Anomalous Temperature-Induced Particle Size Reduction in Manganese Oxide Nanoparticles.

The intricate role of temperature in the structure-property relationship of manganese oxide nanoparticles (Mn3O4 NPs) remains an open question. In this study, we successfully synthesized Mn3O4 NPs using the hydrothermal method with two differing temperatures, namely, 90 and 150 °C. Interestingly, a smaller average particle size is found when Mn3O4 NPs are synthesized at 150 °C compared to 90 °C, corresponding to 46.54 and 63.37 nm, respectively. This was confirmed by the time variation of temperature setting of 150 °C where the size evolution was insignificant, indicating a competing effect of nucleation and growth particles. Under varying NaOH concentrations (2-6 M) at 150 °C, a reduction in the particle size is found at the highest NaOH concentration (6 M). The particle grows slightly, indicating that the growth state is dominant compared to the nucleation state at low concentrations of NaOH. This finding implies that the high nucleation rate originates from the excessive monomer supply in the high-temperature reaction. In terms of crystallinity order, the structural arrangement of Mn3O4 NPs (150 °C) is largely decreased; this is likely due to a facile redox shift to the higher oxidation state of manganese. In addition, the higher ratio of adsorbed oxygen and lattice oxygen in Mn3O4 NPs at 150 °C is indirectly due to the higher oxygen vacancy occupancies, which supported the crystallinity decrease. Our findings provide a new perspective on manganese oxide formation in hydrothermal systems.

Two steps CrAlFeSi coating on low carbon steel prepared by mechanical alloying and its oxidation properties.

Two steps CrAlFeSi coating has been fabricated on low carbon steel via mechanical alloying methods and its oxidation properties have been elucidated thoroughly. First, Al coating was deposited on the low carbon steel substrate via mechanical alloying for 1 h. Afterward, CrAlFeSi coating was deposited on Al coating using the same technique for 2 h. The effect of annealing at 650 °C on the oxidation behavior of two steps CrAlFeSi coatings was examined thoroughly. The microstructure of the coating layer before and after annealing was relatively similar. Microholes and microcracks were found in the coating layer of the substrate before and after annealing. Intermetallic phases were observed in the samples along with the major elements. The mass gain after cyclic oxidation at 800 °C in the air atmosphere for a substrate with two coating layers reduced by a factor of 10 compared to the substrate without coating layer, which is likely due to the formation of Al2O3 on the outer layer during the oxidation process. The thin layer of Al2O3 protects the inner layer from severe oxidation. Therefore, the two steps coating of CrAlFeSi on the low carbon steel can be used as an alternative method for reducing the oxidation at high temperature.