Reaction sintering of dense gadolinium-iron garnet (GdIG) material

Two synthesis methods of gadolinium-iron garnet (GdIG) materials were described. The first one was a typical co-precipitation-calcination method (CP method). In the second one gadolinium-iron perovskite (GdIP, GdFeO3) and iron oxide (α-Fe2O3) were mixed in proper amounts for GdIG stoichiometry (RS method). DSC data for precursor powder mixtures obtained from both synthesis routes allowed to examine heat effects. In the case of the CP method, two crucial reactions were observed. The first one at ab. 767 °C was associated with the formation of GdIP phase, whereas the second one at ab. 1080 °C was attributed to GdIG formation. For the RS method, only formation of GdIG at 1122 °C was observed. Both powder mixtures were sintered at 1200, 1300, and 1400 °C for 2 h. For the CP method, the highest density was below 90 % even at 1400 °C whereas for the RS method the density of materials sintered at the same temperature exceeded 97 %.     

Synthesis of Porous α-Fe2O3 Nanorods as Catalyst Support and A Novel Method to Deposit Small Gold Colloids on Them

Two novel methods, one for preparation of porous α-Fe₂O₃ nanorod catalyst support and another for the deposition of gold (Au) particles on the catalyst support with high efficiency and high dispersion, were reported. In the former, FeO(OH) nanorods were first prepared by a mild hydrothermal synthesis using tetraethylammonium hydroxide (TEAOH) as the structure director. The FeO(OH) product was then converted to porous α-Fe₂O₃ nanorods via calcination at 300 °C. During this calcination, pores with a size distribution in the range of 1–5 nm were generated by removal of TEAOH molecules. By employing our invented Au colloid-based and sonication-assisted method, in which lysine was used as the capping agent and sonication was employed to facilitate the deposition of the Au particles, we were able to deposit very small Au particles (2–5 nm) into these pores. This method is rapid as the reaction/deposition is completed within 1 min. The prepared Au/α-Fe₂O₃-nanorod catalyst exhibited much higher catalytic activity than the Au/commercial α-Fe₂O₃ (Fluka) catalyst.     

Nanosized α-Fe2O3 and Li-Fe composite oxide electrodes for lithium-ion batteries

Nanosized α-Fe₂O₃ (ca. 50 nm) and Li-Fe composite oxides (ca. 29 nm) powders were synthesized via gel polymer route. The gels were obtained with thermal polymerization of acrylic acid solutions of iron and lithium nitrates. The calcination of these gels at temperatures from 300 °C to 500 °C results in α-Fe₂O₃ from Fe(NO₃)₃ precursor and Li-Fe composite oxides Li₂O-Fe₃O₄-LiFeO₂ from a mixed precursors of Fe(NO₃)₃ and LiNO₃. Thermal gravimetric analysis, X-ray diffraction and transmission electron microscopy were used to investigate the precursors and products. The electrochemical performance of the Fe-based oxides was also evaluated. After 200 cycles, their capacity can be as high as 1300 mAh/g for α-Fe₂O₃ and 1400 mAh/g for Li-Fe oxide while the initial capacity loss is as low as 21.8%. The Li-Fe oxide electrodes exhibit better capacity retention than the α-Fe₂O₃ electrodes. They are interesting negative electrodes for high energy density lithium-ion batteries.     

Preparation of highly (001)-oriented α-Fe2O3 film on Si-substrate from drop coated BaFe12O19 via barium diffusion-induced transformation

Oriented iron oxide structure is of particular importance in electronic, magnetic, and catalytic applications. We report on a facile route to prepare (001)-oriented hematite (α-Fe₂O₃) film on a silicon substrate with drop coated BaFe₁₂O₁₉ nanoplates from hydrothermal process as precursor. The obtained α-Fe₂O₃ film displays an ultrahigh (001) orientation of 99.4% when treated at 1300 °C for 5 h. Transformation of BaFe₁₂O₁₉ into α-Fe₂O₃ is driven by directional diffusion of barium into the interface between BaFe₁₂O₁₉ coating and the substrate, where a thermodynamically stable barium silicate was produced. Given the structural similarity between BaFe₁₂O₁₉ and α-Fe₂O₃, the crystallographic direction of BaFe₁₂O₁₉ [001] is topologically converted into α-Fe₂O₃ [001] during the transformation. The ultrahigh degree of orientation of the α-Fe₂O₃ film is attributed to the large aspect ratios of the starting BaFe₁₂O₁₉ nanoplates, which facilitate the realignment of plates, causing them to flatten on the substrate during heat treatment.     

Hydrogen evolution on iron pyrite (FeSy) in sodium chloride solution

The sulphur derivatives of iron rust and a mixed iron oxide from the co-precipitation of Fe^II-Fe^III chlorides in 6M NaOH have been found to posses high hydrogen evolution in sodium chloride solution. The enhanced performance of these suphides for hydrogen evolution is observed to depend on the sulphide constituents, prepared by the combinations of Fe_1-x), α-Fe₂O₃ and Fe₃O₄ oxide mixtures, and reduced by hydrogen gas. Pyrite, prepared from iron rust-H₂S with optimum sulphide mixture, has the best performance for hydrogen evolution reaction (h.e.r.) of 100 MA/cm², -270mV vs scein 3.5% sodium chloride solution at 25°C after ohmic corrections.     

A new slow-releasing iron fertilizer

This paper describes the development of an environment-friendly, slow-release fertilizer of the micronutrient iron. The compound is water insoluble, and is based on a polymeric phosphate structure. Kinetics and solubility of products in the goethite [FeO(OH)]–H₃PO₄ and [FeO(OH)]–MgO–H₃PO₄ systems were studied at 170–300 °C. Polymerization patterns were complex. The presence of Mg as additive, improved product properties. The fertilizer was prepared by polymerization of goethite, magnesium oxide and phosphoric acid to an optimized chain length at 200 °C, followed by neutralization with magnesia. The fertilizer was soluble in citrate and DTPA. A significant increase in the yield of wheat and uptake of iron was observed at a dosage of 2 kg/ha Fe as the slow-release fertilizer.     

Hydrothermal synthesis and visible-light photocatalytic activity of α-Fe2O3/TiO2 composite hollow microspheres

Novel α-Fe₂O₃/TiO₂ composite hollow spheres were successfully synthesized by a template-assisted precipitation reaction using urea as a precipitating agent and carbon spheres as templates in a mixed solvent of water and ethanol, and then calcined at 400 °C for 4 h. The as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption–desorption isotherms, and vibrating sample magnetometer. The influence of calcination temperature and the molar ratio of titanium to iron (R) on the photocatalytic activity of the samples was investigated. The results indicated that the composite spheres show magnetic characteristics at room temperature and good photocatalytic activity under visible-light irradiation compare to the single-component α-Fe₂O₃ particles. This method can be further applied to synthesize nanocomposites of magnetic metal oxide and other metal oxide.     

Review of the hydrolysis of iron(III) and the crystallization of amorphous iron(III) hydroxide hydrate

Hydrolysis of ferric solutions leads initially to mono- and dinuclear species which interact to produce further species of higher nuclearity. These polynuclear species age eventually to either crystalline compounds or to an amorphous precipitate (amorphous iron(III) hydroxide hydrate). Amorphous iron(III) hydroxide hydrate is thermodynamically unstable and gradually transforms to α-FeO(OH) and α-Fe₂O₃. These crystalline products form by competing mechanisms and the proportion of each in the final product depends on the relative rates of formation. The master variable governing the rates at which these compounds form is pH. Other important factors are temperature and the presence of additives. Most additives retard the transformation and by suppressing formation of α-FeO(OH) lead to an increase in the amount of α-Fe₂O₃ in the product; some additives also directly promote formation of the latter compound. Metal ions can oftxen replace a proportion of Fe in the α-FeO(OH) and α-Fe₂O₃ lattices. At high enough concentrations they can induce formation of additional phases. Additives may also modify the morphology of the crystalline products.     

Effects of drying conditions on physicochemical properties of epoxide sol−gel derived α-Fe2O3 and NiO: A comparison between xerogels and aerogels

A simple and easily operated supercritical CO₂ dryer was designed and manufactured with the aim of producing high-surface-area mesoporous α-Fe₂O₃ (hematite) and NiO aerogels. The gels were synthesized by a sol?gel method with the aid of propylene oxide (PO), as the gelation agent, and then dried and calcined at different conditions. The effects of drying and calcination conditions on the physicochemical properties of the final aerogels were investigated using X-ray diffraction (XRD), N₂ adsorption-desorption, Fourier-transformed infrared spectroscopy (FT-IR), and field emission scanning electron microscopy (FE-SEM) analyses. It was demonstrated that α-Fe₂O₃ and NiO aerogels with high surface areas and mesoporosities could be successfully synthesized using the home-made supercritical CO₂ dryer. Supercritical drying of the gels resulted in α-Fe₂O₃ (186 m²/g) and NiO (178 m²/g) aerogels with 186% and 34% higher surface areas, respectively, than xerogels obtained via simple drying at 80°C using a laboratory oven. In addition, the results showed that supercritical CO₂ drying could enhance preservation of the porous network of the oxide nanostructures at high calcination temperatures via suppression of sintering phenomenon. Calcination of α-Fe₂O₃ and NiO aerogels at 600°C yielded 225% and 53% higher surface areas than the corresponding xerogel, confirming the significance of drying step in the sol?gel method. Asphaltene adsorption from a model oil with asphaltene concentration of 3000 ppm indicated that the aerogels possessed higher adsorption capacities for the bulky asphaltene molecules than xerogels calcined at the same temperature of 600°C, which was due to their enhanced textural properties.     

Synthesis of hexagonal pyramidal columnar hematite particles by a two-step solution route and their characterization

Hexagonal pyramidal columnar hematite (α-Fe₂O₃) particles have been synthesized by a hydrolysis method using iron sheets and nitric acid of mildly acidic pH at 90 °C. The morphologies and structures of the resulting products have been characterized by transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FESEM), X-ray powder diffraction (XRD), and Fourier-transform IR spectroscopy (FTIR). An aggregation growth mechanism is proposed to rationalize the formation of α-Fe₂O₃. The influence of pH on the structure and morphology of the as-prepared α-Fe₂O₃ has been investigated. Additionally, magnetic investigations have shown that the hexagonal pyramidal columnar α-Fe₂O₃ exhibited weak ferromagnetism at room temperature.     

Room temperature NO2 gas sensor based on PPy/α-Fe2O3 hybrid nanocomposites

Polypyrrole–iron oxide (PPy/α-Fe₂O₃) hybrid nanocomposite sensor films were prepared by spin coating method on glass substrate and characterized for structural and morphological properties by means of X-ray diffraction (XRD), Fourier transform infra red (FTIR) spectroscopy and scanning electron microscopy (SEM), which proved the strong interaction between polypyrrole and α-Fe₂O₃ nanoparticles. The gas-sensing properties of the hybrid nanocomposites were studied and compared with those of PPy and α-Fe₂O₃. It was found that PPy/α-Fe₂O₃ hybrid nanocomposites can complement the drawbacks of pure PPy and α-Fe₂O₃ to some extent. It was revealed that PPy/α-Fe₂O₃ (50%) hybrid sensor operating at room temperature could detect NO₂ at low concentration (10 ppm) with very high selectivity (18% compared to C₂H₅OH) and high sensitivity (56%), with better stability (85%). The sensing mechanism of PPy/α-Fe₂O₃ materials to NO₂ was presumed to be the synergism of PPy and α-Fe₂O₃ or the effect of p–n heterojunctions.     

EFFECT OF COMPOSITION AND TEMPERATURE ON SOLUBILITY OF IRON OXIDE IN GROUND-COAT ENAMEL GLASS*

The solubility of iron oxide in ground-coat enamel glasses at various temperatures was studied by adding varying amounts of ferric oxide to the milled enamel and giving the mixture a heat-treatment to acquire uniformity without devitrification at the desired temperature. The iron oxide solubility was obtained by finding the breaking point in the curve for iron oxide versus index of refraction. The frit solubilities were obtained at 1400°, 1600°, 1800°, and 2000°F. with variations in Na₂O, B₂O₃, A1₂O₃, CaF₂, CaO, F₂, SiO₂, COO, NiO, MnO₂, BaO, and MoO₃. Data are given on a number of commercial frits.     

Synthesis and characterization of an iron oxide poly(styrene-co-carboxybutylmaleimide) ferrimagnetic composite

In situ precipitation of iron oxide nanoparticles within the cross-linked styrene-(N-4-carboxybutylmaleimide) copolymer was carried out by an ion-exchange method. The resulting composite was studied by X-ray photoelectron (XPS) and Fourier transform infrared (FTIR) spectroscopies. FTIR analysis showed the evolution of iron oxide deposition and the formation of sodium carboxylate due to the deposition treatment. In addition, XPS analysis indicated the complete oxidation of iron(II) to iron(III) by the presence of the representative peaks of iron oxide and iron oxyhydroxide. X-ray diffraction analysis was used to identify the inorganic phases. The results showed the formation of maghemite (γ-Fe₂O₃), and after several deposition cycles, goethite (α-FeOOH). The morphology and spatial distribution of iron oxide particles within the copolymer matrix were determined by transmission electron microscopy. The mean particle size of the iron oxide was of 14 nm as determined from wide-angle X-ray diffraction using the Scherrer equation. The evolution of magnetic properties with the number of deposition cycles was investigated by magnetometry at room temperature. The poly(styrene-co-N-4-carboxybutylmaleimide)/γ-Fe₂O₃/α-FeOOH/composite showed a soft ferrimagnetic behavior and, after the third deposition cycle, showed a saturation magnetization of 8.04 emu/g at 12 kOe and coercivity field of 51 Oe.     

Magnetite/maghemite mixture prepared in benzyl alcohol for the preparation of α″-Fe16N2 with α-Fe

Fine powder of iron oxide has been required in the preparation of ferromagnetic α″-Fe₁₆N₂ powder in low temperature nitridation. Magnetite and maghemite mixture was obtained by the reaction of iron acetylacetonate Fe(acac)₃ in benzyl alcohol. It was reduced to α-Fe in hydrogen at 400 °C. Particle size of the α-Fe was 300 nm in the reduction of iron oxides obtained from 2 g of Fe(acac)₃. It decreased to 100 nm in the preparation using 8 g of Fe(acac)₃. After the successive nitridation under ammonia flow at 160 °C, the highest yield of α″-Fe₁₆N₂ in 66 wt.% was observed on the nitrided product from the latter α-Fe, because of the smallest α-Fe particle size after the reduction in the present study. The yield and reproducibility of α″-Fe₁₆N₂ formation in low temperature nitridation was improved using the iron oxide prepared in non-aqueous benzyl alcohol compared to the use of magnetite obtained from aqueous solution.     

Synthesis of monodispersed micaceous iron oxide by the oxidation of iron with oxygen under hydrothermal conditions

The oxidation behavior of iron powder with oxygen was investigated in 5–25 m NaOH solutions at 5 MPa of oxygen partial pressure and 130–290°C, where m = mol(kg H₂O)^?1. Monodispersed micaceous iron oxide, α-Fe₂O₃, was synthesized by the oxidation of iron powder with 5 MPa of oxygen in 10–16 m NaOH solutions at 250–270°C. The diameter of micaceous iron oxide greatly changed depending on the reaction conditions such as the temperature, reaction time and concentrations of NaOH and coexisting ions.     

Adsorptive removal of Congo red dye from wastewater by mixed iron oxide–alumina nanocomposites

Nanocomposites composed of mixed iron and aluminium oxide (Fe₂O₃–Al₂O₃), have been synthesized by hydrothermal method, and further used as adsorbent for the adsorptive decolorization of Congo red dye from an aqueous system. The as-prepared nanomaterials were sintered at 500 °C and 1000 °C, to obtain pure Fe₂O₃–Al₂O₃ mixed composites. The XRD studies confirmed the formation of pure and crystalline FeOOH–AlOOH (as-prepared), γ-Fe₂O₃–Al₂O₃ at 500 °C and α-Fe₂O₃–Al₂O₃ phases at 1000 °C. The morphology and size of the obtained nanocomposites were characterized by SEM and TEM. Effects of pH, contact time, initial concentration of adsorbate have been studied. The optimum pH for maximum removal of Congo red in all the three phases of nanocomposites was found to be 7. The maximum removal capacity was 498 mg/g for γ-Fe₂O₃–Al₂O₃ phases. Among the three different adsorbents, γ-Fe₂O₃–Al₂O₃ shows complete removal within 15 min of contact time.     

Mechanochemical synthesis of zinc ferrite from zinc oxide and α-Fe2O3

Mechanochemical synthesis of zinc ferrite (ZnFe₂O₄) from a powder mixture of zinc oxide (ZnO) and hematite (α-Fe₂O₃) by room temperature grinding using a planetary ball mill was investigated. The grinding enables us to obtain the amorphous mixture of the starting materials. Most of ZnO reacts with α-Fe₂O₃ to convert into insoluble amorphous zinc and iron compounds within 2h-grinding. Prolonged grinding enhances the crystallization of ZnFe₂O₄ from the amorphous compounds. ZnFe₂O₄ crystallized by the grinding for 3 h or more consists of nanocrystalline particles with high specific surface area.     

Highly stable and selective ethanol sensor based on α-Fe2O3 nanoparticles prepared by Pechini sol–gel method

In the present work, α-Fe₂O₃ nanoparticles were successfully synthesized by Pechini sol–gel (PSG) method following annealing at 550 °C. The morphology and microstructure of the prepared α-Fe₂O₃ nanoparticles were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman analysis. The electrical and sensing properties were also investigated. The α-Fe₂O₃ based sensor showed good sensitivity and selectivity towards ethanol at the optimal temperature of 225 °C. Moreover, the sensor displayed good electrical and sensing stability. These results suggest the potential applications of α-Fe₂O₃ synthesized by Pechini sol–gel method as a sensor material for ethanol detection.     

Synthesis and characterization of nanometric gadolinia powders by room temperature solid-state displacement reaction and low temperature calcination

Nanometric-sized gadolinia (Gd₂O₃) powders were obtained by applying solid-state displacement reaction at room temperature and low temperature calcination. The XRD analysis revealed that the room temperature product was gadolinium hydroxide, Gd(OH)₃. In order to induce crystallization of Gd₂O₃, the subsequent calcination at 600 ∼ 1200 °C of the room temperature reaction products was studied. Calculation of average crystallite size (D) as well as separation of the effect of crystallite size and strain of nanocrystals was performed on the basic of Williamson-Hall plots. The morphologies of powders calcined at different temperatures were followed by scanning electron microscopy. The pure cubic Gd₂O₃ phase was made at 600 °C which converted to monoclinic Gd₂O₃ phase between 1400° and 1600 °C. High-density (96% of theoretical density) ceramic pellet free of any additives was obtained after pressureless sintering at 1600 °C for 4 h in air, using calcined powder at 600 °C.