Guilhem Dezanneau | Ecole CentraleSupelec (original) (raw)

Papers by Guilhem Dezanneau

Double perovskite cobaltites were recently presented as promising cathode materials for solid oxi... more Double perovskite cobaltites were recently presented as promising cathode materials for solid oxide fuel cells. While an atomistic mechanism was proposed for oxygen diffusion in this family of materials, no direct experimental proof has been presented so far. We report here the first study that directly compares experimental and theoretical diffusion pathways of oxygen in an oxide, namely in the double cobaltite compound, NdBaCo 2 O 5+x. Model-free experimental nuclear density maps are obtained from the maximum entropy method combined with Rietveld refinement against high resolution neutron diffraction data collected at 1173 K. They are then compared to theoretical maps resulting from classical molecular dynamics calculations. The analysis of 3D maps of atomic densities allows identifying unambiguously the pathways and the mechanisms involved in the oxide ion diffusion. It is shown that oxygen diffusion occurs along a complex trajectory between Nd-and Co-containing a,b planes. The study also reveals that Ba-containing planes act as a barrier for oxygen diffusion. The diffusion mechanism is also supported through the oxygen sites occupancy analysis that confirms the increase of oxygen vacancies in the cobalt-planes on heating. The use of such combined experimental and theoretical analysis should be considered as a very powerful approach for materials design.

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Electrochemical and Solid State Letters, 2004

Abstract Transport number measurements and impedance spectroscopy in controlled temperature and a... more Abstract Transport number measurements and impedance spectroscopy in controlled temperature and atmosphere were used to investigate the electrical conduction properties of Formula The compound is a pure oxygen ion conductor in the temperature range of 400-800 C with oxide ion transport numbers greater than 0.99. The effect of the oxygen partial pressure on the electrical conductivity of Formula was investigated from 1 to Formula atm, showing that the material is stable and mainly an ionic conductor down Formula atm at ...

The increasing necessity of reducing harming emissions in exhaust gases has spurred on the develo... more The increasing necessity of reducing harming emissions in exhaust gases has spurred on the development of robust and long living gas sensors capable to work in very aggressive environments (Duk-Dong Lee and Dai-Sik Lee, 2001). In this respect, oxygen sensors take a very important role in the control of the oxygen-to-fuel ratio in the mixture of combusting engines (Tifee et al., 2001). High temperature electrochemical sensors based on solid electrolytes have been the subject of much research in the last 20 years, leading to very successful industrial applications. Nevertheless, many improvements are still required in matter of roughness, long term stability, detection range and response speed. In the present work, a novel design and fabrication method of an amperometric oxygen sensor have been developed. Its main originality entails the use of an yttria stabilized zirconia (YSZ) porous layer which acts both as an oxygen conductor (electrolyte) and as a gas diffusion barrier. The electrodes were deposited using a commercial platinum paste. The entire device has been fabricated by means of an optimized screen printing method. The exclusive use of this innovative technique has permitted the implementation of a very compact planar device in an easy, fast and highly economical way. The sensor has been structurally characterized using SEM, revealing a very uniform-shaped micro-stack. Electrical measurements show a behavior typical of that expected from amperometric oxygen sensors (Garzon, 1998). The device developed with a linear output and a good selectivity in a wide range of oxygen partial pressures. Moreover, the response to changes in the atmosphere composition was almost immediate. Those promising results, joined to the ease of fabrication and scalability, demonstrates the viability of the proposed amperometric oxygen sensor design.

Solid State Ionics

Yttria-stabilized zirconia thin films for micro solid oxide fuel cell applications were grown by ... more Yttria-stabilized zirconia thin films for micro solid oxide fuel cell applications were grown by pulsed laser deposition onto silicon-based microfabricated substrates. In order to obtain pinhole-free membranes, the influence of the microstructure of the target on the ejection of particulates to the film was studied. Targets with different contents of yttria (3mol% Y2O3: ZrO2 and 8mol% Y2O3: ZrO2) and fabricated by different sintering methods (conventional sintering and spark plasma sintering) were evaluated. Microstructural and electrical ...

Abstract Recently, La 2 Mo 2 O 9 has been presented as a promising electrolyte material for solid... more Abstract Recently, La 2 Mo 2 O 9 has been presented as a promising electrolyte material for solid oxide fuel cells (SOFCs), and recent results from our group have confirmed that such compound fulfill some of the requirements for SOFCs applications such as a high ion conduction and a wide oxygen partial pressure range of pure ionic conduction. Nevertheless, the mechanism of oxygen conduction in La 2 Mo 2 O 9 is still not understood, in particular some questions remain about the strong decrease of conductivity from the ...

Simulation of the influence of particle size distribution ... Alex Morata, Guilhem Dezanneau , Al... more Simulation of the influence of particle size distribution ... Alex Morata, Guilhem Dezanneau , Albert Tarancbn, Francesca Peir6 and Joan Ramon Morante ... EME (CERMAE), Departament d'Electranica, Universitat de Barcelona, Marti i Franquks, 1, 08028 Barcelona, SPAIN, Ph: ...

Fuel Cells, 2010

The effect of the dopant distribution on the oxygen diffusion in doped fluorites typically used f... more The effect of the dopant distribution on the oxygen diffusion in doped fluorites typically used for solid oxide fuel cells electrolyte applications has been analysed by using molecular dynamics simulations. The oxygen mass transport in both yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria has been studied and compared in the range of temperatures between 1,159 and 1,959 K. A new methodology based on the analysis of local environments is used to describe the diffusion process at an atomic scale. Preferred vacancy migration pathways, most suitable conduction models, energy landscapes and jump efficiency have been detailed for each material. Finally, a particular case of non-random distribution of dopants in YSZ is presented in order to quantitatively evaluate the effect of the dopant pattern on the mass transport properties and the potential of the methodology developed here for understanding and foreseeing real configurations at the nanoscale.

Solid State Ionics, 2006

We study the influence of grain size distributions of 2D and 3D polycrystals on the electrical re... more We study the influence of grain size distributions of 2D and 3D polycrystals on the electrical response of grain boundaries by means of numerical calculations based on Kirchhoff networks. The GB contribution to the overall conductivity is compared to that of the bricklayer model in order to estimate the validity of such model. We thus quantify the error committed when using the BLM to extract the intrinsic GB resistivity of 2D and 3D real polycrystals from impedance measurements.

MRS Proceedings, 2004

ABSTRACT In the present work, the screen printing technique has been used to deposit thick films ... more ABSTRACT In the present work, the screen printing technique has been used to deposit thick films of Zr0. 84Y016O1. 92 (8YSZ). In order to control the final porosity in view of a specific application (SOFCs or gas sensor), an experimental design based on analysis of variances (ANOVA) has been carried out. From this, we were able to determine the influence of several technological parameters on films porosity and grain size. The films obtained have been analysed with both Scanning Electron Microscopy (SEM) and Focused Ion ...

Phys. Chem. Chem. Phys., 2015

Density-functional theory calculations are performed to investigate proton transport in BaSnO3. S... more Density-functional theory calculations are performed to investigate proton transport in BaSnO3. Structural optimizations in the stable and saddle point configurations for transfer (hopping) and reorientation allow description of the high-temperature classical and semi-classical regimes, in which diffusion occurs by over-barrier motion. At lower temperature (typically below 300 K), we describe the thermally-assisted quantum regime, in which protonic motion is of quantum nature and occurs in "coincidence" configurations favored by thermal fluctuations of the surrounding atoms. Both the non-adiabatic and the adiabatic limits are examined. In the adiabatic limit, the protonic energy landscape in the coincidence configuration is very flat. Path-integral molecular dynamics simulations of the proton in the coincidence potential reveal, in the transfer case, that the density of probability of H(+) has its maximum at the saddle point, because the zero-point energy exceeds the coincidence barrier. Arguments are given that support the adiabatic picture for the transfer mechanism. In the case of reorientation, the time scales for the existence of the coincidence and for protonic motion, as estimated from the time-energy uncertainty principle by using a simple one-dimensional model, are of the same order of magnitude, suggesting that the adiabatic limit is not reached. Protonic transfer and reorientation in this oxide are therefore governed by different mechanisms below room temperature.

New Developments on Sensors for Environmental Control (ENVSENS) - Proceedings of the International Workshop, 2003

J. Mater. Chem. A, 2014

ABSTRACT The oxygen transport kinetics of the misfit-layered cobaltite, Ca3Co4O9+d, known for its... more ABSTRACT The oxygen transport kinetics of the misfit-layered cobaltite, Ca3Co4O9+d, known for its thermoelectric properties, was investigated by combined application of 18O/16O isotope exchange and electrical conductivity relaxation techniques. Although oxygen diffusion is found two orders of magnitude lower than in well-investigated lanthanum nickelates, e.g., La2NiO4+d, the mixed ionic-electronic conductor Ca3Co4O9+d is found to exhibit fast surface exchange kinetics (k*=1.6.10-7 cm.s-1 at 700°C to be compared to 1.3.10-7 cm.s-1 for the nickelate), rendering it a promising electrode for application as air electrode in solid oxide cells. In parallel, the chemical nature of the outermost surface of Ca3Co4O9+d was characterized by means of Low Energy Ion Scattering (LEIS) spectroscopy. The absence of cobalt at the sample’s outermost surface suggests that the Ca2CoO3-δ rock salt layers in the structure may play a key role in the oxygen exchange mechanism.

MRS Proceedings, 1999

Abstract Raman scattering from La y R x MnO 3-δ compounds (single crystals, ceramics, films) was ... more Abstract Raman scattering from La y R x MnO 3-δ compounds (single crystals, ceramics, films) was studied as a function of kind (R) and amount (x) of dopant as well as of oxygen deficit (δ). For x= 0.1 to 0.55, a nearly linear Raman shift of the A g mode from 240 to 125 cm− 1 was observed in Srdoped compounds. While the intensity of disorder-induced Raman bands was found to be sensitive to the oxygen content, the symmetry allowed related Raman modes exhibit the changes in the frequency. As an indication of deposition ...

J. Mater. Chem. A, 2014

ABSTRACT The energy landscape of the protonic defect is investigated in acceptor-doped barium sta... more ABSTRACT The energy landscape of the protonic defect is investigated in acceptor-doped barium stannate using density-functional calculations. Several trivalent dopants are studied (Ga, Sc, In, Y, Gd, La), covering a wide range of ionic radii. All the dopants are found attractive with respect to the proton, with (negative) association energies varying from -0.40 to -0.07 eV. A radius r(c1) is defined to separate the "small" dopants that induce tensile stress from the "large" ones that induce compressive stress in the host matrix (r(c1) approximate to 0.72 angstrom). The protonic energy surface exhibits a non-trivial evolution with ionic radius of the dopant: for low dopant radii, the most stable protonic site is the oxygen first-neighbor of the dopant, while for high dopant radii, the most stable position is obtained when the proton is bonded to an oxygen second-neighbor of the dopant. This evolution of the protonic energy surface with dopant ionic radius is smooth and the transition takes place between In and Y, i.e. for a critical radius r(c2) between 0.80 and 0.90 angstrom (r(c2) > r(c1) significantly). The dopant-proton association energy exhibits a minimal value approximate to -0.07 eV (weakest attraction) at this transition, i.e. in the case of yttrium, for which the first-neighbor and second-neighbor positions are almost degenerate. Other dopants, smaller or larger, are more attractive to protons. The present study gives useful information about the modification of the trapping effect according to the dopant ionic size.

Journal of Materials Chemistry A, 2014

ABSTRACT Oxide ion and proton transport properties in the fuel cell electrolyte Gd-doped BaCeO3 a... more ABSTRACT Oxide ion and proton transport properties in the fuel cell electrolyte Gd-doped BaCeO3 are investigated by first-principles density-functional calculations and kinetic Monte Carlo simulations. Behind the apparent complexity of the energy landscape (related to the low-symmetry of the system), general tendencies governing the energy barriers can be extracted. In particular, the set of barriers is tested with respect to the Bell-Evans-Polanyi (BEP) principle that relates the activation energy Ea of a series of similar chemical reactions to their reaction enthalpies [capital Delta]H. This rule is found poorly satisfied in the case of oxide ion migration, but much better satisfied for proton hopping mechanisms. Protonic reorientations, by contrast, do not obey the BEP rule. Kinetic Monte Carlo simulations give insight into the macroscopic transport properties. We observe that dopants act as traps for oxygen vacancies and slow down their motion, whereas their effect on the protonic diffusion coefficient is more complex. This is related to the fact that a two-state picture roughly applies to the oxygen vacancy energy landscape, while it does not apply to the protonic one. Oxide ion migration exhibits strong anisotropy, the motion of the oxygen vacancies being favored in the equatorial planes, while protonic diffusion is found more isotropic.

Density-functional theory calculations are performed to investigate hydrogen transport in the pro... more Density-functional theory calculations are performed to investigate hydrogen transport in the proton conductor BaSnO$_3$. Structural optimizations in the stable and saddle point configurations for transfer and reorientation allow description of the high-temperature classical and semi-classical regimes, in which diffusion occurs by over-barrier motion. At lower temperature (typically below 300 K), we describe a thermally-assisted quantum regime. In this regime, transfer and reorientation occur when the surrounding matrix adopts particular "coincidence" configurations in which quantum tunneling is favored. Both the non-adiabatic and the adiabatic cases are examined. In the adiabatic case, the energy landscape of hydrogen in the coincidence configuration is very flat, with very low coincidence energy barriers. Path-integral molecular dynamics simulations of the H atom in the coincidence potential reveal, in the transfer case, highly quantum behavior up to T=300 K (the density...

Procedia Engineering, 2014

Two cobaltites were studied as air electrodes for Solid Oxide Cells with a Cerium Gadolinium Oxid... more Two cobaltites were studied as air electrodes for Solid Oxide Cells with a Cerium Gadolinium Oxide electrolyte (CGO): Ca 3 Co 4 O 9+ , well known for its thermoelectric properties, and Ba 2 Co 9 O 14 . After optimisation of composition and thickness, a very good ASR of only 0.08 .cm 2 was obtained for the 50 wt% Ba 2 Co 9 O 14 -50 wt% 50CGO composite. In contrast, a value of 0.5 .cm 2 was reached for the 50 wt% Ca 3 Co 4 O 9+ -50 wt% CGO composite. Although, Ba 2 Co 9 O 14 sample contained 18 O after annealing, oxide ion diffusion in this compound still has to be confirmed. In contrast, high surface exchange kinetics were measured for both Ca 3 Co 4 O 9+ and NdBaCo 2 O 5+ , for which mainly calcium and barium/neodymium were evidenced at the uppermost surface of the samples, whose atoms may play a key role in the mechanism of oxygen molecules dissociation.

Faraday Discuss., 2014

Oxide diffusion was studied in two innovative SOFC cathode materials, Ba 2 Co 9 O 14 and Ca 3 Co ... more Oxide diffusion was studied in two innovative SOFC cathode materials, Ba 2 Co 9 O 14 and Ca 3 Co 4 O 9+d derivatives. Although oxygen diffusion was confirmed in the promising material Ba 2 Co 9 O 14 , it was not possible to derive accurate transport parameters because of an oxidation process at the sample surface which has still to be clarified. In contrast, oxygen diffusion in the well-known Ca 3 Co 4 O 9+d thermoelectric material was improved when calcium was partly substituted with strontium, likely due to an increase of the volume of the rock salt layers in which the conduction process takes place. Although the diffusion coefficient remains low, interestingly, fast kinetics towards the oxygen molecule dissociation reaction were shown with surface exchange coefficients higher than those reported for the best cathode materials in the field. They increased with the strontium content; the Sr atoms potentially play a key role in the mechanism of oxygen molecule dissociation at the solid surface.

Journal of the European Ceramic Society, 2015

ZrO 2 and TiO 2 modified lead-free (K 0.5 Na 0.5 )NbO 3 (KNN) piezoelectric ceramics are prepared... more ZrO 2 and TiO 2 modified lead-free (K 0.5 Na 0.5 )NbO 3 (KNN) piezoelectric ceramics are prepared by a conventional solid-state reaction. The effect of acceptor doping on structural and functional properties is investigated. A decrease in the Curie temperature and an increase in the dielectric constant values are observed when doping. More interestingly, an increase in the coercive field E c and remanent polarization P r is observed. The piezoelectric properties are greatly increased when doping with small concentrations dopants. ZrO 2 doped ceramic exhibits good piezoelectric properties with piezoelectric coefficient d 33 = 134 pC/N and electromechanical coupling factor k p = 35%. It is verified that nonlinearity is significantly reduced. Thus, the creation of complex defects capable of pinning the domain wall motion is enhanced with doping, probably due to the formation of oxygen vacancies. These results strongly suggest that compositional engineering using low concentrations of acceptor doping is a good means of improving the functional properties of KNN lead-free piezoceramic system. (X. Vendrell).

Double perovskite cobaltites were recently presented as promising cathode materials for solid oxi... more Double perovskite cobaltites were recently presented as promising cathode materials for solid oxide fuel cells. While an atomistic mechanism was proposed for oxygen diffusion in this family of materials, no direct experimental proof has been presented so far. We report here the first study that directly compares experimental and theoretical diffusion pathways of oxygen in an oxide, namely in the double cobaltite compound, NdBaCo 2 O 5+x. Model-free experimental nuclear density maps are obtained from the maximum entropy method combined with Rietveld refinement against high resolution neutron diffraction data collected at 1173 K. They are then compared to theoretical maps resulting from classical molecular dynamics calculations. The analysis of 3D maps of atomic densities allows identifying unambiguously the pathways and the mechanisms involved in the oxide ion diffusion. It is shown that oxygen diffusion occurs along a complex trajectory between Nd-and Co-containing a,b planes. The study also reveals that Ba-containing planes act as a barrier for oxygen diffusion. The diffusion mechanism is also supported through the oxygen sites occupancy analysis that confirms the increase of oxygen vacancies in the cobalt-planes on heating. The use of such combined experimental and theoretical analysis should be considered as a very powerful approach for materials design.

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Electrochemical and Solid State Letters, 2004

Abstract Transport number measurements and impedance spectroscopy in controlled temperature and a... more Abstract Transport number measurements and impedance spectroscopy in controlled temperature and atmosphere were used to investigate the electrical conduction properties of Formula The compound is a pure oxygen ion conductor in the temperature range of 400-800 C with oxide ion transport numbers greater than 0.99. The effect of the oxygen partial pressure on the electrical conductivity of Formula was investigated from 1 to Formula atm, showing that the material is stable and mainly an ionic conductor down Formula atm at ...

The increasing necessity of reducing harming emissions in exhaust gases has spurred on the develo... more The increasing necessity of reducing harming emissions in exhaust gases has spurred on the development of robust and long living gas sensors capable to work in very aggressive environments (Duk-Dong Lee and Dai-Sik Lee, 2001). In this respect, oxygen sensors take a very important role in the control of the oxygen-to-fuel ratio in the mixture of combusting engines (Tifee et al., 2001). High temperature electrochemical sensors based on solid electrolytes have been the subject of much research in the last 20 years, leading to very successful industrial applications. Nevertheless, many improvements are still required in matter of roughness, long term stability, detection range and response speed. In the present work, a novel design and fabrication method of an amperometric oxygen sensor have been developed. Its main originality entails the use of an yttria stabilized zirconia (YSZ) porous layer which acts both as an oxygen conductor (electrolyte) and as a gas diffusion barrier. The electrodes were deposited using a commercial platinum paste. The entire device has been fabricated by means of an optimized screen printing method. The exclusive use of this innovative technique has permitted the implementation of a very compact planar device in an easy, fast and highly economical way. The sensor has been structurally characterized using SEM, revealing a very uniform-shaped micro-stack. Electrical measurements show a behavior typical of that expected from amperometric oxygen sensors (Garzon, 1998). The device developed with a linear output and a good selectivity in a wide range of oxygen partial pressures. Moreover, the response to changes in the atmosphere composition was almost immediate. Those promising results, joined to the ease of fabrication and scalability, demonstrates the viability of the proposed amperometric oxygen sensor design.

Solid State Ionics

Yttria-stabilized zirconia thin films for micro solid oxide fuel cell applications were grown by ... more Yttria-stabilized zirconia thin films for micro solid oxide fuel cell applications were grown by pulsed laser deposition onto silicon-based microfabricated substrates. In order to obtain pinhole-free membranes, the influence of the microstructure of the target on the ejection of particulates to the film was studied. Targets with different contents of yttria (3mol% Y2O3: ZrO2 and 8mol% Y2O3: ZrO2) and fabricated by different sintering methods (conventional sintering and spark plasma sintering) were evaluated. Microstructural and electrical ...

Abstract Recently, La 2 Mo 2 O 9 has been presented as a promising electrolyte material for solid... more Abstract Recently, La 2 Mo 2 O 9 has been presented as a promising electrolyte material for solid oxide fuel cells (SOFCs), and recent results from our group have confirmed that such compound fulfill some of the requirements for SOFCs applications such as a high ion conduction and a wide oxygen partial pressure range of pure ionic conduction. Nevertheless, the mechanism of oxygen conduction in La 2 Mo 2 O 9 is still not understood, in particular some questions remain about the strong decrease of conductivity from the ...

Simulation of the influence of particle size distribution ... Alex Morata, Guilhem Dezanneau , Al... more Simulation of the influence of particle size distribution ... Alex Morata, Guilhem Dezanneau , Albert Tarancbn, Francesca Peir6 and Joan Ramon Morante ... EME (CERMAE), Departament d'Electranica, Universitat de Barcelona, Marti i Franquks, 1, 08028 Barcelona, SPAIN, Ph: ...

Fuel Cells, 2010

The effect of the dopant distribution on the oxygen diffusion in doped fluorites typically used f... more The effect of the dopant distribution on the oxygen diffusion in doped fluorites typically used for solid oxide fuel cells electrolyte applications has been analysed by using molecular dynamics simulations. The oxygen mass transport in both yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria has been studied and compared in the range of temperatures between 1,159 and 1,959 K. A new methodology based on the analysis of local environments is used to describe the diffusion process at an atomic scale. Preferred vacancy migration pathways, most suitable conduction models, energy landscapes and jump efficiency have been detailed for each material. Finally, a particular case of non-random distribution of dopants in YSZ is presented in order to quantitatively evaluate the effect of the dopant pattern on the mass transport properties and the potential of the methodology developed here for understanding and foreseeing real configurations at the nanoscale.

Solid State Ionics, 2006

We study the influence of grain size distributions of 2D and 3D polycrystals on the electrical re... more We study the influence of grain size distributions of 2D and 3D polycrystals on the electrical response of grain boundaries by means of numerical calculations based on Kirchhoff networks. The GB contribution to the overall conductivity is compared to that of the bricklayer model in order to estimate the validity of such model. We thus quantify the error committed when using the BLM to extract the intrinsic GB resistivity of 2D and 3D real polycrystals from impedance measurements.

MRS Proceedings, 2004

ABSTRACT In the present work, the screen printing technique has been used to deposit thick films ... more ABSTRACT In the present work, the screen printing technique has been used to deposit thick films of Zr0. 84Y016O1. 92 (8YSZ). In order to control the final porosity in view of a specific application (SOFCs or gas sensor), an experimental design based on analysis of variances (ANOVA) has been carried out. From this, we were able to determine the influence of several technological parameters on films porosity and grain size. The films obtained have been analysed with both Scanning Electron Microscopy (SEM) and Focused Ion ...

Phys. Chem. Chem. Phys., 2015

Density-functional theory calculations are performed to investigate proton transport in BaSnO3. S... more Density-functional theory calculations are performed to investigate proton transport in BaSnO3. Structural optimizations in the stable and saddle point configurations for transfer (hopping) and reorientation allow description of the high-temperature classical and semi-classical regimes, in which diffusion occurs by over-barrier motion. At lower temperature (typically below 300 K), we describe the thermally-assisted quantum regime, in which protonic motion is of quantum nature and occurs in "coincidence" configurations favored by thermal fluctuations of the surrounding atoms. Both the non-adiabatic and the adiabatic limits are examined. In the adiabatic limit, the protonic energy landscape in the coincidence configuration is very flat. Path-integral molecular dynamics simulations of the proton in the coincidence potential reveal, in the transfer case, that the density of probability of H(+) has its maximum at the saddle point, because the zero-point energy exceeds the coincidence barrier. Arguments are given that support the adiabatic picture for the transfer mechanism. In the case of reorientation, the time scales for the existence of the coincidence and for protonic motion, as estimated from the time-energy uncertainty principle by using a simple one-dimensional model, are of the same order of magnitude, suggesting that the adiabatic limit is not reached. Protonic transfer and reorientation in this oxide are therefore governed by different mechanisms below room temperature.

New Developments on Sensors for Environmental Control (ENVSENS) - Proceedings of the International Workshop, 2003

J. Mater. Chem. A, 2014

ABSTRACT The oxygen transport kinetics of the misfit-layered cobaltite, Ca3Co4O9+d, known for its... more ABSTRACT The oxygen transport kinetics of the misfit-layered cobaltite, Ca3Co4O9+d, known for its thermoelectric properties, was investigated by combined application of 18O/16O isotope exchange and electrical conductivity relaxation techniques. Although oxygen diffusion is found two orders of magnitude lower than in well-investigated lanthanum nickelates, e.g., La2NiO4+d, the mixed ionic-electronic conductor Ca3Co4O9+d is found to exhibit fast surface exchange kinetics (k*=1.6.10-7 cm.s-1 at 700°C to be compared to 1.3.10-7 cm.s-1 for the nickelate), rendering it a promising electrode for application as air electrode in solid oxide cells. In parallel, the chemical nature of the outermost surface of Ca3Co4O9+d was characterized by means of Low Energy Ion Scattering (LEIS) spectroscopy. The absence of cobalt at the sample’s outermost surface suggests that the Ca2CoO3-δ rock salt layers in the structure may play a key role in the oxygen exchange mechanism.

MRS Proceedings, 1999

Abstract Raman scattering from La y R x MnO 3-δ compounds (single crystals, ceramics, films) was ... more Abstract Raman scattering from La y R x MnO 3-δ compounds (single crystals, ceramics, films) was studied as a function of kind (R) and amount (x) of dopant as well as of oxygen deficit (δ). For x= 0.1 to 0.55, a nearly linear Raman shift of the A g mode from 240 to 125 cm− 1 was observed in Srdoped compounds. While the intensity of disorder-induced Raman bands was found to be sensitive to the oxygen content, the symmetry allowed related Raman modes exhibit the changes in the frequency. As an indication of deposition ...

J. Mater. Chem. A, 2014

ABSTRACT The energy landscape of the protonic defect is investigated in acceptor-doped barium sta... more ABSTRACT The energy landscape of the protonic defect is investigated in acceptor-doped barium stannate using density-functional calculations. Several trivalent dopants are studied (Ga, Sc, In, Y, Gd, La), covering a wide range of ionic radii. All the dopants are found attractive with respect to the proton, with (negative) association energies varying from -0.40 to -0.07 eV. A radius r(c1) is defined to separate the "small" dopants that induce tensile stress from the "large" ones that induce compressive stress in the host matrix (r(c1) approximate to 0.72 angstrom). The protonic energy surface exhibits a non-trivial evolution with ionic radius of the dopant: for low dopant radii, the most stable protonic site is the oxygen first-neighbor of the dopant, while for high dopant radii, the most stable position is obtained when the proton is bonded to an oxygen second-neighbor of the dopant. This evolution of the protonic energy surface with dopant ionic radius is smooth and the transition takes place between In and Y, i.e. for a critical radius r(c2) between 0.80 and 0.90 angstrom (r(c2) > r(c1) significantly). The dopant-proton association energy exhibits a minimal value approximate to -0.07 eV (weakest attraction) at this transition, i.e. in the case of yttrium, for which the first-neighbor and second-neighbor positions are almost degenerate. Other dopants, smaller or larger, are more attractive to protons. The present study gives useful information about the modification of the trapping effect according to the dopant ionic size.

Journal of Materials Chemistry A, 2014

ABSTRACT Oxide ion and proton transport properties in the fuel cell electrolyte Gd-doped BaCeO3 a... more ABSTRACT Oxide ion and proton transport properties in the fuel cell electrolyte Gd-doped BaCeO3 are investigated by first-principles density-functional calculations and kinetic Monte Carlo simulations. Behind the apparent complexity of the energy landscape (related to the low-symmetry of the system), general tendencies governing the energy barriers can be extracted. In particular, the set of barriers is tested with respect to the Bell-Evans-Polanyi (BEP) principle that relates the activation energy Ea of a series of similar chemical reactions to their reaction enthalpies [capital Delta]H. This rule is found poorly satisfied in the case of oxide ion migration, but much better satisfied for proton hopping mechanisms. Protonic reorientations, by contrast, do not obey the BEP rule. Kinetic Monte Carlo simulations give insight into the macroscopic transport properties. We observe that dopants act as traps for oxygen vacancies and slow down their motion, whereas their effect on the protonic diffusion coefficient is more complex. This is related to the fact that a two-state picture roughly applies to the oxygen vacancy energy landscape, while it does not apply to the protonic one. Oxide ion migration exhibits strong anisotropy, the motion of the oxygen vacancies being favored in the equatorial planes, while protonic diffusion is found more isotropic.

Density-functional theory calculations are performed to investigate hydrogen transport in the pro... more Density-functional theory calculations are performed to investigate hydrogen transport in the proton conductor BaSnO$_3$. Structural optimizations in the stable and saddle point configurations for transfer and reorientation allow description of the high-temperature classical and semi-classical regimes, in which diffusion occurs by over-barrier motion. At lower temperature (typically below 300 K), we describe a thermally-assisted quantum regime. In this regime, transfer and reorientation occur when the surrounding matrix adopts particular "coincidence" configurations in which quantum tunneling is favored. Both the non-adiabatic and the adiabatic cases are examined. In the adiabatic case, the energy landscape of hydrogen in the coincidence configuration is very flat, with very low coincidence energy barriers. Path-integral molecular dynamics simulations of the H atom in the coincidence potential reveal, in the transfer case, highly quantum behavior up to T=300 K (the density...

Procedia Engineering, 2014

Two cobaltites were studied as air electrodes for Solid Oxide Cells with a Cerium Gadolinium Oxid... more Two cobaltites were studied as air electrodes for Solid Oxide Cells with a Cerium Gadolinium Oxide electrolyte (CGO): Ca 3 Co 4 O 9+ , well known for its thermoelectric properties, and Ba 2 Co 9 O 14 . After optimisation of composition and thickness, a very good ASR of only 0.08 .cm 2 was obtained for the 50 wt% Ba 2 Co 9 O 14 -50 wt% 50CGO composite. In contrast, a value of 0.5 .cm 2 was reached for the 50 wt% Ca 3 Co 4 O 9+ -50 wt% CGO composite. Although, Ba 2 Co 9 O 14 sample contained 18 O after annealing, oxide ion diffusion in this compound still has to be confirmed. In contrast, high surface exchange kinetics were measured for both Ca 3 Co 4 O 9+ and NdBaCo 2 O 5+ , for which mainly calcium and barium/neodymium were evidenced at the uppermost surface of the samples, whose atoms may play a key role in the mechanism of oxygen molecules dissociation.

Faraday Discuss., 2014

Oxide diffusion was studied in two innovative SOFC cathode materials, Ba 2 Co 9 O 14 and Ca 3 Co ... more Oxide diffusion was studied in two innovative SOFC cathode materials, Ba 2 Co 9 O 14 and Ca 3 Co 4 O 9+d derivatives. Although oxygen diffusion was confirmed in the promising material Ba 2 Co 9 O 14 , it was not possible to derive accurate transport parameters because of an oxidation process at the sample surface which has still to be clarified. In contrast, oxygen diffusion in the well-known Ca 3 Co 4 O 9+d thermoelectric material was improved when calcium was partly substituted with strontium, likely due to an increase of the volume of the rock salt layers in which the conduction process takes place. Although the diffusion coefficient remains low, interestingly, fast kinetics towards the oxygen molecule dissociation reaction were shown with surface exchange coefficients higher than those reported for the best cathode materials in the field. They increased with the strontium content; the Sr atoms potentially play a key role in the mechanism of oxygen molecule dissociation at the solid surface.

Journal of the European Ceramic Society, 2015

ZrO 2 and TiO 2 modified lead-free (K 0.5 Na 0.5 )NbO 3 (KNN) piezoelectric ceramics are prepared... more ZrO 2 and TiO 2 modified lead-free (K 0.5 Na 0.5 )NbO 3 (KNN) piezoelectric ceramics are prepared by a conventional solid-state reaction. The effect of acceptor doping on structural and functional properties is investigated. A decrease in the Curie temperature and an increase in the dielectric constant values are observed when doping. More interestingly, an increase in the coercive field E c and remanent polarization P r is observed. The piezoelectric properties are greatly increased when doping with small concentrations dopants. ZrO 2 doped ceramic exhibits good piezoelectric properties with piezoelectric coefficient d 33 = 134 pC/N and electromechanical coupling factor k p = 35%. It is verified that nonlinearity is significantly reduced. Thus, the creation of complex defects capable of pinning the domain wall motion is enhanced with doping, probably due to the formation of oxygen vacancies. These results strongly suggest that compositional engineering using low concentrations of acceptor doping is a good means of improving the functional properties of KNN lead-free piezoceramic system. (X. Vendrell).