Superplastically foaming method for inclusion of closed pores in fully densified ceramics

Porous ceramics incorporate pores to improve material properties, including thermal insulation, while maintaining inherent ceramic properties such as corrosion resistance and high mechanical strength. Conventional porous ceramics are usually fabricated by insufficient sintering that excludes pores; thus, it must be terminated in an early stage to maintain high porosity. The premature termination leads to degraded strength and durability. We developed a superplastic-foaming method to fabricate ceramic foams in the solid state. In this method, the inserted foam agent evaporates after full densification of the matrix at the sintering temperature. Closed pores expand by superplastic deformation driven by the gas pressure. The pores are introduced after sintering the solid polycrystal. Then, only closed pores are introduced, improving the insulation of gas, sound and heat. The pore walls are fully densified for high mechanical strength. Compared to the melt-foaming method, this technique is practical because the fabrication temperature is far below the material's melting point, and it does not require moulds. In addition, the size and location of the pores can be controlled by the amount and position of the foam agent.