Electrical and Thermal Properties of tpPI/h-BN Composites Fabricated by Electrostatic Adsorption Method Influenced by h-BN Orientation Direction

The electrostatic adsorption method can effectively minimize filler-to-filler contact, which is a critical issue in composite insulating materials. Using this technique, a thermoplastic polyimide (tpPI)/oriented micro-hexagonal boron nitride (h-BN) composite insulating material was fabricated. The flaky surfaces of the h-BN particles were oriented in three distinct directions relative to the sample thickness: perpendicular (CMV sample), parallel (CMP sample), and oblique (CMO sample). To investigate the influence of h-BN orientation on the thermal and electrical properties, the orientation ratio, thermal conductivity, and DC dielectric breakdown strength were evaluated. The thermal conductivity increased with the X-ray diffraction intensity ratio of (100)/(002), indicating enhanced in-plane alignment of the h-BN particles. The DC breakdown strength, at identical h-BN content, decreased in the order of CMV > CMO > CMP. This difference became more pronounced with increasing h-BN content. The lower breakdown strength observed in the CMO and CMP samples is attributed to the alignment of tpPI/h-BN interfaces parallel to the electric field direction (i.e., the sample thickness direction), which may act as electrical weak points.