WO1998036887A1

WO1998036887A1 - Aerosol method and apparatus for making particulate products

Info

Publication number: WO1998036887A1
Application number: PCT/US1998/003621
Authority: WO
Inventors: Mark J. Hampden-Smith; Toivo T. Kodas; Quint H. Powell; Daniel J. Skamser; James Caruso; Clive D. Chandler
Original assignee: Superior Micropowders Llc
Priority date: 1997-02-24
Filing date: 1998-02-24
Publication date: 1998-08-27
Also published as: AU6439098A

Abstract

Provided is an aerosol method, and accompanying apparatus, for preparing powdered products of a variety of materials involving the use of an ultrasonic aerosol generator (106) including a plurality of ultrasonic transducers (120) underlying and ultrasonically energizing a reservoir of liquid feed (102) which forms droplets of the aerosol. Carrier gas (104) is delivered to different portions of the reservoir by a plurality of gas delivered ports (136) delivering gas from a gas delivery system. The aerosol is pyrolyzed to form particles, which are then cooled and collected.

Description

AEROSOL METHOD AND APPARATUS FOR MAKING PARTICULATE PRODUCTS

FIELD OF THE INVENTION

The present invention involves aerosol production of finely-divided particles of a variety of compositions. The present invention also involves the particles so manufactured and electronic devices made using the particles.

BACKGROUND OF THE INVENTION

Powdered materials are used in many manufacturing processes. One large use for powders is for thick film deposition to prepare films of a variety of materials. Some thick film applications include, for example, deposition of phosphor materials for flat panel displays, and patterning of eclectically conductive features for electronic products. For thick film applications, and for other applications, there is a trend to use powders of ever smaller particles. Generally desirable features in small particles include a small particle size; a narrow particle size distribution; a dense, spherical particle morphology; and a crystalline grain structure. Existing technologies for preparing powdered products, however, often could be improved with respect to attaining all, or substantially all, of these desired features for particles used in thick film applications.

One method that has been used to make small particles is to precipitate the particles from a liquid medium. Such liquid precipitation techniques are often difficult to control to produce particles with the desired characteristics. Also, particles prepared by liquid precipitation routes often are contaminated with significant quantities of surfactants or other organic materials used during the liquid phase processing.

Aerosol methods have also been used to make a variety of small particles. One aerosol method for making small particles is spray pyrolysis, in which an aerosol spray is generated and then converted in a reactor to the desired particles. Spray pyrolysis systems have, however, been mostly experimental, and unsuitable for commercial particle production. Furthermore, control of particle size distribution is a concern with spray pyrolysis. Also, spray pyrolysis systems are often inefficient in the use of carrier gases that suspend and carry liquid droplets of the aerosol. This inefficiency is a major consideration for commercial applications of spray pyrolysis systems. There is a significant need for improved manufacture techniques for making powders of small particles for use in thick film and other applications.

SUMMARY OF THE INVENTION The present invention provides an aerosol process for manufacturing finely- divided powders of a variety of materials having desirable properties and at commercially acceptable rates. Apparatus is also provided for implementing the manufacturing method.

An important aspect of the present invention is aerosol generation. An aerosol generator and aerosol generation method are provided that are capable of producing large quantities of a high quality, dense aerosol for spray pyrolysis operations. This is significantly different from aerosol generation that has previously occurred with respect to spray pyrolysis particle manufacture in small-scale, laboratory systems. An aerosol generator is provided including an array of ultrasonic transducers underlying a single reservoir of precursor solution that is ultrasonically energized to produce the aerosol.

Careful distribution of carrier gas to different portions of the reservoir result in an efficient use of carrier gas in making a dense aerosol and at a high rate suitable for commercial applications.

These and other aspects of the invention are discussed in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a process block diagram showing one embodiment of the process of the present invention.

Fig. 2 is a side view in cross section of one embodiment of aerosol generator of the present invention.

Fig. 3 is a top view of a transducer mounting plate showing a 49 transducer array for use in an aerosol generator of the present invention.

Fig. 4 is a top view of a transducer mounting plate for a 400 transducer array for use in an ultrasonic generator of the present invention. Fig. 5 is a side view of the transducer mounting plate shown in Fig. 4. Fig. 6 is a partial side view showing the profile of a single transducer mounting receptacle of the transducer mounting plate shown in Fig. 4.

Fig. 7 is a partial side view in cross-section showing an alternative embodiment for mounting an ultrasonic transducer. Fig. 8 is a top view of a bottom retaining plate for retaining a separator for use in an aerosol generator of the present invention.

Fig. 9 is a top view of a liquid feed box having a bottom retaining plate to assist in retaining a separator for use in an aerosol generator of the present invention.

Fig. 10 is a side view of the liquid feed box shown in Fig. 9. Fig. 11 is a side view of a gas tube for delivering gas within an aerosol generator of the present invention.

Fig. 12 shows a partial top view of gas tubes positioned in a liquid feed box for distributing gas relative to ultrasonic transducer positions for use in an aerosol generator of the present invention. Fig. 13 shows one embodiment for a gas distribution configuration for the aerosol generator of the present invention.

Fig. 14 shows another embodiment for a gas distribution configuration for the aerosol generator of the present invention.

Fig. 15 is a top view of one embodiment of a gas distribution plate/gas tube assembly of the aerosol generator of the present invention.

Fig. 16 is a side view of one embodiment of the gas distribution plate/gas tube assembly shown in Fig. 15.

Fig. 17 shows one embodiment for orienting a transducer in the aerosol generator of the present invention. Fig. 18 is a top view of a gas manifold for distributing gas within an aerosol generator of the present invention.

Fig. 19 is a side view of the gas manifold shown in Fig. 18.

Fig. 20 is a top view of a generator lid of a hood design for use in an aerosol generator of the present invention. Fig. 21 is a side view of the generator lid shown in Fig. 20. Fig. 22 is a process block diagram of one embodiment in the present invention including an aerosol concentrator.

Fig. 23 is a process block diagram of one embodiment of the process of the present invention including a droplet classifier. Fig. 24 is a process block diagram of one embodiment of the present invention including a particle cooler.

Fig. 25 is a top view of a gas quench cooler of the present invention.

Fig. 26 is an end view of the gas quench cooler shown in Fig. 25.

Fig. 27 is a side view of a perforated conduit of the quench cooler shown in Fig. 25.

Fig. 28 is a process block diagram of one embodiment of the present invention including a particle coater.

Fig. 29 is a block diagram of one embodiment of the present invention including a particle modifier. Fig. 30 shows cross sections of various particle morphologies of some composite particles manufacturable according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION In one aspect, the present invention provides a method for preparing a particulate product. A feed of liquid-containing, flowable medium, including at least one precursor for the desired particulate product, is converted to aerosol form, with droplets of the medium being dispersed in and suspended by a carrier gas. Liquid from the droplets in the aerosol is then removed to permit formation in a dispersed state of the desired particles. Typically, the feed precursor is pyrolyzed in a furnace to make the particles. In one embodiment, the particles are subjected, while still in a dispersed state, to compositional or structural modification, if desired. Compositional modification may include, for example, coating the particles. Structural modification may include, for example, crystallization, recrystallization or morphological alteration of the particles. The term powder is often used herein to refer to the particulate product of the present invention. The use of the term powder does not indicate, however, that the particulate product must be dry or in any particular environment. Although the particulate product is typically manufactured in a dry state, the particulate product may, after manufacture, be placed in a wet environment, such as in a slurry.

The process of the present invention is particularly well suited for the production of particulate products of finely divided particles having a weight average size, for most applications, in a range having a lower limit of about 0.1 micron, preferably about 0.3 micron, more preferably about 0.5 micron and most preferably about 0.8 micron; and having an upper limit of about 4 microns, preferably about 3 microns, more preferably about 2.5 microns and more preferably about 2 microns. A particularly preferred range for many applications is a weight average size of from about 0.5 micron to about 3 microns, and more particularly from about 0.5 micron to about 2 microns. For some applications, however, other weight average particle sizes may be particularly preferred.

In addition to making particles within a desired range of weight average particle size, with the present invention the particles may be produced with a desirably narrow size distribution, thereby providing size uniformity that is desired for many applications. In addition to control over particle size and size distribution, the method of the present invention provides significant flexibility for producing particles of varying composition, crystallinity and morphology. For example, the present invention may be used to produce homogeneous particles involving only a single phase or multi-phase particles including multiple phases. In the case of multi-phase particles, the phases may be present in a variety of morphologies. For example, one phase may be uniformly dispersed throughout a matrix of another phase. Alternatively, one phase may form an interior core while another phase forms a coating that surrounds the core. Other morphologies are also possible, as discussed more fully below.

Referring now to Fig. 1, one embodiment of the process of the present invention is described. A liquid feed 102, including at least one precursor for the desired particles, and a carrier gas 104 are fed to an aerosol generator 106 where an aerosol 108 is produced. The aerosol 108 is then fed to a furnace 110 where liquid in the aerosol 108 is removed to produce particles 112 that are dispersed in and suspended by gas exiting the furnace 110. The particles 112 are then collected in a particle collector 114 to produce a particulate product 116. As used herein, the liquid feed 102 is a feed that includes one or more flowable liquids as the major constituent(s), such that the feed is a flowable medium. The liquid feed 102 need not comprise only liquid constituents. The liquid feed 102 may comprise only constituents in one or more liquid phase, or it may also include particulate material suspended in a liquid phase. The liquid feed 102 must, however, be capable of being atomized to form droplets of sufficiently small size for preparation of the aerosol 108. Therefore, if the liquid feed 102 includes suspended particles, those particles should be relatively small in relation to the size of droplets in the aerosol 108. Such suspended particles should typically be smaller than about 1 micron in size, preferably smaller than about 0.5 micron in size, and more preferably smaller than about 0.3 micron in size and most preferably smaller than about 0.1 micron in size. Most preferably, the suspended particles should be able to form a colloid. The suspended particles could be finely divided particles, or could be agglomerate masses comprised of agglomerated smaller primary particles. For example, 0.5 micron particles could be agglomerates of nanometer-sized primary particles. When the liquid feed 102 includes suspended particles, the particles typically comprise no greater than about 25 to 50 weight percent of the liquid feed.

As noted, the liquid feed 102 includes at least one precursor for preparation of the particles 112. The precursor may be a substance in either a liquid or solid phase of the liquid feed 102. Frequently, the precursor will be a material, such as a salt, dissolved in a liquid solvent of the liquid feed 102. Typical precursor salts include nitrate, chloride, sulfate, acetate and oxalate salts, and the like. The precursor may undergo one or more chemical reactions in the furnace 110 to assist in production of the particles 112. Alternatively, the precursor material may contribute to formation of the particles 112 without undergoing chemical reaction. This could be the case, for example, when the liquid feed 102 includes, as a precursor material, suspended particles that are not chemically modified in the furnace 110. In any event, the particles 112 comprise at least one component originally contributed by the precursor.

The liquid feed 102 may include multiple precursor materials, which may be present together in a single phase or separately in multiple phases. For example, the liquid feed 102 may include multiple precursors in solution in a single liquid vehicle. Alternatively, one precursor material could be in a solid particulate phase and a second precursor material could be in a liquid phase. Also, one precursor material could be in one liquid phase and a second precursor material could be in a second liquid phase, such as could be the case when the liquid feed 102 comprises an emulsion. Different components contributed by different precursors may be present in the particles together in a single material phase, or the different components may be present in different material phases when the particles 112 are composites of multiple phases.

When the liquid feed 102 includes a soluble precursor, the precursor solution should be unsaturated to avoid the formation of precipitates. Solutions of salts will typically be used in concentrations in a range to provide a solution including from about

1 to about 50 weight percent solute. Most often, the liquid feed will include a solution with from about 5 weight percent to about 40 weight percent solute, and more preferably to about 30 weight percent solute. Preferably the solvent is aqueous-based for ease of operation, although other solvents, such as toluene or other organic solvents, may be desirable for specific materials. The use of organic solvents, however, can sometimes lead to undesirable carbon contamination in the particles. The pH of the aqueous-based solutions can be adjusted to alter the solubility characteristics of the precursor or precursors in the solution.

The carrier gas 104 may comprise any gaseous medium in which droplets produced from the liquid feed 102 may be dispersed in aerosol form. Also, the carrier gas 104 may be inert, in that the carrier gas 104 does not participate in formation of the particles 112. Alternatively, the carrier gas may have one or more active component(s) that contribute to formation of the particles 112. In that regard, the carrier gas may include one or more reactive components that .react in the furnace 110 to contribute to formation of the particles 112.

The aerosol generator 106 atomizes the liquid feed 102 to form droplets in a manner to permit the carrier gas 104 to sweep the droplets away to form the aerosol 108. The droplets comprise liquid from the liquid feed 102. The droplets may, however, also include nonliquid material, such as one or more small particles held in the droplet by the liquid. For example, when the particles 112 are composite, or multi-phase, particles, one phase of the composite may be provided in the liquid feed 102 in the form of suspended precursor particles and a second phase of the composite may be produced in the furnace 110 from one or more precursors in the liquid phase of the liquid feed 102. Furthermore, the precursor particles could be included in the liquid feed 102, and therefore also in droplets of the aerosol 108, for the purpose only of dispersing the particles for subsequent compositional or structural modification during or after processing in the furnace 110.

An important aspect of the present invention is generation of the aerosol 108 with droplets of a small average size, narrow size distribution. In this manner, the particles 112 may be produced at a desired small size with a narrow size distribution, which are advantageous for many applications. The aerosol generator 106 is capable of producing the aerosol 108 such that it includes droplets having a weight average size in a range having a lower limit of about 1 micron and preferably about 2 microns; and an upper limit of about 10 microns, preferably about 7 microns, more preferably about 5 microns and most preferably about 4 microns. A weight average droplet size in a range of from about 2 microns to about 4 microns is more preferred for most applications, with a weight average droplet size of about 3 microns being particularly preferred for some applications. The aerosol generator is also capable of producing the aerosol 108 such that it includes droplets in a narrow size distribution. Preferably, the droplets in the aerosol are such that at least about 70 percent (more preferably at least about 80 weight percent and most preferably at least about 85 weight percent) of the droplets are smaller than about 10 microns and more preferably at least about 70 weight percent (more preferably at least about 80 weight percent and most preferably at least about 85 weight percent) are smaller than about 5 microns. Furthermore, preferably no greater than about 30 weight percent, more preferably no greater than about 25 weight percent and most preferably no greater than about 20 weight percent, of the droplets in the aerosol 108 are larger than about twice the weight average droplet size.

Another important aspect of the present invention is that the aerosol 108 may be generated without consuming excessive amounts of the carrier gas 104. The aerosol generator 106 is capable of producing the aerosol 108 such that it has a high loading, or high concentration, of the liquid feed 102 in droplet form. In that regard, the aerosol 108 preferably includes greater than about 1 x 10⁶ droplets per cubic centimeter of the aerosol 108, more preferably greater than about 5 x 10⁶ droplets per cubic centimeter, still more preferably greater than about 1 x 10⁷ droplets per cubic centimeter, and most preferably greater than about 5 x 10⁷ droplets per cubic centimeter. That the aerosol generator 106 can produce such a heavily loaded aerosol 108 is particularly surprising considering the high quality of the aerosol 108 with respect to small average droplet size and narrow droplet size distribution. Typically, droplet loading in the aerosol is such that the volumetric ratio of liquid feed 102 to carrier gas 104 in the aerosol 108 is larger than about 0.04 milliliters of liquid feed 102 per liter of carrier gas 104 in the aerosol 108, preferably larger than about 0.083 milliliters of liquid feed 102 per liter of carrier gas 104 in the aerosol 108, more preferably larger than about 0.167 milliliters of liquid feed 102 per liter of carrier gas 104, still more preferably larger than about 0.25 milliliters of liquid feed 102 per liter of carrier gas 104, and most preferably larger than about 0.333 milliliters of liquid feed 102 per liter of carrier gas 104.

This capability of the aerosol generator 106 to produce a heavily loaded aerosol 108 is even more surprising given the high droplet output rate of which the aerosol generator 106 is capable, as discussed more fully below. It will be appreciated that the concentration of liquid feed 102 in the aerosol 108 will depend upon the specific components and attributes of the liquid feed 102 and, particularly, the size of the droplets in the aerosol 108. For example, when the average droplet size is from about 2 microns to about 4 microns, the droplet loading is preferably larger than about 0.15 milliliters of aerosol feed 102 per liter of carrier gas 104, more preferably larger than about 0.2 milliliters of liquid feed 102 per liter of carrier gas 104, even more preferably larger than about 0.2 milliliters of liquid feed 102 per liter of carrier gas 104, and most preferably larger than about 0.3 milliliters of liquid feed 102 per liter of carrier gas 104. When reference is made herein to liters of carrier gas 104, it refers to the volume that the carrier gas 104 would occupy under conditions of standard temperature and pressure.

The furnace 110 may be any suitable device for heating the aerosol 108 to evaporate liquid from the droplets of the aerosol 108 and thereby permit formation of the particles 112. For most applications, maximum average stream temperatures in the furnace 110 will generally be in a range of from about 500°C to about 1500 °C, and preferably in the range of from about 900 °C to about 1300°C. The maximum average stream temperature refers to the maximum average temperature that an aerosol stream attains while flowing through the furnace. This is typically determined by a temperature probe inserted into the furnace.

Although longer residence times are possible, for many applications, residence time in the heating zone of the furnace 110 of shorter than about 4 seconds is typical, with shorter than about 2 seconds being preferred, shorter than about 1 second being more preferred, shorter than about 0.5 second being even more preferred, and shorter than about 0.2 second being most preferred. The residence time should be long enough, however, to assure that the particles 112 attain the desired maximum average stream temperature for a given heat transfer rate. In that regard, with extremely short residence times, higher furnace temperatures could be used to increase the rate of heat transfer so long as the particles 112 attain a maximum temperature within the desired stream temperature range. That mode of operation, however, is not preferred. Also, it is noted that as used herein, residence time refers to the actual time for a material to pass through the relevant process equipment. In the case of the furnace, this includes the effect of increasing velocity with gas expansion due to heating.

Typically, the furnace 110 will be a tube-shaped furnace, so that the aerosol 108 moving into and through the furnace does not encounter sharp edges on which droplets could collect. Loss of droplets to collection at sharp surfaces results in a lower yield of particles 112. More important, however, the accumulation of liquid at sharp edges can result in re-release of undesirably large droplets back into the aerosol 108, which can cause contamination of the particulate product 116 with undesirably large particles. Also, over time, such liquid collection at sharp surfaces can cause fouling of process equipment, impairing process performance. The furnace 110 may be any suitable furnace reactor, which typically includes a tubular furnace through which the aerosol flows. Also, although the present invention is described with primary reference to a furnace reactor, which is preferred, it should be recognized that, except as noted, any other thermal reactor, including a flame reactor or a plasma reactor, could be used instead. A furnace reactor is, however, preferred, because of the generally even heating characteristic of a furnace for attaining a uniform stream temperature. The particle collector 114, may be any suitable apparatus for collecting particles 112 to produce the particulate product 116. One preferred embodiment of the particle, collector 114 uses one or more filter to separate the particles 112 from gas. Such a filter may be of any type, including a bag filter. Another preferred embodiment of the particle collector uses one or more cyclone to separate the particles 112. Other apparatus that may be used in the particle collector 114 includes an electrostatic precipitator. Also, collection should normally occur at a temperature above the condensation temperature of the gas stream in which the particles 112 are suspended. Also, collection should normally be at a temperature that is low enough to prevent significant agglomeration of the particles 112.

The process and apparatus of the present invention are well-suited for producing commercial-size batches of extremely high quality particles. In that regard, the process and the accompanying apparatus provide versatility for preparing powder including a wide variety of materials, and easily accommodate shifting of production between different specialty batches of particles.

Of significant importance to the operation of the process of the present invention is the aerosol generator 106, which must be capable of producing a high quality aerosol with high droplet loading, as previously noted. With reference to Fig. 2, one embodiment of an aerosol generator 106 of the present invention is described. The aerosol generator 106 includes a plurality of ultrasonic transducer discs 120 that are each mounted in a transducer housing 122. The transducer housings 122 are mounted to a transducer mounting plate 124, creating an array of the ultrasonic transducer discs 120. Any convenient spacing may be used for the ultrasonic transducer discs 120. Center-to-center spacing of the ultrasonic transducer discs 120 of about 4 centimeters is often adequate. The aerosol generator 106, as shown in Fig. 2, includes forty-nine transducers in a 7 x 7 array. The array configuration is as shown in Fig. 3, which depicts the locations of the transducer housings 122 mounted to the transducer mounting plate 124.

With continued reference to Fig. 2, a separator 126, in spaced relation to the transducer discs 120, is retained between a bottom retaining plate 128 and a top retaining plate 130. Gas delivery tubes 132 are connected to gas distribution manifolds 134, which have gas delivery ports 136. The gas distribution manifolds 134 are housed within a generator body 138 that is covered by generator lid 140. A transducer driver 144, having circuitry for driving the transducer discs 120, is electronically connected with the . transducer discs 120 via electrical cables 146.

During operation of the aerosol generator 106, as shown in Fig. 2, the transducer discs 120 are activated by the transducer driver 144 via the electrical cables 146. The transducers preferably vibrate at a frequency of from about 1 MHz to about 5 MHz, more preferably from about 1.5 MHz to about 3 MHz. Frequently used frequencies are at about 1.6 MHz and about 2.4 MHz. Furthermore, all of the transducer discs 110 should be operating at substantially the same frequency when an aerosol with a narrow droplet size distribution is desired. This is important because commercially available transducers can vary significantly in thickness, sometimes by as much as 10%. It is preferred, however, that the transducer discs 120 operate at frequencies within a range of 5% above and below the median transducer frequency, more preferably within a range of 2.5%, and most preferably within a range of 1%. This can be accomplished by careful selection of the transducer discs 120 so that they all preferably have thicknesses within 5% of the median transducer thickness, more preferably within 2.5%, and most preferably within 1%.

Liquid feed 102 enters through a feed inlet 148 and flows through flow channels 150 to exit through feed outlet 152. An ultrasonically transmissive fluid, typically water, enters through a water inlet 154 to fill a water bath volume 156 and flow through flow channels 158 to exit through a water outlet 160. A proper flow rate of the ultrasonically transmissive fluid is necessary to cool the transducer discs 120 and to prevent overheating of the ultrasonically transmissive fluid. Ultrasonic signals from the transducer discs 120 are transmitted, via the ultrasonically transmissive fluid, across the water bath volume 156, and ultimately across the separator 126, to the liquid feed 102 in flow channels 150.

The ultrasonic signals from the ultrasonic transducer discs 120 cause atomization cones 162 to develop in the liquid feed 102 at locations corresponding with the transducer discs 120. Carrier gas 104 is introduced into the gas delivery tubes 132 and delivered to the vicinity of the atomization cones 162 via gas delivery ports 136. Jets of carrier gas exit the gas delivery ports 136 in a direction so as to impinge on the atomization cones 162, thereby sweeping away atomized droplets of the liquid feed 102 that are being generated from the atomization cones 162 and creating the aerosol 108, which exits the. aerosol generator 106 through an aerosol exit opening 164.

Efficient use of the carrier gas 104 is an important aspect of the aerosol generator 106. The embodiment of the aerosol generator 106 shown in Fig. 2 includes two gas exit ports per atomization cone 162, with the gas ports being positioned above the liquid medium 102 over troughs that develop between the atomization cones 162, such that the exiting carrier gas 104 is horizontally directed at the surface of the atomization cones 162, thereby efficiently distributing the carrier gas 104 to critical portions of the liquid feed 102 for effective and efficient sweeping away of droplets as they form about the ultrasonically energized atomization cones 162. Furthermore, it is preferred that at least a portion of the opening of each of the gas delivery ports 136, through which the carrier gas exits the gas delivery tubes, should be located below the top of the atomization cones 162 at which the carrier gas 104 is directed. This relative placement of the gas delivery ports 136 is very important to efficient use of carrier gas 104. Orientation of the gas delivery ports 136 is also important. Preferably, the gas delivery ports 136 are positioned to horizontally direct jets of the carrier gas 104 at the atomization cones 162. The aerosol generator 106 permits generation of the aerosol 108 with heavy loading with droplets of the carrier liquid 102, unlike aerosol generator designs that do not efficiently focus gas delivery to the locations of droplet formation.

Another important feature of the aerosol generator 106, as shown in Fig. 2, is the use of the separator 126, which protects the transducer discs 120 from direct contact with the liquid feed 102, which is often highly corrosive. The height of the separator 126 above the top of the transducer discs 120 should normally be kept as small as possible, and is often in the range of from about 1 centimeter to about 2 centimeters. The top of the liquid feed 102 in the flow channels above the tops of the ultrasonic transducer discs 120 is typically in a range of from about 2 centimeters to about 5 centimeters, whether or not the aerosol generator includes the separator 126, with a distance of about 3 to 4 centimeters being preferred. Although the aerosol generator 106 could be made without the separator 126, in which case the liquid feed 102 would be in direct contact with the transducer discs 120, the highly corrosive nature of the liquid feed 102 can often cause premature failure of the transducer discs 120. The use of the separator 126, in combination with use of the ultrasonically transmissive fluid in the water bath volume . 156 to provide ultrasonic coupling, significantly extending the life of the ultrasonic transducers 120. One disadvantage of using the separator 126, however, is that the rate of droplet production from the atomization cones 162 is reduced, often by a factor of two or more, relative to designs in which the liquid feed 102 is in direct contact with the ultrasonic transducer discs 102. Even with the separator 126, however, the aerosol generator 106 used with the present invention is capable of producing a high quality aerosol with heavy droplet loading, as previously discussed. Suitable materials for the separator 126 include, for example, polyamides (such as Kapton™ membranes from

DuPont) and other polymer materials, glass, and plexiglass. The main requirements for the separator 126 are that it be ultrasonically transmissive, corrosion resistant and impermeable.

One alternative to using the separator 126 is to bind a corrosion-resistant protective coating onto the surface of the ultrasonic transducer discs 120, thereby preventing the liquid feed 102 from contacting the surface of the ultrasonic transducer discs 120. When the ultrasonic transducer discs 120 have a protective coating, the aerosol generator 106 will typically be constructed without the water bath volume 156 and the liquid feed 102 will flow directly over the ultrasonic transducer discs 120. Examples of such protective coating materials include platinum, gold, TEFLON™, epoxies and various plastics. Such coating typically significantly extends transducer life. Also, when operating without the separator 126, the aerosol generator 106 will typically produce the aerosol 108 with a much higher droplet loading than when the separator 126 is used. The design for the aerosol generator 106 based on an array of ultrasonic transducers is versatile and is easily modified to accommodate different generator sizes for different specialty applications. The aerosol generator 106 may be designed to include a plurality of ultrasonic transducers in any convenient number. Even for smaller scale production, however, the aerosol generator 106 preferably has at least nine ultrasonic transducers, more preferably at least 16 ultrasonic transducers, and even more preferably at least 25 ultrasonic transducers. For larger scale production, however, the aerosol generator 106 includes at least 40 ultrasonic transducers, more preferably at least 100 ultrasonic transducers, and even more preferably at least 400 ultrasonic transducers.. In some large volume applications, the aerosol generator may have at least 1000 ultrasonic transducers. Figs. 4-21 show component designs for an aerosol generator 106 including an array of 400 ultrasonic transducers. Referring first to Figs. 4 and 5, the transducer mounting plate 124 is shown with a design to accommodate an array of 400 ultrasonic transducers, arranged in four subarrays of 100 ultrasonic transducers each. The transducer mounting plate 124 has integral vertical walls 172 for containing the ultrasonically transmissive fluid, typically water, in a water bath similar to the water bath volume 156 described previously with reference to Fig. 2.

As shown in Figs. 4 and 5, four hundred transducer mounting receptacles 174 are provided in the transducer mounting plate 124 for mounting ultrasonic transducers for the desired array. With reference to Fig. 6, the profile of an individual transducer mounting receptacle 174 is shown. A mounting seat 176 accepts an ultrasonic transducer for mounting, with a mounted ultrasonic transducer being held in place via screw holes 178. Opposite the mounting receptacle 176 is a flared opening 180 through which an ultrasonic signal may be transmitted for the purpose of generating the aerosol 108, as previously described with reference to Fig. 2. A preferred transducer mounting configuration, however, is shown in Fig. 7 for another configuration for the transducer mounting plate 124. As seen in Fig. 7, an ultrasonic transducer disc 120 is mounted to the transducer mounting plate 124 by use of a compression screw 177 threaded into a threaded receptacle 179. The compression screw 177 bears against the ultrasonic transducer disc 120, causing an o-ring 181, situated in an o-ring seat 182 on the transducer mounting plate, to be compressed to form a seal between the transducer mounting plate 124 and the ultrasonic transducer disc 120. This type of transducer mounting is particularly preferred when the ultrasonic transducer disc 120 includes a protective surface coating, as discussed previously, because the seal of the o-ring to the ultrasonic transducer disc 120 will be inside of the outer edge of the protective seal, thereby preventing liquid from penetrating under the protective surface coating from the edges of the ultrasonic transducer disc 120. Referring now to Fig. 8, the bottom retaining plate 128 for a 400 transducer array is shown having a design for mating with the transducer mounting plate 124 (shown in. Figs. 4-5). The bottom retaining plate 128 has eighty openings 184, arranged in four subgroups 186 of twenty openings 184 each. Each of the openings 184 corresponds with five of the transducer mounting receptacles 174 (shown in Figs. 4 and 5) when the bottom retaining plate 128 is mated with the transducer mounting plate 124 to create a volume for a water bath between the transducer mounting plate 124 and the bottom retaining plate 128. The openings 184, therefore, provide a pathway for ultrasonic signals generated by ultrasonic transducers to be transmitted through the bottom retaining plate. Referring now to Figs. 9 and 10, a liquid feed box 190 for a 400 transducer array is shown having the top retaining plate 130 designed to fit over the bottom retaining plate 128 (shown in Fig. 8), with a separator 126 (not shown) being retained between the bottom retaining plate 128 and the top retaining plate 130 when the aerosol generator 106 is assembled. The liquid feed box 190 also includes vertically extending walls 192 for containing the liquid feed 102 when the aerosol generator is in operation. Also shown in Figs. 9 and 10 is the feed inlet 148 and the feed outlet 152. An adjustable weir 198 determines the level of liquid feed 102 in the liquid feed box 190 during operation of the aerosol generator 106.

The top retaining plate 130 of the liquid feed box 190 has eighty openings 194 therethrough, which are arranged in four subgroups 196 of twenty openings 194 each.

The openings 194 of the top retaining plate 130 correspond in size with the openings 184 of the bottom retaining plate 128 (shown in Fig. 8). When the aerosol generator 106 is assembled, the openings 194 through the top retaining plate 130 and the openings 184 through the bottom retaining plate 128 are aligned, with the separator 126 positioned therebetween, to permit transmission of ultrasonic signals when the aerosol generator 106 is in operation.

Referring now to Figs. 9-11 , a plurality of gas tube feed-through holes 202 extend through the vertically extending walls 192 to either side of the assembly including the feed inlet 148 and feed outlet 152 of the liquid feed box 190. The gas tube feed-through holes 202 are designed to permit insertion therethrough of gas tubes 208 of a design as shown in Fig. 11. When the aerosol generator 106 is assembled, a gas tube 208 is inserted through each of the gas tube feed-through holes 202 so that gas delivery ports 136 in the gas tube 208 will be properly positioned and aligned adjacent the openings 194. in the top retaining plate 130 for delivery of gas to atomization cones that develop in the liquid feed box 190 during operation of the aerosol generator 106. The gas delivery ports 136 are typically holes having a diameter of from about 1.5 millimeters to about 3.5 millimeters.

Referring now to Fig. 12, a partial view of the liquid feed box 190 is shown with gas tubes 208A, 208B and 208C positioned adjacent to the openings 194 through the top retaining plate 130. Also shown in Fig. 12 are the relative locations that ultrasonic transducer discs 120 would occupy when the aerosol generator 106 is assembled. As seen in Fig. 12, the gas tube 208 A, which is at the edge of the array, has five gas delivery ports 136. Each of the gas delivery ports 136 is positioned to divert carrier gas 104 to a different one of atomization cones that develop over the array of ultrasonic transducer discs 120 when the aerosol generator 106 is operating. The gas tube 208B, which is one row in from the edge of the array, is a shorter tube that has ten gas delivery ports 136, five each on opposing sides of the gas tube 208B. The gas tube 208B, therefore, has gas delivery ports 136 for delivering gas to atomization cones corresponding with each often ultrasonic transducer discs 120. The third gas tube, 208C, is a longer tube that also has ten gas delivery ports 136 for delivering gas to atomization cones corresponding with ten ultrasonic transducer discs 120. The design shown in Fig. 12, therefore, includes one gas delivery port per ultrasonic transducer disc 120. Although this is a lower density of gas delivery ports 136 than for the embodiment of the aerosol generator 106 shown in Fig. 2, which includes two gas delivery ports per ultrasonic transducer disc 120, the design shown in Fig. 12 is, nevertheless, capable of producing a dense, high-quality aerosol without unnecessary waste of gas.

Referring now to Fig. 13, the flow of carrier gas 104 relative to atomization cones 162 during operation of the aerosol generator 106 having a gas distribution configuration to deliver carrier gas 104 from gas delivery ports on both sides of the gas tubes 208, as was shown for the gas tubes 208A, 208B and 208C in the gas distribution configuration shown in Fig. 11. The carrier gas 104 sweeps both directions from each of the gas tubes

208. An alternative, and preferred, flow for carrier gas 104 is shown in Fig. 14. As shown in Fig. 14, carrier gas 104 is delivered from only one side of each of the gas tubes. 208. This results in a sweep of carrier gas from all of the gas tubes 208 toward a central area 212. This results in a more uniform flow pattern for aerosol generation that may significantly enhance the efficiency with which the carrier gas 104 is used to produce an aerosol. The aerosol that is generated, therefore, tends to be more heavily loaded with liquid droplets.

Another configuration for distributing carrier gas in the aerosol generator 106 is shown in Figs. 15 and 16. In this configuration, the gas tubes 208 are hung from a gas distribution plate 216 adjacent gas flow holes 218 through the gas distribution plate 216.

In the aerosol generator 106, the gas distribution plate 216 would be mounted above the liquid feed, with the gas flow holes positioned to each correspond with an underlying ultrasonic transducer. Referring specifically to Fig. 16, when the ultrasonic generator 106 is in operation, atomization cones 162 develop through the gas flow holes 218, and the gas tubes 208 are located such that carrier gas 104 exiting from ports in the gas tubes 208 impinge on the atomization cones and flow upward through the gas flow holes. The gas flow holes 218, therefore, act to assist in efficiently distributing the carrier gas 104 about the atomization cones 162 for aerosol formation. It should be appreciated that the gas distribution plates 218 can be made to accommodate any number of the gas tubes 208 and gas flow holes 218. For convenience of illustration, the embodiment shown in Figs. 15 and 16 shows a design having only two of the gas tubes 208 and only 16 of the gas flow holes 218. Also, it should be appreciated that the gas distribution plate 216 could be used alone, without the gas tubes 208. In that case, a slight positive pressure of carrier gas 104 would be maintained under the gas distribution plate 216 and the gas flow holes 218 would be sized to maintain the proper velocity of carrier gas 104 through the gas flow holes 218 for efficient aerosol generation. Because of the relative complexity of operating in that mode, however, it is not preferred.

Aerosol generation may also be enhanced through mounting of ultrasonic transducers at a slight angle and directing the carrier gas at resulting atomization cones such that the atomization cones are tilting in the same direction as the direction of flow of carrier gas. Referring to Fig. 17, an ultrasonic transducer disc 120 is shown. The ultrasonic transducer disc 120 is tilted at a tilt angle 114 (typically less than 10 degrees), so that the atomization cone 162 will also have a tilt. It is preferred that the direction of. flow of the carrier gas 104 directed at the atomization cone 162 is in the same direction as the tilt of the atomization cone 162. Referring now to Figs. 18 and 19, a gas manifold 220 is shown for distributing gas to the gas tubes 208 in a 400 transducer array design. The gas manifold 220 includes a gas distribution box 222 and piping stubs 224 for connection with gas tubes 208 (shown in Fig. 11). Inside the gas distribution box 222 are two gas distribution plates 226 that form a flow path to assist in distributing the gas equally throughout the gas distribution box 222, to promote substantially equal delivery of gas through the piping stubs 224. The gas manifold 220, as shown in Figs. 18 and 19, is designed to feed eleven gas tubes 208. For the 400 transducer design, a total of four gas manifolds 220 are required.

Referring now to Figs 20 and 21, the generator lid 140 is shown for a 400 transducer array design. The generator lid 140 mates with and covers the liquid feed box 190 (shown in Figs. 9 and 10). The generator lid 140, as shown in Figs. 20 and 21, has a hood design to permit easy collection of the aerosol 108 without subjecting droplets in the aerosol 108 to sharp edges on which droplets may coalesce and be lost, and possibly interfere with the proper operation of the aerosol generator 106. When the aerosol generator 106 is in operation, the aerosol 108 would be withdrawn via the aerosol exit opening 164 through the generator cover 140.

The design and apparatus of the aerosol generator 106 described with reference to Figures 2-21, as well as a facility including other process equipment described herein for carrying out the process of the present invention for making powders are within the scope of the present invention. Although the aerosol generator 106 produces a high quality aerosol 108 having a high droplet loading, it is often desirable to further concentrate the aerosol 108 prior to introduction into the furnace 110. Referring now to Fig. 22, a process flow diagram is shown for one embodiment of the present invention involving such concentration of the aerosol 108. As shown in Fig. 22, the aerosol 108 from the aerosol generator 106 is sent to an aerosol concentrator 236 where excess carrier gas 238 is withdrawn from the aerosol 108 to produce a concentrated aerosol 240, which is then fed to the furnace 110. The aerosol concentrator 236 typically includes one or more virtual impactors capable of concentrating droplets in the aerosol 108 by a factor of greater than about 2, . preferably by a factor of greater than about 5, and more preferably by a factor of greater than about 10, to produce the concentrated aerosol 240. According to the present invention, the concentrated aerosol 240 should typically contain greater than about 1 x

10⁷ droplets per cubic centimeter, and more preferably from about 5 x 10⁷ to about 5 x

10⁸ droplets per cubic centimeter. A concentration of about 1 x 10 ⁸ droplets per cubic centimeter of the concentrated aerosol is particularly preferred, because when the concentrated aerosol 240 is loaded more heavily than that, then the frequency of collisions between droplets becomes large enough to impair the properties of the concentrated aerosol 240, resulting in potential contamination of the particulate product 116 with an undesirably large quantity of over-sized particles. For example, if the aerosol 108 has a concentration of about 1 x 10⁷ droplets per cubic centimeter, and the aerosol concentrator 236 concentrates droplets by a factor of 10, then the concentrated aerosol 240 will have a concentration of about 1 x 10⁸ droplets per cubic centimeter. Stated another way, for example, when the aerosol generator generates the aerosol 108 with a droplet loading of about 0.167 milliliters liquid feed 102 per liter of carrier gas 104, the concentrated aerosol 240 would be loaded with about 1.67 milliliters of liquid feed 102 per liter of carrier gas 104, assuming the aerosol 108 is concentrated by a factor of 10. Having a high droplet loading in aerosol feed to the furnace provides the important advantage of reducing the heating demand on the furnace 110 and the size of flow conduits required through the furnace. Also, other advantages of having a dense aerosol include a reduction in the demands on cooling and particle collection components, permitting significant equipment and operational savings. Furthermore, as system components are reduced in size, powder holdup within the system is reduced, which is also desirable. Concentration of the aerosol stream prior to entry into the furnace 110, therefore, provides a substantial advantage relative to processes that utilize less concentrated aerosol streams.

The excess carrier gas 238 that is removed in the aerosol concentrator 236 typically includes extremely small droplets that are also removed from the aerosol 108.

Preferably, the droplets removed with the excess carrier gas 238 have a weight average size of smaller than about 1.5 microns, and more preferably smaller than about 1 micron and the droplets retained in the concentrated aerosol 240 have an average droplet size of. larger than about 2 microns. For example, a virtual impactor sized to treat an aerosol stream having a weight average droplet size of about three microns might be designed to remove with the excess carrier gas 238 most droplets smaller than about 1.5 microns in size. Other designs are also possible. When using the aerosol generator 106 with the present invention, however, the loss of these very small droplets in the aerosol concentrator 236 will typically constitute no more than about 10 percent by weight, and more preferably no more than about 5 percent by weight, of the droplets originally in the aerosol stream that is fed to the concentrator 236. Although the aerosol concentrator 236 is useful in some situations, it is normally not required with the process of the present invention, because the aerosol generator 106 is capable, in most circumstances, of generating an aerosol stream that is sufficiently dense. So long as the aerosol stream coming out of the aerosol generator 102 is sufficiently dense, it is preferred that the aerosol concentrator not be used. It is a significant advantage of the present invention that the aerosol generator 106 normally generates such a dense aerosol stream that the aerosol concentrator 236 is not needed. Therefore, the complexity of operation of the aerosol concentrator 236 and accompanying liquid losses may typically be avoided.

It is important that the aerosol stream (whether it has been concentrated or not) that is fed to the furnace 110 have a high droplet flow rate and high droplet loading as would be required for most industrial applications. With the present invention, the aerosol stream fed to the furnace preferably includes a droplet flow of greater than about 0.5 liters per hour, more preferably greater than about 2 liters per hour, still more preferably greater than about 5 liters per hour, even more preferably greater than about 10 liters per hour, particularly greater than about 50 liters per hour and most preferably greater than about 100 liters per hour; and with the droplet loading being typically greater than about 0.04 milliliters of droplets per liter of carrier gas, preferably greater than about 0.083 milliliters of droplets per liter of carrier gas 104, more preferably greater than about 0.167 milliliters of droplets per liter of carrier gas 104, still more preferably greater than about 0.25 milliliters of droplets per liter of carrier gas 104, particularly greater than about 0.33 milliliters of droplets per liter of carrier gas 104 and most preferably greater than about 0.83 milliliters of droplets per liter of carrier gas 104.

As discussed previously, the aerosol generator 106 of the present invention produces a concentrated, high quality aerosol of micro-sized droplets having a relatively narrow size distribution. It has been found, however, that for many applications the process of the present invention is significantly enhanced by further classifying by size the droplets in the aerosol 108 prior to introduction of the droplets into the furnace 110. In this manner, the size and size distribution of particles in the particulate product 116 are further controlled. Referring now to Fig. 23, a process flow diagram is shown for one embodiment of the process of the present invention including such droplet classification. As shown in Fig. 23, the aerosol 108 from the aerosol generator 106 goes to a droplet classifier 280 where oversized droplets are removed from the aerosol 108 to prepare a classified aerosol 282. Liquid 284 from the oversized droplets that are being removed is drained from the droplet classifier 280. This drained liquid 284 may advantageously be recycled for use in preparing additional liquid feed 102.

Any suitable droplet classifier may be used for removing droplets above a predetermined size. For example, a cyclone could be used to remove over-size droplets. A preferred droplet classifier for many applications, however, is an impactor. In a preferred embodiment of the present invention, the droplet classifier 280 is typically designed to remove droplets from the aerosol 108 that are larger than about 15 microns in size, more preferably to remove droplets larger than about 10 microns in size, even more preferably to remove droplets of a size larger than about 8 microns in size and most preferably to remove droplets larger than, about 5 microns in size. The droplet classification size in the droplet classifier is preferably smaller than about 15 microns, more preferably smaller than about 10 microns, even more preferably smaller than about 8 microns and most preferably smaller than about 5 microns. The classification size, also called the classification cut point, is that size at which half of the droplets of that size are removed and half of the droplets of that size are retained. Because the aerosol generator 106 of the present invention initially produces a high quality aerosol 108, having a relatively narrow size distribution of droplets, typically less than about 30 weight percent oi liquid teed 102 in the aerosol 108 is removed as the drain liquid 284 in the droplet classifier 288, with preferably less than about 25 weight percent being removed, even more preferably less than about 20 weight percent being removed and most preferably less than about 15 weight percent being removed. Minimizing the removal of liquid feed 102 from the aerosol 108 is particularly important for commercial applications to increase the yield of high quality particulate product 116. It should be noted, however, that because of the superior performance of the aerosol generator 106, it is frequently not required to use an impactor or other droplet classifier to obtain a desired absence of oversize droplets to the furnace. This is a major advantage, because the added complexity and liquid losses accompanying use of an impactor may often be avoided with the process of the present invention.

Sometimes it is desirable to use both the aerosol concentrator 236 and the droplet classifier 280 to produce an extremely high quality aerosol stream for introduction into the furnace for the production of particles of highly controlled size and size distribution. By using both a virtual impactor and an impactor, both undesirably large and undesirably small droplets are removed, thereby producing a classified aerosol with a very narrow droplet size distribution. Also, the order of the aerosol concentrator 236 and the aerosol classifier 280 could be with either device positioned first. Typically, however, the aerosol concentrator 236 will be positioned ahead of the droplet classifier 280. With some applications of the process of the present invention, it may be possible to collect the particles 112 directly from the output of the furnace 110. More often, however, it will be desirable to cool the particles 112 exiting the furnace 110 prior to collection of the particles 112 in the particle collector 114. Referring now to Fig. 24, one embodiment of the process of the present invention is shown in which the particles 112 exiting the furnace 110 are sent to a particle cooler 320 to produce a cooled particle stream 322, which is then feed to the particle collector 114. Although the particle cooler 320 may be any cooling apparatus capable of cooling the particles 112 to the desired temperature for introduction into the particle collector 114, traditional heat exchanger designs are not preferred. This is because a traditional heat exchanger design ordinarily directly subjects the aerosol stream, in which the hot particles 112 are suspended, to cool surfaces. In that situation, significant losses of the particles 112 occur due to thermophoretic deposition of the hot particles 112 on the cool surfaces of the heat exchanger. According to the present invention, a gas quench apparatus is provided for use as the particle cooler 320 that significantly reduces thermophoretic losses compared to a traditional heat exchanger. Referring now to Figs. 25-27, one embodiment of a gas quench cooler 330 is shown. The gas quench cooler includes a perforated conduit 332 housed inside of a cooler housing 334 with an annular space 336 located between the cooler housing 334 and the perforated conduit 332. In fluid communication with the annular space 336 is a quench gas inlet box 338, inside of which is disposed a portion of an aerosol outlet conduit 340. The perforated conduit 332 extends between the aerosol outlet conduit 340 and an aerosol inlet conduit 342. Attached to an opening into the quench gas inlet box 338 are two quench gas feed tubes 344. Referring specifically to Fig. 27, the perforated tube 332 is shown. The perforated tube 332 has a plurality of openings 345. The openings 345, when the perforated conduit 332 is assembled into the gas quench cooler 330, permit the flow of quench gas 346 from the annular space 336 into the interior space

348 of the perforated conduit 332. Although the openings 345 are shown as being round holes, any shape of opening could be used, such as slits. Also, the perforated conduit 332 could be a porous screen. Two heat radiation shields 347 prevent downstream radiant heating from the furnace. In most instances, however, it will not be necessary to include the heat radiation shields 347, because downstream radiant heating from the furnace is normally not a significant problem. Use of the heat radiation shields 347 is not preferred due to particulate losses that accompany their use.

With continued reference to Figs. 25-27, operation of the gas quench cooler 330 will now be described. During operation, the particles 112, carried by and dispersed in a gas stream, enter the gas quench cooler 330 through the aerosol inlet conduit 342 and flow into the interior space 348 of perforated conduit 332. Quench gas 346 is introduced through the quench gas feed tubes 344 into the quench gas inlet box 338. Quench gas 346 entering the quench gas inlet box 338 encounters the outer surface of the aerosol outlet conduit 340, forcing the quench gas 346 to flow, in a spiraling, swirling manner, into the annular space 336, where the quench gas 346 flows through the openings 345 through the walls of the perforated conduit 332. Preferably, the gas 346 retains some swirling motion even after passing into the interior space 348. In this way, the particles 112 are quickly cooled with low losses of particles to the walls of the gas quench cooler 330. In this manner, the quench gas 346 enters in a radial direction into the interior space 348 of the perforated conduit 332 around the entire periphery, or circumference, of the perforated conduit 332 and over the entire length of the perforated conduit 332. The cool quench gas 346 mixes with and cools the hot particles 112, which then exit through the aerosol outlet conduit 340 as the cooled particle stream 322. The cooled particle stream 322 can then be sent to the particle collector 114 for particle collection. The temperature of the cooled particle stream 322 is controlled by introducing more or less quench gas. Also, as shown in Fig. 25, the quench gas 346 is fed into the quench cooler 330 in counter flow to flow of the particles. Alternatively, the quench cooler could be designed so that the quench gas 346 is fed into the quench cooler in concurrent flow with the flow of the particles 112. The amount of quench gas 346 fed to the gas quench cooler 330 will depend upon the specific material being made and the specific operating conditions. The quantity of quench gas 346 used, however, must be sufficient to reduce the temperature of the aerosol steam including the particles 112 to the desired temperature. Typically, the particles 112 are cooled to a temperature at least below about 200 °C, and often lower. The only limitation on how much the particles 112 are cooled is that the cooled particle stream 322 must be at a temperature that is above the condensation temperature for water as another condensible vapor in the stream. The temperature of the cooled particle stream

322 is often at a temperature of from about 50 °C to about 120°C.

Because of the entry of quench gas 346 into the interior space 348 of the perforated conduit 322 in a radial direction about the entire circumference and length of the perforated conduit 322, a buffer of the cool quench gas 346 is formed about the inner wall of the perforated conduit 332, thereby significantly inhibiting the loss of hot particles

112 due to thermophoretic deposition on the cool wall of the perforated conduit 332. In operation, the quench gas 346 exiting the openings 345 and entering into the interior space 348 should have a radial velocity (velocity inward toward the center of the circular cross-section of the perforated conduit 332) of larger than the thermophoretic velocity of the particles 112 inside the perforated conduit 332 in a direction radially outward toward the perforated wall of the perforated conduit 332. As seen in Figs. 25-27, the gas quench cooler 330 includes a flow path for the particles 112 through the gas quench cooler of a substantially constant cross-sectional shape and area. Preferably, the flow path through the gas quench cooler 330 will have the same cross-sectional shape and area as the flow path through the furnace 110 and through the conduit delivering the aerosol 108 from the aerosol generator 106 to the furnace 110.

Also, particle cooling in the quench cooler is accomplished very quickly, reducing the potential for thermophoretic losses during cooling. The total residence time for the aerosol flowing through both the heated zone of the furnace 110 and through the quench cooler is typically shorter than about 5 seconds, more preferably shorter than about 2 seconds, and most preferably shorter than about 1 second.

In an additional embodiment, the process of the present invention can also incorporate compositional modification of the particles 112 exiting the furnace. Most commonly, the compositional modification will involve forming on the particles 112 a material phase that is different than that of the particles 112, such as by coating the particles 112 with a coating material. One embodiment of the process of the present invention incorporating particle coating is shown in Fig. 28. As shown in Fig. 28, the particles 112 exiting from the furnace 110 go to a particle coater 350 where a coating is placed over the outer surface of the particles 112 to form coated particles 352, which are then sent to the particle collector 114 for preparation of the particulate product 116.

In the particle coater 350, the particles 112 are coated using any suitable particle coating technology, such as by gas-to-particle conversion. Preferably, however, the coating is accomplished by chemical vapor deposition (CVD) and/or physical vapor deposition (PVD). In CVD coating, one or more vapor phase coating precursors are reacted to form a surface coating on the particles 112. Preferred coatings deposited by

CVD include oxides, such as silica, alumina, titania and zirconia, and elemental metals. For example, silica may be deposited using a silane precursor, such as tetrachlorosilane. In PVD coating, coating material physically deposits on the surface of the particles 112. Preferred coatings deposited by PVD include organic materials and elemental metals, such as elemental silver, copper and gold. Another possible surface coating method is surface conversion of the surface portion of the particles 112 by reaction with a vapor phase reactant to convert a surface portion of the particles to a different material than that originally contained in the particles 112. Although any suitable apparatus may be used for the particle coater 350, when a gaseous coating feed involving coating precursors is used, such as for CVD and PVD, feed of the gaseous coating feed is introduced through a circumferentially perforated conduit, such as was described for the quench cooler 330 with reference to Figs. 25-27. In some instances, the quench cooler 330 may also act as the particle coater 350, when coating material precursors are included in the quench gas 346.

With continued reference primarily to Fig. 28, in a preferred embodiment, when the particles 112 are coated according to the process of the present invention, the particles

112 are also manufactured via the aerosol process of the present invention, as previously described. The process of the present invention can, however, be used to coat particles that have been premanufactured by a different process, such as by a liquid precipitation route. When coating particles that have been premanufactured by a different route, such as by liquid precipitation, it is preferred that the particles remain in a dispersed state from the time of manufacture to the time that the particles are introduced in slurry form into the aerosol generator 106 for preparation of the aerosol 108 to form the dry particles 112 in the furnace 110, which particles 112 can then be coated in the particle coater 350. Maintaining particles in a dispersed state from manufacture through coating avoids problems associated with agglomeration and redispersion of particles if particles must be redispersed in the liquid feed 102 for feed to the aerosol generator 106. For example, for particles originally precipitated from a liquid medium, the liquid medium containing the suspended precipitated particles could be used to form the liquid feed 102 to the aerosol generator 106. It should be noted that the particle coater 350 could be an integral extension of the furnace 110 or could be a separate piece of equipment.

In a further embodiment of the present invention, following preparation of the particles 112 in the furnace 110, the particles 112 may then be structurally modified to impart desired physical properties prior to particle collection. Referring now to Fig. 29, one embodiment of the process of the present invention is shown including such structural particle modification. The particles 112 exiting the furnace 110 go to a particle modifier 360 where the particles are structurally modified to form modified particles 362, which are then sent to the particle collector 114 for preparation of the particulate product 116. The particle modifier 360 is typically a furnace, such as an annealing furnace, which may be integral with the furnace 110 or may be a separate heating device. Regardless, it is important that the particle modifier 360 have temperature control that is independent of the furnace 110, so that the proper conditions for particle modification may be provided separate from conditions required of the furnace 110 to prepare the particles 112. The particle modifier 360, therefore, typically provides a temperature controlled environment and necessary residence time to effect the desired structural modification of the particles 112. The structural modification that occurs in the particle modifier 360 may be any modification to the crystalline structure or morphology of the particles 112. For example, the particles 112 may be annealed in the particle modifier 360 to density the particles 112 or to recrystallize the particles 112 into a poly crystalline or single crystalline form. Also, especially in the case of composite particles 112, the particles may be annealed for a sufficient time to permit redistribution within the particles 112 of different material phases.

The initial morphology of composite particles made in the furnace 110, according to the present invention, could take a variety of forms, depending upon the specified materials involved and the specific processing conditions. Examples of some possible composite particle morphologies, manufacturable according to the present invention are shown in Fig. 30. These morphologies could be of the particles as initially produced in the furnace 110 or that result from structural modification in the particle modifier 360. Furthermore, the composite particles could include a mixture of the morphological attributes shown in Fig. 30. When making multi-phase particles, a preferred multi-phase particle includes a metallic phase, such as with at least one of palladium, silver, nickel and copper, and a nonmetallic phase. Preferred for the nonmetallic phase is at least one of silica, alumina, titania and zirconia. Another preferred nonmetallic phase includes a titanate, and preferably a titanate of at least one of barium, strontium, neodymium, calcium, magnesium and lead. Aerosol generation with the process of the present invention has thus far been described with respect to the ultrasonic aerosol generator. Use of the ultrasonic generatox is preferred for the process of the present invention because of the extremely high quality and dense aerosol generated. In some instances, however, the aerosol generator for the process of the present invention may have a different design depending upon the specific application. For example, when larger particles are desired, such as those having a weight average size of larger than about 3 microns, a spray nozzle atomizer may be preferred. For smaller-particle applications, however, and particularly for those applications to produce particles smaller than about 3 microns, and preferably smaller than about 2 microns in size, as is generally desired with the particles of the present invention, the ultrasonic generator, as described herein, is particularly preferred. In that regard, the ultrasonic generator of the present invention is particularly preferred for when making particles with a weight average size of from about 0.2 micron to about 3 microns.

Although ultrasonic aerosol generators have been used for medical applications and home humidifiers, use of ultrasonic generators for spray pyrolysis particle manufacture has largely been confined to small-scale, experimental situations. The ultrasonic aerosol generator of the present invention described with reference to Figures 2-21, however, is well suited for commercial production of high quality powders with a small average size and a narrow size distribution. In that regard, the aerosol generator produces a high quality aerosol, with heavy droplet loading and at a high rate of production. Such a combination of small droplet size, narrow size distribution, heavy droplet loading, and high production rate provide significant advantages over existing aerosol generators that usually suffer from at least one of inadequately narrow size distribution, undesirably low droplet loading, or unacceptably low production rate. Through the careful and controlled design of the ultrasonic generator of the present invention, an aerosol may be produced typically having greater than about 70 weight percent (and preferably greater than about 80 weight percent) of droplets in the size range of from about 1 micron to about 10 microns, preferably in a size range of from about 1 micron to about 5 microns and more preferably from about 2 microns to about 4 microns. Also, the ultrasonic generator of the present invention is capable of delivering high output rates of liquid feed in the aerosol. The rate of liquid feed, at the high liquid loadings previously described, is preferably greater than about 25 milliliters per hour per transducer, more preferably greater than about 37.5 milliliters per hour per transducer, even more preferably greater than about 50 milliliters per hour per transducer and most preferably greater than about 100 millimeters per hour per transducer. This high level of performance is desirable for commercial operations and is accomplished with the present invention with a relatively simple design including a single precursor bath over an array of ultrasonic transducers. The ultrasonic generator is made for high aerosol production rates at a high droplet loading, and with a narrow size distribution of droplets. The generator preferably produces an aerosol at a rate of greater than about 0.5 liter per hour of droplets, more preferably greater than about 2 liters per hour of droplets, still more preferably greater than about 5 liters per hour of droplets, even more preferably greater than about 10 liters per hour of droplets and most preferably greater than about 40 liters per hour of droplets. For example, when the aerosol generator has a 400 transducer design, as described with reference to Figures 3-21, the aerosol generator is capable of producing a high quality aerosol having high droplet loading as previously described, at a total production rate of preferably greater than about 10 liters per hour of liquid feed, more preferably greater than about 15 liters per hour of liquid feed, even more preferably greater than about 20 liters per hour of liquid feed and most preferably greater than about 40 liters per hour of liquid feed. Under most operating conditions, when using such an aerosol generator, total particulate product produced is preferably greater than about 0.5 gram per hour per transducer, more preferably greater than about 0.75 gram per hour per transducer, even more preferably greater than about 1.0 gram per hour per transducer and most preferably greater than about 2.0 grams per hour per transducer. The concentrations of soluble precursors in the liquid feed 102 will vary depending upon the particular materials involved and the particular particle composition and particle morphology desired. For most applications, when soluble precursor(s) are used, the soluble precursor(s) are present at a concentration of from about 1-50 weight percent of the liquid feed. 102. In any event, however, when soluble precursors are used, the precursors should be at a low enough concentration to permit the liquid feed to be ultrasonically atomized and to prevent premature precipitation of materials from the liquid feed 102. The concentration of suspended particulate precursors will also vary depending upon the particular materials involved in the particular application.

Powders of a variety of materials may be made according to the present invention, with the powders so produced being an important aspect of the invention. With the present invention, these various powders may be made with very desirable attributes for a variety of applications. In that regard, the powders are typically made with a small weight average particle size, narrow particle size distribution, spheroidal particle shape, and high density relative to a theoretical density for the material of the particles. Also, the particles of the powder typically are either substantially single crystalline or are polycrystalline and with a large mean crystallite size.

With respect to particle size, the powders are characterized generally as having a weight average particle size that typically is in the range of from about 0.05 micron to about 4 microns, with most powders having a weight average size of from about 0.1 micron to about 3 microns. With the process of the present invention, however, particle size may generally be controlled to provide particles with a desired size. Particle size is varied primarily by altering the frequency of ultrasonic transducers in the aerosol generator and by altering the concentration of precursors in the liquid feed. Lower ultrasonic frequencies tend to produce larger particles, while higher frequencies tend to produce smaller particles. Also, higher precursor concentrations in the liquid feed tend to produce larger particles and lower precursor concentrations in the liquid feed tend to produce smaller particles.

The particles are typically characterized as having a weight average particle size in a range having a lower limit, depending upon the application, of from about 0.1 micron, or about 0.2 micron, or about 0.3 micron, or about 0.5 micron, or about 0.8 micron, or about 1 micron; and having an upper limit, depending upon the application, of about 4 microns, or about 3 microns, or about 2.5 microns, or about 2 microns, or about 1 micron, or about 0.8 micron, or about 0.6 micron. Powders having a weight average size range defined by any combination of one of the specified upper limits and one of the specified lower limits are within the scope of the present invention, so long as the upper limit is larger than the lower limit. Some particularly preferred ranges for weight average particle size are provided below in discussions specific to certain material.

The powders are also characterized as having a narrow particle size distribution, typically with greater than about 75 weight percent, preferably greater than about 90 weight percent, and more preferably greater than about 95 weight percent of the particles in the powder having a size of smaller than two times the weight average particle size, and even more particularly smaller than about 1.5 times the weight average particle size. The powders are also typically characterized as being comprised of spheroidal particles. In that regard, the particles are substantially spherical, in that the particles are not jagged or irregular in shape, although the particles may become faceted as the crystallite size in the particles increases. Spheroidal particles are advantageous because they typically have increased dispersibility and flowability in paste formulations relative to jagged or irregular particles.

Although in some instances the powders may be made as very porous or hollow particles, the powders are usually characterized as being very dense, with the particles typically having a density of at least about 80%, preferably at least about 90% and more preferably at least about 95%, of a theoretical density. The theoretical density is that density that particles would have assuming that the particles included zero porosity. As used herein, the density of a particle is as measured by helium pycnometry. High particle density is particularly advantageous for thick film applications involving a fired film, because higher density particles tend to exhibit reduced shrinkage during sintering than highly porous particles.

The powders are further characterized as typically having a high degree of purity, with generally no more than about 0.1 atomic percent impurities and preferably no more than about 0.01 atomic percent impurities. One significant characteristic of the powders of the present invention is that they may be made to be substantially free of organic materials, if desired, and particularly to be substantially free of surfactants. This is a significant advantage over particles made by a liquid route, which typically include residual surfactants. These residual surfactants can significantly impair the utility of the particles, especially in making thick film pastes. EXAMPLES The following examples are provided to aid in understanding of the present, invention, and are not intended to in any way limit the scope of the present invention. Example 1 This example demonstrates preparation of multi-phase particles of either neodymium titanate or barium titanate with various metals.

A titanate precursor solution is prepared for each of barium titanate and neodymium titanate. The barium titanate precursor solution is prepared by dissolving barium nitrate in water and then, with rapid stirring, adding titanium tetraisopropoxide. A fine precipitate is formed. Sufficient nitric acid is added to completely dissolve the precipitate. Precursor solutions of various metals are prepared by dissolving the metal salt in water. The neodymium titanate precursor solution is prepared in the same way except using neodymium nitrate.

The titanate precursor solution and the metal precursor solution are mixed in various relative quantities to obtain the desired relative quantities of titanate and metal components in the final particles. The mixed solutions are aerosolized in an ultrasonic aerosol generator with transducers operated at 1.6 MHz and the aerosol is sent to a furnace where droplets in the aerosol are pyrolized to form the desired multi-phase particles. Air or nitrogen is used as a carrier gas, with tests involving copper and nickel also including hydrogen in an amount of 2.8 volume percent of the carrier gas.

Results are summarized in Table 2. Example 2

A variety of materials are made according to the process of the present invention, with some materials being made with and some being made without droplet classification prior to the furnace. Various single phase and multi -phase (or composite) particles are made as well as several coated particles. Tables 3 through 8 tabulate various of these materials and conditions of manufacture.

⁽¹⁾ 70:30 Ag:Pd alloy, BaTi03 varied from 5 to 90 weight percent of the composite.

⁽²¹ 30:70 Ag:Pd alloy.

( 1 ) In aqueous solution

(2) Urea addition improves densification of particles

(3) Metal organic sold by DuPont

(4) Some Zn reduced to Zn during manufacture, the amount of reduction being controllable.

( 1 ) Morphology of particles changes from intimately mixed Pd/Si0₂ to Si0₂ coating over Pd as reactor temperature is increased.

(2) Coating of Pd on Si0₂ particles. (3) Titanium tetraisopropoxide.

(4) Metal dispersed on high surface area Ti0₂ support.

(5) Al[OCH (CH₃)C₂H₅]₃.

(6) Metal dispersed on high surface area A1₂0₃ support.

(7) Pd coating on Ti0₂ particles. (8) Ag coating on Ti0₂ particles.

(9) Pt coating on Ti0₂ particles.

(10) Ti0₂ coating on Ag particles.

(11) Ti0₂ coating on Au particles . While various specific embodiments of the process of the present invention and the apparatus of the present invention for preparing powders are described in detail, it should be recognized that the features described with respect to each embodiment may be combined, in any combination, with features described in any other embodiment, to the extent that the features are compatible. For example, any or all of the aerosol concentrator, aerosol classifier, particle cooler, particle coater, particle modifier and other described process/apparatus components may be incorporated into the apparatus and/or process of the present invention. Also, additional apparatus and/or process steps may be incorporated to the extent they do not substantially interfere with operation of the process of the present invention or the apparatus useful therefor.

Also, while various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations to those embodiments will occur to those skilled in the art. It is to be expressly understood, however, that such modifications and adaptations are within the scope of the present invention, as set forth in the claims below. Further, it should be recognized that any feature of any embodiment disclosed herein can be combined with any other feature of any other embodiment in any combination.

Claims

What is Claimed is:

1. An aerosol method for making a particulate product, the method comprising the steps of: generating an aerosol stream including droplets suspended in a carrier gas, the generating comprising sweeping away with said carrier gas said droplets as said droplets are released from a reservoir of an ultrasonically energized flowable medium, said flowable medium comprising a liquid and at least one precursor for a material to be included in the particulate product. removing at least a portion of said liquid from at least a portion of said droplets of said aerosol stream and forming particles including said material; wherein, said flowable medium, during said step of generating said aerosol stream, being ultrasonically energized by a plurality of ultrasonic transducers underlying said reservoir.

2. The method of Claim 1, wherein said plurality of ultrasonic transducers includes at least about 9 ultrasonic transducers.

3. The method of Claim 1, wherein said plurality of ultrasonic transducers includes at least about 25 ultrasonic transducers.

4. The method of Claim 1, wherein said plurality of ultrasonic transducers includes at least about 40 ultrasonic transducers.

5. The method of Claim 1, wherein said plurality of ultrasonic transducers includes at least about 100 ultrasonic transducers.

6. The method of Claim 1 , wherein said plurality of transducers are mounted on a single mounting plate underlying said reservoir.

7. The method of Claim 1, wherein there is a protective coating on said ultrasonic transducers to prevent direct contact between said flowable medium and said ultrasonic transducers.

8. The method of Claim 1, wherein said ultrasonic transducers are ultrasonically coupled with said flowable medium via an ultrasonically transmissive separator, disposed between said ultrasonic transducers and said flowable medium, to prevent said flowable medium from contacting said ultrasonic transducers.

9. The method of Claim 8, wherein said ultrasonic transducers are ultrasonically coupled with said separator via an ultrasonically transmissive fluid, other than said flowable medium, located between said ultrasonic transducers and said ultrasonically transmissive separator.

10. The method of Claim 9, wherein said ultrasonically transmissive fluid flows between said ultrasonic transducers and said separator to cool said ultrasonic transducers during operation.

11. The method of Claim 1 , wherein during said generating step greater than about 25 milliliters per hour of said droplets are swept away in aerosol form per each of said ultrasonic transducers.

12. The method of Claim 1, wherein during said generating step greater than about 50 milliliters per hour of said droplets as swept away in aerosol form per each of said ultrasonic transducers.

13. The method of Claim 1 , wherein during said generating step greater than about 100 milliliters per hour of said droplets are swept away in aerosol form per each of said ultrasonic transducers.

14. An aerosol method for making a particulate product, the method comprising the steps of: generating an aerosol stream including droplets suspended in a carrier gas, the generating comprising sweeping away with said carrier gas said droplets as said droplets are released from a reservoir of an ultrasonically energized flowable medium, said flowable medium comprising a liquid and at least one precursor for a material to be included in the particulate product. removing at least a portion of said liquid from at least a portion of said droplets of said aerosol stream and forming particles including said material; wherein, during said step of generating said aerosol stream, said carrier gas being delivered from a plurality of gas delivery outlets of a gas delivery system, with each of said plurality of gas delivery outlets being positioned to direct a different portion of said canier gas to sweep away a different portion of said droplets released from a different portion of said flowable medium.

15. The method of Claim 14, wherein a plurality of ultrasonic transducers underlie and are ultrasonically coupled with said flowable medium such that each of said ultrasonic transducers ultrasonically energizes a different portion of said flowable medium in said reservoir.

16. The method of Claim 15, wherein there being at least one of said gas outlets per each one of said transducers.

17. The method of Claim 15, wherein each of said gas delivery outlets is located above a different portion of said flowable medium.

18. The method of Claim 17, wherein said flowable medium includes atomization cones, each overlying one of said ultrasonic transducers, with troughs located between said atomization cones; said gas delivery outlets being located above said troughs and adjacent to said atomization cones, such that carrier gas exiting said gas outlets is directed toward said atomization cones to sweep away said droplets as said droplets are released from said atomization cones.

19. The method of Claim 18, wherein at least a portion of at least one of said gas outlets is vertically lower than the top of the atomization cone at which gas exiting said at least one of said gas outlets is directed, whereby at least a portion of said carrier gas exits from said at least one of said gas outlets vertically lower than said top of the atomization cone.

20. The method of Claim 18, wherein exiting from each of said gas delivery outlets is a jet of said carrier gas, said jet being directed at one of said atomization cones of said flowable medium.

21. The method of Claim 20, wherein said jet of said carrier gas is substantially horizontally directed.

22. An aerosol method for making a particulate product, the method comprising the steps of: providing an aerosol stream including droplets suspended in a carrier gas, said droplets including at least one precursor for a material to be included in the particulate product; removing liquid from at least a portion of said droplets and forming particles including said component from said precursor; said aerosol stream characterized as including greater than about 0.5 liter per hour of said droplets at a droplet loading of greater than about 0.083 milliliters of said droplets per liter of said carrier gas, said droplets having an average size and a size distribution such that said particles have a weight average size of from about 0.1 micron to about 4 microns with at least about 75 weight percent of said particles being smaller than about twice said average size

23. The method of any one of Claims 1 through 22, wherein said aerosol stream includes greater than about 2 liters per hour of said droplets.

24. The method of any one of Claims 1 through 22, wherein said aerosol stream includes greater than about 5 liters per hour of said droplets.

25. The method of any one of Claims 1 through 22, wherein said aerosol stream includes greater than about 10 liters per hour of said droplets.

26. The method of any one of Claims 1 through 22, wherein said aerosol stream includes greater than about 40 liters per hour of said droplets.

27. The method of any one of Claims 1 through 22, wherein said aerosol stream has a droplet loading of greater than about 0.083 milliliters of said droplets per liter of carrier gas.

28. The method of any one of Claims 1 through 22, wherein said aerosol stream has a droplet loading of greater than about 0.167 milliliters of said droplets per liter of carrier gas.

29. The method of any one of Claims 1 through 22, wherein said aerosol stream has a droplet loading of greater than about 0.333 milliliters of said droplets per liter of carrier gas.

30. The method of any one of Claims 1 through 22, wherein said aerosol stream has a droplet loading of greater than about 0.833 milliliters of said droplets per liter of canier gas.

31. The method of any one of Claims 1 through 22, wherein said aerosol stream includes said droplets at a density of larger than about 5x10⁶ of said droplets per cubic centimeter of said aerosol stream.

32. The method of any one of Claims 1 through 22, wherein said aerosol stream includes said droplets at a density of larger than about lxl0⁷of said droplets per cubic centimeter of said aerosol stream.

33. The method of any one of Claims 1 through 22, wherein said particles have a weight average size of smaller than about 3 microns.

34. The method of any one of Claims 1 through 22, wherein said particles have a weight average size of smaller than about 2 microns.

35. The method of any one of Claims 1 through 22, wherein said particles have a weight average size of from about 0.5 micron to about 2 microns.

36. The method of any one of Claims 1 through 22, wherein said particles have a weight average size of from about 0.2 micron to about 0.8 micron.

37. The method of any one of Claims 1 through 22, wherein said particles have a size distribution such that at least about 90 weight percent of said particles are smaller than about twice said average size.

38. The method of any one of Claims 1 through 22, wherein said particles have a density, as measured by helium pycnometry, of greater than about 90% of theoretical density.

39. The method of any one of Claims 1 through 22, wherein said particles are substantially spheroidal.

40. The method of Claim 22, wherein said droplets have a weight average size of from about 1 micron to about 5 microns.

41. The method of Claim 40, wherein said droplets have a size distribution such that no greater than about 25 weight percent of said first droplets are larger than about twice the weight average size of said first droplets.

42. The method of Claim 22, wherein the method further comprises, prior to removing liquid from said droplets, concentrating said aerosol stream, including removing carrier gas from said aerosol such that, following the step of concentrating, said aerosol stream has a higher loading of droplets per unit volume of canier gas than prior to the step of concentrating.

43. The method of Claim 42 wherein during said step of concentrating, droplet loading in said aerosol stream per unit volume of said canier gas is increased by a factor of greater than about 2.

44. The method of Claim 42 wherein during said step of concentrating, droplet loading in said aerosol stream per unit volume of said carrier gas is increased by a factor of greater than about 5.

45. The method of Claim 22, wherein the method further comprises, prior to said step of removing liquid from said droplets, subjecting said aerosol stream to classifying droplets by size, including removing from said aerosol stream a first portion of droplets, including larger-size droplets, and retaining in said aerosol stream a second portion of droplets, including smaller-size droplets.

46. The method of Claim 45, wherein the cut point of classification between said first portion of droplets and said second portion of droplets is smaller than about 10 microns.

47. The method of Claim 45, wherein the cut point of classification between said first portion of droplets and said second portion of droplets is smaller than about 5 microns.

48. The method of Claim 45, wherein said first portion of droplets comprises no greater than about 20 weight percent of the total of said first portion of droplets and said second portion of droplets.

49. The method of any one of Claims 1 through 22, wherein said step of removing liquid from said droplets includes vaporization of said liquid.

50. The method of Claim 22, wherein said particles including said material are formed in a furnace at elevated temperature.

51. The method of Claim 50 wherein residence time of said aerosol stream in a heated zone of said furnace is shorter than about 4 seconds.

52. The method of Claim 50, wherein said residence time is shorter than about 2 seconds.

53. The method of Claim 50, wherein said residence time is shorter than about 1 second.

54. The method of Claim 50, wherein said aerosol stream, in said furnace, attains a maximum average stream temperature of from about 500┬░C to about 1500┬░C

55. The method of Claim 22, wherein the method further comprises, after said step of forming said particles, cooling said particles while said particles remain suspended in an aerosol, said cooling including flowing said aerosol having said particles through a cooling conduit while introducing into said cooling conduit a cooling gas that mixes with and cools said particles.

56. The method of Claim 55, wherein said cooling gas is introduced into said cooling conduit in a manner such that a buffer of said cooling gas forms adjacent a wall of said cooling conduit between said wall and said particles, thereby separating said particles from said wall to reduce thermophoretic losses of said particles to said wall.

57. The method of Claim 55, wherein said cooling conduit includes a perforated wall perforated with openings through which said cooling gas enters said cooling conduit.

58. The method of Claim 57, wherein said perforated wall of said conduit extends substantially about the entire circumference of said cooling conduit such that said cooling gas is introduced radially into said conduit through holes of said perforated wall around substantially the entire circumference of said cooling conduit.

59. The method of Claim 57, wherein said cooling gas has a radial velocity entering into said conduit that is larger than a thermophoretic velocity of said particles toward said perforated wall.

60. The method of Claim 22, wherein said particles comprise substantially only a single phase including said material.

61. The method of Claim 22, wherein said particles include said material in a first material phase; and the method further comprises, after said step of forming said particles, forming a second material phase on said particles, said second material phase being compositionally different from said first material phase, whereby multi-phase particles are prepared including both of said first material phase and said second material phase.

62. The method of Claim 61, wherein said second material phase comprises a coating substantially covering said particles.

63. The method of Claim 62, wherein said coating has an average thickness of thinner than about 100 nanometers.

64. The method of Claim 62, wherein said coating has an average thickness that is thinner than about 25 nanometers.

65. The method of Claim 61, wherein the step of forming said second material phase includes physical vapor deposition of said second material phase on said particles.

66. The method of Claim 65, wherein said physical vapor deposition includes deposition from a vapor phase comprising at least one of copper, gold and silver.

67. The method of Claim 61 , wherein the step of forming said second material phase includes chemical vapor deposition of said second material phase on said particles.

68. The method of Claim 61 , wherein said second material phase comprises at least one of silica, alumina, titania and zirconia.

69. The method of Claim 61, wherein said second material comprises a titanate.

70. The method of Claim 61 , wherein said second material phase comprises a titanate of at least one of barium, strontium, neodymium, calcium, magnesium and lead.

71. The method of Claim 22, wherein said precursor is a first precursor for a first material phase including said material and said aerosol stream also includes at least a second precursor for a second material phase; and said particles are multi-phase particles comprising both of said first material phase and said second material phase.

72. The method of Claim 71 , wherein said first material phase is a metallic phase including a metal and said second material phase is a nonmetallic phase substantially free of said metal.

73. The method of Claim 72, wherein said second material phase comprises at least one of silica, alumina, titania and zirconia.

74. The method of Claim 72, wherein said second material phase includes a titanate.

75. The method of Claim 72, wherein said second material phase comprises a titanate of at least one of barium, strontium, neodymium, calcium, magnesium and lead.

76. The method of Claim 71, wherein said first precursor is in solution in liquid of said droplets and said second precursor comprises precursor particles held within said droplets.

77. The method of Claim 76, wherein said precursor particles are smaller than about one micron.

78. The method of Claim 76, wherein said precursor particles are smaller than about 0.5 micron.

79. The method of Claim 76, wherein said precursor particles are smaller than about 0.3 micron.

80. The method of Claim 76, wherein said precursor particles are smaller than about 0.1 micron.

81. The method of Claim 76, wherein said precursor particles are of colloidal size and are present in said first droplets in a colloidal suspension.

82. The method of Claim 71 , wherein both said first precursor and said second precursor are in solution in liquid of said droplets.

83. The method of Claim 71, wherein said particles comprise said second material phase in an interior portion of said particles and said first material phase about the outer surface of said particles.

84. The method of Claim 71, wherein said particles comprise said second material phase as a coating about the surface of a core comprising said first material phase.

85. The method of Claim 22, wherein said precursor is a first precursor including a first metal and said aerosol also includes a second precursor including a second metal that is different than said first metal; and said material in said particles has an alloy including said first metal and said second metal.

86. An aerosol method for making a particulate product, the method comprising the steps of: providing an aerosol stream including droplets suspended in a canier gas, said droplets including at least one precursor for a material to be included in the particulate product; removing at least a portion of said liquid from at least a portion of said droplets of said aerosol stream and forming, in a thermal reactor at an elevated temperature, particles including said material, said particles exiting said reactor in an aerosol flow and at an elevated temperature; cooling said particles while said particles remained suspended in a flowing aerosol, said cooling including flowing said aerosol flow through a cooling conduit while introducing into said cooling conduit a cooling gas that mixes with and cools said particles.

87. The method of Claim 86, wherein said cooling conduit includes a perforated wall perforated with openings through which said cooling gas enters said cooling conduit.

88. The method of Claim 87, wherein said perforated wall of said conduit extends substantially about the entire circumference of said cooling conduit such that said cooling gas is introduced radially into said conduit through holes of said perforated wall around substantially the entire circumference of said cooling conduit.

89. The method of Claim 87, wherein said cooling gas has a velocity in a radially inward direction into said cooling conduit that is larger than a thermophoretic velocity of said particles radially outward toward said perforated wall.

90. The method of Claim 86, wherein said thermal reactor comprises a furnace with a heating zone, the total residence time for aerosol to pass through said heating zone and said cooling conduit is shorter than about 5 seconds.

91. The method of Claim 86, wherein said thermal reactor comprises a furnace with a heating zone, the total residence time for aerosol to pass through said heating zone and said cooling conduit is shorter than about 2 seconds.