Discharge Characteristics of Bubbles at Interface Between AlN Ceramic and FC-72 Liquid

In order to evaluate the insulation of two-phase immersion cooling in the HV power electronic package, the insulation degradation of the dielectric interface induced by bubbles is investigated. In this paper, a test strategy with 50 Hz unipolar DC and AC combined voltage for partial discharge (PD) at boiling interface of AIN ceramic is proposed. The insulation threshold of an AIN ceramic surface is acquired in several dielectric environments, such as air, FC-72 liquid (FC-72, a Fluorinert™ from 3M™), FC-72 vapor, and boiling state of FC-72. This reveals the deterioration of boiling on the insulation of the surface immersed in the dielectric refrigerant. To investigate the mechanism of the PD feature at the boiling interface, the PD patterns of the unrestricted bubble and the accumulated bubble are acquired and contrastively analyzed. Combined with the feature of the back discharge and the bubble behavior, the charged vapor-ceramic interface is relatively stable due to the accumulated vapor layer. This stability of the charged vapor-ceramic interface is broken if the bubble is unrestricted. Besides, it is discovered that the vapor-liquid interface inside the bubble may be another charged interface, which can also trigger a back discharge.