PROCESS FOR DEPOSITING ATOMIC TO NANOMETER PARTICLE COATINGS ON HOST PARTICLES BACKGROUND OF THE INVENTION This invention relates in general to the field of coating a substrate where physical vapor deposition (PVD) techniques for creating extremely small or atomic size species in a flux, such as laser ablation, thermal evaporation or sputtering, are used to deposit coating material onto the substrate. More particularly, the invention relates to the field of material coating where the substrate consists of individual host particles rather than a film or bulk substrate. Even more particularly, the invention relates to depositing a number of discrete particles on the host particles to form generally uniform discontinuous or continuous coatings on the individual host particles, where the coating particles vary in size from atomic to nanometer scale particles.

A well known technique for coating a bulk or thin film substrate with a separate material, known as physical vapor deposition (PVD). is to provide a target object composed of the coating material in relatively close proximity and parallel to the substrate surface, then striking the target with an energy beam to free discrete particles of the coating material from the target, the discrete particles then adhering to the substrate. In the PVD method known as laser ablation, a laser is used to supply the energy to create the coating particles. Laser ablation works well with multicompositional coatings where the composition of the targets are directly transferred onto the substrate. While this technique is very successful for providing continuous thin film coatings on bulk substrates, it is not readily applicable for substrates consisting of discrete host particles. and is

especially not applicable for applying uniform discontinuous coatings where the size of coating particles is to be in the range of atomic size to only a few nanometers. If not properly controlled, the coating particles produced by laser ablation may agglomerate prior to adhesion to the host particles to form coating particles of greater than desired size. As the aggregation strength of the agglomerated coating particles is expected to be very high, the usefulness of these particles as coatings on the host particles is significantly reduced. Additionally, this and other coating techniques often result in the individual host particles agglomerating into a unified mass.

There are many applications where discontinuous or continuous nano-particle or smaller size coatings on host particulates are desirable, such as in flat panel displays, sintering of advanced ceramics and in rechargeable batteries. The nano-particle coatings on the host particles can increase surface area, produce higher catalytic activity, increase adhesion and lower sintering temperatures. Surface properties of the host particles relating to heat transfer, electrical, adsorption, desorption. and reflectivity can be altered.

Through the methodology of the invention, it has been found that the size of the coating particles and the coating parameters can be controlled such that a number of individual host particles can have a controlled number or amount of discrete coating particles, sized from atomic size to a few nanometers, adhered in a well dispersed pattern onto the surface of the host particles.

It is an object of this invention to provide discrete, relatively large, host particles with a discontinuous or continuous surface coating of discrete particles, sized as small as a few nanometers down to atomic scale, through a PVD method such as laser ablation method. It is a further object to provide such coated particles by a method where the cluster size of the individual coating particles and the amount of surface coverage of the individual coating particles can be controlled to prevent agglomeration of the individual coating particles. It is a further object to provide means to prevent the coated host particles from agglomerating.

SUMMARY OF THE INVENTION Host particles, which may range in size for example from several nanometers to several millimeters in diameter, are provided with a relatively uniformly dispersed discontinuous or continuous coating of discrete individual coating particles sized from atomic scale to a few nanometers. The coating particles are created by a PVD process, and preferably by laser ablation, where a pulsed laser beam is aimed at a target composed of the coating material under conditions sufficient to release individual particles from the target in a generally perpendicular ablation flux. The laser ablation technique is especially suited for multi-elemental deposition in which the stoichiometry of the ablated species is maintained. The size of the coating particles can be varied from atomic species to nanometer species by controlling the gas pressure in the system. The chamber pressure can be dynamically varied with time to control the agglomeration zones. During laser ablation, the host particles are kept agitated or fluidized such that there is continual relative movement between all the host particles. The fluidization of the host particles can be accomplished by various means. such as mechanical vibration or impaction of a container designed to provide exposure of the host particles to the coating particles. The degree of coating is controlled by varying the laser parameters. energy density and number of pulses, gas pressure within the treatment chamber, and the treatment time.

DETAILED DESCRIPTION OF THE INVENTION The invention in general comprises a method or process, and the articles produced by this method, for creating host or core particles with discrete particles adhered generally uniformly to the surface of the host particles to form continuous or discontinuous coatings where the individual host particles remain non-agglomerated after the deposition step.

The coating particles are extremely small, being sized from atomic to a few nanometers in diameter, relative to the host particles, which are sized from several nanometers to several millimeters in diameter. The method utilizes physical vapor deposition (PVD) such as thermal evaporation. sputtering, or preferably laser ablation of a target material to produce a flux of coating particles, and fluidization means to agitate the host particles during the coating process to prevent agglomeration.

Laser ablation of a target material to produce free particles of the target material which adhere to a substrate is a well known technique. A sealable chamber is provided so that the atmosphere within the chamber may be controlled as to the particular gases present and as to the partial pressure within the system using common technology. A target composed of the desired coating material is mounted, preferably rotatably, within the chamber and a UV transparent quartz window is provided through which a laser beam can be directed to strike the target. A typical laser which has been used experimentally

for the methodology of this application is a Lambda Physik model 305i pulsed excimer gas laser with an operating waveiength of 248 nanometers. Many other suitable lasers may be substituted therefore. The laser beam will produce a particle flux generally perpendicular to the surface of the target. Laser ablation is preferred since under optimized conditions the removal of species from the target takes place in a stoichiometric manner. Other well known PVD techniques which produce atomic to nanometer scale ablated species in a flux, such as thermal evaporation and sputtering, may also be utilized.

The host or core particles are generally large relative to the size of the coating particles, with the method proven to be very applicable to host particles sized from 0.5 to 100 microns. It is understood that the host particles can be smaller, down to several nanometers in diameter, or larger, up to several millimeters in diameter, than this range if so desired. The host particles are retained within a processing container which has a large enough volume to permit movement of the particles within the container. The top of the container is open and the container maintained in a vertical position during fluidization, or a portion of the processing container, such as a part or all of a side or bottom, is provided with openings or apertures to retain the host particles within the processing container, if the particle deposition is to occur laterally or from below. A suitable construction for the processing container has been found to be a cylindrical glass vial with one open end, the open end being covered, if necessary, by a wire mesh or screen with apertures slightly smaller than the size of the host particles. The processing container is mounted within the treatment chamber with the open end facing the target at a distance of from approximately 3 to 10 centimeters such that the majority of particles in the perpendicular flux from the target will enter the processing container and contact

the host particles. The system may also be constructed with continuous or incremental transport means for the host particles. such as a conveyor system, whereby the host particles can be moved relative to the ablation flux during the coating process so that coating may occur in a continuous manner.

The host particles must be agitated or fluidized in some manner to expose the entire surface of each host particle to the coating particles entering the processing container to insure general uniformity of coating and to assist in the prevention of agglomeration of individual host particles. This fluidization may be accomplished in a number of equivalent manners, such as by mechanical agitation by vibration, rotation or movement of the processing container, by providing a stirring device within the container, or by pneumatic agitation by passing gas flow through the host particles. Another means to accomplish the required fluidization is to intermix magnetic particles, such as iron, with the host particles and then to apply an alternating magnetic field to the processing container during the deposition of the coating particles. The magnetic particles are separated from the host particles after the treatment process.

The percentage of deposition or coverage of the coating particles on the host particles is controlled by controlling the size of the coating particles and the treatment time. The longer the treatment time, the more coating particles will be adhered to the surface of the host particles. increasing both the percentage of coverage and the thickness of the coating layer. Surface coverage can be adjusted from below 1 percent up to 100 percent. The size of the coating particles is controlled by the atmospheric composition and partial pressure within the treatment chamber. By dynamically controlling the gas pressure the reaction zone for forming the coating particles can be controlled. Reactive

gases such as oxygen, ammonia or nitrous oxide produce higher concentrations of molecular, as opposed to atomic. species within the ablated particle flux, and are used if deposition of oxide, nitride or simiiar particles is desired. Pressure within the chamber determines the number of collisions between ablated coating particles, with higher pressure causing more collisions and therefore larger coating particles in the ablated flux. Pressure within the system may vary greatly, from 104 to 10 Torr for example, but production of 1 to 10 nanometer or smaller coating particles typically occurs at approximately 400 mTorr or higher, and production of atomic particles occurs at below approximately 300 mTorr.

EXAMPLE 1 Atomic scale titanium dioxide coating particles were deposited onto silicon dioxide host particles under the following conditions: laser focal lens at 7.5 cm, laser pulse rate at 35 hz for 8 minutes at 600 mJ and energy density of 3 to 6 J/cm-, argon atmosphere, 120 mTorr starting partial pressure and 180 mTorr ending partial pressure. The host particles were retained in a cylindrical glass vial approximately 1 inch in diameter and 2 inches in length with an open end covered by a wire mesh. The host particles were agitated using magnetic stirring. SEM (scanning electron microscope) photomicrography of the host particles and wavelength dispersive x-ray maps of the titanium on the surface of the host particles show a generally uniform but discontinuous deposition of discrete particles of titanium dioxide.

EXAMPLE 2 Nano-scale silver coating particles were deposited onto silica host particles under the following conditions: laser focal lens at 7.5 cm, laser pulse rate at 30 hz for 9 minutes at 450 mJ and energy density of 3 to 6 J/cm-. 17 mTorr starting partial pressure and 10 mTorr ending partial pressure. The host particles were retained in a cylindrical <BR> <BR> <BR> <BR> <BR> glass vial approximately 1 inch in diameter and 2 inches in length with an open end covered by a wire mesh. The host particles were agitated using magnetic stirring. SEM photomicrography of the host particles and wavelength dispersive x-ray maps of the silver on the surface on the silica host particles show a generally uniform but discontinuous deposition of discrete particles of silver.

EXAMPLE 3 Nano-scale silver coating particles were deposited onto silica host particles under the following conditions: laser focal lens at 7.5 ci-n, laser pulse rate at 15 hz for 40 minutes at 450 mJ and energy density of 3 to 6 J/ci-n2,19 mTorr starting partial pressure and 21 mTorr ending partial pressure. The host particles were retained in a cylindrical <BR> <BR> <BR> <BR> <BR> glass vial approximately 1 inch in diameter and 2 inches in length with an open end covered by a wire mesh. The host particles were agitated using magnetic stirring. SEM photomicrography of the host particles and wavelength dispersive x-ray maps of the silver on the surface on the silica host particles. showing a generally uniform but discontinuous deposition of discrete particles ot silver.

It is understood that equivalents and substitutions for elements set forth above may be obvious to those skilled in the art. and therefore the true scope and definition of the invention is to be as set forth in the following claims.