Synthesis, electronic structure and luminescence properties of color-controllable Dy³⁺/Eu³⁺-codoped CaWO₄ phosphors

Du, P.,Wu, S.,Yu, J.S. Elsevier [etc.] 2016 Journal of luminescence Vol.173 No.-

<P>Color-tunable CaWO4:0.05Dy(3+)/xEu(3+) phosphors were synthesized via a solid-state reaction method and the electronic properties of CaWO4 host lattice were analyzed through density function theory calculation. Under the excitations of 250 and 351 nm, characteristic emissions corresponding to D-5(0)-> F-4(J) (J=0, 1, 2) transitions of Eu3+ ions and characteristic blue and yellow emissions corresponding to (F9/2 -> H15/2)-F-4-H-6 and F-4(9/2)-> H-6(13/2) transitions of Dy3+ ions, respectively, were observed in all the Dy3+/Eu3+ ions codoped CaWO4 phosphors. The photoluminescence spectra and decay curves revealed that there existed an energy transfer from Dy3+ to Eu3+ ions and this was certified to be a resonant type via a dipole-dipole interaction. Furthermore, color-tunable emissions were observed by adjusting the relative content between Dy3+ and Eu3+ ions and the excitation wavelength. In addition, the Dy3+/Eu3+ ions codoped CaWO4 phosphors exhibited strong cathodoluminescence properties and their emission intensity increased with the increment of accelerating voltage from 1 to 5 kV and filament current from 34 to 55 mu A. (C) 2016 Elsevier B.V. All rights reserved.</P>

Mesoporous sulfur-modified iron oxide as an effective Fenton-like catalyst for degradation of bisphenol A

Du, J.,Bao, J.,Fu, X.,Lu, C.,Kim, S.H. Elsevier 2016 Applied Catalysis B Vol.184 No.-

<P>A mesoporous sulfur-modified iron oxide (MS-Fe) was prepared as a heterogeneous H2O2 catalyst for degradation of BPA. The physico-chemical properties of MS-Fe and bare M-Fe were characterized by BET surface area measurement, SEM, XRD, MIR and XPS. Both M-Fe and MS-Fe composites appeared as cubic microparticles with abundant pores and cracks as well as large surface area. As depicted by XRD, EDX and XPS, M-Fe is mainly consisted of hematite while MS-Fe is a kind of S-doped iron oxide with about 5-6% of sulfur element in terms of atomic ratio. In contrast to the poor catalytic activity of bare M-Fe, the MS Fe composites showed greatly improved efficiencies for H2O2 activation for BPA degradation. The high catalytic activity of this new Fenton-like catalyst can be obtained at different initial pH in range of 3.0-9.0. The time evolution of degradation of BPA followed pseudo-first-order kinetics, and the first-order rate constants showed a linear relationship with parameters of initial pH, catalyst dosage and concentration of BPA. However, the H2O2 dosage showed a dual effect on BPA degradation because excessive H2O2 addition lead to scavenging of hydroxyl radicals ((OH)-O-center dot). The investigation of working mechanisms of MS Fe suggested a synergistic effect of homogeneous and heterogeneous degradation reaction, wherein a strong acidic environment, abundant surface-bonded hydroxyl group and electron-mediating effect of sulfur all contributed to fast activation of H2O2. Overall, this new material overcomes the limitation of narrow working pH range and shows a fast oxidation of BPA with a low H2O2 and catalyst dosage, would have a good potential for environmental application. (C) 2015 Elsevier B.V. All rights reserved.</P>

AC over-current characteristics of YBCO coated conductor with copper stabilizer layer considering insulation layer

Du, H.I.,Kim, M.J.,Kim, Y.J.,Lee, D.H.,Han, B.S.,Song, S.S. North-Holland 2010 Physica. C, Superconductivity Vol.470 No.20

Compared with the first-generation BSCCO wire, the YBCO thin-film wire boasts low material costs and high J<SUB>c</SUB> and superior magnetic-field properties, among other strengths. Meanwhile, the previous BSCCO wire material for superconducting cables has been researched on considerably with regard to its post-wire quenching characteristics during the application of an alternating over-current. In this regard, the promising YBCO thin-film wire has yet to be further researched on. Moreover, still lacking is research on the YBCO thin-film wire with insulating layers, which is essential in the manufacture of superconducting cables, along with the testing of the application of an alternating over-current to the wire. In this study, YBCO thin-film wires with copper-stabilizing layers were used in testing alternating over-current application according to the presence or absence of insulating layers and to the thickness of such layers, to examine the post-quenching wire resistance increase and quenching trends. The YBCO thin-film wire with copper-stabilizing layers has a critical temperature of 90K and a critical current of 85A<SUB>rms</SUB>. Moreover, its current application cycle is 5.5 cycles, and its applied currents are 354, 517, 712, and 915A<SUB>peak</SUB>. These figures enabled the YBCO thin-film wires with copper-stabilizing layers to reach 90, 180, 250, and 300K, respectively, in this study. These temperatures serve as a relative reference to examine the post-quenching wire properties following the application of an alternating over-current.

Study on stability evaluation and quench characteristics of non-stabilizing layer YBCO thin-film wire wound with the radius of curvature for application of SFCLS

Du, H.I.,Kim, M.J.,Han, B.S.,Song, S.S. North-Holland 2010 Physica. C, Superconductivity Vol.470 No.20

The YBCO thin-film wire for superconducting fault current limiters shows promise with the following characteristics. (1) It has a high index value that enables a quick phase transition. (2) It has a high current density. (3) It experiences little change in the critical current in high magnetic fields. (4) It has varying quenching phenomena according to its stabilizing layers. Thus, it is being actively studied in many countries for application to power transmission and distribution systems. Based on current research results, this study examined the quenching occurrence trends in the YBCO thin-film wire that has been verified as a superconducting fault current limiter element and that has no stabilizing layers. This was done according to the wire's radii of curvature and in terms of the increase in its resistance, initial limited current, and quenching occurrence time. Five kinds of spiral-shaped formers were produced with the radii increasing from the minimum radius by 25%, taking into account their changing critical properties due to bending stress. The YBCO thin-film wire without stabilizing layers was wound around these formers. Then an alternating over-current was applied to the current limiting elements that were produced according to the radii of curvature. The resulting trends in resistance enhancement were examined to evaluate the stability of the produced current limiting elements. Finally, the produced current limiting elements' initial limited current and quenching occurrence time were observed at 90K, 180K, and 250K to evaluate the performance of the current limiting elements.

Study on the application filed of HTS power apparatus of YBCO thin film wire through analysis of current transport characteristics

Du, H.I.,Lee, D.H.,Han, B.S.,Song, S.S.,Lee, J.S.,Yim, S.W.,Sung, T.H. North-Holland 2011 Physica. C, Superconductivity Vol.471 No.21

The advantages of the YBCO thin film wire, which is creating expectations as a next-generation superconducting current-limiting element, are its high index value, current density, and convenience when its capacity is increased by increasing its length. In particular, the stabilization layer of the YBCO thin film wire, which determines its abnormal current protection and excitation characteristics, can diversify its applications to superconducting power devices, depending on the specific resistance of the material of its stabilization layer. Thus, an analysis of the excitation characteristics of the YBCO thin film wire according to the material of its stabilization layer and the existence of the stabilization layer is a very important study subject. Accordingly, this study was conducted to evaluate the capabilities of the YBCO thin film wire as a current-limiting element using a YBCO thin film wire with no stabilization layer. For this purpose, the voltage-current characteristics of the YBCO thin film wire immediately after the quenching, according to the size and time of the fault current applied from the voltage source, were analyzed. Additionally, the wire's resistance-increasing tendency and its tendency to return to its superconducting state immediately after the quenching were examined. The results of this study present the evaluation data for the verification of the phase transition characteristics of the YBCO thin film wire with no stabilization layer and its potential as a superconducting element for the superconducting current limiter.

Evaluation on current-limiting performance of the YBCO thin-film wire considering electric coupling condition

Du, H.I.,Han, B.S.,Kim, Y.J.,Lee, D.H.,Song, S.S.,Han, T.H.,Han, S.C. North-Holland 2011 Physica. C, Superconductivity Vol.471 No.21

The basic way to improve the performance of a superconducting current limiter is to apply and evaluate a superconducting device that is appropriate to the superconducting current limiter. Among the many types of superconducting devices, the YBCO thin film wire has excellent current-limiting performance that is appropriate for actual system application. For the application of the YBCO thin film wire to superconducting current limiters, its current-limiting performance as a unit device must be accurately evaluated, and measures to improve its current-limiting performance must be sought. Accordingly, to evaluate the current-limiting performance of the YBCO thin film wire, this study was conducted to evaluate its resistance-increasing trend, V<SUB>max</SUB>, T<SUB>r</SUB>, I<SUB>max</SUB>, I<SUB>qt</SUB>, and current-limiting rate as a unit device, after which the electric coupling condition that consists of a core and windings was used to evaluate the current-limiting performance of the YBCO thin film wire.

Upconversion emission, cathodoluminescence and temperature sensing behaviors of Yb³⁺ ions sensitized NaY(WO₄)₂:Er³⁺ phosphors

Du, P.,Luo, L.,Yu, J.S. Ceramurgica ; Elsevier Science Ltd 2016 CERAMICS INTERNATIONAL Vol.42 No.5

A series of Yb<SUP>3+</SUP> ions sensitized NaY(WO<SUB>4</SUB>)<SUB>2</SUB>:Er<SUP>3+</SUP> phosphors were synthesized through a solid-sate reaction method. The X-ray diffraction (XRD), upconversion (UC) emission and cathodoluminescence (CL) measurments were applied to characterize the as-prepared samples. Under the excitation of 980nm light, bright green UC emissions corresponding to (<SUP>2</SUP>H<SUB>1½</SUB>,<SUP>4</SUP>S<SUB>3/2</SUB>)→<SUP>4</SUP>I<SUB>15/2</SUB> transitions of Er<SUP>3+</SUP> ions were observed and the UC emission intensities showed an upward trend with increasing the Yb<SUP>3+</SUP> ion concentration, achieving its optimum value at 25mol%. Furthermore, the temperature sensing behavior based on the thermally coupled levels (<SUP>2</SUP>H<SUB>1½</SUB>,<SUP>4</SUP>S<SUB>3/2</SUB>) of Er<SUP>3+</SUP> ions was analyzed by a fluorescence intensity ratio technique. It was found that the obtained samples can be operated in a wide temperature range of 133-773K with a maximum sensitivity of approximately 0.0112K<SUP>-1</SUP> at 515K. Ultimately, strong CL properties were observed in NaY(WO<SUB>4</SUB>)<SUB>2</SUB>:0.01Er<SUP>3+</SUP>/0.25Yb<SUP>3+</SUP> phosphors and the CL emission intensity increased gradually with the increment of accelerating voltage and filament current.

Study on resistance characteristics and operating conditions of YBCO thin-film wire for current limitation considering insulation layer

Du, H.I.,Kim, M.J.,Kim, Y.J.,Lee, D.H.,Han, B.S.,Song, S.S. North-Holland 2010 Physica. C, Superconductivity Vol.470 No.20

The YBCO thin-film wire has a high index value and a fast phase transition speed when a fault current is applied to it. Its critical properties are also superior to those of the existing Bi-Sr-Ca-O wire. Moreover, it can choose its stabilizing layers and control the magnitude of the resistance it generates in with the specific-resistance of the stabilizing layers. Thus, many researchers are studying the application of the Y-Ba-Cu-O(YBCO) thin-film wire to superconducting power machines. Being particularly studied are the properties of the YBCO thin-film wire as a current-limiting element material. In this study, to evaluate the basic properties of superconducting current-limiting elements that contain insulating layers, test samples were manufactured using the YBCO thin-film wire according to the absence and presence as well as the thickness of the insulating layers. Then a fault current was applied to the test samples to examine their quenching resistance trends and operational properties according to the fault angles. Towards those ends, a current was applied to various YBCO thin-film wires with different stabilizing layers to test their properties, and the YBCO thin-film wire that exhibited superior current-limiting performance was selected. Also, a fault current was applied to the selected YBCO thin-film wire with insulating layers to test their properties. Using the test results, the resistance occurrence trends were examined at the critical temperature of 90K, the perfect quenching temperature of 250K, and the middle temperature of 180K. Also, to increase the current limiters' capacity, the element's operational properties, such as its maximum voltage, maximum current, initial limited current, and response time were observed at 90K, 180K, and 250K according to the fault angles.

Nonorientable regular embeddings of graphs of order p²

Du, S.F.,Kwak, J.H. North-Holland Pub. Co ; Elsevier Science Ltd 2010 Discrete mathematics Vol.310 No.12

A map is called regular if its automorphism group acts regularly on the set of all flags (incident vertex-edge-face triples). An orientable map is called orientably regular if the group of all orientation-preserving automorphisms is regular on the set of all arcs (incident vertex-edge pairs). If an orientably regular map admits also orientation-reversing automorphisms, then it is regular, and is called reflexible. A regular embedding and orientably regular embedding of a graph G are, respectively, 2-cell embeddings of G as a regular map and orientably regular map on some closed surface. In Du et al. (2004) [7], the orientably regular embeddings of graphs of order pq for two primes p and q (p may be equal to q) have been classified, where all the reflexible maps can be easily read from the classification theorem. In [11], Du and Wang (2007) classified the nonorientable regular embeddings of these graphs for p<>q. In this paper, we shall classify the nonorientable regular embeddings of graphs of order p<SUP>2</SUP>where p is a prime so that a complete classification of regular embeddings of graphs of order pq for two primes p and q is obtained. All graphs in this paper are connected and simple.

Efficient activation of peroxymonosulfate by magnetic Mn-MGO for degradation of bisphenol A

Du, J.,Bao, J.,Liu, Y.,Ling, H.,Zheng, H.,Kim, S.H.,Dionysiou, D.D. Elsevier Scientific Pub. Co 2016 Journal of hazardous materials Vol.320 No.-

A heterogeneous manganese/magnetite/graphene oxide (Mn-MGO) hybrid catalyst was fabricated through the reduction of KMnO<SUB>4</SUB> by ethylene glycol in the presence of magnetite/GO (MGO) particles. The Mn-MGO catalyst exhibited high efficacy and long-term stability in activating peroxymonosulfate (PMS) to generate sulfate radicals for the removal of bisphenol A (BPA) from water. The results of the batch experiments indicated that an increase in the catalyst dose and solution pH could enhance BPA degradation in the coupled Mn-MGO/PMS system. Regardless of the initial pH, the solution pH significantly dropped after the reaction, which was caused by catalytic PMS activation. The production of sulfate radicals and hydroxyl radicals was validated through radical quenching and electron paramagnetic resonances (EPR) tests. BPA degradation pathways were proposed on the basis of LC@?MS and GC@?MS analyses. Finally, a possible mechanism of catalytic PMS activation was proposed that involved electron transfer from MnO or Mn<SUB>2</SUB>O<SUB>3</SUB> to PMS with the generation of sulfate radicals, protons and MnO<SUB>2</SUB>, as well as the simultaneous reduction of MnO<SUB>2</SUB> by PMS.