Time studies of ionization processes

Summary Measurements of the temporal growth of ionization between parallel plane electrodes in hydrogen have been made. The results show that for low values ( 40 V/cm mm Hg) of the ratio of electric fieldE to gas pressurep the growth times can be short ( 1s for over-voltages V 1 %) while at values ofE/p 300 V/cm mm Hg the times are of the order of milli seconds withV 5%. Comparison of the experimental data with Davidson's mathematical analysis of current growth based upon the action of primary and secondary ionization processes shows that the relative significance of the possible secondary processes changes asE/p is altered. For the low values ofE/p, the predominant secondary process contributing to the growth was found to be photoelectric emission from the cathode, but with increasing values ofE/p the role of positive ion interaction with the cathode becomes increasingly important. No single secondary process was exclusively operating in any of the conditions examined.     

LiCoO(2) Concaved Cuboctahedrons from Symmetry-Controlled Topological Reactions

Morphology control of functional materials is generally performed by controlling the growth rates on selected orientations or faces. Here, we control particle morphology by "crystal templating": by choosing appropriate precursor crystals and reaction conditions, we demonstrate that a material with rhombohedral symmetry-namely the layered, positive electrode material, LiCoO(2)-can grow to form a quadruple-twinned crystal with overall cubic symmetry. The twinned crystals show an unusual, concaved-cuboctahedron morphology, with uniform particle sizes of 0.5-2 μm. On the basis of a range of synthetic and analytical experiments, including solid-state NMR, X-ray powder diffraction analysis and HRTEM, we propose that these twinned crystals form via selective dissolution and an ion-exchange reaction accompanied by oxidation of a parent crystal of CoO, a material with cubic symmetry. This template crystal serves to nucleate the growth of four LiCoO(2) twin crystals and to convert a highly anisotropic, layered material into a pseudo-3-dimensional, isotropic material.     

Isolation of a Novel Polyketide from Neodidymelliopsis sp.

Fungi have become an invaluable source of bioactive natural products, with more than 5 million species of fungi spanning the globe. Fractionation of crude extract of Neodidymelliopsis sp., led to the isolation of a novel polyketide, (2Z)-cillifuranone (1) and five previously reported natural products, (2E)-cillifuranone (2), taiwapyrone (3), xylariolide D (4), pachybasin (5), and N-(5-hydroxypentyl)acetamide (6). It was discovered that (2Z)-cillifuranone (1) was particularly sensitive to ambient temperature and light resulting in isomerisation to (2E)-cillifuranone (2). Structure elucidation of all the natural products were conducted by NMR spectroscopic techniques. The antimicrobial activity of 2, 3, and 5 were evaluated against a variety of bacterial and fungal pathogens. A sodium [1-¹³C] acetate labelling study was conducted on Neodidymelliopsis sp. and confirmed that pachybasin is biosynthesised through the acetate polyketide pathway.     

Surface differences of oxide nanocrystals determined by geometry and exogenously coordinated water molecules

Determining the different surfaces of oxide nanocrystals is key in developing structure–property relations. In many cases, only surface geometry is considered while ignoring the influence of surroundings, such as ubiquitous water on the surface. Here we apply ¹⁷O solid-state NMR spectroscopy to explore the facet differences of morphology-controlled ceria nanocrystals considering both geometry and water adsorption. Tri-coordinated oxygen ions at the 1^st layer of ceria (111), (110), and (100) facets exhibit distinct ¹⁷O NMR shifts at dry surfaces while these ¹⁷O NMR parameters vary in the presence of water, indicating its non-negligible effects on the oxide surface. Thus, the interaction between water and oxide surfaces and its impact on the chemical environment should be considered in future studies, and solid-state NMR spectroscopy is a sensitive approach for obtaining such information. The work provides new insights into elucidating the surface chemistry of oxide nanomaterials.

Both atomic geometry and the influence of surroundings (e.g., exogenously coordinated water) are key issues for determining the chemical environment of oxide surfaces, whereas the latter is usually ignored and should be considered in future studies.     

Antimicrobial Polyketide Metabolites from Penicillium bissettii and P. glabrum

Screening of several fungi from the New Zealand International Collection of Microorganisms from Plants identified two strains of Penicillium, P. bissettii and P. glabrum, which exhibited antimicrobial activity against Escherichia coli, Klebsiella pneumoniae, and Staphylococcus aureus. Further investigation into the natural products of the fungi, through extraction and fractionation, led to the isolation of five known polyketide metabolites, penicillic acid (1), citromycetin (2), penialdin A (3), penialdin F (4), and myxotrichin B (5). Semi-synthetic derivatization of 1 led to the discovery of a novel dihydro (1a) derivative that provided evidence for the existence of the much-speculated open-chained form of 1. Upon investigation of the antimicrobial activities of the natural products and derivatives, both penicillic acid (1) and penialdin F (4) were found to inhibit the growth of Methicillin-resistant S. aureus. Penialdin F (4) was also found to have some inhibitory activity against Mycobacterium abscessus and M. marinum along with citromycetin (2).     

Comparative study of the electronic structure of the ordered and disordered Cu3Au (100) and Cu3Au (110) surfaces

The influence of structural changes on the electronic structure has been investigated by a comparison of the angle-integrated valence and core level photoelectron spectra of ordered and disordered Cu₃Au(100) and (110) surfaces. The total width of the Au 5d and Cu 3d bands does not change with the ordering state or surface orientation. The spectra for the (100) surface are compared with selfconsistent calculations and good agreement is found, for the ordered state, however with a 0.6 eV correction of the calculated Fermi level position. We observe three Au 5d derived bands at 5.1, 6.0 and 6.9 eV, in contrast to previous experimental findings. Our results indicate the existence of shortrange order above the critical temperature.     

New cesium titanate layer structures

A phase study of the Cs₂OTiO₂ system in the composition range 75–100 mole% TiO₂ and the temperature range 850–1200°C revealed the existence of two new cesium titanates, with compositions Cs₂Ti₅O₁₁ and Cs₂Ti₆O₁₃. The former compound undergoes a reversible hydration reaction below 200°C to form Cs₂Ti₅O₁₁ · (1 + x)H₂O, 0.5 < x < 1. The structures of the three phases have been determined. They are based on corrugated layers of edge-shared octahedra, with cesium ions (and H₂O) packing between the layers. In Cs₂Ti₆O₁₃, the layers are continuous in two dimensions, whereas in Cs₂Ti₅O₁₁ and Cs₂Ti₅O₁₁ · (1 + x)H₂O, the layers are periodically stepped to give 5-octahedra wide, corner-linked ribbons.