Influence of annealing treatment on magnetic properties of Fe2O3/SiO2 and formation of ε-Fe2O3 phase

Magnetic properties of Fe₂O₃/SiO₂ samples were studied after being produced by sol-gel synthesis and formation of ε-Fe₂O₃ polymorph. Samples were thermally treated, using different annealing temperatures and annealing times. The size and morphological characteristics of the iron oxide nanoparticles were examined using a TEM microscope. We used the “ellipticity of shapes”, which is a measure of how much the shape of a nanoparticle differs from a perfect ellipse, in order to quantitatively describe morphological properties of nanoparticles. Coercivity measurements were used to identify and monitor the formation of the epsilon-iron oxide phase during the thermal treatments (annealing). Coercivity values were in the range from 1.2 to 15.4 kOe, which is in accordance with previous experience regarding the existence of ε-Fe₂O₃. We have determined the optimal formation conditions for the ε-Fe₂O₃ polymorph (t=1050 °C for 7 h, H_C=15.4 kOe), as well as the narrow temperature interval (1050–1060 °C) in which the polymorph abruptly vanished (H_C=2300 Oe), on the basis of results of the magnetic properties. The threshold temperature for the ε-Fe₂O₃ phase transformation was measured as 1060 °C. We found that different annealing temperatures and annealing times significantly affected magnetic properties of the examined samples.     

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.     

Synthesis and enhanced gas sensing properties of flower-like ZnO/α-Fe2O3 core-shell nanorods

This paper reports a facile two-step synthesis route for the preparation of flower-like ZnO/α-Fe₂O₃ nanorods (NRs). Flower-like ZnO NRs with the average diameter about 810 nm and length about 4.5 µm were firstly synthesized via a chemical solution method, and then ZnO NRs was coated with a continuous α-Fe₂O₃ layer to form ZnO/α-Fe₂O₃ core-shell structure through an ionic-layer adsorption and reaction method. The gas-sensing results show that the ZnO/α-Fe₂O₃ NRs exhibit excellent sensitivity, selectivity, and response-recovery capacity to ethanol vapor at a low optimum temperature of 240 °C. In particular, compared with pure ZnO NRs and α-Fe₂O₃ nanoparticles (NPs), the ZnO/α-Fe₂O₃ NRs show an obvious improvement in gas sensing properties. The substantial improvement of sensing properties may be attributed to the unique microstructure and heterojunction formed between ZnO and α-Fe₂O₃.     

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.     

Revisiting metastable immiscibility in SiO2–Al2O3: Structure and phase separation of supercooled liquids and glasses

Understanding and controlling liquid–liquid phase separation in aluminosilicates is crucial for optimizing glass properties. However, the metastable nature of aluminosilicates’ phase separation has made it difficult to study experimentally, and uncertainty persists regarding the compositional and temperature extents of the miscibility gap. Here, we present new experimental evidence that suggests a consolute temperature between 1440 and 1590°C and endmember compositions of 7 and 62 mol.% Al₂O₃ for the phase-separated glasses. Using containerless melt processing, deeply supercooled liquids over the 0–60 mol.% Al₂O₃ range are probed with in situ small- and wide-angle X-ray scattering, which simultaneously reveals changes in nanoscale density heterogeneity and atomic structure. Correlations between phase separation and atomic coordination environments are compared for liquids and glasses. Pair distribution function analysis shows mean O–(Si + Al) coordination increases with Al₂O₃ content and decreases with temperature.     

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   

Synthesis, Electrical Measurement, and Field Emission Properties of α-Fe2O3 Nanowires

α-Fe₂O₃ nanowires (NWs) were formed by the thermal oxidation of an iron film in air at 350 °C for 10 h. The rhombohedral structure of the α-Fe₂O₃ NWs was grown vertically on the substrate with diameters of 8–25 nm and lengths of several hundred nm. It was found that the population density of the NWs per unit area (D _NWs) can be varied by the film thickness. The thicker the iron film, the more NWs were grown. The growth mechanism of the NWs is suggested to be a combination effect of the thermal oxidation rate, defects on the film, and selective directional growth. The electrical resistivity of a single NW with a length of 800 nm and a diameter of 15 nm was measured to be 4.42 × 10³ Ωcm using conductive atomic force microscopy. The field emission characteristics of the NWs were studied using a two-parallel-plate system. A low turn–on field of 3.3 V/μm and a large current density of 10⁻³ A/cm² (under an applied field of about 7 V/μm) can be obtained using optimal factors of D _NWs in the cathode.     

Synthesis,characterization, and magnetic properties of α-MnS nanocrystals

MnS nanocrystals have been prepared by a colloidal synthesis route through the reaction of MnCl₂ and S[Si(CH₃)₃]₂ in trioctylphosphineoxide. The nanocrystals were characterized using X-ray diffraction and transmission electron microscopy. The magnetic properties were studied with SQUID magnetometry. X-ray diffraction shows that the nanocrystals are of the thermodynamically stable α-MnS (alabandite) structure. Size control was achieved by changing the concentration of the precursors. Nanocrystal sizes were measured by transmission electron microscopy, and three samples of average diameters 20, 40, and 80 nm were obtained, with narrow size distribution (σ˜9%). The zero field cooled magnetization curves for the 80-, 40-, and 20-nm samples showed a cusp at 116 K, 97 K, and 50 K respectively, all smaller than the antiferromagnetic transition temperature, T_N = 130 K, of bulk α-MnS. Below T_N the magnetization exhibits a paramagnetic behavior unlike typical antiferromagnetic materials. These results indicate that there is a mixture of paramagnetic and antiferromagnetic phases in the nanocrystals. The size dependence shows a general trend of decrease of T_N with reduced particle size, indicating size dependent magnetic ordering.     

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.     

Fabrication and characterization of La2O3–Fe2O3–Bi2O3 nanopowders: Effects of La2O3 addition on structure,optical, and radiation-absorption properties

In this study, we fabricated and characterized six new nanopowders representing variations of La₂O₃–Fe₂O₃–Bi₂O₃, i.e., 100Bi₂O₃, 30Fe₂O₃–70Bi₂O₃, 3La₂O₃–27Fe₂O₃–70Bi₂O₃, 7La₂O₃–23Fe₂O₃–70Bi₂O₃, 10La₂O₃–20Fe₂O₃–70Bi₂O₃, and 20La₂O₃–10Fe₂O₃–70Bi₂O₃ (represented by 100B, 30F70B, 3L27F70B, 10L20F70B, and 20L10F70B, respectively). These nanopowders were prepared by the microwave-assisted hydrothermal synthesis method. Saponin extract from soapnuts was used as the nanoparticle capping agent. The structural, optical, and gamma radiation characteristics were measured, calculated, and analysed, respectively. The chemical structures of the nanocomposites influenced their optical and radiation shielding characteristics. The optical bandgaps of the 100B, 30F70B, 3L27F70B, 7L23F70B, 10L20F70B, and 20L10F70B nanopowders were 3.16, 3.13, 3.43, 3.45, 3.46, and 3.58 eV, respectively. The ranges of the mass attenuation coefficients of the nanopowders were computed, using XCOM, to be 0.0412–5.1624, 0.0401–4.5406, 0.0401–4.5285, 0.0401–4.5129, 0.0401–0.5015, and 0.0400–4.4156 cm²/g, respectively, and the ranges of mass energy absorption coefficients were found to be 0.0232–1.7525, 0.0228–1.5484, 0.0228–1.5598, 0.0288–1.5746, 0.0228–1.5853, and 0.0227–1.6192 cm²/g, respectively, for photon energies in the range of 0.1–10 MeV. The order of the dose rate trend was as follows: 30F70B < L27F70B < 7L23F70B < 10L20F70B < 20L10F70B. Analysis of the photon interaction parameters showed that the synthesized nanopowders could function well as fillers in radiation-shielding matrices.     

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.     

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...     

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.     

Facile Synthesis of Fe3O4−SiO2 Nanocomposites for Wastewater Treatment

Water constitutes ≈70–90% of the organism's body by mass and is highly important for its survival. Water contains a variety of chemical contaminants introduced by various sectors, resulting in contamination that has a direct impact on the ecosystem. Various approaches are in practice to tackle these issues. Among these, semiconductor photocatalysis appears to be the cutting-edge technology for the degradation of wastewater contaminants. Herein, the fabrication of Fe₃O₄−SiO₂ nanocomposite via facile co-precipitation and Stober methods are reported. Various characterization techniques are employed for the structural elucidation, morphology, crystallinity, and stability of the as-prepared composite. The nanocomposite is employed in catalytic and photocatalytic applications toward the removal of methylene blue (MB) and methyl orange (MO) dyes from a comparative perspective. It is observed that the composite can remove about 93% of MB and 51% of MO within 7 and 6 h, respectively. These findings indicate that the nanocomposite has a higher MB removal effectiveness than the MO. This trend can be accredited to the difference in the chemical structure of both dyes. The nanocomposite is also evaluated for antioxidant and antileishmanial activity, and it is shown to be quite effective even at very low concentrations.     

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.     

Polyaniline (PANI)–Co0.5Mn0.5Fe2O4 nanocomposite: Synthesis,characterization and magnetic properties evaluation

Polyaniline (PANI)/Cobalt-manganese ferrite, (PANI)/Co_0.5Mn_0.5Fe₂O₄, nanocomposite was synthesized by oxidative chemical polymerization of aniline in the presence of ammonium peroxydisulfate (APS). Microwave assisted synthesis method was used for the fabrication of core CoFe₂O₄ nanoparticles. The structural, morphological, thermal and magnetic properties of the nanocomposite were investigated in detail by X-ray diffraction (XRD), fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). The average crystallite size of (PANI)/Co_0.5Mn_0.5Fe₂O₄ nanocomposite by the line profile method was 20±9 nm. The magnetization measurements revealed that (PANI)/Co_0.5Mn_0.5Fe₂O₄ nanocomposite has superparamagnetic behavior with blocking temperature higher than 300 K. The saturation magnetization of the composite is considerably low compared to that of CoFe₂O₄ nanoparticles due to the partial replacement of Co²⁺ ions and surface spin disorder. As temperature decreases, both coercivity and strength of antiferromagnetic interactions increase which results in unsaturated magnetization of the nanocomposite.     

Ternay Au@TiO2/α-Fe2O3 Nanocomposite with Nanoring Structure: Synthesis,Characterization and Photocatalytic Activity

Journal of Inorganic and Organometallic Polymers and Materials - In the present study, a modified solvothermal reaction of (hematite) with titanium(IV) butoxide and gold(III) chloride produced...     

Phase equilibria in the Nd2O3–BaO–Fe2O3 system: Crystal structure,oxygen content,and properties of intermediate oxides

The phase equilibria in the ½Nd₂O₃–BaO–½Fe₂O₃ system were systematically studied at 1373 K in air and presented in the form of a phase diagram. By X-ray diffraction (XRD) analysis of quenched samples, the homogeneity ranges and crystal structure were determined for the following intermediate oxides: Nd_1-xBa_xFeO_3-δ with 0.0 ≤ x ≤ 0.05 (space group (SG) Pnma) and with 0.6 ≤ x ≤ 0.7 (SG Pm-3m), Ba_6+yNd_2-yFe₄O_15-δ with 0.0 ≤ y ≤ 0.4 (SG P6₃mc), and Ba_1.1Nd_1.9Fe₂O_7-δ (SG P4₂/mnm). The structural parameters of single-phase oxides were refined by the Rietveld method. The changes in oxygen content for Nd_1-xBa_xFeO_3-δ (0.6 ≤ x ≤ 0.7), Ba₆Nd₂Fe₄O_15-δ, and Ba_1.1Nd_1.9Fe₂O_7-δ versus temperature in air were determined by thermogravimetric analysis. Gradual substitution of neodymium by barium in the Nd_1-xBa_xFeO_3-δ (0.6 ≤ x ≤ 0.7) oxides leads to a decrease of oxygen content. Partial ordering via formation of separate domains with a_p × a_p × 5a_p superstructure in Nd_0.4Ba_0.6FeO_3-δ, which cannot be detected by XRD, noticeably influence its thermal expansion and electrical conductivity.     

Pressure-less joining of SiCf/SiC composites by Y2O3–Al2O3–SiO2 glass: Microstructure and properties

In this study, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass was prepared from Y₂O₃, Al₂O₃, and SiO₂ micron powders. Thermal expansion coefficient of as-obtained YAS glass was about 3.9 × 10⁻⁶, matching-well with that of SiC_f/SiC composites. SiC_f/SiC composites were then brazed under pressure-less state by YAS glass and effects of brazing temperature on microstructures and properties of resulting joints were investigated. The results showed that glass powder in brazed seam sintered and precipitated yttrium disilicate, cristobalite, and mullite crystals after heat treatment. With the increase in temperature, joint layer gradually densified and got tightly bonded to SiC_f/SiC composite. The optimal brazing parameter was recorded as 1400 °C/30 min and shear strength of the joint was 51.7 MPa. Formation mechanism of glass-ceramic joints was proposed based on combined analysis of microstructure and fracture morphology of joints brazed at different temperatures. Thermal shock resistance testing of joints was also carried out, which depicted decline in shear strength with the increase of thermal shock times. The strength of the joint after three successive thermal shock cycles at 1200 °C was 35.6 MPa, equivalent to 69% of that without thermal shock.     

SiO2–Al2O3–CaO glass-ceramics: Effects of CaF2 on crystallization,microstructure and properties

The effects of fluorine content on the nucleation and crystallization behavior of SiO₂–Al₂O₃–CaO glass ceramics system have been investigated. The crystalline phases were determined by X-ray diffraction (XRD). The crystallization kinetics was determined by differential thermal analysis (DTA). The microstructures were examined by using scanning electron microscope (SEM). Fourier transformed infrared spectra (FTIR) analysis was used to study the glass structure. The results showed that by increasing the fluorine content, both the crystallization peak temperature (T_p) and activation energy (E) decreased. Wollastonite, anorthite and gehlenite are the main crystalline phases that exist in the glass ceramics system. The study shows that fluorine promoted initial crystallization of glass and can be used as an effective nucleating agent in the SiO₂–Al₂O₃–CaO system.