Crystal Structure of Metastable Tetragonal Zirconia up to 1473 K

The crystal structure of metastable tetragonal zirconia prepared via the alkoxide method has been investigated at temperatures up to 1473 K, to clarify the similarity between this metastable phase and the tetragonal phase at high temperature. The lattice constants, tetragonality, oxygen shift parameter, and equivalent isotropic thermal parameter of the metastable tetragonal phase are proportional to the temperature. These parameters, when extrapolated to the high-temperature range, are very similar to those of the high-temperature tetragonal phase. Present results indicate that the structure of the metastable tetragonal phase is the same as that of the high-temperature tetragonal phase.     

Neutron Diffraction Studies of Phase Transformations between Tetragonal and Orthorhombic Zirconia in Magnesia-Partially-Stabilized Zirconia

Neutron powder diffraction and conventional dilatometry have been used to investigate the tetragonal-to-orthorhombic phase transformation and the orthorhombic-to-tetragonal reversion in a high-toughness magnesia-partially-stabilized zirconia. For this material, the onset temperature on cooling for the tetragonal-to-orthorhombic transformation (determined by dilatometry) was 192 K, and the reversion on subsequent heating occurred between 500 and 620 K. Neutron diffraction patterns were recorded at temperatures down to 19 K then up to 664 K, and analyzed by the multiphase Rietveld method to determine the amounts of different phases as well as their lattice parameters and unit-cell volumes. It is notable that, at its maximum, the orthorhombic phase amounted to 45% of the sample by weight. Length changes were measured, using pushrod dilatometers, in the temperature range 80 to 700 K. Length changes calculated from the neutron diffraction determinations of the proportions and unit-cell volumes of the different phases are in very good agreement with the directly measured values.     

Crystal Structure of Metastable Tetragonal Zirconia by Neutron Powder Diffraction Study

The crystal structure of metastable tetragonal zirconia prepared by the alkoxide method without any dopant has been examined by neutron diffraction at room temperature. The lattice parameters of this sample were a = 0.3591 ± 0.0001 nm and c = 0.5169 ± 0.0001 nm. The oxygen ions are shifted from their lattice sites in the 〈001〉 direction with a magnitude of Δ/ c = 0.046 ± 0.004.     

Measurement of Phase Abundance in Magnesia-Partially-Stabilized Zirconia by Rietveld Analysis of X-ray Diffraction Data

The relative abundance of the cubic ( c ), tetragonal ( t ), monoclinic ( m ), and orthorhombic ( o ) polymorphs of ZrO₂, and the δ phase, Mg₂Zr₅O₁₂, present in samples of 3.4-wt%-magnesia-partially-stabilized zirconia have been determined by Rietveld analysis of X-ray powder diffraction data. The samples studied correspond to the as-fired (AF), and subetectoid-aged maximum-strength (MS) and thermalshock (TS) states, with their surfaces in the ground or polished condition. The polymorph abundances of the bulk and near-surface regions are discussed in relation to the type of surface treatment. Grinding produces significant quantities of both m - and o -ZrO₂ in the near-surface regions of all samples. The m content increases from about 5 wt% in the bulk, to 10, 24, and 33 wt% in AF, MS, and TS material, respectively, while the o content increases from trace amounts to about 11 wt% in all samples. The m and o phases both increase at the expense of t -ZrO₂, and the transformation is accompanied by significant lattice distortion and/or crystal size reduction. Thus, measurement of only the 'ground-surface-monoclinic' content does not give an accurate indication of the total amount of transformable t -ZrO₂ in ceramics of this kind. Polishing removes some of the ground-surface m -ZrO₂ in MS and TS, and all of the m -ZrO₂ in AF material. The o -ZrO₂ produced by grinding also declines substantially in AF and MS, but is not removed by polishing of TS. As a result, the bulk composition cannot be guaranteed, in the general case, to be accessible by X-ray analysis of polished surfaces.     

Nitrogen-Containing Tetragonal Zirconia

The present paper gives evidence for the replacement of oxygen by nitrogen when partially stabilized tetragonal zirconia is reacted with nitrogen above 1400°C to give a tetragonal product but in the t ' (nontransformable) form. No transformation toughening is therefore expected from this nitrogencontaining tetragonal phase. These results may explain why less-effective toughening is obtained in nitrogen-containing ceramic-based composites than in equivalent zirconia-alumina and other oxide-based systems.     

Synthesis of Niobium(V)-Stabilized Tetragonal Zirconia Nanocrystalline Powders

The polymer precursor method is very useful to prepare Nb⁵⁺-stabilized nanocrystalline powders of t -ZrO₂. The precursor solution is composed of zirconium oxalate, niobium tartrate, and poly(vinyl alcohol), which help to form a network matrix to disperse the metal ions homogeneously. Nb⁵⁺ is an effective agent to stabilize t -ZrO₂, and ease of formation of the tetragonal phase increases with increasing dopant concentration. Thermal stability of t -phase is found up to 1700°C having 15 mol% Nb⁵⁺, prepared at 600°C with particle sizes of 35 ± 5 nm.     

Crystallization and Phase Transformation Process in Zirconia: An in situ High-Temperature X-ray Diffraction Study

Amorphous zirconia precursors were made by the precipitation of a zirconium tetrachloride solution with either slow (8 h) or rapid additions of ammonium hydroxide at a pH of 10.5. Following calcination at 500°C for 4 h, the rapidly precipitated precursor exhibited predominantly monoclinic ZrO₂ phase, while the slowly precipitated precursor produced the tetragonal ZrO₂ phase. The crystallization and phase transformations were followed by in situ high-temperature X-ray diffraction (HTXRD) for both specimens in helium and in air. Each amorphous precursor first crystallizes as the tetragonal phase at about 450°C. A tetragonal-to-monoclinic phase transformation of the rapidly precipitated material was observed on cooling at about 275°C. Surface impregnation of sulfate ions following precipitation inhibited the tetragonal-to-monoclinic transformation for the rapidly precipitated ZrO₂ sample. The crystallite size for the t -ZrO₂ of all samples, irrespective of whether they transform to monoclinic, was approximately 11 nm, indicating that the t → m transformation in these materials is not controlled by differences in crystallite size. It is therefore suggested that anionic vacancies control the tetragonal-to-monoclinic phase transformation on cooling, and that oxygen adsorption triggers this phase transformation.     

Tensile Strength of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Tensile strengths of 2.0 to 5.0 mol% Y₂O₃-stabilized ZrO₂ polycrystals were described using the newly developed tensile testing method. The tensile test was conducted by attaching three strain gauges on both sides of a rectangular bar that was 10 mm by 1 mm by 200 mm. The tensile strength of tetragonal ZrO₂ polycrystals (TZP) containing 2.0 mol% Y₂O₃ showed 745 MPa, whereas the bend strength of this material was 1630 MPa. Inelastic behavior of the stress-strain curve was observed at critical stresses and strains of 500 to 700 MPa and 0.25% to 0.35%, respectively. Although deviation from proportionality was observed to be small, it increased with the increase of temperature from −100° to 200°C.     

Water Incorporation in Tetragonal Zirconia

Samples of 3 mol% Y₂O₃-stabilized tetragonal ZrO₂ ceramics were annealed at 250°C in atmospheres of water vapor pressures of 1 bar and 26 mbar. As demonstrated by the water uptake and the lattice expansion, water molecules were incorporated into the ZrO₂ lattice during annealing, and the amount of the incorporated water is determined by the water vapor pressure. Owing to the filling of oxygen vacancies by the incorporated water molecules, part of the tetragonal ZrO₂ transformed to the monoclinic structure, and protonic defects were induced. The expected proton conduction was confirmed by the polarity of the water vapor concentration cells.     

Synthesis of Metastable Tetragonal (t') Zirconia–Ceria Solid Solutions by the Polymerized Complex Method

Homogeneous metastable tetragonal ( t ') solid solutions of ZrO₂— x mol% CeO₂ ( x = 20 and 50) were successfully synthesized by the organic polymerized complex method. The citric acid-ethylene glycol solution containing Zr and Ce ions was polymerized at about 140°C and then heat-treated at about 350°C to obtain a precursor. The black precursor was heated at 450°C and then fired up to 1300° or 1590°C, resulting in the homogeneous solid solutions.     

Critical Particle Size and Phase Transformation in Zirconia: Transmission Electron Microscopy and X-ray Diffraction Studies

A study was undertaken to examine the crystallite size effect on the low-temperature transformation of tettragonal zirconia. Zirconia weas prepared by precipitation from a solution of zirconium tetrachloride by adding ammonium hydroxide to produce a pH of 2.95. Portions of the sample, after drying, were calcined at 500°C for various time intervals. Phase transformation was followed by X-ray diffraction; the data show that the tetragonal phase was initially formed and it was transformed to the monoclinic phase at longer periods of calcination. It was observed by TEM particle size and XRD crystallite size that the transformation does not appear to be due to a critical particle size effect.     

Grinding-Induced Texture in Ferroelastic Tetragonal Zirconia

Ceria- and yttria-doped tetragonal polycrystalline zirconia ceramics were ground at temperatures as high as 1100°C. X-ray diffraction revealed that the intensity ratio I ₍₀₀₂₎/ I ₍₂₀₀₎ increased (to as high as ∼4.5) compared with that from the as-sintered surfaces (∼0.55). The enhancement in I ₍₀₀₂₎/ I ₍₂₀₀₎ at temperatures well above the m → t transition temperature shows that it is not related to transformation, reversible or otherwise, but can be explained by ferroelastic domain switching.     

Cubic-Formation and Grain-Growth Mechanisms in Tetragonal Zirconia Polycrystal

The microstructure in Y₂O₃-stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1300°–1500°C was examined to clarify the role of Y³⁺ ions on grain growth and the formation of cubic phase. The grain size and the fraction of the cubic phase in Y-TZP increased as the sintering temperature increased. Both the fraction of the tetragonal phase and the Y₂O₃ concentration within the tetragonal phase decreased with increasing fraction of the cubic phase. Scanning transmission electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) measurements revealed that cubic phase regions in grain interiors in Y-TZP generated as the sintering temperature increased. High-resolution electron microscopy and nanoprobe EDS measurements revealed that no amorphous layer or second phase existed along the grain-boundary faces in Y-TZP and Y³⁺ ions segregated at their grain boundaries over a width of ∼10 nm. Taking into account these results, it was clarified that cubic phase regions in grain interiors started to form from grain boundaries and the triple junctions in which Y³⁺ ions segregated. The cubic-formation and grain-growth mechanisms in Y-TZP can be explained using the grain boundary segregation-induced phase transformation model and the solute drag effect of Y³⁺ ions segregating along the grain boundary, respectively.     

Enhanced Phase Stability for Tetragonal Zirconia in Precipitation Synthesis

Tetragonal ZrO₂ nanocrystallites—with or without yttria (3 mol%) doping—have been synthesized via a precipitation process in which the hydrous oxide precipitate reacts with hexamethyldisilazane (HMDS) vapor before calcination. The nanocrystallites are formed and retain a tetragonal structure for hours after calcination at temperatures of 300°–1100°C. The enhanced structural metastability has been attributed to the combined effect of suppressed grain growth and reduced surface energy that results from the HMDS treatment.     

Cubic-to-Tetragonal (t') Transformation in Zirconia-Containing Systems

The coexistence of the cubic fluorite and tetragonal phases in rapidly quenched samples was studied in the ZrO₂-MO_1.5 systems for M = Sc, In, Y, and rare earths (R). Spontaneous transformation from metastable cubic phase was triggered at room temperature by a mechanical force. Isolated tetragonal platelets in the cubic matrix were bounded by [101] habit planes and contained anti-phase boundaries. The tetragonality decreased with stabilizer content and vanished at around 18 mol% for M = Y and R, 23 mol% for M = Sc, and 25 mol% for M = In, all at room temperature. With increasing temperature, the tetragonality initially increased because of anisotropic thermal expansion, then decreased rapidly, after reaching a maximum, as the temperature for the tetragonal-to-cubic transformation was approached. Being a first-order martensitic transformation, the cubic-to-tetragonal transformation is accompanied by a discontinuous change of tetragonality and a hysteresis loop as the temperature or composition passes through the equilibrium value.     

Growth of Tetragonal Zirconia Coatings by Reactive Sputter Deposition

Zirconia coatings were produced by reactive dc magnetron sputter deposition, using a system with multiple sputter sources and a biased substrate stage. Tetragonal zirconia with either a random orientation or a highly (111) preferred orientation was formed by applying a substrate bias. Coating grown with no substrate bias had the equilibrium monoclinic structure. X-ray diffraction and transmission electron microscopy analyses revealed that bias sputtering could effectively decrease crystalline size in the as-deposited coating, which resulted in room-temperature stabilization of the tetragonal phase. The fraction of tetragonal phase, the desired phase for transformation-toughening behavior, was strongly dependent on the substrate bias and post-deposition annealing temperature.     

On the Thermoelastic Martensitic Transformation in Tetragonal Zirconia

Recent evidence is summarized showing that the tetragonal ( t ) → monoclinic ( m ) martensitic transformation in ZrO₂ can occur thermoelastically in certain ZrO₂-containing ceramics, and that microcracking accompanying the transformation is more common than had previously been recognized. The implications of these new data for the conditions under which the stress-induced transformation is irreversible, and for the particle size dependence of the transformation start ( M _s), temperature, are discussed.     

两步烧结法制备纳米氧化钇稳定的四方氧化锆陶瓷

采用共沉淀法制备纳米氧化钇稳定的四方氧化锆（yttria stabilized tetragonal zirconia,3Y-TZP）粉体。利用X射线衍射、N2吸附–脱附等温线,透射电子显微镜对3Y-TZP粉体的物理性能和化学性能进行表征。研究了纳米3Y-TZP粉体的烧结曲线,分析了3Y-TZP素坯在烧结过程中的致密化行为和显微结构,探讨了两步烧结工艺对3Y-TZP纳米陶瓷微观结构的影响。结果表明：采用共沉淀法,在600℃煅烧2h后,可获得晶粒尺寸为13nm、晶型发育良好、团聚较少的纳米3Y-TZP粉体;采用两步烧结法,将素坯升温至1200℃保温1min后,再降温到1050℃保温35h,可获得相对密度大于98%,晶粒尺寸约为100nm的3Y-TZP陶瓷。两步烧结法通过控制煅烧温度和保温时间,利用晶界扩散及其迁移动力学之间的差异,使晶粒生长受到抑制,样品烧结致密化得以维持,实现在晶粒无显著生长前提下完成致密化。     

Raman Investigation of the Nitridation of Yttria-Stabilized Tetragonal Zirconia

Raman spectroscopy has been used to obtain Raman spectra of yttria-stabilized tetragonal zirconia subject to surface nitridation induced by contact with zirconium nitride. Raman spectra recorded from regions at increasing distance from the source of nitridation have been used to obtain diffusion profiles from samples treated at different times and temperatures. The coupling of X-ray diffraction data previously taken and of the Raman spectra shows that in the samples there is a two-phase region (tetragonal + cubic) near the nitrided surface and that, at larger distance inside the samples, there is only one phase (tetragonal). Fitting of the diffusive profiles in the single-phase tetragonal region with an appropriate diffusion function allows the determination of the diffusion coefficient of nitrogen in tetragonal zirconia which is expressed in terms of the preexponential factor, D ₀= (3.98 ± 0.5) × 10⁻³ cm²/s, and the activation energy, Q = 170 ± 10 kJ/mol.     

Colloidal Processing and Ionic Conductivity of Fine-Grained Cupric-Oxide-Doped Tetragonal Zirconia

CuO-doped tetragonal ZrO₂ (3-mol%-Y₂O₃-doped tetragonal zirconia, 3Y-TZ) green bodies were consolidated from zirconia slurries with Cu²⁺ by a pressure filtration method. The slurries were prepared by dispersing 3Y-TZ powder in a solution of [NH₄OH + NH₃NO₃] = 0.1 M at pH 11 and adding an appropriate amount of Cu(NO₃)·3H₂O solution. Green bodies with narrow pore-size distribution were obtained after cold isostatically pressing the pressure-filtrated bodies. Small amounts of CuO-doped samples were densified fully at 1200°C. The size of a grain of 0.16-mol%-CuO-doped 3Y-TZ sintered at 1200°C was 84 nm. Bulk and grain-boundary conductivities are measured by a complex impedance method. The bulk conductivity of the CuO-doped 3Y-TZ was almost equal to the undoped one, but the grain-boundary conductivity decreased with CuO addition.