Oxides supported carbon air-electrodes for alkaline solution power devices

Porous carbon oxygen-reducing electrodes incorporated with perovskite oxide catalysts are reported. It has been possible to fabricate high-performance oxygen-reducing electrodes by introducing La0.5Sr0.5CoO3 and La0.99Sr0.01NiO3 with the activated coconut-shell charcoal; these electrodes could sustain load currents as high as 1 A cm−2 without serious degradation. A model to explain oxygen-reducing activity of these oxides has been proposed.

Low Temperature Synthesis of Nano-Crystalline CaCu3Ti4O12 Through aFuel Mediated Auto-Combustion Pathway

A novel, cost effective,environment-friendly and energetically beneficial alternative method for the synthesis of giant dielectric pseudo-perovskite material CaCu3Ti4O12 (CCTO) is presented. The method involved auto-combustion of an aqueous precursor solution in oxygen atmosphere with the help of external fuels and is capable of producing high amount of CCTO at ultra-low temperature, in the combustion residue itself. The amount of phase generated was observed to be highly dependent on the combustion process i.e. on the nature and amount of external-fuels added for combustion. Two successful fuel combinations capable of producing reasonably higher amount of the desired compound were investigated. On a structural characterization grain size was observed to decrease drastically to nano-dimension compared to submicron-size that was obtained in a traditional sol-gel combustion and subsequent cacination method. Therefore, the method reported can produce nano-crystalline CaCu3Ti4O12 ceramic matrix at an ultra-low temperature and is expected to be applicable for other multifunctional perovskite oxide materials.

Photocatalytic Activity of Combustion Synthesized Nanocrystalline CeAlO3

The photocatalytic activity of combustion synthesized nanocrystalline CeAlO3 was determined for the degradation of four anionic and four cationic dyes. The perovskite oxide showed high-photocatalytic activity and a complete degradation of all the dyes was possible within 2 h. The photocatalytic activity of the compound was comparable with the activity of the commercial Degussa P-25 TiO2 catalyst. The degradation of dyes was found to follow first order kinetics and the first order degradation rate constants were determined.

Electronic conduction in LaNiO3-delta: the dependence on the oxygen stoichiometry delta

We report a systematic study of the electronic transport properties of the metallic perovskite oxide LaNiO3-delta as a function of the oxygen stoichiometry delta (delta less than or equal to 0.14). The electrical resistivity, magnetoresistance, susceptibility, Hall effect and thermopower have been studied, All of the transport coefficients are dependent on the value of delta. The resistivity increases almost exponentially as delta increases. We relate this increase in rho to the creation of Ni2+ with square-planar coordination. We find that there is a distinct T-1.5-contribution to the resistivity over the whole temperature range. The thermopower is negative, as expected for systems with electrons as the carrier, but the Hall coefficient is positive. We have given a qualitative and quantitative explanation for the different quantities observed and their systematic variation with the stoichiometry delta.

Probing a spin-glass state in SrRuO3 thin films through higher-order statistics of resistance fluctuations

The complex perovskite oxide SrRuO3 shows intriguing transport properties at low temperatures due to the interplay of spin, charge, and orbital degrees of freedom. One of the open questions in this system is regarding the origin and nature of the low-temperature glassy state. In this paper we report on measurements of higher-order statistics of resistance fluctuations performed in epitaxial thin films of SrRuO3 to probe this issue. We observe large low-frequency non-Gaussian resistance fluctuations over a certain temperature range. Our observations are compatible with that of a spin-glass system with properties described by hierarchical dynamics rather than with that of a simple ferromagnet with a large coercivity.

Structural characterization, stability and electrical properties of strontium niobate ceramic

The double perovskite oxide Sr2CrNbO6 has a cubic structure according to powder X-ray diffraction. After reducing in CO, Sr2CrNbO6 still exhibited a cubic structure refined by Rietveld technique. The TG analysis indicated that Sr2CrNbO6 loses 0.127 oxygen per formula unit from 400 to 700 degrees C in H-2. The morphology and compositions of this ceramic did not significantly change on reduction

Functional alkali tantalate 2D structures for microelectronics and related applications

Alkali tantalates and niobates, including K(Ta / Nb)O3, Li(Ta / Nb)O3 and Na(Ta / Nb)O3, are a very promising ferroic family of lead-free compounds with perovskite-like structures. Their versatile properties make them potentially interesting for current and future application in microelectronics, photocatalysis, energy and biomedics. Among them potassium tantalate, KTaO3 (KTO), has been raising interest as an alternative for the well-known strontium titanate, SrTiO3 (STO). KTO is a perovskite oxide with a quantum paraelectric behaviour when electrically stimulated and a highly polarizable lattice, giving opportunity to tailor its properties via external or internal stimuli. However problems related with the fabrication of either bulk or 2D nanostructures makes KTO not yet a viable alternative to STO. Within this context and to contribute scientifically to the leverage tantalate based compounds applications, the main goals of this thesis are: i) to produce and characterise thin films of alkali tantalates by chemical solution deposition on rigid Si based substrates, at reduced temperatures to be compatible with Si technology, ii) to fulfil scientific knowledge gaps in these relevant functional materials related to their energetics and ii) to exploit alternative applications for alkali tantalates, as photocatalysis. In what concerns the synthesis attention was given to the understanding of the phase formation in potassium tantalate synthesized via distinct routes, to control the crystallization of desired perovskite structure and to avoid low temperature pyrochlore or K-deficient phases. The phase formation process in alkali tantalates is far from being deeply analysed, as in the case of Pb-containing perovskites, therefore the work was initially focused on the process-phase relationship to identify the driving forces responsible to regulate the synthesis. Comparison of phase formation paths in conventional solid-state reaction and sol-gel method was conducted. The structural analyses revealed that intermediate pyrochlore K2Ta2O6 structure is not formed at any stage of the reaction using conventional solid-state reaction. On the other hand in the solution based processes, as alkoxide-based route, the crystallization of the perovskite occurs through the intermediate pyrochlore phase; at low temperatures pyrochlore is dominant and it is transformed to perovskite at >800 °C. The kinetic analysis carried out by using Johnson-MehlAvrami-Kolmogorow model and quantitative X-ray diffraction (XRD) demonstrated that in sol-gel derived powders the crystallization occurs in two stages: i) at early stage of the reaction dominated by primary nucleation, the mechanism is phase-boundary controlled, and ii) at the second stage the low value of Avrami exponent, n ~ 0.3, does not follow any reported category, thus not permitting an easy identification of the mechanism. Then, in collaboration with Prof. Alexandra Navrotsky group from the University of California at Davis (USA), thermodynamic studies were conducted, using high temperature oxide melt solution calorimetry. The enthalpies of formation of three structures: pyrochlore, perovskite and tetragonal tungsten bronze K6Ta10.8O30 (TTB) were calculated. The enthalpies of formation from corresponding oxides, ∆Hfox, for KTaO3, KTa2.2O6 and K6Ta10.8O30 are -203.63 ± 2.84 kJ/mol, - 358.02 ± 3.74 kJ/mol, and -1252.34 ± 10.10 kJ/mol, respectively, whereas from elements, ∆Hfel, for KTaO3, KTa2.2O6 and K6Ta10.8O30 are -1408.96 ± 3.73 kJ/mol, -2790.82 ± 6.06 kJ/mol, and -13393.04 ± 31.15 kJ/mol, respectively. The possible decomposition reactions of K-deficient KTa2.2O6 pyrochlore to KTaO3 perovskite and Ta2O5 (reaction 1) or to TTB K6Ta10.8O30 and Ta2O5 (reaction 2) were proposed, and the enthalpies were calculated to be 308.79 ± 4.41 kJ/mol and 895.79 ± 8.64 kJ/mol for reaction 1 and reaction 2, respectively. The reactions are strongly endothermic, indicating that these decompositions are energetically unfavourable, since it is unlikely that any entropy term could override such a large positive enthalpy. The energetic studies prove that pyrochlore is energetically more stable phase than perovskite at low temperature. Thus, the local order of the amorphous precipitates drives the crystallization into the most favourable structure that is the pyrochlore one with similar local organization; the distance between nearest neighbours in the amorphous or short-range ordered phase is very close to that in pyrochlore. Taking into account the stoichiometric deviation in KTO system, the selection of the most appropriate fabrication / deposition technique in thin films technology is a key issue, especially concerning complex ferroelectric oxides. Chemical solution deposition has been widely reported as a processing method to growth KTO thin films, but classical alkoxide route allows to crystallize perovskite phase at temperatures >800 °C, while the temperature endurance of platinized Si wafers is ~700 °C. Therefore, alternative diol-based routes, with distinct potassium carboxylate precursors, was developed aiming to stabilize the precursor solution, to avoid using toxic solvents and to decrease the crystallization temperature of the perovskite phase. Studies on powders revealed that in the case of KTOac (solution based on potassium acetate), a mixture of perovskite and pyrochlore phases is detected at temperature as low as 450 °C, and gradual transformation into monophasic perovskite structure occurs as temperature increases up to 750 °C, however the desired monophasic KTaO3 perovskite phase is not achieved. In the case of KTOacac (solution with potassium acetylacetonate), a broad peak is detected at temperatures <650 °C, characteristic of amorphous structures, while at higher temperatures diffraction lines from pyrochlore and perovskite phases are visible and a monophasic perovskite KTaO3 is formed at >700 °C. Infrared analysis indicated that the differences are due to a strong deformation of the carbonate-based structures upon heating. A series of thin films of alkali tantalates were spin-coated onto Si-based substrates using diol-based routes. Interestingly, monophasic perovskite KTaO3 films deposited using KTOacac solution were obtained at temperature as low as 650 °C; films were annealed in rapid thermal furnace in oxygen atmosphere for 5 min with heating rate 30 °C/sec. Other compositions of the tantalum based system as LiTaO3 (LTO) and NaTaO3 (NTO), were successfully derived as well, onto Si substrates at 650 °C as well. The ferroelectric character of LTO at room temperature was proved. Some of dielectric properties of KTO could not be measured in parallel capacitor configuration due to either substrate-film or filmelectrode interfaces. Thus, further studies have to be conducted to overcome this issue. Application-oriented studies have also been conducted; two case studies: i) photocatalytic activity of alkali tantalates and niobates for decomposition of pollutant, and ii) bioactivity of alkali tantalate ferroelectric films as functional coatings for bone regeneration. Much attention has been recently paid to develop new type of photocatalytic materials, and tantalum and niobium oxide based compositions have demonstrated to be active photocatalysts for water splitting due to high potential of the conduction bands. Thus, various powders of alkali tantalates and niobates families were tested as catalysts for methylene blue degradation. Results showed promising activities for some of the tested compounds, and KNbO3 is the most active among them, reaching over 50 % degradation of the dye after 7 h under UVA exposure. However further modifications of powders can improve the performance. In the context of bone regeneration, it is important to have platforms that with appropriate stimuli can support the attachment and direct the growth, proliferation and differentiation of the cells. In lieu of this here we exploited an alternative strategy for bone implants or repairs, based on charged mediating signals for bone regeneration. This strategy includes coating metallic 316L-type stainless steel (316L-SST) substrates with charged, functionalized via electrical charging or UV-light irradiation, ferroelectric LiTaO3 layers. It was demonstrated that the formation of surface calcium phosphates and protein adsorption is considerably enhanced for 316L-SST functionalized ferroelectric coatings. Our approach can be viewed as a set of guidelines for the development of platforms electrically functionalized that can stimulate tissue regeneration promoting direct integration of the implant in the host tissue by bone ingrowth and, hence contributing ultimately to reduce implant failure.

AM(1-x)Al(x)O(3-x) (A=Na or K; M=Nb or Ta): New anion-deficient Perovskite oxides exhibiting oxide ion conduction

Anion-deficient perovskite oxides of the formula AM(1-x)Al(x)O(3-x) (A = Na or K; M = Nb or Ta) have been prepared for 0 < x less than or equal to 0.5. Diffraction experiments reveal that while the potassium compounds adopt orthorhombic/cubic perovskite structures similar to the parent KNbO3/KTaO3, the sodium compound, NaNb0.5Al0.5O2.5, possesses a brownmillerite/LaSr-CuAlO5-like superstructure. Al-27 NMR spectra show an exclusive tetrahedral oxygen coordination for AI(III) in Na-Nb0.5Al0.5O2.5 (I) and both tetrahedral and octahedral coordination for Al(III) in KNb0.5Al0.5O2.5 (II). The results suggest a long-range and short-range ordering of oxide ion vacancies in I and II respectively. Electrical conductivity measurements show a significant oxide ion conduction for KNb1-xAlxO3-x, with the conductivity increasing with x up to x = 0.5. The differences in the Arrhenius plots of the ionic conductivity of I and II have been rationalized in terms of the long-range and short-range ordering of oxide ion vacancies in the anion-deficient perovskite oxides.

Studies on the elimination of NO over the La-Ba-Cu mixed oxide catalysts with perovskite structure

The mixed oxides, including YBa2Cu3O7, LaBa2Cu3O7, LaBaCu2O5, La2BaCu3O7, La4BaCu5O12 with perovskite structure, were prepared. The catalysts were characterized by means of chemical analysis, XRD, TPD and TPR method. It was found that they were the active catalysts for the NO decomposition and NO reduction by CO. The existance of Cu3+ is an important factor to give the catalysts a high activity for the NO reduction by CO.

Comparative study of Nickel-based perovskite-like mixed oxide catalysts for direct decomposition of NO

The mixed oxides LaNiO3, La0.1Sr0.9NiO3, La2NiO4 and LaSrNiO4 were prepared and used as catalysts for the direct decomposition of NO. The catalysts were characterized by means of XRD, XPS, O-2-TPD, NO-TPD and chemical analysis. By comparing the physico-chemical properties and catalytic activity for NO decomposition, a conclusion could be drawn as follows. The direct decomposition of NO over perovskite and related mixed oxide catalysts follows a redox mechanism. The lower valent metal ions Ni2+ and disordered oxygen vacancies seem to be the active sites in the redox process. The oxygen vacancy plays an important role favorable for the adsorption and activation of NO molecules on one hand and on the other hand for increasing the mobility of lattice oxygen which is beneficial to the reproduction of active sites. The presence of oxygen vacancies is one of the indispensable factors to give the mixed oxides a steady activity for NO decomposition.

Studies on the adsorption property of NO on the Ni-based mixed oxide catalysts with Perovskite-like (ABO(3) and A(2)BO(4)) structure

The mixed oxides LaNiO3, La0.1Sr0.9NiO3, La2NiO4 and LaSrNiO4 with perovskite (ABO(3)) and related(A(2)BO(4)) structures were prepared and the adsorption property for NO and the catalytic activity for NO decomposition over these oxidse were also tested. The catalysts were characterized by means of BET surface measurement, chemical analysis, NO-TPD etc.. It was shown that the adsorption amount of NO is correlated with the concentration of oxygen vacancy formed and the adsorption type and strength of NO are related to the valence of metallic ion. Generally there are three kinds of adsorption species, NO-, NO+ and NO on the mixed oxides, among them the negative adsorpion species (NO-) are active for NO decomposition. The weaker the adsorption of oxygen on the catalyst is, the faster the mobility of oxygen is and the easier the redox process takes place in reproducing the active sites in which the oxygen species (O-, O2-) would participate.

Epitaxial Magnetic Oxide Nanocrystals Via Phase Decomposition of Bismuth Perovskite Precursors

The phase instability of bismuth perovskite (BiMO₃), where M is a ferromagnetic cation, is exploited to create self-assembled magnetic oxide nanocrystal arrays on oxide supports. Conditions during pulsed laser deposition are tuned so as to induce complete breakdown of the perovskite precursor into bismuth oxide (Bi₂ O₃ ) and metal oxide (M-O_x ) pockets. Subsequent cooling in vacuum volatizes the Bi₂ O₃ leaving behind an array of monodisperse nanocrystals. In situ reflective high energy electron diffraction beam is exploited to monitor the synthesis in real-time. Analysis of the patterns confi rms the phase separation and volatization process. Successful synthesis of M-O_x, where M = Mn, Fe, Co, and Cr, is shown using this template-free facile approach. Detailed magnetic characterization of nanocrystals is carried out to reveal the functionalities such as magnetic anisotropy as well as larger than bulk moments, as expected in these oxide nanostructures.

High performance cobalt-free perovskite cathode for intermediate temperature solid oxide fuel cells

Bi doping of SrFeO3d results in the formation of a structure with high symmetry and extraordinary electrochemical performance for Bi0.5Sr0.5FeO3-d, which is capable of competing effectively with the current Co-based cathode benchmark with additional advantages of lower thermal expansion and cost.

Deactivation and regeneration of oxygen reduction reactivity on double perovskite Ba2Bi0.1Sc0.2Co1.7O6−x cathode for intermediate-temperature solid oxide fuel cells

in situ high-temperature X-ray diffraction and thermal gravimetric- differential thermal analysis on room-temperature powder, as well as X-ray diffraction, Raman spectroscopy, and transmission electron microscopy on quenched powder, were applied to study crystal structure and phase transformations in Ba₂Bi_0.1Sc_0.2Co _1.7O_6-x (BBSC). Heating BBSC in air to over 800 °C produces a pure cubic phase with space group Fm3m (no. 225), and cooling down below 800 °C leads to a mixture of three noncubic phases including an unknown phase between 200 and 650 °C, a 2H hexagonal BaCoO3 with space group P63/mmc (no. 194) between 600 and 800 °C, and an intermediate phase at 800 °C. These three phases exist concurrently with the major cubic phase. The weight gain and loss between 300 and 900 °C suggest the occurrence of cobalt reduction, oxidation, and disproportion reactions with dominant reduction reaction at above 600 °C. The thermal expansion of BBSC was also examined by dilatometry. BBSC has a highly temperature-dependent thermal expansion coefficient which relates well with its structure evolution. Furthermore, the oxygen reduction reaction (ORR) of BBSC was probed by symmetrical cell and three-electrode configurations. The presence of hexagonal phase at 700 °C rarely affects the ORR performance of BBSC as evidenced by a slight increase of its area-specific resistance (ASR) value following 48 h of testing in this three-electrode configuration. This observation is in contrast to the commonly held point of view that noncubic phase deteriorates performance of perovskite compounds (especially in oxygen transport applications). Moreover, cathodic polarization treatment, for example, current discharge from BBSC (tested in three-electrode configuration), can be utilized to recover the original ORR performance. The cubic structure seems to be retained on the cathodic polarization - the normal cathode operating mode in fuel cells. Stable 72-h performance of BBSC in cathodic polarization mode further confirms that despite the presence of phase impurities, BBSC still demonstrates good performance between 500 and 700 °C, the desired intermediate operating temperature in solid oxide fuel cells.