Yttria-doped Zirconia Research Articles

Glycothermal Synthesis and Characterization of 3Y-TZP Nanoparticles

In this study, 3 mol% yttria-tetragonal zirconia polycrystal (3Y-TZP) nanoparticles were synthesized by the glycothermal method under various reaction temperatures and times. The co-precipitated precursor of 3Y-TZP was prepared by adding <TEX>$NH_4OH$</TEX> to starting solutions, and then the mixtures were placed in an autoclave reactor. Tetragonal yttria-doped zirconia nanoparticles were afforded through a glycothermal reaction at a temperature as low as <TEX>$220^{\circ}C$</TEX>, using co-precipitated gels of <TEX>$ZrCl_4$</TEX> and <TEX>$YCl_3{\cdot}6H_2O$</TEX> as precursors and 1,4-butanediol as the solvent. The synthesized 3Y-TZP particles were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Raman spectroscopy. The 3Y-TZP particles have a stable tetragonal phase only at glycothermal temperatures above <TEX>$200^{\circ}C$</TEX>. To investigate phase transition, the 3Y-TZP particles were heat treated from 400 to <TEX>$1400^{\circ}C$</TEX> for 2 h. Raman analysis indicated that, after heat treatment, the tetragonal phase of the 3Y-TZP particles remained stable. The results of this study, therefore, suggest that 3Y-TZP powders can be prepared by the glycothermal method.

Fe 2O 3-doped forsterite ceramics as a joining partner for ZrO 2 in a laser brazing process

Gastight, high-temperature stable sealing between yttria-doped zirconia (ZrO 2) and an electrically insulating ceramic joining partner is necessary for a wide range of applications in oxygen sensing and energy conversion. To accomplish this, laser brazing with glass solders is an attractive alternative to existing joining processes. Due to a near-perfect match of its thermal expansion with the one of ZrO 2 up to high temperatures (∼800 °C) and good thermal and electrical insulation properties, forsterite (Mg 2SiO 4) is chosen as a candidate joining partner. As the absorptivity of pure forsterite at the used diode laser wavelengths of 808 and 940 nm is quite low, increasing the energy absorption by doping forsterite with Fe 2O 3 appears to be a promising refinement of this technique. The optical properties of the resulting olivine ceramics are evaluated. The results show that Fe 2O 3-doped forsterite is both suitable as a joining partner for ZrO 2 and for tuning its absorptivity.

Analysis of reactions during sintering of CuO-doped 3Y-TZP nano-powder composites

3Y-TZP (yttria-doped tetragonal zirconia) and CuO nano powders were prepared by co-precipitation and copper oxalate complexation–precipitation techniques, respectively. During sintering of powder compacts (8 mol% CuO-doped 3Y-TZP) of this two-phase system several solid-state reactions clearly influence densification behaviour. These reactions were analysed by several techniques like XPS, DSC/TGA and high-temperature XRD. A strong dissolution of CuO in the 3Y-TZP matrix occurs below 600 °C, resulting in significant enrichment of CuO in a 3Y-TZP grain-boundary layer with a thickness of several nanometres. This “transient” liquid phase strongly enhances densification. Around 860 °C a solid-state reaction between CuO and yttria as segregated to the 3Y-TZP grain boundaries occurs, forming Y 2Cu 2O 5. This solid-state reaction induces the formation of the thermodynamic stable monoclinic zirconia phase. The formation of this solid phase also retards densification. Using this knowledge of microstructural development during sintering it was possible to obtain a dense nano–nano composite with a grain size of only 120 nm after sintering at 960 °C.

Nanoindentation of yttria-doped zirconia: Effect of crystallographic structure on deformation mechanisms

This article presents a nanoindentation study of polycrystalline and single crystals of yttria-doped zirconia with both tetragonal and cubic phases. Analysis of the deformation mechanisms is performed by both atomic force microscopy (AFM) and micro-Raman spectroscopy. Phase transformation from tetragonal to monoclinic phase is clearly distinguished on tetragonal crystals, whereas in cubic crystals the plastic deformation seems to be controlled by dislocation nucleation and interactions. AFM observations in tetragonal zirconia grains have shown that both grain size and autocatalytic transformation strongly influence the size of the transformed zone. Furthermore, the martensitic phase transformation seems to be also strongly dependent of the indenter shape. Experimental results suggest that a critical contact pressure is necessary to induce the phase transformation.

Study of near surface changes in yttria-doped tetragonal zirconia after low temperature degradation

Abstract Full tetragonal 3 mol.% Y2O3 doped zirconia has been subjected to hydrothermal ageing in water at 131 °C in order to study the change in microstructure just below the surface. The degraded layer was examined by scanning and transmission electron microscopy in a section perpendicular to the surface which was prepared by focus ion beam machining. The existence of microcracks and transformed grains in a thin surface layer of the hydrothermal degraded specimens is shown. Scratch damage in annealed and in thermally degraded specimens is compared.

Enhanced ionic conductivity and phase meta-stability of nano-sized thin film yttria-doped zirconia (YDZ)

Yttria-doped zirconia (YDZ) thin films with nanometric sized grains were prepared by reactive RF sputtering and their oxygen ion conductivities were systematically measured as a function of yttria doping with levels in the range 0.5–9.1 mol.% Y 2O 3. Enhanced oxygen ion conductivities, as derived from impedance spectra, were observed when compared with values reported for bulk YSZ. Furthermore, the peak conductivity for the YDZ films was observed to occur at considerably reduced yttria levels, i.e., at 6.5 mol.% Y 2O 3 (for T > ∼400 °C) and at 3.2 mol.% Y 2O 3 (for T < ∼300 °C) vs. 9 mol.% Y 2O 3 in bulk YSZ. Based on an analysis of the Raman spectra, these results are believed to result from the extended meta-stability of the cubic phase to reduced yttria levels at nanometric grain sizes.

Direct measurement of oxygen incorporation into thin film oxides at room temperature upon ultraviolet photon irradiation

Minute changes in oxygen concentration in complex oxides even of the order of ∼0.001% can significantly influence functional properties ranging from the onset of superconductivity to colossal dielectric constant and ferroic response. We report on direct experimental measurement of enhanced oxygen incorporation into ultrathin oxide films at room temperature under gentle UV photon exposure. Oxygen concentration changes in nanoscale yttria doped zirconia (YDZ) films grown on Ge substrate were quantified using the O16(d,p)O17 nuclear reaction. The oxygen concentration was consistently ∼3% larger in UV irradiated YDZ films compared to as-grown YDZ films. Possible incorporation mechanisms are discussed.

Temperature-dependent interfacial carrier transport in low-dimensional oxides using ionic conductor-insulator (YDZ-SiO2) superlattices

There is a tremendous interest in understanding mechanisms governing ion and electron transport along interfaces in low-dimensional systems. We report on high temperature ion transport using a model superlattice system consisting of oxygen-ion-conducting yttria-doped zirconia (YDZ) and insulating layers (silica and alumina) of varying conducting layer thickness in the range of 3–33 nm. Total in-plane conductivity shows nearly fivefold increase in conductivity when the YDZ layer thickness decreases from 33 to 3 nm while the total film thickness is held constant. Analysis of the conductivity data shows that volume contributions are dominant when film thickness is ≥210 nm. Activation energies for volume and interfacial oxygen-ion transport have been estimated.

Synthesis and Phase Stability in Ultra-thin Solid Electrolytes: The Case of Zirconia Thin Films

Phase stability in 9.5 mol % yttria doped zirconia (YDZ) and pure zirconia thin films have been studied by transmission electron microscopy (TEM). Through detailed in-situ TEM analysis, we found that phase stability in nanoscale zirconia thin film depends on film thickness, grain size, and annealing ambient. Pure zirconia thin films of ~ 52 nm thickness were stabilized without any dopants at room temperature, while it transformed into a tetragonal phase upon heating to 400 oC. On the other hand, 9.5 % Yttria doping enables stabilization of the cubic structure regardless of grain growth. Annealing of poor crystalline YDZ films in air (oxygen rich) leads to tetragonal phase formation, while ultra-high vacuum (oxygen-deficient) annealed samples display cubic phase at high temperature.

An experimental investigation into micro-fabricated solid oxide fuel cells with ultra-thin La 0.6Sr 0.4Co 0.8Fe 0.2O 3 cathodes and yttria-doped zirconia electrolyte films

Thin-film solid oxide fuel cells (SOFCs) were fabricated with both Pt and mixed conducting oxide cathodes using sputtering, lithography, and etching. Each device consists of a 75–150 nm thick yttria-stabilized zirconia (YSZ) electrolyte, a 40–80 nm porous Pt anode, and a cathode of either 15–150 nm dense La 0.6Sr 0.4Co 0.8Fe 0.2O 3− δ (LSCF) or 130 nm porous Pt. Maximum powers produced by the cells are found to increase with temperature with activation energies of 0.94–1.09 eV. At 500 °C, power densities of 90 and 60 mW cm −2 are observed with Pt and LSCF cathodes, respectively, although in some conditions LSCF outperforms Pt. Several device types were fabricated to systematically investigate electrical properties of components of these fuel cells. Micro-fabricated YSZ structures contacted on opposite edges by Pt electrodes were used to study temperature-dependent in-plane conductivity of YSZ as a function of lateral size and top and bottom interfaces. Si/Si 3N 4/Pt and Si/Si 3N 4/Au capacitor structures are fabricated and found to explain certain features observed in impedance spectra of in-plane and fuel cell devices containing silicon nitride layers. The results are of relevance to micro-scale energy conversion devices for portable applications.

Ultra-thin nanocrystalline lanthanum strontium cobalt ferrite (La 0.6Sr 0.4Co 0.8Fe 0.2O 3− δ) films synthesis by RF-sputtering and temperature-dependent conductivity studies

Nanocrystalline lanthanum strontium cobalt ferrite (LSCF) ultra-thin films with high in-plane electrical conductivity have been deposited by RF sputtering from composite targets. The films, with nominal thickness of 54 nm, are crystalline when annealed or deposited at temperatures above 450 °C. Effects of annealing temperature, annealing time, and substrate temperature on crystallization, microstructure, and room temperature lateral electrical conductivity have been systematically studied. No interfacial reaction products between the LSCF and single crystalline yttria-stabilized zirconia (YSZ) were observed from X-ray diffraction studies upon annealing until 750 °C. In-plane electrical conductivity as high as 580 S cm −1 at 650 °C has been observed for LSCF thin films deposited on single crystalline YSZ substrates and sputtered nanocrystalline YSZ thin films; while activation energy for conductivity were determined to be 0.15 eV and 0.10 eV for the former and latter films, respectively, in 650–400 °C range. The high in-plane electrical conductivity for the nanocrystalline LSCF ultra-thin films is likely attributed to their low level of porosity. Micro-solid oxide fuels cells using 15 nm thick LSCF films as cathodes and sub-100 nm yttria-doped zirconia thin film electrolytes have been fabricated successfully and demonstrated to achieve peak power density of 60 mW cm −2 at 500 °C. Our results demonstrate that RF sputtering provides a low-temperature synthesis route for realizing ultra-thin nanocrystalline LSCF films as cathodes for intermediate- or low-temperature solid oxide fuel cells.

Synergetic effect of NiO and SiO 2 on the sintering and properties of 8 mol% yttria-stabilized zirconia electrolytes

Yttria-doped zirconia electrolytes (e.g., 8 mol% yttria-stabilized ZrO 2, 8YSZ) have been considered to be the most promising candidates for applications in solid oxide fuel cells (SOFC). Due to the ubiquitous presence of SiO 2 impurities and wide use of Ni-containing anodes, it is therefore of great technical importance to understand the synergetic effect of NiO and SiO 2 on densification, grain growth and ionic conductivities (especially the grain boundary (GB) conduction) of zirconia electrolytes. In this study, three groups of 8YSZ ceramics, with Si contents of ∼30, ∼500 and ∼3000 ppm, have been designed. 1 at% NiO was added into these materials by a wet chemical method. The addition of SiO 2 has a negative effect on the sintering and densification, while the introduction of 1 at% NiO reduced the sintering temperature and promotes grain growth of the zirconia ceramics. However, the presence of small amount of NiO prevented full densification of 8YSZ ceramics. NiO also led to a decrease by ∼33% in grain interior (GI) conductivity, with little effect on the GB conduction of high-purity 8YSZ (∼30 ppm SiO 2). However, the coexistence of NiO and SiO 2 is extremely detrimental to total conductivity by significantly reducing the GB conduction. Moreover, it is observed that, unlike the 8YSZ-doped SiO 2 with only, whose GB conduction increases greatly with increasing sintering temperature, the GB conduction of the NiO and SiO 2 codoped samples is less sensitive to sintering temperature.

Room temperature tunable blue-green luminescence in nanocrystalline (Pb1−xSrx)TiO3 thin film grown on yttrium-doped zirconia substrate

Room temperature tunable blue-green photoluminescence was observed in nanocrystalline (Pb1−xSrx)TiO3 thin film under UV excitation. Its emission energy increases from 2.42 (at x=0.6) to 2.76 eV (at x=0.4), while the band gap decreases from 3.6 to 3.3 eV. Thin films of (Pb1−xSrx)TiO3 were prepared on yttrium-doped zirconia substrate by a simple sol-gel technique with spinning-coating process. Atom force microscope micrographs and crystallographic studies revealed the polycrystalline perovskite-type structure of the thin films. The observed optical properties are attributed to distorted octahedral due to different cation substitutions. The work shows that this kind of wide band gap and low cost nanocrystalline thin films is a very promising material for flat panel display applications and integrated light emission devices.

Enhanced grain growth in yttria-doped zirconia thin film structures synthesized under photon irradiation

Enhanced grain growth in ultra-thin yttria-doped zirconia (YDZ) films synthesized under ultraviolet (UV) irradiation is reported. The mean grain size in UV-synthesized 7.5 mol% YDZ films nearly 56 nm thick increased to 85 nm upon annealing in an oxygen-rich ambient at 900°C for 1 h, while in thermally grown YDZ they grew only to ∼15 nm under identical annealing conditions. In situ electron microscopy kinetic studies reveal an enhanced kinetic constant and self-limiting grain growth behaviour in the UV-synthesized oxide films. The difference between UV and thermally grown films was not significant in undoped films when compared to the case of 7.5 mol% YDZ.

Photon irradiation-induced structural and interfacial phenomena in pure and alio-valently doped zirconia thin films

We report on the effect of ultra-violet (UV) irradiation on structural and interfacial phenomena in pure and doped zirconia thin film grown by physical vapour deposition. Interfacial layer formation by substrate oxidation and resultant densification of zirconia layer was found in yttria-doped zirconia (YDZ) films grown on Si, while no change was observed in identical films grown on Ge. A comparison of un-doped zirconia and YDZ films indicates yttria-doping significantly assists structural changes during UV irradiation. Interestingly, the effect of UV photons becomes minimal at ∼300°C in films grown on Si, while the effect of UV becomes more pronounced in YDZ films grown on Ge. An interfacial layer was formed between the YDZ and Ge substrate at 300°C in the presence of UV irradiation, in contrast to the sharp interface maintained, even after annealing at 300°C, without UV. The results suggest that photon irradiation may be an elegant approach to tailor structural and interfacial properties at near-atomic length scales.

Nanoscaled ( La0.5Sr0.5 ) CoO3 − δ Thin Film Cathodes for SOFC Application at 500 ° C < T < 700 ° C

Nanoscaled and nanoporous (LSC) thin film cathodes (film thickness ranging from 200 to , grain and pore size in the range of ) were electrochemically characterized to determine their potential for intermediate and low-temperature solid oxide fuel cells (SOFCs). Chemically homogeneous, large area , and nanoporous LSC thin films were derived from metallorganic precursors (metallorganic deposition) and deposited on yttria-doped zirconia [(YSZ) “design 1”] and gadolinia-doped ceria [(GCO) “design 2”]. The area-specific polarization resistance of both designs was evaluated on symmetrical cells with special emphasis on constancy, depending on temperature and time by means of electrochemical impedance spectroscopy. For both designs, we report the capability of low polarization resistances, e.g., at , (LSC/YSZ), respectively, (LSC/GCO). Oxygen reduction reaction was facilitated by a substantial inner surface area of the porous thin-film cathode, as suggested by the application of Adler’s model. Nanoporous LSC thin-film cathodes from this study were compared to alternative design concepts for high-performance porous cathodes and with dense thin-film cathodes. Furthermore, the aim of our study was (i) to find a temperature regime with a perspective for chemical durability of an LSC/YSZ interface. We could prove that at a temperature of and for , polarization resistance of the LSC/YSZ interface remains constant, and at unreported low values, which reopens LSC/YSZ for micro-SOFC application; (ii) to estimate the structural durability of nanoscaled and nanoporous LSC thin-film cathodes. We have been able to demonstrate stable and unreported low polarization resistance for LSC/GCO in the temperature range between 500 and , which is of technical interest for auxiliary-power-unit application.

Effect of thermal treatment on yttria doped zirconia coated DIN 1˙4306

Thin films of 8 wt-% yttria doped zirconia (YZ) sol were deposited by a dip coating process on stainless steel (DIN 1˙4306) substrate for surface protection of this material widely used for corrosion resistance. The YZ sol coated specimens were heat treated in the temperature range of 500–1100°C for 30 min. The characterisation of the thin layer deposited was performed using thermal (TG-DTA) methods. The microstructure and phase identification analysis were carried out using scanning electron microscopy (SEM) having an energy dispersive system (EDS) as an attachment and X-ray diffraction (XRD). The coated surface specimen heat treated at 900°C showed fine grain surface without porosity, while specimens heat treated at 1100°C showed coarse grain structure with porosity. Three different phases, i.e. FeCr2O4, (Fe0˙6Cr0˙4)2O3 and ZrO2, were identified on the surface of coated specimens heat treated at 900 and 1100°C for 30 min. The YZ coated specimens show improved resistance in acidic environment. Tape adhesion testing has shown that YZ coatings on stainless steel are adherent. Good adhesion at the interface indicated that the sol–gel route is a feasible method for applying YZ coating on stainless steel substrates.

Studies on structure–electrochemical conduction relationships in doped-zirconia thin films

Microstructural studies have been carried out on yttria doped zirconia (YDZ) thin films grown on MgO(100) and Ge(100) single crystal substrates using in-situ transmission electron microscopy. Microstructure evolution is strongly impeded in films grown on Ge (100) when thickness is less than 30 nm. Electrical studies show one order of enhancement in total conductivity in films of thickness less than 20 nm grown on MgO substrate. For 17 nm films that show enhanced conductivity, the activation energy obtained from electrical relaxation is ∼ 1.7 eV while it is ∼ 1.1 eV for 933 nm thick films.

Synthesis and interfacial phenomena in ultra-thin yttria-doped zirconia films grown by alloy oxidation under photon irradiation

We report on synthesis, structural evolution, and high temperature properties of ultrathin yttria-doped-zirconia (YDZ) films grown by alloy oxidation under ultraviolet (UV) irradiation. From high-resolution studies on interfacial layer growth kinetics, we found that oxygen diffusivity maximum in UV-YDZ film is located at higher yttria concentration than that in thermally oxidized films. It is likely that the oxygen-rich nature of UV-grown films enabled by photon irradiation is responsible for the observed differences. The results demonstrate routes for synthesis of oxide films with controlled structure and stoichiometry.

Size-dependent phase transformations in nanoscale pure and Y-doped zirconia thin films

Phase stability in nanoscale pure zirconia and 9.5 mol.% yttria-doped zirconia (YDZ) thin films was studied by in-situ transmission electron microscopy. Oxygen vacancies are found to play a significant role in determining the microstructure and phase evolution. Pure zirconia thin films of ∼52 nm thickness were stabilized without any dopants at room temperature, whereas they transformed into a tetragonal phase upon heating to 400°C. On the other hand, 9.5% yttria doping enables stabilization of the cubic structure regardless of grain growth. Annealing of amorphous YDZ films in air (oxygen-rich) leads to tetragonal phase formation, whereas ultrahigh vacuum (oxygen-deficient) annealed samples display a cubic phase at high temperature. Detailed discussions on the effects of initial microstructure, oxygen deficiency, aliovalent doping and thickness are presented.