The use of magnetic field treatment (MFT) was investigated to establish the appropriate temperature needed to enhance the out‐of‐plane effective thermal conductivity (k) of polyimide (PI)‐based composite sheets. Hexagonal boron nitride (hBN) surfaces were decorated with magnetite particles (Fe3O4@hBN) for use as fillers. The magnetite amount was adjusted to a Fe3O4/hBN weight ratio of 0.2 in most cases. When MFT was carried out at the temperature starting from 80, 90, and 100°C, the effective k was enhanced compared with the lack of MFT. On the other hand, the effective k was not enhanced by the use of MFT at the starting temperature of either 150 or 200°C. Moreover, the cross‐sectional scanning electron microscope images and X‐ray diffractometer analyses of the sheets prepared both with and without MFT were consistent with the results of the effective k measurements. Namely, the fillers inside the sheets moved and rotated under MFT at the starting temperature of 80, 90, and 100°C. On the other hand, for MFT starting from either 150 or 200°C, the fillers were restrained due to high viscosity of the composite solutions. The results of measurements of the breakdown voltages also implied the formation of filler paths inside the sheets.
The orientation and distribution of the filler in hexagonal boron nitride (hBN) coated with iron oxide (Fe3O4@hBN)/polyimide (PI) composite sheets were controlled via magnetic field treatment (MFT) to enhance the effective thermal conductivity (TC) in the out‐of‐plane direction. Filler chain structures gradually formed in the direction of the magnetic field in the precursor solution. The effective TC of the Fe3O4@hBN/PI sheet after MFT was much higher than that of pristine hBN/PI. The effective TC of the composites was preferably improved by placing the sample near a one‐sided magnet instead of in the middle of a pair of magnets. The effective TC of 10, 20, and 30 vol% Fe3O4@hBN/PI composites improved by 59%, 53%, and 35%, respectively, compared to that of pristine hBN/PI for a magnet distance of 10 mm and with the sample placed 2 mm from the bottom magnet. These values are in good agreement with those reported in the literature using MFT. The tensile strengths of the 10, 20, and 30 vol% Fe3O4@hBN/PI composites prepared using MFT were 33%, 33%, and 22% lower, respectively, than those of the composites prepared without MFT.
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