Evaporation Of MoO3 Research Articles

Hybrid Physicochemical Synthesis and Electrochromic Performance of WO3/MoO3 Thin Films

AbstractMolybdenum oxide (MoO3) mixed tungsten oxide (WO3) thin films have been prepared by hybrid physico‐chemical synthesis involving microwave assisted sol‐gel synthesis (MW‐SGS) of WO3 followed by vacuum evaporation of MoO3 on fluorine doped tin oxide (FTO) coated glass substrates. The optostructural and morphological properties of WO3 thin films with different thickness of MoO3 layer were investigated and studied for enhanced electrochromic coloration. The electrochromic performance of these samples was studied in LiClO4/PC as electrolyte for Li+ insertion/extraction. Colorimetric analysis was done to define the colors in the bleached and colored states in terms of Lab* values and xy chromaticity diagram. Enhancement in electrochromic properties in terms of optical modulation (ΔOD), coloration efficiency (CE) and switching time (tb and tc) was studied and reported.

Stability of graphene doping with MoO3 and I2

We dope graphene by evaporation of MoO3 or by solution-deposition of I2 and assess the doping stability for its use as transparent electrodes. Electrical measurements show that both dopants increase the graphene sheet conductivity and find that MoO3-doped graphene is significantly more stable during thermal cycling. Raman spectroscopy finds that neither dopant creates defects in the graphene lattice. In-situ photoemission determines the minimum necessary thickness of MoO3 for full graphene doping.

Oxidation mechanisms of lanthanum-alloyed Mo–Si–B

High temperature oxidation behavior of a Mo–9Si–8B (at.%) alloy containing 0.2 and 2at.% La was studied between 750 and 1400°C in static air. Isothermal thermogravimetric analysis for up to 72h indicated a significant improvement in the oxidation resistance of the 2La-alloyed material in the 750–1000°C temperature range. The oxide layers characterized by XRD, SEM, EDS, and FIB-TEM revealed the formation of more stable lanthanum–molybdates (xLa2O3⋅yMoO3), which reduces the evaporation of MoO3 below 1000°C. The thickness data obtained for the external scale and the inner oxidation layers indicated a reduction of inward diffusion of oxygen with higher La concentrations.

Anti-ablation properties of MoSi2–W multi-layer coating system deposited by atmospheric plasma spray

Carbon fibre reinforced carbon (C/C) composites have become an important material for hyper thermal conditions and have been widely used in aerospace and aviation industry. However, the composites will be ablated above 723 K in an oxygen-containing atmosphere. The MoSi2–W multi-layer anti-ablation coating system was deposited by atmospheric plasma spray. The supersonic flame caused by kerosene and oxygen was selected to examine the property of the coatings. Samples of three groups were ablated by the flame at 1600°C. The speed of gas flow was 850 m s–1. The MoSi2–W multi-layer anti-ablation coating system can protect the graphite matrix effectively. The products of ablation include SiO2, Mo5Si3 and MoO3. With the prolongation of time, the aggregates sinter and grow up. The network structure of the coatings is caused by the evaporation of MoO3 and loss of SiO2.

Effect of Ti (Macro-) Alloying on the High-Temperature Oxidation Behavior of Ternary Mo–Si–B Alloys at 820–1,300 °C

The aim of the present investigation was to gain an initial understanding of the effect of (macro-) alloying with Ti on the oxidation behavior of Mo–Si–B alloys in the ternary phase region of Mo_ss–Mo3Si–Mo5SiB2 at 820–1,300 °C. Motivated by recent studies and thermodynamic calculations, the alloy compositions Mo–9Si–8B–29Ti (at.%) and Mo–12.5Si–8.5B–27.5Ti (at.%) were selected and synthesized by arc-melting. Compared to the reference alloy Mo–9Si–8B, superior initial oxidation rates at 1,100–1,300 °C as well as a significant density reduction by nearly 18 % were observed. Due to enhanced initial evaporation of MoO3 and mainly inward diffusion of oxygen, a borosilicate-rutile duplex scale with a continuous TiO2 phase had formed. Detailed investigations of the oxidation mechanism by SEM, EDX, XRD and confocal micro-Raman spectroscopy indicated that Ti alloying is promising with regard to further improvement of the oxidation resistance as well as the strength-to-weight ratio of Mo–Si–B alloys.

Oxidation Behavior of Nb-20Mo-15Si-25Cr and Nb-20Mo-15Si-25Cr-5B Alloys

Nb-20Mo-15Si-25Cr (25Cr alloy) and Nb-20Mo-15Si-25Cr-5B (25Cr/5B alloy) alloys have been subjected to oxidation in air for 24 hours from 973 K to 1673 K (700 °C to 1400 °C). Even though B additions do not improve oxidation resistance at temperatures higher than 1473 K (1200 °C), the lower temperature oxidation resistance is superior with B by influencing the microstructure. Porous oxide scale development at lower temperatures has been attributed to the dominant growth of Nb2O5 and the vaporization of MoO3. An intermediate oxidation layer is developed between the scale and the metal for the 25Cr/5B alloy at temperatures above 1173 K (900 °C). Scale densification at elevated temperatures results in higher stress development as a result of the mismatch of coefficients of thermal expansion, ultimately resulting in oxide spallation.

Effects of 7·5 vol.-% water vapour on oxidation of nickel based alloy between 900 and 1100°C

A Ni based SY 625 alloy was oxidised at 900, 1000 and 1100°C under dry and wet conditions. Water vapour has little effect on the oxidation rate and scale composition. At 900 and 1000°C, the outer scale is composed of Cr2O3, and a continuous NbNi4–Ni3Mo subscale is found at the oxide/alloy interface. At 1100°C, the scale is composed of an outer chromia scale and an internal CrNbO4 subscale. Nevertheless, the oxide scale morphology differs between dry and wet conditions. Under dry conditions, the oxide scale appears to be compact, and chromia pegs are observed at the internal interface. The oxide scales formed under wet conditions show that porosities spread inside the scale, and the chromia grain size is smaller. At 1100°C, some scale spallation is observed under dry and wet conditions probably due to the molybdenum oxidation, leading to MoO3 evaporation and void accumulation at the internal interface.

Oxidation of Mo/Al2O3 hybrid materials at high temperatures

In the present work, the oxidation behaviour of 5 vol% Mo/Al2O3 hybrid materials was investigated at high temperatures in air. At oxidation temperature ranging from 600 to 800°C, Al2(MoO4)3 and MoO3 were developed. The growth of the oxidized zone, at which metallic Mo was not observed, obeyed the parabolic manner. The apparent activation energy of the parabolic rate constant was equal to 95 kJmol−1. The microstructures showed that there were many voids in the oxidized zone. Significant cracks, which would be formed due to volume expansion of Mo particles via inward diffusion of oxygen, were not observed in oxidized zone. They must be the evidence of outward diffusion of cations, most likely Mo6+ at grain boundary. As the temperature over 800°C, the evaporation of MoO3 occurred dramatically.

MoO3 Evaporation Studies from Binary Systems towards Choice of Mo Precursors in EAF

AbstractThe evaporation rate of molybdenum oxide from mixtures with CaO or MgO was studied in the temperature range 300‐1573 K. The investigations were carried out using high temperature X‐ray diffraction and thermogravimetry. Further, additions of these precursors to molten steel in the laboratory scale and the Mo yield achieved were determined. The X‐ray studies show that the calcium molybdate is formed from the oxide mixture in the temperature interval 773‐873 K, which precedes the beginning of evaporation of MoO3. Results of thermogravimetric studies with mixtures CaO and MgO with MoO3 as well as the compounds CaMoO4 and MgMoO4 confirm the above results. Addition of various molybdenum precursors, viz. the mixtures of carbon with pure MoO3, CaMoO4 and MgMoO4, as well as oxide mixtures (CaO + MoO3, MgO + MoO3) show that the highest yield was observed for CaMoO4 + C and MoO3 + C mixtures, while MgO + C + MoO3 mixture showed much lower yield.

Volumetric flame synthesis of well-defined molybdenum oxide nanocrystals

Well-defined faceted inorganic Mo oxide nanocrystals are synthesized in the gasphase using a solid-fed-precursor flame synthesis method. The solid crystals haverectangular cross-section with characteristic size of 10–20 nm and with lengths rangingfrom 50 nm to a few hundred nanometres. A 1 mm diameter high purity Mo probeintroduced in the oxygen-rich part of the flame serves as the material source. Acombination of the strong temperature gradient and varying chemical speciesconcentrations within the flame volume provides the ideal conditions for the rapidand direct formation of these unique nanocrystals. Oxidation and evaporation ofMoO3 in the oxygen-rich zone are followed by reduction toMoO2 in the lower temperature, more fuel-rich zone. TheMoO3 vapours formed are pushed in the direction of the gas flow and transformed into maturewell-defined convex polyhedron nanocrystals bounded with six faces resembling rectangularparallelepipeds.

Fullerene-like MoS2 Nanoparticles and Their Tribological Behavior

Using a new quartz-made reactor, large amounts of fullerene-like (IF) MoS2 nanoparticles were synthesized by reacting MoO3 vapor with H2 Si n a reducing atmosphere. The nanoparticles were found to be of high crystalline order; with an average size of 70 nm and consist of more than 30 closed shells. Extensive tribolog- ical testing of the nanoparticles in two types of synthetic oils- poly-alpha olefins (PAO)- was carried out and com- pared to that of bulk (2H platelets) MoS2 and IF-WS2. These tests indicated that under high pressure and rela- tively low humidity, the IF-MoS2 exhibited a friction coefficient as low as 0.03 and the smallest wear rate of the measured systems. However, its performance was found to be lower in comparison to IF-WS2 after 2500 cycles, due probably to its inferior chemical stability. This study indicates that the tribological performance of the IF nanoparticles depends strongly on their crystalline order and size.

Nanostructures of Crystalline Molybdenum Trioxide Grown by Condensation in a Carrier Gas

Molybdenum trioxide nanostructures were grown by direct evaporation of MoO3 from a tungsten boat resistively heated in the presence of hydrogen or helium as carrier gas at pressures from 100 to 600 Pa. Crystalline structures such as, nanoribbons, nanofibers, nanoneedles and nanoparticles were obtained at source temperatures below 900 degrees C. On the other hand, at source temperatures above 1000 degrees C, nanoporous structures were obtained. The latter were found more often when hydrogen was used as carrier gas.

A study on the anti-electron-bombing life of La 2O 3–Y 2O 3–Mo cermet cathode materials

Anti-bombing insensitivity of La 2O 3–Y 2O 3–Mo cermet cathode material has been studied in this paper. The variation of maximum secondary emission coefficient δ max with time was measured. The cathode after life experiment was analyzed by means of SEM, EDS and XRD. The results showed that δ max of La 2O 3–Y 2O 3–Mo cermet cathode operating at 1100 °C under continuous electron bombardment of 300 W/cm 2 reached the maximum 3.35 at 200 h, and then decreased gradually. After bombardment for 500 h, the maximum secondary electron yield remained nearly the same. The δ max of La 2O 3–Y 2O 3–Mo cermet cathode was still about 2.5 after 1000 h operation, indicating that this kind of cathode had good anti-bombing insensitivity. The high δ max of La 2O 3–Y 2O 3–Mo cathode was related to the rare earth oxide layer on the surface at the high temperature and the amount of nanosized La 2O 3 particles on the Y 2O 3 layer. The decrease of the amount of La 2O 3 in the surface layer and pores in the cathode produced by the evaporation of MoO 2 which is the oxidation product of Mo, led to the decrease of δ max.

The development of protective borosilicate layers on a Mo-3Si-1B (weight percent) alloy

Molybdenum-based alloys with the addition of small amounts of silicon (2 to 4.5 wt pct) and boron (∼1 wt pct) can form a passivating layer that protects the alloy from further rapid oxidation. When such molybdenum-based alloys are exposed to oxidizing environments at high temperatures, a borosilicate glass layer can form that will reduce the transport of oxygen to the alloy to limit further oxidation. Oxidation is then controlled by diffusion through the borosilicate glass layer. The focus of this research was to study the development of the borosilicate layer on a Mo-3Si-1B (wt pct) alloy. The oxidation of this alloy was studied in a variety of gas environments over a range of temperatures in order to elucidate the critical factors that allow it to develop a protective borosilicate glass layer. The borosilicate glass layer is protective when no continuous channels exist in the layer extending from the gas interface to the alloy interface. The borosilicate layer is believed to contain channels in the early stages of development, and the elimination of the channels is obtained by appropriate control of the temperature and gas-flow conditions, whereby MoO3 is removed via vaporization while the borosilicate viscosity is not increased due to loss of B2O3. Once the borosilicate layer is continuous and free of channels, subsequent oxidation occurs by inward diffusion of oxygen and outward diffusion of molybdenum through this layer, with vaporization of MoO3 occurring at the gas/borosilicate-layer interface and MoO2 and additional borosilicate forming at the alloy/MoO2 interface.

Oxidation behavior of multiphase Nb-Mo-Si-B intermetallics

The mixed substitution of Nb and Mo in the ternary systems Mo-Si-B and Nb-Si-B was studied with the goal of balancing oxidation resistance with mechanical behavior. The microstructure and oxidation behavior of six compositions in the Nb-Mo-Si-B system were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy, electron probe microanalysis (EPMA), and thermogravimetric analysis. Proper selection of the total metal content and the Nb/Mo ratio results in the co-existence of a T1 phase, as (Nb, Mo)5Si3Bx, and a solid solution (Nb,Mo) metal phase. At 800 °C, all compositions exhibited catastrophic oxidation, while changing to a quasi-steady-state mass gain at 1200 °C. The high rate constants at 1200 °C indicate that the scales formed were not passivating. A complex scale consisting of four layers formed that was about 350-to 450-µm thick after oxidation for 50 hours at 1200 °C. Borosilicate glass did form within the scale, but the significant prevalence of Nb2O5 within the glass, and the resulting inability of the glass to seal pores formed by the evaporation of MoO3, contributed to the overall poor oxidation resistance compared to the ternary Mo-Si-B system. The Nb and Mo content of the alloy must be further studied and optimized before these alloys may be considered for further development for hightemperature applications.

High temperature oxidation behavior of a Mo–3Si–1B(wt%) alloy

Molybdenum base alloys with the addition of small amounts of silicon (2–4.5 wt.%) and boron (∼1 wt.%) can form a passivating layer protecting the alloy from further rapid oxidation. When such molybdenum base alloys are exposed to oxidizing environments at high temperatures, a borosilicate glass layer can form that will reduce the transport of oxygen to the alloy to limit further oxidation. Oxidation is then controlled by diffusion through the borosilicate glass layer. The focus of this research was to study the mechanisms and kinetics of high temperature oxidation of a Mo–Si–B alloy. The base alloy has a composition of Mo–3Si–1B (wt.%) and was studied in a variety of gas environments over a range of temperatures in order to elucidate the critical factors that allow it to develop a protective borosilicate glass layer. The borosilicate glass layer is protective when no continuous channels exist in the layer extending from the gas interface to the alloy interface. The borosilicate layer is believed to contain channels in the early stages of development and the elimination of the channels is obtained by appropriate control of the temperature and gas flow conditions whereby MoO3 is removed via vaporization while the borosilicate viscosity is not increased due to loss of B2O3. Once the borosilicate layer is continuous and free of channels, subsequent oxidation occurs by inward diffusion of oxygen and the outward diffusion of molybdenum through this layer with vaporization of MoO3 occurring at the gas/borosilicate layer interface, and MoO2 and additional borosilicate forming at the alloy/MoO2 interface.

Surface modification and oxidation kinetics of hot-pressed AlN–SiC–MoSi 2 electroconductive ceramic composite

The effects of oxidation on the microstructural modification and on the electrical resistivity and mechanical strength of a hot-pressed AlN–SiC–MoSi 2 electroconductive ceramic composite were studied. The kinetic of the oxidation was also evaluated. After the oxidation at temperatures below 1000 °C samples do not gain weight, due to simultaneous formation of SiO 2 and evaporation of MoO 3 formed by the oxidation of MoSi 2. However, the AlN/SiC matrix disables the “pesting” phenomena and strength degradation, despite the fact that at these temperatures MoSi 2 oxidizes rapidly. At temperatures above 1000 °C, the composite gains weight due to protective mullite layer formation on the surface, that provides a good oxidation resistance for use at higher temperatures. The kinetics of the oxidation follows the parabolic law. The possible rate-controlling mechanism is the diffusion of oxygen through the mullite-rich surface oxide scale.

Microstructure characterization of sol-gel prepared MoO3–TiO2 thin films for oxygen gas sensors

Binary metal–oxide MoO3–TiO2 films have been prepared using the sol-gel technique. The thin films were annealed at several temperatures including 400, 450, 500, 550, and 600 °C for 1 h. The morphology, crystalline structure, and chemical composition of the films have been analyzed using scanning electron microscopy (SEM) and atomic force microscopy, x-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), and x-ray photoelectron spectroscopy (XPS) techniques. The SEM analysis showed that there are two different sizes of grains in the films annealed at temperatures of 400, 450, 500, and 550 °C. One grain type is small with 20–100 nm; the other is a large grain type several micrometers in length. The XRD analysis revealed that the films annealed at 400 °C were a mixture of orthorhombic and hexagonal MoO3 phases. The films annealed at 450 °C showed an increase in the hexagonal phase. A preferential orientation growth along the (100) plane of the hexagonal phase and the (010) plane of the orthorhombic phase has been found in both samples. RBS and XPS analysis showed that the films were stoichiometric. When the annealing temperature was increased beyond 500 °C, the concentration ratio of MoO3–TiO2 decreased due to the evaporation of MoO3. For the study of the electrical and gas sensing properties, the films were deposited on sapphire substrates with interdigital electrodes on the frontside and a Pt heater on the backside. The MoO3–TiO2 thin films are sensitive to oxygen gas. The film has exhibited the O2 response (S=Rg/Rb) of 2.1, 8.1, and 80 for 120, 1000, and 10 000 ppm concentration of O2, respectively.

Growth Mechanism of MoS2 Fullerene-like Nanoparticles by Gas-Phase Synthesis

Inorganic fullerene-like (hollow onionlike) nanoparticles (IF) and nanotubes have attracted considerable interest in recent years, due to their unusual crystallographic morphology and their interesting physical properties. IF-MoS2 and nanotubes were first synthesized by a gas-phase reaction from MoO3 powder. This process consists of three steps: (1) evaporation of the MoO3 powder as molecular clusters; (2) condensation of the oxide clusters to give MoO3-x nanosize particles; (3) sulfidization of the suboxide nanoparticles to generate IF nanoparticles. The evaporation of MoO3 (step 1) and the IF particle formation from the oxide nanoparticles (step 3) have been investigated already, while the mechanism for the suboxide nanoparticles formation (step 2) has not been studied before and is reported here. According to the present model, a partial reduction of the trioxide molecular clusters (3−5 molecules) leads to the formation of MoO3-x nanoparticles (5−300-nm particles size)the precursor for IF-MoS2. A mathematical model, which takes into account the diffusion of the reactants into the reaction zone, the chemical reactions, and the boundary conditions obtained from the experiments, is established and solved. Based on the comprehensive understanding of the IF-MoS2 growth mechanism from MoO3 powder and the solution of the diffusion equations, a gas-phase reactor, which allowed reproducible preparation of a pure IF-MoS2 powder (50 mg per batch) with controllable sizes, is demonstrated.

Kinetic Study of Vapor-Phase Preparation of Orthorhombic Molybdenum Trioxide

A new kinetic approach to study the vapor-phase preparation of orthorhombic molybdenum trioxide (MoO3) has been established in this work. Using a simple experimental setup, the vaporization and condensation of MoO3 can be investigated with sufficient oxygen background to ensure the stoichiometry of the compound. The empirical condensation rate of the MoO3 obtained in the temperature range 642−660 °C is RE = 1.20 × 10-3ΔP 2.07, where ΔP is the partial pressure difference of MoO3 between the source and condensed crystals. On the basis of the observed crystal morphology and XRD results, a more precise MoO3 condensation kinetic model has been developed to explain the layer-by-layer growth of orthorhombic structure. The staged growth phenomena can be interpreted by this mechanistic model, which correlates the condensed MoO3 weight data to the experimental setup. The driving force of the condensation growth is investigated with respect to the current experimental conditions. For the MoO3 partial pressure ratios...