Molten B2O3 Research Articles

Effect of Carbon and Water on Wetting and Reactions of B2O3‐Containing Glasses on Platinum

Sessile‐drop experiments were conducted to analyze the wetting characteristics of molten B2O3 and B2O3‐containing glasses on Pt. In atmospheres free of carbonaceous gases, the contact angle for all glasses on Pt was small and invariant under ambient pressures of 10‐4 torr to 1 atm for the range 700° to 1000°C. Adsorption of carbon at the metal surface decreased the solid/vapor surface energy and increased the contact angle. Carbon was removed by oxidation at a specific ambient pressure, resulting in a reduction in the contact angle. Temperature‐pressure data are reported for Pt. Absorption of water vapor by the glasses resulted in a reaction at the solid/liquid interface, spreading of the glass, and adherence. The possible reactions that can occur are given.

117. Oxidation of boronated graphite

Abstract The oxidation of boronated graphite occurs by two competing reactions in the temperature range 580–780°C, namely, the oxidation of carbon to yield CO2 and the oxidation of B4C to yield B2O3 and CO2. The former reaction is appreciably faster than the latter. The formation of molten B2O3 yields a physical barrier to reactant diffusion and, in the case of grey boronated graphite, leads to a maximum in the rate as a function of time. Because less B2O3 is formed in black boronated graphite, it behaves much like ordinary nuclear graphite. Upon removal of the B2O3 by leaching in boiling water, the oxidation rate is appreciably increased. This is particularly true for the grey boronated graphite, because the amount of B2O3 formed is greater than in black boronated graphite where the B4C is apparently not as accessible to the oxidant.

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.

Viscosity and Density of Boron Trioxide

Extensive and accurate viscosity values for molten B2O3 are reported over the continuous range from 20 poises (at 1400°C) to 1010 poises (at 318°C). The measurements were made with a wide‐range rotating‐cylinder viscometer. The melts were pretreated by bubbling superdry nitrogen through them to ensure a minimum water content. Above 800°C the temperature dependence of the viscosity curve obeyed the Arrhenius equation; below 800°C the curvature was smooth showing no breaks. At the low temperatures, although its curvature decreased, the viscosity curve did not obey the Arrhenius equation. Densities were determined between 411° and 1400°C. Over this range the volume expansion coefficient changed by 1 order of magnitude from 3.35 × 10−4/°C to 3.34 × 10−5/°C. The liquid volume expansion coefficient above 1200°C was smaller than that of the glass below the glass transition temperature, indicating some type of structural rearrangement in 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.

A Technique for Pulling Single Crystals of Volatile Materials

A technique has been developed to pull single crystals of volatile materials using standard pulling apparatus. This has been accomplished by covering the surface of the material to be pulled with a substance that is liquid at the pulling temperature. Crystals of PbTe and PbSe were pulled using molten B2O3 as the surface liquid.