Isabel Santacruz

ABSTRACT Perovskite powders prepared by combustion synthesis with a nominal composition of La0.800Sr0.200Cr0.920Co0.080O3- were obtained from the corresponding metal nitrates, and characterised in terms of in-situ phase development (crystalline structure and thermal behaviour). Synchrotron X-ray powder diffraction (SXRPD) and Rietveld analyses demonstrated that the as-prepared powder showed an orthorhombic perovskite structure with an estimated composition of La0.803Sr0.197Cr0.966Co0.034O3-. Second phases present in the as-prepared powder were m-LaCrO4, CoLa1.250Sr0.750O4, c-CoCr2.000O4 and m-SrCrO4. Sintering processes, including phase transition (from orthorhombic to rhombohedral) of the main phase and, melting/evaporation of second phases have been studied by DTA/TGA and dilatometric studies; the results were in full agreement with those detected by High-Temperature Synchrotron X-ray powder diffraction (HT-SXRPD). The transition temperature was determined to be 1380ºC by dilatometric studies, and between 1240 and 1405ºC by HT-SXRPD. Dye-pressed samples sintered at 1600ºC/4h showed a density of 98.9 dth%. The phase transition is an irreversible process since the sintered perovskite, measured at room temperature, only showed the rhombohedral phase as main phase. The evaluation of the linearity of the conductivity versus the inverse of temperature and the activation energy values of the sintered material indicated that the electrical conduction occurs via the non-adiabatic small-polaron mechanism in either air or hydrogen atmosphere. In the reducing environment, the perovskite shows significantly reduced electrical conductivity compared with that in air, as it is expected for a p-type conductor. Keywords: Lanthanum chromite; Synchrotron radiation; Rietveld refinement; sintering; Electrical properties; Solid Oxide Fuel Cell.

Commercial tiles are being produced in vast quantities. The main properties of tiles are well established but there is an increasing interest in producing ceramics with tailored-properties and advanced functionalities. One way of adding value to commercial tiles is to deposit a photocatalytic coating to obtain ‘smart’ tiles for environmental reasons, e.g. for the (photo) degradation of organic pollutants in air or in a liquid. Here, we show the manufacture of ‘smart’ tiles by formation of TiO2 coatings onto commercial tiles by a colloidal processing route based on the immersion of the substrate into a homogeneous aqueous ceramic suspension and its consolidation by agar thermogelation. The effect of the processing parameters (withdrawal rate, solid loading and gelling agent content) and the grain size on the photocatalytic activity of the final coated tiles is reported and discussed. Final coatings properties depend on the viscosity of the suspension, particle size, withdrawal rate, ...

Sodalites are tridimensional alumino-silicate materials containing cages where loosely bonded anions are located. Yeelimite, Ca-4[Al6O12]SO4, is outstanding as an aluminate sodalite with a flexible framework accepting several type of dopants with important structural consequences. Moreover, yeelimite is also important from an applied perspective as it is the most relevant phase in calcium sulfoaluminate cements. The crystal structure of stoichiometric yeelimite has recently been unraveled, but the structure of dopant-containing yeelimite, which is present in cements, is not well studied. Here, we report the pseudocubic crystal structure of doped yeelimite, Ca3.8Na0.2Al5.6Fe0.2Si0.2O12SO4, from high-resolution synchrotron powder diffraction data. The powder pattern is indexed with a cubic cell, and a structural model is reported based on the I (4) over bar (3)m space group. However, this compound displays diffraction peak narrowing on heating. Furthermore, some high-angle split peaks...

The availability of cements, including eco-cements, with tailored mechanical properties is very important for special applications in the building industry. Here we report a full study of the hydration of calcium sulfoaluminate eco-cements with different sulfate sources (gypsum, bassanite and anhydrite) and two water/cement ratios (0.50 and 0.65). These parameters have been chosen because they are known to strongly modify the mechanical properties of the resulting mortars and concretes. The applied multi-technique characterization includes: phase assemblage by Rietveld method, evolved heat, conductivity, rheology, compressive strength and expansion/retraction measurements. The dissolution rate of the sulfate sources is key to control the hydration reactions. Bassanite dissolves very fast and hence the initial setting time of the pastes and mortars is too short (20 min) to produce homogeneous samples. Anhydrite dissolves slowly, at 1 hydration-day, the amount of ettringite formed (20...

ABSTRACT Lanthanum tungstates are promising proton conductor materials with applications in several areas, including electrolytes for solid oxide fuel cells and hydrogen gas separation membranes. Here, we report the preparation and optimisation of dense La5.5WO11.25 tapes obtained by the combination of tape casting and reaction sintering. Homogeneous concentrated mixed suspensions of La2O3 and WO3 were prepared in ethanol. All involved parameters, such as zeta potential and particle size of the initial powders, binder/plasticizer content and sintering conditions have been optimised. Sintered tapes were studied by Rietveld analysis of high-resolution laboratory X-ray powder diffraction, scanning electron microscopy and impedance spectroscopy. This study demonstrates that tape casting and reaction sintering is a suitable technique to prepare thin films of lanthanum tungstates in a single step that show good electrical properties.

ABSTRACT Water treatment is a hot topic, and it will become much more important in the decades ahead. Advanced oxidation processes are being increasingly used for organic contaminant removal, for example using photo-Fenton reactions. Here we report the use of an organo-inorganic hybrid, Fe[HO3PCH(OH)COO]·2H2O, as Fenton photocatalyst for phenol oxidation with H2O2 under UVA radiation. Preactivation, catalyst content, and particle size parameters have been studied/optimized for increasing phenol mineralization. Upon reaction, iron species are leached from the catalyst making a homogeneous catalysis contribution to the overall phenol photo-oxidation. Under optimized conditions, the mineralization degree was slightly larger than 90% after 80 min of irradiation. Analysis by X-ray photoelectron spectroscopy revealed important chemical modifications occurring on the surface of the catalyst after activation and phenol photodegradation. The sustained slow delivery of iron species upon phenol photoreaction is advantageous as the mixed heterogeneous–homogeneous catalytic processes result in very high phenol mineralization.

Al2O3–TiO2 coatings were deposited on austenitic stainless steel coupons from nanostructured powders by atmospheric plasma spraying (APS). Commercial suspensions of nanosized Al2O3 and TiO2 particles were used as starting materials. Mixtures of these suspensions and of more concentrated suspensions of Al2O3 and TiO2 were then agglomerated into plasma sprayable feedstock. Agglomeration was performed by spray drying, followed by consolidation thermal treatment.These powders were successfully deposited, yielding coatings that were well bonded to the substrates. The coating microstructure thus consisted of semi-molten feedstock agglomerates surrounded by fully molten particles that acted as binders. Agglomerates from suspensions with higher solids contents yielded coatings with lower porosity and fewer semi-molten areas.

ABSTRACT This work evaluates the potential of zirconium titanate–zirconia composites for thermal shock. Materials with Zr0.97Y0.03O1.985:TiO2 molar ratios 50:50 (Z(Y)T50) and 70:30 (Z(Y)T70) were obtained from Y2O3 (3 mol%)-stabilized ZrO2 and TiO2 mixtures colloidal processed and reaction sintered at 1773 K with low cooling rate (2–5 K/min). The crystalline phases and their unit cell parameters were determined by Rietveld analysis of high resolution X-ray diffraction patterns. The zirconium titanate phase in these materials is o-TiZrO4, being the major phase in Z(Y)T50 in which c-ZrO2 is secondary phase. Z(Y)T70 has t-ZrO2 as main phase, o-TiZrO4 as secondary phase and c-ZrO2 and m-ZrO2 as minor phases. The Hasselman thermal shock resistance factors, calculated using the experimental values of the involved properties, Young's modulus, thermal expansion coefficient, and fracture strength, have demonstrated the high potential of zirconia–zirconium titanate composites for thermal shock applications in oxidizing atmospheres.

The addition of carbon nanotubes (CNTs) is expected to increase the fracture toughness of ceramic matrix composites, but an uniform dispersion of the nanotubes in the matrix is essential. This is a complex issue in aqueous medium because of the nanotubes hydrophobicity. In this ...

This paper deals with the rheological characterization of agar and foaming surfactant-containing suspensions for obtaining stoichiometric cordierite samples with tailored open macroporosity and their characterization through density and microstructural studies. The ...

ABSTRACT Belite calcium sulfoaluminate (BCSA) cements are low-CO2 building materials. However, their hydration behavior and its effect on mechanical properties have still to be clarified. Here, we report a full multitechnique study of the hydration behavior up to 120 days of nonactivated and activated BCSA laboratory-prepared clinkers, with β- or αH-belite as main phase, respectively. The effects of the amount of gypsum added were also studied. The hydration and crystallization processes are reported and discussed in detail. Finally, shrinkage/expansion data are also given. The optimum amount of gypsum was close to 10 wt %. Our study has demonstrated that β-belite reacts at a higher pace than α′H-belite, irrespective of the gypsum content. The hydration mechanism of belite determines the development of the mechanical strengths. These are much higher for activated BCSA cement, 65 MPa at 120 days, against 20 MPa for nonactivated BCSA cement, with the latter having larger amounts of stratlingite.