E. Vila

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.

The effect of the starting solution on the physico-chemical properties of zinc ferrite synthesized at low temperature

Abstract Fine particle zinc ferrite powders (ZnFe2O4) were synthesized by co-precipitation of a bi-ionic Fe3+/Zn2+ solution with 1 M n-butylamine at low temperature. Ferric nitrate and ferrous sulphate solutions were used as the starting material to investigate the effect of the source of iron on particle size, morphology, thermal behaviour and surface area of the final products. ZnFe2O4. In both cases, Zn2+ ions were provided by ZnO. The ferrous ions of the sulphate solution were previously oxidized with H2O2 in sulphuric medium. The cubic spinel-type structure of the ferrite product was obtained at a lower temperature when nitrate solution was used. Zinc ferrite of smaller particle size and higher surface area was obtained when ferrous sulphate was used as the starting solution. The ferrite precursors produced at room temperature and final products were characterized by X-ray powder diffraction, Fourier transformed infrared spectroscopy, thermal analyses, scanning and transmission electron microscopy and nitrogen adsorption volumetry.

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.

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.

Low temperature synthesis of tetragonal nickel manganite spinels: Thermal behaviour and reactivity

A route for the low temperature preparation of nickel manganite spinels is described. This method has allowed to obtain, for the first time, tetragonal nickel manganite spinels NixMn3-xO4, in the composition range 0.6 ≤ x ≤ 1.0. This phase has been prepared by heating at 200°C gels obtained by addition of 1M n-butylamine to mixed Ni2+ and Mn2+ solutions. In this paper, we report about the results obtained from X-ray studies, of the thermal evolution and reactivity for the tetragonal spinel with x = 0.8, between 200° to 1000°C. The formation of a metastable γ-cation deficient cubic spinel is attained at 300°C; it decomposes at ≈ 500°C, giving at 600°C a mixture of NiMnO3 and α-Mn2O3. Above 800°C, the reaction between these two phases leads to the formation of the high temperature cubic spinel, in which (Mn4+Mn3+)B ≈ 1.0. Cell lattice and oxygen positional parameters have been determined from Rietveld refinements. From these data, ionic configuration and cation distribution formulae for cubic spinels have been derived. These results can account for the mechanism of transformations that takes place during the thermal treatments.

Synthesis of mixed ferrite with spinel-type structure from a stainless steelmaking solid waste

Abstract Acid recovery plant powder, a solid by-product of the stainless steel industry with high iron, chromium and nickel content, was used to synthesise a nanocrystalline zinc–chromium–nickel ferrite in order to recover the total metal content of this waste as a valuable ferric product. Zn 2+ was provided by dissolving ZnO in HNO 3 and precipitation with 1M n -butylamine solution. The cubic spinel-type ferrite obtained, at temperature as low as 350°C, was characterised by X-ray powder diffraction, scanning and transmission electron microscopy.

Study of nanocrystalline BiMnO3-PbTiO3: synthesis, structural elucidation, and magnetic characterization of the whole solid solution.

In the last ten years, the study and the search for new multiferroic materials have been a major challenge due to their potential applications in electronic technology. In this way, bismuth-containing perovskites (BiMO(3)), and particularly those in which the metal M position is occupied by a magnetically active cation, have been extensively investigated as possible multiferroic materials. From the point of view of synthesis, only a few of the possible bismuth-containing perovskites can be prepared by conventional methods but at high pressures. Herein, the preparation of one of these potential multiferroic systems, the solid solution xBiMnO(3)-(1-x)PbTiO(3) by mechanosynthesis is reported. Note that this synthetic method allows the oxides with high x values, and more particularly the BiMnO(3) phase, to be obtained as nanocrystalline phases, in a single step and at room temperature without the application of external pressure. These results confirm that, in the case of Bi perovskites, mechanosynthesis is a good alternative to high-pressure synthesis. These materials have been studied from the point of view of their structural characteristics by precession electron diffraction and magnetic property measurements.

High-sensitivity piezoelectric perovskites for magnetoelectric composites

Abstract A highly topical set of perovskite oxides are high-sensitivity piezoelectric ones, among which Pb(Zr,Ti)O3 at the morphotropic phase boundary (MPB) between ferroelectric rhombohedral and tetragonal polymorphic phases is reckoned a case study. Piezoelectric ceramics are used in a wide range of mature, electromechanical transduction technologies like piezoelectric sensors, actuators and ultrasound generation, to name only a few examples, and more recently for demonstrating novel applications like magnetoelectric composites. In this case, piezoelectric perovskites are combined with magnetostrictive materials to provide magnetoelectricity as a product property of the piezoelectricity and piezomagnetism of the component phases. Interfaces play a key issue, for they control the mechanical coupling between the piezoresponsive phases. We present here main results of our investigation on the suitability of the high sensitivity MPB piezoelectric perovskite BiScO3–PbTiO3 in combination with ferrimagnetic spinel oxides for magnetoelectric composites. Emphasis has been put on the processing at low temperature to control reactions and interdiffusion between the two oxides. The role of the grain size effects is extensively addressed.