Background
The main mode of low temperature plasma generation is gas discharge. At present, gas discharge is mainly performed under vacuum, and from the application perspective, a vacuum system required by plasma generated in a vacuum environment is complex in equipment, high in cost and harsh in conditions. The discharge plasma under the atmospheric pressure does not need an expensive and complicated vacuum system, is particularly beneficial to industrial application, and not only improves the efficiency but also obviously reduces the production cost.
The plasma generated under the atmospheric pressure condition at present comprises cold plasma and hot plasma, and the nature of the plasmas is that the mechanism of energy transfer from an electromagnetic field to electrons in the plasma is different. The atmospheric pressure cold plasma belongs to typical non-equilibrium plasma, has low gas temperature even close to room temperature, and is mainly used in the fields of film preparation, material surface modification, biological purification and the like. Since the cold plasma temperature is low, cold plasma cannot be used to treat high melting point materials. The gas temperature of the thermal plasma can reach 3000-10000K, and the thermal plasma can be used in the fields of large-area solid waste treatment, spraying, high-melting-point material synthesis and the like. The common modes for generating the thermal plasma comprise direct current arc and radio frequency induction coupling thermal plasma, the direct current arc torch has the defect of impurity pollution caused by electrode ablation, and the radio frequency thermal plasma has large volume, high energy density and cleanness (no electrode), and is rapidly applied to the fields of powder densification, purification, synthesis, spheroidization and the like. The radio frequency thermal plasma method is used for material synthesis and spheroidization, so that advanced materials with high performance and high quality, such as micro-nano particles of rare metals, high-melting-point metal oxide ceramics, nitrides and hydrides and the like which are difficult to realize by the traditional method can be obtained.
In the prior art, for the rf thermal plasma under atmospheric pressure, the breakdown and sustain voltage is high, and usually an auxiliary ignition device such as a tesla coil needs to be introduced, and a small-area discharge is generated through a coil terminal in a boosting manner; or a metal rod is extended into the discharge area, and the discharge breakdown effect is achieved under the power of hundreds of watts by the mode of metal tip thermionic emission. The existing auxiliary ignition mode has impurity pollution, small auxiliary discharge area and low integral concentration of ionized particles, and manual removal of a Tesla coil or a metal rod placed in a radio frequency thermal plasma discharge area is needed after ignition is finished, which is not practical for some plasma systems which need to control atmosphere and are provided with a cavity, so that the application of the atmospheric pressure radio frequency thermal plasma technology is greatly limited.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides the atmospheric pressure injection frequency thermal plasma generator based on the preionization ignition device, which can realize stable gas discharge difficult to be directly punctured under atmospheric pressure under lower puncture voltage and provides a convenient and practical novel technical scheme for the problem of puncture ignition of the atmospheric pressure injection frequency thermal plasma.
The invention provides an atmospheric pressure jet-frequency thermal plasma generator based on a preionization ignition device, which is an atmospheric pressure jet-frequency induction coupling plasma generator and comprises: the plasma torch comprises a preionization ignition device and an inductive coupling plasma torch tube, wherein the inductive coupling plasma torch tube comprises an inner tube, an intermediate tube, an outer tube and a metal coil surrounding the outer tube; the preionization ignition device is used for providing seed electrons for thermal plasma, and is a double-dielectric barrier discharge structure which is composed of a positive electrode and a ground electrode, wherein the positive electrode is arranged in the inner tube and is wrapped by an insulating medium layer which is coaxial with the inner tube, and the ground electrode surrounds the inner tube.
In one embodiment, the preionization ignition device is arranged at an upstream position of the inner tube by taking the center of the length direction of the inner tube as a reference.
In one embodiment, the electrodes of the pre-ionization ignition device are connected to a radio frequency power supply or a high voltage power supply capable of providing a pre-ionization discharge.
In one embodiment, the density of the seed electrons, the jet length, provided by the preionization ignition device is controlled by adjusting the voltage to which the preionization ignition device is connected, the radio frequency power, or the distance between the positive electrode and the ground electrode.
In one embodiment, the distance between the bottom of the positive electrode and the ground electrode is set to 1cm to 6 cm.
In one embodiment, the ground electrode in the pre-ionization ignition device is provided as at least one.
In one embodiment, the plasma generator further comprises an insulating support for supporting the inductively coupled plasma torch.
In one embodiment, the metal coil is a multi-turn water-cooled coil for connecting to a power supply, and the metal coil is wound around the outer tube at a downstream position thereof in a cylindrical helical cylindrical shape with reference to a center of the outer tube in a length direction.
In one embodiment, the insulating medium layer is a heat-resistant insulating tape attached to the surface of the positive electrode or a silicone grease or quartz glass tube coated on the surface of the positive electrode.
In one embodiment, the positive electrode, the insulating medium layer, the inner tube, the outer tube and the intermediate tube are coaxially arranged.
Compared with the prior art, the plasma generator has the advantages that the preionization ignition device is arranged in the plasma generator, so that stable gas discharge which is difficult to directly breakdown under atmospheric pressure can be realized under lower breakdown voltage, the traditional metal electrode ignition device is replaced, other metal impurities are prevented from being introduced, and the ignition device does not need to be removed after ignition is finished. The invention provides a convenient and practical novel technical scheme for the breakdown ignition problem of atmospheric pressure injection frequency thermal plasma, increases the volume of the plasma and realizes the pretreatment of feeding.
Other features of the present invention and advantages thereof will become apparent from the following detailed description of exemplary embodiments thereof, which proceeds with reference to the accompanying drawings.
Detailed Description
Various exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that: the relative arrangement of the components and steps, the numerical expressions and numerical values set forth in these embodiments do not limit the scope of the present invention unless specifically stated otherwise.
The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses.
Techniques, methods, and apparatus known to those of ordinary skill in the relevant art may not be discussed in detail but are intended to be part of the specification where appropriate.
In all examples shown and discussed herein, any particular value should be construed as merely illustrative, and not limiting. Thus, other examples of the exemplary embodiments may have different values.
It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus, once an item is defined in one figure, further discussion thereof is not required in subsequent figures.
Referring to fig. 1 and 2, the rf thermal plasma generator according to the embodiment of the present invention is an atmospheric pressure rf inductively coupled plasma generator, and includes a preionization ignition device and an inductively coupled plasma torch, wherein the inductively coupled plasma torch is provided with three layers of tubes, an inner tube 10, an intermediate tube 20 and an outer tube 30, respectively, an insulating heat-resistant material 3 and a metal coil 4 for assembling the inner tube, the intermediate tube and the outer tube, and the preionization ignition device includes a positive electrode 1 and a ground electrode 2 (also referred to as a ground electrode or a negative electrode).
In a preferred embodiment, the preionization ignition device structure is formed at the upstream position of the inner tube 10 (i.e. the upper half of the inner tube in the length direction), and the structure is a dual-dielectric barrier discharge structure composed of a positive electrode wrapped by an insulating medium layer 5 disposed in the inner tube coaxially with the inner tube, and a ground electrode surrounding the inner tube, wherein the positive electrode 1 and the ground electrode 2 can both be wrapped by insulating medium layers, for example, the insulating medium layer can be a heat-resistant insulating tape attached to the electrode surface or a silicone grease or quartz glass tube coated on the electrode surface, etc.
The positive electrode 1 in the preionization ignition device can be used for being connected with a high-voltage power supply or a radio frequency power supply (also called as a high-voltage/radio frequency electrode) or other power supplies capable of providing preionization discharge, and the aim of preionizing the working gas can be achieved. For example, using a radio frequency power supply, the frequency is 13.56MHz or other tunable radio frequency range. The ground electrodes 2 may be provided as one or more.
Parameters such as the density of seed electrons or the jet length provided by the preionization ignition device can be controlled by adjusting the voltage and the radio frequency power connected with the preionization ignition device or adjusting the distance between the positive electrode 1 and the ground electrode 2. For example, the distance between the bottommost portion of the positive electrode 1 and the ground electrode 2 may be set to a range of 1cm to 6cm or other values, preferably to 2cm or 3 cm.
In the embodiment of the invention, the pre-ionization ignition device is arranged at the upstream position of the inner tube, and a double-dielectric barrier discharge structure is adopted, namely the structure is a double-dielectric barrier discharge structure which is formed by a high-voltage/radio-frequency electrode and a ground electrode, wherein the high-voltage/radio-frequency electrode is wrapped by a dielectric insulator (or insulating dielectric layer) which is arranged in the inner tube and is coaxial with the inner tube, and the ground electrode surrounds the inner tube. When a sufficiently high voltage is applied between the electrodes, the gas between the electrodes is broken down to generate a discharge, i.e., a dielectric barrier discharge. The dielectric insulator disposed on the outer surface of the electrode is a heat resistant insulating layer that insulates the electrode from air.
The metal coil 4 is used for connecting a radio frequency power supply, and is a multi-turn (e.g. 4-turn or 5-turn) water-cooled coil, for example, and is wound around the outer tube at a position downstream of the outer tube according to a cylindrical ring of a cylindrical spiral.
In an embodiment of the present invention, the inductively coupled plasma torch is preferably a three-layer coaxial quartz glass tube. Specifically, the outer tube 30 may be purged with cooling argon to avoid burning the quartz tube by the plasma torch. The intermediate tube 20 may be purged with argon to maintain the plasma. The inner tube 10 introduces the sample aerosol from the inner tube into the plasma with a carrier gas. When the power supply is connected to a load induction metal coil 4 surrounding the outside of the torch tube, an induced current flows through the coil, generating an axial magnetic field. At the moment, cooling argon is introduced into the outer tube of the torch tube in the tangential direction, auxiliary gas argon is introduced into the middle layer tube in the axial direction (or the tangential direction), and a preionization ignition device is used for exciting to generate seed electrons. It should be noted that, in practical applications, an insulating support (not shown) is further included for supporting the three-layer coaxial quartz glass tube.
In one embodiment, the outer tube, the intermediate tube, and the inner tube of the inductively coupled plasma torch are respectively set to have lengths of 16.5cm, 9cm, and 15cm, outer diameters of 35mm, 22mm, and 6mm, and inner diameters of 31mm,20mm, and 4 mm.
In the embodiment of the invention, the pre-ionization effect of the preposed double-medium barrier discharge on the working gas is utilized to provide seed electrons for the radio frequency thermal plasma discharge. Specifically, for rf thermal plasma discharge, there are two discharge modes, one is E-type discharge and one is H-type discharge, the E-type discharge being a discharge caused by an electrostatic field generated by surface charges of a coil, such as a discharge in a capacitively coupled plasma. The H-discharge is a magnetic field induced by the current in the coil, which in turn induces an electric field, and the plasma is maintained in discharge. The RF thermal plasma is in E-type discharge in the initial discharge stage, and the E-type discharge jumps to H-type discharge after the applied power reaches a certain value along with the increase of the applied power. In the E-type discharge stage, plasma discharge is caused by collision (electrons collide with background gas for multiple times before reaching the wall), the invention provides additional seed electrons through pre-ionization, and can provide effective assistance for breakdown of the radio-frequency thermal plasma, thereby realizing direct realization of radio-frequency thermal plasma discharge which can hardly break down under atmospheric pressure under the conditions of larger discharge gap and lower breakdown voltage; and the metastable state particles in the preionization discharge and the active reaction environment can also play a role in providing pretreatment for the synthesis of materials.
The invention applies the preionization ignition device to the atmospheric pressure injection frequency thermal plasma generator, can work stably under atmospheric pressure, and has the advantages that at least: adopting a radio frequency power supply to achieve uniform and stable H-shaped discharge under atmospheric pressure to form a high-temperature radio frequency thermal plasma torch; the device can realize the discharge of argon, even nitrogen and other gases under lower breakdown voltage, and directly realize the radio frequency thermal plasma discharge which can hardly be broken down under atmospheric pressure; other metal impurities are prevented from being introduced; the volume of the plasma is increased, metastable state particles in the preionization discharge and active reaction environment can also play a role in preprocessing materials in application.
In conclusion, the preionization ignition device is applied to the radio frequency thermal plasma generator, and gas discharge of argon, nitrogen and the like which are difficult to be directly broken down under atmospheric pressure can be realized under lower breakdown voltage. Compared with the radio frequency thermal plasma generator which does not adopt the structural design of the preionization ignition device, the invention reduces the breakdown and maintenance voltage of the thermal plasma, can generate uniform and stable discharge, simultaneously avoids introducing other metal impurities, realizes the radio frequency thermal plasma discharge which can hardly be broken down under the atmospheric pressure by using lower breakdown voltage under large discharge gap directly under the atmospheric pressure, and provides a convenient and practical solution for the breakdown ignition problem of the atmospheric pressure radio frequency thermal plasma. The technical scheme provided by the invention can greatly promote the practical application of the atmospheric pressure radio frequency thermal plasma in the fields of material treatment, powder densification, purification, synthesis, spheroidization and the like, and shows the clean, efficient and rapid treatment capability.
Having described embodiments of the present invention, the foregoing description is intended to be exemplary, not exhaustive, and not limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen in order to best explain the principles of the embodiments, the practical application, or improvements made to the technology in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein. The scope of the invention is defined by the appended claims.