What factors will affect the transparency of alumina ceramics?

One of the key properties of transparent ceramics is its light transmittance. When light passes through a certain medium, light loss and light intensity will be attenuated due to the effects of medium absorption, surface reflection, scattering and refraction (it can be simply understood as a material with poor transparency, relatively more light loss). In addition to the basic chemical composition of the material, these attenuations also depend on the microstructure of the material. The following is a brief summary of the factors that affect the transmittance of precision ceramics.

1. The porosity of ceramics

The preparation process of transparent ceramics is essentially a densification process that completely eliminates microscopic pores during the sintering process. The size, number, and type of pores in the material will have a significant impact on the transparency of the ceramic material.

Small changes in porosity can significantly change the light transmittance of the material. For example, studies have shown that when the closed-mouth porosity in the ceramic body changes from 0.25% to 0.85%, the transparency decreases by 33%. Although this may be the result under certain circumstances, to some extent we can see that the effect of porosity on the transparency of ceramics is a direct and violent manifestation. Research data shows that when the pore volume accounts for 3%, the light transmittance is 0.01%, and when the pore volume is 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 the porosity, the diameter of the pores also has a greater impact on the light transmittance of the ceramic.

2. Grain size

The crystal grain size of ceramic polycrystals also has a great influence on the light transmittance of transparent ceramics. When the incident light wavelength is equivalent to the crystal grain diameter, the light scattering effect is the largest and the light transmittance is the lowest. Therefore, in order to improve the light transmittance of transparent ceramics, the grain size should be controlled outside the wavelength range of incident light.

3. Grain boundary structure

Grain boundaries are one of the important factors that destroy the optical uniformity of the ceramic body, thereby causing light scattering and reducing the light transmittance of the material. The phase composition of ceramic materials usually contains two or more phases, and this multi-phase structure easily causes light to scatter on the surface of the phase boundary. The greater the difference in the composition of materials, the greater the difference in refractive index, and the lower the light transmittance of the entire Shenzhen precision ceramics.

Therefore, the transparent ceramic grain boundary area should be thin, good light matching, free of pores, inclusions, and dislocations. Ceramic materials with isotropic crystals can achieve linear light transmittance similar to that of glass.

4. Surface finishing finish

The transmittance of transparent ceramics is also affected by the surface roughness. The roughness of the ceramic surface is not only related to the fineness of the raw materials, but also related to the machining finish of the ceramic surface. After sintering, the untreated ceramic surface has a relatively large roughness, and light incident on this surface will be diffusely reflected, resulting in light loss. The greater the roughness of its surface, the worse its light transmission performance.

The surface roughness of precision ceramics is related to the fineness of the raw materials. In addition to the selection of high-fineness raw materials, the ceramic surface should also be ground and polished. Grinding and polishing the ceramic surface will greatly increase the light transmittance. The transmittance of the polished alumina transparent ceramic can generally be increased from 40% to 45% to 50% to 60% or more, and the polishing may reach more than 80%. .