Characterization of Nickel Ions in Nickel‐Doped Yttria‐Stabilized Zirconia

The distribution of Ni²⁺ ions in NiO‐doped 10YSZ powder is examined with Superconducting Quantum Interference Device magnetometry, a technique that is able to distinguish between randomly distributed Ni²⁺ ions in solid solution and ordered Ni²⁺ ions within NiO with high precision. Very high purity powders containing 0.01, 0.1, 0.5, and 1.0 mol% NiO in 10YSZ (all levels below the solid solubility limit of NiO in 10YSZ) were made from acetate precursors and a modified EDTA (ethylenediaminetetraacetic acid)‐citrate synthesis method. The powders were calcined in air at either 873 or 1273 K. The 873 K calcination leads to single phase YSZ particles about 10 nm in diameter, and almost all of the NiO dopant exists in complete solid solution. The 1273 K calcination leads to a larger YSZ particle size (55–95 nm), and also to the formation and/or growth of NiO particles, the amount of which depends on the length of time of calcination. Upon sintering the powders in air (1773 K, 1 h), the NiO dissolves back into 10YSZ. The results demonstrate that particle growth during calcination leads to the exsolution of Ni²⁺ ions to form NiO. This has important implications for the synthesis of NiO‐doped 10YSZ from chemical precursors.     

Electro‐Sintering of Yttria‐Stabilized Cubic Zirconia

Electro‐sintering, i.e., electrically enhanced densification without the assistance of Joule heating, has been observed in 70% dense 8 mol% Y₂O₃‐stabilized ZrO₂ ceramics at temperatures well below those for conventional sintering. Remarkably, full density can be obtained without grain growth under a wide range of conditions, including those standard for solid oxide fuel cell (SOFS) and solid oxide electrolysis cell (SOEC), such as 840°C with 0.15 A/cm². Microstructure evidence and scaling analysis suggest that electro‐sintering is aided by electro‐migration of pores, made possible by surface flow of cations across the pore meeting lattice/grain‐boundary counter flow of O^2?. This allows pore removal from the anode/air interface and densification at unprecedentedly low temperatures. Shrinkage cracking caused by electro‐sintering of residual pores is envisioned as a potential damage mechanism in SOFC/SOEC 8YSZ membranes.     

Effects of He Irradiation on Yttria‐Stabilized Zirconia Ceramics

Effects of He irradiation on polycrystalline yttria‐stabilized zirconia (YSZ) are studied with the focus on irradiation‐induced damage buildup, He behavior, and volume swelling. The evolution of irradiation‐induced structural damage in polycrystalline YSZ, which is independent of grain orientation, is described by a multistep damage accumulation model. A three‐step damage evolution process was found, and different types of defects were observed in the different damage steps. Compared with single‐crystal YSZ, the second damage step occurs at a lower dose in polycrystalline YSZ due to the initial defects and strain. The implanted He ions are readily trapped along the grain boundaries and the mobility of He ions is greatly increased. The enhanced He mobility along the grain boundary leads to a lower threshold irradiation dose and a larger penetration depth for bubble formation. Similar morphologies are observed for the He bubbles in the polycrystalline YSZ and in single‐crystal YSZ, and the formation of He bubbles in polycrystalline YSZ is not influenced by grain orientation. As both the extended defects and He bubbles can induce volume swelling, the variation in volume swelling as a function of dose can be divided into a two stage process.     

Controlling the Diameter of Electrospun Yttria‐Stabilized Zirconia Nanofibers

This work reports the precise diameter control of electrospun yttria‐stabilized zirconia (YSZ) nanofibers from 200 to 900 nm after calcination. Fabricated YSZ nanofibers showed porous nanocrystalline structures with high aspect ratios of more than 500:1 and high surface‐to‐volume ratios with a specific surface area of 43.32 m²/g. The diameter of the YSZ nanofibers increased with the viscosity of the precursor solution, which was controlled by the concentrations of either polymers (polyacrylonitrile) or ceramic precursors (YSZ). We present a modified correlation between the diameter of a nanofiber and the synthetic conditions, as the observed behavior for calcined ceramic nanofibers deviated from the expected behavior. Our results demonstrate a modified but simple approach to fabricate ceramic nanofibers with desired diameters, providing a new design guideline for many electrochemical applications.     

Microstructural Properties of Air Plasma Sprayed Coating Consisting of Tungsten Carbide and Yttria‐Stabilized Zirconia

Thermal Spraying technologies are proven to be capable of producing composite materials and structures. In the present work, an innovative composite coating was produced to achieve high wear and thermal resistant properties in a single‐step process using air plasma spraying (APS) technique. Tungsten carbide has shown high wear resistance and zirconia coatings exhibited excellent tribological and insulation properties. It is speculated that a composite material consisting of zirconia and tungsten carbide exhibits excellent thermomechanical properties. A powder mixture of 50wt% WC‐10wt% Ni (WC‐Ni) and 50wt% ZrO₂‐8wt% Y₂O₃ (YPSZ) was deposited on a low carbon steel substrate using APS technique. Important microstructural properties of WC‐Ni/YPSZ coating such as splat boundaries, pore and grain morphology, microcracks, phase composition, elemental distribution of coatings, and lattice parameters of the crystals were investigated using optical microscopy, scanning electron microscopy (SEM), energy dispersive X‐ray (EDS), and X‐ray diffractometry (XRD). A good adhesion was observed between different phases in tungsten carbide mixed with zirconia coatings. Decarburization process which occurred during APS process resulted in formation of tungsten hemi‐carbide (W₂C) phase in plasma sprayed samples. The calculated crystal size for APS‐deposited coating was smaller than those of feedstock powder.     

Ionomigration of Pores and Gas Bubbles in Yttria‐Stabilized Cubic Zirconia

Migration of residual pores in partially sintered 8 mol% yttria‐stabilized zirconia under an electric field was investigated. To avoid shrinkage via sintering, Ar‐filled bubbles introduced to dense ceramic were also studied. Pores/bubbles were found to migrate against the field, e.g., under 1.9 V at 875°C, a temperature when cation diffusion is supposed to have frozen according to the prevailing consensus. Pore/bubble movement left contorted grains in some samples, but not at lower temperatures and higher fields when they apparently pass through grain boundaries without causing any visible distortion. These results are explained by a surface diffusion model and a temperature‐pore‐size map that delineates two distinct modes of pore/boundary pinning and breakaway. The implications of these results to solid oxide fuel cells and electrolysis cells are explored.     

Microstructure and Ionic Conductivity of Yttria‐Stabilized Zirconia Thin Films Deposited on MgO

A number of reports have suggested that nanometric thin films of yttria‐stabilized zirconia (YSZ) deposited on MgO can support high ionic conductivity, but the results remain controversial and difficult to repeat. In this work, sub‐100‐nm‐thick YSZ films have been deposited on single‐crystal MgO substrates with different crystallographic orientations and sourced from different companies. The growth of YSZ on MgO (100) was found to be unstable: both (111)‐oriented films with polycrystalline structure and (100)‐oriented films with cube‐on‐cube epitaxy were observed despite seemingly identical deposition conditions. On MgO (110) and MgO (111) substrates, the growth of YSZ was more stable with high degrees of texture in the (110) and (111) film directions, respectively. Ionic conductivities of the films were measured with impedance spectroscopy, and conductivity values were consistently near or slightly below that of a YSZ single crystal.     

Lattice and Grain‐Boundary Diffusion of Al in Tetragonal Yttria‐Stabilized Zirconia Polycrystalline Ceramics (3Y‐TZP) Analyzed Using SIMS

Aluminum oxide was deposited on the surface of 3 mol% yttria‐stabilized tetragonal zirconia polycrystals (3Y‐TZP). The samples were annealed at temperatures from 1523 to 1773 K. Diffusion profiles of Al in the form of mean concentration vs. depth in B‐type kinetic region were investigated by secondary ion mass spectroscopy. The experimental results for the lattice diffusion (D_B) and grain boundary diffusion (D_GB) are as follows: and where δ is the grain‐boundary width and s is the segregation factor.     

Investigation of Annealing Induced Yttria Segregation in Sputtered Yttria‐Stabilized Zirconia Thin Films

8 mol% yttria‐stabilized zirconia (8YSZ) is an extensively studied solid electrolyte. But there is no consistency in the reported ionic conductivity values of 8YSZ thin films. Interfacial segregation in YSZ thin films can affect its ionic conductivity by locally altering the surface chemistry. This article presents the effects of annealing temperature and film thickness on free surface yttria segregation behavior in 8YSZ thin film by Angle Resolved XPS and its influence on the ionic conductivity of sputtered 8YSZ thin films. Surface yttria concentration of about 32, 20, and 9 mol% have been found in 40 nm 8YSZ films annealed at 1273, 1173, and 1073 K, respectively. Yttria segregation is found to increase with increase in annealing temperature and film thickness. Ionic conductivities of 0.23, 0.16, and 0.08 Scm^?1 are observed at 923 K for 40 nm 8YSZ films annealed at 1073, 1173, and 1273 K, respectively. The decrease in conductivity with increase in annealing temperature is attributed to the increased yttria segregation with annealing. Neither segregation nor film thickness is found to affect the activation energy of oxygen ion conduction. Target purity is found to play a key role in determining free surface yttria segregation in 8YSZ thin films.     

Understanding the Formation of Vertical Cracks in Solution Precursor Plasma Sprayed Yttria‐Stabilized Zirconia Coatings

Yttria‐stabilized zirconia (YSZ) deposition by the solution precursor plasma spraying (SPPS) route has been of interest for potential thermal barrier coating (TBC) applications. It has been surmised that realization of unique microstructural features like vertical cracks, nanosized pores and fine splats in the TBCs can significantly enhance coating durability and performance. However, satisfactory control over the YSZ coating microstructure has been elusive in the absence of an adequate understanding of the mechanism responsible for coating deposition in SPPS. This study demonstrates the ability to tailor microstructure of deposited YSZ coatings over a wide range, from nano‐porous coatings to a vertically cracked microstructure. Varying of precursor flow rate has been shown to dictate the pyrolysis events occurring in situ and, adopting this approach, YSZ coatings with widely varying microstructural features have been developed. The coatings have been characterized in detail and the observations correlated with in‐flight particle generation and splat formation. These studies also provide useful insights into the possible origin of vertical cracks in the coating for which a mechanism is proposed.     

Impedance Spectroscopy and Dielectric Properties of Flash Versus Conventionally Sintered Yttria‐Doped Zirconia Electroceramics Viewed at the Microstructural Level

The defect chemistry‐modulated dielectric properties of dense yttria‐doped zirconia ceramics prepared by conventional sintering (at 1350°C–1500°C) and electric field‐assisted flash sintering (55 V/cm at 900°C) were studied by impedance spectroscopy. While the bulk dielectric properties from both sets of samples showed only small and insignificant changes in conductivity and permittivity, respectively, a huge increase of these properties was measured for the grain boundaries in the flash sintered specimens. A close analysis of these results suggests that flash sintering reduced grain‐boundary thickness (by about 30%), while increasing the concentration of oxygen vacancies near these interfaces (by about 49%). The underlying mechanism proposed is electric field‐assisted generation and accommodation of defects in the space‐charge layers adjacent to the grain surface. The changes in measured permittivity are attributed to the boundary thickness effect on capacitance, while conductivity involved variations in its defect density‐dependent intrinsic value, accounting for changes also observed in grain‐boundary relaxation frequencies. Therefore, in terms of modifications to the specific dielectric properties of these materials, the overall consequence of flash sintering was to considerably lower the semi‐blocking character of the grain boundaries.     

Nano–Nano Composite Powders of Lanthanum Zirconate and Yttria‐Stabilized Zirconia by Spray Pyrolysis

Powders composing of La₂Zr₂O₇ (LZ) and (Zr_0.8Y_0.2)O_1.9 (10YSZ) phases (volume ratio = 1:1) were synthesized by using a sol‐spray pyrolysis method. The effects of annealing temperature on the grain size and lattice parameter of the LZ–10YSZ powders were investigated. XRD results showed that the grain size of LZ and 10YSZ phases gradually grew from 10 to 95 nm and from 5 to 65 nm as the annealing temperature elevated from 900°C to 1200°C. The relative decreasing percentage of grain size comparing to that of the single‐phase LZ and 10YSZ powders were in the range 9%–36% and 37%–86%. The activation energy for grain growth of LZ and 10YSZ phases in the composite powders were 225 ± 12 and 382 ± 17 kJ/mol, which were 20% and 183% higher than that of the single‐phase counterparts. Obvious lattice contraction and lattice expansion for LZ and 10YSZ phases were observed at temperatures below 1100°C, respectively. SEM results revealed that LZ and 10YSZ phases were homogeneously distributed in the sintered bulk. The TEM results suggested that the grain growth was affected by the interaction on nanometer length scales of grain boundaries between LZ and 10YSZ phases in the composite.     

Compatibility Issues of Yttria‐Stabilized Zirconia Solid Oxide Membrane in the Direct Electro‐Deoxidation of Metal Oxides

Direct electro‐deoxidation of metal oxides has become quite popular in the production of metals and alloys. In this process, metal oxide cathode is directly reduced to a metal in a molten CaCl₂ salt bath. The anode material used is graphite. Over the years, graphite is reported to cause numerous process difficulties. Recently, based on the solid oxide membrane technology, yttria‐stabilized zirconia (YSZ) has been tested as oxygen ion conducting membrane for the anode. The success of using a membrane implies its long‐term stability in the bath. In this paper, it is seen that YSZ chemically degrades in a static melt of CaCl₂ or CaCl₂–CaO. The degradation occurs by leaching of yttria into solution leading to the formation of monoclinic zirconia which, being porous, reacts with the molten electrolyte to form calcium zirconate. However, on application of voltage, YSZ degrades via a different mechanism. The metallic calcium produced during electrolysis increases the electronic conductivity of the salt, apparently leading to the electrochemical reduction of zirconia to ZrO_2?x. As a result, localized pores are formed which allow the infiltration of salts. Addition of yttria to the salt is seen to prevent both the chemical and electrochemical degradation of the membrane.     

Oxygen Isotope Transport Properties of Yttria‐Stabilized Zirconia (YSZ) in O2‐ and H2O‐Containing Atmospheres

Oxygen isotope exchange experiments, H₂¹⁸O/H₂¹⁶O (”wet” anneals) and ¹⁸O₂/¹⁶O₂ (”dry” anneals), were performed on single crystal samples of yttria‐stabilized zirconia (YSZ) at a temperature of T = 1073 K with subsequent determination of the oxygen isotope profiles in the solid by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). Such experiments yielded oxygen tracer diffusion coefficients (D*) and oxygen tracer surface exchange coefficients (k*), from both the polished (smooth) and unpolished (rough) sides of single crystal samples, as a function of water partial pressure pH₂O and oxygen partial pressure pO₂. Isothermal values of D* were found to depend on neither pO₂ nor pH₂O (nor surface roughness). Isothermal values of k*, in contrast, displayed a strong dependence on pO₂ or pH₂O; k*_wet was, in addition, 2–3 orders of magnitude higher than k*_dry. Surprisingly, surface roughness had little effect on k*_wet, whereas rough surfaces exhibited much higher k*_dry values than smooth surfaces. Data for k*_wet obtained as a function of temperature at pH₂O = 18 mbar show a change in activation enthalpy at T ≈ 973 K. The behavior of k* is discussed in terms of surface composition, surface area and surface reaction mechanisms.     

Low Thermal Conductivity of SnO2‐Doped Y2O3‐Stabilized ZrO2: Effect of the Lattice Tetragonal Distortion

Considering the phonon scattering effect and the stability of t′ zirconia, Sn⁴⁺ ion is recognized as an appropriate dopant to achieve the best combination of thermal insulating capability and durability of yttria‐stabilized zirconia thermal barrier coatings (TBCs). In this research, unusual lattice expansion and strong structural disordering were observed in a series of SnO₂‐doped Y₂O₃‐stabilized ZrO₂ compounds, which are caused by the tetragonal distortion of oxygen coordination. Phonon scattering due to the structural disordering rather than point defects of Sn⁴⁺ substitutions predominates in reducing the thermal conductivity. However, deterioration of the thermal properties was observed at high doping content, which may be attributed to the t‐m phase transformation during the measurements. Considering the structure stability and thermal properties, SnO₂‐doped Y₂O₃‐stabilized ZrO₂ compounds can be promising candidates for TBCs.     

Mechanical Properties of Plasma‐Sprayed Mullite‐Reinforced Titania–Bioglass Composite

Bioceramics have been extensively used for various medical applications including hip and knee prostheses, tissue engineering scaffolds, and dental implants. Bioceramics, particularly bioglass, are desired because of their bioactivity but are often limited by their inherent brittleness. To compensate, composites have been formed to obtain unique properties where both bioactivity and mechanical integrity can be achieved. Mullite‐reinforced titania–bioglass (TiO₂–BG) composites were therefore deposited using plasma spraying technique. The microstructure of the coating materials were analyzed for their morphology and microstructure using scanning electron microscopy/energy dispersive spectrometry. Mechanical properties of the coatings were tested using three‐point bend test, indentation test, and pin‐on‐disk wear test to determine their fracture strength, fracture toughness, and wear resistance, respectively. The addition of mullite fibers improved the fracture strength and wear resistance of TiO₂–BG composites while having minimal effect on fracture toughness. After the addition of mullite, failure mode was bimodal, failing intergranularly and by fiber pull‐out. Although mullite fibers have not been particularly used for medical applications, fiber reinforcement has shown efficacy in mechanically reinforcing composites of various medical applications.     

Estimation of Stress Exponent and Activation Energy for Rapid Densification of 8 mol% Yttria‐Stabilized Zirconia Powder

The rapid densification behavior of 8 mol% Y₂O₃‐stabilized ZrO₂ polycrystalline (8Y‐SZP) powder compacts at the initial stage of pressure sintering (relative density () below 0.92) has been investigated using an electric current‐activated/assisted sintering (ECAS) system. Data points corresponding to a fixed heating rate were extracted from the densification rate () versus ρ and versus temperature (T) curves. These curves were obtained experimentally by consolidation at a fixed current. Under fixed current ECAS, the heating rate () decreases continuously over sintering time. Using a quasi‐ constant heating rate (CHR) method, data points were extracted to plot vs. ρ, vs. T, and ρ vs. T curves at a fixed . The stress exponent (n), estimated from a log‐log plot of grain size (d)‐corrected /ρ and effective stress (σ_eff) at 1300–1400 K, shows an almost constant value of 1. In addition, the activation energy (Q) for rapid densification, estimated from an Arrhenius plot of d‐corrected /ρ also shows an almost constant value of 350 kJ/mol, which is considerably lower than the previously reported value of the activation energy for Zr⁴⁺ lattice diffusion of about 440 kJ/mol. These results suggest that rapid densification of 8Y‐SZP by ECAS seems to proceed by diffusional creep controlled by grain‐boundary diffusion of Zr⁴⁺ ions.     

Effect of Nano‐Zirconia on the Mechanical and Biological Properties of Calcium Silicate Scaffolds

The calcium silicate (CaSiO₃) scaffolds added with 0, 10, 20, 30, and 40 wt% nano‐zirconia (nano‐ZrO₂) with controlable porous structure were fabricated via selective laser sintering. The effects of nano‐ZrO₂ content on the microstructure, crystalline phase, and mechanical and biological properties were investigated. The results showed that the compressive strength and fracture toughness of the scaffolds were enhanced by the addition of nano‐ZrO₂, and the phase transformation of monoclinic phase (m‐ZrO₂) into tetragonal phase (t‐ZrO₂) occurred, which was favorable for the reinforcing ability of ZrO₂ due to the stress‐induced phase transformation toughening mechanism. However, the excessive amount of nano‐ZrO₂ would cause undesired agglomeration, poor sinterability, and weak apatite‐forming ability. In vitro results showed that there were bone‐like apatite layer formation and MG‐63 cells attachment on the surfaces of the scaffolds, indicating the scaffolds possessed good biological properties.     

Critical analysis of aqueous tape casting,sintering, and characterization of planar Yttria‐Stabilized Zirconia electrolytes for SOFC

Tape casting is an established forming technique for several industries, however, researches focus more on slurry composition. In this work, the combined use of design of experiment and materials characterization techniques showed tape casting process parameters have great influence on the microstructure and mechanical properties of green tapes. Formulation and processing optimization allowed obtaining YSZ green tapes with good mechanical characteristics and homogeneous microstructure without laminating step. The optimized sintering schedule and sintering load allowed obtaining planar electrolytes with high density, tensile strength, and electrical conductivity. This work provides an environmental friendly procedure for large‐scale production of SOFCs planar electrolytes.     

Colloidal Shaping of 8 mol% Yttria‐Stabilized Zirconia Electrolyte Honeycomb Structures by Microwave‐Assisted Thermal Gelation of Methyl Cellulose

Eight mol% Yttria‐stabilized zirconia, the most commonly employed electrolyte material for solid oxide fuel cells (SOFC), was shaped into honeycomb structures by thermally induced gelation of aqueous zirconia slurry containing methyl cellulose using microwave irradiation. The green honeycomb samples were subjected to green density and green compressive strength measurements revealing a uniform gelation and hence a relatively higher strength for microwave irradiated samples. The green honeycomb samples were further sintered to crack‐free dense honeycombs (>99% TD) at 1525°C for 1 h. Honeycomb samples were characterized for their physical, cellular, and electrical properties. A relatively high ionic conductivity value of 0.07 S/cm at 800°C and corresponding low activation energy of 0.61 eV in the temperature range 700–800°C provide opportunities to explore the development of novel designs for SOFC application.