Simultaneous enhancement of mechanical and thermoelectric properties of polycrystalline magnesium silicide with conductive glass inclusion

Thermal fatigue and mechanical failure are two challenging aspects of thermoelectric (TE) module design and fabrication. Among the well-known TE materials with high conversion efficiencies are various brittle semiconductors or ceramics. However, formation of microcracks during fabrication and mechanical failure due to the high stress response in the course of thermal cycles are commonly observed in such material systems. In this work, we report the results of a novel technique to improve the mechanical and TE properties of magnesium silicide (Mg2Si) via addition of a small quantity (0.25-1 vol.%) of conductive glass-frit. Mechanically alloyed and hot-pressed Mg2Si specimens separately doped with 2 at.% Bi and 2 at.% Al were sintered at 1173 and 1123 K, respectively. The TE properties of both compounds were characterized by measurements of electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (κ) in the temperature range 300-970 K. The beneficial effects of addition of a minuscule quantity of Mg-Si-B-R-based (R = rare earth) conductive glass-frit to Al-doped Mg2Si samples were investigated. Both Al-doped and Bi-doped Mg2Si specimen were tested for mechanical reliability using diametric compression tests. Power factors times temperature (S2σT) of >2 W m-1 K-1 were obtained from Al-doped samples containing conductive glass-frit. It was also found that addition of 1% of the conductive glass-frit results in significant improvement of the mechanical properties of Mg2Si by eliminating microcracks in the brittle Mg2Si system. Nearly 150% improvement was observed in the mechanical strength of the Al-doped samples reinforced with conductive glass-frit as compared to the samples without glass-frit.