Influence of Intermediate ZnO on the Crystallization of PbSe Quantum Dots in Silicate Glasses

Glasses containing PbSe quantum dots have important prospective applications in the area of near infrared optoelectronic devices. ZnO plays an important but complicated role in the preparation of quantum dot glass. We gave reasonable elaborations on the color changes of the as‐prepared and annealed glasses containing PbSe quantum dots. The as‐prepared glasses presenting nearly black to transparent straw‐yellow colors with rising ZnO content can be understood in terms of the conversion between PbSe and ZnSe; the colors of all annealed glasses turning very black can be well explained through thermodynamic calculations with the Gibbs–Helmho1tz equation. Next, according to the results of X‐ray diffraction, transmission electron microscopy and optical measurements, the sample with an introduction of 9.4 wt.% ZnO possesses a maximum degree of crystallization and the best optical properties among all samples, which suggests the host glass with addition of 9.4 ± 1 wt.% ZnO is optimum for crystallization of PbSe quantum dots. Finally, we employed classical nucleation theory to quantitatively calculate the nucleation rates and growth rates of PbSe quantum dots, and found that with the growing ZnO content in host glass, the nucleation rate decreases, whereas the growth rate of PbSe quantum dot increases gradually.     

Growth kinetics and optical properties of PbSe quantum dots in dual-phase lithium-aluminum-silicate glass ceramic

Lead selenide quantum dots (QDs) were precipitated in lithium-aluminum-silicate glass-ceramics. Their optical properties and the crystallization behavior of both glass host and PbSe QDs were investigated. When the glasses heat-treated at 480–520 °C for 10 h, the absorption bands from PbSe QDs were tuned from 1051 to 2016 nm, and the PL bands from 1165 to 2095 nm. Upon the dwelling time 10 h, the crystallization of glass host required relatively higher heating temperature (∼490 °C) than the formation of PbSe QDs (∼480 °C). With the heat treatment at 490 °C for10 h, the PLQY from PbSe QDs in glass-ceramics reached ∼38.7 %. Phase separation induced the glass crystallization and had a significant effect on the distribution of PbSe QDs. To study the growth kinetics of PbSe QDs, the glasses were heat treated at 480 °C and 500 °C for various durations. The average diameters of PbSe QDs (D) showed exponential relationship to the growth time (t), described by the equation D(t)∼ t ^x. In rapid growth stage, the growth exponents x is ∼0.56 at 480 °C and ∼0.47 at 500 °C, and then decrease to ∼0.13 and ∼0.10, respectively. The formation of a large number of LiAlO₂ crystals in glass hindered the diffusion of semiconductor ions, which is the main reason for the decreasing in the growth rate of PbSe QDs.     

Phase separation and crystallization in glasses of the lithium silicate system xLi2O · (100 ? x)SiO2 (x = 23.4, 26.0, 33.5)

The phase separation in ultimately homogenized glasses of the lithium silicate system xLi₂O · (100 − x)SiO₂ (where x = 23.4, 26.0, and 33.5 mol % Li₂O) has been investigated. The glasses of these compositions have been homogenized using the previously established special temperature-time conditions, which provide the maximum dehydration and the removal of bubbles from the glass melt. The parameters of nucleation and growth of phase_separated inhomogeneities and homogeneous crystal nucleation have been determined. The absolute values of the stationary nucleation rates I _st of lithium disilicate crystals in the 23.4Li₂O · 76.6SiO₂ and 26Li₂O · 74SiO₂ glasses with the compositions lying in the metastable phase separation region have been compared with the corresponding rates I _st for the glass of the stoichiometric lithium disilicate composition. It has been established that the crystal growth rate have a tendency toward a monotonic increase with an increase in the temperature, whereas the dependences of the crystal growth rate on the time of low-temperature heat treatment exhibit an oscillatory behavior with a monotonic decrease in the absolute value of oscillations. The character of crystallization in glasses with the compositions lying in the phase separation region of the Li₂O-SiO₂ system is compared with that in the glass of the stoichiometric lithium disilicate composition. The inference has been made that the phase separation weakly affects the nucleation parameters of lithium disilicate and has a strong effect on the crystal growth.     

Substructure Classification of Silicate Glasses

It is proposed that silicate glasses be classified in terms of substructures uniquely identified with primary crystallization phases. All glasses having compositions within the ranges or areas covered by a given primary crystallization phase field should constitute a separate glass type or class. This classification is definitive for properties determined principally by the silica frameworks or substructures. Such composition-property relations can be reproduced by independent investigators if the properties are measured under temperature equilibrium conditions.     

Time–Temperature–Transformation Diagrams for Borosilicate Glasses and Preparation of Chemically Durable Porous Glasses

To clarify the effects of glass composition and heat-treatment conditions on phase separation and crystallization, time–temperature–transformation (TTT) diagrams for the system (65 − x )SiO₂·25B₂O₃·(10 − y )Na₂O· y CaO· x ZrO₂, which is used for fabricating alkali-durable porous glasses, were determined. The microstructures and properties, such as resistance to alkaline attack as a function of ZrO₂ content, of the porous glasses fabricated on the basis of the developed TTT diagrams were examined. CaO and ZrO₂ additions were shown to affect the locations of the noses in the TTT curves for phase separation and crystallization. The substituting of CaO for Na₂O was effective for retaining ZrO₂ in the skeleton of the porous glasses. The alkali resistance of ZrO₂-containing porous glasses was 8–10 times superior to that of the porous glass without ZrO₂. The results demonstrate that TTT diagrams can be used as guides for heat-treatment scheduling in processing or predicting of glass-forming ability and the onset of phase separation and crystallization in glass-forming systems.     

Nucleation and Crystallization Kinetics in Fragile Glass-Forming Liquids

Isothermal kinetics of crystallization in the "fragile" Ca(NO₃)₂─KNO₃ melts and in AgI─Ag₂SO₄─Ag₂WO₄ melts of intermediate fragility have been investigated using singlestep and multistep calorimetric techniques. Time-temperature-transformation curves for crystal nucleation and growth have been delineated and the temperature for maximum nucleation rate ( T _NN) identified. The results are compared with the kinetics of nucleation observed in other fragile systems, such as fluoride glass melts, and with classical oxide melts (such as Li₂Si₂O₅) which have "strong" liquid characteristics. Reduced-temperature presentations of nucleation-rate data show qualitative correlations between T _NN/ T _L ( T _L is liquidus temperature) and liquid fragility. These correlations show that strong-liquid glass formers survive much larger supercoolings without nucleation than do fragile liquids.     

Transparent Hafnia-Containing β-Quartz Glass Ceramics: Nucleation and Crystallization Behavior

Heterogeneous nucleation and crystallization of lithium alumosilicate glasses with hafnia-containing nucleation agents was explored. Kinetic, thermodynamic, and structural data were considered to assess nucleation efficiency and to characterize the crystallization process. It is shown how lattice parameters and, particularly, anisotropy of the nuclei phase depend on the amount of ZrO₂–HfO₂ substitution in a specific base glass. For a given crystallization treatment, the size of the derived crystallites of β-quartz structure is used as a measure of nucleation rates. A nonlinear dependence of nucleation efficiency on HfO₂ content was established, with the supposedly most efficient nucleation corresponding to the lowest degree of anisotropy of the nuclei crystallites, i.e., when about 20%–30% of ZrO₂ was substituted by HfO₂. Apparent activation energies and estimates of the Avrami coefficient were determined from nonisothermal crystallization experiments for selected compositions to highlight the differences between hafnia and zirconia. Hafnia can be used alone or in combination with other agents to nucleate nanocrystalline glass ceramics with a low coefficient of thermal expansion.     

Influence of Dopants on Nucleation and Growth of High-Quartz Solid Solution in Lithium Aluminosilicate Glass

The kinetic parameters of nucleation and crystal growth of high-quartz solid solution in multicomponent lithium aluminosilicate glasses doped with various transition-metalions were studied by nonisothermal DTA. The crystallization of glasses nucleated at different temperatures was carried out, and plots of the DTA peak versus the nucleation temperatures were used to determine the maximum nucleation rate temperature. Peak temperature data of nucleated samples at varying heating rates (5–20 K/min) were used to determine the activation energy for crystallization via the JMA equation. The temperature of maximum nucleation rate depends greatly on the doped transition- metal ions present. The activation energy for crystallization obtained for undoped glass or glasses doped with Fe₂O₃ is of the same order as that already published, and the Avrami exponent is consistent with predominantly three-dimensional crystal growth. The much higher activation energy values for glasses doped with CoO could be a consequence of two crystallization processes proceeding simultaneously.     

Dilatometric fragility and prediction of the viscosity curve of glass-forming liquids

Viscosity and coefficient of thermal expansion (CTE) are both crucial properties in the design of new glasses for various applications. In this work, we extend the application of dilatometry to measure two important parameters governing the viscosity of glass-forming systems, viz., glass transition temperature and fragility index. We also describe a method to determine the dilatometric fictive temperature (T_f,DIL) and present data for five unique glass compositions covering a range of fragilities spanning 38-96, which are subjected to cooling and reheating rates in the range 1-30 K/min. The results show that the glass transition temperature obtained from the dilatometric method at 10 K/min (T_g,DIL) is consistent with both viscosity-based (T_g,vis) and DSC-based measurements (T_g,DSC). It is shown that the fragility of a liquid (m_vis) can be determined by calibrating the dilatometric fragility (m_DIL) with the same empirical model as in the calorimetric approach. Put together, we have developed a reliable method to measure the fragility and predict the viscosity curves of glass-forming liquids over a wide range (eg, 10¹-10¹⁶ Pa·s) without direct viscosity measurements, while simultaneously obtaining the CTE of the glass. However, this method is not suitable for glasses with a strong tendency toward phase separation or crystallization.     

Study of Crystallization Kinetics in Glasses along the Diopside–Ca-Tschermak Join

We report on the thermal stability and crystallization kinetics of the glasses in the diopside (CaMgSi₂O₆)–Ca-Tschermak (CaAl₂SiO₆) system. Four glasses with compositions corresponding to different diopside/Ca-Tschermak ratio were studied. Structural investigations on the glasses have been made by employing Infrared spectroscopy (FTIR). Activation energies for structural relaxation and viscous flow have been calculated using the data obtained from differential thermal analysis. The existence of glass-in-glass phase separation was observed in all the glasses. Kinetic fragility of the glasses along with other thermal parameters have also been calculated. Nonisothermal crystallization kinetic studies have been employed to study the mechanism of crystallization in all the four glasses. The Avrami parameter for the glass powders is ∼2, indicating the existence of intermediate mechanism of crystallization. Crystallization sequence in the glasses has been followed by X-ray diffraction analysis, scanning electron microscopy, and FTIR.     

Characterization of New Categories of Bioactive Based Tellurite and Silicate Glasses

New types of tellurite glass ceramics were prepared and studied from the viewpoint of bioactivity. The obtained results were compared with those of silicate glass ceramics. The crystallization behaviors of both silicate and tellurite glass ceramics with equal ratio of CaO/P₂O₅ were investigated. The silicate glass samples were transformed to glass ceramics by a thermal treatment process. While the tellurite glass ceramics were directly obtained without any thermal treatment. The microstructure of these materials was characterized by X-ray diffraction (XRD), Fourier transform infrared absorption spectroscopy (FTIR) and a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (SEM/EDX). The results revealed clear proof that TeO₂ promoted the nucleation and crystallization processes which led to the formation of different crystalline bio-phases. While the silicate glasses showed a much lower degree of crystallinity than that presented by the tellurite glass ceramics. The crystals of tellurite containing glass were needle- like morphology, which is attributed to the one-dimensional rapid growth of the apatite-tellurite phase. On the other hand, a particle-like morphology is shown in the silicate glass matrix. Bioactivity of the glasses in simulated body fluids (SBF) was investigated. Tellurite containing glass ceramics showed a better bioactivity during the in vitro test than that of the silicate one. This was attributed to a great analogous between the morphology of crystals of tellurite glass and the morphology of hydroxyapatite in human bone, since both possess a needle-like morphology.     

The formation mechanism of Cd–S–Se quantum dots with CdSe/CdS core–shell structure within silicate glass

The formation mechanism of Cd–S–Se quantum dots (QDs) with a CdS/CdSe core–shell structure within a silicate glass system was investigated by monitoring the change in the first excitonic absorption peak of CdSe, CdS, and Cd–S–Se QDs as a function of the heat-treatment (HT) duration. When a silicate glass containing QD formation components for CdSe or CdS single-phased QDs was heat-treated, CdSe QDs grew via Ostwald ripening, whereas CdS QDs hardly formed within the given silicate glass matrix. When the glass, including CdO, ZnSe, and ZnS, was utilized for the synthesis of Cd–S–Se QDs within the glass matrix, Cd–S–Se QDs were successfully formed and exhibited similar growth behavior to CdSe QDs during the initial stage of HT. The structural changes in the QD-embedded glasses during HT were also monitored by Raman spectroscopy and discussed, and a possible formation mechanism of CdSe/CdS core–shell-structured QDs within silicate glasses is suggested.     

Glass Formation and Properties in the System Calcia-Gallia-Germania

The critical cooling rate for glass formation, R^c, was measured for four compositions in the system calcia-gallia-germania. The activation energy, E, and frequency factor, u, for the crystallization process were determined by reheating the glasses at varied constant heating rates and measuring the temperature of crystallization. Both E and v increased, with increasing germania content of the glass, whereas R^c decreased. The density, refractive index, and Abbe number were also measured; all decreased with increasing GeO² content. These results are compared with those for calcia-gallia-silica glasses of comparable compositions.     

Dissolution Rates of Silicate Glasses in Water at pH 7

Dissolution rates of different glasses in buffered solutions of constant pH of 7 were measured by weight change, profilometry, and ion implantation with Rutherford backscattering. Rates with different techniques agreed within experimental error. Natural obsidian glass dissolved most slowly, at a rate comparable with those of quartz and crystalline aluminosilicate minerals. Commercial soda–lime glass containing alumina dissolved slowly, at about the same rate as vitreous silica; soda–lime silicate glasses both commercial and laboratory without alumina dissolved much more rapidly. Pyrex borosilicate glass dissolved at a rate intermediate between those of soda–lime silicate glasses with and without alumina; at room temperature Pyrex borosilicate glass dissolved about 100 times faster than a commercial soda–lime glass containing alumina. We suggest that surface structure is the main factor determining the relative dissolution rates of silicate glasses. Glasses with transformed surface layers caused by hydration dissolve most rapidly; phase separation and openness of the glass structure are also important factors.     

Effects of composition and phase relations on mechanical properties and crystallization of silicate glasses

Crystallization, mechanical properties, and workability are all important for the commercialization and optimization of silicate glass compositions. However, the inter-relations of these properties as a function of glass composition have received little investigation. Soda-lime-silica glasses with Na₂O-MgO-CaO-Al₂O₃-SiO₂ compositions relevant to commercial glass manufacture were experimentally studied and multiple liquidus temperature and viscosity models were used to complement the experimental results. Liquidus temperatures of the fabricated glasses were measured by the temperature gradient technique, and Rietveld refinements were applied to X-Ray powder diffraction (XRD) data for devitrified glasses, enabling quantitative determination of the crystalline and amorphous fractions and the nature of the crystals. Structural properties were investigated by Raman spectroscopy. Acoustic echography, micro-Vicker's indentation, and single-edge-notched bend testing methods were used to measure Young's moduli, hardness, and fracture toughness, respectively. It is shown that it is possible to design lower-melting soda-lime-silica glass compositions without compromising their mechanical and crystallization properties. Unlike Young's modulus, brittleness is highly responsive to the composition in soda-lime-silica glasses, and notably low brittleness values can be obtained in glasses with compositions in the wollastonite primary phase field: an effect that is more pronounced in the silica primary phase field. The measured bulk crystal fractions of the glasses subjected to devitrification at the lowest possible industrial conditioning temperatures indicate that soda-lime-silica glass melts can be conditioned close to their liquidus temperatures within the compositional ranges of the primary phase fields of cristobalite, wollastonite, or their combinations.     

T–T–T behaviour of bioactive glasses 1–98 and 13–93

In this work crystallization kinetics of bioactive glasses 1–98 and 13–93 are discussed. Within a certain temperature–time window these glasses can be hot worked into various products without interfering with crystallization. The crystallization was studied isothermally by heating glass plates at different temperatures for different times. Phases in the samples were studied through XRD and SEM analyses. The nucleation-like curves and crystallization characteristics were measured with DTA. The temperature of maximum nucleation was measured for glass 1–98 at 725 °C and for 13–93 at 700 °C. The activation energy of crystallization of both glasses was 280 kJ/mol. The Johnson–Mehl–Avrami exponent and the SEM micrographs of the samples suggested surface crystallization. The primary crystalline phase was wollastonite. The growth rate of the crystallized surface layer was 1 order of magnitude higher in the plates of 1–98 than in 13–93. The results can be utilized to optimize the parameters in hot-working of the glasses.     

Photodarkening and anti-Stokes photoluminescence from PbSe and Sr2+-doped PbSe quantum dots in silicate glasses

Photoluminescence (PL) properties of PbSe and Sr²⁺-doped PbSe quantum dots (QDs) doped in silicate glasses were investigated by changing the excited laser wavelength and intensity. When BaO was replaced by SrO, the Sr²⁺-doped PbSe QDs formed and showed large blue-shifts in both absorption and PL bands compared to the PbSe QDs. It is attributed to the increasing in band gap resulted by the incorporation of Sr²⁺ ions into PbSe QDs. These two kinds of QDs were excited by the laser with the wavelengths of 1319 and 1532 nm. When rising the pumping intensity up to 1000 mW, the PL intensity from Sr²⁺-doped PbSe QDs monotonically increased, while, the PL intensity from PbSe QDs reached maximum at pumping intensity at ~200 mW (λ_ex = 1319 nm) and ~300 mW (λ_ex = 1532 nm). The good resistance to photodarkening of Sr²⁺-doped PbSe QDs is indicated passivation effect on the surface defects from Sr²⁺ ions doping. For Sr²⁺-doped PbSe QDs the anti-Stokes photoluminescence (ASPL) were observed when using both 1319 and 1532 nm excitation. The integrated intensity of the ASPL is linear with the excitation intensity, which indicates that the ASPL belongs to the phonon-assisted one-photon process. And the ASPL intensity as a function of excitation intensity shows a size dependence of the QDs.     

Crystallization and melting behaviors of poly(aryletheretherketone) (PEEK) on origin of double melting peaks

The crystallization and melting behaviors of poly(aryletheretherketone) (PEEK) films were investigated, using differential scanning calorimetry and metallurgy concepts. The shape of the time–temperature–transformation (TTT) diagram, established for PEEK, results from both nucleation and growth phenomena. The double melting behavior exhibited by isothermally crystallized PEEK samples are discussed through the TTT diagram and the influence of the thermal history in the molten state. The upper melting peak arises first and the lower melting peak is developed later. The location of such a second endotherm is shifted toward the higher temperature with increasing either the crystallization temperature or the annealing time while the location of the upper melting peak seems to be unchanged. The double melting behavior is related to a bimodal distribution in size and/or perfection of lamellae developed in a two-step crystallization. With increasing temperature and/or annealing time in the molten state, the pattern of the endothermic curves is modified. The observed changes are discussed through two origins: the progressive disappearance of remnants of the former crystals and a thermal degradation leading to a cross-linking of the polymer. © 1994 John Wiley & Sons, Inc.     

Heat of Annealing in Simple Alkali Silicate Glasses

The enthalpy changes associated with annealing of glass were studied in simple and mixed alkali silicate glasses. The data indicate that during prolonged annealing the glass comes to a metastable equilibrium state and has a unique heat of solution which depends on its fictive temperature. The heats of solution of these glasses show a linear dependence on fictive temperature, and the magnitude of this dependence is related to the molar volume of the glass. The significance of these heat effects is discussed. The maximum heat effects which can occur on annealing sodium silicate glasses were measured and were approximately half as large as the enthalpy changes associated with the structural arrangements that occur during crystallization of these glasses.     

Engineering the crystallization behavior of CsPbBr3 quantum dots in borosilicate glass through modulating the glass network modifiers for wide-color-gamut displays

The in-situ growth of CsPbBr₃ perovskite quantum dots (QDs) inside glass has been regarded as an alternative approach to improve their stability. Alkaline-earth metal oxides has multiple effects on the structure of the glass network. Herein, four types of alkaline-earth metal oxides are introduced into borosilicate glasses to modulate glass network structure, which has quite different effects on the crystallization behavior of CsPbBr₃ QDs. The reason can be ascribed to the different impacts of alkaline-earth metal on phase separation, nucleation, and growth procedure. Moreover, CsPbBr₃ QDs embedded in glass (CsPbBr₃ QD@glass) exhibit superior thermostability and photostability compared with CsPbBr₃ QDs powder. Finally, a white light-emitting diode achieving 124% of National Television System Committee (NTSC) gamut is fabricated using the CsPbBr₃ QD@glass, K₂SiF₆:Mn⁴⁺ phosphor film, and blue chip-on-board. This work provides a reference for modulating the glass network modifiers to regulate the crystallization behavior of perovskite QDs.