The influence of frequency of AC component on space charge behaviours in polyethylene under combined AC and DC electric fields
Combined ac and dc voltages may occur in high voltage power transmission system, which bring challenges to the dielectric materials. Generally speaking, space charge is well recognised under high voltage dc conditions but has received little attention under high voltage ac conditions. Under ac electric fields, the formation space charge in the material depends strongly on the frequency. The knowledge of space charge behaviour in dielectric materials under combined voltages is desirable in terms of both experimental and simulation. In this paper, a numerical model based on the bipolar charge injection/transport model is used to analyse characteristics of space charge in polyethylene under combined electric fields at room temperature. The DC component ratio and the overall root mean square (RMS) are kept the same, while the frequency of the AC component is altered from 0.05 Hz to 50 Hz. It has been found that the amount of the accumulated charges reduces with increasing frequency due to the more frequent polarity reversal. Moreover, the time to reach the steady states of charge amount increases with frequency as well.
Observation of negative differential mobility and charge packet in polyethylene
Charge packet has been observed on many occasions but its physical mechanisms have never been properly understood. One of the models proposed by Lewis et al shows the presence of negative differential mobility with an electric field in semicrystalline polyethylene. In this paper we have observed the negative differential mobility using the transient space charge profile measured by the pulsed electroacoustic technique. By superimposing a short pulse voltage to a dc voltage, it is possible to obtain the velocity of holes at different applied fields. To the best of our knowledge we have for the first time observed negative differential mobility in polyethylene. This observation provides crucial evidence to support Lewis's model and allows one to simulate charge packet and its behaviours.
Beings of nature and reason: mythological masculinities in early 18th-century French art and visual culture
Statement of Originality Acknowledgments List of Illustrations Introduction 11 Chapter One Of Myth and Men 27 Chapter Two In the Likeness of Bacchus 62 Chapter Three Venus and a Satyr 97 Chapter Four Pan and Syrinx 141 Chapter Five Marsyas Flayed by the Order of Apollo 175 Conclusion 212 Catalogue of Images 227 Bibliography 300