Despite that the capacitor is a device that is hundreds of years old, it is still proving to researchers that their understanding of it is nothing but crude. The classic capacitor is quite simple: bring together two conducting plates, separate them with an insulator material, and bingo! You got your capacitor.
Ideally, the insulator material should conduct no current. Practically however, some charges do pass and these represent losses that are not, generally, desired. The latter is true in principle. For some applications however, designers want to control the amount of losses in the insulator material. Because science is not black and white, it turns out that controlling the losses within the insulator is not an easy job! Some researchers have played around with this insulator and tried innovative ways to control the loss and attained some success. When you are capable of controlling the loss to an amount that you desire, the device that you have is referred to as a fractional capacitor.
One group for example has realized a fractional capacitor that is electrochemical based, where electrodes were immersed in a chemical solution, and the losses were governed by the immersion depth of the electrodes. This was one of the earlier designs that deserve to be commended, but good luck putting a chemical solution in your next iPhone or soldering it onto a printed circuit board (PCB)! Further, this capacitor can only function in the kHz range and the loss is not considered to be stable throughout the frequency range of interest.
Here at KAUST, a true success story is to be told regarding the development of fractional capacitors, in which the latter was conceived fruitfully based on collaborative efforts between Dr. Amro Elshurafa from the sensors labheaded by prof. K.N. Salama (electrical) and Mahmoud Almadhoun from SABIC Corporate Research and Innovation Center, Thuwal, Saudi Arabia working at the the nanomaterials lab headed by prof. H. Alshareef (materials). Elshurafa and Almadhoun realized the first compact, microscale, fractional capacitor that can be easily soldered onto a PCB, but how did they do it?
Not surprisingly, the secret lies in the insulator layer that separates the two conducting electrodes. The insulator, which is made out of a polymer, is percolated with tiny sheets of graphene that are only a few nanometers thick. These graphene sheets are conductive and create a very large number of microcapacitors within the polymer. The overall effect of these capacitors that are connected in series and parallel is what creates the fractional response. As one can imagine, by controlling the number of graphene sheets within the polymer, the losses vary accordingly. The patent-pending fractional capacitor is functional up to the low MHz range and the response is also stable, making it much better than its predecessors. But this is just the start. With further development, this capacitor can be further enhanced to add much more versatility to our electronic gadgets that we use in our daily lives.
The work was partially funded by Saudi BasicIndustries Corporation (SABIC) under Grant No. 2000000015. To learn more about this, you are invited to check our U.S. patent 9,305,706 and read the paper, Microscale Electrostatic Fractional Capacitors using Reduced Graphene Oxide Percolated Polymer Composites, that was published in the leading physics venue Applied Physics Letters.