Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials via Bulk Mechanical Alloying and Shear Extrusion

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1 Materials Transactions, Vol. 45, No. 3 (2004) pp. 918 to 924 #2004 The Japan Institute of Metals Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials via Bulk Mechanical Alloying and Shear Extrusion Sang Seok Kim* and Tatsuhiko Aizawa Research Center for Advanced Science and Technology, University of Tokyo, Tokyo , Japan The shear extrusion processing combined with bulk mechanical alloying is proposed to yield the n-type Bi-Te-Se material from elemental granules. It has well-developed texture so as to improve the electric conductivity and thermoelectric properties. The shear extrusion processing of (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 alloy green compact can afford the preferred orientation factor of anisotropic crystallographic structure: F ¼ 0:67. The electric resistivity of (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 is controlled to be 0: (m), which is 0.2 times lower than that of hot-pressed specimen. Maximum power factor is achieved to be 3: (W/mK 2 ) even without any dopants. The bending strength of the material produced in this work is also improved to be 166 MPa, 1.7 times higher than that of conventional hot-extruded specimens. (Received September 4, 2003; Accepted January 6, 2004) Keywords: n-type thermoelectric materials, shear extrusion processing, anisotropy, orientation factor, bismuth telluride, thermoelectric properties 1. Introduction The Bismuth telluride of Bi 2 Te 3 has a rhombohedral structure (a ¼ 0:438 nm and c ¼ 3:049 nm) and belongs to the space group R3m. This crystallographic structure is composed of a five-layer stacking sequence unit of Te ð1þ -Bi- Te ð2þ -Bi-Te ð1þ. Its successive units have the weak van der Waals bonding at the interface of neighboring Te ð1þ planes. 1,2) Since the cleavage fracture easily occurs along this weak bonding plane, the single crystal is difficult to use as the thermoelectric devices. In order to overcome this difficulty, the powder metallurgy method was selected for commercial application of Bi-Te materials. The Bi-Te based compounds have the highest figure of merit at room temperature. Since the thermoelectric properties along the a-axis is superior to those along the c-axis, the single crystal has a preferred anisotropy to increase the thermoelectric properties. 3,4) The polycrystalline material fabricated via the conventional sintering method has homogeneous microstructure. Since each grain has random orientation, the original crystallographic anisotropy disappears in the sintered polycrystalline materials and their figure-of-merit becomes lower than that of the single crystal. In order to attain higher strength and crystallographic anisotropy at the same time, it is important to exploit a new method to control texture formation during sintering. Highly oriented polycrystalline materials by texture formation are thought to have the same crystallographic anisotropy as the single crystal. The alloyed green compacts of (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 is prepared from elemental granule mixture by the bulk mechanical alloying (BMA). As suggested by Ref. 5), submicron compound particles produced via BMA are thought to be effective in texture formation during intense shear straining. The shear extrusion (SE) process is employed to enhance the texture development. This process is directly observed to understand the phenomena of grain rotation and laminated grain growth during intense plastic deformation. *Graduate Student, The University of Tokyo The correlation between the texture and the thermoelectric properties is discussed to evaluate the controllability of thermoelectric properties by shear extrusion. The favored texture formation is attained even by a single-step shearstraining. Higher bending strength than 150 MPa can be obtained by the fine-grained microstructure after shear extrusion. 2. Experimental Procedure Elemental granules of Bi (99.999%, 2 5 mm), Se (99.999%, 1 5 mm) and Te (99.999%, 1 5 mm) were weighed to have the initial molar ratio with the formula of (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95, before blending. The blended granule mixture was subjected to BMA for the specified number of cycles (N). Details of BMA were reported elsewhere. 5) The final product is a high-dense alloyed compact ( ¼ 25 mm, h ¼ 21 mm) with the relative density, 85 90% of the theoretical density, or 85 90% T.D. In the present experiment, as-bma preform was pulverized and sieved to have the average agglomerated particle size of 125 mm. The compacted preform ( ¼ 10:5 mm, h ¼ 15 mm, 85% T.D.) was prepared from the sieved particles for shear extrusion. In the shear extrusion process, the crosshead speed was kept constant at 0.5 mm/min. The holding temperature was varied ranging from 643 to 713 K in argon atmosphere. Crack-free sample was fabricated with the size of (a) L ¼ 45 (mm), W ¼ 10 (mm) and T ¼ 2 (mm), and, (b) L ¼ 70 (mm), W ¼ 10 (mm) and T ¼ 2 (mm), as shown in Fig. 1. The mechanical properties were measured at room temperature by three-point bending test under a crosshead speed of 0.5 mm/min. The van der Pauw method was employed to measure the electric resistivity at room temperature. Temperature difference by 10 K was applied to both ends of the sample to measure the Seebeck coefficient at room temperature. Differential thermal analysis (DTA) was performed at the heating speed of 10 K/min with the Shimadzu Differential Thermal Analyzer. X-ray diffraction analysis (XRD) was used to investigate the rearrangement of crystal

Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials via Bulk Mechanical Alloying and Shear Extrusion 919 (a) (b) Fig.

The grain rotations and texture development during shear extrusion were observed by scanning electron microscopy (SEM). 3. Results and Discussions 3.

In usual, refining and alloying process monotonically advances in BMA with increasing the number of cycles.

2 Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials via Bulk Mechanical Alloying and Shear Extrusion 919 (a) (b) Fig. 1 Fabricated samples via BMA and SE: (a) 45 (mm) 10 (mm) 2 (mm), and (b) 70 (mm) 10 (mm) 2 (mm). orientation. The grain rotations and texture development during shear extrusion were observed by scanning electron microscopy (SEM). 3. Results and Discussions 3.1 Solid-state synthesis (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 solid solution was synthesized by BMA. In usual, refining and alloying process monotonically advances in BMA with increasing the number of cycles. The number of cycles is usually determined when the initial element mixture is fully reacted to solid solution of compounds without any residual elements. Figure 2 compares three DTA curves of (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 : (1) after BMA (N ¼ 400), (2) after BMA (N ¼ 800) and (3) BMA (N ¼ 800) followed by shear extrusion. At N ¼ 400, two endothermic peaks were observed. The endothermic peak between 473 K and 573 K corresponds to the melting of residual Bi. The endothermic peak near 873 K corresponds to the melting of (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 solid solution. When increasing the number of cycles to N ¼ 800, the first endothermic peak completely disappeared. The intensity of the second endothermic peak was enhanced since the solid Fig. 2 Comparison of three DTA curves for (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 : (1) After BMA (N ¼ 400), (2) After BMA (800) and (3) BMA (N ¼ 800) followed by shear extrusion. state synthesis advances and more fraction of solid solution is yielded by further cyclic loading in BMA. When employing shear extrusion after BMA (N ¼ 800), no change was observed in DTA. It is concluded that (Bi 2 Se 3 ) 0:05 - (Bi 2 Te 3 ) 0:95 solid solution is stable as a stoichiometric compound without any dissolution. Hence, the number of cycle, N ¼ 800, was fixed to make the (Bi 2 Se 3 ) 0:05 - (Bi 2 Te 3 ) 0:95 solid solution. After BMA, its green compact was shear extruded. 3.2 Texture formation by shear extrusion Figure 3(a) illustrates the slip line field produced by shear extrusion. After V. M. Segal, 6) the plastic flow in the shear extrusion is described in what follows. The area ABC denotes for a dead metal zone where no plastic deformation takes place. The material plastically deforms at the range from AO to the outlet OD. The triangular area ODE in this crosssectional view corresponds to a rigid zone accompanied by maximum friction along DE. The gray-colored unit cell of area ODE is continuously affected by the deformation on the same slip line located at the area OACD. The deformation history of shear-extruded unit cell is similar to the all elements. At the near of point O, severe shear strain is applied uniformly to the sample, which is pushed down vertically at the constant velocity. The applied true shear strain is estimated, " ¼ 1:16. This value is sufficiently high enough to enforce the fine-grained texture. 7) Figure 3(b) depicts the load-displacement curves measured during shear extrusion. The curves consist of three different steps irrespective of the holding temperature during extrusion. Two frictional forces must be taken into account to consider the application of loading. During the (A)-region, relatively low load is applied against the frictional force f 1 along the vertical channel. In this stage, any unit cells of green compact do not reach the bottom BD. The (B)-region in Fig. 3(b) corresponds to the transient state where the backpressure from the channel begins to increase. In the (C)-region, the applied load monotonically increases with displacement since unit cells are subjected to plastic strain by high backpressure. The plastic work (W p ) was calculated from the on-line measured load-displacement curve. Table 1 lists the calculated plastic work of materials during shear extrusion for

3 920 S. S. Kim and T. Aizawa (a) W p ¼ C f ; ð2þ Fig. 3 Shear extrusion process: (a) Slip line solution, and (b) Loaddisplacement relation. The plastic work by shear straining for various holding temper- Table 1 ature. various holding temperature. W p monotonically decreases with increasing the holding temperature. In general, W p is roughly estimated by Z W p ¼ e _" e dv ð1þ V where e and _" e are the equivalent stress and strain rate, respectively, and V, the volume of a material. In the same extrusion process, _" e might be assumed to be constant, irrespective of the holding temperature. Further assuming that the whole materials except for the dead zone does deform plastically and e is given by the flow stress ( f ), eq. (1) is rewritten by (b) Shear extrusion temperature (K) Plastic work (J) where C is the proper constant. Hence, the above monotonic decrease of W p with increasing the holding temperature mainly comes from the monotonic decrease of the flow stress with temperature. SEM micrographs at the same position, P, above AO line are compared in Fig. 4 between (B)-region and (C)-region in the loading history. In the (B) region, the material has nearly the same microstructure as the green compact. Low densification took place and elongation of grains was hardly observed in Fig. 4(a). On the other hand, densification advanced significantly by the effect of high backpressure. The elongated grains by plastic deformation were observed in the (C)-region, as shown in Fig. 4(b). Significant texture formation is driven by the shear extrusion in the inside of ABDEO area. As shown in Fig. 5, SEM micrographs taken at the different positions in this area are used to describe the texture formation during the shear extrusion at 663 K. In parallel with the macroscopic plastic flow around the L-shaped corner in the shear extrusion process, each constituent grain is well aligned in the intense shear-strained region. The well-aligned grain inevitably leads to the high orientation factor and further results in the low electric resistivity. 3.3 Correlation between the texture and electric resistivity Figure 6 shows the SEM fractographs of the specimens, shear-extruded at various holding temperatures. In all the specimens, the whole grains were aligned to have the preferred orientation along the extruded direction. At the relatively low temperature, T ¼ 643 K, the size of laminated grain is about 4 mm. AtT 663 K, the uniform grain growth was observed; the average grain size is 8.5 mm. Relatively large grain as well as homogeneously laminated grain at T 663 K enhances electric conductivity by the reduction of grain boundary scattering along shear-extruded direction. To measure the bending strength of shear-extruded specimens, three-point bending tests were carried out. Table 2 shows the bending strength at the various holding temperature for the n-type (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 compound. The average bending strength of shear-extruded specimen at 663 K reaches about 166 MPa. This value is 1.7 times higher than that of the conventional hot-extruded n-type (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 compound. 8) This high strength leads to improvement of handling and machinability in fabrication of thermoelectric modules and increases reliability of modules under extreme service condition. Figure 7 compares the XRD profiles of (Bi 2 Se 3 ) 0:05 - (Bi 2 Te 3 ) 0:95 between the powder sample and the extruded specimens. The former sample was made by pulverizing the shear-extruded specimen. Higher intensities are detected along the orientation perpendicular to the shear-extruded direction: e.g., of (0015) or (0018) in R3m for Bi 2 Te 3.This texture development along (00l) direction proves that significant grain rotation should be accompanied with severe shear deformation to make arrangement of crystalline orientation. From this XRD observation, the orientation

4 Comparison of microstructure at the same sampling point: (a) Material in region (b),

4 Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials via Bulk Mechanical Alloying and Shear Extrusion 921 (a) (b) Fig. 4 Comparison of microstructure at the same sampling point: (a) Material in region (b), and (b) Material in the region (c). Fig. 5 Microscopic plastic flow in the shear extrusion along the L-shaped channel at 663 K.

Shear extrusion temperature (K) 643 663 683 713 Bending strength (MPa) 93.1 166.0 118.1 99.0 Fig.

5 922 S. S. Kim and T. Aizawa Fig. 6 Microstructure of the shear extruded (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 compounds at various holding temperatures. Table 2 Bending strength of the (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 compounds fabricated by shear extrusion for various holding temperature. Shear extrusion temperature (K) Bending strength (MPa) Fig. 7 Variation of XRD profiles for the shear extruded (Bi 2 Se 3 ) 0:05 - (Bi 2 Te 3 ) 0:95 specimens with increasing the holding temperature with the reference of the powder sample. factor (F) is calculated as an indicator of crystallographic anisotropy. After Ref. 9), F is defined by F ¼ðp p o Þ=ð1 p o Þ ð3þ where p is the ratio of the integrated intensity along (00l) directions to the whole integrated intensity for all (hkl) directions for the texture-developed bulk specimen, and p o is the ratio of the integrated intensity along (00l) directions to the whole integrated intensity for all (hkl) directions for the powder sample which was used as a reference. The calculated orientation factor for each specimen was shown in Fig. 8. At the relatively low sintering temperature, T ¼ 663 K, the maximum orientation factor of (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 is This high orientation factor leads to minimization of electric resistivity up to 0: (m) by the texture formation along a-axis. In general, the electric resistivity () in (Bi 2 Se 3 ) 0:05 - (Bi 2 Te 3 ) 0:95 solid solution has high sensitivity to the crystallographic orientation or F. In the case of the single crystal of (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 doped with 0.06% HgBr 2, F is 1.0 and is 0: m. 10) On the contrary, the hot pressed sample has F ¼ 0:03 and ¼ random ¼ 2: m. 11) Hence the shear-extruded specimens might further lower the value of electric resistivity by the control of F. Figure 9 depicts the correlation between the orientation factor (F) and the electric resisitivity () in (Bi 2 Se 3 ) 0:05 - (Bi 2 Te 3 ) 0:95 solid solutions. For F > 0:6, the measured

Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials via Bulk Mechanical Alloying and Shear Extrusion 923 Fig.

10 Variation of the electric resistivity and Seebeck coefficient with increasing the holding temperature for the shear extruded (Bi 2 Se 3 ) 0:05 - (Bi 2 Te 3 ) 0:95. Fig.

resistivity becomes nearly equal to or less than that of the single crystals with F ¼ 1.

6 Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials via Bulk Mechanical Alloying and Shear Extrusion 923 Fig. 8 Variation of the orientation factor (F) for the shear extruded (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 compound at various holding temperatures. Fig. 10 Variation of the electric resistivity and Seebeck coefficient with increasing the holding temperature for the shear extruded (Bi 2 Se 3 ) 0:05 - (Bi 2 Te 3 ) 0:95. Fig. 9 Correlation between the orientation factor (F) and electric resistivity () in (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 specimen. resistivity becomes nearly equal to or less than that of the single crystals with F ¼ 1. Hence, the electric resistivity can be sufficiently lowered to that for single crystals when F is increased to be higher than 0.6 by the shear extrusion process, irrespective of the holding temperature. 3.4 Power factor Figure 10 depicts the variation of electric resistivity and Seebeck coefficient with increasing the holding temperature. Variation of the Seebeck coefficient with increasing the holding temperature is similar to that of electric resisitivity. After Ref. 12), when assuming the Boltzmann distribution for the electron, the relationship between Seebeck coefficient and electric resisitivity at the temperature T is given by ¼ B s þ 5 e 2 þ ln 2ð2m k B TÞ 3=2 ð4þ nh 3 ¼ s lnðnþþc ð5þ where k B is the Boltzmann constant, e, the electronic charge, m, the effective mass, n, the carrier concentration, h, the Plank s constant, s, the scattering parameter and C, the constant. The first term s in the right-hand side of eq. (5) depicts the scattering contribution and the second term, lnðnþ, the carrier concentration contribution to Seebeck coefficient (), respectively. When the material has the similar texture, scattering mode is nearly the same. Hence, in this work, the variation of Seebeck coefficient with the extrusion temperature is mainly controlled by the carrier concentration. According to Ref. 13), the Seebeck coefficient is not dependent on the carrier mobility (). By the relationship of ¼ 1=ðneÞ, lnðþ. The Seebeck coefficient is directly related to the electric resistivity when the scattering parameter is almost the same. Thermoelectric relationship by eq. (4) reflects on the result in Fig. 10. The measured power factors are shown in Fig. 11 with increasing of the holding temperature. At T 663 K, the power factors become nearly constant, P ð ) W/mK 2. The maximum power factor is 3: W/ mk 2. Table 3 compares the thermoelectric properties among Fig. 11 Variation of the power factors ( 2 =) for the shear extruded (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 specimen with the holding temperature in the shear extrusion.

7 924 S. S. Kim and T. Aizawa Table 3 Power factors of (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 prepared by various fabrication methods. the single crystal, the hot-pressed sample and the shearextruded sample. Significant reduction of electric resistivity takes place by shear extrusion, resulting in high power factor than that of hot-pressed sample. With doping of BN, Mg 2 Si, or Si 3 N 4, the Seebeck coefficient can be increased by decreasing the carrier concentration and by increasing the electric resistivity to the same level as the doped single crystal. The power factor of shear-extruded sample is improved as comparable to that of the single crystal by optimization of doping. 4. Conclusion P.F. ( 2 =) (10 5 m) (mv/k) (10 3 W/mK 2 ) Single crystal 10) (with doping 0.06% HgBr 2 ) Hot pressing 11) (No doping) Present method (No doping) The crack-free thermoelectric plate with the thickness of 2 mm was successfully produced by the bulk mechanical alloying and shear extrusion process. It is noticeable that the preferred anisotropic crystallography and high strength is attained simultaneously by shear extrusion. The anisotropy of specimen is related to the shear extrusion temperature: F ¼ 0:52 at T ¼ 664 K, or F 0:54, F max ¼ 0:67 at T 663 K. This highest orientation factor (F max ¼ 0:67) leads to the lowest of the electric resistivity, 0: (m). Thermoelectric properties of (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 are enhanced by shear-extrusion: e.g., higher power factor of 3: W/mK 2 than that of hot-pressed specimens. Shear-extruded (Bi 2 Se 3 ) 0:05 (Bi 2 Te 3 ) 0:95 also has high bending strength of 166 MPa, which is 1.7 times higher than that of conventional hot-extruded specimen. The present shear extrusion process is also effective for intense shear-straining of the bulk thermoelectric materials. It is adaptive as a new processing route to control the thermoelectric properties even when starting from the single-crystal rod. Acknowledgments Authors would like to express their gratitude to Mr. J. Niekawa (Tohoku Okano Electric Co. Ltd.) and Mr. Y. Suzuki (Okano Electric Co. Ltd.) for their help in measurement of thermoelectric properties. This study is financially supported in part by the Grand-in-Aid from MEXT with the contract number of # REFERENCES 1) J. R. Drabble and C. H. L. Goodman: J. Phys. Chem. Solids 5 (1953) ) J. D. Shim, T. Y. Seong and J. M. Kim: J. KIMM 27 (1989) ) I. J. Ohsugi, T. Kojima, M. Sakata, M. Yamanashi and I. A. Nishida: J. Appl. Phys. 76 (1994) ) J. Nagao, M. Ferhat, E. Hatta and K. Mukasa: Phys. Status Solidi(b) 219 (2000) ) T. Aizawa, J. Kihara and D. Benson: Mater. Trans., JIM 36 (1995) ) V. M. Segal: Mater. Sci. Eng. A345 (2003) ) Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto and T. G. Langdon: Scr. Mater. 35 (1996) ) J. Seo, K. Park and C. Lee: Materials Research Bulletin 33 (1998) ) F. K. Lotgering: J. Inorg. Nucl. Chem. 9 (1959) ) I. Ohsugi and M. Sakata: J. Mater. Sci. Soc. Jap. 32 (1995) ) J. Yang, T. Aizawa, A. Yamamoto and T. Ohta: J. Alloys Compds. 312 (2000) ) H. J. Goldsmid: Recent Trends in Thermoelectric Materials Research I, Semiconductors and Semimetals 69 (2001) ) W. M. Yim and F. D. Rosi: J. Solid State Electronics 15 (1972) 1121.

Thermoelectric Properties and Scattering Factors of Finely Grained Bi 2 Te 3 -Related Materials Prepared by Mechanical Alloying

Materials Transactions, Vol. 51, No. 5 (21) pp. 863 to 867 Special Issue on Growth of Ecomaterials as a Key to Eco-Society IV #21 The Japan Institute of Metals Thermoelectric Properties and Scattering