Vitreous Materials for Electrooptics

A complex approach to the development of electrooptic glasses is proposed. The approach is based on the use of the traditional methods of structural-physical investigations in combination with the Rayleigh and Mandelstam-Brillouin scattering spectroscopic techniques. The structural features of niobate glasses and their properties are considered. It is shown that niobium selectively enters into the fluctuation microinhomogeneities in the alkali borosilicate vitreous matrix. The conditions for the precipitation of sodium niobate microcrystals are determined, and their influence on the increase in the Kerr constant for glass-ceramic materials is analyzed.     

Vitreous Materials for Electrooptics

A complex approach to the development of electrooptic glasses is proposed. The approach is based on the use of the traditional methods of structural-physical investigations in combination with the Rayleigh and Mandelstam-Brillouin scattering spectroscopic techniques. The structural features of niobate glasses and their properties are considered. It is shown that niobium selectively enters into the fluctuation microinhomogeneities in the alkali borosilicate vitreous matrix. The conditions for the precipitation of sodium niobate microcrystals are determined, and their influence on the increase in the Kerr constant for glass-ceramic materials is analyzed.     

Creep Viscosity of Vitreous China

Creep in four-point bend of a set of vitreous china compositions containing different amounts of mullite and quartz has been measured over the stress range 1.5 to 4 MPa at temperatures between 1035°–1100°C. Creep deformation is treated as the flow under load of a viscous liquid-particulate suspension, and creep rates are related to the measured mullite and quartz contents. Kaolinite relicts, containing the larger proportion of the mullite in the system, are expected to behave as rigid particles and to have a major influence on creep rate. The composition of the primarily feldspathic liquid phase has a lesser influence on creep rate.     

Vitreous carbon micro-lattice structures

A new approach is presented to fabricate open-cellular carbon materials with an ordered, lattice-type micro-scale architecture. The carbon micro-lattice materials were fabricated by pyrolyzing a polymer precursor template formed from an interconnected three-dimensional array of self-propagating photopolymer waveguides. Impregnating the polymer precursor template with acrylonitrile increased the carbon yield of the material from 19% to 46%. Structural analysis and density measurements of the solid carbon phase are consistent with vitreous carbon. Compression experiments yielded a compressive modulus (E) of 1.1 GPa and a failure strength (σ_f) of 10.2 MPa for a structure with relative density of 12.8%.     

Hertzian Fracture of Vitreous Carbon

Hertzian fracture produced by steel ball indenters on vitreous carbon is qualitatively similar to Hertzian fracture of soda-lime glass. The bulk material obeys Auerbach's law, i.e. the critical fracture load is proportional to the ball radius. However, a surface layer of material, which is denser than the bulk, is stronger and does not obey Auerbach's law. The fracture energy calculated from Auerbach's law and the theory of Frank and Lawn is more than an order of magnitude lower than that measured by conventional methods. Part of this discrepancy arises from the fact that the ring crack is 17% larger than the circle of contact. The fracture surface energy of vitreous carbon as determined by a somewhat empirical application of Auerbach's law is ∼3.5 J/m². The Hertzian fracture stress of vitreous carbon, like that of soda-lime glass, approaches the transverse rupture strength (TRS) when large indenters are used.     

Thermal Expansion of Vitreous Silica

The thermal expansion of vitreous silica is negative at low temperatures. This negative expansion can be explained as being caused by the transverse vibrations of the oxygen atoms. The frequency of such vibrations, because of the structure of vitreous silica, decreases as the structure shrinks, and this decrease results in negative thermal expansion.     

The New Leveling Agent Ivalon K. Mechanism of the Leveling Action

The mechanism of the leveling action of Ivalon K was investigated. It was found that the leveling effect of the product is primarily determined by its anion-active constituent, while the high migration of dye into the fibre material is due to its nonionogenic component.     

Neutron Diffraction Investigation of Vitreous Germania

The neturon diffraction pattern of vitreous germania is explained using the model of randomly oriented tetrahedra linked at corner oxygen atoms. The atomic radial distribution function calculated from the data gives 1.73, 2.83, and 3.45 Å as the Ge–O, O–O, and Ge–Ge interatomic distances, respectively. The number of atoms coordinated at these distances is 3.9, 5.9, and 4.3, respectively.     

Vitreous fillers in intumescent coatings

The introduction of vitreous fillers in some intumescent coatings compositions proved to be useful both in improvement of thermal insulation and durability of char under fire and in smoke emission characteristics. In this paper a comprehensive experimental study is presented showing results for solvent-borne, 2K epoxy and water-borne intumescent paints.

Blends of vitreous fillers with different melting temperatures, between 350°C and 850°C, have been tested together with refractory fillers (with melting temperatures between 1100°C and 1350°C). Lead free vitreous fillers with very low melting temperatures (from 350°C to 550°C) and with low water solubility were developed. Different types of paints have been prepared by adding different amounts of vitreous fillers. These compositions were then fire tested after application on steel plates. The effect of vitreous fillers on fire performance and stabilization of char (by means of encapsulation, with or without self-extinguishing additives) has been studied. The fillers’ effect on smoke emission under fire was also examined both in terms of optical density and toxicity index.

A special attention was focused on water-borne intumescent paints modified with vitreous fillers of different solubility and melting temperature. Residual solubility of the vitreous fillers, which appeared as a very important parameter for paint stability, has been assessed. Vitreous fillers releasing alkaline ions in such an amount to give a conductivity higher than 500 μS cm, make the intumescent paint composition unstable.     

Three Anisotropies in Vitreous Silica

Plasma-torch fused-silica boules of low OH content showed an optical anisotropy in the as-grown state which disappeared after annealing in O₂; this behavior was related to an EPR anisotropy which also depended on annealing, probably involving Fe²⁺ and Fe³⁺, respectively. High-temperature (1200° to 1500°C) deformation experiments performed in a modified DTA machine on slabs cut parallel and perpendicular to the preparation symmetry axis also showed an anisotropy, corresponding approximately to a factor of 2 in the viscosity. This anisotropy was also observed in commercial worked fused silica of high and low OH (water) content, made from purified Si compounds or natural quartz, and was not removed by annealing for 5 days at 1200°C in O₂. The deformation rate of fused silica decreases as water content is reduced from high to medium, but there is little further change when water content is decreased to a very low level.     

Lead-Ion Stability in Vitreous Systems

A new glaze composition was characterized and compared with one studied previously in order to evaluate structural model relating lead-ion stability and coloring-ion content to the vitreous system in which they are found. This model is based on the geometrical arrangement of the ionic network, depending in turn on the preferred coordination of the cations involved. The stability of the ionic structure, as indicated by release of Pb²⁺ on acid attack, is correlated with the coordination of sites occupied by network modifiers, i.e. cations producing cubic coordination were associated with increased Pb²⁺ release, whereas the opposite ccurred with those producing anti prismatic coordination.     

Deuteroxyl Groups in Vitreous Silica

Vitreous silica exhibits a characteristic absorption band at 3.81 μm which was attributed previously to a combination frequency of SiO₄ fundamentals. Deuterium treatment of water-containing fused silica also gives rise to an absorption band at 3.81-μm. An analysis of the previously characterized 3.81-μm band and its relation to absorption bands in deuterated specimens is presented.     

Diffusion of Oxygen in Vitreous Silica

The diffusivity of the oxygen ion in vitreous silica has been directly determined by exchange measurements employing the stable isotope ¹⁸O and mass spectrometer analysis. It was found that over the temperature range 925° to 1225°C the results can be represented by the equation D = 1.51 × 10⁻² exp (-71,200/ RT ) cm² sec⁻¹. These results are compared with other measurements of oxygen diffusion in silicate glasses. It is proposed that the controlling diffusion step in silicate glasses and nonstoichiometric silica is the rupture of a single oxygen bond to silicon and that the diffusion mechanism is interstitial motion through voids in the lattice. An analysis of theoretical expressions for the pre-exponential term D _O shows that present theories are unable to predict D _O for oxygen diffusion in glasses. It is also shown that the mechanism for electrical conduction in vitreous silica or in electrolytically purified quartz is not migration of oxygen ions.     

Surface Energy of Crystalline and Vitreous Silica

A variant of evaluation of the theoretical values of the surface energy of quartz glass and various forms of crystalline silica based on analysins of the structures of these materials and experimental determination of the zeta-potential of β-quartz and amorphous silica is considered.     

Resonant Optical Nonlinearity in Vitreous Semiconductors

Upconversion pumping of rare-earth ions in a semiconductor optical medium under particular conditions leads to an increase in the refractive index. The mechanism of the effect observed implies that, upon double or triple excitation by photons with energies less than the band gap, the energy of an Er³⁺ ion becomes sufficiently high for interband generation of free carriers. The resonant internal photoeffect is responsible for the increase in the refractive index.