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Synthesis, sintering and electrical properties of P-doped Bi₄V₂O₁₁ ceramics

Partial substitution of vanadium by phosphorus in Bi4V2O11, compound belonging to the Aurivillius family, leads to the formation of Bi4V2−xPxO11 solid solution with an orthorhombic symmetry and does not stabilize the γ-high temperature form down to room temperature. X-ray diffraction, density measurements, thermal analysis and IR spectroscopy have been used to confirm the formation of the solid solution. The influence of sintering temperature on the microstructure and the grains size of the samples with different compositions was investigated by the scanning electron microscopy (SEM). The evolution of the electrical conductivity with phosphorus ratio has been investigated by impedance spectroscopy and correlated to the structural transformations

Effect of Niobium doping on structural, thermal, sintering and electrical properties of Bi4V1.8Cu0.2O10.7

Doping Bi4V1.8Cu0.2O10.7 with niobium has led to the formation of the Bi4V1.8Cu0.2−xNbxO10.7+3x/2 solid solution. X-ray diffraction and thermal analysis have shown that only the compound with x=0.05 presents a tetragonal symmetry with a γ′ polymorph while the other compositions are of β polymorph. The influence of sintering temperature on the microstructure of the samples was investigated by the scanning electron microscopy (SEM). The ceramics sintered at temperatures higher than 820 °C present micro-craks. The evolution of the electrical conductivity with temperature and the degree of substitution has been investigated by impedance spectroscopy. Among all compositions studied the sample with x=0.05 presents the highest value of the conductivity

Oxygen reduction reaction in Pr2NiO4+δ/Ce0.9Gd0.1O1.95 and La0.6Sr0.4Co0.2Fe0.8O3−δ /La0.8Sr0.2Ga0.8Mg0.2O2.80 half cells: an electrochemical study

The electrochemical properties of La0.6Sr0.4Co0.2Fe0.8O3−δ and Pr2NiO4+δ electrodes screen-printed on La0.8Sr0.2Ga0.8Mg0.2O2.8 and Ce0.9Gd0.1O1.95, respectively, have been investigated by electrochemical impedance spectroscopy (EIS). A study of the heat treatment of La0.6Sr0.4Co0.2Fe0.8O3−δ material used as oxygen electrode associated to La0.8Sr0.2Ga0.8Mg0.2O2.8 electrolyte was performed. The La0.6Sr0.4Co0.2Fe0.8O3−δ porous electrode sintered at 900 °C for 1 h in air exhibits the lowest cathodic polarization resistance; i.e. R P = 0.12 Ω cm2 at 600 °C. The SEM images show that the La0.6Sr0.4Co0.2Fe0.8O3−δ electrode structure is highly porous, facilitating the gas diffusion and maximizing the number of active sites for the oxygen reduction reaction (ORR). Furthermore, it forms good contact with the electrolyte after this heat treatment. In order to characterize the oxygen electrode reaction of La0.6Sr0.4Co0.2Fe0.8O3−δ and Pr2NiO4+δ , the electrochemical impedance spectroscopy (EIS) measurements were performed at temperatures between 400 and 700 °C and at different oxygen partial pressures (pO2) ranging in between 10−3 and 0.21 atm. Analysis of the impedance data revealed that there are two different processes involved in the cathode reaction. The first process in the medium-frequency range is assigned to the oxygen surface exchange reaction at the electrode/gas interface and possibly to the ionic diffusion in the material for La0.6Sr0.4Co0.2Fe0.8O3−δ and to the dissociation of the adsorbed molecular oxygen for Pr2NiO4+δ . The second one at low frequency is associated to the gas phase diffusion for both cathodes. The exchange current density, i 0, allows evaluating the electrocatalytic activity of the cathode materials. The La0.6Sr0.4Co0.2Fe0.8O3−δ /La0.8Sr0.2Ga0.8Mg0.2O2.8 couple shows the largest i 0 value, reaching 432 mA cm−2 at 700 °C and suggesting a high electrochemical activity for the O2 reduction reaction

Study on structural, thermal, sintering and conductivity of Cu-Co doubly substituted Bi4V2O11

Doubly substitution of vanadium by Cu and Co in the limit of 10% in Bi4V2O11, has led to the formation of the Bi4V1.8Cu0.2−xCoxO10.7 solid solution. X-ray diffraction shows that all the compositions present a tetragonal symmetry. The thermal analysis has revealed that the polymorph γ' phase, which is formed by a partial ordering of oxygen ions in the γ high temperature form, is stabilized at room temperature. The influence of sintering temperature on the microstructure of the samples was investigated by the scanning electron microscopy (SEM). The ceramics sintered at 820 °C for more than 3 hours present micro-craks. The evolution of the electrical conductivity with temperature and the degree of substitution has been investigated by impedance spectroscopy. The sample with x=0.1 presents the highest value of the conductivity ≈4.6×10−2 S·cm−1 at 600 °C