The formation and thermostability of MgO and MgAl2O4 nanoparticles in oxidized SiC particle-reinforced Al-Mg composites

Interfacial reactions and their products in oxidized SiC particle-reinforced Al-Mg matrix composites were investigated using X-ray diffraction and Field EmissionScanning Electron Microscopy (FE-SEM). Observation of the interfacial reaction between oxidized SiC particles and aluminum alloys containing Mg showed that nanoparticles of MgO form initially and do not change form when more than 4 wt. % Mg is in the matrix. However, MgO transforms into octahedral MgAl₂O₄ crystals when less than 2 wt. % Mg is in the matrix .Comparison of the amounts and the sizes of the reaction products MgAl₂O₄ and MgO between the Al-Mg alloyswith different matrix compositions shows that fewer MgAl₂O₄ crystals form at the surface of the particles in the 2014Al matrix composite than in the Al-2 wt. % Mg (Al-2Mg) matrix composite. Also, the size of MgAl₂O₄ in the former composite is greater than that of the latter composite under the same conditions. However, the amount and the size of MgO crystals that form in the Al-4 wt. % Mg (Al-4Mg) matrix composite is almost the same as that of the Al-8 wt. % Mg (Al-8Mg) composite, and the size of MgO changes a little during heat-treatment at elevated temperatures. The amount of the reaction product (either MgO or MgAl₂O₄) depends on nucleation rates and density of nucleation sites on the oxidized SiC particles at the initial reaction. The more completely the nuclei cover the surface of the oxidized SiC particles, the smaller the resulting size. According to the results, an addition of Mg into the matrix can be used to control the interfacial characteristics in the oxidized SiC/Al composites. Received: 25 January 2001 / Accepted: 26 January 2001 / Published online: 23 May 2001     

Electrical properties of Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-based composite systems

Composite electrolytes are well-known multiphase systems and exhibit maxima in the conductivity at certain second-phase concentration. An attempt has been made to investigate a number of sodium sulfate (Na₂SO₄)-based composite systems. The dispersoids that have been used are MgO, Al₂O₃, and SiO₂. The samples have been characterized using impedance spectroscopy, X-ray diffraction, and differential scanning calorimetry. The maximum conductivity has been observed for MgO dispersed system, and the percolation threshold has been observed at 30-mol% dispersoid, MgO concentration. Interestingly, two maxima have been observed in case of the Na₂SO₄–SiO₂ and Na₂SO₄–Al₂O₃ composite systems. In the Na₂SO₄–SiO₂ system, the first maximum occurs at lower concentration, i.e., in the range between 10 and 20 mol%, whereas the second occurs at the 40-mol% dispersoid concentration. For the Na₂SO₄–Al₂O₃ system, although slightly indistinguishable, two peaks in the conductivity vs composition plot have been observed around 12- and 30-mol% Al₂O₃ concentrations.     

Synthesis of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel nanoparticles using a mixture of bayerite and magnesium sulfate

This article reports a novel method to prepare MgAl₂O₄ spinel nanoparticles. By calcining a powder mixture of bayerite and magnesium sulfate at 800 °C and washing with water, single-phase MgAl₂O₄ spinel nanoparticles were prepared. The powder mixture and the calcined products were characterized by differential thermal and thermogravimetric analysis (DSC-TG), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Brunauer–Emmett–Teller (BET) nitrogen-gas adsorption method. The obtained MgAl₂O₄ spinel nanoparticles have an average particle size of 12 nm, a narrow size distribution, and weak agglomeration. The specific surface area of the MgAl₂O₄ spinel powder is 110 m²/g. The formation of MgAl₂O₄ spinel is attributed to a solid-state reaction between γ-Al₂O₃ and MgSO₄.     

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.     

Metallic liquid hydrogen and likely Al<Subscript>2</Subscript>O<Subscript>3</Subscript> metallic glass

Dynamic compression has been used to synthesize liquid metallic hydrogen at 140 GPa (1.4 million bar) and experimental data and theory predict Al₂O₃ might be a metallic glass at ∼ 300 GPa. The mechanism of metallization in both cases is probably a Mott-like transition. The strength of sapphire causes shock dissipation to be split differently in the strong solid and soft fluid. Once the 4.5-eV H-H and Al-O bonds are broken at sufficiently high pressures in liquid H₂ and in sapphire (single-crystal Al₂O₃), electrons are delocalized, which leads to formation of energy bands in fluid H and probably in amorphous Al₂O₃. The high strength of sapphire causes shock dissipation to be absorbed primarily in entropy up to ∼400 GPa, which also causes the 300-K isotherm and Hugoniot to be virtually coincident in this pressure range. Above ∼400 GPa shock dissipation must go primarily into temperature, which is observed experimentally as a rapid increase in shock pressure above ∼400 GPa. The metallization of glassy Al₂O₃, if verified, is expected to be general in strong oxide insulators. Implications for Super Earths are discussed.     

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.     

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.     

Structural and catalytic properties of ZnO and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanostructures loaded with metal nanoparticles

Nanostructured zinc oxide (ZnO) nanobelts and aluminum oxide (Al₂O₃) nanoribbons have been grown successfully from the vapor phase. XRD results confirmed the purity and the high quality of the formed crystalline materials. TEM images showed that ZnO nanostructures grew in the commonly known tetrapod structure with nanobelts separated from the tetrapods with an average width range of 10–30 nm and a length of about 500 nm. Al₂O₃ nanostructures grew in the form of nanoribbons with an average width range of 20–30 nm and a length of up to 1 μm. The catalytic oxidation of CO gas into CO₂ gas over the synthesized nanostructures is also reported. Higher catalytic activity was observed for Pd nanoparticles loaded on the ZnO nanobelts (100% conversion at 270 °C) and Al₂O₃ nanoribbons (100% conversion at 250 °C). The catalytic activity increased in the order Cu < Co < Au < Pd for the metal-loaded nanostructures. The preparation methods could be applied for the synthesis of novel nanostructures of various materials with novel properties resulting from the different shapes and morphologies.     

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.     

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.     

Synthesis and nanostructural investigation of TiO<Subscript>2</Subscript> nanorods doped by SiO<Subscript>2</Subscript>

TiO₂ Nano rods can be used as dye-sensitized solar cells, various sensors and photocatalysts. These nanorods are synthesized by a hydrothermal corrosion process in NaOH solution at 200°C using TiO₂ powder as the source material. In the present work, the synthesis of TiO₂ nanorods in anatase, rutile and Ti₇O₁₃ phases and synthesis of TiO₂ nanorods by incorporating SiO₂ dopant, using the sol–gel method and alkaline corrosion are reported. The morphologies and crystal structures of the TiO₂ nanorods are characterized using field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) study. The obtained results show not only an aggregation structure at high calcination temperatures with spherical particles but also Ti–O–Si bonds having four-fold coordination with oxygen in SiO^4 −.     

Tunable bluish green to yellowish green Ca<Subscript>2(1-x)</Subscript>Sr<Subscript>2x</Subscript>Al<Subscript>2</Subscript>SiO<Subscript>7</Subscript>:Eu<Superscript>2+</Superscript> phosphors for potential LED application

A series of solid solutions with a general formula of Ca_2(1-x)Sr_2xAl₂SiO₇:Eu²⁺ were synthesized by a high temperature solid state reaction. The structure, diffuse reflection spectra, photoluminescence spectra, color-coordinate parameters and lifetimes of phosphors were investigated. XRD results show that Ca₂Al₂SiO₇ is totally miscible with Sr₂Al₂SiO₇. These solid solution phosphors show a broad excitation band of 350–450 nm that matches well with the output lights of near-UV LEDs and tunable emission from bluish green to yellowish green. These optical properties originate from the 4f⁷–4f⁶5d transition of Eu²⁺ ions. The crystal field strength was considered to be tailed by controlling the host composition, which leads to the shift of absorption band and emission band, and the varying of color coordinates. PACS  78.55.-m; 42.70.-a; 61.05.C-     

Photoluminescence behavior of pulsed laser deposited ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript> thin-film phosphors grown on various substrates

ZnGa₂O₄ thin-film phosphors have been grown on Si(100), Al₂O₃(0001) and MgO(100) substrates using pulsed laser deposition. The structural characterization was carried out on a series of ZnGa₂O₄ films grown on various substrates under various substrate temperatures and oxygen pressures. The films grown on these substrates not only have different crystallinity and surface morphology, but also different Zn/Ga composition ratio. The crystallinity and photoluminescence (PL) of the ZnGa₂O₄ films are highly dependent on the deposition conditions, in particular the stoichiometry ratio of Zn/Ga and the kind of substrate. The variation of Zn/Ga in the films also depends on not only the oxygen pressure but also the substrate temperature during deposition. The PL properties of pulsed laser deposited ZnGa₂O₄ thin films have indicated that Al₂O₃(0001) and MgO(100) are promising substrates for the growth of high-quality ZnGa₂O₄ thin films and that the luminescence brightness depends on the substrate. The luminescence spectra show a broad band extending from 350 to 600 nm and peaking at 460 nm. Received: 11 July 2002 / Accepted: 31 July 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +82-51-6206356, E-mail: jhjeong@pknu.ac.kr     

Electronic structure of an oxygen vacancy in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> from the results of Ab Initio quantum-chemical calculations and photoluminescence experiments

The electronic structure of an oxygen vacancy in α-Al₂O₃ and γ-Al₂O₃ is calculated. The calculation predicts an absorption peak at an energy of 6.4 and 6.3 eV in α-Al₂O₃ and γ-Al₂O₃, respectively. The luminescence and luminescence excitation spectra of amorphous Al₂O₃ are measured using synchrotron radiation. The presence of a luminescence band at 2.9 eV and a peak at 6.2 eV in the luminescence excitation spectrum indicates the presence of oxygen vacancies in amorphous Al₂O₃.     

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.     

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     

X-ray emission study of the electronic structure of nanocrystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Valence states of metal ions and the phase composition of nanocrystalline Al₂O₃ (of the original oxide and the oxide irradiated by high-energy Fe⁺ ions) are studied by using x-ray emission Al L_{2, 3} and O Kα spectra. It is established that the shape of the Al L_{2, 3} spectra strongly changes as one goes from the original (bulk) Al₂O₃ to nanocrystalline oxide, while the O Kα spectra remain practically unchanged. Moreover, irradiation by high-energy Fe⁺ ions results in slight additional changes in the x-ray spectral characteristics of the aluminum oxides under study. The obtained experimental data are compared with the results of theoretical calculations of the electronic structure of α and γ phases of Al₂O₃ performed using the LDA formalism. Using the results of x-ray spectral studies, electronic structure calculations, and x-ray diffraction analysis, it is shown that the revealed spectral differences between the nanocrystalline state of aluminum oxide and the bulk material can be interpreted as a phase transition from the α phase to the γ phase of Al₂O₃ with an addition of bayerite.     

Up-conversion emissions of Er<Superscript>3+</Superscript>-Yb<Superscript>3+</Superscript> codoped Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles by the arc discharge synthesis method

The Er³⁺-Yb³⁺ codoped Al₂O₃ nanoparticles with an average particle size of about 50 nm have been synthesized by an arc discharge synthesis method. The green and red up-conversion emissions centered at about 526, 547 and 677 nm, corresponding respectively to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺, were detected by a 978-nm semiconductor laser diode excitation. The Annealing has evident effect on the up-conversion emissions of the samples: The red up-conversion emission is noticeable before annealing; however, the green up-conversion emission becomes predominant after annealing. The mixture of (Er,Yb)₃Al₅O₁₂ and α-(Al,Er,Yb)₂O₃ phases is more favorable for green up-conversion emissions due to an enhancement of the ESA (I) of ⁴I_11/2+a photon→⁴F_7/2 and ET (III) of ²F_5/2(Yb³⁺)+⁴I_11/2(Er³⁺)→²F_7/2(Yb³⁺)+⁴F_7/2(Er³⁺) processes. The two-photon absorption up-conversion process is involved in the green and red up-conversion emissions. The results have proved that arc discharge synthesis is a new promising preparation technology for optical materials. Supported by National Natural Science Foundation of China (Grant No. 10804015), the Scientific Research Foundation for Doctor of Liaoning Province (Grant No. 20071095), and the Educational Committee Foundation of Liaoning Province (Grant No. 2008123)     

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.     

On the Coexistence of Chemically Similar Stable and Metastable Phases in the BeO–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> System

The joint crystallization of the stable phase (a number of solid solutions of chromium-containing beryllian indialite) and metastable phases (crystalline modifications) of the compound with β-quartz structure (Si_0.64Al_0.28Mg_0.21Be_0.09)^IVO₂ ~ Mg_1.89Be_0.81Al_2.52Si_5.76O₁₈ admixed with Cr₂O₃ phases of khmaralite Mg_1.21Cr_0.01Be_0.46Al_1.78Si_3.38O₂₀ and spinel {(Mg_0.95Be_0.045Si_0.005)^IV(Al_1.31Cr_0.67Mg_0.02)^VI}O₄ is performed using melted chromium–beryllian indialite preliminarily obtained by solid-phase synthesis as a precursor. Simultaneously, the residual X-ray amorphous melt of composition Mg_1.83Cr_0.01Be_1.04Al_2.64Si_5.57O₁₈ is hardened. Thus, a reconstructive transition from the beryllian indialite melt to a phase with the β-quartz structure is implemented, and the chemical similarity of these compounds is demonstrated. The rate of change in the crystallization isotherm of 2°С/h and increased heat outflow through the highly heat-conducting walls of the Pt–Rh crucible (taper) contribute to this process.