1. What is plasma ？
Plasma, the fourth state of matter, is an ionized gaseous substance composed of atoms with some electrons stripped away and positive and negative ions generated after the ionization of atoms. This ionized gas consists of atoms, molecules, atomic groups, ions, and electrons. Its application on the surface of objects can achieve ultra-clean cleaning, surface activation, etching, and plasma surface coating of objects.
2. How to artificially obtain plasma？
Plasma can be generated through artificial means such as nuclear fusion, nuclear fission, glow discharge, and various types of discharges. Different types of plasma can also be produced through artificial discharge methods, mainly including: glow (fluorescent lamps), arc (electric arc), and corona discharge (often seen around high-voltage lines). For precision surface cleaning, surface activation and modification, as well as surface processing of bioengineering materials, plastics, and paper, low-temperature plasma generated by glow discharge, corona discharge, and dielectric barrier discharge is mostly used.
In a high-frequency electric field, gas molecules such as oxygen, nitrogen, methane, and water vapor under low pressure can decompose into accelerated atoms and molecules during glow discharge. The electrons generated in this way and the atoms and molecules dissociated into positive and negative charges collide with surrounding molecules or atoms, resulting in the excitation of electrons in the molecules and atoms, which themselves are in an excited or ionic state. At this point, the state of matter is in a plasma state.
3. What plasma types are there ?
1) High-temperature plasma and low-temperature plasma. High-temperature plasma refers to fully ionized or locally thermally equilibrated plasma where the electron temperature is exactly equal to the ion and gas temperatures. The temperatures of all particles are almost identical. The temperature is extremely high, typically reaching 106−108 K (approximately 100-100 million degrees Celsius). Low-temperature plasma is in a non-thermal equilibrium state, where the electron temperature is much higher than the temperatures of ions and neutral particles. Due to the excessive intensity of the effect of high-temperature plasma on the surface of objects, it is rarely used in practical applications, and currently only low-temperature plasma is put into use.
2) Plasma of reactive and non-reactive gases. This classification is based on the chemical properties of the gases used to generate plasma. Non-reactive gases include argon (Ar), nitrogen (N2), nitrogen fluoride (NF3), carbon tetrafluoride (CF4), etc., while reactive gases include oxygen (O2), hydrogen (H2), etc. The reaction mechanisms of different types of gases during the cleaning process are different, with plasma of reactive gases exhibiting stronger chemical reactivity.
4. What are the interaction between plasma and object surface?
The chemical reaction between plasma and the surface of a workpiece is quite different from conventional chemical reactions. Due to the bombardment of high-speed electrons, many gases or vapors that are stable at room temperature can react with the surface of the workpiece in the form of plasma, producing many unique and useful effects:
1) Cleaning and etching: For example, during cleaning, oxygen is often used as the working gas. When bombarded by accelerated electrons, it is converted into oxygen ions and free radicals, which exhibit strong oxidizing properties. Contaminants on the surface of the workpiece, such as grease, flux, photoresist, release agent, and punch oil, are quickly oxidized into carbon dioxide and water, which are then pumped away by a vacuum pump, achieving the goal of cleaning the surface and improving wettability and adhesion. Low-temperature plasma treatment only involves the shallow surface (<10nm) of the material and does not affect the properties of the bulk material. Because plasma cleaning is performed under high vacuum, the various active ions in the plasma have a long mean free path, and their penetration and permeation abilities are strong, enabling the treatment of complex structures, including fine tubes and blind holes.
2) Introduction of functional groups: Plasma treatment of polymer materials with gases such as N2, NH3, O2, and SO2 can alter the chemical composition of the surface and introduce corresponding new functional groups: -NH2, -OH, -COOH, -SO3H, etc. These functional groups can transform completely inert substrates such as polyethylene, polypropylene, polystyrene, and polytetrafluoroethylene into functionalized materials, enhancing surface polarity, wettability, bondability, reactivity, and greatly increasing their use value. In contrast to oxygen plasma, low-temperature plasma treatment with fluorine-containing gases can introduce fluorine atoms onto the substrate surface, imparting hydrophobicity to the substrate.
3) Polymerization: Many vinyl monomers, such as ethylene and styrene, can undergo polymerization on the surface of workpieces under plasma conditions without the need for any other catalysts or initiators. Even substances that cannot be polymerized under conventional conditions, such as methane, ethane, and benzene, can undergo cross-linking polymerization on the surface of workpieces under plasma conditions. This polymerized layer can be very dense and strongly bonded to the substrate. In foreign countries, plastic beer bottles and automobile fuel tanks are coated with such a dense layer through plasma polymerization to prevent micro-leakage. The surface of biomedical polymer materials can also be coated with this dense layer to prevent the diffusion of toxic substances such as plasticizers from the plastic into human tissues. Optical components can often be coated with an appropriate optical film on their surface through plasma polymerization measures, in order to enhance the performance of the optical components.
4) Plasma-induced grafting:
The process involves generating active free radicals on the surface of polymer materials through plasma pretreatment, which triggers the polymerization of vinyl monomers on the material surface. Plasma can also induce grafting reactions on certain irregular surfaces, such as the inner walls of bottles. By selecting appropriate grafting monomers and controlling the appropriate grafting reaction conditions, the hydrophilicity or water repellency, adhesion, corrosion resistance, wear resistance, conductivity, permeability selectivity, and biocompatibility of the material can be altered. Therefore, plasma grafting is highly innovative and holds great application prospects.
