Synthesis and magnetic properties of hydrothermal magnesium–zinc spinel ferrite powders

Mg–Zn ferrite powders with the nominal composition Mg_0.5Zn_0.5Fe₂O₄ were synthesized via hydrothermal method, and their synthesis, magnetic properties and microstructures were studied. It was found that the pH value affected the amount of impurity Fe₂O₃ and the purity of ferrites greatly. It was also found excess Zn content (5 at.%) in starting materials was not helpful to synthesize pure spinel ferrite, while the prolonged reaction time was harmful for the formation of pure spinel structure. The specimens presented small coercivity lower than 10 Oe, which showed a typical magnetically soft behavior. With the increase of pH value, the saturation magnetization of specimens with excess Zn ions (5 at.%) kept increasing from 23.90 to 41.82 emu/g due to the decreasing amount of impurity Fe₂O₃. The study of microstructures showed that the large particles in powders were the aggregates of small nanoscale crystallites. The analysis of actual Zn and Mg content in synthesized ferrites confirmed that the best experimental conditions for synthesis of pure spinel Mg–Zn ferrite are the hydrothermal temperature is 200 °C, the reaction time is 8 h, the pH value is 12 and the excess amount of Zn(NO)₃ in starting materials is 5 at.%.     

X-ray powder diffraction and Mössbauer studies on the formation of Cd0.5Ni0.5Fe2O4/Zn0.5Ni0.5Fe2O4 spinel solid solutions

Structural investigations by X-ray powder diffraction, magnetic measurements and by Mössbauer spectroscopy, applied to a series of solid solutions formed between two mixed spinels, zinc–nickel ferrite and cadmium–nickel ferrite, indicates the difference in the cation arrangement in the solid solution obtained hydrothermally, compared to that of ferrites sintered at high temperatures. The ferrimagnetic crystalline Cd–Zn–Ni ferrite series, of a composition of Cd_0.5-αZn_αNi_0.5Fe₂O₄, have been prepared by hydrothermal treatment of the coprecipitated amorphous Cd–Zn–Ni–Fe hydroxide mixtures. The hydrothermally obtained samples displaying a defected spinel structure, with clearly noticeable non-stoichiometry, may be considered as precursors for the preparation of stoichiometric products by further thermal treatment. The first approach to the Mössbauer spectra of the system (Cd_0.5-αZn_αFe_0.5)_tetr[Ni_0.5Fe_1.5]_octO₄ analysis has been undertaken.     

Characteristics of powder and sintered bodies of hydrothermally synthesized Mn-Zn ferrites

To improve the sinterability of powders fabricated by the conventional mixed-oxides method, ultrafine Mn-Zn ferrite powders were hydrothermally synthesized from metal nitrates solution using ammonia as a precipitant. The R value (alkalinity) was introduced to adjust the amount of added OH^– in the reaction suspension. The characteristics of the powders synthesized at different hydrothermal conditions and the properties of the sintered bodies were investigated. The results show that the R value and hydrothermal time have a great effect on the compositions and phases of hydrothermally synthesized Mn-Zn ferrite powders. Powders synthesized from a starting suspension with a higher content of Zn ions (or lower content of Mn²⁺) may approach to a stable spinel structure with a lower Mn/Zn ratio as the hydrothermal time is longer. Factors affecting the position of the diffraction angle (2) of the spinel Mn-Zn ferrite (311) of powders may include both the compositions of spinel ferrite structure and crystallite sizes (or particle sizes) of powders. Some possible reasons were suggested to explain the dependence of composition and phase of hydrothermally synthesized Mn-Zn ferrite powders on the R value and hydrothermal time. The temperature that the green compact begins to shrink at increases with increasing R value, and ranges from 510°C (R = 2) to 650°C (R = 6). After being sintered at 950°C for 2 h in N₂ atmosphere, the relative sintered density of each specimen reaches a value of 94.5–99.8%.     

Self-propagating high temperature synthesis of Ni0.35Zn0.65Fe2O4 ferrite powders

The stoichiometric Ni_0.35Zn_0.65Fe₂O₄ ferrite powders were synthesized by SHS method. In the process of SHS, the effects of the molar ratio Fe/Fe₂O₃ in the starting mixture, oxygen pressure, grain size and relative density of the raw materials on combustion temperature, combustion wave velocity, phase composition and microstructure of the combustion products were investigated. X-ray diffraction, scanning electron microscope, TEM, vibrating sample magnetometry were used to characterize the microstructure and magnetic properties of the products. The results showed that as the molar ratio Fe/Fe₂O₃ increases, the combustion temperature and combustion wave velocity increased. The same results can be observed when the oxygen pressure increased from 0.1 to 0.9 MPa. The increase of grain size and relative density of raw materials resulted in the decrease of combustion temperature and combustion wave velocity. Compared with other methods, SHS process leads to ferrite powders with improved magnetic properties.     

Synthesis of superparamagnetic ZnFe2O4 nanoparticle by surfactant assisted hydrothermal method

Zinc ferrite (ZnFe₂O₄) ultrafine powder was synthesized by hydrothermal method using various amounts of cetyltrymethylammonium bromide (CTAB) as a surfactant. The phase purity, thermal stability, morphology, size and magnetic properties of the final products were studied. All the synthesized products were possessing normal spinel structure without any impurities. The crystalline size of synthesized products decreased with the increasing amounts of surfactant. Particles had narrow size distribution with an average particle size of 6.5 nm for 1.5 g of CTAB. Magnetic characterizations revealed that the synthesized products were superparamagnetic in nature.     

Comparative Studies on the Structure and Magnetic Properties of Ni–Zn Ferrite Powders Prepared by Glycine-Nitrate Auto-combustion Process and Solid State Reaction Method

Ni–Zn ferrite compositions (Ni_1?x Zn _x Fe₂O₄) are well known due to their remarkable soft magnetic properties, which potentially have a broad range of applications in many areas. In this study, Ni–Zn ferrite with the chemical formula of Ni_0.64Zn_0.36Fe₂O₄ was prepared by the glycine-nitrate autocombustion process (GNP) and solid state reaction method (SSRM). In order to achieve a desirable particle size, the SSRM powders were milled for 3 h at a milling rate of 200 rpm. The structure and magnetic properties of the ferrite powders, which were synthesized by both methods, were characterized and their properties were compared. The results indicate that a significant amount (～?90 wt.%) of nanocrystalline Ni_0.64Zn_0.36Fe₂O₄ ferrite with the average crystallite size of 47 nm, particle size of 200 nm, saturation magnetization of 73 emu/g and coercivity of 54 Oe has been formed by means of the glycine-nitrate process. The results also show that not only the saturation magnetization of the GNP ferrite powder is relatively similar to that of the milled SSRM powders, but also it is synthesized at a much shorter duration than that of the solid state reaction method.     

Photochemical hydrogen production from a water-methanol mixture with small particles of iron oxide suspensions

Photochemical hydrogen production has been detected from small particles of doped iron oxide in a methanol-water (1:1) mixture. The systems studied consisted of a pure n-type semiconductor, Fe_2−xNb_xO₃ (x=0.02), a pure p-type semiconductor, La_1−xSr_xFeO₃ (x=0.25), and Mg-doped α-Fe₂O₃. This system was found to be heterogeneous, consisting of both spinel and corundum phase iron oxides, and shows a much higher activity than the pure single phase systems, with either corundum (α-Fe₂O₃) or spinel (Fe_3−xMg_xO_y) structure. The efficiency of the reaction increased substantially when the powders were loaded with Pt. Hydrogen production from these Mg-doped iron oxides is photo-ocatalytic and occurs mainly as a result of bandgap irradiation, but also occurs with sub-bandgap illumination in small amounts. There is a linearly increasing dependence of the H₂ production with increasing light intensity.     

Effect of Substitutions of Zn for Mn on Size and Magnetic Properties of Mn–Zn Ferrite Nanoparticles

In this study Mn?CZn ferrite nanoparticles (Mn_(1?x)Zn _x Fe₂O₄, x=0, 0.3 and 0.5) were produced by a chemical co-precipitation method. The structure and size of the Mn?CZn ferrite nanoparticles were characterized using X-ray diffraction (XRD) and Transmission electron microscopy (TEM). Results show that the ferrite nanoparticles have the spinel structure. It was found that the size of Mn?CZn ferrite nanoparticles decreases by increasing of the Zn concentration. The magnetic properties of Mn?CZn ferrite nanoparticles were investigated with a vibrational sample magnetometer (VSM) and it was observed that Mn_0.7Zn_0.3Fe₂O₃ ferrite nanoparticles have the maximum saturation magnetization and that the initial susceptibility decreases with the increase in Zn concentration.     

0.3–3 GHz magneto-dielectric properties of nanostructured NiZnCo ferrite from hydrothermal process

A simple hydrothermal route with cetyltrimethylammonium bromide was proposed for directly synthesizing single-crystalline NiZnCo ferrite at 160 °C. X-ray diffraction patterns and micrographs indicate that products consist of spinel ferrite nanocrystals. The dielectric constant of NiZnCo ferrite is about 11 and the imaginary part of complex permittivity is 1.3. The saturation magnetization of Ni_0.54Zn_0.48Fe_1.98O₄ increases from 41.36 to 73.9 emu/g for Ni_0.55Zn_0.46Fe_1.98O₄ with a cobalt stoichiometry of 0.01. The real part μ′ of complex permeability for NiZnCo ferrite reaches 3 at 1 GHz. The imaginary part μ″ of NiZnCo ferrite has values higher than 1.2 within 0.7–3 GHz. Through the incorporation of the magnetic fillers, the low dielectric constant of the composites may meet the requirements of impedance matching to achieve maximal absorption of the electromagnetic energy in GHz frequency range.     

A study of sintering and magnetic parameters of spinel lithium ferrite: II. Lithium ferrite containing Bi2O3

High-dispersity powders of spinel lithium ferrite, Li_0.5Fe_2.5O₄, containing different amounts of Bi₂O₃ were prepared by thermal treatment of mixtures of /Fe₃(HCOO)₆(OH)₂/HCOO.4H₂O, LiHCOO.H₂O and Bi(HCOO)₃ obtained by spray drying. It was found that sintering of lithium ferrite in the presence of Bi₂O₃ leads to very high densities of the products at 900 – 1000°C. This is due to the formation, by Li_0.5Fe_2.5O₄ and Bi₂O₃, of an easily melting eutectic. The study of the magnetic properties showed that the presence of Bi₂O₃ ensured the formation of lithium ferrites with good characteristics at considerably lower temperatures than those usually observed.     

Preparation of acicular NiZn-ferrite powders

Acicular NiZn-ferrite powder particles have been prepared from goethite-derived acicular Fe₂O₃ and spherical constituent oxides in the presence of molten chloride or sulphate. The morphology of NiZn-ferrite particles has been studied with reference to the effect of chemical species of molten salts, ferrite composition and particle size of constituent divalent oxides. Hence the best preparation conditions for acicular NiZn-ferrite powder with homogeneous composition have been determined; a mixture of acicular Fe₂O₃ and submicron constituent oxides is heated at 900° C in an amount of potassium chloride with four times the weight of ferrite. Electron diffraction analysis shows that the particle axes of acicular Fe₂O₃ and NiZn-ferrite are parallel to [¯1 1 0 0]_haematite and [¯1 0 1]_ferrite and that a topotactic relation is retained.     

Synthesis of Zn0.5Mn0.5Fe2O4 by low-temperature spray pyrolysis

The effect of synthesis temperature on the structural perfection of the Zn_0.5Mn_0.5Fe₂O₄ ferrite synthesized via spray pyrolysis of a solution of Zn(II), Mn(II), and Fe(III) nitrates has been studied using X-ray diffraction, scanning electron microscopy, and IR spectroscopy. The material obtained at 650°C is shown to have a nanocrystalline structure. IR spectroscopy results indicate that the synthesized Zn_0.5Mn_0.5Fe₂O₄ spinel ferrite is highly homogeneous in composition and structure.     

Synthesis of manganese–zinc ferrite by powder mixing using ferric oxide from a steel mill acid recovery unit

With the aim of producing fine-grained manganese–zinc (Mn–Zn) ferrite at the end of a calcination process at moderate temperatures, this study consisted, at first, of an “electrochemically designed” powder mixing by wet-ball milling a mixture of manganese (MnO₂), zinc (ZnO), and iron (Fe₂O₃ granules produced by an acid recovery unit of a Brazilian steelmaker, milled to fine sizes using alkaline media) –based raw materials. This mixing/milling resulted in improved size reduction when compared to milling without any alkali addition. Further, noticeable size reduction was achieved when elemental Zn was used in place of ZnO, especially when ammonia was used as the medium. Calcination of the alkaline-milled mixture of MnO₂ + ZnO + Fe₂O₃ at 1200 °C allowed obtaining well-crystallized single-phase Mn–Zn ferrite, whereas calcination of the MnO₂ + ZnO + Fe₂O₃ mill-mixed in 100% NH₄OH at 1200 °C produced the highest saturation magnetization in the as-calcined state.     

Preparation and characteristics of Ni-ferrite powders obtained in the presence of fused salts

This paper describes Ni-ferrite formation in the presence of Li₂SO₄-Na₂SO₄ molten salts, and in particular the effects of the raw materials, amount of salts, heating temperature and time on the size and shape of the ferrite powders, as well as on the rate of ferrite formation. The molten salts accelerate ferrite formation and complete ferrite formation is attained at lower temperatures than in solid state reactions. Ferrite powders with two types of shapes are obtained from NiO with different particle sizes and aggregation states; one is similar to the starting Fe₂O₃ particles and the other has an octahedral crystal habit. The difference follows from the different dissolution rates of Fe₂O₃ and NiO. Growing particles in molten salts have a crystal habit, but the most stable particle shape is rounded.     

Titanate coupling agent effects on nonaqueous Co2Z ferrite suspensions dispersion

Co₂Z ferrite powders with the chemical composition 3Ba_0.5Sr_0.5O·2CoO·12Fe₂O₃ have superior high frequency magnetic properties. However, Co₂Z ferrite powders are difficult to apply to practical processes because of agglomeration induced by the strong magnetic attraction between particles. In this study, Co₂Z ferrite powder pretreatment using a titanate coupling agent—Neopentyl (dially)oxy tri(dioctyl)pyrophosphate titanate (Lica 38) on the sedimentation and rheological behavior is investigated. The bonding mechanisms between ferrite powder, Lica 38, and dispersant (KD1) are studied using diffuse reflectance Fourier transform infrared spectroscopy is used to explain the difference in the rheological and sedimentation behaviors of untreated and titanate coupling agent-modified ferrite powders. The affinity of Co₂Z ferrite and dispersants could be substantially enhanced by coating a titanate- coupling agent onto the ferrite surface. The coated layer could prevent particles from agglomerating from magnetic interaction.     

Hydrothermal formation of Ni-Zn ferrite from heavy metal co-precipitates

The hydrothermal synthesis of Ni-Zn ferrite from simulated wastewater containing Ni²⁺ and Zn²⁺ ions has been studied. The influence of co-precipitation order, the existence of Na⁺ in suspension, the hydrothermal reaction time and temperature on the composition, morphology and saturation magnetization (_s) of the hydrothermal products is reported. Adding the simulated wastewater to the NaOH solution can prevent the formation of -Fe₂O₃ in the Ni-Zn ferrite. Increasing the hydrothermal reaction time improved the magnetization of the Ni-Zn ferrite, while the influence of temperature, stirring intensity and Na⁺ in suspension on the hydrothermal formation of ferrite were not obvious. Thermodynamic calculation indicated that under hydrothermal conditions (180–240°C), the order of chemical stability is as follows: NiFe₂O₄ > Fe₂O₃ > Na₂Fe₂O₄. The high Gibbs formation energy of Na₂Fe₂O₄ prevented the incorporation of Na⁺ into the ferrite lattice.     

Synthesis of nano particle of inorganic oxides by polymer matrix

Ultrafine (⩽ 150 nm) powders of spinels [MFe₂O₄ where M = Ni(II), Co(II) and Zn(II)]; rare-earth orthoferrites [RFeO₃ where R = Sm, Nd and Gd], and rare-earth garnets [R₃Fe₃O₁₂ where R = Sm, Nd and Gd] with good purity and chemical homogeneity were prepared through two new versatile chemical routes. The first route involved the coprecipitation of the desired metal nitrates from their aqueous solution, in presence of a water soluble polymer-polyvinyl alcohol (PVA), by triethyl ammonium carbonate solution. The other process involved complete evaporation of a mixture of optimum amounts of PVA and the desired aqueous metal nitrate solutions, with and without the addition of optimum amounts of urea when the mixture was evaporated to a pasty mass. In addition, detailed study on the reported potassium ferricyanide route was also carried out for the production of the rare-earth orthoferrite powders. The various precursor as well as the heat-treated mixed-oxide powders, prepared through each of the routes, were compared by the physical characterization studies involving thermal gravimetry and differential scanning calorimetry, infrared spectroscopy, X-ray powder diffraction, transmission electron microscopy, and room temperature magnetic measurements.     

Investigation of the reactivity of AlCl3 and CoCl2 toward molten alkali-metal nitrates in order to synthesize CoAl2O4

Cobalt aluminate CoAl₂O₄ powder, constituted of nano-sized crystallites, is prepared, involving the reactivity of AlCl₃ and CoCl₂ with molten alkali-metal nitrates. The reaction at 450 °C for 2 h leads to a mixture of spinel oxide Co₃O₄ and amorphous γ-Al₂O₃. It is transformed into the spinel oxide CoAl₂O₄ by heating at 1000 °C. The powders are mainly characterized by XRD, FTIR, ICP, electron microscopy and diffraction, X-EDS and diffuse reflection. Their properties are compared to those of powders obtained by solid state reactions of a mechanical mixture of chlorides or oxides submitted to the same thermal treatment.     

Synthesis of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> powder via PVA assisted sol–gel process

CoFe₂O₄ ferrites were synthesized by sol–gel method, having metal nitrates as precursors and PVA as surfactant, followed by a heat treatment at 960 °C for 2 h. The ultrafine ferrite powders obtained have been characterized by X-ray diffraction, thermal gravimetry, differential scanning calorimetry and room temperature magnetic measurement studies. The morphology of the powder was identified by high resolution-scanning electron microscopy. X-ray diffraction results indicate that the resultant CoFe₂O₄ crystallites consist of spinel phase. Significant differences in magnetic properties of CoFe₂O₄ samples synthesized with various concentrations of PVA were observed. The magnetisation measurements show that when the PVA concentration increased, coercivity initially decreased and then increased where as retentivity and magnetisation decreased. The optimum concentration of PVA for the synthesis of CoFe₂O₄ ferrites is obtained from this investigation. Obviously this material can be used as an efficient candidate for practical recording purpose.     

The cobalt-ferrite,CoxFe3−xO4, activity in the catalytic chemical decomposition op hydrogen peroxide

The cobalt-ferrite spinel oxide series, Co_xFe_3−xO₄,(O≦ x ≦3), have been synthesized by coprecipitation. Kinetic studies nave revealed pronounced activity of these oxides in the catalytic decomposition of hydrogen peroxide. The optimum activity of this decomposition reaction is established experimentally at lattice composition of x = 1.5 in the spinel system. X-ray and scanning electron microscopy (SEM) results indicated that at x = 1.5, the Co⁺⁺ ions are at the octahedral lattice sites of the ferrite support.