A new class of infrared transmitting glass-ceramics based on controlled nucleation and growth of alkali halide in a sulphide based glass matrix

Oxides-based glass-ceramics have been intensively studied and while they exhibit exceptional thermo-mechanical properties, their transparency in the infrared is limited to the 3 μm region. In this paper we describe a new type of glass-ceramics which are transparent up to 11 μm and based on the controlled nucleation and crystallization of cesium chloride sub-micron particles inside a Ge-Sb-S glass matrix. The evolution of the optical transmission versus annealing time and temperature has been investigated. Observations under scanning electronic microscopy as well as X-ray diffraction indicate that the crystalline phase is a primitive cubic cell with a parameter slightly inferior to that of pure CsCl and that the grain sizes are about 100 nm. A preliminary test on fracture propagation shows a much better resistance of glass-ceramics to cracks than the corresponding pure glass matrix.     

Observation of nucleation effect on crystallization in lithium aluminosilicate glass by viscosity measurement

The crystal nucleation effect in lithium aluminosilicate glasses was investigated by the viscosity measurement with aid of the fiber elongation method. The abrupt increase of viscosity due to the crystallization of glass was observed in viscosity-temperature curve but the minimum viscosity temperature (T_η) related with crystallization showed a strong dependence on the nucleation state such as nucleation temperature, nucleation time and heating rate. The results by viscosity agreed well with those of DTA. The nucleation effect on the microstructure of glass-ceramics was also discussed. Finally, the nucleation effect on the crystallization kinetics was approached quantitatively by calculating the crystal volume from viscosity value.     

New insights on the thermodynamic barrier for nucleation in glasses: The case of lithium disilicate

An analysis is performed of the temperature dependence of the thermodynamic barrier to nucleation, W*(T), calculated from a fit of lithium disilicate glass data to the classical theory of nucleation. It is shown that, in order to obtain a satisfactory agreement between experimental and theoretical determinations of W*(T), lower values must be assigned to both the thermodynamic driving force and the surface energy as compared with the corresponding macroscopic values. This finding is consistent with theoretical considerations taking into account the effect that, in general, both the bulk and surface properties of the critical nuclei differ considerably from the respective properties of the newly evolving macroscopic phases. In addition, an anomalous increase of W*(T) with decreasing temperature is found near the glass transition interval. This increase is interpreted as a result of the effect of elastic strain on the thermodynamic driving force. The values of elastic strain energy estimated from the low temperature behavior of W*(T) are congruent with those calculated using the elastic constants of glass and crystal.     

A test of classical nucleation theory: crystal nucleation of lithium disilicate glass

For several inorganic glasses predictions have been made of the homogeneous crystal nucleation rate using classical nucleation theory. However, in none of these cases were comparisons made with experiment, due primarily to the inability of being able to guarantee homogeneous conditions. Evidence has been provided that crystalline formation in Li₂O · 2 SiO₂ glass may initiate by a homogeneous mechanism. Thus, we have computed the nucleation rate curve of lithium disilicate crystals in this glass. It is found that not only do all forms of the classical theory predict nucleation rates many orders of magnitude smaller than those observed, but also that the temperature dependence of the theoretical rate is quite different from that observed.     

Pressure-induced structural changes in a borosilicate glass-forming liquid: boron coordination, non-bridging oxygens, and network ordering

Two of the most important structural controls on the properties of borosilicate glasses and glass melts are the variation between three- and four-coordination of network-forming boron cations, and the extent of mixing of Si and B. The effects of composition on these key parameters are relatively well studied. However, proposed mechanisms could be better constrained by testing with another, independent parameter that can also strongly affect the network. Here we present some of the first quantitative structural data on the effects of high pressure on the network structure of a sodium borosilicate glass. Using high-resolution ¹¹B and ¹⁷O NMR on a sample melted at 5 GPa, we demonstrate that the formation of tetrahedral boron from trigonal boron is indeed closely coupled to the conversion of non-bridging to bridging oxygens. The increased fraction of tetrahedral boron at high pressure also causes increased mixing of boron and silicate structural units, as oxygens bridging between two BO₄ groups are energetically relatively unfavorable.     

Potassium surface enrichment in mixed alkali glass irradiated with electrons

Sodium-potassium-silicate glass was irradiated with electrons of energy of 1600 eV. The changes in the surface composition were analyzed by means of the angular-resolved X-ray induced photoelectron spectroscopy (ARXPS). Low electron dose irradiation enriched the uppermost glass surface with alkali ions, considerably more with potassium than with sodium ions.     

Stress relaxation of a soda lime silicate glass below the glass transition temperature

Stress relaxation is an important effect in the ion-exchange procedure of glasses, as it controls the stress profile and the strength. Creep and stress relaxation tests have been performed to study the viscoelastic behavior of soda-lime silicate glass at typical ion-exchange temperatures. The experimental data of these tests can be fitted well by the Burger model and a comparison between the viscosity data from both tests was made. The strain and temperature dependences of the stress relaxation process were studied and the glass exhibited a non-linear viscoelastic behavior and an anomalous temperature dependence. In addition, it was found there is a relationship between the glass density and the stress relaxation behavior.     

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.     

A molecular dynamics study on vibration spectra of a-SiO2 surface

The charge equilibration (QEq) method was used in the molecular dynamics study of the vibration spectra on the a-SiO₂ surface. The vibrational density of states (VDOS) of a silica glass surface and partial contribution of Si and O atoms were compared with those obtained from the fixed-charge (FQ) model. The VDOS in each of the samples has two groups of vibrational bands, i.e., wide-lower-frequency band (WB) between 0-25 THz and narrow-higher-frequency band (NB) between 25 and 40 THz, which is similar to the bulk VDOS obtained experimentally. The QEq surface exhibits a peak and two shoulders while two peaks are found for the FQ surface. At the surface, an excess peak (at 1-2 THz) was observed by substracting the Debye VDOS from the calculated VDOS, which is regarded as the experimentally observed the boson peak (BP). The position of the BP is shorter than that in the case of the bulk. In the FQ surface, the VDOS consists of narrower bands, while in the QEq sample, wider bands exist due to charge variation.     

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.     

Crystallization kinetics and optical band gap studies of Se96In4 glass before and after slow neutron irradiation

Results of differential scanning calorimetry (DSC) under non-isothermal condition on Se₉₆In₄ semiconducting chalcogenide glass before and after slow neutron irradiation, for different exposure times, have been reported and discussed. Some of Sn atoms have been injected into the glass by nuclear transmutation processes and the binary glass is converted into a ternary. This is accompanied by an increase in the activation energy of crystallization, E_c, and in the glass transition temperature, T_g and a decrease in the glass transition activation energy, E_t, in the onset crystallization temperature, T_c and in the peak temperature of crystallization T_p. Optical band gap measurements have also been carried out, before and after irradiation, on identical thin pellets of Se₉₆In₄ glass. The energy band gap, E_g, is found to decrease upon irradiation. These effects have been attributed to a structural change upon doping and to irradiation induced defects.     

Crystallization of CoFeSiB metallic glass induced by long-time ball milling

Milling up to 800 h causes amorphous Co_70.3Fe_4.7Si₁₀B₁₅ alloy, prepared in the form of thin ribbon, to partially crystallize thus forming a powder material consisting of an amorphous phase and fcc-Co nanocrystals with an average grain size of about 10 nm. A gradual increase of the nanocrystalline fcc-Co fraction, produced by ball milling, was detected. Prolonged milling results in destabilization of the fcc-Co phase and oxidation of the powder material (presence of CoO phase after 1500 h of milling). The thermal stability studies of as-quenched and milled Co_70.3Fe_4.7Si₁₀B₁₅ alloy emphasized a two step crystallization behavior. During the first crystallization event, cobalt rich phases, i.e., fcc-Co and hcp-Co crystallize, whereas after the second crystallization event, Co₂B and Co₂Si are formed.     

Sensitivity of structural results to initial configurations and quench algorithms of lead silicate glass

The sensitivity of resulting structures to starting configurations and quench algorithms were characterized using molecular dynamics (MD) simulations. The classical potential model introduced by Damodaran, Rao, and Rao (DRR) Phys. Chem. Glasses 31 (1990) 212 for lead silicate glass was used. Glasses were prepared using five distinct initial configurations and four glass forming algorithms. In previous MD work of bulk lead silicate glasses the ability of this potential model to provide good structural results were established by comparing to experimental results. Here the sensitivity of the results to the simulation methodology and the persistence of clustering with attention to details of molecular structure are determined.     

Studies of crystal phase formations in high-strength lithium disilicate glass–ceramics

The objective of the study was to analyze the nucleation, primary phase formation and solid state reaction to form lithium disilicate glass–ceramics derived from the SiO₂–Li₂O–Al₂O₃–K₂O–ZrO₂–P₂O₅ system. The concentration of P₂O₅ was increased from zero up to 3.2 wt%. Thermal analysis, scanning electron microscopy and X-ray diffraction were used to characterize the microstructure formation, the nucleation process and the solid state reaction of the crystal phase precipitation in the glass–ceramics. Additives of P₂O₅ allowed the control of bulk crystallization. Nucleation was catalyzed by nano-scaled Li₃PO₄ phases, visualized by HR-SEM. Li₃PO₄ reacts most probably as the heterogeneous catalyst, acting by epitaxy, of both Li₂SiO₃ and Li₂Si₂O₅ crystals. Based on the discussion of the main results, the authors deduced a four-step reaction mechanism. This mechanism demonstrated that after nucleation of lithium metasilicate and lithium disilicate, the latter phase grows as agglomerated nanocrystals, but remained in a relative small amount. By contrast, lithium metasilicate grows rapidly and decomposes at 780–820 °C with the result of a drastic increase of lithium disilicate phase. This was a result of a solid state reaction with the SiO₂-rich glassy phase. In a parallel reaction, cristobalite was formed as a preliminary phase. The final product of a glass–ceramic with 3.2 wt% P₂O₅ shows a highly crystalline interlocking microstructure demonstrating a high-strength of 726 ± 63 MPa and translucency.     

Effects of Sn addition on the glass forming ability and crystallization behavior in Ni-Zr-Ti-Si alloys

The effect of Sn substitution for Si on the glass forming ability (GFA) and crystallization behavior has been studied in Ni₅₉Zr₂₀Ti₁₆Si_5 − xSn_x (x=0, 3, 5) alloys. A bulk amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy with diameter up to 3 mm can be fabricated by injection casting. Partial substitution of Si by Sn in Ni₅₉Zr₂₀Ti₆Si_5 − xSn_x alloys improves the glass forming ability. The improved GFA of the Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy is can be explained based on the lowering of liquidus temperature. The crystallization sequence becomes completely different with addition of Sn. The amorphous Ni₅₉Zr₂₀Ti₁₆Si₅ alloy crystallizes via precipitation of only a cubic NiTi phase in the first crystallization step, whereas the amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy crystallizes via simultaneous precipitation of orthorhombic Ni₁₀(Zr,Ti)₇ and cubic NiTi phases. Addition of Sn in the Ni₅₉Zr₂₀Ti₁₆Si₅ alloy suppresses the formation of the primary cubic NiTi phase. The bulk amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy exhibits high compressive fracture strength of about 2.7 GPa with a plastic strain of about 2%.     

The role of stress development and relaxation on crystal growth in glass

The role of internal stresses energy is usually neglected when crystallization kinetics is considered. The common argument is that stress is relaxing too fast to affect the process. In this article we develop a generalized formalism to describe steady-state growth kinetics in viscoelastic media. The residual stress energy results from interplay between the rate of stress development (due to the propagation of a crystal throughout the matrix) and rate of stress dissipation (due to relaxation of the viscous matrix). The degree to which the stress energy can relax depends on the ratio of τ_c/τ_r the characteristic time for crystal growth, τ_c, and the relaxation time, τ_r. The present results challenge the widespread fallacy that internal stresses relax too fast to affect crystal growth. Our model explains the often observed lack of agreement between the theoretical predictions (without taking into account the stresses energy development) and experimental data.     

In situ crystallization of lithium disilicate glass: Effect of pressure on crystal growth rate

Crystallization of a Li₂O · 2SiO₂ (LS₂) glass subjected to a uniform hydrostatic pressure of 4.5 and 6 GPa was investigated up to a temperature of 750 °C. The density of the compressed glass is ∼2% greater at 4.5 GPa than 1 atm and, depending on the processing temperature, up to 10% greater at 6 GPa. Crystal growth rates investigated as a function of temperature and pressure show that lithium disilicate crystal growth is an order of magnitude slower at 4.5 GPa than 1 atm resulting in a shift of +45 °C (±10 °C) in the growth rate curve at high pressure compared to 1 atm conditions. At 6 GPa lithium disilicate crystallization is suppressed entirely, while a new high pressure lithium metasilicate crystallizes at temperatures 95 °C (±10 °C) higher than those reported for lithium disilicate crystallization at 1 atm. The observed decrease in crystal growth rate with increasing pressure for the lithium disilicate glass up to 750 °C is attributed to an increase in viscosity with pressure associated with fundamental changes in glass structure accommodating densification.     

Free energy profiles of Al and La cation distribution in silica and soda silicate glasses

The factors that control the distribution of Al³⁺ and La³⁺ cations in silica and soda silicate glasses is examined by using molecular dynamics (MD) simulations. In particular, the response of the glass network to the presence of metal oxide is probed using a liquid state theory that treats the glass network as a solvent and the metal cation as a solute. MD simulations are used to obtain the mean solvent-solute and solute-solute force. The trajectory used to determine the free energy is analyzed to determine the stable configurations of a cation pair. Details of determining the potential of mean force for an Al cation pair in silica and silicate glass is presented. A comparison of these results with those previously calculated for a La cation pair in the same glass systems is given. The results reveal that there are differences in how the network accommodates the two different size cations. It is found that for the potential used here, based on the Vessal potential, the network wraps itself around the larger La cation forming a solvent shell, whereas, the smaller Al cation is incorporated into the network backbone. In silica and soda silicate glasses, La ion pairs cluster to form La-O-La linkages. In contrast, the glasses favor a separated state of the Al ion pair.     

Characterization of a mould flux glass

A simplified mould flux glass composition used for the continuous casting of steel was synthesized and then characterized using X-ray powder diffraction (XRD) differential thermal analysis (DTA) and ¹⁹F and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR). DTA showed the glass to have a low glass transition temperature and to crystallize readily at 600 °C. XRD of the heat-treated glass showed it to crystallize to cuspidine. ¹⁹F MAS-NMR showed the principal fluorine species to be F-Ca(n) with no evidence of Si-F or Al-F species. Fluoride ions therefore, complex calcium in this glass, rather than forming non-bridging fluorines. The network connectivity of the glass was calculated on this basis and found to be 2.07 this would be expected to correspond to a Q² Si species which was supported by the ²⁹Si data that gave a chemical shift of −78 ppm corresponding to Q² Si.