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Effects of chemical modifications on photophysics and exciton dynamics on {pi}-conjugation attenuated and metal-chelated photoconducting polymers

Description: Effects of two types of chemical modifications on photoconducting polymers consisting of polyphenylenevinylene (PPV) derivatives are studied by static and ultrafast transient optical spectroscopy as well as semi-empirical ZINDO calculations. The first type of modification inserts 2,2{prime}-bipyridyl-5-vinylene units (bpy V) in the PPV backbone, and the second type involves metal-chelation with the bpy sites. Photoluminescence and exciton dynamics of polymers 1 and 2 with PV:bpyV ratios of 1 and 3 were examined in solution, and compared to those of the homopolymer, poly(2,5-bis(2{prime}-ethylhexyloxy)-1,4-phenylenevinylene) (BEH-PPV). Similar studies were carried out for several metal-chelated polymers. These results can be explained by changes in {pi}-conjugation throughout the polymer backbone. The attenuation in {pi}-conjugation by the chemical modifications transforms a conducting polymer from one-dimensional semiconductor to molecular aggregates.
Date: March 11, 2000
Creator: Chen, L. X.; Jager, W. J. H.; Gosztola, D. J.; Niemczyk, M. P. & Wasielewski, M. R.
Partner: UNT Libraries Government Documents Department

The addition of disilanes to cumulenes

Description: The syntheses of silicon-containing compounds and the studies of their rearrangements have been active research areas in the Barton research group. Previously, the addition of disilanes to acetylenes was studied in the group and an intramolecular 2S + 2A mechanism has been proposed. In this thesis, the work is focused on the addition of disilanes to cumulenes. The syntheses of the precursors are discussed and the possible mechanisms for their thermal, photochemical and catalytic rearrangements are proposed. Conjugated organic polymers have been studied in the group since 1985 because of their potential for exhibiting high electroconductivity, photoconductivity, strong non-linear optical response and intense fluorescence. In the second section of this dissertation, the synthesis and property studies of poly(phenylene vinylene) analogues are discussed.
Date: October 8, 1997
Creator: Chen, Y.
Partner: UNT Libraries Government Documents Department

Twenty years of service at NBNM - Analysis of Spectrolab module

Description: This study of adhesional strength and surface analysis of encapsulant and silicon cell samples from a Natural Bridges National Monument (NBNM) Spectrolab module is an attempt to understand from its success. The module was fabricated using polyvinyl butyral (PVB) as an encapsulant. The average adhesional shear strength of the encapsulant at the cell/encapsulant interface in this module was 4.51 MPa or {approximately} 18% lower than that in currently manufactured modules. Typical encapsulant surface composition was as follows: C 75.0 at.% O 23.2 at.%, and Si 1.6 at.%, with Ag {approximately}0.2 at.% and Pb {approximately} 0.5 at.% with some tin respectively over the grid lines and solder bond. Representative silicon cell surface composition was: K 1.4 at.%, C 20.8 at.%, Sn 0.94 at.%, O 15.1 at.%, Na 2.7 at.% and Si 59.0 at.%. The presence of tin detected on the silicon cell surface may be attributed to corrosion of solder bond. The module differs from typical contemporary modules in the use of PVB, metallic mesh type interconnection, and silicon oxide AR coating.
Date: April 11, 2000
Partner: UNT Libraries Government Documents Department

Improved electrical leads for high pressure systems

Description: Electrical connections from the interior of a pressure vessel to atmospheric pressure typically place many design constraints on high pressure experimentation. A simplified version of older techniques using the cone seal has proven itself repeatedly in both gas and liquid systems with high reliability and ease of fabrication. Previous cone sleeves were made of oil soaked pyrophyllite, ivory and bone as well as certain plastics. All of these sleeves were made by careful machining to very close tolerances. Such machining was a considerable challenge (as well as being time consuming) and is completely eliminated in the technique described here. (auth)
Date: January 1, 1972
Creator: Terry, R.E. & Ruoff, A.L.
Partner: UNT Libraries Government Documents Department

PVT Analysis With A Deconvolution Algorithm

Description: Polyvinyl Toluene (PVT) plastic scintillator is the most common gamma ray detector material used for large systems when only gross counting is needed because of its low cost, robustness, and relative sensitivity. PVT does provide some energy information about the incident photons, as has been demonstrated through the development of Energy Windowing analysis. There is a more sophisticated energy analysis algorithm developed by Symetrica, Inc., and they have demonstrated the application of their deconvolution algorithm to PVT with very promising results. The thrust of such a deconvolution algorithm used with PVT is to allow for identification and rejection of naturally occurring radioactive material, reducing alarm rates, rather than the complete identification of all radionuclides, which is the goal of spectroscopic portal monitors. Under this condition, there could be a significant increase in sensitivity to threat materials. The advantage of this approach is an enhancement to the low cost, robust detection capability of PVT-based radiation portal monitor systems. The success of this method could provide an inexpensive upgrade path for a large number of deployed PVT-based systems to provide significantly improved capability at a much lower cost than deployment of NaI(Tl)-based systems of comparable sensitivity.
Date: February 1, 2011
Creator: Kouzes, Richard T.
Partner: UNT Libraries Government Documents Department

Capillary electrophoretic separation of DNA restriction fragments using dilute polymer solutions

Description: Because the mechanism of DNA separation in capillary electrophoresis is not well understood, selection of polymers is a {open_quotes}trial-and-error{close_quotes} procedure. We investigated dilute-solution DNA separations by capillary electrophoresis using solutions of four polymers that differ in size, shape and stiffness. Hydroxyethylcellulose of high molecular weight provides excellent separation of large DNA fragments (2027 bp - 23130 bp). Polyvinylpyrrolidone separates DNA from 72 bp to 23 kbp and star-(polyethylene oxide), like linear poly (ethylene oxide), provides separation of fragments up to 1353 bp.
Date: February 1, 1997
Creator: Braun, B.; Blanch, W. & Prausnitz, J.M.
Partner: UNT Libraries Government Documents Department

Preliminary Evaluation of a Portable Handheld Combined Gamma and Neutron Directional Isotopic Identifying Detector

Description: Purpose: To simulate the performance of a portable dual gamma and neutron identification detector array with dual directional detection capability for preliminary algorithm evaluation. Experimental procedures: Monte Carlo N-Particle (MCNP) analysis on a compact array of borated polyvinyl toluene light pipes and photomultiplier tubes, standard spectral histogramming, and directional detection capability simulations. Results: Results demonstrate that the detector system is fully capable of correcting for background variations when identifying common gamma and neutron sources while simultaneously providing source location direction estimates. Familiar anthropogenic isotopes are readily identified such as Am-241, Cs-137, Cf-252 and Co-60. Directional resolution is estimated to be within approximately 15 degrees. Specifically, all features expected for the array have been demonstrated to be credible through MCNP analysis. Conclusions: Use of this handheld dual neutron and gamma spectrometer has the promise of widespread applicability. By ultimately correlating MCNP results with empirical measurements, substantial confidence can be placed on predicting detector response to sufficiently similar spectral sources under alternate experimental configurations. Use of the detector has substantial promise for operational health physics applications.
Date: February 14, 2008
Creator: Hayes, Robert
Partner: UNT Libraries Government Documents Department

Evaluation of a Polyvinyl Toluene Neutron Counter Array

Description: The purpose of this article is to simulate the performance of a neutron detector array for empirical configuration optimization and preliminary algorithm evaluation. Utilizing a compact array of borated Polyvinyl Toluene light pipes and Photomultiplier Tubes, pulse shape analysis, standard spectral histogramming, and multiplicity counting can enable neutron measurements for multiple applications. Results demonstrate that analysis with Monte Carlo N-Particle (MCNP) can be used to obtain a better understanding of field measurement results and aid in algorithm development for unfolding in conjunction with detector optimization. Use of a handheld neutron spectrometer has promise of widespread applicability. By correlating MCNP results with empirical measurements, substantial confidence can be placed on predicting detector response to sufficiently similar spectral sources under alternate experimental configurations. In addition, use of the detector has substantial promise for operational health physics applications.
Date: March 1, 2008
Creator: Hayes, Robert
Partner: UNT Libraries Government Documents Department

Characterization, performance and optimization of PVDF as a piezoelectric film for advanced space mirror concepts.

Description: Piezoelectric polymers based on polyvinylidene fluoride (PVDF) are of interest for large aperture space-based telescopes as adaptive or smart materials. Dimensional adjustments of adaptive polymer films depend on controlled charge deposition. Predicting their long-term performance requires a detailed understanding of the piezoelectric material features, expected to suffer due to space environmental degradation. Hence, the degradation and performance of PVDF and its copolymers under various stress environments expected in low Earth orbit has been reviewed and investigated. Various experiments were conducted to expose these polymers to elevated temperature, vacuum UV, {gamma}-radiation and atomic oxygen. The resulting degradative processes were evaluated. The overall materials performance is governed by a combination of chemical and physical degradation processes. Molecular changes are primarily induced via radiative damage, and physical damage from temperature and atomic oxygen exposure is evident as depoling, loss of orientation and surface erosion. The effects of combined vacuum UV radiation and atomic oxygen resulted in expected surface erosion and pitting rates that determine the lifetime of thin films. Interestingly, the piezo responsiveness in the underlying bulk material remained largely unchanged. This study has delivered a comprehensive framework for material properties and degradation sensitivities with variations in individual polymer performances clearly apparent. The results provide guidance for material selection, qualification, optimization strategies, feedback for manufacturing and processing, or alternative materials. Further material qualification should be conducted via experiments under actual space conditions.
Date: November 1, 2005
Creator: Jones, Gary D.; Assink, Roger Alan; Dargaville, Tim Richard; Chaplya, Pavel Mikhail; Clough, Roger Lee; Elliott, Julie M. et al.
Partner: UNT Libraries Government Documents Department

PVT-NG sensor final report.

Description: This document is a final report for the polyvinyl toluene (PVT) neutron-gamma (PVT-NG) project, which was sponsored by the Domestic Nuclear Detection Office (DNDO). The PVT-NG sensor uses PVT detectors for both gamma and neutron detection. The sensor exhibits excellent spectral resolution and gain stabilization, which are features that are beneficial for detection of both gamma-ray and neutron sources. In fact, the ability to perform isotope identification based on spectra that were measured by the PVT-NG sensor was demonstrated. As described in a previous report, the neutron sensitivity of the first version of the prototype was about 25% less than the DNDO requirement of 2.5 cps/ng for bare Cf-252. This document describes design modifications that were expected to improve the neutron sensitivity by about 50% relative to the PVT-NG prototype. However, the project was terminated before execution of the design modifications after portal vendors demonstrated other technologies that enable neutron detection without the use of He-3. Nevertheless, the PVT-NG sensor development demonstrated several performance goals that may be useful in future portal designs.
Date: January 1, 2012
Creator: Mitchell, Dean James & Brusseau, Charles A.
Partner: UNT Libraries Government Documents Department

Muon Fluence Measurements for Homeland Security Applications

Description: This report focuses on work conducted at Pacific Northwest National Laboratory to better characterize aspects of backgrounds in RPMs deployed for homeland security purposes. Two polyvinyl toluene scintillators were utilized with supporting NIM electronics to measure the muon coincidence rate. Muon spallation is one mechanism by which background neutrons are produced. The measurements performed concentrated on a broad investigation of the dependence of the muon flux on a) variations in solid angle subtended by the detector; b) the detector inclination with the horizontal; c) depth underground; and d) diurnal effects. These tests were conducted inside at Building 318/133, outdoors at Building 331G, and underground at Building 3425 at Pacific Northwest National Laboratory.
Date: August 10, 2010
Creator: Ankney, Austin S.; Berguson, Timothy J.; Borgardt, James D. & Kouzes, Richard T.
Partner: UNT Libraries Government Documents Department


Description: A comprehensive series of 16 laboratory experiments demonstrated that the presence of vinyl tubing within a recirculating pipe system was responsible for lowering zebra mussel kill following treatment with the bacterium Pseudomonas fluorescens. All vinyl tubing was replaced in all testing units with silicone tubing, and high mussel kill (>95%) was then obtained.
Date: June 4, 2003
Creator: Molloy, Daniel P.
Partner: UNT Libraries Government Documents Department

Accelerated Exposure Tests of Encapsulated Si Solar Cells and Encapsulation Materials

Description: We have conducted a series of accelerated exposure test (AET) studies for various crystalline-Si (c-Si) and amorphous-Si (a-Si) cell samples that were encapsulated with different superstrates, pottants, and substrates. Nonuniform browning patterns of ethylene vinyl acetate (EVA) pottants were observed for glass/EVA/glass-encapsulated c-Si cell samples under solar simulator exposures at elevated temperatures. The polymer/polymer-configured laminates with Tedlar or Tefzel did not discolor because of photobleaching reactions, but yellowed with polyester or nylon top films. Delamination was observed for the polyester/EVA layers on a-Si minimodules and for a polyolefin-based thermoplastic pottant at high temperatures. For all tested c-Si cell samples, irregular changes in the current-voltage parameters were observed that could not be accounted for simply by the transmittance changes of the superstrate/pottant layers. Silicone-type adhesives used under UV-transmitting polymer top films were observed to cause greater cell current/efficiency loss than EVA or polyethylene pottants.
Date: October 8, 1998
Creator: Pern, F. J. & Glick, S. H.
Partner: UNT Libraries Government Documents Department

Ultra Accelerated Testing of PV Module Components

Description: Using concentrated natural sunlight at the NREL High Flux Solar Furnace, we have exposed several materials to acceleration factors of up to 400 times the normal outdoor UV exposure dose. This accelerated rate allows the exposure of materials such that a year of outdoor exposure can be simulated in about 5 hours. We have studied the solarization of cerium containing glass, the degradation of ethylene vinyl acetate laminated between borosilicate glass, and the yellowing of standard polystyrene test coupons. The first two candidates are of interest to the photovoltaics (PV) program, and the last candidate material is a widely used dosimeter for ultra violet (UV) exposure in accelerated weathering chambers
Date: October 28, 1998
Creator: Pitts, J. R.; King, D. E.; Bingham, C. & Czanderna, A. W.
Partner: UNT Libraries Government Documents Department

Ion beam modification of PVDC and PE polymers

Description: Electronic and nuclear stopping effects produced by MeV ion bombardment in polyvinylidine chloride and polyethylene are separated by stacking thin films of the polymers. Resulting multi-layer laminates of each polymer were bombarded with 3.5-MeV alpha particles. Energy of the incident ions was selected using the TRIM code so that the first layers experienced most of the effects of the electronic energy deposited and the last layers received most of the effects of the nuclear stopping power. Changes in conductance and chemical structure of each layer were measured by direct resistivity measurements and Raman microprobe analysis.
Date: December 31, 1995
Creator: Evelyn, A.L.; Ila, D.; Fisher, J. & Poker, D.B.
Partner: UNT Libraries Government Documents Department

Exciton annihilation studies in poly(p-phenylene vinylene)

Description: To verify the excitonic nature of the light-emitting state in PPV, fluorescence intensities and decay lifetimes were investigated as a function of excitation intensity. The results agree with the behavior predicted by the molecular exciton model. In particular, exciton-exciton annihilation causes the fluorescence intensity to saturate and the fluorescence lifetime to shorten at high exciton densities. In addition, the exciton annihilation, and thus diffusion, coefficients are found to be relatively large, even at low temperatures, indicating that exciton migration is important in PPV. These results indicate that the fluorescent (photoluminescent) state in PPV is excitonic in nature. The results argue against the band model where high mobility at reduced temperatures is not expected because the light-emitting species, neutral bipolarons, are associated with large lattice distortions.
Date: August 1, 1995
Creator: Valencia, V.S.; Kepler, R.G.; Jacobs, S.J.; Beeson, P.M. & Allemond, P.M.
Partner: UNT Libraries Government Documents Department

Synthesis, Characterization, to application of water soluble and easily removable cationic pressure sensitive adhesives

Description: In recent years, the world has expressed an increasing interest in the recycling of waste paper to supplement the use of virgin fiber as a way to protect the environment. Statistics show that major countries are increasing their use of recycled paper. For example, in 1991 to 1996, the U.S. increased its recovered paper utilization rate from 31% to 39%, Germany went from 50% to 60%, the UK went from 60% to 70%, France increased from 46% to 49%, and China went from 32% to 35% [1]. As recycled fiber levels and water system closures both increase, recycled product quality will need to improve in order for recycled products to compete with products made from virgin fiber [2]. The use of recycled fiber has introduced an increasing level of metal, plastic, and adhesive contamination into the papermaking process which has added to the complexity of the already overwhelming task of providing a uniform and clean recycle furnish. The most harmful of these contaminates is a mixture of adhesives and polymeric substances that are commonly known as stickies. Stickies, which enter the mill with the pulp furnish, are not easily removed from the repulper and become more difficult the further down the system they get. This can be detrimental to the final product quality. Stickies are hydrophobic, tacky, polymeric materials that are introduced into the papermaking system from a mixture of recycled fiber sources. Properties of stickies are very similar to the fibers used in papermaking, viz. size, density, hydrophobicity, and electrokinetic charge. This reduces the probability of their removal by conventional separation processes, such as screening and cleaning, which are based on such properties. Also, their physical and chemical structure allows for them to extrude through screens, attach to fibers, process equipment, wires and felts. Stickies can break down and ...
Date: January 30, 2004
Creator: Technology, Institute of Paper Science
Partner: UNT Libraries Government Documents Department

Can Handheld Plastic Detectors Do Both Gamma and Neutron Isotopic Identification with Directional Source Location?

Description: This paper demonstrates, through MCNPX simulations, that a compact hexagonal array of detectors can be utilized to do both gamma isotopic identification (ID) along with neutron identification while simultaneously finding the direction of the source relative to the detector array. The detector array itself is composed of seven borated polyvinyl toluene (PVT) hexagonal light pipes approximately 4 inches long and with a 1.25 inch face-to-face thickness assembled in a tight configuration. The gamma ID capability is realized through judicious windowing algorithms as is the neutron spectral unfolding. By having multiple detectors in different relative positions, directional determination of the source can be realized. By further adding multiplicity counters to the neutron counts, fission events can be measured.
Date: April 18, 2008
Creator: Hayes, Robert
Partner: UNT Libraries Government Documents Department

Use of Melt Flow Rate Test in Reliability Study of Thermoplastic Encapsulation Materials in Photovoltaic Modules

Description: Use of thermoplastic materials as encapsulants in photovoltaic (PV) modules presents a potential concern in terms of high temperature creep, which should be evaluated before thermoplastics are qualified for use in the field. Historically, the issue of creep has been avoided by using thermosetting polymers as encapsulants, such as crosslinked ethylene-co-vinyl acetate (EVA). Because they lack crosslinked networks, however, thermoplastics may be subject to phase transitions and visco-elastic flow at the temperatures and mechanical stresses encountered by modules in the field, creating the potential for a number of reliability and safety issues. Thermoplastic materials investigated in this study include PV-grade uncured-EVA (without curing agents and therefore not crosslinked); polyvinyl butyral (PVB); thermoplastic polyurethane (TPU); and three polyolefins (PO), which have been proposed for use as PV encapsulation. Two approaches were used to evaluate the performance of these materials as encapsulants: module-level testing and a material-level testing.
Date: October 1, 2011
Creator: Moseley, J.; Miller, D.; Shah, Q.-U.-A. S. J.; Sakurai, K.; Kempe, M.; Tamizhmani, G. et al.
Partner: UNT Libraries Government Documents Department

Neutron counting and gamma spectroscopy with PVT detectors.

Description: Radiation portals normally incorporate a dedicated neutron counter and a gamma-ray detector with at least some spectroscopic capability. This paper describes the design and presents characterization data for a detection system called PVT-NG, which uses large polyvinyl toluene (PVT) detectors to monitor both types of radiation. The detector material is surrounded by polyvinyl chloride (PVC), which emits high-energy gamma rays following neutron capture reactions. Assessments based on high-energy gamma rays are well suited for the detection of neutron sources, particularly in border security applications, because few isotopes in the normal stream of commerce have significant gamma ray yields above 3 MeV. Therefore, an increased count rate for high-energy gamma rays is a strong indicator for the presence of a neutron source. The sensitivity of the PVT-NG sensor to bare {sup 252}Cf is 1.9 counts per second per nanogram (cps/ng) and the sensitivity for {sup 252}Cf surrounded by 2.5 cm of polyethylene is 2.3 cps/ng. The PVT-NG sensor is a proof-of-principal sensor that was not fully optimized. The neutron detector sensitivity could be improved, for instance, by using additional moderator. The PVT-NG detectors and associated electronics are designed to provide improved resolution, gain stability, and performance at high-count rates relative to PVT detectors in typical radiation portals. As well as addressing the needs for neutron detection, these characteristics are also desirable for analysis of the gamma-ray spectra. Accurate isotope identification results were obtained despite the common impression that the absence of photopeaks makes data collected by PVT detectors unsuitable for spectroscopic analysis. The PVT detectors in the PVT-NG unit are used for both gamma-ray and neutron detection, so the sensitive volume exceeds the volume of the detection elements in portals that use dedicated components to detect each type of radiation.
Date: June 1, 2011
Creator: Mitchell, Dean James & Brusseau, Charles A.
Partner: UNT Libraries Government Documents Department