Stereoselective polymerization of rac-lactide with a bulky aluminum/Schiff base complex

An aluminum/Schiff base complex {[2,2-dimethyl-1,3-propylenebis(3,5-di-tert-butylsalicylideneiminato)](isopropanolato)aluminum(III) (2)} based on a bulky ligand and aluminum isopropoxide was prepared and employed for the stereoselective ring-opening polymerization (ROP) of rac-lactide (rac-LA). The initiator was characterized with nuclear magnetic resonance (NMR), crystal structure measurements, and elemental analysis. It contained a five-coordinate aluminum atom that was trigonal bipyramidal in the solid state according to the crystal structure measurements. The two conformational stereoisomers of 2 exchanged quickly on the NMR scale. Compound 2 polymerized rac-LA into a crystalline polymer that was characterized with H-1 NMR, wide-angle X-ray diffraction, electrospray ionization mass spectrometry, and gel permeation chromatography. The kinetics of the polymerization were first-order in both the monomer and initiator, and there was a linear relationship between the rac-LA conversion and the number-average molecular weight of poly(rac-LA) with a narrow molecular distribution (1.04-1.08). These features showed that the polymerization was well controlled. The high melting temperature (196-201 degreesC) and isotacticity of poly(rac-LA) indicated that complex 2 was a highly stereoselective initiator for the ROP of rac-LA.

Efficient blue electroluminescence from neutral alcohol-soluble polyfluorenes with aluminum cathode

Efficient blue polymer light-emitting diodes (PLEDs) have been fabricated with a neutral alcohol-soluble polyfluorene, i.e., poly(9,9-bis(6(')-diethoxylphosphorylhexyl)fluorene) (PF-EP), as the emitting layer, high work-function Al as the cathode, and poly(vinyl carbazole) as the hole-transporting layer. The PLEDs display a maximum luminous efficiency of 4.0 cd/A and the luminous efficiency > 2.4 cd/A in a wide range of current densities. It is found that the promising performance of the devices is attributed to the fact that the PF-EP is not only an efficient blue light-emitting polymer, but it also can facilitate efficient electron injection at the Al/PF-EP interface.

Improvement of low-field magnetoresistance and Curie temperature in the hole doping of double perovskites Sr2FeMoO6 polycrystals

Polycrystalline Sr2FeMoO6 compounds with most vacancies at normal Fe sites were fabricated through Mo hole doping; its effect is similar to Fe3+ by our estimation. Sharp increase of magnetoconductance at low field was evidence of spin-polarized tunneling between the grains. The room temperature low-field magnetoresistivity at optimal doping x=0.03 is 8.5% in 3000 Oe and increases to 11.4% in 1 T associated with soft magnetic behaviors; furthermore it exhibits a ferromagnetic Curie temperature of 450 K, connected with hole doping effect. The improved magnetoresistivity behavior was related to Curie temperature.

Performance improvement by charge trapping of doping fluorescent dyes in organic memory devices

We studied the memory effect in the devices consisting of dye-doped N, N'-di(naphthalene-1-yl)-N, N'-diphenyl-benzidine sandwiched between indium-tin oxide and Ag electrodes. It was found that the on/off current ratio was greatly improved by the doped fluorescent dyes compared with nondoping devices. A mechanism of charge trapping was demonstrated to explain the improvement of the memory effect. For the off state, the conduction process is dominated by the trapping current, which is a characteristic of the space-charge limited current, whereas the on state is dominated by the detrapping current, and interpreted by Poole-Frenkel emission.

High current density tris(8-hydroxyquinoline) aluminum-based hybrid transistor in vertical architecture

We report the construction of hybrid permeable-base transistors, in vertical architecture, using tris(8-hydroxyquinoline) aluminum as emitter, a thin gold layer as base, and n-type silicon as collector. These transistors present high common-base current gain, can be operated at low driving voltages, and allow high current density.

Preparation and characterization of a novel solid solution of aluminum in tungsten carbide by mechanically activated high-temperature reaction

In this work, a novel substitutional solid solution (W0.8Al0.2)C was synthesized by mechanically activated high-temperature reaction. X-ray diffraction was used for phase identification during the whole reaction process. Environment scanning electronic microscopy-field emission gun and energy dispersive x-ray were used to investigate the microstructure and the quantitative material composition of the specimen. (W(0.8)A(10.2))C was found to crystallize in the WC-type, and the cell parameters were a = 2.907(1) angstrom and c = 2.837(1) angstrom. The hardness of (W0.8Al0.2)C was tested to be 19.3 +/- 1 GPa, and the density was 13.19 +/- 0.05 g cm(-3).

A dinuclear aluminum 8-hydroxyquinoline complex with high electron mobility for organic light-emitting diodes

A dinuclear aluminum 8-hydroxyquinoline complex (DAlq(3)) with improved electron mobility was designed for organic light-emitting diodes. The electron mobility in DAlq(3) was determined via transient electroluminescence (EL) from bilayer devices with structure of indium tin oxide (ITO)/N,N-'-di(naphthalene-1-yl)-N,N-'-diphenyl-benzidine (NPB)/DAlq(3)/Mg:Ag. It was found that the electron mobility in DAlq(3) is between 3.7-8.4x10(-6) cm(2)/Vs at electric fields ranging between 1.2x10(6) and 4.0x10(6) V/cm, which is a factor of two higher than that in Alq(3). The DAlq(3) also shows a higher EL efficiency of 2.2 cd/A (1.2 Lm/W), as compared to Alq(3) with an EL efficiency of 2.0 cd/A (1.0 Lm/W), which is attributed to more balanced electron and hole recombination due to the improved electron mobility of DAlq(3).

Surface plasmon resonance studies on the electrochemical doping/dedoping processes of anions on polyaniline-modified electrode

The combination of in situ surface plasmon resonance (SPR) with electrochemistry was used to investigate the electrochemical doping/dedoping processes of anions on a polyaniline (PAn)-modified electrode. Electrochemical SPR characteristics of the PAn film before and after doping/dedoping were revealed. The redox transformation between the insulating leucoemeraldine, and the conductive emeraldine, corresponding to the doping/dedoping of anion, can lead to very distinct changes in both the resonance minimum angle and the shape of SPR curve. This is ascribed to the swelling/shrinking effect, and the change of the PAn film in the imaginary part of the dielectric constant resulted from the transition of the film conductivity. In situ recording the time evolution of reflectance change at a fixed angle permits the continuous monitoring of the kinetic processes of doping/dedoping anions. The size and the charge of anions, the film thickness, as well as the concentration of anions are shown to strongly influence the rate of ingress/egress of anions. The time differential of SPR kinetic curves can be well applied in the detecting electroinactive anion by flow injection analysis. The approach has higher sensitivity and reproducibility compared with other kinetic measurements, such as those obtained by amperometry.

Synthesis-dependent luminescence properties of Y3Al5O12 : Re3+ (Re=Ce, Sm, Tb) phosphors

By using metal nitrates and oxides as the starting materials, Y2Al5O12 (YAG) and YAG:Re3+ (Re = Ce, Sm, Th) powder phosphors were prepared by solid-state (SS), coprecipitation (CP) and citrate gel (CG) methods. The resulting YAG and YAG-based phosphors were characterized by XRD, FT-IR, SEM and photoluminescent excitation and emission spectra. The purified crystalline phases of YAG were obtained at 800 degreesC (CG) and 900 degreesC (CP, SS). At an identical annealing temperature and doping concentration, the doped rare-earth ions showed the stronger emission intensity in the CP- and SS-derived phosphors than the CG-derived YAG phosphors. The poor emission intensity for the CG-derived phosphors is mainly caused by the contamination of carbon impurities from citric acid in the starting materials.

Simultaneous spectrophotometric determination of trace iron (III) and aluminum (III) with hybrid linear analysis

Hybrid linear analysis (HLA) was applied to resolution of overlapping spectra of Fe3+-salicylfluorone and Al3+-salicylfluorone complexes and simultaneous spectrophotometric determination of Fe3+ and Al3+. The absorbance matrix of 7 standard mixtures at 41 measuring points ranged from the wavelength of 550 nm to 630 nm was used for calibration. To avoid the effect of interaction between the two components on the determination, the column vector of K matrix obtained from the standard mixtures with least squares was used as the pure spectrum of component. The recoveries of the two elements for the analysis of the synthetic samples were 93.3% similar to 107.5% in the range of the concentration ratio of Fe3+:Al3+ = 10:1 to 1:8. Comparing with the partial least squares (PIS) model, the HLA method was simple, accuracy and precise.

Polymerization of ethylene by zirconocene-B(C6F5)(3) catalysts with aluminum compounds

Ethylene polymerization by zirconocene-B(C6F5)(3) catalysts with various aluminum compounds has been investigated. It is found that the catalytic activity depended on zirconocenes used, and especially on the type of aluminum compounds. For Et(H(4)Ind)(2)ZrCl2 (H(4)Ind : tetrahydroindenyl), the activity decreases in the following order: Me3Al > i-Bu3Al > Et3Al much greater than Et2AlCl. While for Cp2ZrCl2(Cp : cyclopentadienyl), it varies as follows: i-Bu3Al > Me3Al much greater than Et3Al. Furthermore, the activity is significantly affected by the addition mode of the catalytic components, which may imply that the formation of active centers is associated with an existing concentration of catalytic components. Results of thermal behavior of polyethylene (PE) studied by differential scanning calorimetry(DSC) show that crystallinity of the polymer prepared with Et3Al is higher than that with Me3Al or i-Bu3Al. It is also found that the number-average molecular weight ((M) over bar) of the polymers prepared with Me3Al or i-Bu3Al is much higher than that with Et3Al. H-1-NMR studies substantiate that i-Bu3Al is a more efficient alkylation agent of Cp2ZrCl2 in comparison with Me3Al. (C) 1997 John Wiley & Sons, Inc.

Irradiation damage on BaLiF3 crystallites and its suppression by rare-earth ion doping

The x-ray and gamma-ray induced damage in BaLiF3 crystallites and its suppression by rare earth ion doping have been studied by electron spin resonance and thermally stimulated luminescence methods. It has been found that the x-ray irradiation damage is light and can be erased easily. This shows that the BaLiF3 crystallite is an ideal host for x-ray storage material. But the damage induced by gamma-ray has been found to be relatively hard to recover; however the gamma-ray irradiation hardness can be improved by rare earth (e.g., La3+, Yb3+) ion doping. So the BaLiF3 is also promising material for being used in detection of high-energy particles (e.g., gamma-ray).

Hydrothermal synthesis and lanthanide doping of complex fluorides, LiYF4, KYF4 and BaBeF4 under mild conditions

The complex fluorides LiYF4, KYF4, BaBeF4 and AYF(4)Eu(x) (A = Li, K) are hydrothermally synthesized at 140-240 degrees C and characterized by powder X-ray diffraction, thermogravimetric analysis, IR spectroscopy, scanning electron microscopy and luminescence measurements.