| Liquid dielectrics, with advantages of high energy storage density and good self-healing, are widely used as the storage medium in the pulse forming line of high power pulse devices. However, the liquid dielectrics in the engineering are difficult to meet the requirements of high resistivity, dielectric constant and breakdown strength at the same time. Therefore, we firstly study the problem about aqueous mixture resistivity prone to failure when storage. Secondly, the impulse breakdown experiments of high energy storage density liquid dielectrics are carried out. Finally, effect on the pulsed insulating characteristics of liquid dielectrics with additive, pressurized and nano-powder has been studied, too. The research set a good foundation for improving energy storage properties of liquid dielectrics, and developing of compact pulsed power facilities. Contents are shown as follows.First of all, the experiments of resistivity maintaining of ethanol/deionized water mixture and glycol/deionized water mixture are carried out. And all of these two mixtures are of 25% volume fraction. The experiments find that mixtures resistivity would decline when storage. And the resistivity decline process can be divided into three stages. They are decline rapidly stage, decline slowly stage and constant phase, respectively. Moreover, the resistivity variation process is an electrochemical equilibrium process. The resistivity decline of aqueous mixture can be slowed down with vacuum degassing and storage in PET container. If use industrial sample hold a month later, the mixture resistivity in stainless steel container decreased to 6.1% of the initial resistivity value, while the mixture resistivity in PET container reduced to 26% of the initial resistivity value.Although the aqueous mixtures after a certain treatment can prolong the resistivity maintain time, their chemical stability is a bit poor relative to propylene carbonate. Thus, secondly, the pulsed insulating characteristics of propylene carbonate and its mixture with ethylene carbonate have been studied. In addition, we also study on effect of applying mechanical pressure on the impulse breakdown of propylene carbonate. The experimental results show that the propylene carbonate inherent resistivity is higher than 10 M?·cm, and open-air keep two weeks later there is still 6 M?·cm. And average breakdown strength of propylene carbonate is 398.5 k V/cm, and its dielectric constant and energy storage density can be further improved by adding ethylene carbonate or applying pressure. When static pressure of propylene carbonate added from 1 bar up to 11 bar, the effective time increases from 1.24 ?s to 2.79 ?s, and the normalized breakdown strength is up from 480.3 k V/cm to 887.3 k V/cm, at last the energy storage density is up from 0.65 MJ/m~3 to 2.2 MJ/m~3, increased 3.4 times.Finally, we experimentally studied the breakdown properties of a new kind of colloidal dielectric fluids, by dispersing the Ba Ti O3 nano-powder into the castor oil through the ultrasonic treating. The experimental results demonstrate that the permittivity of the castor oil modified by the Ba Ti O3 nano-powder can rise to 4 times that of the pure castor oil, with the breakdown strength and energy storage density improved. For instance, the breakdown strength of the nanofluid, with 6% mass fraction of Ba Ti O3 nano-powder whose average grain size of 100 nm, is 1.55 MV/cm under the microsecond pulse, 25% higher than that of the pure castor oil. In addition, the energy storage density reaches 648 k J/m~3, while that of the pure oil is 402 k J/m~3. It is furthermore found that, the effects of the castor oil modified with the nano-powder are distinct from different mass fractions or grain sizes, and there exists an optimum fraction for these nanofluids under the same average grain size, where the energy storage density reaches the largest. Besides, the optimum fraction varies with the grain size of the nano-powder. |
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