Reactions in the System TiO2-P2O5

The system TiO₂-P₂O₅ was investigated in the compositional range TiO₂.P₂O₅ to 100% TiO₂. Two compounds exist, TiO₂.P₂O₅ and 5TiO₂.-2P₂O₅. TiO₂.P₂O₅ begins to lose P₂O₅ at 1400°C. and both fusion and vaporization proceed rapidly at 1500°C. 5TiO₂.2P₂O₆ melts congruently at 1260°± 3°C. to a glass which can be retained in substantial quantities at room temperature. Physical properties of certain compositions are described.     

Reinvestigation of Al–Si Spinel Phase in Diphasic Al2O3–SiO2 Gel

Mechanical mixture of γ-Al₂O₃ and amorphous SiO₂, and diphasic Al₂O₃/SiO₂ gels of three different compositions were synthesized. They were subjected to heat treatment to various temperatures in the range 900°–1600°C. Qualitative X-ray diffraction data show that these diphasic gels do not crystallize to a combined mixture of θ-Al₂O₃ and α-Al₂O₃ polymorphs at the intermediate stage, prior to mullite formation. Estimated mullite formation data show that the course of its formation from mixed oxides was different from that of diphasic gels. Results are compared with previous findings and the concept of Al–Si spinel formation in the phase transformation of stoichiometric diphasic gel system is substantiated.     

Electrical Conductivity of TiO2-V2O5-P205 Glasses

The dc conductivities (σ) of V₂O₅-P₂O₅ glasses containing up to 30 mol% TiO₂ were measured at T=100° to ∼10°C below the glass-transition temperature. Dielectric constants from 30 to 10⁶ Hz, densities, and the fraction of reduced V ion were measured at room temperature. The conduction mechanism was considered to be small polaron hopping between V ions, as previously reported for V₂O₅-P₂O₅ glass. The temperature dependence of σ was exponential with σ = σ₀ exp(-W/kT ) in the high-temperature range. When part of the P₂O₅ was replaced by TiO_2,σ increased and W decreased. The hopping energy depended on the reciprocal dielectric constant which, in this case, increased with increasing TiO₂ content.     

Crystallization Behavior of CaO–P2O5 Glass with TiO2, SiO2, and Al2O3 Additions

TiO₂ above 4 mol% is an effective nucleating agent for CaO–P₂O₅ glass which also contains substantial SiO₂ and Al₂O₃ additions. Glass ceramics can be made from this glass using a single slow heating ramp with no need for a nucleating heat treatment step. Powder of this composition crystallizes rapidly to β-Ca₂P₂O₇, whereas bulk glass crystallizes from diphasic nuclei consisting of a central cubic Ca-P-Ti-Si-Al oxide phase surrounded by impure AlPO₄ dendrites. Metastable calcium phosphate grows on the AlPO₄ dendrites and later transforms to β-Ca₂P₂O₇.     

Application of Compositionally Diphasic Xerogels for Enhanced Densification: The System Al2O3-SiO2

Stoichiometric mullite (71.38 wt% Al₂O₃-28.17 wt% SiO₂) and 80 wt% Al₂O₃-20 wt% SiO₂ gels were prepared by the single-phase and/or diphasic routes. Dense sintered bodies were prepared from both sets of gels in the Al₂O₃-SiO₂ system. Apparent densities of 96% and 97% of theoretical density were measured for the diphasic (using two sols) mullite samples sintered at 1200° and 1300°C for 100 min, respectively; this compared with 85% and 94% for the single-phase xerogels under the same conditions, and to much lower values for mullite prepared from conventional mixed powders. The microstructure of the mullite pellets from diphasic xerogel precursors is also considerably finer.     

Role of P2O5 in TiO2-Opacified Porcelain Enamels

Phosphorus pentoxide is known to affect the reflectance and color of TiO₂-opacified porcelain enamels. It has been postulated that P₂O₅, impedes the anatase-to-rutile phase transformation. This hypothesis was tested by applying recently developed analytical procedures based on X-ray diffraction analysis to a typical porcelain enamel cover coat system. The inversion of anatase to rutile was not significantly affected by P₂O₅. Instead, in the range 0.5 to 4%, as P₂O₅ concentration was increased in the frit, the rate of crystallization of both anatase and rutile also increased, with the rate of increase of anatase being about twice that of rutile.     

Internal Friction of Network-Forming Oxide Glasses in the System P2O5-(GeO2, TeO2, Sb2O3, V2O5)

The internal friction of only network-forming oxide glasses containing a P₂O₅ component, i.e., in the systems P₂O₅-GeO₂, P₂O₅-TeO₂, P₂O₅-Sb₂O₃, and P₂O₅-V₂O₅, was measured as a function of temperature by a free torsional vibration method. P₂O₅-TeO₂ and P₂O₅-Sb₂O₃ glasses exhibited clearly high-temperature peaks in a plot of internal friction vs temperature in spite of the absence of nonbridging oxygens and network modifiers. Therefore, we conclude that the high-temperature peaks appeared when strong and weak parts coexisted in the network structure.     

Effect of Seeding and Water Vapor on the Nucleation and Growth of α-Al2O3 from γ-Al2O3

Isothermal transformation kinetics and coarsening rates were studied in unseeded and alpha-Al₂O₃-seeded γ-Al₂O₃ powders heated in dry air and water vapor. Unseeded samples heated in dry air transformed to alpha-Al₂O₃ with an activation energy of 567 kJ/mol. Seeding with alpha-Al₂O₃ increased the transformation rates and reduced incubation times by providing low-energy sites for nucleation/growth of the alpha-Al₂O₃ transformation. The activation energy for the transformation was reduced to 350 kJ/mol in seeded samples heated in dry air. Seeded samples completely transformed to alpha-Al₂O₃ after 1 h at 1050°C when heated in dry air compared to 1 h at 925°C when heated in saturated water vapor. The combined effects of a lower nucleation barrier due to seeding and the increased diffusion due to water vapor reduced the activation energy for the transformation by 390 kJ/mol and the transformation temperature by ∼225°C compared to the unseeded samples heated in dry air. The accelerated kinetics is believed to be due to increased surface diffusion.     

X-Ray Study of the Solid Solution of TiO, Fe2O3, and G, O3 in Mullite (3A12O3a2Si02)

A study of the solid solution of TiO₂, Fe₂O₃, and Cr₂0₃ in mullite was made by measuring the changes in lattice parameters and unit-cell volume. Synthetic mullite (3O₃-2SiO₂) was reacted with up to 12 weight % of the oxides at temperatures ranging from 1000° to 17000C. The approximate minimum temperature required for the formation of solid solution was 12000C. for Fe₂0₃ and 1400°C. for Cr₂O₃ and TiO₃. The maximum amount of solid solution found was 2 to 4% TiO₂ at 1600°C., 10 to 12% Fe₂O_s at 1300°C., and 8 to 10% Cr_ZO₃ at 1600OC. Lattice parameters and unit-cell volumes for each solid solution series increased with increasing amounts of foreign oxide. There was good agreement between the calculated and observed increase in cell dimensions for the iron oxide series. Except in the case of titania, there was good agreement between X-ray data and petrographic observations.     

Diffusion and Reaction Studies in the System Al2O3-SiO2

High-temperature diffusion kinetics and phase relations between couples of fused SiO₂ or cristobalite and sapphire or mullite were investigated in air and in helium. Subsolidus liquid formation between sapphire and cristobalite indicates the existence of a metastable system without mullite. A liquid phase is considered to be essential for the nucleation of mullite. The growth rate of mullite exceeded its dissolution rate in semi-infinite fused-SiO₂-sapphire couples above 1634°C. The inter-facial liquid compositions provided data for a minor revision of the mullite liquidus curve. Diffusion coefficients calculated from the Al profiles vary greatly with concentration and temperature, resulting in a large range of values for apparent activation energy, which decreases with increasing Al₂O₃ content (∼310 to ∼60 kcal/mol for ∼4 to ∼22 wt% Al₂O₃). The diffusion process in the liquid is considered to be cooperative movement of oxygen-containing Al and Si complexes whose nature changes with composition and temperature; this change in the diffusing species contributes to the range in the values of experimental apparent activation energies.     

Microstructural Development on Crystallizing Hot-Pressed Pellets of Cordierite Melt-derived Glass Containing B2O3 and P2O5

Comparing the crystallization mechanism of stoichiometric and B₂O₃ and P₂O₅ containing glass reveals that the additives extend the gap between the glass transition and crystallization temperatures and suppress formation of μ, cordierite while promoting direct crystallization of α cordierite. Detailed TEM analysis of nucleation and growth of crystals in hot-pressed pellets of B₂O₃/P₂O₅-containing glass particles shows that nucleation occurs on unidentified heterogeneous nuclei at the sites of the previous particle surfaces. Growth of α cordierite with a cellular morphology or μ cordierite with a dendritic morphology is most likely controlled by the glass composition directly ahead of the growth front.     

Effects of (B2O3, P2O5) Additives on Microstructural Development and Phase-Transformation Kinetics of Stoichiometric Cordierite Glasses

The microstructural development and phase-transformation kinetics of stoichiometric cordierite glasses containing B₂O₃ and/or P₂O₅ additives were highly affected by the microstructural characteristics of the μ-cordierite and the type of additives. The addition of B₂O₃ tended to cause the formation of μ-spherulitic dendrites with thin dendritic arms, which promoted the formation of α-cordierite, either from crystallization of the residual glass or from transformation of μ-cordierite. P₂O₅ had the opposite effect: Increasing the temperature increased the growth rate of α-cordierite more than that of μ-cordierite.     

Ionically Conducting Composite Membranes from the Li2O–Al2O3–TiO2–P2O5 Glass–Ceramic

This paper reports processing of lithium ion-conducting, composite membranes comprised of 14Li₂O·9Al₂O₃·38 TiO₂·39P₂O₅ glass–ceramic and polyethylene. The processing involved tape casting of 14Li₂O·9Al₂O₃·38TiO₂·39P₂O₅ glass powder with organic additives into tapes, subjecting the green tape to binder burnout and thermal soaking in the temperature range of 950°–1100°C, and finally infiltrating the porous tape with polyethylene solution. The ionic conductivity and microstructure of 150–350 μm thick membranes were characterized and are discussed in this paper. The crystallites of the glass–ceramic show liquid-like conductivity at ambient temperature, whereas the grain boundary conductivity is lower by a factor of five. The lower grain boundary conductivity is explained on the basis of crystallographic mismatch and the existence of AlPO₄ at the grain boundary. The polyethylene infiltration in the porous membrane improved mechanical resilience with a minor adverse effect on conductivity.     

Free Volume of Network-Forming Oxide Glasses in the Systems P2O5-(GeO2,TeO2,Sb2O3,V2O5)

The free-volume fraction (V_f) defined by Simha and Boyer was measured for network-forming oxide glasses in the systems P₂O₅-(GeO₂, TeO₂,Sb₂O₃.V₂O₅). The V_f values varied from 0.06 to 0.25. The systems P₂O₅-TeO₂: and P₂O₅-Sb₂O₃ have V_f∼0.1, which is near the magnitude of the free-volume fraction for normal metaphosphate glasses and many organic high polymers.     

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.     

Aluminum Titanate Formation by Solid-State Reaction of Coarse Al2O3 and TiO2 Powders

Reaction kinetics in a coarse equimolar powder mixture were slow enough to allow for the different stages to be identified, notably in the lower and higher temperature ranges, respectively. In the former ( T ≤ 1600 K), Al₂TiO₅ nucleation was hindered by the strain energy contribution to the overall driving force. The setting up of metastable layer sequences Al₂TiO₅/TiO₂/Al₂O₃ was found to occur generally during subsequent growth. The high Al mobility in the TiO₂ provided a rapid aluminum transport from the metastable Al₂O₃/TiO₂ interface to the TiO₂/Al₂TiO₅ front. At temperatures above ∼1700 K the Al₂O₃/TiO₂ interface was very rapidly sealed off by Al₂TiO₅ formation. Reactant transport across the Al₂TiO₅ was slow because of the low mobilities in the product phase. Therefore, much lower product growth velocities were observed at higher temperatures than at lower temperatures.     

Fabrication of Nano-Scaled α-Al2O3 Crystallites Through Heterogeneous Precipitation of Boehmite in a Well-Dispersed θ-Al2O3-Suspension

The possibility of eliminating finger or vermicular growth of α-Al₂O₃ particles obtained by calcination of boehmite was examined. Heterogeneous precipitation of boehmite in a well-dispersed θ-Al₂O₃ suspension was first prepared, in which the mass ratio of boehmite to θ-crystallite was evaluated to form agglomerates of similar sizes that will form α-Al₂O₃ crystallites of <100 nm in diameter. θ- to α-phase transformation of alumina experiences a nucleation and growth mechanism, with the critical size of nucleation being ∼25 nm for θ-Al₂O₃ and the size for accomplishment of transformation followed by finger growth being ∼100 nm. Hence, fabricating agglomerates that would form α-Al₂O₃ crystallites with sizes <100 nm accompanied with appropriate thermal treatments can be a method for obtaining α-Al₂O₃ crystallites free of finger growth. It is found that proper preparation of the agglomerate with appropriate size may initiate a simultaneous and lower temperature θ- to α-Al₂O₃ phase transformation for such powder systems, substantially limiting the mass transfer among the newly formed α-Al₂O₃ particles. Moreover, α-Al₂O₃ crystallites free of finger growth can be obtained.     

The Influence of Sb2O3 Addition on the Properties of Low-melting ZnO–P2O5 Glasses

The influence of 0–16 mol% Sb₂O₃ substitution for P₂O₅ on the properties of ZnO–P₂O₅ glasses has been investigated. It was shown that Sb₂O₃ could participate in the glass network and thermal stability of the glasses decreased with increasing Sb₂O₃ content. Glass transition temperature T _g, softening temperature T _s, and water durability all decreased firstly (up to 6 mol% Sb₂O₃ added) and then increased. Substitution of 12 mol% Sb₂O₃ led to a 16°C decrease in T _g and 30°C decrease in T _s, and weight loss of the glass was only 0.42 mg/cm², which is ∼11 times lower than that of the glass without Sb₂O₃ after immersion in deionized water at 90°C for 1 day. The glass containing 12 mol% Sb₂O₃ might be a substitute for Pb-based glasses in some applications.     

Transformation, Microstructure Development, and Densification in α-Fe2O3-Seeded Boehmite-Derived Alumina

Addition of α-Fe₂O₃ seed particles to alkoxide-derived boehmite sols resulted in a 10-fold increase in isothermal rate constants for the transformation of γ- to α-Al₂O₃. Changes in porosity and surface area with sintering temperature showed no effect of seeding on coarsening of the transition alumina gels, but the 200-fold decrease in surface area associated with transformation to α-Al₂O₃ occurred ∼ 100°C lower in seeded gels compared with unseeded materials. As a result of high nucleation frequency and reduced microstructure coarsening, fully transformed seeded alumina retained specific surface areas >22 m²/g and exhibited narrow pore size distributions, permitting development of fully dense, submicrometer α-Al₂O₃ at ∼ 1200°C.     

Electrical Conductivity of Glasses in the Systems P4O10-V2O5 and P4O10-WO3

The electrical conductivities of P₄O₁₀-V₂O₅ and P₄O₁₀-WO₃ glasses were compared. The P₄O₁₀ content of a glass from each system was replaced by increasing amounts of several oxides. The conductivity of the vanadium phosphate glass was insensitive to oxide replacement, in contrast to the conductivity of the tungsten phosphate glass, which decreased by almost five decades when a small amount of V₂O₅ was introduced. The change was attributed to the effect of oxidation-reduction on the tungsten ions.