Keli Zhang

Wuhan University, Wuhan, Hubei, China

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Publications (34)41.57 Total impact

  • ChemInform 01/2009; 40(50).
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    ABSTRACT: A novel europium(III) complex involving a carbazole fragment as hole-transporting group and an oxadiazole fragment as electron-transporting group was synthesized and used as red light-emitting material in organic light-emitting diodes (OLEDs). The complex is amorphous, and exhibits high glass transition temperature (Tg = 157 °C) and high thermal stability with a 5% weight loss temperature of 367 °C. Two devices, device 1: ITO/NPB (40 nm)/Eu(III) complex (30 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm) and device 2: ITO/NPB (40 nm)/3% Eu(III) complex: CBP (30 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm), were fabricated, where NPB is N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine, Alq3 is tris(8-hydroxyquinoline) Al(III), CBP is 4,4′-bis(carbazole-9-yl)-biphenyl, and BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, respectively. In contrast with device 1, owing to less self-quenching and better charge confinement, device 2 shows improved performances: the maximum luminance of device 2 was dramatically increased from 199 to 1845 cd/m2, the maximum current efficiency was increased from 0.69 to 2.62 cd/A, the turn-on voltage was decreased from 9.5 to 5.5 V, and higher color purity was attained.
    Synthetic Metals 01/2009; 159:72-77. · 2.11 Impact Factor
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    ABSTRACT: A novel quaternary Eu(III) complex containing a carbazole fragment as hole-transporting functional group was synthesized, and its thermal stability, photoluminescent (PL), electroluminescent (EL) properties were studied. Its glass transition temperature (T g) was 131 °C and 5% weight loss temperature was 325 °C. In studies of its EL properties, two devices with the Eu(III) complex as red light-emitting materials were fabricated and measured. Device 1: ITO/NPB (40 nm)/Eu(III) complex (30 nm)/Alq3 (30 nm)/LiF (0.7 nm)/Al (100 nm), NPB was N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine as the hole-transporting layer, Alq3 was tris(8-hydroxyquinoline) aluminum as the electron-transporting layer. Device 1 gave two emission bands of the Eu(III) complex and Alq3 with the maximum luminance of 437 cd/m2 at 17.34 V, and its turn-on voltage was 10 V. In device 2, an electron-transporting/hole-locking layer of BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, 30 nm) was added between the Eu(III) complex and Alq3 layers, only a sharp red emission band of the Eu(III) complex was given with the maximum luminance of 186 cd/m2 at 20.08 V, and its turn-on voltage was 12 V.
    Journal of Materials Science Materials in Electronics 01/2009; 20(7):597-603. · 1.49 Impact Factor
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    ABSTRACT: This paper reports a newly developed method for the shape and size control of transition metal composite oxides, such as LiVMoO6, to obtain significantly enhanced electrode properties for lithium-ion batteries. Rod-like LiVMoO6 nanocrystals were synthesized through a designed route of partial reduction, self-assembly and re-oxidation. V5+ and Mo6+ ions were used with low-grade starting materials to get a mixed valence of V and Mo. It is believed that ion pairs of V5+/V4+ or Mo6+/Mo5+ in the resultant mixture play an important role in the formation of a template precursor by self-assembly during a rheological phase reaction, although further explanation is required. The electrochemical performance of the LiVMoO6 obtained has been much improved due to the increased crystallinity and reduced particle size of this material. 176 mA h g-1 and 166 mA h g-1 capacity was delivered in the initial discharge with a reversible capacity retention of 94.8% and 95.3% after 100 cycles in the range of 3.6-1.80 V versus metallic Li at 1 and 3 C current rate, respectively.
    Nanotechnology 04/2007; 18(13). · 3.84 Impact Factor
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    ABSTRACT: 4ZnO·B2O3·H2O is commonly used as a flame-retardant filler in composite materials. The microstructure of the powder is of importance in its applications. In our study, for the first time, one-dimensional (1D) nanostructure of 4ZnO·B2O3·H2O with rectangle rod-like shape has been synthesized by a hydrothermal route in the presence of surfactant polyethylene glycol-300 (PEG-300). The nanorods have been characterized by X-ray powder diffraction (XRD), inductively coupled plasma with atomic emission spectroscopy (ICP-AES), thermogravimetry (TG) and differential thermal analysis (DTA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) equipped with selected area electron diffraction (SAED) as well as high-resolution transmission electron microscopy (HRTEM). These nanorods are about 70 nm in thickness, 150–800 nm in width and have lengths up to a few microns. 4ZnO·B2O3·H2O nanorods crystallize in the monoclinic space group P21/m, a = 6.8871(19) Å, b = 4.9318(10) Å, c = 5.7137(16) Å, β = 98.81(21)° and V = 191.779(71) Å3.
    Materials Research Bulletin. 01/2007; 42(9):1649-1656.
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    ABSTRACT: CoMoS4 nanoflowers was synthesized by precipitation method from Na2MoO4, CoCl2 6H2O and CH3CSNH2 as starting materials. X-ray diffraction (XRD) analysis showed that the prepared samples is amorphous structure, X-ray photoelectron spectrometer data of Co, Mo and S in CoMoS4 revealed that the valences of the corresponding elements are almost +2, +6, and −2, respectively. Scanning electron microscope gave the result that the morphologies of the as-prepared powder is flower-like nanostructure. Moreover, electrochemical tests were also carried out, the initial discharge was 1547 mA h g−1, even after 20 cycles, it still remains 461 mA h g−1.
    Journal of Nanoparticle Research 01/2007; 9(4):653-656. · 2.18 Impact Factor
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    ABSTRACT: The surface of the spinel LiMn2O4 was modified with zinc oxide by a chemical process to improve its electrochemical performance at high temperatures. The physical properties of the prepared products have been investigated by thermogravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-rays analysis (EDAX). The charge/discharge of the materials was carried at 1mA/cm2 in the range of 3.0 and 4.4V at 55°C. The discharge capacity of ZnO-coated LiMn2O4 (117mAh/g) showed only 3% loss of the initial capacity (121mAh/g) over 60 cycles. The cycle ability improvement of the spinel LiMn2O4 coated with ZnO is demonstrated at high temperatures. From the analysis of electrochemical impedance spectroscopy (EIS), the improvement of cycle ability may be attributed to the suppression on the formation of the passivation film and the reduction of Mn dissolution, which result from the modifying the surface of the spinel LiMn2O4 with zinc oxide.
    Materials Chemistry and Physics - MATER CHEM PHYS. 01/2007; 101(2):276-279.
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    ABSTRACT: Without Abstract
    Journal of Materials Science 01/2006; 41(10):3179-3182. · 2.31 Impact Factor
  • Jianhe Hong, Guanghui Guo, Keli Zhang
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    ABSTRACT: The non-isothermal decomposition of nickel acetate tetrahydrate in air was studied using thermogravimetry (TG)–DTG, differential scanning calorimetry (DSC) and XRD techniques. The decomposition occurs in two major steps and the final product is NiO. The dependence of mass loss on heating rates in TG measurements imply that the dehydration and hydrolysis concur at temperature below 240 °C; the apparent activation energies calculated by Flynn, Wall and Ozawa (FWO) isoconversional method indicate the existence of a consecutive process. The kinetics of the first major decomposition step (below 240 °C) was obtained with multivariate non-linear regression of four measurements at different heating rates. According to the kinetics results from non-linear regression, the dehydration reaction (F1 type with an activation energy E of 167.7 kJ/mol) goes first. After the loss of almost half of water, the retained water and acetate are linked to each other by hydrogen bonding, so dehydration and hydrolysis concur. The pathway with a lower E is related to the hydrolysis process and the other is corresponding to the dehydration process. The simulations of reactants at different heating rates were used to verify the correctness of the reaction model. With the kinetics results, the dehydration mechanism was discussed for the first time.
    Journal of Analytical and Applied Pyrolysis. 01/2006;
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    ABSTRACT: The main purpose of this work is proposing a new method of using non-isothermal formal kinetics analysis to predict the lifetime of luminescent complex materials. The Eu(III)-phenanthroline complex doped xerogel has been in situ synthesized by a catalyst-free sol–gel method. The photoluminescence spectra and TG curves of the xerogel verify the formation and decomposition of Eu(III)-phenanthroline complex in xerogel. The decomposition of the xerogel formally occurs in three steps. The Friedman and FWO isoconversional methods and multivariate non-linear regression method are used for formal kinetic analysis. The overall decomposition process below 800 °C is fitted by three-step consecutive reaction. The best fitted model for each step is Fn (n order reaction, the corresponding function f(α) is (1 − α)n). Correlation coefficient is 0.99956. The lifetime values of xerogel at different temperatures are predicted based on non-isothermal kinetic models by the 5% decomposition of europium organic complex.
    Thermochimica Acta 01/2006; 440(1):31-35. · 1.99 Impact Factor
  • Journal of Materials Science 11/2005; 40(21):5767-5769. · 2.31 Impact Factor
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    ABSTRACT: Lithium trivanadate, LiV3O8, was synthesized using rheological phase reaction method. First, the mixture of LiAc·2H2O, NH3VO3, citric acid and a required amount of water reacted to form the rheological precursor, then the precursor was heated at different temperatures in the range of 350–600 °C and the product LiV3O8 was obtained. Thermogravimetric (TG) was employed to analyze the precursor. The LiV3O8 product which was characterized by inductively coupled plasma (ICP), X-ray diffraction (XRD) and scanning electron microscope (SEM) indicated that LiV3O8 has been obtained by this novel synthesis method. The influence of the preparing conditions including sintering temperature on crystal structure and electrochemical performance has been investigated. As a result, the LiV3O8 obtained at 400 °C for 10 h has excellent electrochemical properties. At 20 °C, the first discharge of which was 272.9 mAh/g in the range of 1.8–4.0 V at 0.3 mA cm− 2. An inexpensive, simple and effective route to synthesis inorganic material was provided.
    Solid State Ionics 01/2005; 176:1549-1554. · 2.05 Impact Factor
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    ABSTRACT: Thermal decomposition taking place in solid state complex, NiC2O4·2H2O, has been investigated in air by means of TG–DTG/DTA, DSC, XRD. TG–DTG/DTA curves showed that the decomposition proceeds through two well-defined steps with DTA peaks closely corresponding to the weight loss obtained. XRD showed that the final decomposition product of NiC2O4·2H2O was NiO. Kinetics analysis of NiC2O4·2H2O decomposition steps was performed under non-isothermal conditions. The activation energies were calculated through Friedman and Flynn–Wall–Ozawa (FWO) methods, and the most possible kinetic model function has been estimated through the multiple-linear regression method. The activation energies for the two decomposition steps of NiC2O4·2H2O were 171.1±4.2 and 174.4±8.1kJ/mol, respectively.
    Thermochimica Acta 01/2005; 430(1):101-105. · 1.99 Impact Factor
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    ABSTRACT: The thermal decomposition of copper(II) acetate monohydrate was studied in air using TG-DTG/DTA, DSC, XRD techniques. TG-DTA curves show that the decomposition occurs in four steps. TG and XRD data indicate the reduction of Cu(II) during the decomposition of CuAc2·H2O. DTG and DSC data imply at least two steps contained in dehydration reaction, which is verified by the activation energy values estimated with Friedman and Flynn–Wall–Ozawa (FWO) methods. The dependence of activation energy on conversions indicates the dehydration reaction contains an additional initial reversible step. The most-probable kinetic model has been estimated with multivariate non-linear regression method assuming a two-step consecutive reaction. Bna (expanded Prout–Tompkins equation) → Cn (n order autocatalytic reaction) model fits the original data best with a high correlation coefficient of 0.9998, and the calculated apparent activation energies of the fitted models are consistent with those calculated by isoconversional methods using original data. The corresponding function f(α), activation energy E and preexponential factor A of Bna, are (1 − α)0.7593α0.2867, 128.5 kJ/mol and 1.6 × 1015, respectively. Those of Cn, are (1 − α)1.0534(1 + 2.712α), 80.5 kJ/mol and 6.9 × 107, respectively. The combination of model free isoconversional methods and multivariate non-linear regression can give more reasonable and applicable models than commonly used model fitting methods.
    Thermochimica Acta. 01/2005;
  • Journal of Materials Science 01/2005; · 2.31 Impact Factor
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    ABSTRACT: Magnesium phthalate dihydrate was synthesized by rheological phase reaction. The crystal structure was determined by XRD. Its thermal decomposition mechanism in N2 was studied by TG DTA. The thermal decomposition products were characterized by XRD, GC–MS. The thermal decomposition of magnesium phthalate dihydrate in N2 proceeded in two stages; it first lost two crystal water to form anhydrate salt, then magnesium phthalate decomposed further, the last product was MgO. The organic compounds are mainly benzophenone and anthraquinone.
    Thermochimica Acta - THERMOCHIM ACTA. 01/2005; 428(1):47-50.
  • Journal of Materials Science 01/2005; 40(9):2557-2559. · 2.31 Impact Factor
  • Kahirou Diakite, Dan Zhan, Keli Zhang
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    ABSTRACT: This paper deals with the synthesis and characterization of strontium salicylate from strontium carbonate. The elaboration method is a soft chemistry one which allows to obtain a pure product. The strontium salicylate was characterized by FTIR spectra, X-ray diffraction and thermal analysis (DTA-TGA). The final decomposition product was found to be SrO. The structure was characterized by XRD. The complex is monoclinic crystal system and the crystal cell parameters are a=13.772(5)(10−10 m), b=4.994(5)(10−10 m), c=9.563(0)(10−10 m), β=101.32(9)°, V=645(10−10 m)3.
    Materials Letters - MATER LETT. 01/2005; 59(18):2243-2245.
  • Chuan Qi Feng, Keli Zhang, Jutang Sun
    Materials Science Forum - MATER SCI FORUM. 01/2005;
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    ABSTRACT: Two europium α-thiophene carboxylic acid (HTPA) compounds, coordination polymer Eu(TPA)3(HTPA)2 (1) (TPA=α-thiophene carboxylate) and supramolecular compound Eu(TPA)3(H2O)3·0.5H2O (2) with luminescence and triboluminescence, have been synthesized and structurally characterized. In 1 each europium is bridged by six oxygen atoms from six carboxylates and coordinated with two carboxyl oxygen atoms from two α-thiophene carboxylic acid molecules, resulting in a coordination number of eight to Eu. For 2 each europium is chelated by six oxygen atoms from six carboxylates and coordinated with three oxygen atoms from three coordinated water generating a coordination number nine to Eu; A supramolecular compound is constructed through hydrogen bonds. Both 1 and 2 display strong characteristic emission of Eu3+ ion radiated by UV light and produce twinkling red light with an external force.
    Inorganica Chimica Acta 01/2004; 357(1):89-94. · 1.69 Impact Factor