Plasma torch

Field of the Invention

The invention relates to a method and apparatus for making nanosize particles of material, typically starting from micron sized feedstock, in a way which excludes atmospheric contamination (oxidation or nitridation) using a conventional plasma spray torch (used in volume industrially for coating of surfaces with melted particles) and associated system. It also concerns means of attaching said torch, or further devices, and other aspects of this integration and process.

Background to the invention

It is known to pass wire or rod or even micron-sized feedstock into a bespoke plasma torch, whether high frequency induction or a simpler HF struck plasma, in a variety of electrode formats. For example US2006/0233965 (Tekna Plasma systems) discloses a method of using a plasma torch in a soft vacuum to coat a substrate.

A problem with such systems found in industry, is that the torches used tend to be proprietary parts of an entirely proprietary system, and thus both exclusive and expensive. Any parts or repairs or changes or improvements to the system are likely to be slowly delivered and also costly, as design costs will need to be borne by the one customer. It means also that improvements do not flow automatically to the user, and product development is dependent on the installed user base for such plant, which can be very limited.

As well as cost, a further problem is that the performance of such torches can be very variable, with unstable plasmas, the torches tend to be one-off designs adapted from other uses, such as spheroidisation or waste vitrification or precious metal recovery. Furthermore, such torches are not designed for high yield of nanosize particles, are often being designed for the feedstock material to be heated to melting point but not necessarily converted into the plasma state.

This latter is true in industrial plasma spray torches used in coating processes, which are designed to convert material from solid into liquid, for long enough for the liquid to be rapidly cooled when the droplets impact on the surface to be coated. Such coatings may be any material which is not greatly harmed by this process of spraying in air, such as oxides or nitrides.

Such plasma spray torches, and their power supplies, control and instrumentation systems, are in large-scale use; they are developed and in a continual state of technology improvement. They are therefore attractive as a source of plasma torch for a system designed to create nanosize material. However, to date, no such plasma spray torch system for nanoparticles production is commercially available.

The reasons for this are many, one being the technical problems involved in making such a composite system.

For example, a problem of some plasma torches is the cooling system, which is not suitable for the production of nanoparticles. It is known for a plasma torch to have a so-called "Hot Zone" which is lined with a ceramic and cooled gently by gas passing through apertures in that ceramic liner. Such cooling is found to be very uneven and too slow for the effective production of nanoparticles, which require rapid cooling to prevent agglomeration of the particles. A further problem with a hot-zone is the design of the zone permits large amounts of product to become built up in the tube, sometimes completely blocking it.

A further problem is the maximising of the yield of the material obtained. A known problem with introducing material directly into a plasma stream is that a plasma is electrically charged, and particles in flight tend equally to become charged, making them potentially mutually repulsive, and therefore difficult to introduce.

Yet another problem with all plasma torches, and especially true of plasma spray torches, is the noise made by the high velocity gas and high temperature. In air, at a distance of approximately 4 metres, the sound level measured from plasma torches is typically above 120 dbA. To comply with Health and Safety laws and for the comfort

of any operators of such a system, it is known for the operators to wear ear defenders which are large and may be uncomfortable during prolonged use.

It is the object of this invention to describe one such system and which is enabled to at least mitigate some of the above problems, and further known problems in the prior art.

Summary of the Invention

An aspect of the invention provides apparatus according to claim 1 appended hereto. Further aspects of the invention and features thereof are set out in the following detailed description and in the appended claims.

According to a first aspect of the invention there is described a combination of a commercially available and standard system for spraying materials using a plasma, with a water-cooled and atmosphere controlled system. The object of this combination is that it allows conversion of feedstock into nanosize material, using a low cost torch and allows use of an advanced power and control system that are widely available.

According to a second aspect of the invention, there is described the means whereby the torch is connected to a temperature and atmosphere controlled system. A problem with such a temperature and atmosphere control system is that the front face of such torches is at some potential above earth (typically 100 volts DC) and the back face is earthed for operator safety (although many are now robot mounted). It is also commonplace for such torches to be designed to be clamped in a handle, which does not lend them to being held onto a flange; which is further complicated by the need for such attachment to be adequate to contain a 9 bar pressure increase, which is desirable for such systems in the event of an explosion. This is particularly important if the system is intended to produce nanomaterials that would oxidise or nitridise or otherwise become degraded in the presence of air. Such materials need a controlled atmosphere and a sealed system to exclude air. The Basis of Safety of such a system includes a requirement that the system has an atmosphere of an inert gas and can contain an explosion of the most hazardous material (for example aluminium, has a

measured pressure wave of approximately 9 bar). The system also has to be built in conformity with the prevailing pressure system regulation (in the European Community this is the Pressure Equipment Directive). For the torch to be attached to the system in conformity with such a regulation, it needs to be fixed and inspected by and external and approved testing authority. This invention describes in one aspect, how this is achieved, with all due regard to the forces, the environment inside the chamber including temperatures of 5,000 to 15,000 ⁰ K, and the need for materials which will provide the required integrity and durability.

It is a further object of this invention to take advantage of the high yield characteristics of plasma spray torches, that is where the majority of the material is exposed to the plasma conditions and atomised. In a preferred embodiment the feedstock is introduced in such a way that it cannot evade the plasma (bearing in mind that a plasma is electrically charged, and particles in flight tend equally to become charged). This problem is mitigated by one aspect of the invention in which a plasma spray torch has one or more points in the torch where material can be introduced. An aspect of the invention describes how to employ these methods and a method which has maximum flexibility and effect.

A further aspect of the invention describes means for allowing the operators a clear view of the plasma to allow imaging so that the operating parameters may be adjusted for maximum stability and yield. This is particularly important when a plasma torch which is designed to operate in free air is operated inside a contained system with very proximate walls. Due to the high temperatures of the plasmas such viewing is inherently fraught with difficulties.

One aspect of this invention is the use of water-cooling on walls and feed tubes and to cool electrically insulating materials (inherently poor conductors of heat). It is this cooling, combined with the gas flow regime and related safety trip systems, which permit the use of a plasma spray system in such a confined space while preserving the integrity of the materials exposed to such high temperatures. The use of insulating materials in the "hot zone" proximate the torch was also tested but with very

unsatisfactory results, which will be described; this led to the use of water-cooling so extensively in the tube area close to the plasma flame.

A further aspect of the invention is to reduce to noise from the plasma torch, to increase the comfort of any operator using the system, and to avoid any problems associated with exposure to loud noises. The present invention provides a method of reducing the noise of the plasma torch system, to well below the level required for continuous working as set out in Health and Safety Laws (80-85 dbA).

In a further aspect of the invention there is provided an assembly for attachment to a plasma torch for use in the conversion of material into nanoparticles, the assembly comprising: a first section for attachment to the plasma torch wherein the first section comprises: an inlet channel; and a feedstock injector, positioned to direct the feedstock into the inlet channel whereby in use the feedstock injector injects the feedstock into a plasma stream from the plasma torch; a second section defining a cooling chamber connected to the inlet channel to enable cooling of the stream of particles in use; the assembly further comprising a cooling arrangement adapted to cool both the feedstock injector and the second section in use.

Brief description of the drawings

An embodiment of the invention will now be described by way of example only, with reference to the following drawings, in which:

Figure 1 is a side elevation of a plasma torch and the attachment according to an embodiment of the invention;

Figure 2 is a front elevation of the apparatus of Figure 1;

Figure 3 shows various elevations showing the cooling elements and powder injector of the first section of the attachment according to an embodiment of the invention;

Figure 4 shows further elevations of the cooling elements and powder injector shown in Figure 3;

Figure 5 shows various elevations of the expansion region in the second section of the attachment according to an embodiment of the invention;

Figure 6 shows further elevations of the expansion region shown in Figure 5;

Figure 7 shows an embodiment of the camera assembly that is attached to the section of the invention.

Detailed description of the invention

Figure 1 is a side elevation of an embodiment of the assembly attached to a known plasma torch.

There is shown the plasma torch 100, comprising a cathode 102, anode 104, gas pipes 106, 108, powder feed connector 110, retaining collar 112 and torch outlet 114.

The plasma torch 100 is coupled to the first section 200 of the plasma torch assembly. The first section 200 comprises an inlet channel 202, a cooling arrangement 204, 206 and a powder injector 208.

The first section 200 is connected to a second section 300 which comprises an expansion region 302, cooling chamber 304, isolation plate 306, adaptor flange 308, a coolant inlet 310, a coolant channel 312, annulus 314, and chamber walls 316. There is also shown an imager 400 which images the plasma stream in the expansion region. This imager 400 is so placed to provide a view of the hot plasma (which produces much visible radiation) to see the effect on it of changes in gas velocity, power setting, plasma gas mixture, and those caused by introducing feedstock. In the preferred embodiment the entire apparatus 100 is sealed and air is excluded, and the system is purged with an inert gas preferably argon. The image of the plasma stream is obtained using the pinhole method, where the image in fact is projected onto a ground glass

screen, which is itself then viewed by an imaging device such as a CCD or video camera; this is partly for safety reasons, to keep the design robust and intact. The pinhole, especially when it is purged by gas, minimise any deposition by plasma materials that might obscure the image.

The plasma torch 100 in the preferred embodiment is a known DC non-transferred arc torch. Other plasma torches or plasma spray torches may also be used. Gas, preferably argon, is passed between the cathode 102 and anode 104 where it is ionised and is turned into a plasma. In further embodiments the argon gas contains up to 30% helium by volume, and/or hydrogen and/or a hydrocarbon gas such as methane or mixtures of these gases may also be used.

Preferably the plasma torch 100 has a flow stabilisation means, such as a vortex flow stabiliser to help define the path of the plasma stream. The torch 100 may also comprise a known powder feed system that is enabled to directly feed powdered material into the gas flow or into the arc of current that is created between the cathode 102 and anode 104. In the preferred embodiment the rate of gas flow is approximately 50 to 80 1/ min, and the plasma produced exits the torch at the torch outlet 114.

The plasma torch 100 is coupled to the first section 200 of the assembly. The plasma stream from the torch 100 passes through the torch outlet 114 and into the inlet channel 202. The first section 200 is described in greater detail with respect to Figures 3 and 4.

In use the plasma stream will traverse the inlet channel 202 and the powder injector 208 injects a powder directly into the plasma stream. In the preferred embodiment the powder feedstock is in the size range 1 to 10 micron and the feed rate is 0.2 to 2 kg/hour. Clearly the feed rate may be adjusted depending on the desired yield, those skilled in the art will also appreciate that a variation in feed rate may also require a variation in the gas flow rate to maximise the yield. The temperature of the plasma stream (up to the order of 10,000 ⁰ K) causes the injected powder to evaporate. The resulting material is entrained in the plasma stream through the inlet channel 202. The width of the inlet channel 202 is preferably similar or identical to the width of the

torch outlet 114. Having similar widths for the torch outlet 114 and inlet channel 202 is found to preferentially stabilise the flow of the plasma stream as the similar widths creates a smooth channel and does not introduce discontinuities (which are known to de-stabilise flow) or ledges (which are known to collect material and act as sites for product build-up). The first section 200 also contains a network of pipes containing coolant ensuring that the high temperatures of the plasma stream (up to 12,000K) do not affect the assembly. The structure of the cooling arrangement and the powder injector are discussed in greater detail with reference to Figures 3 and 4.

The plasma stream now contains the largely vaporised feedstock and traverses the inlet channel 202. Given the temperature and speed of the gas in a typical plasma stream, this process of evaporation and entrainment takes typically circa 0.2 millisecond. The powder is injected at velocity of 100 m/s which is found to allow good injection of the powder into the plasma stream, and mitigate the problem of the introduction of the feedstock into the charged plasma stream. From the inlet channel 202 the plasma stream enters the second section of the assembly 300 via the expansion region 302. The expansion region 302, in the preferred embodiment, is frustro-conical in shape and the base, the widest section, is ~3 times the diameter of the inlet channel 202. The shape and size of the expansion region 302 causes expansion, disrupting the stability of the plasma stream. The expansion region 202 is also cooled by a cooling arrangement which is discussed in greater detail with respect to Figures 5 and 6.

In the preferred embodiment the expansion region 302 is observed by an imaging system 400. The high temperature plasma stream emits a large amount of radiation, especially in the visible band, and certainly needs no illumination; it could be viewed using any number of means, including "directly" using an optic fibre cable. It is preferred, for safety reasons, to view its image as projected onto a ground glass screen, and to do so using a small video camera. The imaging system 400 is discussed in greater detail with reference to Figure 7.

The expansion region 302 leads directly to a cooling chamber 304 which is designed to cool the plasma stream as rapidly as possible so that the nanosized particles (~50nm in diameter) condense out of the plasma stream without having chance to aggregate to

form larger particles. Depending on many factors (feeding point, feed rate, feedstock size and nature, plasma power, gas type & feed rate, etc.) a proportion of the powder fed will become converted to the plasma state. Such material will therefore be in an atomic and possibly ionised state, and these atoms will coalesce into clusters based on proximity, and the thermodynamics of re-combination. If feedstock is a compound, or if it forms a compound with a contaminant or a system gas, then compounds may be formed most of which may or may not be stoichiometric. Typically, this requires cooling timescales of a few milliseconds.

Given the high voltages involved the second section also contains an electrical isolation plate 306. The isolation plate electrically isolates the cooling chamber 304 from the high voltage torch 100. The isolation plate is made from a composite of a polymer and a ceramic material and also isolates the heat from the first section 100 and expansion region 302 from the cooling chamber 304. Other non-conducting materials may also be used to create the isolation plate. The isolator plate has to be in a material which is gas tight and its fitment must also be gas tight; in the preferred embodiment the insulating plate is held in place hermetically using 'O '-ring seals which are water cooled to prevent degradation.

An adaptor flange 308 is coupled to the isolation plate 306 and a coolant system, which forms an annulus of coolant 314 around the chamber walls 316. Heat from the plasma is transferred by radiation and by conduction (the balance depending on temperature and material and the plasma gas) to the surroundings causing the plasma to cool. The coolant is introduced via the inlet 310 and is transported via the coolant channel 312 to create the annulus of coolant 314. The annulus of coolant 314 optimally extends the length of the cooling chamber 304 and is able to provide coolant at a rate of 20-40 1/min at 6 Bar. It is found that to effectively cool the plasma, a heat transfer rate of up to 20 kW is required to maximise the yield. Those skilled in the art will understand that the choice and temperature of coolant at the inlet 310 and the flow speed of the coolant are all variables that may be adjusted to further control the cooling rate of the plasma. It is preferable that the flow of cooling water be balanced through the system; with the cold water being first supplied to the hottest parts of the plant, the cooling area next to the torch including the interface plate.

The cooling chamber 304, may optionally lead to a so-called quench zone where cold inert gas is injected to cool product (mostly effective for lower temperature boiling point materials) in a quench zone where the plasma is further cooled. Such a quench- zones may be one known in the art.

It is beneficially found that the arrangement of the inlet channel 202 and cooling chamber 304 which are surrounded by cooling apparatus herewith described, in an atmosphere from which air has been excluded significantly reduces the noise of the torch 100. It is found that the construction of the assembly from stainless steel and the flow rates of water used for the coolant (typically 20-40 1/min) act as a sound insulator for the torch, thereby reducing the noise levels of the torch to levels deemed safe by the relevant Health and Safety statutes.

Figure 2 is a side elevation of the apparatus described in Figure 1. The features are numerals are identical to those described with reference to Figure 1.

Figures 3 (a) (b) (c) and (d) and Figure 4 (a) (b) and (c) show various elevations of the first section 200 of the attachment for assembly with a plasma torch 100. Figure 3 (a) is the top view and 3 (b) the bottom view of the first section 200. Figure 3 (c) is a cross-sectional view through line A-A' and Figure 3(d) through line C-C. Figure 4 shows the cross-sectional views through lines C-C, D-D' and E-E' for Figures 4 (a), (b) and (c) respectively.

In the preferred embodiment the first section 200 is an annulus and the plasma stream passes through the central space defined by the annulus. The first section 200 comprises the inlet channel 202, cooling arrangement inlet 204, outlet 206 and pipe 210, powder injectors 208, central wall 212 and material 214. The first section 200 is solid and must be able to withstand the temperature of the plasma, in the preferred embodiment the section 200 is made of stainless steel.

As the plasma stream passes through the inlet channel 202 substantial heating of the central wall 212 will occur leading to heating of the entire first section 200. To

minimise the heating a cooling arrangement is incorporated into the section 200. Coolant, preferably water (though other suitable coolants may be used), is introduced in the inlet 204 and passes through pipe 210 and exits via the outlet 206. The rate of coolant flow is typically between 20-40 1/min though this may be varied according to the circumstances. The pressure at which the coolant flows is typically 6 Bar, though for safety reasons the system is designed to withstand 10 Bar. The coolant is typically inputted at room temperature and exits at about 6O ⁰ C. As with the cooling annulus 314 that surrounds the cooling chamber 304 the rate of cooling may be controlled by the rate of flow of the coolant, initial temperature etc.

A hole is bored into the annulus of the first section 200 so that powder may be injected directly into the plasma stream in the inlet channel 202 via the powder injectors 208.

Figures 3 (c) and 4 (a ) (b) and (c) show that the inlet 204 and outlet 206 are angled compared to the transverse pipe 310, at an angle of approximately 10 degrees. This allows for efficient cooling of the entire of the first section 200 and additionally allows for the cooling of and placement of the powder injectors 208. Such an arrangement ensures that the powder injectors 208 are also cooled and subsequently are not damaged by the temperatures of the plasma stream.

Though the preferred embodiment shows the cooling arrangement of the inlet 204, outlet 206 and pipe 210 in a rectilinear arrangement, those skilled in the art will understand that any suitable configuration may be used which is able to provide a coolant flow of up to 40 1/min at the desired pressure. The coolant system preferably must be able to withstand high surface temperatures and cool the first section 200 at a rate of 10 ⁶ K/min.

Figure 4 shows further cross-sectional views of the first section as described with reference to Figure 3. The numerals and features are the same as those described with reference to Figures 3 (a) (b) (c) (d).

Figure 5 shows the cooling arrangement of the second section 300 around the expansion region 302. The high temperatures mean that there will be expansion of gas into the water-cooled section.

There is shown respectively in Figures 5 (a) (b) (c) the bottom, top and side elevation of the second section 300 and in particular the expansion region 302. There is shown the bottom of the inlet channel 202 from the first section 200 (not shown for clarity) the expansion region 302, the walls of the expansion region 330, expansion coolant inlet 332, a coolant collar 334, coolant outlet 336 and a channel 338 for the imaging equipment 400 (not shown).

The expansion region 302 is frustro-conical in shape and the expansion region walls 330 define the frustro-cone hollow. Coolant is required as this region is exposed to very high temperature plasmas and it also forms the beginning of the cooling chamber in which the plasma stream cools. Coolant is inputted via the coolant inlet, in the preferred embodiment the rate is 20-40 1/min at a pressure of 7 Bar, and flows around the expansion region walls 330 in the coolant collar 334 and defines a frusto-cone collar. The coolant collar 334 is typically of a similar width to the thickness of the expansion region walls 330, and extends the height of the expansion region walls 330 to provide optimal cooling in the expansion region 302. In the preferred embodiment the cooling arrangement of the expansion region 302 is separate from that of the first section 100 and is separate from that of the cooling chamber 304 (not shown in Figure 5). Though in further embodiments the cooling arrangement may be integrated for two or more sections.

Figure 6 shows varies cross-sectional views of the second section 300 around the expansion region 302. Figure 6 (a) shows a view through A-A' of Figure 5 (a), 6 (b) shows the images through B-B' of image 5(c), 6(c) through C-C of 5 (a), with references and features as described with reference to Figure 5. Figures 6 (c) and (d) show that the channel 338 for imaging equipment requires that there is a section which does not have any coolant, this can also be seen in Figure 5 (a) and Figure 6 (b) where the coolant collar 334 does not fully close. The channel 338 typically has a conical hole of 85° degrees and extends through the entire section, as the housing of the

camera is kept away form the main body of the apparatus. In the preferred embodiment the channel 338 is not cooled but in further embodiments the cooling collar 334 surrounds the channel 338.

The channel 338 leads to the imaging equipment 400. Due to the high temperatures and high photon count that is emitted from the plasma stream, it is undesirable for a camera to directly image the stream. The high temperatures would potentially damage the equipment and the high counts from the plasma stream would quickly saturate any camera and possibly damage a CCD imager. Figure 7 shows an embodiment of the camera assembly that contains an imager 400 which is able to view the plasma stream without damaging the camera or subjecting it to the high temperatures of the plasma. Additionally, such an arrangement has the advantage of placing the apparatus required to view the stream, away from the stream, thereby increasing the safety of the operator, who is not required to be near the hot plasma torch.

There is shown the imaging assembly 400, comprising the channel 338 leading to the expansion region, pin hole 402, a gas purge 403, lens 404, screen 406, housing unit 408, optical path 410, lens 412, focusing lens 414, camera 416, camera assembly 418, camera unit 420 and connector to computer 422.

The imager 400 uses a pinhole 402 to view the plasma. The pinhole 402 projects an inverted image of the plasma stream on the screen 406. A focusing lens 404 may be used to focus the image on the screen, though in further embodiments lens 404 is not used. The lens 404 advantageously allows for the screen 406 to be placed further away from the plasma stream. In the preferred embodiment, the screen 406 is a glass screen as it is found to best withstand the temperatures and optimally reduce the photon count from the plasma stream and therefore avoid saturation of the camera 416.

The screen, and therefore an image of the plasma is viewed by a second camera 416. The camera 416 is preferably a known digital camera. The screen 406 is placed in a housing unit 408, which prevents any foreign matter, i.e. nanoparticles, entering. The housing unit also allows for the removal of the camera unit 420 for replacement or maintenance. The optical path 410 may be altered by the lenses 412, 414 allowing the

camera 416 to be placed away from the stream, thereby reducing the amount of heating from the plasma stream. The camera unit 420 contains the camera 416 and lenses 412, 414 and comprises a heat insulating material. In a further embodiment the camera unit 420 may have a cooling arrangement to further cool the camera 416. The image of the screen 406 as viewed by the camera 416 is transmitted to a monitor or computer (not shown) via the connector 422. The connector 422, camera 416 and lenses are known in the art.

It is found that the pinhole 402 becomes clogged by the nanoparticles present in the plasma stream. To overcome this, a gas purge 403 is placed between the pinhole 402 and the screen 406. The gas purge 403 is simply an input of compressed gas, preferably argon, that is injected via the purge 403 and into the plasma stream. It is also found that sealing the housing unit 408 prevents contamination from the particles after the screen 406. An inert gas, preferably argon, is continually passed via the gas purge 403 which ensures that the pinhole 402 remains clear. It is found that a rate defined by choked flow is adequate to avoid any blockages of the pinhole 402.

The image of the plasma stream allows a person in control of the machinery to ensure that the plasma region is working as planned. The image of the stream allows the user to determine if the plasma is taking feedstock into the hot zone well, that there is no melting of the material in or near the high temperature region, including the gun electrodes, and that the plasma flame is normal and stable over time and when control parameters are changed. Such information allows the person, or persons, in control of the apparatus to adjust various parameters, such as gas flow rate, feed rate etc., in order to maximise the yields obtained.

Those skilled in the art will understand that the rates of coolant, particle and gas flow may be scaled to increase or decrease the yield to be obtained, without departing from the scope of the invention.