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.     

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.     

Depolymerization of GeO2 and B2O3-GeO2 Network Glasses by Sb2O3

Binary Sb₂O₃-GeO₂ glasses containing 45 mol% Sb₂O₃ and ternary Sb₂O₃-B₂O₃-GeO₂ glasses containing 50 mol% GeO₂ were prepared. Their densities (volumes), refractive indices, and infrared spectra were determined, and their colors and high-temperature viscosities were estimated visually. Small amounts of Sb₂O₃ (∼10 mol%) appear to perturb neither the Ge-O-Ge network nor those B-O-Ge networks with small B/Ge ratios (∼0.2). The B-O-Ge networks with larger B/Ge ratios (∼1.0) depolymerize in the presence of even less Sb₂O₃. Amounts of Sb₂O₃ >10 mol% appear to depolymerize the Ge-O-Ge and Ge-O-B networks progressively, possibly with the formation of chains. A structurally sensitive ir isofrequency contour technique developed for ternary glass systems was applied successfully to these Sb₂O₃-B₂O₃-GeO₂ glasses. These contours can thus readily detect significant network depolymerization in the absence of the usual network modifiers.     

Effect of Sb2O3 on Thermal Properties of Glasses in Bi2O3–B2O3–SiO2 System

Glasses with compositions 50Bi₂O₃– x Sb₂O₃–10B₂O₃–(40– x ) SiO₂ ( x =0, 1, 3, 5, 8, 10) have been prepared by conventional melt quench technique. Substitution of Sb₂O₃ for SiO₂ exerted an obvious effect on properties of glasses, especially, increased glass transition temperature ( T _g) and crystalline temperature ( T _c) greatly. Results of infrared transmission spectra attributed the effect to the formation of new bridging bonds of Sb–O–B and Sb–O–Si in glass network.     

Viscous Relaxation and Non-Arrhenius Behavior in B2O3

Ultrasonic shear and longitudinal measurements were made in Bz03 from 650° to 1000° C. From these data both the shear and volume relaxation time spectra were determined. The spectra had the same temperature dependence, although the volume spectrum was always broader than the shear spectrum. The shear relaxation process can be represented by a single relaxation time above 800° C in the region where the shear viscosity is Arrhenius. Both processes exhibit an increasingly broad distribution of relaxation times in the non-Arrhenius region. The temperature dependence of the shear spectrum was analyzed in terms of a distribution of activation energies. A surprising conclusion of this study is that activation energies at low temperatures are smaller than the activation energy in the Arrhenius region.     

Studies in Lithium Oxide Systems: I, Li2O B2O3–B2O3

Phase relationships in the system Li₂O, B₂O₃-B₂O₃ were studied by the quenching method using twenty compositions. The crystalline phases encountered were (a) Li₂O, B₂O₃, which melts congruently at 849°± 2°C., (b) Li₂O.-2B₂O₃, which melts congruently at 917°± 2°C., (c) a new compound, 2Li₂O-5B₂O₃, which melts incongruently at 856°± 2°C. and dissociates below 696°± 4°C., (d) Li₂O.3B₂O₃, which melts incongruently at 834°± 4°C. and dissociates below 595°± 20°C., and (e) probably Li₂O.4B₂O₃, which melts incongruently at 635°± 10°C. Reactions were sluggish at temperatures near 600°C., resulting in metastable relations. Hence phase equilibrium data relating to the lower stability limit of Li₂O.3B₂O₃ and to the upper stability limit of Li₂O.4B₂O₃ are considered to be tentative. Properties of the glasses and crystalline phases were studied. The refractive index of the glasses increased with the addition of Li₂O up to 22%, but further additions up to 40% had no substantial effect. Glasses containing less than 30% Li₂O were water soluble. Limited data on the density and thermal expansion of the glasses are presented. Li₂OB₂O₃ was euhedral, lath-shaped, length-fast, biaxial negative (2V = 27°), with n_α= 1.540, n_β= 1.612, n_γ= 1.616. Li₂O.2B₂O₃ was uniaxial negative, with n_e= 1.560, n_w= 1.605. Li₂O.3B₂O₃ was biaxial negative (2V = 75° to 80°), with n_α= 1.576, n_β= 1.602, n_γ= 1.605. X-ray powder diffraction data for the five crystalline compounds are presented. Thermal expansion data for Li₂O-B₂O₃ and Li₂O.2B₂O₃ and limited data on the fluorescent properties of the compounds are given. X-ray diffraction data are also presented for Li₂O.B₂O₃.4H₂O and Li₂O.-5B₂O₃. 10H₂O. Li₂O B₂O₃ was obtained by heating the first hydrate at 450° to 680° C. X-ray diffraction showed Li₂O.4B₂O₃ and Li₂O-3B₂O₃ to be the crystalline products obtained during heating the decahydrate at 500°C. and 600°C., respectively.     

Improvement of Crystallinity of KZr2P3O12 by Sol-Gel Synthesis

The X-ray diffraction pattern for potassium-zirconium phosphate is significantly improved when the material is prepared by the sol-gel route rather than the powder method. Peaks at high, low, and intermediate angles are presented and compared for the two methods. It is assumed that the more nearly homogeneous mixing of the elements in the sol-gel process is responsible for the significant improvement in crystallinity.     

Sub-Sub-Tg Enthalpy Relaxation in a B2O3 Glass

Enthalpy relaxation processes in a B,O₃ glass were investigated calorimetrically in the temperature region from well below to near the glass-transition temperature T_g . The low-temperature ( T_aT_g- 100 K) or sub-sub- T_g anneals stabilize the glass structure. On heating, the annealed sample shows an excess endothermic peak above the annealing temperature T_a , and gradually recovers the original enthalpy without heating through T_g . The enthalpy relaxation evolves in a continuous manner with annealing time as seen in the recovery process. As T_a approaches T_g , a gradual transformation of the sub-sub- T_g behavior to that characteristic of the sub- T_g anneal occurs. The relaxation spectrum exhibits a broad distribution of relaxation times with activation energies of ∼ 10⁵ J/mol. The so-called β distribution commonly used in the analysis of the sub- T_g relaxation processes is seen to be too narrow to describe the sub-sub- T_g behavior. Possible mechanisms for the sub-sub- T_g relaxation phenomena are proposed.     

Spectrum of Volume Relaxation Times in B2O3

Time-index of refraction isotherms were measured for B₂O₃ glass starting from both a high and a low temperature in the transformation region. The equilibrium index values at each temperature, obtained from both types of approach curve, were identical. As in the case of the density values, the equilibrium refractive index curve as a function of temperature for this glass is not a straight line. The two-relaxation-times (crossover) model was applied to B₂O₃ glass and fitted the data as well as it did in previous experiments with borosilicate crown and GeO₂ glasses. The reverse crossover which was predicted by the model was experimentally confirmed with the B₂O₃ glass. The spectrum of relaxation times narrowed with decreasing temperature, indicating approach to another region of single relaxation associated with the low-temperature Arrhenius region. The relaxation times for the low-temperature crossover agreed well with those from the high-temperature curves, indicating complete linearity in the experiments. The spectrum of relaxation times was slightly asymmetrical at constant pressure and very asymmetrical at constant volume.     

Raman Spectroscopy of Heat-Treated B2O3-SiO2 Glasses

Raman spectra were measured on B₂O₃-SiO₂ glasses heat-treated at different temperatures and for different periods of time. It was found that the intensity ratio of the 810- and 475-cm⁻¹ bands depended on the thermal history and composition of glass. The intensity vs heat-treatment temperature curve exhibited a maximum in the glass-transition temperature range. High-temperature Raman spectra were measured on 0.5B₂O₃-0.5SiO₂ glass, both with increasing and decreasing temperature, between room temperature and 600°C. Above 300°C with increasing temperature, the intensity of the 810-cm⁻¹ band decreased, whereas the intensity of the 475-cm⁻¹ band increased. The changes reversed with decreasing temperature. The reversible change of the intensity of the two bands was interpreted as a reversible formation and decomposition of boroxy rings with temperature. The reaction becomes very sluggish below the glass transition so that the intensity ratio stayed constant below 300°C.     

Mineralizing Magnesium Aluminate Spinel Formation With B2O3

B₂O₃ mineralizes spinel formation from stoichiometric (1:1 mole ratio) calcined magnesia and alumina. After 3 h at 1100°C, X-ray diffraction (XRD) shows the mineralization effect of B₂O₃ is limited to 1.5 wt% additions with higher B₂O₃ contents leading to Mg₃B₂O₆ formation and reduced spinel content. Boron nuclear magnetic resonance, electron probe microanalysis (EPMA), scanning electron microscopy, transmission electron microscopy (TEM) and XRD reveal formation of a boron-containing liquid. Energy dispersive spectroscopy in the TEM and EPMA of the glassy phases formed from solidification of the liquid reveal that initially it is Mg borate, later becoming a magnesia-modified boroaluminate, composition suggesting dissolution–precipitation as opposed to templated growth as the mechanism of this liquid phase mediated mineralization.