Brief introduction of the first author: Zeng, the first and second vice-chairmen of China Baoxie Artificial Products Professional Committee, the third senior consultant, the former director of Guangxi Gem Research Institute, and a professor-level senior engineer.
I. Introduction
Since the 1960s, man-made products became popular all over the world, man-made products crystals and their ornaments have attracted more and more attention and love. This is because: ① Natural gem resources are increasingly exhausted, especially precious gems with high quality and large particles are rare, which are in short supply and expensive; ② Artificial crystals, especially many gem crystals grown by hydrothermal method, are very similar to natural gem crystals in growth conditions and gemmological characteristics. With the development of social economy and the improvement of people's living standards, especially in economically developed countries and regions, people's jewelry consumption concept has undergone profound changes, and the pursuit of gem cultural taste and jewelry fashion is the main feature of this change. Under this background, we have carried out the research and engineering development of hydrothermal growth technology for colored sapphire and other gemstone crystals, and independently designed φ22mm×250mm and φ30mm×5 10mm which can work safely and reliably for a long time at t≤600℃ and p≤200MPa. Φ φ42mm×760mm and Φ 60 mm× 1100 mm series autoclaves and their matching temperature difference well resistance furnaces have solved many technical problems, such as supersaturation control, slow release of chromonic ions, redox control and so on, and successfully synthesized large and high-quality colored sapphire crystals (Figure1,2,3). Based on the above research results, this paper discusses the principle and technology of hydrothermal growth of gem crystals.
Figure 1 Hydrothermal Growth of Red Series Corundum Gems
Blue corundum sapphire crystal grown by hydrothermal method.
Yellow corundum gem crystal grown by hydrothermal method.
Colorful corundum gemstone faceted ornaments grown by hydrothermal method.
Second, the principle of growth
The principle of hydrothermal growth of gem crystals is as follows: the raw materials needed for gem crystals to grow are dissolved in high-temperature and high-pressure mineralizer aqueous solution to form a saturated solution, and appropriate technical measures are taken to transform the saturated solution into a supersaturated solution, and the gem crystals grow in this supersaturated solution or nucleate or seed crystals, and finally block gem crystals are generated. At present, the temperature difference method is widely used, and most of them are seeds. We call it seed crystal temperature difference hydrothermal method, which is suitable for the growth of gem crystals with large solubility and temperature coefficient, and is an important method for the industrialization of artificial crystal.
The basic principle of seed crystal temperature difference hydrothermal method is to establish a constant and stable temperature gradient in the hydrothermal system of gem crystal growth, that is, to maintain a constant and stable temperature difference between the high temperature zone where raw materials are dissolved and the low temperature zone where seed crystal grows during the whole growth process. Therefore, the saturated solution formed in the dissolution zone is transported to the growth zone by temperature difference convection and transformed into metastable supersaturated solution, in which the seed crystal finally grows into a massive gem crystal. It can be seen that the main points of seed temperature difference hydrothermal method are: ① establishing a constant and stable temperature difference; (2) The solution in the seed crystal growth area is always kept in a suitable and stable metastable supersaturated state.
Third, growth technology.
1. Design and manufacturing technology of autoclave and resistance furnace
(1) φ 60mm×1100mm autoclave
Autoclave is the key equipment for hydrothermal growth of gem crystals, and its performance is directly related to the success or failure of gem crystal growth. See fig. 5 for the structure of φ60mm× 1 100mm autoclave.
The key points in the design and manufacture of φ60mm× 1 100mm autoclave are as follows: ① A carefully selected superalloy should not only have high mechanical strength at high temperature, but also have good plasticity and impact toughness; ② Strict design strength calculation and check (Wang,1986); (3) carrying out strict heat treatment on the selected superalloy; ④ Strict flaw detection should be carried out before and after heat treatment and machining of superalloy; ⑤ Before the autoclave is used, pressure tests at normal temperature and high pressure (100 ~ 220 MPa) and high temperature (490 ~ 600℃) and high pressure (100 ~ 180 MPa) shall be conducted strictly, and the holding time shall be 1h and 34 ~ 35h respectively.
The results show that with the increase of the size of the autoclave reaction chamber, the heat capacity and thermal stability increase, the temperature fluctuation decreases, the crystal size increases, the growth rate accelerates, the daily output increases, and the crystal quality also improves (table 1). Therefore, it is still an important task for R&D to design and manufacture an autoclave with a larger reaction chamber size (maximum working temperature and pressure t≤600℃, p≤200MPa).
Fig. 5 schematic structural diagram of φ 60mm×1100mm autoclave.
1- thermal barrier; 2- pot body; 3-Lower the nut; 4— Lower flange; 5— Upper flange; 6— Upper nut; 7— Stud; 8- pressure pad; 9— Jacking up the screw; 10- connector; 1 1- sealing ring; 12- pressure ring; 13- kettle plug; 14- sealing ring; 15- pressure ring; 16- kettle plug; 17- explosion-proof device
Data unit: mm
(2) Resistance furnace
The temperature difference well resistance furnace is designed and manufactured according to the following technical principles: ① The high-temperature zone in the lower part of the furnace corresponds to the high-temperature dissolution zone in the lower part of the autoclave reaction chamber, and the low-temperature zone in the upper part corresponds to the low-temperature crystallization zone in the upper part of the reaction chamber. There is a positive temperature difference between the two zones, which can be adjusted; ② The crystallization zone should be as long as possible and the temperature gradient should be as small as possible; (3) the heating rate is appropriate, the heat preservation effect is as good as possible, and the heat dissipation on the outer wall of the furnace is as uniform as possible; The key technology is the reasonable distribution of heating power. We design and manufacture according to the scheme of three-stage heating and two-point temperature control.
The temperature curve in the reaction chamber of φ60mm× 1 100mm autoclave is shown in Figure 6, and the temperature curve measured in the gold liner under high temperature and high pressure is shown in Figure 7. Figures 6 and 7 show that the length of crystallization zone is 600 ~ 640 mm, the average temperature gradient is 0. 1 1 ~ 0.22℃/cm, and the temperature fluctuation in the reaction chamber and gold lining is ≤0.2℃, which is suitable for hydrothermal growth of gem crystals.
2. Temperature control and temperature measurement technology
The characteristics of temperature and its temperature field are very important for the hydrothermal growth of gem crystals. The following seven technical measures have been taken to improve the temperature control and temperature measurement technology.
1) Improvement of resistance furnace: ① The furnace is changed into corundum tubes with evenly distributed holes (φ6mm) and equidistantly distributed external thread grooves (R8mm), so that the heat exchange is more sufficient, faster and more timely; (2) The increase of heating power increases the initial heating rate and shortens the heating time, which is also beneficial to adjust the matching relationship of heating power in each section; (3) The whole body is changed into two-body manufacturing process, which is not only convenient to manufacture, but also has better heat preservation effect and more uniform heat dissipation on the furnace wall.
Table 1 Comparison of Hydrothermal Growth Results of Ruby Crystal
Fig. 6 Temperature-height curve in φ 60 mm×1100 mm autoclave.
Fig. 7 Temperature-height curve in gold liner
2) Adding heat insulation layer: adding a layer of heat insulation layer composed of stainless steel heat insulation ring and aluminum silicate fiber felt between the furnace body and the kettle body. It separates the lower high temperature zone from the upper low temperature zone of the furnace, effectively suppresses the thermal convection between the two zones, and ensures the required positive temperature difference and its stability.
3) Adding a thermocouple positioning device: the added thermocouple positioning device makes the hot end of the lower temperature-controlled thermocouple closely contact with the bottom of the kettle body, positioning a fixed point and having good temperature control repeatability; On the other hand, it can greatly reduce the heat loss at the bottom of the resistance furnace and make the furnace temperature more stable.
4) Add a cold-end constant temperature compensation box: the distilled water in the box is constant at 45℃, and its maximum temperature fluctuation is 0.05438 0℃. The cold end of thermocouple is inserted into this thermostat, which avoids the temperature fluctuation in the furnace caused by environmental temperature fluctuation.
5) Replacing single armored thermocouple with two branches: Replacing single armored thermocouple with two branches realizes one point, two measurements and two controls, which greatly improves the safety and reliability of equipment operation.
6) Using UP350 PC monitoring system: Using UP350 PC monitoring system, real-time monitoring and real-time recording of furnace temperature are realized.
7) Internal temperature measurement technology is adopted: under the conditions similar to the hydrothermal growth of gem crystals, the temperature curve is directly measured in the gold pad (Figure 7), which more truly reflects the temperature and temperature field characteristics of crystal growth.
3. Mineralizer selection and solubility determination technology
Selection of (1) mineralizer
Mineralizer is very important for hydrothermal growth of gem crystals. We think that the gem crystal forms coordination ions in mineralizer aqueous solution at high temperature and high pressure, which is similar to the coordination polyhedron structure in gem crystal and is beneficial to the growth of gem crystal. Therefore, the mineralizer must be selected according to the technical principles (Wuhan University, 1983) such as the ionic configuration of the central element, the electronegativity of the coordination atom and the basicity of the ligand. The selected mineralizer should also make the gem crystal have the characteristics of uniform dissolution and large solubility and temperature coefficient.
(2) Determination of solubility
The solubility and temperature coefficient of gem crystal in high temperature and high pressure mineralizer aqueous solution are important basis for designing hydrothermal growth process of gem crystal. In an isothermal furnace, the solubility of gem crystals was determined by quenching method and according to the weight loss before and after. In order to prevent the interference of impurities, a gold lining is adopted, that is, gem crystal fragments (particle size 3 ~ 5 mm) and mineralizer aqueous solution are sealed in the lining; In order to separate the crystal from the solution in time during quenching, a gold basket with crystal fragments and evenly distributed holes was hung on the mineralizer solution. In order to ensure the balance of the dissolution reaction, a kinetic test was carried out in advance, that is, the relationship test between solubility and dissolution duration; In order to determine whether the dissolution is uniform, the solid products after the dissolution reaction are analyzed and identified.
The coordination polyhedron in corundum gemstone crystal is [AlO6], and the center Al3 ++ is 8-electron type, which forms coordination ion [all 6]3- with L as F-, OH- and O2-, among which F- is the most favorable for coordination, but F- generates insoluble compound CrF3 with chromogenic ion Cr3+, so OH- is selected. In addition, most valuable ruby deposits occur in carbonate rocks, so alkali carbonate is finally chosen as mineralizer. The solubility curve of ruby crystal by flame melting method is shown in Figure 8. The results show that it is suitable to grow seed crystal by hydrothermal method with temperature difference.
Fig. 8 solubility curve of ruby crystal in aqueous solution of potassium bicarbonate and sodium bicarbonate (p=200MPa)
4. Supersaturation control technology
It has been pointed out that the key technology of seed hydrothermal growth is that the solution in the growth zone must always be kept in a suitable and stable metastable supersaturated state during the whole growth process. The size and trend of metastable supersaturated region can be estimated by supersaturation (Zhang Ke et al., 1997). In this sense, the control technology of metastable supersaturation is still the control technology of supersaturation. The experimental results show that the crystallization temperature and its temperature difference and the opening rate of baffle are the main external factors affecting supersaturation in the selected hydrothermal growth system.
(1) crystallization temperature and temperature difference control technology
The control of temperature and temperature difference has been mentioned technically, so I won't repeat it, but it needs to be emphasized that it is very important to determine the matching relationship between crystallization temperature and temperature difference. In practical work, we mainly choose the highest temperature in the raw material dissolution zone according to the maximum temperature and pressure for the long-term safe operation of the autoclave and the liquid-solid ratio in the hydrothermal growth system of gem crystals (that is, the ratio of the volume (mL) of the initial mineralizer aqueous solution to the mass (G) of the solid raw material), then choose the crystallization temperature according to the temperature range of the metastable supersaturated zone determined by the solubility curve, and finally determine the best matching relationship between them through experiments. For φ42mm×760mm autoclave and its supporting resistance furnace, the best matching temperature parameters of high-quality and fast-growing ruby crystals are shown in Table 2.
Table 2 Optimal Matching Temperature Parameters of Reaction Chamber
(2) Baffle and its opening ratio
An open baffle is arranged between the dissolution zone and the crystallization zone in the gold liner to adjust the solution convection or mass transfer of the gem crystal hydrothermal growth system, thus controlling the supersaturation of the solution.
In practical work, the correctness and effectiveness of supersaturation control can be judged according to whether nucleation growth occurs or not. If nucleation growth occurs (nucleated fine grains are often attached to the upper part of the inner wall of the gold lining and the upper part of the seed frame), it means that the solution in the crystallization zone has exceeded the metastable supersaturated zone, so it is necessary to reduce the temperature difference or increase the aperture ratio of the baffle to keep the solution in the metastable supersaturated zone.
5. Chromophoric ion slow release technology
In the hydrothermal growth experiment of yellow sapphire crystal, when the chromogenic agent Ni2O3 is directly added to the bottom of the pad, the crystal color changes from black to dark brown and brown with the content of Ni2O3 in the raw material decreasing from 1.66% to 0.05%, which shows that the content of Ni2O3 directly affects the color of sapphire. Therefore, the purity and uniformity of gemstone color can be guaranteed by controlling the release speed and quantity of chromonic ions through special devices.
6. Redox control technology
Yellow sapphire (commonly known as topaz) is a kind of sapphire crystal (Ni3+: α-Al2O3) doped with Ni3+,and the following reactions may occur in the hydrothermal growth system:
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Therefore, it is necessary to adjust its redox ability. In this regard, according to the basic principle of redox, using the potential diagram of related elements, we choose a suitable oxidant (or reductant) to directly add to the hydrothermal growth system to control its redox ability, that is, to control the valence state of chromonic ions, so that topaz crystals appear pure yellow. Otherwise, the crystal appears yellow-green and grass-green, which is the result of doping Ni2+ and Ni3+ into the crystal (Ni3++Ni2+:α-Al2O3), leading to its color.
7. Color mixing technology
In order to study and develop new varieties of ruby crystals, based on crystal chemistry, redox reaction principle and crystal coloring mechanism, the color mixing technology of mixing two or more colors to produce new varieties of ruby crystals was adopted. If it is required to improve the lightness and purity of color varieties, additive mixing technology is adopted; Instead, the subtractive mixing technique is adopted. Using this technology, we have successfully grown a ruby crystal with a new color, which is beautiful bright red, and its brightness and purity have been improved.
8. Phase state of growth system and its discrimination
The phase state of hydrothermal growth system refers to which phase region the system is in under given physical and chemical conditions (such as temperature, pressure, solubility of mineralizer, etc.). ), which is not only the problem of dissolution balance of artificial crystals (including artificial product crystals and artificial functional crystals), but also the key problem of hydrothermal growth of artificial crystals. The results show that: ① the solubility of intraocular lens in liquid phase region and supercritical phase region is large, which often increases with the increase of mineralizer concentration and temperature and pressure, which is beneficial to crystal growth; (2) In the above two homogeneous regions, the transmission of matter and heat is uniform and stable, which is beneficial to the growth of high-quality crystals; (3) Compared with supercritical fluid phase, the growth pressure and temperature of intraocular lens in liquid phase region are lower, so the design and manufacture of large-diameter autoclave is beneficial to the growth of large-size crystals. In practical work, it can be judged according to the type of inclusions in intraocular lens, homogenization temperature (Lu et al., 1990) and P-V-T-C-F curve of hydrothermal growth system.
Four. Concluding remarks
Through the research of the project and the engineering of its achievements, a series of autoclaves and supporting resistance furnaces have been successfully designed and manufactured independently, and a set of technical processes for growing colored sapphires by hydrothermal method have been developed, and high-quality colored sapphires have been produced in small batches, filling the gap in China. At the same time, the equipment and technology of this project have important guidance and reference significance for hydrothermal synthesis of other gems.
refer to
Lu Deng 1990. Geochemistry of inclusions. Beijing: Geological Publishing House.
Editor Wang. 1986. Design and calculation of engineering pressure vessels. Beijing: National Defense Industry Press.
Wuhan university et al. 1983. Inorganic chemistry. Beijing: Higher Education Press.
Cong et al. 1997. Crystal growth science and technology. Beijing: Science Press.