Alumina transparent ceramics are widely used in high-pressure sodium lamps, ceramic metal halide lamps, ceramic discharge tubes, electronic Integrated circuit substrates, high-frequency insulating materials, and infrared detection windows in the industrial field. Alumina transparent ceramic is a polycrystalline inorganic material composed of a variety of microstructures, including grains, grain boundaries, second phases, pores, etc. Due to the difference in their respective refractive indices (theoretical transmittance is 86%), This results in lower light transmittance of the material.

Therefore, in the preparation process of alumina transparent ceramics, in addition to using high-purity raw material powder, it is necessary to add a very small amount of sintering aids to promote sintering densification and eliminate pores during sintering shrinkage. In addition, trace amounts of sintering aids also have the effect of inhibiting the abnormal growth of crystal grains, so the selection and content of sintering aids are very important to finally obtain high-quality transparent ceramics. Today we will take a look at the effect of MgO as a sintering aid on alumina transparent ceramics.

Influence of MgO on Alumina Transparent Ceramics

01 Effect on Relative Density
The relative density of alumina transparent ceramics with an appropriate amount of MgO added will increase, but it should be noted that with the increase of MgO doping amount, the relative density will decrease. This is because too much MgO additive will cause pores. The increase is not conducive to the densification of the sample. When the addition amount of MgO is about 0.05%, the relative density reaches 99.5%, the density of the ceramic sample is very high, and the transmittance also reaches the highest.
At the same time, with the prolongation of holding time, the relative density of ceramic samples also increased. It shows that with the prolongation of the holding time, the ceramic grains are further grown and perfected, the pores in the ceramic samples are further discharged, and the relative density of the samples is increased. Therefore, prolonging the holding time can obtain transparent Al2O3 ceramics with better density and better light transmittance.

02 Effect on Microstructure
With the increase of MgO addition, the ceramic grains become finer. This is because the MgO·Al2O3 spinel phase formed in the ceramic can hinder the movement of grain boundaries and inhibit the growth of grains. However, if the amount of MgO added is too large, the pores will gradually increase. The reason for this phenomenon may be that excessive doping will form a discontinuous liquid phase at the grain boundary, so that tiny pores aggregate into larger pores and remain in them. The removal of such pores is slow, so when the addition amount is too large, the bulk density tends to decrease.

03 Influence 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 grain boundaries, so that the pore discharge is relatively complete, the ceramics are denser, and the transmittance is high. However, with the increase of the doping amount, after 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 transmittance of transparent ceramics. |

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