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Preparation and processing of tungsten

Tungsten exists in wolframite (FeMnWOs) and scheelite (CaWO) in nature. The production of tungsten first involves smelting and chemically treating the ore to obtain light yellow WOs powder, and then using pure hydrogen to reduce WO₃ to obtain tungsten powder. The reaction is as follows:

WO₃ + 3H₂←W + 3H₂O

Add adhesive to the granular tungsten powder, and obtain a tungsten rod with a residual porosity of about 10~15% after compression and sintering. This sintered tungsten rod is very brittle, and the atomic bonding force between the grain interfaces is very weak.

However, the density of tungsten after high-temperature sintering is greatly increased, so the size of the tungsten rod will be reduced, and it has sufficient hardness and good toughness for easy machining.

For the machining of tungsten, the tungsten rod must first be forged in a special rotary hammer machine. After multiple forgings, the tungsten rod is processed into a tungsten rod with a diameter of about 1mm. If finer materials are required, they can be further processed by wire drawing.

There are many types of wire drawing machines, such as chain wire drawing machines for drawing coarse wires and drum wire drawing machines for drawing fine tungsten wires. In order to improve metal properties and eliminate stress, intermediate annealing is often used during wire drawing. Hot drawing is also used to increase elongation and make it easier to pass through the die hole.

The wire drawing die is made of diamond. Tungsten wire produced by die drawing is normally composed of three parts: (1) matrix tungsten, (2) oxide layer, and (3) graphite layer.

Graphite emulsion is a high-temperature lubricant that is used to reduce deformation resistance during wire drawing, increase the service life of the wire drawing die, and prevent tungsten wire from oxidizing when heated. However, graphite emulsion is difficult to apply directly to a smooth tungsten matrix. Even if it can be applied, it is very harmful: there is a risk of carbonizing and brittle tungsten wire during tungsten wire processing and filament hydrogen burning. Therefore, there must be a "medium" between the matrix tungsten and the graphite layer to improve the adhesion of the graphite emulsion and prevent the adverse reaction between tungsten and carbon. The uniform tungsten oxide film physically attached to the substrate tungsten can play this role as a medium, and due to the presence of tungsten oxide, the filament is easy to clean and loses less weight. This layer of tungsten oxide is formed by the oxidation of the substrate tungsten.

The stability of the photoelectric performance of the bulb is closely related to the quality of the tungsten filament. The uneven diameter of the tungsten filament will cause a large fluctuation in the photoelectric parameters of the bulb; the worse the surface finish of the tungsten filament, the larger its actual radiation surface, and the lower the working temperature of the filament, which is one of the reasons for the low luminous flux of the bulb.

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