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CeyFexCo4-xSb12 thermoelectric joints at high temperature
Journal of Alloys and Compounds 671 (2016) 238e244
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Study on the interfacial stability of p-type Ti/CeyFexCo4-xSb12 thermoelectric joints at high temperature Ming Gu a, Xugui Xia a, Xiangyang Huang a, Shengqiang Bai a, Xiaoya Li 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 b State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050, China
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a b s t r a c t
Article history: Received 11 June 2015 Received in revised form 4 February 2016 Accepted 5 February 2016 Available online 9 February 2016
The interfacial stability of thermoelectric (TE) joints is among the key issues determining the durability of thermoelectric generators (TEGs) in practical operation. For skutterudite (SKD) based TEGs, the interdiffusion between the metal electrode and the SKD material at high temperature plays a vital role on the interfacial stability of the joints. In this work, we report the evolution of the interfacial microstructure and the interfacial contact resistivity for a series of p-type SKD TE joints (Ti/CeyFexCo4-xSb12, x ¼ 1.5e4, y ¼ 0.35e1) at high temperature. The growth of the intermetallic compound (IMC) layers at the interface of all the joints was found to be controlled by the diffusion process. In contrast to the binary Ti/CoSb3 joints, the growth of the interfacial IMC layers in the p-type Ti/CeyFexCo4-xSb12 joints was considerably depressed. Moreover, no brittle TiCoSb phase was formed at the interface of the p-type joints, which resulted in good stability of the interfacial contact resistivity at high temperature. Related mechanisms were discussed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Skutterudite Thermoelectric joint Interfacial diffusion Contact resistivity Intermetallic layer
1. Introduction Utilizing the Seebeck effect of thermoelectric (TE) materials, thermoelectric power generators (TEGs) produce an electric voltage from an applied temperature differential across the material. In contrast to traditional power generators, TEGs possess extremely simple structures without moving parts, are especially adjustable in size with a wide range of power densities, and normally need little maintenance because of good reliability. TEGs feature special advantages and have achieved remarkable success in the application of space power sources [1]. Besides, TEGs also have great civil and commercial potential. They can convert part of the waste heat from gas pipe into extra electricity for automobiles [2]. They can be embraced into PV/TE hybrid solar energy systems to improve the overall efficiency [3e5]. And they can also serve as stable power sources for wearable electronic equipments [6]. As the fundamental of the TE technology, numerous TE materials with different preferred performing temperatures have being hotly studied, such as SiGe [7,8] (>700C), half-Heusler [9] (>600C), PbTe
* Corresponding author. E-mail address: [email protected] (L. Chen). http://dx.doi.org/10.1016/j.jallcom.2016.02.041 0925-8388/© 2016 Elsevier B.V. All rights reserved.
[10,11] (>500C), skutterudite [12e15](>400C) and Bi2Te3/Sb2Te3 alloys [16,17] (RT~300C). Particularly, filled skutterudite (SKD) materials are currently among the most promising candidates for practical application due to excellent TE properties (high ZT), relatively low cost, low toxicity and relatively good mechanical properties. In parallel, the study of SKD based TE devices has been actively pursued [18e21]. The main body of a TEG is usually composed of tens or hundreds of p-shape elements which are connected electrically in series (and/or parallel) and thermally in parallel. The p-shape elements consist of coupled n- and p-type TE joints. As the fundamental and key component of the TEGs, the properties of a TE joint, which is made up of TE materials (n- or p-type) and electrodes on both hot and cold sides, greatly affect the overall performance of the system. Basically, the performance of the TE joints is evaluated by the output characteristics (conversion efficiency and power output) and the service behavior. Apart from the TE properties of the bulk material, the output characteristics of the TE joints also rely on the temperature difference between the two sides of the joints and the contact properties of the interface between the electrode and the bulk TE material [22]. Improving the hot side temperature helps to build up a large temperature difference and make full use of the TE
M. Gu et al. / Journal of Alloys and Compounds 671 (2016) 238e244 Table 1 Information of raw materials for the fabrication of TE joints. Element
Manufacturer
Purity
Shape
Co Sb Ce Fe Ti
Alfa Aesar Alfa Aesar Grirem Alfa Aesar Alfa Aesar
4N5 6N 2N5 4N 4N
Slug Shot Ingot Pieces Foil
properties of the bulk materials, both of which are conducive to better output characteristics of the TE joints. However, high operation temperature also lays down great threats to the durability of the TE joints because it accelerates the inter-diffusions and reactions between the metal electrode and the TE material, which results in the formation of thick intermetallic compound (IMC) layers at the interface. It is well known that suitable interfacial IMC layer helps to form a strong bonding. But excessive IMC layer will both damage the structural stability of the joints by pulling down the interfacial strength and degrade the output performance of the joints by pushing up the contact electrical and thermal resistivity at the interface. Therefore, high temperature interfacial stability of the TE joints is one of the key issues determining the service behavior of the joints, and the evolution of the interfacial microstructures and properties of the TE joints at high temperature functions as an important foundation for the evaluation of the serving performance of TEGs. As for the SKD-based TE generators, the hot side is expected to work at 500 C or higher, and the systems are usually required to perform stably for a couple of years or even longer. Unfortunately, antimony, the major component of the SKD material, reacts fiercely at high temperature with almost all the common electrode materials such as Ni, Cu, Al, etc., and the resulting thick interfacial IMC layers lead to quick failure of the interface. To improve the interfacial stability, a barrier layer is commonly introduced between the electrode and the SKD materials. Fan et al. firstly reported the
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application of Ti as the barrier layer in the SKD based TE joints [23]. Zhao et al. afterwards studied the stability of the Ti/CoSb3 interface during accelerated aging [24,25]. Song et al. investigated the effect of sputtered Au, Pt, and Ti as the barrier layer for CoSb3 [26]. Krzysztof Tomasz tried Mo, Ni and Cr80Si20 as the barrier layer in the CueCoSb3 junctions [27]. Guo et al. reported the using of CoeFeeNi alloys [28], and Salvador from General Motors showed the performance of SKD-based TE modules using Mo as the hot side diffusion barrier [21]. Recently, we studied the interfacial stability of n-type Ti1-xAlx/Yb0.6Co4Sb12 (x ¼ 0e0.15) joints and found that the interfacial evolution of the n-type Ti1-xAlx/Yb0.6Co4Sb12 joints is similar to that of the binary Ti/CoSb3 joints [29]. Among all the reported barrier layer materials, Ti and Ti based alloys show the best potential in long-term application of the SKD based TE joints. Currently available reports mainly focused on Ti/CoSb3 and n-type Ti/SKD joints. However, the interfacial stability of p-type Ti/SKD joints is of equal significance for the practical application of SKD based TEGs. Among the p-type SKDs, a number of materials from the family of CeyFexCo4-xSb12 with excellent TE performance have been intensively studied [30e32], which makes this family the most promising p-type SKDs for practical application. In this paper, we selected three typical components from this family, i.e. Ce0.35Fe1.5Co2.5Sb12, Ce0.9Fe3CoSb12 and CeFe4Sb12 to fabricate a series of ptype Ti/CeyFexCo4-xSb12 (x ¼ 1.5e4, y ¼ 0.35e1) joints by one-step SPS method. The Ti/CoSb3 joints were also fabricated following the same process for comparison. The above TE joints were aged at 550 C under vacuum for various durations. The evolution of the interfacial microstructure and the contact electrical resistivity were studied. Significant difference in the interfacial stability between ptype Ti/CeyFexCo4-xSb12 joints and Ti/CoSb3 joints were revealed, and related mechanisms were discussed. 2. Experimental procedures The fabrication of p-type SKD powder and binary CoSb3 powder
Fig. 1. Evolution of the interfacial structure at the Ti/Ce0.35Fe1.5Co2.5Sb12 interface aged at 550 C. (a) as-sintered, (b) 1day, (c) 8 days, (d) 30 days.
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Fig. 2. Spot testing and line scan results of the diffusion layer at the Ti/Ce0.35Fe1.5Co2.5Sb12 interface aged at 550 Cfor 8 days. Clearly it consists of a TiSb2 layer and a thin gradient layer of Ti and Sb.
follow the same process mentioned in references 30 and 24, respectively. To obtain TE joints, the SKD powder was loaded into a graphite die (12.7 mm in diameter) with a piece of Ti foil on the top and consolidated using a spark plasma sintering (SPS) technique. The information of related raw materials is given in Table 1. To make sure the SKD parts have a more than 98% relative density, the sintering temperatures were set at 620 C, 605 C, 585 C and 575 Cfor
Ti/CeyFexCo4-xSb12 joints of x ¼ 0, 1.5, 3 and 4, respectively. All the samples were sintered under the pressure of 60 MPa and held for 10 min. The sintered samples were cut into a final dimension of 3 3 5 mm3. The resulting joints were then sealed in quartz ampoules under vacuum and aged at 550 C for different durations. The phase composition of the CeyFexCo4-xSb12 (x ¼ 0e4) materials of the as-prepared TE joints was determined by X-ray diffraction (XRD) analysis (Rigaku, Rint2000), and all the diffraction peaks belong to the SKD phase. For the as-prepared and aged TE joints, the interfacial microstructures were investigated by scanning electronic microscope (JSM-6700F, JEOL), and the chemical compositions of the interfacial diffusion and reaction layers were analyzed using electron probe microanalysis (JXA-8100, JEOL). The contact resistivity of the joints was measured by a homemade 4probe platform. For each sample, the interfacial diffusion layer thickness is the arithmetic average value in three interfacial areas at intervals of about 0.8 mm. All the data of the diffusion layer thickness, the element distribution and the contact resistivity presented in this paper are the root mean square of three parallel samples, and the error bars are the root mean square errors. 3. Results and discussion 3.1. Evolution of the interfacial microstructure
Fig. 3. Diffusion layer thickness at the Ti/CeyFexCo4-xSb12 interface grows linearly with the square root of aging time, indicating that the interfacial evolution of all the above TE joints is controlled by the diffusion process.
From the Ti side to the CoSb3 side, three diffusion layers consisting of TiSb, TiSb2 and TiCoSb appeared in turn at the interface of the Ti/CoSb3 joints during aging, which agrees Zhao's report [24]. For the p-type TE joints, however, the evolution of the interfacial microstructure was significantly different. According to Fig. 1(b), a two-layer structure appeared between Ti and Ce0.35Fe1.5Co2.5Sb12 after 1 day's aging. This structure grew thicker with the increase of aging time (see Fig. 1(c),(d)). Composition analysis revealed that the
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Fig. 4. (a). Testing spots of the interfacial composition analysis for the Ti/Ce0.35Fe1.5Co2.5Sb12 joints aged at 550C for 30 days. (b). Element distribution of Sb, Co and Ce along the testing spots of Fig.4(a) (The Fe content is standardized as 1.5).
thicker layer is TiSb2 and the thinner one consists of Ti and Sb (see Fig. 2). In p-type TE joints with higher Ce and Fe contents (x ¼ 3, y ¼ 0.9 and x ¼ 4, y ¼ 1), similar two-layer structure appeared at the interface after aging.
3.2. Growth of the interfacial IMC layer Fig. 3 shows that the total thickness of the interfacial IMC layers in all the TE joints increases linearly with the square root of the aging time. It means that the growth of the interfacial IMC layers of all the TE joints is controlled by the diffusion process of interfacial atoms. Meanwhile, the growth rates of the interfacial IMC layers in all the p-type SKD joints are evidently lower than that in the binary
Ti/CoSb3 joints. This means that the p-type SKD joints have better stability at high temperature in terms of the interfacial diffusion and reactions. In reference 24, Zhao explained the formation and growth of IMC layers at the Ti/CoSb3 interface during high temperature aging. The process includes the decomposition of the CoSb3 lattice and the inter diffusion of Ti and decomposed Sb&Co atoms. To analyze the microstructural evolution at the p-type Ti/CeyFexCo4-xSb12 interface, we made detailed composition analyses across the IMC/p-SKD interfaces of different aged samples. Fig. 4(a) shows the testing spots of the interfacial composition analysis for the Ti/Ce0.35Fe1.5Co2.5Sb12 joints aged at 550C for 30 days. Fig. 4(b) illustrates the element distribution of Sb, Co and Ce (The Fe content is
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Fig. 7. Diffusion layers at the Ti/CoSb3 interface aged at 550 C in vacuum for 30 days. The TiCoSb layer became porous and pulverized. Fig. 5. A Schematic diagram of a homemade 4-probe platform for the measurement of the contact resistivity. The inset is an example of the RBC verse the distance between probe 2 and probe 3.
lattices. Detailed mechanism will be studied and reported later. 3.3. Stability of the contact resistivity
Fig. 6. Evolution of the contact resistivity with the aging time for Ti/CeyFexCo4-xSb12 thermoelectric joints. The data connected with dashed line is calculated under the assumption that the contact resistivity is contributed by the electrical resistance of the TiCoSb layer.
standardized as 1.5). A depletion of Sb is found in the interfacial area, which is believed to appear with the growth of the interfacial TiSb2 and TieSb layers. Another thing worth mentioning is that there is no sudden and considerable increase of the Ce and Co contents in the interfacial area. This indicates that there's no severe decomposition in the interfacial Ce0.35Fe1.5Co2.5Sb12 during aging. Similar results were also found in other p-type TE joints. Therefore, it can be deduced that in contrast to the case of the binary CoSb3, less decomposition happened in the interfacial p-type SKD and fewer atoms were decomposed from the lattices to diffuse and react with Ti atoms during aging, so the growth rate of the interfacial IMC layers in the p-type SKD joints were lowered. Based on the above results and analyses, it is clear that the Ce filling and/or Fe doping influences the stability of the interfacial CeyFexCo4-xSb12 during aging. In another word, the existence of Ce and/or Fe atoms may change the chemical environment at the interface and slow down the decomposition of the p-type SKD
As an important character of the TE joints, the contact electrical resistivity has significant influence on the performance of a TE module. Lower contact resistivity corresponds to less Joule heat at the interface, therefore less losses of the efficiency and power output of a TEG. Normally, contact resistivity is determined by the nature of the contacting materials at the interface and the integrity of the interfacial structure. In this paper, the contact resistivity was measured on a homemade 4-probe platform. Fig. 5 shows a schematic diagram of the platform. The inset is an example of the electrical contact resistivity measurement. In Fig. 5, probes A and D, B and C are pressed under constant pressure at the two ends and the polished side surface of a TE joint, respectively. Electrical current from a pulse constant current source flows through the sample via probes A and D. The voltage drop and consequently the electrical resistance between probes B and C, RBC, is detected. See the inset of Fig. 5, RBC increases linearly with increasing BC distance as probe C moves on the polished surface from the SKD side to the Ti side. Then there is a leap of resistance, Rc (Rc ¼ R2 e R1) when probe C is moving across the interface. Rc represents the contact resistance of the interface. Multiply Rc with the contacting area, and the contact resistivity is obtained. Fig. 6 plots the interfacial contact resistivity (solid points) of various joints as a function of the aging time under 550 C. The interfacial contact resistivity of the p-type Ti/CeyFexCo4-xSb12 (x 1.5, y 0.35) joints, all of which were initially around 3 mU$cm2, grew very slowly during aging. As a result, even after 30 days of aging at 550 C, the contact resistivity still remained below 6 mU$cm2. However, in case of the Ti/CoSb3 joints, the contact resistivity rapidly rose from the initial 5 mU$cm2 to 16 mU$cm2 after 1 day's aging. Afterwards, it further increased and reached 94 mU$cm2 on the 30th day. Clearly, the evolution behavior of the contact resistivity of the Ti/ CoSb3 joints and the p-type joints, which initially are comparable, deviated severely during aging. When we compare the microstructure evolution behavior at the interfaces of Ti/CoSb3 and ptype Ti/CeyFexCo4-xSb12 joints, it can be easily found that the essential difference between the interfacial IMC structure of the Ti/ CoSb3 joints and that of the p-type Ti/CeyFexCo4-xSb12 joints is the formation of TiCoSb layer. In the Ti/CoSb3 joints, TiCoSb IMC layer
M. Gu et al. / Journal of Alloys and Compounds 671 (2016) 238e244
was found during high temperature aging. In the p-type Ti/CeyFexCo4-xSb12 joints, however, TiCoSb IMC layer was not observed after aging. The room temperature electrical resistivity of the TiCoSb material is about 500 mU$m [33]. Supposing the TiCoSb layer formed at the Ti/CoSb3 interface has similar electrical resistivity, then 1 mm thick TiCoSb layer with full density will itself induce an extra contact resistivity of about 6 mU$cm2 Fig. 6 also plots the calculated extra contact resistivity (hollow points) simply contributed by the electrical resistance of the grown TiCoSb layer. It was found that after 1 day's aging, the rising of the total contact resistivity was largely contributed by such extra contact resistivity. With increasing aging time, however, the total contact resistivity increased much more rapidly than the TiCoSb layer contribution. After 30 days' aging, the TiCoSb layer itself contributed about 20 mU$cm2 to the total contact resistivity, which was far less than the total contact resistivity (~94 mU$cm2) at the Ti/CoSb3 interface. This indicates that there are extra mechanisms playing a leading role in raising the contact resistivity of the Ti/CoSb3 joints during prolonged aging. It was found that the TiCoSb layer at the interface of the aged Ti/ CoSb3 joints became porous and showed a sign of pulverization. This is especially evident after 30 days' aging (see Fig. 7). This degradation of the interfacial TiCoSb layer was also presented in reference 25. Recently we have observed similar degradation of the TiCoSb layer at the Ti/Yb0.6Co4Sb12 interface after aging and confirmed that it is responsible for the sharp rising of the contact resistivity [29]. Therefore, it is reasonable to conclude that the degradation of the TiCoSb layer at the Ti/CoSb3 interface is the major contributor to the rapid rising of the contact resistivity during prolonged aging. 4. Conclusions In this paper, SKD based binary Ti/CoSb3 and p-type Ti/CeyFexCo4-xSb12 (x ¼ 1.5e4, y ¼ 0.35e1) TE joints were fabricated by a one-step SPS process. The evolution of the interfacial microstructure as well as the stability of the contact resistivity at the Ti/CeyFexCo4-xSb12 (x ¼ 0 to 4, y ¼ 0 to 1) interfaces were investigated after aging at 550 C in vacuum. It was found that the growth of the IMC layers at the interfaces of all the above TE joints is dominated by the diffusion process. In contrast to the binary Ti/CoSb3 joints, the growth of the interfacial IMC layers are evidently depressed and no TiCoSb layer is formed at the interfaces of the p-type Ti/CeyFexCo4-xSb12 (x ¼ 1.5e4, y ¼ 0.35e1) joints after aging. Related mechanisms were discussed. It was found that there is no severe decomposition of the interfacial p-type SKD lattices during aging. Besides, the prohibiting of the TiCoSb layer greatly improved the stability of the contact resistivity for all the p-type Ti/CeyFexCo4xSb12 joints. Based on the above studies, Ti is a promising barrier layer material for the above p-type CeyFexCo4-xSb12 SKDs. Acknowledgments The authors acknowledge the financial supports from National Natural Science Foundation of China (Contract No. 51404236 and 51372261) and National Basic Research Program of China (Contract No. 2013CB632504). References [1] Hamed H. Saber, Mohamed S. El-Genk, Thierry Caillat, Tests results of skutterudite based thermoelectric unicouples, Energy Conversat. Manage 48 (2007) 555e567. [2] Sumeet Kumar, Stephen D. Heister, Xianfan Xu, James R. Salvador, Optimization of thermoelectric components for automobile waste heat recovery
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Study on the interfacial stability of p-type Ti/CeyFexCo4-xSb12 thermoelectric joints at high temperature Ming Gu a, Xugui Xia a, Xiangyang Huang a, Shengqiang Bai a, Xiaoya Li 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 b State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 June 2015 Received in revised form 4 February 2016 Accepted 5 February 2016 Available online 9 February 2016
The interfacial stability of thermoelectric (TE) joints is among the key issues determining the durability of thermoelectric generators (TEGs) in practical operation. For skutterudite (SKD) based TEGs, the interdiffusion between the metal electrode and the SKD material at high temperature plays a vital role on the interfacial stability of the joints. In this work, we report the evolution of the interfacial microstructure and the interfacial contact resistivity for a series of p-type SKD TE joints (Ti/CeyFexCo4-xSb12, x ¼ 1.5e4, y ¼ 0.35e1) at high temperature. The growth of the intermetallic compound (IMC) layers at the interface of all the joints was found to be controlled by the diffusion process. In contrast to the binary Ti/CoSb3 joints, the growth of the interfacial IMC layers in the p-type Ti/CeyFexCo4-xSb12 joints was considerably depressed. Moreover, no brittle TiCoSb phase was formed at the interface of the p-type joints, which resulted in good stability of the interfacial contact resistivity at high temperature. Related mechanisms were discussed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Skutterudite Thermoelectric joint Interfacial diffusion Contact resistivity Intermetallic layer
1. Introduction Utilizing the Seebeck effect of thermoelectric (TE) materials, thermoelectric power generators (TEGs) produce an electric voltage from an applied temperature differential across the material. In contrast to traditional power generators, TEGs possess extremely simple structures without moving parts, are especially adjustable in size with a wide range of power densities, and normally need little maintenance because of good reliability. TEGs feature special advantages and have achieved remarkable success in the application of space power sources [1]. Besides, TEGs also have great civil and commercial potential. They can convert part of the waste heat from gas pipe into extra electricity for automobiles [2]. They can be embraced into PV/TE hybrid solar energy systems to improve the overall efficiency [3e5]. And they can also serve as stable power sources for wearable electronic equipments [6]. As the fundamental of the TE technology, numerous TE materials with different preferred performing temperatures have being hotly studied, such as SiGe [7,8] (>700C), half-Heusler [9] (>600C), PbTe
* Corresponding author. E-mail address: [email protected] (L. Chen). http://dx.doi.org/10.1016/j.jallcom.2016.02.041 0925-8388/© 2016 Elsevier B.V. All rights reserved.
[10,11] (>500C), skutterudite [12e15](>400C) and Bi2Te3/Sb2Te3 alloys [16,17] (RT~300C). Particularly, filled skutterudite (SKD) materials are currently among the most promising candidates for practical application due to excellent TE properties (high ZT), relatively low cost, low toxicity and relatively good mechanical properties. In parallel, the study of SKD based TE devices has been actively pursued [18e21]. The main body of a TEG is usually composed of tens or hundreds of p-shape elements which are connected electrically in series (and/or parallel) and thermally in parallel. The p-shape elements consist of coupled n- and p-type TE joints. As the fundamental and key component of the TEGs, the properties of a TE joint, which is made up of TE materials (n- or p-type) and electrodes on both hot and cold sides, greatly affect the overall performance of the system. Basically, the performance of the TE joints is evaluated by the output characteristics (conversion efficiency and power output) and the service behavior. Apart from the TE properties of the bulk material, the output characteristics of the TE joints also rely on the temperature difference between the two sides of the joints and the contact properties of the interface between the electrode and the bulk TE material [22]. Improving the hot side temperature helps to build up a large temperature difference and make full use of the TE
M. Gu et al. / Journal of Alloys and Compounds 671 (2016) 238e244 Table 1 Information of raw materials for the fabrication of TE joints. Element
Manufacturer
Purity
Shape
Co Sb Ce Fe Ti
Alfa Aesar Alfa Aesar Grirem Alfa Aesar Alfa Aesar
4N5 6N 2N5 4N 4N
Slug Shot Ingot Pieces Foil
properties of the bulk materials, both of which are conducive to better output characteristics of the TE joints. However, high operation temperature also lays down great threats to the durability of the TE joints because it accelerates the inter-diffusions and reactions between the metal electrode and the TE material, which results in the formation of thick intermetallic compound (IMC) layers at the interface. It is well known that suitable interfacial IMC layer helps to form a strong bonding. But excessive IMC layer will both damage the structural stability of the joints by pulling down the interfacial strength and degrade the output performance of the joints by pushing up the contact electrical and thermal resistivity at the interface. Therefore, high temperature interfacial stability of the TE joints is one of the key issues determining the service behavior of the joints, and the evolution of the interfacial microstructures and properties of the TE joints at high temperature functions as an important foundation for the evaluation of the serving performance of TEGs. As for the SKD-based TE generators, the hot side is expected to work at 500 C or higher, and the systems are usually required to perform stably for a couple of years or even longer. Unfortunately, antimony, the major component of the SKD material, reacts fiercely at high temperature with almost all the common electrode materials such as Ni, Cu, Al, etc., and the resulting thick interfacial IMC layers lead to quick failure of the interface. To improve the interfacial stability, a barrier layer is commonly introduced between the electrode and the SKD materials. Fan et al. firstly reported the
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application of Ti as the barrier layer in the SKD based TE joints [23]. Zhao et al. afterwards studied the stability of the Ti/CoSb3 interface during accelerated aging [24,25]. Song et al. investigated the effect of sputtered Au, Pt, and Ti as the barrier layer for CoSb3 [26]. Krzysztof Tomasz tried Mo, Ni and Cr80Si20 as the barrier layer in the CueCoSb3 junctions [27]. Guo et al. reported the using of CoeFeeNi alloys [28], and Salvador from General Motors showed the performance of SKD-based TE modules using Mo as the hot side diffusion barrier [21]. Recently, we studied the interfacial stability of n-type Ti1-xAlx/Yb0.6Co4Sb12 (x ¼ 0e0.15) joints and found that the interfacial evolution of the n-type Ti1-xAlx/Yb0.6Co4Sb12 joints is similar to that of the binary Ti/CoSb3 joints [29]. Among all the reported barrier layer materials, Ti and Ti based alloys show the best potential in long-term application of the SKD based TE joints. Currently available reports mainly focused on Ti/CoSb3 and n-type Ti/SKD joints. However, the interfacial stability of p-type Ti/SKD joints is of equal significance for the practical application of SKD based TEGs. Among the p-type SKDs, a number of materials from the family of CeyFexCo4-xSb12 with excellent TE performance have been intensively studied [30e32], which makes this family the most promising p-type SKDs for practical application. In this paper, we selected three typical components from this family, i.e. Ce0.35Fe1.5Co2.5Sb12, Ce0.9Fe3CoSb12 and CeFe4Sb12 to fabricate a series of ptype Ti/CeyFexCo4-xSb12 (x ¼ 1.5e4, y ¼ 0.35e1) joints by one-step SPS method. The Ti/CoSb3 joints were also fabricated following the same process for comparison. The above TE joints were aged at 550 C under vacuum for various durations. The evolution of the interfacial microstructure and the contact electrical resistivity were studied. Significant difference in the interfacial stability between ptype Ti/CeyFexCo4-xSb12 joints and Ti/CoSb3 joints were revealed, and related mechanisms were discussed. 2. Experimental procedures The fabrication of p-type SKD powder and binary CoSb3 powder
Fig. 1. Evolution of the interfacial structure at the Ti/Ce0.35Fe1.5Co2.5Sb12 interface aged at 550 C. (a) as-sintered, (b) 1day, (c) 8 days, (d) 30 days.
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Fig. 2. Spot testing and line scan results of the diffusion layer at the Ti/Ce0.35Fe1.5Co2.5Sb12 interface aged at 550 Cfor 8 days. Clearly it consists of a TiSb2 layer and a thin gradient layer of Ti and Sb.
follow the same process mentioned in references 30 and 24, respectively. To obtain TE joints, the SKD powder was loaded into a graphite die (12.7 mm in diameter) with a piece of Ti foil on the top and consolidated using a spark plasma sintering (SPS) technique. The information of related raw materials is given in Table 1. To make sure the SKD parts have a more than 98% relative density, the sintering temperatures were set at 620 C, 605 C, 585 C and 575 Cfor
Ti/CeyFexCo4-xSb12 joints of x ¼ 0, 1.5, 3 and 4, respectively. All the samples were sintered under the pressure of 60 MPa and held for 10 min. The sintered samples were cut into a final dimension of 3 3 5 mm3. The resulting joints were then sealed in quartz ampoules under vacuum and aged at 550 C for different durations. The phase composition of the CeyFexCo4-xSb12 (x ¼ 0e4) materials of the as-prepared TE joints was determined by X-ray diffraction (XRD) analysis (Rigaku, Rint2000), and all the diffraction peaks belong to the SKD phase. For the as-prepared and aged TE joints, the interfacial microstructures were investigated by scanning electronic microscope (JSM-6700F, JEOL), and the chemical compositions of the interfacial diffusion and reaction layers were analyzed using electron probe microanalysis (JXA-8100, JEOL). The contact resistivity of the joints was measured by a homemade 4probe platform. For each sample, the interfacial diffusion layer thickness is the arithmetic average value in three interfacial areas at intervals of about 0.8 mm. All the data of the diffusion layer thickness, the element distribution and the contact resistivity presented in this paper are the root mean square of three parallel samples, and the error bars are the root mean square errors. 3. Results and discussion 3.1. Evolution of the interfacial microstructure
Fig. 3. Diffusion layer thickness at the Ti/CeyFexCo4-xSb12 interface grows linearly with the square root of aging time, indicating that the interfacial evolution of all the above TE joints is controlled by the diffusion process.
From the Ti side to the CoSb3 side, three diffusion layers consisting of TiSb, TiSb2 and TiCoSb appeared in turn at the interface of the Ti/CoSb3 joints during aging, which agrees Zhao's report [24]. For the p-type TE joints, however, the evolution of the interfacial microstructure was significantly different. According to Fig. 1(b), a two-layer structure appeared between Ti and Ce0.35Fe1.5Co2.5Sb12 after 1 day's aging. This structure grew thicker with the increase of aging time (see Fig. 1(c),(d)). Composition analysis revealed that the
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Fig. 4. (a). Testing spots of the interfacial composition analysis for the Ti/Ce0.35Fe1.5Co2.5Sb12 joints aged at 550C for 30 days. (b). Element distribution of Sb, Co and Ce along the testing spots of Fig.4(a) (The Fe content is standardized as 1.5).
thicker layer is TiSb2 and the thinner one consists of Ti and Sb (see Fig. 2). In p-type TE joints with higher Ce and Fe contents (x ¼ 3, y ¼ 0.9 and x ¼ 4, y ¼ 1), similar two-layer structure appeared at the interface after aging.
3.2. Growth of the interfacial IMC layer Fig. 3 shows that the total thickness of the interfacial IMC layers in all the TE joints increases linearly with the square root of the aging time. It means that the growth of the interfacial IMC layers of all the TE joints is controlled by the diffusion process of interfacial atoms. Meanwhile, the growth rates of the interfacial IMC layers in all the p-type SKD joints are evidently lower than that in the binary
Ti/CoSb3 joints. This means that the p-type SKD joints have better stability at high temperature in terms of the interfacial diffusion and reactions. In reference 24, Zhao explained the formation and growth of IMC layers at the Ti/CoSb3 interface during high temperature aging. The process includes the decomposition of the CoSb3 lattice and the inter diffusion of Ti and decomposed Sb&Co atoms. To analyze the microstructural evolution at the p-type Ti/CeyFexCo4-xSb12 interface, we made detailed composition analyses across the IMC/p-SKD interfaces of different aged samples. Fig. 4(a) shows the testing spots of the interfacial composition analysis for the Ti/Ce0.35Fe1.5Co2.5Sb12 joints aged at 550C for 30 days. Fig. 4(b) illustrates the element distribution of Sb, Co and Ce (The Fe content is
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Fig. 7. Diffusion layers at the Ti/CoSb3 interface aged at 550 C in vacuum for 30 days. The TiCoSb layer became porous and pulverized. Fig. 5. A Schematic diagram of a homemade 4-probe platform for the measurement of the contact resistivity. The inset is an example of the RBC verse the distance between probe 2 and probe 3.
lattices. Detailed mechanism will be studied and reported later. 3.3. Stability of the contact resistivity
Fig. 6. Evolution of the contact resistivity with the aging time for Ti/CeyFexCo4-xSb12 thermoelectric joints. The data connected with dashed line is calculated under the assumption that the contact resistivity is contributed by the electrical resistance of the TiCoSb layer.
standardized as 1.5). A depletion of Sb is found in the interfacial area, which is believed to appear with the growth of the interfacial TiSb2 and TieSb layers. Another thing worth mentioning is that there is no sudden and considerable increase of the Ce and Co contents in the interfacial area. This indicates that there's no severe decomposition in the interfacial Ce0.35Fe1.5Co2.5Sb12 during aging. Similar results were also found in other p-type TE joints. Therefore, it can be deduced that in contrast to the case of the binary CoSb3, less decomposition happened in the interfacial p-type SKD and fewer atoms were decomposed from the lattices to diffuse and react with Ti atoms during aging, so the growth rate of the interfacial IMC layers in the p-type SKD joints were lowered. Based on the above results and analyses, it is clear that the Ce filling and/or Fe doping influences the stability of the interfacial CeyFexCo4-xSb12 during aging. In another word, the existence of Ce and/or Fe atoms may change the chemical environment at the interface and slow down the decomposition of the p-type SKD
As an important character of the TE joints, the contact electrical resistivity has significant influence on the performance of a TE module. Lower contact resistivity corresponds to less Joule heat at the interface, therefore less losses of the efficiency and power output of a TEG. Normally, contact resistivity is determined by the nature of the contacting materials at the interface and the integrity of the interfacial structure. In this paper, the contact resistivity was measured on a homemade 4-probe platform. Fig. 5 shows a schematic diagram of the platform. The inset is an example of the electrical contact resistivity measurement. In Fig. 5, probes A and D, B and C are pressed under constant pressure at the two ends and the polished side surface of a TE joint, respectively. Electrical current from a pulse constant current source flows through the sample via probes A and D. The voltage drop and consequently the electrical resistance between probes B and C, RBC, is detected. See the inset of Fig. 5, RBC increases linearly with increasing BC distance as probe C moves on the polished surface from the SKD side to the Ti side. Then there is a leap of resistance, Rc (Rc ¼ R2 e R1) when probe C is moving across the interface. Rc represents the contact resistance of the interface. Multiply Rc with the contacting area, and the contact resistivity is obtained. Fig. 6 plots the interfacial contact resistivity (solid points) of various joints as a function of the aging time under 550 C. The interfacial contact resistivity of the p-type Ti/CeyFexCo4-xSb12 (x 1.5, y 0.35) joints, all of which were initially around 3 mU$cm2, grew very slowly during aging. As a result, even after 30 days of aging at 550 C, the contact resistivity still remained below 6 mU$cm2. However, in case of the Ti/CoSb3 joints, the contact resistivity rapidly rose from the initial 5 mU$cm2 to 16 mU$cm2 after 1 day's aging. Afterwards, it further increased and reached 94 mU$cm2 on the 30th day. Clearly, the evolution behavior of the contact resistivity of the Ti/ CoSb3 joints and the p-type joints, which initially are comparable, deviated severely during aging. When we compare the microstructure evolution behavior at the interfaces of Ti/CoSb3 and ptype Ti/CeyFexCo4-xSb12 joints, it can be easily found that the essential difference between the interfacial IMC structure of the Ti/ CoSb3 joints and that of the p-type Ti/CeyFexCo4-xSb12 joints is the formation of TiCoSb layer. In the Ti/CoSb3 joints, TiCoSb IMC layer
M. Gu et al. / Journal of Alloys and Compounds 671 (2016) 238e244
was found during high temperature aging. In the p-type Ti/CeyFexCo4-xSb12 joints, however, TiCoSb IMC layer was not observed after aging. The room temperature electrical resistivity of the TiCoSb material is about 500 mU$m [33]. Supposing the TiCoSb layer formed at the Ti/CoSb3 interface has similar electrical resistivity, then 1 mm thick TiCoSb layer with full density will itself induce an extra contact resistivity of about 6 mU$cm2 Fig. 6 also plots the calculated extra contact resistivity (hollow points) simply contributed by the electrical resistance of the grown TiCoSb layer. It was found that after 1 day's aging, the rising of the total contact resistivity was largely contributed by such extra contact resistivity. With increasing aging time, however, the total contact resistivity increased much more rapidly than the TiCoSb layer contribution. After 30 days' aging, the TiCoSb layer itself contributed about 20 mU$cm2 to the total contact resistivity, which was far less than the total contact resistivity (~94 mU$cm2) at the Ti/CoSb3 interface. This indicates that there are extra mechanisms playing a leading role in raising the contact resistivity of the Ti/CoSb3 joints during prolonged aging. It was found that the TiCoSb layer at the interface of the aged Ti/ CoSb3 joints became porous and showed a sign of pulverization. This is especially evident after 30 days' aging (see Fig. 7). This degradation of the interfacial TiCoSb layer was also presented in reference 25. Recently we have observed similar degradation of the TiCoSb layer at the Ti/Yb0.6Co4Sb12 interface after aging and confirmed that it is responsible for the sharp rising of the contact resistivity [29]. Therefore, it is reasonable to conclude that the degradation of the TiCoSb layer at the Ti/CoSb3 interface is the major contributor to the rapid rising of the contact resistivity during prolonged aging. 4. Conclusions In this paper, SKD based binary Ti/CoSb3 and p-type Ti/CeyFexCo4-xSb12 (x ¼ 1.5e4, y ¼ 0.35e1) TE joints were fabricated by a one-step SPS process. The evolution of the interfacial microstructure as well as the stability of the contact resistivity at the Ti/CeyFexCo4-xSb12 (x ¼ 0 to 4, y ¼ 0 to 1) interfaces were investigated after aging at 550 C in vacuum. It was found that the growth of the IMC layers at the interfaces of all the above TE joints is dominated by the diffusion process. In contrast to the binary Ti/CoSb3 joints, the growth of the interfacial IMC layers are evidently depressed and no TiCoSb layer is formed at the interfaces of the p-type Ti/CeyFexCo4-xSb12 (x ¼ 1.5e4, y ¼ 0.35e1) joints after aging. Related mechanisms were discussed. It was found that there is no severe decomposition of the interfacial p-type SKD lattices during aging. Besides, the prohibiting of the TiCoSb layer greatly improved the stability of the contact resistivity for all the p-type Ti/CeyFexCo4xSb12 joints. Based on the above studies, Ti is a promising barrier layer material for the above p-type CeyFexCo4-xSb12 SKDs. Acknowledgments The authors acknowledge the financial supports from National Natural Science Foundation of China (Contract No. 51404236 and 51372261) and National Basic Research Program of China (Contract No. 2013CB632504). References [1] Hamed H. Saber, Mohamed S. El-Genk, Thierry Caillat, Tests results of skutterudite based thermoelectric unicouples, Energy Conversat. Manage 48 (2007) 555e567. [2] Sumeet Kumar, Stephen D. Heister, Xianfan Xu, James R. Salvador, Optimization of thermoelectric components for automobile waste heat recovery
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