Yttria-stabilized Zirconia Disc Research Articles

Nanocrystalline Yttria-Stabilized Zirconia Ceramics for Cranial Window Applications.

Transparent yttria-stabilized zirconia (YSZ) ceramics are promising for cranial window applications because of their good mechanical and optical properties as well as biocompatibility. YSZ discs with different yttria concentrations were either processed via current-activated pressure-assisted densification (CAPAD) using commercial nanoparticles or densified via spark plasma sintering (SPS) using pyrolysis-synthesized nanoparticles in-house. This study provided critical results to screen composition, processing, microstructure, and cytocompatibility of transparent YSZ discs for cranial window applications. CAPAD-processed YSZ discs with 6 or 8 mol % yttria (6YSZ and 8YSZ) and SPS-densified YSZ discs with 4 mol % yttria (4YSZ_P) showed 200-350 nm polycrystalline grains containing 20-30 nm crystallite domains. SPS-densified YSZ discs with 8 mol % yttria (8YSZ_P) showed larger polycrystalline grains of 819 ± 155 nm with 29 ± 5 nm crystallite domains. CAPAD-processed YSZ discs with 3 mol % yttria (3YSZ) showed 39 ± 9 nm grains. Bone-marrow-derived stem cells (BMSCs) on the polished YSZ discs showed statistically higher spreading areas than those on the unpolished YSZ discs of the same compositions. Generally, polished 8YSZ, 4YSZ_P, and 8YSZ_P discs and unpolished 8YSZ_R, 4YSZ_PR, and 8YSZ_PR discs had lower average cell adhesion densities than other YSZ discs under direct contact conditions. Under indirect contact conditions, all the YSZ disc groups showed similar average cell adhesion densities to the Cell-only control. The groups of polished 4YSZ_P and 8YSZ_P discs, unpolished 4YSZ_PR and 8YSZ_PR discs, and particle control of 8YSZ_Pnp showed higher Y3+ ion concentrations than other groups. No mineral deposition was detected on the polished YSZ discs after cell culture. Considering multiple factors such as cytocompatibility, cell adhesion density, Y3+ ion release, mineral deposition, and optical transparency collectively, 8YSZ may be the best candidate for the cranial window applications. Further studies are needed to evaluate the long-term transparency and biocompatibility of YSZ discs.

Laser surface treatment on Yttria-stabilized zirconia dental implants: Influence on cell behavior.

Yttria-stabilized zirconia (YSZ) is being proposed as an alternative material to Titanium for dental implants due to its aesthetic and biocompatibility properties. However, is it yet to define the optimal surface treatment to improve YSZ bioactivy. Texturization is a promising approach, but the biological role of patterned YSZ surfaces in cell cultures is yet to be determined. Thus, cellular behavior of osteoblasts and fibroblasts in contact with groove-texturized YSZ surfaces was investigated. YSZ discs were groove-textured by conventional milling and Nd:YAG laser. All samples including control were sandblasted and acid-etched. Human osteoblasts and fibroblasts were cultured on discs for 14 days. Morphology and cellular adhesion were observed. Cell viability, interleukin-1β, osteopontin, collagen type I prodution, alkaline phosphatase activity, and interleukin-8 were measured. YSZ texturization by conventional milling improved osteoblasts viability and differentiation when compared to laser texturization. Fibroblasts behavior did not seem to be influenced by the texturing technique. Compared to sandblasting and acid etching currently used as gold standard for zirconia dental implants no superiority of macrotexturization was found.

The effect of temperature and roughness of the substrate surface on the microstructure and adhesion strength of EB-PVD ZrO2-%8wtY2O3 coating

Thermal barrier coatings are of great interest in increasing the working temperature and thus, improving the efficiency of turbine engines. In this study, coating was conducted on nickel-base superalloy substrates using physical vapor deposition process and the electron beam evaporation technique of Yttria-stabilized Zirconia discs. In order to characterize the coatings including microstructure, topography and phase structure, field emission scanning electron microscopy, atomic force microscopy and X-ray diffraction analysis were used, respectively. The scratch test was also used to evaluate the adhesion strength of the coating. It was observed that at the temperature of 470 °C, the coating has no preferential orientation. By increasing the substrate temperature to 750 °C, the intensity of the peak (211) was reduced, while that of the peak (101) was sharply increased, such that at the substrate temperature of 750 °C, the coating had only the preferred orientation (101). Increasing the substrate temperature from 470 °C to 750 °C enhanced the critical force (Lc) from 8.2 N to 25.1 N. Increasing the substrate roughness from 0.55 μm to 3.43 μm enhanced the coating roughness from 0.62 μm to 3.71 μm raised the columnar diameter from 36.98 nm to 120.9 nm and reduced the critical force (Lc) from 18 N to 6.3 N.

Comprehensive Kinetics Model with Competitive Adsorption Reaction on Water Electrode in Reversible Solid Oxide Fuel Cell / Electrolysis

Hydrogen generation technologies by electrolysis (EC) and the reversible operation with fuel cells (FC) are rapidly increased their attention as a large-scale energy storage to solve the gap between the unsteady power supply and the electricity demand due to the expanding installation of renewable energy. For example, if solar cells are installed into a certain microgrid to supply 30% of their electricity demand, large amount of surplus power generated in almost every sunny day. Thus, large scale energy storage by water electrolysis or a reversible FC/EC system will be strongly desired. To apply the reversible FC/EC with unsteady operations, the appropriate and quantitative understanding of the reversible reaction kinetics is required. The kinetics of hydrogen oxidation reaction in solid oxide fuel cell (SOFC) on Ni/ yttria stabilized zirconia (YSZ) have been developed and a number of kinetics models based on the chemical and/or electrochemical reactions have been proposed [1-5]. The rate determining step was described as a surface chemistry of neutral absorbents on triple phase boundary (TPB) under the local equilibrium between the oxygen atom on TPB, oxide ion and electron [1, 2], or the charge transfer reaction of neutral absorbent, surface ion and electron between Ni and YSZ [3-5]. The rate of anode reaction in each model is described chemically such as Langmuir type surface reaction [2] and electrochemically such as Butler-Volmer equation [5, 6], respectively. By applying those models, there are a number of discussion about the reaction of electrolysis, but most of them have suggested that the water reduction reaction on TPB cannot be described as a simple reverse reaction of hydrogen oxidation reaction [1, 5, 6]. Thus, in order to establish the comprehensive redox reaction model on the water electrode of reversible SOFC/EC, it is required to clarify the mechanism of the water reduction reaction and to describe its kinetics model, but there are a limited number of discussion which focus on the mechanism of the electrolysis reaction. To establish the comprehensive redox kinetics model on the water electrode of SOFC/EC, in this work we applied the existing reaction model of SOFC anode which we previously developed and shown in Figure (a) [2]. The model is described the kinetics by competitive adsorption reaction at TPB with oxygen activity (aO). aO is calculated from anode potential (Ea= ohmic free potential difference between anode working electrode and cathode reference electrode). The coverage ratio of adsorbing oxygen on TPB is fixed under the local equilibrium against O2- in YSZ, aO and electron at Ni and YSZ, and the rate determining step was described as Langmuir type surface chemical reaction on TPB. We can separately discuss the effect of the chemical/electrochemical property and the electrode porous structure. This time we applied the model to the reversible SOFC/EC with the series of H2/H2O gas components and discuss the SOEC anode reaction. Electrolyte support type cell were prepared with a YSZ disc as electrolyte, Ni/YSZ with 3:2 weight ratio as a fuel electrode and La0.85Sr0.15MnO3/ScSZ as an air electrode. Reversible SOFC/EC measurement were carried out in 1-5 kPa H2O / 30-99 kPa H2 / Ar balance and the current density was measured under 10-13 < aO < 10-7. As a result, the current density of SOEC was much larger than the extrapolated value by SOFC data at any H2/H2O gas conditions. By reinvestigating the local equilibrium of adsorbing oxygen atoms, oxide ion and electron on Ni as shown in Figure (right), we proposed a modified kinetics model by defining the “electron activity ratio between Ni and YSZ” as another independent variable. The kinetics is discussed with the change of the surface components on TPB by the shift of local oxygen equilibrium due to their shift of the charge balance. Acknowledgement: a part of this work is supported by the New Energy and Industrial Technology Development Organization (NEDO) [1] J. Mizusaki et al., Solid State Ionics, 70/71, 52 (1994). [2] M. Ihara et al., J. Electrochem. Soc., 148(3), A209 (2001). [3] A. Bieberle et al., J. Electrochem. Soc., 148(6), A646 (2001). [4] S. P Jiang et al., Solid State Ionics 123, 209 (1999). [5] D. G. Goodwin et al., J. Electrochem. Soc., 156(9), B1004 (2009). [6] M. Vogler et al., Journal of The Electrochemical Society, 156(5) B663 (2009). Figure 1

Effect of new zirconia surface coatings on the surface properties and bonding strength of veneering zirconia substrate

Zirconia ceramic dental restorations showed a low bond strength to veneering ceramic because of there is no inherent glass content along with a nonpolar covalent bond in its matrix. Two zirconia coatings by airbrush spraying were used as a new surface treatment on zirconia substrate to create surface roughness and improved bond strength to the veneering ceramic. Unsintered yttria stabilized zirconia (YSZ) powder was partially sintered at 1100°C to produce powders with two particle sizes; A=26.0±0.3μm and B=47.0±0.5μm. Unsintered YSZ blocks were sectioned into 50 discs of 25mm in diameter and 2.5mm in thickness. The discs were divided according to the surface treatments into three groups; ten sintered YSZ discs were blasted by airborne particle abrasion (APA) with 50μm aluminium oxide particles as control group; twenty unsintered YSZ discs were coated with a mixture of glaze ceramic and YSZ powders (M1), and twenty unsintered YSZ discs were coated with a mixture of ceramic liner and YSZ powders (M2). Based on the sizes of the YSZ powders, the coated groups subdivided into M1A, M1B, M2A and M2B. The surface area roughness (Sa), surface morphology, elemental composition, phase transition and shear bond strength test (SBS) were assessed. The collected data were analysed with one-way ANOVA and Tukey HSD test at (P<0.05). The Sa results revealed significant differences among all groups (P<0.000). The coated groups showed higher Sa and SBS values with a significant difference than APA. The M1B group exhibited higher values of Sa (10.33μm) and SBS (37.54MPa) with significant differences among the other tested groups. The zirconia specimens treated with new coatings significantly improved the shear bond strength to the veneering ceramic.

Self-heated HTCC-based ceramic disc for mixed potential sensors and for direct conversion sensors for automotive catalysts

Solid electrolyte gas sensors following the mixed potential sensing principle are usually operated at several hundred °C. They are often characterized using half-cell setups where one side of the sensor faces a reference gas atmosphere whereas the other side faces the gas mixture to be analyzed. Since two gas compartments are compared, a high temperature sealing is needed. Typically, such setups require indirect heating, and often, the manufacturing process is complicated and the dismantling of sensor and sealing is only possible by destroying it. To overcome these and other drawbacks of the half-cell setup, a self-heatable, stand-alone ceramic sensor device is developed in this work. We choose high temperature co-firing ceramics technology for the sensor design that bases on a self-heated yttria-stabilized zirconia disc that is hot enough for sensing in the central region and cold enough at the outer radius for contacting and plastic sealing. This work shows how problems that occurred due to the high thermal stress were overcome by employing finite element simulations. After developing and manufacturing the novel device, it is demonstrated that the sensor is capable to compare two gas mixtures electrochemically. For that purpose, the device was used to compare the gas composition up- and downstream of an oxidation catalyst. The sensor signal response correlates well with a theoretically derived dependence on the catalyst conversion.

Investigation of Heterogeneous Catalysts by an Electrochemical Method: Ceria and Titania-Supported Iridium Nanoparticles for Ethylene Oxidation

Electrochemical promotion of catalysis (EPOC) is a well-established phenomenon in heterogeneous catalysis for enhancing catalytic activity through the application of a small electrical stimulus between the catalyst-working and counter electrode deposited on a solid electrolyte (e.g., yttria-stabilized zirconia (YSZ))1. This electrical stimulus causes the backspillover of ionic species, in this case O2-, from the solid electrolyte to the catalyst surface. A resulting change in catalytic activity is due to the modification of the electronic properties of the catalyst. The extent to which these properties can be modified depends on several factors, such as temperature, type of catalyst material and solid electrolyte, catalyst morphology, e.g., particle size and dispersion, ionic conductivity, and gas phase composition 2,3. EPOC is an interfacial phenomenon where two-phase boundaries, i.e., catalyst/gas and catalyst/solid electrolyte, as well as the three-phase boundary (tpb), i.e., electrolyte/catalyst/gas phase play a key role in the reaction. Recent EPOC studies have shown catalytic enhancement of highly dispersed, nano-structured catalysts through the use of mixed ionic electronic conducting (MIEC) materials, i.e., CeO2 4 and TiO2 5, which are in contact with the YSZ solid-electrolyte. This approach allows for considerable reduction of the mass of metal catalyst required, while maintaining the electrical conductivity of the working electrode needed to complete the electrochemical cell 6,7. Both CeO2 and TiO2 are considered reducible supports. More specifically, CeO2, due to its non-stoichiometry, has the ability to undergo conversion between Ce4+ and Ce3+ quite easily 7. TiO2 has also been known to strongly influence the performance of the supported metal catalysts due to this reducibility 8. These properties make the use of ceria- and titania-containing catalysts of interest for many catalytic applications. This study investigates the complete oxidation of ethylene over low particle size (1.1 nm) ruthenium and (1.0 nm) iridium nanoparticles supported on CeO2 and TiO2 under open (o.c.) and closed circuit conditions. Furthermore, detailed electrochemical characterization of the Ir- and Ru- based catalysts was carried out using a steady-state polarization technique. The results obtained were correlated with their o.c. catalytic performance. The discussion of this study includes the effect of temperature and applied potential on the catalytic activity of the supported and freestanding Ir and Ru catalysts. Ru and Ir nanoparticles, synthesized using a polyol reduction method, were supported on CeO2 and TiO2 resulting in a 1 wt% catalyst loading (RuNPs/CeO2, IrNPs/CeO2, RuNPs/TiO2 and IrNPs/TiO2). The supported and free-standing nanoparticle catalysts were deposited on one side of a YSZ solid electrolyte disk in order to apply polarization 4. Gold counter and reference electrodes were applied on the opposite side of YSZ disk 9. Figure 1 shows representative polarization curves of the Ir/CeO2 catalyst at various temperatures. Positive current density (i) is due to electrochemical oxidation of ethylene and oxygen evolution at the tpb, whereas anodic current density is due to O2 electro-reduction. As can be seen, the rate of both positive and negative current densities (i) increases with temperature, indicating an increase in the reaction rates at the tpb as a result of an increase in electrolyte conductivity. i was also found to increase from 1.26, 3.5 to 29.5 µA.cm-2 for 350, 375 and 400 oC, respectively. As a result, this indicates that as temperature is increased, there is more available O2- at the tpb. A combination of open circuit catalytic and electrochemical measurements was used to evaluate and understand the catalytic performance of highly-dispersed Ru- and Ir-based, free-standing, and CeO2 and TiO2-supported catalysts with regards to the effect of O2- ions from the support. The presence of CeO2 and TiO2 was shown to play a significant role in enhancing the catalytic activity of Ru and Ir nanoparticles. Acknowledgment The financial support from Natural Science and Engineering Research Council (NSERC) is acknowledged.

Electronic conductivity measurement of yttria-stabilized zirconia solid electrolytes by a transient technique

A new oxygen permeation technique is developed to measure the electronic conductivity of yttria-stabilized zirconia (YSZ). The permeation cell is a YSZ disc with an embedded Pt probe and a cavity at the center. Two porous platinum electrodes are applied on the disc surfaces. By applying a small DC bias (0.03 V) across one surface electrode and the embedded probe, oxygen is pumped into the YSZ disc and stored in the cavity. In steady state, a stable Nernst potential is developed between the cavity and the outer surfaces. The Nernst voltage is very close to the applied voltage since YSZ is essentially an ionic conductor. When the DC bias is removed, oxygen permeates out of the cavity leading to a decay of the Nernst potential. Electronic conductivity of YSZ corresponding to the ambient oxygen pressure (∼0.21 atm) is determined by analyzing the time dependence of the decay of Nernst potential. The measured electronic conductivity is in good agreement with values reported in the literature.

Quinone-rich polydopamine functionalization of yttria stabilized zirconia for apatite biomineralization: The effects of coating temperature

The use of yttria stabilized zirconia (YSZ) as biomedical implants is often offset by its bioinert nature that prevents its osseointegration to occur. Therefore, the functionalization of YSZ surface by polydopamine to facilitate the biomineralization of apatite layer on top of the coated film has incessantly been studied. In this study YSZ discs were first immersed in 2mg/mL of stirred dopamine solution at coating temperatures between 25 and 80°C. The specimens were then incubated for 7d in 1.5 SBF. The effect of coating temperature on the properties (chemical compositions and wettability) and the apatite mineralization on top of the generated films was investigated. It was found that at 50°C, the specimen displayed the highest intensity of Ca 2p peak (1.55±0.42 cps) with Ca/P ratio of 1.67 due to the presence of abundant quinone groups (CO). However, the hydrophilicity (40.9±01.7°) was greatly improved at 60°C accompanied by the highest film thickness of 306nm. Therefore, it was concluded that the presence of high intensity of quinone groups (CO) in polydopamine film at elevated temperature affects the chelation of Ca2+ ions and thus enhance the growth of apatite layer on top of the functionalized YSZ surface.

Planar Impedancemetric NO Sensor with Thick Film Perovskite Electrodes Based on Samarium Cobaltite

AbstractImpedancemetric NO sensors were fabricated, comprising yttria stabilized zirconia disc with thick film Sm0.9Sr0.1CoO3−δ electrodes of different design. Impedance measurements were performed at 600–800 °C for frequencies 0.01 Hz–100 kHz, in gas mixtures with 0–400 ppm of NO. The smaller arc on Nyquist plots can be associated with solid electrolyte‐electrode interfacial phenomena, while the bigger one to the perovskite electrode reactions. As NO concentration increases, the bigger arc decreases and its maximum shifts toward higher frequencies. Both the sensitivity and the applicable NO concentration range were found to be dependent on the sensor design, temperature and chosen frequency.

Surface modification of yttria stabilized zirconia via polydopamine inspired coating for hydroxyapatite biomineralization

Yttria stabilized zirconia (YSZ) has been widely used as biomedical implant due to its high strength and enhanced toughening characteristics. However, YSZ is a bioinert material which constrains the formation of chemical bonds with bone tissue following implantation. Inspired by the property of mussels, the surface of YSZ ceramics was functionalized by quinone-rich polydopamine to facilitate the biomineralization of hydroxyapatite. YSZ discs were first immersed in 2mg/mL of stirred or unstirred dopamine solution at either 25 or 37°C. The samples were then incubated in 1.5 simulated body fluid (SBF) for 7d. The effect of coating temperature for stirred and unstirred dopamine solutions during substrate grafting was investigated on the basis of chemical compositions, wettability and biomineralization of hydroxyapatite on the YSZ functionalized surface. The results revealed that the YSZ substrate grafted at 37°C in stirred solution of dopamine possessed significantly improved hydrophilicity (water contact angle of 44.0±2.3) and apatite-mineralization ability (apatite ratio of 1.78). In summary, the coating temperature and stirring condition during grafting procedure affected the chemical compositions of the films and thus influenced the formation of apatite layer on the substrate during the biomineralization process.

Microorganism adhesion inhibited by silver doped Yttria-stabilized zirconia ceramics

The aim of this study was to incorporate silver nanoparticles into yttria-stabilized zirconia (YSZ) ceramic for eliminating microorganism adhesion on dental restoration graft. Y 2O 3 (3% mol) partially stabilized ZrO 2 powder with particle size around 80 nm was employed to fabricate tetragonal phase dominated YSZ ceramic block. Silver nanoparticles were efficiently loaded at pH 9.5 by NaBH 4 reducing of AgNO 3. The biocompatibility of silver incorporated YSZ was evaluated by MTT assay. Antimicrobial activities were quantitatively determined by optical density measurement and qualitatively analysed by scanning electro microscope. Inductively coupled plasma-mass spectrometry (ICP-MS) revealed that YSZ block containing 0.0047 wt% nanosilver, which is safe to mammalian cell, can inhibit the growth of Escherichia coli, Streptococcus mitis and Candida albicans. The pristine YSZ disc had no effect on bacterial growth. This study suggests that silver nanoparticles incorporated into YSZ blocks possess a broad spectrum antimicrobial effect and may help prevent biofilm formation on their surfaces to improve implant survival rate.

Investigation of Electrolyte Stability Using Amperometric Sensors: Implications Concerning Electrode Polarization Measurements

Amperometric sensors with yttria-stabilized zirconia (YSZ) discs and YSZ cylinders were made with platinum electrodes applied on the YSZ disc, which served as working (inside) and counter (outside) electrodes, and a platinum wire electrode positioned along the circumference of the disc. Two sets of platinum electrodes were applied on the YSZ cylinder, one inside and the other outside. The YSZ cylinder was attached to the YSZ disc using a sealing glass. A YSZ disc with a tiny diffusion hole was glass-sealed on the other end of the YSZ cylinder. The amperometric sensor was tested at 800°C in air over an applied DC voltage range from zero to 2.5 V. The measured current over a voltage range from ∼0.4 to ∼1.6 V exhibited a plateau. For applied voltage ≥ 1.7 V, the measured current increased, with a rapid increase for applied voltage ≥1.9 V. This rapid rise is consistent with the development of nonstoichiometry or the occurrence of YSZ electrolyte decomposition. The voltage between the reference and the working electrodes was measured. Based on this information, the cathodic activity of the platinum working electrode was estimated. Implications concerning the use of the three electrode system for the study of electrode polarization are discussed.

Fabrication and Testing of a Doped Lanthanum Gallate Electrolyte Thin-Film Solid Oxide Fuel Cell

A supported lanthanum gallate (LSGM) electrolyte thin-film solid oxide fuel cell with Ni-YSZ cermet anode and strontium-doped lanthanum manganite (LSM)-yttria stabilized zirconia (YSZ) composite cathode was, for the first time, fabricated and tested. The cell was prepared by an unconventional approach, in which an LSGM thin film (about 15 μm thick) was first deposited on a porous substrate such as a porous YSZ disk by a wet process and sintered at a high temperature (above 1400°C). NiO was then incorporated into the porous substrate by a carefully controlled impregnation process and fired at a much lower temperature. In this way, the severe reaction between LSGM and NiO at a high temperature, which is required for the full densification of LSGM film, can be avoided. A strontium-doped (LSM)-YSZ composite cathode was screen printed on the surface of the LSGM film and then fired at 1250°C. The electrolyte resistances of the SOFC single cells fabricated by this approach are much lower compared to those of thick LSGM film supported cells. A maximum output power density of over at 800°C with as fuel and air as oxidant for a fabricated cell was achieved. © 2002 The Electrochemical Society. All rights reserved.

Electrochemical cell with two layers cathode for NO decomposition

An electrochemical cell composed of an yttria-stabilized zirconia disk and two layers cathode was used for nitrogen monoxide decomposition. It was found that covering the Pt cathode by a mixture of oxygen ionic conductor (YSZ) and electronic conductor (NiO) leads to enhancement of the performance of the electrochemical cell for NOx decomposition in the presence of excess oxygen. The decomposition activity was measured for the one-compartment cell oxide|(cathode)|YSZ|(anode) by applying a DC voltage lower than 3.7 V in the temperature range 550–700 °C. The microstructure of the YSZ-NiO mixed oxide electrodes was investigated in dependence of the cell operating condition and the working electrode sintering temperature. The correlation between the microstructure of the mixed oxide electrode and conversion rate of NO was studied. The phenomenon of self-optimization of the microstructure of the NiO-YSZ working electrode during the cell operation was observed and investigated.

Voltammetry on BIMEVOX.10 (ME=Cu, Ni, Zn, Mg) microsamples

High temperature cyclic voltammetry on microsamples was used for the investigation of the electrochemical stability of BIMEVOX.10 materials. The electrochemical measurements were performed on a three-electrode cell of the type: Pt|microsample|YSZ|Pt with the interface Pt|YSZ as a pseudo reference electrode. The experiments were performed in nitrogen containing a small quantity of oxygen. The cell consisted of a disc of yttria stabilized zirconia (YSZ) with platinum paste electrodes for the counter and the reference electrode. The working electrode was a small platinum plate which contacts the microsample and presses it onto the surface of the YSZ disc. In general, the stability of BIMEVOX.10 is determined by the stability of the main compound Bi 2O 3. None of the samples exhibited a substantially higher stability against complete reduction than BICUVOX.10. Nevertheless slight differences in the reduction processes at less negative potentials than the standard potential of Bi 2O 3 have been observed. Redox processes in this potential region could be related to valence changes of vanadium from V 5+ to V 4+ and, at more negative potentials, from V 4+ to V 3+.

A limiting current type sensor for hydrocarbons

A hydrocarbon (HC) sensor, composed of yttria-stabilized zirconia (YSZ) as a solid electrolyte and catalytically active and passive material for HC as electrodes, is proposed. The active and passive electrodes were attached to the same side of a YSZ disk as cathodes. A Pt electrode was applied to the other side of YSZ as anode. The diffusion of gas to the cathodes was limited by a pinhole of a cover attached to YSZ and the sensor showed a limiting current behavior by applying voltage. The difference between the limiting current, Δ I, of two cathodes increased linearly with the concentration of HC such as methane and isobutane, which suggests that HC concentration can be obtained by measuring Δ I.

Biaxial strength of advanced materials

The concentric ring test has been proposed as the preferred method for testing the flexural strength of bioceramics. If the apparatus used for such tests is proved to be reliable and accurate, it may be adapted to enable cyclic fatigue testing. However, before such adaptions are made, it is necessary to ensure the apparatus operates satisfactorily. The assessment programme involved strain gauge and finite element analyses of both an advanced titanium alloy (IMI 834) and 3 mol% yttria-stabilized zirconia disc specimens and preliminary fatigue tests on IMI 834 discs. The results of the analyses indicate that concentrically loaded discs are exposed to peak biaxial stresses on the tensile surface of the specimen and that these stresses are uniformly distributed within the peak loaded region of the disc. The fatigue response of IMI 834 discs under concentric loading was similar to that under uniaxial tension and torsional loading. The success of this programme will now enable fatigue tests on bioceramic discs to be performed with confidence in the reliability of the test apparatus.

Sensing Behavior of an Amperometric Hydrogen Sensor: Theoretical Modeling and Experimental Verification

A solid‐state hydrogen sensor was fabricated with a 9% yttria‐stabilized zirconia (YSZ) disk sandwiched between two platinum films; one of which was completely covered by a catalyst with a composition of . Its hydrogen sensing characteristics in a mixture of nitrogen, oxygen, and hydrogen were examined under limiting current conditions with respect to oxygen. The sensor response was adequately correlated against the hydrogen concentration and the operating temperature according to a proposed theoretical model, which relates the response to the rate processes occurring in the catalyst, the solid‐state electrolyte, and the electrodes. The response equation given by the model consists of a first‐order and a half‐order hydrogen concentration terms. The model shows that the first‐order concentration effect would prevail at high hydrogen concentrations using a sensor consisting of a thick compact layer of highly active catalyst and a thick solid‐state electrolyte. On the other hand, the present experimental sensor with a thin porous layer of moderately active catalyst and a thin YSZ disk electrode showed, as described by the model, a response of half‐order hydrogen concentration dependence in a gas mixture containing 0.018 to 0.18 mole percent of hydrogen.

Electrochemical modification of methanol oxidation selectivity and activity on a platinum single-pellet catalytic reactor

It was found that the catalytic activity and selectivity of Pt for the oxidation of methanol to formaldehyde and CO 2 can be altered significantly and reversibly by depositing a Pt catalyst film on an yttria-stabilized zirconia (YSZ) disc and by applying current or potential between the catalyst film and a Ag film deposited on the other side of the O 2 -conducting YSZ disc. Both the catalyst film and the Ag counter and reference electrodes are exposed to the reacting CH 3 OH-O 2 mixture. The observed increase in the rate of H 2 CO production was typically a factor of 100 higher than the rate of O 2 -supply to the catalyst with a concomitant 2-fold increase in selectivity