Potassium Vapor Attack in Refractories of the Alumina–Silica System

A graphite chamber was used for the reaction between samples of 45 or 55 wt% alumina and a mixture of metallurgical coke and potassium carbonate. Thermal treatments were conducted at 1000°C. The results suggest that the potassium attack in silica-alumina bricks is controlled by the following reactions: K₂O + SiO₂→ K₂O → SiO₂ in the glassy matrix; 3(K₂O · 2SiO₂) + 3Al₂O₃→ 2SiO₂· 3(K₂O · Al₂O₃· 2SiO₂) + 2SiO₂ for short times; and K₂O → Al₂O₃· 2SiO₂+ 2SiO₂· K₂O · Al₂O₃· 4SiO₂ for long times. In 55 wt% alumina bricks containing corundum and tridymite, potassium also attacks those phases forming a glassy phase. The formation of kaliophilite at the matrix/mullite grain interface causes a volumetric expansion of 55.5%, resulting in cracks in the matrix. Because the kaliophilite phase is not in equilibrion with mullite, the former will react with free silica to form leucite that is more thermodynamically stable.     

Cassiterite Solubility in High-Silica K2O-Al2O3-SiO2 Liquids

The saturation surface of cassiterite, SnO₂, was determined for liquids in the system K₂O–Al₂O₃–SiO₂ as a function of bulk composition and temperature. At fixed K₂O/Al₂O₃ cassiterite solubility varies weakly with SiO₂ concentration (76 to 84 mol%), temperature (1350° to 1550°C), and log ( f _O2) (−0.7 to −5.3). Cassiterite solubility is also approximately independent of composition in liquids with molar ratios of K₂O/Al₂O₃ lessthan equal to 1 (peraluminous liquids). As K₂O/Al₂O₃ increases from 1 (peralkaline liquids), however, cassiterite solubility increases steeply and approximately linearly with K₂O in excess of Al₂O₃. It is proposed that potassium in excess of aluminum combines with Sn⁴⁺ to form quasi-molecular complexes with an effective stoichiometry of K₄SnO₄.     

Liquid Immiscibility in the System K2O-B2O3-SiO2

The subliquidus miscibility gap in the system K₂O-B₂O₃-SiO₂ has been determined for compositions with molar ratios SiO₂/B₂O₃<2 and T≥550°C. The shape of the miscibility gap is an elongated dome similar in form to, but less extensive than those in the lithium and sodium borosilicate systems. The consolute composition (molar) and temperature are estimated to be 4 ± 1 K₂O -30±8 B₂O₃-66±8 SiO₂ and 629±5°C, respectively .     

Influence of Al2O3, ZnO, and K2O on the Opacity of Fluoride-Opacified Glasses

In the glass SiO₂ 71, Na₂O 17, CaO 12% with 7.5 parts per hundred of fluorine added to the batch, substitution of up to 6% ZnO for CaO produced a great increase in the opacity; substitution of Al₂O₃ for SiO₂ or of K₂O for Na₂O produced much smaller effects, which were dependent on the composition and were inappreciable in the presence of 6% ZnO. Despite the differences in opacity, no differences in fluorine content were detected. No support was found for the belief that Al₂O₃ is essential to the successful opacification of a glass by means of fluorides.     

Properties of Sm2O3─Al2O3─SiO2 Glasses for In Vivo Applications

The compositional range for glass formation below 1600°C in the Sm₂O₃─Al₂O₃─SiO₂ system is (9–25)Sm₂O₃─(10–35)Al₂O₃─(40–75)SiO₂ (mol%). Selected properties of the Sm₂O₃─Al₂O₃─SiO₂ (SmAS) glasses were evaluated as a function of composition. The density, refractive index, microhardness, and thermal expansion coefficient increased as the Sm₂O₃ content increased from 9 to 25 mol%, the values exceeding those for fused silica. The dissolution rate in 1 N HCl and in deionized water increased with increasing Sm₂O₃ content and with increasing temperature to 70°C. The transformation temperature ( T _g ) and dilatometric softening temperature ( T _d ) of the SmAS glasses exceeded 800° and 850°C, respectively.     

Cone Deformation Study of a Portion of the System SrO–Al2O3–SiO2

The results of a study of deformation temperatures and rates within the composition area 25 to 45% SrO, 5 to 25% Al₂O₃, 50 to 70% SiO₂ are presented. A eutectic of the composition 30% SrO, 10% Al₂O₃, 60% SiO₂ is indicated with "1 o'clock" deformation at 1155°C. and "6 o'clock" deformation at 1165°C. A low-temperature area surrounding this eutectic includes 27.5 to 32.5% SrO, 10 to 12.5% Al₂O₃, 57.5 to 62.5% SiO₂. Compositions within this range reach "6 o'clock" deformation at approximately 1180°C.     

The System K2O-BaO-SiO2

Phase equilibrium studies of compound formation and liquidus and solidus surfaces of the system K₂O-BaO-SiO₂ are presented. The system contains 3 ternary compounds: K₄BaSi₃O₉, K₈BaSi₁₀O₂₅, and K₂Ba₃Si₈O₂₀. Both high and low polymorphs of the third have fields on the ternary liquidus surface. Solid solution with SiO₂ depresses the high-low inversion from 1030°C at K₂Ba₃Si₈O₂₀ to 835°C at 70.2 mol% SiO₂. Data for 20 liquidus invariant points were found; 8 are thermal maxima and 12 are eutectics or peritectics. The isofracts of quenched glasses were determined.     

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.     

Reduction of Eu3+ to Eu2+ in Aluminoborosilicate Glasses Prepared in Air

Eu₂O₃-doped aluminoborosilicate glasses were prepared in air at high temperature. Luminescence measurements were used to investigate a valence change from Eu³⁺ to Eu²⁺ ions in the aluminoborosilicate glasses. The results showed that the doped Eu³⁺ ions were partially reduced to Eu²⁺ in the Eu₂O₃:RO–Al₂O₃–B₂O₃–SiO₂ (RO=CaO, SrO, BaO, Li₂O) glasses, but not in the Eu₂O₃:RO–Al₂O₃–B₂O₃–SiO₂ (RO=Na₂O, K₂O) glasses. The changes of Eu reduction with different RO components were discussed with the variation of optical basicity of RO and with different valency of R cations. The effects of co-doping BaO and ZnO in aluminoborosilicate glasses on Eu reduction were also investigated and discussed.     

Densities of SiO2-Al2O3 Melts

The densities of binary aluminosilicate melts were measured X-radiographically as a function of Al₂O₃, concentration between 1800° and 2000°C. Within this temperature range, the density curves vary linearly and are parallel from fused SiO₂ to ≊30 to 45 mol% Al₂O₃, depending on the temperature. At higher Al₂O₃ contents, negative deviation from linearity increases with increasing temperature. Recent supplementary research efforts on various aspects of the system SiO₂-Al₂O₃ indicate that the changing coordination and structural role of the aluminum ion may be a primary factor in determining the shapes of the density curves.     

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.     

Studies in Lithium Oxide Systems: IX, Li2o-Al2O3-TiO2

Compatible phases in the system Li₂O-Al₂O₃-TiO₂ at various temperature levels were determined mainly by solid-state reactions for the portion of the ternary system bounded by Li₂O Al₂O₂, Li₂O.TiO₂, Al₂O, and TiO₂. The existence of a ternary compound, Li₂O.Al₂O₃.4TiO₂, and nine joins was established. The ternary compound has a lower limit of stability at 1090°± 15°C. and dissociates and recombines rapidly at 1380°± 15°C.     

Crystallization Behavior of Al2O3 in the Presence or SiO2

The independent crystallization sequence of an Al₂O₃ component is modified in the presence of SiO₂ and vice versa. Mixed SiO₂-Al₂O₃, gel (28 wt% SiO₂ and 72 wt% Al₂O₃) forms neither cristobalite nor γ-Al₂O₃ and corundum at 1000°C but forms Si-Al spinel; an amorphous aluminosilicate phase invariably also forms after the gel is heated. However, the composition of this amorphous aluminosilicate phase is not as yet known.     

The System CaO-Al2O3-H2O

Phase equilibria have been determined in the system CaO-Al₂O₃-H₂O in the temperature range 100° to 1000°C. under water pressures of up to 3000 atmospheres. Only three hydrated phases are formed stably in the system: Ca(OH)₂, 3CaO·Al₂O₃·6H₂O, and 4CaO·3Al₂O₃-3H₂O. Pressure-temperature curves delineating the equilibrium decomposition of each of these phases have been determined, and some ther-mochemical data have been deduced therefrom. It has been established that both the compounds CaO·Al₂O₃ and 3CaO·Al₂O₃ have a minimum temperature of stability which is above 1000°C. The relevance of the new data to some aspects of cement chemistry is discussed.     

Eletrical Properties of Glasses in the System PbO-Al2O3-B2O3-SiO2

Glasses in the system Pb0–Al₂O₃-B₂O₃-SiO₂ are chemically stable over a wide composition range and have very desirable electrical characteristics such as high electrical resistivities and activation energies for conduction. Variations in these electrical properties were studied as a function of composition changes within the system, the object being to identify the role of the constituent oxides in achieving the highest activation energy and resistivity values consistent with moderate preparation temperatures. Measurements were made in the temperature range 25° to 400°C on carefully prepared glass disks in which the individual oxide components or different oxide ratios such as PbO/SiO₂, Al₂O₃/SiO₂, and B_sO₃/SiO₁ were systematically varied. The activation energy and resistivity values obtained ranged from 1.2 to 1.6 ev and 10° to 10¹⁴ ohm-cm, with dielectric constants ranging from 9 to 19 and densities from 4.30 to 4.50 g/cmY. Indications were that, for the composition range studied, the behavior manifested was basically that of the binary PbO-SO₂ glass with additions of Al₂O₃ or B₂O₃, even in small concentrations, sharply increasing the activation energy for conduction while lowering the density.     

The Al2O3-Nd2O3 Phase Diagram

A tentative phase diagram for the system Al₂0₃-Nd₂O₃ is presented. Three compounds were obtained: a β -A1₂O₃-type compound, the perovskite NdAlO₃, and Nd₄Al₂O₉. The perovskite melts congruently (mp 2090°C), and the two other compounds exhibit incongruent melting behavior: β -Nd/Al₂O₃, mp 1900°C; Nd₄Al₂O₉, mp 1905°C. Two eutectics exist with the following compositions and melting points: 80 mol% Al₂O₃, 1750°C; 23 mol% Al₂O₃,1800°C. Nd₄Al₂O9 decomposes in the solid state at 1780°C.     

Studies on 4CaOAl2.O3.13H2O and the Related Natural Mineral Hydrocalumite

Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al₂O₃. 13H₂O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca₂(OH)₇- 3H₂O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A1₂O₃.13H₂O, from which it is derived by substitution of CO₃^2-for 20H^-+ 3H₂O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al₂O₃.Ca Y ₂- xH₂O and 3CaO Al₂O₃ Ca Y xH₂O. On dehydration, 4CaO.Al₂O₃.13H₂O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)₂ and 4CaO.3Al₂O₃.3H₂O are formed and, by 1000°C., CaO and 12CaO.7Al₂O₈. The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al₂O₃.3H₂O is formed.     

Structure of B2O3 and Binary Aluminoborate Melts at 1600°C

Viscosity and density data were obtained up to 1700°C for a series of binary aluminoborate melts that contained as much as 15 mole% (∼21 wt%) Al₂O₃ and up to 1620°C for pure molten B₂O₃. Large expansion coefficient decreases and a slight activation energy increase for B₂O₃ above 1400°C suggested a tightening of its structure. The addition of Al₂O₃ reduced viscosity and increased activation energy. The decreased compositional dependence of molar volume (compared to SiO₂ additions) and the increased expansion coefficients accompanying Al₂O₃ additions suggested a loosening of the O—B—O structure at 1600°C. Molar volume deviations from ideality were similar to but smaller than those for SiO₂ and GeO₂ additions at 1300°C. Microclustering of aluminum-bearing polyhedra appeared to occur at slightly higher boron atom contents than with SiO₂ and GeO₂ additions.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

Glass Formation and Recrystallization in the Lithium Metasilicate Region of the System Li2O–Al2O3–SiO2

The stability of the vitreous state in the lithium metasilicate region of the system Li₂O–Al₂O₃–SiO₂ was found to be a function of the concentration of lithia. The higher the lithia content, the less stable was the glass. The devitrification of glasses in this system was studied. In addition to the phases present at or near the liquidus, it was found that the β -eucryptite– β -quartz solid solution phase was metastable over most of the region. The Li₂O–SiO₂, β -Li₂O–Al₂O₃–4SiO₂ solid solution, β -Li₂O–Al₂O₃–2SiO₂ solid solution triple point was estimated to be near 62.5% SiO₂, 17% Al₂O₃, and 20.5% Li₂O (by weight). The thermal expansions of bodies in this region were measured and the values obtained are explained in terms of the phases present.