Magnesium Aluminate Spinel Research Articles

Fabrication of periclase and magnesium aluminate spinel refractory from washed residue of secondary aluminum dross

A novel method for fabricating the periclase and magnesium aluminate spinel refractory from the secondary aluminum dross was proposed in the present work by adding magnesium oxide. The fabrication mechanism of the refractory was analyzed by thermogravimetric and differential thermal analysis, scanning electron microscopy, and X-ray Diffraction. The effects of MgO addition and sintering temperature on the mechanical properties and density of refractories were studied. The results showed that with the increase of sintering temperature, the purity, crystallinity, and densification of the refractory were significantly improved, and the porosity of the refractory was decreased. As an obvious second phase in the refractory, periclase can strengthen the grain–grain bonding and inhibit the grain boundary movement. With the increase of MgO addition, due to the significant reduction of porosity, the improvement of grain size uniformity and the absence of microcracks, the flexural strength and the impact toughness were significantly improved. When the MgO addition was 50 wt% at the sintering temperature of 1600 °C, the density and porosity of the refractory were 2.92 g/cm3 and 18.2%, while the flexural strength and impact toughness can reach 270 MPa and 3.7 MPa m1/2, respectively.

Effect of Eu2O3 on sintering densification and corrosion resistance of magnesium aluminate spinel

The effect of the doping amount of Eu2O3 on the densification behaviour of magnesium aluminate spinel (MAS) and its corrosion resistance to aluminium electrolyte were studied. The relative density, phase composition, micro morphology and hardness of the sintered samples were characterised by Archimedes’ drainage method, X-ray diffractometer, scanning electron microscope and automatic micro Vickers hardness tester. Results showed that the doping of Eu2O3 was conducive to the densification of MgAl2O4. When the content of Eu2O3 was 3 wt.%, the relative density of MAS was the largest (99.32%), the microstructure was more compact and the hardness was the largest (2293.4 kgf/mm2). The MAS sample with 3 wt.% Eu2O3 had the best corrosion resistance to aluminium electrolyte, and the corrosion depth was 80.99 μm. It was speculated that the electrolyte may penetrate into the sample through the micropores, and the fluoride salt chemically reacted with MgAl2O4 to form Al2O3, NaF and MgF2.

Effect of Y2O3 on the corrosion resistance of two-step sintered Al5Y3O12-MgAl2O4 sidewalls in the aluminum electrolyte

Magnesium–aluminum spinel (MAS) precursor powder was synthesized through a microwave hydrothermal method. The synergistic effects of sintering process and sintering aids on the densification, hardness and corrosion resistance of MAS were revealed. X-ray diffraction analysis (XRD), Archimedes’ drainage method, fully automatic micro-Vickers hardness test and scanning electron microscopy (SEM) were performed to analyze the phase composition, bulk density, hardness microstructure and corrosion depth of the samples, respectively. Results revealed that the best two-step sintering condition is 1650 °C/3 min/1550 °C/20 h. The MAS products obtained under the best condition have clear grain boundaries, uniform particle size distribution, and few pores. When the amount of Y2O3 added is 4 wt.%, Y2O3 and Al2O3 form the second-phase solid solution Al5Y3O12, which activates the crystal lattice and benefits the sintering densification of MAS. Under these conditions, the relative density of the MAS composite ceramics prepared is relatively large (95.94 %), the grain size is relatively uniform, the hardness is relatively large (1264 HV), and the corrosion depth is relatively small (94.58 μm).

Low-cost preparation and characterization of MgAl2O4 ceramics

Low-cost magnesium aluminate spinel ceramics were successfully synthesized from the minerals of magnesite(M) and bauxite(B) via the one-step sintering method. The effect of the ratio of bauxite to magnesite on the phase evolution, microstructure, as well as the mechanical properties was thoroughly investigated. The phase structural analysis indicated that Mg2+ or Al3+ ions were dissolved in the spinel lattice and formed Mg-rich/Al-rich spinel MgAl2O4 (MA). The microstructure analysis revealed that the liquid phase gradually appeared with the increment of bauxite content. Appropriate but not saturated cation solution had the effects of structural densification, homogenization of crystal grains, and the enhancement of strength. Ceramics with 1.31 bauxite to magnesite stoichimetric ratio exhibited the highest density of 3.43 g/cm3 and flexural strength of 93.5 MPa.

Doped MgAl2O4 semiconductor host nanopowders for tunable primary color emissions for application as white LEDs

The emission of white light from hosts having a single phase and the compromised applicability of a very inert, chemically stable, and structurally diverse MgAl2O4 led to a quest for improving the structural, optical, thermal and electrical properties for more innovative applications. A slight modification in lattice site occupation can lead to antisite defects which alter the physical properties of the materials. The presence of larger alkaline earth Sr2+ cations at the T-sites have been found to improve the behavior of the activator Mn2+ cations which are substituted for Mg2+ at T-sites. In the current study, a series of Mn2+ doped derivatives having a general formula of Mg1−x−0.3Mn x Sr0.3Al2O4 (x = 0.1–0.5) were synthesized using the chemical co-precipitation method. Due to the larger radius of Sr2+ cations, the lattice strain was observed in the cubic crystal structure of magnesium aluminate spinel (MAS). A decrease in the bandgaps of the doped samples indicates the formation of defect states within the bandgap of MAS. Apart from the bandgap transitions, the capture of electrons at oxygen vacancies is also observed in the UV/Vis spectra. The strong tetrahedral site preference of Mn2+ is altered by the presence of Sr2+, hence, some octahedral site occupation of Mn2+ in the lattice is achieved. The antisite defects along with the occupation of both T- and O-sites by Mn2+ resulted in novel emission bands centered at 464 nm (blue), 515 nm (green), and 621 nm (red) at λ ex of 380 nm. The emission of the primary colors in a simple lattice with cost-effective and in-toxic constituents can be a possible alternative to the costly rare-earth ions doped complex materials in use to-date.

Comparative study of filament-fed and blown powder-based laser additive manufacturing for transparent magnesium aluminate spinel ceramics

Magnesium aluminate spinel is of great interest as a transparent ceramic for its excellent mechanical properties and excellent optical transmittance. Additive manufacturing of this desirable material presents several benefits over traditional manufacturing methods, including reduced fabrication time and cost and the potential to fabricate structures with complex geometries and internal cooling networks. Despite the many benefits, the challenges hindering this technology must be overcome. A primary challenge with powder-based laser additive manufacturing of transparent ceramics is a trade-off between densification and cracking. The fabrication of transparent ceramics requires nearly full densification since pores act as light scattering centers. Even relatively small percentages of porosity render ceramics translucent or opaque. Previous studies on powder-based laser direct deposition of spinel ceramics have shown that densification to transparency is possible with high-laser power deposition. While high-laser powers are beneficial for densification, it also produces high thermal gradients that result in significant crack formation. Cracks hinder mechanical properties and transparency, limiting possible applications. Thus, we propose a filament-based deposition strategy to reduce laser power requirements. Filament-fed laser direct deposition, instead of blown powder, dramatically reduced the amount of gas porosity within the melt. Hence, highly densified, transparent, spinel ceramics were fabricated. Through decreased laser power requirements for high densification, cracking was largely reduced. This paper provides a comprehensive comparison between filament- and powder-based laser direct deposition by analyzing important sample characteristics, including porosity, cracking, grain size, and their controlling mechanisms. This paper also presents a laser direct deposition and postprocessing method to manufacture predensified spinel filaments.

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.

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.

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.

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.

Liquid Inclusion Collision and Agglomeration in Calcium-Treated Aluminum-Killed Steel

This work addresses conflicting results in the literature regarding liquid inclusion agglomeration. To assess whether liquid calcium aluminates do agglomerate in liquid steel, laboratory experiments were performed: melting electrolytic iron, deoxidizing the melt with aluminum and subsequently calcium treating the deoxidation products (alumina and magnesia-alumina spinel inclusions). Under laboratory conditions, solid spinels and alumina inclusions were successfully modified, producing a new population of much smaller calcium aluminate inclusions. The new population of inclusions forms because the presence of calcium in the liquid steel destabilizes alumina and MgO-alumina inclusions, which then dissolve into the melt. The liquid inclusions exhibited a weak but statistically significant tendency to agglomerate. Laboratory results were assessed in the light of different collision mechanisms. Agglomeration mainly occurs by Stokes and laminar fluid flow collision when no external stirring is imposed. Monte Carlo simulations of collisions agree reasonably well with experimental results. For industrial conditions, where the liquid steel is agitated by argon bubbling and/or electromagnetic stirring, turbulent collisions dominate.

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.

Preparation of Dense Ceramics Based on Aluminum–Magnesium Spinel by Forming Solid Solutions in the MgAl2O4–Ga2O3 System

Preparation of Dense Ceramics Based on Aluminum–Magnesium Spinel by Forming Solid Solutions in the MgAl2O4–Ga2O3 System

Self-Propagating High-Temperature Synthesis of Ceramic Material Based on Aluminum-Magnesium Spinel and Titanium Diboride

Self-Propagating High-Temperature Synthesis of Ceramic Material Based on Aluminum-Magnesium Spinel and Titanium Diboride

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.