Coupling effect of chromia doping and vacuum on the phase transition of alumina prepared by co-precipitation process

• Cr dopant inhibits the phase transformation to α-Al2O3 in air.
• Vacuum calcination can accelerate the transformation to α-Al2O3.
• Cr dopant coupled with vacuum significantly reduces the transition temperature.
• A novel mechanism was proposed to explain the coupling effect.

Synthesis of thermodynamically stable corundum structure alumina under lower temperature is of great significance in practical applications, exploration on new technique and the involved mechanism therefore becomes increasingly essential. In this work, the coupling effect and mechanism of chromia doping and vacuum condition on the phase transition of Al2O3 prepared via co-precipitation process were investigated and explored. It was found that chromia doping by itself did not promote the nucleation of α-Al2O3, or in some sense inhibited the transformation process. Cr doped γ-Al2O3 powders treated in a vacuum condition (10− 5 Pa) however showed a gradual transition to α-Al2O3 even at low temperature of 600 °C, demonstrating that surface nucleation sites introduced by a coupled effect of vacuum and Cr2O3 doping contributed to the dramatic reduction (200–400 °C) of the phase transformation temperature. Based on the comprehensive characterizations and conventional diffusional nucleation mechanism, it is concluded that the surface nucleation sites for phase change to α-Al2O3 increased significantly during vacuum calcination with the combination of template effects of Cr2O3. This finding may provide a new way in fabrication and understanding of stable alumina formation under relatively moderate temperature condition.

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