Magnetite nanowires in MCM-41 type mesoporous silica templates

MCM-41 mesoporous material was chosen as a template of very small Fe₃O₄ particles. The results of structural and magnetic studies of magnetite nanowires are reported. The average length of these nanowires is about 70 nm and their diameter is 3 nm. Magnetite polycrystalline nanowires were characterized by means of X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy (MS). Almost 80% of the particles exist in a superparamagnetic state at room temperature. Mössbauer measurements also provided evidence that the composites displayed a distribution of magnetic particles by size. As a result, strong changes of superparamagnetic and magnetic relative contributions along with temperature were observed.     

Effect of ferric ions on the morphology and size of magnetite nanocrystals synthesized by ultrasonic irradiation

Two‐dimensional plate‐like Fe₃O₄ nanocrystals and nanoparticles could be synthesized by a simple one‐step sonochemical method through ultrasonic irradiation in reverse co‐precipitation solution at low temperature. This technique provided a facile and rapid way to prepare Fe₃O₄nanocrystals with different morphology and size. Magnetite nanoplates were synthesized with only ferrous salt adding into alkali solution, and adding ferric ions with low molar ratio in the metal salts solution would lead to the formation of very small magnetite nanoparticles (∼10 nm). The size of as‐prepared magnetite nanoparticles increased with increasing reaction temperature and showed narrow size distribution, the standard deviation less than 2 nm. This investigation indicated that ferric ions had significant influence on the morphology of Fe₃O₄ nanocrystals. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hydrothermal synthesis of organic hybrid BaTiO3 nanoparticles using a supercritical continuous flow reaction system

Barium titanate (BaTiO₃: BT) nanoparticles were synthesized by the hydrothermal method in the presence of dispersants using a continuous supercritical flow reaction system. The reactants of TiO₂ sol/Ba(NO₃)₂ mixed solution and KOH solution were used as starting materials and that was heated quickly up to 400 °C under the pressure of 30 MPa for 8 ms as reaction time. The dispersant solution such as polyacrylic acid (PAA) and polyoxyethylene(20) sorbitan monooleate (Tween 80) was injected in the cooling process after the reaction. The crystal phase of the obtained particles was identified as perovskite cubic BaTiO₃ by X-ray diffractometry (XRD) and Raman spectroscopy. Raman spectra and thermogravimetric data revealed that PAA and Tween 80 fabricated hybrid BT nanoparicles. Primarily particle size of the BaTiO₃ nanoparticle was determined by means of BET surface area, as small as less than 10 nm irrespective of dispersants. In contrast, dispersed particle size in solution measured by dynamic light scattering (DLS) technique decreased from 282 nm to less than 100 nm depending on the dispersant. Aggregation of BaTiO₃ nanoparticles might be depressed in the presence of dispersants, especially PAA is the most effective among the dispersants examined.     

Scale-up synthesis of zinc borate from the reaction of zinc oxide and boric acid in aqueous medium

Synthesis of zinc borate was conducted in a laboratory and a pilot scale batch reactor to see the influence of process variables on the reaction parameters and the final product, 2ZnO·3B₂O₃·3.5H₂O. Effects of stirring speed, presence of baffles, amount of seed, particle size and purity of zinc oxide, and mole ratio of H₃BO₃:ZnO on the zinc borate formation reaction were examined at a constant temperature of 85 °C in a laboratory (4 L) and a pilot scale (85 L) reactor. Products obtained from the reaction in both reactors were characterized by chemical analysis, X-ray diffraction, particle size distribution analysis, thermal gravimetric analysis and scanning electron microscopy. The kinetic data for the zinc borate production reaction was fit by using the logistic model. The results revealed that the specific reaction rate, a model parameter, decreases with increase in particle size of zinc oxide and the presence of baffles, but increases with increase in stirring speed and purity of zinc oxide; however, it is unaffected with the changes in the amount of seed and reactants ratio. The reaction completion time is unaffected by scaling-up.     

Fe3+ doped SiO2 nanostructured gel-glasses: Structural,optical and magnetic properties

Transparent crack-free and bubble-free Fe³⁺ doped SiO₂ nanostructured gel-glasses were obtained by the sol–gel process. The process involves the hydrolysis and condensation of an appropriate molar ratio of tetraethoxysilane (TEOS), absolute ethanol, nitric acid and ferric nitrate, followed by stepwise annealing at temperatures ranging from 110 °C to 1000 °C. The structural variation of the gel-glasses and their influence on physical properties during annealing has been studied. It has been observed that monolithicity and chemical environment around Fe³⁺ in the gel-glasses are strongly dependent on the annealing temperatures. The colour of gel-glass samples is different for different annealing temperatures, mainly due to the different co-ordination state of Fe³⁺ and the generation of Fe₂O₃ colloids of size 20–60 nm in the silica matrix. The annealing process facilitates the tuning of the UV–visible transmission cut-off edge in high optical quality Fe³⁺ doped silica gel-glasses. A marked difference in the magnetic properties of these glasses is also observed with annealing temperatures.     

Effect of trace metals on reactive crystallization of gypsum

The effect of Fe²⁺, Fe³⁺, and Cr³⁺ ions on crystallization of calcium sulfate dihydrate (gypsum) produced by the reaction between calcium hydroxide suspension and sulphuric acid solution was investigated at 3.5 pH and 65°C in the absence and presence of 2500 ppm citric acid concentration. Crystal size distributions, filtration rates, and morphology of gypsum were determined and discussed as a function of ion concentration. Average particle size of gypsum was not affected significantly by the presence of Fe²⁺, Fe³⁺, and Cr³⁺ ions individually. Variation of gypsum morphology depending on ion concentration affected the filtration characteristics. The presence of Fe³⁺ or Cr³⁺ ions besides 2500 ppm citric acid influenced both average particle size and filtration characteristics. The effect of citric acid on gypsum morphology was suppressed at high Fe³⁺ and Cr³⁺ ion concentrations. The change of morphology is related to the complex formation between Fe³⁺ or Cr³⁺ ions and citric acid at high ion concentrations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical absorption spectra of LiNbO3, Fe:LiNbO3, and Zn:Fe:LiNbO3 single crystals grown by Bridgman method

The optical absorption spectra of LiNbO₃ (LN), Fe:LiNbO₃ (Fe:LN), and Zn:Fe:LiNbO₃ (Zn:Fe:LN) single crystals grown by Bridgman method were measured and compared. The absorption characteristics of the samples and the effects of growth process conditions on the absorption spectra were investigated. The Fe, Zn and Li concentrations in the crystals were analyzed by inductively coupled plasma (ICP) spectrometry. The results indicated that the overall Fe ion and Fe²⁺ concentration in Fe:LN and Zn:Fe:LN crystals increased along the growing direction. The incorporation of ZnO in Fe:LN crystal induced increase of Fe²⁺ in the crystal. Among Fe‐doped and Zn:Fe‐codoped LN single crystals, 3 mol% ZnO doped Fe:LN had a biggest change of Fe²⁺ ion concentration from bottom to top part of crystal. The effects of technical conditions (atmosphere and thermal history) on Fe²⁺ ion concentration were discussed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Precipitation of calcium carbonate particles by gas–liquid reaction: Morphology and size distribution of particles in Couette-Taylor and stirred tank reactors

The morphology and size of CaCO₃ precipitated by CO₂–Ca(OH)₂ reaction in stirred tank and Couette-Taylor reactors were experimentally investigated. The Taylor vortex in CT reactor encouraged more homogeneous mixing conditions, resulting in the production of smaller particles with a uniform shape throughout the reactor. However, in the stirred tank reactor, the local non-homogeneity of the mixing intensity led to the simultaneous production of cube-like and spindle-like particles at a high reactant concentration. The agglomeration of CaCO₃ resulted in a bimodal size distribution. However, the morphology and size of a single particle were predominantly changed by the excess species in the solution. The largest mean size and cube-like particles were observed under stoichiometric reaction conditions. As the excess species concentration increased, the morphology was transformed to a spindle-like shape and the mean size decreased due to selective adsorption of the excess species on the crystal faces.     

Crystal growth and spectroscopic properties of Yb:CaYAlO4 single crystal

A Yb³⁺-doped CaYAlO₄ laser crystal has been grown by the Czochralski technique. The segregation coefficient was measured by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The cell parameters were analyzed with X-ray diffraction experiments. Color defects in Yb:CaYAlO₄ have been evidenced to be similar to those in undoped CaYAlO₄. The polarized absorption spectra and the fluorescence spectrum of the Yb:CaYAlO₄ crystal were measured at room temperature. The fluorescence decay time of the Yb³⁺ ion was investigated. The results show that Yb:CaYAlO₄ has potential as a laser gain medium for an ultrashort laser system.     

Cs2LiGdCl6 (Ce): New scintillation material

We investigated the scintillation properties of Cs₂LiGdCl₆:Ce³⁺ as a function of the Ce concentration. X-ray excited luminescence spectra of the scintillation material showed broad emission bands between 360 and 460 nm, with two overlapping peaks, due to the d→f transitions on Ce³⁺ ions. The samples provide good scintillation results. The energy resolution was found to be 5.0% (FWHM) at 662 keV for 10% Ce sample. Under γ-ray excitation, Cs₂LiGdCl₆:Ce³⁺ showed three exponential decay time components of about 130–200 ns decay time constant. The light output of the investigated samples was 20,000 photons/MeV for a 10% Ce concentration. The light output deviation from the linear response is within 7% between the energy range of 31 and 1333 keV. Overall, the scintillation properties confirm that Cs₂LiGdCl₆:Ce³⁺ single crystal is a promising candidate for medical imaging and radiation detection.     

Facile synthesis and luminescence properties of octahedral YVO4:Eu microcrystals

Uniform octahedral YVO₄:Eu³⁺ microcrystals have been successfully prepared through a designed two-step hydrothermal conversion method. One-dimensional precursor Y₄O(OH)₉NO₃ was first prepared through a simple hydrothermal process without using any surfactant, catalyst or template. Subsequently, well-defined octahedral YVO₄ was synthesized at the expense of the precursor during a hydrothermal conversion process. XRD results demonstrate that the diffraction peaks of the final product can be well indexed to the pure tetragonal phase of YVO₄. The SEM and TEM images indicate that the as-prepared YVO₄ sample has regular octahedral shape with sharp corners and well-defined edges. The as-obtained YVO₄:Eu³⁺ phosphor shows strong red emission under ultraviolet excitation or low-voltage electron beam excitation. Furthermore, this facile and general conversion method may be of much significance in the synthesis of many other lanthanide compounds with uniform morphology.     

Synthesis and magnetic properties of monodisperse Fe3O4 nanoparticles with controlled sizes

Monodisperse magnetite nanoparticles coated with organic ligands have been chemically synthesised by using the poliol method. Modifying the synthetic conditions, particle diameter can be tuned from 3.5 to 7.1 nm. In order to investigate the critical size effects on the magnetic behavior of the samples, Fe₃O₄ nanoparticles have been fully characterized by X-ray diffraction, thermogravimetric analysis, transmission electron microscopy, SQUID magnetometry and electron paramagnetic resonance spectroscopy.     

Shape evolution of zinc oxide from twinned disks to single spindles through solvothermal synthesis in binary solvents

Shape evolution of ZnO crystals from twinned disks to single spindles was studied through solvothermal synthesis in binary solvents N,N-diethylformamide (DEF) and methanol (MeOH). The MeOH content in DEF had large influence on the morphology of the obtained ZnO crystals. In MeOH-free DEF, well-shaped ZnO twinned disks with perfect mirror symmetry could be formed through the assembly of ZnO₄⁶⁻–julolidinium–ZnO₄⁶⁻ growth units on the (0 0 0 1) growth interfaces. For small amounts of MeOH (MeOH/DEF=0.04), elongated twinned disks were formed since the growth along the polar c-axis was enhanced. With increasing MeOH content (MeOH/DEF=0.1), twinned rods with reduced mirror symmetry were formed. When a large amount of MeOH was added to DEF (MeOH/DEF=0.5), single spindles rather than twinned disks or twinned rods were obtained. A similar shape evolution of zinc oxide was observed in binary solvents DEF and N,N-dimethylformamide (DMF), suggesting that the growth of ZnO crystals with tuneable shape and size can be controlled by the composition of the binary solvent mixture.     

Synthesis, properties, and applications of magnetic iron oxide nanoparticles

Magnetic nanoparticles exhibit many interesting properties that can be exploited in a variety of applications such as catalysis and in biomedicine. This review discusses the properties, applications, and syntheses of three magnetic iron oxides – hematite, magnetite, and maghemite – and outlines methods of preparation that allow control over the size, morphology, surface treatment and magnetic properties of their nanoparticles. Some challenges to further development of these materials and methods are also presented.     

Iron redox equilibrium, structure and properties of zinc iron phosphate glasses

Iron redox equilibrium, structure and properties were investigated for the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses melted at different temperatures. The structure and valence states of the iron ions in these glasses were investigated using Mössbauer spectroscopy, Raman spectroscopy and differential thermal analysis. Mössbauer spectroscopy indicated that the concentration of Fe²⁺ ions increased in the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glass with increasing melting temperature. The Fe²⁺/(Fe²⁺ + Fe³⁺) ratio increased from 0.18 to 0.38 as the melting temperature increased from 1100 to 1300 °C. The measured isomer shifts showed that both Fe²⁺ and Fe³⁺ ions are in octahedral coordination. It was shown that the dc conductivity strongly depended on Fe²⁺/(Fe²⁺ + Fe³⁺) ratio in glasses. The dc conductivity increases with the increasing Fe²⁺ ion content in these glasses. The conductivity arises from the polaron hopping between Fe²⁺ and Fe³⁺ ions which suggests that the conduction is electronic in nature in zinc iron phosphate glasses.     

Superparamagnetism and ferrimagnetism of the small particles of magnetite in a silicate matrix

The magnetic behaviour of a basalt glass and glass-ceramic was studied by magnetization measurements between 4 and 800 K as a function of a wide range of magnetic fields (H) between 0 and 60 kG. For the as-annealed glass it was found that nearly all the iron ions behaved as paramagnetic ions. In the samples heat-treated at 700 and 900°C, the magnetization (M) values showed three magnetic components: paramagnetic Fe²⁺ ions, magnetite in a superparamagnetic and in a ferrimagnetic state. This confirmed our previous Mössbauer results. The superparamagnetic behaviour of the fine particles of magnetite was interpreted by Langevin's theory. From the M (H) and M(T) values we evaluated the percentage of each component as a function of temperature, the magnetization values in the saturated states, the mean particle diameter and the particle size distributions.     

Growth and shape control of orthorhombic Fe5(PO4)4(OH)3·2H2O single crystalline dendrites

Orthorhombic Fe₅(PO₄)₄(OH)₃·2H₂O single crystalline dendritic nanostructures have been synthesized by a facile and reproducible hydrothermal method without the aid of any surfactants. The influences of synthetic parameters, such as reaction time, temperature, the amount of H₂O₂ solution, pH values, and types of iron precursors, on the crystal structures and morphologies of the resulting products have been investigated. The formation process of Fe₅(PO₄)₄(OH)₃·2H₂O dendritic nanostructures is time dependent: amorphous FePO₄·nH₂O nanoparticles are formed firstly, and then Fe₅(PO₄)₄(OH)₃·2H₂O dendrites are assembled via a crystallization-orientation attachment process, accompanying a color change from yellow to green. The shapes and sizes of Fe₅(PO₄)₄(OH)₃·2H₂O products can be controlled by adjusting the amount of H₂O₂ solution, pH values, and types of iron precursors in the reaction system.     

Growth, refined structural and spectroscopic characteristics of Tm-doped NaGd(WO4)2 single crystals

The details of Tm³⁺-doped NaGd(WO₄)₂ single-crystal growth are discussed, the results of precise investigations of its structural and spectroscopic characteristics, as well as the analysis of cross-relaxation process of Tm³⁺ ions (³H₄→³F₄, ³H₆→³F₄) in this crystal are presented. Based on the Judd–Ofelt theory, three intensity parameters, spontaneous emission probabilities, fluorescence branching ratios and fluorescence quantum efficiency from ³H₄ and ³F₄ levels were refined.     

Crystal structure and magnetic properties of the LaCo0.5Fe0.5O3 perovskite

The magnetic and crystal structures of the LaCo_0.5Fe_0.5O₃ perovskite are investigated. It is established that the unit cell of this compound at room temperature is characterized by rhombohedral distortions. As the temperature decreases, the compound undergoes a structural phase transition from the rhombohedral phase to the orthorhombic phase in the temperature range 200–300 K. The LaCo_0.5Fe_0.5O₃ perovskite has an antiferromagnetic structure with the G _z spatial orientation of the antiferromagnetic vector. The magnetic properties of the LaCo_0.5Fe_0.5O₃ perovskite are interpreted within a model according to which the ground state of Co³⁺ ions is a low-spin state and the existence of the weak ferromagnetic component is associated with the exchange interactions between the Fe³⁺ ions.