How does rare earth affect the performance of alumina ceramics?

Alumina ceramics have the characteristics of high mechanical strength, high hardness, wear resistance, corrosion resistance, high temperature resistance, light weight, and good insulation. They are widely used in textile, coal, petroleum, chemical, electronics and construction industries. It is a low-cost and widely used of ceramic materials. However, due to the poor toughness of simple alumina ceramics, the strength and wear resistance need to be improved, which limits the further expansion of its application fields. The use of multi-phase synergistic modification has become a research direction to improve the mechanical properties and wear resistance of alumina ceramics.
 
Rare earth cations have a much larger radius than aluminum ions. The difference in ionic radius makes them difficult to dissolve. Therefore, rare earth elements mainly exist on alumina grain boundaries, and rare earth oxides with a glass network structure are large in volume and difficult to move, which hinders others. Ion migration reduces the migration rate of grain boundaries, inhibits the abnormal growth of crystal grains, and makes the structure compact. Rare earth oxides doped into the glass phase of the grain boundary can improve the strength of the glass phase and thereby enhance the mechanical properties of the ceramic. The more rare earth oxides are added, the more obvious the effect of reducing the sintering temperature, but excessive addition will make the mechanical properties of ceramics worse. 
 
Influence on the microstructure of alumina ceramics
After doping with La2O3, Y2O3, and CeO2, the grain size will decrease. This indicates that the rare earth oxide has the effect of refining the grain. But with the increase of the rare earth oxide content, the ceramic grain size will gradually increase. And at the same time, the amount of liquid phase is also gradually increasing. The explanation maybe: the ionic radius for rare earth ions is much larger than that of aluminum ions. It is hardly dissolved in alumina, and mainly exist in the grain boundary glass phase.At the same time, rare earth oxides with a glass network structure hinder ions. The migration of crystal grains inhibits the growth of crystal grains and refines the crystal grains. However, the excessive addition of rare earth oxides will increase the amount of liquid phase and reduce the viscosity of the liquid phase, promote ion migration, and cause excessive growth of crystal grains and increase the size of the crystal grains.
 
Influence on the hardness of alumina ceramics
The hardness of alumina ceramics doped with rare earth oxides such as La2O3 and Y2O3 shows a trend of first increasing and then decreasing with the increase of the content, and there is a peak value. The possible reason for this phenomenon is that the addition of the appropriate amount of rare earth oxide can be fine. At the same time, increase the amount of liquid phase and fill the gaps between the grains, which will increase the density and increase the hardness. However, with the excessive addition of rare earth oxides, the increase in the size of the grains and the increase in the gaps will hardly have negative effects on the density and hardness. Offset, manifested as a gradual decrease in hardness.
 
Influence on the friction and wear properties of alumina ceramics
The wear surface of alumina ceramics has undergone four processes: the fracture and pull-out of crystal grains, the formation of friction layer, the increase of friction layer area and the increase of cracks. The crystal grains shed during wear of alumina ceramics will remain in At the friction interface, a smooth friction layer is formed under the action of surface stress. The friction layer is composed of two kinds of abrasive debris, which can reduce the wear rate of ceramics.
The wear mechanism of alumina ceramics is mainly abrasive wear, and proper amount of rare earth oxide doping can improve the wear resistance of ceramics.
 
Influence on the relative density of alumina ceramics
The study found that with the different doping amount, the relative density of Y2O3 and CeO2 doped alumina ceramics showed a trend of first increasing and then decreasing. At the same time, it was found that the relative density of alumina ceramics added with Y2O3 was lower than that of alumina doped with CeO2 and La2O3.
The above phenomenon can be explained as follows: (1) Rare earth ions have a large radius and are difficult to form a solid solution with alumina. They mainly exist in the liquid phase, which reduces the viscosity of the liquid phase. At the same time, rare earth oxides can promote alumina and alumina to form a solid solution. The chemical reaction of other additive components increases the amount of liquid phase. Adding a small amount of rare earth oxide is conducive to the formation of liquid phase, speeds up the removal of pores, and increases the density of ceramics. (2) The addition of rare earth oxides with high melting point increases the sintering temperature of ceramics. At the same time, rare earth ions have a larger ion radius, which hinders the migration of other ions. Excessive addition of rare earth oxides is not conducive to ceramic sintering and reduces the degree of densification. .
 
Conclusion
Rare earth is a precious resource and has a special electronic structure. The alumina modified by it has important uses in industries such as catalytic materials and ceramic materials. In rare earth modified alumina, appropriate rare earth elements must be selected according to needs, and at the same time, attention must be paid to the compounding of different rare earth elements to play a coordinated role.
 
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