J.R. Jurado

Combustion synthesis of iron-substituted strontium titanate perovskites

Perovskites in the system SrOTiO2Fe2O3 can be good candidates for electrochemical applications, and the properties of sintered, fully dense bodies depend on the Fe to Ti stoichiometry and the prevailing temperature and oxygen partial pressure. Conductivity studies normally require that the oxides with the desired stoichiometry and structure be synthesized. Solid state synthesis of perovskites requires reasonably high temperatures and full conversion is not guaranteed. Moreover, with this method, it is difficult to obtain a compositionally homogeneous product. The present work discusses a straightforward combustion synthesis technique to prepare submicron iron-substituted strontium titanate, the Sr(Fe0.5Ti0.5)O3 perovskite, using the corresponding metal salts-urea mixtures, at low temperature and short reaction times. The effect of the ratio oxidizerfuel in the redox mixture was investigated, namely to find out how that ratio affected the phase formation and the characteristics, such as morphology and grain size, of the powder produced.

Oxygen ionic conductivity of Ti-containing strontium ferrite

Abstract The total electrical conductivity, oxygen permeability, Faradaic efficiency and thermal expansion of perovskite-type Sr 0.97 Fe 0.80 Ti 0.20 O 3− δ were studied. The oxygen permeation through strontium ferrite–titanate ceramics was found to be limited by both bulk ionic conduction and surface exchange rates, similar to other Fe- and Ti-containing perovskites. Increasing iron content in the system Sr 0.97 (Ti,Fe)O 3− δ leads to a considerable increase in oxygen ionic and electronic conductivities, and thermal expansion. The ion transference numbers of Sr 0.97 Fe 0.80 Ti 0.20 O 3− δ were determined by the Faradaic efficiency measurements, and estimated from the oxygen permeability data. Typical values of the transference numbers vary in the range from 4×10 −3 to 1.5×10 −2 at 973–1223 K, decreasing with reducing temperature. The oxygen ionic conductivity of Sr 0.97 Fe 0.80 Ti 0.20 O 3− δ is close to the conductivity of the Zr 0.92 Y 0.08 O 1.96 solid electrolyte. The average thermal expansion coefficient of strontium ferrite–titanate, calculated from dilatometric data, is (13.8±0.1)×10 −6 K −1 at 300–780 K and (27.0±0.4)×10 −6 K −1 at 780–1040 K.

Titanium oxide as substrate for neural cell growth

Titanium oxide has antiinflammatory activity and tunable electrochemical behavior that make it an attractive material for the fabrication of implantable devices. The most stable composition is TiO2 and occurs mainly in three polymorphs, namely, anatase, rutile, and brookite, which differ in its crystallochemical properties. Here, we report the preparation of rutile surfaces that permit good adherence and axonal growth of cultured rat cerebral cortex neurons. Rutile disks were obtained by sinterization of TiO2 powders of commercial origin or precipitated from hydrolysis of Ti(IV)-isopropoxide. Commercial powders sintered at 1300-1600 degrees C produced rutile surfaces with abnormal grain growth, probably because of impurities of the powders. Neurons cultured on those surfaces survived in variable numbers and showed fewer neurites than on control materials. On the other hand, rutile sintered from precipitated powders had less contaminants and more homogenous grain growth. By adjusting the thermal treatment it was possible to obtain surfaces performing well as substrate for neuron survival for at least 10 days. Some surfaces permitted normal axonal elongation, whereas dendrite growth was generally impaired. These findings support the potential use of titanium oxide in neuroprostheses and other devices demanding materials with enhanced properties in terms of biocompatibility and axon growth promotion.

Microstructure, electrical properties and phase equilibria relationships in the ZrO2Y2O3TiO2 system: The subsolidus isothermal section at 1500°C

The thermodynamic data found in the literature on the binary systems ZrO2Y2O3 and ZrO2TiO2 are briefly reviewed. Scarce information can be found about the ternary system ZrO2Y2O3TiO2. In this study the isothermal section of the ternary system at 1500°C in air in the zirconia rich region is outlined. The isoconductivity lines in air, obtained by using impedance spectroscopy analysis, are drawn on the isothermal section. Results from energy-dispersive X-ray analysis in the scanning electron microscope were used to correlate both, the thermodynamical data and the electrical behaviour. From the isothermal section and the isoconductivity lines it is possible to design materials for, specific applications.

Role of grain boundaries on the electrical properties of titania doped yttria stabilized zirconia

Abstract 10 mol % titania doped YSZ samples were prepared by high temperature firing of mixtures of YSZ and TiO 2 . The role of firing time (2 to 16h) and temperature (1300 to 1600 °C) on the low temperature electrical behavior in air was found significant. For samples sintered for 2h at different temperatures, the grain boundary conductivity increased with firing temperature. For samples sintered at constant temperature (1500 °C) the grain boundary conductivity increased with firing time up to about 8h. The role of grain boundaries on the high temperature electronic conductivity under reducing conditions was found negligible when compared to the low temperature data in air, suggesting that at high temperature the overall conductivity is determined by the bulk grain behavior.

Electrical transport properties in zirconia/alumina functionally graded materials

Functionally graded materials (FGMs) are promising candidates for the fabrication of technological components, not only as structural devices, but also in electrochemical ones, such as solid oxide fuel cells (SOFC), or high-efficiency hybrid direct energy conversion systems. In the present work FGMs were prepared by the sequential slip casting technique, starting with an yttria tetragonal zirconia polycrystalline layer and increasing subsequently the amount of Al2O3 in the following layers. Electrochemical impedance spectroscopy (EIS) analysis was used to evaluate the electrical characteristics of these materials and to compare with those of the monolithic compacts. In general, it was observed that the FGM conductivity is ruled by the conductivity of the layer which contains the highest amount of alumina blocking particles. By EIS no electrical interfaces between adjoining layers were detected and, accordingly, no specific electric ohmic losses were observed. The conductivity of the FGMs is close to that calculated using the normalized thicknesses and the alumina volume fractions of the layers after measuring the conductivity of the monolithic materials with the same composition to what correspond to that of the final layer in the FGM. These results suggest that the gradient structure can be used to control the oxygen vacancy motion, and then applied in electrochemical devices.

Overpotential terms on the electrochemical permeability of Sr0.97(Ti,Fe)O3−y materials

Abstract The transport properties of Sr0.97(Ti,Fe)O3−y materials were reevaluated by a simple electrochemical permeability technique, comprising an electrochemical oxygen pump and a potentiometric oxygen sensor to measure the driving force. The rate of transport through single layer dense samples was significantly lower than through bilayered dense∣porous samples with the porous layer in contact with the oxidizing atmosphere (air); this suggests that the overall resistance to oxygen transfer across the mixed conducting material includes terms related to surface exchange, and mixed (electronic+oxide ion) conduction. The dependence of permeability current on the driving-force also tends to a limiting current, which was attributed to concentration overpotentials due to limitations of transport in dilute gas phases.

High oxygen ion conduction in some Bi2O3-Y2O3 (Er2O3) solid solutions

Abstract Highly densified bodies (~ 98 % theoretical density) of the Bi 2 O 3 -Y 2 O 3 (Er 2 O 3 ) systems containing 30 moles % Y 2 O 3 and 20 moles % Er 2 O 3 respectively were prepared from both mixed oxides and coprecipitated oxalates. DC ionic conductivity and impedance complex plane analysis on sintered samples were performed. Under oxygen partial pressure ranging from 1 to 10 Pa was found that, samples containing 30 moles % yttria showed a pure ionic conductivity up to an oxygen partial pressure of 10 −20 Pa. Samples containing 20 moles % erbia, extended its ionic conductivity up to an oxygen partial pressure of 10 −22 Pa. The impedance analysis shows the presence of only one semicircle at low tempertures and it is attributed to bulk conductivity contribution. The conductivity was higher for the Bi 2 O 3 -Er 2 O 3 sintered solid electrolytes, in such a case, a conductivity as high as 1.38 ohm cm at 700oC was obtained. The activation enthalpy for the conduction process was calculated in the temperature range from 200o to 700oC in all the cases. Microstructural development in the sintered sample was also studied.

Preparation and electrical properties of oxygen ion conductors in the Bi2O3−Y2O3 (Er2O3) systems

Dense bodies (98% of theoretical density Dth) in the compositional range 12–30 mol% Y2O3 and 18–20 mol% Er2O3 of the Bi2O3−Y2O3 (Er2O3) systems were prepared by coprecipitating oxalates. Sintering process, grain growth and microstructural development were also studied. It was found that a very rapid grain growth took place in these ceramics which impeded the migration of the residual porosity located at the interior of the grains. dc ionic conductivity and ac impedance plane measurements on sintered samples were carried out. Erbia-containing samples exhibit the larger ionic conductivity domain, from 1 to 10−23 Pa of oxygen partial pressure and electrode-electrolyte reduction reaction of Bi2O3 took place. The impedance analysis on low yttria-containing samples (12 mol% Y2O3) shows a very high electrical conductivity. In all cases only one semicircle at low temperature was found which was attributed to the bulk conductivity contribution. Only in erbia-containing samples does a grain boundary contribution seem to be detected. Finally, a Warburg diffusion contribution could explain the quarter semicircle of low-frequency arc at the high- and intermediate-temperature regions.

Targeting Neural Stem Cells with Titanium Dioxide Nanoparticles Coupled to Specific Monoclonal antibodies

Aiming to characterize the use of biomaterials in cancer therapy, we took advantage of the n-type semiconductor properties, which upon irradiation excite their electrons into the conduction band to induce photoelectrochemical reactions generating oxygen reactive species (ROS). Indeed, photoactivated TiO2 nanoparticles have been shown to kill in vitro either bacteria or tumor cells in culture following UV irradiation, as a consequence of the ROS levels generated; the killing was highly effective although devoid of specificity. In this report, we have directed the TiO2 nanoparticles to particular targets by coupling them to the monoclonal antibody (mAb) Nilo1, recognizing a surface antigen in neural stem cells within a cell culture, to explore the possibility of making this process specific. TiO2 nanoparticles generated with particular rutile/anatase ratios were coupled to Nilo1 antibody and the complexes formed were highly stable. The coupled antibody retained the ability to identify neural stem cells and upon UV irradiation, the TiO2 nanoparticles were activated, inducing the selective photokilling of the antibody-targeted cells. Thus, these data indicate that antibody-TiO2 complexes could be used to specifically remove target cell subpopulations, as demonstrated with neural stem cells. The possible applications in cancer therapy are discussed.