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.     

Phase Equilibria in the System MgO-B2O3

Phase equilibria in the system MgO-B₂O₃ were investigated using DTA and quenching techniques. The system contains 4 invariant points. The compounds MgO·2B₂O₃ and 2MgO·B₂O₃ melt incongruently at 995° and 1312°C, respectively, whereas 3MgO·B₂O₃ melts congruently at 1410°C. A eutectic occurs at 1333°C and 71% MgO.     

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.     

Effect of B2O3 and hBN Crystallinity on cBN Synthesis

Four kinds of BN powders—amorphous BN with B₂O₃, partially crystallized BN without B₂O₃, well-crystallized hBN with B₂O₃, and well-crystallized hBN without B₂O₃—were prepared to determine the effect of B₂O₃ on the crystallization of amorphous BN and the effect of BN crystallinity on the formation of cBN under high pressure (4–5 GPa) and at high temperature (1350–1450°C). The amorphous BN with B₂O₃ easily crystallized and transformed to cBN in the presence of A1N catalyst, while the partially crystallized BN without B₂O₃ did not. The well-crystallized hBN transformed very slowly to cBN without B₂O₃, in contrast to fast transformation with B₂O₃. It is thus found that the transformation from hBN to cBN in the presence of AIN catalyst is determined by the degree of BN crystallinity as well as the presence of B₂O₃. Cubic BN can be synthesized only from crystallized hBN under the experimental conditions used. The formation of cBN from amorphous BN is possible through its prior crystallization, which can occur in the presence of B₂O₃.     

Structure of Borosilicate and Borogermanate Melts at 1300°C; a Viscosity and Density Study

An improved counterbalanced sphere viscometer-densitometer was used to obtain information at temperatures between 1000° and 1400°C for (a) molten B₂O₃, (b) a series of borosilicate melts containing up to 55 mole % SiO₂, and (c) a series of borogermanate melts containing up to 65 mole % GeO₂. The dependence on composition of these structure-sensitive parameters was used to develop a model for the alteration of molten B₂O₈ by other network-forming species. The B/Si or B/Ge ratio is shown to be a significant factor in determining melt structure. The silicon (or germanium) atoms appear to be widely separated in the B₂O₂ solvent for compositions in the 0 to 10-20 mole % SiO₂ (or GeO₂) region. The evidence suggests that a gradual microclustering of SiO₂ (or GeO₂) accompanies moderate departures from ideality for B₂O₃ in the 10-20 to 60 mole % SiO₂ (or GeO₂) region. Extensive micro-clustering of SiO₂ (or GeO₂), approaching network formation, appears to occur in the 60 to 100 mole % SiO₂ (or GeO₂) region; some of the boron atoms remain in clusters and/or are forced to adopt a tetrahedral configuration in this region.     

Immiscibility and the System Lanthanum Oxide–Boric Oxide

The phase equilibrium diagram for the system La₂O₃-B₂O₃ has been determined experimentally. The compounds La₂O₃-3B₂O₃and La₂O₃-B₂O₃ melt congruently at 1141°± 5°C. and 1660°± 15°C, respectively. At 1488°± 5°C, La₂O₃-B₂O₃ inverts from the aragonite-type structure to a high-temperature form. Trilanthanum borate, 3La₂O₃ B₂O₃, melts incongruently at 1386°± 5°C. to give liquid and La₂O₃. No solid solutions exist in the system. A region of liquid immiscibility exists in the system and extends at 1136°± 5°C. from almost pure B₂O₃ to 21.5 mole % La₂O₃. The experimental value for the extent of immiscibility agrees with that calculated from theoretical considerations. A second method for estimating immiscibility in the system is demonstrated, which requires experimentally only the determination of the index of refraction of the modifier-rich liquid. Principles governing immiscibility are discussed.     

Phase Separation and Crystallization of BiSrCaCu2Al0.5Ox Glass

Al₂O₃ addition to the melt of a BiSrCaCu₂O _x composition was found by TEM observation to cause the liquid-liquid phase separation of the melt-quenched glass, and to result in preferential precipitation of superconducting Bi₂Sr₂Ca₁Cu₂O _x crystals from the melt during the cooling process.     

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.     

Effects of Doping Trivalent Ions in Bismuth Borate Glasses

Bismuth borate glasses from the system: 40Bi₂O₃–59B₂O₃–1Tv₂O₃ (where Tv=Al, Y, Nd, Sm, and Eu) and three glasses of composition: 40Bi₂O₃–60B₂O₃, 37.5Bi₂O₃–62.5B₂O₃ and 38Bi₂O₃–60B₂O₃–2Al₂O₃ were prepared by melt quenching and characterized by density, UV-visible absorption spectroscopy and differential thermal analysis (DTA) studies. Bismuth borate glasses exhibit a very strong optical absorption band just below their absorption edge. Glasses were devitrified by heat treatment at temperatures above their glass transition temperatures and the crystalline phases produced in them were characterized by Fourier transform infrared (FTIR) absorption spectroscopy and X-ray diffraction (XRD). Bi₃B₅O₁₂ was found to be the most abundant phase in all devitrified samples. DTA studies on glasses and FTIR and XRD analysis on crystallized samples revealed that very small amounts of trivalent ion doping causes significant changes in the devitrification properties of bismuth borate glasses; rare-earth ions promote the formation of metastable BiBO₃–I and BiBO₃–II phases during glass crystallization.     

Densification of Calcia-Stabilized Zirconia with Borates

Densification studies of submicrometer ZrO₂ powders stabilized with 6.5 wt% CaO (CSZ) showed borate additions (1 to 10 wt%) to be effective sintering aids. Estimated densities >99% of theoretical were obtained on sintering at 1200°Cfor 4 h with 2 wt% B₂O₃ or 5 wt% CaO·2B₂O₃ additions to the CSZ powders. Average grain sizes obtained were typically <1 μm. Partial development of a monoclinic ZrO₂ phase was observed in the sintered samples. The amount of this phase varied from ∼7 to 75 wt% and was approximately linearly dependent on the additive concentration. The effect was most marked for the B₂O₃ additions. Development of the monoclinic phase was attributed to progressive leaching of Ca from the CSZ phase by B₂O₃, in effect partially destabilizing the ZrO₂.     

Reactions and Microstructure Development in Mullite Fibers

Microstructural and compositional changes during heat treatment of sol–gel-derived mullite fibers with additions of 2 wt% B₂O₃, 2 wt% P₂O₅, 2 wt% Cr₂O₃, and (1 wt% P₂O₅+ 1 wt% Cr₂O₃) were compared with those of undoped mullite fibers. For all compositions the sequence of phase development was the crystallization of a spinel phase (†-Al₂O₃ or Al–Si spinel) from amorphous material, followed by the formation of mullite at higher temperatures. Differential thermal analysis showed that additions of B₂O₃ and P₂O₅ increased the temperature of spinel formation and that B₂O₃ significantly decreased the temperature of mullite formation. After 1 h at 1200°C, the size of mullite grains in fibers that contained B₂O₃ was less than 1000 Å the grains in fibers of other compositions were 6000 to 12000 Å. After 60 h at 1400°C, fibers modified with B₂O₃ had a grain size less than 2000 to 3000 Å the grains in fibers of other compositions were 6000 to 12000 Å. B₂O₃ was the most volatile additive.     

Unusual Immiscibility Structures in Tellurite Glasses

Unusual droplet microaggregates are observed in TeO₂-rich glasses of the systems TeO₂–B₂O₃, TeO₂–GeO₂, TeO₂–B₂O₃–GeO₂, and TeO₂–GeO₂–V₂O₅. A decrease of the TeO₂ content is established in the aggregates in comparison with the matrix. Their appearance is related to the process of metastable liquid-phase separation at high viscosity of the melts.     

Density, Surface Tension, and Viscosity of PbO-B2O3-SiO2 Glass Melts

The density, surface tension, and viscosity of the melts from the PbO-B₂O₃-SiO₂ system have been measured at temperatures in the range 1073–1473 K. The effect of composition on these properties was also investigated. The density of the melt was found to increase linearly with increasing PbO content. Molar volume was derived from the density data, and its deviation from the additivity of partial molar volumes was calculated. These deviations in molar volume from those obtained from additivity rules have been used along with the ratio of various coordination numbers of boron (as reported by Bray) to discuss the structure of the melts. The surface tension was found to decrease with decreasing SiO₂/B₂O₃ ratio, and to increase in the range of the PbO content between 30 and 60 mol%, showing a maximum at ∼60 mol% PbO, and then decreased with further additions. This result suggested that the surface tension would be affected primarily by the B₂O₃ content in the range of the PbO content between 30–60 mol%, and mainly by the PbO content in the range of the PbO content >60 mol%, respectively. The viscosity of the melt was found to decrease linearly with increasing PbO content. The results obtained indicate that the increase in viscosity with B₂O₃ was half that of SiO₂ (on a molar basis), and an empirical equation has been proposed for the viscosity as a function of mole fraction.     

Enhanced Crystallization of a Glassy Phase in Silicon Nitride by the Addition of Scandia

A Sc₂O₃-MgO-Al₂O₃-SiO₂ melt reacted with bulk pieces of hot-pressed Si₃N₄, dissolving Si₃N₄ at the surface and penetrating into the interior via the grain boundaries. During subsequent cooling, the glass phase in the Si₃N₄ easily crystallized and fine crystallites were formed, but no phase separation was observed. This behavior contrasts strongly with that observed for a similar melt reaction without Sc₂O₃.     

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.     

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.     

Interaction between Barium Titanate and Binary Glasses

Interactions between BaTiO₃, and three binary glasses were studied through the reaction of BaTiO₃, powder with glass powder. For PbO–B₂O₃ and PbO–SiO₂ glasses, the reaction led to stable compound formation, the substitution of Pb in the BaTiO₃ structure, and noticeable grain growth of BaTiO₃. The interaction phenomena for these two glass systems were very similar. The substitution of Pb into BaTiO₃ is assisted by chemical reactions in which BaB₂O₄ or Ba₂SiO₄ is formed. The substitution into BaTiO3 also seems to be closely related to the grain growth of BaTiO₃. On the other hand, only compound formation was observed during the processing of BaTiO₃ with Bi₂O₃–B₂O₃ glass. Neither BaTiO₃ grain growth nor Bi substitution took place with the Bi₂O₃–B₂O₃ glass system. Based on the observed reactions and the glass viscosity, several sintering aids for BaTiO₃ ceramic products are suggested in this paper.     

Effect of B2O3 Addition on the Thermal Stability of Barium Phosphate Glasses for Optical Fiber Devices

The effect of B₂O₃ addition on the thermal stability of BaO–P₂O₅ glasses is studied by differential thermal analysis (DTA), X-ray diffraction (XRD) analysis, scanning electron microscopy, and micro-Raman spectroscopy. The difference between glass-transition and onset-crystallization temperatures increases monotonically with increasing B₂O₃ concentration. The DTA result reveals that no exothermic peak due to surface crystallization exists in the BaO–P₂O₅ glass doped with 3 mol% B₂O₃. A single-mode BaO–P₂O₅-B₂O₃ glass fiber could be fabricated by a rod-in-tube technique. The modification of glass structure due to B₂O₃ addition is qualitatively discussed.     

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.     

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.