Papers by Keyword: Silicide

Abstract: The transition metal silicide X₃Si (X = V, Nb, Cr, Mo and W) was characterized by its low density, high melting point, high temperature hardness, high temperature resistance to wear, high temperature oxidation resistance and corrosion resistance in this paper. For the fields such as aerospace, gas turbine etc, with the application of a new generation of high temperature structural materials, transition metal silicide will be one of their candidate materials. The stability, crystal structure, mechanical properties, electronic properties, Debye temperature and hardness of X₃Si(X=V, Nb, Cr, Mo and W) compounds were calculated employing electronic density functional theory (DFT) and the generalized gradient approximation (GGA). The results show that the remaining silicides have stable structures except that W₃Si is a metastable structure in X₃Si compounds. Based on the stress-strain theory, the bulk modulus, shear modulus, Young's modulus and Poisson's ratio of Cr₃Si and Mo₃Si were estimated by Voigt-Reuss-Hill method: 248.7 GPa, 158.9 GPa, 393.0 GPa, 0.24 and 249.2 GPa, 134.6 GPa, 342.1 GPa, 0.27. According to the state density (DOS) analysis, we can see that the valence band of X₃Si compound is a combination of covalent bond and metal bond. The temperature of Debye of Cr₃Si (645.1 K) in X₃Si compound is the highest. The hardness of these silicon compounds is evaluated using a semi empirical hardness theory and the result shows that Cr₃Si (10.96 GPa) is the hardest compound among them.

Abstract: An investigation of the electrical and microstructural properties of gold (Au)/p-type silicon (Si) contact was performed as a function of rapid thermal annealing (RTA) temperatures. Au films reacted with Si and produced Au₂Si and Au₃Si phases during the deposition of the films at room temperature. The electrical properties of the Au contact to p-type Si degraded with increasing RTA temperature. Such a degradation of the electrical properties could be associated with the degradation of the surface and interface morphology caused by the formation of Au-silicide clusters. The RTA process at 500 °C led to an increase in the size of the Au-silicide Island. This led to the further degradation of the electrical properties after annealing at 500 °C.

Abstract: Dynamic recrystallization in Ti-1100 was investigated. Ti-1100 is one of near α titanium alloys and contains Si for improving high temperature mechanical properties. Ti-1100 exhibits martensitic transformation by quenching into iced brine after solid solution treatment. Hereafter specimens subjected to quenching into iced brine and to cooling in air after solid solution treatment are called IBQ specimen and AC specimen, respectively. After tensile test at high temperature, IBQ specimen exhibits morphological change from lath structure to equiaxed structure, but AC specimen does not. It is indicated that dynamic recrystallization occurs during the tensile test of IBQ specimen. Effect of silicide on the dynamic recrystallization was investigated using two specimens: one included more silicide precipitates and the other less. The former specimen shows smaller recrystallized grains than the latter. It is indicated that the specimen including more silicides exhibits smaller recrystallized grains.

Abstract: The effect of alloying element (such as Cr, Zr, and Ir) addition on the high-temperature creep deformation behavior of C40/C11_b lamellar-structured (Mo_0.85Nb_0.15)Si₂ silicide crystals was examined. The results indicated that these additions all lead to a decrease in the steady-state creep strain rate (SSCR) when the applied stress is parallel to the lamellar interface. To clarify the origin of this, the dependence of the creep deformation behavior on the microstructure was determined in detail. As a result, it was found that the C40 phase acts as a strengthening phase during the deformation of the C40/C11_b duplex-phase crystals. The variant-1-type C11_b phase grains, whose loading orientation is parallel to [001], also acts as an effective strengthening component. The decrease in SSCR by Cr or Zr addition is attributed to the increase in volume fraction of those C40 phase and C11_b-V1 grains. The refinement of microstructure by Ir addition was also found to result in a modest decrease in the SSCR.

Abstract: A novel ternary compound, namely U₃Pt₁₂Si_4, which is best described by the crystallographic formula U₃Pt_12+x-ySi_4-x-z (x = 0.15, y = 0.23, z = 0.16), has been discovered in the U-Pt-Si phase diagram. It crystallizes in a hexagonal unit cell (P6₃/mmc space group) with a = 8.7267(1) Å and c = 9.3385(2) Å. Its structure is an ordered variant of the EuMg_5.2 type with partial occupancies of the Si 4e and Pt 2b positions and mixed Si/Pt occupancy of the 6g site. Magnetic properties measurements revealed that the compound is a Curie-Weiss paramagnet with an effective magnetic moment μ_eff = 3.18 μ_B. Its low temperature specific heat is moderately enhanced (its Sommerfeld coefficient γ is equal to 79(1) mJ mol_U^-1 K^-2) and the electrical resistivity exhibits some characteristic features of dense Kondo lattices.

Abstract: A new refractory alloy HfNbSi_0.5TiVZr was synthesized by induction levitation melting with the aim to achieve an excellent strength and toughness balance of the Hf-Nb-Ti-Zr based alloy. The as-cast alloy with density of ρ=7.75g/cm³ and microhardness of H_v=464 had the microstructure consisting of bcc solid solution with little vanadium rich phase and fine intermetallic phase presented dendritic structure. Mixing entropy and formation enthalpy can explain this behavior. After heat treatment at 1373K for 4 h, no new phase come into being but elements solute more fully. Compressive yield strength of the alloy gradually decreased from 1540MPa at room temperature to 371MPa at 1073K in as-cast state and decreased from 1483MPa at room temperature to 102MPa at 1073K after annealed. Comparing with the similar high entropy alloys, the structure combining silicide and continuous solid phase have a great benefit to the balance of strength and ductility.

Abstract: Silicide growth via reaction between a metallic film and a Si substrate has been well documented. In general, atomic transport kinetic during the growth of silicides is considered to be the same as during equilibrium diffusion, despite the reaction and its possible injection of point-defects in the two phases on each side of the interface. To date, the main studies aiming to investigate atomic transport during silicide growth used immobile markers in order to determine which element diffuses the fastest during growth and in which proportion. The quantitative measurements of effective diffusion coefficients during growth was also performed using Deal-and-Groove-type of models, however, these effective coefficients are in general not in agreement with the interdiffusion coefficients calculated using the equilibrium diffusion coefficients measured during diffusion experiments. In general, atomic transport kinetic measurements during growth and without growth are performed using different types of samples for experimental reasons. In this paper, we discuss the possible use of ultrahigh vacuum in situ Auger electron spectroscopy in order to measure the effective diffusion coefficient during growth, as well as the equilibrium self-diffusion coefficients, in the same samples, in the same experimental conditions. The first results on the Pd-Si system show that atomic transport during Pd₂Si growth is several orders of magnitude faster than at equilibrium without interfacial reaction.

Abstract: Nb-base refractory intermetallic materials have potential interest for high temperature applications thanks to their low density and high temperature strength. While advanced intermetallics in monolithic form have limited prospects for providing the required balance of properties for use at high temperatures, two-phase or multicomponent intermetallic systems composed of a ductile, Nb-base refractory phase in equilibrium with one or more silicide intermetallics show promise for further development as structural materials. In the present paper, Nb-base refractory alloys based on Nb-35Ti-15Al (at.%) were doped with small amount of Si (1 and 2 at% of silicon) addition to improve its high temperature strength by keeping an acceptable ductility at room temperature. The samples were prepared by arc-melting starting from pure elements (99.99%). The silicon addition effects on the microstructural features were investigated by using X Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) techniques. Its effects on the mechanical properties were assessed by compression tests at ambient and high temperatures. Compression tests show the beneficial effect of the Si addition on strength.

Abstract: NbSi₂/MoSi₂ duplex silicide crystals are potentially a new-class of ultra-high temperature structural materials. Improvement in the thermal stability of their lamellar microstructure was accomplished by the addition of a minute amount of either Cr or Zr. The mechanical properties of the duplex silicide, such as fracture toughness and high temperature strength, show strong orientation dependence, thereby suggesting the importance of the control of microstructure to improve their properties.

Abstract: Abstract. It is shown, by the combination of SNMS, (Secondary Neutral Mass Spectrometry), XRD, XPS and APT (Atom Probe Technique) that the growth of the Cu₃Si crystalline layer at 408 K between the amorphous Si and nanocrystalline Cu thin films follows a linear law and the shifts of the Cu₃Si/Cu and Cu₃Si/a-Si interfaces approximately equally contributed to the growth of this phase. It is also illustrated that the Si atoms diffuse fast into the grain boundaries of the nanocrystalline Cu, leading to Si segregation. Both the SNMS and APT results indicate that even during the deposition of Cu on the amorphous Si an intermixed region is formed at the interface. This region easily transforms into a homogeneous Cu₃Si crystalline reaction layer subsequently which further grows following apparently an interface controlled linear kinetics. Similar experiments performed in Co/a-Si system to study the formation and growth kinetics of the intermetallic phase. However, interestingly, homogenous formation of the new phase at the Co/a-Si interface was not always observed.