MWCNTs and their use in Al-MMCs for ultra-high thermal conductivity applications: A review

Abstract

Rapid developments in the electronic industry have greatly benefited society. However, more recently some high-end technological applications have seen their advances limited by an inherent inability tofind new materials capable of meeting industrial and consumer demands. One of the most challenging technological barriers that has hindered the progress in electronics is the failure to remove excessive heat produced during the operation of the device. The heat flux in electronic components can be as high as 80 W cm-2 and in the next few years is expected to increase to over 300 W cm-2 for some applications.Traditional materials have serious deficiencies in meeting the requirements for thermal management especially minimization of the thermal stresses in HPLED packaging. Copper (Cu, K = 400 W m-1 K-1), the standard material for applications requiring high thermal conductivity, has a coefficient of thermal expansion (CTE = 17 ppm K-1) that is much larger than those of ceramics and semiconductor materials. This gives rise to thermal stresses when packages are subjected to thermal loads. Aluminium (Al, K = 237 W m-1 K-1) has a lower thermal conductivity and larger CTE (23 ppm K-1) than Cu but it is cheaper and lighter, making its more attractive option, and when produced with high thermal conductivity reinforcement has the potential to be applied as a heat sink for the HPLEDs.Amongst all the C-based materials, multi-walled carbon nanotubes (MWCNTs)are reported to have the highest thermal conductivity (∼ 3000–3500 W m-1 K-1 (experimental value)) with a low CTE (-2.5 ppm K-1 at room temperature). This makes this material an excellent filler candidate tomanufacture composites with ultra-high thermal conductivity (K = 400 W m-1 K-1) in order to solve heat dissipation problems such as those found in HPLED applications. However, harnessing the thermal conductivity potential of the MWCNTs within Al metal matrices has yet to be achieved.The authors aim to give the reader an overview of the limitations in thefield of Al/MWCNTs in order to potentiate the research and development of advanced thermal management materials with ultra-high thermal conductivity (K = 400 W m-1 K-1) which has hindered the rapid developments in the electronic industry.This review exposes the necessity to develop standard procedures for theselection of suitable MWCNTs for high thermal conductivity/thermal management applications that follow the following criteria; MWCNTs with smallest possible lengths and diameters (i.e. contain a low number of walls), low defect concentration and high crystallinity and no Impurities (carbonaceous and/or metal catalyst). The immaturity of the techniques available to measure the thermal conductivity of individual nanotubes, and the necessity of developing less complex and more reliable methods to quickly ascertain the high thermal conductivity of the selected MWCNT sprior to composite processing is vital. The development of such methods would not only benefit the use of MWCNTs but any other nanoparticles with potential high thermal conductivities.The limited number of studies that have focused on the thermal conductivity of AMCs reinforced with MWCNTs clearly demonstrates the undeveloped state of these materials to harness the potential high thermal properties of the MWCNTs. For this reason, a substantial volume of work is still needed to develop processing methods capable of overcoming the inherent challenges faced (i.e. introduction of the MWCNTs into the Al matrix, wettability of the nanotubes, dispersion, subsequent reaction products, alignment and structural damage) during Al/MWCNT processing. If these nanoparticles with outstanding thermal properties can be harnessed to produce ultra-high thermal conductivity (K = 400 W m-1 K-1) materials, they can be the solution to overcome the limitations of important thermal management applications such as HPLEDs.