Chapter 3. Contact Thermal Resistance Between Silver Nanowires with
4.4 Binary Filler Composites
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To fabricate the composite films, purified FWCNTs (Duke University, CH4 CVD Batch) (Qi et al. 2006) were added to an already prepared solution of PVP and AgNWs such that the dried film layers would have an AgNW and FWCNT volume fraction of 0.1 (i.e., the total filler volume fraction was 0.2). Film layers were then cast according to the procedure described in Section 4.1, and an SEM micrograph of a representative sample cross-section is given in Figure 4-6. Note that, due to the increased stiffness of the thin film layers relative to those containing solely AgNWs, trapped air bubbles were more prevalent in the PVP-AgNW-FWCNT composite film cross sections and appear as voids in the SEM images. In order to ensure measurement accuracy, these regions were excluded when characterizing the sample cross-sectional areas.
Figure 4-7: Measured thermal conductivity of the PVP-AgNW-FWCNT composite compared against the measured thermal conductivity of the PVP-AgNW films. The FWCNT volume fraction is 0.1.
The measured thermal conductivity of the PVP-AgNW-FWCNT plotted in Figure 4-7 and compared against the thermal conductivity measured for the PVP-AgNW composite films.
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However, despite the extraordinarily high thermal conductivity of CNTs, the addition of an equal volume of AgNWs and FWCNTs to a composite film resulted in an ~55% decrease in the measured thermal conductivity relative to the composite containing only AgNWs. It is important to note that in many of the previous studies which demonstrated an increase in thermal performance for the binary filler systems, the thermal performance of the binary filler composite is compared to a polymer composite containing only the carbon filler constituent (CNTs or graphene). However, a similar trend was observed by Agari et al. when comparing the thermal performance of a polyethylene (PE)-copper composite, a PE-graphite composite, and a pair of PE-Cu-graphite composites with varying ratios of copper to graphite in the binary systems (Agari et al. 1987).
Across all investigated volume fractions, the copper only composite outperformed both the graphite only and binary, copper-graphite composites. Therefore, while the PE-Cu study investigated micron sized particles where the effects of thermal boundary resistance are less impactful (Every et al. 1992), it is possible that this trend is also valid for binary, nanofiller composites and can explain the observed difference between the PVP-AgNW-FWCNT composite and the previously measured binary filler nanocomposites.
Further, while there is some evidence to suggest that large thermal boundary resistance associated with CNTs dispersed in a polymer matrix can result in small diameter SWCNTs reducing the overall thermal conductivity of polymer composites (Moisala et al. 2006), here it is the magnitude of that reduction that is most surprising. For instance, if the thermal boundary resistance between PVP and the dispersed FWCNTs was high enough such that the surrounding polymer represented the more efficient thermal pathway, the FWCNTs would be unable to contribute to thermal transport through the binary composite material. In this case the FWCNTs would effectively represent voids in the composite, and the expected reduction in the overall
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thermal transport properties would be ~10%, roughly equal to the volume of embedded FWCNTs.
In the binary PVP-AgNW-FWCNT composite, however, the FWCNTs contributed to an outsized reduction in the measured thermal conductivity when compared to a composite containing only AgNWs. This suggests that, in addition to being unable to meaningfully contribute to the thermal conductivity enhancement of the binary composite, the FWCNTs also interfere with thermal transport through the embedded AgNW network. This is a notable deviation from the trend observed by Agari et al. for the micron sized particles where the measured thermal conductivity of the binary Cu-graphite composites increased proportional to total amount of filler such that both copper and graphite contributed to an overall enhancement of the composite thermal conductivity (Agari et al. 1987).
Assuming a homogenous dispersion of both AgNW and FWCNTs, one potential scenario that could lead to this observed phenomenon is that an equivalent volume of FWCNTs is situated between each pair of adjacent AgNWs. In this case, the FWCNTs can increase the thermal resistance of each interface by either physically separating the adjacent AgNWs such that heat must be transferred across a larger polymer gap or by preventing direct AgNW-AgNW contact which has been shown to have up to an order of magnitude lower thermal boundary resistance than PVP-AgNW interfaces (Yang et al. 2020).
Though more work is required to fully understand thermal transport through the binary AgNW-FWCNT filler system, the results of the PVP-AgNW-FWCNT study do provide an interesting strategy for further elucidating thermal transport through PVP-AgNW composites.
Because FWCNTs themselves do not appear to readily contribute to thermal transport through the composite system, the FWCNT concentration can be varied while the AgNW concentration is held fixed. The resulting thermal conductivity trend with varying FWCNT concentration should yield
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some interesting insights into the mechanisms of thermal transport though unary, AgNW filler systems.