Sintering and Characterization of Polycrystalline Monoclinic, Tetragonal, and Cubic Zirconia

Polycrystalline monoclinic ( m ), tetragonal ( t ), and cubic ( c ) ZrO₂, sintered at 1500°C, were annealed in the cubic stability field and rapidly cooled to permit the displacive c → t ' transformation to occur in compositions containing 0–6 mol% Y₂O₃. The bulk fracture toughness of coarse-grained (> 25 μm) m , t ', and c zirconias were compared with conventionally sintered, fine-grained (typically less than 1 μm) materials. The ferroelastic monoclinic and tetragonal zirconias were more than twice as tough as paraelastic cubic zirconia.     

Microindentation-Induced Transformation in 3.5-mol%-Yttria-Partially-Stabilized Zirconia Single Crystals

The temperature dependence of the Vickers microhardness was studied in 3.4-mol%-Y₂O₃-partially-stabilized ZrO₂ (Y-PSZ) single crystals up to 1000°C; the samples had previously been annealed at 1600°C for 150 h to develop "colony" precipitates of tetragonal ZrO₂ in the cubic ZrO₂ matrix. Indentation caused extensive stress-induced martensitic transformation of the colony precipitates to monoclinic symmetry in zones which extended in extreme cases up to several hundred micrometers from the indent. For indents made at 500°C and above, the M _d and M _f temperatures are 450° and 310°C, respectively; A _s is ∼600°C ( M _d is the temperature of initial transformation (the "martensite start temperature") in deformed samples; M _f is the temperature at which the final transformation occurs; A _s is the temperature at which the reverse (monoclinic → tetragonal) transformation begins). However, extensive transformation zones are also found for indents made at 200°, 300°, and 400°C. The dislocation density introduced during indentation is responsible for nucleating the transformation in a zone adjacent to the indent. However, the transformation zone extends further than the plastic zone around the indent, indicating extensive autocatalytic transformation. Transformation within the zone appeared to occur in individual plates with {110} habit planes. The plate dimensions (∼100 μm ×∼175 μm ×∼10 μm) are large compared to the size of the colony precipitates (∼2 μm in maximum dimension).     

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.     

Low-Temperature Aging of t'-Zirconia: The Role of Microstructure on Phase Stability

Polycrystalline, tetragonal ( t ') zirconia samples containing 3 and 4 mol% yttria were fabricated by annealing pressureless-sintered samples in air at ∼ 2100°C for 15 min. The grain size of these fully tetragonal samples was on the order of 100 to 200 μm. Domain structure of the samples and of a 3-mol%-yttria-doped tetragonal zirconia single crystal was examined by transmission optical microscopy under polarized light and by transmission electron microscopy. The orientations of the domain/colony boundaries were in accord with the predictions of group theory. As-polished surfaces of polycrystalline t ' materials showed no monoclinic phase even after 1000 h at 275°C in air. By contrast, conventionally yttria-doped tetragonal zirconia polycrystalline (Y-TZP) ceramics of grain size >0.5 μm showed substantial transformation. Surface grinding enhanced the resistance to degradation of Y-TZP but decreased that of t ' materials. Even then, the t ' materials exhibited better resistance to degradation than the Y-TZP ceramics. Excellent resistance of the t ' materials to low-temperature aging despite a very large grain size and the opposite effect of grinding on phase stability are all explained on the basis of ferroelastic domain structure of these materials.     

Nanometer-Sized ZrO2 Particles Prepared by a Sol–Emulsion–Gel Method

ZrO₂ powder was prepared by a sol–emulsion–gel method at temperatures below 140°C from ZrO(NO₃)₂· n H₂O. The asprepared powder was amorphous, but crystallized into the tetragonal structure by 600°C. The metastable tetragonal powder (600°C) was comprised of ultrafine 4- to 6-nm size particles. On heat treatment, the tetragonal form completely transformed into the monoclinic state at 1100°C. Preliminary studies indicate good sinterability with densities greater than 94% at 1100°C and with a grain size of 0.25 μ.     

Hot Isostatic Pressing of Transparent Nd:YAG Ceramics

This paper demonstrates that fine-grained (2–3 μm), transparent Nd:YAG can be achieved at SiO₂ doping levels as low as 0.02 wt% by the sinter plus hot isostatic pressing (HIP) approach. Fine grain size is assured by sintering to 98% density, in order to limit grain growth, followed by HIP. Unlike dry-pressed samples, tape-cast samples were free of large, agglomerate-related pores after sintering, and thus high transparency (i.e., >80% transmission at 1064 nm) could be achieved by HIP at <1750°C along with lower silica levels, thereby avoiding conditions shown to cause exaggerated grain growth. Grain growth was substantially limited at lower SiO₂ levels because silica is soluble in the YAG lattice up to ∼0.02–0.1 wt% at 1750°C, thus allowing sintering and grain growth to occur by solid-state diffusional processes. In contrast, liquid phase enhanced densification and grain growth occur at ∼0.08–0.14 wt% SiO₂, especially at higher temperatures, because the SiO₂ solubility limit is exceeded.     

Characterization of Barium Titanyl Oxalate Tetrahydrate

The XRD pattern and the thermal behavior of barium titanyl oxalate tetrahydrate (BTOT) were investigated. BTOT crystallizes in the monoclinic system, 2/ m , with unit cell dimensions of a = 14.954 Å, b = 19.332 Å, c = 13.947 Å, and β= 106.43°. Unit cell content ( Z ) is 12 and the Bravais lattice is P. Theoretical density is 2.31 g/cm³. At a relatively low temperature (∼60°C), BTOT starts to dehydrate, resulting in a less-than-calculated weight loss of ignition. Barium titanate powder obtained by calcining the oxalate exists as the cubic perovskite phase, instead of the tetragonal phase at low temperatures. This happens when the particle size of the crystals is smaller than ∼ 30 nm. As the crystal coarsens, barium titanate powder transforms to the tetragonal state.     

Measurements of Excess Enthalpy in Ultrafine-Grained Titanium Dioxide

Differential scanning calorimetry has been used to make direct measurements of the excess enthalpy of TiO₂ (rutile) with an initial grain size of 30–70 nm. When the heat released during grain growth is normalized to the change in grain boundary area, the specific excess enthalpy at low temperatures and fine grain sizes (600°–780°C, 30–200 nm) is found to be 0.5–1 J/m², while values averaged over a larger temperature and size range (600°–1300°C, 30 nm- ∼2 μm) are 1.3–1.7 J/m². After exclusion of extraneous contributions from other heat-dissipating processes, origins of a specific grain boundary enthalpy that increases with grain size or temperature are considered, including solute segregation, changes in grain boundary structure, and contributions from grain boundary triple junctions. It is concluded that the most plausible explanation is a size-dependent nonstoichiometry of TiO₂ due to the impingement of space charge layers in the temperature and grain size range of the experiments.     

Grain-Size Dependence of Thermal-Shock Resistance of Yttria-Doped Tetragonal Zirconia Polycrystals

Thermal-shock fracture behavior of yttria-doped tetragonal zirconia polycrystals (Y-TZP) of various grain sizes was evaluated by the quenching method using water as the quenching solvent. The tetragonal-to-monoclinic phase transformation behavior of Y-TZP around cracks introduced by thermal stress was investigated by using Raman microprobe spectroscopy. The critical quenching temperature difference (Δ T _c ) of Y-TZP ceramics increased from 250° to 425°C with increasing grain size of zirconia from 0.4 to 3.0 μm, while the fracture strength decreased from 900 to 680 MPa. The improvement of Δ T _c of Y-TZP with increasing grain size of zirconia corresponded with the quantity of tetragonal-to-monoclinic phase transformation around cracks introduced by thermal stress.     

Alkoxide Sol-Gel-Processed Cordierite Fiber

An alkoxide sol-gel route was developed to prepare stoichometric cordierite fibers. The influences of the aging treatment and heating rate on the sinterability of the gel fibers were also examined. X-ray diffraction analysis revealed that the unaged and aged fibrous gels all remained amorphous <800&, but began crystallization into μ-cordierite and α-cordierite at ∼900°C and 1050°C, respectively; single-phase α-cordierite fibers were obtained at 1300°C. Heating the unaged fibers yielded denser microstructures, with fine grain sizes of ∼0.2–0.4 μm, whereas the aged fibers exhibited porous microstructures following heating at 1300°C. A higher heating rate and aging treatment resulted in a higher open porosity of the fired fiber.     

Formation and Sintering Characteristics of Aluminum Borate Whiskers

A boric acid-stabilized aluminum acetate powder was decomposed to aluminum borate (Al₁₈B₄O₃₃) by calcining at 1000°C. The powder was then ball-milled, compacted, and fired to temperatures of 1500°, 1600°, and 1700°C for a period of 1 h. The resulting aluminum borate ceramic had a whiskerlike grain morphology, with the whisker length approaching 20 μm and a diameter of 2–3 μm. The sintered compact showed no shrinkage upon firing, had a porosity of ∼50%, a narrow pore size distribution, and an average pore size of 1–3 μm.     

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.     

ZrO2-Transformation-Toughened Glass-Ceramics Prepared by the Sol-Gel Process from Metal Alkoxides

Glasses of composition 3ZrO₂O · 2SiO₂ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by appropriate heat treatment at 1000° to 1200°C. The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO₂, reaching ∼5.0 MN/m^3/2 at a size of ∼40 nm. The higher fracture toughness was attributed to tetragonal → monoclinic ZrO₂ transformation toughening.     

Oxygen Vacancies in Pure Tetragonal Zirconia Powders: Dependence on the Presence of Chlorine during Processing

We have used several experimental methods to study how a large extrinsic oxygen vacancy density in pure tetragonal ZrO₂ powders depends on details of how those powders are made. Samples were made from oxychloride and nitrate precursor solutions. We used perturbed angular correlation spectroscopy to determine in situ phase structure and the density of oxygen vacancies at 1200°C, XRD and SEM to determine the grain size and morphology of samples annealed at temperatures ranging from 200°–1200°C, and neutron activation analysis (NAA) to investigate purity of samples. NAA results showed that samples contain cation impurities at levels <<100 ppm. The XRD and SEM measurements showed that grains were nanometer-size, had a broad distribution, and grew from ∼10 nm at 200°C to ∼1 μm at 1200°C. The most striking process dependence is on presence of chlorine during processing. The grain size and phase above 600°C, and both the morphology and the density of oxygen vacancies at 1200°C were strongly affected by presence of chlorine-containing vapor during annealing. Samples processed in a chlorine-free atmosphere had large well-sintered grains and large (>500 ppm) oxygen vacancy concentrations at 1200°C, whereas samples processed in flowing H₂O/HCl vapor had smaller grains, porous morphology, and small (<100 ppm) vacancy density. All samples were loose powders consisting of single grain particles at <1000°C and multiple-grain particles at 1200°C.     

Measurement of the Crystallographically Transformed Zone Produced by Fracture in Ceramics Containing Tetragonal Zirconia

During fracture of ceramics containing tetragonal zirconia particles, a volume of zirconia material on either side of the crack irreversibly transforms to the monoclinic crystal structure. Transformation zone sizes, measured using Raman microprobe spectroscopy, are presented for three sintered ceramics. In a single-phase ZrO₂−3.5 mol% Y₂O₃ material, an upper bound measurement of 5 μm is obtained for the zone size. In the Al₂O₃/ZrO₂ composites studied, the zone size is deduced to correspond to ∼1 grain in diameter. On the basis of the monoclinic concentrations derived from the Raman spectra it is further concluded that only a fraction of the ZrO₂ grains within the transformation zone transform, providing indirect evidence for the effect of particle size on the propensity for transformation.     

Effect of NiO on the Phase Stability and Microstructure of Yttria-Stabilized Zirconia

Nickel oxide (NiO) was screen printed onto the surfaces of 3 and 8 mol% yttria-stabilized zirconia (YSZ) dense pre-fired substrates and then heat treated at temperatures from 1350° to 1550°C. The effect of NiO was dependent on the yttria content of the substrate. In 3 mol% YSZ, it was found to alter the phase composition from predominantly tetragonal with a small amount of cubic phase to one consisting of approximately equal amounts of cubic and monoclinic phase. The cubic grains were much larger than the monoclinic ones and contained more nickel. Furthermore, nickel was observed to migrate through the thickness of the tile, a distance of approximately 200 μm. In the 8 mol% YSZ substrates, the phase composition was unaltered, although large grains developed under the printed NiO layer and the nickel migration was confined to the extent of these large grains.     

Synthesis and Processing Characteristics of Ba0.65Sr0.35TiO3 Powders from Catecholate Precursors

Powders of composition Ba_0.65Sr_0.35TiO₃ were prepared from catecholate precursor phases, BaTi(C₆H₄O₂)₃ and SrTi (C₆H₄O₂)₃. The physical and chemical properties of the base powders, and those doped with 0.2 wt% manganese, are reported in detail. The dimensions of the primary particles in the starting powders were of the order of 20–50 nm, but the occurrence of abnormal grain growth during sintering promoted grain sizes in the ceramic of up to ∼100 μm. In some microstructures, coarse grains coexisted with a ∼1-μm fraction to produce a characteristic bimodal grain size distribution. By contrast, under comparable sintering conditions, namely 1350° or 1400°C for 1 h, grain growth in Mn-doped samples was suppressed, leading to uniform microstructures with a grain size of only a few micrometers. The pellet densities were nevertheless similar, 97% of theoretical in both doped and undoped samples. No significant difference was observed in the dielectric permittivity of the two compositions: the peak relative permittivity occurred at ∼20°C, with a maximum value of ∼22 000.     

Microstructure of Nanocrystalline Yttria-Doped Zirconia Thin Films Obtained by Sol–Gel Processing

Nano- and microcrystalline yttria-stabilized zirconia (YSZ) thin films with a dopant concentration of 8.3±0.3 mol% Y₂O₃ were prepared with a variation in grain size by two orders of magnitude. A sol–gel-based method with consecutive rapid thermal annealing was applied to fabricate YSZ films, resulting in about 400 nm YSZ on sapphire substrates. The average grain sizes were varied between 5 nm and 0.5 μm by heat treatment in the temperature range of 650°–1350°C for 24 h. High-resolution (HRTEM) and conventional transmission electron microscopy analyses confirmed specimens—irrespective of the thermal treatment—consisting of cubic ( c -)ZrO₂ grains with nanoscaled tetragonal precipitates coherently embedded in the cubic matrix. Energy-dispersive X-ray spectroscopy and HRTEM on a large number of specimens yielded a homogeneous yttria concentration within the grains and at the grain boundaries with the absence of impurities, i.e. silica at the grain boundaries.     

Kinetics and Microstructural Evolution of Heterogeneous Transformation of θ-Alumina to α-Alumina

Ultrafine (<0.1 μm) high-purity θ-Al₂O₃ powder containing 3–17.5 mol%α-Al₂O₃ seeds was used to investigate the kinetics and microstructural evolution of the θ-Al₂O₃ to α-Al₂O₃ transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X-ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al₂O₃ lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al₂O₃ sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm.     

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.