Alumina ceramics are sintered under the temperature of 1450~1800℃, which contain aluminum oxide as the main raw material and 6~30% of sintering additives, such as clay, MgO, CaO, TiO.

The purer alumina ceramics are, the better performance can be obtained, however the manufacturing of alumina ceramics also becomes more difficult with aluminum oxide proportion increases.

Pure alumina ceramic can be used up to temperature of 800℃ and shows the highest mechanical strength performance among all oxide ceramics, also the thermal conductivity is high and thermal shock resistance is strong.

Various sintering methods of alumina ceramics

• Atmospheric pressure sintering

Alumina ceramics atmospheric pressure sintering refers to the process that the alumina green bodies are sintered at ordinary pressure.

This sintering method requires high furnace temperature, which means high requirements of furnace and large energy waste.

• Hot pressing sintering

The required sintering temperature of hot pressing sintering method is lower than that of atmospheric pressure sintering. In addition, the density of sintered alumina ceramic products can theoretically reach up to 99% after hot pressing sintering.

This sintering method is comparatively easy to achieve because it is to fill the powder into molds directly or mainly applies preforming process.

• Sintering on electric field

This sintering method is generally recognized that the alumina ceramic green bodies are sintered on direct current(DC) electric field during the whole process.

• Ultra-high pressure sintering

Ultra-high pressure sintering refers to the process that alumina ceramic green bodies are sintered at the required pressure of over several dozens of gigapascals(GPa).

• Atmosphere sintering

This method mainly serves for the alumina ceramic products which are difficult to sinter in the air.

In order to prevent the products oxidizing, it is suggest that make the furnace chamber fill up with proper gases to create a required atmosphere where the sintering process can be completed.

Three stages of sintering furnace

• Furnace temperature heating-up stage

It is crucial to control the furnace temperature at this stage. The green body of alumina ceramic shrinks due to the continuously rising furnace temperature, but its density and strength basically will not change. However, it is found that the crystalline grain size was actually altered when observed by microscopy.

During the furnace temperature heating-up stage, the green body of alumina ceramic has a high probability to crack as its moisture and binder were evaporated, so it is particularly important to control the furnace temperature in a proper range.

• Furnace temperature maintaining stage

During this stage of sintering process, though the furnace temperature changes slightly, the shrinkage of ceramic green body continues and the change of its density is evident.

Under the microscopic observation of crystalline grains, the grain size does not change distinctly and the state of grains turns into inter-particle debonding, so the pores become smaller.

Furthermore, the cracking and deforming of alumina ceramic green bodies remain during the whole stage due to the evident change of its volume.

• Furnace Cooling Down Stage

The final stage of the sintering process is furnace cooling down stage, which shows a great change of furnace temperature. Besides, the density of the ceramic green body also obviously changes at this stage.

A dramatic grain growth can be observed no matter from microscopic or visual observation. The voids between grains reduce and many single pores are formed, therefore numerous pores appear on the grain surface.