Interface propagation and microstructure evolution in phase field models of stress-induced martensitic phase transformations

Analytical solutions for diffuse interface propagation are found for two recently developed Landau potentials that account for the phenomenology of stress-induced martensitic phase transformations. The solutions include the interface profile and velocity as a function of temperature and stress tensor. An instability in the interface propagation near lattice instability conditions is studied numerically. The effect of material inertia is approximately included. Two methods for introducing an athermal interface friction in phase field models are discussed. In the first method an analytic expression defines the location of the diffuse interface, and the rate of change of the order parameters is required to vanish if the driving force is below a threshold. As an alternative and more physical approach, we demonstrate that the introduction of spatially oscillatory stress fields due to crystal defects and the Peierls barrier, or to a jump in chemical energy, reproduces the effect of an athermal threshold. Finite element simulations of microstructure evolution with and without an athermal threshold are performed. In the presence of spatially oscillatory fields the evolution self-arrests in realistic stationary microstructures, thus the system does not converge to an unphysical single-phase final state, and rate-independent temperature- and stress-induced phase transformation hysteresis are exhibited.     

PhPAd‐DalPhos: Ligand‐Enabled,Nickel‐Catalyzed Cross‐Coupling of (Hetero)aryl Electrophiles with Bulky Primary Alkylamines

The base metal‐catalyzed C?N cross‐coupling of bulky α,α,α‐trisubstituted primary alkylamines with (hetero)aryl electrophiles represents a challenging and under‐developed class of transformations that is of significant potential utility, including in the synthesis of lipophilic active pharmaceutical ingredients. Herein, we report that a new, air‐stable Ni(II) pre‐catalyst incorporating the optimized ancillary ligand PhPAd‐DalPhos enables such transformations of (hetero)aryl chloride, bromide, and tosylate electrophiles to be carried out for the first time with substrate scope rivalling that achieved using state‐of‐the‐art Pd catalysts, including room temperature cross‐couplings of (hetero)aryl chlorides that are unprecedented for any catalyst (Pd, Ni, or other).     

Recognition Properties and Self‐assembly of Planar [M(2‐pyridyl‐1,2,3‐triazole)2]2+ Metallo‐ligands

Molecular recognition continues to be an area of keen interest for supramolecular chemists. The investigated [M( L )₂]²⁺ metallo‐ligands (M=Pd^II, Pt^II, L =2‐(1‐(pyridine‐4‐methyl)‐1 H‐1,2,3‐triazol‐4‐yl)pyridine) form a planar cationic panel with vacant pyridyl binding sites. They interact with planar neutral aromatic guests through π–π and/or metallophilic interactions. In some cases, the metallo‐ligands also interacted in the solid state with Ag^I either through coordination to the pendant pyridyl arms, or through metal–metal interactions, forming coordination polymers. We have therefore developed a system that reliably recognises a planar electron‐rich guest in solution and in the solid state, and shows the potential to link the resultant host–guest adducts into extended solid‐state structures. The facile synthesis and ready functionalisation of 2‐pyridyl‐1,2,3‐triazole ligands through copper(I)‐catalyzed azide–alkyne cycloaddition (CuAAC) “click” chemistry should allow for ready tuning of the electronic properties of adducts formed from these systems.     

Reversible Transformation between a [PdL2]2+ “Figure-of-Eight” Complex and a [Pd2L2]4+ Dimer: Switching On and Off Self-Recognition

Structural changes to metallosupramolecular assemblies resulting in the release or uptake of guests are currently well established, whereas transformations turning on and off specific self-recognition are far less developed. We report a novel ligand (2,6-bis(1-(3-pyridin-4-yl)phenyl-1H-1,2,3-triazol-4-yl)pyridine) possessing a tridentate central metal-binding site flanked by two pendant pyridyl arms. In a 2:1 ratio with Pd^II metal ions, a spiro-type [PdL₂]²⁺ “Figure-of-eight” complex forms with the central tridentate binding pocket unoccupied. The introduction of an additional one equivalent of Pd^II metal ion results in the conversion to a dimeric [Pd₂L₂]⁴⁺ molecule with the tridentate pocket occupied. There is site-specific self-recognition between dimers in solution with strong NOE peaks between adjacent molecules. The self-recognition between dimers can be turned off in two ways: firstly, adding another equivalent of Pd^II metal ion brings about binding to the previously uncoordinated pyridyl arms that are key to the self-recognition event, and; secondly, addition of sufficient ligand to return the stoichiometry to 2:1 regenerates the [PdL₂]²⁺ complex. Hence, the self-recognition event can be turned on or off through simple variation of L:Pd^II stoichiometry.     

Improved measurement of protein synthesis in human subjects using 2H‐phenylalanine isotopomers and gas chromatography/mass spectrometry

Sensitive methods to measure protein synthetic rate in vivo are required to assess changes in protein expression, especially when comparing healthy with infirm subjects. We have previously applied a ‘flooding dose’ procedure using ²H₅‐phenylalanine (²H₅‐phe) and ²H₈‐phe isotopomers as tracers, which has proven successful in measuring albumin and fibrinogen synthesis in response to feeding in cancer patients. Using tert‐butyldimethylsilyl derivatives, we have observed that ²H₇‐phe is formed with time in vivo from ²H₈‐phe, probably during transamination. This increases errors when estimating the fractional synthetic rate (FSR) using the ²H₈‐phe isotopomer compared with the ²H₅‐phe isotopomer. We sought to improve this situation by use of an alternative derivative that overcomes this problem whilst also streamlining sample preparation. When using N‐ethoxycarbonyltrifluoroethyl (ECTFE) amino acid esters, ²H₈‐phe is effectively converted into ²H₇‐phe through fragmentation under electron ionisation (EI), allowing both ²H₈‐phe and ²H₇‐phe isotopomers to be measured as a single intense CH fragment at 98 Th. To illustrate the improved situation, the mean RMS residual was calculated for all albumin data, for each isotopomer and for each derivative. Albumin‐bound Phe was analysed as ECTFE‐phe with improved precision, independent of the isotopomer used, confirming that the new derivative is superior. Copyright © 2010 John Wiley & Sons, Ltd.     

(100) MgAl2O4 as a lattice-matched substrate for the epitaxial thin film deposition of the relaxor ferroelectric PMN-PT

The (100) surface of MgAl₂O₄ is evaluated as a substrate for the thin film deposition of the relaxor ferroelectric PbMg_1/3Nb_2/3O₃(65%)–PbTiO₃(35%). With a lattice mismatch of less than 0.5%, this film-substrate combination presents a geometrical template for growth that is far superior to that formed with other commercially available oxide substrates. Films were deposited using the pulsed laser deposition technique and were characterized in terms of their crystallographic, microstructural, and dielectric properties. From a crystallographic perspective the films show excellent cube-on-cube epitaxy, are highly oriented, and show no evidence of the frequently observed parasitic pyrochlore phase. With the exception of a few faceted surface structures, the film’s microstructure is single-crystal-like, exhibiting a sharp film-substrate interface, a smooth top surface, and no discernable granularity. The dielectric response shows the frequency-dependent diffuse phase transition characteristic of a relaxor material, but with less frequency dispersion and a smaller maximum in the dielectric constant. Taken together, the results suggest that the (100) MgAl₂O₄ substrate could prove to be an effective substrate material, not only for the PbMg_1/3Nb_2/3O₃(65%)–PbTiO₃(35%) system, but also for a number of other important lattice-matched ferroelectric, relaxor, and ferroelectric superlattice systems.     

Gold modified cadmium sulfide aerogels

Monolithic aerogels composed of cadmium sulfide nanoparticles partially modified with metallic gold (CdS-Au) are reported. The semiconductor–metal nanoparticles are synthesized using an inverse micelle media of Bis-(2-ethylhexyl)sulfosuccinate sodium salt (AOT) in heptane, followed by capping with 4-fluorothiophenol and precipitation with triethylamine. The nanoparticles are then dispersed in acetone and gel formation is achieved using nanoparticle condensation strategy. The resultant CdS-Au aerogel materials are mesoporous, with an interconnected network of semiconductor–metal nanoparticles. A detailed microstructure analysis of the semiconductor–metal aerogels via transmission electron microscopy indicates that the final gold concentration significantly impacts the semiconductor–metal aerogel morphology and porosity.