Synthesis and characterization of uranium (VI) chloride flourides

Uranium (VI) chloride fluorides were synthesized by the reaction of liquid HCl and solid UF₆ between −80 and −114°C. These dark red compounds are unstable above −40 to −60°C. The simplest formulas derived from compositional analysis are UF₅Cl and UF₄Cl₂.     

Optimal matrix approximants in structural identification

Problems of model correlation and system identification are central in the design, analysis, and control of large space structures. Of the numerous methods that have been proposed, many are based on finding minimal adjustments to a model matrix sufficient to introduce some desirable quality into that matrix. In this work, several of these methods are reviewed, placed in a modern framework, and linked to other previously known ideas in computational linear algebra and optimization. This new framework provides a point of departure for a number of new methods which are introduced here. Significant among these is a method for stiffness matrix adjustment which preserves the sparsity pattern of an original matrix, requires comparatively modest computational resources, and allows robust handling of noisy modal data. Numerical examples are included to illustrate the methods presented herein.This research was partially supported by the National Science Foundation under Grant DMS-88-07483 and by NASA under Grant NAG-1-960.     

In situ determination of the size and polydispersity of concentrated emulsions

Droplet size distributions of concentrated, polydisperse oil-in-water emulsions have been measured using ultra small angle neutron scattering (USANS). The mean radii calculated by fitting a model for polydisperse hard spheres with excluded volume interactions to the USANS data were consistent with those derived from electroacoustics on diluted emulsions after correction for conductance behind the shear plane. The Porod radii measured by USANS were similarly consistent with the mean surface-area-weighted radii derived from electroacoustics, irrespective of the drop concentration or polydispersity.     

Synchrotron FTIR microscopy of Langmuir-Blodgett monolayers and polyelectrolyte multilayers at the solid-solid interface

Synchrotron FTIR microscopy has been used to probe the structure of model boundary lubricant layers confined at the solid-solid interface. The combination of high brightness of the IR source and a novel contact geometry that uses a hemispherical internal reflection element as the means for light delivery has enabled the detection of <2.5 nm thin monolayer lubricant layers in the solid-solid contact, in addition to allowing for spectral acquisition from specific regions of the contact. Spectra of hydration water from within a confined polyelectrolyte multilayer film have also been acquired, highlighting the altered hydrogen bonding environment within the polymer layer.     

The thermal degradation of polysiloxanes: Part 6—Products of degradation of poly(tetramethyl-p-silphenylene siloxane) and copolymers with dimethylsiloxane

The volatile products of thermal degradation of poly(tetramethyl-p-silphenylene siloxane) and a series of copolymers with dimethylsiloxane have been separated and identified using principally gas chromatography-mass spectroscopy (GC-MS). The effect of temperature of degradation and copolymer composition on the pattern of products has been demonstrated. As the proportion of dimethylsiloxane structures in the copolymers is increased, the predominantly linear products give way to cyclics.     

The laser-induced fluorescence spectrum of chromyl fluoride (chromium dioxidedifluoride) in a seeded molecular beam

We habe obtained part of the laser-induced fluorescence spectrum of chromyl fluoride in a free jet expansion at an instrumental resolution of better than 0.001 cm^?1. This spectrum (together with that of the dispersed fluorescence) allows the identification of the (0,0) band or the absorption spectrum and of several progressions in the lowest bending frequency.     

The Homogeneous rate constant for the recombination of fluorine atoms with F2 as the third body

The rate constant for the homogeneous recombination of F-atoms with F₂ as the third body was found to be (3.6 ± 1.4) × 10¹⁴ cm⁶/mol²s at 300 to 320 K. The corresponding rate constants with He and Ar as third bodies were also measured.     

Direct absorption measurement of the spin—orbit splitting of the ground state in atomic fluorine

We have measured the three hyperfine components of the spin—orbit split (²P_{$\text{3}\text{2}$} → ²P_{$\text{1}\text{2}$}) ground state in atomic fluorine by diode laser absorption spectroscopy. The measurement improves the accuracy of the two lines previously reported (404.175 and 403.969 cm^?1), with the third line at 404.210 cm^?1. This confirms the spacing of the hyperfine components measured by EPR, and establishes diode laser absorption as a viable technique for determining F-atom concentrations.     

Synthesis and surface structure of thymine-functionalized, self-assembled monolayer-protected gold nanoparticles

Thymine-functionalized SAM-protected gold nanoparticles with diameters of 2.2 +/- 0.3 nm and 7.0 +/- 1.0 nm were prepared via a modified two-phase transfer method. UV-vis spectra showed that particle size and solvent type, as well as surface charge, influenced the gold surface plasmon band absorption, along with the interaction between thymine terminal groups in the solution. Although the bulky thymine end groups interacted strongly on the particle surface, a well-ordered monolayer of thyminethiol derivatives with a long hydrocarbon chain was formed on the particle surface, exhibiting an ordered, all-trans conformation of the methylene backbone, similar to those of corresponding self-assembled monolayers (SAMs) generated from normal alkanethiols. A larger particle size and a longer reaction time facilitated the formation of more ordered thymine-terminated thiol SAMs. Thermal analysis indicated that reorientation of the SAMs during heat treatment occurred by two processes, caused possibly by the separate recrystallization of the hydrocarbon long chains and thymine units. More ordered SAMs with a higher thermal stability were formed on the larger particle surfaces when compared with those on the smaller ones. A greater density of molecular packing was found on the smaller particle surfaces. However, SAMs formed on the larger gold particles resembled 2D SAMs on the smooth, flat gold surfaces. XPS results confirmed the thymine structure as well as the chemical bond between gold and sulfur. One type of adsorbed sulfur species was observed for the smaller particles and two for the larger ones, but a slightly higher binding energy of thiolate was found for the smaller ones.     

Direct attachment of well-aligned single-walled carbon nanotube architectures to silicon (100) surfaces: a simple approach for device assembly

A new approach for the attachment of vertically-aligned shortened carbon nanotube architectures to a silicon (100) substrate by chemical anchoring directly to the surface has been demonstrated for the first time. The ordered assembly of single-walled carbon nanotubes (SWCNTs) was accomplished by hydroxylating the silicon surface followed by a condensation reaction with carboxylic acid functionalised SWCNTs. This new nanostructure has been characterised by X-ray photoelectron, Raman and Fourier transform infrared (FTIR) spectroscopy as well as scanning electron and atomic force microscopy. The assembly behaviour of SWCNTs onto the silicon surface shows a fast initial step producing isolated functionalised carbon nanotubes or nanotube bundles anchored to the silicon surface followed by a slower step where the adsorbed nanotubes grow into larger aggregates via van der Waals interactions between adsorbed and solvated nanotubes. The electrochemical and optical properties of the SWCNTs directly attached to silicon have also been investigated. These new nanostructures are excellent electrochemical electrodes. They also fluoresce in the wavelength range 650-800 nm. The successful attachment of the SWCNTs directly to silicon provides a simple, new avenue for fabrication and development of silicon-based nanoelectronic, nano-optoelectronic and sensing devices. Compared to existing techniques, this new approach has several advantages including low operating temperature, low cost and the possibility of further modification.