Scheelite Research Articles

Production of tungsten boride from CaWO 4 by self-propagating high-temperature synthesis followed by HCl leaching

In this study, the conditions of producing tungsten boride powder from calcium tungstate (CaWO 4) by self-propagating high-temperature synthesis (SHS) followed by HCl leaching techniques were investigated. In the first stage of the experimental study, the SHS products consisting of borides and other compounds were obtained starting with different initial molar ratios of CaWO 4, Mg and B 2O 3. It was found that increasing B 2O 3 content in the initial mixture resulted in an increase of W 2B 5/WB ratio along with Mg 3B 2O 6 in the SHS product. In the second step, Mg and Ca containing byproducts (i.e. MgO, Mg 3B 2O 6 and Ca 3B 2O 6) found in the SHS product were leached out by using aqueous HCl solution to obtain a clean tungsten boride product. The effects of particle size, temperature, time, acid concentration and solid/liquid ratio were selected as leaching parameters. The acid leaching experiments of the SHS product synthesized at 1:8:2.5 initial molar ratio of CaWO 4:Mg:B 2O 3 showed that optimum leaching conditions could be achieved by using 2.85 M HCl at 1/10 S/L ratio and the temperature of 80 °C for 60 min.

Experimental Measurement and Calculation of Mole Heat Capacity and Thermodynamic Functions of Wulfenite PbMoO4

The heat capacity of natural lead molybdate (wulfenite, PbMoO4) has been measured by the method of vacuum adiabatic calorimetry over the temperature range of (55 to 320) K, and its thermodynamic functions have been calculated. The obtained standard values are as follows: Cp°(298.15) = (119.41 ± 0.13) J·mol−1·K−1, S°(298.15) = (168.33 ± 2.06) J·mol−1·K−1, ΔH° = (23 095 ± 50) J·mol−1. For extrapolation of heat capacity values to absolute zero, the equation C(T) = (AT−1/αβ + 1)−1/α has been used, where parameter α is the power at which the heat capacity verges toward the Dulong and Petit limiting value, and parameter β reflects structural anisotropy of the compound and can possess values from 1 to 3. The values of parameters found by means of linearization were α = 0.61 and β = 1.9. An error for the extrapolation data to absolute zero has been estimated based on the results of heat capacity measurements of the isostructural compound calcium tungstate, CaWO4. The values of the wulfenite heat capacity over the...

Piezooptical coefficients of La 3Ga 5SiO 14 and CaWO 4 crystals: A combined optical interferometry and polarization-optical study

Paper reports the piezooptical studies on langasit La 3Ga 5SiO 14 (LGS) and calcium tungstate CaWO 4 (CWO) crystals by means of the combined optical interferometry and polarization-optical techniques. Corresponding methodology is supplied by a set of derived equations suitable for the determination of the absolute piezooptical coefficients (POCs) in the crystals of trigonal or tetragonal symmetry. It is shown, that for the determination of all the eight absolute POCs π іm of trigonal LGS or tetragonal CWO crystals by means of polarization-optical technique one must perform additionally at least two optical interferometry measurements in certain experimental geometries. In most cases the combined study considerably improves an accuracy in the determination of the absolute POCs. Due to a large piezooptical effect revealed in CWO crystals they may be considered as quite perspective materials for eventual photoelastic or acoustooptical applications.

The influence on intrinsic light emission of calcium tungstate and molybdate powders by multivalence Pr codoping

For trivalent praseodymium (Pr3+) and quadrivalent praseodymium (Pr4+) codoped CaMO4 (M = W, Mo) powders, the luminescence propriety of matrix is obviously influenced by carrier concentration. The light emission intensity of CaWO4 matrix decreases exponentially with increasing of Pr concentration because oxygen-deficient ( $\mathrm{WO}_{3}\cdot V_{\mathrm{O}}^{\bullet \bullet}$ ) obtains an electron supplied by Pr3+ (5d). However, the light emission intensity of CaMoO4 is enhanced by Pr codoping because the quasi-free electrons increase the probability of radiative combination. The difference of photoluminescence properties in the two materials are attributed to the bonding character of M and O in the CaMO4 structure.

Preparation of CaWO 4:Ln 3+@SiO 2 (Ln=Tb, Dy and Ho) nanoparticles by a combustion reaction and their optical properties

The CaWO 4:Ln 3+@SiO 2 (Ln=Tb, Dy and Ho) nanoparticles were synthesized via a combustion process at 800 °C, using citric acid as chelating agent and fuel, ammonium nitrate as fuel, boric acid as flux material and silica as supports. The persistent phosphor nanoparticles were characterized by X-ray diffraction (XRD), reflectance UV–vis and fluorescence spectroscopy (PL) and transmission electron microscopy (TEM) techniques. XRD patterns indicated that crystalline calcium tungstate with scheelite structure was produced. The reflectance UV–vis spectra showed the broad absorption band of WO 4 2 − groups and the PL spectra showed the WO 4 2 − wide excitation band, broad emission band of WO 4 2 − and characteristic emissions of Ln 3+ ions. The average particle sizes were determined by TEM, which are about 50 nm.

Preparation of Tungstate Nanopowders by Sol-Gel Method

Tungstate crystals are considered high-class scintillators to be used as detector material for high-energy radiation. These crystals are frequently used as references in measurements on new luminescent materials. In this paper, we present manufacturing process of nanocrystalline tungstate materials. Lead tungstate (PbWO4), cadmium tungstate (CdWO4), calcium tungstate (CaWO4), and zinc tungstate (ZnWO4) were synthesized via modifying sol-gel process. An advantage of the developed technology is the possibility of manufacturing tungstate powders in nanocrystalline state not only at elevated temperatures but at room temperatures as well. X-ray diffraction method, scanning electron microscopy, and thermal investigations were done to establish structural and morphological state of these materials. For reaching the most dispersible with the lowest agglomeration single-phase system of tungstate materials, there was an established optimal relationship between the system of starting reagents and solvents.

Synthesis, characterisation and photoluminescence of nanocrystalline calcium tungstate

Calcium tungstate nanocrystalline was successfully synthesised via a microwave irradiation method using Ca(NO3)2 and Na2WO4 in ethylene glycol, without the requirement of any calcination. The product was analysed by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and photoluminescence spectrometry. XRD, SAED and TEM revealed that the product was body-centred tetragonal CaWO4 with an average particle size of 12 nm. Different oscillation modes were detected by Raman spectroscopy and FTIR. They showed the strong W–O stretching in [WO4]2− tetrahedrons at 711–933 cm−1. Photoluminescence (PL) of CaWO4 nanocrystalline showed the maximum emission peak at 422 nm.

Electrochemical Reduction of Tungsten Compounds to Produce Tungsten Powder

The production of tungsten by direct current reduction has been investigated. Experimental studies involved the electrochemical reduction of the solid tungsten compounds tungsten trioxide (WO3) and calcium tungstate (CaWO4) in the form of an assembled cathode of porous pellets attached to a current collector. Molten calcium chloride and a molten solution of calcium chloride and sodium chloride at eutectic composition, 48 pct mol NaCl, were used as the electrolytes. Reduced samples were characterized by means of X-ray diffraction analyses and scanning electron microscopy. The results of X-ray analyses, supported with thermodynamic computations, showed that WO3 cannot be used without loss in processes that involve the use of CaCl2 at high temperatures because it reacts with CaCl2 by releasing volatile tungsten oxychloride. In the electrochemical reduction of CaWO4, X-ray diffraction results indicated the presence of tungsten with significant concentrations of calcium compounds. Metallic tungsten was obtained after treating the reduced samples with dilute hydrochloric acid solutions.

Low Dimensional Effects on Luminescent Properties of CaWO4 : Tb Nanophosphor

Terbium-activated calcium tungstate nanophosphors were synthesized by a hydrothermal method. The particles were proved to be spherelike, about 50 nm in diameter, which behaved in a conspicuously unique nature compared with bulk . In virtue of the quantum size effect, the absorption curve of the nanocrystalline shifted 28 nm to the short wavelength. A higher quenching concentration and a longer decay time of the nanoscaled sample than those of the bulk were observed upon the irradiation of UV and vacuum UV light, and the phenomena could be induced by the limitation of resonant energy transfer. This observation could remedy the deficiency of nanophosphors in emission intensity and show that nanoscaled phosphor has important potential applications.

XANES and XEOL studies of Eu-doped calcium tungstate in silica synthesized by sol-gel method

Europium-doped calcium tungstate (CaWO4:Eu) was synthesized by a modified Pechini method. X-ray absorption near-edge structure (XANES) and X-ray excited optical luminescence (XEOL) was employed to probe the chemical and luminescence properties of CaWO4:Eu. XANES results demonstrate that the chemical environment around calcium in the doped CaWO4 is similar to that of undoped CaWO4. On the other hand, chemical environment around oxygen is very different. Moreover, the results suggest that contributions to luminescence in the sample are from both silica (from silica matrix) and CaWO4. Using XEOL, luminescence pathways and efficiency associated with the system can be identified.

Control of emission peaks of X-ray phosphor using dye-silica nanoparticles

Simple method yielding new emission peaks to the X-ray phosphor and its mechanism were investigated using concentrated-dye-molecule-doped silica nanoparticles (dye-silica nanoparticles) and X-ray phosphors. The dye-silica nanoparticles were coated on the sheet of the X-ray phosphor using 20 wt% polyvinyl alcohol solution. The fluorescence of the dye-silica nanoparticle coated onto X-ray phosphor was successfully observed by X-ray irradiation. The fluorescent cascade from the X-ray-irradiated phosphor could be used in the excitation of the dye-silica nanoparticles by coating on the calcium tungstate (X-ray phosphor). The observed new fluorescence was based on the extent of the overlapping of wavelengths between the emission of the X-ray phosphor and the excitation of the dye-silica nanoparticles. The fluorescent peak of the calcium tungstate as the X-ray phosphor shifted from 434 to 425 nm. The dye-silica particle–calcium tungstate hybrid material showed new emission peaks from 543 to 601 nm due to the addition of fluorescein, rhodamine red-X, or cy5. The new emission peaks changed by the content of dye and the size of the silica nanoparticles. The fluorescence intensity ratio of the new fluorescence peaks at 543 nm against that of the X-ray phosphor depended on the fluorescent cascade from the X-ray-irradiated X-ray phosphor. It can closely match the overall emission of the phosphorous intensity screens to the sensitivity of the film allowing lower dose X-rays to obtain the same image clarity.

Scheelite from the Yaguki mine and its mineralization

Scheelite from the Yaguki mine and its mineralization

Radiopacity of Portland Cement Associated With Different Radiopacifying Agents

This study evaluated the radiopacity of Portland cement associated with the following radiopacifying agents: bismuth oxide, zinc oxide, lead oxide, bismuth subnitrate, bismuth carbonate, barium sulfate, iodoform, calcium tungstate, and zirconium oxide. A ratio of 20% radiopacifier and 80% white Portland cement by weight was used for analysis. Pure Portland cement and dentin served as controls. Cement/radiopacifier and dentin disc-shaped specimens were fabricated, and radiopacity testing was performed according to the ISO 6876/2001 standard for dental root sealing materials. Using Insight occlusal films, the specimens were radiographed near to a graduated aluminum stepwedge varying from 2 to 16 mm in thickness. The radiographs were digitized and radiopacity compared with the aluminum stepwedge using Digora software (Orion Corporation Soredex, Helsinki, Finland). The radiographic density data were converted into mmAl and analyzed statistically by analysis of variance and Tukey-Kramer test (α = 0.05). The radiopacity of pure Portland cement was significantly lower (p < 0.05) than that of dentin, whereas all cement/radiopacifier mixtures were significantly more radiopaque than dentin and Portland cement alone (p < 0.05). Portland cement/bismuth oxide and Portland cement/lead oxide presented the highest radiopacity values and differed significantly from the other materials (p < 0.05), whereas Portland cement/zinc oxide presented the lowest radiopacity values of all mixtures (p < 0.05). All tested substances presented higher radiopacity than that of dentin and may potentially be added to the Portland cement as radiopacifying agents. However, the possible interference of the radiopacifiers with the setting chemistry, biocompatibility, and physical properties of the Portland cement should be further investigated before any clinical recommendation can be done.

Controlled Synthesis of Calcium Tungstate Hollow Microspheres via Ostwald Ripening and Their Photoluminescence Property

Nearly monodispersed CaWO4 hollow microspheres with controlled diameters were successfully prepared by a facile one-step hydrothermal method using CaCl2, Na2WO4, and sodium dodecyl sulfate (SDS) as the starting reaction reagents and distilled water as solvent. The crystal structure, morphology, macroporous and photoluminescence properties of the as-synthesized products are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected-area electron diffraction, nitrogen adsorption−desorption isotherm technique, and fluorescence spectrometer. The hollow microspheres have an average diameter of 2.2 μm and hollow interior of 0.5−1 μm. The shell is consisting of numerous nanoparticles 45 nm in diameter. The effects of SDS amount, pH value, concentrations of reaction reagents, and reaction durations have been studied systematically. The results indicate that a certain SDS amount plays a key important role in the final formation of hollow microspheres. It is also fo...

Carbothermic reduction of calcium tungstate in the presence of various oxides

The possibility of reduction of calcium tungstate in an inert medium (in vacuum and argon) was investigated. As components involving WO3 and B2O3, natural materials—namely, scheelite and datolite concentrates—were used. The main products of reduction are W2B5, WC, W5Si3, and SiC. The apparent activation energy of carbon diffusion in the carbide phase was evaluated based on the data of high-temperature X-ray powder diffraction and chemical analysis for certain tungstate systems. It was shown that reduction of scheelite concentrate could be carried out during electroslag remelting (ESR).

Calcium tungstate coprecipitation for removal of Sr interference with determination of Rb by ID-ICP-MS

A coprecipitation method using calcium tungstate was developed to remove 87Sr isobaric interference with 87Rb prior to measurement of Rb by ID-ICP-MS. Precipitation of calcium tungstate was obtained by adding Ca(NO 3) 2 solution and (NH 4) 2WO 4 solution to the sample, where (NH 4) 2WO 4 was added more than the stoichiometric proportion to precipitate Ca(NO 3) 2 completely and remove Sr effectively. Furthermore, in order to reduce matrix burden to the ICP-MS instrument, the residual (NH 4) 2WO 4 was removed by adding conc. HNO 3. Prior to the application, thorough purification of coprecipitant reagent was carried out to reduce the blank. The effectiveness of the present method was verified by analyzing two brown rice flour certified reference materials (CRMs), NIES CRM 10a and NIES CRM 10b. Finally, the present method was applied to the measurement of Rb in a white rice flour RM sample being developed by National Institute of Metrology of Japan.

Microhardness studies of nanocrystalline calcium tungstate

AbstractNanocrystals of calcium tungstate (CaWO4) of three different grain sizes were synthesized through chemical precipitation technique and the grain sizes and crystal structure were determined using the broadening of X‐ray diffraction patterns and transmission electron microscopy. The microhardness of compacted pellets of nanocrystalline calcium tungstate (CaWO4) with different grain sizes were measured using a Vickers microhardness tester for various applied loads ranging from 0.049 N to 1.96 N. The values of microhardness showed significant reverse indentation size effect at low indentation loads. The microhardness data obtained for samples of different grain sizes showed grain size dependent strengthening obeying normal Hall‐Petch relation. The dependence of compacting pressure and annealing temperature on microhardness of the nanostructured sample with grain size of 13 nm were also studied. The samples showed significant increase in microhardness as the compacting pressure and annealing time were increased. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Inferior Alveolar Nerve Damage Because of Overextended Endodontic Material: A Problem of Sealer Cement Biocompatibility?

Damage to the inferior alveolar nerve is a relatively infrequent complication in dental practice. When root canal treatment of a lower molar or premolar surpasses and/or overextends beyond the apical foramen and invades the periapical zone, the foreign material introduced within such a sensitive anatomical space may mechanically or even chemically affect the inferior alveolar nerve. We describe a case of endodontic treatment of a permanent right lower first molar in which the sealer cement overextended in large amounts and damaged the right inferior alveolar nerve. The condition reverted a few months after the surgical removal of the material. Evaluation of the removed material, using powder x-ray diffraction and scanning electron microscopy with coupled dispersive energy spectroscopy, showed it to consist of calcium tungstate (scheelite [CaWO4]) and zirconium oxide (baddeleyite [ZrO2]), which were chemical components of the sealer cement.

Synthesis of CaWO 4:Eu 3+ phosphor powders via a combustion process and its optical properties

Using citric acid as chelating agent and fuel, ammonium nitrate as fuel, boric acid as flux material and silica as supports, persistent phosphor CaWO 4:Eu 3+ has been prepared via a combustion process, which formed voluminous white powder. The persistent phosphor was further investigated by X-ray diffractometer (XRD), fluorescence spectrophotometer (PL) and transmission electron microscopy (TEM). XRD analyses indicate that the crystalline calcium tungstate has been synthesized when ignited at 500 °C. The PL results showed the highest intensity of emission spectrum obtained at 800 °C, the optimal contents is 0.7CaWO 4:0.3Eu 3+ incorporated into silica which could enhance luminescence intensity of CaWO 4:Eu 3+. The average particle sizes as measured by TEM are ranged in 50 to 100 nm, which could extend the application of this phosphor.

Mechanisms behind blue, green, and red photoluminescence emissions in CaWO4 and CaMoO4 powders

A combined experimental and theoretical study was conducted to analyze the photoluminescence (PL) properties of ordered and disordered CaWO4 (CW) and CaMoO4 (CM) powders. Two mechanisms were found to be responsible for photoluminescence emission in CW and CM powders. The first one, in the disordered powders, was caused by oxygen complex vacancies [MO3∙VOx], [MO3∙VO•] and [MO3∙VO••], where M=W or Mo, which leads to additional levels in the band gap. The second mechanism, in ordered powders, was caused by an intrinsic slight distortion of the [WO4] or [MoO4] tetrahedral in the short range.