583 Matching Results

Search Results

Advanced search parameters have been applied.

Time-Dependent Interfacial Properties and DNAPL Mobility

Description: Interfacial properties play a major role in governing where and how dense nonaqueous phase liquids (DNAPLs) move in the subsurface. Interfacial tension and contact angle measurements were obtained for a simple, single component DNAPL (tetrachloroethene, PCE), complex laboratory DNAPLs (PCE plus Sudan IV dye), and a field DNAPL from the Savannah River Site (SRS) M-Area DNAPL (PCE, trichloroethene [TCE], and maching oils). Interfacial properties for complex DNAPLs were time-dependent, a phenomenon not observed for PCE alone. Drainage capillary pressure-saturation curves are strongly influenced by interfacial properties. Therefore time-dependence will alter the nature of DNAPL migration and penetration. Results indicate that the time-dependence of PCE with relatively high Sudan IV dye concentrations is comparable to that of the field DNAPL. Previous DNAPL mobility experiments in which the DNAPL was dyed should be reviewed to determine whether time-dependent properties influenced the resutls. Dyes appear to make DNAPL more complex, and therefore a more realistic analog for field DNAPLs than single component DNAPLs.
Date: March 10, 1999
Creator: Tuck, D.M.
Partner: UNT Libraries Government Documents Department

Material and time dependence of the voltage noise generated bycathodic vacuum arcs

Description: The high frequency fluctuations of the burning voltage of cathodic vacuum arcs have been investigated in order to extract information on cathode processes. Eight cathode materials (W, Ta, Hf, Ti, Ni, Au, Sn, Bi) were selected covering a wide range of cohesive energy. The voltage noise was recorded using both a broad-band voltage divider and an attenuator connected to a fast oscilloscope (limits 1 GHz analog and 5 GS/s digital). Fast Fourier transform revealed a power spectrum that is linear in log-log presentation, with a slope of 1/f{sup 2}, where f is the frequency (brown noise). The amplitude of the spectral power of the voltage noise was found to scale with the cohesive energy, in agreement with earlier measurements at lower resolution. These basic results do not depend on the time after arc initiation. However, lower arc current in the beginning of the pulse shows greater voltage noise, suggesting an inverse relation between the noise amplitude and number of emission sites (cathode spot fragments).
Date: July 15, 2005
Creator: Rosen, Johanna & Anders, Andre
Partner: UNT Libraries Government Documents Department

Limits on expanding relativistic shells from Gamma-Ray Burst temporal structure

Description: The author calculates the expected envelope of emission for relativistic shells under the assumption of local spherical symmetry. Gamma-Ray Burst envelopes rarely conform to the expected shape, which has a fast rise and a smooth, slower decay. Furthermore, the duration of the decay phase is related to the time the shell expands before converting its energy to gamma rays. From this, one can estimate the energy required for the shell to sweep up the ISM. The energy greatly exceeds 10{sup 53} erg unless the bulk Lorentz factor is less than 75. This puts extreme limits on the {open_quotes}external{close_quotes} shock models. However, the alternative, {open_quotes}internal{close_quotes} shocks from a central engine, has one extremely large problem: the entire long complex time history lasting hundreds of seconds must be postulated at the central site.
Date: December 31, 1996
Creator: Fenimore, E. E.
Partner: UNT Libraries Government Documents Department

Creation and destruction of C{sub 60} and other fullerene solids. Final report

Description: The 1990 announcement of the Huffman-Kratschmer fullerene-production technique set off a world-wide explosion of research into the properties and potential applications of C{sub 60} and C{sub 70}. In the last five years, 4,000+ fullerene articles have appeared in the scientific literature dealing with these fascinating molecules and their condensed phases. They possess a complex chemistry reminiscent of the alkenes, and this has led to the syntheses of numerous new compounds and fullerene-based materials, with suggested applications ranging from medicine to photo-conducting polymers to rocket fuel. The work summarized in this report focused on the creation and destruction of fullerene-based materials, for the purpose of producing new materials of interest. This three year project was supported by a grant from the Advanced Energy Projects Division, Office of Basic Energy Sciences, U.S. Department of Energy (DE-FG03-93ER12133). Following are outlines of the work completed in each of the three years, a section devoted to the professional and educational development of those involved, a brief section on the outlook for fullerene-based materials, and an appendix listing the publications resulting from this project.
Date: June 5, 1996
Creator: Huffman, D. R.
Partner: UNT Libraries Government Documents Department

Retention and Switching Kinetics of Protonated Gate Field Effect Transistors

Description: The switching and memory retention time has been measured in 50 {micro}m gatelength pseudo-non-volatile memory MOSFETs containing, protonated 40 nm gate oxides. Times of the order of 3.3 seconds are observed for fields of 3 MV cm{sup {minus}1}. The retention time with protons placed either at the gate oxide/substrate or gate oxide/gate electrode interfaces is found to better than 96% after 5,000 seconds. Measurement of the time dependence of the source-drain current during switching provides clear evidence for the presence of dispersive proton transport through the gate oxide.
Date: June 27, 2000
Creator: Devine, R. A. B. & Herrera, Gilbert V.
Partner: UNT Libraries Government Documents Department

Accelerating the dynamics of infrequent events: Combining hyperdynamics and parallel replica dynamics to treat epitaxial layer growth

Description: During the growth of a surface, morphology-controlling diffusion events occur over time scales that far exceed those accessible to molecular dynamics (MD) simulation. Kinetic Monte Carlo offers a way to reach much longer times, but suffers from the fact that the dynamics are correct only if all possible diffusion events are specified in advance. This is difficult due to the concerted nature of many of the recently discovered surface diffusion mechanisms and the complex configurations that arise during real growth. Here the authors describe two new approaches for this type of problem. The first, hyperdynamics, is an accelerated MD method, in which the trajectory is run on a modified potential energy surface and time is accumulated as a statistical property. Relative to regular MD, hyperdynamics can give computational gains of more than 10{sup 2}. The second method offers a way to parallelize the dynamics efficiently for systems too small for conventional parallel MD algorithms. Both methods exploit the infrequent-event nature of the diffusion process. After an introductory description of these methods, the authors present preliminary results from simulations combining the two approaches to reach near-millisecond time scales on systems relevant to epitaxial metal growth.
Date: Spring 1998
Creator: Voter, A. F. & Germann, T. C.
Partner: UNT Libraries Government Documents Department

Orientation-dependent shock response of explosive crystals

Description: Some orientations of PETN crystals have anomalously high shock initiation sensitivity around 4 to 5 GPa. Results of a series of laser interferometry experiments at 4.2 GPa show that this is associated with an elastic-plastic, two-wave structure with large elastic precursors. Implications for the initiation mechanism in single crystals is discussed. Initial work on beta phase, monoclinic HMX is also described.
Date: September 1, 1995
Creator: Dick, J.J.
Partner: UNT Libraries Government Documents Department

Microscopic time-reversibility and macroscopic irreversibility: Still a paradox

Description: Microscopic time reversibility and macroscopic irreversibility are a paradoxical combination. This was first observed by J. Loschmidt in 1876 and was explained, for conservative systems, by L. Boltzmann the following year. Both these features are also present in modern simulations of classic many-body systems in steady nonequilibrium states. We illustrate them here for the simplest possible models, a continuous one-dimensional model of field-driven diffusion, the so-called driven Lorentz gas or Galton Board, and an ergodic time reversible dissipative map.
Date: September 13, 1995
Creator: Posch, H. A.; Dellago, Ch.; Hoover, W. G. & Kum, O.
Partner: UNT Libraries Government Documents Department

Time-dependent buoyant puff model for explosive sources

Description: This paper presents a new model for explosive puff rise histories that is derived from the strong conservative form of the partial differential equations of mass, momenta, and total energy that are integrated over space to yield a coupled system of time dependent nonlinear ordinary differential equations (ODEs). By allowing the dimensions of the puff to evolve laterally and horizontally, the initial rising spherical shaped puff evolves into a rising ellipsoidal shaped mushroom cloud. This model treats the turbulence that is generated by the puff itself and the ambient atmospheric turbulence as separate mechanisms in determining the puff history. The puff rise history was found to depend not only upon the mass and initial temperature of the explosion, but also upon the local stability conditions of the ambient atmosphere through which the puff rises. This model was calibrated by comparison with the Roller Coaster experiments, ranging from unstable to very stable atmospheric conditions; the agreement of the model history curves with these experimental curves was within 10%.
Date: October 1, 1997
Creator: Kansa, E.J.
Partner: UNT Libraries Government Documents Department

Long-term (>30,000-40,000 h) creep behavior of an advanced Si{sub 3}N{sub 4} ceramic

Description: Long-term creep data are reported for a hot-isostatically-pressed (HIPed) silicon nitride (Si{sub 3}N{sub 4}) ceramic material tested at 1250 {degrees}C in air for 28,000 to 38,000 h (3.2 to 4.3 4 years) with no creep failure. Two specimens tested at 150 and 175 MPa each showed extensive primary creep in excess of 10,000 h, followed by well-defined steady-state creep. Because the primary creep range was extensively long and reasonably linear, the creep curves appeared to portray two stages of steady-state creep having occurred due to two different creep mechanisms: (1) formation and (2) growth of cavities. Approximately 57 to 70% of measured apparent creep strain was estimated due to the evolution of cavities, while the remaining was attributed to the viscous creep of grain boundary materials.
Date: March 1, 1997
Creator: Liu, K.C.; Lin, H.T.; Stevens, C.O. & Brinkman, C.R.
Partner: UNT Libraries Government Documents Department

Polarization relaxation in zinc oxide varistors from 77 K to 450 K

Description: The time and temperature dependences of polarization currents are investigated to higher temperatures and with greater temperature resolution than has been reported previously for zinc oxide varistors. Arrhenius plots yield thermal activation energies near 140 and 8 meV, which are in reasonable agreement with values reported recently, as well as an additional thermally activated level at 0.66 eV, which emerges in the extension of the measurements to T > 300K. Conductance data in this higher temperature range for the same samples yield the nearly identical energy of 0.68 eV for the intergranular (Schottky) barrier height. The temperature dependence of the exponent m in the power-law expression for the current-time dependence I = I{sub o}t{sup {minus}m} was carefully examined and found to change twice from 0.5 to 1.0. The non-monotonic behavior of m with temperature is interpreted as due to resolution of the polarization currents from two different traps each of which makes the transition from diffusive to dispersive transport as temperature is lowered rather than, for example, a complicated distribution of relaxation times.
Date: October 1, 1994
Creator: Major, R.W.; Werner, A.E.; Wilson, C.B. & Modine, F.A.
Partner: UNT Libraries Government Documents Department

Scale-up of microwave nitridation of sintered reaction bonded silicon nitride parts. Final report

Description: Scale-up were performed in which microwave heating was used to fabricate reaction-bonded silicon nitride and sintered reaction-bonded silicon nitride (SRBSN). Tests were performed in both a 2.45 GHz, 500 liter and a 2.45 GHz, 4000 liter multimode cavities. The silicon preforms processed in the studies were clevis pins for diesel engines. Up to 230 samples were processed in a single microwave furnace run. Data were collected which included weight gains for nitridation and sintering studies were performed using a conventional resistance-heated furnace.
Date: October 1, 1997
Creator: Tiegs, T.N.; Kiggans, J.O. & Garvey, G.A.
Partner: UNT Libraries Government Documents Department

Stress relaxation of silicon nitride at elevated temperatures

Description: The stress relaxation behavior of SN88, SN253, and NCX-5102 silicon nitride materials were experimentally determined in tension at 1300{degrees}C using buttonhead specimens. Specimens were held at constant strain after being loaded at 10 MPa/s to an initial stress of 276 MPa (40 ksi) or 414 MPa (60 ksi). The subsequent decay in tensile stress was measured as a function of time. A non-negative least squares algorithm used in conjunction with a generalized Maxwell model proved to be an efficient means to define characteristic relaxation modulus spectra and stress relaxation behavior. In the last part of this study, the utility of using short-term stress relaxation testing to predict long-term creep performance was examined.
Date: April 1, 1995
Creator: Wereszczak, A.A.; Ferber, M.K.; Kirkland, T.P.; Lara-Curzio, E.; Parthasarathy, V. & Gribb, T.T.
Partner: UNT Libraries Government Documents Department

Results of magnetic measurements and field integral compensation for the Elliptical Multipole Wiggler

Description: A prototype of the Elliptical Multipole Wiggler (EMW) has been assembled, tested and tuned at the APS. This prototype has a period of 160 mm with 7 poles for the hybrid structure and 10 poles for the electromagnet part of the EMW. The hybrid structure of the EMW produces a vertical maenetic field of 0.83 T with K{sub y}= 12 for a cap of 27 mm, and the electromagnetic structure provides a horizontal field chancre up to 100 Hz with a maximum field of 0.12 T (I= 0.6 kA, K{sub x}= 1.6). The current pulse has a trapezium-type shape with a switching time to chancre the current polarity of about 2 ms. The measurements and tuning, were done for direct current (DC) mode and alternating current (AC) mode. Fine adjustment during the test at the NSLS X-ray ring, using the BPMs and active correction system allowed to achieve about 1 {mu}m of beam distortion. It corresponds to the peak-to-peak variations during, the time less than {plus_minus}0.5 G-cm and {plus_minus}100 G-cm{sup 2} of the first and second horizontal field integrals respectively.
Date: June 1, 1995
Creator: Frachon, D.; Ivanov, P.M.; Medvedko, E.A.; Vasserman, I.; Despe, O. & Kang, Y.G.
Partner: UNT Libraries Government Documents Department

Observation of Optical Pulse and Material Dynamics on the Femtosecond Time-Scale

Description: The widespread availability of lasers that generate pulses on the femtosecond scale has opened new realms of investigation in the basic and applied sciences, rendering available excitations delivering intensities well in excess of 10{sup 21} W/cm{sup 2}, and furnishing probes capable of resolving molecular relaxation timescales. As a consequence and a necessity, sophisticated techniques to examine the pulse behavior on the femtosecond scale have been developed and are of crucial importance to gain insight on the behavior of physical systems. These techniques will be discussed with specific application to guided pulse propagation and ionization dynamics of noble gases.
Date: September 13, 1999
Creator: Omenetto, F.; Luce, B.; Siders, C.W. & Taylor, A.J.
Partner: UNT Libraries Government Documents Department

Time Evolution of Beam in the Recycler Ring

Description: We study the time evolution of the beam current in the Fermilab Recycler Ring due to abrupt physical processes (single coulomb scattering, nuclear scattering) that cause sudden loss of beam, and diffusive processes (multiple coulomb scattering, lattice dependence, etc.) which cause emittance growth. This emittance growth combined with finite aperture of the beam pipe will lead to eventual loss of most beam. We develop a fitting technique to the time evolution of beam current to estimate emittance growth. Finally we compare the directly measured growth with the fitted value.
Date: May 7, 2003
Creator: Krish Gounder, John Marriner and Shekhar Mishra
Partner: UNT Libraries Government Documents Department

Time evolution of fields in strontium ferrite permanent magnets

Description: Field strengths for strontium ferrite permanent magnets built for the Fermilab Recycler and 8 GeV transfer line have been measured for the past 4 years. The ferrite magnetization exhibits a time dependence parameterized by M1/M{sub 2} = -9 x 10{sup -4} x log (t{sub 1}/t{sub 2}) as determined from measurements of a gradient magnet. This parameterization has been checked against several other styles of permanent dipoles, and quadrupole magnets with good agreement.
Date: June 11, 2001
Creator: al., James T. Volk et
Partner: UNT Libraries Government Documents Department

Quickest Paths for Different Network Router Mechanisms

Description: The quickest path problem deals with the transmission of a message of size {sigma} from a source to a destination with the minimum end-to-end delay over a network with bandwidth and delay constraints on the links. We consider four basic modes and two variations for the message delivery at the nodes reacting the mechanisms such as circuit switching, Internet protocol, and their combinations. For each of first three modes, we present O(m{sup 2} + mn log n) time algorithm to compute the quickest path for a given message size {sigma}. For the last mode, the quickest path can be computed in O(m + n log n) time.
Date: October 16, 2000
Creator: Rao, NSV
Partner: UNT Libraries Government Documents Department