Degradation of Carbon Nanotube Array Thermal Interface Materials through Thermal Aging: Effects of Bonding, Array Height, and Catalyst Oxidation.

Johan Liu,Josef Hansson,Torbjörn Nilsson,Yifeng Fu,Lilei Ye,Andreas Nylander

doi:10.1021/acsami.1c05685

Abstract

Carbon nanotube (CNT) array thermal interface materials (TIMs) are promising candidates for high-performance applications in terms of thermal performance. However, in order to be useful in commercial applications, the reliability of the interfaces is an equally important parameter, which so far has not been thoroughly investigated. In this study, the reliability of CNT array TIMs is investigated through accelerated aging. The roles of CNT array height and substrate configuration are studied for their relative impact on thermal resistance degradation. After aging, the CNT catalyst is analyzed using X-ray photoelectron spectroscopy to evaluate chemical changes. The CNT-catalyst bond appears to degrade during aging but not to the extent that the TIM performance is compromised. On the other hand, coefficient of thermal expansion mismatch between surfaces creates strain that needs to be absorbed, which requires CNT arrays with sufficient height. Transfer and bonding of both CNT roots and tips also create more reliable interfaces. Crucially, we find that the CNT array height of most previously reported CNT array TIMs is not enough to prevent significant reliability problems.

Highlights

Thermal interface materials (TIMs) are used to enhance heat transfer over an interface between two surfaces
As it can be expected, the thermal interface resistance increases linearly with increasing height, a slope coefficient implies an effective array thermal conductivity of only 4.3 W/mK, while previous measurements of Carbon nanotube (CNT) arrays produced by the same method have found an effective free standing thermal conductivity of around 70 W/mK.[23]
We have investigated parameters influencing the reliability of CNT array TIMs

Summary

Introduction

Thermal interface materials (TIMs) are used to enhance heat transfer over an interface between two surfaces. The TIM conforms to the microscopic surface roughness of the mating surfaces and fills out voids that would otherwise be formed,[1] thereby increasing the effective heat transfer. TIMs are predominantly based on particle laden polymers (PLPs) which consist of thermally conductive filler particles suspended in a polymer matrix. It is difficult to achieve high thermal conductivity at low filler fractions, and increased filler fraction decreases the conformability of the TIM, limiting the total effectiveness of PLP TIMs. It is possible to use solder-based TIMs when the thermal performance is critical, but this might pose other challenges in terms of thermomechanical reliability.[2]. Carbon nanotubes (CNTs) have attracted attention for thermal applications due to their high thermal conductivity of up to 3000 W/mK.[3−5] while there has been a lot of research on CNTs as filler in PLPs, the thermal conductivity of such composites has remained well below what would be expected from a rule of mixtures.[6,7] This is mainly due to CNT matrix thermal boundary resistances and phonon dampening in the CNTs by the surrounding matrix.[8,9]

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Conclusion