Spinel Particles Research Articles

Effect of MgCl2 addition on the sintering behavior of MgAl2O4 spinel and formation of nano-particles

In this paper, the effect of MgCl2 addition on the sintering behavior of MgAl2O4 spinel produced via oxide mixture method was investigated. For this reason, the stoichiometric mixture of magnesite and calcined alumina as raw materials was calcined at 1100?C. The calcined mixture was milled, pressed and then, fired at 1300 and 1500?C after addition of various amounts of MgCl2. Besides, the physical properties, phase composition and microstructure of fired samples were investigated. The results showed that MgCl2 addition has great effect on the densification and particle size of spinel. Besides, MgCl2 addition increases the amount of spinel phase at all firing temperatures. Due to the decomposition of MgCl2 and then formation of ultra-fine MgO particles, the nano-sized spinel is formed on the surface of the larger spinel particles.

The effects of thermal aging on metallic interconnects of solid oxide fuel cells

For solid oxide fuel cells (SOFCs), metallic interconnects (ICs) are crucial components to add to the electrochemical catalyst electrodes. The high temperature operation condition in SOFCs is an extremely degrading factor for metallic ICs. Three metallic ICs have been selected in this thermal aging study. IC-1 and IC-2 contain similar Mn/Cr compositions. The thermal mechanical property of IC-2 is better than that of IC-1, attributed to the inclusion of rare earth elements. IC-3 has lower chromia composition, whereas IC-1 and IC-2 have higher Cr contents. Thermal aging is introduced under ambient atmosphere at 800°C from 0 to 100 h. X-ray diffraction study of the IC oxide scales shows growth of (MnCr)3O4 with the spinel structures. Scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS) reveal the thickness and spinel particles for the three ICs, the oxide layers at about 0·9–1·3 μm thick and the spinel sizes at 1·0–1·3 μm. The oxide weight gain studies have shown that IC-3 is the more readily oxidised IC, with an estimated oxidation rate of ∼1·98×10−7 g2 cm−4 s−1 at 800°C. Thus, IC-3 should be better coated with a protective layer for SOFC applications. For the thermal Cr diffusion study, we applied homemade glass sealant on the surface of IC-1. After thermal aging for 1000 h under ambient atmosphere at 800°C, we examined the sample with micro-Raman. The cross-sectional microstructures were studied with SEM/EDS. Both Raman and SEM/EDS results indicated that the Cr ions penetrated into the glass sealant by ∼65 μm. The oxide scales on ICs are influential on the electrical conductivity. The electrochemical impedance spectroscopy study on IC-2 samples after thermal aging is also shown.

Kinetic model for quartz and spinel dissolution during melting of high-level-waste glass batch

The dissolution of quartz particles and the growth and dissolution of crystalline phases during the conversion of batch to glass potentially affects both the glass melting process and product quality. Crystals of spinel exiting the cold cap to molten glass below can be troublesome during the vitrification of iron-containing high-level wastes. To estimate the distribution of quartz and spinel fractions within the cold cap, we used kinetic models that relate fractions of these phases to temperature and heating rate. Fitting the model equations to data showed that the heating rate, apart from affecting quartz and spinel behavior directly, also affects them indirectly via concurrent processes, such as the formation and motion of bubbles. Because of these indirect effects, it was necessary to allow one kinetic parameter (the pre-exponential factor) to vary with the heating rate. The resulting kinetic equations are sufficiently simple for the detailed modeling of batch-to-glass conversion as it occurs in glass melters. The estimated fractions and sizes of quartz and spinel particles as they leave the cold cap, determined in this study, will provide the source terms needed for modeling the behavior of these solid particles within the flow of molten glass in the melter.

Effect of Sb Addition on the Electrical Characteristics of Bi-Based ZnO Varistors Containing Zr and Y

The effects of adding Sb to a BiMnCoSiCrNiYZr-added ZnO varistor (with the same composition as a commercial varistor) on the varistor voltage, leakage current, and resistance to electrical degradation were investigated. Bi is incorporated in spinel particles, and δ-Bi2O3eventually disappears with the addition of small amounts of Bi, especially as the amount of Sb2O3added increased. Reduction in both the nonlinearity index and the amount of δ-Bi2O3for small amounts of added Bi with the addition of more than approximately 1.25 mol% Sb2O3demonstrates that Sb inhibits Bi2O3from forming deep interfacial impurity levels at the grain boundaries. The sample containing 1.2 mol% Bi2O3, 1.0 mol% ZrO2, 1.0 mol% Y2O3, and 1.5 mol% Sb2O3added exhibits a high varistor voltage (approximately 630 V/mm), high resistance to electrical degradation and low leakage current.

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr2O3 layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr2O4 spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr2O3-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.

Application of Alumina-Magnesia Precast Blocks for LF Refining Ladle

The alumina-magnesia precast blocks developed in authors company have been successfully used in 90-ton LF refining with an average service life 80-85 heats to meet the smelting demand of low carbon steel and ultra low carbon steel. The observation of the residual blocks revealed that the section of the used blocks was divided into four layers from inside to the surface, i.e., slag layer, reaction layer, penetration layer and the original block. It has been found that there was a certain degree of structure spalling of the used blocks. The main factor of blocks damage may be due to the penetration and erosion of molten slag. The results of SEM microstructure analysis indicated that the calcium oxide (CaO) of high basicity slag easily penetrated into the blocks and formed CA6 phase, and then transformed into CA2 or C12A7 phases gradually with the decreasing content of alumina (Al2O3). It has been found that an annular distribution structure, and the silicon dioxide (SiO2) was easily lost together with slag and formed SiO2-rich part in the penetration layer due to the formation of low viscosity and low melting point phase of C2AS. A large number of-5um spinel particles (MgAl2O4) were synthesized at 1600~ 1700, which could absorb FeO, MnO and other impurities in slag to form composite spinel phase. It is believed that the high melting point phases intertwined to form a dense area which was beneficial to improve the bricks' slag resistances to penetration and corrosion

Development of High-Strain-Rate Superplastic Oxide Ceramics Based on Flow Mechanism

In order to attain high-strain-rate superplasticity (HSRS) in ceramics, flow behavior was examined with ZrO2 reference sample. The results suggest that the enhancement of the accommodation processes of grain boundary sliding (GBS) is important in addition to the careful controlling the microstructural factors, such as stable fine grain structure, reducing residual pores and so on. The spinel particles dispersion can simultaneously provide the following positive factors to ZrO2: i) suppressed grain growth due to pinning effect of spinel particles, enhanced accommodation due to ii) accelerated relaxation of stress concentrations exerted by GBS through dislocation motion and iii) accelerated lattice diffusion caused by the dissolution of aluminum and magnesium into ZrO2 from the spinel particles. The positive factors due to spinel dispersion make it possible to attain HSRS in ZrO2 ceramics.

MgAl2O4 spinel particles prepared by metal–chitosan complexation route and used as catalyst support for direct decomposition of methane

Magnesium aluminate particles were prepared from biopolymer (chitosan) and Al–Mg solutions by the metal–chitosan complexation method. The MgAl2O4 spinel obtained was used to prepare a Ni catalyst support. The Ni/MgAl2O4 (20% Ni, w/w) was evaluated as a catalyst in the methane decomposition reaction. The influence of the reaction temperature and the catalyst reduction temperature and time on the methane decomposition, as well as on the properties of the carbon produced, was investigated. The materials were characterized using XRD, SEM, TEM, TPR, TGA and Raman spectroscopy analyses. It was observed that the catalytic behavior was dependent on the reaction operating conditions employed. Under the operating conditions used in this study the carbon byproduct is mainly deposited as multi-walled nanotubes (MWNTs).

Influence of dissolved oxygen concentration on the oxide film formed on Alloy 690 in high temperature water

Characteristics of the oxide films formed on Alloy 690 under different dissolved oxygen (DO) concentrations in 290°C water were examined. Under 3ppm DO, the oxide film mainly consisted of outer spinel particles and porous NiO base layer. A thin Cr-rich film gradually develops with decreasing DO. Fe and Ni levels in the film decrease while Cr level increases concomitantly. The film preformed under high DO changes slightly when further immersed under low DO. The film preformed under low DO becomes much thicker when immersed under high DO, with gradual development of spinel particles and NiO base layer.

Sol–gel mediated surface modification of nanocrystalline NiFe2O4 spinel powders with amorphous SiO2

Abstract Surface modification of nanocrystalline NiFe 2 O 4 spinel particles with amorphous SiO 2 by the sol–gel process at 350 °C was demonstrated. Amorphous phase of the SiO 2 layer was evaluated by X-ray diffraction technique. Structural coordination of the pristine and SiO 2 coated NiFe 2 O 4 particles as investigated by employing FTIR analysis. Thickness of the SiO 2 layer was investigated through transmission electron microscopy and it was identified to be ∼10–23 nm over nanocrystalline NiFe 2 O 4 particles. The magnetic behavior of pristine and surface modified NiFe 2 O 4 particles were investigated using vibrating sample magnetometer (VSM). Magnetic studies showed the retention of magnetic property of surface modified NiFe 2 O 4 particles with the reduced saturation magnetization and coercivity compared to the pristine NiFe 2 O 4 particles, which is respectively due to the lower fraction of the magnetic component and the formation of interfacial structure.

Oxide Coating of Alumina Nanoporous Structure Using ALD to Produce Highly Porous Spinel

AbstractIn this work we show that atomic layer deposition (ALD) stands out as a very promising method for coating nanomaterials and, more specifically, nanoporous materials of technological interest in heterogeneous catalysis. ALD is capable of coating extremely complex shapes with high conformality and high aspect ratios. The current work describes deposition of ZnO and CoO thin films on γ‐alumina nanoporous particles with sizes ranging from 20 to 100 µm in diameter, as well on aluminum membrane discs (anodiscs), and converting the surfaces into nanoporous, mechanically more robust spinel phases. We maintain the porosity of the supporting γ‐alumina particle, despite the addition of material to the 16 nm pores, by utilizing subsequent reactions between the coated layer and γ‐alumina, following the Kirkendall mechanism. The oxide thin film coatings are deposited at 175 and 167°C for ZnO and cobalt oxide, respectively, and thereafter annealed at from 600 to 1000°C for 3 h. This represents a new way to produce highly porous spinel particles and spinel‐coated membrane discs.

Automated quantitative 3D analysis of faceting of particles in tomographic datasets

Characterization of facets of particles is a common problem. In this paper an algorithm is presented which allows automated quantitative 3D analysis of facets of many particles within tomographic datasets. The algorithm is based on the analysis of probability distributions of the orientations of triangle normals of mesh representations. The result consists of lists containing number of detected facets, their size, global orientation and the interplanar angles between facets for each analyzed particle. Characterization of each particle according to any of these facet properties is then possible, e.g. statistics about different crystal shapes or removal of particles that do not show significant faceting. Analyses of a 3D dataset obtained by focused ion beam (FIB) tomography of a sample containing spinel particles are presented.

Microstructure Engineering to Drastically Reduce the Leakage Currents of High Voltage ZnO Varistor Ceramics

The energetic losses associated with the leakage currents can represent a serious problem for the efficient performance of ZnO‐based varistor ceramics. However, by properly tailoring the microstructure of these ceramics, the leakage currents can be drastically reduced. Pursuing this goal, two intended doping strategies have been attempted over a standard high‐voltage varistor formulation based on the ZnO–Bi2O3–Sb2O3 ternary system. The first strategy focuses on the stabilization of the secondary Zn2.33Sb0.67O4 spinel particles; on cooling the ceramic, this spinel phase reversely transforms into a pyrochlore conductive structure whose presence favors the current flow in the varistor prebreakdown region (i.e., high leakage current values). A certain amount of TiO2 incorporated into the spinel lattice impedes its reverse transformation to pyrochlore, thus reducing the varistor leakage current. The second strategy leans on the insertion of an insulating phase in between the skeleton of Bismuth‐rich phases. This skeleton provides a continuous network through the whole varistor microstructure for the current to flow, so the inclusion of an insulating phase may serve as a physical blockade to break off that connectivity. Such a phase is yielded by properly doping with SiO2. Eventually, the combination of both strategies leads to a decrease of two orders of magnitude in the varistor leakage current.

Olivine Coated Spinel: 5-Volt System for High Energy Lithium Batteries

LiMn1.5Ni0.5O4 (LMN) has attracted considerable attention as 5-volt cathode materials for Li-ion batteries. Here, the coating of nano LiFePO4 (LFP) onto the surface of the spinel particles is reported for the first time. Two processes were used to prepare LFP-coated LMN: a traditional sol-gel method in Ar and a mechano-fusion dry process, which provides a simple way to obtain surface coating for battery materials. Surface elemental analyses indicate that nano-LFP is intimately covered on the LMN surface. LiMn1.5Ni0.5O4 showed better rate property especially at high rate (>10C) after LiFePO4 coating. Good performance such as 82 mAh g-1 of capacity was still obtained for the LiFePO4-coated LiMn1.5Ni0.5O4 with 75% of capacity retention after 140 cycles under 1C.

Effect of metal loading processes on the stability and thermal transformation of Co2+- and Cu2+-zeolite Y prepared from Egyptian kaolin

Abstract This paper aims to assess the effect of the transition metals (TM) loading procedure on the incorporation of Co 2 + and Cu 2 + in zeolite Y, and their relevance to stability of the zeolite, particularly with respect to the thermal transformation to the spinel phases. In this work, zeolite Y prepared from Egyptian kaolin was used. XRF, XRD, TEM, UV/visible absorption measurements, and atomic absorption analyses in addition to the visual observations are recorded. XRF has been used to investigate the materials composition. TEM and XRD indicate the presence of nanoparticle spinel upon the calcination of the TM-zeolites at 1000 °C. In addition to spinel particles, XRD shows the formation of metal oxides, SiO 2 and alumino-silicate phases. According to the transition metal and the cation loading process, different phases were detected. UV/visible absorption measurements and the visual observations are used to determine the experimental condition of the highest spinel content. It has been noticed that the experimental conditions of the metal sorption processes greatly affect the phase transformation. Stability and thermal transformation of zeolite depend on the initial concentration of the transition cation solutions and the number of loading cycles.

The effect of SnO2 on the improvement of mechanical properties of MgO–MgAl2O4 composites

Incorporation of SnO2 into MgO–spinel (M–S) increased mechanical properties significantly. Relationships between the parameters improving mechanical properties and microstructural variables were examined. Basic parameters improving the mechanical properties of M–S–SnO2 composites were identified as follows: (a) when microcracks come across with either Mg2SnO4 particles or pores; crack branching and deviation of interlinked microcracks or crack arresting occurred more effectively than those of spinel particles, (b) fracture type was converted to intergranular fracture with incorporation of spinel into MgO, and transgranular fracture with addition of SnO2 to M–S; additionally with the incorporation of additives, (c) critical defect size, (d) work of fracture values increased, and (e) MgO grain size decreased. Rst thermal shock parameter values of M–S–SnO2 composites were markedly higher than those of M–S materials, associated with low strength loss, high thermal shock damage resistance and thus longer service life of M–S–SnO2 composites for high-temperature industrial applications.

Characterization of Nonmetallic Inclusions in High-Manganese and Aluminum-Alloyed Austenitic Steels

The effects of Al and Mn contents on the size, composition, and three-dimensional morphologies of inclusions formed in Fe-xMn-yAl (x = 10 and 20 mass pct, y = 1, 3, and 6 mass pct) steels were investigated to enhance our understanding of the inclusion formation behavior in high Mn-Al–alloyed steels. By assuming that the alumina is a dominant oxide compound, the volume fraction of inclusions estimated from the chemical analysis, i.e., insoluble Al, in the Fe-Mn-3Al steels was larger than the inclusion volume fractions in the Fe-Mn-1Al and Fe-Mn-6Al steels. A similar tendency was found in the analysis of inclusions from a potentiostatic electrolytic extraction method. This finding could be explained from the terminal velocities of the compounds, which was affected by the thermophysical properties of Fe-Mn-Al steels. The inclusions formed in the Fe-Mn-Al–alloyed steels are classified into seven types according to chemistry and morphology: (1) single Al2O3 particle, (2) single AlN or AlON particle, (3) MnAl2O4 single galaxite spinel particle, (4) Al2O3(-Al(O)N) agglomerate, (5) single Mn(S,Se) particle, (6) oxide core with Mn(S,Se) skin (wrap), and (7) Mn(S,Se) core with Al2O3(-Al(O)N) aggregate (or bump). The Mn(S,Se) compounds were formed by the contamination of the steels by Se from the electrolytic Mn. Therefore, the raw materials (Mn) should be used carefully in the melting and casting processes of Fe-Mn-Al–alloyed steels.

Effect of heat treatment on tolerance characteristic for electrical degradation of ZnO varistors with added Sb2O3

AbstractThe effects of the heat treatment of ZnO varistors with added Bi‐Mn‐Co‐Sb on their electrical degradation were investigated. Before heat treatment, the degree of the tolerance characteristics of varistors for electrical degradation was smallest for the sample with 0.12 mole‐% of added Sb2O3. After heat treatment, the electrical degradation tolerance characteristic was markedly improved. It was found that the tolerance characteristics for electrical degradation after heat treatment strongly depend on the amount of added Sb2O3. For the samples with added Sb2O3, the tolerance characteristic for electrical degradation upon heat treatment is not affected by the intensity of the X‐ray diffraction peak of the (004) plane for ZnO. For samples with 0.12 mole‐% or less of added Sb2O3, the value of the nonlinearity index α of the voltage–current (V–I) characteristic after electrical degradation was proportional to the full width at half maximum (FWHM) of the X‐ray diffraction peak at 29.4° for Zn2.33Sb0.67O4 spinel particles in samples subjected to heat treatment under various conditions. Therefore, it is speculated that the increase in the FWHM for the spinel particles is due to the existence of fine spinel particles at grain boundaries or triple points of ZnO grains. Furthermore, it is speculated that the improvement of the electrical degradation tolerance characteristics is due to a decrease in the ion mobility of O2– ions or Zn2+ ions due to their blocking by uniformly dispersed fine spinel particles at grain boundaries. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 179(2): 1–10, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21229

Excellent stability of spinel LiMn2O4-based composites for lithium ion batteries

Spinel LiMn2O4-based composites with cation (Li+)/anion (F−) co-doping and Li2MnO3 thin surface coating are synthesized by one-step solid-state reaction. The composites modified with body-doping and surface-coating exhibits excellent cycling performance and high rate capability. The LF04 sample tested at 1 C rate exhibits only 1.2% capacity loss after 1000 cycles at room temperature; and when cycling at 55 °C, the composite still shows 94.3% capacity retention after 100 cycles. As the charge–discharge rate increases to 7 C, the as-prepared LiMn2O4-based composites deliver more than 95.5% capacity retention after 300 cycles at room temperature and exhibit an average value of coulombic efficiency more than 99.6%. The improved cycling performance of the composites is ascribed to double doping in the spinel structure and the coating on the surface of the spinel particles. The ionic doping could eliminate the problems with cooperative Jahn–Teller distortion and suppress Li+-ion order at 0.5 Li; and the inborn Li2MnO3 coating containing only Mn4+ ions, could prevent the disproportionation of Mn3+ on the surface.

Microwave-hydrothermal synthesis and characterization of nanostructured copper substituted ZnM2O4 (M = Al, Ga) spinels as precursors for thermally stable Cu catalysts

Nanostructured Cu(x)Zn(1-x)Al(2)O(4) with a Cu:Zn ratio of ¼:¾ has been prepared by a microwave-assisted hydrothermal synthesis at 150 °C and used as a precursor for Cu/ZnO/Al(2)O(3)-based catalysts. The spinel nanoparticles exhibit an average size of approximately 5 nm and a high specific surface area (above 250 m(2) g(-1)). Cu nanoparticles of an average size of 3.3 nm can be formed by reduction of the spinel precursor in hydrogen and the accessible metallic Cu(0) surface area of the reduced catalyst was 8 m(2) g(-1). The catalytic performance of the material in CO(2) hydrogenation and methanol steam reforming was compared with conventionally prepared Cu/ZnO/Al(2)O(3) reference catalysts. The observed lower performance of the spinel-based samples is attributed to a lack of synergetic interaction of the Cu nanoparticles with ZnO due to the incorporation of Zn(2+) in the stable spinel lattice. Despite its lower performance, however, the nanostructured nature of the spinel catalyst was stable after thermal treatment up to 500 °C in contrast to other Cu-based catalysts. Furthermore, a large fraction of the re-oxidized copper migrates back into the spinel upon calcination of the reduced catalyst, thereby enabling a regeneration of sintered catalysts after prolonged usage at high temperatures. Similarly prepared samples with Ga instead of Al exhibit a more crystalline catalyst with a spinel particle size around 20 nm. The slightly decreased Cu(0) surface area of 3.2 m(2) g(-1) due to less copper incorporation is not a significant drawback for the methanol steam reforming.