# Progress in Transparent Nano-Ceramics and Their Potential Applications
[PERSON]  
1 Hennan Key Lab of Intelligent Manufacturing of Mechanical Equipment, Zhengzhou University of Light Industry, Zhengzhou 450002, China; [EMAIL_ADDRESS] (W.M. ; j); [EMAIL_ADDRESS] (Z.J. ); [EMAIL_ADDRESS] (XX. ); [EMAIL_ADDRESS] (W.H. ); [EMAIL_ADDRESS] (W.H. ); [EMAIL_ADDRESS] (I.L.) [PERSON]  
1 Hennan Key Lab of Intelligent Manufacturing of Mechanical Equipment, Zhengzhou University of Light Industry, Zhengzhou 450002, China; [EMAIL_ADDRESS] (W.M. ; j); [EMAIL_ADDRESS] (Z.J. ); [EMAIL_ADDRESS] (XX. ); [EMAIL_ADDRESS] (W.H. ); [EMAIL_ADDRESS] (W.H. ); [EMAIL_ADDRESS] (I.L.) Guangdong HQUST Industrial Technology Research Institute, Guangdong Provincial Key Laboratory of Digital Manufacturing Equipment, Dongguan 523808, China; [EMAIL_ADDRESS] (XX. ); [EMAIL_ADDRESS] (XX. ); [EMAIL_ADDRESS] (XX. ); [EMAIL_ADDRESS] (XX. ); xpl836947581@163.  
densification process of transparent nano-ceramics, reduces the impurity content, and is a great breakthrough in the optical quality and various properties of ceramics. Because ceramic is an inorganic polycrystalline material, it generally does not have transparency. This is because there are a lot of defects, such as pores and impurities in the ceramic, which cause scattering and refraction loss to the light incident into the ceramic so that the incident light cannot pass through the ceramic. Figure 1 demonstrates the schematic light transmission phenomena in a polycrystalline ceramic, which has more light scattering sources than a single crystal, such as grain boundaries, pores, impurities, and birefringence [6]. Initially, the incident intensity (\(I_{0}\)) of light depends on the surface roughness and is composed of diffuse reflection (\(R_{D}\)) and diffuse transmission (\(I_{DT}\)). In addition, a certain amount of light is reflected on each surface of the material through specular reflection (\(R_{T}\)). Light passing through polycrystalline materials can also interact with pores (residual pore scattering), grain boundaries (second phase scattering), impurities (inclusion absorption or scattering), and birefringence (birefringence of the non-cubic phase) [6]. The main factors affecting the optical quality of ceramics are light absorption, scattering of pores and the second phase, grain boundary reflection and refraction (birefringence), grain boundary scattering, and light scattering caused by surface roughness. Based on the above factors, the transmittance of ceramics can be calculated by Equation (1):  
\[\frac{I}{I_{0}}=(1-R)^{2}e^{-mx} \tag{1}\]  
where \(I_{0}\) is the incident intensity of light, \(I\) is the transmitted intensity of light, \(m\) is the light absorption coefficient, \(x\) is the sample thickness, and \(R\) is the light reflectivity [7]. The light absorption coefficient \(m\) can be given by Equation (2),  
\[m=\alpha+S_{im}+S_{op} \tag{2}\]  
where \(\alpha\) is the absorption coefficient of electron transition, \(S_{im}\) is the scattering caused by structural heterogeneity (e.g., pores, second phase), and \(S_{op}\) is the scattering caused by optical anisotropy (e.g., hexagonal system and other non-cubic systems). Figure 1: Sketch of light transmission in a polycrystalline ceramic. According to Equations (1) and (2), the necessary conditions for preparing high-quality transparent nano-ceramics are [8]:  
1. High relative density and residual porosity less than 0.01%;
2. No optical anisotropy;
3. The grain boundary is free of impurities and second phase;
4. The selective absorption of the crystal to the incident light is small;
5. The surface is flat and has low roughness. Therefore, polycrystalline transparent nano-ceramics with excellent optical properties can be fabricated by using high-purity raw material powder [9; 10], especially the application of nano powder, reducing impurities and eliminating pores through certain processes, and reducing scattering caused by surface roughness through polishing and other means.