Phase Transition Of Zirconia Research Articles

Mechanism of the Cubic‐to‐Tetragonal Phase Transition in Zirconia and Yttria‐Stabilized Zirconia by Molecular‐Dynamics Simulation

We present a new interatomic potential capable of describing the cubic and tetragonal phases of zirconia. From an analysis of molecular‐dynamics simulations of the structural fluctuations in the cubic phase close to the transition temperature, we show that the cubic‐to‐tetragonal transition is displacive. In addition, the discontinuous change in volume and the latent heat at the transition demonstrate clearly that the transition is first order. The model correctly predicts that doping with yttria tends to stabilize the cubic phase at lower temperatures. A description of the first‐order nature of dopant‐driven transitions is given.

Temperature Dependence of the Raman Spectra and Phase Transition of Zirconia

A newly constructed high-temperature Ramanspectrometer was used to study the temperature-dependence Raman spectra (upto 2023 K) and transformation of zirconia crystal. High-temperature Ramanscattering is a useful tool in characterizing the different structures ofzirconia and offers the possibility of identifying the phase transformation. Itshows that monoclinic zirconia transforms to tetragonal phase at about 1440 Kduring the process of increasing temperature, but shows a lowertransformation temperature from tetragonal to monoclinic phase at about 1323 Kwhile the temperature decreases.

Free energy and molecular dynamics calculations for the cubic-tetragonal phase transition in zirconia

The high-temperature cubic-tetragonal phase transition of pure stoichiometric zirconia is studied by molecular dynamics (MD) simulations and within the framework of the Landau theory of phase transformations. The interatomic forces are calculated using an empirical, self-consistent, orthogonal tight-binding model, which includes atomic polarizabilities up to the quadrupolar level. A first set of standard MD calculations shows that, on increasing temperature, one particular vibrational frequency softens. The temperature evolution of the free-energy surfaces around the phase transition is then studied with a second set of calculations. These combine the thermodynamic integration technique with constrained MD simulations. The results seem to support the thesis of a second-order phase transition but with unusual, very anharmonic behavior above the transition temperature.

Investigation on the zirconia phase transition under irradiation

Zirconia, ZrO 2, produced by the oxidation of zirconium alloys in nuclear reactors, possesses a high stability under neutron irradiation. No amorphisation of yttrium-stabilised zirconia has been observed even at high dpa values (≈100 dpa). In pure monoclinic zirconia, a phase transition monoclinic → cubic (tetragonal) induced by irradiation has already been observed. The aim of this work is to study in detail the mechanism responsible for this transition. For that purpose, different kinds of irradiations with electrons (to study point defects) and low energetic ions (to study clusters due to collision cascades) have been performed on zirconia samples. A local probe (Raman spectroscopy) and a non-local probe (grazing X-ray diffraction) have been used to characterise the phase formed during irradiation, which is clearly the tetragonal phase. For the ionic implantation, the grazing X-ray diffraction permits to separate effects due to the ballistic collisions and the implantation peak. Using this method, it was possible to show that the profile of the tetragonal phase was only linked to the dpa profile. This result associated to the results obtained by the Raman spectroscopy (broadening of Raman peaks) shows that the phase transition may be induced by clusters formed near the collision cascades.

Metastable phase formation in the TiO2-ZrO2 and CdO-ZrO2 systems

Samples of zirconium oxide doped with titanium oxide or cadmium oxide, prepared as a divided state, were examined to investigate the effect of the foreign ions on the zirconia phase transition. Thermal treatments (1173 K in H 2 for TiO 2 -ZrO 2 : 1073 K in air for CdO-ZrO 2 ) caused the incorporation of titanium or cadmium in solid solution in the zirconia structure (tetragonal or cubic metastable phases). The solid solution formation hindered the tetragonal → monoclinic transformation that takes place in undoped ZrO 2 during the cooling step.

Metastable phase formation in the TiO 2ZrO 2 and CdOZrO 2 systems

Samples of zirconium oxide doped with titanium oxide or cadmium oxide, prepared as a divided state, were examined to investigate the effect of the foreign ions on the zirconia phase transition. Thermal treatments (1173 K in H 2 for TiO 2ZrO 2; 1073 K in air for CdOZrO 2) caused the incorporation of titanium or cadmium in solid solution in the zirconia structure (tetragonal or cubic metastable phases). The solid solution formation hindered the tetragonal → monoclinic transformation that takes place in undoped ZrO 2 during the cooling step.

Strain-induced destabilization of crystals: Lattice dynamics of the cubic-tetragonal phase transition in ZrO2.

The concept of the strain-induced interatomic tensions as the most general anharmonic factor responsible for mechanical instability of crystals is discussed. We employ this concept to examine the mechanism of the cubic-tetragonal phase transition in zirconia. According to the model prediction, the ${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$ phonon in the cubic phase would have a low frequency, and vanish when the interatomic separation diminishes. This supports the hypothesis that this phonon's condensation is responsible for the transition in question. The cubic-tetragonal-monoclinic evolution of the lattice and related phenomena are discussed in the light of the results obtained.

Pressure-induced structural phase transitions in zirconia under high pressure.

Angular-dispersive x-ray in situ powder-diffraction experiments have been performed on pure zirconia, ${\mathrm{ZrO}}_{2}$, at room temperature under high pressure up to 50 GPa. Under increasing pressure four phases were successively encountered: baddeleyite (monoclinic, P${2}_{1}$/c) from normal pressure up to about 10 GPa, orthorhombic-I (Pbca) to 25 GPa, orthorhombic-II to 42 GPa, and orthorhombic-III above 42 GPa. The unit-cell parameters and the volume have been determined as a function of pressure. The bulk moduli of the two lower pressure phases have been calculated using Birch's equation of state. The bulk modulus of baddeleyite, 95 GPa, is much lower than expected from bulk modulus-volume systematics, 195 GPa, while for the orthorhombic-I phase, the experimental and calculated values are almost identical. A generalized P-T diagram for ${\mathrm{ZrO}}_{2}$, including an orthorhombic-IV phase, is proposed and discussed. The phase transitions to orthorhombic-II and orthorhombic-III phases can be described by a simple rotation of the unit cell of the orthorhombic-I phase about either the b axis to form the orthorhombic-II phase or a axis to form the orthorhombic-III phase. All high-pressure cells (orthorhombic-I, -II, and -III) have eight formula units (Z=8). The orthorhombic-II phase was found not to have the cotunnite ${\mathrm{PbCl}}_{2}$-type structure which was proposed previously. There is no longer any example of a compound which transforms to such a cotunnite-type structure under high pressure. The behavior of zirconia and hafnia under high pressure is different although they have very close chemical properties at ambient pressure and identical structures in the two lower-pressure phases.

Some Observations on the Role of Dopants in Phase Transitions in Zirconia from Atomistic Simulations

Attempts to calculate the free energy of the polymorphs of zirconia are reported. The results suggest a theoretical basis for the phase transitions in terms of the removal of lattice instabilities (evidenced by imaginary phonon frequencies). Dopant stabilization of the higher symmetry structures is seen to arise because of changes to the phonon frequencies due to the different mass and effective charge of the dopant.

Phonons and phase transitions in zirconia

AbstractThe martensitic‐austenitic phase transition in zirconia was investigated as a function of temperature and pressure using Raman and IR spectroscopy. The frequencies and symmetries of the phonon modes corresponding to the monoclinic (M),C(4 molecules per unit cell) structure were identified at 300 K and 1 atm. The temperatureinduced transition to the tetragonal (TI),Dstructure (2 molecules per unit cell) showed thermal hysteresis (1225 ↔ 1425 K). The corresponding pressure‐induced transition at room temperature showed a pressure hysteresis (3.4 ↔ 3.95 GPa). However, the structural transition was to a different tetragonal modification (TII) withD2horC4vsymmetry with 4 molecules per unit cell.

A Phenomenological Theory of Structural Phase Transitions in Zirconia

The structural phase transitions in zirconia ZrO 2 are interpreted phenomenologically, based upon the Landau-type thermodynamic potential derived on the consideration of the change of translational as well as rotational symmetry of the unit cell. The difference between Chan's model and ours is mentioned.

High‐Pressure Phase Transitions in Zirconia and Yttria‐Doped Zirconia

Raman spectroscopy has been utilized to characterize the phase transformations and transition pressures in pure and doped zirconia containing 3, 4, and 5 wt% Y2O3. The pressure‐induced transformations were investigated to over 6 GPa (at room temperature) using a diamond anvil pressure cell. Pure zirconia single‐crystal samples transformed to a “new” tetragonal phase (different from the one obtained at high temperatures at atmospheric pressure) at about 4 GPa. The pressure transformation, like the temperature transition, was reversible and exhibited an approximately 0.45‐GPa hysteresis at room temperature. The 3 and 4 wt% Y2O3 crystals underwent a monoclinic (P21/b) to tetragonal (P42 nmc) phase transition similar to that observed at high temperatures. This phase change was found to be irreversible on releasing the pressure. The 5 wt% Y2O3 at atmospheric pressure consists of a tetragonal modification in a disordered cubic matrix; a gradual, but reversible, disordering transformation of the tetragonal precipitate takes place with pressure.