Phase Equilibria at Liquidus Temperatures in the System Iron Oxide–Al2O3–SiO3 in Air Atmosphere

Liquidus temperatures, obtained by the quenching technique, are presented for mixtures in the system iron oxide–Al₂O₃–SiO₂ in air atmosphere. Approximate compositions of crystalline phases in equilibrium with liquids have been determined, and paths of equilibrium crystallization are discussed briefly. Application of the phase diagram to refractories problems is indicated.     

Thermodynamic Modeling of the FeO–Fe2O3–MgO–SiO2 System

The liquid and solid phases in the FeO–Fe₂O₃–MgO–SiO₂ system are of importance in ceramics, metallurgy, and petrology. A complete critical evaluation and thermodynamic modeling of the phase diagrams and thermodynamic properties of this system are presented. Optimized equations for the thermodynamic properties of all phases are obtained that reproduce all available thermodynamic and phase equilibrium data within experimental error limits from 25°C to above the liquidus temperatures at all compositions and oxygen partial pressures. The optimized thermodynamic properties and phase diagrams are believed to be the best estimates presently available. The database of the model parameters can be used with software for Gibbs energy minimization to calculate any type of phase diagram section.     

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.     

Phase Equilibrium Relationships in a Portion of the System MgO–Al2O3–2CaO·SiO2

The system MgO–Al₂O₃–2CaO·SiO₂ comprises a plane through the tetrahedron CaO–MgO–Al₂O₃–SiO₂. A total of 108 compositions were prepared having an alumina content below the line joining 2CaO·Al₂O₃SiO₂ (gehlenite) and MgO·Al₂O₃ (spinel). Quenching experiments were carried out on 96 of these compositions at temperatures up to 1590°C. Three binary eutectic systems and two ternary eutectic systems are described. Compositions on this plane are of significance in an investigation of the constitution of basic refractory clinkers made from Canadian dolomitic magnesites. They also concern the compositions of certain blast furnace slags.     

Influence of SO3 on the Phase Relationship in the System CaO–SiO2–Al2O3–Fe2O3

The influence of the additive SO₃ on the phase relationships in the quaternary system CaO-SiO₂-Al₂O₃-Fe₂O₃ was investigated by observing the change of volume ratio of 3CaOSiO₂ (C₃S) to 2CaOSiO₂ (C₂S) + CaO (C) in the sintered material with the increase of SO₃ content. The primary phase volume of C₃S in the quaternary phase diagram shrank with the increase of SO₃ and disappeared when the SO₃ content exceeded 2.6 wt% in the sintered material. Changes in the peritectic reaction relationship between CaO (C), 2CaOSiO₂ (C₂S), 3CaOSiO₂ (C₃S), 3CaOAl₂O₃ (C₃A), 4CaOAl₂O₃Fe₂O₃ (C₄AF), and liquid were also observed and discussed.     

Phase Equilibria in the System NiO–Al2O3–SiO2

Equilibrium diagrams for the systems NiO-SiO₂, NiO-Al₂O₃, NiAl₂O₄-SiO₂, Ni₂SiO₄-NiAl₂O₄, and NiAl₂O₄-Al₆Si₂O₁₃ were drawn from data obtained by quenching and direct observational techniques. The only intermediate compound in the binary system NiO-SiO₂ is Ni₂SiO₄, which has the olivine structure. Unlike other olivines which melt congruently, nickel olivine has an upper temperature of stability (1545°C) and at temperatures between 1545° and 1650°C, NiO and SiO₂ coexist in equilibrium. The only compound in the binary system NiO-Al₂O₃ is NiAl₂O₄, which has a spinel structure. The nickel aluminate spinel varies in composition from 50 to 35 mole % Al₂O₃ at 1800°C, and the stoichiometric NiAl₂O₄ composition has a melting point near 2110°C. Of the joins within the ternary system NiO-Al₂O₃-SiO₂ which were studied, only Ni₂SiO₄-NiAl₂O₄ is not binary. In this join, crystals of NiO exist in equilibrium with liquid and a ternary assemblage of NiO + NiAl₂O₄+ liquid is stable to 1775°C. The decomposition temperature of Ni₂SiO₄ is decreased from 1545°C in the binary system to approximately 1490°C, presumably the result of solubility of NiAl₂O₄ in Ni₂SiO₄. The join NiAl₂O₄-SiO₂ is binary in that the compositions of crystalline phases can be expressed in terms of the chosen components. The eutectic temperature in the system is 1495°C. The join NiAl₂O₄-Al₆Si₂O₁₃ is binary for the same reasons and has a eutectic temperature at 1720°C. Using the data obtained in this study and those published for the well-known system Al₂O₃-SiO₂, a liquidus surface diagram for the system NiO-Al₂O₃-SiO₂ is proposed. Nickel olivine, even though it has an upper limit of stability in the binary system, has a primary field in the ternary system NiO-Al₂O₃-SiO₂. This is the only refractory oxide system known to illustrate this so-called “typical case,” the governing principles of which have been clearly presented in discussions of phase equilibria.     

Determination of CaO–SiO2–MgO–Al2O3–CrOx Liquidus

In this work, the liquidus of synthetic CaO–SiO₂–MgO–Al₂O₃–CrO _x slags is evaluated in the industrially relevant compositional domain. Equilibrium experiments are carried out at 1500°C and partial oxygen pressure ( p _O2) 10^−11.04 atm, and at 1600°C and p _O2=10^−10.16 and 10^−9.36 atm. The studied basicities (CaO/SiO₂) are 1.2 and 0.5. Al₂O₃ levels range from 0 to 30 wt%. Oversaturated liquid is sampled and phase relations are measured with quantitative electron probe microanalysis–wavelength dispersive spectroscopy (EPMA–WDS). The results are compared with the commercially available FactSage thermodynamic databases. Qualitative agreement is always obtained. Also a good quantitative agreement is found at the higher basicity, especially for the spinel liquidus. A minor but systematic deviation can be observed for the eskolaite liquidus. At the lower basicity, the calculated phase diagram deviates strongly from the experimental results, probably due to missing ternary interactions in the database.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.     

Subsolidus Phase Relationships in the System Fe2O3–Al2O3–TiO2 between 1000° and 1300°C

Subsolidus phase equilibria in the system Fe₂O₃–Al₂O₃–TiO₂ were investigated between 1000° and 1300°C. Quenched samples were examined using powder X-ray diffraction and electron probe microanalytical methods. The main features of the phase relations were: (a) the presence of an M₃O₅ solid solution series between end members Fe₂TiO₅ and Al₂TiO₅, (b) a miscibility gap along the Fe₂O₃–Al₂O₃ binary, (c) an α-M₂O₃( ss ) ternary solid-solution region based on mutual solubility between Fe₂O₃, Al₂O₃, and TiO₂, and (d) an extensive three-phase region characterized by the assemblage M₃O₅+α-M₂O₃( ss ) + Cor( ss ). A comparison of results with previously established phase relations for the Fe₂O₃–Al₂O₃–TiO₂ system shows considerable discrepancy.     

Phase Relations in the System Fe2O3–Fe3O4–YFeO3 in Air

Measurements were made of temperature and ternary composition for coexisting liquid and crystalline phases on the air isobar in the system Fe₂O₃-Fe₃O₄-YFeO₃ with particular regard to the stability range and compositional limits of yttrium iron garnet. Phase equilibrium relations were determined by conventional quenching techniques combined with measurements of loss in weight at the reaction temperature to locate true ternary compositions. The intersection of the air isobar with the ternary univariant boundary curve for coexisting magnetite, garnet, and liquid phases results in a eutectic-type situation at the composition Y_0.27Fe_1.73 O_2.87 and 1469°± 2°C. A similar intersection of the isobar with the boundary curve for coexisting garnet, orthoferrite, and liquid phases gives rise to a peritectic-type reaction at 1555° 3°C. and Y_0.44Fe_1.56 O_2.89 The yttrium iron garnet crystallizing from liquids within these temperature and composition limits contains up to 0.5 mole % iron oxide in excess of the stoichiometric formula in terms of the starting composition 37.5 mole % Y₂O₃, 62.5 mole % Fe₂O₃. At 1470° C. the garnet phase in equilibrium with oxide liquid contains 2 mole % FeO in solid solution. The small solubility of excess of iron oxide and partial reduction of the garnet phase in air are unavoidable during equilibrium growth from the melt.     

Liquidus Temperatures in the System FeO—Fe2O3—TiO2

Liquidus temperatures in the system FeO–Fe₂O₃–TiO₂ have been estimated from data in the literature supplemented with experimental determinations of the liquidus temperature for about fifty different compositions within the system using the quenching method.     

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.     

Phase Equilibria in High-Alumina Part of the System CaO–MgO–Al2O3–SiO2

Results are presented of a study of phase equilibria among crystalline and liquid phases in the quaternary system CaO–MgO-Al₂O₃–SiO₂ at Al₂O₃ contents greater than 35%. Equilibrium diagrams shown are for the five triangular joins CaAl₂Si₂O₃-Ca₂Al₂SiO₇-MgAl₂O₄, Ca₂Al₂SiO₇-MgAl₂O₄-Al₂O₃, CaAl₂Si₂O₈-MgO-Al₂O₃, CaAl₂Si₂O₈-Mg₂SiO₄-MgAl₂O₄, and CaAl₂Si₂O₈-MgO-Mg₂SiO₄. The composition and nature of the four quaternary peritectic points and the relationships of univariant lines and primary phase volumes are discussed.     

Phase Relationships in the Si3N4–SiO2–Lu2O3 System

Phase relationships in the Si₃N₄–SiO₂–Lu₂O₃ system were investigated at 1850°C in 1 MPa N₂. Only J-phase, Lu₄Si₂O₇N₂ (monoclinic, space group P 2₁/ c , a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si₃N₄–SiO₂–Lu₂O₃ system. The Si₃N₄/Lu₂O₃ ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu–J-phase, β-Si₃N₄, and a new phase of Lu₃Si₅ON₉, having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si₃N₄–LuN–Lu₂O₃ system. The phase diagram suggests that Lu₄Si₂O₇N₂ is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications.     

Phase Relations in the System Al2O3–B2O3–Nd2O3

The phase relations at a temperature below "subsolidus" in the system Al₂O₃–B₂O₃–Nd₂O₃ are reported. Specimens were prepared from various compositions of Al₂O₃, B₂O₃, and Nd₂O₃ of purity 99.5%, 99.99%, and 99.9%, respectively, and fired at 1100°C. There are six binary compounds and one ternary compound in this system. The ternary compound, NdAl₃(BO₃)₄ (NAB), has a phase transition at 950°C ± 15°C. The high-temperature form of NAB has a second harmonic generation (SHG) efficiency of KH₂PO₄ (KDP) of the order of magnitude of the form which has been used as a good self-activated laser material, and the low-temperature form of NAB has no SHG efficiency.     

Discontinuous Dissolution of Iron Aluminate Spinel in the Al2O3–Fe2O3 System

When sintered 85Al₂O₃–15Fe₂O₃ (in wt%) specimens consisting of corundum grains and spinel particles were annealed at temperature where only a corundum phase was stable, phase transformation of spinel into metastable FeAIO₃ and subsequently complete dissolution of the metastable phase occurred together with the migration of grain boundaries at the surface of the specimens. Since the grain boundary migration was induced by grain boundary diffusion of Fe₂O₃ from the transforming and dissolving particles, the boundary migration by temperature decrease corresponds to a discontinuous dissolution of the spinel particles and a chemically induced grain boundary migration by temperature change. Inside the specimens, however, the transformation—dissolution and the grain boundary migration were suppressed because of unavailable accommodation of the volume expansion due to the transformation.