Preparation And Properties Of Graphene Based Composite

Graphene, one kind of inorganic nanosheet materials consisting of sp² hybridization of carbon atoms arranged in hexagonal cycle formed by two dimensional honeycomb lattice structure, its thickness is only 0.335 nm, which can be considered as the basic construction material of all other carbon dimensionalities, becoming the basis of theoretical calculation and derivation for other material. The unique single atomic layered structure of graphene makes it having many excellent properties, such as the electron mobility at room temperature?2×105cm².V.s?, the strength of 130 GPa, young's modulus is about 1100 GPa, coefficient of thermal conductivity is as high as 5300W/?m.K?, also a great specific surface area?theoretical calculation value is 2630 m²/g?. These superior performances make it showing the important research value and broad application prospects in the energy materials, microelectronics, high-performance composites, information and biotechnology field. In the past decade, the unique nature of graphene makes it applicable to theoretical studies and various technologies and rapidly become as a sharply rising star in many frontier research fields including material science and condensed physics.Graphene intrinsic high mechanical strength, excellent electrical and thermal conductivity makes it being the ideal nano filler used in composite material performance enhancements. At present, the researches mainly focus on the graphene/polymer composites and low dimensional graphene inorganic nanocomposites. Traditional ceramic composite materials usually adopted one dimensional carbon fiber, carbon nanotubes and the ceramic whisker as the strengthen phase to improve the performance of the material, but these adding materials tend to be unequally dispersed in the matrix and easy to agglomeration. Compared with low dimensional nanocomponent, such as CNTs, graphene has a greater advantage, it can be well dispersed in the ceramic matrices, and it is a great application potential that bringing its excellent mechanical, electric and thermal properties to ceramic composite to improve the comprehensive performance of materials, which can be expected to get structure-function integration composite materials with some unique properties. However, compared with graphene/polymer composites and the low dimensional graphene inorganic nanocomposites, research on graphene/ceramic composites started relatively later, however, due to the particularity of preparation technics of ceramic materials, the preparation on graphene requires high yield, lower cost, better dispersibility and more easily to disperse evenly in ceramic. With the deep-going research of the graphene materials, the preparation and application research of graphene and its based composites have made some great progress, but it is still lack of the appropriate technology of graphene for ceramic materials. So it also confront with a lot of problems and challenges to really achieve the large-scale synthesis and industrialization application for graphene based inorganic nanocomposite. To explore the low-cost synthesis of graphene and preparation of ceramic composite materials has very important research significance and application value.Based on the above, we used different ceramic materials as matrices, and the introduction of excellent graphene materials could enhance the graphene/ceramic composites and overcome the shortcomings of ceramic materials with high brittleness and low reliability, but also enhance the comprehensive performance of composites with special performance of functional ceramics or structural ceramics. We proposed a preparation method of graphene composite ceramic, which is low cost and suitable for industrial application. By this method, the GNSs could be prepared, dispersed, and mixed homogeneously in powder mixtures simultaneously can be realized by one-step in the process of the micromechanical. And then we discovered the introduction of GNSs results in the grain refinement of TiC matrix and completely stops the TiC grain growth, meanwhile, the sintering model and sintering mechanism was come up. The main contents are as follows:1. Two kinds of different methods were chosen to prepare graphene material and new method for the preparation of newly graphene material has a preliminary exploration and discussion. Firstly, in order to improve the stripping effect, we started by exfoliating commercial expandable graphite by microwave heating in a home microwave oven at 900 W for 12-20 s. Then, the large scale and high quality graphene nanosheets?GNSs? were respectively fabricated by ultrasonic stripping method and chemical oxidation reduction method, and the microstructure and morphology were characterized in detail. On the other hand, we developed a new method of using cryogenic grinding machine in liquid nitrogen?-195.61 o C? to successfully prepare graphene nanomesh?GNMs? from graphene oxide?GO?, which is never reported. The morphology of obtained GNMs observed by transmission electron microscopy?TEM? clearly showed the holes punched out in the GO sheets, with a size of 3 to 50 nm with a homogeneous distribution. We also analyzed the forming mechanism of GNMs: During the CG process, ice nano columns growing perpendicularly to the surface of GO sheets are expected to, like bears, punch out holes on the GO surfaces, turning the GO to GNMs. As a matter of fact, defects in a GO sheet may be unevenly distributed, indicating many local parts of a GO sheet have much lower strength, and as a result, holes of different size are produced. The above reasonable speculation on the forming mechanism of GNMs is just consistent with its characterization.2. Choosing alumina?Al₂O₃? as the ceramic matrix, we prepared graphene based composite by one-step method. Firstly, we started by commercial graphite by microwave heating to obtain the expandable graphite, which is a good start for the next step of preparation with matrix material. Then, the expandable graphite was ground with Al₂O₃ nano-powder using a planetary mill for 30 h. In order to make the graphite layers can be better stripped and dispersed in matrix, N-methyl-pyrrolidone?NMP? was chosen as the dispersal media. Finally, bulk GNSs/ Al₂O₃ composites were obtained using spark plasma sintering?SPS? apparatus. Results showed that the introduction of GNSs clearly inhibits the agglomeration of matrix nano-powder and meanwhile the added Al₂O₃ powders acted as nano-balls to exfoliate the expanded graphite, and were also loaded on the surface of delaminated GNSs to prevent the aggregation of sheets. By this method, the GNSs could be prepared, dispersed and mixed homogeneously in powder mixtures by one step. Studies on SPSed bulk composits showed that the introduction of GNSs results in the refinement of the grain of Al₂O₃ matrix, from 2.4?m to 230 nm, merely 10% of matrix grain. Compared with monolithic Al₂O₃, the flexural strength, fracture toughness and Vickers hardness of composites have been significantly improved by 103%, 26%, 25%, respectively. Since this method is simple and practicable, it is expected to realize the scale production of the composite in the future.3. In order to keep the excellent electrical properties of graphene from destruction, we resorted to the mechanical grinding method to prepare GNSs/silicon oxide?SiO₂? composites, that is, expanded graphite and amorphous silicon powder were mixed directly via wet ball milling for 30 h, in which, choosing NMP as dispersion medium. This process can make the preparation, dispersion of GNS and its composites with the matrix completed simultaneously. In the final step, bulk GNSs/ SiO₂ composites were obtained using SPS technology. A series of structure and performance characterization on powder and bulk material had been carried out. Studies found that the prepared GNSs by mechanical method retained the excellent electrical and mechanical properties of graphene itself, so introducing the graphene into the perfectly insulated SiO₂ greatly improved the conductivity, dielectric constant and dielectric loss of the composites significantly. The results prove that graphene materials can be used as an ideal electromagnetic wave absorber to add into the inorganic composites, revealing a potential value in the field of absorbing materials. In addition, we also found that the addition of graphene greatly improved the microwave attenuation of its based composites and therefore we can conclude that the graphene is not only an ideal absorber but also a kind of good broadband attenuation agent in electromagnetic microwave shielding and absorbing materials, which expect to broaden the graphene material research and application field.4. We used expanded graphite and micron-sized titanium carbide?TiC? powders as the starting material. Firstly, GO was fabricated by the modified hummers. Then, GO colloid was added drop by drop to the suspensions under ultrasonic stirring. In order to ensure the GO to be more evenly dispersed in matrix, a planetary mill was used for 10 hours. Finally, dense bulk composites were prepared by SPS. During sintering, GO layers were reduced to GNSs at high temperatures. Studies on the microstructure and morphology characterization showed that the introduction of graphene not only significantly refined the average grain size of the matrix, and the more interesting discovery was that with just a small amount of graphene addition, the TiC grains seemed to have completely stopped growing, say, keeping their original size, which is really surprising and not reported previously. The results showed that compared with the pure TiC ceramic materials, the flexural strength and Vickers hardness of GNSs/TiC composites have been improved due mainly to the refinement of matrix grains. Crack deflection, GNSs bridging and pull-out contribute to the increase in the fracture toughness.