Investigations on glass-to-mold sticking in the hot forming process

The sticking behavior of various mold materials and coatings for hot glass melt forming processes, like, e.g. glass container manufacturing, was investigated using a new testing procedure. The mold material specimens under test were subjected to frequent contact with hot viscous glass gobs in a pressing process with presetting well defined non-isothermal pressing parameters to simulate industrial working conditions. Three different glass compositions were used in this investigation, soda-lime silicate glass, lead crystal glass, and borosilicate glass. The sticking characteristics of the tested mold materials and coatings were described by two quantities, a ‘lower’ and an ‘upper’ sticking temperature, which are specific for each mold material and type of glass in the non-isothermal pressing process. The ‘lower’ sticking temperatures of uncoated mold materials were found to depend monotonically on the thermal effusivity (heat penetration coefficient) of the bulk mold materials. All of the coating materials applied to various substrate mold materials were found to reduce the ‘lower’ sticking temperature as compared to the uncoated materials. Most of the coating materials were found to reduce also the ‘upper’ sticking temperature.     

Non-isothermal crystallization kinetics of ZnO-BaO-B2O3-SiO2 glass

Glass of composition 40SiO₂-30BaO-20ZnO-10B₂O₃ (mol%) was made by conventional melting and casting process. Crystallization kinetics of above glass has been investigated under non-isothermal conditions, using the formal theory of transformations for heterogeneous nucleation. The procedure is applied to the experimental data obtained by differential thermal analysis (DTA), using continuous-heating techniques. The crystallization results are analyzed, and both the activation energy of crystallization process as well as the crystallization mechanism is characterized. Dilatometric measurement of this glass was also done and data obtained was used to calculate the viscosity of the formed glass. Development of crystalline phases on thermal treatments of the glass at various temperatures has been analyzed by X-ray diffraction. Microstructure of the crystalline phases was investigated by scanning electron microscopy (SEM) and the development of various structural features with variation in heat treatment cycle was observed. The nucleation and growth of these phases in the matrix of glass has been described and discussed.     

Deduction of activation energy for diffusion by analyzing soft impingement in non-isothermal solid-state precipitation

Physically realistic analytical treatment of soft impingement has been developed for solid-state precipitations in a non-isothermal process following the basic assumptions (i.e., a two-stage transformation including a site saturation of nucleation, an isotropic growth and a linear approximation for a concentration gradient in front of the precipitate/matrix interface). Based on the analytical treatment, Kissinger-like approach has been obtained to deduce the activation energy Q for diffusion during soft impingement in the non-isothermal process.     

The application of differential thermal analysis to the study of isothermal and non-isothermal crystallization kinetics of coal fly ash based glasses

The crystallization kinetics of glasses obtained from coal fly ash was investigated by both isothermal and non-isothermal methods using differential thermal analysis (DTA) data. In DTA experiments, glass samples having coarse (800-1000 μm) and fine (<180 μm) particle sizes were used and the results were compared. The Avrami exponent (n) was calculated by means of Johnson-Mehl-Avrami (JMA) and Ozawa equations. Calculated kinetic parameters indicated that the appropriate crystallization mechanisms were bulk and surface crystallization for coarse and fine particles, respectively. Isothermal and non-isothermal DTA experiments showed that the crystallization activation energies of coarse glasses are changed in the range of 444-578 kJ/mol, while the crystallization activation energies of fine glasses are changed in the range of 610-662 kJ/mol. It was found that crystallization activation energies of fine glasses are higher than those of the coarse glasses. Results showed that isothermal and non-isothermal crystallization kinetics of glasses produced from coal fly ash is in agreement within the experimental error.     

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.     

Kinetics of crystallization of titanium based binary and ternary amorphous alloys

Metallic glasses are kinetically metastable materials. These amorphous materials can be transformed into a crystalline state by both isothermal and isochronal methods. The study of this transformation, and hence the thermal stability of metallic glasses, are important from an application view-point. In the present work, the non-isothermal crystallization kinetics of two titanium-based amorphous alloys namely, Cu₅₀Ti₅₀ and Ti₅₀Ni₃₀Cu₂₀, are reported. The activation energies for crystallization, E_c for both the systems have been evaluated using different non-isothermal methods viz. derived through Kissinger, Augis and Bennet and Ozawa. The values of E_c obtained using these methods are consistent for both the metallic glasses and it is found that E_c for the ternary metallic glass is considerably higher than the binary metallic glass. The increase in the activation energy on the substitution of Ni in the Cu–Ti metallic glass suggests the increase in the thermal stability.     

Thermal stability and crystallization kinetics of new As–Ge–Se–Sb glasses

Thermal stability and crystallization kinetics of As₁₄Ge₁₄Se_72−xSb_x (where x = 3, 6, 9, 12 and 15 at.%) glasses are studied by the differential scanning calorimetry. The values of the glass transition temperature (T_g) and the peak temperature of crystallization (T_p) are found to be dependent on heating rate and antimony content. From the heating rate dependence of T_g and T_p the values of the activation energy for glass transition (E_t) and the activation energy for crystallization (E_c) are evaluated and their composition dependence discussed. Crystallization studies have been made under non-isothermal conditions with the samples heated at several uniform rates. Using a recent analysis developed for non-isothermal crystallization studies, information on some aspects of the crystallization process has been obtained. The stability calculations emphasized that the thermal stability decreases with increasing the Sb content.     

Calorimetric studies of Se80−xTe20Bix bulk samples

The crystallization kinetics of the bulk Se_80−xTe₂₀Bi_x chalcogenide glasses were studied by using differential scanning calorimetry with different heating rates (5,10,15 and 20 K/min) under non-isothermal conditions. The values of glass transition temperature, peak crystallization temperature and melting temperature are found to increase with increase in heating rate as well as with bismuth content. The activation energy for glass transition and that for crystallization have been determined using the Kissinger equation and Matusita equation. The thermal stability and glass-forming tendency have also been studied.     

Effect of substitution of rare earth by mischmetal on the devitrification process of Al-X-Ni-Co (X = Y, Ce, Mm) alloys

The effect of substitution of Ce or Y by mischmetal (Mm) on the glass transition and the kinetics of crystallization of quaternary Al-Ni-Co-(Ce, Y, Mm) alloys was studied using isothermal and non-isothermal calorimetry and X-ray experiments. The alloy containing Mm shows more similarity to Ce containing alloy in all the studied properties than to Y containing alloy. The glass transition is less evident for Ce and Mm containing alloys than for the alloy containing Y. For the two former alloys, the first and the second crystallization stages are strongly overlapped and yield a fully crystallized alloy. Whereas for Y alloy, the first and the second stages are well resolved and the primary crystallization yields nanosized α-Al particles embedded in a residual amorphous matrix.     

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.     

Transport properties of Se80−xTe20Bix (0 ⩽ x ⩽ 1) system

Thermal studies using differential scanning calorimetry under non-isothermal conditions with the samples heated at different rates (5, 10, 15 and 20 K/min) have been reported and discussed. The glass transition activation energy and crystallization activation energy have been determined. Thermal stability and glass-forming tendency have also been studied. The activation energy in dark and optical bandgap have been calculated. It has been found that optical bandgap is equal to twice the dark thermal activation energy.     

Surface oxidation of basalt glass/liquid

To evaluate surface oxidation of Fe²⁺-rich multi-component silicate glass, powder and pieces of natural basalt glass are heat-treated in Ar and subsequently investigated by Mössbauer spectroscopy to monitor the increase of Fe³⁺. Glass pieces show no increase in oxidation with time or temperature, suggesting that the oxygen potential between glass and Ar is insufficient to cause volume oxidation. In contrast, glass powder oxidizes readily to a degree comparable with that of powder oxidation in air, suggesting that surface oxidation does not depend on the oxygen potential. No cation diffusion to the glass surface is detected in Ar, though it is observed upon heat treatment in air; cation diffusion is therefore unlikely to be involved in oxidation. We suggest the following mechanism for surface oxidation: (1) adsorption of water on the glass surface as OH⁻, by exposure to air and (2), a concomitant reaction, i.e., oxidation with the glass, upon heating (chemisorption). Hereby, either oxygen of air or residual oxygen in Ar would react with the hydrogen of the -OH, liberating the oxygen for oxidation of iron. Heat treatment in vacuum of 10⁻⁸ mbar does not result in any oxidation, and we assume that the adsorbed OH is exhausted from the glass surface.     

Various transmission electron microscopic techniques to characterize phase separation - illustrated using a LaF3 containing aluminosilicate glass

Various transmission electron microscopy techniques which are useful for the characterization of phase-separation phenomena in glass materials are exemplarily demonstrated at a 40 SiO₂-30 Al₂O₃-18 Na₂O-12 LaF₃ glass. It is shown that direct imaging of phase-separated regions possesses manifold advantages over the traditional replica technique, particular if the feature size approaches the nano-scale. Although surface replica can be accomplished effortless and made a great leap in glass structure research possible, the method is indirect, i.e. requires a deep understanding of the chemical attack leading to the surface topography eventually imaged. Moreover, surface replicas reach their limits for nanosized features due to the inherent structure of the deposited replica film.     

Influence of structural relaxation on atomic mobility in a Zr41.2Ti13.8Cu125Ni10.0Be22.5 (Vit1) bulk metallic glass

The effect of an annealing at a temperature above or below the glass transition temperature in a Zr_41.2Ti_13.8Cu₁₂₅Ni_10.0Be_22.5 bulk metallic glass was investigated using dynamic mechanical analysis. Structural relaxation influences both the storage modulus (elastic component) and the loss modulus (viscoelastic component). Kinetics can be captured by a stretched exponential relaxation function. Experimental results are correctly described using a physical model based on the concept of defects for the mechanical response of amorphous materials and especially for the characteristic time relative to atomic mobility.     

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.     

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.     

Nanocrystalline Pr-doped ZnO insulator for metal–insulator–Si Schottky diodes

Pr-doped ZnO (ZnO:Pr) insulating thin films were prepared on glass and Si substrates by oxidation in air. The films were characterised by X-ray diffraction (XRD), energy dispersion X-ray fluorescence (EDXRF), and optical absorption spectroscopy. The molar ratio Pr/Zn of the samples was determined by the EDXRF method. The XRD study shows the formation of nanocrystalline (26–50 nm) nc-Pr-doped ZnO. The optical and electrical conduction were explained by a slight change of stoichiometric composition. The nc-ZnO:Pr/Si heterojunctions are being Schottky barrier diodes (SBDs) and exhibited high rectification behaviour. The parameters describe the current pass through those SBDs were determined according to the available models.     

Growth control of stoichiometry in LaMnO3 epitaxial thin films by pulsed laser deposition

We have studied structural, magnetic, and optical transport properties of LaMnO₃ (LMO) thin films grown on SrTiO₃. While the stoichiometric LMO is an insulating antiferromagnet, it tends to be a ferromagnetic insulator when grown as thin films. By exploring the majority of growth parameters, we have found that the bulk-like electronic and magnetic phases can be stabilized by growing thin films under reducing atmospheres and by using more energetic laser processes. These conditions are found to reduce the La deficiency in the film resulting in the greatly improved cation stoichiometry. Since oxides are prone to reduce the oxygen content and to alter the cation ratio under such growth conditions, it suggests that the cation and oxygen stoichiometries in complex oxide thin films can be improved by properly optimizing the growth parameters.     

Fabrication of high-refractive-index glass micron-sized solid immersion lenses by a surface-tension mold technique

A surface-tension mold technique that makes use of the wetting properties between glass melt and a substrate is used to fabricate micrometer-sized solid immersion lens from a high-refractive-index glass. Solid immersion lens can be used to overcome the optical diffraction limit. In this study, glass particles made from two different glasses (n = 1.718 and 1.786) were melted on glassy-carbon substrates having optical-grade surfaces and cooled to room temperature. The optimum conditions for producing a super-spherical shape (i.e., a truncated sphere) while avoiding the crystallization of glass were obtained. The produced glass particles exhibited high transparency and super-spherical shape with smooth surfaces. The variations in the lens thickness and sphericity are submicron or less. Optical lens systems specially designed for these super-spherical lenses are expected to be capable of achieving resolutions that are 2.0 times higher than those attainable with conventional optical systems.     

Fabrication of heavy metal fluoride glass, optical planar waveguides by hot-spin casting

Hot-spin casting is further investigated using a customised rig for making optical planar waveguides from inorganic-compound-glasses. The rig enables a controlled mass of core-glass, held above its liquidus, to be gravity-cast onto the top surface of a spinning cladding-glass substrate that has been pre-heated to around its glass transformation temperature. Spinning encourages the cast liquid to spread as a film over the top surface of the glass substrate. The mass of liquid cast is controlled by the timed opening of an orifice in the base of the core-glass melt-crucible. The resulting step index, slab optical waveguides are annealed, then cooled to room temperature; they comprise a higher refractive index, glass film core, on top of a lower refractive index glass substrate cladding. The glass film core is air-clad. at its upper surface. The process is applied to two heavy metal fluoride core/clad. glass pairs, namely ZBLANPb/ZBLAN and ZBLALiYPb/HBLANY (where ZBLANLiYPb is ZrF₄-BaF₂-LaF₃-AlF₃-LiF-YF₃-PbF₂ and H is HfF₄) to give waveguides of small and large numerical aperture (NA) (e.g. at 643.8 nm wavelength, NA is 0.18 and 0.33, respectively). The Hot-Spin-Cast waveguides exhibit a guiding region whose top surface tends to be parallel to the upper surface of the underlying substrate. However, flatness of the top surface of the guiding region is limited by the flatness of the top surface of the underlying substrate. Multimode slab waveguiding is demonstrated for both NA waveguide types for glass film cores of depths ?10 μm.