300–510°C at steps of 70°C under an extrusion ratioof 20:1 and a ram speed of 50 mm/min.

The density of the hot-extruded compounds was measured with a pycnometer (Micrometrics,Norcross, GA). The microstructure of the compounds was examined by transmission electronmicroscopy (TEM) and X-ray diffraction (XRD). TEM specimens were sectioned from theperpendicular and parallel sections to the extrusion direction with a low-speed diamond saw.TEM specimens were prepared by mechanical grinding, dimpling, and ion milling. Ion millingwas conducted under 1 mA current and 3 kV Ar1 at an incident angle of 12° with liquid nitrogento minimize ion-induced damage. The microstructure of the specimens was investigated with aPhilips CM20 transmission electron microscope. The mechanical properties were measured atroom temperature under a crosshead speed of 0.5 mm/min by three-point bending in accordancewith ASTM D790 [7], using a universal testing machine.

The thermoelectric properties of the hot-extruded compounds were measured at roomtemperature along the direction parallel to the extrusion direction. Specimens with dimen-sions of 23 2 3 15 mm and of 43 4 3 4 mm were cut out of the compounds for themeasurements of the Seebeck coefficienta, thermal conductivityk, and of the electricalresistivity r, respectively. To measure the Seebeck coefficienta, heat was applied to thespecimen, which was placed between two Cu discs. The thermoelectric electromotive forceE was measured on applying small temperature difference (DT , 2°C) between the both endsof the specimen. The Seebeck coefficienta of the compounds was determined from theE/DT.

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The thermal conductivityk was measured by the static comparative method, using atransparent SiO2 (k 5 1.36 W/Km at room temperature) as a standard sample in 53 1025

torr. The Hall coefficientRH was measured by the high speed DC method. Carrier concen-trationnc was calculated from the following equation:nc 5 1/eRH. The electrical resistivityr was measured by the four-probe technique. The repeat measurement was made rapidly witha duration smaller than one second to prevent errors due to the Peltier effect. Carrier mobilitym was calculated from the following relationship:m 5 1/ncer.

RESULTS AND DISCUSSION

High-quality extruded bars without defects, such as tearing, orange peel, and blister, wereobtained in the hot extrusion temperature of 300–440°C. The bonding between the powdersbecame strong by increasing the extrusion temperature, giving rise to an increase in thedensity. The density was achieved up to 99.5% of theoretical density, which was obtained at440°C. However, hot cracks developed during the hot extrusion at 510°C. This resulted fromlocal melting due to the heat formed by the friction between the billet and the die. Thecompounds hot-extruded at 440°C showed the highest bending strength (97 MPa). This valueis much higher than that of the hot-pressed compounds. As an example, the bending strengthof the n-type Bi2Te2.85Se0.15 compounds hot-pressed at 420°C was 51 MPa [8].

Figure 1 shows the TEM bright field images from the perpendicular and parallel sectionsto the extrusion direction, respectively, for the compounds hot-extruded at 440°. It is evidentthat the grains are small equiaxed (;1.0 mm) and contain many dislocations because of thedynamic recrystallization during the extrusion. This grain size is much smaller than that ofthe hot-pressed compounds. The grain size of the n-type Bi2Te2.85Se0.15 compounds hot-pressed at 420°C was;30 mm [9]. The dynamic recrystallization is important for increasingthe ductility during the extrusion and for refining the grains. The fine grains significantlyimproved the mechanical strength and toughness of the extruded compounds. Crystallinedefects such as stacking faults and microtwins were not observed.

The XRD patterns obtained from the compounds hot-extruded at 440°C can be seen in Figure2, which also shows the XRD patterns obtained from the perpendicular and parallel sections tothe extrusion direction, respectively. The intensity of (0 0 6), (0 0 15), and (0 0 18) planes, whichare perpendicular to thec axis, is only observed at the parallel section. This indicates that the hotextrusion gave rise to the preferred orientation of grains. It has previously been reported [10] thatthe preferred orientation of grains is observed in unidirectionally solidified materials, in which thegrowing direction is perpendicular to thec axis. The preferred orientation yields stronglyanisotropic thermoelectricity, which produces high thermoelectric properties along the directionparallel to the extrusion direction. This is due to the fact that the thermal and electrical conduc-tivities along the direction parallel to the cleavage plane are much better than those along thedirection perpendicular to the plane, i.e., thec axis [3,4].

As shown in Figure 3, with increasing extrusion temperature, the mobilitym slightly increased,due to the porosity decrease, and the carrier concentrationnc decreased, probably because of theloss of the volatile SbI3 dopant. The variation of Seebeck coefficienta and electrical resistivityr with the extrusion temperature is shown in Figure 4. This figure shows that the absolute valueof Seebeck coefficienta increased with increasing extrusion temperature because of the decreasein carrier concentration. The relationship between thea andnc can be expressed as follows:a' r 2ln nc, wherer is the scattering factor [11]. The values of Seebeck coefficient for the

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compounds hot-extruded at 300, 370, and 440°C are2138.2, 2143.4, and2161.5 mV/K,respectively. Also, the electrical resistivity increased with the extrusion temperature. The elec-trical resistivity can be expressed as the following relationship:r 5 1/ncem. Consequently, twocompeting factors, carrier concentration and mobility, determine the electrical resistivity. Thus, anincrease in electrical resistivity with the temperature would result from a slight increase inmobility and a decrease in carrier concentration. The electrical resistivity of the compoundsextruded at 440°C is 0.7983 1025 Vm.

The thermal conductivityk significantly decreased with increasing temperature, as shownin Figure 5. The thermal conductivities of the compounds hot-extruded at 300, 370, and440°C were 1.950, 1.755, and 1.249 W/Km, respectively. The thermal conductivity of thecompounds hot-extruded at 440°C was much smaller than that of the hot-pressed compounds.This is due to the fact that the grains were refined through the extrusion. The thermalconductivity of the n-type Bi2Te2.85Se0.15compounds hot-pressed at 420°C was 1.423 W/Km[9]. The fine grains lead to an increase in phonon-grain boundary scattering and, thus, an

FIG. 1TEM bright field images from the sections (a) perpendicular and (b) parallel to the hotextrusion direction for the compounds hot-extruded at 440°C.

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increase in the figure of merit. It has been reported that the phonon-grain boundary scatteringhas a significant effect on reducing the lattice thermal conductivity of SiGe alloys [5,6].

The figure of meritZ was calculated using the following equation:Z 5 a2/r k. Figure 6shows the figure of meritZ of the compounds hot-extruded at various temperatures. Thefigure of merit increased with the extrusion temperature. This is due to the fact that withincreasing extrusion temperature the absolute value of the Seebeck coefficient increased andthe thermal conductivity significantly decreased, although the electrical resistivity increased.The compounds hot-extruded at 440°C showed the highest value ofZ (2.623 1023/K).

FIG. 2XRD patterns obtained from the sections (a) perpendicular and (b) parallel to the hotextrusion direction for the compounds hot-extruded at 440°C.

FIG. 3Carrier concentrationnc and carrier mobilitym as a function of hot extrusion temperature.

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CONCLUSIONS

The n-type 0.1 wt% SbI3-doped Bi2Te2.85Se0.15compounds fabricated by hot extrusion werehighly dense and showed high crystalline quality without any stacking faults or microtwins.The grains were fine equiaxed (;1.0 mm) and contained many dislocations due to thedynamic recrystallization during the extrusion. The grains were preferentially orientedthrough the extrusion. The bending strength and figure of merit of the compounds hot-extruded at 440°C were 97 MPa and 2.623 1023/K, respectively. We believe that the

FIG. 4Variation of the Seebeck coefficienta and electrical resistivityr with hot extrusion temperature.

FIG. 5Variation of the thermal conductivityk with hot extrusion temperature.

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extruded compounds could increase the bonding strength between the thermoelectric mate-rials and metal electrode during soldering for the fabrication of thermoelectric modules.

ACKNOWLEDGMENT

We thank the the University Basic Research Support Fund (1997) of the Korean Ministry ofInformation and Communication for its support.

REFERENCES

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Plastics and Electrical Insulating Materials, American Society for Testing and Materials, Phil-adelphia, PA, 1995.

8. J. Seo, K. Park, and C. Lee, unpublished results.9. J. Seo, K. Park, C. Lee, and J. Kim, inThermoelectric Materials—New Directions and Ap-

proaches, MRS Symp. Proc. Series, Vol. 478, ed. T.M. Tritt, M.G. Kanatzidis, H.B. Lyon, Jr., andG. Mahan, Materials Research Society, Warrendale, PA, In Press.

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11. K. Uemura and I. Nishida,Thermoelectric Semiconductors and Their Application, p. 150,Nikkan-Kogyo Shinbun, Tokyo, (1988).

FIG. 6Figure of meritZ of the compounds hot-extruded at various temperatures.

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