Rapid formation of nanocrystalline HfO2 powders from amorphous hafnium hydroxide under ultrasonically assisted hydrothermal treatment

Peculiarities of hafnium hydroxide hydrothermal decomposition were studied by in situ heat flux calorimetry for the first time. It was shown that this process occurs in one exothermal stage (ΔH = −17.95 kJ mol−1) at 180–250 °C resulting in complete crystallization of amorphous phase with formation of pure monoclinic HfO2. It was found that the rate of m-HfO2 formation can be significantly increased by combining hydrothermal treatment with simultaneous ultrasonic activation.