Extraordinary fast oxide ion conductivity in La1.61GeO5−δ thin film consisting of nano-size grain

Thin films of La_1.61GeO_5−δ, a new oxide ionic conductor, were fabricated on dense polycrystalline Al₂O₃ substrates by a pulsed laser deposition (PLD) method and the effect of the film thickness on the oxide ionic conductivity was investigated on the nanoscale. The deposition parameters were optimized to obtain La_1.61GeO_5−δ thin films with stoichiometric composition. Annealing was found necessary to get crystalline La_1.61GeO_5−δ thin films. It was also found that the annealed La_1.61GeO_5−δ film exhibited extraordinarily high oxide ionic conductivity. Due to the nano-size effects, the oxide ion conductivity of La_1.61GeO_5−δ thin films increased with the decreasing thickness as compared to that in bulk La_1.61GeO_5−δ. In particular, the improvement in conductivity of the film at low temperature was significant .The electrical conductivity of the La_1.61GeO_5−δ film with a thickness of 373 nm is as high as 0.05 S cm^− 1 (log(σ/S cm^− 1) = − 1.3) at 573 K.     

Effect of Eu dopant concentration in SrCe1 − xEuxO3 − δ on ambipolar conductivity

The europium dopant concentration in strontium cerate was studied to achieve maximum hydrogen permeation. In order to determine high ambipolar conductivity, total conductivity and open circuit potential measurements were performed. Among the three different compositions of Eu-doped SrCe_1 ? xEu_xO_3 ? δ (x = 0.1, 0.15 and 0.2) studied, SrCe_0.9Eu_0.1O_3 ? δ showed highest total conductivity between 600 °C and 900 °C. However, transference number measurements showed increasing electronic conductivity with increasing dopant concentration and a stronger temperature dependence for electronic conduction. Therefore, the highest ambipolar conductivity was obtained over the compositional range from SrCe_0.85Eu_0.15O_3 ? δ to SrCe_0.8Eu_0.2O_3 ? δ depending on temperature. Finally, the hydrogen permeation flux was calculated based on the ambipolar conductivity and compared with experimental results.     

Preparation of proton conducting BaCe0.8Gd0.2O3 thin films

Thin films of BaCe_0.8Gd_0.2O₃ were prepared by solid state reaction of two screen-printed layers over porous substrates. The first layer consists of the oxygen ion conductor Ce_0.8Gd_0.2O₂ with a fluorite structure, whereas the top layer consists of BaCO₃. After decomposition of the carbonate, BaO reacts with Ce_0.8Gd_0.2O₂ forming the perovskite oxide BaCe_0.8Gd_0.2O_3−δ with protonic conductivity. The in-situ reaction and densification on the porous substrates results in gastight thin layers of 10 to 50 μm and allows overcoming the problems due to the poor sinterability of the proton conductor. Two different porous substrates prepared by warm-pressing were studied as membrane supports, i.e., (i) porous composite NiO–Zr_0.85Y_0.15O₂, commonly employed as solid oxide fuel cell anode and (ii) porous Ce_0.8Gd_0.2O₂ oxide. The structural properties of the layer, compositional gradients and occurring phases are described, as well as water uptake, gastightness (He leaking rate) and emf measurement. Protonic conducting membranes are particularly suited not only for hydrogen separation combined with reforming and water–gas-shift converters but also as a protonic fuel cell electrolyte.     

Conduction behavior of Pr1−xSrxMn1−yInyO3−δ perovskites

Using a structural field map for ABO₃ perovskites outlining the state of their d-electrons and their conduction behavior, strategies for semi-conducting and metallic conducting perovskites can be investigated. In this study the electrical properties of the Pr_1−xSr_xMn_1−yIn_yO_3−δ perovskites have been characterized. The Pr_1−xSr_xMn_1−yIn_yO_3−δ perovskites show semi-conducting behavior for all compositions. Indium decreases the conductivity and increases the activation energy. The conductivity was found to be independent of oxygen partial pressure in the range from approximately 20% to 20 ppm and at temperatures up to 800 °C. The highest conductivity value of 143 S/cm was recorded for Pr_0.5Sr_0.5MnO_3−δ at 800 °C.     

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.     

Microstructures and electrical conductivity of nanocrystalline ceria-based thin films

Ceria-based thin films are potential materials for use as gas-sensing layers and electrolytes in micro-solid oxide fuel cells. Since the average grain sizes of these films are on the nanocrystalline scale (< 150 nm), it is of fundamental interest whether the electrical conductivity might differ from microcrystalline ceria-based ceramics. In this study, CeO₂ and Ce_0.8Gd_0.2O_1.9−x thin films have been fabrication by spray pyrolysis and pulsed laser deposition, and the influence of the ambient average grain size on the total DC conductivity is investigated. Dense and crack-free CeO₂ and Ce_0.8Gd_0.2O_1.9−x thin films were produced that withstand annealing up to temperatures of 1100 °C. The dopant concentration and annealing temperature affect highly the grain growth kinetics of ceria-based thin films. Large concentrations of dopant exert Zener drag on grain growth and result in retarded grain growth. An increased total DC conductivity and decreased activation energy was observed when the average grain size of a CeO₂ or Ce_0.8Gd_0.2O_1.9−x thin film was decreased.     

Effect of microstructure on chemical stability and electrical properties of BaCe<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3 − <Emphasis Type="Italic">δ</Emphasis></Subscript>

Y-doped barium cerate BaCe_0.9Y_0.1O_3???δ was synthesised by a solid-state reaction method. Materials with different average grain sizes and grain boundary surface areas were obtained. The effect of microstructure on the chemical stability in the CO₂ and H₂O-containing atmosphere and electrical properties was analysed and discussed. To evaluate the chemical stability of BaCe_0.9Y_0.1O_3???δ , the exposure test was performed. Samples were exposed to the carbon dioxide and water vapour-rich atmosphere at 25 °C for 700 h. Thermogravimetry supplied by mass spectrometry was applied to analyse the samples before and after this comprehensive test. The mass loss for samples before and after the test and the amount of BaCO₃ formed during the test were directly treated as the measure of chemical instability of BaCe_0.9Y_0.1O_3???δ in the atmosphere rich in carbon dioxide and water vapour. As it was observed, the BaCe_0.9Y_0.1O_3???δ chemical stability towards CO₂ and H₂O is not affected by the materials’ microstructure. Electrical properties of BaCe_0.9Y_0.1O_3???δ which differs with microstructure were determined using electrochemical impedance spectroscopy (EIS). It was found that the grain interior resistivity and activation energy of grain interior conductivity is microstructure independent. However, the effect on microstructure was seen on the EIS spectra in the range of grain boundary contribution. Therefore, the lowest activation energy and the highest conductivity were observed for a material with the lowest grain boundary surface area.     

On the use of NiO as sintering additive for BaCe0,9Y0,1O3 − α

The effect of nickel oxide addition on the densification behaviour and electrical conductivity of BaCe_0.9Y_0.1O_3 ? α (BCY10) is investigated. Small addition (4 mol%) of this sintering aid reduces the sintering temperature by 200 °C and promotes the densification. 95% of the theoretical density was reached after sintering at 1250 °C for 10 h in air. Addition of NiO has no detrimental effect on the total conductivity of BCY10 in wet hydrogen. Nevertheless, the activation energy of the recorded blocking effect is higher than that of the bulk contribution, indicating different electrical properties between grains and internal interfaces in pure and NiO doped BCY10.     

Structural stability and oxygen permeability of cerium lightly doped BaFeO3−δ ceramic membranes

The structural stability under reducing environment and oxygen permeation fluxes of perovskite-type BaCe_xFe_1−xO_3−δ (0 ≤ x ≤ 0.15) ceramic membranes were investigated. The XRD results showed that 5% of cerium doping into the perovskite B-site can make the BaFeO_3−δ transform from the hexagonal structure to the cubic structure, and make the structure stable under 10% H₂–Ar mixed gas at 900 °C for 1 h, but breaks down after 5 h. Lattice parameters and oxygen non-stoichiometry of the as-prepared and reduced samples were measured. Oxygen permeation fluxes of the membranes were measured between 700 and 950 °C. Oxygen permeation during the cooling and heating circles showed that 5% of cerium doping into the perovskite B-site can avoid the phase transformation. The optimum cerium-doped amount on the B-sites was found to be 10%.     

Transport properties of the Pr2−xSrxNiO4±δ ceramics with x = 0.3 and 0.6

As potential cathode materials Pr_2−xSr_xNiO_4±δ compositions with x = 0.3 and 0.6 were prepared at 1300 °C in air and their electrical conductivity and oxygen non-stoichiometry were investigated in the temperature range 20–1000 °C and oxygen partial pressure (pO₂) 1–21,000 Pa. Sr-doping allows partially to stabilize the Pr₂NiO_4±δ structure, but some phase transitions were observed in spite of that. The electrical conductivity and the oxygen mobility of the Pr- and La-containing ceramic nickelates with K₂NiF₄- type structure are 10–15% higher for Pr_2−xSr_xNiO_4−δ compounds at the same temperature and oxygen partial pressure.     

On the structural stability and oxygen permeation behavior of inorganic SrCo0.8Fe0.2O3−δ membranes

The high oxygen permeability combined with reasonable structural stability of perovskite-type ABO_3−δ compounds is vital for their potential applications in gas separation, solid oxide fuel cells, sensors, etc. Hence, an attempt is made to develop SrCo_0.8Fe_0.2O_3−δ-based dense membranes with sol-gel-derived oxalates and study their phase stability and oxygen permeation. While X-ray diffraction confirms the presence of a perovskite-type cubic phase above 800 °C, X-ray photoelectron spectroscopy reveals the presence of cobalt and iron in 3+ and 4+ oxidation states with O₂ ²⁻, O₂ ⁻ and O⁻ species. The electrical conductivity increases up to a characteristic temperature and decreases slowly thereafter via pronounced carrier scattering. A 1.5-mm-thick membrane displays reasonable oxygen permeability of 1.05 × 10⁻⁶ mol cm⁻² s⁻¹ at 900 °C but has inadequate stability. Partial substitution of iron with zirconium is shown to improve permeability and stability significantly. Thus, SrCo_0.8Fe_0.15Zr_0.05O_3−δ membrane shows promise for oxygen permeation purposes.

     

The chemical stability and conductivity improvement of protonic conductor BaCe0.8 − x Zr x Y0.2O3 − δ

A series of BaCe_0.8???x Zr _x Y_0.2O_3???δ (BCZYx) (x?=?0, 0.2, 0.4, 0.6, 0.8) powders were prepared by EDTA–citrate complexing sol–gel process in this paper. The electrical conducting behavior, as well as chemical stability, was investigated. X-ray diffraction (XRD) results reveal that all samples are homogenous perovskite phases. Observed from XRD patterns and thermogravimetric curves, the samples with x?≥?0.4 survive in the pure CO₂, while samples with various Zr contents all present structurally stable against steam at 800 °C. The Zr-free sample of BaCe_0.8Y_0.2O_3???δ possesses the maximum bulk conductivity, 4.25?×?10^?2 S/cm, but decomposes into Ba(OH)₂ and Ce_0.8Y_0.2O_3???δ in steam. A negative influence of increasing Zr content on the conductivity of BCZYx can be observed by impedance tests. Considering the effect of temperature on the bulk conductivity, BCZY0.4 is preferred to be applied in SOFC as a protonic conductor, ranging from 1.52?×?10^?4 to 1.51?×?10^?3 S/cm (500–850 °C) with E _a?=?0.859 eV, which is proved to be a good protonic conductor with t _H+?≥?0.9.     

Thermoelectric properties of Gd,Y co-doped Ca3Co4O9+δ

The Ca_3−x−yGd_xY_yCo₄O_9+δ precursor powders were synthesized by the polyacrylamide gel method. The powders were sintered by the spark plasma sintering (SPS). The DTA analysis showed that the synthesis temperature of Ca₃Co₄O_9+δ was about 933 K, which is lower than that of the conventional citric acid method. The resistivities, the Seebeck coefficients and the thermal conductivities of the samples were measured from room temperature to 973 K. The Seebeck coefficients and the resistivities of the doped samples were remarkably enhanced due to the decrease of the carrier concentration, whereas the thermal conductivities of them were decreased due to the impurity scattering effect. The maximal ZT value of 0.26 was obtained at 973 K for Ca_2.7 Gd_0.15 Y_0.15Co₄O_9+δ.     

Effect of co-dopant addition on the properties of yttrium and neodymium doped barium cerate electrolyte

The paper discusses the Y and Nd doped barium cerate perovskites prepared by a modified Pechini method. The series prepared is described by the formula BaCe_0.80Y_xNd_0.2−x O_3−δ. The BaCeO₃ based powders, about 10–20 nm sized and uniformly shaped, were obtained through the calcination of the gel at 500 °C for 2 h. Their structures and ionic conductivities were characterized by X-ray diffraction and AC impedance spectroscopy. All the electrolytes were found to be barium cerate based solid solutions of perovskite type structures. Impedance spectra indicate that the grain boundary resistance of the specimen synthesized by this method is smaller than that of the samples prepared by a conventional solid-state method. The ionic conductivities of co-doped barium cerate were a factor of several times than that of the single-doped one at 673–1073 K. Among the electrolytes examined, the one co-doped with 5 mol% Nd and 15 mol% Y shows the best improvement in performance. These co-doped barium cerate are more ideal intermediate temperature (IT) electrolyte materials.     

Ionic conductivity of Ce1−xNdxO2−x / 2

The electrical conductivity of sintered samples of Ce_1−xNd_xO_2−x / 2 (0.01 ≤ x ≤ 0.2) was investigated in air as a function of temperature between 150 and 600 °C using AC impedance spectroscopy. The individual contribution of the bulk and grain boundary conductivities has been discussed in detail. In the low temperature range (< 350 °C), the activation enthalpy for bulk conductivity exhibited a shallow minimum at 3 mol% Nd, with a value of 0.68 eV. The activation enthalpy also produced a shallow minimum at 5 mol% Nd in the high temperature range (> 350 °C), with a value of 0.56 eV. It was shown that Ce_1−xNd_xO_2−x / 2 is an electrolyte that obeys the Meyer Neldel rule. The bulk conductivity data measured by others for the same system has also been recalculated and re-evaluated to facilitate easier comparison with our own data.     

A co-doping approach towards enhanced ionic conductivity in fluorite-based electrolytes

A co-dopant strategy is used to investigate the effect that the elastic strain in the lattice has on the grain ionic conductivity of doped ceria electrolytes. Based on critical dopant ionic radius (r_c), different compositions in the Lu_xNd_yCe_1−x−yO_2−δ (x + y = 0.05, 0.10, 0.15, and 0.20) system are studied. Dopants are added such that the weighted average dopant ionic radius matches r_c for all the compositions. Dense ceramic discs are prepared using conventional solid oxide route and sintering methods. Precise lattice parameter measurements are used to calculate the lattice strain. The ionic conductivity of the samples is measured in the temperature range of 250 °C to 700 °C using two-probe electrochemical impedance spectroscopy technique. The elastic strain present in Lu_xNd_yCe_1−x−yO_2−δ system is found to be negligible when compared to Lu_xCe_1−xO_2−δ (negative) and Nd_xCe_1−xO_2−δ (positive) systems. Grain ionic conductivity of Lu_xNd_yCe_1−x−yO_2−δ (where x + y = 0.05) at 500 °C is observed to be 1.9 × 10^− 3 S/cm which is twice as high as that of Lu_0.05Ce_0.95O_2−δ. These results extend the validity of the r_c concept as a strategy for co-doping ceria electrolytes and open new designing avenues for solid oxide electrolytes with enhanced ionic conductivity.     

A low temperature internal friction study of Y0.85Ca0.15Ba2Cu3O7−δ with different oxygen content

The ceramic sample of Y_0.85Ca_0.15Ba₂Cu₃O_7−δ was prepared by standard solid-state reaction method, and samples with different oxygen concentration were obtained by quenching from high temperature. The internal friction was measured using the vibrating reed method from liquid-nitrogen temperature to room temperature at kilohertz frequency. An internal friction peak was observed around 250 K in Y_0.85Ca_0.15Ba₂Cu₃O_7−δ quenched from 1023 K. The peak is related to the one observed around 220 K (labeled as P3 peak) in undoped YBa₂Cu₃O_7−δ (Y123). This result shows the dependence of P3 peak on carriers density and P3 peak has a strong correlation to the abnormal behavior of Y123 in the underdoped range. The variation of two low temperature thermal activated relaxation peaks (P1 and P2) on oxygen content were also investigated. And consistent explanations were given based on all recent researches.     

Dielectric relaxation in Sr modified PST ceramics

Nanocrystalline powders of strontium modified PbSn_0.15Ti_0.85O₃ (PST) having the formula Pb_0.94Sr_0.06Sn_0.15 have been synthesized by a precursor solution method. The electrical behavior of Pb_0.94Sr_0.06Sn_0.15Ti_0.85O₃ sintered pellets has been studied by complex impedance spectroscopy analysis. The plot of the real and imaginary parts of the impedance shows that the semicircle exhibits a depression degree with a distribution of relaxation time. The modulus curve indicates the possibility of non-exponential type conductivity. The values of the activation energy calculated from both plots of Z” and M”, are 1.06 and 1.09 eV, which reveals that the species responsible for conduction are same. It also confirms that oxygen vacancies play an important role in conduction. The non-overlapping of the peaks in the plot of M”/M”_max and Z”/Z”_max as a function of logarithmic frequency measured at 350 °C indicates short-range conduction. The compounds exhibit a negative temperature coefficient of resistance with an α value of -5×10^-2 °C at 375 °C. The frequency (ω) dependence of conductivity satisfies the ωⁿ power law. The variation of n with temperature suggests that ac conduction is due to small polaron tunneling. PACS 61.28Rx; 77.22Ch; 77.22Gm; 77.80Bh     

Effect of nickel substitution on defect chemistry, electrical properties, and dimensional stability of calcium-doped yttrium chromite

The effect of nickel substitution on defect chemistry, electrical properties, and dimensional stability of calcium-doped yttrium chromite was studied for use as an interconnect material in high temperature solid oxide fuel cells (SOFCs). The compositions of Y_0.8Ca_0.2Cr_1 − xNi_xO_3 ± δ (x = 0-0.15), prepared using the glycine nitrate process, showed single phase orthorhombic perovskite structures over a wide range of oxygen partial pressures (4.6 × 10^− 20 atm ≤ pO₂ ≤ 0.21 atm at 900 °C). X-ray diffraction (XRD) analysis indicated that most of the nickel ions replacing chromium ions are divalent and act as acceptor dopants, leading to a substantial increase in conductivity. In particular, the conductivity at 900 °C in air increased from 10 S/cm to 34 S/cm with 15% nickel substitution, and an increase in charge carrier density was confirmed by Seebeck measurements, which validated the predominant Ni²⁺ oxidation state. A point defect model was derived, and the relationship between electrical conductivity and oxygen partial pressure was successfully fitted into the proposed model. The defect modeling results indicated that nickel substitution improves the stability of calcium-doped yttrium chromite toward reduction and suppresses the oxygen vacancy formation, which results in significantly increased electrical conductivity in reducing environment. The electrical conductivity of Y_0.8Ca_0.2Cr_0.85Ni_0.15O_3 ± δ at 900 °C in reducing atmosphere (pO₂ = 10^− 17 atm) was 5.8 S/cm, which was more than an order of magnitude higher than that of Y_0.8Ca_0.2CrO_3 ± δ (0.2 S/cm). Improved stability in reducing atmosphere was further confirmed by dilatometry measurements showing reduced isothermal “chemical” expansion, and the isothermal expansion in reducing atmosphere (pO₂ = 10^− 17 atm) at 900 °C decreased from 0.07% for Y_0.8Ca_0.2CrO_3 ± δ to 0.03% for Y_0.8Ca_0.2Cr_0.85Ni_0.15O_3 ± δ. Based on these results, enhanced electrical performance and mechanical integrity is expected with nickel substitution on calcium-doped yttrium chromite in SOFC operating conditions.     

Water vapour solubility and conductivity study of the proton conductor BaCe(0.9 − x)ZrxY0.1O(3 − δ)

The perovskite BaCe_(0.9 ? x)Zr_xY_0.1O_(3 ? δ) has been prepared by solid state reaction at 1400 °C and conventional sintering at 1700 °C. Water uptake experiments performed between 400 and 600 °C, at a water vapour pressure of 0.02 atm, provide data on the concentration of protons incorporated in the sample. The direct current conductivity has been measured as a function of oxygen partial pressure, at a water vapour partial pressure of 0.015 atm. The total conductivity has been resolved into a p-type and an ionic component using a fitting procedure appropriate to the assumed defect model. An estimation of the protonic component was made by assuming a conductivity isotope effect between 1.4 and 1.8. The total conductivity, obtained using impedance spectroscopy has been measured as a function of temperature in the water and heavy water exchanged states. The activation energy has been found to be 0.56 eV to 0.59 eV in the water exchanged state with values 0.03 to 0.04 eV higher in the heavy water exchanged state. Impedance spectra measured at 200 °C showed a reduction in grain boundary resistivity with increasing cerium content. The stability of the compounds to carbon dioxide has been studied by thermogravimetry.