Leticia M. Torres-Martínez

Influence of composition on the structure and conductivity of the fast ionic conductors La2/3-xLi3xTiO3 (0.03 ≤ x ≤ 0.167)

The solid solution La2/3−xLi3xTiO3 (0.03<x<0.167) has been investigated by powder X-ray diffraction (XRD), impedance spectroscopy (IS) and 7Li nuclear magnetic resonance (NMR) techniques. In these samples, a change of symmetry from tetragonal to orthorhombic is observed when the lithium content is decreased below x=0.06. Structural modifications produced are mainly due to cation vacancies ordering along the c-axis, which disappear gradually when the Li content increases. Two Li signals with different quadrupole constants are detected in 7Li NMR spectra of orthorhombic/tetragonal perovskites, which have been associated with two crystallographic sites. In La0.5Li0.5TiO3 perovskite, prepared by quenching from 1300°C into liquid nitrogen, an important mobility for Li was detected in 7Li NMR spectra. In perovskites analysed, the dependence of electrical conductivity on Li content departs from that expected on the basis of a random distribution of La and Li in A sites. Dc conductivity increases quickly with the Li content in orthorhombic samples, but changes much more smoothly in tetragonal ones, where vacancies distribution becomes progressively disordered. In all perovskites analysed, the dependence of dc-conductivity with temperature displays a non-Arrhenius behaviour, with activation energies of 0.39±0.02 and 0.29±0.02 eV in 160–250 and 250–360 K temperature ranges respectively.

Engineering of SiO2 Nanoparticles for Optimal Performance in Nano Cement-Based Materials

The reported research examined the effect of 5-70 nm SiO2 nanoparticles on the mechanical properties of nano-cement materials. The strength development of portland cement with nano-SiO2 and superplasticizing admixture was investigated. Experimental results demonstrate an increase in the compressive and flexural strength of mortars with developed nanoparticles. The distribution of nano-SiO2 particles within the cement paste plays an essential role and governs the overall performance of these products. Therefore, the addition of a superplasticizer was proposed to facilitate the distribution of nano-SiO2 particles. Superplasticized mortars with 0.25% of selected nano-SiO2 demonstrated a 16% increase of 1-day compressive strength, reaching 63.9 MPa; the 28-day strength of these mortars was 95.9 MPa (vs. strength of reference superplasticized mortars of 92.1 MPa). Increase of 28-day flexural strength of superplasticized mortars with selected nano-SiO2 was 18%, reaching 27.1 MPa. It is concluded that the effective dispersion of nanoparticles is essential to obtain composite materials with improved performance.

Preparation and characterization of MgO powders obtained from different magnesium salts and the mineral dolomite

Abstract The characterisation of the physical properties of MgO powders, obtained from three commercial magnesium compounds, MgSO 4 ·7H 2 O, MgNO 3 ·6H 2 O and Mg(CH 3 CO 2 ) 2 ·4H 2 O and the mineral dolomite (natural source from Mexico), synthesised by chemical precipitation, is presented. The decomposition of the precipitated Mg(OH) 2 was analysed by DTA/TGA and the crystallisation process was observed by XRD. The variation of the properties with the nature of the precursors at 960°C was studied: as the crystallite size, density, specific surface area, degree of agglomeration, and the total porosity. The microstructural differences between the MgO agglomerates were examined by SEM, at different temperatures.

Photodegradation of indigo carmine and methylene blue dyes in aqueous solution by SiC–TiO2 catalysts prepared by sol–gel

Indigo carmine and methylene blue dyes in aqueous solution were photodegraded using SiC-TiO(2) catalysts prepared by sol-gel method. After thermal treatment at 450°C, SiC-TiO(2) catalysts prepared in this work showed the presence of SiC and TiO(2) anatase phase. Those compounds showed specific surface area values around 22-25 m(2)g(-1), and energy band gap values close to 3.05 eV. In comparison with TiO(2) (P25), SiC-TiO(2) catalysts showed the highest activity for indigo carmine and methylene blue degradation, but this activity cannot be attributed to the properties above mentioned. Therefore, photocatalytic performance is due to the synergy effect between SiC and TiO(2) particles caused by the sol-gel method used to prepare the SiC-TiO(2) catalysts. TiO(2) nanoparticles are well dispersed onto SiC surface allowing the transfer of electronic charges between SiC and TiO(2) semiconductors, which avoid the fast recombination of the electron-hole pair during the photocatalytic process.

Methylene blue degradation by NaTaO3 sol-gel doped with Sm and La.

In this work, NaTaO(3) compounds doped with 1M% of La and Sm, were prepared by the sol-gel (SG) method and solid state (SS) reaction; and tested as photocatalysts on the degradation of methylene blue (MB) under UV light. The structural characterization by X-ray powder diffraction revealed that the crystallization of the NaTaO(3) phase prepared by the sol-gel method started at 600 degrees C, reaching maximum crystallization at 800 degrees C. It was determined that the presence of Sm and La retard the crystallization of the NaTaO(3) phase. On the other hand, the compounds synthesized in this work showed particle sizes in the nanometric scale, as it was observed by scanning electron microscopy (SEM). The specific surface area of the compounds synthesized by the sol-gel method, showed values 4 times higher than those obtained by the solid state reaction, favoring their functional and photocatalytic performance in the methylene blue degradation. In addition, the best photocatalytic performance was shown by the NaTaO(3) doped with Sm and heated at 600 degrees C, having a half-life time of 65 min.

Performance of Cement Systems with Nano-SiO2 Particles Produced by Using the Sol–Gel Method

The effect of 5- to 70-nm SiO2 nanoparticles on the mechanical properties of nanocement materials was examined. The strength development of portland cement with nano-SiO2 and superplasticizing admixture was investigated. Experimental results demonstrate an increase in the compressive and flexural strengths of mortars with SiO2 nanoparticles. Because the distribution of nano-SiO2 particles within the cement paste plays an essential role and governs the overall performance of these products, the addition of a superplasticizer was proposed to facilitate the distribution of nano-SiO2 particles. The application of superplasticizer and high-speed dispergation were found to be effective disagglomeration techniques to improve the strength of superplasticized portland cement mortars, which reached up to 63.9 and 95.9 MPa (Sample 4B3, synthesized by using the sol–gel method in a base reaction medium at ethanol-to-tetraethoxysilane and water-to-tetraethoxysilane molar ratios of 24) at the ages of 1 day and 28 days, respectively. At corresponding ages, the compressive strength of reference portland cement mortars was 53.3 and 86.1 MPa. It is concluded that the effective dispersion of nanoparticles is essential to obtain composite materials with improved performance.

Preparation of ternary compound Ba3Li2Ti8O20 by the sol–gel process

Abstract In this work, we have synthesized the compound Ba 3 Li 2 Ti 8 O 20 by the sol–gel method using titanium isopropoxide, barium isopropoxide and lithium acetate salt as precursors reactants. Ethanol was used as solvent and the reaction was carried out at pH 3 using glacial acetic acid as catalyst. The synthesized products were thermally treated at different temperatures and characterized by infrared (IR) spectroscopy, X-ray diffraction and thermal analysis. The crystalline phase Ba 3 Li 2 Ti 8 O 20 was achieved by heating the obtained gel at 800°C for 6 h, which represents a lower temperature and smaller reaction time than reported through the solid-state reaction method, which use temperatures of 1150–1200°C for 2–10 days.

Electrochemical lithium insertion in some niobates MNb2O6 (M=Mn, Co, Ni, Cu, Zn and Cd)

Abstract Several oxides with the general formula MNb 2 O 6 (M=Mn, Co, Ni, Cu, Zn and Cd) have been tested as cathodes for lithium batteries. When discharging electrochemical cells using these niobates as active materials down 0.5 V, the maximum observed lithium content ( x ) in Li x MNb 2 O 6 was x =0.9, 0.9, 1.15, 1.5, 2.3 and 3.0 for M=Mn, Co, Zn, Cd, Ni and Cu, respectively. In situ X-ray diffraction experiments revealed in all cases a certain loss of crystallinity as the insertion reaction proceeds. Only the copper niobate showed an almost complete amorphization after the initial discharge. The kinetics of lithium insertion in these compounds was studied by electrochemical spectroscopy in a stepping potential mode.

Phase formation and electrical properties in the system BaO–Li2O–TiO2

A survey of compound formation and phase equilibrium in the system BaO–Li2–TiO2 for compositions containing >50% TiO2 has been made at subsolidus temperatures, ca. 1150–1200 °C. Four ternary phases were encountered, all of which have variable composition. These are phase A, centred on Ba3Li2Ti8O20, phase B centred on BaLi2Ti6O14, phase C based on the line Ba2Ti10 –xLi4xO22 and phase E, a hollandite-like phase Ba3xLi(2x+ 4y)Ti(8-2x–y)O16. Phase A is new; phase B is the phase responsible for high Li+ ion conductivity reported by Zheng et al.(Solid State Ionics, 1989, 35, 235); phase C is related to one reported by Tillmanns and Wendt (Z. Kristallogr., 1976, 144, 16) whose formula coincided with x= 0.75; phase E was reported by Sucket et al.(J. Mater. Chem., 1992, 2, 993). In addition, an extensive area of ternary solid solution based on Ba4Ti13O30 is reported. Mechanisms of solid-solution formation are discussed; the two principal mechanisms appear to be Ti4+⇌ 4Li+ and 2Ti4+⇌ 2Li++ 3Ba2+. Electrical conductivity data are reported; phases A, C and D appear to be very modest electronic conductors.

Electrochemical lithium insertion in some nickel, zinc and cadmium vanadates

Abstract In this work we present a study of the electrochemical characteristics of lithium insertion in eight crystalline Ni, Zn and Cd vanadates. During the electrochemical study carried out down to 0.5 V vs. Li + /Li, CdV 2 O 6 and Cd 2 V 2 O 7 were found to be the compounds tested accepting the highest number of lithium atoms per metal atom (Li/∑M=1.75 and 1.63, respectively). The zinc and nickel vanadates tested accepted a smaller number of lithium atoms per formula unit. In situ X-ray diffraction experiments showed an almost complete amorphization of the cadmium vanadates at the end of the first discharge while nickel and zinc vanadates were not amorphous. Therefore, these results are in agreement with previous reports of larger lithium atom intake in amorphous than in crystalline materials. In any case, on cycling none of these vanadates perform as well as previously described vanadium compounds.