Nanoscale surface heterogeneities and glass durability

The surface of a high quality soda-lime silicate glass is found to be inhomogeneous, consisting of nanometer-high ridges that extend for tens of microns. Exposure to mild and aggressive environments results in an increase in the ridge heights as they dissolve much more slowly than the rest of the glass. The ridges also exhibit enrichment of sodium, magnesium, aluminum and oxygen. The parallel nature of the ridges suggests that they originate from compositional heterogeneity during the glass processing.     

Leaching behavior of nuclear waste glass heterogeneities

The results of a six-month burial experiment in granite are discussed. An alkali borosilicate simulated nuclear waste glass was buried in 3 m boreholes at the 345 m level in the Stripa mine. Some glass specimens containing crystallites exhibit preferential attack of the interface between crystalline and glassy phases. The crystalline phase, identified as spinel solid solution, exhibits better chemical resistance than the glassy phase. Results obtained from homogeneous and heterogeneous glasses (i.e., those containing a crystalline phase) are compared.     

Dynamical heterogeneities in an attraction driven colloidal glass

The dynamical heterogeneities (DH) in non-ergodic states of an attractive colloidal glass are studied, as a function of the waiting time. Whereas the fluid states close to vitrification showed strong DH, the distribution of squared displacements of the glassy states studied here only present a tail of particles with increased mobility for the lower attraction strength at short waiting times. These particles are in the surface of the percolating cluster that comprises all of the particles, reminiscent of the fastest particles in the fluid. The quench deeper into the attractive glass is dynamically more homogeneous, in agreement with repulsive glasses (i.e. Lennard-Jones glass).     

Transient nucleation effects in glass formation

We extend to the non-isothermal case a numerical technique that was developed to treat transient homogeneous nucleation in a one-component system by modeling directly the reaction by which clusters are produced. Calculations are presented for the nucleation frequency during the quench and for the number of nuclei produced and the volume fraction transformed at the end of quench for different rates of cooling from the melt. Three model systems are considered: an alkali silicate which is a relatively good glass former, and two metallic glasses. These show a wide range of critical cooling rates for glass formation. In some systems transient effects are predicted to be critical for glass formation. A simple technique is presented for determining when transient effects are important based on a calculation using steady state nucleation frequencies and macroscopic growth velocities.     

Crystallization statistics,thermal history and glass formation

The formal theory of transformation kinetics describes the volume fraction of a phase transformed in a given time at a given temperature. The basic concepts are extended for isotropic crystal growth in a material having a known thermal history T(r, t). A crystal distribution function ψ(r, t, R) is defined such that the number of crystallites in a volume dυ at r having radii between R and R + dR at time t is ψ(r, t, R) dυ dR. The function ψ contains essentially complete statistical information about the state of crystallinity of a material. Formal expressions for ψ are obtained. Applications are discussed, including predictions of crystallinity when T(r, t) is known; predictions of glass-forming tendencies; experimental determination of nucleation rates; and the determination of the thermal history of a sample from post mortem crystallinity measurements. As an example, ψ(r, t, R) is calculated for a lunar glass composition subjected to a typical laboratory heat treatment.     

Multilayer colloid formation in soda lime silica glass

Silver and copper colloids are formed on the tin-rich surface of float glass at elevated temperatures via reaction with the tin present from the float process. Optical absorption measurements of samples initially stained with copper and then subsequently stained with silver indicate that the second treatment with silver eliminates the optical absorption due to the copper colloids formed during the first treatment. If silver is applied before copper, both silver and copper colloid absorption bands increase in intensity for initial heat treatments at lower temperatures and shorter times, but no copper colloids are formed if the initial heat treatment produces a very large amount of silver colloids. It is proposed that copper colloids formed during the initial heat treatment serve as nuclei for the subsequent growth of a layer of silver, eventually covering the nuclei of copper with a layer of silver which is thick enough to mask the optical absorption of the original colloids. The reverse does not occur if silver colloids are formed before copper colloids.     

Role of concentration fluctuations in metallic glass formation

The glass forming composition range in several binary metallic systems is correlated with the composition dependence of S_cc(0), the concentration-concentration fluctuation structure factor at its long wavelength limit. The magnitude of S_cc(0) has been evaluated for the liquid phase from available thermodynamic data. It has been observed that S_cc(0) exhibits a maximum and tends to the ideal value in the glass forming composition range. Significant and systematic negative deviations from ideal values or the tending to zero of S_cc(0) are observed at the stoichiometry corresponding to complexes in the liquid state in the case of compound forming or associated systems. These observations are discussed in terms of the chemical short-range order in the liquid state. It is concluded that while a reasonable degree of order may exist for the compound forming compositions, in the glass forming region itself the liquids are nearly random mixtures of complexes and unassociated component atoms.     

Outer space formation of a laser host glass

Classical crystallization calculations were performed to determine the possibility of forming a particular type of laser glass with the avoidance of devitrification in an outer space laboratory. Although the laser glass in question readily crystallizes in an earth environment, it is demonstrated that under the homogeneous nucleating conditions obtainable in a zero gravity laboratory this laser glass may be easily quenched to a virtually crystal-free product. Use of this material as a host in a neodymium glass laser would result in a more than 10% increase in efficiency when compared to laser glass rods of similar composition currently commercially available.     

Gallium colloid formation during ion implantation of glass

Data are reported on the size and depth distribution of gallium colloids formed by gallium ion implantation at energies of 50 and 60 keV, and nominal doses up to 1.1 × 10¹⁷ ions/cm² into coverlip glass, float glass and white crown glass. Measurement techniques used to reveal colloid-induced changes include the wavelength dependence of optical reflectivity, transmission electron microscopy (TEM) and Rutherford backscattering (RBS). The reflectivity can be controlled by variations in ion dose, implant temperature and ion beam energy. The highest reflectivity is found after implants near 50°C and the level is extremely sensitive to the implant temperature. For controlled beam conditions, the reflectivity data are reproducible, despite there being variations in the colloid size and depth distributions as seen by TEM and RBS. The TEM data reveal that the depth distribution develops in two distinct regions, which at high concentration can precipitate into two layers of large colloids. Subsidiary experiments are reported to attempt to separate the effects of variations in the implant temperature and surface charging which influence the reflectivity, RBS and colloid formation.     

Chemical aspects of glass formation in telluride systems

The compositional dependence of the electrical activation energy, the optical absorption and the glass transition temperature in typical binary, pseudo-binary and ternary glass forming telluride systems has been measured in an effort to correlate chemical binding with physical properties. A modification of the valency satisfaction model to include chemical ordering effects is required to account for the property singularities observed in the GeTe system at the composition GeTe₂. The ordering effects in Te-based alloys containing Ge are further verified by an endothermic disordering reaction in these liquids peaking between 400–475°C depending on composition.     

Contribution to the kinetics of glass formation from solutions

The different reaction steps during the formation of solid materials from metal alkoxides were investigated by IR and DSC methods. The kinetics of hydrolysis of the different components in the system Na₂O/B₂O₃/SiO₂ show, that the hydrolysis of Si(OR)₄ in alcoholic solution is the slowest reaction and may determine the way of cross-linking. The use of alcohol as a solvent leads to incomplete reactions and this, as a consequence, may disturb the process of glass formation. Concerning the glass transition, no difference in transition temperature between gel glasses and conventionally melted glasses could be observed.     

Colloidal silver formation at the surface of float glass

The formation of colloidal silver during the heating in air of silver films on float glass has been observed. The presence of colloidal silver gives to a yellow color and an absorption band centered at 400–420 nm. The results of this study indicate that the formation of colloidal silver is strongly dependent upon the concentration of stannous tin in the glass. The optical density of the absorption band induced in the glass is much greater for samples silvered on the tin-rich face of the glass and varies with the thermal history of the sample. Removal of the outer few micrometers of the glass surface results in a radical change in the amount of colloidal silver formed. Silver colloids are formed only if the sample is heated in an atmosphere containing oxygen, suggesting that the silver must be ionized before it will diffuse into the glass. The colloid formation process has an activation energy of approximately 30 kcal/mol, which is very near that reported for silver-sodium interdiffusion in similar glasses.     

Continuous cooling approximation for the formation of a glass

Non-isothermal equations describing the liquid-crystal transformation are derived using the isothermal Avrami equations. A theoretical expression for the critical cooling rate for the formation of a glass is found. Calculations based on this expression are in better agreement with experimental values than those derived from TTT (time-temperature-transformation) curves. A study performed on typical glass forming materials enables the glass forming ability (GFA) to be determined by experimentally measuring crystallization temperatures at different cooling rates which are easily accessible with commonly available technology. The behaviour of the rate constant for crystallization is also obtained from the same data in the experimental range considered. In both cases no previous knowledge of the parameters involved is needed.With some assumptions the values of the viscosity in the crystallization temperature range can be estimated.Although the study was performed for an Avrami index of 4 an extension to other values of n is made under some restricted conditions and a more general treatment is outlined.     

Hydrogen formation observed during high pressure treatment of silica glass

Silica glass treated at high pressures and temperatures (1-3 GPa, 1100-1250 °C) in a solid-media pressure apparatus incorporated observable quantities of hydroxyl and hydride. The FTIR absorbance signals for these species appeared only when the glass sample was pressurized in the presence of water together with graphite at a high temperature, and the hydroxyl signal disappeared completely when the glass was heat-treated at 1000 °C in air for 1 h. These results indicate that hydrogen diffused into and reacted with the silica glass during the high pressure-temperature treatment. Reduction of H₂O by graphite at high pressure/temperature conditions is the probable source of hydrogen.     

Bulk metallic glass formation: The positive effect of hydrogen

Zr₅₅Cu₃₀Ni₅Al₁₀ alloys with different amounts of hydrogen have been prepared by arc melting under the gaseous mixture of hydrogen and argon. Proper additions of hydrogen have been proved to effectively increase the glass-forming ability (GFA) of this alloy. Positive effect of hydrogen on GFA has been interpreted from the thermodynamic and structural points of view. Proper additions of hydrogen can decrease the liquidus temperature, which leads to more stable glass-forming liquid. Structure analysis by positron annihilation lifetime spectroscopy shows that proper additions of hydrogen can increase the concentration of shortest open volume and decrease the concentration of intermediate and largest open volumes. This leads to formation of a denser random packed structure, and thus increases the GFA of Zr₅₅Cu₃₀Ni₅Al₁₀ alloys.     

Comparison of glass formation by mechanical alloying and solid-state interdiffusion in Ni---Zr composites

The amorphization process in mechanically alloyed Ni---Zr powders has been investigated by optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, differential scanning calorimetry and saturation magnetization measurements. Starting from elemental crystalline Ni and Zr powders, ball milling first produces a characteristically layered microstructure. Further milling leads to an ultrafine composite in which amorphization by solid-state reaction sets in between 4 and 16 h milling time. Longer milling results in fully homogeneous amorphous material. The obtained results corroborate the similarity of the amorphization process during mechanical alloying with the solid-state interdiffusion reaction in artificially modulated multilayer composites. In particular, mechanical alloying prevents intermetallic phase formation during the interdiffusion reaction because of extremely thin starting layers, thus resulting in a wider glass-forming range than obtained by other preparation techniques based on solid-state interdiffusion.     

A kinetic treatment of glass formation V: Surface and bulk heterogeneous nucleation

An analytical method is described for calculating the detailed distribution of crystallite sizes in a supercooled liquid, and the changes in this distribution as a function of temperature (time) while the liquid is cooled from above the melting point.This method, termed the analysis of crystallization statistics, is applied to the calculation of continuous cooling curves for anorthite and o-terphenyl as representative of inorganic and organic systems. In addition to homogeneous nucleation, bulk as well as surface heterogeneous nucleation are considered. The effects of distributions of heterogeneities with contact angles between 40 and 100° as well as overall concentrations of heterogeneities between 10³ and 10⁹ cm^?3 are considered. Heterogeneities with contact angles higher than about 100° are shown not to have an effect on the critical cooling rate for typical concentrations of heterogeneities.For liquids containing distributions of heterogeneities, the nucleation behavior is dominated by small concentrations of heterogeneities having small contact angles. Theoretical log (I_vη) versus (T_r³ΔT_r²)^?1 curves have been constructed for homogeneous nucleation + heterogeneous nucleation with a single type of heterogeneity and for homogeneous nucleation + heterogeneous nucleation with the heterogeneities distributed with regard to contact angle. In the former case, the curve is composed of two linear portions; and in the latter case, the curve shows pronounced curvature. The curvature reflects a continous change in the frequency of heterogeneous nucleation.Surface heterogeneous nucleation was assumed to originate at discrete surface heterogeneities and was shown to give rise to continuous cooling curves similar to those calculated for bulk heterogeneous nucleation.     

Metallic glass formation in the NiBGe system

Metallic glasses may be formed at transition metal/metalloid ratios substantially removed from the usual 80/20 atom fraction for binary systems such as NiB and CoB. The present investigation is of the NiBGe ternary systems in which the Ge is used as a substitutional probe atom for B in a subsequent EXAFS study. Ribbon samples have been fabricated for a wide range of compositions with metalloid content up to 45 at.%, the glass-forming region of which has been determined by bend ductility, XRD, DSC, SEM, and TEM measurements. Samples found to have relatively low thermal stability also tend to crystallize during ion-milling sample preparation for TEM, or during electron irradiation during TEM observation. Some of the ternary ribbon samples are ductile but are partially crystalline on XRD and TEM investigation. Other samples are brittle but show no crystallites, even after careful TEM study. Most ribbon samples made were fully glassy, as determined by XRD and TEM. The dual glass-forming composition range found in the NiB binary system is alternatively interpreted as a single continuous glass-forming regime, interrupted by an Ni₃B intermetallic region which is of such character as to make metallic glass formation very difficult by melt-spinning.