Eu-Mn energy transfer in white-light-emitting T-phase (Ba,Ca)2SiO4:Eu, Mn phosphor

White-light-emitting T-phase Eu²⁺/Mn²⁺-codoped (Ba, Ca)₂SiO₄ phosphors are achieved in terms of the energy transfer between Eu²⁺ and Mn²⁺ ions. All spectra consist of the relatively broad green and the red emission bands, which thus result in a warm-white color with a color temperature of ∼4000 K. With increasing Eu²⁺ ions at fixed Mn²⁺ concentrations, a strong correlation between the luminescence and the electron paramagnetic resonance spectra is observed. This demonstrates that Eu²⁺ doping causes a perturbation of Mn²⁺ sites and the violation of their selection rules that enhances Mn²⁺-related emissions.     

Induction of the Columnar Phase of Unconventional Dendrimers by Breaking the C2 Symmetry of Molecules

Two triazine‐based unconventional dendrimers were prepared and characterized by ¹H and ¹³C NMR spectroscopy, mass spectrometry, and elemental analysis. Differential scanning calorimetry, polarizing microscopy, and powder XRD studies showed that these dendrimers display columnar liquid‐crystalline phases during thermal treatment. This is ascribable to breaking of their C₂ symmetry. The molecular conformations of prepared dendrimers were obtained by computer simulation with the MM3 model of the CaChe program in the gas phase. The simulation showed that the conformations of the prepared dendrimers are rather flat and disfavor formation of the LC phase. However, due to C₂‐symmetry breaking, the prepared dendrimers have structural isomers in the solid state and thus show the desired columnar phases. This new strategy should be applicable to other types of unconventional dendrimers with rigid frameworks.     

Granularity, microstructure, and flux creep in TlBa2Ca2Cu3O9+δ superconducting powder and silver-sheated tapes

Silver-sheathed TlBa₂Ca₂Cu₃O_9+δ (Tl-1223) tapes, with a transport critical current density, J_ct, of 6200 A/cm² at 75 K under zero magnetic field, were fabricated by the oxide-powder-in-tube (OPIT) method and characterized using an X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and superconducting quantum interference device (SQUID) magnetometer. The results of the magnetization and SEM study indicate that, in these tapes, individual grains are distributed randomly in orientation and weak links exist. However, strongly linked percolative current paths within the tape persisting in increasing fields, accompanied by strong intrinsic interplanar coupling, sustain a significant J_ct at high fields and lead to the plateau in the J_ct-H curve. Dislocation networks, which may act as effective pinning centers, are the common features observed in the tapes. To investigate the effects on flux pinning due to thermomechanical processing, magnetic relaxation at 1 and 2 T over 5–50 K was measured. The tape shows slightly lower normalized relaxation rates (S=-(1/M_o)dM/ dln t) than the cauliflower-like precursor powder. Analyses of the relaxation data obtained from the tape, after incorporating temperature dependence and field scaling, yield an expression for the effective pinning energy U_eff(J,T,H) = (U₁/H^0.54)[1-(T/ 72.5)²]⁴(J/J_i)^μ. This result was compared with the prediction of the collective flux-creep model, which suggests that Tl-1223 has a three-dimension-like (3D-like) vortex lattice. Presumably, a more plate-like powder morphology may result in improved texturing by the OPIT process. Tl_0.5Bi_0.5Sr₂CaCu₂O_7+δ ((Tl,Bi)-1212) powder with this morphology was therefore synthesized for comparison.     

Materials characterization of n - ZnO / p - GaN : Mg / c - Al2O3 UV LEDs grown by pulsed laser deposition and metal–organic chemical vapor deposition

n-ZnO/p-GaN:Mg hybrid heterojunctions grown on c-Al₂O₃ substrates showed 375 nm room temperature electroluminescence. It was suggested that the high materials and interface quality obtained using pulsed laser deposition for the n-ZnO growth and metal–organic chemical vapor deposition for the p-GaN:Mg were key factors enabling the injection of holes and the radiative near band edge recombination in the ZnO. In this paper we present the materials characterization of this structure using x-ray diffraction, scanning electron microscopy and atomic force microscopy.     

Green-pumped high-power optical parametric oscillator based on periodically poled MgO-doped stoichiometric LiTaO3

A high-power 532 nm-pumped multikilohertz nanosecond optical parametric oscillator using a periodically poled 1.0 mol.% MgO-doped stoichiometric lithium tantalate crystal that could be operated from room temperature to 200 degrees C without damage is reported. A broad continuous tuning range from 855 to 1410 nm was achieved within a single domain period. Efficient operation of high peak power and watt level average power with a power conversion of 62.5% was measured. These results show that a high-resolution high average power visible tunable source can be realized.     

Unified approach to some geometric results in variational analysis

Based on a study of a minimization problem, we present the following results applicable to possibly nonconvex sets in a Banach space: an approximate projection result, an extended extremal principle, a nonconvex separation theorem, a generalized Bishop-Phelps theorem and a separable point result. The classical result of Dieudonné (on separation of two convex sets in a finite-dimensional space) is also extended to a nonconvex setting.     

Synthesis and characterization of a narrow‐bandgap polymer containing alternating cyclopentadithiophene and diketo‐pyrrolo‐pyrrole units for solar cell applications

We have synthesized a narrow‐bandgap conjugated polymer ( PCTDPP ) containing alternating cyclopentadithiophene (CT) and diketo‐pyrrolo‐pyrrole (DPP) units by Suzuki coupling. This PCTDPP exhibits a low band gap of 1.31 eV and a broad absorption band from 350 to 1000 nm, which allows it to absorb more available photons from sunlight. A bulk heterojunction polymer solar cell incorporating PCTDPP and C₇₀ at a blend ratio of 1:3 exhibited a high short‐circuit current of 10.87 mA/cm² and a power conversion efficiency of 2.27%. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1669–1675, 2010     

Carbazole‐based conjugated polymers incorporating push/pull organic dyes: Synthesis,characterization, and photovoltaic applications

We have synthesized and characterized two new carbazole‐based conjugated polymers, PCDCN and PCDTA , incorporating two strong light‐absorbing organic dyes. These polymers exhibit relatively low band gaps (～1.5 eV) and broad absorption ranges (from 300 to 700 nm). We fabricated polymer solar cells incorporating these polymers as donors and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) as the acceptor. At a blending ratio of 1:4, we obtained power conversion efficiencies, under simulated AM 1.5 (100 mW/cm²) conditions, of 2.31% and 2.47% for the PCDCN ‐ and PCDTA ‐based devices, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Synthesis and properties of new aromatic polyamides with redox‐active 2,4‐dimethoxytriphenylamine moieties

A new triphenylamine‐based diamine monomer, 4,4′‐diamino‐2″,4″‐dimethoxytriphenylamine ( 2 ), was synthesized from readily available reagents and was reacted with various aromatic dicarboxylic acids to produce a series of aromatic polyamides ( 4a–h ) containing the redox‐active 2,4‐dimethoxy‐substituted triphenylamine (dimethoxyTPA) unit. All the resulting polyamides were readily soluble in polar organic solvents and could be solution cast into tough and flexible films. These polymers exhibited good thermal stability with glass transition temperatures of 243–289 °C and softening temperatures of 238–280 °C, 10% weight loss temperatures in excess of 470 °C in nitrogen, and char yields higher than 60% at 800 °C in nitrogen. The redox behaviors of the polymers were examined using cyclic voltammetry (CV). All these polyamides showed two reversible oxidation processes in the first CV scan. The polymers also displayed low ionization potentials as a result of their dimethoxyTPA moieties. In addition, the polymers displayed excellent stability of electrochromic characteristics with coloration change from a colorless neutral state to green and blue‐purple oxidized states. These anodically coloring polyamides showed high green coloration efficiency (CE = 329 cm²/C), high contrast of optical transmittance change (ΔT% = 84% at 829 nm), and long‐term redox reversibility. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3392–3401, 2010