Method to optimize charge distribution in electret films using multiple needle discharges

Abstract

The purpose of this paper was to find the optimal parameters of moving multiple needle corona discharges by looking at the transport velocity and the distance of the needles.The parameters were then used to symmetrically arrange the discharge needles for large-scale mass production. A corona discharge method is typically used to perform charge injection in the creation of electret films.A static single needle corona discharge method typically leads to a non-uniform space charge distribution in electret films. Since obtaining uniformly injected charge distribution and uniform efficiency of large-area electret films have become important research topics recently, methods such as moving multiple needle corona discharges have been developed to improve production efficiency and uniformity in the spatial charge distribution of electret films for industrialapplications. An automated control system was developed using VB (Visual Basic) to create a virtual instrument to perform the corona discharges and to measure the surface potential. Several important processing parameters were identified to enable us to obtain uniform spatial charge distribution in newly created electret films. I. BACKGROUNDIn a previous study conducted by our research group, we found that the optimal parameters of a static multiple needle corona discharge included such parameter information as the exposed length of the needle, spacing of the needles, suitable distance between the needles and plate electrode, etc.[1] The parameters were applied to produce large-scale electret loudspeakers. However, as a static discharge method has been identified to have low production efficiency, a moving multiple needle corona discharge method can circumvent such limitation and allow us to achieve large scale mass production. II. EXPERIMEN TAL SET-UPIn our set-up, both the moving velocity and the spacing of the needles in the moving corona discharge method were varied to tailor to the fmal spatial charge distribution in the electret films. At the beginning, a needle experiment was set up in a large-scale moving X-Y plane to establish and identify the optimal surface potential distribution and the discharge region under a fixed temperature and relative humidity environment. In most situations, a surface potential decay (SPD) measurement technique is extensively employed to investigate the spatial charge distribution of electret materials due to their reliability and low cost. The parameter for a single needle grid plate electrode system was determined with respect to a set of different sample moving velocities. The decay of an average surface potential at different sample moving speeds are detailed. The spacing of the needles in our 978-1-4577-1025-4/11/$26 . 00 ©2011 IEEE mUltiple needle corona discharge method provides us with an effective operating parameter to obtain uniform surface potential distribution in electret films for large-scaleproduction. In a previous study, a COC polymer was found to have better machining properties and better stability to store char...