Efeito do método e da atmosfera de sinterização nas propriedades termoelétricas das cobaltitas de cálcio dopadas com bismuto

Abstract

Electrical power generation from the recovery (or conversion) of waste heat in thermal systems emerges as a sustainable alternative for mitigating the negative impacts of fossil fuel use. The energy conversion performance of thermoelectric materials is considered high when the material exhibits high values of figure of merit (zT > 1.0). Calcium cobaltite (Ca3Co4O9, CCO) stands out among oxides with potential thermoelectric application due to its chemical stability at high temperatures, high electrical conduction, and reduced thermal conduction. The material behaves as a p‑type semiconductor, conducting electricity mainly by electronic holes transported by a hopping mechanism, which migrate from Co4+ to Co3+. Its distinct crystal structure, composed of rock‑salt‑type layers (Ca2CoO32−) and CdI2‑type layers (CoO22+), gives the material morphological characteristics of lamellar particles. Although this complex structure brings benefits to the thermoelectric properties, the morphology of its particles makes it difficult to densify CCO ceramics, which is crucial for the ceramics to exhibit high thermoelectric performance. Additionally, doping with heavy chemical elements such as Bi3+ can induce distortion in the crystal lattice, decreasing the thermal conductivity of the material, as well as increasing the Seebeck coefficient. Thus, aiming to enhance the thermoelectric properties of CCO ceramics, this work combined the use of different sintering techniques and atmospheres. The compositions Ca3Co4O9 and Ca2.8Bi0.2Co4O9 were synthesized by the solid‑state reaction method using the precursors CaCO3, Co(NO3)2·6H2O, and Bi2O3. The ceramics were conventionally sintered in oxidizing atmospheres (air or O2) at 900 °C for 9 h or were sintered by the non‑conventional microwave (MW) technique at 900 °C, with a dwell time of 30 minutes. Alternatively, the MW‑sintered ceramics were subjected to subsequent heat treatments carried out in a conventional furnace, in O2 atmosphere, at 900 °C for 9 h, in order to reduce the concentration of oxygen vacancies and stabilize the CCO phase. X‑ray diffraction (XRD) analyses of the calcined powders revealed that the addition of Bi maximized the formation of the main phase (93.4(4)% of Ca3Co4O9) compared to the undoped composition (81.8(9)%). Scanning electron microscopy (SEM) micrographs showed that the doped ceramics presented larger grain size than those obtained for the undoped ceramics. The undoped samples achieved higher densification (relative geometric densities between about 66.4±0.2% and 69.3±0.7%) and lower apparent porosity (34.7±0.2% to 36.2±0.3%) compared to the doped ceramics, with MW sintering being the most efficient to densify the undoped ceramics. Furthermore, the undoped ceramics presented Seebeck values higher than most doped ceramics. The only exception was the ceramic sintered by MW at 900 °C for 30 minutes, which reached the highest Seebeck value. The undoped samples conventionally sintered in air presented the best thermoelectric results, reaching a power factor of 0.22 mW/m·K² at 600 °C.