Universality of Dielectric Response as an Aid to Diagnostics
- PDF / 820,414 Bytes
- 16 Pages / 414.72 x 648 pts Page_size
- 17 Downloads / 165 Views
Abstract The advent of frequency response analysers and the formulation of the universality of dielectric responses have between them greatly enhanced the usefulness of dielectric measurements as a diagnostic tool. The available frequency range was extended to between eight and twelve decades, making it possible to determine spectra with some precision. In addition, we now have a much more complete understanding than was available previously of the significance of various spectral shapes. For charge carrier systems the universal approach is based on the fractional power law of frequency dependence of the real and imaginary components of the susceptibility i(ow)= '(o))-iX"(wo) (iwo)'-l,where the exponent falls in the range 0 < n < 1. Values of n close to unity correspond to low-loss behaviour, values close to zero to very lossy processes dominated by low-frequency dispersion (LFD). Examples will be presented of both extremes and an indication will be given of the theoretical significance of these results. A brief discussion will be given of the physical principles of low-loss dielectrics showing "flat" or frequency-independent X and of the opposite limit of LFD. It will be shown how the presence of universality simplifies the analysis of data and their interpretation.
Introduction Measurements of dielectric parameters have often been used to characterise a wide range of materials and a considerable volume of literature has appeared on this subject while much work remains unpublished because its presentation does not measure up to the standards required in modem literature. The available range of measurements was expanded in the past twenty years to sub-Hertz frequencies with the advent of Frequency Response Analysers, opening an entirely new territory. Dielectric measurements characterise the capacitance and the loss of a sample and because of the added dimension of frequency in the chosen range, which may encompass between six and ten decades, they provide a very large source of information compared with the conventional static information. Because of this wealth of information, it is essential to use appropriate forms of representation, without which it may be difficult to realise fully the potential of the data. The main purpose of the present paper is to discuss this matter of presentation and to draw attention to considerable advantages which may be gained from its proper use.
167
Mat. Res. Soc. Symp. Proc. Vol. 500 ©1998 Materials Research Society
Dielectric measurements characterise primarily rate processes governing the response of a material to alternating electric fields. The fundamental difference between direct current (dc) and alternating current (ac) or dielectric measurements consists in the fact that the former relate to the steady state condition under a static electric field, while the latter probe the transient response at various frequencies, as it were in the process of reaching the steady state but never quite getting there. Thus the dc conductivity, co is determined by movements of charge
Data Loading...
