NANOCRYSTALLINE Bi₂Te₃ VIA MECHANICAL ALLOYING: EFFECTS OF MILLING SPEED, PROCESS CONTROL AGENTS AND THERMOELECTRIC PERFORMANCE

Abstract

Nanostructured thermoelectric materials have attracted considerable attention as a pathway to enhance device efficiency by reducing lattice thermal conductivity while maintaining favorable electrical transport. In this work, nanocrystalline Bi₂Te₃ was synthesized through mechanical alloying, with systematic evaluation of the effects of milling speed and the introduction of ethanol as a process control agent. Particle size analysis revealed that higher milling speeds promoted significant refinement, with optimal conditions achieved at 600 rpm, though further increases resulted in diminished efficiency due to particle agglomeration and heat generation within the milling vial. The incorporation of ethanol effectively reduced cold welding and particle clustering, yielding finer distributions; however, this came at the expense of powder yield, underscoring the trade-off between structural control and processing efficiency. Microstructural characterization using FESEM confirmed the transformation of bulk Bi₂Te₃ into nanograins with sizes approaching the sub-micron range, accompanied by agglomerated morphologies. Thermal conductivity measurements demonstrated a pronounced reduction for the milled samples compared with unmilled counterparts, a consequence of enhanced phonon scattering at grain boundaries. Importantly, electrical transport properties remained largely preserved, leading to an overall improvement in the thermoelectric figure of merit (ZT). These findings establish mechanical alloying as a cost-effective and scalable strategy to optimize Bi₂Te₃ for thermoelectric applications, particularly in waste heat recovery, low-temperature cooling, and portable energy systems, where efficiency, manufacturability, and scalability are critical.