| As a primary enabling technology, pulsed power generator is of great meaning for vast cutting-edge technologies in both civilian and military. With the progressive development towards ultra-high power density, advanced engineering polymer is urgently needed as vacuum insulation stack. Specifically, apart from the ultra-low dielectric loss at microwave frequency and ultra-high voltage resistance, appropriate mechanical properties over a temperature range up to 100 °C should also be considered which cannot be fulfilled by the typical low loss polymers such as polyethylene (PE), polytetrafluoroethylene (PTFE) and polystyrene (PS). Under this background, the present thesis work aims to develop cross-linked polystyrene and composites that have been always limited by the thermal runaway problem during preparation.;An innovative technology referred to as 3D printing bulk polymerization was developed to solve this limitation. Both field experiments and numerical investigation have been conducted and proved that cross-linked PS bulk can be prepared through the method during which the overall reaction enthalpy releases continuously and gently. The risk of thermal runaway in this technology depends on the in-plane dimension. However, by adjusting the processing parameters such as temperature, feeding rate, convective heat transfer efficiency etc., the large scale preparation of cross-linked PS through this technology is realizable.;The relationship between the structure and properties was also investigated. Owing to the reduced pliability of main-chain deformation and beta relaxation by much higher activation energy, the mechanical properties and thermal resistance of PS were effectively improved after crosslinking with 4 mol % divinylbenzene (DVB). The modulus was increased from 2.43 GPa to 3.06 GPa with the heat distortion temperature from 90.1 °C to 114.4 °C that is satisfactory for targeting application. The dielectric measurement at a microwave frequency range suggested that the excellent dielectric properties of PS were well maintained with a loss factor on the order of 10-4, which was determined mainly by the concentration of carboxyl group introduced by benzoyl peroxide (BPO). While the gamma relaxation frequency was shifted to higher frequency after the crosslinking density, since torsional relaxation of pendent phenyl group was influenced by the interaction with the elastic deformation of C-C in main-chain, which was significantly restricted by crosslinks.;Hybrid of cross-linked PS with nanoparticles was also prepared in order to improve the high voltage resistance. Firstly, study on the mechanism showed that insulating nanoparticle with high polarizability was favorable for the enhancing through "barrier effect", because effective double layers formed around the nanoparticle would suppress the excessive charge injection and conduction through the Coulomb repulsive interaction. Accordingly, nano-MgO was selected and submitted to a routine of treatment, after which cross-linked PS/MgO nanocomposites were successfully prepared through the 3D printing bulk polymerization with breakdown strength increased from 450 kV/mm to over 600 kV/mm while other properties were well maintained. In a word, the products developed in the present research are ideal as insulation stack in high pulsed power generator. |
