Magnesium oxide (MgO) is likely a familiar material to many, given its wide range of applications in daily life. But what exactly is alumina (Al2O3) ceramic, and what is its relationship with magnesium oxide? Let’s explore.
The Fundamentals of Transparent Alumina Ceramics
Transparent alumina ceramics are known for their high-temperature resistance, corrosion resistance, high strength, high toughness, and excellent light transmission. Due to their relatively low production costs, they are widely used in:
- Lighting: Ceramic discharge tubes for high-pressure sodium (HPS) lamps and metal halide (MH) lamps.
- Electronics: Integrated circuit (IC) substrates and high-frequency insulating materials.
- Optics: Infrared detection windows and more.
As a polycrystalline inorganic material, transparent alumina ceramic consists of various microstructures, including grains, grain boundaries, second phases, and pores. Because each of these components has a different refractive index, the material’s natural light transmittance is often reduced (while its theoretical transmittance is 86%).
The Vital Role of Sintering Aids
In the production of transparent alumina ceramics, simply using high-purity raw powders is not enough. A minute amount of sintering aids must be added to promote densification and facilitate the removal of pores during the sintering shrinkage process. Furthermore, trace amounts of these additives inhibit abnormal grain growth. Therefore, the choice and concentration of sintering aids are critical to achieving high-quality transparent ceramics. Today, we will focus on the impact of MgO as a sintering aid.
Impact of MgO on Density and Transparency
Adding an appropriate amount of magnesium oxide to alumina ceramics significantly improves their relative density. However, it is important to note that as the MgO doping level increases beyond a certain point, the relative density begins to decline. This occurs because an excess of MgO leads to an increase in porosity, which hinders the densification of the sample.
Research indicates that when the MgO addition is approximately 0.05%, the relative density reaches 99.5%. At this level, the ceramic achieves extremely high density and its peak transmittance. Additionally, the relative density increases as the holding time (soaking time) is extended. This suggests that longer holding times allow the ceramic grains to further grow and perfect their structure, facilitating the expulsion of remaining pores. Consequently, extending the holding time is an effective way to obtain transparent alumina ceramics with superior density and light transmittance.
The Mechanism Behind MgO Doping
As the amount of MgO increases, the ceramic grains become finer. However, if the addition is excessive, the number of pores gradually increases. This phenomenon occurs because over-doping can form a discontinuous liquid phase at the grain boundaries, which causes micro-pores to aggregate into larger pores that remain trapped within the structure. These larger pores are eliminated much more slowly; thus, when the addition level is too high, the bulk density follows a downward trend.
Conversely, when the MgO doping level is kept at an optimal low level, the light transmittance of the alumina ceramic remains high. This is because a proper amount of MgO inhibits the rapid migration of grain boundaries, allowing pores to be expelled more completely. This results in a denser ceramic structure and higher transparency.