ABSTRACT
Al2O3 is a gangue component in iron ores, significantly influencing the formation and crystallization of calcium ferrite in the sintering process. But the mechanism of the Al2O3 effect on the crystallization of calcium ferrite is rarely reported. In this work, a crystallization device was designed to investigate the crystallization behavior of calcium ferrite in Fe2O3-CaO-SiO2-Al2O3 melt under non-isothermal conditions. XRD, SEM-EDS, and optical microscopy were used to identify the crystalline phase and the microstructure of samples. The result shows that the crystal morphology of SFCA changed in the order of strip, column, and needle as the Al2O3 content increased. The crystallization sequence of samples containing Al2O3 was observed as Ca4Fe14O25 (C4F14) â Fe2O3 â Ca3.18Fe15.48Al1.34O36 (SFCA-I) â CaFe2O4 (CF) â Ca5Si2(Fe, Al)18O36 (SFCA) â γ-Ca2SiO4 (C2S). The generation pathway of SFCA-I was found to be C4F14 + Si4+ + Al3+ â SFCA-I. Increasing the cooling rate can promote the formation of C4F14, SFCA-I, Fe2O3 and the amorphous phase. However, it prevented the crystallization of CF and SFCA while inhibiting the transformation of ß-C2S to γ-C2S. When the Al2O3 content reached or exceeded 2.5 mass pct, the viscosity of Fe2O3-CaO-SiO2-Al2O3 melt increased sharply, resulting in the decrease in the crystal size of calcium ferrite.
ABSTRACT
Metal oxide semiconductor gas sensors are widely used to detect toxic and inflammable gases in industrial production and daily life. The main research hotspot in this field is the synthesis of gas sensing materials. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can exhibit superior gas sensing performance in response and selectivity compared with single phase. This review focuses on mainly the synthesis methods and gas sensing mechanisms of metal oxide heterostructures. A significant number of heterostructures with different morphologies and shapes have been fabricated, which exhibit specific sensing performance toward a specific target gas. Among these synthesis methods, the hydrothermal method is noteworthy due to the fabrication of diverse structures, such as nanorod-like, nanoflower-like, and hollow sphere structures with enhanced sensing properties. In addition, it should be noted that the combination of different synthesis methods is also an efficient way to obtain metal oxide heterostructures with novel morphologies. Despite advanced methods in the metal oxide semiconductors and nanotechnology field, there are still some new issues which deserve further investigation, such as long-term chemical stability of sensing materials, reproducibility of the fabrication process, and selectivity toward homogeneous gases. Moreover, the gas sensing mechanism of metal oxide heterostructures is controversial. It should be clarified so as to further integrate laboratory theory research with practical exploitation.
