Phase Equilibria in the System Na3AlF6−AlF3

The phase diagram for the system Na₃AlF₆−AlF₃ was determined by quenching and microscopy supplemented with DTA and X-ray powder diffraction. Primary crystallization fields for cryolite, chiolite, and aluminum fluoride are defined. Liquid containing 30 wt% AlF₃ is in peritectic equilibrium with solid cryolite and chiolite at 741°C; solid chiolite and aluminum fluoride are in equilibrium with liquid at the eutectic composition 39 wt% AlF₃ and 694°C.     

Phase Diagram of a Portion of the System Na3AlF6-AlF3-Al2O3.

The system Na₃AlF₆-AlF₃-Al₂O₃ was investigated by a combination of quenching, optical microscopy, and X-ray powder diffraction techniques in order to define liquidus temperatures, univariant lines, and invariant points. Phase fields for the primary crystallization of cryolite, chiolite, aluminum fluoride, α-alumina, and η-alumina were located. A ternary peritectic point contained 28.3% AlF₃-4.4% Al₂O₃-67.3% Na₃AlF₆ at 723°C. A eutectic point of composition 37.3% AlF₃-3.2% Al₂O₃-59.5% Na₃AlF₆ occurs at 684°C.     

Formation of α-Si3N4 Solid Solutions in the System Si3N4-A1N-Y2O3

The subsolidus phase diagram of the quasiternary system Si₃N₄-AlN-Y₂O₃ was established. In this system α-Si₃N₄ forms a solid solution with 0.1Y₂O₃: 0.9 AIN. The solubility limits are represented by Y_0.33Si_10.5Al_1.5O_0.5N_15.5 and Y_0.67Si₉A1₃ON₁₅. At 1700°C an equilibrium exists between β-Si₃N₄ and this solid solution.     

Crystallization of Li2O · Al2O3· 6SiO2 Glasses Containing Niobium Pentoxide as Nucleating Dopant

Niobium pentoxide (T form, orthorhombic system) was utilized to promote devitrification in Li₂O · Al₂O₃· 6SiO₂ glasses. Two or more mole percentage of this nucleating dopant enhanced crystallization in these glasses. Glasses containing 4.0 and 8.0 mol% T-Nb₂O₅ exhibited a high tendency to form dispersed TT-Nb₂O₅ (monoclinic system) precipitates during the glass quenching process. The crystallization process in glasses containing 2.0 or 4.0 mol% T-Nb₂O₅ occurred as microphase separation, followed by the formation of dispersed TT-Nb₂O₅ crystalline precipitates (760°C), followed by β-quartz solid-solution ( ss ) formation (850° to 900°C) heterogeneously nucleated from the precipitates. β-quartz( ss ) transformed to β-spodumene( ss ), along with a polymorphic transition from the TT-Nb₂O₅ to M-Nb₂O₅ (tetragonal system) crystalline phase.     

Prediction of a Critical Temperature for Aluminate Formation in Alumina/Copper–Oxygen Eutectic Bonding

A pseudobinary thermodynamic analysis of the alumina/copper–oxygen eutectic bonding process yields a value of approximately 1076°C for the invariant temperature of the {Cu( ss ), Cu( L1 ), CuAlO₂, Al₂O₃} equilibrium. This temperature, which is 11°C above the eutectic, corresponds to a critical temperature for stability of the copper aluminate CuAlO₂ in eutectic bonding. Implications for controlling interface microstructure are highlighted.     

Phase Formation in the System Na2O · Al2O3-CaO · A12O3-Al2O3 at 1200°C in Air

Phase relations in the system Na₂O_· Al₂O₃-CaO· Al₂O₃-Al₂O₃ at 1200°C in air were determined using the quenching method and high-temperature X-ray diffraction. The compound 2Na₂O · 3CaO · 5Al₂O₃, known from the literature, was reformulated as Na₂O · CaO · 2Al₂O₃. A new compound with the probable composition Na₂O · 3CaO · 8Al₂O₃ was found. Cell parameters of both compounds were determined. The compound Na₂O · CaO-2Al₂O₃ is tetragonal with a = 1.04348(24) and c = 0.72539(31) nm; it forms solid solutions with Na₂O · Al₂O₃ up to 38 mol% Na₂O at 1200°C. The compound Na₂O · 3CaO · 8Al₂O₃ is hexagonal with) a = 0.98436(4) and c = 0.69415(4) nm. The compound CaO · 6Al₂O₃ is not initially formed from oxide components at 1200°C but behaves as an equilibrium phase when it is formed separately at higher temperatures. The very slow transformation kinetics between β and β "-Al₂O₃ make it very difficult to determine equilibrium phase relations in the high-Al₂O₃ part of the diagram. Conclusions as to lifetime processes in high-pressure sodium discharge lamps can be drawn from the phase diagram.     

Phase Relations in the Garnet Region of the System Y2O3–Fe2O3–FeO–Al2O3

Liquidus equilibrium relations for the air isobaric section of the system Y₂O₃–Fe₂O₃–FeO–Al₂O₃ are presented. A Complete solid-solution series is found between yttrium iron garnet and yttrium aluminum garnet as well as extensive solid solutions in the spinel, hematite, orthoferrite, and corundum phases. Minimum melting temperatures are raised progressively with the addition of alumina from 1469°C in the system Y–Fe–O to a quaternary isobaric peritectic at 1547°C and composition Y _0.22 Fe _1.08 Al _0.70 O _2.83* Liquidus temperatures increase rapidly with alumina substitutions beyond this point. The thermal stability of the garnet phase is increased with alumina substitution to the extent that above composition Y _0.75 Fe _0.65 Al _0.60 O ₃ garnet melts directly to oxide liquid without the intrusion of the orthoferrite phase. Garnet solid solutions between Y _0.75 Fe _1.25 O ₃ and Y _0.75 Fe _0.32- Al _0.93 O ₃ can be crystallized from oxide liquids at minimum temperatures ranging from 1469° to 1547°C, respectively. During equilibrium crystallization of the garnet phase, large changes in composition occur through reaction with the liquid. Unless care is taken to minimize temperature fluctuations and unless growth proceeds very slowly, the crystals may show extensive compositional variation from core to exterior.     

Chemical Preparation of the Binary Compounds in the Calcia-Alumina System by Self-Propagating Combustion Synthesis

The binary compounds Ca₃Al₂O₆ (C₃A), Ca₁₂Al₁₄O₃₃ (C₁₂A₇), CaAl₂O₄ (CA), CaAl₄O₇ (CA₂), and CaAl₁₂O₁₉ (CA₆) in the CaO-Al₂O₃ system have been synthesized as high-compound-purity ceramic powders by using the self-propagating combustion synthesis (SPCS) method. Materials characterization of the above-mentioned phases was performed via powder X-ray diffractometry (XRD), Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The structural characterization of the C₁₂A₇ phase has been performed via Rietveld analysis on the powdered XRD samples. It has hereby been shown that, by using this synthesis procedure, it should be possible to manufacture high-purity ceramic powders of CA, CA₂, and C₁₂A₇ at 850°C, C₃A at 1050°C, and CA₆ at 1200°C in a dry-air atmosphere.     

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.     

Sintering, Crystallization, and Properties of B2O3/P2O5-Doped Li2O·Al2O3·4SiO2 Glass-Ceramics

Sintering, crystallization, microstructure, and thermal expansion of Li₂O·Al₂O₃·4SiO₂ glass-ceramics doped with B₂O₃, P₂O₅, or (B₂O₃+ P₂O₅) have been investigated. On heating the glass powder compacts, the glassy phase first crystallized into high-quartz s.s., which transformed into β-spodumene after the crystallization process was essentially complete. The effects of dopants on the crystallization of glass to high-quartz s.s. and the subsequent transformation of high-quartz s.s. to β-spodumene were discussed. The major densification occurred only in the early stage of sintering time due to the rapid crystallization. All dopants were found to promote the densification of the glass powders. The effect of doping on the densification can fairly well be explained by the crystallization tendency. All samples heated to 950°C exhibited a negative coefficient of thermal expansion ranging from about −4.7 × 10^-6 to −0.1 × 10^-6 K^-1. Codoping of B₂O₃ and P₂O₅ resulted in the highest densification and an extremely low coefficient of thermal expansion.     

Calcium Phosphate Cements Prepared by Acid–Base Reaction

We investigated the characteristics of calcium phosphate cements (CPC) prepared by an exothermic acid–base reaction between NH₄H₂PO₄-based fertilizer (Poly-N) and calcium aluminate compounds (CAC), such as 3CaO · Al₂O₃ (C₃A), CaO · Al₂O₃ (CA), and CaO · 2Al₂O₃ (CA₂), in a series of integrated studies of reaction kinetics, interfacial reactions, in-situ phase transformations, and microstructure development. Two groups were compared: untreated and hydrothermally treated CPC specimens. The extent of reactivity of CAC with Poly-N at 25°C was in the following order: CA > C₃A ≫ CA₂. The formation of a NH₄CaPO₄· x H₂O salt during this reaction was responsible for the development of strength in the CPC specimens. The in-situ phase transformation of amorphous NH₄CaPO₄· x H₂O into crystalline Ca₅(PO₄)₃(OH) and the conversion of hydrous Al₂O₃ gel →γ-AIOOH occur in cement bodies during exposure in an autoclave to temperatures up to 300°C. This phase transformation significantly improved mechanical strength.     

Effect of Organic Compounds on the Interconversions of Calcium Aluminate Hydrates: Hydration of Tricalcium Aluminate

The paste hydration of tricalcium aluminate (C₃A) in the presence of organic compounds was investigated at several temperatures up to 75°C. The results confirm earlier hypotheses that the hexagonal calcium aluminate hydrates (principally C₄AH₁₃) which are first formed create a protective barrier around the remaining C₃A and severely restrict further hydration. Above 30°C, conversion to C₃AH₆ breaks down this barrier and causes rapid hydration of C₃A. Organic compounds retard the hydration of C₃A by inhibiting the conversion reaction. Experiments with synthetic C₄AH₁₃ showed that organic molecules can form interlayer compounds, and it is considered that random sorption into the C₃AH₁₃ structure restricts the transformation to C₃AH₆. Other aspects of C₃A hydration and of the reactivity of C₄AH₁₃ are also discussed.     

Stability of Mullite as Derived from Equilibria in the System CoO-Al2O3-SiO2

The free energy of reaction for the formation of mullite from its oxide components was derived from equilibrium studies in the system CoO-Al₂O₃-SiO₂. Within this system there appears, at solidus temperature in a certain composition area, the phase assemblage mullite + silica + spinel (= cobalt aluminate) + liquid. Determination of the oxygen pressure of a gas phase at which metallic cobalt precipitates from this phase assemblage and from the phase assemblage spinel (= cobalt aluminate) + corundum in the system CoO-Al₂O₃ permits calculation of ΔG° for the reaction 3Al₂O₃+ 2SiO₂= Al₆Si₂O₁₃. The value obtained at 1422°C is -5.8 kcal.     

Reactions in Silica-Alumina Mixtures

Examination of mixtures of extremely pure silica and alumina shows that the greatest reactivity is not encountered with stoichiometric ratios to form mullite with the formula 3Al₂O₃. 2SiO₂ but rather with the formula 2A1₂O₃-SiO₂, and that reactivity also depends on the crystalline modification of alumina. A sharp exothermic differential thermal peak at 980°C. is attributed to three simultaneous reactions dependent on the silica-alumina ratio of the mixture: (1) the crystallization of gamma alumina, (2) the crystallization of a hydrogen aluminum spinel (HAl₅O₅), and (3) the reaction of silica with the hydrogen aluminum spinel to form mullite.     

Effect of Alumina Additions on Microstructural Aspects of the β to α Transformation in Tantalum (V) Oxide

Tantalum (V) oxide (Ta₂O₅) has potential applications as part of an environmental barrier coating system for Si₃N₄-based turbine components. However, at elevated temperatures, Ta₂O₅ undergoes a phase transformation from the orthorhombic (β) phase to the tetragonal phase (α), which is undesirable because of the associated volume change. The purpose of the present work was to study the effect of alumina additions (0–5 wt%) on the β to α transformation temperature, and associated modifications to the Ta₂O₅ microstructure. Sintered microstructures were characterized using SEM (scanning electron microscopy), and XRD (X-ray diffraction) was used to identify the phases present at room temperature. It was found that for undoped Ta₂O₅, transformation of the low-temperature β-phase begins at ∼1300°C, and leads to extensive microcracking of the sintered sample. For samples containing alumina, an increase in the transformation temperature was observed. The solubility limit of alumina in Ta₂O₅ was between 1 and 3 wt%; for samples in which this was exceeded, the AlTaO₄ second and phase particles were seen to be highly effective at inhibiting grain growth.     

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.     

Forming and Sintering of in Situ Alumina Composite with Hydraulic Inorganic Binder

Two types of barium aluminate binders were prepared by heat treatment of barium aluminate precursors, synthesized by using solution processes, at low temperatures ( T < 500°C). One was barium monoaluminate (BaAl₂O₄), and the other was barium aluminate binder (BAH binder) composed of amorphous phase, barium aluminate monohydrate (BaAl₂O₄·H₂O), and BaAl₂O₄. The setting time of the BAH binder was controlled by adjusting the heat-treatment temperature of the BAH binder precursor. The addition of the synthesized BaAl₂O₄ powders to Al₂O₃ powders improved the bending strength of Al₂O₃ matrix green bodies. The synthesized BaAl₂O₄ powders led to the in situ forming of barium hexaaluminate (BaO· x Al₂O₃, x = 6.9: BA6) platelets in the matrix by reacting with Al₂O₃ during sintering. The formed BA6 platelets inhibited the grain growth of the matrix Al₂O₃ grains.     

Melt Differentiation Induced by Zonal Structure Formation of Calcium Aluminoferrite in a CaO–SiO2–Al2O3–Fe2O3 Pseudoquaternary System

The phase stability in part of the P₂O₅-bearing pseudoquaternary system CaO–SiO₂–Al₂O₃–Fe₂O₃ has been studied by electron probe microanalysis, optical microscopy, and powder X-ray diffractometry. At 1973–1653 K, the α-Ca₂SiO₄ solid solution [α-C₂S(ss)] and melt coexisted in equilibrium, both chemical variations of which were determined as a function of temperature. The three phases of melt, calcium aluminoferrite solid solution (ferrite), and C₂S(ss) coexisted at 1673–1598 K. On the basis of the chemical compositions of these phases, a melt-differentiation mechanism has been, for the first time, suggested to account for the crystallization behavior of Ca₃Al₂O₆ solid solution [C₃A(ss)]. When the α-C₂S(ss) and melt were cooled from high temperatures, the melt would be induced to differentiate by the crystallization of ferrite. Because the local equilibrium would be continually attained between the rims of the precipitating ferrite and coexisting melt during further cooling, the melt would progressively become enriched in Al₂O₃ with respect to Fe₂O₃. The resulting ferrite crystals would show the zonal structure, with the Al/(Al+Fe) value steadily increasing up to 0.7 from the cores toward the rims. The C₃A(ss) would eventually crystallize out of the differentiated melt between the zoned ferrite crystals in contact with their rims.     

Thermodynamic Stability of Potassium-beta-Alumina

The thermodynamic stability of K-beta-Al₂O₃ (KBA) has been determined by characterizing the equilibrium between K-β- and K-β"-Al₂O₃ using a potentiometric technique with yttria-stabilized zirconia (YSZ) as solid electrolyte. The cell allows an in situ check of the establishment and maintenance of the phase equilibrium by changing the composition of the KBA material reversibly. The activity of the potassium oxide dissolved in KBA amounts to (375°–600°C) from which the standard Gibbs free energy of formation of K-β"-Al₂O₃ can be obtained as      

Phase Diagram for the SiF4 Join in the System SiO2-Al2O3-SiF4

Alumina reacts with 1 atm of SiF₄ below 660°± 7°C to form A1F₃ and SiO₂. At higher temperatures the product is a mixture of fluorotopaz and AIF₃. Mixtures of fluorotopaz and AIF3 decompose in 1 atm of SiF₄ at 973°± 8°C and form tabular α-alumina. The equilibrium vapor pressure of SiF₄ above mixtures of fluorotopaz and AlF₃ is log p (atm) = 9.198 – 11460/ T (K). Fluorotopaz itself decomposes at 1056°± 5°C in 1 atm of SiF₄ to give acicular mullite, 2Al₂O₃^.1.07SiO₂. Alumina and mullite are stable in the presence of 1 atm of SiF₄ above 973° and 1056°C, respectively. The phase diagram of the system SiO₂-Al₂O₃-SiF₄ shows only gas-solid equilibria.