(1) Induction coil and crucible
The induction coil of the plasma induction furnace includes a heating coil and a stirring coil. Small-capacity stoves only have heating coils, while large-capacity stoves are equipped with stirring coils in addition to heating coils. The structure of the induction coil is the same as that of an ordinary induction furnace.
(2) Furnace cover
The furnace cover of the plasma induction furnace is a double-layer steel plate structure, which is cooled by water and lined with refractory materials. There is a hole in the center of the furnace cover for the plasma gun body to extend into the furnace, and the gap between the gun body and the furnace cover is sealed with heat-resistant asbestos products. In order to observe the situation in the furnace, a flash observation device or a quartz glass observation window with a brush is also installed on the furnace cover. There is cooling water in the contact part between the furnace cover and the furnace body, and a sealing ring is installed in the middle to prevent the atmosphere from infiltrating into the furnace.
(3) Furnace shell
The structure of the furnace shell is divided into two types: fully enclosed and semi-enclosed. The semi-enclosed furnace shell only seals the upper part of the crucible. The structure of this furnace shell is simple, but the part above the crucible slag line is prone to cracks, which affects the sealing performance. Small test furnaces use this shell. Fully enclosed furnace shell, the crucible and induction coil are all enclosed in the furnace shell. This fully enclosed furnace shell has the advantages of good sealing, easy control of the pressure in the furnace and good refining effect. However, the structure of this furnace is relatively complicated, and special attention should be paid to the insulation of the water and electricity inlets and the furnace shell. And the effect of the magnetic field around the induction coil on the furnace shell and so on.
(4) Plasma gun lifting device
The lifting and lowering of the gun body of the plasma gun is usually driven by hydraulic pressure to ensure the stability of the lifting and lowering of the gun body. Plasma arc is used to dissolve silicon, because it is localized heating and uneven, so an electromagnetic induction coil is added. In this way, the dual heating system heats evenly and has high heating efficiency. The electromagnetic stirring makes the solution flow, which is conducive to the floating of impurities. During the working process, the ambient gas atmosphere can be flexibly changed, and the impurities can be removed by oxidation, reduction and neutral protective gas. For example, the introduction of oxygen or hydrogen can remove carbon and boron, among which boron is volatilized and removed in the form of BHO and BH. The boron was removed with a plasma torch after melting in argon.
Its working principle is as follows: the main gas (hydrogen) and a large flow of secondary gas (nitrogen) are input from the rear gun body, and are ejected through the secondary nozzle of the Laval tube type through the action of the gas swirl ring; the tungsten rod is connected to the negative electrode, When striking the arc, the primary nozzle is connected to the positive pole, the arc is struck by high frequency in the main gas, and the positive pole is connected to the secondary nozzle, that is, an arc is generated between the inner walls of the secondary nozzle. Under the strong action of the rotating secondary gas, the arc is compressed in the nozzle. The center is elongated to the outer edge of the nozzle to form an extended plasma arc with an arc voltage of up to several hundred volts. The high-power extended arc effectively heats the liquid silicon and ionizes and removes boron. Among them, the performance of the plasma gun is the focus of the design.
(5) Plasma arc spray gun
The overall structure design of the spray gun should ensure the long-term stable operation of the spray gun under high power, and the cooling effect of the spray gun should be better. Otherwise, the nozzle is easy to burn out. The installation concentricity of the nozzle and the cathode should be high, and the installation gap should be strictly controlled, otherwise it will easily cause the burning of the nozzle. If the cathode is fixed, the gap between them must be guaranteed by the assembly dimension chain of the entire part; the nozzle design should be reasonable, easy to process, and long service life. The insulation performance should be reliable, especially good insulation between the positive and negative electrodes. The sealing performance of the entire spray gun is better, and there is no water leakage or air leakage during work.
To achieve high-power plasma, the input power must be increased, so that most of the electrical energy input to the spray gun is converted into heat energy. The enthalpy value of the plasma arc depends on the input power of the spray gun, the flow rate of the working gas and the thermal efficiency of the spray gun. When the power is selected, the thermal efficiency of the spray gun and the enthalpy of the plasma arc can be guaranteed by increasing the plasma arc voltage and reducing the current. Due to the existence of the limit working voltage and limit working current, if the limit is exceeded, the nozzle and electrode will be burned out. High-power supersonic plasma arc spraying is different from ordinary plasma arc spraying. It uses the phenomenon of “expanded arc” that occurs when the transfer arc is mixed with high-speed airflow. The secondary nozzle of Laval tube type is used to further improve the plasma arc Compressed to obtain a stable and concentrated high-enthalpy, ultra-high-speed plasma flame. The structure of the high-power plasma arc spray gun is shown in Figure 1. It is divided into front and rear gun bodies. The rear gun body includes a tungsten electrode, a primary nozzle, the front gun body has a secondary nozzle, and a gas swirl ring made of heat-resistant and insulating ceramic materials. And copper powder mouth. Due to the high power, the double water internal cooling method is adopted.