Hong He

Photoluminescent properties of SrSi2O2N2 : Eu2+ phosphor: concentration related quenching and red shift behaviour

Highly efficient green phosphors SrSi2O2N2 : Eu2+, suitable for white light emitting diodes (LEDs), was synthesized by solid-state reaction and their photoluminescence properties were investigated. SrSi2O2N2 : Eu2+ phosphors can be effectively excited by ultraviolet (UV)–Vis light (300–460 nm) and yield green emission with a single, intense, broad band centred at around 540 nm. Concentration quenching occurs in the phosphors when the Eu2+ concentration exceeds 2 at%. The mechanism of concentration quenching is confirmed to be dipole–dipole interaction of Eu2+. With an increase in the Eu2+ concentration, the emission band of Eu2+ shifts to a longer wavelength due to the 5d-orbit hybridization of Eu2+ with the host crystal and the energy transfer between adjacent Eu2+ ions. All the results indicate that SrSi2O2N2 : Eu2+ is a promising green phosphor for UV or blue chip based white LEDs.

Photoluminescence properties of Eu2+-activated CaSi2O2N2: Redshift and concentration quenching

Highly efficient CaSi2O2N2:Eu2+ green phosphors were synthesized via solid-state reaction method. The produced phosphors are effectively excited with UV-vis light of wavelength between 300 and 460 nm and emit a single, intense, and broad emission band centered at 538 nm. The experimental results and their analysis suggest that the energy transfer mechanism should occur due to dipole-dipole interactions among Eu2+ ions, resulting in a shift in emission spectrum toward longer wavelengths with increasing Eu2+ concentration. The quenching concentration of Eu2+ (i.e., where the emission intensity maximizes) is approximately 2 at. %. Accordingly, the produced CaSi2O2N2:Eu2+ green phosphors are qualified for further consideration and experimentation for potential use in white light emitting diodes.

Li(2)SrSiO(4):Eu(2+) phosphor prepared by the Pechini method and its application in white light emitting diode

Eu 2+ -doped Li 2 SrSiO 4 phosphors were prepared by two different synthesis processes, the Pechini sol-gel route and solid-state reaction (SSR) method. Their morphology, crystal structure, and luminescence properties have been characterized. Li 2 SrSiO 4 :Eu 2+ phosphors show broad and intensive excitation in the range of 390–480 nm and emit yellow-orange light extending from 500 to 700 nm. The luminescence efficiency of Li 2 SrSiO 4 : Eu 2+ phosphors synthesized through the Pechini route is much better than that of phosphors prepared by solid-state reaction method. The application of phosphors from the Pechini route in white light emitting diodes (LEDs) has been investigated. The Commission Internationale de L’Eclairage (CIE) chromaticity coordinates and the correlated color temperature of these white LEDs have been calculated; they are comparable to corresponding values of commercial Y 3 Al 5 O 12 :Ce 3+ converted white LEDs.

Luminescence and Energy-Transfer Mechanism in SrSi2O2N2 : Ce3 + , Eu2 + Phosphors for White LEDs

A series of Ce 3+ and Eu 2+ co-doped SrSi 2 O 2 N 2 phosphors, whose features qualify them for consideration in white-light UV or blue light-emitting diodes (LEDs), was synthesized via a high temperature solid-state reaction under a reductive atmosphere. The dependence of luminescence properties of the produced powders on the concentration of an activator (Eu 2+ ) and a coactivator (Ce 3+ ) was investigated. The experimentally recorded luminescence spectra and the calculations of the efficiency of energy transfer from Ce 3+ to Eu 2+ and the critical distance between Ce 3+ and Eu 2+ suggest a resonance-type energy-transfer mechanism from Ce 3+ to Eu 2+ due to dipole-dipole interactions.

High Thermal Conductive Si3N4 Particle Filled Epoxy Composites With a Novel Structure

Traditionally, large quantities of ceramic fillers are added to polymers in order to obtain high thermally conductive polymer composites, which are used for electronic encapsulants. However, that is not cost effective enough. In this study, Si 3 N 4 particle filled epoxy composite with a novel structure was fabricated by a processing method and structure design. Epoxy resin used in particle form was obtained by premixing and crushing. Different particle sizes were selected by sieving. High thermal conductivity was achieved at relative low volume fraction of the filler. The microstructure of the composites indicates that a continuous network is formed by the filler, which mainly completes the heat conduction. Thermal conductivity of the composites increases as the filler content increases, and the samples exhibit a highest thermal conductivity of 1.8 W/mK at 30% volume fraction of the filler in the composites using epoxy particles of 2 mm. The composites show low dielectric constant and low dielectric loss.

Numerical Simulation of Thermal Conductivity of Particle Filled Epoxy Composites

A finite element method was developed to predict the effective thermal conductivity of particle filled epoxy composites. Three-dimensional models, which considered the effect of filler geometry, filler aspect ratio, conductivity ratio of filler to matrix, and interfacial layer were used to simulate the microstructure of epoxy composites for various filler volume fractions up to 30%. The calculated thermal conductivities were compared with results from existing theoretical models and experiments. Numerical estimation of ellipsoids-in-cube model accurately predicted thermal conductivity of epoxy composites with alumina filler particles. The number of length division during mesh process and particle numbers used in the finite element analysis affect the accuracy of calculated results. At a given value of filler content, the numerical results indicated a ratio of conductivity of filler to matrix for achieving the maximum thermal conductivity.

Observation of Fluorescence and Phosphorescence in Ca2MgSi2O7 : Eu2 + , Dy3 + Phosphors

This paper describes the experimental results of the fluorescence and phosphorescence observations in the Ca 2 MgSi 2 O 7 :Eu 2+ ,Dy 3+ phosphors. Discussions are made to elucidate the relationship between the photoluminescence of Eu 2+ and the crystal structure of the host, the luminescent quenching of the emission of Eu 2+ , and the role of Dy 3+ in the afterglow property. There exist two distinguishable Ca 2+ sites in the host, supported by two fluorescent emission bands located in the blue and green regions, respectively, originating from Eu 2+ doped into the host. The emission peak of Eu 2+ at about 474 nm quickly quenches as its concentration increases due to excitation energy transfer between the Eu 2+ ions doped into the two nonequivalent Ca 2+ sites. The answers to the question of why only one emission band was observed in some reports throughout the literature were supplied. Only the emission at 533 nm of Eu 2+ contributes to the long-lasting phosphorescence, whereas Dy 3+ ions do not act as the luminescent center. By investigating the decay time of the phosphorescence at 533 nm, preliminary conclusions were made that the co-doping of Dy 3+ obviously enhances the afterglow of Ca 2 MgSi 2 O 7 :Eu 2+ through electron trap-detrap and recombination processes by its electron trapping role.

Crystal Structure and Luminescent Properties of Eu2 + -Doped Li2BaSiO4 with a Polymorph for White LEDs

The crystal structure and photoluminescence of Li 2 BaSiO 4 :Eu 2+ phosphors were investigated. A preliminary conclusion was reached through X-ray diffraction, Fourier transform infrared absorption, photoluminescent excitation and emission investigations that a Li 2 BaSiO 4 polymorph was found. The Li 2 BaSiO 4 :Eu 2+ characterizes an intense blue emission with a peak at 465 nm and a broad excitation band in the UV/visible range. Warm white light with CIE coordinates (0.334, 0.347) and color temperature of about 5500 K is generated by mixing this blue phosphor with the Li 2 SrSiO 4 :Eu 2+ yellow phosphor. These results indicate that Li 2 BaSiO 4 :Eu 2+ with the polymorph of the host would be a promising blue phosphor candidate for UV-chip-based multiphosphor converted white light-emitting diodes (LEDs).

Luminescence Property of Eu2+ Doped Strontium Silicate Yellow Phosphor for White Light Emitting Diode

Eu2+ activated strontium silicate (Sr3SiO5:Eu2+) yellow phosphor was prepared by the conventional solid-state reaction technique in reduction atmosphere with starting materials SrCO3, SiO2 and Eu2O3. The phase, morphology and luminescence property of the samples were analyzed. The X-ray diffraction analysis showed the main phase in all samples is tetragonal Sr3SiO5. With Eu2+ ions doped into the host lattice, the Sr3SiO5:Eu2+ phosphors absorb light energy in the UV-visible spectrum region and show an intense broad emission band in the yellow colour range (around 570 nm) and a weak band in the blue region (around 470 nm). The excitation and the emission bands are originated from the 4f−5d transition of Eu2+ ions. When the concentration of Eu2+ ions increases, the emission peaks of phosphors shift to longer wavelengths. The effective emission in the yellow colour indicates that phosphor as potential candidates for white light-emitting-diodes.

Structure and Dielectric Properties of AlN Multilayered Film on Al Substrate

AlN multilayered films were deposited on Al substrates using RF reactive magnetron sputtering with Al targets under Ar and N2 atmosphere. Circles of deposition and annealing were repeatedly performed. Macrostructure observations, crystallographic analyses and dielectric property measurements were carried out. The grains of AlN film had a worm-like shape. When the number of layers (and cycles) increased, the (100) and (110) oriented grains weakened and the structure of film changed into (002) and (101) oriented. The capacity–frequency (C-f) curves of Cu-AlN-Al-Cu capacitors, measured at 100 Hz - 1 MHz, showed that the dielectric constant and the dielectric loss of AlN decrease with increasing number of cycles, attributed to annealing processes that influences film microstructure and the orientation of worm-like shape grains.