Papers by Keyword: W

Abstract: In this study, the W-Si-C multi-phase composites were fabricated by an arc melting method. With addition of SiC, the grain size of W is obviously reduced, and the small angle misorientation becomes dominate, which is beneficial for the improvement of deformability. The effects of SiC additions (from 0.5 to 3wt%) on the microstructure and mechanical properties are mainly investigated. With 1 wt% SiC addition, the flexural strength reaches the highest value. The self-generation of W₅Si₃ may enhance the strength and ductility, but too much W₅Si₃ exists as brittle BP (Brittle to Plastic) microstructure. The highest flexural strength is obtained at approximately 1 vol% W₅Si₃.

Abstract: Electroless Ni deposition is often used in presence of Cu, Ti and Au. Recently TiW has also started to be employed, but with this alloy the Ni deposition is not always neat. In our work we investigate the effect of different wet treatments on the Ni growth by means of XPS analyses and SEM inspections. It is found that an oxidized surface inhibits the activator deposition. The de-oxidized Ti atoms in TiW on the other hand are believed to act as the principal sites for Pd seed deposition and subsequent Ni growth.

Abstract: The SCR catalysts were produced with V₂O₅, WO₃, MoO₃ and anatase type TiO₂. The catalyst samples were ground and sieved for 0.3~0.6mm.The NO catalytic efficiency, selectivity against N₂O of the catalysts were investigated on a fixed bed reactor under simulated exhaust gas with a typical gas composition. The addition of W enhanced the catalytic efficiency of V(1)-W(x)-Mo (4.5)/TiO₂ catalysts at high temperature region, while lessened that at low temperature. Increasing the loading of W from 1.5% w/w to 4.5% w/w advanced the maximum catalytic efficiency from 88% to 99% and enlarged the temperature window of the catalyst. The presence of W promoted the N₂O generation. The V(1)-W(4.5)-Mo (4.5)/TiO₂ catalyst showed higher catalytic selectivity for NO compared to the catalysts loading W.

Abstract: 12Cr heat resistant steels with different concentration of Co and W, while Mo equivalent (Mo+1/2W) was fixed at 1.6, were prepared by arc-melting and hot rolling. Mechanical properties were evaluated by tensile tests conducted with the strain rate 2×10-5S-1 at 575oC, 600oC and 625oC instead of time-consuming creep tests. The results show that when Co content is fixed, the steel with 1.5 wt% W is found having the best deformation resistance which is strong work hardening and slow strain softening. Apparent activation energy of the steel with 3.1 wt% Co and 1.5 wt% W is in the range of 370~413 kJ/mol, higher than those of the other steels in our study, which are close to the self-diffusion activation energy of iron (239 kJ/mol). Therefore, the steel with 3.1 % Co and 1.5% W is suggested as a potential candidate material for 625oC~650oC class USC steam turbines.

Abstract: The microstructures and tensile properties of a fully lamellar Ti-48Al-2Cr-2Nb, and two tungsten-modified versions, Ti-48Al-2Cr-2Nb-0.5W and Ti-48Al-2Cr-2Nb-1.0W (atomic percent) are investigated. Gas atomized powders are consolidated by hot isostatic pressing followed by solution treatment and aging. The microstructures are characterized by optical, scanning electron, and transmission electron microscopy and mechanical properties are characterized by room temperature tensile testing. The solution heat treatment, combined with controlled cooling, generates relatively fine, fully lamellar grains. Tungsten reduces the propensity for martensitic gamma formation during cooling, and slows down lamellar coarsening as well as the formation of equiaxed gamma phase during aging. The aging treatment stabilizes the microstructure and, in the tungsten-modified alloys, causes beta phase precipitation at lamellar interfaces and grain boundaries. Both aging and tungsten additions increase the alloy strength and reduce ductility. The fracture morphologies of the alloys are similar and exhibit mixed-mode fracture consisting of inter- and intra-granular cracking, as well as inter-lamellar cracking.

Abstract: The heavy refractory metals and alloys Molybdenum (Mo), Molybdenum – Silicon – Boron (Mo–Si-B; “MoSiBor”), Tungsten (W), Tungsten – Copper (W-Cu), Tungsten – Nickel – Iron (W-Ni-Fe; “Densimet D 176 and 185”) and Tungsten – Nickel – Molybdenum - Iron (W-Ni- Mo-Fe; “Densimet D2M”) were pack-treated at 1100°C with Silicon - powder to form siliconized zones and/or intermetallic phases which are intended to be more oxidation resistant than the plain base materials. These materials (especially the W-based ones) are used at ambient conditions as counterweights, radiation shields etc. because of their high density as well as at high temperatures (600 – 900°C) as metal forming tools, electrodes etc. because of their refractory metal content. In both areas of conditions oxidation of the plain materials occurs and leads to lower functionality or destruction. A suitable oxidation test has been defined to check the presumably enhanced oxidation resistance of the pack-treated materials: an isothermal high temperature oxidation test at 700 and 900°C for one week. At these conditions all untreated materials would have been more or less strongly oxidized. Improved oxidation resistance could be found for the materials with pack-cementation treatment except for sintered Tungsten (92% dense), sharp etched D 185 and D 176 at 900°C and Tungsten – Copper at both temperatures. More stable and dense superficial oxides were formed which led to decreased oxidation rates and could help to increase functional stability and the lifetime of the components. Different pack-treatments e.g. with chromium or silicon plus chromium could improve the behaviour of the materials which failed within this work.

Abstract: Effects of Si, W and Cu on the localized and stress corrosion of 18 % Cr austenitic stainless steel were investigated using experimetal strategy of full factorial design. The resistance to localized and stress corrosion of the alloy was improved with increasing the contents of Si, W and Cu in the range of 0.5 ~ 1.5 % for Si, 0.1 ~ 0.5 % for W and 0.2 ~ 2.0 % for Cu. In addition, we found an interaction effect between Si and Cu on the reistance to localized and stress corrosion of the alloy.

Abstract: Copper is a desirable material to replace aluminum-based alloys in the metallization of very large-scale integrated circuits, due to its high conductivity and reduced electromigration. However, practical ways to grow high quality copper layers on top of the common materials used as barrier layers on silicon is problematic, because of several issues, like poor adhesion and reduced coverage of high aspect-ratio surface features. We will describe efforts in developing procedures and chemical compounds for the growth of high quality films of copper on barrier layers. Our work is based on ab-initio calculations of the energetics and dynamics of the growth processes involved, including the interaction of the chemicals with the surfaces. The calculations presented use density functional theory, and in particular the SIESTA code.

Abstract: Nanometer-sized W-dendritic form structure was fabricated with electron-beam-induced deposition (EBID) in a 200 kV transmission electron microscope. The as-prepared nanodendrites are composed of W-nanocrystals and amorphous. The as-prepared nanodendrites were then irradiated with 1 MeV electron beam in a high voltage transmission electron microscope. The effect of the irradiation is investigated. The irradiation transformed effectively almost all the amorphous part to crystalline state. Morphology of the nanodendrite also changes. The irradiation induced crystallization and morphology change are discussed.

Abstract: Coatings of W on the substrate of oxygen-free Cu have been made by a high efficiency supersonic plasma spray system with a maximum power of 80kW and a maximum working at 6 m3/h flow of gas, and being compared with results of subsonic plasma spray. The bonding strength and thermal shock resistance of the coatings was tested. W/Cu functionally graded coatings is compared with single coatings of W.