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The present work was undertaken to study the chemistry and phase formation mechanism in the salt-controlled MoO3 + Mg + NaCl thermite reaction. It was found that the structure and phase formation mechanism in the studied system primarily depend on the salt content in the initial mixtures. In salt-poor mixtures, nucleation of product particles takes...
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... E eff is the effective value of activation energy of the re- action responsible for the combustion wave propagation rate and R is the universal gas constant. As a result, the slope of a plot of ln(U c /T c ) versus 1/T c can provide the effective value of the activation energy of the process, as depicted in Figure 3. From the slope of the best-fit line of all of the data in Figure 3, a value for the activation energy of 50 kJ/mol was obtained. ...
Context 2
... E eff is the effective value of activation energy of the re- action responsible for the combustion wave propagation rate and R is the universal gas constant. As a result, the slope of a plot of ln(U c /T c ) versus 1/T c can provide the effective value of the activation energy of the process, as depicted in Figure 3. From the slope of the best-fit line of all of the data in Figure 3, a value for the activation energy of 50 kJ/mol was obtained. ...
Citations
... When NaCl is added, a reduction of ∼300°C occurs in the transition temperature, placing the transition temperatures at 525°C, 523°C, 525°C and 514°C for MoO 3 -P, −30, −120 and −300, respectively. Additionally, another transition temperature becomes evident near 600°C for BM samples with NaCl added, which can be accounted for molybdenum oxyhalide species [67]. This high mass flux facilitates the increase of nucleation sites, achieving a quasi-homogeneous dispersion. ...
Single and few-layered MoS2 materials have attracted attention due to their outstanding physicochemical properties with potential applications in optoelectronics, catalysis, and energy storage. In the past, these materials have been produced using the chemical vapor deposition (CVD) method using MoO3 films and powders as Mo precursors. In this work, we demonstrate that the size and morphology of few-layered MoS2 nanostructures can be controlled, modifying the Mo precursor mechanically. We synthesized few-layered MoS2 materials using MoO3 powders previously exposed to a high-energy ball milling (BM) treatment by the salt-assisted CVD method. The MoO3 powders milled for 30, 120, and 300 min were used to synthesize sample MoS2-30, MoS2-120, and MoS2-300, respectively. We found morphologies mainly of hexagons (MoS2-30), triangles (MoS2-120), and fullerenes (MoS2-300). The MoS2 nanostructures and MoO3 powders were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), and thermogravimetric analysis (TGA). It was found that MoO3 milled powders exhibit oxygen loss and decrease in crystallite size as milling time increases. Oxygen deficiency in the Mo precursor prevents the growth of large MoS2 crystals and a large number of milled MoO3-x + NaCl promote greater nucleation sites for the formation of MoS2, achieving a high density of nanoflakes in the 2H and 3R phases, with diameter sizes in the range of ~30-600 nm with 1-12 layers. Photoluminescence characterization at room temperature revealed a direct bandgap and exciting trends for the different MoS2 samples. We envisage that our work provides a route for modifying the structure and optical properties for future device design via precursor engineering.
... Decreasing their melting point can contribute to the evaporation and the release of reactant atoms or clusters. Taking the growth of transition metal dichalcogenides (TMDs) as an example, molten salts (NaCl, KCl, etc.) can reduce the melting point of precursors to promote their evaporation (Manukyan et al., 2011;Gaune-Escard and Haarberg., 2014). A library of 2D TMDs has been successfully synthesized by molten-salt-assisted chemical vapor deposition (CVD) . ...
Two-dimensional atomic single crystals (2DASCs) have drawn immense attention because of their potential for fundamental research and new technologies. Novel properties of 2DASCs are closely related to their atomic structures, and effective modulation of the structures allows for exploring various practical applications. Precise vapor-phase synthesis of 2DASCs with tunable thickness, selectable phase, and controllable chemical composition can be realized to adjust their band structures and electronic properties. This review highlights the latest advances in the precise vapor-phase synthesis of 2DASCs. We thoroughly elaborate on strategies toward the accurate control of layer number, phase, chemical composition of layered 2DASCs, and thickness of non-layered 2DASCs. Finally, we suggest forward-looking solutions to the challenges and directions of future developments in this emerging field. : Atomic Structure; Chemical Synthesis; Materials Characterization Subject Areas: Atomic Structure, Chemical Synthesis, Materials Characterization
... Because of the relative simplicity in using these techniques, they have found widespread use within SHS reactions. These layer velocity approaches have been used to describe the kinetics in the Ni/Al system [69,70], in boride systems, including Nb/B [71][72][73], Ta/B [71], Zr/B [71,74,75], Hf/B [71], Ti/B [76], Mo/B [77], along with other binary systems such as Ti/C [78], Ti/Si [79,80], thermites [81][82][83][84][85][86][87][88][89] and more complex ternary systems [78,[90][91][92][93]. ...
The book on Advanced Chemical Kinetics gives insight into different aspects of chemical reactions both at the bulk and nanoscale level and covers topics from basic to high class. This book has been divided into three sections: (i) "Kinetics Modeling and Mechanism," (ii) "Kinetics of Nanomaterials," and (iii) "Kinetics Techniques." The first section consists of six chapters with a variety of topics like activation energy and complexity of chemical reactions; the measurement of reaction routes; mathematical modeling analysis and simulation of enzyme kinetics; mechanisms of homogeneous charge compression ignition combustion for the fuels; photophysical processes and photochemical changes; the mechanism of hydroxyl radical, hydrate electron, and hydrogen atom; and acceptorless alcohol dehydrogenation. The understanding of the kinetics of nanomaterials, to bridge the knowledge gap, is presented in the second section. The third section highlights an overview of experimental techniques used to study the mechanism of reactions.
https://www.intechopen.com/books/advanced-chemical-kinetics
... Because of the relative simplicity in using these techniques, they have found widespread use within SHS reactions. These layer velocity approaches have been used to describe the kinetics in the Ni/Al system [69,70], in boride systems, including Nb/B [71][72][73], Ta/B [71], Zr/B [71,74,75], Hf/B [71], Ti/B [76], Mo/B [77], along with other binary systems such as Ti/C [78], Ti/Si [79,80], thermites [81][82][83][84][85][86][87][88][89] and more complex ternary systems [78,[90][91][92][93]. ...
... It is noted that all niobium oxides have melting points above 1510°C, which make them difficult to vaporize and react with selenium in the CVD process. The products of reactions between molten salts and metal (Mo 48 , W 28 , and Nb 49 ) oxides have been investigated by several groups and found to be metal oxychlorides, which have much lower melting points compared with corresponding metal oxides 28,48,49 . Thus, it is suggested that in our CVD process, NaCl reacts with niobium oxides to give volatile niobium oxychloride 28 , therefore increasing the vapour pressure of precursor and facilitating the growth of NbSe 2 . ...
The discovery of monolayer superconductors bears consequences for both fundamental physics and device applications. Currently, the growth of superconducting monolayers can only occur under ultrahigh vacuum and on specific lattice-matched or dangling bond-free substrates, to minimize environment-and substrate-induced disorders/defects. Such severe growth requirements limit the exploration of novel two-dimensional superconductivity and related nanodevices. Here we demonstrate the experimental realization of superconductivity in a chemical vapour deposition grown monolayer material-NbSe2. Atomic-resolution scanning transmission electron microscope imaging reveals the atomic structure of the intrinsic point defects and grain boundaries in monolayer NbSe2, and confirms the low defect concentration in our high-quality film, which is the key to two-dimensional superconductivity. By using monolayer chemical vapour deposited graphene as a protective capping layer, thickness-dependent superconducting properties are observed in as-grown NbSe2 with a transition temperature increasing from 1.0 K in monolayer to 4.56 K in 10-layer.
... Manukyan et al. [90] studied the MoO 3 -Mg thermite system using NaCl as a diluent and the Merzhanov model for determination of kinetics. They found that the reaction mechanism changes based on the amount of NaCl, with low salt concentrations, nucleation of the product occurs in liquid Mg, whereas in high salt concentrations the products are formed in molten NaCl. ...
The current state of chemical kinetics for self-propagating high-temperature non-catalytic reactions has been reviewed for results over the past 50 years. Five different characterization techniques are primarily considered: differential thermal analysis (DTA), electrothermal explosion (ETE), electrothermography (ET), combustion velocity/temperature analyses (Merzhanov–Khaikin and Boddington–Laye approaches), and other advanced in-situ diagnostics, including time-resolved X-ray diffraction (TRXRD). Based on the summary of results thus far, recommendations are given for the future of SHS kinetic research.
... To reduce the tungsten and molybdenum oxides, as well as ammonium and sodium tungstates and molybdates by yielding metallic endproducts, the combustion reactions are applied during last two decades [22][23][24][25][26][27][28][29][30][31]. In [25][26][27][28][29][30][31] work reduction of Mo and W is successfully performed in one macroscopic stage using (Mg + C), (Mg + Zn) combined reducing mixtures. ...
... To reduce the tungsten and molybdenum oxides, as well as ammonium and sodium tungstates and molybdates by yielding metallic endproducts, the combustion reactions are applied during last two decades [22][23][24][25][26][27][28][29][30][31]. In [25][26][27][28][29][30][31] work reduction of Mo and W is successfully performed in one macroscopic stage using (Mg + C), (Mg + Zn) combined reducing mixtures. It should be underlined that it is highly challenging to monitor and reveal mechanism of the combustion reaction due to its high velocity. ...
... To reduce the tungsten and molybdenum oxides, as well as ammonium tungstate and molybdate by yielding metallic end-products, the combustion reactions are applied during the last two decades [13][14][15][16][17][18][19][20]. In the [16][17][18][19][20] works reduction of Mo and W is successfully performed in one macroscopic stage using (Mg + C), (Mg + Zn) combined reducing mixtures. ...
... To reduce the tungsten and molybdenum oxides, as well as ammonium tungstate and molybdate by yielding metallic end-products, the combustion reactions are applied during the last two decades [13][14][15][16][17][18][19][20]. In the [16][17][18][19][20] works reduction of Mo and W is successfully performed in one macroscopic stage using (Mg + C), (Mg + Zn) combined reducing mixtures. It should be underlined that it is highly challenging to monitor and reveal mechanism of the combustion reaction due to its high velocity. ...
In this work the reaction mechanism in the WO3–Mg/C systems and ammonium paratungstate (APT)–Mg/C systems are studied. As reducer magnesium, carbon or combinations of both are explored. It is shown that in the WO3–Mg system the reduction undergoes by solid–solid mechanism before melting Mg, where metallic tungsten and MgO are formed. Unlike this system, in the WO3–C system mainly WOx (< 880 °C) and WO2 (> 960 °C) and small amount W is formed. In the WO3–Mg–C ternary system reduction temperature shifts to higher temperature range and depends on amount of carbon. Similar to WO3–Mg system, APT–Mg reaction starts and completes in the solid state. Thus, firstly the APT decomposes, then reduction of formed WO3 takes place at ~ 600 °C yielding W and MgO. Likewise to WO3–Mg system adding carbon into APT–Mg mixture shifts reduction temperature to even higher temperature zone which can exceed melting point of Mg and further reduction undergoes with molten magnesium. It is shown that the reduction products are MgO and W.
... To control the reaction temperature and the microstructure of the products in TRs, several approaches were applied, including excessive use of magnesium in the initial mixture, 23,24,31 the combination of several reducers, e.g., Mg-Zn or Mg-C, 38,39 the dilution of a reactive media with the inert salts (e.g., NaCl). 20,21,39 In the last case, sodium chloride significantly reduces the reduction temperature, and also molten salt prevents intensive agglomeration processes of product particles at elevated temperatures. However, our recent study showed that sodium chloride may not be always considered as an inert media. ...
... However, our recent study showed that sodium chloride may not be always considered as an inert media. 39 In some systems, NaCl intensively reacts with initial oxides forming volatile oxychlorides of metals, which significantly change the reaction mechanism. ...
The influence of calcium fluoride (CaF2) on combustion characteristics of Na2WO4 + 3 Mg system and microstructure of the produced W and WO3/W crystals is investigated. The results of thermodynamic analysis and experimental investigations show that CaF2 simultaneously enhances the conversion of Na2WO4 toward tungsten and binds sodium through the formation of NaF phase. The examination of the microstructure of quenched combustion products and differential scanning calorimetry analysis indicate that at early stages of combustion, a part of Na2WO4 is reduced by Mg to tungsten, whereas another part reacts with CaF2 forming CaWO4 and NaF. Subsequent magnesium reduction of CaWO4 significantly increases the overall temperature of the combustion process. Such modification in reaction mechanism coupled with postcombustion processing (e.g., acid/basic treatment) of the product allows us to produce either pure tungsten nanocrystals or tungsten oxide-tungsten nanostructures consisting of two-dimensional WO3 nanoflakes assembled on a W core. It is found that CaF2 does not influence the sizes of tungsten nanocrystals. However, since the addition of CaF2 leads to the increase of overall reaction temperature, it facilitates the formation of W particles with equilibrium crystal shape by faceting process.
2D materials are enabling disruptive advancements in electronic and photonic devices yielding to the development of sensing and wearable materials and in the field of energy production and storage as key components of photovoltaic technology and batteries. Nevertheless, little attention has been paid to TMDs and oxides that contain vanadium, as it is the case of vanadium disulfide (VS2) and vanadium dioxide (VO2). In this study we review the synthesis and characterization using Raman spectroscopy of VS2 and its oxidized states. Laser-induced oxidation occurring during the Raman experiments in ambient conditions is described and plateau values of laser power levels to induce oxidation are provided. Furthermore, tip-enhanced Raman spectroscopy (TERS) spectra and maps are conducted to reveal at the single flake level the onset of oxidation mechanisms at the surface of the 2D platelets.
