Stoichiometric Spinel Research Articles

Investigation of the process of ferrite formation in the Li2CO3–ZnO–Fe2O3 system under high-energy actions

Results of synthesis of lithium-zinc ferrites under high-energy actions including mechanical activation of the mixture of initial reagents in a planetary mill and heating of reaction mixtures upon exposure to accelerated high-energy electron beams are presented. The initial structure of the reaction mixtures corresponds to the stoichiometric spinel formula: Li0.5(1–x)ZnxFe2.5–0.5xО4, where х = 0.2. To elucidate the effect of radiation on kinetic transformations of the phase composition, analogous investigations are performed with thermal annealing in a furnace. It is established that mechanical activation and radiation-thermal heating of the Li2CO3–ZnO–Fe2O3 mixture of initial reagents increases significantly the reactivity of solid phase systems thereby decreasing strongly the temperature of synthesis and improving the homogeneity of the end product. To synthesize Li0.4Fe2.4Zn0.2O4 lithium-zinc ferrospinels with homogeneous composition, it is suffice to provide a synthesis temperature of 600°С with holding time of 60 min under complex high-energy action including mechanical activation of the initial mixture of reagents and heating upon exposure to a high-energy beam of electrons.

Infrared Spectroscopy Studies of Magnesium Aluminates Spinel Crystals

The reflectance and transmittance spectra for magnesium aluminates spinel single crystals MgO•nAl2O3 and ceramics in the IR spectral range were measured to investigate the nature of vibration modes of constituent ions and incorporated hydrogen. Besides of the earlier observed bands at 545, 710, and 835 cm-1 we registered also two bands 1440 and 1670 cm-1 in single crystals grown by different methods and in spinel optical ceramics. The existence of these bands was supported by measurements of non-stoichiometric spinel crystals and crystals doped with transition metals. The variation of intensity of registered bands in spinel of stoichiometric and non-stoichiometric compositions and spinel ceramics was interpreted in terms of vibration modes of divalent and trivalent cations in different coordination, particularky, at the spatially correlated anti-site defects.

Thermodynamic assessment and experiments in the system MgO–Al2O3

Samples with alumina rich compositions in the system MgO–Al2O3 were investigated with DTA up to 2473K. The peritectic reaction L+Al2O3→ Spinel predicted from calculations was confirmed in this study. A critical literature review was done focusing on the degree of inversion of stoichiometric spinel MgAl2O4. Experimental results and the most reliable literature data were used for re-optimisation of thermodynamic parameters of the quasi binary system.

Gas sensitivity of stoichiometric and excess-iron Ni-Zn ferrite prepared by sol-gel auto-combustion

Application of new materials offers more sensitive, selective and long-term stable sensor materials. The aim of the present work is to compare gas response to acetone of nanostructured sol-gel auto-combustion derived stoichiometric and excess-iron cubic spinel type nickel-zinc ferrite (Ni0.3Zn0.7Fe2+zO4 where z = 0 and 0.1). Detailed synthesis steps and gas sensing measurement methodology were described. The sensor material was characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM) and direct current (DC) resistance measurements. XRD analysis confirms that samples form the single-phased cubic spinel structure, SEM reveals nanosized grains less than 100 nm in diameter. Plots of resistance versus temperature show adsorbed water contribution to the conductance. With excess-iron, Ni-Zn ferrite changes its DC electrical resistivity, type of conductivity, as well as response to reducing gas (more than 2 times). Obtained relationships can be explained with Fe2+ formation in the material, thus increasing charge carrier (electron) concentration. This leads down to higher oxygen adsorption ability which can act with test gas.

Formation of spinel reaction layers in manganese cobaltite – coated Crofer22 APU for solid oxide fuel cell interconnects

The microstructural development of Mn1.5Co1.5O4-coated Crofer22 APU has been studied using cross-sectional transmission electron microscopy. Alloy samples were coated via a slurry process involving consolidation by reduction and re-oxidation, and these samples were then oxidized at 800 °C for times of up to 1000 h. All samples exhibited a thin chromia scale at the alloy/coating interface plus spinel phases as a reaction layer between the chromia and the manganese cobaltite coating. The oxidized samples also exhibited pockets of stoichiometric MnCr2O4 spinel at the chromia/alloy interface and internal Ti-rich oxides in the alloy below the chromia. The reaction layer spinels exhibit remarkable changes in thickness, morphology and composition, and these effects are explained on the basis of changes in the diffusive fluxes during the different stages of coating application and subsequent exposure. The possible consequences of these observations for the degradation mechanisms that could affect SOFC interconnects produced from MCO-coated Crofer22 APU are discussed.

Structure, reactivity and catalytic properties of nanoparticles of nickel ferrite in the dry reforming of methane

Nickel ferrite NiFe2O4 (spinel) was prepared and characterized by several physicochemical methods, and its catalytic properties were investigated in the reforming of methane with CO2 to syngas. Nanoparticles were prepared by coprecipitation (CP), hydrothermal synthesis (HT) and sol–gel process (SG). The methods of preparation influenced strongly the textural and structural properties, as well as their reactivity. X-ray diffraction patterns showed that only HT and SG methods delivered pure NiFe2O4 nanoparticles (6–10nm size). However the cell parameter a calculated by Rietveld analysis being respectively larger (HT) or smaller (SG) than the standard value, a non stoichiometric defective mixed spinel could have been formed. The reducibility of samples was studied by two methods. H2-temperature programmed reduction showed the formation of Ni0 at increased temperature along SG≈HT<CP series, after which step NixFe2−xO4 decomposed to iron suboxides and finally to Fe0. In situ reduction by H2 carried out by HT-XRD showed that the ultimate compound was γ-NixFe1−x alloy, formed above 400 (SG, HT) or 450°C (CP). The catalytic activity in the dry reforming of methane was higher with HT and SG than with CP, but values of conversion of CH4 and CO2 were low as compared with efficient catalysts in the literature. After being pre-reduced by hydrogen at 400°C, a large improvement was observed with SG sample only, in which case the conversions of methane and CO2 at 800°C amounted to 80 and 93mol%, respectively, with H2/CO=1.2.

Influence of Nanocrystalline MgAl2O4 Spinel Addition on the Properties of MgO-C Refractories

Nanopowders of magnesium aluminate (MgAl2O4) spinel, prepared through citrate-nitrate gel method and calcined at 800°C, were incorporated to the matrix of magnesia-carbon (MgO-C) refractory. The physical and chemical properties of the nano-MgAl2O4 spinel-added (0.5 and 1.0 wt%) MgO-C bricks were compared with those of MgO-C bricks containing no spinel and commercially available micron-sized (<45 µm) stoichiometric spinel (AR-78). It was observed that nano-spinel (NS) (1 wt%) containing MgO-C refractories effectively prevents the oxidation, slag corrosion, and penetration resistance compared to MgO-C bricks with or without micron-sized spinel added. The average pore diameter decreases, and the concentration of the fine pore increases with addition of nano-MgAl2O4 spinel in the matrix of MgO-C bricks. Due to the lower pore size and higher concentration of fine pores as well as higher amount of carbon retention, nano-MgAl2O4 spinel-added MgO-C bricks show better physical as well as chemical properties and may have potential applications in both steel and refractory industries.

Oleic-acid-coated CoFe2O4 nanoparticles synthesized by co-precipitation and hydrothermal synthesis

Oleic-acid-coated CoFe2O4 nanoparticles were synthesized by co-precipitation and hydrothermal synthesis. The coprecipitation of the nanoparticles was achieved by the rapid addition of a strong base to an aqueous solution of cations in the presence of the oleic acid surfactant, or without this additive. The nanoparticles were also synthesized by a hydrothermal treatment of suspensions of the precipitates, coprecipitated at room temperature in the presence of the oleic acid, or without it. The influence of the synthesis conditions, such as the valence state of the iron cation in the starting aqueous solution, the temperature of the treatment and the presence of oleic acid, on the particles size was systematically studied. X-ray powder diffractometry (XRD) and transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) revealed that, although spinel forms at room temperature, a substantial amount of Co was incorporated within the secondary, feroxyhyte-like phase when the iron cation was in the 2+ state. In contrast, when iron was in the 3+ state, the spinel forms at elevated temperatures of approximately 60°C. The presence of the oleic acid further increased the formation temperature for the stoichiometric spinel. Moreover, the oleic acid impeded the particles’ growth and enabled the preparation of colloidal suspensions of the nanoparticles in non-polar organic solvents. The nanoparticles’ size was successfully controlled by the temperature of the synthesis in the region where superparamagnetism dominates to the region where mono-domain ferrimagnetism dominates the magnetic properties.

Heat capacity of stoichiometric Al2MnO4 spinel between 2 and 873 K

The heat capacity of stoichiometric Al2MnO4 spinel was measured from 2 to 873 K. These data are used to evaluate the entropy of this compound at 298.15 K and to propose a Gibbs energy function which was extrapolated to higher temperatures. Previously measured and assessed values for this compound are compared and the possible impact on the pseudo-binary Al2O3–MnO phase diagram is discussed. If it is assumed that all solid phases are stoichiometric and described by the compound energy model, and the liquid is described by the Kappor–Frohberg–Gaye quasi-chemical model, the proposed Gibbs energy model of the Al2MnO4 spinel corroborates the formation of two eutectic reactions in the Al2O3–MnO pseudo-binary phase diagram.

Formation of spinel structured compounds in the lithium permanganate thermal decomposition

Products of thermal decomposition of lithium permanganate LiMnO4·3H2O, which are formed in temperature range 160–900 °C, have been characterized by powder XRD and chemical analysis. It has been found that the decomposition of the permanganate results in the formation of an equimolar mixture of manganate(IV) Li2MnO3 and stoichiometric spinel LiMn2O4 at the temperatures above 700 °C. Intermediate products with spinel structure are formed at lower temperatures with oxidation number of manganese being between +4 and +3.5. These compounds can be related to overstoichiometric spinel phases with general formula Li a [Mn(1 + 0.5a)Li(1 − 0.5a)]O4, where a > 1. Electrochemical properties of these intermediates with regard to the reaction of Li extraction were investigated. The data are of interest for the development of synthesis methods for mixed oxides containing lithium and manganese with lithium permanganate as the lithiating reagent.

Preliminary investigations of the production of MgAlON bonded refractories

The aim of the work was to find an appropriate composition for the formation of MgAlON bonding phase for Al2O3 and MgO based refractories. The first step was the preparation of pure MgAlON. AlN and Al2O3 were used as starting powders and either MgO or MgAl2O4 was added as a source of magnesium. The results verified the possibility to produce MgAlON under the prevailing conditions. Afterwards, MgAlON bonded alumina and magnesia refractories were investigated. The obtained results confirmed the possibility of the production of MgAlON bonded alumina refractories. However, in the case of magnesia-based samples MgAlON was not formed and instead stoichiometric spinel, AlN and alumina rich spinel were detected in the bonding phase. Additionally, the joining between the MgO grains and the matrix was poor with wide gaps on the interface.

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.

Characterization of Stoichiometric Nanocrystalline Spinel Ferrites Dispersed on Porous Silica Aerogel

Stoichiometric magnetic nanosized ferrites MFe2O4 (M = Mn, Co, Ni) were prepared in form of nearly spherical nanocrystals supported on a highly porous silica aerogel matrix, by a sol-gel procedure. X-ray diffraction and transmission electron microscopy indicate that these materials are made out of non-agglomerated ferrite nanocrystals having size in the 5-10 nm range. Investigation by Mössbauer Spectroscopy was used to gain insights on the superparamagnetic relaxation and on the inversion degree. Magnetic ordering at room temperature varies from superparamagnetic in the NiFe2O4 sample, highly blocked (approximately 70%) in the MnFe2O4 sample and nearly fully blocked in the CoFe2O4 sample. A fitting procedure of the Mössbauer data has been used in order to resolve the spectrum into the tetrahedral and octahedral components; in this way, an inversion degree of 0.68 (very close to bulk values) was obtained for 6 nm silica-supported CoFe2O4 nanocrystals.

Accumulation of radiation damage and disordering in MgAl2O4 under swift heavy ion irradiation

Abstract Accumulation and recovery of radiation-induced damage with swift heavy ions in stoichiometric magnesium aluminate spinel, MgAl2O4, has been investigated. Microstructural change and atomic disordering was examined through transmission electron microscopy (TEM) techniques, with bright-field (BF) and high-resolution (HR) TEM images, and high angular resolution electron channelling X-ray spectroscopy (HARECXS), for single crystal MgAl2O4 irradiated with 200 MeV Xe, and 340 or 350 MeV Au ions. The density of core damage region, detected by BFTEM with Fresnel-contrast, increased proportionally with ion fluence at the early stage of accumulation and saturated at a fluence higher than 1016 ions m2. This result is discussed with a balance between the formation and recovery of the core damage region under irradiation, and the influence region to induce the recovery was evaluated to be 7 – 9 nm in radius. HARECXS and electron diffraction analysis revealed that cations at tetrahedral sites preferentially occupy octahedral sites to transform to defective rock-salt structure. The structure of the core damage region is found from HR and BFTEM images to be a columnar vacancy-rich region with a low atomic density.

Transparent Solid Solution Magnesium Aluminate Spinel Polycrystalline Ceramic with the Alumina‐Rich Composition MgO·1.2 Al2O3

Solid solution magnesium aluminate spinel with the alumina‐rich composition MgO·1.2 Al2O3 has been prepared as a transparent polycrystalline ceramic with average in‐line transmission at 550 nm of 84.8 ± 2.7% and &gt;82% throughout the visible spectrum. Starting powders are prepared from mixtures of high purity Mg(OH)2 and γ‐Al2O3 thoroughly mixed in an aqueous slurry. Water is removed by rotary evaporation. The solids are collected, dried, calcined, mixed with LiF (as a sintering aid), and sieved. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load followed by hot isostatic pressing at 1850°C under 200 MPa Ar. Final grain sizes ranged between 300 and 1000 μm. Samples exhibited flexural strength of 176.8 ± 46.2 MPa; hardness of 12.3 ± 0.2 GPa; and elastic modulus of 292.9 ± 7.5 GPa. Control samples of stoichiometric magnesium aluminate spinel (MgO·Al2O3) were prepared with the same procedure and exhibited comparable values for transmission and physical properties.

Synthesis and characterization of lithium manganese oxides with core-shell Li 4Mn 5O 12@Li 2MnO 3 structure as lithium battery electrode materials

By a facile LiNO 3 flux method, lithium manganese oxide composites (xLi 4Mn 5O 12∙ yLi 2MnO 3) were synthesized using a hierarchical organization precursor of manganese dioxide. Li 4Mn 5O 12 and Li 2MnO 3 have spinel and rocksalt structures, respectively. The lithiation and structural transformation from the precursor to the composites occurred topotactically from exterior toward interior in the precursor particle with the increase of reaction time, and the composites had core-shell spinel@rocksalt structures in addition to the original hierarchical core-shell organization. The electrochemical measurements at 50 °C after 50 cycles confirmed that a typical spinel@rocksalt cathode had higher capacity retention (87.1%) than that with the composition close to the stoichiometric spinel (64.6%), indicating the Li 2MnO 3 shell can improve cycling stability for the composite electrode at elevated temperature.

Enhancement of MgAl2O4 spinel formation from coprecipitated precursor by powder processing

Although low temperature fast coprecipitation technique has been used to synthesize stoichiometric (MgO-nAl2O3, n = 1) MgAl2O4 spinel forming precursor, delayed spinellization has always been the concern in this process. In this article, the precursor of this ‘fast technique’ has been used for bulk production by further processing by high speed mixing with solvents and mechanical activation by attrition milling in terms of superior spinellization. At 1000°C, MgAl2O4 — γ-Al2O3 solid solution and MgO phases are formed (spinel formed by 1000°C is regarded as primary spinel). At higher temperatures, due to large agglomerate size, MgO can not properly interact with the exsolved α-Al2O3 from spinel solid solution to form secondary spinel; and consequently spinellization gets affected. Solvent treatment and attrition milling of the coprecipitated precursor disintegrate the larger agglomerates into smaller size (effect is more in attrition). Then MgO comes in proper contact with exsolved alumina, and therefore total spinel formation (primary + secondary) is enhanced. Extent of spinellization, for processed calcined samples where some alumina exists as solid solution with spinel, can be determined from the percentage conversion of MgO. Analysis of the processed powders suggests that the 4 h attrited precursor is most effective in terms of nano size (< 25 nm) stoichiometric spinel crystallite formation at ≤ 1100°C.

Synthesis of Nano Crystalline MgAl&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;4&lt;/SUB&gt; Spinel Powder by Microwave Assisted Combustion

Stoichiometric MgAl2O4 spinel nanoparticles were synthesized by microwave assisted combustion reaction from aluminium nitrate nanohydrate (Al(NO3)3·9H2O) and Sol-Gel prepared magnesium hydroxide (Mg(OH)2) in the presence of urea ((NH2)2CO) as a fuel, in about 20 min of irradiation. X-ray diffraction (XRD) studies reveal that mi- crowave assisted combustion synthesis route yields single-phase spinel nanoparticles with larger crystalline size (around 75 nm) than other conventional heating methods. Scanning electronic microscope (SEM) images show nanoparticles with spherical shape and homogenous morphology. The surface area measurements (SBET) show crystals with 2.11 m 2 /g and 0.0033 mL/g pore volume.

Co[V2]O4: A Spinel Approaching the Itinerant Electron Limit

Studies of the structure, magnetization, and resistivity under pressure on stoichiometric normal spinel Co[V(2)]O(4) single crystals show (i) absence of a structural distortion, (ii) abnormal magnetic critical exponents, and (iii) metallic conductivity induced by pressures at low temperatures. All these results prove that Co[V(2)]O(4) sits on the edge of the itinerant-electron limit. Compared with similar measurements on Fe[V(2)]O(4) and other A[V(2)]O(4) studies, it is shown that a critical V-V separation for a localized-itinerant electronic phase transition exists.

Steam reforming of methane to syngas over NiAl 2O 4 spinel catalysts

A series of several mixed oxides close to NiAl 2O 4 (NiAl 2O 4, 5 wt% NiO/NiAl 2O 4, 10 wt% NiO/NiAl 2O 4 and 15 wt% NiO/NiAl 2O 4) were prepared by a pseudo sol gel method using propionic acid then calcinated at 900 °C during 4 h. The catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), surface areas (BET) and temperature-programmed reduction (TPR). The nature of the crystalline structure, the surface area and the reduction temperature depend on the Ni/Al ratio. For the stoichiometric ratio Ni/Al exactly equal to 0.5, homogeneous crystalline spinel phase was formed after temperature of calcination was equal or higher than 725 °C. The spinel structure being a non-tolerant structure for an excess of Ni, when Ni/Al is higher than 0.5, free NiO in strong interaction with the spinel was present and favoured the reduction of the nickel of the structure at lower temperature compared to the spinel. Till 700 °C, the spinel structure was well conserved even if it was partially reduced. For a temperature higher than 700 °C, the structure leads to Ni species formation on γ-Al 2O 3. Comparative steam reforming of methane was conducted to control the formation of Ni and its sintering. The stoichiometric spinel showed relevant catalytic performances: high CH 4 conversion, high selectivities of CO and H 2 and low carbon formation. The presence of nickel inside the spinel structure of NiAl 2O 4 catalyst confers a high stability and strong dispersion for the metallic particles. The low formation of carbon was due to the high dispersion and the limited growing of the Ni particles on γ-Al 2O 3. Over 5 wt% of NiO excess, a rough decrease of the CH 4 conversion, CO and H 2 selectivities were observed. The x wt% NiO/NiAl 2O 4 systems give an important carbon formation due to a large Ni particle formation after rapid sintering.