The oxidation resistance of black tungsten wire is relatively poor, particularly in high-temperature oxygen-containing environments, where it is prone to oxidation and failure. Therefore, protective measures such as vacuum encapsulation, inert gas filling, or surface coatings are necessary in practical applications to extend its service life.
Tungsten is a high-melting-point metal (approximately 3422°C), but when exposed to oxygen at high temperatures, it readily reacts to form tungsten trioxide (WO₃) or other oxides. This oxidation reaction creates a yellow or green oxide layer on the wire’s surface, which degrades its mechanical properties and lifespan.
Below are key factors affecting the oxidation resistance of black tungsten wire:
- Temperature: At room temperature, tungsten wire oxidizes slowly, almost negligibly. However, in high-temperature environments (e.g., above 600°C), oxidation occurs rapidly, especially in the presence of oxygen or water vapor.
- Surface Condition: The unpolished surface of black tungsten wire has high roughness, and surface defects or impurities may accelerate oxidation. In contrast, polished tungsten wire (cleaned tungsten wire) has a smoother surface with slightly better oxidation resistance, though the difference is limited.
- Environment: In a vacuum or inert gas environment (e.g., argon, nitrogen), oxidation of black tungsten wire can be effectively controlled. This is why tungsten wire is commonly used in vacuum light bulbs or inert gas-protected light sources.
- Doping and Modification: To enhance oxidation resistance, industrial practices often involve doping (e.g., with potassium, aluminum, or silicon) or applying surface coatings (e.g., zirconia or silicide coatings). These methods can slow oxidation but cannot fully prevent it.
Practical Application Measures:
- Vacuum or Inert Gas Protection: In applications like incandescent lamps and electron tubes, tungsten wire operates in a vacuum or inert gas environment to prevent oxidation.
- Surface Treatment: Applying anti-oxidation coatings via chemical vapor deposition (CVD) or physical vapor deposition (PVD) enhances resistance to oxidation.
- Temperature Control: Operating below the oxidation threshold temperature of tungsten wire can extend its lifespan.
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