Near infrared transmittance of translucent Si₃N₄ sinteredceramics

Si₃N₄ was sintered with AlN-MgO additives by spark plasma sintering. The optical transmittances of sintered bodies were studied in the wavelength range of 0.4-5.5 μm, especially with emphasis on near infrared region. When the additive content was large, the sintered body was composed of α-SiAlON and showed high transmittance in both visible and near infrared regions. When the additive content was small, the sintered body was composed of β-Si₃N₄ and exhibited high transmittance in visible region but low transmittance in near infrared region. The electron spin resonance spectra indicated that the reduction of near infrared transmittance was due to the absorption by unpaired electrons derived from nitrogen vacancies.