Featured Researcher
UC Santa Cruz: Jiankuai Diao and Deepak Srivastava
Molecular Dynamics Simulations of Carbon Nanotube/Silicon Substrate Interfacial Thermal Conductance
Our group is using the Teragrid to perform molecular dynamics simulations to study the thermal conductance at the interface of carbon nanotube and silicon substrate. Carbon nanotubes (CNTs) have been theoretically predicted and experimentally measured to have high thermal conductivity along the axial direction. This high axial thermal conductivity, along with the fine structure of CNTs and their compliance due to high aspect ratio, make vertically aligned CNTs a strong candidate for thermal interface materials between electronic devices and heat sink materials. The overall performance of vertically aligned CNTs as thermal interface materials is determined by the vertically aligned CNTs themselves and the two interfaces formed with the electronic devices and with the heat sink materials. The thermal conductivity of CNT has been well studied, but the interfacial thermal conductance is less so and since CNT itself has high axial thermal conductivity, the interfacial thermal conductance might be a limiting factor for the overall performance. This study helps to identify the key factors that contribute to the interfacial thermal conductance and constitutes the very first step towards designing and optimizing interfacial structures for better thermal conductance.
We have simulated the interfacial thermal conductance at the CNT and silicon interface at different pressures. Our results show an increase in the interfacial thermal conductance with applied pressure for both capped and uncapped CNTs, which is mainly due to an increased bonding or conducting area at the interface between the CNT and silicon substrate. With an increase in the applied pressure, the cap of the capped CNT becomes more and more flattened, facilitating the formation of more bonds at the interface. At low pressure, the interfacial thermal conductance for the system with uncapped CNT can be three times higher than the systems with the capped CNT due to larger conducting area at the interface. At higher applied pressure, the interfacial thermal conductance for the system with capped CNT is higher due to the fact that bonds formed at the interface are more efficient in transferring heat between the two materials. The inverse of the interfacial thermal conductance is the interfacial thermal resistance and the range of values of interfacial thermal resistance simulated in this work corresponds to the resistance of a CNT with a length ranging from 100 micrometer to 700 micrometer if a thermal conductivity of 6600 W/m K is used for CNT. These lengths are usually larger than those of the vertically aligned CNTs themselves, implying that the interfacial thermal resistance is indeed a limiting factor for the overall performance of vertically aligned CNTs as thermal interface materials. This work suggests that better interfacial thermal conductance can be achieved by opening the CNT caps and by applying pressure.
Acknowledgement:This work is supported as a part of a UC Discovery Grant Award from UC Santa Cruz and Nanoconduction to Deepak Srivastava.
