Yttria-doped Zirconia Research Articles

Effect of gadolinia addition on the mechanical and physical properties of zirconia/ceria ceramics

In this work, zirconia powders containing 5 and 10 mol % ceria were prepared by co-precipitation method followed by doping with small amounts (< 1 mol %) of gadolinia which stabilized the tetragonal phase. X-ray diffraction and scanning electron microscopy were used for phase analysis and microstructure examination, respectively. The sintered compacts showed fine grained, dense, tough and strong ceramics. Strength up to 795.8 MPa and toughness of 17 MPa√m have been obtained. These ceramics showed higher toughness than yttria-doped tetragonal zirconia polycrystals (Y-TZP) and a higher strength than the ceria tetragonal zirconia polycrystals (Ce-TZP). The high fracture toughness was attributed to the tetragonal-to-monoclinic phase transformation which was associated with ferroelastic domain switching. The domain switching toughening has been discussed in view of Chevalier's model. Four-point bending strengths for the samples tested in the as cut state are higher than those tested in the as-sintered or the ground states.

Evaluation of fracture toughness of zirconia ceramics with heterogeneous yttrium distribution microstructures

Yttrium stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics containing 2 mol% and 2.5 mol% yttria were prepared by mixing 3 mol% yttria-doped zirconia powders with yttria-free zirconia powders. The microstructures of the sintered samples were studied using SEM, XRD, and Raman spectra. In addition, an ultra-sharp V-notch (ρ < 0.5 μm) was induced by femtosecond laser to measure its actual fracture toughness by single-edge V-notched beam (SEVNB) method. The results reveal that mixture-based Y-TZP with 2.5 mol% yttria has deeper phase transformation depth, and fracture toughness of 6.4 MPa.m1/2 is obtained. The heterogeneous distribution of the yttrium is taken account for the excellent properties.

Microstructure and mechanical properties of 4YTZP-SiC composites obtained through colloidal processing and Spark Plasma Sintering

Bulk composites of silicon carbide reinforced zirconia were obtained through colloidal processing and spark plasma sintering, and their microstructural and mechanical properties were investigated. Mixtures of powders of 4mol% yttria-doped zirconia and silicon carbide, with percentages of 15 and 20wt% of silicon carbide, were prepared by colloidal processing and freeze-drying to guarantee the homogeneity of powders, and sintered by SPS at temperatures ranging from 1300 to 1400°C to obtain dense composites. The spark plasma sintering technique makes it possible to control sintering energy and speed, and impedes the oxidation of silicon carbide to ensure larger reproducibility. The microstructure of the composites reveals that some defects arise during sintering due to the differences of coefficient of thermal expansion between zirconia and silicon carbide. Mechanical properties were studied, such as Vickers hardness, Young's modulus, and bending strength. Compared with 4YTZP monolithic material, composites have better fracture strength values (∼588MPa and 492MPa, for 15wt% and 20wt% of silicon carbide, respectively) when sintered at 1400°C. It is also demonstrated that silicon carbide particles act as an absorption centre for propagating cracks.

Few-monolayer yttria-doped zirconia films: Segregation and phase stabilization.

For most applications, zirconia (ZrO2) is doped with yttria. Doping leads to the stabilization of the tetragonal or cubic phase and increased oxygen ion conductivity. Most previous surface studies of yttria-doped zirconia were plagued by impurities, however. We have studied doping of pure, 5-monolayer ZrO2 films on Rh(111) by x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). STM and LEED show that the tetragonal phase is stabilized by unexpectedly low dopant concentrations, 0.5 mol % Y2O3, even when the films are essentially fully oxidized (as evidenced by XPS core level shifts). XPS also shows Y segregation to the surface with an estimated segregation enthalpy of -23 ± 4 kJ/mol.

Grain Size-Effect on Intergranular Cracking in Shape Memory Zirconia During Cyclic Martensitic Transformations

Bulk polycrystalline zirconia-based shape memory ceramics (SMCs) generally crack as they undergo martensitic transformations, largely due to the transformation mismatch stresses. Polycrystalline SMCs subjected to cyclic transformations bodily lose individual grains and progressively comminute, but it is not yet clear how the grain size of a polycrystal affects such cyclic degradation. We explore this issue in 1.5 mol% yttria-doped zirconia SMCs by varying the grain size from 0.6 to 7.9 µm in pellets of fixed composition and sample size and subjecting them to multiple thermal cycles through the transformation. A smaller grain size is found to increase the number of cycles required to disaggregate the pellet because of the larger amount of grain boundary area that must crack. Calorimetry analysis shows that the energy relieved through cracking decreases with increasing grain size and reveals an apparent material length scale of ~2 micrometers for the stress relief zone. For grain diameters below this critical length scale, complete stress relief is suggested, while at larger grain sizes, intergranular cracking apparently does not fully relieve the transformation mismatch stresses. Alternate accommodation mechanisms are required including the formation of multiple variants and even some transgranular fracture.

Enhancing the flame stability in a slot burner using yttrium-doped zirconia coating

Quenching experiments using a slot burner and surface analysis were conducted to analyze the effects of two materials (ZrO2 and 13.5 wt%Y2O3-doped ZrO2 (13.5YSZ)) deposited on stainless steel 304 (STS304) substrate on the flame stability of premixed methane/air mixtures of varying equivalence ratio and flow velocity at a wall temperature range of 373 K to 773 K. Results showed that these two types of coatings are beneficial for decreasing the quenching distance and extending the flame stability limit. Moreover, doping 13.5 wt%Y2O3 into ZrO2 coating was more conducive to reduce quenching data and broaden lower flammability limits in the fuel-lean side. Surface analyses reflected that the superior mobility of lattice oxygen for 13.5YSZ coating plays a crucial role in improving the quenching characteristics due to the formation of extrinsic active oxygen vacancies. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) verified that the methane oxidation process can be accelerated over the two coating surfaces due to the high catalytic performance. Finally, one migration route of oxygen ions consisting of the dynamic oxygen diffusion and replenishment pathways was proposed to discuss the underlying mechanism of flame stability improvement when using yttrium-stabilized zirconia composite coatings in a narrow channel.

Influence of Aging on Mechanical Properties of Yttria-Doped Zirconia

We evaluated the influence of aging on mechanical properties of 8% yttria-doped zirconia (8YSZ) from room temperature to 1200 K. The temperature dependence of the dynamic Young’s and shear moduli of 8YSZ with and without the aging treatment was investigated by using a resonance method. The dynamic Young’s and shear moduli of 8YSZ without the aging treatment decreased by 33% below 700 K and gradually increased at higher temperatures with increasing temperature. On the other hand, those with the aging treatments decreased by around 20% below 600 K while did not significantly change above 600 K with increasing temperature. These demonstrated the effect of aging on the dynamic Young’s and shear moduli of 8YSZ was most remarkable at intermediate temperatures (600~1000 K). Although it was suggested that the existence ratio of the metastable tetragonal phase was increased during the aging treatment, it is likely that the influence of this phase transition on the dynamic Young’s and shear moduli was not significant. It seemed that the difference in the dynamic Young’s and shear moduli of 8YSZ with and without the aging treatment at intermediate temperatures was due to the local ordering of the oxygen vacancies.

Effect of withdrawal rate on the evolution of optical properties of dip-coated yttria-doped zirconia thin films

Abstract In this work, the Swanepoel method is described and applied for determining various optical parameters and thicknesses of dip–coated yttria–doped zirconia thin films. Using this method the influence of the withdrawal rate on optical parameters was studied. The characterization of the deposited thin films was carried out by optical microscopy and FT–IR spectrophotometry. As expected, coating thickness was closely related to the withdrawal rate and consequently influenced optical parameters such as refractive index, extinction coefficient, and absorption coefficient. Regarding the average refractive index of the prepared thin films, n is in the 2.0 – 2.2 range, the higher refractive index average value being obtained with films deposited at 25 mm min −1 (n = 2.19). The value of the optical band gap was also studied, this increased with withdrawal rate and was quite similar to values reported by other investigators at 50, 25 and 10 mm min −1 . Thus, this study proposes analysing the influence of the withdrawal rate for the manufacture of different types of thin films with previously specified optical parameters.

Enhanced ionic conductivity in phase stabilized yttria-doped zirconia nanowires

Yttria-doped zirconia (YSZ) is a remarkable functional material with wide applications, including solid oxide fuel cells (SOFCs) and oxygen sensors, due to its excellent mechanical and electrical properties. However, the elevated operating temperatures using the conventional YSZ electrolytes for the solid ionic devices yield high cost and limited lifetime. Here we report YSZ nanowires with doping content rang of 0–10mol% prepared by electrospinning. The variety of tetragonal and cubic phases in the nanowires has been detected, which demonstrates that in comparison to bulk counterparts, the high-temperature phases in zirconia-based nanowires could be stabilized at lower doping levels, owing to the critical crystallite size effect. The electrical conductivity of uniaxially aligned nanowires with respect to yttria content has been investigated. The highest ionic conductivity of 0.01Scm−1 is achieved at a low temperature of 375°C for the YSZ nanowires with 6.5mol% yttria, which is 290 times enhancement over corresponding bulk.

Current progress and perspectives of applying cold sintering process to ZrO2-based ceramics

Zirconia-based ceramics are normally sintered at high temperatures to obtain dense compacts that can deliver good mechanical and/or electrical performance. We recently developed a cold sintering process to achieve dense ceramics at incredibly low temperatures, or an improved densification after a second-step conventional sintering. Here we briefly review the current progress on its relevance to zirconia ceramics as exemplified by yttria-doped zirconia. We will contrast the processing of compositions for a structural-ceramic and an electroceramic. Together with enhanced densification, the processing-structure-property relationship is outlined. The cold sintering technique can provide a cost-effective and energy-saving processing for fabrication of zirconia-based materials.

Oxygen transport in epitaxial SrTiO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;/SrTi&amp;lt;sub&amp;gt;1 − &amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt;Fe&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; multilayer stacks

Abstract. Nano-ionic materials made of strontium titanate (SrTiO3, STO) and solid solutions of strontium ferrite in STO (SrTi1 − xFexO3, STF) are grown on single crystalline STO substrates and characterized. Since STF exhibits an oxygen deficiency and, simultaneously, enables oxygen interstitial defects, a space charge area close to the STO | STF interface is present. Oxygen tracer diffusion experiments and impedance spectroscopy at temperatures from 500 to 700 °C and at oxygen partial pressure ranging from 10−3 to 10−23 bar confirm fast oxygen transport caused by enhanced ionic conductivity at the interface. There, an oxygen diffusion coefficient of 3. 4 × 10−10 m2 s−1 at 600 °C and a p type conductivity of about 360 S m−1 at 700 °C are calculated. Such structures open new options in design of nano-ionic materials for oxygen sensors and energy conversion at temperatures lower than those of conventional materials such as yttrium-doped zirconia.

Thermal stability and field assisted sintering of cerium-doped YSZ ceramic nanoparticles obtained via a hydrothermal process

Owing to its extraordinary range of properties, yttria-doped zirconia holds a unique place among the ceramic oxide systems. To improve the properties for some specific custom design applications, co-doping with other rare earth oxides such as ceria is needed. The aim of this paper is to identify the correlations between the phase composition evolution with increasing thermal treatment temperature in order to establish the thermal stability in connection with the ceria content and how does it influence the yttria-stabilised zirconia microstructure. The ZrO 2 –3Y 2 O 3 – n CeO 2 ( n = 3, 6 and 9 wt.%) samples were obtained by a hydrothermal process and submitted to a thermal treatment up to 1600 °C. Intensive characterization was performed via X-ray powder diffraction and EDX analysis. It was found that up to 400 °C, a monophasic structure was formed. At higher temperatures tetragonal zirconia is formed as a major phase with the presence of secondary monoclinic and cubic phases, depending on the Ce content and thermal treatment temperature. Sintered compacts with densities up to 99.5% from the theoretical density were obtained starting from the 6%CeO 2 –3%Y 2 O 3 –ZrO 2 -nanostructured powders using a special field-assisted (FAST) sintering process. With increasing CeO 2 content to 9% only, tetragonal zirconia with 6–9 nm crystallite sizes is formed during the FAST sintering process.

Isolated hydrogen configurations in zirconia as seen by muon spin spectroscopy andab initiocalculations

We present a systematic study of isolated hydrogen in diverse forms of ${\mathrm{ZrO}}_{2}$ (zirconia), both undoped and stabilized in the cubic phase by additions of transition-metal oxides $({\mathrm{Y}}_{2}{\mathrm{O}}_{3},{\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$, MgO, CaO). Hydrogen is modeled by using muonium as a pseudoisotope in muon-spin spectroscopy experiments. The muon study is also supplemented with first-principles calculations of the hydrogen states in scandia-stabilized zirconia by conventional density-functional theory (DFT) as well as a hybrid-functional approach which admixes a portion of exact exchange to the semilocal DFT exchange. The experimentally observable metastable states accessible by means of the muon implantation allowed us to probe two distinct hydrogen configurations predicted theoretically: an oxygen-bound configuration and a quasiatomic interstitial one with a large isotropic hyperfine constant. The neutral-oxygen-bound configuration is characterized by an electron spreading over the neighboring zirconium cations, forming a polaronic state with a vanishingly small hyperfine interaction at the muon. The atom-like interstitial muonium is observed also in all samples but with different fractions. The hyperfine interaction is isotropic in calcia-doped zirconia $[{A}_{\mathrm{iso}}=3.02(8)$ GHz], but slightly anisotropic in the nanograin yttria-doped zirconia $[{A}_{\mathrm{iso}}=2.1(1)$ GHz, $D=0.13(2)$ GHz] probably due to muons stopping close to the interface regions between the nanograins in the latter case.

Operation of Thin-Film Electrolyte Metal-Supported Solid Oxide Fuel Cells in Lightweight and Stationary Stacks: Material and Microstructural Aspects.

In this study we report on the development and operational data of a metal-supported solid oxide fuel cell with a thin film electrolyte under varying conditions. The metal-ceramic structure was developed for a mobile auxiliary power unit and offers power densities of 1 W/cm2 at 800 °C, as well as robustness under mechanical, thermal and chemical stresses. A dense and thin yttria-doped zirconia layer was applied to a nanoporous nickel/zirconia anode using a scalable adapted gas-flow sputter process, which allowed the homogeneous coating of areas up to 100 cm2. The cell performance is presented for single cells and for stack operation, both in lightweight and stationary stack designs. The results from short-term operation indicate that this cell technology may be a very suitable alternative for mobile applications.

Mechanism of Enhanced Pt ORR Catalyst By Atomic Layer Deposition-Based Oxide Functionalization

In a recent study, ultrathin oxide overcoat was proven to have significant impact on the ORR kinetics of nanoporous Pt catalyst on yttria-doped zirconia (YSZ).1 In the report, a few nm thick ultrathin yttria-stabilized zirconia (uYSZ) was coated on the porous Pt cathode by atomic layer deposition (ALD). The coating was found not only to preserve the overall Pt morphology but also to improve the electrode reaction. The uniform and conformal geometry of the as-deposited overcoat became percolated at the operational temperature (~500°C), which is conjectured beneficial to the catalytic activity due to enlarged Pt/YSZ interfaces. In this presentation, we present our revised understanding of how the ultrathin oxide overcoat improved the ORR catalysis through a parametric study. Geometrically well-defined Pt/uYSZ electrode structures were prepared by accumulating Pt nanoparticles through a colloidal process. We controlled the Pt nanoparticle size, dominant surface facets, overall electrode thickness, YSZ overcoat thickness and stoichiometry of overcoat. Pt nanocrystals with different sizes and controlled surface facets were prepared through wet chemical and electrochemical processes. We then performed electrochemical analysis at various temperature (400 – 700 °C) and oxygen partial pressure (0.01 – 1 atm). A series of characterization indicated that the enhanced ORR activity is most likely originated from the enhanced ‘chemical’ processes such as dissociation and transport. However, in samples with thin Pt electrode layer, the enhancement is largely ascribed to increased triple phase boundary areas. This project is funded by the NASA MUREP Institutional Research Opportunity (MIRO) Program (Grant No. NNX15AQ01A). 1 I. Chang, S. Ji, J. Park, M.H. Lee, and S.W. Cha, Adv. Energy Mater. 5, 1402251 (2015).

Electrical conductivity of zirconia and yttrium-doped zirconia from Indonesian local zircon as prospective material for fuel cells

In this research, zirconium dioxide, ZrO2, was synthesized from high-grade zircon sand that was founded from Bangka Island, Sumatra, Indonesia. The zircon sand is a side product of Tin mining plant industry. The synthesis was conducted by caustic fusion method with considering definite stoichiometric mole at every reaction step. Yttrium has been doped into the prepared zirconia by solid state reaction. The prepared materials were then being analyzed by X-ray diffraction equipped with Le Bail refinement to study its crystal structure and cell parameters. Electrical conductivity was studied through impedance measurement at a frequency range of 20 Hz- 5 MHz. Morphological analysis was conducted through Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray (EDX) for elemental analysis. The results show that the prepared yttrium stabilized zirconia, YSZ, was crystallized in the cubic structure with a space group of P42/NMC. The sintered zirconia and yttrium stabilized zirconia at 8 mol% of yttrium ions (8YSZ) show dense surface morphology with a grain size less than 10 pm. Elemental analysis on the sintered zirconia and 8YSZ show that sintering at 1500°C could eliminate the impurities, and the purity became 81.30%. Impedance analysis shows that ZrO2 provide grain and grain boundary conductivity meanwhile 8YSZ only provide grain mechanism. The yttrium doping enhanced the conductivity up to 1.5 orders. The ionic conductivity of the prepared 8YSZ is categorized as a good material with conductivity reach 7.01 x10-3 at 700 °C. The ionic conductivities are still lower than commercial 8YSZ at various temperature. It indicates that purity of raw material might significantly contribute to the electrical conductivity.

Microstructure, crystallization and shape memory behavior of titania and yttria co-doped zirconia

Small volume zirconia ceramics with few or no grain boundaries have been demonstrated recently to exhibit the shape memory effect. To explore the shape memory properties of yttria doped zirconia (YDZ), it is desirable to develop large, microscale grains, instead of submicron grains that result from typical processing of YDZ. In this work, we have successfully produced single crystal micro-pillars from microscale grains encouraged by the addition of titania during processing. Titania has been doped into YDZ ceramics and its effect on the grain growth, crystallization and microscale elemental distribution of the ceramics have been systematically studied. With 5mol% titania doping, the grain size can be increased up to ∼4μm, while retaining a large quantity of the desired tetragonal phase of zirconia. Micro-pillars machined from tetragonal grains exhibit the expected shape memory effects where pillars made from titania-free YDZ would not.

Nanocrystalline yttria-doped zirconia sintered by fast firing

Sintering of powders commonly leads to simultaneous densification and grain growth, particularly for nanocrystalline materials. Currently, methods such as spark plasma sintering (SPS), hot pressing (HP), two-step sintering (TSS) and fast firing (FF) are employed to hinder grain growth while maintaining a high densification. In this work, FF consisting in thermal treatments with high heating rates (>500° C/min) and shorter holding times (10min or less) and conventional sintering (CS) approaches were experimentally compared in the sintering of commercial yttria doped zirconia (3YSZ and 8YSZ) compacts. CS-samples presented larger grain sizes by a factor of ~2 and ~4 in comparison to the initial 3YSZ and 8YSZ powders. Conversely, FF method significantly suppressed grain growth with a growth factor of ~1. Those results and comparison with previous work indicated that high heat inputs could indeed minimize grain growth.

Estimating Joule heating and ionic conductivity during flash sintering of 8YSZ

Sintering is a crucial processing step used in a wide variety of fields. Flash sintering is a recent technique to quickly densify conductive ceramics in the presence of an electric field. Full densification can occur in seconds and at low temperatures. However, the mechanisms responsible for such extreme densification rates remain a matter of debate. We provide evidence that Joule heating sets off a chain of events resulting in flash sintering of 8% yttria-doped zirconia (YSZ). We show that prior to flash onset, Joule heating raises sample temperature relative to the thermal equilibrium established with the furnace. Flash sintering occurs once the sample temperature is high enough to cause densification. We derive an empirical model to estimate the conductivity of 8YSZ during flash sintering. We show that the model is equally valid under constant heating rate and in isothermal conditions, regardless of applied electric field strength.

Relation between microstructures and electric properties for 3Y-TZP/Ba0.5Sr0.5Fe12O19 composites prepared by two different methods

Relation between microstructures and electric properties of 3-mol%-yttria-doped zirconia (3Y-TZP)/Ba0.5Sr0.5Fe12O19 composites prepared by two different methods were investigated. Ba0.5Sr0.5Fe12O19 ferrite powders were firstly prepared by solid reaction and sol–gel process, respectively. They were then combined with 3Y-TZP. Finally, the mixture powders were molded and sintered by a conventional ceramic method. The electric properties of the composites were studied through the impedance and dielectric spectroscopy. A corresponding equivalent-circuit model including the constant phase element of the composites was obtained using Zview software. The grain boundary morphology of the both composites was analyzed using fractal theory. A mathematical evaluation of the grain boundary morphology of the both composites can be given by the box-dimension. The information of the microstructure features of the composites can be indirectly reflected by the obtained parameters of the equivalent-circuit model and dielectric loss peak.