Diffusion charging

The diffusion charging describes the electrical charging of gas-borne particles ( aerosol ) by collisions between particles and charge carriers ( ions and electrons ) due to the Brownian motion . Depending on whether charge carriers with only one or both signs are present, one also speaks of unipolar or bipolar diffusion charging.

Unipolar diffusion charging

When an initially neutral particle collides with a charge carrier, the particle is charged by charge transfer. Since it now has a charge of the same name, the likelihood of further charge transfers through collision decreases as the number of charges increases , since the speed difference between the collision partners must be large enough to overcome the now larger repulsive Coulomb forces. The resulting charge number of the particle, which is established after a period of time , can be calculated using the kinetic gas theory ${\displaystyle t}$

${\displaystyle n={\frac {2\pi \varepsilon _{0}d_{\text{P}}kT}{e^{2}}}\cdot \ln \left(1+{\frac {d_{\text{P}}{\bar {v}}_{\text{i}}e^{2}N_{\text{i}}t}{8\varepsilon _{0}kT}}\right)}$

Here is the permittivity of the vacuum, the particle diameter, the Boltzmann constant , the gas temperature, the elementary charge , the mean thermal velocity of the ions and the ion concentration. ${\displaystyle \varepsilon _{0}}$ ${\displaystyle d_{\text{P}}}$ ${\displaystyle k}$ ${\displaystyle T}$ ${\displaystyle e}$ ${\displaystyle {\bar {v}}_{\text{i}}}$ ${\displaystyle N_{\text{i}}}$

Bipolar diffusion charging

In contrast to unipolar diffusion charging, electrically charged particles can be discharged again by colliding with charge carriers of opposite polarity. In the stationary case, a Boltzmann charge distribution is established in a particle collective , in which a symmetrical charge distribution is assumed and most of the particles remain electrically neutral.

See also

• Particle charge
• Boltzmann load distribution
• Differential mobility analyzer

literature

• William C. Hinds: Aerosol Technology . John Wiley & Sons, New York 1982, ISBN 0-471-08726-2 .

Individual evidence

1. ↑ Uwe Sievert: Development of a unipolar aerosol charger with high dynamics for determining the particle size distribution in the flowing aerosol . Herbert Utz Verlag, 1998, p. 15 ( limited preview in Google Book search).
2. ↑ Klaus Görner, Kurt Hübner: Gas cleaning and air pollution control . Springer-Verlag, 2013, p. F-36 ( limited preview in Google Book search).