Interfacial Reaction Between Nb Foil and n-Type PbTe Thermoelectric Materials During Thermoelectric Contact Fabrication

Abstract

PbTe is a high-conversion-efficiency thermoelectric (TE) material that is commonly used in space exploration applications. Integration of PbTe in TE devices has a significant impact on the conversion efficiency and reliability of TE devices. Hence, our effort focuses on developing novel approaches for bonding metallic contacts to PbTe to improve device performance and reliability. In this study, pure Nb foil was directly bonded to PbTe-based TE materials to fabricate the hot-side contacts of TE elements using a rapid hotpress. The materials were sintered at 700°C under pressure of 40 MPa for various holding times. We found that a reaction layer of needle-like Nb 3 Te 4 mixed with Pb forms at the interface of the Nb/PbTe joints and that Pb is distributed in the gaps of the Nb 3 Te 4 grains. We analyze the resulting microstructure and finally calculate the time exponent of the growth kinetics of the Nb 3 Te 4 layer. Fracture surface analysis showed that the Nb/PbTe joint fractures at the interface between Nb and Nb 3 Te 4 and within the PbTe matrix, indicating that the bonding between Nb and Nb 3 Te 4 is weak.

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

“…The composition of the generated interphase is consistent with Ni 3 Te 2 as the only intermetallic formed during the reaction. An earlier study [11] on the one-step bonding of Ni to n-type PbTe by rapid hot pressing found defects at the junction with the nickel electrode, which increased in number with the sintering time, allowing PbTe to diffuse and react inside the cracks. The main reported composition is Ni 3±x Te 2 (38.6-41.0 at.% Te), and a ternary phase of Ni 5 Pb 2 Te 3 is observed together with it via a eutectic reaction at the Ni/PbTe interface at 923 K. The continuous diffusion of elements into the defects in Ni electrode, where they can react at the device operating temperature, was introduced as the major limitation of this sort of fabricated interface.…”

“…This makes it more difficult to bond PbTe to an electrode due to the stress caused by thermal expansion [18]. Nevertheless, Ni (CTE = 13.4 × 10 − 6 /K) and Fe (CTE = 11.8 × 10 −6 /K) were studied as electrode materials for PbTe [11,19]. The Fe/PbTe joint was found to be successful for n-type material, revealed as a mechanically stable joint with a low electrical resistance at the junction [19].…”

“…Nickel was joined to PbTe thermoelectric material by plasma activated sintering for the first time by Orihashi et al [21], and the joint showed low electrical resistance at the contacts. Recently, in a study by Xia et al [11], the same elements were bonded by a one-step hot press process, simultaneously consolidating and bonding the thermoelectric material to the electrode, and the detailed microstructures and composition of the interface were reported. It appears that Ni is a more viable electrode for PbTe than Fe.…”

Thermoelectric Device: Contact Interface and Interface Materials

“…PbTe 热电材料用于温差发电技术可以追溯到 上世纪 60 年代, 为宇宙深空探测器供给持续、 稳定、 可靠的电能, 它以同位素衰减热为热源, 实现了 5%~6%热电转换效率 [15] 。铁用作 PbTe 的电极材料, 与 n 型 PbTe 接触电阻率小于 10 μΩ•cm 2 , 结合强度 甚至大于 PbTe 材料本身; 而 p 型 PbTe 采用 SnTe 半金属作为中间层与 Fe 连接后, 其接触电阻率也小 于 10 μΩ•cm 2 [35] 。Singh 等 [36] 同样采用 Fe 作为接触 材料, 添加 200 μm 厚 0.5PbTe-0.5Fe 混合物作为缓 冲层 600 ℃真空热压制作 n-PbTe 热电臂, 添加 SnTe 中间层形成 p-TAGS-85 热电臂, 通过测量热电偶臂 的内阻推导出平均的界面电阻率小于 7.6 μΩ•cm 2 , 界面电阻占 3.5%的总内阻, 并且界面元素分布清晰 未发生扩散生成化合物。Leavitt 等 [37] 申请的专利中 考虑了 Fe 和 PbTe 的热膨胀系数差异(12×10 −6 和 18×10 −6 K −1 ), 采用 1 mm 厚的 0.75PbTe-0.25Fe 混合 粉末层作为 p 型、 n 型 PbTe 和 Fe 之间的缓冲, 然后 采用真空热压粉末冶金工艺。Xia 等 [38][39][40] 尝试了 Fe、Mo、Ni、NiFeMo、Nb 箔与 n 型 PbTe 粉末热 压 连 接 , Mo 未 能 成 功 键 合 ; 扫 描 电 镜 观 察 了 PbTe/Fe 界面, 发现 Fe 在 PbTe 内能有 20 m 扩散深 度, 浓度依指数关系递减, 但未反应形成铁碲化合 物, 两相界面清晰, 无过渡层; 而 Ni、Nb 连接生成 了 Ni 3 Te 2 和 Nb 3 Te 4 , 但研究未给出这些界面的接触 电阻数据。 此外, Ni 也被尝试用作 PbTe 的接触材料。 Orihashi 等 [41] 发现 Ni/n-PbTe 无明显接触电阻, 而 Ni/p-Pb 0.5 Te 0.5 界面可能由于生成了 Ni 施主能级有 着较大的接触电阻, 通过增加 SnTe 中间层可以降低 该 电 阻 。 放 电 等 离 子 技 术 也 用 于 一 步 烧 结 键 合 n-PbTe 粉末和 Ni 片, 形成 27 μm 厚的 Ni 3 Te 2 中间层, 由 Te 扩散进入 Ni 反应生成, Pb 元素基本无迁移 [4142] 。 Hu 等 [11] [46] 采用电弧喷涂法形成 SKD 的 Mo 金属 化层, 制造了 32 对偶臂的热电器件。但是, Mo 或 Mo-Cu 很难与 SKD 直接键合, 需要引入 Ti 粘附层 [47] , Ti 的热膨胀系数为 8.6×10 −6 K −1 , 比较接近 CoSb 3 的热膨胀系数。Zhao 等 [48][49] [50] 。 Fan 等 [45] [13] Fig. 9 (a) Power generation efficiency of segmented BT/SKD modules and (b) scanning electron microscopy image of SKD/Ti 0.88 Al 0.12 /Ni interface and electrode on hot side [13] 前景。他们设计了…”

Interfacial reactions in thermoelectric modules

“…While bonding, the liquid Pb forms inside the PbTe, and penetrates into NiFeMo film, which leads to the joint failure. As switching to Nb electrode, two reaction products, which are of Nb 3 Te 4 and Pb, are observed at the interface of Nb/PbTe [48]. However, the large mismatch in CTE between Nb (7.3 × 10 −6 /K) and PbTe (19.8 × 10 −6 /K) limits the use of Nb in the PbTe-based modules.…”