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Improved thermal conductivity of carbon-based thermal interface materials by high-magnetic-field alignment

Improved thermal conductivity of carbon-based thermal interface materials by high-magnetic-field alignment
Chung, Seok-HwanKim, Dong HwanKim, HoyoungJeong, Sang Won
DGIST Authors
Chung, Seok-HwanKim, Dong HwanKim, HoyoungJeong, Sang Won
Issue Date
2020 대한금속재료학회 춘계학술대회
Compared to the conventional inorganic fillers for thermal interface materials (TIMs), carbon-based fillers are attractive due to their advantages such as high thermal conductivity, low thermal expansion, mechanical strength, flexibility, and lightweight. To improve carbon-based TIMs, the alignment of crystalline carbon fillers in a polymer matrix has become a recent strategy that takes advantage of their anisotropy in thermal conductivity. In this work, we applied a high magnetic field to magnetically feeble graphite fillers in a polymer solution to remotely control and vertically orient the fillers. High-purity natural graphite powder (2-15 um) was treated with HNO3 for 12 h and dried after rinsing with DI water. Then the powder was dispersed in anhydrous ethanol by ultrasonic treatment for 3 h. For the binding polymer, poly(vinyl pyrrolidone) (PVP, (C6H9NO)n, Mw=360k) powder was dissolved in ethanol using a magnetic stirrer. Subsequently, the graphite suspension and the PVP solution were mixed, stirred, and degassed in a vacuum desiccator. Then the mixture was cast in a polystyrene mold and dried at the center of a cryogen-free 10T superconducting magnet. The thickness of the TIM films was between 0.25 and 0.8 mm, and the weight fraction of graphite filler was between 10 and 80 wt%. The thermal conductivity for both in-plane and through-plane direction was evaluated using the thermal diffusivity obtained by a laser flash method. The XRD pattern analysis and the FE-SEM images confirmed that the crystalline c-axis of the graphite plates is oriented perpendicular to the magnetic field direction. The filler alignment is based on the large anisotropy in the magnetic susceptibility of the graphite platelets. As a result, we observed about 330% enhancement of the through-plane thermal conductivity (kth) (from 2.4 to 7.9 W/mK) of a graphite-PVP composite TIM film with 80 wt% filler loading. The through-plane thermal conductivity increased from that of the polymer matrix (0.27 W/mK) to 4.5 W/mK for 60 wt% of the graphite filler. The geometric mean model for composite fits well with the experimental result at high filler contents. We also studied the anisotropic thermal transport of the carbon-based TIMs as a function of the filler content. After applying the perpendicular high magnetic field, kth increases sharply up to 8.9 W/mK at 60 wt% while the in-plane thermal conductivity (kin) increases slowly with the filler content. As the filler content increases from 10 to 60 wt%, the anisotropy of thermal conductivity (kth/kin) increases from 1.2 up to 2.3 for perpendicular magnetic field alignment, whereas it remains the same for parallel magnetic field alignment. The increased anisotropy is associated with enhanced filler alignment due to the structural reorganization to reduce particle-particle interaction at high filler loadings. This work provides a simple and effective solution to improve the thermal properties of composite films by controlling their microstructure using a high magnetic field.
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