Plasma, also known as the "fourth state of matter," is a fully or partially ionized gas state of matter under high temperature or specific excitation conditions. It is composed of ionized conductive gas and includes six typical particles: electrons, cations, anions, excited atoms or molecules, ground state atoms or molecules, and photons.

Low temperature plasma

Low temperature plasma, also known as non-equilibrium plasma, generally refers to the temperatures Te, Ti, Tn of electrons, ions, and neutral particles, satisfying Te>>Ti and Te>>Tn. The reason why it is called low-temperature plasma is mainly because the massive ions and neutral particles as the plasma body are generally at room temperature, and it exhibits a very low temperature to the outside world as a whole; Also known as non-equilibrium plasma, it is mainly due to the temperature of electrons reaching several eV (tens of thousands of degrees) and not reaching thermal equilibrium with ions and neutral particles.

Low temperature plasma generation method

Generally, low-temperature plasma can be generated by gas discharge under low pressure. According to the different forms of applied voltage, discharge can be divided into DC discharge, high-frequency (RF) discharge, and microwave discharge. When a voltage is applied to a gas, some free electrons begin to accelerate under the action of an external electric field, and high-speed electrons collide with atoms or molecules of the gas. If the energy of electrons does not reach the minimum ionization energy of atoms, they will undergo elastic collisions with atoms. Due to the fact that the mass of electrons is much smaller than that of atoms, electrons hardly lose energy in elastic collisions and are then accelerated by an electric field. When the energy of electrons reaches the lowest ionization energy of atoms, inelastic collisions occur. Atoms absorb the energy of electrons and undergo ionization. In addition to ionization, there are also excitation transitions and demagnetization radiation, producing a large number of photons and atoms in metastable states; If the gas contains molecules and compounds, due to the less intense collision of low-temperature plasma, free unpaired atoms and neutral groups will be generated, all of which have high chemical activity.

The manifestation of the duality of low-temperature plasma is not only its classical physical properties, but also its chemical activity. In fact, its chemical activity is determined by the inelastic collisions in its microstructure, ultimately the components of atomic physics, including the excitation, ionization, and dissociation of molecules and atoms. However, the core of its chemical activity is the products of these inelastic collisions - active groups (high-energy electrons, metastable atoms, neutral groups, photons), which are extremely unstable and easily recombine into new substances.

Taking oxygen plasma as an example, oxygen molecules undergo ionization, excitation, and dissociation under the inelastic collision of electrons. Oxygen ions O2+, excited oxygen molecules O2 *, and free oxygen atoms O are generated, all of which are highly chemically active groups.

Ionization: e+O2 →+O2++2e

Excitation: e+O2 → O2 *+e

Dissociation: e+O2 → O+O+e

Moreover, the occurrence of ionization generates many charged groups such as electrons e and ions O2+, which accelerate under the action of an electric field. High speed electrons and ions collide with the material surface and undergo some chemical reactions, and their role cannot be ignored.

Advantages of low-temperature plasma material modification treatment

Low temperature plasma is known for its active wave chemical activity, so its applications are mainly chemical applications, mainly in the preparation of new materials and surface modification of materials. Moreover, there are many advantages to using low-temperature plasma to treat materials compared to traditional chemical methods. Low temperature plasma treatment technology has the advantages of simple process, convenient operation, fast processing speed, good treatment effect, low environmental pollution, and energy saving. By using low-temperature plasma technology to modify materials, the performance of conventional materials can be improved and their application scope can be expanded. Moreover, the surface modification technology of low-temperature plasma can greatly improve the surface properties (hydrophilicity, adhesion) of some materials, which cannot be achieved by other chemical methods.