Narottam P. Bansal

The influence of glass composition on the crystal growth kinetics of heavy metal fluoride glasses

Abstract The kinetics of crystallization of two heavy metal fluoride glasses, ZrF 4 ue5f8BaF 2 ue5f8LaF 3 ue5f8AlF 3 ue5f8LiF and ZrF 4 ue5f8BaF 2 ue5f8LaF 3 ue5f8AlF 3 ue5f8LiFue5f8PbF 2 , which are of potential use as core and clad materials for fiber optic waveguides, have been studied by differential scanning calorimetry. The devitrification processes follow the Johnson-Mehl-Avrami relation, and the values of the kinetic parameters for isothermal and non-isothermal techniques are in excellent agreement. The crystallization activation energies for these glasses are about two-thirds those previously reported for a ternary ZrF 4 ue5f8BaF 2 ue5f8LaF 3 glass. The implications of the experimental results for glass stability against devitrifications are discussed.

Solid state synthesis and properties of monoclinic celsian

Monoclinic celsian of Ba0.75Sr0.25Al2Si2O8 (BSAS-1) and Ba0.85Sr0.15Al2Si2O8 (BSAS-2) compositions have been synthesized from metal carbonates and oxides by solid state reaction. A mixture of BaCO3, SrCO3, Al2O3, and SiO2 powders was precalcined at ∼900–940 °C to decompose the carbonates followed by hot pressing at ∼1300 °C. The hot pressed BSAS-1 material was almost fully dense and contained the monoclinic celsian phase, with complete absence of the undesirable hexacelsian as indicated by X-ray diffraction. In contrast, a small fraction of hexacelsian was still present in hot pressed BSAS-2. However, on further heat treatment at 1200 °C for 24 h, the hexacelsian phase was completely eliminated. The average linear thermal expansion coefficients of BSAS-1 and BSAS-2 compositions, having the monoclinic celsian phase, were measured to be 5.28 × 10−6 °C−1 and 5.15 × 10−6 °C−1, respectively, from room temperature to 1200°C. The hot-pressed BSAS-1 celsian showed room temperature flexural strength of 131 MPa, elastic modulus of 96 GPa and was stable in air up to temperatures as high as ∼1500 °C.

Phase transformations in xerogels of mullite composition

Monophasic and diphasic xerogels have been prepared as precursors for mullite (3Al2O3-2SiO2). Monophasic xerogel was synthesized from tetraethyl orthosilicate and aluminium nitrate nanohydrate and the diphasic xerogel from colloidal suspension of silica and boehmite. The chemical and structural evolutions, as a function of thermal treatment in these two types of sol-gel-derived mullite precursor powders, have been characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and infrared spectroscopy (IRS). Monophasic xerogel transforms to an aluminium-silicon spinel from an amorphous structure at ∼980 ° C. The spinel then changes into mullite on further heating. Diphasic xerogel forms mullite at ∼1360 ° C. The components of the diphasic powder react independently up to the point of mullite formation. The transformation in the monophasic powder occurs rapidly and yields strongly crystalline mullite with no other phases present. The diphasic powder, however, transforms rather slowly and contains remnants of the starting materials (α-Al2O3, cristobalite) even after heating at high temperatures for long periods (1600 ° C, 6 h). The diphasic powder could be sintered to high density but not the monophasic powder, in spite of its molecular-level homogeneity.

Crystallization of fluorozirconate glasses

The crystallization of a number of glasses of the fluorozirconate family has been studied using powder X-ray diffraction and differential scanning calorimetry, as a function of time and temperature of heating. The main crystalline phases were ..beta..-BaZrF/sub 6/ and ..beta..-BaZr/sub 2/F/sub 10/. Stable and metastable transformations to the low-temperature ..cap alpha..phases were also investigated. The size of crystallites in fully devitrified glasses was calculated to be about 600 A from line broadening of the X-ray diffraction peaks.

Synthesis and thermal evolution of structure in alkoxide-derived niobium pentoxide gels

Niobium pentoxide gels in the form of transparent monoliths and powder have been synthesized from the controlled hydrolysis and polycondensation of niobium pentaethoxide under different experimental conditions using various mole ratios of Nb(OC2H5)5:H2O:C2H5OH:HCl. Alcohol acted as the mutual solvent and HCl as the deflocculating agent. In the absence of HCl, precipitation of colloidal particles was encountered on the addition of any water to the alkoxide. The gels were subjected to various thermal treatments and characterized by differential thermal analysis, thermogravimetric analysis, X-ray diffraction and infra-red spectroscopy. After drying at 400°C, the gels were amorphous to X-rays. The amorphous powder crystallized into the low temperature orthorhombic form of Nb2O5 at ~ 500°C, which transformed irreversibly into the high temperature monoclinic α-Nb2O5 between 900 to 1000°C. The kinetics of crystallization of the amorphous niobium pentoxide have been investigated by non-isothermal differential scanning calorimetry. The crystallization activation energy was determined to be 399 kJ mol−1.

Reaction of zirconium fluoride glass with water: kinetics of dissolution

When liquid water contacts a zirconium-barium-lanthanum fluoride glass, at least three different processes occur. Barium and zirconium fluoride dissolve into the water, water penetrates into the glass, and zirconium fluoride crystals grow on the glass surface, in static solution. The rate of dissolution, as measured by solution analysis, is possibly controlled by diffusion in the solid surface; surface blockage and surface reactions are other possible kinetic steps involved. Diffusion in solution is not the controlling mechanism. Hydrogen profiles in the glass surface suggest that the penetration rate of water into the glass is controlled by diffusion and a surface reaction.

Zirconium fluoride glass: surface crystals formed by reaction with water

The hydrated surfaces of a zirconium barium fluoride glass, which has potential for application in optical fibers and other optical elements, were observed by scanning electron microscopy. Crystalline zirconium fluoride was identified by analysis of X-ray diffraction patterns of the surface crystals and found to be the main constituent of the surface material. It was also found that hydrated zirconium fluorides form only in highly acidic fluoride solutions. It is possible that the zirconium fluoride crystals form directly on the glass surface as a result of its depletion of other ions. The solubility of zirconium fluoride is suggested to be probably much lower than that of barium fluoride (0.16 g/100 cu cm at 18 C). Dissolution was determined to be the predominant process in the initial stages of the reaction of the glass with water. Penetration of water into the glass has little effect.

Low temperature synthesis of CaO-SiO2 glasses having stable liquid-liquid immiscibility by the sol-gel process

Calcium silicate glass compositions lying within the liquid-liquid immiscibility dome of the phase diagram, which could not have been prepared by the conventional melting method, have been synthesized by the sol-gel process. Hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) solutions containing up to 20 mol% calcium nitrate, resulted in the formation of clear and transparent gels. The gel formation time decreased with increase in water:TEOS mole ratio, calcium content and the reaction temperature. Smaller values of gel times in the presence of calcium nitrate are probably caused by lowering of the ionic charge on the sol particles by the salt present. The gelation activation energy,Egel was evaluated from the temperature dependence of the gel time. Neither the presence of Ca2+ ions nor the water:TEOS mole ratio had any appreciable effect on the value ofEgel The presence of glycerol in the solution helped in the formation of crack-free monolithic gel specimens. Chemical and structural changes occurring in the gels, as a function of the heat treatments, have been monitored using differential thermal analysis, thermogravimetric analysis, infrared spectroscopy, X-ray diffraction, surface-area and pore-size distribution measurements.

X-ray diffraction studies of phase transformations in heavy-metal fluoride glasses

The crystallization of five ZrF4-based glasses has been investigated using powder X-ray diffraction and differential scanning calorimetry. The crystalline phase in Zr-Ba-La-Pb fluoride glass was found to beβ-BaZrF6. In other glasses the crystal phases could not be identified. Reversible polymorphic phase transformations occur in Zr-Ba-La-Li and Zr-Ba-La-Na fluoride glasses when heated to higher temperatures.

Crystallization of heavy metal fluoride glasses

The kinetics of crystallization of a number of fluorozirconate glasses were studied using isothermal and dynamic differential scanning calorimetry, and X-ray diffraction. The addition of the fluorides LiF, NaF, AiF3, LaF3 to a base glass composition of ZrF4-BaF2 reduced the tendency to crystallize, probably by modifying the viscosity-temperature relation. ZrF4-BaF2-LaF3-A2,F3-NaF glass was the most stable against devitrification and perhaps is the best composition for optical fibers with low scattering loss. Some glasses first crystallize out into metastable --BaZr2F10 and ,.---BaZrF6 phases, which transform into the most stable a-phases when heated to higher temperatures. The size of the crystallites was estimated to be (≈ 600 Å from X-ray diffraction.