Abstract
It has been more than a decade since the thermal conductivity of vertically aligned carbon nanotube (VACNT) arrays was reported possible to exceed that of the best thermal greases or phase change materials by an order of magnitude. Despite tremendous prospects as a thermal interface material (TIM), results were discouraging for practical applications. The primary reason is the large thermal contact resistance between the CNT tips and the heat sink. Here we report a simultaneous sevenfold increase in in-plane thermal conductivity and a fourfold reduction in the thermal contact resistance at the flexible CNT-SiO2 coated heat sink interface by coupling the CNTs with orderly physical overlapping along the horizontal direction through an engineering approach (shear pressing). The removal of empty space rapidly increases the density of transport channels, and the replacement of the fine CNT tips with their cylindrical surface insures intimate contact at CNT-SiO2 interface. Our results suggest horizontally aligned CNT arrays exhibit remarkably enhanced in-plane thermal conductivity and reduced out-of-plane thermal conductivity and thermal contact resistance. This novel structure makes CNT film promising for applications in chip-level heat dissipation. Besides TIM, it also provides for a solution to anisotropic heat spreader which is significant for eliminating hot spots.
Highlights
For perfectly aligned Vertically aligned carbon nanotube (VACNT) array-based systems, heat transport is one-dimensional and heat dissipation will not occur in plane[3,4]
VACNT could be used as thermal interface material (TIM), its variant horizontally aligned carbon nanotube (HACNT) can be used as micro/nanoscale anisotropic heat spreader, which is significant for retarding thermal fatigue, slimming down design and reducing weights
Experimental observations revealed that parallel CNTs inevitably partly contacted with adjacent ones either by forming joints or micro-connections after shear pressing as pointed out above (Fig. 2f), which created numerous contacts enabling heat transfer. This is totally different from the situation in VACNT arrays which display no heat transfer between separated CNTs4 as no interactions can help to convey vibrational modes crossing the large gap between adjacent CNTs, and the emerging in-plane thermal transport is a unique feature for HACNTs
Summary
The flexible horizontally aligned CNT array films were fabricated by a similar mechanical method with our previous work[6,9], as shown by Fig. 1. The as-grown VACNT array was shear pressed into a HACNT array at an optimized angle of 35°9 in relation to the Si plane. It is to be noted that the lower pressing angle was desired to increase the horizontal shear imparted to the CNT array. The top surface of the HACNT array was carefully polished to have acceptable surface evenness and roughness to ensure success in deposition of a 200 nm thick metal sensor for thermal characterization. This approach involved only mechanical processing of the CNT arrays, avoiding additional change in defects or impurity concentration which might accompany heat treatment[11] or chemical processing[12]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
