Effect Of Cation Nonstoichiometry Research Articles

Effect of Cation Nonstoichiometry on Hydration and Charge Transport Processes in Yb-Doped SrZrO3 Perovskite-Type Proton Conductor for Ceramic Electrochemical Cells

The effect of Sr deficiency on the hydration process and ionic and electronic conductivity of Yb-doped SrZrO3 proton conductors with a perovskite-type structure was investigated. Dense SrxZr0.95Yb0.05O3-δ (x = 0.94–1.00) ceramics were prepared using solution combustion synthesis. Thermogravimetry and Raman spectroscopy methods were used to determine the concentration of bulk protonic species. Sr deficiency was found to enhance the hydration ability of the zirconate; however, lowering of Sr content to x = 0.94 deteriorated the proton uptake. The conductivity of the SrxZr0.95Yb0.05O3-δ series depending on the oxygen partial pressure at different humidities was studied by the four-probe direct current technique. Sr-deficient ceramics with x = 0.96 and 0.98 were shown to become purely protonic conductors in humid atmospheres at a temperature close to 500 °C. The ionic conductivity reaches its highest value at a Sr content of x = 0.98 (2 × 10−4 S cm−1 at 500 °C and pH2O = 3.17 kPa). The hydration behavior and transport properties of SrxZr0.95Yb0.05O3-δ are discussed in terms of the defect chemistry model that assumes the distribution of Yb ions over Sr and Zr sites at a large Sr deficiency.

High surface reactivity of La/Sr–Co perovskite based cathode with cation nonstoichiometry

Abstract The effect of cation nonstoichiometry on the surface reactivity of mixed ionic and electronic conducting (La,Sr)CoO 3 (LSC113)-based bi-layer thin film cathodes is investigated by ac impedance method as a function of thin film surface composition. The bi-layer thin film cathodes are fabricated by an RF-sputtering method. The results of X-ray diffraction and transmission electron microscopy observation prove that LSC thin films of A-site and B-site deficient composition crystallize in a perovskite structure, which is attained by retained supersaturated metastability from the fabrication process. Both A-site and B-site deficient LSC113 phases have surface reaction performance significantly higher than the cation-stoichiometric LSC113 phase, which is attributed to the increase of oxygen vacancy in charge-compensation for cation vacancy. The “cation nonstoichiometry” is one of high surface reaction models to improve solid oxide fuel cells' performance.

Effect of Cation Nonstoichiometry on Surface Reactivity of LaCoO3-Based Cathode

The effect of cation nonstoichiometry on the enhancement of surface reaction kinetics of oxide cathode has been studied with bi-layer (La,Sr)CoO3 and LaCoO3 dense thin film electrodes fabricated by an RF-sputtering method for improving the solid oxide fuel cells’ performance. The bi-layer electrodes with cation (A-site or B-site) deficiency at surface show higher surface reaction kinetics than the cation-stoichiometric electrode, which is related to the increase of oxygen vacancy induced by cation vacancy. Additionally the P O2 dependence of the interfacial conductivity is analyzed based on a defect chemical model in consideration of “cation deficiency”, which supports the effect of cation nonstoichiometry.

The effect of cation non-stoichiometry in LaNbO4 materials

The effect of cation non-stoichiometry in LaNbO4 was investigated by impregnating nano-crystalline LaNbO4 with small amounts of La3+, Nb5+ and Ca2+ oxide precursors. The sintering properties of the modified LaNbO4 powders were investigated by dilatometry, and the microstructure and phase composition were studied by electron microscopy and X-ray diffraction. The electrical properties of the materials were studied by 4-point DC-conductivity and 2-point 4-wire AC-conductivity at elevated temperatures in controlled atmosphere. Minor variations in the cation stoichiometry were shown to have a pronounced effect on both the sintering properties as well as the electrical conductivity. Addition of CaO, which introduced secondary phases above 0.25mol% CaO, increased the sintering temperature and improved the conductivity of the materials. La2O3- and Nb2O5-excess materials did not show large variation in the electrical conductivity relative to pure LaNbO4, while the sintering properties were strongly affected by the nominal La/Nb ratio in LaNbO4. The present findings demonstrate the sensitivity of cation non-stoichiometry in materials with limited solid solubility.

Effect of cation nonstoichiometry on Gd123 thin films

We investigated the cation-nonstoichiometric Gd123 thin films grown by a conventional pulsed laser deposition (PLD) technique, using a Raman spectrometer. Although XRD patterns show that all the thin films are completely oriented to c-axis and have no impurity phase, the superconducting property is dependent on the target composition. Raman spectra for the Gd123 films show the phonon peaks peculiar to 123-structure and some of them also include additional peaks. We discuss that the difference of the phononic Raman spectra is closely connected with the origin of the suppression of T c and J c.

The effect of cation nonstoichiometry on the electrical conductivity of acceptor-doped CaZrO 3

The electrical properties of acceptor-doped Ca 1− x Zr 0.99M 0.01O 3− δ (M = Mg 2+, In 3+) 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.

The effect of cation nonstoichiometry on the electrical conductivity of CaZrO 3

The electrical conductivities of Ca 1− x ZrO 3− δ systems were measured by using impedance spectroscopy as a function of Ca/Zr ratio (0 ≤ x ≤ 0.1) and temperature (550–1100 °C). Stoichiometric CaZrO 3 revealed poor sinterability due possibly to the free CaO observed at grain junctions. With decreasing Ca/Zr ratio, the free CaO disappeared and the sintered density increased. The total electrical conductivity rapidly decreased, ∼100 times at 1100 °C, when the Ca content decreased below 0.98. The impedance measurement revealed that the decrease is due mostly to the increase in the grain boundary resistivity. The activation energy of grain boundary resistivity also rapidly increased with decreasing Ca/Zr ratio. In other words, the grain boundary resistivity was very sensitive to Ca/Zr ratio.

Sintering Behavior, Microstructure, and Phase Composition of Sr(Fe,Co)O3−δ Ceramics

SrFe1−xCoxO3−δ (x= 0, 0.33) ceramics have been prepared in the temperature interval 900–1350°C in air, nitrogen, and oxygen atmosphere. The effect of cation nonstoichiometry on densification behavior, microstructure, and phase composition has been investigated. Densification of Sr‐deficient SrFeO3−δ initiates at a lower temperature than near stoichiometric SrFeO3−δ probably due to enhanced diffusion of Sr. However, for Sr‐deficient samples Sr4Fe6O13 is formed above ∼775°C, causing a significant decrease in the sintering rate. It is therefore necessary to avoid Sr deficiency to obtain dense SrFeO3−δ ceramics. The densification rate was significantly increased by Co substitution. Dense ceramics (>95% of theoretical density) of Sr‐excess and Co‐substituted SrFeO3−δ were obtained by sintering in the temperature region 1000–1200°C. The grain size increases by increasing temperature, decreasing partial pressure of oxygen and Co substitution. Exaggerated grain growth in the Co‐substituted material occurred at high temperatures. Sintering above 1200°C caused all materials, apart from Co‐substituted SrFeO3−δ in oxygen, to swell and develop a porous microstructure. The swelling mechanism was related to heterogeneous phase equilibria, which is reductive in nature and leads to evolution of oxygen gas. The phase equilibria are governed by the cation nonstoichiometry of the materials. The present findings demonstrate the importance of controlling the cation stoichiometry of ternary transition‐metal oxides to achieve ceramics with the desired homogeneous microstructure. The mixed valence state of the transition metal may lead to both swelling during sintering and cracking during cooling due to reduction/oxidation during heating/cooling.

Sintering of LaFeO3 Ceramics

The sintering of LaFeO3 has been studied in the temperature interval 1100–1600°C in air. The effect of cation nonstoichiometry on densification, microstructure, and phase composition is emphasized. La2O3 was observed to inhibit both sintering and grain growth. In Fe‐excess materials, exaggerated grain growth occurred, particularly above 1430°C, where a liquid phase was formed. Postsintering swelling was observed in Fe‐excess materials above 1430°C. The swelling mechanism is related to phase equilibria, which are reductive in nature and lead to the evolution of oxygen gas. The density in La‐excess materials remains high up to 1600°C, but the ceramics might disintegrate in air.