CO oxidation on Pd(100) and Pd(111): A comparative study of reaction pathways and reactivity at low and medium coverages

We have performed density functional theory calculations with the generalized gradient approximation to investigate CO oxidation on a close-packed transition metal surface, Pd(lll), and a more open surface, Pd(100), aiming to shed light on surface structure effects on reaction pathways and reactivity, an important issue in catalysis. Reaction pathways on both surfaces at two different coverages have been studied. It is found that the reaction pathways on both surfaces possess crucial common features despite the fact that they have different surface symmetries. Having determined reaction barriers in these systems, we find that the reaction on Pd(lll) is strongly coverage dependent. Surface coverages, however, have little effect on the reaction on Pd(100). Calculations also reveal that the low coverage reactions are structure sensitive while the medium coverage reactions are not. Detailed discussions on these results are given.

Deficiencies of double-strand break repair factors and effects on mutagenesis in directly gamma-irradiated and medium-mediated bystander human lymphoblastoid cells

Using RNA interference techniques to knock down key proteins in two major double-strand break (DSB) repair pathways (DNA-PKcs for nonhomologous end joining, NHEJ, and Rad54 for homologous recombination, HR), we investigated the influence of DSB repair factors on radiation mutagenesis at the autosomal thymidine kinase (TK) locus both in directly irradiated cells and in unirradiated bystander cells. We also examined the role of p53 (TP53) in these processes by using cells of three human lymphoblastoid cell lines from the same donor but with differing p53 status (TK6 is p53 wild-type, NH32 is p53 null, and WTK1 is p53 mutant). Our results indicated that p53 status did not affect either the production of radiation bystander mutagenic signals or the response to these signals. In directly irradiated cells, knockdown of DNA-PKcs led to an increased mutant fraction in WTK1 cells and decreased mutant fractions in TK6 and NH32 cells. In contrast, knockdown of DNA-PKcs led to increased mutagenesis in bystander cells regardless of p53 status. In directly irradiated cells, knockdown of Rad54 led to increased induced mutant fractions in WTK1 and NH32 cells, but the knockdown did not affect mutagenesis in p53 wild-type TK6 cells. In all cell lines, Rad54 knockdown had no effect on the magnitude of bystander mutagenesis. Studies with extracellular catalase confirmed the involvement of H2O2 in bystander signaling. Our results demonstrate that DSB repair factors have different roles in mediating mutagenesis in irradiated and bystander cells. (C) 2008 by Radiation Research Society.

High-gain subwavelength resonant cavity antennas based on metamaterial ground planes

Planar metarnaterial Surfaces with negative reflection phase values are proposed as ground planes in a high-gain resonant cavity antenna configuration. The antenna is formed by the metarnaterial ground plane (MGP) and a superimposed metallodielectric electromagnetic band gap (MEBG) array that acts as a partially reflective surface (PRS). A single dipole positioned between the PRS and the ground IS utilised as the excitation. Ray analysis is employed to describe the functioning of the antennas and to qualitatively predict the effect of the MGP oil the antenna performance. By employing MGPs with negative reflection phase values, the planar antenna profile is reduced to subwavelength values (less than lambda/6) whilst maintaining high directivity. Full-wave simulations have been carried out with commercially available software (Microstripes (TM)). The effect of the finite PRS size on the antenna radiation performance (directivity and sidelobe level) is studied. A prototype has been fabricated and tested experimentally in order to validate the predictions.

Artificial magnetic conductor surfaces and their application to low-profile high-gain planar antennas

Planar periodic metallic arrays behave as artificial magnetic conductor (AMC) surfaces when placed on a grounded dielectric substrate and they introduce a zero degrees reflection phase shift to incident waves. In this paper the AMC operation of single-layer arrays without vias is studied using a resonant cavity model and a new application to high-gain printed antennas is presented. A ray analysis is employed in order to give physical insight into the performance of AMCs and derive design guidelines. The bandwidth and center frequency of AMC surfaces are investigated using full-wave analysis and the qualitative predictions of the ray model are validated. Planar AMC surfaces are used for the first time as the ground plane in a high-gain microstrip patch antenna with a partially reflective surface as superstrate. A significant reduction of the antenna profile is achieved. A ray theory approach is employed in order to describe the functioning of the antenna and to predict the existence of quarter wavelength resonant cavities.

Sampled-data gain scheduling of continuous LTV plants

This paper discusses the design of gain- scheduled sampled-data controllers for continuous-time polytopic linear parameter-varying systems. The scheduling variables are assumed to available only at the sampling instants, and a bound on the time-variation of the scheduling parameters is also assumed to be known. The resultant gain-scheduled controllers improve the maximum achieveable delay bound over previous constant-gain ones in the literature.