Investigations on the MnO2-Fe2O3 system roasted in air atmosphere

Solid-state reaction method is a common and effective technique to synthesize ferrites. This work investigated the phase transformation of MnO₂ and Fe₂O₃ system roasted at 500–1400 °C in air atmosphere to understand the formation process of manganese ferrite. The results showed that the formation of manganese ferrite (Mn_xFe_3?xO₄) was derived from the reaction between Fe₂O₃ and Mn₃O₄ (the decomposition product of MnO₂). Below 900 °C, MnO₂ firstly decomposed to Mn₂O₃ and then to Mn₃O₄, and Fe₂O₃ was seldom reacted with Mn₂O₃ and Mn₃O₄. When the temperature went up to 1000 °C, Fe₂O₃ easily reacted with Mn₃O₄ to generate manganese ferrite. The reaction degree was enhanced dramatically with the rising of temperature. Moreover, the x value in the Mn_xFe_3?xO₄ increased from 0 to 1 from 900 °C to 1400 °C. In other words, the higher the temperature was, the closer the Mn_xFe_3?xO₄ was to MnFe₂O₄. Thermodynamic analysis of MnO₂-Fe₂O₃ system under different O₂ partial pressures was carried out to further explain the formation mechanism.     

New understanding on formation mechanism of CaFe2O4 in Fe2O3 -Fe3O4-CaO-SiO2 system during sintering Process: Phase transformation and morphologies evolution

The complex calcium ferrite is the main binder phase of high basicity, high iron and low silicon sinter used in blast furnace (BF), and its formation amount and structure are important factors affecting sinter quality. This work research on the phase transformation and morphologies evolution of Fe₂O₃ -Fe₃O₄-CaO-SiO₂ systems roasted 900 °C ? 1200 °C in air atmosphere to understand the formation process of calcium ferrite. In CaO-Fe₃O₄ system, the prolonged roasting time and higher temperature promotes that CaFe₅O₇, CaFe₃O₅, and Ca₂Fe₂O₅ gradually transformed into the stable existence of CaFe₂O₄. In CaO-Fe₃O₄ system, the higher temperature promotes that the combination of Fe₃O₄ and CaO formed the stable CaFe₃O₅ with orthorhombic structure. The replacement reaction between the newly formed CaFe₃O₅ phase and the unreacted CaO phase occur to form Ca₂Fe₂O₅. In CaO-Fe₃O₄-SiO₂ system, FeSiO₃ can be combined with Fe₃O₄ to form Fe₂SiO₄ and Fe₂O₃. Under the action of high temperature, FeSiO₃ and CaO undergo displacement reaction to form the unstable CaSiO₃, the new formation of CaSiO₃ can be easily combined with CaO to form the stable Ca₂SiO₄. With the further increase of temperature, the complex calcium ferrite is formed in calcium silicate layer. The final product complex calcium ferrite and calcium silicate exist at the same time. The formation of calcium silicate has an unfavorable effect on formation of complex calcium ferrite in the sintering process.     

玻璃基TiO_2-Fe_2O_3-CeO_2复合纳米薄膜的光催化性能研究

在溶胶-凝胶法制备的TiO2胶膜表面涂覆金属离子,通过焙烧制备TiO2-Fe2O3-CeO2复合纳米薄膜。以甲基橙为目标降解物,讨论过渡金属离子Fe3+和稀土金属离子Ce3+的掺杂对TiO2薄膜光催化活性的影响。采用SEM、XRD、EDS等表征手段对复合氧化物薄膜进行表征。结果表明:所制备的薄膜具有纳米结构;Fe3+、Ce3+单掺和Fe3+/Ce3+共掺均可提高TiO2薄膜的光催化性能,但相同条件下共掺离子的光催化活性更高。     

Metastable phase relation and phase equilibria in the Cr2O3-Fe2O3 system

In order to ascertain the metastable phase relation in the Cr₂O₃-Fe₂O₃ system, the existing phases were investigated by X-ray analysis using samples obtained by heating the coprecipitated powders for 1 h at 600–1000°C. There was a metastable two-phase region of Cr₂O₃-rich (CC) and Fe₂O₃-rich (FC) phases below about 940°C. Equilibrium state of 1:1 composition at 600–900°C was considered to be a single phase of the corundum solid solution. The metastable two-phase CC + FC region was suggested to appear probably due to the compositional inhomogeneity in the coprecipitated powders.     

Glass formation and immiscibility in the TeO2-B2O3-Fe2O3-MnO system

The glass formation in the quaternary TeO₂-B₂O₃-MnO-Fe₂O₃ system and in its ternary systems was investigated. A range of liquid immiscible phases, located near to the binary TeO₂-B₂O₃ and B₂O₃-MnO systems was established. Using transmission electron microscopy, a trend to metastable liquid-phase separation in the single-phase glasses, located near to the boundary of immiscibility was observed. With an increase in the Fe₂O₃ and MnO content still in the process of cooling of the melts, it was possible for a fine glassy crystalline structure to be formed in them. It was shown that by changing the upper limit of the melting temperature and the cooling rate, the glassy crystalline structure and the Fe₃O₄ content could be modified.     

Electrical behaviour of the MnO2-doped ZnO-Bi2O3 system

The electrical behaviour of xwt% MnO₂-(100-x) (95wtx% ZnO-5wt% Bi₂O₃) is investigated. Addition of MnO₂ retards grain growth in the ZnO-Bi₂O₃ system. It is observed that the MnO₂ dopant decreases the leakage current, while it increases the non-linearity coefficient and the grain boundary resistance of ZnO-Bi₂O₃ ceramics. In addition, the noise spectrum of MnO₂-doped ZnO-Bi₂O₃ deviates from 1/f behaviour. The deviation is, however, less enhanced for samples with more MnO₂ dopant.     

An infrared study of the Na2O-V2O5-Fe2O3 glass system

Structural studies of Na₂O-V₂O₅-Fe₂O₃ glasses have been made from their IR spectra which show that vibrational bands characteristic of the vanadium-oxygen bonds in V₂O₅ are maintained in these glasses, but the addition of Na₂O to these glasses results in a shifting of the higher frequency peaks towards lower wave number due to structural changes produced in V₂O₅. It is inferred that Na⁺ ions make bonds interstitially with isolated V=O bonds and VO₅ polyhedra are destroyed, resulting in the formation of VO₄ polyhedra through intermediate complexes. The variation of Fe₂O₃, however, produces an insignificant structural change in these glasses. The IR spectra of samples heated to their temperature of crystallization confirm the formation of a series of complexes with several isolated V=O bonds.     

纳米氧化铝的相变研究

以硫酸铝为原料,用沉淀法制取纳米氧化铝,研究了其相变过程。用XRD、SEM、AFM及IR等手段对不同温度下煅烧所得的产品进行了表征。结果表明,该方法制备的氧化铝粉体呈球形、团聚程度轻、粒度分布较均匀、γ相和δ相平均粒径20-30nm、α相平均粒径53nm,其物相变化次序为：非晶态Al2O3→γ-Al2O3→δ-A12O3→α-Al2O3。     

Crystallization behavior of amorphous solid solutions and phase sepration in the Cr2O3-Fe2O3 system

Fine particles of the amorphous Cr₂O₃-Fe₂O₂ solid solutions were prepared by dehydration of coprecipitated hydroxides and their crystallization behavior was studied by differential thermal analysis and X-ray diffraction. The peak temperature for crystallization attained a maximum at a composition near Fe₂O₃ content of about 60 mol % and the activation energy for crystallization attained a minimum at a composition near Fe₂O₃ content of about 50 mol % in this quasibinary system. Phase separation occurred in a range of Fe₂O₃ content from about 35 to 80 mol % in the corundum-type solid solutions heat treated at 600 °C for 2 h. Crystallization behavior was discussed briefly related with phase separation and diffusion in fine particles.     

Effect of heat treatment and irradiation on electrical conductivity and crystallization in the BaO-B2O3-Fe2O3 glass system

A glass system was prepared according to the formula 75 mol % B₂O₃-(25 –x) mol % BaO –x mol % Fe₂O₃, wherex = 0, 1, 2.5, 5, 7.5 and 10. The glasses were subjected to heat treatment at 550° C for 2, 6, 12, 18 and 24 h. The glasses were also irradiated using-rays at a dose of 4.805 × 10⁴ rad h^–1 for 12, 18 and 24 h. An X-ray diffraction technique was used to identify the separated crystalline phases. The electrical conductivity and activation energy of untreated, heat-treated and irradiated samples were measured and calculated. The rate and the dimensions of crystallization were also calculated by using the Avrami equation. It was found that-Fe₂O₃ is the separated phase when a sample containing 7.5 mol% Fe₂O₃ is heat treated for 24h;-Fe₂O₃ and Fe₂O₃ are the separated phases when the sample containing 10 mol% Fe₂O₃ is heat treated for 6, 12 and 18 h, with the addition of BaO when the sample is heat treated for 24 h. A miminum value for the electrical conductivity of glass samples was found to occur around an Fe₂O₃/BaO ratio of 0.425. The rate of crystallization in the sample containing 10 mol% Fe₂O₃ is 1.30607 × 10^–3 and the geometry of crystallizationn is 1.2238, which indicates that the crystallization was in one dimension.