Study On Novel Functionalized Graphene Hybrids And Their Composites Based On Epoxy Resin With High Dielectric Constant And Low Dielectric Loss

The great potentials of applying in many cutting-edge fields, such as electronics, energy conversion and storage materials, make high dielectric constant(High-k) polymeric composites become one of hottest functional materials, and thus have attracted wide and great attentions of scientists and engineers worldwide. Electric conductor/polymer composites based on the percolation theory is one important form for fabricating High-k composites. However, generally, they also exhibit high dielectric loss due to the occurrence of the large leakage current near the percolation threshold(fc). How to decrease dielectric loss of the conductor/polymer composites is a key issue for developing High-k materials. To solve this issue, many researchers tried to form insulating coatings on the surfaces of the conductors, and found that this route can effectively reduce the dielectric loss. But, the insulating layer coating on the conductors could inevitably hinder the formation of conductive network. Moreover, high contents of the fillers are often needed to obtain high dielectric constant, which not only will cause the increase of fc, but also tend to degrade the processability and other properties such as mechanical properties of composites. Therefore, developing a kind of composites with high dielectric constant, low dielectric loss and low fc has been an important project. This thesis is focused on this goal.Starting from the design and preparation of novel functional graphene, we have done researches based on the relationship between structure and performance. Firstly, a novel hybrid with extraordinary microcapacitor structure(Ti O2@Ti B2-r GO) was synthesized by decorating core-shell Ti O2@titanium diboride(Ti B2), Ti O2@Ti B2, on reduced graphene oxide(r GO) through the chemical coupling. And the Ti O2@Ti B2-r GO hybrids were embodied into epoxy(EP) resin to produce a series of Ti O2@Ti B2-r GO/EP composites. By adjusting the weight ratio of Ti O2@Ti B2 to r GO, the electric and dielectric properties of Ti O2@Ti B2-r GO/EP composites were intensively studied. Results show that Ti O2@Ti B2-0.75 r GO/EP composites have much higher dielectric constant, lower dielectric loss and low fc(0.758wt%). When the mass ratio of Ti O2@Ti B2 to r GO is 0.1:1, the dielectric constant and loss of the Ti O2@Ti B2-0.75 r GO/EP composite are 77 and 0.55 at 100 Hz, about 1.14 and 5.8×10-2 times of those of 0.75 r GO/EP composite at the same content of r GO, respectively. These results can be attributed to the construction of a large number of microcapacitors in the composites, where Ti B2 and r GO conductors act as the electrodes, while Ti O2 insulating shell acts as the dielectric medium. And the core-shell structure of Ti O2@Ti B2 on r GO can block the connection of r GO sheets, so the leakage current is suppressed by isolating the r GO sheets in EP matrix, resulting in a lower loss tangent for the Ti O2@Ti B2-0.75 r GO/EP composites. The impedance spectra of Ti O2@Ti B2-0.75 r GO/EP composites are simulated using equivalent circuit model, which deeply explains the influence of unique micro-capacitor structure of Ti O2@Ti B2-r GO hybrid on the dielectric properties of Ti O2@Ti B2-0.75 r GO/EP composites.Secondly, in order to keep the excellent electrical property of r GO from being damaged and acquire composite materials with a higher dielectric constant, a novel hybridized conductor with “sandwich” structure(r PANI@CNT-r GO) was designed and prepared by introducing polyaniline(r PANI) coated carbon nanotube(CNT) on the surface of reduced graphene oxide(r GO) through double driving forces(electrostatic and π-π conjugate). On this basis, fixing the r GO loading in epoxy resin(EP) at 0.75wt%, and then r PANI@CNT-r GO hybrids with different loadings of r PANI coated CNT(r PANI@CNT) were introduced into EP to prepare a series of r PANI@CNT-0.75 r GO/EP composites. At the same time, r PANI@CNT and r GO were used as conductors and added into EP to prepare r PANI@CNT/0.75 r GO/EP composites by the mechanical blending method for comparison. The electrical and dielectric properties of r PANI@CNT-0.75 r GO/EP and r PANI@CNT/0.75 r GO/EP composites were studied systematically. Results show that r PANI@CNT/0.75 r GO/EP composites have low dielectric constant(10-20), whereas the dielectric constant of r PANI@CNT-0.75 r GO/EP composites increases with the increasing content of r PANI@CNT, and the dielectric constant of the 7r PANI@CNT-0.75 r GO/EP composite with 0.75wt% is as high as 210 at 100 Hz, much larger than those of r PANI@CNT/EP, 0.75 r GO/EP and r PANI@CNT/0.75 r GO/EP composites. Meanwhile, the dielectric loss of 7r PANI@CNT-0.75 r GO/EP composite at 100 Hz is 208, 0.18 and 22 times of that of r PANI@CNT/EP, 0.75 r GO/EP and r PANI@CNT/0.75 r GO/EP composites, respectively, indicating that the dielectric behavior of r PANI@CNT-0.75 r GO/EP composites does not originate from a simple addition of the basic components, but has an obvious synergistic effect. Besides, the percolation threshold of r PANI@CNT-0.75 r GO/EP composites is only 1.1 wt%.