Calcium Tungstate Phosphors Research Articles

Synthesis and photoluminescence studies of Sm3+ activated calcium tungstate phosphor for illuminating devices

Synthesis and photoluminescence studies of Sm3+ activated calcium tungstate phosphor for illuminating devices

Spectroscopic analysis of calcium tungstate and Eu doped calcium tungstate phosphor

Calcium Tungstate and Eu3+ doped calcium tungstate nanocrystals were effectively synthesized by co-precipitation technique. The crystal structure and particle size of CaWO4 and Eu doped CaWO4 phosphors were elucidated by powder X-ray diffraction analysis. The photoluminescence analysis were carried out by recording the excitation and emission spectra of samples. Furthermore, the emission spectrum is used to determine the samples' emission color coordinates and is found in the red region when excited at 394 nm and in the blue region when excited at 256 nm. The Judd-Ofelt theoretical analysis was carried out to find out the various spectroscopic parameters of the tungstate matrix. The photoluminescenceinvestigation reveals that the material is suitable for a range of opto-electronic applications.

Eu3+ doped CaWO4 nanophosphor for high sensitivity optical thermometry

Eu3+ doped calcium tungstate phosphors were obtained by using sodium citrate as chelating agent in hydrothermal process. The structure and morphology of the samples were indicated by XRD and the FE-SEM. The samples prepared by us are scheelite structure. In addition, the particle size of sample decreases with sodium citrate dosage increasing, and finally reaches the nanoscale. The average particle size is 90 nm. The temperature measurement properties of phosphors were tested. It can be seen from test results that the thermal quenching behavior of Eu3+ and WO42- luminescence has obvious difference. Hence, the FIR of Eu3+ and WO42- can be used to express temperature. The maximum relative sensitivity increases with the decrease of particle size and the maximum is 4.3% K−1 (303 K, 90 nm). Moreover, the color of sample luminescence altered continuously from blue to pink-red as the temperature increased. The luminous color of the sample can be used to roughly estimate the temperature. Therefore, the CaWO4: Eu3+ nanophosphor are promising materials for optical thermometry.

Photoluminescence properties of red europium doped calcium tungstate phosphors for blue-pumped light-emitting diodes

A series of stable CaWO4:Eu3+ phosphors have been prepared by the hydrothermal method. Eu2O3 (99.99%), CaCl2 (A.R.), Na2WO4·2H2O (A.R.) are raw materials. The luminescent properties of the phosphors have been investigated by the different contents of the dopants in CaWO4 and the optimum concentration of as-synthesized phophors is 1.5wt%. The structure, morphology and luminescent properties of the phosphors are characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and spectroscopy technology. The results showed that introduction of Eu did not change the crystal structure and morphology of CaWO4:Eu3+ phosphors. The strongest emission at 614nm is corresponding to 5D0→7F2 transition of Eu3+ under 464nm blue light excitation. The result reveals that CaWO4:Eu3+ is a promising red-emitting phosphor for blue LED applications.

Luminescence properties of calcium tungstate activated by lanthanide(III) ions

Calcium tungstate phosphors activated by the Ln3+ ions (Ln=Pr, Nd, Tb, Yb) were synthesized by a traditional high-temperature solid-state method. The crystal structures and morphologies of the products were characterized by scanning electron microscopy (SEM), X-ray powders diffraction (XRD) and infrared spectra (FT-IR). The samples were found to show luminescence properties (down-conversion, DC, at excitation wavelength 254 nm and up-conversion, UC, at excitation wavelength 980 nm). CaWO4 doped with Tb3+/Yb3+ showed green DC and UC luminescence characteristic of Tb(III) ion in the range of 470–660 nm, corresponding to the 5D4→7F6,5,4,3,2 electronic transition. CaWO4 doped with Pr3+/Yb3+ showed week blue, green and red (DC and UC) luminescence of Pr(III) ion, in the wavelength region of 450–700 nm. Emission peaks were ascribed to the 3P1→3H4,5,6, 3P0→3H4,5,6, 3P1→3F2 and 3P0→3F2 transitions, respectively. CaWO4 doped with Nd3+/Yb3+ phosphor emitted orange UC luminescence at 450–690 nm (2P3/2→4I15/2, 4G7/2→4I9/2,11/2,13/2) and strong near-infrared UC luminescence at 720–900 nm (4F7/2+4S3/2→4I9/2, 4F5/2+2H3/2→4I9/2, 4F3/2→4I9/2) which is the characteristic of Nd(III) ion.

Ca1−x−yDyxKyWO4: A novel near UV converting phosphor for white light emitting diode

A series of Dy3+/K+ doped calcium tungstate phosphors were synthesized by solid state reaction method. The crystal structure, surface morphology, chemical composition and photoluminescence properties of the prepared phosphors were investigated. The luminescence decay curves of doped/codoped phosphors were recorded. The X-ray diffraction analysis shows that the phosphors are of tetragonal structure. SEM studies confirm the particle size in the micro-meter (μm) range. The photoluminescence results indicate that these phosphors could be efficiently excited by the near-ultraviolet radiation which causes the emission in the blue and yellow regions. The white light was achieved by tailoring this yellow to blue ratio varying the Dy3+/K+ dopant concentration. The quality of emitted white light was checked by calculating different CIE parameters of these phosphors. Decay kinetics studies confirms the life time of activator ions in micro-second (μs) range.

Polarization selection rules for the allowed optical transitions in europium–terbium double activated calcium tungstate phosphor

The paper is devoted to the problem of the optical anisotropy of the rare-earth ions occupying low-symmetry positions in crystals. The crystal field multiplets arising from LSJ terms of Eu 3+ and Tb 3+ ions in the crystal field of calcium tungstate scheelite (CaWO 4) are analyzed ( S 4 point symmetry). The selection rules, in particular, polarization rules for the allowed electric dipole optical transitions in the electronic shells of the Eu 3+ and Tb 3+ in CaWO 4 host lattice are discussed. Special attention is paid to the study of the angular (polarization) dependence of the two-photon absorption that seems to be an effective tool for the understanding of the complicated optical pattern. The peculiarities of the anisotropy of the two-photon absorption prove to be specific for each allowed dipole transition in S 4 symmetry center.

Thoracic radiology: the past 50 years.

The radiology of 50 years ago was a primitive science compared with the radiology of today. Hospital departments were small and radiologists few in number. Night call was uncommon. Examinations consisted primarily of radiographs of the chest, bones, and gastrointestinal tract, although some early neuroradiologic studies were performed. Chest fluoroscopy was common. Film processing was done manually, often with poor results. Radiographic examinations of the chest were likewise unsophisticated by today's standards. Chest radiographs were made with low-kilovoltage, calcium tungstate phosphors and relatively large focal spots. There were no image intensifiers, nuclear medicine studies, ultrasonography, computed tomography, or magnetic resonance studies. How far we have come!

Effective Debye-Waller parameter and emission intensity of calcium tungstate phosphors

The relation between emission intensity under ultraviolet excitation and Debye-Waller parameter was studied on calcium tungstate phosphors. The samples were prepared by precipitation from aqueous solition of calcium chloride and ammonia-alkaline solution of ammonium paratungstate and by firing at 400°–1000°C in air. The precipitating temperature has a strong influence on the form of precipitated particles, being spherical at 0°C and spindle-shaped at 90°C, and also on the preferred orientation of crystallites in precipitates. The change of emission intensity under ultraviolet excitation with firing temperature shows a correspondence to the change of effective Debye-Waller parameter, B eff, determined from X-ray diffraction intensity measurements. On the ground samples, of which the original is fired at 900°C and has spherical particles, a linear relation was found between the emission intensity under ultraviolet excitation and the dynamic component B d in B eff, which is due to thermal vibration of constituent atoms.

Luminescence of alkali earth tungstate

Calcium tungstate and the solid solution between calcium and barium tungstates were prepared by the precipitation from their aqueous solutions. The precipitate has a spherical form and the resultant phosphor did not change its form. By the grinding the luminescent intensity of pure calcium tungstate phosphor decreased linearly with the increase of the effective value of Debye-Waller parameter. Calcium tungstate and its solid solution activated by Eu 2+ were prepared and their luminescent properties were studied. It is possible to prepare the more efficient phosphor for X-ray intensify screen by activating the solid solution of alkali earth tungstate by Eu 2+.

On the energy storage of calcium tungstate phosphor

On the energy storage of calcium tungstate phosphor

On the Preparation of Calcium Tungstate Phosphors

On the Preparation of Calcium Tungstate Phosphors

Physical Properties of Calcium Tungstate X-Ray Screens

The absolute energy of the light from a fluorescent calcium tungstate x-ray screen, its spectral distribution, and the efficiency of conversion from x-rays to light have been determined. The absolute energy and its spectral distribution were obtained spectrographically by comparison with a standard tungsten filament lamp. Equations are given for determining the distribution of light in the diffusing screen material, and calculation of the light losses are made for varying screen thickness and x-ray absorption. The data for energy distribution of the ingoing x-rays were taken from measurements obtained in this laboratory by a new method to be published elsewhere. Allowance for scattering and secondary x-radiation was made and a conversion efficiency of 5.0 percent was obtained for this calcium tungstate phosphor.