E. V. Esquivel

Carbon nanotubes, nanocrystal forms, and complex nanoparticle aggregates in common fuel-gas combustion sources and the ambient air

Aggregated multiwall carbon nanotubes (with diameters ranging from ∼3 to 30nm) and related carbon nanocrystal forms ranging in size from 0.4 to 2 μm (average diameter) have been collected in the combustion streams for methane/air, natural gas/air, and propane gas/air flames using a thermal precipitator. Individual particle aggregates were collected on carbon/formvar-coated 3mm nickel grids and examined in a transmission electron microscope, utilizing bright-field imaging, selected-area electron diffraction analysis, and energy-dispersive X-ray spectrometry techniques. The natural gas and propane gas sources were domestic (kitchen) stoves, and similar particle aggregates collected in the outdoor air were correspondingly identified as carbon nanocrystal aggregates and sometimes more complex aggregates of silica nanocrystals intermixed with the carbon nanotubes and other carbon nanocrystals. Finally, and in light of the potential for methane-series gas burning as major sources of carbon nanocrystal aggregates in both the indoor and outdoor air, data for natural gas consumption and corresponding asthma deaths and incidence are examined with a degree of speculation regarding any significance in the correlations.

Characterization of nanostructure phenomena in airborne particulate aggregates and their potential for respiratory health effects

Airborne aggregates of nanoparticulates were collected on carbon/form-coated, 100-mesh Ni TEM grids in a thermal precipitator and observed in an analytical TEM utilizing a BF-SAED-DF-EDS characterization protocol to identify the nanocrystalline or nanoparticulate components, especially their degree of crystallinity, size, structural/morphologic features, and chemistries. Reference aggregates of TiO2 rutile and anatase as well as Si3N4 nanoparticles were used to establish these characterization protocols, which were applied to several hundred individual particulates: homogeneous aggregates of carbonaceous/diesel particulate matter, complex mixtures of carbonaceous matter, including carbon nanocrystals, and inorganic nanocrystals; and heterogeneous, nanocrystal/nanoparticulate aggregates. Most airborne particulates were aggregates ranging in aerodynamic diameters from a few nanometers to a few microns; containing as few as 2 nanocrystals to several thousand nanocrystals or nanoparticulates such as carbonaceous spherules arranged in complex branched homogeneous aggregates composing diesel exhaust, with spherule diameters ranging from 10 to 30 nm. The potential for ultrafine airborne aggregates to fragment into hundreds or thousands of nanoparticulate components in human airways and act as toxic agents in deep lung tissue is demonstrated.

Deformation effects in shocked metals and alloys

Abstract Plane wave shock loading produces twins or twin faults in many metals and alloys, and, especially for fcc materials with decreasing stacking fault energy (SFE), a critical twinning pressure (CTP), and crystallographic orientations which control the onset and extent of twinning. The CTP increases with increasing SFE, and is lowest in [001] orientations for fcc. The deformation associated with plane shock is generally within the realm of the plastic regime of the stress–strain diagram. Correspondingly, spherical shock, characteristic of impact cratering, produces large strains through solid state flow and sliding of overlapping shear bands composed of dynamically recrystallised grains, and is therefore only partly encompassed in the realm of the stress–strain diagram. Below this zone for impact craters, and similar to plane shock loading, there is a slip–twinning or slip–microbanding transition in fcc materials, which depends on the SFE; twinning persists for low SFE while microbands dominate in high SFE fcc materials such as nickel or copper. A simple model is used to explain differences in residual microstructures in plane shock loading and spherical shock or impact cratering as they relate to SFE and shock wave geometry. Simple schematics are used to explain the connection between plane shock and spherical shock loading in relation to the stress–strain diagram as a paradigm.

Comparison of microstructures for plane shock-loaded and impact crater-related nickel: the microtwin-microband transition

Plane-wave shock-loaded Ni exhibits {111} microtwins which increase in frequency with increasing peak shock pressure above a critical twinning pressure of ∼30 GPa. In contrast, microbands coincident with traces of {111} are produced below impact craters in Ni targets by stainless steel projectiles at velocities up to 3.5 km/s. The microband widths are ten times the 0.02 μm twin widths and are characterized by misorientations of roughly 2°. Both shock-loaded and impacted Ni have similar dislocation cell structures which decrease in cell size with increasing pressure or equivalent stress. The exclusive formation of microbands in connection with impact craters in Ni is expected on the basis of its high SFE (∼130 mJ/m2), and a simple dislocation model is developed for the microtwin-microband transition based on graphical summaries which include shock (stress) geometry and SFE effects in FCC metals and alloys.

Microstructural development during aging of 2014 aluminum alloy composite

The 2014 aluminum alloy reinforced with 0.1 and 0.15 volume fraction of alumina particles (VFAP) have been solutionized for a range of time from 1.5 to 20 h at 813 K. The effect of solutionizing time (ST) on the age hardening response of the composites has been studied and compared with the characteristics exhibited by the monolith. The results indicate that increasing the ST decreases the time required to get the peak hardness (TPH) values in the monolith but the composites do not show a systematic monotonic behavior. The TPH values first decrease and then increase with an increase in ST at an aging temperature of 473 K for the composite. It has been speculated that he ST influences the concentration of quenched-in vacancies and continued heating may affect the bonding between particles and matrix which can generate additional dislocations throughout the solutionizing process due to curvature effects.

Comparison of flow and shear band structures in oriented, columnar tungsten, single crystal tungsten-tantalum ands intered tungsten heavy alloy ballistic penetrators

Abstract The penetration of long rods (L/D10) into standard RHA targets differs with rod failure and flow, which produces erosion and deceleration. Oriented, columnar grained W and [001] single crystal W-4%Ta rods exhibit dense shear/shear band flow phenomena consisting of overlapping bands ofpredominantly dynamically recrystallised (DRX) grain structures observed by optical metallography and transmission electron microscopy. This flow is contrasted withWHA (93%W, 5%Ni, 2%Fe) sintered rod penetration characterised by penetrator nose failure in blocks which move on narrow shear bands between the blocks and function like microstructural lubricants. From these comparative observations of residual, penetrated rods, strategies to promote rod penetration seem to involve the development of wide, overlapping bands or layers of equiaxed or preferentially oriented, refined microstructures, which facilitate material flow at high strain rates. Microstructural precursors, such as deformation twinning or the enhancement of recrystallisation or related microstructural issues, promoting frequent or overlapping shear bands, either through alloying or processing routes to control the shear instabilities, seem to provide the best strategies to enhance long rod penetration.