One-step bonding of Ni electrode to n-type PbTe — A step towards fabrication of thermoelectric generators

Abstract

PbTe-based thermoelectric materials are good candidates for harvesting waste heat at mid-range temperatures due to their high thermoelectric efficiencies. Excellent quality and reliability of the bonding between the thermoelectric material and the electrode at high temperatures are essential for manufacturing thermoelectric generators. Here, a technique has been developed to achieve high-quality bonding between PbTe and the electrode. We have successfully performed one-step sintering of nickel electrode to n-type PbTe powder using spark plasma sintering. The fabricated interphase, composed of nickel telluride, is continuous and homogeneous across the junction, without visible flaws on the electrode or in the interphase and PbTe. To evaluate the long-term thermal stability of the fabricated bond, an aging test was conducted at 823 K for 360 h under vacuum. The microstructures and chemical composition of the fabricated bonding and the aged sample were investigated in detail by scanning electron microscopy equipped with energy dispersive X-ray spectroscopy analysis. No excess reaction was observed between the electrode and the thermoelectric material after aging, supporting the formation of a chemically stable interphase, which acts as a diffusion barrier. Degradation of the PbTe was detected after aging, however. The fabricated interface meets the required criteria for maximum efficiency of PbTe materials.

Solid-State Bonding of Bulk PbTe to Nickel Electrode for Thermoelectric Modules

“…The CTE of PbTe (20 × 10 −6 K −1 ) 33 and Ni (13.4 × 10 −6 K −1 ) have shown to reasonably hold at high temperatures. 22,34 Panels a and b of Figure 2 show SEM micrographs of the fabricated interphase between PbTe and Ni plate, generated by sintering at 623 K, 20 MPa for 5 min. Figure 2b delineates a new phase at the PbTe/Ni interface, indicating the occurrence of a reaction between the initial counterparts.…”

Interfacial reactions in thermoelectric modules

“…Instead, an interlayer of Ni with CTE of 13.4 × 10 −6 /K should be inserted to minimize the CTE mismatch between PbTe (19.8 × 10 −6 /K) and Fe (11.7 × 10 −6 /K), in order to reduce thermal stress during the thermal cycling. Further investigations upon the interfacial reactions between PbTe and Ni is therefore of interests [45,46]. In the beginning of aging (600°C), the β 2 phase (Ni 3±x Te 2 ) precipitates out from the PbTe nearing PbTe/Ni interfaces, and grows as a continuous layer with increasing aging time.…”

Al–Si Alloy as a Diffusion Barrier for GeTe-Based Thermoelectric Legs with High Interfacial Reliability and Mechanical Strength

“…TE devices are generally composed of a couple of p- and n-type TE materials as TE legs, ,, which are connected with electrodes. The traditional soldering and brazing methods have been widely used for TE-device connection, , and hot pressing and spark plasma sintering (SPS) have also been developed as efficient jointing technologies for TE devices. − To fabricate high-performance TE devices, it is especially important to obtain good interfacial connection between TE materials and electrodes with high mechanical strength, high stability, but low interfacial electrical and thermal contact resistivity. ,, The connection technology is mature for the commercial low-temperature Bi 2 Te 3 -based TE devices, which generally use Cu or Ni as electrodes and Sn or Sn-based alloys as soldering materials . However, for those mid- and high-temperature TE devices, many problems still need to be solved, especially the problem of interdiffusion between the TE materials and electrodes at the high-temperature end, which can usually degenerate the performance of TE devices.…”

“…However, for those mid- and high-temperature TE devices, many problems still need to be solved, especially the problem of interdiffusion between the TE materials and electrodes at the high-temperature end, which can usually degenerate the performance of TE devices. Therefore, a suitable diffusion barrier is usually required to be inserted between the electrodes and thermoelectric legs. , The diffusion-barrier layers generally need to have the following properties: ,,, matched coefficient of thermal expansion (CTE) with that of TE materials to avoid cracks in the joint, high bonding strength with TE materials during the entire service, stabilized interdiffusion between electrodes and TE materials, high electrical and thermal conductivity to reduce energy loss, and excellent thermal and chemical stability. Thus, it is usually a big challenge to find suitable diffusion-barrier materials to concurrently meet all these requirements …”

“…Therefore, a suitable diffusion barrier is usually required to be inserted between the electrodes and thermoelectric legs. 4,12 The diffusion-barrier layers generally need to have the following properties: 4,12,15,19 with TE materials during the entire service, stabilized interdiffusion between electrodes and TE materials, high electrical and thermal conductivity to reduce energy loss, and excellent thermal and chemical stability. Thus, it is usually a big challenge to find suitable diffusion-barrier materials to concurrently meet all these requirements.…”