–––EXECUTIVE SUMMARY–––
Our basic concept in the project was as follows: for a fundamental discussion on the phenomena of electrostatic charging of powder, it is essential to study the charge generation due to a single impact/contact on a single particle. To realize the concept we proposed two directions:
Approach 1: Impact Charging Experiment:
To extend the original (previous version of the) impact charging experiments to the actual pow- der particle size region, a new equipment was designed and built in the previous project years. The sensitivity of the charge measurement was extremely improved, and this allowed particles from 50 to 500 μm to be applied to the experiments. Although the actual data were dispersed, almost all of them fell in a certain region explained by the charge relaxation model, which determines the amount of impact charge non-empirically, with an assumption of localized initial charge. Particles which were plated with nickel to make its surface electro conductive lost their all the initial charge at the impact onto a metal target as the same to the cases of 3 mm metal particles. Experiments with different kind of metal targets did not show any significant differ- ence due to the difference of work function of the metals whilst it is expected that the impact charging behavior may depend on it if theprocess of charge relaxation due to gas discharge is not considered. These facts encouraged us to say that the charge relaxation model works in this particle size region of hundred micro-meter as well as the case of bigger particle with 1-3 mm in diameter in the previous work. In the future study, more improvement of the sensitivity of the charge measurement will be tried to allow particles under 10 μm to be applied. As well, an experimental study chasing process of successive impacts will be interesting.
Approach 2:
Development of the Method of Electrostatic Adhesive Force Measurement: This is the completely new approach to determine the amount of charge transferred onto a particle due to a contact from a measurement of adhesive force curve at approaching and separating particle against a metal target. We have actually launched AFM study: force curves of particles due to electrostatic attraction were successfully observed with PS particles of 30 and 100 μm, and with glass bead with 40μm. In the cases of smaller particle, the force curves on approach and separation were almost same after enough repeated contact, whilst they were not in the case of bigger particle: the 100 μm particle case showed that the force in the case of separation was significantly bigger than the case of approach even after enough repeated contacts. This proved that some kind of charge relaxation takes place: this must be investigated in more detail in the future study. To evaluate the force curve quantitatively, a disk charge interaction gave a good account, and it was indicated that the amount of charge and the contact area can be estimated by the method.