Kinetics of Crystallization of ZrF4-Ba2-LaF3 Glass by Differential Scanning Calorimetry

The kinetics of devitrification in ZrF₄-BaF₂-LaF₃ (62–33–5 mol % ) glass were studied by isothermal and nonisothermal methods using differential scanning calorimetry. The crystallization reaction followed the Avrami equation, x =1-exp[—( kt) ], where x is the fraction crystallized after time t . The rate con stant k obeyed an Arrhenius equation, k(s⁻¹)=6.69 × 10²¹ exp

The value of the Avrami exponent, n , bas 3.2×0.2, indicating three-dimensional crystal growth at a constant number of nucleation sites. Values of kinetics parame ters obtained from isothermal and nonisothermal techniques were in excellent agreement. The heat of crystallization measured from isothermal peaks was computed to be 38.5×3.5 J/g.     

Nucleation and Crystallization of Na2O · 2CaO · 3SiO2 Glass by Differential Thermal Analysis

The nucleation and crystallization of the Na₂O · 2CaO · 3SiO₂ (NC₂S₃) glass were studied by differential thermal analysis (DTA), and a (nucleation rate—temperature)-like curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T_p , or the height of the crystallization peak, (δ T ) _p , as a function of nucleation temperature, T_n. The temperature where nucleation can occur for this glass ranges from 550° to 650°C and the temperature for maximum nucleation is 600°± 5°C. Both temperatures are in excellent agreement with those determined by the classical technique of nucleation followed by isothermal crystallization. The activation energy for crystallization, E_c , for this glass is the same for surface and/or bulk crystallization, and is 370 ± 15 kJ/mol. The analysis of the crystallization data with the Kissinger equation yelds the correct value for E_c only when crystal growth occurs on a fixed number of nuclei. When a majority of the nucleation occurs during the DTA measurements, a modified Kissinger equation must be used to calculate E_c . E_c is also independent of the heating rate when determined using a single-crystallization-peak analysis technique. The single-peak analysis technique is useful for a rapid determination of E_c or when only a small amount of sample is available.     

Crystallization Kinetics of α-Agl in Ag I-Based Silver Orthoborate Glasses

Kinetics for nucleation and growth of α-AgI crystals in AgI-based silver orthoborate glasses, in which only α-AgI crystals were found to crystallize in the heating process of the glasses, were studied by isothermal and nonisothermal measurements using differential scanning calorimetry and by observations of the microstructure of the glasses using scanning electron microscopy. The values of both n and m , dependent on the crystallization mechanism, were found to be 3, indicating that a constant number of nuclei of α-AgI precipitated in a glass matrix grew three-dimensionally. The preannealing of the glasses at temperatures between glass transition and crystallization did not affect the DSC isothermal curves in the crystal growth process, which suggested that the number of α-Agl nuclei would have been saturated in the glass when the glasses were prepared by quenching the AgI-based melts.     

Controlled Crystallization and Properties of Zirconium Fluoride-Based Glass-Ceramics

51ZrF₄16BaF₂, 5LaF₃.3AlF₂,.20Li.F5PbF₂, glasses were pre-pared with CdF, additions of 0, 2, 5, and 7.5 mol%. The glasses with 2 and 5 mol% exhibited controllable nucleation upon heat treatment above the glass transformation tem-perature. The nucleation and crystallization of glasses with 5 mol% CdF, were characterized using bulk heat treat-ments, differential scanning calorimetry, and scanning elec-tron microscopy. The isothermal crystallization of nucleated glasses resulted in the formation of a glass-ceramic that was transparent in the infrared (>70% transmission) and more resistant to fracture than typical fluoride glasses.     

Microstructural Development on Crystallizing Hot-Pressed Pellets of Cordierite Melt-derived Glass Containing B2O3 and P2O5

Comparing the crystallization mechanism of stoichiometric and B₂O₃ and P₂O₅ containing glass reveals that the additives extend the gap between the glass transition and crystallization temperatures and suppress formation of μ, cordierite while promoting direct crystallization of α cordierite. Detailed TEM analysis of nucleation and growth of crystals in hot-pressed pellets of B₂O₃/P₂O₅-containing glass particles shows that nucleation occurs on unidentified heterogeneous nuclei at the sites of the previous particle surfaces. Growth of α cordierite with a cellular morphology or μ cordierite with a dendritic morphology is most likely controlled by the glass composition directly ahead of the growth front.     

Effect of Temperature and Water-Solid Ratio on Growth of Ca(OH)2 Crystals Formed During Hydration of Ca3SiO5

The growth behavior, time of nucleation, and morphology of Ca(OH)₂ crystals formed during the hydration of Ca₃SiO₅, at 15°, 25°, and 35°C at water-solid ratios ( w/s ) from 0.3 to 5.0 were studied by optical microscopy. In samples with w/s >0.5 growth of Ca(OH)₂ in the c -axis direction is initially dominant. Growth in this direction ends after a few hours, but growth perpendicular to the c axis continues for several days and produces a dendritic morphology. Growth behavior is not so well defined for w/s <0.5, in part because of the large number of unhydrated particles engulfed. Increasing temperature resulted in an increase in the number of Ca(OH)₂ nuclei and a decrease in nucleation time and crystal size. Increasing the w/s ratio improved the euhedral character of the Ca(OH)₂ crystals, decreased the number of engulfed Ca₃SiO₅ particles, and increased the nucleation time. Dendritic morphology was most pronounced in the samples for which w/s = 1. Growth rates and the ultimate size of the Ca(OH)₂ crystals varied within a given sample. The effects of temperature and the w/s ratio on the heat evolved during the hydration were studied by isothermal calorimetry. The times of nucleation of crystalline Ca(OH)₂ estimated from calorimetry were similar to those derived from growth curves determined by optical microscopy.     

Direct Observation of the Crystallization of Li2O-Al2O3-SiO2 Glasses Containing TiO2

The nucleation and crystallization of Li₂O-Al₂O₃-SiO₂ glasses containing TiO₂ were investigated using transmission electron microscopy of thin sections produced from bulk samples. Phase separation occurs during cooling from the melt, and on heating, a large number of titanium-aluminum crystals approximately 50 A in diameter are formed. These crystals are the heterogeneous nuclei for the crystallization of the remaining glass. Photomicrographs of various stages of crystallization show the development of the fine-grained glass-ceramic.     

Effect of Heat Treatment on Rapidly Quenched AgI-Based Silver Orthoborate Glasses Containing Large Amounts of AgI

The effect of heat treatment on the twin-roller rapidly quenched 75AgI·18.7Ag₂O6.3B₂O₃ glass was investigated by differential scanning calorimetry (DSC), high-temperature X-ray diffraction (XRD), and field-emission-type scanning electron microscopy. The glass had an inhomogeneous microstructure with dispersed particles 40-60 nm in diameter at room temperature. On the other hand, island regions of several hundred nanometers with fine dispersed particles about 20-30 nm in diameter were observed in the glass after heating to 120°C. DSC and high-temperature XRD measurements revealed that crystallization occurred at around 120°C, which is lower than the α-β phase transformation temperature (147°C), to form α-AgI in the glass. The crystallization of α-AgI from the glass below the α-ß phase transformation temperature strongly supports the possibility of the existence of α-AgI nuclei in AgI-based silver orthoborate glasses. Validating the existence of AgI microcrystals supports the microdomain model for superionic AgI-based glasses.     

In Situ Whisker-Reinforced AlPO4-Modified β-Eucryptite Glass-Ceramic: I, Morphology and Crystallization Kinetics

A whisker-reinforced glass-ceramic composite in the Li₂O-Al₂O₃-SiO₂-P₂O₅ system has been fabricated by a single-stage process that simultaneously forms the glass-ceramic material and whiskers of TiO₂ in situ . The method utilizes typical glass-ceramic processing techniques and requires precise addition of a TiO₂ nucleation agent during the glass-melting operation, followed by controlled nucleation and crystallization. The maximum nucleation temperature, 740°C, was determined by differential thermal analysis (DTA), and the result was confirmed by scanning electron microscopy (SEM) of the microstructures formed by nucleation/isothermal crystallization heat treatments. The apparent activation energy for crystallization of the material was determined to be 285 ± 3 kj/mol. The average aspect ratio of the TiO₂ whiskers depends on temperature and time during crystallization. The X-ray diffraction patterns of the in situ composite show that eucryptite(ss) and rutile exist as two different phases; no additional phases were observed. Elemental X-ray mapping by electron microprobe indicates that these highly crystallized composites consist of modified β-eucryptite glass-ceramic matrix and acicular grains of TiO₂.     

Determining the Nucleation Rate Curve for Lithium Disilicate Glass by Differential Thermal Analysis

The crystallization of lithium disilicate (Li₂O·2SiO₂) glass nucleated at various temperatures was studied by differential thermal analysis (DTA). A plot of the DTA crystallization peak height versus nucleation temperature closely resembles the classical nucleation rate curve for lithium disilicate glass whose maximum is at 453°C. The glass becomes saturated with internal nuclei when heated at 453°C for 10 h. The DTA technique is a rapid, alternative method for determining the temperature for maximum nucleation. The activation energy for crystallization, E , and the heat of crystallization, H , are independent of the concentration of nuclei and are 249±10 and 67±3 kJ/mol, respectively. The Avrami exponent, n , depends strongly on the concentration of nuclei in the glass.     

Nonisothermal crystallization behavior of a biodegradable segmented copolymer constituted by glycolide and trimethylene carbonate units

Nonisothermal crystallization of a segmented copolymer constituted by glycolide and trimethylene carbonate units was studied from both the melt and the glass state by optical microscopy, differential scanning calorimetry and time‐resolved X‐ray diffraction techniques. Positive spherulites with a fibrilar appearance were always obtained and corresponded to the crystallization of the polyglycolide hard segments. A single crystallization regime and the kinetic parameters were inferred from optical microscopy data on crystallizations performed at different cooling/heating rates. The parameters were in good agreement with values previously deduced from isothermal experiments. Isoconversional data of melt and glass nonisothermal crystallizations were combined to obtain the Lauritzen and Hoffman parameters from calorimetric data. Results revealed again the existence of a single crystallization regime with a secondary nucleation constant close to that deduced from isothermal DSC experiments. Morphological changes occurring during the hot and cold crystallization were evaluated by time‐resolved SAXS/WAXD experiments employing synchrotron radiation. Measurements showed that significant differences on the lamellar thicknesses exist depending on the crystallization process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Crystallization characteristics of isotactic polypropylene with and without nucleating agents

Nucleation effects of two sorbitol derivatives on the crystallization of isotactic polypropylene (iPP) were studied by means of differential scanning calorimetry (DSC) and polarized optical microscopy (POM). A nonisothermal crystallization kinetic equation was employed to analyze the crystallization characteristics of iPP with or without the nucleating agents from DSC crystallization thermograms. The equilibrium melting temperature of iPP necessary for the kinetic study was obtained by the extrapolation method to be 209°C. The nonisothermal crystallization kinetic analysis for the unnucleated iPP at different cooling rates was possible by assuming the spherulite growth initiated simultaneously by heterogeneous and homogeneous nucleation. On the other hand, the crystallization kinetics of the nucleated iPP could be described by the heterogeneous nucleation and growth process alone. The addition of the nucleating agents up to their saturation concentrations in iPP increased the crystallization peak temperature by 17°C, and the number of effective nuclei by three orders of magnitude. A high concentration of the nucleating agents caused agglomeration of the agents to lower the number of effective nuclei.     

Crystallization Behavior of a High-Zinc-Content Li2O-ZnO0-SiO2 Glass-Ceramic and the Effect of K2O Additions

The crystallization behavior of an Li₂O-Zn0-SiO₂ glass with a ZnO content of −28.5 wt% and nucleated with P₂O₅ was investigated by infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. A three-stage process, consisting of the nucleation, crystallization, and transformation of Li₃Zn_0.5Si0₄ and the emergence of a silica phase, occurs during heat treatment of this system. A small addition of K₂O varies the sequence of crystallization and the phase composition of the resulting glass-ceramic.     

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.     

Spherulitic Growth from a Phase-Separated Vitreous Matrix in a Cordierite–Y-Stabilized Zirconia Glass-Ceramic

Crystallization behavior, especially spherulitic growth of a cordierite–3 mol% Y₂O₃–ZrO₂ glass composition annealed at 950° to 1370°C was studied by optical and electron microscopy. Microstructural analysis revealed that glass-inglass phase separation occurred prior to the crystallization of β-quartz solid solution ( ss ). ZrO₂ was involved in promoting the nucleation of β-quartz ss ; the crystallization evolved from a cellular to a spherulitic morphology, and β-quartz transformed to α-cordierite as the temperature was raised. ZrO₂ was eventually expelled intergranularly and intragranularly to form slightly misoriented domains upon annealing, and the dendritic or clustered tetragonal t -ZrO₂ crystals transformed to monoclinic ( m ) in symmetry. A possible dragging force exerted between α-cordierite and a glassy droplet phase is proposed to explain the corrugated cordierite/glass interface.     

On the Determination of the Concentration of Crystal Nuclei in Glasses by DSC

An interesting technique was proposed by Ray et al. over a decade ago to determine the number density of nucleation sites, N_q, or the nucleation rate, I(T), in glasses that exhibit internal nucleation. Their approach is based on the measurement of the areas under the differential scanning calorimetry (DSC) crystallization peaks of partially crystallized glass samples. In this study, we review their method and test a modified equation recently proposed by some of us, which also takes into account the crystal morphology and impingement. We compare N_q obtained with both methods for a Li₂O·2SiO₂ glass. Small glass monoliths were treated at 620°C for different time periods for crystal growth, without any nucleation treatment, and subsequently analyzed by DSC up to 800°C. We thus estimated N_q from the area under the DSC crystallization peaks. The corrected approach resulted in N_q values which were not affected by the pair of growth times chosen, as expected, but the obtained values were two‐ to fivefold lower than those calculated with the Ray model. Taking into account previously reported nucleation rates and the corresponding induction periods as a function of temperature (for specimens of the same glass batch), we estimated the number of nuclei formed during the DSC heating/cooling steps, and also measured them by optical microscopy (OM). Finally, we compared the obtained values from OM with the N_q values determined by the DSC method. The N_q resulting from the original and new equations were approximately two orders of magnitude larger than those experimentally determined for the same glass using optical microscopy. This difference is attributed to the formation of new nuclei during the heating and cooling paths of the DSC runs and to surface crystallization, which are not taken into account in the DSC expressions.     

Infrared Glass–Ceramics With Fine Porous Surfaces for Optical Sensor Applications

GeS₂–Sb₂S₃–CsCl glass–ceramics with fine porous surfaces were synthesized and tested as optical elements. The porosity is obtained through a two-step process, including controlled nucleation of CsCl nuclei in the glass matrix followed by selective etching of the nuclei with an acid solution. The porous surface is several hundred nanometers thick and results in a surface area increase of almost four orders of magnitude. The pores size is approximately 150 nm and can be tailored by controlling the nucleation process and the etching time. It is shown that the creation of the porous surface does not critically affect the optical transmission of these infrared (IR) transparent glass–ceramics. These materials can therefore be used for the design of optical elements and an attenuated total reflexion plate with porous surface was fabricated and tested as an optical IR sensor. The porous element shows higher detection sensitivity in initial experiments with an analyte sprayed at the plate surface and a coating of silane molecules.     

Formation of Nepheline Glass-Ceramics Using Nb2O5 as a Nucleation Catalyst

The crystallization of several Nb₂O₅-catalyzed glasses in the Na₂O.Al₂O.SiO₂ system was studied using DTA, X-ray diffraction, and electron microscopy. The Nb₂O₅ was an effective nucleation catalyst; fine-grained body-nucleated glass-ceramic materials containing hexagonal nepheline and NaNbO₃ were obtained. The crystallization sequence and final crystalline phases in these compositions were quite different from those found in the equivalent TiO₂ analogs. The addition of small amounts of carbides and nitrides at the expense of oxides in the initial glass batch markedly affected the final crystalline phases.     

Surface Nucleation and Crystal Orientation in Lithium Silicate Glass Fibers

The effect of inorganic surface treatments on the orientation of lithium disilicate crystals formed in Li₂O·2.75SiO₂ glass fibers was studied. Glass fibers 0.5 mm and 9μ in diameter were subjected to various surface treatments at room temperature and then heated between 550O and 800°C. It was observed that metal salt solutions applied to the fiber surfaces at room temperature decreased the degree of orientation during heating only if the metal entered the glass and formed nucleation sites within the fibers. Orientation could also be decreased by removing lithium from the glass surfaces. Comparison of the crystallization characteristics of untreated and AgNO₃-treated fibers indicated that the crystallization behavior was controlled by either a growth process or a nucleation process, depending on whether the temperature was below or above 625°, respectively.     

Further Studies of the Crystallization of a Lithium Silicate Glass

Rates of crystallization of Li₂O · 2SiO₂, from an Li₂O · 4SiO₂ glass as a function of platinum concentration at 600° and 650° C were determined. At both temperatures a maximum rate was obtained in the region of 0.005% Pt. Calculations of the activation energies for the crystallization process showed that 0.005%, Pt lowered it to less than half its original value of 120 kcal per mole. Electron microscopy of this glass quenched from 1450OC revealed a granular structure 250 A in size which was believed to be due to lithium-rich clusters in the glass, svpporting the concepts of other investigators. In glasses containing 0.025%, an additional phase identified as silica O was found. Explanations for the effectiveness of platinum as a nucleating agent in this glass are offered.