Magnesium Aluminate Spinel Powder Research Articles

Effect of Yb2O3 Addition and Sintering Temperature on the Densification Behavior of Magnesium Aluminate Spinel Powder

The effect of Yb2O3 and sintering temperature on the densification behavior of magnesium aluminate spinel powder was studied. Results showed that Yb2O3 was beneficial to the sintering densification of MgAl2O4. When the content of Yb2O3 was 3 wt%, the relative density of magnesium aluminium spinel (MAS) was 98.54%, and the microstructure was more compact. The maximum hardness was 21.48 GPa, and the second phase Yb3Al5O12 appeared in the 3 wt% Yb2O3 samples. The mechanism of Yb2O3 in promoting the sintering densification of MAS is due to the appearance of Yb3Al5O12. When the sintering temperature is 1700°C, the relative density of MAS reaches a maximum of 98.54%, the hardness also reaches a maximum value of 21.48 GPa, and the grain size is more uniform. When the temperature exceeds 1700°C, the relative density and hardness of MAS decrease. The best sintering aid content and temperature synergistically worked to promote the densification and increased the hardness of MAS.

Transparent LiOH-doped magnesium aluminate spinel produced by spark plasma sintering: Effects of heating rate and dopant concentration

The effects of LiOH doping of magnesium aluminate spinel powders and various Spark Plasma Sintering (SPS) schedules on densification behavior and final transparency of polycrystalline magnesium aluminate spinel were studied. Two commercial magnesium aluminate spinel powders, with different specific surface areas, were doped with up to 0.6 wt% of LiOH and consolidated using SPS with slow (2.75 °C/min) and fast (100 °C/min) heating rates. The slow heating rate was optimal for undoped magnesium aluminate spinel (LiOH-free) with the best real in-line transmittance (RIT) of 84.8% (measured at 633 nm on a disc 0.8 mm thick). For the magnesium aluminate spinel doped with 0.3 wt% of LiOH, the fast heating rate was beneficial, and an RIT of 76.5% was achieved. μ-Raman analysis confirmed that the addition of LiOH suppressed carbon contamination.

Comparative Study on Compaction and Sintering Behavior of Spray and Freeze Granulated Magnesium Aluminate Spinel Powder

Commercially procured magnesium aluminate (MgAl2O4) spinel powder with average particle size of 300 nm and irregular morphology were spray and freeze granulated. The granules were characterized for the texture, strength, granule size and its distribution. In order to evaluate the flowability of granules, rheological parameters, such as Carr index and Hausner ratio were measured. The granules were further subjected to quasi-static compaction under loading, unloading conditions and compaction curves were constructed. Compaction coefficients thus obtained were correlated with granulation conditions and other parameters including green densities. Compacted green samples were also subjected to flexural strength measurements following ASTM standards and fractographs were observed under FESEM. A Carr index of 13.7 and Hausner ratio of 1.16 were obtained for freeze dried granules indicating the superiority in flow behavior as compared to its spray granulated counterpart, thereby showing a better compaction behavior: compaction coefficient of 5.9 in combination with higher true and predicted green density values and lower granule strength of 1.22 MPa. The green samples exhibited a flexural strength of 11 and 12.5 MPa and compressive strength of 2.44 and 1.22 MPa for spray and freeze granulated sample, respectively. The above observations were also complemented by the fractographs.

Direct 3D printing of transparent magnesium aluminate spinel ceramics

Transparent spinel ceramics were additively manufactured by laser direct deposition, a blown powder additive manufacturing (AM) process. With a laser melt-growth process, the need for powder binders and postprocessing procedures was eliminated. Transparent spinel ceramic samples were directly fabricated from micrometer-size magnesium aluminate spinel powders. The optical transparency of the printed spinel samples was mainly affected by residual porosity and cracking. Among other processing conditions, powder flow rate and laser power showed the most significant effects. An obvious transition from opaqueness to transparency was observed after reducing the powder flow rate below 0.1 g/min. The optical transmittance, morphology, and phase composition of the printed spinel samples were systematically investigated. Microstructural aspects including porosity, cracking, and grain size were also characterized. A maximum optical transmittance of 82% was obtained at a wavelength of 632.8 nm, which was nearly comparable to that of their traditionally sintered counterparts. As the laser power increased, residual porosity steadily decreased, with a minimum porosity of 0.2% achieved. Meanwhile, the total crack length obviously increased, while a relatively smaller variation of the average crack length was observed. The measured mechanical properties, including hardness and fracture toughness, were found to be nearly comparable to those of their sintered counterparts. These well demonstrated the potential of the proposed laser AM method in the direct fabrication of transparent ceramics.

Studies on correlation of surface properties, colloidal shaping and transparency of magnesium aluminate spinel powder

Aqueous colloidal processing of magnesium aluminate spinel (MAS) is cumbersome due to its basic nature which induces hydrolysis leading to poor solid loading. In order to address this issue, an attempt has been made to develop a simple and effective thermal passivation process by calcining the powder in the temperature ranging from 1100 to 1350 °C without affecting sintering kinetics. Powder under uncalcined (MAS-UC) and calcined (MAS-C) conditions were characterized for its physico-chemical properties and slurries were prepared using these powders in aqueous medium. The rheological behaviour and zeta potential of the slurries were correlated with optimum dispersant concentration. The solid loading of the slurry could be increase by 16% as a result of thermal passivation obtaining a maximum of 65 wt% for MAS-C. The rate of cast formation is found to be higher with MAS-C1350 with the maximum green density of 53% theoretical density (TD). Further, the pressure-less sintering temperature was optimized based on the dilatometric studies and a sintered density of 3.54 g/cm3 (99% TD), which, on hot isostatic pressing, resulted in ~100% TD and after optical polishing sample exhibited close to theoretical transmission of 80–86% in the 3–5 μm wavelength. Thermal passivation is found to be a simple and effective process for MAS powder processing to transparent ceramic parts, as demonstrated in the current study.

СПОСОБИ ОТРИМАННЯ ПОРОШКІВ АЛЮМОМАГНЕЗІАЛЬНОЇ ШПІНЕЛІ.ОГЛЯД

The main methods of synthesis of magnesium-aluminate spinel powder their features and advantages were analyzed. The character of the physicochemical interaction of components in theMgO–Al2O3 system physical and crystallographic characteristics and structural parameters of thecrystalline lattice of magnesium-aluminate spinel wereresearched. The synthesis conditions arepresented

THE EFFECT OF Mg-Al Wt% FOR PHASE FORMATION OF SPINEL MgAl2O4 PRODUCED BY METAL DISSOLVED METHOD

<p class="Abstract">Synthesis of magnesium aluminate spinel powder (MgAl<sub>2</sub>O<sub>4</sub>, abbreviated as MA) were prepared by the liquid mixing method. The synthesis of MA involved Mg powders with various weight compositions (4.8; 10; 20; 30; 40; and 60%) and Al powders (95.2; 90; 80; 70; 60 and 40%) as the raw materials, which were independently dissolved in 37% HCl to form MgCl<sub>2</sub> and AlCl<sub>3</sub> solutions. Both solutions were then mixed and stirred for 5 hours and dried to a temperature of about 100-105°C to produce powders with different weight compositions. Each powder resulted from drying was characterized using DTA-TGA, and then calcined at 650 °C; 750 °C and 850 °C for 1 hour. The calcined powder was characterized by XRD to qualitative and quantitative analyses using Rietica. It was found that MA samples contained only MgAl<sub>2</sub>O<sub>4</sub> and MgO as the impurity phase. The relative weight fraction of MgAl<sub>2</sub>O<sub>4</sub> increased up to 99% for 95.2 wt% Al. Using an extrapolative approach to determine the Mg-to-Al composition, nearly pure MA, as high as 99%, was achieved at 95.2% Al and 4.8% Mg.</p>

Synthesis of magnesium aluminate spinel nanocrystallites by co-precipitation as function of pH and temperature

Magnesium aluminate (MgAl2O4) spinel nanocrystallites are prepared through nitrate route with liquor ammonia as a precipitant by co-precipitation process. By varying the solution pH (8–10) and bath temperature (10–40°C), the precipitated gel materials are obtained. Yield of each batch is determined by firing at 1000°C with 2 hours (h) soaking. The yield is maximum at 30°C and pH 9.2. Chemical analysis indicates near stoichiometric spinel at pH 9.2. Below pH 9.2, Al2O3:MgO molar ratio shifts to alumina side; beyond pH 9.2, it is towards magnesia side. Stoichiometric spinel-forming precursor material is characterized by various techniques. X-ray diffraction (XRD) analysis indicates spinel formation at 600°C and its crystallinity increases with rise of temperature. Hydroxyl group and absorbed water in gel substrate disappear with variation of temperature as observed by Fourier transform infrared (FTIR). Only one exothermic response of the precursor of spinel formation is demonstrated through differential scanning calorimetry (DSC). Studies reveal formation of near stoichiometric magnesium aluminate spinel powder having average crystalline size in the range 15–38 nm. HR-transmission electron microscopy studies also confirm this nano-size crystallite formation.

Effect of cation ratio on microstructure and optical absorbance of magnesium aluminate spinel

This work is pertaining to the synthesis of fine magnesium aluminate spinel (MgAl2O4) powders of varied trivalent:bivalent cation ratio along line of homogeneity of the solid solution (MgO. xAl2O3, x = 1, 1.25, 1.50, 1.75, and 2) via gel combustion method. Magnesium- and aluminum- nitrate were used as the oxidants in combustion reaction fuelled by urea in combination with stoichiometric formaldehyde solution acting as reductant. Synthesized powders were characterized in terms of microscopic analysis and optical absorbance measurements. The cation ratio, through a change in gel structure influences the nature of crystallization of the product, while on the other hand does not affect grain shapes and sizes. Distinct enhancement in both absorption intensity and the corresponding estimated energy band gap has been observed against increasing excess than stiochiometric alumina concentrations. Evaluated optical band gaps were widened in proportion to the Al: Mg ratio which may be attributed to Burstein-Moss effect in consequence of substitutional insertion of introduced Al3+ ions in spinel lattices.

Study on Sinterability of Magnesium Aluminate Spinel Powders Prepared by Different Technologies

In this paper, the phase, structure and sinterability of magnesium aluminate spinel powders prepared by the chemical co-precipitation, low temperature combustion and solid phase reaction methods were studied. The results show that the uniform nanometer magnesium aluminate spinel powders are prepared from precursor via the chemical co-precipitation, calcined at 700°C. Using the low temperature combustion method, the agglomeration, complete crystalline nanometer magnesium aluminate spinel powders are prepared at the thermal explosion temperature of 400°C. For the magnesium aluminate spinel micrometre powders synthesized by the solid phase reaction method, the phases are mainly spinel with a little of periclase and corundum phase. The sinterability of powders prepared by chemical co-precipitation method is the best, but that of powders synthesized by low temperature combustion and solid phase reaction methods are poor.

Synthesis of Magnesium Aluminate Spinel Powder from the Purified Sodium Hydroxide Leaching Solution of Black Dross

Synthesis of magnesium aluminate spinel (MgAl2O4) was investigated by employing ball milling and co-precipitation methods. The starting materials (aluminum hydroxides) were obtained from the purified sodium hydroxide leaching solution of black dross. The characteristics of the synthesized spinel was analyzed through X-ray diffraction (XRD), scanning electron microscopy (SEM) images. In this work, the effect of calcination temperature and time on the formation of spinel by the two methods was compared. Calcination temperature showed a great effect on the formation of spinel in both methods. The results showed that the co-precipitation method has many advantages over the ball milling method. In ball milling method, complete conversion of the starting materials to spinel was impossible even at 1500 °C, while complete conversion to spinel was accomplished at 1000 °C for 5 h by the co-precipitation method. The average size of the spinel synthesized at these optimum conditions of the co-precipitation method was about 17 nm. A process can be developed to synthesize spinel from the black dross which is regarded as hazardous materials.

Impact of high energy ball milling on densification behaviour of magnesium aluminate spinel evaluated by master sintering curve and constant rate of heating approach

The present study investigates the influences of high energy ball-milling on densification of a commercial sub-micron magnesium aluminate spinel powder. High energy milling decreases the crystallite size and modifies the particle size distribution of powder resulting in better compaction of green bodies. Milling, also, increases the number of structural defects.The concept of master sintering curve (MSC) and the constant rate of heating (CRH) approach were employed to evaluate the activation energy of sintering. It was demonstrated that, according to MSC, the milled samples exhibit lower densification activation energy compared to the as-received powder (530 kJ mol−1 vs 750 kJ mol−1). Analysing the densification using CRH approach reveals that the sintering activation energy of milled samples remains constant during the sintering process, while that of as-received samples increases with densification. The estimated activation energy using MSC method is thus an average value of the values measured over the whole process of sintering.

Tailorable nano MgO.xAl2O3 spinel compositions along line of homogeneity via gel combustion route: Influence on phase evolution

Very fine sized (∼5–15 nm) magnesium aluminate spinel (MgO.xAl2O3) powder of different degree of solid solubility (x = 1, 1.25, 1.50, 1.75, and 2) have been attempted for synthesis following a gel combustion method to meet its high demand in the fields of refractory, photo catalysis, sensor and strategic applications, etc. Nitrates of magnesium and aluminum were used as oxidants for exothermic reactions fuelled by urea added with formaldehyde solution acting as reductant. Thermal decomposition reactions of polymer gels resulting in phases containing varied cationic ratio (Mg2+:Al3+) were investigated through DTA-TGA studies. Phase identification of synthesized powders derived from varying mole ratio of metal nitrates was performed by X-ray diffractometry (XRD) through Rietveld refinement using HighScore Plus software and was reconfirmed by Fourier Transform Infrared Spectroscopy (FTIR). Particle morphology was examined under Field Emission Scanning Electron Microscope (FESEM) & Transmission Electron Microscope (TEM). XRD pattern revealed the as-synthesized stoichiometric magnesium aluminate spinel (MgAl2O4) powder to be in amorphous state. In contrast, increasing trends in the extent of crystallinity in the as-synthesized form was obvious for alumina richer compositions. Dependence of development of crystallinity was explained in the light of thermal stability of polymer containing varied valent cations. Unlike crystallinity, a reverse trend is observed in case of cell parameter of the as-synthesized different non-stoichiometric spinel varieties where lattice parameter decreased with increasing alumina concentrations following Vegard's law. Variation in cell dimension against alumina concentration is caused by the insertion of smaller substituting Al3+ ions (0.054 nm) for Mg2+ ions (0.072 nm) in the unit cell. TEM observation reveals that the synthesized powder is aggregate of fine nano crystals. Crystallites were larger with increasing alumina concentration. An initial sintering behavior of the powder synthesized at 1400 °C has also been reported. Initial sintering trials were attempted under spark plasma sintering process.

Morphology and luminescence of MgAl2O4 ceramics obtained via spark plasma sintering

Spark plasma sintering cycles were applied to a commercial magnesium aluminate spinel powder to evaluate its suitability for producing small-grained optical ceramics. Translucent stoichiometric samples with a crystalline grain size as small as ∼0.5 μm were obtained. Structural, microstructural (XRD and SEM), and correlated spectroscopic (transmission and cathodoluminescence) analyses were used to further investigate the defect structure and inversion degree of the material. We argue that the transparency is closely related to F and F+ centers, which can be distinguished in the absorption and luminescence spectra of the stoichiometric compositions.

A novel technique of gel-casting for producing dense ceramics of spinel (MgAl2O4)

In this study, magnesium aluminate spinel (MgAl2O4) powder was synthesized at the temperature of 1300 °C with an average particle size of 90 nm and 99.99% purity. 0.8 wt% LiF was added as a sintering aid. XRD, BET, ICP, FESEM and FTIR analyses used for characterization of prepared powder. In order to investigate the effect of de-airing and homogenization of suspension on Gel-casted samples, three methods of Shahbaz machine, Ultrasonic, and Ball-mill were used. The green and final body (after sintering at 1600 °C for 1 h) relative densities of samples were 67, 63, 58% and 97, 94, and 92%, respectively. Furthermore, the average pore size of the samples was 25, 35 and 125 nm, respectively. The strength of samples before and after sintering was 13.32, 12.16, 7.78 and 95.41, 92.83, 84.21 MPa, respectively. It was illustrated that the presence of entrapped gases results in the reduction of green and final body density as well as the strength of the samples.

Soild-phase synthesis of MgAl2O4 powder in reducing atmosphere: effects of alumina sources and addition of carbon black

Magnesium aluminate spinel powder was prepared by one-step carbon-bed sintering at 1400 °C for 4 h with light magnesium oxide as magnesia source, alumina/boehmite/alumina-sol as alumina source, respectively. The phase composition of synthesized powders was characterized by X_ray diffraction, and the microstructure was depicted by field-emission scanning electron microscopy. The effects of alumina type and carbon black (CB) on the phase composition and morphology of synthesized powders were studied. The results show that the type of alumina source has remarkable influence on the morphology of spinel powder. The average particle sizes of the product with both alumina and boehmite as alumina source are about 1.5 μm. However, the shape of the former is cobblestone while that of the latter is regular rhombic plate. Additionally, the morphology of boehmite was preserved in its corresponding product, implying that the spinel grows by template mechanism. The average particle sizes of the product with alumina-sol as alumina source were about 0.6–2.1 μm, and it possesses a regular octahedral structure. CB can be distributed more uniformly in the powder when polyvinylpyrrolidone (PVP) solution was used as a dispersant. The uniformly dispersed CB could significantly decrease the grain size of the product when alumina-sol was used as alumina source, but it had little effect on the particle size of products with alumina or boehmite as alumina source.

The Effect of Synthesis Temperature and Gallium Oxide Additive on the Structure and Phase Composition of Magnesium Aluminate Spinel Powder

The possibility of producing optically transparent ceramics from magnesium aluminate spinel (MAS) is considered. The conditions of preparing the precursor powder and the MAS powder were investigated along with the effect of synthesis temperature and gallium oxide additive on the properties of the powder.

Influence of magnesium aluminate spinel powder content on cement clinker corrosion and adherence properties of lightweight periclase-spinel refractories

The effect of the amount of magnesium aluminate spinel powder (MASP) in lightweight periclase-spinel refractories on the cement clinker corrosion and adherence properties was investigated by the static crucible test and sandwich test. The results showed that MASP addition significantly affected the cement clinker corrosion resistance and adherence properties although it had little effect on the apparent porosity and pore size of refractories. The spinel in the refractories reacted with the cement clinker rather than periclase to generate the glass phase. Its amount and viscosity increased from 0 to 13wt% MASP addition in refractories, which brought the benefit to the adherence strength. However, the excessive glass phase deteriorated the corrosion and penetration resistances of refractories as MASP was added up to 19wt%. So, the addition amount from 9 to 13wt% MASP into periclase-spinel refractories balanced the cement clinker resistance and adherence strength.

Synthesis and characterization of porous magnesium aluminate spinel by hydrothermal process

Magnesium aluminate spinel has a great prospect in catalyst supports due to porous structure, good catalytic activity, high thermal stability and two active centers as acid and alkaline. The magnesium aluminate spinel powders synthesized by hydrothermal process were characterized by scanning electron microscope, transmission electron microscope, X-ray diffraction and N2 adsorption-desorption isotherms. The particle surface is made up of laminated structure and it grows bigger with the increase of hydrothermal temperature and hydrothermal time. The spinel has wormlike porous structures and these structures grow smaller and better distributed at Hexadecyl trimethyl ammonium Bromide (CTAB) condition. The spinel phase emerges at 450 °C and the perfect crystalline structure emerges at 600 °C. The percent of MgAl2O4 spinel goes up with the increase of heat treatment temperature. The spinel has a large BET surface area and its special surface area diminishes continuously from 245.68 m2/g to 58.65 m2/g when the calcinating temperature increases from 500 °C to 1200 °C. Moreover, it reaches the maximum surface area of 195.1 m2/g with the addition of 1% CTAB according to BET analysis, yet excess of it has an adverse effect.

Effect of annealing at different time intervals on the structure, morphology and luminescent properties of MgAl2O4:0.3% In3+ nanophosphor prepared by citrate sol-gel method

Indium-doped magnesium aluminate (MgAl2O4:03% In3+) spinel powders were prepared by citrate sol-gel method. The effect of annealing period (AP) at a fixed annealing temperature (800 °C) and dopant concentration (0.3% In3+) on the structure, particle morphology and photoluminescent properties of the powders were investigated. The X-ray powder diffraction (XRD) results showed that the crystalline quality and crystallite sizes of the powders were influenced by the AP. Doping with 0.3% In3+ did not influence the major crystal structure of the MgAl2O4 (host) material. The scanning electron microscope (SEM) image suggested that the AP influences the particle morphology of the nanophosphor. Transmission electron microscopy (TEM) image suggested that the crystallites sizes were in the region of 10 nm. The Photoluminescence (PL) results showed that the violet emission at 388 nm and green emission at 560 nm both originated from the host material and there was no evidence of any emission from the dopant (In3+). Increasing the AP up to 2 h lead to luminescence enhancement, while further increase in AP lead to luminescence quenching. The Commision Internationale de l’Eclairage (CIE) co-ordinates results showed that the emission colour of all samples was in the blue region and it was not tuned by varying the AP.