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Articles 31 - 40 of 40
Full-Text Articles in Physics
Low-Fluence Electron Yields Of Highly Insulating Materials, Ryan Hoffmann, Jr Dennison, Clint D. Thomson, Jennifer Albretson
Low-Fluence Electron Yields Of Highly Insulating Materials, Ryan Hoffmann, Jr Dennison, Clint D. Thomson, Jennifer Albretson
Journal Articles
Electron-induced electron yields of high-resistivity, high-yield materials - ceramic polycrystalline aluminum oxide and the polymer polyimide (Kapton HN), - were made by using a low-fluence, pulsed incident electron beam and charge neutralization electron source to minimize charge accumulation. Large changes in energy-dependent total yield curves and yield decay curves were observed, even for incident electron fluences of <3 fC/mm2. The evolution of the electron yield as charge accumulates in the material is modeled in terms of electron re-capture based on an extended Chung-Everhart model of the electron emission spectrum. This model is used to explain anomalies measured in highly insulating, …
Methods For High Resistivity Measurements Related To Spacecraft Charging, Jr Dennison, Jerilyn Brunson, Prasanna Swaminathan, Nelson Green, A Robb Frederickson
Methods For High Resistivity Measurements Related To Spacecraft Charging, Jr Dennison, Jerilyn Brunson, Prasanna Swaminathan, Nelson Green, A Robb Frederickson
Journal Articles
A key parameter in modeling differential spacecraft charging is the resistivity of insulating materials. This parameter determines how charge will accumulate and redistribute across the spacecraft, as well as the time scale for charge transport and dissipation. ASTM constant voltage methods are shown to provide inaccurate resistivity measurements for materials with resistivities greater than ~1017 Ω-cm or with long polarization decay times such as are found in many polymers. These data have been shown to often be inappropriate for spacecraft charging applications, and have been found to underestimate charging effects by one to four orders of magnitude for many …
Extended Beg Model Of Halogenated Methanes Physisorbed On Ionic Crystals, T. E. Burns, Jr Dennison, Jason Kite
Extended Beg Model Of Halogenated Methanes Physisorbed On Ionic Crystals, T. E. Burns, Jr Dennison, Jason Kite
Journal Articles
The 2D dielectric phases and phase transitions of adsorbed dipolar molecules are modeled using a dilute spin-one Ising model. This model is studied in the Blume-Emery-Griffiths formalism, using a mean-field approximation, where the interaction parameters are related to system interaction energies using a unique averaging procedure. The model is applied to four halogenated methane species physisorbed on MgO(100) and NaCl(100) surfaces using previous experimental and theoretical studies to estimate the interaction energy parameters. We find that temperature- and coverage-dependent antiferroelectric to ferroelectric, coverage-dependant ferroelectric up to ferroelectric down, reentrant ferroelectric to ferroelectric, and order-disorder dipole phase transitions can occur. Phase …
Measurement Of Conductivity And Charge Storage In Insulators Related To Spacecraft Charging, A Robb Frederickson, Jr Dennison
Measurement Of Conductivity And Charge Storage In Insulators Related To Spacecraft Charging, A Robb Frederickson, Jr Dennison
Journal Articles
Improved experimental methods are discussed for laboratory measurement of conductivity and electric field in insulating spacecraft material intended for space radiation and plasma environments. These measurement techniques investigate the following features: 1) Measurements of conductivity are up to four orders of magnitude smaller than those determined by existing standard methods. 2) Conductivity is altered as radiation accumulates and trapping states fill with electrons. 3) With intense keV electron irradiation, electrons are continually emitted for hours from the irradiated surface after the irradiation ceases. 4) Charging induced by electron irradiation is strongly modified by the electron-hole pairs that the irradiation generates …
Evolution Of Secondary Electron Emission Characteristics Of Spacecraft Surfaces: Importance To Spacecraft Charging, R. E. Davies, Jr Dennison
Evolution Of Secondary Electron Emission Characteristics Of Spacecraft Surfaces: Importance To Spacecraft Charging, R. E. Davies, Jr Dennison
Journal Articles
Secondary electron emission (SEE) plays a key role in spacecraft charging [Garrett, 1981; Frooninckx and Sojka, 1992] . As a result, spacecraft charging codes require knowledge of the SEE characteristics of various materials in order to predict vehicle potentials in various orbital environments [Katz, et. al., 1986]. Because SEE is a surface phenomenon, occurring in the first few atomic layers of a material, the SEE characteristics of a given surface are extremely sensitive to changes in surface condition—e.g., the addition or removal of surface contaminants, or changes in surface morphology. That spacecraft surfaces …
Physisorbed Co On Ionic Crystals: An Extended Beg Spin-Lattice Model Of Adsorbed Dipolar Molecules, T. E. Burns, Jr Dennison
Physisorbed Co On Ionic Crystals: An Extended Beg Spin-Lattice Model Of Adsorbed Dipolar Molecules, T. E. Burns, Jr Dennison
Journal Articles
The 2D dielectric phases and phase transitions of adsorbed dipolar molecules are studied using a dilute spin-one Ising model. The spin-one formulation assigns a spin Si = ±1 to a (up/down) dipole occupying a lattice site i adsorbed perpendicular to the substrate surface and Si = 0 to unoccupied sites. We relate the mean field expression for the spin-model interaction energy to a more detailed microscopic model involving dipolar, quadrupolar and dispersive interactions beyond nearest neighbor. Analytic solutions in the mean field approximation are discussed for dipole-orientation order-disorder transitions and ferroelectric-to-antiferroelectric transitions as a function of temperature and …
Raman Spectroscopy Of Carbon Materials, Jr Dennison, Mark Holtz, Greg Swain
Raman Spectroscopy Of Carbon Materials, Jr Dennison, Mark Holtz, Greg Swain
Journal Articles
Use of carbon materials is no longer limited to diamond jewelry or graphite pencils and lubricants. The last decade has witnessed an explosion of technological applications driven by the development of fabrication methods and the discovery of several new classes of pure carbon. Structural diversity exhibited by the carbon atoms, from local chemical order to long-range crystalline order, is key to understanding their physical and chemical properties and in future materials development. This article summarizes the use of Raman spectroscopy as a principal tool to investigate the vibrational dynamics of carbon materials and to provide indirect structural characterization of their …
Application Of (E,2e) Spectroscopy To The Electronic Structure Of Valence Electrons In Crystalline And Amorphous Solids, Jr Dennison, A. L. Ritter
Application Of (E,2e) Spectroscopy To The Electronic Structure Of Valence Electrons In Crystalline And Amorphous Solids, Jr Dennison, A. L. Ritter
Journal Articles
This review presents theoretical and experimental aspects of (e,2e) spectroscopy specific to the study of crystalline and amorphous solids. The cross section for (e,2e) scattering is proportional to the spectral momentum density of the ejected electron under certain approximations. The theoretical framework for interpreting (e,2e) measurements is summarized here and general properties of the spectral momentum density of solids are discussed. Different designs of the (e,2e) spectrometer are described and sample preparation techniques are briefly reviewed. A summary of recent (e,2e) experiments on the electronic structure of valence electrons in graphite, ion sputtered and evaporated amorphous carbon, and aluminum-aluminum oxide …
Ultrahigh Vacuum Chamber For Synchrotron X-Ray Diffraction From Films Adsorbed On Single-Crystal Surfaces, Jr Dennison, S. K. Wang, S. N. Ehrlich
Ultrahigh Vacuum Chamber For Synchrotron X-Ray Diffraction From Films Adsorbed On Single-Crystal Surfaces, Jr Dennison, S. K. Wang, S. N. Ehrlich
Journal Articles
An ultrahigh vacuum chamber has been developed for structural analysis of adsorbed films and single-crystal surfaces using synchrotron x-ray diffraction. It is particularly well suited for investigations of physisorbed and other weakly bound films. The chamber is small enough to transport and mount directly on a standard four-axis diffractometer and can also be used independently of the x-ray diffractometer. A low-current, pulse-counting, low-energy electron diffraction/Auger spectroscopy system with a position-sensitive detector enables in situ characterization of the film and substrate while the sample is located at the x-ray scattering position. A closed-cycle He refrigerator and electron bombardment heater provide controlled …
Spectral Momentum Density From Graphite From (E,2e) Spectroscopy: Comparison With First Principles Calculations, Chao Gao, A. L. Ritter, Jr Dennison, N. A. W. Holzwarth
Spectral Momentum Density From Graphite From (E,2e) Spectroscopy: Comparison With First Principles Calculations, Chao Gao, A. L. Ritter, Jr Dennison, N. A. W. Holzwarth
Journal Articles
We have measured the spectral momentum density p(E,q) of graphite by (e,2e) spectroscopy for momentum parallel and perpendicular to the crystal c axis. In the independent-electron approximation, p(E,q) = ΣG |Uk(G)|2 δ(q-k-G)δ(E-E(k)) where the one-electron wave function is Ψk(r) = eik•rΣGUk(G)eiG•r and G is a reciprocal-lattice vector. The measurements covered a range of momentum parallel to the c axis equal to 0 ≤ |q| ≤ 1.84Å-1 and a …