Research Paper. Isothermal solidification bonding of Bi2Te2.55Se0.45 thermoelectric material with Cu electrodes

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1 Engineering & Technology Research 3(3): , February 2019 DOI: /etr Academia Publishing Research Paper Isothermal solidification bonding of Bi2Te2.55Se0.45 thermoelectric material with Cu electrodes Accepted ABSTRACT Yan-Cheng Lin, Chun-HaoChen*, Jui-Hung Yuan,Yu-Kai Sun, Kuan-Yu Chiu, Pei-Ing Lee, Wun-Hua Chang and Tung-Han Chuang Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan. *Corresponding author. Isothermal solidification bonding has previously been used to join a Bi 2Te 2.55Se 0.45 TE material with Cu electrodes at 250 C using a Sn thin film, and the bond had a maximal shear strength of 21.8 MPa. In the present study, the Sn interlayer was replaced with indium (In) thin film to decrease the bonding temperature. The results indicated that Ag 3In/Ag 2In intermetallic compounds formed during bonding at 200 to 250 C for 30 min under a bonding pressure of 3 MPa had satisfactory shear strengths of 15.0 to 19.7 MPa. However, the maximal shear strength achieved with bonding at 175 C for 30 min was only 10.5 MPa. Increasing the external pressure from 3 to 9.8 MPa improved the bonding strength of Bi 2Te 2.55Se 0.45/Cu joints bonded at 175 C for 30 min from 10.5 to 14.3 MPa. The bonding strength of Bi 2Te 2.55Se 0.45/Cu joints was further improved to 19.3 MPa by isothermal solidification bonding using an In interlayer at 250 C under external pressure of 9.8 MPa. Key words: Bi 2Te 2.55Se 0.45 thermoelectric material, intermetallic compounds, indium thin film, isothermal solidification bonding. INTRODUCTION Thermoelectric (TE) materials, which can transform thermal energy into electrical energy or vice versa, have been applied to the generation of electricity from waste heat below 300 C in steel-making plants. Among the numerous TE materials, the bismuth telluride (BiTe) intermetallic compound possesses the maximal TE efficiency of about 100 C and is the most common system to be used in industry. A conventional method to assemble BiTe modules is to solder multiple elements of N-Type Bi 2Te 2.55Se 0.45 and P-Type Bi 0.5Sb 1.5Te 3 with metallic electrodes (Zhang et al., 2013). However, the reliability of such soldered TE modules raises concerns due to the low melting point of the solder alloy. In addition, the excessive molten solder may wick up the sides of TE pellets and cause electrical shortage between TE couples (Ritzer et al., 1997). Isothermal solidification bonding, also called solid liquid inter-diffusion bonding or diffusion soldering, was developed for the manufacturing of thermal stable interconnections in electronic packaging through sandwiching a low-melting metallic thin film between high temperature metallic layers for reacting at the interfaces to form intermetallic compounds (Khanna et al., 1995). It has also been successfully applied to produce Bi 2Te 2.55Se 0.45 (Chuang et al., 2016), Bi 0.5Sb 1.5Te 3 (Yang et al., 2013a), (Pb,Sn)Te (Chuang et al., 2014), GeTe (Yang et al., 2013b), and Zn 3Sb 4 (Lin et al., 2016a) thermoelectric (TE) modules using a Sn thin interlayer to react completely with an Ag layer on TE elements and Cu electrodes and thereby form high-melting Ag 3Sn intermetallic compounds. It has been found that bonding P-Type Bi 0.5Sb 1.5Te 3 with a Cu electrode using a Sn interlayer at 300 C for 30 min resulted in a maximal shear strength of 10.7 MPa (Yang et al., 2013a). To lower the bonding temperature of this Bi 0.5Sb 1.5Te 3/Cu joint, SLID was performed using an In interlayer in previous study (Lin et al., 2016b). Due to the low melting point of In at 157 C, the bonding temperature could be reduced to 175 C. The resultant Ag 2In and Ag 3In intermetallic layers at the Ag/In/Ag interfaces had melting points of 320 and 690 C, respectively, and the bonded Bi 0.5Sb 1.5Te 3/electrode joints could endure operating temperatures higher than

2 Figure 1: Intermetallic compounds at the interfaces of the Bi2Te2.55Se0.45/Cu joints with In interlayers bonded with the modified isothermal solidification bonding process under an external pressure of 3 MPa at 175 C for various times: (a) 5 min, (b) 10 min, (c) 30 min. 300 C. However, voids and cracks often appeared at the Ag 2In/Ag 3In interfaces after isothermal solidification bonding at a low bonding temperature of 175 C under a pressure of 3 MPa, leading to insufficient bonding strengths of 7.1 to 8.5 MPa. To solve this problem, the external pressure was increased to 9.8 MPa, and the bonding strengths were effectively improved to a range of 10.6 to 11.7 MPa. Unfortunately, the improvement in bonding strengths from increasing the external pressure did not occur for a Bi 0.5Sb 1.5Te 3/Cu joint bonded at temperatures of 200 to 250 C. For the production of a bismuth telluride (BiTe) thermoelectric module, isothermal solidification bonding of another N-type Bi 2Te 2.55Se 0.45material at 250 C for 30 min with a Cu electrode and a Sn interlayer achieved a maximal shear strength of 21.8 MPa (Chuang et al., 2016). The bonding strength of this Bi 2Te 2.55Se 0.45/Cu joint using an In interlayer at 175 C can also be improved from 10.5 to 14.3 MPa through the employment of an external pressure of 3 MPa. In contrast to the Bi 0.5Sb 1.5Te 3/Cu joint, isothermal solidification bonding of a Bi 2Te 2.55Se 0.45/Cu joint at 250 C with an In interlayer can increase the bonding strength to 19.3 MPa. The discrepancy in the improvement effects of external applied pressure is attributed to the lower compressive strength of Bi 0.5Sb 1.5Te 3in comparison with that of Bi 2Te 2.55Se MATERIALS AND METHODS For this experiments, an N-type Bi 2Te 2.55Se 0.45 thermoelectric material was produced with vacuum melting and zone refining processes similar to those used for Bi 2Te 2.55Se 0.45 and Bi 0.5Sb 1.5Te 3 in previous studies (Chuang et al., 2016; Yang et al., 2013a). For the isothermal solidification bonding of this Bi 2Te 2.55Se 0.45 thermoelectric element with Cu electrode, the specimens were pre-coated with a 1 μm thickness of Sn film and subsequently heated at 250 C for 3 min, after which they were coated with an additional 4 μm thick Ni barrier layer and a 10 μm thick Ag reaction layer. The pre-treated Bi 2Te 2.55Se 0.45 thermoelectric elements were bonded with an Ag-coated Cu electrode using a 4 μm In thin film interlayer in a vacuum furnace of 5.3 x 10-4 Pa at temperatures of 175 to 250 C for durations of 5 to 30 min under a pressure of 3 MPa. To verify the effect of external pressure on the bonding strength, the applied pressure was increased to 9.8 MPa. The interfaces of Bi 2Te 2.55Se 0.45/Cu joints were observed by scanning electron microscopy (SEM) and analyzed with energy dispersive X-ray spectroscopy (EDX). The shear strengths of the SLID-bonded specimens under various bonding conditions were measured using a DAGE 4000 Bond Tester at a speed of 0.3 mm/s. Finally, the fractured surfaces of specimens after shear testing were also observed by SEM to identify the fracture mode. RESULTS AND DISCUSSION Figure 1 shows the microstructures of the interfaces after bonding at 175 C for various times. Similar to those of interfaces observed in a previous study, in which Sn thin

3 Figure 2: Intermetallic compounds at the interfaces of the Bi2Te2.55Se0.45/Cu joints with In interlayers bonded with the modified isothermal solidification bonding process under an external pressure of 3 MPa at various temperatures for 30 min: (a) 200 C, (b)225 C, (c) 250 C. film was used as the interlayer for bonding a Bi 2Te 2.55Se 0.45/Cu joint (Zhang et al., 2013), a very thin layer of Bi-rich phase (white in the figure) with a composition (at.%) of Bi : Te : Se = 52.2 : 35.9 : 11.9 appeared on the surface of the Bi 2Te 2.55Se 0.45 thermoelectric material. In addition, a thick layer of Sn-rich phase (gray in the figure) is embedded with Bi-rich islands (white in the figure) with compositions (at.%) of Sn : Te : Se : Bi = 43.0 : 36.5 : 9.8 : 10.7 and Bi : Te : Se : Sn = 53.7 : 37.5 : 5.2 : 3.6, respectively, formed between the Bi-Te-Se thin layer and the Ni barrier layer. In contrast to the formation of a Ag 3Sn intermetallic layer in the bonded Bi 2Te 2.55Se 0.45/Cu joints using a Sn interlayer (Chuang et al., 2016), the reaction of the In thin film interlayer with the Ag layer formed a double layer of Ag 3In and Ag 2In intermetallic compounds in the present study. However, it should be noted that, as shown in Figure 1, many voids and cracks were found between the Ag 3In and Ag 2In intermetallic layers of Bi 2Te 2.55Se 0.45/Cu joints bonded at 175 C. Figure 2 shows that after the bonding temperatures were raised above 200 C for 30 min, far fewer voids and cracks appeared at the Ag 3In/Ag 2In interfaces as compared with those in specimens bonded at 175 C. As shown in Figure 3, the shear strengths of the Bi 2Te 2.55Se 0.45 thermoelectric materials bonded with Cu electrodes increased as the bonding times and temperatures increased, rising to satisfactory values of 12.8 to 19.8 MPa at bonding temperatures of 200 to 250 C and bonding times of 10 to 30 min. Figure 4a indicates that the shear tested Bi 2Te 2.55Se 0.45/Cu joints fractured mainly in the interior of the Bi 2Te 2.55Se 0.45 thermoelectric material. However, Figure 3 also shows that the strengths of the Bi 2Te 2.55Se 0.45/Cu joints bonded at 175 C were much lower (5.4 to 10.5 MPa) than those of joints bonded at temperatures of 200 to 250 C. It is evidenced in Figure 4(b) that the shear tested specimens bonded at 175 C fractured along the interfaces between the Ag 3In and Ag 2In intermetallic phases. The insufficient bonding effect of Bi 2Te 2.55Se 0.45/Cu joints at 175 C could be linked to the more frequent appearance of voids and cracks at the Ag 3In/Ag 2In interfaces in these specimens than in specimens bonded at higher temperatures, as shown from the comparison of Figures 1 and 2. Figure 5 shows that the defects were eliminated by increasing the external pressure from 3 to 9.8 MPa for the SLID bonding process of Bi 2Te 2.55Se 0.45/Cu joints at 175 C. Accompanied with the disappearance of interfacial defects under higher pressure were drastic increases in the shear strengths of the specimens to values of 10.2 to 14.1 MPa after bonding at 175 C for various times, as shown in Figure 6. In addition, Figure 4(c) shows that the fracture path changed from the Ag 3In/Ag 2In interface to the interior of the Bi 2Te 2.55Se 0.45 thermoelectric material. Higher shear strengths could also be obtained in the Bi 2Te 2.55Se 0.45/Cu joints with bonding temperatures of 200 to 250 C. It can be seen from Figure 6 that the shear strengths of Bi 2Te 2.55Se 0.45/Cu joints bonded at temperatures of 200 to

4 Figure 3: Bonding strengths of the Bi2Te2.55Se0.45/Cu joints with In interlayers after the modified isothermal solidification bonding process under an external pressure of 3 MPa. Figure 4: Typical fracture surfaces of shear-tested Bi2Te2.55Se0.45/Cu joints with In interlayers bonded under various conditions: (a) 225 C, 30 min, 3 MPa, (b) 175 C, 30 min, 3 MPa, (c) 175 C, 30 min, 9.8 MPa.

5 Figure 5: Intermetallic compounds at the interfaces of the Bi2Te2.55Se0.45/Cu joints with In interlayers bonded with the modified isothermal solidification bonding process under an external pressure of 9.8 MPa at 175 Cfor various times: (a) 5 min, (b) 10 min, (c) 30 min. Figure 6: Bonding strengths of the Bi2Te2.55Se0.45/Cu joints with In interlayers after the modified isothermal solidification bonding process under an external pressure of 9.8 MPa. 250 C for 10 to 30 min increased slightly to 15.0 to 19.3 MPa under such a high bonding pressure. In a previous study, the shear strengths of the Bi 0.5Sb 1.5Te 3/Cu joints with an In interlayer bonded at 175 C

6 Table 1: Compressive strengths of N-Type Bi2Te2.55Se0.45 and P-Type Bi0.5Sb1.5Te3 thermoelectric materials measured in 5.3 x 10-4 Pa vacuum. Temperature ( C) N-Type Bi2Te2.55Se0.45 (MPa) P-Type Bi0.5Sb1.5Te3 (MPa) were also effectively improved when the applied pressure was increased from 3 to 9.8 MPa (Lin et al., 2016b). However, increasing the bonding pressure to 9.8 MPa did not improve the shear strengths of Bi 0.5Sb 1.5Te 3/Cu specimens bonded at higher temperatures of 200 to 250 C, although the voids and cracks at the Ag 3In/Ag 2In interfaces were indeed diminished. This discrepancy can be attributed to the weak mechanical properties of Bi 0.5Sb 1.5Te 3; damage may have occurred in the interior of the thermoelectric element during the bonding process under high external pressure at elevated temperatures. In comparison with the Bi 0.5Sb 1.5Te 3 intermetallic compound, the Bi 2Te 2.55Se 0.45 thermoelectric material in this study was much stronger, as shown in Table 1, so it could endure the bonding pressure of 9.8 MPa at higher temperatures, leading to improved Ag 3In/Ag 2In interfaces and higher bonding strengths. CONCLUSIONS Isothermal solidification bonding has been employed to produce Bi 2Te 2.55Se 0.45 thermoelectric modules at low temperatures of 175 to 250 C using In interlayers. The Ag/In/Ag interfacial reactions resulted in the formation of Ag 3In/Ag 2In intermetallic compounds with voids and cracks at the interfaces. Shear strengths of 12.8 to 19.8 MPa were achieved with bonding at temperatures of 200 to 250 C for 10 to 30 min under external pressure of 3 MPa, and the shear-tested specimens fractured through the interior of the Bi 2Te 2.55Se 0.45 thermoelectric material. In contrast, in Bi 2Te 2.55Se 0.45/Cu joints bonded at 175 C, the interfacial defects resulted in much lower strengths of 5.4 to 10.5 MPa and fracture paths along the Ag 3In/Ag 2In interfaces. The insufficient shear strengths at 175 C were greatly improved to values of 10.5 to 14.3 MPa by increasing the external bonding pressure from 3 to 9.8 MPa, which effectively eliminated the defects at the Ag 3In/Ag 2In interfaces. The higher external pressure during the bonding process also increased the shear strengths of Bi 2Te 2.55Se 0.45/Cu joints bonded at temperatures of 200 to 250 C for 10 to 30 min to values of 15.0 to 19.3 MPa. ACKNOWLEDGEMENT This study was sponsored by the Ministry of Science and Technology, Taiwan, under Grant No. MOST E MY3. REFERENCES Chuang CH, Lin YC, Lin CW (2016). Intermetallic Reactions during the Solid-Liquid Inter-diffusion Bonding of Bi2Te2.55Se0.45 Thermoelectric Material with Cu Electrodes Using a Sn Interlayer. Metals 6(4): 92. Chuang TH, Lin HJ, Chuang CH, Yeh WT, Hwang JD, Chu HS (2014). Solid Liquid Inter-diffusion Bonding of (Pb, Sn)Te Thermoelectric Modules with Cu Electrodes Using a Thin-Film Sn Interlayer. J. Electron. Mater. 43: Khanna P, Bhatnagar SK, Dalke G, Brunner D, Gust W (1995). Development os thermally stable Ni-Ag interconnections for application to resistive films, Mater. Sci. Eng. B33: L6-L9. Lin YC, Lee KT, Hwang JD, Chu HS, Hsu CC, Chen SC, Chuang TH (2016a). Solid Liquid Inter-diffusion Bonding of Zn4Sb3 Thermoelectric Material with Cu Electrode. J. Electron. Mater. 45: Lin YC, Yang CL, Huang JY, Jain CC, Hwang JD, Chu HS, Chen SC, Chuang TH (2016b). Low-Temperature Bonding of Bi0.5Sb1.5Te3 Thermoelectric Material with Cu Electrodes Using a Thin-Film In Interlayer. Metall. Mater. Trans. 47A: Ritzer TM, Lau PG, Bogard AD (1997). A Critical Evaluation of Today's Thermoelectric Modules. Proc. 16th Int. Conf. Thermoelectrics, Aug (Dresden, Germany),Institute of Electrical and Electronics Engineers,1997. Yang CL, Lai HJ, Hwang JD, Chuang TH (2013a). Diffusion Soldering of Bi0.5Sb1.5Te3 Thermoelectric Material with Cu Electrode. J. Mater. Eng. Perform. 22: Yang CL, Lai HJ, Hwang JD, Chuang TH (2013b). Diffusion Soldering of Pb- Doped GeTe Thermoelectric Modules with Cu Electrodes Using a Thin- Film Sn Interlayer. J. Electron. Mater. 42: Zhang H, Jing HY, Han YD, Xu LY (2013). Interfacial reaction between n- and p-type thermoelectric materials and SAC305 solders. J. Alloys Compd. 576: