Thermoelectric performance of Ni,Co, and Fe nanoparticles incorporated into their metal borates glassy matrices

Here, we present our current attempt to intrinsically dope Ni⁰, Co⁰, and Fe⁰ nanoparticles within Ni^II-, Co^II-, and Fe^II-borate glassy matrices, respectively. The system was prepared by one-pot reaction of the desired M_T^II salt with excess NaBH₄ through an in-situ reduction and hydrolysis processes to afford metallic M_T⁰ nanoparticles dispersed into the M_T-BO₃ matrix. The composition and structural characteristics of these M_T⁰:M_T-BO₃ materials were identified by thermal oxidation, ATR-IR, X-ray powder diffraction, and magnetic techniques as glassy/amorphous borate matrices containing magnetic nanoparticles. The electrical conductivity (σ) of cold-pressed discs of these metal-doped composites shows that they behave as nonohmic semiconductors within the temperature range of 303 ≤ T ≤ 373 K suggesting a mixed electronic-ionic conduction. However, their thermal conductivity (κ) occurs through phonon lattice vibration dynamics rather than electronic. The σ/κ ratio shows a steep non-linear increase from 9.4 to 270 KV^?2 in Ni⁰:Ni-BO₃. In contrast, a moderate-weak increase is observed for Co⁰:Co-BO₃ and Fe⁰:Fe-BO₃ analogs. The obtained materials are examined for thermoelectric (TE) applications by determining their Seebeck coefficient (S) power factor (PF), figure of merit (ZT), and conversion efficiency (η%). All the TE data shows that Ni⁰:Ni-BO₃ (S, 80 μVK^?1; PF, 97.7 mWm^?1 K^?1; ZT 0.54; η, 2.15%) is a better TE semiconductor than the other two M_T⁰:M_T-BO₃. This finding shows that Ni⁰:Ni-BO₃ is a promising candidate to exploit low-temperature waste heat from body heat, sunshine, and small domestic devices for small-scale TE applications.