Energy efficient synthesis of porous ZrO2 with fine closed pores by microwave irradiation

Partially stabilized ZrO2 balls with a diameter of 5 mm and a relative density of over 99%, containing 3.7 mass% Y, 0.11 mass% Si, 590 ppm of Ti, 580 ppm of P, and 150 ppm of Ca as impurities were used. When the ZrO2 balls were placed in a BN crucible and heated by microwave irradiation at 2.45 GHz to 1400  °C for 10 min, each ball expanded by 25% in volume due to the formation of numerous fine closed pores and connected with strong linkage and strong bonds formed between the balls. The surface of each ZrO2 ball was a dense layer approximately 100 μm thick, and closed pores 5–20 μm in diameter formed inside the balls. After heating, the open pore porosity was 2% and the closed pore porosity was 22.2% (total: 24.2%). Superplastic behavior of ZrO2 and generated gas species in the system P–O with a high pressure during heating contribute to show the strong bonds and the closed pores, respectively.

► Porous ZrO2 could be fabricated by a microwave irradiation.
► Most of the pores were closed pores.
► Superplasticity and generated gases during heating could make the micromorphology.
► Thermodynamics consideration for the generated gaseous species was performed.
► When irradiated with microwave, the total energy consumption decreased to one tenth.