US20060144972A1

US20060144972A1 - Apparatus and method for generating fine particulates

Info

Publication number: US20060144972A1
Application number: US11/301,362
Authority: US
Inventors: Arnold Kelly
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-12-13
Filing date: 2005-12-09
Publication date: 2006-07-06

Abstract

A method and apparatus to provide charges to a flow, or stream, of fluent material droplets, or a planar array of parallel droplet streams of fluent material, wherein said fluent material may be electrically conductive, to form smaller droplets thereof is described. In one embodiment, an apparatus is configured to generate a stream, such as a coaxial stream, of droplets of fluent material that are electrically isolated, about uniform in size, and are spaced about equidistantly apart. In one aspect, an electron emissive device is employed to irradiate the stream of droplets to inject charges therein. The charges atomize the droplets into smaller droplets to form a plume of the fluent material particulates having an approximately uniform size. In some embodiments, the individual particulates of fluent material freeze, thereby forming powder particulates having predictable characteristics.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application claiming priority from Provisional Application Ser. No. 60/635,605 entitled “Apparatus and Method For Generating Fine Particulates” filed Dec. 13, 2004.

BACKGROUND OF THE INVENTION

Embodiments of the present invention generally relate to dispersion of fluent materials. More specifically, the present invention relates to electrostatic atomization of fluent materials.
Generally, industries employing processes such as spray painting, chemical treatment of liquids, combustion, and the like often utilize a fine discharge of fluent material. The industry has provided several techniques to generate such a discharge of fluent material. For example, atomization systems are often employed to provide a fine and uniform discharge of fluent material where it is desirable to provide such fluent material in uniform droplets. One conventional atomization technique involves mechanically forcing fluent material though a small orifice under high pressure. The fluent material output is often used for combustion systems. Such fluent materials may be mixed with other effluents such as gas flowing at a high velocity to kinetically disperse the fluent material. Unfortunately, such mechanical systems generally provide a mixture of dispersed particulates of varying sizes and are often prone to problems such as clogging and frictional degradation.
Other conventional atomization systems employ electrostatic atomization in which different portions of a precursor material (e.g., a fluent material) are electrically charged as they are discharged from an orifice. These charged portions are directed toward an electrode having an opposite charge potential to induce a charge therein. Because the portions of fluent material are generally charged with the same polarity, they tend to repel each other causing such portions of the fluent material to disperse. For example, such electrostatic atomization techniques are often used in spray paint systems. Although such single electrode systems are capable of providing high charging levels, they are subject to severe flow rate limitations. Some conventional systems use a pair of electrically opposed electrodes to provide a greater net charge thereby increasing the dispersion of the fluent material. In such systems, injecting such charge may atomize and disperse virtually all fluent material.
Conventionally, one technique used to impart a charge to a fluent material uses a submerged field-emission electron gun. Such an electron gun uses an emitter electrode positioned upstream in proximity to a grounded plate having an orifice though which fluid is forced therethrough. Generally, the emitter electrode is maintained at an elevated potential relative to the orifice plate to enable charges to be forced into and trapped within the fluent material prior to passing though the orifice. Once the charged fluid passes through the orifice, the fluent material vigorously atomizes into a spray of uni-polarly charged small droplets of varying sizes. Unfortunately, such techniques only work with non-conductive fluent material such as oils, but do not work with conductive fluent material such as water or metal, as such conductive fluent material tends to short out the charges.
The aforementioned industries have provided several similar techniques to atomize conductive fluent material. One such technique includes a non-contact system in which charges are injected into an electrically isolated fluent material to atomize some low temperature conductive fluids such as water. Unfortunately, current systems employing such non-conductive techniques require the use of electrical insulating materials and complicated configurations such as insulated equipment mountings to increase the ground path resistance for such trapped charges within the conductive fluent material to electrical ground. Such an increase in ground path resistance is designed such that the conductive fluid material is allowed to build up a charge faster than the charges flow to available grounds.
The complexity of systems for atomizing conductive materials generally increases as the fluent material temperatures increases. Consequently, vacuum and heat shields are often used to protect charging equipment such as electron guns in proximity with higher temperature conductive fluent material such as molten metal. Unfortunately, such heat shields are often contaminated by the higher temperature fluent material, especially if the fluent material is processed under low pressure conditions or a vacuum, wherein the gaseous cooling of the pre-atomized particles is limited and therefore their trajectory is less predictably constrained. For example, such high temperature unrestrained pre-atomized particles of fluent material often foul electron transparent membranes used to isolate the electron gun from such particles, which thereby decreases the efficiency of the process and increases the maintenance of such membranes and associated equipment.
Another problem inherent to conventional electrostatic atomization systems is Coulomb shielding. Coulomb shielding limits coupling of an irradiating electron beam with the majority of the precursor material stream regardless of whether such a multiple particle precursor material stream is solid, liquid, conductor, or insulator. Charging of portions of the precursor material stream closest to the irradiating device, such as an electron gun, forms a Coulombic “mirror”. Such a Coulombic mirror on the stream surface redirects or reflects the incoming irradiating beam thus effectively shielding the interior of the precursor material stream from the irradiating beam. Unfortunately, even under conditions where irradiating beam energy is sufficient to cross a precursor material plume, differential charging due to beam energy degradation with passage distance results in the generation of a broad, non-optimal spectrum of secondary droplet sizes.

BRIEF SUMMARY OF THE INVENTION

Disclosed is an apparatus for generating a plurality of particulates including at least one dispensing device for generating at least one of the group consisting of a stream of droplets, a coaxial stream of droplets, a planar array of parallel droplet streams, and combinations thereof, and at least one charge injection device adjacent a discharge of the at least one dispensing device for inducing at least one charge to at least a portion of the plurality of droplets dispensed from the at least one dispensing device, wherein the at least one charge is of sufficient magnitude to initiate electrostatic atomization as per at least one of the group consisting of an equilibrium spray theory, a Coulomb bursting process, and combinations thereof.
Also disclosed is a method for generating a plurality of particulates including receiving at least one fluent material, generating at least one of the group consisting of a stream of droplets, a coaxial stream of droplets, a planar array of parallel droplet streams, and combinations thereof from said fluent material, inducing at least one charge to a plurality of droplets, the charge having sufficient magnitude to initiate electrostatic atomization as per at least one of the group consisting of an equilibrium spray theory, a Coulomb bursting process, and combinations thereof, and forming a plurality of particulates via electrostatic atomization, wherein the plurality of particulates have approximately uniform, predetermined diameters. All measurements are in SI units unless otherwise specified.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments that are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:
FIG. 1 is a high level illustration of one embodiment of a particulate generation apparatus in accordance with embodiments of the invention;
FIG. 2 is a high level illustration of a discharge assembly in accordance with embodiments of the invention;
FIG. 3 is a flow diagram of one embodiment of a method to generate particulates in accordance with embodiments of the invention; and
FIG. 4 is a graph illustrating a profile of a beam of charges in accordance with embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As required, a detailed illustrative embodiment of the present invention is disclosed herein. However, techniques, systems and operating structures in accordance with the present invention may be embodied in a wide variety of forms and modes, some of which may be quite different from those in the disclosed embodiment. Consequently, the specific structural and functional details disclosed herein are merely representative, yet in that regard, they are deemed to afford the best embodiment for purposes of disclosure and to provide a basis for the claims herein, which define the scope of the present invention. The following presents a detailed description of the preferred embodiment (as well as some alternative embodiments) of the present invention.
In the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention. However, it will be apparent to one of skill in the art that the present invention may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring the present invention. For clarity, embodiments of the present invention are described in terms of conductive fluent material. However, it is contemplated that virtually any fluent material including materials such as non-conductive fluids, and the like, may be used to advantage within the scope of the present invention. In addition, both fluent and non-fluent materials may be produced using the methods and apparatus of the present invention.
An aspect of the present invention is an apparatus configured to generate a plurality of fine particulates, such as powder, from a fluent material. Such fluent material may be a high temperature liquid material, however, the present invention is not so limited. The apparatus includes a means to provide a stream of individual droplets, or a planar array of parallel droplet streams, of the fluent material. The droplets are electrically isolated from each other, are substantially uniform in size, and are about equidistantly separated. The apparatus further includes a charge injection device configured to provide a charge to each of the individual droplets that is of sufficient magnitude to initiate electrostatic atomization as per the equilibrium spray theory or at least one Coulomb (Rayleigh) bursting process (i.e., a process whereby an outwardly directed electrostatic force within each charged droplet overwhelms a cohesive surface tension force associated therewith, thereby forming smaller droplets therefrom).
Another aspect of the present invention is a method of generating fine particulates, such as powder, from a fluent material. The method includes generating a stream of fluent material droplets, such as a coaxial stream, or a planar array of parallel droplet streams, that are electrically isolated, substantially uniform in size, and about equidistantly separated. The method further includes independently irradiating each of the droplets with charges for a predetermined amount of time to charge each of the droplets sufficiently to initiate electrostatic atomization as per the equilibrium spray theory or at least one Coulomb (Rayleigh) bursting process. Through this process, smaller droplets are formed from the fluent material droplets.
Other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of the structure, and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following detailed description with reference to the accompanying drawings, all of which form a part of this specification.
Referring first to FIG. 1, illustrated is a high level illustration of one embodiment of a particulate generation apparatus 100 in accordance with embodiments of the invention. Particulate generation apparatus 100 includes a fluent material processing system 102 coupled to a discharge assembly 120, the latter of which is discussed in greater detail below with respect to FIG. 2. Discharge assembly 120 is configured to charge and atomize a stream 140 of fluent material 111 provided by fluent material processing system 102 such that, when charged, fluent material 111 atomizes to form a plume 150 as described herein. Such plume 150 may be used for coating a surface 160, for example, with such atomized fluent material 111.
In one embodiment, fluent material processing system 102 includes a reservoir 110 disposed adjacent to a heating assembly 116. Reservoir 110 is configured to hold fluent material 111, such as conductive fluids, therein for processing within a predetermined range of processing temperatures. For example, reservoir 110 may be configured to hold molten conductive fluids such as Wood's metal, Inconel®, Cobalt, Chromium, Nickel, Titanium, Octoil®, alloys, superalloys, refractory metals, and the like, for processing thereof. However, the invention is not so limited.
If particle generation apparatus 100 is utilized to spray the resulting daughter droplets 201′, fluent material 111 preferably will atomize to conform to the restraints of a maximum entropy, low charge density atomization process when the parameter −α′ is approximately 0.1 as calculated from the following equation:
−α′=3.1(√ρ)/ (Eq. 1)
wherein ρ is the density of fluid material 111′ and μ is the viscosity of fluid material 111.
When the above criterion is satisfied (i.e., −α′ is approximately 0.1), the mean size of daughter droplet 201′ may be calculated from the following equation:
d=75/√ρ_e (Eq. 2)
wherein d is the diameter of daughter droplet 201′ and ρ_eis the charge per unit volume of fluent material 111.
In one embodiment, reservoir 110 is coupled on a lower end to stream generator 105 and on an upper end to a vacuum port 104, a fluent material inlet port 106, and a pressurization port 108. Vacuum port 104 couples reservoir 110 to an external vacuum source 112. In one embodiment, reservoir 110 is evacuated to the micron range to avoid oxidation of and to thoroughly degas fluent material 111. Fluent material inlet port 106 couples reservoir 110 to an external fluent material source 113. Pressurization port 108 couples reservoir 110 to an external gas source 124 of at least one gas, such as dry nitrogen and the like. In one embodiment, external gas source 124 pressurizes reservoir 110 to pressures ranging as high as the maximum pressure rating of reservoir 110 and fittings, if any, associated therewith.
Discharge assembly 120 receives fluent material 111 from a stream generator 105. Preferably, stream generator 105 generates individual, isolated, co-streaming parent droplets 201 of fluent material 111. In one embodiment, stream generator 105 is coupled to the downwardly facing side of reservoir 110 when reservoir 110 is in its upright position as depicted in FIG. 1, allowing gravitational forces to assist in generation of stream 140. In one embodiment, stream generator 105 consists of valve driver 117 and valve 115, the latter of which is coupled to reservoir 110 via a fitting such that valve 115 may control the gravitational flow of fluent material 111 from reservoir 110 to discharge assembly 120 as further described herein.
Generally, valve 115 and valve driver 117 may be virtually any type of valve or valve driver system, respectively. More specifically, valve 115 may be a chromatographic grade, fast acting valve driven by a repetitious valve driver 117 configured to set the valve 115 opening duty cycles over the entire range of zero percent open (i.e., 100% closed) to one hundred percent open to output a stream 140 of fluent material 111 from discharge assembly 120 to be tailored within a predetermined flow rate range. Although this embodiment configures valve 115 to cycle over the entire range of zero percent open to one hundred percent open, the scope of the present invention is not so limited.
In one embodiment, fluent material processing system 102 includes a heating assembly 116 disposed about reservoir 110 to maintain fluent material 111 at or near a predetermined temperature. One embodiment of heating assembly 116 includes a container 118 which houses heater 121 and a heat transfer liquid 119, such as oil, within cavity 109. In this embodiment, container 118 supports reservoir 110 in an upright position as depicted in FIG. 1 such that heater 121 maintains contact with heat transfer liquid 119 to impart heat energy thereto providing a heating bath for reservoir 110 and the fluent material 111 contained therein. Heater 121 is generally an electrical heater as known, however, other types of heaters and heating systems are contemplated that may be used to advantage. Heater 121 may be coupled to a heat control 131 configured to control the electrical power provided thereto for operation thereof. In another embodiment of heating assembly 116, an insulating jacket 122 encircles the outer sides and bottom of container 118. Insulating jacket 122 is configured to insulate container 118 to help maintain heat transfer liquid 119 within a predefined temperature range and may also be configured to protect a user of particle generation apparatus 100 from heat radiated from heating assembly 116.
In one embodiment, particulate generation apparatus 100 is surrounded by an atmosphere 123, such as an inert atmosphere, to minimize interaction between atmospheric elements and stream 140. For example, atmosphere 123 may include a dry nitrogen gas purged of oxygen below one percent to minimize atmospheric contamination and interference with an atomization process of fluent material 111 described herein. In one embodiment, atmosphere 123 is maintained at a vacuum pressure, however, such atmosphere 123 may range from vacuum to supra-atmospheric pressures without departing from the scope of the present invention.
Turning next to FIG. 2, depicted is a more detailed illustration of a discharge assembly, such as the discharge assembly 120 depicted in FIG. 1, in accordance with embodiments of the invention. In one embodiment, discharge assembly 120 includes a droplet nozzle 221 coupled to a stream generator 105. Discharge assembly 120 further includes a charging apparatus 210 disposed adjacent the area below droplet nozzle 221 through which the parent droplets 201 generated by stream generator 105 pass. Charging apparatus 210 may be configured from one or more devices configured to irradiate material, such as parent droplets 201, with a beam 219 of charged particles, such as free electrons. A power supply 214, as known, may be configured to power such charging apparatus 210.
In one configuration, a droplet nozzle 221 is included. Droplet nozzle 221 includes an orifice 223. Droplet nozzle 221 may be formed of materials such as sapphire configured to permit fluent material 111 to issue as a stream 140 of parent droplets 201 from orifice 223, wherein parent droplets 201 have approximately equal volume within a range of flow rates. Droplet nozzle 221 is configured with sufficient length and diameter to receive fluent material 111 from stream generator 105 and output parent droplets 201 at equidistant intervals within the range of predetermined flow rates such that stream 140 is formed.
In an illustrative embodiment, droplet nozzle 221 may be formed from a single tube having an inside diameter of about 5 mil or 127 μm. When such a droplet nozzle 221 is incorporated, providing fluent material 111 to droplet nozzle 221 at a velocity limited to less than about 10 meters per second (“m/s”) results in a laminar flow creating a stream 140 of about equidistantly spaced parent droplets 201 having diameters of about 250 μM. At such a flow rate, about 15,000 parent droplets 201 may be generated per second with a volumetric throughput of 0.12 milliliters per second (“mL/s”). This results in the potential generation of 3.66×10⁶40 μm daughter droplets 201′. However, the invention is not so limited.
In one embodiment of the present invention, one or more heaters 232 are incorporated to ensure that parent droplets 201 remain in molten form while passing through and after discharge from nozzle 221. In one embodiment, heaters 232 extend coaxially along the flow path of parent droplets 201 of stream 140. For example, heaters 232 may be cylindrical band heaters, and the like, positioned coaxially along droplet nozzle 221 and extending downwardly below orifice 223. Heaters 232 are configured to provide an about uniform radiated heat to stream 140. Heat control 233 may be used to provide power to and control a radiant heat output of such heaters 232.
Charging apparatus 210 may be selected or configured to provide sufficient energy to charge parent droplets 201 within a predetermined range of charge values such that plume 150 of fluent material 111 includes daughter droplets 201′ having an approximately uniform predetermined diameter. The approximate charge that must be acquired by the daughter droplet 201′ portions of parent droplets 201 to create daughter droplets 201′ having the predetermined diameter may be calculated based upon the following equilibrium spray theory equation:
N _cd =D _d /a ₀ (Eq. 3)
wherein N_cdis a net charge that daughter droplets 201′ must acquire, D_dis a diameter of a daughter droplet 201′, and a₀is the first Bohr radius.
For example, to generate daughter droplets 201′, each having a diameter of about 40 μm, charging apparatus 210 must charge each daughter droplet 201′ portion of parent droplet 201 with 7.56×10⁵charges 213 (i.e., electrons). At the flow rate example presented above, wherein about 3.66×10⁶40 μm daughter droplets 201′ are created per second, this required net charge N_cdresults in a charge requirement of 2.769×10¹²electrons per second, which equates to a current requirement greater than or equal to 0.44 microamperes (“μA”). If the effect of space charge is ignored, as discussed herein, for a beam 219 having a radial distance from beam 219 centerline of 5 millimeters (“mm”), as discussed in greater detail below, approximately 2% of the current of beam 219 will impinge on parent droplets 201 entering beam 219 perpendicularly. Consequently, a beam 219 having a minimum current requirement of about 23 μA is required to generate 3.66×10⁶40 μm daughter droplets 201′.
In another embodiment, the current requirements of beam 219 may be reduced if the angle between beam 219 and the centerline of stream 140 is less than 90 degrees as the exposure of beam 219 to stream 140 is more protracted. In yet another embodiment, beam 219 is aligned with stream 140 such that stream 140 undergoes maximum exposure to beam 219. In this latter embodiment, parent droplets 201, and sub-droplets associated therewith, may undergo multiple Coulomb (Rayleigh) bursting events.
Calculation of the approximate charge that must be acquired by parent droplets 201 to created daughter droplets 201′ having approximately a predetermined diameter may be calculated as per the following equation:
N _cp=(D _p /D _d)³(N _cd) (Eq. 4)
wherein N_cpis a net charge that parent droplets 201 must acquire, D_pis a diameter of parent droplet 201, Dd is a diameter of a daughter droplet 201′, and N_cdis a net charge that daughter droplets 201′ must acquire.
In one illustrative embodiment, generation of daughter droplets 201′, each having a diameter of about 40 μm, from parent droplets 201 having a diameter of approximately 250 μm, requires charging apparatus 210 to charge each parent droplet 201 with approximately 1.84×10⁸electrons to undergo electrostatic atomization as per the equilibrium spray theory.
In this illustrative embodiment, the electrostatic atomization charge requirement for parent droplets 201 is 43% of the Coulomb (Rayleigh) bursting limit based upon a surface tension of 0.45 newtons per meter (“N/m”). If the parent droplet 201 acquires the additional 2.4×10⁸ charges 213 to reach the Coulomb (Rayleigh) bursting limit, the outwardly directed electrostatic force shall overwhelm the cohesive surface tension causing parent droplet 201 to disrupt. However, when parent droplets 201 are driven to the Coulomb (Rayleigh) bursting limit, the size of the resulting daughter droplets 201′ decreases to about 10 μm and parent droplets 201 lose about 25% of their acquired charges 213 and about 5% of their mass.
Continuing this illustrative embodiment, a parent droplet 201 having a diameter of 250 μm and supporting the approximate charges 213 required for electrostatic atomization on its surface (i.e., about 1.84×10⁸charges 213), will exhibit a surface electric field of 17 megavolts per meter (“MV/m”). Such a surface electric field corresponds to a potential barrier of approximately 2.13 kilovolt (“kV”) for delivery of the last charge to the surface of parent droplet 201. Consequently, charging apparatus 210 must have sufficient energy to overcome the surface electric field created by partial charging of parent droplet 201.
An additional factor to be considered for sizing the energy requirements of charging apparatus 210 is space charge (i.e., the acquired charges 213 of parent droplet 201). As parent droplet 201 penetrates beam 219 and begins acquiring charges 213, the surface electric field of parent droplet 201 increases. As such surface field increases, its ability to deflect charges 213 increases, causing progressive distortion and deflection of beam 219 away from parent droplet 201. In addition, if parent droplets 201 become fully charged and generate daughter droplets 201′ in the vicinity of beam 219, additional distortion of beam 219 is likely to occur. To avoid the negative effects of space charge, the energy of beam 219 created by charging apparatus 210 may be increased above the level required to achieve generation of daughter droplets 201′.
A meaningful estimate of the effect space charge has upon the energy requirement of beam 219 may be garnered by an idealized model of stream 140. For example, given a diameter of parent droplet 201 of 250 μm, and where each parent droplet 201 is spaced approximately 0.67 mm from adjacent parent droplets 201, the discrete nature of the stream 140 is not likely to influence beam 219 behavior beyond a radial distance of several millimeters from a centerline of such stream 140. Thus, for this example, given such line charge is taken to be infinite in both directions to place an upper bound on space charge effects, such stream 140 may be approximated by an infinitely long line charge λ, wherein λ has an about similar charge per unit length as parent droplet 201, or 4.436×10⁻⁸coulomb per meter (“C/m”). Consequently, a potential required to move a charge from infinity to the surface of parent droplet 201 is calculated to be about 7.2 kV. Therefore, conservatively sizing the energy of charging apparatus 210 to overcome the space charge effects of all charges associated with an infinitely long line should be more than sufficient to overcome the space charge effects of charges 213 associated with individual parent droplets 201, as each parent droplet 201 is independently charged to the same level. Inter-droplet shielding, should it occur, can be reduced to a trivial level by adjusting the energy of charging apparatus 210.
In an operational embodiment such as that illustrated in FIG. 2, charging apparatus 210 includes an electron gun 211 (e.g., a 100 kV class electron gun), or similar device, positioned to direct a beam 219 of charges 213 toward stream 140 of parent droplets 201. In another configuration, charging apparatus 210 may be a thirteen millimeter (“mm”) electron beam gun operating at about 30 kV to provide beam 219 of electrons (i.e., charges 213) with an energy approximating 100 electron volts (“eV”) to each parent droplet 201 of stream 140.
To help direct and accelerate such charges 213, discharge assembly 120 includes at least one electrode 220. Electrode 220 is configured to provide an electric field 222 to attract, accelerate, and direct charges 213 emitted from charging apparatus 210 toward the parent droplets 201 of stream 140. Electrode 220 is electrically connected to an electrode control 225, which is configured to provide power to electrode 220 and control at least one electric field emitted therefrom.
In one operational embodiment, electrode 220 is disposed adjacent to charging apparatus 210 about crosswise to a centerline of beam 219 at sufficient distance to allow stream 140 to pass through the area between electrode 220 and charging apparatus 210. Furthermore, parent droplets 201 are created having diameters of 250 μm and flowing coaxially at about 10 m/s, causing the centers of each parent droplet 201 to be spaced about 0.67 mm apart. In this operational embodiment, atmospheric operation of a 30 kV charging apparatus 210 may result in an about conically shaped beam 219 profile having a half angle of about 45°. In alternate embodiments, the orientation of electrode 220, as well as the voltage differential between electrode 220 and charging apparatus 210, may be adjusted to modify the angle of beam 219. In one embodiment, such a profile of beam 219 may be characterized with respect to the current of beam 219. For example, the current of beam 219 may be used to focus beam 219 within a desired radial distance from a centerline of such beam 219 to maximize charging of parent droplets 201 and spraying of daughter droplets 201′.
A window barrier 212 may be positioned between charging apparatus 210 and parent droplets 201 of stream 140 to protect charging apparatus 210 from contamination by parent droplets 201, daughter droplets 201′, and other particulates associated therewith. In one embodiment, window barrier 212 is a charge permeable membrane configured to physically separate charging apparatus 210 from parent droplets 201 while allowing charges 213 to pass therethrough. In one configuration, window barrier 212 and electrode 220 form a charging space 230 (i.e., a charging zone for parent droplets 201) wherein parent droplets 201 of stream 140, and portions thereof, are irradiated with charges 213 from beam 219.
Charging space 230 may alternatively be created between the discharge end of charging apparatus 210 and electrode 220 if window barrier 212 is not incorporated in particle generation apparatus 100. In either scenario, charging space 230 may be filled with a gas such as dry nitrogen at about atmospheric pressure. In an alternative embodiment, charging space 230 may be filled with sulfur hexafluoride (“SF₆”) to prevent loss of charges 213, if any, from parent droplet 201 caused by Corona discharge (i.e., an electrical discharge initiated by the ionization of a fluid surrounding a conductor that occurs when the potential gradient exceeds a certain value). Similarly, Corona discharge may be minimized, or entirely eliminated, by conducting charging in charging space 230 under vacuum conditions.
In the absence of drag, the voltage required to achieve a predetermined velocity of daughter droplets 201′ charged in accordance with the equilibrium spray theory may be calculated by the following equation:
V=πa ₀ρ(dv)²/(12q) (Eq. 5)
wherein V is the voltage required to achieve the predetermined velocity v, a₀is the first Bohr radius, ρ is a density of daughter droplet 201′, d is a diameter of daughter droplet 201′, v is the velocity of daughter droplet 201′, and q is an elemental charge of daughter droplet 201′.
Biasing of the target of the daughter droplets 201′ of plume 150 using a conventional power supply supplying a voltage in the range of about 10 kV to about 30 kV may aid in the distribution pattern of daughter droplets 201′. However, since the space charge electric field strength of plume 150 is multiple MV/m, the electric field generated by such biasing of the target may be ignored during calculation of the velocity of daughter droplets 201′. In an illustrative embodiment, the space charge electric field of plume 150 varies from a magnitude of 5 MV/m to 10 MV/m at the point at which daughter droplets 201′ are created to a lesser magnitude at points ranging from a radial distance of 10 to 20 centimeters (“cm”) from the centerline of plume 150. In this embodiment, daughter droplets 201′ will be subjected to a voltage, V, of 1.5 MV within 0.5 m of the centerline of plume 150. In an embodiment wherein fluent material 111 is Wood's metal, such a voltage V results in a velocity of daughter droplet 201′, v, of 33 m/s. However, in alternate embodiments, smaller parent droplets 201 or higher charging levels of charging apparatus 210, may be incorporated to achieve a greater velocity v of daughter droplets 201′. In yet another alternate embodiment, parent droplets 201 may be charged to the Coulomb (Rayleigh) bursting limit to achieve greater velocities v of daughter droplets 201′.
In summary, particulate generation apparatus 100 (FIG. 1) includes a stream generator 105 to provide parent droplets 201 of fluent material 111 in a stream 140 that are electrically isolated from each other, are about uniform in size, and are separated about equidistantly apart. Particulate generation apparatus 100 further includes a charging apparatus 210 configured to irradiate and individually charge each of the parent droplets 201 with a beam 219 of charges 213 sufficient to initiate electrostatic atomization as per the equilibrium spray theory or at least one Coulomb (Rayleigh) bursting process to atomize parent droplets 201 into daughter droplets 201′ forming a plume 150 of fluent material 111.
Referring now to FIG. 3, illustrated is a flow diagram of one embodiment of a method 300 to generate particulates in accordance with embodiments of the invention. Method 300 may start at 302, for example, by a user operating an apparatus such as the particulate generation apparatus 100 depicted in FIG. 1. At 306, a supply of fluent material 111 is received for processing.
At 310, such fluent material 111, or a portion thereof, is formed into a stream, possibly a coaxial stream 140, of parent droplets 201. Parent droplets 201 may be formed in a variety of ways. For example, a fast acting valve drip pressure feed system as described above with respect to FIGS. 1 and 2 may be incorporated. Alternatively, a vibrating nozzle may be configured to introduce instabilities into a stream of parent droplets 201. In yet another embodiment, an oscillatory pressure feed system may generate a stream of parent droplets 201. Or, unstable fluidic nozzles capable of providing streams 140 of parent droplets 201 without utilizing moving parts may be used. In yet another alternate embodiment, the stream generator includes a plurality of stream generators to create a planar array of parallel streams 140 of parent droplets 201 to achieve faster creation of larger quantities of fluent material 111 particulates. In essence, the present invention may be utilized with any method of forming a stream of parent droplets 201, coaxial or otherwise, without departing from the scope of the present invention.
In one configuration, a stream 140 of parent droplets 201 of fluent material 111 is provided below a laminar flow limit of about 10 m/s to allow such fluent material 111 to break up into approximately uniform parent droplets 201 forming a stream 140 of such parent droplets 201. In one configuration, such stream 140 includes catenary-like fluid filaments that are detached from parent droplets 201 and flow coaxially within stream 140. Such catenary-like fluid filaments, when irradiated by a source such as beam 219, contribute to the formation of daughter droplets 201′.
In one illustrative example at 310, a stream 140 of parent droplets 201 is positioned in a vertical flow though a charging space such as charging space 230 (FIG. 2). Such charging space 230 may be filled with a gas to create a dry nitrogen atmosphere at about atmospheric pressure. However, the present invention is not so limited. Alternate embodiments are envisioned in which the stream 140 of parent droplets 201 is positioned for horizontal flow. In addition, charging space 230 may be filled with a gas other than dry nitrogen, or not filled with a gas at all. Furthermore, the ambient atmospheric pressure may range from a vacuum to supra-atmospheric pressures.
At 314, the stream of parent droplets 201 is irradiated with charges (e.g., charges 213) via a charge source (e.g., charging apparatus 210) to charge at least some parent droplets 201 with a predetermined net charge. In one embodiment, providing such predetermined net charge is accomplished by incorporating a charging source sufficient to charge each parent droplet 201 to initiate electrostatic atomization. In another embodiment, such predetermined net charge is accomplished by incorporating a charging source sufficient to charge each parent droplet 201 to selectively initiate electrostatic atomization or at least one Coulomb (Rayleigh) bursting process.
At 318, individual parent droplets 201 are independently charged with a sufficient net charge to initiate electrostatic atomization and/or one or more Coulomb (Rayleigh) bursting processes, breaking such charged parent droplets 201 apart into daughter droplets 201′ to form a plume 150 of particulates. For example, as discussed above with respect to equation 4, generation of daughter droplets 201′, each having a diameter of about 40 μm, from parent droplets 201 having a diameter of approximately 250 μm, via electrostatic atomization requires each parent droplet 201 to be charged with approximately 1.84×10⁸electrons.
To maximize charge injection, a profile of the charging source may be graphed. In one embodiment, a radial spread of beam 219 may be characterized relative to the normalized current of the charging source to determine the optimum current range for the charging source (i.e., the current range required to achieve maximize charge injection). For example, FIG. 4 is a graph 400 illustrating a profile 403 of a charging source in atmospheric conditions in accordance with one embodiment of the invention. In the instant case, such beam 219 is provided by a charging apparatus, such as charging apparatus 210 discussed with respect to FIG. 2, set to a voltage of beam 219 of about 20 kV. Graph 400 is defined by an x-axis 401 indicative of a radial distance from about a centerline of beam 219. A Y-axis 402 is indicative of normalized beam current. As illustrated, a graphical line 403 graphs the change in radial distance of beam 219 from a centerline of such beam 401 as normalized beam current 402 is varied. For example, at a normalized beam current 402 of about 1 μA, a radial distance of charges 213 about a centerline of beam 219 is about 0 mm. Whereas, at a normalized beam current of about 0.1 μA, the radial distance of beam 219 is about 7 mm from such centerline.
Referring back to FIG. 3, at 322, parent droplet 201 has received a net charge at 318 capable of causing electrostatic atomization of such charged parent droplets 201 into daughter droplets 201′ to form a plume 150 of daughter droplets 201′ of fluent material 111, such as plume 150 illustrated in FIGS. 1 and 2. In one embodiment, daughter droplets 201′ freeze to form powder. Alternatively, if, after completion of step 322, daughter droplets 201′ have not been completely atomized, such daughter droplets 201′ may be subjected to further charging (such as that discussed with respect to 318) to perform further atomization of such daughter droplets 201′. Such recharging may be performed one or more times, as required to fully atomize the resulting droplets. Method 300 then proceeds to 326. At 326, if method 300 is finished, method 300 proceeds to 330 and ends. However, if at 326, method 300 is not finished (i.e., the user desires generation of additional particulates), method 300 proceeds to 306.
In summary, method 300 includes processing received fluent material 111 to generate a stream 140 of parent droplets 201. Parent droplets 201 are electrically isolated and about uniform in size and distance apart. Method 300 further includes independently irradiating about each of the parent droplets 201 with charges 213. Method 300 further includes charging each of the parent droplets 201 a net charge. Such net charge N_cis sufficient to cause electrostatic atomization of parent droplets 201 to form daughter droplets 201′ therefrom. Such daughter droplets 201′ form a plume 150 of fluent material 111.
It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.

Claims

1. An apparatus for generating a plurality of particulates comprising:

at least one dispensing device for generating at least one of the group consisting of a stream of droplets, a coaxial stream of droplets, a planar array of parallel droplet streams, and combinations thereof; and

at least one charge injection device adjacent a discharge of said at least one dispensing device for inducing at least one charge to at least a portion of said plurality of said droplets dispensed from said at least one dispensing device;

wherein said at least one charge is of sufficient magnitude to initiate electrostatic atomization as per at least one of the group consisting of an equilibrium spray theory, a Coulomb bursting process, and combinations thereof.

2. An apparatus according to claim 1, wherein said droplets are formed from at least one of the group consisting of Wood's metal, Inconel®, cobalt, chromium, nickel, titanium, Octoil®, alloys, superalloys, refractory metals, a high temperature liquid metal, a high temperature fluent material, and combinations thereof.

3. An apparatus according to claim 1, wherein said droplets are at least one of the group consisting of electrically isolated from each other, substantially uniform in size, about equidistantly separated, and combinations thereof.

4. An apparatus according to claim 1, wherein said electrostatic atomization of said droplets causes at least a portion of said particulates to disperse.

5. An apparatus according to claim 1, wherein said electrostatic atomization of said droplets results in said plurality of said particulates having approximately uniform predetermined diameters.

6. An apparatus according to claim 1, wherein said particulates are formed of at least one of the group consisting of powder, fluent materials, and combinations thereof.

7. An apparatus according to claim 1, wherein said inducing at least one charge to said plurality of said droplets includes independently irradiating each of said droplets with said charges.

8. An apparatus according to claim 1, wherein said at least one dispensing device comprises the sub-components of:

at least one reservoir for holding or storing a fluent material; and

at least one stream generator coupled to said reservoir for receiving said fluent material from said reservoir and generating at least one of the group consisting of said stream of droplets, said coaxial stream of droplets, said planar array of parallel droplet streams, and combinations thereof.

9. An apparatus according to claim 8, wherein said at least one stream generator controls a gravitational flow of said fluent material from said reservoir to create said at least one of the group consisting of said stream of droplets, said coaxial stream of droplets, said planar array of parallel droplet streams, and combinations thereof.

10. An apparatus according to claim 1, said apparatus further comprising:

at least one vacuum source coupled to said at least one dispensing device for evacuating an area surrounding a fluent material contained in said dispensing device.

11. An apparatus according to claim 1, said apparatus further comprising:

at least one pressurization source coupled to said at least one dispensing device for pressurizing an area surrounding a fluent material contained in said dispensing device.

12. An apparatus according to claim 1, said apparatus further comprising:

at least one heating assembly for maintaining a fluent material contained in said at least one dispensing device at or near a predetermined temperature setpoint.

13. An apparatus according to claim 12, wherein said heating assembly includes at least one of the group consisting of a heater, a heat transfer liquid, an insulating jacket, and combinations thereof.

14. An apparatus according to claim 1, wherein an atmosphere surrounding said apparatus is at least one of the group consisting of dry nitrogen containing less than one percent oxygen, an atmosphere maintained at a vacuum pressure, sulfur hexafluoride, and combinations thereof.

15. An apparatus according to claim 1, said apparatus further comprising:

at least one stream heater for heating at least one of said droplets, a fluent material contained within said at least one dispensing device, and combinations thereof;

wherein said heating creates or retains said droplets in molten form prior to said inducing of said at least one charge.

16. An apparatus according to claim 1,

wherein said at least one charge injection device includes at least one device for generating at least one beam of charged particles; and

wherein said at least one beam of charged particles induces said charge to said at least a portion of said droplets.

17. An apparatus according to claim 16, wherein said charged particles are free electrons.

18. An apparatus according to claim 16, wherein said at least one device for generating at least one beam of charged particles is an electron gun.

19. An apparatus according to claim 1,

wherein said at least one charge injection device includes at least one electrode; and

wherein said at least one electrode provides an electric field for performing at least one of the group consisting of attracting at least one of said charges, accelerating at least one of said charges, directing at least one of said charges, and combinations thereof.

20. An apparatus according to claim 1, said apparatus further comprising:

at least one barrier positioned between said at least one charge injection device and said at least one of the group consisting of said stream of droplets, said coaxial stream of droplets, said planar array of parallel droplet streams, and combinations thereof;

wherein said at least one barrier protects said at least one charge injection device from contamination caused by at least a portion of said droplets, at least a portion of said particulates, and combinations thereof.

21. An apparatus according to claim 20, wherein said at least one barrier is a charge permeable membrane.

22. An apparatus according to claim 20, said apparatus further comprising:

at least one electrode;

wherein said droplets are induced with said charge while passing between said at least one barrier and said at least one electrode.

23. An apparatus according to claim 1, said apparatus further comprising:

at least one electrode;

wherein said droplets are induced with said charge while passing between said at least one charge injection device and said at least one electrode.

24. An apparatus according to claim 1, wherein said at least one charge injection device is configured to minimize inter-droplet shielding.

25. A method for generating a plurality of particulates comprising:

receiving at least one fluent material;

generating at least one of the group consisting of a stream of droplets, a coaxial stream of droplets, a planar array of parallel droplet streams, and combinations thereof from said fluent material;

inducing at least one charge to a plurality of said droplets, said charge having sufficient magnitude to initiate electrostatic atomization as per at least one of the group consisting of an equilibrium spray theory, a Coulomb bursting process, and combinations thereof; and

forming said plurality of particulates via said electrostatic atomization;

wherein said plurality of said particulates have approximately uniform, predetermined diameters.

26. A method according to claim 25, wherein said droplets are formed from at least one of the group consisting of Wood's metal, Inconel®, cobalt, chromium, nickel, titanium, Octoil®, alloys, superalloys, refractory metals, a high temperature liquid metal, a high temperature fluent material, and combinations thereof.

27. A method according to claim 25, wherein said droplets are at least one of the group consisting of electrically isolated from each other, substantially uniform in size, about equidistantly separated, and combinations thereof.

28. A method according to claim 25, wherein said particulates are formed of at least one of the group consisting of powder, said fluent material, and combinations thereof.

29. A method according to claim 25, wherein said inducing at least one charge to said plurality of said droplets includes independently irradiating each of said droplets with charges.

30. A method according to claim 25,

wherein said fluent material is received from at least one reservoir; and

wherein said at least one of the group consisting of said stream of droplets, said coaxial stream of droplets, said planar array of parallel droplet streams, and combinations thereof are generated by at least one stream generator.

31. A method according to claim 30, wherein said at least one stream generator controls a gravitational flow of said fluent material from said reservoir to create said at least one of the group consisting of said stream of droplets, said coaxial stream of droplets, said planar array of parallel droplet streams, and combinations thereof.

32. A method according to claim 25, said method further comprising:

evacuating an area surrounding said at least one fluent material.

33. A method according to claim 25, said method further comprising:

pressurizing an area surrounding said at least one fluent material.

34. A method according to claim 25, said method further comprising:

maintaining said at least one fluent material at or near a predetermined temperature setpoint.

35. An method according to claim 25, wherein at least a portion of an atmosphere in which at least a portion of said method is performed is at least one of the group consisting of dry nitrogen containing less than one percent oxygen, an atmosphere maintained at a vacuum pressure, sulfur hexafluoride, and combinations thereof.

36. A method according to claim 25, said method further comprising:

heating at least one of said at least one fluent material, said droplets, and combinations thereof,

37. A method according to claim 25, wherein said inducing of said at least one charge is performed by irradiating at least a portion of said droplets with at least one beam of charged particles.

38. A method according to claim 37, wherein said charged particles are free electrons.

39. A method according to claim 37, wherein said at least one beam of charged particles is generated by an electron gun.

40. A method according to claim 25, said method further comprising:

performing at least one of the group consisting of attracting at least one of said charges, accelerating at least one of said charges, directing at least one of said charges, and combinations thereof via at least one electrode.