Electrostatic charging of micro- and nano-particles for use with highly energetic applications

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Abstract

Electrostatically charging spherical and cylindrical conducting particles against an electrode with an applied electric field is investigated both theoretically and experimentally as a method of charging micro- and nano-particles for use with a variety of high energy and high velocity applications, which include nano-particle electrostatic space propulsion systems, materials processing, and nano-printing. Increasing the particles' charge-to-mass ratios is critical for maximizing their velocities when accelerated with applied electric fields, which requires minimizing the particles' sizes down to the micro- and nano-meter ranges for some applications. An analysis reveals that the charge-to-mass ratio is maximized with low aspect ratio particles when the maximum electric field strength, which is at the top of the particles, is held constant. Experimental results of charging titanium and aluminum spherical and cylindrical particles are presented, which suggest that under appropriate conditions, the particles are charged as predicted by theory. But that electrical contact resistance between the particles and electrode can influence the charging time. An analysis of the expected particle charging times is presented, which shows a strong dependence on the conductivity and thickness of the oxide layer coating the particles.

Introduction

A variety of applications requiring high energy and high velocity charged micro- and nano-particles are under development. One such application is a proposed electrostatic thruster concept that uses charged nano-particles as propellant and is termed the nano-particle field extraction thruster, or nanoFET for short [1], [2], [3]. Other applications include materials processing and nano-printing [4]. These applications would all require particles in the micro- and nano-meter ranges for various requirements, which include maximizing the particles' velocities. These applications may electrostatically charge particles by inducing a charge transfer from an electrode surface onto the particles with an applied high strength electric field as shown by the model in Fig. 1.

Once charged, the particles can be removed from the surface of the bottom electrode if the electrostatic force is greater than the sum of the gravitational force and any adhesion forces. Note that the relative magnitude of the gravitational force and the adhesion forces is dependent on the particle's size, the materials in contact, and the environmental conditions [5]. Gravity is the dominant force for the experiments presented in this paper. Next, the particles can be accelerated to the desired velocities using the large electric potential drop between the parallel electrodes. Once reaching the desired velocities, the particles can be delivered to a substrate mounted on the bottom surface of the top electrode for materials processing or nano-printing, or ejected through a small orifice in the top electrode (not shown) to produce thrust for a propulsion system. Note that the model shown in Fig. 1 is only a top-level overview, and many important aspects needed for the various applications are not shown. Depending on the application, the final velocities of the particles may need to be precisely controlled. The ability to accurately charge the conducting particles is at the core of controlling the particles' velocities and is the focus of this paper.

This paper first presents an overview of the theoretical models used to determine the charge acquired by individual conducting spherical and cylindrical particles. Next, spherical and cylindrical particles are compared as potential shapes for use with high energy and high velocity applications. Here, experimental results of charging scaled-up spherical titanium and aluminum micro-particles with diameters ranging from approximately 800 μm to 4000 μm and cylindrical aluminum particles with diameters of approximately 300 μm and lengths of 1.3 mm and 1.7 mm are presented and compared to the models.

Section snippets

Theory of particle charging

This section presents an overview of the charge acquired by spherical and cylindrical particles when in contact with a planar electrode in the presence of an electric field. The obtainable charge-to-mass ratios of the spherical and cylindrical particles are presented and compared as possible shapes for use with a variety of applications.

Experimental setup

To experimentally investigate the charging models described by Eqs. (3), (4) for spherical and cylindrical conducting particles, respectively, when in contact with a planar electrode and exposed to an electric field, this section presents the experimental setup. A system similar to the geometry depicted in Fig. 1 with parallel electrodes was constructed with 10 cm square, planar, stainless steel electrodes. The electrode surface was polished to a mirror finish, and the separation was controlled

Experimental results

Fig. 8, Fig. 9, Fig. 10 are plots of the average charge acquired by various sizes and shapes of titanium and aluminum particles. The error is a result of the current measurement and the numerical integration, which is estimated to provide a total uncertainty of approximately 10%.

The experimental data, represented as the points, are plotted against the theoretical particle charge, represented as the lines, according to Eqs. (3), (4). The theoretical charge of the titanium spherical particle (

Particle charging time

It is possible that the aluminum spherical particles did not acquire the charge predicted by the model when relatively high strength electric fields were applied because they were lifted from the electrode surface once the electrostatic force was strong enough to overcome the gravitational and any adhesion forces. This may have occurred before the charging process was completed due to contact resistance between the particle and the electrode. This section investigates the total time that the

Conclusion

The results from the experimental section indicate that spherical conducting particles are charged as predicted as long as they remain in contact with the electrode much longer than the charging time constant. This result may impact the ability to use highly energetic charged micro- and nano-particles if it is important that all of the particles are charged to the same state. Therefore, a good electrical contact between the particles and the electrode needs to be established, which means that

Acknowledgements

The authors acknowledge the financial support of the NASA Institute for Advanced Concepts (NIAC). We would also like to thank others who contributed to the paper. Dr. Michael Keider provided technical support and ElectroDynamic Applications, Inc. (EDA), specifically Dr. David Morris, who reviewed the paper and graciously provided additional support.

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