共查询到20条相似文献,搜索用时 15 毫秒
1.
The phase transformation of strontium hexagonal ferrite (SrFe12O19) to magnetite (Fe3O4) as main phase and strontium carbonate (SrCO3) as secondary phase is reported here. SrFe12O19 powder was obtained by a heat treatment at 250 °C under controlled oxygen flow. It was observed that the phase transformation occurred when the SrFe12O19 ferrite was heated up to 625 °C in confinement conditions. This transformation took place by a combination of three factors: the presence of stresses in the crystal lattice of SrFe12O19 due to a low synthesis temperature, the reduction of Fe3+ to Fe2+ during the heating up to 625 °C, and the similarity of the coordination spheres of the iron atoms present in the S-block of SrFe12O19 and Fe3O4. X-ray diffraction analysis confirmed the existence of strain and crystal deformation in SrFe12O19 and the absence of them in the material after the phase transformation. Dispersive X-ray absorption spectroscopy and Fe57 Mössbauer spectroscopy provided evidences of the reduction of Fe3+ to Fe2+ in the SrFe12O19 crystal. 相似文献
2.
《Journal of magnetism and magnetic materials》2001,234(3):366-370
Strontium hexaferrite nanoparticles are prepared by the chemical sol–gel route. Specific saturation magnetization σs and coercive field strength Hc are determined depending on the heat treatment of the gel and iron/strontium ratio in the starting solution. These ultrafine powders with single-domain behavior have specific saturation magnetization σs=74 emu/g and coercive field strength Hc=6.4 kOe. Experimental results show that it is necessary to preheat the gel between 400 and 500°C for several hours . It can prevent the formation of intermediate γ-Fe2O3 and help to obtain ultrafine strontium ferrite single phase with narrow size distribution at a low annealing temperature. Additionally, the magnetic properties of sol–gel derived strontium ferrite with iron substituted by Zn2+, Ti4+ and Ir4+ are discussed. For an amount of substitution 0<x⩽0.6, the (Zn, Ti)x substituted strontium ferrite shows higher values of both coercive field strength and saturation magnetization than the (Zn, Ir)x substituted phase. 相似文献
3.
Preparing M-type barium hexaferrite and improving the magnetic response of natural ferrites by incorporating barium carbonate (BaCO3) is ever-demanding. Series of barium carbonate doped ferrites with composition (100−x)Fe3O4·xBaCO3 (x=0, 10, 20, 30 wt%) are prepared through solid state reaction method and sintered gradually at temperatures of 800 and 1000 °C. Nanoparticles of natural ferrite and commercial BaCO3 are used as raw materials. Impacts of BaCO3 on structural and magnetic properties of these synthesized ferrites are inspected. The obtained ferrites are characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) at room temperature. Uniform barium hexaferrite particles in terms of both morphology and size are not achieved. The average crystallite size of BaFe12O19 is observed to be within 30–600 nm. The sintering process results phase transformation from Fe3O4 (magnetite) to α-Fe2O3 (hematite) and the formation of hexagonal barium ferrite crystals. The occurrence of barium crystal is found to enhance with the increase of BaCO3 concentrations up to 20 wt% and suddenly drop at 30 wt%. Saturation and remanent magnetization of the doped ferrites are significantly augmented up to 16.37 and 8.92 emu g−1, respectively compared to their pure counterpart. Furthermore, the coercivity field is slightly decreased as BaCO3 concentrations are increased. BaCO3 mediated improvements in the magnetic response of natural ferrites are demonstrated. 相似文献
4.
《Solid State Ionics》2006,177(13-14):1199-1204
Perovskite oxides of the composition BaxSr1−xCo1−yFeyO3−δ(BSCF) were synthesized via a modified Pechini method and characterized by X-ray diffraction, dilatometry and thermogravimetry. Investigations revealed that single-phase perovskites with cubic structure can be obtained for x ≤ 0.6 and 0.2 ≤ y ≤ 1.0. The as-synthesized BSCF powders can be sintered in several hours to nearly full density at temperatures of over 1180 °C. Thermal expansion curves of dense BSCF samples show nonlinear behavior with sudden increase in thermal expansion rate between about 500 °C and 650 °C, due mainly to the loss of lattice oxygen caused by the reduction of Co4+ and Fe4+ to lower valence states. Thermal expansion coefficients (TECs) of BSCF were measured to be 19.2–22.9 × 10− 6 K− 1 between 25 °C and 850 °C. Investigations showed further that Ba0.5Sr0.5Co0.8Fe0.2O3−δ is chemically compatible with 8YSZ and 20GDC for temperatures up to 800 °C, above which severe reactions were detected. After being heat-treated with 8YSZ or 20GDC for 5 h above 1000 °C, Ba0.5Sr0.5Co0.8Fe0.2O3−δ was completely converted to phases like SrCoO3−δ, BaCeO3, BaZrO3, etc. 相似文献
5.
Maike Lübbe Alexander M. Gigler Robert W. Stark Wolfgang Moritz 《Surface science》2010,604(7-8):679-685
We report on the identification of Fe3O4 (magnetite) and α-Fe2O3 (hematite) in iron oxide thin films grown on α-Al2O3(0 0 0 1) by evaporation of Fe in an O2-atmosphere with a thickness of a few unit cells. The phases were observed by Raman spectroscopy and confirmed by X-ray diffraction (XRD). Magnetite appeared independently from the substrate temperature and could not be completely removed by post-annealing in an oxygen atmosphere as observed by X-ray diffraction. In the temperature range between 400 °C and 500 °C the X-ray diffraction shows that predominantly hematite is formed, the Raman spectrum shows a mixture of magnetite and hematite. At both lower and higher substrate temperatures (300 °C and 600 °C) only magnetite was observed. After post-annealing in an O2-atmosphere of 5 × 10?5 mbar only hematite was detectable in the Raman spectrum. 相似文献
6.
Nan Wang Lihua Zhu Dali Wang Mingqiong Wang Zhifen Lin Heqing Tang 《Ultrasonics sonochemistry》2010,17(3):526-533
Fe3O4 magnetic nanoparticles (Fe3O4 MNPs) with much improved peroxidase-like activity were successfully prepared through an advanced reverse co-precipitation method under the assistance of ultrasound irradiation. The characterizations with XRD, BET and SEM indicated that the ultrasound irradiation in the preparation induced the production of Fe3O4 MNPs possessing smaller particle sizes (16.5 nm), greater BET surface area (82.5 m2 g?1) and much higher dispersibility in water. The particle sizes, BET surface area, chemical composition and then catalytic property of the Fe3O4 MNPs could be tailored by adjusting the initial concentration of ammonia water and the molar ratio of Fe2+/Fe3+ during the preparation process. The H2O2-activating ability of Fe3O4 MNPs was evaluated by using Rhodamine B (RhB) as a model compound of organic pollutants to be degraded. At pH 5.4 and temperature 40 °C, the sonochemically synthesized Fe3O4 MNPs were observed to be able to activate H2O2 and remove ca. 90% of RhB (0.02 mmol L?1) in 60 min with a apparent rate constant of 0.034 min?1 for the RhB degradation, being 12.6 folds of that (0.0027 min?1) over the Fe3O4 MNPs prepared via a conventional reverse co-precipitation method. The mechanisms of the peroxidase-like catalysis with Fe3O4 MNPs were discussed to develop more efficient novel catalysts. 相似文献
7.
《Ultrasonics sonochemistry》2014,21(4):1366-1373
Porous (Ce0.5Zr0.5)O2 solid solutions were prepared by thermolysis (T = 285 °C) or sonolysis (20 kHz, I = 32 W cm−2, Pac = 0.46 W mL−1, T = 200 °C) of Ce(III) and Zr(IV) acetylacetonates in oleylamine or hexadecylamine under argon followed by heat treatment of the precipitates obtained in air at 450 °C. Transmission Electron Microscopy images of the samples show nanoparticles of ca. 4–6 nm for the two synthetic approaches. The powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray and μ-Raman spectroscopy of solids obtained after heat treatment indicate the formation of (Ce0.5Zr0.5)O2 solid solutions with a metastable tetragonal crystal structure for the two synthetic routes. The specific surface area of the samples varies between 78 and 149 m2 g−1 depending on synthesis conditions. The use of Barrett–Joyner–Halenda and t-plot methods reveal the formation of mixed oxides with a hybrid morphology that combines mesoporosity and microporosity regardless of the method of preparation. Platinum nanoparticles were deposited on the surface of the mixed oxides by sonochemical reduction of Pt(IV). It was found that the materials prepared by sonochemistry exhibit better resistance to dissolution during the deposition process of platinum. X-ray photoelectron spectroscopy analysis shows the presence of Pt(0) and Pt(II) on the surface of mixed oxides. Porous (Ce0.5Zr0.5)O2 mixed oxides loaded with 1.5 %wt. platinum exhibit high activity in catalytic wet air oxidation of formic acid at 40 °C. 相似文献
8.
《Journal of Electron Spectroscopy and Related Phenomena》1998,97(3):225-234
Stoichiometric Ni-bearing ferrite was formed by air oxidation of an iron(II) hydroxide suspension at an initial Ni : Fetot mol ratio (rNi) of 0.20 : 2.80 at pH 10.0 and 65°C. Most of products formed at rNi=0.40 : 2.60 and 0.60 : 2.40 were Ni-bearing ferrites, of which vacancies of Fe3+ ion on the lattice points may be considered. Only Ni, Zn-bearing ferrites were formed in the suspensions at initial (Ni + Zn) : Fetot mol ratios (rNi + Zn) of 0.20 : 2.80–0.60 : 2.40 at pH 10.0 and 65°C. At higher rNi or rNi + Zn by-products containing Ni, Fe and O42− were formed. The formation of the by-products was depressed in the suspensions containing chloride ions in the place of sulfate ions. 相似文献
9.
《Ultrasonics sonochemistry》2014,21(4):1358-1365
Ultrasonic-assisted heterogeneous Fenton reaction was used for degradation of nitrobenzene (NB) at neutral pH conditions. Nano-sized oxides of α-Fe2O3 and CuO were prepared, characterized and tested in degradation of NB (10 mg L−1) under sonication of 20 kHz at 25 °C. Complete degradation of NB was effected at pH 7 in presence of 10 mM H2O2 after 10 min of sonication in presence of α-Fe2O3 (1.0 g L−1), (k = 0.58 min−1) and after 25 min in case of CuO (k = 0.126 min−1). α-Fe2O3 showed also effective degradation under the conditions of 0.1 g L−1 oxide and 5.0 mM of H2O2, even though with a lower rate constant (0.346 min−1). Sonication plays a major role in enhancing the production of hydroxyl radicals in presence of solid oxides. Hydroxyl radicals-degradation pathway is suggested and adopted to explain the differences noted in rate constants recorded on using different oxides. 相似文献
10.
Mohammad Ghaffari Mark Shannon H. Hui Ooi Kiang Tan Ahmad Irannejad 《Surface science》2012,606(5-6):670-677
In this report, SrTi(1 ? x)Fe(x)O(3 ? δ) photocatalyst powder was synthesized by a high temperature solid state reaction method. The morphology, crystalline structures of obtained samples, was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM), respectively. The electronic properties and local structure of the perovskite STFx (0 ≤ x ≤ 1) systems have been probed by extended X-ray absorption fine structure (EXAFS) spectroscopy. The effects of iron doping level x (x = 0–1) on the crystal structure and chemical state of the STFx have been investigated by X-ray photoelectron spectroscopy and the valence band edges for electronic band gaps were obtained for STFx by ultraviolet photoelectron spectroscopy (UPS). A single cubic perovskite phase of STFx oxide was successfully obtained at 1200 °C for 24 h by the solid state reaction method. The XPS results showed that the iron present in the STFx perovskite structure is composed of a mixture of Fe3+ and Fe4+ (SrTi(1 ? x)[Fe3+, Fe4+](x)O(3 ? δ)). When the content x of iron doping was increased, the amount of Fe3+ and Fe4+ increased significantly and the oxygen lattice decreased on the surface of STFx oxide. The UPS data has confirmed that with more substitution of iron, the position of the valence band decreased. 相似文献
11.
Michael Bowker Graham Hutchings Philip R. Davies Dyfan Edwards Robert Davies Shamil Shaikhutdinov Hans-Joachim Freund 《Surface science》2012,606(21-22):1594-1599
The surface structure of γ-Fe2O3(111) has been investigated with a range of surface techniques. Two different surface structures were discovered depending upon surface preparation techniques. Sputtering followed by annealing in vacuum produced a reduced surface characterised by a (2 × 2) LEED pattern, whereas sputtering followed by annealing in 1 × 10? 6 mbar oxygen produced a surface characterised by a (√3 × √3)-R30° LEED pattern. The latter appears to be a very low conductivity surface, whereas the former has the band gap expected for maghemite (~ 2.0 eV). We propose that the reduced surface is a magnetite-like layer, whereas the oxidised surface is an Fe2O3-like layer. 相似文献
12.
Study on the optically linear birefringence and dichroism properties of Fe3O4 magnetic nanoparticles
We adopt an improved co-precipitation method to prepare the Fe3O4 magnetic nanoparticles (MNPs). Influence factors such as the reaction temperature, the pH value of the solution, and the Fe3+/Fe2+ molar concentration are considered. Via the transmission electronic microscope and X-ray diffractometry, we characterize the dispersibility and size of the products. The reaction temperature and the pH value of the solution have a great influence in the dispersibility and size of MNPs. The diameter of Fe3O4 MNP, produced under Fe3+/Fe2+ molar concentration of 0.25 mole/l and molar ratio of 1.9:1, the reaction temperature is 80 °C, and the pH value reaches 9, is close to 11 nm. Above all, considering the variation of molar concentrations in Fe3+/Fe2+, the linear birefringence and dichroism of the kerosene-based ferrofluids are investigated by a Stokes polarimeter. 相似文献
13.
Fe3O4 nanoparticles and thin films were prepared on the Au(1 1 1) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Fe3O4 was formed by annealing α-Fe2O3(0 0 0 1) structures on Au(1 1 1) at 750 K in ultrahigh vacuum (UHV) for 60 min. Transformation of the α-Fe2O3(0 0 0 1) structures into Fe3O4 nanoparticles and thin films was supported by XPS. STM images show that during the growth procedure used, Fe3O4 initially appears as nanoparticles at low coverages, and forms thin films at ~2 monolayer equivalents (MLE) of iron. Two types of ordered superstructures were observed on the Fe3O4 particles with periodicities of ~50 and ~42 Å, respectively. As the Fe3O4 particles form more continuous films, the ~50 Å feature was the predominant superstructure observed. The Fe3O4 structures at all coverages show a hexagonal unit cell with a ~3 Å periodicity in the atomically resolved STM images. 相似文献
14.
We report on the preparation of Eu2+-doped BaSi2O5 glass-ceramics by crystallizing an Eu3+-doped barium-silicate glass at temperatures in the range from 750 to 1100 °C. Single phase BaSi2O5 glass ceramics can be obtained by thermal annealing at temperatures of about 950 °C. The luminescence intensity of Eu2+ increases dramatically if monoclinic BaSi2O5 is formed. Monoclinic Eu2+:BaSi2O5 shows efficient, broad band luminescence between 450 and 550 nm by excitation in the near UV. Annealing at temperatures >1000 °C leads to orthorhombic BaSi2O5 with much smaller Eu2+ luminescence. Static and time-resolved luminescence measurements indicate that Eu2+ ions are incorporated into the BaSi2O5 crystallites while Eu3+ ions remain in the amorphous phase. 相似文献
15.
Michael A. Henderson 《Surface science》2010,604(13-14):1197-1201
Temperature programmed desorption (TPD), electron energy loss spectroscopy (ELS) and low energy electron diffraction (LEED) were used to study the interaction of molecular oxygen with the (2 × 1) reconstructed surface of hematite α-Fe2O3(0112) under UHV conditions. The (2 × 1) surface is formed from vacuum annealing of the ‘ideal’ (1 × 1) surface and possesses Fe2+ surface sites based on ELS. While O2 does not stick to the (1 × 1) surface at 120 K, the amount of O2 that can be reversibly adsorbed at 120 K on the (2 × 1) surface was estimated to be ~ 0.5 ML (where 1 ML is defined as the Fe3+ surface coverage on the ideal (1 × 1) surface), with additional O2 that is irreversibly adsorbed based on subsequent H2O TPD. Molecularly and dissociatively adsorbed O2 modifies the surface chemistry of H2O both in terms of enhanced OH stability (relative to either the (1 × 1) or (2 × 1) surfaces) and in the blocking of H2O adsorption sites. While O2 adsorption at 120 to 300 K does not transform the (2 × 1) surface into the (1 × 1) surface, the influence of O2 on the (2 × 1) surface involves both charge transfer from surface Fe2+ sites and formation of an ordered c(2 × 2) structure resulting from O2 dissociation. 相似文献
16.
Razvan Stefan Petru Pascuta Adriana Popa Oana Raita Emil Indrea Eugen Culea 《Journal of Physics and Chemistry of Solids》2012,73(2):221-226
Glasses in the system xFe2O3·(100?x) [45ZnO·55B2O3] (0≤x≤10 mol%) have been prepared by melting at 1200 °C and rapidly cooling at room temperature. The obtained samples were submitted to an additional thermal treatment at 570 °C for 12 h in order to relax the glass structure as well as to improve the local order. The as cast and heat treated samples were investigated using X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) measurements. The XRD patterns of all the studied samples show their vitreous nature. Structural modifications occurring in the heat treated samples compared to the untreated ones have been pointed out. EPR spectra of untreated and heat treated samples revealed resonance absorptions centered at g≈2.0, g≈4.3 and g≈6.4. The compositional variation of the line intensity and linewidth of the absorptions from g≈4.3 and g≈2.0 have been interpreted in terms of the variation in the concentration of the Fe3+ ions and the interaction between the iron ions. The EPR spectra of the untreated samples containing 5 mol% Fe2O3 have been studied at different temperatures (110–290 K). The line intensity of the resonance signals decreases with increase in temperature whereas the linewidth is found to be independent of temperature. It was also found that the temperature variation of reciprocal line intensity obeys the Boltzmann law. 相似文献
17.
T.K. Kundu S. Mishra N. Karak P. Barik 《Journal of Physics and Chemistry of Solids》2012,73(4):579-583
Nanostructured nickel ferrites (NiFe2O4) were prepared by doping with Ti4+ ions using solid-state reaction route. Lowest grain size of 55 nm was achieved in the specimens with 20 mole% TiO2 doping. Magnetization in the specimens decreases with decreasing grain sizes. Lower volume fractions of ferrite phase due to dissociation of the magnetic phase into smaller particles by the disruption of super exchange interaction by the titanium substitution results a decrease in magnetizations. Coercivity showed an increasing trend. This was explained as arising due to multidomain/monodomain magnetic behavior of magnetic nanoparticles. Small polaron hopping conduction between Fe2+ and Fe3+ sites controls the dc electrical properties of the specimens. The presence of an interfacial amorphous phase between the sites is evident from Mott's analysis. Specimens containing 10 mole or more TiO2 and sintered at 1350 °C contain NiTiO3 as a secondary phase and show unusual dc conductivity. 相似文献
18.
《Journal of Physics and Chemistry of Solids》2003,64(6):1021-1025
Transparent glass–ceramics containing zinc–aluminum spinel (ZnAl2O4) nanocrystals doped with tetrahedrally coordinated Co2+ ions were obtained by the sol–gel method for the first time. The gels of composition SiO2–Al2O3–ZnO–CoO were prepared at room temperature and heat-treated at temperature ranging 800–950 °C. When the gel samples were heated up to 900 °C, ZnAl2O4 nanocrystals were precipitated. Co2+ ions were located in tetrahedral sites in ZnAl2O4 nanocrystals. X-ray diffraction analysis shows that the crystallite sizes of ZnAl2O4 crystal become large with the heat-treatment temperature and time, and the crystallite diameter is in the range of 10–15 nm. The dependence of the absorption and emission spectra of the samples on heat-treatment temperature were presented. The difference in the luminescence between Co2+ doped glass–ceramic and Co2+ doped bulk crystal was analysed. The crystal field parameter Dq of 423 cm−1 and the Racah parameters B of 773 cm−1 and C of 3478.5 cm−1 were calculated for tetrahedral Co2+ ions. 相似文献
19.
《Current Applied Physics》2010,10(3):724-728
Fe3+ doped δ-Bi2O3 thin films were prepared by sol–gel method on quartz glass substrate at room temperature and annealed at 800 °C. The thin films were then characterized for structural, surface morphological, optical and electrical properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption measurements and d.c. two-probe, respectively. The XRD analyses revealed the formation δ-Bi2O3 followed by a mixture of Bi25FeO40 and Bi2Fe4O9. SEM images showed reduction in grain sizes after doping and the optical studies showed a direct band gap which reduced from 2.39 eV for pure δ-Bi2O3 to 1.9 eV for 10% Fe3+ doped δ-Bi2O3 thin film. The electrical conductivity measurement showed the films are semiconductors. 相似文献
20.
Sono-dispersion of Ni, Co and Ni–Co over Al2O3–MgO with Al/Mg ratio of 1.5 was prepared and tested for dry reforming of methane. The samples were characterized by XRD, FESEM, PSD, EDX, TEM, BET and FTIR analyses. In order to assess the effect of ultrasound irradiation, Ni–Co/Al2O3–MgO with Co content of 8% prepared via sonochemistry and impregnation methods. The sono-synthesized sample showed better textural properties and higher activity than that of impregnated one. Comparison of XRD patterns indicated that the NiO peaks became broader by increasing Co content over the support. The FESEM images displayed the particles are small and well-dispersed as a result of sonochemistry method. Also, EDX analysis demonstrated better dispersion of Ni and Co as a result of sonochemistry method in confirmation of XRD analysis. The sono-synthesized Ni–Co/Al2O3–MgO as a superior nanocatalyst with Co content of 3% illustrates much higher conversions (97.5% and 99% for CH4 and CO2 at 850 °C), yields (94% and 96% for H2 and CO at 850 °C) and 0.97 of H2/CO molar ratio in all samples using an equimolar feed ratio at 850 °C. During the 1200 min stability test, H2/CO molar ratio remained constant for the superior nanocatalyst. 相似文献