bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Molecular & Crystal Structures of Polycyclic Energetic Materials
Authors: Gilardi Richard 
Published: March 2011

AbstractEven before the ‘Buckey-ball’ (or fullerene) era, organic chemists were interested in, and attracted to, the synthesis of compounds containing many rings fused together - especially the symmetric ones. By virtue of their balanced linkages, such compounds may be kinetically stable despite inclusion of massive deviations from normal bond distances and angles. These distortions, in compounds such as cubanes, tetrahedranes, dodecahedranes and prismanes, comprise a storehouse of chemical potential energy known to the chemist as ‘strain energy’ (unrelated to overall crystal strain discussed in many materials studies).Besides the obvious benefit of including more potential energy in an energetic material, the arrangement of atoms in ring assemblies (rather than chains) leads to a material with a high intrinsic crystal density - a sine qua non for an explosive or propellant. It has long been recognized that the next generation of energetics - better in performance and/or safety - will contain many polycyclic compounds. Hundreds of model compounds were synthesized in the last decade, and their molecular structures identified or verified at the Naval Research Lab. Some of the most promising, beautiful, and unusual ones, including many energetic cubanes, will be discussed.
NTO Polymorphs
Authors: Lee Kien-yin , Giloxdi Richard 
Published: March 2011

AbstractNTO is an explosive of current interest. It has been evaluated as an insensitive component to replace RDX in the bomb fill, and as a major ingredient for the auto air bag system. The crystal structure of the β from of NTO has been determined by single-crystal X-ray methods. The unit cell is monoclinic, space group P21/c, with a = 9.326, b = 5.515, c = 9.107 Å, β = 100.77°. There are four molecules in the cell, density 1.878 g/cm3. Infinite extension of H-bonding in two-dimensional sheets occurs in the monoclinic form. Bond lengths and angles all have normal values.Efforts have been made to determine the crystal structure of α-NTO. However, a structure refined only to R = 17% was obtained, probably due to some kind of twinning about the crystal needle axis. The unit cell is triclinic, space group P1, with a = 5.12, b = 10.30, c = 17.9 Å, α = 106.7°, β= 97.7°, γ = 90.2°. There are eight molecules in the cell, density 1.92 g/cm3. Ribbons of NTO molecules formed by a relatively strong network of hydrogen bonds are observed. It was found that α-NTO is the stable, dominating form. A variety of techniques have been chosen to identify the two polymorphs.
Surface Chemical Characterization Methods Applied to Energetic Materials
Authors: Beard B. C. , Sharma I. 
Published: March 2011

AbstractThe reaction chemistry of energetic materials is often considered only with respect to the types and quantities of gaseous products formed. For a thorough understanding of the initi reaction steps, that largely determine the sensitivity of the material, both gaseous and solid products must be determined. In addition, it is the solid state partial decomposition intermediates remaining in a damaged material that can lead to increased sensitivity. The preference for the initial reactions to take place at the surface of particles and the low concentrations of intermediates formed demands the use of highly sensitive surface specific chemical probe techniques. State of the art surface chemical techniques will be discussed, focusing on x-ray photoelectron spectroscopy and surface mass spectrometry. Principles of operation and performance will be highlighted, comparisons will be made to bulk chemical analysis, and examples of applications will be presented.
Quantum Dynamical Studies of the Decomposition of Energetic Materials
Authors: Rabitz Herschel , Vilallonga Eduardo 
Published: March 2011

AbstractWe present a theoretical approach aimed at elucidating the microscopic dynamics of energetic materials fragmentation. Two interdependent components are combined to bridge the atomic and bulk realms. A hierarchy of quantal multiple-scattering expansions is first introduced to decompoce the complex reaction dynamics into coupled sequences of fewatom interactions, each of which is more amenable to calculation. This microscopic component is complemented by a continuum-mechanical process to account for energy flow into the as yet unreacted bulk material. Such a dual approach allows for quantum-mechanical treatment of coupled atomic-bulk dynamics in a self-consistent way that incorporates large thermal gradients. The analysis, in terms of coupled few-atom interactions, also yields additional insight into the various pathways for reactivity and energy transfer.
Hot Spot Histories in Energetic Materials
Authors: Mellor A. M. , Wiegand D. A. , Isom K. B. 
Published: March 2011

AbstractInterest in the mechanisms by which hot spots either grow to sustained reaction or are quenched results from the observation that the energy required to ignite a propellant or explosive can be significantly less than that needed to bulk heat a test specimen uniformly to its ignition temperature. This result is independent of the original form of non-thermal energy and has been used to interpret data for shock, impact, friction and electrostatic discharge (ESD) stimuli. We present new flowcharts which include 1) events resulting in hot spot formation and 2) subsequent pathways which lead to sustained reaction or quenching. The mechanism appears capable of categorizing and demonstrating the similarities and differences between hot spot growth or quenching, for impact and ESD stimuli. Sample confinement and temperature and stimulus duration are the independent variables whose roles are particularly clarified in the mechanism.
Reaction Kinetics of Metallized Explosives
Authors: Jones Hermenzo D. , Zerilli Frank J. 
Published: March 2011

AbstractTime dependence of the energy release of metallized explosives has been observed and must be accounted for in predicting their performance. However, there is no complete theoretical description of this phenomenon. A preliminary investigation of the combustion of aluminum indicates that it probably takes place behind the reaction zone. A flexible time dependent model for the reaction of Al in explosive mixtures has been implemented in EPIC2, a Lagrangian material dynamics computer program. Exploratory calculations for nitromethane/PMMA/Al mixtures indicate that close in pressure measurements could reveal the presence of Al reaction behind the detonation front.
Interrelationship Between Solid Propellant Combustion and Materials Behavior
Authors: Kuo Kenneth K. , Litzinger Thomas A. , Hsieh Wen H. 
Published: March 2011

AbstractThermochemical and physical processes which occur in various aspects of solid-propellant combustion including steady-state burning, ignition, transient burning, cook-off and crack propagation are presented in this paper. For each case, the various thermochemical and physical properties involved are summarized. In addition, the current direction of solidpropellant research is highlighted. The intent of this paper is to stimulate the interest of materials scientists and engineers in the field of solid propellants so that they will contribute their unique expertise to the advancement of current research. It is also the hope of the authors that the paper will serve as a useful summary of important relationships and parameters for scientists and engineers currently involved in solid-propellant research.
Laboratory Testing and Consitiutive Modeling of Coal Including Anisotropy
Authors: Chen Dar-Hao , Zaman Musharraf M. , Kukreti Anant R. 
Published: March 2011

AbstractIn this study, the stress-deformation response of coal in the laboratory under threedimensional (3-D) loading conditions similar to those existing in an actual coal mine is investigated, and a constitutive model, including the effects of anisotropy, is developed. The coal samples were obtained from a mine in LeFlore County, Oklahoma, at a depth of approximately 25–30 ft below the ground surface. A High Capacity Cubical Device with servo-controlled independent loading along three axes of a cubical specimen and a computerized data acquisition and monitoring system were used to conduct the tests. A total of 21 tests under 4 different confining pressures and 5 different stress paths were conducted. The influence of the degree of anisotropy was investigated by comparing the transversely isotropic and isotropic idealizations for different stress paths (Triaxial Compression, Triaxial Extension and Simple Shear) at different confining pressures (1,600, 3,200 and 5,600 psi). The experimental results demonstrated that the coal exhibits inherent anisotropy and that it can be treated approximately as a transversely isotropic material. Also, the Young's moduli were found to be dependent on the confining pressure. The experimental data were used to evaluate the material constants associated with the elasto-plastic constitutive model developed in the study.
Infrared Fiber Optic Diagnostic for Solid Propellant Combustion
Authors: Wormhoudt J. , Kebabian P. L. 
Published: March 2011

AbstractWe report on a program to develop and demonstrate a diagnostic technique using infrared fiber optics to probe the decomposition processes in burning gun propellant strands. The present experimental configuration involves measuring the absorption through a small gap between two embedded fibers. The gap can be filled with propellant, or left open to fill with gaseous decomposition products. Spectroscopic detection is presently achieved using pairs of bandpass filters. The absorption record can be correlated with readings from an embedded thermocouple and with a high resolution video recording of the bum. We also report on preliminary experiments in which an electrically heated filament is used to melt the infrared fiber as its transmission and physical appearance are monitored.The goal of this program is to develop a fast-response probe of solid phase processes which can support the development of a predictive modeling capability for gun propellant combustion. We present examples of data for the atmospheric pressure burning now under investigation. A full assessment of the usefulness of this technique will require further observations and analysis.
The Electron-Beam Sensitivity of Binary Metal Azides
Authors: Herley Patrick J. , Jones William 
Published: March 2011

AbstractThe metal azides (or azido complexes) of La, Ce, Ti, Fe(III), Co, Ni, Pd, Ag, Tl, Cd, Sn, Sb and Bi have been synthesised and subjected in situ within a high resolution transmission electron microscope, to electron-beam fluxes (ca 0.1–1.0 A/cm2) to induce rapid decomposition. The materials, which are highly energetic and some are reportedly explosively sensitive, were carefully handled during synthesis to avoid mechanical shock, and only microgram quantities were decomposed whilst on holey carbon support films. In all cases beam-induced decomposition was very rapid. The exothermicity of the reaction induced in the beam promotes vigorous melting which either precedes or is accompanied by radiolysis. The net result is a rapid evolution of nitrogen (product) gas, which causes atomisation and spallation of the metal products. These molten metal particles are hard landed on a 300–500 μm 2 area of the support film with diameters in the nanometre size range. Data is presented on the synthesis of the azides, the nature of the beam-induced process as well as representative electron micrographs of some of the pristine starting materials and the nanoparticulate products.
Comparison of Deformation and Shock Reactivity for Single Crystals of RDX and Ammonium Perchlorate
Authors: Sandusky H. W. , Beard B. C. , Glancy B. C. , Elban W. L. , Armstrong R. W. 
Published: March 2011

AbstractDeformation of cyclotrimethylenetrinitramine (RDX) and ammonium perchlorate (AP) crystals at low strain rates was studied by diamond pyramid (Vickers and Knoop) microindentation hardness testing. RDX is two to three times harder than AP and has relatively limited slip system activity. While both crystals readily crack, cracking did not reduce hardness in RDX but did in AP. Strain fields surrounding the hardness impressions in RDX were extremely localized while in AP they extended well beyond the impressions. Shock experiments were conducted on large (5–9 mm), single crystals in a fluid-filled tank designed to permit high-speed photography and sample recovery. Reaction threshold was obtained by varying the shock pressure entering the crystals. Shock-entry orientation and large hardness impressions were used to alter microstructural responses. High-speed photography showed luminous crack propagation and reaction in both materials and the same slip deformation in AP as from hardness testing. Orientation affected the microstructural response and reaction threshold for AP, and hardness impressions sensitized chemical decomposition far from the impressions. Recovered AP crystals were much more plastically deformed than RDX crystals and were often still transparent in the region opposite shock entry. Recovered RDX crystals, at even the lowest shock pressure of 8.6 kbar, were uniformly white from a high density of fine cracks. RDX reaction threshold was ∼62 kbar versus 17 to 24 kbar for AP, depending on crystal orientation to the shock wave.
Centrifugaily-Assisted Size Classification and Immobilization of Silicon Crystallites in Gels
Authors: Duval D.J. , Risbud S.H. , Munir Z.A. , Mccoy B.J. 
Published: March 2011

AbstractStokes settling in viscous gels was used as a method for classifying mixed powders of silicon crystallites and amorphous silicon; Quantum-size materials derived from porous silicon were encapsulated and trapped in the viscous gels. After complete gelation the matrix retained size gradations developed during Stokes settling, thus yielding a new variant of functionally gradient materials (FGM). Production and rheological properties of the sol and preparation of specimens for centrifuging are discussed. The relative position of particles with various sizes are quantitatively interpreted in relation to the increase in gel viscosity with time.
Raman Scattering Spectroscopy of GeSi/Si Strained Layer Superlattice
Authors: Rong Zhang , Youdou Zheng , Shulin Gu , Liqun Hu 
Published: March 2011

AbstractRaman scattering measurements have been carried out on Si1-xGex/Si SLS. It is found that the Ge–Ge optic phonon frequency shift is proportional to strain in the SiGe film, and the Ge–Ge strain shift coefficient is 408cm−1. Based on these study a new method for analyzing the Raman spectra of SiGe/Si SLS has been proposed. Using the new method we can obtain the composition of the alloy sublayers as well as the strain in SLS. The strain distribution in the SiGe/Si SLS has been discussed, and strain in both SiGe and Si sublayers of the SLS have been calculated.
Visible Photoluminescence from SI1-xGEx Quantum Wells
Authors: Steiner T.W. , Lenchyshyn L.C. , Thewalt M.L.W. , Houghton D.C. , Noël J.-P. , Rowell N.L. , Sturm J.C. , Xiao X. 
Published: March 2011

AbstractWe have observed photoluminescence from strained SiGe quantum well layers at energies approximately equal to twice the SiGe band-gap energy. This luminescence is caused by the simultaneous recombination of two electron hole pairs yielding a single photon. Detection of luminescence at twice the band-gap has been previously used in Si to observe luminescence originating from electron-hole droplets, biexcitons, bound multiexciton complexes and polyexcitons. Time resolved spectra at twice the band-gap have been obtained from our SiGe samples prepared by molecular beam epitaxy (MIRE) as well as rapid thermal chemical vapor deposition (RTCVD). This new luminescence clearly distinguishes multiexciton or dense e-h plasma processes from single exciton processes such as bound excitons, free excitons or localized excitons, which are difficult to separate in the usual nearinfrared luminescence.
Hydrogen Radical Annealing Effect on the Growth of Microcrystalline Silicon
Authors: Jun Jung Mok , Park Kyu Chang , Kim Sung Ki , Lee Kyung Ha , Chu Mi Kyung , Han Min Koo , Lee Young Hee , Jang Jin 
Published: March 2011

AbstractWe have studied the growth of microcrystalline silicon (μc-Si) and amorphous silicon (a-Si:H) by layer by layer deposition technique, where the deposition and the radical exposure are done alternatively. He or hydrogen plasma exposure gives rise to the etching effect of both μc-Si and a-Si:H even though the etch rate by He plasma is much smaller. The long exposure of hydrogen radical on a-Si:H gives rise to the formation of μc-Si at low substrate temperature (Ts), whereas the hydrogen content decreases at high Ts. The growth mechanism of the crystallite is proposed on the basis of experimental results.
A Study on Michanism of VisibLe Luminescence From Fbrous SiLicon
Authors: Yongdong Zhou , Yixin Jin , Yongqiang Ning , Linshan Yuan 
Published: March 2011

AbstractThe efficient visible tight emitting porous silicon was made using the standard method of anodic oxidation. The characteristic photoluminescence spectra and Raman Scattering spectra of the porous silicon were obtained. A kind of stable yellow powder (not the fragments of the porous silicon thin film the porous structure was still on the crystalline silicon substrate) was taken off from the visible light emitting porous silicon with nonchemical method. The powder can not dissolve in water, alcohol, acetone and some other common solvents, and it still emits efficient visible light after further grinding. The PL spectrum of the powder shows the same peak position, the same shape and the same FW-M as that of the porous silicon wafer. The microstructure of the porous silicon wafer and the microscopic shape of thepowder were studied using the scanningelectron microscopy (SEM) The X–ray photoelectron spectroscopy MSP) shows that the fluorescent powder in the surface layer of the porous silicon is composed of many kinds of elements, such as Si, Q a N and so on. The Si content is only 50% or less in the surface layer of porous silicon. The above–mentioned experiments were performed again on the unpolished surface of the single crystal silicon substrate. So we suggest that the visible luminescence of the porous silicon is from the fluorescent powder maybe not due to the quantum confinement effect in the nm–scate crystalline silicon pillars.
Light-Emitting Porous Silicon After Standard Microelectronic Processing
Authors: Peng C. , Tsybeskov L. , Fauchet P.M. , Seiferth F. , Kurinec S.K. , Rehm J.M. , Mclendon G.L. 
Published: March 2011

AbstractWe have investigated the properties of light-emitting porous silicon (LEpSi) after standard microelectronic processing steps such as annealing, thermal and chemical oxidation, ion implantation, and reactive ion etching. The nature of the physical and chemical changes induced by these processing steps is studied. After thermal or chemical oxidation, the photoluminescence (PL) from LEpSi is blue shifted and more stable. Low dose dopant implantation essentially keeps the PL spectrum unchanged. Thermal annealing after ion implantation affects the PL intensity differently, depending on the type of ions. Reactive ion etching changes the surface morphology and shifts the PL peak to blue.
The Relationship of Porous Silicon Film Morphology to The Photoluminescence Spectra
Authors: Penczek John , Smith R. L. 
Published: March 2011

AbstractPhotoluminescence (PL) spectra are presented for porous silicon samples formed under various formation conditions in aqueous HF solutions. Formation conditions are chosen that effect maximal changes in morphology of resultant films and that correspond to varying formation electrochemistry. The trends in PL center wavelength and full width at half maximum (FWHM) with formation conditions are examined and compared to the resulting morphology. The PL spectra were observed to be most affected by changes in formation conditions when porous films are formed where the electrochemical process of silicon dissolution changes from a 2e- to 4e- (oxide production) reaction. Under these conditions, decreasing HF concentration and/or increasing current density produces a spectral blue-shift which is proportional to the narrowing of the FWHM. This behavior corresponds to morphology changes which are consistent with the quantum confinement model.
Initial Stage Carrier Dynamics in Porous Silicon Using Ultrafast Spectroscopy
Authors: Matsumoto T. , Wright O. B. , Futagi T. , Mimura H. , Kanemitsu Y. 
Published: March 2011

AbstractWe have studied the electronic relaxation processes in porous silicon using a femtosecond pump and probe pulse–correlation technique at 440–nm wavelength. We have observed photoinduced absorption with a response time on the order of 5 ps. From picosecond luminescence decay measurements and the femtosecond pump and probe experiments, the carrier dynamics corresponding to our excitation conditions in porous Si is clarified as follows: carriers are excited in Si microcrystals and then rapidly thermalize to the surface state within 5 ps. After this, strong luminescence occurs from this state with decay components on the order of 1 ns.
Visible Photoluminescence From Rapid-Thermal-Oxidized Porous Silicon
Authors: Kanemitsu Y. , Matsumoto T. , Futagi T. , Mimura H. 
Published: March 2011

AbstractWe have studied the origin of the visible photoluminescence (PL) from oxidized porous Si. The hydrogen–passivated surface of porous Si prepared by electrochemical etching is converted to stable silicon oxides by rapid–thermal–oxidization processes. At low oxidation temperature (Tox), the PL spectrum with a peak near 700 nm is observed. At high Tox above 800 °C, a strong blue PL is observed near 400 nm. We discuss the origin of blue and red PL by employing the results of ab initio electronic structure calculations of silicon–oxygen compounds.
Si3 N4/SiO2 Multi-Layer Reflecting Stacks for Photonic Switching Applications
Authors: Stephens D.J. , He S.S. , Lucovsky G. , Mikkelsen H. , Leo K. 
Published: March 2011

AbstractWe have fabricated stacked-structures comprised of i) fused silica substrates, and ii) near-periodic Si3N4/SiO2 bi-layers by low-temperature, 250°C, remote plasmaenhanced chemical-vapor deposition. Comparing the reflectance of these structures with model calculations, we have been able to identify the effects on the reflectance spectra of departures from i) exact periodicity, ii) not having the constituent dielectric layers each posses an ideal optical path length, OPL, exactly equal to λcentral/4, and iii) the intrinsic dispersion in the dielectric functions of the oxide and nitride materials. We have prepared quasi-periodic structures in which the OPL of the higher index Si3N4 layer was > λcentral/4, and in which the OPL of the lower index SiO2 layer was < λcentral/4. This promotes a second strong reflectance band at an energy that is approximately two times that of the primary band. Calculations have shown that the reflectance values in this band, and near a reflectance minimum on the high energy side of the band, are both very sensitive to changes in the optical properties of the nitride film. We present calculations that demonstrate the effects on the reflectance of this band by a temperature-induced modulation of the optical properties of the oxide and nitride layers.
The Effects of Halogen Exposure on the Photoluminescence of Porous Silicon
Authors: Lauerhaas Jeffrey M. , Sailor Michael J. 
Published: March 2011

AbstractInteraction of I2 vapor with luminescent porous silicon (PS) results in a mixed hydrogen/iodine terminated surface. The photoluminescence (PL) of PS is completely quenched upon I2 exposure. Excess iodine can be removed by pumping on the PS for several hours, resulting in partial recovery of the original PL spectrum. Exposing the hydrogen/iodine terminated PS to air results in the accelerated growth of an oxide layer and increased PL. The adsorption of iodine is postulated to induce surface traps that are responsible for the luminescence quenching observed.