Materials Science Forum Vols. 654-656

Abstract: In this study, it was aimed to develop a novel interconnect material simultaneously possessing high electrical conductivity and strength. Combined structural Cu sheets were fabricated by forming the high electrical conduction paths of Ag on the surface of high strength Cu alloy substrate by damascene electroplating. As a result, the electrical conductivity increased by 40%, while the ultimate tensile strength decreased by only 20%. When the depth of Ag conduction path was deep, fracture zone ratio as well as roll-over zone increased.

Abstract: Brazing with active filler alloys containing some active elements, which promotes wetting of ceramics surfaces, is one of the most widely methods for joining ceramics to metals. The joints formed by brazing A12O3 to metal by using copper-titanium-nickel (Cu-Ti-Ni) as brazing filler were investigated. The metals/ceramics joints were produced at a vacuum level of 10-2-10-3 Pa at 1273K, using a constant holding time of 10 min. The surfaces were studied both morphologically and structurally using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). In the brazing process, copper and titanium in the braze alloy form a series of reaction products. The formation of Ti3Cu3O and Ti2Ni at the interface is characteristic of these joints. The estimated free energies of formation of the Ti3Cu3O and Ti2Ni are -119kcal/mol and -245.92 kJ ~-263.78kJ/mol at 1200~1288K respectively. The highly negative values for the free energies of formation suggest that these compounds are thermodynamically stable.

Abstract: Composite nanoparticles (anatase-type Ti1-2XNbXAlXO2 solid solution/SiO2) were directly synthesized from precursor solutions in the presence of urea under mild hydrothermal conditions at 180 °C for 5 h. The crystallite size of anatase was gradually decreased with increased silica content in the composite nanoparticles formed under weakly basic hydrothermal conditions via the hydrolysis of urea. The composite nanoparticles with composition Ti0.9Nb0.05Al0.05O2/SiO2 = 100/10 mol% showed good photocatalytic activity. The anatase-to-rutile phase transformation in the course of heating in air was retarded to maintain the anatase-type structure up to more than 1050 °C without a trace of rutile phase by the formation of composite nanoparticles (Ti0.90Nb0.05Al0.05O2/SiO2 = 100/20 mol%) with amorphous silica.

Abstract: Intermetallic nanocomposites of nickel aluminide (Ni3Al) alloy and nanosized zirconia (ZrO2) were fabricated using combined powder metallurgy technique and reaction synthesis. Nanosized zirconia as dispersed phase at 2 and 5 weight percent were pre-mixed with nickel, aluminum powders and other alloying elements in a planetary ball mill for 18 hours at 175 rpm to achieve mechanical alloying effect. The mixture was then compacted using a hydraulic press at 400MPa for 15 minutes. Sintering was done under inert condition (flowing Argon gas) in a tube furnace at 850oC with 3 hours holding time. The saturation magnetization (Ms) values of nickel aluminide nanocomposites containing 2wt% ZrO2 (ICZ2) and 5wt% ZrO2 (ICZ5) were 7.94 and 3.65 emu/g respectively. Reduced elastic modulus (Er) for ICZ5 was lower than ICZ2. Isothermal oxidation/sulfidation test in 1%SO2/air gas mixture at 800 and 1000oC were performed using a thermogravimetric analyzer (TGA) for up to 24 hours duration. The isothermal kinetic results for ICZ2 and ICZ5 are parabolic indicating rate limiting step. Reaction rates increased with increasing temperature. At this low concentration of sulfur, the test specimen only exhibited adsorption of sulfur in the vicinity of the surface region and no sulfide phase was observed.

Abstract: Carbon fiber reinforced SiCN matrix composite (C/SiCN) was used in present investigation, its amorphous SiCN matrix was derived from the hexamethyldisilazane by chemical-liquid and vapor-infiltration into the carbon fiber weaving preform. The tension-tension fatigue was conducted at 1300°C in vaccum both for the as-received and 1500°C-annealed C/SiCN with 60Hz and 41MPa. The microstructure evolution of SiCN during fatigue was examined by X-Ray diffraction and transmission electron microscopy (TEM). The results indicate that pronounced crystallization takes place in the as-received C/SiCN during fatigue, and only β-SiC crystallites are detected within amorphous SiCN matrix, Si3N4 can’t be detected. This is different from the literature that reports that there isn’t any crystallization when C/SiCN is annealed at 1300°C under nitrogen or argon atmosphere. As for 1500°C-annealed C/SiCN, there isn’t any crystallization during fatigue, and it can be attributed to the phase stabilization in the SiCN matrix after annealing. Both vacuum and fatigue stress promote the crystallization course because they accelerate the decomposition of amorphous SiCN and atomic diffusion. Degradation – crystallization mechanism is used to explain the crystallization behavior of amorphous SiCN under condition of vacuum and tension-tension fatigue.

Abstract: Density functional theory calculations are performed on the monometallic (Fe or Ni) bilayer modified α-Al2O3(0001) surface. Comparison has been made to their structural and electronic behaviors upon CH4 adsorption and dissociation. Local density of states and frontier orbital analysis show that C-H activation proceeds through weak chemical interactions with the metallic 3d electrons. It was found that electron transport within the sp and 3d type orbitals of the catalyst is important for the equilibration of the system. Such electron transport also promotes electron donation to the σ*(C-H) antibonding orbital for C-H bond activation. The calculated adsorption energies showed that the CH+H intermediate is most stable on the Fe/α-Al2O3 catalyst and is suspect to deactivation via carburization. Furthermore, C-H bond activation is most pronounced in cases where the CH4 molecule has one or two H atoms directed towards the catalyst surface.

Abstract: An approach for three-dimensional modelling of thermo-mechanical responses in an arc welding process is developed using the Smoothed Particle Hydrodynamics (SPH) method. It is demonstrated for a simple arc welding configuration by solving the fully coupled three-dimensional elastoplastic and heat transfer analysis. The temperature distribution of the metal in the weld pool and the surrounding parent material are analysed using SPH, and the resulting residual thermal stresses are evaluated. This work establishes the capability of SPH as a tool for simulating the long-term thermo-mechanical responses, including heat transfer and residual stresses in a welded joint, and gaining insights into post-welding structural behaviour of joints during cooling stages.

Abstract: Carbon nanotubes (CNTs) have remarkable mechanical strength, electrical conductivity, and thermal conductivity in spite of low density. Recently, CNT / epoxy composite have been widely investigated in terms of fabrication process and material characterizations. However, there have been few previous studies on B-stage film type CNT / epoxy composites for electronic packaging applications. B-stage film type CNT / epoxy composite films were fabricated and their properties were characterized for electronic packaging applications. The most important issue on fabrication on B-stage epoxy based films were uniform dispersion of CNTs in an epoxy resin. In this study, using optimized dispersion process, CNT / epoxy films were coated on a releasing film and subsequently dried by the comma roll coating method. Curing behavior of B-stage films, mechanical properties and electrical properties of fully cured films were characterized as a function of CNT contents. According to experimental results, CNTs lowered the curing activation energy of epoxy resin and increased electrical conductivity of epoxy resin.

Abstract: Superconducting wires have been applied for the fabrication of superconducting magnets in nuclear magneto-resonance (NMR), Magneto-resonance imaging (MRI) and so on. MgB2 has the highest critical temperature of superconducting transition (TC39K) among intermetallic compound superconductive materials. This means that MgB2 Superconductive wire doesn’t need expensive liquid He for cooling. We used the original method of the three-dimensional penetration casting (3DPC) in this laboratory to fabricate the MgB2/Al composite. Our 3DPC method for fabricating composite materials can disperse particles in the matrix homogenously without any aggregation and control volume fractions of composites within the range of 4 – 40%, even when particle size is less than 1 m. Thus, these composite materials can be processed by machining, extrusion and rolling. In the composite material we made, MgB2 particles dispersed to the Al matrix uniformly. The TC was determined by electrical resistivity and magnetization to be about 37 – 39K. We succeeded in extruding MgB2/Al composite billet to 1mm wire. Microstructures of these samples have been confirmed by SEM method. MgB2/Al composite billet and extruded wire were showed there no cracks inside the materials.

Abstract: The tribological properties of polyimide (PI) composites reinforced with graphite or MoS2 sliding in liquid alkali and water as well as dry friction were investigated using a ring-on-ring tester. The results show that the friction coefficient (μ) and wear rate (W) for both graphite/PI and MoS2/PI composites in different liquid mediums are μdry>μwater >μalkali and Wwater>Wdry >Walkali. Results also indicate that the friction coefficient and wear rate of the PI composites filled with different solid lubricants are μMoS2 >μgraphite and W MoS2 >Wgraphite in different liquid mediums. In addition, the hydrophobic inorganic fillers are fit for the reinforcement of polymer-based composites sliding in liquid mediums. It is also concluded from the authors’ work that the wear rate and friction coefficient of polymer-based (such as PI, PTFE) composites in the alkali lubricated conditions is lowest among all the friction conditions. This may be attributed to the ionic hydration in the alkaline solution.