Phonon Dispersions in Calcium Tungstate

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Category: Tungsten Information
Published on Thursday, 09 July 2015 16:15
Written by xinyi
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In the studies on phonons in CaWO4, Kanamori et al. investigated the phonon dispersion of translational modes alone, ignoring the finite moment of inertia of tungstate ions and Coulomb interactions. In our present investigations, based on external mode formalism (Venkataraman and Sahni), we consider the coupling of the translational and librational modes and include the long range interactions. The effective ionic charges of the Coulomb interactions and the effective ionic radii of Born–Mayer short range potential arc determined so that the dynamical equilibrium conditions are satisfied and lattice energy is of the right order in comparison with other complex ionic crystals (Rao et al.). The phonon dispersion relations in CaWO4 along [001] and [110] directions arc calculated and found to be in reasonable agreement with available neutron data (Steinman et al.). The generalized LST relations (Venkataraman et al.) computed with our model agree with the IR data of Barker. In the long wavelength limit, the acoustic branches fit well with experimental data, indicating that this model can explain the acoustic properties as well. Zone centre phonons in CaMoO4, SrWO4, SrMoO4, BaWO4, BaMoO4, PbWO4, and PbMoO4 are also investigated and compared with experimental data to study the influence of polarizability and a possible breakdown of external mode formalism.


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Luminescence Properties of Tm3+/Yb3+, Er3+/Yb3+ and Ho3+/Yb3+ Activated Calcium Tungstate

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Category: Tungsten Information
Published on Thursday, 09 July 2015 15:49
Written by xinyi
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CaWO4 phosphor activated by the Tm3+/Yb3+,Er3+/Yb3+ and Ho3+/Yb3+ ions were synthesized by a traditional high-temperature solid-state method.The crystal structures and morphologies of the products were characterized by X-ray powders diffraction method (XRD),infrared spectra (FT-IR) and scanning electron microscopy (SEM).The samples were found to show up-conversion luminescence properties.CaWO4 doped with Tm3+/Yb3+ showed blue luminescence characteristic of Tm(Ⅲ) ion in the range of 460-485 nm,corresponding to the 1G4→3H6 electronic transition.CaWO4 doped with Er3+/Yb3+ showed strong green luminescence at 510-565 nm (2H11/2,4S3/2→4I15/2) and weak red luminescence at 640-685 nm (4F9/2→4I15/2) of Er(Ⅲ) ion.CaWO4 doped with Ho3+/Yb3+ phosphor emitted green luminescence at 525-560 nm (5S2,5F4→5I8) and red luminescence at 630-670 um (5F5→5I8) and at 730-770 nm (5S2,5F4→5I7),which is the characteristic of Ho(Ⅲ) ion.

 

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Ammonium Paratungstate SEM Micrograph

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Category: Tungsten Information
Published on Thursday, 09 July 2015 14:44
Written by Cristina
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Ammonium paratungstate (APT) is crystalline material with white powder. The production method of obtaining pure paratungstate crystal is by evaporation of purified ammonium tungstate solution. Factors influencing the quality and physical properties of APT are controlling time, temperature, concentration and pH. The Chinese national standard of manufacturing this product is GB/T 10116-2007, which is a revision of GB/T 10116-1988. Its CAS no is 11120-25-5; 14311-52-5, EINECS 234-364-9, formula of H8N2O4W and molecular weight of 283.9145. It is the raw material of manufacturing ammonium metatungstate, and also widely used for producing tungsten metal products.

The SEM micrograph of ammonium paratungstate is multiplying the product under scanning electron microscope, which can be 10 to 100,000 multiplied. The scanning micrograph of APT is really different from AMT. The shape showing from SEM of APT is with cubic shape, but round for AMT.




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Identification of Three Trapping Centers in Calcium Tungstate

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Category: Tungsten Information
Published on Thursday, 09 July 2015 14:54
Written by xinyi
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Partial thermal annealing of nominally pure calcium tungstate, after gamma irradiation at 78 °K, provided evidence for identifying three major 4900 Å thermoluminescence peaks at 0.36, 0.55, and 0.72 eV with three groups of electron spin resonance lines of paramagnetic tungsten at g001 = 2.009 and 1.843 and niobium at g001 = 2.021, respectively. The impurity center identified with the paramagnetic Nb+4 ion is assigned to a tungsten site and a NbOmath imagecomplex. Arguments are presented for the assignment of two lattice defect models, a WOmath image complex with nearby WO3 and NbOmath image complexes, and a WOmath image complex with a nearby calcium deficiency, to the paramagnetic (W+5)A and (W+5)B ions, respectively.

Teilweise Temperung von gamma-bestrahlten (78 °K), sogenannten reinen CaWO4-Einkristallen ermöglichte die Identifizierung von drei ausgeprägten 4900 Å Thermolumineszenzmaxima bei 0,36, 0,55 und 0,72 eV mit drei Gruppen von ESR-Spektren, die mit paramagnetischem Wolfram mit g001 = 2,009 und 1,843 bzw. Niobium mit g001 = 2,021 verbunden sind. Die Fremdstörstelle wird als Nb+4 Ion auf Wolfram-Platz identifiziert und einem NbOmath image-Komplex zugeordnet. Argumente für die Zuordnung der paramagnetischen (W+5)A- bzw. (W+5)B-Ionen zu zwei Eigenstörstellenmodellen, einem WOmath image-Komplex mit benachbarten WO3- und NbOmath image-Komplexen und einem WOmath image-Komplex mit einer benachbarten Kalzium-Fehlstelle, werden angeführt.

 

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Ammonium Metatungstate SEM Micrograph

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Category: Tungsten Information
Published on Thursday, 09 July 2015 14:42
Written by Cristina
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Ammonium metatungstate is also named AMT, which is a kind of white crystal powder. AMT is the raw material for making tungsten product, and also widely used in alloyed steel, fireproof industries. Further more, it is a good tungsten based flux in oil chemical and ceramic industries. The manufacturing standard of AMT in China is GB/T 23368, which is also used for measuring the content of ammonium metatungstate. Based on different manufacturing degree, the products manufactured out by factories have performed different chemical contents. Standard used manufacturing ammonium metatunstate, AMT is GB/T 23368. The content of WO3 is 90.00% in most Chinese manufacturers, but 91.4~91.5% for Chinatungsten Online. For the different crystalized conditions, AMT can be analyzed as (NH4)6H2W12OnH2O, (NH4)6H2W12O40.XH2O and (NH4)6H2W12O40.

Scanning Electron Microscope (SEM) is a kind of scanning equipment using for micrograph by multiplying objects, which can be metal, powder, vegetables etc. Ammonium metatungstate is one of can-be scanned products. After scanning, the powder of AMT is showing ball shape with inner empty. All the balls are gathered together when the multiply value is low, but with gap when the value large enough, which means that the molecule of AMT are separate actually.




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