scholarly journals Activation Energies and Temperature Dependencies of the Rates of Crystallization and Melting of Polymers

Polymers ◽  
2020 ◽  
Vol 12 (5) ◽  
pp. 1070 ◽  
Author(s):  
Sergey Vyazovkin
Keyword(s):  
Experimental Data ◽  
Phase Transition ◽  
Activation Energy ◽  
Temperature Dependence ◽  
Kinetic Analysis ◽  
Review Paper ◽  
Isoconversional Methods ◽  
Activation Energies ◽  
Phase Transition Kinetics ◽  
Kinetic Treatment

The objective of this review paper is to survey the phase transition kinetics with a focus on the temperature dependence of the rates of crystallization and melting, as well as on the activation energies of these processes obtained via the Arrhenius kinetic treatment, including the treatment by isoconversional methods. The literature is analyzed to track the development of the basic models and their underlying concepts. The review presents both theoretical and practical considerations regarding the kinetic analysis of crystallization and melting. Both processes are demonstrated to be kinetically complex, and this is revealed in the form of nonlinear Arrhenius plots and/or the variation of the activation energy with temperature. Principles which aid one to understand and interpret such results are discussed. An emphasis is also put on identifying proper computational methods and experimental data that can lead to meaningful kinetic interpretation.

Study on the Melting Mechanism of Maleic Anhydride

2020 ◽  
Vol 10 (1) ◽  
pp. 65-78
Author(s):  
Bratati Das ◽  
Ashis Bhattacharjee
Keyword(s):  
Phase Transition ◽  
Activation Energy ◽  
Maleic Anhydride ◽  
Temperature Dependence ◽  
Growth Models ◽  
Melting Process ◽  
Nucleation And Growth ◽  
Crystalline Solid ◽  
Scanning Calorimetry ◽  
Melting Mechanism

Background: Melting of a pure crystalline material is generally treated thermodynamically which disregards the dynamic aspects of the melting process. According to the kinetic phenomenon, any process should be characterized by activation energy and preexponential factor where these kinetic parameters are derivable from the temperature dependence of the process rate. Study on such dependence in case of melting of a pure crystalline solid gives rise to a challenge as such melting occurs at a particular temperature only. The temperature region of melting of pure crystalline solid cannot be extended beyond this temperature making it difficult to explore the temperature dependence of the melting rate and consequently the derivation of the related kinetic parameters. Objective: The present study aims to explore the mechanism of the melting process of maleic anhydride in the framework of phase transition models. Taking this process as just another first-order phase transition, occurring through the formation of nuclei of new phase and their growth, particular focus is on the nucleation and growth models. Methods: Non-isothermal thermogravimetry, as well as differential scanning calorimetry studies, has been performed. Using isoconversional kinetic analysis, temperature dependence of the activation energy of melting has been obtained. Nucleation and growth models have been utilized to obtain the theoretical temperature dependencies for the activation energy of melting and these dependencies are then compared with the experimentally estimated ones. Conclusion: The thermogravimetry study indicates that melting is followed by concomitant evaporation, whereas the differential scanning calorimetry study shows that the two processes appear in two different temperature regions, and these differences observed may be due to the applied experimental conditions. From the statistical analysis, the growth model seems more suitable than the nucleation model for the interpretation of the melting mechanism of the maleic anhydride crystals.

scholarly journals Транспортные процессы с участием атомов углерода между поверхностью и объемом родия при образовании и разрушении графена

2020 ◽  
Vol 54 (6) ◽  
pp. 552
Author(s):  
Е.В. Рутьков ◽  
Е.Ю. Афанасьева ◽  
Н.Р. Галль
Keyword(s):  
Phase Transition ◽  
Activation Energy ◽  
Temperature Dependence ◽  
Atomic Carbon ◽  
Graphene Growth ◽  
Carbon Dissolution ◽  
The Difference ◽  
Kinetics Of

Equilibrium transport of atomic carbon between Rh surface and bulk has been studied. This transport controls the kinetics of the phase transition resulting in graphene growth or destruction. The difference ΔE=0.7 eV has been measured between the activation energy of atomic carbon dissolution E1s and that of its segregation from the bulk to the surface E1s. The temperature dependence of chemisorbed carbon critical cover Neq = Neq(T) has been measured, that is the cover when 2D phase transition takes place and graphene islands start to grow. E.g., Neq = 7.7•1014 cm-2 at T = 1800 K, and Neq = 3.1•1014 cm-2 at T = 1000 K.

Diffusion von In und Cu in ZnS-Einkristallen

1969 ◽  
Vol 24 (9) ◽  
pp. 1301-1306 ◽  
Author(s):  
H. Nelkowski ◽  
G. Bollmann
Keyword(s):  
Experimental Data ◽  
Activation Energy ◽  
Diffusion Equation ◽  
Temperature Dependence ◽  
Single Crystals ◽  
Diffusion Constant ◽  
Pair Formation ◽  
Concentration Profiles ◽  
Radioactive Tracers ◽  
Calculated Concentration

Abstract The diffusion of In and Cu in ZnS single crystals was investigated by means of radioactive tracers. The diffusion constant Do and the activation energy EA of the diffusion equation D = D0 · exp(- EA/kT) were determined by analysing the temperature dependence of the concentration profiles. The result for ln EA = 2,2 eV, D0 = 30cm2 s-1) is interpreted as a diffusion via Zn vacancies and that for Cu (EA - 0,79 eV, D0 = 2,6 · 10-3 cm2 s-1) as a diffusion via interstitial sites which probably is influenced by Zn vacancies. - Preceding doping with In significantly retards the Cu diffusion, consistent with the model of Cu-In pair formation. The reasons for some deviations of the experimental data from the calculated concentration profiles are discussed. - Diffusion measurements with high Cu-concentrations yield a Cu-solubility of 300 ppm at 840 K and 1000 ppm at 950 K.

scholarly journals An Analysis of the Time and Temperature Dependence of the Upper Yield Point in Iron

1961 ◽  
Vol 83 (4) ◽  
pp. 557-564 ◽  
Author(s):  
P. E. Bennett ◽  
G. M. Sinclair
Keyword(s):  
Experimental Data ◽  
Activation Energy ◽  
Strain Rate ◽  
Temperature Dependence ◽  
Yield Point ◽  
Strain Rates ◽  
Influence Of Temperature ◽  
State Of Stress ◽  
The Relationship ◽  
Tensile Test Data

The influence of temperature and strain rate on the upper yield point of ingot iron was studied. Torsion tests were conducted using strain rates of 12.5/sec, 0.25/sec, and 0.0001/sec over the temperature range 77 to 525 deg K. The upper yield point showed a rapid increase as the temperature was lowered. An increase in the strain rate also caused an increase in the yield point. An apparent activation energy can be associated with the strain rate and temperature dependence of the yield point. This energy is influenced by stress level, and it appears from the present study that the relationship can be described by an equation of the form ΔH=ΔH¯τ¯−ττ¯b. If this relationship is substituted for ΔH in a modification of the Boltzmann relation, the following result is obtained: logγ˙γ˙1=MΔH¯RT1τ¯−τ1τ¯b1−T1Tτ¯−ττ¯−τ1b. This equation describes the experimental data within ± 3000 psi. The results of this investigation compared with tensile test data from other investigators confirm that state of stress is an important factor in determining whether a material will behave in a ductile or brittle fashion.

Influence of Er and O doses in Er-related emission in Al0.70Ga0.30As:Er

2001 ◽  
Vol 692 ◽  
Author(s):  
Shin-ichiro Uekusa ◽  
Tomoyuki Arai
Keyword(s):  
Activation Energy ◽  
Temperature Dependence ◽  
Room Temperature ◽  
Thermal Quenching ◽  
Activation Energies ◽  
Optimum Dose ◽  
Radiative Efficiency ◽  
Gaas Substrates ◽  
Pl Intensity

AbstractEr ions with doses ranging from 1×1013 cm−2 to 1×1015 cm−2 were implanted into Al0.70Ga0.30As on GaAs substrates. at 800 °C. Photoluminescence (PL) intensity of Er-related emission around 1.54 μm was enhanced by co-implanted oxygen (O). The optimum dose of Er ion was 1×1014 cm−2 and O ion was 1×1015 cm−2, respectively. Furthermore, from the temperature dependence of the PL intensity of sample implanted with the optimum dose, we estimated the values of E1, E2, and E3, the activation energies in order to investigate the rapid thermal quenching of Er ion in Al0.70Ga0.30As. We found that PL intensity of Er-related emission, in addition to O dose, was enhanced approximately twenty two times at room temperature. And from the temperature dependence of the lifetime of the optimum dose of Er and O, the value 245meV of EA, the activation energy for the decrease of the lifetime, was nearly equal to the value 235meV of E3. Based on the result, the decrease of the lifetime confirms that the radiative efficiency is lower; therefore, we propose that rapid thermal quenching occurs at temperatures above 200 K due to the decrease of the radiative efficiency.

scholarly journals Determining Preexponential Factor in Model-Free Kinetic Methods: How and Why?

Molecules ◽  
2021 ◽  
Vol 26 (11) ◽  
pp. 3077
Author(s):  
Sergey Vyazovkin
Keyword(s):  
Activation Energy ◽  
Kinetic Analysis ◽  
Rate Constant ◽  
Condensed Phase ◽  
Compensation Effect ◽  
Preexponential Factor ◽  
Isoconversional Methods ◽  
Activation Entropy ◽  
Model Free ◽  
Kinetics Of

The kinetics of thermally stimulated processes in the condensed phase is commonly analyzed by model-free techniques such as isoconversional methods. Oftentimes, this type of analysis is unjustifiably limited to probing the activation energy alone, whereas the preexponential factor remains unexplored. This article calls attention to the importance of determining the preexponential factor as an integral part of model-free kinetic analysis. The use of the compensation effect provides an efficient way of evaluating the preexponential factor for both single- and multi-step kinetics. Many effects observed experimentally as the reaction temperature shifts usually involve changes in both activation energy and preexponential factor and, thus, are better understood by combining both parameters into the rate constant. A technique for establishing the temperature dependence of the rate constant by utilizing the isoconversional values of the activation energy and preexponential factor is explained. It is stressed that that the experimental effects that involve changes in the preexponential factor can be traced to the activation entropy changes that may help in obtaining deeper insights into the process kinetics. The arguments are illustrated by experimental examples.

Generalised kinetics of overall phase transition useful for glass crystallisation when assuming non-isothermal conditions

2020 ◽  
Vol 61 (6) ◽  
pp. 209-212 ◽  
Author(s):  
Isak Avramov ◽  
◽  
Jaroslav Šesták ◽  
Keyword(s):  
Experimental Data ◽  
Phase Transition ◽  
Activation Energy ◽  
Heating Rate ◽  
Characteristic Temperature ◽  
Isothermal Conditions ◽  
Constant Rate ◽  
Avrami Parameter ◽  
Kinetics Of ◽  
Glass Crystallisation

The modelling of reaction kinetics is a fashionable subject of publications. We developed an analogue of the KJMA equation under non-isothermal conditions α(T)=1−exp(−(T/θ)N) that describes the dependence of degree of transformation α(T) at a constant rate, q, of heating with characteristic temperature θ(q) and power N, proportional to the Avrami parameter, n. This equation is valid even when the activation energy of the process is not constant. We demonstrate that reliable information about the activation energy is obtained when the experimental data are plotted in coordinates: logq (heating rate) against logTp (peak temperature).

Heat Capacity of K3LnCl6 Compounds with Ln = La, Ce, Pr, Nd

1999 ◽  
Vol 54 (3-4) ◽  
pp. 229-235 ◽  
Author(s):  
M. Gaune-Escard ◽  
L. Rycerz
Keyword(s):  
Experimental Data ◽  
Phase Transition ◽  
Differential Scanning Calorimetry ◽  
Heat Capacity ◽  
Phase Transitions ◽  
Liquid Phase ◽  
Temperature Dependence ◽  
Heat Capacities ◽  
Scanning Calorimetry ◽  
The Enthalpy Of Formation

The heat capacities of the solid and liquid K3LnCl6 compounds (Ln = La, Ce, Pr, Nd) have been determined by differential scanning calorimetry (DSC) in the temperature range 300 -1100 K. Their temperature dependence is discussed in terms of the phase transitions of these compounds as reported in literature. The heat capacity increases and decreases strongly in the vicinity of a phase transition but else varies smoothly. The Cp data were fitted by an equation which provides a satisfactory representation up to the temperatures of Cp discontinuity. The measured heat capacities were checked for consistency by calculating the enthalpy of formation of the liquid phase, which had been previously measured. The results obtained compare satisfactorily with these experimental data.

Analyzing the discrepancies in the activation energies of the backbiting and β-scission reactions in the radical polymerization of n-butyl acrylate

2016 ◽  
Vol 7 (11) ◽  
pp. 2069-2077 ◽  
Author(s):  
S. Hamzehlou ◽  
N. Ballard ◽  
Y. Reyes ◽  
A. Aguirre ◽  
J. M. Asua ◽  
...  
Keyword(s):  
Experimental Data ◽  
Activation Energy ◽  
Mathematical Model ◽  
Radical Polymerization ◽  
Butyl Acrylate ◽  
Activation Energies

Activaton energies for backbiting and β-scission reactions for the polymerization of n-BA were determined by fitting experimental data to a mathematical model. The activation energy for backbiting was higher and that for β-scission lower than those accepted in the literature.

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