Arc discharge in aqueous electrolytes (for example, welding under seawater), is widely used in engineering and construction, and is at present the only known form of stationary plasma discharge in liquid media. In recent years, such discharge was also used in different physicochemical studies and in the synthesis of various materials. The specific feature of arc discharge in liquid media is the localization of a plasma region near the electrode ends and a “falling” form of volt-ampere characteristic as illustrated in FIG. 1.
In a gaseous phase, different kinds of discharges can be implemented, the external manifestation and electrical parameters of which are connected with a wide range of technical characteristics for devices used in their implementation and a variety of elementary processes determining the conditions of current passage through gas. The essential feature of electric discharge development in the gaseous phase is a profound effect of the properties of the gas medium on the current passages through the gas.
Under usual conditions, the concentration of charge carriers (electrons and ions) in the gas is very low: a gas is a very good dielectric. For a gas to have a high electrical conductivity (as a result of ionization) it is necessary for a high quantity of charge carriers to be present, requiring in turn a great quantity of energy. Gases have a steady electric conductivity when there is equilibrium between the origination and disappearance of charges. Thus, to create a means by which high electrical conductivity in a gas can be achieved through substantially lower energy requirements than has been taught in the prior art is highly desirable.
If the rate of movement of electrical charges is proportional to the field strength, the conductivity of gas approximately obeys Ohm's law (FIG. 2, section a). With increasing field strength, the decrease of electrical charges begins to have an influence (FIG. 2, section b) because of the migration of the charges to the electrodes. Further increases of the electrical field strength result in a steep increase of current due to the start of collision ionization (FIG. 2, section c). In spite of the avalanche-like character of current increases, the existence of external ionizer(s) is needed to sustain the electrical discharge, and the discharge remains being as not self-sustained (region 1). Eventually, a point is reached where for each electron leaving the cathode, one or more electrons arrive at the anode, in a phenomenon known as breakdown discharge (glow discharge or plasma discharge). This causes a self-sustained electrical current from the cathode to the anode. However, the current state-of-the-art process requires a large amount of energy to reach this self-sustaining threshold. Since high energy requirements directly and indirectly decrease the overall economy of the model, the requirement of high energy is undesirable. Therefore, it is highly desirable to have a new process having low energy demand in which the transition from non self-sustained discharge to self-sustained discharge (glow discharge) would occur with a low-energy input.
Again referring to FIG. 1, which illustrates the prior art, the voltage-current characteristic curve for glow discharge preferably comprises three sections, referred to for the sake of clarity as subnormal section or subnormal mode (FIG. 2, section d), normal section or normal mode (FIG. 2, section e) and abnormal section or abnormal mode (FIG. 2, section f).
Further increase of current density on the cathode causes the appearance of electric arc, as well as a drastic change of the main characteristics of the discharge (FIG. 2, section g).
It should be noted that the appearance or threshold of discharges in the gas phase depends considerably on the pressure of the gas. Thus, in the case of a uniform field of breakdown voltage (self-maintained discharge initiation voltage) the threshold is determined by the product of pressure by the distance between the electrodes, according to Paschen's Law. Pachen determined that breakdown voltage is determined by the following equation:
  V  =            a      ⁡              (        pd        )                            ln        ⁡                  (          pd          )                    +      b      where V is the breakdown voltage in Volts, p is the pressure in atmospheres, d is the gap distance in meters, and a and b are constants that depend upon the particular gas between the electrodes. Thus, in contrast to liquids, which are relatively incompressible, different forms of electric discharge can be implemented in gases by varying the pressure of the gas between the electrodes.
Moreover, when ultrasonic cavitation, a sort of “cold boiling” resulting from the creation and collapse of zillions of microscopic bubbles in the liquid caused by ultrasonic waves, is implemented within a liquid, its phase composition and physical properties abruptly change, which can lead to some specific features for the formation of electric discharges within the liquid. In the region of intense cavitation, a gaseous component is formed which represents a significant fraction of the liquid. Therefore it can be assumed that the conditions for electric breakdown into the cavitation region should become easier, and the initiation of different forms of discharge could start through use of this invention. By varying the parameters of an ultrasonic field, it is possible to influence the processes of plasma glow within a cavitating liquid.
The prior art has several examples of attempts to resolve this problem.
However, few patent applications or patents work in the abnormal mode. In abnormal mode, also known as abnormal glow, effectively all of the gas molecules must be ionized to provide charge carriers for the current. Typically, the gas molecules are ionized multiple times meaning that more than one electron has been freed for most of the gas molecules. This creates a relatively uniformly distributed plasma across the electrodes. A higher density (or pressure) of gas molecules, on the other hand, would lead to a normal mode, or normal glow discharge. In this region, fewer than all of the molecules are ionized. This creates a situation where plasma forms in a relatively small region between the electrodes. A plasma discharge of this type can lead to concentrated energy in a relatively small area and possibly lead to electrode damage. Therefore, it is preferable to work in the abnormal mode.
Those patent applications or patents that do work in the abnormal mode, like U.S. Pat. No. 5,068,002, to Monroe, do not use an electrode as the radiator, in the same way that the instant application uses it, whereby the current application discloses a very low energy consumption jointly with a very low voltage to initiate and maintains a volumetric discharge which generates operational advantages in term of achieving the goals of this application. Monroe describes an ultrasonic glow discharge surface cleaning apparatus for abrading contaminants from the surface of a work piece using plasma glow discharge.
For example, in US Patent Application 2004/0265137 A1 to Bar-Gadda, a method is proposed for hydrogen production from water or steam by means of plasma discharge excited in the UHF, radio- or low-frequency range, as well as with arc discharge. This application describes the injection of water molecules into plasma discharge.
U.S. Pat. No. 7,070,634 B1 A1 to Wang describes a plasma apparatus for converting a gaseous mixture of water vapor and hydrocarbons into hydrogen.
US Patent Application 2006/0060464 to Chang teaches a fluid phase contained in a reactor, within which electrodes (anode and cathode) are placed. A flow of gas bubbles is introduced or generated in the medium in the region adjacent to the cathode. The potential difference necessary for the initiation of glow discharge and for the ionization of gas molecules in the bubbles is applied between the cathode and the anode.
U.S. Pat. No. 7,067,204 to Nomura et al., describes an apparatus comprising an ultrasonic generator for creation of bubbles within a liquid, and a generator providing the excitation of electromagnetic waves in the liquid phase, for the implementation of the plasma discharge.
Japanese Application JP2006273707 to Shibata et al. relates to the publication, “Synthesis of amorphous carbon nanoparticles and carbon-encapsulated metal nanoparticles in liquid benzene by an electric plasma discharge in ultrasonic cavitation field,” Ultrasonic Sonochemistry 13 (2006) 6-12, Institute of Multidisciplinary Research for Advanced Material (IMRAM), Tohoku University. This application illustrates a method and a device for producing a nanocarbon material that does not require an expensive production facility such as the ones normally required for dry treatment. It can easily produce the nanocarbon material because the application of high voltage is not needed and neither worsens nor deteriorates the working environment in a production premise, and at the same time considers safety factors. This method can remarkably reduce production costs by improving production efficiency because of its continuous production and recovery, and providing an alternative for mass productivity. The method comprises a process (A) for arranging electrodes, one cathode and one anode, connected to the power source; an ultrasonic horn connected to an ultrasonic generator within an organic solvent that fills a container; and a process (B) for generating an ultrasonic cavitation field by ultrasonic waves into the organic solvent, around the head of the ultrasonic horn; and effecting the thermal decomposition of the molecules in the organic solvent by applying a voltage to the electrodes so as to generate plasma discharge within the ultrasonic cavitation field adequate for the production of the nanocarbon material.
U.S. Pat. No. 6,835,523 to Yamazaki et al. describes a “Method for fabricating with ultrasonic vibration a carbon coating,” which is a process for fabricating a carbon coating in a medium disposed on one side of an electrode connected to a high-frequency power supply. Ultrasonic vibrations are then supplied to the object.
None of the prior art, however, either individually or in combination, provides a method by which initiating and maintaining an abnormal glow volumetric sonoplasma discharge can be performed using a substantially lower amount of electrical power.
Thus there has existed a long-felt need for a method by which the sonoplasma discharge can be initiated and maintained with substantially less electrical power than is currently needed to accomplish the same result using the prior art. This is accomplished with this invention.
The current invention provides just such a solution by having a method and apparatus for initiating and maintaining an abnormal glow volumetric sonoplasma discharge (VSPD). With certain parameters of the electrical discharge and of the intensity of elastic vibrations, it is possible to initiate VSPD within a cavitating liquid medium. The mechanism for the initiation of VSPD is related to the breakdown of gas-phase microchannels formed by the growth cavitation bubbles. The method uses elastic vibrations (EV) in the frequency range 1,000-100,000 Hz with enough intensity for the development of cavitation phenomena; these vibrations are introduced into the liquid-phase working medium, and a source of direct, alternating (hertz and kilohertz range), high frequency (HF) (megahertz range) and ultrahigh frequency (UHF) (gigahertz range) electric field in liquid (DPS) provides the initiation and stable glow of VSPD. Resulting VSPD is characterized by volumetric glow in the frequency range of visible light and ultraviolet radiation in the entire cavitation-electric field, and is characterized by a rising volt-ampere characteristic curve.
When a high-intensity ultrasonic field exceeding a cavitation threshold is induced within liquids, a new form of electric discharge is obtained, characterized by a volumetric glow electrical discharge throughout the space between the electrodes, having a rising volt-ampere characteristic curve that is inherent to abnormal glow discharge in gas. Such discharge within the liquid has the surface characteristic of micro bubbles, and can be used for the design of novel sonoplasma-chemical processes because of the extensive interface plasma. The heterogeneous liquid/gas-vapor system leads to a rise in diffusion rates of chemically active particles in the system and a more economical method to achieve the desired result(s).