Tailorable magnetic properties of ε-Fe2O3/SiO2 hybrid via alkaline etching

In this study, conventional silicon alkaline-etching procedure was utilized to tailor magnetic properties of ε-Fe₂O₃/SiO₂ hybrid. It was found that the saturation magnetization, coercivity and exchange bias field can be readily changed and tailored by altering the etching time and frequency in a set of sodium hydroxide solutions. The relative quantity of ε-Fe₂O₃ phase, the proximity or pinning effect derived from SiO₂ phase as well as the phase transformation from ε-Fe₂O₃ to α-Fe₂O₃ during etching treatment were three main factors to its controllable magnetic properties. This work will shed new light on the development of functional ε-Fe₂O₃/SiO₂ composites with tailorable magnetism in practical magnetically-relevant applications.     

Re-formation of metastable ε-Fe2O3 in post-annealing of Fe2O3/SiO2 nanostructure: Synthesis,computational particle shape analysis in micrographs and magnetic properties

Several Fe₂O₃/SiO₂ nanostructures were synthesized by the combination of the microemulsion and a sol-gel methods. Based on X-ray powder diffraction (XRD) and magnetic measurements (giant coercivity ~2.13 T) we identified ε-Fe₂O₃ (hard magnet) as the dominant crystalline phase. TEM analysis showed a wide size distribution of iron oxide nanoparticles (from 4 to 50 nm) with various morphologies (spherical, ellipsoidal and rod-like). We quantitatively described (computational analysis, MATLAB code) morphological properties of nanoparticles using the ellipticity of the shapes. The as-synthesized hard magnetic material was subjected to a post-annealing treatment at different temperatures (200, 500, 750, 1000 and 1100 °C) in order to investigate stability, formation and transformation of the ε-Fe₂O₃ polymorph. We found decreasing coercivity in the thermally treated samples up to the temperature of 750 °C (H_c=1245 Oe), followed by an observation of a surprising jump in coercivity H_C~1.5 T after post-annealing at 1000 °C. We conclude that the re-formation of the ε-Fe₂O₃ structure during post-annealing at 1000 °C is the origin of the observed phenomena. The phase transformation ε-Fe₂O₃→α-Fe₂O₃ and crystallization of amorphous silica in quartz and cristobalite were observed in the sample treated at 1100 °C.     

Effect of annealing on the structure and magnetic properties of mechanically milled TiO2–Fe2O3 mixture

A mixture of Fe₂O₃ and TiO₂ oxides has been mechanically milled to form TiFe₂O₄ spinel phase. X-ray diffraction (XRD) pattern of the as-milled mixture shows the presence of both Fe₂O₃ and TiO₂ phases. The diffraction peaks become broader and their relative intensity drastically decreases due to the particle size reduction and accumulation of strains. The milled powder was then subjected to annealing at different temperatures (600, 750, 900, 1200 °C). Annealing at 600 °C and 750 °C does not show any significant change in the phase formation. Nonetheless, XRD patterns show a narrowing and an increase in the intensity of Fe₂O₃ peaks with respect to TiO₂, which was reflected by an evolution in particle nano-structure following SEM analysis. An increase in the intensity ratio of the major peaks belonging to Fe₂O₃ relative to the as-milled mixture, which was associated with a reduction of the amount of TiO₂, suggested a possible insertion of Ti into the Fe₂O₃ crystal lattice. However, in VSM measurements, annealing at 600 °C and 750 °C does not change the ferromagnetic phase but the effect of annealing was a notable reduction in the values of both Ms and Mr (saturation magnetization and remanence magnetization respectively) Ultimately, as the powder was heated to 900 °C a new phase seemed to have emerged, this phase was confirmed by SEM, XRD, and magnetic measurements (VSM) where it change phase from ferromagnetic to paramagnetic phase.     

Influence of La3+ additions on the phase behaviour and antibacterial properties of ZrO2–SiO2 binary oxides

The present study reports the influence of lanthanum (La³⁺) content on the phase stability and antibacterial activity of ZrO₂–SiO₂ binary oxides. Four different concentrations of La³⁺ additions in ZrO₂–SiO₂ binary oxides were synthesized using a sol–gel technique. Heat treatment of the synthesized powders resulted in the formation of t-ZrO₂ phase at 1000 °C. Heat treatment beyond 1000 °C resulted in the phase degradation of t-ZrO₂ to yield m-ZrO₂ and ZrSiO₄. Results from antibacterial tests confirmed the potential activity of La³⁺ doped ZrO₂–SiO₂ binary oxides in countering the microbial invasion.     

Preparation and photocatalytic properties of a novel kind of loaded photocatalyst of TiO2/SiO2/γ‐Fe2O3

A novel kind of loaded photocatalyst of TiO₂/SiO₂/γ‐Fe₂O₃ (TSF) that can photodegrade effectively organic pollutants in the dispersion system and can be recycled easily by a magnetic field is reported in this paper. The γ‐Fe₂O₃ cores in TiO₂/γ‐Fe₂O₃ are found to reduce the activity of the TiO₂ photocatalyst in the photodegradation of dyes under either UV or visible light irradiation. Addition of a SiO₂ membrane between the γ‐Fe₂O₃ core and the TiO₂ shell weakens efficiently the influence of the γ‐Fe₂O₃ cores on the TiO₂ photocatalytic activity and leads to a highly active and magnetically separable photocatalyst on TSF. Comparison of the photodegradation processes of dyes under UV and visible irradiation is also carried out. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of sintering temperature on heterogeneous-interface structural evolution and magnetic properties of Fe–Si soft magnetic powder cores

Ceramic oxides have attracted considerable research attention as ideal coating layers for novel Fe-based soft magnetic powder cores (SMPCs). However, maintenance of the integrity and uniformity of Fe-based/MO_x core–shell heterostructures is challenging. The mechanism underlying the evolution of the core–shell heterostructure is a key determinant of the performance of Fe-based SMPCs. Herein, the laws governing the evolution of the core–shell structure of and heterogeneous interface in Fe–Si/SiO₂ SMPCs with temperature and the influence of this evolution on SMPCs performance were investigated. The results revealed that at the sintering temperature of 1093–1183 K, the core–shell heterostructure gradually integrated, while SiO₂ insulating coatings underwent amorphous-to-crystalline state transformation. When the sintering temperature was >1243 K, Fe–Si particles melted partially, and the core–shell heterostructure collapsed owing to the overheating induced by the gradient temperature field during the hot-pressing sintering process. When the sintering temperature was 1153 K, the core–shell heterostructure was intact, and the Fe–Si/SiO₂ SMPCs had a saturation magnetisation of 245.5 emu/g, resistivity of 0.42 mΩ cm, and total loss of 923.2 kW/m³ at 10 mT and 100 kHz. When the core–shell heterostructure was destroyed, the resistivity dropped drastically, and the total loss increased by approximately 36.7% and 41.8%. Based on these results, the relationship among the core–shell heterostructure of Fe–Si/SiO₂ SMPCs, sintering temperature, and magnetic properties was established, which is instrumental in achieving high power density, high efficiency, and miniaturisation in SMPCs.     

Preparation of α-Fe2O3/Ag core/shell nanocomposites and SERS properties

用籽晶法,以甲醛为还原剂、3-氨丙基三乙氧基硅烷(APS)为改性剂,在Ag［(NH₃)₂］⁺溶液中制备α-Fe₂O₃/Ag核壳结构复合粉体。采用XRD、TEM和EDX对样品进行表征,系统研究了APS改性剂、醇水比等对复合纳米颗粒包覆效果及性能的影响;并用吡啶(Py)为探针,研究了α-Fe₂O₃/Ag核壳纳米颗粒作为拉曼衬底时的拉曼增强性能相似文献   

Relationship of structural phase transformations and properties of sol-gel films in the Bi2O3 — TiO2 — Fe2O3 system

It is shown that the structure and properties of gel-sol oxide films depend on the total mass content of film-forming oxides in the solution and are determined by the different degrees of nonequilibrium of the physicochemical processes. The existence of quite clear relationships between the solution viscosity, the specifics of the microstructure, and certain properties of the film is established. Translated from Steklo i Keramika, No. 4, pp. 11 – 14, April, 2000.     

Fabrication and Formation Mechanism of Hollow-Structure Supermagnetic α-Fe2O3/Fe3O4 Heterogeneous Nanospindles

Journal of Inorganic and Organometallic Polymers and Materials - A simple process for the fabrication of hollow-structure supermagnetic α-Fe2O3/Fe3O4 heterogeneous nanospindles was introduced...     

Influence of Fe3+-doping on optical properties of CeO2−y nanopowders

Ce_1?xFe_xO_2?y (0≤x≤0.05) nanopowders were synthesized using hydrothermal method at low calcination temperature and low doping regime. Structural and morphological characterization has been carried out by the X-ray diffraction method and non-contact atomic force microscopy. Vibrational properties were investigated by Raman spectroscopy. It was observed that the content of oxygen vacancies increased significantly with Fe doping up to 3 mol%. For higher dopant concentration, phase separation was detected. The optical properties of pure and Fe³⁺-doped CeO_2?y samples were investigated by spectroscopic ellipsometry. Several analytical models were applied to analyze the optical absorption onset of ceria defective structure. It was found that, Cody–Lorentz model most suitably described the sub-band gap region of CeO_2?y nanopowders and consequently gave more accurate band gap values, which are closer to the direct band gap transitions than to the indirect ones. The increased content of localized defect states in the ceria gap and corresponding shift of the optical absorption edge towards visible range in Fe-doped samples can significantly improve the optical activity of nanocrystalline ceria.     

The Preparation and Gas-sensing Property of Nanosize γ-Fe2O3／SiO2

Nanostructured γ-Fe2O3/SiO2 complex oxide was prepared by sol-gel method with tetraethoxysilane and iron nitrate as precursors. The particle size distribution, thermal and phase stabilities and gas sensing properties were systematically characterized by TEM, granularity distribution, TG-DTA, XRD and gas sensitivity measurements. The particle size is about 10 nm and size distribution is very narrow. The sensitivity of the sensing element to CO, H2, C2H4, C6H6 and the effects of calcination temperature on the sensitivity and conductance of gases were examined. The combination of excellent thermal stability and tunable gas sensing properties through careful control of the preparation and judicious selection of material compositions gives rise to novel nanocomposites, which is attractive for the sensitive and selective detection of reducing gases and some hydrocarbon gases.     

Enhancement of magnetic and dielectric properties of KNbO3–CoFe2O4 multiferroic composites via thermal treatment

In this work, multiferroic composites were produced from CoFe₂O₄ and KNbO₃ mixtures via control of the heat treatment temperature. For this, CoFe₂O₄ nanoparticles were produced by sol-gel method, while KNbO₃ was synthesized by microwave-assisted hydrothermal synthesis. The powders were homogenized and subjected to heat treatment at 300, 400 and 500 °C for 5 h. The structural, electrical and magnetic properties were characterized. The results of X-ray diffraction indicated that there was no formation of secondary phases with heat treatment. Raman vibrational modes confirmed the presence of KNbO₃ and CoFe₂O₄ in the prepared composites. SEM analysis showed that the composite microstructure consists of smaller ferrite particles arranged on the surface of largest cubic KNbO₃ particles. The improvement of coercivity (H_C = 382.1Oe) and dielectric constant (?’~7860) was observed for the composite thermally treated at 300 °C. The obtained results show the potential application of KN:CFO composites for multifunctional devices.     

Effect of CaO doping on the properties and crystallization behavior of Y2O3–Al2O3–SiO2 glass

Nowadays, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass joining is considered to be a promising scheme for nuclear-grade continuous silicon carbide (SiC) fiber reinforced SiC matrix composites (SiC/SiC). CaO has great potential for nuclear applications since it has low reactivity and low decay rate under nuclear irradiation. In this paper, the effect of CaO doping on the structure, thermophysical properties, and crystallization behavior of YAS glass was systematically studied. As the CaO doping content increased, the number of bridge oxygens and the viscosity at high temperatures reduced gradually. After heat treatment at 1400 °C, the main phases in YAS glass were β-Y₂Si₂O₇, mullite, and SiO₂ (coexistence of crystalline and glass phases), while that with 3.0% CaO doping turned into a single glassy phase under the same treatment conditions. Moreover, a structural model and the modification mechanism were proposed, which provided a theoretical basis for the subsequent component design and optimization.     

Influence of Cr2O3 on highly nonlinear properties and low leakage current of ZnO–Bi2O3 varistor ceramics

ZnO–Bi₂O₃–Sb₂O₃–Co₂O₃–MnO₂–xCr₂O₃ (ZBSCM–xCr₂O₃, 0≤x≤0.6 mol%) varistors were fabricated through the conventional solid state method, and the effects of Cr₂O₃ on the microstructures and electrical properties were investigated. Results showed that the secondary phases CrBi₁₈O₃₀ and Co₂Cr_0.5Sb_0.5O₄ emerged when x ranges from 0.2 to 0.4. In these compositions, Cr₂O₃ acted as a donor and decreased the electrical properties of ZBSCM. For samples with x=0.5, the secondary phases transformed to MnCr₂O₄ and the electrical properties increased significantly: the nonlinear coefficient α sharply increased up to 80.71 and the barrier height ϕ_b reached 3.88 eV. This indicates that the donor effect of Cr₂O₃ disappeared. In addition, with the increase of Cr₂O₃, the average grain size of ZnO decreased from 7.48 μm to 5.46 μm, which in turn resulted in an increase of breakdown voltage E_1mA from 216.17 V/mm to 362.50 V/mm. Besides, all the samples showed the low value of leakage current of lower than 0.1 μA. This varistor might be a promising candidate for highly effective applications.     

Sonochemical synthesis and characterization of magnetic separable Fe3O4–TiO2 nanocomposites and their catalytic properties

A novel sonochemical method is described for the preparation of Fe₃O₄–TiO₂ photocatalysts in which nanocrystalline titanium dioxide particles are directly coated onto a magnetic core. The Fe₃O₄ nanoparticles were partially embedded in TiO₂ agglomerates. TiO₂ nanocrystallites were obtained by hydrolysis and condensation of titanium tetraisopropyl in the presence of ethanol and water under high-intensity ultrasound irradiation. This method is attractive since it eliminated the high-temperature heat treatment required in the conventional sol–gel method, which is important in transforming amorphous titanium dioxide into a photoactive crystalline phase. In comparison to other methods, the developed method is simple, mild, green and efficient. The magnetization hysteresis loop for Fe₃O₄–TiO₂ nanocomposites indicates that the hybrid catalyst shows superparamagnetic characteristics at room temperature. Photocatalytic activity studies confirmed that the as-prepared nanocomposites have high photocatalytic ability toward the photodegradation of RhB solution. Furthermore, the photodecomposition rate decreases only slightly after six cycles of the photocatalysis experiment. Thus, these Fe₃O₄–TiO₂ nanocomposites can be served as an effective and conveniently recyclable photocatalyst.     

Influence of nucleating agents on crystallization and electrical properties of SiO2–MgO–MgF2–K2O mica glass-ceramics

The effects of adding ZrO₂ and TiO₂ at the expense of MgF₂ on the crystallization, microstructure, mechanical properties, thermal properties and electrical properties of mica glass-ceramics based on the SiO₂–MgO–MgF₂–K₂O system were investigated by the differential scanning calorimetry (DSC), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), microhardness tester and resistivity tester. The electrical properties were discussed emphatically. The results showed that the additions of ZrO₂ and/or TiO₂ at the expense of MgF₂ effectively increase the viscosity, the glass transition temperatures (T_g) and the crystallization temperatures (T_p) of the glasses. The crystallization activation energy (E_c) of the amorphous glasses varied with the nucleating agents was discussed in depth. It was discovered that the nucleating agents had no effect on the crystal phase type but had a certain effect on the crystallinity and microstructure. Tetrasilicic fluoromica and enstatite were precipitated at different crystallization temperatures. Due to the non-stoichiometric ratio of tetrasilicic fluoromica crystal, the prepared glass-ceramics had high dielectric constant (24.4–34.3) and volume resistivity (>2 × 10¹¹ Ω cm) at 25 ^°C, and the dielectric loss was almost zero.     

Ionothermal synthesis of aggregated α-Fe₂O₃ nanoplates and their magnetic properties

Aggregated α-Fe(2)O(3) nanoplates have been successfully synthesized under ionothermal conditions through the self-assembly of nanoplatelets in a side-to-side manner. During the formation process of aggregated α-Fe(2)O(3) nanoplates, pure ionic liquid media is essential for the assembly and coalescence of small nanoplatelets into final nanoplates. Moreover, the magnetic properties of the aggregated α-Fe(2)O(3) nanoplates are strongly correlated to their unique structural characteristics.     

Influence of synthesis conditions on the properties of Y2O3–MgO nanopowders and sintered nanocomposites

In this study, a citrate–nitrate combustion method was applied to synthesize composite Y₂O₃–MgO nanopowders. In order to optimize the synthesis condition to support sufficient combustion, the molar ratio of citric acid to nitrate (c/n molar ratio) used in the reaction mixtures was varied between 0.17 and 0.34. Nanopowders with an average particle size of 17 nm were achieved. The properties of these nanopowders indicated that the higher molar ratios decreased the unreacted organic components and increased the amount of carbide on the surface of the oxides, which helped to inhibit the formation of carbonate groups. The amount of carbonate groups was reduced with the increasing c/n molar ratio. Y₂O₃–MgO nanocomposites fabricated through hot-isostatic-pressing sintering showed a uniform distribution of Y₂O₃ and MgO grains, which had an average size of ∼180 nm. In addition, the absorption peaks at 1410 and 1511 cm⁻¹ disappeared until the c/n molar ratio reached 0.28. A high average infrared transmittance of 83% in the range of 4000–1667 cm⁻¹ (2.5–6 μm) was obtained in the nanocomposites.     

Effect of Al2O3 and K2O content on structure,properties and devitrification of glasses in the Li2O–SiO2 system

The effect of Al₂O₃ and K₂O content on structure, sintering and devitrification behaviour of glasses in the Li₂O–SiO₂ system along with the properties of the resultant glass–ceramics (GCs) was investigated. Glasses containing Al₂O₃ and K₂O and featuring SiO₂/Li₂O molar ratios (3.13–4.88) far beyond that of lithium disilicate (Li₂Si₂O₅) stoichiometry were produced by conventional melt-quenching technique along with a bicomponent glass with a composition 23Li₂O–77SiO₂ (mol.%) (L₂₃S₇₇). The GCs were produced through two different methods: (a) nucleation and crystallization of monolithic bulk glass, (b) sintering and crystallization of glass powder compacts.Scanning electron microscopy (SEM) examination of as cast non-annealed monolithic glasses revealed precipitation of nanosize droplet phase in glassy matrices suggesting the occurrence of phase separation in all investigated compositions. The extent of segregation, as judged from the mean droplet diameter and the packing density of droplet phase, decreased with increasing Al₂O₃ and K₂O content in the glasses. The crystallization of glasses richer in Al₂O₃ and K₂O was dominated by surface nucleation leading to crystallization of lithium metasilicate (Li₂SiO₃) within the temperature range of 550–900 °C. On the other hand, the glass with lowest amount of Al₂O₃ and K₂O and glass L₂₃S₇₇ were prone to volume nucleation and crystallization, resulting in formation of Li₂Si₂O₅ within the temperature interval of 650–800 °C.Sintering and crystallization behaviour of glass powders was followed by hot stage microscopy (HSM) and differential thermal analysis (DTA), respectively. GCs from composition L₂₃S₇₇ demonstrated high fragility along with low flexural strength and density. The addition of Al₂O₃ and K₂O to Li₂O–SiO₂ system resulted in improved densification and mechanical strength.