Production of superconductor/carbon bicomponent fibers

Certain materials are unable to be drawn or spun into fiber form due to their improper melting characteristics or brittleness. However, fibrous samples of such materials are often necessary for the fabrication of intricate shapes and composites. In response to this problem, a unique process, referred to as the piggyback process, was developed to prepare fibrous samples of a variety of nonspinnable ceramics. In this technique, specially produced C shaped carbon fibers serve as micromolds to hold the desired materials prior to sintering. Depending on the sintering atmosphere used, bicomponent or single component fibers result. While much has been shown worldwide concerning the YBa2Cu3O(7-x) superconductor, fabrication into unique forms has proven quite difficult. However, a variety of intricate shapes are necessary for rapid commercialization of the superconducting materials. The potential for producing fibrous samples of the YBa2Cu3O(7-x) compound by the piggyback process is being studied. Various organic and acrylic materials were studied to determine suspending ability, reactivity with the YBa2Cu3O(7-x) compound during long term storage, and burn out characteristics. While many questions were answered with respect to the interfacial reactions between YBa2Cu3O(7-x) and carbon, much work is still necessary to improve the quality of the sintered material if the fibers produced are to be incorporated into useful composite or cables

Production of superconductor/carbon bicomponent fibers

Certain materials are unable to be drawn or spun into fiber form due to their improper melting characteristics or brittleness. However, fibrous samples of such materials are often necessary for the fabrication of intricate shapes and composites. In response to this problem, a unique process, referred to as the piggyback process, was developed to prepare fibrous samples of a variety of nonspinnable ceramics. In this technique, specially produced C-shaped carbon fibers serve as micromolds to hold the desired materials prior to sintering. Depending on the sintering atmosphere used, bicomponent or single component fibers result. While much has been demonstrated worldwide concerning the YBa2Cu3O(7-x) superconductor, fabrication into unique forms has proven quite difficult. However, a variety of intricate shapes are necessary for rapid commercialization of the superconducting materials. The potential for producing fibrous samples of the YBa2Cu3O(7-x) compound by the piggyback process is being investigated. Various organic and acrylic materials were investigated to determine suspending ability, reactivity with the YBa2Cu3O(7-x) compound during long term storage, and burn out characteristics. While many questions were answered with respect to the interfacial reactions between YBa2Cu3O(7-x) and carbon, much work is still necessary to improve the quality of the sintered material if the fibers produced are to be incorporated into useful composites or cables. Additional research is necessary to evaluate quality of the barrier layer during long soakings at the peak temperature; adjust the firing schedule to avoid microcracking and improve densification; and increase the solids loading in the superconductive suspension to decrease porosity

Lecteur de thermoluminescence permettant l'analyse des spectres d'émission

Nous présentons un appareil conçu dans un but de recherche fondamentale pour obtenir des données quantitatives sur la thermoluminescence. Il peut être utilisé pour la mesure intégrale des émissions lumineuses et pour leur analyse en longueur d'onde. Nous indiquons les performances et le domaine d'emploi de l'appareil. Un exemple d'étude de spectre est donné

Internal cardiac defibrillation: Histopathology and temporal stability of defibrillation energy requirements

The automatic implantable cardioverter/defibrillator is tested intraoperatively to ensure effectiveness by performing a number of induced fribrillation-defibrillation trials. The temporal stability of defibrillation energy requirements and the histopathologic effects of multiple defibrillating shocks were studied in 12 dogs chronically instrumented with an internal spring-patch lead system identical to that used in humans. Dogs were studied on days 1, 11, 18, 25 and 32. Data were analyzed by logistic regression and the energy required for 50% (E50) and 80% (E80) success was compared. On day 32 the dogs were killed and the heart was removed for gross and microscopic pathologic examination.There was a significant decrease in energy requirements from day 1 to day 11, as the E50decreased from 6.9 ± 4.5 to 4.9 ± 2.5 J (p < 0.02) and the E80decreased from 8.5 ± 5.2 to 6.1 ± 3.4 J (p < 0.02). The energy requirements then remained stable over the remainder of the experiment. The dogs were administered 209 ± 18 shocks (range 1 to 24 J) for a total cumulative dose of 1,524 ± 571 J. In all cases, both grossly and microscopically, there was no evidence of pathologic changes in the myocardium or coronary vessels. In all cases there was a fibrous plaque beneath the patch electrodes, at times containing an area of patchy hemorrhage; in a single specimen a mixed inflammatory infiltrate accompanied the hemorrhage. Endothelialization of the spring electrode with mild right atrial endocardial fibrosis was also observed.Therefore, the energy requirements for successful defibrillation decrease during the first 11 days after testing at implantation, and then remain stable. Also, multiple, closely spaced defibrillating shocks applied through the spring-patch electrode system do not cause myocardial damage or changes other than those associated with the implantation

Theoretical and experimental estimation of self-attenuation corrections in determination of 210Pb^{210}Pb by gamma-spectrometry with well Ge detector

article accepté par "Radiation Measurements", sous presseThis paper aims at giving a practical method for routine measurements of the activity of 210Pb in solids by γ-spectrometry with a well Ge crystal. Since the γ-rays emitted by 210Pb have a low energy, 46.5 keV, the proportion of pulses counted in the crystal depends strongly on the sample composition trough self-attenuation within the sample itself. Self-attenuation can be calculated by use of a Monte Carlo method based on a model. In the present work two codes, namely GEANT 4 and a Laboratory code, gave results in agreement. However, both necessitate a good knowledge of the sample composition, of the detector geometry and of the computing programs. In order to circumvent such drawbacks, an experimental technique was developed, where the overall counting efficiency can be derived directly from the transmission of 46.5 keV γ-rays through the sample, by means of a linear relationship. Such correlation was observed experimentally with samples of known activity and the approach was validated by verification that a linear relation preliminary exists between the transmission and the calculated self-absorption

Constraints, Histones, and the 30 Nanometer Spiral

We investigate the mechanical stability of a segment of DNA wrapped around a histone in the nucleosome configuration. The assumption underlying this investigation is that the proper model for this packaging arrangement is that of an elastic rod that is free to twist and that writhes subject to mechanical constraints. We find that the number of constraints required to stabilize the nuclesome configuration is determined by the length of the segment, the number of times the DNA wraps around the histone spool, and the specific constraints utilized. While it can be shown that four constraints suffice, in principle, to insure stability of the nucleosome, a proper choice must be made to guarantee the effectiveness of this minimal number. The optimal choice of constraints appears to bear a relation to the existence of a spiral ridge on the surface of the histone octamer. The particular configuration that we investigate is related to the 30 nanometer spiral, a higher-order organization of DNA in chromatin.Comment: ReVTeX, 15 pages, 18 figure

Fluorescence from a few electrons

Systems containing few Fermions (e.g., electrons) are of great current interest. Fluorescence occurs when electrons drop from one level to another without changing spin. Only electron gases in a state of equilibrium are considered. When the system may exchange electrons with a large reservoir, the electron-gas fluorescence is easily obtained from the well-known Fermi-Dirac distribution. But this is not so when the number of electrons in the system is prevented from varying, as is the case for isolated systems and for systems that are in thermal contact with electrical insulators such as diamond. Our accurate expressions rest on the assumption that single-electron energy levels are evenly spaced, and that energy coupling and spin coupling between electrons are small. These assumptions are shown to be realistic for many systems. Fluorescence from short, nearly isolated, quantum wires is predicted to drop abruptly in the visible, a result not predicted by the Fermi-Dirac distribution. Our exact formulas are based on restricted and unrestricted partitions of integers. The method is considerably simpler than the ones proposed earlier, which are based on second quantization and contour integration.Comment: 10 pages, 3 figures, RevTe

Three-dimensional harmonic oscillator and time evolution in quantum mechanics

The problem of defining time (or phase) operator for three-dimensional harmonic oscillator has been analyzed. A new formula for this operator has been derived. The results have been used to demonstrate a possibility of representing quantum-mechanical time evolution in the framework of an extended Hilbert space structure. Physical interpretation of the extended structure has been discussed shortly, too.Comment: 14 pages; submitted to Phys Rev

Conformations of closed DNA

We examine the conformations of a model for a short segment of closed DNA. The molecule is represented as a cylindrically symmetric elastic rod with a constraint corresponding to a specification of the linking number. We obtain analytic expressions leading to the spatial configuration of a family of solutions representing distortions that interpolate between the circular form of DNA and a figure-eight form that represents the onset of interwinding. We are also able to generate knotted loops. We suggest ways to use our approach to produce other configurations relevant to studies of DNA structure. The stability of the distorted configurations is assessed, along with the effects of fluctuations on the free energy of the various configurations.Comment: 39 pages in REVTEX with 14 eps figures. Submitted to Phys. Rev. E. This manuscript updates, expands and revises, to a considerable extent, a previously posted manuscript, entitled "Conformations of Circular DNA," which appeared as cond-mat/970104