Patent Grant
7214255
The present invention involves an aerosol manufacture facility, operation of that aerosol manufacture facility, and automated control of various operations involving the aerosol manufacture facility for producing a particulate product.
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 electrically 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. Moreover, spray pyrolysis systems are frequently operated in batch mode, and there is a significant potential for inefficiency during transient periods during the early and late stages of particle production. During these periods, variations in particle properties may degrade the quality of the entire batch.
There is a significant need for improved manufacture techniques for making powders of small particles for use in thick film and other applications.
It is an object of the present invention to provide an aerosol method of manufacture suitable for commercial production of particles. Is also an object to provide an aerosol method to produce a particulate product at a high yield and of a high quality. It is also an object to provide an aerosol method involving significant process control for efficient operation, especially for batch processing. It is another object of the present invention to provide an aerosol method that is at least partially automated, to improve efficiency and yield. It is a further object to provide an aerosol manufacture facility in which the aerosol method may be conducted. These and other objects of the present invention are addressed by the present invention as described herein.
In one aspect, the present invention provides an automated aerosol method for processing batches of precursor liquid to manufacture batches of particles of a selected composition. The method involves automation of at least a portion of the process, with automated features being controlled at the direction of an electronic processor that processes instructions for manufacture of the particles of the selected composition. The method is often operated in batch mode. In that case, the batch processing begins with batch initiation operations, during which aerosol generation is commenced, proceeds through intermediate operations, during which the bulk of particle production occurs, and ends with batch termination operations, during which aerosol generation is terminated. As used herein, batch mode refers to processing of a discrete quantity, or batch, of a precursor liquid prepared in a single preparation. The batch mode processing of the present invention includes processing which might be considered as semi-batch or semi-continuous operation because of the length of the batch run involved and/or the manner of product removal. Batch mode processing includes, during the intermediate operations, generating, in an aerosol generator, an aerosol stream from carrier gas supplied and precursor liquid supplied to the aerosol generator and processing the aerosol stream through an aerosol heater to form particles of the selected composition. In a preferred embodiment, droplets of the aerosol are produced, in the aerosol generator, from a reservoir of circulating precursor liquid that overlies a plurality of ultrasonic transducers, which energize precursor liquid in the reservoir to produce the droplets.
In one embodiment of the automated aerosol method of the present invention, an operator instructs the electronic processor to direct processing of a precursor liquid batch to prepare particles of a selected composition. The electronic processor then processes instructions concerning manufacture of particles of the selected composition and, based on the instructions, the electronic processor directs, during batch initiation operations, automatic control in the aerosol manufacture facility of one or more of commencement of precursor liquid supply to the aerosol generator, commencement of carrier gas supply to the aerosol generator, commencement or increase of heat input into the aerosol heater, and activation of ultrasonic transducers in the aerosol generator. During intermediate operations, the electronic processor directs automatic control of one or more of carrier gas supply to the aerosol generator, precursor liquid supply to the aerosol generator, and heat input into the aerosol heater. During batch termination operations, the electronic processor directs automatic control of one or more of deactivation of the ultrasonic transducers, termination of carrier gas supply to the aerosol generator, termination of precursor liquid supply to the aerosol generator, and reduction or termination of heat input into the aerosol heater. In a preferred embodiment, all of these noted operations are automatically controlled at the direction of the electronic processor.
The method of the present invention includes significant flexibility to accommodate automation in a variety of different processing embodiments. For example, the method may include automated cooling of one or more process stream or piece of equipment during the method. In one embodiment, the aerosol stream, after passing through the aerosol heater, passes to an aerosol cooler where a cooling gas is mixed into the aerosol stream to lower the temperature of the aerosol stream, to permit subsequent collection of the particles, the supply of the cooling gas to the aerosol cooler being automatically controlled at the direction of the electronic processor. In one embodiment, the aerosol generator includes a pathway for circulation of a cooling liquid adjacent to ultrasonic transducers to cool the ultrasonic transducers during operation. The cooling liquid pathway is typically interposed between the reservoir of precursor liquid and the ultrasonic transducers, so that ultrasonic signals energizing the precursor liquid first pass through the cooling liquid. Supply of the cooling liquid is automated at the direction of the electronic processor. In another embodiment, a cooling liquid is supplied to the vicinity of electronic driver circuits driving the ultrasonic transducers to cool the circuits, with the supply of the cooling liquid being automatically controlled at the direction of the electronic processor. In another embodiment, a cooling liquid is supplied to end caps adjacent entrance and exit ends of the aerosol heater, with supply of the cooling liquid being automatically controlled at the direction of the electronic processor.
In one aspect, the present invention addresses a significant problem of precursor liquid tending to become more concentrated over time when aerosol generation is from a recirculating precursor liquid. The precursor liquid includes at least one precursor material dissolved or suspended in a liquid vehicle, typically water. Over time, the precursor liquid tends to become more concentrated in the precursor material. This concentration of the precursor liquid over time can result in an undesirable lack of uniformity in properties of particles that are produced. The present invention addresses this problem through the addition of additional liquid vehicle to the aerosol manufacture facility, during generation of the aerosol stream, in a manner to at least partially counteract the tendency of the precursor liquid to otherwise become more concentrated. The additional liquid vehicle may be added, for example, to the aerosol generator, to the liquid supply system and/or to the carrier gas supply system.
In one embodiment, the liquid supply system includes two liquid containment tanks, or vessels, to facilitate control of the precursor liquid concentration in the liquid supply system and regulation of supply of the precursor liquid to the aerosol generator. A first, larger vessel acts as the primary supply vessel for the precursor liquid, and a second, smaller vessel acts as a control vessel. During generation of the aerosol stream, precursor liquid is transferred from the first vessel to the second vessel. Precursor liquid is then supplied to the aerosol generator from the second vessel. Effluent precursor liquid from the aerosol generator is returned to the second vessel for recirculation. Additional liquid vehicle may be added to the second vessel to at least partially offset the tendency of the precursor liquid to become more concentrated in the precursor material over time.
Furthermore, in one embodiment control of the concentration of the precursor material in the precursor liquid is automated. For example, the electronic processor may monitor, at some location in the precursor liquid supply system, a property or properties of precursor liquid indicative of the concentration of the precursor material in the precursor liquid at that location. Based at least in part on the monitored property or properties, the electronic processor then directs automatic addition, as necessary, of additional precursor liquid to the precursor liquid supply system to at least partially offset the tendency of the precursor liquid to become more concentrated over time. A convenient location to monitor the property or properties is in the second vessel or in the precursor liquid stream being supplied from the second vessel to the aerosol generator.
In another aspect, the aerosol manufacture method of the present invention addresses detrimental effects on particle quality of transient conditions that may occur during manufacture, and especially during initial stages of particle production during batch processing. The effects of process transients occurring during the initial stages of particle manufacture are, at least in part, addressed with the present invention by conditioning equipment of the manufacture facility during batch initiation operations, prior to particle manufacture. During the conditioning, the temperature of certain equipment is increased to simulate conditions that will exist later during steady state particle manufacture during intermediate operations. The conditioning involves flowing a carrier gas, prior to particle production, through the aerosol heater at an elevated temperature to simulate temperature and flow conditions that will exist when the aerosol stream is flowing through the aerosol heater during steady state particle manufacture. In a preferred embodiment, the heated carrier gas exiting the aerosol heater then passes through the aerosol cooler, where it is mixed with cooling gas and conditions the aerosol cooler. Following the aerosol cooler, the mixture of cooling gas and carrier gas then flows through the particle collector to condition the particle collector. With respect to the aerosol generator, conditioning may include, in addition to flow of the carrier gas, heating the precursor liquid supplied to the aerosol generator prior to commencement of aerosol generation. The heating of the precursor liquid simulates heating that occurs during aerosol generation due to operation of the ultrasonic transducers.
In one aspect, the present invention provides an automated facility for aerosol manufacture of particles according to the method of the present invention. The facility includes an aerosol generator, capable of producing an aerosol stream from carrier gas and precursor liquid, a carrier gas supply system capable of supplying carrier gas to the aerosol generator, a precursor liquid supply system capable of supplying precursor liquid to the aerosol generator, an aerosol heater capable of heating the aerosol stream to form particles of the desired composition, and an electronic processor capable of processing instructions concerning manufacture of particles of the selected composition and capable of communicating, for the purpose of automated control, with one or more of the aerosol generator, the carrier gas supply system, the precursor liquid supply system and the aerosol heater during manufacture of particles in the facility.
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
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, maybe 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 1×106 droplets per cubic centimeter of the aerosol 108, more preferably greater than about 5×106 droplets per cubic centimeter, still more preferably greater than about 1×107 droplets per cubic centimeter, and most preferably greater than about 5×107 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 0.25 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 10 seconds is typical. 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
With continued reference to
During operation of the aerosol generator 106, as shown in
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
Another important feature of the aerosol generator 106, as shown in
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.
As shown in
A preferred transducer mounting configuration, however, is shown in
Referring now to
Referring now to
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
Referring now to
Referring now to
Referring now to
An alternative, and preferred, flow for carrier gas 104 is shown in
Another configuration for distributing carrier gas in the aerosol generator 106 is shown in
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
Referring now to
Referring now to
The design and apparatus of the aerosol generator 106 described with reference to
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
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 1×107 droplets per cubic centimeter, and more preferably from about 5×107 to about 5×108 droplets per cubic centimeter. A concentration of about 1×108 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 1×107 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 1×108 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 110 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
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 of liquid feed 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 110. 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
Referring now to
With continued reference to
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
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
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
With continued reference primarily to
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
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 polycrystalline 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
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 generator 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
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
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. The particles may include, for example, single phase or multi-phase particles. Also, the particles may include a metallic phase or a nonmetallic phase.
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.
As will be appreciated by those skilled in the art, the size of particles as referred to herein is the size of what is often referred to as the primary particles. As is known in the art, it is common for particles manufactured by an aerosol route to form loosely bound, or “soft,” agglomerates when the particles are collected. These soft agglomerates are easily dispersed back to the loose primary particles, such as by sonication, sieving or low shear milling. A preferred method for determining particle size is to first disperse the particles in a liquid medium, such as water, by sonication in an ultrasonic bath or horn to disperse soft agglomerates that may have formed, and to then determine primary particle size attributes by light scattering, such as in a Microtrac™ or other analytical equipment.
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.
Efficient manufacture of particulate product through the aerosol manufacture method of the present invention requires control of a number of flow streams and heat input.
The aerosol manufacture facility, as shown in
During operation of the aerosol manufacture facility to produce a particulate product, precursor liquid is circulated to the aerosol generator 600 from the precursor liquid supply system 608. Circulation of the precursor liquid includes supplying a precursor liquid feed 620 from the precursor liquid supply system 608 to the aerosol generator 600 and removing a precursor liquid effluent 622 from the aerosol generator 600 and returning the precursor liquid effluent 622 to the precursor liquid supply system 608, for recirculation to the aerosol generator 600 as part of the precursor liquid feed 620. Carrier gas 624 is supplied to the aerosol generator 600 from the carrier gas supply system 610. In the aerosol generator 600, there is a reservoir of the precursor liquid overlying ultrasonic transducers, as previously described with reference to
As can be seen in
Manufacture of particles in the aerosol manufacture facility may occur in a batch mode or a continuous mode of operation. In most instances, however, the aerosol manufacture facility will be operated in a batch mode. As used herein, unless otherwise noted, operating in a batch mode refers to processing of a batch of precursor liquid to produce particles, and includes processing that could be considered as being technically semi-batch or semi-continuous in nature. A batch of precursor liquid refers to a discrete volume of precursor liquid to be processed. The particles produced from processing of a batch of precursor liquid are generally referred to as a batch of particles or a powder batch, even though the particles may be removed from the system periodically at different times during processing of the precursor liquid batch. When operated in a batch mode, the aerosol manufacture facility may be designed with appropriately sized equipment to accommodate any desired precursor liquid batch size. In some instances, when processing large precursor liquid batches, processing of the precursor liquid batch in batch mode may require a batch run time of a week or more.
Referring now to
In a second stage, the intermediate operations involve manufacture of particles after the commencement of generation of the aerosol stream. Referring briefly to
Referring again to
As noted, the intermediate operations are generally analogous to operations in a continuous mode. Likewise, the batch initiation operations are analogous to start-up operations that may precede continuous mode operation. Also, the batch termination operations are analogous to a periodic shut-down that may be required even when the aerosol manufacture facility is operated in a continuous mode. Therefore, the discussions herein, although focused primarily on batch mode of operation, apply equally to a continuous mode of operation.
Efficient control of the batch initiation operations, the intermediate operations, the batch termination operations, and transitions between these stages is an important aspect of the present invention for efficient operation of the aerosol manufacture facility.
One major aspect of the present invention is control of the concentration of the precursor material in the precursor liquid being processed in a batch, especially during the intermediate operations. This is important because, over time, the circulating precursor liquid has a tendency to become more concentrated in the precursor material. Referring to
The problem with concentration of the precursor material over time results primarily from vaporization, in the aerosol generator 600, of some of the liquid vehicle from the precursor liquid circulating through the aerosol generator 600. The tendency of the circulating precursor liquid to become more concentrated in the precursor material can be a serious problem when it is desired to make a uniform batch of particles, as is usually the case. This significant problem is addressed with the present invention through the addition of additional liquid vehicle to the aerosol manufacture facility during generation of the aerosol stream to at least partially offset the tendency of the precursor liquid to otherwise become more concentrated in the precursor material. The additional liquid vehicle may be added at any convenient location within the aerosol manufacture facility to effect the desired concentration. Preferred locations for adding the additional liquid vehicle include the aerosol generator 600, the carrier gas supply system 610, and the precursor liquid supply system 608.
In addition to becoming more concentrated in the precursor material, it is possible also that the precursor liquid may become concentrated in or depleted in some other component, for which appropriate process adjustments may be made. For example, when making some materials, the precursor liquid comprises an acidic aqueous nitrate solution. In that situation, significant nitric acid can be lost to volatilization in the aerosol generator and the precursor liquid will, therefore, become depleted in nitric acid over time. Additional nitric acid may, however, be added, to at least partially offset the depletion. The additional nitric acid could be added together with the additional liquid vehicle 636, as an aqueous solution of nitric acid, or could be added separately.
Another significant aspect of the present invention concerning efficient control and operation of batch processing in the aerosol manufacture facility is that the processing may, at least in part, be automated, with automated control of at least a portion of one of the batch initiation operations, the intermediate operations and the batch termination operations. In preferred process embodiments, all three of the stages of batch processing are significantly automated. In one preferred automated mode of operation, an operator directs an electronic processor to process a batch of precursor liquid to prepare a batch of particles of a selected composition. The processor then processes instructions concerning manufacture of particles of the selected composition and automatically directs the aerosol manufacture facility to manufacture a batch of particles of the selected composition.
Referring now to
It will be appreciated that the electronic processor 640 is ultimately responsible for directing the process control, even though the actual control signals to effectuate the control come from the controller 642. The controller 642 merely facilitates communication between the electronic processor 640 and actuatable equipment through which process control is effectuated. For example, the controller 642 is capable of converting analog signals received from process equipment into digital signals to send to the electronic processor 640 for processing. Also, the controller 642 is capable of converting digital signals received from the electronic processor to analog control signals to send to actuatable equipment. The controller 642 is also capable of relaying a signal without conversion. The controller 642 may comprise a single unit or may comprise a plurality of components that are coordinated to facilitate communication between the electronic processor 640 and different portions of the aerosol manufacture facility. Moreover, the actuatable equipment may be actuated electronically or pneumatically. As will be appreciated, the use of pneumatically actuated equipment may require transducers to convert electronic signals from the controller 642 into pneumatic signals to actuate the equipment.
The electronic processor 640 may be any suitable processor, such as a microprocessor or a computer. Typically, the electronic processor will be a programmable logic control microprocessor. Also, the electronic processor 640 includes, or is connected to, memory including instructions for manufacture of particles of the desired composition, which instructions are processable by the electronic processor 640. Furthermore, the memory may include instructions for manufacture of particles of a number of different compositions. An operator could then instruct the electronic processor 640 as to which composition is desired, and the electronic processor 640 could select and process the appropriate set of instructions for the desired composition. In this way, the aerosol manufacture facility could be used to manufacture batches of particles of different compositions, although thorough cleaning of process equipment would be required between batches of different compositions.
Also, although
When operation of the aerosol generator 600 is automated, automatic control in the aerosol generator 600 typically includes automatically activating, at the direction of the electronic processor 640, the ultrasonic transducers in the aerosol generator 600 during batch initiation operations and automatically deactivating, at the direction of the electronic processor 640, the ultrasonic transducers during batch termination operations. The timing for activation and deactivation of the ultrasonic transducers in relation to other operations is very important, as discussed more fully below.
As noted previously, control of the concentration of the precursor material in the precursor liquid through the addition of additional liquid vehicle is an important aspect of the present invention. In a preferred embodiment, the concentration control is automated. This automation may be accomplished, for example, through monitoring, by the electronic processor 640, the concentration of the precursor material in the precursor liquid supply system 608, or monitoring of one or more properties of the precursor liquid which are indicative of concentration or from which concentration may be calculated.
During the intermediate operations, with reference to
Also, during the intermediate operations, precursor liquid is withdrawn from the second vessel 652 by a pump 665 for supply to the aerosol generator 600 as the precursor liquid feed 620, after passing through a flow control valve 660 and a flow element 668. The precursor liquid effluent 622 from the aerosol generator is returned to the second vessel 652. A side stream 670 is withdrawn from the bottom portion of the second vessel 652 by a pump 672. The side stream 670 passes through a monitor element 674 and is recirculated to the top portion of the second vessel 652. The recirculation of the side stream 670 helps to keep the precursor liquid in the second vessel 652 well mixed and to avoid the development of concentration gradients within the second vessel 652. The additional liquid vehicle 636 passes through a flow control valve 676 and a flow element 678 prior to entering the second vessel 652.
The precursor liquid effluent 622 typically involves significant flow. This is because, typically, only a small portion of the precursor liquid feed 620 is converted to droplets in the aerosol stream during a single pass through the aerosol generator 600. The recycle ratio for the precursor liquid fed to the aerosol generator, is typically larger than about four to one, more typically larger than about six to one, even more typically larger than about eight to one and still more typically larger than about ten to one. The recycle ratio is the volumetric ratio of recycled precursor liquid to fresh precursor liquid in the precursor liquid feed 620 (i.e., ratio of rate of flow of precursor liquid effluent 622 to rate of transfer of fresh precursor liquid from the first vessel 650 to the second vessel 652). The portion of the precursor liquid feed 620 that exits the aerosol generator 600 in the aerosol stream is typically less than about twenty volume percent of the precursor liquid feed 620, and more typically less than about fifteen volume percent, even more typically less than about ten volume percent, and still more typically less than about five volume percent of the precursor liquid feed 620. The portion of the precursor liquid feed 620 exiting the aerosol generator 600 as the precursor liquid effluent 622 is typically larger than about eighty volume percent of the precursor liquid feed 620, more typically larger than about eighty-five volume percent, even more typically larger than about ninety volume percent and still more typically larger than about ninety-five volume percent of the precursor liquid feed 620.
As shown in
One of the important control features shown in
Automated process control during intermediate operations of batch processing will now be described, with reference to
The flow rate of the precursor liquid feed 620 is controlled by automatic actuation of the flow control valve 666. The electronic processor 640 monitors flow rate, via the flow element 668, and responsively directs control of the flow control valve 666 to maintain the flow rate to the aerosol generator 600 within a desired range.
In one embodiment, not shown in
Referring again to
Automated process control during batch initiation operations will now be described, with continued reference to
After there is an adequate quantity of precursor liquid in the second vessel 652, circulation of precursor liquid is established through the aerosol generator 600, with the ultrasonic transducers being deactivated so that no aerosol is being generated in the aerosol generator 600. To establish the circulation, the pump 665 is activated to commence the flow of the precursor liquid feed 620 which circulates through the aerosol generator 600 and returns to the second vessel 652 as the precursor liquid effluent 622. Also, the pump 672 is activated to commence recirculation of precursor liquid through the recirculation stream 673. In a preferred embodiment, the pump 654, pump 665, pump 672, and mixer 662 are all automatically actuated at the direction of the electronic processor 640. This embodiment is shown in
Another embodiment of the precursor liquid supply system 608, including major process components, is shown in
Referring now to
Referring once again to
After the batch termination operations are commenced, the pump 654 is automatically shut off at the direction of the electronic processor to terminate transfer of precursor liquid from the first vessel 650 to the second vessel 652. Precursor liquid, however, continues to be withdrawn from the second vessel 652 by the pump 665 to supply the precursor liquid feed 620. Because there is no fresh precursor liquid being introduced into the second vessel 652, however, the tendency of the precursor liquid to become more concentrated in the precursor material over time becomes an even bigger problem. Therefore, during the batch termination operations, the rate of addition of the additional liquid vehicle 636 will typically be accelerated relative to the rate of addition during the intermediate operations. When the level of the precursor liquid in the second vessel 652 drops below a certain level, as monitored by either level indicator 680 or level indicator 682, the electronic processor automatically turns off pump 665 and pump 672 and closes the flow control valve 676, if the flow control valve 676 is not already closed. Alternatively, the pump 665 could be automatically turned off, at the direction of the electronic processor 640 when the electronic processor 640 determines that the concentration of precursor material in the liquid vehicle has reached an undesirably high level.
Although the precursor liquid supply system 608 has been described in
With continued reference to
Also, although a preferred embodiment of the method of the present invention includes a circulating precursor liquid, it is possible to operate without precursor liquid circulation. For example, when the carrier gas 624 is saturated with vapor of the liquid vehicle, as just described, then the loss of liquid vehicle from precursor liquid in the aerosol generator 600 may be small enough to avoid circulation altogether. In one embodiment, the precursor liquid could be fed to the aerosol generator 600 at a rate substantially equal to the rate of droplet generation to form the aerosol stream, with no precursor liquid effluent stream 622 exiting the aerosol generator. As another example, circulation could also be avoided by processing a more dilute precursor liquid, that then concentrates, in a steady state fashion in the aerosol generator 600 to a desired concentration. The precursor liquid would be fed to the aerosol generator 600 at a rate substantially equal to the rate consumption in the aerosol generator 600, including consumption to produce droplets for the aerosol stream and to saturate the carrier gas in the aerosol generator 600 with vapor of the liquid vehicle.
As one alternative to the embodiment described with reference to
With reference to
Referring now to
Referring now to
In one embodiment for the cooling liquid supply system 630, cooling liquid could be supplied, during the intermediate operations, to electronic driver circuits driving the ultrasonic transducers of the aerosol generator 600, to cool the driver circuits to prevent overheating. Cooling liquid to the driver circuitry may be automatically controlled at the direction of the electronic processor 640, in a manner similar to control of the cooling liquid feed 632 to cool the ultrasonic transducers.
Referring now to
With continued reference to
Although the aerosol manufacture method has been described as generally ending with collection of the particles in the particle collector, in some embodiments additional processing may be performed after particle collection. For example, if it is desired to further modify the composition or the morphology of the particles, the particles may be subjected to a post-collection anneal, or other operation at elevated temperature. During the post-collection anneal, components in the particles may react to alter the chemical composition of the particle, or one or more phases within the particles may be recrystallized or reconfigured. The anneal may be performed, for example in a rotary kiln.
Referring again to
The next step shown in
The next steps, as shown in
The next step shown in
The next step, as shown in
The second pressure test step shown in
Referring now to
Referring now to
With reference to
Referring to
Referring now to
The next step, as shown in
The next step, as shown in
The next steps, as shown occurring substantially simultaneously in
Referring now to
Referring now to
The sequences of steps for batch initiation operations and batch termination operations as shown in
As described previously, the intermediate operations typically include steady state or quasi-steady state manufacture of particles. As noted, however, it may be desirable to have periodic planned, or unplanned, interruptions of the steady state or quasi-steady state conditions to correct a problem identified with particle manufacture or for periodic maintenance or removal of accumulated particulate product. When the manufacture is interrupted during the intermediate operations, initiating the interruption is somewhat analogous to batch termination operations and resuming production following the interruption is somewhat analogous to batch initiation operations. Therefore, the prior discussions concerning batch termination operations and batch initiation operations are relevant to temporary interruptions occurring during intermediate operations, suitably modified to fit the particular situation. For example, when interrupting production during intermediate operations, it will be necessary to deactivate the ultrasonic transducers, and typically also to temporarily terminate the supply of carrier gas to the aerosol generator and of cooling gas to the aerosol cooler. Circulation of precursor liquid and of cooling liquid to the generator, if used, could also be temporarily terminated. Furthermore, it may be desirable to lower the temperature in the furnace below the operating temperature present during particle manufacture. When commencing production at the end of an interruption, the system would be conditioned to bring it up to temperature, if necessary, and the flows of all the fluid streams would be reinitiated and the ultrasonic transducers again activated.
As noted previously, a significant aspect of the present invention is the automation of one or more operations during the batch processing. In that regard, any of the steps occurring during the batch initiation operations, intermediate operations or batch termination operations can be automatically controlled at the direction of the electronic processor. Although it is only necessary, within the scope of the present invention, that at least one operation be automated, it is preferred that substantially all of the operations be automated. In that regard, any or all of the steps shown in any of
It should be recognized that
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.
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.
A variety of materials are made, 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, BaTiO3 varied from 5 to 90 weight percent of the composite.
(2)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/SiO2 to SiO2 coating over Pd as reactor temperature is increased.
(2)Coating of Pd on SiO2 particles.
(3)Titanium tetraisopropoxide.
(4)Metal dispersed on high surface area TiO2 support.
(5)Al[OCH (CH3)C2H5]3.
(6)Metal dispersed on high surface area Al2O3 support.
(7)Pd coating on TiO2 particles.
(8)Ag coating on TiO2 particles.
(9)Pt coating on TiO2 particles.
(10)TiO2 coating on Ag particles.
(11)TiO2 coating on Au particles.
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.
This application is a continuation of U.S. patent application Ser. No. 09/786,999 filed on Feb. 11, 2002 now U.S. Pat. No. 6,685,762, which is a United States National Stage under 35 U.S.C § 371 of PCT/US99/19163 filed Aug. 23, 1999, which claims priority to U.S. Provisional Patent Application No. 60/098,174 filed Aug. 26, 1998, the contents of each and every one of which are incorporated herein as if set forth herein in full.
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| Number | Date | Country | |
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| Number | Date | Country | |
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| Parent | 09786999 | US | |
| Child | 10761809 | US |
