Yttria-stabilized Zirconia Composites Research Articles

Ni migration on porous yttria-stabilized zirconia and yttria-micropatterned fuel electrodes in solid oxide fuel cells

This study investigates the mechanism of Ni migration and methods for preventing Ni migration by utilizing ceramic obstacles that can block or slow Ni movement during solid oxide fuel cell (SOFC) operation. A patterned thin-Ni model anode on mirror-polished yttria-stabilized zirconia (YSZ) substrate exhibited dynamic movement during operation, as reported previously. In this study, we attempt to elucidate the mechanism of Ni migration that occurs only on the surface of solid-state Ni by blocking or slowing this movement using porous patterns of different materials. Porous micropatterns of 100 % YSZ and 80 wt% yttria (Y2O3) + YSZ composites were fabricated on a YSZ electrolyte, and velocity changes and blocking of Ni were observed. The porous YSZ structure changed the velocity of the Ni. In contrast, no Ni migration occurred in the Y2O3/YSZ composite porous structure, despite migration occurring in the neighboring smooth YSZ area in the same cell. The Ni-migrated areas were separated by the Y2O3/YSZ porous area, indicating that Ni migration was not caused by microscale differences in temperature or supplied gas diffusion, and Ni moved only on the cell surface, not via Ni-containing gases because temperatures and gases cannot form micropatterns.

Exploration into the microstructural, mechanical, and biological characteristics of the functionally graded 3Y-TZP/Ti6Al4V system as a potential material for dental implants

This study investigated the mechanical, microstructural, and biological properties of 3Y-TZP/Ti6Al4V functionally graded material (FGM) fabricated by the spark plasma sintering (SPS) method. For this purpose, 11 layers of 100-x vol% Ti6Al4V/x vol% Yttria stabilized zirconia (YSZ) (x = 0 to 100) were sintered at 1450 °C and a pressure of 30 MPa for 8 min. To investigate the properties of each layer in more detail, 11 batches of 100-x vol% (Ti6Al4V)/x vol% YSZ (x = 0 to 100) composites were sintered separately with the same sintering conditions mentioned for the FGM sample. Phase identification of the FGM sample showed the formation of Ti3O, c-ZrO2, and Zr3O phases as by-products. A schematic model was proposed for the formation of the mentioned phases with the aid of thermodynamic calculations. The formation of these phases was confirmed by microstructural and elemental tests. The results of the relative density of the samples showed that these values were obtained for each layer above 99%. The microhardness of 590 ± 18 Vickers was obtained for Ti6Al4V; by increasing the amount of 3Y-TZP, this value reached 1510 ± 24 Vickers for the YSZ sample. The fracture toughness value for Ti6Al4V was 39.2 ± 2 MPa m0.5, which was significantly reduced to 4.84 ± 1 MPa m0.5 by adding 10 vol% YSZ. After that, with the further increase of YSZ, this value increased slowly. A similar trend was observed for the bending strength of the samples. By increasing 3Y-TZP from 0 to 30 vol%, the bending strength was decreased from 1556 ± 32 to 272 ± 62 MPa. By further increasing the amount of 3Y-TZP from 30 to 100 vol%, an increase in the bending strength was observed in the samples, which reached 1180 ± 71 MPa for the YSZ sample. The FGM sample showed a brittle fracture despite a metal layer, but a higher bending strength (982 ± 44 MPa) was obtained for this structure than the composite samples. The biological results show that increasing YSZ content leads to a decrease in antimicrobial activity. Additionally, all samples demonstrated high biocompatibility based on MTT cytotoxicity tests after 1 and 7 days of culture.

Enhancing the Properties of Yttria-Stabilized Zirconia Composites with Zeolitic Imidazolate Framework-Derived Nanocarbons.

Ceramic matrix composites (CMCs) reinforced with nanocarbon have attracted significant interest due to their potential to enhance mechanical, thermal, and electrical properties. Although the investigation of carbon-based materials such as graphene and carbon nanotubes as additives for advanced ceramics has been widespread, the utilization of metal-organic framework (MOF)-derived nanocarbons in CMCs remains largely unexplored. We extended our previous proof-of-concept investigations by demonstrating the effectiveness of a different type of MOF-derived carbon as a reinforcing phase in an alternative ceramic matrix. We employed spark plasma sintering (SPS) to consolidate yttria-stabilized zirconia (YSZ) and zeolitic imidazolate framework (ZIF-67) powder blends at 1300 °C and a uniaxial pressure of 50 MPa. YSZ serves as the ceramic matrix, whereas ZIF-67 serves as the nanocarbon source. The composite exhibits a highly significant improvement in fracture toughness with an increase of up to 13% compared to that of the YSZ monolith. The formation of ZIF-derived nanocarbon interlayers is responsible for the observed enhancement in ductility, which can be attributed to their ability to facilitate energy dissipation during crack propagation and inhibit grain growth. Furthermore, the room-temperature electrical conductivity of the sintered samples demonstrates a substantial improvement, primarily due to the in situ formation of nanocarbon-based fillers, reaching an impressive 27 S/m with 10 wt % ZIF-67 content. Based on the results, it can be inferred that the incorporation of in situ MOF-derived nanocarbons into CMCs leads to a substantial improvement in both the mechanical and electrical properties.

Electrochemical characteristics of limiting current sensors with LSM-YSZ and LSM-CGO-YSZ composite electrodes

Limiting current gas sensors with Pt electrodes are widely used to detect gaseous species, such as O2, CO, HC, and NOx, from exhaust gases generated by the incomplete combustion of fossil fuels. In this study, aperture-type limiting current sensors were fabricated based on La0.8Sr0.2MnO3 (LSM)–yttria-stabilized zirconia (YSZ) and LSM–Gd0.1Ce0.9O2-d (CGO)–YSZ composites. The potential of these composites as alternative electrodes for O2 and CO sensing was evaluated. The composite electrodes exhibited current–voltage curves that are typical for limiting current sensors. LSM–CGO–YSZ registered higher limiting current values than those of the commercial Pt and LSM–YSZ electrodes in O2 and CO atmospheres. The formation of a resistive secondary La2Zr2O7 phase at the LSM/YSZ interface of LSM–YSZ deteriorates the electrochemical reaction and increases the polarization resistance of the electrode in the CO environment. The addition of CGO prevented the formation of the secondary phase, which lowered the ohmic and polarization resistances of LSM–CGO–YSZ compared to that of the LSM–YSZ composite. However, increasing the CGO concentration from 10 to 40 wt% weakened the adhesion of LSM and reduced the triple phase boundaries, which increased the electrode polarization resistance in both O2 and CO atmospheres.

Microstructure and biocorrosion studies of spark plasma sintered yttria stabilized zirconia reinforced Ti6Al7Nb alloy in Hanks' solution

A study of spark plasma sintered Ti6Al7Nb alloy reinforced with yttria stabilized zirconia (YSZ) has been carried out to assess the biocorrosion response in Hank's solution. The results indicate that the microstructural changes resulting from YSZ addition to Ti6Al7Nb were considerably different from pure Ti6Al7Nb. All the Ti6Al7Nb + YSZ composites showed a higher Vickers microhardness value with Ti6Al7Nb + 10%YSZ exhibiting the most enhanced hardness value of 955.3 HV0.1. Furthermore, all Ti6Al7Nb + YSZ present higher corrosion resistance in Hank's solution than Ti6Al7Nb alloy except Ti6Al7Nb + 1YSZ which shows a significant drop in Ecorr and Icorr values. This could be due to grain refinement with less stable oxide film growth kinetics. The combination of mechanical property and corrosion resistance of the fabricated Ti6Al7Nb + YSZ alloy could make it a suitable material for dental implant in the nearest future.

Sintering and thermal expansion behaviors of glass and glass–YSZ composites as self‐repairable seals for SOFC

AbstractGlass and glass–ceramics are used as sealants in solid oxide fuel cell (SOFC) because their thermophysical properties can be tailored to meet the stringent requirements of the SOFC stack. The processing, sintering, and thermal expansion behaviors of self‐healing and non‐crystallizing glass and glass containing 10%–30 wt.% non‐reacting yttria‐stabilized zirconia (YSZ) are studied. The addition of inert YSZ to glass significantly retarded the sintering behavior. Thermal expansion behaviors of glass and glass–YSZ are also measured to study the role of YSZ addition on the glass transition, softening point, and coefficient of thermal expansion (CTE). It is shown that the densification is controlled by the viscous sintering mechanism, in which the addition of YSZ increased the effective viscosity of the glass–YSZ as evident from higher glass transition and softening temperatures and decreased CTE. These results demonstrated that the addition of YSZ to glass is promising for achieving optimum thermophysical properties useful as seals for SOFC.

Constructing high-performance radiation-resistant ternary YSZ-MgO-CNT nanocomposites via tailored nanostructures

Developing long-lifetime bulk-form ceramic-based materials with high irradiation resistance is crucial for advanced nuclear systems. Here, we incorporated carbon nanotubes (CNTs) into yttria-stabilized zirconia (YSZ) and magnesia (MgO) nanocrystals to fabricate bulk YSZ-MgO-CNT nanocomposites with abundant ternary nanostructures by spark plasma sintering. To understand the role of tailored ternary nanostructure on irradiation, we investigated the microstructure and mechanical properties evolutions of the YSZ-MgO-CNT nanocomposites irradiated by multi-energy He+ ions at high temperature to different fluences. Compared with the single-phase YSZ and ultrafine-grained YSZ-MgO composites, the YSZ-MgO-CNT nanocomposites possessed higher ability to manage irradiation-induced He bubbles/defects via the defect-interface interactions of proposed “loading-unloading” and “loading-transporting-unloading” mechanisms for controlling the dynamical behaviors of He atoms/defects in the CNT-doped ternary nanostructures, thereby presenting more stable microstructure and better performance in resisting irradiation hardening. This work provides insight into the design of advanced inert matrix nuclear fuel and new nuclear waste management materials.

Mechanical and Tribological Properties of Alumina Toughened Zirconia Composites through Conventional Sintering and Microwave Sintering

The present study investigates the relative density, mechanical and tribological properties of 3 mol% yttria-stabilized zirconia composites reinforced with 10 vol% of alumna through both conventional sintering and microwave sintering (CS and MW). The variations in mechanical and tribological properties were correlated to the corresponding microstructures. Wear tests were carried out on the composite samples against EN30 grade stainless steel disk with 320-grade emery paper for a sliding distance of 1500 m under dry sliding condition. Wear rate and coefficient of friction of the developed composite were evaluated with varying normal loads in the range of 20–200 N with corresponding sliding velocities of 0.79 m/s, 1.4 m/s, 1.8 m/s and 2.4 m/s, respectively. The maximum value of wear rate and COF of the MW samples were 51.21% and 24.59%, respectively, which were lower compared to that of CS.

Porous NiO–YSZ and Ni–YSZ composites fabricated using NiO–YSZ composite nanopowders and WO3 additive

NiO–YSZ composite nanopowders were synthesized through a hydrothermal process without an additional mixing of NiO and YSZ nanopowders. Porous Ni–yttria stabilized zirconia (YSZ) composites, Ni–YSZ composites, with high porosity (25.0–32.8%) were fabricated by reducing NiO-YSZ composites in (94% Ar + 6% H2) gas. The electrical conductivity (14,221 cm−1 at 30 °C) and bending strength (130 MPa) of Ni–YSZ composites were enhanced by the addition of WO3 (0.1 wt% WO3) to NiO–YSZ composite nanopowders. Furthermore, the Y2(WO4)3 with a negative coefficient of thermal expansion (CTE) reduced the CTE of Ni–YSZ composites. The CTE values of 0 and 0.6 wt% WO3–added Ni–YSZ composites were 13.8 × 10−6 and 12.4 × 10−6 °C−1, respectively. We demonstrate that the fabrication of NiO–YSZ composite nanopowders and the addition of WO3 to the NiO–YSZ composite nanopowders are highly effective methods for the fabrication of high-quality Ni–YSZ anode materials in solid oxide fuel cells.

Continuous alumina fiber-reinforced yttria-stabilized zirconia composites with high density and toughness

Continuous alumina fiber-reinforced yttria-stabilized zirconia (YSZ) composites with a LaPO4 fiber coating were fabricated by slurry infiltration and spark plasma sintering (SPS). The LaPO4 coating was deposited on the reinforcement alumina fabrics by a modified sol-gel method. The YSZ slurry with good dispersion and stability was prepared by optimizing the pH value, dispersant addition and ball milling time. The fabricated composite with a high density of ∼ 92 % has a good flexural strength of 277 ± 43 MPa, and a superior fracture toughness of 15.93 ± 0.75 MPa·m1/2 exhibiting a non-brittle failure behavior. It was found that the LaPO4 coating reduced the residual stress near the fiber/matrix interface to 131 ± 41 MPa, which was 369 ± 63 MPa in the composite without the fiber coating. The LaPO4 coating renders a weak interphase to improve the composite toughness by activating several toughening mechanisms including crack deflection, fiber debonding and pullout, and delamination behavior.

Electrochemical Performance of a Na-β”-Alumina+ YSZ Composite As a Mixed Ionic Conductor

Most of the ceramic ionic conductors typically conduct one ion. For example, AgI conducts Ag+ ion, Sodium β”-alumina conducts Na+ ion and Yttria-stabilized zirconia (YSZ) conducts O2- ions. Some selected materials such as alkaline earth cerates or zirconates conduct both protons and O2- ions. In such materials also, conductivity of one species is not independent of the other species. Thus, temperature and atmosphere determine the total conductivity. All of such materials are single phase materials. We have been investigating two phase materials such that one phase conducts one ion (Na+) while the other phase conducts another ion (O2-). The materials in such cases is a composite with the two phases forming an interpenetrating network. The volume ratios of the two phases can be adjusted as necessary. Since the two phases essentially constitute a mixture, conduction of one species occurring through one phase is independent of the conduction of the other species through the second phase. Yet, the condition of electroneutrality is applicable globally across multiple phases. Transport equations can then be written down in the usual manner but accounting for different volume fractions for the two phases. A mixed Na+ ion and O2- ion two phase conductor is fabricated by first making a sintered dense composite containing α-alumina and YSZ such that both phases are contiguous. YSZ and α-alumina are immiscible. This allows the fabrication of fine-grained, dense, strong composites. Subsequently, the sample is packed in Na-β”-alumina + YSZ powder and heat treated in air over a range of temperatures from 1200oC to 1400oC. During this process, α-alumina is converted into Na-β”-alumina by the incorporation of Na2O into α-alumina by coupled transport in which Na+ transports through the formed Na-β”-alumina and O2- ions transport through YSZ. The reaction front over time sweeps through the sample forming a two phase composite of Na-β”-alumina + YSZ. Such a material is now capable of conducting two ions; Na+ and O2-. Materials capable of transporting other alkali ions can be made by an ion exchange process in molten salts. For example, by ion exchanging in lithium salts, it is possible to make mixed Li-ion and O2- ion conducting materials. The oxygen ion conductivity is negligible at low temperatures. Thus, the materials at room temperature are predominantly alkali ion conductors with possible use in all solid state lithium ion and sodium ion batteries. Another mixed ion conductor was made by a similar process using a composite of Ga2O3 and YSZ. It was similarly packed in Na-β”-alumina powder and heat treated. In this case a two phase mixture containing YSZ and sodium zirconium gallate formed, the latter with a chemical formula of Na0.7Ga4.7Zr0.3O8 with a monoclinic structure. This compound is isostructural with Na0.7Ga4.7Ti0.3O8 which is known to be one dimensional sodium ion conductor. Measurement of ionic conductivity on Na0.7Ga4.7Zr0.3O8 + YSZ showed it to be a mixed Na+ ion and O2- ion conductor. Sodium ion conductivity of Na0.7Ga4.7Zr0.3O8 is much lower than that of Na-β”-alumina. The activation energy for Na+ ion transport is much lower than for O2- ion transport in YSZ. As a result, the two phase mixture is predominantly a Na+ ion conductor at low temperatures and an O2- ion conductor at elevated temperatures. Both Na-β”-alumina + YSZ and Na0.7Ga4.7Zr0.3O8 + YSZ are mixed Na+ ion and O2- ion conductors. This allows one two explore electrochemical transport of two species and examine the applicability of Goldman-Hotchkin-Katz trype of an equation, routinely used in aqueous electrochemistry, for solid-state systems. Preliminary results of experiments conducted by maintaining different chemical potentials of Na2O at the two electrodes and different chemical potentials of oxygen (by testing in a fuel cell mode) will be described. Possible degradation of Na-β”-alumina in reducing atmosphere will also be addressed. Acknowledgements: This work was supported by the National Science Foundation under Grant Number DMR-1407048.

Porosity, microstructure and mechanical behavior of NiO–YSZ composite anode for solid oxide fuel cells

Abstract In this study, NiO–yttria stabilized zirconia (YSZ) composite anodes were prepared by using ball-milling followed by a sintering process. For this purpose, two different weight percent ratios of NiO and YSZ powders (50 : 50 and 60 : 40) were mixed by the cost-effective ball-milling method. The morphology and particle size distribution of the mixed powders were investigated using scanning electron microscopy and dynamic light scattering methods, respectively. The powders were then uniaxially pressed to form pellets with a thickness and diameter of around 1 and 22 mm, respectively. Sintering of the pellets was conducted at 1 100 °C and the microstructure and porosity, as well as phase analysis of bulk composites, were then investigated using scanning electron microscopy and X-ray diffraction, respectively. More importantly, the microhardnesses of the prepared pellets were obtained and compared with literature values to evaluate the accuracy of the sintering process. The results showed that the open porosity of the samples varied between 30 to 40 percent, which makes them suitable for anode application in solid oxide fuel cells. The prepared NiO–YSZ composites with a uniform distribution of Ni particles showed a value of microhardness above 0.5 GPa, and these results were discussed in detail.

Fabrication of Li-β" Alumina + Yttria-Stabilized Zirconia Composites and Their Application in Lithium Battery

There has been considerable interest in developing either all-solid-state lithium battery or batteries with a solid electrolyte to replace flammable liquids in conventional lithium ion batteries. Here we report on the fabrication of Li-β”-alumina containing materials as solid electrolytes by pressure less sintering for lithium-based batteries and conduct preliminary studies on a possible room temperature lithium-based battery. The resultant material is a two phase composite containing Li-β”-alumina and YSZ, which also makes it very strong. At low temperatures (room temperature), the oxygen ion conductivity of YSZ is negligible. Thus, the two phase composite is predominantly a Li+-ion conductor. A two phase composite disc containing α-alumina and yttria-stabilized zirconia (YSZ) was die pressed and sintered at 1600oC first. The disc was then converted to β"-alumina and YSZ composite by the previously reported vapor phase method in which the sintered samples of α-alumina + YSZ were exposed to Na2O vapor [1, 2]. After the vapor phase conversion treatment, the samples were ion-exchanged in molten LiNO3 to form Li-β”-alumina + YSZ composites. Fig.1a shows the measured ion exchange fraction (Li+/(Li++Na+)) vs. ion exchange time at 300oC and 370oC. The measured ion exchange fraction agrees relatively well with that by Yao and Kummer in single phase β-alumina [3]. The partially ion-exchanged Li-β”-alumina is expected to be a predominantly Li+-ion conductor [4] Ionic conductivity of both Na-β”-alumina + YSZ and Li-β”-alumina +YSZ was measured using electrochemical impedance spectroscopy (EIS) with Au electrodes. Fig. 1b shows the Arrhenius plots of the total ionic conductivity measured by EIS on a fully converted sample and after ion exchange for 24 h at 300oC. The left inset shows EIS spectra (10 Hz to 1 MHz) at different scales. Over the range of temperatures investigated, the grain boundary arc was not observed. Thus, the measured conductivity is attributed to the total conductivity. Both samples exhibit an Arrhenius behavior. The activation energy for the converted sample is 0.13 eV while that for the ion exchanged sample is 0.18 eV. The room temperature conductivities were estimated by extrapolation. The estimated total ionic conductivities at room temperature are 3.6 x 10-3 and 7.5 x 10-4 Scm-1 for the sample before and after ion exchange, respectively. Fig. 1c shows the measured OCV of a LMO+C||Li+- β"-alumina + YSZ ||Li cell at room temperature over a period of 12 hours using a LiNO3 ion exchanged disc (0.4 exchange fraction, 1 mm thickness). The OCV was measured on the cell assembly without any charging process. The measured initial OCV of 3.4 V was slightly lower than theoretical OCV of ~3.5 V reported for this type of cathode. Since the electronic conductivity of this type of solid electrolyte is expected to be extremely low, self-discharge is not expected. The voltage drop here thus indicates possibly changing activity of the anode due to non-optimized cell assembly, such as the sealing method. EIS spectra obtained on the cell at room temperature over a frequency range from 1 Hz to 1 MHz showed a very large and incomplete arc. The high frequency intercept is ~350 Ω. This is expected to be mainly the electrolyte resistance. From the disc dimensions, the estimated ionic conductivity is ~1.78 x 10-4 Scm-1. The measurements indicates the battery performance was predominantly limited by the electrode polarization. Future work will involve the fabrication of an all solid-state battery with cathode containing a mixture LMO (or other materials such as sulfur), powder of Li- β”-alumina and carbon. Future work will also involve using a much thinner electrolyte disc, made by tape-casting, sintering, vapor phase conversion, and ion exchange. Acknowledgments: This work was supported by the National Science Foundation under Grant Number DMR-1407048.

Fabrication of Li-β" Alumina + Yttria-Stabilized Zirconia Composites and Their Application in Lithium Battery

Samples of Na-β”-alumina + yttria-stabilized (YSZ) were made by vapor phase conversion of sintered α-alumina + YSZ. The samples were then treated in molten LiNO3 to convert Na-β”-alumina into Li-β”-alumina by ion exchange. The resulting two phase material is a Li+-ion conductor with room temperature ionic conductivity of ~2 x 10-4 to 7.5 x 10-4 Scm-1. The corresponding activation energy is 0.18 eV. Two lithium batteries of LMO+C||Li+/Na+- β"-alumina + YSZ ||Li structure were constructed using the same ion exchanged disc of 1 mm thickness over a period two months apart. Both batteries displayed a stable OCV of around 3.2 V at room temperature. No chemical and mechanical degradation of the solid electrolyte disc was observed over the testing period, indicating a high stability of the reported solid electrolyte against lithium metal. Thermodynamic activity of lithium is estimated to be ≈ 7.6 x 10-58 in LMO, and ≈ 8.8x 10-50 in Li2O.

High performance zirconia-bismuth oxide nanocomposite electrolytes for lower temperature solid oxide fuel cells

We develop a novel nanocomposite electrolyte, consisting of yttria-stabilized zirconia (YSZ) and erbia-stabilized bismuth oxide (ESB). The 20 mol% ESB-incorporated YSZ composite (20ESB-YSZ) achieves the high density (>97%) at the low sintering temperature of 800 °C. The microstructural analysis of 20ESB-YSZ reveals the characteristic nanocomposite structure of the highly percolated ESB phase at the YSZ grain boundaries (a few ∼ nm thick). The ionic conductivity of 20ESB-YSZ is increased by 5 times compared to that of the conventional YSZ due to the fast oxygen ion transport along the ESB phase. Moreover, this high conductivity is maintained up to 580 h, indicating high stability of the ESB-YSZ nanocomposite. In addition, the oxygen reduction reaction at the composite electrolyte/cathode interface is effectively enhanced (∼70%) at the temperature below 650 °C, mainly due to the fast dissociative oxygen adsorption on the ESB surface as well as the rapid oxygen ion incorporation into the ESB lattice. Thus, we believe this ESB-YSZ nanocomposite is a promising electrolyte for high performance solid oxide fuel cells at reduced temperatures.

(Invited) High Performance Nano-Ceria Electrodes for Solid Oxide Cells

Solid oxide electrochemical cells (SOCs) show great promise to become a cornerstone of a renewable energy future, based on their ability to reversibly store electrical energy as green chemical fuels and to produce electricity from fuels. The conventional nickel and yttria-stabilized zirconia composite (Ni-YSZ) fuel-electrode provides high reaction rates for the H2O/CO2 reduction and H2/CO oxidation reactions, but it is often a major source of long-term performance degradation due to poisoning of reaction sites, carbon deposition, Ni mobility, and other degradation mechanisms. Alternative electrodes made of mixed ionic and electronic conducting (MIEC) oxides have been shown to avoid these issues, however they also typically show much lower electrochemical activity than that of state-of-the-art Ni-YSZ electrodes. During the last 10 years, DTU Energy has developed MIEC electrodes with nanostructured doped ceria (NDC) electrocatalysts as the active component. As doped ceria has low electronic conductivity, the initial electrodes were comprised of a small amount of NDC coated by infiltration onto the internal surfaces of porous electron-conducting backbones made of Sr(Ti,Nb)O3 and/or stainless steel. Although the electrochemical activity matches or even exceeds that of Ni-YSZ, insufficient long-term stability has proven to be a persistent issue with these electrodes. Recently we have developed new NDC based electrodes with no additional electron conductor or only a small volume fraction. Nanostructure optimization, primarily by adjusting heat treatment parameters, have yielded an unprecedented electrode polarization resistance (Rp) of ~0.01 Ω cm2 at 650 °C in a fuel gas mixture of H2/H2O 0.8/0.2 atm for a nanoporous Ce0.9Gd0.1O1.95-d thin film prepared by spin-coating. Initial results show highly promising long-term stability of the electrochemical reaction rates, although providing and maintaining sufficient electronic current collection is challenging due to the low in-plane conductivity of the film. We found that adding a small amount of a better electron conductor to the film minimizes ohmic resistance and maintains electronic contact. Similarly, more conventional structured porous electrodes with NDC surfaces and only a small amount of electronic conductor were found to show high activity and negligible electronic resistivity. We have also applied the same design principles to obtain NDC based electrodes that work almost equally well as either a fuel-electrode or oxygen-electrode, using CexPr1-xO2-d as an active surface component. High activity for both fuel and oxidant electrochemical reactions is possible because CexPr1-xO2-d is a MIEC in both the low oxygen partial pressure (pO2) of the fuel gas and high pO2of the oxidant gas. Electrochemical impedance and structural analyses of these highly defective, highly active MIEC electrodes will be presented, together with the impact of current collector components and a discussion of known limitations and future outlook. Figure: (a) Impedance spectra measured 42 h apart at 650 °C in a fuel gas mixture of H2/H2O 0.8/0.2 atm on a nanoporous Ce0.9Gd0.1O1.95-d thin film electrode. In between the measurements, the operating temperature was kept at 550 °C to 650 °C. (b) Scanning electron micrograph with backscattered electron detector on the same type of electrode. Figure 1

Synthesis of Ion Conducting Sodium Zirconium Gallate + Yttria-Stabilized Zirconia by a Vapor Phase Process

Two phase samples containing ∼70 vol.% Ga2O3 and ∼30 vol.% yttria-stabilized zirconia (YSZ) were fabricated by sintering powder compacts in air at 1500°C for 1 h and at 1600°C for 1 h and 3 h. Single phase samples of Ga2O3 were fabricated by sintering at 1500°C for 1 h. Sintered samples were packed in Na-β″-alumina powder of composition ∼8.85 wt% Na2O, 0.75 wt% Li2O and 90.4 wt% Al2O3, and were heat treated at 1250°C for up to 20 h. The two phase Ga2O3 + YSZ samples converted into a sodium zirconium gallate of composition Na0.7Ga4.7Zr0.3O8 and balance yttria-stabilized zirconia. The reaction of single phase Ga2O3 was limited to a thin surface layer in which Na-β″-gallate formed. Rapid conversion of Ga2O3 + YSZ samples into Na0.7Ga4.7Zr0.3O8 + YSZ occurred by coupled diffusion of O2− ions through YSZ and of Na+ ions through the formed Na0.7Ga4.7Zr0.3O8, similar to the conversion of α-Al2O3 + YSZ into Na-β″-alumina + YSZ. The phase Na0.7Ga4.7Zr0.3O8 is isostructural with Na0.7Ga4.7Ti0.3O8 which is a one dimensional sodium ion conductor. The conductivity of two phase Na0.7Ga4.7Zr0.3O8 + YSZ composite was measured by Electrochemical Impedance Spectroscopy.

Machine-able Yttria Stabilized Zirconia Composites for Thermal Insulation in Nuclear Reactors

Ceramics are a promising insulating material for high temperature environment. To qualify for in-core use in nuclear reactors, there are many other materials requirements to be met, such as neutron irradiation resistance, corrosion resistance, low thermal conductivity, high coefficient of thermal expansion, high strength, high fracture toughness, ease of fabricability, etc. And among the promising ceramics meeting most of the requirements, with the exception of fabricability, is yttria-stabilized zirconia (YSZ). Like all ceramics, YSZ is hard, brittle and difficult to machine. At CanmetMATERIALS, YSZ-based composites for in-core insulation that are machine-able and capable of being formed into complex shapes have been developed. In this paper, the focus is geared towards the fabrication and property evaluation of such composites. In addition, the machinability aspect of the YSZ composites was addressed with a demonstration of a machined component.

Surface modification of La(0.8)Sr(0.2)CrO(3-δ)-YSZ dual-phase membranes for syngas production.

Oxygen permeation fluxes were studied in Mixed Ionic and Electronic Conducting (MIEC) membranes based on composites of 40 vol% La(0.8)Sr(0.2)CrO3 (LSCr) and 60 vol% yttria-stabilized zirconia (YSZ), using ambient air and flowing CO to establish a P(O2) gradient. The ambipolar conductivity of the dense LSCr-YSZ composite was determined for membranes with dense layers that were 115 μm and 650 μm thick. Other parts of the investigation focused on how modifications to the surface on the CO side affected the fluxes. Using a porous LSCr-YSZ composite on the surface as the base case, oxygen fluxes were shown to increase dramatically upon addition of 5 wt% CeO2 as a catalyst and an additional increase was observed with 1 wt% Pt. Changes in the structure of the porous composite LSCr-YSZ surface to improve connectivity of the YSZ phase also led to large increases in the oxygen fluxes.

Processing and characterization of WC–10Ni/–7Ni–3Cr based yttria stabilized zirconia spark plasma sintered composites

The mechanical properties and wet sliding wear behavior of spark plasma sintered WC based composites consolidated using 10Ni/7Ni–3Cr (metallic binders) and varied wt.% (15 and 20%) zirconia stabilized with yttria (ceramic binder) as mixed binder additives were investigated. Relative density measurement, SEM analysis, hardness and Palmquist toughness characterization were used to assess the properties of the produced composites. The results show that the use of mixtures of 10Ni, 7Ni–3Cr and yttria stabilized zirconia binder additives for the consolidation of the WC based composites yielded relative densities above 94%. Significant drop in hardness values was observed in the sintered composites with the addition of 15 and 20% zirconia. But the Palmquist toughness improved with the addition of zirconia for both composite grades. The fracture toughness for the WC–10Ni based yttria stabilized zirconia composites appeared to be invariant to the addition of zirconia while the WC–7Ni–3Cr composite grades showed a slight improvement in the fracture toughness with an increase in the zirconia content. The fracture toughness of the WC–10Ni based yttria stabilized zirconia composites was slightly superior to that of the WC–7Ni–3Cr based yttria stabilized zirconia composites. The addition of zirconia resulted in a significant reduction of the coefficient of friction of the WC–10Ni based yttria stabilized zirconia composites but resulted in the increase of the coefficient of friction of the WC–7Ni–3Cr based yttria stabilized zirconia composites.