Crystal Growth in the System PbO-B2O3

The temperature dependence of the rate of growth of PbO/2B₂O₃ from lead borate melts with PbO/B₂O₃ mole ratios of 1/3,1/2, 2/3, and 1 was determined. The maximum growth rates occurred from those melts which contained a slight excess of PbO. The results are explained on the basis of a modification in the structure of the melt in the interfacial zone as the PbO content is changed. Such changes markedly affect the fluidity of the melt, alter the concentration of crystallizable species, and may reduce the driving force for crystallization.     

Boron Coordination in PbO.2B2O3 and BaO·2B2O3 Melts

A partial molar volume technique was used to estimate the coordination number of oxygen ions around a boron ion in PbO·2B₂O₃ and BaO·2B₂O₃ melts. The boron coordination number appeared to be lower in the melt than in the crystal of PbO·2B₂O₃, whereas it was nearly the same in the melt and crystal of BaO·2B₂O₃.     

Preparation and Characterization of High-Density PbO–Bi2O3–B2O3 Glasses

Transparent glasses with densities greater than 8 g/mL, which are resistant to nuclear radiation damage and can serve as a host for rare-earth scintillators, are being developed. Glasses in the PbO–Bi₂O₃ system are known to have some of the highest densities yet reported. In this study, a thorough reexamination of these glasses has been performed to examine their optical-transmission windows and densities. The observed glass-forming range found in this work agrees well with that reported in the literature. A minimum of ≅ 10 mol% B₂O₃ was required in some cases to vitrify the compositions. Glasses were examined in which the PbO:Bi₂O₃ ratios ranged from 10:90 to 90:10 with a maximum total B₂O₃ content of ≅50 mol%. Glasses with higher B₂O₃ contents were not examined because of the rapidly decreasing density with increasing B₂O₃ content. The densities of the glasses ranged from a low of 6.0 g/mL to a high of 8.1 g/mL. The highest densities were observed for glasses with the greatest PbO content. Ultraviolet/visible and infrared spectra were recorded for these glasses. The transmission window lies between ≅430 and 3850 nm. A systematic increase in the cutoff wavelength was found to occur with an increase in the density. Water-absorption peaks were found in the infrared spectra at ≅3000 nm.     

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.     

Volume Relations in Na2O·B2O3 and Na2O·SiO2 B2O3, Melts at 1300·C

Density (and some viscosity) data are presented for binary sodium borate melts containing as much as 60 mole % Na₂O and for ternary sodium silicoborate melts with B/Si <2.0 between 1000°C and 1300°C. The high-temperature partial molar volume analysis of the binary sodium borate melts reveals about 50% BO₄ tetrahedra at the 40 mole % Na₂O composition, in agreement with recent NMR estimates for the binary glasses. No "boron anomaly" was found near 18 mole % Na₂O at high temperature. The synthetic partial molar volume model that agrees best with experiment for all ternary melts studied involves the presence of some BO₄ tetrahedra, the percentage of which varies with composition. This ternary model involves a high degree of internal consistency. No tendency toward extensive micro-immiscibility was observed for ternary melts near the SiO₂·B₂O₃ binary.     

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.     

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.     

Low-Temperature Sintering and Microwave Dielectric Properties of the Zn2SiO4 Ceramics

B₂O₃ was added to nominal composition Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature for application to low-temperature cofired ceramic (LTCC) devices. B₂O₃ reacted with SiO₂ to form a liquid phase containing SiO₂ and B₂O₃. The composition and melting temperature of the liquid phase depended on the sintering temperature and the B₂O₃ content. The specimen containing 20.0 mol% of B₂O₃ sintered at 900°C exhibited high microwave dielectric properties of Q × f =53 000 GHz, ɛ _r=5.7, and τ_f=−16 ppm/°C, confirming the promising potential of the B₂O₃-added ZS ceramics as candidate materials for the LTCC devices.     

Thermodynamic Activities in B2O3-SiO2 Melts at 1475 K

Eight glass samples in the B₂O₃-SiO₂ system with compositions from 20 to 90 mol% B₂O₃ were prepared. The equilibrium vaporization was studied by Knudsen effusion mass spectrometry at temperatures between 1450 and 1500 K. B₂O₃ ( g ) was the most abundant boron-containing species in the vapor; no silicon-containing gaseous species were detected. Thermodynamic activities of B₂O₃ in the liquid were determined at 1475 K. Thermodynamic activities of SiO₂ and integral excess Gibbs energies were estimated from the thermodynamic activities of B₂O₃. The thermodynamic data support the results obtained by other methods indicating the existance of a miscibility gap in the metastable liquid.     

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.     

Thermodynamics of Nitrogen in B2O3, B2O3─SiO2, and B2O3─CaO Systems

The BN solubilities for B₂O₃, B₂O₃─SiO₂, and B₂O₃─CaO systems have been measured mainly at 1823 K using a graphite crucible. The capability of the systems for nitrogen dissolution is compared with that of silicate systems in terms of nitride capacity. The dependence of nitrogen solubility in molten CaO containing 15 mol% of B₂O₃ on oxygen and nitrogen partial pressures is also investigated. It has been found that there are two mechanisms for nitrogen dissolution, namely as chemically bonded nitrogen and as physically dissolved nitrogen gas.     

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 .     

Properties and Structure of Glasses in the System Bi2O3-SiO2-GeO2

In the system Bi₂O₃-SiO₂-GeO₂, good glasses can be formed only from limited compositional regions consisting of 2 narrow strips along the lines x Bi₂O₃-(100-:t) GeO₂ ( x ≤40) and 40Bi₂O₃ y SiO₂ (60- y )GeO₂ (mol%); such glass is dark brown. Compositions from a large region (Bi₂O₃ content <40 mol%) showed immiscibility. In the binary system Bi₂O₃-GeO₂, density and refractive index vary linearly with composition (mol%). Negative deviations of molar volume from ideality suggest that the coordination of a significant number of Ge ions is changing from 4-fold to 6-fold. Thermal expansion and electrical resistivity data are also reported.     

Structure of Glasses and Melts in the Na2O·GeO2·B2O3 System

Density and viscosity results are presented for ternary Na₂O·GeO₂·B₂O₃ melts (∼600° to 1300°C) and glasses containing as much as 35 mole % Na₂O. Synthetic partial molar volume models indicate a fairly broad stability region for BO₄ tetrahedra in the B₂O₃-rich melts. Similar models for GeO₂-rich melts reveal a more limited stability region for GeO₆ octahedra. The expansion coefficient contours and viscosity isotherms confirm the volume-based conclusions for the liquid state. The high-temperature volume models were used to develop glass volume models that agree to within several percent of experiment. It has been concluded that the melts and glasses possess similar structures. The relatively greater compositional stability of GeO₆ octahedra in the presence of B₂O₃ (compared to Al₂O₃) can be related to the smaller average number of oxygens around boron (III), at a fixed O/Ge ratio, compared to aluminum (III). Evidence is presented for a slight decrease of the thermal stability of GeO₆ octahedra in the GeO₂-rich melts above about 1000°C.     

Elastic Modulus and Fracture Toughness of Ternary PbO-ZnO-B2O3 Glasses

The elastic moduli and fracture toughnesses of a series of PbO-ZnO-B₂O₃ glasses were measured for different PbO/ZnO ratios and for B₂O₃ contents from 30 to 70 mol%. Substituting ZnO for PbO increased both the elastic modulus and fracture toughness at all B₂O₃ levels with the fracture toughness being related to the elastic modulus. Structural effects on these properties are discussed.     

Indirect Dissolution of (Al, Cr)2O3 in CaO—MgO—Al2O3—SiO2 (CMAS) Melts

The dissolution of (Al, Cr)₂O₃ into CaO—MgO—Al₂O₃—SiO₂ melts, under static and forced-convective conditions was investigated at 1550°C in air. With sufficient MgO in the melt, or sufficient Cr₂O₃ in (Al, Cr)₂O₃, a layer consisting of a spinel solid solution, Mg(Al, Cr)₂O₄, formed at the (Al, Cr)₂O₃/melt interface. The dissolution kinetics of 1.5 and 10 wt% Cr₂O₃ specimens were determined as a function of immersion time, specimen rotation rate, and magnesia content of the melt. Electron microprobe analysis was used to characterize concentration gradients in the (Al, Cr)₂O₃ sample, the Mg(Al, Cr)₂O₄ spinel, or in the melt after immersion of specimens containing 1.5 to 78 mol% Cr₂O₃. The dissolution kinetics and microprobe analyses indicated that a steady-state condition was reached during forced-convective, indirect (Al, Cr)₂O₃ dissolution such that spinel layer formation was rate limited by solid-state diffusion through the spinel layer and/or through the specimen, and spinel layer dissolution was rate limited by liquid-phase diffusion through a boundary layer in the melt. This is consistent with a model previously developed for the indirect dissolution of sapphire in CMAS melts.     

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 Separation of Glasses in the System SrO-B2O3-SiO2

The region of stable liquid-phase separation in the system SrO-B₂O₃SiO₂ was found to extend from the SrO-SiO₂ binary join, across the ternary region, to the SrO-B₂O₃ join. The boundary of the region rich in network-forming oxides lies between the SiO₂-B₂O₃ boundary and the 2.5-mol% SrO isopleth, whereas the maximum SrO content is between 30.3 and 35.2 mol% (42 to 48 wt%) SrO at an SiO₂: B₂O₃ ratio of 4:1. The microstructures of glasses quenched from the immiscibility region were characterized by scanning electron microscopy. Evidence is presented of both stable and metastable phase separation and of microseparation and coring in disperse-phase spheres rich in network-forming oxides.     

Sintering and Microwave Dielectric Properties of the LiNb0.63Ti0.4625O3 Ceramics with the B2O3–SiO2 Liquid-Phase Additives

The effect of B₂O₃–SiO₂ liquid-phase additives on the sintering, microstructure, and microwave dielectric properties of LiNb_0.63Ti_0.4625O₃ ceramics was investigated. It was found that the sintering temperature could be lowered easily, and the densification and dielectric properties of LiNb_0.63Ti_0.4625O₃ ceramics could be greatly improved by adding a small amount of B₂O₃–SiO₂ solution additives. No secondary phase was observed for the ceramics with B₂O₃–SiO₂ additives. With the addition of 0.10 wt% B₂O₃–SiO₂, the ceramics sintered at 900°C showed favorable microwave dielectric properties with ɛ_r=71.7, Q × f =4950 GHz, and τ_f=−2.1 ppm/°C. The energy dispersive spectra analysis showed an excellent co-firing interfacial behavior between the LiNb_0.63Ti_0.4625O₃ ceramic and the Ag electrode. It indicated that LiNb_0.63Ti_0.4625O₃ ceramics with B₂O₃–SiO₂ solution additives have a number of potential applications on passive integrated devices based on the low-temperature co-fired ceramics technology.     

Studies in Lithium Oxide Systems: VII, Li2O–B2O2–SiO2

Phase relations in the system Li₂O–B₂O₃–SiO₂ were studied by quenching and solid-state reactions. No ternary compounds were detected in the portion of the system containing less than 53% Li₂O. Compatibility triangles were formed from the binary borate and silicate compounds. Liquidus data obtained by quenching are reported for four joins, Li₂O·2SiO₂–Li₂O·2B₂O₃, Li₂O·SiO₂-Li₂O·2B₂O₃, Li₂O·SiO₂-Li₂O·B₂O₃, and Li₂O·2B₂O₃-SiO₂. The last join cuts across the two-liquid region and is not a true binary system. Some probable ternary invariant points were located in the portion of the system which was quenchable to glass and adjacent to the two-liquid region. Further data on the previously reported immiscible liquid formation are given and the significance is discussed. Data on the thermal expansion behavior of certain glasses are presented.