Similar trends as for the electrical conductivities were observed for the thermal conductivities in the transverse and through-thickness directions, see Table
3 and Figs.
8 and
9. Epoxy-rich interlaminar regions decrease the through-thickness thermal conductivity relative to the transverse thermal conductivity by approximately 12%, which can be interpreted as the rate of improvement that needs to be achieved to obtain the same thermal conductivities in the through-thickness and transverse directions. As indicated by the measurements in the through-thickness direction, significant thermal conductivity improvements are only observed when high amounts of conducting filler are added to the epoxy matrix, i.e. 6.3 vol% GO. The associated mechanism is the enhancement of thermal conduction along the platelet-shaped GO inclusions that occur in the interlaminar region of the thermally insulating polymer matrix. Unlike the electrical conductivity, improvements in thermal conductivity of a polymer require significantly higher filler loadings, which is not surprising since the values of thermal conductivities for GO and epoxy are only two orders of magnitude apart [
18]. Hence, improved thermal conductivities can only be seen for 3.78 and 6.3 vol%. Although the addition of a thermally conducting filler can assist the thermal conduction process, particular fillers and manufacturing/dispersion processes can further affect the rate of improvement, for example by aligning GO inclusions in a particular direction which is consistent with the conventional effective medium field theory [
44]. In the case of GO filler, the oxygen content represents impurities in the crystalline structure of the filler and can greatly affect the thermal conductivity of the filler itself [
22,
23]. For the as-received GO used in this study, the oxygen content was between 5 and 10%, which can be considered as low, and, thus, the thermal conductivity of the filler will not be affected drastically. Molecular dynamics has shown that the thermal conductivity of GO can be reduced down to 8.8 W/mK when the oxygen coverage is close to 20% [
23]. Similar results were reported after characterising the thermal conductivity of free-standing GO films [
22]. For low filler contents, up to 2.52 vol%, the added filler is not sufficient to provide additional conduction in the interlaminar region, and the measured thermal conductivities are of the same magnitude as for the neat CFRP laminate. However, for 6.3 vol% filler content, the through-thickness thermal conductivity increased up to a point of reaching almost identical values as observed in the transverse direction of the neat CFRP. This is probably related to a non-uniform distribution of GO flakes, as at high GO vol% the flakes mainly accumulate in the interlaminar region making the local vol% of GO higher than the average in the laminate’s bulk. The highest thermal conductivity provided in the present study is 0.83 W/mK; it is the case that higher values have been reported in the literature [
35]. However, many other factors have an effect on the thermal conductivity of nano-reinforced laminates such as the carbon fibre type, the fibre volume fraction, the thermal conductivity of the epoxy, the manufacturing method, the method of adding the nano-filler into the composite and the measurement temperature. Here, the important factor is the suitability of the materials and the process for large volume manufacturing (as discussed in the previous section of the paper). Leaving that aside, similar values, 0.86 W/mK, have been reported for laminates containing GNPs, for the same filler content [
16], but also for a significantly lower filler content of 0.5 wt%, 0.84 W/mK [
45], but the GNPs were deposited directly onto the carbon fabric, adding both to cost and the complexity of the manufacturing process. Significantly lower values, approx. 0.53 W/mK, were observed in laminates containing both GNPs and SNPs, however, at a much lower fibre volume fraction [
7].
The addition of GO makes little difference to the transverse direction thermal conductivity (Fig.
8). Unlike the through-thickness direction, in the transverse direction, there are no interlaminar resin-rich regions, which means that the heat flows mainly through the fibres passing small epoxy gaps between them. The main thermal resistance mechanism in this case would be heat flux funnelling towards fibre contacts. Hence, the addition of GO filler cannot provide the noticeable improvements observed for the through-thickness direction. The marginal decrease with increasing GO filler content, although within the experimental uncertainty, can be attributed to fibre contact topology changes caused by fibre volume fraction variations during the infusion process.