Papers by Author: Jorge R. Frade

Abstract: Dense ceramic anodes of perovskite-type La1-x-ySrxCo1-zAlzO3-δ ( x = 0.45-0.70; y = 0- 0.05; z = 0-0.20) and K2NiF4-type La2Ni1-xMexO4+δ (Me = Co, Cu; x = 0-0.20), synthesized by the glycine-nitrate technique, were assessed for oxygen evolution in alkaline media. The lowest overpotentials are observed for (La0.3Sr0.7)0.97CoO3-δ, which exhibits a significant oxygen deficiency in combination with high conductivity associated with the A-site cation nonstoichiometry compensation mechanism via Co4+ formation. Perovskite-type cobaltite anodes are essentially stable in alkaline solutions, whilst La2NiO4-based electrodes exhibit degradation at the potentials where the oxygen evolution occurs, probably due to the electrochemical oxygen intercalation in the lattice.

Abstract: Impedance spectroscopy was used to monitor the non-isothermal crystallisation of a phosphate phase in bulk samples of a glass with the molar composition 0.45SiO2–0.36MgO– 0.09K2O–0.1(3CaO⋅P2O5), intended for biomedical applications. The K+ alkali ions are probably the main charge carriers in glass of this composition [1]. Impedance spectra were obtained as a function of temperature, following non-isothermal heat treatments at 2°Cmin-1. It is shown that suitable analysis of impedance spectra can be used to evaluate the crystallization peak of a orthophosphate phase, Ca9MgK(PO4)7. Results obtained by this method are comparable to those obtained by differential thermal analysis (DTA) of the corresponding glass frits. Impedance spectroscopy is thus suitable to study the crystallization of bulk samples, and can be used both with variable temperature and under isothermal conditions.

Abstract: Impedance spectroscopy was used to measure the changes of electrical properties during the isothermal crystallisation of the phosphate phase in bulk samples of a glass with nominal molar composition (3CaO.P2O5)20(SiO2)35(MgO)38.354(K2O)6.646, developed for biomedical applications. Crystallization studies were performed in isothermal conditions at 800°C and 860°C, and were monitored by impedance spectroscopy. XRD shows that a stable orthophosphate phase, Ca9MgK(PO4)7 crystallizes at about 860°C, after formation of earlier phosphate precursors formed at lower temperatures, namely oxyapatite (Ca5(PO4)3O) and tricalcium phosphate (Ca3(PO4)2). The conductivity of bulk glass samples decreases at 800°C, as the phosphate precursors phases crystallize. The opposite trend is observed on crystallizing the orthophosphate phase, at 860°C.

Abstract: The dependence of electrical conductivity of pyrochlore type Yb2+xTi2-xO7-x/2 materials shows that these materials are predominantly ionic conductors and are potential solid electrolytes for high temperature electrochemical applications. However, a detailed impedance study as a function of temperature revealed that a proper assessment of these materials must take into account significant effects of microstructure, composition and thermal history. At intermediate temperatures, the impedance spectra can be described by a series association of well separated contributions, and these could be ascribed to the properties of grain interiors, internal interfaces, and external electrode/material interfaces. The bulk and grain boundary properties were thus assessed and related to changes in microstructure and composition. The firing conditions also exert significant effects on the transport properties, especially for compositions which might deviate from the expected single phase range.

Abstract: Due to relatively high oxygen permeability, lanthanum-strontium ferrite phases are of interest as ceramic membrane materials for the partial oxidation of natural gas. This work was focused on the study of perovskite-type ferrites co-doped with Sr2+ and Ce4+ or Nb5+, with particular emphasis on the ionic transport and thermodynamic stability limits at low oxygen chemical potentials. Dense membranes of La0.5-2xCexSr0.5+xFeO3-δ (x = 0 - 0.2) and La0.5-2ySr0.5+2yFe1-yNbyO3-δ (y = 0 - 0.1) were characterized employing X-ray diffraction (XRD), scanning electron microscopy (SEM), dilatometry, oxygen permeation and faradaic efficiency studies, and the measurements of total conductivity and Seebeck coefficient in the oxygen partial pressure range from 10-20 to 0.5 atm. The incorporation of Ce4+ or Nb5+ was found to decrease thermal expansion and electronic transport parameters, whereas the ionic conductivity behavior is complex, indicating the relevance of redox interactions of the variable-valence cations and the concentration of mobile oxygen vacancies.

Abstract: The p(O2)-T- diagrams of La2Ni1-xMxO4+ (M=Co and Cu, x= 0-0.20), determined by the coulometric titration technique at 923-1223 K in the oxygen partial pressure range 10-4 to 0.6 atm, can be adequately described by equilibrium processes of oxygen intercalation into the rock-salt type layers and hole localization on B-site cations forming 3+ oxidation states. For the hole activity, a non-ideal solution model taking into account the repulsion of p-type electronic charge carriers can be used. The electrostatic repulsion excludes occupation of nearest neighboring sites and leads to splitting of the energy levels for more distant sites. The affinity of Ni and Cu cations with respect to the hole localization is similar and cannot be statistically separated analyzing the oxygen nonstoichiometry data only. On the contrary, cobalt cations tend to remain in the trivalent state and Co3+ should be treated as a separate type of charged point defect. Oxygen vacancies formed in the perovskite-like layers due to intrinsic Frenkel disorder have no essential effect on the oxygen thermodynamics. As expected, the thermodynamic functions governing the intercalation-related processes are independent of defect concentrations.

Abstract: Oxygen transport properties of perovskite-type SrCe1-xYxO3-δ (x = 0.05–0.10), exhibiting protonic transport in hydrogen-containing reducing atmospheres and mixed oxygen-ionic and ptype electronic conductivity at oxygen partial pressures close to atmospheric, were studied at 973– 1223 K under oxidizing conditions. The oxygen transference numbers of SrCe(Y)O3-δ in air vary in the range 0.37–0.80, decreasing when temperature increases. The oxygen permeability is significantly affected by the hole conduction, which influences both bulk ambipolar conductivity and surface exchange kinetics. The average thermal expansion coefficients of SrCe1-xYxO3-δ ceramics, calculated from dilatometric data in air, are (11.1–11.3)×10-6 K-1 at 373–1373 K.

Abstract: Ceramic anodes, made of perovskite-type rare-earth and strontium cobaltites substituted in both sublattices, exhibit a high electrocatalytic activity towards oxygen evolution in alkaline media. This work analyzes the relationships between cation composition, defect structure, electronic conductivity and electrochemical performance for a wide group of perovskite-like cobaltites, including Ln1-yAyCoO3-δ (Ln= Pr, Nd, Sm; A= Sr, Ca; y= 0-0.4), La1-x-ySrxBiyCoO3-δ (x= 0-0.6, y= 0-0.1), La0.7-xSr0.3CoO3-δ (x= 0-0.10), Sr1-xBaxCoO3-δ (x= 0.1-0.2) and SrCo1-yMyO3-δ (M=Fe, Ni, Ti, Cu; y= 0.1-0.6). The materials were prepared by the standard ceramic technique and characterized employing XRD, TGA, iodometric titration, and total conductivity measurements. A relatively high electrochemical performance in alkaline solutions was observed for (La,Sr)CoO3-based compositions with a moderate A-site deficiency. For SrCoO3-based materials, an increase in the oxygen evolution rate was found when co-substituting cobalt with several transition metal cations, such as Fe3+/4+ and Cu2+/3+. The results show that, in general, the key composition-related factors influencing electrochemical activity in alkaline media include the oxygen vacancy concentration, the average positive charge density in the crystal lattice, and possible blocking of active sites on the electrode surface.

Abstract: Several alkaline baths based on different complexing agents were examined for iron electroplating. The resultant films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was shown that adherent and smooth iron coatings with uniform microstructure can be obtained using alkaline Fe (II) baths containing pyrophosphate and tartrate ions as complexing agents. The average grain size can be substantially decreased by glycine additions in the pyrophosphate bath. The faradaic efficiency in these electrolytes may achieve up to 40-50%. The tartrate-containing baths are characterized with a higher throwing power and an increased buffer capacity with respect to the pyrophosphate-based electrolytes. The resultant Fe coatings are single-phase, whilst substantial broadening of the XRD peaks indicates nano-scale grain size. The alkaline baths based on EDTA complexes of iron (III) give black dull iron deposits and are characterized by rather low cathodic current efficiencies, especially at low current densities.