About the author: Shen, the first vice-chairman of insurance institute of china Artifacts Professional Committee, the second and third executive vice-chairmen and secretary-general, and a senior engineer in the metallogenic simulation laboratory of Beijing Institute of Geology, nuclear industry.
I. Characteristics of diamonds
The chemical composition of diamond is carbon (C), which may contain impurities such as boron and nitrogen.
Crystallization state: crystals.
Crystal system: equiaxed crystal system, common octahedron, rhombic dodecahedron, cube and other crystal forms, and the crystals often develop stepped growth lines, growth cones or etched images.
Common colors: ① white series, colorless to light yellow and light brown; ② Color series: dark yellow, brownish gray, light blue to dark blue, green, orange yellow, pink, red and purplish red, with occasional black.
Gloss: Diamond luster.
Mohs hardness: 10.
Density: (3.52 0.01) g/cm3.
Optical characteristics: isotropic, with occasional abnormal extinction.
Refractive index: 2.4 17.
Birefringence: None.
Deviation rate: 0.044.
Ultraviolet fluorescence: Long-wave fluorescence is from zero to strong, and the fluorescence colors are blue, yellow, orange and pink. Short-wave fluorescence is weak.
Special properties: The thermal conductivity of diamond is higher than that of all other substances (except the recently synthesized moissanite), and it emits special light. Diamonds will emit light blue phosphorescence after being exposed to the sun; Under X-ray irradiation, most of them are sky blue or light blue fluorescent, and a few of them are not fluorescent. Emit blue or green light under cathode ray.
Both natural diamonds and synthetic diamonds are very stable to all acids, and even at high temperature, acids can't have any effect on diamond crystals. However, diamond is easily corroded in alkali, oxygen-containing salts and metals. Because the composition of diamond is carbon, it can burn at 700 ~ 780℃ in pure oxygen. It can also burn when heated to 800 ~ 1000℃ in air; Under the vacuum of 800 ~ 1700℃, only the thin layer on the crystal surface is graphitized, and there is no change inside; In inert gas, when the temperature is above about 1700℃, the whole crystal will be graphitized rapidly and eventually become graphite powder. The starting temperature of graphitization varies with different crystals, ranging from 1600 ~ 1800℃. The melting temperature of diamond is (3700100)℃. Defective diamond crystals are often broken when heated, but well-crystallized diamond crystals can be heated to 1800 ~ 1850℃ and cooled quickly. At this time, they are not destroyed, but strengthened by eliminating local stress.
The most common diamond crystal is octahedron, followed by oblique dodecahedron, and there are few real cubes. Diamond has the highest hardness, but it is easy to crack. It is easy to crack along the (1 1 1) plane with the largest crystal plane spacing. This plane is also called the "cleavage plane" of a diamond. The original stone of the famous diamond "Cullinan" weighs 3 106.9 carats. The cleavage plane of the diamond is used to split it into many small pieces. For diamond with complete crystal and no visible defects, the pressure of cleavage crystal is between 300 ~ 1000 N/cm2.
Second, the history, methods and principles of synthetic diamonds.
1. The history of synthetic diamonds
1953 synthetic diamond was successfully tested for the first time in Swiss ASEA company, but it was not reported. 1955 February 15 General Electric Company of the United States first reported the successful growth of synthetic diamonds and obtained the right of invention. Since then, all countries in the world have carried out experiments and development of artificial diamonds. At first, people could only synthesize a large number of small and low-quality industrial-grade diamonds, which were mainly used in industrial applications. But people have been trying to grow high-quality diamond large single crystals. Finally, in 1970, the American General Electric Company announced that 5 ~ 6 mm gem-grade diamonds could be grown in seven days by seed crystal method, and the crystal weight reached about 1 carat. Later, they devoted themselves to the study of improving the growth rate of crystals, and it took only a few dozen hours to grow diamonds of the same size as the above. 1992, the company synthesized super diamonds with thermal conductivity twice that of natural diamonds, and the particle weight reached 3 carats. South Africa's De Beers Company was able to grow gem-grade diamonds in the early 1970s. In 1987, a large single crystal of1.14 carat was light yellow and transparent. In 1990, it was announced that it had grown a diamond of 14.3 carat. In 1990, the Siberian branch of the former Soviet Academy of Sciences announced that it had grown 7.5 mm gem-grade diamonds of different colors and weighing 1.5 carats. At present, they are the only country in the world that can use synthetic diamonds (usually processed diamonds are called diamonds) to enter the market. Today, Tyrus, a joint venture between Russia and Thailand, produces synthetic diamonds, including loose diamonds and inlaid diamonds. It is reported that the United States bought the technology of synthesizing gem-grade diamonds from Russia, so there are also synthetic diamonds produced in the United States on the market.
2. The history of synthetic diamonds in China.
China synthetic diamond 1963 succeeded. Due to the mature technology, there are manufacturers specializing in the production of equipment, and the supply and demand are large, many township enterprises can produce. According to the statistics of 1998, there are about 3,000 synthetic diamond factories in China, with an annual output of about 500 million carats. However, these synthetic diamonds are relatively small and can only be used in industry, and their quality belongs to industrial grade. For large-particle diamond, 1974 grows high-quality diamond single crystal by metal film method in Shanghai Silicate Research Institute, 1977 grows boron-containing semiconductor diamond single crystal with a maximum of 4mm and a weight of 0.29 carat, and then 1985 obtains high-quality synthetic diamond single crystal with a diameter of 3.2mm and a weight of 0.2 carat by seed method. But until now, China has not entered the ranks of commercial production of artificial gem-class diamonds, that is to say, there are no artificial diamonds produced in China in the jewelry market. According to incomplete statistics in mid-2002, there are 4,000 ~ 5,000 synthetic diamond factories in China, but only about 450 factories produce single-particle industrial synthetic diamonds, and others mainly produce polycrystalline diamonds or diamond products. However, the output of these 450 synthetic industrial diamond factories is relatively large. According to the estimation of the consumption of raw materials and catalysts (there is a certain proportional relationship between the consumption of raw materials and catalysts and the output of synthetic diamonds), the annual output of synthetic industrial diamonds in China should be around 65.438+0.2 billion carats, and the estimated annual production capacity can reach 65.438+0.5 billion to 2 billion carats. Through strong alliance or merger, there are about 10 factories with an annual output of 20 million carats of synthetic industrial diamonds in China, and the largest factory can produce 1 to 200 million carats of synthetic industrial diamonds. The appearance of pyrophyllite used in the cubic diamond press is shown in Figure 1, and the synthetic diamond raw material separator is shown in Figure 2.
Figure 1 pyrophyllite shape used in hexagonal top diamond machine
3. Advantages and disadvantages of artificial industrial diamonds in China.
The annual output of synthetic industrial-grade diamonds in China is about 65.438+0.2 billion carats, but at present, the annual output of synthetic industrial-grade diamonds in the world (except China) is 700-800 million carats, among which the main producing countries and companies are: Russia, the United States and De Beers, with an annual output of about 200 million carats, which shows that China has great advantages in annual output. However, the domestic production of synthetic industrial-grade diamonds is also at a great disadvantage. The main gaps are as follows: ① the output of each synthetic diamond (output per unit area): the output per unit area abroad is 600 ~ 700 carats; More than 97% synthetic industrial diamond factories in China use cubic diamond presses, and the minimum output is only about 10 carat. Good output can reach about 30 carats, and the best output can reach about 40 carats. The yield of double-sided diamond press is high, which can reach about 60 carats, indicating that the yield of double-sided diamond press is far from that of foreign synthetic diamonds. ② Quality gap of synthetic industrial diamond: The quality of synthetic industrial diamond mainly includes the following aspects: compressive strength, crystal morphology, thermal stability, impact strength, particle size, etc. Compared with the synthetic industrial diamonds produced by major foreign producing countries, the quality of synthetic industrial diamonds produced in China is relatively poor. Compared with similar products in China, the quality of artificial industrial diamond produced by double-sided diamond press is better than that of hexagonal diamond press. (3) Price gap: China's synthetic industrial diamonds are mainly raw materials, and the average price per carat is about 10 cent; The average price of synthetic industrial diamond raw materials abroad is 70 ~ 80 cents, and the highest price can reach 1 ~ 2 dollars. The price is determined by the product quality, which also confirms the evaluation of the poor quality of synthetic industrial diamonds produced in China. (4) Equipment gap: Double-sided diamond press is the main production equipment abroad, and its pressure is equivalent to 6000 ~ 10000 t, so the volume of synthesis chamber is large, so the yield is high; 97% of synthetic diamonds in China are produced by cubic diamond press, which has the advantages of less investment and low technical difficulty. But the disadvantages are small synthesis pressure chamber, low output and poor quality; For double-sided diamond press, the pressure is higher than that of hexagonal diamond press, but the pressure is equivalent to 2500 tons, which is much lower than that of foreign double-sided diamond press, and the synthesis chamber is also smaller than that of foreign countries, so the output per unit area is relatively low. Can we increase the pressure? ! Difficult. It is said that the quality of the synthetic cavity materials mainly produced in China equivalent to 6000 tons of pressure can not meet the requirements. At present, some domestic units have imported diamond presses equivalent to 6000 tons from abroad to produce high-quality synthetic industrial-grade diamonds.
Fig. 2 Artificial diamond raw material sorting machine
4. The latest development of synthetic diamonds in China.
1) In the 1990s, the Institute of Artificial Crystals of the former Ministry of Building Materials used chemical vapor deposition (CVD) to grow the surface of a black diamond ring with a thickness of 2mm and a length of 5mm for market use. According to Professor Chen Biankun of Beihang University, in 2006, a domestic company was able to grow a diamond block with a thickness of about 1mm, an area of about 100cm2 and a weight of 150 carats by this method, but the price was still high, so the price of such a diamond raw material was about 1 10,000 yuan.
2) On August 14, 2003, Gem Weekly published the news that "I successfully synthesized diamond at 440℃". The research group led by Professor Chen of China University of Science and Technology successfully synthesized 250μm diamond at 440℃ with CO2 as carbon source in the research of "synthesizing diamond by reducing CO2 at low temperature", which realized the transformation from CO2 to diamond for the first time, and caused great repercussions in international academic circles. Professor Chen and his colleagues developed their own high-pressure reactor for experiments. Using safe and nontoxic CO2 as raw material and metal Na as reducing agent, CO2 was finally reduced to diamond through 12h chemical reaction at 440℃ and 80MPa. At present, 1.2mm diamond has been grown, which is expected to reach the gem level. The yield of CO2-converted diamond is 8.9%. X-ray diffraction and Raman spectrum analysis show that these synthetic particles are diamonds, colorless and transparent, which can be comparable to natural diamonds. The process has good repeatability, and other carbon sources and reducing agents have also been successful, and the related achievements have applied for international patents.
5. The principle of synthetic diamond
As we all know, the chemical composition of diamond is carbon (C) just like graphite, but graphite is soft and diamond is hard. The difference is that graphite has a hexagonal structure and diamond has a cubic structure. In order to transform the hexagonal structure of graphite into the cubic structure of diamond, the conditions are very harsh, and the temperature of 2700℃ and the pressure of 12.5GPa are needed. Such high temperature and high pressure bring considerable difficulties to the manufacture of production equipment and the conversion rate is not high. Later, people adopted a "catalyst" composed of transition metal elements such as iron, cobalt, nickel, chromium and manganese, which can convert graphite into diamond at 1200℃ and 4GPa. The structural diagram of graphite transformed into diamond under the action of catalyst is shown in Figure 3.
Fig. 3 Structure diagram of graphite transformed into diamond under the action of catalyst.
Comparing the structural changes before and after the transformation, it can be seen that the spacing between graphite layers has decreased by about1.3×10-10m. The adjacent atoms in the graphite layer are shifted upward and downward by about 2.5× 10- 10m respectively with respect to the vertical direction of the layer plane, forming a double layer with a distance of 5.0×10-1m.. The atoms in the bilayer are connected by * * * valence bonds to form a twisted hexagonal lattice, and the elongation of the atomic spacing is1.54×10-10m. In this way, the atoms of the lower layer of the upper double layer and the atoms of the lower double layer are completely corresponding, and the distance is also1.54x10-10m. As long as the original free 2Pz electrons are concentrated in pairs between these corresponding atomic pairs to form a vertical valence bond with the bond length of1.54×1-kloc-0/0m, it can become a diamond structure. This transformation is obviously much easier than the transformation of disassembling carbon atoms in graphite and reassembling them into diamonds. At present, this method is used in the artificial synthesis of industrial diamonds all over the world. In operation, a piece of high purity graphite and a piece of metal catalyst are alternately stacked and assembled, and then put into a special device to be synthesized in a diamond press with double top or hexagonal top (Figure 4). However, up to now, there are no manufacturers producing gem-grade synthetic diamonds (generally considering 5mm crystals) in China, and almost all of them are industrial-grade synthetic diamonds and deep processing enterprises of diamond products.
Fig. 4 Double-sided diamond press and products.
In the artificial synthesis of gem-grade large-particle diamond, diamond is generally used as seed crystal, diamond powder is used instead of graphite as carbon source, and the temperature in the middle of the growth chamber is higher than that at both ends, so metal catalyst must be used. Fig. 5 shows two different synthesis cavity structures for growing gem-grade diamond by seed catalysis.
Fig. 5 Two different synthesis cavity structures for synthesizing gem-grade diamonds.
The growth process is as follows: diamond powder (or the mixture of spectrally pure graphite and diamond powder) is placed in the middle (hot zone) of the cavity, nickel-iron (1: 1) alloy is used as the catalyst, and diamond seeds are placed in the cold zones at both ends, driven by temperature gradient of 30 ~ 50℃ at high temperature and ultra-high pressure (5.5 GPA,130). The carbon in the hot zone diffuses to the diamond seed crystal in the cold zone, and some supersaturated carbon will inevitably appear during the cooling process, which will be deposited on the diamond seed crystal, making the seed crystal grow into a large diamond crystal until the carbon source is consumed. If some impurities are artificially added to the raw material, the diamond can be colored, such as adding nitrogen (absorbing nitrogen by adding a small amount of titanium) to get yellow or green; Blue color can be obtained by adding boron, which has semiconductor properties; Adding enough titanium can make synthetic diamonds colorless; Adding a certain amount of iron can also make synthetic diamonds obtain nearly colorless synthetic diamonds. Here, the catalyst not only plays the role of dissolving carbon, but also plays the role of accelerating the growth of diamond.
6. Overview of synthetic diamond methods
There are many ways to synthesize diamonds, and the two methods mentioned above are the most commonly used. There are different methods for different purposes. With the development of science and technology, some new synthetic methods have been invented, and there are dozens of them. Here are five:
(1) explosion method
Graphite is transformed into diamond by the high temperature and high pressure produced by the explosion of high explosives. However, due to the short holding time, the diamond particles formed are very small, the average particle size is less than 10μm, and the maximum particle size is about 40μ m. In the best case, 60 carats of diamond powder can be synthesized per kilogram of explosives. The product is suitable for making grinding paste and can also be used as raw material of polycrystalline diamond. The biggest advantages of this method are cheapness, less investment and high yield (up to 500 carats).
(2) Discharge method in liquid
The graphite electrode containing catalyst metal and hollow cylindrical graphite (or metal) are made into two electrodes, which are immersed in a liquid medium with low vaporization heat (such as carbon tetrachloride). The hollow cylindrical electrode and the graphite electrode are coaxial. When high current and high voltage are switched on, a spark discharge is generated between the two electrodes, which causes the liquid to generate shock waves, forming a high temperature and high pressure region, and graphite can be transformed into diamond. 0.5mm diamond powder can be obtained by this method, but the main disadvantage is that the yield is not high.
(3) atmospheric pressure and high temperature synthesis method
Also known as CVD method, it is a method of synthesizing diamond under normal pressure. In this method, methane gas containing carbon or liquid with alcohol concentration is used as raw material, and carbon atoms (plasma) are decomposed by heating under normal pressure. Under the action of electric field, free carbon atoms are deposited on the surface of diamond seed crystal to grow diamond, and diamond particles can also be plated on the non-diamond surface. The diamond grown by this method is very slow and the particles are very fine, so it is often used for surface coating, for example, a thin layer of diamond is coated on the missile head by this method. In recent years, the international research on this method has made a technical breakthrough and the growth rate has been greatly improved. Large single crystal diamond with the size exceeding 10 carat has been grown, which has become a hot technology developed by various countries, and China is also catching up.
(4) atmospheric pressure vacuum synthesis method
Put the catalyst metal into a vacuum furnace, then sprinkle graphite powder, and then vacuum heat it at a constant temperature of 900℃ 10h. Industrial-grade diamond, which can be used for drill bits and abrasives, is crystallized in the heated mixture and can be used after separation.
(5) reducing carbon dioxide to synthesize diamond.
On August 14, 2003, Gem Weekly published the news that "I successfully synthesized diamonds at 440℃". The research group led by Professor Chen of China University of Science and Technology successfully synthesized 250μm diamond at 440℃ with CO2 as carbon source in the research of "synthesizing diamond by reducing CO2 at low temperature", which realized the transformation from CO2 to diamond for the first time, and caused great repercussions in international academic circles.
Third, the use and prospect of synthetic diamond
Synthetic diamonds are widely used.
1) Our common diamond bit for geological exploration and diamond saw blade for cutting stones and roads (Figure 6). Diamond grinding disc, diamond micropowder polishing paste, diamond wire drawing die, etc. It is indispensable to process precious stones, and the consumption is very large. According to 1975, the annual consumption of diamonds in the world is12.5 billion carats, most of which are synthetic diamonds.
Fig. 6 synthetic diamond saw blade
In addition, synthetic diamonds are also useful in high-tech and national defense industries.
2) Using the high thermal conductivity of diamond, it can be used as a heat sink for solid-state microwave devices and solid-state laser devices, creating favorable conditions for manufacturing micro-radar and communication equipment.
3) Using the semiconductor characteristics of Ⅱ A diamond, it has high temperature resistance, heat dissipation, high hardness and corrosion resistance, and can be used as diamond rectifier, diamond triode, diamond thermometer, etc. It can play a great role in space navigation.
4) Kitchen utensils revolution: In the field of daily consumer goods, artificial diamond films can be coated on the surfaces of various kitchen utensils, so that the low friction coefficient of diamond makes food not easy to stick to the bottom of the pot; The high hardness of diamonds makes kitchen utensils not easy to be damaged.
5) Oil-free bearing: plating synthetic diamond film on the surface of the existing bearing can greatly reduce the friction coefficient, and it is not easy to be damaged without oil, and at the same time, it can protect the bearing from seawater corrosion.
6) Diamond window: Diamonds are completely transparent to electromagnetic radiation in the visible and infrared spectrum, have strong resistance to high-speed raindrops and dust, and can quickly conduct heat generated by air friction. These characteristics make diamonds of great significance in space exploration. For example, in 1978, the American Pioneer space probe installed a diamond window when exploring Venus. Since the atmospheric pressure of Venus is nearly 100 times that of the earth, when the probe descends in Venus' atmosphere, the diamond window can not only bear huge heat and pressure, but also make the infrared rays in Venus' atmosphere pass through the diamond window without being absorbed, so that the probe can successfully measure the infrared radiation in Venus' atmosphere. At that time, this diamond window was cut from gem-grade natural diamonds, and now diamond windows with similar or larger diameters can be artificially synthesized by CVD.
7) Supercomputer application: The operation speed of large computers using digital integrated circuits depends on the transmission speed of signals between chips. People use three-dimensional multi-chip modules, but the high-speed transmission of signals between chips will release a lot of heat, which was previously solved by liquid nitrogen. Now the chip is directly placed on the high-purity synthetic diamond film to dissipate heat, which can greatly improve the running speed of the supercomputer.
It can be seen that synthetic diamonds play an important role in the development of industry, science and technology and national defense industry. From here, we also see that the prospect of synthetic diamonds or diamonds is very broad.
refer to
Shen, man. 1994. artificial gem. Beijing: China Geo University Press.
Guo Yongcun et al. 1984. Artificial synthesis and application of diamond. Beijing: Science Press.
He, Shen. Wu Guozhong. 1997. Artificial synthesis and identification of gems. Beijing: Aviation Press.
He, Shen. 2005. Gemstone synthesis technology. Beijing: Chemical Industry Press.
Zhang Beili et al. 1997. Systematic gemmology. Beijing: Geological Publishing House.
Gem Weekly (newspaper), August 2003, 14.