Anisotropy of transport properties and microstructure of Bi_0,5Sb_1,5Te₃ thermoelectric composites with reduced graphene oxide inclusions

The development of efficient thermoelectric materials for low-grade waste heat recovery is a critical challenge in modern energetics. In this study, bulk p-type nanocomposites based on the Bi0,5Sb1,5Te3 solid solution reinforced with reduced graphene oxide (rGO) nanosheets (concentrations of 1,0; 2,5 and 5,0 wt. %) were successfully fabricated. The synthesis approach combined high-energy ball milling for precursor homogenization and spark plasma sintering (SPS) for rapid consolidation. The impact of filler concentration on the microstructure evolution, phase composition, and transport properties was systematically evaluated in the 300-575 K temperature range.
Detailed microstructural analysis revealed that rGO inclusions are chemically inert and uniformly dispersed within the matrix. A key finding is the induction of a strong crystallographic texture during the SPS process: the 2D rGO sheets tend to align perpendicularly to the pressing direction, thereby blocking carrier transport along the parallel axis and inducing significant anisotropy in both electrical and thermal properties. Hall effect measurements confirmed that the hole concentration remained nearly constant (approx. 2,0 · 1019 cm-3) across all compositions, ruling out a doping effect. However, a sharp degradation in carrier mobility (decreasing from 267 to 83 cm2/V · s) was observed, attributed to severe scattering of charge carriers at the incoherent matrix/filler interfaces.
Conversely, the rGO network proved effective in scattering heat-carrying phonons, leading to a substantial reduction in total thermal conductivity (up to 23% reduction in the parallel direction compared to the pristine sample). Despite this beneficial thermal suppression, the deterioration of the electrical conductivity dominated the overall performance. Consequently, the peak dimensionless figure of merit (ZT) for the composites was lower than that of the unfilled matrix (ZT_max ≈ 1,0 at 420 K). This research highlights the critical trade-off between phonon blocking and electron transmitting mechanisms, suggesting that future strategies must focus on interface engineering to preserve carrier mobility in carbon-reinforced bismuth telluride thermoelectrics.

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