Study on crystallization kinetics of Al65Cu20Ti15 amorphous alloy

The crystallization behavior and thermal stability of amorphous phases of Al₆₅Cu₂₀Ti₁₅ alloy obtained by mechanical alloying were investigated by using in-situ X-ray diffraction and differential scanning calorimetry (DSC) under non isothermal and isothermal conditions. The result of a Kissinger analysis shows that the activation energy for crystallization is 1131 kJ/mol. The higher stability against crystallization of Al₆₅Cu₂₀Ti₁₅ amorphous alloy is attributed to the stronger interaction of atoms in the Al-Cu-Ti system and formed of complicated compound like Al₅CuTi₂ and Al₄Cu₉ as primary phases. The isothermal crystallization was modeled by using the Johnson-Mehl-Avrami (JMA) equation. The Avarami exponents suggest that the isothermal crystallization is governed by a three-dimensional diffusion-controlled growth.     

The formation of stereocomplex in asymmetric blends of PLLGA and PDLA and the effect of stereocomplex on PLLGA crystallization

The isothermal and non‐isothermal crystallization behaviors of asymmetric poly(L‐ lactic acid‐ co‐ glycolic acid) (PLLGA)/poly(D‐lactic acid) (PDLA) blends was investigated at PDLA loadings of up to 10 wt%. A simple solvent‐casting procedure, was developed for the preparation of PLLGA/PDLA stereocomplex crystallites dispersed in a PLLGA matrix. The formation of stereocomplexes in the PLLGA matrix was verified by differential scanning calorimetry (DSC) and X‐ray diffraction (XRD). Non‐isothermal crystallization measurement demonstrated that the addition of PDLA significantly accelerates the crystallization of PLLGA. Subsequent assessment of the influence of PDLA content on the isothermal crystallization kinetics of PLLGA by the Avrami equation confirmed that the crystallization rate of PLLGA is enhanced by the addition of PDLA contents, with a reduction in the half crystallization time (t_1/2) and increase in the crystallization rate constant (k) values. A PDLA content of 1–7 wt% was found to particularly enhance the crystallization rate of PLLGA, from which it is inferred that the PLLGA/PDLA stereocomplex crystals act as effective nucleus to enhance the crystallization of PLLGA.     

A Study of the Nucleation Kinetics,Linear Growth and the Overall Crystallization Kinetics of Meltspun Pd80Si20 Amorphous alloy

Nucleation kinetics, linear growth rate and overall isothermal crystallization kinetics of Pd Si amorphous alloy are studied by means of DSC and optical microscopy. It is found that both the histograms of the crystallite sizes and the DSC isotherms (resp. the time dependences of the degree of transformation α(t)) are best described by the model comprosing the simultaneous occurrence of two reactions of crystallization: heterogeneous as well as homogeneous non-stationary nucleation with subsequent three-dimensional growth in both cases. From the fit of the experimentally measured and the calculated according the model thus proposed distribution curves N_i(Φ) the stationary rates of heterogeneous and homogeneous nucleation, the number of quenched-in nucleation sites and the transient time lags of heterogeneous and homogeneous nucleation have been estimated. Using the values of these parameters kintic curves degree of transformation vs. time have been generated, which are closed to the exerimental x(t) curves obtained by means of DSC at isothermal conditions.     

Isothermal crystallization behavior and crystal structure of poly(ethylene terephthalate‐co‐1,4‐cyclohexylene dimethylene terephthalate) (P(ET/CT)) copolyesters

Poly(ethylene terephthalate) (PET) and a series of poly(ethylene terephthalate‐co‐1,4‐cyclohexylene dimethylene terephthalate) (P(ET/CT)) copolyesters with different molar ratios of ethylene glycol (EG) to 1,4‐cyclohexanedimethanol (1,4‐CHDM) were investigated by their isothermal crystallization behavior, transparency, crystal structure and morphology changes in different isothermal crystallization process using differential scanning calorimetry (DSC), digital photos, wide‐angle X‐ray diffraction (WAXD) and polarized optical microscopy (POM). The results revealed that P(ET/CT) copolyesters with ≤ 15 mol% 1,4‐CHDM and ≥ 50 mol% 1,4‐CHDM, such as P(ET/CT)(85/15), P(ET/CT)(50/50) and P(ET/CT)(30/70), were crystallizable, while that with 30 mol% 1,4‐CHDM, namely P(ET/CT)(70/30), was amorphous. The crystallization rate, crystallinity and transparency of these copolyesters underwent the same isothermal crystallization process, i.e. it first decreased and then increased remarkably with the increase of 1,4‐CHDM content. Accordingly, the crystal structure of these copolyesters changed from PET‐type lattice to PCT‐type lattice when the copolyester with around 30 mol% 1,4‐cyclohexylene dimethylene terephthalate (CT) unit. Interestingly, the small incorporation of 1,4‐CHDM into PET could lead to the formation of larger spherulite crystals than that of PET. But small and grainy crystallites appeared with further increase in the 1,4‐CHDM content.     

In situ monitoring of cyclic butylene terephtalate polymerization by dielectric sensing

The in situ monitoring of the polymerization of cyclic butylene terephtalate (CBT) oligomers by dielectric sensing has been investigated under isothermal conditions at different processing temperatures. The change in conductivity depends on the increase of molecular weight but is also influenced by crystallization of the obtained c-PBT during or after polymerization. Dielectric sensing investigations are in good agreement with SEC and DSC analysis which confirm the observation of polymerization and crystallization kinetics. Unfortunately, the conductivity signal could not distinguish between the polymerization and crystallization when they occur simultaneously at lower temperatures (<210 °C) but the crystallization remains detectable by the MWS relaxation which is related to the crystalline/amorphous interface.     

Influence of the B and P content on the thermal stability and crystallization of cast iron based bulk amorphous alloys

The possibility of synthesizing bulk amorphous alloys with additions of B and P to commercial cast iron (Ci) of the chemical composition Fe_81.5Si_3.8C₁₄Tm_0.7 (at.%) was investigated. The effect of the B and P concentration on thermal stability, bulk glass forming ability (BGFA) and microstructure was studied by calorimetric (DSC and DTA) measurements as well as by X-ray diffraction. With the addition of 8.42 and 12.17 B to the commercial Ci bulk amorphous alloys with very wide supercooled liquid regions (77 and 81 K, respectively) could be produced. The partial replacement of Ci by B and P atoms increases the thermal stability and the BGFA. The kinetics of crystallization was investigated both by linear heating and by isothermal DSC measurements. The changes in the microstructure of the crystalline phases formed during linear heating were studied during further isothermal annealing at 833 K.     

Crystallization kinetics of electro-deposited amorphous CoP alloys

The crystallization characteristics of amorphous Co-15.2 at.% P and Co-18.4 at.% P alloys were studied by means of a differential scanning calorimeter (DSC) and a transmission electron microscope (TEM). The analysis of DSC data obtained in the isothermal operation was performed by means of the Johnson-Mehl-Avrami (J-M-A) rate equation. The values of the J-M-A time index, n, for the crystallization of Co-15.2 at.% P and Co-18.4 at.% P alloys were 3.3 ± 0.1 and 2.2±0.1, respectively. It has been revealed through these experimental results and TEM observation during the crystallization process of the amorphous alloys that the crystallization of the former alloy results in interface-controlled three-dimensional spherical growth (n = 3) of Co₂P crystal, whereas the crystallization of the latter alloy results in two-dimensional growth (n = 2) of Co₂P disks having constant thickness. It has also been found that the apparent activation energy for the crystallization is 195 kJ/mol. and 190 kJ/mol. for Co-15.2 at.% P and Co-18.4 at.% P alloys, respectively.     

A comparison of the crystallization behavior of liquid-quenched and vapor-quenched amorphous Cu60Ti40

The crystallization behavior of two Cu₆₀Ti₄₀ amorphous alloys, one prepared by a vapor-quench (VQ) process and one by a liquid-quench (LQ) process, has been studied using differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. Microstructural studies show that the two alloys have the same transformation sequence under similar annealing conditions. However DSC measurements of the crystallization temperature during isochronal annealing and of the incubation time for crystallization during isothermal annealing show that the LQ alloy is more thermally stable and therefore more structurally relaxed. TEM analysis of the partially annealed microstructures gives some insight into the mode of crystal nucleation and growth as a function of temperature. During isothermal annealing significant differences are observed for the two alloys in the shape and orientation of the crystals and in the crystal structure formed. Whereas the VQ alloy transforms polymorphicaly into the intermetallic Cu₃Ti₂, the LQ alloy forms the Ti-rich phase, Cu₄Ti₃. The two phases are closely related in structure and composition. The differences observed for the two alloys are discussed in terms of greater structural relaxation and short range order in the LQ alloy resulting from the mode of synthesis.     

Thermal stability of a low Tg phosphate glass investigated by DSC, XRD and solid state NMR

The thermal stability of a low Tg phosphate glass (Tg = 339 °C), formulated in the ZnO-Na₂O-P₂O₅ system, is investigated in this contribution by Differential Scanning Calorimetry (DSC), X-Ray diffraction (XRD) and solid state Nuclear Magnetic Resonance (NMR). DSC measurement indicates a very important T_x − T_g value (197 °C) for the investigated composition compared to other low Tg phosphate glasses found in literature. XRD and 1D ³¹P solid state NMR were used to monitor the isothermal crystallization process occurring at 60 °C above Tg (400 °C). The mixture of phases formed after crystallization cannot be identified by XRD but 2D MAS-NMR experiment provides valuable information about the composition and the structure of the unknown sample.     

Kinetic theory of crystallization of amorphous materials

Careful analysis of the Avrami equation [x = 1 ?exp(?Atⁿ)] shows that an activation energy for crystallization (E_c) for amorphous materials can be defined over a selected range of temperatures. This activation energy can be determined experimentally using non-isothermal differential scanning calorimetry (DSC) by determining the crystallization temperature (T_c) as a function of heating rate (φ). A plot of (ln(φ/T_c) has a slope equal to E_c/n. The activation energy for the crystallization of amorphous arsenic obtained by this non-isothermal method is found to be in fair agreement with that obtained from an isothermal DSC experiment.     

Crystallization behaviour of an amorphous Ti50Be40Zr10 alloy

The crystallization behaviour of an amorphous Ti₅₀Be₄₀Zr₁₀ alloy during a continuous heating mode from room temperature to 973 K and isothermal annealing at temperatures above the glass transition temperature is examined by differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS) measurement and large-angle X-ray diffractometry (LAXD). DSC indicated two well-defined exothermic peaks, a slight shoulder at the higher temperature side of the second peak and a small heat evolution at higher temperature. The Kissinger plot for the first and the second peak gives a straight line, from which the apparent activation energy is estimated to be 269 and 413 kJ/mol respectively; the enthalpies for the first and second crystallization process are 1.04 kJ/mol and 4.39 kJ/mol for a heating rate of 20 K/min. The SAXS intensities increase sharply after annealing at about 673 K (corresponding to the first peak in the DSC curves); the scattering is due to the formation of fine-scale crystalline Ti particles by the LAXD. The size of the particles does not change significantly while the number of scattering particles increases, indicating that the reaction is almost nucleation controlled and the growth is very limited. Another crystalline phase would appear in addition to the Ti particles on annealing at temperatures above about 753 K (corresponding to the second peak in the DSC curves), where the SAXS intensities decrease compared with those for only the first-stage of crystallization. The crystalline phase might be a metastable cubic phase with the lattice parameter a₀?0.2994 nm.The sequence in the crystallization of the initial non-crystalline material is amorphous → microcrystalline (MS I) → crystalline (MS II; S III), although the structure of crystalline phase in the final stage (S III) was not identified. It is also likely that cold-rolling does not have a perceptible effect on the crystallization behaviour of the present amorphous alloy.     

Crystallization kinetics in Cu46Zr45Al7Y2 bulk metallic glass by differential scanning calorimetry (DSC)

Crystallization transformation kinetics in isothermal and non-isothermal (continuous heating) modes were investigated in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass by differential scanning calorimetry (DSC). In isochronal heating process, activation energy for crystallization at different crystallized volume fraction is analyzed by Kissinger method. Average value for crystallization in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass is 361 kJ/mol in isochronal process. Isothermal transformation kinetics was described by the Johnson-Mehl-Avrami (JMA) model. Avrami exponent n ranges from 2.4 to 2.8. The average value, around 2.5, indicates that crystallization mechanism is mainly three-dimensional diffusion-controlled. Activation energy is 484 kJ/mol in isothermal transformation for Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass. These different results were discussed using kinetic models. In addition, average activation energy of Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass calculated using Arrhenius equation is larger than the value calculated by the Kissinger method in non-isothermal conditions. The reason lies in the nucleation determinant in the non-isothermal mode, since crystallization begins at low temperature. Moreover, both nucleation and growth are involved with the same significance during isothermal crystallization. Therefore, the energy barrier in isothermal annealing mode is higher than that of isochronal conditions.     

A multi-peak transformation kinetics model and its application to the isothermal crystallization of Mg65Cu25Y10 amorphous alloy

Transformation kinetic analytical model plays an important role in the prediction of the microstructural evolution. In this paper, a simple formula has been developed for isothermal mixed nucleation transformation as the kinetic parameters of its JMAK-form formula vary upon time. The explored multi-peak transformation kinetics shows that each peak can be treated as a JMAK case, which is consistent with the classical JMAK model in only one peak case. Thereafter, a method has been developed to deal with the isothermal DSC data of multi-peak overlapping transformation. The isothermal crystallization process of Mg₆₅Cu₂₅Y₁₀ amorphous alloy has been explored and fitted well with the multi-peak kinetics model, which indicates a continuous nucleation, three dimensional interface-controlled growth mechanism with three crystallization peaks overlapping each other.     

Computer modelling of the non-isothermal crystallization kinetics of electroless nickel-phosphorus deposits

Non-isothermal differential scanning calorimetry (DSC) data of electroless nickel-phosphorus (EN) samples with three different phosphorus contents (9, 12 and 16 wt%) has been used in the computer modelling of crystallization kinetics based upon the Johnson-Mehl-Avrami (JMA) theory. The major crystallization processes of the EN samples are complex and could involve multiple reactions, as indicated by the corresponding DSC curves obtained at the heating rates of 5-50 °C/min. The degrees of transformation for major crystallization process of the samples were calculated from those DSC experimental data. Computer simulations of the degree of transformation as well as the corresponding DSC curves were performed using the JMA models, and the results were compared to those from the experiments. It was found that the JMA models could perform within the experimental accuracy of the samples with high (12 and 16 wt%) phosphorus contents. However, satisfactory application of the model to a sample with medium (9 wt%) phosphorus content was not achieved, probably due to the presence of crystalline phase in the deposit prior to crystallization process. The derived JMA kinetic exponents (n) are compared and discussed upon their kinetic interpretation to the non-isothermal crystallization processes of the samples.     

Determination of activation energies for nucleation and growth from isothermal differential scanning calorimetry data

Differential scanning calorimetry (DSC) is usually adopted to analyze solid-state phase transformation incorporating nucleation, growth and impingement. Then, for isothermal transformation, time-dependent Avrami exponent and overall effective activation energy can always be deduced using recipes, which are derived from an analytical phase transformation model. On this basis, a concise and reliable approach to determine time-independent activation energies for nucleation and growth is proposed. Numerical calculations have demonstrated that the new approach is sufficiently precise under different conditions of transformation (e.g. nucleation: mixed nucleation and Avrami nucleation; growth: interface-controlled growth and diffusion-controlled growth; impingement: randomly nuclei dispersed, anisotropic growth and non-random nuclei distributions). Application of the approach in crystallization of Zr₅₅Cu₃₀Al₁₀Ni₅, Zr₅₀Al₁₀Ni₄₀ and Cu₄₆Zr₄₅Al₇Y₂ bulk amorphous alloys as measured by isothermal DSC was performed.     

Phase transformation and crystallization kinetics of melt-spun Ni60Nb20Zr20 amorphous alloy

The glass transition and crystallization kinetics of melt-spun Ni₆₀Nb₂₀Zr₂₀ amorphous alloy ribbons have been studied under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC). The dependence of glass transition and crystallization temperatures on heating rates was analyzed by Lasocka's relationship. The activation energies of crystallization, E_x, were determined to be 499.5 kJ/mol and 488.6 kJ/mol using the Kissinger and Ozawa equations, respectively. The Johnson–Mehl–Avrami equation has also been applied to the isothermal kinetics and the Avrami exponents are in the range of 1.92–2.47 indicating a diffusion-controlled three-dimensional growth mechanism. The activation energy obtained from the Arrhenius equation in the isothermal process was calculated to be E_x = 419.5 kJ/mol. The corresponding three dimensional (3D) time–temperature–transformation (TTT) diagram of crystallization for the alloy has been drawn which provides the information about transformation at a particular temperature. In addition, the intermetallic phases and morphology after thermal treatment have been identified by X-ray diffraction (XRD) and scanning electron microscope (SEM).     

Semifluorinated Methacrylate Random Copolymers: Phase Transitions and Molecular Dynamics

Random copolymers of methyl methacrylate (MMA) and sermifluorinated methacrylate (sfMA), with constant side chain length (H₁₀F₁₀), as comonomers and various sfMA molar contents were studied by Dielectric Relaxation Spectroscopy (DRS) technique with respect to their phase transitions and molecular dynamics. DRS technique was proven a suitable technique for the detection of the phase transitions that take place in the systems under investigation, as it follows from the comparison with Differential Scanning Calorimetry (DSC) technique, which is traditionally used. Regarding molecular mobility, molecular motions of both the main chain and the sf side chains were followed, while different dynamics was recorded depending on the structure of the copolymers.     

Uncertainties in crystallization of hen‐egg white lysozyme: reproducibility issue

The reproducibility of biomacromolecular crystallization (tetragonal and orthorhombic lysozyme crystals) was studied by monitoring the evolution of protein concentration during the crystallization process using Mach‐Zehnder interferometer. It was found that formation of both tetragonal and orthorhombic crystals exhibited poor reproducibility. When the crystallization occurred under isothermal conditions, the protein concentration in the solution varied differently in different experiments under identical conditions (for both types of crystals). Moreover, in the case of orthorhombic lysozyme crystallization (under either isothermal or thermal gradient conditions), it is clear that the crystals could not be always readily formed. When formation of tetragonal lysozyme crystals was conducted at a temperature gradient condition, however, the evolution of concentration was reproducible. The phenomena found in this study revealed that biomacromolecular crystallization can be uncertain, which is probably caused by the process of nucleation. Such uncertainties will be harmful for the efforts of screening crystallization conditions for biomacromolecules. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of thermo-electro-optic properties of some mixed nematic liquid crystals

The 6CB/8CB/8OCB liquid crystal mixture has been studied by Differential Scanning Calorimetry (DSC), polarised optic microscopy (POM), Semiconductor Characterization System, and Ultra-violet spectrophotometry (UV). DSC and POM results indicate that the 6CB/8CB/8OCB mixture exhibits liquid crystalline properties. The capacitance-voltage and conductance-voltage measurements were performed in the frequency range of 200-500 kHz and in the temperature range of 30oC–50oC. The 6CB/8CB/8OCB mixture showed an extremely large positive dielectric anisotropy. The molar absorptivity ? for the 6CB/8CB/8OCB mixture was calculated and found to be higher than the absorptivity values of the binary mixtures due to the alkyl chain length with H-aggregation.     

A modeling approach for the non‐isothermal antisolvent crystallization of a solute with weak temperature dependent solubility

Antisolvent crystallization is a crystallization technique, which is normally operated isothermally. However, non‐isothermal operation is on occasions performed for solutes that have temperature dependent solubility. This paper shows that it is beneficial to operate antisolvent crystallizers non‐isothermally even for solutes whose solubility is weakly dependent on temperature. In this context, it demonstrates the joint control of particle mean size and size distribution coefficient of variation. A non‐isothermal crystallization model‐based framework is developed for the sodium chloride‐ethanol‐water system and validated for both isothermal and non‐isothermal operations. This framework was used to systematically determine both the optimal antisolvent feed rate and temperature profiles that minimize the coefficient of variation while producing a specified mean crystal size. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)