The extremely high thermal conductivity of individual carbon nanotubes, predicted theoretically and observed experimentally, has not yet been achieved for large nanotube assemblies. Resistances at tube-tube interconnections and tube-electrode interfaces have been considered the main obstacles for effective electronic and heat transport. Here we show that, even for infinitely long and perfect nanotubes with well-designed tube-electrode interfaces, excessive radial heat radiation from nanotube surfaces and quenching of phonon modes in large bundles are additional processes that substantially reduce thermal transport along nanotubes. Equivalent circuit simulations and an experimental self-heating 3omega technique were used to determine the peculiarities of anisotropic heat flow and thermal conductivity of single MWNTs, bundled MWNTs and aligned, free-standing MWNT sheets. The thermal conductivity of individual MWNTs grown by chemical vapor deposition and normalized to the density of graphite is much lower (kappa(MWNT) = 600 +/- 100 W m(-1) K(-1)) than theoretically predicted. Coupling within MWNT bundles decreases this thermal conductivity to 150 W m(-1) K(-1). Further decrease of the effective thermal conductivity in MWNT sheets to 50 W m(-1) K(-1) comes from tube-tube interconnections and sheet imperfections like dangling fiber ends, loops and misalignment of nanotubes. Optimal structures for enhancing thermal conductivity are discussed.

One important application of nanocomposites is their use in engineered structural composites. Among the wide variety of structural applications, fiber-reinforced composites for aerospace structures have some of the most demanding physical, chemical, electrical, thermal, and mechanical property requirements. Nanocomposites offer tremendous po tential to improve the properties of advanced engineered composites with modest additional weight and easy integration into current proc essing schemes. Sig nificant progress has been made in fulfilling this vision. In particular, nanocomposites have been applied at numerous locations within hierarchical composites to improve specific properties and optimize the multifunctional properties of the overall structure. Within this ar ticle, we review the status of nanocomposite incorporation into aerospace composite structures and the need for continued development.

The room temperature tensile properties of Ti-6Al-4V alloy prepared under two different processing routes were evaluated and compared. One group of samples was prepared by conventional casting-forging-rolling into flat plates. The other group was prepared by using TritonÕs Laser Free-Form Fabrication (LF3)Ô processes, i.e., a laser was used to melt pre-alloyed powders of the required metallic composition as they were dropped onto a moveable substrate programmed to move in such a manner as to form a solid alloy plate. Five populations of Ti-6Al-4V were evaluated: a standard wrought form, an as-deposited form, a machined as-deposited form, a heat-treated as-deposited form, and a machined as-deposited and heattreated form. The poorest mechanical properties occurred with the rough surfaces, likely due to existing microcracks and stress concentrations. The LF3Ô as-deposited material had mechanical properties comparable to, if not higher than, the mechanical properties of the wrought material. Further evaluations of the laser-formed material for complex spacecraft piece parts were warranted, specifically in regards to improving the surface finish of the materials.

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Impact and flexural creep testing were conducted at temperatures between −22°F (−30°C) and 250°F (121°C) to evaluate and compare the end‐use performance of continuous long glass fiber‐reinforced thermoplastic sheet composites to that of short glass fiber‐reinforced thermoplastics. The matrices studied consisted of amorphous (polycarbonate and acrylonitrile‐butadiene‐styrene) and semicrystalline (polypropylene) polymers. Data were obtained from both injection‐molded specimens (short fibers), and from specimens machine‐cut from compression‐molded test panels (continuous long fibers). The creep results of this study demonstrated that continuous long fibers are more efficient than short fibers in reinforcing the thermoplastic matrices, resulting in enhanced load‐bearing ability at elevated temperatures. The addition of continuous long glass fibers to the thermoplastic matrices led to a significant increase in the notched Izod impact strengths between the temperatures of −22°F (−30°C) and 77°F (25°C), and only slight improvement in the drop‐weight impact strengths. The lack of correlation between notched Izod impact and drop‐weight strengths is largely due to the difference in crack propagation and fracture initiation energies. Results of the Rheometrics instrumented impact test indicated a higher total fracture energy for the long glass‐reinforced thermoplastic sheet composites than for the short glass‐reinforced injection‐molded thermoplastics. The decreased ease of crack propagation in thermoplastic sheet composites is associated with the high energy‐absorbing mechanisms of fiber debonding and interply delamination. The results of this study point to the significant property improvement of continuous long fibers vs. short fibers. The creep strength of short fiber‐reinforced thermoplastics are greatly affected by the nature of the stress transfer which in turn is influenced by the critical fiber length and temperature, which is not the case for the long fiber‐reinforced thermoplastic sheet composites. Long fibers dramatically increase the impact resistance of thermoplastics. The retention of toughness at low temperatures coupled with elevated temperature performance greater than similar short glass fiber‐reinforced thermoplastics effectively extends the capabilities of thermoplastic sheet composites at both temperature extremes.

The interaction of oxygen with a Ni 76%/Fe 24% (100) surface has been studied at 293 K. XPS measurements were made as a function of emission angle to obtain depth distributions. Slight Fe segregation is observed for a clean annealed surface. It is enhanced by oxygen adsorption. By 0.7 monolayers oxidized Fe species are detected, but the Ni remains metallic until well above one monolayer. The Fe concentration in the outer 4 Å has then increased twofold. At about ∠2.7 monolayers (passivation) the surface Fe concentration is nearly three times the original and entirely oxidized. Heating causes further Fe segregation which reduces the Ni2+ to yield Ni0. LEED shows a c (2×2) pattern which persists above one monolayer. Correlations of the XPS and LEED data and comparison with XPS/LEED studies of oxygen on Fe(100) and Ni(100) show that between 0.7–2 monolayers, the surface must be heavily patched with Fe oxide and metallic Ni aggregations, the Ni being covered with a c (2×2) oxygen superstructure.

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Adsorption and decomposition of formic acid on clean and potassiumdosed Rh(111) surfaces J.