Preparation of graphene oxides with different sheet sizes by temperature control

The sheet size of a graphene oxide （GO） can greatly influence its electrical, optical, mechanical, electrochemical and catalytic property. It is a key challenge to how to control the sheet size during its preparation in different application fields. According to our previous theoretical calculations of the effect of temperature on the oxidation process of graphene, we use Hummers method to prepare GOs with different sheet sizes by simply controlling the temperature condition in the process of the oxidation reaction of potassium permanganate （KMnO4） with graphene and the dilution process with deionized water. The results detected by transmission electron microscopy （TEM） and atomic force microscopy （AFM） show that the average sizes of GO sheets prepared at different temperatures are about 1 μm and 7 μm respectively. The ultraviolet-visible spectroscopy （UV-vis） shows that lower temperature can lead to smaller oxidation degrees of GO and less oxygen functional groups on the surface. In addition, we prepare GO membranes to test their mechanical strengths by ultrasonic waves, and we find that the strengths of the GO membranes prepared under low temperatures are considerably higher than those prepared under high temperatures, showing the high mechanical strengths of larger GO sheets. Our experimental results testify our previous theoretical calculations. Compared with the traditional centrifugal separation and chemical cutting method, the preparation process of GO by temperature control is simple and low-cost and also enables large-size synthesis. These findings develop a new method to control GO sheet sizes for large-scale potential applications.

Study of Magnetic Force Distribution for Electromagnetic V-shape Bending Forming of Small Size Flat Sheet

Electromagnetic V-shape bending of small size sheet blank is investigated numerically and experimentally. Three-dimensional electromagnetic field models are established to calculate the magnetic force distribution on the sheet by software ANSYS / EMAG. Series of electromagnetic V-shape bending forming experiments are presented,in which small size uniform pressure coil and big size round flat spiral coil are used. The results show that small size uniform pressure coil is not suitable for electromagnetic forming of small size flat sheet,and the coil is susceptible to failure such as bulging,ablation and cracking. When the plane dimension of round flat spiral coil is bigger than sheet blank sizes,the induced current crowding effect will be resulted which seriously influence the magnetic force distribution on the sheet. In this case,magnetic force distribution can be adjusted through the change of the relative position between coil and sheet,the desired deformation can be obtained finally. Therefore,big size round flat spiral coil can be well applied to electromagnetic V-shape bending forming of small size flat sheet.