Study Of Crystallization Kinetics Research Articles

Crystal G phase induced through intermolecular hydrogen bonding IV: a study of crystallization kinetics

A systematic crystallization kinetic study using thermal microscopy and differential scanning calorimetry has been carried out on two novel liquid crystalline compounds, DBA:MHB and DBA:ACP. These involve intermolecular hydrogen bonding between 4-n-decyloxybenzoic acid (DBA) and methyl 4-hydroxybenzoate (MHB); and between DBA and 2-amino-5-chloropyridine (ACP). The kinetics experiments were performed from the crystal G phase, which is a common induced kinetophase in both the compounds. Further, the proton donor and acceptor capabilities of the -COOH group of DBA towards the -OH group of MHB and -N atom of ACP were studied in the light of mesomorphism and rate of crystallization. The dimensionality in the crystal growth and the sporadic nucleation were estimated from the Avrami exponent, n. A similar type of crystallization mechanism is predicted to operate for all the crystallization temperatures. The characteristic crystallization time (t*) at each crystallization temperature is deduced from the individual plots of log t vs. ΔH (change in enthalpy).

00/02887 Kinetic studies of crystallization in Ca 1−xMg x-(PO 3) 2

00/02887 Kinetic studies of crystallization in Ca 1−xMg x-(PO 3) 2

Kinetic analysis of crystallization processes in amorphous materials

Thermal analysis methods are widely used to study crystallization kinetics in amorphous solids. The experimental data is frequently interpreted in terms of the Johnson–Mehl–Avrami (JMA) nucleation-growth model. This paper discusses the limits of such an approach. A simple and convenient method is proposed to verify the applicability of the JMA model as well as the basic assumptions of kinetic analysis. It is shown that the two parameter autocatalytic model includes the JMA model and that is a plausible description of the crystallization kinetics. The main advantage of this model is its flexibility in describing quantitatively the kinetics of complex crystallization processes. The experimental data for crystallization of a chalcogenide glass and zirconia gel analyzed in this paper clearly demonstrate the rather complex nature of these processes. As a consequence, it is very difficult to explore the real mechanism of the crystallization unless some complementary studies are made.

Synthesis and characterization of glass-ceramic Bi2-xGaxSr2Ca2Cu3O10+y superconductors

We have melt quenched a series of Ga substitutedBi2-xGaxSr2Ca2Cu3O10+y samples with the compositionswith x = 0.2, 0.4, 0.6, 0.8 and 1.0. Microstructure, thermal propertiesincluding crystallization activation energy, magnetic and superconductingproperties were investigated. We have found that Ga3+ ions have a solidsolubility limit in the BSCCO system and this limit was passed at x = 0.6.Below this limit HTc BSCCO n = 3 phase was obtained, but after the solidsolubility limit samples yield multiphase superconducting properties. Extendedheat treatment at 840°C displayed no suppression of the solidsolubility limit. The crystallization kinetic studies were investigated usingnon-isothermal kinetic models of Kissinger and Augis-Bennet. Highercrystallization activation energies, Ea, were obtained compared tounsubstituted BSCCO materials, glassification improved and glass handling timeincreased. The substitution had a slight effect on Jc, the flux pinning inthe material was improved and better Jc values were obtained below thesolid solubility limit.

Crystallization kinetics study on N-(p-n-alkyloxybenzylidene)-p-n-alkylanilines (nO.m compounds)

A comparative systematic kinetic study of crystallization among various smectogens of higher homologues of the benzylidene aniline nO.m series (9O.2, 13O.2, 14O.2, 15O.2, 16O.2, 2O.16, 11O.16, 16O.16 and 18O.16) has been carried out by thermal microscopy and differential scanning calorimetry (DSC). The DSC thermograms were run from crystallization temperature to the isotropic melt for different time interval. The liquid crystalline behaviour together with rate of crystallization of smectic ordering in newly synthesized nO.m compounds (16O.2, 2O.16, 11O.16, 16O.16, 18O.16) with respect to their lower homologues are discussed in relation to the kinetophase (which occurs prior to the crystallization), end chain lengths, the odd-even effect and chain length ratio. The molecular mechanism and dimensionality in the crystal growth were computed from the Avrami equation. The characteristic crystallization time (t*) at each crystallization temperature was deduced from the individual plots of log t vs. DeltaH. Further, a qualitative approach was made to the crystallization process in smectic layers.

Quantitation of crystallinity in substantially amorphous pharmaceuticals and study of crystallization kinetics by X-ray powder diffractometry

The first object was to develop an X-ray diffractometric method for the detection and quantification of crystalline sucrose when it occurs as a mixture with amorphous sucrose. Standards consisting of amorphous sucrose physically mixed with 1 to 5 weight percent crystalline sucrose were prepared. The sum of the background subtracted integrated intensities of the 12.7°2θ (6.94 Å) and 13.1°2θ (6.73 Å) sucrose diffraction peaks were linearly related to the weight percent crystalline sucrose. The limits of detection and quantitation of crystalline sucrose were 0.9% and 1.8% w/w, respectively. The second object was to study the kinetics of crystallization of sucrose as a function of temperature (at 102, 105 and 110 °C under a water vapor pressure of 0 Torr) and water vapor pressure (17.4, 19.8 and 21.4 Torr at 27 °C). In all cases, the crystallization kinetics was best described by the Avrami-Erofe’ev model (three-dimensional nucleation).

Study of crystallization kinetics by temperature-modulated DSC

A method of temperature-modulated differential scanning calorimetry (TMDSC), proposed by Toda et al., is used to study the crystallization kinetics of a polyester-imid, namely poly(4,4′-phthaloimidobenzoyldodecamethyleneoxycarbonyl). The polymer shows one irreversible melting/crystallization transition and one smaller, partially reversible transition. The experimental results prove that with this method one can obtain an important parameter that characterizes polymer crystallization via the temperature-dependence of the crystal-growth rate. The TMDSC data consist of the total heat flow, the magnitude of the apparent heat capacity, and the change of the phase lag of the heat flow in the transition region. No specific assumptions are made about the crystallization kinetics.

Internal Friction and Elastic Modulus Change Associated with Structural Relaxation and Crystallization in Amorphous Mg-Ni-Y Alloys.

AbstractMechanical spectroscopy technique has been applied to investigation of structural relaxation and crystallization of Mg-Ni-Y amorphous alloys of different composition. The composition of the alloys was varied due to change of yttrium concentration from 0 to 7 at.%. The internal friction (IF) and Young's modulus have been measured by a vibrating reed method at frequency of ∼200 Hz at heating and cooling runs in the temperature range from 300K to 650K. Crystallization kinetic studies of the alloys by DSC and XRD showed that the transition from amorphous to crystalline state in the Mg-Ni-Y alloys has gone through several stages with origination of intermediate phase. IF spectra and Young's modulus of the alloys have been analyzed and the effect of yttrium content on the stability of intermediate phase was investigated.

Study of electrical properties, crystallization kinetics and structure of the glassy Cu 35As 20Te 45 semiconductor system

Preparation of glassy arsenic telluride with high Cu content has been carried out. The prepared Cu 35As 20Te 45 glassy semiconducting system shows many interesting characteristics. It has a low melting point (325°C), high density (6.2 gm cm −3) and low resistivity (2.86 Ω cm at room temperature). Studies of the crystallization kinetics using the non-isothermal single scan technique shows that random nuclei are already present in the as-quenched alloy and that the growth occurs in three dimensions. Thermal annealing of the alloy at 160°C gives crystalline material of large grains (∼150 μm) embedded in less crystalline matrix. Some of the phases at which the alloy crystallizes have been identified by X-ray diffraction.

Polymorphism in Polyamide of Dytek®-A and Dodecanedioic Acid

Our X-ray work of Dytek®-A, 2-methyl-pentamethylenediamine, containing polyamides shows polymorphism, whereas the polyamides with linear diamines do not. The polyamide of Dytek®-A and dodecanedioic acid, MPMD-12, is singled out for discussion and compared with the unbranched analogs of polyamides 6,12 and 5,12. Due to the presence of the -CH 3 side group in the 2-position of the diamine, the polyamide MPMD-12 exhibits two stable crystal conformations. The new δ polymorph is not seen in linear polyamides 6,12 and 5,12. Studies by DSC polyamide MPMD-12 clearly illustrates at least two crystal forms, γ and δ, coexisting over a wide temperature range, and the isolation of each phase is possible by controlling temperature and time. The DMA modulus in the temperature region between the glass transition (or alpha relaxation) and melting transition shows strong dependence on the thermal history as demonstated in a study of crystallization kinetics.

Non-isothermal Crystallization of UHMWPE

Processing of Ultra High Molecular Weight Polyethylene (UHMWPE) parts involves non-isothermal cooling leading to crystallinity variations, which cause variations in the mechanical properties. Study of non-isothermal crystallization kinetics of UHMWPE forms the basis for process modelling. The crystallization of UHMWPE was studied at seven different cooling rates. The crystallization onset and peak temperatures were linearly related to the cooling rate. The crystallization of UHMWPE was concluded to be a nucleation dominated process with small contribution from growth of nuclei. Differences in ultimate crystallinity (≈11%) were produced due to different cooling rates. A significant portion of the change in ultimate crystallinity occurred at lower cooling rates (<6°C min−1). At higher cooling rates (6–22°C min−1) the change in ultimate crystallinity was insignificant.

Estudo cinético da cristalização dinâmica de um poliéster líquido-cristalino termotrópico com espaçadores flexíveis na cadeia principal

Foi realizado o estudo cinético da cristalização dinâmica de um poliéster líquido-cristalino termotrópico com espaçadores flexíveis na cadeia principal. O objetivo deste estudo foi verificar o mecanismo da cinética de cristalização do poliéster termotrópico semi-flexível através de um método não-isotérmico. O método empregado (método de Ozawa) permitiu descrever a cinética de cristalização dinâmica do poliéster e os valores de n (expoente de Avrami) encontrados correspondem a um crescimento de cristais em forma de feixe sem crescimento esferulítico posterior.

Crystallization and phase behavior in nylon 6/aromatic polyimide triblock copolymers

Several series of new b-nylon 6-b-polyimide-b-nylon 6 triblock copolymers were synthesized via a condensation polymerization of polyimide segments and anionic polymerization of the nylon 6 components. Crystallization and phase behavior of these triblock copolymers have been studied with varying polyimide block molecular weights and copolymer compositions as well as chemical structures (and therefore, different solubility parameters) of the polyimide components. Furthermore, in-situ and solution blends have also been prepared in order to compare the phase behaviors of the triblock copolymers. Wide angle X-ray diffraction experiments indicate that in these triblock copolymers, only nylon 6 components crystallize and polyimides do not take part in the crystals. From isothermal crystallization kinetics studies carried out in differential scanning calorimetry experiments, the miscibility of the polyimide components with nylon 6 critically affects the crystallization rate of these triblock copolymers. When the polyimide composition is low and its solubility parameter is similar to that of the nylon 6 components, the polyimides mix with the amorphous part of the nylon 6 components and are located in the inter-lamellar region. This can be identified by monitoring the change of α-relaxation temperature and activation energy measured using a dynamic mechanical method and by examining the crystalline lamellar morphology under small angle X-ray scattering experiments. Microphase separation can also be recognized by transmission electron microscopy using an OsO4 vapor staining technique. As the difference of solubility parameters between these two components as well as the block molecular weight of the polyimides increase, microphase separation between the two components in the triblock copolymers can be found.

Crystallization and phase behavior in nylon 6/aromatic polyimide triblock copolymers

Several series of new b-nylon 6-b-polyimide-b-nylon 6 triblock copolymers were synthesized via a condensation polymerization of polyimide segments and anionic polymerization of the nylon 6 components. Crystallization and phase behavior of these triblock copolymers have been studied with varying polyimide block molecular weights and copolymer compositions as well as chemical structures (and therefore, different solubility parameters) of the polyimide components. Furthermore, in-situ and solution blends have also been prepared in order to compare the phase behaviors of the triblock copolymers. Wide angle X-ray diffraction experiments indicate that in these triblock copolymers, only nylon 6 components crystallize and polyimides do not take part in the crystals. From isothermal crystallization kinetics studies carried out in differential scanning calorimetry experiments, the miscibility of the polyimide components with nylon 6 critically affects the crystallization rate of these triblock copolymers. When the polyimide composition is low and its solubility parameter is similar to that of the nylon 6 components, the polyimides mix with the amorphous part of the nylon 6 components and are located in the inter-lamellar region. This can be identified by monitoring the change of α-relaxation temperature and activation energy measured using a dynamic mechanical method and by examining the crystalline lamellar morphology under small angle X-ray scattering experiments. Microphase separation can also be recognized by transmission electron microscopy using an OsO 4 vapor staining technique. As the difference of solubility parameters between these two components as well as the block molecular weight of the polyimides increase, microphase separation between the two components in the triblock copolymers can be found.

The use of metastable phase diagrams in primary crystallization kinetics study

Metastable multiphase materials are currently obtained by controlled crystallization of amorphous samples. This paper deals with the use of metastable equilibrium phase diagrams and related thermodynamic quantities to the kinetic analysis of primary crystallization of an undercooled liquid solution as obtained by calorimetric means. The analysis is performed as a function of the processing conditions, assuming thermodynamic and kinetic competition between several crystalline phases for nucleation and growth. The hierarchy of crystalline phases formation on both heating an initial amorphous alloy or on cooling the liquid solution follows a sequence mostly consequent with the different temperature dependence of the several quantities driving crystal formation.

Method to Study Crystallization Kinetics by Electrical Resistivity

A procedure of transformed material fraction evaluation from electrical resistivity isochronal measurements was characterized. The amorphous-to-crystalline transformation in Fe-B alloys was taken as the model eutectic and primary reaction. The results were applied to the experimental resistivity data obtained by Dehghan [9]. The interpretation was done with help of the latest ideas of activation energy spectrum developed for primary reaction.

Measurement of Atomic Fractions in Multi-Phased Materials of Limited Mass via an Empirical Approach to EXAFS Modeling

A least-square fitting analysis of EXAFS data collected from partially-crystallized Fe8uB2u thin films (t = 15 um)~ using data collected from pure phase standards of the crystalliza- tion products, was found effective in determining the relative atomic fraction of each crystalline phase present This fitting scheme provides a means for the quantitative treatment of crystall- lization and precipitation kinetics in thin films and multilayered structures A long standing limitation of extended X-ray absorption fine structure (EXAFS) as a tool for quantitative materials science has been its inability to measure the relative fraction of phases in multiphased materials. This is a prerequisite~ for example, in the study of crystallization and precipitation kinetics. Although there exist techniques which are able to perform these tasks on bulk materials, e.g. X-ray diffraction and digital scanning calorimetry~ recent trends toward the design and fabrication of low dimensional devices has made the study of thin films~ which cannot be readily measured by these techniques because of their small masses~ of particular importance. Of the popular local probes~ EXAFS is largely insensitive to small masses (lj, for example~ the signal-to-noise ratio of the EXAFS collected in total electron yield mode does not deteriorate appreciably for thin films until the sampled mass approaches Ge 10~~ grams. In an attempt to illustrate the usefulness of EXAFS in performing quantitative materials science we have applied EXAFS to study the crystallization of a model transition metal metalloid amorphous system in Fe-B. A least-square fitting analysis of the EXAFS data collected from partially-crystallized Fe-B thin films~ using data collected from pure phase standards of the crystallization products~ was found effective in determining the relative atomic fraction of each crystalline phase present. The samples used in this study are from a single 15 nm film (50 mm x 50 mm area) which was ion beam sputter-deposited from a pressed-powder target having the stoichiometry of Fe80B20. Individual pieces (7 mm x 7 mm) were encapsulated in evacuated glass ampoules and annealed at temperatures ranging from 473 K to 823 K for a period of 60 minutes. The X-ray absorption

A kinetics study of crystallization from discotic mesophases

The polymorphism of hexa- n -octanoate of rufigallol (RHO) was used to investigate the crystallization kinetics when starting from two different discotic mesophases, namely, a hightemperature enantiotropic mesophase and a low-temperature monotropic mesophase. In all cases, an Avrami exponent of n 3 was obtained, suggesting two-dimensional sporadic nucleation and growth. Although the same overall mechanism was predicted to operate for all crystallization temperatures investigated, the Avrami constant b has a temperaturedependence which is different for different starting columnar phases, indicating a strong influence on the rates of nucleation and of growth of the different molecular arrangements of the starting mesophases. Differential scanning calorimetry was used for the studies, supplemented by direct observations through optical polarizing microscopy.

Thermal stability and crystallization kinetics study of some Se-Te-Ge glassy alloys

Thermal stability and crystallization kinetics of (Se 85Te 15) 100 − x Ge x ( x = 0, 2, 5 and 10) glasses have been studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD), optical microscopy (POM) and scanning electron microscopy (SEM). The variation of the glass transition temperature with composition suggests that a small amount of Ge ( ≤ 5 at.%) leads to an increase in the length of the Se-Te chains, whereas further enrichment in Ge (10 at.%) increases the number of Se-Te chains in the alloys. Thermal stability of these samples is discussed. For glassy alloys prone to crystallize, the emerging crystalline phase reflections correspond to h.c.p. Se(Te). Surface and bulk crystallization can be found and in both cases crystalline nuclei develop as spherulites. The activation energies for the glass transition and for crystallization have been calculated. The crystallization kinetics has been established and a discussion of the possible mechanisms involved in the crystallization process is presented.

Simultaneous recording of Hv and Vv small-angle light-scattering patterns for crystallization kinetics studies

Simultaneous recording of Hv and Vv small-angle light-scattering patterns for crystallization kinetics studies