Mass production of graphene nanosheets by multi-roll milling technique

Recently, the liquid phase exfoliation of graphite has widely been used to produce graphene due to the economic profitability of the current method. However, the compressive load that appears during the exfoliation process leads to the formation of small graphite fragments in the graphite suspension. In addition, the applied shear load (responsible for delamination of graphene) has a chaotic nature as a result of random collisions of the traditionally used balls or beads what adversely affects the structure of graphene, in particular its size and purity. This work aims to develop a new effective graphene exfoliation process with a uniform shearing rate through replacing the spherical surface of the balls used in ball mill by an involute-shaped surface of gear teeth which have non conformal surfaces similar to the ones of multi-rollers. The new design consisted of two units: mechanical mixer unit that was used to prepare low-temperature expandable graphite (LT-EG)/Dimethylformamide (DMF) suspensions at 100 rpm for 30 min with a LT-EG to DMF ratio 1:3 (W/W), while the second unit multi-roller wet milling (MR-WM) was used to synthesize the as-prepared graphene from LT-EG/DMF at constant time 120 min and different speeds in the range 300-600 rpm. The MR-WM unit was divided into three sections: a) three rollers connected together in parallel (installed at the beginning of MR-VVM unit) and used to unite the path of flow of LTEG/DMF, b) two meshing spur gears with a reduction ratio 1:1 used as an exfoliating element, and c) anther roller installed at the end to repeat the exfoliation process and thus to produce as-prepared graphene/DMF. Rotary decompression evaporator was used to extract graphene and regenerate DMF. Finally, ultrasonication treatment followed by centrifugation was used to remove exfoliated particles and then obtain graphene after drying. The obtained graphene was examined using TEM, SEM, XRD, FTIR, and TGA.