Crystallization and Seeding Effect in BaAl2Si2O8 Gels

Two sol-gel routes have been used to prepare celsian (BaAl₂Si₂O₈) precursors. The crystallization behaviors of unseeded gels and the gels seeded with 5 wt% feldspar, rutile, LiAISi₂O₂, and ZrO₂ crystals as well as SrAl₂Si₂O₈ gel have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermal gravimetric and differential thermal analyses (TGA and DTA). It was found that metastable hexacelsian, rather than the preferred monoclinic celsian, was the major crystallization product in the two unseeded gels. Seeding largely enhanced the crystallization of monoclinic celsian, but the mechanisms involved were different for seeding with different crystals. SrAl₂Si₂O₈ gel and ZrO₂ crystals acted as normal heterogeneous centers and had little catalytic effect on monoclinic celsian crystallization. Feldspar and rutile crystals worked as epitaxial substrates for monoclinic celsian and produced pure monoclinic celsian in some temperature regions. LiAISi₃O₈ seeds lowered the glass transition temperature of the immediately adjacent gel matrix and led to the crystallization of pure monoclinic celsian.     

Synthesis of Celsian (BaAl2Si2O8) from Solid Ba-Al-Al2O3-SiO2 Precursors: I, XRD and SEM/EDX Analyses of Phase Evolution

An intimate Ba-Al-Al₂O₃-SiO₂ powder mixture, produced by high-energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edge-to-edge width). Heat treatments conducted from 300° to 1650°C in pure oxygen or air were used to transform these solid-metal/oxide precursors into BaAl₂Si₂O₈. Barium oxidation was completed, and a binary silicate compound, Ba₂SiO₄, had formed within 24 h at 300°C. After 72 h at 650°C, aluminum oxidation was completed, and an appreciable amount of BaAl₂O₄ had formed. Diffraction peaks consistent with hexagonal BaAl₂Si₂O₈, BaAl₂O₄, β-BaSiO₃, and possibly β-BaSi₂O₅ were detected after 24 h at 900°C. Diffraction peaks for BaAl₂O₄ and BaAl₂Si₂O₈ were observed after 35 h at 1200°C, although SEM analyses also revealed fine silicate particles. Further reaction of this silicate with BaAl₂O₄ at 1350° to 1650°C yielded a mixture of hexagonal and monoclinic BaAl₂Si₂O₈. The observed reaction path was compared to prior work with other inorganic precursors to BaAl₂Si₂O₈.     

Crystallization Mechanism of MAS-osumilite with Composition Mg2Al4Si11O30 from Glass

MAS-osumilite is a metastable double-ring silicate which is attractive for use in glass-ceramics because of its low thermal expansion coefficient. The phase can be crystallized from glass powders of MAS-osumilite composition containing small amounts of BaO. The nucleation and crystallization process has been investigated by means of TEM and SEM. Crystallization starts with the precipitation of β-quartz^ss of composition MgAl₂O₄· n SiO₂. BaO concentrates in a residual glass phase between the β-quartz_ss crystals. In a second stage small amounts of Ba-osumilite (BaMg₂Al₆-Si₉O₃₀) nucleate and crystallize from this glass phase, In a third stage BaO-free MAS-osumilite crystals are formed by epitactic growth on the Ba-osumilite surfaces and grow into adjacent β-quartz_ss grains. Two factors are essential for the suppression of undesired further phases to develop a monophase (besides some Ba-osumilite) MAS-osumilite glassceramic: a residual glass phase with a composition close to Ba-osumilite, and β-quartz_ss crystals having a composition close to that of MAS-osumite.     

The Binary System Anorthite (CaAl2Si2O8)—Åkermanite (Ca2MgSi2O7)

A study of the equilibrium relations in the system anorthite-åkermanite has been made as a part of an investigation of the ternary system diopside-anorthite-åkermanite. The technique employed was the well-known quenching method. The liquidus relations indicate that anorthite and åkermanite form a simple binary system, with a eutectic located at a composition 46 anorthite, 54 åkermanite (by weight) and at a temperature of 1234°± 2°C. There is strong reason to believe that pure åkermanite, rather than an aluminous melilite, crystallizes with anorthite from intermediate compositions. No evidence to support the alleged decomposition of åkermanite at or below 1325°C. was obtained, since åkermanite gave no indication of instability in the liquidus range of temperatures. The study sheds no light on the relations between anorthite and its hexagonal and orthorhombic polymorphs.     

Synthesis and Consolidation of BaAl2Si2O8:Eu: Development of an Integrated Process for Luminescent Smart Ceramic Materials

A novel, integrated, fast, and inexpensive process for the preparation of dense Ba_{(1− x )}Eu _x Al₂Si₂O₈ thin ceramic specimens for damage sensor applications is reported. The processing approach involves a combination of combustion synthesis for the preparation of the powders and spark plasma sintering (SPS) for the consolidation of the specimens to densities close to 100% of relative density. The synthesis of the porous powders by combustion resulted in particle (agglomerate) sizes that were on average 421 nm, as determined from dynamic light scattering, and in the almost complete reduction of the initial Eu³⁺ activators to Eu²⁺. The powders densified to grain sizes of around 250 nm due to a collapse of the porous powder structure and minimal grain growth during SPS. Thermal treatment of the powders and sintered specimens improved the intensity of the emissions at 373 and 745 nm and diminished the emission at 485 nm. The luminescence phenomena from the specimens were a result of two mechanisms: (1) the removal of strain in the lattice due to thermal treatment, and (2) a charge transfer mechanism between Eu²⁺ and Eu³⁺.     

Crystallization of MgO-CaO-SiO2-P2O5 Glass

The crystallization behavior of a glass with a composition of 40 wt% 3CaO · P₂O₅−60 wt% CaO · MgO · 2SiO₂ was investigated. The primary crystalline phase was apatite with a dendritic form and ellipsoidal shape. β-(3CaO · P₂O₅) and CaO · MgO · 2SiO₂ were crystallized as samples heated to 990°C, and a three-layer structure was obtained. The development and morphology of this construction were explained by both the surface crystallization of the apatite and CaO · MgO · 2SiO₂ and the bulk crystallization of apatite and the CaO · MgO · 2SiO₂-β-(3CaO · P₂O₅) composite.     

Kinetics of Hexacelsian-to-Celsian Phase Transformation in SrAl2Si2O8

The kinetics of hexacelsian-to-celsian phase transformation in SrAl₂Si₂O₈ have been investigated. Phase-pure hexacelsian was prepared by heat treatment of glass flakes at 990°C for 10 h. Hexacelsian flakes were isothermally heat-treated at 1026°, 1050°, 1100°, 1152°, and 1200°C for various times. The amounts of monoclinic celsian formed were determined using quantitative X-ray diffraction. Values of reaction rate constant, k , at various temperatures were evaluated from the Avrami equation. The Avrami parameter was determined to be 1.1, suggesting one-dimensional growth with the interface rather than a diffusion-controlled transformation mechanism. From the temperature dependence of k , the apparent activation energy for this reaction was evaluated to be 527 ± 50 kj/mol (126 ± 12 kcal/mol). This value is consistent with a mechanism involving the transformation of the layered hexacelsian structure to a three-dimensional network celsian structure which necessitates breaking of the strongest bonds, the Si─O bonds.     

Silica Glass Segregation in 3 wt% LiF-Doped Hot-Pressed Y2Si2O7

Hot-pressed yttrium disilicate ceramics have been characterized using analytical transmission electron microscopy (TEM). The microstructure consists of large grains of the γ phase of stoichiometric γ-Y₂Si₂O₇ containing rounded glassy Y-doped SiO₂ inclusions; excess glassy SiO₂-rich material is also found at the grain boundaries. Two main reasons are found for the inhomogeneity: a slight SiO₂ excess is inferred from the composition measurements, and the LiF flux used in hot pressing would also promote glass formation. Improved high-temperature mechanical properties would only be possible if residual glass formation was minimized, strategies for doing so are discussed, and the importance of analytical TEM for studying such submicron scale inhomogeneity is underlined.     

Electrochemical Studies in Glass: I, The System NiO-Na2Si2O5

A solid electrolyte electrochemical cell of the type Pt|Ni:NiO _{a =1}∥ZrO₂+7.5% CaO∥Ni:NiO _{a <1}+glass|Pt was used to measure the activities of NiO in sodium disilicate glass from 750° to 1100°C. The data indicate a solubility varying from 11 mol% (5.0 wt%) at 800° to 20 mol% (9.3 wt%) at 1100°C. From the variation in NiO activity, the activity of sodium disilicate in glass solution was estimated; from these combined data partial molar free energies and entropies of solution of NiO and Na₂Si₂O₅ and free energies and entropies of mixing were calculated. A partial phase diagram for the system NiO-Na₂Si₂O₅ proposed from solubility data indicates a eutectic at ∼12 mol% (5.3 wt%) NiO at 830°C.     

Polar-Oriented Crystallization of Fresnoite (Ba2TiSi2O8) on Glass Surface Due to Ultrasonic Treatment with Suspensions

Improved polar ( c -axis) oriented growth of fresnoite (Ba₂TiSi₂O₈) was observed for surface crystallization of a glass 33.3BaO· 16.7TiO₂· 50SiO₂ (in mol%) due to ultrasonic surface treatment with an aqueous suspension of Ba₂TiSi₂O₈, and an oriented film more than 50 μm in thickness was prepared. The effect of ultrasonic treatment on the polar orientation depends on the suspending particles; of these, foreign particles of a phase other than the crystallizing Ba₂TiSi₂O₈ give poorer efficiency than the Ba₂TiSi₂O₈ particles. It is assumed that fresnoite has a tendency toward preferred polar growth on the glass, and seeding of a large number of fine particles through ultrasonic bombardment realizes the tendency. The polar-oriented growth kinetics of Ba₂TiSi₂O₈ were also discussed.     

Structure of Liquid Al6Si2O13 (3:2 Mullite)

We report the first measurements of the structure factor, S ( Q ), and the pair distribution function, G ( r ), of Al₆Si₂O₁₃ (3:2 mullite) in the normal and supercooled liquid states in the temperature range 1776–2203 K. Measurements are obtained by synchrotron X-ray scattering on levitated, laser-heated liquid specimens. The S ( Q ) shows a prepeak at 2.0 Å⁻¹ followed by a main peak at 4.5 Å⁻¹ and a weak feature at 8 Å⁻¹. The G ( r ) shows a strong (Si,Al)–O correlation at 1.80 Å at high temperature that moves to 1.72 Å as the liquid is supercooled. The second and third nearest neighbor peaks at 3.0 and 4.25 Å sharpen with supercooling. The short-range structure of the high-temperature liquid is similar to the corresponding glasses produced by rapid quenching. Supercooling causes an increase in the concentration of tetrahedral Si⁴⁺ ions, which is manifested by the large shift in the first peak to lower ionic distance, r , values in G ( r ). The increase in tetrahedrally coordinated Si⁴⁺ ions is offset by an increase in octahedral Al³⁺ ions. The clustering of the SiO₄⁴⁻ tetrahedral units results in increased viscosity of the liquid at temperatures below the melting point, which is consistent with Al₆Si₂O₁₃ being a fragile liquid.     

Crystallization of a PbO-BaO-TiO2-Al2O3-SiO2 Glass

The phase development, microstructure development, and Curie temperature of PbO-BaO-TiO₂-Al₂O₃-SiO₂ glass- ceramics were investigated by X-ray diffractometry, electron microscopy, and dilatometry. The primary crystalline phase was perovskite titanate, which exhibited bulk nucle- ation. When the samples were heated at low temperatures, the Curie inversion was hindered and cubic perovskite was obtained. Increased heating temperature/time resulted in tetragonal perovskite. Growth of a surface layer due to the crystallization of the secondary crystalline phase PbO- Al₂O₃-2SiO₂ resulted in the coexistence of coarse and fine perovskite particles. The increase in the relative number of coarse-to-fine perovskite particles with the heating temperature/time was the major factor responsible for increased spontaneous deformation and higher Curie temperature of the perovskite phase. Substitution of barium for lead in the lead titanate phase probably occurred and is explained by the use of SiO₂ instead of B₂O₃ as the glass former in the present study.     

Near-Net-Shaped (Ba,Sr)Al2Si2O8 Bodies by the Oxidation of Machinable Metal-Bearing Precursors

A novel process has been used to fabricate refractory aluminosilicate bodies of near net shape: the oxidation of machinable, alkaline-earth-metal-bearing precursors. Ba–Sr–Al–Al₂O₃–SiO₂ precursors of similar molar volume as strontium-doped celsian, (Ba,Sr)Al₂Si₂O₈, were prepared by powder metallurgical processing. Precursor powder mixtures, obtained by high-energy vibratory ball milling, were compacted and formed into bars or disks by uniaxial pressing. The bar-shaped precursors were machined on a metal-working lathe using hardened steel tooling. The shaped precursors were oxidized and converted into celsian. The resulting celsian bodies retained the shapes, and fine machined features, of the precursors. The measured changes in dimensions were small and in good agreement with calculated values.     

Crystallization Behavior of CaO–P2O5 Glass with TiO2, SiO2, and Al2O3 Additions

TiO₂ above 4 mol% is an effective nucleating agent for CaO–P₂O₅ glass which also contains substantial SiO₂ and Al₂O₃ additions. Glass ceramics can be made from this glass using a single slow heating ramp with no need for a nucleating heat treatment step. Powder of this composition crystallizes rapidly to β-Ca₂P₂O₇, whereas bulk glass crystallizes from diphasic nuclei consisting of a central cubic Ca-P-Ti-Si-Al oxide phase surrounded by impure AlPO₄ dendrites. Metastable calcium phosphate grows on the AlPO₄ dendrites and later transforms to β-Ca₂P₂O₇.     

Seeding Effects on Crystallization of KAlSi3O8, RbAlSi3O8, and CsAlSi3O8 Gels and Glasses

Crystallization of stoichiometric KAlSi₃O₈, RbAlSi₃O₈, and CsAlSi₃O₈ gels (single-phase and multiphasic gels) and glasses with and without seeding has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). The pure gels of K-, Rb-, and Cs-feldspar compositions crystallized mainly to leucite (KAlSi₂O₆), a Rb analogue of leucite (RbAlSi₂O₃), and pollucite (CsAlSi₂O₆), respectively. The KAlSi₃O₈ glass also yielded leucite. No feldspar phases crystallized in the temperature region studied. The leucite crystallization temperature was lowered by using multiphasic gels. The crystallization sequence of the KAlSi₃O₈ glasses and single-phase gels did not change upon seeding with isostructural feldspar crystals. No significant crystallization effects were detected with the use of RbAlSi₃O₈ and CsAlSi₃O₈ multiphasic gels. However, K-feldspar (sanidine) was crystallized by using both compositionally and structurally different gels of KAlSi₃O₈, i.e., multiphasic gels seeded With feldspar crystals. The microstructure of oriented sanidine crystals around the seeds indicated an epitaxial crystallization mechanism.     

Phase Relations in the System NaF-CaF2-NaAlSiO4-CaAl2Si2O8

The phase diagram of the pseudoternary reciprocal system NaF–CaF₂–NaAlSiO₄–CaAl₂Si₂O₈ is reported in this paper. The phase relations in the system have been investigated by differential thermal analysis, quenching melts, X-ray diffractometry, and optical and electron microscopies. The stable diagonal CaF₂–NaAlSiO₄ divides the system in two pseudoternary systems. The solidus temperatures in the two subsystems NaF–CaF₂–NaAlSiO₄ and CaF₂–NaAlSiO₄–CaAl₂Si₂O₈ are 805°± 2°C and 1095°± 4°C, respectively. An extensive region of liquid–liquid immiscibility is evident in the NaF–CaF₂–NaAlSiO₄ subsystem. The compositions of the two liquids fall outside the compositional surface NaF–CaF₂–NaAlSiO₄–CaAl₂Si₂O₈, but only a small deviation from the ternary behavior is observed.     

Crystallization of Hexagonal BaNb2O6

Pure hexagonal BaNb₂O₆ crystallizes at 640° to 700°C from an amorphous material prepared by simultaneous hydrolysis of barium and niobium alkoxides. The structure is characterized by layer networks of NbO₆ octahedra. Crystallization isotherms are described by the Avrami equation In (1—f) = -ktⁿ with n=2; the activation energy is 268 kJ-mol-'. Hexagonal BaNb₂O₆ transforms to the ortho-rhombic modification one at 12.50° to 1310°C.     

Hydrolysis and Dispersion Properties of Aqueous Y2Si2O7 Suspensions

The dispersion of aqueous γ-Y₂Si₂O₇ suspensions, which contain only one component but have a complex ion environment, was studied by the introduction of two different polymer dispersants, polyethylenimine (PEI) and polyacrylic acid (PAA). The suspension without any dispersant remains stable in the pH range of 9–11.5 because of electrostatic repulsion, while it is flocculated upon stirring due to the readsorption of hydrolyzed ions on the colloid surface. However, suspensions with 1 dwb% PEI exhibit greater stability in the pH range of 4–11.5. The addition of PEI shifts the isoelectric point (IEP) of the suspensions from pH 5.8 to 10.8. Near the IEP (pH_IEP=10.8), the stability of the suspensions with PEI is dominated by the steric effect. When the pH is decreased to acid direction, the stabilization mechanism is changed from steric hindrance to an electrosteric effect little by little. PAA also has the effect of reducing the hydrolysis speed via a "buffer effect" in the basic pH range, but the lack of adsorption between the highly ionized anionic polymer molecules and the negative colloid particle surfaces shows no positive effect on hydrolysis of colloids and on the stabilization of Y₂Si₂O₇ suspensions.     

Coalescence and Crystallization in Powdered High-Cordierite (2MgO·2Al2O3·5SiO2) Glass

Coalescence and crystallization in powdered high-cordierite glass were investigated by DTA, SEM, and XRD methods, using also thermal expansion and viscosity data. The apparent activation energy for crystallization obtained from the DTA experiments was about half that for viscous flow estimated from the viscosity. Crystallization in such a system is believed to be controlled by a surface nucleation mechanism.