Molybdate Compounds Research Articles

Microstructure and wear characteristics of laser-clad Ni-based self-lubricating coating incorporating MoS2/Ag for use in high temperature

Nickel-based self-lubricating cladding containing Ag and MoS2 was prepared on Ti6Al4V alloy using laser cladding. The microstructure, microhardness and wear behaviour were measured and analysed. The coating was mainly composed of Ti2Ni and TiNi as the matrix, and TiC and TiB2 as the reinforcements. A new phase of Ti2SC was synthesized in situ in the coating, whereas Mo was consolidated in the matrix. The microhardness of the coating is greatly increased by in-situ generation of reinforcement phase and solution strengthening, with an average hardness of 823.6HV0.2, which is about 2–3 times that of the substrate. When the temperature increased to 600 °C, the coefficient of friction and wear rate of the coating decreased. The friction coefficient and the wear rate of the coating were greatly reduced at 800 °C. The wear rate of the coating decreased to 4.85 × 10−6 mm3, which decreased by nearly an order of magnitude compared with the coating's wear rate at room temperature. The anti-friction effect of the lubricating phase not only counteracted the effect of the decreased hardness on the wear rate, but also played a more obvious anti-friction effect. In the high-temperature stage, MoO3 and silver molybdate compounds produced by the tribological chemical reaction of wear surfaces could significantly improve the density of the enamel layer on the wear surface.

Microwave assisted preparation of ternary scheelite CaMoO4: Er3+/Yb3+ nano-phosphors for up/down-conversion photoluminescence, temperature sensing and antibacterial properties

Due to their exceptional intrinsic optical characteristics, molybdate compounds have aroused significant attention in recent years for their effective up and downconversion luminescence. In the present study, a series of xEr3+/Yb3+ (x = 0.5 %, 1 %, 2 %, 4 % and Yb3+ =15 %) doped CaMoO4 nanophosphors synthesized through a microwave assisted approach and subsequently the comprehensive analysis of the functionalities such as optical properties, antibacterial traits and their integration with temperature sensing, have been explored. Furthermore, the structural refinement confirms the occurrence of tetragonal scheelite phase of the prepared materials. The morphology of the material was identified by FE-SEM and HR-TEM analysis which reveals particles are in spherical shape. Up-conversion luminescence intensities were found to be dependent on doping concentration, laser excitation power and external temperature upon 980 nm light excitation. The temperature sensing response of the optimized sample was examined based on intensity ratio of two emission bands that involves thermally connected energy levels of Er3+ ion. The maximum absolute sensitivity of 10.74 × 10−3 K−1 (at 500 K) was found, demonstrating its potential application in high temperature thermometry. The comprehensive analysis and the outcome of the present study suggests the potential prospect of the as-synthesized material in food preservation, medical equipment, water treatment, and as an optical coating agent for solid state luminous devices.

A novel dielectric ceramic Na0·48La0·52MoO4 with excellent thermal stability for LTCC applications

This article reports the fabrication of a novel molybdate compound, Na0·48La0·52MoO4 (NLM), through the solid-state reaction method. The structural characterization and dielectric properties of NLM were conducted via powder X-ray diffraction, SEM, laser Raman spectroscopy, and vector network analysis. The research results indicate that the sintering temperature directly affects the firing process and microwave dielectric properties. NLM ceramics sintered at 920 °C demonstrate excellent microwave dielectric properties of εr = 8.86, Q × f = 61,378 GHz (at 13.5 GHz) and τf = −10.8 ppm/°C. Furthermore, NLM ceramics exhibit favorable chemical compatibility with Ag. These findings underscore the significant potential of NLM ceramics for utilization in LTCC applications.

Crystal Chemistry and Structural Complexity of the Uranyl Molybdate Minerals and Synthetic Compounds

This paper reviews not the largest, but at the same time quite an interesting, group of natural and synthetic uranyl molybdate compounds. Nowadays, nine minerals of U and Mo are known, but the crystal structures have only been reported for five of them. Almost an order of magnitude more (69) synthetic compounds are known. A significant discrepancy in the topological types for natural and synthetic phases is shown, which is most likely due to elevated temperatures of laboratory experiments (up to 1000 °C), while natural phases apparently grow at significantly lower temperatures. At the same time, the prevalence of dense topologies (with edge-sharing interpolyhedral linkage) among natural phases can be noted, which is fully consistent with other recently considered mineral groups. Uranyl molybdates demonstrate several similarities with compounds of other U-bearing groups; however, even topological matches do not lead to the appearance of completely isotypic compounds. Structural complexity calculations confirm, in general, crystal chemical observations. Considering the prevalence of dense structures in which coordination polyhedra of uranium and molybdenum are connected through common edges as well as framework architectures, one can expect a less significant influence of interlayer species on the formation of the crystal structure than the main U-bearing complexes. The more structural complexity of the uranyl molybdate units, the more complex of the entire crystal structure is. In addition, there is a tendency for complexity to increase with increasing density of the complex; the simplest structures are vertex-shared, while the complexity increases with the appearance of common edges.

Solid-Phase Synthesis of Nickel-Molybdenum Catalysts for Metathesis of Propylene under Mechanical Action

The solid-phase synthesis of model aluminum-molybdenum (Al-Mo) and aluminum-nickel-molybdenum (Al-Ni-Mo) compositions constituting the catalysts for metathesis of propylene was carried out under mechanical action. The structure of model Al-Mo and Al-Ni-Mo compositions was studied using X-ray diffraction analysis, high-resolution transmission electron microscopy, infrared spectroscopy, and diffuse reflectance electron spectroscopy (UV-vis DRS). The latter method revealed the presence of isolated monomeric and oligomeric molybdate compounds in the model Al-Ni-Mo compositions. Granulated catalysts for metathesis were obtained by molding the model Al-Mo and Al-Ni-Mo compositions with aluminum hydroxide followed by calcination. Most active in the metathesis of propylene was the aluminum-molybdenum catalyst containing 2.6 wt.% Ni, 13.0 wt.% Mo and 32.7 wt.% Al. At the process temperature of 200 °С, pressure 0.1 MPa, and weight space velocity of propylene 1 h–1, the conversion of propylene on this catalyst reached 33.7 %, which makes the catalyst promising for practical application. Therewith, the weight fraction of ethylene in the reaction products was 17.5 %, and butenes – 71.3 %.

Investigations of structure-improper ferroelectricity relationships to enhance the multifunctional applications of the β′-Y2(MoO4)3 phase

Polycrystalline samples of the compound Y2(MoO4)3 have been synthesized by solid-state reaction starting from the corresponding yttrium and molybdenum oxides, and the pure ferroelectric phase has been successfully isolated. Both β’ (ferroelectric) and β (paraelectric) phases were studied by conventional X-ray diffraction at different temperatures, performing the corresponding Rietveld refinements before and after the phase transition. An analysis of the displacive symmetry modes involved was carry out. The thermal dependence of the crystal structure was also studied, confirming that it is an improper ferroelectric with a primary order parameter that does not correspond to the spontaneous polarization. To corroborate the purity of the β′-phase, TG and SEM-EDX analyses were performed, demonstrating the absence of the γ-phase. Moreover, DSC analysis and impedance spectroscopy confirmed the Curie temperature of the β′-β phase transition (Tc around 378 ​K). Ferroelectric hysteresis cycles were recorded as a function of temperature and at different maximum applied electric fields. The values of the maximal and remanent polarization were correlated with the structural study. Finally, anomalies in the electrical permittivity and conductivity were observed between 600 and 877 ​K, which may be associated with the evolution of the lattice parameter c at high temperatures. With this ferroelectric characterization we want to highlight this still unexploited crystal matrix in comparison to other rare earth molybdates with the β′-phase and other yttrium molybdate compounds.

Structural change by phosphorus addition to borosilicate glass containing simulated waste components

Simulated nuclear waste glass samples containing phosphorus, which increase the solubility of molybdenum, were prepared and analyzed using synchrotron X-ray Absorption Fine Structure (XAFS) analysis for some constituent elements and Raman spectroscopic analysis of their complex structure. Changes in local structure and chemical state due to different phosphorus additions and waste loading rates were systematically studied. Consequently, no crystalline phase due to the molybdate compound was observed even at a maximum waste content of 30 wt% (corresponding to 1.87 mol% MoO3). Oxidation proceeded when the waste-loading rate was increased, whereas the reduction proceeded when phosphorus was added. In some cases, the effects of oxidation and reduction were offset. The local structure around specific elements can be classified as follows; Zn that is affected mainly by the waste-loading rate, Ce that is affected by both the waste-loading rate and phosphorus addition, and Zr element that is not affected by either of them. From the comparison between the analytical results of Mo and other elements, it was considered that the added phosphorus exists as a free PO4 structural unit and may deprive the alkali metal coordinated to the molybdate ion.

Study of structural, dielectric and AC conductivity properties of SrMoO4

Strontium molybdate (SrMoO4) compound is synthesized using a solid-state reaction route. SrMoO4 was observed to form in tetragonal Scheelite structure with I41/a space group and is confirmed with the help of powder X-ray diffraction studies. The temperature-dependent dielectric constant measured at different frequencies indicated a maximum at around 795 K. The frequency-dependent dielectric maximum reveals the presence of a dielectric relaxation near 795 K. Further, the Nyquist plot analysis shows a depressed semi-circle and manifest the non-Debye type dielectric relaxation in SrMoO4. The SrMoO4 compound possesses a negative temperature coefficient of resistance and is confirmed by fitting the Nyquist plot with a suitable equivalent circuit. The AC conductivity studies ascertain that the observed conduction mechanism in the SrMoO4 as a translational type correlated barrier hopping. The temperature-dependent AC conductivity reveals that the obtained activation energies are comparable with the oxygen vacancies and are responsible for observed dielectric relaxation.

Cobalt Molybdate Incorporated Thermoset Composites: Preparation and Characterization

Piezo-chromic oxides are able to mark pressure or strain by a drastic color change. In this paper, CoMoO4 molybdate compound and related thermoset composites were prepared in lab. The distribution of CoMoO4 oxide in the thermoset matrix was first investigated via SEM. Moreover, the effects on the thermal and mechanical properties with the addition of CoMoO4 oxide were further analyzed. The piezo-chromic characteristic for CoMoO4 incorporated thermoset composites were verified with compression test. The color change can be clear visualized at a strain level close to the yield point of the composite blends. This study has the potential to open new directions to piezo-chromic polymeric materials as early damage detector.

Growth, spectroscopy and laser operation of monoclinic Nd:CsGd(MoO4)2 crystal with a layered structure

We report on the growth, structure, polarized spectroscopy and first laser operation of a double molybdate compound featuring a layered structure – the monoclinic Nd:CsGd(MoO4)2. Single-crystals of 3 at.% Nd:CsGd(MoO4)2 are grown from the flux using cesium trimolybdate (Cs2Mo3O10) as a solvent. They crystallize in the monoclinic system (sp. gr. P2/c) with lattice constants a = 9.5239(4) Å, b = 5.0752(5) Å, c = 8.0580(7) Å and β = 91.157(9)°. Nd:CsGd(MoO4)2 exhibits a perfect cleavage along the (100) plane. Polarized absorption and luminescence spectra of the Nd3+ dopant ion are measured. The maximum stimulated-emission cross-section is 29.0 × 10−20 cm2 at 1065.9 nm for light polarization E ||b. The luminescence lifetime is 145 μs. The transition probabilities are determined using the modified Judd-Ofelt theory yielding the intensity parameters Ω2 = 26.736, Ω4 = 12.736 and Ω6 = 17.771 [10−20 cm2] and α = 0.122 [10−4 cm]. Continuous-wave laser operation is achieved in cleaved single-crystal plates of Nd:CsGd(MoO4)2 yielding a maximum output power of 0.54 W at 1066 nm with a laser threshold of 70 mW, a slope efficiency η of 60.4% and a linearly polarized laser output (E ||b).

Construction of p-n type heterojunction for effective photo-generated electron separation and visible light hydrogen evolution

With the development of research on the photocatalytic water splitting and hydrogen (H2) precipitation, cobalt ferrite and molybdate have begun to appear as catalysts for the study of photocatalytic hydrogen precipitation. In this paper, the p-n heterojunction CoFe2O4/NiMoO4 (CFO/NMO) is prepared by hydrothermal method. The constructed p-n type (type Ⅱ) heterojunction CFO/NMO has the highest hydrogen evolution (Eosin Y as a sensitizer), which is 7.39 times and 1.95 times that of elemental CFO and NMO, respectively. After the CFO and NMO form a heterojunction, a built-in electric field is formed inside to effectively separate the photogenerated carriers. At the same time, the dense interface of the heterojunction also promotes the transfer of photogenerated carriers and promotes the reduction of H+ protons on this catalyst surface. In short, this work provides more possibilities for the field of photocatalytic hydrogen evolution from molybdate and cobalt ferrite compounds.

Cerium molybdate nanocrystals: Microstructural, optical and gas-sensing properties

Metal molybdate compounds have attracted considerable attention due to their technological applications. Herein, we demonstrate the controllable synthesis of cerium molybdate (Ce2(MoO4)3) nanocrystals via the co-precipitation method followed by microwave-assisted hydrothermal (MAH) treatment at 150 °C during different times (15, 30 and 60 min). The effect of MAH treatment time on the microstructural, optical, and ozone gas-sensing properties of these nanocrystals was investigated. X-ray diffraction (XRD) and Raman spectroscopy measurements revealed that the samples presented a single-crystalline phase with scheelite-type tetragonal structure. Field emission gun scanning electron microscopy (FEG-SEM) images showed that the MAH conditions favored changes in the Ce2(MoO4)3 nanocrystal morphology. Photoluminescence (PL) measurements indicated a significant enhancement of PL emission with MAH time, suggesting an increase in the intrinsic defects formed during the MAH treatment. The gas-sensing performance of cerium molybdate nanocrystals towards sub-ppm ozone levels was also investigated. The experiments revealed complete recovery and good repeatability as well as good sensor response, which was improved in the sample synthesized at longer MAH time.

Controllable synthesis of cobalt molybdate nanoarrays on nickel foam as the advanced electrodes of alkaline battery-supercapacitor hybrid devices

Abstract Metal molybdate compounds have attracted considerable attentions for their applications in supercapacitors. Herein, we demonstrate the controllable synthesis of cobalt molybdate (CoMoO4) nanoarrays on nickel foam (NF). The synthesis mechanism and the influencing factors of the morphology and electrochemical properties of CoMoO4 were investigated. It is found that the growth of CoMoO4 nanosheets need sufficient heat and a ‘contact surface’ provided by the conductive substrate for nucleation. Different fabricated conditions make the electrodes exhibit different morphologies and compositions so as to different performances. The CM-140 electrode prepared at 140 °C exhibits the best electrochemical performance due to its unique chemical composition and appropriate nanostructure. It delivers a high capacity of 527.3C g−1 at the current density of 3 mA cm−2. The alkaline battery-supercapacitor hybrid (BSH) device assembled with CM-140 electrode and activated carbon (AC) electrode exhibits high energy density of 40.0 Wh kg−1 at the power density of 685.8 W kg−1 and excellent cycling stability with 86.9% capacitance retention after 5000 successive charge-discharge cycles at the current density of 10 mA cm−2. This work reveals the growth mechanism of CoMoO4 nanosheet arrays, and realizes the controllable synthesis of CoMoO4 nanoarray electrodes with exceptional electrochemical properties for advanced energy storage devices.

Laser operation of cleaved single-crystal plates and films of Tm:KY(MoO4)2.

We report on the crystal growth, spectroscopy and first laser operation of a novel double molybdate compound - Tm:KY(MoO4)2. This orthorhombic (sp. gr. Pbna) crystal exhibits strong anisotropy of the spectroscopic properties due to its layered structure. The maximum stimulated emission cross-section for the 3F4 → 3H6 transition is 2.70×10-20 cm2 at 1856nm with a bandwidth of >110 nm (for E || b). The lifetime of the 3F4 state is 2.29 ms. Crystalline films and plates (thickness down to 70 µm) of high optical quality are obtained by mechanical cleavage along the (100) plane. Continuous-wave diode-pumped laser operation is achieved in such thin films and plates yielding a maximum output power of 0.88 W at ∼1.9 µm with a slope efficiency of 65.8% and a linearly polarized laser output. Vibronic lasing is demonstrated at ∼2.06 µm. Tm:KY(MoO4)2 is promising for microchip and thin-disk lasers.

Effect of W doping on phase transition behavior and dielectric relaxation of CuMoO4 obtained by a modified sol-gel method

Tungsten (W)-doped copper molybdate (CuMo1−xWxO4, x = 0∼0.12) compounds were prepared by the sol-gel method. The effects of doping content on the phase transition behavior and dielectric properties of CuMo1−xWxO4 (x = 0∼0.12) were investigated. X-ray diffraction and UV–vis diffuse reflectance spectroscopy clarified that W doping facilitated the transition of green α-phase to brown γ-phase. The chemical composition and structure of the CuMo1−xWxO4 (x = 0∼0.12) were investigated by Fourier transform infrared (FT-IR) spectroscopy and Raman spectra. The dielectric spectra of CuMo1−xWxO4 at −130 °C∼150 °C and the color-change of CuMo0.94W0.06O4 at 20 °C∼50 °C illustrated that the phase-transition temperature moves toward high temperature with increasing W. The functional relationship between the electrical modulus M’ and the frequency (1 Hz∼10 MHz) indicated that there are two dielectric relaxation mechanisms for CuMo1−xWxO4, which correspond to the polarization relaxation caused by hopping motion of polaron at low-temperature region (R1) and the relaxation dominated by oxygen vacancies at high temperatures (R2). It has also been confirmed that the phase transition of relaxation type exists in CuMo1−xWxO4, and that R1 occurs in γ-phase and R2 occurs in α-phase. It is of great significance to establish the dielectric relationship between phase transition and relaxation. With the content of W, the intensity of relaxation peak and activation energy of R1 did not change too much, but the relaxation behavior of R2 was inhibited and the activation energy increased gradually. The above results show that dielectric spectra are an important discovery as a new method to study the phase transition of materials, and is conducive to exploring the motion state of micro-particles. The control of phase-transition temperature is of great significance for this thermochromic material as a temperature sensor.

Carbon-Free, High-Capacity and Long Cycle Life 1D-2D NiMoO4 Nanowires/Metallic 1T MoS2 Composite Lithium-Ion Battery Anodes.

Both metallic 1T MoS2 and conductive molybdate compounds exhibit interesting electrochemical properties, however, the properties of composite electrodes based on these materials have not been investigated. Here, 1T MoS2 single crystal nanosheets and NiMoO4 single crystal nanowires are synthesized and formed into a carbon-free composite lithium-ion anode using blade- and spray-coating. The composite anodes deliver charge mass specific capacity of 940.1 mAh g-1, while the discharge mass specific capacity is up to 941.6 mAh g-1, with a capacity retention ratio of 84.2% after 750 cycles. The charge and discharge volumetric capacity (porosity of 15.6%, full electrode basis, excluding the current collector) are 1238.7 mAh cm-3 and 1240 mAh cm-3, respectively, and the active materials volume fraction is 82.5%. These capacities significantly exceed that of single 1T MoS2 or single NiMoO4 anodes we reported. We calculate if matched vs a cathode with an average discharge voltage of 4.0 V the gravimetric energy density of the composite electrodes would be 3389.8 Wh kg-1. Electrochemical measurements indicate that the composite electrode has excellent electrochemical reversibility, suggesting that the structure has played a crucial role in the cycling process.

Ferroelectric properties and alternative current conduction mechanisms of lithium rubidium molybdate

The lithium rubidium molybdate compound LiRbMoO4 was prepared by the solid-state reaction method. XRD spectra revealed a single phase material with an orthorhombic structure. The internal Raman modes were observed at room temperature. The dielectric measurement exhibited three subsequent phase transitions: two structural transitions at the neighborhood of 397 K and 420 K followed by a ferro-paraelectric transition at 424 K. The Nyquist plot was proved to be a non-Debye relaxation mechanism. The combined spectroscopic plots of the imaginary part of electric impedance and modulus in the temperature range of 581–718 K at various frequencies confirmed the non-Debye behavior. The AC conductivity was found to follow the Jonscher’s universal dynamic law ωS, and the overlapping large polaron tunneling model (OLPT) was proposed to describe the conduction mechanism.

Highly Efficient Transparent Nanophosphor Films for Tunable White-Light-Emitting Layered Coatings

Bright luminescence in rare-earth (RE) nanocrystals, the so-called nanophosphors, is generally achieved by choosing a host that enables an effective excitation of the RE activator through charge or energy transfer. Although tungstate, molybdate, or vanadate compounds provide the aforementioned transfer, a comparative analysis of the efficiency of such emitters remains elusive. Herein, we perform a combined structural and optical analysis, which reveals that the tetragonal GdVO4 matrix gives rise to the highest efficiency among the different transparent nanophosphor films compared. Then, we demonstrate that by a sequential stacking of optical quality layers made of Eu3+- and Dy3+-doped nanocrystals, it is possible to attain highly transparent white-light-emitting coatings of tunable shade with photoluminescence quantum yields above 35%. Layering provides a precise dynamic tuning of the chromaticity based on the photoexcitation wavelength dependence of the emission of the nanophosphor ensemble without altering the chemical composition of the emitters or degrading their efficiency. The total extinction of the incoming radiation along with the high quantum yields achieved makes these thin-layered phosphors one of the most efficient transparent white converter coatings ever developed.

Comparing the Efficiency of N-Doped TiO2 and N-Doped Bi2MoO6 Photo Catalysts for MB and Lignin Photodegradation

In this study, we tested the efficiency of nitrogen-doped titanium dioxide (N-TiO2) and nitrogen-doped bismuth molybdate (N-Bi2MoO6) compounds as photocatalysts capable of degrading methylene blue and lignin molecules under irradiation with ultraviolet (UV) and visible light (VIS). Moreover, we compared TiO2 and Bi2MoO6 catalysts with N-TiO2 and N-Bi2MoO6 compounds using chemical coprecipitation. The catalysts were prepared starting from Ti(OCH2CH2CH3)4, Bi(NO3)3·5H2O, and (NH4)6Mo7O24 reagents. N-doping was achieved in a continuous reflux system, using ethylene diamine as a nitrogen source. The resulting materials were characterized using Scanning Electron Microscopy (SEM), X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Additionally, we observed the decrease in particle size after processing the compounds in the reflux system. The results regarding photocatalytic degradation tests show a remarkable effect for nitrogen doped samples, achieving 90% of lignin degradation.

Passivation of T2 Cu and QCr0.5 Cu-alloy with Chromate-free Solutions of Molybdate Compound

Passivation of T2 Cu and QCr0.5 Cu-alloy with Chromate-free Solutions of Molybdate Compound