Dielectric particle injector for material processing

A device for use as an electrostatic particle or droplet injector is disclosed which is capable of injecting dielectric particles or droplets. The device operates by first charging the dielectric particles or droplets using ultraviolet light induced photoelectrons from a low work function material plate supporting the dielectric particles or droplets, and then ejecting the charged particles or droplets from the plate by utilizing an electrostatic force. The ejected particles or droplets are mostly negatively charged in the preferred embodiment; however, in an alternate embodiment, an ion source is used instead of ultraviolet light to eject positively charged dielectric particles or droplets.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to electrostatic particle or 
droplet injectors, and more particularly to an apparatus and related 
method for injecting dielectric particles or droplets by first charging 
the dielectric particles or droplets using ultraviolet light induced 
photoelectrons, and then ejecting the charged particles or droplets by 
utilizing an electrostatic force. 
2. Description of Background Information 
Charged droplet atomizers are well known in the art, and have been used for 
a wide variety of applications. Commonly, such atomizers use electrostatic 
force to form an atomized field of small droplets, which droplets are 
preferably of relatively uniform size. This technique involves the use of 
a high voltage to electrostatically atomize the fluid into small droplets. 
Two typical examples of such electrostatic atomizers are found in U.S. Pat 
Nos. 4,255,777, to Kelly, and in 4,748,043, to Seaver et al. The Kelly 
patent induces a free excess charge on fluid contained within a housing 
chamber using at least two electrodes. The fluid containing the free 
excess charge is supplied to a spray mechanism, and is accelerated 
outwardly into small droplets by a strong electrostatic field generated by 
a ground electrode. The droplets are accelerated toward the ground 
electrode, and pass through one or more apertures in the ground electrode. 
The Seaver et al. patent uses a first electric field between a plurality of 
needles and a plate, with the needles being disposed concentrically with 
respect to holes in the plate to cause a mist of highly charged droplets 
to be emitted from the needles. A second electric field is used to draw 
the droplets to the surface of an object to be coated with a thin but 
uniform coating. Both the Kelly patent and the Seaver et al. patent thus 
use an electrostatic atomizer to generate a mist of droplets. 
The point which needs to be made is that both of these references are 
limited to the use of generating a mist of conductive droplets for the 
desired purpose. If the liquid is not conductive, the atomizers will not 
work. Similarly, it would not be possible using conventional electrostatic 
techniques to inject particles, unless the particles are made of a 
conductive material. 
In most situations, the use of particles or droplets which are made of a 
conductive material has not presented a problem. However, recently a 
problem occurred which made it desirable to be able to inject particles 
made of nonconductive dielectric material. A brief description of the 
nature of the problem encountered is helpful to the understanding of the 
necessity of a dielectric particle injector. 
During the Magellan mission to Venus, a number of anomalous events were 
observed in the use of the star scanner. The star scanner is a light 
sensitive device used to calibrate the attitude control system of the 
spacecraft. The events involved the detection of false incidences in which 
the star scanner indicated the detection of a star when in fact no star 
was in a position to be detected. 
After a number of other possible causes for the false incidences were 
identified and ruled out, the possibility of particulates released from 
the surface of the spacecraft reflecting sunlight into the star scanner 
was indicated as the most likely possibility. The outer surface of the 
spacecraft was astroquartz, and it was suspected that these particles were 
the cause of the false incidences detected by the star scanner. It was 
hypothesized that the particles were released from the astroquartz surface 
of the spacecraft due to thermal shock when the astroquartz was exposed to 
the sunlight. 
In order to confirm the theories advanced as to the cause of the star 
scanner anomalies, it was necessary to run several experiments in which 
dielectric particles were charged and released by a particle injector 
device. 
Accordingly, it is the primary objective of the present invention that it 
provide an apparatus and a method for injecting dielectric particles in a 
manner analogous to conventional particle injection of conductive droplets 
and particles. 
Thus, it is an objective of the present invention that a charge must 
initially be placed on the dielectric particles or droplets in order to 
provide a manner of controlling the ensuing movement of the dielectric 
particles or droplets. Appropriate apparatus and a suitable method must be 
developed to accomplish this objective. 
It is a further objective that the appropriately charged dielectric 
particles or droplets be ejected into a desired area using an 
electrostatic force. The charged dielectric particles or droplets may then 
be maintained in the desired area through electrostatic confinement. 
It is a still further objective of the present invention that the 
dielectric particles or droplets may be charged either negatively or 
positively by varying the charging technique. It is another objective of 
the present invention that the apparatus used be relatively compact and 
inexpensive, both to construct, as well as to operate and maintain. 
Finally, it is also an objective that all of the aforesaid advantages and 
objectives of the present invention be achieved without incurring any 
substantial relative disadvantage. 
SUMMARY OF THE INVENTION 
The disadvantaqes and limitations of the background art discussed above are 
overcome by the present invention. With this invention, a particle 
injector and related method are disclosed which are suitable for charging 
and injecting dielectric particles or droplets. The present invention 
operates by first using ultraviolet (UV) light induced photoelectrons to 
charge the dielectric particles. 
The dielectric particles or droplets to be injected are placed on the 
surface of a flat metallic plate made of material having a low work 
function, such as zinc or nickel. A UV source having a wavelength of 
between 2000 and 3000 Angstroms is used to illuminate the surface of the 
flat plate. Photoelectrons emitted from the surface of the flat plate will 
charge the dielectric particles or droplets. 
The present invention next uses electrostatic force to eject the charged 
dielectric particles or droplets from the flat plate. This is accomplished 
by connecting the side of a high voltage DC power source having the same 
charge as that of the charged dielectric particles or droplets to the flat 
plate. The other side of the DC power source is connected to a metallic 
screen spaced away from the flat plate. The charged dielectric particles 
or droplets will be ejected from the surface of the flat plate through the 
screen and into a desired area. Electrostatic confinement, such as 
electrostatic levitation techniques well known in the art, may then be 
used to maintain the charged dielectric particles in the desired area. 
It may therefore be seen that the present invention teaches an apparatus 
and a method for injecting dielectric particles in a manner analogous to 
conventional particle injection of conductive droplets and particles. 
The present invention accomplishes this in two stages. Initially, a charge 
is placed on the dielectric particles or droplets in order to provide a 
manner of controlling the ensuing movement of the dielectric particles or 
droplets. 
Next, the appropriately charged dielectric particles or droplets are 
ejected into a desired area using electrostatic force. The charged 
dielectric particles or droplets are then maintained in the desired area 
through electrostatic confinement. 
The present invention allows the dielectric particles or droplets to be 
charged either negatively or positively by varying the charging technique. 
The apparatus of the present invention is relatively compact and 
inexpensive, both to construct, and to operate and maintain as well. 
Finally, all of the aforesaid advantages and objectives of the present 
invention are achieved without incurring any substantial relative 
disadvantage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the present invention is schematically 
illustrated in its simplest form in FIG. 1. For purposes of the example 
used herein in FIG. 1 (and in the example used in FIG. 2 as well), the 
particle injector will be used to inject dielectric particles 20 rather 
than dielectric droplets, although the teachings of the present invention 
are equally applicable to the injection of dielectric droplets. 
The dielectric particles 20 used both in FIG. 1 and in FIG. 2 may be, for 
example, mioroballoons, which are small, hollow, spherical balls made of 
ordinary, normal density glass. Such microballoons would preferably be 
approximately 60 microns in diameter, and would float with neutral 
buoyancy in water. 
Referring now to FIG. 1, the dielectric particles 20 are placed on the top 
surface of a flat plate 22, which plate 22 is made from a low work 
function material. In the preferred embodiment, the material of the plate 
22 is zinc (nickel may also be used). A flat, highly transparent, coarse 
wire grid 24 is located above and parallel to the top surface of the plate 
22. The wire grid 24, which is made of a conductive metal, is located 
approximately one to two centimeters above the surface of the plate 22, 
although this distance may vary with the characteristics of the dielectric 
particles 20 to be injected. 
A high voltage DC power source 26 has its negative side electrically 
connected to the plate 22, and its positive side connected to the wire 
grid 24. The high voltage DC power source 26 preferably has an adjustable 
voltage between zero and 10,000 Volts. In the preferred embodiment, the 
voltage supplied by the high voltage DC power source 26 will be 
approximately 1000 Volts, although the voltage may vary with the 
characteristics of the dielectric particles 20 to be injected. 
A UV source 28 producing UV light having a wavelength in the 2000 to 3000 
Angstrom range is used to illuminate the top surface of the plate 22. 
Since the preferred material of the plate 22, zinc, has a low work 
function characteristic, photoelectrons are readily emitted from the 
surface of the plate 22. Of course, since the dielectric particles 20 are 
made of a dielectric material with a work function higher than than the 
energy of the UV source 28, no photoelectrons will be emitted from the 
surfaces of the dielectric particles 20. 
The photoelectrons emitted from the surface of the plate 22 will become 
attached to some of the dielectric particles 20, causing these dielectric 
particles 20 to become negatively charged. Immediately as soon as these 
dielectric particles 20 become negatively charged, the external electric 
field caused by the relative negative charge of the plate 22 will tend to 
repel these negatively charged dielectric particles 20 away from the top 
surface of the plate 22. 
The positive charge on the wire grid 24 will also tend to attract the 
negatively charged dielectric particles 20 in an upward direction. Thus, 
the negatively charged dielectric particles 20 will be repelled from the 
plate 22 and toward the wire grid 24. However, due to the coarseness of 
the wire grid 24 and the small size of the dielectric particles 20, most 
of the negatively charged dielectric particles 20 will pass upwardly 
through the wire grid 24, as shown in FIG. 1. 
The negatively charged dielectric particles 20 passing upwardly through the 
wire grid 24 may then be trapped in a confined area by using the proper 
electrostatic levitation and confinement field geometry. The principles of 
electrostatic levitation and confinement fields are well known in the art. 
It should be noted that in testing, the dielectric particles 20 were 
ejected from the top surface of the plate 22 at 1000 Volts. This voltage 
was required to overcome both gravity and adhesion forces, which tend to 
hold the dielectric particles 20 to the top surface of the plate 22. In a 
microgravity environment, the electrostatic force required for particle 
injection would be significantly reduced. 
Referring next to FIG. 2, the dielectric particles 20 are again placed on 
the top surface of the plate 22, which is again preferably made of a low 
work function material such as zinc. The wire grid 24 is again placed over 
and parallel to the top surface of the plate 22. The wire grid 24 is 
preferably spaced away from the top surface of the plate 22 by 
approximately two centimeters. 
The plate 22 and the wire grid 24 are located inside a metallic vacuum 
chamber 30, which has a hollow cylindrical neck 32 defining an opening 
into the vacuum chamber 30. First and second hermetically sealed 
electrical feedthroughs 34 and 36 extend through the wall of the vacuum 
chamber 30. 
The negative side of the high voltage DC power source 26 is electrically 
connected to one side of an ammeter 38. The other side of the ammeter 38 
is electrically connected through the first hermetically sealed 
feedthrough 34 to the plate 22. The negative side of the high voltage DC 
power source 26 is also electrically connected to the wall of the vacuum 
chamber 30, which is made of electrically conductive material. 
The positive side of the high voltage DC power source 26 is electrically 
connected to one side of a resistor 40. The other side of the resistor 40 
is electrically connected through the second hermetically sealed 
feedthrough 36 to the wire grid 24. In the preferred embodiment, the value 
of the resistor 40 is approximately 1M Ohm. 
The particle injector illustrated in FIG. 2 includes a flat, highly 
transparent, coarse shield grid 42, which is located parallel to and above 
the top of the wire grid 24. The shield grid 42, which is made of a 
conductive metal, is located approximately two centimeters above the top 
of the wire grid 24. The shield grid 42 is electrically connected to the 
wall of the vacuum chamber 30, and is thus electrically connected to the 
negative side of the high voltage DC power source 26. 
A flat, conductive metal plate 44 is located parallel to and below the 
bottom of the plate 22. The plate 44 is electrically connected to the wall 
of the vacuum chamber 30, and is thus electrically connected to the 
negative side of the high voltage DC power source 26. 
The cylindrical neck 32 of the vacuum chamber 30 has an annular flange 46 
located on the top thereof. A quartz vacuum window 48 is located on top of 
the flange 46, and is sealingly held in place by an annular cap member 50. 
The quartz vacuum window 48 is essentially transparent to UV light. The 
vacuum chamber 30 is sealed, in the preferred embodiment with a vacuum of 
approximately 10.sup.-6 Torr. 
The UV light is supplied in the preferred embodiment from a 250 Watt 
Mercury arc lamp 52 operated at 30 Volts and 8 Amps. The light from the 
lamp 52 is focused by a flat-convex quartz lens 54, and is directed by a 
front surface mirror 56 through the quartz vacuum window 48 and onto the 
top surface of the plate 22. A removable glass plate 58 may optionally be 
placed in the path of the UV light between the flat-convex quartz lens 54 
and the front surface mirror 56 to alter the characteristics of the UV 
light. 
In operation, the particle injector of FIG. 2 is similar to the device 
shown in FIG. 1 and discussed above. The dielectric particles 20, which 
each weigh approximately 0.1 micrograms, have an initial charge to mass 
ratio of approximately 0.0004 Coulombs per kilogram. This corresponds to 
the field necessary to levitate the dielectric particles 20: the 50,000 
Volt per meter field obtained with a 1000 Volt output from the high 
voltage DC power source 26 and the 2 centimeter spacing used. 
The existence of positively charged dielectric particles 20 above the 
shield grid 42 as shown is most likely caused by two different factors. 
First, negatively charged dielectric particles 20 floating near the wire 
grid 24 may experience field emission and become positively charged. 
Second, when the UV energy is sufficiently high, photoemission from the 
dielectric particles 20 directly will increase, resulting in some 
positively charged dielectric particles 20. 
Referring next to the alternate embodiment of FIG. 3, a particle injector 
is illustrated which will produce positively charged dielectric particles. 
In the embodiment of FIG. 3, small grains 60 of dielectric material are 
used instead of the dielectric particles 20 such as microballoons, 
although this is irrelevant to the technique used to produce positively 
charged particles instead of negatively charged particles. 
The plate 22 is again used, and is again made of zinc in the preferred 
embodiment. The wire grid 24 is also used again, and is mounted in 
parallel fashion over the top surface of the plate 22. The space between 
the wire grid 24 and the top surface of the plate 22 is approximately two 
centimeters in the embodiment of FIG. 3. 
A high voltage DC power source 62 having a variable output of up to 1000 
Volts is used. The positive side of the high voltage DC power source 62 is 
electrically connected to the wire grid 24. The negative side of the high 
voltage DC power source 62 is electrically connected to one side of the 
ammeter 38. The other side of the ammeter 38 is electrically connected to 
the plate 22. 
A Kaufman ion source 64 is used in the particle injector of FIG. 3 to 
provide a beam of Argon or Helium ions. The negative side of the high 
voltage DC power source 62 is electrically connected to the Kaufman ion 
source 64. The Argon or Helium ion beam from the Kaufman ion source 64 is 
directed onto the top surface of the plate 22 at a slight downward angle 
between the bottom of the wire grid 24 and the top of the plate 22, and 
also onto the side of the plate 22 as shown. 
In the preferred embodiment, the width of the plate 22 and the wire grid 24 
shown in FIG. 3 are approximately 25 centimeters. In this preferred 
embodiment, the distance from the Kaufman ion source 64 to the furthest of 
the grains 60 on the top surface of the plate 22 which are in the beam 
from the Kaufman ion source 64 is approximately 45 centimeters. It should, 
however, be noted that these dimensions are not presently viewed as being 
critical. The preferred voltage from the high voltage DC power source 62 
is set to produce a reading on the ammeter 38 of approximately 140 
microamps. 
It may therefore be appreciated from the above detailed description of the 
preferred embodiment of the present invention that it teaches an apparatus 
and a method for injecting dielectric particles in a manner analogous to 
conventional particle injection of conductive droplets and particles. 
The present invention accomplishes this in two stages. Initially, a charge 
is placed on the dielectric particles or droplets in order to provide a 
manner of controlling the ensuing movement of the dielectric particles or 
droplets. 
Next, the appropriately charged dielectric particles or droplets are 
ejected into a desired area using electrostatic force. The charged 
dielectric particles or droplets are then maintained in the desired area 
through electrostatic confinement. 
The present invention allows the dielectric particles or droplets to be 
charged either negatively or positively by varying the charging technique. 
The apparatus of the present invention is relatively compact and 
inexpensive, both to construct, and to operate and maintain as well. 
Finally, all of the aforesaid advantages and objectives of the present 
invention are achieved without incurring any substantial relative 
disadvantage. 
Although an exemplary embodiment of the present invention has been shown 
and described, it will be apparent to those having ordinary skill in the 
art that a number of changes, modifications, or alterations to the 
invention as described herein may be made, none of which depart from the 
spirit of the present invention. All such changes, modifications, and 
alterations should therefore be seen as within the scope of the present 
invention.