MgAl2O4 Grains Research Articles

Effect of Al2O3 nano/micro powder ratio on microstructures and properties of microporous MgO-MgAl2O4 refractory aggregates

In this work, the microporous MgO-MgAl2O4 refractory aggregates were prepared by in-situ decomposition synthesis method. The effect of Al2O3 nano/micro powder ratio on the microstructures and properties was studied under the premise that the fixed total content of Al2O3 was 15wt%. The results demonstrated that the aggregates were composed of microporous MgO microparticles with nano pores as well as MgAl2O4 grains located among them. The ratio not only influenced the volume expansion through changing the lattice constant of in-situ MgAl2O4, but also affected the formation of two different positions of MgAl2O4 bonds, thereby influencing the microstructures and properties. When the ratio was 4:6, the combined effect of the shrinkage of MgO microparticles and the expansion of in-situ MgAl2O4 grains promoted the formation of the bonds between MgAl2O4 grains and MgO microparticles, as well as among the MgAl2O4 grains, which resulted in the best comprehensive properties of the microporous MgO-MgAl2O4 refractory aggregates, characterized by micro-nano pores, an apparent porosity of 26%, a flexural strength of 23.2 MPa, a flexure strength after thermal shock of 20.4 MPa and a low thermal conductivity of 2.99 W/(m·K) at 800°C.

Formation of TiB2-MgAl2O4 Composites by SHS Metallurgy.

TiB2-MgAl2O4 composites were fabricated by combustion synthesis involving metallothermic reduction reactions. Thermite reagents contained Al and Mg as dual reductants and TiO2 or B2O3 as the oxidant. The reactant mixtures also comprised elemental Ti and boron, as well as a small amount of Al2O3 or MgO to serve as the combustion moderator. Four reaction systems were conducted and all of them were exothermic enough to proceed in the mode of self-propagating high-temperature synthesis (SHS). The reaction based on B2O3/Al/Mg thermite and diluted with MgO was the most exothermic, while that containing TiO2/Al/Mg thermite and Al2O3 as the diluent was the least. Depending on different thermites and diluents, the combustion front temperatures in a range from 1320 to 1720 °C, and combustion wave velocity from 3.9 to 5.7 mm/s were measured. The XRD spectra confirmed in situ formation of TiB2 and MgAl2O4. It is believed that MgAl2O4 was synthesized through a combination reaction between Al2O3 and MgO, both of which can be totally or partially produced from the metallothermic reduction of B2O3 or TiO2. The microstructure of the TiB2-MgAl2O4 composite exhibited fine TiB2 crystals surrounded by large densified MgAl2O4 grains. This study demonstrated an energy-saving and efficient route for fabricating MgAl2O4-containing composites.

Preparation and high-temperature oxidation behaviors of MgAl2O4-MgAlON composites by in-situ nitriding

In this study, MgAl2O4-MgAlON composites with Al, α-Al2O3, MgO and MgAl2O4 raw materials were fabricated through in-situ nitriding and reaction sintering, with a tunable MgAlON composition from 10 wt% to 25 wt%. Subsequently, the sintered samples were calcined at 1500 ℃ for 3 h in an oxidation atmosphere. The densification, mechanical properties, phase assemblage, microstructures, and oxidation behaviors of MgAl2O4-MgAlON composites were investigated. Results reveal that the MgAlON grains were well-crystallized and exhibited a spinel structure in sintered body. The mechanical properties were markedly improved with the increase of MgAlON mass fraction and reached the maximum at 25 wt%. After oxidation, the densification and mechanical properties have both declined, despite the composites were oxidized to a core-shell structure. Additionally, the weight gain rate and linear expansion rate of oxidized samples simultaneously exhibited a gradual rise with the increase in MgAlON contents. More specifically, the MgAlON grains were transformed into many submicron MgAl2O4 and α-Al2O3 grains with particle sizes< 1 µm after oxidation, while the oxidation reaction was carried out along the particle gaps created by MgAlON grains oxidized from surface to interior. Our findings provide some scientific guidance for enhancing the oxidation resistance and application potentials of MgAlON composites in high-temperature.

In-situ synthesis of magnesium aluminate spinel – Zirconium diboride composite powder in magnesium chloride melt

Phase pure MgAl2O4–ZrB2 composite powder was successfully synthesized at 1200 °C for 4 h from ZrO2, B2O3, and Al, in molten MgCl2 which acted as not only an Mg-bearing reactant but also a liquid reaction medium. Mediated by such a liquid medium, Al initially reduced B2O3, forming B and Al2O3 which further reacted with ZrO2 and MgCl2, forming ZrB2 and MgAl2O4, respectively. As prepared powder comprised angular MgAl2O4 grains with sizes from ∼100 nm to ∼2 μm, and ZrB2 particles of ∼100 nm in average. The molten salt synthesis strategy demonstrated here is believed to be universally applicable to preparations of a range of MgAl2O4-boride composite powders for future in-depth exploration.

Mechanical Properties of Al Matrix Composite Enhanced by In Situ Formed SiC, MgAl2O4, and MgO via Casting Process.

Al matrix composite, reinforced with the in situ synthesized 3C–SiC, MgAl2O4, and MgO grains, was produced via the casting process using phenolic resin pyrolysis products in flash mode. The contents and microstructure of the composites’ fracture characteristics were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Mechanical properties were tested by universal testing machine. Owing to the strong propulsion formed in turbulent flow in the pyrolysis process, nano-ceramic grains were formed in the resin pyrolysis process and simultaneously were homogeneously scattered in the alloy matrix. Thermodynamic calculation supported that the gas products, as carbon and oxygen sources, had a different chemical activity on in situ growth. In addition, ceramic (3C–SiC, MgAl2O4, and MgO) grains have discrepant contents. Resin pyrolysis in the molten alloy decreased oxide slag but increased pores in the alloy matrix. Tensile strength (142.6 ± 3.5 MPa) had no change due to the cooperative action of increased pores and fine grains; the bending and compression strength was increasing under increased contents of ceramic grains; the maximum bending strength was 378.2 MPa in 1.5% resin-added samples; and the maximum compression strength was 299.4 MPa. Lath-shaped Si was the primary effect factor of mechanical properties. The failure mechanism was controlled by transcrystalline rupture mechanism. We explain that the effects of the ceramic grains formed in the hot process at the condition of the resin exist in mold or other accessory materials. Meanwhile, a novel ceramic-reinforced Al matrix was provided. The organic gas was an excellent source of carbon, nitrogen, and oxygen to in situ ceramic grains in Al alloy.

Improved properties of low‐carbon MgO‐C refractories with the addition of multilayer graphene/MgAl2O4 composite powders

AbstractThe paper investigated the effects of different amounts (0%, 3%, 6%, 9%) of in situ multilayer graphene/MgAl2O4 composite powders on the slag resistance, thermal shock resistance, and oxidation resistance of low‐carbon MgO‐C refractories. Comparing with commercial MgAl2O4, the MgAl2O4 in in situ multilayer graphene/MgAl2O4 composite powders has higher lattice strain of crystal, which can trap more Mn and Fe ions, resulting in the better slag resistance. The oxidation decarbonation layer of MgO‐C specimen with 3% composite powders is 9.71 mm, which is lower than not only the specimen with other contents but also specimen containing carbon black/MgAl2O4 powders. Moreover, the residual strength ratio of the specimen C/MA‐3 was 47.47%, which is 28.5% and 8.08% higher than specimens with no additive and with carbon black/MgAl2O4 powders, respectively. Both improving thermal shock and oxidation resistance properties are related with the unique nano structure, multilayer graphene in situ formed between MgAl2O4 grains, of added composite powders. The former is due to higher strain energy consumed by multi‐deflection of cracks inside the multilayer graphene/MgAl2O4 composite powders. And the latter is due to the higher energy of oxidation activation of multilayer graphene/MgAl2O4 composite powders due to effective protection of multilayer graphene by MgAl2O4.

Preparation and thermal shock behavior of nanoscale MgAl2O4 spinel-toughened MgO-based refractory aggregates

Abstract Nanoscale MgAl2O4 (MA) spinel-toughened MgO-based refractory aggregates were prepared by firing lightly calcined magnesia powder and some nanoscale Al2O3 (≤50 nm) particles at high temperature. The effects of various amounts of nanoscale Al2O3 powder on the phase composition, microstructure, physical properties and thermal shock resistance (TSR) of the prepared MgO-based refractory aggregates were investigated. The results showed that the optimum nanoscale Al2O3 powder addition was 10 wt%, and the nanoscale Al2O3 particles reacted with MgO during firing and formed nanoscale MA creating reactive sintering, which contributed to significant increasing in density and cold strength of the prepared aggregates. The thermal mismatch between nano-MA and MgO could produce microcracks created toughening effect, leading to improvement in TSR of the prepared aggregates. And the formed nano MA grains pinned in MgO grain boundaries and on MgO grain surfaces would lead to microcracks deflection and absorbing much more fracture energy, further improving the TSR. The obtained aggregates were trial used in MgO-C slide plate materials, and thermal shock resistance of the materials was improved.

One-step synthesis of in situ multilayer graphene containing MgAl2O4 spinel composite powders

Owing to its exceptional properties, graphene has been considered to be a new and promising reinforcement for oxide ceramics over the last few years. Herein, in situ multilayer graphene/MgAl2O4 composite powders with homogeneously distributed graphene were synthesized via one-step carbon-bed sintering with magnesium citrate serving as the source of both MgO and carbon. The products were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and Raman spectroscopy. The microstructure and synthesis mechanism of the products were characterized and discussed. The results indicated that graphene with 6–10 layers grows between or outside the MgAl2O4 grains. The carbon content of the products was 5.96 wt%. The crystallite size of carbon (La) of in situ graphene increased as temperature rose to 1400 ℃. The composite powders synthesized retained the size and morphology of the Al2O3 raw material, confirming the template mechanism of spinel synthesis.

Preparation and properties of MgAl2O4 based ceramics reinforced with rod-like microcrystallines by co-doping Sm2O3 and La2O3

A rod-like microcrystalline reinforced MgAl2O4 (MA) based ceramic with high strength and good thermal shock resistance has been successfully prepared by co-doping Sm2O3 and La2O3 via a single-stage solid-state reaction sintering (SRS) process. The effects of addition amounts of Sm2O3 and La2O3 on the phase compositions, microstructures, shrinkage ratio, apparent porosity, bulk density, high temperature compressive strength and thermal shock resistance of the MA based ceramics have been investigated. The results showed that MA, SmAlO3, La10Al4O21, Sm4.67(SiO4)3O and La4.67(SiO4)3O phases could be found in the ceramics after co-doping additives. The new formed rare earth compounds could prevent the growth of MA grains leading to a densification microstructure of the MA based ceramics. The Sm4.67(SiO4)3O and La4.67(SiO4)3O were formed by the reaction between additives and impurities of raw materials, which presented as rod-like microcrystallines to effectively clean the impurities in the grain boundaries of the MA based ceramics. Accordingly, the sintering properties, high temperature compressive strength and thermal shock resistance of the MA based ceramics were improved markedly.

In-situ formation and densification of MgAl2O4-Y3Al5O12 and MgAl2O4-MgNb2O6 ceramics via a single-stage SRS process

MgAl2O4 (MA)-Y3Al5O12 (YAG) and MA-MgNb2O6 (MN) ceramics with high density were successfully fabricated via a single-stage solid-state reaction sintering (SRS) process at 1580?C for 4 h. The effect of Y2O3 or Nb2O5 additions from 2.5 wt% to 7.5 wt% on the phase compositions, microstructures, shrinkage ratio, apparent porosity, bulk density and cold compressive strength of MA-YAG and MA-MN ceramics has been investigated. It was found that MgO and Y2O3 reacted with Al2O3 to form MA and YAG during sintering while Nb2O5 reacted with MgO to form MN. YAG and MA grains in the MA-YAG ceramics exist as granular shape, and their average grain size is about 1 ?m and 5 ?m, respectively. YAG grains distribute on the intergranular space of MA particles. Polygonal MA particles can be observed in the MA-MN ceramics, and MN grains distribute on the intergranular space of MA particles as well as on MA particles. Rod-like MN grains can be formed in the MA-MN ceramics by addition of 7.5 wt% Nb2O5. The diameter shrinkage ratio, volume shrinkage ratio, bulk density and cold compressive strength of MA-YAG and MA-MN ceramics are greatly improved by doping Y2O3 and Nb2O5, respectively.

The structure of a propagating MgAl2O4/MgO interface: linked atomic- and μm-scale mechanisms of interface motion

To understand how a new phase forms between two reactant layers, MgAl2O4 (spinel) has been grown between MgO (periclase) and Al2O3 (corundum) single crystals under defined temperature and load. Electron backscatter diffraction data show a topotaxial relationship between the MgO reactant and the MgAl2O4 reaction product. These MgAl2O4 grains are misoriented from perfect alignment with the MgO substrate by ~2–4°, with misorientation axes concentrated in the interface plane. Further study using atomic resolution scanning transmission electron microscopy shows that in 2D the MgAl2O4/MgO interface has a periodic configuration consisting of curved segments (convex towards MgO) joined by regularly spaced misfit dislocations occurring every ~4.5 nm (~23 atomic planes). This configuration is observed along the two equivalent [1 0 0] directions parallel to the MgAl2O4/MgO interface, indicating that the 3D geometry of the interface is a grid of convex protrusions of MgAl2O4 into MgO. At each minimum between the protrusions is a misfit dislocation. This geometry results from the coupling between long-range diffusion, which supplies Al3+ to and removes Mg2+ from the reaction interface, and interface reaction, in which climb of the misfit dislocations is the rate-limiting process. The extra oxygen atoms required for dislocation climb were likely derived from the reactant MgO, leaving behind oxygen vacancies that eventually form pores at the interface. The pores are dragged along by the propagating reaction interface, providing additional resistance to interface motion. The pinning effect of the pores leads to doming of the interface on the scale of individual grains.

High temperature deformation and fracture of tribo-layers on the surface of AA5083 sheet aluminum–magnesium alloy

Abstract Plastic deformation and fracture of tribo-layers covering surfaces of aluminum AA5083 alloy sheets subjected to tensile tests at elevated temperatures and different strain rates were studied. When tested at 420 °C and a strain rate of 4 × 10−2 s−1, tribo-layers failed predominantly by brittle fracture but about 10% of the fractured oxide patches were found to be held together by fibrous structures. These fibres maintained uniform cross-sections and were as long as 2.5 μm at higher temperatures consistent with superplastic behaviour. Typically each oxide fibre had a diameter of 200 nm and consisted of nanocrystalline MgO and MgAl2O4 grains of 4.5 ± 0.7 nm in diameter as revealed by high resolution TEM. It was suggested that a Coble type creep was responsible for the superplasticity of the oxide fibres.

Constraints on Heterogeneous Galactic Chemical Evolution from Meteoritic Stardust

Meteorites contain presolar grains that formed in stars prior to solar system formation and preserve a record of stellar and galactic evolutionary processes. We consider a stochastic model of heterogeneous mixing of supernova ejecta into a homogeneous interstellar medium (ISM), first presented by Lugaro et al. The predictions of the model for Si, Ti, and O isotopic ratios are compared with the compositions of presolar SiC, Al2O3, and MgAl2O4 grains. Model parameters that reproduce the range of Si isotopic ratios in SiC grains fail to reproduce the strong observed correlation between SiC Si and Ti isotopic ratios and the range of 18O/16O ratios in the oxide phases. The Si-Ti correlation in SiC implies that heterogeneous mixing of SNe ejecta can account for at most 40% of the range of grain isotopic compositions. This result further implies that the ejecta from diverse SNe are well mixed to the percent level in the ISM, consistent with recent results based on stellar and interstellar abundance measurements. However, the presolar grain data provide much more stringent limits on the range of relative element ratios (e.g., Mg/Fe) than can be derived from astronomical observations. The large scatter in metallicity observed for disk stars of a given age cannot be explained by heterogeneous mixing of stellar ejecta. The range of 18O/16O ratios measured in presolar oxide grains implies either that they formed in stars with a wider range of metallicities than the SiC progenitor stars or that their parent stars experienced moderate amounts of cool bottom processing.

High‐Strain‐Rate Superplasticity in Y2O3‐Stabilized Tetragonal ZrO2 Dispersed with 30 vol% MgAl2O4 Spinel

High‐strain‐rate superplasticity is attained in a 3‐mol%‐Y2O3‐stabilized tetragonal ZrO2 polycrystal (3Y‐TZP) dispersed with 30 vol% MgAl2O4 spinel: tensile elongation at 1823 K reached &gt;300% at strain rates of 1.7 × 10−2– 3.3 × 10−1 s−1. The flow behavior and the microstructure of this material indicate that the MgAl2O4 dispersion should enhance accommodation processes necessary for grain boundary sliding. Such an effect is assumed to arise from an enhancement of the cation diffusion by the dissolution of Al and Mg ions into the ZrO2 matrix and from stress relaxation due to the dispersed MgAl2O4 grains.

Cesium Ion Implantation in Single Crystal Yttria-Stabilized Zirconia (YSZ) and Polycrystalline MgAl2O4-YSZ

ABSTRACTYSZ and MgAl2O4-YSZ composite are two promising materials as in the inert matrix nuclear fuel for the incineration of plutonium. In this study, 400 keV Cs ions were implanted to a fluence of 1×1017 ions/m2 in both materials in order to investigate the retention behavior of fission product at elevated temperature. The implantations were completed at room temperature for the YSZ and 973 K for MgAl2O4-YSZ, respectively. Subsequent annealing at 1273 K was performed after a room temperature implantation of YSZ. After annealing, a high density of gas bubbles formed with diameter from 3 to 40 nm from the surface to a depth of 150 nm in the YSZ matrix. The swelling due to bubbles formation was estimated to be 2.5%. In situ TEM was employed during high temperature ion implantation of the MgAl2O4-YSZ composite. A high density of small bubbles with 5 nm in diameter formed in the MgAl2O4 and YSZ grains after an ion dose of 1×1017 ions/cm2. There was no preferential precipitation along grain boundaries. Electron Paramagnetic resonance (EPR) spectroscopy was used to investigate the type of defects that formed in the YSZ matrix.

Synthesis of MgAl2O4 (Spinel) Powder Using MgCl2.

In order to develop a novel synthetic method for MgAl2O4 powder, a mixture consisting of Al2O3 and MgCl2 (1:10 molar ratio) was heated. As raw materials, two types of Al2O3; α-Al2O3 platelets and γ-Al2O3 powder and MgCl2 were used. When the mixture consisting of α-Al2O3 platelets and MgCl2 was heated at 750°C for 2h, MgAl2O4 powder with high crystallinity was formed. The formed Mg2O4 grains had a polyhedral shape (octahedron or pentahedron) 1-2μm in size and a maximum size of 3μm. When the mixture consisting of γ-Al2O3 powder and MgCl2 was heated at 750°C for 2h, MgAl2O4 powder was formed, but its crystallinity was low. After heating the mixture at 1000°C for 2h, the crystallinity of MgAl2O4 became high and the size of the formed polyhedral MgAl2O4 grains was 3-4μm and they attained a maximum size of 10μm.

Radiation effects on MgAl2O4-stabilized ZrO2 composite material under He+ or Xe2+ ion irradiation

The damage evolution processes in polycrystalline MgAl2O4–yttria-stabilized ZrO2 (YSZ) composite materials were studied in a TEM. The irradiations were performed with 60 keV Xe2+ ions at a flux of 5×1012 ions/cm2 s or 35 keV He+ ions at a flux of 5×1013 ions/cm2 s at room temperature and 650°C, respectively. Preferential bubble formation at grain boundaries was observed in all irradiation conditions. Stress associated with the different swelling rate between MgAl2O4 and YSZ grains would cause the grain boundary cracking. Differences in damage accumulation process between MgAl2O4 and YSZ grains were clearly shown under He+ ion irradiation at 650°C. Bubbles in YSZ grains remarkably grew large by coalescence during annealing process compared to those in MgAl2O4 grains.

Shape of MgAl2O4 Grains in a CaMgSiAlO Glass Matrix

When sintered Al2O3 is annealed with CaMgSiAlO glass at 1600°C, polyhedral MgAl2O4 grains form and glass pockets are entrapped within the grains. After annealing for 13 h at 1600°C, the liquid pockets show a regular octahedron shape which is expected to represent the equilibrium shape. All grain surfaces in contact with the glass matrix show the same shape. The small grains, which must be shrinking, thus have the equilibrium shape, because their shrinkage shape is is identical to the equilibrium and growth shape. However, the octahedral shape also represents the growth shape for the growing large grains. The grains also form grain boundaries with neighboring grains.