The effects of amorphous phase separation on crystal nucleation kinetics in BaO-SiO2 glasses

The nucleation kinetics of the barium disilicate crystal phase were determined in BaO-SiO₂ glasses containing 25.3 to 33.1 mol % BaO at temperatures from 673 to 807°C, using quantitative optical microscopy. The highest nucleation rates were found in the 33.1 mol % BaO glass close to the stoichiometric BaO·2SiO₂ composition, which was just outside the immiscibility region. Much lower rates were observed in the glasses with lower BaO contents including those exhibiting amorphous phase separation. However phase separation had a marked indirect effect on crystal nucleation. Crystal nucleation was strongly dependent on the composition of the baria-rich matrix phase in the phase separated glasses, so that after amorphous phase separation had occurred at a given temperature the crystal nucleation rates of different glasses tended to converge to similar values. There was no evidence from the present results for a significant enhancement of crystal nucleation rates at the interfaces between the amorphous phases. However, nucleation was significantly influenced by certain impurities, particularly alumina, in the glasses. The early stages of crystallization in the glasses were studied by TEM. Immiscibility temperature measurements and results of DTA and X-ray diffraction are also reported.     

The effects of amorphous phase separation on crystal nucleation kinetics in BaO-SiO2 glasses

The nucleation kinetics of the barium disilicate crystal phase were determined in BaO-SiO₂ glasses containing 25.3, 28.5 and 30.4 mol % BaO at 700° C. In the 25.3 and 28.5 glasses, which exhibited amorphous phase separation, marked differences in crystal nucleation occurred in each glass at 700° C, depending on the previous heat treatment, for example whether the glass had been heated at a higher temperature to induce phase separation or whether it had been quenched to suppress phase separatiocn. Moreover, at 700° C phase separation and crystal nucleation occurred simultaneously over an extended period and an increase in crystal nucleation rate with time was observed. This was reflected in curved plots ofN _v (number of crystals per unit volume) against time. In contrast, for glass 30.4, which did not phase separate, the plots were linear. The results in glasses 25.3 and 28.5 could be completely explained by changes in composition of the baria-rich amorphous phase. Crystal nucleation rates rose with increase in baria content, i.e. as the composition became closer to BaO·2SiO₂. Prior heat treatments produced major shifts in composition of the phases. Also, at 700° C the average baria content of the baria-rich phase increased gradually towards the equilibrium value given by the immiscibility boundary.     

The effects of amorphous phase separation on crystal nucleation kinetics in BaO-SiO2 glasses

The nucleation kinetics of barium disilicate spherulites were determined by optical microscopy in BaO-SiO₂ glasses containing 27.0 to 33.3 mol % BaO for isothermal treatments at 718 to 760‡ C. Amorphous phase separation in two glasses was studied by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The average diameter, number and surface area of amorphous droplets were obtained by SAXS as a function of time at 743 and 760‡ C. In glasses not showing amorphous phase separation the crystal nucleation rate,I, was constant at a given temperature. In glasses undergoing phase separation,I increased with time approaching a constant value, which was identical for different phase-separated glasses. There was a striking correlation between the time to reach a constantI and the time required to attain the equilibrium composition of the amorphous (baria-rich) matrix phase, as revealed by a constant integrated SAXS intensity. The crystal nucleation rate,I, depended primarily on the composition of the baria-rich phase. There was no apparent relationship betweenI and the surface area or number of droplets from SAXS. However, possible evidence of additional crystal nucleation at droplet interfaces was found, although the effect was small. Viscosities and crystal growth rates for the phase-separated glasses are also discussed.     

Amorphous phase separation of ionomer glasses

Ionomer glasses of generic composition SiO₂-Al₂O₃-P₂O₅-CaO-CaF₂ were studied using differential scanning calorimetry (DSC) and high temperature dynamic-mechanical thermal analysis (DMTA). High temperature DMTA was used to measure the glass transition temperatures (T _g) of the original starting glass compositions, as well as being able to follow amorphous phase separation (APS) within the glass. High temperature DMTA traces of all the glasses studied exhibited two maxima in tan . These maxima correspond to two glass transition temperatures and demonstrate that amorphous phase separation of the parent glass into two glass phases had occurred. A DMTA study of a Sodium-Boro-Silicate glass, which is known to undergo amorphous phase separation yielded similar results. DSC studies showed that the ionomer glasses underwent a nucleation process at temperatures just above the glass transition temperature which is probably associated with APS. The glasses exhibited optimum nucleation temperatures which moved to lower temperatures with longer hold times indicating the time dependency of the APS process.     

Glass formation and phase selection of melt-spun Al–Zn–Ce alloys

The glass-formation characteristics and phase-selection behavior of Al–Zn–Ce alloys have been studied by X-ray diffraction (XRD) and different scanning calorimetry (DSC). As the concentration of Ce increases, an intermetallic compound Al₂Zn₂Ce appears, which prevents the occurrence of phase separation, and improves the forming ability of the single amorphous phase. In Al₈₃Zn₁₀Ce₇ alloys, the precipitation of fcc-Al was accompanied with the Al₂Zn₂Ce phase and Al₄Ce phase. Moreover, the presence of fcc-Al appears to favor the nucleation and growth of the Al₂Zn₂Ce and Al₄Ce phase. However, it seems that the Al₂Zn₂Ce and Al₄Ce nucleate competitively with the fcc-Al phase and the growth of fcc-Al and Al₂Zn₂Ce prefers to that of Al₄Ce. The competitive nucleation and growth limitation of the various phases are favorable to the formation of Al–Zn–Ce amorphous alloys. For the amorphous Al–Zn–Ce alloys, the glass formation is not controlled by nucleation restrictions but largely by the suppression of growth of nuclei formed during rapid melt quenching.     

Nucleation sites of the T2 phase in alloy 2090

Alloy 2090 fabricated as O-Temper sheet exhibited the presence of the T₂ phase. The morphology and nucleation characteristics of the T₂ phase in alloy 2090 were investigated by transmission electron microscopy. Also, the crystal structure was documented by electron and X-ray diffraction analyses. It was found that T₂ precipitates formed preferentially at the interface of heterogeneities in the alloy. Several types of heterogeneities were identified. From these results it was proposed that as the impurities are eliminated, the propensity for nucleation of the T₂ phase should be reduced.     

Study of kinetics of the phase separation in sodium borate glasses

The kinetics of the phase separation in 15Na₂O-85B₂O₃ binary glasses was investigated using ¹¹B nuclear-magnetic-resonance (NMR) spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM) observation. It was found that the equilibrium of the phase separation took long time due to the growth of a boron-rich phase and the composition fluctuation in Na-rich phase. Although the XRD results showed that the development of the boron-rich phase was through nucleation and growth, the NMR spectra indicated that the sodium-rich phase always occurred as amorphous glasses with fluctuation in composition during the phase separation. This may result from that the over-coordination of oxygen atoms in the sodium-rich phase, which renders difficulties in the crystallization of the sodium-rich phase with less than 75 mol% sodium oxides. It was found that, at 500°C, the 15Na₂O-85B₂O₃ glasses was finally separated into Na₂O·9B₂O₃ and 3Na₂O·B₂O₃ rather than those being proposed by the conventional phase diagram.     

Structure of plasma-sprayed oxides in the MgO-Al2O3-SiO2 system

Cordierite (2MgO · 2Al₂O₃-SiO₂), forsterite (2MgO · SiO₂) and spinel (MgO · Al₂O₃) were rapidly solidified into a thick film through plasma spraying. The crystal structure of the assprayed deposits was examined using X-ray diffractometry and transmission electron microscopy, and shows that as-sprayed cordierite is amorphous, as-sprayed spinel is a crystalline phase, and as-sprayed forsterite is a mixture of amorphous and crystalline phases. Moreover, transmission electron microscopy reveals uniform amorphous cordierite, a nonuniform grainsize distribution of crystalline spinel and a twinned texture of as-sprayed forsterite.     

Phase equilibria in Ag-Ga-S films and kinetics of AgGaS<Subscript>2</Subscript> crystallization

The phase compositions of films produced by coevaporation and sequential evaporation of Ag, Ga, and S have been determined. The phases identified in the films grown at room temperature are Ag₂S, GaS, Ga₂S₃, and AgGaS₂. The kinetics of phase transformations in thin amorphous AgGaS₂ films have been studied by dynamic electron diffraction. The dimensionality of growth during amorphous AgGaS₂ crystallization and the activation energies for nucleation and crystallite growth have been evaluated.     

Plasma arc Synthesis of Nano-Boron Towards the Synthesis of MgB2 Superconductors

Since the discovery of superconductivity in the tempting binary intermetallic compound MgB₂, the solid-state synthesis technique is highly dominated by the usage of amorphous boron as one of the precursor powders. The formation of MgB₂ phase proceeds through the diffusion of Mg into B powder mainly driven by the low melting point of Mg as compared to B. Once the nucleation is achieved, the progress of polycrystalline MgB₂ phase occurs due to the out diffusion of boron through the MgB₂ layer and by the inward diffusion of Mg. This growth is impeded due to the presence of certain oxide phases or formation of Mg deficient phases. It is speculated that the probability for the inclusion of Mg(B)–O phases is higher for crystalline boron precursor as compared to the amorphous B. Thus, the use of nanosized amorphous boron may lead to larger nucleation centers, smaller grain size and consequently higher packing density in the polycrystalline MgB₂, which will in turn provide optimum superconducting properties. Hence an attempt to synthesize amorphous nano-boron powders is presented. Plasma arc discharge technique was successfully employed to produce nano-boron powder. The XPS analysis was carried out to inveterate the formation of boron. The as-synthesized powder had a uniform average particle size distribution of around 20 nm as confirmed by TEM measurements. The selected area electron diffraction pattern composed of diffused ring clearly depicts the amorphous nature of boron powder.     

Anomalous phase formation during annealing of La2O3 thin films deposited by ion beam assisted electron beam evaporation

The fifty seven nm thick La₂O₃ thin films were deposited on Si (100) substrates. After deposition, the amorphones thin films, were amorphous, were annealed at 750 and 900 °C for 1 h. It was found that their amorphous structure had been crystallized to hexagonal and cubic structures, respectively. The phase formation of the La₂O₃ thin films was anomalous at higher annealing temperatures. The theory of heterogeneous nucleation was used to interpret the anomalous phase formation of La₂O₃ films. To investigate the effects of the phase structure on these properties, Refractive indexes and dielectric constants of different structures of La₂O₃ films were measured.     

Crystallization kinetics and structural properties of nanocrystalline europium-yttrium-titanate (Eu0.5Y0.5)2Ti2O7

We present a versatile sol–gel approach for nanocrystalline (Eu_0.5Y_0.5)₂Ti₂O₇. We determined the crystallization kinetics of the nucleation and the nucleation mechanism. The crystallization temperature was 1050.1 ± 0.8 K, and the activation energy of crystallization was 605 kJ mol^?1. The nanocrystal growth started by homogenous nucleation with a constant nucleation rate, and the nanocrystal growth was limited by mass transfer through the phase boundary. The crystal structure of (Eu_0.5Y_0.5)₂Ti₂O₇ was refined from the powder diffraction data using the Rietveld method, and the results were compared with the data recorded for the isostructural compounds, Eu₂Ti₂O₇ and Y₂Ti₂O₇. We proved the existence of a single phase of (Eu_0.5Y_0.5)₂Ti₂O₇ and the regular distribution of Eu³⁺ and Y³⁺ ions inside the crystal lattice. The results provide key information regarding the crystallization properties and crystal structure of nanocrystalline (Eu_0.5Y_0.5)₂Ti₂O₇. This knowledge is necessary for preparing pure nanocrystalline powders with tailored structural properties that are suitable for photonic applications.     

Influence of alumina content on the nucleation crystallization and microstructure of barium fluorphlogopite glass-ceramics based on 8SiO2·YAl2O34MgO2MgF2BaO Part I Nucleation and crystallization behaviour

The effect of varying the aluminium oxide content on the nucleation and crystallization behaviour of barium containing glasses based on 8SiO₂·YAl₂O₃4MgO2MgF₂BaO was investigated in order to develop novel, high strength, machinable glass-ceramics. Nine glasses were synthesized and characterized by Differential Scanning Calorimetry (DSC) Combined Differential Thermal Analysis Thermal Gravimetric Analysis (DTA/TGA), X-ray diffraction (XRD) and dilatometry. The glass transition temperature (T _g) and first peak crystallization temperature (Tp1) reduced with reducing alumina content. Glasses with Y > 1.5 exhibited a second peak crystallization temperature (Tp2). Tp1 was shown to correspond to the crystallization of barium fluorphlogopite (BaSi₆Al₂Mg₆F₂O₁₉) and Tp2 to the crystallization of cordierite (Mg₂Al₂Si₅O₁₈). The thermal expansion coefficient (TEC) was insensitive to alumina content. All the glasses exhibited an optimum nucleation temperature just above T _g, which was thought to be a result of amorphous phase separation (APS). DTA/TGA showed the glasses to undergo weight loss corresponding to silicon tetrafluoride volatilization from the surface, which resulted in a fluorphlogopite deficient surface layer.     

Crystallization behavior of Ti50Ni25Cu25 amorphous alloy

Crystallization behavior of the Ti₅₀Ni₂₅Cu₂₅ alloy was studied by means of scanning and isothermal differential calorimetry, X-ray diffraction, conventional and high-resolution transmission electron microscopy. A single stage polymorphic-type transformation of the amorphous phase forming a Ti₂CuNi crystalline phase was observed. The activation energy for such a single stage crystallization of the amorphous phase was determined by Kissinger analysis. Kinetics of the crystallization was analyzed on the basis of Johnson-Mehl-Avrami equation and discussed regarding to the value of Avrami exponent obtained. Relatively high average value of Avrami exponent of 5.5 at the range from 702 to 709 K suggests nucleation with an increasing nucleation rate.     

Nucleation behavior and microstructure of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-poor LAS glass–ceramics

Influence of MgO and K₂O on the nucleation behavior of Al₂O₃-poor LAS (Li₂O–Al₂O₃–SiO₂) base glasses was investigated by thermal analysis and, the effect on microstructure and surface topography of glass–ceramics was also examined by SEM, AFM and TEM. According to results of thermal analysis, the exothermic peak temperature of the glass showed a decrease with increase of nucleation temperature to nucleation time of 6 h. But some glasses nucleated for 9 h showed nucleation rate-like curve with maximum point. The dependence of reciprocal value of the exothermic peak temperature on the nucleation temperature indicated that an introduction of MgO might accelerate the nucleation of the base glass and thus result in rough surface topography of glass–ceramics. On the other hand, in the case of glass–ceramics containing K₂O the main crystalline phase was lithium metasilicate and they showed fine microstructure resulting in smooth surface topography. TEM micrographs of as-quenched and nucleated glasses showed no trace of phase separation affecting nucleation or final microstructure.     

The crystallisation of glasses based on eutectic compositions in the system Li2O-Al2O3-SiO2

Structural transformations have been studied in glasses related in composition to the binary eutectic between lithium metasilicate and -spodumene. Crystallisation processes and changes in microstructure during the controlled heating of the glasses have been followed using X-ray diffraction, electron microscopy, high temperature microscopy, thermal analysis and electron spin resonance spectroscopy.The influence exerted by titanium dioxide on the phase relationships, crystal growth rates and micromorphology of the polycrystalline products of heat-treatment has been investigated and the findings used as a basis for proposals on the rôle of TiO₂ during nucleation and crystal growth.     

Initial nucleation of amorphous Si–B–C–N ceramics derived from polymer-precursors

Nucleation behavior of amorphous Si–B–C–N ceramics derived from boron-modified polyvinylsilazane procusors was systematically investigated by transmission electron microscopy(TEM) combined with spatially-resolved electron energy-loss spectroscopy(EELS) analysis. The ceramics were pyrolyzed at1000?C followed by further annealing in N2, and SiC nano-crystallites start to emerge at 1200?C and dominate at 1500?C. Observed by high-angle annular dark-field imaging, bright and dark clusters were revealed as universal nano-structured features in ceramic matrices before and after nucleation, and the growth of cluster size saturated before reaching 5 nm at 1400?C. EELS analysis demonstrated the gradual development of bonding structures successively into SiC, graphetic BNCxand Si_3N_4 phases, as well as a constant presence of unexpected oxygen in the matrices. Furthermore, EELS profiling revealed the bright SiC clusters and less bright Si_3N_4-like clusters at 1200–1400?C. Since the amorphous matrix has already phase separated into SiCN and carbon clusters, another phase separation of SiCN into SiC and Si_3N_4-like clusters might occur by annealing to accompany their nucleation and growth, albeit one crystallized and another remained in amorphous structure. Hinderance of the cluster growth and further crystallization was owing to the formation of BNCxlayers that developed between SiC and Si_3N_4-like clusters as well as from the excessive oxygen to form the stable SiO_2.     

Crystallization process in Ge2Sb2Te5 amorphous films

The aim of this work is to investigate the isokinetic and isothermal amorphous-to-crystalline phase transformation process in Ge₂Sb₂Te₅ ternary alloys. The experiments were carried out using electrical impedance, X-ray diffraction and reflection measurements. The results have shown that, upon annealing, the crystallization process in amorphous Ge₂Sb₂Te₅ films starts with nuclei which were identified as the Ge₁Sb₄Te₇ crystalline phase. As temperature increases (or time of isothermal annealing) these nuclei are transformed into the fcc-Ge₂Sb₂Te₅ phase. In order to establish the mechanism of crystallization for this system, a stochastic lattice model was implemented to analyze nucleation and growth of the two phases involved (i.e., the metastable Ge₁Sb₄Te₇ nuclei followed by the stable fcc-Ge₂Sb₂Te₅). The results of the simulations demonstrate close agreement with experimental results. Furthermore, the crystallization process in amorphous films with the Ge₁Sb₄Te₇ composition shows the existence of only one phase during the whole process and can be described by the classical Johnson-Mehl-Avrami-Kolmogorov model.     

Effect of ZrO2 and SiO2 on glass forming ability of Zr57Cu20Al10Ni8Ti5 alloy

Zr₅₇Cu₂₀Al₁₀Ni₈Ti₅ has very high glass forming ability (GFA) due to its low crystal growth rate which limits development of crystallites arising from container walls or nuclei. Effect of ZrO₂ and SiO₂ on GFA was studied by investigating the solidification microstructures of alloys containing different amount of the oxides. Amorphous ingots were formed by melting and freezing alloys without addition of the oxides using an arc furnace. Partially amorphous ingots were formed for addition of the oxides up to 0.2 wt %. Amorphous forming conditions of alloys containing greater concentration of the oxides were studied using a drop tube. GFA decreases with increasing concentration of the oxides. However, superheating of melts eliminates some effects of the oxides. Different types of oxides leads to different microstructures due to their different heterogeneous nucleation rates and nucleation temperature.     

Phase transitions of liquid crystalline polyesters: Poly(heptane-1,7-diyl-4,4′-biphenyldicarboxylate)

Wide-angle X-ray scattering was applied to investigate the non-isothermal and isothermal phase transitions of poly(heptane-1,7-dyil-4,4′-biphenyldicarboxylate). It is suggested that the isotropic–smectic (I–S) transition is controled by a nanophase separation with a critical point T_ns=151 °C. Nanophase separation is followed by a first-order S–Cr (crystal) transition. At isothermal conditions above T_ns only S phase exists, whereas below it the S phase appears first and the Cr phase grows from the S by nucleation and three-dimensional crystal growth.