Adsorption as a Method of Doping 3-mol%-Yttria-Stabilized Zirconia Powder with Copper Oxide

The adsorption behavior of Cu²⁺ on 3-mol%-yttria-stabilized tetragonal zirconia polycrystalline (3Y-TZP) powder was studied. There is a window of ph values (10 < pH < 11) where adsorption may be used as a method of doping 3Y-TZP with Cu²⁺. The maximum mole percent of the CuO additions is determined by the specific surface area of the 3Y-TZP powder; a powder with a specific surface area of 16.1 m²/g is limited to about 1 mol% CuO. Compacts made from powders doped with CuO using this method exhibited an enhancement in superplasticity comparable to that observed in other studies using samples doped with CuO by attrition milling.     

Accelerated Aging in 3-mol%-Yttria-Stabilized Tetragonal Zirconia Ceramics Sintered in Reducing Conditions

The aging behavior of 3-mol%-yttria-stabilized tetragonal zirconia (3Y-TZP) ceramics sintered in air and in reducing conditions was investigated at 140°C in water vapor. It was observed by X-ray diffraction (XRD) that 3Y-TZP samples sintered in reducing conditions exhibited significantly higher tetragonal-to-monoclinic transformation than samples with similar density and average grain size values but obtained by sintering in air. This fact is explained by the increase of the oxygen vacancy concentration and by the presence at the grain boundary region of a new aggregate phase formed because of the exolution of Fe²⁺ ions observed by X-ray photoelectron spectroscopy.     

Structural Changes by Compressive Stresses of 2.0-mol%-Yttria-Stabilized Tetragonal Zirconia Polycrystals

Reorientation of the tetragonal (002) peak in tetragonal ZrO₂ polycrystals (TZP) (so-called domain switching) was studied by XRD and residual stress measurement using TZP specimens containing 2.0 mol% Y₂O₃ that had undergone mechanical and thermal treatments and compressive stress. The observed domain switching was due to a preferred transformation of the tetragonal phase caused by compressive stress above 70 MPa leading to a remnant c-axis orientation normal to the compressive direction. Domain switching did not depend on thermal stress but arose directly from the tetragonal phase with little relation to monoclinic phase.     

Anisotropic Thermal Expansion of Tetragonal Zirconia Polycrystals

Thermal expansion coefficients (α _a and α _c ) in two crystallographic axes ( a and c ) of the tetragonal phase are measured at 25°–1200°C in ZrO₂–M₂O₃ (M = Sc, In, Yb) and in ZrO₂–YTaO₄. The difference between these two thermal expansion coefficients, α _c –α _a , decreases with M₂O₃ or YTaO₄ composition even though the tetragonality ( c/a ) behaves differently in these two systems. The locus of α _c =α _a represents a maximum tetragonality for the tetragonal phase, but not the phase boundary for the cubic phase. The relationships among thermal expansion, temperature, and composition are discussed.     

De-Agglomeration Kinetics of Feedstocks with Granule Tetragonal Zirconia Polycrystalline Powder

This study establishes the relationship between homogeneity of tetragonal zirconia polycrystalline (TZP) feedstocks and the kneading time in order to understand the kneading energy adsorption during the de-agglomeration period of pre-existing ceramic granules. Two spray-dried Y-TZP powders with different compress strengths were investigated using torque rheometer and transmission optical microscopy. Several properties, kneading torque ( T ), Casson viscosity (μ), Reynolds number ( Re ), and dispersive index ( D _s) of the feedstocks were introduced, and monitored as a function of kneading time. Optimal processing windows were identified from the dependence of dispersive index with a power number ( N _p). The best shear-kneading procedure was proposed in consideration of best feedstock homogeneity and shortest kneading time. One homogenous feedstock and two comparable cases were prepared for subsequent injection molding, debinding, sintering, and mechanical testing. The fracture strength of sintering parts by a partially homogeneous feedstock can be improved from 520 to 680 MPa by a homogeneous feedstock.     

Thermal Expansion Anisotropy of Ceria-Stabilized Tetragonal Zirconia

Ceria-stabilized tetragonal zirconia polycrystals were obtained by thermal treatment of amorphous powder prepared by the sol–gel method. Detailed XRD profile analysis was employed to study microstructural disorder and crystallite size and shape; in particular, no fluctuation of stoichiometry was found, the main cause of disorder being attributable to dislocations. Thermal expansion measurements were carried out by high-temperature XRD at 294, 473, 673, 873, and 1073 K using silicon as an internal standard. Thermal expansion coefficients are anisotropic and changes in the stabilizer content have little effect on them. A mean value, α _a = 10.6 × 10⁻⁶ (K⁻¹) and α _c = 13.5 × 10⁻⁶ (K⁻¹), can be assumed for Zr_{1− x} Ce _x O₂ with x in the range 0.12–0.18.     

Influence of Amorphous Grain Boundary Phases on the Superplastic Behavior of 3-mol%-Yttria-Stabilized Tetragonal Zirconia Polycrystals (3Y-TZP)

Amorphous silicate grain boundary phases of varying chemistry and amounts were added to 3Y-TZP in order to determine their influence on the superplastic behavior between 1200° and 1300°C and on the room-temperature mechanical properties. Strain rate enhancement at high temperatures was observed in 3Y-TZP containing a glassy grain boundary phase, even with as little as 0.1 wt% glass. Strain rate enhancement was greatest in 3Y-TZP with 5 wt% glass, but the room-temperature hardness, elastic modulus, and fracture toughness were degraded. The addition of glassy grain boundary phases did not significantly affect the stress exponent of 3Y-TZP, but did lower the activation energy for superplastic flow. Strain rate enhancement was highest in samples containing the grain boundary phase with the highest solubility for Y₂O₃ and ZrO₂, but the strain rate did not scale inversely with the viscosity of the silicate phases. Grain boundary sliding accommodated by diffusional creep controlled by an interface reaction is proposed as the mechanism for superplastic deformation in 3Y-TZP with and without glassy grain boundary phases.     

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.     

Strength Analysis of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Tensile strength of Y₂O₃-stabilized ZrO₂ polycrystals (Y-TZP) was measured by a newly developed tensile testing method with a rectangular bar. The tensile strength of Y-TZP was lower than that of the three-point bend strength, and the shape of the tensile strength distribution was quite different from that of the three-point bend strength distribution. It was difficult to predict the distribution curve of the tensile strength using the data of the three-point bend strength by one-modal Weibull distribution. The distribution of the tensile strength was analyzed by two- or three-model Weibull distribution coupled with an analysis of fracture origins. The distribution curve of the three-point bend strength which was estimated by multimodal Weibull distribution agreed favorably with that of the measured three-point bend strength values. A two-modal Weibull distribution function was formulated approximately from the distributions of the tensile and three-point bend strengths, and the estimated two-modal Weibull distribution function for the four-point bend strength agreed well with the measured four-point bend strength.     

Elastic Constants of Tetragonal Zirconia Measured by a New Powder Diffraction Technique

Elastic constants for 12 mol% Ce-doped tetragonal zirconia have been determined from peak shifts in neutron diffraction patterns recorded under applied uniaxial stress. When these diffraction data are combined with a measured value of Young's modulus, a complete set of elastic constants is obtained. The values are c ₁₁= 327, c ₁₂= 100, c ₁₃= 62, c ₃₃= 264, c ₄₄= 59, and c ₆₆= 64 (units: GPa). These are the first reported results using a new technique for the measurement of elastic constants for anisotropic materials via neutron diffraction measurements on polycrystalline samples.     

Metastability of Tetragonal Zirconia Powders

The formation of metastable t -ZrO₂ by thermal treatment was studied in a systematic way on two different samples: a gel (G) and zirconyl acetate (A). The results show that initially nucle-ation of t -ZrO₂ is favored by creation of anionic vacancies with trapped electrons. At higher temperatures the electronic defects disappear, the crystallites grow, and the m -phase can nucleate. This evolution is observed perfectly on sample A because of its special morphological characteristics.     

Subeutectoid Degradation of Yttria-Stabilized Tetragonal Zirconia Polycrystal and Ceria-Doped Yttria-Stabilized Tetragonal Zirconia Polycrystal Ceramics

Yttria-doped tetragonal zirconia containing 2 and 3 mol% Y₂O₃ (Y-TZP), and CeO₂-doped Y-TZP containing 0 to 12 mol% CeO₂ were sintered at 1350°C in a tetragonal single-phase field for 2 h in air, and the degradation behavior at low temperature in air and in hot water was observed. X-ray photoelectron spectroscopy studies on the surface of hydrothermally treated samples show evidence for the formation of a YO(OH) species, along with the simultaneous formation of purely tetragonal zirconia nuclei that retain their coherence in the Y-TZP matrix. Above a critical size, the tetragonal nuclei spontaneously transform to a monoclinic structure, giving rise to macro- and microcracking. The strong tetragonal grains degrade to produce a spalling phenomenon that facilitates further degradation. Y-TZP ceramics alloyed with adequate amounts of CeO₂ show no tetragonal-to-monoclinic transformation after hydrothermal treatment.     

Tetragonal Zirconia Polycrystals Reinforced with SiC Whiskers

The microstructure and the mechanical properties of hot-pressed tetragonal ZrO₂ polycrystals (TZP) reinforced with up to 30 vol% SiC whiskers were studied. The SiC whisker-TZP composites were stable under the hot-pressing conditions at 1450°C. Annealing in an oxidizing atmosphere at ∼1000°C resulted in glass formation and microcracking caused by whisker oxidation and transformation of the ZrO₂ grains near the whiskers to monoclinic symmetry. The fracture toughness was markedly improved by the dispersed whiskers (∼12 Mpa·m^1/2 at 30 vol% SiC) compared to the values measured for the matrix (∼6 Mpa·m^1/2). The flexural strength of the hot-pressed TZP-30 vol% SiC whisker composite at 1000°C (∼400 MPa) was twice that of the TZP matrix.     

Low-Temperature Ionic Conductivity of 9.4-mol%-Yttria-Stabilized Zirconia Single Crystals

Direct current and alternating current electrical conductivity measurements are used to determine the activation energies for the formation of oxygen vacancies by breakup of bound defect complexes (0.32 eV) and their migration (0.84 eV) at low temperatures (<600°C) in Y₂O₃-fully-stabilized ZrO₂ single crystals. The defect clusters break up between 360° and 450°C.     

Fracture Resistance and Stable Crack-Growth Behavior of 8-mol%-Yttria-Stabilized Zirconia

The crack-growth behavior of a yttria-stabilized zirconia ceramic (8 mol% of cubic-phase yttria) was studied at room temperature. Double-cantilever-beam specimens were loaded with pure bending moments in a specially designed loading fixture inside an environmental scanning electron microscope. Crack-growth data were obtained from truly sharp (arrested) cracks, bypassing interpretation problems that involve crack initiation from a machined notch. The crack-growth study was conducted over a range of applied energy-release rates that allowed crack arrest on one hand and fast fracture on the other. Three energy-release-rate values were relevant: initiation of crack growth (3.5 J/m²), crack arrest (2.8 J/m²), and fast fracture (8.0 J/m²). At the macroscopic scale, subcritical crack growth occurred as a continuous process. In situ observations revealed that, at the microscopic scale, crack growth occurred in small jumps. The fracture mode for stable crack growth was identified to be transgranular.     

Evaluation of the Cyclic Fatigue Life of 3-mol%-Yttria-Stabilized Zirconia Bioceramic Using Biaxial Flexion

The cyclic fatigue life of zirconia bioceramic was determined under biaxial flexion using disk-shaped specimens. The fatigue life was not reduced by subjecting the material to a simulated physiological environment nor was it affected by testing at different loading frequencies. Tests performed at different stress ratios produced a large degree of scatter in the data but a statistical analysis proved that there was no significant effect on the fatigue life of the material. Failure of disk specimens was initiated either by flaws introduced during manufacturing or by the more typical inherent flaws commonly observed in ceramics. The former generally resulted in premature, low-stress failures on initial loading whereas the latter acted as initiating defects for fatigue cracks that then propagated to failure. The distributions of the two categories of flaw were analyzed using two- and three-parameter Weibull probability functions. It became clear that the Weibull modulus for the fatigue failures was consistent with previously reported work on other ceramics.     

Raman Spectroscopy of Tetragonal Zirconia Solid Solutions

Raman spectra of tetragonal zirconia were studied for the systems ZrO₂-Y₂O₃, ZrO₂-CeO₂, ZrO₂-Y₂O₃-Nb₂O₅, and ZrO₂-Y₂O₃-Ta₂O₅ There was no change in the Raman bands for tetragonal ziraconia in the ZrO₂-Y₂O₃ system as a function of Y₂O₃ content . On the other hand, in the ternary systems, the Raman lines corresponding to the stretching modes of two sets of cation-oxygen bonds shifted to greater wave numbers as either Nb₂O₅ Contents increased in Y₂O₃-stabilized tetragonal zirconia. This difference was interpreted by local bonding environments of the pentavalent cations, which were supposed to occupy tetrahedral sites in teteragonal zirconia solid solutions.     

高性能热接口材料的研究

对高性能热接口材料进行了研究。分析了材料的导热系数、击穿电压、拉伸强度等性能的变化关系，并对其变化趋势进行了探讨。最终给出了该类热接口材料的半定量配比及性能指标。     

Method of Fabricating Ceria-Stabilized Tetragonal Zirconia Polycrystals

Most zirconia-based toughened ceramics need specialized processes to achieve their desired properties. In this paper, we report the fabrication of toughened ceria-stabilized tetragonal zirconia polycrystal by conventional processing, i.e., ball milling and cold-pressing followed by sintering. We believe that ball milling works here because a somewhat coarser particle size is actually beneficial in this case. Although the samples were not fully dense (they need not be), a composition of ZrO₂-12 mol% CeO₂ yielded a fracture toughness value of 14.1 MPa·m^1/2. This is comparable to values reported for materials processed by specialized techniques and can be rationalized in terms of R-curve behavior.     

Aging Behavior of Ceria-Stabilized Tetragonal Zirconia Polycrystals

A commercial ceria-stabilized tetragonal zirconia polycrystal is shown not to degrade at low temperatures. Slight strength decrease is found for elevated-temperature treatments, which is not related to formation of the monoclinic phase. A possible mechanism to explain the behavior based upon the ferroelastic nature of the tetragonal phase is proposed.