Littérature scientifique sur le sujet « Nanometric spinel ferrites »

The mixed ferrite nanometric Co – Zn ferrites [ Co 1-x Zn x Fe 2 O 4] with x = 0, x = 0.35, x = 0.5, x = 0.65, and x = 1 having particle size down to ~20 nm, synthesized through chemical sol–gel route have been found to have higher Curie temperature and higher resistivity as compared to the ones derived from conventional processes. Studies were made to estimate the effect of Co concentration (x) on these mixed ferrite nanometric Co – Zn ferrites having a AB2O4 -type cubic inverse spinel structure. Investigation of magnetic properties of all Co – Zn nanoferrites with varying Co content has been made using a VSM at room temperature. The saturation magnetization and coercive field have been observed to be highest for x = 0.35 and x = 0, respectively. The magneto-impedance characteristics and the complex impedance spectroscopy [Z(ω,T) = Z′+iZ″] have also been investigated in detail for all the samples at different frequencies.

The synthesis by combustion reaction stands as an alternative technique for preparing powders with high purity level, nanometric particle size and low cost. Therefore, this study had as objective the synthesis and characterization of ferrite powders with nominal composition NI1-xZnxFe2O4(x = 0.6, 0.7 and 0.8 mol) prepared by combustion reaction using urea as fuel. The influence of the quantity of zinc in the final characteristics of the powders was also investigated. The powders were characterized by X-ray diffraction (XRD), nitrogen adsorption (BET) and scanning electron microscopy (SEM). All compositions resulted in nanometric powders of Ni-Zn ferrites with direct formation of the inverse spinel phase, showing the effectiveness of the synthesis method applied. The increase of zinc's concentration caused an increase in surface area, ranging 4 m2g-1from 23 m2g-1. And the SEM micrographs show that the powders have thinner particle morphology with increasing zinc content.

Ferrites are a broad class of ceramic oxides, possessing intriguing physico-chemical properties, mainly due to their unique structural features, that, during the last 50–60 years, made them the materials of choice for many different applications. They are, indeed, applied as inductors, high-frequency materials, for electric field suppression, as catalysts and sensors, in nanomedicine for magneto-fluid hyperthermia and magnetic resonance imaging, and, more recently, in electrochemistry. Ferrites are commonly broadly divided into three groups based on the crystal structure: garnets, hexaferrites and cubic spinels with AB2O4 stoichiometry, that are undoubtedly the most known and exploited ferrites, so that worldwide the term ferrite is synonymous for the AB2O4 compounds. In particular, ZnFe2O4 (ZFO) and its solid solutions attracted the researchers’ attention for the application as anode materials in lithium-ion batteries (LIBs). ZFO is a normal cubic spinel, with Zn2+ ions on tetrahedral sites and Fe3+ on octahedral ones, with an anti-ferromagnetic character. The reasons of the interest for electrochemical applications can be found in the low cost, abundance, and environmental friendliness of both Zn and Fe precursors, high surface-to-volume ratio, relatively short path for Li-ion diffusion, low working voltage of about 1.5 V for lithium extraction, and high theoretical specific capacity (1072 mAh g-1). However, some drawbacks are represented by fast capacity fading and poor rate capability, resulting from a low electronic conductivity, severe agglomeration, and large volume change during lithiation/delithiation processes. ZnFe2O4, unlike other oxides, has a lithium insertion mechanism that involves both conversion and alloying reactions. After the conversion reaction of ZFO with lithium ions and the formation of metallic Zn, Fe, and Li2O, the resulting Zn can further react with lithium to form a LixZn alloy, thus contributing additional capacity. ZFO experiences first-cycle irreversibility and fast decay in capacity with cycling, mainly resulting from poor electrical conductivity and large volumetric changes associated with the conversion reaction. However, thanks to the downsizing of the particles, the addition of proper carbon sources, and to peculiar morphologies, the long-term cycling and the capacity values of ZnFe2O4 are, nowadays, very appealing. The peculiar ferrite properties are mainly related to the cation distribution (affected by the eventual substitutions of other elements onto tetrahedral and/or octahedral sites) and nano-dimensions. In particular, this is true for magnetic, optical, and electrical properties but also for the electrochemical ones. The determination of sample purity is also mandatory, because even small amount of iron oxides such as hematite Fe2O3 or magnetite Fe3O4, easily stabilized during the synthesis processes, heavily influence the intrinsic ZFO properties. To determine all the cited characteristics, the combined use of diffraction, morphological and spectroscopic techniques is required for the whole sample characterization. In this work, ZnFe2O4 was synthesized by two different methods, co-precipitation route and by using a template synthesis from MOF, to obtain samples with different crystallite sizes. Then, the obtained samples were thoroughly characterized: structural, morphological and vibrational properties were detected by combining X-ray powder diffraction (XRPD) with Rietveld structural refinement, SEM analysis with EDS and micro-Raman spectroscopy. In this way, the samples’ purity, the lattice parameters and crystallite sizes, the particles’ morphology and the eventual spinel inversion degree were determined. Furthermore, Mössbauer and EPR spectroscopies were carried out to verify the possible presence of impurity phases, particularly iron oxides, hard to be determined by XRPD only. The electrochemical properties were measured by using galvanostatic cycling at different C-rates and cyclic voltammetry. Then, in-situ diffraction measurements were applied on electrochemical cells and the results were discussed on the base of the physico-chemical characterizations.

Spinel phases, with unique and outstanding physical properties, are attracting a great deal of interest in many fields. In particular, MgFe2O4, a partially inverted spinel phase, could find applications in medicine thanks to the remarkable antibacterial properties attributed to the generation of reactive oxygen species. In this paper, undoped and Ag-doped MgFe2-xAgxO4 (x = 0.1 and 0.3) nanoparticles were prepared using microwave-assisted combustion and sol–gel methods. X-ray powder diffraction, with Rietveld structural refinements combined with micro-Raman spectroscopy, allowed to determine sample purity and the inversion degree of the spinel, passing from about 0.4 to 0.7 when Ag was introduced as dopant. The results are discussed in view of the antibacterial activity towards Escherichia coli and Staphylococcus aureus, representative strains of Gram-negative and Gram-positive bacteria. The sol–gel particles were more efficient towards the chosen bacteria, possibly thanks to the nanometric sizes of metallic silver, which were well distributed in the powders and in the spinel phase, with respect to microwave ones, that, however, acquired antibacterial activity after thermal treatment, probably due to the nucleation of hematite, itself displaying well-known antibacterial properties and which could synergistically act with silver and spinel.

Nanometric laminar two-dimensional artificial multiferroic oxide thin films can be elaborated using spinel ferrites and perovskite ferroelectrics like CoFe2O4 and BaTiO3. Such materials can retain their individual ferromagnetic or ferroelectric properties. In the thin epitaxial film regime a cross coupling of these properties is possible thanks to strain engineering. After introducing the concepts supporting artificial multiferroic laminar structures, the growth of strained BaTiO3 thin films and the growth of subsequent Co-ferrites layers will be detailed. With respect to the relative film thickness, a detailed understanding of the elastic behavior of these films will be proposed based on the characterization using several synchrotron radiation techniques including x-ray specular and off-specular diffraction, x-ray absorption spectroscopy, as well as x-ray magnetic circular dichroism.

Abstract: Aluminium-doped Nickel-Cobalt ferrite nanoparticles with general formula Ni0.5Co0.5AlxFe2-xO4 (x = 0 and x = 0.5) were synthesized by microwave-assisted sol-gel auto-combustion method. The X-ray diffraction analysis of the samples confirms single phase with cubic spinel structure belonging to space group Fd3m. The average crystallite size calculated using Debey-Scherrer formula was found to be in the range of 19-36 nm. XRD studies revealed that the lattice parameter (a), the particle size (D) and X-ray density (dx) decrease, whereas porosity increases with Al substitution. Energy Dispersive X-ray (EDX) was used to confirm the elemental composition of synthesized powders. TEM micrograph suggests that the particle size is in nanometric range, which confirms the nanocrystalline nature of the samples. The electrical resistivity and dielectric constant have been studied as functions of temperature. It was observed that the electrical resistivity decreases with increasing temperature, exhibiting the semiconducting nature of the sample. The dielectric constant increases with increase in temperature up to transition temperature and then decreases, which is explained on the basis of hopping mechanism. Keywords: Ni-Co ferrites, Sol-Gel, XRD, Resistivity, Dielectric constant.