Preparation and properties of nickel ferrite (NiFe2O4) nanoparticles via sol-gel auto-combustion method

Using nickel and ferric nitrates and citric acid, NiFe₂O₄ nanoparticles are prepared by a simple and cost-effective polyvinylpyrrolidone (PVP) assisted sol-gel auto-combustion method. The synthesised nanoparticles consist of single phase inverse spinel structure of NiFe₂O₄. The particles are in spherical shape with an average size of ∼8 nm. The vibrational properties show tetrahedral and octahedral sites of NiFe₂O₄ nanoparticles. The super-paramagnetism is observed with magnetic saturation (M_s) of 50.4 emug^−1.     

Synthesis and characterization of FeVO4 nanoparticles

Iron vanadate (FeVO₄) nanoparticles were synthesized by simple co-precipitation method using various surfactants such as ethylene glycol, polyethylene glycol 200 and polyethylene glycol 400 as the structure directing agents. Systematic investigations on the structural, morphological and magnetic properties of the materials have been studied. The lattice constants of the triclinic structure of FeVO₄ were calculated from the X-ray diffraction (XRD) analyses. The average grain size was estimated to be around 35 nm, which increased with increasing the calcination temperature. The stretching and bending vibrations of Fe-O were evaluated from the FT-IR spectra. Using VSM magnetometer, magnetic property was investigated through magnetic susceptibility and magnetization measurements. FeVO₄ exhibits two magnetic ordering temperatures at T ≈ 20 K and 14 K, which is due to two different chemical environments of Fe ligands such as octahedral FeO₆ and trigonal bipyramidal FeO₅ in a six-column doubly bent chain, respectively.     

Effect of PbO-SiO2 and PbO-B2O3 flux systems on the crystalline and magnetic properties of Ni0.5Zn0.5Fe2O4 ferrite prepared from the mixed powders

Lead borate and lead silicate were added to lower the sintering temperature of a Ni_0.5Zn_0.5Fe₂O₄ ferrite prepared from the blend of two types of powders and to homogenize the grain size. 5PbO·SiO₂ and 5PbO·B₂O₃ flux systems were added to lower the sintering temperature and diminish the magnetic loss at high frequencies. The ferrites were studied by bulk density, scanning electron microscopy and impedance analysis. It was found that the addition of PbO markedly accelerated the grain growth, while SiO₂ and B₂O₃ were found to be effective to obstruct the movement of grain boundaries and to minimize the grain size. Doping with PbO in the mixed powders appropriately increased the densification and initial permeability. The ferrite doped with 1% of 5PbO·SiO₂ possessed the lowest loss tangent (tgδ) in the range of 5 M-40 MHz and the highest threshold frequency.     

Hydrothermal fabrication of various morphological α-Fe2O3 nanoparticles modified by surfactants

Two kinds of various morphological α-Fe₂O₃ nanoparticles modified by anionic surfactant (sodium dodecylsulfonate, SDS) and cationic surfactant (hexadecyipyridinium chloride, HPC), respectively, have been synthesized via hydrothermal method, using simple inorganic salt (NH₄)₃Fe(C₂O₄)₃ and alkali NaOH as starting precursors. Meanwhile, α-Fe₂O₃ nanoparticles without surfactant are also fabricated under the same conditions for comparison. The resultant products were characterized by means of Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron micrograph (TEM) combined with electron diffraction (ED) and magnetization measurements. It is interesting that the obtained α-Fe₂O₃ nanoparticles without surfactant are polyhedral with average particle size of 90 ± 35 nm; while the obtained α-Fe₂O₃ nanoparticles modified by SDS are ellipsoidal with mean particle size of major axis: ca. 420 nm; minor axis: ca. 205 nm and those modified by HPC are spherical with mean particle size of ca. 185 nm observed from TEM. In addition, magnetic hysteresis measurements reveal that the α-Fe₂O₃ nanoparticles modified by two surfactants show enhancement in coercivity (H_c) and the remanent magnetization (M_r) compared with those of the obtained α-Fe₂O₃ nanoparticles without surfactant at room temperature. The experimental results suggest that the surfactants not only significantly influence the size and shape of the particles, but also their magnetic properties.     

Microwave-hydrothermal synthesis of γ-Fe2O3 nanoparticles and their magnetic properties

Nanosized γ-Fe₂O₃ is synthesized by the microwave-hydrothermal method. Powder X-ray diffraction and transmission electron microscopic studies showed that the average particle size is 10 nm. Magnetic studies reveal that the γ-Fe₂O₃ nanoparticles are superparamagnetic at room temperature, with a superparamagnetic blocking temperature of 200 K. The magnetic characteristics of the nanoparticles indicate their strongly interacting nature.     

One-step hydrothermal process to prepare highly crystalline Fe3O4 nanoparticles with improved magnetic properties

Hydrothermal process was successfully used to synthesize Fe₃O₄ powder using ferrous chloride (FeCl₂) and diamine hydrate (H₄N₂·H₂O) as starting materials by carefully controlling the reaction conditions. The as-prepared Fe₃O₄ sample was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), and its magnetic properties were evaluated on a vibrating sample magnetometer (VSM). The nanoscale (40 nm) Fe₃O₄ powder obtained at 140 °C for 6 h possessed a saturation magnetization of 85.8 emu/g, a little lower than that of the correspondent bulk Fe₃O₄ (92 emu/g). It is suggested that the well-crystallized Fe₃O₄ grains formed under appropriate hydrothermal conditions should be responsible for the increased saturation magnetization in nanosized Fe₃O₄.     

Preparation of polycrystalline BaTi2O5 by pressureless sintering

Polycrystalline BaTi₂O₅ (BT₂) was prepared by pressureless sintering in air using BaCO₃ and TiO₂ as starting materials. XRD results of the calcined powder showed BaCO₃ and TiO₂ reacted completely after being calcined above 950 °C, showing a mixture of BaTiO₃ (BT), BT₂, BaTi₄O₉ and Ba₄Ti₁₃O₃₀. A small amount of ZrO₂ (less than 0.1 wt%) was effective to prepare BT₂ in a single phase and the second phase of BT and B₆T₁₇ increased with ZrO₂ content. BT₂ sintered body in a single phase was obtained at 1175-1300 °C when ZrO₂ content was 0.08 wt%. The maximum permittivity of BT₂ sintered body was 340 at the Curie temperature (T_c) of 463 °C and the frequency of 100 kHz.     

The preparation of magnetite Fe3O4 and its morphology control by a novel arc-electrodeposition method

Magnetite Fe₃O₄ ultrafine powder, a magnetic material, was synthesized by a novel arc-electrodeposition method. The products were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results showed that the morphologies of the Fe₃O₄ produced were greatly influenced by the diameters of the metallic iron filament electrodes and the electrolyte composition.     

Hydrothermal synthesis and magnetic properties of Co0.2Cu0.03Fe2.77O4 nanoparticles

Co_0.2Cu_0.03Fe_2.77O₄ nanoparticles with different morphologies have been synthesized directly via a simple hydrothermal method. The effects of pH value, precursor concentration, reaction temperature and surfactant on the particle size were discussed. X-ray diffraction analyses showed that the as-synthesized Co_0.2Cu_0.03Fe_2.77O₄ nanoparticles possessed typical spinel structure. Scanning electron microscope images showed different morphologies of the particles, including truncated octahedron and octahedron. It was indicated that well-dispersed Co_0.2Cu_0.03Fe_2.77O₄ nanoparticles can be synthesized at pH values ranging from 11 to 13, and reaction temperature of 160 °C. The particle size decreased from 18 to 10 nm after the addition of sodium dodecyl sulphate at the pH value of 9. The magnetic measurement showed that the as-prepared Co-Cu spinel ferrite nanoparticles possessed hard magnetic property.     

Preparation and luminescence properties of Mn-doped ZnGa2O4 nanofibers via electrospinning process

One-dimensional Mn²⁺-doped ZnGa₂O₄ nanofibers were prepared by a simple and cost-effective electrospinning process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. SEM results indicated that the as-formed precursor fibers and those annealed at 700 °C are uniform with length of several tens to hundred micrometers, and the diameters of the fibers decrease greatly after being heated at 700 °C. Under ultraviolet excitation (246 nm) and low-voltage electron beams (1–3 kV) excitation, the ZnGa₂O₄:Mn²⁺ nanofibers presents the blue emission band of the ZnGa₂O₄ host lattice and the strong green emission with a peak at 505 nm corresponding to the ⁴T₁–⁶A₁ transition of Mn²⁺ ion.     

Preparation and characterization of hollow glass microspheres coated by CoFe2O4 nanoparticles using urea as precipitator via coprecipitation method

The composite of hollow glass microspheres coated by CoFe₂O₄ nanoparticles has been successfully prepared using urea as precipitator via coprecipitation method. The resultant composites were characterized by X-ray diffraction, field emission scanning electron microscope and vibrating sample magnetometer. The results showed that the slow decomposition of urea could be beneficial to form uniform and entire cobalt ferrite coating layer on the surface of hollow glass microspheres. The smoothest morphology was obtained for the sample prepared from 0.7 M urea, while the sample prepared from 1.0 M urea had the thickest shell. This indicated that there was a competition between the morphology and thickness of the coated microspheres. A possible formation mechanism of hollow glass microspheres coated with cobalt ferrite was proposed. The magnetic properties of the samples were also investigated.     

Photoluminescence and thermoluminescence properties of Sr3Al2O6:Tb

Photoluminescence and thermoluminescence characteristics of the SrO-Al₂O₃ system doped with terbium have been studied and reported here. The phosphor was synthesized by a reflux sol-gel technique. With a view to study the dosimetric properties, the effect of beta irradiation has been studied in detail; further, concentration quenching effects observed in the emission of various terbium doped phosphors have led to optimization of dopant content. A case is made to project the phosphor as an efficient green light emitting material with good dosimetric properties.     

Hydrothermal synthesis, characterization and magnetic properties of (N4C6H21)·(Co(H2PO4)(HPO4)2)

The title compound, (N₄C₆H₂₁)·(Co(H₂PO₄)(HPO₄)₂), was prepared hydrothermally (473 K, 10 days, autogenous pressure), in the presence of the tris(2-aminoethyl)amine as organic template. Its structure is built up from a network of four membered-rings, formed by the vertex linkages between [CoO₄] and [H₂PO₄] tetrahedra with [HPO₄] moieties hanging from the Co center. Hydrogen bonds involving the cobalt phosphate units and the triply protonated amine molecule, contribute to the stability of the structure. The IR spectrum shows bands characteristic of the (N₄C₆H₂₁)³⁺ cations and the (H₂PO₄)⁻ and (HPO₄)²⁻ phosphate anions. The UV-Visible-NIR spectrum confirms the tetrahedral coordination of Co²⁺ ions. The TGA analysis indicates that the dehydration of (N₄C₆H₂₁)·(Co(H₂PO₄)(HPO₄)₂) occurs in one step. Magnetic measurements from 4.5 to 305 K show a weak antiferromagnetic character of this compound.     

Structural and magnetic properties of Mn nanoparticles prepared by arc-discharge

Mn nanoparticles are prepared by arc discharge technique. MnO, α-Mn, β-Mn, and γ-Mn are detected by X-ray diffraction, while the presence of Mn₃O₄ and MnO₂ is revealed by X-ray photoelectron spectroscopy. Transmission electron microscopy observations show that most of the Mn nanoparticles have irregular shapes, rough surfaces and a shell/core structure, with sizes ranging from several nanometers to 80 nm. The magnetic properties of the Mn nanoparticles are investigated between 2 and 350 K at magnetic fields up to 5 T. A magnetic transition occurring near 43 K is attributed to the formation of the ferrimagnetic Mn₃O₄. The coercivity of the Mn nanoparticles, arising mainly from Mn₃O₄, decreases linearly with increasing temperature below 40 K. Below the blocking temperature T_B ≈ 34 K, the hysteresis loops exhibit large coercivity (up to 500 kA/m), owing to finite size effects, and irreversibility in the loops is found up to 4 T, and magnetization is not saturated up to 5 T. The relationship between structure and the magnetic properties are discussed.     

Comparative study of nanocrystalline Zr0.85Ce0.15O2 powders synthesised by spray-pyrolysis and gel-combustion methods

Zr_0.85Ce_0.15O₂ nanopowders synthesised by gel-combustion and spray-pyrolysis methods were comparatively studied by means of X-ray diffraction, Raman spectroscopy, thermogravimetric and differential thermal analyses, specific surface area measurements, scanning and transmission electron microscopies and chemical analysis. Fully tetragonal powders were obtained by both methods, as determined by X-ray diffraction and Raman spectroscopy. Both materials exhibited extremely small crystallite sizes (about 6 nm) and high specific surface areas (93 m²/g and 42 m²/g for gel-combustion and spray-pyrolysis powders, respectively). In both cases, no tetragonal-to-monoclinic transition was observed in the whole temperature range up to 1300 °C by differential thermal analysis. The amounts of the expected impurities (Si, B, C) were acceptable and comparable in both cases.     

Fine Fe16N2 powder prepared by low-temperature nitridation

α-Fe was prepared by reduction of a fine γ-Fe₂O₃ powder under hydrogen at 500 °C for 8 h. The α-Fe fine powder, about 100 nm in crystallite size, was then nitrided under an ammonia flow at 130 °C for 100 h. X-ray single-phase Fe₁₆N₂ was obtained with a magnetization value of 225 emu/g at room temperature under a magnetic field of 15 kOe. The Mössbauer spectrum at room temperature could be resolved into three sets of hyperfine fields with an average magnetic moment of 2.52 μ_B. An additional paramagnetic component was present in the spectrum with an area ratio of 19%.     

Properties of TiO2 prepared by acid treatment of BaTiO3

TiO₃ powders were prepared by acid treatment of BaTiO₃ and their properties were investigated. The BaTiO₃ powder was subjected to HNO₃ in concentrations ranging from 10⁻³ to 8 M at 90 °C for 0.5-6 h. Dissolution of BaTiO₃ and precipitation of TiO₂ occurred at acid concentrations of 2-5 M. BaTiO₃ dissolves completely to form a clear solution at reaction times of 0.5-1 h, but a rutile precipitate is formed after 2 h of acid treatment. By contrast, anatase is precipitated by adjusting the pH of the clear solution to 2-3 using NaOH or NH₄OH solution. The rutile crystals were small and rod-shaped, consisting of many small coherent domains connected by grain boundaries with small inclination angles and edge dislocations, giving them a high specific surface area (S_BET). With increasing HNO₃ concentration, the S_BET value increased from 100 to 170 m²/g while the crystallite size decreased from 25 to 11 nm. The anatase crystals obtained here were very small equi-axial particles with a smaller crystallite size than the rutile and S_BET values of about 270 m²/g (higher than the rutile samples). The photocatalytic activity of these TiO₂ was determined from the decomposition rate of Methylene Blue under ultraviolet irradiation. Higher decomposition rates were obtained with larger crystallite sizes resulting from heat treatment. The maximum decomposition rates were obtained in samples heated at 500-600 °C. The photocatalytic activity of the TiO₂ was found to depend more strongly on the sample crystallite size than on S_BET.     

A study of the influence of crystallite size on the electrical and magnetic properties of CuFe2O4

An attempt has been made to clarify the fundamental assumption that the properties of materials change as the crystallite size of the material is reduced below 100 nm. CuFe₂O₄ samples of different crystallite sizes were prepared by the sol–gel and combustion methods and then analyzed by X-ray diffraction (XRD), thermal analyses (TGA/DTG) and scanning electron microscopy (SEM) techniques. The magnetic properties were studied by measuring the AC magnetic susceptibility (χ) and the Mössbauer spectroscopy. The DC electrical resistivity, dielectric constant, dielectric loss tangent, Curie temperature and hyperfine splitting of the samples change with the crystallite size. The change in the electrical properties is attributed to the formation of discrete energy levels instead of the bands. However, the magnetic parameters change due to the existence of non magnetic surface layers. The isomer shift and the hyperfine splitting show gradual increase with the increase in crystallite sizes.     

Hydrothermal synthesis, characterization and magnetic properties of NaVGe2O6 and LiVGe2O6

Supercritical fluids are shown to be an excellent reaction media for the synthesis of novel solid state phases at intermediate temperatures. LiVGe₂O₆ and NaVGe₂O₆ have the common pyroxene structure composed of VO₆ linear chains. NaVGe₂O₆ crystallizes in the monoclinic space group C2/c with four formula units having cell dimensions a = 9.960(4) Å, b = 8.853(10) Å, c = 5.4861(10) Å, β = 106.403(3)°. The structure was refined until R = 0.0290 and R_w = 0.0370. For LiVGe₂O₆ in space group P2₁/c: a = 9.8508(7) Å, b = 8.754(3) Å, c = 5.3948(13) Å, β = 108(3)°, R = 0.0240 and R_w = 0.0250. The compounds contain edge-shared VO₆ octahedral chains and corner-shared GeO₄ tetrahedral chains. The presence of these VO₆ chains results in spin-Peierls distortion. Structural and physical characterization of the compounds are reported.     

Study on the structures and magnetic properties of Ni, Co-Al2O3 electrodeposited nanowire arrays

We report on the direct electrodeposition of nickel and cobalt nanowire arrays within the nanopores of ordered porous alumina films prepared by a two-step anodization. SEM and TEM images reveal that the pore arrays are regularly arranged throughout the alumina film. X-ray diffraction and TEM analysis show that the nickel and cobalt nanowires are single crystalline with highly preferential orientation. The aspect ratio of nanowires is over 300. M-H hysteresis loops determined by VSM indicate that the nanowire arrays obtained possess obvious magnetic anisotropy. Because of proper square ratio and coercivity the nanowire arrays of nickel seem to be more suitable candidates for perpendicular magnetic recording medium than those of cobalt.