Alumina-rich Spinel Research Articles

The effect of the alumina-rich spinel exsolution on the microstructure of CAC-bonded castables

According to the magnesia-alumina phase diagram, the alumina-rich spinel may undergo exsolution of corundum during firing when incorporated into the castable. This research aimed to explore the phenomenon of alumina-rich spinel exsolution and its subsequent impact on the distribution of CaO·6Al2O3 inside the spinel and the strength of the castables when subjected to a firing temperature of 1600 °C. The results indicated that the amount of exsolved corundum escalated as the alumina content within the spinel rose after heat treatment, and this exsolved corundum was predominantly found within the original spinel particle. The CaO·2Al2O3, which formed when calcium aluminate cement was heated above 1200 °C, reacted preferentially with the exsolved corundum rather than with pre-added α-Al2O3 leading to the formation of CaO·6Al2O3 inside the spinel after firing at temperatures exceeding 1400 °C. This reaction formed an interpenetrated "spinel-CaO·6Al2O3" structure during firing, which enhanced the strength of the castables.

A novel low thermal conductivity refractory aggregate for high-temperature applications: Lightweight microporous alumina-rich spinel (Mg0.4Al2.4O4)

This study aimed to develop lightweight refractory aggregates with high closed porosity and low thermal conductivity. We report the preparation of a series of lightweight microporous alumina-rich spinel (Mg0.4Al2.4O4) aggregates with low open porosity, high closed porosity, small intragranular pore size, and low thermal conductivity. The sintering activity of the nano-zirconia sol increased the grain boundary migration rate under the action of surface tension during sintering; therefore, the pores were closed before they were eliminated. Simultaneously, Zr4+ replaced Mg2+ and Al3+, which increased the number of cation vacancies in the crystal and accelerated the ion migration, resulting in a faster formation of Mg0.4Al2.4O4. When 0.75 wt% nano-zirconia sol was added, it increased the diffusion rate of ions and the migration rate of grain boundary, which improved the sintering reactivity. The pores were mainly small-sized intragranular pores (median size: 1.89 μm), which reduced the thermal conductivity (2.5 W•m−1K−1 at 1000 °C).

Enhanced corrosion resistance by simultaneous refinement of pore structures and phase composition: A novel lightweight microporous alumina-rich spinel (Mg0.4Al2.4O4)

A lightweight microporous alumina-rich spinel aggregate with a single phase (Mg0.4Al2.4O4) was prepared to fabricate lightweight refractories with guaranteed corrosion resistance. To explore the corrosion behaviour of slags with different compositions, four types of slags were tested and compared with commercial sintered alumina and lightweight corundum-spinel aggregates. The slag corrosion depth of the lightweight microporous alumina-rich spinel aggregates was significantly lower than that of the other two aggregates. The alumina aggregate formed a calcium aluminate isolation layer with a thickness of 100–150 µm at the interface. The lightweight corundum-spinel aggregate did not form any isolation layer; however, a 50 µm reaction layer, composing of spinel and calcium aluminate, exists between aggregate and slag. Nevertheless, at the contact interface between the lightweight microporous alumina-rich spinel aggregate and slag, a uniform isolation layer (approximately 50 µm thick) forms, preventing direct contact and ensuring excellent corrosion resistance.

The microstructure evolution and performance enhancement of MgO-C refractories by the addition of MA90 spinel micro-powder

Spinel has been widely used in refractories because of its excellent properties, but conventional spinel has the problem of poor activity and high cost. In this study, a highly reactive alumina-rich spinel micro-powder was synthesized at a low temperature by solid-phase reaction using commercial alumina and commercial light-burned magnesia. The MA90 spinel micro-powder was added to MgO-C refractories for the slag line of steelmaking ladles to investigate its effect on the microstructure, physical properties, and corrosion resistance of MgO-C refractories. The results showed that the addition of MA90 spinel micro-powder improved the performance of MgO-C refractories significantly, and its optimal addition was 3 wt%. The addition of MA90 spinel micro-powder optimized the microstructure of the matrix and enhance the mechanical strength and corrosion resistance of the MgO-C refractories. The formation and growth of SiC whiskers in MgO-C refractories facilitated by the addition of MA90 spinel micro-powder.

Effect of alumina content on the crystal structure, lattice thermal expansion and thermal conductivity of aluminium-rich spinel solid solutions

Magnesium alumina spinel solid solutions with different chemical compositions were synthesized and their crystal structures were refined using the Rietveld method based on synchrotron radiation X-ray diffraction (XRD). A linear relationship between the lattice parameters a0 and x in Mg1-xAl2+2/3x□1/3xO4 was established, the thermal stability and lattice thermal expansion of the solid solutions by in-situ high temperature XRD were investigated. Alumina was eliminated from the high-solubility alumina-rich spinel solid solution to form corundum and a lower-solubility alumina-rich spinel solid solution. The relationship between a0 and temperature is expressed by a quadratic polynomial. The thermal conductivity of Mg1-xAl2+2/3x□1/3xO4 decreased almost linearly with an increase in x, which is attributed to the stronger point-defect scattering effect induced by the difference in atomic mass and cation radii and the increase in cation vacancies as alumina dissolves in spinel.

Precipitation and sintering behaviour of non-stoichiometric MgAl2O4 solid solution powder prepared by combustion synthesis

In this work, non-stoichiometric MgAl2O4 spinel solid solution powders with excess magnesia or alumina were synthesized by combustion synthesis, and their precipitation and sintering behaviour were investigated. The dissolution of MgO or Al2O3 in the lattice of spinel formed interstitial or vacancy solid solutions, respectively. During annealing, the powders with different solid solubility exhibited various precipitation and phase transition behaviour. After hot-press sintering with the as-prepared powders, there were many spheres with 100 nm evenly precipitated in the MgAl2O4 ceramic matrix, which improved mechanical performance. This work can not only provide comprehensive information about the precipitation of magnesia- and alumina-rich MgAl2O4 spinel solid solutions during annealing or sintering, but also offer an easy and convenient way for the industrial preparation of nanocomposite MgAl2O4 ceramics.

Elasticity of Nonstoichiometric Alumina-Rich Spinel Determined by Bond Valence Theory and Brillouin Scattering.

An accurate knowledge of the elastic properties of materials is essential for material science and engineering applications. Four single crystals of nonstoichiometric alumina-rich spinel [Mg1-xAl2(1+x/3)O4] were obtained from sintered transparent ceramics for the investigation of its elastic properties. The disordered crystal structures were fully resolved by combining single-crystal structure refinement and a quadratic programming approach for the first time. The bond valence model and Brillouin scattering experiments were used to evaluate the bulk modulus (K), shear modulus (G), Young's modulus (E), and Poisson's ratio. The discrepancy between the theoretical and experimental results is <2.6%. The independent elastic constants (C11, C12, and C44) were determined from Brillouin scattering experiments. A negative Poisson's ratio, υ(110, 11̅0), was found to exist in all alumina-rich spinels, which means it is a partially auxetic material. Blackman diagram analysis was introduced to identify the interrelationships and trends in mechanical and bonding properties in alumina-rich spinels. The bond valence model was suggested to be an effective and accurate approach for predicting the elastic modulus of spinels, which provides a useful tool for the study of the composition-structure-property relationship of materials.

A case study on mineralogy and physico-mechanical properties of commercial bricks produced in Nepal

Uses of clay brick have practiced since ancient times in the Kathmandu valley of Nepal as structural materials for the construction of historical monuments, temples, and even modern buildings so on. However, their durability and sustainability properties are less studied scientifically in Nepal. Taking into consideration, analysis of mineralogical phases and physico-mechanical properties of modern clay bricks of Nepal was carried out using X-ray powder diffraction as well as Fourier transform infra-red techniques, and American Standards for Testing Materials standards, respectively. Quartz, feldspars, alumina-rich spinel, muscovite, primary mullite, and hematite are constituent in the bricks. Appearance of the spinel and primary mullite phases in burnt brick samples with diminishing feldspar peaks were to be different, indicating that the temperature applied for the making of these clay bricks presumed to be between 900 and 1050 °C. All the commercially used contemporary brick samples analyzed herein could be of II and III classes based on the estimated physical and mechanical properties. They showed the water absorption capacity (WAC), apparent porosity (PA), bulk density (DB), and compressive strength (SC) in the range of 5–30%, 10–40%, 1.1–2.15 g/cm3, and 3.35–10.53 MPa, respectively, and these properties have a reasonable correlation. The SC is to be decreased exponentially with increasing both the WAC and PA and increased exponentially with DB.

Effect of firing temperature on physico-mechanical properties of contemporary clay brick productions in Lalitpur, Nepal

Effect of firing temperature on some physico-mechanical properties of ten brick samples, those were composed by feldspars, quartz, alumina-rich spinel, primary mullite and hematite phases, was investigated in accordance with ASTM standards. The brick samples fired between 700° to 1100° C showed 11-23 % water adsorptivity (WA), 19-37 % apparent porosity (AP) and 1.50-1.65 g/cm3 bulk density (BD) indicate good physical properties. The maximum compressive strength (CS) of the fired-bricks at 950° to 1000° C was found to be between 15.6 and 17.1 MPa. At 700°-1000° C firing temperatures, the CS of these bricks is found to be increased exponentially with decreasing of both WA and AP, however it is found to be increased with increasing the BD. Consequently, it can be said that there is good correlation between mechanical and physical properties of the fired-brick samples up to the firing temperature of 1000° C.&#x0D; Bangladesh J. Sci. Ind. Res.55(1), 43-52, 2020

Corrosion of MgO-C with Magnesium Aluminate Spinel Addition in A Steel Casting Simulator

For more than 20 years, the sidewalls and bottom of steel ladles have been lined with carbon-bonded magnesia (MgO-C) and magnesia-alumina bricks (MAC). The alumina raw materials react with magnesia forming a spinel, which decreases open porosity and slag infiltration. The amount, grain size, and chemistry of the added spinel impact the properties of spinel-containing MgO-C. Corrosion tests have been performed in a steel casting simulator at 1580 °C using 18CrNiMo7-6 steel and Fe-rich slag as corrosion medium. Digital light microscopy and SEM/ EDS (scanning electron microscope with energy dispersive spectroscopy) were used to evaluate the corrosion mechanisms. The metal casting simulator test showed that the addition of CaO-MgO-Al2O3 aggregates results in the highest corrosion resistance against molten steel and synthetic basic slag compared to alumina-rich spinel aggregates.

Effect of Seawater, Aluminate Cement, and Alumina-Rich Spinel on Pelletized CaO-Based Sorbents for Calcium Looping

Calcium looping (CaL) is considered as an emerging technology to reduce CO2 emissions in power generation systems and carbon-intensive industries. The main disadvantage of this technology is reactivity decay over carbonation/calcination cycles due to sintering. The main objective of this study was to evaluate the performance of novel sorbents for CaL. Three types of pelletized CaO-based sorbents for CO2 capture were developed by adding aluminate cement, aluminate cement with seawater, or alumina-rich spinel to calcined limestone. Different concentrations of seawater in deionized water solutions were tested: 1, 10, 25, and 50 vol %. All samples were tested in a thermogravimetric analyzer (TGA) under two different calcination conditions: mild (N2 atmosphere and 850 °C during calcination) and realistic (CO2 atmosphere and 950 °C during calcination). The samples were characterized using SEM and EDX. Aluminate cement CaO-based sorbents exhibited better performance in the TGA tests (25% conversion after 20 cycl...

Investigation of the firing temperature effects on clay brick sample; Part-I: Mineralogical phase characterization

Present research work was focused to investigate the firing temperature effects on mineralogical phase composition of nine clay bricks collected from the brick factory of Kathmandu valley using X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra analyses. Main mineralogical phases of quartz, feldspars, spinel, mullite and hematite in the brick specimens fired at different firing temperatures including muscovite type of mica mineral in the sun-dried brick specimen are identified from XRD and FTIR analyses. Disappearance of the muscovite type of mica clay mineral with feldspars enhanced to form alumina rich spinel phase at firing 900° to 1000° C, and finally the primary mullite phase in the fired clay brick samples is clearly observed at 1100° C firing temperature.BIBECHANA 16 (2019) 122-130

Effect of CeO2 addition on the sintering behavior of pre-synthesized magnesium aluminate spinel ceramic powders

In this work, the effects of 1 wt%, 2 wt%, and 3 wt% CeO2 as an additive on the sintering behavior of alumina-rich spinel and magnesia-rich spinel powders subjected to sintering at temperatures of 1600 °C, 1650 °C, 1700 °C, and 1750 °C were investigated. The sintering behavior of the ceramics was investigated according to dilatometry measurements, linear shrinkage, bulk density, phase analysis, and microstructure. It was demonstrated that CeO2 hindered the sintering process in alumina-rich spinel by reacting with Al2O3 exsolved from the spinel to form platelet-shaped particles of CeAl11O18 interspersed between the spinel grains. Meanwhile, the presence of CeO2 promotes the sintering process in magnesia-rich spinel by being distributed in an isolated form among the spinel grains.

Magic Angle Spinning NMR Study on Inversion Behavior and Vacancy Disorder in Alumina-Rich Spinel.

Solid state magic angle spinning nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) were used to investigate the inverse behavior and vacancy disorder in alumina-rich spinel, Mg1- xAl2(1+ x/3)O4 (0 ≤ x ≤ 0.86). Simulation and integration of NMR spectra have been developed for probing the population of Al located in different coordinated environments. Rietveld profile refinements were performed for powder XRD spectra by combining with NMR analysis. With changes in the composition, inversion disorder and cationic vacancies coexisting in tetrahedral and octahedral coordinations fluctuate in amount in the crystal lattice. The coordination polyhedra in the crystal structure can adjust the volume to variations of composition, anion parameter, and inverse parameter. This opens a window to the design and functionalization of spinel materials.

Theoretical predictions of composition-dependent structure and properties of alumina-rich spinel

Alumina-rich spinel (MgO·nAl2O3, n≥1) was described as a solid solution of MgAl2O4 and Al8/3O4 (γ-Al2O3) with formula of Mg1–xAl2(1+x/3)O4 (0≤x≤1). Five redefined supercell models of MgO·nAl2O3 were constructed to investigate the composition-dependence of structural, electronic, optical and mechanical properties. The first-principles calculations were developed with high accuracy to evaluate the lattice constant, electronic structure, dielectric function and elastic constants. The detailed structural information was presented by analyzing the variations in lattice constant, interstitial volume and anion parameter with the content of γ-Al2O3. The composition-dependent bulk modulus, shear modulus and electronic structure of alumina-rich spinel were well discussed. As increased the content of Al, the mechanical properties showed an increasing trend, while the optical absorption in the UV region presented a blue-shift of ∼1.2eV. These theoretical results provided a basis for understanding the crystal structure and intrinsic properties of alumina-rich spinel.

The Influences of Zirconia on the Properties of Magnesia Alumina Spinel Bricks

Using alumina-rich MgAl2O4 spinel AR78 and 97 fused magnesite as raw materials, brine as the binder, magnesia alumina spinel bricks close to theoretical composition and spinel bricks with zirconia have been prepared. Then the influences of zirconia on the properties of MgAl2O4 spinel bricks are researched. The results show that: (1) After the addition of zirconia, the direct bonding structure formed between spinel and zirconia can strengthen matrix structure and promote sintering, so the structural properties, the alkali resistance performance and the creep property of MgAl2O4 spinel bricks are all improved. (2) The optimal addition of zirconia is 6%, with which the spinel bricks have well alkali resistance performance and creep property of which the final creep rate is -1.46% after creep experiment for 50h.

Investigation on the Creep property of Theoretical Composition Magnesia Alumina Spinel Bricks

Using alumina-rich MgAl2O4 spinel AR78 and 97 fused magnesite as raw materials, brine as the binder, magnesia alumina spinel bricks close to theoretical composition have been prepared. Then the high temperature creep experiments are carried on. The creep property is analyzed by microstructure and creep curve. The results show that: (1) The magnesia alumina spinel bricks close to theoretical composition prepared have well sintering performance and compact structure.(2) The specimen 1# has the best creep property, of which the final creep rate was -1.45% after creep experiment for 50h. (3) Under high temperature and pressure for a long time in creep experiment, the internal of spinel is further sintered, the pores of the specimens are rearranged.

Sintering Performance of MgAl&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; Spinel with Various Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Content

Magnesium aluminate spinel specimens with various Al2O3 content were prepared. Sintering performance and microstructure of different kinds of MgAl2O4 spinel were studied. It is found that the sintering of materials becomes more difficult with the increase in their Al2O3 contents. Corundum begins to precipitate from the magnesium aluminate spinel with 85wt% Al2O3 at 1400. However, with further increase in the sintering temperature, the corundum dissolves into magnesium aluminate spinel (above 1600). The precipitation and solid solution of corundum hinder the sintering of materials. In alumina-rich spinel, if the content of Al2O3 in alumina-rich spinel materials is more and the content of CA6 is also more. At the same time CA6 crystal is bigger in the alumina-rich spinel.

Sintering Mechanism of MgAl&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;-SiC Composites in Reduction Atmosphere

MgAl2O4-SiC Composites were prepared by using magnesium aluminate spinel and silicon carbide powder as starting materials and the sintering mechanism was investigated at 1600°C in reduction atmosphere. Sintered samples were analyzed using XRD, SEM and EDS. The results show that Alumina-rich spinel and SiCN are formed due to the reactions between spinel and SiC. Weight loss of samples was detected due to SiO formation and volatilization. The bulk density of samples decreases and apparent porosity increases as increasing the content of SiC.

The Effect of Varied Amounts of LiF Sintering Aid on the Transparency of Alumina Rich Spinel Ceramic with the Composition MgO · 1.5 Al2O3

Magnesium aluminate spinel with the alumina rich composition MgO · 1.5 Al2O3 has been prepared as a transparent polycrystalline ceramic with average in‐line transmission at 550 nm of 83.3 ± 0.9% and &gt;80% throughout the visible spectrum. This finding significantly increases the compositional range over which polycrystalline magnesium aluminates can be prepared as fully dense ceramics with high transparency to visible light. Starting powders are prepared from combinations of high purity Mg(OH)2 and γ‐Al2O3 thoroughly mixed in an aqueous slurry, and the solids are collected, dried, calcined, mixed with LiF sintering aid, and sieved. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load, followed by hot isostatic pressing at 1850°C under 200 MPa Ar. The crucial parameter for forming highly transparent MgO · 1.5 Al2O3 ceramic from this procedure is to hold the amount of LiF to 0.25 wt%.