Elastic behavior of La0.67Ca0.33MnO3:ZrO2 composites

A series of Colossal Magneto Resistance materials, with compositional formula (1 − x) La_0.67Ca_0.33MnO₃ + xZrO₂ (where x = 0%, 10%, 20%, 40%, 60%, 80%) were prepared by sol–gel technique. When characterized structurally by X-ray diffraction they are found to have cubic structure. After measuring their bulk densities, the ultra sonic longitudinal (V_l) and shear velocities (V_s) were measured at room temperature using the pulse transmission technique. Using the ultrasonic data, the values of Young's and rigidity moduli along with Poisson's ratio and Debye temperatures have been calculated. As the materials are porous, zero porous elastic moduli have also been arrived at using a well-known model. The observed variation of elastic moduli with varying ZrO₂ concentration has been explained qualitatively.     

Synthesis and structure screening of nanostructured chromium oxide powders

The evolution with calcinations of chromium oxide (Cr₂O₃) nanocrystals of catalytic interest, prepared from reduction of K₂Cr₂O₇ with maleic acid has been studied. The characterization of the materials was confirmed by X-ray diffraction (XRD), Fourier transformation infrared (FT-IR), transmission electron microscope (TEM), N₂ adsorption–desorption isotherms, and thermo-analytical methods. The influence of the operating variables such as molar ratio, pH, gelation time and specific surface area was investigated and discussed. The results showed that the K₂Cr₂O₇/maleic acid of 1:1.7 molar ratio at pH 7 and 15 days gelation time were considered to be the best conditions for producing Cr₂O₃ of the pore diameter distribution in the range of 2–6 nm and specific surface area of 123 m²/g. The particle size and phases of chromium oxide were affected by the ratio of K₂Cr₂O₇ to maleic acid, temperature, and time of calcination. The amorphous phase appeared at room temperature, whereas it turned to crystalline phase when the calcination temperature increased to about 400 °C.     

Effect of the synthesis route on the microstructure and the reducibility of LaCoO3

The effect of the synthesis route on the microstructure and reducibility of lanthanum cobaltates (LaCoO₃) perovskites was examined. Two synthesis methods were used: thermal decomposition of freeze-dried La–Co-citrates and the Pechini method. The crystal structure, morphology and defect structure of LaCoO₃ were characterized by XRD powder diffraction, TEM and SEM analyses and electron paramagnetic resonance spectroscopy. The reducibility was tested by thermal programmed reduction with hydrogen. The intermediate stage of reduction was determined by ex situ XRD experiments. LaCoO₃ powders obtained by the Pechini method were reduced relatively easier as LaCoO₃ obtained from freeze-dried citrates. The LaCoO₃ reduction yielded Co metal and La₂O₃ via the formation of oxygen deficient Brownmillerite-type La₃Co₃O₈ and La₂Co₂O₅ oxides. For LaCoO₃ obtained from freeze-dried citrates and annealed at higher temperatures, Co metal, in addition to oxygen deficient perovskites, was formed at the initial stage of the reduction. The different reducibility of LaCoO₃ obtained by the Pechini method and that from the freeze-dried citrates was discussed taking into account the formation of oxygen-deficient phases from the Brownmillerite and Ruddlesden–Popper series during the reduction.     

Synthesis of Er2Ti2O7 nanocrystals and its electrochemical hydrogen storage behavior

Ultrafine Er₂Ti₂O₇ was synthesized at 700 °C within 2 h by a soft-chemistry route named citric acid sol–gel method (CAM). The obtained Er₂Ti₂O₇ with high dispersibility was square-like and the average size was about 70 nm. The prepared Er₂Ti₂O₇ nanocrystals in 6 M KOH aqueous solutions were investigated as a hydrogen storage material. It was found that the Er₂Ti₂O₇ powders would function as electrochemical hydrogen storage, showed fair electrochemical reversibility, and considerably high charge–discharge capacity. The reversible discharge capacity of the Er₂Ti₂O₇ electrode was found to exceed 320 mAh/g and adsorption capability of hydrogen is up to 1.27% at a current rate of 100 mA/g. In addition, the cycling ability and high rate capability of the Er₂Ti₂O₇ electrode are fairly good with only 4% capacity decay after 25 cycles. Cyclic voltammograms (CVs) were carried out to further examine the electrochemical hydrogen storage mechanism of Er₂Ti₂O₇.     

Polymer-assisted sol–gel synthesis and characterization of Zn2SiO4:Eu powders

The synthesis of nanophosphors for various applications and the improvement of their working parameters constitute one of the most topical problems of nowadays technology. Zinc-silicate has been extensively used as host material for phosphors in cathode ray tubes, and nowadays for phosphors in electroluminescent devices. In this work we present novel synthesis procedure for obtaining Zn₂SiO₄:Eu³⁺ powder, based on the combination of standard sol–gel and modified combustion method. Influence of preparation conditions on the structure and morphology are analyzed using X-ray diffraction and scanning electron microscopy. Luminescence properties of Eu³⁺ are investigated with fluorescence emission and lifetime measurements.     

Synthesis and luminescent properties of GdSrAl3O7:Tb phosphor under VUV/UV excitation

Superfine powder of Tb³⁺ ion-doped aluminates phosphors, GdSrAl₃O₇:Tb³⁺ was synthesized with a precursor prepared by an EDTA-sol–gel method at 900 °C. Field-emission scanning electron-microscopy (FE-SEM) observation indicated a narrow size-distribution of about 150–300 nm for the particles with elliptical shape. Upon excitation with vacuum ultraviolet (VUV) and near UV light excitation, the phosphors show a strong emission line at around 542 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺, and the highest PL intensity at 542 nm was found at a content of about 12 mol% Tb³⁺. As the Tb³⁺ concentration increases, Tb ions strongly cross-relaxation interact resulting in a decrease of the lifetime. The results reveal that GdSrAl₃O₇:Tb³⁺ would be a promising green phosphor for PDP application.     

The wide band gap of highly oriented nanocrystalline Al doped ZnO thin films from sol–gel dip coating

Undoped and Al doped ZnO thin films were prepared on glass substrate by sol–gel dip coating from PVP-modified zinc acetate dihydrate and aluminium chloride hexahydrate solutions. The XRD patterns of all thin films indexed a highly preferential orientation along c-axis. The AFM images showed the average grain size of undoped ZnO thin film was about 101 nm whereas the smallest average grain size at 8 mol% Al was about 49 nm. The values of direct optical band gap of thin films varied in the range of 3.70–3.87 eV.     

New process for synthesis of ZnO thin films: Microstructural, optical and electrical characterization

Nanocrystalline ZnO thin films were prepared on glass substrates by using spin coating technique. The effect of annealing temperature (400-700 °C) on structural, compositional, microstructural, morphological, electrical and optical properties of ZnO thin films were studied by X-ray diffraction (XRD), Energy dispersive Spectroscopy (EDS), Atomic Force Microscopy (AFM), High Resolution Transmission Microscopy (HRTEM), Scanning Electron Microscopy (SEM), Electrical conductivity and UV-visible Spectroscopy (UV-vis). XRD measurements show that all the films are nanocrystallized in the hexagonal wurtzite structure and present a random orientation. The crystallite size increases with increasing annealing temperature. These modifications influence the optical properties. The AFM analysis revealed that the surface morphology is smooth. The HRTEM analysis of ZnO thin film annealed at 700 °C confirms nanocrystalline nature of film. The SEM results shows that a uniform surface morphology and the nanoparticles are fine with an average grain size of about 40-60 nm. The dc room temperature electrical conductivity of ZnO thin films were increased from 10⁻⁶ to 10⁻⁵ (Ω cm)⁻¹ with increase in annealing temperature. The electron carrier concentration (n) and mobility (μ) of ZnO films annealed at 400-700 °C were estimated to be of the order of 4.75-7.10 × 10¹⁹ cm⁻³ and 2.98-5.20 × 10⁻⁵ cm² V⁻¹ S⁻¹.The optical band gap has been determined from the absorption coefficient. We found that the optical band gap energy decreases from 3.32 eV to 3.18 eV with increasing annealing temperature between 400 and 700 °C. This means that the optical quality of ZnO films is improved by annealing.It is observed that the ZnO thin film annealing at 700 °C has a smooth and flat texture suited for different optoelectronic applications.     

Electrodeposition of sol–gel films on Al for corrosion protection

Sol–gel films were electrodeposited on aluminum electrodes following the methodology we have developed and is based on applying negative potentials. This increases the pH at the surface, causing acceleration of the polymerization. Our process follows the “two step method”, in which the monomer is first hydrolyzed in acidic solution (pH 4) and only then the negative potential is applied, which consumes protons and releases hydroxyl ions, thus enhancing the condensation.Films made of different monomers, i.e., tetraethoxysilane (TEOS), methyl trimethoxysilane and phenyl trimethoxysilane (PTMOS), were prepared, characterized and examined for their corrosion inhibition properties. Potentiodynamic polarization, electrochemical impedance spectroscopy, optical and scanning electron microscopy as well as atomic force microscopy have been used as a means of film characterization. Hydrophobic and steric silanes, such as PTMOS showed a considerable corrosion inhibition capacity as compared to the capacity exhibited by less hydrophobic and steric derivatives such as TEOS. The difference between the conventional dip-coating method and the electrodeposition approach for depositing sol–gel films was also examined, indicating a clear advantage of the latter.     

Tailored magnetic properties of Sm(Zn) substituted nanocrystalline barium hexaferrites

Sm and SmZn substituted nanocrystalline barium hexaferrites (Ba_1−xSm_xFe₁₂O₁₉ and Ba_1−xSm_xFe_12−xZn_xO₁₉, x = 0–0.6) were prepared by the sol–gel autocombustion process. X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) were used to characterize the phase composition, crystal structure and magnetic properties of the as-prepared barium hexaferrites. All results indicated that the substitution content (x) critically influenced on the phase composition and magnetic properties. When the substitution content x > 0.2, impurity phases such as α-Fe₂O₃, SmFeO₃ and ZnFe₂O₄ were detected, which diluted and weakened saturation magnetization (M_s). For the Sm-doped samples, owing to the hyperfine field, canting spin, magnetic dilution and impurity phases, M_s increased with x firstly, and then decreased when x > 0.03. On the other hand, the coercivity (Hc) continuously increased with x. Nevertheless, for the SmZn-doped samples, M_s reached maximum when x = 0.06 and Hc almost unchanged with x. Compared the magnetic properties of Sm- and SmZn-doped samples, it was proved that Zn²⁺ substituted Fe³⁺ at 4f₂ sites and Sm³⁺ substituted Ba²⁺, which changed Fe³⁺ to Fe²⁺ at 2a sites in order to satisfy the electroneutrality principle.     

Structural evolution of alumina membrane prepared on an alumina support using a sol–gel method

We prepared supported and unsupported alumina membranes using a sol–gel method. The supported membrane system consisted of an alumina support, two intermediate α-Al₂O₃ layers, and a top alumina membrane. The θ- to α-Al₂O₃ transformation in supported and unsupported alumina membranes was investigated using X-ray diffraction (XRD) and scanning electron microscopy. XRD patterns showed that the supported membrane had a 100°C higher θ- to α-Al₂O₃ transformation temperature than the unsupported one. A similar effect was observed for microstructures of the membranes. We explained their transformation-temperature difference with a stress generated in the supported top membrane using a theoretical approach.     

Effects of sucrose concentration on morphology and luminescence performance of Gd2O3:Eu nanocrystals

Red Gd₂O₃:Eu nanophosphors were prepared by novel sucrose-assisted combustion method. The sucrose hydrolysis and complexing procedures were discussed by the Fourier transform infrared spectroscopy (FTIR) analysis. The effect of sucrose-to-metal ratio (S:M) on morphology and luminescence intensity of Gd₂O₃:Eu nanocrystals were investigated. And the optimum ratio was found to be S:M = 7:1, as confirmed by the results of scanning electron microscope (SEM), transmission electron microscope (TEM) and photoluminescence (PL) measurements. The highest photoluminescence emission from the particles obtained at S:M = 7:1 was about 85% of the commercial red phosphors.     

Synthesis and characteristics of silica-modified ZnS nanoparticles by sol–gel-hydrothermal method

Monodisperse silica-modified ZnS nanoparticles were prepared by sol–gel-hydrothermal method. XRD, XPS and FT-IR analysis results proved that there existed strong interaction between SiO₂ and ZnS. ZnS was covered with silica. The crystallinity and sizes of ZnS nanoparticles depended on the silica content, and the addition of silica could effectively suppress the growth of ZnS crystals. A considerable blue shift in the absorbing band edge significantly was observed for the silica-modified ZnS, and the blue shift became more remarkable as increase of silica content. The room temperature photoluminescence showed two UV emission peaks (at 304 and 364 nm, respectively) and a broad green peak.     

Morphology, optical and magnetic properties of Zn1−xNixO nanorod arrays fabricated by hydrothermal method

Zn_1−xNi_xO (x = 0, 0.05, and 0.1) nanorod arrays were prepared by hydrothermal method. Morphology and structure analysis indicate that the nanorods have single-crystalline wurtzite structure, and no metallic Ni or NiO phase exists. Room temperature ferromagnetism (RTFM) was observed in the Zn_0.9Ni_0.1O nanorods. In addtion, photoluminescence spectra of Zn_1−xNi_xO (x = 0, 0.05, 0.1 and 0.2) samples exhibit near band edge UV emissions and orange-red emissions. And the orange-red emission is confirmed to have originated from the interstitial oxygen defects.     

Stress analysis of ceramic insulation coating on Cu/MgB2 wires for W&R MgB2 coils

Ceramic insulation coatings were produced on Cu/MgB₂ wires, which were fabricated by Hyper Tech Research Inc., using Continuous Tube Forming and Filling (CTFF) process, from the solution of Zr, and Y based organometalic compounds, solvent and chelating agent using reel-to-reel sol–gel technique for MgB₂ coils. Y₂O₃–ZrO₂/Cu/MgB₂ wires were annealed at 700 °C for 30 min with 5.8 °C/min heating rate under 4% H₂–Ar gas flow. Residual stresses were examined for Cu/MgB₂ wire and YSZ coatings with varying thicknesses. It was observed that displacement values are independent from YSZ thicknesses and the maximum effective stress value is in the Cu region. The surface morphologies and microstructure of samples were characterized using SEM. SEM micrographs of the insulation coatings revealed cracks, pinholes and mosaic structure.     

Facile autocombustion synthesis of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) perovskite via a modified complexing sol–gel process with NH4NO3 as combustion aid

Ammonia nitrate was applied as an oxidizer and combustion trigger to modify the normal combined EDTA-citrate complexing method into a process with autocombustion and low ignition temperature properties. Therefore, the synthesis procedure was greatly simplified. The effect of NH₄NO₃/metal ions to organic mole ratios and the heating temperature on the autocombustion behavior and the properties of the powders derived were investigated in detail. The critical amount of NH₄NO₃ for the autocombustion to occur was identified at the NO₃⁻ to citric acid to EDTA mole ratio of around 10:2:1. After the experimental optimization, well-crystallized nanostructured La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) powder with a specific surface area as high as 21 m²/g was obtained; it is comparable with that obtained from a normal complexing process. By adjusting the combustion parameters, the properties of the powders then derived can be tailored for different applications, such as nanograined dense membrane for oxygen separation membrane, and porous cathode for fuel cells and sensors.     

Fabrication of Dy2Ti2O7 nanocrystalline at 700 °C and its photocatalytic activity

Dy₂Ti₂O₇ nanocrystalline was fabricated by a soft-chemistry route named as citric acid sol–gel method (CAM). The fabricating process was monitored by XRD, FT-IR and TG-DTA methods. It was found that compared with traditional solid state reaction (SSR), Dy₂Ti₂O₇ was synthesized at a relatively low temperature (700 °C) and with shortened reaction time (2 h). The morphology and surface composition of obtained products were determined by TEM and SEM-EDX experiments. Results showed that the obtained Dy₂Ti₂O₇ with good dispersibility were all square-like; the average size was about 50 nm and there was ample oxygen-deficient (40%) on the product interfaces. Also, the obtained products had higher BET surface area (25.10 m²/g). These properties are very helpful for a catalyst to achieve excellent photocatalytic activity. Photodecomposition of methyl orange was used as the model system to evaluate its photocatalytic property. It was found that Dy₂Ti₂O₇ showed good photocatalytic activity and the decomposition rate of methyl orange was about 99% within 60 min.     

Structure and electrical properties of BaTiO3 prepared by sol–gel process

BaTiO₃ nanopowders were synthesized from alkoxide solution precursor by sol–gel process. Attempts had been made to understand the synthesis process with the help of thermo gravimetric (TG) and differential scanning calorimetry (DSC). XRD and SEM results showed that the BaTiO₃ powders calcined at 700 °C for 2 h were maintained in cubic phase and the average size was 25 nm. The effects of the initial grain sizes of BaTiO₃ powders on the phase structures, microstructures and dielectric properties of the ceramics were investigated. The results indicated that both the grain size and the relative density of the ceramics increased with the increase in sintered temperature. The permittivity at room temperature was not sensitive to the grain size.     

Structure and optical properties of nanocrystalline NiO thin film synthesized by sol–gel spin-coating method

NiO thin film was prepared by sol–gel spin-coating method. This thin film annealed at T = 600 °C. The structure of NiO thin film was investigated by means of X-ray diffraction (XRD) technique and scanning electron microscopy (SEM). The optical properties of the deposited film were characterized from the analysis of the experimentally recorded transmittance and reflectance data in the spectral wavelength range of 300–800 nm. The values of some important parameters of the studied films are determined, such as refractive index (n), extinction coefficient (k), optical absorption coefficient (α) and band energy gap (E_g). According to the analysis of dispersion curves, it has been found that the dispersion data obeyed the single oscillator of the Wemple–DiDomenico model, from which the dispersion parameters and high-frequency dielectric constant were determined. In such work, from the transmission spectra, the dielectric constant (_∞), the third-order optical nonlinear susceptibility χ⁽³⁾, volume energy loss function (VELF) and surface energy loss function (SELF) were determined.