Crystallization Behavior of Al2O3 in the Presence or SiO2

The independent crystallization sequence of an Al₂O₃ component is modified in the presence of SiO₂ and vice versa. Mixed SiO₂-Al₂O₃, gel (28 wt% SiO₂ and 72 wt% Al₂O₃) forms neither cristobalite nor γ-Al₂O₃ and corundum at 1000°C but forms Si-Al spinel; an amorphous aluminosilicate phase invariably also forms after the gel is heated. However, the composition of this amorphous aluminosilicate phase is not as yet known.     

Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.     

Quaternary System Al2O3–CaO–MgO–SiO2: I, Study of the Crystallization Volume of Al2O3

Compatibility relations of Al₂O₃ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching followed by microstructural and energy-dispersive X-ray examination. A projection of the liquidus surface of the primary phase volume of Al₂O₃ was constructed in terms of the CaO, SiO₂, and MgO contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, were defined, and the positions of the isotherms were tentatively established. The effect of SiO₂, MgO, and CaO impurities on Al₂O₃ growth also was studied.     

Revised Phase Diagram for the System Al2O3—SiO2

On the basis of 190 runs made up to 1860°C in sealed noble-metal containers the following revisions have been made in the equilibrium diagram for the system A1₂O₃–SiO₂. Mullite melts congruently at 1850°C. The extent of equilibrium solid solution in mullite at solidus temperature is from approximately 60 mole % Al₂O₃ (3/2 ratio) to 63 mole % A1₂O₃. Metastable solid solutions can be prepared up to about 67 mole % Al₂O₃. There is no evidence for stable solubility of excess SiO₂ beyond the 3/2 composition at pressures below 3 kbars. Refractive indices are presented for glasses containing up to 60 mole % Al₂O₃ and from them the composition of the eutectic is confirmed at 5 mole % SiO₂. The variation in lattice constants of the mullite solid solution is not an unequivocal guide to composition since mullites at one composition produced at different temperatures show differences in spacing, no doubt reflecting Al-Si ordering phenomena. The possibility of quartz and corundum being the stable assemblage at some low temperatures and pressures cannot be ruled out. A new anhydrous phase in the system is described, which was previously thought to be synthetic andalusite; it is probably a new polymorph of the Al₂SiO₅ composition with ortho-rhombic unit-cell dimensions a =7.55 A, b =8.27 A, and c = 5.66 A.     

Homogeneous ZrO2–Al2O3 Composite Prepared by Nano-ZrO2 Particle Multilayer-Coated Al2O3 Particles

ZrO₂–Al₂O₃ nanocomposite particles were synthesized by coating nano-ZrO₂ particles on the surface of Al₂O₃ particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al₂O₃ surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO₂ in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO₂ particles, which contained about 12 wt% ZrO₂, were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO₂ was well dispersed in the Al₂O₃ matrix.     

Wet Milling of Fe/Al/Al2O3 and Fe2O3/Al/Al2O3 Powder Mixtures

Wet milling of Al₂O₃-aluminide alloy (3A) precursor powders in acetone has been investigated by milling Fe/Al/Al₂O₃ and Fe₂O₃/Al/Al₂O₃ powder mixtures. The influence of the milling process on the physical and chemical properties of the milled powders has been studied. Particle refinement and homogenization were found not to play a dominant role, whereas plastic deformation of the metal particles leads to the formation of dislocations and a highly disarranged polycrystalline structure. Although no chemical reactions among the powder components in Fe₂O₃/Al/Al₂O₃ powder mixtures were observed, the formation of a nanocrystalline, ordered intermetallic FeAl phase in Fe/Al/Al₂O₃ powder mixtures caused by mechanical alloying was detected. Chemical reactions of Fe and Al particle surfaces with the atmosphere and the milling media lead to the formation of highly porous hydroxides on the particle surfaces. Hence the specific surface area of the powders increases, while the powder density decreases during milling. The fraction of Fe oxidized during milling was determined to be 0.13. The fraction of Al oxidized during milling strongly depends on the metal content of the powder mixture. It ranges between 0.4 and 0.8.     

Structure of Incoherent ZrO2/Al2O3 Interfaces

High-resolution electron microscopy was used to image incoherent ZrO₂/Al₂O₃ interfaces in ZrO₂-toughened Al₂O₃ containing intragranular ZrO₂. These particles are generally spherical but are sometimes faceted. High-resolution electron micrographs provide atomic-level information on the interfacial structure. For spherical particles, both ledgelike images and misfit dislocation-like images accommodated the lattice misfit, depending on the orientation of the interface, while faceted particles imply at least one low-energy ZrO₂/Al₂O₃ interface.     

Properties of Sm2O3─Al2O3─SiO2 Glasses for In Vivo Applications

The compositional range for glass formation below 1600°C in the Sm₂O₃─Al₂O₃─SiO₂ system is (9–25)Sm₂O₃─(10–35)Al₂O₃─(40–75)SiO₂ (mol%). Selected properties of the Sm₂O₃─Al₂O₃─SiO₂ (SmAS) glasses were evaluated as a function of composition. The density, refractive index, microhardness, and thermal expansion coefficient increased as the Sm₂O₃ content increased from 9 to 25 mol%, the values exceeding those for fused silica. The dissolution rate in 1 N HCl and in deionized water increased with increasing Sm₂O₃ content and with increasing temperature to 70°C. The transformation temperature ( T _g ) and dilatometric softening temperature ( T _d ) of the SmAS glasses exceeded 800° and 850°C, respectively.     

Densities of SiO2-Al2O3 Melts

The densities of binary aluminosilicate melts were measured X-radiographically as a function of Al₂O₃, concentration between 1800° and 2000°C. Within this temperature range, the density curves vary linearly and are parallel from fused SiO₂ to ≊30 to 45 mol% Al₂O₃, depending on the temperature. At higher Al₂O₃ contents, negative deviation from linearity increases with increasing temperature. Recent supplementary research efforts on various aspects of the system SiO₂-Al₂O₃ indicate that the changing coordination and structural role of the aluminum ion may be a primary factor in determining the shapes of the density curves.     

Quaternary System Al2O3–CaO–MgO–SiO2: II Study of the Crystallization Volume of MgAl2O4

Solid-state compatibility and melting relations of MgAl₂O₄ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching selected samples located in the 65 wt% MgAl₂O₄, plane followed by microstructural and energy dispersive X-ray analysis. A projection of the liquidus surface of the primary crystallization volume of MgAl₂O₄ was constructed from CaO, SiO₂ and exceeding Al₂O₃, not involved in stoichiometric MgAl₂O₄ formation; those three amounts were recalculated to 100 wt%. The temperature and character of six invariant points, where four solids co-exist with a liquid phase, were defined. One maximum point was localized and the positions of the isotherms were tentatively established. The effect of CaO, SiO₂, and Al₂O₃ impurities on the high temperature behavior of spinel materials was also discussed.     

Metastable Phase Formation in Plasma-Sprayed ZrO2 (Y2O3)–Al2O3

Rapidly solidified ZrO₂ (Y₂O₃)–Al₂O₃ powders were prepared by melting fine-particle aggregates in a high-enthalpy plasma flame and then rapidly quenching them in cold water or on a copper chill plate. To ensure complete melting and homogenization of all the particles before quenching, the water-quenching treatment was often repeated two or even three times. The resulting melt-quenched powders and splats displayed a variety of metastable structures, depending on composition and cooling rate. ZrO₂-rich material developed an extended solid solution phase, whereas eutectic material formed a nanofibrous or amorphous structure. Under high cooling rate conditions, the ZrO₂-rich material developed a nanocomposite structure ( t -ZrO₂+α-Al₂O) directly by melt-quenching, whereas, more typically, such a structure was developed only after postannealing of the as-quenched metastable material.     

The System Al2O3-Cr2O3-Sio2

Liquidus phase equilibrium data are presented for the system Al₂O₃-Cr₂O₃-SiO₂. The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al₂O₃-ICr₂O₃-93SiO₂) coexists with a mullite solid solution (61Al₂O₃-10Cr₂O₃-29SiO₂), a corundum solid solution (19Al₂O₃-81Cr₂O₃), and cristobalite (SO₂). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases.     

Reinvestigation of Al–Si Spinel Phase in Diphasic Al2O3–SiO2 Gel

Mechanical mixture of γ-Al₂O₃ and amorphous SiO₂, and diphasic Al₂O₃/SiO₂ gels of three different compositions were synthesized. They were subjected to heat treatment to various temperatures in the range 900°–1600°C. Qualitative X-ray diffraction data show that these diphasic gels do not crystallize to a combined mixture of θ-Al₂O₃ and α-Al₂O₃ polymorphs at the intermediate stage, prior to mullite formation. Estimated mullite formation data show that the course of its formation from mixed oxides was different from that of diphasic gels. Results are compared with previous findings and the concept of Al–Si spinel formation in the phase transformation of stoichiometric diphasic gel system is substantiated.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

Structural Analysis of SiO2-Al2O3 Glasses

The structure of SiO₂-Al₂O₃ glasses with up to 60 wt% Al₂O₃ was investigated using the radial distribution function together with the correlation method based on X-ray scattering intensity data. Radial distribution curves are interpreted on the basis of glass-in-glass separation with the constituents of SiO₂-rich and Al₂O₃-rich glasses. The structure of the Al₂O₃-rich glass has a short-range ordering similar to the crystal structure of mullite. The calculated S- i (S) curve of this model gives good agreement with the observed one.     

Cone Deformation Study of a Portion of the System SrO–Al2O3–SiO2

The results of a study of deformation temperatures and rates within the composition area 25 to 45% SrO, 5 to 25% Al₂O₃, 50 to 70% SiO₂ are presented. A eutectic of the composition 30% SrO, 10% Al₂O₃, 60% SiO₂ is indicated with "1 o'clock" deformation at 1155°C. and "6 o'clock" deformation at 1165°C. A low-temperature area surrounding this eutectic includes 27.5 to 32.5% SrO, 10 to 12.5% Al₂O₃, 57.5 to 62.5% SiO₂. Compositions within this range reach "6 o'clock" deformation at approximately 1180°C.     

Glass Formation and Recrystallization in the Lithium Metasilicate Region of the System Li2O–Al2O3–SiO2

The stability of the vitreous state in the lithium metasilicate region of the system Li₂O–Al₂O₃–SiO₂ was found to be a function of the concentration of lithia. The higher the lithia content, the less stable was the glass. The devitrification of glasses in this system was studied. In addition to the phases present at or near the liquidus, it was found that the β -eucryptite– β -quartz solid solution phase was metastable over most of the region. The Li₂O–SiO₂, β -Li₂O–Al₂O₃–4SiO₂ solid solution, β -Li₂O–Al₂O₃–2SiO₂ solid solution triple point was estimated to be near 62.5% SiO₂, 17% Al₂O₃, and 20.5% Li₂O (by weight). The thermal expansions of bodies in this region were measured and the values obtained are explained in terms of the phases present.     

Application of Compositionally Diphasic Xerogels for Enhanced Densification: The System Al2O3-SiO2

Stoichiometric mullite (71.38 wt% Al₂O₃-28.17 wt% SiO₂) and 80 wt% Al₂O₃-20 wt% SiO₂ gels were prepared by the single-phase and/or diphasic routes. Dense sintered bodies were prepared from both sets of gels in the Al₂O₃-SiO₂ system. Apparent densities of 96% and 97% of theoretical density were measured for the diphasic (using two sols) mullite samples sintered at 1200° and 1300°C for 100 min, respectively; this compared with 85% and 94% for the single-phase xerogels under the same conditions, and to much lower values for mullite prepared from conventional mixed powders. The microstructure of the mullite pellets from diphasic xerogel precursors is also considerably finer.     

Effects of Amorphous and Crystalline SiO2 Additives on γ-Al2O3-to-alpha-Al2O3 Phase Transitions

This paper focused on the effects of various phases of SiO₂ additives on the γ-Al₂O₃-to-α-Al₂O₃ phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta-to-α phase transition was elevated by adding amorphous SiO₂, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO₂, such as quartz and cristobalite. Amorphous SiO₂ was considered to retard the γ-to-α phase transition by preventing γ-Al₂O₃ particles from coming into contact and suppressing heterogeneous nucleation on the γ-Al₂O₃ surface. On the other hand, crystalline SiO₂ accelerated the α-Al₂O₃ transition; thus, this SiO₂ may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO₂ additives, especially amorphous and crystalline phases, largely influenced the temperature of γ-Al₂O₃-to-α-Al₂O₃ phase transition.     

Change in Chemical Composition of Mullite Formed from 2SiO2°3Al2O3 Xerogel During the Formation Process

The chemical composition of mullite which was termed from 2SiO₂3Al₂O₃ xerogel by firing was examined by analytical TEM. Mullite formed at 950°C firing showed around 66 mol% Al₂O₃, which was fairly Al₂O₃ rich compared with the bulk composition. The chemical composition of mullite gradually approached the bulk composition as the firing temperature increased to 1400°C and slightly departed again above that firing temperature.