311,708 research outputs found
Atomistic Simulations of Basal Dislocations Interacting with MgAl Precipitates in Mg
The mechanical properties of Mg-Al alloys are greatly influenced by the
complex intermetallic phase MgAl, which is the most dominant
precipitate found in this alloy system. The interaction of basal edge and
30 dislocations with MgAl precipitates is studied by
molecular dynamics and statics simulations, varying the inter-precipitate
spacing (), and size (), shape and orientation of the precipitates. The
critical resolved shear stress to pass an array of precipitates
follows the usual proportionality. In all cases but the
smallest precipitate, the dislocations pass the obstacles by depositing
dislocation segments in the disordered interphase boundary rather than shearing
the precipitate or leaving Orowan loops in the matrix around the precipitate.
An absorbed dislocation increases the stress necessary for a second dislocation
to pass the precipitate also by absorbing dislocation segments into the
boundary. Replacing the precipitate with a void of identical size and shape
decreases the critical passing stress and work hardening contribution while an
artificially impenetrable MgAl precipitate increases both. These
insights will help improve mesoscale models of hardening by incoherent
particles.Comment: 13 pages with 9 figures and 2 tables. Supplementary materia
An \u3cem\u3eIn Vitro\u3c/em\u3e Spectroscopic Analysis to Determine the Chemical Composition of the Precipitate Formed by Mixing Sodium Hypochlorite and Chlorhexidine
Introduction—The purpose of this in vitro study was to determine the chemical composition of the precipitate formed by mixing sodium hypochlorite (NaOCl) and Chlorhexidine (CHX), and relative molecular weight of the components.
Methods—Using commercially available chlorhexidine gluconate (CHXg), a 2% solution was formed and mixed in a 1:1 ratio with commercially available NaOCl producing a brown precipitate. The precipitate as well as a mixture of precipitate and pure chlorhexidine diacetate (CHXa) was then analyzed using 1D and 2D NMR spectroscopy.
Results—The 1D and 2D NMR spectra were fully assigned, in terms of chemical shifts of all proton and carbon atoms in intact CHX. This permitted identification of CHX breakdown products with and without the aliphatic linker present, including lower molecular weight components of CHX that contained a para-substituted benzene that was not para-chloroaniline (PCA).
Conclusions—Based on this in vitro study, the precipitate formed by NaOCl and CHX is composed of at least two separate molecules, all of which are smaller in size than CHX. Along with native CHX, the precipitate contains two chemical fragments derived from CHX, neither of which are PCA
Solvation agent for disulfide precipitates from inhibited glycol-water solutions
Small additions /0.01 percent or less/ of triethanoloamine sodium sulfite adduct to mercapto benzothiazole inhibited glycol water heat transfer solutions containing disulfide precipitate produce marked reduction in amount of precipitate. Adduct is useful as additive in glycol base antifreezes and coolants
An \u3cem\u3eIn Vitro\u3c/em\u3e Spectroscopic Analysis to Determine Whether Para-Chloroaniline Is Produced from Mixing Sodium Hypochlorite and Chlorhexidine
Introduction: The purpose of this in vitro study was to determine whether para-chloroaniline (PCA) is formed through the reaction of mixing sodium hypochlorite (NaOCl) and chlorhexidine (CHX).
Methods: Initially, commercially available samples of chlorhexidine acetate (CHXa) and PCA were analyzed with 1H nuclear magnetic resonance (NMR) spectroscopy. Two solutions, NaOCl and CHXa, were warmed to 37ºC, and when mixed they produced a brown precipitate. This precipitate was separated in half, and pure PCA was added to 1 of the samples for comparison before they were each analyzed with 1H NMR spectroscopy.
Results: The peaks in the 1H NMR spectra of CHXa and PCA were assigned to specific protons of the molecules, and the location of the aromatic peaks in the PCA spectrum defined the PCA doublet region. Although the spectrum of the precipitate alone resulted in a complex combination of peaks, on magnification there were no peaks in the PCA doublet region that were intense enough to be quantified. In the spectrum of the precipitate to which PCA was added, 2 peaks do appear in the PCA doublet region. Comparing this spectrum with that of precipitate alone, the peaks in the PCA doublet region are not visible before the addition of PCA.
Conclusions: On the basis of this in vitro study, the reaction mixture of NaOCl and CHXa does not produce PCA at any measurable quantity, and further investigation is needed to determine the chemical composition of the brown precipitate
Ab initio Modelling of the Early Stages of Precipitation in Al-6000 Alloys
Age hardening induced by the formation of (semi)-coherent precipitate phases
is crucial for the processing and final properties of the widely used Al-6000
alloys. Early stages of precipitation are particularly important from the
fundamental and technological side, but are still far from being fully
understood. Here, an analysis of the energetics of nanometric precipitates of
the meta-stable phases is performed, identifying the bulk, elastic
strain and interface energies that contribute to the stability of a nucleating
cluster. Results show that needle-shape precipitates are unstable to growth
even at the smallest size formula unit, i.e. there is no energy
barrier to growth. The small differences between different compositions points
toward the need for the study of possible precipitate/matrix interface
reconstruction. A classical semi-quantitative nucleation theory approach
including elastic strain energy captures the trends in precipitate energy
versus size and composition. This validates the use of mesoscale models to
assess stability and interactions of precipitates. Studies of smaller 3d
clusters also show stability relative to the solid solution state, indicating
that the early stages of precipitation may be diffusion-limited. Overall, these
results demonstrate the important interplay among composition-dependent bulk,
interface, and elastic strain energies in determining nanoscale precipitate
stability and growth
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