The self-consistent structure of light nuclei (II)

Using methods developed in an earlier work, the ground state structures of a number of light nuclei A 40 are studied. All calculations are based on the Reid soft-core potential from which a local density-dependent nuclear-matter approximation to the Brueckner-Bethe-Goldstone reaction matrix has been calculated by Siemens. The Kuo and Brown approximation to the effective interaction is found not to produce sufficient saturation. Both the exact and an effective charge approximation effect of the static Coulomb interaction are studied.

Binding energies which are in agreement with the experimental results are obtained (except for the very light nucleus ⁴He); however, the density distributions are considerably more compact than those currently predicted by electron scattering experiments unless the density dependence of the reaction matrix is included self-consistently.     

Experimental confirmation of the predicted shallow donor hydrogen state in zinc oxide

We confirm the recent prediction that interstitial protium may act as a shallow donor in zinc oxide, by direct spectroscopic observation of its muonium counterpart. On implantation into ZnO, positive muons--chemically analogous to protons in this context--form paramagnetic centers below about 40 K. The muon-electron contact hyperfine interaction, as well as the temperature and activation energy for ionization, imply a shallow level. Similar results for the cadmium chalcogenides suggest that such shallow donor states are generic to the II-VI compounds. The donor level depths should serve as a guide for the electrical activity of interstitial hydrogen.     

B site doped strontium titanate as a potential SOFC substrate

Excellent thermal and mechanical stability coupled with low cost have attracted interest in the application of the cubic perovskite SrTiO₃ as a substrate material in supported SOFC designs. For such designs increased substrate conductivity is beneficial. A method of improving conductivity is by cation substitution. Due to the constraint of electro-neutrality, oxygen ion vacancies can be generated in strontium titanate by successful substitution of tetra-valent titanium ions with divalent metal ions (M) to produce materials of stoichiometry SrTi_(1−x)M_xO_(3−x). By raising the intrinsic oxygen vacancy concentration in this manner there is an increase in available hopping sites. The increase in vacant sites facilitates oxygen transport through the crystal and hence increases the potential for oxide ion conductivity. The synthesis of such materials was carried out by standard solid-state techniques using calcium and magnesium as dopants. B site solubility limits for both species were obtained by powder X-ray diffraction. The conductivity behaviour of successful phase pure compounds was investigated using AC impedance spectroscopy and four point DC measurements across a range of pO₂ values. The B site solubility limit for magnesium was found to lie between 5 and 7 %. SrTi_0.95Mg_0.05O_2.95 exhibited increased conductivity and reduced activation energy for conduction as compared to undoped strontium titanate. DC measurements for the same material confirmed the increased p-type behaviour of the system associated with magnesium doping at high oxygen partial pressures. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France Oct. 1–7, 2000.     

Fabricating protein immunoassay arrays on nitrocellulose using dip-pen lithography techniques

Advancements in lithography methods for printing biomolecules on surfaces are proving to be potentially beneficial for disease screening and biological research. Dip-pen nanolithography (DPN) is a versatile micro and nanofabrication technique that has the ability to produce functional biomolecule arrays. The greatest advantage, with respect to the printing mechanism, is that DPN adheres to the sensitive mild conditions required for biomolecules such as proteins. We have developed an optimised, high-throughput printing technique for fabricating protein arrays using DPN. This study highlights the fabrication of a prostate specific antigen (PSA) immunoassay detectable by fluorescence. Spot sizes are typically no larger than 8 μm in diameter and limits of detection for PSA are comparable with a commercially available ELISA kit. Furthermore, atomic force microscopy (AFM) analysis of the array surface gives great insight into how the nitrocellulose substrate functions to retain protein integrity. This is the first report of protein arrays being printed on nitrocellulose using the DPN technique and the smallest feature size yet to be achieved on this type of surface. This method offers a significant advance in the ability to produce dense protein arrays on nitrocellulose which are suitable for disease screening using standard fluorescence detection.     

On DABAL-Me3 promoted formation of amides

The range and utility of DABAL-Me₃ couplings of methyl esters and free carboxylic acids with primary and secondary amines under a variety of conditions (reflux, sealed tube, microwave) has been compared for a significant range of coupling partners of relevance to the preparation of amides of interest in pharmaceutical chemistry. Commercial microwave reactors promote the fastest couplings and allow the use of significantly sterically hindered amines (primary and secondary) and carboxylic acids derivatives. The influence of microwave energy on the reaction system was shown to be typically related to thermal effects (over-pressuring and superheating).     

NbTi0.5Ni0.5O4 as anode compound material for SOFCs

NbTi_0.5Ni_0.5O₄ (NTNO) has been prepared using solid state synthesis and investigated as a potential anode material. The oxide form of NTNO has single phase rutile-type structure with tetragonal (P42/mnm) space group. The reduced form is a composite of nano-scaled particles of metallic Ni and Nb_1.33Ti_0.67O₄ phase. Reduced NTNO showed high electronic conductivity up to 280 S.cm^− 1 at 900 °C in reducing atmosphere, but suffers from low CTE equal to 3.78 10^− 6 K^− 1. Studies of NTNO as anode material were carried out in a three electrode - electrochemical half cell configuration under pure humidified H₂ at 900 °C using a 2 mm thick zirconia electrolyte and without any additional current collector material. The results show a reasonable series resistance (R_s) equal to 2.7 Ωcm² (about 50% higher than for metallic gold layers) indicating a good current collection performance for a 10 μm layer of material. The polarization resistance (R_p) was equal to 33 Ωcm² and is attributed to a poor density of three phase boundaries (TPB) and shortage of oxide ion conduction in the anode layer. The results show the potential of NTNO as an anode material, especially after optimization of the microstructure towards the increase of TPB length.     

Sol–gel preparation and characterisation of dense proton conducting Sr3CaZr0.5Ta1.5O8.75 thin films

Dense proton conducting Sr₃CaZr_0.5Ta_1.5O_8.75 films (1.25 μm, with grain size in the 200–400 nm range) were deposited, using the sol–gel method, on Al₂O₃–8%Y₂O₃-stabilised ZrO₂ plates. The obtained gels were characterised by differential and thermogravimetric thermal analysis (DTA–TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results of a study of the structural and electrical properties of Sr₃CaZr_0.5Ta_1.5O_8.75 films deposited on the mentioned substrates are presented herein. The structural data for the gels and films were compared with those obtained for the same material prepared by solid state synthesis. Electrical properties of the sandwich-type structure were investigated by AC impedance conductivity measurements at different temperatures, in both dry and wet 5% H₂/Ar atmospheres. A careful analysis of the impedance spectra for this complex structure was performed, using a model with a series of five electrical circuits, having resistance and capacitance coupled in parallel. The specific responses observed in the impedance spectra were assigned to the corresponding substrate and layer contributions. A significant improvement, by an order of magnitude, in the electrode response was observed in the presence of the interleaving Sr₃CaZr_0.5Ta_1.5O_8.75 proton conducting layer, between the electrode and electrolyte. This enhancement is lost at temperatures above that at which the Sr₃CaZr_0.5Ta_1.5O_8.75 dehydrates and its protonic conductivity diminishes. Considering the structural and electrical characterisation results, these Sr₃CaZr_0.5Ta_1.5O_8.75 sol–gel derived films have a potential use for proton conducting electrolyte or intermediate layer in fuel cells.