Spinel Particles Research Articles

Selective oxidation of styrene over nanosized spinel-type Mg xFe 3− xO 4 complex oxide catalysts

Nanosized spinel-type Mg x Fe 3− x O 4 complex oxide catalysts were prepared by coprecipitation and citrate gel methods and were characterized. The crystallization temperature of spinel particles prepared by citrate gel method is about 300 °C lower than that of those prepared by coprecipitation method. Mg x Fe 3− x O 4 catalysts prepared by citrate gel method have higher dispersity and smaller particle size, leading to higher activity for styrene oxidation with H 2O 2 as oxidant. The nonstoichiometric Mg x Fe 3− x O 4 catalysts ( x<1) are more active for styrene oxidation than the pure spinel MgFe 2O 4. The major reaction (∼65–70 mol%) is oxidative CC cleavage into benzaldehyde, and the minor reaction (10 mol%) is epoxidation and its further isomerization into phenylacetaldehyde. The effects of solvent, styrene:H 2O 2 molar ratio, reaction temperature and time on the conversion and product distribution were also studied.

Sulfur-Doped Aluminum-Substituted Manganese Oxide Spinels for Lithium-Ion Battery Applications

Spinels with nominal composition and have been evaluated for their suitability as positive electrode materials in rechargeable lithium-ion batteries for electric (EV) and hybrid electric vehicle (HEV) applications. magic angle spinning, nuclear magnetic resonance, X-ray diffraction, (XRD) and energy-dispersive spectroscopy experiments indicate that sulfur is most likely present as a trace impurity on the surface of the spinel particles rather than substituting for oxygen ions in the bulk, so it is unlikely to account for the previously reported enhanced cyclability of this material. Rather, the unusual particle morphology produced during calcination of some samples in the presence of sulfur compounds appears to impede (but does not completely prevent) conversion to the tetragonal phase that occurs at 3 V vs. Li and ameliorates the capacity fading associated with it. These materials exhibit reduced rate capability and capacity at 4 V, making them unsuitable for high energy density EV, or high power density applications. © 2003 The Electrochemical Society. All rights reserved.

Work of fracture and fracture surface energy of magnesia-spinel composites

This study used a model system of fully dense high purity magnesia, incorporating fine grain spinel prepared by hot-pressing. Work of fracture ( γ WOF) and fracture surface energy ( γ i ) were determined to be a function of spinel particle size and volume fraction. Predictions made from the thermal shock parameter, R⁗, were tested by measurements made on γ WOF γ i ratios of magnesia and magnesia-spinel composites. The results obtained from γ WOF γ i ratios matched the predictions. On the basis of γ WOF γ i ratios, the optimum spinel content and particle size were determined. Thermal shock data confirmed that higher values of the γ WOF γ i ratios were considered to be a good indicator for thermal shock resistance. Higher values of γ WOF were associated with the occurrence of more intergranular fracture with increasing spinel additions. Crack propagation was found to be much more difficult in magnesia-spinel composites, than the initiation of cracks.

Young’s modulus measurements of magnesia–spinel composites using load–deflection curves, sonic modulus, strain gauges and Rayleigh waves

Abstract The extent of interlinking of the microcracking and a decrease in strength and modulus values were determined to be a function of both spinel particle size and volume fraction to allow calculation of thermal shock parameter, R‴. Measurements of Young’s modulus were carried out both at room temperature and after thermal shock testing by using the load–deflection curves (defined as “mechanical” modulus) and by the sonic modulus technique. The values obtained from these methods were significantly different for quenched/unquenched samples. To understand the basis for these differences, strain gauge and Rayleigh wave methods were also used to determine Young’s modulus of the composites. Modulus values obtained from these methods confirmed the differences measured, and provided a guide to the values to be used in calculating thermal shock parameters. The mechanical modulus technique was considered the most meaningful indicator of Young’s modulus for a situation in which large mechanical strains were to be applied to the materials during thermal shock.

Effect of the particle size distribution of spinel on the mechanical properties and thermal shock performance of MgO–spinel composites

Abstract The influence of varying the amounts of spinel with a similar median particle size, but with different distribution, on the mechanical properties and thermal shock performance of MgO–spinel composites was investigated. Mechanical properties of composites decreased significantly with increasing spinel content due to the thermal expansion mismatch. However, γWOF values of composites increased markedly, because of a significant change in the fracture mode from transgranular to intergranular fracture. A narrow distributed spinel A (Alcoa MR66) particles resulted in shorter initial crack propagation distances from the spinel particles, but spinel B (Britmag 67) particles with a significantly broader distribution were the origins of longer interlinked cracks. The improved resistance to thermal shock in MgO–spinel composites can therefore be attributed to the microcrack networks developed around the spinel particles, associated with the high values of γWOF, and not to an increased K1c. On the basis of theoretically calculated R‴ values and experimentally found γWOF/γi ratios, resistance to thermal shock damage would be more strongly favoured with materials containing spinel B particles, rather than spinel A, for which a much larger volume% was required to achieve a similar improvement.

6Li MAS NMR study of stoichiometric and chemically delithiated LixMn2O4 spinels

6Li magic angle spinning (MAS) nuclear magnetic resonance spectroscopy has been used to study the structure of a series of LiMn2O4 spinel samples prepared using different annealing methods. The results show that the annealing conditions affect the number of defect sites within the structure; higher annealing temperatures leading to a spinel sample with fewer defects. Analysis of the peak intensities in the NMR spectra revealed that the structural evolution of the spinel samples with temperature is dependent upon particle size. A series of chemically delithiated samples have also been studied using a combination of 6Li MAS NMR, X-ray diffraction and infrared spectroscopy in order to follow the structural changes that occur upon delithiation. The NMR spectra of the delithiated samples, LixMn2O4, exhibit a gradual shift in the position of main spinel resonances (i.e. Li in 8a tetrahedral sites) with decreasing lithium content for 0.3 < x < 1.0. For x < 0.3, the delithiated samples exhibit a new 6Li resonance at ∼950 ppm. This resonance is assigned to lithium present in 8a sites in a Mn4+-rich phase at the centre of the spinel particles. This assignment is supported by FT-IR data and crystallite size measurements (XRD), which suggest that HMn2O4 forms on the surface of the particles in the heavily delithiated samples.

Preparation and performance of spinel LiMn 2O 4 by a citrate route with combustion

Nano- and submicrometer LiMn 2O 4 spinel particles were successfully synthesized by a modified citrate route with spontaneous combustion at 250 °C followed by a calcination process. Citric acid was used as the chelating and combustion-assistant agent. The synthesized LiMn 2O 4 showed a high electrochemical performance compared with commercial LiMn 2O 4 materials. The initial discharge specific capacity reached 130 mAh/g. The effects of the processing factors and element substitution on the microstructure and electrochemical performance of LiMn 2O 4 were also examined.

TEM investigations of intergranular stress corrosion cracking in austenitic alloys in PWR environmental conditions

Analytical transmission microscopy has been used to investigate the initiation of stress corrosion cracking in Inconel 600 subjected to constant load testing under simulated pressured water reactor primary water conditions. The observations revealed that intergranular attack proceeded by the development of a zone of polycrystalline chromia along the boundary plane intersecting either the free surface or a blunted, open crack in contact with the free surface. Ni-rich metal particles were interspersed within the chromia. Conversely, open cracks were filled with nanocrystalline NiO and large compound particles of spinel and NiO, indicating a difference in potential between closed, attacked boundaries and open cracks. Open cracks appeared to have initiated by fracture of the chromia zones, such fracture being strongly dependent on boundary geometry with respect to loading direction. The observations suggest that stress corrosion crack initiation and propagation is dependent on diffusion of oxygen through the porous oxides. Dislocations and stress could enhance diffusion as chromia was observed along slip planes at the arrested tips of blunt cracks.

Glassy Magnetic Behavior in a Nanostructured Cu–Fe–O System

A nanostructured material in which copper ferrite grains are in close contact with antiferromagnetic α-Fe2O3 and CuO particles was obtained by ball milling an equimolar mixture of α-Fe2O3 and CuO. On cooling down from room temperature, Mossbauer and magnetization measurements reveal the progressive blocking of the α-Fe2O3 and spinel particles magnetic moments, followed by a sudden freezing below 60 K. This temperature indicates the start of a new magnetic regime that displays spin-glass-like magnetic properties at the lowest temperatures.

High precision isotope micro-imaging of materials

Secondary ion mass spectrometry (SIMS) is widely used to identify the isotope ratio of the micro-area of materials. We demonstrate that the precision and spatial resolution of an ion microscope using SIMS coupled with a newly developed solid-state ion imager can be extended to two-dimensional isotope ratio imaging with permil-precision. An isotopic map for oxygen clearly reveals the distribution of 4% enrichment of 16 O in spinel particles embedded in melilite from a Ca–Al-rich inclusion of a carbonaceous chondrite. This characterization technique called isotopography thus provides the eyes to see small heterogeneity of isotope abundance in micro-scale. It is likely to find broad application in earth, material, and life science research.

Hydrothermal Synthesis and Low‐Temperature Sintering of Zinc Gallate Spinel Fine Particles.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Thermal shock parameters [R, R‴ and R‴′] of magnesia–spinel composites

Abstract Thermal shock parameters (R, R‴ and R‴′) have been calculated using measured strength and modulus values for model magnesia-spinel composite materials. The R‴ and R‴′ parameters vary with both spinel content (0–30%) and spinel particle size (3–22 μm). In general, the larger the spinel content, the higher the values of R‴ and R‴′. It is predicted that the coarsest (22 μm) spinel provides a significant improvement in resistance to thermal shock damage through maximised difficulty of crack propagation, with a maximum at ∼20% addition. These predictions were also matched by thermal shock testing. After quenching from 1000 °C, the 20% 22 μm spinel composite had a retained strength ∼4.5 times higher than that for similarly quenched pure magnesia.

A SAXS/WAXS XAFS study of crystallisation in cordierite glass.

New Cr X-ray absorption fine structure (XAFS) data have been combined with the results of small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) experiments to probe in detail the crystallisation mechanism in cordierite (Mg2Al4Si5O18) glass doped with 0.34 mol% Cr2O3. By direct comparison with chromo-aluminate spinels (MgCr2xAl2(1 - x)O4) Cr XAFS is used to determine the composition of the devitrified Cr species. This is identified as MgCr(0.18)Al(1.82)O4, which can be directly related to the Cr content in the starting glass and as a result the total crystalline volume in the fully developed ceramic is predicted to be 4%. In situ WAXS not only reveals the presence of the spinel phase but also a silica-rich stuffed quartz phase. This grows independently of the spinel and is probably nucleated from the glass surface. From our knowledge of the compositions of both crystalline phases we are able to deduce that the SAXS contrast between the surrounding glass and the spinel crystallites is 30 times greater than that between the quartz crystallites and the glass matrix, and therefore dominates the measured scattered intensity and the SAXS invariant that is derived from it. As a consequence we are able to show that the spinel crystalline volume fraction inherent in the SAXS is in close agreement with the 4% value obtained from the Cr XAFS. Furthermore in situ SAXS reveals the gradual development of the spinel particle size and shape during heat treatment. This is conducted in the super-cooled region just above the glass transition temperature, Tg. By employing a two-step annealing process nucleation can be separated from growth and from time-resolved SAXS measurements the alumino-chromate nanocrystals are found to be closely monodispersed. Over a total time course of 600 min they grow from rough crystallites to smooth spherical particles of radius 21 +/- 2 nm, with a final density of (1.2 +/- 0.4) x 10(21) m(-3). As the process of ceramic formation takes place in the viscous melt, growth is indeed found to be limited by diffusion and is complete when all the Cr is exhausted. We use this comprehensive in situ study of crystallisation in cordierite glass to demonstrate the advantages of combining SAXS, WAXS and XAFS for probing the time-resolved chemistry, the microstructure and its development from nucleation sites, that underpins the processing of nanoparticle ceramics.

Oxidation–decomposition facilitated reorientation of nanoparticles in reactively sintered (Ni 0.33Co 0.67) 1− δO polycrystals

(Ni 0.33Co 0.67) 1− δ O polycrystals with rock salt structure and a bimodal size distribution due to reactive sintering at 1000 °C were subject to annealing at 720 °C for 2–72 h in air and studied by analytical electron microscopy with regard to the effect of oxidation decomposition on the reorientation of nanoparticles in host grains. Upon annealing, the nanoparticles rapidly oxidized as spinel structure progressively Co-richer, whereas the host protoxide grains with rock salt-type structure progressively Ni-richer. The spinel particles less than 100 nm in size readily detached from grain boundaries and fell into parallel epitaxial relationship with respect to the host protoxide grains sharing a coherent interface. Such a Brownian-type reorientation process, in terms of anchorage release at interphase interface and driven by epitaxy energy cusp, at a rather low apparent homologous temperature ( T/ T m=0.45) was facilitated by oxidation decomposition process and nanometer-size effect.

Fabrications and electrochemical properties of fluorine-modified spinel LiMn 2O 4 for lithium ion batteries

Spinel LiMn 2O 4 was modified by an aqueous fluoride modification method (AFMM). It is found that tiny crystals of potassium manganese trifluoride (KMnF 3) grew on the surface of the spinel particles during the electrochemical process. The quantity and the integrality of the KMnF 3 crystals were controlled by varying current density and processing time. The mechanisms of the formation of KMnF 3 were discussed. The harmful two-phase reaction of LiMn 2O 4 upon cycling was suppressed after the cathode was processed by AFMM. The chemical diffusion coefficients ( D s) of Li + were measured by a potential relaxation technique (PRT). The values of D s obtained in this way are of the order of 10 −10–10 −9 cm 2/s.

Synthesis andcharacterisation of nanocrystalline MgAl2O4ceramic powders by use of molten salts

AbstractNanocrystalline MgAl2O4 powders were prepared by a thermal decomposition method, i.e. by use of molten salts. This method involves co-melting stoichiometric amounts of magnesium nitrate hydrate Mg(NO3)2.6H 2O and aluminium nitrate hydrate Al(N O3) 3.9H2O at 500°C. The spinel content of the co-melted and calcined powders at different firing temperatures up to 1000°C was determined by chemical analysis and the powders were characterised with respect to spinel formation, crystallite and particle sizes by X RD, T EM , and IR spectroscopy. The results obtained revealed that the co-melted materials were amorphous. After heat treatment of the amorphous materials at up to 1000°C, pure spinel powder was obtained, reaching over 98% spinel content. During calcination at different firing temperatures up to 1000°C the amorphous material progressively crystallised, forming nanocrystalline spinel with a maximum crystallite size of about 10 nm and particle size of around 300 nm. Bands in IR spectra were observed ...

Hydrothermal Synthesis and Low‐Temperature Sintering of Zinc Gallate Spinel Fine Particles

Nanosized powders of single‐phase zinc gallate (ZnGa2O4) spinel were hydrothermally synthesized from solutions in the presence of NaOH over the pH range of 1.9 to 7.0 and from solutions above pH 7.0, i.e., the very basic medium (pH of 13.85), by removing the residual ZnO phase by washing with aqueous H2SO4 from the precipitate mixtures of zinc gallate spinel particles and ZnO. A very wide compositional range (Zn/2Ga = 0.705–1.157) of zinc gallate spinel solid solutions could be hydrothermally synthesized in the form of nanosized particles from acid and very basic mediums (pH of 2.4–13.85) in the presence of NaOH. These hydrothermally synthesized spinel powders showed good sinterability and almost full densification at 1100°C for 1 h. Dense sintered bodies consisting of single‐phase zinc gallate spinel were fabricated at 1100°C using zinc gallate spinel powders having almost stoichiometric composition formed from the solution at pH 9.95 in the presence of aqueous ammonia.

Chemical synthesis and thermal evolution of MgAl 2O 4 spinel precursor prepared from industrial gibbsite and magnesia powder

A chemical route, using industrial raw materials, was developed for the synthesis of MgAl 2O 4 spinel precursor. The as-obtained powder was then calcined in air, up to 1200 °C for a period of 1 h. It was found that spinel single phase is formed at several hundred degrees lower than temperatures reported for the conventional powder preparation methods. Moreover, at 500 °C the degree of crystallinity is higher than that reported by other chemical processes at this temperature. The calcined powder reaches 83% of relative density at 1200 °C, indicating reactivity and good sintering behaviour. Scanning electron microscopy images revealed that the product consisted of fine spherical particles of magnesium aluminate spinel.

Ion exchange at a metal–ceramic interface

We present the results for an interfacial reaction between magnesium aluminate spinel and aluminum metal at 640 °C which, for the first time, show the occurence of ion exchange at a metal–ceramic interface. The exchange reaction occurs by the diffusion of magnesium and aluminum ions across the interface in opposite directions. The enrichment of the spinel in aluminum cations, and the metal in magnesium is confirmed by the appearance of phases of aluminum oxide and magnesium–aluminum intermetallics in the X-ray diffraction spectra. Electron spectroscopic images of spatial elemental distribution provide further evidence for the reaction. A drop in the thermal and electronic conductivities are explained by enhanced scattering of the electrons by solid solution Mg in the aluminum metal. The experiments were carried out on composites consisting of an aluminum matrix with a fine dispersion of spinel particles, whose size was varied while the volume fraction was held constant. Measurable ion exchange reaction was seen only in the composites containing the smaller (one micrometer size) particles. The influence of surface to volume ratio of the particles on the reaction rate confirms that the reaction was interface controlled. The possibility of a nucleation and growth process for the conversion of spinel into alumina and solid solution magnesium was ruled out by quantitative thermodynamic analysis.

Arrays of magnetic nanoparticles capped with alkylamines

Magnetic metal and metal oxide nanoparticles capped with alkylamines have been synthesized and characterized by transmission electron microscopy. X-ray diffraction, energy dispersive X-ray analysis and magnetization measurements. Core-shell Pd-Ni particles with composition, Pd561Ni3000, (diameter ∼3.3 nm) are superparamagnetic at 5 K and organize themselves into two-dimensional crystalline arrays. Similar arrays are obtained with Pd561Ni3000Pd1500 nanoparticles containing an additional Pd shell. Magnetic spinel particles of γ-Fe2O3, Fe3O4 and CoFe2O4 of average diameters in the 4–6 nm range coated with octylamine are all supermagnetic at room temperature and yield close-packed disordered arrays. Relatively regular arrays are formed by dodecylaminecapped Fe3O4 nanoparticles (∼8.6 nm diameter) while well-ordered hexagonal arrays were obtained with octylamine-covered Co3O4 nanoparticles (∼4.2 nm diameter).