Aluminate Spinel Research Articles

From nanoporous to transparent MgAl2O4 spinel—Nanostructural flexibility by reaction densification of metastable powders

Polycrystalline magnesium aluminate spinel (MgAl2O4) is one of the most important structural ceramics with proven applications as a hard, temperature resistant material and as a high strength optical window. Here, we present a study of the reaction densification of nano-sized MgO (nMgO) and metastable γ-Al2O3 to form MgAl2O4 instead of the typical stable α-Al2O3 and micrometer-sized MgO (μMgO). We show that the reaction kinetics of γ-Al2O3 – nMgO are substantially faster and occur at significantly lower temperatures compared to previous kinetic studies of α-Al2O3 – μMgO, revealing that they are not controlled by the same mechanism. We propose that the enhanced reaction rates are caused by short diffusion distances, faster diffusion through defected γ-Al2O3 as well as similarities in crystal structure between products and reactants. Lastly, we show that understanding the reaction and densification kinetics of this reactant combination can be leveraged to produce a wide range of microstructures leading to nanoporous high temperature energy absorbing materials and highly dense optically transparent spinel.

Effect of Reaction Time on the Synthesis and Sintering of Magnesium-Aluminium Spinel by Microwave Hydrothermal Synthesis

In this study, microwave hydrothermal synthesis of magnesium-aluminium spinel from pseudo-boehmite (AlOOH.nH2O) (n=0.62) and magnesium hydroxide (Mg(OH)2) for different reaction times is researched. X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) are used to find out how the reaction time affects the synthesis of magnesium aluminium spinel precursors. The superficial properties of magnesium aluminium spinel precursors are investigated with scanning electron microscopy (SEM). According to XRD analysis, magnesium hydroxide reacts with pseudo-boehmite to form magnesium aluminium hydroxide hydrate. The median diameter of the samples obtained at 200 min was 5.958 μm. The agglomeration phenomenon was very serious especially when the synthesis time is greater than 200 min. The sintered sample obtained at 150°C for 200 min had more dense structure and smaller average grain size. The average grain size of the sintered sample was 0.527 μm. The best reaction time for microwave isothermal synthesis and sintering of magnesium aluminium spinel is 200 min.

Synthesis of cobalt aluminate spinel by sol-gel process: Investigation of starting reagents and precursors obtained after pyrolysis

In this study, cobalt aluminate was prepared by a sol-gel method. XRD and FTIR analyses of the precursors obtained at 300 °C for 12 h revealed that the type of complexing agent used in the synthesis affects the pyrolysis step. An amorphous phase was obtained by using citric acid as the complexing agent. A beginning of crystallization was observed by using complexing agents such as polyacrylic acid and glycine. The structural investigation of the starting reagents revealed that the observed lines exhibit characteristics of Co3O4 derived from cobalt nitrate. The crystallinity of the samples increased with the increase in the annealing temperature, with no effect observed by the change in the complexing agent. The appearance of the (331) reflection for the heat-treated powders was characteristic of the blue spinel compound.

Influence of presintering temperature on magnesium aluminate spinel transparent ceramics fabricated by solid‐state reactive sintering

AbstractTransparent magnesium aluminate spinel (MgO·1.1Al2O3) ceramics were successfully prepared by the solid‐state reactive sintering method using commercial γ‐Al2O3 and MgO powders as raw material. The extra alumina compared with stoichiometric spinel was found efficient to improve the optical quality of transparent spinel ceramics. The influence of presintering temperature on the phase formation, densification process, microstructure evolution, and optical quality of MgO·1.1Al2O3 was systematically investigated. The α‐Al2O3 phase was found in the ceramics calcined at 800°C. With the calcination temperature exceeding 1000°C, the magnesium aluminate spinel ceramics were all pure spinel phase. The density increased from 3.05 to 3.53 g/cm3 and the average grain size increased from 0.5 to 1.9 μm with the increase of presintering temperature from 1450°C to 1600°C. The transparent MgO·1.1Al2O3 ceramic sample presintered at 1550°C for 3 h and hot isostatic pressing posttreated at 1600°C for 3 h showed the best in‐line transmittance of 86.5% at 1100 nm and 84.3% at 800 nm.

Study of the structure and properties of cermets based on the NiAl–Al<sub>2</sub>O<sub>3</sub> system

The powder metallurgy method including mechanical activation of powders in a planetary mill and spark plasma sintering at 1470 °C in an inert atmosphere was used to obtain NiAl–45vol.%Al2O3 cermet samples with the addition of nanoparticles of magnesium aluminum spinel in an amount of 0.05 vol.%. The features of their microstructure were investigated. Spinel nanoparticles are located at the boundaries between the grains of composite components. The results of X-ray phase analysis at t = 25 and 800 °C were obtained. The main components of the material at t = 20 °C are α-Al2O3 and NiAl. The dependence of internal friction on temperature in the range of 20–900 °C was studied, and the influence of magnesium aluminum spinel nanoparticles on the nature of its change was established. The internal friction curve shows that vibration damping occurs up to 600 °C. Dependences of the ultimate bending strength of cermets at t = 20÷750 °C were determined. The positive effect of introducing a small amount of magnesium aluminum spinel on the elastic properties of composites was established. The best mechanical properties were demonstrated for NiAl–42vol.%Al2O3–0.05vol.%MgAl2O4 samples. On average, the ultimate bending strength of this material was 8–15 % higher compared to samples without nanoparticles. The materials obtained in this research had an ultimate bending strength under normal conditions of 460–490 MPa. A summarizing analysis of NiAl–Al2O3 cermet researches was carried out to determine the nature of the ultimate bending strength dependence on the ratio of components. It was found that it has an extreme nature: the maximum is observed when using the ratio of aluminum oxide to aluminum nickel equal to 0.5.

Synthesis of 3-hydroxybutyraldehyde over highly stable solid base catalysts prepared from layered double hydroxides

3-Hydroxybutyraldehyde (3-HBA) is an important intermediate for the manufacture of biodegradable plastics (polyhydroxyalkanoate), and it is of great significance to find an efficient way for the catalytic synthesis of 3-HBA from surplus acetaldehyde. In this work, a series of MgxAlO(x+1.5)-t (t referred to the calcination temperature) solid bases were prepared via the controlled calcination of MgxAl layered double hydroxides with different ratio of Mg/Al and tested in the aldol condensation of acetaldehyde to 3-HBA. Characterization results revealed that the formed MgO particles were embedded in magnesium aluminate spinel (MgAl2O4) in Mg3AlO4.5–1100, which exhibited excellent activity and stability in the continuous liquid flow condensation of acetaldehyde. The detected selectivity of 3-HBA reached 89.0% with a 73.8% conversion of acetaldehyde at 25 °C and WHSV−1 = 2.8 h. Meantime, Mg3AlO4.5–1100 catalyst could retain its activity within 300 h on stream. What's more, the reaction mechanism over Mg3AlO4.5–1100 catalyst was proposed according to the results of several experiments.

Experimental study on the utilization of magnetic nanofluids in an air-to-air heat pipe heat exchanger

Nanofluids have often been utilized lately for enhancing the thermal performance of heat transfer devices operating with a working fluid. In this study, using iron aluminate spinel nanoparticles, deionized water-based nanofluids at various mass concentrations (0.5%, 1.0%, and 2.0%) of nanoparticles were prepared and tested as the test fluid inside the heat pipes of the air-to-air heat exchanger operating between hot and cold air passages. The efficiency, as well as thermal resistance of the heat pipes, were investigated under varying Reynolds number combinations of hot and cold air passages. An increase up to 87.2% in efficiency and a decrease up to 71.6% in thermal resistance of the heat pipes were obtained with the utilization of magnetic nanofluid. Furthermore, the effect of nanoparticle concentration on performance was discussed.

Immobilization of nickel ions by the confinement of surface aluminate spinel at low temperature

The accumulation of heavy metal on catalysts during petrochemical process produces deactivated catalysts that cannot be directly disposed in nature, due to the high toxicity. The elimination of the detrimental effect of heavy metal ions can be achieved by immobilizing these ions in the form of spinel structure, yet this requires high-temperature calcination. Herein, Ni2+ ions are stabilized and confined within the surface NiAl2O4 on Al2O3 through hydrothermal process and subsequent low-temperature thermal treatment. The formation of nickel aluminum layered double hydroxide (NiAl-LDH) as intermediate favors the generation of surface aluminate spinel at the temperature as low as 450 ​°C. The formation of surface spinel is beneficial to restricting the mobility of Ni2+ ions. The growth and immobilization mechanisms of the surface spinel structure were proposed, which contributes to alternative and promising route for the low-temperature approach of the retention of the heavy metal ions mobility.

Structural, photocatalytic and electrochemical studies on facile combustion synthesized low-cost nano chromium (III) doped polycrystalline magnesium aluminate spinels

An economic low-temperature self-ignition solution combustion method was used to synthesize 1–11 mol% chromium ion (Cr 3+ ) doped MgAl 2 O 4 nanoparticles (NPs). The structural, morphological and energy gap variations due to the influence of Cr 3+ on the host matrix were evidenced by powder X-ray diffraction (PXRD), Fourier transformed infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS), and transmission electron microscopy (TEM). Under visible light irradiation, the as-prepared NPs showed excellent photocatalytic activities (PCA) for the industrial dye Acid Red-88 (AR-88) and pyrocatechol (PY) - an emerging pollutant (EP). The experiments were carried out at room temperature in an aqueous solution at a concentration of 20 ppm. Among others, MgAl 2 O 4 :Cr 3+ (5 mol%) showed 91.38% degradation within 150 min exhibiting zero-order kinetics and 82% degradation in 6 h displaying the first order kinetics for AR-88 dye and PY EP, respectively. In this case, a greater number of hydroxyl (OH . ) radicles dominate over other reactive oxygen species (ROS), such as holes and singlet oxygens ( 1 O 2 ) under different pH conditions. Electrochemical studies have yielded the proton diffusion coefficient (D) value as large as 2.145 × 10 −4 cm 2 s −1 for the optimized NP electrode, which is substantially greater than 1.0935 × 10 −5 cm 2 s −1 for the pure MgAl 2 O 4 one owing to the intercalated anions and structurally disordered density of the electrode material. This superior electrochemical behaviour suggests the potential application of the optimized NP material as an anodic electrode for supercapacitors.

Femtosecond laser surface modification of magnesium aluminate spinel and its effect on polishing

Magnesium aluminate spinel is a promising material for optical windows due to its outstanding comprehensive properties. However, the highly efficient polishing of spinel by traditional processing is difficult due to its high hardness, high strength and chemical inertness. In this study, a femtosecond laser was used to induce micro-nanostructures on the spinel surface to improve its machinability. The effects of laser power, scanning speed and scanning interval on the surface morphology of the spinel were studied. The experimental results indicate that the femtosecond laser modification reduced the surface hardness of the spinel, and the critical cutting depth was increased. Moreover, the micro-nanostructures resulting from the laser modification improved the contact area between the slurry and the substrate, which improved the polishing performance. Experiments were carried out to prove that the chemical mechanical polishing (CMP) process of spinel can be promoted by femtosecond laser surface modification. This method provides a new perspective for high-performance spinel ultra-precision manufacturing. • A highly efficient femtosecond laser surface modification assisted polishing for spinel is proposed, the improved mechanisms were analyzed. • The effect of laser modification parameters on the evolution of micro-nanostructures on the spinel was investigated. • This method has obvious advantages such as simplicity, flexibility, thus making it a very promising machining method.

Slag erosion-resistant coating for periclase-magnesia-aluminum spinel brick

The magnesia-carbon bricks are used at the ladle slag line adds carbon to molten steel and thus has an adverse effect on the smelting process. Steel production enterprises hope to remove carbon from the ladle slag line. This study presents a Silica sol (SS)–magnesium aluminate spinel(MA) coating that can be applied to the surfaces of periclase–MA spinel brick (spinel brick) to improve their slag erosion resistance, facilitating the elimination of carbon from the ladle slag line. The slag corrosion resistance mechanism of a spinel brick covered with the SS–MA coating was analyzed through scanning electron microscopy and simulation using FactSage software. The results show that increasing the SS content (CSS) led to the formation of fine cracks during the coating-drying process and was detrimental to the slag erosion resistance of the coating. The optimum CSS was 20%. In the high-temperature slag erosion experiment, the MA in the coating was capable of forming solid solutions with Fe and Mn in the slag, and the nano-SiO2 in the coating interacted with the slag, increasing the viscosity of the slag and reducing its penetration. In addition, the presence of the coating extended the penetration path of the slag and reduced the contact area between the slag and the spinel brick and their chances to undergo chemical reactions, thereby protecting the spinel brick from slag erosion. The coated spinel bricks outperformed the magnesia-carbon brick in terms of slag erosion resistance, providing a basis for selecting carbon-free refractory materials for the ladle slag line.

Sulfuric Acid Etching for Surface Modification and Improved Polishing of Magnesium Aluminate Spinel

Magnesium aluminate spinel is a material with high optical transparency and great potential for optical applications. However, its high hardness, high strength and chemical inertness make obtaining the required surface quality quickly through traditional processing a challenge. Existing assisted machining methods are complicated and costly, which limits their application to high-efficiency and ultra-precision manufacturing. In this study, etching was employed as a polishing preprocess owing to its obvious advantages of simplicity, practicality and low cost. Sulfuric acid was applied to etching spinel, and the influences of the etching temperature and time on the surface morphology and mechanical properties of the spinel were evaluated. Surface analytical techniques were performed to clarify the mechanism for improved polishing. Experimental results indicated that sulfuric acid etching significantly reduced the surface hardness. The surface quality and efficiency were significantly improved with the etching pretreatment compared with traditional chemical mechanical polishing. Thus, the proposed method of sulfuric acid etching-assisted polishing can reduce the difficulty of spinel machining to realize the high-efficiency and precision manufacturing for a wider range of applications.

Morphological Characterzations of Spinel Nanoparticles Synthesized by Sol- Gel Method

In this paper, pure magnesium alumina (MgAl 2 O 4) and magnesium alumina spinel nanoparticles doped iron (MgAl 2-X Fe X O 4) where (X= 0.01, 0.02, 0.03, 0.04, 0.05) were by sol-gel method. The calcination operation was performed at a temperature of 800 ° C at a rate of 5 ° C/min. The resulting materials of MgAl2-XFeXO4 nanoparticles were determined using,, XRD, FT-IR, EDX, SEM methods. XRD results show that MgAl 2 O 4 and Fe +3:MgAl 2 O 4 nanoparticles have a single-phase at 800 ° C. FTIR analyze confirmed the presence of MgAl 2 O 4 and Fe +3:MgAl 2 O 4 as well as FTIR analysis shows its composition and structure. Surface morphology examinations by scanning electron microscopy (SEM) show that the nanoparticles synthesized in this way are estimated to be angular, uniform and about 50-70 nm in size.

Effect of Calcium Treatment on Inclusions in H08A Welding Rod Steel

The effect of calcium treatment on inclusions in H08A welding rod steel was studied by industrial experiment and using thermodynamics theory. The effects of inclusion composition, morphology, quantity, and size in H08A welding rod steel before and after calcium treatment were studied by metallographic microscope, scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). Thermodynamic studies show that the addition of calcium can form various forms of xCaO·yAl2O3, under the condition that the composition of molten steel remains unchanged, the control of calcium content is the key to generate low melting point calcium-aluminate complex non-metallic inclusions and improve the quality of molten steel. The production practice in steel plant shows that for welding rod steels, the calcium content in a suitable range can meet the requirements of calcium treatment. Effective calcium treatment can not only transform the high melting point Al2O3 inclusions into the low melting point complex non-metallic inclusions between 3CaO·Al2O3 and 12CaO·7Al2O3, but also make the original shape-diversified inclusions into the spherical calcium-aluminate complex non-metallic inclusions. Meanwhile, the total number of inclusions and large-scale inclusions in welding rod steel are reduced, and the inclusions tend to disperse in the steel, which is very conducive to the improvement of steel quality. The results show that the modification path of magnesium aluminate spinel in steel is as follows: Al2O3 → MgO-Al2O3 → MgO-CaO-Al2O3. In addition, calcium treatment can modify MgO-Al2O3 spinel in steel into liquid MgO-CaO-Al2O3 complex non-metallic inclusions with low melting point.

Evolution of Inclusions in Magnesium–Calcium-Treated Liquid Iron

With the development of clean steel technology, the control of non-metallic inclusions in steel is of increasing importance. Magnesium–calcium treatment can effectively balance the castability of molten steel and the control on inclusion size, which is an inclusion modification approach with application prospect. In view of this, how three addition methods (i.e., adding Mg before Ca, adding Mg after Ca, and adding Mg together with Ca) influenced the modification effect of inclusions in liquid iron was experimentally studied, and how these inclusions evolved with time was discussed in this paper. The results demonstrated that despite the sharp difference in their inclusion evolution, composite inclusions with a magnesium aluminate spinel (MAS) core and an outer CaO–Al2O3–MgO layer were formed by all the three addition methods, with the average inclusion size of 1–2 μm. Furthermore, thermodynamic calculation was adopted to reveal the transformation relationship between MAS and calcium aluminate in each of the three addition methods, and clarify the formation and disappearance mechanisms of the intermediate product CaS in the process of Mg–Ca treatment. The thermodynamic calculation results agreed well with the experiment data.

Mechanical behavior in the interior and boundary of magnesium aluminate spinel (MgAl2O4) grain under nanoindentation.

The understanding of mechanical behavior in magnesium aluminate spinel (MgAl2O4) at the nanoscale lays a foundation for its material removal mechanism in ultraprecision machining. Nanoindentation tests are carried out in the interior and boundary of spinel grain with different loads. An obvious indentation size effect exhibits in both of these areas. First, the nano-hardness and elastic modulus decrease, followed by stabilization due to an increase of pressure. The measured elastic modulus, hardness, and fracture toughness of the grain interior are 277.7±8.4GPa, 19.79±0.83GPa, and 1.12±0.02MPa⋅m1/2, respectively. Deformation of spinel transits from elastic to plastic at approximately 0.8 mN load, which corresponds to the discontinuous steps of load-displacement curves. By comparing the fracture toughness and the residual indent morphology, the grain boundary exhibits lower brittleness than the grain interior. Radial cracks form on the grain surface as indentation load exceeds 29 mN, whose propagation is influenced by the loading conditions and the grain boundary effect.

Enhanced dispersion of nickel nanoparticles on SAPO-5 for boosting hydroisomerization of n-hexane

The nickel based bifunctional catalyst with enhanced hydroisomerization performance was developed using an in-situ solid synthesis method. It was achieved to stabilize smaller Ni active sites on SAPO-5 using ethylenediaminetetraacetic acid (EDTA) ligands. The role of EDTA ligands was clarified by controlling the molar ratio of EDTA to Ni2+ (EDTA/Ni2+) over Ni/SAPO-5 catalysts. EDTA ligands inhibited the formation of nickel aluminate spinel and aggregation of NiO species during calcination, which dispersed Ni nanoparticles in a mean size of 4.7 nm on SAPO-5. The size of Ni nanoparticles could be controlled by regulating EDTA/Ni2+ ratio in [Ni-EDTA]2- complex. The prepared catalyst exhibited high yield of isomers (54.0%) and di-branched isomers selectivity (18.0%) in the n-hexane hydroisomerization, which was approximately 2 times higher than that of the Ni/SAPO-5 catalyst without EDTA ligands at similar conversion. These results are important to propose a facile approach for the preparation of highly dispersed non-noble metal based bifunctional catalysts at a high loading.

Influence of Additive’s Concentration on the Temperature Conditions for the Formation of the Solid Solutions of the System MgAl<sub>2</sub>O<sub>4</sub>-Ga<sub>2</sub>O<sub>3</sub>

The production of a precursor of magnesium aluminate spinel (AMS) by thermic synthesis to obtain a material with high-performance properties is reviewed in the work. The temperature synthesis of AMS is determined, the introduction of the additive is described in the range of concentrations of 3, 5, and 7 mol.%. The current processes during the firing of products have been investigated with a view to the further selection of the firing mode of the ceramic samples, which can be obtained.

Effect of Nanocrystalline Magnesium Aluminate Precursor and Powder Granulometryon Spinellization and Densification to Develop Magnesium Aluminate Spinel Aggregates

Nanocrystalline spinel forming precursor spinel hydrate (PSH), prepared by co-precipitation technique with controlled concentration of mixed salt, pH, temperature and time, was used as additive in calcined alumina-dead burnt magnesia (DBM) admixture to develop spinel (MgAl2O4) aggregates. Powder granulometry, mixing and dispersion of additive PSH were controlled during milling. Three batches of varying particle size distribution with same proportion of additive were used for body making. Firing was conducted in 1500°-1650°C temperature range. True specific gravity (TSG) of alumina, DBM, spinel and individual fired pellets were used to estimate wt% spinel formation based on the derived mass balance equation and subsequently verified by X-ray diffraction (XRD) analysis. Fired properties, i.e. bulk density, apparent porosity, closed porosity, %linear shrinkage and densification rate were measured. XRD was conducted for phase analysis. SEM and HR-TEM were performed to study the grain/ pore morphology and elemental analysis was done to calculate the spinel stoichiometry. PSH was found to have positive effect both in spinellization and densification; the effect gradually decreased with finer particle size distribution and increasing firing temperatures. The maximum fired density achieved for three batches were 89.21, 92.84 and 96.2% ρth, respectively. Based on the requirements of specific powder granulometry, addition of PSH and firing schedule can be used for bulk production of spinel aggregates.

Synthesis of magnesium aluminate spinel doped TiO2 from magnesite waste

Synthesis of magnesium aluminate spinel doped TiO2 from magnesite waste