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SrCo1−xSbxO3−δ cathode materials prepared by Pechini method for solid oxide fuel cell applications

Authors Sea-Fue Wang, Hsi-Chuan Lu, Yung-Fu Hsu, Chien-Chung Huang, Chun-Ting Yeh
Source
Ceramics International
Volume: 38, Issue: 7, Pages: 5941–5947
Time of Publication: 2012-09
Abstract In this study, SrCo1−ySbyO3−δ powders were prepared by a modified Pechini method. According to the study results, the cubic Pm3m phase of the SrCo1−ySbyO3−δ ceramics was obtained as 10% of cobalt ions were substituted by antimony ions. Doping of Sb3+ ions appeared both to stabilize the Pm3m phase of the SrCo1−ySbyO3−δ ceramics and to enhance densification and retard grain growth. The coefficient of thermal expansion of the SrCo1−xSbxO3−δ ceramics increased with the content of the antimony ions, ranging from 10.17 to 15.37 ppm/C at temperatures lower than the inflection point (ranging from 450 C to 550 C) and from 22.16 to 29.29 ppm/C at higher temperatures. For the SrCo0.98Sb0.02O3−δ ceramic, electrical conductivity reached a maximum of 507 S/cm at 450 C. The ohmic and polarization resistances of the single cell with the pure SrCo0.98Sb0.02O3−δ cathode at 700 C read respectively 0.298 Ω cm2 and 0.560 Ω cm2. The single cell with the SrCo0.98Sb0.02O3−δ-SDC composite cathode appeared to reduce the impedances with the R0 and RP at 700 C reading respectively 0.109 Ω cm2 and 0.127 Ω cm2. Without microstructure optimization and measured at 700 C, the single cells with the pure SrCo0.98Sb0.02O3−δ cathode and the SrCo0.98Sb0.02O3−δ-SDC composite cathode, demonstrated maximum power densities of 0.100 W/cm2 and 0.487 W/cm2. Apparently, SrCo1−ySbyO3−δ is a potential cathode for use in IT-SOFCs.
Keywords Solid oxide fuel cell; Cathode; Impedance; Cell performance
Remark Available online 19 April 2012
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Synthesis, Sintering, Transport Properties, and Surface Exchange of La2Ni0.9Cu0.1O4+δ

Authors Zuoan Li, Truls Norby, Reidar Haugsrud
Source
Journal of the American Ceramic Society
Volume: 95, Issue: 6, Pages: 2065–2073
Time of Publication: 2012-06
Abstract Dense La2Ni0.9Cu0.1O4+δ ceramics were sintered from powders synthesized through a wet-chemical citrate nitrate route with optimized ratios of citrate to nitrate. Less citrate decreases the required sintering temperature and improves the oxygen permeativity. The oxygen permeation was measured as a function of oxygen activity gradient, membrane thickness (0.4–2.6 mm) and temperature (800C–950C). The oxygen self diffusion coefficient DO and the surface exchange coefficient k show Arrhenius-type behaviors with activation energies of ~50 and ~100 kJ/mol, respectively. The oxygen chemical diffusion coefficient Dchem and surface exchange coefficient kchem, measured by conductivity relaxation, exhibit Arrhenius-type behaviors with activation energies of 62 and 104 kJ/mol, respectively. Dchem and kchem are related to DO and k through the thermodynamic factor ωO.
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LaCoO3 ceramics obtained from reactive powders

Authors L. Predoana, B. Malic, D. Crisan, N. Dragan, M. Anastasescu, J. Calderon-Moreno, R. Scurtu, M. Zaharescu
Source
Ceramics International
Volume: 38, Issue: 7, Pages: 5433–5443
Time of Publication: 2012-09
Abstract The aim of the present study was to establishing the correlation between the structure and properties of the LaCoO3 powders obtained by aqueous sol–gel method with citric acid and their sintering behavior in order to obtain fully densified ceramics with perovskite structure. Two types of cobalt and lanthanum reagents were used in synthesis, namely nitrates and acetates. The sintering was realized at temperatures ranging between 800 and 1200 C for 2 h. The sintered samples were investigated by classical ceramic methods (shrinkage, density, porosity) and by structural and morphological investigations: XRD, SEM, AFM and XPS. The electrical properties of the samples were determined by impedance spectroscopy. The ceramics obtained with powders starting with acetates have presented a lower sintering ability as compared with the samples obtained from powders starting with nitrates. LaCoO3 ceramics with best properties was obtained from powders starting with nitrates sintered at 1100 C.
Remark Available online 11 April 2012
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Preparation and characterization of composite membranes based on sulfonated PEEK and AlPO4 for PEMFCs

Authors Vijay Shankar Rangasamy, Savitha Thayumanasundaram, Niels De Greef, Jin Won Seo, Jean-Pierre Locquet
Source
Solid State Ionics
Volume: 219, Pages: 83–89
Time of Publication: 2012-05
Abstract Sulfonated poly(ether ether ketone) (PEEK) and their composites are considered one of the most promising alternatives for Nafion, the industry benchmark for electrolytic membranes in proton exchange membrane (PEM) fuel cells. In the present study, PEEK was non-homogeneously sulfonated using concentrated H2SO4 at different temperatures (room temperature, 60 C, and 80 C) and time durations (5, 7, 48, and 72 h). Composite membranes of SPEEK with different weight ratios of AlPO4 synthesized by sol–gel were also prepared. Depending on the degree of sulfonation (DS), the Ion Exchange Capacity (IEC) of the membranes varied from 1.06 to 2.9 meq g− 1. XRD results show the increasing amorphous nature of the membranes with increase in IEC and DS value. The water uptake of the membranes also increased with DS. Simultaneous TGA–FTIR measurement of the composite membranes showed better thermal stability compared to pure SPEEK membranes. The water uptake and proton conductivity of the composite SPEEK membranes were found to be lower than that of pure SPEEK membranes, while the composite membranes exhibited a better swelling behavior and mechanical stability than the pure SPEEK samples.
Keywords Proton exchange membrane (PEM); Composite membranes; Sulfonated poly(etheretherketone) (SPEEK); Proton conductivity; Ion exchange capacity (IEC); Sol–gel
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Synthesis and electrical properties of a new fluorite-like anionic conductor in the Nd2O3–MoO3 system (43–47 mol% Nd2O3)

Authors V.I. Voronkova, , E.P. Kharitonova, D.A. Belov
Source
Solid State Ionics
Volume: 225, Pages: 654–657
Time of Publication: 2012-10
Abstract This work addresses phase relations in the Nd2O3–MoO3 system (25–50 mol% Nd2O3) and properties of a fluorite-like compound existing in this system in the composition range of 43–47 mol% Nd2O3. We describe for the first time the crystal growth of this compound. The composition of the crystals is Nd10Mo6O33 (Nd2O3:MoO3 = 5:6, 45.5 mol% Nd2O3), which falls within the above composition range. Dielectric spectroscopy data for polycrystalline Nd14Mo8O45 (within the homogeneity range of the fluorite-like compound) suggest a phase transition at 650–700 C. The exact mechanism of the transition remains to be clarified. Nd14Mo8O45 has rather high bulk conductivity, approaching 10− 2 S/cm at 800 C. The low activation energy and key structural features of the compound in question indicate that it is an anionic conductor.
Keywords Fluorite-like compounds; Rare-earth molybdates; Single crystals; Relaxation; Anionic conductor
Remark Available online 22 March 2012
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Temperature dependent thermoelectric material power factor measurement system

Authors Jonathan D'Angelo, Adam Downey, Timothy Hogan
Source
Review of scientific instruments
Volume: 81, Issue: 075107 Publisher: American Institute of Physics, Time of Publication: 2010-06
Abstract Thermoelectric materials can be used for cooling/heating applications, or converting waste heat into electricity. Novel thermoelectric materials have been discovered in recent years. Characterization of an electrical conductivity and thermopower of a sample from room temperature to ≥ 900 K is often necessary for thermoelectric materials. This paper describes a system built for measurement of the power factor of thermoelectric materials from 300 to 1273 K. Characterization results of the system are also presented.

High temperature electronic properties of BaTiO3 – Bi(Zn1/2Ti1/2)O3 – BiInO3 for capacitor applications

Author Natthaphon Raengthon and David P. Cann
Source
Journal of Electroceramics
Volume: 28, Issue: 2-3, Pages: 165-171
Time of Publication: 2012-03
Abstract Solid solutions xBaTiO3 – (1-x)(0.5Bi(Zn1/2Ti1/2)O3 – 0.5BiInO3), where x = 0.95–0.60, were prepared by conventional mixed oxide method. The single phase perovskite structure was obtained for the composition with x ≥ 0.75. Phase transformation from tetragonal to pseudocubic was observed from x-ray diffraction patterns when x decreased from 0.95 to 0.75. In tetragonal phase region, x ≥ 0.90, the increase of Bi(Zn1/2Ti1/2)O3 – BiInO3 content decreased the tetragonality and the temperature at which the relative permittivity is maximum (Tmax). The increase in lattice parameter and Tmax were observed in the pseudocubic phase region, x < 0.90. Additionally, a highly broad and diffuse phase transition was observed from the dielectric data in the pseudocubic phase region. The introduction of Ba vacancies in compositions with x = 0.80 and 0.75 also improved dielectric loss at high temperatures. The incorporation of BiInO3 into the BaTiO3 – Bi(Zn1/2Ti1/2)O3 compound was also found to improve the temperature coefficient of the relative permittivity, with values as low as approximately −1,000 ppm/K. Overall, ternary perovskite solid solutions based on adding Bi(Zn1/2Ti1/2)O3 – BiInO3 to BaTiO3 shows excellent potential for high temperature capacitor applications
Keywords High temperature capacitor – Bi-based perovskite – BiInO3 – Bi(Zn1/2Ti1/2)O3 – BaTiO3 – Insulation resistance
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Stability of (Ln0.8Ca0.1Ln′0.1)2Ti2O7−δ (Ln=Dy, Yb; Ln′=Ce, Tb) and (Tb0.9Ca0.1)2Ti2O7−δ pyrochlores under redox conditions

Authors S.N. Savvin, A.V. Shlyakhtina, D.A. Belov, J.C. Ruiz-Morales, L.G. Shcherbakova, P. Nuez
Source
Solid State Ionics
Volume: 225, Pages: 457–463
Time of Publication: 2012-10
Abstract The stability of (Ln0.8Ca0.1Ln′0.1)2Ti2O7−δ (Ln=Dy, Yb; Ln′=Ce, Tb) pyrochlores under different redox conditions (air, dry and wet H2) has been studied. The bulk conductivity of the terbium-containing materials is slightly higher under reducing conditions (5% H2 + 95% Ar) than in air, reaching ~ 6 10− 2 and 2.5 10− 2 S/cm at 800 C for (Yb0.8Ca0.1Tb0.1)2Ti2O7−δ and (Dy0.8Ca0.1Tb0.1)2Ti2O7−δ, respectively. In air–hydrogen–air cycles, the bulk conductivity returns to its original level in air: ~ 2 10− 2 and 1 10− 2 S/cm at 800 C in (Yb0.8Ca0.1Tb0.1)2Ti2O7−δ and (Dy0.8Ca0.1Tb0.1)2Ti2O7−δ, respectively. Exposure of (Dy0.8Ca0.1Tb0.1)2Ti2O7−δ to a flowing mixture of 5% H2 and 95% Ar for 100 h slightly reduces its conductivity: from 0.029 to 0.023 S/cm at 900 C. The cerium-containing materials (Yb0.8Ca0.1Ce0.1)2Ti2O7−δ and (Dy0.8Ca0.1Ce0.1)2Ti2O7−δ were found to be unstable under reducing conditions. The bulk conductivity of (Yb0.8Ca0.1Ce0.1)2Ti2O7−δ is practically independent on the atmosphere while the grain boundary conductivity increased from 5.6 10− 8 in air to ~ 1 10− 5 S/cm at 425 C on reducing the sample in 5%H2–Ar gas mixture. However, after two redox cycles air–5%H2–Ar–air the sintered pellet of (Yb0.8Ca0.1Ce0.1)2Ti2O7−δ fractured whereas (Dy0.8Ca0.1Ce0.1)2Ti2O7−δ degraded immediately under reducing conditions. It was observed that the size mismatch between Ln and Ln´ cations strongly affects the bulk to grain boundary conductivity ratio in (Ln0.8Ca0.1Ln′0.1)2Ti2O7−δ (Ln=Dy, Yb; Ln′=Ce, Tb) pyrochlores.
Keywords Pyrochlore; Donor doping; High-temperature conductivity; Ionic conductivity; Solid electrolyte; Electronic conductivity
Remark Available online 8 March 2012; http://dx.doi.org/10.1016/j.ssi.2012.02.009
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Spark Plasma Sintering and Hot Pressing of Hetero-Doped LaNbO4

Authors Guttorm E. Syvertsen, Claude Estourns, Harald Fjeld, Reidar Haugsrud, Mari-Ann Einarsrud, Tor Grande
Source
Journal of the American Ceramic Society
Volume: 95, Issue: 5, Pages: 1563–1571
Time of Publication: 2012-05
Abstract LaNbO4/La3NbO7 and LaNbO4/LaNb3O9 cer-cer composites were prepared by impregnating Ca-doped LaNbO4 powder, synthesized by spray pyrolysis, with La- or Nb-precursor solutions. The sintering of the calcined powders was investigated by dilatometry, and dense composites were prepared by conventional sintering, hot pressing, and spark plasma sintering. The particle size of the starting powders was about 50 nm, and the average grain size of the dense materials ranged from 100 nm and upwards, depending on the sintering temperature, sintering procedure, and the phase composition. The unit cell parameters of LaNbO4 showed a finite size effect and approached the cell parameters of tetragonal LaNbO4 with decreasing crystallite size, both for the single-phase material and the composites. The minority phase (La3NbO7 or LaNb3O9) were observed as isolated grains and accumulated at triple points and not along the grain boundaries, pointing to a large dihedral angle between the phases. The calcium-solubility in the minority phases was larger than in LaNbO4, which corresponds well with previous reports. The electrical conductivity of the hetero-doped materials was similar to, or lower than, that for Ca-doped LaNbO4.
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Sandvik Sanergy HT – A potential interconnect material for LaNbO4-based proton ceramic fuel cells

Authors Anders Werner Bredvei Skilbred, Reidar Haugsrud
Source
Journal of Power Sources
Volume: 206, Pages: 70–76
Time of Publication: 2012-05
Abstract High temperature properties of Sandvik Sanergy HT have been studied to evaluate the alloy's suitability as an interconnect material for LaNbO4 based proton ceramic fuel cells (PCFCs). The thermal expansion behavior of the alloy deviates from LaNbO4 at higher temperatures which may be unfavorable, however the average values for the two materials over the whole temperature region are rather similar. The oxidation kinetics was parabolic and the rate constants were low at temperatures below 1000 C. Accelerated oxidation was encountered after 300 h at 1000 C revealing that the material may undergo severe degradation at sufficiently high temperatures. A complex oxide scale containing an inner layer of chromium oxide and an outer layer of various spinel phases containing chromium, manganese and iron was formed at all temperatures. As a consequence of high oxidation resistance and an oxide with relatively high electronic conductivity, the area specific resistance (ASR) of Sandvik Sanergy HT measured at 700 C proved to be low.
Keywords Proton ceramic fuel cell; Interconnect; Sandvik Sanergy HT; Oxidation kinetics; Thermal expansion; Area specific resistance
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A novel perovskite-based proton conductor for solid oxide fuel cells

Authors Ramya Hariharan, T.R.S. Prasanna, Prakash Gopalan
Source
Scripta Materialia
Volume: 66, Issue: 9, Pages: 658–661
Time of Publication: 2012-05
Abstract A perovskite-based electrolyte, Ca-substituted YAlO3, has been synthesized by the citrate gel process. The conductivity of Y0.9Ca0.1AlO3-δ has been studied by AC impedance spectroscopy in controlled atmospheres in the temperature range 300–800 C. The material has been found to be a mixed conductor with dominant p-type electronic conduction at elevated temperatures and a combined protonic conduction below 600 C in wet conditions. The H/D isotope effect on conductivity confirms the presence of protonic conductivity between 400 and 600 C.
Keywords Proton conduction; Electrical properties; Solid electrolyte; Perovskite oxide; Fuel cell materials
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50 mol% indium substituted BaTiO3: Characterization of structure and conductivity

Authors S.M.H. Rahman, C.S. Knee, I. Ahmed, S.G. Eriksson, R. Haugsrud
Source
International Journal of Hydrogen Energy
Volume: 37, Issue: 9, Pages: 7975–7982
Time of Publication: 2012-05
Abstract BaTi0.5In0.5O3−δ was prepared by solid state reaction at 1400 C. Rietveld analysis of high resolution X-ray powder diffraction data indicated phase pure as-prepared material that adopts a cubic perovskite structure with a = 4.1536(1) . Thermogravimetric analysis revealed the presence of significant levels of protons in the as-prepared material and 57% of the theoretically achievable protonation was attained on exposure to a humid environment at 185 C. After hydration the cell parameter increased to 4.1623(1) . Electrical conductivity was measured both with fixed and variable frequency ac impedance methods as a function of temperature, oxygen-, water vapour- and heavy water vapour partial pressures. In the temperature range 400–800 C a slight increase in the total conductivity with increasing oxygen partial pressure is encountered, characteristic of a contribution from p-type charge carriers. The effect of the water vapour pressure on conductivity below 600 C is much more prominent indicative of dominant proton conduction. At 300 C the total conductivity in wet O2 was estimated to be 9.30 10−5 S/cm. At T > 800 C the material is a pure oxide ion conductor.
Keywords Barium titanate; Perovskite; Brownmillerite; Proton conductivity; X-ray diffraction; Impedance spectroscopy
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Ion mobility, phase transitions, and conductivity of crystal phases in KF-CsF-SbF3-H2O system according to data of NMR and impedance spectroscopy

Authors V. Ya. Kavun, L. A. Zemnukhova, A. I. Ryabov, A. B. Podgorbunskii, S. V. Gnedenkov, S. L. Sinebryukhov and V. I. Sergienko
Source
Russian Journal of Electrochemistry
Volume: 48, Issue: 1, Pages: 104-110
Time of Publication: 2012-01
Abstract The methods of NMR, thermogravimetric analysis, and impedance spectroscopy were used to study ion mobility, phase transitions, and ion conductivity in crystal phases in the KF-CsF-SbF3-H2O system. Analysis of 19F NMR spectra allowed tracing the dynamics of ion movement in the fluoride sublattice under temperature variations, determining their types and temperature ranges, in which they are implemented. The observed phase transitions in potassium-cesium fluoroantimonates(III) are phase transitions to the superionic state. It is found that the predominant form of ion movement in the high-temperature modifications formed as a result of phase transitions becomes diffusion of fluoride ions. According to the results of electrophysical studies the K1−xCsx SbF4 (x ≤ 0.2) high-temperature phases are superionic. Their conductivity reaches the values of ∼10−2 to 10−3 S/cm at 463–483 K. The high-temperature phases are stabilized under cooling, which results in a significant increase in conductivity at the room temperature.
Keywords potassium-cesium tetrafluoroantimonates(III) – ion mobility – phase transitions – conductivity – NMR spectra
Remark DOI: 10.1134/S1023193512010090
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Defect structure and its nomenclature for mixed conducting lanthanum tungstates La28–xW4+xO54+3x/2

Authors Skjalg Erdal, Liv-Elisif Kalland, Ragnhild Hancke, Jonathan Polfus, Reidar Haugsrud, Truls Norby, Anna Magras
Source
International Journal of Hydrogen Energy
Volume: 37, Issue: 9, Pages: 8051–8055
Time of Publication: 2012-05
Abstract Based on results from experimental and theoretical studies of the crystal structure of lanthanumtungstateView the MathML source, we present a defect model comprising an inherently disordered and partially occupied oxide ion sublattice, which rationalizes hydration and ionic conduction of the materials in the undoped state. Applying the model to experimental conductivity data enables extraction of defect thermodynamics and transport parameters of protons, oxide ions and electronic defects. The standard enthalpy and entropy changes of the hydration of inherent oxygen vacancies are estimated to be −83 kJ/mol and −125 J/mol K (per mole of H2O), respectively.
Keywords Lanthanum tungstate; Defect structure; Inherent disorder; Partial occupancy; Mixed conductor
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The role of B-site cations on proton conductivity in double perovskite oxides La2MgTiO6 and La2MgZrO6

Authors Camilla K. Vigen, Tor Svendsen Bjrheim, Reidar Haugsrud
Source
International Journal of Hydrogen Energy
Volume: 37, Issue: 9, Pages: 7983–7994
Time of Publication: 2012-05
Abstract Acceptor-doped La2MgTiO6 (LMTO) and La2MgZrO6 (LMZO) have been investigated to contribute to clarify the role of the B-site cations in A2B′B″O6 double perovskite oxides on formation and mobility of protons. Thermogravimetry and a.c. conductivity measurements in the temperature range 1000-300 C, as well as DFT-calculations of LMTO, have been the basis for evaluating hydration thermodynamics and transport parameters of the materials. Both compounds show exothermic hydration of oxygen vacancies, but low concentrations of protons. The proton transport is limited by relatively high activation energies of mobility; 0.84 eV and 0.78 eV for LMTO and LMZO, respectively. This is suggested to reflect association to effectively negative charged defects formed by site exchange among the B-site cations. Consequently, the maximum proton conductivity of LMTO and LMZO is in the order of 10−5 S/cm.
Keywords Double perovskite; La2MgTiO6; La2MgZrO6; Proton mobility; Hydration thermodynamics
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Influence of Microwave-Assisted Pechini Method on La0.80Sr0.20Ga0.83Mg0.17O3–δ Ionic Conductivity

Authors S. Boldrini, C. Mortal, S. Fasolin, F. Agresti, L. Doubova, M. Fabrizio, and S. Barison
Source
Fuel Cells
Volume: 12, Issue: 1, Pages: 54–60
Time of Publication: 2012-02
Abstract With the aim of investigating the microwave influence on the electrolyte material properties, La0.80Sr0.20Ga0.83Mg0.17O2.815 was prepared by both a conventional and a microwave-assisted sol–gel Pechini method. With respect to the conventional Pechini method (hereafter SGP), the microwave assisted process (hereafter MWA-SGP) guaranteed a faster procedure, reducing the time needed to remove the excess solvents to complete the polyesterification reaction from some days to a few hours. In fact, when a MWA-SGP method was used, powders having higher phase purity were obtained. The sintering process at 1,450 C of the powders prepared by both methods yielded pellets with similar density values (≥92% of theoretical). Nevertheless, only by microwave-assisted process single-phase products were obtained and no secondary phases such as tetragonal LaSrGaO4 and LaSrGa3O7 were detected. These by-products have been demonstrated to be detrimental for conductivity. Indeed, pellets obtained by MWA-SGP method showed oxygen ionic conductivity values higher (about 30–40%) than those checked for SGP samples, thus demonstrating the important role of the microwave process on reducing time and costs and on improving the electrolyte properties.
Keywords Ionic Conductivity;IT-SOFC;Microwave Processing;(Sr, Mg)-Doped LaGaO3;Sol–Gel
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Impact of Parylene-A Encapsulation on ZnO Nanobridge Sensors and Sensitivity Enhancement via Continuous Ultraviolet Illumination

Authors C.-C. Huang, A.D. Mason, J.F. Conley, C. Heist, M.T. Koesdjojo, V.T. Remcho and T. Afentakis
Source
Journal of Electronic Materials
Volume: 41, Issue: 5, Pages: 873-880
Time of Publication: 2012-05
Abstract The impact of parylene-A encapsulation and the effect of continuous ultraviolet (UV) exposure on ZnO nanobridge sensor response are investigated. ZnO nanowire (NW) devices are fabricated using a novel method that involves selective growth of ZnO nanobridges between lithographically defined pads of carbonized photoresist (C-PR). We find that a thin coating of parylene-A effectively attenuates the response of NW devices to O2, H2O vapor, and UV illumination. The accessibility of the amine group on parylene-A for chemical functionalization is verified by transforming the amine groups on the surface of the parylene-A coating into aromatic imine groups, followed by UV–Vis absorption. Our results suggest that, in addition to modulating environmental sensitivity and providing protection of the ZnO NWs for liquid- and vapor-phase sensing, the parylene-A encapsulation may also serve as an activation layer for further specific functionalization targeting selective sensing. We also found that the sensitivity and response time of ZnO nanobridge devices to O2 are dramatically improved by continuously exposing the nanobridge devices to UV illumination. Finally, we show that the C-PR directed growth method can also be used to isolate free-standing NW carpet.
Keywords ZnO – nanowire – parylene – CVD – nanobridge – sensor – functionalization – directed integration
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On the hydration of grain boundaries and bulk of proton conducting BaZr0.7Pr0.2Y0.1O3-δ

Authors Kristine Bakkemo Kostl, Anna Magras, Truls Norby
Source
International Journal of Hydrogen Energy
Volume: 37, Issue: 9, Pages: 7970–7974
Time of Publication: 2011-12
Abstract We report here for the first time bulk and grain boundary conductivities from impedance spectra of a ceramic proton conductor (BaZr0.7Pr0.2Y0.1O3-δ) taken during hydration and H/D isotope exchange transients (at 400 C). The results suggest that water moves quickly along grain boundary cores, and then interact from there with the space charge layers and, in turn, grain interiors. Hydration and H/D isotope exchange have simple monotonic effects on the bulk conductivity in line with what is expected from it being dominated by protons. The transients for grain boundary conductivity exhibit however hysteresis: During hydration, the core charge and grain boundary resistance appear to go through transient minima related to non-equilibrium distributions of defects between the core and grain interior – notably because protons diffuse faster than oxygen vacancies between the grain boundary and grain interior. At equilibrium, hydration increases the core charge and the depletion of positive charge carriers in the space charge layers. During H/D isotope exchange relatively fast hysteretic transients indicate that the space charge layers experience changes in charge carrier (D+ vs. H+) mobility as well as in D2O vs. H2O hydration thermodynamics.
Keywords BaZrO3, Pr-substituted, acceptor-doped; BaZr0.7Pr0.2Y0.1O3-δ; Proton conductivity; Grain boundaries, space charge layer; Hydration; Impedance spectroscopy
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Synthesis, structural and electrical properties of double perovskite Sr2NiMoO6 ceramics

Authors A. Prasatkhetragarn, S. Kaowphong and R. Yimnirun
Source
Applied Physics A: Materials Science & Processing
Volume: 107, Issue: 1, Pages: 117-121
Time of Publication: 2012-01
Abstract The double perovskite Sr2NiMoO6 powders and ceramics were prepared by two different (conventional and precursor) solid-state reaction methods. The phase structure was characterized by XRD and TEM techniques. It has been indicated that single-phase perovskite powders were obtained when calcined in air at 1300C. However, nano-particles of the size 30–60 nm have been found in powders prepared with the precursor method, while those from the conventional route exhibit large irregular shaped particles with aggregation. The dielectric properties (ε r and tanδ) were also examined in the sintered ceramics. The results showed the transition point at 280C for conventional route, while no clear phase change was observed in ceramics from the precursor route. These observations clearly indicate that the different starting processes affected the phase formation behavior and the electrical properties of Sr2NiMoO6 ceramics.
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Dielectric Relaxation in BaTiO3–Bi(Zn1/2Ti1/2)O3 Ceramics

Authors Natthaphon Raengthon, David P. Cann
Source
Journal of the American Ceramic Society
Volume: 95, Issue: 5, Pages: 1604–1612
Time of Publication: 2012-05
Abstract A dramatic improvement in the dielectric and electrical properties has been observed in ceramics of 0.8BaTiO3–0.2Bi(Zn1/2Ti1/2)O3 through the introduction of Ba vacancies. It possesses a high relative permittivity (εr > 1150) along with a low dielectric loss (tan δ < 0.05) that is maintained up to temperatures as high as 460C. It is also characterized by a high resistivity of 70 GΩ-cm, which remains constant up to 270C. Analysis of complex impedance (Z*) and complex electric modulus (M*) data, measured over the frequency range of 1–106 Hz, revealed a number of important findings. At high temperatures (T > 255C), a complex plane analysis of Z″ versus Z′ and the frequency dependence of Z″ suggests an electrically inhomogeneous microstructure for the stoichiometric composition. The stoichiometric composition exhibited activation energies of ~1 eV which suggests an extrinsic conduction mechanism. However, the introduction of Ba vacancies resulted in electrically homogeneous microstructure. An overlap of the Z″ and M″ peaks in the frequency domain and much larger activation energies were observed, on the order of half of the band gap, suggesting an intrinsic conduction mechanism. A more detailed analysis of the data reveals insights into the physical mechanisms underpinning the dielectric and ac conductivity.
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Fabrication and electrochemical properties of cathode-supported solid oxide fuel cells via slurry spin coating

Authors Min Chen, Jing-Li Luo, Karl T. Chuang, Alan R. Sanger
Source
Electrochimica Acta
Volume: 63, Pages: 277–286
Time of Publication: 2012-02
Abstract A cathode-supported SOFC consisting of LSM (La0.8Sr0.2MnO3-δ) cathode supporter, LSM-Sm0.2Ce0.8O2-δ (SDC) cathode functional layer (CFL), yttria stabilized zirconia (YSZ)/SDC bi-layered electrolyte and Ni-YSZ anode layer was fabricated by a slurry spin coating technique. The influence of the porosity in both the CFL and cathode supporter on the electrochemical properties of the cells has been investigated. It was found that properly controlling the porosity in the CFL would improve the performance of the cells using O2 in the cathode side (O2-cells), with a maximum power density (MPD) value achieving as high as 0.58 W•cm−2 at 850 C. However, this improvement is not so evident for the cells using air in the cathode side (air-cells). When increasing the porosity in the cathode-supporter, a significant increase of the power density for the air cells due to the decreasing Rconc,c(concentration polarization to the cell resistance) can be ascertained. In terms of our analysis on various electrochemical parameters, the Ract (activation polarization to the cell resistance) is assumed to be mainly responsible for the impedance arcs measured under the OCV condition, with a negligible Rconc,cvalue being able to be detected in our impedances. In this case, a significant decreasing size of the impedance arcs due to the increasing porosity in the cathode supporter would correspond to a decrease of the Ract values, which was proved to be induced by the decreasingRconc,c.
Keywords Slurry spin coating; Cathode-supported SOFC; Concentration polarization; Activation polarization; Power density
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The effect of cation non-stoichiometry in LaNbO4 materials

Authors Guttorm E. Syvertsen, Anna Magras, Reidar Haugsrud, Mari-Ann Einarsrud, Tor Grande
Source
International Journal of Hydrogen Energy
Volume: 37, Issue: 9, Pages: 8017–8026
Time of Publication: 2012-05
Abstract The effect of cation non-stoichiometry in LaNbO4 was investigated by impregnating nano-crystalline LaNbO4 with small amounts of La3+, Nb5+ and Ca2+ oxide precursors. The sintering properties of the modified LaNbO4 powders were investigated by dilatometry, and the microstructure and phase composition were studied by electron microscopy and X-ray diffraction. The electrical properties of the materials were studied by 4-point DC-conductivity and 2-point 4-wire AC-conductivity at elevated temperatures in controlled atmosphere. Minor variations in the cation stoichiometry were shown to have a pronounced effect on both the sintering properties as well as the electrical conductivity. Addition of CaO, which introduced secondary phases above 0.25 mol% CaO, increased the sintering temperature and improved the conductivity of the materials. La2O3- and Nb2O5-excess materials did not show large variation in the electrical conductivity relative to pure LaNbO4, while the sintering properties were strongly affected by the nominal La/Nb ratio in LaNbO4. The present findings demonstrate the sensitivity of cation non-stoichiometry in materials with limited solid solubility.
Keywords LaNbO4; Proton conductivity; Phase purity; Solid solubility
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Effects of surface coatings on the determination of Dchem and kchem in La2NiO4+δ by conductivity relaxation

Authors Zuoan Li, Reidar Haugsrud
Source
Solid State Ionics
Volume: 206, Pages: 67-71
Time of Publication: 2012-01
Abstract In this work, we utilize surface modification to accurately determine oxygen diffusivity and surface exchange in La2NiO4 + δ by transient conductivity. To achieve ‘instantaneous’ gas exchange in the reaction cell, a total pressure change was adopted instead of exchange of gas mixtures at 1 atm. Conductivity relaxation measurements were performed in the temperature range of 600–900 C and at oxygen partial pressures of 0.2, 1.0 and 1.9 atm. Due to the large uncertainties associated with deriving oxygen chemical diffusion and surface coefficients simultaneously, we limited the relaxation to pure bulk diffusion control by coating nano-grained La2NiO4 + δ particles on the sample surface. After determining Dchem, kchem was derived by fitting the relaxation data of the uncoated sample. The transient experiments via both oxidation (a step change of pO2 from 0.2 to 1.0 atm) and reduction (1.9 to 1.0 atm) give consistent results of Dchem and kchem showing Arrhenius-type behaviour with activation energies of ~ 90 kJ/mol and ~ 150 kJ/mol, respectively.
Keywords Conductivity relaxation; Surface modification; Surface exchange; Oxygen diffusion; La2NiO4 + δ
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Hydration and proton conductivity in LaAsO4

Authors Tor S. Bjrheim, Truls Norby and Reidar Haugsrud
Source
Journal of Materials Chemistry
Volume: 22, Issue: 4, Pages: 1652-1661
Time of Publication: 2012-04
Abstract Incorporation and transport of protonic defects have been studied in nominally undoped and 1 and 3 mol% Sr-doped LaAsO4 prepared by a co-precipitation route. AC impedance of the materials was measured as a function of temperature (1150 to 400 C), pO2 (1 to 1 10−5 atm) and pH2O (0.025 to 3 10−5 atm). The bulk conductivities generally decrease with decreasing temperature and moreover with decreasing pH2O within the whole temperature range. At the highest temperatures, a small decrease in the conductivity with decreasing pO2 was also observed. The defect structure of Sr-doped LaAsO4 appears to be dominated by oxygen vacancies in the form of pyroarsenate ions, As2O4−7, in dry atmospheres at high temperatures and by protonic defects in the form of hydrogen arsenate ions, HAsO2−4, in wet atmospheres. A significant isotope effect shows that protons contribute to the total conductivity at all temperatures under wet conditions and predominate at temperatures below [similar]850 C. The remaining contributions are attributed to oxide ions and electron holes. The extracted hydration thermodynamics are comparable to those determined for other LnXO4 (X = P, V, Nb, Ta) compounds, and the enthalpy of mobility of protons (86 5 and 88 5 kJ mol−1 for the 1 and 3 mol% doped samples, respectively) follows an apparent trend for the isostructural LaXO4 (X = P, As, V) series with the enthalpy of mobility of protons decreasing with increasing radius of the X-site cation. However, the partial proton conductivities of Sr-doped LaAsO4 are lower than those determined for acceptor doped LaPO4 and LaVO4 for which the possible reasons are discussed.
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Evaluation of the electrode/electrolyte contact quality in solid oxide fuel cells

Authors Jacqueline Amanda Figueiredo dos Santos, Michel Kleitz, Tulio Matencio, Rosana Zacarias Domingues
Source
Electrochimica Acta
Volume: 60, Pages: 224–229
Time of Publication: 2012-01
Abstract Symmetrical cells have been prepared by depositing suspensions with different active powder concentrations (50% and 60% by weight respectively) of lanthanum strontium cobalt iron oxide (La0.6Sr0.4Co0.2Fe0.8O3-δ) on identical yttria stabilized zirconia pellets (LSCF/YSZ/LSCF). Their impedance characteristics are compared to that of a symmetrical cell with platinum electrodes deposited on a similar zirconia pellet (Pt/YSZ/Pt). The LSCF cells show different values of the electrolyte resistance. Referring to the electrolyte resistance obtained with the Pt cell and assuming that this value corresponds to almost perfect electrode/electrolyte contacts, it is possible to estimate the Effective Conducting Area (ECA) of the electrodes. The use of the ECA parameter allows a better comparison of the electrode polarizations.
Keywords solid oxide fuel cells; porous electrodes; cathode
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