Preparation and characterization of carbon micro/nano hybrids and their functional composites

Due to the outstanding mechanical electrical and thermal properties, carbon nanotubes (CNTs) received worldwide attentions and intensive investigations in last decades. CNTs are greatly potential in applications such as energy storage and microelectronics. The one dimensional structure, high aspect ratio and low density, promote CNTs serving as the excellent fillers in composites field. However, due to the strong interactions, CNTs are usually difficult to be dispersed and aligned in a polymer matrix. Designing the CNTs construction reasonably is an effective way to ameliorate the dispersion states of CNTs in matrix. These specific hybrid constructions allowed CNTs arrays synthesized vertically onto the substrates through catalyst chemical vapor deposition method. These CNT arrays effectively overcome the problem of CNTs aggregation and promote the interconnection among CNTs, leading to a considerable improvement of multi-functional properties of composites. Graphite nanoplatelets (GNPs) served as substrate make their synthesizing products-GNP-CNTs hybrids (GCHs) possess distinct merits of all-carbon composition, totally-conductive coupling structure and the low intrinsic density. These GCHs constructions provide a great improvement in the dielectric and electrical properties of composites. However, the relationship between GCHs organization and synthesizing conditions during CVD process and the influence of the addition of GCHs to internal conductive networks have not been reported in detail. These mentioned issues will be investigated and discussed in this thesis, which is divided into four chapters:The first chapter makes a general review of the structure, properties, application and synthesis of CNTs and GNP substrates, and the main procedures of fabricating composites and surface functionalization of CNTs. Moreover, a short introduction of the development of micro-nano hybrids applied to the functional composites is made. Most importantly, the developing electrical states and (di) electrical characteristics of composites with ever-increasing conducting filler loading are reviewed in detail at the last part.The second chapter discusses firstly the synthesis process through the CCVD approach and the relationship between CVD parameters and the corresponding construction of GCHs, where the temperature, gas composition and reaction time were controlled. The constructions CNT arrays are dependent on the synthesis conditions. Furthermore, the results obtained from analysis can provide a structural foundation for the huge application potential of GCHs constructions. The third chapter introduces the poly(vinylidene fluoride)-based nanocomposites containing GCH particles, the dielectric properties of which are improved more greatly than the ternary composites loading equivalent mixture of GNPs and CNTs. The composites achieved by dispersing GCH particles into matrix using the mechanical melt-mixing process, showing a strongly reduced percolation threshold (5.53 vol %) and the relatively high thermal stability. Their improved dielectric properties can be attributed to the formed microcapacitor networks and the change of crystalline formation of matrix, caused by well-designed CNT arrays constructions. The fourth chapter investigates the advanced GCHs/ polydimethylsilicone (PDMS) composites with high piezo-resistive performance at wide temperature range. The synthesized GCHs can be well dispersed in the matrix through the mechanical blending process. The flexible composite shows an ultra-low percolation threshold (0.64 vol%) and high piezo-resistive sensitivity (gauge factor ~103 and pressure sensitivity ~ 0.6 kPa-1). Particularly, the much improvements of electrical properties achieved in GCHs/PDMS composites compared with composites filled with equivalent CNT, GNP or mixture of CNTs/GNPs. Slight motions of finger can be detected and distinguished accurately using the composites film as typical wearable sensor.