Hsin-jay Wu

Liquidus Projection of the Ternary Bi-Sb-Te Thermoelectric Material System

The ternary Bi-Sb-Te system is important for thermoelectric applications. Its liquidus projection is determined, except for the Bi-rich corner with various unconfirmed binary Bi-Te compounds. Fifty Bi-Sb-Te ternary alloys are prepared. Primary solidification phases are identified using metallographic observations and X-ray diffraction techniques. Phase transformation temperatures are determined using differential thermal analysis. No ternary compound is found. (Bi,Sb)2Te3 and (Bi,Sb) are continuous solid solutions. The Bi2Te3 and Sb2Te3 phases form a pseudo-binary section. Alloys of the primary solidification phases of Te, (Bi,Sb)2Te3, γ, δ, and (Bi,Sb) are determined and the boundaries of various primary solidification phases are delineated. The Bi-Sb-Te liquidus projection is also calculated with the CALPHAD method using preliminary thermodynamic models, and the calculated results are qualitatively in agreement with the experimental determinations.

Phase Equilibria of the Sn-Bi-Te Ternary System

Bi2Te3 is one of the most promising thermoelectric materials, and Sn is the primary constituent of most electronic solders. Knowledge of phase equilibria of the Sn-Bi-Te ternary system is important for thermoelectric applications. Twenty-seven Sn-Bi-Te alloys were prepared and equilibrated at 160°C and 500°C. The equilibrium phases were determined, and the isothermal sections were constructed based on three binary constituent phase diagrams and ternary phase equilibria results. No ternary compounds were observed in the Sn-SnTe-Bi region. The SnTe phase is very stable and has tie-lines with the Sn, liquid, and Bi phases at 160°C. At 500°C, in addition to the already known SnBi2Te4 and SnBi4Te7 ternary phases, two new ternary compounds (Sn2Bi2Te5 and SnBiTe2 phases) were found. SnTe and Bi2Te3 have significant mutual solubilities, and the ternary compounds SnBi2Te4, SnBi4Te7, and Sn2Bi2Te5 are all in the SnTe-Bi2Te3 pseudobinary section.

Interfacial reactions in thermoelectric modules

ABSTRACT 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. GRAPHICAL ABSTRACT

Interfacial reaction at 250°C in the Sn/Ni-7wt.%V couple

Sn is the base element of all the promising electronic solders, and Ni-7wt.%V is the major diffusion barrier layer material of flip chip technology. Interfacial reactions in the Sn/Ni-7wt.%V couple at 250°C are examined in this study. The Ni3Sn4 phase is observed in the Sn matrix. Two phase layers, Sn-Ni-V ternary phase (T phase) and a mixture of the nano-crystalline V2Sn3 phase and Sn phase (T2 phase), are observed at the Sn/Ni-7wt.%V interface. The micro structures of T phase and T2 phase was analyzed by TEM, T phase has a dense structure and T2 phase has a loose structure. The loose structure of Sn-V phase can not only cause a harmful effect on mechanical properties, but also on electrical properties.