CO oxidation on ceria studied by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy technique has been widely used to analyze the electrical properties of a large number of materials. In this study, the electrical properties of CeO2 pellets under CO oxidation conditions have been analyzed by electrochemical impedance spectroscopy. CeO2 pellets have been prepared by a conventional precipitation method and sintered at low temperature to satisfy a compromise between large surface area and a high relative density of the pellet. The electrical properties of CeO2 have been investigated under different atmosphere conditions such as N2, O2, CO, CO2, or selected combinations. The electrical sensitivity of CeO2 to the surrounding atmosphere allows to follow the catalytic reaction as a function of the CO : O2 ratio and temperature. The appropriate analysis of the electrical response by electro-chemical impedance spectroscopy could open a new insight to monitor the catalytic response and behavior of any catalyst

Efecto de la sustitución catiónica y aniónica en las transiciones de fase del LiNH4SO4

El ß LÍNH4SO4 (ß LAS) presenta dos transiciones de fase, una alrededor de 10 °C y la otra a 186 °C. La fase intermedia es ferroeléctrica. En este trabajo se estudia el efecto que produce la sustitución parcial del catión amonio por rubidio y del anión sulfato por seleniato en dichas transiciones de fase. Se establece la zona de existencia de las soluciones sólidas Li(NH4)-^_^Rb^S04 y LiNH4(SO^)^_^(Se04)^ en las que se mantiene la estructura del ß LAS. La presencia de aniones seleniato o de cationes rubidio afecta a las transiciones de fase de alta y baja temperatura, por lo que, tanto los tetraedros sulfato como el catión amonio intervienen en dichas transiciones. Hay un efecto cooperativo entre el desorden de los grupos sulfato y las distorsiones de los tetraedros amonio. El mecanismo que puede explicar estas transiciones es del tipo orden-desorden

Induced p-type semiconductivity in Mg-doped Nd2Zr2O7 pyrochlore system

Heterovalent B-site MgO substitution in the Nd2Zr2O7-system (Nd2Zr2−xMgxO7−x) has been explored. The pyrochlores were synthesized by a polymeric sol-gel method and characterized by X-ray diffraction (XRD), Raman spectroscopy and Scanning Electron Microscopy to determine structure, phase composition and microstructure. Impedance Spectroscopy (IS) was employed to study the electrical behavior of the ceramics over the ranges 200–800 °C and under pure N2 and O2. The XRD showed that the solid solution limit was x > 0.02 and all the materials show a cubic ̅ structure. The Raman results confirm the structural disorder created by the introduction of Mg2+ and the subsequent generation of oxygen vacancies. The IS data shows a dramatical increase of the oxide-ion conductivity when doping and that the conductivity depends strongly on the atmosphere, leading to p-type semiconductivity under pure O2 atmosphere. The present study highlights the use of heterovalent dopants to drastically increase the oxide-ion conductivity of pyrochlore-like materials

Structural and electrical properties of Zr-doped K0.48 Na 0.52 NbO 3 ceramics: “Hard” lead-free piezoelectric

The structural and electrical properties of K0.48Na0.52Nb1−xZrxO3−δ (x = 0–0.04) ceramics prepared by the conventional solid-state reaction method were studied. Pellets with composition x ≤ 0.03 sintered at 1125 °C for 2 h showed single-phase of potassium sodium niobate (KNN) perovskite structure. Based on X-ray diffraction and Raman results, a mixture of orthorhombic and monoclinic phases was observed in intermediate compositions. The addition of Zr improved the sinterability and the “hard” piezoelectric properties of KNN, increasing the Ec and Qm values. The composition with x = 0.03 presented the highest permittivity at room temperature, ɛr′ = 363 and the lowest dielectric losses, tan δ = 0.027. Moreover, it was the sample with the highest Qm and d33 values, with Qm = 1781 and d33 = 82 pC/N. It was therefore the best compositions to obtain a “hard” piezoelectric material based on Zr-doped KNN, which makes it promising candidate for use as “hard” lead-free piezoelectric material for high power applications

Tailoring the microstructure by a proper electric current control in flash sintering: The case of barium titanate

Flash sintering is arousing growing interest because high-density ceramics can be obtained at lower temperatures and shorter dwell times than conventional sintering. However, not only temperature and dwell times should be controlled during flash sintering but also parameters such as the electric field and electric current should be considered. Controlling all the parameters during the processing allows comprehensive control of the microstructure and, consequently, functional properties can be improved. In this work, it is evidenced that an exhaustive control of the flash electric current is a crucial factor for tailoring the microstructure of BaTiO3 ceramics. The results reveal that the most suitable way to control the sintering process is by using nonlinear current profiles because better densification and improved grain growth is achieved. Although the results focus on BaTiO3, this work offers a new pathway to tailor the microstructure of flash sintered ceramics, which may be extended to other materials

Particle size effect on the microstructure and the aging process of flash-sintered barium titanate from micro and nanopowders

Flash sintering is a novel sintering technique that allows high-density ceramics to be obtained at lowtemperatures and using short dwell times, thus providing an energy-efficient alternative to conventionalsintering. The microstructure of flash-sintered samples can be fine-tuned by a proper control ofelectrical parameters such as current density, electric field, and current profile, yielding significantimprovements of functional properties. The starting powder should also be carefully selected sincebetter sintering results are reported for smaller green grain sizes. However, this work evidences timeevolution of electrical properties of flash-sintered BaTiO3 ceramics from submicron powders. Theresults reveal that these transformations greatly depend on powder grain size and can be furtheradjusted with an adequate selection of electric power profiles. This work provides new insights intoongoing phenomena during field-assisted sintering, such as grain growth and defect formationdynamics. Although the results focus on BaTiO3, it offers a new pathway to tailor the microstructure offlash-sintered ceramics, which may be extended to other electronic materials

Growth and physical properties of highly oriented La-doped (K,Na)NbO3 ferroelectric thin films

Lead-free (K,Na)NbO3 (KNN) and La doped (K,Na)NbO3 (KNN-La) thin films were grown on SrTiO3 substrates by chemical solution deposition method. The effect of adding different amounts of Na and K excess (0-20 mol%) has been investigated. The results confirm the necessity to add 20 mol% excess amounts of Na and K precursor solutions in order to avoid the formation of the secondary phase, K4Nb6O17, as is confirmed by X-ray diffraction and Raman spectroscopy. Moreover, when adding a 20 mol% of alkaline metal excess the thin films are highly textured with out-of-plane preferential orientation in the [100] direction following the [100] orientation of the substrate. Doping with lanthanum resulted in the decrease in the leakage current density at low electric field, and an increase in the dielectric permittivity across all the temperatures range (80-380 K). Although the (100)-oriented KNN and KNN-La films exhibited rounded hysteresis loops, at low temperature the films showed the typical ferroelectric hysteresis loops

Improving the functional properties of (K0.5Na0.5)NbO3 piezoceramics by acceptor doping

ZrO2 and TiO2 modified lead-free (K0.5Na0.5)NbO3 (KNN) piezoelectric ceramics are prepared by conventional solid-state reaction. The effect of acceptor doping on structural and functional properties are investigated. A decrease in the Curie temperature and an increase in the dielectric constant values are observed when doping. More interestingly, an increase in the coercive field Ec and remanent polarization Pr is observed. The piezoelectric properties are greatly increased when doping with small concentrations dopants. ZrO2 doped ceramic exhibits good piezoelectric properties with piezoelectric coefficient d33=134 pC/N and electromechanical coupling factor kp=35%. It is verified that nonlinearity is significantly reduced. Thus, the creation of complex defects capable of pinning the domain wall motion is enhanced with doping, probably due to by the formation of oxygen vacancies. These results strongly suggest that compositional engineering using low concentrations of acceptor doping is a good means of improving the functional properties of KNN lead-free piezoceramic system

X-ray diffraction, thermal analysis, and Raman scattering study of K2BeF4 and comparation to other member of the (beta)-K2SO4 family with ferroelectric -paraelectric transition

Thermal analysis, powder diffraction, and Raman scattering as a function of the temperature were carried out on K2BeF4. Moreover, the crystal structure was determined at 293 K from powder diffraction. The compound shows a transition from Pna21 to Pnam space group at 921 K with a transition enthalpy of 5 kJ/mol. The transition is assumed to be first order because the compound shows metastability. Structurally and spectroscopically the transition is similar to those observed in (NH4)2SO4, which suggests that the low-temperature phase is ferroelectric. In order to confirm it, the spontaneous polarization has been computed using an ionic model

A joint experimental and theoretical study on the electronic structure and photoluminescence properties of Al2(WO4)3 powders

In this paper, aluminum tungstate Al2(WO4)3 powders were synthesized using the co-precipitation method at room temperature and then submitted to heat treatment processes at different temperatures (100, 200, 400, 800, and 1000 °C) for 2 h. The structure and morphology of the powders were characterized by means of X-ray diffraction (XRD), Rietveld refinement data, and field emission scanning electron microscopy (FE-SEM) images. Their optical properties were examined with ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy and photoluminescence (PL) measurements. XRD patterns and Rietveld refinement data showed that Al2(WO4)3 powders heat treated at 1000 °C for 2 h have a orthorhombic structure with a space group (Pnca) without the presence of deleterious phases. FE-SEM images revealed that these powders are formed by the aggregation of several nanoparticles leading to the growth of microparticles with irregular morphologies and an agglomerated nature. UV-vis spectra indicated that optical band gap energy increased from 3.16 to 3.48 eV) as the processing temperature rose, which was in turn associated with a reduction in intermediary energy levels. First-principle calculations were performed in order to understand the behavior of the PL properties using density functional theory at the B3LYP calculation level on periodic model systems and indicate the presence of stable electronic excited states (singlet). The analyses of the band structures and density of states at both ground and first excited electronic states provide insight into the main features, based on structural and electronic order-disorder effects in octahedral [AlO6] clusters and tetrahedral [WO4] clusters, as constituent building units of this material