Why does the volume of ceramics shrink during sintering?

During the sintering process of ceramics, the mass changes very little, but the volume reduction ratio can exceed 40%, which is the key factor that causes the density of ceramics to increase. So, why does the volume of ceramics shrink during sintering?

Gas escape and pore reduction: Ceramics are sintered from raw material powders, and both the raw material powders and the ceramic body contain a certain amount of gas and pores. Under high-temperature sintering conditions, a large amount of gas in the body will escape, and the pores will decrease or even disappear, thereby reducing the volume of the ceramics and increasing the density.

Moisture and impurity volatilization: The raw material powders used to fire ceramics vary, and the amount of impurities they contain is also different, but the impurity content is usually lower. Some impurities will decompose and volatilize in a high-temperature environment, causing the ceramic raw material particles to combine more tightly, thereby causing the ceramic volume to shrink.

Particle movement and structural reorganization: During high-temperature sintering, the crystal structure of the ceramic will change to a more stable state, and the mobility of the raw material particles will gradually increase. During this process, the raw material particles will spontaneously fill the original voids in the green body and the holes left after the volatilization of gas, impurities, and water, resulting in a decrease in the volume of the ceramic and an increase in density.

During the ceramic sintering process, although the loss of gas, water, and impurities will cause a certain degree of decline in the quality of the ceramic, the reduction in quality is very small. In comparison, the reduction ratio of the ceramic volume can reach 40%, so the density of the ceramic will increase significantly during the sintering process, and density has therefore become an important indicator of the degree of ceramic sintering.