The effect of particle morphology on unipolar diffusion charging of nanoparticle agglomerates in the transition regime

We investigated the effect of particle morphology on unipolar diffusion charging of nanoparticle agglomerates consisting of multiple primary spheres. In the unipolar diffusion charging of non-spherical agglomerates, geometric surface area and electrical capacitance of particles, which are related to particle morphology, are known as important parameters to determine mean charge per particle. From mobility analysis we found that the geometric surface area of chain-like agglomerates is only larger than that of spherical particles with the same mobility diameter for mobility size range below d_m=80nm. We estimated the electrical capacitance of agglomerates with a newly developed model based on electrostatics and mobility theories. The results show that the electrical capacitance of chain-like agglomerates becomes significantly larger compared to that of spheres with the same mobility diameter as particles become larger. Our analysis results indicate that loose agglomerates have larger mean charge per particle compared to compact particles with the same mobility diameter because the electrical capacitance of agglomerates becomes larger as particle morphology becomes looser. Our experimental data show that mean charge per particle for silver agglomerates is larger than that for fully coalesced silver spheres with the same mobility diameter as agglomerates by about 24%. The experimental data is in good agreement with estimates of mean charge per particle for silver agglomerates.