Effect of composition on rheological behavior of iron oxides produced by hydrothermal method

The aim of the present work was to investigate the rheological properties of different iron oxides (Fe₃O₄, NiFe₂O₄, ZnFe₂O₄ and Ni_0.5Zn_0.5Fe₂O₄) aqueous suspensions. The oxides were produced through mixing the respective metallic sulfates within a closed isothermal reactor at 100 °C and at pH ≈12, in an oxidant environment (provided by H₂O₂ 0.63% w/v). The reactor was coupled with an adequate real-time data (RTD) acquisition system enabling measurement of temperature, pH and pressure. Obtained RTD data showed that once the isothermal conditions are reached, the pressure slowly decreases over time, which is a result of O₂ consumption through oxidation of Fe²⁺ to Fe³⁺. To characterize the suspensions as a function of temperature and shear rate, the steady rheology was used. The results revealed that the effect of temperature on viscosity of all suspensions was insignificant while steady rheology showed pseudoplastic behavior for all ferrites. The magnitude of viscosity and pseudoplasticity turned out to be in agreement with the hydrodynamic diameters of particles complying with the order: NiFe₂O₄>Fe₃O₄>Ni_0.5Zn_0.5Fe₂O₄>ZnFe₂O₄. Finally, the rheological behavior of suspensions was attributed to the concentration of OH groups on the surface of particles and this hypothesis was effectively supported by DRX, FTIR and TGA/DTA measurements.     

Corundum ceramics based on aluminum oxide obtained by a plasma chemical method

Problems of sintering aluminum oxide consisting of highly disperse α-Al₂O₃ prepared by the plasma chemical method are considered. It is shown that this material possesses high sinterability even in the case of considerable pressing pressures. The introduction of MgO improves sintering of plasma chemical aluminum oxide and the addition of partially stabilized zirconia increases the ultimate strength of the ceramics in three-point bending to 450 MPa.     

Electrochemical synthesis and the physicochemical properties of a nanodispersed system based on iron and aluminum oxides

An Al₂O₃–Fe₂O₃ dispersed oxide system has been obtained electrochemically in aqueous solutions containing chloride ions with the subsequent thermal treatment. The phase and elemental composition and structural characteristics of the samples have been studied using X-ray phase analysis, Mössbauer spectroscopy, and scanning electron microscopy. The effect of the electrolysis mode and thermal treatment on the characteristics of the samples which were synthesized is shown. It is found that a high-temperature treatment (1100°C) facilitates the formation of a complex oxide material whose phase composition is a combination of the α-Al₂O₃ (corundum) and α-Fe₂O₃ (hematite) crystalline phases and whose particles have nanometer sizes.     

Influence of fuel on phase formation,morphology, electric and dielectric properties of iron oxides obtained by SCS method

Nanopowders of iron oxides were obtained by Solution Combustion Synthesis (SCS) method from sol-gel compositions containing iron nitrates and soluble organic reducing agents (glycine, urea, citric acid). The synthesis processes and their intensity depending on the type of fuel and fuel/oxidizer ratio (?) were investigated. It was established that the combustion regime affects the phase formation of the obtained powders, their morphology, the color of the final Fe₂O₃ powders, dielectric properties and etc. It was shown that iron oxides with a preferred morphology and high dielectric properties (ε = 44.5 at ? = 0.6 vs ε = 4.0 at ? = 1.4 using urea as fuel) could be produced by the SCS method.     

水热法制备纳米氧化物的研究进展

简述了水热法的原理和特点.介绍了水热晶化法、水热氧化法、水热还原法、水热沉淀法、水热分解法、水热合成法制备纳米氧化物的特点和现状,并介绍了水热法与其他方法的联合应用,如:微波-水热法、微乳液-水热法、溶胶(凝胶)-水热法等制备纳米氧化物的研究进展.最后对水热法制备纳米氧化物进行了展望.     

Tunable morphology of aluminum oxide whiskers grown by hydrothermal method

Hydrothermal method was used to grow α-Al₂O₃ whiskers by using hydrated aluminum sulfate, urea and poly ethylene glycol as precursors. X-ray diffraction (XRD), selected area electron diffraction (SAED), and high resolution transmission electron microscope (HRTEM) were used to characterize morphology of the whiskers. By increasing the pH of the solution (by adding extra NaOH), by adjusting calcination times and atmospheres it was possible to tune the whiskers morphology and their aspect ratio. Aspect ratio as high as 25 was obtained after hydrothermal treatment of a solution having pH 3 followed by calcination in Ar or N₂ atmosphere at 1200?°C for 6 h.     

Synthesis and characterization of monetite and hydroxyapatite whiskers obtained by a hydrothermal method

High temperature hydrothermal syntheses, using calcium nitrate tetrahydrate, sodium dihydrogen phosphate and urea as precursors, and characterization of hydroxyapatite (HAp) whiskers are reported herein. The morphology and chemical composition of the crystals from a monetite to a hydroxyapatite phase were controlled by varying the starting concentrations of the precursors and the solution pH through the amount of urea that is decomposed during heating. X-ray diffraction (XRD) analysis, infrared spectroscopy (IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were used to investigate the products of the syntheses in order to find the optimum reaction conditions for obtaining the desired morphology and phase composition. Different morphologies ranging from single crystals of monetite through rods and plates of hydroxyapatite with different size distribution to whisker-like single hydroxyapatite crystal were achieved by simply varying the starting concentration of urea. Structural refinement of the hydroxyapatite whiskers confirmed a strong preferential orientation along the c-axis direction of the hexagonal crystal structure, which was significantly different from the usually observed random crystal orientation. TEM and SEM analysis of the apatite whiskers confirmed single crystal structure with the a c-axis orientation parallel to the long axis of the whiskers, with sizes up to 150 μm in length, 10 μm in width and with a thickness of about 300 nm, that grew from the same centre of nucleation, forming flaky-like particles.     

Effect of iron on the microstructure of basalt glass-ceramics obtained by the petrurgic method

Basalt glasses may form glass-ceramic materials upon a thermal treatment (TT) if the Fe₂O₃/FeO ratio is greater than 0.5. In this work, a Serra Geral Formation basalt was used to produce different glass-ceramic materials by changing its chemical composition, removing Fe₂O₃ via magnetic separation and adding 20 wt% Fe₂O₃. Glass-ceramics were prepared by melting at 1500°C, followed by a crystallization plateau at 950°C. This petrurgic TT yields dendritic structures upon surface crystallization. With the increase of the iron content, crystallization was more pronounced, as observed in thermal analysis results and optical/scanning electron microscopy. The microstructure analysis shows that these glass-ceramics exhibit at least two different types of crystallization. On the surface, the predominant crystallization was dendritic, while in bulk, crystallization occurred in the form of randomly distributed submicrometric spheroid crystals.     

Probing structure of ytterbium stabilized Pr-doped zirconia obtained by microwave hydrothermal method

Nanoparticles of ZrO₂:Pr stabilized with ytterbium in the range 0.5%mol.-20%mol. were synthesized. Crystallite sizes depended on Yb concentration. Red luminescence resulting from ¹D₂→³H₄ transition of Pr³⁺ is present in all the samples. Additionally in the samples calcined at 1200 °C blue-green luminescence from ³P₀ level of Pr³⁺ was found. This emission was more intense than red one. Intensity of the ³P₀→³H₄ luminescence depended on ytterbium content and was found to be the highest at 20% mol. Yb. Luminescence study had shown, that ytterbium introduced additional lattice defects. Some quantity of praseodymium activator ions were found to be present at +4 state and changed its valence to +3 as ytterbium content had risen.     

Role of iron and aluminum oxides as fluxes during the burning of Portlamd cement

Phase equilibrium data show that A?₂O₃ and Fe₂O₃ act as fluxes during the burning of Portland cements. In principle, interactions between the four major components of cement can be determined directly from the relevant phase diagrams but in practice, supplementary constructions are almost essential to show the quantitative relationship between temperature, chemical composition and the amount of liquid formed. The concept of ‘clinkering diagrams’ is developed and applied. The principles underlying the selection of fluxes to assist clinkering reactions are discussed.     

Composite materials based on sitall glass and technical aluminum and titanium oxides

The possibility of fabrication of glass-ceramic composite materials based on sitall glass of the SiO₂-TiO₂-Al2O₃-CaO-BaO-B₂O₃ nonalkaline system and fillers-powdered industrial oxides with low sintering and crystallization temperatures and high wear resistance — was investigated. Industrial aluminum and titanium oxides were used as the fillers.     

Combustion of highly exothermic mixtures based on iron and chromium oxides

The combustion of highly exothermic heterogeneous mixtures containing metal oxides (FeO, Cr₂O₃) and metallic (Al) or nonmetallic (C) reducing agents has been studied under dynamic conditions. A relation between the temperature of ignition and the ratio of the components of the mixture has been established. The ignition temperature was compared with the theoretical value derived from the quasisteady theory of ignition of condensed systems of hot material with a high thermal conductivity and a variable surface temperature. The theoretical values were in agreement with the experimental results.Chernogolovka. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 4, pp. 46–48, July–August, 1993.     

Perovskite nanostructures obtained by a hydrothermal electrochemical process

Increasing the miniaturisation is a key factor in the manufacturing process of the new, high-performances electronic devices. Thin/thick films represent a solution for the miniaturisation and the films can be obtained by different techniques such as RF sputtering, vacuum evaporation, laser ablation, CVD, sol–gel processing, hydrothermal electrochemical procedures. The aim of the paper is to investigate the hydrothermal electrochemical synthesis of perovskite nanostructured thin films. The influence of the synthesis parameters (pH, substrate, current density, temperature) on the composition, morphology and microstructure of the films was studied.     

I. Adsorption mechanism of chlorophenols on iron oxides, titanium oxide and aluminum oxide as detected by infrared spectroscopy

The adsorption of 2-chlorophenol, 2,3- and 2,4-dichlorophenols and 2,4,6-trichlorophenol in liquid and gas phase on iron, titanium and aluminum oxides seem to proceed in a similar way. Higher adsorption of chlorophenols either from gas phase or from aqueous solution was observed on -Fe₂O₃ than on -FeOOH. The low adsorption of chlorophenols from aqueous solution on oxide surfaces suggests that hydrophobic chlorophenols cannot effectively compete with water for the absorption on hydrophilic oxide surface sites. The adsorption of chlorophenols on iron, titanium and aluminum oxides was followed by the adsorption isotherm, HPLC and diffuse reflectance FT-IR (DRIFT) spectroscopy. The adsorption of the chlorophenols on the oxides under study is related to the amount of interfacial water content on the iron oxide. The alumina–chlorophenolate surface complex was found to be weak when compared with either the iron or titanium analogs as seen by the C---O stretching vibrations, leading to a lower adsorption on alumina than on iron and titanium oxides.     

A highly refractory material based on zirconia stabilized with yttrium and aluminum oxides

Conclusions With combined additives (Y₂O₃ + Al₂O₃), zirconium dioxide forms solid solutions which resist thermal decomposition.A material was synthesized containing from 90–93 mol. % ZrO₂, from 3.5 to 5 mol.% Y₂O₃, and from 3.5 to 5% mol Al₂O₃, possessing an average coefficient of thermal expansion which is lower, and a thermalshock resistance which is higher, than the double solid solutions in the system ZrO₂-Y₂O₃ and ZrO₂-CaO.Translated from Ogneupory, No. 4, pp. 42–45, April, 1973.     

X-ray photoelectron spectroscopy of nitrogen oxides adsorbed on iron oxides

Adsorption of N₂O, NO, and NO₂ on Fe₂O₃ and Fe₃O₄ was studied by means of X-ray photoelectron spectroscopy (XPS) using an AEI ES 100 spectrometer at temperatures from ?100 to +250 °C. Adsorption isotherms and isobars of NO and NO₂ have been plotted for Fe₃O₄ and Fe₂O₃. Isotherms are Langmuir curves, and the adsorption isobar of nitrogen dioxide adsorbed on Fe₂O₃ shows a maximum at room temperature. No adsorption of nitrogen monoxide is detected with Fe₂O₃ (the detection limit is about 0.1 monolayer). With adsorbed nitrous oxide a simple peak is recorded in contrast to what is observed in the vapor phase. The N 1s binding energy of adsorbed NO is lower than that of the gaseous species, suggesting that significant electron transfer from the iron oxides to the adsorbed nitrogen oxide occurs; however, this back donation is not observed with adsorbed nitrogen dioxide. Adsorption does not cause changes in the binding energies of Fe₂O₃ core electrons, but shifts the Fe 3p line of Fe₃O₄ toward the higher binding energies, suggesting that Fe₃O₄ is oxidized by the adsorption of nitrogen oxides.