Antonio F. Fuentes

Enhanced radiation resistance of nanocrystalline pyrochlore Gd2(Ti0.65Zr0.35)2O7

The radiation response of nanostructured materials is of great interest because of the potential of nanoscale materials design for mitigating radiation damage. We report a greatly enhanced resistance to radiation-induced amorphization in nanocrystalline Gd2(Ti0.65Zr0.35)2O7 at a particle size less than 20 nm while larger crystals with the size >100 nm are radiation sensitive. The grain size of pyrochlore, Gd2(Ti0.65Zr0.35)2O7, can be controlled by mechanical milling and subsequent thermal treatment (from 800 to 1500 °C), offering the possibility of designing pyrochlore materials at the nanoscale with enhanced performance for specific radiation environments.

Tailoring Disorder and Dimensionality: Strategies for Improved Solid Oxide Fuel Cell Electrolytes

Reducing the operation temperature of solid oxide fuel cells is a major challenge towards their widespread use for power generation. This has triggered an intense materials research effort involving the search for novel electrolytes with higher ionic conductivity near room temperature. Two main directions are being currently followed: the use of doping strategies for the synthesis of new bulk materials and the implementation of nanotechnology routes for the fabrication of artificial nanostructures with improved properties. In this paper, we review our recent work on solid oxide fuel cell electrolyte materials in these two directions, with special emphasis on the importance of disorder and reduced dimensionality in determining ion conductivity. Substitution of Ti for Zr in the A(2)Zr(2-) (y)Ti(y)O(7) (A = Y, Dy, and Gd) series, directly related to yttria stabilized zirconia (a common fuel cell electrolyte), allows controlling ion mobility over wide ranges. In the second scenario we describe the strong enhancement of the conductivity occurring at the interfaces of superlattices made by alternating strontium titanate and yttria stabilized zirconia ultrathin films. We conclude that cooperative effects in oxygen dynamics play a primary role in determining ion mobility of bulk and artificially nanolayered materials and should be considered in the design of new electrolytes with enhanced conductivity.

Preparation of multicomponent oxides by mechanochemical methods

A large variety of synthesis strategies and processing techniques are currently being used to obtain new multicomponent oxides and/or modify existing ones. Among them, mechanochemical processing has become very popular because it is simple to implement, solvent free, and capable of providing enough volume of the target material in an economically viable manner. The preparation of complex oxides can benefit from mechanochemical methods for two important reasons: First, it is not a diffusion-controlled process and thus, high-rate solid state reactions can be promoted between oxides with different physical and chemical properties without using high temperatures; secondly, because reactants are processed under non-equilibrium conditions, uncommon metastable phases are frequently obtained featuring flexible crystal structures, small particle size, high concentration of defects, and off-stoichiometry. Furthermore, conversion to the “true” equilibrium phases induced by additional processing (e.g., firing) offers the possibility of isolating fairly stable intermediate states with unusual and desirable properties that are inaccessible for more conventional processing techniques. As oxide particles are hard and brittle, the number of oxide systems prepared by means of mechanochemical methods grew rapidly only in recent years when more powerful milling devices and abrasion-resistant milling tools became available. This article summarizes recent work carried out in the field; only dry milling of oxides (and occasionally carbonates) in the absence of additives is considered. Some of the main challenges of mechanochemical processing are also highlighted and discussed.

EFFECT OF GEOTHERMAL WASTE ON STRENGTH AND MICROSTRUCTURE OF ALKALI-ACTIVATED SLAG CEMENT MORTARS

Abstract Mortars of blast furnace slag replaced with 10% of a geothermal silica waste were cured for 90 days. The binder was activated by 6 wt.% Na 2 O equivalent of NaOH and water glass. The presence of the silica enhanced the formation of hydration products as shown by nonevaporable water (NEW) results. Backscattered electron images indicated that the microstructures of blended slag had less porosity than those of neat slag mortars and the interfacial zone between aggregate and hydration products was dense and of homogeneous composition similar to the matrix of hydration products. The main hydration products were C-S-H and for NaOH a hydrotalcite type phase was found as finely intermixed with the C-S-H.

Vibrational studies of A(B'2/3B''1/3)O3perovskites (A = Ba, Sr; B' = Y, Sm, Dy, Gd, In; B'' = Mo, W)

The Raman- and IR-active phonons were studied in ordered Ba(Y2/3Mo1/3)O3, Ba(Y2/3W1/3)O3, Ba(Gd2/3W1/3)O3, Ba(Sm2/3W1/3)O3, Ba(Dy2/3W1/3)O3, Ba(Dy2/3Mo1/3)O3 and Ba(In2/3W1/3)O3 as well as disordered Ba(In2/3Mo1/3) O3 and Sr(In2/3W1/3)O3. The assignment of the observed modes was given on the basis of lattice dynamical calculations. The studies performed revealed the presence of forbidden by the selection rules Raman bands for the Ba(In2/3Mo1/3)O3 sample. This result suggests that the Ba(In2/3Mo1/3)O3 contains 1:1 ordered domains of symmetry embedded in the disordered matrix and that Ba(In2/3Mo1/3)O3 and Ba(In2/3W1/3)O3 may be obtained in both ordered and disordered structures, depending on the thermal treatment. This behaviour resembles that of well known Pb(In1/2Nb1/2)O3, Pb(Sc1/2Nb1/2)O3 and Pb(Sc1/2Ta1/2)O3 relaxors.

Lithium insertion in two tetragonal tungsten bronze type phases, M8W9O47 (M=Nb and Ta)

A study of lithium insertion in two tetragonal tungsten bronze (TTB) type phases of general formula M 8 W 9 O 47 (M=Nb and Ta), is presented. The electrochemical insertion of up to 20 lithium atoms per formula unit in Nb 8 W 9 O 47 (Li/ΣM=1.2) proceeds through a reversible reaction with several single phase and one two-phase domains, while in Ta8W9O47 the reversibility of lithium insertion is limited to 15 atoms (Li/Sigma;M=0.9). Structural changes on Nb 8 W 9 O 47 as a function of the number of lithium atoms inserted have been studied by X-ray powder diffraction.

High-pressure behavior of A2B2O7 pyrochlore (A=Eu, Dy; B=Ti, Zr)

In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A2B2O7 pyrochlore (A = Eu, Dy; B = Ti, Zr) up to ∼50 GPa. Based on X-ray diffraction results, all compositions transformed to the high-pressure cotunnite structure. The B-site cation species had a larger effect on the transition pressure than the A-site cation species, with the onset of the phase transformation occurring at ∼41 GPa for B = Ti and ∼16 GPa B = Zr. However, the A-site cation affected the kinetics of the phase transformation, with the transformation for compositions with the smaller ionic radii, i.e., A = Dy, proceeding faster than those with a larger ionic radii, i.e., A = Eu. These results were consistent with previous work in which the radius-ratio of the A- and B-site cations determined the energetics of disordering, and compositions with more similarly sized A- and B-site cations had a lower defect formation energy. Raman spectra revealed dif...

Synthesis and structural characterization of Ba(LnIII2/3BVI1/3)O3 (LnIII=Dy, Gd and Sm; BVI=Mo or W) complex perovskites

Abstract We describe in this work the synthesis and crystal structure of five rare earth and Mo(VI) or W(VI) containing complex perovskites. The compounds studied are Ba(Dy 2/3 Mo 1/3 )O 3 , Ba(Dy 2/3 W 1/3 )O 3 , Ba(Gd 2/3 Mo 1/3 )O 3 , Ba(Gd 2/3 W 1/3 )O 3 and Ba(Sm 2/3 W 1/3 )O 3 and were prepared starting from solutions, by the polymeric precursors method. Structural characterization by HREM, SAED and powder XRD revealed the five compounds to be ordered cubic perovskites, SG Fm -3 m (225), with a cell parameter double of that of a simple perovskite cell and increasing as the size of the trivalent lanthanide ion increases (Dy

Pressure-induced structural modifications of rare-earth hafnate pyrochlore

Complex oxides with the pyrochlore (A2B2O7) and defect-fluorite ((A,B)4O7) structure-types undergo structural transformations under high-pressure. Rare-earth hafnates (A2Hf2O7) form the pyrochlore structure for A  =  La-Tb and the defect-fluorite structure for A  =  Dy-Lu. High-pressure transformations in A2Hf2O7 pyrochlore (A  =  Sm, Eu, Gd) and defect-fluorite (A  =  Dy, Y, Yb) were investigated up to ~50 GPa and characterized by in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD). Raman spectra at ambient pressure revealed that all compositions, including the defect-fluorites, have some pyrochlore-type short-range order. In situ high-pressure synchrotron XRD showed that all of the rare earth hafnates investigated undergo a pressure-induced phase transition to a cotunnite-like (orthorhombic) structure that begins between 18 and 25 GPa. The phase transition to the cotunnite-like structure is not complete at 50 GPa, and upon release of pressure, the hafnates transform to defect-fluorite with an amorphous component. For all compositions, in situ Raman spectroscopy showed that disordering occurs gradually with increasing pressure. Pyrochlore-structured hafnates retain their short-range order to a higher pressure (30 GPa vs.  <10 GPa) than defect-fluorite-structured hafnates. Rare earth hafnates quenched from 50 GPa show Raman spectra consistent with weberite-type structures, as also reported for irradiated rare-earth stannates. The second-order Birch-Murnaghan equation of state fit gives a bulk modulus of ~250 GPa for hafnates with the pyrochlore structure, and ~400 GPa for hafnates with the defect-fluorite structure. Dy2Hf2O7 is intermediate in its response, with some pyrochlore-type ordering, based on Raman spectroscopy and the equation of state, with a bulk modulus of ~300 GPa. As predicted based on the similar ionic radius of Zr4+ and Hf4+, rare-earth hafnates show similar behavior to that reported for rare earth zirconates at high pressure.

Preparation and magnetic properties of Zn–Ti subtituted Ba-ferrite powders

Abstract Zn–Ti-substituted barium ferrite powders were prepared by the sol–gel method and characterized. The room temperature magnetic properties were evaluated for different levels of dopant, Fe/Ba ratios, and heat-treatment temperatures. The results showed that M s was not very much influenced by the substitution level up to 0.6 at.%. Contrarily, H ci did show a marked decrease, owing to the reduction of the magnetocrystalline anisotropy of the BaM phase. On the other hand, an excess of Ba led to the decrease of M s and the increase of H ci . Additionally, Zn–Ti substitutions were effective in decreasing crystallite sizes below 100 nm.