Rosa M. Rojas

Synthesizing nanocrystalline LiMn2O4 by a combustion route

Nanocrystalline samples of lithium manganese oxide with cubic spinel structure have been prepared by combustion of reaction mixtures containing Li(I) and Mn(II) nitrates that operate as oxidisers, and sucrose that acts as fuel. The samples were characterised by X-ray diffraction, transmission electron microscopy, thermal analysis, and impedance and electrochemical measurements. The effect of the fuel content on the purity and morphology of the products was analysed. The samples as prepared showed small amounts of Mn2O3 and Mn3O4 as impurities, depending on the amount of sucrose used in the synthesis. Annealing at 700 °C led to single-phase cubic spinels. In these phases, the smallest average particle size (ca. 30 nm) corresponded to the sample obtained with a hyperstoichiometric amount of fuel. This sample showed the Li1.05Mn1.95O4 composition as deduced from the thermal and electrochemical data. No variation in conductivity associated with the cubic⇔orthorhombic phase transition was observed. The electrochemical behaviour as positive electrode showed good cyclability at high current densities (reversible capacity of 73 mAh g−1 at 2.46 mA cm−2).

Preparation and characterization of spinel-type Mn–Ni–Co–O negative temperature coefficient ceramic thermistors

Powders of ternary Mn–Ni–Co oxide negative temperature coefficient thermistors were synthesized by a low-temperature alternative route. The procedure allowed straightforward preparation without the addition of any binder, of highly densified Mn–Ni–Co–O semiconducting ceramics as cubic single-phase spinels, at relatively moderate temperature (≈1000 °C). A tentative cation distribution for the Mn1.5Ni0.6Co0.9O4 spinel oxide has been proposed, and its variation with temperature has also been considered. The dilatometric and X-ray powder diffraction studies carried out for Mn1.5Ni0.6Co0.9O4 showed that sintering takes place in a single stage between 900 and 1000 °C, and yields highly densified ceramic with an apparant density larger than 96% of the calculated X-ray density. Scanning electron microscopy showed different microstructures for the Mn1.5Ni0.6Co0.9O4 spinel oxide, depending on sintering conditions. The value of the sensitivity index, β=3068 K, indicates a good technological thermistor performance for this material.

Nanosize LiNiyMn2 −yO4(0 < y≤ 0.5) spinels synthesized by a sucrose-aided combustion method. Characterization and electrochemical performance

Nanosize crystalline cathode materials of LiNiyMn2 − yO4 (0 < y ≤ 0.5) composition and spinel-type structure have been obtained by a single-step sucrose-aided self-combustion method. The as-prepared samples contained some amorphous organic impurities that were removed after a short period of heating at 500 °C. The pure single-phase spinels have been characterized by X-ray diffraction, transmission electron microscopy, chemical analysis, and nitrogen sorption isotherms. The samples consist of particles (ca. 24 nm size) that are aggregated in clusters (ca. 1 µm size) in which mesopores (10–80 nm size) appear among the particles. Additional heating at 800° and 1000 °C produces a slight increase in the cubic lattice parameter and a pronounced increase in particle size (>100 nm). Electrical conductivity decreases as the Ni content increases in accordance with an electron hopping mechanism between Mn3+ and Mn4+ ions. The 500 °C- and 800 °C-heated LiNi0.5Mn1.5O4 samples show good electrochemical behaviour at 4.7 V as cathode materials. The capacity (132.7 mA h g−1) found is close to the nominal capacity (146.7 mA h g−1) and remains constant for current densities in the range C/24–2C (where C = 2.6 mA cm−2). At higher current densities (2C–10C) the capacity decreases progressively. The cyclability at the C current density is ca. 99.7% for both samples.

Microstructural characterization of nanocrystals of ZnO and CuO obtained from basic salts

Abstract ZnO and CuO powders have been obtained by reacting β-Zn(OH)Cl, Zn5(OH)8Cl2·H2O and γ-Cu2(OH)3Cl with n-butyl-amine at room temperature. Microstructural characterization of both ZnO and CuO oxides have been carried out by X-ray powder diffraction, using the Rietveld method and convolutive X-ray line broadening analysis. Results indicate very small crystallite sizes: v ,h00 =79 A and v ,00l =39 A for ZnO obtained from β-Zn(OH)Cl, v ,h00 =102 A and v ,00l =105 A for ZnO from Zn5(OH)8Cl2·H2O and v ,hkl =115 A for CuO. ZnO and CuO nanocrystals are slightly microstrained. TEM st udies confirms the crystallite size and morphology obtained from X-ray peaks analysis.

Conductivity relaxation parameters of some Ag+ conducting tellurite glasses containing AgI or the (AgI)0.75 (T1I)0.25 eutectic mixture

Abstract The conductivity relaxation parameters of some Ag + conducting tellurite glasses containing either AgI or the (AgI) 0.75 -(T1I) 0.25 eutectic mixture have been determined from an analysis of ac conductivity data measured in a wide temperature range. Transport properties in these materials appear as due to an Ag + ion hopping mechanism. The stretched exponential function exp-( t /τ σ ) β has been used to describe the conductivity relaxation. The relaxation parameters have been investigated as a function of the AgI rate. The results obtained are shown to be in good agreement with the predictions of the Ngai coupling model. A correlation has been also established for the glasses investigated between the stretched exponent β of conductivity relaxation function with the decoupling index R τ ( T g ), proposed by Angell, where R τ ( T g ) is the τ s /τ σ ratio of the average structural and conductivity relaxation times at the glass transition temperature T g .

Electrochemical characteristics of cobalt-doped LiCoyMn2−yO4 (0≤y≤0.66) spinels synthesized at low temperature from CoxMn3−xO4 precursors

Abstract A series of Co-doped LiMn 2 O 4 spinels of general formula LiCo y Mn 2− y O 4 (0≤ y ≤0.66) has been synthesized by a new procedure, i.e. by reacting Co x Mn 3− x O 4 (0≤ x ≤0.99) precursors with LiOH·H 2 O at 600°C for 4 h. Structural and electrochemical characterization has been carried out. The spinels have been obtained as single-phase compounds, with lattice parameters decreasing from 8.2090(4) A for the LiMn 2 O 4 to 8.0664(4) A for LiCo 0.66 Mn 1.34 O 4 . The LiCo y Mn 2− y O 4 compounds have small crystallite size, that ranges from 250 to 400 A. The first cycle of the Li//LiCo y Mn 2− y O 4 cells has been studied by step potential electrochemical spectroscopy (SPECS). The lithium extraction/insertion mechanism has been studied. A linear diminution of the spinel capacity on increasing the Co-dopant content is observed for the first cycle. For the doped spinels a remarkable enhancement of the cyclability with a retention of the initial capacity >90% after 12 cycles at low current density (C/30) is observed.

Antimonic acid and sulfonated polystyrene proton-conducting polymeric composites

Abstract Polymer composites formed of a proton conductor and polyvinylidene fluoride (PVDF) are prepared. Sulfonated polystyrene (SPS) and antimonic acid (AAc) are used as organic and inorganic proton conductor, respectively. PVDF is the insulating matrix used as a binder to improve the mechanical stability of the composites compared with the pure proton conductors. The effect of the SPS and/or AAc on both the glass transition temperature, T g , and the melting temperature, T m , of the PVDF is studied. The ionic conductivity of the composites, after water vapor exposure at r.t., and after water immersion at 50°C, is measured at room temperature. The overall conductivity of the composite increases with time up to saturation, which is attained: (i) after 17 h for the water vapor exposure experiment and (ii) after ≈10 min, for the water immersion experiment. The ionic conductivity of the composites is smaller than that of the pure protonic conductors, but the composites show high-dimensional stability. The shape of the conductivity curve depends on the proton conductor chosen.

Thermal behaviour and reactivity of manganese cobaltites MnxCo3 –xO4(0.0 ⩽x⩽1.0) obtained at low temperature

The thermal behaviour and reactivity in air, N2 and vacuum of manganese cobaltites MnxCo3 –xO4(0 ⩽x⩽1.0) with a cubic spinel-type structure obtained at low temperature have been studied by thermal analysis, X-ray powder diffraction, IR spectroscopy, transmission electron microscopy and X-ray energy-dispersive spectroscopy. In air, the formation of mixed Co–Mn oxides with rock-salt-type structure as a single phase is attained between 1100–1300 °C, depending on x. In a vacuum, the rock-salt-type mixed oxide is formed at ca. 700 ° C; Co metal and MnO are the products obtained at 800 °C. The change with temperature of the lattice constant in the cubic spinel-type phase MnCo2O4, has been also determined. It increases from a= 8.167(2)A at 450 °C to 8.2626(3)A at 950 °C. These results are interpreted with regard to the progressive reduction of manganese in the cubic spinel lattice.

Low temperature preparation of manganese cobaltite spinels [MnxCo3−xO4 (0 ≤ x ≤ 1)]

Abstract A route of low temperature preparation of manganese cobaltite spinels Mn x CO 3−x O 4 (0 ≤ x ≤ 1) is described. Gels obtained by addition of n-butylamine to mixed Co 2+ and Mn 2+ solutions, after aged at 80° and heated at 200°C correspond to the spinel-type material. Manganese cobaltite MnCo 2 O 4 obtained by this procedure shows surface area (BET) of 44 m 2 /g, and a mean crystallite size of 11 nm. Cell parameter of MnCo 2 O 4 is smaller that the reported for this material obtained by the ceramic procedure; it seems to indicate the existence of Mn 3+ in the spinel-type phase when it is prepared by this low temperature method.

On the thermal decomposition of the zinc(II) hydroxide chlorides Zn5(OH)8Cl2·H2O and Β-Zn(OH)Cl

The thermal decomposition of Zn5(OH)8Cl2·H2O and β-Zn(OH)Cl materials under several experimental conditions has been studied by thermal analysis, X-ray powder diffraction and X-ray high-temperature powder diffraction techniques. Several reaction schemes are proposed to account for thermal decomposition reactions undergone by both zinc hydroxide chlorides.