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Yb0.6Co4Sb12 thermoelectric joints at high temperature
Journal of Alloys and Compounds 610 (2014) 665–670
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructural evolution of the interfacial layer in the Ti–Al/Yb0.6Co4Sb12 thermoelectric joints at high temperature Ming Gu a, Xugui Xia a, Xiaoya Li a, Xiangyang Huang a, Lidong Chen b,⇑ a
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050, China State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050, China b
a r t i c l e
i n f o
Article history: Received 20 January 2014 Received in revised form 6 May 2014 Accepted 7 May 2014 Available online 22 May 2014 Keywords: Filled-skutterudite Ti–Al barrier layer Interfacial diffusion Shear strength Contact resistivity
a b s t r a c t A thermoelectric (TE) device is basically fabricated by joining p- and n-type thermoelectric materials with electrodes. For skutterudite (SKD) based TE devices, Ti and alloys like Co–Fe–Ni are widely used as the barrier layer joining the SKD and the electrode. In this paper, TE joints composed of Yb0.6Co4Sb12 (Yb-SKD) and a Ti–Al barrier layer with various Al contents (0–15 at.%) were fabricated by a one-step SPS process. The influence of the Al content on the evolution of the interfacial microstructure during accelerated aging at 600 °C was studied. During sintering and aging, Ti and Al diffused into each other and formed inert Ti–Al alloys, which hindered the interfacial diffusion and resulted in a lower growth rate of the diffusion layers. On the other hand, excess Al at the Ti–Al/Yb-SKD interface encouraged the interfacial diffusion. The interplay of the above two mechanisms decides the inter-diffusion behaviors at the interface. In this work, the two mechanisms reached balanced when the Al content was 6 at.%, which resulted in the best interfacial stability and the highest shear strength after aging. At the same time, with the increase of the Al content in the Ti–Al barrier layer, the CTE mismatch between the Yb-SKD and the Ti–Al barrier layer was reduced, consequently the interfacial integrity after aging was improved. As a result, the contact resistivity of the Ti–Al/Yb-SKD joints (Al: 6–12 at.%) remained below 12 lX cm2 after 16 days of aging. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric (TE) technology converts thermal energy directly into electrical power, and thermoelectric generators (TEGs) have special advantages in applications such as space power, waste heat recovery, and high-efficient utility of the solar energy. Among the state-of-the-art TE materials, the CoSb3-based filled skutterudites (SKDs) have attracted extensive attention in the past decade for the outstanding TE properties in the medium temperature range [1–4], and great efforts have been made to embrace these materials into next generation TEGs for the deepspace exploration and the waste heat recovery [5–8]. The basic component of a SKD based TEG is TE joints, which consist of the SKD bulks and the hot/cold side electrodes. In practical applications, the temperature at the hot side of the joints usually reaches higher than 500 °C, and the joints are required to perform stably for a couple of years or more. Long term service at such high temperatures could induce evident inter-diffusion and/or reactions between the SKDs and the electrodes. As a result, the performance ⇑ Corresponding author. Tel.: +86 021 52412612. E-mail address: [email protected] (L. Chen). http://dx.doi.org/10.1016/j.jallcom.2014.05.087 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
of the TEGs would be greatly degraded. Therefore, the interfacial stability at the hot side of the TE joints is always one of the key issues in the reliability design of the TEGs. Because antimony, the major component in the SKDs, reacts with most electrode materials such as Cu, Mo, Ni and their alloys, a barrier layer is needed between the electrode and the SKD. At the same time, the barrier layer is also expected to have certain ‘‘bonding’’ function so that the electrode and the SKDs can be jointed together with high adhesive strength. Fan et al. firstly reported the application of Ti as the barrier layer in the SKD based TE joints in 2004 [9]. Zhao et al. then studied the stability of the Ti/CoSb3 interface during accelerated aging [10,11]. Recently, Krzysztof Tomasz tried Mo, Ni and Cr80Si20 as the barrier layer in the Cu–CoSb3 junctions [12], and Guo et al. reported the use of Co–Fe–Ni alloys [13]. Comparatively, Ti turns out to be one of the most feasible barrier layer materials for the SKD based TE joints in terms of the interfacial stability at high temperatures. However, according to Zhao’s reports, there is still considerable interfacial diffusion at the Ti/CoSb3 interface during accelerated aging at high temperatures, and the growth of the diffusion layer leads to evident reduction of the interfacial shear strength. [10,11] Therefore, the SKD-metal joint is still the
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critical component dominating the life period of the SKD based TE devices. The present work aims to further improve the interfacial stability of the SKD based TE joints. In contrast to pure Ti, Ti–Al alloys such as TiAl and Ti3Al present much better stability at high temperatures, therefore might be better candidates of the barrier layer. However, the sintering temperature (600 °C) of the SKD powders is unfortunately not high enough for the Ti–Al alloy powders to be densified. So it is difficult to fabricate TE joints with SKD and Ti–Al alloy powders through the one-step SPS process. To achieve both an ‘‘active’’ bonding layer during sintering and an ‘‘inert’’ barrier layer during operation, we proposed a modified bonding method by adopting the mixture of Ti and Al powders in the SPS process. n-type skutterudite Yb0.6Co4Sb12 (Yb-SKD) was used as the thermoelectric material due to its excellent TE performance among the single filled skutterudites. It is found that the mixture of the Ti and Al powders can be easily densified during SPS sintering. During aging at 600 °C, the solid reactions between Ti and Al result in the formation of the TiAl and Ti3Al compounds. It is also found that suitable Al content can effectively reduce the inter-diffusion between the Ti–Al layer and the Yb-SKD at 600 °C, and the stability of the bonding strength is evidently improved as compared to the Ti/Yb-SKD joints. Increasing Al content also relieves the mismatch of the coefficient of thermal expansion (CTE) between the Ti–Al barrier layer and the Yb-SKD bulk, which greatly reduces the interfacial damages and results in much better stability of the contact resistivity.
2. Experimental procedures Mixed powders of Ti (99.99%, 325 mesh) and Al (99.97%, 325 mesh) with different Al content (0–15 at.%) were ball milled in inert gas for 5 min. The Yb-SKD powders and the ball milled Ti–Al powders were then loaded in turn in a graphite die. The Ti–Al/Yb-SKD joints (cylinder: U10 mm 8 mm) were fabricated by sintering the above powders at 620 °C under 64 MPa for 10 min with a spark plasma sintering system. The sintered joints were supersonically cleaned in alcohol, sealed in vacuum in quartz tubes, and then kept in a furnace at 600 °C for accelerated aging. The microstructure of the Ti–Al/Yb-SKD interface was observed with a SEM (JSM6700F, JEOL). The chemical compositions of the interfacial diffusion layers were analyzed with an EPMA (JXA-8100, JEOL). The phases of the Ti–Al alloys formed during aging were determined using the X-ray diffraction (XRD) analysis (Rigaku, Rint2000, 4°/min). The interfacial shear strength was tested in a tensile tester (INSTRON 5566R) at room temperature, and the load speed was 0.2 mm/min. The contact resistivity was measured by a homemade 4-probe platform, and the CTEs were measured with a Linseis-L75VS dilatometer. All the data of the diffusion layer thickness, the shear strength and the contact resistivity in this paper were the average value of three parallel samples.
3. Results and discussion 3.1. Interfacial microstructure Fig. 1(a–d) presents the microstructural evolution at the Ti/YbSKD interface during aging at 600 °C. After 4 days of aging, the initially negligible diffusion layer gradually grew into a three-layer sandwich-like structure. Composition analyses and line scan (Fig. 3(a)) revealed that these layers were TiCoSb, TiSb2 and TiSb in turn from the Yb-SKD toward the Ti side. The observed threelayer structure is similar to that at the Ti/CoSb3 interface after aging [10]. This implies that the filling of the Yb atoms into the CoSb3 lattice has little influence on the inter-diffusion behavior between the Ti barrier layer and the Yb-SKD. Another thing worth mentioning is that there appeared some chips within the diffusion layer after 4 days of aging, and more chips were noticed with increasing aging time. This indicates that the Ti/Yb-SKD interface became increasingly damaged during aging, and that similar to the Ti/CoSb3 system [11], the diffusion layer is the weak part of the interface.
Fig. 2(a–c) and (d–f) shows the evolution of the interfacial microstructures in the Ti94Al6/Yb-SKD and Ti88Al12/Yb-SKD systems aged at 600 °C, respectively. It was noticed that in both systems there already existed a thin diffusion layer after sintering, and higher Al content resulted in thicker initial diffusion layer. In contrast to the case of the Ti/Yb-SKD system, the above diffusion layers grew much more slowly during aging, while the interfacial chips were evidently reduced. According to the composition analyses and line scan (Fig. 3(b)) results, the diffusion layers were AlCo, TiCoSb, TiSb2 and TiSb in turn from the Yb-SKD toward the Ti–Al side. Fig. 4 summarizes the evolution of the diffusion layer thickness in different joints as a function of the aging time. The inset in Fig. 4 gives the change of the diffusion layer thickness with the Al content in the barrier layer aged for 16 days. Apparently, with increasing Al content, the diffusion layer thickness decreased until the Al content reached 6 at.%, then it increased again. Based on the above experimental results, it is clear that the interfacial diffusion and reactions are sensitive to the Al content in the Ti–Al barrier layer, and suitable Al content can effectively depress the interfacial diffusion between the Ti–Al barrier layer and the YbSKD. In order to reveal related mechanisms, microstructures and phase compositions of the Ti–Al barrier layers were examined. Fig. 5(a and b) shows the XRD spectra of the Ti–Al barrier layers (polished surface) before and after aging at 600 °C for 16 days, respectively. According to Fig. 5(a), before aging, for Ti97Al3, strong diffraction peaks of elemental Ti with weak TiAl3 diffraction peaks were observed. With the increase of the Al content in the Ti– Al layer, peaks of the TiAl3 phase became stronger. Further increasing the Al content to 12 at.% and diffraction peaks of Ti3Al, TiAl and elemental Al appeared. Fig. 5(b) reveals that after 16 days of aging at 600 °C, diffraction peaks of the TiAl3 and elemental Al phases disappeared while peaks of the Ti3Al and TiAl phases became considerably stronger than they were before aging. According to the Ti–Al phase diagram, there are three kinds of Ti–Al alloy phases: Ti3Al, TiAl and TiAl3. The above results show that during sintering the elemental Al phase in the Ti–Al layer mostly transferred into the TiAl3 phase, which then transferred into the TiAl and Ti3Al phases after long term aging. This evolution was also confirmed by the BES observation and EPMA analysis results. See from Fig. 6(a), Ti and TiAl3 phases with trace Al phase were observed in the as-sintered samples, and TiAl3 and Al form Al-core/TiAl3shell grains dispersed in the Ti matrix. This implies that during sintering there were inter-diffusion and solid reactions between the Al and Ti phases, and the diffusion of Ti toward Al dominates the formation process of the TiAl3 phase. During aging, more Ti diffused into the TiAl3 phase, converting it into the TiAl phase. The TiAl phase then continued to react with coming Ti to form the Ti3Al phase. After being aged at 600 °C for 16 days, almost all the Al and TiAl3 phases were converted into the TiAl and/or Ti3Al phases (Fig. 6(b)). In areas with initially small Al particles, there was only Ti3Al phase left, while in areas with initially large Al particles, the TiAl-core/Ti3Al-shell structure was frequently observed. Similar results were also obtained in Ti–Al layers with other Al contents. The above analysis is also true for the Ti and Al phases at the Ti– Al/Yb-SKD interface, and it can be deduced that higher Al content will result in higher fraction of the TiAl and Ti3Al alloy phases in the Ti–Al barrier layer. Since these alloys are much more ‘‘inactive’’ (or ‘‘inert’’) than pure Ti, increasing Al content will lead to lower formation rate of the diffusion layer between the Ti–Al barrier layer and the Yb-SKD, and therefore better interfacial thermal stability of the Ti–Al/Yb-SKD joints. Meanwhile, it is worth noting that Al itself is an active element, which does harm to the interfacial stability. The appearing of the AlCo compound at the Ti–Al/YbSKD interfaces during sintering (Fig. 2(a) and (d)) implies strong inter-diffusion and reaction between the interfacial Al and the
M. Gu et al. / Journal of Alloys and Compounds 610 (2014) 665–670
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Fig. 1. Evolution of the Ti/Yb-SKD interfacial microstructure aged at 600 °C for different time: (a) as sintered; (b) 4 days; (c) 8 days; (d) 16 days.
Fig. 2. Evolution of the Ti(1 x)Alx/Yb-SKD interfacial microstructure aged at 600 °C for different time: (a) x = 6, as sintered; (b) x = 6, 8 days; (c) x = 6, 16 days; (d) x = 12, as sintered; (e) x = 12, 8 days; (f) x = 12, 16 days.
Yb-SKD. After sintering, there are still certain amount of Al particles remained (Figs. 5 and 6), and if the remained Al particles contacted directly with the Yb-SKD (or in case that the surrounding TiAl3 or Ti layer is not thick enough), diffusion of Al toward the Yb-SKD and reaction between them will continue to proceed during aging. In that case, more and more AlCo will be formed at the interface. Formation of such loose layer will damage the interfacial stability. In real systems, the above two processes, formation of the inert Ti–Al alloys (TiAl and Ti3Al) and the reactions of the elemental Al with the Yb-SKD, compete with each other and determine the structure and evolution of the Ti–Al/Yb-SKD interface. In case of low Al content, the first process dominates the overall diffusion, and therefore the interfacial diffusion is reduced with increasing Al content. Further increasing the Al content, however, the second process becomes dominant and the addition of Al on the contrary damages the interfacial stability. In the present work, when the Al
content was about 6 at.%, the above two processes balanced each other and the best interfacial stability was achieved. 3.2. Interfacial strength The interfacial strength between the barrier layer and the YbSKD is an important parameter of the TE joints. It represents the ability of the joints to remain mechanically intact during service, which is usually the premise of the stable performance for the TEGs. In this paper, the interfacial shear strength were measured for the above Ti–Al/Yb-SKD joints before and after aging at 600 °C for 16 days, and the results are listed in Table 1. The shear strength of the as-sintered joints varied within a limited range with increasing Al content. After aging, all the samples exhibited considerable loss of shear strength. Fig. 7 plots the reduction rate of the shear strength corresponding to the diffusion layer thickness of the aged
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Fig. 3. Interfacial line scans of the TE joints aged at 600 °C for different time: (a) the Ti/Yb-SKD joints, 4 days; (b) the Ti88Al12/Yb-SKD joints, 8 days.
joints. It is clear that the reduction rate of the shear strength increased monotonously with the increase of the diffusion layer thickness. Similar results were also reported in the Ti/CoSb3 joints by Zhao [10,11]. The possible reason is that some of the diffusion layer structures such as the TiCoSb and AlCo are brittle and/or loose, which degrades the interfacial strength of the joints. 3.3. Contact resistivity Contact resistivity is another important interfacial property of the TE joints, which is sensitive to the integrity of the interfacial structure as well as the nature of the interfacial materials. Lower contact resistivity indicates less Joule heat at the interface, therefore less loss of the efficiency and power output of the TEGs. Fig. 8 shows the evolution of the contact resistivity as a function of aging time for the above TE joints. The initial contact resistivity
of all the joints were around 10 lX cm2. During aging, the contact resistivity of the Ti/Yb-SKD joints rapidly increased. After 16 days, it rose up to 53 lX cm2. With 3 at.% Al content in the barrier layer, however, the rising of the contact resistivity during aging was evidently reduced. Further increasing Al contents (6–15 at.%) in the barrier layer, the contact resistivity became much more stable and remained below 12 lX cm2 after 16 days of aging. Clearly, suitable Al content in the Ti–Al barrier layer greatly improves the stability of the contact resistivity. Zhao et al. reported that the growth of diffusion layer from 0 lm to 20 lm in thickness led to an increase of 9 lX cm2 in the contact resistivity of the Ti/ CoSb3 interface [10]. Therefore, in the Ti/Yb-SKD joints, the increase of the contact resistivity from 4 to 12 lX cm2 during the first 4 days of aging might be from the interfacial diffusion. Afterwards, however, the accelerated increase of the contact resistivity despite the slowing down of the interfacial diffusion suggests that
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Table 1 Change of the joint strength with the increase of the Al content in the Ti–Al barrier layer before and after 16 days of aging at 600 °C.
25
Al content/at.%
20 15 10 5 0
3
6
9
12
30
25
20
15 0
-5
0
Average shear Before aging 15 ± 3 12 ± 4 12 ± 3 13 ± 1 15 ± 3 strength/MPa After aging 2.1 ± 0.6 5.3 ± 1.7 8.4 ± 2.6 5.7 ± 2.1 2.3 ± 1.1 Diffusion layer thickness / µm
Diffusion layer thickness / µm
30
0
100
200
2 4 6 8 10 12 Al content in interlayer / at.%
300
400
14
500
Isothermal aging time / hour Fig. 4. Diffusion layer thickness vs the aging time for different Ti–Al/Yb-SKD joints aged at 600 °C. The inset is the diffusion layer thickness vs the Al content aged at 600 °C for 16 days.
extra mechanisms are playing a vital role in further pushing up the contact resistivity. As has been mentioned before, with the prolonging of the aging time, more interfacial chips appeared at the Ti/Yb-SKD interface. With the increase of the Al content in the Ti–Al barrier layer, however, the chips were evidently reduced. Normally the interfacial chips appear when the interface is undermined and no longer strong enough to resist the interfacial stress. The chips degrade the integrity of the interfacial structure and naturally result in rapid increase of the contact resistivity. As the interfacial stress usually comes from the CTE mismatch between the bonding materials, we then measured the CTEs of the Yb-SKD and the Ti–Al barrier layer bulks. According to Fig. 9, there are CTE mismatches
between the Yb-SKD and the Ti–Al barrier layers. Obviously, the CTE mismatch between Yb-SKD and Ti is the largest, consequently the interfacial stress at the Ti/Yb-SKD interface may be the highest. With the increase of Al content in the Ti–Al barrier layer, the CTE mismatch between Yb-SKD and the Ti–Al barrier layer is reduced. As a result, lower interfacial stress can be achieved at corresponding interfaces. The evolution of the contact resistivity is largely the result of the competition between the interfacial strength and the interfacial stress. During aging, the interfacial strength of the Ti/Yb-SKD joints suffered heavy loss, while the interfacial stress of this system was the highest among the above 6 kinds of TE joints. As a result, the interface was seriously damaged, which then led to rapid increase of the contact resistivity during aging. With the increase of the Al content in the Ti–Al barrier layer, the joint strength increased and the interfacial stress decreased, both of which helped to reduce the interfacial chips, therefore the stability of the contact resistivity was continuingly improved in the Ti97Al3/ Yb-SKD and Ti94Al6/Yb-SKD systems. Further increasing the Al content in the barrier layer, the interfacial stress continued to decrease. Although the joint strength began to decrease again, the reduction of the interfacial stress outweighed that of the interfacial strength. As a result, there were even less interfacial chips at the Ti88Al12/Yb-SKD interface after aging, and the contact resistivity remained stable.
Fig. 5. XRD spectra of the Ti–Al barrier layers: (a) as sintered; (b) aged at 600 °C for 16 days.
Fig. 6. BES images and composition analysis of the Ti94Al6 barrier layer: (a) as sintered; (b) aged at 600 °C for 16 days.
M. Gu et al. / Journal of Alloys and Compounds 610 (2014) 665–670
Reduction rate of shear strength
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4. Conclusions 100% 80% 60% 40% Ti Ti97Al3 Ti94Al6 Ti91Al9 Ti88Al12
20% 0% 14
16
18
20
22
24
26
28
Diffusion layer thickness / um Fig. 7. Evolution of the shear strength reduction rate with the diffusion layer thickness of the TE joints with different Ti–Al barrier layer aged at 600 °C for 16 days.
In the present work, the Ti–Al/Yb0.6Co4Sb12 TE joints were isothermal aged in vacuum at 600 °C, and the influence of the Al content in the Ti–Al barrier layer on the interfacial evolution behavior was investigated. With comparison to the Ti/Yb-SKD system, the interfacial diffusion of the Ti–Al/Yb-SKD system was effectively reduced at an appropriate Al content of about 6 at.%. Further increasing the Al content, however, the interfacial diffusion was promoted. XRD and composition analyses showed that during sintering and aging, Ti and Al diffused into each other and formed the Ti3Al and TiAl phases, which depressed the interfacial diffusion. However, diffusion of Al into the Yb-SKD intensified the interfacial diffusion, which damaged the high temperature stability of the joints. The competition of the above two mechanisms decided the inter-diffusion at the interface, and TE joints with less interfacial diffusion achieved higher interfacial shear strength after aging. The Al content also influenced the CTE of the Ti–Al barrier layer. More Al content led to less CTE mismatch between the barrier layer and the Yb-SKD, which brought down the interfacial stress and helped to relieve the interfacial damages during aging. As a result, the stability of the contact resistivity was significantly improved. In brief, the Al content in the Ti–Al barrier layer influences both the interfacial diffusion and the CTE mismatch of the Ti–Al/YbSKD interface, which work together to determine the stability of the interface and therefore construct the basis of the optimized design of TE joints. Acknowledgements The authors acknowledge the financial supports from National Natural Science Foundation of China (Contract No. 51372261), International S&T Cooperation Program of China (Contract No. 2011DFB60150) and National Basic Research Program of China (Contract No. 2013CB632504).
Fig. 8. Evolution of the contact resistivity with aging time for the Ti–Al/Yb-SKD joints aged at 600 °C.
CTE / 10-6/ K
14 Yb-SKD Ti88Al12 Ti94Al6 Ti
12
10
8
6 0
100
200
300
400
500
600
700
T / oC Fig. 9. Variation of the CTE with temperature for the Yb-SKD and the Ti–Al barrier layer materials.
References [1] Shengqiang Bai, Xun Shi, Lidong Chen, Appl. Phys. Lett. 96 (2010). 202102-1–3. [2] Xun Shi, Jiong Yang, James R. Salvador, Miaofang Chi, Jung Y. Cho, Hsin Wang, Shengqiang Bai, Jihui Yang, Wenqing Zhang, Lidong Chen, J. Am. Chem. Soc. 133 (2011) 7837–7846. [3] Sedat Ballikaya, Ctirad Uher, J. Alloys Comp. 585 (2014) 168–172. [4] Gerda Rogl, Andriy Grytsiv, Peter Rogl, Esmaeil Royanian, Ernst Bauer, Jelena Horky, Daria Setman, Erhard Schafler, Michael Zehetbauer, Acta Mater. 61 (2013) 6778–6789. [5] Hamed H. Saber, Mohamed S. El-Genk, Thierry Caillat, Energy Convers. Manage. 48 (2) (2007) 555–567. [6] M.S. El-Genk, H.H. Saber, AIP Conf. Proc. 746 (2005) 485–494. [7] J. LaGrandeur, D. Crane, S. Hung, B. Mazar, A. Eder, in: Proceedings of the 25th International Conference on Thermoelectrics, 2006, pp. 343–348. [8] K. Salzgeber, P. Prenninger, A. Grytsiv, P. Rogl, E. Bauer, J. Electron. Mater. 39 (9) (2010) 2074–2078. [9] Junfeng Fan, Lidong Chen, Shengqiang Bai, Xun Shi, Mater. Lett. 58 (2004) 3876–3878. [10] Degang Zhao, Xiaoya Li, Lin He, Wan Jiang, Lidong Chen, J. Alloys Comp. 477 (2009) 425–431. [11] Degang Zhao, Xiaoya Li, Lin He, Wan Jiang, Lidong Chen, Intermetallics 17 (3) (2009) 425–431. [12] Krzysztof Tomasz Wojciechowski, Rafal Zybala, Ryszard Mania, Microelectron. Reliab. 51 (2011) 1198–1202. [13] J.Q. Guo, H.Y. Geng, T. Ochi, S. Suzuki, M. Kikuchi, Y. Yamaguchi, S. Ito, J. Electron. Mater. 41 (6) (2012) 1036–1042.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructural evolution of the interfacial layer in the Ti–Al/Yb0.6Co4Sb12 thermoelectric joints at high temperature Ming Gu a, Xugui Xia a, Xiaoya Li a, Xiangyang Huang a, Lidong Chen b,⇑ a
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050, China State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050, China b
a r t i c l e
i n f o
Article history: Received 20 January 2014 Received in revised form 6 May 2014 Accepted 7 May 2014 Available online 22 May 2014 Keywords: Filled-skutterudite Ti–Al barrier layer Interfacial diffusion Shear strength Contact resistivity
a b s t r a c t A thermoelectric (TE) device is basically fabricated by joining p- and n-type thermoelectric materials with electrodes. For skutterudite (SKD) based TE devices, Ti and alloys like Co–Fe–Ni are widely used as the barrier layer joining the SKD and the electrode. In this paper, TE joints composed of Yb0.6Co4Sb12 (Yb-SKD) and a Ti–Al barrier layer with various Al contents (0–15 at.%) were fabricated by a one-step SPS process. The influence of the Al content on the evolution of the interfacial microstructure during accelerated aging at 600 °C was studied. During sintering and aging, Ti and Al diffused into each other and formed inert Ti–Al alloys, which hindered the interfacial diffusion and resulted in a lower growth rate of the diffusion layers. On the other hand, excess Al at the Ti–Al/Yb-SKD interface encouraged the interfacial diffusion. The interplay of the above two mechanisms decides the inter-diffusion behaviors at the interface. In this work, the two mechanisms reached balanced when the Al content was 6 at.%, which resulted in the best interfacial stability and the highest shear strength after aging. At the same time, with the increase of the Al content in the Ti–Al barrier layer, the CTE mismatch between the Yb-SKD and the Ti–Al barrier layer was reduced, consequently the interfacial integrity after aging was improved. As a result, the contact resistivity of the Ti–Al/Yb-SKD joints (Al: 6–12 at.%) remained below 12 lX cm2 after 16 days of aging. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric (TE) technology converts thermal energy directly into electrical power, and thermoelectric generators (TEGs) have special advantages in applications such as space power, waste heat recovery, and high-efficient utility of the solar energy. Among the state-of-the-art TE materials, the CoSb3-based filled skutterudites (SKDs) have attracted extensive attention in the past decade for the outstanding TE properties in the medium temperature range [1–4], and great efforts have been made to embrace these materials into next generation TEGs for the deepspace exploration and the waste heat recovery [5–8]. The basic component of a SKD based TEG is TE joints, which consist of the SKD bulks and the hot/cold side electrodes. In practical applications, the temperature at the hot side of the joints usually reaches higher than 500 °C, and the joints are required to perform stably for a couple of years or more. Long term service at such high temperatures could induce evident inter-diffusion and/or reactions between the SKDs and the electrodes. As a result, the performance ⇑ Corresponding author. Tel.: +86 021 52412612. E-mail address: [email protected] (L. Chen). http://dx.doi.org/10.1016/j.jallcom.2014.05.087 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
of the TEGs would be greatly degraded. Therefore, the interfacial stability at the hot side of the TE joints is always one of the key issues in the reliability design of the TEGs. Because antimony, the major component in the SKDs, reacts with most electrode materials such as Cu, Mo, Ni and their alloys, a barrier layer is needed between the electrode and the SKD. At the same time, the barrier layer is also expected to have certain ‘‘bonding’’ function so that the electrode and the SKDs can be jointed together with high adhesive strength. Fan et al. firstly reported the application of Ti as the barrier layer in the SKD based TE joints in 2004 [9]. Zhao et al. then studied the stability of the Ti/CoSb3 interface during accelerated aging [10,11]. Recently, Krzysztof Tomasz tried Mo, Ni and Cr80Si20 as the barrier layer in the Cu–CoSb3 junctions [12], and Guo et al. reported the use of Co–Fe–Ni alloys [13]. Comparatively, Ti turns out to be one of the most feasible barrier layer materials for the SKD based TE joints in terms of the interfacial stability at high temperatures. However, according to Zhao’s reports, there is still considerable interfacial diffusion at the Ti/CoSb3 interface during accelerated aging at high temperatures, and the growth of the diffusion layer leads to evident reduction of the interfacial shear strength. [10,11] Therefore, the SKD-metal joint is still the
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critical component dominating the life period of the SKD based TE devices. The present work aims to further improve the interfacial stability of the SKD based TE joints. In contrast to pure Ti, Ti–Al alloys such as TiAl and Ti3Al present much better stability at high temperatures, therefore might be better candidates of the barrier layer. However, the sintering temperature (600 °C) of the SKD powders is unfortunately not high enough for the Ti–Al alloy powders to be densified. So it is difficult to fabricate TE joints with SKD and Ti–Al alloy powders through the one-step SPS process. To achieve both an ‘‘active’’ bonding layer during sintering and an ‘‘inert’’ barrier layer during operation, we proposed a modified bonding method by adopting the mixture of Ti and Al powders in the SPS process. n-type skutterudite Yb0.6Co4Sb12 (Yb-SKD) was used as the thermoelectric material due to its excellent TE performance among the single filled skutterudites. It is found that the mixture of the Ti and Al powders can be easily densified during SPS sintering. During aging at 600 °C, the solid reactions between Ti and Al result in the formation of the TiAl and Ti3Al compounds. It is also found that suitable Al content can effectively reduce the inter-diffusion between the Ti–Al layer and the Yb-SKD at 600 °C, and the stability of the bonding strength is evidently improved as compared to the Ti/Yb-SKD joints. Increasing Al content also relieves the mismatch of the coefficient of thermal expansion (CTE) between the Ti–Al barrier layer and the Yb-SKD bulk, which greatly reduces the interfacial damages and results in much better stability of the contact resistivity.
2. Experimental procedures Mixed powders of Ti (99.99%, 325 mesh) and Al (99.97%, 325 mesh) with different Al content (0–15 at.%) were ball milled in inert gas for 5 min. The Yb-SKD powders and the ball milled Ti–Al powders were then loaded in turn in a graphite die. The Ti–Al/Yb-SKD joints (cylinder: U10 mm 8 mm) were fabricated by sintering the above powders at 620 °C under 64 MPa for 10 min with a spark plasma sintering system. The sintered joints were supersonically cleaned in alcohol, sealed in vacuum in quartz tubes, and then kept in a furnace at 600 °C for accelerated aging. The microstructure of the Ti–Al/Yb-SKD interface was observed with a SEM (JSM6700F, JEOL). The chemical compositions of the interfacial diffusion layers were analyzed with an EPMA (JXA-8100, JEOL). The phases of the Ti–Al alloys formed during aging were determined using the X-ray diffraction (XRD) analysis (Rigaku, Rint2000, 4°/min). The interfacial shear strength was tested in a tensile tester (INSTRON 5566R) at room temperature, and the load speed was 0.2 mm/min. The contact resistivity was measured by a homemade 4-probe platform, and the CTEs were measured with a Linseis-L75VS dilatometer. All the data of the diffusion layer thickness, the shear strength and the contact resistivity in this paper were the average value of three parallel samples.
3. Results and discussion 3.1. Interfacial microstructure Fig. 1(a–d) presents the microstructural evolution at the Ti/YbSKD interface during aging at 600 °C. After 4 days of aging, the initially negligible diffusion layer gradually grew into a three-layer sandwich-like structure. Composition analyses and line scan (Fig. 3(a)) revealed that these layers were TiCoSb, TiSb2 and TiSb in turn from the Yb-SKD toward the Ti side. The observed threelayer structure is similar to that at the Ti/CoSb3 interface after aging [10]. This implies that the filling of the Yb atoms into the CoSb3 lattice has little influence on the inter-diffusion behavior between the Ti barrier layer and the Yb-SKD. Another thing worth mentioning is that there appeared some chips within the diffusion layer after 4 days of aging, and more chips were noticed with increasing aging time. This indicates that the Ti/Yb-SKD interface became increasingly damaged during aging, and that similar to the Ti/CoSb3 system [11], the diffusion layer is the weak part of the interface.
Fig. 2(a–c) and (d–f) shows the evolution of the interfacial microstructures in the Ti94Al6/Yb-SKD and Ti88Al12/Yb-SKD systems aged at 600 °C, respectively. It was noticed that in both systems there already existed a thin diffusion layer after sintering, and higher Al content resulted in thicker initial diffusion layer. In contrast to the case of the Ti/Yb-SKD system, the above diffusion layers grew much more slowly during aging, while the interfacial chips were evidently reduced. According to the composition analyses and line scan (Fig. 3(b)) results, the diffusion layers were AlCo, TiCoSb, TiSb2 and TiSb in turn from the Yb-SKD toward the Ti–Al side. Fig. 4 summarizes the evolution of the diffusion layer thickness in different joints as a function of the aging time. The inset in Fig. 4 gives the change of the diffusion layer thickness with the Al content in the barrier layer aged for 16 days. Apparently, with increasing Al content, the diffusion layer thickness decreased until the Al content reached 6 at.%, then it increased again. Based on the above experimental results, it is clear that the interfacial diffusion and reactions are sensitive to the Al content in the Ti–Al barrier layer, and suitable Al content can effectively depress the interfacial diffusion between the Ti–Al barrier layer and the YbSKD. In order to reveal related mechanisms, microstructures and phase compositions of the Ti–Al barrier layers were examined. Fig. 5(a and b) shows the XRD spectra of the Ti–Al barrier layers (polished surface) before and after aging at 600 °C for 16 days, respectively. According to Fig. 5(a), before aging, for Ti97Al3, strong diffraction peaks of elemental Ti with weak TiAl3 diffraction peaks were observed. With the increase of the Al content in the Ti– Al layer, peaks of the TiAl3 phase became stronger. Further increasing the Al content to 12 at.% and diffraction peaks of Ti3Al, TiAl and elemental Al appeared. Fig. 5(b) reveals that after 16 days of aging at 600 °C, diffraction peaks of the TiAl3 and elemental Al phases disappeared while peaks of the Ti3Al and TiAl phases became considerably stronger than they were before aging. According to the Ti–Al phase diagram, there are three kinds of Ti–Al alloy phases: Ti3Al, TiAl and TiAl3. The above results show that during sintering the elemental Al phase in the Ti–Al layer mostly transferred into the TiAl3 phase, which then transferred into the TiAl and Ti3Al phases after long term aging. This evolution was also confirmed by the BES observation and EPMA analysis results. See from Fig. 6(a), Ti and TiAl3 phases with trace Al phase were observed in the as-sintered samples, and TiAl3 and Al form Al-core/TiAl3shell grains dispersed in the Ti matrix. This implies that during sintering there were inter-diffusion and solid reactions between the Al and Ti phases, and the diffusion of Ti toward Al dominates the formation process of the TiAl3 phase. During aging, more Ti diffused into the TiAl3 phase, converting it into the TiAl phase. The TiAl phase then continued to react with coming Ti to form the Ti3Al phase. After being aged at 600 °C for 16 days, almost all the Al and TiAl3 phases were converted into the TiAl and/or Ti3Al phases (Fig. 6(b)). In areas with initially small Al particles, there was only Ti3Al phase left, while in areas with initially large Al particles, the TiAl-core/Ti3Al-shell structure was frequently observed. Similar results were also obtained in Ti–Al layers with other Al contents. The above analysis is also true for the Ti and Al phases at the Ti– Al/Yb-SKD interface, and it can be deduced that higher Al content will result in higher fraction of the TiAl and Ti3Al alloy phases in the Ti–Al barrier layer. Since these alloys are much more ‘‘inactive’’ (or ‘‘inert’’) than pure Ti, increasing Al content will lead to lower formation rate of the diffusion layer between the Ti–Al barrier layer and the Yb-SKD, and therefore better interfacial thermal stability of the Ti–Al/Yb-SKD joints. Meanwhile, it is worth noting that Al itself is an active element, which does harm to the interfacial stability. The appearing of the AlCo compound at the Ti–Al/YbSKD interfaces during sintering (Fig. 2(a) and (d)) implies strong inter-diffusion and reaction between the interfacial Al and the
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Fig. 1. Evolution of the Ti/Yb-SKD interfacial microstructure aged at 600 °C for different time: (a) as sintered; (b) 4 days; (c) 8 days; (d) 16 days.
Fig. 2. Evolution of the Ti(1 x)Alx/Yb-SKD interfacial microstructure aged at 600 °C for different time: (a) x = 6, as sintered; (b) x = 6, 8 days; (c) x = 6, 16 days; (d) x = 12, as sintered; (e) x = 12, 8 days; (f) x = 12, 16 days.
Yb-SKD. After sintering, there are still certain amount of Al particles remained (Figs. 5 and 6), and if the remained Al particles contacted directly with the Yb-SKD (or in case that the surrounding TiAl3 or Ti layer is not thick enough), diffusion of Al toward the Yb-SKD and reaction between them will continue to proceed during aging. In that case, more and more AlCo will be formed at the interface. Formation of such loose layer will damage the interfacial stability. In real systems, the above two processes, formation of the inert Ti–Al alloys (TiAl and Ti3Al) and the reactions of the elemental Al with the Yb-SKD, compete with each other and determine the structure and evolution of the Ti–Al/Yb-SKD interface. In case of low Al content, the first process dominates the overall diffusion, and therefore the interfacial diffusion is reduced with increasing Al content. Further increasing the Al content, however, the second process becomes dominant and the addition of Al on the contrary damages the interfacial stability. In the present work, when the Al
content was about 6 at.%, the above two processes balanced each other and the best interfacial stability was achieved. 3.2. Interfacial strength The interfacial strength between the barrier layer and the YbSKD is an important parameter of the TE joints. It represents the ability of the joints to remain mechanically intact during service, which is usually the premise of the stable performance for the TEGs. In this paper, the interfacial shear strength were measured for the above Ti–Al/Yb-SKD joints before and after aging at 600 °C for 16 days, and the results are listed in Table 1. The shear strength of the as-sintered joints varied within a limited range with increasing Al content. After aging, all the samples exhibited considerable loss of shear strength. Fig. 7 plots the reduction rate of the shear strength corresponding to the diffusion layer thickness of the aged
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Fig. 3. Interfacial line scans of the TE joints aged at 600 °C for different time: (a) the Ti/Yb-SKD joints, 4 days; (b) the Ti88Al12/Yb-SKD joints, 8 days.
joints. It is clear that the reduction rate of the shear strength increased monotonously with the increase of the diffusion layer thickness. Similar results were also reported in the Ti/CoSb3 joints by Zhao [10,11]. The possible reason is that some of the diffusion layer structures such as the TiCoSb and AlCo are brittle and/or loose, which degrades the interfacial strength of the joints. 3.3. Contact resistivity Contact resistivity is another important interfacial property of the TE joints, which is sensitive to the integrity of the interfacial structure as well as the nature of the interfacial materials. Lower contact resistivity indicates less Joule heat at the interface, therefore less loss of the efficiency and power output of the TEGs. Fig. 8 shows the evolution of the contact resistivity as a function of aging time for the above TE joints. The initial contact resistivity
of all the joints were around 10 lX cm2. During aging, the contact resistivity of the Ti/Yb-SKD joints rapidly increased. After 16 days, it rose up to 53 lX cm2. With 3 at.% Al content in the barrier layer, however, the rising of the contact resistivity during aging was evidently reduced. Further increasing Al contents (6–15 at.%) in the barrier layer, the contact resistivity became much more stable and remained below 12 lX cm2 after 16 days of aging. Clearly, suitable Al content in the Ti–Al barrier layer greatly improves the stability of the contact resistivity. Zhao et al. reported that the growth of diffusion layer from 0 lm to 20 lm in thickness led to an increase of 9 lX cm2 in the contact resistivity of the Ti/ CoSb3 interface [10]. Therefore, in the Ti/Yb-SKD joints, the increase of the contact resistivity from 4 to 12 lX cm2 during the first 4 days of aging might be from the interfacial diffusion. Afterwards, however, the accelerated increase of the contact resistivity despite the slowing down of the interfacial diffusion suggests that
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Table 1 Change of the joint strength with the increase of the Al content in the Ti–Al barrier layer before and after 16 days of aging at 600 °C.
25
Al content/at.%
20 15 10 5 0
3
6
9
12
30
25
20
15 0
-5
0
Average shear Before aging 15 ± 3 12 ± 4 12 ± 3 13 ± 1 15 ± 3 strength/MPa After aging 2.1 ± 0.6 5.3 ± 1.7 8.4 ± 2.6 5.7 ± 2.1 2.3 ± 1.1 Diffusion layer thickness / µm
Diffusion layer thickness / µm
30
0
100
200
2 4 6 8 10 12 Al content in interlayer / at.%
300
400
14
500
Isothermal aging time / hour Fig. 4. Diffusion layer thickness vs the aging time for different Ti–Al/Yb-SKD joints aged at 600 °C. The inset is the diffusion layer thickness vs the Al content aged at 600 °C for 16 days.
extra mechanisms are playing a vital role in further pushing up the contact resistivity. As has been mentioned before, with the prolonging of the aging time, more interfacial chips appeared at the Ti/Yb-SKD interface. With the increase of the Al content in the Ti–Al barrier layer, however, the chips were evidently reduced. Normally the interfacial chips appear when the interface is undermined and no longer strong enough to resist the interfacial stress. The chips degrade the integrity of the interfacial structure and naturally result in rapid increase of the contact resistivity. As the interfacial stress usually comes from the CTE mismatch between the bonding materials, we then measured the CTEs of the Yb-SKD and the Ti–Al barrier layer bulks. According to Fig. 9, there are CTE mismatches
between the Yb-SKD and the Ti–Al barrier layers. Obviously, the CTE mismatch between Yb-SKD and Ti is the largest, consequently the interfacial stress at the Ti/Yb-SKD interface may be the highest. With the increase of Al content in the Ti–Al barrier layer, the CTE mismatch between Yb-SKD and the Ti–Al barrier layer is reduced. As a result, lower interfacial stress can be achieved at corresponding interfaces. The evolution of the contact resistivity is largely the result of the competition between the interfacial strength and the interfacial stress. During aging, the interfacial strength of the Ti/Yb-SKD joints suffered heavy loss, while the interfacial stress of this system was the highest among the above 6 kinds of TE joints. As a result, the interface was seriously damaged, which then led to rapid increase of the contact resistivity during aging. With the increase of the Al content in the Ti–Al barrier layer, the joint strength increased and the interfacial stress decreased, both of which helped to reduce the interfacial chips, therefore the stability of the contact resistivity was continuingly improved in the Ti97Al3/ Yb-SKD and Ti94Al6/Yb-SKD systems. Further increasing the Al content in the barrier layer, the interfacial stress continued to decrease. Although the joint strength began to decrease again, the reduction of the interfacial stress outweighed that of the interfacial strength. As a result, there were even less interfacial chips at the Ti88Al12/Yb-SKD interface after aging, and the contact resistivity remained stable.
Fig. 5. XRD spectra of the Ti–Al barrier layers: (a) as sintered; (b) aged at 600 °C for 16 days.
Fig. 6. BES images and composition analysis of the Ti94Al6 barrier layer: (a) as sintered; (b) aged at 600 °C for 16 days.
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Reduction rate of shear strength
670
4. Conclusions 100% 80% 60% 40% Ti Ti97Al3 Ti94Al6 Ti91Al9 Ti88Al12
20% 0% 14
16
18
20
22
24
26
28
Diffusion layer thickness / um Fig. 7. Evolution of the shear strength reduction rate with the diffusion layer thickness of the TE joints with different Ti–Al barrier layer aged at 600 °C for 16 days.
In the present work, the Ti–Al/Yb0.6Co4Sb12 TE joints were isothermal aged in vacuum at 600 °C, and the influence of the Al content in the Ti–Al barrier layer on the interfacial evolution behavior was investigated. With comparison to the Ti/Yb-SKD system, the interfacial diffusion of the Ti–Al/Yb-SKD system was effectively reduced at an appropriate Al content of about 6 at.%. Further increasing the Al content, however, the interfacial diffusion was promoted. XRD and composition analyses showed that during sintering and aging, Ti and Al diffused into each other and formed the Ti3Al and TiAl phases, which depressed the interfacial diffusion. However, diffusion of Al into the Yb-SKD intensified the interfacial diffusion, which damaged the high temperature stability of the joints. The competition of the above two mechanisms decided the inter-diffusion at the interface, and TE joints with less interfacial diffusion achieved higher interfacial shear strength after aging. The Al content also influenced the CTE of the Ti–Al barrier layer. More Al content led to less CTE mismatch between the barrier layer and the Yb-SKD, which brought down the interfacial stress and helped to relieve the interfacial damages during aging. As a result, the stability of the contact resistivity was significantly improved. In brief, the Al content in the Ti–Al barrier layer influences both the interfacial diffusion and the CTE mismatch of the Ti–Al/YbSKD interface, which work together to determine the stability of the interface and therefore construct the basis of the optimized design of TE joints. Acknowledgements The authors acknowledge the financial supports from National Natural Science Foundation of China (Contract No. 51372261), International S&T Cooperation Program of China (Contract No. 2011DFB60150) and National Basic Research Program of China (Contract No. 2013CB632504).
Fig. 8. Evolution of the contact resistivity with aging time for the Ti–Al/Yb-SKD joints aged at 600 °C.
CTE / 10-6/ K
14 Yb-SKD Ti88Al12 Ti94Al6 Ti
12
10
8
6 0
100
200
300
400
500
600
700
T / oC Fig. 9. Variation of the CTE with temperature for the Yb-SKD and the Ti–Al barrier layer materials.
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