Tetragonal MoSi2 Research Articles

The topological nodal lines and drum-head-like surface states in semimetals CrSi2, MoSi2 and WSi2

The structural and electronic properties of I4/mmm XSi2 (X = Cr, Mo and W) are systematically studied by first-principles calculation. First, the dynamical stability of XSi2 (X = Cr, Mo and W) is verified by the phonon spectrums, the thermal stability is confirmed by the ab initio molecular dynamics simulations (AIMD) at 300 K and the mechanical stability is affirmed by the calculated elastic constants. Electronic structure computations uncover the clear drum-head-like surface states and the topological nodal lines (TNLs) which are close to the Fermi level EF and Γ centered in the Brillouin zone (BZ). Further investigations indicate the band structures and the TNLs are robust against the spin-orbit coupling (SOC), the biaxial and uniform strains, which is beneficial to experimental observation. Our results suggest tetragonal CrSi2, MoSi2 and WSi2 are ideal transition metal silicides to study the TNLs and drum-head-like surface states for topological physics.

Oxidation Behavior and Microstructural Evolution of ZrB2–35MoSi2–10Al Composite Coating

The problem of creating and implementing high-temperature coatings for the protection of carbon–carbon (C/C) composites remains relevant due to the extremely low or insufficient heat resistance of C/C composites in an oxygen-containing environment. In the present work, detonation spraying was used for preparing new ZrB2–35MoSi2–10Al coatings on the surface of C/C composites without a sublayer. As a stabilizer of high-temperature modification of zirconia, and to increase the wettability of the surface of C/C composites, 5 wt.% Y2O3 and 10 wt.% Al were added to the initial powder mixture, respectively. The structure of the as-sprayed coating presents many lamellae piled up one upon another, and is composed of hexagonal ZrB2 (h- ZrB2), tetragonal MoSi2 (t-MoSi2), monoclinic ZrO2 (m-ZrO2), tetragonal ZrO2 (t-ZrO2), monoclinic SiO2 (m-SiO2), and cubic Al phases. The oxidation behavior and microstructural evolution of the ZrB2–35MoSi2–10Al composite coating were characterized from RT to 1400 °C in open air. During oxidation at 1400 °C, a continuous layer of silicate glass was formed on the coating surface. This layer contained cubic ZrO2 (c-ZrO2), m-ZrO2, and small amounts of mullite and zircon. The results indicated that a new ZrB2–35MoSi2–10Al composite coating could be used on the surface of C/C composites as a protective layer from oxidation at elevated temperatures.

A high-entropy silicide: (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2

A high-entropy metal disilicide, (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2, has been successfully synthesized. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and electron backscatter diffraction (EBSD) collectively show the formation of a single high-entropy silicide phase. This high-entropy (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2 possesses a hexagonal C40 crystal structure with ABC stacking sequence and a space group of P6222. This discovery expands the known families of high-entropy materials from metals, oxides, borides, carbides, and nitrides to a silicide, for the first time to our knowledge, as well as demonstrating that a new, non-cubic, crystal structure (with lower symmetry) can be made into high-entropy phase. This (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2 exhibits high nanohardness of 16.7 ± 1.9 GPa and Vickers hardness of 11.6 ± 0.5 GPa. Moreover, it has a low thermal conductivity of 6.9 ± 1.1 W m−1 K−1, which is approximately one order of magnitude lower than that of the widely-used tetragonal MoSi2 and ∼1/3 of those reported values for the hexagonal NbSi2 and TaSi2 with the same crystal structure.

Microstructure Evolution of Plasma-Sprayed MoSi2 Coating at RT-1200 °C in Air

In this work, the phase composition and microstructure evolution of vacuum plasma-sprayed MoSi2 coating between room temperature and 1200 °C in air was evaluated and characterized. The results showed that hexagonal MoSi2 (h-MoSi2) became the main phase in the deposited coating, which remained even after 50 h oxidation at 500 °C, exhibiting excellent thermal stability. MoO3 bundles and SiO2 clusters were generated by consuming tetragonal MoSi2 (t-MoSi2) after 1 h, and white powders formed on the coating’s surface after 10-h exposure to air at 500 °C. Most h-MoSi2 transformed to t-MoSi2 at 800 °C; moreover, a protective silica layer formed on the coating surface. Similar phenomenon was observed for the coating exposed to 1000 °C where grain growth also occurred. Vacuum heat treatment at 900 °C effectively improved the thermal stability of the MoSi2 coating. The formation of silica layer alleviated negative effects of structural defects and helped the MoSi2 coating serve as a protective coating for varied substrates.

Oxidation response determined by multiphase-dependent melting degree of plasma sprayed MoSi2 on Nb-based alloy

Multiphase constituent is a typical characteristic of plasma sprayed MoSi2 coatings. In present work, the in-situ multiphase doping effect on oxidation behavior of plasma-sprayed MoSi2 compound coating on Nb-based alloy was comparatively investigated through two different fabrication methods: suspension plasma spray (SPS) and supersonic atmospheric plasma spray (SAPS). For the first time, the melting index, M.I., was proposed based on formation feature of each phase to illustrate the multiphase effect. Multi-phases including tetragonal MoSi2 (α), metastable MoSi2 (β), Mo5Si3, the amorphous and nanoparticle precipitates were found in both plasma sprayed MoSi2 coatings. The SAPS-MoSi2 coating exhibited a higher M.I. value than that of SPS-MoSi2 coating. Cyclic oxidation behavior at 1200 °C demonstrated that higher M.I. could bring better oxidation resistance of MoSi2 coating on Nb-based alloy. SAPS-MoSi2 coating possessed an extremely low thermally parabolic rate constant of 1.39 × 10−4 mg2cm−4h−1 under long-duration exposure in air at 1200 °C. The phase-dependent M.I. on thermally sprayed MoSi2 can balance negative factor of pesting oxidation and positive factor of forming protective glassy film in high-temperature cyclic oxidation experiment. High M.I.-value MoSi2 coating on Nb-based alloy could easily achieve a continuous, adhesive and protective surface scale as well as the absence of pesting oxidation.

First-principles study of magnetic properties of ultra-thin MoSi2 films

The magnetic properties of ultra-thin tetragonal MoSi2 thin films were investigated by the first-principles method. Our results indicate that the Si terminated MoSi2 film is always metallic independent of its thickness and non-magnetic when its thickness is larger than three atomic layers. However, the three-atomic-layer MoSi2 film (1L MoSi2) exhibits magnetism with magnetic moments of ∼0.274 μB/atom for Mo atoms and ∼0.096 μB/atom for Si atoms. The system shows weaker magnetism with magnetic moments of ∼0.184 μB/atom for Mo and ∼0.079 μB/atom for Si after unilateral surface hydrogenation and becomes non-magnetic after bilateral hydrogenation. By comparing the ferromagnetic (FM) configuration with antiferromagnetic configurations, we found that the FM order is the ground state with the lowest energy. Furthermore, it is found that the magnetic properties of 1L MoSi2 can be tuned effectively by strain.

Surface Metallization of SiC Ceramic by Mo-Ni-Si Coatings for Improving Its Wettability by Molten Ag

Abstract Three kinds of Mo-Ni-Si metallized coatings with various chemical compositions were deposited on SiC ceramic substrates by vacuum fusion sintering process, and the phase compositions of the coatings and their interface microstructures were analyzed. The wetting and spreading properties of molten Ag on the coated SiC ceramic substrates were investigated by the sessile drop technique, and the interfacial behavior of the Ag/coated SiC systems was analyzed. The results show that the coatings are mainly composed of Mo5Si3, MoSi2, Ni2Si, NiSi2 and MoNiSi. The tetragonal MoSi2 grains on the coating surface disappear gradually with the concentration of Mo increasing from 20 at% to 40 at%. The final contact angles of Ag/coated SiC systems at 1000 °C for holding 30 min are 45°, 79° and 85° for the coating compositions of Mo20-Ni32-Si48, Mo30-Ni28-Si42 and Mo40-Ni24-Si36, respectively. This result could be closely related to the interactions between the Ag drop and the microstructures of the three Mo-Ni-Si coatings. No obvious reaction layers are found at all the coating/substrate interfaces before and after the wetting tests.

Comparison of ZrB2-MoSi2 Composite Coatings Fabricated by Atmospheric and Vacuum Plasma Spray Processes

In this work, ZrB2-20 vol.% MoSi2 (denoted as ZM) composite coatings were fabricated by atmospheric plasma spray (APS) and vacuum plasma spray (VPS) techniques, respectively. Phase composition and microstructure of the composite coatings were characterized. Their oxidation behaviors and microstructure changes at 1500 °C were comparatively investigated. The results showed that VPS-ZM coating was composed of hexagonal ZrB2, tetragonal and hexagonal MoSi2, while certain amount of ZrO2 existed in APS-ZM coating. The oxide content, surface roughness and porosity of VPS-ZM coating were apparently lower than those of APS-ZM coating. The mass gain of APS-ZM coating was maximum at the beginning (1500 °C, 0 h) and then decreased with the oxidation time extending, while the mass of VPS-ZM coating gradually increased with increasing the oxidation time. The possible reasons for the different oxidation behaviors of the two kinds of coatings were analyzed.

Fabrication and Composition Investigation of WSi2/MoSi2 Composite Powders Obtained by a Self-Propagating High-Temperature Synthesis Method

In this paper, MoSi2/WSi2 composite powders with different phase compositions were fabricated via a self-propagating high-temperature synthesis (SHS) method. The influence of initial raw material compositions on the formation and the phase evolution of the MoSi2/WSi2 composite powders were investigated. Thermodynamic analysis indicated that the incorporation of W in the starting materials decreased the adiabatic temperature during the SHS reaction, and self-sustainable reaction can be hindered when the W content reached up to 40%. For samples with raw material composition of (\(1-x){\rm Mo}{\cdot} {\rm xW}{\cdot} {\rm 2Si}\) (X = 0.10, 0.20, 0.30, 0.40), composite MoSi2/WSi2 powders can be prepared, which were confirmed by the existence of two fitted peaks that can be assigned to tetragonal MoSi2 (103) and WSi2 (103) crystallographic planes in the XRD fine scan patterns. SEM images also confirmed the existence of two maxima in particle size distributions for samples with high W addition contents. The formation of MoSi2/WSi2 composite powders and the evolution of composite phases reported in this paper may provide an effective strategy for the synthesis of silicide based composite materials.

First-principles study of electronic properties of MoSi2 thin films

Electronic properties of tetragonal MoSi2 thin films are studied by the first-principles method. The results show that the MoSi2 film is always metallic, and its density of states and electronic structure are gradually close to their bulk counterpart as the film thickness increases. We further show that the three-atomic-layer film with the lowest energy is magnetic and has a magnetic moment of 0.33 B for its unit cell, and the film becomes non-magnetic when its thickness is more than three atomic layers. Moreover, we investigate the electronic properties of the three-atomic-layer MoSi2 films under unilateral and bilateral hydrogenation and find that the film with unilateral hydrogenation is magnetic and has a magnetic moment of 0.26 B, while the film with bilateral hydrogenation is non-magnetic. The spin polarizations for the films without hydrogenation and unilateral hydrogenation are 30% and 33%, respectively. These results suggest that three-atomic-layer MoSi2 film is metallic or magnetic when it is under suspension or grown on substrate, indicating its potential applications in nanoscale electronic and spintronic devices.

SEM/EDS/EBSD study of the behaviour of Ge, Mo and Al impurities in complex-doped crystals of higher manganese silicide

The structure of Al, Ge, Mo doped higher manganese silicides (HMS) grown by the Bridgman technique has been studied by SEM/EDS/EBSD methods. It is shown that dopants are partially integrated into the HMS crystal lattice. Some inclusions with sizes of 0.1-100 μm and different shapes (round, irregular, elongated) are formed. The precipitation of tetragonal MoSi2 and Si-Ge solid-solution has been observed. MoSi2 inclusions hundreds of microns in size form a multicomponent texture. The inclusions of Si-Ge solid solution have an irregular shape. The orientation relationship between these inclusions and matrix crystal is determined.

Microstructure Characteristics and Oxidation Behavior of Molybdenum Disilicide Coatings Prepared by Vacuum Plasma Spraying

In this study, molybdenum disilicide (MoSi2) coatings were fabricated by vacuum plasma spraying technology. Their morphology, composition, and microstructure characteristics were intensively investigated. The oxidation behavior of MoSi2 coatings was also explored. The results show that the MoSi2 coatings are compact with porosity less than 5%. Their microstructure exhibits typical lamellar character and is mainly composed of tetragonal and hexagonal MoSi2 phases. A small amount of tetragonal Mo5Si3 phase is randomly distributed in the MoSi2 matrix. A rapid weight gain is found between 300 and 800 °C. The MoSi2 coatings exhibit excellent oxidation-resistant properties at temperatures between 1300 and 1500 °C, which results from the continuous dense glassy SiO2 film formed on their surface. A thick layer composed of Mo5Si3 is found to be present under the SiO2 film for the MoSi2 coatings treated at 1700 °C, suggesting that the phenomenon of continuous oxidation took place.

SHS of ultrafine and nanosized MoSi2 powders

Ultrafine MoSi2 powders were prepared by SHS reaction from the elements (under an Ar pressure of 5 atm) followed by optimized diminution process and chemical dispersion (hot alkali etching). Raw SHS products (cakes) were found to contain tetragonal MoSi2 and an admixture of Mo4.8Si3C0.6. Chemical dispersion of ground product yielded MoSi2 powders in the form of porous agglomerates. The precipitates containing 50% MoSi2 and 50% K2MoO4 were separated from etching solutions. The size of MoSi2 particles was below 100 nm.

Arrays of Si cones prepared by ion beams: growth mechanisms

AbstractArrays of uniformly distributed Si cones with controlled orientations are fabricated on Si substrates by an ion beam technique. The axes of produced Si cones are well aligned with the direction of ion beams. Each cone essentially consists of a metal containing tip and a single crystalline Si base with the crystallographic orientation identical to the Si substrate. TEM studies show tapered pit interface between the tip and the base, visualizing the ion impinging process. The tip comprises tetragonal MoSi2, domains of Mo‐modulated Si ordered superstructures and a few Mo nanoparticles. Experimental evidence indicates that the cone formation follows the left‐standing model. Since the cones progressively are sharpened with the elapsed time of their formation, they might develop into whiskers if the ion bombardment and supply of sputtered metal clusters are prolonged.

MoSi2 oxidation resistance coatings for Mo5Si3/MoSi2 composites

In order to improve the oxidation resistance properties of 30 at.% Mo5Si3/MoSi2 composite at high temperature in air, a molybdenum disilicide coating was prepared on its surface by a molten salt technology. XRD and SEM analysis showed that only tetragonal MoSi2 phase existed in the coating after being siliconized for 5 h at 900°C. The oxidation film formed on the uncoated sample was not dense, so that oxygen diffused easily through it. The volatilization of MoO3 resulted in the oxidation film separating from the substrate. The MoSi2 coating was proved to be an effective method to prevent 30 at.% Mo5Si3/MoSi2 composites from being oxidized at 1200°C. A dense glassy SiO2 film was formed on the MoSi2 coating surface, which acted as a barrier layer for the diffusion of oxygen atoms to the substrate. The 30at.% Mo5Si3/MoSi2 composites with a MoSi2 coating showed much better oxidation resistance at high temperature.

Phase stability, electronic structure and mechanical properties of molybdenum disilicide: a first-principles investigation

The phase stability, electronic structure and mechanical properties of MoSi2 at different phases were systematically investigated by first-principles density functional theory calculations. The results indicated that both tetragonal and hexagonal MoSi2 are thermodynamically and mechanically stable. The formation energy of the hexagonal phase is 6.27 kJ mol−1 smaller than that of the tetragonal one. In tetragonal MoSi2, Mo 4dxz, 4dyz and orbitals overlap effectively with Si , px and py ones, while interactions between Mo (4dxy) and Si 2p orbitals are confirmed in the hexagonal phase. However, the bond strengths of the hexagonal phase are smaller, leading to changes in the mechanical properties. Young's modulus decreases from 443.33 to 341.37 GPa as the phase transforms from the tetragonal to the hexagonal phase. The weakness of the Si–Mo bonds along the [0 0 1] direction and the Si–Si bonds within the (0 0 1) plane make the shear deformations of the hexagonal phase much easier to occur, and the G/B ratio correspondingly decreases, suggesting improvement in ductility. Moreover, the calculated Vicker's hardness of the hexagonal phase is 10.15 GPa, 48% smaller than the value in the tetragonal one. Besides the structural transformation, the external pressure can also affect the mechanical properties of the system. Different from the structural change, the external pressure enhances the Si–Si interactions while it reduces the Si–Mo (II) bond populations. Both the Vicker's hardness and ductility are improved as the hydrostatic pressure increases. The present calculations confirmed that the Si–Si (I) interactions play a central role in the hardness and ductility of MoSi2 materials.

High-pressure synthesis of novel compounds in an Mg–Ni system

High-pressure works are attractive techniques to obtain new compounds, such as alkali or alkaline earth metal-based systems. The atomic radius of Mg under GPa pressure is considerably smaller compared with transition metals; as such, it may be preferable to synthesize novel intermetallic compounds and hydrides by using high-pressure techniques. In this study, novel compounds were synthesized in an Mg–Ni system by a high-pressure technique using a cubic-anvil-type apparatus. A novel Mg 6Ni intermetallic compound was obtained by exposing a mixture of Mg and Ni to 6 GPa at 900 °C for 2 h. The crystal structure of the compound is a tetragonal F-43m structure with a lattice parameter of a=1.9987(1) nm. This compound decomposed to Mg and Mg 2Ni phases at 278 °C with exothermic reaction. As is well known, MgNi 2 does not form hydrides under conventional hydrogenation conditions, hence we investigated the reactivity of MgNi 2 with highly pressurized hydrogen. It was found that the MgNi 2 was able to form MgNi 2H 3.2 by treatment at 700 °C for 2 h under 5 GPa with a hydrogen source, leading to a hydrogen capacity of 2.23 mass%. This novel hydride was found to be a tetragonal MoSi 2-type structure (I4/mmm) with lattice parameters of a=0.327(3) nm and c=0.878(9) nm. The dehydrogenation of this hydride occurred at 187 °C with endothermic reaction, and caused decomposition into C36-type MgNi 2. This hydride had solubility of Ni content and its thermal stability decreased with increasing Ni content.

The Thermal Stability of Mo(Al,Si)<sub>2</sub> Phase

It has been reported that formation of Mo(Al,Si)2 phases by the addition of aluminum can improve the mechanical and oxidation properties of MoSi2, however, the constancy of the Mo(Al,Si)2 phase was not explored. In this study, MoSi2 preforms were added with Al whose contents were 1, 5, 10wt%, respectively, and these performs were sintered at 1150°C-1550°C, respectively. These MoSi2 specimens with the Al addition were heat-treated at 1350°C-1550°C in N2 atmosphere. Results showed that single Mo(Al,Si)2 phase existed only when the aluminum content was above 5 wt%. It was difficult to form single Mo(Al,Si)2 phase for the samples with 1wt% Al addition. For the samples with 5wt% Al, only the single Mo(Al,Si)2 phase was formed at 1350°C; and a large number of the tetragonal MoSi2 were re-appeared at 1450°C, 1550°C. For the samples with 10wt% Al addition, the single Mo(Al,Si)2 phase would always formed at individual sintering temperatures. The formation temperature of the single Mo(Al,Si)2 phase was decreased with increment of aluminum content. For the samples composed of the single Mo(Al,Si)2 phase, the Mo(Al,Si)2 phase disappeared gradually and was substituted for MoSi2 and a little of other phases, such as Mo3Al4Si2, by heat treatment at temperatures mentioned above for 40 min. This indicated that the Mo(Al,Si)2 phase was unstable at the temperatures above 1350°C.

Vertical section of Au 2Y–Ag 2Y in Au–Ag–Y system

A vertical section at 33.3 at.% Y in the Au–Ag–Y ternary system was studied using X-ray diffraction analysis, differential thermal analysis, and optical microscopy. An allotropic phase transformation of the Au 2Y phase in the Au–Y binary system is found. The Au 2Y phase transforms from a room temperature phase α to a high-temperature modification β at about 778 °C. The α-Au 2Y has a tetragonal MoSi 2-type structure with the lattice parameters a = 0.3692 nm, c = 0.8981 nm; the β-Au 2Y has an orthorhombic CeCu 2-type structure with the lattice parameters a = 0.4658 nm, b = 0.7340 nm, and c = 0.8200 nm. No allotropic phase transformation of the Ag 2Y phase in the Ag–Y binary system is found. The vertical section of the Au 2Y–Ag 2Y in the Au–Ag–Y ternary phase diagram established on the basis of these studies consists of seven phase regions: L (liquid phase region), α (MoSi 2-type structure), β (CeCu 2-type structure), α + β, α + L, β + L, and α + β + L.

Thermodynamic modeling of the Pd–Zr system

The thermodynamic properties and phase diagram of the Pd–Zr system have been analyzed by the CALPHAD technique using a computational optimization procedure. Based on the experimental data, the solution phases (liquid, body-centered cubic (Zr), face-centered cubic (Pd) and hexagonal close-packed (Zr)) were modeled with the Redlich–Kister equation. Both compounds Pd 2Zr and PdZr 2 having a tetragonal MoSi 2-type structure were treated as one phase with the formula PdZr(Pd, Zr) by a three-sublattice model with Pd on the first sublattice, Zr on the second and Pd and Zr on the third one, respectively. A three-sublattice model (Pd,Zr) 0.5(Pd,Zr) 0.5(V a) 3 is applied to describe the compound γ PdZr in order to cope with the order–disorder transition between body-centered cubic solution (A2) and γ PdZr with CsCl-type structure (B2). Another three-sublattice model (Pd,Zr) 0.75(Pd,Zr) 0.25(V a) 0.5 is applied to describe the compound Pd 3Zr in order to cope with the order–disorder transition between hexagonal close-packed (A3) and Pd 3Zr with Ni 3Ti-type structure (D0 24). The other compounds were treated as stoichiometric compounds. A set of self-consistent thermodynamic parameters of the Pd–Zr system was obtained.