Low-Temperature Structural Phase Transition in ZrO2–12 mol% CeO2 Studied by Infrared Spectroscopy and X-ray Diffractometry

The low-temperature tetragonal ( t ) to monoclinic ( m ) phase transition in ZrO₂–12 mol% CeO₂ was studied by Fourier-transform infrared spectroscopy and X-ray diffractometry (XRD). When the ZrO₂–12 mol% CeO₂ was cooled, the t → m phase transition occurred at about 120 K; this caused significant changes in the infrared reflectance spectra and X-ray diffractograms. As the phase transition proceeded, "difference spectra" obtained by subtracting the infrared spectra before and after the phase transition revealed two infrared modes at 575 and 740 cm⁻¹ which were assigned to the m phase. The volume fraction of the m phase (determined by XRD) and the subtraction coefficient (defined such that the difference spectra reproduced the infrared spectrum of m -ZrO₂) were consistent with each other; this revealed a marked discontinuity at about 120 K. Full width at half maximum of the ( 1 11) _m and (111) _t reflections also indicated a similar discontinuity at about 120 K.     

Low-Temperature Phase Equilibria by the Flux Method and the Metastable–Stable Phase Diagram in the ZrO2–CeO2 System

Low-temperature phase equilibria ranging from 1000° to 1200°C in the ZrO₂–CeO₂ system were investigated by annealing compositionally homogeneous ZrO₂–CeO₂ solid solutions in a Na₂B₂O₇.1 NaF flux. The 5 mol% CeO₂ samples decomposed into monoclinic ( m ) and tetragonal ( t ) phases during annealing at 1100°2 and 1120°C, and the t -phase transformed diffusionlessly into monoclinic ( m ') symmetry during quenching. A eutectoid reaction, t → ( m + c ), was confirmed to occur at 1055°± 10°C, where the equilibrium compositions of the t -, m -, and c -phases were 11.2 ± 2.8, 0.9 ± 0.9, and 84 ± 1 mol% CeO₂, respectively. The equilibrium phase boundaries were almost independent of the annealing time and/or the flux:sample ratio, which indicates that the flux accelerates the reaction rate withouts affecting the equilibration. The previous data are discussed using metastable–stable phase diagrams. The discrepancies of the low-temperature phase diagram in the literature are attributable to either regarding the metastable phase boundaries as stable ones or ignoring the sluggish kinetics.     

Fracture Strength–Fracture Resistance Response and Damage Resistance of Sintered Al2O3–ZrO2 (12 mol% CeO2) Composites

The fracture strengths of sintered Al₂O₃ containing 20 and 40 vol% ZrO₂(12 mol% CeO₂)—zirconia-toughened alumina (ZTA)—composites along with the fracture resistance can be increased (e.g., to ∼900 MPa and >12 Mpa·m^1/2, respectively), by increasing the mean grain size of the t -ZrO₂ (and the Al₂O₃) from ∼0.5 μm to ∼3 μm. At these lower t -ZrO₂ contents, the fracture strength-fracture resistance curves show a continuous rise as opposed to the strength maxima observed in polycrystalline t -ZrO₂(12 mol% CeO₂), CeTZP, and ZrO₂(12 mol% CeO₂) ceramics containing ≤20 vol% Al₂O₃. The toughened composites also exhibit excellent damage resistance with fracture strengths of 500 MPa retained with surfaces containing ∼150- N Vickers indentations which produce cracks of ∼160-μm radius. Greater damage resistance correlates with an increase in the apparent R -curve response of these composites.     

Cubic-to-Tetragonal Displacive Transformation in Gd2O3–Bi2O3 Ceramics

Gd₂O₃-doped Bi₂O₃ polycrystalline ceramics containing between 2 and 7 mol% Gd₂O₃ were fabricated by pressureless sintering powder compacts. The as-sintered samples were tetragonal at room temperature. Hightemperature X-ray diffraction (XRD) traces showed that the samples were cubic at elevated temperatures and transformed into the tetragonal polymorph during cooling. On the basis of conductivity measurements as a function of temperature and differential scanning calorimetry (DSC), the cubic → tetragonal as well as tetragonal → cubic → teansition temperatures were determined as a function of Gd₂O₃ concentration. The cubic → tetragonal transformation appears to be a displacive transformation. It was observed that additions of ZrO₂ as a dopant, which is known to suppress cation interdiffusion in rare-earth oxide–Bi₂O₃ systems, did not suppress the transition, consistent with it being a displacive transition. Annealing of samples at temperatures 660°C for several hundred hours led to decomposition into a mixture of monoclinic and rhombohedral phases. This shows that the tetragonal polymorph is a metastable phase.     

Thin-Foil Phase Transformations of Tetragonal ZrO2 in a ZrO2-8 wt% Y2O3 Alloy

Tetragonal zirconia ( t -ZrO₂) grains in an annealed ZrO₂ 8 wt% Y₂O₃ alloy transformed to orthorhombic ( o ) or monoclinic ( m ) symmetry by stresses induced by localized electron beam heating in the transmission electron microscope. Different transformation mechanisms were observed, depending on foil thickness and orientation of individual grains. In thicker grains (≥150 nm), the transformation proceeded by a burst-like growth of m laths, and this is believed to approximate bulk behavior. In thinner grains near the edge of the foil, usually those with a [100], orientation perpendicular to the thin-foil surface, "continuous" growth of an o or m phase with an antiphase-boundary-containing microstructure was observed. The o phase is believed to be a high-pressure poly-morph of ZrO₂, which forms (paradoxically) as a thin-foil artifact because it is less dense than t -ZrO₂, but more dense than m -ZrO₂. In some very thin grains, the t → m transformation was thermoelastic. Furthermore, a mottled structure often occurred just before the t → m or t → o transformation, which is attributed to surface transformation. Aside from the lath formation, the observed transformation modes are a result of the reduced constraints in thin foils.     

Residual α–Si3N4 in O' Crystals in CeO2-Doped O'+β' SiAlON Ceramics

The microstructure of a pressureless sintered (1605°C, 90 min) O'+β' SiAlON ceramic with CeO₂ doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O' grains (20–30 μm long), and a continuous matrix of small rodlike β' grains (< 1.0 μm in length). Many α-Si₃N₄ inclusions (0.1–0.5 μm in size) were found in the large O' grains. CeO₂-doping and its high doping level as well as the high Al₂O₃ concentration were thought to be the main reasons for accelerating the reaction between the α-Si₃N₄ and the Si-Al-O-N liquid to precipitate O'–SiAlON. This caused the supergrowth of O' grains. The rapid growth of O' crystals isolated the remnant α–Si₃N₄ from the reacting liquid, resulting in a delay in the α→β-Si₃N₄ transformation. The large O' grains and the α-Si₃N₄ inclusions have a pronounced effect on the strength degradation of O'+β' ceramics.     

Sintering, Microstructure, Hardness, and Fracture Toughness Behavior of Y2O3-CeO2-ZrO2

A wet-chemical approach is applied to derive fine powders with compositions 11 mol% CeO₂-ZrO₂, 1 mol% YO_1.5-10 mol% CeO₂-ZrO₂, 12 mol% CeO₂-ZrO₂, and 2 mol% YO_1.5-10 mol% CeO₂-ZrO₂ by the coprecipitation method. The characteristics of the as-derived powders are evaluated through thermal analysis and electron microscopy. The sintering behavior of the calcined powders is carried out at 1400° and 1500°C for 1 to 10 h. Sintered density higher than 98% of theoretical is achieved for sintering at 1400°C for several hours. The as-sintered density dependence on the sintering condition is related to the extent of tetragonal-to-monoclinic phase transformation as well as the associated microcracks. Partial substitution by Y₂O₃ in CeO₂-ZrO₂ results in reduced grain size and tends to stabilize the tetragonal structure. Y₂O₃ is more effective than CeO₂ with respect to the grain size refinement and tetragonal stability. In addition, Y₂O₃ substitution in CeO₂-ZrO₂ increases the hardness, while it decreases the fracture toughness.     

Transformation and Microcrack Toughening as Complementary Processes in ZrO2-Toughened Al2O3

Both tetragonal ( t ) and monoclinic ( m ) ZrO₂ particles in ZrO₂-toughened Al₂O₃ can give rise to toughening. In the stress field of propagating cracks, the t -ZrO₂ particles can undergo the stress-induced t → m transformation, and the residual stresses around already-transformed m -ZrO₂ particles can cause microcracking. The t -ZrO₂ particles transformed in crack tip stress fields do not, however, also cause appreciable microcracking. The toughening increments via these distinct mechanisms are comparable. It appears that optimally fabricated Zr0₂-toughened Al₂O₃'s should contain a mixture of t - and m -ZrO₂.     

Effect of Nd2O3 Concentration on the Defect Structure of CeO2–Nd2O3 Solid Solution

An extensive X-ray study of CeO₂–Nd₂O₃ solid solutions was performed, and the densities of solid solutions containing various concentrations of NdO_1.5 were measured using several techniques. Solid solutions containing 0–80 mol% NdO_1.5 were synthesized by coprecipitation from Ce(NO₃)₃ and Nd(NO₃)₃ aqueous solutions, and the coprecipitated samples were sintered at 1400°C. A fluorite structure was observed for CeO₂–NdO_1.5 solid solutions with 0–40 mol% NdO_1.5, which changed to a rare earth C-type structure at 45–75 mol% NdO_1.5. The change in the lattice parameters of CeO₂–NdO_1.5 solid solutions, when plotted with respect to the NdO_1.5 concentration, showed that the lattice parameters followed Vegard's law in both the fluorite and rare earth C-type regions. The maximum solubility limit for NdO_1.5 in CeO₂ solid solution was approximately 75 mol%. The relationship between the density and the Nd concentration indicated that the defect structure followed the anion vacancy model over the entire range (0–70 mol% NdO_1.5) of solid solution.     

Influence of ZrO2 Grain Size and Content on the Transformation Response in the Al2O3─ZrO2 (12 mol% CeO2) System

Based on experimental and modeling studies, the rate of increase in the martensite start temperature M _s for the tetragonal-to-monoclinic transformation with increase in zirconia grain size is found to rise with decrease in ZrO₂ content in the zirconia-toughened alumina ZTA system. The observed grain size dependence of M _s can be related to the thermal expansion mismatch tensile (internal) stresses which increase with decrease in zirconia content. The result is that finer zirconia grain sizes are required to retain the tetragonal phase as less zirconia is incorporated into the alumina, in agreement with the experimental observations. At the same time, both the predicted and observed applied stress required to induce the transformation are reduced with increase in the ZrO₂ grain size. In addition, the transformation-toughening contribution at temperature T increases with increase in the M _s temperature brought about by the increase in the ZrO₂ grain size, when T > M _s. In alumina containing 20 vol% ZrO₂ (12 mol% CeO₂), a toughness of ∼10 MPa. √m can be achieved for a ZrO₂ grain size of ∼2 μm ( M _s∼ 225 K). However, at a grain size of ∼2 μm, the alumina–40 vol% ZrO₂ (12 mol% CeO₂) has a toughness of only 8.5 MPa. √m ( M _s∼ 150 K) but reaches 12.3 MPa. ∼m ( M _s∼ 260 K) at a grain size of ∼3 μm. These findings show that composition (and matrix properties) play critical roles in determining the ZrO₂ grain size to optimize the transformation toughening in ZrO₂-toughened ceramics.     

In Situ Martensitic Transformation in a Ternary MgO-Y2O3-ZrO2 Alloy: II, Transformation in Tetragonal ZrO2 Precipitates

The stress-induced martensitic transformation of t -ZrO₂ precipitates in a ternary MgO-Y₂O₃-ZrO₂ alloy has been studied in situ in the transmission electron microscope. The transformation occurs autocatalytically and takes place by piecewise growth of two twin-related m -ZrO₂ variants. Unloading causes retransformation of partially transformed precipitates, but this reverse ( m → t ) transformation of fully transformed precipitates only occurs on heating. The martensitic transformation in this system is clearly thermoelastic .     

Raman Scattering Study of Cubic–Tetragonal Phase Transition in Zr1−xCexO2 Solid Solution

A structural phase transition between the cubic (space group, Fm 3 m) and tetragonal (space group, P 4₂ /nmc) phases in a zirconia–ceria solid solution (Zr_1−xCe_xO₂) has been observed by Raman spectroscopy. The cubic–tetragonal ( c–t" ) phase boundary in compositionally homogeneous samples exists at a composition X₀ (0.8 < X₀ < 0.9) at room temperature, where t " is defined as a tetragonal phase whose axial ratio c/a equals unity. The axial ratio c/a decreases with an increase of ceria concentration and becomes 1 at a composition X'₀ (0.65 < X'₀ < 0.7) at room temperature. The sample with a composition between X₀ and X'₀ is t " ZrO₂. By Raman scattering measurements at high temperatures, the tetragonal ( t" ) → cubic and cubic → tetragonal phase transitions occur above 400°C in Zr_0.2 Ce_0.8O₂ solid solution.     

High-Toughness Ce-TZP/Al2O3 Ceramics with Improved Hardness and Strength

Simulataneous additions of SrO and Al₂O₃ to ZrO₂ (12 mol% CeO₂) lead to the in situ formation of strontium aluminate (SrO · 6Al₂O₃) platelets (∼0.5 μm in width and 5 to 10 μm in length) within the Ce-TZP matrix. These platelet-containing Ce-TZP ceramics have the strength (500 to 700 MPa) and hardness (13 to 14 GPa) of Ce-TZP/Al₂O₃ while maintaining the high toughness (14 to 15 MPa ± m^1/2) of Ce-TZP. Optimum room-temperature properties are obtained at SrO/Al₂O₃ molar ratios between 0.025 and 0.1 for ZrO₂ (12 mol% CeO₂) with starting Al₂O₃ contents ranging between 15 and 60 vol%. The role of various toughening mechanisms is discussed for these composite ceramics.     

Metastable Phase Selection and Partitioning in ZrO2—MgO Processed from Liquid Precursors

Aqueous mixtures of either zirconium acetate or zirconium nitrate and magnesium nitrate were dried and subsequently pyrolyzed at fast heating rates (upquenching) to form metastable crystalline phases of ZrO₂ with various degrees of MgO supersaturation. The crystallization temperature was determined to be 380°C for the zirconium acetate, and 270°C for the zirconium nitrate at a heating rate of 5°C/min. The crystalline structures were characterized as a function of MgO content and thermal history for specimens containing 0 to 30 mol% MgO. Upquenching to 900°C, where monoclinic ( m ) ZrO₂ and MgO are the equilibrium phases, yielded single-phase tetragonal ( t ) ZrO₂ (<8 mol% MgO), single-phase cubic ( c ) ZrO₂ (9 to 17 mol% MgO), and two-phase c -ZrO₂+ MgO structures (>17 mol% MgO). The composition for which T ₀( t/c ) = 900°C was estimated as 9 ± 1 mol% MgO. Compositions crystallizing as metastable t -ZrO₂ (<8 mol% MgO) partitioned at higher temperatures and/or longer times into two-phase mixtures, following the general sequence t → t + m → m + MgO. Similarly, compositions forming metastable c -ZrO₂ (10 to 30 mol% MgO) partitioned in the following sequence: c → c + t + MgO → t + MgO → t + m + Mgo → m + Mgo. The initial phase selection and subsequent partitioning sequence are discussed in light of phase hierarchies predicted from thermodynamic concepts and kinetic constraints which are introduced by the solute partitioning required to achieve equilibrium.     

Phase Equilibria in the System ZrO2─InO1.5

Phase equilibria in the system ZrO₂─InO_1.5 have been investigated in the temperature range from 800° to 1700°C Up to 4 mol%, InO_1.5 is soluble in t -ZrO₂ at 1500°C. The martensitic transformation temperature m → t of ZrO₂ containing InO_1.5 is compared with that of ZrO₂ solid solutions with various other trivalent ions with different ionic radii. The diffusionless c → t ' A phase transformation is discussed. Extended solid solubility from 12.4 ± 0.8 to 56.5 ± 3 mol% InO_1.5 is found at 1700°C in the cubic ZrO₂ phase. The eutectoid composition and temperature for the decomposition of c -ZrO₂ solid solution into t -ZrO₂+InO_1.5 solid solutions were determined. A maximum of about 1 mol% ZrO₂ is soluble in bcc InO_1.5 phase. Metastable supersaturation of ZrO₂ in bcc InO _1.5 and conditions for phase separation are discussed.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

Formation, Characterization, and Hot Isostatic Pressing of Cr2O3-Doped ZrO2 (0,3 mol% Y2O3) Prepared by Hydrazine Method

In the ZrO₂-Cr₂O₃ system, metastable t -ZrO₂ solid solutions containing up to 11 mol% Cr₂O₃ crystallize at low temperatures from amorphous materials prepared by the hydrazine method. The lattice parameter c decreases linearly from 0.5149 to 0.5077 nm with increased Cr₂O₃ content, whereas the lattice parameter a is a constant value ( a = 0.5077 nm) regardless of the starting composition. At higher temperatures, transformation (decomposition) of the solid solutions proceeds in the following way: t (ss)→ t (ss) + m + Cr₂O₃→ m + Cr₂O₃. Above 11 mol% Cr₂O₃ addition, c-ZrO₂ phases are formed in the presence of Cr₂O₃. The t -ZrO₂ solid solution powders have been characterized for particle size, shape, and surface area. They consist of very fine particles (15–30 nm) showing thin platelike morphology. Dense ZrO₂(3Y)-Cr₂O₃ composite ceramics (∼99.7% of theoretical) with an average grain size of 0.3 μm have been fabricated by hot isostatic pressing for 2 h at 1400°C and 196 MPa. Their fracture toughness increases with increased Cr₂O₃ content. The highest K _Ic value of 9.5 MPa·;m^1/2 is achieved in the composite ceramics containing 10 mol% Cr₂O₃.     

Preparation of an Alkali-Element-Doped CeO2-Sm2O3 System and Its Operation Properties as the Electrolyte in Planar Solid Oxide Fuel Cells

The ionic conductivity of the ceria-samaria (CeO₂-Sm₂O₃) system is higher than that of yttria-stabilized zirconia and other CeO₂-based oxides. In this study, a small amount of alkali-element-doped CeO₂-Sm₂O₃ solid solution was prepared. This solid solution was characterized by measuring the powder density and the chemical composition. Moreover, its electrochemical properties were investigated in the temperature range from 700° to 1000°C. It was found that a small amount of alkali-element-doped CeO₂ solid solution enhanced the ionic conductivity. The power density of an oxygen-hydrogen fuel cell for alkali-element-doped CeO₂-Sm₂O₃ ceramics exhibited high values at low temperatures such as 700° to 800°C. It is concluded that the improved fuel cell performance can be attributed to the high stability of this composition in the fuel atmosphere.     

Thermal Expansion of CeO2, Ho2O3, and Lu2O3 from 100° to 300°K by an X-Ray Method

Thermal expansion of CeO₂, Ho₂O₈, and Lu₂O₃ was determined from 100° to 300°K by a back-reflection X-ray technique. The variation of thermal expansion with temperature is the same as that of specific heat for CeO₂ and Ho₂O₃; these oxides obey the Grueneisen model of thermal expansion in the temperature range studied.     

Zirconium–Hafnium Interdiffusion in Polycrystalline Fluorite-Cubic CeO2─ZrO2─HfO2 Solid Solution

Zr–Hf interdiffusion was studied in the temperature range of 1650° to 1850°C in air for polycrystalline fluorite-cubic systems of 90CeO₂·10(Zr_{1- x} Hf _x )O₂ and 60CeO₂·40(Zr_{1- x} Hf _x )O₂. Lattice and grain-boundary diffusion parameters were calculated from the Zr–Hf concentration distributions by using the grain-boundary diffusion equation of Oishi and Ichimura. The cation iattice diffusivity was close to that in the fluorite-cubic Y₂O₂-ZrO₂ solid solution.