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Zirconia
Kinds of Zirconia Terms modified by Zirconia Selected AbstractsTris(2,2,-bipyridyl)ruthenium(II) Electrogenerated Chemiluminescence Sensor Based on Platinized Carbon Nanotube,Zirconia,Nafion Composite FilmsELECTROANALYSIS, Issue 12 2010Hyun Yoon Abstract Mesoporous films of platinized carbon nanotube,zirconia,Nafion composite have been used for the immobilization of tris(2,2,-bipyridyl)ruthenium (II) (Ru(bpy)32+) on an electrode surface to yield a solid-state electrogenerated chemiluminescence (ECL) sensor. The composite films of Pt,CNT,zirconia,Nafion exhibit much larger pore diameter (3.55,nm) than that of Nafion (2.82,nm) and thus leading to much larger ECL response for tripropylamine (TPA) because of the fast diffusion of the analyte within the films. Due to the conducting and electrocatalytic features of CNTs and Pt nanoparticles, their incorporation into the zirconia,Nafion composite films resulted in the decreased electron transfer resistance within the films. The present ECL sensor based on the Pt,CNT,zirconia,Nafion gave a linear response (R2=0.999) for TPA concentration from 3.0,nM to 1.0,mM with a remarkable detection limit (S/N=3) of 1.0,nM, which is much lower compared to those obtained with the ECL sensors based on other types of sol-gel ceramic,Nafion composite films such as silica,Nafion and titania,Nafion. [source] Continuum Mechanical Approach to Sintering of Nanocrystalline Zirconia,ADVANCED ENGINEERING MATERIALS, Issue 10 2005R. Zuo Nanocrystalline 3,mol,% yttria-stabilized zirconia was sinter-forged isothermally under varying external uniaxial stresses. The applied uniaxial stresses were relatively low, compared to the intrinsic sintering stress of the material studied. Uniaxial sintering stresses and uniaxial viscosities were experimentally determined as function of density by means of a continuum mechanical approach which involves measuring the sintering rate of a free-sintered specimen, and a specimen sintered under the application of an external uniaxial stress. The uniaxial viscosity increased strongly with density only in the final stage sintering regime. The magnitude of the uniaxial sintering stress exhibited a decrease with density. [source] Low-Temperature and High-Strain Rate Superplastic ZirconiaADVANCED ENGINEERING MATERIALS, Issue 3 2003Y. Sakka A superplastic ceramic with a strain rate comparable to metals based on 3% Y2O3 -doped tetragonal zirconia (3YTZ) was synthesized at relatively low temperature, based on the doping with MgO and TiO2. The desirable material properties result from both a refined preparation procedure for the zirconia, and from enhanced cation lattice diffusion due to the MgO and TiO2 doping. [source] Low-Temperature Superionic Conductivity in Strained Yttria-Stabilized ZirconiaADVANCED FUNCTIONAL MATERIALS, Issue 13 2010Michael Sillassen Abstract Very high lateral ionic conductivities in epitaxial cubic yttria-stabilized zirconia (YSZ) synthesized on single-crystal SrTiO3 and MgO substrates by reactive direct current magnetron sputtering are reported. Superionic conductivities (i.e., ionic conductivities of the order ,1 ,,1cm,1) are observed at 500,°C for 58-nm-thick films on MgO. The results indicate a superposition of two parallel contributions , one due to bulk conductivity and one attributable to conduction along the film,substrate interface. Interfacial effects dominate the conductivity at low temperatures (<350,°C), showing more than three orders of magnitude enhancement compared to bulk YSZ. At higher temperatures, a more bulk-like conductivity is observed. The films have a negligible grain-boundary network, thus ruling out grain boundaries as a pathway for ionic conduction. The observed enhancement in lateral ionic conductivity is caused by a combination of misfit dislocation density and elastic strain in the interface. These very high ionic conductivities in the temperature range 150,500,°C are of great fundamental importance but may also be technologically relevant for low-temperature applications. [source] Microstructure Control of Sintered Porous Yttria-Stabilized Zirconia as a Durable Thermal Shielding MaterialINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 3 2009Kazuya Sasaki The microstructure of a thermal shielding material affects its thermal conductivity and mechanical property. In this study, the effects of the sintering temperature and the polymethyl methacrylate powder as a pore-former on the microstructure of a sintered porous yttria-stabilized zirconia (YSZ), which is used as a durable thermal shielding material, were investigated. It became clear that the microstructure of the sintered YSZ could be controlled by the particle size and the amount of the pore-former and the sintering temperature. The effect of the yttria amount in the YSZ on the microstructure was also clarified. [source] A New Method for Post-Synthesis Coating of Zirconia on the Mesopore Walls of SBA-15 Without Pore Blocking,ADVANCED MATERIALS, Issue 11 2008Cheralathan Kanakkampalayan Krishnan Zirconia coating of the mesopore walls of the mesoporous silica material SBA-15 is achieved by internal hydrolysis of a zirconia precursor, which is loaded inside the mesopores, using NH3/H2O vapor at elevated temperature and subsequent calcination (see figure). High loadings of zirconia, more than 30 wt%, can be coated on the mesopore walls without any pore blocking. [source] Thermal Barrier Coatings Design with Increased Reflectivity and Lower Thermal Conductivity for High-Temperature Turbine ApplicationsINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 2 2006Matthew J. Kelly High reflectance thermal barrier coatings consisting of 7% Yittria-Stabilized Zirconia (7YSZ) and Al2O3 were deposited by co-evaporation using electron beam physical vapor deposition (EB-PVD). Multilayer 7YSZ and Al2O3 coatings with fixed layer spacing showed a 73% infrared reflectance maxima at 1.85 ,m wavelength. The variable 7YSZ and Al2O3 multilayer coatings showed an increase in reflection spectrum from 1 to 2.75 ,m. Preliminary results suggest that coating reflectance can be tailored to achieve increased reflectance over a desired wavelength range by controlling the thickness of the individual layers. In addition, microstructural enhancements were also used to produce low thermal conductive and high hemispherical reflective thermal barrier coatings (TBCs) in which the coating flux was periodically interrupted creating modulated strain fields within the TBC. TBC showed no macrostructural differences in the grain size or faceted surface morphology at low magnification as compared with standard TBC. The residual stress state was determined to be compressive in all of the TBC samples, and was found to decrease with increasing number of modulations. The average thermal conductivity was shown to decrease approximately 30% from 1.8 to 1.2 W/m-K for the 20-layer monolithic TBC after 2 h of testing at 1316°C. Monolithic modulated TBC also resulted in a 28% increase in the hemispherical reflectance, and increased with increasing total number of modulations. [source] Structure of nanoporous zirconia-based powders synthesized by different gel-combustion routesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2007Jorge R. Casanova Zirconia-based ceramics that retain their metastable tetragonal phase at room temperature are widely studied due to their excellent mechanical and electrical properties. When these materials are prepared from precursor nanopowders with high specific surface areas, this phase is retained in dense ceramic bodies. In this work, we present a morphological study of nanocrystalline ZrO2,2.8 mol% Y2O3 powders synthesized by the gel-combustion method, using different organic fuels , alanine, glycine, lysine and citric acid , and calcined at temperatures ranging from 873 to 1173,K. The nanopore structures were investigated by small-angle X-ray scattering. The experimental results indicate that nanopores in samples prepared with alanine, glycine and lysine have an essentially single-mode volume distribution for calcination temperatures up to 1073,K, while those calcined at 1173,K exhibit a more complex and wider volume distribution. The volume-weighted average of the nanopore radii monotonically increases with increasing calcination temperature. The samples prepared with citric acid exhibit a size distribution much wider than the others. The Brunauer,Emmett,Teller technique was used to determine specific surface area and X-ray diffraction, environmental scanning electron microscopy and transmission electron microscopy were also employed for a complete characterization of the samples. [source] Techniques for oxygen transfer measurement in bioreactors: a reviewJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 8 2009S Suresh Abstract Oxygen is the most essential requirement for aerobic bioprocesses. The microbial growth in a bioreactor depends upon the oxygen transfer rate (OTR). The OTR is widely used to study the growth behavior of microbial and plant cell cultures. The mass transfer coefficient (kLa) determines the magnitude of the OTR. There are many techniques for measuring oxygen concentration and OTR in bioreactors. Zirconia, electrochemical, infrared, ultrasonic and laser cells are used to measure oxygen concentration in the liquid medium. Optical sensors are better alternatives to measure oxygen concentration in small bioreactors. Sulfite oxidation and gassing-out methods with a Clark-type electrode have been used for OTR measurements in bioreactors. Many new novel techniques have evolved recently for intermittent and continuous online measurement of OTR/kLa in various types of bioreactors. The present paper gives an overview of various measurement techniques and their limitations and/or suitability for measurement of OTR/kLa in various kinds of bioreactors, especially small bioreactors. Copyright © 2009 Society of Chemical Industry [source] The ZiReal Post: A New Ceramic Implant AbutmentJOURNAL OF ESTHETIC AND RESTORATIVE DENTISTRY, Issue 1 2003URS BRODBECK DMD ABSTRACT Restorations in the anterior esthetic zone present significant challenges in both the surgical and prosthetic phases of implant dentistry. Titanium has been established as the material of choice for endosseous implants, resulting in a high degree of predictability. Many types of implants require transmucosal abutments to retain implant restorations. Ceramics may be the ideal material to replace natural teeth, but most transmucosal abutments are made of titanium. However, ceramics may also be used as abutments in implant restorations. This combination of ceramics for abutment and crown provides better translucency for the implant restoration than is available with metal abutments and porcelain-fused-to-metal crowns. Ceramic abutments and implant restorations also minimize the gray color associated with metal components that is transmitted through the peri-implant tissues. Customized emergence profiles also may be obtained with ceramic abutments; this generally improves the predictability and consistency of the esthetics obtainable in implant restorations. Zirconia as a ceramic material offers not only outstanding material properties but also a well-documented biocompatibility. CLINICAL SIGNIFICANCE This article discusses the clinical and laboratory features of a new ceramic abutment, ZiRealÔ Post (Implant Innovations, Inc., Palm Beach Gardens, Florida). [source] Clinical Success of Zirconia in Dental ApplicationsJOURNAL OF PROSTHODONTICS, Issue 1 2010Zeynep Özkurt DDS Abstract The application of ceramic materials for the fabrication of dental restorations is a focus of interest in esthetic dentistry. The ceramic materials of choice are glass ceramics, spinel, alumina, and zirconia. Zirconia was introduced into dentistry in the 1990s because of its good mechanical and chemical properties and is currently being used as a material for frameworks, dowels, implants, abutments, and orthodontic brackets. Many in vitro studies about zirconia use have been published, but clinical long-term studies are very important. This article presents data regarding the incidence of clinical success and complications of zirconia in these dental applications. Clinical studies published to date seem to indicate that zirconia is well tolerated and sufficiently resistant. [source] Sintering of Transparent Yttria Ceramics in Oxygen AtmosphereJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2010Yihua Huang A novel method is reported for the preparation of transparent polycrystalline yttria ceramics in oxygen atmosphere. Zirconia and other additives were added to control the grain growth. Pores can be eliminated clearly at a temperature <1650°C with the grain size around 1 ,m. The grain growth kinetics and the mechanisms controlling grain growth were studied. Sintering in oxygen atmosphere is beneficial for making samples with a big size at low cost and avoiding the posttreatment of samples sintered in vacuum or hydrogen atmosphere. [source] Surface Enthalpy, Enthalpy of Water Adsorption, and Phase Stability in Nanocrystalline Monoclinic ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2009A. V. Radha A fundamental issue that remains to be solved when approaching the nanoscale is how the size induces transformation among different polymorphic structures. Understanding the size-induced transformation among the different polymorphic structures is essential for widespread use of nanostructured materials in technological applications. Herein, we report water adsorption and high-temperature solution calorimetry experiments on a set of samples of single-phase monoclinic zirconia with different surface areas. Essential to the success of the study has been the use of a new ternary water-in-oil/water liquid solvothermal method that allows the preparation of monoclinic zirconia nanoparticles with a broad range of (BET) Brunauer,Emmett,Teller surface area values. Thus, the surface enthalpy for anhydrous monoclinic zirconia is reported for the first time, while that for the hydrous surface is a significant improvement over the previously reported value. Combining these data with previously published surface enthalpy for nanocrystalline tetragonal zirconia, we have calculated the stability crossovers between monoclinic and tetragonal phases to take place at a particle size of 28 ± 6 nm for hydrous zirconia and 34 ± 5 nm for anhydrous zirconia. Below these particle sizes, tetragonal hydrous and anhydrous phases of zirconia become thermodynamically stable. These results are within the margin of the theoretical estimation and confirm the importance of the presence of water vapor on the transformation of nanostructured materials. [source] Low-Temperature Aging Behavior of Alumina-Toughened ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2008Jens Schneider The corrosion of alumina-toughened zirconia (ATZ) as a consequence of hydrothermal treatment was investigated, especially the transformation kinetics from tetragonal zirconia to the monoclinic phase. For this purpose, polished ATZ specimens (Ra<5 nm) were aged in water vapor at different temperatures ranging from 70° to 134°C. The fraction of the monoclinic phase was determined using X-ray diffraction and Rietveld refinement. The isothermal transformation curves obtained were fitted to the Mehl,Johnson,Avrami equation by least squares. An Arrhenius plot of the fitted transformation rates was used to determine the activation energy and the pre-exponential factor. Following this procedure, the kinetic parameters of the phase transformation were extrapolated down to body temperature and the formation of the monoclinic phase was simulated. In addition, optical interferometry on well-polished specimens (Ra<2 nm) was alternatively used to calculate the monoclinic fraction from the histogram dataset. The results agree very well with those of the X-ray measurements. Additionally, the development of surface roughness with increasing aging time is discussed. [source] Pressureless Densification of Zirconium Diboride with Boron Carbide AdditionsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2006S. C. Zhang Zirconium diboride (ZrB2) was densified (>98% relative density) at temperatures as low as 1850°C by pressureless sintering. Sintering was activated by removing oxide impurities (B2O3 and ZrO2) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (,150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B4C were evaluated as additives to facilitate the removal of ZrO2. Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB2 (surface area ,1 m2/g) or attrition-milled ZrB2 (surface area ,7.5 m2/g) with WC and/or B4C as a sintering aid. ZrB2 containing only WC could be sintered to ,95% relative density in 4 h at 2050°C under vacuum. In contrast, the addition of B4C allowed for sintering to >98% relative density in 1 h at 1850°C under vacuum. [source] Temperature-Dependent Optical Reflectivity of Tetragonal-Prime Yttria-Stabilized ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2006John A. Nychka The optical reflectance of dense, metastable, tetragonal-prime zirconia plates, made by densifying electron beam physical vapor-deposited powder, is reported as a function of temperature up to 1673 K (1400°C) over the range of 400,1500 cm,1 (6.67,25 ,m). Curve fitting of the reflectance as a function of temperature was performed using two different damped oscillator models, each with three infrared (IR)-active modes. Oscillator parameters were then used to calculate the values of the indices of refraction and absorption as a function of temperature using the classical dispersion theory. The reflectance data of tetragonal-prime yttria-stabilized zirconia at room temperature are qualitatively similar to that reported for the equilibrium tetragonal phase in that it can be fit with three IR-active modes. [source] Oxygen Diffusion in Yttria-Stabilized Zirconia: A New Simulation ModelJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2004R. Krishnamurthy We present a multiscale modeling approach to study oxygen diffusion in cubic yttria-stabilized zirconia. In this approach, we employ density functional theory methods to calculate activation energies for oxygen migration in different cation environments. These are used in a kinetic Monte Carlo framework to calculate long-time oxygen diffusivities. Simulation results show that the oxygen diffusivity attains a maximum value at around 0.1 mole fraction yttria. This variation in the oxygen diffusivity with yttria mole fraction and the calculated values for the diffusivity agree well with experiment. The competing effects of increased oxygen vacancy concentration and increasing activation energy and correlation effects for oxygen diffusion with increasing yttria mole fraction are responsible for the observed dopant content dependence of the oxygen diffusivity. We provide a detailed analysis of cation-dopant-induced correlation effects in support of the above explanation. [source] Zirconia,Silica,Carbon Coatings on Ceramic FibersJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2004Emmanuel E. Boakye Precursors for zircon,carbon mixtures were made to coat fibers for ceramic-matrix composites. Precursors were characterized using XRD, TGA, and DTA. Zircon formed from vanadium- or lithium-doped precursors after heat treatments at ,900°C in air, but it did not form at 1200°,1400°C in argon when large amounts of carbon were added. Some precursors were used to coat NextelÔ 720 and Hi-NicalonÔ fibers. The coatings were characterized using SEM and TEM, and coated-fiber tensile strengths were measured. Although zircon formed in powders, only tetragonal-zirconia,silica mixed phases formed in fiber coatings at 1200°C in air. Loss of vanadium oxide flux to the fibers may have caused the lack of conversion to zircon. The strengths of the coated fibers were severely degraded after heat treatment at ,1000°C in air, but not in argon. The coated fibers were compared with zirconia,carbon-coated fibers made using similar methods. Mechanisms for fiber strength degradation are discussed. [source] Mechanical Properties of Monoclinic ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2004Jens Eichler Fracture toughness and fracture strength data are presented for the first time for monoclinic zirconia. An undoped nanocrystalline zirconia powder was sintered at 1100°C and yielded a theoretical density of more than 90% with a grain size of about 150 nm. The surface crack in flexure (SCF) technique was deemed most suitable for nanocrystalline materials. Measurements of Young's modulus and the determination of the fracture origin are also provided. [source] Pressureless Sintering and Mechanical and Biological Properties of Fluor-hydroxyapatite Composites with ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2003Hae-Won Kim Fluor-hydroxyapatite (FHA) fabricated by a reaction between fluorapatite (FA) and hydroxyapatite (HA) was mixed with ZrO2 to produce FHA,ZrO2 composites. When the relative amount of FA to HA increased, the decomposition of the composite was decreased gradually because of the formation of thermally stable FHA solid solutions. With such suppression of decomposition, the FHA,ZrO2 composites retained fully densified bodies. As a result, significant enhancements in mechanical properties, such as hardness, flexural strength, and fracture toughness, were achieved as the relative amount of FA to HA increased. The highest values in strength and toughness were 220 MPa and 2.5 MPa·m1/2, respectively, with FHA,40 vol% ZrO2 composites. In vitro proliferation of osteoblast-like cells (MG63) on the composites showed behavior similar to that observed on pure HA and FHA. Alkaline phosphatase (ALP) activity of the growing cells (HOS) on the composites was slightly down-regulated compared with that on pure HA and FHA at prolonged periods. [source] Synthesis of Highly Porous Yttria-Stabilized Zirconia by Tape-Casting MethodsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2003Martha Boaro Porous ceramics of Y2O3 -stabilized ZrO2 (YSZ) were prepared by tape-casting methods using both pyrolyzable pore formers and NiO followed by acid leaching. The porosity of YSZ wafers increased in a regular manner with the mass of graphite or polymethyl methacrylate (PMMA) to between 60% and 75% porosity. SEM indicated that the shape of the pores in the final ceramic was related to the shape of the pore formers, so that the pore size and microstructure of YSZ wafers could be controlled by the choice of pore former. Dilatometry measurements showed that measurable shrinkage started at 1300 K, and a total shrinkage of 26% was observed, independent of the amount or type of pore former used. Temperature-programmed oxidation (TPO) measurements on the green tapes demonstrated that the binders and dispersants were combusted between 550 and 750 K, that PMMA decomposed to methyl methacrylate between 500 and 700 K, and that graphite combusted above 900 K. The porosity of YSZ ceramics prepared by acid leaching of nickel from NiO,YSZ, with 50 wt% NiO, was studied as a function of NiO and YSZ particle size. Significant changes in pore dimension were found when NiO particle size was changed. [source] Crystallite and Grain-Size-Dependent Phase Transformations in Yttria-Doped ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2003Arun Suresh In pure zirconia, ultrafine powders are often observed to take on the high-temperature tetragonal phase instead of the "equilibrium" monoclinic phase. The present experiments and analysis show that this observation is one manifestation of a much more general phenomenon in which phase transformation temperatures shift with crystallite/grain size. In the present study, the effect of crystallite (for powders) and grain (for solids) size on the tetragonal , monoclinic phase transformation is examined more broadly across the yttria,zirconia system. Using dilatometry and high-temperature differential scanning calorimetry on zirconia samples with varying crystallite/grain sizes and yttria content, we are able to show that the tetragonal , monoclinic phase transformation temperature varies linearly with inverse crystallite/grain size. This experimental behavior is consistent with thermodynamic predictions that incorporate a surface energy difference term in the calculation of free-energy equilibrium between two phases. [source] Interface Structure of an Epitaxial Cubic Ceria Film on Cubic ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2003Chong-Min Wang A cubic CeO2 (001) film with a thickness of ,58 nm was grown epitaxially on Y2O3 -stablized cubic ZrO2 by oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE). The interface was characterized using high-resolution transmission electron microscopy (HRTEM). The interface exhibited coherent regions separated by equally spaced misfit dislocations. When imaged from the [100] direction, the dislocation spacing is 3.3 ± 0.5 nm, which is slightly shorter than the expected value of 4.9 nm calculated from the differences in lattice constants given in the literature, but is fairly consistent with that of 3.9 nm which was calculated using the lattice mismatch measured by electron diffraction. Thus, the results presented here indicate that the lattice mismatch between the film and the substrate is accommodated mainly by interface misfit dislocations above some critical thickness. [source] New Route for the Extraction of Crude Zirconia from ZirconJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2002Nicholas J. Welham A commercial grade of zircon (ZrSiO4) concentrate was mechanically milled with MgO for up to 100 h in a laboratory-scale mill. The resultant powders were subjected to thermal processing, chemical leaching, and X-ray diffraction (XRD). There was no direct evidence of reaction during the milling step, with no new phases evident from XRD. Leaching of the powder showed that a reaction had occurred, and increased solubility with milling time was attributed to the formation of a nanostructured Mg-Zr-Si oxide, which dissolved congruently. Heating the powders resulted in a number of thermal events, including the formation/crystallization of ZrO2 and Mg2SiO4. Thermal treatment of the milled powders allowed selective chemical leaching of the magnesium and silicon, leaving a powder containing ,90% ZrO2. [source] Mechanically Stable Monoclinic Zirconia,Nickel CompositeJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2002José S. Moya A dense (>98% theoretical density), residual-stress-free m-ZrO2/40 vol% nickel composite with K IC, 5.4 MPa·m1/2 and ,f, 225 MPa has been obtained using a simple wet processing route and subsequent sintering at 1430°C in a 90% argon/10% hydrogen atmosphere. The mechanism for release of internal stresses by the composite developed during the t , m transformation on cooling is explained in terms of plastic flow in the infinite nickel cluster formed at the percolation threshold. [source] Comparative Lattice-Dynamical Study of the Raman Spectra of Monoclinic and Tetragonal Phases of Zirconia and HafniaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2002Pierre E. Quintard An interpretation of the Raman spectra of monoclinic ZrO2 and monoclinic HfO2 is made by analyzing the results of the zirconia,hafnia substitution jointly with a lattice dynamical treatment of both structures. The Raman spectra of tetragonal ZrO2 and tetragonal HfO2 are also interpreted. Emphasis is put on their relations to the spectrum of the parent cubic structure and on the position of the soft mode. The band assignment proposed earlier by other researchers is critically reconsidered. [source] Ab Initio Calculations of Pristine and Doped Zirconia ,5 (310)/[001] Tilt Grain BoundariesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2002Zugang Mao The structure of the cubic-ZrO2 symmetrical tilt ,5 (310)/[001] grain boundary is examined using density functional theory within the local density and pseudopotential approximations. Several pristine stoichiometric grain-boundary structures are investigated and compared with Z-contrast scanning transmission electron microscopy and electron energy loss spectroscopy results. The lowest-energy grain-boundary structure is found to agree well with the experimental data. When Y3+ is substituted for Zr4+ at various sites in the lowest-energy grain-boundary structure, the calculations indicate that Y3+ segregation to the grain boundary is energetically preferred to bulk doping, in agreement with experimental results. [source] Effect of Nitrogen Atmosphere on the Densification of a 3-mol%-Yttria-Doped ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2002Yeong-Kyeun Paek The densification behavior of a 3-mol%-Y2O3 -doped ZrO2 (3Y-ZrO2) has been investigated under N2 and O2 atmospheres. Powder compacts have been sintered at 1550° and 1400°C for various times. The density of the specimen sintered at 1550°C is higher in N2 than in O2, while the contrary result is obtained in the case of the specimen sintered at 1400°C. Such results can be explained in terms of nitrogen solubility and oxygen vacancy in a ZrO2 matrix. Because nitrogen solubility into the ZrO2 increases with an increase in heat-treatment temperature, leading to the formation of oxygen vacancy, the densification rate becomes higher. The present study thus shows evidence of nitrogen solubility into the ZrO2 and its role on the densification behavior of 3Y-ZrO2. [source] Cosintering Simulation and Experimentation: Case Study of Nanocrystalline ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2001Johannes Kanters Laminates consisting of two different nanocrystalline zirconia materials (undoped and 3Y-TZP doped) were sintered, and their densification and curvature were quantified using optical dilatometry. The experimental results agreed well with a continuum-mechanical description for the cosintering of laminates with diffusion coefficients and activation energies determined using densification of the free-sintering materials. The comparison covered various relative layer thicknesses and heating rates, but was developed only for the intermediate and late stages of sintering. Sintering stresses and compatibility stresses also were determined from the model. [source] Mechanism of Thermal Transport in Zirconia and Yttria-Stabilized Zirconia by Molecular-Dynamics SimulationJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2001Patrick K. Schelling We present results of molecular-dynamics simulations of the thermal conductivity, ,, of ZrO2 and Y2O3 -stabilized ZrO2 (YSZ). For both pure ZrO2 and YSZ with low concentrations of Y2O3, we find that the high-temperature , is typical of a crystalline solid, with the dominant mechanism being phonon-phonon scattering. With increasing Y2O3 concentration, however, the mechanism changes to one more typical of an amorphous system. In particular, phononlike vibrational modes with well-defined wave vectors appear only at very low frequencies. As in amorphous materials, the vast majority of vibrational modes, while delocalized, do not propagate like ordinary phonon modes but transport energy in a diffusive manner. We also find that the few highest frequency modes are localized and do not contribute to ,. [source] | |||||||||||||||||||||||||||||||||||||