Preparation of a Calcium Titanium Phosphate Glass–Ceramic with Improved Chemical Durability

A calcium titanium phosphate glass–ceramic for use as a dental material with excellent chemical durability was derived from a mother glass with a small amount of fluorine. The laser Raman spectroscopic analysis showed that 35CaO–10CaF₂–30P₂O₅–25TiO₂ glass, as the nominal composition, consists of ortho-, pyro-, and meta-phosphate groups. On heating the glass at 865°C, orthophosphate crystals, such as fluorine-containing oxyapatite (Ca₁₀(PO₄)₆(O,F₂)) and the Nasicon-type phase (CaTi₄(PO₄)₆), were preferentially precipitated; the apatite particles of several tens of nanometers in size were embedded in the CaTi₄(PO₄)₆ phase. The pale bluish color of the glass–ceramic indicated that titanium ions were included in the residual glassy phase. When the glass–ceramic was treated with dilute hydrochloric acid, only the apatite particles at the surface were leached out, while no CaTi₄(PO₄)₆ phase was etched; the dissolution of the glass–ceramic was effectively controlled. Almost no dissolution of ions from the glass–ceramic occurred in water. It was suggested that the behavior is a result of the microstructure of the glass–ceramic, which consists of crystalline and glassy phases with excellent chemical durability.     

Preparation of Porous Glass-Ceramics with a Skeleton of NASICON-Type Crystal CuTi2(PO4)3

A novel porous glass-ceramic with a skeleton of CuTi₂(PO₄)₃ was prepared by controlled crystallization of a glass and subsequent chemical leaching of the resulting dense glass-ceramic. A volume-crystallized dense glass-ceramic composed of CuTi₂(PO₄)₃ and Cu₃(PO₄)₂ whose surface was covered by a CuO thin layer was prepared by reheating a glass with a nominal composition of 50CuO·20TiO₂30P₂O₅ (in mol%) glass in air. When the resultant glass-ceramic was leached with dilute H₂SO₄, Cu₃(PO₄)₂ and CuO phases were dissolved out selectively, leaving a crystalline CuTi₂(PO₄)₃ skeleton. The specific surface area and the average pore radius of the porous glass-ceramic obtained were approximately 45 m₂g_-1 and 9 nm, respectively. The porous glass-ceramic showed catalytic activity in the conversion reaction of propene into acrolein.     

Stress in Leached Phase-Separated Glass

When a phase-separated glass is leached, stresses develop because of release of thermal stresses, creation of surface area, ion exchange, and hydration. Analyses are presented for the thermal stresses, including the portion that develops on cooling from the heat-treatment temperature to the setting temperature of the less viscous phase. During leaching, the interfacial energy of the residual phase increases, so that phase tends to contract. A more important effect is the contraction caused by removal of alkali and B₂O₃ from the residual phase during leaching. The extent of removal of B₂O₂ decreases with heat-treatment time, t_H , because the scale of the microstructure increases as t^1/3_H. The change in residual B₂O₂ content with t_H is shown to be consistent with diffusion-controlled ion exchange. The dependence of stress on t_H in partially leached glasses, measured by Drexhage and Gupta, results principally from the change in extent of ion exchange; the reduction in surface area with increasing t_H also has a significant effect on the stresses.     

Molten Glass Corrosion Resistance of Immersed Combustion-Heating Tube Materials in Soda-Lime-Silicate Glass

The corrosion resistance of molybdenum, molybdenum disilicide, and a SiC_(p)/Al₂O₃ composite to molten soda-lime-silicate glass was studied. The ASTM-C621–84 corrosion test method was modified because of inherent inaccuracies in the method and Si attack of platinum crucibles. Specimen-glass interfacial regions were characterized using XRD, SEM, and EDS. After 48 h of exposure at 1565°C, the half-down corrosion recessions of Mo, MoSi₂, and SiC_(P)/Al₂O₃ were 0.11, 0.316, and 0.26 mm, respectively. Mo oxidized to form a MoO₂ surface scale which cracked, allowing glass seepage and further oxidation. Silicon was leached out of MoSi₂ into the glass, leaving a Mo₅Si₃ interface and particles of Mo near the interface. For the SiC_(P)/Al₂O₃ composite, bubbles observed at the interfacial regions formed from oxidation of SiC to form CO. Thermodynamic modeling corroborated these experimental observations.     

Effects of the Weak Absorption Tail on the Transmission Loss of Ge–Sb–Se Optical Fibers

Selenide glass optical fibers were fabricated for Ge₃₀Sb₁₀Se₅₈S₂ and Ge₂₀Sb₁₀Se₇₀ glasses. Their transmission loss has been measured and compared with the theoretical attenuation loss that was calculated taking into account the electronic transition absorption, Rayleigh scattering, and multiphonon absorption. A low attenuation loss of the Ge₂₀Sb₁₀Se₇₀ glass composition in 1.2–1.7 μm range has been expected due to its high optical band gap energy compared with the Ge₃₀Sb₁₀Se₅₈S₂ glass. However, the measured attenuation loss of the Ge₃₀Sb₁₀Se₅₈S₂ glass fiber was ∼13 dB/m at 1.5 μm while Ge₂₀Sb₁₀Se₇₀ glass showed ∼82 dB/m. An enhanced weak absorption tail due to the localized states of the Ge₂₀Sb₁₀Se₇₀ glass was responsible for this behavior. Structural defects are related to the localized states and discussed for the present glass compositions.     

Microporous Glass-Ceramics in NASICON-Type Copper(II) Titanium Phosphate

A novel porous glass-ceramic with a skeleton of a NASICON-type copper(II) titanium phosphate was prepared via the controlled crystallization of a glass and the subsequent chemical leaching of the resulting dense glass-ceramic. A volume-crystallized dense glass-ceramic comprised of CuTi₂(PO₄)₃ and Cu₃(PO₄)₂, whose surface was covered by a thin layer of CuO, was prepared by reheating a glass with a nominal composition of 50CuO20TiO₂30P₂O₅ (in mol%) in air. When the resulting glass-ceramic was leached with dilute HCl, the Cu₃(PO₄)₂ and CuO phases were dissolved out selectively, and a cuprous NASICON crystal of CuTi₂(PO₄)₃ was converted to its cupric type, CuTi₄(PO₄)₆, which was left as a skeleton of the porous materials. The specific surface area and the average pore radius of the porous glass-ceramic obtained were ∼70 m²/g and ∼7 nm, respectively. The porous glass-ceramic showed high catalytic activities for the dehydration of 2-propanol.     

Characterization and Sintering of a Porous Glass-Ceramic in the System Na2O-B2O3-Sc2O3

Clear glasses which included droplet-like microphases were produced when SiO₂ in sodium borosilicate glasses was replaced by Sc₂O₃. Phase separation and/or crystallization occurred after heat treatment. The porous skeleton of leached glasses consisted of hexagonal ScBO₃. The specific surface areas and pore radii are comparable to those of porous SiO₂ glass. The sintering temperature of porous Sc-based material is higher than that of porous SiO₂. Alumina contamination influenced the structure of the porous material.     

Melting and Interface Reaction of Mn–Si–O Glass on BaTiO3

A glass with the eutectic composition 3MnO_1.5–2SiO₂ was used to simulate the formation of a liquid phase during sintering of BaTiO₃. Two oxide additives (Mn₂O₃ and SiO₂) performing various functions of the properties of BaTiO₃ were investigated for their crystallization and thermal characteristics at temperatures ≤1400°C. The wetting behavior of the glass, the dissolution of BaTiO₃ in glass melt, the identification of newly formed phases, and the sequential reaction kinetics of the glass/BaTiO₃ system, especially when isothermally treated at 1150°C, were investigated by electron microscopy with quantitative X-ray energy dispersive spectroscopic (Q-EDS) analysis. The evolution of the interfacial reaction of the glass/BaTiO₃ at 1150°C is reported and discussed.     

Mixed-Alkali Effect on Rayleigh Scattering in K2O–Na2O–MgO–SiO2 Glasses

The Rayleigh scattering of the mixed-alkali glass system K₂O–Na₂O–MgO–SiO₂ (KNMS) was investigated, both experimentally and theoretically. The lowest Rayleigh scattering coefficient (38% of that for pure SiO₂ glass) was obtained when the glass composition was 22K₂O–8Na₂O–10MgO–60SiO₂ (in mol%). These values are equal to or less than the minimum values reported for the ternary sodium silicate glass Na₂O–MgO–SiO₂. The Rayleigh scattering caused by concentration fluctuation was believed to have been reduced greatly in this KNMS glass, because the mobility of the alkali-metal ions was reduced by the mixed-alkali effect.     

Influence of Fluorine Content in Apatite–Mullite Glass-Ceramics

The nucleation and crystallization of a series of glasses based on 4.5SiO₂·3Al₂O₃·1.5P₂O₅·(5 − z )CaO · z CaF₂ with a Ca:P ratio corresponding to apatite were studied. In these glasses, the objective was to investigate the influence of fluorine content and z was varied from 3 to 0. All the glasses studied crystallized to fluorapatite (FAP) and mullite with the exception of the glass containing no fluorine, which crystallized to β-tricalcium phosphate (Ca₃(PO₄)₂) and anorthite (CaAl₂Si₂O₈). Glasses that contained sufficient fluorine to form FAP bulk nucleated to give FAP without a nucleation hold. Thermal gravimetric analysis demonstrated a significant weight loss corresponding to the crystallization of mullite, which increased with the fluorine content of the glass and also with decreased particle size. The loss was attributed to volatile SiF₄. The glass transition temperature decreased with increased fluorine content of the glass.     

Equilibrium Amorphous Silicon–Calcium–Oxygen Films at Interfaces in Copper–Alumina Composites Prepared by Melt Infiltration

The microstructure of copper–alumina (Cu-Al₂O₃) composites that have been prepared via the melt infiltration of liquid copper into porous alumina preforms was studied in detail, using various transmission electron microscopy (TEM) techniques. Two different samples—with open pore diameters of 0.2 and 0.8 μm—were investigated. For both specimens, a single crystalline copper network that extended throughout the open porosity of the alumina preform was observed. An amorphous glass phase that contained silicon and calcium was observed at the Al₂O₃/Cu/Al₂O₃ triple junctions. The diameters of these amorphous pockets, which were strongly faceted along the Al₂O₃ grains, were up to 20 and 100 nm for the initial pore sizes of 0.2 and 0.8 μm, respectively. A glass phase that contained silicon and calcium also was present at the Cu/Al₂O₃ interfaces, whereas the Al₂O₃ boundaries remained dry. Detailed high-resolution transmission electron microscopy investigations have shown that the interfacial glass phase at the Cu/Al₂O₃ interfaces exhibited a uniform equilibrium film thickness along the interface region. However, the interfacial film thickness was dependent on the orientation of the Al₂O₃ grain, and its value varied from 0.4 nm for Al₂O₃ rhombohedral-plane termination ((1¯012)) up to 1 nm for Al₂O₃ basal-plane termination ((0001)).     

Effect of Glass Additions on BaO–TiO2–WO3 Microwave Ceramics

The effect of glass addition on the properties of BaO–TiO₂-WO₃ microwave dielectric material N-35, which has Q = 5900 and K = 35 at 7.2 GHz for samples sintered at 1360°C, was investigated. Several glasses including B₂O₃, SiO₂, 5ZnO–2B₂O₃, and nine other commercial glasses were selected for this study. Among these glasses, one with a 5 wt% addition of B₂O₃ to N-35, when sintered at 1200°C, had the best dielectric properties: Q = 8300 and K = 34 at 8.5 GHz. Both Q and K increased with firing temperature as well as with density. The Q of N-35, when sintered with a ZnO–B₂O₃ glass system, showed a sudden drop in the sintering temperature to about 1000°C. The results of XRD, thermal analysis, and scanning electron microscopy indicated that the chemical reaction between the dielectric ceramics and glass had a greater effect on Q than on the density. The effects of the glass content and the mixing process on the densification and microwave dielectric properties are also presented. Ball milling improved the densification and dielectric properties of the N-35 sintered with ZnO–B₂O₃.     

Porous Glass-Ceramic in the CαO–TiO2–P2O5 System

A porous glass-ceramic in the CaO–TiO₂—P₂O₅ system has been prepared by crystallization and subsequent chemical leaching of the corresponding glass. By applying a two-step heat treatment to 45CaO · 25TiO₂· 30P₂O₅ glasses containing a few mol% of Na₂O, volume crystallization results in the formation of dense glass-ceramics composed of CaTi₄(PO₄)₆ and β-Ca₃(PO₄)₂ phases. By leaching the resultant glass ceramics with HCI, β-Ca₃(PO₄)₂ is selectively dissolved out, leaving a crystalline CaTi₄(PO₄)₆ skeleton. The surface area and mean pore radius of the porous glass-ceramics were approximately 40 m²/g and 13 nm, respectively.     

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.     

Reaction of CaTiO3 with PbO–B2O3 and PbO–SiO2 Glasses

Interfacial and powder reactions between CaTiO₃ and 90PbO–10B₂O₃ and 75PbO–25SiO₂ binary glasses were studied. The reaction has been analyzed as the effect of B₂O₃ and SiO₂ additions on the interaction between CaTiO₃ and PbO, and discussed from thermodynamic and kinetic points of view. For a fixed CaTiO₃/PbO ratio₂ the product perovskite phase became enriched with lead as the amount of additives increased, which is more pronounced with B₂O₃ addition. The reaction of CaTiO₃ with the lead–boron glass was controlled by a dissolution-precipitation mechanism, and that with the lead-silica glass by a diffusion mechanism.     

Effect of Molybdenum on the Structure and on the Crystallization of SiO2–Na2O–CaO–B2O3 Glasses

Crystallization of the poorly durable Na₂MoO₄ phase able to incorporate radioactive cesium must be avoided in SiO₂–Al₂O₃–B₂O₃–Na₂O–CaO glasses developed for the immobilization of Mo-rich nuclear wastes. Increasing amounts of B₂O₃ and MoO₃ were added to a SiO₂–Na₂O–CaO glass, and crystallization tendency was studied. Na₂MoO₄ crystallization tendency decreased with the increase of B₂O₃ concentration whereas the tendency of CaMoO₄ to crystallize increased due to preferential charge compensation of BO₄⁻ entities by Na⁺ ions. ²⁹Si MAS NMR showed that molybdenum acts as a reticulating agent in glass structure. Trivalent actinides surrogate (Nd³⁺) were shown to enter into CaMoO₄ crystals formed in glasses.     

Strengthening of Glass Surfaces by Sulfur Trioxide Treatment

Removal of soda from a glass surface at high temperature can, on cooling, give appreciable strengthening due to lowering of the expansion coefficient at the surface. Soda depletion has been accomplished previously by treating a high-expansion soda-lime glass with SO₂ in the presence of oxygen and water. The depletion rate can be doubled by using dispersed platinum to catalyze the oxidation of SO₂ to SO₃ before the gas reacts with soda from the glass. The data are consistent with the soda depletion being Fickian both with and without the catalyst. Alumina in the glass composition markedly increases the strength which can be attained by sulfur oxide treatment. Although the percentage of soda retained in the leached layer increases, the total amount of soda removed from the glass also increases as alumina increases. It must follow that the depth of the layer is increased.     

BiNbO4-Based Glass–Ceramic Composites for Microwave Applications

The effect of a bespoke glass sintering aid, 0.3Bi₂O₃–0.3Nb₂O₅–0.3B₂O₃–0.1SiO₂ (BN1), developed from the base ceramic composition, BiNbO₄ (BN), on the sinterability, microstructure, and microwave (MW) dielectric properties of BN ceramics has been investigated. Densities >97% theoretical could be achieved at 1020°C for samples with up to 15% BN1 additions. The resulting microstructure was composed of BN laths surrounded by a residual glass phase that contained small fibrous crystals. Some evidence of dissolution of BN crystals was observed. Optimum properties were exhibited for samples with 15 wt% of glass addition sintered for 4 h at 1020°C with a relative permittivity ɛ_r=38, a MW quality factor Q × f ₀=17 353 at 5.6 GHz, and a temperature coefficient of resonant frequency τ_f=−10 ppm/°C. The high Q × f ₀, ɛ_r, and low τ_f, coupled with a relatively low sintering temperature, suggest that the use of bespoke glass sintering aids of this type may have great potential for the fabrication of MW ceramics.     

Phase Evolution and Dielectric Properties of MgTiO3–CaTiO3-Based Ceramic Sintered with Lithium Borosilicate Glass for Application to Low Temperature Co-Fired Ceramics

Effects on phase evolution caused by the addition of a new sintering agent, lithium borosilicate, Li₂O·B₂O₃·SiO₂ (LBS) glass to 0.9MgTiO₃–0.1CaTiO₃ ceramic and resultant dielectric properties were investigated. The added LBS glass, a liquid phase sintering agent, significantly lowered the densification temperature from 1300° to about 950°C, while yielding decomposition of MgTiO₃ into MgTi₂O₅ and Mg₂TiO₄. At the same time, the by-products of the decomposition reaction, MgO and TiO₂, were dissolved into the glass network. Such phase evolution partly compensated the influence of deleterious glass addition so that the specimen demonstrated fairly good apparent dielectric properties.     

Effect of Alkali Metal Oxide Addition on the Sinterability of MgO–B2O3–Al2O3 Glass–Al2O3 Filler Composites

The sintering of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler is terminated due to the crystallization of Al₄B₂O₉ in the glass. The densification of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO–B₂O₃–Al₂O₃ glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT—200°C) of 4.4 × 10⁻⁶ K⁻¹, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.