Because of its high temperature resistance, corrosion resistance, high strength, high toughness and good light transmittance, and because of its low preparation cost, transparent alumina ceramics are widely used in high-voltage sodium lamps in the lighting field, ceramic discharge tubes for ceramic metal halide lamps, integrated circuit substrates in the electronic industry, high-frequency insulating materials, infrared detection windows and other fields. Transparent alumina ceramic is a kind of polycrystalline inorganic material, which is composed of a variety of microstructures, including grains, grain boundaries, second phases, pores, etc. Due to the difference of their respective refractive index (theoretical transmittance is 86%), the light transmittance of the material is low.
Therefore, in the preparation process of transparent alumina ceramics, in addition to using high-purity raw powder, a small amount of sintering additives should be added to promote sintering densification and eliminate pores during sintering shrinkage. In addition, the micro amount of sintering aids can also inhibit the abnormal growth of grains, so the selection and content of sintering aids are crucial for obtaining high-quality transparent ceramics. Today, we will learn about the influence of MgO as sintering additive on transparent alumina ceramics.
Effect of MgO on Transparent Alumina Ceramics
01 Effect on relative density
The relative density of transparent alumina ceramics added with appropriate amount of magnesium oxide will increase, but it should be noted that with the increase of MgO doping amount, the relative density will decrease, because excessive MgO additives will cause the increase of pores, which is not conducive to the densification of samples. When the addition of MgO is about 0.05%, the relative density reaches 99.5%, the density of ceramic sample is very high, and the transmissivity reaches the highest.
At the same time, the relative density of ceramic samples increases with the increase of holding time. It shows that with the prolongation of the holding time, the ceramic grains get further growth and improvement, and the pores in the ceramic samples get further discharged, which increases the relative density of the samples. Therefore, the transparent Al2O3 ceramics with better density and light transmittance can be obtained by prolonging the holding time.
02 Effect on microstructure
With the increase of MgO content, the ceramic grains become finer. This is because the MgO · Al2O3 spinel phase formed in ceramics can hinder the movement of grain boundaries and inhibit the growth of grains. However, if the amount of MgO added is too large, the porosity will gradually increase. This phenomenon may be caused by the formation of discontinuous liquid phase at the grain boundary when the doping amount is too high, so that the small pores will converge into larger pores and remain in them. This kind of pore is eliminated slowly, so when the addition amount is too large, the bulk density will show a downward trend.
03 Effect on optical performance
When the doping amount of MgO is low, the light transmittance of transparent ceramics is relatively high. This is because an appropriate amount of MgO can inhibit the rapid movement of the grain boundary, so that the pores are completely discharged, the ceramics are more dense, and the transmittance is high. However, with the increase of doping amount, when the MgO content exceeds the solid solubility in Al2O3, a second phase will be formed locally, forming a light scattering center, resulting in a decrease in the transmissivity of transparent ceramics.