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.     

Hybrid Gels for Homoepitactic Nucleation of Mullite

Hybrid gels, defined as gels from mixtures of polymerically and colloidally derived sols, offer many opportunities for crystalline microstructure development upon heating. In this study, hybrid mullite gels are formed by mixing a colloidal boehmite—silica sol with a polymeric aluminum nitrate—tetraethoxysilane-derived sol. The polymeric gel crystallizes in situ to form mullite that acts as seed crystals for homoepitactic nucleation during the subsequent transformation of the colloidal component of the hybrid gel. Compared with the entirely colloidal gel, the introduction of a 30 wt% polymeric gel fraction results in an increase in apparent nucleation frequency from ∼5x10¹¹ to ∼1x10¹⁴ nuclei / cm³ at 1375°C, a reduction in high-temperature grain size from 1.4 to 0.4 μm at 1550°C, and an increase in the degree of microstructural homogeneity, as evidenced by intragranular pore removal.     

Effect of Physical Structure on the Phase Development of Aluminosilicate Gels

The surface area and density of aluminosilicate xerogels containing a one-to-one Al/Si molar ratio (47 wt% alumina) can be varied dramatically by changing the pore fluid prior to drying. The surface area of ethanol-washed 47 wt% alumina gels was more than 500 m²/g, while gels dried from the mother liquor (approximately 75 vol% ethanol, 25 vol% water) had less than 1 m²/g surface area. Changes in the physical structure of the dried gels had dramatic effects on subsequent phase evolution and densification behavior during heat treatment. NMR, X-ray diffraction, and DTA were used to follow the phase evolution of different gels. Differences in the amorphous gel structure were identified using ²⁷Al and ²⁹Si MAS NMR. Gels of identical composition prepared from the same precursor solutions crystallized to different phases, depending upon the surface area of the gel prior to heating. The high surface material (ethanol washed) formed mullite and amorphous silica, while the low surface area gel (unswashed) crystallized to mullite and cristobalite. These gels were prepared from alkoxide precursors. A low surface area gel with a different degree of chemical homogeneity was prepared by the nitrate method for comparison. Results indicate that the physical structure of aluminosilicate gels, i.e., pore structure and chemical homogeneity, has a dramatic influence on phase evolution.     

Effects of Hydrolysis on the Kinetics of High-Temperature Transformations in Aluminosilicate Gels

The reaction kinetics for the formation of mullite (3Al₂O₃· 2SiO₂) from sol-gel-derived aluminosilicate gels prepared under various hydrolysis conditions were studied using dynamic X-ray diffraction (DXRD) and differential thermal analysis (DTA). The DXRD experiments showed that the apparent single-phase gels aluminosilicate gels were not completely single-phase gels but a composite of a single-phase gel (which has molecular-scale mixing) and a diphasic gel (which has nanometer-scale mixing). Mullite formation from these composite gels exhibits a two-stage conversion, the first at about 980°C and the second at about 1220°C. Gels prepared by a slower hydrolysis rate tend to have a higher conversion after the first stage, therefore, better molecular-scale mixing. Simultaneous formation of Al-Si spinel and mullite was also observed at 980°C. This coincidence of mullite and spinel formation could explain some of the controversy in the literature concerning the 980°C thermal event.     

Transformation Kinetics of Diphasic Aluminosilicate Gels

The formation of mullite in diphasic aluminosilicate gels of stoichiometric composition was studied by quantitative X-ray diffraction (QXRD) and electron microscopy (AEM and SEM). The transformation is preceded by an incubation time which is apparently an activated process with an activation energy of 236 ± 15 kcal/(g·mol) (∼987 ± 63 kJ/(g·mol)). Microscopic observations show that mullitization occurs via nucleation and growth as a mechanism either interface-controlled or controlled by short-range diffusion near the interface. The kinetic time exponent ( n ), determined by microstructural and QXRD observations, is close to 1.3. The transformation kinetics involve a nearly constant nucleus density, and time-dependent growth rate for nearly spherical grains. A linear time-temperature-transformation diagram is observed.     

Mullite Formation from Nonstoichiometric Diphasic Precursors

Diphasic gels with Al/Si atomic ratios of 6/1, 3.1/1, 3/1, 2/1, and 1/1 were used to study the effect of precursor composition on mullite formation process and the resulting microstructure. Mullite formation initiated at about 1300°C for all samples, with only some slight differences in temperatures. The mullite formation temperature was a minimum for an Al/Si ratio of 3.1/1 and increased as the Al/Si ratio of the gels increased or decreased. For the 6/1 gel, dissolution of alumina into mullite solid solution was observed after the initial mullite formation. The dissolution process was reversed above 1450°C with the formation of θ-Al₂O₃ and then α-Al₂O₃. Change of microstructures from equiaxed to elongated mullite grain structures was found within a narrow range of composition near the nominal Al/Si ratio of 3/1.     

Phase Transformation of Diphasic Aluminosilicate Gels

Aluminosilicate gels with compositions Al₂O₂/SiO₂ and 2 were prepared by gelling a mixture of colloidal pseudo-boehmite and a silica sol prepared from acid-hydrolyzed Si(OC₂H₅)₄. Upon heating the pseudo-boehmite transforms to γ-Al₂O₃ around 400°C, then to δ-Al₂O₃ at 1050°C, and at 1200°C reacts with amorphous SiO₂ to form mullite. Some twinned θ-Al₂O₃ forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al₂O₃ or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape.     

Kaolinite–Mullite Reaction Series: The Development and Significance of a Binary Aluminosilicate Phase

The semiquantitative estimations of 980°C exothermic reaction products of kaolinite by quantitative X-ray diffraction (QXRD) and chemical leaching techniques show the formation of a significant amount of amorphous aluminosilicate phase (∼ 30 to 40 wt%). The theoretically expected AlO₄/AlO₆ ratio in the 980°C reaction is in close agreement with the value measured by the X-ray fluorescence (XRF) technique and the experimental radial electron distribution (RED) profile agrees with the suggested 980°C formation of Si-Al spinel with mullite-like composition. Mullitization of kaolinite has been compared with a synthetic Al₂O₃—SiO₂ mixture. In synthetic mixtures development of an intermediate amorphous aluminosilicate phase is an essential step prior to mullitization. Kaolinite forms mullite in two ways: (i) by polymorphic transformation of cubic mullite at 1150° to 1250°C and (ii) by nucleation of mullite in the amorphous aluminosilicate phase and its subsequent growth above 1250°C. Thus chemical continuity is maintained throughout the reaction series and the intermediate spinel phase is silicon bearing and its subsequent transformation to mullite confirms the topotactic concept in the kaolinite transformation.     

红柱石的莫来石化动力学

通过对红柱石在１３００℃、１３５０℃和１４００℃的等温转化试验，用ＸＲＤ外标法定量测定了各温度下不同时间所取样品的莫来石含量，并绘制出转化率对时间的动力学曲线。求出该过程的表现活化也能为４６７．６ｋＪ·ｍｏｌ－１，并确定过程的表观反应速度常数与绝对温度的关系为ｋ＝２．０３×１０１３ｅｘｐ（５．５６×１０４／Ｔ）。     

氟化铝对红柱石莫来石化的影响

本文主要研究了氟化铝的加入对红柱石矿物转化为莫来石的温度,转化程度以及生成相莫来石的形态的影响.结果表明,加入6wt％氟化铝可以使红柱石矿物在1300℃完全莫来石化;同时氟化铝的加入可以促进莫来石晶体各向异性生长,得到直径为0.5～2.0 μm,长径比为20～40的莫来石晶须,可以用于制备自增韧的莫来石陶瓷或莫来石晶须增强的复合材料.     

Microstructures and Properties of Three Composites of Alumina, Mullite, and Monoclinic SrAl2Si2O8

Three composites that were 96% alumina were mixed and uniaxially dry-pressed into bars and pellets; all had monoclinic SrAl₂Si₂O₈ as an intergranular phase. The diffraction patterns, microstructure, density, dielectric properties, strength, and toughness were measured. The first composition, which contained crystalline SrCO₃, Al₂O₃, and SiO₂, in a 1:1:2 molar ratio, as the 4% component, densified but was generally inferior to the second and third compositions, which contained strontium aluminosilicate (SrAl _x Si _y O _z , SAS) glass as the 4% component, in terms of mechanical properties, defects, and monoclinic SrAl₂Si₂O₈ transformation. The second composition, which lacked B₂O₃, was very tough and was comparable to commercial alumina, in terms of the dielectric constant. The third, which contained 0.068% of B₂O₃ that was dissolved in the SAS glass as a sintering aid, had high strength and toughness and no macroscopically visible defects. Mullite formed, in addition to monoclinic SrAl₂Si₂O₈ in all three composites. Alumina–monoclinic SrAl₂Si₂O₈ composites of the third composition had room-temperature properties that were comparable to commercial aluminas that contained 96% alumina and also had potential for mechanical and refractory applications.     

Structure of Sodium Aluminosilicate Glasses

A series of sodium aluminosilicate glasses composed of varying ratios ( R ) of Al₂O₃/Na₂O (0.25 R 2.0) has been simulated with the molecular dynamics technique using a tetrahedral form of a three-body interaction potential. Extrema in the activation energies for sodium diffusion and in the diffusion constants for all of the atomic species were observed for glasses with equal concentrations of Al₂O₃ and Na₂O ( R = 1.0). These changes corresponded to the minimum observed experimentally in the activation energy for electrical conductivity and to the maximum observed in the viscosity for glasses with compositions of R = 1.0. The coordination of aluminum remained 4 over the entire compositional range, negating the need to invoke a coordination change of aluminum to explain the changes in the physical properties. The changes to the simulated physical properties as R passed through the equivalence point were attributed to the elimination of nonbridging oxygen, to the introduction of oxygen triclusters, and to changes in the distribution of ring structures within the glass networks.     

High-Temperature Hydroxylation of Mullite

A plane-parallel, polished, 0.9 mm thick, single-crystal (001) plate of 2:1 mullite was treated for 6 h at 1600°C in an Ar/H₂O (90/10) gas mixture at 100 kPa. Optical microscopy studies and infrared (IR) reflection spectroscopy studies of the lattice vibrations yielded no evidence for change with respect to the untreated reference crystal. However, IR absorption spectroscopy showed that structurally bound OH groups were formed by the heat treatment in the Ar/H₂O gas mixture. IR absorption depth profile analysis showed a rather homogeneous OH distribution through the crystal. Five different hydroxyl groups were separated according to dipole orientations and peak positions: E ‖ a , ω _{a 1}= 3447 cm⁻¹, ω _{a 2}= 3579 cm⁻¹; E ‖ b , ω _{b 1}= 3456 cm⁻¹, ω _{b 2}= 3544 cm⁻¹; and E ‖ c , ω _{c 1}= 3498 cm⁻¹. All IR peaks were strongly broadened (between 90 and 150 cm⁻¹) because of a distribution in O-H binding distances caused by the real structure of mullite.     

Fracture Behavior of SiC-Whisker-Reinforced Barium Aluminosilicate Glass-Ceramic Matrix Composites

Barium aluminosilicate (BAS) glass-ceramic composites reinforced with various volume percents (0, 10, 20, 30, 40 vol%) of SiC whiskers were fabricated by hot pressing. The microstructure, the whisker/matrix interface structure, the phase constitution, and the mechanical properties of the composites were systematically studied by means of SEM, TEM, and XRD techniques as well as by indentation crack microfracture and single-edge-notched-beam bend testing. It was demonstrated that the incorporation of SiC whiskers could significantly increase the flexural strength and fracture toughness of BAS glass-ceramic matrices. The addition of active Al₂O₃ to the BAS matrix reduced the amount of SiO₂ in the matrix, forming needlelike mullite, which further improved the mechanical properties.     

Structural Control of Elastic Constants of Mullite in Comparison to Sillimanite

Nine elastic stiffness coefficients, c_ij, of a mullite single crystal (2Al₂O₃·SiO₂) are measured using acoustic resonance spectroscopy. The obtained values are similar to those of the structurally related aluminosilicate phase sillimanite (Al₂O₃·SiO₂). Characteristic elastic properties of the two minerals are interpreted with the help of their crystal structures and atomic force constants for sillimanite. The high longitudinal stiffness coefficients, c₃₃, of mullite (∼352 GPa) and sillimanite (∼388 GPa) are caused by continuous “stiff” load-bearing tetrahedral chains parallel to c-axis, while the “soft” octahedral chains have minor direct influence. They stabilize the tetrahedal chains against tilting. The lower c₃₃ value of mullite in comparison to the sillimanite value may be caused by a weakening of the load-bearing tetrahedral chains which are parallel to c-axis because of partial replacement of silicon by the weaker-bonded aluminum. The longitudinal stiffness coefficients perpendicular to c-axis are significantly lower, because of sequences of alternating “soft” octahedral and “stiff” tetrahedral units. Within the plane (001), the compliant octahedra exhibit stiffness-controlling influence with coefficients parallel to b-axis (c₂₂∼ 233 GPa) being somewhat lower than parallel to a-axis (c₁₁∼ 291 GPa). This is explained with the occurrence of compliant octahedral Al(1)–O(D) bonds, which are more effective parallel to b-axis rather than to a-axis. Because octahedra are unaffected by the aluminum to silicon substitution, c₁₁ and c₂₂ coefficients of mullite and sillimanite are very similar. Shear stiffness coefficients of mullite increase from c₅₅ (∼77 GPa) to c₆₆ (∼80 GPa) to c₄₄ (∼110 GPa), indicating increasing resistance against shear deformation within the planes (010), (001), and (100). The lattice plane of the highest shear stiffness (100) is built up of an oxygen-oxygen network, diagonally braced along 〈011〉 (“Jägerzaun”). This network with short oxygen–oxygen distances can be sheared by compression and elongation along oxygen–oxygen interaction lines only which is rather unlikely. Because of the lack of such networks in the planes (010) and (001), bending and deformation of structural units become easier, and consequently c₅₅ and c₆₆ are <c₄₄. All three shear stiffness coefficients of mullite are slightly lower than those of sillimanite because of the reduction of the mean tetrahedral bond strength in mullite caused by partial substitution of silicon by aluminum.     

Rare-Earth Aluminosilicate Glasses

Rare-earth aluminosilicate glasses of the general formula 20R₂O₃· 20Al₂O₃· 60SiO₂ have been formed for 10 of the 14 possible rare-earth oxides. Two series of "mixed-rare-earth" glasses were also formed (Nd/Er and Nd/Y). These glasses exhibit exceptionally high glass transformation temperatures, moderate thermal expansion coefficients, and refractive indices of approximately 1.65. The glass transformation temperature and thermal expansion coefficients vary linearly with the field strength of the rare-earth ion. No evidence of a "mixed-rare-earth effect" was observed. MAS-NMR indicates that the aluminum ions are tetrahedrally coordinated in at least some of these glasses.     

Internal Reduction of an Iron-Doped Magnesium Aluminosilicate Melt

Optical and electron microscopies are used to analyze the mechanism and kinetics of internal reduction of an Fe²⁺-doped magnesium aluminosilicate melt. Melt samples are heated to temperatures in the range of 1300°–1400°C under a flowing gas mixture of CO/CO₂, which corresponds to a p _O2 range of 1 × 10⁻¹³–4 × 10⁻¹³ atm. The melt experiences an internal reaction in which a dispersion of nanometer-scale iron-metal precipitates forms at an internal interface. The metal precipitates show no signs of coarsening within the samples; however, the crystals at the surface (which formed in the initial part of the reaction) do grow via vapor phase transport. The overall reaction is characterized by parabolic kinetics, which is indicative of chemical diffusion being the rate-limiting step. The diffusion of network-modifier divalent cations—particularly Mg²⁺ cations—is demonstrated to be the rate-limiting factor, and its diffusion coefficient is calculated to be ∼1 × 10⁻⁶ cm²/s within the temperature range of the experiments.     

Crystallization Kinetics and Mechanism of Low-Expansion Lithium Aluminosilicate Glass-Ceramics by Dilatometry

The crystallization kinetics and mechanism of a precursor glass of lithium aluminosilicate (LAS)-based commercial low-expansion glass-ceramics were investigated using a dilatometer. The isothermal crystallization behavior of β-quartz solid solution (ss) was found to obey the Avrami equation. Nonisothermal crystallization data were analyzed by the Ozawa method and modified Kissinger equation. The value of the Avrami exponent ( n ) was ∼1.5, and the apparent activation energy ( E _a) was ∼500 kJ/mol, which was close to that of the diffusion of silicon and aluminum ions as well as metal–oxygen bond strengths, suggesting a three-dimensional (3D) diffusion-controlled reaction mechanism.     

Infrared Spectroscopy of Lead and Alkaline-Earth Aluminosilicate Glasses

Infrared spectroscopic studies of lead and alkaline-earth aluminosilicate glasses in the series x MO- x Al₂O₃ (1 - 2 x )SiO₂ M = Mg, Ca, Sr, Ba, and Pb; 0.05 x 0.275] were carried out in the range 2000 to 200 cm⁻¹. Three major absorption bands were observed in the 1100-, 800-, and 500-cm⁻¹ regions. The frequency and the intensity of the 1100-cm⁻¹ band varied linearly with composition. For a specific value of x , the changes in the frequency, intensity, and bandwidth of this band decreased in the order Mg > Ca > Sr > Pb > Ba and the apparent disorder in the glass structure, effected by the substitution of aluminum, increased in the direction Mg < Ca < Sr < Ba, with Pb in between Mg and Ca. The force constants and the bond orders of the Si-O and Al-O stretching vibrations in each of the glasses increased monotonically as Al/(Al + Si) decreased.     

Effects of Water Content on the Properties of Sodium Aluminosilicate Glasses

Sodium aluminosilicate glasses of the general formula 25Na₂O. x Al₂O₃.(75 – x )SiO₂ were prepared with a range of hydroxyl contents. Both the glass transformation and isokom temperatures decrease as the hydroxyl content increases. The magnitude of the decrease in each property is a function of the alumina content of the glass, with the largest effects occurring for the glass containing 25 mol% Al₂O₃. The magnitude of the effect of water as a function of alumina content reflects the decrease in the concentration of nonbridging oxygens in the glass with increasing Al₂O₃ content, since the effect of the terminal hydroxyl species on the connectivity of the glass is enhanced by the elimination of nonbridging oxygens. The results indicate that the large effect of water content on the properties of these glasses must be considered when discussing the role of alumina in glasses.