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 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.     

Crystallization of Celsian (BaAl2Si2O8) Glass

The fabrication and crystallization of stoichiometric and near-stoichiometric celsian, BaAl₂Si₂O₈ (BAS), glass made via melt and sol-gel routes is reviewed. After considering celsian polymorphism and methods of stabilizing monoclinic celsian, some new experimental results are presented which show that melt glass crystallizes predominantly via surface nucleation, but sol-gel glass via volume nucleation. Possible reasons for this behavior and the kinetics of the crystallization process are examined.     

Low-Temperature Synthesis of BaAl2O4/Aluminum-Bearing Composites by the Oxidation of Solid Metal-Bearing Precursors

BaAl₂O₄/aluminum-bearing composites have been synthesized via the low-temperature oxidation of Ba-Al precursors. Ba-Al powder mixtures that were prepared via high-energy vibratory milling were uniaxially pressed into bar-shaped specimens that were then exposed to a series of heat treatments in pure, flowing oxygen at temperatures up to 640°C. Oxidation at a temperature of 300°C resulted in the formation of barium peroxide (BaO₂). Additional heat treatment at a temperature of 550°C resulted in the consumption of BaO₂ and some aluminum to yield BaAl₂O₄ and Al₄Ba. The oxidation of Al₄Ba at a temperature of 640°C yielded additional BaAl₂O₄. Microstructural analyses revealed that a well-dispersed, co-continuous mixture of Al₂O₃-excess BaAl₂O₄ and 99.5% pure aluminum was produced.     

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⁺.     

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.     

Subsolidus Phase Equilibria in the System SrO-B2O3-SiO2

The subsolidus compatibility relations in the system SrO-B₂O₃- SiO₂ were determined by solid-state reaction techniques and X-ray powder diffraction methods. The system was found to contain 11 subsolidus compatibility relations, one stable ternary compound (Sr₃B₂SiO₈), and one metastable ternary compound with a probable composition SrB₂Si₂O₈.     

Phase Equilibrium Studies in the System CaO-Cr2O3-SiO2

Results are presented of a study in air of mixtures in the system CaO-Cr₂O₃-SiO₂. The phase equilibrium diagram shows relations at liquidus temperatures for all but the high-lime part of the system. In this omitted part chromium in the mixtures oxidizes in air to higher valence forms. The compound Ca₃Cr₂Si₃O₁₂ (uvarovite) occurs at subsolidus temperatures, decomposing at 1370°C. to α-CaSiO₃ and Cr₂O₃. The inhibiting action of chromium oxide on the inversion of high-temperature forms of Ca₂SiO₄ to the low-temperature γ-Ca₂SiO₄ is discussed in the light of new data. Evidence is presented for the existence of a pentavalent chromium compound, Ca₃(CrO₄)₂, having solid-solution relations with Ca₃SiO₄.     

The System Bi2O3-SiO2

The Bi₂O₃-rich side of the system Bi₂O₃-SiO₂ was studied with powder X-ray diffraction and differential thermal analysis. In the composition 6Bi₂O₃. x SiO₂, the metastable γ phase (bcc) was observed to exist over the range of 0 < x ≤ 1. In most of the compositions studied, metastable phases of water-quenched melts transformed into another metastable phase before reaching stable phases. A modification of the phase diagram is proposed.     

Phase Diagram of Na2O-B2O3-SiO2 System

The binary isopleth Na₂O.B₂O₃-SiO₂ of the Na₂O-B₂O₃ SiO₂ ternary system has been investigated. A phase diagram is presented based upon data from differential thermal analysis studies of prepared glasses and direct observation of the melting behavior using solid-state video imaging. Phase equilibria relations in the Na₂O-B₂O₃-SiO₂ ternary system have been reassessed by combining information from this study with existing data from the literature. A revised liquidus surface for the ternary is presented in which the form of the isotherms is updated.     

Phase Separation of Glasses in the System SrO-B2O3-SiO2

The region of stable liquid-phase separation in the system SrO-B₂O₃SiO₂ was found to extend from the SrO-SiO₂ binary join, across the ternary region, to the SrO-B₂O₃ join. The boundary of the region rich in network-forming oxides lies between the SiO₂-B₂O₃ boundary and the 2.5-mol% SrO isopleth, whereas the maximum SrO content is between 30.3 and 35.2 mol% (42 to 48 wt%) SrO at an SiO₂: B₂O₃ ratio of 4:1. The microstructures of glasses quenched from the immiscibility region were characterized by scanning electron microscopy. Evidence is presented of both stable and metastable phase separation and of microseparation and coring in disperse-phase spheres rich in network-forming oxides.     

Effects of MoO3, on Phase Separation of Na2O-B2O3-SiO2 Glasses

Immiscibility temperatures of Na₂O-B₂O₃-SiO glasses, with andwithout 1 mol% MoO₃, additions, were determined and the effect of MoO₃ additions on the 65O°C immiscibility isotherms was established. In addition, immiscibility temperature and phase-separation morphology of an Na₂O-B₂O₃-SiO₂ glass with progressive additions of MoO₃, were investigated. It was found that the addition of small amounts of MoO₃ extends the immiscibility boundary of the system and raises the immiscibility temperature by ∼l8°C for each mol % MoO₃, addition. Analysis of phase-separation morphology suggests that the MoO₃, additions do not significantly alter the tie lines of phase separation in the system, although such additions cause a lowering of the viscosities and the glass-transition temperatures of these glasses.     

Phase Relations in the System FeO-Fe2O3-ZrO2-SiO2

Phase equilibrium relations in the liquidus-temperature region of the system iron oxide-ZrO₂-SiO₂ in air were delineated by the quenching technique. Additional equilibration runs were made at other controlled oxygen pressures as well as in sealed containers. The results of all runs have been combined with literature data for the system in contact with metallic iron to locate approximately the quaternary liquidus invariant points within the system FeO-Fe₂O₃-ZrO₂-SiO₂.     

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.     

Phase Equilibria in the System Li2O-Cr2O3-SiO2

The system Li₂O-Cr₂O₃–SiO₂ contains one previously reported ternary compound, LiCrSi₂O₆. Six subsolidus compatibility triangles and six ternary invariant points were located. The highest solidus, temperature is 1283°C, but liquidus temperatures are much higher for many compositions.     

Reactions in the System Li2O-MgO-Al2O3-SiO2: II, Phase Equilibria in the High-Silica Region

Liquidus and subsolidus relationships were determined for compositions lying on the 65 and 75% SiO₂ planes in the quaternary tetrahedron. From these data and from additional data on several compositions not lying on these two planes, a picture of the probable positions of the phase volumes and quaternary invariant points was formulated. Solid solution effects in quartz and in β-spodumene, the polymorphism of MgSiO₃, the phase relationships along the join Li₂O -MgO ° SiO₂-Li₂O. Al₂O₃.2SiO₂, and the location of the β-spodumene-mullite boundary were briefly investigated as subsidiary problems during the course of the work. The significance of the data with respect to ceramic bodies and glazes is discussed.     

Phase Relations in the System CaO-Ta2O5-SiO2

The system CaO-Ta₂O₃-SiO₂ was studied using a combination of hot-stage microscopy and the quenching technique. Primary crystallization fields were defined for the various calcium silicates, and for the calcium tantalates: CaO·2Ta₂O₅, CaO·Ta₂O₅, 2CaO·Ta₂O₅, and 4CaO·Ta₂O₅. A congruently melting ternary compound 10CaO·Ta₂O₅·6SiO₂, isostructural with the mineral niocalite, is the only ternary phase in the system. A large twoliquid region extends across the system from the CaO-SiO₂ binary to within 1 wt% of the Ta₂O₃-SiO₂ binary but does not cut it, in marked contrast to the analogous CaO-Nb₂O₅-SiO₂ system. Other somewhat unexpected differences were noted between these systems.     

Phase Equilibria at Liquidus Temperatures in the System MgO-FeO-Fe2O3-SiO2

Liquidus temperatures are presented for mixtures in the system MgO-FeO-Fe₂O₃-SiO₂. The standard quenching technique was modified for work under controlled atmospheres of varying O₂ pressures. Data were obtained for the temperature range 1159° to 1775°C., and with O₂ pressures ranging from 1 to 10^-8.⁹ atm. Approximate compositions of crystalline phases were determined, and paths of equilibrium crystallization were derived for selected mixtures under idealized conditions. Application of the phase diagrams to steel-plant refractories problems is indicated.