Diamonds are long-lasting, and a eternal rumor, diamonds have almost become the most suitable equivalent of eternal love. More than half a century ago, the sexy goddess Marilyn Monroe also told us that "diamond is the woman's best friend." For women, in love, nothing can be solved by a diamond. If so, please use two.
In May of this year, the famous diamond brand announced that it would open its own brand of synthetic diamonds. The price of 1 carat is equivalent to about RMB 5,500, which is less than 1/8 of the price of natural diamonds of the same weight. China is the world's largest producer of synthetic diamonds. In 2017, China's synthetic diamond production accounted for more than 90% of the world.
Some people say: "Artificial diamonds" are the catastrophe of the diamond giants, and Chinese manufacturing has sounded the death knell for the diamond industry. Today we will talk about the topic of diamonds.
(1) Why do diamonds look so dazzling?
The scientific name of diamonds is diamond, which has two major categories: natural mining and artificial synthesis. Its English name, diamond, is derived from the Arabic word "almas" ("the hardest") or the Greek word "Adamas" ("invincible"). Obviously, from the beginning of human understanding of diamonds, it is extremely hard.
However, although diamond is the hardest natural substance in the world, it does not mean that it is “indestructibleâ€. On the contrary, you can smash diamond with a hammer. Unbelieving readers can try with their wife's diamond ring.
This is because the hardness of diamond refers to "the ability of the material to partially resist the pressing of hard objects into its surface", while the hardness of diamond is high, but the toughness is extremely poor, and the ability to withstand impact loads is extremely weak, saying that human words are "crunchy".
The hard and brittle nature of diamond is determined by its crystal structure. Although diamond is the same as graphite, it is elemental carbon, but the atom of diamond is sp3 hybridized and covalently bonded to three adjacent carbon atoms to form a tetrahedral structure. It is a typical face-centered cubic lattice structure. Under this structure, the carbon atoms are always rapidly diffused and accumulated along the crystal plane with low free energy, so that the diamond shape is a regular polyhedron, and the diamond crystal is easily broken under the applied load at a specific angle. This is also the principle of diamond cutting.
Moreover, natural diamond is a single crystal with a very low doping level, that is, the atomic arrangement is very regular, which causes the light to refract in the diamond, resulting in a "dispersion phenomenon", showing a ray of light, called "fire" in jewelry appreciation. color". Every time a diamond is machined, the effect of reflection and refraction increases, and it looks even more awkward. However, the more diamonds are not the better, the best "fire" is the value of jewelry diamond cutting.
The critical angle of total reflection of light in a diamond is 24°26′. To eliminate the light leakage area inside the diamond, it is necessary to measure the proportion, shape and size, shape and size of the diamond pavilion and crown, and then pass the most exquisite The process can achieve the best reflection of the incoming light to achieve the best effect of diamond fire.
(2) China is the world's largest producer of synthetic diamonds
In today's diamond jewellery market, although there is no authoritative data on the proportion of synthetic diamonds, as early as 2014, the diamond industry began to worry about the use of synthetic diamonds for natural diamond jewellery. Because synthetic diamonds are no better than natural diamonds under laboratory conditions, synthetic diamonds are only half the cost of natural diamonds compared to the scarcity of natural diamonds and the increasing cost of mining.
Tom Moses, chief laboratory and research officer at the Gemological Institute of the United States, once said, "The natural, diamond and synthetic diamonds have exactly the same chemical, physical and optical properties; the only difference is the way in which natural diamonds are formed underground for billions of years. It takes only two or three days for the laboratory to produce synthetic diamonds."
Where is the world's largest manufacturer of artificial diamonds? I believe you have already guessed - China.
In the field of synthetic diamond production, as of 2016, China has ranked first in the world for 14 consecutive years. China's synthetic diamond production accounts for more than 90% of the global production of synthetic single crystal diamonds, and the output in 2016 is close to 20 billion carats.
Let's count this account: In 2016, there were 11.33 million new couples registered in China. If we want to average the artificial diamond production this year to about every new couple, it is about 1800 carats/person. Looking at it this way, isn't every bride able to bring a big diamond ring like Fan Ye? Obviously not.
In fact, less than 10% of the world's natural and synthetic diamonds are used as jewellery each year, and most of them flow to the industry, playing the excellent material properties of this pure carbon crystal.
(3) Except for appreciation, most of the diamonds flow to the industry.
The optical performance of diamonds is just a pity to watch the flash. In fact, one of the big uses of diamonds is the use of optics.
Diamond has a high transmittance from X-ray to microwave, and has excellent hardness, high thermal conductivity, high chemical stability and low expansion coefficient. It is an excellent optical material. Lenses made of natural diamonds have excellent performance and have been used in the aerospace industry. Single crystal diamond can also be used as a window for high-power lasers, with high laser transmittance, high temperature resistance and thermal shock resistance, which greatly improves the output power of the laser.
Of course, diamond is also a common application for making tools as hard materials.
At the same time, diamond has high thermal conductivity, chemical stability, low expansion coefficient and other properties, and has unique advantages in some special occasions. For example, in the processing of nuclear reactors, mirrors of precision optical instruments, computer hard drives, navigation gyros of missiles or space vehicles, accelerator electron guns, etc.
In addition, synthetic diamond can artificially control the synthesis process relative to natural diamond, so the composition and microstructure of synthetic diamond can be changed. In particular, diamond has a wide band gap width, high carrier mobility, high breakdown voltage, and high thermal conductivity. It is an ideal semiconductor material. If n-type doping is realized, the industrialization of diamond semiconductor devices will become may.
(4) When can artificial diamonds replace natural diamonds?
According to news reports, it is true that a company in the United States has used the carbon in the human body as a raw material to make ashes diamonds.
This is about the technical methods of synthetic diamonds.
1. How is synthetic diamond synthesized?
Nowadays, there are many methods for synthesizing synthetic diamond. The technologies that can be industrialized mainly include high temperature and high pressure (HTHP) and chemical vapor deposition (CVD). Industrial synthetic diamond, HTHP is basically used at home and abroad.
China's synthetic diamonds have an absolute advantage in the world and are inseparable from the industrialization of large-scale synthetic presses, high-quality hard alloy top hammers, powder catalysts and indirect heating processes (collectively referred to as powder processes).
The large-scale synthetic press is the core equipment for synthetic diamond. It is to create a high-temperature (>1400°C), high-pressure (>5GPa) synthesis chamber. According to the carbon element equilibrium phase diagram, the carbon atoms are stabilized in this cavity. Diamond crystals. Commonly used in foreign countries are uniaxial presses, commonly known as two-sided presses, while in China, they use a 90-degree three-axis press. This six-sided press presses the three-axis pressure by a hinged structure.
For presses, the most critical part is the hammer that makes up the synthesis chamber. The top hammer is subjected to complex and severe compression, shearing and tensile stresses. In addition, the instantaneous hammer surface temperature reaches 500-600 °C during heating, requiring nearly 10,000 repeated service life, and the service conditions are very demanding. At present, the top hammer material is made of WC-Co alloy.
The hinged six-face press has two advantages: the first one is to design and install a high-pressure cylinder at the center of the hinge beam, the cylinder has a built-in piston, and the high-pressure oil pressure is pushed from the three axial directions to the synthesis cavity, and the three-axis pressure ratio is two sides. The uniaxial pressure of the top is more favorable for the growth of single crystal diamond. Another feature is that it requires only six identical alloy top hammers, and does not require a hard alloy cylinder of a two-sided press. The hard alloy cylinders are difficult to manufacture and costly.
China designed and manufactured this six-face press from 1965. So far, the entire industry has almost adopted a six-face press, and it has begun to be introduced by foreign manufacturers.
With synthetic tools, the next step is to solve the problems of raw materials and processes. Materials required for synthetic diamonds include graphite and metal catalysts. Since 2005, the entire synthetic diamond industry has begun to use a powder process, which is a process in which a mixture of graphite powder and catalyst powder is cold pressed into a composite block, followed by deoxidation. The treated synthetic block is loaded into the synthetic cavity of the pyrophyllite, and a large current is introduced into the thin piece heating element of carbonaceous material wound around the synthetic block by the conductive caps at both ends, and a heating is formed around the synthetic block. furnace.
From this technical point of view, it is completely feasible to synthesize diamonds using the human body as a carbon source. However, the proportion of carbon powder made of ashes that can be finally synthesized into diamonds is very small, and most of them will be combined with catalysts. After mixing, it is washed by subsequent pickling as sewage. The carbon content of adult ashes is not too large. Considering the yield, the final diamond will not be too large.
2. How dirty is the synthetic diamond industry?
As mentioned before, diamond is the growth of graphite in the synthetic cavity under the action of metal catalyst. When this furnace "popcorn" comes out, I am afraid that I can't see the "diamond" at all, but one by one. Blackbird. This is because, in the synthesis of synthetic diamonds, the catalyst powder and the graphite powder are thoroughly mixed uniformly. After the high temperature and high pressure synthesis process, the molten catalyst metal fuses the diamond and the graphite together to form a metal catalyst coating. So why do the diamonds we finally see are crystal clear?
There is also a separate step in the synthesis of diamonds, which is the process of “taking†the diamond out of the metal catalyst coating. A lot of pollution and hazards are mainly concentrated in this step.
In the final process of separating diamond, concentrated sulfuric acid and concentrated nitric acid are used to remove graphite and catalytic metal. The function of concentrated sulfuric acid is to remove graphite. The function of nitric acid is to remove the catalyst metal and also participate in the reaction of removing graphite. The catalytic metals used in the synthesis of diamonds are generally Fe (iron) and Ni (nickel). Therefore, the substance after the chemical reaction is the ionic heavy metal Co (cobalt, one of the iron-based elements), Ni and the human respiratory tract. A large amount of nitrogen oxide gas and sulfur dioxide gas, a carcinogen HNO2 (nitrous acid), which irritate the symptoms.
The treatment of industrial sewage containing a large amount of waste acid and heavy metals Ni and Co is a very big challenge for the synthetic diamond industry.
In order to be able to produce purer diamonds, chemical vapor deposition (CVD) provides another technical approach. It mainly utilizes chemical reactions occurring in high temperature space (also included in the substrate) and in the activation space. The gas raw materials used in the preparation of diamond are generally methane and hydrogen, which are decomposed by excitation under high temperature conditions to form a gas. The active particles of the carbon group, and finally a diamond film is deposited on the substrate material.
3. There is still a difference between synthetic diamonds and natural diamonds.
Although humans have mastered the method of synthesizing diamonds, synthetic diamonds and natural diamonds still have a "natural" difference.
Unlike natural diamond, synthetic diamond has inclusions and bubbles in the single crystal, and there are also substitutions, vacancies, dislocations, etc. of carbon atoms. The inclusions are residues that are held by the HTHP synthetic catalyst and the carbon eutectic during crystal growth, which is the accumulation region of impurity elements in the diamond crystal.
At the same time, synthetic diamonds are more yellowish in color than natural diamonds due to the introduction of impurities such as oxygen and nitrogen and the influence of Fe, Ni, etc. in the catalyst.
At present, the content of impurities in diamonds at home and abroad is very close, which is also reflected in the compressive failure strength (CFS) and thermal shock strength index (TTI). The average values ​​measured are about 42% and 89% respectively. That is to say, China's synthetic diamonds have been evaluated in terms of crystal shape, strength, and impurity content, and the quality has reached the world's advanced level.
Conclusion
The technological progress of China's synthetic diamond has greatly improved the cost performance of diamonds, and the cost has been declining. The application of industrial diamonds will become wider and wider. In the future, diamonds, diamond core CPUs, diamond core mobile phones and diamond core bank cards will be used extensively in daily life. Maybe not far away.
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