Influence of porosity on transparency behavior of MgAl2O4 spinel, experiment vs Mie theory

Abstract The effect of porosity on the transmission behavior of MgAl2O4 was modeled using Mie and Rayleigh-Gans-Debye (RGD) single-scattering theories. The RGD theory showed that the porosity less than 0.01 percent (especially for pore radius less than 70 nm) had no remarkable effect on the transparency of this ceramic. While Mie theory showed that this limit is less than 0.1 percent (especially for pore radius less than 70 nm). At the wavelengths of 550 and 1100 nm respectively and for the porosity higher than 0.01 percent, the pore size had to be smaller than 70 and 90 nm in order to achieve the maximum transmission. To verify the simulation results, the transparent spinel ceramic was used which made by two techniques, gel-casting and cold isostatic pressing (CIP). The strength of the final gel-casted and CIPed sample was 15.32 and 2.47 MPa, respectively. After spark plasma sintering (SPS), the density of sintered sample was 98.8 and 99.8% respectively with a transmission of 74.1 and 86.8% at the wavelength of 1100 nm. It was found that RGD theory is not as suitable as Mie theory for simulating the transmission of spinel ceramics, because of the high difference between refractive indexes of medium (spinel) and pore. However at shorter wavelength, by considering the scattering coefficient relation of RGD theory, two theories agree well.