Thermal analyses of bulk amorphous oxides and silicates of zirconium and hafnium

Amorphous pure and Y-doped ZrO₂ and HfO₂ were prepared by precipitation with ammonia or hydrazine. Amorphous zirconium and hafnium silicates with SiO₂ content from 10 to 90 mol % were prepared by sol-gel. Crystallization was studied by differential scanning calorimetry (DSC) at 20 °C/min and crystallite size after crystallization was determined from XRD data. ZrO₂ crystallized into the tetragonal phase with ΔH -21 ±2 kJ/mol and HfO₂ into the monoclinic phase with ΔH -31 ±2 kJ/mol. Doping with 20 at% Y decreased crystallite size after crystallization. Crystallization temperatures for pure and Y-doped ZrO₂ samples were in the range 420-440 °C. Crystallization temperatures for pure and Y-doped HfO₂ samples varied from 470 to 570°C and correlate with surface area. Crystallization onset temperature in silicates increased with silica content from about 650 to 950 °C for ZrO₂·SiO₂ and from 740 to 1030 °C for HfO₂·SiO₂. Tetragonal zirconia and hafnia were the only crystalline phases formed below 1100 °C in all zirconium silicates and in hafnium silicates with more than 10 mol% SiO₂. Crystallite size after crystallization decreased with increase in silica content. In hafnium silicate, a decrease in HfO₂ crystallite size from 5 to 2.5 ±1 nm corresponds to a crystallization enthalpy change from -22 to -15 ±2 kJ/mol. The tetragonal HfO₂/amorphous SiO₂ interface energy can be calculated from calorimetric data as -0.25 J/m². The critical particle size for the tetragonal to monoclinic transformation of HfO₂ in HfO₂-SiO₂ system is about 6 nm. We predict that tetragonal HfO₂ will be stabilized in films thinner than 2 nm.