Electrohydrodynamic spraying to produce ultrafine particles

Amorphous or microcrystalline alloy powder is prepared by the rapid quenching of ultrafine metallic spheroids generated from the molten metal state. The molten metal droplets are formed when an intense electric field (10.sup.5 V/cm) is applied to the surface of liquid metal held in a suitable container. The interactions between the intense electric field and liquid surface tension disrupts the metal surface, resulting in a beam of positively charged droplets. The liquid metal spheres generated by this electrohydrodynamic process are subsequently cooled by radiative heat transfer. Rapid cooling of the droplets may be accomplished by heat transfer to a low pressure gas by free molecular heat conductivity. Quenching rates exceeding 10.sup.6 .degree.K./sec are possible using this technique. Thin film coatings are prepared by electrohydrodynamically spraying a beam of charged droplets against a target (substrate). The target can be electrically controlled to effect the charged particles impact. The materials to be sprayed electrodynamically can be varied in both throughput and species such that a target can have multimaterial layers being deposited coincidentally or sequentially. The ultra small droplet size will enhance the physical properties by reducing skin stresses and enhance the optical properties by reducing the growth of crystallites in the film. Precise layers can be deposited from extremely thin films to thick filters for optical characteristics into the infrared. All materials that can be molten and contained can be electrohydrodynamically sprayed and controlled for depositions upon a substrate material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the electrohydrodynamic generation of sub-micron 
to micron size particles, to the rapid cooling and solidification of such 
particles, the preparation of amorphous metals, alloys, ceramics and the 
like from such particles, the application of thin film coatings from such 
particles, and the apparatus for electrohydrodynamically generating the 
sub-micron to micron size particles and for carrying out the rapid cooling 
and solidification and thin film coating applications as well as other 
applications. 
2. Discussion of the Prior Art 
The production of fine metal powders has been an active field of technology 
and many different approaches have been taken for the production of these 
metal as well as non-metal particles. For example, in U.S. Pat. No. 
3,830,603 to Blucher, et al. the patentees mention many prior art 
techniques such as the atomization of molten metal by gas jets or by high 
pressure water; spraying molten metal into a vacuum to form discrete 
particles; the vaporization of metal in a vacuum followed by condensation; 
the fusion of metal by an electric arc followed by the formation of 
condensed droplets which may be forced out of the arc zone either by means 
of a gas stream or by centrifugal force either alone or coupled with the 
influence of the magnetic repulsion inherent in the arc; forming a molten 
surface on a metal rod and agitating the molten metal at ultrasonic 
frequency generated either by an ultrasonic transducer or by use of a high 
frequency electric current coupled with a strong direct current magnetic 
field. The processes of these prior art references are carried on either 
in the presence of an inert gas or in a vacuum. The Blucher, et al. 
apparatus produces fine metal powder from a wire or rod by forming 
particles of molten metal in an electric arc and removing the formed 
particles by the interaction of two magnetic fields operating at right 
angles to one another. 
While these devices have achieved some degree of success, there is still a 
need for a more efficient means of obtaining even finer size particles 
from a broader range of materials than can be accomplished by any of the 
above mentioned techniques. 
It has been possible by electrohydrodynamic spraying techniques to produce 
singly charged ions and heavier sub-micron particles as described, for 
example, by Swatik and Hendricks, "Production of Ions by 
Electrohydrodynamic Spraying Techniques" J. AIAA, Vol. 6, 1596-7 (August 
1968); Mahoney, et al., "Electrohydrodynamic Ion Source", J. Applied 
Physics, Vol. 40, 5101-06, (December 1969). However, the 
electrohydrodynamic (EHD) spraying technique has principally been used 
only as a research tool for testing properties of singly charged and 
multiply charged ions and as an electric propulsion source. The full 
potential for the electrohydrodynamic spraying technique for forming new 
types of materials and ultra-fine particles has neither been fully 
appreciated nor developed to the extent where the potential applications 
of this technique could be applied in a commercially useful sense. 
In particular, it has now been found that electrohydrodynamic spraying can 
be used to form sub-micron to micron size particles from metal alloys, 
ceramic materials and similar high melting point composite materials which 
never previously existed in that form. It has also been discovered that 
very thin coatings having superior properties can be formed from such 
materials. 
Electrohydrodynamic Technology 
The electrohydrodynamic (EHD) method for droplet generation involves the 
use of very intense electric fields which are obtained by exposing, at 
electrodes, a liquid having high curvature geometry using moderate 
voltages of about 3 to about 20 kilovolts. The electric field intersecting 
a liquid meniscus produces the very high electric fields needed for 
charged droplet generation. After generation, the droplets are then 
accelerated by the applied voltage. 
The EHD technique for charged droplet generation initiates with a 
conductive liquid exposed to a high electric field at the tip of a small 
capillary tube. The interaction of the electrostatic stresses applied by 
the electric field with the surface tension forces on the liquid at the 
capillary tip results in a highly dynamic process at the charged liquid 
surface. When the outward force exerted on the liquid meniscus exceeds the 
surface tension forces, the surface disrupts by expelling droplets. 
Relatively intense fields on the order of about 10.sup.5 V/cm or greater 
are required for droplet production and capillary needle emitters having 
an orifice on the order of about 75 microns are typically employed to 
yield the necessary field strengths to produce micron size particles. In 
contrast, using hydrostatic jets with no electric fields on similar 
emitters and under vacuum conditions will produce particles typically 
having diameters of between 110 and 150 microns. The field strength at the 
tip of a capillary electrode is directly related to the applied voltage 
and the capillary emitter dimensions. 
The controllable variables in electrohydrodynamic droplet generation 
include, for example, the acceleration voltage, the electric field at the 
emitter, the emitter and extractor geometries, the conductive liquid feed 
rate and the temperature. The properties of the liquid metal feed is also 
a variable effecting the results. Electrohydrodynamically generated 
droplets are controlled in terms of radius, charge-to-mass ratio and 
velocity. The droplet trajectory and impact energy are controllable by 
means of electric and magnetic fields. In general, the droplet size can be 
increased by decreasing the electric field (voltage) and/or by increasing 
the flow rate of the liquid feed. With increasing size, the charge-to-mass 
ratio and the droplet velocity decrease and the time-of-flight from the 
source to a collector increase accordingly. The droplet velocity (v) can 
be defined in terms of the charge-to-mass ratio (q/m) and the acceleration 
voltage (V) according to the energy relation; 
EQU qV =1/2 mv.sup.2. 
In general, if the EHD source current (I) and the mass flow-rate (m) are 
measured, then the charged-to-mass ratio (q/m) of the emitted droplets can 
be determined by the following expression; 
EQU I= (q/m) m. 
The electrohydrodynamic droplet formation method is highly versatile with 
respect to the types of feed materials, particle sizes, particle size 
distribution, feed material and particle temperature control, coating 
characteristics and the like. 
The feed material may be substantially any material: metallic or 
non-metallic, inorganic or organic, single element of alloy, mixtures, 
compounds, etc., the only requirements being that it can exist in the 
liquid (molten) state and be capable of being electrically charged. 
A novel aspect of the present invention is the application of 
electrohydrodynamics to non-wetting liquids, such as metal alloys. The 
early studies with EHD spraying for the production of singly charged ions, 
as a propulsion source, in the nuclear field for studying thin film 
cross-sections, etc., involved feed materials which wetted the walls of 
the EHD spraying apparatus. In these prior art systems, therefore, flow 
control of the feed material could be accomplished, for example, by 
capillary action, by gravity flow or by simple mechanical pressure, such 
as pressure exerted on the liquid feed material by a piston. Also, totally 
different surface phenomena was observed, especially with respect to the 
geometry of the liquid meniscus at the capillary needle emitter, a concave 
geometry observed in a wetting system and a convex geometry being observed 
in a non-wetting system. The different geometrical patterns account for 
very significant differences in droplet formation as a result of 
differences in the interaction with the electrostatic field. 
Particle sizes can be achieved for electrohydroynamically formed particles 
ranging from several microns (up to the dimension of the orifice of the 
capillary needle emitter) to sub-micron (down to singly charged ions). 
Particle size variables include the mass flow rate of the feed material to 
the capillary needle emitter, and the applied voltage. For fixed mass flow 
rate, as the applied voltage increases the droplets formed from the 
meniscus at the capillary tip break off faster, i.e. smaller size particle 
droplets having a higher charge-to-mass ratio are formed. For a fixed 
applied voltage, as the mass flow rate is decreased, so that the rate at 
which the liquid meniscus forms at the capillary tip decreases, the size 
of the formed droplet decreases, i.e. charge-to-mass ratio increases. 
Other processing variables which effect particle size and particle size 
distribution of electrohydrodynamically formed particles include the 
position of the extractor or accel electrode relative to the tip of the 
capillary needle emitter and the electrostatic field. With metals and 
alloys it is possible to prepare substantially all singly charged ions. 
With other materials, e.g. semiconductors, ceramics, etc., narrow 
distributions of particle sizes can be achieved. Particle sizes ranging 
from about 0.01 micron to about 100 microns are possible. 
Another novel aspect of the EHD spraying technique for fine size particle 
generation according to the present invention, involves the treatment of 
metals, metal alloys, ceramics and other high melting temperature 
materials, especially metal alloys such as iron alloys, aluminum alloys, 
rare earth metal alloys, transition metal alloys and the like having 
melting temperatures greater than about 500.degree. C., especially greater 
than about 1000.degree. C., and most especially greater than about 
1500.degree. C. For example, a Fe-P-C alloy at a weight ratio of iron to 
phosphorous to carbon of about 80-13-7 can be formed into ultrafine 
particles without phase separation. 
In fact, it is now possible for the first time to process virtually any 
metal alloy into very fine particles ranging in size from about 100 
microns to sub-micron size. Because of the extremely rapid quenching, on 
the order of about 10.sup.6 .degree.K/sec or more, which can be achieved 
for these ultrafine size particles, simply by radiative cooling, or 
radiative cooling combined with conductive and/or free molecular flow, the 
particles can be solidified without separation of the different metal 
elements of the metal alloy. Iron, nickel, copper, aluminum and similar 
metal alloys can be electrohydrodynamically sprayed. 
Furthermore, temperature control of the feed material in the processing 
apparatus provides for variations in material viscosity and therefore more 
precise control of feed material mass flow rate. Temperature also effects 
the thermal as well as electrical conductivities of the feed material so 
that control of temperature permits still further controls of particle 
sizes, particle size distributions, particle trajectories, particle 
cooling, etc. 
With respect to formation of coatings the materials to be sprayed 
electrohydrodynamically can be varied in both throughput and species such 
that a target can have multilateral layers deposited coincidentally or 
sequentially. The ultra small droplet size enhances the physical 
properties of the coating by reducing skin stresses and enhances the 
optical properties by reducing the growth of crystallites in the film. 
Precise layers can be deposited from extremely thin films to relatively 
thick filters for optical characteristics into the infrared. 
As previously stated, the most important applications of the 
electrohydrodynamic spraying technique of the present invention involve 
the formation of new alloys as well as new forms (ultrafine particle 
sizes, amorphous or crystalline) of old alloys and ceramics especially 
alloys and ceramics composed of high and low melting temperature elements, 
and in formation of thin film coatings for such uses as antireflection 
coatings, dichroic beam splitters, color and bandpass filters, color 
selective beam filters, narrow pass-band (interference) filters 
semi-transparent mirrors, heat control filters, high reflectivity mirrors, 
polarizers, reflection filters, semiconductor films, integrated circuits, 
and the like. 
Accordingly, it is a principal objective of the present invention to 
provide a method for preparing amorphous or microcrystalline alloy and 
ceramic powders and products therefrom which are characterized by their 
formation at rapid cooling rates exceeding 10.sup.6 .degree.K/sec and up 
to 10.sup.10 .degree.K/sec. The desirability of a system that generates 
molten material microdroplets by the controlled stressing of a molten 
metal surface by intense electrostatic fields is apparent. The small 
metallic or ceramic spheroids possess a large surface-to-volume ratio and, 
additionally, the cooling rate is inversely proportional to the droplet 
radius. It is a further object of this invention to provide an improved 
system for the generation and control of high temperature, non-wetting 
molten metal droplets in the size range conductive to rapid quenching 
rates. It is a further object of the present invention to provide a novel 
apparatus having a single or plurality of refractory nozzles (capillary 
needle emitters) for the production of molten metal droplets. Further, it 
is an object of the present invention to provide an improved means of 
droplet generation that relates specifically to the quenching from the 
liquid state to produce nonequilibrium effects in solids, i.e., 
noncrystalline (amorphous) structures, extended solid solubilities, new 
metastable phases or distinctive microstructures and alloys composed of 
high and low melting temperature constituents difficult to alloy by any 
other means. It is an object of the present invention to provide an 
atomization system that does not require the formation of liquid metal 
streams or use of a complex gas system, requires no moving parts, and 
eliminates the need for the stock or feed material to be reduced to the 
powder form before processing. Further, an object of this invention is to 
provide a means of continuously processing material, a clear advantage 
over batch ststems. An object of the present invention is to provide a 
means for eliminating the possibility of contamination or oxidation of the 
feed material droplets in transition between generation and collection 
(impact). Further, an object of the present invention is to extend the 
lower range of particle sizes presently achievable by the above-mentioned 
schemes. It is an object of this invention to provide higher quenching 
rates by providing smaller particles and to utilize the transition time of 
the droplets between generation and impact as an effective means of 
cooling by radiative heat transfer. It is a further object of this 
invention to provide a means of injecting the droplets into a background 
of low pressure inert gas such as argon to utilize the quenching effect of 
free molecular conductivity. In this case, provided the droplets are small 
enough, the gas atoms incident on the droplet surface afford a method of 
heat transfer by collision and emission at the surface temperature of the 
droplet. The present invention can also utilize a cooled substrate for 
quenching after impact similar to such conventional systems as described 
in the Background of the Invention, including, for example, the gun 
technique of splat cooling, gas atomization, centrifugal or rotary 
atomization, and plasma jet or arc spraying. Still further object of the 
present invention is to provide an apparatus for processing amorphous 
materials that is less complex and more economical than prior art devices. 
It is another principal objective of the present invention to provide a 
method for preparing materials especially high melting temperature and 
non-wetting materials and depositing the material on a substrate by 
electrohydrodynamic spraying of materials in the liquid state. 
Further, it is an object of the present invention to provide an improved 
means of droplet generation that relates specifically to the deposition 
from the liquid state to produce thin film on metallic or other material 
substrate. It is an object of the present invention to provide an 
electroatomization system that does not require the formation of 
evaporative liquid material streams or use of a complex boiler system, 
requires no moving parts, and eliminates the need for the stock or feed 
material to be evaporated from a boiler, elimination of carrier or 
catalyst, the reductions of contaminants in vacuo. 
It is still another object of this invention to provide stable films by 
providing smaller particles and to utilize the rapid cooling of the 
droplets during flight and upon impact as an effective means of reducing 
crystallite buildup. The present invention can also utilize multiple 
sources for multimaterial deposition on the substrate. Still further 
object of the present invention is to provide an apparatus for processing 
thin films that is less complex and more economical than prior art 
devices. 
SUMMARY OF THE INVENTION 
These and other objectives of the present invention are accomplished with 
apparatus and method for electrohydrodynamically producing a beam of 
charged particles from a molten feed stock of a high melting temperature, 
non-wetting metal, metal alloy, semiconductor, ceramic or similar material 
contained in a refractory or other high temperture resistant non-corrosive 
container which is in flow communication with a refractory or similar 
material nozzle, the molten feed stock being subjected to precise 
temperature and mass flow rate control. The nozzle is a capillary needle 
emitter the tip of which is located in the same vertical plane as, or in a 
vertical plane slightly forward of or slightly behind an extractor or 
accelerating electrode; the tip is also located with its axis coinciding 
with the midpoint of an aperture in the extractor electrode. The droplet 
sources, including the crucible, feedstock, nozzle, extractor or accel 
electrodes, heating and temperature control elements as well as the 
droplet collection system are all, preferably, housed in a vacuum housing 
maintained at low pressure (about 10.sup.-6 torr. to about 0.5 torr). The 
metal alloy or other source material is fed into the refractory crucible 
in the evacuated chamber where it is melted and maintained at a precise 
temperature. 
Pneumatic pressure, using an inert gas, such as argon, forces the molten 
feed material from the crucible to the capillary needle emitter which is 
positively charged using moderate voltages of about 3 to 20 kilovolts and 
also temperature controlled. The liquid meniscus which forms at the 
orifice of the capillary needle emitter is intersected by the field lines 
of the electrostatic field. The interaction of the electrostatic stresses 
with surface tension force results in a highly dynamic process at the 
charged liquid surface. When the outward force exerted on the liquid 
meniscus exceeds the surface tension forces the surface disrupts by 
expelling droplets. In this process the electric field causes the liquid 
meniscus to form liquid jets or spikes which in turn produce the very high 
electric fields (about 10.sup.5 volts/cm) needed for charged droplet 
generation. After generation, the positively charged droplets are then 
accelerated by the applied electric field, across the gap between the 
orifice of the nozzle tip and accel electrode to form a diverging beam. 
The angle of divergence of the droplet beam is a function of the applied 
voltage, charge-to-mass ratio, position of the accel electrode and the 
electrical repulsion between the like-charged droplets. 
The droplets are eventually collected either by impinging on a 
collector-target and forming a thin film coating or by simple 
solidification as discrete particles. The target may be electrically 
controlled to effect the impact of the charged particles. Cooling rates 
are affected by particle sizes and ambient conditions. Radiation, 
convection, conduction and free molecular flow are all available means of 
heat transfer. Droplet cooling by radiation is a function of several 
parameters, of which the following are the most important: droplet size 
and material; heat of fusion; spectral emissivity; temperature; thermal 
conduction; velocity; and cooling media. 
For a typical feed material, assuming an applied voltage in the range of 
about 2 to about 10 kilovolts and a distance of about 1 meter between the 
capillary needle emitter and the target, cooling rates on the order of 
10.sup.6 to 10.sup.8 .degree.K/sec, mass flow rates of about 10.sup.-5 to 
10.sup.-2 g/sec, charge-to-mass ratios of about 10.sup.-1 to 10.sup.5 
coulomb/kilogram, droplet velocities of about 10.sup.4 to 10.sup.6 cm/sec 
and droplet time-of-flight (TOF) of about 10.sup.-4 sec can be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, the apparatus includes a refractory nozzle 10, 
which can operate singularly or ganged together to form a plurality of 
nozzles. For the high melting temperature materials of interest 
commercially available nozzle materials include, for example, tungsten, 
tungsten carbide, alumina, beryllia, thoria, yttria, zirconia and the 
spinels. The refractory material must be chemically inert with respect to 
the sources material. The source material will also be non-wetting with 
respect to the refractory material. A molten metal alloy 12 or other 
material such as a molten metal, or molten ceramic, et cetera flows 
through the nozzle 10 where it is kept in the molten condition at a fixed 
temperature by a nozzle heater 14, which can be one of several types; for 
example, R-F, radiation or the preferred type as illustrated in FIG. 1, a 
metal sheathed, ceramic insulated heater wire held in intimate thermal 
contact with the nozzle 10 affixed to the nozzle by vacuum furnace 
brazing. A heat shield 13 helps to maintain constant temperature 
conditions. An accel or extractor electrode 16 is located in proximity to 
nozzle 10. The accel electrode 16 is a thin metallic plate, preferably 
also made from a refractory element to avoid high temperature problems. 
For symmetry considerations, the accel electrode 16 is usually circular, 
if the source is tubular, but it is possible to utilize other shapes. The 
accel electrode 16 contains a circular aperture 17, if tubular. In the 
preferred embodiment of this invention, the circular opening has a 
diameter whose dimensions are on the order of 1/16 inch to 1/8 inch. When 
a plurality or array of nozzles are employed, a single accel electrode 16 
can be employed with multi-apertures to provide the individual electrode 
systems required to establish the intense electric field at each of the 
nozzles comprising the array. The nozzle 10, or tubular droplet source 
tapers from a large diameter and terminates in a short capillary tip 15. 
In the preferred form of this invention, the capillary tip 15 should taper 
down to an outer diameter in the range of about 0.0010 to about 0.020 
inch, preferably about 0.003 to about 0.01 inch at the tip. The orifice 
diameter has dimensions at the tip typically on the order of about 0.0003 
to about 0.012 inch, preferably about 0.0005 to about 0.008 inch. These 
small dimensions are necessary to enhance the electric field applied to 
the molten alloy-vacuum interface. The refractory nozzle 10 is positioned 
coaxially with the extractor or accel electrode 16 with the nozzle tip 15 
centered in the aperture 17. The preferred alignment of the 
nozzle-electrode system consists of placing the nozzle tip 15 in the accel 
aperture 17 such that the nozzle tip and front surface of the accel 
electrode 16 lie in the same plane. Although this alignment is preferred, 
satisfactory operation can be obtained if the nozzle tip 15 is positioned 
either directly behind the rear surface of the accel electrode 16 or is 
allowed to extend slightly forward of the front surface of the accel 
electrode. The particular alignment of the nozzle tip and accel electrode 
can be varied to modify the electrostatic field and hence, particle size 
distribution and particle trajectory. The nozzle 10, is constructed 
preferably from a refractory metal or oxide material resistant to any 
corrosion by the type of molten material processed and which is able to 
withstand temperatures of 1700.degree. C. and more, especially 
2000.degree. C. or more. 
As seen in FIG. 1, according to a particularly preferred geometry, the 
capillary nozzle tip should taper at an angle of about 15.degree. and the 
aperture 17 should similarly taper at an angle of about 15.degree. to 
about 25.degree. from its back surface to its front surface (from left to 
right as seen in FIG. 1). It has also been found that best results are 
obtained when the nozzle tip at the orifice is slightly rounded to provide 
a cylindrical outlet. 
With the aid of FIG. 2, the features of the materials processing apparatus 
according to the present invention, will now be described. A metal alloy 
12, ceramic charge, etc. is placed in the refractory reservoir 23, or 
crucible which may be formed from the same material as nozzle 10. The 
charge is melted by means of the crucible heater 25. The molten charge is 
then delivered to the capillary nozzle 10, by means of applying a positive 
pressure through the pressurizing tube 18 from an inert gas supply source 
21, for example, argon. The pressurizing tube 18 serves the dual purpose 
of vacuum (not shown) or pressure line. When the molten charge approaches 
the nozzle tip 15, it enters a region of intense electrostatic field 
established by the application of high positive voltage to nozzle 10, by 
means of power supply 11. The electric field is maintained between the 
positively charged nozzle tip 15 and the extractor or accel electrode 16, 
held at a negative potential of about -500V provided by means of power 
supply 13. The negative potential at the extractor electrode 16 also 
serves to control the amount of backstreaming electrons which bombard the 
tip 15 during operation. The relatively intense fields generated at tip 15 
(&gt;10.sup.5 V/cm) result in electrostatic stresses at the exposed molten 
alloy meniscus. The resulting interaction with the opposing surface 
tension forces produces a highly-dynamic process at the charged liquid 
metal alloy surface. When a critical value of electric field is 
approached, nominally at voltages of 2 to 5 kilovolts, the electrostatic 
forces overcome the surface tension forces tending to hold the liquid 
together. The liquid surface disrupts into an aggregate of positively 
charged droplets as depicted in FIG. 2. The droplet beam 26, is 
subsequently accelerated across the gap and impinges on the collector or 
cooling substrate 30. In one configuration of the present invention, the 
droplet collector 30 can be stationary, thereby fixing the droplet transit 
distance. An alternate embodiment of the invention allows for a deployable 
collector capable of translating in either direction along the elongated 
axis of the nozzle. This allows the flexibility to collect the emitted 
droplets in the molten, partially solidified or solidified phase as 
desired by providing controlled variations in the cooling time before 
collection. A further embodiment of the present invention accomplishes the 
variations in the time-of-flight of the molten microdroplets before 
impingement by means of applying a positive deceleration voltage to 
collector 30 provided by power supply 19. A novel feature of the present 
invention becomes apparent by replacing the electrically conducting 
substrate or collector by a nonconducting substrate. When positively 
charged droplets impinge on the collector, the resulting buildup of charge 
generates a floating potential at the collector surface, providing a novel 
means for decelerating the microdroplets. In addition to varying the 
transit time, a further advantage of decelerating the microdroplets is to 
reduce the energy of impact which avoids unwanted heating of the 
substrate. 
The particle dimensions generated by this process range from singly charged 
ions to droplet sizes on the order of the tip orifice dimensions. Droplet 
sizes are controlled by adjusting either the pressure supplied by an inert 
gas from supply 21, through the pressurizing line 18, or the voltage 
applied to nozzle 10, by means of power supply 11. The entire droplet 
source, comprised of reservoir 23, nozzle 10, and electrode 16, is housed 
in the vacuum chamber 22. Operation of the system has been performed at 
vacuums down to about 10.sup.-5 to about 10.sup.-6 torr. Since the present 
invention is capable of generating smaller metallic or ceramic spheres in 
a controlled fashion than is possible by any other technology, the 
formation of the microspheres can be controlled to allow the droplets to 
impinge in the molten form on substrate 30, where quenching occurs by 
direct thermal contact, that is, by heat conduction into the substrate 30, 
cooled by a suitable means. 
The system can also be operated at higher pressures up to atmospheric 
pressure or even slightly higher than atmospheric. However, operation in a 
vacuum assures greatest purity of the resulting product since the 
probability of entrapping residual gases becomes remote in high vacuums. 
On the other hand, there will be certain applications where it may be 
desirable to entrap impurity or dopant molecules in the molten droplets 
prior to solidification and impingement. The present invention offers this 
versatility by providing an optional gas supply 32 and conduit 33. 
While the system provides sufficient flexibility to adjust the process 
variables such that quenching of the molten particles takes place by 
direct thermal contact with the target 30, it is also possible that the 
particles can be rapidly quenched from the liquid to solid state by 
radiative heat transfer in flight between nozzle 10 and collector 30. For 
radiative heat transfer to be effective, it is essential that spheres of 
small dimensions be produced to experience cooling rates on the order of 
10.sup.6 .degree.K/sec and higher. The versatility of the present 
invention is readily apparent when it is understood that "in-flight" 
radiative cooling is not the only means by which rapid cooling to produce 
amorphous or microcystalline structures can be accomplished. The formation 
of the microspheres can be controlled to allow the droplets to impinge in 
the molten form on substrate 30 where quenching occurs by direct thermal 
contact, i.e., by heat conduction into the substrate 30 cooled by a 
suitable means. While it is presently contemplated that the preferred form 
of the invention would utilize radiative cooling to accomplish the 
processing of rapidly quenched material, it will be appreciated that 
another form of the invention allows for rapid cooling by free-molecular 
heat transfer. In this form, an inert gas such as argon provided by means 
of gas supply 32 is bled into the chamber 22 by means of the chamber 
pressurizing line 33. The chamber pressure is maintained at a pedetermined 
level which satisfies the requirement that the dimensions of the droplets 
generated by the nozzle 10 are small compared with the mean free path of 
the background gas atoms. Other advantages of the application of 
electrohydrodynamic spraying for ultrafine droplet formation and 
preparation of thin film coatings include that the material source does 
not need any catalyst or carrier material added such that no material 
other than the desired material evolves from the source which is directed 
in a narrow electrostatically focused beam and virtually all of the source 
material will impact on the target (substrate). The narrow beam is from a 
narrow emission aperture such that there will be nil neutral efflux so 
that high vacuum may be maintained in a practical manner, and thereby 
reduce entrapped gases in the film. Thermal control of the material may 
permit sticking characteristics that are different from normal vapor 
condensation phenomena. This temperature control of the beam material may 
also enhance crystalline or amorphous structure of the film.