Tetragonal MoSi2 Research Articles

Diffusion in molybdenum disilicide

Abstract The diffusion behaviour of the high-temperature material molybdenum disilicide (MoSi2) was completely unknown until recently. In this paper we present studies of Mo self-diffusion and compare our present results with our already published studies of Si and Ge diffusion in MoSi2. Self-diffusion of molybdenum in monocrystalline MoSi2 was studied by the radiotracer technique using the radioisotope 99Mo. Deposition of the radiotracer and serial sectioning after the diffusion anneals to determine the concentration-depth profiles was performed using a sputtering device. Diffusion of Mo is a very slow process. In the entire temperature region investigated (1437 to 2173 K), the 99Mo diffusivities in both principal directions of the tetragonal MoSi2 crystals obey Arrhenius laws, where the diffusion perpendicular to the tetragonal axis is faster by two to three orders of magnitude than parallel to it. The activation enthalpies for diffusion perpendicular and parallel to the tetragonal axis are Q⊥ = 468 kJ ...

A Surface Chemistry Route to Molybdenum Sulfide and Germanide Films Using the Single-Source Precursor Tetrakis(diethylaminodithiocarbomato)molybdate(IV)

We report for the first time the direct deposition of crystalline molybdenum sulfide (MoS2) using a single-source precursor based on tetrakis(diethylaminodithiocarbomato)molybdate(IV) (abbreviated as Mo(Et2NCS2)4). The chemistry of this precursor adsorbed on a range of substrates (silicon, germanium, gold-coated germanium, nickel, etc.) has been studied using in situ X-ray photoelectron spectroscopy. The Mo(Et2NCS2)4 precursor can be evaporated at 300 °C; its vapor adsorbs on most surfaces at room temperature and decomposes by 400 °C to form crystalline MoS2. Using this method, high-quality, basal plane-oriented MoS2 can be grown on nickel by a one-step thermal evaporation process for the first time. Interestingly, choosing elemental substrates which form eutectic alloys with gold favors the elimination of sulfur from the MoS2 film. This results in Mo intermetallic compound formation at the eutectic temperatures of the Au and substrate element. Unpredecented low-temperature growth of tetragonal MoSi2 or o...

Self-diffusion of silicon in molybdenum disilicide

The self-diffusion of silicon in single crystal MoSi2 was studied by means of a radiotracer technique using the short-lived radioisotope 31Si (half-life ), which was produced and implanted into the samples at the ion-guide isotope separator on-line device at the University of Jyväskylä in Finland. Diffusion annealing and subsequent serial sectioning of the specimens were performed immediately after the radiotracer implantation. In the entire temperature region investigated (835–1124 K), the 31Si diffusivities in both principal directions of the tetragonal MoSi2 crystals obey Arrhenius laws, where the diffusion perpendicular to the tetragonal axis is faster than parallel to it. In previous studies the same features were observed for the 71Ge diffusivities in MoSi2, except that these are somewhat higher than those of 31Si. Furthermore, it is noteworthy that in MoSi2 the diffusivities of 31Si and 71Ge are orders of magnitude faster than the diffusivity of 99Mo. This large difference suggests that silicon diffusion and molybdenum diffusion are completely decoupled and that silicon diffusion takes place exclusively on the silicon sublattice. Literature data on the phase growth of MoSi2 are in accordance with the present results on the 31Si diffusivities; Monte Carlo simulations of the correlation effects of vacancy-mediated diffusion on the silicon sublattice of MoSi2 lead to their rationalization.

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.

Formation of MoSi 2 by rapid thermal annealing in vacuum of CVD – Mo films on silicon substrate

Silicide formation due to thermal treatment of thin (200–1000 Å) molybdenum films on single-crystal silicon substrates in the temperature range of 800–1000°C was studied. Rapid thermal annealing (RTA) in vacuum was performed on Mo/Si system to form silicides. Molybdenum films were deposited on Si (1 0 0) by chemical vapor deposition (CVD), using Mo(CO) 6 (molybdenum hexacarbonyl) as precursor. The as-deposited films with thicknesses of 200 and 1000 Å were heated in vacuum at RTA temperatures of 800 and 1000°C for time durations of 15 and 30 s, 1 and 3 min. The results from the X-ray diffraction analysis and the Reflection Infrared Spectroscopy measurements show that Mo films after annealing transform into Mo silicides, preferably tetragonal MoSi 2 crystal phase. The presence of Mo 5Si 3 formation is also found. There is a change in the electrical resistance of the films before and after annealing. For the molybdenum silicides obtained the resistivity is in the range 0.9–9.8 mΩ cm.

MoSi 2 coating on molybdenum using molten salt

For obtaining oxidation resistance of molybdenum in air, molybdenum silicide was non-electrolytically coated in the molten salt, where a disproportion reaction occurs between the salt composed of NaCl–KCl–NaF–Na 2SiF 6–SiO 2 and the Si powder. Hexagonal MoSi 2 was formed as single phase with homogeneous thickness of tens of μm above 1073 K, while the tetragonal MoSi 2 phase was additionally formed at 973 K. The growth rate of the MoSi 2 layer and its morphology at the sample corner were affected by this phase formation. The protective layer on the siliconized sample proved to be effective for preventing oxidation that occurs for pure molybdenum at low temperatures. During the oxidation at the high temperature, Mo 5Si 3 was formed at the interface between the Mo substrate and the MoSi 2 layer.

Sintering of molybdenum disilicide doped with nickel : I.H.Moon et al. (Hanyang University, Seoul, Korea.)

The nanocomposite concept is briefly reviewed at first, and the approach to fabricating strong and tough MoSi2-based materials through the nanocomposite technology is demonstrated. Strong MoSi2-based nanocomposites such as MoSi2/SiC and MoSi2/Mo(Si,Al)2/SiC were developed by conventional powder metallurgical techniques. Effects of grain boundary and interface structure on mechanical properties were investigated for these composites. In the MoSi2/SiC nanocomposites, both MoSi2 matrix and siliceous grain boundary phases were simultaneously controlled, nano-sized SiC particles (50–200 nm) were dispersed into both MoSi2 matrix grains and glassy SiO2 located at triple junctions. High-resolution TEM observation revealed that the intragranular SiC particles directly bonded to MoSi2 without any impurity phases, resulting in toughening by the thermal expansion mismatch between MoSi2 and SiC. The intergranular SiC particles played important roles to inhibit the grain growth of MoSi2 matrix, and they concurrently toughened the glassy SiO2. For Al-alloyed systems, α-Al2O3 was dispersed in the compacts instead of glassy SiO2. The matrices became di-phasic, that is, mixture of tetragonal MoSi2 and hexagonal Mo(Si,Al)2. SiC dispersion in the Mo–Si–Al alloys effectively improved their mechanical properties. By means of such material designs, all composites showed very high strength (more than 1 GPa by the three-point bending test). In particular, MoSi2/Mo(Si,Al)2/SiC composites possessed the improved high-temperature strength of about 1 GPa at 1100°C.

Effect of pressure on the ferroquadrupolar compound TmAu 2

TmAu 2 with a simple body-centered tetragonal MoSi 2-type structure undergoes an electric transition to a ferroquadrupolar ordered state at T Q of 7.0 K and a magnetic transition to an antiferomagnetic state at T N of 3.2 K. We have measured pressure dependence of electrical resistivity on single-crystalline TmAu 2 sample. The results reveal that the T N decreased with increasing pressure (∂ T N/∂ P=−4.54 mK/kbar), while the T Q increased with increasing pressure (∂ T Q/∂ P=21.5 mK/kbar).

Pseudobinary intermetallic compounds in Hf2M′–Hf2M″ (M′, M″=Mn, Fe, Ni, Cu) systems and their interaction with hydrogen at high pressures

Abstract Ternary alloys in the Hf 2 Ni–Hf 2 Mn, Hf 2 Ni–Hf 2 Fe, Hf 2 Ni–Hf 2 Cu and Hf 2 Fe–Hf 2 Cu systems have been prepared and characterised by X-ray diffraction and pressure–composition isotherm measurements. Single phase compounds have been obtained for all Ni-containing alloys with a general composition of Hf 2 Ni 0.5 M 0.5 . These compounds retained the structure type of parent Hf 2 M intermetallics: cubic Ti 2 Ni type for Hf 2 Ni 0.5 Fe 0.5 and Hf 2 Ni 0.5 Mn 0.5 and tetragonal MoSi 2 type for Hf 2 Ni 0.5 Cu 0.5 . The new hydrides obtained from ternary intermetallics had the composition of Hf 2 Ni 0.5 M 0.5 H x where x =3.1 (M=Cu) and 4.9–5.0 (M=Mn, Fe). Formation of hydride phases from cubic Mn- or Fe-containing compounds occurred without any change in structure type but was accompanied by unit cell volume expansion of 23–24%. In the case of Cu, there was a tetragonal to orthorhombic transformation. An absence of phase transformations in high pressure range of 100 to 2000 atm for new compounds unlike the earlier studied Hf 2 M–H 2 systems has to be noted.

Metal silicides synthesized by high current metal–ion implantation

High current metal–ion implantation by a metal vapor vacuum arc ion source was conducted to synthesize some metal silicides, which are important candidates as materials in microelectronics. It was found that C54-TiSi2, ZrSi2, NiSi2, CoSi2, β-FeSi2, NbSi2, and TaSi2 layers on Si wafers with good electric properties could be obtained directly after implantation at relatively low formation temperature and that the formation of α-FeSi2, NbSi2, TaSi2, tetragonal WSi2, and tetragonal MoSi2 required additional postannealing to improve their crystallinity as well as their electric properties. Interestingly, NiSi2 layers of superior crystallinity and thus electric property were obtained for the first time by Ni ion implantation with a selected current density of 35 μA/cm2, which heated the Si wafers to a specific temperature of 380 °C. Under such formation conditions, the lattice mismatch between the growing NiSi2 and the Si substrate was calculated to be zero. The resistivity of the NiSi2 layers obtained was much lower than that of the Ni disilicide synthesized by a solid-state reaction that required a formation temperature of over 750 °C. The formation mechanism of the metal silicides studied and their associated electrical properties are also discussed.

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.

Strong monolithic and composite MoSi2 materials by nanostructure design

The nanocomposite concept is briefly reviewed at first, and the approach to fabricating strong and tough MoSi2-based materials through the nanocomposite technology is demonstrated. Strong MoSi2-based nanocomposites such as MoSi2/SiC and MoSi2/Mo(Si,Al)2/SiC were developed by conventional powder metallurgical techniques. Effects of grain boundary and interface structure on mechanical properties were investigated for these composites. In the MoSi2/SiC nanocomposites, both MoSi2 matrix and siliceous grain boundary phases were simultaneously controlled, nano-sized SiC particles (50–200 nm) were dispersed into both MoSi2 matrix grains and glassy SiO2 located at triple junctions. High-resolution TEM observation revealed that the intragranular SiC particles directly bonded to MoSi2 without any impurity phases, resulting in toughening by the thermal expansion mismatch between MoSi2 and SiC. The intergranular SiC particles played important roles to inhibit the grain growth of MoSi2 matrix, and they concurrently toughened the glassy SiO2. For Al-alloyed systems, α-Al2O3 was dispersed in the compacts instead of glassy SiO2. The matrices became di-phasic, that is, mixture of tetragonal MoSi2 and hexagonal Mo(Si,Al)2. SiC dispersion in the Mo–Si–Al alloys effectively improved their mechanical properties. By means of such material designs, all composites showed very high strength (more than 1 GPa by the three-point bending test). In particular, MoSi2/Mo(Si,Al)2/SiC composites possessed the improved high-temperature strength of about 1 GPa at 1100°C.

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,

Structural, electrical, magnetic and low-temperature specific heat studies of PrPb 2

Investigations of the structure, the electrical resistivity, the magnetic properties and the low-temperature specific heat of PrPb 2 have been made. The crystal structure identified by X-ray diffraction for PrPb 2 is the body-centered tetragonal MoSi 2-type structure with a=3.305 Å and c=16.060 Å. The magnetic part of the electrical resistivity for PrPb 2 shows a −log T dependence similarly to that of CePb 2, but its temperature region is not as wide as that of CePb 2. The temperature derivative of the low-temperature magnetic electrical resistivity d ρ mag/d T and the magnetic susceptibility show a broad peak around 6 K. The magnetic susceptibility exhibits Curie–Weiss behavior at high temperatures, and the effective magnetic moment is 3.68 μ B/Pr-mol and practically equals the value of the free trivalent Pr 3+ ion. A linear extrapolation in zero field of the C/ T versus T 2 plot for low-temperature specific heat gives a large electronic specific heat coefficient γ of 656 mJ·mol −1·K −2. Such a large value may be attribute to a Schottky specific heat. The peak around 4.5 K is too small and too broad, compared with a peak representative of a magnetic phase transition. Furthermore, this peak is apparently reduced by applied magnetic fields. Therefore, the peaks observed in the magnetic susceptibility and the specific heat are not correlated with the Néel temperature.

Elastic properties of C40 transition metal disilicides

Room-temperature and low temperature elastic properties of hexagonal C40 transition metal disilicides, NbSi2 and TaSi2, have been studied using Resonant Ultrasound Spectroscopy (RUS). All five independent elastic stiffness constantscij for NbSi2 and single crystals have been obtained. The temperature dependence of thecij is normal but not large. The orientation dependence of the Young's and shear moduli was examined in comparison with other transition metal disilicides. The room temperature shear moduli in the {0001} plane, with values of 145.3 and 143.7 GPa for NbSi2 and TaSi2 respectively, are low relative to those in the equivalent pseudo hexagonal {110} close-packed plane for tetragonal C11b MoSi2 and WSi2. The isotropic elastic constants for polycrystalline materials were also calculated. The results show that the various moduli are all much higher than those of the constituent elements. The room temperature Poisson's ratios of NbSi2 and TaSi2 are 0.18 and 0.19, respectively, which are smaller than those of the constituent elements and smaller than most materials. The Debye temperatures,θD, were estimated to be 688 K for NbSi2 and 552 K for TaSi2. The elastic properties of C40 VSi2, NbSi2, TaSi2, and CrSi2 and C11b and WSi2 are compared and the possible influence on mechanical behavior discussed.

Interfacial reactions and thermal stability of ultrahigh vacuum deposited multilayered Mo/Si structures

Interfacial reactions and thermal stability of ultrahigh vacuum deposited multilayered Mo/Si structures have been investigated by high resolution transmission electron microscopy in conjunction with fast Fourier transform and autocorrelation function analysis. For samples with nominal atomic ratios Mo:Si=1:2 and 3:1, well defined multilayered Mo/Si structures were obtained after annealing at 250 °C for 30 min. On the other hand, distinct multilayered MoSi2/Si structure was formed only for Mo:Si=1:2 samples after annealing at 650 °C for 1 h. Multiphases were observed to simultaneously form in samples annealed at 400–500 °C. After 600 °C annealing for 1 h, tetragonal MoSi2 was the only silicide phase observed for the Mo:Si=1:2 samples, whereas both tetragonal and hexagonal MoSi2 were present in Mo:Si-3:1 samples. The stability of the multilayered Mo/Si structures was found to depend critically on the atomic ratios of constituent elements, bilayer period, and annealing conditions. The results are interpreted in terms of the delicate balance among intermixing of constituent atoms, silicide formation, and crystallization of silicon.

Induction brazing of Ti-6Al-4V alloy with amorphous 25Ti-25Zr-50Cu brazing filler metal

Ti-6Al-4V alloy was induction brazed with 25Ti-25Zr-50Cu amorphous filler metal using argon as a shielding gas. The brazing cycles were rather short: RF inductor power was supplied for only 40–60 s. The tensile strength and ductility of the brazed joint are at the level of that of Ti-6Al-4V base metal. According to SEM/EDAX and STEM phase analysis, two main joint microstructures are observed: either a very fine lamellar/cellular eutectoid consisting of mixture of α-Ti + γ-(Ti,Zr) 2Cu tetragonal MoSi 2 type phase or martensitic α′-Ti with dispersed γ-phase. The final copper concentration in the joint area is considered one of the critical parameters for joint microstructure formation. The optimal, eutectoidal microstructure characteristic of a ductile joint is obtained under brazing conditions that should result in average copper joint concentration within the 10–12 wt.% range. It is also found that the postbrazing natural cooling rate of 12.5 mm diameter rod samples is sufficient to suppress the formation of undesirable brittle λ-Cu 2TiZr Laves phase appearing in vacuum furnace brazing due to inherently low cooling rates. A variety of metallurgical paths along which base and filler metal may interact is proposed, explaining the mechanisms of formation of various braze microstructures and mechanical properties related to them. From the practical point of view, it is proved that induction brazing of compatible samples located in a simple close chamber may be carried out as an effective and inexpensive process.

Refractory metal silicides synthesized by metal vapor vacuum arc ion source implantation

Refractory metal silicides, namely NbSi2, TaSi2, WSi2, and MoSi2, were successfully synthesized by using a metal vapor vacuum arc (MEVVA) ion source to implant the respective metal ions with high current density into Si(111) and Si(100) wafers. The implantation was conducted at room temperature with an extracted voltage of 40 kV. When the current densities of the refractory metal ions were up to 65 μA/cm2, the equilibrium hexagonal NbSi2 and TaSi2 phases were formed at an implantation dose of 3×1017 ions/cm2, while the hexagonal WSi2 and MoSi2 phases were formed at a dose of 5×1017 ions/cm2. With increasing the current density up to 90 μA/cm2, the transition of the hexagonal WSi2 and MoSi2 phases to their most stable tetragonal structures was observed. Postannealing at 750 and 950 °C resulted in the formation of the unique tetragonal WSi2 and MoSi2 phases, respectively. The electrical property of the MEVVA-synthesized refractory metal silicides was measured for both as-implanted and postannealed wafers. In addition, the formation of the refractory metal silicides by MEVVA implantation is discussed in terms of the beam heating effect caused by high current ion implantation.

Interfacial Reactions and Thermal Stability of Ultrahigh Vacuum Deposited Multilayered Mo/Si Structures

ABSTRACTInterfacial reactions and thermal stability of ultrahigh vacuum deposited multilayered Mo/Si structures have been investigated by high resolution transmission electron microscopy in conjunction with fast Fourier transform and auto–correlation function analysis. For samples with nominal atomic ratios Mo:Si = 1:2 and 3:1, well defined multilayered Mo/Si structures were obtained after annealing at 250 °C for 30 min. On the other hand, distinct multilayered MoSi2/Si structure was formed only for Mo:Si = 1:2 samples after annealing at 650 °C for 1 h.Multiphases were observed to form simultaneously in samples annealed at 400–500 °C. After 650 °C annealing for 1 h, tetragonal MoSi2 was the only silicide phase observed for the Mo:Si = 1:2 samples, whereas both tetragonal and hexagonal MoSi2 were present in Mo:Si = 3:1 samples. The stability of the multilayered Mo/Si structures was found to depend critically on the atomic ratios of constituent elements, bilayer period and annealing conditions. The results are interpreted in terms of the delicate balance between intermixing of constituent atoms and silicide formation.

Nucleation and ordering of MoSi 2 on Si(100)

Molybdenum deposition on clean Si( 100) with a fluence as low as 1–4 monolayers suffices to nucleate molybdenum disilicide. At 640°C, disilicide growth is initiated by the formation of small crystallites of hexagonal MoSi 2, separated by clean, but roughened, Si(100)−2 × 1, as revealed by scanning tunneling microscopy, Auger electron spectroscopy, and glancing angle X-ray diffraction. At 770°C, molybdenum deposition leads to tetragonal MoSi 2 preferentially oriented with respect to the silicon substrate.