WO2006012165A2

WO2006012165A2 - Plasma jet generating apparatus and method of use thereof

Info

Publication number: WO2006012165A2
Application number: PCT/US2005/022147
Authority: WO
Inventors: Oleg Pavlovich Solonenko; Victor Ivanovich Kuzmin; Vlaadimir E. Belashchenko
Original assignee: H.C. Starck Inc.
Priority date: 2004-06-25
Filing date: 2005-06-22
Publication date: 2006-02-02
Also published as: WO2006012165A3

Abstract

The present invention is a plasma jet generating apparatus that includes a cathode module having a cathode, an anode module having a first passage therethrough and an intermediate module positioned between the cathode and the anode module. The intermediate module includes at least one insert electrically isolated from the anode and cathode modules and defining a second passage in fluid communication between the cathode and the first passage. The first passage can initially converge and then, optionally, diverge in a direction away from the intermediate module. Also or alternatively, the second passage can optionally diverge in a direction toward the anode module. A pilot insert can be positioned between the anode module and the interelectrode insert for the purpose of initiating an electrical arc. A feeding module can be positioned to feed material adjacent the end of the anode module opposite the intermediate module for treatment by the plasma jet.

Description

PLASMA JET GENERATING APPARATUS AND METHOD

OF USE THEREOF

BACKGROUND OF THE INVENTION

[0001] Field of the Invention

[0002] The present invention relates to plasma treatment of materials and, more particularly, to a plasma jet generating apparatus and method of use thereof for plasma treatment of materials. [0003] Description of Related Art

[0004] Plasma jet generating apparatus, or plasma guns, are well known in the art. In operation of these plasma guns, a plasma gas is introduced into a cylindrical passage having a cathode and an anode on or adjacent opposite ends thereof. In the presence of the plasma gas in the passage, an electrical arc is generated between the anode and the cathode. In response to this electrical arc, the plasma gas in the passage ionizes thereby forming a heated plasma jet that exits the anode end of the passage. This heated plasma jet can be utilized for treatment of material brought into close contact with the plasma jet or for a thermal spraying of material. [0005] Generally speaking, it would be desirable to provide a plasma gun having the following advantages over prior art plasma guns: reduced pulsation of the plasma jet; stable operation within a wide range of plasma gas flow rates; extended operating life of the parts of the plasma gun that are subject to erosion and wear; reduced tangential component of plasma velocity; and improved controllability of material injection. The present invention is a plasma gun, or plasma jet generating apparatus, having one or more of these advantages.

SUMMARY OF THE INVENTION

[0006] The invention is a plasma jet generating apparatus that includes a cathode module having a cathode, an anode module having a first passage therethrough, and an intermediate module positioned between the cathode module and the anode module. The intermediate module has at least one insert electrically

ancfthe cathode modules and defining a second passage in fluid communication between the cathode module and the first passage. The first passage converges in a direction away from the intermediate module. [0007] The apparatus can include a feed module for feeding material to an end of the first passage opposite the intermediate module. The feed module can either feed material to inside the first passage or outside the first passage. [0008] Desirably, the convergence of the first passage resides closer to the intermediate module than an end of the first passage opposite the intermediate module. The first passage can initially converge and then diverge in a direction away from the intermediate module. Where the first passage converges, a wall of the first passage converges at an angle between 10° and 20° with respect to an axis of the first passage. More desirably, the wall of the first passage converges at an angle between 12.5^° and 17.5° with respect to the axis of the first passage. [0009] The apparatus can also include a plasma gas passage for introducing a plasma gas into the first and second passages. Means can be provided for causing an electrical arc to extend between a wall of the anode module that defines the first passage and the cathode via the second passage in the presence of the plasma gas thereby forming a plasma jet. [0010] The apparatus can also include a secondary gas feeding passage for introducing into the first passage a secondary gas which causes diffusion of the electrical arc on the wall of the first passage. The apparatus can include means for causing swirling of the secondary gas in the first passage. The secondary gas can be either an inert gas, such as argon, or a hydrocarbon gas such as methane or natural gas. [0011] The intermediate module can include a fluid passage for receiving a flow of cooling fluid for cooling each insert of the intermediate module. [0012] The second passage can be defined by a plurality of inserts that are electrically insulated from each other. The second passage can be generally cylindrical adjacent the cathode module and increase in diameter in a direction toward the anode module. The second passage can either step increase in diameter or have a generally conical profile. [0013] The cathode can either be rod-shaped or flat.

©M"ferther include a shielding gas passage for introducing a shielding gas, such as argon, into contact with the cathode. Means can be provided for causing swirling of the shielding gas adjacent the cathode. [0015] The invention is also a plasma jet generating apparatus that includes a cathode module, an anode module defining a first passage therethrough and an intermediate module positioned between the cathode module and the anode module. The intermediate module can include at least one interelectrode insert defining a second passage in fluid communication between the cathode module and the first passage. The second passage diverges in the direction toward the anode module.

[0016] A feed module can be provided for feeding material to an end of the first passage opposite the intermediate module. The feed module can feed the material to either the inside or outside of the first passage. [0017] The first passage can converge and then, optionally, diverge in a direction away from the intermediate module.

[0018] A pilot insert can be positioned between the cathode module and the interelectrode insert. The pilot insert can define a third passage in fluid communication with the first and second passages. Means can be provided for feeding a plasma gas into the first, second and third passages. An electrical arc generating means can be connected to the cathode module, the pilot insert and the anode module. In the presence of plasma gas in at least the second and third passages, the electrical arc generating means can cause a pilot arc to be generated between the cathode module and the pilot insert. Once the pilot arc is generated, the electrical arc generating means can cause a primary electrical arc to be generated between the cathode wall and a wall of the anode module that defines the first passage.

[0019] Lastly, the invention is a method of plasma treating material that includes (a) introducing a plasma gas into a passage defined by a cathode and a pilot insert adjacent one end of the passage and an anode adjacent the other end of the passage; (b) igniting a pilot electrical arc between the cathode and the pilot electrode thereby ionizing the plasma gas; (c) causing a main electrical arc to form between the cathode and the anode from the pilot electrical arc thereby generating . .

^La

a material into the plasma jet adjacent the end of the passage opposite the cathode.

[0020] A shielding gas, such as argon, can be introduced adjacent the cathode, at a rate between 0.1 grams per second and 0.5 grams per second, during generation of the plasma jet. The plasma gas can be one or a combination of argon, nitrogen, air, hydrogen, helium and CO₂.

[0021] The anode can include an interior wall that defines the anode's portion of the passage. At least a portion of the interior wall of the anode at least partially faces the cathode. The main electrical arc contacts the anode on the portion of the wall that at least partially faces the cathode.

[0022] A secondary gas can be introduced into at least the anode portion of the passage, at a rate between 10% and 40% of a mass flow of the plasma gas, for causing diffusion of the main electrical arc on a wall of the anode portion of the passage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Fig. 1 is an end view of a plasma jet generating apparatus in accordance with the present invention;

[0024] Fig. 2 is a sectional view taken along lines H-II in Fig. 1 ; [0025] Fig. 3 is a close-up view of a portion of an intermediate module of the plasma jet generating apparatus shown in Fig. 2;

[0026] Fig. 4 is a partial sectional view taken along lines IV-IV in Fig. 1 ;

[0027] Fig. 5 is a block diagram of an ignition circuit coupled to a schematic illustration of the plasma jet generating apparatus shown in Fig. 2; [0028] Fig. 6a is a cross-sectional side view of a prior art anode body of the type that can be utilized in the plasma jet generating apparatus shown in Fig. 2;

[0029] Fig. 6b is a cross-sectional side view of a first embodiment anode body in accordance with the present invention that can be utilized with the plasma jet generating apparatus shown in Fig. 2; [0030] Fig. 6c is an enlarged view of a portion of the anode body of the plasma jet generating apparatus shown in Fig. 2; _ .

^■[0t)3lp %ilr 7a "'"afil" W ''ire cross-sectional side views of two alternate embodiments of interelectrode inserts that can be utilized in place of the interelectrode inserts of the plasma jet generating apparatus shown in Fig. 2; [0032] Fig. 8 is an alternate embodiment cathode and cathode holder that can be utilized in place of the cathode and cathode holder shown in the plasma jet generating apparatus shown in Fig. 2;

[0033] Fig. 9 is a cross-sectional side view of an alternate embodiment anode module for feeding secondary gas(es) into a passage of the anode module; and [0034] Fig. 10 is a cross-sectional side view of an alternate embodiment cathode module for feeding shielding gas(es) to a cathode of the plasma jet generating apparatus shown in Fig. 2.

DETAILED DESCRIPTION OF THE INVENTION [0035] The present invention will be described with reference to the accompanying figures where like reference numbers correspond to like elements. [0036] With reference to Figs. 1 and 2, a plasma gun 2 for generating a plasma jet includes a cathode module 4, an intermediate module 6, an anode module 8 and a feeding module 10. [0037] Anode module 8 includes a passage 12 therethrough defined by an inner wall 14 of an anode body 16 of anode module 8. Passage 12 defines a first passage of plasma gun 2.

[0038] Intermediate module 6 includes a pilot insert 20, having a passage 21 therethrough, positioned adjacent cathode module 4 and one or more interelectrode inserts 22, each including a passage 24 therethrough, positioned between pilot insert 20 and anode module 8. In Fig. 2, intermediate module 6 includes interelectrode inserts 22a-22d having passages 24a-24d, respectively, therethrough. The number of interelectrode inserts 22 shown in Fig. 2, however, is not to be construed as limiting the invention. [0039] Intermediate module 6 includes an internal housing 26 that holds and aligns pilot insert 20 and interelectrode inserts 22a-22d with passages 21 and 24a- 24d in axial alignment with each other and passage 12 of anode body 16. Passages 24a-24d of interelectrode inserts 22a-22d define a second passage of

'Wii^'pissltgM'2'i^' of pilot insert 20 defines a third passage of plasma gun 2. As shown best in Fig. 3, interelectrode inserts 22 are electrically insulated from each other by rings 28 made from electrically insulating material, such as ceramic, and by sealing rubber O-rings 29. [0040] The number of interelectrode inserts 22 and the related length of cathode module 4 can be adjusted based on the desired voltage and power to be utilized with a particular plasma gas and related plasma gas flow rate. For example, an 80-120 IcW plasma gun operating at a voltage between 300 to 400 volts and a current up to 300 amps may include between 7 and 8 interelectrode inserts 22; a 40 to 80 kW plasma gun operating at the same voltage may include between 4 and 6 interelectrode inserts 22; and a 10 to 40 kW plasma gun operating at the same voltage may have up to 3 interelectrode inserts 22. Suitable plasma gases include argon, nitrogen, air, hydrogen, helium, CO₂, or some combination thereof. [0041] With reference to Fig. 4, and with continuing reference to Figs. 1 and 2, cathode module 4 includes a cathode 30 that is braised or press-fit into a cathode holder 32 positioned on an end of a cathode body 36. Cathode holder 32 is desirably made from copper. However, this is not to be construed as limiting the invention. [0042] Cathode body 36 includes a passage 40 therethrough.. Cathode holder 32 is positioned at an end of passage 40 adjacent intermediate module 6. A cylinder 38 including a passage 44 therethrough is positioned in spaced relation with passage 40 of cathode body 36 whereupon an annular passage 42 is formed between cathode body 36 and cylinder 38. [0043] During operation of plasma gun 2, a cooling fluid, such as water, can be introduced into passage 44 via a hose 46. This cooling fluid enters passage 40 and contacts a side of cathode holder 32 opposite cathode 30 thereby cooling cathode 30 and cathode holder 32. This cooling fluid then flows away from cathode holder 32 via annular passage 42. From annular passage 42, the cooling fluid flows through one or more of passages (not numbered) until it enters an annular passage 48 defined by internal housing 26 and an intermediate housing 52 of intermediate module 6. Cooling fluid flowing through annular passage 48 cools pilot insert 20 and interelectrode inserts 22 during operation of plasma gun 2. ψθύl ^!'' ^!?Mi?ahM3 |afei'a|e 48, the cooling fluid flows through one or more passages (not numbered) until it enters an annular passage 50 defined between anode body 16 and an anode housing 54. Cooling fluid flowing through annular passage 50 cools anode module 8 during operation of plasma gun 2. Finally, cooling fluid flows from annular passage 50 to a discharge pipe 56 via one or more passages (not numbered). The flow of cooling fluid through the various passages of cathode module 4, intermediate module 6 and anode module 8 enables these various modules to be maintained at a suitable operating temperature during operation of plasma gun 2. [0045] Feeding module 10 includes a plurality of feeding hoses 60 for feeding material to be treated to a like plurality of injectors 62. Feeding hoses 60 and injectors 62 co-act to feed a material, such as a powdered material, to an end of anode module 8 opposite intermediate module 6. Injectors 62 can be configured to feed the material into passage 12 of anode body 16 or to a point outside of passage 12 of anode body 16 opposite intermediate module 6.

[0046] A flow of plasma gas is supplied to the cathode 30 side of cathode holder 32 via a hose 66 coupled in fluid communication with a pipe 68 which is connected by passages (not numbered) of cathode module 4 to a distribution ring 70. Distribution ring 70 includes tangential slots 71 for swirling the plasma gas introduced into a space 72 formed between cathode holder 32 and pilot insert 20. [0047] With reference to Fig. 5, and with continuing reference to Figs. 1 and 2, an ignition circuit 80 includes a DC power supply 82, a high frequency oscillator 84, capacitors 86 and 88, resistor 90, diode 92, inductor 94, choke 96, contact 98 and resistor 100 connected in the illustrated manner to cathode holder 32, pilot insert 20 and anode body 16.

[0048] The process of forming a main arc 102 between cathode 30 and anode body 16 in the presence of a plasma gas in passage 12 of anode body 8 (the first passage of plasma gun 2), passage 24 of cathode module 4 (the second passage of plasma gun 2) and passage 21 of pilot insert 20 (the third passage of plasma gun 2) will now be described. Initially, contact 98 is moved to its closed position whereupon the positive terminal of DC power supply 82 is connected to pilot insert 20 via choke 96. At a suitable time, oscillator 84 commences oscillating ~wlierettpϋή"ah AC 's^gnaiHs^superimposed between pilot insert 20 and the positive terminal of DC power supply 82, and cathode holder 32 and the negative terminal of power supply 82. In response to this oscillation, a pilot arc 104 (shown in phantom) forms between cathode 30 and pilot insert 20. Once established, pilot arc 104 ionizes the plasma gas that is present thereby forming a low resistance path between cathode 30 and anode body 16 via passages 12, 24 and 21 of anode body 16, interelectrode insert(s) 22 and pilot insert 20, respectively. In response to forming this low resistance path, a high current main arc 102 forms between cathode 30 and anode body 16. After main arc 102 is established, pilot arc 104 may be extinguished by moving contact 98 to its open position whereupon pilot insert 20 is disconnected from power supply 82.

[0049] With reference to Fig. 6a, a prior art anode body 16a has passage 12a therethrough in the form of a cylinder. Figs. 6b and 6c, however, disclose two embodiments of anode body 16 in accordance with the present invention. In the anode body 16b shown in Fig. 6b, an interior wall 14b and, hence, passage 12b of anode body 16b adjacent one end thereof converge with increasing distance from said end. When anode body 16b is part of plasma gun 2, the converging portion of inner wall 14b and, hence, passage 12b of anode body 16b is positioned adjacent intermediate module 6 whereupon inner wall 14b and, hence, passage 12b converge in a direction away from intermediate module 6. It has been observed that the convergence of inner wall 14b improves the stability of main arc 102, especially the root of main arc 102 which will locate at this convergence, thereby improving the homogeneity of the plasma treatment of materials. Desirably, inner wall 14b converges at an angle 106 between 10° and 20°, and more desirably between 12.5°and 17.5°, relative to an axis 108 of passage 12b.

[0050] The portion of passage 12b other than where inner wall 14b converges can be cylindrical as shown in Fig. 6b. However, as shown in Fig. 6c, inner wall 14c and, hence, passage 12c of anode body 16c can converge adjacent the end thereof to be positioned adjacent intermediate module 6, and diverge adjacent the end thereof to be positioned adjacent feeding module 10. The divergence of inner wall 14c and, hence, passage 12c decreases a velocity of the plasma jet where material injection occurs. This lower plasma jet velocity can improve the njedtibn'^:of'^Jm^;aterral^i!;mtfe ^s'tW' plasma jet and increase the efficiency of plasma treatment of the material injected into the plasma jet. Anode body 16 shown in Fig. 2 is the same as anode body 16c shown in Fig. 6c.

[0051] Each anode body 16 is desirably made from copper. However, this is not to be construed as limiting the invention. Each anode body 16 can also have a tungsten sleeve (not shown) lining passage 12 for increasing the usable life of anode body 16.

[0052] With reference to Figs. 7a and 7b, and with continuing to reference to Fig. 2, the first embodiment intermediate module 6 shown in Fig. 2 includes interelectrode inserts 22a-22d having passages 24a-24d, respectively, therethrough. As shown in Fig. 2, the diameter of passage 24d is greater than the diameter of passage 24c which is greater than the diameter of passage 24b which is greater than the diameter of passage 24a. Thus, the passage defined by passages 24a-24d step diverges in a direction away from pilot insert 20 toward anode module 8.

[0053] A second embodiment intermediate module 6 shown in Fig. 7a includes interelectrode inserts 22e-22h having passages 24e-24h, respectively, therethrough which gradually diverge in a direction away from pilot insert 20 toward anode module 8. As shown in Fig. 7a, the gradual divergence of passages 24e-24h has a generally coaxial profile.

[0054] Lastly, a third embodiment intermediate module 6 shown in Fig. 7b includes interelectrode inserts 22i-22p having passages 24i-24p which step diverge in the direction away from pilot insert 20 toward anode module 8, wherein the number of steps is less than the number of interelectrode inserts. Specifically, passages 24i-241 of interelectrode inserts 22i-221 have a first diameter Dl; while passages 24m and 24n of interelectrode inserts 22m and 22n have a second diameter D2; and passages 24o and 24p of interelectrode inserts 22o and 22p have a third diameter D3. Thus, from the pilot insert 20 side of intermediate module 6 to the anode module 8 side of intermediate module 6, passages 24i-24p of interelectrode inserts 22i-22p step diverge in three discrete steps. The foregoing description of gradual and step divergences of passages 24 of interelectrode inserts 22 is not to be construed as limiting the invention since the use of the combination" of gradual WS step divergence of passages 24 is envisioned. Moreover, the number of step divergences of passages 24 of interelectrode inserts 22 in Figs. 2 and 7b is not to be construed as limiting the invention. [0055] It has been determined that the relationships disclosed in the following equations EQ1-EQ4 are desirable for determining the dimensions of pilot insert 20, interelectrode inserts 22 and the length of the passage defined by interelectrode inserts 22:

EQ3: L₀ < (Y)(D_c); and

EQ4: Δ./D, = Z where L₁, = axial length of passage 21 of pilot insert 20; X = between 1 and 3.5; D_p = diameter of passage 21 of pilot insert 20; D_c = diameter of the passage 24 of the interelectrode insert 22 adjacent pilot insert 20;

Y = between 10 and 12;

D₁ = diameter of interelectrode insert i, where i begins at 1 for the interelectrode insert adjacent pilot insert 20 and increases by 1 for each insert in the direction of anode module 8;

Δ, = thickness of interelectrode insert i;

L₀ = length of the passage formed by the passages 24 of all the inserts 22 of the intermediate module 6; and

Z = between 0.5 and 3.0.

[0056] The ratio of Δ,/D, generally depends on the plasma gas being used. For example, Δ,/D, is between 0.5 and 1 for argon; around 1 for hydrogen; between 1 and 2 for nitrogen; up to about 3 for air.

[0057] With reference to Fig. 8, and with continuing reference to Fig. 2, in accordance with the present invention, cathode 30 can have two different embodiments. In the embodiment shown in Fig. 8a, cathode 30 is rod-shaped and extends" "Φfrάy frofh ^"a ^ta^efed surface 110 of cathode holder 32. In the embodiment shown in Fig. 2, the exposed end of cathode 30 is flush with a surface 112 of cathode holder 32. The embodiment of cathode 30 shown in Fig. 2 has better cooling properties and, therefore, a longer life than the embodiment of cathode 30 shown in Fig. 8. The rod-shaped cathode 30 shown in Fig. 8 enables some decrease in the flow of a shield gas (described hereinafter) in comparison with the flat cathode 30 shown in Fig. 8b.

[0058] As discussed above, cathode holder 32 is desirably made from copper. However, cathode 30 can be made from hafnium or zirconium when air or another plasma gas containing oxygen is used without special protection of cathode 30 by a shielding gas, such as argon. Cathode 30 can also be made from tungsten (lanthanated or thoriated tungsten) if an inert gas or nitrogen, with or without hydrogen, is used as a plasma gas. A tungsten cathode 30 can also be used with an air-based plasma gas if a shielding gas, such as argon, is used to protect cathode 30.

[0059] With reference to Fig. 9, and with continuing reference to Figs. 1 and 2, increasing the life of anode body 16 and cathode 30 as well as minimizing a swirl component of the plasma jet produced by plasma gun 2 are desirable goals of the invention. These goals can be achieved by introducing one or more secondary gases into passage 12 of anode body 16. Fig. 9 illustrates an alternate embodiment of anode module 8 where a secondary gas can be fed via a hose 114 and a pipe 116 to passage 12 of anode body 16 via a distribution ring 118. Distribution ring 118 has one or more passages 119 for feeding the secondary gas into passage 12. The one or more passages 119 in distribution ring 118 can be configured to provide homogeneous distribution of the secondary gas into passage 12 via an annular slot 120 formed by anode body 16 and the interelectrode insert 22 adjacent anode body 16.

[0060] The use of argon, methane or other hydrocarbon gas, such as natural gas, as the secondary gas causes diffusion of the root of main arc 102 on the converging portion of wall 14 of anode body 16. This diffused main arc root minimizes or avoids tangential flow of the plasma gas that results in lower interaction of the plasma jet with ambient air. Higher flow rates of secondary gas re§ultS4n^::_' ^!b^re«eϊ''tiifMsϊ^k^;iϊf4he root of main arc 102, and, correspondingly, a longer life of inner wall 14 of anode body 16. Desirably, secondary gas is introduced into passage Yl ak z. rate between 10% and 40% of the mass flow rate that the plasma gas is introduced into space 72 between cathode holder 32 and pilot insert 20.

[0061] With reference to Fig. 10, and with continuing reference to Figs. 1 and 2, a shielding gas can be fed to space 72 between cathode holder 32 and pilot insert 20 via a hose 122, a pipe 124, a passage 125 and a passage 127 of a swirl nut 126. Desirably, the shielding gas is fed into space 72 at a flow rate between 0.1 grams per second and 0.5 grams per second. However, this is not to be construed as limiting the invention since the use of other shielding gas flow rates is envisioned. [0062] Lastly, plasma gas can be fed to space 72 via a hose 128, a pipe 130 and one or more passages (not numbered) in pilot insert 20. From space 72, plasma gas flows through passage 21 of pilot insert 20, passages 24 of interelectrode inserts 22 and passage 12 of anode body 16. Exposure of the plasma gas to main arc 102 ignites the plasma gas thereby forming a plasma jet which exhausts from the end of anode module 8 adjacent feeding module 10. The material introduced via feeding module 10 into the plasma jet is treated thereby for subsequent use. [0063] The plasma generating apparatus is particularly useful in making various materials that are useful in the art. In one embodiment, for instance, the plasma gun can be used to manufacture spheridized low oxygen, refractory metal powders. To make such materials, conventional irregularly shaped powders can be fed into the gun. Upon entering the region of the plasma these powders are melted and accelerated away from the gun. While molten, three separate events can occur: 1. If temperatures are high enough surface oxides are broken down releasing the oxygen from the metal, 2. entrapped interstitial gases such as oxygen and nitrogen are released from the molten droplets and 3. surface tension shape the powder particles into near perfect spheres. Once the oxygen has been released from the droplets it must be swept out of their immediate vicinity to prevent it from recombining with the metals particles as they cool down. This can be accomplished by spraying the droplets into an inert gas chamber that is continuously flushed with fresh inert gas to prevent build up of the contaminating _ - gases bϊ lϊβfΘ'Wders -may also be sprayed into a low pressure or vacuum chamber that is continuously pumped to remove the contamination gases. To retain the spherical shape the powders take on while molten, they must be allowed to fully solidify before contacting any hard surfaces. This is accomplished by using an inert gas or vacuum chamber of sufficient size that the molten droplets are solidified before reaching the walls of the chamber.

[0064] In another embodiment the plasma gun can be used to manufacture low oxygen refractory metal coatings on base metal substrates such as carbon steel or stainless steel. Doing so provides a bimetal with the desirable properties of a refractory metal at a small fraction of the cost of a pure refractory metal structure. Conventional irregularly shaped powders are fed into the gun and sprayed onto a substrate. Upon entering the region of the plasma these powders are melted and accelerated away from the gun. In order to provide a coating a base metal substrate is placed in front of the spray so that the droplets strike and adhere to the base metal substrate while they are still molten, or at least still partially molten. By transversing the gun across the substrate at rates known to those skilled in the art, coherent, dense, refractory metal coatings may be put on the substrate. Placing the substrate and spraying the droplets into an inert gas chamber that is continuously flushed with fresh inert gas to prevent build up of the contaminating gases or spraying into a low pressure or vacuum chamber that is continuously pumped will reduce the contamination gas content before the droplets solidify as a coating.

[0065] In another embodiment the plasma gun can be used to manufacture three dimensional, low oxygen refractory metal structures. Conventional irregularly shaped powders are fed into the gun and sprayed onto a substrate. Upon entering the region of the plasma these powders are melted and accelerated away from the gun. In order to provide a structure a substrate, which may or may not be the same as the powder being sprayed, is placed in front of the spray so that the droplets strike and adhere to the substrate while they are still molten, or at least still partially molten. By transferring the gun across the substrate at rates known to those skilled in the art, coherent, dense, refractory metal coatings may be put on the substrate. The three dimensional structure is built by continuing this process

'd&vm iτamittple^'irre^'fractory metal coatings such that the total coating becomes very thick and results in the final desired structure. Placing the substrate and spraying the droplets into an inert gas chamber that is continuously flushed with fresh inert gas to prevent build up of the contaminating gases or spraying into a low pressure or vacuum.

The invention has been described with reference to the preferred embodiments. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

THE MVENTIΘHOtAI-KffiD IS:

1. A plasma jet generating apparatus comprising: a cathode module having a cathode; an anode module having a first passage therethrough; and an intermediate module positioned between the cathode and the anode module, the intermediate module having at least one insert electrically isolated from the anode and cathode modules and defining a second passage in fluid communication between the cathode module and the first passage, wherein the first passage converges in a direction away from the intermediate module.

2. The apparatus of claim 1, further including a feed module for feeding material to an end of the first passage opposite the intermediate module.

3. The apparatus of claim 2, wherein the feed module feeds the material to one of inside and outside the first passage.

4. The apparatus of claim 1, wherein the convergence of the first passage resides closer to the intermediate module than an end of the first passage opposite the intermediate module.

5. The apparatus of claim 1, wherein the first passage initially converges and then diverges in a direction away from the intermediate module.

6. The apparatus of claim 1, wherein a wall of the first passage converges at an angle between 10 and 20 degrees with respect to an axis of the first passage.

7. The apparatus of claim 1, wherein a wall of the first passage converges at an angle between 12.5 and 17.5 degrees with respect to an axis of the first passage.

8. The apparatus of claim 1, further including: ή-plaΛiia-gal^:pass^fagfe^" for introducing a plasma gas into the first and second passages; and means for causing an electrical arc to extend between a wall of the anode module that defines the first passage and the cathode via the second passage in the presence of the plasma gas thereby forming a plasma j et.

9. The apparatus of claim 8, further including: a secondary gas passage for introducing into the first passage a secondary gas which causes diffusion of the electrical arc on the wall of the first passage.

10. The apparatus of claim 9, further including means for causing swirling of the secondary gas in the first passage.

11. The apparatus of claim 9, wherein the secondary gas is one of an inert gas and a hydrocarbon gas.

12. The apparatus of claim 9, wherein: the inert gas is argon; and the hydrocarbon gas is one of methane and natural gas.

13. The apparatus of claim 1, wherein the second passage is defined by a plurality of inserts that are electrically insulated from each other.

14. The apparatus of claim 1, wherein the intermediate module includes a fluid passage for receiving a flow of cooling fluid for cooling the insert.

15. The apparatus of claim 1, wherein the second passage is generally cylindrical adjacent the cathode module and increases in diameter toward the anode module.

16. The apparatus of claim 15, wherein the second passage has a generally conical profile.

17. The apparatus of claim 1, wherein the cathode is one of rod-shaped and flat.

18. The apparatus of claim 1, further including a shielding gas passage for introducing a shielding gas into contact with the cathode.

19. The apparatus of claim 18, wherein the shielding gas is argon.

20. The apparatus of claim 18, further including means for causing swirling of the shielding gas adjacent the cathode.

21. A plasma jet generating apparatus comprising: a cathode module; an anode module defining a first passage therethrough; and an intermediate module positioned between the cathode module and the anode module, the intermediate module having at least one interelectrode insert defining a second passage in fluid communication between the cathode module and the first passage, wherein the second passage diverges in a direction toward the anode module.

22. The apparatus of claim 21, further including a feed module for feeding material to an end of the first passage opposite the intermediate module.

23. The apparatus of claim 22, wherein the feed module feeds the material to one of inside and outside the first passage.

24. The apparatus of claim 21, wherein the first passage converges in a direction away from the intermediate module.

25. The apparatus of claim 21, wherein the first passage initially converges and then diverges in a direction away from the intermediate module.

26. ■%IeNkpρdratu§^;»oϊ^" "rifaim 21, further including a pilot insert positioned between the cathode module and the interelectrode insert, the pilot insert defining a third passage in fluid communication with the first and second passage.

27. The apparatus of claim 26, further including: means for feeding a plasma gas into the first, second and third passages; and means for generating an electrical arc connected to the cathode module, the pilot insert and the anode module, wherein, in the presence of the plasma gas in at least the second and third passages, the electrical arc generating means causes a pilot electrical arc to be generated between the cathode module and the pilot insert and, once the pilot electrical arc is generated, the electrical arc generating means causes a primary electrical arc to be generated between the cathode module and a wall of the anode module that defines the first passage.

28. A method of plasma treating a material comprising:

(a) introducing a plasma gas into a passage defined by a cathode and a pilot insert adjacent one end of the passage and an anode adjacent the other end of the passage; (b) igniting a pilot electrical arc between the cathode and the pilot electrode thereby ionizing the plasma gas;

(c) causing a main electrical arc to form between the cathode and the anode from the pilot electrical arc thereby generating a plasma jet; and

(d) introducing a material into the plasma jet adjacent the end of the passage opposite the cathode.

29. The method of claim 28, further including introducing a shielding gas adjacent the cathode during the generation of the plasma jet.

30. The method of claim 29, wherein: the shielding gas is argon; and . - to Mifeldmg gas- is -introduced adjacent the cathode at a rate between 0.1 grams/sec and 0.5 grams/sec.

31. The method of claim 28, further including extinguishing the pilot electrical arc after step (c).

32. The method of claim 28, wherein the plasma gas is at least one of argon, nitrogen, air, hydrogen, helium and CO₂.

33. The method of claim 28, wherein: the anode includes an interior wall that defines the anode's portion of the passage; at least a portion of the interior wall at least partially faces the cathode; and the main electrical arc contacts the anode on the portion of a wall that at least partially faces the cathode.

34. The method of claim 28, further including introducing a secondary gas into at least the anode portion of the passage, at a rate between 10% and 40% of the mass flow of the plasma gas, for causing diffusion of the main electrical arc on a wall of the anode portion of the passage.