Why is the diameter of tungsten wire uneven?
The non-uniform diameter of tungsten wire can be attributed to several factors related to the manufacturing process and material properties:
1. Wire Drawing Process: Tungsten wire is typically produced through a process called wire drawing, where a thicker tungsten rod is pulled through a series of dies to reduce its diameter. Variations in the wire drawing process, such as inconsistent tension, worn dies, or improper alignment, can cause variations in the diameter of the tungsten wire.
2. Material Characteristics: Tungsten is a very hard and brittle material, which makes it very difficult to process. During the stretching process, if the material is not heated evenly or the microstructure of the tungsten is inconsistent, it will cause uneven stretching and diameter variation.
3. Temperature effect: The processing temperature of tungsten affects its ductility. If the tungsten wire is heated unevenly during the drawing process, some parts may be more ductile than others, resulting in uneven diameter of the tungsten wire when drawn.
4. Surface defects: Any surface defects or inclusions in the tungsten will also cause diameter changes. These defects will cause local defects and affect the drawing of the wire.
5. Post-processing: After drawing, the tungsten wire may undergo additional processes such as annealing or coating. If the treatment is uneven, it will also cause diameter changes.
Why does the conductivity of tungsten wire decrease?
The conductivity of tungsten wire decreases due to several factors:
1. Temperature Effect: As temperature increases, the resistivity of tungsten increases. This is a common phenomenon in metals, where increased temperature leads to increased lattice vibrations, which scatter electrons and reduce conductivity.
2. Impurities: Impurities present in tungsten can significantly affect its conductivity. Impurities disrupt the metal’s regular lattice structure, forming scattering centers that hinder the flow of electrons.
3. Grain Boundaries: Tungsten is usually processed into wire by methods such as wire drawing and annealing, which will produce grain boundaries. These boundaries can act as barriers to the flow of electrons, reducing conductivity.
4. Mechanical deformation: If the tungsten wire is subjected to mechanical stress or deformation, it will cause dislocations and defects in the crystal structure. These defects scatter electrons and reduce conductivity.
5. Oxidation and surface contamination: Tungsten will oxidize when exposed to air at high temperatures, forming tungsten oxide on the surface. These oxides can act as insulators, reducing the effective conductivity of the wire.
6. Phase change: Under certain conditions, tungsten will undergo phase change, which will affect its electrical properties. For example, at extremely high temperatures, tungsten may transform into a different crystal structure, changing its conductivity.
