Enhancement of mechanical performance and reduction in thermal conductivity of Mg2Si-based thermoelectric nanocomposites through rGO addition

Thermoelectric materials-based devices are used to convert heat energy into electrical energy. Magnesium silicide-based thermoelectric-based devices are considered commercially viable due to their low cost compared to other contemporary materials. The current study investigates the influence of Sb doping on the thermoelectric properties of the Mg_2.15Si_0.28Sn_0.714Sb_0.006 (Sample-A) compound with an excess Mg content (7.5 mol %). The excess Mg induces point defects through interstitial Mg and Si/Sn vacancies, significantly enhancing the electron concentration (n_e). Moreover, Sb is recognized as an effective single-electron donor in Mg₂Si-based materials, leading to notable increases in n_e and electrical conductivity. Consequently, in the current investigation, excess Mg combined with appropriate Sb doping, resulted in the selection of Mg_2.15Si_0.28Sn_0.714Sb_0.006 (Sample-A), which exhibited high n_e and superior thermoelectric performance. Further, the current study was extended by incorporating 3 vol.% of reduced graphene oxide (rGO) into Mg_2.15Si_0.28Sn_0.714Sb_0.006 + 3 vol.% rGO (Sample-B) to enhance mechanical performance and reduce thermal conductivity (k). Consequently, Sample-B showed a ∿ 28% increase in fracture toughness (from 1.48 to 1.9 MPa√m) and a ∿ 137% improvement over conventional Mg₂Si. Moreover, the inclusion of rGO resulted in a substantial reduction in k ∿ 40% in the mid-temperature range, due to intensified phonon scattering caused by the higher interface density within the matrix. However, adding more than 3 vol.% rGO negatively impacts both thermoelectric and mechanical properties by obstructing the charge carriers. Therefore, achieving an optimal balance between rGO addition and compositional modulation is essential to enhance both thermoelectric and mechanical performance in these composites.