2O3 Phases Research Articles

Effect of Co2+ion on the magnetic properties of sol–gel cobalt ferrite thin films

The effect of Co2+ ion on the magnetic properties of the sol–gel derived cobalt ferrite thin films (400 nm thickness) was studied. Films with different solution composition ([Co2+]/[Fe3+] = 0.5–1.0) were deposited on a Si substrate from the precursor containing an aqueous solution of FeCl3·6H2O and CoCl2·6H2O. Deposited films were heat treated at different temperatures (700–1100 °C) and were characterized by X-ray diffraction using Cu Kα radiation. The films containing a nonstoichiometric α-(Fe1−xCox)2O3 phase were obtained in addition to a small amount of CoO at relatively lower temperatures. With increasing heat treatment temperature, nonstoichiometric α-(Fe1−xCox)2O3 was decomposed to stoichiometric α-Fe2O3 and the CoFe2O4 phase was intensified. Only the films obtained from the solution enriched with Co showed the single phase of CoFe2O4, though the Co to Fe content ratio analyzed by inductive coupled plasma spectroscopy (ICP) and energy dispersive X-ray spectroscopy (EDX) was nearly consistent with that of the starting solutions. The study of surface morphology of the films by using field emission scanning electron microscope (FESEM) showed that films were homogeneously distributed throughout the surface of the substrate and the grain sizes were in the range of 60 to 140 nm. Magnetic properties of the films, including the Curie temperature, were studied by using a vibrating sample magnetometer (VSM) and conversion electron Mossbauer spectroscopy (CEMS). The Curie temperature and the magnetic hyperfine fields of the films slightly decreased with increasing molar ratio of the Co2+ and Fe3+ ions while the saturation magnetization reached a maximum value with the film composition of x = 0.28.

Phase transformation of Er3+-doped Al2O3 powders prepared by the sol-gel method

Er3+-doped Al2O3 powders were prepared by the sol-gel method, using aluminum isopropoxide [Al(OC3H7)3]-derived Al2O3 sols. The phase content diagram for the Er-doped Al–O system with the doping concentration up to 20 mol% is presented for sintering temperature from 550 to 1250 °C. The different crystalline types of (Al,Er)2O3 phases—γ, θ, α—and two Er-Al-O phases—ErAlO3 and Al10Er6O24—were detected. The Er3+ doping in the Al2O3 phases has a significant effect on phase transformations for γ → θ and θ → α, indicating the enhancement for stability of the γ and θ phases. Compared with the γ–Al2O3 phase, the γ–(Al,Er)2O3 phase can be obtained at high temperatures up to about 1100 °C. The θ–(Al,Er)2O3 phase, together with the α–(Al,Er)2O3 phase, was still observed at 1200 °C under the high Er3+ doping concentration. The Er3+ doping suppressed the crystallization of the γ and θ phases. At a low Er3+ doping concentration from about 2 to 5 mol%, the ErAlO3 phase precipitated out around 1100 °C. The Al10Er6O24 phase was observed at all sintering temperatures; the precipitation was carried out to all Er3+ doping concentrations with increasing sintering temperatures up to about 1200 °C.

The influence of thermal treatment on phase development in ZrO 2–Fe 2O 3 and HfO 2–Fe 2O 3 systems

The effects of thermal treatment (500, 600, 800 and 1100°C) of the amorphous precursors of ZrO 2–Fe 2O 3 and HfO 2–Fe 2O 3 systems, coprecipitated from aqueous solutions of the corresponding salts, were studied at room temperature (RT) by X-ray powder diffraction (XRD) and, in some cases, by laser Raman spectroscopy. The incorporation of Fe 3+ ions partially stabilized the high temperature polymorphs of zirconia (ZrO 2) and hafnia (HfO 2). The stabilization of the cubic polymorphs of ZrO 2 and HfO 2 occurred in the solid solutions with an Fe 2O 3 content of ≥10 mol%. The presence of Fe 3+ ions above their solid solubility limit caused the transition of metastable polymorphs of ZrO 2 and HfO 2 to monoclinic polymorphs. The terminal solid solubility limit of Fe 2O 3 in ZrO 2, as estimated at RT, was ∼33 mol% after calcination at 600°C, ∼9 mol% after calcination at 800°C and ∼2 mol% after calcination at 1100°C. After the same thermal treatment, the terminal solid solubility limits of Fe 2O 3 in HfO 2 proved to be very similar only a little smaller. The unit-cell volume of the cubic polymorphs of ZrO 2 and HfO 2, determined by powder-pattern-fitting methods, decreased linearly with the increase in iron content, but the rate of decrease was greater in the ZrO 2 polymorph. The thermal behavior of the amorphous precursors was also followed by differential thermal analysis and thermogravimetric analysis. The crystallization temperature of both systems increased with an increase in the Fe 2O 3 content, from 405 to 730°C in the ZrO 2–Fe 2O 3 system, and from 520 to 720°C in the HfO 2–Fe 2O 3 system. The results indicated that the amorphous precursors with more than 30 mol% of Fe 2O 3 were single co-gels. The status of the Fe 3+ ions in the ZrO 2-type lattice and the process of their segregation into the α-Fe 2O 3-phase were examined by 57Fe Mössbauer spectroscopy. Significantly lower values of the hyperfine magnetic fields compared to that of α-Fe 2O 3 indicated the incorporation of Zr 4+ ions into the α-Fe 2O 3 lattice, the amount of which decreased with the increasing temperature of treatment.

Effect of extended annealing cycles on the thermal conductivity of AlN/Y 2O 3 ceramics

The effect of extended annealing cycles (up to 50 h at 1800°C) on the thermal conductivity of polycrystalline AlN, doped with 5 wt% Y 2O 3, has been studied. The microstructural evolution upon annealing has also been characterized in detail, using quantitative scanning electron microscopy (SEM) observation and energy dispersive X-ray analysis (EDX). As-sintered AlN/Y 2O 3 composites typically contained a dilute yttrium aluminate secondary phase well distributed and completely wetting the AlN grains. Upon annealing, the AlN matrix grains isotropically grew, while the grain-boundary yttrium aluminate phase tended to segregate to triple grain junctions. This segregation process produced a collapse of the grain-boundary film thickness, thus resulting in a completely different AlN microstructure dispersed with isolated yttrium aluminate grains. Equilibrium of the microstructural morphology was achieved after annealing times in the interval 5–10 h. As a consequence of microstructural changes, the thermal conductivity of the annealed AlN polycrystal exceeded that of the as-sintered material. A discussion is given about the variation of thermal properties in terms of both segregation to the triple-grain junctions of the intergranular Y 2O 3-phase and grain-growth of the bulk AlN grains.

Towards Microwave Absorbing Metal-Oxide Nanocomposites: The Problem of Metal/Matrix Reaction

Thermostable microwave absorbing materials are of considerable interest for the applications with the tailored radar cross-section. Nanocomposites metal (Co, Fe)-dielectric (Al2O3, Al2O32SiO2, 3Al2O32SiO2) prepared by sol-gel route have been studied by electron microscopy and 57Fe Mossbauer spectroscopy. The relationships between the micro/meso porosity, the composition and the state of the matrix precursor and the metal particle size are discussed with emphasis on the oxidation of nanoparticles. The size are discussed with emphasis on the oxidation of nanoparticles. Fe-based nanocomposites are obtained only using microporous host matrices. The interfacial reaction between iron nitrate solution precursor and host matrix promotes the formation of γ-(Fe1-XAl9)2O3 phase. The metal content is maximized by the use of highly concentrate iron solution and/or mixing with cobalt nitrate, in which case alloying particles are obtained.

Effect of hydrogen ion implantation on the stability of ODS ferritic steel

The effect of hydrogen ion implantation on the stability of ODS ferritic steel was studied by means of ion implantation at room temperature and in situ observation in a low voltage electron microscope. The energy and the fluence of hydrogen ion were 50 keV and 1 × 1021ions/m2, respectively. The results indicated that a certain amount of (Fe, Cr)2O3 phases existed in the original ODS alloy. They did not change observably after hydrogen ion implantation at room temperature. During heating the specimen implanted with hydrogen ions, however, the (Fe, Cr)203 phase was dissolved gradually and the non-equilibrium segregation of Cr and C occurred, resulting in the precipitation of Cr carbides. On the contrary, the microstructure of the specimen which was not implanted with hydrogen ions did not change obviously during heating. Neither was (Fe, Cr)203 phase dissolved, nor was new phase precipitated.

Study of the Ti/Al2O3 interface

The Ti/Al2O3 (1 ¯1 0 2) interface formation has been investigated by X-ray photoelectron spectroscopy and Auger electron spectroscopy (AES). The results showed that when an active metal titanium was evaporated on to a room-temperature Al2O3 (1 ¯ 1 0 2) surface in ultrahigh vaccum, a Ti/Al2O3 interface region of about 200 nm was formed, and in the first several monolayers of titanium, the titanium was oxidized due to the active oxygen anions on the surface. Therefore, the pure Ti/Al2O3 interface was replaced gradually by a titanium oxides/Al2O3 interface, which has a stronger interaction than the former. The change of shape of the photoemission lines and the shift of binding energy of aluminium, oxygen and titanium with increasing coverage of titanium showed that the formation of the Ti-O bond at the interface is due to titanium transferring its electrons to Al3+ via O2− anions in the Al-O bond, whereby the Al3+ was reduced to metallic aluminium, Al0. The AES intensity profile also proved the existence of the reduced species Al0. This suggests that the reaction layer consists of a multiphasic mixture: the Ti-O type phase, the (Ti, Al)2O3 phase and metallic aluminium phase.

X-ray diffraction and Mössbauer spectra of the system Fe2O3-SnO2

The influence of gallium substitution on the chemical and structural properties of haematite, α-Fe2O3, has been studied using X-ray diffraction and57Fe Mossbauer spectroscopy. The presence of only α-(Ga x Fe1−x)2O3 phase is detected for the compositions withx between 0.01 and 0.90. A gradual decrease of the unit-cell parameters of α-(Ga x Fe1−x)2O3 with the increase of gallium substitution is measured.57Fe Mossbauer spectra showed that the value of the magnetic hyperfine field of pure α-Fe2O3 decreases with increasing gallium for iron substitution. The hyperfine magnetic structure, which is observed for α-(Ga x Fe1−x)2O3 at room temperature, collapsed for the composition withx≃0.50. The changes in the57Fe Mossbauer spectra of the α-(Ga x Fe1−x)2O3 phase are discussed in the sense of the electronic relaxation and the superparamagnetic effects.

Sublimation of oxygen and lithium from Li 0·5Fe 2·5O 4 during heat-treatment

In the heat-treatment of spinel type lithium ferrite, Li 0·5Fe 2·5O 4, the sublimation of lithium and oxygen and the precipitation of second phases were investigated. In the heat-treatment in an oxygen atmosphere, the sublimation of lithium and oxygen became measurable at 850° ∼ 900°C and increased with increasing temperature. The amount of lithium sublimed was twice that of oxygen in atomic ratio, and the oxygen content of the resulting spinel phase was necessarily increased. In a nitrogen atmosphere, however, more lithium was sublimed than in oxygen atmosphere, the amount being less than twice that of oxygen, and the comparatively large amount of precipitation of LiFeO 2-phase was observed. The precipitation of α-Fe 2O 3-phase was also observed by heat-treating at temperatures of several hundred degrees, to a greater extent in oxygen than in nitrogen, but this process was considered to occur independently of the sublimation reaction of lithium and oxygen at much higher temperatures.