Optical properties and dependence of optical transmittance on the particle sizes of nano-structured translucent γ-Al2O3 ceramics

Nano-size gamma-Al2O3 particles of four different sizes were synthesized by MOCVD (Metal Organic Chemical Vapor Deposition) method and then translucent nano-structured gamma-Al2O3 ceramics are pelletized under a pressure of 2.5 GPa by a uniaxial compaction-method. Although these nano-structured gamma-Al2O3 ceramics showed low density, they were fairly translucent. The relationship between transmittance and thickness of these specimens can be described by Lambert-Beer's law. The nano-structured gamma-Al2O3 ceramics with different particle diameter and thickness have been prepared in order to clarify the dependence of particle size on the optical transmittance. The optical transmittance decreased with increasing initial particle diameter. In the case of 0.20 mm thickness samples, the gamma-Al2O3 with 5.6 nm initial particle diameter showed an optical transmittance of about 60%. However, the optical transmittance decreases less than 5% when initial particle diameter is increased to 34.4 nm. This phenomenon could be explained by Rayleigh scattering, which is the scattering due to the particle diameter being much smaller than the wavelength of light. In these visible regions (500-800 nm), a scattering coefficient related with Rayleigh scattering was inversely related to the fourth power of the wavelength and directly proportional to the square of the grain size.