Electric heating – Metal heating – By arc
Reexamination Certificate
2003-06-05
2004-08-17
Van, Quang T. (Department: 3742)
Electric heating
Metal heating
By arc
C219S121470, C219S121520
Reexamination Certificate
active
06777639
ABSTRACT:
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 (~15,000PSI) 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.
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Weihua Jiang and Kiyoshi Yatsui, “Pulsed Wire Discharge for Nanosize Powder Synthesis”, IEEE Transactions On Plasma Science, Oct. 1998, pp. 1498-1501, vol. 26, No. 5, IEEE, United States.
A. Weisenburger, “ Application of Pulsed Power and Microwave Techniques for the Production of Nano-crystalline Powders and Ceramic Bulk Materials”, Published in Metastable, Mechanically Alloyed and Nanocrystalline Materials, year 2000, pp 545-550, Materials Science Forum, vols. 343-346, Trans Tech Publishers Ltd., Switzerland.
Jackson Doug K.
Schroder Kurt A.
Lester Gerald E.
Nanotechnologies, Inc.
Ross Spencer Garsson Winstead Sechrest & Minick P.C.
Van Quang T.
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