Thermoelectric and Mechanical Properties of Angular Extruded Bi 0:4 Sb 1:6 Te 3 Compounds

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1 Materials Transactions, Vol. 48, No. 10 (2007) pp to 2728 #2007 The Japan Institute of Metals Thermoelectric and Mechanical Properties of Angular Extruded Bi 0:4 Sb 1:6 Te 3 Compounds Takahiro Hayashi 1, Masayoshi Sekine 1, Junya Suzuki 1, Yuma Horio 1 and Hirotsugu Takizawa 2 1 Productive Technology Business Development Division, YAMAHA Corporation, Iwata , Japan 2 Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Sendai , Japan P-type Bi 0:4 Sb 1:6 Te 3 thermoelectric materials were prepared by an angular extrusion technique with rapidly solidified and stacked foils in a temperature range from 643 K to 838 K. Textures of the angular-extruded specimens were observed by Orientation Imaging Microscopy (OIM). The average grain size was monotonously enlarged from 4.7 to 16.1 mm while increasing the extrusion temperature. The texture of the angularextruded specimens shows that the basal planes are preferably aligned along the extrusion direction. Strong textures were observed in specimens extruded at a temperature range from 683 K to 803 K. The carrier mobility of the extruded specimens depends strongly on both the texture strength and grain size. As the extrusion temperature rises, the bending strength decreases. This change in bending strength is in good agreement with the Hall-Petch relationship. A maximum Z value of 3: K 1 and bending strength of 80.3 MPa were achieved in a specimen angular-extruded at 773 K. The Z value and the bending strength were sufficiently high compared with conventional hot-extruded or hot-pressed specimens. These results indicate that the angular extrusion technique is effective in improving the thermoelectric and mechanical properties of bismuth-telluride based thermoelectric materials. [doi: /matertrans.mra ] (Received May 18, 2007; Accepted July 23, 2007; Published September 5, 2007) Keywords: bismuth-telluride, thermoelectric properties, mechanical properties, texture, angular extrusion, rapid solidification 1. Introduction Bismuth-telluride based material is well known as a thermoelectric material that yields the highest cooling performance at room temperature. This material is widely used as thermoelectric coolers (TEC). TECs are the focus of much recent attention because of applications in laser diodes in telecommunications, biomedical devices, and microprocessors. Unique features that TECs offer include cooling of localized sections, and sizes as small as 1 mm 2 in commercially available TECs. Several research papers have been devoted to the study of thin-film thermoelectric devices. 1,2) Industrial TECs however are still fabricated by mounting thermoelectric elements on a substrate. These thermoelectric elements are made by slicing and cutting thermoelectric materials fabricated by conventional hot-pressing or unidirectional solidification techniques such as zone melting. Unidirectional solidified elements have coarsely orientated grains which cause them too weak to cut out or machine as elements for miniature TECs. Hot-pressed elements on the other hand, are polycrystalline aggregates with a weak orientated texture which gives them superior mechanical properties, yet they do not offer much in the way of thermoelectric properties. Recently, hot deformation techniques for bismuth-telluride based materials such as hot extrusion 3,4) or hot forging, 5,6) have attracted much attention as methods for fabricating thermoelectric materials that possess both superior mechanical properties and excellent thermoelectric properties. Recent observations suggest that dynamic recrystallization and formation of orientated texture take place in the materials during hot deformation. This means that hot deformation can yield both excellent thermoelectric properties and superior mechanical properties. Equal channel angular extrusion (ECAE) for structural materials such as iron, copper and aluminum has been intensively studies and is one of the most well known severe plastic deformation techniques. In the last few years, several studies have been made on applying ECAE or similar methods to the bismuth-telluride based materials. 7 9) We also had reported that applying the angular extrusion technique with rapidly solidified and stacked foils of bismuth-telluride based material was effective in improving the carrier mobility of the bismuth-telluride based material because of the fine orientated texture that is formed. 10) The resultant of angular extrusion had excellent thermoelectric properties. Angular extrusion is clearly a promising technique for fabricating bismuth-telluride based material with superior thermoelectric properties. In this study, the angular extrusion technique is applied to rapidly solidified and stacked foils of P-type Bi 0:4 Sb 1:6 Te 3 compounds, and the extrusion temperature then varied to determine the relation between the texture and mechanical properties. This study aims at producing P-type bismuthtelluride based material possessing both excellent mechanical and thermoelectric properties. 2. Experimental Procedure Alloy ingots of Bi 0:4 Sb 1:6 Te 3 were prepared by melting a mixture of elements (=99:99%) in a sealed evacuated silica tube. Rapidly solidified foils were prepared from alloy ingots using the single-roller melt-spinning technique in argon atmosphere. These foils were then crushed into a foil size of less than 1 mm. These crushed foils were stacked into an angular extrusion die. The die for angular extrusion was made from SKD 61 tool steel. The shape of the die inlet is a circle of 30 mm in diameter, with a rectangular outlet of 30 6 mm. Angular extrusions were conducted at temperatures ranging from 643 K to 838 K in an argon atmosphere. The ram speed was a constant 0.3 mm/min. Angularextruded specimens were hot-pressed at 100 MPa and a temperature of 653 K in an argon atmosphere to increase their density. All the specimens were fully densified (R.D. = 99:2%) after hot-press treatment. A theoretical density of

2 Thermoelectric and Mechanical Properties of Angular Extruded Bi 0:4 Sb 1:6 Te 3 Compounds 2725 Hot-Pressed Direction direction across the extrusion direction but along the hotpressed direction. The direction B represented a direction across the extrusion and hot-pressed directions. Extrusion direction Direction A Direction B Fig. 1 Schematic diagram of a relation between angular extrusion and bending test g/cm 3 was used in this experiment. 11) The thermoelectric properties S, and were measured at room temperature in a direction parallel to the extrusion direction. Electrical transport measurements from the Hall effect were carried out under a magnetic field of 0.5 T using a Toyo Technica ResiTest Textures were characterized using a JEOL JSM 5610 SEM equipped with a TSL EBSD system. Angular-extruded and hot-pressed specimens were cut into mm in size. Bending strengths were tested with a Shimadzu AG-20KNG by the 3-point bending method with a span of 15 mm and a crosshead speed of 0.5 mm/min. Figure 1 shows a schematic diagram illustrating the relation between the angular extrusion direction and the direction of bending test. Two specific directions were defined on the specimens for bending tests. The direction A referred to a 3. Results and Discussions 3.1 Texture formed by Angular Extrusion technique Figure 2 shows inverse pole figure (I.P.F.) maps and, pole figures measured in cross sections along the extrusion direction and hot-pressed direction. The pole marked is in parallel with the extrusion direction and is in parallel with the hot-pressed direction in these pole figures. As seen in Fig. 2, all textures exhibit preferred orientations. The plane of the angular-extruded specimen is preferably in parallel with the extrusion direction. Grains of the angular-extruded specimen become larger as the extrusion temperature rises. Figure 3 shows the cumulative distribution of crystal directions in the I.P.F. map of the specimen angular-extruded at 773 K. EBSD measurements indicate a crystal directions at each measurement points of specimen. Here, angle () indicates the angle between the c- axis and the extrusion direction at each measurement point in the I.P.F. map. That is, a measurement point where is equal to 90 degrees means that basal plane is parallel to the extrusion direction. The preferred orientation angle (P) is defined as the value obtained from subtracting the value of from 90 degrees when the surface area ratio is equal to 0.2 in (a) (b) (c) (d) (010) Fig. 2 Inverse pole figure maps and, pole figures of the angular-extruded specimens extruded at (a) 643 K, (b) 738 K, (c) 773 K and (d) 838 K.

3 2726 T. Hayashi, M. Sekine, J. Suzuki, Y. Horio and H. Takizawa [001] θ Extrusion direction : P-type Bi 0.4 Sb 1.6 Te 3 : N-type Bi 1.9 Sb 0.1 Te 2.7 Se 0.3 Fig. 3 Distribution of the angle () of the specimen angular-extruded at 773 K. Fig. 4 The effect of angular extrusion temperature on preferred orientation angle (P) and average grain size (d). Table 1 Thermoelectric properties of angular-extruded specimens. Extrusion Temperature T/K S/VK 1 /10 5 m /Wm 1 K 1 PF/10 3 Wm 1 K 2 Z/10 3 K Fig. 3. Namely, as the P value is close to 0 degree, the orientated texture is formed. Figure 4 shows the effect of the extrusion temperature on both P and average grain sizes (d) of the angular-extruded P-type specimens in comparison with those of N-type Bi 1:9 Sb 0:1 Te 2:7 Se 0:3. 10) Filled symbols indicate P and d values of angular-extruded P-type specimens, open symbols indicate P and d values of angular-extruded N- type specimens. P values of the both type specimens clearly show minimum points. This phenomenon indicates the occurrence of the abnormal grain growth. The change of P value in the P-type was gradual, and the minimum P value of the P-type appears at lower temperature than that of the N- type. In the P-type specimen, the abnormal grain growth strongly affects the formation of the texture in comparison with the N-type as the extrusion temperature rises. Therefore, this is why the formation of the orientated texture in the P- type specimen is difficult. 3.2 Thermoelectric properties Thermoelectric properties of specimens after angular extrusion at various temperatures are described in Table 1. In the table, the Seebeck coefficient, electric resistivity, thermal conductivity, power factor and figure of merit were expressed respectively as; S,,, PF and Z. A maximum Z value of 3: K 1 was reached at an angular extrusion temperature of 773 K. The value exhibits a minimum at the extrusion temperature of 773 K. The change in Z value was mainly due to the change in. Effects of extrusion temperature on the carrier concentrations (n) and the carrier mobilities () were plotted in Fig. 5 to allow describing the change in detail. Here, n was determined by measuring the Hall effect, and was calculated by following eq. (1): ¼ 1=ne ð1þ where e is the electron charge. Although the obtained n values tend to increase monotonically as the angular extrusion temperature increases, the variation of S seems to be rather temperature independent. A maximum value was obtained at an angular extrusion temperature of 773 K. There is clearly a strong correlation between and. Thatis, improving the enhances the Z value. The value appears to be dependent on both P and d. As the extrusion temperature rises, the value increases because of grain growth and the formation of an orientated texture below 773 K. Above 773 K, grain growth without formation of an orientated texture is predominant. This indicates that the abnormal grain growth becomes dominant in the formation of the texture in this temperature range. 3.3 Mechanical properties Bending strengths of the specimens angular-extruded at each temperature were plotted in Fig. 6. SEM images of the fractured surfaces of the specimens tested in the direction A are shown in Fig. 7. As can be seen, although the grain shapes are in flakes, bending strength doesn t seem to be anisotropic between direction A and direction B. As the extrusion temperature rises, the bending strength monotonously decreases. The maximum bending strength of 97.0 MPa was obtained at an angular extrusion temperature of 643 K, which is lower than that of 120 MPa reported by Kim et al. 7) This result is attributable to the difference of ram

4 Thermoelectric and Mechanical Properties of Angular Extruded Bi 0:4 Sb 1:6 Te 3 Compounds 2727 Fig. 5 Extrusion temperature dependence of carrier mobility () and carrier concentration (n). Fig. 6 Variation in the bending strength with extrusion temperature. (a) (b) (c) Load 50 μ m Fig. 7 SEM images of the fractured surface of angular-extruded specimens extruded at (a) 643 K, (b) 773 K and (c) 838 K. speed. The bending strength of the specimen with the maximum Z value was 80.3 MPa. On the other hand, a bending strength of up to 52 MPa in conventional hotextruded Bi 0:5 Sb 1:5 Te 3 specimens was reported by Seo et al. 12) Also, a bending strength of up to 57 MPa in conventional hot-pressed Bi 0:4 Sb 1:6 Te 3 specimens was reported by Yashima et al. 13) The bending strength of angular-extruded specimens was significantly higher than that of conventional hot-extruded specimens, although the extrusion ratio of the angular extrusion is lower and the ram speed is slower than that of hot-extruded specimens. The angular extrusion technique is considered to produce one type of severe plastic deformation. The effect of grain size on yield stress is well known in terms of the Hall-Petch relationship: ¼ 0 þ Kd 1=2 ð2þ where is the yield stress of a material with a grain size d, and 0 is the yield stress of a single crystal of material. The parameter K defines the grain boundary resistance. Figure 8 shows the Hall-Petch plot of specimens angular-extruded at various temperatures. Open circles indicate the average bending strengths of angular-extruded specimens tested in

5 2728 T. Hayashi, M. Sekine, J. Suzuki, Y. Horio and H. Takizawa or hot-extruded materials. The present study allows concluding that the angular extrusion technique is effective in improving both the thermoelectric and mechanical properties of bismuth-telluride based thermoelectric materials. Acknowledgements The authors would like to express our gratitude to Dr. S. Yin of Tohoku University for experimental assistance with the 3-point bending tests. The authors would also like to express our gratitude to T. Hoshi and K. Iijima for their cooperation in this study. Fig. 8 Hall-Petch plot for angular-extruded specimen. REFERENCES direction A and in direction B. The filled circle indicates the bending strength of a single crystal of Bi 0:5 Sb 1:5 Te 3 loaded in [001] direction. 14) The 0 value was extrapolated from plots of angular-extruded specimens and is in good agreement with the bending strength of the single crystal. This plot therefore shows that angular extrusion on Bi 0:4 Sb 1:6 Te 3 thermoelectric material results in the consistency with the expectations of the Hall-Petch relationship. 4. Summary P-type Bi 0:4 Sb 1:6 Te 3 compounds were prepared by an angular extrusion technique with rapidly solidified and stacked foils. A maximum Z value of 3: K 1 was achieved at the angular extrusion temperature of 773 K. The enhanced Z value clearly depends on improvement of the value which is dependent on both the oriented texture formed by angular extrusion and the grain size. The bending strength of the angular-extruded specimen at 773 K was 80.3 MPa. This value was higher than that of conventional hot-pressed 1) R. Venkatasubramanian E. Siivola, T. Colpitts and B. O Quinn: Nature 413 (2001) ) L. W. da Silva and M. Kaviany: J. Microelectromechanical Systems 14 (2005) ) J. Seo, C. Lee and K. Park: Mater. Sci. Eng. B54 (1998) ) S. Miura, Y. Sato, K. Fukuda, K. Nishimura and K. Ikeda: Mater. Sci. Eng. A277 (2000) ) T. Kajihara, K. Fukuda, Y. Sato and M. Kikuchi: Proc. 17th Int. Conf. on Thermoelectrics, (Nagoya Japan, 1998) pp ) E. J. Gonzalez, J. E. Blendell, J. P. Cline, J. J. Ritter, P. Maruthamuthu, E. H. Nelson and S. B. Horn: J. Mater. Res. 13 (1998) ) S. S. Kim, S. Yamamoto and T. Aizawa: J. Alloys Compd. 375 (2004) ) J.-T. Im, K. T. Hartwig and J. Sharp: Acta Metarialia 52 (2004) ) Z. M. Sun, H. Hashimoto, N. Keawprak, A. B. Ma, L. F. Li and M. W. Barsoum: J. Mater. Res. 20 (2005) ) T. Hayashi, M. Sekine, J. Suzuki and Y. Horio: Mater. Trans. 47 (2006) ) K.-H. Kim and S. Katayama: Applied Physics 39 (1970) ) J. Seo, D. Cho, K. Park and C. Lee: Mater. Res. Bull. 35 (2000) ) I. Yashima, R. Tsukuda, T. Sato and Y. Tochio: J. Ceram. Soc. Japan 105 (1997) ) L. D. Ivanova, I. M. Kop ev, N. Kh. Abrikosov, S. N. Chizhevskaya, V. A. Klim and I. P. Petryuk: Izv. Akad. Nauk SSSR, Neorg. Mater. 7 (1989)