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

Here, we present our current attempt to intrinsically dope Ni-0, Co-0, and Fe-0 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-T(0) nanoparticles dispersed into the M-T-BO3 matrix. The composition and structural characteristics of these M-T(0):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 (sigma) 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 (kappa) occurs through phonon lattice vibration dynamics rather than electronic. The sigma/kappa ratio shows a steep non-linear increase from 9.4 to 270 KV-2 in Ni-0:Ni-BO3. In contrast, a moderate-weak increase is observed for Co-0:Co-BO3 and Fe-0: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 (eta%). All the TE data shows that Ni-0:Ni-BO3 (S, 80 mu VK-1; PF, 97.7 mWm(-1) K-1; ZT 0.54; eta, 2.15%) is a better TE semiconductor than the other two M-T(0):M-T-BO3. This finding shows that Ni-0: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.