| Transparent ceramics have recently attracted appreciable attention of research in optoelectronic material, because of their excellent optical performance and mechanical properties. Nowadays, researches for the optical properties of transparent ceramics are mainly on the experiment and processing technology, the absence of the physical mechanism study limited the development and applications of transparent ceramics.The transmission properties of polycrystalline transparent ceramics are influenced by the chemical composition and the microstructure of the material. The fundamental photophysical mechanism of transparent ceramics and the influencing factors on transmission properties were discussed in this paper. Models have been established to describe the effects of microstructure such as pores, grains and grain boundaries on the in-line transmission of transparent ceramics. The aim of this paper is to study correlations between microstructure contents and optical properties of transparent ceramics in order to find the ways leading to high transparency.Firstly, the basic scattering theory is illustrated; research has been done on the Mie theory and Discrete Dipole Approximation (DDA) method. The scattering efficiency factors of cuboid, ellipsoid and cylinder particles were calculated by using the DDA method under different size parameters, and compared with the results of the equivalent-volume spheres based on Mie theory. The qualifications of treating the non-spherical particles as sphere obtained as the foundation of analyzing the scattering in transparent ceramics.Secondly, the characteristics of transparent ceramics with cubic crystal structure were analyzed. Models have been established on the scattering of pores and grain boundaries. Effects of pores on transmission were studied using the Mie theory. The in-line transmissions were calculated as a function of pore size, width of distribution and porosity. Porosity has great impact as the transmission decreases with the increasing of porosity. The minimum of the transmissions were observed when the pore size close to the optical wavelength. The effects of grain boundary were evaluated by core-shell model. The results show that the grain boundary which has grain boundary phase causes scattering. The higher the difference between the refractive indexes of the grain and grain boundary is, the lower the transmission. The transmission is affected strongly by the amount of the grain boundary in the visible range. We can conclude that the elimination of residual pores and the preparation of grains with uniform size are essential to improve transparency of ceramics. Meanwhile, keep the compositions of both the grain and grain boundary same and reduce the amount of grain boundary phase are desired to eliminate the scattering of grain boundary in cubic ceramics. Thirdly, the characteristics of transparent ceramics with non-cubic crystal structure were analyzed. A light scattering model has been established for the birefringence of uniaxial crystal ceramic. Effects of grain size and size distribution on transmission were studied using the Mie theory. Then, the effect of orientation was discussed by the developed Rayleigh-Gans-Debye approximation. The results show that the transparency of non-cubic ceramics depends not only the maximum grain size but also on the preferential orientation of their texture.Finally, the characteristics of transparent composite ceramics were analyzed. A model has been established for the ceramics with two phase based on the scattering theory with high particle concentrations. Effects of phase fraction and grain size on transmission were studied using Interference Approximation. The results show that the transmission reaches the maximum with a phase volume ratio of 50:50. Furthermore, the composite ceramics exhibit excellent transmission in both the visible part and infrared part of spectrum only when controlling the grain size in nanometer range. |
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