Two kinds of surface-modified MWCNT/Al2O3/ETDS composites, cured pad and uncured grease, were fabricated for high-thermal-conductivity materials application. MWCNTs were modified using aminoproplytriethoxysilane (APTES) and dodecylamine after acid treatment. The reaction mechanism of APTES and the surface structure of MWCNT was controlled using different reaction conditions. The effect of surface modification on the thermal conductivity was confirmed based on a 70 wt% Al2O3/ETDS composite. The thermal conductivity was found to be dependent on the composite type and surface structure of the modified MWCNTs; the amine-terminated MWCNTs showed the highest thermal conductivity among the pad type composites, while long carbon chain-terminated MWCNTs showed outstanding performance among the grease type composites. Moreover, the maximum processable MWCNT content was also influenced by the surface modification; structurally similar silanol groups affected the molecular mobility of the ETDS resin and they had the highest MWCNT content under the same process conditions.

고방열 복합소재를 위하여 고분자 경화 유무에 따른 두 가지 종류의 표면처리된 탄소나노튜브/알루미나/ETDS(epoxy-terminated dimethylsiloxane)복합체를 제작하였다. 탄소나노튜브는 산처리 후 실란커플링제 및 dodecylamine을 이용하여 표면처리를 진행하였다. 실란커플링제와의 반응 메커니즘은 반응조건에 따라 조절되었다. 탄소나노튜브 표면처리에 따른 열전도도 특성 변화를 70 w% 알루미나/ETDS 복합체를 기반으로 확인하였다. 탄소나노튜브 표면처리의 효과는 복합소재의 종류에 따라 상이한 경향을 보이는데, 패드 형태의 복합소재에서는 아민기가 말단으로 위치할 때 가장 우수한 열전도도 특성을 보이는 반면 그리스 형태의 복합소재에서는 긴 탄소사슬이 말단에 위치한 구조가 가장 높은 열전도 특성을 보였다. 또한, 실란올기가 말단에 위치할 때 탄소나노튜브의 복합화 가능한 함량이 가장 높아져, 표면처리 방법에 따른 탄소나노튜브의 최대 함량 및 열전도도 차이가 나타나는 것을 확인하였다.

Keywords: multi-walled carbon nanotube, surface modification, thermal conductivity, surface structure, matrix

Multi-walled carbon nanotubes (MWCNTs) exhibit outstanding thermal and electrical properties, mechanical strength, and optical properties, and the development of mass production processes has led to an increase in the applications of MWCNTs in various fields not only academic but also industrial, such as electronic devices, electrodes for energy storage devices, catalysts, and transparent devices.^1-4 Among them polymeric composite materials were considered one of the most promising application, consisting of small amounts of MWCNTs have been reported to exhibit enhanced thermal and mechanical properties.⁵ To realize the true potential of MWCNTs, it is important to overcome their extreme hydrophobicity and high surface energy, as these factors lead to aggregation of MWCNTs in polar liquids and polymer matrices. Therefore, proper dispersion and good interfacial bonding between the CNTs and the polymer matrix are essential requirements for utilizing CNTs as effective reinforcements in polymer composites.^6,7 Many researchers have focused on surface modification of MWCNTs via functionalization or surface coating using various organic or inorganic materials to obtain highly dispersed MWCNT composites. These surface modification processes improve the intermolecular interactions, both chemically and physically, preventing aggregation of MWCNTs and ensuring uniform dispersion. However, an allpowerful surface modification method does not exist; the introduced surface structure and applied polymeric material hardly influence the properties of composite materials. Therefore, finding a suitable combination of surface modifications and matrices is challenging.^8,9 Generally, wet chemical oxidation using strong acid solutions is recognized as an efficient method for MWCNT purification, promoting dispersion and surface activation at the same time. The introduced functional groups enable further modification via chemical reactions at the MWCNT surface; it is the general starting point for proper surface modification with various matrices.¹⁰
Owing to miniaturization and multi-functionalization of electronic devices, MWCNTs have great application potential as high-thermal-conductivity materials. Especially, the development of light emitting diodes has accelerated research on high-thermal-conductivity materials to improve the lifespan and the reliability of the diodes. Most of the thermally conductive materials are composed of high-thermal-conductivity ceramic particles and polymeric materials.^11,12 Despite the ultra-high thermal conductivity of MWCNTs, their applications are very limited owing to their high electrical conductivity that causes leakage of electrical signals. However, thermally conductive materials containing small amounts of MWCNTs are promising because the nanotubes form heat conducting bridges between the other fillers. In this case, uniform dispersion of MWCNTs is very important as only a small amount of MWCNTs is used. The dispersed MWCNTs should not form aggregated bundles, instead they should connect with other particles of the matrix; therefore, appropriate surface modification is necessary.^13,14
In this study, epoxy-terminated polydimethylsiloxane based thermally conductive composites were fabricated using Al₂O₃ and MWCNTs. The MWCNTs were chemically modified using a strong acid solution to introduce oxygen functional groups, and the resulting MWCNTs were further modified using the silane coupling agent aminopropyltriethoxysilane (APTES) and dodecylamine. The APTES treatments were performed at two reaction conditions to control the surface structure. The Al₂O₃ concentration was fixed at 70 wt%, and the effect of surface modification of MWCNTs on the thermal conductivity was studied. Furthermore, two types of composite materials were fabricated: uncured grease type and thermally cured pad type with hardener. The thermal conductivity of the different composite materials was correlated with the surface structure of the MWCNTs and thermal curing.