Structure Of MoSi2 Research Articles

Electrodepositing a Mo layer and composite siliconized coating on Nb-based alloy for high temperature protection

In order to protect Nb-based alloy from oxidation attack, a Mo layer was first electrodeposited under different current densities and then its siliconized coating was obtained via halide activated pack cementation (HAPC) process. Microstructure, chemical composition, hardness and adhesive strength of the Mo pre-layer on the Nb-based alloy were analyzed and oxidation behavior of the siliconized coating exposed at 1200 °C and 1300 °C was investigated. The results showed that the electrodeposited Mo layer was composed of amorphous Mo phase and MoO3. The oxygen content in the Mo layer was low as 38.2 at% when the current density during electrodeposition was 400 mA/cm2 with the microhardness as 238 ± 18 HV. The HAPC siliconized coating exhibited a multilayer structure of MoSi2/(Mo, Nb)5Si3/NbSi2, which showed an increase in the cohesive/adhesive strength compared to the Mo layer. Mainly Nb2O5 and SiO2 were formed on the exposed siliconized coating. An accelerated mass gain and a thicker oxidation layer were found as the exposure temperature increased with a thinning of NbSi2 inner layer. The siliconized coating provided an effective oxidation resistance for the Nb-based alloy and the increased exposure temperature made the protection of the coating deteriorate.

The Crystal Structure and Phase Transition of Hf2Pt3

The structure of Hf2Pt3 has been re-investigated using transmission electron microscopy and Rietveld refinement of neutron powder diffraction data at temperatures up to 1500 °C. This compound does not belong to the MoSi2 structure type as previously reported, but instead is isostructural with Ti2Pd3 and the low-temperature form of Ti2Ni3. The crystal structure is orthorhombic (pseudo-tetragonal), Cmcm, Z = 4, with a = 14.653(1) A, b = 4.8741(3) A, and c = 4.8671(3) A at room temperature. The b/c ratio decreases from 1.0014(2) at room temperature to 1.0005(3) at 1500 °C, but the material does not transform to the high-temperature form of Ti2Ni3 (I4/mmm) under the conditions studied. The electron diffraction data indicate multiple stacking faults as well as short-range order. On heating above 1630 °C there is a phase transition to a B2-related structure with a ≈ 3.315(1) A at 1659 °C.

Calculation and analysis of the valence electron structure of MoSi2 and (Mo0.95, Nb0.05)Si2

Based on the empirical electron theory (EET) of solids and molecules, the valence electron structure (VES) and theoretical bond energy of MoSi2 was calculated by the bond length difference method (BLD). According to the average atomic model in substitutional solid solution of EET, the VES and theoretical bond energy of (Mo0.95, Nb0.05)Si2 were analyzed. The results indicated that the Nb alloying into MoSi2 changed the hybridization state of Mo and Si atom, which correspondingly changed the parameters of VES. The ratio of covalent valence electron number to total number of valence electron in MoSi2 is 65.87%, compared with that of MoSi2, the ratio increase to 80.61% in (Mo0.95, Nb0.05)Si2, thus the addition of niobium can improve the strength of MoSi2. The lattice electron decrease from 4.7141 to 2.5526, which reduced the plasticity of MoSi2 by niobium microalloying.

Re4Si7, First in a New MoSi2‐Based Family of 14‐Electron Phases

Magic number 14! Re4Si7 is the first complex structure outside of the Nowotny Chimney Ladder series recognized as adhering to the 14-electron rule. Its structure is based on Si vacancies in the MoSi2 structure pattern clustering into periodic edge-dislocations (see figure). Coincident with its 14-electron count is a pseudo-gap in its electronic density of states.

Micro and nano-indentation of MoSi2

Molybdenum disilicide based materials are candidate for high temperature structural applications, such as a furnace heating element and an electrical conductor in silicon intergrated circuit design or parts of engines [1]. MoSi2 exhibits a high melting point (2030 ◦C), excellent high temperature oxidation resistance, and posseses many convenient properties such as high stiffness, high thermal conductivity, relatively low density, and high strength at elevated temperatures. However, a major difficulty in application of these materials is the lack of adequate ductility and fracture toughness below 1000 ◦C. Only toward the higher temperatures (between 1000 and 1400 ◦C), with the onset of dislocation climb and diffusional creep processes, does MoSi2 exhibit significant plasticity in compression, bending, and tension in both single crystals and polycrystalline materials [2, 3]. During the last decade many approaches had been applied to reduce the brittle-to-ductile transition temperature (BDTT) of these materials, to overcome the limitation of dislocation mobility and dislocation density, and to enhance the capability for plastic flow and increase the fracture toughness. The main approaches for ductility enhancement are solid solution alloying, second phase microstructure control, ductile phase toughening, and high temperature prestrain [4]. The crystal structure of MoSi2 is tetragonal (C11b type), space group 14/mmm. The lattice parameters are a = 0.3205 nm and c = 0.7845 nm with c/a = 2.45 (Fig. 1). MoSi2 is also reported to have hexagonal C40 structure above 1900 ◦C [5]. There exists an absence of knowledge concerning the relative mobility of edge and screw dislocations and information about different dislocation types 〈100〉, 〈110〉, 1/2〈111〉, and 1/2〈331〉, their glide planes; furthermore the operative slip systems as a function of temperature, strain rate, and crystallographic orientation are only partially understood. Studies of the slip systems by means of hardness indentation for MoSi2 single crystal has found that the primary and secondary slip systems were {100} 〈001〉 and {110} 〈001〉, respectively [4]. Berkowitz et al. [6] reported that {110} is the slip plane in MoSi2 single crystal deformed between 625 and 1125 ◦C under compressive load along three different directions. They con-

Silicon diffusion in molybdenum disilicide: correlation effects

Correlation factors for silicon diffusion by a vacancy mechanism in the silicon sublattice of the tetragonal MoSi2 structure have been calculated by combining an analytical and a Monte Carlo approach. The ratio of the silicon diffusivity perpendicular to the tetragonal axis to that parallel to the tetragonal axis is also deduced. An effect of forward correlation of tracer atom jumps in the silicon sublattice with the corresponding partial correlation factor of 1.5 appears at small frequencies of silicon atom jumps along the tetragonal axis with respect to the jump frequencies in the silicon layer perpendicular to the tetragonal axis of the MoSi2 structure. The anisotropy of silicon diffusion in MoSi2 measured by Salamon et al. is explained in terms of correlation effects of silicon diffusion on its own sublattice.

Fermi surface and electrical characteristics of molybdenum disilicide

Semirelativistic self-consistent calculations of the electronic structure of MoSi2 are performed within the framework of the linearized augmented-plane-wave (APW) method in the local density functional approximation. The results of investigations of the band structure, the Fermi surface, and electrical characteristics (effective cyclotron masses, the conductivity anisotropy constant, the mean free path, and the coefficient γ of the heat capacity component linear in temperature) are reported. The Fermi surface consists of two sheets, namely, an electron sheet and a hole sheet. The extreme sectional areas of the Fermi surface agree well with the experimental data on the de Haas-van Alphen effect. The results of first-principles calculations need no additional correction.

Incommensuration and other structural anomalies in rhenium disilicide

Abstract The structure of solidification-processed rhenium disilicide is investigated by X-ray diffraction and transmission electron microscopy. It is usually reported to have the tetragonal C11b MoSi2 structure and the stoichiometry of ReSi2. In the present study it is found that rhenium disilicide exhibits an off-stoichiometric Si-deficient composition of Re4Si7. In addition, several structural instabilities are observed by electron microscopy including firstly an orthorhombic distortion of about 0.6% from the tetragonal C11b MoSi2 structure with spot splitting across the {110} twin plane, secondly an incommensuration along the a axis with a periodicity of 1.3nm, thirdly another longer period modulation along the c axis, fourthly a weak monoclinic distortion (β ≈ 90.4°) in some specimens, as revealed by spot splitting in the high-order Laue zone reflections of [100] diffraction patterns, accompanied by twinning on the (001) planes, and fifthly much larger distortions in some [010] patterns plus a chevron structure in the image which may result from a displacive transformation. These structural anomalies are probably related to the stoichiometry of Re4Si7 which may result in a composite modulated structure where the Si sublattice has a larger average periodicity along the a axis than the Re sublattice, as is the case with manganese silicides.

Incommensurate Structures in Rhenium Disilicide

Abstract The binary ReSi2 and ternary (Mo,Re)Si2 alloys are of interest due to potential applications as infrared detectors [1]. Also, Re may be a beneficial alloying element to MoSi2 which is a potential high temperature structural material [2]. A recent study of the structure of ReSi2 by X-ray diffraction (XRD) revealed an orthorhombic distortion (a= 0.3121 nm, b= 0.3139 nm and c= 0.7670 nm) from the tetragonal CI lb MoSi2 structure and a Si-deficient composition of Re4Si7 [3]. Further, a weak monoclinic distortion (β=89.87°) was also inferred from high-resolution XRD [3]. In the present investigation, the orthorhombic distortion and the Si-deficient stoichiometry were confirmed by XRD and the structure of arc-melted Re4Si7 studied by transmission electron microscopy (TEM). A bright field (BF) TEM image and the corresponding [010] zone axis selected area diffraction pattern (SADP) are shown in Fig. 1. In addition to the fundamental reflections for tetragonal Cllb MoSi2 structure,

Refinement in crystal structure of MoSi2

Abstract The crystal structure of MoSi2 was investigated using a synchrotron X-ray powder diffraction method. From the Rietveld analysis it was shown that MoSi2 had a tetragonal structure of the space group I4/mmm and the lattice parameters were a = 0.32064(2) nm and c = 0.78478(8) nm. The position of Si atoms in the 4e site was determined to be (0, 0, 0.3353). There was a small shift in the z position, as compared to the previously reported value, 0.3333. The electronic structure of MoSi2 was calculated by the DV-Xα molecular orbital method employing the refined structural parameters, and this showed that there was a strong covalent bond between Si atoms aligned along the c axis in MoSi2.

Study of the Effects of Al, Ta, W Additions on the Microstructure and Properties of Mosi2 Base Alloys

AbstractSolidification microstructures and phase selection in ingots and melt-spun ribbons of MoSi2, MoSi2-W-Ta and MoSi2-Al alloys were studied. Rapid quenching from the melt refined the grain size of all alloys by two orders of magnitude compared to the ingots. Zone A microstructures were formed in the wheel side of all alloy ribbons. Addition of W and Ta and of Al suppressed the formation of Mo5Si3 in the ingots and ribbons. Tungsten exhibited complete solid solubility in MoSi2. In the ribbons the solubility of Ta in MoSi2 was extended to 2 at% in Zone A, and in the ingots and zone B of the ribbons hexagonal (C40) TaSi2 and tetragonal (C11b) MoSi2 structures were formed. A fine lamellar microstructure between the Cllb and C40 phases was formed at 1350°C after 48h. Addition of Al changed dramatically the solidification microstructure which consisted of two phases C40 Mo(Al0.5Si0.5)2 and C54 MoAl1.3Si0.7. In the ingots the room temperature hardness decreased with addition of W and Ta and of Al. Addition of Al increased the thermal stability of the MoSi2-Al alloy.

Fracture Modes in MoSi2

The room‐temperature fracture behavior of polycrystalline MoSi2 was characterized using Vickers indentation fracture. Fracture analysis was aided by the optically active grain structure of MoSi2 revealed under polarized light. Radial crack propagation from indentations was found to be predominantly transgranular. The approximate indentation fracture toughness of MoSi2 was 3 MPa.m1/2, while the measured hardness was 8.7 GPa. Fracture behavior is believed to be controlled by anisotropy and cleavage energy of the tetragonal MoSi2 crystal structure.

Optical Properties of Growth Phases Formed by Rapid Thermal Annealing of MoSix thin Films on Si (100)

AbstractAmorphous thin films were deposited by co-sputtering Mo and Si on Si(100) single crystals. Rapid thermal annealing at 700, 800 and 1050°C for 30 sec. resulted in the growth of hexagonal and tetragonal phases of MoSi2 as revealed by glancing angle x-ray diffraction. The optical properties of these films were studied by spectroscopic ellipsometry in the range 1.0 to 5.0 eV. As deposited films showed both metallic and semiconducting features in the <ε1> and <ε2> spectra. As a result of annealing at 700 and 800°C, pronounced increase in <ε1> was obtained. Annealing at 1050°C resulted in decrease of the values of <ε2>. Surface morphology, density changes and stress variations associated with recrystallization and compound formation are proposed to account for these observations. The peaks in <ε2> spectra are found to be interband transitions in the MoSi2 structures. These results are in confirmity with the EELS and UPS studies and theoretically calculated band structure of MoSi2.

Transport properties of hexagonal and tetragonal MoSi2 thin films

The electrical resistivity and the Hall coefficient of thin polycrystalline MoSi2 films were measured as a function of temperature from 4.2 to 300 K. The transverse and longitudinal magnetoresistances have been determined at temperatures between 2 and 50 K at fields up to 10 T. Samples of both the high-temperature tetragonal MoSi2 phase and the low-temperature hexagonal phase have been prepared. The Hall coefficient of tetragonal MoSi2 changes sign with temperature, which means that this compound is a multiband conductor, and thus no reliable value for the carrier concentration can be derived from Hall measurements alone. Magnetoresistance data have therefore been analyzed with a simple two-band model in order to determine the dominant carrier type in both MoSi2 structures. In tetragonal MoSi2, electrons and holes are present in equal concentrations, whereas in hexagonal MoSi2 holes are the dominating type of carrier.

Studies of the electronic structures of MoSi2

The electronic states of MoSi2 in the body-centred tetragonal CaC2 structure have been studied using the self-consistent linear muffin-tin orbital method. The core states are treated in a fully relativistic way, whereas for the valence states a semirelativistic approximation is utilised. The results show that the majority of Mo d electrons interact with Si p electrons; therefore there exists a strong p-d hybridisation state in MoSi2. The energy band structure is compared with that calculated by the fully relativistic LCGO (linear combination of Gaussian orbitals) method; it was found that the spin-orbit coupling has little effect on the band structure of MoSi2. The total and the projected densities of states are also calculated and compared with the photo-emission data.

Effects of hydrogen on magnetic properties of amorphous zirconium alloys

The magnetic susceptibilities χ(T) of the amorphous ternary hydrides of the paramagnetic metallic glasses a-Zr2Pd and a-Zr3Rh have been measured between 7 and 300 K. It has been found that χ(T) of both a-Zr2PdHx and a-Zr3RhHx decrease substantially with increasing hydrogen content. This behavior is primarily attributed to reductions in the densities of the Fermi level electronic states. Although χ(T) for crystalline Zr2Pd (MoSi2 structure type) is smaller than the susceptibility of the a-Zr2Pd glass, χ(T) for a crystallized Zr3Rh sample was found to increase relative to the initial susceptibility for this glass.

The effect of phosphorus ion implantation on molybdenum/silicon contacts

Formation of Mo/Si contacts by implantation of phosphorus ions was studied. The implantation was carried out at temperatures of −196, 25, and 150 °C and a fluence ranging between 1015 and 1017 ions cm−2. The morphological and structural characterizations were done with scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. Hexagonal MoSi2 phase was identified in samples implanted with more than 1016 ions cm−2 at all three implantation temperatures. Traces of MoP were found in the sample implanted with 1017 ions cm−2 at 150 °C. Measured effective contact resistance showed ohmic behavior in as-implanted samples except for samples implanted with 1015 ions cm−2 at 150 °C. Smooth surfaces of implanted MoSi2 structures remained after post-implant annealing at 850 °C for 1/2 h in H2 ambient. The effect of post-implant annealing is also discussed in terms of doping, microstucture, and contact resistance.

反応性スパッタ蒸着法によるモリブデンシリサイド薄膜形成

Recently, metal silicides have been noticed as a gate and an interconnection for the fabrication of high performances MOS devices. In this paper, a new method of depositing molybdenum suicide films is presented, in which the films are deposited by the reactive sputtering of a Mo target in a mixture of argon and silane. Using a mass spectrometer, it was confirmed that most of the silane was decomposed by applying a sputtering power above 0.6 kW. The crystal structure and chemical composition of the films have been determined by X-ray diffraction and an Auger electron spectrometer, respectively. It was found that the Si/Mo atomic ratio of the deposited films could change in the range 0.026 to 1.9 by controlling the silane partial pressure and the sputtering power. It was possible to obtain the Mo3Si, Mo5Si3 and MoSi2 structures by this method. On increasing Si/Mo atomic ratio in the ranee 0.026 to 1.9. the electrical resistivity of the films increase gradually.