TEM studies on phases and phase stabilities of zirconia ceramics (original) (raw)
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TEM-Studies of Phases and Phase Stabilities in Zirconia Ceramics
Physica B 150 (1988) 86-98
Transmission electron microscopy (TEM) is a powerful tool to study defects and structures of materials. A comparison between quantitative evaluation of micrographs and diffraction patterns with results of contrast simulations allows a quantitative determination of many parameters. TEM is applied to study phase stability and phase transformations in ZrO 2. Number densities of point defects can be evaluated for cubic zirconia, the nucleation of stable m-ZrOz at stress singularities of grain boundaries can be studied in situ. A quantitative evaluation of displacement fields in m-ZrO 2 which exist close to terminating twins can be measured and stress concentrations can be evaluated. The information is useful for a better understanding of phase stability in ZrOz.
Effect of the energy deposition modes on the structural stability of pure zirconia
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2008
One of the most important goals in materials science is to be able to design materials with specific properties. Irradiation seems to be a powerful tool for the design of advanced materials because of its ability to modify over different scales the microstructure of solids. Nowadays, it is clearly proved that irradiation induces order-disorder phase transitions in metallic alloys and in some ceramics. Recent investigations on pure monoclinic zirconia have clearly shown that a displacive phase transition can be induced by irradiation. In this work, the impact of the energy deposition modes on the structural stability of pure monoclinic ZrO 2 is discussed in detail. Based on experimental evidences, a microscopic model is proposed to explain the displacive phase transition observed in this material after irradiation by low and high energy ions within the Landau theory framework. Even if defects generated by low and high energy ions are quite different, these defects are able to quench the same tetragonal phase in pure zirconia.
A study of phase transformation at the surface of a zirconia ceramic
2014
applications is the remarkable fracture toughness of YPSZ that arises from its ability to undergo martensitic transformation, a phase transformation that is dependent on stress, temperature, time, humidity, grain size, and the proximity of an interface. The present study was aimed at revealing the influence of the thermal ageing on the tetragonal to monoclinic phase transformation in the near-surface regions of YPSZ. In order to perform qualitative and quantitative characterisation of the phase composition, three principal microscopic techniques were employed: atomic force microscopy, depth resolved Raman micro-spectroscopic scanning, and synchrotron X-ray diffraction. Satisfactory consistency was achieved between the results obtained using different techniques. Moreover, the data obtained in this way displayed complementarity that provided valuable input for the development of thermodynamic modelling of the complex interdependence between phase state and processing history of zirco...
A structural study of metastable tetragonal zirconia in an Al2O3-ZrO2-SiO2-Na2O glass ceramic system
Journal of Materials Science, 1980
The structural and microstructural properties (crystalline system at the beginning of crystallization, lattice disorder and crystallite size) of metastable zirconia have been studied by an X-ray line broadening analysis using simplified methods based on suitably assumed functions describing the diffraction profiles. Metastable tetragonal zirconia has been crystallized at 970, 1000 and 1050 ~ C, respectively, starting from an AI203-ZrO=-SiO2-Na20 glassy system with a chemical composition very close to that of well known electromelted refractory materials. In the present work we have definitely shown the presence, inside the crystallized zirconia phase, of internal microstrains having values ranging approximately between 2 and 4 • 10-3. Moreover, we have confirmed the peculiar smallness in size of precipitated zirconia crystallites (~< 200A). Therefore, in the present system, the stabilization of the tetragonal form of ZrO2 with respect to the stable monoclinic one can be explained in terms of a contribution to the amount of free energy due to strain energy, in addition to the previously hypothesized surface energy. The observed strong line broadening for some samples treated at lower temperatures (970 and 1000 ~ C) gives rise to an apparent cubic lattice pattern; but the asymmetry of each apparent single line masks unequivocally a tetragonal doublet. This latter conclusion disagrees with some hypotheses on the existence of a cubic metastable form of ZrO2 which could originate at the beginning of zirconia crystallization.
Mechanisms of room temperature metastable tetragonal phase stabilisation in zirconia
International Materials Reviews, 2005
Mechanisms of tetragonal phase stabilisation, at room temperature, in nanocrystalline (,100 nm), submicrometre-sized (100 nm-1 mm), and bulk zirconia (ZrO 2) (.1 mm) are reviewed in detail. The merits, demerits and scope of each individual model are outlined. The analysis of the literature shows that, although the mechanism of tetragonal phase stabilisation in bulk ZrO 2 is well understood, the room temperature tetragonal phase stabilisation mechanism in undoped, nanocrystalline ZrO 2 is controversial. Various proposed models, based on surface energy (nanocrystallite size), strain energy, internal and external hydrostatic pressure, structural similarities, foreign surface oxides, anionic impurities, water vapour and lattice defects (oxygen ion vacancies), are discussed in detail. It is proposed that generation of excess oxygen ion vacancies within the nanocrystalline ZrO 2 is primarily responsible for the room temperature tetragonal phase stabilisation, below a critical size. Hence, the mechanism of tetragonal phase stabilisation in nanocrystalline ZrO 2 appears to be the same as that in doped ZrO 2 (at room temperature) and undoped ZrO 2 (at higher temperature).
Metallographic Observation of the Monoclinic-Tetragonal Phase Transformation in ZrO2
Journal of the American Ceramic Society, 1965
The monoclinic-tetragonal phase transformation of ZrOn was examined with a vacuum hot-stage microscope. Polished sections of vacuumsintered 99.7y0 pure ZrOp were observed from room temperature to 13OO0C. The rapid formation of platelet substructure within ZrOB grains in the temperature range 1050" to 115OOC was associated with the heating transformation. Photomicrographs and motion pictures were taken of the specimen as the transformation progressed. The surface deformation was irreversible, preventing observation of the phase inversion on cooling. From these hot-stage studies, supplementary DTA, X-ray, and thermal expansion data, and other existing information, it is concluded that the transformation is of the diffusionless, athermal type, characteristic of Fe-Ni martensitic transformations.
Grain-size/( t ″ or c )-phase relationship in dense ZrO 2 ceramics
Journal of the European Ceramic Society, 2016
Zirconia-based materials are widely investigated and used as electrolytes in solid-oxide fuel cells, oxygen sensors and electrochemical devices. These materials present polymorphism, which has a critical effect on their technologically important properties. The polymorphism is influenced by, among other factors, aliovalent dopant nature and content, grain size and interfacial energy. In this work, we investigated the crystal structure of ZrO 2-12 mol% CaO and −9 mol% Y 2 O 3 dense ceramics as a function of grain size. We found that the samples undergo a phase transition from the t form of the tetragonal phase to the cubic phase with an increase in grain size. This transition is directly detected by Raman spectroscopy and further evidence is given by a change in the activation energy for bulk ionic conduction. The transition occurs at an average grain size greater than 500 nm for both systems.
Journal of Applied Physics, 2008
Local environment surrounding Zr atoms in the thin films of nanocrystalline zirconia ͑ZrO 2 ͒ has been investigated by using the extended x-ray absorption fine structure ͑EXAFS͒ technique. These films prepared by the ion beam assisted deposition exhibit long-range structural order of cubic phase and high hardness at room temperature without chemical stabilizers. The local structure around Zr probed by EXAFS indicates a cubic Zr sublattice with O atoms located on the nearest tetragonal sites with respect to the Zr central atoms, as well as highly disordered locations. Similar Zr local structure was also found in a ZrO 2 nanocrystal sample prepared by a sol-gel method. Variations in local structures due to thermal annealing were observed and analyzed. Most importantly, our x-ray results provide direct experimental evidence for the existence of oxygen vacancies arising from local disorder and distortion of the oxygen sublattice in nanocrystalline ZrO 2 . These oxygen vacancies are regarded as the essential stabilizing factor for the nanostructurally stabilized cubic zirconia.