Patent Application
20050227115
The present invention relates to a method of generating plasma in a liquid and an apparatus therefor.
Conventionally, the deposition technique with vapor phase plasma is widely made use of as a deposition technique with the use of plasma. For example, JP-A-10-81589 describes formation of a diamond film on surfaces of silicon or cubic silicon carbide with the plasma CVD method. However, with such method, it is difficult to compose a large amount of deposited substance. When a source material such as methane, etc. is quickly supplied in order to increase the deposition rate, there is a fear of incurring a danger. Also, when high-energy plasma is generated in vapor phase, the temperature rises and it is not possible to perform deposition on a substrate material having a low degree of heat resistance.
On the other hand, formation of submerged plasma by ultrasonic cavitation and superposition of local electromagnetic fields described on page 56 of the proceeding in Fourth All Ehime University Symposium is based on an ATTACHMENT B exceedingly promising thought directed to generation of plasma in a liquid, but the document and the presentation present a principle at the start of such study and in no way describe how to concretely carry out such formation.
Subsequently, conventional methods of garbage disposal are described. It is general to burn garbage in an incinerator to discharge exhaust gases to the atmosphere after soot particle is removed by an electric precipitator or an adsorption filter. There have been tested a method of absorbing and removing gaseous dioxin present in exhaust gases by blowing-in of active carbon and an active-carbon adsorption tower, a method of decomposition removal by the use of a catalytic conversion tower, but these methods of removal involve problems that after-treatment is necessary and the effect of treatment is dependent upon temperatures. That is, decomposition of dioxin is performed in high-temperature combustion reaction and so is exceedingly decreased in decomposing ability in case of low combustion temperature. Also, in currently leading methods with the use of a bag filter, it is likewise necessary to perform treatment of decreasing gaseous dioxin in toxicity after the treatment of exhaust gases. Also, PCB having been used as an insulating oil for transformers in the past is also accumulated and stored in large amount because of the absence of effective processing means.
There is described a method of decomposing dioxin, in which electromagnetic waves are irradiated on a liquid containing dioxin while the liquid is dripped so that plasma is generated for decomposition (for example, JP-A-2002-336650).
Conventional methods of treatment with the use of an incinerator involve a problem of the necessity of the treatment of finally making dioxin harmless. However, dioxin and PCB are substances, decomposition of which is exceedingly difficult, and even if it is possible to experimentally decompose a small amount of such substances, it is difficult to treat them in large amounts. A technique described in JP-A-10-81589 has its limit in throughput when a liquid is decomposed while it is dripped. Accordingly, there are desired chemical reactors and methods of cracking treatment, in which such harmful substances are effectively decomposed and can finally be made harmless. Also, it is desirable to realize chemical reactors capable of effectively composing those substances, such as diamond or the like, which cannot be composed only at high temperature/high pressure.
It is an object to provide a submerged plasma generating apparatus and a submerged plasma generating method, in which high-energy plasma macroscopically easy to handle at low temperatures is generated by generating plasma in a liquid and to provide a technique capable of effectively performing decomposition of harmful substances, which are difficult to decompose, and composition of substances, which are difficult to compose, with the use of submerged plasma having a high energy.
A submerged plasma generating apparatus according to the invention comprises bubble generating means to generate bubbles in a liquid, and an electromagnetic wave generator to irradiate electromagnetic waves in the liquid. It is possible to select a heating device, a decompression device, an ultrasonic wave generator, which irradiates ultrasonic waves in a liquid, or the like, as the bubble generating means. As the liquid, it is also possible to select one containing hydrocarbon, in particular, dodecane. A substrate mount for mounting of a substrate being an object being formed with a thin film is directed, and a thin film such as an amorphous carbon film, a silicon carbide film, etc. can be formed on the surfaces of the substrate.
The apparatus may comprise a vessel to accommodate therein a liquid, supply means to continuously supply the liquid to the vessel, and discharge means to discharge the liquid from the vessel, and may cause chemical reaction due to plasma in the liquid. When the liquid contains harmful substances, plasma may be generated in the liquid to decompose the harmful substances. In particular, when the liquid contains dioxin or polychlorinated biphenyl (PCB), the apparatus may decompose dioxin or PCB.
A submerged plasma generating method according to the invention comprises generating bubbles in a liquid, and irradiating electromagnetic waves on the bubbles to generate plasma in the bubbles. It is also possible to irradiate ultrasonic waves in a liquid containing harmful substances to generate bubbles in the liquid and to irradiate electromagnetic waves on the bubbles to generate plasma in the bubbles. In particular, the liquid may contain dioxin or polychlorinated biphenyl.
The invention will be described in more detail with reference to the drawings.
Embodiments of the invention will be described below.
A tip end of an ultrasonic wave generator 4 making use of a horn type oscillator is put in a liquid 3 to irradiate ultrasonic waves in the liquid. The ultrasonic waves generate numerous bubbles cloudily. That is, the ultrasonic wave generator 4 acts as means that generates bubbles in the liquid. As the bubble generating means, a heating device to heat a liquid, a pressure reduction device, such as vacuum pump, etc., to reduce pressure in the vessel 2, or the like may be used in addition to an ultrasonic wave generator. A substance of the liquid 3 such as dodecane, etc. in gaseous phase is contained in bubbles and ultrasonic waves cause gases in the bubbles to quickly repeat expansion and contraction. A substantially adiabatic compression is performed at the time of contraction, so that superhigh pressure and high temperature are established in the bubbles to be liable to generate plasma.
An electromagnetic wave generator 5 is provided in the submerged plasma generating apparatus according to the invention to irradiate electromagnetic waves to a position in the liquid 3, where bubbles are generated. It suffices to select frequency and output of electromagnetic waves according to type and intensity of plasma being generated, and microwave in the order of 2 GHZ or higher is mainly used. High-energy plasma is generated in bubbles by superposition of electromagnetic waves in a state of high temperature and high pressure, which state is caused by ultrasonic waves. In addition, the vessel 2 in the embodiment is received in an external vessel 6 and can be reduced in pressure by means of a vacuum pump 7. Pressure reduction makes it easy for bubbles to be generated in the liquid, thus enabling to facilitate the starting a plasma generation. When the ultrasonic wave generator 4 is used as bubble generating means and plasma is once generated to be made stable, plasma continues in most cases even when the vacuum pump is stopped and atmospheric pressure is restored.
In the above manner, it is possible to generate high-energy plasma having a high density in the liquid. Since plasma has been already confined in bubbles, confinement of plasma thus generated, which presents an important problem in plasma technology, is not problematic in the invention. While plasma of high temperature and high pressure is generated locally, it is confined in a liquid having a large thermal capacity, so that temperature is macroscopically low. Accordingly, outside thereof and things in contact with the plasma are not heated. Since plasma thus generated is high in temperature, pressure, and energy density and easy to handle, it is usable for vapor deposition, chemical compounding of substances, nuclear fusion, or the like. Further, it is applicable to electrolysis in a high-energy state and usable for decomposition of toxic substance such as dioxin, etc. Bubbles generated by acoustic cavitation include single bubbles and multi-bubbles, and the invention is applicable to the both. While single bubbles are small as a whole in energy, a high-energy state of 5000 K to 100000 K is obtained in the bubbles only by irradiation of ultrasonic waves, so that they are effective for reactors of nuclear fusion, or the like. On the other hand, multi-bubbles are a little low in temperature and in the order of 5000 K or so in case of only irradiation of ultrasonic waves, but are large as a whole in energy and advantageous in industrial use such as deposition processing, or the like.
While the plasma generating apparatus according to the invention is simple and small in size and can be formed in a size to be placed on a desk, it can be also large-scaled with the use of an ultrasonic wave generator and an electromagnetic wave generator, which are high in output.
Subsequently, the invention will be further described on the basis of embodiments.
After irradiation of ultrasonic waves and microwaves, there existed a brown dissolved material and a black material of poor solubility generated from dodecane. Since the molecular density is very high in liquid phase as compared with that in vapor phase, a far higher rate of composition than that in a vapor deposition method is obtained. Also, the liquid produces a cooling effect owing to deposition in the liquid, a material having a low degree of heat resistance can be used as the substrate, and deposition can be processed on a surface of a material, such as plastic, paper, wood, etc., which has not been conventionally conceivable. High-energy plasma is generated in hydrocarbon such as dodecane or the like to be able to perform composition and deposition of diamond and to perform composition of fullerene and carbon nanotube. Further, coating of TiC, SiN, TiN, etc. can be performed by means of various liquids other than dodecane. In addition, since plasma can be continued, the substrate mount is made movable to enable performing deposition in a wide range on surfaces of the substrate.
Plasma can be generated with the use of the submerged plasma generating apparatus 1 shown in
While it suffices to select electromagnetic waves as irradiated among ones having a frequency of 13 MHz to 2.5 GHz depending on the uses, electromagnetic waves having a frequency of 2.5 GHz and an output of 100 W were used in the embodiment because of being effective in formation of a diamond film and advantageous in generation of bubbles by heating.
Plasma generated in bubbles rose together with bubbles to come into contact with the substrate 9. Thereby, carbon making high-energy plasma formed a film on surfaces of the substrate 9.
According to the embodiment, the electrode 10 unite s the functions of bubble generating means and electromagnetic wave generating means (electromagnetic wave probe), and a set of electromagnetic-wave supply circuits is suffice, so that there is produced an advantage that the construction becomes simple. Also, since the substrate 9 being an object for film formation is not directly charged with electricity and heated, it is possible to form a film on surfaces of a non-conducting material, such as wood, paper, etc., and a material, such as synthetic resin, biomaterial, etc., having a low degree of heat resistance.
According to the embodiment, a mixture of dodecane and a silicone oil with a ratio of 1:1 was used as the liquid 3 and a silicon carbide film was formed on the substrate 9 made of a silicone plate. Electromagnetic waves having a frequency of 2.5 GHz and an output of 100 W were irradiated for 3 minutes, and a silicon carbide film having a thickness of 1.2 mm could be formed on the substrate 9. Accordingly, the forming rate of the film reached 0.4 mm/in. According to the embodiment, a vacuum pump was actuated to reduce pressure to around 104 Pa in the vessel 2. The silicon carbide film thus formed has a high hardness and can be applied in increasing the hardness of a cutting tool to extend its service life and improve its sharpness.
A tank 13, which stores a liquid containing harmful substances, is provided and a supply pipe 14, through which the liquid is supplied to the vessel 2 from the tank, is connected to an upper portion of the vessel 2. Also, the supply pipe 14 is provided with a first control valve 15. On the other hand, a discharge pipe 16 as discharge means for discharging of the liquid from the vessel 2 is provided on a lower portion of the vessel 2 to afford discharging the liquid to a waste liquid port 17. The discharge pipe 16 is provided with a second control valve 18.
Thus, the embodiment is of a sequential processing system, in which the supply pipe 14 and the discharge pipe 16, respectively, are provided with the openable control valve, and a process is repeated, in which a predetermined amount of the liquid is supplied to the vessel 2 from the tank 13 to carry out decomposition processing, and the liquid having been made harmless is discharged from a discharge port 11.
The vessel 2 is in the form of a closed tank provided with an intake pipe 19, through which a carrier gas is introduced from outside, and an exhaust pipe 20 connected to the vacuum pump 7. Harmless gases generated in the decomposition processing of harmful substances are discharged outside the vessel 2 through the exhaust pipe 20 by the vacuum pump 7. The vacuum pump 7 can be also used to reduce pressure in the vessel 2.
In the embodiment, plasma having a temperature of 5000 K was generated. When decomposition processing was performed in a solution, in which 1 μg of OCCD (octachlorodibenzoparadioxin) was added to 80 ml of n-dodecane, all of OCCD in the solution could be decomposed in 30 seconds. In addition, the ambient atmospheric pressure was regulated to 500 Pa.
According to the invention, bubbles are generated in a liquid and electromagnetic waves are irradiated on the bubbles whereby it is possible to safely and readily generate high-energy plasma in the liquid. The submerged plasma is widely usable for formation of a film on a substrate, effective decomposition processing of recalcitrant, harmful substances such as dioxin, PCB, etc., or the like.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2002-98193 | Apr 2002 | JP | national |
| 2002-313979 | Oct 2002 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP03/04139 | 3/31/2003 | WO | 6/6/2005 |
