Synthesis and characterization of mono-dispersed Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Er<Superscript>3+</Superscript>-coated SiO<Subscript>2</Subscript> nanoparticles by co-precipitation process

In this paper, a facile co-precipitation process for preparing mono-dispersed core–shell structure nanoparticles is reported. The 110 nm SiO₂ cores coated with an yttrium aluminum garnet (Y₃Al₅O₁₂) layer doped with Er³⁺ were synthesized and the influence of the concentration ratio of [urea]/[metal ions] on the final product was investigated. The structure and morphology of samples were characterized by the X-ray powder diffraction, Fourier transform IR spectroscopy and transmission electron microscopy, respectively. The results indicate that a layer of well-crystallized garnet Y₃Al₅O₁₂:Er³⁺ were successfully coated on the silica particles with the thickness of 20 nm. The near infrared and upconversion luminescent spectra of the SiO₂@Y₃Al₅O₁₂:Er³⁺ powders further confirm that a Y₃Al₅O₁₂:Er³⁺ coating layer has formed on the surface of silica spherical particles.     

Optical and magnetooptical properties of CoFeB/SiO<Subscript>2</Subscript> and CoFeZr/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> granular magnetic nanostructures

The optical and magnetooptical properties of the new granular nanocomposites (CoFeB)/(SiO₂) and (CoFeZr)/(Al₂O₃), which are grains of amorphous ferromagnetic alloys embedded in dielectric matrices, have been studied. The dependence of the optical, magnetooptical, and magnetic properties of the nanocomposites on their qualitative and quantitative composition, as well as on the conditions of their preparation, was investigated. Spectra of the dielectric functions ε = ε₁ ? iε₂ were obtained by the ellipsometric method in the range 0.6–5.4 eV. Above 4.2 eV, the absorption coefficient of the (CoFeB)/(SiO₂) composites was found to be close to zero for all magnetic-grain concentrations. The polar Kerr effect measured at a photon energy of 1.96 eV in dc magnetic fields of up to 15 kOe reaches values as high as 0.25°–0.3° for these nanocomposites and depends only weakly on the conditions of preparation. On the other hand, the (CoFeZr)/(Al₂O₃) nanostructures reveal a considerable difference in the concentration dependences of the Kerr effect between samples prepared in a dc magnetic field and in zero field.     

UV-protection properties of electrospun polyacrylonitrile nanofibrous mats embedded with MgO and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

This article describes the ultraviolet (UV) protection of MgO and Al₂O₃ nanoparticles embedded electrospun polyacrylonitrile (PAN) nanofibrous mats. UV radiation is a harmful part of sunlight and prolonged exposure to it can cause serious skin damages. In this research, nanofibrous mats consisting of nanofibers with different diameters containing different amounts of MgO, Al₂O₃, MgO Plus, and Al₂O₃ Plus nanoparticles were produced, and their UV-protection was measured. The specific surface area of MgO, MgO Plus, Al₂O₃, and Al₂O₃ Plus nanoparticles was 230, 600, 275, and 550 m²/g, respectively. The mean diameter of electrospun PAN nanofibers embedded with metal oxide nanoparticles was in the range of 665–337 nm. The results showed that the UV-protection (shielding) capability of the mats strongly depends on fiber diameter; in fact a thin mat of nanofibers has a much stronger UV-protection in comparison to a thicker mat composed of regular fibers. UV transmission is reduced as a result of embedding MgO and Al₂O₃ nanoparticles in the electrospun PAN nanofibrous mats. MgO Plus and Al₂O₃ Plus show higher UV-protection than MgO and Al₂O₃.     

One-step synthesis of colloidal Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> and γ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles at room temperature

A facile room-temperature synthesis has been developed to prepare colloidal Mn₃O₄ and γ-Fe₂O₃ nanoparticles (5 to 25 nm) by an ultrasonic-assisted method in the absence of any additional nucleation and surfactant. The morphology of the as-prepared samples was observed by transmission electron microscopy. High-resolution transmission electron microscopy observations revealed that the as-synthesized nanoparticles were single crystals. The magnetic properties of the samples were investigated with a superconducting quantum interference device magnetometer. The possible formation process has been proposed.     

Fabrication of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al structure by nitric acid oxidation at room temperature

A thick Al₂O₃/aluminum (Al) structure has been fabricated by oxidation of Al with 68wt% and 98wt% nitric acid (HNO₃) aqueous solutions at room temperature. Measurements of the Al₂O₃ thickness vs. the oxidation time show that reaction and diffusion are the rate-determining steps for oxidation with 68wt% and 98wt% HNO₃ solutions, respectively. Observation of transmission electron micrographs shows that the Al₂O₃ layer formed with 68wt% HNO₃ has a structure with cylindrically shaped pores vertically aligned from the Al₂O₃ surface to the Al₂O₃/Al interface. Due to the porous structure, diffusion of HNO₃ proceeds easily, resulting in the reaction-limited oxidation mechanism. In this case, the Al₂O₃/Al structure is considerably rough. The Al₂O₃ layer formed with 98wt% HNO₃ solutions, on the other hand, possesses a denser structure without pores, and the Al₂O₃/Al interface is much smoother, but the thickness of the Al₂O₃ layer formed on crystalline Al regions is much smaller than that on amorphous Al regions. Due to the relatively uniform Al₂O₃ thickness, the leakage current density flowing through the Al₂O₃ layer formed with 68wt% HNO₃ is lower than that formed with 98wt% HNO₃.     

Multinuclear MAS NMR Investigation of Sol-Gel and Ball-Milled Nanocrystalline Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>

⁷¹Ga magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to characterize the structural evolution of nanocrystalline Ga₂O₃ samples prepared by sol-gel and ball-milling techniques. ²⁹Si and ²⁷Al MAS NMR have also been used to characterize silica and alumina Zener pinning phases. ⁷¹Ga NMR parameters are reported for the α- and β-Ga₂O₃ phases, and more tentatively for the δ-Ga₂O₃ phase. By simulating the octahedrally coordinated gallium NMR line of β-Ga₂O₃ using Gaussian distributions in χ_Q, the extent of disorder in the Ga₂O₃ crystallites has been quantified. The ball-milled samples contain much more inherent disorder than the sol-gel samples in the nano-phase, which was observed from simulations of the ⁷¹Ga MAS NMR spectra. The silica pinning phase produced highly crystalline and densely aggregated nanocrystalline Ga₂O₃, as well as the smallest nanocrystal sizes. Authors' address: Mark E. Smith, Department of Physics, University of Warwick, Coventry CV4 7AL, UK     

Metastable superconductivity of the Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> interface

Superconductivity in the interface region between metallic Al and its oxide, Al₂O₃, has been detected at about 45 K in the measured dynamic magnetic susceptibility.     

X-ray diffraction and calorimetric studies of powder nanocrystalline systems based on ZrO<Subscript>2</Subscript>(Y) and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with second insoluble component

Four nanocrystalline ZrO₂(Y)- and Al₂O₃-based powders synthesized by plasma spray pyrolysis have been studied. It has been shown that the ZrO₂(3Y)-based system with second component Al₂O₃ forms a nonequilibrium solid solution ZrO₂(3Y, Al) with the tetragonal structure. It has been found that the existence of an component (ZrO₂(Y)) insoluble in the coarse-grained state in Al₂O₃-based systems causes the delay of the γ → α transformation and decreases the size of the coherently scattering domains of formed nanosized modifications of Al₂O₃.     

Crystal structure and magnetic properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles by <Superscript>27</Superscript>Al NMR data

The magnetization of a series of Al₂O₃ with different particle sizes and their ²⁷Al NMR spectra have been studied at room temperature. The field dependence of the magnetization demonstrated the existence of a long-range ferromagnetic order in a small part of the sample at room temperature; however, the relative volume of this contribution was very small (less than 1%), and this seems likely due to an impurity phase. The NMR spectra did not contain any lines of metallic aluminum the existence of which in these nanooxides was assumed before in a surface layer of the nanoparticles, according to the data of other techniques. The data on the phase composition and the charge distribution in different phases of the Al₂O₃ nanoparticles have been obtained. The change in the mean particle size (by a factor of almost three) only insignificantly changed their phase composition.     

Phase composition and structure of nanocrystalline powders based on ZrO<Subscript>2</Subscript>(Y) and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> obtained by plasma spray pyrolysis

It is shown that a nonequilibrium solid solution ZrO₂(3Y, Al) with tetragonal structure is formed in systems based on ZrO₂(3Y) with Al₂O₃ as a second component. A delay in the γ → α Al₂O₃ transformation and a reduction in the size of the coherently scattering domain of modifications are observed in systems based on Al₂O₃ with ZrO₂(Y) as a second component.     

Properties of an Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Si interface

The phase chemical composition of an Al₂O₃/Si interface formed upon molecular deposition of a 100-nm-thick Al₂O₃ layer on the Si(100) (c-Si) surface is investigated by depth-resolved ultrasoft x-ray emission spectroscopy. Analysis is performed using Al and Si L_{2, 3} emission bands. It is found that the thickness of the interface separating the c-Si substrate and the Al₂O₃ layer is approximately equal to 60 nm and the interface has a complex structure. The upper layer of the interface contains Al₂O₃ molecules and Al atoms, whose coordination is characteristic of metallic aluminum (most likely, these atoms form sufficiently large-sized Al clusters). The shape of the Si bands indicates that the interface layer (no more than 10-nm thick) adjacent to the substrate involves Si atoms in an unusual chemical state. This state is not typical of amorphous Si, c-Si, SiO₂, or SiO_x (it is assumed that these Si atoms form small-sized Si clusters). It is revealed that SiO₂ is contained in the vicinity of the substrate. The properties of thicker coatings are similar to those of the 100-nm-thick Al₂O₃ layer and differ significantly from the properties of the interfaces of Al₂O₃ thin layers.     

Structural investigation on PEO-based polymer electrolytes dispersed with Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

Thin solid polymer electrolytes based on polyethylene oxide (PEO) and silver triflate (AgCF₃SO₃) dispersed with various concentrations of aluminum oxide (Al₂O₃) nanoparticles have been prepared by solution casting technique. These thin polymer films are found to have thickness of the order of 30 to 100 μm. The X-ray diffraction (XRD) patterns have indicated the amorphous nature of the polymer electrolyte. The differential scanning calorimeter (DSC) traces showed slight change in the glass transition temperature (T _g) whereas the degree of crystallization (X _c) decreases markedly due to the addition of alumina nanoparticles. Fourier transform infrared (FTIR) spectral analysis of all these samples has revealed the presence of absorption bands around 1,000 cm⁻¹; thus indicating the complexation of silver ions with oxygen in PEO. Employing the Wagner’s polarization technique as the standard method, the total ionic transference number for the complexed polymer electrolyte was found to be approximately unity thereby revealing that the significant contribution to electrical conduction was due to ions only. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, December 7–9, 2006     

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.     

Studies on toxicity of aluminum oxide (Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) nanoparticles to microalgae species: <Emphasis Type="Italic">Scenedesmus</Emphasis> sp. and <Emphasis Type="Italic">Chlorella</Emphasis> sp.

In view of increasing commercial applications of metal oxide nanoparticles their toxicity assessment becomes important. Alumina (Al₂O₃) nanoparticles have wide range of applications in industrial as well as personal care products. In the absence of prior report on toxicological impact of alumina nanoparticles to microalgae, the principal objective of this study was to demonstrate the effect of the nanoparticles on microalgae isolated from aquatic environment (Scenedesmus sp. and Chlorella sp.). The growth inhibitory effect of alumina nanoparticles was observed for both the species (72 h EC₅₀ value, 45.4 mg/L for Chlorella sp.; 39.35 mg/L for Scenedesmus sp.). Bulk alumina also showed toxicity though to a lesser extent (72 h EC₅₀ value, 110.2 mg/L for Chlorella sp.; 100.4 mg/L for Scenedesmus sp.). A clear decrease in chlorophyll content was observed in the treated cells compared to the untreated ones, more effect being notable in the case of nanoparticles. Preliminary results based on FT-IR studies, optical and scanning electron microscopic images suggest interaction of the nanoparticles with the cell surface.     

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.     

Self-propagating high-temperature synthesis and magnetic features of Fe-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> granulated films

The classical solid-phase reaction between Fe₂O₃ and Al layers in thin films is initiated. It is shown that, in the reaction products, Fe granulated films are formed in the Al₂O₃ nonconducting matrix. Analysis of the reaction equation demonstrates that the volume fraction of iron in the granulated films is less than the percolation threshold. This determines the magnetic properties of iron clusters in a superparamagnetic state. It is assumed that the nanocrystalline microstructure exists in thin films after solid-phase reactions proceeding under conditions of self-propagating high-temperature synthesis.     

Investigation on structural characteristics of PVDF–AgCF<Subscript>3</Subscript>SO<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposite solid polymer electrolyte system

There has been an increasing interest towards the incorporation of nanosize ceramic fillers in polymer electrolytes. Solid polymer electrolytes based on polyvinylidene fluoride (PVDF), silver triflate (AgCF₃SO₃), and x wt% of aluminum oxide (Al₂O₃) nanopowders (where x = 1, 3, 5, and 10, respectively) have been prepared using solution casting technique. The structural characteristics of these thin film specimens were studied using Fourier transform infrared (FTIR) and X-ray diffraction (XRD) patterns at room temperature. The appearance of new absorption bands and gradual shifts observed in some characteristic peaks confirmed the complex formation between polyvinylidene fluoride and silver triflate. Furthermore, the addition of nanosized filler Al₂O₃ has also indicated the interaction of the filler with the polymer salt complex. The XRD patterns obtained for all these samples in the 2θ range 10° to 70° showed the amorphous nature of these samples. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, December 7–9, 2006.     

Structural and magnetic properties of layered Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript> thin films

Layered cobalt oxides Ca₃Co₄O₉ thin films have been grown directly on c-cut sapphire substrates using pulsed laser deposition. X-ray diffraction and transmission electron microscopy characterizations show that the deposited films present the expected monoclinic structure and a texture along the direction perpendicular to the Al₂O₃(001) plane. The Ca₃Co₄O₉ structure presents six variants in the film plane. Rutherford backscattering spectroscopy shows that the films are stoichiometric and that the film thickness agrees with the nominal value. The susceptibility χ of the films, recorded along the c-axis of the substrate, after field cooling and zero field cooling in an applied field of 1 kOe shows two magnetic transitions at 19 and 370 K which agree well with previous findings on single crystal samples. In turn, at low temperature (5 K), the magnetization curve along the c-axis exhibits coercive field and remanent magnetization much smaller than those reported for bulk samples, which can be related to the influence of structural variants and structural defects.     

Magnetic properties of LiNi<Subscript>0.5</Subscript>Mn<Subscript>0.47</Subscript>Al<Subscript>0.03</Subscript>O<Subscript>2</Subscript> as positive electrode for Li-ion batteries

The layered LiNi_0.5Mn_0.47Al_0.03O₂ was synthesized by wet chemical method and characterized by X-ray diffraction and analysis of magnetic measurements. The powders adopted the α-NaFeO₂ structure. This substitution of Al for Mn promotes the formation of Li(Ni_0.47²⁺Ni_0.03³⁺Mn_0.47⁴⁺Al_0.03³⁺)O₂ structures and induces an increase in the average oxidation state of Ni, thereby leading to the shrinkage of the lattice unit cell. The concentration of antisite defects in which Ni²⁺ occupies the (3a) Li lattice sites in the Wyckoff notation has been estimated from the ferromagnetic Ni²⁺(3a)–Mn⁴⁺(3b) pairing observed below 140 K. The substitution of 3% Al for Mn reduces the amount of antisite defects from 7% to 6.4–6.5%. The analysis of the magnetic properties in the paramagnetic phase in the framework of the Curie–Weiss law agrees well with the combination of Ni²⁺ (S = 1), Ni³⁺ (S = 1/2) and Mn⁴⁺ (S = 3/2) spin-only values. Delithiation has been made by the use of K₂S₂O₈. According to this process, known to be softer than the electrochemical one, the nickel ions in the (3b) sites are converted into Ni⁴⁺ in the high spin configuration, while Ni²⁺(3a)–Mn⁴⁺(3b) ferromagnetic pairs remain, as the Li⁺(3b) ions linked to the Ni²⁺(3a) ions in the antisite defects are not removed. The results show that the antisite defect is surrounded by Mn⁴⁺ ions, implying the nonuniform distribution of the cations in agreement with previous NMR and neutron experiments.     

NMR spectrum in a Mn<Subscript>3</Subscript>Al<Subscript>2</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> noncollinear antiferromagnet

The dependence of the NMR frequencies on the external magnetic field in a Mn₃Al₂Ge₃O₁₂ non-collinear 12-sublattice antiferromgnet is calculated using the exchange approximation for the spin dynamics.