DIFFRACTION STUDIES OF SOME CONVENTIONAL AND NOVEL MICROPHONE BAFFLES

A directional microphone system for field recording of sounds in the air often involves a parabolic reflector to focus the sound waves on the microphone (transducer) element. Some deficiencies of such a system are noted with respect to reproduction of spectra. The reflector system, involving as it does a structure comparable to a wavelength in linear dimension, is not susceptible to traditional high- or low-frequency approximate methods of computation. Modern numerical techniques now permit precise calculation of the directional responses of small reflectors of various shapes. One result is a proposal for a very economical and effective system involving a plane reflector. Other baffle shapes are also investigated, which may be of interest in special applications.     

Chiral ethylhexyl substituents for optically active aggregates of pi-conjugated polymers

We report an efficient synthesis of chiral (2S)-ethylhexanol for functionalizing and solubilizing conjugated polymers. The alpha-substituted chiral ethylhexyl side chains were obtained through a powerful and flexible asymmetric synthesis using pseudoephedrine as a chiral auxiliary. The dependence of the properties of conjugated polymers on molecular structure is investigated by circular dichroism, fluorescence, and absorption spectroscopy on two new chiral conjugated polymers, poly(3,3-bis((S)-2-ethylhexyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT((2S)-ethylhexyl)(2)) and poly(3,3-bis((S)-2-methylbutyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT((2S)-methylbutyl)(2)). The properties of PProDOT((2S)-ethylhexyl)(2)) differ significantly from those of its methylbutyl analog as investigated by chiral aggregation providing insight into the role of interchain interactions in these subsecond switching electrochromic polymers.     

Effect of structural distortions on the magnetism of doped spin-Peierls CuGeO3

The chemical selectivity and great sensitivity of the Extended X-ray Absorption Spectroscopy technique allowed the determination, in the paramagnetic phase, of the structural distortions induced by doping in the spin-Peierls CuGeO₃ compound. The distorted environments were analyzed as a function of concentration, magnetic nature of impurity and the substitution site (Ni, Mn and Zn impurities on the Cu site, Si impurity on the Ge site). This has led to estimate the variation of the angles and pair distances, and hence to evaluate the magnetic coupling along the Cu chains in the vicinity of the impurities. The antiferromagnetic interaction between Cu first neighbors in the pure sample is found to be weakened around Ni, almost cancelled in the case of Mn doping, and even to change sign, producing a ferromagnetic coupling for Si doping. More generally, the structural distortions on a local scale are shown to be key parameters for the understanding of the magnetic properties of doped spin-Peierls compounds.     

Recent Developments in Solid-Propellant Binders

Abstract

The complex and important role of the propellant binder in solid-propellant rockets is described. The severe weight problems of space exploratrion cause high propellant performance to be of major concern in a highly competitive field. Binders contribute to performance, not only in terms of fuel value, but by being compatible with energetic components and by providing good mechanical properties. Both sterilization required for planetary landings and compatibility problems of new energetic oxidizers generate new requirements too stringent for existing binders; only binders composed essentially of saturated aliphatic hydrocarbon are predicted to be suitable. Several saturated-binder developments are in progress. The achievements of one of these programs, based on free-radical synthesis, are evaluated against the goals of the new binder.     

Compact High-Order Finite Differences for Interface Problems in One Dimension

This paper is concerned with the construction and analysis ofcompact finite difference approximations to the model linearsource problem –(pu')' + qu = f where the functions p,q, and f can have jump discontinuities at a finite number ofpoints. Explicit formulae that give O(h²) O(h³) and O(h⁴) accuracyare derived, and a procedure for computing three-point schemesof any prescribed order of accuracy is presented. A rigoroustruncation and discretization error analysis is offered. Numericalresults are also given.     

Microdomain texture and oxygen excess in the calcium-lanthanum ferrite: Ca2LaFe3O8

The analysis by TEM and electron diffraction of the anion-deficient perovskite Ca₂LaFe₃O₈ confirms the model previously proposed by J. C. Grenier et al. (Mater. Res. Bull.11, 1219 (1976)) with a structure intermediate between perovskite and brownmillerite. The unit cell parameters are ～√2a_c, 3a_c, √2a_c (where a_c is the cubic perovskite unit cell parameter). However, the unit cell is sometimes doubled along the b axis. When the sample is treated in air at temperatures around 1400°C, an oxidation process is observed and the unit cell becomes cubic (a_c = 3.848(3) Å). Nevertheless, electron diffraction investigations suggest the existence of a much more complex situation in which three-dimensional microdomains intergrow within one crystal. Each of these microdomains appears to have a structure clearly related to the low-temperature sample, but the superstructure is randomly found along each of the three cubic subcell directions (i.e., the unit cell √2a_c, √2a_c, 3a_c alternates randomly with 3a_c, √2a_c, √2a, and with √2a_c, 3a_c, √2a_c). High-resolution electron microscopy allows one to ascertain this microdomain texture of the real crystal.     

Review of Chemometrics Applied to Spectroscopy: 1985-95, Part 3 — Multi-way Analysis

INTRODUCTION

In this third part of a review on chemometrics in spectroscopy we will describe a recent methodology that has attracted increasing interest in spectroscopy. namely multi-way analysis. The application of multi-way analysis in spectroscopy is still relatively new. hence many methodological improvements are being investigated currently. Part of thls review will also be used to describe the algorithmic improvements gained the last decade.     

Asymptotic behaviour of the ratio of density gradient to electron density for atomic systems

The asymptotic behaviour of the negative ratio of the density gradient to the electron density, q(r), for neutral and singly charged atoms has been investigated. The results show that, for large r, q(r) can approach its limit √8?, where ? is the first ionization potential, from either above or below depending on the specific system considered.     

Oscillatory structure of confined fluids

The confinement of fluids in pores and wedges is associated with exponentially damped oscillatory packing structure, as observed with the surface forces apparatus. This paper reviews the statistical mechanics of confined fluids and then illustrates the results with density functional data for hard-sphere solvent. The free energy of the pore fluid and its functional derivatives with respect to thermodynamic fields all oscillate, as a function of pore width, with a wavelength close to the solvent diameter. In contrast, the density at the centre of pores oscillates with twice this wavelength, as a function of pore width. The development of a unified physics of confined fluids is considered. Approximations based on one-dimensional physics do extraordinarily well in planar symmetry at three-dimensions.