MgAl2O4 Spinel Research Articles

Rare-Metal-Free Ultrabroadband Near-Infrared Phosphors.

Trivalent chromium (Cr3+) is an attractive near-infrared (NIR) emitter, but its ultrabroadband NIR emission is limited to host crystals containing large amounts of rare-metal elements and usually suffers from low internal quantum efficiency (IQE) and poor thermal stability. Here, a class of high-performance, rare-metal-free ultrabroadband NIR phosphors, are reported by revealing that weak-field Cr3+ centers featuring broadband NIR emission with near-unity IQEs are intrinsic, though in trace quantities, to Cr3+ doped MgAl2O4 spinel (MAS) and its derivatives well-known for their narrowband far-red emission. It is shown that such weak-field Cr3+ centers stem from cation inversion ubiquitous in spinel compounds, and their quantity can be increased simply by superstoichiometric Al2O3/Ga2O3. Then SiO2 is introduced into Al2O3-excess MAS to break the inversion symmetry of Cr3+ centers for greatly improving the probabilities of their otherwise parity-forbidden 3d-3d transitions. The as-fabricated phosphor-converted light-emitting diodes are capable of emitting ultrabroadband NIR light with high photoelectric efficiency (16.0%) and optical power (180.8mW), and excellent spectral stability, which apparently outperforms existing state-of-the-art devices.

REACTIVITY OF ALUMINUM OXIDE PRECURSORS FOR SOLID-PHASE FORMATION OF MgAl2O4 SPINEL

The reactivity of various alumina precursors in the reaction of magnesia spinel formation is compared: industrial powders (corundum powder KP, alumina: metallurgical GC, non-metallurgical G-00, reactive RG) and the xerogel combustion product from aluminum nitrate with citric acid under conditions of mechanical activating treatment and without it. The infrared spectra of corundum and periclase after mechanical treatment of various types (impact-abrasion in a planetary mill, abrasion in a ball-ring mill) were analyzed. It has been found that short-term action not only activated the components, but also contributed to the destruction of adsorption compounds at the preparation stage. The effect of mechanical activation of reagents on spinel yield was analyzed. It has been established that the alumina xerogel combustion product without annealing was X-ray amorphous, which suggested its high chemical activity. When using it, a more complete binding of initial reagents into the product was found compared to a mixture of periclase and corundum. Even in the absence of mechanical activation of the components, it was possible to achieve a high spinel yield (up to 80%). The obtained values of the effective reaction rate constants indicated a greater efficiency of mechanical treatment, including an impact component, with the combined activation of periclase and corundum/alumina. Co-treatment of reagents in a planetary mill (PM) made it possible to increase the reaction rate by 6-6.5 times, while abrasion in a ball-ring mill was only 1.7 times. Pretreatment of one component of the initial mix in PM was the most expedient for periclase, since it allowed to speed up the reaction by ~ 5 times, and is also the most profitable energetically and technologically due to the treatment of only one component of reduced hardness. The use of xerogel combustion product in the spinel synthesis was very effective, because it accelerated the process by ~4 times even without preliminary activation by mechanical means. For citation: Filatova N.V., Kosenko N.F., Artyushin A.S., Maloivan M.S., Zonina I.I., Vlasenkov A.S. Reactivity of aluminum oxide precursors for solid-phase formation of MgAl2O4 spinel. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 12. P. 15-24. DOI: 10.6060/ivkkt.20246712.7152.

The particle size effect of CeO2 on the structure and properties of MgAl2O4 spinel ceramics

Undoped and CeO2-doped transparent magnesium aluminate spinel ceramics were successfully fabricated by the Spark Plasma Sintering (SPS) method. The effect of the CeO2 with different dispersions on the phase evolution, microstructure, as well as the optical and mechanical properties was thoroughly investigated. Discrete-element modeling of MgAl2O4 spinel powder mixtures with cerium oxide of various dispersions and their imitation consolidation were conducted. It was found that the sintering of MgAl2O4 and CeO2 nanopowders mixture occurs with the lowest energy cost. The apparent activation energy of intensive sintering is 77 kJ/mol, the relative shrinkage is 64.7 %. The optimal CeO2 concentrations were determined, that guarantees the possibility of spinel ceramics consolidation with in-line transmittance value of at least 5 %. It has been explored that the introduction of CeO2 nanopowder into MgAl2O4 ceramics not only does not have a negative effect on its optical properties, but also leads to an increase in its mechanical properties.

Study of structural and mechanical properties of composite ceramics of the AlMgB14–TiB2 system

AlMgB14 ceramics is known as a material characterized by increased hardness in combination with a low friction coefficient. Composite structures based on this ceramics have even higher strength characteristics. In the present work, we investigate the structural-phase states and physicomechanical properties of composite ceramics of the AlMgB14–TiB2 system with a variable TiB2 content, obtained by hot pressing the initial batch based on AlMgB14 and TiB2 ceramic powders preliminarily synthesized by the method of self-propagating high-temperature synthesis. It was found that the resulting materials are characterized by a composite structure represented by TiB2 inclusions distributed in the AlMgB14 matrix. The phase composition of the resulting composites is similar to the phase composition of the initial batch, with 5 to 9 wt. % of the MgAl2O4 spinel phase being formed. The microhardness of AlMgB14–TiB2 composites is up to 19.9 GPa (the hardness of AlMgB14 ceramics obtained by a similar method without additives is 7 GPa). The three-point bending strength of AlMgB14–TiB2 composite materials is 309 MPa.

Exploring the potential of cobalt-based catalysts in the methane dry reforming for sustainable energy applications

Cobalt-based powder catalysts with different Co loadings were synthesized via wet impregnation using MgAl2O4 spinel as support. The catalytic properties of the solids were evaluated in the dry-reforming methane at different reaction temperatures and two CH4:CO2 ratios. The catalyst with higher cobalt content (13 %) presented superior performance due to increased Co0 species availability, confirmed by XPS analysis. Besides, this catalyst displayed remarkable resistance to carbon deposition at 550 °C and CH4:CO2 1: 1 ratio. In-situ DRIFT measurements demonstrated the formation and transformation of carbonate and hydrogen carbonate species during the DRM reaction, highlighting efficient CO2 and CH4 activation on the catalyst surface. Based on this data, a structured catalyst was deposited on top of the FeCrAlloy monolith, which was active, stable, selective for H2, and resistant against on/off cycles. Compared with the powder catalyst, the structured system displayed higher catalytic activity, possibly due to higher reducibility of metal species and improved heat transfer provided by the FeCrAlloy substrate. The results highlight the potential of Co-based structured catalysts for H2 or synthesis gas production processes.

Facile one-pot synthesis of CuO–Bi2O3/MgAl2O4 catalyst and its performance in the ethynylation of formaldehyde

The CuO–Bi2O3/MgAl2O4 catalyst was synthesized via one-pot synthesis and used to catalyze formaldehyde (HCHO) ethynylation. Coprecipitation using Cu2+, Bi3+, Mg2+, and Al3+ nitrates and NaOH generated Cu and Bi oxides and spinel MgAl2O4 phase. The catalyst precursor was calcined at 450 °C. The catalytic performance of CuO–Bi2O3/MgAl2O4 in the synthesis of 1,4-butynediol via HCHO ethynylation was investigated. The presence of a new spinel phase enhanced the acid–base properties on the catalyst surface and prevented the aggregation of CuO particles. These properties resulted in improved CuO dispersion during calcination and CuO particle growth suppression, affording smaller CuO crystals. The MgAl2O4 support facilitated the reduction of Cu2+ to Cu+ and formation of abundant active species during the reaction. The catalyst exhibited abundant weakly basic, fewer strongly basic, and least acidic sites, which facilitated the adsorption of HCHO and acetylene. The catalytic performance of CuO–Bi2O3/MgAl2O4 demonstrated 97 % conversion and 80 % selectivity after the online monitoring of the ethynylation reaction for 6 h. The leaching of Cu during the reaction, as analyzed by inductively coupled plasma spectroscopy, was extremely low. Moreover, conversion and selectivity did not substantially change after eight cycles. In addition, the catalyst exhibited superior activity and long-term stability in the ethynylation reaction.

The synthesis and application of Ni2+-doped NIR-II Phosphors composed of MgAl2-xGaxO4 solid-solution

Near-infrared (NIR) light has been widely used in fields, such as biometrics recognition, security monitoring, lighting for indoor agriculture, food safety inspection and biomedical imaging. NIR phosphors are crucial components for NIR phosphor-conversion light-emitting diodes (NIR pc-LED). Compared with the traditional high-temperature solid phase method for spinel-type phosphors, the synthesis by the sol-gel combustion method is rapid, more efficient, and requires lower temperatures, with good homogeneity and uniformed nano-size particle. Here we report the synthesis of a panel of NIR phosphors that are composed of nano-sized Ni2+-doped MgAl2-xGaxO4 solid-solution of spinal crystals with high phase purity. By adjusting the ratios of Al/Ga, the Ni2+-doped phosphors with spinel (MgAl2O4) to hybrid-spinel (MgGa2O4) crystal structures were obtained. The phosphor with the Al/Ga ratio of 0.8–1.2 showed the maximal NIR emission intensity, which was 5.9 times higher than that of the MgAl2O4 spinel structure, with an internal photoluminescence quantum efficiency of 31.63 %. The phase transformation of the crystal form from spinel to hybrid-spinel resulted in a shift of the emission peak from 1223 nm to 1287 nm, realizing a ∼ 64 nm structure-dependent tunability of NIR luminescence. We assembled a NIR pc-LED device using the optimal phosphor and a commercially available near ultraviolet LED, and experimentally demonstrated its application for night-vision monitoring and bioimaging of veins in a human hand.

Synergistic effect of YAG particulate reinforcement on microstructural and thermo-mechanical properties of pressureless sintered MgAl2O4 spinel ceramic composites

Yttrium aluminum garnet (YAG) powders, synthesized through a partial wet chemical process, exhibit controlled morphology, high phase purity, and uniform homogeneity etc. This process involves the precipitation of aluminum ions onto Y2O3 particles. The transformation process of the Al-compound/Y2O3 precursor structure leads to the direct crystallization of pure YAG at 900 °C, passing through an intermediate stage where hexagonal YAH phase forms. A comprehensive study was conducted on the impact of preformed YAG as a reinforcing phase on the densification, mechanical, and thermo-mechanical properties of MgAl2O4-YAG composite, sintered under pressureless sintering in temperature range of 1500 °C–1600 °C. The inclusion of YAG particulate up to 5.0 wt% significantly enhances the densification, hardness, fracture toughness, flexural strength and high-temperature mechanical behavior of magnesium aluminate spinel (MAS) ceramics, however, increasing the YAG content beyond 5.0 wt% causes a reduction in these properties. The optimized addition of YAG particulate contributes to reducing the average grain size and refining the microstructure of the spinel ceramic matrix.

Phase compositions, microstructures, and microwave dielectric properties of novel high-entropy spinel-structured MAl2O4 ceramics

High-entropy design for MgAl2O4 spinel has rarely been reported. In this study, six novel high-entropy spinel-structured (HES) MAl2O4 ceramics were prepared through the conventional solid-state reaction method. The effect of high-entropy design on the crystal structure, microscopic morphology, and microwave dielectric performance of HES ceramics was investigated based on X-ray diffraction (XRD), Raman spectra, and scanning/transmission electron microscopy (SEM/TEM). The analysis shows that the difference of one element among the high-entropy components can lead to an obvious variation in the lattice parameters, microstructures, and performance. The differences in microstructure, especially grain size, are the joint effects of particle size distribution, porosity, and composition. Specifically, (Mg0.2Mn0.2Co0.2Ni0.2Zn0.2)Al2O4 (HES04) ceramic sintered at 1625 °C for 4 h exhibits superior properties: εr=8.78 ± 0.03, Q×f=34,022 ± 910 GHz (@12.62 GHz), and τf=−29.3 ± 0.6 ppm/°C. These results suggest that the high-entropy strategy is an effective method to tune the dielectric properties of MgAl2O4 ceramic.

Molecular dynamics simulation of the orientation and temperature dependence in MgAl2O4 spinel

Molecular dynamics simulation of the orientation and temperature dependence in MgAl2O4 spinel

The Cerium-Modified MgAl2O4 Spinel and Its Supported PdCu Three-Way Catalyst with Improved Performance

The Cerium-Modified MgAl2O4 Spinel and Its Supported PdCu Three-Way Catalyst with Improved Performance

Interaction between refractory for refining ladle and low-carbon low-silicon Al-killed molten steel

To investigate the interaction between low-carbon low-silicon Al-killed molten steel and the refractory materials employed in refining ladles, the study focused on the interface reaction between molten steel with a composition of Fe-0.03C-0.01Si-0.2Mn-0.03Al (wt%) and the four types of refractory materials used in different positions of refining ladles at 1873 K. The results show that different interface layers form after 60 min of contact between the refractory materials and the molten steel. The MgO-C refractory generates a dense MgO layer, the MgO-Al2O3 refractory produces a (Fe, Mg)Al2O4 layer, the Al2O3-SiC refractory forms a Mg Al2O4 layer rich in Si, and the MgO-Al2O3-C refractory generates a MgAl2O4 spinel layer. The average content of Mg at the interface decreases in the following order: MgO-C, MgO-Al2O3-C, Al2O3-SiC and MgO-Al2O3 refractory, and SiC available in Al2O3-SiC refractory supplies more Si to the interface. The thermodynamic simulation is consistent with the actual results.

Exploring nanovoids in spinel ceramics using positron–positronium trapping model

The investigation focused on nanovoids (free-volume defects) in spinel MgAl2O4 ceramics, synthesized at various temperatures, both before and after saturation (water-immersion), utilizing the positron annihilation spectroscopy method. The findings were analyzed within the framework of a multi-channel model. It was demonstrated that this model integrates channels capturing positrons by volumetric defects and the decay of ortho-positronium atoms. Within this refined approach, the first component reflects the microstructural characteristics of the spinel structure, the second component corresponds to volumetric defects near grain boundaries, while the third pertains to the “pick-off” process of ortho-positronium annihilation within water-filled nanopores of the ceramics.

Sintering and exploitation properties of humidity-sensitive nanostructured ceramics and thick films based on MgAl2O4 spinel

The electrophysical properties of nanostructured MgAl2O4 ceramics sintered at 1300 °C for 5 h, as well as thick films were obtained and investigated. The study of adsorption-desorption cycles was carried out after sintering and aging test. It is shown that ceramic has a sensitivity to humidity in the range of 25–95% with minimal hysteresis of the characteristic in adsorption-desorption cycles, which can be eliminated after the aging test. Thick films based on it are characterized by hysteresis and loss of sensitivity at low values of relative humidity, which is caused by the clogging of small nanopores.

Effect of in-situ generated MgAl2O4 spinel on thermal shock resistance of magnesia-zirconia refractories

This study aims to advance the application of magnesia-zirconia (MgO–ZrO2) refractories in non-ferrous metal smelting furnaces by enhancing their thermal shock resistance. To fabricate MgO–ZrO2–MgAl2O4 refractories, tabular corundum particles and activated α-Al2O3 powder are integrated into MgO–ZrO2 refractories. The analysis of thermal shock resistance, phase composition, and microstructure of the samples was conducted to gain insights into the toughening mechanisms. The results reveal a substantial enhancement in thermal shock resistance with the addition of 15 wt% tabular corundum (1–0.5 mm) and activated α-Al2O3. Compared to samples without these additives, a notable increase of over 50.0 % in the ratio of residual cold modulus of rupture is shown. The enhancement in thermal shock resistance is primarily attributed to the in-situ generated MgAl2O4 spinel. This process involves volume expansion and increased thermal expansion mismatch, which induce microcrack toughening. Additionally, larger tabular corundum particles form in-situ MgAl2O4 spinel, causing crack deflection and branching, thus extending the crack propagation pathway. Furthermore, the presence of micropores in the spinel zone absorbs the energy required for crack propagation, thereby improving toughness and thermal shock resistance. Consequently, the MgO–ZrO2–MgAl2O4 refractories containing in-situ MgAl2O4 spinel with micropores are promising candidate for chrome-free refractories used in non-ferrous metal smelting furnaces.

Improving the Transparency of a MgAl2O4 Spinel Damaged by Sandblasting through a SiO2-ZrO2 Coating

Transparent materials in contact with harmful environments such as sandstorms are exposed to surface damage. Transparent MgAl2O4 spinel used as protective window, lens or laser exit port, among others, is one of the materials affected by natural aggressions. The impact of sand particles can cause significant defects on the exposed surface, thus affecting its optical and mechanical behavior. The aim of this work is to improve the surface state of a spinel damaged surface by the deposition of a thin layer of SiO2-ZrO2. For this purpose, spinel samples obtained from different commercial powders sintered by Spark Plasma Sintering were sandblasted and further coated with a SiO2-ZrO2 thin layer. The coating was successfully synthesized by the sol/gel method, deposited on the sandblasted samples and then treated at 900 °C, reaching a final thickness of 250 nm. The results indicated that sandblasting significantly affects the surface of the spinel samples as well as the optical transmission, confirmed by UV-visible spectroscopy and profilometry tests. However, the deposition of a SiO2-ZrO2 coating modifies the UV-visible response. Thus, the optical transmission of the S25CRX12 sample presents the best transmission values of 81%, followed by the S25CRX14 sample then the S30CR sample at 550 nm wavelength. An important difference was observed between sandblasted samples and coated samples at low and high wavelengths. At low wavelengths (around 200 nm), sandblasting tends to improve significantly the transmission of spinel samples, which exhibit a low transmission in the pristine state. This phenomenon can be attributed to the healing of small superficial defects responsible for the degradation of transmission such as pores or flaws. When the initial transmission at 200 nm is high, the sandblasting worsens the transmission. Sandblasting reduces slightly the transmission values for long wavelengths due to the formation of large superficial defects like chipping by creation and propagation of lateral cracks. The coating of the sandblasted samples exhibits some healing of defects induced by sandblasting. The deposition of the SiO2-ZrO2 layer induces a clear increase in the optical transmission values, sometimes exceeding the initial values of the transmission in the pristine state.

Facile fabrication of MgAl2O4/MWCNT nanocomposite for efficient and stable solar-driven degradation of organic dye

This work reports the synthesis and characterization of Magnesium Aluminate (MgAl2O4) and Magnesium Aluminate/Multi Walled Carbon Nanotube (MgAl2O4/MWCNT) nanocomposite by facile chemical co-precipitation method for the dye degradation application. MgAl2O4 and MgAl2O4/MWCNT nanocomposite are characterized by Scanning Electron Microscopy (SEM), x-ray Diffractometry (XRD), Raman Spectrometry, UV–vis Spectrophotometry (UV–vis), Fourier Transform Infrared Spectroscopy(FTIR), and x-ray Photoelectron Spectroscopy (XPS). Surface morphology of MgAl2O4/MWCNT nanocomposite exhibits entangled needle-like structures while MgAl2O4 spinel comprises agglomerated nanoparticles of different sizes. XRD confirms the formation of MgAl2O4. XPS identifies the chemical states and binding energies of constituent elements present in the sample. Optical properties reveal that addition of MWCNTs in MgAl2O4 decreases the optical bandgap energy from 3.02 eV to 2.78 eV. MgAl2O4/MWCNT nanocomposite shows reduced bandgap compared to pristine MgAl2O4 due to increased chemical defects or vacancies in intergranular regions and chemical interaction between MgAl2O4 and MWCNT, leading to the formation of new energy levels in MgAl2O4/MWCNT nanocomposite. Addition of MWCNTs provides a large surface area, more active sites, and enhances electron mobility between energy levels. MgAl2O4/MWCNT nanocomposite proves itself a better photocatalyst due to the fast degradation of Methyl Blue (MB) in 65 min as compared to MgAl2O4 which degrades the dye in 90 min. MgAl2O4/MWCNT nanocomposite also shows good stability and reusability even after performing the six cycles of dye degradation.

Modification effect of Mg(OH)2–2Al(OH)3 composite gels on Y-PSZ ceramics

A Mg(OH)2–2Al(OH)3 composite gel, synthesized via the sol-gel method, served as a modifier in the preparation of modified yttria partially stabilized zirconia (Y-PSZ) ceramics. The volume density, apparent porosity, compressive strength, thermal shock resistance, phase composition, and microstructure of the ceramics were systematically characterized. The findings reveal that, upon sintering, the Mg(OH)2–2Al(OH)3 composite gel decomposes into MgO–Al2O3 powder, which in turn in-situ generates a MgAl2O4 spinel reinforcing phase, characterized by its uniform size and distribution. The presence of this MgAl2O4 spinel applies compressive stress to the adjacent zirconia grains, effectively inhibiting their abnormal growth. By occupying the grain boundaries and establishing a mosaic structure, the compressive strength and thermal shock resistance of the Y-PSZ ceramics are significantly enhanced. Notably, an increase in the composite gel quantity correlates with a reduction in both volume density and apparent porosity of the Y-PSZ samples, whereas the compressive strength initially rises before diminishing, and the thermal shock resistance markedly improves. When the amount of Mg(OH)2–2Al(OH)3 composite gel introduced reaches 9 %, the comprehensive performance of the modified Y-PSZ ceramics is optimized. At this point, its compressive strength is increased by 1.26 times compared to the unmodified Y-PSZ ceramics, reaching 732 MPa; moreover, after undergoing 60 thermal shocks, its residual strength retention rate still remains as high as 59.15 %.

Kinetic analysis of low-temperature MgAl2O4 formation from brucite with α-Al2O3 nanoparticles

This paper presents a detailed analysis of the synthesis of spinel MgAl2O4, a material with key industrial applications, through the calcination of brucite and alumina. The study reveals that a longer heat treatment time at 960°C favors the formation of spinel with spherical morphology and reduces the presence of MgO in irregular agglomerates. XRD analyses confirm the progressive phase transformation, while SEM-EDS provides evidence of chemical composition. These findings support the use of the Avrami equation to model spinel formation kinetics and highlight the importance of optimizing calcination parameters to improve material properties. The study concludes with an improved route for the efficient production of MgAl2O4, which could significantly influence its application in demanding environments.

Interfacial characteristics, interfacial oxide evolution and bonding mechanism of 2219 aluminum alloy joints manufactured by hot-compression bonding

The bonding joints of 2219 aluminum alloy with different deformations of 30%, 45% and 55% were fabricated using hot-compression bonding, and subsequent subjected to homogenization treatment. The interfacial microstructure and interfacial oxide evolution of the bonding joints were systematically investigated. The results showed that increasing deformation can effectively reduce the formation of interfacial voids and promote the interfacial grain boundary migration (IGBM). As the deformation increased from 30% to 55%, the flat bonding interface transformed into curved grain boundaries, and ultimately being occupied by dynamically recrystallized (DRXed) grains. The development of IGBM and interfacial DRXed grains is crucial for ensuring high-quality bonding joints. At 55% deformation, the interface bonding strength reached 156.2 MPa, corresponding to 98.1% of the matrix bonding strength. In addition, the residual interfacial oxide film Al2O3 reacted with MgO and transformed into MgAl2O4 spinel. The final evolution products at the bonding interface were the mixture of MgO, Al2Cu, Al2O3, and MgAl2O4. The bonding interface at 55% deformation successfully achieved relatively clean atomic bonding after homogenization. Based on interfacial void closure, interfacial oxide film fragmentation and evolution, and interfacial recrystallization, the bonding mechanism of 2219 aluminum alloy joints was proposed. These findings contribute to our understanding of the interfacial bonding process in metal solid bonding.