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High temperature and high pressure method-synthetic diamond
I. Overview

As we all know, diamond is a gem made of single element carbon. In nature, the formation of diamonds is the result of geological processes under high temperature and high pressure. Synthetic diamond artificially simulates the conditions for the formation of natural diamond, so that carbon with non-diamond structure can be transformed into carbon with diamond structure. As early as 1953, the Swiss engineering company (ASEL) successfully synthesized 40 small diamond crystals by using a device called "pressure ball", but it was not until 1955 that the American General Electric Company (gE) announced that it had successfully manufactured diamonds for the first time by using a device called "pressure belt" that their research results were made public. Not to be outdone, De Beers mastered the complex technology of diamond synthesis in 1959. Their method is very similar to that of General Electric Company, which is strictly kept secret by the US government. In the early 1960s, De Beers and General Electric Company began to produce industrial synthetic diamond powder. In 1960s, China also successfully synthesized abrasive grade diamond and put it into production. 197 1 year, General Electric Company announced the synthesis of diamond crystals with an average diameter of 6mm. These diamond crystals are not only yellow and brown, but also almost colorless, with low nitrogen content. They are blue diamonds containing boron. Sumitorno Electric Industries began to join the ranks of synthetic diamonds in 1985, and produced high-purity industrial diamonds in 1993. 1990 Novosibirsk, Russia, announced that they have successfully synthesized diamonds using Spl(it-sphere) or BARS devices.

Due to the limitation of ultra-high pressure equipment and high temperature conditions, the production cost is high, so gem-grade synthetic diamonds are still expensive and scarce, but we believe that one day there will be gem-grade synthetic diamonds with reasonable prices.

Second, the principle of artificial diamonds

1. Carbon compounds

Diamond, graphite and amorphous carbon are all composed of carbon atoms. Their different appearances and completely different physical properties depend on their completely different atomic structures (Figure 9-7- 1).

Fig. 9-7- 1 equidistantly arranged diamond structure (left); Layered graphite (right)

2. Transformation from graphite to diamond

The temperature and pressure of synthetic diamond are very high. Even in the presence of catalyst, the pressure is (50 ~ 80) × 108 Pa and the temperature is 1350 ~ 1800℃. The most commonly used metal fluxes (catalysts) for diamond synthesis in high-pressure equipment are iron, nickel, cobalt and palladium. Figure 9-7-2 shows the relationship between the crystal shape of synthetic diamond region and temperature and pressure in the carbon stable phase diagram. In the field of synthetic diamonds, the crystal morphology of diamonds is different under different temperatures and pressures. Therefore, synthetic diamonds are greatly influenced by temperature and pressure. When the temperature is low, the growth of cube is dominant; When the temperature is high, octahedral growth is dominant. Therefore, the artificially grown diamonds are mostly cubic-octahedral aggregates.

Generally, the carbon source of synthetic diamonds is graphite, so the production of synthetic diamonds is the process of converting graphite into diamonds. However, the synthesis of gem-grade diamonds is divided into two steps. Firstly, diamond powder (industrial abrasive) was synthesized from graphite, and then gem-grade diamond was synthesized from diamond powder. Diamond powder can keep the pressure stable and grow large crystals. If graphite is used, when the broken carbon bond becomes diamond, there will be volume loss, which will reduce the system pressure and affect the formation of larger crystals.

Third, the technology and equipment for synthesizing diamond.

1. Hexagon press

At present, the equipment for synthesizing diamond mostly adopts high temperature and high pressure press. Domestic synthetic diamonds are mainly industrial diamonds (industrial diamonds). This equipment is a kind of press called "cubic ultra-high temperature and high pressure device". The press adopts oil pressure and vertical solid pressure transmission device, which can be divided into double-sided top, four-sided top and six-sided top according to the number of top hammers. At present, the cubic press is widely used in China (up and down, back and forth, left and right). Its working pressure is 1000 ~ 5000 tons, and its working space is 640 mm× 600 mm× 500 mm Generally, the upper and lower jacks are electrified and heated, and the temperature can reach about 1900℃.

2. "Belt pressing" method

The belt pressing device is shown in Figure 9-7-3. This method is similar to the top hammer press. As a carbon source, diamond powder is placed in a growth chamber, which is placed in a washer made of special material and located between two anvils. Then, the raw materials in the growth chamber are subjected to extremely high temperature and pressure, and the temperature at the bottom of the growth chamber is lower than that at the top, forming a temperature gradient, so that the diamond powder at the top can be fully melted and migrated to the bottom of the growth chamber through the flux. At the bottom of the low-temperature growth chamber, diamond grows around the seed crystal into diamond crystal.

Figure 9-7-2 Relationship between Crystal Shape of Synthetic Diamond Region and Temperature and Pressure in Carbon Stable Phase Diagram

Figure 9-7-3 Belt Pressing Device

3. Bar equipment

BARS is an acronym for Russian "pressureless ball splitting device". The pressure required in this device is obtained by injecting liquid into the pressure tank hydraulically, rather than by using a huge hydraulic press. Hydraulic pressure brings the eight parts of the ball device together and produces pressure on the octahedron composed of six pistons. There is a cubic growth chamber in the octahedron, which is equipped with heating equipment, seed crystal, carbon source (diamond powder) and metal flux. In this device, nickel is often used as a flux. Fig. 9-7-4 is the growth chamber of the BARS device and its sectional view.

Fig. 9-7-4 Growth chamber of rod device and its cross section.

The above different equipment and methods should belong to different static pressure catalysis methods. In addition, there are many synthetic methods of industrial diamonds, but some methods are immature and some methods have been eliminated. Such as: explosion method, drainage method, vapor deposition method, underground nuclear explosion method and so on. Vapor deposition has made great progress in recent years.

Four. Identification characteristics of synthetic diamonds

Due to the high technical requirements and high cost of synthetic diamonds, large-scale industrial production cannot be carried out at present, and diamonds sold in the market generally do not need to declare their natural attributes. However, there are still some ways to distinguish between synthetic diamonds and natural diamonds.

1. Color of synthetic diamonds

Because it is difficult to eliminate nitrogen in the growth chamber, most synthetic diamonds are Ib diamonds with solitary nitrogen impurities, which are often yellow to brownish yellow. Sometimes, boron atoms are introduced into the growth chamber to randomly replace carbon atoms in the diamond structure, thus producing blue IIb diamonds with conductivity. In order to grow colorless synthetic diamonds, a metal called "nitrogen absorbent", such as zirconium or aluminum, is usually used. Because nitrogen is easier to combine with these elements and no longer replaces the carbon atoms in diamonds, colorless IIa diamonds are produced. Therefore, there are few Ia diamonds in synthetic diamonds (this kind of diamonds accounts for about 98% of natural diamonds).

2. Absorption spectrum

Most natural diamonds (type Ia) show an absorption line of 4 15nm, while synthetic diamonds do not have such a characteristic absorption line.

3. Ultraviolet fluorescence

Generally, the fluorescence of synthetic diamonds under short-wave ultraviolet is stronger than that under long-wave ultraviolet, and the fluorescence color is yellow or yellow-green, not blue or blue-green of natural diamonds. The cubic-octahedral pattern displayed by the ultraviolet fluorescent ribbon pattern of synthetic diamond is also completely different from the octahedral pattern of natural diamond.

Step 4 include

Synthetic diamonds sometimes have metal flux, dust, crumb inclusions and "sand leakage" zones.

5. Tools

According to the characteristics of synthetic diamonds, De Beers has developed two instruments to identify synthetic diamonds. Rattlesnakes and rattlesnakes. The absorption line of 4 15nm in most natural diamonds can be observed by diamond spectrum analyzer. If the absorption line of 4 15nm is found, no further detection is needed. Diamond structure fluorometer can be used to observe the cubic octahedral pattern of ultraviolet fluorescence of synthetic diamond, which is caused by different impurity concentrations in different growth regions and growth zones.

Think about a problem

First of all, is it right or wrong?

1. The most common synthetic gem in the jewelry market is glass.

2. The code name of yttrium aluminum garnet is GGG.

3. The presence or absence of gas-liquid inclusions is the main basis for distinguishing crystals from synthetic crystals.

4. Emerald and ruby can be produced by hydrothermal method.

5. Arc-shaped growth line is one of the characteristics of gem synthesis by flux method.

6. There is SrTiO3 _ 3 _ 3 in man-made products with molar hardness greater than 7.

7. At present, synthetic metamorphic rocks on the market are all hydrothermal products.

8. See synthetic gemstones with flaky platinum or alloy inclusions, that is, hydrothermal products.

9. The common needle-like and columnar inclusions in synthetic emeralds can be calcite.

10. The ribbon of artificial ruby is always bent.

1 1. The code name of yttrium aluminum garnet is GGG.

12. The presence or absence of vapor-liquid inclusions is the main basis for distinguishing crystals from synthetic crystals.

Second, multiple choice questions

1. When distinguishing green tourmaline from synthetic green crystal, you should use: ()

A. color filter

B. polarizer

C. refractometer

2. Generally speaking, the water-drop inclusion in the gem synthesized by flux method is ().

A. Residual flux

B. Captured liquid

C. gas-fluid inclusions after the gap

3. When distinguishing between ruby synthesized by hydrothermal method and ruby, we should observe: ()

A. refractive index

B. Is there a syngenetic fluid inclusion?

C. Whether there are rutile or zircon equivalent biological inclusions.

4. The Chinese name of "YAG" is: ()

A. yttrium aluminum garnet

B. garnet

C. strontium titanate

5. Which of the following artificially cultivated gems must begin with the word "synthesis": ()

A. golden green jade

B. strontium titanate

C. yttrium aluminum garnet

6. The colored ions added to the synthetic color-changing corundum are: ()

A. vanadium

A. Chromium

C. titanium

7. Any artificially grown crystal with the same chemical composition, atomic structure and physical properties as natural inorganic gemstones should be called: ()

A. artificial gem

B. man-made products

C. synthetic gemstones

8. Flame melting method can be used to synthesize ().

A. yttrium aluminum garnet

B. emerald

C. spinel

D. cubic zirconia

9. The heat required to produce cubic zirconia by cold crucible (molten shell) method comes from ().

A. liquefied petroleum gas

B. propane and chlorine

C. high frequency current

D. high temperature resistance

10. Synthesis of green crystal: ()

A. strong dichroism

B. Monochrome

C. weak dichroism

1 1. At present, the most dispersed synthetic gem or artificial gem is: ().

α-silicon carbide (α-moissanite)

B. strontium titanate

C. titanium dioxide (rutile)

12. Artificial products crystallized from the melt will not contain ().

A. gas-liquid two-phase inclusions

B. Syngas-liquid two-phase inclusions

C. Gas-liquid two-phase inclusions in post-gas.

13. At present, artificial GGG is formed in the following ways: ()

A. crystallization from the melt

B. crystallization from solution

C. gas crystallization

14. The characteristic inclusions in emerald synthesized by flux method are: ()

A. gas-liquid two-phase inclusions in the same gas B. solid-gas two-phase inclusions C. fingerprint gas-liquid two-phase inclusions

15. The characteristic inclusions of metamorphic rocks synthesized by Czochralski method are: ()

A. Three syngenetic inclusions in healing fracture

B. Fingerprint gas-liquid two-phase inclusions

C. Curved growth line

16. The white diamond marked "GE POL" on the waist is: ()

A.it was replaced with a brown diamond.

B.it was replaced by a yellow diamond

C.it was exchanged with inferior green diamonds.

17. Among the following diamond-like materials, the thermal conductivity is closest to that of diamond: ()

A. synthesizing CZ

B.α- synthesis of α -silicon carbide

C. synthesizing corundum

Third, multiple choice questions

1. The physical properties of synthetic moissanite (α-SiC) are: ().

A. there is an n value of 2.4 17.

B.it has birefringence.

C. the thermal conductivity is higher than that of diamond.

D. vickers hardness is very similar to diamond.

E. the reflectivity is higher than that of diamonds.

2. Imitation stone refers to ()

A. Artificial products imitate the color and appearance of natural gemstones.

B. Artificial products imitate the special optical effects of natural gemstones.

C. a natural gem imitates the characteristics of another natural gem.

3. Crystal and artificial crystal: ()

A there may be crystal shapes composed of rhombohedral, hexagonal column and other simplex.

B.its chemical formula is SiO2 nH2O.

C. it can be very polychromatic.

D. the crystal plane stripes are parallel to the c axis.

E there is a fixed refractive index 1.544 on any cross section.

Fourth, fill in the blanks

The density and refractive index of spinel synthesized by 1. flame melting method are higher than that of magnesia-alumina spinel ().

2. The synthetic methods of corundum gems mainly include (), () and ().

3. In artificial and synthetic gems, the code CZ is () and GGG is ().

4. To synthesize amethyst, we should not only add coloring elements () to the raw materials, but also treat them with ().

5. Synthetic gems mean that all or part of the process is controlled by people. Their (), () and their corresponding natural gems are basically the same.

6. The manufacturing methods of artificial products can be divided into: crystallization or condensation from melt, crystallization from () and crystallization from (), and (), etc.

7. The typical inclusion shapes of emerald synthesized by hydrothermal method are (), () and ().

8. Compared with hydrothermal method, the advantage of flux method in synthesizing gems is that it can heat flux to melt various raw materials under the condition of (), so that crystals can crystallize in the melt.

9. Ruby synthesized by hydrothermal method, which can be seen as (), (), (), metallic inclusions and ().