In China, the research on hydrothermal synthesis of crystals began at the beginning of 1958, and 1964 was put into trial production. Later, due to the wide application of piezoelectric crystals in the radio industry, the production capacity of synthetic crystals continued to expand, reaching 1760t in 2002. Due to the rise of Chinese jewelry industry, a large number of colorless synthetic crystals are used for decoration, and the introduction of irradiation technology makes a large number of smoke crystals used in glasses industry. In recent years, there has been a major breakthrough in the production of color synthetic crystals in China, which can grow almost all colors of rock crystals, such as purple, yellow, brown, blue, bicolor, green and black. , greatly enriched the decoration market, widely used for export.
In addition to hydrothermal synthesis of crystals, 1987, China successfully synthesized emeralds and put them into production. 1993 and 1999 successfully grew synthetic rubies and synthetic sapphires of various colors, and improved the original process of synthesizing emeralds. Hydrothermal synthetic gemstones occupy an important position in China synthetic gemstone market.
First, the principle, device and characteristics of hydrothermal method
The basic device of hydrothermal method includes autoclave, heater and temperature control part, as shown in Figure 4- 1- 16.
Figure 4- 1- 16 Typical configuration of electric furnace and autoclave for hydrothermal crystal growth
The autoclave is a round steel cylinder with pressure resistance, heat resistance and corrosion resistance, and the end cover can be opened and sealed. The kettle body is generally made of special alloy steel with high strength, heat resistance, corrosion resistance and creep resistance.
Kettle end sealing is one of the key technologies, and there are many design schemes, including pressure sealing and pull sealing. The sealing materials include silver, pure iron, graphite, copper and other soft materials.
The heater is generally heated by resistance wire, and the furnace wire is wound on the insulating bracket, and the insulation material is used as the sheath; According to the requirements of temperature field, it is designed to grow synthetic crystals from hot to cold. Power supply and heating generally adopt SCR automatic temperature controller to ensure temperature gradient on the one hand and control accuracy (0.5℃) on the other.
The mineralizer solution changes with the growth of the crystal. For synthetic crystals, alkaline aqueous solution, namely Na2CO3 or Na OH and KOH solution, is commonly used. The raw material is broken crystal.
The growth principle is to grow crystals in supersaturated solution. In the lower part of the kettle, due to the high temperature, SiO _ 2 gradually dissolved into the solution, while in the upper part, due to the low temperature, SiO _ 2 slowly precipitated and SiO _ 2 molecules slowly grew on the seeds.
The advantage of this method is that it is suitable for growing some crystals with phase transition at high temperature (such as α-time), which is difficult to grow from melt. In addition, some substances or decomposed substances with high vapor pressure near the melting point are also suitable for hydrothermal method. Hydrothermal method belongs to solution growth, which can grow large and perfect crystals. Because this method is very similar to the conditions of growing crystals in nature, the grown gem crystals are the closest to natural gem crystals. The disadvantage is that the equipment is expensive and the safety is poor, the growth process is not intuitive, and the size of the grown crystal is limited by the size of the autoclave container.
Second, the growth of synthetic crystals.
Taking color synthetic crystal as an example, the technological conditions of synthesizing gemstones are explained.
The autoclave used is an improved bridgman sealed autoclave with D = =200mm, and the temperature control system adopts DW702 precision temperature controller.
Process conditions: the crystal grown by hydrothermal method is α-time. Since the reaction time will be converted into β-reaction time at 573℃, the temperature of hydrothermal crystal growth should be lower than 573℃. The temperature of the growth zone is 300 ~ 340℃, and the temperature gradient is 20 ~ 60℃. Mineralized liquid x (NaOH) = 0.5 ~ 0. 1 mol, x (KOH) = 0.5 ~ 1.0 mol, and x (Na2CO3) = 0.25 ~ 0.5 mol; The filling degree is 75% ~ 85%; Raw materials are timely for smelting; The orientation of the seed crystal is y or z lamellae.
Dyeing additives: 2coco3 3co (OH) 2 NH2O, cocl2 6h2o, Co (NO3) 2 6H2O, kmno4, K2Cr2o7, Fe (OH) 3, Fe2 (SO4) 3 NH2O, etc.
Practice shows that the growth of colored synthetic crystals is different from that of colorless synthetic crystals. NaOH and Na2CO3 are used as mineralizers in the production of colorless crystals, while KOH and K2CO3 are used as mineralizers in colored crystals.
In the process of synthesizing amethyst, using 5% mass fraction of (K2CO3+KOH) as mineralizer and adding Fe(OH)3 at a dose of 5 ~ 7g/L, the crystal grown is lemon yellow, which turns purple after irradiation with 60Co, and turns lemon yellow after heating amethyst.
The color of colored synthetic crystals is related to the type and content of impurities and irradiation dose. Table 4- 1-9 is for reference only.
Table 4- 1-9 Comparison Table of Doping and Color of Synthetic Crystals
3. Hydrothermal synthesis of jadeite crystals
Emerald is an beryl mineral, which is green because of its Cr content. Emerald has always been the goal pursued by scientists because of its attractive color and scarce mineral resources. Emerald crystals were synthesized by hydrothermal method from Austria's1928 r nakken, 196 1 Lechleiter and America's 1965 Linde, and some products were supplied to the market. 65438-0988 China Guangxi Gemological Research Institute also has hydrothermal synthesis of emeralds, which has been produced and supplied to the market in small batches.
The molecular formula of emerald is Be3Al2Si6O 18, and its theoretical chemical composition is w (SiO2) = 67%, w (BeO) = 14. 1%, and w (Al2O3) = 18.9%. Natural emeralds contain water and w (H2O).
There is no difference in principle between the equipment for hydrothermal synthesis of emerald and the equipment for crystal synthesis, except that it is smaller in size and is lined with precious metals such as gold and platinum, including autoclave, heating system and temperature control system. Its structure is shown in Figure 4- 1- 17.
Figure 4- 1- 17 Hydrothermal Synthesis of Emerald
Culture medium SiO _ 2 (small crystal block), w (w(SiO _ 2)= 64% ~ 67%；%; Al2O3, w (Al2O3) =17% ~19% (AP grade Al (OH) 3); BeO，w(BeO)= 14%~ 15.5%。
Mineralizer solution acid solution 4 ~ 1.2 mol HCl.
Seed crystal natural aquamarine, (000 1) or slice with cylinder 35.
The culture medium and seeds are put into a kettle, sealed with nuts and heated in an autoclave. SiO2 _ 2 decomposes at the top, other media dissolve and rise at the bottom, and the components meet in the middle, and grow on the seed under appropriate temperature gradient and supersaturation, with an average growth rate of 0.50 ~ 0.80 mm/d..
Emerald grown by hydrothermal method has good color and few inclusions, which is very similar to natural high-grade emerald.
Fourthly, the corundum crystal was synthesized by hydrothermal method.
Through continuous exploration, China Guilin Gemological Research Institute has improved the process, suspended multiple seed crystals with a new trapezoidal gold seed crystal frame, and grown thick synthetic corundum crystals of various colors in a newly designed large autoclave by using redox buffering technology and slow release technology of different chromogenic ions or chromogenic ion pairs. The main process conditions are as follows.
The process equipment for synthesizing corundum polycrystal by trapezoidal hydrothermal method is mainly composed of 38mm(d)×700mm(h) autoclave and its supporting temperature difference well resistance furnace. The design of autoclave adopts double cone sealing ring and flange self-sealing structure, which is simple in processing and convenient in operation. The temperature difference well resistance furnace adopts three-stage temperature control mode, which is convenient for different requirements of different sections on temperature field. The gold lining is used as a protective lining in the autoclave.
Temperature and temperature difference: 550 ~ 580℃ in the dissolution zone, 505 ~ 5 15℃ in the growth zone, and 45 ~ 65℃ in the temperature difference.
Working pressure (1.5 ~ 2.0) × 108 Pa.
The composite solution of mineralizer alkali metal carbonate has a total concentration of 2 ~ 3 mol/L.
The seed crystal is tangentially parallel [2243].
The opening rate of baffle is 5% ~ 10%.
The liquid-solid ratio is1.8 ~ 2.0 ml/g.
The filling degree is 55% ~ 65%.
The average growth rate of single crystal is 6.5 ~ 7.5 CT/d.
It takes 10h for the vertical speed of furnace temperature to rise from room temperature to the predetermined temperature, and 24h for the temperature to drop to room temperature after growth.
According to the different color requirements of crystals, oxides containing chromonic ions such as Cr3++, V3+, Mn3++, Co3++, Ni2 ++ and Ni3 ++ or the mixture of two chromonic ion oxide powders are added. In addition to adding Cr3 ++ as a colorant for synthetic rubies and pink synthetic sapphires, and colorless synthetic corundum without any colorant, it is necessary to control the valence state of the colorant when growing synthetic sapphire crystals of other colors. Therefore, in addition to the corresponding colorant, a redox buffer, usually a combination of Cu2O-Cu O or PbO-Pb2O, should be added to make the colorant ions effectively enter the crystal lattice in the required valence state. The redox buffer solution was put into a small platinum tube of 8mm(d)×50mm(h), and the addition amount was 5 ~ 10 times of the added color developer. There is a small hole with a certain opening ratio on the surface of the platinum tube, which is usually placed at the bottom of the liner.
The medium used to synthesize several colored corundum single crystals by trapezoidal hydrothermal method is a mixture of a certain number of colorless corundum crystal fragments with a particle size of 5 ~ 7 mm synthesized by flame melting method and a small amount of Al(OH)3 powder. Put the culture material into the bottom of the gold liner, and then add mineralizer according to the filling degree.
The trapezoidal seed crystal frame is made of gold wire, and the seed crystals cut in a certain direction are connected with gold wire and fixed on the frame. A trapezoidal frame can hang 6 ~ 10 seeds at a time. The swinging directions of the seed crystals should be perpendicular to each other, so the purpose of placement is to make the dissolved mass reaching the surface of each seed crystal as consistent as possible and prevent some crystals from growing unevenly, as shown in Figure 4- 1- 18.
The growth period of several kinds of colored corundum single crystals synthesized by trapezoidal hydrothermal method is 7 ~ 10d, the single furnace grows 350 ~ 450 ct, and the single crystal weighs 60 ~ 90 ct. The grown crystal is in the shape of a thick plate, and its size is about 30mm×25mm× 10mm (see Figure 4- 1- 19).
Colorants are not needed to grow colorless synthetic sapphire crystals, but the mineralizer alkali carbonate solution needs to be purified. Under the same conditions, the growth rate of colorless synthetic sapphire single crystal is 2 ~ 3 times that of corundum crystal synthesized with other colors.
Figure 4- 1- 18 Synthesis of Ruby by Trapezoidal Hydrothermal Method
Figure 4- 1- 19 Hydrothermal Synthesis of Ruby
Identification of Hydrothermal Gemstones Synthesized by verb (abbreviation of verb)
1. seed crystal
Seed crystal must be used to grow crystals by hydrothermal method, but there are always differences between seed crystal and gem crystal in optical characteristics. Therefore, whether there is seed crystal can be used as evidence to determine whether the gem crystal is a natural product or a synthetic product. The color of the seed crystal is different from that of the grown crystal, which is very easy to identify; If the homochromatic boundary is not obvious, it can be observed in the immersion liquid and identified according to the characteristics of irregular wavy growth boundary between the seed crystal and the growth layer.
Step 2 include
Gas-liquid inclusions and solid inclusions will appear in gem crystals grown by hydrothermal method.
1) gas-liquid inclusions Thermal liquid crystal growth is the only method in which water participates in all crystal growth methods, so gas-liquid inclusions can often be seen in the grown crystals, which is very similar to the gas-liquid inclusions of natural gemstones; The difference is that the gas-liquid inclusions in the synthetic crystals have a strong stereoscopic effect and are relatively regular, mainly appearing on the growth interface of the seed crystals.
2) Common solid inclusions in hydrothermal synthetic gemstones include "crumb" inclusions in synthetic crystals composed of ordinary pyroxene or timely microcrystal nuclei, beryl inclusions in synthetic emeralds (sometimes forming nail inclusions with gas-liquid inclusions), and gold microcrystal aggregates in synthetic corundum gemstones with flocculent or flocculent distribution.
Planting textures and ribbons
Most gem crystals grown by hydrothermal method have obvious wavy growth lines or serrated growth lines.