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Joining of thermoelectric material with metallic electrode using Spark Plasma Sintering (SPS) technique
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 10277–10282
www.materialstoday.com/proceedings
ECT 2016
Joining of thermoelectric material with metallic electrode using Spark Plasma Sintering (SPS) technique K. Kaszycaa, M. Schmidta, M. Chmielewskia, K. Pietrzaka, R. Zybalab* a
Departament of Ceramic-Metal Composites and Joints, Intitute of Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland b Faculty of Materials Science and Engineering, Warsaw Univesity of Technology, Woloska 141, 02-507 Warsaw, Poland
Abstract One of the most important challenges faced while producing elements of a thermoelectric module (TEM) is the development of an appropriate joining technology affecting the performance of the whole thermoelectric generator. The produced joints must be characterized not only by high electrical conductivity, high heat conductivity and good adhesion, but also proper mechanical strength and, due to the working conditions, good chemical and temperature stability. This work presents lead-free junctions between a skutterudite material (CoSb3) and a metallic electrode for middle-temperature range applications. The production process and the properties of the obtained junctions are described. The materials were joined by the resistance soldering technique using the Spark Plasma Sintering (SPS) apparatus. The properties of the samples, including their electrical conductivity and Seebeck coefficient were determined and presented. Scanning electron microscopy (SEM) was used to investigate the microstructure, whereas electron dispersive spectroscopy (EDS) was employed to analyze the distribution of the elements. The research was aimed at examining the progress of diffusion between the bounded surfaces. The obtained junction was characterized by a very low resistivity. This work summarizes the properties of the produced junctions and the possibility of their application. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS. Keywords:electrical properties; energy harvesting; thermoelectric materials; Spark Plasma Sintering (SPS); junction
* Corresponding author. Tel.: +48222348144; fax: +48222348514. E-mail address:[email protected] 2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS.
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1. Introduction Nowadays, constantly increasing energy consumption forces humanity to develop new power sources intended for waste energy harvesting. One of the possible solutions are thermoelectric modules [1] (TEM) that can directly convert heat energy into electricity. This feature can be used to recover waste heat energy from engine exhaust gases [2,3]. TEMs can be applied also as long-life, emergency electrical power supplies. In this case the heat of human body [4, 5] or radioisotope elements [6] serves as a heat source. Optimization of the joining technology used for connecting thermoelectric materials with electrodes is still a principal topic [7]. That is why, this work focuses on a CoSb3-based material as a very promising lead-free, middle temperature range thermoelectric material [1,8]. It is known that the efficiency of all heat engines is limited by the Carnot cycle and decreased by the thermoelectric properties of the used material and imperfect contacts [9,10]. Increasing the hot side temperature will surely improve electrical parameters, but it also necessitates the use of a thermoelectric and solder material that can operate in the requested temperature range, and sometimes the use of diffusion barriers [11-14]. One of the major problems to be faced is the production of a junction with low electrical resistance, low thermal resistance and proper chemical stability, all of which are directly connected with the quality of contacts between the semiconductor material and the electrode. Junctions that do not meet the abovementioned requirements reduce TEM efficiency and power density. Mechanical parameters such as tensile strength must be also taken into account. Another important aspect to be considered is the type of the electrical conduction of the produced junction, because the connection between the metal and the semiconductor (m-s) can possibly have rectifying properties. If the m-s junction has linear, symmetrical current-voltage (U-I) characteristics, it maintains the Ohm’s law. Obtaining the m-s Ohm-law junction is really challenging and possible only in the case of heavily doped materials such as certain thermoelectric materials. The RoHS directive [15] issued by the European Union prohibits the use of lead. This paper shows lead-free solders based on silver, copper, tin and antimony. The development of ecological, innovative technology is the main purpose of modern science. 2. Experimental part 2.1. Sample preparation The n-type In0.4Co4Sb12thermoelectric material (TE) was synthesized by the direct fusion technique. The weighed amounts of pure elements were prepared in quantities corresponding to the stoichiometric composition of a final compound. As initial materials, pure elements of In, Co and Sb, with purity higher than 99.99 % (Alfa Aesar) were used. The synthesis was performed in quartz ampoules closed in vacuum (Pvac=1×10-3 mbar). The reagents were heated to the temperature of 690°C in the rocking furnace, held under these conditions for 1 week and next slowly cooled down. The prepared ingot was crushed in an agate mortar, and the process was repeated once again. The fine powders were produced through mechanical milling using an agate mortar for 2 h with a rotational speed of 300 rpm. The densification process was conducted under vacuum at the uniaxial compressive pressure of 50 MPa in the SPS apparatus (Spark Plasma Sintering) [16-19]. The powder was placed in a cylindrical graphite die with an inner diameter of 10 mm and pressed between two graphite punches. Next, the pressed powder was heated up to 650°C with a heating rate of 50°C×min-1 and then held under these conditions for 20 min and cooled down at a rate of 25°C×min-1. The relative density of the samples, determined by Archimedes method, was estimated at 98÷99 % of the theoretical value. The Ag30Cu60Sn10 solder was also synthesized by the direct fusion technique from pure elements. In this case the vacuum ampule with reagents was heated to the temperature of 700°C and held like this for 2 h. The antimony powder used as solder was obtained by mechanical milling in a ball mill. Electrolytic copper and Fe82Cr18 stainless steel were the two materials tested for application in electrodes.
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2.2. Diffusion barrier preparation In order to reduce the degradation rate of thermoelectric segments, diffusion barriers are required. This paper presents the properties of two types of diffusion barriers, i.e. Mo deposited using PVD magnetron sputtering and a chemically deployed nickel-phosphorus barrier. The molybdenum diffusion barriers were deposited under vacuum on the flat, mat surfaces of the TE samples. The employed apparatus was equipped with a WMK-50 magnetron gun with replaceable cathodes made of molybdenum. The deposition of the Mo coating on the CoSb3 samples was performed as follows: the specimens were placed on a heating table and heated up to 200°C, under vacuum at 7.0×10-5 mbar for 1 h. The magnetron with Mo target having 2 inches in diameter was power-supplied with a DORA POWER DC pulsed current source with the frequency of 160 kHz. The process of layer deposition was performed with the cathode current of 0.70 A at the power of about 500 W, with the pressure of Ar stabilized at the level of 4.5×10-3 mbar. Under these conditions, the average rate of the layer deposition was approximately 40 nm×min-1. The molybdenum coating obtained after 75 minutes was about 3 μm thick. The next tested nickel-phosphorus layers were produced during the chemical reduction of nickel ions with hydrogen on the metal surface. The basic advantages of these barriers are chemical stability, high electrical conductivity and, when compared to the PVD methods, simple deployment technology. Before Ni-P layer deployment, the surface has to be activated by deposition of palladium precursors coming from palladium (II) chloride. The layers were deposited in a bath at the temperature of 80°C and pH equal to 4.6. The main substrates used in the bath were nickel salts and the reducing agent was sodium hypophosphite and other compounds preventing spontaneous decomposition and stabilizing the pH of the solution. The analysis of the obtained junctions showed that the layers deposited for 40 minutes were about 20 μm thick, so the growth rate of the layer was about ~0.5 μm×min-1. The deployment procedure of the Ni-P layer is protected by patent [20]. 2.3. Junction preparation One of the key steps in the manufacture of the thermoelectric module is joining of the segments of thermoelectric materials with metal electrodes. To produce these junctions, the resistance soldering method was used.
Fig. 1a) Cross section of the graphite die used for joining materials using resistance-soldering technique in SPS apparatus. 1 – force direction, 2 – graphite punch, 3 – graphite die, 4 – electrode, 5 – solder, 6 – TE material, 1b) Example of typical soldering process.
Soldering was carried out inside a graphite die in a vacuum chamber. Figure 1a shows the idea of resistance soldering. During this process the solder (5) is placed between a metallic electrode (4) and a thermoelectric material. The connected parts were placed inside the graphite die (3), between punches (2). The system was compressed by uniaxial force (1) and heated by the DC current flow. Figure 1b shows an exemplary run of the soldering process. At the first stage the die with elements was heated up at the rate of 50°C×min-1, near the melting point of the used solder. After that, the rate was decreased to 5°C×min-1. The last step, which was cooling at a rate of 50°C×min-1, started when contraction significantly changed. The soldering process controlled by the additional observation of contraction increases precision and reproducibility of the obtained joints. It also prevents overheating, which significantly improves the quality of the obtained junctions.
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3. Results and discussion The obtained junctions were characterized using the EDS line analysis to examine the progress of diffusion inside the TE material. The properties of the thermoelectric material depend heavily on their composition [6], so the EDS observable diffusion of any elements into the thermoelectric material will surely exclude the examined junction from further analysis. Figure 2 shows three SEM images with the EDS line analysis of the cross sections of the obtained junctions. Figure 2a and figure 2b show the cross section of samples with a copper electrode and Ag30Cu60Sn10 solder. Although the obtained junctions were continuous, thus resulting in an excellent electrical connection, the solder ingredients diffused to the TE material in both samples. The reason for this were the insufficient properties of the diffusion barrier. The Ni-P barrier was dissolved and the Mo barrier had many cracks, caused by significant differences between the coefficients of thermal expansion.
Fig. 2. SEM images of examples of junctions a) Cu-Ag30Cu60Sn10 solder – Ni barrier – CoSb3; b) Cu – Ag30Cu60Sn10 solder – Mo barrier – CoSb3; c) Fe82Cr18 – Sb solder – CoSb3.
Due to these problems the next tested materials were stainless steel as an electrode and pure antimony as solder. Figure 2c shows a cross section of the junction of the Fe82Cr18 electrode and the TE material obtained using antimony as solder. The application of antimony allowed dropping out of the diffusion barriers, as a result of there being no possible element to diffuse, which simplified the fabrication process. The EDS analysis shown in figure 2c confirms no significant diffusion of elements inside the sample. The process of soldering was optimized to decrease junction thickness, thus improving the electrical parameters of the sample junction. The outcome junction was about 50µm in width. SEM images shown in figure 3a present a continuous Sb solder connecting the TE material with the Fe82Cr18 electrode.
Fig. 3a) Cross section of final Sb junctions, 3b) EDS line analysis of obtained junction.
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To examine the diffusion progress, the EDS analysis of the obtained junction was conducted. Figure 3b shows the EDS analysis of the cross section of the final Sb junction. The performed detailed EDS analysis confirmed no significant diffusion between the joined elements, as it happened in samples connected using the Ag30Cu60Sn10 solder. It additionally showed a small diffusion of indium inside the Sb solder and an intermetallic phase on the Sb and Fe82Cr18 electrode boundary. The created intermetallic phase resulted in an excellent contact between the solder and the electrode, but its stability must be examined by carrying out long-term high temperature tests. In order to determine the electrical parameters of the obtained junction, the conductivity and rectifying properties of samples must be examined.
Fig. 4. Ideas for electrical conductivity (a, b) and Seebeck coefficient (c) measurement for sample with XY-movable probe. 1 – electrode, 2 – voltmeter, 3 – junction, 4 – thermoelectric material, 5a – alternating current source, 5b – adjustable current source, 5c – heated voltage XY probe.
Figure 4a presents the idea for an electrical conductivity measurement along the examined samples. The usage of alternating current allows one to separate voltage-drop caused by the flow of current from the Seebeck effect. The XY-movable probe enables the determination of conductivity changes along the sample, thus making it possible to define the electrical properties of the examined sample, including its resistance and rectifying properties. The resistance and the conductivity character of the produced joints was measured using the technique shown in figure 4b. Using an adjustable current source allows the determination of both the resistance value and rectifying properties. Further work confirmed that the produced joints showed no rectifying properties. The last calculated electrical property was the Seebeck coefficient. Figure 4c presents the idea for measuring the Seebeck parameter distribution on the sample surface. The heated probe scans the sample point by point in order to create the Seebeck coefficient maps showing the practical usage of the produced TEMs. The examination of the rectifying properties was performed following the idea shown in figure 4a, c.
Fig. 5a) Electrical characteristic of obtained junction, 5b) U-I characteristic of the area containing obtained junction.
Figure 5a shows an example of the electrical characteristic of the Sb junction. There is a noticeable resistance and Seebeck coefficient change connected with a chemical composition of the present phase. The resistance of the junction, calculated during the measurement was estimated at 1.6×10-5 Ω, whereas for the area of the sample resistivity was 1.8×10-7 Ωcm2, which compared to other publications (6.9×10-4 Ωcm2 [21], 2.0×10-4 Ωcm2 [22]) is a very low value. Figure 5b shows the U-I characteristic for the area with the obtained junctions. The presented result
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proves that the junction has a symmetrical, linear U-I characteristic, therefore not having rectifying properties. Conducted research confirmed excellent electrical properties of the examined Sb junctions. Further work is aimed at carrying out durability tests at high temperatures. 4. Summary The present study shows the structure of junctions obtained using the resistance soldering technique inside the SPS apparatus. During the study, two types of diffusion barriers with the Ag30Cu60Sn10 solder were tested, but their diffusion–preventing properties were not sufficient to meet the requirements of thermoelectric modules. A significant diffusion of solder elements into the TE material was observed in both cases. The use of the pure antimony solder allowed creating a continues connection between the solder and the TE material and there was no need to use diffusion barriers. The use of the full-automated SPS apparatus for the application of the vacuum resistance soldering technique allowed us to obtain reproducible junctions, characterized by very low resistance and good mechanical properties in all cases (Ag30Cu60Sn10 and Sb solders). In accordance with the RoHS directive, the presented work focuses on lead-free solders, and therefore it is possible to use the results of this work for commercialization purposes. Because of very low contact resistance of the Sb junction further research works will be continued to examine the high temperature stability and degradation progress of the obtained junctions. Acknowledgements Work presented in this paper was supported by the National Centre for Research and Development (NCBR, Poland) under the project “Innovative thermoelectric modules for energy harvesting” (project no. PBS3/A5/49/2015). This scientific work has been partially financed as a research postdoctoral project no. DEC2014/12/S/ST8/00582 from the resources assigned for science by National Science Centre (NCN, Poland). References [1] C. Gayner, K. Kar, Prog Mater Sci 83 (2016) 330-382. [2] F.P. Brito, J. Martins, E. Hancer, N. Antunes, L.M. Goncalves, J. Electron. Mater. 44, 6 (2015) 1984-1997. [3] K.T. Wojciechowski, M. Schmidt, R. Zybala, J. Merkisz, P. Fuc, P. Lijewski, J. Electron. Mater. 39, 9 (2010) 2034-2038. [4] Z. Lu, H. Zhang, C. Mao, C. M. Li, Applied Energy 164 (2016) 57-63. [5] A.R.M. Siddique, R. Rabari, S. Mahmud, B. Heyst, Energy 115 (2016) 1081-1091. [6] M.S. El-Genk, H.H. Saber, Energ. Convers. Manage. 46 (2005) 1083-1105. [7] D.K. Aswal,R. Basu, A. Singh, Energ. Convers. Manage. 114 (2016) 50-67. [8] P. Nieroda, K. Kutorasinski, J. Tobola, K.T. Wojciechowski, J. Electron. Mater. 43, 6 (2014) 1681-1688. [9] D.C. Agrawal, V.J. Menon, J. Appl. Phys. 30 (1997) 357-359. [10] R. Zybała, M. Schmidt, K. Kaszyca, Ł. Ciupiński, M.J. Kruszewski, K. Pietrzak, J. Electron. Mater. 45, 10 (2016) 5223-5231. [11] B. Song, S. Lee, S. Cho, M.J. Song, S.M. Choi, W.S. Seo, Y. Yoon, W. Lee, Journal of Alloys and Compounds 617 (2014) 160-162. [12] D. Zhao, H. Geng, X. Teng, J. Alloy. Compd. 517 (2012) 198-203. [13] H. Li, H. Jing, Y. Han, G-Q.Lu, L. Xu, T. Liu, Mater. Design. 89 (2016) 604-610. [14] J. Piekoszewski, W. Olesinska, J. Jagielski, D. Kalinski, M. Chmielewski, Z. Werner, M. Barlak, W. Szymczyk, Sol. State. Phen. 99, 100 (2004) 231-234. [15] Directive 2011/65/EU of the European Parliament and of the Council of 8 June 2011 on the restriction of the use of certain hazardous substances in electrical and electronic equipment, OJ L 174 of 1 July 2011. [16] R. Zybala, K.T. Wojciechowski, AIP Conf. Proc. 1449 (2012) 393-396, doi: 10.1063/1.4731579. [17] M.J. Kruszewski, R. Zybala, L. Ciupinski, M. Chmielewski, B. Adamczyk-Cieslak, A. Michalski, M. Rajska, K.J. Kurzydlowski, J. Electron. Mater. 45, 3, (2016) 1369-1376. [18] P. Nieroda, R. Zybala, K.T. Wojciechowski, AIP Conf. Proc. 1449, (2012) 199-202, doi: 10.1063/1.4731531. [19] K. Pietrzak, N. Sobczak, M. Chmielewski, M. Homa, A. Gazda, R. Zybała, A. Strojny-Nędza, J. Mater. Eng. Perform. 25, 8, (2016) 30773083. [20] A. Malecki, J. Postolko, R. Gajerski, B. Prochowska-Klisch, S. Labus, S. Mrowiec, M. Danielewski, Patent Application 11.12.1989 BUP 25/89 (1992). [21] M.S. El-Genk, H.H.Saber, Energ. Convers. Manage. 44 (2003) 1069-1088. [22] D. Zhao, X. Li, L. He, W. Jiang, L. Chen, Intermetallics 17 (2009) 136-141.
ScienceDirect Materials Today: Proceedings 5 (2018) 10277–10282
www.materialstoday.com/proceedings
ECT 2016
Joining of thermoelectric material with metallic electrode using Spark Plasma Sintering (SPS) technique K. Kaszycaa, M. Schmidta, M. Chmielewskia, K. Pietrzaka, R. Zybalab* a
Departament of Ceramic-Metal Composites and Joints, Intitute of Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland b Faculty of Materials Science and Engineering, Warsaw Univesity of Technology, Woloska 141, 02-507 Warsaw, Poland
Abstract One of the most important challenges faced while producing elements of a thermoelectric module (TEM) is the development of an appropriate joining technology affecting the performance of the whole thermoelectric generator. The produced joints must be characterized not only by high electrical conductivity, high heat conductivity and good adhesion, but also proper mechanical strength and, due to the working conditions, good chemical and temperature stability. This work presents lead-free junctions between a skutterudite material (CoSb3) and a metallic electrode for middle-temperature range applications. The production process and the properties of the obtained junctions are described. The materials were joined by the resistance soldering technique using the Spark Plasma Sintering (SPS) apparatus. The properties of the samples, including their electrical conductivity and Seebeck coefficient were determined and presented. Scanning electron microscopy (SEM) was used to investigate the microstructure, whereas electron dispersive spectroscopy (EDS) was employed to analyze the distribution of the elements. The research was aimed at examining the progress of diffusion between the bounded surfaces. The obtained junction was characterized by a very low resistivity. This work summarizes the properties of the produced junctions and the possibility of their application. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS. Keywords:electrical properties; energy harvesting; thermoelectric materials; Spark Plasma Sintering (SPS); junction
* Corresponding author. Tel.: +48222348144; fax: +48222348514. E-mail address:[email protected] 2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS.
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1. Introduction Nowadays, constantly increasing energy consumption forces humanity to develop new power sources intended for waste energy harvesting. One of the possible solutions are thermoelectric modules [1] (TEM) that can directly convert heat energy into electricity. This feature can be used to recover waste heat energy from engine exhaust gases [2,3]. TEMs can be applied also as long-life, emergency electrical power supplies. In this case the heat of human body [4, 5] or radioisotope elements [6] serves as a heat source. Optimization of the joining technology used for connecting thermoelectric materials with electrodes is still a principal topic [7]. That is why, this work focuses on a CoSb3-based material as a very promising lead-free, middle temperature range thermoelectric material [1,8]. It is known that the efficiency of all heat engines is limited by the Carnot cycle and decreased by the thermoelectric properties of the used material and imperfect contacts [9,10]. Increasing the hot side temperature will surely improve electrical parameters, but it also necessitates the use of a thermoelectric and solder material that can operate in the requested temperature range, and sometimes the use of diffusion barriers [11-14]. One of the major problems to be faced is the production of a junction with low electrical resistance, low thermal resistance and proper chemical stability, all of which are directly connected with the quality of contacts between the semiconductor material and the electrode. Junctions that do not meet the abovementioned requirements reduce TEM efficiency and power density. Mechanical parameters such as tensile strength must be also taken into account. Another important aspect to be considered is the type of the electrical conduction of the produced junction, because the connection between the metal and the semiconductor (m-s) can possibly have rectifying properties. If the m-s junction has linear, symmetrical current-voltage (U-I) characteristics, it maintains the Ohm’s law. Obtaining the m-s Ohm-law junction is really challenging and possible only in the case of heavily doped materials such as certain thermoelectric materials. The RoHS directive [15] issued by the European Union prohibits the use of lead. This paper shows lead-free solders based on silver, copper, tin and antimony. The development of ecological, innovative technology is the main purpose of modern science. 2. Experimental part 2.1. Sample preparation The n-type In0.4Co4Sb12thermoelectric material (TE) was synthesized by the direct fusion technique. The weighed amounts of pure elements were prepared in quantities corresponding to the stoichiometric composition of a final compound. As initial materials, pure elements of In, Co and Sb, with purity higher than 99.99 % (Alfa Aesar) were used. The synthesis was performed in quartz ampoules closed in vacuum (Pvac=1×10-3 mbar). The reagents were heated to the temperature of 690°C in the rocking furnace, held under these conditions for 1 week and next slowly cooled down. The prepared ingot was crushed in an agate mortar, and the process was repeated once again. The fine powders were produced through mechanical milling using an agate mortar for 2 h with a rotational speed of 300 rpm. The densification process was conducted under vacuum at the uniaxial compressive pressure of 50 MPa in the SPS apparatus (Spark Plasma Sintering) [16-19]. The powder was placed in a cylindrical graphite die with an inner diameter of 10 mm and pressed between two graphite punches. Next, the pressed powder was heated up to 650°C with a heating rate of 50°C×min-1 and then held under these conditions for 20 min and cooled down at a rate of 25°C×min-1. The relative density of the samples, determined by Archimedes method, was estimated at 98÷99 % of the theoretical value. The Ag30Cu60Sn10 solder was also synthesized by the direct fusion technique from pure elements. In this case the vacuum ampule with reagents was heated to the temperature of 700°C and held like this for 2 h. The antimony powder used as solder was obtained by mechanical milling in a ball mill. Electrolytic copper and Fe82Cr18 stainless steel were the two materials tested for application in electrodes.
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2.2. Diffusion barrier preparation In order to reduce the degradation rate of thermoelectric segments, diffusion barriers are required. This paper presents the properties of two types of diffusion barriers, i.e. Mo deposited using PVD magnetron sputtering and a chemically deployed nickel-phosphorus barrier. The molybdenum diffusion barriers were deposited under vacuum on the flat, mat surfaces of the TE samples. The employed apparatus was equipped with a WMK-50 magnetron gun with replaceable cathodes made of molybdenum. The deposition of the Mo coating on the CoSb3 samples was performed as follows: the specimens were placed on a heating table and heated up to 200°C, under vacuum at 7.0×10-5 mbar for 1 h. The magnetron with Mo target having 2 inches in diameter was power-supplied with a DORA POWER DC pulsed current source with the frequency of 160 kHz. The process of layer deposition was performed with the cathode current of 0.70 A at the power of about 500 W, with the pressure of Ar stabilized at the level of 4.5×10-3 mbar. Under these conditions, the average rate of the layer deposition was approximately 40 nm×min-1. The molybdenum coating obtained after 75 minutes was about 3 μm thick. The next tested nickel-phosphorus layers were produced during the chemical reduction of nickel ions with hydrogen on the metal surface. The basic advantages of these barriers are chemical stability, high electrical conductivity and, when compared to the PVD methods, simple deployment technology. Before Ni-P layer deployment, the surface has to be activated by deposition of palladium precursors coming from palladium (II) chloride. The layers were deposited in a bath at the temperature of 80°C and pH equal to 4.6. The main substrates used in the bath were nickel salts and the reducing agent was sodium hypophosphite and other compounds preventing spontaneous decomposition and stabilizing the pH of the solution. The analysis of the obtained junctions showed that the layers deposited for 40 minutes were about 20 μm thick, so the growth rate of the layer was about ~0.5 μm×min-1. The deployment procedure of the Ni-P layer is protected by patent [20]. 2.3. Junction preparation One of the key steps in the manufacture of the thermoelectric module is joining of the segments of thermoelectric materials with metal electrodes. To produce these junctions, the resistance soldering method was used.
Fig. 1a) Cross section of the graphite die used for joining materials using resistance-soldering technique in SPS apparatus. 1 – force direction, 2 – graphite punch, 3 – graphite die, 4 – electrode, 5 – solder, 6 – TE material, 1b) Example of typical soldering process.
Soldering was carried out inside a graphite die in a vacuum chamber. Figure 1a shows the idea of resistance soldering. During this process the solder (5) is placed between a metallic electrode (4) and a thermoelectric material. The connected parts were placed inside the graphite die (3), between punches (2). The system was compressed by uniaxial force (1) and heated by the DC current flow. Figure 1b shows an exemplary run of the soldering process. At the first stage the die with elements was heated up at the rate of 50°C×min-1, near the melting point of the used solder. After that, the rate was decreased to 5°C×min-1. The last step, which was cooling at a rate of 50°C×min-1, started when contraction significantly changed. The soldering process controlled by the additional observation of contraction increases precision and reproducibility of the obtained joints. It also prevents overheating, which significantly improves the quality of the obtained junctions.
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3. Results and discussion The obtained junctions were characterized using the EDS line analysis to examine the progress of diffusion inside the TE material. The properties of the thermoelectric material depend heavily on their composition [6], so the EDS observable diffusion of any elements into the thermoelectric material will surely exclude the examined junction from further analysis. Figure 2 shows three SEM images with the EDS line analysis of the cross sections of the obtained junctions. Figure 2a and figure 2b show the cross section of samples with a copper electrode and Ag30Cu60Sn10 solder. Although the obtained junctions were continuous, thus resulting in an excellent electrical connection, the solder ingredients diffused to the TE material in both samples. The reason for this were the insufficient properties of the diffusion barrier. The Ni-P barrier was dissolved and the Mo barrier had many cracks, caused by significant differences between the coefficients of thermal expansion.
Fig. 2. SEM images of examples of junctions a) Cu-Ag30Cu60Sn10 solder – Ni barrier – CoSb3; b) Cu – Ag30Cu60Sn10 solder – Mo barrier – CoSb3; c) Fe82Cr18 – Sb solder – CoSb3.
Due to these problems the next tested materials were stainless steel as an electrode and pure antimony as solder. Figure 2c shows a cross section of the junction of the Fe82Cr18 electrode and the TE material obtained using antimony as solder. The application of antimony allowed dropping out of the diffusion barriers, as a result of there being no possible element to diffuse, which simplified the fabrication process. The EDS analysis shown in figure 2c confirms no significant diffusion of elements inside the sample. The process of soldering was optimized to decrease junction thickness, thus improving the electrical parameters of the sample junction. The outcome junction was about 50µm in width. SEM images shown in figure 3a present a continuous Sb solder connecting the TE material with the Fe82Cr18 electrode.
Fig. 3a) Cross section of final Sb junctions, 3b) EDS line analysis of obtained junction.
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To examine the diffusion progress, the EDS analysis of the obtained junction was conducted. Figure 3b shows the EDS analysis of the cross section of the final Sb junction. The performed detailed EDS analysis confirmed no significant diffusion between the joined elements, as it happened in samples connected using the Ag30Cu60Sn10 solder. It additionally showed a small diffusion of indium inside the Sb solder and an intermetallic phase on the Sb and Fe82Cr18 electrode boundary. The created intermetallic phase resulted in an excellent contact between the solder and the electrode, but its stability must be examined by carrying out long-term high temperature tests. In order to determine the electrical parameters of the obtained junction, the conductivity and rectifying properties of samples must be examined.
Fig. 4. Ideas for electrical conductivity (a, b) and Seebeck coefficient (c) measurement for sample with XY-movable probe. 1 – electrode, 2 – voltmeter, 3 – junction, 4 – thermoelectric material, 5a – alternating current source, 5b – adjustable current source, 5c – heated voltage XY probe.
Figure 4a presents the idea for an electrical conductivity measurement along the examined samples. The usage of alternating current allows one to separate voltage-drop caused by the flow of current from the Seebeck effect. The XY-movable probe enables the determination of conductivity changes along the sample, thus making it possible to define the electrical properties of the examined sample, including its resistance and rectifying properties. The resistance and the conductivity character of the produced joints was measured using the technique shown in figure 4b. Using an adjustable current source allows the determination of both the resistance value and rectifying properties. Further work confirmed that the produced joints showed no rectifying properties. The last calculated electrical property was the Seebeck coefficient. Figure 4c presents the idea for measuring the Seebeck parameter distribution on the sample surface. The heated probe scans the sample point by point in order to create the Seebeck coefficient maps showing the practical usage of the produced TEMs. The examination of the rectifying properties was performed following the idea shown in figure 4a, c.
Fig. 5a) Electrical characteristic of obtained junction, 5b) U-I characteristic of the area containing obtained junction.
Figure 5a shows an example of the electrical characteristic of the Sb junction. There is a noticeable resistance and Seebeck coefficient change connected with a chemical composition of the present phase. The resistance of the junction, calculated during the measurement was estimated at 1.6×10-5 Ω, whereas for the area of the sample resistivity was 1.8×10-7 Ωcm2, which compared to other publications (6.9×10-4 Ωcm2 [21], 2.0×10-4 Ωcm2 [22]) is a very low value. Figure 5b shows the U-I characteristic for the area with the obtained junctions. The presented result
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proves that the junction has a symmetrical, linear U-I characteristic, therefore not having rectifying properties. Conducted research confirmed excellent electrical properties of the examined Sb junctions. Further work is aimed at carrying out durability tests at high temperatures. 4. Summary The present study shows the structure of junctions obtained using the resistance soldering technique inside the SPS apparatus. During the study, two types of diffusion barriers with the Ag30Cu60Sn10 solder were tested, but their diffusion–preventing properties were not sufficient to meet the requirements of thermoelectric modules. A significant diffusion of solder elements into the TE material was observed in both cases. The use of the pure antimony solder allowed creating a continues connection between the solder and the TE material and there was no need to use diffusion barriers. The use of the full-automated SPS apparatus for the application of the vacuum resistance soldering technique allowed us to obtain reproducible junctions, characterized by very low resistance and good mechanical properties in all cases (Ag30Cu60Sn10 and Sb solders). In accordance with the RoHS directive, the presented work focuses on lead-free solders, and therefore it is possible to use the results of this work for commercialization purposes. Because of very low contact resistance of the Sb junction further research works will be continued to examine the high temperature stability and degradation progress of the obtained junctions. Acknowledgements Work presented in this paper was supported by the National Centre for Research and Development (NCBR, Poland) under the project “Innovative thermoelectric modules for energy harvesting” (project no. PBS3/A5/49/2015). This scientific work has been partially financed as a research postdoctoral project no. DEC2014/12/S/ST8/00582 from the resources assigned for science by National Science Centre (NCN, Poland). References [1] C. Gayner, K. Kar, Prog Mater Sci 83 (2016) 330-382. [2] F.P. Brito, J. Martins, E. Hancer, N. Antunes, L.M. Goncalves, J. Electron. Mater. 44, 6 (2015) 1984-1997. [3] K.T. Wojciechowski, M. Schmidt, R. Zybala, J. Merkisz, P. Fuc, P. Lijewski, J. Electron. Mater. 39, 9 (2010) 2034-2038. [4] Z. Lu, H. Zhang, C. Mao, C. M. Li, Applied Energy 164 (2016) 57-63. [5] A.R.M. Siddique, R. Rabari, S. Mahmud, B. Heyst, Energy 115 (2016) 1081-1091. [6] M.S. El-Genk, H.H. Saber, Energ. Convers. Manage. 46 (2005) 1083-1105. [7] D.K. Aswal,R. Basu, A. Singh, Energ. Convers. Manage. 114 (2016) 50-67. [8] P. Nieroda, K. Kutorasinski, J. Tobola, K.T. Wojciechowski, J. Electron. Mater. 43, 6 (2014) 1681-1688. [9] D.C. Agrawal, V.J. Menon, J. Appl. Phys. 30 (1997) 357-359. [10] R. Zybała, M. Schmidt, K. Kaszyca, Ł. Ciupiński, M.J. Kruszewski, K. Pietrzak, J. Electron. Mater. 45, 10 (2016) 5223-5231. [11] B. Song, S. Lee, S. Cho, M.J. Song, S.M. Choi, W.S. Seo, Y. Yoon, W. Lee, Journal of Alloys and Compounds 617 (2014) 160-162. [12] D. Zhao, H. Geng, X. Teng, J. Alloy. Compd. 517 (2012) 198-203. [13] H. Li, H. Jing, Y. Han, G-Q.Lu, L. Xu, T. Liu, Mater. Design. 89 (2016) 604-610. [14] J. Piekoszewski, W. Olesinska, J. Jagielski, D. Kalinski, M. Chmielewski, Z. Werner, M. Barlak, W. Szymczyk, Sol. State. Phen. 99, 100 (2004) 231-234. [15] Directive 2011/65/EU of the European Parliament and of the Council of 8 June 2011 on the restriction of the use of certain hazardous substances in electrical and electronic equipment, OJ L 174 of 1 July 2011. [16] R. Zybala, K.T. Wojciechowski, AIP Conf. Proc. 1449 (2012) 393-396, doi: 10.1063/1.4731579. [17] M.J. Kruszewski, R. Zybala, L. Ciupinski, M. Chmielewski, B. Adamczyk-Cieslak, A. Michalski, M. Rajska, K.J. Kurzydlowski, J. Electron. Mater. 45, 3, (2016) 1369-1376. [18] P. Nieroda, R. Zybala, K.T. Wojciechowski, AIP Conf. Proc. 1449, (2012) 199-202, doi: 10.1063/1.4731531. [19] K. Pietrzak, N. Sobczak, M. Chmielewski, M. Homa, A. Gazda, R. Zybała, A. Strojny-Nędza, J. Mater. Eng. Perform. 25, 8, (2016) 30773083. [20] A. Malecki, J. Postolko, R. Gajerski, B. Prochowska-Klisch, S. Labus, S. Mrowiec, M. Danielewski, Patent Application 11.12.1989 BUP 25/89 (1992). [21] M.S. El-Genk, H.H.Saber, Energ. Convers. Manage. 44 (2003) 1069-1088. [22] D. Zhao, X. Li, L. He, W. Jiang, L. Chen, Intermetallics 17 (2009) 136-141.