| Heat conducting materials have been extensively applied in the fields of heat exchange engineering, heat collection engineering and electronic information engineering. With the development of industrial production and technology, heat conducting materials are demanded to possess good combined properties. The study on thermally conductive polymer-matrix composites(PMCs) is attractive. Multi-walled carbon nanotubes(MWCNTs) are considered to be a type of very ideal nano-sized filler for high-performance heat conducting PMCs due to their large aspect ratio, extremely high thermal conductivity, excellent mechanical properties and stable chemical behaviour. Epoxy resins are dominant in the field of thermosetting resins owing to their high adhesion, good heat resistance, superior chemical corrosion resistance, high mechanical strength and relatively low cost. However, the main shortcomings of epoxy resins are low thermal conductivity and poor toughness for the application of thermally conductive PMCs. In the present work, an idea that the design of MWCNT surface functionalization is combined with the improvement of allround properties of epoxy composites is presented. The aim is to obtain uniform dispersion of MWCNTs in epoxy matrix and effectively improve MWCNT-epoxy interfacial interaction. The effects of functionalized MWCNTs on the thermal conduction improvement, reinforcing and toughening of epoxy composites are investigated. In order to further optimize the combined properties of epoxy composites, hybrid filler system consisting of functionalized MWCNTs and modified nano-sized silicon carbide particles(SiCnp) is utilized. Epoxy composites filled with hybrid filler system are prepared and the mechanism for combined property optimization of the composites is further elucidated.Surface functionalization of MWCNTs was firstly performed. The synthetic procedure is shown as follows: MWCNTs were first treated by a 3:1(v/v) mixture of concentrated H2SO4/HNO3, and then reacted with SOCl2, finally triethylenetetramine(TETA) grafting was carried out. The final product was TETA-functionalized MWCNTs, namely defined as T-MWCNTs. X-ray photoelectron spectroscopy analysis proves that TETA has been successfully grafted onto the MWCNT surface. The quantitative analysis of the crystalline content of different MWCNTs was performed by wide-angle X-ray diffraction and the results indicate that T-MWCNTs maintain good microstructure. Field emission scanning electron microscope(FESEM) and UV-vis-near IR analyses denote that T-MWCNTs are looser and possess better dispersion state in absolute ethyl alcohol than as-received MWCNTs. High-resolution transmission electron microscope analysis provides the direct evidence that TETA is effectively grafted onto the MWCNT wall to form a thin layer of thickness 3nm calculated by thermogravimetric analysis. T-MWCNTs can be regarded as the'core-shell'structure.The curing process is closely related to the quality of final products. The weight ratio of epoxy/curing agent is 100:6 in neat epoxy system and MWCNT/epoxy systems. The curing kinetics of all systems follows the autocatalytic kinetic mechanism. MWCNTs delay the cure reaction of epoxy due to their steric hindrance. Therefore, the onset cure temperature Ti, the peak temperature Tp and the activation energy Eαincrease and the heat of cure△H decreases. After surface functionalization, TETA functional groups on the MWCNT surface can play the role of curing agents and facilitate the primary amine-epoxide reaction, which means that they weaken the retardation effect caused by MWCNTs on the cure reaction of epoxy. Consequently, Ti, Tp and Eαfall and△H rises a little. According to the fact that MWCNTs have the retarding effect on the cure reaction of epoxy, with the adding of fillers, the curing process of MWCNT/epoxy systems can be based upon that of neat epoxy system(precured at 80℃for 1h, cured at 120℃for 1.5h and postcured at 140℃for 1.5h in a vacuum oven) and simultaneously the postcure time should be extended.The preparation of MWCNT/epoxy composites was carried out by solution blending-in situ polymerization method. FESEM, Raman microscope and differential scanning calorimetry analyses indicate the homogeneous dispersion of T-MWCNTs and the improvement of interfacial interaction between T-MWCNTs and epoxy matrix. T-MWCNTs are more effective heat conducting fillers than as-received MWCNTs. When nanotube volume fraction is 1%, the thermal conductivity of T-MWCNT/epoxy composite is 1.3W·m-1·K-1, 3.7times that of epoxy matrix; when nanotube volume fraction is in the range of 1%～4%, with the increase of fillers, thermal conductivity of T-MWCNT/epoxy composites is enhanced and it reaches the maximum value of 3.9W·m-1·K-1, furthermore, thermal conductivity calculation can be performed by the rule of mixture; when nanotube volume fraction is continuously increased, thermal conductivity of T-MWCNT/epoxy composites shows a descending tendency. Likewise, T-MWCNTs are more efficient reinforcing and toughening fillers. When nanotube volume fraction is 1%, the impact toughness, bending strength and bending modulus of T-MWCNT/epoxy composite reach the maximum values of 22.3kJ·m-2, 119.7MPa and 2.9GPa, respectively. Compared with the maxima of corresponding performance indices of as-received MWCNT/epoxy composite, the three performance indices of the composite increase by 47%, 25% and 21%, respectively. With the continuous adding of fillers, mechanical properties of the composites reveal a falling tendency.In order to further optimize the combined properties of epoxy composites, hybrid filler system consisting of T-MWCNTs and silane-modified SiCnp was applied. In hybrid filler/epoxy composites, nanotube volume fraction maintains 1%. Epoxy composites filled with hybrid filler system possess better heat-conducting property and greater mechanical performance indices, compared with those filled with single filler system. When filler volume fraction is 2%, the mechanical properties of hybrid filler/epoxy composite are the best and its thermal conductivity, impact toughness, bending strength and bending modulus are 2.6W·m-1·K-1, 23.9kJ·m-2, 130.5MPa and 3.8GPa, respectively; when filler volume fraction is 5%, the heat-conducting property of the corresponding composite is the best and its performance indices are 6.1W·m-1·K-1, 12.7kJ·m-2, 93.9MPa and 2.3GPa, respectively。MWCNT surface functionalization could establish the covalent bonding between nanotube wall and epoxy matrix, which decreases interfacial thermal resistance and facilitates the dispersion of MWCNTs in epoxy matrix and the formation of heat-conducting network. Therefore, effective thermal conductivity improvement of epoxy composites is shown. The'core-shell'structure endows T-MWCNTs with the special mechanism of reinforcing and toughening. This soft layer establishes good connection of MWCNT wall to epoxy matrix, so it can efficiently transfer loading between the reinforcer and the matrix and absorb impact energy. Thereby, mechanical properties of the composites are greatly enhanced. The filling of SiCnp further improves MWCNT-to-MWCNT network and effectively inhibits the agglomeration of fillers, which is beneficial to the optimization of combined properties of epoxy composites.
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