Hardening of Bi-Te based alloys by dispersing B4C nanoparticles

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 Bi0.4Sb1.6Te3 matrix on the mechanical properties. X-ray diffraction (XRD) result confirms that B4C-dispersed Bi0.4Sb1.6Te3 has a single phase. We observe that the grain size of Bi0.4Sb1.6Te3 becomes decreased with the B4C nanoparticle concentration by electron backscatter diffraction (EBSD) technique. Hardness, Young's modulus, and flexural strength of B4C-dispersed Bi0.4Sb1.6Te3 are enhanced, compared to the B4C-free Bi0.4Sb1.6Te3 polycrystals. On the other hand, the thermoelectric figure-of-merit of B4C-dispersed Bi0.4Sb1.6Te3 is almost identical to that of the pure Bi0.4Sb1.6Te3. Such enhancements of the mechanical properties of the B4C-dispersed Bi0.4Sb1.6Te3 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. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.