1. Field of the invention
The present invention relates to an apparatus for generating low-temperature plasma in a high density at atmospheric pressure with low discharge initiation and maintenance voltages.
2. Description of the Prior Art
Generally, plasma is defined as a partially ionized gas composed of a nearly equal number of positive and negative free charges so that it is electrically neutral. Subgrouped into high-temperature and low-temperature plasma according to the temperature at which it undergoes ionization, plasma is of very high reactivity, chemically and physically.
Low-temperature plasma is used to synthesize various materials, such as metals, semiconductors, polymers, nylon, plastic, paper, fiber, and ozone, or to modify surface properties of materials with a concomitant improvement in various physical and chemical properties such as junction strength, dyeing properties, printability, etc. Accordingly, low-temperature plasma finds numerous applications in semiconductor, metal, ceramic thin film synthesis, and cleaning fields.
Typically, low-temperature plasma can be generated in a vacuum vessel of low pressure. In order to maintain such a vacuum, there is needed an apparatus, which is expensive on the whole. Additionally, if materials to be treated are large in size, it is difficult to apply plasma to them. Another problem with plasma treatment is difficulty in automation of plasma processes. Further to these, plasma has difficulty in treating materials which show high vapor pressures or are degassed, such as rubber, biomaterials, etc.
To avoid these problems, there have been developed various techniques, exemplified by corona discharge, dielectric barrier discharge and glow discharge, by which low-temperature plasma can be generated at atmospheric pressure. These techniques are now applied to a broad range of fields, including synthesis of chemicals, such as ozone, sterilization, detoxification, and synthesis of materials which are difficult to treat with plasma in vacuo, in addition to the fields mentioned above.
A corona discharge is a discharge of electricity appearing as a bluish-purple glow on the surface of and adjacent to a conductor when the voltage gradient exceeds a critical value. In general, by applying a high voltage across two pointed electrodes made of conductive materials, such as metal, streamer plasma is generated from the electrodes. When a voltage is applied across two electrodes with a very short distance therebetween, an arc is generated, forming linear plasma with a very small diameter. At this time, to prevent the plasma from being converted to arc discharge, the voltage is intermittently applied or a resistance is provided to the electrodes.
A dielectric barrier discharge utilizes the charge accumulation resulting from dielectric polarization to form a reverse potential at which the discharge is halted, that is, it takes advantage of a pulse discharge, thereby preventing the development of arc discharges.
In the case of a corona discharge, plasma is generated in the form of a streamer that is not homogeneous and is low in density. Additionally, because the gap between two electrodes is narrow, a corona discharge is difficult to apply to targets of three-dimensional shape. Also, other problems with the coronal discharge include noise generation and a short electrode lifetime.
Although providing homogenous plasma, the dielectric barrier discharge does not ensure the generation of homogenous, diffused plasma over a large area, as in the corona discharge. Where an additional means is provided for preventing the development to an arc discharge, the dielectric barrier discharge is low in plasma density, and the distance between two electrodes is so narrow as to limit the size and shape of a target to be treated.
When gases with high discharge initiation and maintenance potentials, such as argon, oxygen and nitrogen, are used, both the corona discharge and the dielectric barrier discharge techniques require a high-voltage power supply. However, the power supply is difficult to operate and manage because of its being expensive and high in electricity consumption.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide an apparatus for generating low-temperature plasma at atmospheric pressure, which has such a novel structure of electrodes as to prevent the conversion of the plasma to an arc discharge.
It is another object of the present invention to provide a plasma-generating apparatus, which is so low in discharge voltage as to greatly reduce the operating and installment cost and electricity consumption of the power supply equipped.
It is a further object of the present invention to provide a plasma-generating apparatus, which can take advantage of alternating currents and pulse direct currents in a broad bandwidth of frequencies.
It is still a further object of the present invention to provide a plasma-generating apparatus, which can discharge in gases of high discharge initiation potentials, such as nitrogen, oxygen and the air.
It is still another object of the present invention to provide a plasma-generating apparatus, which can generate a homogeneous, high-density, low-temperature plasma at a low discharge voltage over a large area.
Based on the present invention, the above objects could be accomplished by a provision of an apparatus for generating low-temperature plasma at atmospheric pressure, comprising: a couple of electrodes facing each other at a distance, one of them being connected to a power supply, the other being grounded; a couple of dielectrics with a thickness of 25 xcexcm-10 mm, positioned on the facing surfaces of the electrodes in such a way as to face each other, one of them having at least one discharge gap therein; and a conductor electrode having at least one tip positioned within the discharge gap, in which an electric field is applied at an intensity of 1-100 KV/cm through the power supply across the electrodes by use of a pulse direct current or an alternating current in a frequency bandwidth of 50 Hz-10 GHz while a reaction gas is fed between the electrodes.
The plasma generated from the apparatus of the present invention is suitable to form radicals of high energy, which have numerous applications in various fields, including bonding, polishing, cleaning, thin films deposition, sterilization, disinfection, ozone generation, printing, dyeing, etching of various materials such as metal, rubber, fibers, paper, synthetic resins and semiconductors. Also, application fields of the plasma include purification of tap water and waste water, purification of air and automobile exhaust gas such as SOx and NOX, combustion of fuels, manufacture of highly luminous lamps, etc.