Plasma cleaning instrument Atmospheric pressure DIELECTRIC barrier discharge plasma

Plasma cleaning instrument Atmospheric pressure DIELECTRIC barrier discharge plasma:

Dielectric barrier discharge (DBD), referred to as DBD discharge, is a kind of gas discharge with insulating medium inserted between discharge electrodes. The medium may be overlaid on the electrode or suspended in the discharge space. In this way, when a high enough ac voltage is applied to both ends of the electrode, the gas between the electrodes can be broken down under high pressure, forming a so-called DBD discharge. The plasma washer discharge is similar to glow discharge, which is uniform, diffuse and stable. It is actually composed of many small fast pulse discharge channels. Generally, the pressure of discharge gas can reach 1 ATM (1 ATM = 1.013X 10^5 Pa), so DBD discharge belongs to non-thermal equilibrium discharge under high pressure, which is also known as silent discharge.

The common DBD discharge device of plasma cleaning instrument is usually composed of two parallel electrodes, and at least one electrode is covered by dielectric material. In order to ensure the stability of discharge, the two electrodes are spaced at several millimeters apart, and sinusoidal or pulsed high-voltage power supply is needed to realize atmospheric discharge. DBD discharge reactor consists of three parts: high voltage electrode, electric dielectric and ground electrode. The structure of single-gap single-dielectric barrier discharge reactor is characterized by that the dielectric is connected with the high-voltage electrode and the discharge area is between the grounding electrode and the dielectric. The structure is simple and is often used to produce ozone. Double gap single dielectric barrier discharge reactor is characterized by two different reaction zones formed between the upper and lower electrodes of the plasma cleaning apparatus and the medium, which are generally used to produce plasma with two different components. The structure of single-gap dual-dielectric barrier discharge reactor is characterized by the fact that the reaction takes place between two layers of dielectric, so as to avoid the influence of electrodes on the reaction, especially for corrosive gases and the reaction that needs to produce high-purity plasma in a closed environment. This structure has outstanding advantages. In addition to parallel plate structure, DBD discharge device also has line simple structure and surface structure.

Atmospheric DBD discharge plasma usually presents filament discharge or glow discharge characteristics. When high pressure is applied at both ends of the electrode, the gas near the cathode ionizes under the action of electric field to produce electrons. These electrons accelerate in the electric field before the gas is completely broken down. When the energy reaches or exceeds the ionization energy of the gas, the electrons multiply in each ionization collision and form an electron avalanche. Electrons are more mobile than ions, allowing them to pass through gas gaps in the measurable nanosecond range. When the electron avalanche is formed in the gas gap and generates directional movement, ions will be trapped behind due to slow movement speed and will form accumulation in the discharge space. The generation of space charge distorts the electric field in the discharge space, so that the electric field intensity of the air gap between electrodes equals or exceeds the breakdown field intensity of the surrounding gas. Therefore, the gas ionization increases sharply in a short time, leading to the occurrence of a single filament discharge.

A single filamentation discharge occurs at one location in the discharge gas gap and at other locations at the same time. It is the insulating nature of the medium that enables this filament discharge to occur independently in many discharge Spaces. When the voltage at both ends of the filamentous discharge is lower than the breakdown voltage, the current is cut off. Only when the breakdown voltage is reached again at the same position can the plasma cleaner re-breakdown and a second filameter discharge occur at the original place. Each filamentous discharge is only a few dozen to a few hundred nanometers in diameter, and the roots of these filaments are attached to the dielectric layer and create bumps and bumps on the surface. The existence of concave-convex points on the surface of the dielectric layer increases the local electric field intensity and makes the discharge more likely to occur, which is commonly referred to as the cusp discharge. A micro-discharge process is actually a process in which streamer discharge occurs and disappears. The so-called streamer discharge is a discharge phenomenon in which a local area of the discharge space is highly ionized and rapidly transmitted. In DBD discharge, it is usually divided into three stages: discharge breakdown, streamer development and discharge disappearance.

As a simple and easy to operate atmospheric pressure plasma method, DBD discharge has been used in material preparation, surface modification and biomedicine. Kim et al. used atmospheric DBD discharge plasma system to prepare the supported catalytic materials. Jeon and Lee have successfully prepared Au nano-catalysis materials. Pd^2+ was effectively reduced to Pd elements by atmospheric pressure DBD discharge hydrogen cold plasma. For example, Pd/TiO2 prepared by Xu et al through atmospheric pressure cold plasma treatment has high photocatalytic activity. Qi et al. obtained Pd/C catalytic materials by DBD discharge plasma method at atmospheric pressure, and the obtained samples showed relatively small particle size and high catalytic activity at relatively low temperature.