Gamma Irradiation Studies of Some Borate Glasses

The gamma-ray-induced optical absorption in a series of cabal (calcium-boron-aluminum) glasses was studied and is interpreted, wherever possible, in terms of structural concepts. A resolution of the observed absorption spectra showed that three Gaussian-shaped bands were induced with their maxima at about 2.3, 3.5, and 5.0 e.v. (550, 350, and 250 m_μ). The 2.3-e.v. band decreased in intensity with increasing CaO content, reaching a minimum intensity at a composition corresponding to the four-coordination of about 20% of the boron. Further increase in CaO content was associated with an increase in the intensity of this band. The intensity of the 3.5-e.v. band decreased gradually with increased mole per cent of CaO and increased with increased Al₂O₃. The 5.0-e.v. band showed an abrupt increase in intensity which corresponded to the appearance of non-bridging oxygens in the network. Replacing Ca²⁺ by Mg²⁺, Sr²⁺, or Ba²⁺ or replacing Li⁺ by Na⁺ or K⁺ showed that glasses containing large ions of low field strength give less induced absorption than glasses containing small ions of high field strength. A potassium alumina borate glass melted under reducing conditions gave a considerably higher ultraviolet transmission, before irradiation, as compared with the same glass melted under normal conditions. The gamma-induced absorption of these two glasses showed that reducing conditions resulted in a decrease in the intensity of the 2.3- and 3.5-e.v. bands, whereas it caused an increase in the far-ultraviolet-induced absorption. The effect of additions of arsenic, thallium, titanium, germanium, and some rare-earth oxides is discussed.     

Spinodal Decomposition in Tellurite-Based Glasses Induced by Excimer Laser Irradiation

Tellurite-based glasses in the TeO₂–K₃Li₂Nb₅O₁₅, TeO₂–Ba₅Li₂Ti₂Nb₈O₃₀, and V₂Te₂O₉ were fabricated by the conventional melt-quenching technique. Amorphous and glassy characteristics of the as-quenched samples were established via the X-ray powder diffraction technique and differential thermal analysis, respectively. The as-quenched samples were irradiated by an excimer laser (248 nm). The effect of laser power, duration of irradiation, and the frequency of the laser pulses on the surface features of the above glasses were studied. The optical microscopic studies carried out on the above systems revealed the presence of quasi-periodic and periodic structures on their surfaces. The local compositional variations of these structures were confirmed by back-scattered electron imaging using scanning electron microscope accompanied by energy-dispersive X-ray analysis. These results were convincing enough to state that the glasses in the present investigations had undergone spinodal decomposition on laser irradiation. The incidence of the interconnected texture of two different phases was observed owing to the quenching effect produced by the heating and cooling cycle of the successive laser pulses. Ring- and line-shaped patterns were also observed, respectively, when the pulse frequency of the laser and the duration of irradiation were increased.     

A Gamma-Ray Induced Absorption Band in Some Lead Borate Glasses

Observations made on some lead borate, lead aluminoborate, and lead boroaluminosilicate glasses have shown that an absorption band is induced at 1.5 e.v. (825 mμ) by gamma irradiation. Experimental evidence indicates that this band is associated with Pb²⁺ ions and boron in the structure of these glasses. On replacing PbO by Tl₂O in a borate glass, two strong bands were observed; one band corresponds to the 1.5 e.v. band induced in lead borate glasses and a completely new band at 1.0 e.v. (1235 mμ). A postulate for the center responsible for the absorption band at 1.5 e.v. is proposed.     

Analysis of Complex ESR Spectra Induced in Potassium Borate Glasses

Electron spin resonance studies showed that gamma-irradiated potassium borate glasses display complex spectra. These were resolved into five overlapping resonances by a thermal bleaching technique. The resonances differ in nature and stability, suggesting that they belong to different paramagnetic centers (I, II, III, IV, and V) induced by irradiation. The relative concentrations of these centers change with K₂O concentration; the stability increases in the order V1⋍2.002, g₂⋍2.010, g₃⋍2.034; (II) g_avg⋍2.0087; (III) g⋍2.008; (IV) g⋍2.002; and (V) g⋍2.000. These values suggest that centers I, II, and III may be caused by trapped-hole centers, whereas centers IV and V are attributed to trapped electron centers. Centers I and II have hyperfine structures with an ¹¹B nucleus producing a five-line-plus-a-shoulder spectrum and a four-line spectrum, respectively. The resonance attributed to center V was very weak and was observed only in glasses which contained more than 20 mol% K₂O.     

Role of Cerium in Suppression of Gamma-Ray Induced Coloring of Borate Glasses

The role of cerium in the suppression of gamma-ray induced coloration in glass has been found to depend on the relative concentration of Ce³⁺ to Ce⁴⁺ ions as well as on the total cerium content. In a borate glass having high ultraviolet transmission, it has been found that both Ce³⁺ and Ce⁴⁺ ions are necessary to suppress the optical absorption bands induced in the visible region. The role of cerium can be explained on the basis of a change in its oxidation state as a result of gamma irradiation. It is postulated that the cerous ions, by the reaction Ce³⁺→ Ce⁴⁺+ e , suppress the induced visible band at 2.36 ev (525 mμ), which may result from positive hole centers. High cerous ion concentration results, however, in an induced center (Ce³⁺+ e ) which absorbs in the visible at about 1.9 ev (650 mμ). The presence of Ce⁴⁺ ions near Ce³⁺ prevents the formation of this center possibly by the reaction Ce⁴⁺+ e → Ce³⁺. These induced opposite changes in the oxidation state of cerium tend to maintain a balance in the ratio of Ce³⁺ to Ce⁴⁺ ions in the glass during irradiation, and the suppression of the visible bands depends on this ratio.     

New Sodium Borate Glasses

The glass-forming regions for the sodium borate system were determined using a roller-quencher. The new regions are R=0 to 0.8 and R=1.2 to 4.9for samples prepared from sodium carbonate (where R=mol% Na₂O/mol% B₂O₃). Above R=2.0 retention of'C02 occurs. Quantitative measures of CO₂ retention as a function of R are reported. The results using sodium carbonate extend by a considerable amount the limits reported by Imaoka and are comparable to the recent data of Ota and Soga.     

Microseparation in Lead Borate Glasses

The microstructure of a series of lead borate glasses was studied by electron microscopy and small-angle X-ray scattering techniques. Although the glasses were optically homogeneous, microheterogeneities of the order of 2000 to 3000 A were detected in glasses with compositions near the region of two-liquid stability. The size of these heterogeneities increased as the composition approached the region of immiscibility.     

Structure of Borate Glasses

Anomalous thermal expansion and other properties of borate glasses have previously been given a structural explanation by Abe. This explanation is here reinterpreted and revised in terms of the structon theory of Huggins. In agreement with Warren, it is postulated that, in glasses containing, in addition to boric oxide, small relative amounts of an alkali or alkaline-earth oxide, some of the boron atoms are surrounded by four oxygen atoms and all the oxygen atoms bridge between two boron atoms. At concentrations beyond that of the thermal expansion minimum, some oxygens are assumed to have only one boron neighbor. These ideas are formulated quantitatively and are used to interpret pertinent experimental data. From the concentrations where the property-composition curves show breaks, conclusions may be drawn as to the average number of oxygens surrounding each metal atom.     

Structure-Property Relations in Lanthanide Borate Glasses

Glass formation in the system Ln₂O₃-B₂O₃ (Ln = Nd, Sm) was studied. Glasses could be formed in the range from 0 to -28 mol% rare-earth oxide (Ln₂O₃), but liquid immiscibility in these systems limits the range of homogeneous glasses to 0 to 1.5 and 25 to 28 mol% Ln₂O₃. The infrared spectra indicate that the rare-earth-rich glasses are structurally similar to rare-earth metaborates (LnB₃O₆) which contain (B₃O₆)- chains. The variation in density, transformation temperature, thermal expansion coefficient, and transformation-range viscosity of these glasses with the size of the rare-earth ion is discussed. Glasses near the metaborate composition have a transformation temperature of =700°C, which is high for binary borate glasses. Glasses could not be formed in the systems EU₂O₃-, Gd₂O₃-, HO₂O₃-, and Er₂O₃-B₂O₃, even by quenching at =1300°C/s. The sudden lack of glass formation in the system Ln₂O₃-B₂O₃ with Ln³⁺ ions smaller than Sm³⁺ is explained on the basis of the size effect of the Ln³⁺ ion on the stability of (B₃O₆)- chains in these metaborates.     

Manganous-Manganic Equilibrium in Alkali Borate Glasses

The manganous-manganic equilibrium was studied in potassium, sodium, and lithium borate glasses. The glasses were analyzed chemically for total and trivalent manganese. The manganese equilibrium shifts more toward the oxidized state when the alkali concentration in the glass is increased and/or when the molar equivalent of lithia is replaced by soda and soda is replaced by potash. A linear relation between log (Mn³⁺/Mn²⁺) and mole percent alkali oxide was observed. Straight lines were obtained for lithium, sodium, and potassium borates. The disadvantages of using a concentration equilibrium constant where the glass composition is varied are discussed.     

Effect of CoO and Gamma Irradiation on the Infrared Absorption Spectra of Lithium Borate Glasses

Undoped and cobalt-doped lithium borate glasses (LBG) of various compositions, by varying cobalt contents were prepared by a conventional melt quenching technique. The density and molar volumes of the glass samples were estimated and; infrared absorption spectra were measured in the spectral range 400–1600 cm^?1before and after an irradiation dose of 50 kGy and 200 kGy. Experimental results showed that the density of studied samples increased as CoO increased while the molar volume decreased. FT-IR spectra of the prepared samples have been analyzed by the deconvolution of the spectra. A deconvolution technique is presented to make use of the BO₄ data and follow the change in the modifier and former fractions of CoO. FTIR was also used to study the glass system before and after gamma irradiation. The experimental results clearly indicate that after irradiation a significant change in the structure of the LBG glass network is observed. The modifying action of CoO on the glass composition is also studied. The glasses doped with 5 wt% of CoO are relatively more radiation resistant than the other compositions.     

Properties and Structure of Lithium Borate and Strontium Borate Glasses

Experimental results for some physical properties of Li₂O-B₂O₃ and SrO-B₂O₃ glasses with high modifier content showed anomalies in the composition dependence of many properties such as thermal expansion coefficient, transformation temperature, softening temperature, density, and refractive index at 30 to 33 mol% modifier oxide in these glasses. These anomalies are attributed to the formation of nonbridging oxygens near this composition.     

Secondary Phase Separation in Lead Borate Glasses

The effect of secondary phase separation on the apparent volume fraction of second phase was examined in the immiscible region of the PbO-B₂A₃ system. The volume fraction, V₂, of the PbO-rich phase agreed well with theoretical expressions relating V₂ to density and composition, except for 0.2< V₂ <0.5. The discrepancy in this region was explained in terms of the peculiar shape of the immiscible phase boundary and the resulting mutual supersaturation of the immiscible phases with respect to each other.     

States of Sulfur in Alkali Borate Glasses

Simple binary alkali borate glasses of differing alkali content (alkali = Li, Na, or K) containing sulfur were prepared and their absorption spectra were measured in the range of 200 to 700 nm. The absorption bands obtained were used to determine the stable states of sulfur in these glasses by directly comparing them with published data. The following states of sulfur were identified and their ranges of stability were determined: the S₂ molecule in glasses containing at least 15 mol% alkali oxide, the S₃⁻ and S₂⁻ in glasses containing 20 to 30 mol% Na₂O or K₂O, and the polysulfide ion (S x ²⁻) in glasses of 30 to 35 mol% Na₂O or K₂O.     

Low-Melting Glasses Based on Borate Systems

The results of analyzing systems of low-melting glasses and their application areas based on published data are used to select an optimum system. The melting properties of low-melting glasses based on the borate system ZnO – SrO – B₂O₃ are studied. The dependences of the main properties of glasses (TCLE, softening temperature, microhardness, chemical resistance to water) on their compositions are determined. The optimum low-melting glass compositions with required physicochemical and technological parameters are identified.     

Thermal Expansion of Alkali Borate Glasses

The thermal expansion behavior of lithium, sodium, potassium, and rubidium borate glasses was measured. The results indicate that the "borate anomaly" in the thermal expansion coefficient occurs at ∼20 mol% alkali oxide. A maximum in the glass transformation and dilatometric softening temperatures also occurs at ∼27 mol% alkali oxide. No evidence for phase separation was observed in this study. These effects are related to the structural changes reported for these glasses by Bray and coworkers.