Stability of Hercynite at High Temperatures

The free energy of formation of the spinel FeAl₂O₄ from its constituent oxides has been determined in the temperature range 1550° to 1700°C. The method used involved determination of the gas ratio P_aiO/p_H2 in equilibrium with liquid iron, corundum, and hercynite and previously published data on the same gas equilibrium with liquid iron and FeO. The values of AF° at 1550° and 1700°C. are respectively -3690 and – 3490 cal. per mole.     

Phase Equilibria in the System CaO-Yb2O3

Phase equilibrium relations in the system CaO-Yb₂O₃ were studied. Results of this work demonstrated the existence of four crystalline phases: Yb₂O₃.3CaO, Yb₂O₃.2CaO, Yb₂O₃°CaO, and 2Yb₂O₃°CaO. The 2Yb₂O₃°CaO phase is metastable at all temperatures and was obtained only by rapid quenching from the melt. The crystalline solubility limit of Yb_aO₃ in CaO at 1850°C is slightly greater than 8 mole %, whereas no solubility of CaO in Yb₂O₃ was detected. All four compounds have subsolidus minimums of stability and dissociate into the component oxides below 1800°C. Data are also presented for the systems CaO-Gd₂O₃ and CaO-La₂O₃.     

Stability of the Aluminum Silicates

From the ternary phase diagrams of Al₂O₃–SiO₂ with CaO, MgO, or FeO, it can be concluded that the free energies of formation of kyanite, andalusite, and sillimanite, according to the reaction Al₂O₃+ SiO₂= Al₂O₃.SiO₂, are of the order of magnitude of 0 to –10 kcal. rather than the previously accepted value of –40 to –45 kcal. However, this result may be expected from the general variation of free energies of formation with the ionic potential of the silicate-forming cation; this conclusion is supported by a plot for some silicates, carbonates, and sulfates.     

Preparation of Bi4Ti3O12 Powders in the Presence of Molten Salt Containing LiCl

The reaction between Bi₂O₃ and TiO₂ in molten LiCl-KCl was examined with special emphasis on the reaction mechanism and the size of Bi₄Ti₃O₁₂ particles. The oxides reacted with LiCl to form an intermediate compound, which changed into Bi₄Ti₃O₁₂ on extended heating. Potassium chloride retarded the reaction between the oxides and LiCl and promoted the change from the intermediate compound to Bi₄Ti₃O₁₂. Bi₄Ti₃O₁₂ particles prepared in the flux were platelike, irrespective of the preparation conditions, but their size depended on reaction temperature and time, the ratio of LiCl to KCl, and the amount of flux.     

High-Temperature Stability of Ca2ZrSi4O12

The standard Gibbs free energy of formation of orthorhombic Ca₂ZrSi₄O₁₂ from component oxides ZrO₂ (monoclinic), CaO (rock salt), and SiO₂ (quartz) has been determined in the temperature range 973 to 1273 K using a solid-state cell incorporating single-crystal CaF₂ as the electrolyte: This is the only quantitative information now available on the stability Ca₂ZrSi₄O₁₂.     

Observation and Control of Deviations in Molar Composition in Single-Crystal Growth of Mixed Oxides

A technique is described for the preparation of optically transparent crystals of CaO.2Al₂O₃, 12CaO.7Al₂O₃, and 3Y₂O₃.5Al₂O₃. The main difficulties in obtaining optically transparent crystals of these materials by direct Czochralski growth from the component oxides arise from deviations in molar composition.     

Viscosity of Molten Na2O─B2O3 Slags

The viscosity of sodium borate slags at high Na₂O concentrations (37.3 to 49.4 mol%) and high temperatures (1000° to 1300°C) follows an Arrhenius-type relationship. This relationship was also observed for sodium borate slags (mass% Na₂O/mass% B₂O₃= 0.86) containing CaO and CaF₂ for the same temperature range. There has been a reduction in viscosity of the sodium borate slags (mass% Na₂O₃mass% B₂O = 0.53 to 0.86) with increase in Na₂O concentration. On adding CaO (10 to 50 mass%) to the sodium borate slag (mass% Na₂O/mass% B₂O₃= 0.86), the viscosity increased considerably, while an addition of CaF₂ (S to 15 mass%) to the slag (30.9 mass% Na₂O₃ 35.8 mass% B₂O₃, 33.3 mass% CaO) decreased the viscosity. The average activation energies of Na₂O─B₂O₃, Na₂O─B₂O₃─CaO₃ and Na₂O─B₂O₃─CaO─CaF₂ slag systems have been estimated as 14.6, 124.7, and 41.4 kJ/mol, respectively, for the given composition ranges and 1000° to 1300°C temperature range.     

Phase Relations in the System Iron Oxide—Cr2O3 in Air

The quenching method has been used to determine approximate phase relations in the system iron oxide-Cr₂O₃ in air. Only two crystalline phases, a sesquioxide solid solution (Fe₂O₃–Cr₂O₃) with corundum structure and a spinel solid solution (approximately FeO ·Fe₂O₃–FeO – Cr₂O₃), occur in this system at conditions of temperature and O₂ partial pressure (0.21 atm.) used in this investigation. Liquidus temperatures increase rapidly as Cr₂O₃ is added to iron oxide, from 1591°C. for the pure iron oxide end member to a maximum of approximately 2265°C. for Cr₂O₃. Spinel(ss) is the primary crystalline phase in iron oxide-rich mixtures and sesquioxide (ss) in Cr₂O₃–rich mixtures. These two crystalline phases are present together in equilibrium with a liquid and gas (po₂= 0.21 atm.) at approximately 2075°C.     

Crystal Chemistry of Hydrous Calcium Silicates: I, Substitution of Aluminum in Lattice of Tobermorite

Mixtures of 0.8 moles of CaO per mole of SiO₂ plus Al₂O₃ were prepared from lime, kaolin, and tripoli (microcrystalline quartz); the amounts of SiO₂ to Al₂O₃ were varied to give from 0.2 to 20.7% Al₂O₃ by weight of dry solids. After hydrothermal treatment (170° to 175°C.), the products were examined by differential thermal analysis and by X-ray diffraction. A homogeneous solid identified as the mineral tobermorite (4CaO.5SiO₂.5H₂O) and containing up to 4 or 5% Al₂O₃ was obtained. Increasing the amount of Al₂O₃ in the raw mixture above about 5% resulted in the formation of the hydrogarnet 3CaO.Al₂O₃.SiO₂.4H₂O as a second phase. Allowing for the Al₂O₃ combined in this solid, it was indicated that slightly more Al₂O₃ was substituted in the tobermorite as the amount was increased in the raw mixture. It is suggested that the Al³⁺ ions probably assume tetrahedral coordination when substituting for the Si⁴⁺ ions.     

Oxidation Behavior of Liquid-Phase Sintered Silicon Carbide with Aluminum Nitride and Rare-Earth Oxides (Re2O3, where Re = Y, Er, Yb)

Silicon carbide (SiC) ceramics have been fabricated by hot-pressing and subsequent annealing under pressure with aluminum nitride (AlN) and rare-earth oxides (Y₂O₃, Er₂O₃, and Yb₂O₃) as sintering additives. The oxidation behavior of the SiC ceramics in air was characterized and compared with that of the SiC ceramics with yttrium–aluminum–garnet (YAG) and Al₂O₃–Y₂O₃–CaO (AYC). All SiC ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C. The SiC ceramics sintered with AlN and rare-earth oxides showed superior oxidation resistance to those with YAG and Al₂O₃–Y₂O₃–CaO. SiC ceramics with AlN and Yb₂O₃ showed the best oxidation resistance of 0.4748 mg/cm² after oxidation at 1400°C for 192 h. The minimization of aluminum in the sintering additives was postulated as the prime factor contributing to the superior oxidation resistance of the resulting ceramics. A small cationic radius of rare-earth oxides, dissolution of nitrogen to the intergranular glassy film, and formation of disilicate crystalline phase as an oxidation product could also contribute to the superior oxidation resistance.     

Hercynite-Free Ceramic Liner for Composite Steel Pipe Made Using a Self-Propagating High-Temperature Synthesis Gravitational-Thermite Process

A ceramic-lined steel pipe was fabricated using a self-propagating high-temperature synthesis gravitational-thermite process. Some rare-earth oxides and glass powders were added to the aluminothermite. The experimental results showed that the ceramic liner consisted of three phases: corundum (Al₂O₃), glass, and iron particles. Neither hercynite (FeO·Al₂O₃) nor iron ions existed in the liner of the composite pipe.     

Spray Pyrolysis of Fe3O4–BaTiO3 Composite Particles

Fe₃O₄–BaTiO₃ composite particles were successfully prepared by ultrasonic spray pyrolysis. A mixture of iron(III) nitrate, barium acetate and titanium tetrachloride aqueous solution were atomized into the mist, and the mist was dried and pyrolyzed in N₂ (90%) and H₂ (10%) atmosphere. Fe₃O₄–BaTiO₃ composite particle was obtained between 900° and 950°C while the coexistence of FeO was detected at 1000°C. Transmission electron microscope observation revealed that the composite particle is consisted of nanocrystalline having primary particle size of 35 nm. Lattice parameter of the Fe₃O₄–BaTiO₃ nanocomposite particle was 0.8404 nm that is larger than that of pure Fe₃O₄. Coercivity of the nanocomposite particle (390 Oe) was much larger than that of pure Fe₃O₄ (140 Oe). These results suggest that slight diffusion of Ba into Fe₃O₄ occurred.     

Protonic Conduction in Acceptor-Doped Cubic Rare-Earth Sesquioxides

Using ac conductivity and the concentration cell emf method, conductivity contributions from protons, native ions, and electrons have been measured as a function of temperature (560° to 1156°C) in wet oxygen/air for the cubic systems Y₂O₃+ 1 mol% MgO, Sm₂O₃+ 1 mol% CaO, Gd₂O₃+ 1 mol% CaO, and YYbO₃+ 5 mol% CaO. All exhibit significant proton conductivities as well as native-ion and electronic conductivities at all temperatures. In wet atmospheres and reduced temperatures, the oxides dissolve protons to compensate for the acceptor doping, at the expense of native positive defects. This dissolution of protons seems to be relatively more favorable in oxides of smaller rare-earth cations (e.g., Y³⁺ and Yb³⁺). On the other hand, larger cations (e.g., Sm³⁺ and Gd³⁺) give higher proton mobilities. As compared to oxides of the true lanthanides with similar cationic radii, yttrium-containing oxides have lower proton mobilities.     

Synthesis of Nanosize-Necked Structure α- and γ-Fe2O3 and its Photocatalytic Activity

Highly dispersed nanometer-sized α-Fe₂O₃ (hematite) and γ-Fe₂O₃ (maghemite) iron oxide particles were synthesized by the combustion method. Ferric nitrate was used as a precursor. X-ray diffractometer study revealed the phase purity of α- and γ-Fe₂O₃. Both the products were characterized using field emission scanning electron microscope and transmission electron microscope for particle size and morphology. Necked structure particle morphology was observed for the first time in both the iron oxides. The particle size was observed in the range of 25–55 nm. Photodecomposition of H₂S for hydrogen generation was performed using α- and γ-Fe₂O₃. Good photocatalytic activity was obtained using α- and γ-Fe₂O₃ as photocatalysts under visible light irradiation.     

Melting Relations of the Common Rock-Forming Oxides

Geological science is concerned with the nature of the materials of the earth and particularly with the processes by which earth materials have been changed and modified. Laboratory studies of the melting behavior of the common rock-forming oxides have been an important adjunct to the observations of the field geologist. For the past fifty years investigators at the Geophysical Laboratory have been obtaining quantitative information on the melting relations of many of the important rock-forming minerals. These studies of the fundamental chemistry of igneous and metamorphic rocks have yielded much information of value to ceramists, metallurgists, and mineral technologists. This paper summarizes the most important phase-equilibrium studies of unary, binary, ternary, quaternary, and portions of quinary systems of the common rock-forming oxides-SiO₂, Al₂O₃, FeO, Fe₂O₃, CaO, MgO, Na₂O, and K₂O.     

The System CaO-Ga2O3

CaO and Ga₂O₃ form three compounds: 3CaO-Ga₂O₃, CaO-Ga₂O₃, and CaO-2Ga₂O₃. 3CaO-Ga₂O₃ melts incongruently to CaO plus liquid at 1263°C.; CaO Ga₂O₃ and CaO 2Ga₂O₃ melt congruently at 1369° and 1504°C. respectively. Eutectics are located at the following temperatures and compositions (in mole% Ga₂O₃): between 3CaO Ga₂O₃ and CaO Ga₂O₃, 1245°C. and 37.5%; between CaO Ga₂O₃ and CaO-2Ga₂O₃,1323^oC. and 57.0%; and between CaO -2Ga₂O₃ and β-Ga₂O₃,1457°C. and 68.0%. There is a peritectic at 1263°C. and 36.0%. Three polymorphs of CaO Ga₂O₃ are described. Compositions from approximately 35 to 70 mole% Ga₂O₃ can be quenched to yield homogeneous glasses.     

Water of Hydration in the System Fe–O

Fe or Fe + Fe₂O₃ was heated in a stream of carefully dried oxygen, and the amount of water carried by the gas was determined as a function of time and temperature in the range 300° to 1100°C. Water was evolved as a result of a reaction with the crucible. This water was absorbed by FeO when FeO was present, and was released when the FeO was converted to Fe₃O₄. The apparent excess of O or deficit of Fe in FeO, reported in the literature, therefore may be due to the presence of water or hydroxide in the FeO. The other oxides, which occur in the phase diagram with exact stoichiometry, absorbed and held much less water, if any.     

Synthesis of Microcrystalline Hematite and Magnetite in Organic Solvents and Effect of a Small Amount of Water in the Solvents

Microcrystalline magnetite (Fe₃O₄) particles having diameters of 7-34 nm were synthesized by thermal treatment of iron(III) acetylacetonate in organic solvents at high temperatures (423-573 K) and under autogenous pressures of the solvents. When the treatment was conducted in 1-propanol containing a small amount of water (1-3 vol%), nanosized hematite (α-Fe₂O₃) crystallized, which then was reduced to magnetite after a prolonged reaction time, indicating that a small amount of water in 1-propanol drastically changed the reaction mechanism for the formation of iron oxides.     

Li2O─SiO2─Al2O3─MeIIO Glass-Ceramic Systems for Tile Glaze Applications

In order to verify the possibility of using glass-ceramic materials as tile coatings, the devitrification processes of three industrial formulations belonging to the Li₂O─Al₂O₃─SiO₂ glass-ceramic system were investigated by differential thermal analysis, X-ray diffractometry, scanning electron microscopy, and IR spectroscopy. Compositional variations were made by addition of large amounts of MgO or CaO or PbO (ZnO) oxides as well as through smaller additions of other oxides. In these systems the surface crystallization contributes appreciably to the bulk crystallization mechanism. All the systems investigated show a high tendency toward crystallization even at very high heating rates, developing a very close network of interlocked crystals of synthetic β-spodumene-silica solid solutions (LiAlSi₄O₁₀). The results of this research are expected to establish the conditions under which these glass-ceramic systems can be practically used as tile glazes.     

Microwave Synthesis of Yttrium Iron Garnet Powder

A 28 GHz microwave heating method was used to react an Fe₂O₃+ Y₂O₃ powder mixture to form yttrium iron garnet (YIG, Y₃Fe₅O₁₂) powder. The minimum temperature to form YIG was lower than the conventional (external) heating method. YIG began to form after only 70 s of irradiation, which means that the solid-state reaction proceeded very rapidly. The amounts of byproducts were controlled by the starting composition and by the Y₂O₃ particle size. The resultant YIG particle size also was controlled by the Y₂O₃ particle size.