Microstructural Engineering for Power Factor and ZT Optimization in Cu Alloyed n-Type Bi _{2– <i>x</i>} Sb _<i>x</i> Te ₃ Thermoelectric Materials

Bismuth telluride (Bi₂Te₃) alloys are widely used thermoelectric (TE) materials for near-room-temperature energy conversion. The performance of TE devices in this range critically depends on n-type Bi₂Te₃ alloys for performance parity with that of their p-type counterparts. This study presents a strategy for tuning the thermoelectric properties of selenium-free n-type Bi_2-<i>x</i> Sb _<i>x</i> Te₃ (BST) through controlled copper (Cu) alloying. Cu incorporation via ball milling enhanced the Seebeck coefficient but reduced electrical conductivity due to the formation of coarse Cu-rich precipitation. Subsequent melt-spinning recovered conductivity and promoted uniform Cu dispersion while inducing beneficial point defects and lowering lattice thermal conductivity. These combined effects shifted the peak <i>ZT</i> value toward room temperature, achieving better alignment with the operational range of the p-type materials. A four-pair TE device fabricated with 0.1 wt % Cu alloyed BST (melt-spun) demonstrated a 20% increase in output power at <i>T</i> _hot = 450 K compared to the pristine sample. This work demonstrates the role of Cu alloying and microstructural engineering in optimizing n-type BST for efficient thermoelectric power generation.