Stoichiometric MgAl2O4 Spinel Research Articles

Synthesis of rGO-reinforced MgAl2O4 spinel composites by solid-state powder route mechanism: A comparative analysis between non-reinforced and rGO-reinforced spinel composites

Reduced Graphene Oxide (rGO)-reinforced MgAl2O4 spinel composite was successfully synthesized by a solid-state powder route mechanism via high-energy planetary ball milling of corresponding metal oxide precursors followed by a high-temperature single-stage sintering process. The effect of rGO on the homogeneity, phase development, morphology, chemical composition, rate of formation of spinel, and cation anti-site formation of the composites was methodically investigated. For comparison, a stoichiometric non-reinforced MgAl2O4 spinel was also synthesized. The addition of 0.2 wt% and 0.4 wt% of rGO as reinforcement increased the homogeneity of the ball-milled composite powder, with Polydispersity Index (PDI) values of 0.19 and 0.15 respectively, as compared to 0.24 obtained in non-reinforced spinel composite. The X-ray diffraction pattern and IR spectrum of 1650 °C sintered composites confirmed the formation of highly crystalline phases with spinel structure. The microstructural analysis revealed that the addition of rGO led to a reduction in porosity and improved the process of sintering. Calorimetric studies showed that spinelization started to occur at 858 °C and 880 °C for 0.2 wt% and 0.4 wt% rGO-reinforced composites respectively, which was faster than non-reinforced composite. 27Al solid-sate nuclear magnetic resonance (SS-NMR) analysis depicted that ceramic composite reinforced with rGO exhibited lesser inversion parameter of 0.09 (for 0.2 wt% rGO) and 0.055 (for 0.4 wt% rGO), as compared to 0.145 obtained in non-reinforced composite, indicating lesser cation anti-sites formation in rGO-reinforced spinel composite.

Atomic-scale structure and non-stoichiometry of meteoritic hibonite: A transmission electron microscope study

Abstract Hibonite (CaAl12O19) is a common refractory mineral in Ca-Al-rich inclusions (CAIs) in primitive meteorites. Transmission electron microscope (TEM) studies have identified enigmatic planar defects in different occurrences of hibonite in the Allende meteorite that give rise to strong streaking along c* in electron diffraction patterns. Atomic resolution high-angle annular dark-field (HAADF) imaging and energy-dispersive X-ray (EDX) analyses were used to determine the nature and origin of these planar features. HAADF images of hibonite grains reveal lamellar intergrowths of common 1.6 nm spacing, and less commonly 2.0 and 2.5 nm spacings, interspersed in stoichiometric hibonite showing 1.1 nm (002) spacing. Stoichiometric hibonite consists of alternating Ca-containing (“R”) and spinel-structured (“S”) blocks stacked in a sequence RS. In contrast, the 1.6 nm layers result from a doubled S block such that the stacking sequence is RSS, while in the widest defect observed, the stacking sequence is RSSSS. These intergrowths are epitaxial and have coherent, low-strain boundaries with the host hibonite Meteoritic hibonite shows common Ti and Mg substitution for Al in its structure. Atomic-resolution EDX maps of hibonite grains in the Allende CAI confirm the preferred site occupancy of Mg on tetragonal M3 sites in S blocks and of Ti on trigonal bipyramidal M2 and octahedral M4 sites in R blocks. Mg is highly concentrated, but Ti is absent in the planar defects where wider S blocks show Al-rich compositions compared to stoichiometric MgAl2O4 spinel. Therefore, Mg likely played the major role in the formation and metastability of planar defects in hibonite. Electron energy loss spectroscopy data from the Ti L2,3 edge show the presence of mixed Ti oxidation states with ~15–20% of Ti as Ti3+ in hibonite, suggesting a direct substitution of Ti3+ ↔ Al3+ in hibonite. The remaining ~80–85% of Ti is present as Ti4+ and corresponding EDX analyses are consistent with the well-known coupled substitution 2Al3+ ↔ Ti4+ + Mg2+ being the major mechanism for Ti and Mg substitution in hibonite. The formation of planar defects in hibonite occurred during high-temperature nebular condensation or melting/crystallization processes. The occurrence of non-stoichiometric hibonite in the Allende CAI deviates from the mineral formation sequence predicted from equilibrium condensation models. Overall, our atomic resolution TEM observations signify non-equilibrium, kinetic-controlled crystal growth during the high-temperature formation of refractory solids in the early solar nebula.

Distinctive features of diffusion-controlled radiation defect recombination in stoichiometric magnesium aluminate spinel single crystals and transparent polycrystalline ceramics

MgAl2O4 spinel is important optical material for harsh radiation environment and other important applications. The kinetics of thermal annealing of the basic electron (F, F+) and hole (V) centers in stoichiometric MgAl2O4 spinel irradiated by fast neutrons and protons is analyzed in terms of diffusion-controlled bimolecular reactions. Properties of MgAl2O4 single crystals and optical polycrystalline ceramics are compared. It is demonstrated that both transparent ceramics and single crystals, as well as different types of irradiation show qualitatively similar kinetics, but the effective migration energy Ea and pre-exponent D0 are strongly correlated. Such correlation is discussed in terms of the so-called Meyer-Neldel rule known in chemical kinetics of condensed matter. The results for the irradiated spinel are compared with those for sapphire, MgO and other radiation-resistant materials.

Mining tailings as a raw material for glass-bonded thermally sprayed ceramic coatings: Microstructure and properties

Magnesium aluminate, MgAl2O4, spinel powders for thermal spraying, were synthesized from secondary raw materials by spray drying and subsequent reaction sintering. Talc ore mining tailings and aluminium hydroxide precipitate from aluminium anodizing process were studied. A stoichiometric MgAl2O4 spinel coating was prepared as a reference using pure raw materials. Atmospheric plasma spraying resulted in the formation of ceramic coatings. Microstructural investigations revealed that the reference coatings exhibited crystalline lamellar microstructure of MgAl2O4 but secondary coatings contained amorphous areas between the crystalline MgAl2O4 clusters. Abrasive wear test results revealed considerably lower wear rate for secondary coatings. It is suggested that the different structure of coatings, particularly the high degree of amorphous phase between the isolated crystalline MgAl2O4 clusters caused the higher abrasive wear resistance by changing the wear mechanism. The dielectric breakdown strength of the secondary coatings were at the same level, 24 V/μm, as compared to reference coating, 23 V/μm.

Enhancement of MgAl2O4 spinel formation from coprecipitated precursor by powder processing

Although low temperature fast coprecipitation technique has been used to synthesize stoichiometric (MgO-nAl2O3, n = 1) MgAl2O4 spinel forming precursor, delayed spinellization has always been the concern in this process. In this article, the precursor of this ‘fast technique’ has been used for bulk production by further processing by high speed mixing with solvents and mechanical activation by attrition milling in terms of superior spinellization. At 1000°C, MgAl2O4 — γ-Al2O3 solid solution and MgO phases are formed (spinel formed by 1000°C is regarded as primary spinel). At higher temperatures, due to large agglomerate size, MgO can not properly interact with the exsolved α-Al2O3 from spinel solid solution to form secondary spinel; and consequently spinellization gets affected. Solvent treatment and attrition milling of the coprecipitated precursor disintegrate the larger agglomerates into smaller size (effect is more in attrition). Then MgO comes in proper contact with exsolved alumina, and therefore total spinel formation (primary + secondary) is enhanced. Extent of spinellization, for processed calcined samples where some alumina exists as solid solution with spinel, can be determined from the percentage conversion of MgO. Analysis of the processed powders suggests that the 4 h attrited precursor is most effective in terms of nano size (< 25 nm) stoichiometric spinel crystallite formation at ≤ 1100°C.

Synthesis of Nano Crystalline MgAl&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;4&lt;/SUB&gt; Spinel Powder by Microwave Assisted Combustion

Stoichiometric MgAl2O4 spinel nanoparticles were synthesized by microwave assisted combustion reaction from aluminium nitrate nanohydrate (Al(NO3)3·9H2O) and Sol-Gel prepared magnesium hydroxide (Mg(OH)2) in the presence of urea ((NH2)2CO) as a fuel, in about 20 min of irradiation. X-ray diffraction (XRD) studies reveal that mi- crowave assisted combustion synthesis route yields single-phase spinel nanoparticles with larger crystalline size (around 75 nm) than other conventional heating methods. Scanning electronic microscope (SEM) images show nanoparticles with spherical shape and homogenous morphology. The surface area measurements (SBET) show crystals with 2.11 m 2 /g and 0.0033 mL/g pore volume.

Aqueous slip casting and hydrolysis assisted solidification of MgAl2O4 spinel ceramics

This paper reports on synthesis of MgAl2O4 spinel (MAS) powders with six different chemical compositions and the consolidation of the synthesised MAS powders following an aqueous slip casting and hydrolysis assisted solidification (HAS) routes. The synthesised MAS powders were surface passivated against hydrolysis before being dispersed in distilled water to obtain suspensions with 41–45 vol.‐% solid loading using suitable dispersing agents. In the case of the HAS process, the consolidation of suspensions was achieved in non‐porous moulds under ambient conditions by the incorporation of AlN equivalent to 1–5 wt‐%Al2O3 into the suspension. The stoichiometric MAS powder consolidated by slip casting and dry pressing routes was sintered along with those consolidated by HAS route at 1550–1650°C for 1 h. Various characterisation techniques were utilised to evaluate the effect of composition and consolidation technique on slurry characteristics and sintered properties of MAS ceramics.

The effect of non-stoichiometry on the high-temperature behaviour of MgAl2O4 spinel

AbstractThe effect of cation vacancies upon the thermal expansion and crystal structure of a synthetic defect spinel with composition Mg0.4Al2.4☐0.2O4 was investigated by X-ray diffraction, in situ, at temperatures up to 1273 K. No evidence of symmetry violations from the Fd3m evenat the highest temperature were noted. The volume thermal expansion is markedly less than that of stoichiometric MgAl2O4 spinel, regardless of the degree of inversion. The u oxygen atomic coordinate remains constant throughout the temperature range investigated, with the M–O an dT–O bond lengths showing identical rates of expansivities. An analysis of the evolution of polyhedral volumes with temperature indicates that at 1273 K the octahedron inflates by 0.099 Å3 and the tetrahedron by 0.056 Å3. The expansion of the octahedron is significantly greater than in stoichiometric MgAl2O4 spinel, whereas the expansion of the tetrahedron is similar. The results demonstrate that an excess of Al in the spinel structure accompanied by the formation of cation vacancies strongly affect an important thermodynamic property, in this case, thermal expansion. Such an effect must be considered for those phases stable inthe Earth’s mantle where 4–5 wt.% Al2O3 is thought to be present.

Gelcasting of Magnesium Aluminate Spinel Powder

A stoichiometric MgAl2O4 spinel (MAS) powder was processed in aqueous media and consolidated by gelcasting from suspensions containing 41–45 vol% solids loading. The MAS powder was first obtained by heat treating a compacted mixture of α‐Al2O3 and calcined caustic MgO at 1400°C for 1 h, followed by crushing and milling. Then, its surface was passivated against hydrolysis using an ethanol solution of H3PO4 and Al(H2PO4)3. The as‐treated surface MAS powder could then be dispersed in water using tetra methyl ammonium hydroxide and an ammonium salt of poly‐acrylic acid (Duramax D‐3005) as dispersing agents. The as‐obtained stable suspensions were gelcast, dried, and sintered at 1650°C for 1–3 h. For comparison purposes, the treated powder was also compacted by die pressing of freeze‐dried granules and sintered along with gelcast samples. Near‐net‐shape MAS components with 99.55% of the theoretical density could be fabricated by aqueous gelcasting upon sintering at 1650°C for 3 h. The MAS ceramics fabricated by gelcasting and die pressing exhibited comparable properties.

Effect of fuel type on morphology and reactivity of combustion synthesised MgAl2O4powders

AbstractTwo 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 ...

Cation ordering and structural variations with temperature in MgAl2O4spinel: An X-ray single-crystal study

The equilibrium distribution of Mg and Al between the tetrahedral and octahedral sites of a flux grown, stoichiometric MgAl2O4 spinel was investigated between 600 and 1100 °C by single-crystal X-ray diffraction of quenched samples. The cation distribution for both ordering and disordering runs was obtained by minimizing accurate crystallographic parameters and effective ionic radii. Along with the variation of the degree of inversion from 0.18 to 0.29 between 600 and 1100 °C, both unit cell and oxygen positional parameter decreased linearly. Multiple non-linear least-squares fit of our data with the thermodynamic model of O’Neill and Navrotsky (1983) gave α = 23 ± 2 KJ/mol and β = 13 ± 4 KJ/mol. The influence of both cation inversion and thermal expansion on T-O and M-O bond length variation was determined by comparison of our data with previous in situ studies. In the thermal interval investigated, the inversion accounts for change of −0.014 A for T-O and +0.007 A for M-O. Mean linear polyhedral thermal expansion coefficients of 6.5 × 10−6 °C−1 and 8.9 × 10−6 °C−1 were calculated for T and M sites, respectively.

Epitaxial growth of Pb(Zr,Ti)O3 films on MgAl2O4 by pulsed laser deposition

Stoichiometric MgAl2O4 spinel single crystals were used as the substrates for deposition of lead zirconate titanate (PZT) thin films because of their excellent lattice matching. Epitaxial growth of (001) PZT films with a perovskite structure was achieved by means of the pulsed laser deposition technique but there exists a stringent restriction on the substrate temperature and oxygen pressure. PZT films could also be grown epitaxially on spinel substrates with an odd cut surface indicating that other growth directions can be obtained if the substrates are prepared with suitable orientation.

Knoop Microhardness Anisotropy of Single‐Crystal Stoichiometric MgAl2O4 Spinel

Microhardness anisotropy profiles for the (100) and (111) planes of single‐crystal stoichiometric MgAl2O., spinel were determined at room temperaturé. The (100) microhardness profile has ahardness maximum in tiie [001] and a minimum in the [O11], which supports the previous suggestion that the primary slip system is the {111}〈11¯0〉. The microhardness of the (111) plane is independent of indenter orientation, also consistent, with a {111}〈11¯0〉 primary slip system. It is concluded that these microhardness profiles are in accord with other experimental observations that the {111}〈11¯0〉 is the primary slip system in stoichiometric MgAl2O4 spinel.

Magnesium depletion in spinel during H.V.E.M. irradiations

Abstract The magnesium content of stoichiometric MgAl2O4 spinel is found to decrease progressively with electron dose during irradiation at elevated temperatures in a hgh voltage electron nucroscope (HVEM). The rate of decrease vanes with the electron beam energy. Diffraction patterns from the irradiated regions suggest that the end product is cubic γ-alumina rather than the equilibrium trigonal α-alumina. Additional reflections present in the patterns indicate a long-range ordering of the aluminium ions consistent with a true lattice parameter of 23.70 A, a three-fold increase over the tabulated value of 7.90 A for γ-alumina. A mechanism for the (spinel← γ-alumina) transformation is proposed. Microcracking is observed in some irradiated regions as a consequence of the 6.5% volume contraction associated with the transformation.

Dislocation dissociation in low-angle spinel boundaries

It is well known that in the spinel structure a perfect dislocation can dissociate into two partial dislocations with collinear Burgers vectors :(1)The two partial dislocations, which have the same character as the perfect dislocation, are separated by a stacking fault in the cation sublattice only. In ceramic materials, unlike fcc metals, dissociation of dislocations can occur by a climb mechanism in addition to, or instead of, dissociation by glide. Climb dissociation of perfect dislocations into two or more partials with collinear Burgers vectors has been observed in e.g. yttrium iron garnet and sapphire. Previous studies in spinels, deformed at high temperatures, emphasize climb dissociation and dissociation of mixed character. Dissociation of screw dislocations in stoichiometric MgAl2O4 spinel could not be resolved and hence no experimental measurements of the dissociation width have been reported.

Fracture of single crystal MgAl2O4

The fracture of single crystal, stoichiometric MgAl2O4 spinel, was investigated for the (1 0 0), (1 1 0), and (1 1 1) from room temperature to 1500° C. Two regions of fracture behaviour were observed; a low temperature elastic region where K lc decreased with increasing temperature, and an elevated temperature region where K lc increased rapidly with increasing temperature. The elastic region is explained primarily by the decrease of elastic modulus with increasing temperature, whereas the rapid increase of K lc at elevated temperature is attributed to plastic flow in the vicinity of the crack tip

Deformation in spinel

Stoichiometric MgAl2O4 spinel was deformed in compression at temperatures from 1790 to 1895‡ C and the dislocation structures analysed by transmission electron microscopy. {1 1 1}〈1 1 0〉 slip was observed on both the primary and cross-slip systems, and there was much secondary slip as well; all six 〈1 1 0〉 Burgers vectors were present in electron micrographs. This secondary slip leads to very high work-hardening rates, approximatelyΜ/70 at 1790‡ C, whereΜ is the shear modulus. Since it is known that deformation in nonstoichiometric (alumina-rich) spinel crystals occurs by {1 1 0}〈1 1 0〉 slip, the electrostatic and geometric aspects of 1/4 〈110〉 dislocations moving on {1 1 1} and {1 1 0} planes are considered in some detail. It is porposed that the octahedral cation vacancies present in non-stoichiometric spinel diffuse to dislocations during deformation and thus favour {1 1 0} slip.

X-ray topographic analysis of dislocations in Czochralski-grown stoichiometric MgAl2O4 spinel single crystals

Large stoichiometric MgAl2O4 spinel single crystals of high crystalline perfection have been grown by the Czochralski method. By means of X-ray diffraction topography the dislocation arrangement, Burgers vector directions and glide planes were investigated in relation to crystal growth conditions. Fast cooling rates generated prismatic dislocation hexagons in the central region of as-grown crystals. For the first time a new glide system in spinel following {311} planes was observed.

Growth of nearly stoichiometric MgAl2O4 spinel single crystals by a gradient furnace technique

Nearly stoichiometric MgAl2O4 spinel single crystals can be grown by a gradient furnace technique in molybdenum crucibles with an inert-gas atmosphere of industrial-grade argon, high-purity argon, or technical-grade helium. There is a reaction between MgO in the melt and the molybdenum crucibles which results in bands of molybdenum metal inclusions in the boule, although molybdenum concentration away from the bands is less than 100 ppm. This reaction can be minimized by ensuring that the starting materials are well reacted by high-temperature heat treatment to form MgAl2O4 before melting. Similarly, a greenish-brown discoloration which arises from oxygen deficiency in the boules may be eliminated by reacting the starting materials completely to MgAl2O4. A loss of nearly 2 mole% MgO generally occurs during the growth of a boule. This loss results in the formation of a ``skin'' of nonstoichiometric spinel on the outside of the boule. This ``skin'' is the last material to solidify and is highly strained upon cooling. It cracks and these cracks propagate throughout the boule. The loss of MgO also causes the formation of a second phase, α-Al2O3, when the spinel is annealed at 1150 and/or 1550°C for periods of time that are less than 30 h. This second-phase precipitate causes variations in the Knoop100 hardness of spinel between 1360 and 1820 kg/mm2. Because of the difficulty in controlling composition, cracking, and the changes in hardness upon annealing at temperatures where silicon is commonly deposited on single-crystal insulating substrates, it is questionable whether spinel will find widespread applicability as an insulating substrate material.