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一． What materials are synthetic diamonds made of?
Synthetic diamonds are polycrystalline diamonds made from the aggregation of diamond crystals with a diameter of 10 to 30 nanometers. The manufacturing process is similar to the process by which diamonds form naturally.There are two main types of materials: the first is to mix raw materials such as silicon carbide or alumina in a certain proportion, and then conduct a chemical reaction under high temperature and pressure to generate crystal raw materials.The second is to put the crystal raw material into a diamond grower, and use the vapor deposition method under high temperature and high pressure to gradually grow the crystal raw material into the shape of a diamond. The material properties of synthetic diamonds are as follows:
1. Silicon carbide
Diamonds made of silicon carbide are often called "CVD diamonds". Silicon carbide is a high-hardness, high-melting-point material with good heat resistance and chemical stability. Silicon carbide crystals prepared through chemical reactions can achieve optical and physical properties similar to those of natural diamonds.
2. Alumina
Diamonds made of aluminum oxide are often called "HPHT diamonds". Aluminum oxide is also a good material choice primarily due to its chemical stability and high melting point properties. Alumina diamonds prepared through high-temperature and high-pressure reactions can achieve optical and physical properties similar to those of natural diamonds.
二. How are artificially grown diamonds grown?
The technology of synthetic diamonds is mainly to cultivate diamonds by imitating the environment when natural diamonds grow. In terms of the growth methods of crystal formation, synthetic diamonds can be roughly divided into three methods:
1. High temperature and high pressure method
The materials used in this method include coal, coke, graphite, paraffin, sugar, etc. One report listed more than 20 materials that have been successfully synthesized.
The catalysts used can be iron, cobalt, nickel, rhodium, ruthenium, palladium, osmium, iridium, chromium, tantalum, magnesium, or mixtures of these metal elements.It is required to reach at least 75,000atm (atmospheric pressure), preferably at a high pressure state of 80,000atm-110,000atm, and the formation temperature should be between 1200℃-2000℃, preferably between 1400℃-1800℃. The middle part of the reaction chamber is the high-temperature area, the carbon source is placed in this area, and the seed crystal is placed in the low-temperature area at the bottom of the reaction chamber.Using graphite as the carbon source, the seed crystal is fixed in the sodium chloride (salt) crystal bed. A special crystal face of the seed crystal faces the metal catalyst. A metal catalyst cylinder with a diameter of 6mm and a thickness of 3mm is placed between the seed crystal and the carbon source. The metal catalyst is an iron-nickel alloy.
After combination, it is placed in a one-way pressurized, four-slope three-dimensional ultra-high-pressure and high-temperature device, and then placed in a 1000-ton hydraulic press. The temperature of the reaction chamber is about 1450°C, the pressure is controlled at about 6GPa, and the growth time is 22~52 Hour.
2. Shock wave method
It mainly uses the instantaneous high-temperature and high-pressure conditions generated during explosions to synthesize diamonds. The synthesized diamond particles are very small (commonly known as diamond powder) and are only suitable for industrial applications.
3. Chemical vapor precipitation method
First, hydrogen and hydrocarbon-containing gas (usually methane CH4) are passed through a set of regulators to adjust the ratio of the two gases, and then a microwave source or electric heating wire is used to heat the mixed gas until the temperature reaches about 2000°C. Hydrogen and methane will decompose into a plasma flow of hydrogen atoms and carbon atoms, and then nodules will grow into a thin film on a substrate heated to 600~1000°C.
