Analysis of the formation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> + Fe nanocomposites

The formation of Al₂O₃ + Fe nanocomposites (in the range 0–20 wt % Fe) in the course of three sequential processes, such as dispersion, compaction, and sintering at a temperature of 1573 K, is investigated. It is revealed that the sintering is accompanied by the formation of the spinel phase at interfaces. It is demonstrated that the composition of the sintered samples corresponds to an equilibrium composition at a temperature of approximately 1073 K and that the spinel phase serves as a barrier layer preventing oxidation of iron     

Formation,conversion, and elimination of intermediate spinel phase in MgO/kaolin mixture firing for cordierite

Cordierite ceramic is usually used for diesel particulate filter owing to its excellent low thermal expansion coefficient and high thermal shock resistance properties. However, the co-exited intermediate spinel phase can deteriorate the thermal and mechanical performances of cordierite ceramic product, because the spinel phase has much higher thermal expansion coefficient comparing to that of cordierite. In this study, two methods are utilized to reduce the spinel impurity in the cordierite ceramic. On the one hand, rational reaction resources were introduced to decrease spinel production. The formation of intermediate spinel phase is systematically researched by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman characterizations and the results clarified the preference of path “Enstatite + Mullite → Cordierite” for less spinel production in comparison to path “Enstatite + Al₂O₃ → Cordierite + Spinel.” Additionally, MgO was introduced as fluxing agent to promote liquid-phase sintering, thus facilitating the conversion of spinel. On the other hand, the sintering schedule was improved by introducing a holding temperature gradient to promote the diffusion of Si⁴⁺ and further promote the conversion of spinel into cordierite. With these methods, the residual spinel phase is minimized, the resulting high-purity cordierite has a 47% reduction in the thermal expansion coefficient from 3.07 × 10⁻⁶/K to 1.63 × 10⁻⁶/K compared to the original cordierite sample.     

Phase equilibria studies in the CaO–SiO2–Al2O3–MgO system with CaO/SiO2 ratio of 0.9

Phase equilibria and liquidus temperatures in the CaO–SiO₂–Al₂O₃–MgO system at a CaO/SiO₂ weight ratio of 0.9 in the liquid phase have been experimentally determined employing high-temperature equilibration and quenching technique followed by electron probe X-ray microanalysis. Isotherms at 1573, 1623, 1673, and 1773 K were determined and the primary phase fields of wollastonite, melilite, olivine, periclase, spinel, and corundum have been located. Compositions of the olivine and melilite solid solutions were analyzed and discussed. Comparisons between the newly constructed diagram, existing data, and FactSage predicted phase diagrams were performed and differences were discussed. The present study will be useful for guidance of industrial practices and further development of thermodynamic modeling.     

Diffusion limited precipitation of alumina in magnesium aluminate spinel

Precipitation of Al₂O₃ from nonstoichiometric, dense spinel was examined with the intent to design strengthened transparent ceramics. Powders of magnesium aluminate spinel, MgO·nAl₂O₃, with compositions n=1 and 2 were uniaxially hot‐pressed at 1873 K, hot isostatically pressed at 2073 K, and heat treated in air or vacuum at 1573 K for 1, 5, 10, 15, and 20 hour. It was observed that precipitation of α‐Al₂O₃ in n=2 material initiated from the surface and progressed to various depths, with greater depths corresponding to treatments in air. The kinetics are shown to be limited by the diffusion of oxygen through the reacted layer. The results reveal that the environment used to densify spinel has a large influence on the evolution of the two phase microstructure during subsequent heat treatment.     

Experimental determination of phase equilibria in the CaO-BaO-SiO2-12 mol pct. Al2O3-13 mol pct. MgO system at 1573 K and 1623 K

Phase equilibria of the CaO-BaO-SiO₂-12mol pct. Al₂O₃-13mol pct. MgO system with a wide substitution range of CaO with BaO have been experimentally determined at 1623 K (1350°C) and 1573 K (1300°C) using high-temperature equilibration followed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) analysis. The (Ca,Ba)₃MgSi₂O₈, BaAl₂O₄, BaAl₂Si₂O₈ and Ca₂(Al_0.46Mg_0.54)(Al_0.46Si_1.54)O₇ phases have been designated within the phase diagram. The liquidus temperature initially decreased and then increased for the samples substituting CaO with BaO at a constant (C + B)/S ratio between 1.50 and 1.86. For samples with a constant BaO/CaO ratio, the liquidus temperature showed similar trends with a lower (C + B)/S ratio from 1.22 to 0.67. The volume fraction of crystal phases of the samples as-quenched from 1573 K (1300°C) correlated well with the variation of liquidus and the primary phases. In addition, the change in the Gibbs free energy of the reactions and the bond parameter (X_p × Z/R_k) of the cations were analyzed, where the maximum change in the Gibbs free energy was found for the formation of Ca₃MgSi₂O₈; furthermore, the stronger basic tendency of Ba²⁺ than Ca²⁺ facilitates Ba²⁺ substitution for Ca²⁺ and bonding with acidic tendency cations to form Ba-containing phases.     

Enhancement of reaction-sintering of alumina-excess magnesium aluminate spinel in presence of titania

Alumina-excess magnesium aluminate spinel finds use in different high temperature applications including steel ladles. Alumina-excess spinel was prepared by solid oxide reaction using magnesia (MgO=10?wt%) and calcined alumina (Al₂O₃ = 90?wt%), in the sintering temperature range of 1500–1700?°C. The role of titania on the densification, spinelisation, evolution of microstructure and phase assemblage was investigated in this MgO-Al₂O₃ system. Titania addition increased the rate of densification 20x compared to undoped composition at 1500?°C under dynamic heating condition. However, under static firing, the beneficial effect of titania on densification could only be discerned at lower temperatures. The microstructure of titania doped sintered alumina-excess spinel compacts contain magnesium aluminium titanate phase in the grain boundary of corundum and spinel grains. The beneficial effect of titania on densification is attributed to magnesium aluminium titanate phase (Mg_xAl_2(1-x)Ti_(1+x)O₅) development and also by incorporation of Ti⁴⁺ into the spinel structure.     

Preparation and ladle slag resistance mechanism of MgAlON bonded Al2O3 -MgAlON-Zr2Al3C4-(Al2CO)1-x(AlN)x refractories

MgAlON bonded Al₂O₃-MgAlON-Zr₂Al₃C₄-(Al₂CO)_1-x(AlN)_x refractories were prepared at high temperatures from 1300?℃ to 1600?℃ in N₂-flowing based on the design of Al-AlN core-shell. The refractory prepared at 1500?℃ was chosen to conduct the ladle slag resistance test at 1600?℃ in air. All the refractories are composed of MgAlON, Zr₂Al₃C₄, (Al₂CO)_1-x(AlN)_x and Al₂O₃. The ladle slag resistance test for the chosen refractory presents a good result and there exist two different reaction layers —— the reacted slag layer and the spinel solid solution layer. The reacted slag layer consists of spinel solid solution, Ca-ZrO₂ and gehlenite/gehlenite solution, where the formation of those phases has changed the chemical composition and phase composition of the original ladle slag. The compact spinel solid solution layer forms by the decomposition of MgAlON into rich-Al₂O₃ spinel and the incorporation of Mn²⁺ and Fe^2+/3+ into rich-Al₂O₃ spinel, which plays an important role in slag penetration resistance.     

Understanding the solidification and leaching behavior of synthesized Cr-containing stainless steel slags with varying Al2O3/SiO2 mass ratios

This study investigated the effect of Al₂O₃/SiO₂ mass ratios on the equilibrium crystallization behavior of synthesized CaO–SiO₂–MgO–Al₂O₃–Cr₂O₃ stainless steel slags to understand the selective concentration behavior of Cr into a primary Mg(Cr,Al)₂O₄ spinel phase during slag solidification and to determine the leaching stability of Cr-containing slags. The spinel solid solution was precipitated within the temperature range of 1600-1400 °C, where the Cr/(Cr+Al) mole ratio in the Mg(Cr,Al)₂O₄ spinel phase gradually decreased for slags with higher Al₂O₃/SiO₂ mass ratios. When the Al₂O₃/SiO₂ mass ratio increased from 0.125 to 0.5, the Cr content in the amorphous glass phase gradually decreased, with a subsequent increase in the Cr content in the crystalline phase. For slags with a unit Al₂O₃/SiO₂ mass ratio and MgO mole percent comprising less than the combined sum of the Cr₂O₃ and Al₂O₃ mole percents, the Cr content in the amorphous glass phase increased, which was correlated with the enhanced substitution of Cr³⁺ with Al³⁺ in the spinel. The trend of the amount of Cr-related ions in the leachate was consistent with the trend of Cr in the amorphous glass phase: the amount decreased for slags with Al₂O₃/SiO₂ mass ratios from 0.125 to 5 and then increased for slags with an Al₂O₃/SiO₂ mass ratio of 1. The results suggest that the addition of appropriate amounts of Al₂O₃ to stainless steel slags could be conducive to stabilizing Cr into the primary spinel phase to minimize Cr leaching into the environment.     

Investigation of the microstructure,cation distribution and optical properties of nanoscale NixMg1-xAl2O4 spinel pigments

Nanoscale Ni_xMg_1-xAl₂O₄ spinel pigments were synthesized by a citric acid precursor combined with the gel-casting method. The microstructure, cation distribution and optical properties as a function of calcining temperature and nickel content were investigated by the X-ray diffraction (XRD) Rietveld refinement, transmission/field emission scanning electron microscopy (TEM/FESEM), X-ray photoelectron spectroscopy (XPS), colour measurement and UV–vis–NIR spectrophotometry. Upon increasing the calcining temperature, both Ni²⁺ and Mg²⁺ hindered the migration of Al³⁺ to octahedral sites. When the Ni content increased, the cation site percentage of Ni²⁺ in the tetrahedral and octahedral sites varied slightly while that of Al³⁺ and Mg²⁺ change substantially. The cation exchange resulted in an increase in the inversion parameters and a decrease in the lattice parameters with increasing temperature or Ni content. Furthermore, Rietveld refinement also showed a shrinkage of the tetrahedra and an expansion of the distorted octahedra in the spinel structure. Short-range information based on optical spectra suggests that variation in the splitting energy of tetrahedra and octahedra caused the change in the spectral absorption. This study may deepen the understanding of the structural-optical property relationship of Ni_xMg_1-xAl₂O₄ spinel, which is vital to the further colour modification of ceramics and glazes.     

Research on high-temperature creep properties of Al2O3-MgAl2O4 refractory

In this study, the Al₂O₃-MgAl₂O₄ refractory samples were prepared with tabular alumina and fused magnesia-alumina spinel as the raw materials. A creep resistance test was performed at 1400°C and studied. The tested samples after the creep resistance test were characterized and analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy to investigate the effect of adding the amount and particle size of MgAl₂O₄ on high-temperature creep resistance of Al₂O₃-MgAl₂O₄ refractory, as well as its mechanism. As indicated by the results, under the same amount of spinel added, the sample with 3-1 mm spinel had higher creep resistance than the sample with 1-0 mm spinel. In the creep test, spinel aggregates would react with the alumina matrix to form a secondary spinel layer around the spinel particles, thereby connecting the matrix and the aggregates and improving the creep resistance of the specimens. As clearly indicated by the observed result of the microstructure, in the formation of the secondary spinel layer, Mg²⁺ had a higher migration rate and showed the secondary spinel layer's higher thickness ratio on both sides of the reaction interface, which would induce the Kirkendall effect. Accordingly, considerable pores generated and accumulating at the interface tended to reduce the creep resistance of the material, and this effect was highly dependent on the size of the spinel and the activation solid solution reaction intensity.     

High-temperature oxidation behavior of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramics in air

High-entropy metal carbides have recently been arousing considerable interest. Nevertheless, their high-temperature oxidation behavior is rarely studied. Herein the high-temperature oxidation behavior of (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy metal carbide (HEC-1) was investigated at 1573-1773 K in air for 120 minutes. The results showed that HEC-1 had good oxidation resistance and its oxidation obeyed a parabolic law at 1573-1673 K, while HEC-1 was completely oxidized after isothermal oxidation at 1773 K for 60 minutes and thereby its oxidation followed a parabolic-linear law at 1773 K. An interesting triple-layered structure was observed within the formed oxide layer at 1673 K, which was attributed to the inward diffusion of O₂ and the outward diffusion of Ti element and CO or CO₂ gaseous products.     

Effect of the calcination temperature on the cation distribution and optical properties of Mg0.5Co0.5CrAlO4 spinel pigment

In this study, a blue-green pigment has been prepared by partially replace Co²⁺ and Cr³⁺ in CoCr₂O₄ spinel structure with Mg²⁺ and Al³⁺ using a gel casting method The gel precursor was calcined at various temperatures (900–1400 °C) to obtain Mg_0.5Co_0.5CrAlO₄ spinel pigment. Combining the Rietveld refinement method of XRD and peak fitting of XPS high-resolution spectra, the relationships between the cation distributions (Co²⁺, Mg²⁺, Al³⁺, and Cr³⁺) in the tetrahedron and octahedron of the spinel structure and the calcination temperature were examined. In the octahedron, the contents of Co²⁺ and Al³⁺ decreased with increasing calcination temperature, and the Mg²⁺ and Cr³⁺ contents exhibited the opposite trend. The bond lengths of A-O and B–O change with increasing calcination temperature, thereby leading to a change in the unit cell. The optical performance of the pigments was investigated via UV–vis and CIE L*a*b* spectrophotometry, and the study shows that the blue-green hue of the pigment powder is caused by the absorption at υ3～υ8 (370 nm–640 nm) in the visible light region. The varied contents of Co²⁺ and Cr³⁺ in the spinel structure among calcination temperatures cause the absorption spectrum intensity change, thereby resulting in various blue-green tones. This study lays the foundations for subsequent investigations of colour modification in Mg_xCo_1-xAl_yCr_2-yO₄ spinel.     

In situ synthesis of Al2O3–SiC powders via molten-salt-assisted aluminum/carbothermal reduction method

Al₂O₃–SiC composite powder (ASCP) was successfully synthesized using a novel molten-salt-assisted aluminum/carbothermal reduction (MS-ACTR) method with silica fume, aluminum powder, and carbon black as raw materials; NaCl–KCl was used as the molten salt medium. The effects of the synthesis temperature and salt-reactant ratio on the phase composition and microstructure were investigated. The results showed that the Al₂O₃–SiC content increased with an increase in molten salt temperature, and the salt–reactant ratio in the range of 1.5:1–2.5:1 had an impact on the fabrication of ASCP. The optimum condition for synthesizing ASCP from NaCl–KCl molten salt consisted of maintaining the temperature at 1573 K for 4 h. The chemical reaction thermodynamics and growth mechanism indicate that the molten salt plays an important role in the formation of SiC whiskers by following the vapor-solid growth mode in the MS-ACTR treatment. This study demonstrates that the addition of molten salt as a reaction medium is a promising approach for synthesizing high-melting-point composite powders at low temperatures.     

Determination of interdiffusion coefficients of Si and Al in Ti3SiC2–Ti3AlC2 diffusion couple at 1373-1673 K

In the diffusion couple of Ti₃SiC₂ and Ti₃AlC₂, only interdiffusion of Si and Al occurred during diffusion treatment process. Based on the concentration profiles of Si and Al measured by electron probe microanalysis (EPMA), the interdiffusion coefficients of Si and Al at 1373-1673 K in Ti₃SiC₂–Ti₃AlC₂ diffusion couple were determined by both the Boltzmann-Matano (B-M) method and the Saucer-Freise (S-F) method. At the position of Matano plane with the composition of Ti₃Al_0.5Si_0.5C₂, the interdiffusion coefficient could be expressed as D_int (m²/s) = 5.6 × 10⁻⁴⋅exp [−246 ± 14 (kJ/mol)/RT]. Based on the two methods, the calculated interdiffusion coefficients increased with increasing temperature, and the magnitudes of their absolute values were on the order of 10^–13-10^–11 m²/s at 1373-1673 K. At 1373-1573 K, the calculated interdiffusion coefficients decreased monotonously with the increase of Si concentration, that is, x_Si/(x_Al + x_Si). But at 1673 K, the variation trend of interdiffusion coefficients with x_Si/(x_Al + x_Si) was no longer monotonous, probably due to the presence of Ti₅Si₃ phase and voids on Ti₃AlC₂ side.     

Atomic Scale Mechanisms of the Reduction of Nickel–Magnesium Aluminate Spinels

The dynamics of the reduction reaction of Ni_xMg_1?xAl₂O₄ to form nickel metal and a remnant oxide was quantified to understand spinel behavior in catalysis applications. X‐ray diffraction, thermogravimetry, and pycnometry were employed to track the evolution of high‐Ni spinels to metastable nonstiochiometric spinels during reduction, but before the phase transformation to theta alumina. Rietveld refinements of X‐ray diffraction data were used to quantify structural changes in the spinel and the phase fraction, crystallite size, and microstrain of all phases during H₂ reduction. During reduction, one O^2? is lost for each Ni²⁺ reduced to Ni metal. Ni_0.25Mg_0.75Al₂O₄ and Ni_0.5Mg_0.5Al₂O₄ were shown to form Ni metal and a non‐stoichiometric spinel of the same Mg‐Al ratio as the starting composition. NiAl₂O₄ and Ni_0.75Mg_0.25Al₂O₄ were found to become unstable as full reduction was approached, and metastable spinel, Θ‐Al₂O₃, and α‐Al₂O₃ formed sequentially given sufficient time at temperature.     

Kinetic analysis of magnesium aluminate spinel formation: Effect of MgO:Al2O3 ratio and titania dopant

The mechanistic pathway of MgO-Al₂O₃ reaction in solid state to form MgAl₂O₄ spinel was investigated to correlate the kinetic parameters with ratio of reactants (MgO:Al₂O₃) and with the presence of a doping agent, TiO₂. The time-temperature-expansion data of oxide compacts was analyzed using several model free analyses and model based (linear and non-linear) kinetic algorithms. These indicated that spinel formation process can be best described by single step with n-dimensional Avrami equation for every MgO:Al₂O₃ ratio, irrespective of titania dopant. The activation energy (E_a) of the process was proportional to % spinel formed in each system and validated with quantitative XRD analysis. The higher value of Avrami coefficient (n) in 90 wt% Al₂O₃ compositions has been explained with geometric considerations of powder packing. Incorporations of 1% TiO₂ in the MgO: Al₂O₃ oxide compact did not markedly affect the reaction model, frequency factor and Activation energy.     

Effect of Ni additives on pressureless sintering of SHS ZrB2

Abstract

Pressureless sintering of ultrafine zirconium diboride ZrB₂ produced by self-propagating high temperature synthesis (SHS) was carried out over a temperature range of 1573 to 1873 K using a nickel additive. The additive improved densification behaviour and a maximum densification of 88% was achieved at 1873 K. The XRD pattern showed formation of Ni₃Zr phase during pressureless sintering. The microhardness of sintered ZrB₂ was found to increase with Ni content and a maximum hardness of 1150 kg mm^?2 was found at 40 wt-%Ni addition. The coefficient of thermal expansion of different sintered samples was also investigated.     

Interfacial Reactions Between Anorthite (CaAl2Si2O8) and Al 7075 Alloy at 850°C and 1150°C

The present work reports an investigation of the interactions of Al 7075 alloy and anorthite at 850°C (150 h) and 1150°C (24 h). Transmission electron microscopy, electron probe microanalysis, X‐ray diffraction, and scanning electron microscopy coupled with energy‐dispersive spectroscopy were used to identify the mineralogical and microstructural changes at the metal–ceramic interface. At 850°C, the phase formation mechanisms were (a) Si⁴⁺–Al³⁺ interdiffusion between the Al alloy and anorthite to form calcium dialuminate (CA₂) and Ca²⁺–Mg²⁺ interdiffusion between the Al alloy and calcium dialuminate to form spinel. At 1150°C, spinel + Al₂O₃ and calcium hexaluminate (CA₆) + CA₂ were the major and minor phase mixtures, respectively in the corroded area. A thin layer of calcium monoaluminate (CA), gehlenite, and Si was present in the immediate vicinity of anorthite. The early stages of corrosion at 1150°C and 850°C were identical. However, due to thickening of the corroded region (viz., spinel formation) and enhanced evaporation of Mg at the higher temperature, the interdiffusion path evolves from Si⁴⁺–Al³⁺ + Ca²⁺–Mg²⁺ to Si⁴⁺–Al³⁺ + Ca²⁺–Al³⁺, thus establishing the following phase evolution path at the interface:      

Mechanical and optical properties in precipitated regions of alumina‐rich magnesium aluminate spinel

The toughening and strengthening of transparent ceramics is challenging because microstructural alterations typically lead to light scattering. Here, controlled precipitation of α‐Al₂O₃ from nonstoichiometric spinel is explored to demonstrate unique control over the evolution of second phase Al₂O₃ and how the microstructure might be altered to enhance fracture toughness while minimizing light scatter. Alumina‐rich magnesium aluminate spinel, MgO·nAl₂O₃, where n=2, was hot pressed and HIPed to produce fully dense, single‐phase material. The material was then heat treated in air at 1573 K for up to 20 hours to create a two‐phase spinel‐Al₂O₃ composite. The fracture toughness varies from 0.88 to 2.47 MPa√m depending on the microstructure; enhanced toughness at the surface was due to increased crack tortuosity at phase boundaries, but residual tensile stresses were observed in the interior of the material. Precipitation causes local volume contraction and the formation of porosity, decreasing optical transmission, especially for heat treatment times longer than 5 hours.     

Effect of potassium on the high pressure kinetics of ammonia synthesis over fused iron catalyst

Measurements were performed of reaction rate in the process of ammonia synthesis (T=370–470°C) on doubly promoted (DP) (Al₂O₃, CaO) and triply promoted (TP) (K₂O, Al₂O₃, CaO) iron catalysts. The latter were obtained by impregnation of the reduced and subsequently passivated DP precursors with alcoholic solution of KOH. The studies were carried out under high total pressure (10 MPa) in a wide range of ammonia partial pressure in the gas phase: from 0.25 to about 7 bar. The results are shown to be authoritative for the so-called kinetic regime. The effect of the presence of K⁺ cations in the catalyst was the stronger, as the temperature of the reaction was the lower and, in particular, the ammonia pressure in the gas phase the higher. The obtained results are in good accordance with the results of Somorjai's studies on activity of iron single crystal surfaces both clean and covered with (K+O) adlayer.