Google Search
A2: Medium temperature materials
In recent years, it has been reported that increasing grain boundary scattering can be beneficial to reduce the lattice thermal conductivity whereas an appropriately reduced grain size can barely affect the electron transport for both of low-dimensional and bulk materials for many alloys. In this work, the effect of introducing fine grain boundaries using ball-milling on the thermoelectric properties was investigated. The samples were synthesized by a combination of arc-melting, ball-milling, and spark plasma sintering (SPS). The average grain size of the milled and sintered bulk sample (100–300 nm) was decreased dramatically by ball-milling compared with that of the unmilled and sintered bulk sample (more than 1 μm). The maximum dimensionless figure of merit, ZTmax, of ball-milled and sintered RuGa2 was 0.58 at 773 K; 7% enhancement on the value of 0.54 for unmilled RuGa2 due to the reduction of the lattice thermal conductivity. The average grain size of the sintered bulk sample was 10-20 times larger than that of the ball-milled nanopowder, ranged from 5 to 30 nm. Therefore, further enhancement in ZT can be expected by preserving the nano-scale sized powder during SPS process [1].
In addition to the experimental studies, we performed the calculation of the phonon dispersion relations for RuGa2 using the real space force constant method. The calculated frequencies at Γ point were comparable to the experimental Raman wavenumbers. Likewise, the group velocity from calculation were in good agreement with the experimental transverse and longitudinal speed of sound measured by the ultrasonic pulse echo method. The relation between the reduction of lattice thermal conductivity and the grain size will be clarified quantitatively by calculating the distribution of the phonon mean free path.
[1] N. Sato et al., J. Alloy. Compd. 585, 455 (2014).