Y2O3-doped ZrO2 Research Articles

Fatigue behavior of 3%Y2O3-doped ZrO2 ceramics

The objective of this work was to evaluate the cyclic fatigue strength of a commercial pre-sintered tetragonal zirconia ceramics for dental systems, and to determine the main mechanical properties. Samples were sintered in air at 1600°C for 120min with heating and cooling rate of 10°C/min. The sintered specimens were characterized by X-ray diffraction and scanning electron microscopy. Hardness and fracture toughness were determined using the Vicker's indentation method. The strength was determined by four-point bending tests. The cyclic fatigue tests were realized as four-point bending tests within a frequency of 25Hz and a stress ratio R of 0.1. The Weibull analysis was employed in order to perform failure probability calculations. Sintered specimens presented average values of hardness, fracture toughness and bending strength near to 13.5GPa, 8MPam1/2 and 900MPa, respectively. The fatigue tests results allow concluding that the fatigue strength limit over 5×106 stress cycles is about 550MPa or around 63% of the static strength of this material. The tetragonal–monoclinic (t–m) zirconia transformation observed by X-ray diffraction of fractured surfaces occurs during cyclic loading and the fracture of specimens. The 3Y-TZP samples clearly present a range of loading conditions where cyclic fatigue can be detected. The fatigue strength limit around 550MPa is appropriate for application in dental implant parts.

Oxygen Exchange Kinetics of (Bi,Sr)(Co,Fe)O3-δ Thin-Film Microelectrodes

Mixed-conducting (Bi,Sr)(Co,Fe)O3-δ (BiSCF) perovskites are considered to be promising candidates for the application as cathode materials in solid oxide fuel cells (SOFCs) operating in the intermediate-temperature regime. In this study, the effective rate constants of surface oxygen exchange of both Co-free and Co-containing BiSCF perovskites were measured by electrochemical impedance spectroscopy (EIS) on geometrically well-defined thin-film microelectrodes on Y2O3-doped ZrO2 (YSZ) single crystal substrates. Oxygen exchange rate constants in the range of 3.0 × 10−7 cm s−1 to 2.4 × 10−6 cm s−1 at 750°C and an oxygen partial pressure p(O2) of 0.2 bar were obtained, which are higher in particular for Co-containing BiSCF than those of La-containing perovskites. The interrelation between the oxygen exchange rate constant and other material properties was investigated to derive information about the oxygen exchange mechanism. A correlation between the oxygen exchange rate constant and the ionic conductivity from thin-film electrochemical polarization experiments and measurements on bulk samples in the range of 1.4 × 10−2 S cm−1 to 2.6 × 10−1 S cm−1 at 650°C was observed that is similar to the correlation found for Ba-containing perovskites. This suggests oxygen vacancy diffusion to be involved in the rate-determining step (rds) of the oxygen exchange reaction.

Superplastic deformation of directionally solidified nanofibrillar Al2O3–Y3Al5O12–ZrO2 eutectics

Nanofibrillar Al2O3–Y3Al5O12–ZrO2 eutectic rods were manufactured by directional solidification from the melt at high growth rates in an inert atmosphere using the laser-heated floating zone method. Under conditions of cooperative growth, the ternary eutectic presented a homogeneous microstructure, formed by bundles of single-crystal c-oriented Al2O3 and Y3Al5O12 (YAG) whiskers of ≈100 nm in width with smaller Y2O3-doped ZrO2 (YSZ) whiskers between them. Owing to the anisotropic fibrillar microstructure, Al2O3–YAG–YSZ ternary eutectics present high strength and toughness at ambient temperature while they exhibit superplastic behavior at 1600 K and above. Careful examination of the deformed samples by transmission electron microscopy did not show any evidence of dislocation activity and superplastic deformation was attributed to mass-transport by diffusion within the nanometric domains. This combination of high strength and toughness at ambient temperature together with the ability to support large deformations without failure above 1600 K is unique and shows a large potential to develop new structural materials for very high temperature structural applications.

Influence of partial substitution of Sc2O3 with Gd2O3 on the phase stability and thermal conductivity of Sc2O3-doped ZrO2

Multi-rare earth elements doping is one of effective methods to improve the phase stability and thermal barrier property of zirconia for thermal barrier coatings (TBCs) application. In this paper, the phase stability and thermo-physical properties of 7.5mol% Sc2O3 and Gd2O3 co-doped ZrO2 (ScGdSZ) were investigated. ScGdSZ after 150h heat-treatment at 1400°C exhibited no monoclinic phase, showing high phase stability. The substitution of Sc2O3 with Gd2O3 increased the fraction of c-phase, however, t′-phase was still the dominant phase when the substitution amount was less than 2mol%. The thermal conductivities of ScGdSZ gradually decreased with the increasing Gd2O3 proportion. 3.7 Sc2O3 and 3.7 Gd2O3 co-doped ZrO2 (in mol%) showed the lowest thermal conductivity, which was 20% lower than 7.5 Sc2O3-doped ZrO2 and 40% lower than 4.5 Y2O3-doped ZrO2.

Characterization of the thermal conductivity of EB-PVD ZrO2–Y2O3 coatings with a pulsed thermal imaging method

ZrO2-(2–8mol%)Y2O3 coatings were deposited on zirconia substrates by EB-PVD. The coated layer had a columnar microstructure with intercolumnar gaps between columnar grains. Nanosized pores could be observed around feather-like grains as well as inside the columnar grains. The main phase of all coated specimens was the tetragonal phase. However, the monoclinic and cubic phase of the second phase could be detected for ZrO2-2mol%Y2O3 coatings and ZrO2-(6–8mol%)Y2O3 coatings, respectively. The thermal conductivity of coatings was successfully evaluated with the pulsed thermal imaging method using a combined sample of coatings and a substrate. Both the thermal conductivity and heat capacity of Y2O3-doped ZrO2 coatings tended to decrease with increasing amounts of Y2O3.

Accelerated degradation of 8.5 mol% Y2O3-doped zirconia by dissolved Ni

This study focuses on the clarification of the accelerated degradation of the ionic conductivity of Ni-containing 8.5mol% Y2O3-doped ZrO2 (8YDZ) under reducing atmosphere, a transition that proceeds 50 times faster in comparison to pure 8YDZ. A comparative study of high-performance anodes and NiO/YDZ thin-film specimens (NiO thin film on thick 8YDZ and 10YDZ electrolytes) has been carried out to characterize the microstructural and chemical evolution of the YDZ under operating conditions (reducing atmosphere). The extremely deep indiffusion of Ni (>100μm) into microcrystalline YDZ was revealed after sintering at 1400°C for 5h (air). Hence, such Ni-containing 8YDZ is expected to form whenever NiO and 8YDZ are co-sintered at high temperature, e.g., during half-cell (anode with thin-film electrolyte) fabrication. Moreover, the spinodal decomposition of the Ni-containing 8YDZ on the short time scale was identified and studied by conventional and analytical transmission electron microscopy (TEM). This comprises both the microstructural coarsening as well as the strong chemical decomposition on the nanoscale. Since the decomposition rate is directly related to the mobilities of the host cations these have to be enhanced by the dissolved Ni. The explanation therefore is based on a localized strain field that is expected to arise in the vicinity of individual Ni species due to electron capture under reducing atmosphere. It is proposed that the coulomb interactions are weakened (due to the expansion of the YDZ lattice) while lattice vibrations are enhanced leading to lower potential barriers for ion hopping and thus to higher cation mobilities.

Broadband spectroscopy of the complex conductivity of polycrystalline yttria-stabilized zirconia

Abstract Broadband conductivity spectra from 100 to 1014 Hz (100 THz) were acquired for yttria-stabilized zirconia (10 mol% Y2O3-doped ZrO2, 10YSZ) to quantify contributions from conduction due to the electrolyte–electrode interface, grain boundaries, universal dielectric response (UDR), and optical phonons. The UDR contribution governed the intrinsic conductivity at all frequencies except specific frequencies in the terahertz range, where phonon contributions governed conductivity for both ceramics and single crystals. UDR parameters σ0 and σdc increased with increasing temperature, resulting in increased microwave conductivity. The complex conductivity converged at frequencies of hundreds of gigahertz due to a decrease in the power-law constant, s, with increasing temperature. The optical phonon contribution to the total conductivity, due to an increase in the damping factor γ1TO with increasing temperature, was small, while the phonon-mode frequency ω1TO affected the microwave conductivity of 10YSZ.

Impurity and vacancy segregation at symmetric tilt grain boundaries in Y2O3-doped ZrO2

Segregation energies of impurity ions and oxygen vacancies at grain boundaries in Y2O3-doped ZrO2 as calculated from atomistic simulations using energy minimization and Monte Carlo methods are reported. Based on these energies, local defect equilibrium concentrations have been estimated. It is found that it is more energetically favorable for an yttrium ion to be accompanied by an oxygen vacancy at grain boundaries, although decrease in energy when associated with an oxygen vacancy differs from boundary to boundary. The segregation energy for a neutral defect complex consisting of a two yttrium ions and an oxygen vacancy at infinitely dilute concentration is highly correlated with the coordination environment of each site in the vicinity of the grain boundary (GB), and, in turn, GB energy. Although the estimated local equilibrium concentrations of these defects are similar, detailed analysis of the atomic coordination and defect distributions in the vicinity of a GB reveal that defect distributions, especially of oxygen vacancies, are dependent on the characteristics of the particular GB and that segregation in effect reduces lattice strains at the GB. Equilibrium concentration distributions of yttrium at grain boundaries are also given as a function of spatial resolution, and are useful for interpretation of experimental results.

Linking origin of the electric field-assisted β-PbF2 crystallization in lead oxyfluoroborate glasses below T g to simultaneous cathode/anode-compensated electrochemical reactions

Full validation of the electrochemical mechanisms so far postulated as driving force of electric field-assisted non-spontaneous crystallization development in given glasses has suffered experimental restrictions. In this work, we looked into origin of this phenomenon in lead oxyfluoroborate glasses, resulting in β-PbF2 growth even below the corresponding glass transition temperatures, through achieving a systematic study of not only Pt,Ag/Glass/Ag,Pt- but also Pt,Ag/Glass/YSZ:PbF2/Ag,Pt-type cells, where YSZ:PbF2 represents a two-phase system (formed by Y2O3-doped ZrO2 and PbF2). It is demonstrated that crystallization induction in these glasses involves Pb2+ ions reduction at the cathode, the phenomenon being, however, confirmed only when the F− ions were simultaneously also able to reach the anode for oxidation, after assuring either a direct glass–anode contact or percolation pathways for free fluoride migration across the YSZ:PbF2 mixtures. A further support of this account is that the electrochemically induced β-PbF2 phase crystallizes showing ramified-like microstructure morphology that arises, accordingly, from development of electroconvective diffusion processes under electric field action.

Studies on chemical stability in CO2 and H2O and electrical conductivity of perovskite-type Ba3In2Zr1−x Ce x O8 (x = 0, 0.5, 1)

We report the synthesis, structure, microstructure, chemical stability in H2O and CO2, and electrical transport properties of an oxide ion-conducting perovskite-related structure Ba3In2MO8 (M = Zr, Ce, Zr0.5Ce0.5). Powder X-ray diffraction confirmed the formation of a simple cubic perovskite-like structure for Ba3In2ZrO8 (a = 4.205(9) A), Ba3In2CeO8 (a = 4.234(1) A), and Ba3In2Zr0.5Ce0.5O8 (a = 4.285(8) A). The increase in lattice constant is consistent with the Shannon’s ionic radius trend. Among the three samples investigated, Ba3In2ZrO8 was found to be stable against reaction with pure CO2 at elevated temperature, while the Ce and 1:1 Zr and Ce compounds were unstable at 600 °C. Ba3In2ZrO8, Ba3In2CeO8, and Ba3In2Zr0.5Ce0.5O8 were found to be chemically unstable in H2O at about 50 °C. The bulk electrical conductivity of the samples prepared at different temperatures was found to be nearly the same; the total conductivity (bulk + grain–boundary + electrode) seems to change with sintering temperature. Both Ba3In2ZrO8 and Ba3In2CeO8, prepared at 1,400 °C, exhibited comparable electrical conductivity of about 6 × 10−3 S cm−1 at 800 °C, which is comparable to that of conventional Y2O3-doped ZrO2 electrolyte. These compounds are very promising electroltes, provided that their chemical and mechnical stabitities are improved without losing any ionic conductivity.

Electrode Properties of Pr0.7Sr0.3Fe0.8Al0.2O3-δ Thin Film Prepared by Pulsed Laser Deposition

A mixed conductive Pr0.7Sr0.3Fe0.8Al0.2O3-δ (PSFA) thin film has been prepared by using pulsed laser deposition (PLD) and its electrochemical properties have been investigated. A dense PSFA target for PLD was prepared at 1130 oC for 10 h. By using the target, 285 nm-thick PSFA thin film was deposited on an 8mol%Y2O3-doped ZrO2 substrate at 625 oC. The PSFA thin film was well crystallized and its grain size was in the range of 5 to 10 nm. The PSFA thin film showed an area specific resistance (ASR) of 0.37 Ω•cm2 at 900 oC. The activation energy of ASR was approximately 1.6 eV at temperature range of 600 to 900 oC.

Intense Atomic Oxygen Emission from Incandescent Zirconia

Particle emissions from an oxide-ion conducting solid electrolyte (tetragonal zirconia polycrystal, 3% Y2O3-doped ZrO2) have been examined using a high vacuum evaluation system. Oxygen gas with a pressure range from 10 to 3 × 103 Pa was fed into a 2 mm outer diameter zirconia tube, whose central region of 10 mm length was heated to 1400−1800 °C by direct Joule heating. The emission species were identified by appearance potential spectroscopy using a quadrupole mass spectrometer. Atomic oxygen (AO) is the dominant species that is radiantly emitted from the high-temperature region. Temperature dependence of the AO emission intensity exhibits far less activation energy (∼0.3−1.0 and 0.7 eV, on average) than electric field-assisted thermal O− ion (∼2 eV) and electron (∼6 eV) emission from zirconia reported before. High AO emission flux (in the order of 1017 atoms·cm−2·s−1) is promising for use as practical AO sources.

Temperature programmed desorption of oxygen from Pd films interfaced with Y2O3-doped ZrO2

The origin of the effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) or Electrochemical Promotion was investigated via temperature-programmed-desorption (TPD) of oxygen, from polycrystalline Pd films deposited on 8 mol%Y2O3–stabilized–ZrO2 (YSZ), an O2− conductor, under high-vacuum conditions and temperatures between 50 and 250 °C. Oxygen was adsorbed both via the gas phase and electrochemically, as O2−, via electrical current application between the Pd catalyst film and a Au counter electrode. Gaseous oxygen adsorption gives two adsorbed atomic oxygen species desorbing at about 300 °C (state β1) and 340–500 °C (state β2). The creation of the low temperature peak is favored at high exposure times (exposure >1 kL) and low adsorption temperatures (Tads < 200 °C). The decrease of the open circuit potential (or catalyst work function) during the adsorption at high exposure times, indicates the formation of subsurface oxygen species which desorbs at higher temperatures (above 450 °C). The desorption peak of this subsurface oxygen is not clear due to the wide peaks of the TPD spectra. The TPD spectra after electrochemical O2− pumping to the Pd catalyst film show two peaks (at 350 and 430 °C) corresponding to spillover Oads and \( O^{{\delta - }}_{{\rm ads}} \) according to the reaction: $$ O^{{2 - }}_{{\rm (YSZ)}} \to {\text{O}}^{{\delta - }}_{{{\text{ads}}}} \to {\text{O}}_{{{\text{ads}}}} $$

The effects of LSM coating on 444 stainless steel as SOFC interconnect

The effect of oxide (La0.9Sr0.1MnO3, LSM) coating on the commercial stainless steel (STS444) interconnect as SOFC interconnect was examined by measuring the polarization resistance (Rp) of LSCF (La0.6Sr0.4Co0.2Fe0.8) cathodes of various electrolyte-supported cells (La0.9Sr0.1Ga0.8Mg0.2 [LSGM], Ce0.9Gd0.1O2 [GDC10], or 8 mol% Y2O3-doped ZrO2 [8YSZ]). The electrochemical impedance of LSCF cathodes was monitored during ∼140 h in air at 600 and 700°C to determine the cathodic Rp values. With or without interconnect contacts, the magnitude of cathodic Rp value of LSGM electrolyte was similar to that of GDC electrolyte and much smaller than that of YSZ electrolyte. However, no apparent difference in the rate of increase was observed among the cathodes on the different electrolytes. Although the Rp value of the LSCF cathode in contact with LSM-coated STS444 was much reduced from that with uncoated STS444, the coating was not perfect to prevent the Cr evaporation from the interconnect and thus to avoid the degradation of LSCF cathode. Thus new coating methods or materials are needed to protect the LSCF cathode from the Cr poisoning.

스테인리스 스틸 연결재의 Cr이 LSCF 양극의 분극저항에 미치는 영향

STS444 with or without La0.9Sr0.1MnO3 (LSM)-coating was contacted to La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O₃ (LSCF) cathode on various electrolyte materials and the polarization resistance (R p ) was measured by impedance spectroscopy. By making a symmetric half-cell and contacting only one side of the cathode with the interconnect, the effect of chromium (Cr) poisoning was separated from the aging effects. When the LSCF cathode was contacted with LSM-coated STS (stainless steel), Rp of LSCF was lower than that contacted with the uncoated STS. Impedance patterns measured for the working electrode (W.E.), the counter electrode (C.E.) at 600oC in air were analyzed. Normalized data of net Cr effect showed that Ce 0.9 Gd 0.1 O₂ (GDC) electrolyte is more tolerant to the chromium poisoning than La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 (LSGM) or 8 ㏖% Y₂O₃-doped ZrO₂ (YSZ) electrolytes.

Synthesis and microstructure of Y2O3-doped ZrO2–CeO2 composite nanoparticles by hydrothermal process

Y2O3-doped ZrO2–CeO2 nanoparticles were prepared under high temperature and pressure conditions by precipitation from metal nitrates with aqueous potassium hydroxide. Y2O3-doped ZrO2–CeO2 particles were obtained in the temperature range of 190–210°C after 6h. TEM and X-ray diffraction patterns showed that the synthesized particles were crystalline. The average size and size distribution of the synthesized particles were below 20nm and narrow, respectively.

Thermal Conductivity of Zirconia for Thermal Barrier Coatings: A Perturbed Molecular Dynamics Study

Thermal conductivity of pure and Y2O3-doped ZrO2 was calculated using a perturbed molecular dynamics method in order to analyze phonon scattering mechanism which is responsible for the reduction of thermal conductivity. Although absolute values of thermal conductivity were overestimated due to a simple model used in this study, relative values were in good agreement with experiment, which indicates that phonon scattering due to Y2O3 addition is reproduced well. It is found from quantitative analysis of the phonon scattering using the mean field theory that decrease of the thermal conductivity upon Y2O3 addition is attributed not only to the introduction of O2- vacancies but also to substitution of Y3+ ions for Zr4+ ions.

Evaluation of Transformation Zone Around Propagating Cracks in Zirconia Biomaterials Using Raman Microprobe Spectroscopy

Structural reliability, biocompatibility and bioinertness are fundamental prerequisites for bioceramics used in artificial hip and knee joints. Among structural properties, superior fracture toughness is necessary for guaranteeing high reliability during implantation lifetime. Bioinert ceramics employed in artificial joints are mainly limited to alumina and zirconia materials. In this paper, the critical crack-tip stress intensity factor, KI0, and the tetragonal-to-monoclinic phase-transformation behavior of a 3 mol % Y2O3-doped tetragonal ZrO2 polycrystals (3Y-TZP) were studied as a function of grain size. 3Y-TZP’s with four different grain sizes were prepared and the size and morphology of the monoclinic transformation zone generated around the tip of an indentation crack were analyzed by quantitative Raman microprobe spectroscopy. The stress intensity factor, KI0, was evaluated by the crack opening displacement (COD) method using a recently proposed equation for calculating the compliance of an indentation crack.

Development of Yttria-doped Zirconia Thin Films on Mild Steel by a Sol-Gel Process

SUMMARYAn homogeneous transparent sol of 8 wt% Y2O3-doped ZrO2 (YZ) was prepared by the sol gel process using inorganic salts and anion exchange resins. This sol provided a crack-free fine-grained layer. Thin films of yttria-doped zirconia sol were deposited by a dip coating technique on mild steel (MS) substrates for the protection of this widely used material from chemical attack. The coated specimens were heat treated in the temperature range 773–1123 K. Characterizations of the thin-layers deposited were performed adopting thermal, morphological analysis and thickness measurement methods. The phase identification and analysis were carried out using X-ray diffraction. The coatings obtained after the heat treatment to 1070 K developed fine grain structure with a smooth surface. Heat treatment at 1070 K gave good resistance to chemical attack along with improved microhardness properties.

Electrosteric Stabilization of Al2O3, ZrO2, and 3Y–ZrO2 Suspensions: Effect of Dissociation and Type of Polyelectrolyte

The mechanisms of eight anionic polyelectrolytes stabilizing colloidal sized α-Al2O3, pure ZrO2, and Y2O3-doped ZrO2 particles in aqueous solution are discussed. The polyelectrolytes studied were the Na+ and NH4+ salts of polyacrylic acid and polymethacrylic acid having different molecular weights. The particle–dispersant interactions were studied by measuring adsorption isotherms, particle size, thickness of adsorbed layer, and zeta potentials by elektrokinetic sonic analysis at different powder volume fractions (φ=0.01–0.3), pH, and electrolyte (KCl) content. The dissociation of the polyelectrolytes was studied by potentiometric titrations. The dissociation constant of the polymethacrylates was found to be 0.6 pH unit higher than that for the polyacrylates. High-affinity adsorption isotherms were observed over the pH range when the polyelectrolytes were fully ionized. The results show good correlation between adsorption isotherms and zeta potential data in systems of dispersed, dilute alumina particles. When particles and polymers were of equal charge (the same sign of charge) the polymer shell was thicker. At higher volume fractions (φ=0.3), and when alumina particles/added ammonium polyelectrolyte were of equal charge, a maximum in the absolute value of zeta potential resulted. Due to adsorption all the anionic polyelectrolytes studied provided electrosteric stabilization of the α-Al2O3, and Y2O3-doped ZrO2 suspensions by enhancing the zeta potential to 40 mV or over and by shifting the isoelectric point to lower pH, the low-molecular-weight polyelectrolytes decreasing the isoelectric point more than the polyelectrolytes having higher molecular weight. The polyelectrolytes studied failed to stabilize pure monoclinic ZrO2 particles. Due to the shortness of the chain of polyelectrolytes studied, no bridging was observed between oppositely charged polyelectrolyte/alumina particles.