Graded n-type β-FeSi 2 /Bi 2 Te 3 thermoelectric materials have been prepared by dip coating procedure using Sn 95 Ag 5 as bridge material. It was found that an apparent Seebeck coefficient is in between those of β-FeSi 2 and Bi 2 Te 3 below 150 °C and higher than both of them between 150 and 300 °C. A measurement shown that the maximum power output of the functionally graded material (FGM) is approximately 2.5–3 times that of monolithic material β-FeSi 2 at the same temperature difference, which suggests that not only do the graded material benefit the superior performance of Bi 2 Te 3 from low temperature side, but also make fully use of the properties of β-FeSi 2 at high one. Besides, the FGM revealed rather optimistic thermal stability with almost no power output loss even after annealing at 192 °C for 200 h, if a nickel layer was sandwiched between Sn 95 Ag 5 and Bi 2 Te 3 layers. An obvious diffusion of nickel element towards both Sn 95 Ag 5 and Bi 2 Te 3 sides was observed by EMPA, with the former being most likely occurred during long time annealing, and latter being irrelevant to annealing procedure applied.

Preparation, thermoelectric properties and interface analysis of n-type graded material FeSi2/Bi2Te3

Engineering transport properties of thermoelectric (TE) materials leads to incessantly breakthroughs in the zT values. Nevertheless, modular design holds a key factor to advance the TE technology. Herein, we discuss the structures of TE module and illustrate the inter-diffusions across the interface of constituent layers. For Bi2Te3-based module, soldering is the primary bonding method, giving rise to the investigations on the selections of solder, diffusion barrier layer and electrode. For mid-temperature PbTe-based TE module, hot-pressing or spark plasma sintering are alternative bonding approaches; the inter-diffusions between the diffusion barrier layer, electrode and TE substrate are addressed as well.

https://www.tandfonline.com/doi/pdf/10.1080/21663831.2018.1436092

Interfacial reactions in thermoelectric modules

In this study, electroless-plating of a nickel-phosphor (Ni–P) thin film on surface-controlled thermoelectric elements was developed to significantly increase the bonding strength between Bi–Te materials and copper (Cu) electrodes in thermoelectric modules. Without electroless Ni–P plating, the effect of surface roughness on the bonding strength was negligible. Brittle SnTe intermetallic compounds were formed at the bonding interface of the thermoelectric elements and defects such as pores were generated at the bonding interface owing to poor wettability with the solder. However, defects were not present at the bonding interface of the specimen subjected to electroless Ni–P plating, and the electroless Ni–P plating layer acted as a diffusion barrier toward Sn and Te. The bonding strength was higher when the specimen was subjected to Ni–P plating compared with that without Ni–P plating, and it improved with increasing surface roughness. As electroless Ni–P plating improved the wettability with molten solder, the increase in bonding strength was attributed to the formation of a thicker solder reaction layer below the bonding interface owing to an increase in the bonding interface with the solder at higher surface roughness.

https://www.mdpi.com/2079-6412/9/3/213/pdf

Effect of Surface Roughness and Electroless Ni–P Plating on the Bonding Strength of Bi–Te-based Thermoelectric Modules

Enhanced thermoelectric properties of metal film on bismuth telluride-based materials

We present detailed studies of the mechanical behavior of electrical contacts since they have a strong effect on the performance and long term reliability of thermoelectric (TE) modules both for cooling and electricity generation. We have studied the mechanical properties of electroplated and electroless plated Ni contacts on n-type and p-type (Bi1?xSbx)2(Te1?ySey) 3, polycrystalline quaternary alloys. These alloys were obtained by mechanical alloying followed by hot extrusion. A variety of surface preparation methods and metallization processes were applied and compared. As soldering alloys we used Sn(60%) Pb(40%) or Sn(95%) Sb(5%) on top of the Ni contacts. Our results show that it is possible to reach mechanical strength above 350 kg/cm2 when proper metallization and soldering technologies are applied. Contact reliability becomes especially important for short-legged modules, which provide higher density of cooling and generating power. Extruded TE materials may be the only ones suitable for devices where the required mechanical stress is very high.

Mechanical Properties of the Interface between Nickel Contact and Extruded (Bi1-xSbx)2(Te1-ySey)3 Thermoelectric Materials

In this letter homogeneous electroless nickel plating to sintered Bi-Te alloy was tested by the heat-shock test and it was shown to function as a barrier of the diffusion of impurities

T. Nishizaka

Papers

Electroless nickel plating to Bi-Te sintered alloy and its properties