Study on the synthesis of NiZnCu ferrite nanoparticles by PVA sol–gel method and their magnetic properties

Ni_0.65Zn_0.35Cu_0.1Fe_1.9O₄ nanoparticles fabricated by a polyvinyl alcohol (PVA) sol–gel process have been investigated by infrared spectra (IR), X-ray diffraction (XRD), transmission electron microscope (TEM) and Mössbauer spectroscopy measurements. The particles annealed at and above 250 °C are single phase NiZnCu ferrite with spinel structure. Particles annealed at and above 350 °C behave ferrimagnetically, while sample annealed at 250 °C is simultaneously paramagnetic and ferrimagnetic. In addition, the transition from the paramagnetic to the ferrimagnetic state can be observed in sample annealed at 250 °C as the measuring temperature decreases from 260 °C to liquid nitrogen temperature. The magnetic properties of NiZnCu ferrite nanoparticles are clearly size-dependent. The saturation magnetization increases with the annealing temperature. The coercivity of NiZnCu ferrite nanoparticles reaches a maximum when the annealing temperature is 550 °C.     

Photophysics of the porphyrins; unusual fluorescence of europium porphyrin complex entrapped in sol–gel silica matrix

In this paper, the study on encapsulation of water-soluble cationic porphyrins: methyl-pyridyl porphyrin (H₂TMePyP) and its Eu(III) complex in the monolith gels prepared by sol–gel method are reported. The samples doped with the porphyrins were prepared by tetraethoxysilane (TEOS) hydrolysis and condensation. Their absorption and emission spectroscopic properties in comparison with the spectra of the same compounds in various solvents are investigated. The spectra of metal complex were compared with those of free-base porphyrin. The strong fluorescence of europium porphyrin in the silica matrix is observed under excitation in Q-band (530 nm), while at the same time emission neither free-base porphyrin nor europium chloride does not occur. It can be explained by the strong interaction of the Eu(III)TMePyP(acac) with the silica.     

Encapsulation of protein molecules in transparent porous silica matrices via an aqueous colloidal sol–gel process

Encapsulation of several biologically important proteins, cytochrome c, catalase, myoglobin, and hemoglobin, into transparent porous silica matrices by an aqueous colloidal sol–gel process that requires no alcohol is reported. Optical characterization indicates a successful retention of protein conformation after encapsulation. The conformation retention is strongly correlated to both the rate of gelation and the subsequent drying speed. Using hemoglobin as a model protein, a higher colloidal solid concentration and a lower synthesis pH were found, both causing faster gelation, resulting in a better retention of conformation. Hemoglobin encapsulated in a thin film, which dries faster, also showed a better retention than in the bulk. This is attributed to the fact that when a protein is isolated, and especially when it is confined to a space close to its own dimensions, conformational changes are sterically hindered, hence the structural stability. Enzymatic activity of bovine liver catalase was also monitored and showed a remarkable improvement when encapsulated using the aqueous colloidal process, compared to using the conventional alkoxide-based process. Thus, the aqueous colloidal sol–gel process offers a promising alternative to the conventional sol–gel process for encapsulating biomolecules into transparent, porous matrices.     

Electric and magnetic properties of sol–gel silica powders doped with ferrofluid

SiO₂ powders with magnetic properties have been prepared via the sol–gel method. Morphology of the magnetic powders has been investigated by scanning electron microscope (SEM). The magnetic susceptibility and electric permittivity measurements of the obtained materials have been performed by means of the lumped-capacitance method. The complex magnetic permeability measurements have been performed by means of the inductance method using toroidal-shaped samples.     

Non-hydrolytic sol–gel synthesis and characterization of Eosin-Y doped titanosilicate thin films for waveguiding applications

We report a non-hydrolytic sol–gel method by which dye doped inorganic binary oxide thin films were prepared and characterized. Here the problem of incorporating organic dyes in an inorganic host without the degradation of organic active species is surpassed. This method avoids the hydroxyl quenching and the presence of titanium content in varying concentration will tailor the refractive index of the films. The prepared films were characterized using FTIR, scanning electron microscopy, m-line spectroscopy and absorption and emission spectroscopy. Highly uniform, homogenous, amorphous and crack-free dip-coated films were produced by this synthesis method. The films showed a refractive index of 1.5856, 1.6147 and 1.6392 with increasing titanium content.     

Er-doped SiO2–Al2O3 thin films prepared by the sol–gel route

The synthesis of homogeneous and pure silica–alumina binary glasses doped with rare-earth (RE) ions such as Er³⁺ is currently a key challenge for the development of integrated optics devices such as lasers, optical amplifiers or waveguides. In this study Er³⁺-doped SiO₂–Al₂O₃ films were prepared by the sol–gel route. Aluminium sec-butoxide, Al(O-sec-C₄H₉)₃ (ASB), and tetraethoxysilane, Si(OC₂H₅)₄ (TEOS), were used as glass oxide precursors, whereas erbium was introduced as Er(NO₃)₃. The alumina content in the silica matrix was 10 at.%, while erbium doping ranged between 200 and 5000 ppm. The preparation of the starting sol–gel solution and the layer deposition by a dip-coating procedure were performed in dry-box under nitrogen atmosphere. The obtained films were subsequently annealed in air between 300 and 1000 °C. After treatment at 500 °C, layers 200 nm thick were obtained. The composition, microstructure and surface morphology of the films were investigated by X-ray photoelectron spectroscopy (XPS), secondary-ion mass spectrometry (SIMS), glancing incidence X-ray diffraction (GIXRD) and atomic force microscopy (AFM). Crack-free, transparent, high purity films were obtained, characterised by compositional and microstructural homogeneity.     

The size-effect on luminescence properties of BaTiO3:Eu nanocrystallites prepared by the sol–gel method

Luminescence investigations of nanocrystalline (nc) BaTiO₃ powders doped with Eu³⁺-ions prepared by the sol–gel method is described. A stability of Eu³⁺-ions replacing Ba²⁺-sites up to 1100 °C is reported. The effect of BaTiO₃ grain sizes on luminescence properties of Eu³⁺ was noticed. To explain these differences a detailed analysis of luminescence spectra by the Judd–Ofelt theory has been performed.     

On the synthesis and crystallization process of nanocrystalline PZT powders obtained by a hybrid sol–gel alkoxides route

Sol–gel synthesis has been a very successful and efficient route for obtaining high quality ferroelectric materials. Particularly, the alkoxides based route has proven its advantages due to the better homogeneity and easier controlled gelation rate. In this work, butoxides and propoxides are used to obtain nanocrystalline PZT powders in order to determine the influence of each B-site precursor on crystallization. Along with this, we study the involved chemical reactions (reactants and intermediates) and the crystallization process by using several spectroscopic techniques (Raman, FT-IR and XRD) as well as thermogravimetric (TGA) and calorimetric (DTA) analysis. The evolution of the initial gel solution and the amorphous powder into the final perovskite PZT phase is traced. According to our results, the propoxy route tends to be the more suitable for good PZT crystallization under our experimental conditions.     

Structural, dielectric and piezoelectric properties of aluminium doped PLZT ceramics prepared by sol–gel route

Nanosized piezoelectric ceramics for vibration sensor applications have been prepared by mixing the ferroelectric PLZT (8:60:40) with variable doping fractions of trivalent aluminium ion (Al³⁺). Samples have been prepared through a standard sol–gel route. X-ray diffraction and scanning electron microscopy (SEM) have been used to determine the phase and morphological modifications. Transmission electron microscopy (TEM) studies reveal the microstructure with nanosized well-dispersed homogeneous spherical particles. The vibrational infra-red (IR) spectroscopy record is taken to locate the position of the doping Al³⁺ ion. Using electrical impedance spectroscopy, the resonance and anti-resonance frequencies of the Al modified PLZT system have been determined and analysed. Al addition in PLZT has left a profound effect in its dielectric and piezoelectric properties. An interpretation of the role of Al addition is proposed in terms of structure modification. The sensing power of the investigated material was found useful for the vibration control of a cantilever beam.     

Characterization of particulate sol–gel synthesis of LiNi0.8Co0.2O2 via maleic acid assistance with different solvents

Particulate sol–gel LiNi_0.8Co_0.2O₂ has been synthesized by a maleic-acid-assisted process using de-ionized water or ethanol as the solvent. A comparison of the effect on these two different solvents was made on the basis of thermal studies, Fourier transform infrared spectroscopy, X-ray diffraction analysis, chemical diffusion coefficients measurement, and electrochemical cyclability tests. An esterification reaction occurred on the xerogel prepared with ethanol as solvent, reducing Ni and Co from their nitrate salts. LiNi_0.8Co_0.2O₂ grew at the expense of Li₂CO₃, NiO, and CoO during calcination. Better results of capacity and cyclability were obtained in a DI-water-solvent sample associated with a larger interslab thickness between OLiO and lower Ni occupancy on the Li site. The activation energy for the calcinations of DI-water-solvent sample is one-half of that of the ethanol-solvent one, which could be the reason for its better properties. Chemical diffusion coefficients of Li⁺ ion are of the same order 10⁻¹⁰ cm²/s, is not affected by the solvents used and/or the temperature raise to 55 °C.     

Immobilization of Mo(IV) complex in hybrid matrix obtained via sol–gel technique

A molybdenum(IV) complex, trans-bis-[1,2-bis(diphenylphosphino)ethane]-fluoro-(diazopropano)-molybdenum tetraphenylborate, [MoF(DIAZO)(dppe)₂][BPh₄], was prepared and immobilized in a hybrid matrix synthesized by the sol–gel process. The host matrix, designated as U(500), is an organic–inorganic network material, classed as ureasil, that combines a reticulated siliceous backbone linked by short polyether-based segments. Urea bridges make the link between these two components, and the polymerization of silicate substituted terminal groups generates the inorganic network. The free Mo(IV) complex and all new materials were characterized by spectroscopic techniques (FT-IR and UV–Vis) and thermal analysis (DSC). The ionic conductivity of the resulting material was also studied. The results indicate that immobilized Mo(IV) complex has kept its solid-state structure, although there is evidence of inter-molecular interactions between the Mo(IV) complex and some groups/atoms of the hybrid host matrix.     

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.     

Evolution of corrosion protection performance of hybrid silica based sol–gel nanocoatings by doping inorganic inhibitor

Silica based hybrid sol–gel coatings were developed to protect AA2024 alloy from corrosion. In order to have an active protection, cerium nitrate corrosion inhibitor was introduced into the coating system. The anti‐corrosion properties of the coatings were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) methods. The structure of the films was studied by scanning electron microscopy (SEM) after corrosion. The results indicate that the improvement of the protection properties of the films occurred with immersion time. This would imply that cerium ions could reach the defects, hindering the corrosion reactions and thus reduces the corrosion rate.     

Low-temperature synthesis and characterization of (Zn,Ni)TiO3 ceramics by a modified sol–gel route

Hexagonal ilmenite-type (Zn_1−xNi_x)TiO₃ (x = 0, 0.85–1.0) ceramic powders were successfully synthesized by a sol–gel route with low temperature (800 °C) sintering, which was modified by using the two-step heat treatment so as to obtain pure products. The thermal stability of the hexagonal (Zn,Ni)TiO₃ was enhanced with the increasing amount of nickel addition. FE-SEM observations demonstrated that the average crystallite sizes of (Zn_1−xNi_x)TiO₃ remarkably decreased from more than 200 nm to less than 100 nm with the increasing solubility x. The dielectric properties of (Zn_1−xNi_x)TiO₃ were measured at different frequencies and the results showed that there existed maximum values both for the dielectric constants and the loss tangents at x = 0.85.     

Yttrium sol–gel coating effects on the cyclic oxidation behaviour of 304 stainless steel

The present results reveal the interest of sol–gel coating technique to improve 304 steel high temperature oxidation resistance. An yttrium sol–gel coating appears to enhance the oxidation resistance during isothermal oxidation test, to decrease widely the oxide weight gain and to reduce the initial transient oxidation stage generally observed in the case of blank steels. Moreover, the experimental results confirm that yttrium sol–gel coating also plays a significant role on the cyclic oxidation behaviour of the 304 steel. In fact, the yttrium addition promotes remarkably the prolongation of the period during which the oxide scale still remains adherent to the substrate.     

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.     

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.     

Synthesis and characterization of LiGaxMn2−xO4 (0 ≤ x ≤ 0.05) by triethanolamine-assisted sol–gel method

Spinel LiGa_xMn_2−xO₄ (0 ≤ x ≤ 0.05) cathode materials with phase-pure particles and nano-sized distribution were synthesized by sol–gel method using triethanolamine as the chelating agent. The effects of heat treatment on the physicochemical properties of the spinel LiGa_xMn_2−xO₄ powders were examined with thermogravimetric and differential thermal analysis (TG/DTA), powder X-ray diffraction (XRD) and scanning electron micrograph (SEM). The LiGa_xMn_2−xO₄ (0 ≤ x ≤ 0.05) electrodes were characterized electrochemically by charge/discharge experiments under a current rate of 0.5C at 55 °C. Although the Ga-doped spinel electrode showed smaller initial discharge capacity, it exhibited better cycling performance than the undoped-LiMn₂O₄ electrode. The dQ/dV versus potential plots at 55 °C revealed that the improvement in cycling performance of the Ga-doped spinel electrode is attributed to stabilization of the spinel structure by the presence of gallium ion.     

Correlation between dielectric properties and strain in Pb0.5 Sr0.5TiO3 thin films prepared by using the sol–gel method

Pb_0.5Sr_0.5TiO₃ (PST) thin films were fabricated by the alkoxide-based sol–gel process using spin-coating method on Pt/Ti/SiO₂/Si substrate. The PST films annealed from 500 °C to 650 °C for 1 h show a perovskite phase and dense microstructure with a smooth surface. The grain size and dielectric constant of PST films increase with the increase in annealing temperature, which reduces the SiO₂ equivalent thickness of the PST film. The crystallinity or internal strain in the PST thin films analyzed from the diffraction-peak widths correlates well with the decrease in the dielectric losses. The dielectric constants and dielectric loss (%) of the PST films annealed at 650 °C (t_eq=0.89 nm) were 549 and 0.21%, respectively.