J. L. Martín de Vidales

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.

Structural and microstructural features of pyrite FeS 2− x thin films obtained by thermal sulfuration of iron

Structural and microstructural properties of synthetic thin films of pyrite (FeS 2− x ), prepared by thermal sulfuration of iron layers, were investigated from Rietveld refinements of x-ray diffraction data, collected by step/scan mode. From this refinement lattice constant, a , and sulfur position parameter, u , nearest neighbor Fe–S and S–S bond distances and tetrahedral and octahedral bond angles have been determined. Moreover, sulfur deficit in the samples, surface and volume-weighted crystallite size and microstrains were also obtained. From these data, the influence of temperature and time of sulfuration and sulfur pressure on their structural and microstructural properties has been established. Stoichiometric pyrite thin films are obtained by sulfurating the iron films at low temperatures ( T s ∼ 600–700 K) during short times ( t s ∼ 0.5–2 h). These experimental conditions yield films with the highest a , u , Fe–S bond distance, and microstrains, as well as S/Fe ratios about 2.00, i.e., null sulfur vacancies, the smallest S–S bond distances, and crystallite size. Finally, the possible influence of these structural and microstructural characteristics on some physical properties (optical absorption, electrical resistivity …) of the films is discussed.

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.

Structural characterization of CuIn2Se3.5, CuIn3Se5 and CuIn5Se8 compounds

Abstract Polycrystalline CuIn2Se3.5, CuIn3Se5 and CuIn5Se8 have been synthesized in vacuum under different cooling conditions resulting in different phases. X-ray diffraction and Rietveld refinement of the diagrams have been used to characterize the different structures obtained. The two former compounds were refined using two different types of structure: stannite (S.G.: I42m) and the proposed P–chalcopyrite (S.G.: P42c). The determined bond distances after refinements indicate a higher tetragonal distortion for stannite type structure than P–chalcopyrite structure. These results seem to indicate that the Cu–In–Se structure with these two stoichiometries belongs to the P42c S.G. On the other hand, for the CuIn5Se8 compound a far different XRD diagram was obtained where all the reflections have been assigned to two structures, named γ-phases, one trigonal (γ) and one hexagonal (γH).

COMPOSITION EFFECTS ON THE CRYSTAL STRUCTURE OF CUINSE2

By x‐ray powder diffraction and the Rietveld refinement method, the atomic positions in CuInSe2 were determined for compositions close to stoichiometry. The Se position, x(Se), was found to be correlated to the Cu content. According to a model proposed by Jaffe and Zunger [Phys. Rev. B 29, 1882 (1984)], changes in x(Se) induce a variation in the optical band gap, Eg. This could explain the spread in energy gap, Eg, found by many authors for this compound. The increase in the lattice parameter, a, correlated with the difference between the energy dispersive analysis of x rays and x‐ray diffraction determined Cu contents, suggests the presence of a fraction of Cu atoms as interstitials.

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.

Crystal structure of synthesized CuGaTe2 determined by X-ray powder diffraction using the Rietveld method

A full profile X-ray powder diffraction structure refinement has been carried out on a sample of synthesized CuGaTe2 using graphite monocromatized CuKα step-scan data and a profile shape of the Pearson VII type. The most satisfactory convergence was achieved at Rp = 0.0666, Rwp = 0.0884, RB = 0.0106 and RF = 0.0102. The derived structural parameters at 26.5°C are: a = 0.602348(7), c = 1.193979(2) nm and x(Te) = 0.256(6). The ratio between lattice parameters, η = c/2a = 0.9911 (0), differs from 1.0, indicating a tetragonal distortion, and non-ideal anion displacements, x(Te)≠1/4, is manifested by the existence of bond alternation of Cu-Te and Ga-Te with interatomic distances of 0.262(5) and 0.2578(5) nm, respectively. These results show a light tetrahedral deformation produced by four-fold tetrahedra of the copper cation in the CuGaTe2 chalcopyrite-type structure.

Synthesis of nickel–chromium–zinc ferrite powders from stainless steel pickling liquors

A low-temperature method was used to synthesize a nickel–chromium–zinc ferrite from stainless steel pickling liquor, a waste product of the steel industry, which is listed in most industrialized countries as a toxic and hazardous waste. This article reports the recovery (as a valuable ferric product) of the total metal content of this waste (namely, iron, chromium, nickel, and minor manganese), by coprecipitation of the multi-ionic solution and Zn 2+ (provided by ZnO) with 1 M n-butylamine at room temperature. The spinel-type ferrite produced was characterized by x-ray diffraction, thermogravimetric analysis and differential scanning calorimetry, and scanning electron microscopy. Its recorded magnetization of 2600 emu cm −3 allows its use in different magnetic applications. Furthermore, the synthesis method is a low-cost technology that yields a more environmentally friendly final effluent.

Quantification curves for XRD analysis of mixed-layer 14Aa/10Aa clay minerals

Using theoretical profiles of diffracted X-ray intensity for interstratification between layers having d-spacings around 14.3 Å and 10.1 Å, a series of diagrams was derived from which the proportion of 14.3 Å layers (W14) and the probability of passing from a 14.3 Å layer to a 10.1 Å layer (P14/10) can be derived. W14 can be derived independently of P14/10 using the angular distance between reflections situated at 18.2° and 25.4° 2θ (CuKα). Once W14 is determined, P14/10 may be obtained using the angular width of the diffuse reflections between 27° and 34° 2θ. In this case, two different diagrams are proposed for P14/10 determination because experimental X-ray patterns show either one or two diffuse reflections. Comparison of five experimental patterns with theoretical patterns calculated using W14 and P14/10 obtained using these diagrams indicates that the method can be useful for determining W14 and P14/10 in unknown samples. Moreover, the method described is independent of the Lorentz polarization factor and the layer type. The d-spacings associated with the two kinds of layers, however, should be similar (± 1%) to those for which the determinative diagrams were calculated.

Composition effects on structural and optical infrared properties of CuIn0.5Ga0.5Se2

The dependence of structural parameters and force constants for Cu–Se, Ga–Se, and In–Se bonds on compositional deviations in CuIn0.5Ga0.5Se2 have been studied. The composition gradient along the ingot was obtained by a single fusion at 1150 °C of the components and subsequent slow cooling in a still ampoule placed in a vertical furnace. All along the sample, a single chalcopyrite phase is present and its composition along its length was found by energy dispersive analysis of x-ray measurements on slices. Unit cell parameters, anion displacement, and Cu occupation fraction in its sublattice were analyzed by x-ray powder diffraction and Rietveld refinement methods. The anion displacement found is a function of the Cu defect in its sublattice. The existence of associated defects, i.e., two Cu vacancies and one Ga in Cu site, [2V(Cu)+GaCu], is proposed to explain the Cu defect in its sublattice and the changes in lattice parameters. This leads to the existence of BIII vacancies (BIII=In+Ga), and interstitial ...