In this study, we investigated the heat transfer behavior of thermally conductive networks with carbon materials to confirm effective heat transfer pathways of polymer matrix composites.
In chapter 1, the processing method, analysis, and applications of thermally conductive polymer composites were discussed.
In chapter 2, nano-sized few-walled carbon nanotubes (FWCNTs) and micro-sized mesophase pitch-based carbon fibers (MPCFs) were used as the thermally conductive materials. The bulk density and thermal conductivity of the FWCNT films increased proportionally with the ultrasonication time, owing to the enhanced dispersibility of the FWCNTs in ethanol solvent. The ultrasonication-induced densification of the FWCNT films led to the effective formation of filler-to-filler connections, resulting in improved thermal conductivity. The thermal conductivity of the FWCNT-MPCF hybrid films was proportional to the MPCF content (maximum thermal conductivity at an MPCF content of 60 wt%), indicating the synergistic effect on the thermal conductivity enhancement. Moreover, the MPCF-to-MPCF heat transfer pathways in the FWCNT-MPCF hybrid films were the most effective for achieving high thermal conductivity, owing to the smaller interfacial area and shorter heat transfer pathway of the MPCFs. The FWCNTs could act as thermal bridges between neighboring MPCFs for effective heat transfer.
Furthermore, the incorporation of Ag nanoparticles of approximately 300 nm into the FWCNT-MPCF hybrid film dramatically enhanced the thermal conductivity, which was closely related to a decreased thermal interfacial resistance at the intersection points between the materials. Epoxy-based composites loaded with the FWCNTs, MPCFs, FWCNT-MPCF hybrids, and FWCNT-MPCF-Ag hybrid fillers were also fabricated. A similar trend in thermal conductivity was observed in the polymer matrix composite with carbon-based hybrid films.
In chapter 3, micro-sized MPCFs with different lengths (50 ㎛, 200 ㎛, and 6 mm) and nano-sized reduced graphene oxides (rGOs) were used as the thermally conductive fillers used for the preparation of the heat-dissipation polymer composites. For all MPCF fillers with a different length, the thermal conductivity values of the MPCF/epoxy composites were proportional to the MPCF length and loading amount (0 to 50 wt%) of MPCFs. For an MPCF:rGO weight ratio of 49:1 (total loading amount of 50 wt%), the thermal conductivity values of MPCF-rGO/epoxy composites loaded with MPCFs of 50 ㎛, 200 ㎛, and 6 mm increased from 5.56 to 7.98 W/mK (approximately 44% increase), from 7.36 to 9.80 W/mK (approximately 33% increase), and from 11.53 to 12.58 W/mK (approximately 9% increase) compared to the MPCF/epoxy composites, respectively, indicating the synergistic effect on the thermal conductivity enhancement. The rGOs in the MPCF-rGO/epoxy composites acted as thermal bridges between neighboring MPCFs, resulting in the formation of effective heat transfer pathways. In contrast, the MPCF-rGO/epoxy composites with MPCF:rGO weight ratios of 48:2 and 47:3 decreased the synergistic effect more significantly compared to rGO content of 1 wt%, which is associated with the agglomeration of rGO nanoparticles. The synergistic effect was inversely proportional to the MPCF length. A theoretical approach, the modified Mori-Tanaka model, was used to estimate the thermal conductivity values of the MPCF-rGO/epoxy composites, which were in agreement with the experimentally measured values for MPCF-rGO/epoxy composites loaded with short MPCF lengths of 50 and 200 ㎛.
In chapter 4, we implemented high-temperature heat treatment to improve the quality of reduced graphene oxide (rGO). And the change of rGO was observed according to the heat treatment temperature. The rGO was heat-treated at 1000 ℃ to 2500 ℃ in 500 ℃ increments. The high-temperature heat-treated rGOs were observed structure change by XRD, XPS, and Raman spectroscopy. As a result of the analysis, it was confirmed that as the oxygen groups of rGO decreased after heat treatment, defects decreased, and crystallinity increased. Besides, the high-temperature heat-treated rGOs exhibited an electrical conductivity similar to commercial graphite over 2000 ℃ heat treatment. The pristine rGO and high-temperature heat-treated rGOs were used as a filler to make epoxy composites, and the change of thermal conductivity was observed. When the contents of fillers were 10 wt%, the thermal conductivity of 2500 ℃ heat-treated rGO composite increase of approximately 1064% (0.22 to 2.56 W/mK) compared with that of pristine rGO composite. This result is approximately 212 and 99% higher than that of commercial graphite (0.82 W/mK) or commercial MPCF (1.29 W/mK) composite, respectively. Based on these results, it can be seen that the improvement of rGO quality is closely related to the improvement of conductivity.