Browsing by Author "Kazilas, Michalis C."
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Item Open Access Acquisition and interpretation of dielectric data for thermoset cure monitoring(Cranfield University, 2003-09) Kazilas, Michalis C.; Partridge, Ivana K.The interpretation and modelling of the dielectric response of thermosetting materials during cure was the main focus of this study. The equivalence of complex permittivity and complex impedance in terms of information content was outlined in a series of case studies covering the separate effects of dipolar movements and charge migration as well as the combined effect of the two polarisation mechanisms. Equivalent electrical circuits were used in order to model the evolution of the complex impedance during cure. A numerical method that can model consecutive spectra throughout the cure was developed. The method is based on Genetic Algorithms and requires only input from the modelling of the initial spectra. Complex impedance spectra were collected during the cure of a commercial epoxy resin formulation under isothermal and dynamic heating conditions. The spectra were analysed and modelled. The modelling was successful over the whole frequency range of the measurements (1 Hz – 1 MHz). The analysis of the estimated model parameters showed that charge migration dominates the dielectric response in a wide frequency range. In addition, the modelling algorithm also distinguished between the effects of electrode polarisation and dipolar movements in the signal. A new equivalent circuit was used in order to map the frequency regions where the each one of the three phenomena that together comprise the dielectric signal can be monitored most effectively. A chemical cure kinetics model was developed for the studied system. A correlation between the maximum point of the imaginary impedance spectrum and the reaction conversion was established. A mathematical model, based on a simple linear dependence of the dielectric signal on conversion and temperature, was built. The model predictions agreed well with the experimental data. The aim of simplifying the interpretation of the dielectric signals led to the development of a new experimental technique. Temperature Modulated Dielectric Analysis employs temperature modulations superimposed on an underlying thermal profile in order to separate the influence on the signal of the temperature alone from that of the cure reaction. The early study carried out here shows that such measurements are feasible and reveals important issues for its further development.Item Open Access Feasibility study on Temperature Modulated Dynamic Dielectric Analysis(2004-12-01T00:00:00Z) Partridge, Ivana K.; Kazilas, Michalis C.The project concerned the development of a novel measurement and analysis tool, designed to aid the study of cure in thermosetting polymers. Cure is the irreversible process whereby an initially liquid (unreacted) resin turns into a rubbery solid and eventually into a rigid crosslinked glass. The process usually requires the application of heat to proceed to completion. The changes in structure are reflected in changing mobilities of electrically charged species in the resin and dielectric signals can therefore be used to monitor the progress of cure. In this project, the normal heating–up of the resin was accompanied by a superimposed sinusoidal temperature variation of +/- 1°C. Deconvolution of the resulting dielectric signal into its reversible and irreversible components made it possible to distinguish between the signal contributions arising from the temperature change alone and from those directly contributable to the irreversible chemical chain extension and crosslinking. This proves the original hypothesis put forward in the proposal. The new ideas generated during the course of the project enabled us to obtain significant follow-on funding from the EPSRC. A new dielectric cell has been designed, which will be used in attempting to track particle dispersion in thermosetting nanocompositItem Open Access Parameter estimation in equivalent circuit analysis of dielectric cure monitoring signals using genetic algorithms.(Taylor & Francis, 2005-04-01T00:00:00Z) Kazilas, Michalis C.; Skordos, Alexandros A.; Partridge, Ivana K.This communication concerns the treatment of dielectric data obtained from experiments following the chemical hardening process (cure) in thermosetting resins. The aim is to follow, in real time, the evolution of the individual parameters of an equivalent electrical circuit that expresses the electrical behavior of a curing thermoset. The article presents a methodology for the sequential inversion of impedance spectra obtained in cure monitoring experiments. A new parameter estimation technique based on genetic algorithms is developed and tested using different objective functions. The influence of the objective functions on the modelling performance is investigated. The new technique models successfully spectra contaminated with high noise levels. The introduction of regularization in the optimization function rationalizes the effects of outliers usually detected in cure monitoring dielectric spectra. The technique was successfully applied to the analysis of a series of spectra obtained during the cure of an epoxy thermosetting resin.