Use of dielectric spectroscopy to assess adhesively bonded composite structures,Part III: Investigation of mechanical strength

This paper explores the way in which the data obtained from the dielectric study (reported in Part II of this 3-part series of articles), can be related to the mechanical strength of the bonded structure. Exposure of the adhesive bonded joints to a hot and wet environment leads to a loss of strength. Changes are observed in the dielectric properties on exposure to the humid environment, and an attempt is made to relate these observations to the mechanisms leading to loss in the mechanical strength ofthe bond. The observation of a correlation between the changes in these physical properties indicates the possibility ofusingdielectricspectroscopyas a powerful nondestructive evaluation (NDE) technique for bonded composite structures.     

Whither Physical Organic Chemistry? Wither? Or Wider?

This article presents a personal view of the field of physical organic chemistry (broadly construed), its history, its current status, and what it may become in the future.     

Identification of explosives with two-dimensional ultraviolet resonance Raman spectroscopy

The first two-dimensional (2D) resonance Raman spectra of TNT, RDX, HMX, and PETN are measured with an instrument that sequentially and rapidly switches between laser wavelengths, illuminating these explosives with forty wavelengths between 210 nm and 280 nm. Two-dimensional spectra reflect variations in resonance Raman scatter with illumination wavelength, adding information not available from single or few one-dimensional spectra, thereby increasing the number of variables available for use in identification, which is especially useful in environments with contaminants and interferents. We have recently shown that 2D resonance Raman spectra can identify bacteria. Thus, a single device that identifies the presence of explosives, bacteria, and other chemicals in complex backgrounds may be feasible.     

Reactions of conjugated fat acids

Summary 1. Addition products of ethyl crotonate and crotononitrile with conjugated methyl linoleate, derived from soybean methyl esters conjugated with a nickelcarbon catalyst, were prepared. 2. The addition product of ethyl crotonate and nonconjugated methyl linoleate was prepared and compared with that from conjugated methyl linoleate. 3. The addition product of ethyl crotonate and conjugated methyl linoleate was saponified to a dibasic acid which was used to prepare a decamethylene glycol polyester and an ethylene diamine polyamide. 4. The compatibility of the addition product of ethyl crotonate and conjugated methyl linoleate with various synthetic resins was determined. One of the laboratories of the Bureau of Agricultural and Industrial Ohemistry, Agricultural Research Administration, U. S. Department of Agriculture.     

Single p-type/intrinsic/n-type silicon nanowires as nanoscale avalanche photodetectors

We report the controlled synthesis of axial modulation-doped p-type/intrinsic/n-type (p-i-n) silicon nanowires with uniform diameters and single-crystal structures. The p-i-n nanowires were grown in three sequential steps: in the presence of diborane for the p-type region, in the absence of chemical dopant sources for the middle segment, and in the presence of phosphine for the n-type region. The p-i-n nanowires were structurally characterized by transmission electron microscopy, and the spatially resolved electrical properties of individual nanowires were determined by electrostatic force and scanning gate microscopies. Temperature-dependent current-voltage measurements recorded from individual p-i-n devices show an increase in the breakdown voltage with temperature, characteristic of band-to-band impact ionization, or avalanche breakdown. Spatially resolved photocurrent measurements show that the largest photocurrent is generated at the intrinsic region located between the electrode contacts, with multiplication factors in excess of ca. 30, and demonstrate that single p-i-n nanowires function as avalanche photodiodes. Electron- and hole-initiated avalanche gain measurements performed by localized photoexcitation of the p-type and n-type regions yield multiplication factors of ca. 100 and 20, respectively. These results demonstrate the significant potential of single p-i-n nanowires as nanoscale avalanche photodetectors and open possible opportunities for studying impact ionization of electrons and holes within quasi-one-dimensional semiconductor systems.     

The Electrical Response of the Human Eye to Sinusoidal Light Stimulation

In order to extract maximum information from electroretingraphic waveforms obtained under clinical conditions, it is necessary to have reliable quantitative methods to characterize both the amplitude and shape of these waveforms. When sinusoidally modulated light is used to stimulate the retina, the resulting ERG potentials are, in general, not sinusoidal due to the nonlinearities in the system. However, the responses are very reproducible and can easily be characterized by a few parameters based on a Fourier analysis. The more conventional flash ERG, although usually of higher amplitude, is much less reproducible in shape and needs many more parameters to characterize completely. Amplitude and phase characteristics can be understood on the basis of a simple model for the scotopic B-wave system and additive interaction by the photopic system. Changes in amplitude and phase characteristics with various experimental conditions could be predicted and were confirmed by subsequent experiments. The ultimate goal of this work is to improve the quantitative basis for clinical electroretinography and to provide the clinician with additional data which can be useful in the diagnosis of retinal diseases.     

DLTS study of the influence of annealing on deep level defects induced in xenon ions implanted n-type 4H-SiC

In this study, nitrogen-doped 4H-SiC samples were bombarded with 167 MeV xenon ions to a fluence of 1?×?10⁸ cm^?2 at 300 K prior to the fabrication of Schottky barrier diodes. The implanted samples were annealed at approximately 900 °C for 1 h before the resistive evaporation of nickel Schottky barrier diodes. In comparing the current–voltage results of the implanted devices with as-deposited ones, generation-recombination took place in the implanted Schottky barrier diodes. Four defects (100, 120, 170, and 650 meV) were present in as-deposited Schottky barrier diodes when characterized by deep level transient spectroscopy (DLTS). In addition to the defects observed in the as-deposited samples, two additional defects with activation energies of 400 and 700 meV below the conduction band minimum were induced by Xe ions implantation. The two deep level defects present have signatures similar to defects present after irradiated by MeV electron. The two defects present after irradiation disappeared after annealing at 400 °C which indicate instability of the defects after annealing implanted samples.