Analysis of multiple FSS screens of unequal periodicity using an efficient cascading technique

In this paper, we present an efficient cascading procedure for analyzing frequency selective surface (FSS) systems consisting of multiple FSS screens of unequal periodicity embedded in multiple dielectric layers. In this procedure, we first find a global period for the FSS system by studying the composite in its entirety. Next, we compute the scattering matrix [S] of each of the FSS subsystems for the global Floquet harmonics by applying a relationship we establish that maps the [S] matrix of the subsystem for the individual Floquet harmonics to that for the global harmonics. This mapping-cum-filling process substantially reduces the effort needed to compute the [S] matrix of a subsystem. Finally, we compute the [S] of the entire system by applying a modified cascading formulation, in which one matrix inversion step is eliminated, resulting in a reduction in the total computing resource requirement as well as time. Two numerical examples are given to illustrate the efficiency and effectiveness of the technique.     

Edge-to-edge interfaces in Ti?Al modeled with the embedded atom method

The atomistic structure and energies of high-index interphase boundaries are explored using a combination of molecular statics and dynamics simulations with embedded atom potentials. We investigate planar boundaries between the α₂ and γ phases in the Ti−Al system. The class of boundaries considered has a high-index boundary orientation; the orientation relationship between the α₂ and γ phases also is high index, and a set of planes from each phase meet edge to edge at the boundary plane. For the particular case of a boundary that is commensurate in one direction and coincides with a moiré plane given by the so-called “Δg” diffraction condition, the boundary is not structurally singular, but it is energetically stable and does not appear to dissociate into other low-energy configurations. Misfit compensating defects are not observed; misfit in directions other than the commensurate one appears to be distributed uniformly. The boundary energy is evaluated as a function of the orientation of the boundary plane, and the edge-to-edge (moiré) boundary is found to lie in an energy cusp. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committee.     

Low-thermal expansion polycrystalline tantalum tungstate ceramics

The synthesis of thermal-shock-resistant materials from the system Ta₂O₅WO₃ was investigated. Ta₂WO₈ had a very low unit-cell thermal expansion coefficient (+0.5 X 10^–6° C^–1). Ta₃₀W₂O₈₁ also had a relatively low coefficient (+4.0 X 10^–6 ° C^–1) and a thermal durability over 1600° C. The thermal expansion curves of these polycrystalline ceramics were lowered because of microcracks caused by the large thermal expansion anisotropy of the crystal axes and were accompanied by hysteresis loops. The densification of Ta₂WO₈ ceramic was promoted by the addition of some metal oxides, and the strong ceramic of Ta₃₀W₂O₈₁ was obtained by controlling grain growth.     

Microbial sensor for preliminary screening of mutagens utilizing a phage induction test

For the preliminary screening of mutagens, a novel microbial sensor system was developed utilizing a phage induction test. Escherichia coli lysogenic strain GY5027 and nonlysogenic strain GY5026 were used in this study. The number of living cells was determined by measuring the respiration of cells immobilized onto an oxygen electrode. The injection of a mutagen, such as AF-2 and MNNG, caused the phage induction in the lysogenic strain, resulting in the decreased respiration of only the lysogenic strain immobilized onto the oxygen electrode but not of nonlysogenic strain. The rate of current increase correlated well with the concentration of mutagens. The sensor responses to the antibiotics and bactericides were definitely different from those of mutagens. Therefore, utilization of this microbial sensor system makes possible the estimation of a substrate's mutagenicity.     

Interfacial strength and chemistry of additive-free silicon nitride ceramics brazed with aluminium

Two kinds of additive-free silicon nitride ceramics were brazed with aluminium; one was with as-ground faying surfaces and the other was with faying surfaces heat-treated at 1073K for 1.8 ksec in air. The heat-treatment of the silicon nitride ceramics formed a silicon oxynitride layer on the faying surfaces and increased the brazing strength of the joints. A silica-alumina non-crystalline layer and a β′-sialon layer were formed successively from the aluminium side at the interface of the joints. The heat-treatment which made the former layer thicker is a necessary process in making reliable, strong brazed joints.     

Study of dielectric properties of RbNO3 single crystals

Dielectric properties, dielectric constant (k), dielectric loss (tan δ) and a.c. conductivity (σ) in the solution-grown single crystals of RbNO₃ are presented from room temperature to about 200°C covering the frequency range 10² to 10⁵ Hz. A broad peak observed in tan δ-frequency data between 10³ and 10⁵Hz is thought to be due to impurity-vacancy dipoles. The sudden rise of three parameters near 160°C is attributed to the known phase transition from trigonal to CsCl structure.