Zhongtai Zhang

Preparation of long-afterglow Sr4Al14O25-based luminescent material and its optical properties

Abstract Sr 4 Al 14 O 25 –Eu,Dy photoluminescent material with high brightness and long afterglow was prepared by sintering at high-temperature and weak-reducing atmosphere. The luminescent properties of Sr 4 Al 14 O 25 -based luminescent materials have been studied systematically. The analytical results indicated that the emission spectra of Sr 4 Al 14 O 25 –Eu,Dy are similar to that of Sr 4 Al 14 O 25 –Eu. Both of them have two main peaks near 424 and 486 nm, respectively, which are ascribed to two types of Eu 2+ that existed in the Sr 4 Al 14 O 25 matrix crystal. However, the Sr 4 Al 14 O 25 –Eu phosphor does not have the long-afterglow phenomenon; the Sr 4 Al 14 O 25 –Eu,Dy showed the obvious long afterglow. As for the mechanism of the long afterglow of this phosphor, Dy 3+ acted as the trap level and captured the free holes in the system. It is the trapped–detrapped recombination process that resulted in the long-afterglow phenomenon of this phosphor.

Anomalous luminescence in Sr4Al14O25:Eu, Dy phosphors

Sr4Al14O25:Eu, Dy material with an extraordinarily long afterglow was synthesized via a traditional ceramic processing. Such a long afterglow observed can last over 20 h at recognizable intensity level (0.32 mcd/m2), which is attributed to energy exchange between the traps and emission states resulting from Dy and Eu doping. When the Eu2+ concentration doped in the host is less than 1.2 mol %, the emission spectra of Sr4Al14O25:Eu, Dy show two main peaks at 407 and 494 nm, ascribed to two types of Eu2+ centers existing in the Sr4Al14O25 host. The emission peak at 407 nm disappears slowly with increasing concentration of Eu2+ due to the energy exchange between two types of Eu2+ centers.

Luminescence of Eu2+ and Dy3+ activated R3MgSi2O8-based (R=Ca, Sr, Ba) phosphors

Abstract Long afterglow R 3 MgSi 2 O 8 :Eu 2+ , Dy 3+ (R=Ca, Sr, Ba) phosphors were prepared at high temperature and weak reductive atmosphere. The excitation and emission spectra of these phosphors showed that all are broadband due to 4f 7 –4f 6 5d transitions of Eu 2+ , and the main emission peaks at 471.3, 458.2 and 439.1 nm, respectively. The decay curves indicated that all of these phosphors have long afterglow features, and the Sr 3 MgSi 2 O 8 -based phosphor has better afterglow properties than the other phosphors.

Influence of co-doping different rare earth ions on the luminescence of CaAl2O4-based phosphors

Abstract CaAl 2 O 4 : Eu, RE 3+ (RE: Dy, Nd, La) photoluminescent materials with high brightness and long afterglow are prepared by sintering at high temperature and in a reductive atmosphere. The influence of co-doping rare earth ions (Dy, La and Nd) on the luminescence of CaAl 2 O 4 : Eu was studied. The excitation and emission spectra of these three kinds of phosphors are very similar to that of CaAl 2 O 4 : Eu phosphor, and all of them have long afterglow phenomenon except for CaAl 2 O 4 : Eu. Both of initial brightness and persistent afterglow time of CaAl 2 O 4 : Eu, Nd is better than those of CaAl 2 O 4 : Eu, Dy, and CaAl 2 O 4 : Eu, La, which is agreement with the results of thermoluminescence curves. The low temperature photoluminescent spectra indicate that Eu ions occupied only one kind of Ca site in CaAl 2 O 4 crystal lattice.

Surface modification of Li1.03Mn1.97O4 spinels for improved capacity retention

Li1.03Mn1.97O4 spinel material was coated with a thin layer of SiO2 by a sol–gel method. The surface modification was found to reduce the Mn2+ dissolution into the electrolyte according to the inductively coupled plasma (ICP) analysis results and improve the capacity retention of the Li1.03Mn1.97O4 spinel. The structure and properties of the coating materials were also investigated by X-ray photoelectron spectra (XPS) analysis, X-ray diffraction (XRD) patterns and Transmission electron microscope (TEM).

Luminescent properties of SrAl2O4: Eu, Dy material prepared by the gel method

SrAl2O4:Eu,Dy materials were first prepared by the gel method. Compared with samples prepared by solid state reactions, the grain size of the gel method is greatly reduced to nanometer grade. A clear blue shift occurs in the excitation and emission spectra of nano SrAl2O4:Eu,Dy, of which the peak of the excitation and emission spectra are at 323 and 500 nm respectively. The brightness of nano SrAl2O4:Eu,Dy is greatly reduced. The blue shift and the change of luminescent intensity in nano SrAl2O4:Eu,Dy materials can be attributed to the effect of surface energy.

Preparation of Ni/YSZ materials for SOFC anodes by buffer-solution method

NiO/YSZ composites were fabricated using mixed nickel oxideyttria stabilized zirconia powders (NiO/YSZ) with different Ni contents prepared by an improved co-precipitation technique, named the buffer-solution method. The composites were then exposed in a reducing gas mixture of CO and CO2 to form Ni/YSZ cermets which may be used as anode materials in solid oxide fuel cell (SOFC) technology. The conductivities of the resultant cermets were measured over the temperature range from 800 to 1200 degreesC and were found to be always much higher than those of samples with the same Ni contents but prepared by the traditional mechanical mixing method. This may be owing to the fact that the buffer-solution method can assure more homogeneous distribution of Ni in YSZ matrix

Preparation and characterization of a new long afterglow indigo phosphor Ca12Al14O33:Nd,Eu

A new phosphor Ca12Al14O33:Nd,Eu was synthesized by a traditional solid-state reaction method and the luminescent properties were investigated. The phosphors are well crystallized by calcination at 1200 °C. The excitation and the emission spectra show the characteristic broadband of the Eu2+ ion and the emission light is indigo. Doping Nd3+ ion in the phosphor results in indigo long afterglow phosphorescence (the time for the phosphorescence intensity to decrease to 10% of its initial value is about 50 s) when the excitation light is cut off. The emission intensity decreases with the increase of Nd3+ ion concentration. Whereas, the phosphorescence intensity and afterglow time improve when the concentration of Nd3+ ion increases.

Low-temperature combustion synthesis and characterization of nanosized tetragonal barium titanate powders

Nanosized tetragonal barium titanate has been directly prepared by low-temperature combustion synthesis process (LCS). According to thermochemical calculations of propellant chemistry theory, the optimum molar ratio of reactants was obtained with the Ba(NO3)2-TiO(NO3)2-citric acid-NH4NO3 system. The influence of the ratio of reactants on the properties of the final products was investigated. The results showed that tetragonal BaTiO3 with high purity was produced at low ignition temperature (∼300 °C). The crystalline size of the powder obtained is less than 50 nm. The crystal structure, crystalline size and morphology were analyzed by XRD, SEM and TEM.

Role of second phase in (Nb,Ce,Si,Ca)-doped TiO2 varistor ceramics

Abstract A low-voltage TiO 2 varistor ceramics doped with Nb 2 O 5 , SiO 2 , CeO 2 and CaCO 3 was systematically researched. The effects of different dopants on varistor voltage V 1 mA were investigated by orthogonal test method. SEM, XRD and EDAX were carried out to study the change of microstructure. The results show that there exist second phase on the surface of TiO 2 grains, which can facilitate an increasing varistor voltage. The second phase is proved to be Perrierite phase (Ce 2 Ti 2 Si 2 O 11 ) by EDAX and the content varies with sintering temperature. It is suggested that the second phase segregates at the grain surface during sintering process and makes an insulating layer which result in a higher V 1 mA .