UV assisted local crystallization in Er3+ doped oxy-fluoride glass

We report on fabrication of Er³⁺-activated LaF₃ nanocrystals in transparent glasses using an original technique, which combines both heat treatment, below glass crystallization temperature, and ultraviolet laser irradiation at 244 nm. The main advantage of this method is to control the spatial localization of the nanoparticles in the glass sample, whereas annealing solely at the crystallization temperature leads to a fully crystallized glass sample. Thermal differential analysis was used to determine the crystallization temperature of the sample. The photoluminescence spectra behaviour of Er³⁺ ions, collected from the UV-irradiated and unirradiated regions, allowed us to follow and to distinguish the structural changes in the glass network under heat treatment and ultraviolet exposure.     

A kinetic treatment of glass formation VII: Transient nucleation in non-isothermal crystallization during cooling

The analysis of crystallization statistics has been modified to allow for time-dependent (transient) nucleation. To establish its accuracy, the numerical analysis has been applied to isothermal crystallization kinetics and shown to yield crystallization versus time curves which compare very closely with curves calculated analytically with or without the inclusion of transient nucleation.The numerical analysis including transient has been used to calculate the critical cooling rates for glass formation in anorthite and o-terphenyl considering (1) only homogeneous nucleation and (2) homogeneous nucleation + heterogeneous nucleation for 10⁷ heterogeneities cm^?3 with contact angles between 40° and 100°. It has been shown that inclusion of time-dependent nucleation in the calculations does not change the critical cooling rates for glass formation calculated assuming steady-state homogeneous nucleation in both materials. The critical cooling rate in anorthite calculated including steady-state heterogeneous nucleation was found to be decreased only slightly by the inclusion of time-dependent nucleation; while the critical cooling rates calculated for o-terphenyl were not change at all by the inclusion of time-dependent nncleation.The lack of an effect of time-dependent nucleation on the critical cooling rates calculated assuming only homogeneous nucleation is explained by the relatively small transient times on the high temperature side of the nucleation peak (a temperature range which has an overwhelming effect on the overall crystallization process because of the relatively high crystal growth rates in this range).Although the critical cooling rates associated with heterogeneous nucleation are large, the nucleation here takes place at relatively small undercooling where the transient times are relatively small. Thus, transient nucleation causes only a temporary delay in the over all crystallization, and its effect on the critical cooling rate is small.     

Re-examination of effects of nucleation temperature and time on glass crystallization

The effects of nucleation temperature and time on the kinetics of non-isothermal glass crystallization have been re-examined to demonstrate the limitations of some approximate solutions used to extract kinetic parameters from differential thermal analysis (DTA) experiments. Those features were analyzed by numerical solutions of equations describing the dependence of fraction crystallized on the rates of nucleation and growth, and the corresponding transient time, reported for lithium disilicate. It was shown that the temperature of maximum nucleation rate varies on changing the nucleation time. Some guidelines were established to assist the selection of suitable conditions to perform crystallization studies by DTA, and to extract the values of activation energy and dimensionality of growth from the dependences of crystallization peak temperature on heating rate, and nucleation time. The main limitations of these methods were identified and discussed.     

Viscosity of metglas 2826 near the glass transition using rapid heating

An attempt was made to measure the temperature dependence of viscosity of Metglas 2826 near the glass temperature using rapid heating. This temperature region is normally inaccessible to viscous flow measurements due to crystallization. The melt viscosity, heat of fusion at the melting temperature, and the melting temperature itself were also measured. These parameters are used to calculate the critical cooling rate needed to avoid crystallization via standard nucleation/crystal growth/transformation theory.     

On the possibility of devitrification of ancient glass

The question whether some crystallization will take place in old glass during long-time storage (up to 1000 years) at low temperatures well below T_g is considered theoretically. Evaluations for the systems Na₂O · SiO₂ and Li₂O · 2SiO₂ are carried out using the model of homogeneous and heterogeneous nucleation theory. For these systems a barely observable crystallized volume fraction will be reached at 300 K only after storage for astronomical periods (10⁶ ? 10¹³ years), depending on the number of heterogeneities and the contact angle of heterogeneity. If the observed strong influence of H₂O content in glass on crystallization is also taken into account, crystallization will not take place for at least 1000 years of storage at 300 K. This limiting value is close to archaeological periods and will therefore merit careful attention in further investigations.     

An investigation of the kinetic transformation mechanism of Ge12.5Te87.5 chalcogenide glass under non-isothermal regime

Differential scanning calorimetry (DSC) technique was used to study the kinetics of amorphous to crystalline transformation in Ge_12.5Te_87.5 chalcogenide glass. The kinetic parameters of glassy Ge_12.5Te_87.5 under non-isothermal conditions are analyzed by the model-free and model-fitting approaches from a series of experiments at different constant heating rates (5-50 K/min). The effective activation energy of crystallization was determined by analyzing the data using the isoconversional methods of Kissinger-Akahira-Sunose (KAS), Tang, Starink, Flynn-Wall-Ozawa (FWO) and Vyazovkin. The analysis of the present data shows that the effective activation energy of crystallization is constant throughout the entire interval of conversions and hence with temperature. The transformation mechanism examined using the local Avrami exponents indicates that one mechanism (three-dimensional growth) is responsible for the transformation process for all heating rates used. The reaction model that may describe the transformation process of the Ge_12.5Te_87.5 chalcogenide glass is the Avrami-Erofeev model (g(α) = [− ln(1 − α)]^1/n) with n = 3 for all heating range at the whole range of crystallized fraction (α = 0.05-0.95). A good agreement between the experimental and the reconstructed (α-T) curves has been achieved. The transformation from amorphous to crystalline phase in Ge_12.5Te_87.5 chalcogenide glass demonstrates a single-step mechanism.     

Kinetics of relaxation and crystallization of sodium metaphosphate glass

Kinetics of structural relaxation and crystallization of NaPO₃ glassforming melt is studied by means of thermal analyses. It is demonstrated that activation energy depends strongly on fictive temperature T_f. The dependence of the onset temperature T_o of the glass transition interval on the heating rate q⁺ is investigated for samples that were previously cooled down at a rate q⁻ of about 850 K/min. The dimensionless fragility F is a measure of the dependence of the activation energy for spatial rearrangement on the changes of structure. According to the present results F is large for NaPO₃ (i.e. phosphates are ‘fragile’ substances).     

Glass formation and non-isothermal crystallization of Zr62.5Al12.1Cu7.95Ni17.45 bulk metallic glass

Glass forming ability (GFA) and non-isothermal crystallization kinetics of Zr_62.5Al_12.1Cu_7.95Ni_17.45 bulk metallic glass were investigated. Its critical dimension is up to 7.5 mm and its critical cooling rate is less than 40 K s⁻¹, indicating its better GFA. It manifests two crystallization procedures and the second crystallization peak is more sensitive to heating rate than the first crystallization peak. The glass transition and crystallization both have remarkable kinetics effects. The apparent activation energies derived from the Kissinger plots are 175.24 ± 27.59 KJ mol⁻¹ for glass transition E_g, 212.84 ± 15.84 KJ mol⁻¹ for onset crystallization E_x, 230.51 ± 23.85 KJ mol⁻¹ for the first crystallization peak E_p1 and 124.85 ± 15.15 KJ mol⁻¹ for the second crystallization peak E_p2.     

Nucleating heterogeneities and glass formation

The kinetic treatment of glass formation is extended by the introduction of continuous cooling (CT) curves to estimate the cooling rates required to form glasses of various materials. The CT curves may be constructed from isothermal time-temperature-transformation curves following the approach originally suggested by Grange and Kiefer. The modified analysis is used to evaluate the effects of nucleating heterogeneities on glass formation. It is found that for the concentrations of such heterogeneities found in most liquids, those characterized by contact angles greater than about 100° have a negligible effect on the cooling rate required to form glasses. Heterogeneities with smaller contact angles, can, however, have a significant effect on glass formation, with the critical cooling rate increasing with decreasing contact angle. The effects on glass formation of changes in the contact angle of nucleating heterogeneities are also compared with the effects of changes in the thermodynamic barrier to nucleation (in the crystal-liquid surface energy).     

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.     

Non-isothermal crystallization kinetic studies on MgO–Al2O3–SiO2–TiO2 glass

Thermal stability and crystallization kinetics of the glass 21% MgO, 21.36% Al₂O₃, 53.32% SiO₂ and 4.11% TiO₂ (mol%) has been studied using differential thermal analysis (DTA), dilatometry and X-ray diffraction (XRD). Glass in both bulk and frit forms were produced by melting in platinum crucible at 1600 °C for 1–2 h. From variation of DTA peak maximum temperature with heating rate, the activation energies of crystallization were calculated to be 340 kJ mol⁻¹ and 498 kJ mol⁻¹ for first and second crystallization exotherms, respectively. Crystallization of bulk glass was carried out at various temperatures and for different time durations in the range of 850–1000 °C. The influence of the addition of TiO₂ on the crystallization sequence of the glass was experimentally determined and discussed.     

Structural characterization of light-induced crystal growth in Se98Sb2 chalcogenide glass

The present paper reports the detailed study of crystallization morphology of light-induced crystal growth in Se₉₈Sb₂ chalcogenide glass using DSC, XRD and SEM techniques. Thermally-activated crystallization of the samples in powder form is analyzed by Differential Scanning Calorimetry (DSC) at different heating rates under non-isothermal conditions. The activation energy of crystallization has been calculated by analyzing the data using the classical Johnson–Mehl–Avrami (JMA) model.Amorphous thin films of Se₉₈Sb₂ are used for light-induced crystal growth. The d.c. conductivity of the films is taken as a characteristic quantity to measure the extent of light-induced crystal growth. X-ray diffraction (XRD) analysis has been carried out on Se₉₈Sb₂ samples for different illumination time and their diffractograms are analyzed to obtain information about various crystallographic aspects. Scanning electron microscopy has been used to confirm light-induced crystal growth.     

The ionic transition and the glass transition in ion-conducting glasses

In an ion-conducting glass, there may be the ionic transition which characterizes ‘melting’ of the modifying ions in a similar way as the glass transition characterizes ‘melting’ of the glass network former. The ionic transition can be observed, electrically, by the thermally stimulated depolarization current technique. It is demonstrated that the relaxation time for the ionic transition is the same (near 100 s) with that for the glass transition in so far as the heating/cooling rate lies within, say, 10⁻¹-10⁻⁵ K/s as adopted in our routine works.     

Structure and non-linear optical properties of BaO–TiO2–SiO2 glass containing Ba2TiSi2O8 crystal

The BaO–TiO₂–SiO₂ crystallized glass containing Ba₂TiSi₂O₈ crystal phase was examined using X-ray diffraction (XRD) measurement and Maker fringe technique. It has been found that the crystallized glasses with a composition of close to the stoichiometric Ba₂TiSi₂O₈ showed bulk crystallization, whereas other crystallized glasses with non-stoichiometric composition of Ba₂TiSi₂O₈ showed surface crystallization. The parameter ΔT, the difference in temperature between the crystallization onset and the glass transition temperature, strongly affects the crystallization behaviors of the present BTS glasses. The 30BaO–20TiO₂–50SiO₂ transparent surface crystallized glass with strong c-oriented Ba₂TiSi₂O₈ phase showed its second-order non-linear optical constant, d₃₃ = 6.1 pm/V.     

Kinetics of non-isothermal crystallization and glass transition phenomena in Ga10Se87Pb3 and Ga10Se84Pb6 chalcogenide glasses by DSC

The crystallization process affects solid properties through the crystal structure and morphology established during the transition process. An important aspect of the crystallization process is its kinetics, both from the fundamental point of view of amorphous material as well as the modeling and phase transition. In the present research work, non-isothermal crystallization data in the form of heat flow vs. temperature curves has been studied by using some well known models for amorphous Ga₁₀Se₈₇Pb₃ and Ga₁₀Se₈₄Pb₆ chalcogenide glasses, prepared by the melt quenching technique. The glass transition phenomena and crystallization of these glasses have been studied by using non-isothermal differential scanning calorimetery (DSC) measurements at constant heating rates of 5, 10, 15, 20, 25 and 30 K/min. The glass transition temperature (T_g), crystallization temperature (T_c), and melting temperature (T_m) were determined from DSC thermograms. The dependence of T_g and T_c on the heating rate was used to determine different crystallization parameters such as the order parameter (n), the glass transition energy (ΔE_g) and the crystallization activation energy (ΔE_c). The results of crystallization were discussed on the basis of different models such as Kissinger's approach and the modification for non-isothermal crystallization in addition to Johnson, Mehl, Ozawa and Avrami.     

Glass formation and crystallization in highly undercooled Te-Cu alloys

The large undercoolings required for glass formation have been achieved by the slow cooling (10-20°C/min) of liquid Te-Cu alloys in the form of a fine droplet emulsion. Within the region of glass formation, between 19 and 39 at.% Cu, DTA measurements indicate that the glass (T_g) and crystallization (T_c) temperatures during heating exhibit a broad maximum at the eutectic. During slow cooling of Te-rich alloy droplets, the maximum undercooling for nucleation increases from 213°C for pure Te to 264°C for Te-12.5 at.% Cu. An enhanced depression of the nucleation (T_n) temperature compared with the change of the liquidus develops in Te-rich alloys upon approaching the glass forming composition range and can be a useful feature in assessing the glass forming tendency. Thermal cycling experiments indicate that even at an undercooling of 181°C crystallization in an eutectic Te-29 at.% Cu alloy is limited by an inadequate nucleation rate in clean droplet samples. For a eutectic alloy, at undercoolings in excess of 200°C crystal nucleation does develop in the droplet samples, but complete crystallization is hindered by a rapidly rising liquid viscosity with increased undercooling.     

Phosphate ion removal from a solution by soda-lime borosilicate glass

The excessive phosphate ion content in industrial wastewater causes water eutrophication. The objective of this study is to develop a new method of phosphate ion removal from wastewater using soda-lime borosilicate glass. Soda-lime borosilicate glass with high content of modifiers is reacted in a solution containing phosphate ions. Ca²⁺ ions that are leached out of the glass combine with P⁵⁺ ions in a solution to form hydroxyapatite crystals on the glass surface. The phosphate ions can thereby be removed from the solution. The phosphate ion removal rate was highest when glass containing 20–30 mol% of CaO was reacted in a solution with pH of 3.0. The phosphate ion removal rate was faster when the glass was reacted with the solution under a dynamic condition than under a static condition. The elimination rate also strongly depended on the surface area of the glass.     

Crystallization kinetics in Cu46Zr45Al7Y2 bulk metallic glass by differential scanning calorimetry (DSC)

Crystallization transformation kinetics in isothermal and non-isothermal (continuous heating) modes were investigated in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass by differential scanning calorimetry (DSC). In isochronal heating process, activation energy for crystallization at different crystallized volume fraction is analyzed by Kissinger method. Average value for crystallization in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass is 361 kJ/mol in isochronal process. Isothermal transformation kinetics was described by the Johnson-Mehl-Avrami (JMA) model. Avrami exponent n ranges from 2.4 to 2.8. The average value, around 2.5, indicates that crystallization mechanism is mainly three-dimensional diffusion-controlled. Activation energy is 484 kJ/mol in isothermal transformation for Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass. These different results were discussed using kinetic models. In addition, average activation energy of Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass calculated using Arrhenius equation is larger than the value calculated by the Kissinger method in non-isothermal conditions. The reason lies in the nucleation determinant in the non-isothermal mode, since crystallization begins at low temperature. Moreover, both nucleation and growth are involved with the same significance during isothermal crystallization. Therefore, the energy barrier in isothermal annealing mode is higher than that of isochronal conditions.     

Simulated effects of transient nucleation on the crystallization of glass samples

The kinetics of crystallization have been re-examined by taking into account the transient nucleation time and numerical methods were used to obtain the dependence of fraction crystallized on temperature, heating rate and nucleation conditions (temperature and time). These solutions were used to analyze the applicability of approximate models which were based on the assumption of steady state nucleation. The transient nucleation time, which precedes nucleation in as quenched samples, was taken into account to re-examine the deviations expected for relatively short nucleation time, and/or for cases when the actual nucleation temperature is displaced from the corresponding nucleation peak. Additional solutions were computed to obtain the dependence of crystallization peak temperature on the nucleation temperature. It was found that the crystallization peak temperature reaches its lowest value after nucleation at a temperature which can be significantly higher than the peak temperature of steady state nucleation.     

Structural order and crystallization of an iron-rich aluminosilicate liquid under oxidizing condition

An iron-rich aluminosilicate liquid is studied in different atmospheres by using differential scanning calorimetry. The results show that the oxidation state of iron and the maximum temperature of the scan have an effect on the liquid structure and on the crystallization behavior of the liquid during both cooling and reheating processes. Increase in Fe²⁺/Fe³⁺ enhances crystallization and favors formation of a stable crystal structure. The structural order conserved in the liquid is an intrinsic phenomenon, which is not related to the oxidation state of iron.