Differences In Crystallization Behavior Research Articles

A novel polymer-derived high-temperature SiC/SiAlCN nanocrystalline glass ceramic

A polymer alloy (CA) composed of polycarbosilane (PCS) and polyaluminasilazane (PASZ) was used as precursor for polymer-derived ceramics. Due to the difference in crystallization behavior between PCS-derived and PASZ-derived ceramics, a novel non-oxide SiC/SiAlCN nanocrystalline glass ceramic was successfully fabricated via in situ crystallization of the PCS domain in CA. Ceramic yield of the CA, which was up to 89.9% at 1000°C, was very high because of interaction between PCS and PASZ during pyrolysis. Micro-morphology of the novel glass ceramic indicated that nanocrystalline SiC was uniformly dispersed in an amorphous phase. The SiC/SiAlCN nanocrystalline glass ceramic may be a high-performance potential material for use in high temperature environments.

Development of A Manufacturing Process for An Organicinorganic Nanocomposite and Its Physical Property Characterization

The nonisothermal crystallization kinetics of a polyolefin terpolymer (poly(propylene-co-ethylene-co-1butene) and its expanded foam were investigated by differential scanning calorimetry. A kinetic equation for nonisothermal crystallization was employed to analyze the crystallization characteristics of the terpolymer and its foam. The Avramic exponent, n, can be reasonably well determined from the nonisothermal crystallization exotherm. The polarized optical microscopy showed that pristine terpolymer had a well developed spherulite morphology whereas the foamed terpolymer consisted of elongated entities that subsequently developed as more bundle-like entities. The latter morphology is typical one of the γ-phase. The difference in crystallization behavior observed for the pristine and foamed terpolymer samples is attributed to the formation of different morphologies during the foam expansion process. We also studied application of a foaming process to prepare an inorganic-organic nanocomposite. The foaming process enabled better dispersion of clay particles as well as their intercalation and exfoliation in the polypropylene matrix.

Hierarchical Orientational Relaxation Inducing Shish-Kebab Formations in Polymer Melt Crystallization

We studied athermal relaxation of bulk extended chains by means of dynamic Monte Carlo simulations, and we got intermediately relaxed melts with a memory of chain orientations but no more crystalline order. The orientational memory in the melts dominated the crystal orientation and nucleation types. The difference in crystallization behaviors induced by orientational relaxation suggested the mechanism of hierarchical crystallization. Thus, we studied the isothermal crystallization of a binary blend of different relaxed chains. We observed the prior crystallization of less relaxed chains could act as a shish to induce epitaxial crystallization of more relaxed chains to form kebabs. This mechanism had demonstrated the structure of precursors and gave insight into the formation of shish-kebab crystals in polymer melts. The results suggested that in flow-induced polymer crystallization the hierarchical orientational relaxation of chains decided the formation of shish-kebabs.

Glass forming ability and stability: Ternary Cu bearing Ti, Zr, Hf alloys

Four Cu bearing alloys of nominal composition Zr 25Ti 25Cu 50, Zr 34Ti 16Cu 50, Zr 25Hf 25Cu 50 and Ti 25Hf 25Cu 50 have been rapidly solidified in order to produce ribbons. All the alloys become amorphous after melt-spinning. In the Zr 34Ti 16Cu 50 alloy localized precipitation of cF24 Cu 5Zr phase can be observed in the amorphous matrix. The alloys show a tendency of phase separation at the initial stages of crystallization. The difference in crystallization behavior of these alloys with Ni bearing ternary alloys can be explained by atomic size, binary heat of mixing and Mendeleev number. It has been observed that both Laves and Anti-Laves phase forming compositions are suitable for glass formation. The structures of the phases, precipitated during rapid solidification and crystallization can be viewed in terms of Bernal deltahedra and Frank–Kasper polyhedra.

Effect of Geometrical Confinement on the Nucleation and Crystallization Behavior of n-Alkane Mixtures

The condensed structure of normal alkane (n-alkane) mixtures in confined geometry is an interesting topic concerning the difference in crystallization behavior of odd and even alkanes. In the present work, the crystallization of mixtures of normal octadecane (n-C18H38) and normal nonadecane (n-C19H40) in microcapsules with narrow size distribution was investigated using the combination of differential scanning calorimetry (DSC) and X-ray diffraction (XRD). A surface freezing monolayer for microencapsulated n-C18H38, n-C19H40, and their mixture was detected by DSC, which for the mixture is a mixed homogeneous crystalline phase with continuous change in the composition. A more stable rotator phase (RI) was observed for the microencapsulated n-C18H38/n-C19H40 = 95/5 (molar ratio) mixture, confirmed by an increased supercooling of the transition from RI to stable phase compared to that of the mixture in bulk. Two nucleation mechanisms were speculated as "liquid-to-solid" heterogeneous nucleation and "solid-to-solid" homogeneous nucleation, which occur at different crystallization stages in microcapsules and might be attributed to the surface effect and confinement effect, respectively, in the confined geometry.

Nonisothermal crystallization behaviors of a polyolefin terpolymer and its foam

The nonisothermal crystallization kinetics of a polyolefin terpolymer (poly(propylene- co-ethylene- co-1-butene) and its expanded foam was investigated by differential scanning calorimetry. A kinetic equation for nonisothermal crystallization was employed to analyze the crystallization characteristics of the terpolymer and its foam. The Avrami exponent, n, can be reasonably well determined from the nonisothermal crystallization exotherm. The polarized optical microscopy showed that pristine terpolymer had a well-developed spherulite morphology whereas the foamed terpolymer consisted of elongated entities that subsequently developed as more bundle-like entities. The latter morphology is typical one of the γ phase. The difference in crystallization behavior observed for the pristine and foamed terpolymer samples is attributed to the formation of different morphologies during the foam expansion process.

Effect of solid‐state shearing of poly(ethylene terephtalate) on isothermal crystallization and fiber structure formation

AbstractSolid‐state shearing was applied on recycled poly(ethylene terephtalate) (PET) using twin‐screw extruder. The shearing effect on isothermal crystallization was investigated by DSC. Structure formation in melt spinning was also studied by characterizing the spun fibers. It was found that the solid‐state shearing imposes a great influence on its crystallization behavior. Crystallization peak temperature of the sheared PET (199°C) was much higher than that of the PET without shearing (188°C). Overall isothermal crystallization rate was found to be accelerated to more than 50 times by the shearing. The shearing effect remained even after fiber spinning (melt processing). Differences in crystallization behavior, fiber structure formation, and tensile properties of the fibers were found. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 787–791, 2007

Strain Rate Induced Crystallization in Bulk Metallic Glass-Forming Liquid

We report on the solidification of Au49, Ag5.5, Pd2.3, Cu26.9, Si16.3 bulk metallic glass under various strain rates. Using a copper mold casting technique with a low strain rate during solidification, this alloy is capable of forming glassy rods of at least 5 mm in diameter. Surprisingly, when the liquid alloy is splat cooled at much higher cooling rates and large strain rates, the solidified alloy is no longer fully amorphous. Our finding suggests that the large strain rate during splat cooling induces crystallization. The pronounced difference in crystallization behavior cannot be explained by the previously observed strain rate effect on viscosity alone. A strain rate induced phase separation process is suggested as one of the explanations for this crystallization behavior. The strain-rate-dependent critical cooling rate must be considered in order to assess the intrinsic glass forming ability of metallic liquid.

Phase Transformation in Fe81.0Zr9.0B10.0 Metallic Glass during Thermal Annealing and Electron Irradiation

Solid-state phase transitions in Fe 81 Zr 9 B 10 alloy were investigated focusing on crystallization and amorphization introduced by thermal process and mechanical process of electron irradiation. Melt-spun amorphous phase can not maintain their original structure under thermal heated state at 828 K and electron irradiation at 103 and 298 K, crystallization of this phase occurred. There was a great difference in crystallization behavior between thermal annealing and electron irradiation. Metastable α-Mn type crystalline phase precipitated through thermal crystallization, while nano-crystalline b.c.c. solid solution was formed through electron irradiation induced crystallization. The α-Mn type crystalline phase underwent solid-state amorphization by electron irradiation at 103 and 298 K. Both amorphization and crystallization were observed in Fe 81 Zr 9 B 10 alloy under electron irradiation in spite of same irradiation conditions, resulting in the occurrence of crystal-to-amorphous-to-crystal (C-A-C) transition.

Study of the crystallization behavior of Ag-In-Sb-Te phase change optical recording film

Recently the demand for high-speed and high-density optical recording media using a direct overwrite scheme is very high. Among some of the potential candidates, the AgInSbTe alloy appears to be one of the promising materials that has drawn worldwide attention. It can give direct overwrite capability within a short period of time and is reported to give a well-defined shape with sharp edges, leading to intrinsically lower jitter values, thereby increasing the linear density. Hence, considerable interest has been focused on the study of the crystallization behavior of AgInSbTe alloy. Different crystalline phases are observed due to thermal annealing of AgInSbTe four-element alloy films. Results of the x-ray diffraction analysis of amorphous and crystalline (AgSbTe) x (In 1–y Sb y ) 1–x films with x = 0.2, 0.4 and y = 0.7, deposited by thermal evaporation technique are presented. The difference in crystallization behavior of the crystalline phases formed after 1 h of isothermal annealing at temperature between 200 and 400°C are studied through x-ray diffraction analysis. The experimental results are presented for a composition close to eutectic Sb 69 Te 31 in which some of the Te is replaced by Ag and In. The results indicate that the growth of the crystalline phases depends on the annealing temperature and it is also affected by the change in the composition.

Compositional Effects on Milk Fat Crystallization

Seasonal and regional variability in milk fat composition causes differences in crystallization behavior, which can, for example, result in variability in fractionation efficiency and physical properties of butter. However, the specific compositional causes of variability in milk fat crystallization behavior are still only poorly understood. In this work, the seasonal and regional variations in composition of anhydrous milk fat were analyzed and related to crystallization behavior. Although there were no clear-cut trends in chemical composition (triacylglycerol, fatty acid, and minor lipid profiles) among the milk fats, significant differences in crystallization behavior were observed. For anhydrous milk fat samples made from sweet cream and obtained from the same milk supply through a period of 14 mo, no significant trends in either composition or crystallization behavior were observed. This suggests that seasonal variability of milk fat may be reduced by current feeding strategies in the United States. More detailed examination of the triacylglycerol profiles led to the conclusion that the relative contents of certain triacylglycerol combinations correlated well with crystallization behavior. In particular, the ratio of higher-melting to lower-melting triacylglycerols could be used to predict crystallization behavior. Higher ratios of higher-melting triacylglycerols led to higher crystallization rate.

Transmission Electron Microscopy and Electron Energy‐Loss Spectroscopy Study of Nonstoichiometric Silicon‐Carbon‐Oxygen Glasses

Crystallization behavior of Si‐C‐O glasses in the temperature range of 1000°–1400°C was investigated using transmission electron microscopy (TEM) in conjunction with electron energy‐loss spectroscopy (EELS). Si‐C‐O glasses were prepared by pyrolysis of polysiloxane networks obtained from homogeneous mixtures of triethoxysilane, TH, and methyldiethoxysilane, DH. Si‐C‐O glass composition depended on the molar ratio of the precursors utilized. At a ratio of TH/DH= 1, the formation of a carbon‐rich glass was observed, whereas a ratio of TH/DH= 9 yielded a Si‐C‐O glass with excess free silicon. Both materials were amorphous at 1000°C, but showed a distinct difference in crystallization behavior on annealing at high temperature. Although TH/DH= 1 revealed a small volume fraction of SiC precipitates in addition to a very small amount of residual free carbon at 1400°C, TH/DH= 9 showed, in addition to SiC crystallites, numerous larger silicon precipitates (20–50 nm), even at 1200°C. Both materials underwent a phase separation process, SiCxO2(1‐x)→xSiC + (1 ‐ x)SiO2, when annealed at temperatures exceeding 1200°C.

Miscibility and Crystallization of an Amine-Cured Epoxy Resin Modified with Crystalline Poly(ɛ-caprolactone)

Blends of diglicidyl ether of bisphenol-A, DGEBA, and poly(ε-caprolactone), PCL, were cured with 4,4′-diaminodiphenyl sulfone, DDS, and analyzed by differential scanning calorimetry, DSC, in order to study the effect of curing on the miscibility and crystallization behavior of the crystallizable component. The spherulitic morphology of PCL was investigated by thermal microscopy. The results indicate a change in the miscibility state resulting from curing. Likewise, at all the temperatures studied a marked difference in crystallization behavior and morphology between the uncured and the cured systems was observed. These differences are attributed to the multiphase nature of the cured blends versus the homogeneous character of the uncured ones.

Miscibility and Crystallization of an Amine-Cured Epoxy Resin Modified with Crystalline Poly(-caprolactone)

Blends of diglicidyl ether of bisphenol-A, DGEBA, and poly(ε-caprolactone), PCL, were cured with 4,4′-diaminodiphenyl sulfone, DDS, and analyzed by differential scanning calorimetry, DSC, in order to study the effect of curing on the miscibility and crystallization behavior of the crystallizable component. The spherulitic morphology of PCL was investigated by thermal microscopy. The results indicate a change in the miscibility state resulting from curing. Likewise, at all the temperatures studied a marked difference in crystallization behavior and morphology between the uncured and the cured systems was observed. These differences are attributed to the multiphase nature of the cured blends versus the homogeneous character of the uncured ones.

Photoelectrochemical Properties of the Sol-Gel Derived Ti1-xVxO2 Thin Film Electrodes.

The origin of the photoresponse in the visible region observed for the sol-gel derived Ti1-xVxO2 thin films has been studied in relation to the doping level, x, which is ranging from 0.03 to 0.12. Effects of the postheating atmosphere and the substrate nature on the crystallization behavior and photoelectrochemical properties of the films were examined. X-ray diffraction analysis revealed that thin films with x≤0.06 consisted only of anatase type TiO2 phase regardless of the post-heating atmosphere and substrate nature, whereas for the samples with x≥0.07, VO2 or V2O3 phases have been detected in addition to anatase phase depending on the post-heating atmosphere. Differences in photoelectrochemical characteristics between the thin film electrodes with x≤0.06 and x≥0.07 and the origin of the visible photoresponse are discussed on the basis of the difference in crystallization behavior of the Ti1-xVxO2 electrodes.

Characterization of Amorphous Phases of Ge2Sb2Te5 Phase-Change Optical Recording Material on Their Crystallization Behavior

The difference in crystallization behavior between the as-deposited amorphous state and the melt-quenched amorphous state of Ge2Sb2Te5 alloy has been studied using a dynamic tester. The melt-quenched amorphous alloy showed much shorter crystallization time than did the as-deposited one. We laser-annealed the as-deposited amorphous alloy under a certain condition so as to make it exhibit the same crystallization property as that of the melt-quenched one. The crystallization kinetics of this laser-annealed amorphous alloy and the as-deposited amorphous alloy has been explained by applying transmission electron microscopy (TEM) and continuously heating the specimens in a differential scanning calorimeter (DSC). From the TEM results, it has been shown that preexisting atomic clusters accelerate the crystallization speed of the amorphous alloy. The two states have been found to have different crystallization temperatures and activation energies.

Differences in crystallization behavior between quenched and ground amorphous ursodeoxycholic acid.

To study the crystallization of ground and quenched ursodeoxycholic acid (UDCA) and to characterize their amorphous states. Amorphous UDCA was prepared by grinding and also by rapid cooling of the melt. These samples were characterized by powder X-ray diffraction (XRD), near IR spectra and dynamic water sorption. The heat associated with crystallization was measured in an isothermal microcalorimeter at 25 degrees C at various relative humidities (RH) (50%-100%) and, in the presence of the vapour from a mixed solvent of ethanol and water (ethanol conc. 10%-100%). The specific surface area was calculated from krypton adsorption. Contact angles were measured by using a Wilhelmy plate to calculate the surface energy of the samples. Ground and quenched samples yielded amorphous XRD patterns. Differential scanning calorimetry thermographs of the milled sample revealed that crystallization occurred at around 80 degrees C, whereas the quenched sample did not crystallize. Exposure to humid air did not result in crystallization of either amorphous sample during the microcalorimetric experiments. In the presence of ethanol vapour, the ground sample did, but the quenched sample did not, crystallize. The amount of water sorption into the quenched sample was larger than that of the ground sample at low RH. The surface energy of the quenched material was different to that of the ground. Peak shifts were observed in the NIR spectra at around 1450, 2100 nm, allowing differentiation between the ground and quenched samples. It can be concluded that different molecular states of amorphous UDCA were obtained depending on the preparation method. The crystallisation of amorphous UDCA was related to the molecular state of disorder.

Short-Range Order and Nanocrystallization in Amorphous Zr-Ti-Cu-Ni-Al

AbstractWe have examined the effect of Ti content on the crystallization kinetics of Zr-based amorphous alloys. In Zr59Ti3Cu20Ni8Al10, annealing above the glass transition temperature produces 50-100 nm crystalline precipitates, as seen in transmission electron microscope images. In contrast, TEM images and diffraction patterns from annealed Zr54.5Ti7.5Cu20Ni8Al10 show no evidence of crystalline phase formation. Structural changes upon annealing do occur in this alloy, however, as revealed by field ion microscopy. The effect of Ti is to favor the formation of clusters of short-range order; this tendency for clustering is apparently the cause of difference in crystallization behavior between the two alloys.

Crystallization of Whitlockite from a Glass in the System CaOP2O5SiO2MgO

The kinetics of crystallization of a glass with composition 30SiO2·24.14P2 O5·28.61CaO·17.25MgO (wt%) was studied with variable temperature, by differential thermal analysis. These results were interpreted according to revised theoretical models which describe the dependence of fraction crystallized on temperature and on the heating rate, and the dependence of the peak temperature on the rate of change in temperature. These models emphasize the differences in crystallization behavior between “as-prepared” glass samples, and the corresponding previously nucleated samples. These differences in behavior suggest that the kinetics of crystallization comprises a nucleation stage at temperatures of about 730°C, and particle growth for temperatures higher than 800°C. The temperature dependence of the fraction crystallized corresponds to a very high value of activation energy for growth (≈763 kJ/mol), but this is still lower than the activation energy evaluated from the temperature dependence of viscosity.

A study of PBT/PC blends by modulated DSC and conventional DSC

Two poly(butylene terephthalate)/polycarbonate (PBT/PC) blends with different formulations were analyzed by modulated DSC (MDSC) and conventional DSC to determine differences in crystallization behavior. A significant difference (30°C in cold crystallization temperature) between the two samples was detectable by MDSC while no significant difference was seen by conventional DSC. That indicatesthe total heat flow from MDSC is not always equivalent to the heat flow from conventional DSC as we have assumed or seen before. The reason has not been fully understood, but may be related to unusual nucleation and crystallization induced by modulation. Alternative conventional DSC methods were developed and compared to the MDSC results.