27 research outputs found
On the role of selective nucleation and growth to recrystallization texture development in a Mg-Gd-Zn alloy
One of the main material properties altered by rare earth additions in
magnesium alloys is texture, which can be specifically adjusted to enhance
ductility and formability. The current study aims at illuminating the texture
selection process in a Mg-0.073at%Gd-0.165at%Zn alloy by investigating
recrystallization nucleation and early nucleus growth during static
recrystallization. An as-cast sample of the investigated alloy was deformed in
uniaxial compression at 200{\deg}C till 40% strain and was then cut into two
halves for subsequent microstructure characterization via ex-situ and quasi
in-situ EBSD investigations. In order to gain insights into the evolution of
texture during recrystallization, the contributions from dynamic and static
recrystallization were initially separated and the origin of the non-basal
orientation of recrystallization nuclei was traced back to several potential
nucleation sites within the deformed matrix. Considering the significant role
of double-twin band recrystallization in determining the recrystallization
texture, this type of recrystallization nucleation was further investigated via
quasi-in-situ EBSD on a deformed sample, annealed at 400{\deg} for different
annealing times. With progressive annealing a noticeable trend was observed, in
which the basal nuclei gradually diminished and eventually vanished from the
annealed microstructure. In contrast, the off-basal nuclei exhibited continuous
growth, ultimately becoming the dominant contributors to the recrystallization
texture. The study therefore emphasizes the importance of particular nucleation
sites that generate favorably oriented off-basal nuclei, which over the course
of recrystallization outcompete the neighboring basal-oriented nuclei in terms
of growth, and thereby dominate the recrystallization texture
TOF-SIMS depth profiling of multilayer amino-acid films using large Argon cluster Arn
The performance of Cs+, C60 + and Ar n + (with n ≈ 1700) sputtering ions have been compared for depth profiling multilayer films made from three evaporated phenylalanine layers sandwiched between four thicker evaporated tyrosine layers. Using Cs +, the ion signals and depth resolution degrade with depth and were significantly affected beyond a 200-nm depth. The depth profiling quality was more successful using C60 +. However, in this case, the depth resolution and the layer width values still degrade with the sputtered depth and are particularly poor after reaching a depth of about 400 nm. When Ar1700 + clusters were used, a depth resolution as low as 6 nm was obtained, and this value never exceeds 9 nm. Moreover, the experimental layer width is found to be of the same order of magnitude as the real value. Copyright © 2012 John Wiley & Sons, Ltd. Copyright © 2012 John Wiley & Sons, Ltd
Molecular depth profiling of organic photovoltaic heterojunction layers by ToF-SIMS: comparative evaluation of three sputtering beams
With the recent developments in secondary ion mass spectrometry (SIMS), it is now possible to obtain molecular depth profiles and 3D molecular images of organic thin films, i.e. SIMS depth profiles where the molecular information of the mass spectrum is retained through the sputtering of the sample. Several approaches have been proposed for "damageless" profiling, including the sputtering with SF5(+) and C60(+) clusters, low energy Cs(+) ions and, more recently, large noble gas clusters (Ar500-5000(+)). In this article, we evaluate the merits of these different approaches for the in depth analysis of organic photovoltaic heterojunctions involving poly(3-hexylthiophene) (P3HT) as the electron donor and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as the acceptor. It is demonstrated that the use of 30 keV C60(3+) and 500 eV Cs(+) (500 eV per atom) leads to strong artifacts for layers in which the fullerene derivative PCBM is involved, related to crosslinking and topography development. In comparison, the profiles obtained using 10 keV Ar1700(+) (∼6 eV per atom) do not indicate any sign of artifacts and reveal fine compositional details in the blends. However, increasing the energy of the Ar cluster beam beyond that value leads to irreversible damage and failure of the molecular depth profiling. The profile qualities, apparent interface widths and sputtering yields are analyzed in detail. On the grounds of these experiments and recent molecular dynamics simulations, the discussion addresses the issues of damage and crater formation induced by the sputtering and the analysis ions in such radiation-sensitive materials, and their effects on the profile quality and the depth resolution. Solutions are proposed to optimize the depth resolution using either large Ar clusters or low energy cesium projectiles for sputtering and/or analysis
Mechanical Unfolding of an Autotransporter Passenger Protein Reveals the Secretion Starting Point and Processive Transport Intermediates
The backbone of secreted autotransporter passenger proteins generally attains a stable β-helical structure. The secretion of passengers across the outer membrane was proposed to be driven by sequential folding of this structure at the cell surface. This mechanism would require a relatively stable intermediate as starting point. Here, we investigated the mechanics of secreted truncated versions of the autotransporter hemoglobin protease (Hbp) of Escherichia coli using atomic force microscopy. The data obtained reveal a β-helical structure at the C terminus that is very stable. In addition, several other distinct metastable intermediates are found which are connected during unfolding by multiroute pathways. Computational analysis indicates that these intermediates correlate to the β-helical rungs in the Hbp structure which are clamped by stacked aromatic residues. Our results suggest a secretion mechanism that is initiated by a stable C-terminal structure and driven forward by several folding intermediates that build up the β-helical backbone
Molecular depth profiling of organic photovoltaic heterojunction layers by ToF-SIMS: comparative evaluation of three sputtering beams
Temperature Dependence of Arn + Cluster Backscattering from Polymer Surfaces: a New Method to Determine the Surface Glass Transition Temperature
Systematic study of magnetization reversal in square Fe nanodots of varying dimensions in different orientations
Improving Secondary Ion Mass Spectrometry C<sub>60</sub><sup><i>n</i>+</sup> Sputter Depth Profiling of Challenging Polymers with Nitric Oxide Gas Dosing
Organic depth profiling using secondary
ion mass spectrometry (SIMS)
provides valuable information about the three-dimensional distribution
of organic molecules. However, for a range of materials, commonly
used cluster ion beams such as C<sub>60</sub><sup><i>n</i>+</sup> do not yield useful depth profiles. A promising solution
to this problem is offered by the use of nitric oxide (NO) gas dosing
during sputtering to reduce molecular cross-linking. In this study
a C<sub>60</sub><sup>2+</sup> ion beam is used to depth profile a
polystyrene film. By systematically varying NO pressure and sample
temperature, we evaluate their combined effect on organic depth profiling.
Profiles are also acquired from a multilayered polystyrene and polyvinylpyrrolidone
film and from a polystyrene/polymethylmethacrylate bilayer, in the
former case by using an optimized set of conditions for C<sub>60</sub><sup>2+</sup> and, for comparison, an Ar<sub>2000</sub><sup>+</sup> ion beam. Our results show a dramatic improvement for depth profiling
with C<sub>60</sub><sup>2+</sup> using NO at pressures above 10<sup>–6</sup> mbar and sample temperatures below −75 °C.
For the multilayered polymer film, the depth profile acquired using
C<sub>60</sub><sup>2+</sup> exhibits high signal stability with the
exception of an initial signal loss transient and thus allows for
successful chemical identification of each of the six layers. The
results demonstrate that NO dosing can significantly improve SIMS
depth profiling analysis for certain organic materials that are difficult
to analyze with C<sub>60</sub><sup><i>n</i>+</sup> sputtering
using conventional approaches/conditions. While the analytical capability
is not as good as large gas cluster ion beams, NO dosing comprises
a useful low-cost alternative for instruments equipped with C<sub>60</sub><sup><i>n</i>+</sup> sputtering