The production of tungsten wire mostly uses APT as raw materials. The general process is that ammonium paratungstate is roasted into tungsten wire in the air about 500 ℃. Or it was slightly reduced into blue tungsten oxide in the hydrogen about 450 ℃.
The tungsten wire used for incandescent lamp filament production requires the incorporation of small amount of potassium oxide, silicon oxide and alumina in tungsten oxide or blue tungsten oxide, the using total amount of three can't exceed 1%, and this is the tungsten doping process invented by Baz in 1922.
The tungsten oxide with doping processing can be reduced into tungsten powder by hydrogen. The reduction process is generally divided into two steps: the first step is that it can be reduced into tungsten dioxide (brown tungsten oxide) at about 630℃, the second step is that it can be reduced to metallic tungsten powder at about 820℃. The aim of two step reduction is to enable full play the role of potassium mixed and control particle size of powder. In this case, the doped tungsten powder will be pressed into elongated square bar in a special mold. The square bar get electricity in hydrogen, sintering with the method of the self resistance heating ( the temperature is about 3000℃), the density of tungsten bars after sintering can achieve more than 85% of the theoretical value.
The tungsten bars can be processed into tungsten rod with 3mm diameter in the method of rotary forging, and then further processed into a variety of tungsten wire with different thickness in the method of mold drawing. For example, the diameter of tungsten wire used in incandescent lamp with 220V, 15W is about 15µm, but the diameter of tungsten wire used in bromine tungsten lamp with 10000W is about 1.25mm.There is finer tungsten wire, such as the diameter of 220V, 10W incandescent bulb is about12µm, in this case, you should use the method of electrolytic corrosion to make.
When the diameter of tungsten wire can reach micron level, the conventional caliper is difficult to accurately measure its diameter. Therefore, the thickness of tungsten filament with diameter below 0.2mm is often expressed with the weight of wire section with the cutting length of 200mm, for example, the diameter of tungsten wire used in 15W incandescent lamp can be expressed with 0.679mg/200mm.