High temperature and ultra-high pressure method is also called HTHP. Due to the limitation of ultra-high pressure equipment and high temperature technology, diamond synthesis progressed slowly at first. It was not until 1970 that GE announced the birth of the first gem-grade synthetic diamond, and countries have been secretly studying it for several years. In the 1990s, there was a breakthrough in synthetic diamonds. Sumitomo, De Beers and GE announced their artificial gem diamonds one after another, which shocked the jewelry industry.
The methods of synthesizing diamond can be divided into static pressure method, dynamic pressure method and gas phase epitaxial growth method. Among the HTHP static pressure method and chemical vapor deposition method (CVD method) reported by many media recently, large gem-grade diamonds are mainly synthesized by seed catalyst method (including belt pressing method and rod method). This section and section 7 will focus on them respectively.
First, the principle of diamond synthesis by HTHP method
1. Transformation between graphite and diamond
Synthetic diamond artificially simulates the formation conditions of natural diamond, so that carbon with non-diamond structure can be transformed into carbon with diamond structure.
W.L.Bragy determined the crystal structure of diamond in 19 13. Most diamonds are cubic, while graphite is layered. See the "Diamonds" section of this book for the structure of diamonds, and the graphite structure is shown in the figure (4- 1-20).
Figure 4- 1-20 Graphite Structure Diagram
The four orbitals 2s, 2px, 2py and 2pz of carbon atoms in diamond form four sp3 hybrid orbitals, forming tetrahedral coordination, and each carbon atom forms a saturated bond with valence of * * * with the bond length of 0. 154nm.
The carbon atoms of graphite are distributed on the hexagonal ring, and each carbon atom is surrounded by three adjacent carbon atoms with an interval of 0. 142nm. The carbon atoms of two adjacent layers are stacked alternately, and the spacing between layers is 0.34nm, so the binding force is much weaker, so graphite has a perfect cleavage of polarity and can slip and separate. Graphite can be transformed into diamond at high temperature and high pressure.
As shown in fig. 4- 1-20, graphite is arranged between layers with a spacing of 0.34nm, and carbon atoms are stacked in dislocation mode. Under high pressure, the layers in the Z-axis direction are close to each other. Due to the staggered accumulation of carbon atoms, 1', 3' and 5' move upward, and 1, 3, 5, 2', 4' and 6' move downward, so the graphite structure becomes a diamond structure.
Figure 4- 1-2 1 carbon phase diagram
2. Growth mechanism of synthetic diamond
For a long time, scientists all over the world have been trying to find the conditions for the growth of diamond crystals. Fig. 4- 1-2 1 is the graphite-diamond phase diagram. According to the phase diagram, the solid phase zone I is graphite zone, II is diamond zone, III is metallic carbon zone, and there is also liquid phase zone. In the low pressure and high temperature region, it mainly exists as graphite phase, and diamond is a stable phase only in the range of high pressure and high temperature. In addition to degassing phase method and epitaxial growth method, diamond crystal growth is in a high pressure range, and catalyst method can reduce the pressure.
It can also be seen from the phase diagram 4- 1-2 1 that in the upper part of the phase diagram, the carbon atoms in the carbonaceous raw materials are compressed, sheared and thermally vibrated, so that the non-sp3 hybrid atomic orbitals are transformed into sp3 hybrid orbitals, and the diamond nucleates and grows. Under the above pressure, the pressure and temperature are not enough to make carbon atoms reach the diamond structure on the boundary of diamond and graphite stability zone. However, if the compound action of flux and catalyst is used, the purpose can still be achieved, because the melting temperature of these fluxes is relatively low, and they melt with carbon, so that carbon atoms and flux diffuse with each other to form two-dimensional and three-dimensional interstitial phases, and finally form diamond phase.
Under the condition of modern science and technology, it is easy to realize stable and reliable technical equipment and experimental conditions, so it is possible to grow gem-grade diamonds. In recent years, scientists from all over the world have done a lot of research, and gained a lot of experimental data and experience in experimental conditions such as temperature, pressure and time, the types of melting media, the types of carbon-containing raw materials and the influence of impurities, thus further improving the growth theory of synthetic diamonds.
2. Equipment and technology for synthesizing gem-grade diamond by 2.HTHP method.
(1) equipment for synthesizing diamond by HTHP method
The equipment for synthesizing diamond by static pressure method can be roughly divided into four parts, namely, large-tonnage hydraulic press, high-temperature and high-pressure container (i.e. mold) for synthesizing diamond, heating system and control and detection system.
Due to the use of ultra-high pressure equipment, there are many technical problems, such as high mechanical properties of materials, high machining accuracy, long-term pressure stability of the press, and the ability to step up and step down. This puts forward high requirements for hydraulic cylinders, seals, hydraulic components and machining accuracy. It is not easy to meet these requirements, which is related to the level of the whole machinery industry.
In addition, the requirements for pressure vessels are higher. The first is the material problem. At high temperature, there are few materials that can bear the pressure above 500× 108Pa, and the price is also expensive. Under high pressure, the properties of materials may change and even explode. At present, the heating system and measuring system have been automated.
There are many equipment schemes to realize HTHP method, including six-sided roof, four-sided roof and double-sided roof. Here, taking double-sided annual rings as an example, the principle of the equipment is introduced (see Figure 4- 1-22). 1 is the frame of the oil press, which can be integrally cast. For presses with tonnage less than 1000, castings can be used. If the tonnage is large, a winding frame can be used, that is, a frame formed by winding steel wires or steel belts. 2. High pressure vessel, which is the key part of diamond synthesis, has strict requirements on its material, machining accuracy and shape design; 3 is an oil cylinder, and the internal piston moves up and down by high-pressure oil, which makes the mold compressed, which is similar to other types of oil presses.
The high-pressure die for the annual ring is shown in Figure 4- 1-23.
Figure 4- 1-22 Schematic diagram of main press
Fig. 4- 1-23 wheel high-pressure die
In Figure 4- 1-23, 1 is a pressure cylinder made of cemented carbide, generally W, Co and C alloys, with W (Co) =15%; 2 is anvil and cemented carbide, generally w (co) = 6%; 3 is a heat-resistant gold-bearing steel ring; The chamber 4, which consists of a pressure cylinder and an anvil, is a chamber for synthesizing diamond. The annual ring high-pressure die can also be wound with steel wire, which makes the stress distribution more reasonable, thus improving the service life of the die. There are many structures of the growth chamber for synthesizing diamond, and the simple growth chamber structure is shown in Figure 4- 1-24.
In Figure 4- 1-24, 1 is pyrophyllite, which is an ideal solid pressure transmission medium and insulating medium. Because it contains crystal water, it affects the synthesis of diamond. At present, most of them are made of pyrophyllite powder after combustion, which not only reduces the cost, improves the utilization rate of materials, but also meets the requirements of synthesis process. Pyrophyllite is a key auxiliary material in the process of diamond synthesis, and its functions include pressure transmission, heat preservation, heat insulation and sealing.
Figure 4- 1-24 diamond growth chamber
In fig. 4- 1-24, 2 is graphite flake, and synthetic diamond converts carbon with graphite structure into carbon with diamond structure, so carbonaceous material is the key material. Theoretically, all forms of carbon can be converted into diamond, but the research shows that different carbonaceous materials have considerable influence on the quantity, quality and particle size of diamond, and the free energy of graphite to diamond is low, so it is widely used. GAI (SK-2) is a commonly used graphite material in China, which is made of cooked petroleum coke powder, pitch coke powder and flake graphite as raw materials and molten asphalt as binder.
Carbonaceous materials are one of the important factors affecting the quality and output of synthetic diamonds. In order to obtain better diamond, there are the following requirements for graphite: ① graphite has a certain porosity, which can increase the reaction area; ② The carbon of synthetic diamond must contain a small amount of elements such as Ni, Fe, n a and Co, because these elements can promote the activation of carbon atoms, destroy the original structure and create conditions for the growth of diamond; ③ There are also requirements for the crystallinity of graphite. The number and arrangement of crystals have an influence on the transformation of diamond, and the degree of graphitization is high. From a kinetic point of view, it is relatively easy to convert into diamonds.
In fig. 4- 1-24, 3 denotes a metal alloy, that is, a catalyst sheet. According to the phase diagram of carbon, the transformation of graphite into diamond requires a pressure of1.25×10/0pb and a temperature above 2700℃. In order to reduce the synthesis temperature, carbon is added to the molten alloy. During the research, various metals were used in experiments. At present, most of them are alloys of Ni, Mn, Co and Fe, and there are even alloy sheets specially used for diamond synthesis, such as Ni95Co5, Ni65Mn35, Fe73Co27, etc. The research shows that nickel, manganese, cobalt, iron, chromium and other elements or their binary, ternary and multicomponent alloys are the basic and effective catalyst alloys for diamond synthesis. If a small amount of copper, niobium, magnesium, boron and aluminum are added, not only the conditions of diamond nucleation and growth can be changed, but also different diamonds can be grown.
There are many arrangements and combinations of crystal growth chambers. Figure 4- 1-25 shows the structure of a large diamond growth chamber. As shown in the figure, the raw materials are loaded into the growth chamber (that is, the cylinder of the press), the press is started, the two pressure heads are sealed, and electricity is applied for heating. Diamonds larger than 1mm can be grown by this pressure and heating method, but the single output is not high.
Figure 4- 1-25 Large-grain diamond growth chamber
As for the high-temperature heating system, there are two kinds of static pressure methods: direct heating and indirect heating. The direct heating is through the heating of the reaction material itself, and the indirect heating is through the graphite tube sheathed outside (insulated from the cylinder).
(2) The process of synthesizing gem-grade diamonds by HTHP method.
The most common methods for synthesizing gem-grade diamonds are the belt pressing method and the rod method.
1, diamond synthesis process by pressure method
1955, General Electric Company (GE) of the United States announced the first successful manufacture of synthetic diamonds with belt devices. Until 1970, GE used the seed catalyst method for seven days to obtain a diamond single crystal with a weight of about 1ct and a diameter of more than 5 mm. Its growth chamber is shown in Figure 4- 1-26.
Fig. 4- 1-26 growth chamber (a) and improved growth chamber (b) for artificial gem grade diamonds.
The growth chamber shown in Figure 4- 1-26 is divided into two parts. As a carbon source, diamond powder is placed in the central area of the pressure chamber, and seed crystals are placed at both ends. The catalyst metal (iron or nickel) is placed between the carbon source and the seed, and the temperature gradient in the chamber is maintained by the resistance heating of carbon tubes (the temperature gradient can also be changed with the thickness of carbon tubes or other hot materials placed in different parts). The temperature in the central carbon source region is the highest, and the end is crystallized. When heated to 1700℃, the metal catalyst melts, and the diamond powder in the central carbon source area continuously dissolves into the metal catalyst and becomes free carbon atoms. At first, the density of carbon is lower than that of metal, so the seed crystal tends to float from the bottom crystal bed to the central area of the chamber (seed crystal dissolves) or from the central area to the upper crystal bed, and reaches equilibrium after about1h. The top crystal bed contains many fine diamond crystals, while a small amount of diamond cores remain on the bottom crystal bed. Because the carbon in the metal has reached saturation, the diamond core no longer dissolves, and the carbon in the metal melt begins to slowly diffuse. Due to the high temperature in the central area and low temperature at both ends, more carbon atoms are dissolved in the central area than at both ends, which diffuse to both ends and deposit on the diamond crystal nucleus. This process continues until the fine diamond powder in the central area is used up. If the temperature gradient in the middle and tail of the cabin can be kept at 30℃/cm, the crystal can grow into gem-grade diamond stably. Because there are few crystal nuclei in the bottom crystal bed, large gem-grade diamonds can be obtained.
Experiments show that gem-grade diamond with a size of 5mm (about lct) can be obtained at the temperature of 1370℃ and the pressure of 6.0× 109Pa for one week. If proper trace elements are added to the cabin, the performance of the diamond can be improved and the diamond can be colored. If nitrogen is added, the diamond crystal can be yellow. Add boron, which is blue and has semiconductor properties.
2. The "rod" method of diamond synthesis
1990, Russia announced their achievements in planting synthetic diamonds with the BARS system, which means a pressure-free ball splitting device. In recent years, the technicians of American Gemesis Company have improved Russian technology and designed a new bar "separator" device. The pressure in the compound chamber (about 2.5 cm thick) of the device is obtained from a continuous carbide steel anvil. The inner cabin is equipped with six pressure anvils, which are located on the front of the cube and surround the synthetic cabin; The outer cabin is equipped with eight anvils, which are located on the octahedral surface and surround the inner cabin. The whole arranged multi-anvil parts are placed in two cast steel hemispheres (these two hinged hemispheres are called "separated bodies" and can be used as channels for anvil and synthesis chamber), and two large steel chisels connect these parts together, as shown in Figure 4- 1-27. This "stick" device uses graphite tubes to heat the synthesis chamber.
Figure 4- 1-27 Improved diamond synthesis device with "rod" method
The improved equipment has the characteristics of longer service life, higher productivity, simpler operation and easier maintenance. What is important is that its operation is very safe, and the probability of danger caused by leakage of high-pressure vessels during operation is also very small. In addition to purity, concentration and initial crystal growth, the key to the growth of commercial gem-grade synthetic diamonds is to carefully control the temperature and pressure in the whole crystal growth process by computer to ensure a continuous and stable growth environment. Another technological innovation is that the casting hemisphere can be opened and closed, which is convenient for loading and unloading samples.
Using this improved equipment, it takes about 80 hours to grow a 3.5-carat synthetic diamond crystal. The concentration of yellow in synthetic diamonds and the shape, symmetry and transparency of crystals can be controlled within a certain range. The device grows many crystals in a cabin by experimental method, and the growth period of crystals is 36 hours. However, due to the limitation of volume, these crystals grow very small. If there are four crystals in the cabin, each crystal is only 0.6ct in size; If eight crystals grow in the cabin, each crystal is only 0.35ct.
The technological conditions of diamond synthesis by "rod" method are as follows:
1) pressure is 5.0 ~ 6.5 GPA (equivalent to 50,000 ~ 65,000 atmospheres).
2) The temperature is 1350 ~ 1800℃.
3) Catalyze various transition metals (such as iron, nickel, cobalt, etc. ).
4) Planting natural diamonds or artificial diamonds.
5) Carbon source graphite powder or diamond powder.
The orientation of the seed crystal determines the crystal shape of the grown crystal. There is a small but important temperature difference between the top (also called "hot end", where the carbon source is placed) and the bottom (also called "cold end", where the seed crystal is placed) of the synthesis chamber. This temperature difference provides power for the growth of diamond crystals, so this technology is also called "temperature gradient" method. Under the condition of high temperature and high pressure, the graphite powder in the raw material area melts rapidly in the metal flux at the hot end. Driven by the temperature gradient, the carbon atoms in the hot zone diffuse to the cold end of the cabin through the flux, and finally deposit on the seed crystal and crystallize into single crystal.