Chapter 1: Introduction
1.3 Thermal Boundary Resistance in Polymer Nanocomposites
1.3.4 Binary Filler Systems
Another interesting class of composites are those with binary filler systems in which two different types of fillers are embedded in a polymer matrix in an effort to enhance the thermal transport properties of the material beyond what either constituent filler could achieve in a single filler system. Some binary composites, for instance, seek to combine high aspect ratio materials, which can percolate at lower volume fractions, and two-dimensional materials, which have larger surfaces areas, such as those described in a pair of studies which investigated the thermal properties of an epoxy-graphene oxide (GO)-MWCNT composite (Im and Kim 2012) and an epoxy-boron
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nitride nanosheet (BNNS)-BNNT composite (Su et al. 2013). In the case of the epoxy-GO- MWCNT composite, the authors were able to demonstrate that adding just 0.36% by weight of MWCNTs to the composite increased the thermal conductivity of the systems by as much as 150%
relative to the epoxy-GO composite (Im and Kim 2012). Similarly, the thermal conductivity of the epoxy-BNNS-BNNT composite outperformed both an epoxy-BNNS and an epoxy-BNNT composite at the same total filler volume fraction (Su et al. 2013). However, as with the unary nanotube or two-dimensional filler systems described above, the enhancement of the composite thermal conductivity relative to that of the neat polymer remained modest.
More interesting, perhaps, are those binary composites which seek to combine metallic fillers with phonon dominant fillers, again such as CNTs or BNNTs, either through direct mixing or surface functionalization of the latter. For instance, Jouni et al. prepared PE-MWCNT-AgNP composites by incorporating a fixed amount of AgNPs (φ = 0.03) into molten mixtures of PE and MWCNTs with varying volume fractions of MWCNTs (Jouni et al. 2014). At the maximum MWCNT volume fraction of 0.05, the measured thermal conductivities of the PE-MWCNT and the PE-MWCNT-AgNP composites were 0.782 W m-1 K-1 and 0.924 W m-1 K-1, respectively, representing a thermal conductivity enhancement of 113% for the AgNP containing material.
Similarly, Wang et al. investigated the effects of adding 1% by weight MWCNTs to PVDF composites containing nano-aluminum particles (n-Al) and found that at n-Al concentrations of 59% by weight, the addition of 1 wt% MWCNTs increased the thermal conductivity of the composite to 1.44 W m-1 K-1 from 1.32 W m-1 K-1 (Wang et al. 2015).
Later, Zhang et al. measured the thermal conductivity of an epoxy-AgNW-graphene oxide (GO) composite and demonstrated that the optimal weight ratio of AgNWs relative to the total filler concentration was 0.75 (Zhang et al. 2019). Further, it was shown that at all filler volume
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fractions, the epoxy-AgNW-GO composites outperformed both the epoxy-AgNW and epoxy-GO composites at the same filler volume concentrations, achieving a peak value of 1.2 W m-1 K-1 as shown in Figure 1-12 (Zhang et al. 2019). While in each of these binary filler studies the total thermal conductivity enhancement of the composite material was modest, the results offer some suggestion that by simply mixing stiff, phonon dominant materials with metallic nanofillers, the thermal conductivity of a polymer composite can be improved over what could be achieved in a single filler system at the same loading level.
Figure 1-12: Thermal conductivities of AgNW/GO composites filled with different amounts of hybrid fillers (Zhang et al. 2019).
Another, more common, strategy for incorporating both metallic and non-metallic fillers into a polymer composite is to functionalize the surface of the non-metallic filler with metallic nanoparticles. Wang et al. adopted this strategy in the fabrication of an epoxy composite filled with AgNP decorated boron nitride nanosheets (BNNS) and measured a peak thermal conductivity of 3.06 W m-1 K-1 at a volume fraction of 0.251 compared to just 1.66 W m-1 K-1 for the epoxy- BNNS composite (Wang, F. et al. 2015). Interestingly, however, the AgNP decorated BNNS demonstrated no improvement over the un-decorated BNNS at volume fractions below 0.117 as
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the BNNS fillers are below the percolation limit. In this way, it is thought that, rather than directly contributing to an increased thermal conductivity of the composite, the decorated AgNPs allow for more effective junctions to form between adjacent BNNS in a fully percolated network (Wang, F.
et al. 2016).
In a similar study, AgNP decorated BNNTs were incorporated into a matrix of cellulose nanofibers (CNFs), and the resulting composite was found to have an in-plane thermal conductivity of 20.9 W m-1 K-1 at a BNNT weight fraction of 0.25 compared to 12.9 W m-1 K-1 for the un-decorated BNNT composite (Fu et al. 2018). In this case, the weight fraction of silver in the composite was just 1.99 x 10-3, and in order to maximize the benefit of the decorated AgNPs, the samples were hot pressed after their initial fabrication. Here, hot pressing serves to both increase the in-plane alignment of the embedded BNNTs and to take advantage of the low melting point of the AgNPs which, upon melting, could further enhance the effectiveness of BNNT-BNNT junctions (Fu et al. 2018).
Finally, Suh et al. measured the thermal conductivity of epoxy-Ag flake composites with a number of secondary fillers including both AgNP functionalized and un-functionalized MWCNTs (Suh et al. 2016). While, in this case, the composite containing the AgNP functionalized MWCNTs is most correctly classed as a trinary filler system due to the two distinct phases of the embedded silver, the epoxy-Ag flake-AgNP functionalized MWCNT composite achieved a peak thermal conductivity of 45.5 W m-1 K-1 at an Ag flake volume fraction of 0.367 and a secondary filler volume fraction of 0.023 as shown in Figure 1-13. Though this represents a significant improvement over the thermal conductivity of the epoxy-Ag flake composite (1.64 W m-1 K-1), perhaps most interesting about this result is that the epoxy-Ag flake-MWCNT composite had a
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thermal conductivity of ~5 W m-1 K-1, indicating that a significant majority of the thermal conductivity enhancement can be attributed to the AgNP functionalization (Suh et al. 2016).
Figure 1-13: Thermal conductivity of TIMs cured at 180 °C for 1 h. The mass of primary filler (Ag flakes) was fixed at 7.9 g (36.7 vol%), and the concentration of secondary fillers was varied (0–3.5 vol%). ■: Ag flake, : Ag Flake + AgNP Functionalized MWCNTs, ▼: Ag Flake + MWCNTs (Adapted from Suh et al. 2016).