One of the key properties of transparent ceramics is their light transmission. When the light passes through a medium, due to the absorption of the medium, surface reflection, scattering and refraction effects will cause light loss, light intensity attenuation (can be simply understood as, poor transparency of materials, light loss is relatively large). In addition to the basic chemical composition of the material, these decays also depend on the microstructure of the material. The following will make a brief summary of the factors that affect the transmittance of ceramics.
1. Porosity of ceramics
The preparation process of transparent ceramics is essentially the densification process of completely eliminating micro-pores in the sintering process, and the size, number and type of pores in the material will have a significant impact on the transparency of ceramic materials.
A small change in porosity can significantly change the light transmittance of the material. For example, studies have shown that when the closed porosity in the ceramic body changes from 0.25% to 0.85%, the transparency is reduced by 33%. Although this may be the result in certain circumstances, it can be seen that the effect of porosity on the transparency of ceramics can be a direct manifestation of violence to some extent. Some research data show that when the pore volume accounts for 3%, the light transmittance is 0.01%, and when 0.3%, the light transmittance is 10%. Therefore, transparent ceramics must increase the density and reduce the porosity, usually the density is greater than 99.9%. In addition to porosity, the diameter of porosity also has a great influence on the light transmittance of ceramics. As shown in the figure below, we can see that the transmittance is lowest when the diameter of the pore is related to the wavelength of the incident light.
2. Grain size
The grain size of ceramic polycrystal also has a great influence on the transmittance of transparent ceramics. When the wavelength of incident light is equivalent to the grain diameter, the scattering effect of light is the largest and the transmittance is the lowest. Therefore, in order to improve the transmittance of transparent ceramics, the grain size should be controlled outside the wavelength range of incident light.
What factors affect the transparency of alumina ceramics?
3. Grain boundary structure
Grain boundary is one of the important factors that damage the optical uniformity of ceramic, which leads to the scattering of light and the decrease of the transmittance of the material. The phase composition of ceramic materials usually consists of two or more phases, which easily leads to light scattering on the surface of the phase boundary. The greater the difference in the composition of the material, the greater the difference in refractive index, and the lower the transmittance of the whole ceramic.
Therefore, the grain boundary area of transparent ceramics should be thin, good light matching, no porosity, inclusions, dislocation and so on. Ceramic materials with isotropic crystals can achieve linear light transmittance similar to that of glass.
4. Surface processing finish
The transmittance of transparent ceramics is also affected by surface roughness. The roughness of ceramic surface is not only related to the fine divergence of raw materials, but also related to the machining finish of ceramic surface. The surface of untreated ceramic after sintering has a large roughness, and light incident on this surface will diffuse reflection, resulting in light loss. The greater the roughness of its surface, the worse its light transmission performance.
The surface roughness of ceramics is related to the fine divergence of raw materials. In addition to the selection of raw materials with high fine divergence, the ceramic surface should also be ground and polished. The grinding and polishing of the ceramic surface will greatly improve the transmittance, and the transmittance of the polished alumina transparent ceramics can generally be increased from 40% to 45% to more than 50% to 60%, and the polishing may reach more than 80%.
