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Effective Activation Energy (effective + activation_energy)

Distribution by Scientific Domains
Polymers and Materials Science71%
Physics and Astronomy28%

Selected Abstracts

Variation of the Effective Activation Energy Throughout the Glass Transition

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 19 2004
Sergey Vyazovkin
Abstract Summary: An advanced isoconversional method has been applied to determine the effective activation energies (E) for the glass transition in polystyrene (PS), poly(ethylene terephthalate) (PET), and boron oxide (B2O3). The values of E decrease from 280 to 120 kJ,·,mol,1 in PS, from 1,270 to 550 kJ mol,1 in PET, and from 290 to 200 kJ mol,1 in B2O3. It is suggested that a significant variation in E should be observed for the fragile glasses that typically include polymers. Variation in the effective activation energy of PS, PET, and B2O3 with temperature. [source]

Moisture curing kinetics of isocyanate ended urethane quasi-prepolymers monitored by IR spectroscopy and DSC

JOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2008
Ana Luísa Daniel-da-Silva
Abstract The study of the kinetics of the curing of isocyanate quasi-prepolymers with water was performed by infrared spectroscopy and differential scanning calorimetry. The influence of the free isocyanate content, polyol functionality, and of the addition of an amine catalyst (2,2,-dimorpholinediethylether) in the reaction kinetics and morphology of the final poly(urethane urea) was analyzed. A second-order autocatalyzed model was successfully applied to reproduce the curing process under isothermal curing conditions, until gelation occurred. A kinetic model-free approach was used to find the dependence of the effective activation energy (Ea) with the extent of cure, when the reaction was performed under nonisothermal conditions. The dependence of Ea with the reaction progress was different depending on the initial composition of the quasi-prepolymer, which reveals the complexity of the curing process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008 [source]

Temperature Dependence of Sol-Gel Conversion Kinetics in Gelatin-Water System

MACROMOLECULAR BIOSCIENCE, Issue 4 2009
Kai Chen
Abstract The conversion kinetics of an aqueous gelatin solution to gel was studied by temperature modulated and regular DSC under isothermal and continuous cooling conditions. Isothermal runs revealed a decrease in the quasi-static heat capacity primarily associated with syneresis (phase separation) of the gel. Above 19,°C the isothermal process demonstrated negative effective activation energy that turned positive below 14,°C. Continuous cooling runs detected a reversing heat flow apparently related to the continuing formation and melting of new gel structures. Isoconversional kinetic analysis of continuous cooling measurements yielded negative activation energy for the whole range of conversions and temperatures suggesting that nucleation remained a rate controlling step throughout the whole gelation process. [source]

Variation of the Effective Activation Energy Throughout the Glass Transition

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 19 2004
Sergey Vyazovkin
Abstract Summary: An advanced isoconversional method has been applied to determine the effective activation energies (E) for the glass transition in polystyrene (PS), poly(ethylene terephthalate) (PET), and boron oxide (B2O3). The values of E decrease from 280 to 120 kJ,·,mol,1 in PS, from 1,270 to 550 kJ mol,1 in PET, and from 290 to 200 kJ mol,1 in B2O3. It is suggested that a significant variation in E should be observed for the fragile glasses that typically include polymers. Variation in the effective activation energy of PS, PET, and B2O3 with temperature. [source]

Study of the mechanical and thermal properties of Sn,5 wt% Sb solder alloy at two annealing temperatures

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003
M. M. EL-Bahay
Abstract Sn,5 wt% Sb alloy is one of the materials considered for replacing lead-containing alloys for soldering in electronic packaging. Differential thermal analysis (DTA) and specific heat of the sample were studied. Wetting contact angle measurements of the alloy on different substrates were carried out at high temperature. Microhardness tests as a function of temperature were performed to calculate the effective activation energy of the solder alloy Sn,Sb and compared with the pure elements Sn and Sb. Isothermal creep curves for alloy samples were obtained under different constant applied stresses at different working temperatures ranging from 463 K to 503 K, followed by annealing the samples at two different temperatures before and above the threshold value (Tm/2). The transient creep parameters and the values of the stress exponent n were calculated for the two annealing temperatures. Microstructure examinations of the as-cast alloy at room temperature and after the two annealing treatments with the effect of the cold work deformation and creep test on the structure change and properties of Sn,Sb alloys were reported. The stress rupture test was also measured. [source]

Temperature dependence effective activation energy in Tl2Ba2CaCu2O8 thin film

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2006
S. L. Liu
Abstract By considering the competition of thermal activation and effective pinning, a general description of temperature dependence resistivity under magnetic fields has been deduced on high temperature superconductors. The temperature dependence resistivity is a function of the energy ratio of thermal actviation and pinning. Based on vortex glass phase transition analysis, the temperature dependence activation energy can be obtained from the resistivity transition under magnetic fields. It is found that the activation energy can be scaled onto a single parabola. The experiments on Tl-2212 thin film and other superconductors support our model. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]