Year: 2026
Type: article
Abstract: This study investigates the effects of reducing (hydrogen) and oxidizing (air or oxygen) atmospheres during the calcination of calcium manganite (CaMnO 3 or CMO), synthesized via solid-state reaction (SSR) and modified Pechini method MPM). CMO is an n-type thermoelectric material that exhibits low thermal conductivity and favorable electronic properties. The crystalline phase evolution and particle morphology were assessed under distinct calcination conditions: time (3 or 24 h), temperature (800 °C or 1000 °C), and atmosphere. Crystallization temperature was lower for powders calcined in hydrogen (720 °C) than in air (755 °C), highlighting the atmosphere's influence. Also, hydrogen atmosphere inhibited particle growth in SSR-derived powders, yielding smaller particles versus oxidizing conditions. At 800 °C, CMO yields were generally lower: SSR powders reached 70% of CMO phase, while MPM-derived powders achieved between 19% and 35% under most conditions, with Mn 2 O 3 and Mn 3 O 4 as secondary phases in SSR and additional Ca 2 Mn 3 O 8 in MPM; the exception was the MPM powder calcined for 3 h in hydrogen, which reached 79% CMO. At 1000 °C in oxygen, SSR-derived powders formed 100% CMO, whereas MPM-derived powders attained 70% CMO, with Ca 2 MnO 4 and residual Mn 3 O 4 as the main secondary phases. Under hydrogen at 1000 °C, MPM-derived CMO formation was strongly suppressed, ranging from 0% (3 h) to 50.2% (24 h), with a (MnO) 0.441 (CaO) 0.559 solid solution forming instead. Orthorhombicity reached 0.366% in the hydrogen-calcined MPM powder at 1000 °C, compared to about 0.16% in SSR-derived powders under the same conditions, a difference consistent with a higher Mn 3+ /Mn 4+ ratio, although this ratio was not directly measured and is inferred from lattice distortion. Reducing atmospheres (hydrogen) are associated with increased Mn 3+ content, enhancing lattice distortion and orthorhombicity, while oxidizing conditions (oxygen) are associated with a lower Mn 3+ /Mn 4+ ratio and reduced distortion. SSR with oxygen calcination minimized secondary phases, optimizing CMO formation, whereas MPM requires further refinement for higher purity. MPM combined with hydrogen calcination induced greater lattice distortion, which is expected to influence electrical conductivity through the small-polaron hopping mechanism, although this was not directly measured in the present study.
Source: Open Ceramics
Institution: Universidade Federal de Itajubá
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $2,000
OpenAlex ID: W7171465929