Tungsten filaments are mostly produced with ammonium paratungstate (APT) as raw material. The general process is to roast ammonium paratungstate in air at about 500℃ to
tungsten trioxide, or slightly reduce it to blue tungsten oxide in hydrogen at about 450℃. The tungsten filaments for making incandescent lamp filaments need to be doped with a small amount of potassium oxide, silicon oxide and aluminum oxide in tungsten trioxide or blue tungsten oxide. The total amount of the three should not exceed 1%. This is the tungsten filament doping process invented by Baz in 1922. The tungsten oxide after doping is reduced to metal tungsten powder with hydrogen. The reduction process is generally carried out in two steps: the first step is reduced to tungsten dioxide (brown tungsten oxide) at about 630℃, and the second step is reduced to metal tungsten powder at about 820℃. The purpose of the two-step reduction is to make the added potassium fully play its role and control the powder particle size. The doped tungsten powder obtained in this way is then pressed into a long and thin square bar in a special mold. The square bar is energized in hydrogen and sintered by self-resistance heating (temperature up to about 3000℃). After sintering, the density of the tungsten bar can reach more than 85% of the theoretical value. This tungsten bar can be processed into a tungsten rod with a diameter of about 3mm by rotary forging, and then further processed into tungsten wires of various thicknesses by die drawing. For example, the diameter of the tungsten wire used in a 220V, 15W incandescent lamp is about 15?m, while the diameter of the tungsten wire used in a 10000W bromine tungsten lamp is about 1.25mm. Thinner tungsten wires, such as the tungsten wire of a 220V, 10W incandescent lamp with a diameter of about 12?m, must be made by electrolytic corrosion.
When the diameter of the tungsten wire reaches the micron level, it is difficult to accurately measure its diameter with a conventional caliper. Therefore, internationally, the thickness of tungsten wire with a diameter of less than 0.2mm is usually expressed by the weight of a 200mm long wire segment. For example, the diameter of the 15W incandescent lamp tungsten wire mentioned above can be expressed as 0.679mg/200mm.