Thermal transformations in ultrafine plasmochemical zirconium dioxide powders

The methods of thermal analysis and X-ray diffraction are used to investigate polymorphic transformations taking place under heating and cooling in nonstabilized ultrafine ZrO2 powders (ZrO2 UFP) synthesized in a plasmochemical process. It is found out that ZrO2 UFP is characterized by an increased content (up to 55 mass%) of tetragonal-phase particles, which is associated with the size effect of its stabilization. Heating of UFP within T = (25–700) °C, which is followed by the release of H2O, CH2O, and CO2, does not result in a change in its structural-phase state, while annealing within the temperature interval T = (700–1,000) °C gives rise to an increased growth of the size of t-ZrO2 crystallites and results in an elevated tetragonality of the crystal lattice (c/a). A complete t-ZrO2 → m-ZrO2 transition occurs as a result of heating the powder up to T = 1,300 °C. The effect of the dimensional factor on temperature characteristics of polymorphic m ↔ t transitions and the value of their temperature hysteresis is established. It is shown that the powder particle size exerts the most pronounced influence on the temperature-dependent position of the point of martensitic transformation Ms. As this influence is increased, Ms is shifted toward the region of higher temperatures. This is followed by a decreased temperature hysteresis of the m ↔ t martensitic transformations.