Microwave-assisted synthesis of Co0.4Ni0.6Al2O4 - Evaluating the effect of practical parameters on material properties

Abstract

In this study, microwave-assisted combustion synthesis (MACS) was used to prepare Co_0.4Ni_0.6Al₂O₄ spinel powders from respective metal nitrate salts and urea fuel using a domestic microwave oven. The study systematically varied the fuel-to-oxidizer (F/O) molar ratio and microwave output power to determine their influence on combustion behavior, phase formation, crystallinity, yield and morphology. Results show that the fuel content strongly controls combustion intensity and homogeneity, and a molar ratio Al³⁺:urea = 1:6 (F/O = 1.8) produced the most uniform, dark-blue spinel with sharper X-ray diffraction (XRD) peaks and fewer secondary phases compared with both fuel-rich and fuel-poor conditions. Furthermore, microwave power critically affected ignition, conversion and phase purity. The insufficient power (770 W) failed to trigger self-propagating combustion, while excessive power (1100 W) induced local overheating and more secondary phases (α-Al₂O₃ and NiO). The optimal condition identified in this work was 990 W at Al³⁺:urea = 1:6, which provided the highest mass conversion (91%) and well-crystallized Co_0.4Ni_0.6Al₂O₄ with porous morphology characteristic of combustion synthesis. The study demonstrates that control of F/O ratio and microwave power in MACS enables rapid, energy-efficient synthesis of Co–Ni aluminate spinels.