WO2011099247A1 - Electrode for plasma in liquid, plasma in liquid generator device, and plasma generation method - Google Patents

Abstract

An electrode for plasma in liquid (1) comprises an extended inner conductor (2); a dielectric (3) disposed upon the outer circumference of the inner conductor (2); and an outer conductor (4) disposed upon the outer circumference of the dielectric (3). The leading end portion (2a) of the inner conductor (2) is covered by the dielectric (3), electromagnetic waves are transmitted in either TEM mode or quasi-TEM mode, allowing stable generation of plasma in liquid even at low power levels, reducing the risk of defects occurring, and ensuring that the metallic components of the inner conductor (2) will not escape into the liquid.

Description

Electrode for plasma in liquid, plasma generator in liquid, and plasma generation method

The present invention relates to a plasma generation apparatus and a plasma generation method for generating high energy plasma in a liquid or a supercritical fluid.

Non-Patent Document 1 describes a method in which the surface of an inner conductor of a coaxial electrode is covered with a dielectric, and surface wave plasma is generated in a gas phase on the surface of the dielectric.

On the other hand, Patent Documents 1 to 4 and Non-Patent Documents 2 to 4 describe generating plasma in a liquid. In these submerged plasmas, a high voltage is applied to the electrode of the conductor. As an application method, Patent Document 1 uses high frequency, and Patent Document 2 uses microwave. Also, in order to facilitate the generation of plasma in liquid, a technique of devising the electrode shape, irradiating ultrasonic waves as in Patent Document 3, or irradiating laser as in Patent Document 4 has been proposed. Yes. Furthermore, in Non-Patent Document 2, a slot antenna is used to generate plasma in liquid without contact with the metal surface.

International Publication No. 2006/059808 Pamphlet Japanese Unexamined Patent Publication No. 2009-181960 Japanese Patent No. 3624238 Japanese Unexamined Patent Publication No. 2005-235661

M.Fuener, C.Wild, P.Koidl, Numerical simulationsbof microwave plasma reactors for diamond CVD, Surface and Coatings Technology 74-75 (1995) 221-226 T.Ishijima, H.Hotta, and H.sugai, Multibaubble plasma and solvent decomposition in water by spot-excited microwave discharge, Applied Physics letters 91, 121501 (2007) Yoshinori Hattori, Shinobu Mukagasa, Nobufuku Nomura, Hiromichi Toyota, Bubble Behavior and Ambient Temperature of Liquid Plasma, Thermal Science & Engineering Vol.15 No. (2007) Maehara T. et. Al, Plasma Chem. Plasma Process. 28 (4) 467 (2008)

The invention described in Non-Patent Document 1 relates to a technique for generating plasma in a gas phase. Since the gas has a low material density, and the reaction rate is low even if plasma is generated there, the processing speed is limited even if coating is performed.

On the other hand, according to the method of generating plasma by irradiating electromagnetic waves in a liquid as described in Patent Documents 1 to 4 and the like, the molecular density in the liquid phase is extremely high compared to the gas phase, Compared with the conventional gas-phase plasma CVD method, a higher reaction rate can be obtained in the vapor deposition processing using plasma in liquid. However, in order to generate plasma, it was necessary to supply high frequency with high power. In addition, the conventional methods for generating plasma in liquid have a problem that the plasma is generated on the surface of the metal electrode, so that the metal electrode is damaged when continuously used. This is a problem of the life of the electrode, and also includes a problem that it flows out into a liquid such as a metal component and becomes an impurity in the application of plasma in liquid.

It is an object of the present invention to provide an in-liquid plasma electrode, an in-liquid plasma generator, and a plasma generation method that can stably generate plasma in liquid even with a small electric power and are less likely to be damaged. .

In order to solve the above problems, an in-liquid plasma electrode according to the present invention includes an extending inner conductor, a dielectric provided on the outer periphery of the inner conductor, and an outer conductor provided on the outer periphery of the dielectric. The tip of the inner conductor is covered with a dielectric. Furthermore, the dielectric has a first dielectric on the inner circumference side and a second dielectric provided on the outer circumference of the first dielectric, and the dielectric constants of the first dielectric and the second dielectric are different. preferable. In the case of using a liquid having a high dielectric constant such as an electrolyte, water, ethanol, etc., it is particularly preferable that the dielectric constant of the first dielectric is higher than that of the second dielectric. More preferably, the height of the outer conductor tip is substantially the same as the height of the inner conductor tip.

A submerged plasma generator according to the present invention includes the above-described submerged plasma electrode, a power supply for electromagnetic waves connected to the submerged plasma electrode, and a liquid container, and the submerged plasma electrode is in liquid contact. Part is inserted into the liquid container, and the other end is connected to the power supply for electromagnetic wave supply, and the liquid container is irradiated with electromagnetic waves from the liquid contact part of the liquid plasma electrode, and plasma is generated in the liquid. It has been made to generate.

Furthermore, the plasma generation method according to the present invention includes an extending inner conductor, a dielectric provided on the outer periphery of the inner conductor, and an external insulating member provided on the outer periphery of the dielectric, and the tip of the inner conductor. A plasma electrode whose part is covered with a dielectric is brought into contact with a liquid or a supercritical fluid at its tip, and electromagnetic waves are supplied in a TEM mode or a quasi-TEM mode which is a transmission mode slightly deviated from the TEM mode, and plasma Electromagnetic waves are applied to the liquid or supercritical fluid from the tip of the electrode for generation, and are generated in the liquid or supercritical fluid.

The electrode for submerged plasma, the submerged plasma generation device, and the submerged plasma generation method according to the present invention have an effect that plasma can be generated in the liquid even with low power. Further, the electrode is not damaged and the metal component does not flow into the liquid.

It is a conceptual diagram which shows the 1st example of the electrode for plasma in a liquid. It is a conceptual diagram which shows the 2nd example of the electrode for plasma in a liquid. It is a graph which shows the numerical calculation result of the electric field of the dielectric material surface in the case of microwave transmission. It is a conceptual diagram which shows the example of an in-liquid plasma generator. It is a conceptual diagram which shows the example which uses a microwave. It is a graph which shows the relationship between plasma generation time and metal concentration. It is a conceptual diagram which shows the example applied to vapor deposition processing. It is a photograph which shows the vapor deposition film formed by the 4th Example. It is a photograph which shows the vapor deposition film formed by the comparative example.

EMBODIMENT OF THE INVENTION The form for implementing this invention is demonstrated based on drawing. FIG. 1 is a conceptual diagram showing a first example of an in-liquid plasma electrode, which is a schematic longitudinal sectional view.

The plasma electrode 1 has an extending inner conductor 2, a dielectric 3 provided on the outer periphery of the inner conductor 2, and an outer conductor 4 provided on the outer periphery of the dielectric 3. The tip 2 a of the inner conductor 2 is covered with the dielectric 3.

The plasma electrode 1 has an inner conductor 2 at the center of the cross section, and has a coaxial line structure in which an outer conductor 4 is disposed coaxially with the inner conductor 2. One end (the lower side in FIG. 1) is connected to the electromagnetic wave supply source 5. The opposite end (upper side in FIG. 1) is the liquid contact portion 1a.

The material for the inner conductor 2 is not particularly limited as long as it is a highly conductive substance. Since the inner conductor 2 is structured to be covered with the dielectric 3 as described later, the inner conductor 2 is not necessarily solid. It can also be used by filling a liquid metal such as mercury in a closed tubular dielectric. When a brittle material is used as the dielectric 3, a material having a low coefficient of thermal expansion is preferable for the inner conductor 2. The narrower inner conductor 2 can generate plasma with lower power. The shape is not particularly limited as long as the electromagnetic wave can be transmitted in the TEM mode or the quasi-TEM mode.

In the plasma electrode 1, plasma is generated on the surface of the dielectric 3. Here, it is not a thermal equilibrium plasma in which the gas temperature reaches several thousand degrees, but a non-equilibrium plasma in which the electron temperature and the gas temperature do not coincide with each other. This plasma is a glow discharge with a low gas temperature. Therefore, as the material of the dielectric 3, a material that can withstand the plasma gas temperature, such as heat-resistant glass or alumina, is used.

The outer conductor 4 is not particularly limited as long as it is a highly conductive material. Moreover, if electromagnetic waves can be transmitted in TEM mode or quasi-TEM mode, the shape is not particularly limited. Since high frequency electromagnetic waves are transmitted in the coaxial line of the plasma electrode, current flows almost only on the conductor surface due to the skin effect. Therefore, the outer conductor 4 does not need to be particularly thick as long as it covers the periphery of the dielectric, and a thin coaxial outer conductor or mesh outer conductor is used to construct a flexible coaxial line to transmit electromagnetic waves. Is also possible.

An electromagnetic wave is transmitted to the coaxial line of the plasma electrode 1 in a TEM mode or a quasi-TEM mode to generate plasma. It is preferable that the positions (heights) of the tips of the outer conductor 4, the dielectric 3, and the inner conductor 2 are substantially the same.

Next, a second example of the plasma electrode 1 will be described. FIG. 2 is a conceptual diagram showing a second example of the electrode for plasma in liquid, and is a schematic longitudinal sectional view. Note that detailed description of matters common to the first example is omitted here.

The plasma electrode 1 of the first example can also generate plasma on the surface of the dielectric. However, if the supplied power is increased or the diameter of the inner conductor 2 is reduced, the air contact surface between the inner conductor 2 and the dielectric 3 will be reduced. Phase plasma may be generated. In order to solve this problem, the dielectric of the plasma electrode 1 of this example has a plurality of dielectric layers. Here, it has the 1st dielectric material 3a of the inner peripheral side, and the 2nd dielectric material 3b provided in the outer periphery of the 1st dielectric material, and the dielectric constants of the 1st dielectric material 3a and the 2nd dielectric material 3b differ.

In the plasma electrode 1 of this example, the electric field locally increases at the boundary between the first dielectric 3a and the second dielectric 3b. Thereby, plasma can be generated with lower power. This is due to the action caused by the difference in dielectric constant of the medium. FIG. 3 is a numerical calculation that calculates the electric field on the dielectric surface when a 2.45 GHz microwave is transmitted to the coaxial electrode. It was confirmed that the electric field increased at the boundary between the first dielectric 3a and the second dielectric 3b. It is particularly preferable that the dielectric constant of the first dielectric is higher than that of the second dielectric.

It is preferable to align the positions (heights) of the tips of the outer conductor 4 and the second dielectric 3b to substantially the same height. In addition, it is preferable that the tip of the inner conductor 2 is aligned with the same position (height) of the tip of the outer conductor 4.

Next, an in-liquid plasma generator and generation method will be described. FIG. 4 is a conceptual diagram showing an example of the in-liquid plasma generator. The in-liquid plasma generator 10 includes an electromagnetic wave supply power source 5 and a liquid container 11 in addition to the plasma electrode. The liquid contact portion 1 a of the in-liquid plasma electrode 1 is inserted into the liquid container 11, and the other end is connected to the electromagnetic wave supply power source 5. A liquid or a supercritical fluid can be placed in the liquid container 11, and at this time, the liquid contact portion 1a is in contact with the liquid or the supercritical fluid.

The liquid container 11 only needs to be capable of holding a liquid or a supercritical fluid during and before and after plasma generation. The present invention may be applied not only to a normal liquid but also to a supercritical fluid. Therefore, in the present invention, the term “liquid” includes supercritical fluid. The in-liquid plasma electrode 1 is inserted into the liquid container 11, but the number, position, or orientation of attachment can be arbitrarily selected.

The inside of the liquid container 11 may be atmospheric pressure, but generally, plasma is more easily generated when the pressure is lower. Therefore, here, a sealed container is used, and a pump 12, a pressure regulating valve 13 and a pressure gauge 14 are provided so that pressure can be reduced or increased.

The frequency of the electromagnetic wave may be appropriately selected according to the application of the liquid or plasma used, and may be used in the range of about 3 MHz to 200 GHz. The circuit is configured such that alternating current such as high frequency or microwave is transmitted through the electrode 1. In order to supply energy efficiently, it is preferable to configure so as to facilitate matching.

FIG. 4 is a conceptual diagram showing an example using microwaves. A solid circuit for transmitting a microwave to a coaxial line is shown. In this example, matching is performed by the stub tuner 15 and the plunger 16, and the coaxial waveguide converter 17 is adjusted so that energy is efficiently sent from the microwave generator 18 to the coaxial line.

A first embodiment of the present invention will be described. Plasma was generated using pure water as the liquid. The pressure inside the liquid container 11 was 6 kPa, and the electromagnetic wave used was 2.45 GHz. The in-liquid plasma electrode uses the example shown in FIG. The inner conductor 2 is zinc having a diameter of 3 mm. The first dielectric 3a is heat-resistant glass (Pyrex, registered trademark) having an inner diameter of 3 mm and an outer diameter of 6 mm, and the second dielectric 3b is a fluororesin and has an inner diameter of 6 mm and an outer diameter of 9 mm. The outer conductor 4 is brass.

Plasma was stably generated by a microwave of 75 W power. This is about one-tenth the electric power compared to Non-Patent Document 2 that generates plasma in microwave liquid at about 750 W. Although plasma was generated continuously for 1 hour, no damage was observed in the electrodes.

The emission spectrum of plasma generated in pure water was examined with a spectrometer. In this emission spectrum, emission due to H _α , H _β and OH radicals was observed. This is because water molecules are turned into plasma. From this, it can be seen that the in-liquid plasma electrode of the present invention is effective for decomposing harmful substances in water.

Moreover, the same measurement was performed using the electrode for plasma in a liquid described in the nonpatent literature 4 as a comparative example. In the emission spectrum of the plasma in this comparative example, a luminescent component attributable to the conductor metal was observed. That is, it was confirmed that the surface of the conductive metal of the liquid plasma electrode of the comparative example was damaged by being turned into plasma.

A second embodiment of the present invention will be described. In this example, damage to the plasma electrode was measured in more detail. In the conventional liquid plasma electrode, it is considered that the electrode surface is damaged by the generation of plasma, and the material components of the electrode flow out into the liquid. Therefore, the metal concentration in the liquid is measured by emission spectroscopic analysis.

Plasma was generated using pure water as the liquid. The pressure inside the liquid container 11 was 6 kPa, and the electromagnetic wave used was 2.45 GHz. The in-liquid plasma electrode uses the example shown in FIG. The inner conductor 2 is zinc with a diameter of 3 mm, and the tip is conical. The first dielectric 3a is a heat-resistant glass having an outer diameter of 6 mm, and the second dielectric 3b is a fluororesin having an outer diameter of 15 mm. The material of the outer conductor 4 is copper. Further, as a comparative example, the same measurement was performed using an in-liquid plasma electrode exposed in a metallic inner conductor described in Non-Patent Document 3. In the liquid plasma electrode of this comparative example, the inner conductor is zinc with a diameter of 3 mm having a conical tip, the dielectric is a fluororesin with an outer diameter of 15 mm, and the outer conductor is copper.

FIG. 6 is a graph showing the relationship between plasma generation time and metal concentration. The horizontal axis indicates the time during which plasma is generated, and the vertical axis indicates the concentration of zinc ions that are the material of the inner conductor. The dotted line is the data of this example, and the solid line is the data of the comparative example. In the submerged plasma electrode of the comparative example, the concentration of zinc ions increases with the plasma generation time. That is, it was confirmed that the electrode was damaged by the generation of plasma. On the other hand, in the submerged plasma electrode of the example, the concentration of zinc ions does not change with the passage of time and remains at a low value. Therefore, even if the electrode for plasma in liquid of this invention is used, the material component of the electrode does not flow out into the liquid as impurities. For example, a high-purity vapor deposition film can be formed when used for vapor deposition as in the third embodiment described later.

A third embodiment of the present invention will be described. In this embodiment, the same plasma electrode as in the first embodiment is used, and a carbon film is deposited on the substrate. Ethanol was used as the liquid. FIG. 7 shows a schematic diagram in which the electrode of FIG. 2 is used for vapor deposition. Moreover, the board | substrate which is the object of a vapor deposition process is a copper plate with a thickness of 0.3 mm. Also here, as a comparative example, a carbon film vapor deposition was carried out under the same conditions using a plasma electrode with a tip portion of a metallic inner conductor described in Non-Patent Document 3 exposed.

FIG. 8 is a photograph showing a deposited film formed according to this example. The carbon film appears white. Metal components are not included. By generating plasma in ethanol with the plasma electrode of the present invention, a good carbon film could be deposited on the substrate. Therefore, it was shown that this invention is applicable to vapor deposition processing. On the other hand, FIG. 9 is a photograph showing the deposited film formed by this comparative example. There was no carbon film at the center of the electrode, instead copper was deposited as the electrode material. From these results, it was confirmed that when a conventional bare electrode was used, the electrode material was deposited on the substrate, and a high-purity film could not be deposited.

A fourth embodiment of the present invention will be described. In this embodiment, the structure is the same as that of the first embodiment, but plasma in liquid is generated by a plasma electrode using alumina (ceramic) having plasma resistance instead of heat resistant glass as the first dielectric 3a. Generated. Other conditions are the same as in the first embodiment.

Also in this example, plasma in liquid could be generated. By using alumina as a dielectric, plasma resistance is improved and higher power can be supplied.

1. Electrode for plasma in liquid 2. 2. Inner conductor Dielectric 4. 4. outer conductor 10. Power supply for electromagnetic wave supply 10. In-liquid plasma generator Liquid container

Claims (7)

An in-liquid plasma having an extending inner conductor, a dielectric provided on the outer periphery of the inner conductor, and an outer conductor provided on the outer periphery of the dielectric, the tip of the inner conductor being covered by the dielectric Electrode. The dielectric according to claim 1, wherein the dielectric has a first dielectric on the inner circumference side and a second dielectric provided on the outer circumference of the first dielectric, and the dielectric constants of the first dielectric and the second dielectric are different. The electrode for plasma in liquid described. The liquid plasma electrode according to claim 2, wherein a dielectric constant of the first dielectric is higher than a dielectric constant of the second dielectric. The electrode for submerged plasma according to any one of claims 2 to 3, wherein the height of the tip of the outer conductor is practically the same as the height of the tip of the inner conductor. A liquid plasma electrode according to any one of claims 1 to 4, an electromagnetic wave supply power source connected to the liquid plasma electrode, and a liquid container,
The liquid contact portion of the liquid plasma electrode is inserted into the liquid container, and the other end is connected to the electromagnetic wave supply power source,
A submerged plasma generator that generates a plasma in a liquid by irradiating a liquid container with an electromagnetic wave from a liquid contact portion of the submerged plasma electrode. For plasma that has an extending inner conductor, a dielectric provided on the outer periphery of the inner conductor, and an external insulating member provided on the outer periphery of the dielectric, the tip of the inner conductor being covered by the dielectric The electrode is brought into contact with the liquid or supercritical fluid at the tip, electromagnetic waves are supplied in the TEM mode or quasi-TEM mode, and the liquid or supercritical fluid is irradiated with the electromagnetic waves from the tip of the plasma electrode. A method for generating plasma in a critical fluid. The dielectric has a first dielectric on the inner circumference side and a second dielectric provided on the outer circumference of the first dielectric, and a plasma electrode having a different dielectric constant between the first dielectric and the second dielectric The plasma generation method according to claim 6 to be used.

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