Magnetron, also known as microwave generator, magnetron microwave tube, is a kind of electronic tube, mainly composed of two parts: tube core and magnet. The tube core is composed of a filament, a cathode, an anode and a microwave energy output device. The filament, made of thoriated tungsten wire (ThO2 wire) or pure tungsten wire wound into a spiral shape, is used to heat the cathode.
In order to improve the stability of magnetron emission, it is required to carbonize the magnetron wound with ThO2 wire. Carbonization is to place the magnetron that has been shaped and assembled in the vapor of hydrocarbons and heat it to 2200-2500°C. When the vapor of hydrocarbons is in contact with high-temperature tungsten, carbon and hydrogen are decomposed, and the carbon is attached to the thorium. The surface of the tungsten wire forms tungsten carbide with tungsten and diffuses into the tungsten.
Because the bonding force between thorium atoms and tungsten carbide is stronger than that of tungsten, the wire after carbonizing will reduce thorium oxide to thorium atoms at the working temperature, and thus a stable single-atom covering layer is formed on its surface to produces stable electron emission.
In order to prolong the service life of the magnetron, the quality of the wire is very important. Its manufacturing processes are as following:
1,Preparing thoriated tungsten powder. Thorium nitrate solution, potassium nitrate solution, aluminum silicate solution and blue tungsten powder are mixed and dried in a doping pot, and sent to a reduction furnace for reduction to obtain thorium tungsten powder containing trace potassium elements. The reduction adopts a five-stage temperature-controlled process, which are 550°C, 650°C, 750°C, 850°C, and 950°C. The holding time of each section is 60-80min, the loading capacity of the boat is 300-350g/boat, the reducing atmosphere is hydrogen, and the hydrogen flow rate is 3-5m3/h.
2,High-energy ball milling. Put the thoriated tungsten powder into a ball mill jar containing a polyurethane lining for high-energy ball milling for 3-5 hours, and the ball milling medium is tungsten balls.
3,Press. Isostatic pressing is adopted, and the pressing force is 145-250 MPa to obtain thoriated tungsten bars with a unit weight of 1.5kg-2.5kg.
4,Temporary sintering and electrification sintering. Direct current sintering is adopted. The sintering process is that the current is increased from 0A to 900A in 2 minutes, increased to 3100A-3500A in 18 minutes, and finally increased to 4000A-4500A in 20 minutes to obtain sintered bars with 17.5-18g/cm3 density.
5,Swaging. Using a rotary forging machine, the vertical melting bar is processed from about 17mm to a diameter of 3.3mm. During this process, a high-temperature annealing is performed on the vertical melting bar of 9mm and 5mm respectively. The annealing temperature is between 2000-2300℃, and the annealing speed is 0.5-2m/min.
6,Drawing. The doped thorium tungsten bar swaged into 3.3mm is drawn into the wire drawing machine, and the die sequence is 2.8-2.4-2.0-1.52-1.24-1.08-0.93-0.77-0.7-0.63-0.57-0.51mm, to obtain 0.51mm doped thorium tungsten wire with black surface.
7,Cleaning. The black wire with a diameter of 0.51 mm is electropolished to 0.50 mm to obtain a thorium tungsten wire for winding a microwave oven magnetron.
In practical application, in order to improve the performance of thoriated tungsten wire, a small amount of precious metal rhenium can also be added in the first step, and the doping amount is about 0.3-0.5% to make thorium tungsten rhenium wire. After the magnetron coils for microwave ovens are carbonized at high temperature, the grains are not easy to grow, and the structure can be kept stable.
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