Strength Improvement of Yttria-Partially-Stabilized Zirconia by Flaw Elimination

A systematic improvement of the flexural strength of conventionally sintered 4.5-wt%-Y₂O₃-partially-stabilized zirconia was realized by identifying the fracture origins of four-point-bend bars and eliminating them by process modification. The average flexural strength increased from 880 to 1380 MPa, a value comparable with that of isostatically hot-pressed or slip-cast material. A part-through crack extension model was used to correlate the fracture stresses and the fracture origin sizes and a "fracture origin severity parameter" was identified.     

Phase Transformations in Monoclinic Zirconia Caused by Milling and Subsequent Annealing

Monoclinic ZrO₂ has been milled for various times down to a crystallite size of 145 Å. Using the time-differential perturbed-angular-correlation technique, it was possible to associate the resulting growing amount of disordered material with two different, very distributed and milling-time-dependent hyperfine interactions: one ( Y ) of quadrupole frequency similar to that of tetragonal zirconia after long milling, the other ( X ) one of a quadrupole frequency similar to that of monoclinic zirconia at shorter milling times. Upon annealing, all samples showed the recovery of the crystalline monoclinic phase at the expense of the disordered structures. In samples milled for shorter times, the ( Y ) interaction emerges as an ordered crystallization product, upon annealing at temperatures which depend nearly linearly on the crystallite size.     

Analytical Electron Microscopy of Yttria Partitioning in the Yttria-Partially-Stabilized Zirconia Crystal

Yttria-partiaUy-stabilized zirconia was grown frcm the melt by the arc-image floating zone technique and annealed at 1700'C. The yttria concentration of the crystal was measured by analytical electron microscopy. The crystal, which contained 8.6 mol% YO_{1. 5}, consists of tetragonal and cubic phases with yttria concentrations of 3.9 and 9.7 mol% YO_1.5, respectively. There is a small difference between this result and the composition expected from the ZrO₂-Y₂O₃ phase diagram.     

Ambient and High-Temperature Stable Fracture Tests in Ceramics: Applications to Yttria-Partially-Stabilized Zirconia

A new technique that provides stable fracture tests in brittle ceramic materials at room and at high temperature is presented. This technique uses the crack mouth opening displacement signal from a laser extensometer to feed the servo control of a hydraulic testing machine. In this way it is possible to obtain—in addition to the fracture toughness—the fracture energy and the R -curve, in a single test.     

Phase Transformation in EB-PVD Yttria Partially Stabilized Zirconia Thermal Barrier Coatings during Annealing

Electron-beam physical-vapor-deposited thermal barrier coatings consisting of ZrO₂ stabilized by 7 wt% Y₂O₃ were investigated in regard to phase transformation after annealing. Free-standing ceramic layers were heat-treated in air, for up to 200 h, in the temperature range 1200°—1400°C and then analyzed by X-ray diffractometry. Based on information obtained from the {111} and {400} peaks, the phase composition and the Y₂O₃ content in the phases were calculated. At the start of transformation, small grains of a low-Y₂O₃ t phase and a c phase formed. After >30 h at 1300°C and at 1400°C, a mixture of a t phase deficient in Y₂O₃, an m phase, and a c phase formed after cooling, with the Y₂O₃ contents in the phases roughly predicted by the phase diagrams. The results of the present study are discussed here in detail and compared with data for plasma-sprayed coatings.     

结晶性高聚物作为相变材料的应用

本文研究了结晶性高聚物聚乙烯及其共混物作为相变材料对模拟发热体铜片热量的吸收作用，由于相变的吸热，铜片的温度在一定时间范围内保持在一个定值。选择不同的相变材料，可使这种温度平台出现在不同的温度范围，不共晶的共混物可有两个以上的温度平台，提高了降温效果。     

聚合物共混体系中分散相在物理老化过程的尺寸变化

文章研究了退火过程对乙烯-辛烯共聚物/热塑性聚氨酯不相容共混物的相聚集现象的影响,利用扫描电子显微镜分析分散相的粒径,结果表明,随着退火温度的提高和时间的延长,分散相的粒径变大、分布变宽,聚集长大机理起关键作用。     

Low-Temperature Hydrothermal Synthesis of Yttrium-Doped Zirconia Powders

The feasibility of low-temperature synthesis of yttrium-doped zirconia (Y-ZrO₂) crystalline powders in aqueous solutions at lessthan equal to100°C has been evaluated, and the hydrothermal crystallization mechanism for Y-ZrO₂ powders also has been investigated. Coprecipitated (Y,Zr) hydroxide gel, mechanical mixtures of Y(OH)₃ and Zr(OH)₄ gel, and Y(OH)₃ gel have been reacted in boiling alkaline solutions. Coprecipitated (Y,Zr) hydroxide gel crystallized to cubic or tetragonal Y-ZrO₂ at pH 13.9. The yttrium content in the powder synthesized from coprecipitated (Y,Zr) hydroxide is consistent with the initial precursor solution composition, as expected from the similarity in solubility of Zr(OH)^-₅ and Y(OH)^-₄. A diffusionless mechanism for the transformation of the (Y,Zr) hydroxide gel to Y-ZrO₂ is proposed, and the phase stability in aqueous solution is discussed in terms of an in situ crystallization model. It is also demonstrated through thermodynamic arguments with experimental verification that the stable form of the Y-ZrO₂ at 25°C is the anhydrous phase, not the metal hydroxide as previously thought.     

Dielectric Relaxation of La-Doped Zirconia Caused by Annealing Ambient

La-doped zirconia films, deposited by ALD at 300°C, were found to be amorphous with dielectric constants (k-values) up to 19. A tetragonal or cubic phase was induced by post-deposition annealing (PDA) at 900°C in both nitrogen and air. Higher k-values (~32) were measured following PDA in air, but not after PDA in nitrogen. However, a significant dielectric relaxation was observed in the air-annealed film, and this is attributed to the formation of nano-crystallites. The relaxation behavior was modeled using the Curie–von Schweidler (CS) and Havriliak–Negami (HN) relationships. The k-value of the as-deposited films clearly shows a mixed CS and HN dependence on frequency. The CS dependence vanished after annealing in air, while the HN dependence disappeared after annealing in nitrogen.     

Prevention of Low-Temperature Surface Transformation by Surface Recrystallization in Yttria-Doped Tetragonal Zirconia

The low-temperature (100° to 400°C) tetragonal to monoclinic transformation in yttria-doped tetragonal zirconia (Y-TZP) can be inhibited by a postsintering grinding and annealing treatment. The surface region so treated contains fine tetragonal grains which have recrystallized from the severely damaged ground surface. The various features of the recrystallized surface that may affect the lowtemperature transformation are analyzed.     

High-Pressure Phase Transitions in Zirconia and Yttria-Doped Zirconia

Raman spectroscopy has been utilized to characterize the phase transformations and transition pressures in pure and doped zirconia containing 3, 4, and 5 wt% Y₂O₃. The pressure-induced transformations were investigated to over 6 GPa (at room temperature) using a diamond anvil pressure cell. Pure zirconia single-crystal samples transformed to a "new" tetragonal phase (different from the one obtained at high temperatures at atmospheric pressure) at about 4 GPa. The pressure transformation, like the temperature transition, was reversible and exhibited an approximately 0.45-GPa hysteresis at room temperature. The 3 and 4 wt% Y₂O₃ crystals underwent a monoclinic ( P 2_1/b) to tetragonal ( P 4_{2 nmc}) phase transition similar to that observed at high temperatures. This phase change was found to be irreversible on releasing the pressure. The 5 wt% Y₂O₃ at atmospheric pressure consists of a tetragonal modification in a disordered cubic matrix; a gradual, but reversible, disordering transformation of the tetragonal precipitate takes place with pressure.     

Low-Temperature Aging of Zirconia Ferrules for Optical Connectors

Zirconia ferrules for optical connectors were examined after aging at 85°C and 95% relative humidity. Two degradation mechanisms were the roughening and the deformation of the zirconia ferrule surface. Raman microscopy revealed that this relatively low-temperature degradation of zirconia ferrules is caused by the tetragonal to monoclinic transformation of zirconia, and is accelerated by stress relief during polishing. The surface upheavals associated with low-temperature aging may significantly degrade the performance of optical connectors over time.     

氧化锆陶瓷低温老化研究进展

氧化锆陶瓷具有优异的力学性能、化学稳定性和白色美学特性,已广泛用于与牙齿相关的修复体中,如牙冠、牙桥、基台以及最近的种植体。然而,氧化锆陶瓷的生物安全性受到低温老化(LTD)的威胁。LTD现象发生在低温潮湿环境中,例如在人体环境中,氧化锆陶瓷的强度在短期内迅速降低,进而导致早期失效。本文从表征方法、影响因素以及老化理论模型等角度,对近年来各国学者对氧化锆陶瓷LTD现象开展的相研究进行了综述,并对相关研究结果进行梳理了及归纳。     

Low-Temperature Processing of Dense Hydroxyapatite–Zirconia Composites

Hydroxyapatite (HA)–YTZP (2, 5, 7.5, and 10 wt% ZrO₂) composite powders prepared from inorganic precursors were characterized by FTIR, DSC/TG, XRD, and TEM. The calcined powders had HA and t / c -ZrO₂, which undergo structural changes between 650°C and 1050°C. TEM of calcined powder showed larger HA particles (100 nm) and smaller ZrO₂ particles (≤50 nm). HA and HA–2 wt% ZrO₂-sintered samples had 98% density and it was (90–95%) for HA–5, 7.5, and 10 wt% ZrO₂. The bending strength of HA–2wt% ZrO₂ composites was 72 MPa. The grain sizes of HA showed a refinement with ZrO₂ addition.     

Improved Low-Temperature Environmental Degradation of Yttria-Stabilized Tetragonal Zirconia Polycrystals by Surface Encapsulation

An encapsulating layer was deposited on the surface of tetragonal zirconia polycrystals doped with 3 mol% of yttria (3Y-TZP), to prevent low-temperature environmental degradation (aging) of the material. The layer, which was composed of silica and zircon, was formed on the surface by exposing the specimens next to a bed of silicon carbide powder in a flowing hydrogen atmosphere that contained ∼0.1% water vapor at 1450°C. The layer was ∼0.5 µm thick and is expected to be under strong residual compressive stress. This encapsulation process remarkably improved the low-temperature degradation of the material. The strength of the specimens also was improved by this process.     

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.     

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.     

Phase Morphology in Electrospun Zirconia Microfibers

Electrospinning of sol–gels has been used to produce zirconium-doped polymer microfibers from zirconyl chloride and poly(vinylpyrollidone) precursors. Calcination of these structures between temperatures of 370° and 930°C resulted in the formation of zirconia nanograined microfibers whose diameters ranged from 1200 to 800 nm at the higher temperatures and whose average grain size ranged from 9 to 33 nm. X-ray diffraction analysis revealed varying amounts of monoclinic and tetragonal zirconia present in the fibers and established how this varied with calcination temperature and time. The tetragonal phase was shown to be unstable and disappeared on heating the material beyond around 750°C. The amount of zirconia yielded from the precursor material was measured and was found to be consistently greater than the theoretical yield. Average grain size within the microfibers increased with increasing calcination temperature and is effectively doubled when a 10 kPa pressure was applied. The effect of pressure also results in the creation of new crystal structures within the nanofibers and, as with traditional zirconia processing, the addition of impurity ions was found to stabilize the tetragonal phase.     

Annealing of Test Specimens of High-Toughness Magnesia-Partially-Stabilized Zirconia

The peak toughness of a commercial magnesia-partially-stabilized zirconia was strongly dependent on a postprocessing, postspecimen preparation, and modest annealing (1000°C for 20 min), increasing it from ∼9 to almost ∼18 MP a·m^1/2. Further study of this surprising phenomenon revealed that increases in toughness occurred for annealing temperatures as low as 275°C. The low toughness of the as-received materials is due to a post-manufacture room-temperature aging.