Executive Summary
The basic concept in the project research to address the electrostatic charging of powder was that it is essentially important to study the impact (contact) charging of a single particle due to a single collision. To realize the concept, we proposed two approaches. One was a development of our original ‘impact charging experiments method’ (approach 1). The other was a new technique involving the measurement of the electrostatic adhesive force curve (approach 2).
Approach 1:
- A new equipment was designed and successfully constructed.
- The sensitivity of the charge measurement was improved by 1000 times, which allowed impact charging experiments to be made on particles from 50 to 500 µm.
- Although the actual data were scattered, almost all of them fell in the region that can be explained by the ‘charge relaxation model,’ with an assumption of localized initial charge.
- Surface conductive particles lost their all the initial charge at the impact onto a metal target: this was the same as found with 3 mm metal particles in the previous work.
- Experiments with different kinds of metal targets with different work functions did not show any signifi cant difference; this is not expected from the conventional model regardless of the charge relaxation path.
- From the results, it was shown that the charge relaxation model works at this particle size range of hundred microns as well as the case of bigger particle with 1-3 mm in diameter in the previous work.
Approach 2:
- The force curve was successfully measured with atomic force microscopy (AFM).
- By detailed analysis of the ‘force curve’ measurement, rather than only looking at the maximum adhesive force, the electrostatic interaction was successfully observed by separating other interactions such as liquid bridge and intermolecular force.
- A theory was developed to evaluate the force curve based on an image force method using an approximation of disk-to-disk interaction.
- The good agreement between the observed force curve and theory showed that the force curve observed can be surely attributed to the electrostatic interaction, and that the amount of charge on the particle and the radius of the charged (contact) area can be estimated from the analysis.
- The order of magnitude of the measured charge density was 10-2 C/m2, which is much greater than that obtained with impact charging experiments (10-4 C/m2). The force curve measurement with AFM can catch the net amount of the charge generated before charge relaxation due to gas discharge or charge relaxation takes place.
- In the experiment using 8 kind of metal targets, the net charge generated showed fairly good agreement with the conventional simple condenser model, but the data dispersed.
- Behind the apparent random data scatter, a strong relationship was found between the charge density and contact area. The mechanism of the relationship is not know at this moment, however the fi nding is a good starting point for future work.