ZnWO4 Crystals Research Articles

Effect of Zn Contents of W/Zn-Beta Catalysts on Their Catalytic Performance in the Selective Hydrocracking of Tetrahydronaphthalene into Benzene, Toluene, and Xylene

Selective hydrocracking (HDC) of tetrahydronaphthalene (THN) into light aromatic hydrocarbons, such as benzene (B), toluene (T), and xylene (X), was performed over bifunctional catalysts with different ZnO contents (W/Zn-Beta) at 400 °C and 6 MPa. In this work, the change of catalytic performance of the bifunctional catalysts with different ZnO contents in selective HDC of THN and the nine-lump reaction kinetic model was investigated. It was found for the first time that ZnO can not only effectively regulate the acid properties of Beta zeolite but also react with WO3 to form non-active ZnWO4 crystals, resulting in low HDC activity of the catalyst. At low ZnO contents of the catalyst (<1 wt %), as the interaction of ZnO–Beta is stronger than that of WO3–Beta, the interaction of ZnO–Beta would replace the interaction of WO3–Beta, leading to the agglomeration of WO3 and the dispersions of WO3 on Beta zeolite decrease. As a result, WO3 would be easily reduced to WS2, and the BTX selectivity would increase. At high ZnO contents of the catalyst (≥1 wt %), the strong acid sites of Beta zeolite would be covered by ZnO. In addition, the excess ZnO would react with WO3 to form non-active ZnWO4 crystals, and the BTX selectivity would decrease. The W(25)/Zn(1)-Beta-40 catalyst exhibited maximal BTX selectivity (45 wt %) at 94% THN conversion. The kinetic study further indicated that with the increase of ZnO contents, the ratio of formation reaction rate of BTX to the over-HDC rate of BTX increases first and then decreases, and the reaction path selectivity of THN isomerization increases.

Effective Adjustment of Crystallinity and Regular Anti-Corrosion Activity of Zinc Tungstate Pigments

It is important to reduce costs and improve performance by microstructure-control of materials. The direct precipitation method and hydrothermal method were adopted to adjust the crystallinity of zinc tungstate (ZnWO4) pigments. Correlation between the crystallinity of ZnWO4 and its anti-corrosion performance is explored. The results confirm that ZnWO4 crystals prepared by hydrothermal reaction for 12 h have high crystallinity, and the corrosion resistance is enhanced by 737.0% compared with blank coating in the epoxy resin system. The improvement in corrosion inhibition is due to the enhancement of the shielding effect of the multilayer passivation film and electronic suppression effect resulted from the high ZnWO4 crystallinity. The low ZnWO4 crystallinity can weaken the corrosion resistance because of the defect-enriched effect. Advancement in anticorrosive performance by crystallinity-control meets the reduction of costs and energy consumption in the coatings industry.

Luminescence of ZnWO4 crystals under X-ray excitation

Zinc tungstate crystal scintillator is an interesting detector material for double beta decay, dark matter, and rare alpha decay experiments. The luminescence under X-ray excitation in the temperature region from 85 K to 420 K was studied with five samples of ZnWO4 crystals developed by different methods: with a stoichiometric composition, and with excess of Zn or W. It is found that doping a ZnWO4 crystal with 0.2–0.3% Zn and W does not affect the luminescence properties of the material. A theoretical dependence of the sample luminescence intensity on the competing nonradiative recombination centers and traps is obtained. The main center of the recombination luminescence in a ZnWO4 crystal with a spectral maximum at 2.5 eV is the tungsten complex with a structural defect located next to it.

Luminescence and scintillation properties of ZnMo1-xWxO4 crystal

The luminescence and scintillation properties of ZnMo1-xWxO4 (x = 0, 0.0022, 0.0088) crystals grown by low thermal gradient Czochralski technique were studied in a temperature range of 10–300 K. The good optical transparency above the fundamental absorption of crystals were obtained for all the samples. The luminescence light yield enhancement and slowing of the decay time were observed with lowering the temperature for W doped samples under 4.42 eV excitation. Improvement in scintillation light yield with W doping in ZnMoO4 crystals was observed and found 76% that of ZnWO4 crystal at 10 K for 0.88 mol% W doped ZnMoO4 sample. An additional TSL band was observed for the W doped samples in the temperature range of 70–100 K. The enhancement in scintillation light yield at low temperature with W doping in ZnMoO4 crystal are quite encouraging to use it as a cryogenic phonon scintillation detector for the neutrinoless double beta decay search of 100Mo.

Growth of samarium doped zinc tungstate crystals by the Czochralski method

The growth conditions to produce a high quality ZnWO4:Sm single crystals by the Czochralski method were determined in this work. The role of the Sm impurity in the formation of bubbles in a ZnWO4 single crystal is discussed. The Sm3+ ions segregation coefficient in the ZnWO4 single crystal was estimated to be 0.023. The effect of non-isomorphic substitution of Sm3 + for Zn2 + ions on the ZnWO4 crystal lattice parameters has been investigated. The X-ray luminescence spectra of ZnWO4:Sm3+ crystals are discussed. Two ZnWO4 crystals doped with samarium enriched in 148Sm and 149Sm isotopes were obtained following the established growth technique, which demonstrates its feasibility and reliability for production of detector for rare nuclear processes studies.

Effect of Li+ codoping on the mechanical strength of YB:ZnWO4 single crystals

ZnWO4 single crystals both undoped and doped with various concentrations of Yb3+ and Li+ ions have been grown. The actual concentrations of dopants had been measured by atomic emission spectroscopy with inductively coupled plasma, as well as by polarized optical absorption spectroscopy. The segregation coefficients of the dopants between the crystal and melt have been evaluated. Hardness and fracture toughness of the crystals were measured by indentation method. The dependencies of the measured values on the dopants concentrations were analyzed. The optimal dopants concentrations, providing the best mechanical strength of Yb,Li:ZnWO4 crystal have been found.

Precise measurement of the scintillation decay constant of the ZnWO4 crystal

Abstract The scintillation decay time constant of a ZnWO4 crystal irradiated with α-particles from 241Am was precisely investigated, and was found to depend on the incident direction of the α-particles on the crystal. The longest decay time constant (24.3 ± 0.6 μs) was obtained on the surface perpendicular to the b-axis of the crystal (surface B). On surfaces A and C, the decay constants were 20.0 and 21.3 ± 0.2 μs, respectively. The scintillation yield of ZnWO4 was also anisotropic and depended on the incident direction of the heavy particles. The maximum yield was achieved on surface B, suggesting a correlation between the light yield and the scintillation decay time constant of ZnWO4 crystals.

Preparation and luminescence properties of ZnWO4: Tb3+/Yb3+ glass ceramics

Tb3+/Yb3+ co-doped glass ceramics containing ZnWO4 crystals were prepared by melting crystallization method. The effects of Tb3+ and Yb3+ doping on the crystal phase and luminescence properties of glass ceramics were studied. The XRD analysis confirmed the existence of ZnWO4 crystals in glass ceramics, with the increase of Tb3+ doping concentration, the crystal phase changed to Na0.5Tb0.5WO4. The effect of heat treatment time and doping concentration of Tb3+ on the luminescence performance of the glass ceramics were studied to determine the optimal doping concentration of Tb3+. When the concentration of Tb2O3 is 0.1 mol%, with the increase of Yb2O3 doping concentration, the crystal phase of glass ceramics changes from ZnWO4 to Na0.5Yb0.5WO4. The up-conversion luminescence performance of Tb3+/Yb3+ co-doped glass ceramics was studied, and the optimal doping concentration ratio between Tb2O3 and Yb2O3 is determined to be 1:7.

Monoclinic zinc monotungstate Yb3+,Li+:ZnWO4: Part I. Czochralski growth, structure refinement and Raman spectra

We report on the growth of a co-doped divalent-metal monotungstate crystal, Yb3+,Li+:ZnWO4, its structure refinement, thermo-mechanical properties and polarized Raman spectra. Yb3+,Li+:ZnWO4 crystals are grown by the Czochralski method using Pt/Rh and Pt crucibles. The nature of crystal coloration is discussed and assigned to color centers based on oxygen vacancies as well as Fe2+(Fe3+) species and, possibly, other uncontrolled impurities of transition metal ions. Oxidizing annealing at 800 °C helps to improve the transparency of Yb3+,Li+:ZnWO4 leading to a weak residual pink coloration. Yb3+,Li+:ZnWO4 was confirmed to be monoclinic (sp. gr. P2/c – C42h, No. 13) with lattice constants a = 4.702(2) Å, b = 5.718(6) Å, c = 4.930(4) Å and β = 90.713(5)°. The coefficients of linear thermal expansion are α[100] = 11.71, α[010] = 9.90 and α[001] = 7.01 [10−6 K−1]. The melting point of the crystal is at 1166 °C and no thermal effects are found below the melting point. The Yb3+,Li+:ZnWO4 crystal shows intense and strongly polarized Raman spectra with the most intense mode at 906.0 cm−1 assigned to the W – O stretching vibrations in the [WO6] octahedra. It is thus a promising gain medium for laser emission at ~1 μm and for self-Raman frequency conversion.

Measurement of the Anisotropic Response of the ZnWO4 Crystal for Developing the Direction-Sensitive Dark Matter Detector

We measured the scintillation light yield of ZnWO4 crystals using a quasi-monochromatic neutron beam energy of 865 keV. Irradiating the neutron beam on two surfaces, which are almost perpendicular to the crystal axis, the light yield of oxygen recoil was measured. The obtained light yield ratio of a neutron recoil to gamma-ray-induced electron recoil for two surfaces was 0.235 ± 0.026 and 0.199 ± 0.020, respectively, which corresponds to 15.3% of anisotropy for ~200-keV nuclear recoils of this crystal. This property can be applied to a direction-sensitive dark matter (DM) detector.

Study of charge carrier trapping by EPR and TSL methods in ZnxMg1-xWO4 single crystals

Charge carrier trapping was studied using EPR and TSL methods in ZnWO4, MgWO4 and mixed ZnxMg1-xWO4 crystals. An electron trapped at a perturbed W site was detected in MgWO4. It was also shown that holes are trapped at oxygen ions thus forming O− centers. Two different O− centers, one perturbed by a cation vacancy, another by Al impurity, were found in MgWO4, while a single hole center perturbed by Cd impurity and hydroxyl group nearby was detected in ZnWO4. The number of non-equivalent hole centers increases up to five for the mixed crystals due to the perturbations induced by statistically distributed substitutional cations. It is shown that the thermal release of holes is accompanied by thermostimulated luminescence (TSL). The broadening of the TSL peaks typically observed in mixed crystals is connected with the increase of the number of non-equivalent trapping centers of the same type.

Doping controlled oxygen vacancies of ZnWO4 as a novel and effective sensing platform for carbendazim and biomolecule

Oxygen vacancy (VO) is one of the most important defects and acts as the reactive sites in a variety of catalytic reactions. In this work, doping method is employed to generate controllable VO in ZnWO4 crystal and the existence of VO was validated by TEM, XPS and UV–vis absorbance spectra. A novel electrochemical sensor based on Ce-doped ZnWO4 modified carbon paste electrode (CPE) has been fabricated and the electrochemical performance of the proposed sensor is verified by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV). The as-prepared oxygen vacancy-rich ZnWO4 shows excellent electrocatalytic performance in sensing carbendazim (CBZ) compared to the virgin CPE. The results demonstrate that VO can effectively enhance electronic conductivity and narrow the band gap, leading to superior performances of 2%Ce-doped ZnWO4 in electrocatalytic reactions. Low detection limits (LOD) of CBZ is obtained to be 0.0033 μmol L−1 via DPV method (S/N = 3). The impressive detection performance is even spread to the simultaneous determination of common neurotransmitters, DA an UA, with detection limits of 0.0033 μmol L−1 and 0.17 μmol L−1 and wide linear range from 0.01 to 30 μmol L−1 and 0.5 to 100 μmol L−1 for DA and UA, respectively.

The Influence of Chromium Impurity on the Spectral Characteristics of ZnWO4 Crystals

The Influence of Chromium Impurity on the Spectral Characteristics of ZnWO₄ Crystals

Toward Bi3+ Red Luminescence with No Visible Reabsorption through Manageable Energy Interaction and Crystal Defect Modulation in Single Bi3+-Doped ZnWO4 Crystal

The last decades have witnessed the discovery of tens of thousands of rare earth (RE) (e.g., Eu2+) and non-RE (e.g., Mn2+) doped photonic materials for near-ultraviolet (NUV) and blue converted white light-emitting diodes (wLEDs), but the future development of wLEDs technology is limited greatly by the intrinsic problems of these traditional dopants, such as the insurmountable visible light reabsorption, the weak absorption strength in NUV or blue region, and so on. Here we report a feasible strategy guided by density functional theory (DFT) calculation to discover novel Bi3+ red luminescent materials, which can solve the above problems eventually. Once the untraditional ion of bismuth is doped into ZnWO4 crystal, multiple defects can be possibly created in different charge states such as BiZn, BiW, interstitial Bi, and even defect complexes of 2 BiZnVW among others, and they, as DFT calculated results illustrate, have the potential to produce emission spanning from visible to near-infrared. As confirmed ...

A facile microwave-hydrothermal method to fabricate B doped ZnWO4 nanorods with high crystalline and highly efficient photocatalytic activity

A series length of 50∼200nm B doped ZnWO4 nanorods with high crystallinity were fabricated via a fast microwave-hydrothermal method. The effects of B doping on the physical properties of ZnWO4 nanorods were intensively characterized by N2 physical adsorption, XRD, TEM, FT-IR, XPS, ultraviolet-visible diffuse reflection spectrum (UV–vis DRS), photolunminescence spectrum (PL), transient photocurrent response (TPR). The results indicated that the doped B could replace the lattice tungsten ion in ZnWO4 crystals, which caused the lattice spacing contraction, particle size decrease, specific surface area increase. More importantly, the separation of the photogenerated electrons and holes in ZnWO4 was significantly enhanced by B doping. With doped optimum B concentration (2.42wt%), B/ZnWO4 displayed super photocatalytic performance in dyes decomposition. The degradation rate constant (0.065h−1) of Rhodamine B over B (2.42wt%)/ZnWO4 is 4 times of that over ZnWO4 (0.016h−1).

Characterization of ZnWO_4 Raman crystal

Stimulated Raman scattering (SRS) is an important technique for broadening the laser spectra. The Raman crystals with large dimensions and high thermal conductivity are favorable by SRS. Zinc tungstate (ZnWO4) belonging to a monoclinic system has been grown by the Czochralski method with the dimensions of Φ 35 mm × 70 mm. The thermal properties were evaluated and analyzed in several aspects, including the density, specific heat, thermal expansion coefficient, and thermal conductivity. Thermal conductivity was calculated to be 5.412 Wm−1K−1 along the c-axis, which is about two times larger than the famous BaWO4 and SrWO4. Meanwhile, the optical properties including transmission and Raman spectra and Raman shift were measured and the stimulated Raman scattering performance of ZnWO4 crystal was demonstrated. The results show that the thermal conductivity decreases with increasing temperature. Furthermore, the thermal focal lengths were theoretically estimated. Along the a direction, the light with a wavelength of 532 nm can be shifted to the light at 558.95 nm, which is in good accordance with the spontaneous Raman spectrum. All the analyses and results above show that ZnWO4 is a promising solid-state Raman laser crystal.

Step-height standards based on the rapid formation of monolayer steps on the surface of layered crystals

Metrology is essential for nanotechnology, especially for structures and devices with feature sizes going down to nm. Scanning probe microscopes (SPMs) permits measurement of nanometer- and subnanometer-scale objects. Accuracy of size measurements performed using SPMs is largely defined by the accuracy of used calibration measures. In the present publication, we demonstrate that height standards of monolayer step (∼1 and ∼0.6nm) can be easily prepared by cleaving Bi2Se3 and ZnWO4 layered single crystals. It was shown that the conducting surface of Bi2Se3 crystals offers height standard appropriate for calibrating STMs and for testing conductive SPM probes. Our AFM study of the morphology of freshly cleaved (0001) Bi2Se3 surfaces proved that such surfaces remained atomically smooth during a period of at least half a year. The (010) surfaces of ZnWO4 crystals remained atomically smooth during one day, but already two days later an additional nanorelief of amplitude ∼0.3nm appeared on those surfaces. This relief, however, did not further grow in height, and it did not hamper the calibration. Simplicity and the possibility of rapid fabrication of the step-height standards, as well as their high stability, make these standards available for a great, permanently growing number of users involved in 3D printing activities.

End pumped yellow laser performance of Dy3+:ZnWO4

We report an end pumped single transverse-mode (TEM00) yellow laser based on Dy3+:ZnWO4 crystal. The pump light is InGaN blue laser diode at 450nm with the maximum power of 1.5W. A 3cm length Dy3+:ZnWO4 crystal served as the gain medium. By way of the square filter, we discuss the TEM00 properties. In continuous-wave operation and single transverse-mode, a maximum output power of 0.110W and a slope efficiency of 13.0% are achieved at an emission wavelength of 575nm.

Nonlinear Interaction of Electronic Excitations Created at High-Densities in Luminescent Materials and Scintillators

Nonlinear interaction of electronic excitations has been shown to play a remarkable role in luminescent material performance. Recently, it has been intensely studied in semiconductors and wide-gap scintillators. The nonlinear interaction is a cause of the nonproportionality of scintillator response to the energy of the absorbed ionizing radiation and can limit its energy resolution, which results in crucial deterioration of the scintillator functionality. The type and degree of nonproportionality are material specific and depend little on impurity content or sample treatment. Theoretical models have been developed, which explain the effect of scintillator response nonproportionality by the interplay between the density and mobility of charge carriers created in a track of ionizing radiation [1-3]. Energy losses in relaxation processes and the final distribution of excited luminescence centres in a material are determined by a number of processes including thermalisation of hot carriers, their recombination, interaction, trapping at defects and impurities and exciton formation. In the present contribution, we demonstrate how the mutual interaction of small-radius Frenkel excitons created at high densities by ionizing radiation or intense laser pulses can reveal itself in the luminescence characteristics of a luminescent material. It is shown that the related phenomena observed in various materials under X or γ rays are similar to those detected under UV-XUV excitation, which allows their theoretical description proceeding from the same principles and their experimental study in substantially better controlled experimental conditions of excitation by ultrashort laser pulses with well-defined timing and spatial parameters. A remarkable effect of saturation of excitonic absorption was discovered under excitation by ultrashort UV pulses from the energy region of phonon-assisted electronic transitions. The changes in the luminescence decay kinetics and various saturation effects in excitonic and impurity luminescence induced by high-density or high-energy excitation are demonstrated and theoretically modelled for several wide-gap materials. The experimental results of this contribution originate mainly from the study of time-resolved luminescence in tungstate crystals of different structures, wolframites CdWO4 and ZnWO4, and homologous scheelites CaWO4, SrWO4, BaWO4, by using femtosecond UV-XUV sources such as free-electron lasers, tuneable femtosecond lasers, high-order harmonic generation systems. It is shown that the emission decay and saturation can be described within the frame of Förster’s dipole-dipole interaction model on the basis of theoretical statements developed in [1]. The importance of studying exciton-exciton interactions is demonstrated by revealing a direct link between the Förster interaction radius and nonproportionality of the material light response. A good correlation between the radius of cation, radius of exciton and radius of Förster interaction is shown for the sequence of homologous scheelites CaWO4, SrWO4, BaWO4.

Improvement of radiopurity level of enriched 116CdWO4 and ZnWO4 crystal scintillators by recrystallization

As low as possible radioactive contamination of a detector plays a crucial role to improve sensitivity of a double beta decay experiment. The radioactive contamination of a sample of 116CdWO4 crystal scintillator by thorium was reduced by a factor ≈10, down to the level 0.01mBq/kg (228Th), by exploiting the recrystallization procedure. The total alpha activity of uranium and thorium daughters was reduced by a factor ≈3, down to 1.6mBq/kg. No change in the specific activity (the total α activity and 228Th) was observed in a sample of ZnWO4 crystal produced by recrystallization after removing ≈0.4mm surface layer of the crystal.