Abstract:
A system and method for synthesizing nanopowder which provides for precursor material ablation from two opposing electrodes that are spaced apart within a gaseous atmosphere, where a plasma is created by a high power pulsed electrical discharge between the electrodes, such pulse being of short duration to inertially confine the plasma, thereby creating a high temperature and high density plasma having high quench and/or reaction rates with the gaseous atmosphere for improved nanopowder synthesis.

Description:
PRIORITY DATE CLAIM 
     This patent application is a continuation of U.S. patent application Ser. No. 10/455,292, “RADIAL PULSED ARC DISCHARGE GUN FOR SYNTHESIZING NANOPOWDERS,” with named inventors Kurt Schroder and Doug Jackson, filed Jun. 5, 2003, now U.S. Pat. No. 6,777,639 which claims the benefit of the earlier filing date of the U.S. Provisional Patent Application Ser. No. 60/388,200, filed Jun. 12, 2002, with named inventors Kurt Schroder and Doug Jackson, and all of which are assigned to the assignee of the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Systems for producing nanopowders are known which ionize an inert gas to create a plasma in a reactor vessel that vaporizes a precursor material. A quench and/or reaction gas is injected into the vaporized precursor material to produce nanopowders having a desired composition. U.S. Pat. No. 5,514,349 discloses a transferred arc system similar to a tungsten inert gas (TIG) welder, wherein a single rod of metal precursor material acts as an anode, and is vaporized by feeding the anode past a nonconsummable tungsten cathode to expose the anode to a discharge arc. A gas in then injected into the vaporized material to quench and form the nanopowder. The transferred arc system is designed to avoid erosion of the tungsten electrode. 
     U.S. Pat. No. 6,472,632 discloses another method to produce nanopowders which uses a prior art axial electrothermal gun, as illustrated in  FIG. 1 . The axial electrothermal gun or axial gun  10  is shown with a breech electrode  11 , an annular muzzle electrode  12 , and a barrel  13  having a hollow bore  14 . The breech electrode  11  fits into and pneumatically seals one end of the hollow bore  14 . The muzzle electrode  12  is attached to and substantially axially aligned with the barrel  13 . The breech electrode  11  further is connected by way of a conducting wire  15  to the negative terminal of a high-power, pulsed discharge power supply  16 , the positive terminal of which is connected by way of a conducting wire  17  to the muzzle electrode  12 . Unlike the transferred arc system, the polarity of the electrodes in the axial gun  10  is not important, and the device can be operated with the polarity reversed. This axial gun embodiment has been successful in producing moderate volumes of nanopowder in the 10–100 nanometer range. 
     In operation, the power supply  16  is energized to create an electric field between the breech electrode  11  and the muzzle electrode  12 , and thereby discharge a high power pulsed arc  18  between the electrodes. The discharge of the pulsed arc  18  ablates the muzzle electrode  12 , which is the primary source for plasma. More particularly, the material removal rate from the muzzle electrode  12  is a factor of 10–100 greater than the material removal rate from the breech electrode  11 . 
     The pulsed arc discharge in the axial gun  10  occurs in an interval of time too short to allow the plasma to fully expand and equilibrate in the surrounding atmosphere (inertial confinement), and in an area physically confined by the walls of the bore  14  (physical confinement). The combined confinement creates a high temperature (˜50,000° K.), high density (˜10 20 /cm 3 ) plasma which is not ordinarily obtainable in other plasma based systems such as those disclosed by U.S. Pat. Nos. 5,514,349; 5,874,684; and 5,851,507. The plasma acts to ablate the muzzle electrode  12  by means of the rapidly exiting high-temperature plasma. The plasma together with the ablated material exits the muzzle  12  under high pressure (˜15,000 PSI) and supersonic velocity. The ablated material thereafter may be quenched by and/or reacted with a surrounding gas such as one or more of air, oxygen, nitrogen, or helium to produce a cloud of nanopowder. 
     Referring to  FIG. 2 , a prior art transferred-arc-discharge process is illustrated in which a tungsten electrode  20  is shielded in a flow of pure inert gas  21  such as Argon, and is principally aligned with a rod  22  of precursor material. The inert gas shield protects the tungsten electrode  20  from erosion and oxidation. The inert gas ionizes to sustain the arc, but does not act to quench or react with material removed from rod  22 . 
     Rod  22  is connected by way of a conducting wire  23  to the positive terminal of a DC power supply  24 , the negative terminal of which is connected by way of a conducting wire  25  to the tungsten electrode  20 . The tungsten electrode  20  is charged negatively with respect to the rod  22  to retard the absorption of heat and rate of erosion of the tungsten electrode. With these polarities, the material removal rate from the rod  22  is a factor of 100–1000 times greater than that of the tungsten electrode  20 . 
     In operation, the DC power supply  24  is energized to effect a continuous DC low power arc discharge between the tungsten electrode  20  and the rod  22 . The arc discharge erodes rather than ablates the rod  22 . The material so produced is conveyed away from the vicinity of the arc discharge by the flow of the pure inert gas  21 , and injected with a quench and/or reaction gas(es)  21 , such as argon, helium and oxygen, to form the nanopowder. 
     From the above, it should be readily apparent that material removal in the operation of a transferred arc process, or an axial gun process is primarily from a single electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the novel aspects and features of the invention are defined in the appended claims, the principles of the invention, illustrative embodiments, and preferred modes of use are best understood by reference to the Description Of Preferred Embodiments in conjunction with the following drawings, in which: 
         FIG. 1  is a graphic diagram of an axial electrothermal gun in the prior art, which is comprised of a breech electrode, a muzzle electrode, and a hollow bore; 
         FIG. 2  is a graphic diagram of a transferred-arc-discharge process in the prior art, which is comprised of a tungsten electrode that is charged negative with respect to an anode electrode composed of precursor material; 
         FIG. 3  is a graphic diagram of a radial electrothermal gun in accordance with the present invention, wherein an anode and a cathode of the gun are each composed of a precursor material, and are substantially axially aligned but spaced apart opposite to each other; and 
         FIG. 4  is a graphic diagram of an alternative embodiment of the present invention wherein the anode and cathode electrodes are placed within an ablative body, but are separated from the ablative electrodes by an insulator. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following terms shall have the definitions given below when used in either lower case or with capitalizations in this specification: 
     “Ablation” shall mean the removal of material from a body of precursor material due to the combined effects of heat transfer and mechanical shear caused by high fluid velocities. 
     “Erosion” shall mean the removal of material from a body of precursor material through localized heat transfer, as occurs when an electrical discharge arc attaches to a surface. 
     “Axial Gun” shall mean a pulsed power electrothermal gun, wherein a breech electrode and a muzzle electrode are composed of a precursor material, and are separated by a barrel with a bore. 
     “Radial Gun” shall mean a pulsed power electrothermal gun wherein two opposing electrodes of the gun are composed of a precursor material which is to be ablated, and the electrodes are substantially axially aligned opposite to each other within a gaseous atmosphere. The term “radial gun” is used for convenience only, and is not meant to imply any limitation of the present invention. 
     “Nanopowder” shall mean nanomaterial primarily comprised of nanoparticles which are of a size of 1–500 nanometers (nm). 
     Referring to  FIG. 3 , a radial gun  30  in accordance with the present invention is illustrated, wherein an anode electrode  31  and a cathode electrode  32  (each formed of a precursor material and of uniform cross section) are spaced apart opposite to each other, and substantially axially aligned for maximal production of nanopowders. The electrode composition is typically the metal of the nanopowder being produced, such as but not limited to aluminum, tantalum, titanium, or zirconium. The nanopowder itself may be, but is not limited to, a metal such as aluminum or copper; an oxide such as Al 2 O 3  or TiO 2 ; a nitride such TiN, ZrN, or Ta 2 N; or other metals and metal compounds. Please refer to the article, “Pulsed Wire Discharge for Nanosize Powder Synthesis”, by Weihua Jiang and Kiyoshi Yatsui, IEEE Transactions On Plasma Science, vol. 26, No. 5, pp. 1498–1501 (October 1998). 
     The cathode electrode  32  is connected by way of a conducting wire  33  to the negative terminal of a pulsed power supply  34 , the positive terminal of which is connected by way of a conducting wire  35  to the anode electrode  31 . The pulsed power supply  34  may be of any of plural well known designs (which may vary depending upon the material and material size being produced) that are offered by and ordered from any of numerous well known manufacturers. Except for the pulsed power supply  34 , the radial gun  30  may but need not be contained within a chamber or reactor vessel such as that disclosed in U.S. Pat. No. 5,514,349. The reactor vessel is filled with inert and/or reactive gases including but not limited to Argon, Nitrogen, Oxygen, or a combination thereof. The selection of the gaseous atmosphere is based upon the nanopowder that is desired, and the effect of the gas is to control the expansion and quench rate of the plasma that is created by a high-power pulsed electrical discharge between the anode electrode  31  and the cathode electrode  32 . 
     In operation, the pulsed power supply  34  is energized to effect a high-power pulsed electrical discharge  36  between the cathode electrode  32  and anode electrode  31 . For the current invention, the connection to the electrodes could easily be reversed in polarity without impacting the process. The energy from the discharge melts, vaporizes and ionizes material from the two electrodes to create a high temperature (of the order of 50,000° K), high density metal plasma which continues to sustain the electrical discharge from the pulsed power supply. The combined physical axial confinement provided by the electrodes and the inertial confinement resulting from the short term pulse of the electrical discharge, act to increase the temperature and density of the plasma to heights not ordinarily obtainable by other nanopowder synthesizing processes or apparatus. Since the electrodes impede axial expansion of the plasma, expansion occurs primarily radially as indicated by arrows  37  and  38  of  FIG. 3 . As the plasma expands, additional material is ablated from the electrodes. This additional material also contributes to the overall plasma and helps sustain the electrical discharge between the electrodes. The expanding plasma is forced out of the confines of the radial gun at supersonic speeds. It subsequently undergoes expansion, mixes with the chamber gases, and quenches at a rate of the order of 10 6 ° K/sec to form nanopowder. Depending on the ambient gas, the plasma may also react with the gaseous atmosphere to form compounds as it is being quenched. 
     After each high power electrical discharge, the pulsed power supply  34  is reenergized. Further, after nanopowder is produced from one or more high power, pulsed electrical discharges between the electrodes, the anode electrode  31  and the cathode electrode  32  are indexed toward each other only as required to maintain the production rate. The gas surrounding the electrodes is replenished as necessary to sustain the atmosphere within the reactor vessel. 
     It is to be understood that the above process may occur without the benefit of a reactor vessel or chamber, as the plasma created by the present invention need only be quenched by or reacted with a surrounding gaseous atmosphere to produce a desired nanopowder. 
     
       
         
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                   
                 PRIMARY 
                 RELATIVE 
               
               
                   
                   
                 MATERIAL 
                 AMOUNTS 
               
               
                 SYNTHESIZING 
                   
                 REMOVAL 
                 OF MATERIAL 
               
               
                 DEVICE 
                 ELECTRODE 
                 MECHANISM 
                 REMOVED 
               
               
                   
               
             
             
               
                 Transferred Arc 
                 Tungsten 
                 Arc Erosion 
                   1 
               
               
                   
                   
                 (Minimized) 
               
               
                   
                 FeedStock 
                 Arc Erosion 
                 100–1000 
               
               
                 Axial Gun 
                 Breech 
                 Arc Erosion 
                   1 
               
               
                   
                 Muzzle 
                 Ablation 
                 10–100 
               
               
                 Radial Gun 
                 Electrode 1 
                 Ablation 
                 ~1 
               
               
                   
                 Electrode 2 
                 Ablation 
                 ~1 
               
               
                   
               
             
          
         
       
     
     In Table I above, relative removal rates between electrodes of a same nanopowder synthesis device are shown. Table I is not intended to show comparisons between synthesis devices. 
     Referring to Table I, it is seen that the two electrodes of the radial gun each serve almost equally as sources of ablative material. By way of contradistinction, both the axial gun and the transferred arc synthesizing devices have only one electrode that contributes to any substantial degree to the production of nanopowder. 
     Readily apparent differences between the transferred-arc processes and the radial gun are: (1) polarity reversal is of little to no effect in the radial gun, because the electrode material removal caused by erosion is negligible compared to the material removal from ablation; (2) material removal with a radial gun occurs at two electrodes rather than a single electrode; (3) no inert gas shielding is required by the radial gun to protect against electrode erosion; and (4) the power supply of the radial gun is pulsed rather than continuous. 
     The radial electrothermal gun also provides the following additional advantages over the transferred-arc processes for synthesizing nanopowder: 
     i. Increased quench rate of 10 6 –10 8  degrees Kelvin/sec. with the radial electrothermal gun (as compared to 10 4  degrees Kelvin/sec. with the forced convection quenching used with the transferred arc processes) allows smaller sized nanopowder to be synthesized in higher aerosol densities; 
     ii. More uniform time-temperature nanoparticle synthesis, i.e. nucleation and growth, histories are provided with a radial gun which result in more uniform nanoparticle sizes; 
     iii. The increased plasma temperature occurring in a radial gun allows the synthesis of nanopowder from material having very high melting and boiling points; 
     iv. Less energy is used to produce greater quantities of nanopowder because the short duration of the pulsed discharge does not allow sufficient time for thermal diffusion. Consequently, there are fewer thermal (energy) losses to the surrounding environment; and 
     v. Lower agglomeration of nanopowder occurs with the radial gun. 
     vi. Since both the cathode and the anode of the radial gun are made out of the precursor or ablative material, no contamination is introduced from the electrodes. Inherent to the transferred arc process is the contamination that comes from the tungsten electrode. 
     The radial electrothermal gun provides advantages including the following over the axial electrothermal gun in synthesizing nanopowder: 
     i. No high pressure seals are needed since the only physical confinement comes from the electrodes; 
     ii. No insulating bore is required. Thus, a source of both major cost, and impurities in the production of nanopowder is removed; 
     iii. The electrodes can be large diameter rods requiring almost no machining, thus further reducing cost; and 
     iv. The electrodes are nearly all consumed. 
     A further comparison among the radial gun of the present invention, and prior art axial guns and transferred arc processes appears, in Table II below. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 COMPARISON BETWEEN PRIOR ART AND RADIAL GUN 
               
             
          
           
               
                   
                   
                   
                 TRANSFERRED 
               
               
                   
                 RADIAL GUN 
                 AXIAL GUN 
                 ARC DISCHARGE 
               
               
                   
               
               
                 Peak Discharge 
                    10 8   
                    10 8   
                    10 5   
               
               
                 Power 
               
               
                 (Watts) 
               
               
                 Duty Cycle 
                 Pulse 
                 Pulse 
                 Continuous 
               
               
                 Pulse Length (sec.) 
                    10 −3   
                    10 −3   
                 Infinite 
               
               
                 Bore 
                 None 
                 Yes 
                 None 
               
               
                 Arc Confinement 
                 Physical In Axial 
                 Physical In Radial 
                 None 
               
               
                   
                 Direction, Inertial In 
                 Direction, Inertial In 
               
               
                   
                 Radial Direction 
                 Axial Direction 
               
               
                 Arc Temperature 
                 50,000 
                 50,000 
                 10,000 
               
               
                   0 K 
               
               
                 Plasma Quench 
                 Supersonic Expansion 
                 Supersonic 
                 Externally Forced 
               
               
                 Mechanism 
                 Into Surrounding 
                 Expansion 
                 Convection 
               
               
                   
                 Atmosphere 
                 Into Surrounding 
               
               
                   
                   
                 Atmosphere 
               
               
                 Plasma Expansion 
                 Primarily Radial 
                 Axial 
                 N/A 
               
               
                 Between The 
               
               
                 Electrodes 
               
               
                 Electrode Ablation 
                 Both Electrodes 
                 Primarily Muzzle 
                 None. Tungsten 
               
               
                   
                   
                 Electrode, Lesser 
                 Electrode Erosion Is 
               
               
                   
                   
                 Extent Breech 
                 Minimized. Precursor 
               
               
                   
                   
                   
                 Material is eroded. 
               
               
                 Inert Gas Electrode 
                 No 
                 No 
                 Yes 
               
               
                 Shielding 
               
               
                 Effect Of Electrode 
                 Little To No Effect 
                 Little To No Effect 
                 Non-Consumable 
               
               
                 Polarity Reversal 
                   
                   
                 Electrode Erosion 
               
               
                   
                   
                   
                 And Nanopowder 
               
               
                   
                   
                   
                 Contamination. 
               
               
                   
               
             
          
         
       
     
     The radial gun embodiment of  FIG. 3  provides a substantial improvement in cost and efficiency. The high power (megawatt), electric discharge of the radial gun, however, may occur from time to time at other than the center of the electrodes. In this event, the resistance of the arc is decreased, and less energy may be added to the arc discharge for a given arc current. The resulting plasma may be at a lower temperature and/or lower density than otherwise could be achieved, production rates are decreased, and the material produced is of a non-uniform quality. A solution to this problem is provided by the radial gun embodiment of  FIG. 4 . 
     Referring to  FIG. 4 , an alternative embodiment of the present invention is illustrated in which a first composite electrode  40  is comprised of a solid anode electrode  41  which is seated within a wider annular ablative body  42 . The anode electrode  41  is electrically isolated from the annular ablative body  42  by an annular insulator body  43 . Similarly, a second composite electrode  46  is comprised of a solid cathode electrode  47  which is seated within a wider annular ablative body  48 . As before, the cathode electrode  47  is electrically isolated from the annular ablative body  48  by an annular insulator body  49 . 
     The anode electrode  41  is connected electrically by way of a conducting wire  50  to the positive terminal of a high power, pulsed power supply  51 , the negative terminal of which is electrically connected by way of a conducting wire  52  to the cathode electrode  47 . The composite electrodes  40  and  46  are separated by an axial distance  53 . 
     The anode electrode  41 , cathode electrode  47 , annular ablative body  42 , and annular ablative body  48  may be of the same material. The annular ablative bodies  42  and  48  also may be of materials different from that of the anode electrode  41  and the cathode electrode  47 , and even may be nonconductors since their purpose is to provide axial physical confinement of the plasma and to be ablated. In the event the annular ablative bodies  42  and  48  are nonconductors, no annular insulator is required between the ablative bodies and the electrodes. 
     As illustrated in  FIG. 4 , the solid anode electrode  41  and the solid cathode electrode  47  have round uniform cross-sections. The electrodes may be composed of metals including, but not limited to, aluminum, copper, or iron. The annular ablative bodies  42  and  48  are hollow, and may be composed of the same materials as the electrodes, as well as nonconductive materials. The annular insulator bodies  43  and  49  respectively electrically isolate the anode electrode  41  and the cathode electrode  47  from the annular ablative bodies  42  and  48 . 
     In operation, the pulsed power supply  51  is energized to cause a high-power, pulsed electrical discharge between the cathode electrode  47  and the anode electrode  41 . The discharge arc will attach to the cathode and anode because they are conductors. As the cathode electrode  47  and anode electrode  41  are respectively located in the center of the composite electrodes  46  and  40 , which are characterized by diameters that are larger than those of the cathode and anode, the discharge arc can be said to be physically confined to the center of the composite electrodes to a greater extent than provided by the anode and cathode electrodes of the same diameter in the radial gun embodiment of  FIG. 3 . The construction of the composite electrodes  40  and  46  thus dramatically increase the probability of the discharge arc attaching to the tip areas of both the anode electrode  41  and the cathode electrode  47 . The added confinement of the electrical discharge increases its energy to melt, vaporize and ionize material ablated from the anode electrode  41 , the cathode electrode  47 , and the two annular ablative bodies  42  and  48 . Further, the composite electrodes  40  and  46  prevent the axial expansion of the plasma, and the inertial confinement afforded by the pulsed electrical discharge deters expansion of the plasma radially outward as indicated by arrows  54  and  55  of  FIG. 4 . A high temperature, high density plasma thus is created. The ablation of the anode electrode  41 , the cathode electrode  47 , and the annular ablative bodies  42  and  48 , by the high velocity exit of the plasma radially outward, produces nanopowder at an even higher rate per discharge than that of the invention embodiment illustrated in  FIG. 3  above. Further, the ablated material is forced out of the radial gun by the plasma at supersonic speed to be quickly quenched by and/or quickly react with the surrounding gas within the reactor vessel to produce nanopowder. 
     Although the preferred embodiments of the invention have been disclosed in detail, various substitutions, modifications, and alterations can be made without departing from the spirit and scope of the invention as defined in the claims.