On-line measurement of dielectric relaxation : use of unorthodox stimuli

Chowdhry, Bhawani Shankar (1989) On-line measurement of dielectric relaxation : use of unorthodox stimuli. University of Southampton, Doctoral Thesis.

Abstract

This thesis describes a method, improved in speed and accuracy, for low frequency dielectric relaxation measurement and the design of an instrument incorporating this. The method implemented is based on the application of a perfectly low-pass filtered step (the Si function) as the stimulus, the current response being captured by an on-line computer. A discrete Fourier transform can then be performed on the acquired data, leading to the direct computation of the dielectric constant and loss. The generation of unorthodox stimuli (e.g. Si and sinc functions) which were not available by continuous linear systems, is discussed. A novel data storage technique is introduced for such slowly converging `non-causal' waveforms. As it stands the resulting instrument gives sufficient bandwidth (up to 120Hz) to allow cross-calibration with bridge methods, but is also valid down to 0.01Hz. The whole system was initially tested on a lumped circuit - a known RC-series model. The experimental results obtained were found to be in good agreement with those predicted theoretically. The system was also tested with a real polymer dielectric sample, and the results obtained correlated in form with those obtained using other methods, confirming the success of the technique. In addition, a unique form of potential error has been identified and corrected. Finally, the equipment needed was reduced to a personal computer with a data acquisition card and the function generator. As a simple computer-aided measurement method it should prove useful in the laboratory, particularly in the study of polymers.

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