Luminescence of MgAl2O4 and ZnAl2O4 spinel ceramics containing some 3d ions

The luminescence properties of ceramic phosphors based on two spinel hosts MgAl₂O₄ and ZnAl₂O₄ doped with manganese ions have been studied. It has been found that the spectral properties of these phosphors can be strongly varied by changing synthesis conditions. Both types of doped ceramic spinel can serve as efficient Mn²⁺ green-emitting phosphors having peak emissions at 525 and 510 nm, respectively. Mn-doped MgAl₂O₄ spinel can also be prepared as an efficient Mn⁴⁺ red-emitting phosphor having peak emission at ~651 nm by using specific temperatures of heat treatment in air. It has also been shown that the conversion of Mn²⁺ to Mn⁴⁺ and viсe versa, as well as the coexistence of Mn²⁺ green and Mn⁴⁺ red emissions, can be accomplished by properly chosen annealing conditions of the same initially synthesized MgAl₂O₄:Mn sample. Manganese doped MgAl₂O₄ spinel with an optimal intensity ratio of green and red emissions can be a promising single-phase bicolor phosphor suitable for the development of warm white phosphor-converted LED lamps. On the other hand, it has been determined that perfectly normal ZnAl₂O₄ spinel cannot be doped with Mn⁴⁺ ions in contrast to partially inverse MgAl₂O₄ spinel. However, ZnAl₂O₄ samples unintentionally doped with impurity Cr³⁺ ions show emission spectra in the far-red region with well pronounced R, N and vibronic lines of Cr³⁺ luminescence due to the perfect normal spinel structure of synthesized ZnAl₂O₄ ceramics. Also, by partially substituting Al³⁺ cations for Mg²⁺ in ZnAl₂O₄ there is an opportunity to obtain Mn⁴⁺ doped or Mn⁴⁺/Cr³⁺ codoped far-red emitting phosphors which can be suitable for indoor plant growth lighting sources.     

Reduced grain boundary energies in rare-earth doped MgAl2O4 spinel and consequent grain growth inhibition

Grain growth inhibition in MgAl₂O₄ spinel nanostructure was achieved by grain boundary (GB) segregation of rare-earth dopants. Microcalorimetric measurements showed that dense spinel compacts doped with 3 mol% of R₂O₃ (R = Y, Gd, and La) had decreased GB energies as compared to the undoped spinel, representing reduction in the driving force for grain growth. Segregation energies of the three dopants to the Σ3 (111) GB were calculated by atomistic simulation. The dopants with higher ionic radius tend to segregate more strongly to GBs. The GB energies were calculated from atomistic simulation and, consistent with experiments, a systematic reduction in GB energy with dopant ionic size was found. High temperature grain growth experiments revealed a significant reduction of grain growth in the doped nanostructures as compared to the undoped one, which was attributed to increased metastability or possibly also a GB dragging originated from the dopant segregation.     

Impact on aggregate/matrix bonding when a refractory contains zinc aluminate instead of spinel and magnesia-chrome

The high hot strength of MgO–Cr₂O₃ refractory is often ascribed to its intimate aggregate/matrix bonding. For a fundamental comparison with it, ∼2 mm aggregates of MgO and Al₂O₃ were separately embedded in ZnAl₂O₄ and MgAl₂O₄ matrices, sintered at 1600°C, and examined. It was found that similarity of thermal expansion coefficient (TEC) between the aggregate and the matrix is critical to achieve good bonding and this is more important than the extent of interdiffusion. The TEC mismatch of ≥5.7 × 10⁻⁶ K⁻¹ caused significant undesirable debonding in MgO aggregate/MgAl₂O₄ matrix sample and MgO/ZnAl₂O₄ despite >736 μm Zn²⁺ diffusion depth in the latter. Direct bonding, as inferred from a thicker interfacial reaction layer and a greater shift of the aggregate/matrix interface before and after firing, was better in MgAl₂O₄/ZnAl₂O₄ combination, followed by tabular Al₂O₃/ZnAl₂O₄ and Al₂O₃/MgAl₂O₄. Powder X-ray diffraction indicated that the volatilization of ZnAl₂O₄ at 1600°C in air was negligible compared to MgO–Cr₂O₃.     

Gel-casting of MgAl2O4 transparent ceramics using a common dispersant

The ethanolaminic salt of citric acid (commercial name Dolapix CE 64) has commonly been used as a dispersant for colloidal based ceramic forming process. In this paper, a surprise was presented that MgAl₂O₄ spinel slurries consisting of MgAl₂O₄ spinel nanoparticles and Dolapix CE 64 gelled in air at room temperature spontaneously. The MgAl₂O₄ spinel slurries with high solid loading (54 vol%) were prepared with Dolapix CE 64 and the green body with up to 57% relative density was obtained. MgAl₂O₄ transparent ceramics with small grain size (0.92 μm) and high transmittance (81.7% at 600 nm) were fabricated after pre-sintering at 1500°C and hot-isostatic sintering at 1550°C.     

Sintering and grain growth behaviour of magnesium aluminate spinel: Effect of lithium hydroxide addition

Lithium hydroxide, LiOH, in the amounts ranging from 0.1 to 1.2 wt% has been used as a sintering aid to improve the densification of MgAl₂O₄. The addition of 0.3 wt% LiOH promotes densification and limits grain growth. The activation energy of sintering, calculated using master sintering curve approach, decreases from 790 ± 20 kJ.mol^?1 to 510 ± 20 kJ.mol^?1 with the addition of 0.3 wt% of LiOH. In addition, MgAl₂O₄ was also mixed with 10 wt% of LiOH to amplify the formation of reaction products. High-temperature XRD results showed that secondary phases (MgO and LiAlO₂) are produced above 1040 °C. The secondary phases start to disappear at T > 1200 °C, and MgAl₂O₄ is produced. While adding small amounts of LiOH, up to ca. 0.3 wt%, is beneficial for densification and suppressing grain growth, there exists a critical concentration of Li⁺ that is accounted for by the preferential incorporation of lithium ions into MgAl₂O₄ crystal lattice.     

Metastability in the MgAl2O4–Al2O3 System

Aluminum oxide must take a spinel form (γ‐Al₂O₃) at increased temperatures in order for extensive solid solution to form between MgAl₂O₄ and α‐Al₂O₃. The solvus line between MgAl₂O₄ and Al₂O₃ has been defined at 79.6 wt% Al₂O₃ at 1500°C, 83.0 wt% Al₂O₃ at 1600°C, and 86.5 wt% Al₂O₃ at 1700°C. A metastable region has been defined at temperatures up to 1700°C which could have significant implications for material processing and properties. Additionally, initial processing could have major implications on final chemistry.     

Syntheses and characterization of highly transparent ZnAl2O4 ceramic films using poly(acrylamide-co-acrylic acid) assisted microwave approach

Zinc aluminate nanopowders were synthesized via poly(acrylamide-co-acrylic acid) assisted microwave approach. The as-synthesized ZnAl₂O₄ nanopowders were characterized using X-ray diffraction (XRD), High resolution transmission electron microscopy (HRTEM) and selected area of electron diffraction (SAED). The prepared ZnAl₂O₄ nanopowders exhibited a spinel cubic polycrystalline structure. The increase of poly(acrylamide-co-acrylic acid) amounts decreased the particle size of the ZnAl₂O₄ nanopowders. The poly(acrylamide-co-acrylic acid) enhanced the densification rate of ZnAl₂O₄. The increasing of poly(acrylamide-co-acrylic acid) amount decreased the sintering temperature from 1300 °C to 950 °C. The hot-compressed ZnAl₂O₄ nanopowders in the existence of 2 wt% of poly(acrylamide-co-acrylic acid) exhibited full density at 950?C in just 20 min. The ZnAl₂O₄ ceramic films revealed a high transparency of 83 ± 1% at a wavelength range from 450?1200 nm.     

SO2 abatement over nanocrystalline MgAl2O4 spinel-supported catalysts

Thermally stable magnesium-rich MgAl₂O₄ spinel with mesoporous nanostructures and high surface area has been prepared by co-precipitation and post hydrothermal treatment, using glucose as organic template. Physical and chemical properties were characterized by XRD, N₂ sorption, TG, FTIR, SEM, and TEM. The synthesized MgAl₂O₄ showed a surface area of 324 m² g^?1 and centralized mesopore distribution (ca. 3.3 nm pore width) after calcination at 700 °C for 3 h. The prepared MgAl₂O₄ were impregnated with metal oxides as sulfur transfer catalysts for high-temperature SO₂ adsorption reaction. The results showed that ferric doped MgAl₂O₄ had the highest SO₂ pick-up capacity up to 58 % and best regeneration up to 81 %. These results showed that thermally stable nanostructured MgAl₂O₄ are a promising candidate as catalyst for desulfurization in fluid catalytic cracking process.     

Thermodynamic evaluation of SiC oxidation in Al2O3–MgAl2O4–SiC–C refractory castables

The literature suggests that MgAl₂O₄ can accelerate SiC oxidation in Al₂O₃–MgAl₂O₄–SiC–C refractory castables. Thus, in this work thermodynamic calculations have been carried out using FactSage^® software in order to explore, search for and understand the role of MgAl₂O₄ on the SiC oxidation. According to the thermodynamic predictions, at 1500 °C and under a reducing atmosphere, there is no evidence that spinel might directly affect SiC oxidation. The increase of SiC content in an Al₂O₃–SiC–C (AL) castable composition was mainly related to the reaction between mullite and carbon. On the other hand, the SiC generation in the Al₂O₃–MgAl₂O₄–SiC–C (SP) composition was a result of the reaction involving liquid SiO₂ and carbon from the refractory. Therefore, the lower SiC content in the SP castable resulted from the refractory's phase transformations. It was also suggested that the samples thermally treated 15 times at 1500 °C did not reach the equilibrium condition, which explains the differences between experimental and thermodynamic results.     

Poly(N-isopropylacrylamide) assisted microwave synthesis of magnesium aluminate spinel oxide for production of highly transparent films by hot compression

Magnesium aluminate spinel oxides have been prepared via poly(N-isopropylacrylamide) assisted microwave technique. The prepared MgAl₂O₄ powders showed a crystalline cubic structure with spinel phase after calcination at 600 °C only. The poly(N-isopropylacrylamide) amount showed a high effect on the crystallite size and the densification behavior of MgAl₂O₄. The increase of the amount of poly(N-isopropylacrylamide) reduced the sintering temperature of MgAl₂O₄ from 1400 °C to 1050 °C. The hot-pressed of MgAl₂O₄ powders in the presence of 3 wt% of poly(N-isopropylacrylamide) exhibited a full density at sintering temperature 1100 °C in 15 min only. The sintered films showed high transparency (81 ± 2%) in the wavelength range 500–1000 nm.     

Effect of Al(H2PO4)3/Zn/B4C doped resin on properties and microstructure of unfired Al2O3–C slide plate materials

The unfired Al₂O₃–C slide plates have the advantages of energy saving, environment friendly, efficiency and relatively low-cost. However, the decomposition and oxidation of the phenol-formaldehyde (PF) resin at elevated temperatures deteriorate the properties and decrease service life of the unfired slide plates. In order to improve the property of resin, the doped PF resin is prepared by incorporating Al(H₂PO₄)₃, Zn and B₄C powders. The effects of the doped resin on medium-high temperature properties and microstructure of the unfired slide plate materials have been investigated. The results show that Zn and B₄C doped resin contributes to notable increasing the density and strength properties at medium temperature, because Zn and B₄C easily oxide and thus protect resin from oxidation, leading to form a dense structure. Zn and B₄C doped resin can significantly improve hot modulus of rupture of the materials at 1400 °C, which is due to Zn and B₄C react with oxidative gases leading to increase in concentration of C(g), CO and N₂, Al and Si would react with C(g), CO(g) and N₂(g) to form AlN and SiC whiskers creating strengthening effect. Specimens with Zn and B₄C doped resin addition have good oxidation resistance at 1500 °C, because Zn and B₄C in the surface of the material react with O₂ to form ZnAl₂O₄ or mullite containing dense glass film, which would retard O₂ diffusion into the inner of the specimens.     

Origin of Violet‐Blue Emission in Ti‐Doped Gahnite

ZnAl₂O₄ doped with Ti⁴⁺ (2%) was synthesized by the hydrothermal method at 220°C at pressure of 25 bars. An average grain size of the as‐prepared sample was 3 nm, the samples with biggest grain size were obtained after annealing at 300°C, 500°C, 600°C, 700°C, and 900°C, diameter of the latter was about 33 nm. IR spectroscopy indicated that ZnAl₂O₄ was partially inverted. The degree of the inversion decreases with increase in the annealing temperature but increases with increasing Ti⁴⁺ content. Absorption and emission spectra as well as emission decay profiles were recorded at 300 and 77 K. The observed spectra are due to charge‐transfer O^2??Ti⁴⁺ transitions. Color of the emission depends on the nanocrystal size and with increase in its diameter changes from violet to blue, accordingly the absorption bands exhibit redshift. The calculations based on Density Functional Theory confirmed the experimental results. 3d electrons of titanium ions form the bottom of the ZnAl₂O₄:Ti⁴⁺ conduction band, oxygen, aluminum or zinc vacancies create additional levels in the gahnite energy band gap. It was also found that in ZnAl₂O₄ aluminum or zinc vacancy induces magnetism with relatively high magnetic moment close to 1 μ_B per vacancy.     

Preparation of MgAl2O4 solar thermal storage ceramics from different magnesium sources

In order to study the performance and feasibility of magnesia-alumina spinel (MgAl₂O₄) ceramics for thermal storage in solar thermal power generation, MgAl₂O₄ was prepared by theoretical composition using α-Al₂O₃ as aluminium source, fused magnesia, magnesite, and light burned magnesia as different magnesium sources and kaolin as additive. The effects of magnesium source and the additive on sintering properties, thermal shock resistance and thermal properties of MgAl₂O₄ ceramics were researched. The results shown sample A1 (with fused magnesia) sintered at 1670°C possessed the optimum comprehensive properties, the bending strength increased by 7.71% after 30 thermal shock times (room temperature-1000°C, air cooling), the specific heat capacity was 1.05 J/ (g·K). Therefore, the MgAl₂O₄ ceramics exhibited great potential in high-temperature thermal storage material.     

Highly selective and stable bimetallic catalysts supported on different materials for n-butane dehydrogenation

A study about the performance of Pt(0.3 wt%)Sn(0.3 wt%) catalysts supported on different materials in n-butane dehydrogenation is reported in this paper. The materials used as supports were γ-Al₂O₃, ZnAl₂O₄ spinel, MgAl₂O₄ spinel and spheres of α-Al₂O₃ with a washcoating of γ-Al₂O₃. The syntheses of both spinels leaded to very pure ZnAl₂O₄ and MgAl₂O₄ supports. The material prepared by washcoating showed the presence of an uniform and homogeneous layer of γ-Al₂O₃ (with a thickness between 12 and 18 μm) deposited on the spheres of α-Al₂O₃.The best behavior in activity, selectivity and stability through five severe cycles was achieved by bimetallic PtSn catalysts supported on MgAl₂O₄ spinel and on the material prepared by washcoating. The very good performance of these catalysts through the different cycles of reaction-regeneration can be due to the presence of metallic phases which preserve the strong intermetallic interaction along the different treatments, thus avoiding segregation processes.     

Synthesis of aluminomagnesian spinel with excess magnesium oxide under varying flow rates of cation mass transfer

The synthesis of MgAl₂O₄ with 10 mol. % excess of MgO by heating a mixture of highly dispersed magnesium and aluminum oxide powders to 800 and 1100°C is investigated. The diffusion mass transfer rate of aluminum cations in the synthesis of spinel was accelerated by introducing a TiO₂ additive into Al₂O₃, whereas the mass transfer rate of magnesium cations was delayed by introducing a Na₂O additive into MgO. Simultaneously, spinel was synthesized without additives. The products of synthesis were investigated using petrography, x-ray phase analysis, and dissolution in HCl. The use of the additives and increasing the temperature of synthesis decrease the solubility of powders in HCl, which facilitates bringing initially formed solid solutions of Al₂O₃ in MgAl₂O₄ closer to the composition of stoichiometric spinel. __________ Translated from Steklo i Keramika, No. 2, pp. 14–19, February, 2006.     

Synthesis and Sintering Behavior of Ultrafine (<10 nm) Magnesium Aluminate Spinel Nanoparticles

This article reports a comparative characterization of ultrafine MgAl₂O₄ spinel nanoparticles synthesized by polymeric precursor (Pechini) and coprecipitation methods. The nanoparticles were evaluated in terms of purity and surface cleanliness, size distribution, state of agglomeration, and sintering behavior. Powders synthesized by the Pechini technique were highly agglomerated and revealed a bimodal particle size distribution centered around 12 and 27 nm. Thermal analysis and infrared spectroscopy measurements indicated that carbon species remained on the surface of the powders only to be released when temperatures exceeded 1000°C. Isothermal sintering of such nanopowders at 1300°C showed a maximum relative density of only 54%. MgAl₂O₄ synthesized via coprecipitation created small nanoparticles, around 5–6 nm after calcination at 800°C, with significantly less agglomeration. Compared with the precursor‐derived powders, excellent sinterability of the coprecipitated powders was obtained under the same sintering conditions. Relative densities above 90% were obtained after only 10 min, which further increased to greater than 95% after 20 min with no sintering aids or dopants. The results highlight the importance of purity and processing control to exploit the beneficial high sinterability of nanoparticles.     

Preparation,characterisation, and growth mechanism of mesoporous petal-like MgAl2O4 spinel

Petal-like MgAl₂O₄ spinel was successfully prepared using a novel inorganic salt-assisted nonhydrolytic sol-gel method without a template and was employed as absorbent in the removal of the Congo red (CR). The effects of the inorganic salt type, heat-treatment temperature, and dwelling time on the morphology and phase composition of the petal-like MgAl₂O₄ spinel were investigated systematically. Results indicated that when Na₂MoO₄ was employed as the salt and the heat-treatment temperature and dwelling time were 600 °C and 5 h, respectively, the as-obtained petal-like MgAl₂O₄ spinel exhibited a highly uniform morphology with a thickness of 19–23 nm and a length of 240–280 nm. The N₂ adsorption-desorption results revealed that the petal-like MgAl₂O₄ exhibited a large BET specific surface area of 161 m²g^-1 with a pore volume of 0.24 cm³g^-1. The growth mechanism of the petal-like MgAl₂O₄ is believed to be the formation of a two-dimensional layered network structure by the coordination between the condensation product of the magnesium aluminium bimetallic alkoxides and the ions in the salt. The as-prepared MgAl₂O₄ petal exhibited an effective adsorption capacity toward anionic dyes CR. The maximum adsorption capacity of CR onto the mesoporous MgAl₂O₄ petal was found to be 572.01 mg/g, it is showed the petal-like MgAl₂O₄ exhibit huge potential of application in the field of environmental remediation.     

Study of CO2 permeation in Zn2+-modified Al2O3-carbonate membrane

Molten carbonate-based membranes for CO₂ capture have received attentions because of their high CO₂ selectivity, potential energy-saving capability and environmental friendliness. Zn²⁺-modified Al₂O₃/carbonates membranes with the enhanced CO₂ permeability have been developed in this work. Interfaces of LiAlO₂ were formed on the surface of Al₂O₃ due to the carbonates incorporation. Microstructural and interfacial characterisation of the membrane revealed that the outermost LiAlO₂ layer was due to the reactions between Li₂CO₃ and ZnAl₂O₄, resulting in the dissolution of ZnO in the molten carbonate. CO₂ permeability of 0.5% ZnAl₂O₄/Al₂O₃/carbonates reached 9.12 × 10⁻¹² mol.m⁻¹s⁻¹ Pa⁻¹ at 700°C, higher than that of Al₂O₃/carbonates, because of the dissolved ZnO. With the increase of ZnAl₂O₄, CO₂ permeability was decreased. The dissolved ZnO in the molten carbonates could enhance the ionic conductivity, whereas a higher amount of ZnO than its solubility will attenuate its effects on CO₂ permeation.     

Effect of fuel type on morphology and reactivity of combustion synthesised MgAl2O4 powders

Abstract

Two types of stoichiometric MgAl₂O₄ spinel powders were prepared by combustion synthesis routes, using sucrose (SCS) or urea (UCS) as fuel. For comparative purposes a stoichiometric MgAl₂O₄ powder was also prepared by solid state reaction synthesis (SS powder). Pressed compacts of the three powder types were sintered at various temperatures ranging from 1575 to 1625°C for 2 h. After grinding, SCS and SS powders had very narrow particle size distributions, with average particle sizes of 3·17 and 4·13 μm respectively, whereas UCS powder showed a wide particle size distribution with an average particle size of 37·76 μm. Their corresponding surface areas were found to be 65·8, 8·67, and 8·06 m² g^-1. The SCS MgAl₂O₄ powder sintered at 1625°C for 2 h had a bulk density of 3·44 g cm^-3 (96% of theoretical), an apparent porosity of 1·76%, and water absorption of 0·519%. The superior properties of SCS powders compared with other spinel powders are attributed to the higher surface area induced by the larger size of the sucrose molecule and the greater amount of gas evolved during sucrose combustion.     

Fabrication of Mn–Co Spinel Coatings on Crofer 22 APU Stainless Steel by Electrophoretic Deposition for Interconnect Applications in Solid Oxide Fuel Cells

This study investigates the microstructure, oxidation kinetics, and electrical behavior of Mn–Co spinel coating for interconnect applications in solid oxide fuel cells. A relatively dense, uniform, and well‐adherent Mn–Co (Mn_1.5Co_1.5O₄) spinel coating with good oxidation resistance and stable conductivity was successfully prepared on the surface of Crofer 22 APU stainless steel using electrophoretic deposition followed by sintering at 1150°C. During further thermal treatment at 800°C, the chromium oxide (Cr₂O₃) sublayer formed at the substrate/coating interface during sintering showed a very slow growth, and no chromium penetration was detected in the Mn–Co coating. The oxidation kinetics of the Mn–Co‐coated substrate obeyed the parabolic law with the a parabolic rate constant k_p of 5.20 × 10^?15 g²/cm⁴/s, which was 1–2 orders of magnitude lower than that of the uncoated Crofer 22 APU stainless steel substrate. For oxidation (at 800°C) times ≥50 h, the area‐specific resistance of the Mn–Co‐coated Crofer 22 APU substrate became ~17 mΩ·cm² and was almost constant after further oxidation.