Spinel Particles Research Articles

Spark Plasma Sintering of fine-grained YAG:Nd+MgO composite ceramics based on garnet-type oxide Y2.5Nd0.5Al5O12 for inert fuel matrices

Abstract Applicability of Spark Plasma Sintering has been studied for production of high-density (98.6–99.5%) composite ceramics Y2.5Nd0.5Al5O12 (YAG:Nd) – x MgO (x = 5, 10, 20 vol %). Powder YAG:Nd-MgO compositions have been obtained by precipitation of the MgO phase from Mg(NO3)2 water solution of magnesium nitrate onto the surface of garnet nanoparticles YAG:Nd. It has been demonstrated that the intensity of sintering fine-grained YAG:Nd-MgO composites is governed by volume diffusion at low temperatures and by grain boundary diffusion at high temperatures. Faster sintering at low temperatures is caused by formation of MgAl2O4 spinel particles and internal stress fields that are manifested as broadening and shifting of garnet peaks in the sintered composites.

Synergistic effects of temperature and polarization on Cr poisoning of La0.6Sr0.4Co0.2Fe0.8O3−δ solid oxide fuel cell cathodes

A new Cr incorporation mechanism involving preferential formation of nanometre size Fe–Co–Cr–O spinel particles was found to be responsible for a reduced Cr poisoning effect at 900 °C under cathodic polarization for the La0.6Sr0.4Co0.2Fe0.8O3−δ solid oxide fuel cell cathode.

Catalytic performance of Cu-Ni-Al spinel for methanol steam reforming to hydrogen

Using copper hydroxide, nickel acetate and pseudoboehmite as materials, the Cu-Ni-Al spinel catalysts were synthesized by the solid-phase method. The effects of Cu/Ni/Al molar ratio and calcination temperature on specific surface area, phase composition, reduction performance and surface property of Cu-Ni-Al spinel catalysts were characterized by BET, XRD, H2-TPR and XPS. Moreover, the sustained release catalytic performances of Cu-Ni-Al spinel samples for methanol steam reforming were tested. The obtained results indicated that with increasing the calcination temperature, the content of Cu-Ni-Al spinel increased, but the size of spinel particles increased and the specific surface area decreased. Change of the calcination temperature and Cu/Ni/Al molar ratio led to different specific surface area, reduction performance and surface property of Cu-Ni-Al spinel catalysts, thus showing different sustained release catalytic performance. Comparing with those of stoichiometric ratio of Cu/Al=1:2, spinel solid solutions with smaller particle size, higher specific surface area and pore volume, more hardly-reducible spinel and better sustained release catalytic performance were obtained with the nonstoichiometric ratio of Cu/Al=1:3. The results of catalyst evaluation indicated that active copper species were released from Cu-Ni-Al spinel lattice and thus took part in the catalytic action. Among the prepared catalysts, CNA3-1000 catalyst showed the highest catalytic activity and stability.

Efficient humidity-sensitive electrical response of annealed lithium substituted nickel ferrite (Li–NiFe2O4) nanoparticles under ideal, real and corrosive environments

The Li–NiFe2O4 nanoparticles have been prepared via simple cost effective chemical co-precipitation method. X-ray diffraction analysis affirms the cubic spinel structure and particle size is ~ 32 nm. SEM and TEM analysis were revealed the needle shape of nanoparticles with agglomeration. XPS and FT-IR spectrum confirmed composition and usual behaviour of spinel ferrites. Band gap energy of material is 3.62 eV that imply semiconducting nature. Humidity sensor analysis is carried out three different environments in order to test the influence of medium stress factors on sensors parameters. Under these environments, Li–NiFe2O4 nanoparticles exhibit well sensing nature. Besides, the material displays high sensitivity at ideal environments and good stability in real environments. The results also show interesting characteristics of the maturing and aging process of humidity sensors.

Ni Substitution Effect on the Fe Position of Spinel Fe2MnO4 Particles for Chemical Looping Combustion.

The purpose of this study was to use a spinel structure to improve the performance and stability of chemical looping combustion processes. The oxygen carrier employed was Fe2MnO4, in which Ni was substituted at the Fe sites. Fe2-xNixMnO4 spinel particles were successfully synthesized by a sol-gel method. The obtained particles were characterized by X-ray diffraction (XRD), scanning electron microscopy, and CH4-/CO-temperature programmed desorption experiments. The XRD analysis confirmed that all the synthesized particles presented spinel structure. The performance of the particles was evaluated in redox cycle experiments under H2/air and CH4/air at 850 °C using a thermogravimetric analyzer. The Ni-substituted particles exhibited a higher performance than Fe2Mn1, being Fe1Ni1Mn1 the sample with the highest oxygen transfer capacity (19.68 wt% in H2/air and 15.90 wt% in CH4/air).

Enhancement of the nickel converter slag-cleaning operation with the addition of spent potlining

The slag cleaning (or matte settling) process was experimentally investigated at 1573 K using a fayalitic nickel converter slag containing spinel and matte/alloy particles. The addition of various amounts of spent potlining (SPL) was studied in terms of its influence on matte settling and the overall metal recoveries. The slags produced were characterized by scanning electron microscopy, energy-dispersive spectroscopy, and wet chemical analysis using inductively coupled plasma optical emission spectrometry. The presence of solid spinel particles in the molten slag hindered coalescence and settling of matte/alloy droplets. Matte settling was effectively promoted with the addition of as little as 2wt% SPL because of the reduction of spinel by the carbonaceous component of the SPL. The reduced viscosity of the molten slag in the presence of SPL also contributed to the accelerated matte settling. Greater metal recoveries were achieved with larger amounts of added SPL. Fast reduction of the molten slag at 1573 K promoted the formation of highly dispersed metal particles/clusters via accelerated nucleation in the molten slag, which increased the overall slag viscosity. This increase in viscosity, when combined with rapid gas evolution from accelerated reduction reactions, led to slag foaming.

Characterization of nano-scale oxides in austenitic stainless steel processed by powder bed fusion

Nano-scale MnSiO3 Rhodonite particles were observed in 316L stainless steel fabricated by selective laser melting (SLM). These oxide particles act as Zener-pinning particles providing dragging force to retard grain growth, and enhance strength through the comparison of Hall-Petch effects between SLM 316L and commercial wrought 316L under 1200 °C isothermal heat treatment. The metastable Rhodonite particles were observed to in-situ convert to stable MnCr2O4 Spinel particles at 1200 °C, as predicted by computational thermodynamics. This work introduces nano-scale oxides, which are normally detrimental inclusions, as a new type of particle for grain refinement and strengthening through their refinement in additive manufacturing.

Structural Changes and Material Transport in Al2O3-Supported Cu/Fe Spinel Particles in a Simulated Chemical Looping Combustion Environment

Alumina-supported Cu/Fe spinel particles were exposed to oxidation/reduction atmospheres at 800°C. Structural changes of the particles subjected to gas cycles between air and 10 vol.% CO-90 vol.% Ar were studied from physical data and real-time images collected using a confocal scanning laser microscope equipped with a heating chamber. Overall particle volume slowly expanded with cycles while surface roughness decreased. Cross-sections of the exposed particles showed segregation of Cu and Fe to the edges of inner grains, which may have acted as oxygen carriers during the exposures. The particles remained whole during the cyclic exposures without any noticeable structural breakdown.

Effect of oxygen partial pressure on grain-boundary transport in alumina

The oxidation of the bond coat in a thermal barrier coating results in the formation of a protective alumina scale. Such scales experience large gradients in oxygen partial pressure (PO2) during formation, but it is unclear how PO2 affects grain-boundary (GB) transport in alumina. To study the effect of PO2 on GB kinetics in alumina, the rate of oxygen penetration was measured at 1400 °C over a wide range of PO2. This was accomplished using a novel reduction experiment using nickel aluminate spinel particles. The results indicated that the dependence of the boundary transport on PO2 is mainly due to the driving force, namely the gradient in PO2 across the alumina. Point-defect models were then invoked to interpret the measurements, and it was found that transport in an alumina scale is controlled by extrinsic defects.

Growth of diopside crystals in CMAS glass‐ceramics using Cr2O3 as a nucleating agent

AbstractCaO–Al2O3–MgO–SiO2 (CAMS)‐based glass‐ceramics were prepared using body crystallization method. Adding Cr2O3 into the ceramics not only effectively lowered the crystallization temperature, but also led to significant grain refinement of diopside that crystallized in the CAMS glass‐ceramic after crystallization treatment at 900°C for 2 hours. Experimental work verified that the epitaxial growth of the diopside on the spinel particles, which formed during nucleation treatment when fabricating the glass‐ceramics, facilitated the heterogeneous nucleation of diopside on the spinel and refined the diopside. In addition, two energetically favored crystallographic orientation relationships between the epitaxial growth diopside and spinel were experimentally observed. They are //[001]diopside,////(200)diopside and //[101]diopside, (311)spinel//. These two novel results can be potentially used to develop new glass‐ceramic materials with improved performance.

Real-Time Analysis on Structural Variations of Alumina-Supported Cu/Fe Spinel and Natural Hematite Particles in Cyclic Redox Environments

Oxygen carrier materials in chemical looping applications exhibit physical and chemical variations during cyclic oxidation-reduction exposure at elevated temperature, which often lead to material degradation. While ideal chemical reactions of pure components may be predicted thermodynamically, structural changes of materials in chemical looping systems are complex and continuous due to the redox nature of the process. In this work, structural variations in two types of oxygen carrier particles (alumina-supported Cu/Fe spinel and natural hematite) during repeated redox exposures at 800°C were investigated in situ using a customized heat chamber attached to a confocal scanning laser microscope. Volume changes of the micron particles corresponding to each exposure cycle were recorded real time. Volume changes along with other structural degradation influence oxygen accessibility into the particles; impacting the oxygen exchange capability of a material. Structural degradation will be discussed based on a lattice structure changes, void formation in inner grains and particles, diffusion of the constituents, and sintering/densification with different structures.

Can (Mg,Y)‐PSZ—Spinel composites be a valuable option for dental application?

AbstractTwenty‐year‐old samples of (Mg,Y)‐PSZ—Spinel composite were characterized to assess their microstructure and mechanical properties after such a long period of natural aging. The amount of the different phases was determined by XRD, whereas a detailed observation of the microstructural features was carried out by high‐resolution SEM and FIB/SEM. Bending strength, hardness, and toughness were also measured. Translucency was quantified by measuring the contrast ratio (CR). After comparison of the results obtained in this study with those obtained 20 years ago, the studied material reveals a phase stability superior to that exhibited by conventional 3Y‐TZP, good strength, and hardness and a remarkable toughness. The value of CR is comparable with those exhibited by the materials commonly used in dentistry. Thanks to the presence of spinel particles, grains are smaller than in conventional Mg‐PSZ. In addition, we show here that grains can be further refined through spark plasma sntering. For all these reasons (Mg,Y)‐PSZ‐spinel composite could be proposed as valid alternative to 3Y‐TZP for dental applications.

Bi-metal organic framework derived nickel manganese oxide spinel for lithium-ion battery anode

Ternary transition metal oxides offer several advantageous features compared to the binary analogs due to presence of hetero transition metals with multiple valence states and the possible synergistic effects. Recently, metal organic frameworks have emerged as exotic template for synthesis of morphologically pre-designed metal oxide nanostructures. Nonetheless, synthesis of heteroatom ternary metal oxides through bi-metal organic framework (BMOF) route has proved to be challenging. Herein, we report a scalable single-step synthesis protocol for obtaining Ni/Mn-1,3,5-benzenetricarboxylate BMOF from which interconnected multi-faceted particles of NiMn2O4 spinel could be derived by thermal calcination at 800 °C. When applied as an anode for lithium-ion battery, the BMOF-derived mesoporous NiMn2O4 delivered a high reversible capacity (1049 mAh g−1), good rate performance (413 and 258 mAh g−1 at 503 and 1257 mA g−1) and good electrochemical stability highlighting the MOF-derived morphological advantages. The present results would help designing ternary metal oxide nanostructures for electrochemical energy storage application.

Effect of Mn Doping on Fe3O4 Nanoparticles Synthesized by Wet chemical Reduction Technique

The effect of Mn doping on Fe3O4 nanocrystalline spinel particles is studied. Two doping concentrations of 10 and 15% Mn were employed. The nanoparticles synthesis was carried out by wet chemical reduction technique. The energy dispersive analysis of X-ray confirmed the stoichiometry of the samples. The X-ray diffraction technique was used to determine the crystal structure and particles size of the synthesized nanoparticles. The electron microscopy revealed that both the synthesized nanoparticles surfaces have flower-like patterns and the particles are spherical. The optical absorption study showed that the absorption is more in case of 10% Mn doped compared to 15% Mn doped Fe3O4 nanoparticles. The Fourier transform infra-red spectroscopy revealed that both the samples contain characteristics bands. The magnetization variation with temperature with zero field cooled and field cooling conditions for two different applied magnetic fields of 500 Oe and 1000 Oe as well as the hysteresis study was carried out by vibrating sample magnetometer technique. The obtained results are discussed in details.

X-Ray Excited Optical Luminescence and Portable Electron Probe Microanalyzer-Cathodoluminescence (EPMA-CL) Analyzers for On-Line and On-Site Analysis of Nonmetallic Inclusions in Steel.

The potential of the application of an X-ray excited optical luminescence (XEOL) analyzer and portable analyzers, composed of a cathodoluminescence (CL) spectrometer and electron probe microanalyzer (EPMA), to the on-line and on-site analysis of nonmetallic inclusions in steel is investigated as the first step leading to their practical use. MgAl2O4 spinel and Al2O3 particles were identified by capturing the luminescence as a result of irradiating X-rays in air on a model sample containing MgAl2O4 spinel and Al2O3 particles in the size range from 20 to 50 μm. We were able to identify the MgAl2O4 spinel and Al2O3 particles in the same sample using the portable CL spectrometer. In both cases, not all of the particles in the sample were identified because the luminescence intensities of the smaller Al2O3 in particular were too low to detect. These problems could be solved by using an X-ray tube with a higher power and increasing the beam current of the portable CL spectrometer. The portable EPMA distinguished between the MgAl2O4 spinel and Al2O3 particles whose luminescent colors were detected using the portable CL spectrometer. Therefore, XEOL analysis has potential for the on-line analysis of nonmetallic inclusions in steel if we have information on the luminescence colors of the nonmetallic inclusions. In addition, a portable EPMA-CL analyzer would be able to perform on-site analysis of nonmetallic inclusions in steel.

Metal Droplet Entrainment by Solid Particles in Slags: An Experimental Approach

This study investigates the origin of the attachment of metal droplets to solid spinel particles in liquid slags. Previous research hinted a reactive origin: the spinel particles form by a chemical reaction together with a new droplet or alongside a droplet that was already present in the system. In this study, a smelting experiment was used to investigate this hypothesis. For such a study of the mechanism, a simple chemical system was used to avoid complex reactions. However, performing smelting experiments in simple slag systems requires an adaptation of the previously developed experimental methodology, resulting in a new ‘partial melting’ methodology. During the experiment, the atmosphere of the system was first set as oxidative, to dissolve the metallic copper in the slag and then a reductive atmosphere was used to actuate the reaction. Moreover, Ag was added to the metallic phase to act as a tracer element. The results show that the amount and size of copper droplets increase over the duration of the experiment. The fact that silver is present in the attached copper droplets in a smaller concentration than in the master alloy in this study indicates that the origin of the attachment is not purely dispersive, and either a purely reactive or a dispersion–reaction combination is possible, which confirms the hypothesis.

Effect of Na2S treatment on the structural and electrochemical properties of Li1.2Mn0.54Ni0.13Co0.13O2 cathode material

The electrochemical performances of Li1.2Mn0.54Ni0.13Co0.13O2 cathode material are enhanced through hydrothermal approach using Na2S solution as a medium. Furthermore, the influence of contents of Na2S solution on the morphology, structure, and electrochemical performances is fully investigated. The results indicate that a high content of Na2S solution results in the formation of spinel particles which are detected by SEM, XRD, and HR-TEM measurements. The formation of spinel particles leads to a deterioration of electrochemical performances. However, a low content of Na2S solution results in a slight structure change from layered phase to spinel one near the edge of Li1.2Mn0.54Ni0.13Co0.13O2 particles. This slight structure change facilitates the decrease of charge transfer resistance, which contributes to the enhanced rate capability of Na2S-treated Li1.2Mn0.54Ni0.13Co0.13O2.

Matrix microstructure optimization of alumina-spinel castables and its effect on high temperature properties

In this paper, submicron spinel-containing calcium aluminate (SP-CA) cements were prepared at temperatures of 1200°C (RA12), 1300°C (RA13), and 1450°C (RA14) using reactive alumina, magnesium hydroxide, and calcium carbonate as raw materials. The microstructural characteristics and high temperature properties of refractory castables bonded by SP-CA cement were investigated. The results indicated that the mineralogical compositions of SP-CA were spinel (MgAl2O4, MA) and CaAl2O4 (CA). The spinel phases presented no definite borders or shapes, with average size ranging from 0.1μm to 1.2µm at different sintering temperatures. After heating alumina-spinel castables, the submicron spinel particles bonded with corundum and hibonite (CA6) grains could strengthen the castable body. However, the large sized (< 88µm) spinel did not form links between spinel and alumina in the castable with added preformed spinel (castable AS). Submicron spinel can decrease the pore sizes of refractory castables from 2µm to 0.5µm. Extended finite element method (XFEM) calculations showed that smaller pores prevent stress concentration and result in increased deformation with limited crack propagation. Hence, large numbers of 0.5µm pores can act as crack-stoppers to improve the thermal shock resistance of castables with added SP-CA cements. Thermal shock resistance tests showed that R'''' of castable RA12 was 39.7m, which is significantly higher than that of castable AS (19.8m). The corrosion rate of castable RA12 was 5mm2/min, which is significantly lower than that of castable AS (7.4mm2/min). Small pore size and submicron spinel played relevant roles in corrosion behavior.

Investigation of Origin of Attached Cu-Ag Droplets to Solid Particles During High-Temperature Slag/Copper/Spinel Interactions

This study investigates the origin of mechanically entrained metal droplets in liquid slag due to their interaction with solid spinel particles. Two possible mechanisms were proposed previously: separately formed droplets and spinel particles get attached to each other due to agitation of the slag and metal phases; or the spinel particles form by a chemical reaction together with a new droplet or alongside a droplet that was already present in the system. In this study, an inert tracer element was added to the metallic phase in adapted sessile drop experiments. For this purpose, Cu-Ag alloys, with various Ag-contents, were produced. The results showed that the small entrained metal droplets within the slag droplet contained Ag, but in very low amounts with respect to the amount of Ag in the Cu-Ag alloy. This indicates that the entrained metal droplets are formed due to a sequential combination of the two origins: first, very small metal droplets are dispersed in the slag drop, due to the emulsification process. Then, these metal droplets are nucleation sites for the Cu-spinel reactive formation.

Dissolution of MgO based refractories in CaO-Al2O3-MgO-SiO2 slag

Dissolution of different types of MgO based refractories into molten CaO-Al2O3-MgO-SiO2 slag was studied. Formation of micro spinel particles by addition of either colloidal alumina or micro alumina powder in the MgO matrix efficiently reduced the dissolution. The dissolution of MgO was found to be controlled by the slag penetration into the MgO matrix. In the case of stagnant slag, the slag-penetration would lead to the final dissociation of the decarbonized commercial MgO-carbon cube in less than 6min. The major dissolution took place between the MgO grains and the liquid in this penetrated layer. The increase of the thickness slowed down the dissolution. In the case of stagnant slag and slag stirred at low speed, the internal mass transfer in the slag-penetrated layer is the controlling step. At higher stirring speeds, the slag-penetrated layer was removed. The removal of the penetrated layer would enhance the dissolution process.