Transparent alumina ceramic was first invented by Dr. Coble of the United States, and has good heat resistance, corrosion resistance and mechanical properties. Transparent alumina ceramic is the first advanced ceramic material to achieve transparency and has been widely used.
Factors affecting the transparency of alumina ceramics
(1) Porosity
The most important factor affecting the transparent performance of transparent ceramics is porosity. The preparation of transparent ceramics is essentially a densification process that completely excludes microscopic pores during the sintering process. Stomatal size, number, and type all have a significant effect on the transparency of ceramic materials.
(2) Grain boundary structure
The grain boundary structures of transparent and opaque ceramics are different. The crystal boundaries of transparent materials are clean and clear. The phase composition of ceramic materials usually contains two or more phases. This multiphase structure will cause light to scatter on the phase boundary surface. When incident light enters the crystal grains, it will meet the grain boundaries, causing refraction and reflection. If the grain boundaries have the same refractive index as the grains, no refraction or reflection will occur. If there is a certain difference in refractive index between the grain boundaries and the grains, it will affect the transmittance.
(3) Grain size of ceramics
The transmittance of transparent ceramics depends on the crystal size of the ceramic polycrystals. When the wavelength of the incident light is equal to the crystal diameter, the light scattering effect is the largest and the transmittance is the lowest. The grain size is controlled outside the wavelength range of the incident light.
(4) Surface finish
The transmittance of transparent ceramics is also limited by the surface finish. The surface of the untreated ceramic after sintering has a high roughness, and the light will diffusely reflect on this surface. The greater the roughness of a ceramic, the lower its transparency. Grinding and polishing the ceramic surface will greatly increase the transmittance.
Preparation of transparent alumina powder
Compared with ordinary alumina ceramics, transparent alumina ceramics have extremely high requirements for alumina powder.
The purity of the Al2O3 powder used must be above 99.99% and be in the alpha phase. The fineness of the powder is generally controlled below 0.3 μm. This is because impurities in the raw materials easily generate heterogeneous phases, forming a light scattering center, reducing the intensity of the projected light in the incident direction, and reducing the transparency of the product. In addition to the high purity and small particle size of the powder, the particles should be highly dispersed to ensure high sintering activity.
The preparation of alumina powder can be divided into three methods, namely solid phase method, liquid phase method and gas phase method. The solid-phase method has the disadvantages of poor alumina purity, difficult to control particle size distribution, and high calcination temperature. Therefore, the solid-phase method is not suitable for preparing transparent alumina ceramic powder. The advantages of the gas-phase method are high alumina purity and fine particles. This method is relatively expensive equipment, complicated operation, and low production efficiency, so the gas phase method is not suitable for large-scale industrial production.
The liquid phase process is relatively simple, and the powder prepared is of high purity and small particle size. Therefore, the liquid phase method is currently the main method for preparing transparent alumina ceramic powder in the laboratory and industry.
Sintering of transparent alumina ceramics
(1) Sintering aids and functions
In order to sinter the Al2O3 ceramic body into a completely dense body without pores, a small amount of sintering aid must be added to the Al2O3 powder, usually MgO. Y2O3, La2O3, ZrO2, ThO2, etc. can also be used. These oxides and MgO can also be used. Mixed use. Compared with MgO, additives such as Y2O3, La2O3, ZrO2, and ThO2 have a wider concentration range. Within this concentration range, the maximum light transmittance can remain unchanged.
(2) Sintering method
Atmospheric and vacuum sintering
Alumina in sintering in air often has 1% to 3% of residual pores. The generation of these pores is mainly due to the pores being closed in the alumina ceramics in the later stage of sintering. The further shrinkage of pores needs to diffuse to the surface by means of grain boundaries; in the air Nitrogen is insoluble in alumina grains at the sintering temperature, so the pores shrink only when the internal pressure is in equilibrium with the surface shrinkage energy. The sintering of transparent alumina ceramics in a hydrogen atmosphere can effectively eliminate the remaining pores.
Combining atmospheric sintering with hot isostatic pressing (HIP)
By combining nanometer alumina powder with atmospheric sintering and hot isostatic pressing, micron or submicron fine crystal transparent alumina ceramics can be prepared with few remaining pores, clean grain boundaries, and high linear transmittance. Because the grains are small, the mechanical properties are also significantly improved, even close to that of alumina single crystal materials.
Spark Plasma Sintering (SPS)
Michae et al. Used SPS sintering to prepare transparent alumina ceramics, and studied the effects of the type and content of additives on the light transmission of Al2O3 during the SPS process.
Microwave rapid sintering
Microwave sintering is a rapid sintering technology that uses the dielectric loss of the material in the microwave electromagnetic field to heat the ceramic and its composite material as a whole to the sintering temperature to achieve densification. The microwave sintering speed is fast and the time is short, thereby avoiding the abnormal growth of ceramic grains during the sintering process, and finally obtaining transparent ceramics with high strength and high density.
The good performance of transparent alumina ceramics makes it applicable in high lighting, light source materials, aerospace, military and other fields, especially in high-pressure sodium lamps and halogen lamps.