Spherical shape BaO-ZnO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> glass powders prepared by spray pyrolysis

Fine-sized BaO-ZnO-B₂O₃-SiO₂ (BZBS) glass powders were directly prepared by high temperature spray pyrolysis. The hollow glass powders prepared at low preparation temperature of 1000 °C had a low density of 2.65 g/cm³. However, the densities of the BZBS powders obtained at preparation temperatures of 1200 and 1400 °C were each 3.92 and 4.13 g/cm³. The mean size of the BZBS glass powders prepared by spray pyrolysis at preparation temperature of 1400 °C was 0.98 μm. The glass transition temperature (T_g) of the prepared BZBS glass powders was 518.9 °C. The dielectric layers formed from the prepared BZBS glass powders with a dense structure had a clean surface and a dense inner structure without voids at the firing temperature of 580 °C. The transparencies of the dielectric layers formed from the prepared BZBS glass powders were higher than 90% within the visible range. PACS 42.70.Ce; 85.60.Pg; 71.55.Jv     

Formation and mechanical properties of alumina ceramics based on Al<Subscript>2</Subscript>O<Subscript>3</Subscript> micro- and nanoparticles

Alumina micro- and nanopowders with the particle size from 200 μm to 40 nm synthesized by the sol-gel method are studied. The particle size dependence of γ-Al₂O₃→α-Al₂O₃ phase transformation is studied by differential thermal analysis, X-ray diffraction method, and transmission electron microscopy. X-ray diffraction data show that for alumina nanoparticles γ-Al₂O₃→θ-Al₂O₃ phase transformation occurs at 900°C, and for micro-particles it occurs in the temperature range 1150–1200°C. The alumina ceramics produced of alumina nanoparticles is shown to have higher flexural strength under three-point bending than the ceramics produced of micro-particles. The obtained results demonstrate that alumina particle size reduction stabilizes the formation of α-Al₂O₃ at lower temperatures, due to which the grain growth rate decreases and the flexural strength of monolithic oxide ceramics increases.     

Synthesis and magnetic properties of FeNi<Subscript>3</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> core-shell nanocomposites

In this study, FeNi₃/Al₂O₃ core-shell nanocomposites, where individual FeNi₃ nanoparticles were coated with a thin layer of alumina, were fabricated by a modified sol-gel method. Several physical characterizations were performed on the samples of FeNi₃/Al₂O₃ nanocomposites with different thickness of Al₂O₃ shell. The encapsulation of FeNi₃ nanoparticles with alumina stops FeNi₃ agglomeration during heat treatment, and prevents interaction among the closely spaced magnetic FeNi₃ nanoparticles. The Al₂O₃ insulating shell improves the soft magnetic properties of FeNi₃. The study of the complex permeability of the samples shows that the real part μ’ of the permeability of the sample with Al molar content of 20% (Al/(Fe+Ni)) is as high as 12, and independent of frequency up to at least 1 GHz. The tunneling magnetoresistance arising from the presence of the Al₂O₃ shell have also been studied.     

Surface morphology of PrBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-δ</Subscript> single crystals after the long-lasting high-temperature reduction

The results of the detailed scanning electron microscopy study of PrBa₂Cu₃O_7-δ single crystals after the long-lasting high-temperature post-growth treatment are reported. The presence of the unstable decomposition products on the crystal surface indicates that the onset of superconductivity must be related to the structural transformations in the bulk (approximately 20% of the total volume of the examined crystals). The time-dependent character of the superconductivity in the Pr-123 crystals could be attributed to the migration of defects in the Pr−Ba sublattice from the bulk toward the surface.     

Structural Aspects of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Magnetic Nanoparticles According to X-Ray and Neutron Scattering

Structural aspects of powders containing magnetic nanoparticles Fe₃O₄/CoFe₂O₄ with the anticipated “core-shell” structure are considered by means of comparative analysis with individual particles of Fe₃O₄, CoFe₂O₄ in accordance of data obtained from X-ray powder diffraction and small-angle scattering of X-ray (synchrotron) radiation and neutrons. It is shown that magnetic particles in the powders under study have a strong polydispersity and form complex aggregates. Characteristic sizes of the crystallites, as well as a ratio of magnetite to cobalt-ferrite in the composition of the Fe₃O₄/CoFe₂O₄ particles were evaluated from the analysis of the diffraction peaks. Аnalyzing the data on small-angle scattering, the dimensional characteristics of particles and aggregates, as well as the volume fraction of the last ones in the powders, have been obtained. Fractal dimensions of aggregates are determined. A significant difference is observed in the scattering on Fe₃O₄/CoFe₂O₄ particles and the total scattering consisting of partial contributions to scattering on individual magnetite (Fe₃O₄) and cobalt-ferrite (CoFe₂O₄) powders, which does not exclude the formation of the “core-shell” structure.     

The influence of the specific surface of grains on the luminescence properties of Nd<Superscript>3+</Superscript>-doped Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> nanopowders

Nd³⁺:Y₃Al₅O₁₂ (Nd:YAG) powders were prepared by the Pechini method in the temperature range of 800 to 1400 °C. The pure garnet phase of the obtained materials was confirmed by XRD studies. The size of the grains was controlled by the annealing temperature of the samples. Their morphologies were investigated by TEM and porosity measurements (BET). The effect of annealing temperature on the morphology and luminescence properties of Nd:YAG nanocrystallites was studied, and the results were compared to the properties of a Nd:YAG single crystal. A significant enhancement of the ⁴F_3/2→⁴I_9/2/⁴F_3/2→⁴I_11/2 intensity ratio with decreasing grain size was observed. It was found that the decay times of the Nd³⁺ luminescence depends on the specific surface and is significantly longer for well crystallized nanocrystalline grains than for single crystals having the same concentration of Nd³⁺ ions. The role of crystallinity and specific surface on the radiative processes is analyzed. PACS 78.55.-m; 78.20.Ci; 78.67.Bf; 78.68.+m     

Hydrothermal synthesis and magnetic properties of CuSb<Subscript>2</Subscript>O<Subscript>6</Subscript> nanoparticles and nanorods

Nanoparticles and nanorods of CuSb₂O₆ are prepared by hydrothermal method and its high temperature α-phase is stabilized at room temperature. The average size of the nanoparticles is ca. 13.7 nm. The nanorods, with a width of ca. 20 nm and an aspect ratio of ca. 5, are the agglomerates composing of smaller nanoparticles with an average size of ca. 8.3 nm. Compared with the high temperature α-phase of bulk sample at 400 K, the lattice of nanophases elongated in ab plane and compressed along c direction. The CuSb₂O₆ nanoparticles exhibit predominant paramagnetic phenomenon. The difference in magnetic properties of the nanoparticles and nanorods indicates the interfacial interaction of agglomerated nanoparticles.     

Synthesis,sintering, and conductivity of Mn<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>

Studies on the sintering of manganese pyrovanadate depending on the temperature and the crystallite size show that we are prevented from obtaining a bulk ceramic sample by the anisotropic growth of grains. Investigation of the electrical properties of Mn₂V₂O₇ in the temperature range of 250–800°C reveals the activation energy at which bulk conductivity is 0.62 eV.     

Magnetic properties of BiFe<Subscript>0.93</Subscript>Mn<Subscript>0.07</Subscript>O<Subscript>3</Subscript> powders obtained by ultrasonic spray pyrolysis

Samples of BiFe_0.93Mn_0.07O₃ with different specific surface area were synthesized for the first time by ultrasonic spray pyrolysis. The resulting powders consist of porous particles of a spherical shape of medium size ~0.5 μm and have record values of residual magnetization and coercive force. It is found that the magnetic properties of the porous powder particles are determined by the distortion of the crystal lattice and the presence of uncompensated magnetic moments of iron ions on the surface.     

Magnetic properties and local configurations of <Superscript>57</Superscript>Fe atoms in CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> powders and CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/PVA nanocomposites

The composition and magnetic properties of the powders extracted from CoFe₂O₄ aqueous suspensions and the CoFe₂O₄/PVA (PVA is polyvinyl alcohol) nanocomposites with a cobalt ferrite content of 10–30 wt % have been investigated using Mössbauer spectroscopy, transmission electron microscopy, and vibration magnetometry. The cationic formulas of the cobalt ferrites synthesized have been determined. The differences between samples synthesized at temperatures of 72.5 and 82.5°C have been revealed. The specific features of the observed changes in the agglomeration of CoFe₂O₄ particles after introducing into the PVA matrix have been studied. It has been shown that the iron ion distribution determined by Mössbauer spectroscopy in octahedral and tetrahedral lattice sites correlates with vibration magnetometry data.     

Photolumiscent Properties of Nanorods and Nanoplates Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript>

Nanorods and nanoplates of Y₂O₃:Eu³⁺ powders were synthesized through the thermal decomposition of the Y(OH)₃ precursors using a microwave-hydrothermal method in a very short reaction time. These powders were analyzed by X-ray diffraction, field emission scanning electron microscopy, Fourrier transform Raman, as well as photoluminescence measurements. Based on these results, these materials presented nanoplates and nanorods morphologies. The broad emission band between 300 and 440 nm ascribed to the photoluminescence of Y₂O₃ matrix shifts as the procedure used in the microwave-hydrothermal assisted method changes in the Y₂O₃:Eu³⁺ samples. The presence of Eu³⁺ and the hydrothermal treatment time are responsible for the band shifts in Y₂O₃:Eu³⁺ powders, since in the pure Y₂O₃ matrix this behavior was not observed. Y₂O₃:Eu³⁺ powders also show the characteristic Eu³⁺ emission lines at 580, 591, 610, 651 and 695 nm, when excited at 393 nm. The most intense band at 610 nm is responsible for the Eu³⁺ red emission in these materials, and the Eu³⁺ lifetime for this transition presented a slight increase as the time used in the microwave-hydrothermal assisted method increases.     

Transport characteristics of phonons and the specific heat of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:ZrO<Subscript>2</Subscript> solid solution single crystals

The temperature dependences of the specific heat and transport characteristics of phonons in single crystals of yttrium-stabilized zirconium dioxide Y₂O₃:ZrO₂ solid solutions have been studied. It has been shown that the temperature dependences of the specific heat at T > 5 K are almost identical at the degree of stabilization of a solid solution with an Y₂O₃ content of 5–20 mol %. Differences in the temperature dependences of the specific heat of samples from different sources at T < 5 K are due to the presence of low-energy two-level systems. The features of the transport characteristics of thermal phonons at liquid helium temperatures reflect not only the presence of two-level systems but also the scattering of phonons on low-dimensional domains of another phase coherently conjugate to the main phase of the Y₂O₃:ZrO₂ solid solution.     

ZnO-coated LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode material for lithium-ion batteries synthesized by a combustion method

ZnO-coated LiMn₂O₄ cathode materials were prepared by a combustion method using glucose as fuel. The phase structures, size of particles, morphology, and electrochemical performance of pristine and ZnO-coated LiMn₂O₄ powders are studied in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge test, and X-ray photoelectron spectroscopy (XPS). XRD patterns indicated that surface-modified ZnO have no obvious effect on the bulk structure of the LiMn₂O₄. TEM and XPS proved ZnO formation on the surface of the LiMn₂O₄ particles. Galvanostatic charge/discharge test and rate performance showed that the ZnO coating could improve the capacity and cycling performance of LiMn₂O₄. The 2 wt% ZnO-coated LiMn₂O₄ sample exhibited an initial discharge capacity of 112.8 mAh g^?1 with a capacity retention of 84.1 % after 500 cycles at 0.5 C. Besides, a good rate capability at different current densities from 0.5 to 5.0 C can be acquired. CV and EIS measurements showed that the ZnO coating effectively reduced the impacts of polarization and charge transfer resistance upon cycling.     

Electronic excitations and defects in nanostructural Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al₂O₃ (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al₂O₃ nanopowders probed before and after Fe⁺ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ～17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al₂O₃ nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ₁ = 0.5 ns, τ₂ = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P^? centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F⁺ center (F _S ⁺ -type centers). In addition to the fast luminescence, nano-Al₂O₃ was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al₂O₃ suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.     

Synthesis and characterization of hollow nanoparticles of crystalline Gd<Subscript>2</Subscript>O<Subscript>3</Subscript>

Although Gd₂O₃ (gadolinia) nanoparticle is the subject of intense research interest due to its magnetic property as well as controllable emission wavelengths by doping of various lanthanide ions, it is known to be difficult to prepare monodisperse crystalline gadolinia nanoparticles because it requires high temperature thermal annealing process to enhance the crystallinity. In this article, we demonstrate the synthesis of hollow nanoparticles of crystalline Gd₂O₃ by employing poly(N-vinylpyrrolidone) (PVP) to stabilize the surface of Gd(OH)CO₃·H₂O nanoparticles and to successively form SiO₂ shell as a protecting layer to prevent aggregation during calcinations processes. Silica shells could be selectively removed after calcinations by a treatment with basic solution to give hollow nanoparticles of crystalline Gd₂O₃. The formation mechanism of hollow nanoparticles could be suggested based on several characterization results of the size and shape, and crystallinity of Gd₂O₃ nanoparticles by TEM, SEM, and XRD.     

Properties of powders produced by evaporating CeO<Subscript>2</Subscript>/Gd<Subscript>2</Subscript>O<Subscript>3</Subscript> targets exposed to pulsed-periodic radiation of a CO<Subscript>2</Subscript> laser

The characteristics (phase composition, grain shape, grain size distribution, and specific surface area) of Ce_0.78Gd_0.22O_2-δ nanopowders produced by exposing the target to pulsed CO₂ laser radiation are reported. Reasons for a threefold increase in the output of the experimental powder-preparation unit (up to 60 g/h) with the characteristic grain size (≈10 nm) remaining unchanged are discussed.     

Elevated electrochemical property of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> originated from nano-sized Mn<Subscript>3</Subscript>O<Subscript>4</Subscript>

By employment of nano-sized pre-prepared Mn₃O₄ as precursor, LiMn₂O₄ particles have been successfully prepared by facile solid state method and sol-gel route, respectively. And the reaction mechanism of the used precursors of Mn₃O₄ is studied. The structure, morphology, and element distribution of the as-synthesized LiMn₂O₄ samples are characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Compared with LiMn₂O₄ synthesized by facile solid state method (SS-LMO), LiMn₂O₄ synthesized by modified sol-gel route (SG-LMO) possesses higher crystallinity, smaller average particle size (~175 nm), higher lithium chemical diffusion coefficient (1.17 × 10^?11 cm² s^?1), as well as superior electrochemical performance. For example, the cell based on SG-LMO can deliver a capacity of 85.5 mAh g^?1 at a high rate of 5 °C, and manifests 88.3% capacity retention after 100 cycles at 0.5 °C when cycling at 45 °C. The good electrochemical performance of the cell based on SG-LMO is ascribed mainly to its small particle size, high degree of dispersion, and uniform element distribution in bulk material. In addition, the lower polarization potential accelerates Li⁺ ion migration, and the lower atom location confused degree maintains integrity of crystal structure, both of which can effectively improve the rate capability and cyclability of SG-LMO.     

Synthesis of ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript>/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> complex substrates and growth of GaN films

With the solid phase reaction between pulsed-laser-deposited (PLD) ZnO film and α-Al₂O₃ substrate, ZnAl₂O₄/α-Al₂O₃ complex substrates were synthesized. X-ray diffraction (XRD) spectra show that as the reaction proceeds, ZnAl₂O₄ changes from the initial (111)-oriented single crystal to poly-crystal, and then to inadequate (111) orientation. Corresponding scanning electron microscope (SEM) images indicate that the surface morphology of ZnAl₂O₄ transforms from uniform islands to stick structures, and then to bulgy-line structures. In addition, XRD spectra present that ZnAl₂O₄ prepared at low temperature is unstable at the environment of higher temperature. On the as-obtained ZnAl₂O₄/α-Al₂O₃ substrates, GaN films were grown without any nitride buffer using light-radiation heating low-pressure MOCVD (LRH-LP-MOCVD). XRD spectra indicate that GaN film on this kind of complex substrate changes fromc-axis single crystal to poly-crystal as ZnAl₂O₄ layer is thickened. For the single crystal GaN, its full width at half maximum (FWHM) of X-ray rocking curve is 0.4°. Results indicate that islands on thin ZnAl₂O₄ layer can promote nucleation at initial stage of GaN growth, which leads to the (0001)-oriented GaN film.     

Low-temperature sintering effects on NASICON-structured LiSn<Subscript>2</Subscript>P<Subscript>3</Subscript>O<Subscript>12</Subscript> solid electrolytes prepared via citric acid-assisted sol-gel method

LiSn₂P₃O₁₂ with sodium (Na) super ionic conductor (NASICON)-type rhombohedral structure was successfully obtained at low sintering temperature, 600 °C via citric acid-assisted sol-gel method. However, when the sintering temperature increased to 650 °C, triclinic structure coexisted with the rhombohedral structure as confirmed by X-ray diffraction analysis. Conductivity–temperature dependence of all samples were studied using impedance spectroscopy in the temperature range 30 to 500 °C, and bulk, grain boundary and total conductivity increased as the temperature increased. The highest bulk conductivity found was 3.64?×?10^?5 S/cm at 500 °C for LiSn₂P₃O₁₂ sample sintered at 650 °C, and the lowest bulk activation energy at low temperature was 0.008 eV, showing that sintering temperature affect the conductivity value. The voltage stability window for LiSn₂P₃O₁₂ sample sintered at 600 °C at ambient temperature was up to 4.4 V. These results indicated the suitability of the LiSn₂P₃O₁₂ to be exploiting further for potential applications as solid electrolytes in electrochemical devices.     

Gas sensor based on nanosize In<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> film

Nanosize films of In₂O₃:Ga₂O₃ (96:4 weight %) have been deposited on a glassceramic substrate by the method of rf magnetron sputtering. The surfaces of fabricated films were studied with use of a scanning electron microscope; sizes of grains were determined and the thicknesses of films were measured. In order to prepare a gas-sensitive structure, a thin catalytic palladium layer and ohmic comb contacts were deposited on the In₂O₃:Ga₂O₃ film surface by the method of ion-plasma sputtering. The sensitivity of sensors based on the glassceramic/In₂O₃:Ga₂O₃ (96:4 weight %)/Pd structure to different concentrations of propane and butane gas mixture, as well as to methane was investigated at temperatures of working substance from 250 to 300°C.