The growth of ZrO2 _ 2 crystal by cold crucible method can be traced back to 1969, when French scientist Lu Lin and others used high frequency power supply to heat the cold crucible for crystal growth. Although the equipment is simple, cubic zirconia small crystals with Y2O3 stabilizer of 12.5% have been grown, but this research has not been carried out. 1972 The research team led by V.I.Alekasadrov of the Lebedev Institute of Physics in the former Soviet Union improved Lu Lin's technology, grew larger crystals, and applied for patents from the United States and other countries. 1976 ~ 1979 has successively obtained patents from Britain, Germany and the United States. After 1976, the former Soviet Union gradually sold this crystal to the gem market to replace natural diamonds. During this period, Ceres Company of the United States also conducted research, improved the cold crucible system and applied for a patent, and produced a large number of synthetic cubic zirconia crystals for gemstones. China started research from 1982, and mass production was successful soon. It is one of the countries in the world that can supply Czochralski crystals in large quantities.
CZ crystals are easy to grow into crystals of various colors, and the colors are gorgeous. CZ crystal is not only used as a gem, but also an excellent optical material and laser matrix material.
First, the working principle of high frequency cold crucible technology
1. Basic principles
As we all know, general high-temperature nonmetallic materials are dielectric materials at room temperature, with high resistivity and low dielectric loss, and it is difficult to melt directly with high-frequency electromagnetic fields. However, experiments show that the melt conductivity of these materials is good, which provides conditions for high frequency heating technology.
The high-frequency cold crucible technology does not use a special crucible, but directly uses the crystal material to be grown as a "crucible", so that its interior melts and its shell does not melt; Its ingenious point is that a cooling device is added outside to absorb the heat of the surface layer, so that the surface layer does not melt and forms an unmelted shell, which acts as a crucible. This is the "cold crucible". The crystal material that has melted inside is crystallized and grown by the crystal growth principle of crucible descent method.
2. Melting process
To melt high-temperature nonmetallic materials, a small melting zone must be formed by a method to be used as a conductive "seed" melt. The simplest method is to put a small amount of corresponding metal sheet or powder in the center of the material, turn on the high-frequency power supply, and use metal induction heating and rapid oxidation to release heat to melt a small amount of raw materials first (see Figure 4- 1-2).
Fig. 4- 1-2 schematic diagram of melting process
In the local melting zone, the sum of absorbed power and heat released by oxidation is greater than (at least equal to) the sum of heat emitted by the melting zone and heat absorption required for melting materials, so the melting zone can be expanded. Practice shows that this period of time is very short, and the metal is oxidized quickly, forming a hollow sphere melting zone (as shown in Figure 4- 1-2(a)). The size of melting zone is related to the input power. When the input power is increased (mainly by increasing the voltage at the initial stage), the melting zone expands until it is balanced (as shown in Figure 4- 1-2(b)). For different materials, the greater the thermal conductivity, the greater the required input power.
Second, the high frequency cold crucible technology and equipment
The complete equipment of high-frequency cold crucible technology consists of three parts: high-frequency oscillation device, cold crucible system and crystal growth pull-down device (see Figure 4- 1-3).
Fig. 4- 1-3 schematic diagram of crystal growth device by cold crucible method
1. High frequency power supply
According to the principle and technological requirements of melting nonmetallic materials (taking zirconia as an example), high-frequency power supply should have the following characteristics:
1) 1 ~ 6 MHz working frequency is stable and adjustable.
2) Good working coordination, adapting to the change requirements from light load (rated value 10%) to heavy load (1 10%), and working under overvoltage will not damage or breakdown components.
3) The power is adjustable, that is, the anode voltage can be adjusted evenly within the range of 30% to 130% (rated value), and it is best to have a reliable voltage stabilizing function.
4) It can run continuously for a long time. Due to the particularity of the requirements, there is no ready-made high-frequency power supply to choose from. The equipment used for high frequency cold crucible technology is specially designed. The power supply has the following characteristics: the oscillation loop adopts a capacitor three-point oscillator with relatively stable frequency. This circuit has the advantages of stable frequency, good waveform and simple structure. It is not easily affected by parasitic circuits at higher frequencies, but it is difficult to start vibration.
2. Cold crucible system
Cold crucible system is one of the key equipment for crystal growth. It must be able to support the melt with internal temperature above 3000℃ without being melted by high frequency electric field. Its composition is:
1) The water-cooled copper tube is bent into a double "U" shape with a gap of 1 ~ 1.5 mm, which ensures the smooth penetration of high-frequency electromagnetic energy. Because of the smooth internal water flow, it is easy to form a "cold shell" to support the melt.
2) The water-cooled base consists of three parts, the middle of which is insulated, which effectively cuts off the high-frequency induced current and improves the efficiency. The base is divided into an upper cavity and a lower cavity, the upper cavity supplies water and the lower cavity drains water, and the upper cavity and the lower cavity are welded with both ends of the upper copper tube respectively.
3) Below is an insulating bracket made of glass fiber reinforced plastic, which is insulated from the metal part of the pull-down mechanism.
3. Pull-down device and speed control system
The pull-down mechanism adopts lead screw and worm gear transmission mechanism, which is dragged by DC torque generator and motor set. The speed of the motor is adjustable, and the speed regulation accuracy is high, which ensures the stability of crystal growth. The DC torque unit is powered by a specially designed pulse width modulation controller, which has two closed-loop adjustments of speed feedback and voltage feedback to ensure constant speed requirements.
Thirdly, the growth of synthetic cubic zirconia crystals.
1. Preparation of raw materials
The main raw material for producing cubic zirconia is ZrO2 powder, and the content of TiO2 and Fe2O3 in the raw material is relatively high, generally less than 0.03%, so as to ensure that the grown crystal is yellow and has good transparency. The stabilizer is high-purity Y2O3, and the color is white and delicate.
In order to grow colored crystals, it is necessary to dope a certain amount of impurity elements, such as rare earth oxide and transition group oxide, as shown in Table 4- 1-4. ZrO2 _ 2 and Y _ Y2O3 _ 3 are mixed according to the ratio of 9∶ 1mol, and the corresponding impurity elements are added and evenly mixed for later use.
Table 4 Common Dopants and Colors in-1-4cz Crystals
2. Crystal growth process
Put the mixed raw materials into a cold crucible, put a small amount of zirconium tablets on it, and turn on the power supply to boost the pressure to melt the raw materials. After the raw materials are melted, the melt is stable for a period of time, and after the voltage, current and gate current are basically stable, the crucible is slowly put down. At this time, spontaneous nucleation will occur at the bottom due to the cooling of the lower part. As the crucible descends, some crystal nuclei with growth advantages grow rapidly, crowding out other small crystals to grow into crystal arrays. Generally speaking, the typical working parameters of a crucible with a diameter of d=250mm are: voltage 9 ~ 10 kV, current 7 ~ 10A, grid current1~/0.5A, and crucible descending speed 3 ~ 15 mm/h ... Click to open and separate the complete crystal block.
Other applications and latest development of cold crucible technology
Cold crucible technology has developed greatly since 1976 was put into industrial application.
In terms of high-frequency cold crucible equipment, following Russian, the United States, China and other countries have successfully developed and expanded their production capacity, and put it into commercial production, producing hundreds of tons of ZrO2 _ 2 crystals every year. The diameter of the cold crucible is expanded to more than 400mm, and the charging capacity is expanded from several kilograms to 1200 kilograms, and nearly 400 kilograms of zirconia crystals can be produced each time. The stability of the equipment is greatly improved and automatic control is realized.
Great progress has also been made in the study of Czochralski crystals. Now almost all kinds of crystals can be produced, especially the blue-green crystal recently put on the market, the blue of sapphire and the green of emerald can be imitated.