Enhanced figure of merit in Mg2Si0.877Ge0.1Bi0.023/multi wall carbon nanotube nanocomposites

Abstract

The effect of multi wall carbon nanotubes (CNT) on the thermoelectric properties of Mg₂Si_0.877Ge_0.1Bi_0.023 was examined. While introducing CNTs increases the electrical conductivity from around 450 Ω⁻¹ cm⁻¹ to 500 Ω⁻¹ cm⁻¹ at 323 K, the increase is neutralized at higher temperature, with the conductivity resulting to be 440 Ω⁻¹ cm⁻¹–470 Ω⁻¹ cm⁻¹ at 773 K. The Seebeck coefficient of all nanocomposites is enhanced at 773 K due to energy filtering that stems from the introduction of CNTs–Mg₂Si_0.877Ge_0.1Bi_0.023 interfaces. The combined effect of CNTs on Seebeck coefficient and electrical conductivity leads to an approximately 20% power factor improvement, with the best sample reaching a maximum value of ∼19 μW cm⁻¹ K⁻² at 773 K. The lattice thermal conductivity of the nanocomposites is reduced due to the phonon scattering by nanodomains and grain boundaries, particularly at medium temperatures, resulting in a slight reduction in total thermal conductivity. According to high resolution transmission electron microscopy studies, bismuth is homogenously distributed within the grains, while germanium is accumulated at the grain boundaries. All in all, the enhanced thermoelectric figure of merit of 0.67 at 773 K for the sample containing 0.5 weight% MWCNT as compared to 0.55 for the pristine sample, demonstrates the promising effect of CNTs on the thermoelectric properties of Mg₂Si_0.877Ge_0.1Bi_0.023.