Study On Novel Ceramic-carbon Nanotube Hybrids And Their Composites Based On Epoxy Resin With High Dielectric Constant And Low Dielectric Loss

The new dielectric material with high dielectric constant has important applications inthe field of information technology, microelectronics, power engineering, defense andscience and technology, thus causing widespread concern. Main methods for preparinghigh dielectric constant material is by adding a conductor or a dielectric ceramic, which isthe filler to the polymer matrix composite material. But the conductor/polymer compositesand ceramic/polymer composites have different features. When the added amount reachesthe percolation threshold, the former principal conductor material can get a high dielectricconstant, but also has a high dielectric loss. It will not only result in waste of energy, andwill also shorten the life of the equipment. The dielectric ceramic/polymer composites toobtain high dielectric constant should generally be added to the high content of ceramicdielectric, which is not conducive to processing, and may reduce the flexibility andmechanical properties of materials. Therefore, people started to combine theconductor/polymer and ceramic/polymer composites, in order to integrate the advantagesof two types of binary composites. Recently, developing such ternary composites with highdielectric constant and low dielectric loss by designing novel spatial structure is a veryinteresting and challenging subject. And this thesis is focused on this theme.Firstly, a novel type of hybrid filler (LTNOx-CNT) was synthesized using a simplemethod. The configuration was that Li and Ti doped NiO (LTNO) in nano-scale weregrafted onto the surfaces of carbon nanotubes (CNTs). This hybrid filler is not madethrough simple physical blending, but by two inorganic particles chemically linked. Thisdesign can not only be effective in two different inorganic particles bonded into a wholeorganic filler, but is more interesting that the surface of the inorganic particles with organicgroups can help combined well with the polymer matrix, and to reduce voids between thephase interface. It is worth mentioning that the grafted surface of the CNT can beeffectively isolated by nano-ceramic LTNO and it can prevent CNT in contact with eachother to improve its dispersibility in a resin matrix, thus facilitating preparation of the composites with high dielectric constant and low dielectric loss.Then based on this, these synthesized fillers were embedded in epoxy resin (EP),whereby composites (LTNOx-CNT/EP) with high dielectric constant and low dielectricloss were developed. In order to evaluate the function of LTNOx-CNT and the interactionand synergistic effect between LTNO and CNTs, composites fabricated with simpleblending method (LTNOx/CNT/EP) and carboxylated carbon nanotubes (aCNT)/EPcomposites and pure ceramic composites (LTNOx/EP) were developed, while theirdielectric properties were studied. The results showed that the graft of LTNO couldimprove the dispersion of CNTs and the degree of improvement was related to the loadingsof LTNO. Interestingly, when the weight ratio of aCNT is0.5%, the same as the graftingamount of LTNO, the composite was1.37and0.00068times than the pure aCNT/EPcomposite in dielectric constant and dielectric loss, respectively. As for the simpleblending composite with the same composition, it is0.12and0.18times. Thisphenomenon states that the dielectric properties of (LTNOx-CNT)/EP are not simply thesum of LTNOx/EP and aCNT/EP, but show significant synergistic effects. The compositesbased on the hybrid fillers exhibit excellent dielectric properties due to the differentstructure of (LTNOx-CNT)/EP. Specifically, the spatial separation owing to LTNO graftedon CNTs improves the dispersion of CNTs, thereby the increase of the number ofmicro-capacitance due to isolated CNTs in the matrix results in the increase of dielectricconstant of the composites, in the mean time the ceramic particles on the surface reducesthe dielectric loss generated by the direct contact of CNTs. An equivalent circuit modelwas set up for aCNT/EP and LTNOx-CNT/EP and it proves that the different structures ofthe composites cause the significant difference of the dielectric behavior. The outstandingdielectric properties of LTNOx-CNT/EP indicate a new way for the preparation ofcomposites with high dielectric constant and dielectric loss.Finally, to further increase the dielectric constant, we use the energy generated bymicrowave curing which can fast curing epoxy and thus can reduce the settlement of theCNT, designing a way to use microwave curing of m-LTNOx-CNT/EP resin compositematerials. While dielectric properties of microwave curing of contrast materialm-aCNT/EP, m-LTNO/CNT/EP and m-LTNO/EP composites are studied. The resultsshow that the when adding the same amount LTNO, the dielectric constantm-(LTNOx-CNT)0.5/EP can be increased to26.5times of m-LTNOx/EP, and the dielectric loss of only11.5times increase in the same the amount of aCNT. The dielectricconstant of m-(LTNOx-CNT)EP can be increased to1.04times m-aCNT0.5/EP, while thedielectric loss was only0.0076times of m-aCNT0.5/EP. With grafting of LTNO, thecomposite can get a higher dielectric constant, while the dielectric loss is maintained lowvalue. Significantly improved the dielectric properties of the system only from spacebarrier effect LTNO graft after CNT surface, that a small amount of graft LTNO ceramicparticles can effectively reduce the reunion between the CNT, it is worth mentioning that,microwave curing which fast curing epoxy resin can reduce the settling time CNT CNT inthe matrix to improve the dispersibility of the composite, greatly improving the dielectricproperties. While largely enhancing the number of micro-capacitance, it reduces thedielectric loss by shielding the conduction current and decreasing the tunneling current.