WO2009098662A1

WO2009098662A1 - Long lifetime system for the generation of surface plasmas

Info

Publication number: WO2009098662A1
Application number: PCT/IB2009/050489
Authority: WO
Inventors: Samantha Pavon; Jean-Luc Dorier; Christoph Hollenstein; Penelope Leyland; Peter Ott; Alban Sublet
Original assignee: Ecole Polytechnique Federale De Lausanne (Epfl)
Priority date: 2008-02-08
Filing date: 2009-02-06
Publication date: 2009-08-13

Abstract

The invention concerns a system of electrodes for generating a surface dielectric barrier discharge totally encapsulated in a ceramic dielectric, using co-fireable ceramic technology (ceramic tapes, LTCC, HTCC, green tapes). The plasma generation device can be built not only flat but also curved, for integration on flat or curved surfaces of any material, even metallic. The system can sustain voltages of the order of 1OkV or higher, and can be run continuously for very long times thanks to its high thermal and chemical resistance (typically several tens of hours). More specifically, the system can generate plasmas in gases, including air, at atmospheric pressures down to pressures of about 0.1 bars. The plasma can also be sustained in high speed gas flows, up to Mach 1.2. This aspect allows the system to be used for airflow control or plasma-aided combustion in addition to the surface or gas treatment applications.

Description

LONG LIFETIME SYSTEM FOR THE GENERATION OF SURFACE PLASMAS

FIELD OF INVENTION

The invention relates to the generation of surface plasmas, in particular on surfaces which have, at least partially, a curved shape.

BACKGROUND OF THE INVENTION

A plasma, in the physical sciences terminology, is described as a partially ionized gas composed of ions, electrons and neutral species. This state of matter can be produced by the action of either very high temperatures, strong direct currents (DC) or an alternating current, such as by a radio frequency (RF) electric field. Stars, electric arcs or fusion plasmas for example represent the family of so-called hot plasmas, which are plasmas in thermodynamic equilibrium. On the other hand, cold plasmas are discharges which are thermally not in equilibrium, and are usually generated electrically. In this latter case, free electrons are energized by an imposed DC or RF electric field and then collide with neutral molecules. These neutral-molecule collisions transfer energy to the molecules and form a variety of active species which can include photons, excited atoms, metastables, individual atoms, free radicals, molecular fragments, monomers, electrons and ions.

Low power plasmas, such as glow discharges and coronas, have been used in the surface treatment of various materials. Because of their relatively low energy content, they can alter the properties of a material surface without damaging the surface. Moreover, in recent years, investigations have suggested that creating weak ionization, i.e., low-density non-equilibrium plasma, can modify aerodynamic properties of gas flows. One possible method of creating a weakly ionized surface plasma is the dielectric barrier discharge (DBD) set-up, in which two electrodes, separated by a dielectric material, are submitted to an alternating voltage, typically HF. DBDs have been known for more than a century and are widely used in many different applications such as ozone generation, decontamination of gas streams and surface treatments. They have been utilized as sources for CO₂ lasers, excimer and fluorescent lamps and plasma display panels. During the last decade that their applications in aerodynamics have started to be investigated (plasma assisted combustion and airflow control).

The very first experiments on dielectric barrier discharges were reported by Siemens in 1857. Thorough research with new diagnostic techniques and numerical modeling started in thel970s. DBDs have been studied mostly in the volume discharge configuration, at sub-atmospheric or atmospheric pressures, using different working gases including air. It is believed that metastable states as well as impurities etched from the dielectric surface play a key role in the behavior of the plasma. The surface discharge configuration has been less investigated due to its complexity. Indeed, such an arrangement generates a very thin plasma layer, smaller than one millimeter, which disqualifies a certain number of standard plasma diagnostics generally used in the volume arrangement. However, different geometries of the surface electrodes have been compared. It turns out that the asymmetric electrode DBDs can induce a surface gas flow; this effect is called ionic wind. The use of surface plasmas in low speed airflows has become a recent topic of interest in the field of airflow control. For supersonic and hypersonic flows, other types of plasmas have been tested, mostly volume plasmas. Nevertheless, surface arc plasmas begin to be tested. The complexity of the physical phenomena involved in DBDs implies that models are still at an early stage. DBDs in low-speed gas flows are starting to be studied theoretically and numerically.

Up to now, atmospheric pressure surface DBDs in air have been difficult to maintain for long operation times, typically several hours, due to chemical attack of materials (dielectric and electrodes) directly in contact with the plasma. Indeed, very corrosive species are generated in the discharge (O₃, O, OH for example). Usually the dielectric used is polymer based, such as Kapton. This makes the device easy to machine in any shape or thickness, and flexible sheets can be fabricated. However, the lifetime of such systems is very restricted due to etching of the dielectric (shorter than an hour typically). On the other hand, systems using massive ceramics (alumina plates, MACOR) as a dielectric have longer lifetimes. However, the electrodes are the limiting factor. They are quickly oxidized by the plasma, even when covered with a dielectric paste. Moreover, machining and integration is problematic, mostly on curved surfaces. These lifetime and integration problems represent technical barriers not only to scientific investigations, but also to the application possibilities.

Prior art references :

US 6570333, US 5669583, WO 2007/133239, WO 99/35893, WO 91/06117.

GENERAL DESCRIPTION OF THE INVENTION

The invention describes structures and a method for generating discharge plasmas over the surface of a flat or curved structure and, more particularly, a method for integrating such plasma generators in objects of any material including metals. The extended operating conditions in terms of voltage, pressure, temperature and gas flow speed make the system applicable to (1) plasma processing of gases or surfaces (air purification, surface treatments, fabrication of microelectronic devices), (2) to cover the surface of a body to affect the aerodynamic properties of the body and the flow surrounding it, and (3) to aid ignition and combustion in engines. The present invention provides a solution to two important problems simultaneously, i.e. maximizing the lifetime of a device generating surface plasmas be maximized and integrating such a device on a surface of any shape and any material.

In one embodiment of the invention a protective layer is used on the top of the dielectric and electrodes.

Advantageously LTCC is used as dielectric matrix. First of all, LTCC provide a very efficient way to encapsulate electrodes inside ceramics, laminating all dielectric layers together without air trapped between them, therefore protecting them in the most efficient manner and allowing for a maximized lifetime. It also is thermally stable and most important, has a very high dielectric strength (of the order of 3kV per layer). Furthermore, LTCC allow to address in a very elegant way the second problem, in particular when the surface is curved. Ceramic is intrinsically rigid and friable, not like the Kapton foils that are usually used to create surface dielectric barrier discharge, allowing only a short time of operation of the reactor. Alumina or MACOR elements are not machinable in thin layers and are very delicate and breakable. Sputtered or sprayed ceramics are very porous and too thick layers are needed if a DBD wants to be sustained. LTCC however is prepared in thin layers that are flexible before firing, so curved substrates can be covered with them. The layers can be laminated together to make thicker elements. When the LTCC is fired, most the polymer binders are evaporated. The system becomes very dense but still not as breakable as MACOR for example because some ratio of binders stays trapped in the ceramic. A specific zero-x-y-shrink LTCC tape can be chosen in order to facilitate the process.

GENERAL DESCRIPTION OF THE INVENTION

The invention will be better understood below with a detailed description including some embodiments. Brief description of the drawings

Figure 1 shows the system before stacking:

- Base ceramic (LTCC) layer 1 to isolate the system from the object it will be integrated on. The number of such ceramic tape layers can be varied depending on the desired thickness.

Bottom electrode 2 printed on the second ceramic (LTCC) layer Central ceramic (LTCC) layer(s) 3 , the number of such layers can be varied depending on the desired thickness of the central ceramic layer.

Top electrode (several lines, shape of a grid, see Figure 7) 4 printed on the last but one ceramic (LTCC) layer

Last ceramic (LTCC) layer 5 covering the upper electrode and protecting it from chemical attack from the plasma. The number of such ceramic tape layers can be varied depending on the desired thickness.

The first and last ceramic (LTCC) layers 1,5 are preferably completely encapsulated in the system after stacking. The number encapsulating layers can also be varied if necessary. To summarize, dimensions "a" to "g" can be varied, changing the number of ceramic layers and the geometry of the electrodes. After firing, individual ceramic tape layers are 0.1 mm thick, and conducting layers are 15 μm thick.

Figure 2 shows the system after stacking and flat firing. The two electrodes are completely embodied in the ceramics which has become one piece 6. They are therefore completely protected from the plasma, there is no air voids in the ceramics.

Figure 3 shows the flat system with the circuit for the creation of the surface plasma 7.

An alternative voltage is applied to the top electrodes 4 and the bottom electrode 2 is connected to the ground. The plasma will form on the upper surface. Figure 4 shows a curved system before stacking with the flexible ceramic (LTCC) layers, which number can be varied depending on the desired thickness.

Figure 5 shows the system after stacking and curved firing. After stacking, the system is still flexible and can be given a curved shape. This shape has to be maintained during firing. After firing the system becomes rigid, but has the desired curved shape. The two electrodes are completely embodied in the ceramics, therefore completely protected from the plasma. There is no air voids in the ceramics.

Figure 6 shows the curved DBD system with the circuit for the creation of the plasma 7. An alternative voltage is applied to the top electrodes 4 and the bottom electrode 2 is connected to the ground. The plasma will form on the upper surface.

Figure 7 shows the two electrodes 2,4 from top, printed on the ceramic (LTCC layers), the inner central layer design 3, and the encapsulating layers 1,5 with an insert to allow access to the connection point outside of the ceramic.

Figure 8 shows top and cross-sectional views of the DBD system after stacking.

Figure 9 shows the wiring of the system with conductive paste, before firing. An additional conductive line is added 8, surrounding the stack edge, in order to have both connection sites on the same side.

Figure 10 shows top and bottom view of the system just before firing. The system can be cut to the desired dimensions before firing. It can also be adjusted after firing (by grinding).

Figure 11 is an example of a mockup of a curved shape made of MACOR 10, with the exact same dimension of the real object onto which the plasma system will be integrated. The system will be laid on it during firing to and will take the shape of the surface. Figure 12 and Figure 13 are examples of integration of the system 11 on a metallic airfoil 12. A MACOR piece 13 is machined with the same curvature as the surface and allows the isolation of connections from the metallic body and the gas flow.

Figure 14 is an alternative electrode and circuit geometry to Figure 3, with a single upper stripe 14. It is a simpler version of Figure 3 In the same way it can be fabricated in a curved shape.

Figure 15 is an alternative electrode and circuit geometry to Figure 3, with upper and lower stripes 4 arranged with an offset. This arrangement is a typical ion-wind generation device. It can also be fabricated in a curved shape.

Figure 16 is an alternative electrode and circuit geometry to Figure 3, with an upper and a lower stripe 4 arranged with an offset. This arrangement is a typical ion-wind generation device. It can also be fabricated in a curved shape.

Figure 17 is an alternative electrode and circuit geometry to Figure 3, with only one plane of stripes but alternatively connected to the positive or negative polarity of the voltage generator. This is achieved by placing two electrodes 15 and 16 interlaced. The geometry is shown from top on Figure 19 The system can also be fabricated in a curved shape.

Figure 18 is an alternative electrode and circuit geometry to Figure 3, with only one plane of two stripes. One stripe is connected to the positive polarity of the voltage generator and the other one to the negative polarity of the voltage generator. It can also be fabricated in a curved shape.

Figure 19 is a top view of an electrode . Description of specific embodiments

The list of the following embodiments is not exhaustive, the invention is of course not limited to those examples.

Flat systems

The basic configuration before stacking is presented in Figure 1. First a layer (or more) of ceramic (LTCC) is used to encapsulate the bottom electrode. LTCC layers are delivered in rolls. LTCC is attached to a PET layer for facilitating handling and cutting at the desired shape. In our case, we have squares of 5x5 cm. Then this layer is removed by heating (10 minutes at about 80⁰C) and then peeling it off. After the first LTCC layer comes a second LTCC layer on which the bottom electrode is printed via a serigraphy method. This method makes use of conducting pastes containing the desired electrode material and a mesh with the desired electrode pattern which will allow to deposit the paste on the substrate. The first electrode can be made of gold for reducing electrode material migration in the ceramic. Then a certain number of LTCC layers are introduced to build the dielectric barrier between the two electrodes. The number of layers will depend on several parameters, like the space available on the object to cover or the desired voltage between the electrodes influencing the plasma properties. The top electrode is printed on the last but one LTCC layer. One or more LTCC layer encapsulates the top electrode.

All the layers are then aligned and stacked together. According to the manufacturer's process guidelines, the layers are compressed and heated for lamination. In our case, due to equipment availability, the stack is first compressed and then heated. For 5x5 cm layers, the stack is compressed at 2000 to 4000 psi under a hydraulic press, between two metallic blocks. Then blocks are then screwed together to maintain the pressure. After that, the blocks with the LTCC stack in between is heated, about 1.5 hours at 80⁰C, and then cooled down for about 5.5 hours. In order to prevent the encapsulated LTCC layers to stick to the metallic block surfaces, PET layers are just inserted between the block surfaces and the LTCC stack. Also, for a 4x4 cm final device, the layers have been dimensioned 5x5cm, because the borders of the reactor usually crack during compression and then breakdown can occur through the cracks between the two electrodes. The extra 0.5 cm on each side can be removed either after lamination, with a cutter for example, or after firing by grinding. Both processes are critical and must be done extremely carfully to avoir cracking or breaking.

After lamination, the layers are bound together but the system is not finished yet. It has to be fired at high temperature to evaporate the plastics it contains. The system is heated from room temperature up to 850⁰C, over about 6 hours. Then it is cooled down during about 1.5 days. The surface plasma generation device is now rigid.

Figure 2 shows the system after co-lamination and firing. Electrodes are completely encapsulated in the ceramics and there are no voids inside the system. The plasma reactor has now to be connected to a voltage generator in the way shown in Figure 3. This electrode geometry and electric set-up permit to generate plasma on top of the last LTCC layer, just over the second electrode that is stripe-shaped. The first electrode is just a rectangle connected to mass so that no plasma is created on the side that will be in contact with the object it will be integrated on. This electrode also allows controlling the shape of the plasma, which does not depend only on the top electrode. A top view of the layers is shown in Figure 7. The system is electrically isolated from the object, thanks to the first LTCC layer covering the first electrode. Therefore, the system can be integrated on any surface, plastic based, metallic, conducting, non-conducting.

Curved systems and process

When the PET packaging the LTCC is removed, the LTCC layer itself is very flexible. After stacking and lamination, the whole stack is still flexible. It can then be laid on a mock-up of the object surface, made preferably of MACOR to reduce material contamination and big differences in thermal dilatation. The system takes the shape of the mock-up. The mock-up and LTCC stack are then place in the oven for firing. The degree of flexibility after lamination decreases with the number of layers though. Depending on the shape of the object to be covered with the plasma generator, the number of layers has to be considered. In our specific case, 6 layers have been used, and the flexibility was still sufficient to cover the surface presented in Figure 12. Also, a "zero-x-y-shrink" material has been used, which prevents the difficulties related to retraction of the material during firing. The LTCC shrinks mostly in the z direction (orthogonal to the LTCC layer, about 32%), but only 0.2% in the x and y directions.

Firing the system in a sandwich between two MACOR pieces cracks the stack. Indeed the systems become too constrained, even with only a few % of retracting.

Integration on a surface of any material

In order to create the plasma, the electrodes have to be connected to a voltage generator. Since the system is completely encapsulated, there has to be connection holes or inserts made in the encapsulating layers. This is shown in Figure 7. A hole is cut in the encapsulating layers so that the connection sites will be accessible after lamination and firing. Figure 8 shows a top view and a cross view of the stack with the connection sites.

If the plasma generator has to be integrated on a surface without having connections sticking out and modifying the surface shape, it is better to have both connection sites below the object. To do this, the top connection site has to be prolonged down to the bottom side. This can be achieved as pictured in Figure 9. After lamination, the stack is cut along the A-A line so that both connection sites are close to the edge. Then, conducting paste is added on the edge and bottom surface to displace the upper connection site down. Top and bottom sides of the stack then look like the drawing of Figure 10. After drying of the new connection (10 min at 80⁰C), the stack can be laid on the MACOR mock-up of the surface to be covered and fired. In our specific case, the surface to cover is the suction side of a NACA profile. The mock-up is depicted in Figure 11. Then when the stack is rigid it can be integrated on the real object. Figure 12 and Figure 13 depict metallic airfoils with an upper insert to house the plasma reactor. An additional lateral insert has been accommodated. This has been done to introduce a lateral MACOR piece that allows the isolation of the connections from each other and from the metallic object. Also, it allows the connections to be protected from the high-speed airflow in which we mount the profile. The connections between the connection sites on the plasma reactor and the high-voltage wires are made mechanically. The wires are soldered to gold or copper springs that will be introduce in the holes made in the MACOR piece and slightly push against the connection sites of the LTCC stack.

Transonic speed flow control

One promising field of use is transonic speed flow control using weakly ionized surface plasmas. For such a use the inventors have operated the plasma reactor in a wide range of conditions. The first system has been built exactly as described before, with 6 layers of LTCC, giving a final thickness of about 0.6 mm. It has been operated mainly in air but it can be operated in other gases. The working voltage frequency has been varied between 1 and 20 kHz, and for voltage rms values of the order of 1OkV or more. The airflow surrounding the system had velocities from zero to 1.2 times the speed of sound, i.e. Mach 1.2. The plasma could be sustained at atmospheric pressure down to about 0.3 bars. Ambient temperature would range from 20 to 60 ⁰C. The same plasma generator could be used for several tens of hours or more. The plasma is stable under all the conditions states before.

The system is very sensitive to the parameters of the fabrication process. Any bubbles forming in the stack (due to un-proper compression and heating) will prevent formation of plasma on the surface. Also, if the stack is too compressed and the connection sites too much deformed, the connection is insured anymore because micro-cracks have formed.

The plasma generated by such a system is composed of a glow and a filamentary component. When the speed of the gas flow is increased, the plasma becomes more and more filamentary and the physical phenomena involved in the creation of the plasma appear modified by this external flow. A study of the effects of such a surface plasma on transonic airflows, and more specifically on aerodynamic shocks and detachment of boundary layers is being carried on at the moment. Thanks to the long lifetime of our plasma generator and possibility of integration on a curved surface, such investigations are made possible.

Alternate geometries

A wide range of alternate geometries are possible, thanks to the great modularity and versatility of LTCC systems, serigraphy of electrodes and thick film technology in general (as shown in microelectronics applications of LTCC as well).

Figure 14 shows a simplified version of the system presented above. The distance to which the plasma extends around the top electrode can be regulated by the shape of the bottom electrode.

A very different set-up and its simplified version are presented in Figure 15 and Figure 16. The top as well as the bottom electrodes are stripe- shaped. The stripes from the top and bottom electrodes are offset-ed. These set-ups are generally used to generate the so- called ion wind in the surrounding gas. Forming of plasma on the side of the object can be prevented by adding more LTCC layers above the electrode on one side.

In order to try and maximize the plasma-gas flow interaction, it seems that filamentary plasma is to be preferred. In order increase the production of filaments, still another arrangement can be thought of. This is depicted in Figure 17 and the simpler version Figure 18, where two electrodes are placed on the same plane. There is only one row of electrodes encapsulated in ceramics. Stripes are alternatively connected to the positive and negative polarity of the voltage generator. The interlaced electrodes seen from top are shown in Figure 19. Again the ceramic thickness on the side where plasma is unwanted can be increased by adding more LTCC layers.

Other applications

The invention may be used in a wide range of domains, given the extended possibilities of plasma generation thanks to LTCC and the extended range of operating conditions.

A first group of applications include devices related to plasma processing. Up to now, mostly volume DBDs have been used in this area, since the set-up is simpler than surface discharge. Indeed, in the volume set-up, placing the dielectric between the electrodes is relatively simple. With our system, surface DBDs could also be used, for ozone generation, air purification and surface treatment (textile) for example. Also, they could be very efficient for scrubbing and effluent destruction (waste, NOx, toxic gases).

A second group of applications is the field of light emission. Long duration surface plasmas could now be used in lamps and displays, but also sources for coherent light emission (lasers).

In addition to that, the fields of flow control and plasma assisted combustion at low speeds but also at high speeds can greatly profit from this invention. Problems that can now start to be addressed on longer-time studies are for example, drag reduction on slow moving aircraft (at take-off and landing), sonic boom mitigation on supersonic aircraft, plasma-assisted combustion, reduction of aerodynamic losses in turbo-machinery or absorption of electromagnetic waves on curved surfaces. Finally, the system could also be integrated on spacecraft for shielding during (re-)entry phases in planet atmospheres.

The above discussed applications show that the invention opens a wide range of possibilities in different industries like plasma processing, light generation, automotive, aeronautics, energy production (turbomachinery) and space.

Claims

1. Structure having a surface on which a plasma may be generated, said structure including at least a pair of electrodes (2,4) which are adapted to generate a plasma on said surface.

2. Structure according to claim 1 wherein said electrodes are embedded in the structure.

3. Structure according to claim 2 which is made, at least partially made of LTCC.

4. Structure according to claim 3 comprising several layers.

5. Structure according to claim 4 successively comprising a first base LTCC layer

(1), a bottom electrode layer (2), a central LTCC layer (3), a top electrode layer (4) and a top LTCC layer (5).

6. Structure according to claim 5 wherein said top electrode layer (4) is made of several separate and distant electrodes.

7. Structure according to claim 5 or 6 comprising several central LTCC layers (3).

8. Structure according to anyone of the previous claims wherein said surface is curved.

9. Structure according to claim 8 forming part of a moving object.

10. Airplane wing including a structure according to claim 8.