Crystallization and Transformation of Zirconia Under Hydrothermal Conditions

During constant-rate heating to 350°C in concentrated NaOH solutions, cubic ZrO₂ crystallized at ∼120°C from hydrated amorphous ZrO₂; these One cubic ZrO₂ particles abruptly changed into needlelike monoclinic ZrO₂ single crystals at 300°C. Crystallization and phase transformation were studied by XRD, TEM, and EPMA. Cubic ZrO₂ appears to crystallize via collapse of the ZrO₂−nH₂O structure and subsequent slight rearrangement of the lattice. The abrupt formation of mono-clinic ZrO₂ was considered to result when the very fine cubic ZrO₂ particles coagulated in a highly oriented fashion.     

Effect of YO1.5 Dopant on Unit-Cell Parameters of ZrO2 at Low Contents of YO1.5

The effect of YO_1.5 dopant on unit-cell parameters of ZrO₂ (YO_1.5=0 to 14.6 mol%) were examined by the X-ray whole-powder-pattern decom-position technique. The unit cell of monoclinic ZrO₂ has the largest expansion along the direction perpen-dicular to (100). The rate of increase of the unit-cell volume of monoclinic ZrO₂ with YO_1.5 content is greater than that of tetragonal ZrO₂ and comparable to that of cubic ZrO₂.     

Phase Relations in the System ZrO2-Y2O3 at Low Y2O3 Contents

Subsolidus phase relations in the low-Y₂O₃ portion of the system ZrO_2-Y₂O₃ were studied using DTA with fired samples and X-ray phase identification and lattice parameter techniques with quenched samples. Approximately 1.5% Y₂O₃ is soluble in monoclinic ZrO₂, a two-phase monoclinic solid solution plus cubic solid solution region exists to ∼7.5% Y₂O₃ below ∼500°C, and a two-phase tetragonal solid solution plus cubic solid solution exists from ∼1.5 to 7.5% Y₂O₃ from ∼500° to ∼1600°C. At higher Y₂O₃ compositions, cubic ZrO₂ solid solution occurs.     

Thermodynamic Assessment of the ZrO2─YO1.5 System

An optimal set of thermodynamic functions for the ZrO₂─YO_1.5 system are obtained using phase diagram and thermodynamic data. The liquid is described by a subregular solution model. Both cubic ZrO₂ and YO_1.5 solid solutions are regarded as one cubic solution, which is also treated as a subregular solution. The ordered Zr₃Y₄O₁₂ phase is treated as a stoichiometric compound. A regular solution model is applied to the other solid solutions. Tentative equilibrium boundaries between monoclinic and tetragonal ZrO₂ solid solutions are evaluated from information about the T ₀ line. The calculated phase diagram and thermodynamic functions agree well with experimental data.     

Comparative Lattice-Dynamical Study of the Raman Spectra of Monoclinic and Tetragonal Phases of Zirconia and Hafnia

An interpretation of the Raman spectra of monoclinic ZrO₂ and monoclinic HfO₂ is made by analyzing the results of the zirconia–hafnia substitution jointly with a lattice dynamical treatment of both structures. The Raman spectra of tetragonal ZrO₂ and tetragonal HfO₂ are also interpreted. Emphasis is put on their relations to the spectrum of the parent cubic structure and on the position of the soft mode. The band assignment proposed earlier by other researchers is critically reconsidered.     

Phase Analysis in Zirconia Systems

Linear calibration curves were developed for determining the content of free ZrO₂ in partially stabilized zirconia ceramics by X-ray diffraction techniques. Two methods were studied. The matrix method, in which free ZrO₂ was considered to be distributed in a matrix (the cubic phase), gave approximately equal mass absorption coefficients for the monoclinic and cubic phases. The polymorph technique, in which the cubic phase was considered to be a polymorph of ZrO₂ and in which integrated intensities were used, gave the better results.     

Microstructures of Simulated Nuclear Waste

The microstructures of simulated tailored ceramic forms for the Idaho Chemical Processing Plant (ICPP) high-level waste were characterized by TEM. The ceramic forms were consolidated from simulated ICPP waste calcines with either silicayttria-based or silica–lithia-based additives by hot isostatic pressing. The hot isostatically pressed ceramic waste forms are composed of cubic CaF₂, monoclinic ZrO₂, stabilized cubic ZrO₂, tetragonal ZrSiO₄, and amorphous silicate phases which can be phase separated. The phaseseparated glass is a result of phase immiscibility in the soda aluminosilicate glass to the compositions of albite and mullite.     

Effect of MgO Addition on the Microstructure Development of 3 mol% Y2O3—ZrO2

MgO addition to 3 mol% Y₂O₃–ZrO₂ resulted in enhanced densification at 1350°C by a liquid-phase sintering mechanism. This liquid phase resulted from reaction of MgO with trace impurities of CaO and SiO₂ in the starting powder. The bimodal grain structure thus obtained was characterized by large cubic ZrO₂ grains with tetragonal ZrO₂ precipitates, which were surrounded by either small tetragonal grains or monoclinic grains, depending on the heat-treatment schedule.     

Phase Relations and Ordering in the System ZrO2-MgO

The Phase relationships in the system ZrO₂-MgO were reinvestigated over a wide range of temperatures and compositions. The extent of the cubic solid solution field was determined with precise lattice parameter measurements and a high-temperature X-ray furnace using analyzed samples. DTA results show that the addition of MgO to ZrO₂ decreases the transition temperature for monoclinic ⇌ tetragonal ZrO₂ and 1 mol% of MgO is soluble in the monoclinic zirconia at ∼1070°C.The invariant eutectiod point is at 13.5 ± 0.3 mol% MgO at 1406°± 7°C, which is in fair agreement with previous results by Grain. The ordered phase Mg₂Zr₅O₁₂ (δ-phase) can form metastably in cubic solid solutions at temperatures as low as 800°C after prolonged annealing. Evidence for the existence of the ordered phase MgZr₆O₁₃(γ-phase) was obtained by electron diffraction technique. Conditions for the formation of this phase are described. The ordered phases in this system are metastable and their formation is an intermediate step in the eutectoid decomposition of the cubic phase.     

Revised Phase Diagram of the System ZrO2-CeO2 Below 1400°C

The phase diagram of the system ZrO₂-CeO₂ was rein-vestigated using hydrothermal techniques. Cubic, tetragonal, and monoclinic solid solutions are present in this system. The tetragonal solid solution decomposes to monoclinic and cubic solid solutions by a eutectoid reaction at 1050°50°C. The solubility limits of the tetragonal and cubic solid solutions are about 18 and 70 mol% CeO₂, respectively, at 1400°C, and about 16 and 80 mol% CeO₂, respectively, at 1200°C. Solubility limits of the monoclinic and cubic solid solutions are about 1.5 and 88 mol% CeO₂ at 1000°C, and 1.5 and 98 mol% CeO₂ at 800°C, respectively. The compound Ce₂Zr₃O₁₀ is not found in this system.     

Precipitation in Partially Stabilized Zirconia

Transmission electron microscopy was used to study the sub-structure of partially stabilized ZrO₂ (PSZ) samples, i.e. 2-phase "alloys" containing both cubic and monoclinic modifications of zirconia, after various heat treatments. Monoclinic ZrO₂ exists as (1) isolated grains within the polycrystalline aggregate (a grain-boundary phase) and (2) small plate-like particles within cubic grains. These intragranular precipitates are believed to contribute to the useful properties of PSZ via a form of precipitation hardening. These precipitates initially form as tetragonal ZrO₂, with a habit plane parallel to the {100} matrix planes. The orientation relations between the tetragonal precipitates and the cubic matrix are
and      

Polymorphic Behavior of Thin Evaporated Films of Zirconium and Hafnium Oxides

Polymorphism in thin evaporated films of zirconium and hafnium oxides was investigated from 100° to 1500°C by electron diffraction and transmission electron microscopy. The films have metastable cubic structures at room temperature and at moderate temperatures. Zirconium oxide, depending on temperature, exists in cubic, tetragonal, and monoclinic forms, whereas hafnium oxide transforms directly from the cubic to the monoclinic structure. The transformation temperatures depend on the oxygen partial pressure. Air annealing of thin films of ZrO₂ and HfO₂ lowered the temperature of transformation of the tetragonal and the cubic structure into the monoclinic structure by about 150° and 100°C, respectively. The cubic/tetragonal transformation of ZrO₂ is monotropic, whereas the tetragonal monoclinic transformation occurs by the typical nucleation and growth mechanism. Determination of grain size in ZrO₂ at the tetragonal/monoclinic transformation temperature showed that the transformation occurs when a constant grain size of about 800 Å is reached. The oxygen partial pressure, grain size, and temperatures at which the metastable phases exist were correlated. The rate of grain growth is enhanced by increase in oxygen partial pressure. The accelerated transformation in air is attributed to rapid attainment of the critical size; grain boundary energy is an important controlling factor in transformation.     

Structural Changes of Scandia-Doped Zirconia Solid Solutions: Rietveld Analysis and Raman Scattering

Powder X-ray diffractometry (XRD) and Raman scattering measurements were used to study the structural changes of compositionally homogeneous metastable ZrO₂ solid solutions induced by ScO_1.5 doping. The crystal structures of monoclinic, tetragonal, cubic, and rhombohedral (Sc₂Zr₇O₁₇, ß-phase) solid-solution phases have been refined by using the Rietveld analysis of the XRD data at room temperature of arc-melted ZrO₂- X ScO_1.5 ( X = 0, 2, …, 20, and 22 mol%) samples. The results can be interpreted as indicating that the structures of the monoclinic and tetragonal phases approach those of the tetragonal and cubic phases, respectively, by ScO_1.5 doping. Raman scattering, as well as XRD, was useful to investigate the phase assemblage. Moreover, we could obtain Raman spectra of the ß-phase probably for the first time.     

Phase Relations in the ZrO2-MgO System

Phase relations were studied in the system ZrO₂-MgO with emphasis on the range 1350° to 1600°C. A phase relation was determined from samples, using precision lattice parameters, X-ray diffraction line intensities, and petrographic observations, and from dynamic observations of the phases present using high-temperature X-ray diffraction techniques. Limits were established for the solubility of MgO in tetragonal ZrO₂ and for the range of the cubic solid solution. The phase relations below 1240°C were complicated by hysteresis in the monoclinic to tetragonal inversion of ZrO₂.     

Preparation of Fully Cubic Calcium-Stabilized Zirconia With 10 mol% Calcium Oxide Dopant Concentration by Microwave Processing

A novel, fast, and cheaper method has been developed for the synthesis of fully cubic calcium-stabilized zirconia (ZrO₂) of composition Ca_0.1Zr_0.9O_1.9 by dissolution of calcium oxide in monoclinic ZrO₂ for the first time using microwave energy. In this process, the precursors have been prepared by the mixed-oxide method taking the constituents in their stoichiometric ratio. The samples have been allowed to absorb microwave radiation in the presence of a polymeric susceptor. The susceptor absorbs the microwave radiation at room temperature and increases the temperature of the sample, where it starts interacting with microwave radiation. The susceptor burns off at a higher temperature without reacting with the sample. The cubic Ca_0.1Zr_0.9O_1.9 has been prepared at a temperature of 1100°C within 5 min.     

Formation of Acicular Monoclinic Zirconia Particles under Hydrothermal Conditions

Acicular monoclinic ZrO₂, particles were prepared by hydrothermal treatmentat 250°C using sulfuric acid solutions containing zirconium ions. The formation process and morphology of the ZrO₂, particles were investigated. Two types of acicular monoclinic Zr0₂, particle morphologies were obtained, both elongated in the 〈001〉 direction, and the range of acicular ZrO₂, particle sizes changedfrom 0.3 to 1.3 μ m with hydrothermal conditions. Addition of MgSO₄, to the starting solution promoted the crystallization of the monoclinic ZrO₂, particles.     

Toughening by Monoclinic Zirconia

The toughening induced by monoclinic ZrO₂ in the absence of microcracking was investigated, using ZnO as the host material. Toughness levels K_c in excess of the host toughness K_c^M were achieved, attaining a peak toughness K_c/K_c^M ∼1.7, at monoclinic ZrO₂ volume concentrations 0.2. This toughening is attributed to crack/particle interactions, associated with the deflection and bowing of the crack by the residual strain field around the monoclinic ZrO₂ particles.     

The Zirconia–Hafnia System: DTA Measurements and Thermodynamic Calculations

The equilibrium temperature ( T ₀) at which the tetragonal and monoclinic phases of either ZrO₂ or HfO₂ coexist is generally defined by the middle temperature of A _s (the onset transformation temperature on heating) and M _s (the onset transformation temperature on cooling). It cannot be directly determined due to the athermal nature of the martensitic transformation. Practically, the determination of T ₀ is important for the prediction of A _s and M _s in ZrO₂ or HfO₂-based materials. In this work, the ZrO₂–HfO₂ system was studied experimentally by differential thermal analysis (DTA) to obtain the martensitic tetragonal ⇔ monoclinic transformation temperatures in the temperature range of 1273–1973 K. The T ₀ temperatures obtained for ZrO₂ and HfO₂ are 1367±5 and 2052±5 K, respectively. They are adopted for the assessments of the Gibbs energy parameters of these two oxides. A reasonably calculated ZrO₂−HfO₂ phase diagram is presented.     

Structural Evolution and Electrical Properties of Sc2O3-Stabilized ZrO2 Aged at 850°C in Air and Wet-Forming Gas Ambients

X-ray diffraction (XRD) and electron microscopy investigations have been performed on Sc₂O₃-stabilized ZrO₂ as-sintered and after aging in air or in wet-forming gas at 850°C for 1000 h. Some tetragonal to monoclinic transformation had occurred in the near-surface regions of 4 mol% Sc₂O₃ samples after aging; the phase transition was more severe for samples aged in the forming gas ambient. A decrease of ∼20% in electrical conductivity accompanied the aging. In 6 mol% Sc₂O₃ samples, although no cubic to tetragonal transformation was detected, both the electrical conductivity and the activation energy for ionic conductivity decreased significantly during aging. Ten mole percent Sc₂O₃ samples did not show appreciable change in electrical conductivity due to aging, although some near-surface cubic to rhombohedral transformation did occur. Sharpening of the (400)_t XRD peak of Sc₂O₃-stabilzed tetragonal ZrO₂ accompanies the change(s) in the electrical conductivity.     

An Unusual Twin Structure in Transformed Precipitates in Y-PSZ Single Crystals

Mosaic-like monoclinic ZrO₂ particles form in TEM thin foils in 3.4-mol%-Y₂O₃-partially-stabiIized ZrO₂ (Y-PSZ) single crystals aged for 150 h at 1600°C. These particles transformed martensitically during TEM foil preparation; the parent phase existed as internally twinned tetragonal ZrO₂ precipitates which had formed during aging. These monoclinic particles contain (100)_m and {110}_m transformation twins, and the twin variants have uniform thickness. The origin of this transformation microstructure, the lattice correspondences, and the applicability of the invariant plane strain model of martensitic transformations are discussed.