Pulsed Breakdown Characteristics And Application Of Propylene Carbonate Modified By Nano-particles

High power and compact structure are the important development directions of pulsed power technology,the key of which is to raise the energy-storage density.Propylene carbonate?PC?,due to its great permittivity,high dielectric insulating strength and wide range of environmental adaption,has a great potential in improving the compact level of pulse power systems.However,continuous improvements in the energy storage density and dielectric insulating stability of PC in the large size under microseconds pulse have been restricted by traditional liquid processing technology,which constrains the development of propylene carbonate based pulsed power system.Nano science and technology is considered as one of the six major disruptive basic research field in the future by United States Department of Defense,which has become one of hot spots in domestic and foreign research by far.In this dissertation,the energy storage characteristics of PC under microseconds pulse have been significantly improved by adding nano-particles.Then nano-fluids?NFs?with high energy storage density are successfully developed and are used in pulse forming line.At the mean time,the physical mechanism leading to the enhanced dielectric breakdown performance of NFs under microseconds pulse is developed.The main work of the dissertation is as follows:?1?The dielectric insulating characteristics of PC modified by nano-particlesThe dielectric breakdown experiments of PC modified by various nano-particles under microseconds pulse are carried out.The effect of nano species,volume fraction and nano size on the breakdown strength of PC and the best parameters for nano-particles modifying the base liquid are obtained.On the basis of these experimental results,the nano-particles with best parameters are surface-coated in order to ensure the long-term suspension stability of nano-liquid.The results show that the pulsed dielectric performance of PC can be largely increased by adding the insulation or semiconductor nano-particles and the breakdown stability of PC can be enhanced by adding the conductive nano-particles.Under the condition of the same volume fraction,the effect of nano-particles with small particle size modifying PC's dielectric strength is superior to that of the nano-particles with large size,due to the larger particle/liquid interface area.NFs containing TiO₂ nano-particles coated by silane coupling agent KH550 can be stably suspended for a long time.In this case,the dielectric breakdown performance and breakdown stability of the base liquid can also be largely enhanced.?2?Streamer discharge characteristics of PC modified by nano-particlesThe streamers'generation,transmission and arriving at the opposite electrode of PC and NFs are systematically studied including negative polarity,normal polarity and quasi homogeneous field case.The results show that the dielectric strength of PC in the normal polarity is about two times than that of the negative polarity under the same experimental conditions.The streamer in normal polarity is filamentary and the streamer in negative polarity is dendritic.Compared with that of PC,the dielectric strength of NFs in negative polarity is largely increased and the transient resistance increases with the increase of the applied electric field.The number of major branches of NFs increases and the radial branches are more obvious.On the other hand,compared with that of PC,the dielectric strength of NFs in positive polarity is largely reduced,and the streamer is dendritic with large radius.In the quasi homogeneous field case,nano-particles reduce the dielectric strength difference between the cathode and anode.The multiple effects of nano-particles on charge carriers make the breakdown current waveform and the conduction current changing along with the voltage complex.?3?Mechanisms leading to the enhanced dielectric performance of PC modified by nano-particlesMechanisms leading to the enhanced dielectric performance of PC modified by TiO₂nano-particles have been studied experimentally,including space charges and trap distribution of PC/NFs.On the basis of experimental results,energy band theory of NFs and the mechanical stress model generated by the electric field have been established to analyze the breakdown characteristics of NFs in normal/negative polarity.The results show that the trapped charges may largely weaken the field strength in the electrode/NFs interface and increase the charge injection barrier height,which will suppress the further charge injection in the form of resonant tunneling,effectively.As a result,the dielectric strength of the base liquid can be enhanced.In bulk liquid,the nano-particles'trapping and scattering effects on charge carriers may hinder their acceleration process under the external electric field.So the energy of charge carriers can be largely reduced and the formation of low-density region can also be hindered.The gathered electrons at the head of negative streamers can be effectively captured by nano-particles.When charging nano-particles get saturated,scattering effect of charge carriers will play the main part,leading to the diversification of high-energy electrons'migration path and enhancing the dielectric strength of the base liquid in negative polarity.On the other hand,the density of holes near the anode can be largely increased by nano-particles and the holes will enter the bulk liquid by the effect of electric field,which may extend the mainstream branch of the base fluid,lead more bypass branch developing across the electrodes and reduce the dielectric strength of the base liquid in normal polarity.?4?The pulse forming line application of PC modified by nano-particlesThe integrative treatment system for NFs'consummating and filling into the pulse forming line through a vacuuming procedure is conducted.Then the dielectric strength of PC/NFs based pulse forming line is experimentally tested.The results show that the electrical resistivity of NFs can be raised from 20 k?·cm to 2 M?·cm with the method of membrane replacement of liquid molecular.On this basis,the electrical resistivity of NFs can be raised to 50 M?·cm by ion exchange,which can also be integrated into the NFs'vacuum filling system.This may effectively remove the residual impurities such as gas,water and solid particles without affecting the suspension stability of NFs.Compared with that of pure PC,NFs with 1500 ppm TiO₂ nano-particles based pulse forming line possesses 60%higher dielectric strength and 150%higher energy storage density with much higher insulation stability.