Hardening of Bi-Te based alloys by dispersing B₄C nanoparticles

Abstract Thermoelectric devices have attracted a great attention for renewable energy harvesters and solid-state coolers. For practical applications, the mechanical properties of thermoelectric materials become critical for the device reliability, a persistent performance with a long time and high operation cycles. Bi-Te based single-crystals, mostly used in commercial thermoelectric devices, are intrinsically brittle with weak van der Waals bonding, often leading to device failures such as crack and debonding during fabrication and operation. Thus, it is highly desirable to enhance the mechanical property of Bi-Te based alloys as well as the thermoelectric property. Here, we investigate the effect of B₄C nanoparticles (less than 0.5 wt%) dispersed in p-type Bi_0.4Sb_1.6Te₃ matrix on the mechanical properties. X-ray diffraction (XRD) result confirms that B₄C-dispersed Bi_0.4Sb_1.6Te₃ has a single phase. We observe that the grain size of Bi_0.4Sb_1.6Te₃ becomes decreased with the B₄C nanoparticle concentration by electron backscatter diffraction (EBSD) technique. Hardness, Young's modulus, and flexural strength of B₄C-dispersed Bi_0.4Sb_1.6Te₃ are enhanced, compared to the B₄C-free Bi_0.4Sb_1.6Te₃ polycrystals. On the other hand, the thermoelectric figure-of-merit of B₄C-dispersed Bi_0.4Sb_1.6Te₃ is almost identical to that of the pure Bi_0.4Sb_1.6Te₃. Such enhancements of the mechanical properties of the B₄C-dispersed Bi_0.4Sb_1.6Te₃ are attributed to the grain boundary hardening and second-phase hardening. Beyond thermoelectric materials, our result implies that the grain refinement by nanoparticle dispersion is a simple and promising way to strengthen the mechanical properties of other brittle materials with layered structure.