ABO3 Structure Research Articles

Composition Dependence of Crystal Structure, Electronic Structure, Ferroelectric and Piezoelectric Properties of (K,Na,Li)NbO3

INTRODUCTION Pb(Zr,Ti)O3-based piezoelectric ceramics have been widely used in electronic devices because of their excellent piezoelectric properties. Recently, owing to growing concern about environmental problems, the development of alternative piezoelectric ceramics is highly expected because Pb(Zr,Ti)O3 (PZT) contain a toxic element. (K,Na)NbO3(KNN) is one of the promising candidates.1) Superior characteristics of PZT is considered to be due to the morphotropic boundary (MPB), and KNN is also known to have a phase boundary similar to MPB. However, the characteristics and the crystal structure in the phase boundary are not well understood at the moment. It was also reported that substituting Li to the A-site of the perovskite structure ABO3 induces phase transition to tetragonal the KNN.2) From such backgrounds, we synthesized (K0.45Na0.55Lix)NbO3 and tried to reveal the composition dependence of the properties, the crystal and electronic structures in the phase boundary. In addition, we replaced the B-site by Ta, V and Mo partially, and then examined effects of the substitutions on the properties. EXPERIMENTAL Starting materials were wet-mixed in appropriate proportions, and then calcined in air for 2 h at 900 ℃. Thereafter, the samples were ground by a ball mill, and then sintered by the spark plasma sintering (SPS) method under a uniaxial pressing at 50 MPa for 5 min at 1050 ℃ in vacuum. For the obtained products, an recovery annealing treatment was performed at 900 ℃ for 4 h in O2. The identification of the phases and the evaluation of the lattice parameters were performed by XRD. Metal compositions of the ceramics were measured by ICP. Relative densities of the pellets were estimated by the Archimedes method. P-E hysteresis loop measurements were carried out at various frequencies using a TF-2000FE device (aixACCT) with the virtual ground mode. The surface and cross-section morphologies of the samples were observed by SEM. The temperature dependences of the dielectric permittivity (ε s ) and dielectric losses (tanδ) of the samples were measured by a LCR meter (HP-4284A). For average crystal structure analysis, we performed Rietveld analysis (RIETAN-FP) using synchrotron X-ray diffraction patterns.(BL19B2, spring-8) The electron densities were determined by the maximum entropy method (MEM) using the Dysnomia program. RESULTS AND DISCUSSION Figure 1 shows XRD patterns of (K0.45Na0.55Lix)NbO3 (x=0~0.07). These samples could be identified as the perovskite structures with the tetragonal (P4mm) and/or the orthorhombic (Amm2) symmetries. According to the crystal structure analysis by Rietveld method, in the composition of x=0.04~0.07, good fit can be obtained in a two-phase analysis of Amm2 and P4mm. These results indicate that there is a phase boundary around the composition range. It was found that all of the sample were sufficiently dense by the Archimedes method and the SEM observation. Measurements of P-Ehysteresis loop, the relative dielectric constant and dielectric loss demonstrated that the properties were improved in the x = 0.05~ 0.07. Such a change of the properties were examined from the electronic structure analysis by MEM and crystal structure analysis by Rietveld method. Further, we also investigated B-site substitution with a phase boundary, and evaluated their effect of the ferroelectric properties. REFERENCE 1)Y. Saito, et al. Nature, 432,84 (2004). 2) Hongliang Du, et al. Appl. Phys. Lett. 91, 182909 (2007). Figure 1

Catalytic activity of gold-perovskite catalysts in the oxidation of carbon monoxide

Perovskites (ABO3 structures), which can be manipulated by partial substitution, are reported to be active supports for CO oxidation, but only at high temperatures, with no activity being shown for temperatures below 200 °C. In this study, these perovskites were investigated at low temperatures (below 100 °C) with improved activity found upon gold deposition. The presence of gold nanoparticles therefore significantly enhanced the catalytic activity, while the support itself was suspected to be involved in the reaction mechanism. A series of perovskites of the type ABO3 (LaMnO3, LaFeO3, LaCoO3, and LaCuO3) were prepared using the citrate method, while the gold was deposited on them using the deposition-precipitation method. The supports were calcined at different temperatures for optimization. With the support calcined at 800 °C, the best catalyst was 1 wt% Au supported on LaFeO3. Calcium-doping of this system showed decreased surface area, poorer crystallinity, and a drop in catalytic activity relative to the Au-LaFeO3. In addition, Au-LaFeO3 showed online stability over 21 h. Calcining the support improved the incorporation of gold nanoparticles into the perovskite lattice, resulting in superior catalytic activity. Nevertheless, at higher calcination temperatures, the catalytic activity of Au-CaTiO3 was depressed while that of Au-LaFeO3 was enhanced. XPS revealed that in the active catalysts, both cationic and metallic gold coexisted, while in the inactive catalysts, the gold existed predominantly either as cationic or metallic gold.

Surface Tuning of La0.5Sr0.5CoO3 Perovskite Catalysts by Acetic Acid for NOx Storage and Reduction.

Selective dissolution of perovskite A site (A of ABO3 structure) was performed on the La1-xSrxCoO3 catalysts for the NOx storage and reduction (NSR) reaction. The surface area of the catalysts were enhanced using dilute HNO3 impregnation to dissolve Sr. Inactive SrCO3 was removed effectively within 6 h, and the catalyst preserved the perovskite framework after 24 h of treatment. The tuned catalysts exhibited higher NSR performance (both NOx storage and NO-to-NO2 oxidation) under lean-burn and fuel-rich cycles at 250 °C. Large amounts of NOx adsorption were due to the increase of nitrate/nitrite species bonding to the A site and the growth of newly formed monodentate nitrate species. Nitrate species were stored stably on the partial exposed Sr(2+) cations. These exposed Sr(2+) cations played an important role on the NOx reduction by C3H6. High NO-to-NO2 oxidation ability was due to the generation of oxygen defects and Co(2+)-Co(3+) redox couples, which resulted from B-site exsolution induced by A-site dissolution. Hence, our method is facile to modify the surface structures of perovskite catalysts and provides a new strategy to obtain highly active catalysts for the NSR reaction.

Perovskite-Based Oxide Catalysts to Improve the Oxygen Evolution and Reduction Activity

Unitized regenerative fuel cells (URFCs), which include water electrolysis and fuel cells, have received many attentions due to the production of hydrogen gas and electricity in one electrochemical device. Water electrolysis is one of the most pollution-free and renewable ways to produce hydrogen. However, the slow kinetics of oxygen evolution reactions (OERs) in the URFC anode are limited by high polarization resistance for oxidation of water. Likewise, oxygen reduction reactions (ORRs) for the fuel cell cathode also show slow rates due to the multi-step electrochemical reactions demands of 4-electrons in reactions, limiting the overall reaction rate of the fuel cells. Up to date, the noble metal-based catalysts, such as carbon-supported iridium, ruthenium, platinum and alloys, have still used as OER and ORR catalysts. The use of precious metal-based materials is not suitable for the commercialization of the electrochemical cells. In order to replace these noble metal catalysts, studies of perovskite-based oxide catalysts for URFCs have carried out intensively these days. In particular, various dopants in the perovskite materials may help to improve the electrochemical properties significantly. Herein, the perovskite-based catalysts are synthesized by the glycine-nitrate combustion method with various A site material (Nd, Sm and Gd) in ABO3 structure. Then, the ashes are calcined at 900 oC for 4h in electric furnace at a heating rate of 5 oC min-1. The physicochemical properties of the final products are characterized by several analytic tools such as X-ray diffraction, scanning electron microscope, transmission electron microscope and BET. For electrochemical analysis, a rotating disk electrode system is used with an Hg/HgO and a Pt wire and a 0.1 M KOH solution for reference, counter and electrolyte, respectively. The pre-conditioning of catalysts is carried out by cyclic voltammetry (CV) at a scan rate of 100 mV s-1 for 50 cycles with nitrogen-purged electrolyte. The OER (1.2 V ~ 1.7 V) and ORR (0.05 V ~ 1.2 V) activities are measured by linear sweep voltammetry (LSV) at a scan rate of 5 mV s-1 and for the performance and long-term stability. A. Grimaud, K.J. May, C.E. Carlton, Y.-L. Lee, M. Risch, W.T. Hong, J. Zhou and Y. Shao-Horn, Nat. Comm., 4, 2439 (2013).Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Nature Materials, 10, 780 (2011).J. Kim, X. Yin, K.-C. Tsao, S. Fang and H. Yang, J. Am. Chem. Soc., 136, 14646 (2014).J.-I. Jung, H.Y. Jeong, M.G. Kim, G. Nam, J. Park and J. Cho, Adv. Mater., 27, 266 (2015).J. Suntivich, K.J. May, H.A. Gasteiger, J.B. Goodenough, Y. Shao-Horn, Science, 334, 1383 (2011).J. Suntivich, H. A. Gasteiger, N. Yabuuchi, Y. Shao-Horn, Nature Chemistry , 3, 546 (2011).T. Reier, M. Oezaslan, P.Strasser, ACS Catal., 2, 1765 (2012).I. C. Man, H. Y. Su, F. C. Vallejo, H. A. Hansen, J. I. Martínez, N. G. Inoglu, J. Kitchin, T. F. Jaramillo, J. K. Nørskov, J. Rossmeisl, ChemCatChem, 3, 1159 (2011)

The phase diagram of K0.5Na0.5NbO3–Bi1/2Na1/2TiO3

The phase diagram of the lead-free piezoelectric (1 − x)K0.5Na0.5NbO3–xBi1/2Na1/2TiO3 system has been studied by high-resolution synchrotron powder diffraction, neutron powder diffraction and selected area electron diffraction. The two lead-free piezoelectric materials K0.5Na0.5NbO3 and Bi1/2Na1/2TiO3 form an infinite substitution solid solution. The orthorhombic (O), monoclinic (M), tetragonal (T) and rhombohedral (R) phases and the phase coexistence of M (Pm) + T (P4mm) for 0.02 < x ≤ 0.14, T (P4bm) + pseudocubic (Pm{\overline 3}m) for 0.14 < x ≤ 0.87 and T (P4bm) + R (R3c) for 0.87 < x ≤ 0.96 have been investigated at room temperature, with a subtle change in the structure observed. The oxygen octahedral tilt system has been mapped as a function of composition and temperature. The results indicate that K0.5Na0.5NbO3–Bi1/2Na1/2TiO3 does not display a morphotropic phase boundary like lead zirconate titanate, and that the most significant structural changes as a function of composition occur near x = 0.14 and x = 0.87 as a result of ionic disorder at the A and B sites in the perovskite ABO3 structure at room temperature.

Thin perovskite-type ferromagnetic film (La,Sr)CoO3

Properties of materials having a perovskite-type oxide crystal structure ABO3 can easily be modified by means of partial substitution of cations located at A or B sites. Nonstoichiometric lanthanum-strontium cobalt oxides (La,Sr)CoO3 also referred to as LSCO exhibit electrical conductivity and ferromagnetic properties. Perovskite-type compounds LaCoO3 and SrCoO3 can form solid solutions in every ratios to each other. Notation (La,Sr)CoO3, or La(1-x)SrxCoO3 means that some lanthanum ions are substituted with the strontium ones at A sites of crystal cell. This paper describes a sol-gel manufacturing process of LSCO thin films, presents measurements of their resistivity vs. composition, shows thermal coefficient of selected composition resistivity as well as its magnetoresistive properties.

Influence of Aliovalent Doping on Dielectric Properties of Ba0.6Sr0.4TiO3 Thin Film for Voltage Tunable Applications

Ba0.6Sr0.4TiO3 thin film with aliovalent doping of acceptor nature at ‘A’ lattice sites of ABO3 structure were prepared by a metal organic solution deposition (MOSD) technique on low loss quartz substrate. Oxide thin film having perovskite structure, namely, lanthanum nicklate (LaNiO3), also deposited using chemical solution deposition technique (CSD), was used as conductive bottom electrode. Solution properties and deposition process parameters were optimized for crack-free, uniform and smooth, pure and doped BST films on LNO electrode. The structural and microstructural analyses of the film were done using X-ray diffraction (XRD) and atomic force microscopy (AFM). Dielectric properties of the deposited film were studied using C-V and C-F electrical characterizations. The results indicate high tunability of 46%, very low loss tangent of 0.0065 with corresponding figure of merits of 71 for 2% K doped BST films at 1 MHz. The considerable improvement of dielectric properties of doped films compare to pure BST film is due to combined effect of perovskite lattice matched conductive bottom electrode and optimum doping level. These films had moderate dielectric constant, very low loss with good tunability which is suitable for tunable microwave applications.

Effect of Bismuth Doping on the Dielectric and Piezoelectric Properties of Ba1-xBixTiO3 Lead-Free Ceramics

In this paper we report the dielectric and piezoelectric properties of Bi modified Ba1-xBixTiO3 lead-free ceramics. These ceramic samples were synthesized by conventional solid state reaction method. The XRD spectra recorded at room temperature (RT) suggested decrease in the tetragonality (c/a) for Bi doped derivatives with the presence of minor rhombohedral phase for heavily doped specimens. The grain boundary pinning occurring in slightly Bi-doped derivative (x = 0.025) was found to reduce the grain size to submicron regime. However, for higher Bi doping the presence of liquid phase led to the grain coarsening. The doped derivatives exhibited shifting of the paraelectric-ferroelectric (PE-FE) phase transition towards RT with rather diffused Curie peak. The substitution of Bi3+ (with different ionic radii and electronegativity) on A-site of ABO3 structure resulted in regions with modified polar structure giving rise to diffused PE-FE phase transition. The specimen with x = 0.025 exhibited enhanced planar electro-mechanical coupling constant (kp) ∼ 25% with longitudinal piezoelectric constant (d33) of ∼112 pC/N. The sample with x = 0.025 also exhibited 0.085% electric-field induced strain along with the remnant polarization (2Pr) of ∼15 μC/cm2 and coercive field (2Ec) of ∼22 kV/cm. The increased Ec with reasonable Pr is an important material characteristics for the stable functionality of the memory devices.

Dielectric and Polarization Properties of Na0.5Bi0.5TiO3-BaTiO3 Solid Solutions with Na and K Niobates

The role of substitution in A- and B-sites of perovskite ABO3 structure in phase transitions is evaluated, comparing (1-y)(0.939Na0.5Bi0.5TiO3-0.061BaTiO3)-yKxNa1-xNbO3 solid solutions with x = 0.0, x = 0.5 and x = 1.0. The influence of deviation from a stoichiometric Na/Bi ratio is also studied. The influence of K0.5Na0.5NbO3, NaNbO3 or KNbO3 reduces mainly to suppressing of ferroelectric properties, like in other perovskite ferroelectrics where Ti is replaced by Nb, and development of the relaxor state. Besides the frequency-dependent dielectric permittivity, the relaxor state in the studied compositions is characterized also by a temperature dependence of critical electric fields, corresponding to phase transitions between relaxor and ferroelectric states. The influence of Nb in the above mentioned compositions is compared with the one of compositions far from the morphotropic phase boundary by a study of solid solutions (1-y)(0.80Na0.5Bi0.5TiO3-0.20BaTiO3)-yKxNa1-xNbO3.

Catalytic Activity of Double-Perovskite Type BalnMn2O5 for Oxygen Reduction Reaction Using in Rechargeable Zinc-Air Batteries

There have been increased demands for the development of high energy efficient and affordable energy storage devices as the alternatives to lithium-ion secondary batteries for electrical vehicles, portable electronic devices and so on. Recently, rechargeable zinc-air batteries have attracted much attention from the point of view of high-capacity and safety batteries. To realize the rechargeable zinc-air batteries, active cathode catalysts for oxygen reduction reaction and evolution reaction (ORR and OER) play important roles in the overall battery performance because of the majority of energy losses occurring at the cathode reactions. It is well known that the transition-metal oxide family with a perovskite-type ABO3 structure is one of the promising catalysts for ORR and OER. Especially, manganese oxides such as LaMnO3+δ and La1-xCaxMnO3 showed high ORR activities. Some researchers reported that the ORR activity for oxide catalysts primarily correlates to the electronic states and valence of Mn. However, a clear strategy has not been revealed to prepare active catalysts for ORR. On the other hand, more recently, in the case of high-temperature oxygen intake/release reactions of the double-perovskite type BaLnMn2O5+δ (Ln = Y, Gd, Nd and La; Fig. 1a), Motohashi et al. reported that the redox characteristics could be controlled by the radius of Ln ions, even though the valence of Mn remained unchanged [1]. Therefore, in this study, we focused on the ORR activity of the double-perovskite type BaLnMn2O5+δ (Ln: Y, Gd, Nd and La). BaLnMn2O5+δ (Ln: Y, Gd, Nd, and La) powder catalysts were synthesized via a citrate precursor route combined with the oxygen-pressure-controlled encapsulation technique. Ln 2O3 (fired at 1000°C overnight prior to use), Ba(NO3)2 and Mn(NO3)2·6H2O were used as starting materials. Appropriate amounts of these reagents were dissolved in diluted HNO3 (for Ln 2O3) or Milli-Q water [for Ba(NO3)2 and Mn(NO3)2·6H2O] to prepare Ln, Ba, and Mn nitrate solutions. These solutions were mixed in a crucible in which equimolar citric acid was subsequently added as a complexing agent. The citrate solution was stirred and heated at 60-70°C to promote polymerization. The gelatinous product was prefired at 450°C in air for 1 h and then at 1000°C in flowing N2 gas for 24 h. The resulting precursor powder was pressed into pellets and placed in an evacuated silica ampule together with an equal amount of FeO powder, which acts as a getter for excess oxygen. The silica ampule was heated at 1100°C for 24 h, followed by quenching into ice water. The crystallinity, morphology and particle size of the resultant samples were characterized with XRD and SEM. Electrochemical measurements were carried out using a rotating ring disk electrode (RRDE, Pt ring) with a conventional three electrode system. The catalyst ethanol ink (BaLnMn2O5: acetylene black: Nafion = 5: 1: 1) was dropped and dried on a glassy carbon electrode as a working electrode. A Pt plate and a Hg/HgO electrode were used as a counter electrode and a reference electrode, respectively. Argon- or oxygen-saturated 4 mol dm-3 potassium hydroxide aqueous solution was used as an electrolyte. The scan speed and rotating speed of the RRDE were 1 mV sec-1 and 1600 rpm, respectively. The XRD data indicate that all the as-synthesized BaLnMn2O5+δ products are essentially phase-pure of the fully reduced δ ≈ 0 form with a tetragonal unit cell. Both the a- and c-axis lengths are in agreement with those in the previous literature. As expected, these values systematically increase with the increasing ionic radius of Ln 3+ in the order of Y, Gd, Nd, and La. The oxygen content (5+δ) values determined by iodometry are close to 5.00 for all the Ln-products. The particle sizes of the samples were almost the same as 1~2 μm. Fig 1b shows onset potentials for ORR as a function of the radius of Ln ions of the samples. It can be seen that the onset potential is shifted toward positive side as the radius of the Ln ions increases, in spite of the fixed Mn valence for all the samples. The most active catalysts for ORR were BaNdMn2O5 and BaLaMn2O5, which contain larger Ln ions. The result implied that the size of A site ion played an important role in the catalytic activity for ORR.Fig. 1 (a) the crystal structure and (b) the onset potentials for ORR as a function of the radius of Ln ions of BaLnMn2O5+δ (Ln: Y, Gd, Nd, and La).[1] T. Motohashi, M. Kimura, T. Inayoshi, T. Ueda, Y. Masubuchi and S. Kikkawa, Dalton Trans. in press. Figure 1

Strong emissions of blue–yellow–red regions of La and Ti modified KNaNbO3 ferroelectric ceramics

In this work, the results of the study on the optical properties of the perovskite structure ABO3 with La 3+ substitution for ions K + and Na + in the A site and Ti 4+ substitution for ion Nb 5+ in the B site are presented. The ceramics were sintered at 1100 ℃ and 1190 ℃ and formed at 10 MPa and 80 MPa. Dense ceramics were obtained with 94% of the theoretical density. The piezoresponse force microscopy (PFM) showed needle-shaped grains with a size of 30 nm for the samples formed at 10 MPa at both sintering temperatures. Apparently, the high temperature and high pressure used in formation reduced the energy of the band gap (Eg) from 3.36 to 3.09 eV. Strong emissions to 2.19, 1.86, 2.5, and 2.31 eV were obtained by exciting the samples at 325, 373, 457, and 500 nm, respectively; these emissions corresponded to blue-yellow-red regions of the visible spectrum.

ChemInform Abstract: Double Cationic‐Anionic Ordering in Ba‐Based Oxygen‐Deficient Perovskites

AbstractReview: 64 refs.

Influence of Bi chemical state on the photocatalytic performance of Bi-doped NaTaO3

NaBiO3 and Bi(NO3)3 were used to synthesize Bi-doped NaTaO3. The influence of the Bi chemical state on the photocatalytic activity was investigated using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and diffused reflectance spectroscopy to study the structure, chemical state and light absorption characteristics, respectively. The photocatalytic activity was evaluated by the H2 evolution water splitting reaction. The monoclinic phase of NaTaO3 remained intact for the two Bi-doped samples, but the Ta–O–Ta bond was distorted from 180° after Bi doping. XPS results indicated that Bi3+ was doped into NaTaO3 with the Bi(NO3)3 precursor, while Bi5+ and Bi3+ were doped into NaTaO3 with the NaBiO3 precursor. The two samples showed identical light absorption, where doping with Bi extended the light absorption to long wavelength light as expected. However, Bi3+ doping did not promote the photocatalytic activity of NaTaO3, while Bi5+ and Bi3+ doping did. The distorted Ta–O–Ta bond from 180° due to doping with Bi was detrimental for charge carrier transfer in the photocatalytic process. In contrast, the vacancies or defects in the NaTaO3 lattice induced by Bi doping for charge balance were beneficial for charge carrier separation. The opposing action of these two factors resulted in the activity of the Bi3+-doped sample being comparable with pristine NaTaO3. For Bi5+- and Bi3+-doped NaTaO3, a high concentration of defects was induced by the high valence Bi5+ ion and this led to its higher photocatalytic activity. Our results indicated that charge carrier transfer is a priority factor in the photocatalytic process and the doping of a high valence ion in the ABO3 structure is a way to promote the separation of charge carriers.

Double Cationic‐Anionic Ordering in Ba‐Based Oxygen‐­Deficient Perovskites

AbstractComplex cationic‐anionic ordering in barium‐based oxygen deficient perovskites is reviewed. Considering the parent ABO3 structure, the ordering between barium and a rare earth cation in the A site generates three classes of oxides, called double, triple and nanoscale ordered quintuple perovskites. From the ordering between a transition element (Fe, Co) and a larger cation on the B site two structural families are obtained, containing calcium and yttrium respectively. These oxides offer a vast playground for the investigation of magnetic and transport properties.

Invited: Unraveling the Oxygen Reduction Reaction Mechanism and Activity of d-Band Perovskite Electrocatalysts for Low Temperature Alkaline Fuel Cells

Perovskites have recently shown the potentials of relatively high electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline media.[1] Therefore they can represent potential low cost cathode materials for low temperature alkaline fuel cell applications.The basic perovskite oxide structure can be represented as ABO3, where A is the larger cation, such as a rare earth or an alkaline earth element, and B is the smaller cation, generally a transition metal. The ABO3 structure can accommodate cation substitution in a wide range by partial substitution of either the A and the B-site cation with another element giving (AxA`1-x)(ByB`1-y)O3 compositions. Such substitution leads to modification of the perovskite band structure which in turn modifies the electrical, optical and magnetic properties of the oxides, and, thus, may also have a significant effect on their electrocatalytic activity.Generally, the perovskite electronic properties are considered to be determined mostly by the B-site cation. When the B-site cation is a transition metal, the major contribution to the material physical properties derives from the B-site cation d-band electrons; for this reason perovskites having B-site transition metals are generally regarded as d-band perovskites.[2] However, the A-site cation can also play an important role in the physicochemical properties of d-band perovskites. Its size determines whatever the crystal structure is deviated from the ideal cubic form and doping the A-site with an aliovalent element can lead to the formation of oxygen vacancies or electron holes.It has recently been shown that the electronic structure of perovskites (total number of occupied states) can be correlated to the oxygen adsorption energy on their surface.[3] The surface binding energy of oxygenated species is generally considered as one of a most crucial parameters for the catalytic activity toward the ORR.[4] Following the Sabatier principle,[5] the best perovskite catalyst would present an electronic (surface) structure which leads to an optimum surface-oxygen interaction energy able to maximize the overall ORR rate.In the present contribution, different perovskite series have been taken into account systematically varying either the B-site or the A-site cation. This results in a tailored electronic (surface) structure, and thus in different ORR activities which have been investigated by rotating disk electrode (RDE) technique.Perovskite powders have been synthesized using a modified sol gel process.[6] To obtain single phase materials all the powder precursors have been treated in oxygen atmosphere at high temperature (between 700-1000 °C), and the surface area has been subsequently measured by Brunauer-Emmett-Teller (BET) analysis. As generally reported in the literature, most of the prepared perovskites show low surface area (around 10 m2g-1).[7] Furthermore, some of them present a relatively low conductivity. To overcome these issues, several studies on perovskite oxide catalysts use different carbons as additives. However, carbon is known to show a not negligible activity towards ORR in alkaline media reducing O2 to hydroperoxide via a 2-electron process. Therefore, the addition of carbon should be systematically studied in order to verify whether the carbon plays only a simple role as conductive enhancer in the catalytic layer or if it is actually involved in the ORR kinetics.In the present work we have investigated the ORR mechanism and activity of composite electrodes made of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) perovskite and acetylene black carbon by thin film rotating ring disk electrode (RRDE) technique. The RRDE technique allows a fundamental understanding of the ORR mechanism discriminating between a four or a two-electron process. The ratio of BSCF and acetylene black has been systematically varied and the onset potential for the ORR and the hydroperoxide formation has been determined for each composition. The RRDE measurements showed that composite electrodes possess a more positive onset potential for ORR and a lower hydroperoxide production in the whole tested potential range compared to pure BSCF and acetylene black, clearly pointing towards a synergistic effect between BSCF and acetylene black.

Local-site cation ordering of Ln3+ions in doped PbTiO3

The study of doped PbTiO3 with lanthanide (Ln3+) elements is an alternative way in order to obtain high density samples with near optimal piezoelectric properties, avoiding at the same time a high porosity microstructure and a crystal structure with a large c/a ratio. X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), and scanning electron microscopy experiments were carried out in the (Pb0.88Ln0.08)TiO3 ferroelectric system with a perovskite type structure (Ln = La, Sm; Eu, Gd and Dy). Qualitative EXAFS analysis has shown that Ln ions substitute to Pb2+ and Ti4+ ions at A and B sites of the ABO3 structure, respectively except for La doped PbTiO3. The XRD pattern refinement was consistent with the Ln ion substitution at both A and B sites, which provides the formation of donor and acceptor type defects. The shape of the observed X-ray lines profiles has shown features, which are known for this kind of ferroelectric material to be typical of the ferroelectric domains microstructure. A phenomenological model has been used for fitting the diffraction profiles by the Rietveld method [1]. A two phase model considering isotropic size and anisotropic strain broadening was used as a suitable approximation for describing the peculiar peak shapes of the diffraction pattern[2], resulting in a useful approach for analyzing the effects of the domains microstructure. As example, the figure 1 shows the result of the XRD pattern refinement for the sample (Pb0.88Eu0.08)TiO3 taking into account asymmetric and anisotropic effects [3]. The observed and calculated patterns are denoted with dots and continuous lines, respectively. Bragg reflections are represented by vertical marks. The difference between the observed and the calculated data is plotted at the bottom.

Interfacial structure of multi-layered thin-films produced by pulsed laser deposition for use in small-scale ceramic capacitors

The aim of this study was to develop thin film capacitors with superior properties that could provide an alternative to materials currently used in conventional multi-layer ceramic capacitors fabricated by sintering. To this end, an artificial dielectric super lattice technique, incorporating pulsed laser deposition, was applied to improving the dielectric properties of thin film capacitors. This method permits the A-site atoms of a perovskite ABO3 structure to be selected layer by layer at a nanoscopic scale; consequently, multi-layer BaTiO3- SrTiO3 thin films were produced on Pt(111)/Ti/SiO2/Si(100) and SrTiO3(111) substrates. Hetero-epitaxial grain growth was observed between BaTiO3 and SrTiO3, with the lattice mismatch between them introducing a compressive residual strain at the interface. The dielectric properties of these multi-layer thin-film capacitors were found to be superior to those of conventional solid-solution thin films once the thickness of the layers and the ratio of the two oxides were optimized.

Synthesis and Evaluation Catalytic Efficiency of Perovskite-Type Oxide Nanopowders in Removal of Bromocresol Purple from Aqueous Solution

A1-xAxBO3 belongs to the perovskite oxides of the ABO3 structure. In this study, nanoparticles of La0.5Ca0.5FeO3 were fabricated by sol-gel citrate technique. A series of common analytical techniques were used to characterize the crystallinity, morphology, specific surface area, and grain size of the nanopowders. These properties were characterised by means of XRD, SEM, EDX and FTIR. The calculated particles size using the Scherrer's formula was about 20 nm. Moreover, the family of perovskite-type oxides could be considered as an adsorbent/catalyst material for the removal of dyes. This study has also investigated the efficiency of La0.5Ca0.5FeO3, as an adsorbent for removal of dye (Bromocresol Purple (BP)), from an aqueous solution. The adsorption studies were carried out at different pH values, various adsorbent dosages and contact time in a batch experiments. The kinetic studies indicate that the removal process obeys the Pseudo-first-order kinetic equation. Also, the isotherm evaluations reveal that the adsorption of BP by the nanoparticles follows the Langmuir model. In addition, this nanoparticle with good specific affinity towards dye molecules was a promising adsorbent for dye removal from natural water.

Influence of excess Ba concentration on the dielectric nonlinearity in Mn and V-doped BaTiO3 multi layer ceramic capacitors

The effect of excess Ba concentration on the dielectric nonlinearity was investigated in Mn and V-doped BaTiO3 multi layer ceramic capacitors (MLCC) under the same grain size condition, which was described by the Preisach model utilizing the first order reversal curve (FORC) distribution. The high-field dielectric constant and its ac field dependence dramatically changed increasing to a maximum and then decreasing with the increase of Ba concentration. The saturation polarization which scales to the magnitude of spontaneous polarization also showed similar behavior. These results indicate that the dependence of the dielectric constant on the Ba concentration is associated with the variation of both domain wall contribution and the magnitude of the spontaneous polarization, which could be correlated with the same dependence on the Ba concentration of the reversible FORC distribution at zero bias and the irreversible FORC distribution near origin. In the corresponding bulk specimens of the dielectrics of MLCC, almost the same amount of the Ba2TiSi2O8 second phases were detected irrespective of Ba concentration, which shows that the excess Ba incorporate into BaTiO3. Thus, low and high Ba concentration corresponds to Ba-deficient and Ba-rich or Ti-deficient BaTiO3, respectively, which results in a small spontaneous polarization and low domain wall density. The intermediate Ba concentration for the maximum dielectric constant is supposed to be near stoichiometric condition in the ABO3 structure corresponding to large spontaneous polarization and high domain wall density. The excess Ba concentration and its resultant A/B stoichiometry is a crucial factor controlling dielectric properties.

Synthesis, Microstructure and the Crystalline Structure of the Barium Titanate Ceramics Doped with Lanthanum

Abstract In the present study BaTiO3 and Ba1-xLaxTiO3 (0.1-0.4mol.% La) ceramic powders were synthesized by the conventional mixed oxide method (MOM). The characterization of the ceramic powders was carried out using a simultaneous thermal analysis (STA), with a combined DTA/TG/DTG system (Netzsch STA409). The results of thermal analysis allowed to determine the optimal temperature of synthesis. Microstructure was investigated by scanning electron microscopy (SEM), crystalline structure was studied by X-ray diffraction method (XRD). The EDS investigations showed that samples exhibited conservation of stoichiometry according to the chemical composition formula. The X-ray diffraction analysis confirmed formation of the desired crystalline structure both pure and La3+-doped BaTiO3 ceramics exhibiting a perovskite-type structure ABO3 with tetragonal symmetry P4 mm.