A novel charger for nanometer aerosols

1. Aerosol Ski. Vol. 29, Suppl. 1, pp. S1023-SlO24, 1998 0 1998 Published by Elsewer Science Ltd. All rights reserved Printed in Great Britain

Pergamon

OLX-8502/98

A NOVEL CHARGER FOR NANOMETER DA-REN

$19.00 + 0.00

AEROSOLS

CHEN and DAVID Y. H. PUI

Particle Technology Laboratory, Church St., S.E., University

Mechanical Engineering Department, of Minnesota, Minneapolis, MN 55455

111

KEYWORDS Unipolar

charging;

Nanometer

aerosol;

Charging

efficiency

The importance of nanometer particle research is well documented in recent publications, e.g., Pui and Chen (1997). Due to the high electric mobility and high diffusivity of nanometer aerosol particles, electric field is often used to classify and deposit nanoparticles. One of the principal instruments for measuring and classifying nanometer aerosols is the differential mobility analyzer (DMA). To operate the DMA, aerosols must first be electrically charged. The challenges in designing a charger for nanometer aerosols are two folds: 1) the particle charging efficiency is low because of the small cross section of nanoparticles, and 2) the charged particle loss is high because the same electric field used to direct the ions also causes precipitation of the high mobility nanoparticles. The objective of the present study is to design and characterize a charger with high charging efficiency and low particle loss. Further, the charging parameters can be measured and controlled accurately for comparing with existing theories. Bipolar chargers have been used for nanoparticle charging because electric field is not needed to direct the bipolar ions. The upper theoretical limit for charging nanometer aerosols is poor with charging efficiency below 5% for particles smaller than 10 nm. A variety of unipolar chargers, e.g., Biischer et al. (1994). have been designed in order to achieve high charging efficiency. However, high particle loss within the charger results in a small percentage of charged particles exiting the charger, i.e., low extrinsic charging efficiency. In the present study, a novel charger is developed for nanoparticles. Unipolar charging is used to obtain high charging efficiency for particles down to 3 nm. Both the aerosol stream and the ion stream flow in the longitudinal direction of a cylindrical charger. The aerosol stream therefore flows in the same direction as the electric field used to direct the ions. There are three sections in the charger: the inlet, the charging core and the outlet. In the inlet section, unipolar ions are generated using a PO-210 radioactive source with the electric field designed to separate the positive and negative ions, and to focus selected ions into the core section of the charger. Ions of the selected polarity are then attracted to the charging core by a uniform electric field created by a series of ring electrodes with a linearly ramped voltage. The aerosol entering the charger is also surrounded by a clean air sheath in order to confine the aerosol in the core region. A new exit design with a reversed electric field is incorporated in order to minimize the loss of charged nanoparticles. S1023

Abstracts

S1024

of the 5th International

Aerosol

Conference

1998

The design is first optimized using a computer simulation and then constructed for performance evaluation. Experiment data show that the charger achieves 95% penetration efficiency for singly charged particles and 22% extrinsic charging efficiency at 3 nm. Figure 1 shows the extrinsic charging efficiency of this newly developed charger as a function of particle size. Significant improvement in performance is shown over the previous The detailed design concepts and characterization of this chargers. nanoparticle charger will be discussed in this presentation.

1 .oo

1 o

Present Charger (Positive Ions) Present Charger (Negative Ions) - -Q - Buscher et al, 1994 (Positive Ions) - Bipolar-Fuchs

I

+

0.80

8 f [;I

T

m

I

g 3

g 0.60& % 5F 0.40-

d

1.0 Particle Figure

1. Extrinsic

10.0 Diameter

100.0 (nm)

charging efficiency of the present as a function of particle size.

We are grateful for the financial Foundation Grant CTS-9304 152.

support

provided

charger

by the National

Science

Btischer, P., A. Schmidt-Ott and A. Wiedensohler (1994) Performance of a Unipolar “Square Wave” Diffusion Charger with Variable nt-Product, J. Aerosol Sci. 25:651-663. Pui, D.Y.H. and Multidisciplinary

D.R. Chen (1997) Nanometer Particles: Research, J. Aerosol Sci. 28, 539-542.

A New Frontier

For