Explore the secret and reveal the difference between HPHT diamond and CVD diamond

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The progress of science and technology provides more possibilities and convenience for beautifying people’s lives. It will provide more market supervision and technology identification for the development of the market. Cultivating diamonds, also known as synthetic diamonds, especially synthetic colour diamonds, enables beauty lovers to enjoy the decorative beauty of colour diamonds with lower consumption.

Cultivated diamonds are synthesized in the laboratory, which has the same physical properties, chemical composition and crystal structure as natural diamonds. At present, the main synthetic methods of gem grade diamond are high temperature and high-pressure synthesis (HPHT synthesis) and chemical vapour deposition (CVD).

At present, there are two methods of diamond cultivation in the world: high pressure high temperature (HPHT) and chemical vapour deposition (CVD)

1.High temperature and high-pressure method (HPHT for short) to synthesize diamonds

The principle of diamond synthesis under high temperature and high pressure:

High temperature and high pressure synthesis method are also called seed catalyst method. Graphite is a low pressure stable phase, and diamond (the mineralogical name of diamond) is a high pressure stable phase.

The transformation from graphite to diamond requires very high pressure and temperature conditions, which generally requires pressure and temperature of 10 GPa and above 3000 ℃. The metal catalysts are involved in the synthesis of the diamond by high temperature and high pressure. The metal catalyst is between the carbon source (usually graphite) and the diamond seed as a solvent.

The carbon source is at the high temperature end, and the seed crystal is at the low temperature end. Because the solubility of the carbon source at high temperature is greater than that at low temperature.

The solubility difference caused by the temperature difference becomes the driving force for diffusing the carbon source from the high to the low temperature end. The carbon source gradually precipitates at the seed crystal, and the diamond crystal grows up slowly. Because the driving force of crystal growth caused by the temperature difference, this method is also called the temperature difference method.

HTHP synthetic diamonds are usually cubic, octahedral, and the combination of the two; the colour is usually yellow and tawny, and there are color bands and metal inclusions inside. The unmelted metal inclusions are needle-like, sheet-like, columnar or irregular in appearance, with a metallic lustre, which makes synthetic diamonds magnetic. Synthetic diamonds usually have no fluorescence under long-wavelength ultraviolet, but yellow, green, yellow and orange-yellow fluorescence under short wavelength. Synthetic diamonds have characteristic banding phenomenon under ultra short-wavelength ultraviolet or cathode ray, and different growth areas show different colour fluorescence banding.

High temperature and high pressure synthetic diamond applications:

High temperature and high pressure synthetic diamonds are mainly used in drilling (from oil and deep crust drilling to drilling holes in teeth); cutting (from marble cutting to gem cutting); aerospace (from cabin windows to space probes); electronic industry (from electronic instruments to supercomputer chips); heat exchange (from large boilers to kitchen utensils); jewelry and many other fields.

synthetic diamond applications

2. Synthesis of diamond by chemical vapor deposition

The principle of CVD diamond synthesis is as follows:

Chemical vapour deposition (CVD) is when the carbon-containing gas is dissociated, and the carbon atoms are deposited on the substrate to form diamond films. The substrate can be non-diamond material, but a single crystal diamond is usually formed by carbon atoms depositing on the diamond substrate. Carbon-containing gas is typically a mixture of nitrogen, methane and hydrogen. Methane is the source of carbon atoms in diamond synthesis.

Nitrogen can increase the growth rate, and hydrogen can inhibit the formation of graphite. Generally, CVD diamond synthesis is carried out under low pressure and high temperature, the pressure is usually less than one atmosphere, and the temperature is about 1000 ℃.

The basic characteristics of CVD synthetic diamond are as follows:

CVD synthetic diamond single crystals are mostly plate-shaped. The colour of early CVD synthetic diamonds is mostly dark brown or light brown, while the recently discovered CVD synthetic diamonds have been proved to be colourless or nearly colourless after high temperature heat treatment.

There are few inclusions in CVD synthetic diamond. However, it can not be used as a basis for distinguishing natural diamonds from HTHP synthetic diamonds. There is no inclusion in CVD synthetic diamond, so it is not magnetic.

Ordinary magnifying glass can’t recognize CVD synthetic diamond

Early CVD synthetic diamonds show orange to orange fluorescence. Short wave is more stronger than longwave; recently, CVD synthetic diamonds have yellow-green fluorescence, the short wave is stronger than the longwave, and most of them have glowing. Under ultrashort wave ultraviolet lamp, CVD synthetic diamond has a typical layered growth structure, which is the primary identification feature of CVD synthetic diamond.

3.Views of diamond experts on synthetic diamonds

As one of the only three formal members of the international gemological Conference (IGC) in four places on both sides of the Taiwan Strait, Dr Qiao Chuyuan, who is in charge of the diamond field, pointed out in his lecture at Guangdong jewelry, jade and precious metals testing centre (GTC): the mining of natural diamonds will damage the earth’s environment, consume a lot of resources, and have the moral problem of “blood diamonds”, which is expensive; synthetic diamonds must grow in the laboratory A green operation will not damage the ecology, the use of energy is limited, there is no problem of blood diamonds, and the price is low. The future price of synthetic diamonds is only a fraction of that of natural diamonds. The colourless diamond grown by high temperature, high pressure and constant temperature method will be supplied to the market far lower than that of natural diamond.

Although the road ahead is blocked and long, Dr Yuan pointed out the direction and path of future development for the industry, which provided a high reference value for promoting synthetic diamond identification and testing industry. In the near future, consumers will be able to treat synthetic diamonds correctly and be willing to buy synthetic diamonds, an excellent substitute for natural diamonds, at a reasonable price to alleviate the contradiction between supply and demand in the natural diamond market.

AttributeHPHT diamondHPHT diamondNatural diamondDifference
Chemical compositionCarbon (C)Carbon (C)Carbon (C)NO
The refractive index2.422.422.42NO
Relative density3.523.523.52NO
The dispersion0.0440.0440.044NO
Hardness90GPA90GPA90GPANO
Thermal conductivity2X103 W/(MK)2X103 W/(MK)2X103 W/(MK)NO
Thermal expansion0.8X10ˉ6K0.8X10ˉ6K0.8X10ˉ6KNO
Light transmittanceDeep UV to FAR TRDeep UV to FAR TRDeep UV to FAR TRNO
The resistivity1016 CHM-CM1016 CHM-CM1016 CHM-CMNO
Compressibility8.3 X 10-13 M2/N8.3 X 10-13 M2/N8.3 X 10-13 M2/NNO
Differences between HPHT and CVD cultivation and underground mining of natural diamonds
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