No abstract

In the present work, sandwich‐structured composite sheets consisting of a core layer of carbon nanofiber (CNF)/polyimide (PI) and an outer layer of either pristine PI or hexagonal boron nitride (hBN)/PI were proposed and characterized for the possibility that they could enhance the effective thermal conductivity (TC) of PI composite sheets while also retaining their electrical insulation properties. The sandwiched structures were prepared using a dip coating method. The electrical insulation properties were evaluated via the breakdown voltage (BDV) of direct current. By using hBN with an average size of 0.5 μm as filler for the outer layer, a composite sheet with a highly effective level of TC was prepared. The BDV achieved more than 2 kV, when the outer layer had a total thickness of approximately more than 60 μm even at 30 vol% of hBN_0.5 content. The tensile strengths of the sandwich‐structured sheets indicated the values between each single layer of the 10 vol%‐CNF/PI and hBN_0.5/PI composites.

Effective thermal conductivity (TC) was experimentally measured for single‐walled carbon nanotube (SWCNT)/polyimide (PI) composites with a filler content up to 8 vol% and for carbon nanofiber (CNF)/PI composites with a filler content up to 24 vol%. The measurement results indicated that the values for effective TC for SWCNT/PI and CNF/PI composites were similar up to a filler content of 8 vol%. The experimental results were then correlated via thermal conductive models proposed by Xue and by Hamilton and Crosser, and reasonable correlation results were obtained for both the SWCNT/PI and CNF/PI composite sheets. The Nielsen model capably reproduced the experimental TC values without parameter‐fitting operations. Moreover, correlations using the effective medium approach (EMA) were also performed by adjusting the interfacial resistance. Good correlation results were obtained for both the SWCNT/PI and CNF/PI composites, and the correlated interfacial resistance showed reasonable values that compared to those found in the literature. Furthermore, the effect that the coadditions of CNF and hexagonal boron nitride (hBN) exerted on the effective TC of PI was investigated for a wide range of ratios of CNF to hBN. A small amount of CNF significantly increased the effective TC of CNF/hBN/PI composites. POLYM. COMPOS., 40:3032–3039, 2019. © 2018 Society of Plastics Engineers