Charging Mechanisms and Models for Nanoparticles Suspended in Liquid Dielectrics

Liquid dielectrics filled with nanoparticles have drawn huge attentions due to enhanced dielectric and thermal properties. Despite of great achievements have been made, the charging mechanisms of nanoparticles suspended in liquid dielectrics are still open questions. It is previously argued that field charging with Maxwell–Wagner relaxation fails to explain the enhanced breakdown strength when nanoparticles have longer relaxation time constant than the timescale of streamer development in liquid dielectrics. However, it is proven in this article when the condition that the charging time constant $\tau _{\mathrm {pc}} \ll $ the charging time $t_{\mathrm {pc}}$ is fulfilled, nanoparticles can acquire significant number of charges even if $t_{\mathrm {pc}} \ll $ relaxation time constant $\tau _{\mathrm {MW}}$ . It is also showed that the field charging models for nanoparticles suspended in gases and liquids share the same formulation and thus can be unified. In addition, diffusion charging is proposed to be one of the charging mechanisms for nanoparticles in liquid dielectrics, and Fuchs’s model is extended from aerosol science to liquid dielectrics. It is demonstrated the diffusion charging could be far more important than field charging in liquid dielectrics. This work may bridge the gaps of charging mechanisms and models between gaseous and liquid dielectrics.