Controllable syntheses of Mo5Si3 and Mo3Si by silicothermic reduction of MoS2 in the presence of lime

Molybdenum silicides (Mo–Si) have attracted great interest owing to their good electrical conductivity, ultra-high melting point and well oxidation resistance. In this work, the controllable syntheses of Mo₅Si₃ and Mo₃Si powders by silicothermic reduction of MoS₂ in the presence of lime are studied for the first time. The internal mixture composed of MoS₂ and Si is wrapped by sufficient CaO. After the reaction is finished, the synthesized molybdenum silicides can be easily separated from the desulfurization layer by peeling off the desulfurization product. The results show that Mo₃Si and Mo₅Si₃ can be prepared at 1500 °C for the samples with the MoS₂:Si molar ratio of 1:1.7 and 1:2.33, respectively. In addition, the results show that both SiS₂ and SiS can be formed during the heating process. The thermodynamic analysis about the disproportionation reaction of SiS indicates that the existence of SiS₂ depends on the partial pressure of SiS (P_SiS) and temperature. Moreover, it is demonstrated that SiS₂ is preferentially formed under low temperature conditions. As the temperature rises, it is much easier for SiS₂ to react with Si to form SiS. Additionally, this work experimentally proves for the first time that SiS gas can directly reduce MoS₂ to form MoSi₂ and SiS₂. Subsequently, the recovery of S in the desulfurization product was studied. S in MoS₂ was first captured by CaO in the form of CaS (accompanied by Ca₂SiO₄ and Si). In order to solve this CaS containing desulfurization product which is detrimental to the environment, CaS in the desulfurization product is react with Fe₂O₃ and C to form FeS. This work provides new insights into the closed-loop capture and recovery of S in the process of extracting metals or metal compounds from sulfide ores.     

Fabrication of a SiC/Si/MoSi2 multi-coating on graphite materials by a two-step technique

A SiC/Si/MoSi₂ multi-coating for graphite materials was prepared by a two-step technique. SiC whisker reinforcement coating was produced by pyrolysis of hydrogen silicone oil (H-PSO) at 1600 °C, and then the dense coating was formed by embedding with the powder mixture of Si, graphite and MoSi₂ at 1600 °C in argon atmosphere. The microstructure, thickness, phase and oxidation resistance of the coating were investigated. Research results showed that, the phase of multi-coating was composed of SiC, Si and MoSi₂. The thickness of the coating was about 300 μm. In addition, the coating combined with matrix well, and surface was continuous and dense. The oxidation pretreatment experiment was carried out in the static air at 1400 °C for 4 h before thermal failure tests and the specimens had 0.045% weight gain. Subsequent thermal failure tests showed that, the SiC/Si/MoSi₂ multi-coating had excellent anti-oxidation property, which could protect graphite materials from oxidation at 1000 °C in air for 12 h and the corresponding weight loss was below 1 wt%. Based on the surface morphology changes, oxidation pretreatment experiment and thermal failure tests enhanced densification of multi-coating and the coating had a certain self-healing ability.     

In situ reactive spark plasma sintering of WSi2/MoSi2 composites

MoSi₂ based materials have the potential for use in high temperature structural parts. In this work, WSi₂ reinforced MoSi₂ composites were successfully prepared by mechanical activation followed by in situ reactive spark plasma sintering of Mo, Si, and W elemental powders. Benefiting from the high energy raw materials prepared through ball milling, these mechanically activated reactants started to transform into MoSi₂ at 1000 °C. Full density composites were obtained at a low sintering temperature (1200 °C) within 5 min. The addition of W to the reactants led to a finer microstructure than that obtained using pure MoSi₂, resulting in a significant improvement of mechanical properties. The Vicker's hardness of 20 vol% WSi₂/MoSi₂ was as high as 16.47 GPa.     

Preparation of MoSi2@ZrO2 core-shell powders by hydrothermal-calcination and evaluation of low-temperature oxidation resistance

MoSi₂ is a promising candidate for high-temperature, structural materials. However, their oxidation resistance is poor below 600 °C. To prevent MoSi₂ from being oxidized at low temperatures, core-shell structured MoSi₂@ZrO₂ powder were prepared by hydrothermal-calcination approach. First, MoSi₂@Zr(OH)₄ core-shell composite powder with nano-sized Zr(OH)₄ shell particles were synthesised using a hydrothermal method. Subsequently, the MoSi₂@ZrO₂ core-shell structured powder were obtained by calcination of the MoSi₂@Zr(OH)₄ powders at 900 °C for 2h. Finally, microstructure and oxidation behaviour of the MoSi₂@ZrO₂ powder at 400 °C–600 °C in air were investigated systematically. Microstructural analysis revealed that all samples had a distinct core-shell structure, and the ZrO₂ shells coated the surface of the MoSi₂ core. Oxidation behaviour studies showed that the dense ZrO₂ shell layer could isolate the MoSi₂ surface from oxygen, improving the low-temperature oxidation resistance and providing better low-temperature antioxidant properties than those of the MoSi₂ and MoSi₂@Zr(OH)₄ core-shell structures.     

In situ pressureless sintering of SiC/MoSi2 composites

SiC-reinforced MoSi₂ composites have been successfully prepared by in situ pressureless sintering from elemental powders of Mo, Si and C. Meanwhile, the evolutions of the samples’ microstructure and phase at different temperatures were investigated by using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS). It can be seen that at the temperature of 1100 °C, the main phases were Mo and Si, accompanying with a small amount of rich molybdenum products Mo₅Si₃ and Mo₃Si. Then the main phases changed to MoSi₂ and SiC when the sintering temperature reached 1300 °C. Finally we obtained MoSi₂/SiC composites with well-dispersed SiC particles after sintering at the temperature of 1550 °C for 120 min. The evolution of porosity in these composites fits the porosity reduction model well developed by Pines and Bruck, which revealed the particle agglomeration in the composites. The flexural strength and fracture toughness of 10% SiC/MoSi₂ composites were up to 274.5 MPa and 5.5 MPa m^1/2, increased by approximately 40.8% and 30.6% compared with those of monolithic MoSi₂, respectively.     

Synthesis of Ti3SiC2 coatings onto SiC monoliths from molten salts

Coatings with composition close to Ti₃SiC₂ were obtained on SiC substrates from Ti and Si powders with the molten NaCl method. In this work, the growth of coatings by reaction in the salt between monolithic SiC substrates and titanium powder is obtained between 1000 and 1200 °C. At 1000 °C, a coating of 8 µm thickness is formed in 10 h whereas a thin coating of 0.5 µm has been grown in 2 h. A lack in silicon was first found in the coatings prepared at 1100 and 1200 °C. For these temperatures, the addition of silicon powder in the melt had a favorable effect on the final composition, which is found very close to the composition of Ti₃SiC₂. The reaction mechanism implies the formation of TiC_x layers in direct contact with the SiC substrate and the presence of more or less important quantities of Ti₃SiC₂ and Ti₅Si₃C_x in the upper layers.     

Surface strengthening of Ti3SiC2 through magnetron sputtering of Mo and Zr and subsequent annealing

Magnetron sputtering deposition of Mo and Zr and subsequent annealing were conducted with the motivation to modify the surface hardness of Ti₃SiC₂. For Mo-coated Ti₃SiC₂, Si diffused outward into the Mo layer and reacted with Mo to form molybdenum silicides in the temperature range of 1000–1100 °C. The MoSi₂ layer, however, cracked and easily spalled off. For Zr-coated Ti₃SiC₂, Si also diffused outward to form Zr–Si intermetallic compounds at 900–1100 °C. The Zr–Si compounds layer had good adhesion with Ti₃SiC₂ substrate, which resulted in the increased surface hardness.     

Preparation,properties and high-temperature oxidation resistance of MoSi2-HfO2 composite coating to protect niobium using spent MoSi2-based materials

Industrial spent MoSi₂-based materials and HfO₂ were recycled as raw materials to fabricate MoSi₂-HfO₂ composite coating by spark plasma sintering (SPS). The microstructural evolution of the coatings was characterized and the 1500 °C oxidation behavior was explored. Cracks penetrated through the MoSi₂ coating while no cracks can be found in the HfO₂-containing composite coating owing to the reduction of the mismatch of thermal expansion coefficient (CTE). Good metallurgical bonding was exhibited since (Mo,Nb)₅Si₃ diffusion layer was found in the HfO₂-containing coating by the diffusion of Nb and Si across the interface without gaps. After 1500 °C oxidation of 20 h, cracks appeared in the surface of SiO₂ layer on MoSi₂ coating while the HfO₂-containing composite coating possessed crack-free oxide scale. HfSiO₄ with high temperature (>2900 °C) is formed during oxidation and it inlays in the silica oxide scale to improve the stability. Compared to MoSi₂ coating, Nb coated MoSi₂-HfO₂ has thinner oxide scale with lower mass gain during oxidation, thus presenting better high-temperature anti-oxidation properties.     

Field-activated pressure-assisted combustion synthesis of MoSi<Subscript>2</Subscript>

The simultaneous synthesis and densification of MoSi₂ starting from elemental Mo and Si was carried out through a process of field-activated pressure-assisted combustion synthesis (FAPACS). After heating up to 922°C, the synthesis reaction occurs in the mode of thermal explosion but is ceased subsequently due to a sudden shock. The product consists of MoSi₂, residual Mo and amorphous Si with precipitation of very small MoSi₂ primary crystals containing. There is no transition phase layer between Mo and MoSi₂ as well as Si and MoSi₂. This indicates that metallic Mo reacts with molten Si directly forming MoSi₂ in the FAPACS process, without any intermediate reaction steps involved. The formation of MoSi₂ occurs via dissolution of Mo into Si melt followed by MoSi₂ precipitation from the melt.      

High-temperature oxidation resistance of spent MoSi2 modified ZrB2–SiC–MoSi2 coatings prepared by spark plasma sintering

The spent MoSi₂ modified ZrB₂–SiC–MoSi₂ coatings were prepared on carbon matrixes by spark plasma sintering. A continuous metallurgical bonding was formed at the interface between the coating and matrix, and no obvious defects such as pores and cracks were observed inside. The effects of spent MoSi₂ content and trace doping in the spent powder on the oxidation behavior of the coatings in air at 1700 °C were investigated. During the active oxidation stage, the spent MoSi₂ promoted the densification of the coating and enhanced the structural oxygen barrier properties. With the increase of service time, during the inert oxidation stage, doping an appropriate amount of spent MoSi₂ helped to increase the fluidity of the rich-SiO₂ protective layer so that the Zr oxides fully dispersed in the generated Zr–B–Si–O–Al multiphase glass layer, which could impede the penetration of oxygen and enhance the oxidation protection efficiency. However, excessive spent MoSi₂ exacerbated the volatilization of gas by-products, forming pores and cracks in the glass layer and rising the oxidation loss. When the content of spent MoSi₂ was 20 vol%, the glass layer is dense and uniform, with few defects and the best oxygen resistance property. Moreover, compared with commercial powders, spent MoSi₂ contained Al₂O₃ and SiO₂. Al₂O₃ had an excellent modification effect, while SiO₂ glass can promote liquid phase sintering and seal the defects in the coatings. By adding spent MoSi₂, the modified ZrB₂–SiC–MoSi₂ composite coatings could inhibit the formation of defects and improve the dynamic stability of the coatings effectively.     

Enhanced nitridation of silicon compacts by Yb2O3 addition

The catalytic effect of ytterbium oxide (Yb₂O₃) on the nitriding reaction of Si compacts was investigated. Si powder mixtures containing Yb₂O₃ were prepared and nitrided in the form of compacts with a multi-step heating schedule over the range of 1200 °C–1450 °C. The nitriding profiles of the powder mixture with increasing temperature indicated that Yb₂O₃ clearly promoted the nitridation of Si compacts at 1200 °C compared with the pure Si compact containing no additives. The critical role of Yb₂O₃ on the nitridation of Si, was elucidated that Yb₂O₃ promotes the loss of initial SiO₂ of the raw Si powder via the measurement of the weight changes at low temperature (1100 °C) and thermogravimetric analysis under N₂ atmosphere. It was also found that the β-ratio of fully nitrided Si was closely related to the intermediate degree of nitridation at 1200 °C and 1300 °C.     

High temperature oxidation behavior of MoSi2–Al2O3 composite coating on TZM alloy

A novel MoSi₂–Al₂O₃ composite coating was prepared on Mo-based TZM alloy by slurry sintering method. The oxidation behavior of the coating was evaluated at 1600 °C in static air. Microstructure and phase composition of the as-prepared and oxidized coatings were characterized, and the antioxidant mechanism of the coating at high temperature was discussed. A three-layer structure was observed in the as-prepared coating, consisting of a ～2 μm thick Mo₅Si₃ diffusion layer, a ～65 μm thick MoSi₂ inner layer and a ～36 μm thick outer layer of mixture of MoSi₂ and Al₂O₃. After oxidation at 1600 °C for 5 h, all MoSi₂ phases were completely converted to intermediate silicide Mo₅Si₃ by solid-state diffusion, and the formed Mo₅Si₃ phase would be transformed into Mo₃Si phase with further extending the oxidation time. Furthermore, a dense oxide layer of SiO₂-mullite was formed on the specimen surface, which can effectively protect the material to further oxidation. The MoSi₂–Al₂O₃ coating could protect the substrate effectively at 1600 °C for 20 h without failure. The enhanced oxidation resistance of MoSi₂–Al₂O₃ coating is due to the formation of multi-layer structure containing a SiO₂-mullite composite oxide outer layer with high thermal stability and low oxygen permeability.     

Low Temperature Growth of In2O3 and InN Nanocrystals on Si(111) via Chemical Vapour Deposition Based on the Sublimation of NH4Cl in In

Indium oxide (In₂O₃) nanocrystals (NCs) have been obtained via atmospheric pressure, chemical vapour deposition (APCVD) on Si(111) via the direct oxidation of In with Ar:10% O₂ at 1000 °C but also at temperatures as low as 500 °C by the sublimation of ammonium chloride (NH₄Cl) which is incorporated into the In under a gas flow of nitrogen (N₂). Similarly InN NCs have also been obtained using sublimation of NH₄Cl in a gas flow of NH₃. During oxidation of In under a flow of O₂ the transfer of In into the gas stream is inhibited by the formation of In₂O₃ around the In powder which breaks up only at high temperatures, i.e. T > 900 °C, thereby releasing In into the gas stream which can then react with O₂ leading to a high yield formation of isolated 500 nm In₂O₃ octahedrons but also chains of these nanostructures. No such NCs were obtained by direct oxidation for T _G < 900 °C. The incorporation of NH₄Cl in the In leads to the sublimation of NH₄Cl into NH₃ and HCl at around 338 °C which in turn produces an efficient dispersion and transfer of the whole In into the gas stream of N₂ where it reacts with HCl forming primarily InCl. The latter adsorbs onto the Si(111) where it reacts with H₂O and O₂ leading to the formation of In₂O₃ nanopyramids on Si(111). The rest of the InCl is carried downstream, where it solidifies at lower temperatures, and rapidly breaks down into metallic In upon exposure to H₂O in the air. Upon carrying out the reaction of In with NH₄Cl at 600 °C under NH₃ as opposed to N₂, we obtain InN nanoparticles on Si(111) with an average diameter of 300 nm.     

Mechanochemical synthesis of MoSi2–SiC nanocomposite powder

MoSi₂–25 wt.%SiC nanocomposite powder was successfully synthesized by ball milling Mo, Si and graphite powders. The effect of milling time and annealing temperature were investigated. Changes in the crystal structure and powder morphology were monitored by XRD and SEM, respectively. The microstructure of powders was further studied by peak profile analysis and TEM. MoSi₂ and SiC were synthesized after 10 h of milling. Both high and low temperature polymorphs (LTP and HTP) of MoSi₂ were observed at the short milling times. Further milling led to the transformation of LTP to HTP. On the other hands, an inverse HTP to LTP transformation took place during annealing of 20 h milled powder at 900 °C. Results of peak profile analysis showed that the mean grain size and strain of the 20 h milled powder are 31.8 nm and 1.19% that is in consistent with TEM image.     

Contact interaction and hot pressing of ZrB2-MoSi2 in CO/CO2 atmosphere

ZrB₂-15 vol% MoSi₂ ceramics were hot pressed in CO/CO₂ atmosphere in the 1700–1900^oС temperature range. During hot pressing, MoSi₂ decomposes into Mo and Si and the phase composition of the as-sintered ceramic results in ZrB₂, (Zr, Mo)B₂, SiC, SiO₂, and MoB. Contact melting between ZrB₂ and MoSi₂ was observed at 1800^oC, corresponding to the formation of (Zr, Mo)B₂. Ceramics obtained at1800–1850^oС had ∼ 500 МPа and 200 MPa strength at room at 1800^oC in vacuum, respectively. The thickness of the oxidized scales upon exposing the samples at 1600 ^oC for 120 min was 30–80 µm and depended on the amount of residual MoSi₂ and (Zr, Mo)B₂. The highest oxidation resistance was observed for the ceramic sintered at 1850 °C.     

Oxidation behavior of Al2O3 reinforced MoSi2 composite coatings fabricated by vacuum plasma spraying

MoSi₂, MoSi₂–10 vol.% Al₂O₃, MoSi₂–30 vol.% Al₂O₃ (denoted as MA0, MA1, MA3, respectively) coatings were fabricated by vacuum plasma spraying (VPS), and their oxidation behavior was examined at low temperature (500 °C) and high temperature (1500 °C). The test at 500 °C showed that the addition of Al₂O₃ effectively restrained the pest oxidation of MoSi₂. The MA1 coating had satisfactory fluid surface and presented good oxidation resistance at 1500 °C. However, the MA3 coating showed worse oxidation resistant behavior compared with the MA0 coating because of mullite formation.     

Enhanced thermoelectric properties of 2H–MoS2 thin film by tuning post sulfurization temperature

Two-dimensional transition metal dichalcogenide semiconductors (TMDCs) like MoS₂ are becoming more popular as thermoelectric materials because they are abundant, nontoxic, and have good performance. In the study, the MoS₂ thin films have prepared by the sputtering and post-sulfurization process at various temperatures 450 °C, 550 °C, 650 °C, and 750 °C. The XRD data exhibits the formation of the 2H phase of MoS₂ thin film with (002), (004), and (006) planes. The Raman spectroscopy has confirmed the 2H–MoS₂ thin films with 2LA (M), A_1g, E_2g, and E_g vibrational modes. The SEM images have shown the thin MoS₂ flakes. The Seebeck and electrical conductivity data indicated an enhancement in Seebeck coefficient and electrical conductivity from 20 to 31 μV/^°C and 26–53 S/m, respectively, as the post sulfurization temperature increased from 450 °C to 750 °C. The enhancement of the Seebeck coefficient and electrical conductivity have been linked to the perfection of the 2H phase of MoS₂ film. The improved crystal structure has increased carrier mobility, leading to a high power factor of the 5.09 μWm^?1C^?2.     

In situ single-step reduction and silicidation of MoO3 to form MoSi2

Nanocrystalline molybdenum disilicide (MoSi₂) is synthesized in a specially designed autoclave at 900°C. The XRD results revealed that the formation of MoSi₂ is favorable with the blend of MoO₃, Si, and Mg powders. The HR-TEM and SAED patterns confirm the formation of MoSi₂ phase. The structural parameters (crystallite size, strain, stress, and deformation energy density) are calculated using the Williamson-Hall (W-H) analysis. The formation mechanism involved in the synthesis of MoSi₂ is proposed. The nonisothermal oxidation kinetics (~1200°C) of MoSi₂ phase is examined through the thermal analysis techniques. The activation energy is determined by the Kissinger-Akahira-Sunsose isoconversional kinetic model. Finally, the reaction mechanism involved during the oxidation of MoSi₂ phase is identified using the integral master-plots method.     

Synthesis and Properties of MoSi2–MoB–SiC Ceramics

MoB and SiC particulate reinforced MoSi₂ matrix composites were synthesized in situ from Mo, Si, and B₄C powder mixtures by self‐propagating high‐temperature synthesis (SHS). The SHS MoSi₂–MoB–SiC products were vacuum hot‐pressed (HPed) at 1400°C for 90 min to fabricate high‐density (> 97.5% relative density) bulk composites. Microstructure refinement and improvements in the Vickers hardness and fracture toughness of the HPed composites were observed with increasing B₄C content in the reaction mixture. The HPed composite of composition MoSi₂–0.4MoB–0.1SiC exhibited grain size of 1–5 μm, Vickers hardness of 12.5 GPa, bending strength of 537 MPa, and fracture toughness of 3.8 MPa.m^1/2. These excellent mechanical properties indicate that MoB and SiC particulate reinforced MoSi₂ composites could be promising candidates for structural applications.     

Behaviors of oil-soluble molybdenum complexes to form very fine MoS2 particles in vacuum residue

Dispersion and thermal behaviors of oil-soluble Mo-dithiocarbamate (Mo-DTC) and Mo-dithiophosphate (Mo-DTP) as the MoS₂ catalyst precursors were studied in petroleum vacuum residue (VR) using FT-Far IR, XRD and TEM. FT-Far IR was proved to detect the Mo complexes and their derived MoS₂ in VR without the interference of the complicated organic matrix. Their transformation into MoS₂ was identified by detecting the changes in the ligand bonds and crystal structure. These complexes were found to be distributed in asphaltene (or maltene) by fractionation with hexane due to their solubility in the heavy oil, ruling out any chemical interaction of the complex with asphaltene. Mo-DTC was found to be decomposed at 350 °C to form definite MoS₂ in VR. Mo-DTP started its decomposition around 200 °C, and however, no definite formation of MoS₂ was confirmed by heating up to 500 °C. Dispersion of the complexes in VR and asphalthene was always good as indicated by TEM. Both complexes in VR at 380 °C under hydrogenation (HYD) conditions provided more or less MoS₂. H₂S and reactive sulfur species were assumed to accelerate the transformation of the Mo complex to MoS₂ during HYD reaction. Some difficulty of Mo-DTP to be transformed quantitatively into definite MoS₂ in VR may explain its poor activity for up-grading VR.