Low-temperature, shear-induced tribofilm formation from dimethyl disulfide on copper

The frictional properties of a sliding copper-copper interface exposed to dimethyl disulfide (DMDS) are measured in UHV under conditions at which the interfacial temperature rise is <1 K. A significant reduction in friction is found from the clean-surface values and sulfur is found on the surface and below the surface in the wear scar region by Auger spectroscopy. Because the interfacial temperature rise under the experimental conditions used to measure friction is very small, tribofilm formation is not thermally induced. The novel, low-temperature tribofilm formation observed here is ascribed to a shear-induced intermixing of the surface layer(s) with the subsurface region as suggested using previous molecular dynamics simulations. Although the tribofilm contains predominantly sulfur, a small amount of carbon is also found in the film.     

The influence of sulphur on the processing of zirconia based ceramics

Yttria stabilized zirconia powders were synthesized by the coprecipitation route. Zirconium oxychloride containing sulphur as contamination and analytical grade yttrium chloride were used as raw materials. Powders were calcined at temperatures between 600 and 1100 °C and ground by ball and attrition milling. The ceramic bodies were sintered at 1350 and 1550 °C for 1 h and the apparent density was measured. In the present work it is shown that the most deleterious effect of sulphur was observed in 3 mol% yttria stabilized zirconia, especially when the pellets were obtained at high pressures and sintered at 1500 °C. The elimination of sulphur at higher calcination temperatures minimizes the effects caused by this contamination, despite the reduction of powder surface area. The best processing condition to obtain high density zirconia ceramics from powders contaminated with sulphur was established.     

Structural, magnetic and electrical properties of (La0.70−xNdx)Sr0.30Mn0.70Cr0.30O3 with 0 ≤ x ≤ 0.30

The structural, magnetic and electrical properties of (La_0.70−xNd_x)Sr_0.30Mn_0.70Cr_0.30O₃ perovskites (0 ≤ x ≤ 0.30) prepared by the usual ceramic procedure were investigated. Structural Rietveld refinement revealed that these compounds crystallize in a rhombohedral perovskite structure when x = 0, 0.10 and 0.20, while for x = 0.30 the structure becomes orthorhombic (Pbnm). It was found that the substitution of La by Nd reduces the Curie temperature (T_C). The FC, ZFC, M(H) and AC susceptibility measurements show typical canted-antiferromagnetism for the Nd-doped samples, in which a ferromagnetic component coexists with predominant antiferromagnetic interactions. The values of the magnetization (M(H)) decrease very slightly when increasing the Nd content, compared to the undoped sample (M_S values at 5 T and 2 K are, respectively, 47.9, 47.3 and 47.5 emu/g for x = 0.10, 0.20 and 0.30, compared to 48.2 emu/g for x = 0), indicating that the Nd³⁺ contribution is negligible compared to the total moment of the ferromagnetic (Mn/Cr) network. The resistivity increases by several orders of magnitude with Nd-doping and the semi-conducting behaviour persists in the whole temperature range. The interaction between Mn⁴⁺–O–Cr³⁺and Cr³⁺–O–Cr³⁺ is responsible for the semi-conducting state.     

Carbon black filled PET/PMMA blends: Electrical and morphological studies

In this work, the electrical and morphological properties of blends of poly(ethylene terephthalate) (PET), poly(methyl methacrylate) (PMMA), and carbon black (CB) were analyzed. Resistivity decreases similarly in both PET and PMMA with CB concentration. Similarly in the PET/PMMA blend, extensive modification to this behavior occurs, since resistivity becomes a function of morphology and specific location of CB in the polymers. A minimum in the resistivity of the blend with 5% CB (PET basis) is observed at 100% PET, whereas with an increase in the CB content to 20%, the minimum in the resistivity shifts to 60% PET. High conductivity is observed when PET is the continuous phase (having the larger viscosity). Large stresses lead to a large dispersion of CB and a high deformation and rupture of the dispersed PMMA phase. This situation itself promotes an increase of surface area of droplets and high CB concentrations at the interface. Consideration is given to models that predict a selective location of conductive particles in the PET matrix based on its lower interfacial tension with CB.     

Computational Technique to Assess Warhead Lethality against Ballistic Missile

This paper addresses a computational procedure to assess the lethality of kinetic energy projectile warheads against tactical ballistic missile payloads with an innovative and rational damage metric. The individual lethality of different projectile geometries impacting at hypervelocity at different configurations is estimated via a numerical damage index computed from hydrocode simulations. The highest count of possible impacts to the payload is achieved by optimizing the warhead’s configuration and time of detonation, a priori knowledge of the target’s location and speed. The total damage to the payload characterizes the warhead’s lethality for the particular engagement conditions. An example shows the application of the process and potential advantages.     

Ceramic processing and multiferroic properties of the perovskite YMnO3-BiFeO3 binary system

The perovskite (1−x)YMnO₃−xBiFeO₃ binary system is very promising because of its multiferroic end members. Nanocrystalline phases have been recently obtained by mechanosynthesis across the system, and the perovskite structural evolution has been described. Two continuous solid solutions with orthorhombic Pnma and rhombohedral R3c symmetries were found, which coexist within a broad compositional interval of 0.5 ≤ x ≤ 0.9. This might be a polar-nonpolar morphotropic phase boundary region, at which strong phase-change magnetoelectric responses can be expected. A major issue is phase decomposition at moderate temperatures that highly complicates ceramic processing. This is required for carrying out a sound electrical characterization and also for their use in devices. We present here the application of Spark Plasma Sintering to the ceramic processing of YMnO₃-BiFeO₃ phases. This advanced technique, when combined with nanocrystalline powders, allowed densifying phases at reduced processing temperatures and times, so that perovskite decomposition was avoided. Electrical measurements were accomplished, and the response was shown to be mostly dominated by conduction. Nonetheless, the intrinsic dielectric permittivity was obtained, and a distinctive enhancement in the phase coexistence region was revealed. Besides, Rayleigh-type behavior characteristic of ferroelectrics was also demonstrated for all rhombohedral phases. Magnetic characterization was performed in this region, and antiferromagnetism was shown.     

Analysis of the flow behavior of irradiated polyethylene through transient network models

The chemical and physical properties of polymers can be dramatically affected by exposure to high-energy radiation. Two polyethylene samples (HDPE and LDPE) were irradiated with a ⁶⁰Co source. Radiated samples with doses ranging between 0.5 and 5 Mrad were melted and their viscoelastic properties characterized. Results show the normal drastic changes in their flow behavior: higher viscosity and interesting changes induced in their elasticity, particularly in the case of LDPE. An explanation of the change in flow behavior is forwarded in terms of transient network models using the concept of creation and destruction of entanglements.