Scheelite Research Articles

Precision multi-mode microwave spectroscopy of paramagnetic and rare-earth ion spin defects in single crystal calcium tungstate

We present experimental observations of dilute ion spin ensemble defects in a low-loss single crystal cylindrical sample of CaWO4 cooled to 30 mK in temperature. Crystal field perturbations were elucidated by constructing a dielectrically loaded microwave cavity resonator from the crystal. The resonator exhibited numerous whispering gallery modes with high Q-factors of up to 3×107, equivalent to a loss tangent of ∼3×10−8. The low loss allowed precision multi-mode spectroscopy of numerous high Q-factor photon–spin interactions. Measurements between 7 and 22 GHz revealed the presence of Gd3+, Fe3+, and another trace species, inferred to be rare-earth, at concentrations on the order of parts per billion. These findings motivate further exploration of prospective uses of this low-loss dielectric material for applications regarding precision and quantum metrology, as well as tests for beyond standard model physics.

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

Manipulation of Multimodal and Multicolor Luminescence via Interplay of Traps and Rare Earth Emission Centers in Calcium Tungstate.

Multimodal luminescence involves color-tunable and wavelength manageable photon emissions upon variable luminescence pathways in response to different external stimuli, which provides clear visualization and high-level confidentiality for information encryption technologies. Integrating multimodal luminescence into a single matrix is regarded as a feasible strategy but remains a big challenge. In this work, multimodal (photoluminescence, persistent luminescence, upconversion luminescence, and thermally stimulated luminescence) and multicolor luminescence (green, yellow, orange, pink to red) is achieved in CaWO4:Yb3+,Er3+,Eu3+ phosphor by employing an interplay of traps and rare earth emission centers. Bright emission in a wide color gamut is achieved dynamically in response to thermal disturbance and light illumination, which further allows for on-demand emission manipulation in space and time dimensions. The compatible coexistence of multiple rare earth emissive centers together with abundant photoactive traps contributes to the excellent integration of multimodal photon emissions in calcium tungstate. This work provides a good example of integrating multimodal luminescence into one single matrix and indicates potential in advanced high-level information encryption applications.

Construction of scheelite CaWO4 with natural diatomite for boosted photocatalytic degradation of tetracycline and doxycycline antibiotics under natural conditions

Fabrication of efficient and recyclable catalysts with natural supporting materials is still a challenge in the field of photocatalysis. In the present work, scheelite type calcium tungstate was successfully constructed with natural diatomite via in-situ co-precipitation method for effective photocatalytic degradation of tetracycline (TC) and doxycycline (DC) antibiotics. The as-synthesized photocatalysts were characterized by SEM-EDX, TEM, BET, XRD, XRF, XPS, FTIR, UV–vis DRS, PL and EIS techniques. The morphological and physiochemical analyses confirmed the successful hybridization of CaWO4 with natural diatomite. The optical, photoluminescence and electrochemical tests confirmed that the composite material displayed narrower band gap energy, lower recombination rate and facilitated separation of photogenerated carriers. After construction of CaWO4 with the diatomite structure, the TC and DC decomposition efficiencies were found 4.70 and 2.54 times higher than that of the pristine sample, respectively. Furthermore, the composite catalyst showed superior degradation performances (91.8 % for TC and 90.7 % for DC) in the presence of hydrogen peroxide as a Fenton-agent. The enhanced photocatalytic performance was assigned to the increased specific surface area and pore volume, decreased band gap energy, limited recombination rate of photoinduced carriers, decreased charge transfer resistance as well as the abundant of hydroxyl radicals. The degradation reaction was found to be mainly driven by photogenerated holes and superoxide radicals. The CWO@DT/Vis/Fenton catalytic system exhibited good performance at near-neutral pH, also the composite catalyst was sufficiently utilized in the presence of co-existing anions. Furthermore, the photocatalytic efficiency was also tested under natural sunlight irradiation and the TC degradation remained 71.4 % after four cycles, indicating the outstanding feature of the composite catalyst in outdoor process conditions. Under these circumstances, the present work demonstrated the utilization potential of natural diatomite for effective photocatalytic remediation of antibiotic molecules under natural sunlight irradiation.

Evaluation of photoluminescence and colorimetric performance of Tb3+-induced calcium tungstate ceramic

Evaluation of photoluminescence and colorimetric performance of Tb3+-induced calcium tungstate ceramic

Investigations of structural, photoluminescence, colorimetric, lifetime, and luminous efficiency of Tb3+ and Sm3+ co-doped calcium tungstate for WLEDs

Structure, photoluminescence, colorimetric, and thermoluminescence analysis of Ca1-x-yTb2x/3Sm2y/3WO4; x = 0.01; y = 0.01–0.05 ceramics are reported in this manuscript. The materials are made by the use of the solid-state method. Reitveld refinements of the obtained X-ray diffraction patterns are done by using the Fullprof Suite software. The obtained results denoted that the synthesized materials have a tetragonal structure and I41/a space group. Energy bandgaps are found in the UV–visible absorbance spectra. Photoluminescence investigations of the phosphors are done. The critical quenching concentrations of the phosphor are found to be x = 0.01; and y = 0.04. Moreover, the critical distance of energy shift with the adjacently presented ions is computed. Dexter’s theory is employed to elucidate that the dipole-dipole electronic interactions are the imperative reasons for the transfer of energy in the phosphor. The white color of emission is achieved for x = 0.01, y = 0.04 Tb, and Sm doped CaWO4 material. Moreover, great values in color purity, correlated color temperature, color rendering index as well as luminous efficiency of radiation are obtained from the CIE Chromaticity coordinates. Quantum yield efficiency of x = 0.01, y = 0.04 Tb & Sm codoped CaWO4 is measured. Photoluminescence decay analysis of all the samples is done to study the long-after glow properties and also the average lifetime values are evaluated. This is an indication of the suitable usage of the prepared materials in the white light-emitting diodes. Concentration as well as dose-dependent Thermoluminescence spectroscopy is investigated thoroughly. Kinetic parameters are calculated by deconvoluting the spectra of x = 0.01; y = 0.01 Tb & Sm codoped CaWO4 irradiated with 15 min of UV excitation dose. These results are denoted by high trap depth, second-order kinetics, and low UV dosimetry of the phosphor.

FTIR analysis and antimicrobial activity of sodium tungstate and calcium tungstate

In recent years, many antibacterial agents have been produced with the aim of eradicating infectious diseases, but many of these agents are ineffective against the resistance presented by bacteria. It is currently estimated that more than 60 % of current antibiotics are ineffective, so the discovery of new drugs is vital. Among the compounds studied in recent years are polyoxotungstates, inorganic compounds targeted for their pharmacological properties. The aim of this study was therefore to chemically characterize two tungstates: calcium and sodium, and to evaluate their microbiological properties, both in combination with antibiotics and due to their ability to reverse the resistance process represented by the expression of the enzyme betalactamase. The microbiological tests were carried out using the microdilution technique, with colorimetric disclosure, using resazurin, and the chemical characterization and vibrational modes of the compounds were evaluated using Fourier transform infrared spectroscopy with attenuated total reflectance. Calcium tungstate showed four spectroscopic bands, located between 84 and 1915 cm−1, while sodium tungstate showed two bands at 335 and 935 cm−1. Calcium tungstate intensified the effect of gentamicin against the bacterium Escherichia coli 06, as well as reversing the mechanism of enzymatic resistance presented by the bacteria Staphylococcus aureus K-4100 and K-4414. Given the current scenario of resistance, these results represent new alternatives for the treatment of bacterial infections, allowing a better understanding of the properties of polyoxometalates. These results are unprecedented as far as the literature is concerned.

Tailoring the surface features of CaWO4 by coupling with tubular g-C3N4 for enhanced solar photocatalysis and thermal energy storage

Rising energy and environmental crisis has led researchers to develop novel bifunctional materials. Herein, we report the construction of bifunctional hybrid materials for thermal energy storage and solar photocatalytic systems. Such a novel type of green material including calcium tungstate (CaWO4) and tubular graphitic carbon nitride (g-C3N4) was successfully fabricated via thermal mixing method. X-ray diffraction analysis confirmed the formation of both CaWO4 and g-C3N4 in the composite while morphological analyses verified the successful attachment of CaWO4 particles over the tubular g-C3N4 structure. The wide band gap energy (3.6 eV) of the pristine CaWO4 significantly decreased to 1.85 eV after coupling with g-C3N4. The hybrid catalyst showed superior photocatalytic degradation of Allura red (79.3 % within 120 min) and the rate constant was found 31.3-folds higher compared to pristine CaWO4 (13.3 % removal), which was ascribed to strong light harvesting, increased surface area, smaller crystallite size and fast electron transfer rate. In addition to catalytic functionality, the synthesized pristine and hybrid catalysts were also used in the preparation of lauric acid-based phase change materials (PCMs) and thermal properties of different fillers were examined. The 1 wt% CaWO4/g-C3N4 filler loaded composite displayed 13.03 % increment in heat storage rate (oC/s) when compared with lauric acid, demonstrating the essential role of the hybrid material over the PCM structure. The as-prepared hybrid materials displayed a bifunctional activity derived from incorporation of carbon nitride and thus verified a great potential for different applications due to their enhanced features.

Fabrication of chitosan-coated calcium tungstate (CaWO4/Chitosan) and its Antioxidant, antimicrobial and phocatalytic activity

In the present era, the applicability of natural bioactive polymers, such as chitosan, has become a significant concern due to its biologically compatible range, ease of biodegradability, and lower hazardous traits. In our investigation, the production of chitosan-coated calcium tungstate (CaWO4/Chitosan) was carried out using a sol–gel protocol. The resulting chitosan-coated calcium tungstate (CaWO4) was subjected to FTIR spectral analysis to identify the functionally active groups in chitosan interacting with calcium tungstate. X-ray analysis revealed the amorphous nature of chitosan-coated calcium tungstate (CaWO4), with a crystallite size ranging from 14.2 nm to 32.0 nm. The surface morphology and size of the CaWO4/Chitosan was analysed using SEM and TEM studies. The chitosan-coated calcium tungstate (CaWO4) exhibited a profound dose-dependent antioxidant activity, as determined by the DPPH assay. Both E. coli and S. aureus were susceptible to chitosan-coated calcium tungstate (CaWO4), showing inhibition zones of 19 mm and 20 mm, respectively. Additionally, chitosan-coated calcium tungstate (CaWO4) demonstrated antifungal activity against pathogenic fungi (A. flavus and A. niger) with inhibitory zones of 11 mm and 15 mm. UV-DRS analysis yielded a unique peak at 234 nm, corresponding to a band gap of 3.9093 eV. The chitosan-coated calcium tungstate (CaWO4) exhibited outstanding photocatalytic activity on alizarin, influenced by dye catalyst and concentration.

Theoretical Study of Efficient Photon-Phonon Resonance Absorption in the Tungsten-Related Vibrational Mode of Scheelite.

Tungsten (W) is an extremely rare and vital metal extensively used in metallurgy, the chemical industry, optoelectronic devices, and machinery manufacturing. In this work, an environmentally friendly and efficient physical method based on photon-phonon resonance absorption (PPRA) is proposed for separating W from scheelite. We calculated the vibrational spectrum of calcium tungstate (CaWO4) and assigned the infrared (IR) absorption and Raman scattering peaks through a dynamic analysis of the normal modes. We focused on the strong IR absorption peaks related to W and identified three high-intensity IR-active modes at around 830 cm-1, corresponding to the stretching of the W-O bonds. Therefore, we propose the use of high-power terahertz (∼25 THz) laser radiation to facilitate W extraction from compounds, leveraging the high efficiency of PPRA. Experimental testing is required to determine the precise absorption frequency under industrial production conditions.

In silico toxicity and immunological interactions of components of calcium silicate-based and epoxy resin-based endodontic sealers.

The present study aimed to determine in silico toxicity predictions of test compounds from hydraulic calcium silicate-based sealers (HCSBS) and AH Plus and computationally simulate the interaction between these substances and mediators of periapical inflammation via molecular docking. All chemical information of the test compounds was obtained from the PubChem site. Predictions for bioavailability and toxicity analyses were determined by the Molinspiration Cheminformatics, pkCSM, ProTox-II and OSIRIS Property Explorer platforms. Molecular docking was performed using the Autodock4 AMDock v.1.5.2 program to analyse interactions between proteins (IL-1β, IL-6, IL-8, IL-10 and TNF-α) and ligands (calcium silicate hydrate, zirconium oxide, bisphenol-A epoxy resin, dibenzylamine, iron oxide and calcium tungstate) to establish the affinity and bonding mode between systems. Bisphenol-A epoxy resin had the lowest maximum dose tolerated in humans and was the test compound with the largest number of toxicological properties (hepatotoxicity, carcinogenicity and irritant). All systems had favourable molecular docking. However, the ligands bisphenol-A epoxy resin and dibenzylamine had the greatest affinity with the cytokines tested. In silico predictions and molecular docking pointed the higher toxicity and greater interaction with mediators of periapical inflammation of the main test compounds from AH Plus compared to those from HCSBS. This is the first in silico study involving endodontic materials and may serve as the basis for further research that can generate more data, producing knowledge on the interference of each chemical compound in the composition of different root canal sealers.

The microstructure, composition, physical properties, and bioactivity of calcium silicate cement prototypes for vital pulp therapies.

Hydraulic calcium silicate cements (HCSCs) are valuable for various dental procedures. However, several reports document inherent limitations and complaints about their high costs, hindering accessibility in low-and middle-income countries. This study aimed to characterize four low-cost HCSC prototypes to show their microstructure, composition, and fundamental physical properties. Four HCSC prototypes were formulated: 1- calcium silicate powder with 17.5 wt. % replacement of calcium tungstate, 2- calcium silicate powder with 17.5 wt. % replacement of zirconium oxide, 3- calcium silicate powder with 17.5 wt. % replacement of calcium tungstate and 2.5 wt. % of zirconium oxide and 4- calcium silicate powder with 10 wt. % replacement of calcium tungstate and 10 wt. % replacement of zirconium oxide. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction were used to assess their microstructure and composition. Additionally, radiopacity, setting time, solubility, pH, and in vitro bioactivity were evaluated at different time points and contrasted with controls (Mineral trioxide aggregate -MTA Angelus- and Intermediate restorative material -IRM-). Their production cost was significantly lower than commercially available HCSCs. All prototypes exhibited a microstructure and composition comparable to MTA Angelus. All the prototypes exhibited radiopacity exceeding 3 mm of aluminum and shorter initial and final setting times than MTA Angelus. The solubility of some prototypes closely adhered to the ISO standard recommendation of 3% after 1 day, and all promoted an alkaline pH and the formation of calcium/phosphate precipitates. These promising findings suggest the potential clinical application of these prototypes. However, further research is necessary to evaluate their mechanical and biological properties for definitive clinical use.

ELECTROCHEMICAL PRODUCTION OF TUNGSTEN: STATUS AND PROSPECTS

An overview of studies of the electroche­mical reduction of tungsten compounds of different composition in various reaction media is presented. It is shown that among the variety of existing scientific and technical methods for obtaining tungsten, there are attractive prospects for the creation and development of a new industrial process that would ensure the direct electrochemical release of oxygen from its oxygen-containing compounds into molten mixtures based on chloride and calcium oxide. This scientific and technical solution is known in the literature as the FFC Cambridge process (FFC process).In contrast to the known methods of electrochemical reduction of tungsten compounds, this process allows the reduction of oxygen-containing tungsten compounds in the solid state and does not depend on the course of acid-base equilibria at the electrode/electrolyte phase separation boundary. The most favorable conditions for the reduction of oxygen-containing tungsten compounds are provided by electrolysis using a liquid gallium cathode in both galvanostatic and potentiostatic modes, and it is advisable to use the initial tungsten compounds in a finely dispersed state. The electrochemical reduction of tungsten trioxide in thee utectic melt of sodium and calcium chlorides occurs through the intermediate stage of calcium tungstate formation, so it is advisable to use CaWO4 instead of WO3 as the starting compound for reduction. Electrochemical reduction on a liquid gallium cathode in a molten eutectic mixture of sodium and calcium chlorides allows obtaining highly dispersed tungsten powder (11–35 nm) of high purity (99.9%) with a degree of extraction of at least 90.0% from both tungsten trioxide and from calcium tungstate. In addition, in this way it is possible to obtain not only pure tungsten, but also metal alloys and composites based on it.

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.

Effect of radiopacifier and liquid in the physicochemical and biological properties of calcium silicate clinker Angelus: A laboratory investigation.

The aim of this study was to evaluate the effect of radiopacifier calcium tungstate and manipulation with distilled water (DW) or liquid with additives (LA) on calcium silicate clinker Angelus (CL) properties, compared with MTA (Angelus, Brazil) and MTA Repair HP (MTAHP, Angelus, Brazil). The physicochemical properties, cellular viability and bioactivity were evaluated. ANOVA/Tukey and Bonferroni tests were performed (α = 0.05). There was no difference in material setting time (p > 0.05). MTA and MTAHP were similar (p > 0.05) and had greater radiopacity than CL + DW and CL + LA (p < 0.05). All experimental materials showed mass increase, alkalinisation capacity, besides biocompatibility and bioactivity at 3 and 7 days. The different liquids had no influence in the biological properties and bioactivity of the calcium silicate clinker Angelus. Calcium tungstate provided radiopacity, without changing the setting time, maintaining the mass increase and alkalinisation ability of the calcium silicate materials.

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.

White light-emitting calcium tungstate microspheres synthesized via co-precipitation at room temperature and application to UV-LED chip

Crystalline CaWO4 was synthesized by a co-precipitation method at room temperature. XRD peak at a position like that in the reference pattern, and the signal was detected from the (112) plane. Raman signals suggested the presence of symmetrical vibrations of the WO4 bond. The absorption was observed in the UV region, and the bandgap calculated from the Tauc plot was 4.47 eV. When the sample was excited at 230 nm, blue emission with a peak at 430 nm was observed. Crystalline CaWO4 was doped with Dy3+ ions to synthesize a white light-emitting phosphor. The addition of Dy3+ ions altered the crystal structure and shifted the Raman signals slightly. The sample was excited at 230 nm, both 430 nm and 575 nm emissions were observed, and the intensity of the 575 nm emission increased with increasing Dy3+ concentration. The CaWO4:0.1Dy3+ sample showed the highest luminescence intensity, and excessive doping decreased the luminescence intensity owing to concentration quenching. The photoluminescence decay lifetime initially increased with increasing Dy3+ concentration but decreased at concentrations greater than 0.1 mol% because of concentration quenching. The synthesized white light emitting CaWO4:Dy3+ powder was mixed with PDMS to form a flexible composite. The composite could be easily bent with a finger and emitted white light when applied on a UV-LED chip.

High performance inorganic fullerene cage WS2 enhanced cement

An original cement based material enhanced with inorganic fullerene tungsten disulfide (IF-WS2) nanoparticles has been engineered with superb shock absorbing properties. Physical properties were attributed to the IF-WS2 nano-hollow multiple layered onion-like structure. The effect of IF-WS2 concentration at 0.1 wt%, 1 wt% and 5 wt% on the hydration kinetics of ordinary Portland cement (CEM1), electrical impedance, thermal stability, rheology and strength development was thoroughly evaluated. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies confirmed the formation of the new phase, calcium tungstate (CaWO4), at the nano-particle/cement matrix interface during early hydration. 1 wt% IF-WS2 additions enhanced the impact energy of CEM1 by 89% compared to the control. An IF-WS2 cementitious mixture was developed for 3D printing based on the 1% WS2-CEM composition. The mix exhibited excellent workability and buildability enabling the creation of a layer-by-layer printed component. Intimate interlayer adhesion minimized the presence of voids leading to a high flexural strength of 6.7 MPa, which equated to an over 86% improvement compared to plain CEM1 printed components. This study showcases IF-WS2 nanoparticles as a new ground-breaking additive enabling the production of high-performance cementitious construction materials, for use under extreme environments demanding high strength and impact resistance.

Controlled drug delivery from metronidazole-containing bioactive endodontic cements.

This study aims to formulate metronidazole liquid nanocapsules (MTZLNC) and evaluate their effect on the physicochemical and biological properties of calcium silicate-based bioactive endodontic cements, in vitro. A MTZLNC suspension was formulated by deposition of the preformed polymer and characterized by laser diffraction and high-performance liquid chromatography (HPLC). Calcium silicate (CS) was mixed with a radiopaque agent (calcium tungstate - CaWO4), at 10wt%, to produce the cement powder. Cements liquids were used with two concentrations of MTZLNC suspension: 0.3mg/ml and 0.15mg/ml. Cements prepared with distilled water were used as the control. The radiopacity, setting time, and flow were evaluated following ISO 6876:2012. The compressive strength analysis was conducted according to ISO 9917:2007. pH and mineral deposition were evaluated after immersion in simulated body fluid (SBF). Cell behavior was evaluated by the viability of pre-osteoblastic cells and pulp fibroblasts by SRB and MTT and the antibacterial activity against Enterococcus faecalis was analyzed immediately and after nine months of water storage. MTZLNCs were formulated with a median diameter of 148nm and 83.44 % load efficiency. Increased flow and reduced strength were observed for both MTZLNCs concentrations. The incorporation of MTZLNCs maintained the ability of cements to increase pH media and promote mineral deposition over the samples, without promoting cytotoxicity. A 2log10 reduction in E. faecalis CFU was observed immediately and after nine months in water storage. The formulation of MTZLNCs allowed the development of antibacterial calcium silicate-based-cements with suitable physicochemical properties and bioactivity, with a reduction in mechanical strength. The 0.3mg/ml concentration in cements liquid promoted effective and sustainable antibacterial activity.

Fiber Optical Temperature Sensor Based on 980nm Excitation of CaWO4: Yb3+, Er3+ with Up-conversion Fluorescence Intensity Ratio

The Yb3+, Er3+ co-doped calcium tungstate materials were prepared. The temperature-sensitive fiber optic probes made from CaWO4:Yb3+, Er3+ were prepared by the high-temperature solid-phase sintering method. The XRD lattice structure of CaWO4: Yb3+, Er3+ was tested. The fluorescence spectra of the probe under 980nm excitation were tested, and the variable temperature profile of the probe was analyzed to derive its two-photon absorption process. The temperature characteristics of the up-conversion fluorescence intensity ratio (FIR) of the probe in the temperature range of 303-673K were investigated, and the results showed that this optical fiber probe can be used as a fiber optic temperature sensor with a linearity of 5.8% and a temperature measurement return difference of only 1.5%; it was found that the temperature characteristic curve of FIR of this fiber optic probe was reproducible. These properties show that CaWO4: Yb3+, Er3+ is feasible to be made into a fiber optic sensor.