Thermoelectric properties of group VI metal silicide semiconductors

First-principles study of electronic structures and elasticity of Al2Fe3Si3

Al₂Fe₃Si₃intermetallic compound shows promising application in low-cost and non-toxic thermoelectric device because of its relatively high power factor of ∼700μW m^-1 K^-2at 400 K. Herein we performed the first-principles calculations with the projector augmented-wave (PAW) method to study the formation energies, elastic constants, electronic structures, and electronic transport properties of Al₂Fe₃Si₃. We discussed the thermodynamical stability of Al₂Fe₃Si₃against other ternary crystalline compounds in Al-Fe-Si phase. The band gap of Al₂Fe₃Si₃was particularly examined using the semilocal and hybrid functionals and the on-site Hubbard correction, which were also applied to β-FeSi₂to calibrate the prediction reliability of our employed computational methods. Our calculations show that Al₂Fe₃Si₃is a narrow-gap semiconductor. The semilocal functional within generalized gradient approximation (GGA) shows an exceptional agreement between the predicted band gap of Al₂Fe₃Si₃and the available experiment data, which is in contrast to the typical trend and rationally understood through a comprehensive comparison. We found that both HSE06 and PBE0 hybrid functionals with a standard setup overestimated the band gaps of Al₂Fe₃Si₃and β-FeSi₂too much. The underlying reasons may be ascribed to a large electronic screening, which arises from the unique characteristics of Fe 3dstates appearing in both sides of band gaps of Al₂Fe₃Si₃and β-FeSi₂, and to a reduced delocalization error thanks to the covalent Fe-Si and Si-Si bonding nature. The chemical bonding and elasticity of Al₂Fe₃Si₃were compared with those of β-FeSi₂and FeAl₂. In Al₂Fe₃Si₃the Fe-Al bonding is more ionic and the Fe-Si bonding is more covalent. The elastic moduli of Al₂Fe₃Si₃are comparable to those of β-FeSi₂and larger than those of FeAl₂. Our calculation results indicate that the mechanical strength of Al₂Fe₃Si₃could be strong enough for the practical application in thermoelectric device.

Machine-Learning-Assisted Development and Theoretical Consideration for the Al2Fe3Si3 Thermoelectric Material

Chemical composition alteration is a general strategy to optimize the thermoelectric properties of a thermoelectric material to achieve high-efficiency conversion of waste heat into electricity. Recent studies show that the Al₂Fe₃Si₃ intermetallic compound with a relatively high power factor of ∼700 μW m^-1 K^-2 at 400 K is promising for applications in low-cost and nontoxic thermoelectric devices. To accelerate the exploration of the thermoelectric properties of this material in a mid-temperature range and to enhance its power factor, a machine-learning method was employed herein to assist the synthesis of off-stoichiometric samples (namely, Al_{23.5+ x}Fe_36.5Si_{40- x}) of the Al₂Fe₃Si₃ compound by tuning the Al/Si ratio. The optimal Al/Si ratio for a high power factor in the mid-temperature range was found rapidly and efficiently, and the optimal ratio of the sample at x = 0.9 was found to increase the power factor at ∼510 K by about 40% with respect to that of the initial sample at x = 0.0. The possible mechanism for the enhanced power factor is discussed in terms of the precipitations of the metallic secondary phases in the Al_{23.5+ x}Fe_36.5Si_{40- x} samples. Furthermore, the maximum achievable thermal conductivity of Al₂Fe₃Si₃ estimated by the Slack model is ∼10 W m^-1 K^-1 at the Debye temperature. An avoided-crossing behavior of the acoustic and the low-lying optical modes along several crystallographic directions is found in the phonon dispersion of Al₂Fe₃Si₃ calculated by ab initio density functional theory method. These preliminary results suggest that Al₂Fe₃Si₃ can have a low thermal conductivity. The calculated formation energies of point defects suggest that the antisite defects between Al and Si are likely to cause the Al and Si off-stoichiometries in Al₂Fe₃Si₃. The theoretically obtained insight provides additional information for the further understanding of Al₂Fe₃Si₃.