Various substances that exist in nature can be divided into three states based on their temperature, namely solid, liquid, and gas. These states refer to the actual aggregation state of large objects composed of microscopic particles, which will manifest different states of existence depending on the environment they are in.

Among the various states of matter, the particles that combine to form a solid state are the most tightly bound, followed by the liquid state, and the gas state is the most dispersed. To transform a substance from a tightly aggregated state to a dispersed aggregated state, additional energy needs to be provided. For example, when a substance transitions from a solid state to a liquid state, sufficient energy needs to be provided to make the average kinetic energy of the particles that make up the solid state exceed the binding energy of the particles in the solid state. Only when the applied energy is large enough to destroy the crystal structure of the solid state can the transition from solid to liquid be achieved. The same applies to the change from liquid to gas. When a substance becomes a gas state, if energy continues to be applied, the particles that make up the gas state will ionize and become ions and electrons with positive and negative charges, respectively.

For gases, as long as the ambient temperature is not zero, there will always be trace particles that are naturally ionized. However, the number of ionized particles will not change the properties of the gas. If the external environment changes or human influence is applied, causing the concentration of charged particles in the gas to exceed a certain value (usually one thousandth), although the gas still behaves relatively stably, the entire system's motion has been dominated by charged particles. When an electromagnetic field is applied from the outside, a series of new properties will be exhibited. When the gas is completely or partially ionized, the total charge carried by free electrons and ions cancels each other out. We call this state of gas plasma, which can also be called the fourth state of matter. State.

According to thermodynamic equilibrium, plasma can be divided into high-temperature plasma and low-temperature plasma.

(1) The ionization rate of high-temperature plasma is very high, and the electron temperature, ion temperature, and gas temperature are basically the same, such as nuclear fusion plasma.

(2) Low temperature plasma only ionizes a portion of the gas, as it is difficult for the temperature of various particles in the plasma to be completely consistent. Therefore, it can be further divided into two types:

In a thermally balanced plasma, the electron temperature, ion temperature, and gas temperature reach local thermal equilibrium, and both the electron density and gas temperature are high.

In non thermal equilibrium plasma (also known as cold plasma), the electron temperature is very high, while the ion and gas temperatures are close to room temperature.

Low temperature plasma can be generated under simple conditions at room temperature and pressure, with low energy consumption and low environmental requirements. It can be achieved in atmospheric environments and is suitable for industrial production.

High energy electrons in low-temperature plasmas can collide to produce free radicals, excited molecules, atoms, ions, and other chemically active species under low-temperature conditions. By using low-temperature plasma to treat materials, surface molecules can be excited to ionize, while the treated material remains at a relatively low temperature, ensuring that the overall properties and composition of the material remain unchanged. Therefore, low-temperature plasma technology is widely used in fields such as pollutant treatment, nanomaterial synthesis, and surface modification.