Structural Control of Phase Formation in Low-Tem perature AIPO4—Sio2 Reactions

The phase relation along the binary join of AIPO₄-SiO₂ were investigated up to 400°C using starting materials made by a solution route. Precursor structures used were boehmite (AIOOH), H₃PO₄, noncrystalline silica, and quartz. The silica precursors acted as structural seeds for the epitaxial growth of AIPO₄. Studies showed that SiO₂ and AIPO₄ were the only crystalline and noncrystalline phases present along the binary join, and no substantial crystalline solution or any ternary phase was observed. Three polymorphic forms of AIPO₄, i.e., berlinite, tridymite, and cristobalite, coexisted as low as 200°C. The nature of the silica precursor greatly influenced the development of the polymorphic phases of AIPO₄. The low-quartz precursor suppressed the formation of the cristobalite form of AIPO₄ and favored berlinite (AIPO₄ quartz) production. On the other hand, noncrystallin silica with a cristobalite-like broad XRD peak suppressed the formation of berlinite and enhaned that of the cristobalite form of AIPO₄. These precursor effects indicate that heteroepitaxy is very significant during the nucleation and growth of AIPO₄ phases on the surface of SiO₂ particles even in these low-temperature reactions.     

Effects of Amorphous and Crystalline SiO2 Additives on γ-Al2O3-to-alpha-Al2O3 Phase Transitions

This paper focused on the effects of various phases of SiO₂ additives on the γ-Al₂O₃-to-α-Al₂O₃ phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta-to-α phase transition was elevated by adding amorphous SiO₂, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO₂, such as quartz and cristobalite. Amorphous SiO₂ was considered to retard the γ-to-α phase transition by preventing γ-Al₂O₃ particles from coming into contact and suppressing heterogeneous nucleation on the γ-Al₂O₃ surface. On the other hand, crystalline SiO₂ accelerated the α-Al₂O₃ transition; thus, this SiO₂ may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO₂ additives, especially amorphous and crystalline phases, largely influenced the temperature of γ-Al₂O₃-to-α-Al₂O₃ phase transition.     

Phase Composition of Hydrated DSP Cement Pastes

Cement pastes densified with small particles (DSP) containing up to 48% silica fume by weight of cement, and hydrated to up to 180 d at room temperature, have been analyzed using TMS-GPC, TGA, and ²⁹Si NMR to quantitatively estimate the amount of unreacted cement, Ca(OH)₂, and residual silica fume, respectively. Using a mass balance approach, the CaO/SiO₂ and H₂O/SiO₂ molar ratios of the C-S-H in the samples were calculated. For samples containing silica fume, the values of CaO/SiO₂ lie between 0.9 and 1.3, depending on the degree of hydration and silica fume content, whereas for samples without silica fume they were 1.6. Silicate polymerization analysis using TMS-GPC suggests that the molecular structure of the C-S-H is similar to that formed in conventional hydration. No cross-linking species were found, but the fraction of higher polymers (above octamer) increases as the CaO/SiO₂ ratio decreases.     

Pressure and Temperature Dependence of the Elastic Moduli of Na2O-TiO2-SiO2 Glasses

Ultrasonic interferometry was used to measure elastic-wave velocities and moduli in six Na₂O-TiO₂-SiO₂ glasses; three glasses contained 20 mol% TiO₂ and three 25 mol% TiO₂. The elastic moduli and their pressure derivatives varied systematically with the SiO₂/Na₂O molar ratio of the glasses. In the group of glasses which contained 20 mol% TiO₂, dK/dP ( K =bulk modulus) decreased linearly from 4.85 to 2.59 as the SiO₂/Na₂O ratio increased; in the other group, dK/dP decreased from 4.00 to 3.05. Similarly, dμ/dP (μ=shear modulus) decreased with the SiO₂/Na₂O ratio, but somewhat non-linearly. The extrinsic and intrinsic contributions to the temperature dependencies of the elastic moduli are evaluated in light of the measured pressure dependencies of these moduli.     

Elastic Properties of 0.98Li2O-1.0Al2O3-nSiO2 Glasses and Keatite-Phase Glass-Ceramics

Bulk physical properties such as elastic moduli, thermal expansions, and moduli of rupture were measured for a series of 0.98Li₂O-1.0ALO₃- n SiO₂ glasses and the corresponding keatite solid-solution-phase glass-ceramics. The SiO₂ content ranged from n =4 to 12. The magnitude of the elastic properties of the glasses changed monotonically with increasing SiO₂ content. The properties of keatite-phase glass-ceramics depended almost linearly on SiO₂ content, but their behavior differed qualitatively from that of the glasses.     

Fabrication of Bifunctional Titania/Silica-Coated Magnetic Spheres and their Photocatalytic Activities

A magnetic photocatalyst is prepared by coating a magnetic core with a dual layer of inert silica (SiO₂) and photoactive titania (TiO₂). The complete photoactive TiO₂ shell is directly formed on the SiO₂-coated magnetic spheres. Subsequent hydrothermal treatment improves the degree of crystallinity of TiO₂. TiO₂/SiO₂-coated magnetic spheres show good photocatalytic activities in the degradation of methylene blue in the aqueous solution. The photocatalytic TiO₂ surface shell is electrically insulated by the intermediate SiO₂ layer from the magnetic core to maintain good photocatalytic activity of the TiO₂ shell. The magnetic photocatalyst can be easily recovered by applying external magnetic field.     

Stability of Molybdenum Disilicide in Combustion Gak Environments

The stability of MoSi₂ in combustion gas at 1370° and 1600°C was evaluated using SOLGASMIX-PV thermodynamic modeling, periodic weight measurements, and characterization via XRD, SEM, EDS, and image analysis. Passive oxidation occurred at both temperatures. During an initial stage of exposure, specimen surfaces oxidized to form MoO_3(g) and amorphous SiO₂ via reduction of CO₂ and H₂O. After a short time (<6.5 min at 1370°C, <1 min at 1600°C), the oxidation mechanism switched; Mo₅Si₃ and amorphous SiO₂ formed as oxidation products. The first mechanism esulted in the formation of 46.1 vol% at 1370°C and 42.6 vol% at 1600°C of the amorphous silica surface coating. The attainment of a near-terminal weight gain implied silica formation was limited by H₂O and CO₂ diffusion through the silica coating.     

Elastic Properties of Mullite

Using ultrasonic methods, we determined the ambient-temperature elastic constants of dense (99.7%) hot-pressed polycrystalline 3:2 mullite (3Al₂O₃2SiO₂). We report the usual polycrystal elastic constants: Young's, shear, and bulk moduli, and the Poisson's ratio (nu). The Poisson's ratio (nu = 0.280) suggests interatomic bonding that is very different from that of either alumina or silica. Temperature dependences of the elastic constants show a strong irregularity; for example, nu decreases as the temperature increases. This irregularity suggests an internal-state change that is perhaps related to the incommensurate structural modulation.     

Reaction Between K2O and Al2O3-SiO2 Refractories as Related to Blast-Furnace Linings

An examination was conducted to determine the mechanism of peeling of fire-clay brick in the low-temperature region of a blast furnace where 3 to 10% K₂O is the principal contaminant. In laboratory tests, as-received high-duty and superduty fire-clay brick and 70% alumina brick treated with KCl-K₂CO₃ mixtures showed no peeling at a temperature of 1600°F. Cracks were found in high-duty brick that were treated with KCN at 1500°F. under partially reducing conditions. X-ray diffraction studies of mixtures of crushed brick and K₂CO₃ indicated the formation of leucite (K₂O.Al₂O₃.4SiO₂) and kaliophilite (K₂O.-Al₂O₃.2SiO₂) at temperatures below 1700°F. These latter data, confirmed by specimens from used blast-furnace linings, showed that silica is the first constituent attacked by alkali. Since the formation of leucite and kaliophilite in fire-clay brick is the probable cause of peeling, the increased reaction of silica, in a dense Al₂O₃.SiO₂ refractory of higher silica content than fire-clay brick, should confine the alkali attack to the surface of the brick in low-temperature applications.     

Effect of Precursor Particle Size on the Densification and Crystallization Behavior of Mullite

Stoichiometric mullite, 3Al₂O₃·2SiO₂, has been prepared from a variety of colloidal and solution precursors. In order to change the level of mixing, the size of the silica particles has been varied while keeping the alumina (boehmite) particle size constant. The effect of varying particle size on the crystallization and densification behavior has been studied. Densification behavior was characterized by measuring the bulk density of pellets as a function of heat treatment temperature and by dilatometry. Phase development was examined by thermal analysis and X-ray diffraction. Results showed that the formation of crystalline phases including mullite inhibited densification. The formation of mullite was controlled by the initial segregation of alumina and silica in the gel. A heat treatment time-temperature profile designed to optimize the densification of the colloidal powders was investigated in order to compare the ability of the different gels to densify.     

Influence of SiO2 Addition on the Dielectric Properties and Microstructure of (Ba0.96Ca0.04)(Ti0.85Zr0.15)O3 Ceramics

In this study, the effect of SiO₂ doping on the sintering behavior, microstructure, and dielectric properties of BaTiO₃-based ceramics was investigated. Silica was added to (Ba_0.96Ca_0.04)(Ti_0.85Zr_0.15)O₃ (BCTZ) powder prepared using the solid-state method. SiO₂-doped BCTZ ceramics with a high density and a uniform grain size were obtained and sintered at 1220°C in a reducing atmosphere. A second phase (BaTiSiO₅) existed in samples when SiO₂ was added in excess of 1%. The amount of the second phases was observed to increase as the number of SiO₂ additives increased. It was found that BCTZ ceramics sintered with SiO₂ are helpful in reducing the sintering temperature for a typical thick film and MLCC applications. However, there were disadvantageous effects on the dielectric properties with mere addition of SiO₂ addition (3% and 5%) due to higher formation of BaTiSiO₅. Doping with a small amount of silica can improve the sintering and dielectric properties of BCTZ ceramics. In addition, to understand the effect of the BaTiSiO₅ phase on the dielectric properties of BCTZ ceramics, the BaTiSiO₅ composition was synthesized from individual BaCO₃, TiO₂, and SiO₂ powders using conventional solid-state methods. X-ray diffraction results show the presence of mainly the crystalline phase, BaTiSiO₅, in the sintered ceramics.     

High-Temperature Oxidation of Molybdenum Disilicide

The oxidation of MoSi₂ in air at atmospheric pressure was studied by electron diffraction, X-ray diffraction, and thermogravimetric analyses. The oxidation process occurs in two parts: (1) formation of MoO₃ and SiO₂ at temperatures below the boiling point of MoO₃, and (2) formation of Mo₅Si₃ and SiO₂ at higher temperatures. Evidence is presented which indicates that oxygen permeation through a silica layer, which may be of a mixed crystalline-glassy nature, controls reaction rate at high temperatures and that Mo₅Si₃ is present directly beneath the protective oxide. The activation energy for oxidation of MoSi₂ above 1200°C was calculated as 81.3 kcal mole⁻¹.     

Thermodynamic Modeling of Alkali Metal Oxide-Silica Binary Melts

The evaluation of the thermodynamic properties as well as the phase diagrams for the binary Na₂O–SiO₂, K₂O–SiO₂, and Li₂O–SiO₂ systems are carried out with a structural model for silicate melts and glasses. This thermodynamic model is based on the assumption that each metallic oxide produces the depolymerization reaction of silica network with a characteristic free-energy change. A least squares optimization program permits all available thermodynamic and phase diagram data to be optimized simultaneously. In this manner, data for these binary systems have been analyzed and represented with a small number of parameters. The resulting equations represent the thermodynamic and phase diagram data for these alkali metal oxide–silica systems within experimental error limits. In particular, the measured limiting liquidus slope at     is well reproduced.     

Desiliconization of Mullite Felt

High-temperature evaporation from 80% porous, rigid mullite (3Al₂O₃·2SiO₂) whisker felt was studied under vacuum and at various helium pressures using gravimetry, X-ray diffractometry, SEM, and EDS. Heat treatments at 1350° to 1550°C resulted in evaporation of SiO₂ from mullite with a rate strongly dependent on temperature and pressure. A concentration gradient of SiO₂ was observed through the cross section of samples after heating under vacuum, indicating that SiO₂ preferentially evaporated from whiskers on the periphery of the samples. The SiO₂ concentration gradient was accompanied by sharp microstructural changes across the specimens. The gradient was decreased by raising the ambient helium pressure during heat treatment. A mathematical model was developed to predict the SiO₂ concentration profile. The model agrees well with experimental results and demonstrates that the diffusivity of SiO₂ in the vapor phase controls the gradient of SiO₂ through the cross section of mullite felt.     

Crystallization Behavior of Potassium Niobium Silicate Glasses

Crystallization behavior of potassium niobium silicate (KNS) glasses having compositions expressed by the general formula x K₂O· x Nb₂O₅·(1 - 2 x )SiO₂, with x = 0.167, 0.182, 0.200, 0.220, and 0.250, has been studied by DTA, X-ray diffraction, second harmonic optical generation (SHG), and electron microscopy. Bulk crystallization of potassium niobates in glasses with compositions near K₂O·Nb₂O₅·2SiO₂, as well as surface crystallization of KNbSi₂O₇ phase, has been established. Transparent glass-ceramics, based on potassium niobates with remarkable SHG signal values, can be obtained from glasses with the lowest silica content, by heat treatment at temperatures just above T _g, while at higher temperatures from all of the glasses under investigation the main crystallizing phase is KNbSi₂O₇ ferroelectric. Applying a dc electric field, grain-oriented crystallization is produced in KNS glasses with development of significantly anisotropic arrangements of KNbSi₂O₇ crystallites.     

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.     

Dependence of Oxidation Modes on Zirconia Content in Silicon Carbide/Zirconia/Mullite Composites

Two basic oxidation modes of silicon carbide/zirconia/mullite (SiC/ZrO₂/mullite) composites were defined based on the plotted curve of the gradient of the silica (SiO₂) layer thickness (formed on individual SiC particles) versus depth. Mode I, where oxygen diffusivity was much slower in the matrix than in the SiO₂ layer, exhibited a relatively large gradient and limited oxidation depth. Mode II, where oxygen diffusivity was much faster in the matrix than in the SiO₂ layer, displayed a relatively small gradient and an extensive oxidation depth. When the volume fraction of ZrO₂ was below a threshold limit, the composites exhibited Mode I behavior; otherwise, Mode II behavior was observed. For composites with a ZrO₂ content above the threshold limit, the formation of zircon (ZrSiO₄), as a result of the reaction between ZrO₂ and the oxidation product (i.e., SiO₂), might change the oxidation behavior from Mode II to Mode I.     

Preparation of CdS-doped Glasses from Gels Containing Diethyldithiocarbamatocadmium

CdS-doped SiO₂ glasses were prepared via silica gels containing diethyldithiocarbamatocadmium Cd(S₂CN(C₂H₅)₂)₂. Heat treatment of the gels gave transparent yellow SiO₂glasses doped with hexagonal CdS crystals. In optical absorption and fluorescence spectra, the optical absorption edge and the emission peak clearly exhibited a blue shift, which was attributed to the quantum size effect of the carrier confinement, as the CdS content was decreased. In the fluorescence spectra of the CdS-doped silica glasses, the emission peak was observed only near 500 nm and not observed at longer wavelengths which were known to be present if there were sulfur vacancies.     

Anisotropic Abnormal Grain Growth in TiO2/SiO2-Doped Alumina

The effect of TiO₂/SiO₂ addition on the grain growth of alumina was reinvestigated. TiO₂ promoted the grain growth, but there was no abnormal grain growth. However, codoping of TiO₂ and SiO₂ resulted in a duplex microstructure consisting of large platelike grains, ∼800 μm long and ∼100 μm thick, and fine matrix grains. The observed anisotropic abnormal grain growth was explained in terms of liquid formation during heat treatment.     

Effect of Variations in Polymerized Oxides on Sintering and Crystalline Transformations

This work shows that molecular structural variations can be introduced in oxide systems, including AI₂O₃, ZrO₂, TiO₂, and SiO₂, by controlled hydrolysis and polymerization reactions during the formation of oxides from metal alkoxides. The chemical and stoichiometric makeup of hydroxides and oxides is not fixed and changes rapidly as a function of molecular size in the region where the size is extremely small. It was also observed that internal structural variations significantly alter the sintering behavior of oxide powders and affect their subsequent crystalline transformations.