Yttria-stabilized Zirconia Matrix Research Articles

Effects of structural strain and phase transformations on the mechanical properties of alumina-zirconia-graphene composites

Yttria-stabilized-zirconia (YSZ) plus graphene-based-materials composites may present phase transitions. Consequently, it may affect the composite properties. Thus, this work discusses the causality between lattice strain, phase transition, fracture toughness (Kc) and hardness (H) values in alumina+YSZ matrix reinforced with 0 wt.% up to 1.25 wt.% multilayered-graphene (MLG). The conventional sintering was made in a reducing atmosphere. H and Kc were obtained by Vickers indentation. The structural characterization shows that the MLG addition promoted an increase in YSZ lattice parameters and a phase transition, from tetragonal to cubic. As a result, the optimum reinforcement peak in the H and in the Kc happens in different MLG concentrations. The Kc optimum is in lower MLG amount, explained by microstructural features. On the other hand, the optimum H displaced to higher MLG concentration may be effect of increasing lattice parameters and the cubic-YSZ phase proportion.

Aligned nanofibers incorporated composite solid electrolyte for high-sensitivity oxygen sensing at medium temperatures

Potentiometric oxygen sensors have been widely used in internal combustion engines, industrial boilers, and metallurgical heat treatment furnaces. However, traditional oxygen sensors based on yttria-stabilized zirconia (YSZ) electrolyte can only be operated at elevated temperatures (> 750 °C) due to their relatively low ionic conductivity. In this study, we present a highly efficient micro-oxygen sensor that can be operated at a temperature as low as 300 °C. This micro-oxygen sensor incorporates a composite solid electrolyte, i.e., well-aligned gadolinium-doped cerium oxide (CGO) nanofibers embedded within a YSZ matrix (YSZ/CGOf). The arrays of CGO nanofibers in the YSZ matrix are parallel to the conduction direction, providing rapid conducting channels for oxygen ions. Benefitting from this design, the composite electrolyte leads to a conductivity of four times higher than that of traditional YSZ solid electrolytes at low temperatures. This enhancement in conductivity is attributed to the presence of a defective interfacial region between CGOf and YSZ, which promotes the mobility of oxygen ions. The strategy of constructing fast ionic conduction in the composite electrolyte by using well-aligned nanofibers may be considered for the design and optimization of other micro/nano-devices including sensors, batteries, and fuel cells.

Development of yttria-stabilized zirconia and graphene coatings obtained by suspension plasma spraying: Thermal stability and influence on mechanical properties

This study investigated the feasibility of depositing graphene nanoplatelet (GNP)-reinforced yttria-stabilized zirconia (YSZ) composite coatings. The coatings were deposited from an ethanol-based mixed YSZ and GNP suspension using suspension plasma spraying (SPS). Raman spectroscopy confirmed the presence of GNPs in the YSZ matrix, and scanning electron microscopy (SEM) analysis revealed a desired columnar microstructure with GNPs distributed predominantly in the inter-columnar spacing of the YSZ matrix. The as-deposited YSZ-GNP coatings were subjected to different isothermal treatments—400, 500, and 600 °C for 8 h—to study the thermal stability of the GNPs in the composite coatings. Raman analysis showed the retention of GNPs in specimens exposed to temperatures up to 500 °C, although the defect concentration in the graphitic structure increased with increasing temperature. Only a marginal effect on the mechanical properties (i.e., hardness and fracture toughness) was observed for the isothermally treated coatings.

Degradation and lifetime of self-healing thermal barrier coatings containing MoSi2 as self-healing particles in thermo-cycling testing

Yttria-stabilized zirconia (YSZ) is the state-of-the-art top coat material for thermal barrier coatings (TBCs) applied on highly loaded gas turbine parts. During operation at high temperatures, stresses are induced by the thermal expansion coefficient mismatch between the ceramic TBC and the metallic substrate. As a consequence cracks can grow, propagate and finally lead to a spallation of the top coat. Using atmospheric plasma spraying (APS), so-called self-healing MoSi2 particles can be incorporated into the YSZ matrix to mitigate the propagation of cracks leading to a lifetime gain and possibly higher temperature capability of the TBC. In the present work, the healing process is realized by the oxidation of the self-healing particles, which introduces a volume expansion by a formation of reaction products, which can seal the cracks. The self-healing particles were introduced within the first 150 μm of the YSZ coating matrix immediately on top of the bond coat. The degradation and lifetime of such systems were studied in furnace cycling and in burner rig tests, in which a temperature gradient through the sample was applied. The lifetime of the self-healing coatings was then compared to the lifetime of an YSZ coating without self-healing particles. In burner rig tests a clear lifetime extension of the self-healing TBCs was observed. The origin of this different behavior was investigated by microstructural analysis in scanning electron microscopy. A further insight into the failure mechanisms was gained by the analysis of a self-healing TBC cycled in a furnace cycling test only for about 55% of its expected lifetime.

Supporting ionic conductivity of Li2CO3/K2CO3 molten carbonate electrolyte by using yttria stabilized zirconia matrix

It is proposed that dual conductive electrolytes have the potential to improve properties of MCFC by using oxygen ion conductors as matrix material, creating what is termed composite electrolyte. In this work we present the results of testing a novel type of electrolyte of molten carbonate fuel cell which consists of Yttria Stabilized Zirconia impregnated by Lithium/Potassium carbonates. Polarization curves were obtained for temperatures between 500 °C and 800 °C. Without very accurate measurements it is not easy to determine the additional electric charge provided by oxygen ions (O=). We made an attempt to build a mathematical model which takes into account both conductivities and, based on the model, estimated the ratio of electric charge provided by oxygen ions and carbonate ions.

Compositional and operational impacts on the thermochemical reduction of CO2 to CO by iron oxide/yttria-stabilized zirconia.

Ferrites have potential for use as active materials in solar-thermochemical cycles because of their versatile redox chemistry. Such cycles utilize solar-thermal energy for the production of hydrogen from water and carbon monoxide from carbon dioxide. Although ferrites offer the potential for deep levels of reduction (e.g., stoichiometric conversion of magnetite to wüstite) and correspondingly large per-cycle product yields, in practice reactions are limited to surface regions made smaller by rapid sintering and agglomeration. Combining ferrites with zirconia or yttria-stabilized zirconia (YSZ) greatly improves the cyclability of the ferrites and enables a move away from powder to monolithic systems. We have studied the behavior of iron oxides composited with YSZ using thermogravimetric analysis under operando conditions. Samples in which the iron was fully dissolved within the YSZ matrix showed greater overall extent of thermochemical redox and higher rate of reaction than samples with equal iron loading but in which the iron was only partially dissolved, with the rest existing as agglomerates of iron oxide within the ceramic matrix. Varying the yttria content of the YSZ revealed a maximum thermochemical capacity (yield per cycle) for 6 mol% Y2O3 in YSZ. The first thermochemical redox cycle performed for each sample resulted in a net mass loss that was proportional to the iron oxide loading in the material and was stoichiometrically consistent with complete reduction of Fe2O3 to Fe3O4 and further partial reduction of the Fe3O4 to FeO. Mass gains upon reaction with CO2 were consistent with re-oxidation of the FeO fraction back to Fe3O4. The Fe dissolved in the YSZ matrix, however, is capable of cycling stoichiometrically between Fe3+ and Fe2+. Varying the re-oxidation temperature between 1000 and 1200 °C highlighted the trade-off between re-oxidation rate and equilibrium limitations.

Tailoring electrostatic surface potential and adsorption capacity of porous ceramics by silica-assisted sintering

In this study, we apply silica-assisted sintering to develop porous yttria stabilized zirconia (YSZ) ceramics with tailored electrostatic surface potential and adsorption capacity as a promising alternative to chemical functionalization. The porous bodies were formed by partial sintering at 1050 °C and were investigated regarding the influence of admixtures of silica particles on sintering behavior, microstructural evolution and the resulting mechanical and surface properties of the material, particularly the surface potential. With increasing silica concentration, the sintering mechanism was gradually changed from solid state to liquid phase sintering, due to the wetting of YSZ by liquid silica and a resulting inhibition of mass transport, particle growth and diffusion-induced densification. Most importantly, due to the silica layer development, the isoelectric point (IEP) of the YSZ/silica material surfaces was systematically shifted towards the IEP of silica from pH 9.4 to 1.2 resulting in a more pronounced negative surface potential at neutral pH. The relationship between surface IEP and silica concentration was mathematically described using the IEPs of the starting materials, the YSZ particle radius and the glass layer thickness. This estimation allows us to tailor the surface coverage of the YSZ matrix with silica as well as the resulting electrostatic surface potential. We further demonstrate how the applied processing route can be effectively used to develop ceramics with specified adsorption capacities for protein immobilization for use in filtration, bioprocessing or biomaterial applications.

Preparation and up-conversion luminescence of Er-doped yttria stabilized zirconia single crystals

High-quality yttria-stabilized zirconia (YSZ) single crystals doped with different concentrations of Er2O3 were grown by the optical floating zone method. The cubic phase of ZrO2 was retained when Er3+ partially replaced Y3+ in the YSZ matrix. Up-conversion luminescence was observed when the crystals were excited by a 980 nm laser diode. The optical power curve was obtained by plotting the up-conversion luminescence intensity against the laser power, and both the green and red light up-conversion were shown to involve two photons. Seven characteristic visible and three infrared absorption peaks were observed in the absorption spectrum of the 1.50 mol% Er2O3: YSZ. The up-conversion spectra produced three prominent peaks at 547(green), 561 (green) and 679 (red) nm. Green and red light maxima were observed with Er2O3 concentrations of 1.5 mol% and 0.5 mol%, respectively. Oscillator strength parameters Ωt (t = 2, 4, 6) were determined using the Judd-Ofelt theory and Ω2 = 9.90 × 10−20 cm2, Ω4 = 2.92 × 10−20 cm2, Ω6 = 3.32 × 10−20 cm2. The Ω4/Ω6 ratio (0.88), which is the key parameter for laser performance of reactive matrix materials, indicates that Er3+-doped YSZ has potential laser applications.

Preparation and optical properties of samaria‐doped yttria‐stabilized zirconia single crystals

AbstractSingle crystals of yttria‐stabilized zirconia (YSZ) doped with various concentrations of Sm2O3 were synthesized by the optical floating zone method, and their structure and spectroscopic properties characterized using powder X‐ray diffraction (XRD), optical absorption, and luminescence measurements. XRD showed that the cubic phase of ZrO2 was stabilized by addition of Y2O3 and a series of absorption peaks characteristic of Sm3+ was detected by UV‐visible absorption spectroscopy. Three phenomenological J‐O oscillator strength parameters Ωt (t = 2, 4, 6) were calculated to evaluate the local structure and bonding in the vicinity of the Sm3+ for the 0.75 mol% Sm2O3: YSZ sample. These were in order of Ω2 < Ω4 < Ω6 with values of Ω2 = 5.46 × 10−20 cm2, Ω4 = 10.70 × 10−20 cm2, and Ω6 = 12.67 × 10−20 cm2, and indicate a relatively high symmetry and low covalent character for the Sm‐O bond at the Sm3+ sites in the YSZ matrix. Photoluminescence spectra recorded under an excitation of 404 nm showed four emission bands centered at 571 nm, 621 nm, 652 nm, and 716 nm corresponding to the transitions 4G5/2→6Hj (j = 5/2, 7/2, 9/2, 11/2). The transition probability, lifetime, and branching ratio obtained from the emission spectrum of the 0.75 mol% Sm2O3: YSZ crystal showed that YSZ is a potential host for Sm3+ to achieve a reddish‐orange laser output, and the Sm3+‐modified YSZ crystals have a possible use in reddish‐orange laser and lighting devices.

Ion- and electron-beam-induced structural changes in cubic yttria-stabilized zirconia

Thin yttria-stabilized zirconia (YSZ) (100) transmission electron microscopy (TEM) samples prepared using 5 kV Ar ion sputtering and subsequent annealing with 5 kV electron beams under ultrahigh vacuum (UHV) environment were observed with TEM. A new phase which is electron conducting and has high electron beam tolerance is formed. The phase grows epitaxially with the YSZ matrix and has a cubic structure. It is estimated that substrate YSZ reacted with surface contaminants to from ZrC during electron beam annealing. When the substrates were thoroughly cleaned before ion beam irradiation, no new phase formation was observed. Instead, formation of faceted holes was identified.

Plasmon Resonance Induced Photoconductivity in the Yttria Stabilized Zirconia Films with Embedded Au Nanoclusters

We report on the investigation of the photoconductivity (PC) in the yttria stabilized zirconia (YSZ) films with embedded Au nanoclusters (NCs). A peak in the PC spectrum corresponding to the plasmon optical absorption resonance in the Au NCs was observed. The temperature dependence of PC near 300 K obeyed Mott law. In this case, the PC was attributed to the heating of the YSZ matrix due to the plasmon optical absorption in the Au NCs (bolometric effect). Near 77 K, the PC did not depend on temperature and was attributed to plasmon-assisted electron transport between the NCs via the vacancy α-band in YSZ.

Plasmon resonance induced photoconductivity in the yttria stabilized zirconia films with embedded Au nanoclusters

AbstractWe report on the investigation of the photoconductivity (PC) in the yttria stabilized zirconia (YSZ) films with embedded Au nanoclusters (NCs). A peak in the PC spectrum corresponding to the plasmon optical absorption resonance in the Au NCs was observed. The temperature dependence of PC near 300 K obeyed Mott law. In this case, the PC was attributed to the heating of the YSZ matrix due to the plasmon optical absorption in the Au NCs (bolometric effect). Near 77 K, the PC did not depend on temperature and was attributed to plasmon-assisted electron transport between the NCs via the vacancy α-band in YSZ.

Electrical conductivity of YSZ-SDC composite solid electrolyte synthesized via glycine-nitrate method

Abstract Yttria-stabilized zirconia (YSZ) is a common solid electrolyte for solid oxide fuel cells (SOFCs) because of its high electrical conductivity and high ionic transference number in both oxidizing and reducing atmospheres. Samarium doped ceria (SDC) has also been considered as an alternative electrolyte material to YSZ for intermediate temperature SOFC because of its high conductivity at relatively low temperatures. Due to improved ionic conductivity of YSZ at high temperature (~ 800 °C) and good conductivity of SDC in the intermediate temperature range (600–800 °C), the electrical properties of YSZ-SDC composites were investigated. Composites of YSZ and SDC with weight ratio 9.5:0.5, 9:1 and 8.5:1.5 were synthesized via glycine-nitrate route. XRD pattern of the systems revealed the formation of composite phases. Biphasic electrolyte microstructures were observed, in which SDC grains are dispersed in YSZ matrix. Relative density of the compositions was found to be more than 92% to the theoretical density. It was observed that the interface provides a channel for ionic transport, leading to a notable ionic conductivity. With increase in SDC weight ratio the electrical conductivity was found to increase. For weight ratio 8.5:1.5 the electrical conductivity was found to be greater than that of YSZ in the temperature range 400–700 °C. Further, for weight ratio more than 8.5:1.5, conductivity was found to decreases due to the formation of a few other insulating impurity phases. The electrode polarisation was also found to reduce significantly with SDC in the composite electrolyte system. Thus, such composite system may be useful for improving the ionic conductivity of the composite electrolytes.

Degradation of SPS-Fabricated YSZ and Nd2O3-YSZ Ceramics in Supercritical Water

Zirconia (ZrO2) ceramics are being considered as a candidate material for thermal insulating barriers in pressure tubes used in the supercritical water (SCW) nuclear reactors. However, the literature suggests that zirconia may undergo a detrimental phase transformation which is accelerated in aqueous environments. In this research, 8 mol% Yttria-Stabilized Zirconia (YSZ) ceramics with the addition of 5 and 10 mol% Nd2O3 were manufactured via spark plasma sintering (SPS) process and subsequently subjected to a SCW environment. The weight losses and microstructural evolutions of these materials during SCW exposure were studied. The results suggest that doping YSZ with Nd2O3 significantly decreased the degradation rate of the YSZ ceramic and improved its structural stability. X-ray diffraction studies revealed that after degradation testing, the Nd2O3 helped to retain the desirable cubic phase of YSZ matrix. In the case of pure YSZ ceramic, a phase change of the matrix toward the monoclinic lattice was observed and likely contributed to the ceramic’s disintegration in SCW environment.

Formation of Au4Zr nanocrystals in yttria stabilized zirconia in the course of implantation of gold ions

High-resolution transmission microscopy and optical absorption spectroscopy have been used to examine how metallic nanocrystals (NCs) are formed in the course of implantation of Au ions into thin films of yttria stabilized zirconia (YSZ) and subsequent thermal annealing. It was found that, under certain implantation conditions, Au4Zr NCs are formed in addition to Au NCs in the YSZ matrix. The conditions in which Au4Zr NCs are formed in the course of implantation and decompose under the subsequent annealing were determined.

Impregnation of La0.4Ce0.6O1.8–La0.4Sr0.6TiO3 as solid oxide fuel cell anode in H2S-containing fuels

Active anodes were fabricated via wet chemical impregnation of optimized amount of La0.4Sr0.6TiO3 (L4ST) into La0.4Ce0.6O1.8 (LDC) pre-infiltrated porous yttria-stabilized zirconia (YSZ) matrix. Impregnations of 10 wt% LDC with 16 wt% L4ST significantly improved the performance of the fuel cell from 48 mW cm−2 for pure L4ST to 161 mW cm−2 for LDC–L4ST at 900 °C in H2. The contribution of the pre-loaded LDC to this improvement was investigated using electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). The measurement results indicated that pre-infiltrated LDC increased the activity of the anode more effectively by decreasing the total polarization resistance of the cell from 3.3 Ω cm2 to 1.0 Ω cm2 in humidified H2 at 900 °C. More importantly, the LDC nano-deposits (<20 nm) behaved as an effective “adhesive” that substantially enhanced the wettability of L4ST on YSZ matrix, resulting in finer and more uniform structure of L4ST infiltrates. The LDC–L4ST cells also demonstrated significantly improved performances in 0.5% H2S–H2 and 0.5% H2S–CH4 with higher stability than cells with pure L4ST anode.

X-ray absorption spectroscopy studies on the carbothermal reduction reaction products of 3 mol% yttria-stabilized zirconia

Extended X-ray absorption spectroscopy (EXAFS) at the ZrKedge has been used to determine changes in various bond lengths in 3 mol% yttria-stabilized zirconia (YSZ) during zirconium carbide (ZrC) formation. The principal objective of this study was to determine if ZrC formation at the YSZ/carbon interface alters the zirconia structure. A mixed-phase sample (YSZ and graphite) was carbothermally reduced to form ZrC. X-ray diffraction phase quantification by Rietveld analysis confirmed the formation of ∼50% ZrC in the analyzed sample volume. EXAFS data of ZrC and YSZ powders and a sintered YSZ pellet (∼96.7% density) were used as standards to compare with the carbothermally reduced sample.Ab initocalculations using these spectra quantified various Zr—O, Zr—C and Zr—Zr bond distances in the system. Best fit results revealed Zr—OI(tetragonal), Zr—O (monoclinic), Zr—Zr (tetragonal) and Zr—Zr (monoclinic) bond length values of 2.10, 2.25, 3.65 and 3.52 Å, respectively, in the YSZ powder, Zr—OI(tetragonal) and Zr—Zr (tetragonal) bond length values of 2.12 and 3.62 Å, respectively, in the sintered pellet, and Zr—C and Zr—Zr bond lengths of 2.32 and 3.33 Å, respectively, in the ZrC powder. Similar fitting procedures were carried out on the carbothermally reduced pellet, with measured Zr—O, Zr—Zr (of YSZ), Zr—C and Zr—Zr (of ZrC) bond lengths of 2.13, 3.62, 2.36 and 3.33 Å, respectively. These bond lengths indicate that the formation of ZrC in the YSZ matrix does not influence the local structure when compared to pure standards. Therefore, carbothermal reduction does not induce any apparent strain or thermally induced effects on the first and second coordination shells of Zr as measured by the X-ray absorption spectra of the carbothermally reduced sample. Interestingly, the results indicated that sintering of the YSZ powder into pellets did not result in any significant change in the Zr—O and Zr—Zr distances for tetragonal YSZ.

Local modification of the microstructure and electrical properties of multifunctional Au-YSZ nanocomposite thin films by laser interference patterning.

Nanocomposite films consisting of gold nanoparticles embedded in an yttria-stabilized zirconia matrix (Au-YSZ) have been synthesized with different gold loadings by reactive magnetron sputtering followed by ex situ annealing in air or laser interference patterning (LIP) treatment. It is shown that the electrical conductivity of the nanocomposite films can be modified to a large extent by changing the gold loading, by thermal annealing, or by LIP. The structural and microstructural analyses evidenced the segregation of metallic gold in crystalline form for all synthesis conditions and treatments applied. Thermal annealing above 400 °C is observed to trigger the growth of pre-existing nanoparticles in the volume of the films. Moreover, pronounced segregation of gold to the film surface is observed for Au/(Au + Zr + Y) ratios above 0.40, which may prevent the use of thermal annealing to functionalize gold-rich Au-YSZ coatings. In contrast, significant modifications of the microstructure were detected within the interference spot (spot size close to 2 × 2 mm) of LIP treatments only for the regions corresponding to constructive interference. As a consequence, besides its already demonstrated ability to modify the friction behavior of Au-YSZ films, the LIP treatment enables local tailoring of their electrical resistivity. The combination of these characteristics can be of great interest for sliding electrical contacts.

Aramid‐Al2O3 Nano‐Needle Scaffold as Pore Former for Optimal Mechanical and Permeable Properties of Ceramic Matrix

An adroit design of pore‐forming system is required to achieve a porous ceramic matrix with high permeability and adequate mechanical strength. This work developed a hybrid pore‐forming regime constructed in the green body (dense powder packing) of yttria stabilized zirconia (YSZ) through the condensation polymerization to form poly(p‐phenylene terephthalamide) (PPTA) chains and their in‐situ grafting with alumina nano‐needles with pendant–NH2 groups (Sasol®). A PPTA‐Sasol pore‐former scaffold was resulted and distributed uniformly in the green body. It was then subjected to calcination treatment in Ar, through which fibers and ribbons consisting of carbon and alumina needles were derived and penetrated throughout the partially sintered YSZ matrix as the final form of pore‐former. This work further investigates the impacts of the PPTA‐Sasol pore‐former, including the morphologies and loadings, on the Darcy's permeability, pore‐size distribution and flexural strength, displayed by the porous YSZ matrices ultimately attained. It is showed that the vital role of Sasol needles in the present pore‐forming regime lies in significant enhancement on mechanical strength accompanying an improved permeability contrary to the best result achieved without them.

Optimization of Infiltration Techniques Used to Construct Ni/YSZ Anodes

A range of Ni-containing solutions, including an aqueous solution containing the Triton-X surfactant, and aqueous or ethanolic solutions containing urea, have been infiltrated into a symmetrical tubular half-cell composed of a slip-casted, porous yttria-stabilized zirconia (YSZ) anode support, Ni-YSZ functional layers, a YSZ electrolyte, and a second, outer porous YSZ anode support layer, aiming at the development of high performance anodes. A combination of surface wettability experiments and optical and electron microscopy imaging has been used to determine how well these solutions penetrate the porous YSZ matrix, then correlating these results with the electrochemical performance in humidified H2 environments at 800°C. It is shown that the addition of Triton-X-100 surfactant to the infiltration solution results in excellent penetration of the Ni phase deep into the YSZ matrix, while the use of urea as a Ni complexing agent leaves a Ni-rich layer on the outer anode surface. Importantly, the use of a two-step process, involving several vacuum-infiltrations with Ni nitrate solutions containing Triton-X-100, followed by several infiltrations with urea-containing solutions, leads to the best cell performance as well as the best Ni distribution inside the anode layers.