Explore the mysterious world of atmospheric plasma and the plasma technology hidden behind high technology (Principles)

2023/10/12

TechNews
April 24, 2023
Author Hongkang Technology

Atmospheric plasma (or atmospheric plasma) originates from the generated chemical substances, ions, radiation, and even electric fields. It has a significant influence and effect on the modification of the material surface, the overall reaction and doping, making the atmospheric electricity Pulp has many applications in various materials processing, biochemical or micro-manufacturing fields.

(This article was written by Professor Du Zhenggong and Dr. Lai Yuantai from the Department of Materials Science and Engineering, National Tsinghua University, for the "New Channel of Science and Technology | Cooperation Column" article "The Savior to Prevent Nitrogen Fertilizer from Destroying Ecology? Atmospheric Plasma Technology", introducing the application technology of atmospheric plasma and Future development trends. Edited by Science and Technology News into two parts, this one is the first part.)

Atmospheric plasma does not require a fixed or closed cavity, and the size of the object to be tested is not limited to the size of the cavity. It also has many advantages, such as low equipment and operation costs, fast operation speed, and can be suitable for continuous process operations. Easily combined with other equipment to significantly improve production efficiency, it is currently a topic of active research in the industry. The dielectric discharge plasma process using atmospheric plasma technology can produce atmospheric plasma activated water, demonstrating its strength in agricultural seedling technology. For example, it can reuse organic waste into water-soluble nitrogen fertilizer and open up a new era of circular agriculture. vision.

Let’s start from the beginning: What is plasma?
For the entire universe, almost 99.9% of the matter exists in the form of plasma, such as stars and interplanetary space, etc. are composed of plasma. Plasma can be produced by artificial methods, such as nuclear fusion, nuclear fission, glow discharge and other discharge methods.

The internal structure of a molecule or atom is mainly composed of electrons and atomic nuclei. The relationship between electrons and atomic nuclei is relatively fixed. Electrons exist around the nuclear field at different energy levels, with little potential energy or kinetic energy. (, electricity, heat), the potential energy of the outer electrons in the atom drops rapidly, and finally breaks away from the constraints of the nuclear field and escapes to a distant place, which is the so-called ionization. At this point the atom becomes two charged particles, a negatively charged electron and a positively charged ion. If all the molecules or atoms that make up matter are completely ionized into ions and electrons, they will change their original form and become the fourth form of matter - plasma.

The plasma state is mainly a collection of gases ionized under high electric and magnetic fields, including electrons, positive ions and neutral molecules. Plasma matter has extremely high activity and energy, which can trigger a series of chain reactions, including ionization, excitation, recombination, dissociation, and charge transfer. Utilizing the characteristics of high-energy cleavable gas, the plasma process creates unlimited possibilities. Due to its high energy density and reaction characteristics, people have begun to apply plasma to various industries.

How to improve the application of plasma in various fields?
Plasma treatment is used in many fields, and the conditions for generating plasma are also very wide. Application fields, equipment costs and gas pressure requirements, it is not difficult to find that processes such as special films used in semiconductors, magnetic media, and architectural glass need to be carried out in high equipment costs and high vacuum environments, which is the role of vacuum plasma. , but this is only part of the application of plasma surface treatment technology. Due to the limitation of process costs, plasma treatment is prohibitive in the water treatment industry and food processing fields. In industrial cleaning and food processing, plasma treatment is also limited by the gas pressure of the vacuum process and cannot be applied. It can be seen that if the plasma processing technology can be changed to work in a normal pressure environment, the application space can be improved.

The difference between vacuum plasma and normal pressure plasma
Generally, vacuum plasma treatment is expensive and the equipment is complicated, but atmospheric plasma treatment does not require a vacuum chamber and vacuum system matching. It can be performed in an atmospheric pressure environment and has more possible potential applications, such as in the fields of water and sewage treatment. In addition, atmospheric plasma processing costs are low and the processing speed is fast, so it is also suitable for application in the food processing industry. In summary, compared with vacuum plasma, atmospheric pressure plasma treatment has a wider range of application fields and more outstanding application potential.

Principle and structure of dielectric atmospheric plasma
In the design of atmospheric plasma, there are dielectric barrier discharge (DBD), corona discharges (Corona Discharges) and other types. However, due to the weak processing effect of corona discharge and the easy destruction of electrodes, it is limited. In order to expand this technology, the following introduction will focus on DBD.

Dielectric discharge is also called Silent Discharge, that is, one or two dielectric materials (usually glass, quartz or ceramics) are placed between two electrodes. When a high voltage is applied, plasma will be generated. The gap between the electrode and the dielectric material, or two dielectric materials. A dielectric layer is introduced between the two electrodes, and the entire device will be circuit matched by Capacitive Coupling. Due to the introduction of the dielectric layer, dielectric shielding discharge cannot use DC power supply. Generally, pulsed DC, radio frequency or microwave can be selected. power supply. Dielectric shielding discharge plasma is basic. Dielectric shielding discharge has flat and cylindrical forms. The flat shape can modify the surface of large-area materials, while the cylindrical shape can produce higher density of excited particles.

Dielectric shielding discharge: insulation, breakdown, discharge
Dielectric shielding discharge is usually driven by a sinusoidal (Alternating Current, AC) high-voltage power supply. As the supply voltage increases, the state of the reactive gas in the system will undergo three stages of change, that is, from the insulation state ( Insulation) gradually reaches breakdown (Breakdown), and finally discharge occurs. When the supplied voltage is low, some gases will have some ionization and free diffusion, but because the content is too small and the current is too small, it is not enough to cause a plasma reaction of the gas in the reaction zone, and the current at this time is zero.

As the supply voltage gradually increases, the number of electrons in the reaction area also increases, but it does not reach the breakdown voltage (Breakdown Voltage; Avalanche Voltage) of the reaction gas. At this time, the electric field between the two electrodes is low and cannot provide sufficient energy for the electrons. To allow gas molecules to undergo inelastic collisions, this will prevent the number of electrons from increasing significantly, so the reacting gas is still in an insulating state and cannot produce discharge. At this time, the current increases slightly as the applied voltage increases, but is almost zero.

If the supply voltage continues to be increased, when the electric field between the two electrodes is large enough to cause inelastic collision of gas molecules, the gas will increase significantly due to the inelastic collision of ionization. When the electron density in the space is higher than the critical value (i.e. Paschen When the voltage breaks down), many microdischarge wires (Microdischarge) will be generated to conduct between the two poles. At the same time, the phenomenon of luminous (Luminous) can be obviously observed in the system. At this time, the current will increase rapidly as the applied voltage increases. .

Paschen’s Law
Paschen’s Law provides the relationship between gas collapse voltage, discharge gas pressure and distance between two electrodes:

V = APd/ln(Pd)+B………………………….(1)

Among them, P is the discharge gas pressure, d is the distance between electrodes, and A and B are constants that vary with the type of gas. According to this formula, when the P*d value becomes larger, the gas collapse voltage is proportional to the P*d value; when the P*d value becomes smaller, a Vmin value will appear, and gases below this Vmin value will not collapse.

Dielectric shielding discharge plasma—forms and structures produced
The dielectric shielding discharge can work under normal pressure and a wide frequency range. The usual operating pressure is 1~10 atmospheres, and the power frequency can be from 50Hz to 1MHz. As mentioned earlier, the basic structure of dielectric shielding discharge has various electrode designs. Different DBD electrode structures can be designed for different applications to improve plasma processing efficiency. It is mainly divided into three deformations: flat plate array, cylindrical plasma beam, and cylindrical plasma beam array.

1.Plate array
Generally, the electrode is in direct contact with the plasma, and the high-energy particles in the plasma will cause electrode consumption by etching the electrode surface. In order to cope with the above problems, a dielectric shielding discharge plasma generating device was developed, which introduced a dielectric layer between the two electrodes. Due to the introduction of the dielectric layer, the size of the current can be limited and the generation of arcs can be suppressed. In addition, the entire device uses capacitive coupling for circuit matching, so DC power cannot be used for dielectric shielding discharge. Generally speaking, pulsed DC, radio frequency or microwave power supply methods are chosen.

2. Cylindrical plasma beam
Plasma beam is a widely used electrode design that can handle various forms of materials, whether solid or liquid. The advantage of the plasma beam is that it can produce high-density excited particles and perform strong and effective plasma treatment on the surface. At the same time, the temperature of the plasma generated is low and less energy is dissipated in the form of heat.

3. Cylindrical plasma beam array
Plasma beams are arranged in an array to improve processing efficiency . This method creates bubbles by introducing air from below the water surface, and ignites plasma inside the bubbles when the bubbles pass through the electrodes. Finally, the bubbles float to the water surface, and complete the reaction with water during the floating process to prevent active substances from escaping in the air. , to achieve maximum processing efficiency.

In practical applications, cylindrical or tubular electrode structures are widely used in various chemical reactors, while flat electrode structures are used in industrial modification of plates, powders, polymer grafting, and metal films. , Surface tension improvement and cleaning, hydrophilic modification, etc.

Advantages and research directions of atmospheric pressure plasma
If plasma can be generated in the normal temperature and pressure environment that humans live in, it will be an economical and efficient technology, which can eliminate many systems for maintaining high vacuum, such as chambers, pumps, etc., and also save maintenance costs. Cost and time. Since there are no cavity restrictions, size limitations are relatively reduced, and the process is easy to operate continuously, which can greatly improve processing efficiency. Not only that, atmospheric pressure plasma is uniquely environmentally friendly. Plasma can be excited by just using the surrounding air, and can even decompose environmentally polluting substances into non-polluting gases, reducing environmental problems.

The generation of plasma must have sufficient power to stimulate the reaction. After the electrons absorb the energy of the electric field, if the energy is sufficient, they will dissociate from the colliding gas molecules. At the same time, the number of electrons will increase, and the newly born electrons will produce similar reactions again, forming a chain reaction. However, when the pressure is one atmosphere, there are so many gas molecules that collisions are quite frequent. At this time, the mean free path of the gas (the distance between effective collisions of gas molecules) is quite small, and it is difficult to accumulate energy, making it difficult to excite the plasma. The main solution is There are two types: first, increasing the potential of the external power supply; second, increasing the current of the path. Both of these ideas are to increase the input energy. While increasing the energy supply, establishing low-cost and high-efficiency atmospheric pressure plasma technology is a topic that academicians continue to study. In the next article, we will further explore how atmospheric plasma can be used in practical applications to solve the problem of "nitrogen" deficiency and create environmentally friendly sustainable agriculture.

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(First picture source: Freepik; Article picture source: Hongkang Technology)

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