About the properties of CVD diamond

As early as 700 BC, diamond was known to be very hard, and the English name diamond comes from the Greek word "adamas", meaning "invincible". If you open the material physical properties manual, you will find that many properties of diamond are Z large and Z high: diamond has Z high hardness in all known substances, Z high thermal conductivity at room temperature, it is completely transparent to light from the far infrared region to the deep ultraviolet region, has Z low compressibility, better chemical inertness, its biological compatibility exceeds titanium alloy and so on.

However, due to the scarcity of natural diamonds, high price, limited size and other factors, it is difficult to use the above excellent properties of diamond. In 1796, S. Tennant burned diamond into CO2, proving that diamond is composed of carbon. Later, it was known that natural diamond is the transformation of carbon under high temperature and pressure deep in the Earth's crust, so people have been trying to synthesize diamond through another allotrope of carbon - graphite. From the thermodynamic point of view, at room temperature and pressure, graphite is the stable phase of carbon, and diamond is the unstable phase of carbon. Moreover, there is a huge energy barrier between diamond and graphite (see Figure 1), and this energy barrier must be overcome to convert graphite into diamond. Table 1 is a comparison table of the thermodynamic properties of graphite and diamond.

According to these data in Table 1, a rough calculation is required to convert graphite into diamond at least 15,000 ATM pressure, and to make the conversion speed meet the requirements of industrial production, the pressure used is generally more than 50,000 ATM. Based on thermodynamic data and the fact that natural diamond exists, people began to imitate nature's research on converting graphite into diamond under high temperature and pressure conditions, the so-called high temperature and high pressure (HTHP) technology. The United States General (GE) Electric Company was the first to make HTHP diamond in 1955, and HTHP diamond was used in the tool processing field in the 1960s. By adding metal catalysts such as Fe, Co, Ni, Mn, Cr, etc., the conversion temperature and pressure can be reduced from 3,000K, 150,000 atm to 1,600K, 60,000atm. Figure 2 shows the thermodynamic stable phase diagram of carbon in the main methods of diamond production.

At present, using HTHP growth technology, generally can only synthesize small particles of diamond; In the synthesis of large particle diamond single crystal, the main use of crystal seed method, under high pressure and high temperature (60000atm,1800K), in a few days to grow the crystal seed into a particle size of several millimeters, weighing several carats of gem-grade artificial diamond, a longer time of high temperature and high pressure makes the production cost expensive, equipment requirements are demanding. Moreover, HTHP diamond due to the use of metal catalysts, so that there are trace metals remaining in the diamond, so there is still a considerable distance to completely replace natural diamond. And the size of HTHP diamonds produced with current technology can only range from a few microns to a few millimeters, which also limits the large-scale application of HTHP diamonds. Therefore, it is necessary to develop a new method, using this method to produce diamond, whose form can make those excellent properties of diamond fully reflected, this form is the diamond film prepared by chemical vapor deposition.