283 research outputs found
Donor- and transition metal-substituted SrTiO3: relevant issues in the development of ceramic components for SOFC anodes
Due to phase stability requirements, the choice of alternative perovskite-type materials for ceramic SOFC anodes is essentially limited to chromite-, molybdate-, vanadate- or titanate-based systems. Donor-doped SrTiO3 combines remarkable stability in both oxidizing and reducing conditions, substantial electronic conductivity, and good prospects for inhibition of carbon deposition and sulfur-tolerance.
The present lecture gives a brief overview and a comparative assessment of a variety of strontium titanate-based solid solutions as fuel electrode materials with emphasis on the defect chemistry, reducibility and corresponding effects on the electrical properties, thermomechanical compatibility with solid electrolytes, and electrochemical activity. The exemplified oxide systems are (Sr,Ln)TiO3 (Ln = La-Yb), (Sr,Pr)TiO3, Sr(Ti,Ta)O3 and Sr(Ti,V)O3 series including nomically cation-stoichiometric and A-site deficient formulations [1-4].publishe
Pyrochlore-type Y2Ti2O7-based titanates as buffer layer materials for fuel-assisted solid oxide electrolysis cells
No abstract available.publishe
Structural Design of 5 mol.% Yttria Partially Stabilized Zirconia (5Y-PSZ) by Addition of Manganese Oxide and Direct Firing
In this study, 5Y-PSZ-based ceramics with 15 mol.% of manganese oxide were obtained from PSZ + MnO2 powders mixtures by pressing and direct firing. The resulting materials show a stable cubic fluorite structure with only minor traces of segregated manganese oxides and relative density from 90% to 98%. The linear thermal expansion coeffcient is in the order of 10^-5 K^-1 at 500 K and increases gradually with temperature, due to the onset of a contribution of chemical expansion, reaching about 13x10^-6 K^-1 at 1100 K. These results are suitable for prospective applicability as buffer layers to minimize degradation and delamination of electrolyte/oxygen electrode interfaces in solid electrolyte cells. The electrical conductivity remains close to 1 S/m at 973 K and close to 7 S/m at 1273 K, suggesting mixed conductivity with a prospective contribution to electrode processes occurring at electrode/electrolyte interfaces. Guidelines for further improvement were also established by a detailed analysis of the impact of heating/cooling rate, firing temperature, and time on those properties, based on Taguchi planning.publishe
Numerical Analysis of Stress-Strain State of Orthotropic Plates in the Form of Arbitrary Convex Quadrangle
A numerical and analytical approach to solving problems of the stress-strain state of quadrangular orthotropic
plates of complex shape has been proposed. Two-dimensional boundary value problem was solved using spline
collocation and discrete orthogonalization methods after applying the appropriate domain transform.
The influence of geometric shape of plate in different cases of boundary conditions on the displacement and
stress fields is considered according to the refined theory. The results were compared with available data from
other authors
Composite LnNiO3+PrOx oxygen electrodes for solid oxide cells
Ln2NiO4+δ and its derivatives with perovskite-related K2NiF4-type structure demonstrate high mixed
ionic-electronic conductivity, moderate thermal and negligible chemical expansion. As a result, these
phases attracted significant attention as prospective cathode materials for intermediate-temperature solid
oxide fuel cells (IT-SOFC). At the same time, perovskite-like LnNiO3 has not been considered for these
applications, mostly due to the limited phase stability under ambient oxygen pressures. On heating in air,
LaNiO3 decomposes at ~ 1000°C; cathodic polarization can be expected to induce the decomposition of
perovskite phase at lower temperatures characteristic for IT-SOFC operation. On the contrary, redox
changes imposed by anodic polarization (in solid oxide electrolysis cell mode) under oxidizing conditions
should not be of risk for the phase stability of LaNiO3. The goal of the present work was the evaluation of
LnNiO3-based oxygen electrodes for solid oxide fuel/electrolysis cells.
The LnNiO3-δ ceramic powders with perovskite-like structure was prepared by glycine-nitrate combustion
synthesis followed by calcinations in oxygen atmosphere at 800-1000°C. Porous ceramic samples for
electrical and dilatometric studies were sintered in oxygen at 950-1050°C.
Porous LaNiO3-δ samples were found to exhibit favorably high p-type metallic-like electrical
conductivity, 400-500 S/cm at 800-600°C in air. These ceramics demonstrated also a moderate thermal
expansion, with average CTE ~ 13.0 ppm/K at 25-800°C, ensuring thermomechanical compatibility with
solid electrolytes.
As a first step, the electrochemical performance of LaNiO3-δ electrodes was assessed in contact with three
common electrolytes including (ZrO2)0.92(Y2O3)0.08 (8YSZ), Ce0.9Gd0.1O2-δ (CGO10) and
(La0.8Sr0.2)0.98Ga0.8Mg0.2O3-δ (LSGM). The electrode layers were sintered at 1050°C for 2 h under oxygen
flow. The studies of symmetrical cells by EIS demonstrated that the electrochemical activity of LaNiO3-δ
electrodes increases in the sequence 8YSZ < CGO10 < LSGM; the corresponding values of electrode
polarization resistance (Rη) at 800°C were 1.4, 0.8 and 0.25 Ohm×cm2, respectively. Significant
variations of Rη with electrolyte composition correlate with the extent of chemical reactivity between
LaNiO3-δ and electrolyte materials during the electrode fabrication.
The Rη values of LaNiO3-δ electrodes in contact with LSGM electrolyte were further reduced to 0.03
Ohm×cm2 at 800°C and 0.11 Ohm×cm2 at 700°C by the surface modification with PrOx which is known
for its electrocatalytic activity. At 750°C and current density of 0.5 A/cm3, LaNiO3+PrOx (~20 wt.%)
electrodes in contact with LSGM solid electrolyte demonstrate the overpotentials of ~60 mV under
cathodic polarization and ~40 mV under anodic polarization (Fig.1).
The impact of substitution of lanthanum by praseodymium (in order to improve the chemical
compatibility and electrochemical activity) on the relevant properties of LnNiO3 is briefly discussed.publishe
Perovskite-like LaNiO3-δ as oxygen electrode material for solid oxide electrolysis cells
Perovskite-like LaNiO3-δ was evaluated as potential oxygen electrode material for solid oxide electrolysis cells. Compared to the Ruddlesden-Popper Lan+1NinO3n+1 (n = 1,2,3) counterparts, LaNiO3-δ exhibits higher p-type metallic-like conductivity under oxidizing conditions (450 S×cm-1 at 800°C for highly porous ceramics) together with a moderate thermal expansion coefficient (13.7 ppm×K-1 in air at 25-800°C) compatible with common solid electrolytes. The measured electrode polarization resistance (Rη) in contact with YSZ, CGO and LSGM solid electrolytes was 1.4, 0.77 and 0.22 Ω×cm2 at 800°C, and 208, 123 and 7.1 Ω×cm2 at 600°C, respectively, under zero-current conditions in air. Surface modification of via PrOx infiltration resulted in lower values of Rη (0.024 Ω×cm2 at 800°C and 0.76 Ω×cm2 at 600°C) and low anodic overpotentials (20 mV at 800°C and 500 mA×cm-2) in contact with LSGM.publishe
Investigation of static and dynamic behavior of anisotropic inhomogeneous shallow shells by Spline approximation method
The present report proposes an efficient approach to solving within the framework of the classic and refined models the stress‐strain problems of shallow shells as well as the problems on free vibrations. In accordance with the approach the initial system of partial differential equations is reduced to one‐dimensional problems by using approximation of the solution in terms of basic splines in one coordinate. The boundary‐value problems obtained and eigenvalue boundary‐value problems for systems of ordinary differential high‐order equations are solved by the stable numerical method of discrete ortogonalization.
Santrauka
Nagrinėjamas efektyvus lėkštųjų kevalų įtempių, deformacijų ir laisvųjų svyravimų nustatymo algoritmas, pagrįstas klasikiniais ir tobulesniais skaičiavimo modeliais. Algoritme dalinių diferencialinių lygčių sistema yra transformuojama į vienmatį uždavinį, sprendžiamą naudojant pagrindinių splainų aproksimaciją į vieną koordinatę. Gaunamas kraštinis uždavinys, kuris sprendžiamas kaip kraštinis savųjų reikšmių nustatymo uždavinys. Uždavinio sąlygas atitinka įprastų aukštesnės eilės diferencialinių lygčių sistema, kuriai spręsti taikomi patikimi diskrečiosios ortogonalizacijos skaitiniai metodai.
First Published Online: 14 Oct 2010
Reikšminiai žodžiai: lėkštieji kevalai, kintamasis storis, modifikuotoji ir klasikinė formuluotės, įtempių ir deformacijų būvis, laisvieji svyravimai
Characterization of ZrO2-Y2O3-MnO2 solid solutions as components for reversible solid oxide cells
Long-term degradation remains the main issue for the viability of reversible solid oxide
fuel/electrolysis cell (SOFC/SOEC) technology as practical hydrogen production and energy
storage systems. While some lifetime-limiting factors are common in both regimes, the major
specific degradation mechanism in SOEC regime relates to the delamination phenomena. The
experimental and modelling results suggest that high oxygen pressures can develop in electrolyte
near the anode/electrolyte interface resulting in formation of voids at the grain boundaries,
intergranular fractures, cracks in anode, and anode delamination; all factors contribute to
irreversible degradation. The objective of this work was the characterization of ZrO2-Y2O3-MnO2
solid solution in order to design a functional material with oxygen storage ability that may be used
as inclusion into electrolyte membrane or as interlayer between electrolyte and oxygen electrode
with the purpose of delay or prevent degradation and irreversible changes. ((ZrO2)1-x(Y2O3)x)1-y(MnOn)y ceramics (x = 0.02-0.05, y = 0.05-0.15) were prepared by solid-state
reaction route and sintered in air at 1400-1600°C. XRD results showed the formation of singlephase solid solutions with cubic fluorite-type structure for the compositions with x = 0.05, while the
ceramics with lower yttria content comprised 2 or more phases based on different polymorphs of
zirconia. The characterization of materials included microstructural studies (SEM/EDS), thermal
analysis (thermogravimetry, dilatometry), measurements of electrical conductivity as function of
temperature and oxygen partial pressure, and determination of ionic transference numbers by
modified e.m.f. method. Increasing Mn content was found to results in increase of the total
electrical conductivity and electronic contribution under oxidizing conditions, while ionic transport
dominates under reduced oxygen partial pressures. Electrical measurements showed also a slow
relaxation of electrical conductivity on redox cycling that possibly can be attributed to a variable
solubility of Mn cations in fluorite lattice.publishe
Perovskite-Like Pr(A)MnO3 (A = Ca, Sr) as Anode Materials for Solid Oxide Fuel-Assisted Electrolysis Cells
Long-term degradation remains the main issue for the viability of solid oxide electrolysis cell (SOEC) technology as a practical hydrogen production system. One of the main specific degradation mechanisms in SOECs relates to the delamination phenomena at or near the electrolyte/anode interface. The principle of so-called fuel-assisted electrolysis is to supply the carbon-containing species which can react with oxygen at the anode side thus bringing down the oxygen chemical potential at the electrolyte/anode interface and improving its stability. The present work is aimed at the characterization of PrMnO3-based perovskites for potential application as anodes in solid oxide fuel-assisted electrolysis cells.
Pr0.60-xA0.40MnO3±δ (A = Sr, Ca; x = 0 and 0.05) were synthesized by glycine-nitrate combustion technique. The characterization included XRD, SEM/EDS, XPS, dilatometry and thermogravimetry, measurements of electrical properties and oxygen permeability, and determination of oxygen nonstoichiometry. XRD analysis confirmed the formation of solid solutions with orthorhombic perovskite structure. The oxides exhibit negligible variations of oxygen content under oxidizing conditions while reducing p(O2) below 10-4 atm results in oxygen losses from the lattice and reduction of Mn cations. XPS results suggest that praseodymium remains in a 3+ oxidation state in the bulk of ceramics but may adopt a mixed 3+/4+ oxidation state at the surface. The lowp(O2) stability boundary of the perovskite phase at 800°C corresponds to ~10-17-10-16 atm; the stability domain is wider for Ca-substituted compositions and narrows with the introduction of A-site vacancies. Dilatometric studies confirmed good thermomechanical compatibility with common solid electrolytes under oxidizing conditions; however, reduction at operation temperatures (800°C) leads to undesirable chemical expansion. The electrical conductivity of Pr0.60-xA0.40MnO3±δ ceramics is p-type electronic and decreases with reducing p(O2) but still exceeds 40-50 S/cm under anticipated oxygen electrode operation conditions. The electrochemical activity of Pr0.6-xA0.4MnO3±δ electrodes was evaluated in contact with yttria-stabilized zirconia solid electrolyte as a function of relevant parameters (fabrication conditions, with and without buffer layers, with modifications via infiltration of praseodymia and/or doped ceria). The best performance was obtained for the cells with Pr0.55A0.40MnO3±δ electrodes (gadolinia-doped ceria buffer layers, PrOy load of ~33 wt.%) that showed anodic overpotential of around 50 mV under 500 mA/cm2 at 800°C in air.publishe
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