Brief introduction of the first author: He, the third senior consultant of insurance institute of china Artificial Products Professional Committee, and an associate researcher of Shanghai Institute of Silicate, China Academy of Sciences.
I. Introduction
Condensed shell smelting method was originally called cold crucible method. The crystal was invented by French scientist Y.Roulin in 1969, and developed by using high-frequency induction heating and using the unmelted ceramic shell of furnace charge on the inner wall of water-cooled crucible as a container. Only small cubic zirconia crystals were obtained at that time. 1972 The research team led by P.N. Lebedev Institute of Solid State Physics of the Soviet Academy of Sciences, V.V.Osiko, improved the growth technology and equipment of synthetic cubic zirconia crystals, and grew larger synthetic cubic zirconia crystals, and named the improved cold crucible method as shell melting technology.
The characteristics of shell melting technology are as follows: ① High-frequency induction power supply is used as heating source, and the temperature is not limited, which can reach above 3000℃; (2) Using water-cooled crucible as the outer shell and unmelted burden as the inner shell crucible can solve the problem of high-temperature corrosion-resistant container and protect the melt from the pollution of crucible materials; ③ Able to work under vacuum or various atmospheric conditions; ④ Non-metallic materials that conduct electricity in molten state can be prepared by this method; ⑤ The cold crucible can be reused many times.
Second, the high-frequency heating equipment melting shell method
The high-frequency heating shell melting equipment consists of high-frequency induction heating equipment system, cold crucible system and lifting mechanism system, as shown in figure 1.
1. Innovation and development of high frequency induction heating equipment system
Domestic large-scale high-frequency heating shell melting equipment was successfully developed by Tieling High-frequency Equipment Factory of Liaoning Province, and then produced and sold by Tieling High-frequency Equipment Factory of Liaoning Province and Tieling Julong High-frequency Equipment Co., Ltd. At present, the power of domestic high-frequency induction heating equipment is 400kW, and the largest is 600kW. The diameter of cold crucible of 600kW equipment produced by Liaoning Tieling High Frequency Equipment Factory is greatly increased. The working frequency of high frequency induction generator is 800 kHz ~ 1 MHz, and the power consumption of melting crystal can be reduced by using the working frequency of kHz.
Figure 1 High-frequency heating shell melting equipment
The producers of synthetic cubic zirconia crystals are big consumers, and the energy consumption per ton of crystals is 50 ~ 60 thousand kW·h, and it takes about 50 hours to produce each furnace. In the production process, stable power is needed to ensure the normal growth of crystals, so sufficient power and stable power supply are needed.
To reduce costs, we must first reduce energy consumption and transform high-frequency induction heating equipment. Therefore, the transistor high-frequency generator is studied to replace the high-frequency generator of the electron tube oscillator as the power supply to reduce energy consumption.
2. Cold crucible system
The cold crucible has simple manufacturing process and convenient maintenance, and is widely used in equipment for synthesizing cubic zirconia crystals with large capacity. At present, the diameter of large-scale cold crucibles in China is over 800mm, and the maximum diameter is 1m, which can hold more than 400 ~ 800 kg and 1000kg respectively, and the yield is about 45%. Fig. 2 is an unfilled cold crucible, fig. 3 is a cold crucible in high-temperature operation, and fig. 4 is a synthetic cubic zirconia ingot melted by a large-diameter cold crucible. Fig. 5 is a purple-blue synthetic cubic zirconia ingot melted by a large-diameter cold crucible. Ordered purple-blue crystals can be seen from the opened gap.
Fig. 2 Cold crucible without charge
Fig. 3 Cold crucible in high temperature operation
Fig. 4 Large diameter synthetic cubic zirconia ingot
The cooling water of the cold crucible is provided by a group of circulating water units with efficient cooling, and it is necessary to ensure the efficient cooling of the cold crucible in the whole production process.
3. Lifting mechanism system
The requirements for the lifting mechanism system are: the frame should be stable, the vertical speed should be fast or slow, and the control of lifting should be uniform. In particular, there should be no periodic vibration in the crystal growth stage, otherwise the crystal will produce defects such as color bands, growth stripes, polycrystals and faults, and it is impossible to obtain a complete and easy-to-peel large single crystal.
Fig. 5 Large diameter purple-blue synthetic cubic zirconia ingot
Thirdly, the growth of special zirconia crystals.
Cubic zirconia crystals synthesized by blue and emerald are very popular in the market, but it is difficult to obtain ZrO2-Y2O3 with the general formula of 80:20. The content of Y2O3 must be increased. With the increase of Y2O3 content, the crystal size becomes smaller, which is a contradiction. How to formulate a reasonable formula with the best of both worlds, which can not only obtain satisfactory color, but also obtain larger crystals, increase output and reduce costs, should be studied in depth.
Color "milk zirconium" (opaque color synthetic cubic zirconia crystal) is a new variety appearing in the market recently. It can imitate natural jade and make various handicrafts. In the conventional production of synthetic cubic zirconia, it is necessary to avoid the turbidity and opacity of zirconia crystals, and "milk zirconia" just needs the opacity of crystals. To make the color synthesized cubic zirconia crystal opaque, it can be achieved by reducing the amount of Y2O3. In the research experiment, the transparent synthetic cubic zirconia crystal with Y2O3 mole fraction less than 0. 12 was treated at 900℃ for a long time, and the synthetic cubic zirconia crystal was opaque and turbid. The answer can be found in the phase diagram of ZrO2-Y2O3. Fig. 6 is a phase diagram of ZrO2-Y2O3. When Y2O3 content is 12% ~ 4% (mole fraction), tetragonal zirconia and cubic zirconia * * * exist in the range of 900℃ to room temperature.
Fig. 6 Zr-O2-Y2O3 phase equilibrium diagram
(Srivastava et al., 1974)
Fourth, the preparation of the burden
There are three crystal forms of ZrO2, namely cubic crystal form (Fm3m) at 2300 ~ 2750℃, tetragonal crystal form (P42/nmc) at 2300 ~100℃ and monoclinic crystal form (P2 100℃). Cubic zirconia crystals grown at high temperature are cooled to room temperature and undergo two phase transitions, resulting in volume changes and stress, which leads to crystal cracking. The results show that using Y2O3 as stabilizer, cubic zirconia (YZrO2) large single crystal with complete shape and easy peeling can be easily obtained (how,1984; Zhang et al., 1986). The results also show that with the increase of Y2O3 content, the crystal color deepens, showing from colorless to yellowish to light brown. According to the phase equilibrium diagram of ZrO _ 2-Y2O3 (Figure 6), a reasonable proportion of ZrO _ 2 and Y2O3 should be selected.
When synthesizing white cubic zirconia crystals, the Y2O3 content should be 10% ~ 12% (mole fraction), and when synthesizing color cubic zirconia crystals, the Y2O3 content can be higher, especially when synthesizing dark green, emerald green or dark blue cubic zirconia crystals, more Y2O3 is needed.
When the cubic zirconia crystal is produced by shell melting method, the yield of each furnace charge is only 40% ~ 55%, and about 50% of the charge and crystal block are recovered, so the use of recovered charge is one of the key problems in shell melting batching process. In the process of melt growth of synthetic cubic zirconia, the stabilizer Y2O3 volatilizes. The Y2O3 content in the grown synthetic cubic zirconia crystal is lower than that in the original batch, and the Y2O3 content in the slag layer and the return charge is even lower. Therefore, when using the return burden, the deficiency of stabilizer must be supplemented.
In the production process of color synthetic cubic zirconia crystal, the distribution coefficient of some colorants in the crystal growth process is less than 1, and the colorants that can enter the crystal are less than those that remain in the melt. When recycling recycled materials, attention should be paid to the enrichment of colorants in recycled materials and the dosage of colorants should be adjusted in time.
We should also pay attention to the enrichment of harmful impurities in the returned materials, especially some low-melting-point colorants and harmful impurities, which gather into low-melting-point slag during the crystal growth process, occupying the core of the melt, making it impossible for the crystal to continue growing and affecting the yield and quality. This backflow charge block should be removed in time.
New application of verb (abbreviation of verb)
Because the synthesized cubic zirconia has high hardness (Mohs hardness 8), high refractive index (2. 16) and high dispersion (0.06), it is rich in color, good in chemical stability, easy for industrial mass production, low in price and good in quality, and it is an incomparable crystal material for artificial products, which can replace diamonds and various gems. Its application in industry and optical components also has broad prospects. As a high-grade decorative gem, it is difficult to find a superior substitute for synthesizing cubic zirconia crystal so far, even for a long time to come. Although synthetic moissanite gemstones (synthetic silicon carbide single crystal, moissanite, Mosanite) have been put on the market in recent years, their gemmological indicators are very good, but it is difficult to mass produce, and the cost is high, so it is difficult to popularize. Therefore, in terms of cost performance, the synthetic cubic zirconia industry will be prosperous.
Because the application temperature range of melting shell technology is wide, which can be as low as several hundred degrees Celsius, as high as 3,000 degrees Celsius or even higher, it does not need special containers, can maintain the purity of materials, and can be melted in vacuum and various atmospheres, so it is widely concerned. There are many new applications of shell melting method: some are used to melt glass materials, some are used to cast titanium composite materials, and some are used to grow other crystal materials. This shows that this high-frequency induction heating cold crucible shell melting equipment is a good melting furnace with great innovation space.
At present, the cold crucible system is only used for the production operation of tilting the ingot out of the crucible after the melt cools and solidifies. If a mechanism for tilting the furnace body can be added, the molten melt can be cast and the multifunctional function of the equipment can be exerted.
Some titanate crystals and zirconate crystals are high refractive index materials and have good application prospects. They can also be prepared by melting shell method.
Polycrystalline silicon is the raw material of solar cells, which is in short supply in the market at present. It is possible to discuss the possibility of purifying and producing polysilicon and monocrystalline silicon by melting shell method under vacuum or controlled atmosphere conditions.
Molten shell method can be operated in a closed environment and can be used for nuclear waste treatment.
Attachment: Color synthetic cubic zirconia crystals grown by shell melting method in China.
Fig. 7 Color cubic zirconia crystal produced by Tianfu Gem Company.
Blue and green cubic zirconia crystals produced by Lantian Gem Factory.
refer to
He, Tang Yuanfen, Tan Yueyi, etc. Study on crystal growth of cubic zirconia for decoration. A new inorganic material, 12 (3 ~ 4): 7 ~ 12.
Alexanderov V, Osico V, prokhorov Morning, Tatar Lin Tsev. v . m 1978。 In:Kaldis E(Ed), the current top ics in the field of materials science, 1:42 1~480 (north of Amsterdam, Netherlands), Srivastava K K, Patil R N, Choudhary C B.Gokhale K V G K, Subbarao, E C.199. Trans.J.Br，Ceram，Soc。 ,73(3):85.
Zhang Dabin, He Xiaoming, Chen Jianping, et al. Study on crystal growth and defects of cubic zirconia. Journal of crystal growth, 79:336~340 (North Holland. Amsterdam)