The behavior of polymer-based dielectrics under cryogenic conditions

Abstract

Dielectrics is ubiquitous in all electrical systems. In this chapter we introduce many aspects in a systematic manner in order for the reader to be able to follow it in a sequence. Firstly, the various media and the salient features of each are discussed. The media range from vacuum to highly compressed gases, liquids, and solids. The basic mechanism of dielectric behavior is discussed in each different case when subject to electric field. The imposed electric field can be of different forms such as steady-state AC and DC voltages. It can also be due to transients in the system brought about by switching operations or they could be due to naturally occurring phenomena namely lightning. The configuration of electrode geometries and the polarity of the electrodes bring about phenomena within the media that need to be understood in order to design electrical systems for a variety of applications. We also focus our attention particularly when the media are at cryogenic temperatures. The electronic and ionic reaction mechanics change drastically at low temperatures. In the early part of this chapter the discussion is centered around the basics such as partial discharge, electrical breakdown, dielectric losses, and permittivity. This is followed by applications of power cable design and operation at or near the boiling point of liquid helium. The reader is directed to some real life experiences of such cable systems. Once again the emphasis here has been on the dielectric aspects and the role that materials play in enabling such technology. With the advent of high temperature superconductors (HTS) the outlook was more promising as superconductivity could be achieved at around the boiling point of liquid nitrogen (77 K). Most dielectric systems were designed with the cryogen playing a dual role of being the cooling medium and also being an integral part of the dielectric. As HTS became more widespread cold compressed gaseous helium is now considered viable for some special applications. Advances in the development of polymeric materials for cryogenic applications have largely kept up with HTS technology. However, there are problems that have to be overcome, in particular mechanical strength at low temperatures. Another problem inherent to devices such as cables and coils is winding gaps and inclusions. This presents opportunities for partial discharges to start and if not avoided leads to aging of the device and finally failure. Important dielectric properties such as permittivity and loss tangent have been discussed at some length. The measurement of these parameters not only gives their numerical values but also provides insight into the behavior of the material properties in a general sense. Composites is another area that is being actively pursued and this topic is discussed in particular the differences between micro- and nano-fillers in polymer resins. The large increase in surface area with the reduction in particle size to the nano-scale has opened up great opportunities for advancement. The interfacial region is one that holds the key to future advancement in this technology. Cryogenic nanocomposites are a fascinating technology that has opened up new horizons, and many laboratories are making great progress in understanding the behavior of these materials both theoretically and with experiments.