At the end of 18, it was discovered that expensive diamonds were originally allotropes of carbon. Since then, the preparation of synthetic diamond has become the glory and dream of many scientists. A century later, another simple form of carbon, graphite, was discovered, and people tried to simulate the natural process and transform graphite into diamond under the environment of ultra-high temperature and high pressure. In order to shorten the reaction time, special conditions of 2000℃ and 55000 atmospheres are needed.

1955, General Electric Company of the United States specially manufactured high-temperature and high-voltage electrostatic equipment, obtained the first batch of small-scale industrial synthetic diamond crystals in the world, and initiated the industrial scale production of synthetic diamond abrasives, with an annual output of about 20 tons; Soon, DuPont invented the explosion method, using the high pressure and rapid temperature rise generated by the instantaneous explosion, and also obtained artificial diamonds with a size of several millimeters.

The performance of diamond film is slightly inferior to that of diamond particles, and its density and hardness are lower. Even so, its wear resistance is one of the best, and the life of a film only 5 microns thick is more than 10 times longer than that of hard alloy steel. We know that the stylus of a record has to bear great pressure on the tiny contact surface, and at the same time it requires extremely long wear-resistant life. As long as a diamond film is deposited on the needle tip, it can easily go into battle. If diamond film is used as wear-resistant coating on plastic and glass, its application range can be greatly expanded and products with superior performance and economy can be developed.

More importantly, the appearance of thin film makes the application of epigraphy break through the barrier that it can only be used as a cutting tool, and gives full play to its excellent thermal, electrical, acoustic and optical properties. Diamond films have been used in semiconductor electronic devices, optical and acoustic devices, pressure processing and cutting tools. And their development speed is amazing, and they are more attractive in the high-tech field.

Carbon with non-diamond structure is transformed into carbon with diamond structure by artificial method, and single crystal and polycrystalline diamond are formed by nucleation growth, or fine diamond is sintered into polycrystalline diamond at high pressure and high temperature. This is an important example of the application of high pressure research in production.

From the thermodynamic point of view, the phase transition condition that determines whether carbonaceous raw materials with non-diamond structure such as graphite can be transformed into diamond is that the free energy of the latter must be smaller than that of the former. This phase change process is carried out under the conditions of high pressure, high temperature or other components. Certain pressure, temperature and component concentration can change the internal energy of the system, thus changing the statistical weight of the accessible energy levels of valence electrons accordingly. This may lead to electron transfer and electronic structure forming a new bonding state, that is, phase transition. If the energy change in the system is beneficial to the change of the electronic structure in the solid, the high-pressure and high-temperature phase transition will occur in the solid, otherwise it may occur in the molten state or vapor state. The conditions for this change in melt are that the statistical weight of valence electron distribution of bonding characteristics decreases accordingly, the remote ordering effect tends to disappear and the atomic coordination number changes; However, the statistical weight of excited electrons tends to increase, and the short-range order effect also increases accordingly. The condition of this change in gas is that the electronic energy levels between atoms or bonding molecules of a single substance tend to disappear, and all electrons are transferred to the energy levels of a single atom or molecule, making the statistical weight of excited electrons greater. Therefore, synthetic diamond can be carried out in solid state, molten state and vapor state, which depends on the changes of internal energy in the system caused by pressure, temperature and component concentration. From the kinetic point of view, it is also required that carbon-containing raw materials such as graphite should have a suitable conversion rate when they are converted into diamond. When the nucleation rate and growth rate of diamond reach the maximum at the same time, the phase transformation rate is the largest.

Since18th century, it was confirmed that diamond is made of pure carbon, and the research on synthetic diamond began. It was not until 1950s that the real success and rapid development were achieved through the progress of high-pressure research and high-pressure experimental technology. There are more than a dozen specific methods of synthetic diamond. According to the characteristics of the technology used, it can be summarized into three methods: static pressure, dynamic pressure and low pressure. According to the characteristics of diamond formation, it can be summarized into three methods: direct method, molten medium method and epitaxial method. The picture shows the pressure-temperature (□ -□) phase diagram of carbon and the experimental area of diamond synthesis by three methods. The 1 zone is the experimental zone for direct diamond synthesis, the zone 2 is the experimental zone for diamond synthesis in molten medium, and the zone 3 is the experimental zone for diamond synthesis by epitaxy.