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Microstructure control and thermoelectric properties improvement to n-type bismuth telluride based materials by hot extrusion
Journal of Alloys and Compounds 429 (2007) 156–162
Microstructure control and thermoelectric properties improvement to n-type bismuth telluride based materials by hot extrusion Junyou Yang ∗ , Rougang Chen, Xi’an Fan, Wen Zhu, Siqian Bao, Xingkai Duan State Key Laboratory for Plastic Forming Simulation and Dies Technology, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, PR China Received 6 February 2006; received in revised form 12 April 2006; accepted 12 April 2006 Available online 15 May 2006
Abstract In this paper, hot extrusion process of Bi2 Te3 based thermoelectric materials was simulated with FEM tools firstly, it is found that the friction condition between the extrusion die and the sample has large influence on the surface stress state of the sample, making the surface region under more tension stress, thus increases the possibility of surface crack. Extrusion ratio has significant influence on the degree of deformation. Extrusion angle also shows effect on the deformation, low extrusion angle die has longer or larger regions of deformation, which means lower deformation rate and will cause less micro-pores. Then n-type Bi2 Te3 based materials were also hot extruded under different experiment conditions, and effect of extrusion parameters on microstructure and thermoelectric properties of the materials was studied, and a (0 0 l) preferred orientation microstructure was formed in the hot extruded samples. With increasing the extrusion ratio, the Lotgering orientation factor of the extruded sample increases, and the power factor also shows a magnificent increase. The experimental results are in consistence with the results of FEM analysis. © 2006 Elsevier B.V. All rights reserved. Keywords: Thermoelectric materials; Hot extrusion; Finite element analysis; Bismuth telluride; Orientated microstructure
1. Introduction Bismuth telluride based materials are widely used as thermoelectric materials for electronic cooling devices. Bi2 Te3 belongs ¯ space group, in which the hexagonally five-atomic to R3m series stacks along the c-axis with weak van der Waals bonding, and thermoelectric properties show a large anisotropy, with TE properties of the normal direction of c axis twice that of the parallel direction [1]. Conventional preparation method is melting-based, yet materials prepared by this way are inclined to fracture when subjected to machine. In order to achieve materials with good mechanical properties, powder metallurgy (PM) are commonly used. Traditional PM route consists melting, pulverization, and consolidation, and the procedure are too long and the cost is high. Many researchers utilized the more costeffective mechanical alloying (MA) method to prepare the alloy ∗ Corresponding author at: School of Materials Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, PC 430074, PR China. Fax: +86 27 87543776. E-mail address: [email protected] (J. Yang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.04.030
powder. Some of the typical work includes, PIES by Tokiai and Koumoto [2], BMA by Yang et al. [3,4], MA by Hyun et al. [5], Martin-Lopez et al. [6] and Yang et al. [7], etc. Yet thermoelectric properties of materials obtained by PM are not so satisfying due to the loss of anisotropy characteristic, especially for n-type materials. In order to achieve a good combination of mechanical and thermoelectric properties, one feasible way is to control the microstructure through plastic deformation, e.g. to develop texture or refine the grains by means of hot extrusion (HE) [8], powder extrusion [9], equal channel angular extrusion (ECAE) [10] or shear extrusion [11]. The ECAE method gained attention because of its ability to fine grain and achieve texture without area reduction like hot extrusion. Yet for the as-MAed powders, there is no need to refine the grains again, and commonly used HE route are more meaningful for practical production. Seo et al. have examined the HE research of pulverized melting materials, and a figure of merit of 2.62 × 10−3 K−1 for n-type [8] and 3.05 × 10−3 K−1 for p-type [12] Bi2 Te3 -based materials have been achieved. In order to obtain Bi2 Te3 -based materials with good thermoelectric materials, we tried the MA + HE route in this work. To optimize the die design and reduce the possibil-
J. Yang et al. / Journal of Alloys and Compounds 429 (2007) 156–162
Fig. 1. Schematic setup of hot extrusion dies.
157
Fig. 2. Flow stress–strain model used in our simulation.
ity of defects in the hot extruded sample, finite element method (FEM) was conducted to simulate the effect of HE parameters. Materials obtained by HE shows an orientated microstructure, and a great improvement to the power factor was achieved. 2. Finite element analysis Fig. 1 shows the schematic design of the hot extrusion die. The extrusion ratio is assumed to be D2 /d2 , and θ is the extrusion angle, and the work hole length is l. This configuration is used in FEM simulation and subsequent HE experiments. Here D is set to be 20 mm, l is 26 mm, and θ is 60◦ and 45◦ respectively. A DEFORM2D FEM tool is used to conduct the HE simulation. The parameters used in simulation are listed in Table 1. The Young’s modulus, Poisson’s ratio and UTS are required in DEFORM2D to conduct simulation. According to Ref. [13], the values are set to be 40 GPa, 0.3, and 105 MPa respectively. There is little work about the strain–stress analysis on plastic deformation of Bi2 Te3 -based materials [14], and flow stress data is unavailable, thus we used a presumed flow stress according to UTS. The flow stress and strain relation is assumed and shown in Fig. 2. Table 1 Parameters investigated in simulation process Simulation no.
Extrusion angle (◦ )
Extrusion ratio
Friction coefficient, f
S1 S2 S3 S4
60 60 60 45
100:9 100:9 25:4 25:4
0 0.04 0.04 0.04
The ram speed and friction coefficient are set to be 0.1 mm/s and 0.04, respectively, which are in consistence with the hot extrusion experiments. The number of the initial mesh is 3500. For fracture analysis, the Max (effective stress/UTS) model is used, and the critical value was set to be 150 MPa, which is higher than 105 MPa for simulation considerations and are in good accordance with the hot extrusion experiments. 2.1. Simulation analysis of with/without friction extrusion case In order to illustrate the influence of friction on the extrusion and deformation behavior, two simulations, S1 and S2, with no friction and 0.04 friction coefficient respectively, are conducted for comparison. Fig. 3a1 and a2 show the simulation results for effective strain at S1 and S2; Fig. 3b1 and b2 show the maximal principal stress field for S1 and S2. As shown in Fig. 3a1 and a2, friction condition causes great difference in the effective strain distribution. For the f = 0.04 case, there is an ‘H’-zone (see Fig. 3a2), which lies along the extrusion direction. This suggests that friction force will drag the workpiece in the surface during the deformation process, causing tension stress in this region. In Fig. 3b1, there is an ‘F’ region with tension stress (positive value for stress means tension). Such a surface tension region is due to the different deformation speed of the work piece during the extrusion, with the central part of the workpiece deforms faster, thus dragging the low-speed surface region and causing tension. The maximal tensile stress in the frictionless S1 case (Fig. 3b1) is 67.7 MPa, which is far lower than that in the S2 case (338 MPa). Apparently the deformation speed caused surface tension is less than that induced by the combination of the friction and deformation speed. This surface tensile stress
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Fig. 3. (a) Effective strain distribution for the cases of 1: S1, 2: S2. (b) Principal stress field distribution for the cases of 1: S1, 2: S2.
should be the main reason for the annular cracks in the workpiece surface along the extrusion direction, therefore extruding in a low speed and using a lubricant and keeping a good surface finish to the extrusion die are very necessary to get a crack-free workpiece. Fig. 4 shows the simulation results for the extrusion conditions of S3 and S4. In Fig. 4a1 and a2, the effective strains show similar patterns in B, C, D, E curves for the extrusion angle of 60–45◦ , however, the F, G, H, I curves show apparent difference in shape. Furthermore, the maximal effective strain value in the
45◦ extrusion angle case is lower than those in the 60◦ case. In Fig. 4b1 and b2, we can see that the principal stress field distribution in S3 case is quite similar with that in S4 case, however, the maximal principal stress in S3 case is 234 MPa and it is quite larger than that in S4 case. Fig. 5 shows the flow net of S4 during the extrusion process. It is necessary to mention that, the frictionless simulation results show the same pattern of flow net, suggesting that the deformation is mainly controlled by the die geometry rather than the friction effect.
J. Yang et al. / Journal of Alloys and Compounds 429 (2007) 156–162
159
Fig. 4. (a) Effective strain distribution for the cases of 1: S3, 2: S4. (b) Principle stress distribution for the cases of 1: S3, 2: S4.
3. Experimental Elemental powders of Bi (99.99%, 200 mesh), Te (99.99%, 200 mesh) and Se (99.99%, 100 mesh) are weighed according to the composition of (Bi2 Te3 )0.95 (Bi2 Se3 )0.05 , and charged into the stainless steel vial with stainless steel balls under Ar atmosphere. The ball-to-material weight ratio is 15:1. Mechanical alloying was conducted in a planetary ballmill at a speed of 400 rpm for 10 h. The workpiece used for HE is preformed by hot press in Ar atmosphere at 500 ◦ C for 2 h. For HE process, temperature is also set as 500 ◦ C, the ram speed is 1 mm/s. After the HE process, a bar of 4 mm × 4 mm × 17 mm was cut out from the hot extruded sample for thermoelectric property measurement. Seebeck coefficient was tested by applying a 10 K temperature difference between the two ends of the bar specimen and measuring the output voltage between them and then calculating the value by α = V/T, and the electrical resistivity was tested by using the standard four-probe method.
In order to study the influence of different HE parameters on microstructure and thermoelectric properties of the hot extruded sample, a FEI sirion 200 FE-SEM apparatus was used to observe the microstructure, and X-ray diffraction was performed with a PANalytical X’pert PRO diffractometer by using Cu K␣ radiation. Density was measured using the well know Archimedes method.
4. Results and discussion 4.1. Microstructure analysis To study the influence of hot extrusion parameters on the microstructure and thermoelectric properties of n-type
160
J. Yang et al. / Journal of Alloys and Compounds 429 (2007) 156–162
Fig. 5. The flow net of S4.
(Bi2 Te3 )0.95 (Bi2 Se3 )0.05 materials, we conducted the extrusion experiments with three different dies, corresponding to different extrusion conditions shown in Table 1. Fig. 6 shows the XRD patterns of the hot extruded samples and the corresponding MA powders. It is clearly seen that the (0 0 6) and (0 0 15) peaks get stronger in the hot extruded samples, indicating the formation of a (0 0 l) preferred orientated microstructure during the
HE process, and SEM micrographs shown in Fig. 7 also give a direct proof to the orientation effect induced by the hot extrusion process. The orientation factor f can be calculated using the Lotgering method [15]: f = P − P0 /1 − P0 , where P is the fraction of (0 0 l) plane diffraction intensity, P = I(0,0,l)/I(h,k,l), P0 is the value of P for isotropic powder samples.
J. Yang et al. / Journal of Alloys and Compounds 429 (2007) 156–162
161
clearly seen that the hot pressed sample shows a microstructure with random grains, while the hot extruded sample shows orientated stripe-like grains, which is in good accordance with the result of XRD analysis, obviously, crystalline reorientation was induced and developed during the hot extrusion process. 4.2. Thermoelectric properties
Fig. 6. XRD patterns of the as-HEed samples and the as-MAed powders.
The respective Lotgering factors of S2, S3 and S4 are listed in Table 2. It can be seen that a Lotgering factor of 0.169 has been achieved for S2 sample, which is in good accordance with the results by Kim et al. [11]. Fig. 7 shows the SEM micrographs of the hot extruded samples parallel to the extrusion direction, and the micrograph of the hot pressed sample was also given for comparison. It can be
Thermoelectric properties along with the relative densities of the extruded samples were shown in Table 2. According to the above simulation, considering the large extrusion ratio for S2, the amount of plastic deformation should be S2 > S3 ≈ S4, thus we can expect that there will be such a relation for the orientation factor in the extruded samples: S2 > S3 ≈ S4. We can clearly see that, the Lotgering factors shown in Table 2 for S2, S3 and S4 are 0.169, 0.134 and 0.136, respectively, which is in very good accordance with the expectations from the simulation. As mentioned above, thermoelectric properties of single crystal Bi2 Te3 show a large anisotropy, the thermoelectric properties in the normal direction of c axis is twice that of the parallel direction. As shown in Fig. 6, the hot extruded sample forms a (0 0 l) preferred orientation, this is just the base plane that is perpendicular to the c-axis of Bi2 Te3 rhombohedral structure, therefore the orientated hot extruded samples show lower electrical resistivity and thus lower Seebeck coefficient than the randomly orientated hot pressed sample in Table 2, and the total power factor is greatly improved owing to the formation of orientated microstructure. With the exception between S3 and S4, we could conclude that
Fig. 7. SEM fractographs of the hot pressed and the hot extruded samples: (a) 1, (b) S2, (c) S3, and (d) S4.
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Table 2 Thermoelectric properties, density and Lotgering factor of the extruded sample under different extrusion conditions (where 1 denotes the data of the correspondent HP sample used for hot extrusion) Sample
Extrusion ratio
Extrusion angle (◦ )
Resistivity ( m)
Seebeck (V/K)
Relative density (%)
Lotgering factor
Power factor (×10−3 W/m K2 )
1 S2 S3 S4
– 100:9 25:4 25:4
– 60 60 45
24.6 6.73 12.53 12.03
−178.0 −131.6 −171.7 −147.0
90.3 96.9 95.7 98.7
– 0.169 0.134 0.136
1.29 2.57 2.35 1.80
the power factor increases with increasing of the Lotgering factor of the hot extruded sample. For the sample of S3 and S4, they have almost the same Lotgering factor, however, there is a large difference between their power factors, the S3 sample has more micro-pores (see Fig. 7c) and thus lower relative density than that of the S4 sample, which may originate from the larger extrusion angle in S3, which means shorter or smaller regions of deformation and higher deformation rate and thus more micro-pores formed during the hot extrusion process (Fig. 7), and these micro-pores augment the electron scattering and thus increases the Seebeck coefficient and the resultant power factor. 5. Conclusion Bismuth telluride based n-type thermoelectric materials were prepared using mechanical alloying (MA) and the subsequent hot extrusion (HE) process. Firstly we used FEM tools to simulate the HE process, and it is found that friction condition has large influence on the surface stress state, making the surface region under more tension stress, thus increase the possibility of surface crack. At the same time, extrusion ratio has significant influence on the degree of deformation. Extrusion angle also shows effect on the deformation, with longer or larger regions of deformation in the case of low extrusion angle, which means lower deformation rate and will cause less micro-holes. Then n-type Bi2 Te3 based materials were hot extruded, and effect of extrusion parameters on microstructure and thermoelectric properties of the materials was studied, and a (0 0 l) preferred orientation microstructure was formed in the hot extruded sample. With increasing the extrusion ratio, the Lotgering orientation factor of the extruded sample increases, and the power factor
also shows a magnificent increase. The experimental results are in consistence with the results of FEM analysis. Acknowledgements This work is co-financed by the Prophasic Project of National Key Basic Research Program (2004CCA03200) and Natural Science Foundation of China (Grant No. 50401008). References [1] R.T. Delves, A.E. Bowley, D.W. Hazelden, H.J. Goldsmid, Proc. Phys. Soc. 78 (1961) 838–844. [2] T. Tokiai, K. Koumoto, Proceedings of the 17th International Conference on Thermoelectrics, Yokohama, Japan, 1998, pp. 170–173. [3] J.Y. Yang, T. Aizawa, A. Yamamoto, T. Ohta, Mater. Chem. Phys. 70 (2001) 90–94. [4] J.Y. Yang, T. Aizawa, A. Yamamoto, T. Ohta, J. Alloys Compd. 312 (1/2) (2000) 326–330. [5] D.B. Hyun, T.S. Oh, C.W. Hwang, J. Mater. Sci. 33 (1998) 5598–5600. [6] R. Martin-Lopez, A. Dauscher, H. Scherrer, Appl. Phys. A 68 (1999) 597–602. [7] J.Y. Yang, R.G. Chen, X.A. Fan, S.Q. Bao, W. Zhu, J. Alloy Compd. 407 (1–2) (2006) 330–333. [8] J. Seo, K. Park, C. Lee, Mater. Res. Bull. 33 (4) (1998) 553–559. [9] S.J. Hong, B.S. Chun, Mater. Sci. Eng. A 356 (2003) 345–351. [10] J.T. Im, K.Ted. Hartwig, J. Sharp, Acta Mater. 52 (2004) 49–55. [11] S.S. Kim, S. Yamamoto, T. Aizawa, J. Alloy Compd. 375 (2004) 107– 113. [12] J. Seo, C. Lee, K. Park, Mater. Sci. Eng. B 54 (1998) 135–140. [13] J.M. Simard, D. Vasilevskiy, F. Belanger, Proceedings of the 20th International Conference on Thermoelectrics, 2001, pp. 132–134. [14] O.B. Sokolov, S.Ya. Skipidarov, N.I. Duvankov, Proceedings of the Second European Conference on Thermoelectrics, 1996 see: http://galaxy.uci.agh.edu.pl/∼ets2004/proceedings. [15] F.K. Lotgering, J. Inorg. Nucl. Chem. 9 (1959) 113.
Microstructure control and thermoelectric properties improvement to n-type bismuth telluride based materials by hot extrusion Junyou Yang ∗ , Rougang Chen, Xi’an Fan, Wen Zhu, Siqian Bao, Xingkai Duan State Key Laboratory for Plastic Forming Simulation and Dies Technology, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, PR China Received 6 February 2006; received in revised form 12 April 2006; accepted 12 April 2006 Available online 15 May 2006
Abstract In this paper, hot extrusion process of Bi2 Te3 based thermoelectric materials was simulated with FEM tools firstly, it is found that the friction condition between the extrusion die and the sample has large influence on the surface stress state of the sample, making the surface region under more tension stress, thus increases the possibility of surface crack. Extrusion ratio has significant influence on the degree of deformation. Extrusion angle also shows effect on the deformation, low extrusion angle die has longer or larger regions of deformation, which means lower deformation rate and will cause less micro-pores. Then n-type Bi2 Te3 based materials were also hot extruded under different experiment conditions, and effect of extrusion parameters on microstructure and thermoelectric properties of the materials was studied, and a (0 0 l) preferred orientation microstructure was formed in the hot extruded samples. With increasing the extrusion ratio, the Lotgering orientation factor of the extruded sample increases, and the power factor also shows a magnificent increase. The experimental results are in consistence with the results of FEM analysis. © 2006 Elsevier B.V. All rights reserved. Keywords: Thermoelectric materials; Hot extrusion; Finite element analysis; Bismuth telluride; Orientated microstructure
1. Introduction Bismuth telluride based materials are widely used as thermoelectric materials for electronic cooling devices. Bi2 Te3 belongs ¯ space group, in which the hexagonally five-atomic to R3m series stacks along the c-axis with weak van der Waals bonding, and thermoelectric properties show a large anisotropy, with TE properties of the normal direction of c axis twice that of the parallel direction [1]. Conventional preparation method is melting-based, yet materials prepared by this way are inclined to fracture when subjected to machine. In order to achieve materials with good mechanical properties, powder metallurgy (PM) are commonly used. Traditional PM route consists melting, pulverization, and consolidation, and the procedure are too long and the cost is high. Many researchers utilized the more costeffective mechanical alloying (MA) method to prepare the alloy ∗ Corresponding author at: School of Materials Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, PC 430074, PR China. Fax: +86 27 87543776. E-mail address: [email protected] (J. Yang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.04.030
powder. Some of the typical work includes, PIES by Tokiai and Koumoto [2], BMA by Yang et al. [3,4], MA by Hyun et al. [5], Martin-Lopez et al. [6] and Yang et al. [7], etc. Yet thermoelectric properties of materials obtained by PM are not so satisfying due to the loss of anisotropy characteristic, especially for n-type materials. In order to achieve a good combination of mechanical and thermoelectric properties, one feasible way is to control the microstructure through plastic deformation, e.g. to develop texture or refine the grains by means of hot extrusion (HE) [8], powder extrusion [9], equal channel angular extrusion (ECAE) [10] or shear extrusion [11]. The ECAE method gained attention because of its ability to fine grain and achieve texture without area reduction like hot extrusion. Yet for the as-MAed powders, there is no need to refine the grains again, and commonly used HE route are more meaningful for practical production. Seo et al. have examined the HE research of pulverized melting materials, and a figure of merit of 2.62 × 10−3 K−1 for n-type [8] and 3.05 × 10−3 K−1 for p-type [12] Bi2 Te3 -based materials have been achieved. In order to obtain Bi2 Te3 -based materials with good thermoelectric materials, we tried the MA + HE route in this work. To optimize the die design and reduce the possibil-
J. Yang et al. / Journal of Alloys and Compounds 429 (2007) 156–162
Fig. 1. Schematic setup of hot extrusion dies.
157
Fig. 2. Flow stress–strain model used in our simulation.
ity of defects in the hot extruded sample, finite element method (FEM) was conducted to simulate the effect of HE parameters. Materials obtained by HE shows an orientated microstructure, and a great improvement to the power factor was achieved. 2. Finite element analysis Fig. 1 shows the schematic design of the hot extrusion die. The extrusion ratio is assumed to be D2 /d2 , and θ is the extrusion angle, and the work hole length is l. This configuration is used in FEM simulation and subsequent HE experiments. Here D is set to be 20 mm, l is 26 mm, and θ is 60◦ and 45◦ respectively. A DEFORM2D FEM tool is used to conduct the HE simulation. The parameters used in simulation are listed in Table 1. The Young’s modulus, Poisson’s ratio and UTS are required in DEFORM2D to conduct simulation. According to Ref. [13], the values are set to be 40 GPa, 0.3, and 105 MPa respectively. There is little work about the strain–stress analysis on plastic deformation of Bi2 Te3 -based materials [14], and flow stress data is unavailable, thus we used a presumed flow stress according to UTS. The flow stress and strain relation is assumed and shown in Fig. 2. Table 1 Parameters investigated in simulation process Simulation no.
Extrusion angle (◦ )
Extrusion ratio
Friction coefficient, f
S1 S2 S3 S4
60 60 60 45
100:9 100:9 25:4 25:4
0 0.04 0.04 0.04
The ram speed and friction coefficient are set to be 0.1 mm/s and 0.04, respectively, which are in consistence with the hot extrusion experiments. The number of the initial mesh is 3500. For fracture analysis, the Max (effective stress/UTS) model is used, and the critical value was set to be 150 MPa, which is higher than 105 MPa for simulation considerations and are in good accordance with the hot extrusion experiments. 2.1. Simulation analysis of with/without friction extrusion case In order to illustrate the influence of friction on the extrusion and deformation behavior, two simulations, S1 and S2, with no friction and 0.04 friction coefficient respectively, are conducted for comparison. Fig. 3a1 and a2 show the simulation results for effective strain at S1 and S2; Fig. 3b1 and b2 show the maximal principal stress field for S1 and S2. As shown in Fig. 3a1 and a2, friction condition causes great difference in the effective strain distribution. For the f = 0.04 case, there is an ‘H’-zone (see Fig. 3a2), which lies along the extrusion direction. This suggests that friction force will drag the workpiece in the surface during the deformation process, causing tension stress in this region. In Fig. 3b1, there is an ‘F’ region with tension stress (positive value for stress means tension). Such a surface tension region is due to the different deformation speed of the work piece during the extrusion, with the central part of the workpiece deforms faster, thus dragging the low-speed surface region and causing tension. The maximal tensile stress in the frictionless S1 case (Fig. 3b1) is 67.7 MPa, which is far lower than that in the S2 case (338 MPa). Apparently the deformation speed caused surface tension is less than that induced by the combination of the friction and deformation speed. This surface tensile stress
158
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Fig. 3. (a) Effective strain distribution for the cases of 1: S1, 2: S2. (b) Principal stress field distribution for the cases of 1: S1, 2: S2.
should be the main reason for the annular cracks in the workpiece surface along the extrusion direction, therefore extruding in a low speed and using a lubricant and keeping a good surface finish to the extrusion die are very necessary to get a crack-free workpiece. Fig. 4 shows the simulation results for the extrusion conditions of S3 and S4. In Fig. 4a1 and a2, the effective strains show similar patterns in B, C, D, E curves for the extrusion angle of 60–45◦ , however, the F, G, H, I curves show apparent difference in shape. Furthermore, the maximal effective strain value in the
45◦ extrusion angle case is lower than those in the 60◦ case. In Fig. 4b1 and b2, we can see that the principal stress field distribution in S3 case is quite similar with that in S4 case, however, the maximal principal stress in S3 case is 234 MPa and it is quite larger than that in S4 case. Fig. 5 shows the flow net of S4 during the extrusion process. It is necessary to mention that, the frictionless simulation results show the same pattern of flow net, suggesting that the deformation is mainly controlled by the die geometry rather than the friction effect.
J. Yang et al. / Journal of Alloys and Compounds 429 (2007) 156–162
159
Fig. 4. (a) Effective strain distribution for the cases of 1: S3, 2: S4. (b) Principle stress distribution for the cases of 1: S3, 2: S4.
3. Experimental Elemental powders of Bi (99.99%, 200 mesh), Te (99.99%, 200 mesh) and Se (99.99%, 100 mesh) are weighed according to the composition of (Bi2 Te3 )0.95 (Bi2 Se3 )0.05 , and charged into the stainless steel vial with stainless steel balls under Ar atmosphere. The ball-to-material weight ratio is 15:1. Mechanical alloying was conducted in a planetary ballmill at a speed of 400 rpm for 10 h. The workpiece used for HE is preformed by hot press in Ar atmosphere at 500 ◦ C for 2 h. For HE process, temperature is also set as 500 ◦ C, the ram speed is 1 mm/s. After the HE process, a bar of 4 mm × 4 mm × 17 mm was cut out from the hot extruded sample for thermoelectric property measurement. Seebeck coefficient was tested by applying a 10 K temperature difference between the two ends of the bar specimen and measuring the output voltage between them and then calculating the value by α = V/T, and the electrical resistivity was tested by using the standard four-probe method.
In order to study the influence of different HE parameters on microstructure and thermoelectric properties of the hot extruded sample, a FEI sirion 200 FE-SEM apparatus was used to observe the microstructure, and X-ray diffraction was performed with a PANalytical X’pert PRO diffractometer by using Cu K␣ radiation. Density was measured using the well know Archimedes method.
4. Results and discussion 4.1. Microstructure analysis To study the influence of hot extrusion parameters on the microstructure and thermoelectric properties of n-type
160
J. Yang et al. / Journal of Alloys and Compounds 429 (2007) 156–162
Fig. 5. The flow net of S4.
(Bi2 Te3 )0.95 (Bi2 Se3 )0.05 materials, we conducted the extrusion experiments with three different dies, corresponding to different extrusion conditions shown in Table 1. Fig. 6 shows the XRD patterns of the hot extruded samples and the corresponding MA powders. It is clearly seen that the (0 0 6) and (0 0 15) peaks get stronger in the hot extruded samples, indicating the formation of a (0 0 l) preferred orientated microstructure during the
HE process, and SEM micrographs shown in Fig. 7 also give a direct proof to the orientation effect induced by the hot extrusion process. The orientation factor f can be calculated using the Lotgering method [15]: f = P − P0 /1 − P0 , where P is the fraction of (0 0 l) plane diffraction intensity, P = I(0,0,l)/I(h,k,l), P0 is the value of P for isotropic powder samples.
J. Yang et al. / Journal of Alloys and Compounds 429 (2007) 156–162
161
clearly seen that the hot pressed sample shows a microstructure with random grains, while the hot extruded sample shows orientated stripe-like grains, which is in good accordance with the result of XRD analysis, obviously, crystalline reorientation was induced and developed during the hot extrusion process. 4.2. Thermoelectric properties
Fig. 6. XRD patterns of the as-HEed samples and the as-MAed powders.
The respective Lotgering factors of S2, S3 and S4 are listed in Table 2. It can be seen that a Lotgering factor of 0.169 has been achieved for S2 sample, which is in good accordance with the results by Kim et al. [11]. Fig. 7 shows the SEM micrographs of the hot extruded samples parallel to the extrusion direction, and the micrograph of the hot pressed sample was also given for comparison. It can be
Thermoelectric properties along with the relative densities of the extruded samples were shown in Table 2. According to the above simulation, considering the large extrusion ratio for S2, the amount of plastic deformation should be S2 > S3 ≈ S4, thus we can expect that there will be such a relation for the orientation factor in the extruded samples: S2 > S3 ≈ S4. We can clearly see that, the Lotgering factors shown in Table 2 for S2, S3 and S4 are 0.169, 0.134 and 0.136, respectively, which is in very good accordance with the expectations from the simulation. As mentioned above, thermoelectric properties of single crystal Bi2 Te3 show a large anisotropy, the thermoelectric properties in the normal direction of c axis is twice that of the parallel direction. As shown in Fig. 6, the hot extruded sample forms a (0 0 l) preferred orientation, this is just the base plane that is perpendicular to the c-axis of Bi2 Te3 rhombohedral structure, therefore the orientated hot extruded samples show lower electrical resistivity and thus lower Seebeck coefficient than the randomly orientated hot pressed sample in Table 2, and the total power factor is greatly improved owing to the formation of orientated microstructure. With the exception between S3 and S4, we could conclude that
Fig. 7. SEM fractographs of the hot pressed and the hot extruded samples: (a) 1, (b) S2, (c) S3, and (d) S4.
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Table 2 Thermoelectric properties, density and Lotgering factor of the extruded sample under different extrusion conditions (where 1 denotes the data of the correspondent HP sample used for hot extrusion) Sample
Extrusion ratio
Extrusion angle (◦ )
Resistivity ( m)
Seebeck (V/K)
Relative density (%)
Lotgering factor
Power factor (×10−3 W/m K2 )
1 S2 S3 S4
– 100:9 25:4 25:4
– 60 60 45
24.6 6.73 12.53 12.03
−178.0 −131.6 −171.7 −147.0
90.3 96.9 95.7 98.7
– 0.169 0.134 0.136
1.29 2.57 2.35 1.80
the power factor increases with increasing of the Lotgering factor of the hot extruded sample. For the sample of S3 and S4, they have almost the same Lotgering factor, however, there is a large difference between their power factors, the S3 sample has more micro-pores (see Fig. 7c) and thus lower relative density than that of the S4 sample, which may originate from the larger extrusion angle in S3, which means shorter or smaller regions of deformation and higher deformation rate and thus more micro-pores formed during the hot extrusion process (Fig. 7), and these micro-pores augment the electron scattering and thus increases the Seebeck coefficient and the resultant power factor. 5. Conclusion Bismuth telluride based n-type thermoelectric materials were prepared using mechanical alloying (MA) and the subsequent hot extrusion (HE) process. Firstly we used FEM tools to simulate the HE process, and it is found that friction condition has large influence on the surface stress state, making the surface region under more tension stress, thus increase the possibility of surface crack. At the same time, extrusion ratio has significant influence on the degree of deformation. Extrusion angle also shows effect on the deformation, with longer or larger regions of deformation in the case of low extrusion angle, which means lower deformation rate and will cause less micro-holes. Then n-type Bi2 Te3 based materials were hot extruded, and effect of extrusion parameters on microstructure and thermoelectric properties of the materials was studied, and a (0 0 l) preferred orientation microstructure was formed in the hot extruded sample. With increasing the extrusion ratio, the Lotgering orientation factor of the extruded sample increases, and the power factor
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