Carbon-based materials for thermal management applications

• Main Author: Kong, Qinyu
• Other Authors: Tay Beng Kang
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Electrical and electronic engineering
• Online Access: http://hdl.handle.net/10356/73962
• Institution: Nanyang Technological University

The scaling down of device sizes and the increasing in power dissipation make heat transport and removal a major technological impediment to the future development for electronic applications. Limited heat conduction within devices causes severe localized self-heating problems. Temperature increase significantly degrades the device performance, and the thermal stress caused by the mismatch in the coefficient of thermal expansion shortens the device lifetime drastically. To solve these issues, the development of highly effective heat transfer materials, such as heat sinks, heat spreaders, and thermal interface materials, has received extensive attention. Carbon-based materials, such as carbon nanotubes (CNTs) and diamonds, become attractive for thermal management applications due to their superior thermal properties. In this dissertation, I have focused my attention on the thermal property investigation of polycrystalline diamonds (PCDs) and CNTs for thermal management applications. For the PCD and CNT thermal property investigation, it includes the growth of PCDs and CNTs, the thermal measurement (technique development), the building up of the thermal model, and the thermal conductivity fitting based on the thermal model. Diamonds have been proposed as advanced heat spreaders and heat sinks for high power density devices. Polycrystalline diamond stands out among all diamond isotopes for its versatility, low production cost and high thermal conductivity. However, due to the evolutionary growth mode of PCDs, the thermal properties of the top surfaces (growth surfaces) and the bottom surfaces (nucleation surfaces) are different. Besides, a limitation in PCDs as thermal management materials is the dependence of their thermal properties on defects. In this study, the thermal conductivity comparison between top and bottom surfaces of PCDs due to various defects has been investigated. For the top surfaces, the heat transport is limited by the presence of the Ns0 defect. For the bottom surface, the non-diamond carbon phase, Si vacancy, C-H stretch and Ns0 defects all lead to an obvious reduction in the thermal conductivity. The correlation between the thermal conductivity and the optical transmission of a PCD is studied, making the fast estimation of thermal conductivity by optical inspection available. Carbon nanotubes show a high potential for a thermal interface material (TIM) due to its high thermal conductivity and mechanical compliance. A novel three dimensional carbon nanotube (3D CNT) network has been proposed to improve the thermal performance of currently used CNT-based TIMs. The 3D CNT network is comprised of vertically aligned CNT arrays (primary CNT arrays) bridged with randomly-oriented secondary CNTs. These linkages between the primary CNTs can be postulated to enhance the thermal performance of CNT-based TIMs. Because of the large surface roughness of the CNT array (in micrometers), the thermal characterization of the CNT array using state-of-the-art techniques remains a challenge. In this dissertation, two novel thermal characterization techniques, (1) the free-standing sensor-based 3ω technique and (2) the infrared thermal imaging technique, have been well developed for the characterizations of the cross-plane and the in-plane thermal conductivities of the CNT-based TIMs. Both of the theoretical thermal models and the experimental investigations have been reported. The cross-plane thermal conductivity of the 3D CNT network has been measured to be 56% higher than that of the primary CNT array when the array density is 5.6×10^8/cm2. The in-plane thermal conductivity of the 3D CNT network has been estimated to be 5.4 W/mK, which is more than 50 times higher than that of the CNT array alone.