Characterisation and processing of carbon-based reinforced Al-MMCs for thermal management applications”

Abstract

This thesis examines the possibility of using multi-walled carbon nanotubes (MWCNTs) (as fillers), known for their unusual exceptional high thermal conductivity (𝐾 ~ 3000 to 3500 W m-1 K-1), to produce ultra-high thermal conductivity (𝐾  400 W m-1 K-1) aluminium matrix composites (Al/MWCNTs). Composites were processed via a combination of rheocasting and equal channel angular extrusion (ECAE) techniques, for use in advanced thermal management applications such as in high power light-emitting diodes (HPLEDs). Al matrix composites reinforced with Cu-coated pitch-based carbon fibres (Al/Cu-CFs) were first produced to test the processing method selected. Rheocasting allowed the introduction and dispersion of 2 wt.% of Cu-CFs within the Al3Mg matrix. The subsequent ECAE processing of the composites reduced the porosity from 3 to 0.03 % and induced a high degree of fibres alignment (øED-DD ~ 2.69º) within the matrix. However, this resulted in considerable damage on the fibres. The rheocasting alone did not improve the 𝐾 of the composites as the addition of 2 wt.% of fibres showed a value of 𝐾𝑐,𝐶2 = 134.9 ± 4.1 W m-1 K-1, 9% lower in comparison with the matrix, 𝐾𝑚 = 148.4 ± 4.5 W m-1 K-1. After ECAE, for 6 iterations in the in-plane direction, composite with the highest degree of fibre alignment, showed a 𝐾 improvement of ~ 20 % (𝐾𝑐,𝐶1.5,6𝑖,1 = 153.7 ± 4.6 W m-1 K-1) with respect to the “as-rheocasted” composite (𝐾𝑐,𝐶1.5 = 128.5 ± 3.9 W m-1 K-1), and a 3.6% increase with respect to the matrix. The improvement is believed to be due to porosity reduction, fibre alignment and forced intimate contact of clean CF surfaces with the matrix. Rheocasting of the Al/MWCNTs allowed the introduction of up to 0.35 wt.% of MWCNTs (embedded in pure Cu) within the Al matrix. However, the MWCNTs were found in agglomerates. Their introduction within the matrix was aided by the pure Cu, which was further improved after the Cu solubility in Al was reached resulting in the formation of AlCuMg intermetallics which surrounded the agglomerated MWCNTs. ECAE processing reduced the composites porosity (from 1.5 % to 0.03%) and induced a high degree of nanotube bundle alignment (3.24º < øED-DD < 3.62º). Aligned individual nanotubes with a good nanotube matrix interface surface contact were also found. However, damage on the nanotubes was also observed. The SThM+FEM technique developed in this study allowed the acquisition of the 𝐾 of an individual MWCNT that resulted in a combined (in-and out-of-plane) thermal conductivity value of 𝐾𝑀𝑊𝐶𝑁𝑇,1,2 ~ 20 W m-1 K-1. The low value is due the long length and large outer diameter of the nanotube which increases the probability of an increase in the defect content and consequently a lower thermal conductivity. The 𝐾 results of the Al/MWCNTs composites processed via rheocasting+ECAE showed an improvement of ~ 5.7 % (𝐾𝑀1,4𝑖,1 = 156.9 ± 4.7 W m-1 K-1) for an addition of 0.3 wt.% of MWCNTs, in comparison to the matrix (𝐾𝑚 = 148.4 ± 4.5 W m-1 K-1). This finding may be related to porosity removal and MWCNTs bundle alignment forming a percolation network. Comparison with various thermal conductivity theoretical models, taking into account the models limitations, the characteristics of the microstructure of the composites, the MWCNTs quality and purity, and the SThM+FEM results, supports the hypothesis that the 𝐾 of the MWCNTs used for the composite processing is much lower than that claimed by the manufacturer (2000 Wm-1 K-1). However, its value is higher than 𝐾𝑚, which is possible as the value obtained by the SThM+FEM is a combined value, and thus its in-plane value is higher than 20 W m-1 K-1. The theoretical models also showed that it should be possible to obtain 𝐾 values between ~ 351 W m-1 K-1 to ~ 497 W m-1K-1 for Al/MWCNTs composites processed via rheocasting+ECAE with a maximum filler volume content of 𝑣𝑓 = 0.3. Nevertheless, to process ultra-high (𝐾  400 W m-1 K-1) thermal conductivity it is critical that the 𝐾𝑀𝑊𝐶𝑁𝑇𝑠,1  2000 W m-1 K-1.