Scheelite Research Articles

Investigation of the effect of the water to powder ratio on hydraulic cement properties

ObjectiveThe use of rheological properties to determine the optimal water: powder ratio of tricalcium silicate-based prototype materials incorporating alternative radiopacifiers and fillers. Determination of how the proportion of water incorporated affected the physicochemical behaviour of the materials. MethodsEndodontic cements replaced with 30% radiopacifier, and additions of calcium phosphate and micro-silica were tested. The unmodified cements were mixed with a 0.35 water: powder ratio which served as control. At this water: powder ratio, unmodified Portland cement without any addition had an adequate consistency and furthermore it has been well characterized. Assessment of material rheological properties enabled adjustment of the water: powder ratio in each material to provide comparable viscosity values to those of the pure cement. The flowability, phase analysis and calcium release were measured for both viscosity-matched and the standard 0.35 water: powder ratio blends. The prototype materials with the adjusted water: powder ratios were also characterized by scanning electron microscopy, energy-dispersive spectroscopy and evaluated for radio-opacity. ResultsThe use of the 0.35 water: powder ratio is not appropriate when changing the radiopacifier and incorporating additives. Zirconium oxide did not vary the water: powder ratio but tantalum oxide and calcium tungstate resulted in an increase and decrease in water demand respectively. Using the standard 0.35 ratio when the mixture had a low water demand resulted in higher flowability values and calcium release in solution. Micro-silica and calcium phosphate altered the hydration of the materials. All materials were adequately radiopaque. SignificanceRheological assessment is an easy reproducible way to determine the water: powder ratios of materials with varying amounts of additives and radiopacifiers during development. Modifications to the water: powder ratio affects material properties.

Comprehensive Study on the Combined Effect of Laser-Induced Heating and Laser Power Dependence on Luminescence Ratiometric Thermometry.

Luminescence ratiometric thermometry, on the basis of nonthermally linked states of lanthanides, became a hot research issue recently because of its several attractive features. Here, the 5F4,5S2/5F5–5I8 transitions of Ho3+ embedded in calcium tungstate host are taken as an example to show the influence of laser pump power on this temperature detection technology. The luminescence intensity ratio between the 5F4,5S2/5F5–5I8 upconversion emission lines was found to respond monotonously to the temperature between 303 and 603 K and could be fitted well with the use of an empirical function. It suggested that this ratio might be suitable for temperature measurement. However, at 303 K, the temperature readout derived from this ratio decreased from 303 to 248 K on increasing the laser pump power from 35 to 205 mW (the irradiated spot’s area: ca. 2 mm2). This uncommon phenomenon differs from the conventional laser-induced heating effect. With the help of the Boltzmann distribution based on the two Stark components of the 5F5 state of Ho3+, the laser-induced heating was calculated to be ca. 20 K when the excitation power was 205 mW. Thus, this suggested that there should be a mechanism responsible for the gradually decreasing temperature readout. It was then confirmed that this mechanism was the different dependences for the 5F4,5S2/5F5–5I8 transitions on laser pump power, which was much stronger than the laser-induced heating effect. A calibration method to eliminate the influence of laser power dependence on luminescence ratiometric thermometry was then proposed.

Antimicrobial and anti-inflammatory drug-delivery systems at endodontic reparative material: Synthesis and characterization

ObjectiveThe aim of this study was to synthesize and characterize an experimental endodontic paste. MethodsAn experimental endodontic paste (EX) was characterized by its particle size, zeta potential, drug content and morphology. The powder of EX is composed of amoxicillin microspheres, calcium tungstate and α-tricalcium phosphate, mixed with an indomethacin nanocapsules suspension. Ultracal® (Ultradent), an iodoform-based paste (GP) and the EX were evaluated by its physical properties (flow, film thickness and radiopacity). The cytocompatibility was performed by MTT and SRB-colorimetric assays; the cell-migration was tested with scratch assay and cell-ability to remineralization with ALP and Alizarin Red S, with fibroblastic cell line. The antibacterial activity was assessed by the formation of inhibition zones and against planktonic bacteria. ResultsThe EX and UL flow achieved ISO6876 standard, and GP was lower than 17mm. All pastes achieved the film thickness required. Radiopacity was equivalent to 1.81±0.25mmAl for EX, which did not differ from GP group 1.39±0.33mmAl (p>0.05). The UL presented 3.04±0.33mmAl. The values for SRB showed better citocompatibility in comparison with MTT for all materials. The ALP activity and formation of mineralized nodules demonstrated the remineralization potential for UL and EX. Cell migration showed continuous wound closure until complete cell healing, however, the EX accelerated the process (p<0.05). The EX showed the greatest inhibition zone (p<0.05) and was the only group with antibacterial activity against planktonic bacteria. SignificanceThe synthesized endodontic paste demonstrated reliable physical and biological properties and could be a promising material for periapical tissue repair.

Evaluation of color stability of different repairing materials

Evaluate the potential for dental color alteration of high-plasticity cements, MTA HP and MTA Flow, in comparison with MTA Angelus with calcium tungstate as radiopacifier; and alternative the formulations proposed, MTA Flow + 5% zinc oxide (ZnO) and an Experimental cement (powder of tricalcium silicate, bismuth oxide, ZnO and liquid of distilled water and water-soluble polymer). Thirty-five bovine teeth were prepared and filled with the cements (n=5). Triple antibiotic paste and unfilled samples were the positive and negative controls, respectively. The specimens were immersed in water and the color assessed with a spectrophotometer: before filling (B), 24 hours (24h), 28 days (28d) and 90 days after filling. The color change (ΔE) and the luminosity (*L) were calculated. The statistical analysis was performed using nonparametric Kruskal-Wallis and Dunn test (p &lt; 0.05). The analysis of delta E values showed that all the materials presented color alteration after the evaluated periods. High *L values were found for MTA HP and Experimental, with statistical difference in relation to MTA Flow at 90d (p &gt; 0.05). The addition of ZnO inhibited dental staining of MTA Flow after 90d of contact with dentine. The MTA Flow presented potential for dental discoloration, which is inhibited by the addition of zinc oxide. The experimental cement presented similar luminosity values than that presented by MTA and MTA HP.

Limits on dark matter effective field theory parameters with CRESST-II

CRESST is a direct dark matter search experiment, aiming for an observation of nuclear recoils induced by the interaction of dark matter particles with cryogenic scintillating calcium tungstate crystals. Instead of confining ourselves to standard spin-independent and spin-dependent searches, we re-analyze data from CRESST-II using a more general effective field theory (EFT) framework. On many of the EFT coupling constants, improved exclusion limits in the low-mass region (< 3–4 GeV/c^2) are presented.

Coherence times of Ce3+ spin states in CaWO4 crystal

We study the coherence times and perform manipulations on the lowest-energy states of trivalent cerium ion in calcium tungstate crystal. We find the phase memory time reaching 14.2 ${\mu}$s and the time of coherent manipulations reaching 0.3 ${\mu}$s in the low-temperature limit, the latter can potentially be elongated by using the rotation angle and off-resonance error correction schemes.

Sequential extraction of tungsten from scheelite through roasting and alkaline leaching

Extraction of tungsten from scheelite is important for the processing of tungsten resources for tungsten metallurgical companies. However, there are many difficulties to leach scheelite compared to wolframite. Compared with calcium tungstate (CaWO4), magnesium tungstate (MgWO4) is a readily soluble substance in NaOH solution. In this paper, conversion of CaWO4 to MgWO4 through roasting a mixture of scheelite and MgCl2 was investigated systematically. The leaching performance of the roasted products in NaOH solution was also studied. Thermodynamic analyses and experimental results showed that MgWO4 could be prepared through reaction of CaWO4 with MgCl2. Increasing roasting temperature and MgCl2 dosage could increase the extent of conversion of CaWO4 to MgWO4. Additionally, leaching experiments showed a good leachability of the roasted products with a tungsten leaching efficiency of 98.79% in 4 M NaOH solution at 90 °C. This work may provide an alternative approach for the extraction of tungsten from scheelite.

Radiation protective characteristics of some selected tungstates

Abstract The mass attenuation coefficients (μ/ρ) of calcium tungstate, ammonium tungsten oxide, bismuth tungsten oxide, lithium tungstate, cadmium tungstate, magnesium tungstate, strontium tungsten oxide and sodium dodecatungstophosphate hydrate were measured at 14 photon energies in the energy range of 81–1333 keV using 22Na, 54Mn, 57Co, 60Co, 133Ba and 137Cs radioactive sources. The measured μ/ρ values were compared with those obtained from WinXCOM program and the differences between the experimental and theoretical values were very small. The bismuth tungsten oxide has the highest μ/ρ among the present samples in the studied energy region. From the μ/ρ values, we calculated the half value layer, tenth value layer and mean free path, and the results showed that ammonium tungsten oxide (which has the lowest density) and bismuth tungsten oxide (which has the highest density) possess the highest and lowest values of these three parameters, respectively. Additionally, from the incident and transmitted photon intensities, we calculated the radiation protection efficiency (RPE). The bismuth tungsten oxide was found to have RPE 98.53 % at 81 keV, which has the maximum value among the present samples and this suggested that bismuth tungsten oxide is the best to be chosen as the γ radiation shielding material candidate among the selected samples.

The improvement of rhenium recovery technology from W-Re alloys

The lack of rhenium raw materials sources on the territory of Russia leads to the necessity of this valuable metal recovery from wastes and secondary raw materials, in particular, from tungsten-rhenium alloys. The using of hydrometallurgical and electrochemical technologies is the most promising way to dissolve and recovery these metals in the form of prospective precursors. It is considered, that the fundamentals of an improved technology of rhenium recovery in the form of perrhenic acid from binary W-Re alloys. It is shown the desirability of avoiding the precipitation of metal in the form of potassium perrhenate. A basic technological scheme of the Re recovery process is proposed. It includes anodic dissolution of W-Re alloys, precipitation of calcium tungstate from the electrolyte and electrodialysis of rhenium in the form of perrhenic acid.

Boosting the Temperature Detection Accuracy of Luminescent Ratiometric Thermometry via a Multifunction Fit Strategy

The influence of fit strategy on the temperature detection accuracy of luminescence ratiometric thermometry is investigated here. In the range of 303–783 K, the typical 2H11/2/4S3/2–4I15/2 upconversion emission lines of Er3+ embedded in calcium tungstate are investigated as a function of temperature. It is found that using the conventional one-function fit method over the whole temperature range leads to a mean error of 3.47 K. By contrast, the strategy, namely, dividing the whole board temperature range into several narrow ones and using several fit functions over separate temperature scopes, is proposed and studied. It is demonstrated that as the number of the divided temperature scopes is increased, the measured temperature readout gradually approaches to the true value of the temperature. When the number is set to be six, the temperature error sharply goes down to 0.41 K, suggesting that the newly proposed multi-function fit strategy is superior to the traditional one-function fit method, especially when a board temperature scope is involved. The reason responsible for this phenomenon is also discussed.

Strategies toward catalytic biopolymers: Incorporation of tungsten in alginate aerogels

We report the synthesis of W-doped biopolymer aerogels with potential applications to catalysis. The biopolymers were based on alginate, a cost-efficient natural material, and were prepared in environmentally friendly water/ethanol solutions. Gelation of alginate was induced by crosslinking with Ca2+. The resulting wet-gels were impregnated with an ethanolic solution of the ditungsten complex [W2(μ-OEt)2(OEt)2(EtOH)2Cl4] ({W = W}8+) and were dried with supercritical fluid (SCF) CO2 to yield W-doped alginate aerogels containing 10% w/w tungsten. Dry materials were characterized with FTIR, SEM/EDS, TGA, N2 porosimetry and He pycnometry. The bulk densities were low (<0.1 g cm−3), porosities were high (96% v/v), as well as the BET surface areas (380 m2/g). Pyrolysis of those aerogels at 800 °C under Ar yielded carbon along with a mixture of calcium tungstates, i.e., CaWO4 and Ca3WO6 (weight ratio: 70:30), while pyrolysis under O2 yielded the same tungstates, but with a different weight ratio (10:90). This can be a new process for Ca3WO6, which, due to its ordered double perovskite structure, is ideal for doping with metal ions, at relatively low temperature (800 °C versus >1000 °C in the literature).

Effects of fabrication method on initial powder characteristics and liquid phase sintering behaviour of tungsten

Direct electrochemical production of tungsten involves removal of oxygen in solid state from an important tungsten compound, calcium tungstate, in molten salts. A detailed experimental study was performed to characterize the liquid phase sintering behaviour of the tungsten powder (WE), which was produced by the direct electrochemical reduction technique. All of the tungsten heavy alloy (WHA) compositions covered in this study, contained four parts nickel, one part cobalt and over 90 wt% tungsten. The pellets were sintered at 1500 and 1550 °C under continuous flow of an equimolar hydrogen-argon gas mixture. A commercially available tungsten powder (WC) was also used to prepare the same tungsten heavy alloy compositions as those prepared by the electrochemically produced tungsten powder to act as a control group. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF) and hardness measurements were employed for characterization. The analyses provided that; the tungsten powder as obtained from the direct electrochemical reduction process contained some unreduced CaWO4 particles and they limited the grain growth and mass transport in the liquid phase. Although calculated relative densities were a bit lower than desired, the tungsten heavy alloys produced by the electrochemical reduction technique was promising for use when average grain size, dissolved tungsten contents and general/matrix hardness values are considered.

Effect of Different Synthesis Methods on the Textural Properties of Calcium Tungstate (CaWO4) and Its Catalytic Properties in the Toluene Oxidation

Calcium tungstate (CaWO4) crystals were prepared by microwave-assisted hydrothermal (MAH) and polymeric precursor methods (PPM). These crystals were structurally characterized by X-ray diffraction (XRD), N2 adsorption, X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. The morphology and size of these crystals were observed by field emission scanning electron microscopy (FE-SEM). Their optical properties were investigated by ultraviolet visible (UV-Vis) absorption and photoluminescence (PL) measurements. Moreover, these materials were employed as catalysts towards gas phase toluene oxidation reaction. XRD indicates the purity of materials for both preparation methods and MAH process produced crystalline powders synthesized at lower temperatures and shorter processing time compared to the ones prepared by PPM. FE-SEM images showed particles with rounded morphology and particles in clusters dumbbells-like shaped. PL spectra exhibit a broad band covering the visible electromagnetic spectrum in the range of 360 to 750 nm. XANES and EXAFS results show that preparation method does not introduce high disorders into the structure, however the H2-TPR results indicated that the catalyst reducibility is affected by the preparation method of the samples.

Physicochemical properties of calcium silicate-based formulations MTA Repair HP and MTA Vitalcem.

ObjectiveThis study aimed to analyze the following physicochemical properties: radiopacity, final setting time, calcium release, pH change, solubility, water sorption, porosity, surface morphology, and apatite-forming ability of two calcium silicate-based materials.Material and methodsWe tested MTA Repair HP and MTA Vitalcem in comparison with conventional MTA, analyzing radiopacity and final setting time. Water absorption, interconnected pores and apparent porosity were measured after 24-h immersion in deionized water at 37°C. Calcium and pH were tested up to 28 d in deionized water. We analyzed data using two-way ANOVA with Student-Newman-Keuls tests (p<0.05). We performed morphological and chemical analyses of the material surfaces using ESEM/EDX after 28 d in HBSS.ResultsMTA Repair HP showed similar radiopacity to that of conventional MTA. All materials showed a marked alkalinizing activity within 3 h, which continued for 28 d. MTA Repair HP showed the highest calcium release at 28 d (p<0.05). MTA Vitalcem showed statistically higher water sorption and solubility values (p<0.05). All materials showed the ability to nucleate calcium phosphate on their surface after 28 d in HBSS.ConclusionsMTA Repair HP and MTA Vitalcem had extended alkalinizing activity and calcium release that favored calcium phosphate nucleation. The presence of the plasticizer in MTA HP might increase its solubility and porosity. The radiopacifier calcium tungstate can be used to replace bismuth oxide.

Effect of organic fuels on surface area and photocatalytic activity of scheelite CaWO4 nanoparticles

Discrete nanoscale calcium tungstate (CaWO4) nanoparticles with exquisite photocatalytic activities were synthesized through ultra-rapid solution combustion route. Here, we aim to study the effect of different fuels on the synthesis of CaWO4 nanoparticles which lead to improve the characteristic properties and morphological evolution of the powders. From BET surface area measurement, it is observed that CaWO4 nanoparticles synthesized by using citric acid as fuel exhibits relatively large surface area (31.78 m2 g−1) as compared to other fuels. The powder x-ray diffraction (PXRD) studies reveal that CaWO4 nanoparticles belong to scheelite type tetragonal system. The morphology of CaWO4 nanoparticles investigated using scanning electron microscopy (SEM) reveals that the powders are highly porous and agglomerated. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of the CaWO4 nanoparticles show that a well-dispersed nearly oval-shaped nanoparticles with variable dimensions and lattice spacing that depends on the type of fuels used in the synthesis. The selected area electron diffraction (SAED) patterns of CaWO4 nanoparticles exhibit several concentric rings with bright spots indicating the polycrystalline nature of the powders. Investigation on photocatalytic activity of CaWO4 nanoparticles synthesized using citric acid shows highest (∼93%) degradation of methylene blue (MB).

PEG-PLA-Coated and Uncoated Radio-Luminescent CaWO4 Micro- and Nanoparticles for Concomitant Radiation and UV-A/Radio-Enhancement Cancer Treatments.

Currently, there is great interest in the development of ways to achieve the benefits of radiation treatments with reduced negative effects. The present study demonstrates the utilization of radio-luminescent particles (RLPs) as a means to achieve radio-sensitization and enhancement and their ability to affect head- and neck-cancer-cell cultures (in vitro) and xenografts (in vivo). Our approach utilizes a naturally abundant radio-luminescent mineral, calcium tungstate (CaWO4), in its micro or nanoparticulate form for generating secondary UV-A light by γ ray or X-ray photons. In vitro tests demonstrate that unoptimized RLP materials (uncoated CaWO4 (CWO) microparticles (MPs) and PEG-PLA-coated CWO nanoparticles (NPs)) induce a significant enhancement of the tumor-suppressive effect of X-rays and γ rays in both radio-sensitive- and radio-resistant-cancer models; uncoated CWO MPs and PEG-PLA-coated CWO NPs demonstrate comparable radio-sensitization efficacies in vitro. Mechanistic studies reveal that concomitant CaWO4 causes increased mitotic death in radio-resistant cells treated with radiation, whereas CaWO4 sensitizes radio-sensitive cells to X-ray-induced apoptosis and necrosis. The radio-sensitization efficacy of intratumorally injected CaWO4 particles (uncoated CWO MPs and PEG-PLA-coated CWO NPs) is also evaluated in vivo in mouse head- and neck-cancer xenografts. Uncoated CWO MPs suppress tumor growth more effectively than PEG-PLA-coated CWO NPs. On the basis of theoretical considerations, an argument is proposed that uncoated CWO MPs release subtoxic levels of tungstate ions, which cause increased photoelectric-electron-emission effects. The effect of folic acid functionalization on the in vitro radio-sensitization behavior produced by PEG-PLA-coated CWO NPs is studied. Surface folic acid results in a significant improvement in the radio-sensitization efficiency of CaWO4.

Luminescence efficiency of calcium tungstate (CaWO4) under X-ray radiation: Comparison with Gd2O2S:Tb

The aim of the present study was to investigate the absolute luminescence efficiency (AE) of a CaWO4 screen, under X-ray irradiation and to compare it with a custom made PMMA/Gd2O2S:Tb composite film screen. The emitted light was evaluated by performing measurements of the AE under X-ray exposure conditions, with tube voltages ranging from 50 to 125 kV. The spectral compatibility of the CaWO4 screen, with various existing optical detectors, was investigated after emission spectra measurements. AE was found maximum at 50 kVp (2.34 Efficiency Units-E.U) which was slightly lower than the corresponding “gold standard” Gd2O2S:Tb (2.67 E.U), at the same X-ray energy. The emission spectrum of CaWO4 is excellent matched with the spectral sensitivities of photocathodes and silicon photomultipliers often employed in radiation detectors, and with good matching with amorphous silicon photodiodes. Considering the adequate luminescence efficiency values and the spectral compatibility with various photodetectors, CaWO4 could be also considered for use in X-ray imaging devices such as charged-coupled devices (CCD) and complementary metal oxide semiconductors (CMOS).

Correction to: Research of optical absorption and luminescence spectra of double-perovskite calcium tungstate co-doped with Yb3+/Ho3+

The original version of this article unfortunately contained a typo in the Fig. 4 caption. The unit “Er3+” should read as “Ho3+” in the caption of Fig. 4.

SURVEY OF IMAGING TECHNOLOGY AND PATIENT DOSE RECORDING PRACTICE IN DEVELOPING COUNTRIES

A survey was conducted through the International Atomic Energy Agency (IAEA) to assess the use of older imaging technologies and availability of dose information in radiography, computed tomography (CT), mammography and interventional machines. Responses from 55 countries indicated 55.8% having non-digital radiography systems, only 9.1% having all systems that are digital (CR and DR), nearly one-third (36.4%) still use calcium tungstate screens in radiography cassettes and only 16.9% having all mammography systems as digital. On the dose features, not all CT machines provide CTDI, DLP values directly as only about half (50.6%) stated all machines having CTDI and DLP display, nearly half (46.8%) indicated the absence of KAP meters in radiography systems. Only 53.2% stated all angiography machines having DAP/KAP display. Only about a quarter (23.4%) having picture archiving and communication system in all hospitals in the country. Nearly one-third (36.4%) have no difficulty in maintaining patient identification number while sending dose data.

Synthesis and characterization of rare-earth-doped calcium tungstate nanocrystals

In this paper, we report synthesis and characterization of rare-earth-ion-doped calcium tungstate (CaWO4) nanocrystals (NCs). Rare-earth ions, such as gadolinium (Gd), neodymium (Nd), praseodymium (Pr), samarium (Sm) and holmium (Ho), were successfully doped in the CaWO4 NCs by changing the synthesis conditions. The adopted synthesis route was found to be fast and eco-friendly. Structural characterizations, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and compositional analysis, were performed using energy dispersive analysis of X-rays (EDAX) on as-synthesized NCs. The results indicate the size of the NCs ranging between 47 to 68nm and incorporation of rare-earth ions in CaWO4 NCs.