Homogeneous Mullite-Forming Powders from Spray-Drying Aqueous Solutions

A mullite precursor solution was made by hydrolyzing TEOS in a diluted aqueous solution of aluminum nitrate. This solution was spray-dried at a relatively low temperature (200°C), producing a chemically homogeneous powder crystallizing completely into mullite at 975°C. By aging this solution at 100°C, a diphasic precursor sol made of colloidal silica and aluminum nitrate was made. The resulting spraydried powder was less homogeneous, crystallizing into mullite and spinel or only spinel. The aging by thermal treatment may thus be a way to control the heterogeneity of the powder.     

Formation of NiAl2O4 by Solid State Reaction

The growth of nickel-aluminum spinel, NiAl₂O₄, in diffusion couples of polycrystalline Al₂O₃ and NiO was investigated between 1200° and 1500°C. The growth kinetics for the spinel layer obeyed a parabolic rate law in this temperature range. Marker experiments showed that the spinel layer formed by counterdiffusion of nickel and aluminum ions. Comparison of experimental and theoretical values of the parabolic rate constants suggests that the diffusion of aluminum ions through the spinel layer is rate controlling.     

The KaoIinite-MuIIite Reaction Series: Ill, The High-Temperature Phases

X-ray diffraction data for the high-temperature phases show that a spinel-type structure develops with marked orientation at about 925°C. This phase is considered to be an aluminum silicon spinel with vacant cation sites. Mullite is thought to be formed by the decomposition of the spinel. Silica is eliminated progressively as metakaolin transforms to the spinel phase and thence to mullite. The X-ray data show variation in the mullite parameters between 1200° and 1400°C.; at 1400°C., the composition probably approximates 3A1₂O₃·2SiO₂. The nature of the intermediate spinel-type phase is discussed in relation to the crystal chemistry of spinels.     

Nucleation of Solid-State Reactions between Nickel Oxide and Aluminium Oxide

The initial stages of the solid-state reaction between high-quality thin films of nickel oxide and single-crystal aluminum oxide have been investigated. Thin films were grown on specially cleaned and annealed basal-plane (0001) alumina substrates by pulsed-laser deposition. The thin-film reaction couples were then heat-treated in air at 1080° and 1100°C for different times in order to induce the formation of the nickel aluminate spinel. The reaction couples were characterized in cross section by high-resolution scanning electron microscopy and conventional transmission electron microscopy. The films of nickel oxide consisted of two twin variants with a common {111} plane, parallel to the basal plane of alumina, but rotated by 60° or 180° about the 〈111〉 direction normal to that plane. The spinel nucleated where the twin boundaries in the nickel oxide film met the alumina substrate and contained the same twin variants as the original NiO film.     

Chemical Resistance of Refractories to AI and AI-Mg Alloys

The chemistry of the reaction between molten aluminum alloys and some refractory oxides is discussed. High-alumina brick containing 85, 94, and 99% Al₂O₃ periclase brick, and magnesia spinel brick were immersed for 48 hours in three aluminum alloys containing 0, 2.4, and 7.7% magnesium, respectively. All brick were discolored, although most of the discoloration disappeared after reheating at 1000°C in an oxidizing atmosphere. Only the 85 and 94% brick, which contained silica, showed permanently darkened reaction rims. X-ray diffraction analysis of these darkened areas showed that the alloy containing 7.7% magnesium caused periclase formation in the rim whereas the alloy containing 2.4% magnesium produced spinel. Commercial aluminum caused only a decrease in the mullite content of the darkened area. Magnesia spinel and periclase were unaffected by all the alloys tested.     

Thermal Decomposition of Alkoxides in an Inert Organic Solvent: Novel Method for the Synthesis of Homogeneous Mullite Precursor

Thermal decomposition of a mixture of aluminum iso-propoxide and tetraethoxysilane (Al/Si = 3) in toluene at 300°C was examined. The reaction completely proceeded and yielded an amorphous product. The DTA profile of the product showed a sharp peak at ∼1000°C which was associated with crystallization of mullite and a small amount of the spinel phase. The mullite prepared by calcination of the product at 1300°C was composed of micro-crystals having an average diameter of 30 nm and had a large surface area of 41 m²g.     

Fabrication of Translucent Magnesium Aluminum Spinel Ceramics

A precursor for magnesium aluminum spinel powder, composed of crystalline ammonium dawsonite hydrate (NH₄Al(OH)₂CO₃·H₂O) and hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆·4H₂O) phases, was synthesized via precipitation, using ammonium bicarbonate as the precipitant. The precursor was characterized by differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. Reactive spinel powder, which could be densified to translucency under vacuum at 1750°C in 2 h without additives, was obtained by calcining the precursor at 1100°C for 2 h.     

Phase Relations and Reaction Sintering of Transparent Cubic Aluminum Oxynitride Spinel (ALON)

A new isobaric, condensed phase diagram in the region of stability of cubic aluminum oxynitride spinel (ALON) along the pseudobinary AI₂O₃-AIN composition join is presented, deduced primarily from various analytical measurements and microstructural observations. It is shown that cubic aluminum oxynitride spinel melts incongruently at ≊2050°C and is compositionally centered at ≊35.7 mol% AIN, which is equivalent to the following stoichiometric composition: AI₂₃O₂₇N₅ or 5AIN·9AI₂O₃. Single-phase ALON material sintered to nearly full density exhibits transparency in visible light.     

Grain-Boundary Reactions in Magnesia-Chrome Refractories: Application of the Electron Probe, II

When a magnesia-chrome refractory is heated in air a reaction layer develops around the chromite grains. This layer is magnesioferrite at 800°C; above 800° it comprises a solid solution of spinel of the type Mg(Al,Cr,Fe³⁺)₂O₄ and a magnesiowustite solid solution. As the temperature increases, the composition of the spinel in the reaction layer changes toward enrichment in chromium and aluminum and impoverishment in iron. A direct-bond chromite-periclase, well defined at about 1750°C, is formed essentially by diffusion. In slowly cooled specimens the average composition of the spinel in the reaction layer in the direct bond approximates to Mg(Al_0.05Cr_0.40-Fe_0.10)₂O₄. The order of diffusion of the individual ions from chromite to periclase is: Fe ≫ Cr > Al. This order can be explained by considering the charge and size of ions involved and the energy required to create cation vacancies. In specimens quenched from high temperatures the concentration of the magnesium-rich phases in the silicate pockets increases in the direction of the periclase grains whereas the calcium-rich phases are concentrated at the center of the pockets.     

Mechanical Activation of Heterogeneous Sol–Gel Precursors for Synthesis of MgAl2O4 Spinel

MgAl₂O₄ spinel precursor was prepared using a heterogeneous sol–gel process. The effect of high-energy milling on the precursor decomposition and spinel formation was investigated. The milling decreased the Al(OH)₃ dehydroxylation temperature from 190° to about 130°C. The activation energy for spinel formation decreased from 688 kJ/mol for the as-prepared precursors to 468 kJ/mol for the precursors milled for 5 h. Milling of the precursor lowered the incipient temperature of spinel formation from 900° to 800°C, and the temperature of complete MgAl₂O₄ spinel formation from >1280° to ∼900°C.     

Equilibrium Relations in the System NiO–TiO2 in the Temperature Range 1300° to 1750°C

Phase relations in the system NiO–TiO₂ have been determined by heating oxide mixtures in air at selected temperatures in the range 1300° to 1750°C for sufficient periods of time to attain equilibrium, followed by rapid quenching to room temperature. The phases have been characterized by optical microscopy, X-ray diffraction, and electron microprobe analysis. The most striking feature is the presence, above 1430°C, of a spinel-type phase that decomposes below this temperature to a mixture of remnant spinel, NiO of periclase-type structure, and NiTiO₃ of ilmenite-type structure. There are two peritectic points in the system, one at 1730°C where spinel, NiO, and liquid coexist in equilibrium, and one at 1610°C where spinel, NiTiO₃, and liquid are the coexisting phases. A eutectic is present at 1570°C, with NiTiO₃, rutile, and liquid coexisting in equilibrium. Rapid transformation of the spinel phase, even during rapid quenching, imposes uncertainties on the interpretation of the experimental data obtained, but the equilibrium-phase relations are deduced essentially as shown in the phase diagram presented. Results of a small number of calculated activities of NiO in oxide-phase assemblages involving the spinel phase at high temperatures (∼1500°C) lend support to the interpretation of the phase relations as presented in this paper.     

Activity–Composition Relations in NiAl2O4─MnAl2O4 Solid Solutions and Stabilities of NiAl2O4 and MnAl2O4 at 1300° and 1400°C

Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al₂O₃ at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl₂O₄–NiAl₂O₄ solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl₂O₄ from its oxide components (MnO + Al₂O₃) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol⁻¹, respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models.     

Reactions in Silica-Alumina Mixtures

Examination of mixtures of extremely pure silica and alumina shows that the greatest reactivity is not encountered with stoichiometric ratios to form mullite with the formula 3Al₂O₃. 2SiO₂ but rather with the formula 2A1₂O₃-SiO₂, and that reactivity also depends on the crystalline modification of alumina. A sharp exothermic differential thermal peak at 980°C. is attributed to three simultaneous reactions dependent on the silica-alumina ratio of the mixture: (1) the crystallization of gamma alumina, (2) the crystallization of a hydrogen aluminum spinel (HAl₅O₅), and (3) the reaction of silica with the hydrogen aluminum spinel to form mullite.     

Young's Modulus of Various Refractory Materials as a Function of Temperature

Young's modulus as a function of temperature was determined by a dynamic method for single-crystal sapphire and ruby and for polycrystalline aluminum oxide, magnesium oxide, thorium oxide, mullite, spinel, stabilized zirconium oxide, silicon carbide, and nickel-bonded titanium carbide. For the single crystals, Young's modulus was found to decrease linearly with increasing temperature from 100°C. to the highest temperature of measurement. For all the polycrystalline materials, except silicon carbide, stabilized zirconium oxide, and spinel, Young's modulus was found to decrease approximately linearly with increasing temperature until some temperature range characteristic of the material was reached in which Young's modulus decreased very rapidly and in a nonlinear manner with increasing temperature. This rapid decrease at high temperature is attributed to grain-boundary slip. Stabilized zirconium oxide and spinel were found to have the same rapid decrease in Young's modulus at high temperature, but they also had a decidedly nonlinear temperature dependence at low temperature.     

Low-Temperature Chemical Synthesis of Nanocrystalline MgAl2O4 Spinel Powder

Nanocrystalline MgAl₂O₄ spinel powder was synthesized by pyrolysis of complex compounds of aluminum and magnesium with triethanolamine (TEA). The soluble metal ion–TEA complexes formed the precursor material on complete dehydration of the complexes of aluminum–TEA and magnesium–TEA. Single-phase MgAl₂O₄ spinel powder resulted after heat treatment of the precursor material at 675°C. The precursor and the heat-treated powders were characterized by X-ray diffractometry (XRD), differential thermal and thermogravimetric analysis, and transmission electron microscopy (TEM). The average crystallite size as measured from the X-ray line broadening was around 14 nm and the average particle size from TEM studies was around 20 nm.     

Reaction Between Nitrogen and Spinel in Chromium

The reaction between nitrogen dissolved in chromium and a dispersion of magnesium aluminate particles in chromium at high temperatures is discussed. The equilibrium state for various nitrogen concentrations in a 94 chromium-6 spinel mixture was investigated between 1200° and 1400°C. Experimental techniques included electron-beam microprobe analysis and mass spectrometry. Partitioning of nitrogen between chromium and spinel was strongly in favor of the spinel. Mass spectrometry indicated coordination of nitrogen with magnesium in the spinel lattice. The spinel-nitrogen reaction is important because it can prevent the embrittling of chromium by nitrogen.     

Epitactic Nucleation of Spinel in Aluminosilicate Gels and Its Effect on Mullite Crystallization

Control over the structure of hybrid (colloidal + molecular) aluminosilicate gels was utilized to demonstrate that precursor chemistry has a direct and controllable effect on the ∼1000°C crystallization of spinel and mullite in molecular precursor systems. Synthesis or preparation conditions leading to the development of a cubic, transition alumina result in the epitactic nucleation of spinel at ∼1000°C in gels that otherwise crystallize directly to mullite at ∼1000°C. Thus, the preference for spinel nucleation in gels derived from solution precursor systems whose chemistries promote formation of transition alumina readily explains the reported inability to obtain substantial mullite yields at ∼1000°C. Isothermal transformation kinetics of colloidal and hybrid gels show that in the absence of direct mullite formation at ∼1000°C, the release of alumina from the spinel-type crystal structure becomes the rate-controlling step in the transformation. This necessitates higher temperatures for mullite formation and limits the kinetic enhancement possible with extrinsic increases in mullite nucleation frequency.     

Point Defect Hardening in MgO·3Al2O3

Vickers hardness in spinel increases by 23% after neutron irradiation to a fluence of 8.3 × 10²² n/m² at 100°C. Annealing at high temperatures above 100°C gradually decreases the hardness. Above 500°C, the hardness is reduced to almost the same value as that of unirradiated material. The hardness of spinel, irradiated at 470°C to a fluence of 2.4 × 10²⁴ n/m², is unchanged both after irradiation and after annealing up to 1000°C. The length change during annealing was also measured and is similar to the hardness change. Frank dislocation loops with a density of 3 × 10¹⁴/cm³ are induced by neutron irradiation at 470°C but they apparently do not affect the hardening in spinel. Thus, point defects are concluded to act as obstacles against dislocation movement. The yield stress measured at 1400°C is also unchanged after irradiation. It is believed that not only the point defects but also the loops are annihilated by annealing at 1400°C.     

MAS NMR Evidence for the Presence of Silicon in the Alumina Spinel from Thermally Transformed Kaolinite

Georgia kaolinite dehydroxylated at 950°C for 24 h, then leached with KOH at 90°C for 1 h, shows a well-resolved ²⁹Si MAS NMR resonance at −77.5 ppm, within 2 ppm of the calculated position for Si incorporated in γ-alumina spinel. From a knowledge of the total SiO₂ in the leached sample and its partitioning between the spinel and residual grain boundary phases, a maximum of 3.9 wt% is estimated to be present in the spinel.     

Synthesis and Densification of Transparent Magnesium Aluminate Spinel by SPS Processing

Mixtures of elementary oxides, MgO–Al₂O₃, were used to fabricate transparent polycrystalline magnesium aluminate spinel specimens by means of the spark plasma sintering technique. A sintering aid, 1 wt% of LiF, was added to the mixed powder. The presence of the additive promotes the synthesis of spinel that starts at 900°C and is completed at 1100°C. The LiF additive wets spinel on its melting and promotes densification, which is completed at 1600°C. LiF vapor plays a cardinal role in eliminating residual carbon contamination and in the fully dense state, allows attaining a 78% level of optical transmittance. The optimal conditions for achieving adequate transparency were determined and the role of the LiF addition in the various stages of the process is discussed.