Densification and grain growth of 8YSZ containing NiO

The effects of NiO addition on sintering yttria-stabilized zirconia were systematically studied to understand the role of the additive in the sintering process of the solid electrolyte. Specimens of 8 mol% yttria-stabilized zirconia with NiO contents up to 5.0 mol% were prepared using different Ni precursors and sintered at several dwell temperatures and holding times. Densification and microstructural features were studied by apparent density measurements and scanning electron microscopy observations, respectively. The sintering dynamic study was carried out by following the linear shrinkage of powder compacts containing 0-0.75 mol% NiO. Small (up to 1.0 mol%) NiO addition was found to improve the sinterability of yttria-stabilized zirconia. The activation energy for volume diffusion decreases with increasing NiO content, whereas the grain boundary diffusion seems to be independent on this additive. The grain growth of yttria-stabilized zirconia is found to be enhanced even for small NiO contents.     

Magnetic field oriented tetragonal zirconia with anisotropic toughness

(0 0 1)-oriented 3 mol% yttria stabilized tetragonal zirconia (3Y-TZP) has been developed by reactive synthesis of undoped pure monoclinic zirconia and co-precipitated 8 mol% yttria-stabilized zirconia (8Y-ZrO₂). The dispersed pure monoclinic ZrO₂ powder, having magnetic anisotropy, was first aligned in a strong magnetic field and co-sintered in a randomly distributed cubic 8Y-ZrO₂ fine matrix powder. The reactive sintering resulted in a 3Y-TZP ceramic with a (0 0 1) orientation. The (0 0 1)-oriented 3Y-TZP showed a substantial toughness anisotropy, i.e. the toughness along the [0 0 1] direction is 54% higher than that of its perpendicular direction. Moreover, the toughness along the [0 0 1] direction is 49% higher than that of a non-textured isotropic reactively synthesized 3Y-TZP and 110% higher than that of an isotropic co-precipitated powder based 3Y-TZP. The substantially enhanced toughness was interpreted in terms of the tetragonal to monoclinic martensitic phase transformability.     

Investigation of Plasma-Sprayed Dysprosia Coatings

Plasma-sprayed coatings of dysprosia were made and characterized. The coatings consisted mostly of the metastable, monoclinic phase and minor amounts of the cubic phase. Grinding did not transform the high-temperature monoclinic phase, but, on annealing in air at 800° for 5 h, 50% reversion to the cubic phase occurred. Commercial yttria-stabilized zirconia thermal barrier coatings out-performed unannealed plasma-sprayed dysprosia coatings in terms of wear resistance.     

CHARACTERIZATION OF ZIRCONIA POWDER SYNTHESIZED VIA REVERSE MICROEMULSION PRECIPITATION

A two-reverse-microemulsion precipitation technique was applied to synthesize nanocrystalline tetragonal zirconia using H₂O solution/CTAB/hexanol as the microemulsion system. Two solutions of reverse microemulsion, one containing Zr⁴⁺ aqueous droplets and the other aqueous ammonia droplets with the same water/surfactant ratio, were prepared separately and mixed together to form a slurry of nanosized ZrO₂ precursors, which filled the matrix of the surfactant, CTAB. The precursors were recovered, calcined to form nanocrystalline zirconia powder, and then characterized by using a transmission electron microscope to determine the particle size, a scanning electron microscope to examine the microstructure of the zirconia powder, and an X-ray diffractometer to determine the crystal phase and crystallite size. It is concluded that the primary particle size of the precursor determines the transformation temperature of the precursor and the crystal structure of the calcined zirconia.     

Preparation of Monodisperse and Spherical Zirconia Powders by Heating of Alcohol–Aqueous Salt Solutions

A novel method is demonstrated which yields a spherical ZrO₂ powder of narrow size distribution through heating of a zirconyl chloride solution with an alcohol–water mixture as the solvent. The kind and composition of the solvent mixture greatly influenced the behavior of the precipitation and the morphology of the resulting particles. When 1-propanol or 2-propanol was employed as the alcohol of the solvent mixture, the resulting particles had a spherical shape and a narrow size distribution. The particle size and the particle agglomeration level could be controlled by the amount of hydroxypropyl cellulose (HPC) in the solution. As-prepared amorphous powder was crystallized to a mixture of metastable tetragonal phase and monoclinic phase at about 460°C. The metastable tetragonal phase was converted to the monoclinic phase as the calcination temperature was increased. After calcination, the spherical shape of the zirconia powder was retained, while its particle size was decreased slightly.     

Solid Yttria-Stabilized Zirconia Films by Pulsed Chemical Vapor Deposition from Metal-organic Precursors

A pulsed chemical vapor deposition from metal-organic precursors (MOCVD) system was used to produce solid zirconia, and yttria-stabilized zirconia (YSZ) films. A total of six candidate metal-organic precursors for zirconia and three for yttria were investigated. Three precursor solutions for YSZ proved suitable for pulsed-MOCVD processing. Layers were deposited on metal, alumina, and porous nickel cermet substrates. Under optimal deposition conditions, precursor conversion efficiency of 90% was achieved using a solution of 3.74 vol% zirconium 2-methyl-2-butoxide + 0.42% yttium methoxyethoxide in toluene. The film growth rate was 7.5 μm·h⁻¹ at 525°C deposition temperature. Two alkoxide precursors produced YSZ layers with material costs under $0.50/(μm·cm²).     

CHARACTERIZATION OF ZIRCONIA POWDER SYNTHESIZED VIA REVERSE MICROEMULSION PRECIPITATION

A two-reverse-microemulsion precipitation technique was applied to synthesize nanocrystalline tetragonal zirconia using H₂O solution/CTAB/hexanol as the microemulsion system. Two solutions of reverse microemulsion, one containing Zr⁴⁺ aqueous droplets and the other aqueous ammonia droplets with the same water/surfactant ratio, were prepared separately and mixed together to form a slurry of nanosized ZrO₂ precursors, which filled the matrix of the surfactant, CTAB. The precursors were recovered, calcined to form nanocrystalline zirconia powder, and then characterized by using a transmission electron microscope to determine the particle size, a scanning electron microscope to examine the microstructure of the zirconia powder, and an X-ray diffractometer to determine the crystal phase and crystallite size. It is concluded that the primary particle size of the precursor determines the transformation temperature of the precursor and the crystal structure of the calcined zirconia.     

Microstructure and synthesis mechanism of dysprosia-stabilized zirconia nanocrystals via chemical coprecipitation

In this study, zirconia (ZrO₂) and dysprosia-stabilized zirconia (DySZ) nanocrystals were synthesized using a chemical coprecipitation method. The crystal structure and micromorphology of the as-synthesized powders, as well as the structural evolution from precursors to oxides were investigated, and the synthesis mechanism was also examined. Results show that pure ZrO₂ powders mainly comprise the monoclinic ZrO₂ phase with trace tetragonal ZrO₂, while the DySZ powders exhibit a tetragonal ZrO₂ structure. In addition, the crystal growth rate of DySZ is far slower than that of the pure ZrO₂ under elevated calcination temperature. The addition of Dy could significantly improve the phase stability of DySZ powder and effectively inhibit the crystal growth of DySZ. In the DySZ precursor, the binding energy of chemical bonds is significantly difference than in the ZrO₂ precursor. A composite hydroxide can be formed with -Zr-OH-Dy- and -Zr-OH-Zr- units in the tetramer structure because of the in-situ substitution of Zr by Dy atoms. Both the ZrO₂ and DySZ precursors exhibit analogous dehydration and crystallization behaviours in calcination process. Dy-doping plays a significant role in stabilizing both the intermediate product and the DySZ crystal.     

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.     

Destabilization of Zirconia Thermal Barriers in the Presence of V2O5

Impurities, such as vanadium, degrade the operating performance of yttria-stabilized zirconia thermal-barrier coatings. V₂O₅ reacts preferentially with Y₂O₃, forms YVO₄, and leads to the destabilization of zirconia thermal-barrier material. A model experiment has been designed to monitor the destabilization of zirconia thermal barriers by directly exposing thin films of yttria-stabilized zirconia to V₂O₅ vapor. The growth of YVO₄ from yttria-stabilized zirconia and the destabilization of cubic yttria-stabilized zirconia into tetragonal and/or monoclinic zirconia polymorphs are monitored by selected-area diffraction and energy-dispersive X-ray spectroscopy in the transmission electron microscope. A special crystallographic orientation relation between YVO₄ and cubic zirconia is discussed.     

Sintering behavior and phase transformation of YSZ-LZ composite coatings

Sintering behavior and phase transformations in yttria-stabilized zirconia (YSZ)-based thermal barrier coatings (TBCs) control their applications in gas turbines operated at high working temperatures required for improved fuel efficiency. In this work, to control the sintering behavior and reduce phase transformations in YSZ-based TBCs, lanthanum zirconate (LZ) powder was blended with the YSZ feedstock powder, and YSZ-LZ composite coatings were fabricated using the air plasma spraying method. The influence of mixture weight ratio of YSZ to LZ (75:25, 50:50, and 25:75) on the sintering behavior and phase stability of the composite coatings was investigated through the isothermal exposure test at 1100, 1300, and 1400 °C. The as-coated composites showed the pyrochlore and tetragonal phases, indicating that the phases are LZ and YSZ, respectively. As the exposure temperature was increased, the phase transformation of YSZ from the tetragonal phase to the monoclinic phase was accelerated. The content of monoclinic phase was changed with the increasing LZ content after thermal exposure at 1300 and 1400 °C. In addition, the composites showed different sintering and bridging behaviors at the adjacent splats with the LZ content. The composites prepared with the blended feedstock powders of LZ and YSZ produced an obvious effect on the phase stability and mechanical properties.     

Influence of zirconium salt precursors on the crystal structures of zirconia

Procedures have been developed for preparing zirconia to yield either tetragonal or monoclinic phases following low (400–600 °C) temperature calcination. The data in this note emphasize the need for paying attention to the precursor zirconium salt in preparing the monoclinic or tetragonal phases. A salt precursor obtained from a supplier at different times may produce different results. Generally, it is more difficult to obtain a precursor that will produce the monoclinic phase than the one that will produce the tetragonal form.     

Effect of iron oxide coloring agent on the sintering behavior of dental yttria-stabilized zirconia

To explore the use of yttria-stabilized zirconia (YSZ) for applications in dentistry, the effect of iron oxide coloring agent on the sintering behavior of YSZ is investigated. Through the use of a small amount of iron nitrate, the color of YSZ can be tailored. The iron nitrate starts to decompose to result in iron oxide, then to dissolve into zirconia grains before the shrinkage is even started. The iron solutes enhance the sintering activity of zirconia in terms of the temperatures at the start of shrinkage and at the maximum shrinkage rate. However, the size of zirconia grains is also increased along with Fe content. More monoclinic phase is found in the specimens with higher Fe content. The formation of m-phase is detrimental to both hardness and toughness of zirconia, limiting the amount of coloring agent can be added.     

Preparation of zirconia powders by sol-gel route of sodium glycozirconate complex

Zirconia powders were prepared by a sol-gel method, using sodium glycozirconate complex as precursor synthesized via the Oxide One Pot Synthesis (OOPS) process. Gelation of this precursor was achieved through the variation of the hydrolysis ratio without the use of the dopants. The gel samples were also calcined at different temperatures. The resulting zirconia was characterized using X-ray diffraction (XRD), SEM and nitrogen adsorption/desorption. The solid materials obtained after heat treatment at 500 °C by varying the hydrolysis ratio have large surface areas of 154-220 m² g⁻¹ and a narrow pore size distribution in the mesopore region. By variation of the heat treatment, the zirconia xerogels existed in either an amorphous, tetragonal, or monoclinic form at room temperature. Based on XRD data the first identifiable crystalline structure developed from the amorphous phase was the tetragonal polymorph, which was formed between 500 and 800 °C. When the temperature was raised to 1000 °C, zirconia powder with a monoclinic structure was obtained. Surface areas about 280 m² g⁻¹ was obtained after calcination at 400 °C, which drop to ca. 70 m² g⁻¹ following treatment at 1000 °C.     

Electrospinning of Y2O3- and MgO-stabilized zirconia nanofibers and characterization of the evolving phase composition and morphology during thermal treatment

The current paper focuses on the fabrication of yttria and magnesia stabilized zirconia nanofibers via electrospinning from zirconyl chloride octahydrate and polyvinylpyrrolidone precursors with minor additions of yttrium nitrate hexahydrate (3 mol.%) or magnesium nitrate hexahydrate (10 mol.%). The precursor materials were dissolved in an ethanol-water mixture in a ratio of 75:25. After successful fiber preparation, the thermal decomposition behavior of the starting materials and the subsequent phase evolution at elevated temperatures were studied. Pure tetragonal zirconia nanofibers were obtained for the composition stabilized with 3 mol.% yttria when the thermal treatment was conducted with a heating rate of 10 K/min up to 1100 °C. In future research work, these tetragonal zirconia nanofibers will be used as reinforcing material in metal matrix composites based on metastable austenitic steel. The combination of the TRIP/TWIP-effect in the steel matrix with the stress-assisted tetragonal to monoclinic phase transformation in the tetragonal stabilized zirconia will lead to a composite material with outstanding mechanical properties.     

Hydroconversion of n-hexadecane with Pt-promoted monoclinic and/or tetragonal sulfated zirconia catalysts

A series of mixed Pt-promoted monoclinic and tetragonal sulfated zirconia catalysts were prepared and characterized. The catalysts contained 27–86% of the monoclinic phase. The zirconia samples were prepared by varying the speed of precipitation of the hydrous zirconia and the pH of the final solution. The hydrous zirconia was then calcined prior to promotion with Pt and sulfate. The catalysts were activated just prior to activity studies with n-hexadecane. This synthesis route and pretreatment produced mixed-phase catalysts which yielded comparable conversion and selectivity data to that of a purely tetragonal catalyst using optimum conditions. The data show that an active catalyst can be obtained with the monoclinic phase present and that addition of sulfate after development of the crystalline phases can yield an active catalyst.     

Zirconia Nanoparticles Made in Spray Flames at High Production Rates

Synthesis of zirconia nanoparticles by flame spray pyrolysis (FSP) at high production rates is investigated. Product powder is collected continuously in a baghouse filter unit that is cleaned periodically by air-pressure shocks. Nitrogen adsorption (BET), X-ray diffractometry (XRD), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) are used to characterize the product powder. The effect of powder production rate (up to 600 g/h), dispersion gas flow rate, and precursor concentration on product particle size, crystallinity, morphology, and purity is investigated. The primary particle size of zirconia is controlled from 6 to 35 nm, while the crystal structure consists of mostly tetragonal phase (80–95 wt%), with the balance monoclinic phase at all process conditions. The tetragonal crystal size is close to the primary particle size, which indicates weak agglomeration of single crystals.     

Phase Evolution in Yttria-Stabilized Zirconia Thermal Barrier Coatings Studied by Rietveld Refinement of X-Ray Powder Diffraction Patterns

One failure mechanism of thermal barrier coatings composed of yttria-stabilized zirconia (YSZ) has been proposed to be caused, in part, by the transformation of the tetragonal phase of YSZ into its monoclinic phase. Normally, studies of phase evolution are performed by X-ray diffraction (XRD) and by evaluating the intensities of a few diffraction peaks for each phase. However, this method misses some important information that can be obtained with the Rietveld method. Using Rietveld's refinement of XRD patterns, we observed, upon annealing of YSZ coatings, an increase of cubic phase content, a reduction in as-deposited tetragonal phase content, and the appearance of a new tetragonal phase having a lower yttria content that coexists with the as-deposited tetragonal phase of YSZ.     

Solid-Solution Effects of a Small Amount of Nickel Oxide Addition on Phase Stability and Mechanical Properties of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Stability and mechanical properties of the tetragonal phase were investigated for NiO-doped yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) systems. Only 0.3 mol% of NiO in solid solution could be added to the Y-TZP while maintaining the tetragonal phase. Fracture toughness improved remarkably on addition of a small amount of NiO. Raman spectroscopy analysis around cracks introduced by Vickers indentation revealed that the amount of monoclinic phase transformed from tetragonal phase was increased. It was confirmed that fracture toughness improvement was due not only to increased grain size, but also to Y-TZP destabilization by solid solution of NiO.     

The influence of stresses on ageing kinetics of 3Y- and 4Y- stabilized zirconia

Hydrothermal ageing is one of the most important limiting factors for the use of yttria-stabilized zirconia ceramics in contact with water-containing environments. It consists in the transformation of tetragonal phase to monoclinic phase, initiates on the surface of zirconia in the presence of water, and leads to roughening and potentially to micro-cracking and loss of integrity. The present work seeks to explore the influence of applied and residual mechanical stresses on the ageing kinetics of 3Y- and 4Y-TZP. Residual stresses were obtained by rough polishing. A subsequent Annealing step was employed for the preparation of samples free of residual stresses. All samples were submitted to in situ 3-points bending tests in water vapour atmosphere inside an autoclave at 134 °C, allowing surfaces with a mechanical stress gradient to be exposed to hydrothermal ageing. The evolution of the monoclinic fraction with time and stress was then analyzed using Mehl-Avrami-Johnson equation.