Evaporation Of MoO3 Research Articles

Hot corrosion behavior of Mo–9.0Si–8.0B

AbstractIn recent years, interest in Mo‐based alloys has increased significantly due to their promising high‐temperature behavior and their possible uses for industrial high‐temperature applications such as within turbines. An important corrosion mechanism in turbines is the sulfate salt‐induced attack of the structural materials, known as hot corrosion. Therefore, the Type I and II hot corrosion behavior of Mo–9.0Si–8.0B was investigated at 700°C and 900°C for 24 and 100 h. The Mo‐based alloy showed severe degradation, but not by sulfates. Thus, the underlying hot corrosion mechanism through molybdate fluxing, which is accompanied by Mo oxide evaporation, is discussed in detail. At the higher temperature, the MoO3 evaporation becomes predominant, leaving behind a Si‐glass with molybdate inclusions.

Development of molybdenum trioxide based modified graphite sheet electrodes for enhancing the electrochemical sensing of dopamine

Electrochemical detection of dopamine is crucial in the early stages of Parkinson's and Alzheimer's disease diagnosis. In this paper, we have developed modified electrodes based on molybdenum oxide (MoO3) grown on graphite sheet (GS), which was investigated for its ability to detect dopamine sensitively and selectively. The MoO3/GS electrodes were prepared by thermal evaporation of MoO3 on GS sheets followed by annealing at different temperatures (RT and annealing temperatures of 100 °C, 200 °C, 300 °C, 400 °C and 500 °C). The electrodes were analysed using XRD, Raman, FESEM and HRTEM. Compared to other electrodes, the modified MoO3/GS electrode annealed at 500 °C showed more significant charge transfer and increased selectivity and sensitivity in the electrochemical detection of dopamine. The findings showed a significant sensitivity of 85.71 μA. μM−1·cm−2 for the linear range of 1–10 nM and the lowest detection limit of 2.71 nM were achieved from the dopamine sensing. The developed electrode successfully detected dopamine in human blood serum and urine samples with respectable recovery rates.

Improved corrosion resistance of TiAlCrNbMo alloy to lead-bismuth eutectic by pre-oxidation

A Ti48.5Al46Cr3Nb0.5Mo2 alloy was designed and fabricated to investigate its corrosion resistance to static liquid lead-bismuth eutectic (LBE). The microstructure and corrosion behavior of un-oxidized and pre-oxidized samples were compared. The results show that LBE preferentially penetrated into the alloy along the CrMo-rich σ phase formed at high-temperature. A TiO2 and Al2O3 oxide layer was formed on the surface of the matrix after 2 h pre-oxidized leading to remarkable LBE resistance. When extended the pre-oxidized time to 4 h, the exfoliation of the oxide layer caused by the evaporation of MoO3 significantly weakens the corrosion resistance.

Large‐area growth of monolayer MoS2 by using atmospheric‐pressure chemical vapor deposition with nucleation controlling process

The epitaxial growth of high‐quality and large‐area monolayer (ML) MoS2 has attracted widespread attention in recent years. Here, MoS2 on Al2O3 substrate was prepared by using atmospheric‐pressure chemical vapor deposition. The process parameters such as temperature difference and distance between MoO3 and Al2O3, nucleation temperature, and heating rate were optimized. The high MoO3 evaporation temperature facilitates Mo vapor transport to grow ML‐MoS2. The introduction of nucleation temperature facilitates the deposition of MoS2, and the morphology of the nucleation point is related to the amount of MoO3‐x deposited on Al2O3. This process optimization method is also applicable to growth MoS2 on SiO2. The ML‐MoS2 with a size greater than 1 cm2 was successfully fabricated on Al2O3 and SiO2, which laid a foundation for the practical application of ML‐MoS2.

Optimized Solid-State Synthesis of Sr2Fe1.5Mo0.5O6−δ Perovskite: Implications for Efficient Synthesis of Mo-Containing SOFC Electrodes

Sr2Fe1.5Mo0.5O6−δ (SFMO) perovskite has been considered as a promising anode candidate for solid oxide fuel cells. However, the significant inconsistency in the conductivity properties of SFMO perovskite has been reported in the literature through various synthesis procedures, highlighting the necessity of a standard and unified synthesis process. In this work, we propose an optimized solid-state synthesis process of SFMO perovskite based on the thermal properties of the precursors. Our TG analysis indicates that the evaporation of MoO3 during sintering over 752 °C may affect the synthesis of the expected SFMO perovskite. The presence of Fe2O3 has a trap effect on MoO3, based on the TG analysis of the binary mixture. A cubically structured SFMO perovskite without a secondary phase is obtained from the as-proposed stepwise sintering program while an impurity phase of SrMoO4 is observed when adopting a direct sintering program. The as-synthesized SFMO perovskite exhibits high stability in a reducing atmosphere, which is attributed to the self-adjustment of the overall valence states of molybdenum ions and iron ions. Many pure cubically structured perovskites have been successfully synthesized using the as-proposed solid-state synthesis process, suggesting its universality for the synthesis of other Mo-containing SOFC perovskite electrodes.

Shape-controlled monolayer MoSe2 flakes by chemical vapor deposition towards tuning the photoluminescence emission

In this study, we report on the controllable chemical vapor deposition (CVD) synthesis of monolayer MoSe2 flakes with different shapes such as hexagons, triangles, sawtooth hexagons, dendrites and fractals deposited on SiO2/Si substrates. This broad range of morphologies is due to the change in the vapor composition resulting from the confinement of the MoO3 and Se vapor and to the variation in the growth rate of the variable shaped MoSe2 crystals. It is also revealed that the photoluminescence (PL) response of the MoSe2 flakes is strongly affected by their shape and size. Our findings will open new avenues towards achieving morphology-controlled monolayer MoSe2 flakes for optoelectronic and energy harvesting systems.

Temperature dependence of Raman and photoluminescence spectra of pure and high-quality MoO3 synthesized by hot wall horizontal thermal evaporation

In this work, a novel system for the hot wall evaporation of MoO3 in a horizontal furnace and at low vacuum atmosphere was settled. Pure MoO3 material was obtained without the presence of other phases and with a strong preferential orientation, as revealed by electron microscopy, micro-Raman spectroscopy and x-ray diffraction. Photoluminescence and Raman spectra were measured as a function of temperature. As a result, the first-order temperature coefficient of the fifteen measured vibrational modes is reported and compared with the scarce literature data available. The relation of these coefficients with the thermal expansion coefficient is discussed in the framework of the Grüneisen model. Moreover, for the first time, the temperature dependence of the different contributions of photoluminescence spectra for MoO3 is reported, and an anomalous increase in PL intensity of all transitions is observed in the range of 40–140 K, followed by a normal quenching at high temperatures. The Shibata model was used to interpret the origin of this anomalous effect.

Novel cordierite-acicular mullite composite for diesel particulate filters

Cordierite-acicular mullite composites containing 0, 25, 50, 75 and 100 wt% of mullite were fabricated from waste MoSi 2 and commercial powders of Al 2 O 3 and spinel (MgAl 2 O 4 ). Careful oxidation of pulverised waste MoSi 2 rendered a precursor mixture of MoO 3 and amorphous SiO 2 , which served as pore forming agent and SiO 2 source, respectively. Evaporation of MoO 3 at ∼750 °C allowed production of highly porous cordierite-mullite ceramic composite after sintering in air at 1350 °C for 4 h. The combination of equiaxed cordierite grains and elongated (prism-like) mullite grains, resulted in unique microstructure with open porosity between 53.3 and 55.6 vol% which makes the obtained composite convenient for application as diesel particulate filter material. The presence of mullite affected four key thermo-mechanical properties which determine the thermal shock resistance of cordierite-mullite composite. The best thermal shock resistance was measured in composite containing 75 wt% of mullite. It was a result of improved thermal conductivity (1.081 W/mK) and bending strength (3.62 MPa) and relatively low values of coefficient of thermal expansion (3.8 × 10 −6 K −1 ) and elastic modulus (2.27 GPa).

Oxidation mechanism of refractory high entropy alloys Ta-Mo-Cr-Ti-Al with varying Ta content

The effect of Ta on the oxidation resistance of the alloys Tax-(Mo-Cr-Ti-Al)1−x (x=0; 5; 10; 15; 20 at%) in air at 1200 °C was examined. The oxidation behavior improves continuously with the increasing Ta concentration. Alloys with Ta concentrations Ta≤10 at% exhibit poor oxidation. Severe evaporation of MoO3 occurred which was quantitatively estimated. The evaporation of MoO3 is drastically reduced in alloys with the higher Ta concentrations. At 15 and 20 at% Ta, protective CrTaO4 scales are found. The CrTaO4 scale formed on Ta20-(Mo-Cr-Ti-Al)80 grows according to the parabolic rate law as a result of oxygen inward diffusion.

MoO3 films grown on stepped sapphire (0001) by molecular beam epitaxy

MoO3 films were grown on stepped c-plane sapphire substrates by molecular beam epitaxy using MoO3 vapor from a conventional Knudsen cell. Stepped sapphire (0001) substrates were prepared by ex situ annealing at 1100–1300 °C in dry air. Step bunching typically resulted in multistepped surfaces with wide atomically smooth terraces. Ex situ annealing at 1100 °C followed by in vacuo annealing at 700 °C provided clean substrates for growth. Ultrathin films were grown at 450 °C via a self-limiting process that represents a balance between the incident MoO3 flux and the desorption flux. Elongated bilayer islands (0.7-nm thick) were formed on sapphire (0001) terraces. Monocrystalline α-MoO3 (010) thin films [(010)α-MoO3∥(0001)sapphire] were grown at 450 °C using a higher incident MoO3 flux and characterized by atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and cross-sectional transmission electron microscopy. The step-terrace surface morphology of the monocrystalline films strongly suggests multilayer growth.

Chemical vapor deposition of clean and pure MoS2 crystals by the inhibition of MoO3−x intermediates

A sublimated sulfur vapor versus evaporated MoO3 vapor with transition from low to high sulfur supersaturation would cause different CVD products transformed from MoS2–MoO3−x composites to clean and pure MoS2 crystals accordingly.

High-Temperature Oxidation Resistance of CrMoN Films

The CrMoN film is deposited on the surface of M2 tool steel using the closed-field unbalanced magnetron sputtering technology. We measured the surface morphology, composition, phase structure, hardness and film/base bonding strength of CrMoN film using a scanning electron microscope, x-ray diffractometer, nano-indenter and scratch tester after oxidation at room temperature, 500, 600, and 700 °C for 1 h. The results of this study demonstrated that the crystal structure and mechanical property of CrMoN film at room temperature are maintained after oxidation at 500 °C for 1 h. In addition, with the increase in oxidation temperature, there is an increase in the evaporation of MoO3 on the film surface and the oxidation degree of the film along with the appearance of fine pores. Moreover, the mechanical property and film/base bonding strength of CrMoN film significantly decrease after oxidization at 700 °C for 1 h.

Sulfidation characteristics of amorphous nonstoichiometric Mo-oxides for MoS2 synthesis

In this study, the thermal sulfidation characteristics of two amorphous nonstoichiometric Mo-oxide (MoO3−x) films were investigated that were deposited using e-beam evaporation of MoO3 and MoO2 powders. It was observed that evaporation of MoO2 produced an amorphous MoO3−x film that exhibited a greater number of low oxidation states than that obtained by MoO3 evaporation. Moreover, subsequent sulfidation allowed the formation of a continuous MoS2 film; meanwhile, the MoO3-evaporated sample transformed into a discontinuous MoS2 film. Both the initial thickness of the MoO2-evaporated film and the sulfidation temperature were varied to determine their effects on the final MoS2 film quality. The sulfidation of an approximately 1-nm-thick MoO2-evaporated film at 780 °C produced a predominantly monolayer MoS2 film. Further, an operation of the bottom-gated transistor arrays using the synthesized MoS2 film was demonstrated along with a preliminary experimental result for potential application in three-dimensional device integration.

Facile synthesis of hetero-structured few-layer MoS2-coated MoO2 as superior anode materials of lithium ion batteries

A hetero-structured few-layer MoS2-coated MoO2 (MoS2@MoO2) has been fabricated and applied as an anode material for LIBs. The MoS2@MoO2 was synthesized via two-step fire process: reducing MoO3 vapor with H2 for producing Mo powder, and oxidizing-sulfurizing Mo powder with SO2. With the aid of thermodynamic computations, it is found that Mo oxidation under SO2 atmosphere produces limited amounts of S2 vapor, which will locally sulfurize the particles nearby, resulting in finite and even sulfurization. The as-prepared MoS2@MoO2 includes few-layer MoS2 (about 3 nm thick) covering the surface, which serves a well-spreading and active material that synergizes with inner MoO2 to boost the performance for LIBs. Thus, the MoS2@MoO2 could deliver a high reversible specific capacity of 1263 mA h g−1 after 40 cycles at 0.1 A g−1, and maintain a specific capacity of 1052 mA h g−1 after 150 cycles at 0.2 A g−1. Even at 0.5 A g−1, the specific capacity could still keep above 650 mA h g−1 for over 500 cycles. The new method for MoS2@MoO2 synthesis is facile and effective, and may be extended to produce hybrids composed of other transition metal oxides and sulfides.

Spectroscopic properties of nanostructured molybdenum oxysulfide deposits fabricated by MoO3 evaporation in H2S

Nanostructured molybdenum oxysulfide layers were prepared via MoO3 evaporation in H2S at 800 °C. Transmission and scanning electron microscopy (SEM, TEM) and UV/Vis spectroscopy revealed a nano- and microstructure nature of the layers. Despite the significant oxygen content (up to 30 at. %), the qualitative estimation of some analytical measurements (UV/Vis, Raman and X-ray photoelectron spectroscopy) showed a close link to MoS2 structure. As the elemental ratio Mo:(O + S) was ≈2, most of the oxygen atoms substituted sulphur in MoS2 structure. On the other hand, luminescence spectra addressed defects and a band gap structure similar to MoO3.

Thermodynamics guided ultrafast and continuous preparation of Mo2C nanocrystals for hydrogen evolution electrocatalysis

Existing nano-Mo2C preparation methods suffer from expensive raw materials, subtle operation demands and complex synthesis steps. Herein, both theoretical and experimental studies were performed to develop a more efficient and economical method for nano-Mo2C preparation. Reaction thermodynamics of gas-vapor system of CH4-MoO3-N2 indicates that high-proportion inert gas significantly increases the required carbonization potential (CO/CO2) for Mo2C formation, but surplus carbon produced from methane decomposition can trigger water gas producing reaction and Boudouard reaction to help meet the requirement. Nano-Mo2C was then synthesized by mixing N2-carried MoO3 vapor with CH4 under varying conditions including temperature, gas flowrate and vapor concentration. It is found that MoO3 vapor experienced conversions to monoclinic MoO2 to cubic Mo to hexagonal Mo2C, and all these stages could be finished in as short as about 6 s at 1100 °C with initial molar ratio of CH4 to MoO3 over 3.5. The final products were present as porous plate-like single crystals or chain-like polycrystals, which are determined by initial vapor concentration and gas flowrate. Furtherly, electrocatalytic properties of Mo2C nanocrystals for hydrogen evolution reaction were measured, and the sample with the largest specific surface area and cleaner surface showed competitive performance over reported nano-Mo2C.

Cyclic oxidation behavior of MoAlB in the temperature range 450–850 °C

Abstract Mo-Si-B alloys suffer from the materials disintegration below 800 °C due to the formation of a porous oxide scale and the evaporation of MoO3, known as Mo pesting. The nanolaminated ternary boride compound of MoAlB contains both Mo and B elements, hence, pesting is expected at temperatures below 800 °C as in Mo-Si-B alloys. However, the oxidation behavior of MoAlB at moderate temperatures has been less studied so far. This work reports on the cyclic oxidation behavior of MoAlB in air in the temperature range 450–850 °C over a time of up to 100 h. The results showed an excellent oxidation resistance of MoAlB at temperatures of 750 °C and 850 °C due to the formation of a dense and continuous Al2O3 scale, which acts as a barrier layer preventing oxygen inward diffusion and the evaporation of MoO3 and B2O3 oxides. No pesting was found in the MoAlB material during oxidation at temperatures below 850 °C. The phase composition and microstructure of the oxidized samples were characterized by x-ray diffraction and scanning electron microscopy techniques.

Unprecedented oxidation resistance at 900 °C of Mo–Si–B composite with addition of ZrB2

The oxidation behavior of Mo–12Si-8.5B and Mo–12Si-8.5B–ZrB2 composites was studied at 900 °C in air. The Mo–12Si-8.5B composite, presented the significant weight loss of 180 mg/cm2 during oxidation for 30 h at this low temperature. This was largely attributed to the formation of low viscosity borosilicate glass scale on its surface, which allowed the continuous evaporation of MoO3. However, such degree of significant oxidation was controlled dramatically by the addition of ZrB2 to the Mo–12Si-8.5B and it further significantly improved the oxidation resistance of the composite. In particular, the Mo–12Si-8.5B–ZrB2 composite presented the small weight gain of 10 mg/cm2 after oxidation for 30 h; moreover the oxidation trend of Mo–12Si-8.5B–ZrB2 was stable and its behavior was opposed to that of the ZrB2-free composite. In addition, the drastically decreased thicknesses of the silicate scale and internal oxide layer were observed in the ZrB2-contained composite. Owing to the addition of ZrB2, which increased the viscosity of the silica glass, a protective Zr-rich silicate glass scale formed and passivated the composite against further oxidation. This was supplemented by the ZrO2/ZrSiO4 products that further prevented the Mo or Mo oxides from evaporating and hindered the inward diffusion of oxygen.

From nanoparticles to nanofilms: exploring effects of Zn addition for nanostructure modification and photoluminescence intensification of MoO3−x nanomaterials

Exploring the fast synthesis of Zn-doped MoO3−x nanostructures in a N2 environment by the hot filament chemical vapour deposition using a mixture of Zn and MoO3 powders is reported, whereas targeted transformation of nanomaterials is done via Zn addition in MoO3 powder. During this process, MoO3−x nanostructures convert from nanoparticles to nanofilms. The structural conversion of MoO3−x nanostructures is the results of the predominant diffusion of MoO3−x and MoO3 molecules along the substrate surface caused by reduced evaporation of MoO3 due to the melting of Zn particles. At room temperature, the pure and Zn-doped MoO3−x nanostructures generate the photoluminescence (PL) emission in the ultraviolet to infrared range, while the PL emission in the visible range has a great change at low temperature. The PL emission is related to the bandgap transition, Mo5+ d–d transition, intervalence charge transfer transition, and transition between the intermediate and valence bands. Furthermore, the PL emission of MoO3−x nanostructures is enhanced by Zn doping. The strong PL emission from Zn-doped MoO3−x nanostructures compared to pure MoO3−x nanostructures results from the increase of oxygen vacancies caused by the Zn incorporation. The enhancement of PL emission at low temperature compared to room temperature is due to the increase of vacancy concentration, weak lattice vibration and relaxation of polarons. Our present results could be used to control the structures of MoO3−x nanomaterials and impact the development of optoelectronic devices based on MoO3 nanomaterials, such as organic solar cells, photovolatic devices and light-emitting diodes.

Fabrication of Y6MoO12 molybdate ceramics: From synthesis of cubic nano-powder to sintering

The family of cubic yttrium molybdate (Y6MoO12) has been chosen due to the possibility of substituting Y3+ cations by luminescent trivalent rare earth (RE3+) ones and to obtain transparent optical ceramics. The goal of the paper is to find a route towards the fabrication of dense Y6MoO12 ceramics from nano-powders and using well-known ceramic sintering techniques such as Spark Plasma Sintering (SPS) and Hot Isostatic Pressing (HIP). We have investigated in detail process of the powder preparation by a combustion method as well as the sintering processes taking into account all important sintering parameters. Evaporation of MoO3 was found to be correlated to reductive conditions or heating rate. Sintering was optimized in terms of phase composition, microstructure and porosity.