Stoichiometric Ferrite Research Articles

Phase Formation, Microstructure and Permeability of Fe-Deficient Ni-Cu-Zn Ferrites, (I): Effect of Sintering Temperature

We have studied the densification, phase formation, microstructure, and permeability of stoichiometric and Fe-deficient Ni-Cu-Zn ferrites of composition Ni0.30Cu0.20Zn0.50+zFe2-zO4-(z/2) with 0 ≤ z ≤ 0.06 sintered at temperatures from 900 °C to 1150 °C. The shrinkage is shifted from 1000 °C for z = 0 towards lower temperatures and reaches its maximum rate at 900 °C for z = 0.02. Stoichiometric ferrites show regular growth of single-phase ferrite grains if sintered at Ts ≤ 1100 °C. Sintering at 1150 °C leads to the formation of a small amount of Cu2O, triggering exaggerated grain growth. Fe-deficient compositions (z > 0) form Cu-poor stoichiometric ferrites coexisting with a minority CuO phase after sintering at 900–1000 °C. At Ts ≥ 1050 °C, CuO transforms into Cu2O, and exaggerated grain growth is observed. The formation of Cu oxide second phases is investigated using XRD, SEM, and EDX. The permeability of the ferrites increases with sintering temperature up to a maximum permeability of µ = 230 for z = 0 or µ = 580 for z = 0.02, respectively, at Ts = 1000 °C. At higher sintering temperatures, the permeability decreases, which is due to the formation of a microstructure with intra-crystalline porosity in large grains, and a non-magnetic Cu oxide grain boundary phase.

Resonant Inelastic Soft X-ray Scattering Study of Co-Doped Maghemite Nanoparticles.

Cobalt ferrite nanoparticles have been attracting considerable interest in the recent years because of the large number of potential applications, including magnetic storage, magnetic fluid hyperthermia and as contrast agents for magnetic resonance imaging. Physical properties of this class of materials depend critically on a number of parameters, including crystallinity, stoichiometry and cation distribution. In this work we have performed a Resonant Inelastic soft X-ray Scattering (RIXS) study on a series of 5 nm cobalt-doped maghemite nanoparticles to obtain direct quantitative information on cation distribution as a function of cobalt doping. We found that the distribution of divalent cobalt is stable in the investigated doping range and slightly different from that of bulk, stoichiometric cobalt ferrite. These results confirm that cobalt doping can be used to finely tune the magnetic properties of nanostructured ferrites without modifying their structural integrity.

Influence of the stoichiometry and grain morphology on the magnetic properties of Co substituted Ni–Zn nanoferrites

A set of Co substituted Ni–Zn nanoferrites with a nominal composition Co0.2Ni0.3Zn0.5Fe2O4 was prepared by the polyol method. The influence of a number of synthetic parameters on the structure, microstructure, and the magnetic properties was investigated. X-ray diffraction, infrared, energy dispersive X-ray, transmission electron microscopy, and vibrating sample magnetometry were employed for this purpose. The X-ray diffraction results confirmed the formation of a single phase nanocrystalline spinel-type ferrite powders. In addition, the cell parameter and the integrated intensity ratio I220/I422 was found to vary with the synthesis conditions suggesting deviation from the nominal chemical composition and/or a probable deviation from the preferential (that of the bulk) cations occupancy of the tetrahedral (A) and the octahedral (B) spinel sites. For the so-called bulk ferrite obtained by moderate sintering nanoparticles of the stoichiometric ferrite, a cation distribution similar to that of the bulk was suggested. Transmission electron microscopy analysis of the as-produced ferrites revealed the formation of nanoparticles with mean particle size in the range ~4–12nm. The magnetic properties of both as-prepared nanoparticles and the so-called bulk ferrite were studied. All the as-produced particles exhibited superparamagnetic behavior at room temperature with a gradual decrease of the blocking temperature with the decrease of crystallite size. Additionally, the saturation magnetization, the coercivity, and the Curie temperature were found to be clearly dependent on the stoichiometry, the cations occupancy, and/or the grains morphology. For the stoichiometric ferrites the relatively higher Curie temperature values measured for the smaller particles was interpreted on the basis on the cations distribution change; for the as-produced nanoparticles a fraction of Zn2+ ions is expected to migrate from A to B sites accompanied with a reverse transfer of an equal amount of the paramagnetic cations from B to A sites.

Crystalloquasichemical Model of Spinel CoFe2O4 Formation, Obtained by Chemical co-Precipitation Method

In the work the spinel cobalt ferrite synthesized by chemical co-precipitation method. The X-ray diffraction analysis and thermal analysis confirmed the crystalline structure and phase purity of the prepared ferrite. The crystalloquasichemical mechanism of formation stoichiometric cobalt ferrite through interaction by defective hydrooxide phases was described. The surface phenomena that occur on the boundary phase distribution was described. There formation of cationic and anionic vacancies, rooted cobalt atoms or oxygen atoms. Crystalloquasichemical modeling of surface interactions between Co(OH)2 and Fe(OH)3 can trace the formation of spinel structure with the participation of antistructure of matrix and reduced to the corresponding stoichiometric hydroxides species. Shows the reaction of four types of crystal impurity clusters on the surface of the hydroxide matrix and their interaction with each other to form spinel СоFe2O4.

Polyol-made stoichiometric Co<SUB align="right">0.2Ni<SUB align="right">0.3Zn<SUB align="right">0.5Fe<SUB align="right">2O<SUB align="right">4 nanoparticles: synthetic optimisation, structural, and microstructural studies

Crystalline and stoichiometric Co0.2Ni0.3Zn0.5Fe2O4 ferrite nanoparticles were successfully synthesised in polyol medium starting from the low-cost iron (III) chloride and acetates as precursors by the optimisation a number of synthetic parameters. The crystalline structure, the chemical composition, the microstructure, and the thermal stability of the as-produced powders were investigated by X-ray diffraction, infrared spectroscopy, energy dispersive X-ray, X-ray fluorescence spectroscopy, and transmission electron microscopy. We demonstrated that in contrast to the stoichiometric ferrite, which showed excellent thermal stability, the non-stoichiometric ferrites thermally decompose resulting in a stoichiometric ferrite as a major phase and hematite. The degree of deviation from the stoichiometry was calculated on the basis of a proposed decompositional thermal scheme and a quantitative analysis using Rietveld refinement.

Influence of the temperature in the electrochemical synthesis of cobalt ferrites nanoparticles

A new electrochemical method to synthesize cobalt ferrite nanoparticles has been developed. Magnetic measurement, Mössbauer spectroscopy, X-ray diffraction, inductive coupled plasma spectroscopy, and transmission electron microscopy were carried out to characterize the cobalt ferrites synthesized at different temperatures between 25°C and 80°C. These techniques confirm the efficiency of the electrochemical method. At room temperature a mixture of different compounds was obtained with a particle diameter around 20nm, while at 80°C the synthesis of cobalt ferrite leads to a stoichiometric spinel, with a crystallite size of 40nm measured by Scherrer equation. The temperature was defined as an important parameter to obtain stoichiometric ferrites and different diameters.

Low temperature sintering of sub-stoichiometric Ni–Cu–Zn ferrites: Shrinkage, microstructure and permeability

We have studied sub-stoichiometric Ni–Cu–Zn ferrites with iron deficiency (i.e., <50mol% Fe2O3) of composition Ni0.20Cu0.20Zn0.60+zFe2−zO4−(z/2) with 0≤z≤0.06. The temperature of maximum shrinkage rate is shifted from T=1000°C for z=0 towards lower temperatures down to T=900°C for a sub-stoichiometric ferrite with z=0.02. Dense samples are obtained after firing at 900°C for z>0 only. Sub-stoichiometric compositions (z>0) do not form single-phase spinel ferrites after sintering at 900°C, but rather represent mixtures of CuO and a stoichiometric ferrite with slightly modified composition. The formation of small amounts of CuO at grain boundaries is demonstrated by XRD and SEM. The permeability is increased from μ=80 for stoichiometric ferrites (z=0) to μ=660 for z=0.02. The formation of CuO during sintering of sub-stoichiometric ferrites supports densification and is a prerequisite for low temperature firing of multilayer inductors. Addition of 1wt% Bi2O3 as liquid phase sintering aid is required to provide sufficient densification of the stoichiometric ferrite (z=0) at 900°C. Addition of 0.37wt% Bi2O3 to a sub-stoichiometric ferrite (z=0.02) results in dense samples after firing at 900°C; however, the microstructure formation is dominated by heterogeneous grain growth.

The metamorphosis of heterometallic trinuclear antiferromagnetic complexes into nano-sized superparamagnetic spinels

Thermal decomposition of the trinuclear heterometallic oxoacetates [Fe 2M(μ 3-O)(CH 3COO) 6(H 2O) 3] has been used as a single-precursor method for synthesis of the spinel-structured ternary oxides MFe 2O 4 (M = Mn II, Co II, and Ni II). This facile process occurring at 320 °C results in the formation of nanocrystalline (7–20 nm) highly pure stoichiometric ferrites in quantitative yield. The magnetic properties of these nanoparticulate ferrites were studied in the 10–300 K temperature range, revealing superparamagnetic behavior for the Ni and Mn particles and ferrimagnetic behavior for the Co ones at room temperature. Their blocking temperatures follow the order: CoFe 2O 4 > MnFe 2O 4 > NiFe 2O 4.

Oxidation of Stoichiometric Nickel Ferrites Used as Inert Anodes for Aluminium Electrolysis in Molten Cryolite Mixtures

Thanks to their good electronic conductivity and their low solubility in cryolite melts, nickel ferrites are considered to be among the most suitable ceramic materials that could be used as inert anodes in the electrowinning of aluminium. In this work, electrodes composed of single phased and stoichiometric nickel ferrite NixFe3-xO4 (0≤x&lt;1) have been studied by electrochemical techniques (linear voltammetry and potentiostatic electrolysis) in a molten cryolite mixture at 960°C. The aim was to understand the oxidation reactions susceptible to take place inside the material under anodic polarization and oxygen evolution. Ex situ characterization of the electrodes by SEM-EDX and microprobe analysis allows proposing a degradation mechanism of the pure nickel ferrites: the formation of haematite Fe2O3 at the grain boundaries was evidenced and it resulted in a slow degradation. The influence of the Ni content in the ceramic phase was investigated, and it was shown that rich-Ni compositions exhibit a better resistance to corrosion.

Non-stoichiometric NiZn ferrite by sol-gel processing

Non-stoichiometric Ni 0.5Zn 0.5Fe 2− x O 4−3/2 x ferrites over a wide range of x = 0∼0.8 are synthesized by a sol-gel processing. Phase evolution, crystal structure and crystallite size of spinel ferrites are dependent on annealing temperature and the amount of Fe deficiency. The crystallite size of spinel increases with annealing temperature and grows faster in stoichiometric ferrites than that of non-stoichiometric. Fe deficiency results in the partial reduction of spinel ferrite to zincite ZnO. XRD indicates that the crystallization temperature of ZnO is increased to about 700 °C. Zincite reduces the number of ferrite crystallites and disfavors the growth of spinel ferrites. The lattice parameters decrease with Fe deficiency and are insensitive to the variation in composition in the samples annealed at lower temperature due to the segregation of ZnO and lattice expansion in the ultrafine crystallites.

Preparation Conditions of Pure and Stoichiometric NixFe3‐xO4 Bulk Ceramics.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Preparation conditions of pure and stoichiometric Ni xFe 3− xO 4 bulk ceramics

The Ni x Fe 3− x O 4 (0< x<1) phases are spinels which are difficult to obtain under pure, dense and conductive ceramics forms. Treatment under inert atmosphere leads to a biphasic system of spinel and NiO monoxide. Treatment under air does not allow obtaining an optimised conductive ceramic because of oxidation during the cooling or quenching. Then, two step processes is proposed to obtain pure and stoichiometric nickel ferrites. At first, a treatment under air is carried out to synthesise dense and pure spinel ceramics. This treatment is followed by an annealing under inert atmosphere to reduce a part of Fe 3+ and to form stoichiometric ferrites with semiconducting properties.

Nanohybrids of non-stoichiometric zinc ferrite in amorphous silica

Nanohybrids containing non-stoichiometric zinc ferrite of spinel structure in an amorphous silica matrix exhibit a fundamentally different structure to that formed by stoichiometric zinc ferrite. A unique cluster glass structure, where nanocrystallites of zinc ferrite exist in amorphous Fe-rich pockets, occurred at the significantly high Fe/Zn molar ratio of 10, as supported by the magnetic behaviour and characterisation by XRD and TEM. The occurrence of zinc ferrite crystallites due to the Zn2+ deficiency, together with the confinement of the silica matrix, suppresses the nucleation and crystallisation of α-Fe2O3 to a temperature above 900 °C, preserving the cluster glass structure. The Fe/Zn molar ratio also affects the optical properties of nanohybrids consisting of the zinc ferrite nanophase in amorphous silica, which demonstrate a high absorption coefficient to yellow light. The absorption edge was blue-shifted by 0.25 eV when the Fe/Zn ratio was increased from 2 to 10.

Comparative Study of the Effects of Rare Earth Ions in a High Frequency Ni-Zn Ferrite

Some effects of the rare-earth ions on the properties of the polycrystalline stoichiometric ferrite with formula Ni 07 Zn 0.3 Fe 1.98 R 0.02 O 4 (R=Yb, Er, Dy, Tb, Gd, Sm and Ce) are described. The results obtained reveal that by introducing a relatively small amount of R 2 O 3 instead of Fe 2 O 3 , an important modification of both structure and properties can be obtained. R 2 O 3 facilitates the formation of the crystalline secondary phases (orthoferrite, garnet) on the grain boundary which inhibit the ferrite grain growth. The R ions tend to flatten the μi -T curve, shift the Curie point to lower temperature and increase the electrical resistivity. The variation of these properties with the R species has been explained as an effect of the ionic radius size which varies from 0.86 for Yb to 1.07 for Ce.

Effects of Cr2O3 or Y2O3 doping in barium ferrite thin-film medium for high-density recording

The effects of Cr2O3 or Y2O3 doping on the surface morphology and magnetic properties in barium ferrite thin films have been studied. Thin films were deposited by co-sputtering a barium ferrite target with a Cr2O3 or Y2O3 target. The deposited amorphous films were homogenized at a temperature of 600 °C for over 10 h followed by annealing at 800 °C in a tube furnace. It was observed that, whereas stoichiometric barium ferrite shows 5000 Å×500 Å acicular grainlike features, films doped with Cr2O3 or Y2O3 had equiaxed topographic features with an average size of 500 Å. Magnetic studies showed that the coercivity of the doped films was over 3500 Oe and was larger than that of stoichiometric films. A weak magnetostatic interaction was found in all the films. Studies of the magnetic after effect indicated excellent thermal stability in all the films.

高温Ｓｃｈｉｋｏｒｒ反応による水酸化鉄（ＩＩ）からのスピネル型フェライトの合成と水素発生挙動

The preparation of ferrite particles from Fe (OH)2 aqueous suspension is studied in high temperaturehydrothermal condition accompanied with the Schikorr reaction. The well crystalized magnetite particle grew in larger size in this reaction system, compared to the usual co-precipitating or wet-oxidizing methods.The addition of KNO3 as an oxidizing reagent prior to the hydrothermal treatment was useful to control the nucleation of magnetite and this led to reduce the particle size. The addition of excess amount of KNO3 led to hematite formation.When Mn2+ or Ni2+ ion was mixed in starting ferrous solution, respective ion partially entered into spinel phase. In this case, manganese or nickel ferrite with stoichiometric composition was not obtained and the maximum value of x in the MxFe3-xO4 (M: divalent metal ion) seemed to be 0.4 for Mn, and very small for Ni. The amount of H2 generated during the reaction decreased by adding Ni2+. Also, Ni metal phase was observed by XRD in the series of Ni adding experiments.Although, when KNO3 solution was added into the starting solution, Mn2+ or Ni2+ were easily incorporated into the spinel phase to give respective stoichiometric ferrite.

An anomalous behaviour in the phase stability of the system Fe2O3 and NiO

Iron-nickel mixed oxides containing up to 50 mol% of NiO were prepared by firing the corresponding co-precipitated hydrous oxides; characterization was performed by X-ray diffraction, infrared spectroscopy, magnetic susceptibility, electrical conductivity and thermoelectric power measurements. A non-stoichiometric ferrite phase was formed when a sample containing 20 mol% NiO was sintered at 1050°C. This phase had two- to three-fold higher conductivity than either Fe2O3 or the stoichiometric ferrite (NiFe2O4). The thermoelectric power of this phase indicated a sharp change of charge carriers from n- to p-type near 350°C. This non-stoichiometric ferrite phase was stable only in a small temperature range and dissociated into α-Fe2O3 and stoichiometric ferrite above 1200°C. Samples containing 5 and 10 mol% NiO also had small fractions of this non-stoichiometric ferrite phase when sintered at 1050°C.

The mechanism of ferrite formation from iron sulfides during zinc roasting

The formation of zinc ferrite (ZnFe2O4) during the fluidized-bed roasting of zinc concentrates presents subsequent processing difficulties both for zinc recovery and for iron separation and disposal. A major source of iron in these concentrates is from the iron sulfides — pyrite and pyrrhotite. This study examined the changes undergone by these iron minerals when roasted together with sphalerite at 1223 K in a fluidizing gas mixture of 3 pct oxygen and 97 pct nitrogen. Optical microscopy and electron microprobe analysis were employed to identify the three stages that lead to ferrite formation and to examine the processes that occur within each stage. The first stage is oxidation of the sulfides to highly vesicular, amorphous magnetite particles containing small amounts of zinc. The second stage involves both densification of these particles by sintering and counterdiffusion of iron and zinc cations to form a continuous phase of homogeneous zinc-rich spinel and a precipitate of hematite. In the third stage, continuation of cation diffusion and increasingPo 2 results in the formation of stoichiometric zinc ferrite. These observations have been interpreted by reference to the established phase relationships that occur in the Zn-Fe-O system, and a detailed, solid state reaction mechanism for the formation of zinc ferrite has been proposed.

Changes in the Chemical Properties of Li-Mn-Zn-Ferrite during Heat-Treatment

Li-Mn-Zn-ferrite (Li0.33 Mn0.42 Zn0.20 Fe2.05 O4+δ) was heat-treated at various temperatures between 450 and 1200°C, and the changes of cation valence, crystal lattice constant and oxygen content in the ferrite by this heat-treatment were measured. The oxygen and Mn3+ ion contents in the ferrite decreased with increasing temperature, while the crystal lattice constant increased. The stoichiometric ferrite (δ=0) was obtained by heat-treating at about 900°C in oxygen and air, and at about 700°C in nitrogen.The relationship between the crystal lattice constant a0 and an oxygen content deviation from the stoichiometry δ, was found to be as follows:a0=8.371-0.37δ forδ>0a0=8.371-0.37δ forδ>0.

Development of temperature-independent square-loop ferrites

A series of cobalt-substituted nearly stoichiometric nickel-zinc ferrites was investigated in order to develop a temperature independent square-loop ferrite which also had minimum magnetic noise. In particular the effect of nickel-zinc ratio, cobalt content, and sintering conditions on the temperature stability and on Barkhausen noise were studied. The optimal material showed no temperature dependence of coercive force between 0 and +75°C and had less noise than manganese-magnesium and lithium ferrites. A marked difference was observed between quenched and slow-cooled samples. Interpretation of the experimental results is based on the effects of cobalt on the crystalline and induced anisotropies in nickel-zinc ferrites.