Technology of Spinel-Bonded Periclase Brick

This study covers the development of a new basic brick with outstanding resistance to load deformation, slagging, and spalling. This brick is made of 92% high-purity periclase and 8% high-purity alumina. During firing for a ceramically bonded brick or in service for a chemically bonded brick, the fine periclase and alumina react to form a spinel (MgO.Al₂O₃) bond. As the magnesia content of the periclase is increased, the properties of this brick become more outstanding.     

Influence of Magnesium-Aluminum Spinel on the Directed Oxidation of Molten Aluminum Alloys

The directed oxidation of some molten aluminum alloys was studied. Alloys containing Si, Zn, and Mg are readily converted into Al₂O₃/Al composites with the characteristic directed metal oxidation process (DIMOX) metal-channel structure. No reaction took place when alloys without Mg were used. However, when Mg-free melts containing Si and Zn were in contact with bulk spinel (MgAl₂O₄), the typical reaction product grew from the metal pool, although without formation of the spinel starter layer which was thought to be a prerequisite for DIMOX growth. Furthermore, the growth front was more planar than when Mg-containing alloys were used.     

Preparation of Magnesium Aluminate Spinel Containing Controlled Amounts of 17O Isotope

Magnesium aluminate spinel (MgAl₂O₄) with an ¹⁷O enrichment (¹⁷O/O_Tot) of about 23 at.% was prepared by reacting fine mixtures of aluminum hydroxide (enriched with ¹⁷O) and magnesium oxide of normal isotopic content. The material was prepared for experiments in which the radiation damage produced in a fusion reactor is simulated by fission reactor exposures. The powder mixtures were obtained by hydrolyzing, with water containing the ¹⁷O iostope, a mixture of aluminum isopropoxide and magnesium oxide powder. The mixture was converted into pure spinel by a series of heat treatments and grindings. Essentially fully dense bodies, which contained about 45% of the ¹⁷O isotope initially present in the water, were successfully fabricated provided that all thermal treatments were conducted in argon or vacuum atmospheres.     

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.     

The System MgO–MgAl2O4

In the determination of the liquidus, solidus, and subsolidus of the system MgO-MgAl₂O₄ the limits of the solid solution of A1 ions in periclase and Mg ions in spinel were measured. By using both X-ray diffraction and optical techniques, the maximum periclase solid solution was found at 82 wt% MgO, 18 wt% A12O3 (9.5% A1³⁺) and maximum spinel solid solution at 39% MgO, 61 % A1203 (6% Mg⁺⁺). Periclase and spinel solid solutions existed stably in easily detectable amounts at temperatures above approximately 1500°C.     

Microstructural Characterization of Cofired Tungsten-Metallized High-Alumina Electronic Substrates

Microstructural characterization of a high-Al₂O₃ substrate containing cofired thick-film tungsten metallization, with particular emphasis on the metal/ceramic interface, was conducted. The substrate contained tabular Al₂O₃ grains surrounded by a continuous calcium magnesium aluminum silicate glass containing particles of monoclinic ZrO₂ and reduced rutile (TiO_{2- x} ). The metal/ceramic adhesion was caused by mechanical interlocking between the W and Al₂O₃ grains by the glass phase which penetrated the porous W layers during sintering; there was no interfacial reaction or diffusion zone. The mechanical properties of the W metallization did not limit interfacial strength. Heat treatments of the substrate at 1400 K in air and under vacuum resulted in the devitrification of the intergranular glass. The most abundant devitrification product was anorthite (CaAl₂Si₂O₈), accompanied by magnesium aluminate titanate, magnesium aluminate spinel, α-cristobalite (SiO₂), and α-cordierite (Mg₂Al₄Si₅O₁₈). In addition, small rutile particles precipitated within the Al₂O₃ grains.     

Effect of Rapid Solidification on Microstructural Evolution in MgO–MgAl2O4

The response of the MgO–20 and 30 mol% Al₂O₃ compositions to rapid solidification has been studied. The oxides were twin-roller quenched and the resulting flakes were characterized by X-ray diffraction and transmission electron microscopy. The results indicate that metastable extensions of spinel and periclase occurred and the microstructural pathway was determined from the final microstructure. The flakes having MgO–20 mol% Al₂O₃ show a dendritic structure consisting of periclase and spinel. In the MgO–30 mol% Al₂O₃ composition, the liquid transforms to spinel partitionlessly. The spinel is believed to undergo decomposition by a modulation in composition, and the resulting microstructure consists of spinel and periclase. Kinetic and thermodynamic aspects of phase selection have been rationalized based on the metastable extensions of the different phase fields in the phase diagram. It has been proposed that composition fluctuations in spinel are stabilized because of the formation of disordered phases with a continuous range of order parameter on the tetrahedral sublattice.     

Precipitation of Magnesium Aluminum Spinel from Alumina-Matrix Solid Solution: I, Fundamental Concept and Precipitation Behavior

Al₂O₃ ceramics with magnesium aluminum spinel dispersion particulates were produced by a precipitation treatment of Al₂O₃-matrix solid solution, (Al_1—2x,Ti _x ,Mg _x )₂O₃. We successfully constructed the precipitation process for synthesizing the nanocomposite, using the solubility dependence of titanium and magnesium in Al₂O₃ on the valence of titanium. Changing of the valence of titanium from 4+ to 3+ was accomplished by controlling the heating atmosphere, and, thereby, magnesium precipitation was promoted. The precipitation behavior was characterized using X-ray diffractometry, and the microstructure was observed using transmission electron microscopy. We confirmed that magnesium aluminum spinel nanosized particulate were precipitated in the Al₂O₃ grain.     

Reaction Characteristics of Magnesia–Spinel Refractories with Cement Clinker

The adherence ability of cement clinker on magnesia–spinel refractories is investigated, using a sandwich test, at 1550°C for 30 min under a load of 5.3 kPa. Fractional factorial experiments determine that the silica ratio (SR)—SiO₂/(Al₂O₃+Fe₂O₃) and particle size of raw meal, as well as heating rate, have a significant effect on adherence ability. Microstructural analyses indicate that the adherence ability depends upon reactions between clinker and refractories at high temperature. Only spinel reacts with CaO and 3CaO·SiO₂ from clinker to form n -calcium aluminate (such as 3CaO·Al₂O₃, 12CaO·7Al₂O₃, CaO·Al₂O₃), but there is no reaction between MgO and the clinker. Fine crystalline spinel, evenly distributed in magnesia-based brick, is prone to reacting with lime-containing phases from clinker to form low melting phases and a belite-enriched zone at the clinker/brick interface. This reaction positively contributes to the high adherence on a magnesia−spinel brick. The high content of liquid in clinker with low SR accelerates reactions between spinel and clinker, while a limited reaction occurs at the brick/clinker interface with high silica.     

Melting of Forsterite, Monticellite, Merwinite, Spinel, and Periclase Assemblages

Experimental data which define melting temperatures and crystalline solution compositions show that the assemblage forsterite+monticellite+spinel+periclase melts near 1425°C and that the assemblage monticellite+merwinite+spinel+ periclase melts between 1406° and 1420°C. The melting of the former assemblage is a double reaction point. The forsterite crystalline solution contains 13 mol% monticellite and the monticellite crystalline solution contains 22 mol% forsterite. A projection system shows that the new data agree with much previous data but highlight inconsistencies in previous data for assemblages containing more CaO, such as merwinite+dicalcium silicate+spinel+periclase. These assemblages are encountered in magnesite refractories with CaO:SiO₂<2.0 during firing and in service.     

Preparation and Properties of Spinel Made by Vapor Transport and Diffusion in the System MgO-Al2O3

In a hydrogen atmosphere, MgO was vaporized by heating MgO (periclase) in the range 1500° to 1900° C, and the vapor products diffused into Al₂O₃ (sapphire) to form a uniform outer layer of spinel on all surfaces. At 1900°C. a spinel layer 48 mils thick could be obtained in 16 hours. The rates of spinel formation were determined and the activation energy was calculated to be about 100 kcal. per mole. In the spinel layer, the lattice constant, Vickers hardness, and the refractive index varied from the outer surface to the inner boundary in a fairly uniform manner, indicating a continuous change in composition from 1MgO: 1Al₂O₃ to 1MgO: 2–3Al₂O₃. In conversion to spinel a volume increase of 47% over that of the original sapphire took place. Transparent clear shapes of spinel such as disks and rods were obtained from clear sapphire and from translucent polycrystalline alumina. Clear rods of spinel with polished ends acted as thin lenses, round rods as convex lenses, and flat rods as planocylindrical lenses owing to the increase in refractive index from the outer surface to the inner central portion. Objects made of opaque polycrystalline alumina were also converted to spinel. The MgO periclase blocks were etched in the hydrogen atmosphere, and the vapor products of Al₂O₃ entered the MgO to form tiny spinel droplets in an opalescent border. In an oxidizing atmosphere, spinel was formed only on the surface of the sapphire directly in contact with periclase, in the range 1500° to 1900°C.     

Epitaxial Growth of Spinel by Reaction in the Solid State

The condensed-state reaction between single crystals of Al₂O₃ and MgO was conducted in air at 1560°C. Spinel formed as two product layers which were shown to agree with the proposed mechanism. The crystallographic orientation of the parent crystals had no effect on the rate of spinel growth. The orientation of the spinel grown from sapphire was the same as that found by other investigators for isomorphic crystals. The orientation of spinel grown from periclase was largely dependent on the transport mechanism across the periclase-spinel interface.     

Observation of Potassium βm-Alumina in Sintered Alumina

Potassium-containing plate-like precipitates having the β^m-alumina structure were identified within intergranular spinel inclusions in MgO-doped sintered aluminum oxide ceramics. Transmission electron microscopy revealed the corresponding epitaxial relation: (0001) β^m{111} spinel and 1120 β^m110 spinel. Chemical analyses and quantitative metallography suggested that in α-Al₂O₃ the 1800^°C solubility limits of magnesium and potassium do not exceed 300 to 400 ppm and 5 to 10 ppm by weight, respectively.     

Crystalline Solution in the System MgO-Mg,SiO4,-MgAl2,O4,

Crystalline solubility relations in the system MgO-Mg₂SiO₄MgAl₂O₄ (periclase-forsterite-spinel) were studied using coprecipitated gels as starting materials. The substitution 2Al = Mg + Si was investigated along the join Mg₂SiO₄-Mg-Al₂O₄,. At 1720°C the maximum crystalline solution in forsterite is about 0.5 mole % MgAI₂O₄, and in spinel it is slightly more than 5.0 mole % Mg₂SiO₄. The solubility of MgO in forsterite was 0.5 mole % at 1860°C, whereas more than 11 mole % Mg₂SiO₄ can be dissolved in the periclase structure at this temperature. Ternary crystalline solution exists in the periclase structure to a composition of Mg_0.853Al_0.063Si_0.026O at 1710°C.     

Synthesis and Characterization of Catalysts in the System A12O3-MgO-NiO-Ni for Methane Reforming with CO2

Compositions to yield solid solutions were prepared by coprecipitation and firing. Fine active clusters of nickel were obtained by reduction. Phase changes were followed by thermal analysis, X-rays, electron microscopy, surface area measurements, and reforming methane with CO₂. Nickel on periclase remained active over 125 h with nearly 100% conversion. The catalysts suppressed carbon deposition which occurred only immediately upstream and downstream of the catalyst. Nickel on spinel solid solution caused the catalyst to shatter with a drastic increase in surface area due to formation of NiAl₂O₄. The pressure drop decreased the flow rate, making this catalyst infeasible.     

Reducing the Sintering Temperature for MgO–Al2O3Mixtures by Addition of Cryolite (Na3AlF6)

The preparation of near stoichiometric spinel and alumina-rich spinel composites from Al₂O₃and MgO powders with the addition of Na₃AlF₆up to 4 wt% in the temperature range 700°–1600°C was studied; 98 wt% spinel containing 72 wt% Al₂O₃can be produced from the mixture of 72 wt% (50 at.%) Al₂O₃+ 28 wt% (50 at.%) MgO powders with the addition of 1 wt% Na₃AlF₆fired at 1300°C for 1 h. Spinels containing 81–85 wt% Al₂O₃can be produced from either the mixture of 90 wt% (78 at.%) Al₂O₃+ 10 wt% (22 at.%) MgO or the mixture of 95 wt% (88 at.%) Al₂O₃+ 5 wt% (12 at.%) MgO powders with the addition of 4 wt% Na₃AlF₆in the temperature range 1300°–1600°C by using a torch-flame firing for 3 min, followed by quenching in water, while the same system under slow cooling in a furnace results in spinel containing 74–76 wt% Al₂O₃. Microscopic studies indicate that the alumina-rich spinel composites consist of a continuous majority spinel phase and an isolated minority corundum phase, regardless of slow cooling in a furnace or quenching in water.     

Early Stages of Composite Formation by Oxidation ofLiquid Aluminum Alloys

Microstructure evolution during the early stages in the directed oxidation of molten Al-Mg and Al-Mg-Si alloys was investigated to provide needed insight into the origins of the incubation period and its practical elimination by SiO₂ additions. Oxidation experiments were performed primarily in thermogravimetric balances and microstructures were analyzed by optical and scanning electron microscopy. Continuous heating above the alloy liquidus produces first a thin MgO layer and then a brief rapid growth of a spinel + metal mixture within a temperature range which depends on the alloy Mg content and the heating rate. The initial rapid oxidation terminates abruptly with the formation of a dense spinel layer at the surface, leading to a long incubation period of negligible weight gain. The surface MgO regenerates in this regime, while the metal channels slowly advance upward by dissolution of the dense spinel, eventually reaching the MgO and inducing the formation of composite nodules. These consist initially of spinel + metal upon which the conventional A1₂O₃+ metal growth starts after the Mg in the near-surface alloy is depleted to a critical level. SiO₂ surface additions promote composite nucleation by locally hindering surface passivation, acting as an O source for continued spinel growth, and modifying the local chemistry to facilitate the formation of A1₂O₃.     

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.     

TiO2加入量对铝镁质干式捣打料性能的影响

以电熔白刚玉、电熔镁砂、活性Al2 O3微粉为主要原料,以TiO2为烧结剂,分别在1300℃、1400℃、1500℃和1600℃热处理3 h制备铝镁质干式捣打料.研究了TiO2加入量、热处理温度对捣打料物相、烧结性能、力学性能及显微结构的影响.结果表明,适量TiO2的引入能显著促进铝镁质干式捣打料的反应烧结,常温和高温...     

Microstructure Control and Wear of Al2O3-SiC-(Al, Si) Composites Made by Melt Oxidation

Ceramic matrix composites of Al₂O₃-SiC-(Al, Si) have been fabricated by directed melt oxidation of aluminum alloys into SiC particulate preforms. The proportions of AI₂O₃, alloy, and porosity in the composite can be controlled by proper selection of SiC particle size and the processing temperature. The wear resistance of composites was evaluated in pin-on-disk experiments against a hard steel substrate. Minimum wear rate comparable to conventional ceramics such as ZTA is recorded for the composition containing the highest fraction of alloy, owing to the development of a thin and adherent tribofilm with a low coefficient of friction.