MgAl2O4 Spinel Phase Research Articles

TiB2 effects on the AlMgB14–TiB2 ceramics structure and properties

This paper studied titanium diboride (TiB2) effects on the structure and properties of AlMgB14 ceramics. AlMgB14–TiB2 samples with 30 wt% and 50 wt% TiB2 were obtained by the self-propagating high-temperature synthesis. The addition of 30 wt% TiB2 did not significantly change the phase composition of the AlMgB14 ceramics, which remained comparable with the initial powder mixture composition. However, increasing the TiB2 amount to 50 wt% resulted in the MgAl2O4 spinel phase appearance. The addition of TiB2 significantly increased the microhardness of AlMgB14 from 7 GPa to 19.9 GPa, compared to the initial material, and also increased its flexural strength to 314 MPa.

Microstructure and microhardness of the Al-10Mg alloy processed by the mechanical alloying technique

In this work, the Al-10Mg nanostructured alloy was synthesized by high-energy mechanical milling. Subsequently, the powders consolidated under a uniaxial pressing in the air. The as milled powders were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). SEM and XRD also characterized the sintered samples. Vickers microhardness of sintered Al-10Mg alloy was measured. SEM, XRD, and TEM characterizations show that the Al-10Mg alloy was synthesized after 10 h milling. The X-ray powder analysis of the structural parameters showed the increment with the time of the lattice parameter, strain as well as the solubility of Mg in Al. Besides, XRD and TEM studies showed that the crystallite size was reaching an average value of 19 nm after 10 h milling. XRD patterns of the all sintered specimens show the formation of the MgAl2O4 spinel phase. After powders compaction, the specimen sintered at 420?C for 2 h shows microhardness of 125 HV.

Effect of CeO2 and MgO promoters on the performance of a Ni/Al2O3 catalyst in the steam reforming of biomass pyrolysis volatiles

A Ni/Al2O3 catalyst has been modified incorporating CeO2 and MgO promoters in order to improve its performance in the steam reforming of biomass pyrolysis volatiles. Ni/Al2O3, Ni/CeO2-Al2O3 and Ni/MgO-Al2O3 catalysts have been prepared and fresh and deactivated catalysts have been characterized by N2 adsorption/desorption, X-ray Fluorescence (XRF), Temperature Programmed Reduction (TPR), X-ray powder diffraction (XRD), Temperature Programmed Oxidation (TPO), Transmission Electron Microscopy (TEM) and a technique based on Fourier Transform Infrared Spectroscopy-Temperature Programmed Oxidation (FTIR-TPO). The results obtained revealed a similar initial activity for the three catalysts tested (conversion higher than 98%), whereas stability has been greatly improved by incorporating CeO2 as promoter, as it enhances the gasification of coke precursors. However, Ni/MgO-Al2O3 catalyst is slightly less stable than Ni/Al2O3, presumably as a result of its lower reducibility due to the formation of MgAl2O4 spinel phase. Catalysts deactivation has been associated with coke deposition, although sintering phenomenon became also evident when the Ni/CeO2-Al2O3 catalyst was tested. The coke deposited on the catalysts does not present any specific morphology, which is evidence of its amorphous structure in the three catalysts studied.

Catalysis: An Integrated Textbook

Pd/Al2O3 catalysts modified by different metal oxides (Mn, Fe, La, Mg and Ni oxides) were prepared and tested for fuel-lean methane/air catalytic combustion. It was found that the support material influenced the combustion performance significantly. The addition of NiO or MgO effectively improved the activity and hydrothermal stability of catalysts. X-ray diffraction (XRD) results showed the presence of spinel phase in the Pd/Al2O3–NiO and Pd/Al2O3–MgO catalysts. Temperature-programmed desorption of ammonia (NH3-TPD) measurements indicated that the supports became weakly acidic because of the formation of NiAl2O4 or MgAl2O4 spinel. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed the well dispersion of Pd particles on the Al2O3–NiO and Al2O3–MgO supports. It was concluded that the formation of the NiAl2O4 or MgAl2O4 spinel phase during preparation would be beneficial for the methane catalytic performance of Pd catalysts.

High temperature carbon dioxide capture on nano-structured MgO-Al2O3 and CaO-Al2O3 adsorbents: an experimental and theoretical study.

Nano-structured alkaline-earth metal oxide adsorbents (denoted as MgO-Al2O3 and CaO-Al2O3) were prepared by an epoxide-driven one-pot sol-gel method, and they were applied to the dynamic and static CO2 adsorption. For comparison, a nano-structured aluminum oxide adsorbent (denoted as Al2O3) was also prepared by a similar method. MgO-Al2O3 adsorbent exhibited a well-developed mesopore structure through the formation of MgAl2O4 spinel phase, whereas CaO-Al2O3 adsorbent was composed of nano-sized CaO and CaAl2O4, resulting in a pore plugging. It was revealed that total basicity increased in the order of Al2O3 (0.11 mmol-CO2/g) < MgO-Al2O3 (0.37 mmol-CO2/g) < CaO-Al2O3, (1.21 mmol-CO2/g), which is in concurrent with adsorption energy obtained from DFT calculations. However, it was found that both basicity and base strength of the adsorbents played an important role in determining the CO2 adsorptive performance at different operating temperature. Among the adsorbents tested, MgO-Al2O3, which mostly retained medium basic sites, exhibited a best CO2 adsorptive performance at 200 degrees C. Furthermore, the experimental results are well supported by theoretical estimation, suggesting a useful design method of adsorbents for facile and regenerative adsorption in the applications of CO2 capture.

Laser sintering of magnesia with nanoparticles of iron oxide and aluminum oxide

Nanoparticles of iron oxide (Fe2O3, 20–40nm) and aluminum oxide (Al2O3, 50nm) were mixed in different concentrations (3, 5 and 7wt%) in a magnesium oxide (MgO) matrix. The mixture pellet was irradiated with 532nm output from a Q-switched Nd:YAG laser using different laser fluence and translation speed for sintering. The refractory samples obtained were analyzed using X-ray diffraction technique, scanning electron microscopy and X-ray photoelectron spectroscopy. The results showed that the samples irradiated at translation speed of 110μm/s and energy fluence of 1.7J/cm2 with a concentration of 5 and 7wt% of Fe2O3 presented the MgFe2O4 spinel-type phase. With the addition of Al2O3 nanoparticles, at a translation speed of 110μm/s and energy fluence of 1.7J/cm2, there were the formations of MgAl2O4 spinel phase. The changes in morphologies and microstructure due to laser irradiation were analyzed.

Influence of metal oxides on the performance of Pd/Al2O3 catalysts for methane combustion under lean-fuel conditions

Pd/Al2O3 catalysts modified by different metal oxides (Mn, Fe, La, Mg and Ni oxides) were prepared and tested for fuel-lean methane/air catalytic combustion. It was found that the support material influenced the combustion performance significantly. The addition of NiO or MgO effectively improved the activity and hydrothermal stability of catalysts. X-ray diffraction (XRD) results showed the presence of spinel phase in the Pd/Al2O3–NiO and Pd/Al2O3–MgO catalysts. Temperature-programmed desorption of ammonia (NH3-TPD) measurements indicated that the supports became weakly acidic because of the formation of NiAl2O4 or MgAl2O4 spinel. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed the well dispersion of Pd particles on the Al2O3–NiO and Al2O3–MgO supports. It was concluded that the formation of the NiAl2O4 or MgAl2O4 spinel phase during preparation would be beneficial for the methane catalytic performance of Pd catalysts.

Effects of chelation reactions between metal alkoxide and acetylacetone on the preparation of MgAl2O4 powders by sol–gel process

Magnesium aluminate spinel (MgAl2O4) were synthesized via the sol–gel method, using magnesium aluminum double n-butoxide as precursors. The n-butoxide, prepared by the direct reaction of stoichiometric magnesium, aluminum and adequate n-butanol, was modified by acetylacetone as the chelating agent to control the speed of hydrolysis. Gel samples were identified as the mixture of Mg2Al(OH)7 and MgAl2(OH)8 phases, which converted to pure MgAl2O4 spinel phase at 800°C. Then dense spinel ceramics with 96% of relative density were prepared by sintering in air at 1600°C. Besides, the morphology of the powders and sintered ceramics has been investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM).

Influence of Oxide Nanoparticles of Fe, Al and Si on the Sintered Magnesia for the Production of Refractory Material to Be Used in Secondary Ladle Metallurgy

ABSTRACTIron oxide (Fe2O3, 20-40 nm), aluminum oxide (Al2O3, 50 nm) and silicon oxide (SiO2, 20-60 nm) nanoparticles were mixed in different concentrations (1 to 5 wt %) in a magnesium oxide matrix to develop new refractory matrixes as candidates in the lining of secondary ladle metallurgy. To avoid agglomeration of nanoparticles in the magnesium oxide (MgO) matrix, it was carried out a dispersion method of nanoparticles with different dispersants. After that, the powder mixture was sintered at a temperature of 1300 and 1500 °C for 4 hours. The refractory samples obtained were studied using X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray spectrometer (SEM-with EDX) and also measured their density and porosity. The results showed that the samples sintered at 1500 °C with 5 wt % of Fe2O3 reached the highest density and presented the MgFe2O4 spinel-type phase. With the addition of Al2O3-nanoparticles in the MgO matrix, there were the formation of MgAl2O4 spinel phase and in the case of SiO2-nanoparticles addition it was observed the formation of Mg2SiO4 forsterite phase. It is well known that with the increase in spinel phase in the matrix, there is a significant help to retain quantities of ions of iron and nickel due to the dissolution of the slag into the refractory material extending their lining life.

The effect of boron oxide on the crystallization behavior of MgAl2O4 spinel phase during the cooling of the CaO-SiO2-10 mass.% MgO-30 mass.%Al2O3 systems

The microstructural characteristics of the CaO-SiO2-B2O3-10 mass.% MgO-30 mass.% Al2O3 systems solidified during slow cooling from 1600 °C were investigated using SEM-EDS and a thermochemical computation package. The effect of boron oxide on the crystallization behavior of the spinel in the aluminosilicate system was observed because boron oxide is believed to become a potential flux to reduce the melting point of the liquid oxides. The primary crystalline phase was spinel, mainly MgAl2O4, irrespective of the boron content. The liquidus temperature TL continuously decreased as the boron oxide content increased, indicating that the boron oxide decreased the activity of the MgAl2O4 spinel phase in liquid melts at high temperatures. The size of the spinel crystals increased as the temperature range for the solid + liquid coexisting region, viz. the mushy zone, increased. In the present systems, because the TL continuously decreased with the increase in the boron oxide content, the viscosity of the liquid oxide may have affected the crystallization behavior of the spinel during cooling. Based on these results, an injection of a small amount of B2O3 flux into molten steel containing liquid aluminosilicate inclusions is not recommended because large spinel crystals can originate from the changes in the thermophysical properties of the liquid inclusions due to the incorporation of boron oxide into the aluminosilicate networks.

Coke study on MgO-promoted Ni/Al2O3 catalyst in combined H2O and CO2 reforming of methane for gas to liquid (GTL) process

MgO-promoted Ni/Al2O3 catalysts have been investigated with respect to catalytic activity and coke formation in combined steam and carbon dioxide reforming of methane (CSCRM) to develop a highly active and stable catalyst for gas to liquid (GTL) processes. Ni/Al2O3 catalysts were promoted through varying the MgO content by the incipient wetness method. X-ray diffraction (XRD), BET surface area, H2-temperature programmed reduction (TPR), H2-chemisorption and CO2-temperature programmed desorption (TPD) were used to observe the characteristics of the prepared catalysts. The coke formation and amount in used catalysts were examined by SEM and TGA, respectively. H2/CO ratio of 2 was achieved in CSCRM by controlling the feed H2O/CO2 ratio. The catalysts prepared with 20 wt.% MgO exhibit the highest catalytic performance and have high coke resistance in CSCRM. MgO promotion forms MgAl2O4 spinel phase, which is stable at high temperatures and effectively prevents coke formation by increasing the CO2 adsorption due to the increase in base strength on the surface of catalyst.

Reactions in the sintering of MgAl2O4 spinel doped with LiF

Abstract Reactions between MgAl2O4 and LiF during low-temperature sintering have been examined by transmission electron microscopy. It is shown that the presence of LiF at elevated temperatures leads to the formation of a defect MgAl2O4 spinel phase containing oxygen vacancies due to the incorporation of both Li and F ions into the crystal lattice. Consequently, densification is promoted via the diffusion of oxygen vacancies. Note that neither of the lithium aluminate phases, LiAlO2 and Li2Al4O7, were detected.

Synthesis and characterization of co-precipitated magnesium-aluminium hydroxides hydrogel containing minor phosphate additive

Coprecipitated Mg-Al hydroxides hydrogels in presence of minor phosphate additive were prepared in cold condition using different basic media viz. ammonium hydroxide and ammonium hydroxide-triethanolamine. DTA, TGA, particle size analyses and IR studies were performed for characterization of co-precipitates. Finer particle size was noticed for mixed hydroxides formed by using ammonium hydroxide-tirethanolamine as basic medium. Co-precipitates were also subjected to thermal treatment at different temperatures and corresponding crystalline phases developed were identified by XRD analysis. Thermal treatment led to the dehydroxylation and subsequent collapse of structure of Mg-Al hydroxides hydrogel and ultimately to the formation of MgAl2O4 spinel phase at temperature as low as 600degrees.

Chemical Bonding, Structure, and Morphology of Mg/∝-Al2O3 and MGO / ∝-Al2O3 Interfaces

AbstractUltra-thin films of Mg and MgO were grown on ∝-Al2O3 (1012) surfaces (r-cut sapphire) and studied using reflection high energy electron diffraction (RHEED) and x-ray photoelectron spectroscopy (XPS). When Mg is deposited at 30°C in ultra-high vacuum (UHV), the first monolayer of Mg atoms chemically bonds to the oxygen anions of the sapphire surface. At Mg coverages above a monolayer, a polycrystalline metallic Mg overlayer is formed. Annealing above 250°C in UHV causes the metallic Mg to desorb from the surface. However, annealing above 250°C in 10−6 torr O2 produces a polycrystalline MgO film. This MgO film recrystallizes after annealing in O2 at 900°C for 60 minutes and exhibits a crystallographic orientation of MgO (100) // ∝-Al2O3 (1012). RHEED indicates that the recrystallized MgO layer dewets the sapphire surface and forms islands. When Mg is deposited at 30°C in 10−6 torr O2, a polycrystalline MgO layer is created. This layer also becomes recrystallized and dewets the sapphire surface after extended annealing in O2 at 900°C. No evidence for a MgAl2O4 spinel phase was observed.

Interfaces and fracture surfaces in Saffil/Al-Mg-Cu metal-matrix composites

The interfacial phases were studied in pressure-cast Saffil/Al-4.5Cu-3Mg composite material using a variety of characterization techniques. The magnesium- and copper-rich phases were found to segregate near the fibre-matrix interfaces. The major phases were identified as MgAl2O4, Al2CuMg and CuAl2. Significant diffusion of silicon from the fibres into the matrix took place during the pressure-casting. A conclusion was drawn that solidification started with the less alloyed aluminium in the bulk of the matrix and proceeded towards the fibres. Auger and XPS analyses of composite materials fracturedin situ showed the fracture surface to lie within magnesium-rich (and not copper-rich) phases, most likely within the MgAl2O4 spinel phase. The fibre surface treatments which are being developed to improve interfacial wetting may also reverse the direction of crystallization and prevent formation of brittle phases in the vicinity of fibres, thereby improving the toughness of the composite materials.