Doping effect of Mn on the magnetic behavior in Ni-Zn ferrite nanoparticles prepared by sol-gel auto-combustion

The ferrite samples of a chemical formula Ni_0.5−xMn_xZn_0.5Fe₂O₄ (where x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) were synthesized by sol-gel auto-combustion method. The synthesized samples were annealed at 600 °C for 4 h. An analysis of X-ray diffraction patterns reveals the formation of single phase cubic spinel structure. The lattice parameter increases linearly with increase in Mn content x. An initial increase followed by a subsequent decrease in saturation magnetization with increase in Mn content is observed showing inverse trend of coercivity (H_c). Curie temperature decreases with increase in Mn content x. The initial permeability is observed to increase with increase in Mn content up to x=0.3 followed by a decrease, the maximum value being 362. Possible explanation for the observed structural, magnetic, and changes of permeability behavior with various Mn content are discussed.     

Optical properties of sol-gel fabricated Ni/SiO2 glass nanocomposites

Nickel nanoparticles were grown in silica glass by annealing of the sol-gel prepared silicate matrices doped with nickel nitrate. TEM characterization of Ni/SiO₂ glass proves the formation of isolated spherical nickel nanoparticles with mean sizes 6.7 and 20 nm depending on annealing conditions. The absorption and photoluminescence spectra of Ni/SiO₂ glasses were measured. In the absorption spectra, we observed the band related to the surface plasmon resonance (SPR) in Ni nanoparticles. The broadening of SPR was observed with decrease of Ni nanoparticle size. The width of the surface plasmon band decreases 1.5 times at the lowering of temperature from 293 to 2 K because of strong electron-phonon interaction. The spectra proved the creation of nickel oxide NiO clusters and Ni²⁺ ions in silica glass as well.     

Mesoporous TiO2-B nanowires synthesized from tetrabutyl titanate

Ultralong mesoporous TiO₂-B nanowires were synthesized via a hybrid hydrothermal-ion exchanging-thermal treatment using tetrabutyl titanate (TBOT) as a raw material. The phase transformations and porous structures of TiO₂-B nanowires were characterized and studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and N₂ adsorption-desorption measurement. Mesoporous TiO₂-B nanowires showed a length of several micrometers and diameter of about 25 nm. The porous structures of obtained TiO₂-B nanowires were demonstrated by BJH pore distribution measurement. The wirelike morphologies and porous structures of monodisperse nanowires calcined at 600 °C showed little change, which indicated that such nanowires possessed high thermal stability. The formation mechanism of TiO₂-B nanowires with mesoporous structures were also discussed based on our experimental results.     

Synthesis, structure and magnetic properties in the Nd2O3-Gd2O3 mixed system synthesized at 1200 °C

Phase relation studies in the Gd₂O₃-Nd₂O₃ system have been performed on (Gd_1−xNd_x)₂O₃ samples (0?x?1) with the purpose of performing a systematic study of the composition effects on their structural and magnetic properties. All the samples were synthesized by calcination of the related oxalates at 1200 °C in order to ensure the complete decomposition of the oxalates. Five phase regions, namely an A-type hexagonal, a B-type monoclinic, a C-type cubic solid solution and two biphasic mixtures of the former three phase fields were detected in this system. The magnetic susceptibility measurements showed the presence of antiferromagnetic interactions in all samples. The Curie-Weiss temperature shows a nonlinear dependence on concentration. Deduced effective magnetic moments are close to the free ion values.     

Improvement in electrical and magnetic properties of mixed Mg-Al-Mn ferrite system synthesized by citrate precursor technique

In the present work, mixed magnesium-manganese ferrites of composition Mg_0.9Mn_0.1Al_0.3Co_zFe_1.7−zO₄ where z=0.3, 0.5 and 0.7 have been synthesized by the citrate precursor technique. X-ray diffraction patterns of the samples confirmed the formation of single-phase spinel structure. The ferrites have been investigated for their electric and magnetic properties such as dc resistivity, Curie temperature, saturation magnetization, initial permeability and relative loss factor (RLF). Fairly constant value of initial permeability over a wide frequency range (0.1-20 MHz) and low values of the relative loss factor of the order of 10⁻⁴-10⁻⁵, in the frequency range 0.1-30 MHz, are the cardinal achievements of the present investigation. In addition to this, initial permeability was found to increase with an increase in temperature while RLF was observed to be low at these temperatures. The dc resistivity and Curie temperature were found to increase with an increase in cobalt content. The mechanisms contributing to these results are discussed in detail in this paper.     

Spinel-type solid solutions involving Mn and Ti:Crystal chemistry, magnetic and electrochemical properties

This study reports the structural and magnetic properties of spinel systems Li₄Mn_5−xTi_xO₁₂ (“4-5-12” series) and LiNi_0.5Mn_1.5−xTi_xO₄ (“LNMTO” series), both based on Mn⁴⁺ substitution by Ti⁴⁺. Intermediate compositions covering the whole range of compositions (0≤x≤5 and 0≤x≤1.5, respectively) were prepared by solid state reaction. The 4-5-12 system forms a continuous spinel solid solution, whereas the spinel phase range in LNMTO stops before the end member “LiNi_0.5Ti_1.5O₄”, which is multi-phased with a major hexagonal phase component. Cell parameters and (Mn,Ti)-O distances increase monotonically with titanium content in both series. In the LNMTO series, the end member LiNi_0.5Mn_1.5O₄ is known to form a superstructure with Ni/Mn cation ordering. Neutron diffraction and Raman spectroscopy show that this order is lost when Ti is substituted, even at low level (x=0.15). The LNMTO crystal chemistry is also complicated by the presence of partial cation inversion, and the presence of a secondary rocksalt-type phase that modifies the spinel stoichiometry. Magnetic properties are characterized by a competition between ferromagnetic and antiferromagnetic interactions; no magnetic ordering is achieved, in agreement with B-site cation frustration and disorder. Electrochemical measurements show that the Ti^3+/4+ and Mn^3+/4+ redox couples behave independently in the 4-5-12 series, and that titanium decreases the high-potential electrochemical redox activity of LNMTO because of its blocking character for electron transfer to and from the nickel sites in the spinel structure.     

Densification, microstructural evolution, and dielectric properties of CaTiO3-LaGaO3 microwave ceramics

The relations among the densification, microstructural evolution, and microwave dielectric properties of the (1−x)CaTiO₃-xLaGaO₃ ceramics with x=0.34 and 0.36 were investigated in this study. The results indicated that (1−x)CaTiO₃−xLaGaO₃ ceramics can be densified at 1300 °C with at least 97% of the theoretical value. The ceramics reported an orthorhombic perovskite structure, and no other detectable phases were found. Both ε_r and Q×f values can be improved by slowing the cooling rate during sintering. The ε_r and Q×f values of the 0.64CaTiO₃-0.36LaGaO₃ ceramics at cooling rates of >10 °C/min and 0.1 °C/min are 48.1 and 27,500 and 48.7 and 38,000, respectively. The higher densification obtained at a slower cooling rate plays an important role in improving the microwave dielectric properties.     

Evolution of pore and surface characteristics of silica xerogels during calcining

This paper has investigated the effect of calcining temperature (600-1000°C) on the mesopore structure and surface fractal dimension of silica xerogels derived from two-step acid-base-catalyzed TEOS (tetraethyl orthosilicate) systems. Two base contents were adopted in the base step. FE-SEM (field emission gun-scanning electron microscopy) observation and XRD (X-ray diffraction) analysis indicate that gel coarsening, agglomeration and sintering occurred successively with increasing temperature, and those prepared with higher base content exhibit easier agglomeration, lower sintering temperature and weaker crystallizability. Percent mesoporosity and surface fractal dimensions evaluation via image analysis methods shows that the mesoporosity of those prepared with lower base content increases below 700°C and decline at higher temperatures, while the surface fractal dimension appears to decrease above 800°C.     

Solvothermal synthesis and luminescent properties of YAG:Tb nano-sized phosphors

Nano-sized Tb-doped YAG phosphor particles were synthesized by a mixed solvo-thermal method using stoichiometric amounts of inorganic aluminum and yttrium salts. The formation of YAG:Tb was investigated by means of XRD and IR spectra. The pure crystalline-phase YAG was prepared under moderate synthesis conditions (300 °C and 10 MPa), indicating that ethanol partly replaces water as the solvent, thus favoring the formation of YAG. TEM images showed that YAG:Tb phosphor particles sintered at 300 °C were basically of spherical shape, with good dispersion about a particle size of around 80 nm. The crystalline YAG:Tb showed green emission with ⁵D₄−⁷F₆ (544 nm) as the most prominent group. The PL intensity and crystallinity of YAG:Tb phosphors increases with increasing synthesis temperature, and reaches maximum brightness at 300 °C, which is lower than that exhibited by a commercial product.     

Effect of electric-field-assisted thermal treatment on the strain and ferroelectric properties of (100)-oriented ferroelectric Pb(Zr0.52Ti0.48)O3 thin films

Ferroelectric Pb(Zr_0.52Ti_0.48)O₃ thin films were deposited on the Pt/Ti/SiO₂/Si substrate by a sol-gel method. As a direct electric field was applied on the films during thermal treatment, strain behavior and ferroelectric properties have been investigated. X-ray diffraction patterns show that great tensile strain exists nearby the interface of the 250 nm thin film while thermal treatment assisted with direct electric field can obviously relax it. The analysis of hysteresis loops indicates that the remnant polarization increases with the thermal treatment time. These results suggest that electric-field-assisted thermal treatment is an effective way to reduce films' tensile strain through the local plastic deformation in Pt layer and enhance the remnant polarization.     

Characterizations of octahedral zinc oxide synthesized by sonochemical method

The ultrasonic reaction of zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetramine (C₆H₁₂N₄) was investigated by varying the concentration of the reactants, the irradiation time, and the type of sonicator. The morphology, composition, and phase structure of the products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) and ultraviolet-visible (UV-vis) spectroscopy. Octahedral zinc oxide (ZnO) micropowders were formed at low concentrations, 0.05 M, of Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ in both lab-made sonicator and commercial ultrasonic bath. However, at concentrations between 0.1 and 1.0 M Zn(NO₃)₂-C₆H₁₂N₄ mainly plate-like zinc hydroxide nitrate hydrate (Zn₅(OH)₈(NO₃)₂(H₂O)₂) resulted with only a small fraction of ZnO, irrespective of the irradiation time employed, highlighting the sensitivity of the system to the concentration of the starting materials. Heat treatment of Zn₅(OH)₈(NO₃)₂(H₂O)₂ at 350 °C in air affords a ZnO phase of irregular morphology. Octahedral ZnO is found to exhibit slightly lower IR absorption and similar UV absorption to that of commercial prismatic hexagonal ZnO, although an extra peak due to small quantities of Zn₅(OH)₈(NO₃)₂(H₂O)₂ is observed.     

Nanocrystalline Sr2FeMoO6 prepared by citrate-gel method

Double perovskite Sr₂FeMoO₆ powders with small crystallite size have been synthesised with citrate-gel method. The starting solution pH was varied between 1.5 and 9.0 resulting in large differences in the phase composition and ordering of B^′/B^″ sites. The samples prepared at 975 °C had crystallite sizes under 40 nm whereas crystallite sizes of the samples prepared at 1050 °C were between 78 and 239 nm. The XRD patterns were refined with spacegroup I 4/m, which gave good results for both batches, although clearly better results were obtained with monoclinic P 2₁/n spacegroup for the 975 °C batch. The ordering and the saturation magnetization agreed well with each other after treatment at 1050 °C, but the samples prepared at 975 °C had a strongly reduced saturation magnetization from that given by the ordering.     

Electrochemical capacitance from carbon nanotubes decorated with titanium dioxide nanoparticles in acid electrolyte

The electrochemical activity of an electrode of carbon nanotubes (CNTs) attached with TiO₂ nanoparticles was investigated. A chemical-wet impregnation was used to deposit different TiO₂ particle densities onto the CNT surface, which was chemically oxidized by nitric acid. Transmission electron microscopy showed that each TiO₂ nanoparticle has an average size of 30-50 nm. Nitrogen physisorption measurement indicated that the porosity of CNTs is partially hindered by some titania aggregations at high surface coverage. Cyclic voltammetry measurements in 1 M H₂SO₄ showed that (i) an obvious redox peak can be found after the introduction of TiO₂ and (ii) the specific peak current is proportional to the TiO₂ loading. This enhancement of electrochemical activity was attributed to the fact that TiO₂ particles act as a redox site for the improvement of energy storage. According to our calculation, the electrochemical capacitance of TiO₂ nanocatalysts in acid electrolyte was estimated to be 180 F/g. Charge-discharge cycling demonstrated that the TiO₂-CNT composite electrode maintains stable cycleability of over 200 cycles.     

Effect of annealing temperature on the optical constants of zinc oxide films

In this paper, the effect of annealing temperature on optical constants was studied. The ZnO films were deposited on microscopic glass substrates using the sol-gel method for various annealing temperatures. The deposited zinc oxide (ZnO) films were characterized by an X-ray diffractometer (XRD), a spectrophotometer and scanning electron microscopy (SEM). The transmittance spectra recorded through the spectrophotometer exhibits 90% transmittance. The XRD spectra showed polycrystalline nature of ZnO film. Optical constants were determined through transmittance spectra using an envelope method. It was found that there was a significant effect of annealing temperature on the refractive index and extinction coefficient of deposited ZnO films. In this experiment, the optimum refractive index value of 1.97 was obtained at 350 °C annealing temperature at visible (vis) wavelength. The optical energy gap was found to be of ∼3.2 eV for all the samples. The top view of SEM showed the ZnO grain growth on the glass substrates.     

One-pot hydrothermal synthesis of β-MnO2 crystals and their magnetic properties

β-MnO₂ nanorods and flower-like structures were synthesized via a one-pot hydrothermal route at 160 °C without using any template and surfactant. It is demonstrated that H⁺ in the solution has a significant effect on the morphology and resultant magnetic property of β-MnO₂ crystals. Magnetic measurements indicate that the Néel temperature of the β-MnO₂ flower-like structures is 98 K, which is about 2 K higher than that of the nanorods counterpart.     

Magnetic properties of prussian blue modified Fe3O4 nanocubes

Size controlled cubic Fe₃O₄ nanoparticles in the size range 90–10 nm were synthesized by varying the ferric ion concentration using the oxidation method. A bimodal size distribution was found without ferric ion concentration and the monodispersity increased with higher concentration. The saturation magnetization decreased from 90 to 62 emu/g when the particle size is reduced to 10 nm. The Fe₃O₄ nanoparticles with average particle sizes 10 and 90 nm were surface modified with prussian blue. The attachment of prussian blue with Fe₃O₄ was found to depend on the concentration of HCl and the particle size. The saturation magnetization of prussian blue modified Fe₃O₄ varied from 10 to 80 emu/g depending on the particle size. The increased tendency for the attachment of prussian blue with smaller particle size was explained based on the surface charge. The prussian blue modified magnetite nanoparticles could be used as a radiotoxin remover in detoxification applications.     

Drude's model calculation rule on electrical transport in Sb-doped SnO2 thin films, deposited via sol-gel

The evaluation of free carrier concentration based on Drude's theory can be performed by the use of optical transmittance in the range 800-2000 nm (near infrared) for Sb-doped SnO₂ thin films. In this article, we estimate the free carrier concentration for these films, which are deposited via sol-gel dip-coating. At approximately 900 nm, there is a separation among transmittance curves of doped and undoped samples. The plasma resonance phenomena approach leads to free carrier concentration of about 5×10²⁰ cm⁻³. The increase in the Sb concentration increases the film conductivity; however, the magnitude of measured resistivity is still very high. The only way to combine such a high free carrier concentration with a rather low conductivity is to have a very low mobility. It becomes possible when the crystallite dimensions are taken into account. We obtain grains with 5 nm of average size by estimating the grain size from X-ray diffraction data, and by using line broadening in the diffraction pattern. The low conductivity is due to very intense scattering at the grain boundary, which is created by the presence of a large amount of nanoscopic crystallites. Such a result is in accordance with X-ray photoemission spectroscopy data that pointed to Sb incorporation proportional to the free electron concentration, evaluated according to Drude's model.     

Effect of pH of colloidal suspension on crystallization and activation energy of deep levels in SnO2 thin films obtained via sol-gel

Colloidal suspensions of tin dioxide (SnO₂) are prepared by the sol-gel method from suspensions with distinct pHs. The particles in solution are embedded by an electrical layer. Decreasing the pH contributes to the destruction of this layer, leading to a high degree of aggregation among particles (clusters) due to the generation of cross-linked bonds (Sn-O-Sn) between them. The aggregation affects the electrical properties of films deposited by dip-coating from these solutions, due to the higher packing produced by acid pH. The X-ray diffractograms of films indicate higher crystallinity for lower pH. The Arrhenius plot leads to activation energies of the deepest level, which was between 67 and 140 meV, for the films prepared from suspensions with pH 6-11. Lower pH films also presented higher electrical conductivity. Obtained activation energies may be related to different types of defects, which could be associated with oxygen vacancies with distinct neighborhoods, influenced by the pH and potential barriers between grains, due to distinct packing caused by cross-linked bonds. TGA/DTA results indicate an easier crystallization process for lower pH, which ends at lower temperature, in good agreement with X-ray diffraction data.     

Structure and magnetic properties of Ca1−x−yMgxCu2+yO3 influenced by isoelectronic substitution x

Phases of the composition Ca_1−x−yMg_xCu_2+yO₃ have been prepared for the first time. The compounds are isostructural with the known end-members CaCu₂O₃ and MgCu₂O₃ showing a two-leg spin-ladder-like connection of copper and oxygen atoms within the Cu₂O₃-layer. Opposite the spin ladders this layer is folded, which results in a long-range antiferromagnetic ordering of these phases. The Néel temperature can be adapted by variation of x in Ca_1−x−yMg_xCu_2+yO₃ between 24 and 80 K. Several structural features, which influence the magnetic ordering, are discussed.     

Effects of annealing temperature on the structural and photoluminescence properties of nanocrystalline ZrO2 thin films prepared by sol-gel route

Highly transparent nanocrystalline zirconia thin films were prepared by the sol-gel dip coating technique. XRD pattern of ZrO₂ thin film annealed at 400 °C shows the formation of tetragonal phase with a particle size of 13.6 nm. FT-IR spectra reveal the formation of Zr-O-Zr and the reduction of OH and other functional groups as the temperature increases. The transmittance spectra give an average transmittance greater than 80% in the film of thickness 262 nm. Photoluminescence (PL) spectra give intense band at 391 nm and a broad band centered at 300 nm. The increase of PL intensity with elevation of annealing temperature is related to reduction of OH groups, increase in the crystallinity and reduction in the non-radiative related defects. The luminescence dependence on defects in the film makes it suitable for luminescent oxygen-sensor development. The “Red shift” of excitation peak is related to an increase in the oxygen content of films with annealing temperature. The “Blue shift” of PL spectra originates from the change of stress of the film due to lattice distortion. The defect states in the nanocrystalline zirconia thin films play an important role in the energy transfer process.