- Email: [email protected]
Joint properties enhancement for PbTe thermoelectric materials by addition of diffusion barrier
Materials Chemistry and Physics 246 (2020) 122848
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Joint properties enhancement for PbTe thermoelectric materials by addition of diffusion barrier Hsien-Chien Hsieh a, Chun-Hsien Wang a, Tian-Wey Lan b, Tse-Hsiao Lee c, Yang-Yuan Chen b, Hsu-Shen Chu c, Albert T. Wu a, * a b c
Department of Chemical and Materials Engineering, National Central University, Taoyuan City, Taiwan Institute of Physics, Academia Sinica, Taipei City, Taiwan Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan
H I G H L I G H T S
� Thick IMC formed in Cu/p-PbTe and melting of joints for Cu/p-PbTe after bonding. � Niddle-like IMCs formed in Ni/p-PbTe; Pb and Te penetrated in Ni for Ni/n-PbTe. � Ni/Co–P/Ag/p-PbTe and Ni/Co–P/n-PbTe solved the problems. � Improved mechanical strength and reasonable contact resistance were obtained. � zT values increased 27% and 109% for coated p- and n-type joints, respectively. A R T I C L E I N F O
A B S T R A C T
Keywords: Thermoelectric materials PbTe Diffusion barrier Interfacial reaction Mechanical strength
PbTe-based alloys are potential mid-temperature thermoelectric materials due to their excellent thermoelectric properties. Formation of intermetallic deteriorates mechanical joint strength and thermoelectric performance as well. In the present work, interfacial reaction, electrical and mechanical behaviors for both p- and n-PbTe joints with an addition of diffusion barrier layer are investigated. The results show that Co–P is a suitable barrier layer for PbTe-based thermoelectric devices with Cu or Ni electrode to inhibit the growth of massive intermetallic compound formation caused by fast interdiffusion in Cu/p-PbTe joints and melting of the joints in Cu/n-PbTe joints. Directly bonded Ni electrode induced formation of needle-like IMCs and disintegration of the elec trodes in p- and n-PbTe modules respectively. Severe interfacial problems were overcome by adding a Co–P diffusion barrier, mechanical strength was improved. In addition, the Co–P layers enhanced the thermoelectric properties of the joints. The addition of the barrier layer increased 27% and 109% of the zT for p- and n-type joints, respectively.
1. Introduction Under the global climate change scenario, thermoelectric (TE) power generation based on Seebeck effect has attracted global attention to harvest waste heat and convert to electricity to lower the carbon foot print. Hence, it is critical to develop a high-efficiency power-generation thermoelectric device with the improved figure of merit (zT ¼ S2 T= ρκ) and good contact between TE materials and electrode. For mediumtemperature power generation, PbTe-based TE materials were widely used in NASA radioisotope satellites and some generators were embedded in vehicles or in the steel industry [1]. Over the years,
high-performance PbTe-based materials have shown low thermal con ductivity and maximum zT value in medium-temperature range of 400–800 K [2–5]. I2 is widely used as a dopant in n-type PbTe and Sn frequently in p-type PbTe to lower thermal conductivity and enhance zT [1,6–9]. Hence, in this work, we have adopted both these elements as the dopants for the TE materials. In addition to the enhancement of the thermoelectric features, the assembling behavior of the modules is also important. The assembling stability of the metal/thermoelectric joints is usually assessed through interfacial reaction, electrical behavior, and mechanical property. The quality of the joints directly influences the thermoelectric performance.
* Corresponding author. E-mail address: [email protected] (A.T. Wu). https://doi.org/10.1016/j.matchemphys.2020.122848 Received 20 November 2019; Received in revised form 10 February 2020; Accepted 23 February 2020 Available online 24 February 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
Unlike the well-investigated BiTe-based thermoelectric modules, studies on PbTe-based joints in the modules are still limited. Currently, several bonding methods, including brazing [10,11], solid-liquid interdiffusion [12–15], spark plasma sintering [16,17], and hot press [18–22] are employed to assemble PbTe-based thermoelectric modules. However, fierce interdiffusion problems were observed. For instance, poor bonding and severe coefficient of thermal expansion (CTE) mismatch were observed. Further, high bonding temperature (approximately 800 � C) is required for Fe/PbTe joints [23]. Element penetration into metal electrodes was found in Ni/PbTe and NiFeMo/PbTe joints [18,22]. Thick reaction layer which formed at the metal/thermoelectric interface depletes thermoelectric elements and may affect the thermoelectric module [17,19]. Moreover, any severe reaction will consume the metal electrode with Ag and Cu as electrodes [20,21]. Thus, the insertion of a diffusion barrier layer was proposed in several studies to solve the above problems. Hu at al. used a Co0.8Fe0.2 alloy as the diffusion barrier and fabricated a PbTe-based module with the highest conversion efficiency [10]. The ability of the electroplated Co layer to serve as an excellent barrier between PbTe and Ag-24.5 at.% Ge braze alloy has also been proved [11]. In our previous study, we have shown that the n-PbTe joints using Cu and Ni electrodes with electroless Co–P diffusion barrier successfully block interdiffusion and solve the melting problem [24]. A clean interface between the Co-based barrier and PbTe has been shown in the literature [11,24]. Therefore, the electroless Co layer was applied on both p- and n-type thermoelectric modules in this study. The electroless process is low cost, has high aspect ratio, and can deposit on the non-active surface. In addition, Ag was chosen as a CTE-modified layer between p-PbTe and Co layers since the CTE of Ag (18.9 � 10 6 K 1) is close to that of p-PbTe (20 � 10 6 K 1). Ag also dissolves in PbTe and contributes electrons by filling Pb vacancies or diffusing into interstitial sites [25,26]. Addi tionally, the reaction of Ag and PbTe would form Ag2Te intermetallic phase. Many studies have shown that Ag2Te precipitates can enhance thermoelectric properties by combining band structure complexity and nanostructure effects [20,27]. This study has comprehensively investigated the insertion of Co–P diffusion barrier on both p- and n-type thermoelectric modules with Cu and Ni electrodes. The interfacial reaction, contact resistance, and me chanical properties of the joints were also studied. Most importantly, the influence of the above properties on thermoelectric performance was studied. The results indicate that the proposed joints significantly enhance the properties of thermoelectric power generation modules.
Bonded PbTe joints were embedded in epoxy and cross-sectioned; a scanning electron microscope (SEM, Hitachi S-4700) was used to observe the cross-sectional microstructure. Field-emission electron probe microanalysis (FE-EPMA, JEOL JXA-8500F) was used to deter mine the composition of the compounds. The crystal structure was identified using the X-ray diffractometer at BL17B1 at the National Synchrotron Radiation Research Center in Taiwan. A fixed-exit dou ble-crystal Si (111) monochromator (λ ¼ 1.55 Å) with 0.6 � 0.5 mm2 spot size was used. Data were collected at a 2θ range of 20� –80� with a scan step size of 0.05� . The contact resistance of the joints was measured by four-probe method using Keithley 2400 series instrument. To eval uate the mechanical behavior of the joints, both p- and n-type PbTe thermoelectric substrates of 0.6 � 0.4 cm2 were used; the Cu and Ni electrodes were of dimensions of 1 � 0.5 cm2. The mechanical strength was evaluated by the die-shear test with Dage series 4000. The shear height and speed were 200 μm and 50 μm/s, respectively. The ther moelectric properties of bare and Co–P-coated thermoelectric materials were investigated to assess the influence of the diffusion barrier. The Seebeck coefficient and resistivity were measured from room tempera ture to 500 � C with commercial equipment (ZEM-3, ULVAC-RIKO) in He atmosphere. To confirm the reproducibility on the thermoelectric per formance, each sample was scanned thrice. 3. Results and discussion 3.1. Interfacial reaction of bare p- and n-PbTe on Cu or Ni electrode Fig. 1(a) shows the interfacial reaction of the Cu/p-PbTe joints after bonding at 650 � C for 60 min. Severe interfacial reaction and depletion of the Cu electrode were observed. A thick reaction layer of Cu3Sn IMC formed between p-PbTe and Cu electrodes, and the 300 μm Cu foil was fully consumed. Large amounts of Sn came from the 40 vol% Sn dopant in the PbTe substrates. The results indicate that the rapid reaction be tween these elements prevented from forming a strong bond in the thermoelectric device. Voids and spalled IMC appeared in the p-PbTe substrate. Fig. 1(b) presents the enlarged microstructure at the boundary of the IMC. Massive dark region consisted of Cu3Sn, while lamellar light gray phases were formed above the Cu3Sn phase. The composition was characterized to be Cu2Te (Table 1). Moreover, approximately 6 at.% Cu was detected in the p-PbTe substrate region. The two observed phases, i. e., Cu3Sn and Cu2Te, had been confirmed by two-phase equilibria of Cu–Sn and Cu–Te at 650 � C isothermal condition. XRD techniques were used to identify the phases of Cu3Sn, Cu2Te, PbTe, and Cu peaks, as shown in Fig. 1(c). Cu diffuses from the electrode to the thermoelectric, and the
2. Materials and methods Polycrystalline PbTe ingots were synthesized by melting pure Pb and Te by adding pure Sn and 1 wt% I2 in p- and n-type PbTe, respectively, in a vacuum-sealed quartz tube at 1000 � C for 2 h. 40 vol% of Pb was replaced by Sn as dopant in p-type materials, which resulted in a composition of Pb0.6Sn0.4Te. The obtained PbTe ingots were ground to powder and the average size was less than 500 μm. The PbTe powders were then sintered in a graphite fixture by hot pressing at 350 � C under 50 MPa for 15 min to obtain a 3 � 1.5 cm2 substrate with a thickness of 0.5 cm. The PbTe substrates were cut into pad shape with a dimension of 0.5 � 0.5 cm2 using a diamond saw. The surface of the substrates was cleaned by grinding and polishing using 0.1 μm Al2O3 suspension. Possible contaminants on the surface were removed by ultrasonic cleaning with acetone, isopropanol, and deionized water. Both p- and ntype PbTe substrates were directly bonded with 300 μm Cu and Ni foils using a high-temperature Ni-based solder paste (2554-N-1, ESL) under 0.55 MPa at 650 � C for 60 min. The reflow temperature of Ni-based solder paste is above 580 � C, and the temperature is almost 200� higher than the working temperature range for PbTe materials. Co–P diffusion barrier was electroless-plated on both p- and n-type PbTe substrates that were bonded with the Cu and Ni foils at the same con ditions. The plating parameters are referred to our previous study [24].
Fig. 1. (a) BEI micrographs of the Cu/p-PbTe joints bonded at 650 � C for 60 min; (b) Enlarged BEI micrographs for the edge of the reaction layer in the Cu/ p-PbTe joint; (c) XRD spectra of the Cu/p-PbTe joint. 2
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
of the Ni foils. The penetration problem would deteriorate the continuity and integrity of the Ni electrode. Therefore, the n-PbTe joint bonded directly with Ni foil may be suitable only for limited temperature or short working time.
Table 1 The atomic composition (at.%) of each phase for Cu/p-PbTe. Pb
Sn
Te
Cu
Ni
Phase
0.02 – 35.49
26.17 – 10.02
0.25 63.40 47.98
72.57 36.59 6.47
0.99 – 0.04
Cu3Sn Cu2Te p-PbTe
3.2. Interfacial reaction after applying Co–P diffusion barrier
following reaction occurred resulting in lamellar structure: e : L1’ ↔ L’2 þ Cu2 Te þ PbTe ð625 � CÞ
Severe reactions, fast diffusion and large amounts of IMCs were found in the joints of both p- and n-type PbTe substrates on Cn and Ni electrodes, which would cause the decline of mechanical stability and deterioration of thermoelectric properties. A barrier layer is needed to resolve the issues. However, cracks can be found between electroless Co–P layer and p-PbTe legs. This finding indicates that the adhesion of the electroless Co–P layer on the p-PbTe substrate is poor after assembly into modules. It is possible that a crack might be formed due to the mismatch in the coefficient of thermal expansion (CTE). Thus, Ag was selected as an intermediate because its CTE (18:9 � 10 6 =KÞ is close to that of p-PbTe (20 � 10 6 =K). This small CTE difference was expected to release the stresses caused by CTE mismatch. Thus, electroless Ag coatings were first deposited onto the surface of p-PbTe. The substrate was sensitized in Sn(II) solution (40 g/L SnCl2.2H2O with 40 ml/L HCl) for 5 min, rinsed with distilled water, activated in AgNO3 solution (10 g/ L) for 5 min, and then immersed in the electroless silver plating solution. The pH value and solution temperatures were 11 and 50 � C, respec tively. Table 3 lists the composition of the electroless Ag plating solution and plating parameters. The as-deposited Ag/p-PbTe substrates were subsequently plated by electroless Co–P coatings. Fig. 5(a) presents the interfacial microstructure between Cu and pPbTe after the insertion of the Ag adhesion layer and the Co–P diffusion barrier layer. Notably, no massive Cu3Sn and eutectic Cu2Te phases, which were detected in the directly bonded sample, were observed at the interface. In contrast to the severe interfacial reaction of the Cu/p-PbTe joint, the integrity of the Cu electrode and the thin reaction layer observed in the Cu/Co–P/Ag/p-PbTe joint indicates the success of the incorporation of the Co–P diffusion barrier. The gray reaction layers formed between p-PbTe and Co–P layer were analyzed to be the Ag2Te and CoSn layer. The composition of each IMCs is listed in Table 4. The deposited electroless Ag layers (approximately 18.8 μm) were fully exhausted and reacted with p-PbTe to form Ag2Te intermetallic com pounds. Some Ag2Te phases spalled into the p-PbTe substrate as shown in the inserted image in Fig. 5(a). Notably, the thickness of the CoSn and Ag2Te layers was approximately 1.4 and 4.6 μm, respectively. The exhaustion of the thermoelectric element was significantly improved after applying plating techniques compared with the directly bonded samples. Fig. 6(a) shows the interfacial results of the Ni/Co–P/Ag/p-PbTe joint bonded at 650 � C for 60 min. No needle-like Ni3 xSnTe2 phases, which were observed in the directly bonded Ni/p-PbTe joint, formed at the interface, indicating that Ni diffusion had been blocked by the Co–P layer. A thin reaction layer (Ag2Te and CoSn) was also observed between Co–P and p-PbTe. The composition of each phase obtained by EPMA is listed in Table 5. Similar to the results of the Cu/Co–P/Ag/p-PbTe joint, the spalling phenomenon of Ag2Te phases was observed in the p-PbTe matrix, as shown in the inserted image in Fig. 6(a). The exact phases were observed at the Cu/Co–P/Ag/p-PbTe and Ni/Co–P/Ag/p-PbTe interface using XRD techniques. After removing the Cu or Ni electrode by polishing, the Ag2Te, CoSn, p-PbTe, and Ni-based solder paste peaks were fully confirmed in the pattern, which is consistent with the observed microstructures, as shown in Figs. 5(b) and 6(b). Although the study of the thickness for Ag2Te and CoSn needs further investigation, previous study shows that Ag–Te compound could be a self-barrier formed during bonding [32]. It is possible that the Ag2Te layer could prevent the growth of CoSn. Fig. 7(a) and (b) present the interfacial microstructure of Cu/Co–P/ n-PbTe and Ni/Co–P/n-PbTe bonded at 650 � C for 60 min, respectively.
(1)
The same eutectic reaction, which causes the melting problem for the Cu/n-PbTe joint, was observed. Fig. 2 shows the microstructure of Cu/nPbTe joint bonded at 650 � C for 60 min. The thermoelectric joint was damaged as the massive diffusion of Cu led to the melting of n-PbTe. The lamellar structure (PbTe, Cu2Te) and Cu-rich regions were also identi fied. Its detailed mechanism has been published in a previous report [24]. The Cu/p-PbTe joint remained in the solid state (Fig. 1). The melting reaction of the Cu/p-PbTe joint was less severe than that of the Cu/nPbTe joint because of its low Cu concentration. More Cu elements were exhausted in the formation of the Cu3Sn reaction layer. In addition to the thick reaction layer and electrode exhaustion, some voids were observed at the bottom of the p-PbTe substrate. The liquid reaction became severe because of the increase in Cu concentration. The volume difference in liquid-state and solid-state PbTe in the p-PbTe matrix would cause void formation during solidification. These voids would severely deteriorate the bonding strength. Fig. 3(a) shows the interfacial microstructure of Ni/p-PbTe joints after bonding at 650 � C for 60 min. Large amounts of needle-like IMCs were detected near the p-PbTe substrate. The close-up image (Fig. 3(b)) shows two morphologies of IMCs with the same contrast, i.e., continuous layer- and needle-type intermetallic compounds. The composition of the continuous IMC layer and needle-like IMCs between Ni-based solder paste and p-PbTe measured by EPMA is listed in Table 2. The compo sition of the IMCs was 51.3 at.% Ni–22.2 at.% Sn–24.2 at.% Te, which was the Ni3 xSnTe2 phase. This ternary phase had been confirmed by Jandl et al. in 600 � C isothermal sections of the Ni–Sn–Te ternary alloy phase diagram [28]. They proposed that Ni3 xSnTe2 was the stable phase at less than 715 � C approximately, which was consistent with our results for the sample bonded at 650 � C. Synchrotron XRD, which has a high resolution for each step, was employed to confirm the Ni–Sn–Te ternary phase, as shown in Fig. 3(c). Given the lack of Ni3 xSnTe2 standard card, PowderX software, which can calculate the theoretical peaks of the identified phase, was used [29]. Ni3 xSnTe2 (space group P63/mmc, Pearson symbol hP12) with lattice constants of a ¼ 3.9585 and c ¼ 15.6194 Å had been reported in the literature [28,30]. The formula in the literature [31] was used to confirm the diffraction peaks of Ni3 xSnTe2 phase. Thus, the X-ray peaks of PbTe, Ni, and Ni3 xSnTe2 were confirmed and are consistent with the observed microstructures (Fig. 3(c)). Considerable exhaustion of thermoelectric elements (i.e., Sn and Te) was observed and should be avoided during the assembly of the module as the properties of the thermoelectric materials would deteri orate. Thus, the Ni/p-PbTe joint underwent severe reaction depleting the thermoelectric matrix, which was considered an unsuitable ther moelectric module structure. Fig. 4 shows the interfacial microstructure of the Ni/n-PbTe joint bonded at 650 � C for 60 min. Some white PbTe phases were observed inside the Ni electrode that indicated the Pb and Te atoms would diffuse through the solder paste layer and penetrate along the grain boundaries
Fig. 2. Optical images of the Cu/n-PbTe joints bonded at 650 � C for 60 min. 3
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
Fig. 3. BEI micrographs of the Ni/p-PbTe joints bonded at 650 � C for 60 min; (b) Enlarged microstructure of (a); (c) XRD spectra of the Ni/p-PbTe joints.
Clear interfaces were observed in both Cu and Ni electrodes. Notably, the severe melting reaction which observed in directly bonded Cu/nPbTe joint and the penetration of PbTe phase in Ni/n-PbTe joint were successfully avoided by utilizing the Co–P diffusion barrier. As a result, the insertion of Co–P layer between n- or p-PbTe materials and Cu or Ni electrodes could effectively inhibit fast interdiffusion and the deterio ration of the joints.
Table 2 The atomic composition (at.%) of IMCs for Ni/p-PbTe joint. Region
Pb
Continuous layer Needle-like IMCs
0.06 0.13
Sn
Te 22.20 22.34
Ni 24.24 23.31
B 51.39 52.36
Phase 1.55 1.78
Ni3-xSnTe2 Ni3-xSnTe2
3.3. Electrical contact In addition to the interfacial reaction, electrical properties are crit ical for the evaluation of the performance of a module. Notably, the contact resistance of the Cu/p-PbTe and Cu/n-PbTe joints could not be measured because of full exhaustion of the electrode. The contact resistance of the Ni/p-PbTe joint is higher than that of the Ni/n-PbTe joint. The data are shown in Table 6 and Table 7. The massive Ni3 xSnTe2 phases observed in the p-PbTe matrix, which resulted in the severe depletion of Te, were considered to be the main cause for the increase in contact resistance. A similar increase caused by Te deficiency in Ni/Bi2Se0.3Te2.7 has been reported [33]. Literature mentions that an insertion of Co–P in the Ni/n-PbTe joints increases the contact resistance from 11.75 to 21.75 μΩ cm2 [24]. Compared with the directly bonded Ni/p-PbTe joint, the increases in contact resistance were observed in the Cu/Co–P/Ag/p-PbTe and Ni/Co–P/Ag/p-PbTe samples from 18.00 to 27.35 and 28.08 μΩ cm2, respectively (Table 7). It is possible that the Co–P layer is amorphous and may increase the contact resistance. Furthermore, the formation of the Ag2Te layer in the joints of p-PbTe would cause Te depletion in the substrate. Limited data could be found in the medium-temperature thermoelectric joints. The contact resistance of Fe/n-PbTe and Fe/Ni/n-PbTe joints are 86.4 and 31.4 μΩ cm2, respectively [34]. CoSb3-based joints with several metallic electrodes are reported to be in the range of 14.6–20 μΩ cm2. Most importantly, Liu et al. proposed that the contact resistance of the thermoelectric module should be less than 10 4 Ω cm2 [33]. The results of the p- and n-type PbTe joints reported in this study are in the acceptable range when applied to the thermoelectric modules.
Fig. 4. BEI micrograph of Ni/n-PbTe joints bonded at 650 � C for 60 min. Table 3 Compositions of electroless plating Ag bath and plating parameters. Composition
Parameter
AgNO3 (NH4)2SO4 25% NH3 CoSO4⋅7H2O pH Plating Temperature Plating Time
6.78 g/L 99 g/L 425 g/L 31.3 g/L 11.0 50 � 2 � C 180 min
3.4. Mechanical test Sufficient strength of the thermoelectric module is required for commercial thermoelectric applications. The shear test has been widely employed for analyzing the bonding strength of solder joints or die-todie joints [35,36]. However, only a few studies have focused on the mechanical behavior of the thermoelectric joints. This study focuses on evaluating the effects of the incorporation of the Co–P diffusion barrier and different metal electrodes. Fig. 8(a) shows the shear force of the p-PbTe joints with Co–P diffusion barrier and Ag adhesion layer. Notably, the directly bonded joints could not be tested successfully. The exhaustion of the Cu electrode and melting reaction leads to the obvious module failure in the Cu/p-PbTe and Cu/n-PbTe joints, respectively. The Ni/p-PbTe joint was weak as the samples moved along the shear force.
Fig. 5. (a) BEI micrographs of the Cu/Co–P/Ag/p-PbTe joint bonded at 650 � C for 60 min; (b) XRD spectra of the Cu/Co–P/Ag/p-PbTe joint.
4
Materials Chemistry and Physics 246 (2020) 122848
H.-C. Hsieh et al.
Table 4 The atomic composition (at.%) of IMCs for Cu/Co–P/Ag/p-PbTe joint. Pb
Sn
Te
Ag
Co
P
Ni
B
Cu
Phase
0.05 0.05 – 34.53
– 47.51 0.07 9.87
32.54 0.66 – 47.64
60.01 0.16 0.17 2.14
1.26 46.47 97.56 0.16
0.14 0.01 1.26 0.15
0.64 4.14 0.65 0.32
2.67 0.45 – 3.56
2.64 7.5 0.44 1.59
Ag2Te CoSn Co–P p-PbTe
Fig. 6. (a) BEI micrographs of the Ni/Co–P/Ag/p-PbTe joint bonded at 650 � C for 60 min; (b) XRD spectra of the Ni/Co–P/Ag/p-PbTe joint.
The massive needle-like IMCs (Ni3 xSnTe2) that formed at the interface could be the main cause for the weak strength. Weakening joints caused by the needle-like IMC in the Nb/n-PbTe joint have been reported [19]. The bonding force of Cu/Co–P/Ag/p-PbTe and Ni/Co–P/Ag/p-PbTe joints were improved by the Co–P/Ag coating layers, indicating that the incorporation of the Co–P and Ag layers will improve the joint strength. Both Co–P/n-PbTe and Co–P/Ag/p-PbTe joints with Cu electrodes show higher shear force than those bonded with Ni electrodes (Fig. 8). A fracture surface analysis was conducted for analyzing the fracture mechanism. Fig. 9(a–d) shows the BEI images of the fracture surfaces for TE joints. The large portions of PbTe and solder paste regions implied that a sufficient bonding strength at the interface (Co–P/n-PbTe, Co–P/Ag/p-PbTe) could be obtained, which improves the mechanical behavior of the joints. In summary, the shear strengths of the p-PbTe and n-PbTe joints with Co–P layer are higher than those of the directly bonded samples. The addition of the Co–P diffusion barrier layer did not decrease the joint strength but enhanced the reliability of the n-PbTe- based joints.
Table 5 The atomic composition (at.%) of IMCs for Ni/Co–P/Ag/p-PbTe joint. Pb
Sn
Te
Ag
Co
P
Ni
B
Phase
0.09 0.08 0.02 31.61
– 48.33 0.29 14.99
33.55 0.66 – 47.61
62.89 0.18 – 1.19
0.67 47.53 96.05 0.62
0.09 – 1.80 0.19
0.04 1.87 1.81 0.33
2.63 1.32 – 3.43
Ag2Te CoSn Co–P p-PbTe
Fig. 7. BEI micrographs of (a) Cu/Co–P/n-PbTe and (b) Ni/Co–P/n-PbTe bonded at 650 � C for 60 min.
3.5. Thermoelectric property Considering that Co–P layers have been proved to be an appropriate diffusion barriers for p-PbTe modules because of their effect on inter facial reactions, contact resistance, and mechanical strength, the effect of the insertion of Co–P diffusion barrier layer and Ag adhesion layer in the p-PbTe module on thermoelectric performance were studied. Fig. 10 (a) and (b) show the original Seebeck coefficient and resistivity of the bare and coated p-PbTe materials with three times scan. The p-type conduction was identified within the whole measuring temperature range. Similar performance and even exactly the same features after two scans were identified in both bare and coated samples. It implies that the thermoelectric properties of p-PbTe materials exhibited reproducibility, and the addition of extra layers (Ag and Co–P) did not deteriorate the thermoelectric performance after cycling. Likewise, a comparison was also made between bare and Co–P/n-PbTe materials to confirm the ef fect of the additional layer on n-PbTe thermoelectric materials. Fig. 10 (c) and (d) present the temperature-dependent thermoelectric perfor mance of the bare and coated n-PbTe materials. The n-type conduction was identified within the entire temperature range. The Seebeck coef ficient gradually increased with temperature, and reproducibility was observed after two scans. It is noted that the electrical resistivity of the bare n-PbTe increased dramatically after approximately 600 K (327 � C)
Table 6 Contact resistance of various as-boned n-PbTe joints [24]. Assemblage
Voltage step (mV)
Contact resistivity (μΩ⋅cm2)
Cu/n-PbTe Ni/n-PbTe Cu/Co–P/n-PbTe Ni/Co–P/n-PbTe
The joint is melted. 0.047 0.085 0.087
� 11.75 21.25 21.75
Table 7 Contact resistance of various as-boned p-PbTe joints. Assemblage
Voltage step (mV)
Contact resistivity (μΩ⋅cm2)
Cu/p-PbTe Ni/p-PbTe Cu/Co–P/Ag/p-PbTe Ni/Co–P/Ag/p-PbTe
The electrode is exhausted. 0.072 0.109 0.112
� 18.00 27.35 28.08
5
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
Fig. 8. Shear loads of (a) the proposed p-PbTe joints and (b) the proposed n-PbTe joints.
the n-PbTe was coated with Co–P, the sudden increase of resistivity could not be seen. Although the differences need further investigation, the reproducibility is significant. This reproducibility indicates that the addition of Co–P layer in the n-PbTe module caused no deterioration during the module operation. The improvement is significant. Fig. 11(a) compares the Seebeck coefficient, resistivity and thermal conductivity of the bare and coated p-PbTe. Similar thermoelectric features between bare and coated samples were observed. The Seebeck coefficient of the Co–P/Ag/p-PbTe was slightly higher than that of the bare p-PbTe. In addition, there is a slight enhancement in electrical re sistivity after applying Co–P and Ag layers. The thermal conductivities of the bare and coated p-PbTe show similar features. Slight differences were confirmed above 600 K, which may be contributed by the forma tion of Ag2Te phases in p-PbTe matrix. Moreover, the Co–P/Ag/p-PbTe sample also exhibited higher power factors (S2/ρ) and zT values than those of the bare one (Fig. 11(c)). The slight enhancement of Seebeck coefficient and resistivity observed in the Co–P/Ag/p-PbTe indicates the improvement of thermoelectric performance. A peak zT of 0.94 was observed at about 650 K for the sample with coating layers, while the bare sample shows 0.74. Fig. 11(b) compares the Seebeck coefficient, resistivity and thermal conductivity between bare n-PbTe and Co–P/n-PbTe samples. Both samples presented n-type features in the entire temperature range. Better thermoelectric properties, including higher Seebeck coefficient, lower resistivity, and lower thermal conductivity were observed. The Pb precipitates observed in the bare n-PbTe increased resistivity. With the improvement of thermoelectric features, the Co–P/n-PbTe sample exhibited higher power factor and zT value. (Fig. 11(d)). A peak zT of 1.15 was observed at about 650 K for the sample with coating layers, while the bare sample shows 0.55. Notably, the incorporation of extra layer proposed in this study did not deteriorate the performance but enhance the thermoelectric performance of both p- and n-type PbTe samples. A stable and better performance was obtained with the incor poration of the Co–P layer.
Fig. 9. BEI images of the fracture surfaces for (a) Cu/Co–P/Ag/p-PbTe (b) Ni/ Co–P/Ag/p-PbTe (c) Cu/Co–P/n-PbTe (d) Ni/Co–P/n-PbTe.
(Fig. 10(c)). According to the elemental quantitative analysis using EPMA, a relatively high Pb content of 57.3 at.% was confirmed in nPbTe. Based on Pb–Te phase diagram, when the system was heated above the melting point of Pb (327.5 � C), excess Pb content would cause partial melting in n-PbTe and lead to a sudden rise of resistivity. Once the sample was cooled down after first scan, approximately 17.1 wt% of Pb, calculated by the lever rule, would precipitate. Pb-rich region was observed in the fracture surface of n-PbTe after three scans. The size of the Pb-rich area was too small to be distinguished by EDS analysis that has a submicron spot size. However, the Pb content was above 80 at.%, and it indicated segregation of Pb in the n-PbTe matrix. These Pb pre cipitates produced extra scattering center for electron transport and thus significantly increases the resistivity after first scan. It is proposed in the literature that a similar scattering effect of the precipitates reduces the Hall mobility and therefore increases resistivity [25,27]. Notably, when
4. Conclusion In summary, we have observed fast interdiffusion and massive for mation of intermetallic compounds in both Cu/p-PbTe and Ni/p-PbTe joints. Melting of the thermoelectric materials and discontinuous elec trodes due to penetration were observed in Cu/n-PbTe and Ni/n-PbTe, respectively. Electroless Co–P diffusion barrier is added to prevent se vere reactions between medium-temperature PbTe TE materials and electrodes. Clear interfaces were observed in p- and n-type PbTe joints. Thin reaction layer contained Ag2Te and CoSn IMCs was formed at the interface in p-PbTe joints. The results of contact resistance proved that Co–P diffusion barrier only slightly increased the electrical contact and the values are in acceptable range for producing a practical thermo electric module. Module failure with Cu electrode and weak strength 6
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
Fig. 10. Thermoelectric properties as a function of temperature for Seebeck coefficient and electrical resistivity or (a) bare p-PbTe (b) Co–P/Ag/p-PbTe (C) bare nPbTe and (d) Co–P/n-PbTe.
Fig. 11. Comparison of Seebeck coefficient, electrical resistivity, and thermal conductivity as a function of temperature for (a) bare p-PbTe and Co–P/Ag/p-PbTe, (b) bare n-PbTe and Co–P/n-PbTe; Comparison of power factor and zT value for (c) bare p-PbTe and Co–P/Ag/p-PbTe, (d) bare n-PbTe and Co–P/n-PbTe.
with Ni were observed in both p-PbTe and n-PbTe joints. Adding the Co–P layer improved the mechanical reliability significantly. The addi tional Co–P or Co–P/Ag layer enhanced the thermoelectric performance and maintained the reproducibility in the measurement temperature range. The results show that Co–P is a candidate of diffusion barrier layer for PbTe-based modules with Cu and Ni electrodes.
CRediT authorship contribution statement Hsien-Chien Hsieh: Conceptualization, Methodology, Investiga tion, Writing - original draft, Validation. Chun-Hsien Wang: Data curation, Writing - review & editing, Visualization. Tian-Wey Lan: Resources. Tse-Hsiao Lee: Resources. Yang-Yuan Chen: Resources. Hsu-Shen Chu: Resources. Albert T. Wu: Conceptualization, Writing review & editing, Supervision.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments The authors would like to acknowledge the support received from the Ministry of Science and Technology of Taiwan for this study under Contract No. 104-2221-E-008-065-MY3. We also thank the Industrial 7
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
Technology Research Institute of Taiwan for supplying the instrument to produce the substrate. The help received from Prof. Chen and Dr. Lan of Academia Sinica, Taiwan, for measuring thermoelectric properties is also appreciated.
[18] H. Xia, F. Drymiotis, C.L. Chen, A. Wu, G.J. Snyder, Bonding and interfacial reaction between Ni foil and n-type PbTe thermoelectric materials for thermoelectric module applications, J. Mater. Sci. 49 (4) (2013) 1716–1723, https://doi.org/10.1007/s10853-013-7857-9. [19] H. Xia, C.-L. Chen, F. Drymiotis, A. Wu, Y.-Y. Chen, G.J. Snyder, Interfacial reaction between Nb foil and n-type PbTe thermoelectric materials during thermoelectric contact fabrication, J. Electron. Mater. 43 (11) (2014) 4064–4069, https://doi.org/ 10.1007/s11664-014-3350-8. [20] C.C. Li, F. Drymiotis, L.L. Liao, M.J. Dai, C.K. Liu, C.L. Chen, Y.Y. Chen, C.R. Kao, G.J. Snyder, Silver as a highly effective bonding layer for lead telluride thermoelectric modules assembled by rapid hot-pressing, Energy Convers. Manag. 98 (2015) 134–137, https://doi.org/10.1016/j.enconman.2015.03.089. [21] C.C. Li, F. Drymiotis, L.L. Liao, H.T. Hung, J.H. Ke, C.K. Liu, C.R. Kao, G.J. Snyder, Interfacial reactions between PbTe-based thermoelectric materials and Cu and Ag bonding materials, J. Mater. Chem. C 3 (40) (2015) 10590–10596, https://doi.org/ 10.1039/C5TC01662B. [22] H. Xia, F. Drymiotis, C.-L. Chen, A. Wu, Y.-Y. Chen, G. Jeffrey Snyder, Bonding and high-temperature reliability of NiFeMo alloy/n-type PbTe joints for thermoelectric module applications, J. Mater. Sci. 50 (7) (2015) 2700–2708, https://doi.org/ 10.1007/s10853-015-8820-8. [23] M. Weinstein, A.I. Mlavsky, Bonding of lead telluride to pure iron electrodes, Rev. Sci. Instrum. 33 (10) (1962) 1119–1120, https://doi.org/10.1063/1.1717707. [24] H.C. Hsieh, C.H. Wang, W.C. Lin, S. Chakroborty, T.H. Lee, H.S. Chu, A.T. Wu, Electroless Co-P diffusion barrier for n-PbTe thermoelectric material, J. Alloys Compd. 728 (2017) 1023–1029, https://doi.org/10.1016/j.jallcom.2017.09.051. [25] Y. Pei, J. Lensch-Falk, E.S. Toberer, D.L. Medlin, G.J. Snyder, High thermoelectric performance in PbTe due to large nanoscale Ag2Te precipitates and La doping, Adv. Funct. Mater. 21 (2) (2011) 241–249, https://doi.org/10.1002/ adfm.201000878. [26] Y. Pei, A.F. May, G.J. Snyder, Self-tuning the carrier concentration of PbTe/Ag2Te composites with excess Ag for high thermoelectric performance, Adv. Energy Mater. 1 (2) (2011) 291–296, https://doi.org/10.1002/aenm.201000072. [27] Y. Pei, N.A. Heinz, A. LaLonde, G.J. Snyder, Combination of large nanostructures and complex band structure for high performance thermoelectric lead telluride, Energy Environ. Sci. 4 (9) (2011) 3640–3645, https://doi.org/10.1039/ C1EE01928G. [28] I. Jandl, F. Boero, H. Ipser, K.W. Richter, Phase equilibria and structural investigations of the general NiAs-type in the ternary system Ni-Sn-Te, Intermetallics 46 (2014) 199–210, https://doi.org/10.1016/j. intermet.2013.11.018. [29] C. Dong, PowderX: windows-95-based program for powder X-ray diffraction data processing, J. Appl. Crystallogr. 32 (4) (1999) 838, https://doi.org/10.1107/ S0021889899003039. [30] H.J. Deiseroth, F. Spirovski, C. Reiner, M. Schlosser, Crystal structure of trinickel tin ditelluride, Ni3-xSnTe2 (x¼0.13), Z. Krist.-New Cryst. Struct. 222 (3) (2007) 169–170, https://doi.org/10.1524/ncrs.2007.0070. [31] W.D. Callister, D.G. Rethwisch, Materials Science and Engineering, Wiley, 2014. [32] C.-N. Liao, Y.-C. Huang, Effect of Ag addition in Sn on growth of SnTe compound during reaction between molten solder and tellurium, J. Mater. Res. 25 (02) (2011) 391–395, https://doi.org/10.1557/JMR.2010.0040. [33] W. Liu, H. Wang, L. Wang, X. Wang, G. Joshi, G. Chen, Z. Ren, Understanding of the contact of nanostructured thermoelectric n-type Bi2Te2.7Se0.3 legs for power generation applications, J. Mater. Chem. 1 (42) (2013), https://doi.org/10.1039/ c3ta13456c. [34] C. Long, Y. Yan, J. Zhang, B. Ren, Z. Wang, New integration technology for PbTe element, in: 2006 25th International Conference on thermoelectrics, 2006, https:// doi.org/10.1109/ict.2006.331278. [35] Y.S. Chien, Y.P. Huang, R.N. Tzeng, M.S. Shy, T.H. Lin, K.H. Chen, C.T. Chuang, W. Hwang, J.C. Chiou, C.T. Chiu, H.M. Tong, K.N. Chen, Low temperature (<180 o C) wafer-level and chip-level In-to-Cu and Cu-to-Cu bonding for 3D integration, in: 2013 IEEE 63rd Electronic Components and Technology Conference, 2013, pp. 1146–1152, https://doi.org/10.1109/EDSSC.2015.7285131. [36] H. Chen, Y.L. Tsai, Y.T. Chang, A.T. Wu, Effect of massive spalling on mechanical strength of solder joints in Pb-free solder reflowed on Co-based surface finishes, J. Alloys Compd. 671 (2016) 100–108, https://doi.org/10.1016/j. jallcom.2016.02.027.
References [1] A.D. LaLonde, Y. Pei, H. Wang, G. Jeffrey Snyder, Lead telluride alloy thermoelectrics, Mater. Today 14 (11) (2011) 526–532, https://doi.org/10.1016/ S1369-7021(11)70278-4. [2] K. Biswas, J. He, I.D. Blum, C.I. Wu, T.P. Hogan, D.N. Seidman, V.P. Dravid, M. G. Kanatzidis, High-performance bulk thermoelectrics with all-scale hierarchical architectures, Nature 489 (7416) (2012) 414–418, https://doi.org/10.1038/ nature11439. [3] F. Haass, in: B. SE (Ed.), Doped Lead Tellurides for Thermoelectric Applications, US patent, 2009. [4] B. Paul, P.K. Rawat, P. Banerji, Dramatic enhancement of thermoelectric power factor in PbTe:Cr co-doped with iodine, Appl. Phys. Lett. 98 (26) (2011), https:// doi.org/10.1063/1.3603962, 262101. [5] Y. Pei, X. Shi, A. LaLonde, H. Wang, L. Chen, G.J. Snyder, Convergence of electronic bands for high performance bulk thermoelectrics, Nature 473 (7345) (2011) 66–69, https://doi.org/10.1038/nature09996. [6] Y. Gelbstein, Z. Dashevsky, M.P. Dariel, Powder metallurgical processing of functionally graded p-Pb1 xSnxTe materials for thermoelectric applications, Phys. B Condens. Matter 391 (2) (2007) 256–265, https://doi.org/10.1016/j. physb.2006.10.001. [7] A.D. LaLonde, P.D. Moran, Synthesis and characterization of p-type Pb0.5Sn0.5Te thermoelectric power generation elements by mechanical alloying, J. Electron. Mater. 39 (1) (2010) 8–14, https://doi.org/10.1007/s11664-009-0989-7. [8] M. Orihashi, Y. Noda, L.D. Chen, T. Goto, T. Hirai, Effect of tin content on thermoelectric properties of p-type lead tin telluride, J. Phys. Chem. Solid. 61 (6) (2000) 919–923, https://doi.org/10.1016/S0022-3697(99)00384-4. [9] M. Orihashi, Y. Noda, L.D. Chen, T. Hirai, Effect of Sn content on the electrical properties and thermal conductivity of Pb1-xSnxTe, Mater. Trans., JIM 41 (9) (2000) 1196–1201, https://doi.org/10.2320/matertrans1989.41.1196. [10] X.K. Hu, P. Jood, M. Ohta, M. Kunii, K. Nagase, H. Nishiate, M.G. Kanatzidis, A. Yamamoto, Power generation from nanostructured PbTe-based thermoelectrics: comprehensive development from materials to modules, Energy Environ. Sci. 9 (2) (2016) 517–529, https://doi.org/10.1039/C5EE02979A. [11] S.W. Chen, J.C. Wang, L.C. Chen, Interfacial reactions at the joints of PbTe thermoelectric modules using Ag-Ge braze, Intermetallics 83 (2017) 55–63, https://doi.org/10.1016/j.intermet.2016.11.011. [12] C.L. Yang, H.J. Lai, J.D. Hwang, T.H. Chuang, Diffusion soldering of Pb-Doped GeTe thermoelectric modules with Cu electrodes using a thin-film Sn interlayer, J. Electron. Mater. 42 (3) (2012) 359–365, https://doi.org/10.1007/s11664-0122345-6. [13] T.H. Chuang, H.J. Lin, C.H. Chuang, W.T. Yeh, J.D. Hwang, H.S. Chu, Solid liquid interdiffusion bonding of (Pb, Sn)Te thermoelectric modules with Cu electrodes using a thin-film Sn interlayer, J. Electron. Mater. 43 (12) (2014) 4610–4618, https://doi.org/10.1007/s11664-014-3430-9. [14] T.H. Chuang, W.T. Yeh, C.H. Chuang, J.D. Hwang, Improvement of bonding strength of a (Pb,Sn)Te–Cu contact manufactured in a low temperature SLIDbonding process, J. Alloys Compd. 613 (2014) 46–54, https://doi.org/10.1016/j. jallcom.2014.06.020. [15] C.C. Li, S.J. Hsu, C.C. Lee, L.L. Liao, M.J. Dai, C.K. Liu, Z.X. Zhu, H.W. Yang, J. H. Ke, C.R. Kao, G.J. Snyder, Development of interconnection materials for Bi2Te3 and PbTe thermoelectric module by using SLID technique. https://doi.org/10.110 9/ECTC.2015.7159791, 2015, 1470-1476. [16] M. Orihashi, Y. Noda, L. Chen, Y. Kang, A. Moro, T. Hirai, Ni/n-PbTe and Ni/pPb0.5Sn0.5Te joining by plasma activated sintering, Seventeenth International Conference on Thermoelectrics, in: Proceedings ICT98 (Cat. No.98TH8365), 1998, pp. 543–546. [17] X.R. Ferreres, S. Aminorroaya Yamini, M. Nancarrow, C. Zhang, One-step bonding of Ni electrode to n-type PbTe — a step towards fabrication of thermoelectric generators, Mater. Des. 107 (2016) 90–97, https://doi.org/10.1016/j. matdes.2016.06.038.
8
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Joint properties enhancement for PbTe thermoelectric materials by addition of diffusion barrier Hsien-Chien Hsieh a, Chun-Hsien Wang a, Tian-Wey Lan b, Tse-Hsiao Lee c, Yang-Yuan Chen b, Hsu-Shen Chu c, Albert T. Wu a, * a b c
Department of Chemical and Materials Engineering, National Central University, Taoyuan City, Taiwan Institute of Physics, Academia Sinica, Taipei City, Taiwan Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan
H I G H L I G H T S
� Thick IMC formed in Cu/p-PbTe and melting of joints for Cu/p-PbTe after bonding. � Niddle-like IMCs formed in Ni/p-PbTe; Pb and Te penetrated in Ni for Ni/n-PbTe. � Ni/Co–P/Ag/p-PbTe and Ni/Co–P/n-PbTe solved the problems. � Improved mechanical strength and reasonable contact resistance were obtained. � zT values increased 27% and 109% for coated p- and n-type joints, respectively. A R T I C L E I N F O
A B S T R A C T
Keywords: Thermoelectric materials PbTe Diffusion barrier Interfacial reaction Mechanical strength
PbTe-based alloys are potential mid-temperature thermoelectric materials due to their excellent thermoelectric properties. Formation of intermetallic deteriorates mechanical joint strength and thermoelectric performance as well. In the present work, interfacial reaction, electrical and mechanical behaviors for both p- and n-PbTe joints with an addition of diffusion barrier layer are investigated. The results show that Co–P is a suitable barrier layer for PbTe-based thermoelectric devices with Cu or Ni electrode to inhibit the growth of massive intermetallic compound formation caused by fast interdiffusion in Cu/p-PbTe joints and melting of the joints in Cu/n-PbTe joints. Directly bonded Ni electrode induced formation of needle-like IMCs and disintegration of the elec trodes in p- and n-PbTe modules respectively. Severe interfacial problems were overcome by adding a Co–P diffusion barrier, mechanical strength was improved. In addition, the Co–P layers enhanced the thermoelectric properties of the joints. The addition of the barrier layer increased 27% and 109% of the zT for p- and n-type joints, respectively.
1. Introduction Under the global climate change scenario, thermoelectric (TE) power generation based on Seebeck effect has attracted global attention to harvest waste heat and convert to electricity to lower the carbon foot print. Hence, it is critical to develop a high-efficiency power-generation thermoelectric device with the improved figure of merit (zT ¼ S2 T= ρκ) and good contact between TE materials and electrode. For mediumtemperature power generation, PbTe-based TE materials were widely used in NASA radioisotope satellites and some generators were embedded in vehicles or in the steel industry [1]. Over the years,
high-performance PbTe-based materials have shown low thermal con ductivity and maximum zT value in medium-temperature range of 400–800 K [2–5]. I2 is widely used as a dopant in n-type PbTe and Sn frequently in p-type PbTe to lower thermal conductivity and enhance zT [1,6–9]. Hence, in this work, we have adopted both these elements as the dopants for the TE materials. In addition to the enhancement of the thermoelectric features, the assembling behavior of the modules is also important. The assembling stability of the metal/thermoelectric joints is usually assessed through interfacial reaction, electrical behavior, and mechanical property. The quality of the joints directly influences the thermoelectric performance.
* Corresponding author. E-mail address: [email protected] (A.T. Wu). https://doi.org/10.1016/j.matchemphys.2020.122848 Received 20 November 2019; Received in revised form 10 February 2020; Accepted 23 February 2020 Available online 24 February 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
Unlike the well-investigated BiTe-based thermoelectric modules, studies on PbTe-based joints in the modules are still limited. Currently, several bonding methods, including brazing [10,11], solid-liquid interdiffusion [12–15], spark plasma sintering [16,17], and hot press [18–22] are employed to assemble PbTe-based thermoelectric modules. However, fierce interdiffusion problems were observed. For instance, poor bonding and severe coefficient of thermal expansion (CTE) mismatch were observed. Further, high bonding temperature (approximately 800 � C) is required for Fe/PbTe joints [23]. Element penetration into metal electrodes was found in Ni/PbTe and NiFeMo/PbTe joints [18,22]. Thick reaction layer which formed at the metal/thermoelectric interface depletes thermoelectric elements and may affect the thermoelectric module [17,19]. Moreover, any severe reaction will consume the metal electrode with Ag and Cu as electrodes [20,21]. Thus, the insertion of a diffusion barrier layer was proposed in several studies to solve the above problems. Hu at al. used a Co0.8Fe0.2 alloy as the diffusion barrier and fabricated a PbTe-based module with the highest conversion efficiency [10]. The ability of the electroplated Co layer to serve as an excellent barrier between PbTe and Ag-24.5 at.% Ge braze alloy has also been proved [11]. In our previous study, we have shown that the n-PbTe joints using Cu and Ni electrodes with electroless Co–P diffusion barrier successfully block interdiffusion and solve the melting problem [24]. A clean interface between the Co-based barrier and PbTe has been shown in the literature [11,24]. Therefore, the electroless Co layer was applied on both p- and n-type thermoelectric modules in this study. The electroless process is low cost, has high aspect ratio, and can deposit on the non-active surface. In addition, Ag was chosen as a CTE-modified layer between p-PbTe and Co layers since the CTE of Ag (18.9 � 10 6 K 1) is close to that of p-PbTe (20 � 10 6 K 1). Ag also dissolves in PbTe and contributes electrons by filling Pb vacancies or diffusing into interstitial sites [25,26]. Addi tionally, the reaction of Ag and PbTe would form Ag2Te intermetallic phase. Many studies have shown that Ag2Te precipitates can enhance thermoelectric properties by combining band structure complexity and nanostructure effects [20,27]. This study has comprehensively investigated the insertion of Co–P diffusion barrier on both p- and n-type thermoelectric modules with Cu and Ni electrodes. The interfacial reaction, contact resistance, and me chanical properties of the joints were also studied. Most importantly, the influence of the above properties on thermoelectric performance was studied. The results indicate that the proposed joints significantly enhance the properties of thermoelectric power generation modules.
Bonded PbTe joints were embedded in epoxy and cross-sectioned; a scanning electron microscope (SEM, Hitachi S-4700) was used to observe the cross-sectional microstructure. Field-emission electron probe microanalysis (FE-EPMA, JEOL JXA-8500F) was used to deter mine the composition of the compounds. The crystal structure was identified using the X-ray diffractometer at BL17B1 at the National Synchrotron Radiation Research Center in Taiwan. A fixed-exit dou ble-crystal Si (111) monochromator (λ ¼ 1.55 Å) with 0.6 � 0.5 mm2 spot size was used. Data were collected at a 2θ range of 20� –80� with a scan step size of 0.05� . The contact resistance of the joints was measured by four-probe method using Keithley 2400 series instrument. To eval uate the mechanical behavior of the joints, both p- and n-type PbTe thermoelectric substrates of 0.6 � 0.4 cm2 were used; the Cu and Ni electrodes were of dimensions of 1 � 0.5 cm2. The mechanical strength was evaluated by the die-shear test with Dage series 4000. The shear height and speed were 200 μm and 50 μm/s, respectively. The ther moelectric properties of bare and Co–P-coated thermoelectric materials were investigated to assess the influence of the diffusion barrier. The Seebeck coefficient and resistivity were measured from room tempera ture to 500 � C with commercial equipment (ZEM-3, ULVAC-RIKO) in He atmosphere. To confirm the reproducibility on the thermoelectric per formance, each sample was scanned thrice. 3. Results and discussion 3.1. Interfacial reaction of bare p- and n-PbTe on Cu or Ni electrode Fig. 1(a) shows the interfacial reaction of the Cu/p-PbTe joints after bonding at 650 � C for 60 min. Severe interfacial reaction and depletion of the Cu electrode were observed. A thick reaction layer of Cu3Sn IMC formed between p-PbTe and Cu electrodes, and the 300 μm Cu foil was fully consumed. Large amounts of Sn came from the 40 vol% Sn dopant in the PbTe substrates. The results indicate that the rapid reaction be tween these elements prevented from forming a strong bond in the thermoelectric device. Voids and spalled IMC appeared in the p-PbTe substrate. Fig. 1(b) presents the enlarged microstructure at the boundary of the IMC. Massive dark region consisted of Cu3Sn, while lamellar light gray phases were formed above the Cu3Sn phase. The composition was characterized to be Cu2Te (Table 1). Moreover, approximately 6 at.% Cu was detected in the p-PbTe substrate region. The two observed phases, i. e., Cu3Sn and Cu2Te, had been confirmed by two-phase equilibria of Cu–Sn and Cu–Te at 650 � C isothermal condition. XRD techniques were used to identify the phases of Cu3Sn, Cu2Te, PbTe, and Cu peaks, as shown in Fig. 1(c). Cu diffuses from the electrode to the thermoelectric, and the
2. Materials and methods Polycrystalline PbTe ingots were synthesized by melting pure Pb and Te by adding pure Sn and 1 wt% I2 in p- and n-type PbTe, respectively, in a vacuum-sealed quartz tube at 1000 � C for 2 h. 40 vol% of Pb was replaced by Sn as dopant in p-type materials, which resulted in a composition of Pb0.6Sn0.4Te. The obtained PbTe ingots were ground to powder and the average size was less than 500 μm. The PbTe powders were then sintered in a graphite fixture by hot pressing at 350 � C under 50 MPa for 15 min to obtain a 3 � 1.5 cm2 substrate with a thickness of 0.5 cm. The PbTe substrates were cut into pad shape with a dimension of 0.5 � 0.5 cm2 using a diamond saw. The surface of the substrates was cleaned by grinding and polishing using 0.1 μm Al2O3 suspension. Possible contaminants on the surface were removed by ultrasonic cleaning with acetone, isopropanol, and deionized water. Both p- and ntype PbTe substrates were directly bonded with 300 μm Cu and Ni foils using a high-temperature Ni-based solder paste (2554-N-1, ESL) under 0.55 MPa at 650 � C for 60 min. The reflow temperature of Ni-based solder paste is above 580 � C, and the temperature is almost 200� higher than the working temperature range for PbTe materials. Co–P diffusion barrier was electroless-plated on both p- and n-type PbTe substrates that were bonded with the Cu and Ni foils at the same con ditions. The plating parameters are referred to our previous study [24].
Fig. 1. (a) BEI micrographs of the Cu/p-PbTe joints bonded at 650 � C for 60 min; (b) Enlarged BEI micrographs for the edge of the reaction layer in the Cu/ p-PbTe joint; (c) XRD spectra of the Cu/p-PbTe joint. 2
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
of the Ni foils. The penetration problem would deteriorate the continuity and integrity of the Ni electrode. Therefore, the n-PbTe joint bonded directly with Ni foil may be suitable only for limited temperature or short working time.
Table 1 The atomic composition (at.%) of each phase for Cu/p-PbTe. Pb
Sn
Te
Cu
Ni
Phase
0.02 – 35.49
26.17 – 10.02
0.25 63.40 47.98
72.57 36.59 6.47
0.99 – 0.04
Cu3Sn Cu2Te p-PbTe
3.2. Interfacial reaction after applying Co–P diffusion barrier
following reaction occurred resulting in lamellar structure: e : L1’ ↔ L’2 þ Cu2 Te þ PbTe ð625 � CÞ
Severe reactions, fast diffusion and large amounts of IMCs were found in the joints of both p- and n-type PbTe substrates on Cn and Ni electrodes, which would cause the decline of mechanical stability and deterioration of thermoelectric properties. A barrier layer is needed to resolve the issues. However, cracks can be found between electroless Co–P layer and p-PbTe legs. This finding indicates that the adhesion of the electroless Co–P layer on the p-PbTe substrate is poor after assembly into modules. It is possible that a crack might be formed due to the mismatch in the coefficient of thermal expansion (CTE). Thus, Ag was selected as an intermediate because its CTE (18:9 � 10 6 =KÞ is close to that of p-PbTe (20 � 10 6 =K). This small CTE difference was expected to release the stresses caused by CTE mismatch. Thus, electroless Ag coatings were first deposited onto the surface of p-PbTe. The substrate was sensitized in Sn(II) solution (40 g/L SnCl2.2H2O with 40 ml/L HCl) for 5 min, rinsed with distilled water, activated in AgNO3 solution (10 g/ L) for 5 min, and then immersed in the electroless silver plating solution. The pH value and solution temperatures were 11 and 50 � C, respec tively. Table 3 lists the composition of the electroless Ag plating solution and plating parameters. The as-deposited Ag/p-PbTe substrates were subsequently plated by electroless Co–P coatings. Fig. 5(a) presents the interfacial microstructure between Cu and pPbTe after the insertion of the Ag adhesion layer and the Co–P diffusion barrier layer. Notably, no massive Cu3Sn and eutectic Cu2Te phases, which were detected in the directly bonded sample, were observed at the interface. In contrast to the severe interfacial reaction of the Cu/p-PbTe joint, the integrity of the Cu electrode and the thin reaction layer observed in the Cu/Co–P/Ag/p-PbTe joint indicates the success of the incorporation of the Co–P diffusion barrier. The gray reaction layers formed between p-PbTe and Co–P layer were analyzed to be the Ag2Te and CoSn layer. The composition of each IMCs is listed in Table 4. The deposited electroless Ag layers (approximately 18.8 μm) were fully exhausted and reacted with p-PbTe to form Ag2Te intermetallic com pounds. Some Ag2Te phases spalled into the p-PbTe substrate as shown in the inserted image in Fig. 5(a). Notably, the thickness of the CoSn and Ag2Te layers was approximately 1.4 and 4.6 μm, respectively. The exhaustion of the thermoelectric element was significantly improved after applying plating techniques compared with the directly bonded samples. Fig. 6(a) shows the interfacial results of the Ni/Co–P/Ag/p-PbTe joint bonded at 650 � C for 60 min. No needle-like Ni3 xSnTe2 phases, which were observed in the directly bonded Ni/p-PbTe joint, formed at the interface, indicating that Ni diffusion had been blocked by the Co–P layer. A thin reaction layer (Ag2Te and CoSn) was also observed between Co–P and p-PbTe. The composition of each phase obtained by EPMA is listed in Table 5. Similar to the results of the Cu/Co–P/Ag/p-PbTe joint, the spalling phenomenon of Ag2Te phases was observed in the p-PbTe matrix, as shown in the inserted image in Fig. 6(a). The exact phases were observed at the Cu/Co–P/Ag/p-PbTe and Ni/Co–P/Ag/p-PbTe interface using XRD techniques. After removing the Cu or Ni electrode by polishing, the Ag2Te, CoSn, p-PbTe, and Ni-based solder paste peaks were fully confirmed in the pattern, which is consistent with the observed microstructures, as shown in Figs. 5(b) and 6(b). Although the study of the thickness for Ag2Te and CoSn needs further investigation, previous study shows that Ag–Te compound could be a self-barrier formed during bonding [32]. It is possible that the Ag2Te layer could prevent the growth of CoSn. Fig. 7(a) and (b) present the interfacial microstructure of Cu/Co–P/ n-PbTe and Ni/Co–P/n-PbTe bonded at 650 � C for 60 min, respectively.
(1)
The same eutectic reaction, which causes the melting problem for the Cu/n-PbTe joint, was observed. Fig. 2 shows the microstructure of Cu/nPbTe joint bonded at 650 � C for 60 min. The thermoelectric joint was damaged as the massive diffusion of Cu led to the melting of n-PbTe. The lamellar structure (PbTe, Cu2Te) and Cu-rich regions were also identi fied. Its detailed mechanism has been published in a previous report [24]. The Cu/p-PbTe joint remained in the solid state (Fig. 1). The melting reaction of the Cu/p-PbTe joint was less severe than that of the Cu/nPbTe joint because of its low Cu concentration. More Cu elements were exhausted in the formation of the Cu3Sn reaction layer. In addition to the thick reaction layer and electrode exhaustion, some voids were observed at the bottom of the p-PbTe substrate. The liquid reaction became severe because of the increase in Cu concentration. The volume difference in liquid-state and solid-state PbTe in the p-PbTe matrix would cause void formation during solidification. These voids would severely deteriorate the bonding strength. Fig. 3(a) shows the interfacial microstructure of Ni/p-PbTe joints after bonding at 650 � C for 60 min. Large amounts of needle-like IMCs were detected near the p-PbTe substrate. The close-up image (Fig. 3(b)) shows two morphologies of IMCs with the same contrast, i.e., continuous layer- and needle-type intermetallic compounds. The composition of the continuous IMC layer and needle-like IMCs between Ni-based solder paste and p-PbTe measured by EPMA is listed in Table 2. The compo sition of the IMCs was 51.3 at.% Ni–22.2 at.% Sn–24.2 at.% Te, which was the Ni3 xSnTe2 phase. This ternary phase had been confirmed by Jandl et al. in 600 � C isothermal sections of the Ni–Sn–Te ternary alloy phase diagram [28]. They proposed that Ni3 xSnTe2 was the stable phase at less than 715 � C approximately, which was consistent with our results for the sample bonded at 650 � C. Synchrotron XRD, which has a high resolution for each step, was employed to confirm the Ni–Sn–Te ternary phase, as shown in Fig. 3(c). Given the lack of Ni3 xSnTe2 standard card, PowderX software, which can calculate the theoretical peaks of the identified phase, was used [29]. Ni3 xSnTe2 (space group P63/mmc, Pearson symbol hP12) with lattice constants of a ¼ 3.9585 and c ¼ 15.6194 Å had been reported in the literature [28,30]. The formula in the literature [31] was used to confirm the diffraction peaks of Ni3 xSnTe2 phase. Thus, the X-ray peaks of PbTe, Ni, and Ni3 xSnTe2 were confirmed and are consistent with the observed microstructures (Fig. 3(c)). Considerable exhaustion of thermoelectric elements (i.e., Sn and Te) was observed and should be avoided during the assembly of the module as the properties of the thermoelectric materials would deteri orate. Thus, the Ni/p-PbTe joint underwent severe reaction depleting the thermoelectric matrix, which was considered an unsuitable ther moelectric module structure. Fig. 4 shows the interfacial microstructure of the Ni/n-PbTe joint bonded at 650 � C for 60 min. Some white PbTe phases were observed inside the Ni electrode that indicated the Pb and Te atoms would diffuse through the solder paste layer and penetrate along the grain boundaries
Fig. 2. Optical images of the Cu/n-PbTe joints bonded at 650 � C for 60 min. 3
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
Fig. 3. BEI micrographs of the Ni/p-PbTe joints bonded at 650 � C for 60 min; (b) Enlarged microstructure of (a); (c) XRD spectra of the Ni/p-PbTe joints.
Clear interfaces were observed in both Cu and Ni electrodes. Notably, the severe melting reaction which observed in directly bonded Cu/nPbTe joint and the penetration of PbTe phase in Ni/n-PbTe joint were successfully avoided by utilizing the Co–P diffusion barrier. As a result, the insertion of Co–P layer between n- or p-PbTe materials and Cu or Ni electrodes could effectively inhibit fast interdiffusion and the deterio ration of the joints.
Table 2 The atomic composition (at.%) of IMCs for Ni/p-PbTe joint. Region
Pb
Continuous layer Needle-like IMCs
0.06 0.13
Sn
Te 22.20 22.34
Ni 24.24 23.31
B 51.39 52.36
Phase 1.55 1.78
Ni3-xSnTe2 Ni3-xSnTe2
3.3. Electrical contact In addition to the interfacial reaction, electrical properties are crit ical for the evaluation of the performance of a module. Notably, the contact resistance of the Cu/p-PbTe and Cu/n-PbTe joints could not be measured because of full exhaustion of the electrode. The contact resistance of the Ni/p-PbTe joint is higher than that of the Ni/n-PbTe joint. The data are shown in Table 6 and Table 7. The massive Ni3 xSnTe2 phases observed in the p-PbTe matrix, which resulted in the severe depletion of Te, were considered to be the main cause for the increase in contact resistance. A similar increase caused by Te deficiency in Ni/Bi2Se0.3Te2.7 has been reported [33]. Literature mentions that an insertion of Co–P in the Ni/n-PbTe joints increases the contact resistance from 11.75 to 21.75 μΩ cm2 [24]. Compared with the directly bonded Ni/p-PbTe joint, the increases in contact resistance were observed in the Cu/Co–P/Ag/p-PbTe and Ni/Co–P/Ag/p-PbTe samples from 18.00 to 27.35 and 28.08 μΩ cm2, respectively (Table 7). It is possible that the Co–P layer is amorphous and may increase the contact resistance. Furthermore, the formation of the Ag2Te layer in the joints of p-PbTe would cause Te depletion in the substrate. Limited data could be found in the medium-temperature thermoelectric joints. The contact resistance of Fe/n-PbTe and Fe/Ni/n-PbTe joints are 86.4 and 31.4 μΩ cm2, respectively [34]. CoSb3-based joints with several metallic electrodes are reported to be in the range of 14.6–20 μΩ cm2. Most importantly, Liu et al. proposed that the contact resistance of the thermoelectric module should be less than 10 4 Ω cm2 [33]. The results of the p- and n-type PbTe joints reported in this study are in the acceptable range when applied to the thermoelectric modules.
Fig. 4. BEI micrograph of Ni/n-PbTe joints bonded at 650 � C for 60 min. Table 3 Compositions of electroless plating Ag bath and plating parameters. Composition
Parameter
AgNO3 (NH4)2SO4 25% NH3 CoSO4⋅7H2O pH Plating Temperature Plating Time
6.78 g/L 99 g/L 425 g/L 31.3 g/L 11.0 50 � 2 � C 180 min
3.4. Mechanical test Sufficient strength of the thermoelectric module is required for commercial thermoelectric applications. The shear test has been widely employed for analyzing the bonding strength of solder joints or die-todie joints [35,36]. However, only a few studies have focused on the mechanical behavior of the thermoelectric joints. This study focuses on evaluating the effects of the incorporation of the Co–P diffusion barrier and different metal electrodes. Fig. 8(a) shows the shear force of the p-PbTe joints with Co–P diffusion barrier and Ag adhesion layer. Notably, the directly bonded joints could not be tested successfully. The exhaustion of the Cu electrode and melting reaction leads to the obvious module failure in the Cu/p-PbTe and Cu/n-PbTe joints, respectively. The Ni/p-PbTe joint was weak as the samples moved along the shear force.
Fig. 5. (a) BEI micrographs of the Cu/Co–P/Ag/p-PbTe joint bonded at 650 � C for 60 min; (b) XRD spectra of the Cu/Co–P/Ag/p-PbTe joint.
4
Materials Chemistry and Physics 246 (2020) 122848
H.-C. Hsieh et al.
Table 4 The atomic composition (at.%) of IMCs for Cu/Co–P/Ag/p-PbTe joint. Pb
Sn
Te
Ag
Co
P
Ni
B
Cu
Phase
0.05 0.05 – 34.53
– 47.51 0.07 9.87
32.54 0.66 – 47.64
60.01 0.16 0.17 2.14
1.26 46.47 97.56 0.16
0.14 0.01 1.26 0.15
0.64 4.14 0.65 0.32
2.67 0.45 – 3.56
2.64 7.5 0.44 1.59
Ag2Te CoSn Co–P p-PbTe
Fig. 6. (a) BEI micrographs of the Ni/Co–P/Ag/p-PbTe joint bonded at 650 � C for 60 min; (b) XRD spectra of the Ni/Co–P/Ag/p-PbTe joint.
The massive needle-like IMCs (Ni3 xSnTe2) that formed at the interface could be the main cause for the weak strength. Weakening joints caused by the needle-like IMC in the Nb/n-PbTe joint have been reported [19]. The bonding force of Cu/Co–P/Ag/p-PbTe and Ni/Co–P/Ag/p-PbTe joints were improved by the Co–P/Ag coating layers, indicating that the incorporation of the Co–P and Ag layers will improve the joint strength. Both Co–P/n-PbTe and Co–P/Ag/p-PbTe joints with Cu electrodes show higher shear force than those bonded with Ni electrodes (Fig. 8). A fracture surface analysis was conducted for analyzing the fracture mechanism. Fig. 9(a–d) shows the BEI images of the fracture surfaces for TE joints. The large portions of PbTe and solder paste regions implied that a sufficient bonding strength at the interface (Co–P/n-PbTe, Co–P/Ag/p-PbTe) could be obtained, which improves the mechanical behavior of the joints. In summary, the shear strengths of the p-PbTe and n-PbTe joints with Co–P layer are higher than those of the directly bonded samples. The addition of the Co–P diffusion barrier layer did not decrease the joint strength but enhanced the reliability of the n-PbTe- based joints.
Table 5 The atomic composition (at.%) of IMCs for Ni/Co–P/Ag/p-PbTe joint. Pb
Sn
Te
Ag
Co
P
Ni
B
Phase
0.09 0.08 0.02 31.61
– 48.33 0.29 14.99
33.55 0.66 – 47.61
62.89 0.18 – 1.19
0.67 47.53 96.05 0.62
0.09 – 1.80 0.19
0.04 1.87 1.81 0.33
2.63 1.32 – 3.43
Ag2Te CoSn Co–P p-PbTe
Fig. 7. BEI micrographs of (a) Cu/Co–P/n-PbTe and (b) Ni/Co–P/n-PbTe bonded at 650 � C for 60 min.
3.5. Thermoelectric property Considering that Co–P layers have been proved to be an appropriate diffusion barriers for p-PbTe modules because of their effect on inter facial reactions, contact resistance, and mechanical strength, the effect of the insertion of Co–P diffusion barrier layer and Ag adhesion layer in the p-PbTe module on thermoelectric performance were studied. Fig. 10 (a) and (b) show the original Seebeck coefficient and resistivity of the bare and coated p-PbTe materials with three times scan. The p-type conduction was identified within the whole measuring temperature range. Similar performance and even exactly the same features after two scans were identified in both bare and coated samples. It implies that the thermoelectric properties of p-PbTe materials exhibited reproducibility, and the addition of extra layers (Ag and Co–P) did not deteriorate the thermoelectric performance after cycling. Likewise, a comparison was also made between bare and Co–P/n-PbTe materials to confirm the ef fect of the additional layer on n-PbTe thermoelectric materials. Fig. 10 (c) and (d) present the temperature-dependent thermoelectric perfor mance of the bare and coated n-PbTe materials. The n-type conduction was identified within the entire temperature range. The Seebeck coef ficient gradually increased with temperature, and reproducibility was observed after two scans. It is noted that the electrical resistivity of the bare n-PbTe increased dramatically after approximately 600 K (327 � C)
Table 6 Contact resistance of various as-boned n-PbTe joints [24]. Assemblage
Voltage step (mV)
Contact resistivity (μΩ⋅cm2)
Cu/n-PbTe Ni/n-PbTe Cu/Co–P/n-PbTe Ni/Co–P/n-PbTe
The joint is melted. 0.047 0.085 0.087
� 11.75 21.25 21.75
Table 7 Contact resistance of various as-boned p-PbTe joints. Assemblage
Voltage step (mV)
Contact resistivity (μΩ⋅cm2)
Cu/p-PbTe Ni/p-PbTe Cu/Co–P/Ag/p-PbTe Ni/Co–P/Ag/p-PbTe
The electrode is exhausted. 0.072 0.109 0.112
� 18.00 27.35 28.08
5
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
Fig. 8. Shear loads of (a) the proposed p-PbTe joints and (b) the proposed n-PbTe joints.
the n-PbTe was coated with Co–P, the sudden increase of resistivity could not be seen. Although the differences need further investigation, the reproducibility is significant. This reproducibility indicates that the addition of Co–P layer in the n-PbTe module caused no deterioration during the module operation. The improvement is significant. Fig. 11(a) compares the Seebeck coefficient, resistivity and thermal conductivity of the bare and coated p-PbTe. Similar thermoelectric features between bare and coated samples were observed. The Seebeck coefficient of the Co–P/Ag/p-PbTe was slightly higher than that of the bare p-PbTe. In addition, there is a slight enhancement in electrical re sistivity after applying Co–P and Ag layers. The thermal conductivities of the bare and coated p-PbTe show similar features. Slight differences were confirmed above 600 K, which may be contributed by the forma tion of Ag2Te phases in p-PbTe matrix. Moreover, the Co–P/Ag/p-PbTe sample also exhibited higher power factors (S2/ρ) and zT values than those of the bare one (Fig. 11(c)). The slight enhancement of Seebeck coefficient and resistivity observed in the Co–P/Ag/p-PbTe indicates the improvement of thermoelectric performance. A peak zT of 0.94 was observed at about 650 K for the sample with coating layers, while the bare sample shows 0.74. Fig. 11(b) compares the Seebeck coefficient, resistivity and thermal conductivity between bare n-PbTe and Co–P/n-PbTe samples. Both samples presented n-type features in the entire temperature range. Better thermoelectric properties, including higher Seebeck coefficient, lower resistivity, and lower thermal conductivity were observed. The Pb precipitates observed in the bare n-PbTe increased resistivity. With the improvement of thermoelectric features, the Co–P/n-PbTe sample exhibited higher power factor and zT value. (Fig. 11(d)). A peak zT of 1.15 was observed at about 650 K for the sample with coating layers, while the bare sample shows 0.55. Notably, the incorporation of extra layer proposed in this study did not deteriorate the performance but enhance the thermoelectric performance of both p- and n-type PbTe samples. A stable and better performance was obtained with the incor poration of the Co–P layer.
Fig. 9. BEI images of the fracture surfaces for (a) Cu/Co–P/Ag/p-PbTe (b) Ni/ Co–P/Ag/p-PbTe (c) Cu/Co–P/n-PbTe (d) Ni/Co–P/n-PbTe.
(Fig. 10(c)). According to the elemental quantitative analysis using EPMA, a relatively high Pb content of 57.3 at.% was confirmed in nPbTe. Based on Pb–Te phase diagram, when the system was heated above the melting point of Pb (327.5 � C), excess Pb content would cause partial melting in n-PbTe and lead to a sudden rise of resistivity. Once the sample was cooled down after first scan, approximately 17.1 wt% of Pb, calculated by the lever rule, would precipitate. Pb-rich region was observed in the fracture surface of n-PbTe after three scans. The size of the Pb-rich area was too small to be distinguished by EDS analysis that has a submicron spot size. However, the Pb content was above 80 at.%, and it indicated segregation of Pb in the n-PbTe matrix. These Pb pre cipitates produced extra scattering center for electron transport and thus significantly increases the resistivity after first scan. It is proposed in the literature that a similar scattering effect of the precipitates reduces the Hall mobility and therefore increases resistivity [25,27]. Notably, when
4. Conclusion In summary, we have observed fast interdiffusion and massive for mation of intermetallic compounds in both Cu/p-PbTe and Ni/p-PbTe joints. Melting of the thermoelectric materials and discontinuous elec trodes due to penetration were observed in Cu/n-PbTe and Ni/n-PbTe, respectively. Electroless Co–P diffusion barrier is added to prevent se vere reactions between medium-temperature PbTe TE materials and electrodes. Clear interfaces were observed in p- and n-type PbTe joints. Thin reaction layer contained Ag2Te and CoSn IMCs was formed at the interface in p-PbTe joints. The results of contact resistance proved that Co–P diffusion barrier only slightly increased the electrical contact and the values are in acceptable range for producing a practical thermo electric module. Module failure with Cu electrode and weak strength 6
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
Fig. 10. Thermoelectric properties as a function of temperature for Seebeck coefficient and electrical resistivity or (a) bare p-PbTe (b) Co–P/Ag/p-PbTe (C) bare nPbTe and (d) Co–P/n-PbTe.
Fig. 11. Comparison of Seebeck coefficient, electrical resistivity, and thermal conductivity as a function of temperature for (a) bare p-PbTe and Co–P/Ag/p-PbTe, (b) bare n-PbTe and Co–P/n-PbTe; Comparison of power factor and zT value for (c) bare p-PbTe and Co–P/Ag/p-PbTe, (d) bare n-PbTe and Co–P/n-PbTe.
with Ni were observed in both p-PbTe and n-PbTe joints. Adding the Co–P layer improved the mechanical reliability significantly. The addi tional Co–P or Co–P/Ag layer enhanced the thermoelectric performance and maintained the reproducibility in the measurement temperature range. The results show that Co–P is a candidate of diffusion barrier layer for PbTe-based modules with Cu and Ni electrodes.
CRediT authorship contribution statement Hsien-Chien Hsieh: Conceptualization, Methodology, Investiga tion, Writing - original draft, Validation. Chun-Hsien Wang: Data curation, Writing - review & editing, Visualization. Tian-Wey Lan: Resources. Tse-Hsiao Lee: Resources. Yang-Yuan Chen: Resources. Hsu-Shen Chu: Resources. Albert T. Wu: Conceptualization, Writing review & editing, Supervision.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments The authors would like to acknowledge the support received from the Ministry of Science and Technology of Taiwan for this study under Contract No. 104-2221-E-008-065-MY3. We also thank the Industrial 7
H.-C. Hsieh et al.
Materials Chemistry and Physics 246 (2020) 122848
Technology Research Institute of Taiwan for supplying the instrument to produce the substrate. The help received from Prof. Chen and Dr. Lan of Academia Sinica, Taiwan, for measuring thermoelectric properties is also appreciated.
[18] H. Xia, F. Drymiotis, C.L. Chen, A. Wu, G.J. Snyder, Bonding and interfacial reaction between Ni foil and n-type PbTe thermoelectric materials for thermoelectric module applications, J. Mater. Sci. 49 (4) (2013) 1716–1723, https://doi.org/10.1007/s10853-013-7857-9. [19] H. Xia, C.-L. Chen, F. Drymiotis, A. Wu, Y.-Y. Chen, G.J. Snyder, Interfacial reaction between Nb foil and n-type PbTe thermoelectric materials during thermoelectric contact fabrication, J. Electron. Mater. 43 (11) (2014) 4064–4069, https://doi.org/ 10.1007/s11664-014-3350-8. [20] C.C. Li, F. Drymiotis, L.L. Liao, M.J. Dai, C.K. Liu, C.L. Chen, Y.Y. Chen, C.R. Kao, G.J. Snyder, Silver as a highly effective bonding layer for lead telluride thermoelectric modules assembled by rapid hot-pressing, Energy Convers. Manag. 98 (2015) 134–137, https://doi.org/10.1016/j.enconman.2015.03.089. [21] C.C. Li, F. Drymiotis, L.L. Liao, H.T. Hung, J.H. Ke, C.K. Liu, C.R. Kao, G.J. Snyder, Interfacial reactions between PbTe-based thermoelectric materials and Cu and Ag bonding materials, J. Mater. Chem. C 3 (40) (2015) 10590–10596, https://doi.org/ 10.1039/C5TC01662B. [22] H. Xia, F. Drymiotis, C.-L. Chen, A. Wu, Y.-Y. Chen, G. Jeffrey Snyder, Bonding and high-temperature reliability of NiFeMo alloy/n-type PbTe joints for thermoelectric module applications, J. Mater. Sci. 50 (7) (2015) 2700–2708, https://doi.org/ 10.1007/s10853-015-8820-8. [23] M. Weinstein, A.I. Mlavsky, Bonding of lead telluride to pure iron electrodes, Rev. Sci. Instrum. 33 (10) (1962) 1119–1120, https://doi.org/10.1063/1.1717707. [24] H.C. Hsieh, C.H. Wang, W.C. Lin, S. Chakroborty, T.H. Lee, H.S. Chu, A.T. Wu, Electroless Co-P diffusion barrier for n-PbTe thermoelectric material, J. Alloys Compd. 728 (2017) 1023–1029, https://doi.org/10.1016/j.jallcom.2017.09.051. [25] Y. Pei, J. Lensch-Falk, E.S. Toberer, D.L. Medlin, G.J. Snyder, High thermoelectric performance in PbTe due to large nanoscale Ag2Te precipitates and La doping, Adv. Funct. Mater. 21 (2) (2011) 241–249, https://doi.org/10.1002/ adfm.201000878. [26] Y. Pei, A.F. May, G.J. Snyder, Self-tuning the carrier concentration of PbTe/Ag2Te composites with excess Ag for high thermoelectric performance, Adv. Energy Mater. 1 (2) (2011) 291–296, https://doi.org/10.1002/aenm.201000072. [27] Y. Pei, N.A. Heinz, A. LaLonde, G.J. Snyder, Combination of large nanostructures and complex band structure for high performance thermoelectric lead telluride, Energy Environ. Sci. 4 (9) (2011) 3640–3645, https://doi.org/10.1039/ C1EE01928G. [28] I. Jandl, F. Boero, H. Ipser, K.W. Richter, Phase equilibria and structural investigations of the general NiAs-type in the ternary system Ni-Sn-Te, Intermetallics 46 (2014) 199–210, https://doi.org/10.1016/j. intermet.2013.11.018. [29] C. Dong, PowderX: windows-95-based program for powder X-ray diffraction data processing, J. Appl. Crystallogr. 32 (4) (1999) 838, https://doi.org/10.1107/ S0021889899003039. [30] H.J. Deiseroth, F. Spirovski, C. Reiner, M. Schlosser, Crystal structure of trinickel tin ditelluride, Ni3-xSnTe2 (x¼0.13), Z. Krist.-New Cryst. Struct. 222 (3) (2007) 169–170, https://doi.org/10.1524/ncrs.2007.0070. [31] W.D. Callister, D.G. Rethwisch, Materials Science and Engineering, Wiley, 2014. [32] C.-N. Liao, Y.-C. Huang, Effect of Ag addition in Sn on growth of SnTe compound during reaction between molten solder and tellurium, J. Mater. Res. 25 (02) (2011) 391–395, https://doi.org/10.1557/JMR.2010.0040. [33] W. Liu, H. Wang, L. Wang, X. Wang, G. Joshi, G. Chen, Z. Ren, Understanding of the contact of nanostructured thermoelectric n-type Bi2Te2.7Se0.3 legs for power generation applications, J. Mater. Chem. 1 (42) (2013), https://doi.org/10.1039/ c3ta13456c. [34] C. Long, Y. Yan, J. Zhang, B. Ren, Z. Wang, New integration technology for PbTe element, in: 2006 25th International Conference on thermoelectrics, 2006, https:// doi.org/10.1109/ict.2006.331278. [35] Y.S. Chien, Y.P. Huang, R.N. Tzeng, M.S. Shy, T.H. Lin, K.H. Chen, C.T. Chuang, W. Hwang, J.C. Chiou, C.T. Chiu, H.M. Tong, K.N. Chen, Low temperature (<180 o C) wafer-level and chip-level In-to-Cu and Cu-to-Cu bonding for 3D integration, in: 2013 IEEE 63rd Electronic Components and Technology Conference, 2013, pp. 1146–1152, https://doi.org/10.1109/EDSSC.2015.7285131. [36] H. Chen, Y.L. Tsai, Y.T. Chang, A.T. Wu, Effect of massive spalling on mechanical strength of solder joints in Pb-free solder reflowed on Co-based surface finishes, J. Alloys Compd. 671 (2016) 100–108, https://doi.org/10.1016/j. jallcom.2016.02.027.
References [1] A.D. LaLonde, Y. Pei, H. Wang, G. Jeffrey Snyder, Lead telluride alloy thermoelectrics, Mater. Today 14 (11) (2011) 526–532, https://doi.org/10.1016/ S1369-7021(11)70278-4. [2] K. Biswas, J. He, I.D. Blum, C.I. Wu, T.P. Hogan, D.N. Seidman, V.P. Dravid, M. G. Kanatzidis, High-performance bulk thermoelectrics with all-scale hierarchical architectures, Nature 489 (7416) (2012) 414–418, https://doi.org/10.1038/ nature11439. [3] F. Haass, in: B. SE (Ed.), Doped Lead Tellurides for Thermoelectric Applications, US patent, 2009. [4] B. Paul, P.K. Rawat, P. Banerji, Dramatic enhancement of thermoelectric power factor in PbTe:Cr co-doped with iodine, Appl. Phys. Lett. 98 (26) (2011), https:// doi.org/10.1063/1.3603962, 262101. [5] Y. Pei, X. Shi, A. LaLonde, H. Wang, L. Chen, G.J. Snyder, Convergence of electronic bands for high performance bulk thermoelectrics, Nature 473 (7345) (2011) 66–69, https://doi.org/10.1038/nature09996. [6] Y. Gelbstein, Z. Dashevsky, M.P. Dariel, Powder metallurgical processing of functionally graded p-Pb1 xSnxTe materials for thermoelectric applications, Phys. B Condens. Matter 391 (2) (2007) 256–265, https://doi.org/10.1016/j. physb.2006.10.001. [7] A.D. LaLonde, P.D. Moran, Synthesis and characterization of p-type Pb0.5Sn0.5Te thermoelectric power generation elements by mechanical alloying, J. Electron. Mater. 39 (1) (2010) 8–14, https://doi.org/10.1007/s11664-009-0989-7. [8] M. Orihashi, Y. Noda, L.D. Chen, T. Goto, T. Hirai, Effect of tin content on thermoelectric properties of p-type lead tin telluride, J. Phys. Chem. Solid. 61 (6) (2000) 919–923, https://doi.org/10.1016/S0022-3697(99)00384-4. [9] M. Orihashi, Y. Noda, L.D. Chen, T. Hirai, Effect of Sn content on the electrical properties and thermal conductivity of Pb1-xSnxTe, Mater. Trans., JIM 41 (9) (2000) 1196–1201, https://doi.org/10.2320/matertrans1989.41.1196. [10] X.K. Hu, P. Jood, M. Ohta, M. Kunii, K. Nagase, H. Nishiate, M.G. Kanatzidis, A. Yamamoto, Power generation from nanostructured PbTe-based thermoelectrics: comprehensive development from materials to modules, Energy Environ. Sci. 9 (2) (2016) 517–529, https://doi.org/10.1039/C5EE02979A. [11] S.W. Chen, J.C. Wang, L.C. Chen, Interfacial reactions at the joints of PbTe thermoelectric modules using Ag-Ge braze, Intermetallics 83 (2017) 55–63, https://doi.org/10.1016/j.intermet.2016.11.011. [12] C.L. Yang, H.J. Lai, J.D. Hwang, T.H. Chuang, Diffusion soldering of Pb-Doped GeTe thermoelectric modules with Cu electrodes using a thin-film Sn interlayer, J. Electron. Mater. 42 (3) (2012) 359–365, https://doi.org/10.1007/s11664-0122345-6. [13] T.H. Chuang, H.J. Lin, C.H. Chuang, W.T. Yeh, J.D. Hwang, H.S. Chu, Solid liquid interdiffusion bonding of (Pb, Sn)Te thermoelectric modules with Cu electrodes using a thin-film Sn interlayer, J. Electron. Mater. 43 (12) (2014) 4610–4618, https://doi.org/10.1007/s11664-014-3430-9. [14] T.H. Chuang, W.T. Yeh, C.H. Chuang, J.D. Hwang, Improvement of bonding strength of a (Pb,Sn)Te–Cu contact manufactured in a low temperature SLIDbonding process, J. Alloys Compd. 613 (2014) 46–54, https://doi.org/10.1016/j. jallcom.2014.06.020. [15] C.C. Li, S.J. Hsu, C.C. Lee, L.L. Liao, M.J. Dai, C.K. Liu, Z.X. Zhu, H.W. Yang, J. H. Ke, C.R. Kao, G.J. Snyder, Development of interconnection materials for Bi2Te3 and PbTe thermoelectric module by using SLID technique. https://doi.org/10.110 9/ECTC.2015.7159791, 2015, 1470-1476. [16] M. Orihashi, Y. Noda, L. Chen, Y. Kang, A. Moro, T. Hirai, Ni/n-PbTe and Ni/pPb0.5Sn0.5Te joining by plasma activated sintering, Seventeenth International Conference on Thermoelectrics, in: Proceedings ICT98 (Cat. No.98TH8365), 1998, pp. 543–546. [17] X.R. Ferreres, S. Aminorroaya Yamini, M. Nancarrow, C. Zhang, One-step bonding of Ni electrode to n-type PbTe — a step towards fabrication of thermoelectric generators, Mater. Des. 107 (2016) 90–97, https://doi.org/10.1016/j. matdes.2016.06.038.
8