Papers by Keyword: Diffusion Barrier

Abstract: First-principles calculations within the density functional theory (DFT) have been carried out to study the interaction of hydrogen atom with transition metals (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) doped Mg (0001) surfaces. First we have calculated the stability of the transition metals atom on the Mg surface, On the basis of the energetic criteria, all the elements except Zn prefer to substitute one of the Mg atoms from the second layer, while Zn tend to substitute one of the Mg atoms from the first layer. In the second step, we have studied the interaction between hydrogen atom and the transition metals doped Mg (0001) surface.we have studied the interaction of a hydrogen atom with the transition metals doped Mg (0001) surface. The results show that for transition metals atoms doped Mg (0001) surface in the second layer, it not only enhances the chemisorption interaction between hydrogen atom and Mg surface, but also it benefits hydrogen atom diffusion in Mg bulk with relatively more diffusion paths. However, when the Mg surface doped by elements such as Sc, Ti, V, Cu and Zn, hydrogen atom chooses to bond with transition metals atom and block the diffusion of hydrogen atom into Mg bulk, while when the Mg surface doped by elements such as Cr, Mn, Fe, Co and Ni, hydrogen atom chooses to leave from transition metals atom thereby promoting the diffusion of hydrogen atom diffusion into Mg bulk. Charge density difference plots shows that electrons are transferred from electronic states of transition metals atom to the orbital of hydrogen atom which cause attractive interactions between hydrogen atom and transition metals atom and reduce the energy barrier of the hydrogen atom diffusion into Mg bulk. Our results show that useing transition metals (Cr, Mn, Fe, Co and Ni) as catalysts for the hydrogenation/dehydrogenation of Mg bulk samples and provide more diffusion paths of hydrogen atom, they are beneficial for the diffusion of hydrogen atom to Mg bulk and improve significantly the hydrogenation kinetics property of Mg surface.

Abstract: An arc-sprayed oxidation protective barrier coating system on austenitic stainless steel is composed of an inner Ni-Cr-Ti alloy layer and a pure aluminum outer layer. The high-temperature oxidation behavior of the coatings on 304N substrate was investigated during isothermal exposures in air at 1100°C. Experiments showed that interdiffusion of alloying elements within the protective coatings occurred, and the elements, Cr,Ni and Al, were also diffusing into the substrate metal. As test time proceeded, aluminum nitrides were generated in situ in the substrate, which had a layer-like distribution. The nitrides generated play the roles as element diffusion barriers, and together with the surface alumina they provide a long-term effective anti-oxidant protection to the steel substrate. The coatings on 304N substrate were not deteriorated and the substrate was protected well, being exposed to 1100°C for 350 hours. The analysis results of oxidation kinetics showed that the specific mass changes of the coated specimens were much smaller than those without coatings.

Abstract: In order to increase the failure temperature of Zr-N diffusion barrier for Cu, the effect of insertion of a thin Zr layer into Zr-N film on Zr-N diffusion barrier performance in Cu metallization was investigated by means of X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, and 4-point probe technique. XRD,SEM ,AES and FPP results show that the insertion of a thin Zr layer into Zr-N film improves barrier properties significantly when the ZrN / Zr/ZrN barrier layers are deposited by RF reactive magnetron sputtering and Zr-N(10nm)/Zr (5nm)/Zr-N(10nm) barrier tolerates annealing at 700°C for 1 h without any breaking and agglomerating Cu film. This interpretes that insertion of a thin Zr layer into Zr-N film is attributed to the densification of grain boundaries in ZrN/Zr/ZrN films followed by the reduction of fast diffusion of Cu through ZrN /Zr/ ZrN multilayered films.

Abstract: A solid-to-solid, U-7wt.%Mo vs. Mg diffusion couple was assembled and annealed at 550°C for 96 hours. Themicrostructurein the interdiffusion zone and the development of concentration profiles were examined via scanning electron microscopy, transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy. A TEM specimen was prepared at the interface between U-7wt.%Mo andMgusing focused ion beam in-situ lift-out. The U-7wt.%Mo alloy was bonded well tothe Mg at the atomic scale, without any evidence of oxidation, cracks or pores.Despite the good bonding, very little or negligible interdiffusion was observed.This is consistent with the expectation based on negligible solubilities according to the equilibrium phase diagrams. Along with other desirableproperties, Mgis a potential inert matrix or barrier materialfor U-Mo fuel alloy systembeing developed forthe Reduced Enrichment for Research and Test Reactor (RERTR) program.

Abstract: To decrease the interdiffusion, CrON interlayer as a diffusion barrier was introduced into the interface of NiCrAlY overlayer and DSM11 substrate. The microstructure and effect of the diffusion barrier were investigated. It was found that the as-deposited CrON diffusion barrier was comprised of Cr₂O₃ and CrN. During thermal treatment (including vacuum heat treatment and thermal exposure), the diffusion barrier was first transformed to Al₂O₃ and Cr₂N, then to Al₂O₃ dominant interlayer, and porous mixed-oxides including TiO₂ and NiCr₂O₄ spinels were detected in the interdiffusion zone between the diffusion barrier and the substrate. The presence of α- Al₂O₃ in the diffusion barrier was the main reason for suppressing the interdiffusion. The interfacial reaction mechanism in the diffusion barrier was discussed by elemental diffusion and chemical reaction thermodynamics.

Abstract: The evolution of designed [(Ti-Te)]x[(Sb-Te)]y, [(Bi-Te)]x[(Sb-Te)]y, [(Ti-Te)]w[(Bi-Te)]x[(Sb-Te)]y and [(Ti-Te)]w[(Bi-Te)]x[(Ti-Te)]y[(Sb-Te)]z precursors were followed as a function of annealing temperature and time using both low and high angle x-ray diffraction techniques to probe the self assembly into nanolaminate materials. The [(Bi-Te)]x[(Sb-Te)]y precursors were found to interdiffuse at low temperatures to form a (BixSb1-x)2Te3 alloy. The [(Ti-Te)]x[(Bi-Te)]y and [(Ti-Te)]x[(Sb-Te)]y precursors formed ordered nanolaminates [{(TiTe2)}1.35]x[Bi2Te3]y and [{(TiTe2)}1.35]x[Sb2Te3]y respectively. The [(Ti-Te)]w[(Bi-Te)]x[(Sb-Te)]x precursors formed [{(TiTe2)}1.35]w[(Bi0.5Sb0.5)2Te3]2x nanolaminates on annealing, as the bismuth and antimony layers interdiffused. Over the range of TiTe2 thicknesses used in [(Ti-Te)]w[(Bi-Te)]x[(Ti-Te)]y[(Sb-Te)]z precursors, Bi and Sb were found to interdiffuse through the 2-4 nm thick Ti-Te layers, resulting in the formation of (BixSb1-x)2Te3 alloy layers as part of the final nanolaminated products. When the Bi-Te and Sb-Te thicknesses were equal in the amorphous precursors, symmetric [{(TiTe2)}1.35]m[(Bi0.5Sb0.5)2Te3]n nanolamiantes were formed. When the thicknesses of Bi-Te and Sb-Te layers were not equal in the amorphous precursor, asymmetric [(TiTe2)1.35]m[(BixSb1-x)2Te3]n[(TiTe2)1.35]m[(BixSb1-x)2Te3]p nanolaminates were formed. These results imply that to form (A)w(B)x(C)y nanolaminates using designed layered precursors all three components must be immiscible. To form (A)x(B)y(A)x(C)z nanolaminates, the components must be immiscible or the precursor to the A component and the A component itself must be an effective interdiffusion barrier preventing B and C from mixing.

Abstract: TiAl based alloys are promising candidates for structural applications at high temperature. However, the poor oxidation resistance above 800oC obviously restrains their applications. Although NiCrAlY overlay coatings can remarkably improve the high temperature oxidation resistance of TiAl, serious inward diffusion of Ni from the coating to the substrate occurs which could reduce the lifetime of the coating/substrate system. Apparently, the development of interdiffusion barrier could overcome the disadvantage of the NiCrAlY/TiAl system. In this work, Ta, TiN and Cr2O3 interlayers were deposited between NiCrAlY coating and γ-TiAl substrate as diffusion barrier (DB). The interdiffusion behavior of the TiAl/DB/NiCrAlY system was investigated at 1000°C. The results showed that the metallic and nitride interlayers cannot retard the interdiffusion of Ni effectively. As an active diffusion barrier, the oxide interlayer obviously suppressed the inward diffusion of Ni from the coating to the substrate by the formation of alumina-rich layers at both the TiAl/DB and DB/NiCrAlY interfaces.

Abstract: The reliability of Ni2Si/n-SiC ohmic contacts with Au overlayer either without or with Ta-Si-N diffusion barrier was investigated after long-time aging in air at 400oC and rapid thermal annealing in Ar up to 800oC. It is shown that aging of the Au/Ni2Si/n-SiC contacts in air at 400oC resulted in complete degradation due to both oxygen penetration and interdiffusion/reaction processes. In contrast, only a small change in properties was detected on the contacts annealed in Ar at 800°C. The stability of both electrical and structural properties of Au/TaSiN/Ni2Si/n-SiC thermally stressed contacts at different conditions points out their superior thermal stability.

Abstract: Titanium nitride (TiN) film has been widely used as a diffusion barrier layer for VLSI contact metallization because TiN is an excellent barrier against inter-diffusion between Al and Si substrate or silicide. In this work, we studied the properties of TiN films deposited by DC magnetron sputtering with varying N2:Ar flow rate ratio in order to optimize growth conditions and film properties provided for Al diffusion barrier purpose. The TiN films were deposited at the constant pressure level and sputtering time. The crystalline orientation, composition and electrical properties of deposited TiN films were characterized by XRD, AES-depth profile and Four Point Probe measurement, respectively. The XRD results show that the deposited TiN film has two preferred orientations of TiN(111) and TiN(200) planes. The highest intensity of the TiN(111) plane was obtained when the N2:Ar flow rate ratio was 3:1. The electrical resistivity was increased when the N2:Ar flow rate ratio was decreased. The minimum electrical resistivity is 127.8 μΩ-cm when the N2:Ar flow rate ratio is 3:1.

Abstract: Nickel silicide Schottky contacts were formed on 4H-SiC by consecutive deposition of a titanium adhesion layer, 4 nm thick, and nickel, 100 nm thick, followed by annealing at temperatures from 600 to 750 °C. It was found that contacts with barrier heights of 1.45 eV, consisting mainly of NiSi phase, formed in the 600-660 °C temperature range, while annealing at around 750 °C led to the formation of Ni2Si phase with barrier heights of 1.1 eV. Annealing at intermediate temperatures resulted in the nucleation of Ni2Si grains embedded in the NiSi film which were directly observed by micro-Raman mapping. It was concluded that the thermodynamically unfavourable NiSi phase appeared in the 600-660 °C temperature range due to the fact that the solid state chemical reaction between Ni and SiC at these temperatures is controlled by nickel diffusion through the titanium barrier.