Diamond Synthesis Method
Diamonds can be synthesized by gas-phase methods (e.g., PVD or CVD), liquid-phase methods (e.g., hydrostatic, catalytic, or CVD liquid-phase methods), or solid-phase methods.
The solid-phase method is the synthesis of diamonds in a material with no fluid. The solid carbon atoms cannot diffuse through the fluid to reorganize into the structure of a diamond, but must be converted directly into diamonds without catalytic catalysts. This phase transition is Displacive, which is very different from the Reconstructive phase transition in which the atoms are dispersed and then reorganized. Displacive phase transitions occur very quickly because the atoms do not need to diffuse and reorganize. Since the synthesis time is short, it can be synthesized instantaneously (a few microseconds) by capacitive discharge under static pressure, or instantaneously by the short-term high pressure and high temperature generated by a bomb explosion. The former is limited by the volume of the high-pressure chamber, so it is not practical, so industrial diamond powder can be produced in large quantities by explosive method.
Types of Explosive Methods
Explosive method is divided into two categories, to produce high-pressure explosives as raw materials directly synthesized, or shock waves generated by the high pressure of graphite into diamonds. Explosive gasification, the residual carbon and other elements of the atoms will collide with each other into the nanometer level (3-10 nm) of the slag (Detonation Soot). This detonation soot contains Diamond Like Carbon (DLC). This process is similar to the PVD process. The former involves a gas explosion that crashes a large number of carbon atoms into each other to form nanoparticles, while the latter involves an electric field that continuously crashes vaporized carbon ions onto the substrate to form a thin film of nanoparticle compositions.
Explosives Synthesis
Explosive method of explosives used to high carbon content, and oxygen and other impurities to be less (such as TNT, RDX, HMX, etc.). This kind of explosives in the oxygen-free closed chamber detonation can make the residual carbon in the instantaneous compression into slag, slag for the diamond-like carbon containing a lot of impurities (about 10%) and defects. Due to the extremely small particles (e.g. 411 m). The specific surface area is extremely high (amount of impurities, e.g. 300 M2 /g). e.g. 300 M2 / g), a large number of impurities are adsorbed.
Due to the low rate of conversion into diamonds after the explosion of explosives and the high cost of cleaning and grading of nanodiamonds, this product is only used for specific applications, such as precision polishing, surface hardening of engine pistons, and PVDD/CVDD crystallization, etc. The price is high (e.g. $3 carat). Due to the high price (e.g. $3 carat), there is not much demand in the market at present.
supersonic shockwave
Another explosive method is indirectly supersonic shock waves to hit the graphite, so that it is instantly converted into micron-sized diamonds. This kind of Shock Synthesis (Shock Synthesis) method by the manufacture of explosives in the United States DuPont (DuPont) in 1960, and began mass production in the 1970s. Graphite is mixed with copper powder (92%, 1mm) and then pressed into a five-meter-long round rod by the Cold isostatic Pressing (CIP) method. The rods were sealed with steel tubes containing a vacuum barrier. Prior to the explosion, several tubes were placed in a batch in the pit and surrounded by several metric tons of explosive fill.
When the explosive is ignited from one end of the detonator, it will squeeze the steel tube from one end to the other in an instant, and the graphite inside the tube will be briefly pressurized to about 200,000 atmospheres and heated to over 1,000 degrees Celsius as the shock wave passes through it. Due to the extremely high pressure, the diamonds nucleate in large numbers, and within a few microseconds part of the graphite is transformed into diamond microcrystals (containing thousands to millions of atoms) of about 1-20 nanometers. These crystals are bonded together to form micron-sized diamond powders. Several kilograms of diamond powder can be produced at a time by this method. After the shock wave passes through, the pressure decreases dramatically. If the temperature is still high at this point, the resulting diamonds will immediately turn into amorphous carbon slag. Since the graphite is mixed with a large number of copper particles as Heat Sink, the resulting diamond powder can be quenched rapidly to avoid carbonization. The diamond powder is oxidized by PbO at 400°C after a series of acid washing to remove metals. It is then cleaned and graded into products under the Mypolex trademark. Shockwave synthesis method of raw materials must first be cold isostatic pressure treatment, so that it is dense. If there are too many pores, shock wave compression pressure is not enough, and the temperature is too high, can not reach the diamond stabilization zone, so it is difficult to synthesize the diamond. Mypolex diamond is a polycrystalline body containing a large number of defects. So its specific surface area than the same particle size grinding and into a single crystal is about three times larger, so it may absorb more impurities. However, because its shape is like a potato, it does not have the sharp corners often found in monocrystalline diamonds, so it will not scratch the substrate during polishing. When polishing alternating hard and soft workpieces, it will not cause pits. In particular, the polycrystalline micro-powder can be gradually disintegrated, reducing the contact area at the cutting point. This self-sharpening characteristic reduces the polishing power but increases the rate. Polycrystalline powders thus combine the efficiency of large-grain grinding with the quality of small-grain polishing.
The graphite structure directly converted to diamond must be either hexagonal (Hexagonal) or rhombic (Rhombohedral). The former, ordered AAA, can be converted into a hexagonal diamond (Lon- sdaleite); the latter, ordered ABC, can be formed into a cubic diamond. The majority of graphite is sorted into ABA…and therefore cannot be directly converted into diamonds. However, it also contains a small amount (10%) of rhombic graphite. The low conversion rate of diamonds synthesized by shockwave is limited by the amount of rhombic graphite. Because of the low conversion rate, coupled with the micropowder acid washing grading procedures are complicated, so the high cost of diamonds produced by the explosion method is generally more than four times the price of crushed and ground monocrystalline diamond micropowder. Therefore, although the explosion method of polycrystalline diamonds can be polished at high speeds without the risk of scratching the workpiece, but is currently only used in precision polishing such as scratching out the patterns of hard disk drives or smoothing the surface of the magnetic head or high-value polishing (such as gemstones or wafers). The annual global consumption is about one ton, with an output value of about 10 million US dollars.
