Studies On Thermal Conductivity Of Carbon Nanotubes Decorated Pitch-based Carbon Fibers And Their Epoxy Composites

Pitch-based carbon fiber（PCF）,with its neatly arranged carbon atoms,has excellent properties such as high strength,high modulus and high thermal conductivity.PCF is widely used in aerospace and electronic packaging fields as a thermal conductivity composite material.However,PCF/epoxy resin（EP）composite is difficult to form a high thermal conductivity path due to fiber discontinuity and weak bonding between fiber and resin in the vertical fiber direction,so the thermal conductivity in the vertical fiber direction is much lower than that in the fiber direction,which limits the application of PCF/EP thermal conductivity composite.Therefore,it is urgent to construct a thermal conduction pathway between fibers to improve the thermal conductivity of PCF/EP composites.In this study,carbon nanotubes（CNTs）are grown on the surface of PCF by chemical vapor deposition（CVD）and microwave method.The mechanism of CNT growth by CVD and microwave method and the optimal growth conditions of CNT are investigated respectively,and the composites with high vertical thermal conductivity are prepared.The main research contents are as follows:（1）Study on cCNT@PCF prepared by CVD method and thermal conductivity of c CNT@PCF/EP composites:In order to enhance the heat conduction between PCFs,high-density carbon nanotube（c CNT）with a large aspect ratio（250:1）is uniformly grown on PCF surface by CVD method.By adjusting the temperature of CVD and the concentration of catalyst precursor（Ni Cl₂·6H₂O）,the optimum growth conditions（650℃,0.32 M）for large aspect ratio of CNT are obtained,and the growth mechanism of CNT is clarified.The presence of carbon nanotubes not only enhance the conduction of phonons and electrons between fibers,but also effectively inhibit the propagation of cracks when the composites are subjected to external forces.Therefore,compared with the unmodified PCF/EP composites,the interlaminar shear and flexural strengths of the prepared c CNT@PCF_-650/EP composites are 41 and 515 MPa,respectively,which are increased by 64.0 and 66.1%,respectively.The thermal conductivity of vertical fiber and parallel fiber is 3.06 and 307.8 W/（m K）,increasing 59.4 and 124.2%,respectively.The electrical conductivity of vertical fiber and parallel fiber is 0.39 and 53.8 S/cm.By evenly dispersing carbon nanotubes in the resin matrix,the thermal and electrical conductivity network between the fibers is densified,which significantly improve the mechanical,thermal and electrical conductivity properties of epoxy composites.When the addition amount of carbon nanotubes is 0.5 wt.%,the interlaminar shear strength and flexural strength of c CNT@PCF_-650/EP_-0.5 composites are increase to 46 and 546 MPa,respectively,and the thermal conductivity of vertical and parallel fibers are increased to 3.73 and 330.1 W/（m K）,respectively.The electrical conductivities of vertical and parallel fibers are increased to 0.49 and70.9 S/cm,respectively.（2）Study on mCNT@PCF prepared by microwave method and thermal conductivity of m CNT@PCF/EP aerogel composites:Short-cut PCF aerogel is prepared by directional freezing,and the variation of thermal conductivity of its epoxy composites with the content of pitch-base carbon fiber is investigated.The thermal conductivity of oriented PCF aerogel/epoxy composites is the best（5.77 W/（m K））when the carbon fiber content is 10 vol%.Based on the optimal carbon fiber content,carbon nanotube（m CNT）is grown using microwave after loading granular polypyrrole（PPy）on the surface of carbon fiber,and the effect of irradiation time on the morphology of carbon nanotube is investigated under the microwave irradiation power of 700 W.The length and uniformity of the grown carbon nanotube are the best when microwave irradiation is performed for 30 s.The thermal conductivity of m CNT@PCF/EP_-30 aerogel composite in parallel and vertical fiber directions are 8.9 and 2.5 W/（m K）,respectively,which are much higher than that of pure epoxy（0.2 W/（m K））.