Synthesis and characterization of BaZrO3-doped YBa2Cu3O7−δ microtapes with improved critical current densities

We have recently demonstrated that through a sol–gel route, superconductor crystallization in the presence of simple biopolymers results in a drastic alteration of morphology, producing technologically useful nanowires and porous architectures. Morphological control is of the utmost importance to bulk high-temperature superconductors, as grain boundaries act as weak links in limiting the achievable critical current density (J_c). Here we show that, as expected, the incorporation of nanoparticulate barium zirconate (BaZrO₃) species into a biopolymer-mediated synthetic protocol for YBa₂Cu₃O_7?δ (Y123) leads to a significantly improved in-field J_c compared to that observed in a sample without BaZrO₃ additions. To ameliorate degradation of the BaZrO₃ species in this protocol, we demonstrate that by drawing the precursor sol into fibers, a microtape architecture is able to be formed, leading to lengthy, anisotropic structures having enhanced J_c through the retention of the BaZrO₃ species.     

Polyamine-functional sterically stabilized latexes for covalently cross-linkable colloidosomes

Sterically stabilized polystyrene latexes were prepared by aqueous emulsion polymerization using a poly(ethylene imine) (PEI) stabilizer in the presence of 4-vinylbenzyl chloride (4-VBC; 1.0 wt % based on styrene). Partial quaternization of the amine groups on the PEI chains by 4-VBC occurs in situ, hence producing a chemically grafted steric stabilizer. Such 4-VBC-modified PEI chains were grafted more efficiently onto the polystyrene particles than unmodified PEI, as judged by aqueous electrophoresis, XPS, and nitrogen microanalysis. Moreover, partially quaternized PEI gave significantly smaller polystyrene particles than those synthesized in the absence of any PEI stabilizer or those synthesized using unmodified PEI. The partially quaternized PEI-stabilized polystyrene latex proved to be an effective emulsifier at pH 9, forming stable oil-in-water Pickering emulsions when homogenized (12,000 rpm, 2 min, 20 °C) with four model oils, namely, n-dodecane, methyl myristate, isononyl isononanoate, and sunflower oil. The primary and/or secondary amine groups on the PEI stabilizer chains were successfully cross-linked using three commercially available polymeric reagents, namely, tolylene 2,4-diisocyanate-terminated poly(propylene glycol) (PPG-TDI), poly(propylene glycol) diglycidyl ether (PPG-DGE), or poly(ethylene glycol) diglycidyl ether (PEG-DGE). Cross-linking with the former reagent led to robust colloidosomes that survived the removal of the internal oil phase on washing with excess alcohol, as judged by optical microscopy and SEM. PPG-TDI reacted very rapidly with the PEI stabilizer chains, with cross-linking being achieved during homogenization. Well-defined colloidosomes could be formed only by using sunflower oil and isononyl isononanoate with this cross-linker at 20 °C. However, cooling to 0 °C allowed colloidosomes to be formed using n-dodecane, presumably because of the slower rate of cross-linking at this reduced temperature. PPG-DGE proved to be a more generic cross-linker because it formed robust colloidosomes with all four model oils. However, cross-linking was much slower than that achieved using PPG-TDI, with intact colloidosomes being formed only after ～12 h at 20 °C. The PEG-DGE cross-linker allowed cross-linking to be conducted at 20 °C from the aqueous phase (rather from within the oil droplets for the oil-soluble PPG-TDI or PPG-DGE cross-linkers). In this case, well-defined colloidosomes were obtained at 50 vol % with surprisingly little intercolloidosome aggregation, as judged by laser diffraction studies.     

Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

The geometric structures of small cationic rhodium clusters Rh(n)(+) (n = 6-12) are investigated by comparison of experimental far-infrared multiple photon dissociation spectra with spectra calculated using density functional theory. The clusters are found to favor structures based on octahedral and tetrahedral motifs for most of the sizes considered, in contrast to previous theoretical predictions that rhodium clusters should favor cubic motifs. Our findings highlight the need for further development of theoretical and computational methods to treat these high-spin transition metal clusters.     

The nonribosomal peptide synthetase enzyme DdaD tethers N(β)-fumaramoyl-l-2,3-diaminopropionate for Fe(II)/α-ketoglutarate-dependent epoxidation by DdaC during dapdiamide antibiotic biosynthesis

The gene cluster from Pantoea agglomerans responsible for biosynthesis of the dapdiamide antibiotics encodes an adenylation-thiolation didomain protein, DdaD, and an Fe(II)/α-ketoglutarate-dependent dioxygenase homologue, DdaC. Here we show that DdaD, a nonribosomal peptide synthetase module, activates and sequesters N(β)-fumaramoyl-l-2,3-diaminopropionate as a covalently tethered thioester for subsequent oxidative modification of the fumaramoyl group. DdaC catalyzes Fe(II)- and α-ketoglutarate-dependent epoxidation of the covalently bound N(β)-fumaramoyl-l-2,3-diaminopropionyl-S-DdaD species to generate N(β)-epoxysuccinamoyl-DAP (DAP = 2,3-diaminopropionate) in thioester linkage to DdaD. After hydrolytic release, N(β)-epoxysuccinamoyl-DAP can be ligated to l-valine by the ATP-dependent ligase DdaF to form the natural antibiotic N(β)-epoxysuccinamoyl-DAP-Val.     

The deposition of nanostructured β-PbO2 coatings from aqueous methanesulfonic acid for the electrochemical oxidation of organic pollutants

Highly crystalline, nanostructured, three-dimensional β-PbO₂ coatings were successfully obtained by galvanostatic deposition from baths containing aqueous lead(II) and methanesulfonic acid (CH₃SO₃H). This constitutes a much more environmentally friendly methodology compared to plating of β-PbO₂ in HNO₃. The deposits exhibited high quality and good adherence. The crystallite size was in the range 20–30 nm and AFM imaging revealed very uniform, rough deposits (i.e., 255–275 nm rms). The oxidative destruction of Methyl Orange azo dye was studied by electrochemical advanced oxidation processes (EAOPs). An electro-Fenton process with a high surface area carbon-felt cathode performed better than the single anodic oxidation. Rapid and complete decolorisation was achieved following pseudo first-order kinetics. The stability of the β-PbO₂ electrodes during the electrolyses was also demonstrated.     

Fluid limits of many-server queues with abandonments,general service and continuous patience time distributions

This paper extends the works of Kang and Ramanan (2010) and Kaspi and Ramanan (2011), removing the hypothesis of absolute continuity of the service requirement and patience time distributions. We consider a many-server queueing system in which customers enter service in the order of arrival in a non-idling manner and where reneging is considerate. Similarly to Kang and Ramanan (2010), the dynamics of the system are represented in terms of a process that describes the total number of customers in the system as well as two measure-valued processes that record the age in service of each of the customers being served and the “potential” waiting times. When the number of servers goes to infinity, fluid limit is established for this triple of processes. The convergence is in the sense of probability and the limit is characterized by an integral equation.