Fig. 1 Schematic diagrams of graphene and its derivatives[1]

Fig. 2 FESEM micrographs of high energy ball milled graphite powder (a) As-milled graphite powder; (b) Graphite layers with a thickness of 20-50 nm that were highly distorted due to heavy impact of milling balls; (c) and (d) Carbon tubes with a diameter of 80-300 nm and a length of several microns produced during the milling[15]

Fig. 3 FESEM micrographs showing nanosized graphene sheets and carbon tubes on fractured surface of TiC/C composite and pores of an average size of about 3 μm with a porosity of about 15%[15]

Fig. 4 FESEM micrographs showing: (a) the raw MWCNTs used as the starting material; (b-c) polished surface with a scratch of MWCNTs ((b) a shiny surface comprised of high- density aggregation CNTs; (c) transition from the dense to the loose CNTs); (d) polished surface with a scratch on it, MWCNTs in the surface have changed their structure and the outer graphene walls have come off the tubes[16]

Fig.5 SEM image of the fracture surface of SiOC/GNS composite, with graphene forming a lamellar structure of a certain aligned distribution in the ceramic matrix[20]

Fig. 6 Schematic diagram of the preparation process of Al2O3/GNS bulk composites

Fig. 7 SEM images of fractured surfaces of a pure Al2O3 ceramic (a) and a GNS/Al2O3 composite containing 1vol% GNSs (b-d) (c) and (d) are magnified parts of (b)[22]

Fig. 8 TEM and HRTEM images of a GNS/Al2O3composite containing 5vol% GNSs (a) GNSs surrounding Al2O3 nanoparticles; (b) A magnified image of GNSs with a thickness of about 10 nm in (a); (c) GNSs with a thickness of 2.5 nm; (d) Overlap of GNSs between Al2O3 nano-particles[22]

Fig. 9 A proposed scheme of the fabbrication of Si3N4/graphene bulk composites

Fig. 10 Toughening mechanisms of composites (a) Crack deflection on a plane with larger size and crack plane orientation almost parallel (fracture line)[29]; (b) Crack branching during crack propagation in the nanographene platelet-reinforced composite (fracture line)[34]; (c) Sheet put-out with a GPL on the fracture surface, with the plane of the sheet perpendicular to the plane of the fracture surface[28]; (d) Crack bridging by GPLs on the fracture line with the plane orientated of sheet nearly perpendicular to the plane of the polished surface[34]

Fig. 11 Diagram of experimental setup of dc and ac electrical conductivity (σdc ,σac) measurements along different directions[35] (a) perpendicular (defined by superscript┴) and (b) parallel (superscript″) to the SPS pressing axis; (c) sample and graphite die in the SPS furnace

Fig. 12 FESEM micrographs of the fracture surface of the Si3N4/GNP composite where nanoplatelets protrude from the surface (a) and high magnification image showing the zigzag bending of nanoplatelets along the matrix grain boundaries (b)[36]

Fig. 13 Raman images built from G (a and c) and D peak intensities (b and d) for both composite orientations: parallel (a and b) and perpendicular (c and d) to the SPS pressing axis. Average spectra of both scanned regions (e). Example of a single spectrum extracted from the collection of image (c) with the intense 2D peak of few layer graphene (f). SEM micrograph of the fracture surface (parallel orientation) (g), showing the preferential orientation of projected GNPs (pointed by arrows). (h and i) Optical images of the scanned zones (outlined) for both specimen orientations parallel (left) and perpendicular (right) to the SPS axis[38]

Fig. 14 Simplified schematics of the measuring geometry (counter-electrode at the sample surface edge) to illustrate the conducting paths for the orientations perpendicular (a) and parallel (b) respect to the SPS pressing axis[36]

Fig. 15 TEM images of the α-SiC composite sample, showing several regions with different number of graphene layers. Region 1: (a) low magnification TEM image of the region of interest. (b) is a higher magnification image of the P1 region (inside the highlighted rectangular region) which is rich in sp2 carbon, showing different cross-section views of few-layers graphene flakes (2L, 3L and 5L); the spacing between dark contrast lines is consistent with the spacing between graphene sheets. (e) are electron energy loss spectroscopy spectra (after multiple-scattering background removal) of the carbon K edge at positions P1 and P2, respectively. Region 2: (c) low magnification TEM image of a grain boundary with formation of a graphite island, (d, g) high-resolution TEM images of the same region, the inset is SEAD of the right side SiC grain. Region 3: (f) graphitic formation of several tens of nanometers. Region 4: (h and i) two different grain boundaries presenting bi-layer graphene[40]