Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum cobaltite

Oxygen nonstoichiometry, structure and transport properties of the two compositions (La_0.6Sr_0.4)_0.99CoO_3−δ (LSC40) and La_0.85Sr_0.15CoO_3−δ (LSC15) were measured. It was found that the oxygen nonstoichiometry as a function of the temperature and oxygen partial pressure could be described using the itinerant electron model. The electrical conductivity, σ, of the materials is high (σ > 500 S cm^− 1) in the measured temperature range (650–1000 °C) and oxygen partial pressure range (0.209–10^− 4 atm). At 900 °C the electrical conductivity is 1365 and 1491 S cm^− 1 in air for LSC40 and LSC15, respectively. A linear correlation between the electrical conductivity and the oxygen vacancy concentration was found for both samples. The mobility of the electron-holes was inversely proportional with the absolute temperature indicating a metallic type conductivity for LSC40. Using electrical conductivity relaxation the chemical diffusion coefficient of oxygen was determined. It was found that accurate values of the chemical diffusion coefficient could only be obtained using a sample with a porous surface coating. The porous surface coating increased the surface exchange reaction thereby unmasking the chemical diffusion coefficient. The ionic conductivity deduced from electrical conductivity relaxation was determined to be 0.45 S cm^− 1 and 0.01 S cm^− 1 at 1000 and 650 °C, respectively. The activation energy for the ionic conductivity at a constant vacancy concentration (δ = 0.125) was found to be 0.90 eV.     

Transport properties and structural stability of tetragonal CeNbO4+δ

The electrical properties of CeNbO_4+δ have been investigated at 1073–1223 K in the oxygen partial pressure range 10^− 17 to 0.36 atm. The conductivity and Seebeck coefficient behaviour indicates that, at oxygen chemical potentials close to atmospheric, tetragonal CeNbO_4+δ possesses a mixed ionic and p-type electronic conductivity. The ion transference numbers under the p(O₂) gradient of 0.93/0.21 atm, measured by the modified e.m.f. technique, are close to 0.4 decreasing in more reducing environments. The variations of partial ionic and electronic conductivities can be described in terms of the oxygen intercalation into the scheelite-type lattice, which results in increasing concentrations of both dominant charge carriers, oxygen interstitials and holes, when p(O₂) increases. Reduction leads to p(O₂)-independent electrical properties, followed by a drastic decrease in the conductivity at oxygen pressures below 10^− 15–10^− 9 atm due to a reversible transition into the monoclinic phase. Contrary to the zircon-type CeVO_4±δ, no traces of the parent binary oxides were detected in the reduced cerium niobate.     

The effect of cation nonstoichiometry on the electrical conductivity of acceptor-doped CaZrO3

The electrical properties of acceptor-doped Ca_1−xZr_0.99M_0.01O_3−δ (M = Mg²⁺, In³⁺) systems were investigated as a function of cation nonstoichiometry (0 ≤ x ≤ 0.05). The characterization was carried out using the impedance spectroscopy between 550 °C and 1100 °C in dry air. The contributions of the grain and grain boundary conductivity to the total conductivity were obtained from the impedance data. When the Ca deficiency (x) increased, the total conductivity rapidly decreased with the corresponding increase in activation energy. Although the grain conductivity increased slightly with increasing x, the total conductivity is mostly determined by the highly resistive grain boundary. With varying x, the activation energy of total conductivity showed the percolation behavior. The percolation threshold values vary according to the doped species. It may be due to the difference in concentration of oxygen vacancies of the specimens.     

Mixed conductivity and oxygen permeability of doped Pr2NiO4-based oxide

Pr₂NiO₄-based oxide was studied as a new mixed electronic and oxide ionic conductor for the oxygen permeation membrane. It was found that Pr₂NiO₄ doped with Cu and Fe for Ni site exhibits the relatively high oxygen permeation rate. Doping second cation to Ni site is effective for improving the oxygen permeation rate and the trivalent cation seems to be effective for increasing the oxygen permeation rate. Among the examined cation, the highest oxygen permeation rate was obtained by doping 5 mol% Fe. The oxygen permeation rate was also significantly affected by the surface catalyst and the highest oxygen permeation rate of 80 μmol·min^− 1·cm^− 2 at 1273 K was achieved by using La_0.1Sr_0.9Co_0.9Fe_0.1O₃ for the surface catalyst. Since the electrical conductivity slightly decreased with decreasing P_O2 and it dropped significantly at P_O2 = 10^− 19 atm, chemical stability of Pr₂NiO₄-based oxide seems to be reasonably high. Application of this new mixed conductor for the oxygen permeation membrane under the CH₄ partial oxidation was also studied and it was confirmed that the oxygen permeation rate much improved under the CH₄ oxidation condition and this Pr₂NiO₄ can be used for the oxygen permeation membrane for the CH₄ partial oxidation.     

Ionic and electronic conductivities,stability and thermal expansion of La10 − x(Si,Al)6O26 ± δ solid electrolytes

Apatite-type La_10 − xSi_6 − yAl_yO_{27 − 3x/2 − y/2} (x = 0–0.33; y = 0.5–1.5) exhibit predominant oxygen ionic conductivity in a wide range of oxygen partial pressures. The conductivity of silicates containing 26.50–26.75 oxygen atoms per formula unit is comparable to that of gadolinia-doped ceria at 770–870 K. The average thermal expansion coefficients are (8.7–10.8) × 10^− 6 K^− 1 at 373–1273 K. At temperatures above 1100 K, silicon oxide volatilization from the surface layers of apatite ceramics and a moderate degradation of the ionic transport with time are observed under reducing conditions, thus limiting the operation temperature of Si-containing solid electrolytes.     

Mixed conductivity,stability and thermomechanical properties of Ni-doped La(Ga,Mg)O3−δ

Perovskite-type LaGa_0.65Mg_0.15Ni_0.20O_3−δ exhibiting oxygen transport comparable to that in K₂NiF₄-type nickelates was characterized as a model material for ceramic membrane reactors, employing mechanical tests, dilatometry, oxygen permeability and faradaic efficiency measurements, thermogravimetry (TG), and determination of the total conductivity and Seebeck coefficient in the oxygen partial pressure range from 10^− 15 Pa to 40 kPa. Within the phase stability domain which is similar to La₂NiO_4+δ, the defect chemistry of LaGa_0.65Mg_0.15Ni_0.20O_3−δ can be adequately described by the ideal solution model with oxygen vacancies and electron holes to be the only mobile defects, assuming that Ni²⁺ may provide two energetically equivalent sites for hole location. This assumption is in agreement with the density of states, estimated from thermopower, and the coulometric titration and TG data suggesting Ni⁴⁺ formation in air at T < 1150 K. The hole conductivity prevailing under oxidizing conditions occurs via small-polaron mechanism as indicated by relatively low, temperature-activated mobility. The ionic transport increases with vacancy concentration on reducing p(O₂) and becomes dominant at oxygen pressures below 10^− 7–10^− 5 Pa. The average thermal expansion coefficients in air are 11.9 × 10^− 6 and 18.4 × 10^− 6 K^− 1 at 370–850 and 850–1270 K, respectively. The chemical strain of LaGa_0.65Mg_0.15Ni_0.20O_3−δ ceramics at 1073–1123 K, induced by the oxygen partial pressure variations, is substantially lower compared to perovskite ferrites. The flexural strength determined by 3-point and 4-point bending tests is 167–189 MPa at room temperature and 85–97 MPa at 773–1173 K. The mechanical properties are almost independent of temperature and oxygen pressure at p(O₂) = 1–2.1 × 10⁴ Pa and 773–1173 K.     

Ionic conductivity of brownmillerite-type calcium ferrite under oxidizing conditions

The thermogravimetric and Mössbauer spectroscopy studies showed that, at atmospheric oxygen pressure, the oxygen content in Ca₂Fe₂O₅ brownmillerite is very close to stoichiometric at 300–1270 K. The orthorhombic lattice of calcium ferrite undergoes a transition from primitive (space group Pnma) to body-centered (I2mb) at 950–1000 K, which is accompanied with decreasing thermal expansion coefficient (TEC) and increasing activation energy for the total conductivity, predominantly p-type electronic. The steady-state oxygen permeation through dense Ca₂Fe₂O₅ ceramics is limited by the bulk ionic conduction. The ion transference numbers in air vary in the range 0.002–0.007 at 1123–1273 K, increasing with temperature. Analysis of stereological factors, which may affect oxygen diffusivity, suggests a dominant role of the ion jumps along octahedral and, possibly, tetrahedral layers of the brownmillerite structure. The ionic conductivity of calcium ferrite is higher than that of Ca₂FeAlO_5+δ, but lower compared to the oxygen-deficient perovskite phases based on SrFeO_3−δ where the diffusion pathways form a three-dimensional network. The average TECs of Ca₂Fe₂O₅ ceramics, calculated from dilatometric data in air, are 13.1 × 10^− 6 K^− 1 at 370–950 K and 11.3 × 10^− 6 K^− 1 at 970–1270 K.     

Ceria based mixed conductors with adjusted electronic conductivity in the bulk and/or along grain boundaries

Ceramic samples of Ce_1 ? xPr_xO_2 ? δ (CPO) and Ce_1 ? xGd_xO_2 ? δ (CGO) were obtained by different sintering schedules, including the use of cobalt as a sintering aid, added by mixing the precursor powders with cobalt nitrate solution; this allowed one to obtain different microstructural features and to change the transport properties, with emphasis on changes in grain boundary behaviour. Cobalt plays a double effect as sintering aid and also to induce important changes of grain boundary properties. Specific changes of grain boundary properties were ascribed by de-convolution of impedance spectra.Relatively high levels of mixed conductivity could be attained by adding a lanthanide species to yield ionic transport, whereas electronic conduction was promoted by the mixed valence character of PrO_x, combined with the additional contribution of Co-rich grain boundaries. These effects can be used to tune preferential electronic conductivity at bulk or grain boundary level. Oxygen permeability and a modified e.m.f. method were used to obtain the overall ionic transport number under oxidising conditions and its dependence on processing conditions. Additions of PrO_x induce bulk electronic conduction which assumes a greater role at lower temperatures. Further enhancement of electronic conductivity is attained by effects of Co-addition. Though Co-rich grain boundaries also yield significant levels of electronic conductivity in CGO, this contribution becomes minor at intermediate temperatures, due to differences between the activation energies for electronic and ionic conduction.     

Ion and electron conduction in SrFe1−xScxO3−δ

The oxygen ion and electron transport in SrFe_1−xSc_xO_3−δ  (x = 0.1–0.3) system at 700–950 °C were studied analyzing the total conductivity dependencies on the oxygen partial pressure, pO₂. The conductivity measurements were performed both under reducing conditions (10^− 19 ≤ pO₂ ≤ 10^− 8 atm) comprising the electron-hole equilibrium point, and in oxidizing atmospheres (10^− 5 ≤ pO₂ ≤ 0.5 atm) which are characterized by extensive variations of the oxygen content studied by coulometric titration technique. The incorporation of 10% Sc³⁺ cations into the iron sublattice suppresses transition of the cubic perovskite phase into vacancy-ordered brownmillerite, thus improving ion conduction at temperatures below 850 °C. When scandium content increases, the ion conductivity becomes considerably lower. The hole mobility is thermally-activated and varies in the range of 0.001 to 0.05 cm² V^− 1 s^− 1, increasing with oxygen concentration and decreasing on Sc doping.     

Oxygen ionic conductivity of La2NiO4+δ via interstitial oxygen defect

The ionic conduction properties of La₂NiO_4+δ were studied from oxygen permeation flux and defect-related transport properties. The effects of the applied oxygen chemical potential gradient and temperature on the oxygen permeability of La₂NiO_4+δ at various thickness are reported. The thermally activated oxygen permeation flux increased monotonically with increasing oxygen chemical potential gradient, yielding a maximum of 0.15 cc min^?1 cm^?2 under air/N₂ conditions for the 0.95 mm-thick La₂NiO_4+δ specimen at 900 °C. The oxygen ion conductivity of La₂NiO_4+δ was calculated as a function of temperature and oxygen partial pressure by differentiating the chemical diffusion equation for the oxygen permeation flux based on the dominant electronic transference number. In addition, the oxygen ion conductivity was extracted successfully by solving the Nernst–Einstein equation combining with the calculated self-diffusion coefficient of oxygen from the chemical diffusivity and thermodynamic enhancement factor from the equilibrium oxygen nonstoichoimetry of a La₂NiO_4+δ specimen, and a deviation of the OPP dependence of 1/6 power was observed.     

Kinetic studies of electronically conducting Yb3Al5O12 garnet

Upon reduction, originally fully transparent and insulating ytterbium alumina garnet single crystals, Yb₃Al₅O₁₂, become deeply colored and electrically conducting with a conductivity of the order of 10^− 3 Ω^− 1 cm^− 1 in the temperature range of 550 °C to 1000 °C. The redox kinetics of the material is studied by means of conductivity relaxation experiments performed at oxidising and reducing conditions. Good agreement is obtained with an optical study into the redox kinetics of Yb₃Al₅O₁₂.     

Sonochemical synthesis,structural, magnetic and grain size dependent electrical properties of NdVO4 nanoparticles

NdVO₄ nanoparticles are successfully synthesized by efficient sonochemical method using two different structural directing agents like CTAB and P123. The phase formation and functional group analysis are carried out using X-ray diffraction (XRD) and fourier transform infra red (FT-IR) spectra, respectively. Using Scherrer equation the calculated grain sizes are 27 nm, 24 nm and 20 nm corresponding to NdVO₄ synthesized by without surfactant, with CTAB and P123, respectively. The TEM images revealed that the shape of NdVO₄ particles is rice-like and rod shaped particles while using CTAB and P123 as surfactants. The growth mechanism of NdVO₄ nanoparticles is elucidated with the aid of TEM analysis. From electrical analysis, the conductivity of NdVO₄ nanoparticles synthesized without surfactant showed a higher conductivity of 5.5703 × 10⁻⁶ S cm⁻¹. The conductivity of the material depends on grain size and increased with increase in grain size due to the grain size effect. The magnetic measurements indicated the paramagnetic behavior of NdVO₄ nanoparticles.     

Mesopore size dependence of the ionic diffusivity in alumina based composite lithium ionic conductors

Ordered-mesoporous Al₂O₃ was synthesized by a sol–gel method using neutral copolymer surfactants as structure-directing agents. The pore size was controlled over the 3–15 nm range by the use of various surfactants. Composites composed of the synthesized mesoporous Al₂O₃ and a lithium ion conductor (LiI) were prepared. The maximum dc electrical conductivity, 2.6 × 10^− 4 S cm^− 1 at 298 K, was observed for 50 LiI·50 Al₂O₃ composite with 4.2 nm average mesopore size, which was considerably higher than the previously reported LiI-alumina composites. A systematic dependence of conductivity upon pore size was observed, in which conductivity increased with decreasing pore size, except for samples with a pore size of 2.8 nm. The lithium ion diffusion coefficient determined by the ⁷Li pulsed field gradient nuclear magnetic resonance (PFG-NMR) showed excellent agreement with the measured conductivity calculated by the Nernst-Einstein equation. On the other hand, lithium migration activation energies obtained by quasielastic neutron scattering (QENS) and ⁷Li NMR spin-lattice relaxation time (T₁) were considerably smaller than those obtained from electrical conductivity and PFG-NMR. This could be explained by the ion migration mechanism in heterogeneous composites and a possible enhancement of conductivity in mesoscopically confined spaces.     

Structural and optical properties of La-doped BaSnO3 thin films grown by PLD

In this study the structural and optical properties of lanthanum-doped BaSnO₃ powder samples and thin films deposited on fused silica were investigaed using laser ablation. Under an oxygen pressure of 5×10⁻⁴ mbar, phase pure BaSnO₃ films with a lattice constant of 0.417 nm and grain size of 21 nm were prepared at 630 °C. The band gap of BaSnO3 powder sample and thin films was calculated to be 3.36 eV and 3.67 eV, respectively. There was a progressive increase in conductivity for thin films of BaSnO₃ doped with 0~7 at% of La. The highest conductivity, 9 Scm⁻¹, was obtained for 7 at% La-doped BaSnO₃. Carrier concentration, obtained from Burstein-Moss (B-M) shift, nearly matches the measured values except for 3 at% and 10 at% La-doped BaSnO₃ thin films.     

Sensor application-related defect chemistry and electromechanical properties of langasite

The operation of langasite (La₃Ga₅SiO₁₄) resonators as sensors at elevated temperature and controlled atmospheres is examined. This paper focuses on mapping the regimes of gas-insensitive operation of uncoated langasite resonators and the correlation to langasite's defect chemistry for temperatures up to 1000 °C. As a measure of sensitivity, the fundamental resonant mode at 5 MHz is estimated to be determined to within ± 4 Hz by network analysis for resonators operated in air at temperatures below 1000 °C. The calculated frequency shift induced by redox-related reactions in langasite only exceeds the limit of ± 4 Hz below p_O2 ≈ 10^− 17 bar at 1000 °C, below 10^− 24 bar at 800 °C and below 10^− 36 bar at 600 °C. Water vapor is found to shift the resonance frequency at higher oxygen partial pressures. In the hydrogen-containing atmospheres applied here, langasite can be regarded as a stable resonator material above oxygen partial pressures of about 10^− 13 and 10^− 20 bar at 800 and 600 °C, respectively.     

Charge transport in polycrystalline titanium dioxide☆

This work reports semiconducting properties of undoped polycrystalline TiO₂ studied using the measurements of the electrical conductivity (EC) and thermopower as a function of oxygen partial pressure and temperature in the ranges of p(O₂) between 10 Pa and 70 kPa and temperature 1173–1273 K. The width of the band gap, determined from the minimum of EC, is equal to 3.055±0.012 eV. It was found that the apparent concentration of negatively charged defects, involving both acceptor-type aliovalent ions and Ti vacancies, increases with temperature from 0.6 at% at 1173 K to the level of 0.9–1.4 at% at 1273 K. This effect is considered in terms of Schottky-type defects. It was observed that the minimum of EC at the n–p transition is lower than that for TiO₂ single crystal thus suggesting that grain boundaries are responsible for the formation of conductivity weak links.     

Unusual oxygen re-equilibration kinetics of TiO2−δ

Oxygen re-equilibration kinetics, along with the equilibrium conductivity, have been measured on undoped, single-crystal TiO_2−δ, by a four-probe d.c. conductivity relaxation technique, against oxygen partial pressure in the range of − 16 < log(P_O2/atm) ≤ 0 at different temperatures in the range of 1173 ≤ T/K ≤ 1373. The isothermal conductivity varies as σ ∝ P_O2^m with m ≈ − 1/4, − 1/5 and − 1/6 in turn with increasing P_O2 up to 1 atm, suggesting a sequential variation of the majority ionic disorder types from Ti_i to Ti_i to V_O, respectively. Contrary to the conventional knowledge that due to the local (defect) equilibrium postulate there should be one and only one chemical diffusivity or single relaxation time for a binary oxide, the oxygen re-equilibration kinetics has turned out to be twofold with two different relaxation times depending on oxygen activities. This is interpreted as being due to the independent relaxation of each sublattice of TiO₂ in an oxygen activity gradient applied, indicating a failure of local equilibrium during oxygen re-equilibration. From the two different relaxation times the chemical diffusion coefficients of component Ti and O are separately evaluated and subsequently, their self-diffusion coefficients. The latter are found to be in a good agreement with the literature data.     

Studies on the percolation limit of Ce0.9Gd0.1O1.95 in La0.6Sr0.4Co0.2Fe0.8O3−δ–Ce0.9Gd0.1O1.95 nanocomposites for solid oxide fuel cells application

A large difference in thermal expansion coefficient of electrode and electrolyte leads to imperfect electrode/electrolyte interface and hence significant polarization losses in solid oxide fuel cells. To overcome the difficulties associated with electrode and electrode/electrolyte interface, there is need to fabricate the composite cathode. Thus the present paper deals with study of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ(LSCF)–Ce_0.9Gd_0.1O_1.95(GDC) nanocomposite with different fractions of GDC obtained by physical mixing of combustion synthesized nanopowders. No secondary phases were observed upon sintering at 1100 °C for 2 h affirming the chemical compatibility between LSCF and GDC. The composites with relatively high GDC% have higher density as a consequence of rapid grain growth and less conductivity. The nanocomposite with 50% of GDC showed electric conductivity of 30 Scm⁻¹ at 500 °C and low area specific resistance of 106 Ω cm² with 10 μs relaxation time at 200 °C.     

Ultra-fast grain boundary diffusion and its contribution to surface segregation on a martensitic steel. Experiments and modeling

Phosphorus surface segregation was measured by Auger Electron Spectroscopy on a 17-4 PH martensitic stainless steel at 450, 550 and 600 °C. Surface segregation was shown to be much faster than expected which was attributed to a high contribution of phosphorus diffusion along the former austenitic grain boundaries. A model of surface segregation was developed following the Darken–du Plessis approach and taking account of both bulk and grain boundary solute diffusion. The phosphorus grain boundary diffusion coefficient in 17-4 PH was estimated: D_GB^< = 6.2 10⁴ exp(? 157 kJ mol^? 1/RT)cm² s^? 1. It is found to be more than three orders of magnitude higher in 17-4 PH steel than in α-iron.     

Synthesis of thin diamond films from faceted nanosized crystallites

Diamond films consist of crystallites having nanometer grains were deposited using low methane concentration by hot filament chemical vapor deposition (HFCVD). The results show that films consist of nanodiamond grains with grain sizes ranging from 20 nm to 200 nm having thickness dependent size. Increasing the deposition time, the grain size increases and hence the thickness of the film increases. The diamond nucleation (nucleation density 10¹⁰ cm⁻²) is comparable to that obtained by biasing the substrate. The use of low methane concentration for the formation of nano crystallites improves the quality of the film as indicated by Raman spectroscopy. The distance between the filament and substrate is increased while maintaining the substrate temperature. The effects of this large separation on the gas phase chemistry are discussed which helps to understand the reduced size of the crystallites under input gas ratios when microcrystallines are obtained.