Search

The influence of aerosol concentration on the charge per particle is investigated in post-DC corona diffusion chargers for particle diameter between 10 nm and 1 µm. Particles are charged as they pass through the charger volume by collection of ions; the ion concentration decreases along the same path due to unipolar space charge repulsion and collection on walls. With crossflow of ions and aerosols, the final mean charge per particle decreases by up to one order of magnitude with increasing aerosol concentration from 109 to 1011 m−3. The evolution of the ion density profile along particle trajectories with aerosol concentration is shown to be due to the consumption of ions by aerosol charging and to ion dispersion by unipolar space charge repulsion including ions and charged particles. Both are investigated using a simplified 1.5 D model with axial symmetry and homogeneous mixing of ions and aerosols. It is shown that, for a given operating condition, the aerosol space charge is responsible for the modification of the spatial ion distribution and the related lower Ni × τ and charge per particle reported at higher aerosol concentration. From calculations, we define the maximal aerosol concentration to keep the mean charge per particle unaffected by aerosol space charge in the optimal case of homogeneous mixing of ion and aerosol. Finally, the comparison of different ion-aerosol mixing conditions highlights that an axial symmetry of the charger reduces the influence of aerosol space charge. This effect of aerosol concentration on the particle charge is critical for aerosol chargers devoted to electric mobility selection for size measurements or focused electro-deposition, as well as for concentration measurements as performed using emerging low cost sensors based on corona chargers. Copyright © 2021 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

This article focuses on the mean charge per particle of monodisperse submicron aerosols charged by the diffusion of unipolar ions in post-corona discharge. It aims to confirm and discuss the limits of considering a single value of Ni·t to describe aerosol charging and then to present methods to control the size-charge relation. Three aerosol chargers with different mixings of ion and aerosol flows are investigated. Despite comparable ion sources with discharge currents of a few tens of µA, the size-charge relations differ from one charger to another due to different ion-aerosol mixing conditions and subsequent differences in ion density along the particles' trajectories. Discrepancies are even more noticeable as the particle size increases. Discharge current, velocities of ion and aerosol flows and the electric field control post-discharge ion density at each point of the charging zone. The control of particle trajectory in expanding unipolar ion clouds leads to tuneable size-charge relations. Aerosol inertia and charging dynamics both depend on particle size, affect the Ni·t experienced by the particle and thus the final charge of the particle. Operating conditions to reach a constant mean charge for particles larger than 200 nm are reported. Conclusions provide a basis to design aerosol chargers devoted to electric mobility selection for aerosol deposition, separation or electrical measurements, especially to overcome the limits of mobility-to-size data inversion due to the multiple charge ambiguity inherent to diffusion chargers. Copyright © 2020 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The plasma-based aerosol process developed for the direct coating of particles in gases with silicon oxide in a continuous chemical vapor deposition (CVD) process is presented. It is shown that non-thermal plasma filaments induced in a dielectric barrier discharge (DBD) at atmospheric pressure trigger post-DBD gas phase reactions. DBD operating conditions are first scanned to produce ozone and dinitrogen pentoxide. In the selected conditions, these plasma species react with gaseous tetraethyl orthosilicate (TEOS) precursor downstream of the DBD. The gaseous intermediates then condense on the surface of nanoparticles and self-reactions lead to homogeneous solid SiO x coatings, with thickness from nanometer to micrometer. This confirms the interest of post-DBD injection of the organo-silicon precursor to achieve stable production of actives species with subsequent controlled thickness of SiO x coatings. SiO x coatings of spherical and agglomerated metal and metal oxide nanoparticles (Pt, CuO, TiO₂) are achieved. In the selected DBD operating conditions, the thickness of homogeneous nanometer sized coatings of spherical nanoparticles depends on the reaction duration and on the precursor concentration. For agglomerates, operating conditions can be tuned to cover preferentially the interparticle contact zones between primary particles, shifting the sintering of platinum agglomerates to much higher temperatures than the usual sintering temperature. Potential applications for enhanced thermal stability and tunable photoactivity of coated agglomerates are presented.