Formation of oxides on tungsten conductors heated by electric current

High-temperature oxidation of tungsten is studied experimentally with the hot-filament technique. Steady-state oxidation of tungsten conductors (d = 300 μm, L=0.1 m) heated electrically is examined using optical pyrometry and electrothermography. Temperature profiles along the conductor and its average steady-state temperatures are measured for different current intensities. It is found out that tungsten trioxide filiform crystals show up on surface at 800 K. They grow up and gradually transform into acicular and lamellar crystals. Their size, morphology, and surface density depend on oxidation temperature and duration and on gas-phase pressure and composition. After the tungsten conductor is oxidized in air at atmospheric pressure (T = 1400 K) for 30 minutes, the mean lamellar crystals are 60 μm in diameter, the coarsest crystals reaching 320 μm.