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Optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures forphotovoltaic applications

Description: Although ZnO and ZnS are abundant, stable, environmentallybenign, their band gap energies (3.44 eV, 3.72 eV) are too large foroptimal photovoltaic efficiency. By using band-corrected pseudopotentialdensity-functional theory calculations, we study how the band gap,opticalabsorption, and carrier localization canbe controlled by formingquantum-well like and nanowire-based heterostructures ofZnO/ZnS andZnO/ZnTe. In the case of ZnO/ZnS core/shell nanowires, which can besynthesized using existing methods, we obtain a band gap of 2.07 eV,which corresponds to a Shockley-Quiesser efficiency limitof 23 percent.Based on these nanowire results, we propose that ZnO/ZnScore/shellnanowires can be used as photovoltaic devices with organic polymersemiconductors as p-channel contacts.
Date: May 1, 2007
Creator: Schrier, Joshua; Demchenko, Denis O.; Wang, Lin-Wang & Alivisatos,A. Paul
Partner: UNT Libraries Government Documents Department

Nanometer-scale metal dispersions in polymeric matrices

Description: Rutherford backscattering spectrometry was used to measure the depth distribution of gold nanoparticles within thin layers of poly(t-butyl acrylate)(PTBA). The gold nanoparticles were created by evaporation of a discontinuous gold layer onto a thin film of PTBA. A second PTBA film was placed onto these samples to create ``sandwiches`` in which the gold existed between two PTBA films. Gold particle diffusion coefficients were measured from gold particle depth distributions in annealed samples for which the molecular weights of the two PTBA layers were identical. The experiments revealed that particle mobility was decreased by 2 to 3 orders of magnitude compared with predictions of the Stokes-Einstein model of particle diffusion. This is attributed to bridging interactions between particles arising from slow exchange kinetics of polymer segments at the polymer/metal interface. Experiments for which the molecular weights of the two polymer films are different, are sensitive to the ability of polymer molecules to pass through the gold particle layer. Experiments done with thermally evaporated particles are consistent with a picture in which polymer molecules are able to freely pass through the gold particle layer. Results with gold deposited by electron-beam evaporation are different: the gold is not able to diffuse and polymer molecules not able to penetrate the gold layer. These results, combined with optical absorption experiments, indicate that much smaller particles are obtained by electron-beam evaporation than by thermal evaporation.
Date: January 1, 1997
Creator: Shull, K.R.; Cole, D.H.; Rehn, L.E. & Baldo, P.M.
Partner: UNT Libraries Government Documents Department

Study of anisotropy of spin cast and vapor deposited polyimide films using internal reflection techniques

Description: We have compared anisotropy of spin cast and vapor deposited polyimide (VDP) films, using internal reflection infrared spectroscopy. The films were deposited directly on the internal reflection element. We find that spin cast films are more anisotropic than their VDP counterparts, with the polyimide chains tending to align parallel to the substrate. Both films are found to contain more and less ordered regions. Within the ordered regions, the plane of the phenyl ring tends to align parallel to the substrate.
Date: November 1, 1996
Creator: Liberman, V.
Partner: UNT Libraries Government Documents Department

Optically detected magnetic resonance studies on {pi}-conjugate polymers and novel carbon allotropes

Description: This report describes the following: introduction to photoluminescence detected magnetic resonance (PLDMR); introduction to {pi}-conjugated systems; PLDMR measurements on poly(p-phenylene)-type ladder polymers; PLMDR measurements on poly(p-phenylene ethylene); and PLDMR measurements on C{sub 70}, polythiophene, poly(p-phenylene vinylene) and Dan-40. Appendices to this report describe: Operation of ODMR (optically detected magnetic resonance) spectrometer; ODMR system parameters; and Special purpose circuitry.
Date: February 12, 1999
Creator: Partee, J.
Partner: UNT Libraries Government Documents Department

Polymethylsilsesquioxanes through base-catalyzed redistribution of oligomethylhydridosiloxanes

Description: There has been an increasing amount of interest in silsesquioxanes and polysilsesquioxanes. They have been used as models for silica surfaces and have been shown to have great potential for several industrial applications. Typical synthesis of polysilsesquioxanes involves the hydrolysis of organotricholorosilanes and/or organotrialkoxysilanes in the presence of acid or base catalysts, usually in the presence of organic solvents.
Date: April 4, 2000
Partner: UNT Libraries Government Documents Department

Direct Measurement of Extension and Force in Conductive Polymer Gel Actuators

Description: The synthesis of a polythiophene-based conductive polymer gel is described. Preliminary measurements of the electrochemically driven extension and force response of this gel are reported when driven under the action of an applied square-wave potential. Over each square wave interval (i.e., one oxidation pulse followed by one reduction pulse), the axial change in dimension was found to be approximately 2%. Some hysteresis was noted in that the cylindrical specimens did not return to their original axial dimension. The axial pressure generated by the expansion of the gel against a fixed surface was also measured and found to be on the order of 15 kPa.
Date: February 1, 2001
Creator: Goods, S.H.; Whinnery, L.L.; Irvin, D.J. & Korellis, J.S.
Partner: UNT Libraries Government Documents Department

Characterization of microenvironment polarity and solvent accessibility of polysilsesquioxane xerogels by the fluorescent probe technique

Description: Poly (1, 4 bis(triethoxysilyl)benzene) (PTESB), a representative of a new type of organic-inorganic hybrid polysilsesquioxane material, was characterized by fluorescence spectroscopy for both microenvironmental polarity and solvent accessibility. A dansyl fluorescent molecule was incorporated into the bulk as well as onto the surface of both PTESB and silica materials. Information about the microenvironment polarity and accessibility of PTESB to various organic solvents was determined and compared to that of silica gel. This study found that both the bulk and surface of PTESB are less polar than that of the silica material. The silica material is accessible to polar solvents and water, while YMB is accessible to polar solvents but not to water. The hydrophobicity of PTESB differentiates these new materials from silica gel.
Date: May 1, 1995
Creator: Shea, K.J.; Zhu, H.D. & Loy, D.A.
Partner: UNT Libraries Government Documents Department

2001 Gordon Research Conference on Applied and Environmental Microbiology. Final progress report [agenda and attendee list]

Description: The Gordon Research Conference on Applied and Environmental Microbiology was held at Connecticut College, New London, Connecticut, July 22-27, 2001. The conference was attended by 121 participants. The attendees represented the spectrum of endeavor in this field, coming from academia, industry, and government laboratories, and included US and foreign scientists, senior researchers, young investigators, and students. Emphasis was placed on current unpublished research and discussion of the future target areas in this field. There was a conscious effort to stimulate discussion about the key issues in the field today. Session topics included the following: Environmental and applied genomics, Cell-to-cell signaling and multicellular behavior, Emerging technologies and methods, Novel metabolisms and ecosystems, Directed evolution of enzymes and pathways, Symbiotic and trophic relationships, Synthesis and application of novel biopolymers, and Microbes at the oxic-anoxic interface. There was also a special lecture titled ''Under the umbrella of the big tree: microbial biology into the 21st century.''
Date: July 26, 2001
Creator: Drake, Harold
Partner: UNT Libraries Government Documents Department

The degradation of TPX components by oxygen, elevated temperature, and ionizing radiation

Description: TPX is PMP or poly(4-methyl-1-pentene). It has several commercially important characteristics such as high optical transparency, high crystalline melting point, etc., leading to numerous applications including infrared windows, lenses, membranes, food packaging. The life components fabricated from this material may be limited by thermal oxidative and radiation-induced degradation. A preliminary review of the scientific literature was conducted to obtain relevant information on the effects of oxygen, moisture elevated temperature, and radiation on the chemical, thermodynamic, mechanical, and electrical properties of this material. Refs, figs, tabs.
Date: May 31, 1996
Creator: Farmer, J.C.
Partner: UNT Libraries Government Documents Department

Role of dopant counter-anion functionality in polyaniline salts/blends and implications to morphology

Description: Polyanilines are of particular current interest primarily due to their relative ease of synthesis, low cost and stable conductivity in air. The insulating, polyaniline emeraldine base (PANI-EB) form becomes electrically conducting by preferential protonation or doping the imine nitrogen sites to yield an electrically conducting polyaniline emeraldine salt (PANI-ES). In this paper, wide and small angle X-ray scattering techniques (i.e., WAXS and SAXS) and light microscopy are used to characterize the influence of the dopant`s structure on the morphology of both polyaniline salt and blend. In an attempt to modify the morphology of the PANI-ES, the authors have evaluated a number of doping acids (i.e., methane sulfonic acid (HMSA), butane sulfonic acid (HBSA), dodecyl benzene sulfonic acid (HDBSA) and camphor sulfonic acid (HCSA)) that vary in size and polarity to better understanding the role of the acid in PANI-ES`s morphology and the resulting electrical conductivity. The other goal was to investigate the effect of the counter-anion structure on the nature of the phase separated PANI-ES network. The shape of the PANI-ES network in the host polycaprolactam has important implications on the nature of conduction behavior and the final electrical conductivity of the blend.
Date: April 1, 1997
Creator: Hopkins, A.R.; Rasmussen, P.G.; Basheer, R.A.; Annis, B.K. & Wignall, G.D.
Partner: UNT Libraries Government Documents Department

Bridged polysilsesquioxanes: A molecular based approach for the synthesis of functional hybrid materials

Description: Bridged polysilsesquioxanes (BPS) are a family of hybrid organic-inorganic materials prepared by sol-gel polymerization of molecular building blocks that contain a variable organic component and at least two trifunctional silyl groups. The resulting xerogels and aerogels have physical and mechanical properties that are strongly influenced by the organic bridging group. This talk focuses on the synthesis of functional bridged polysilsesquioxanes. Incorporation of functional groups that respond to chemical, photochemical, or thermal stimuli can provide handles for modifying bulk morphology and/or provide function. These materials can find use as ion exchange media, chromatographic stationary phases, photoresists and high capacity selective chemical absorbents.
Date: May 9, 2000
Partner: UNT Libraries Government Documents Department

Polysilsesquioxanes through base-catalyzed redistribution of oligohydridosiloxanes

Description: Organopolysilsesquioxanes have recently gained much interest as materials for low-K dielectrics, ceramic precursors and photoresists. Typical sol-gel synthesis of polysilsesquioxanes involves the hydrolysis of organotricholorosilanes and/or organotrialkoxysilanes in the presence of acid or base catalysts and organic solvents. However, under sol-gel conditions most organotrialkoxysilanes do not afford silsesquioxane gels. This limits the range of organic functionalities that can be introduced into these hybrid organic-inorganic materials. An alternative route to polysilsesquioxanes is through oligohydridosiloxanes. Catalytic disproportionation, by titanium complexes, of linear or cyclic oligomers of methylhydridosiloxanes can lead to polymethylsilsesquioxanes. The authors have shown that disproportionation of oligomethylhydridosiloxanes can also be catalyzed by tetrabutylammonium hydroxide to yield polymethylsilsesquioxanes (scheme 1). This replaces the step-growth sol-gel polymerization process of organotrialkoxysilanes, which requires solvent, stoichiometric water and produces alcohol and water condensation by-products. Tetraalkylammonium hydroxides, as catalysts, are also attractive because they readily decompose by heating above 150 C; thus, they can be easily removed from the final materials. In this paper the authors report on both the catalytic and stoichiometric redistribution of organohydridosiloxanes to produce polysilsesquioxane foams and gels of the formula (RSiO{sub 1.5}){sub n} which otherwise cannot be obtained through traditional sol-gel means.
Date: May 1, 2000
Partner: UNT Libraries Government Documents Department

Porosity in polysilsesquioxane xerogels

Description: Polysilsesquioxanes, [RSiO{sub 1.5}]{sub n} are a class of hybrid organic-inorganic materials in which silicon atoms are linked with up to three siloxane bonds to other monomer units in the polymer and the organic group is a pendent functionality. Polysilsesquioxanes are prepared by the hydrolysis and condensation of organotrialkoxysilanes (Scheme l). Organotrialkoxysilanes RSi(OR{prime}){sub 3}, have been extensively used as coupling agents for composites or surface treatments for materials. Polysilsesquioxanes have become increasingly popular for generating specialty coatings such as low k dielectric materials for microelectronic applications. While there is extensive information on the formation of polysilsesquioxanes, there has not been a survey of the ability of organotrialkoxysilanes to form gels until recently. The formation of polysilsesquioxanes gels has been shown to be very sensitive to the nature of the organic group. Many monomers will only form soluble oligomers or polymers upon hydrolysis and condensation, even when the reaction is conducted solvent-free with neat monomer and aqueous catalyst. Furthermore, there is little information concerning the influence of the organic group, R, on the porosity of the polysilsesquioxanes gels that are formed. In this paper the authors describe the preparation of polysilsesquioxane gels where R = H, methyl, ethyl, cyanoethyl, vinyl, dodecyl, hexadecyl, octadecyl, chloromethyl, and chloromethylphenyl, and the characterization of the porosity of the respective xerogels. Gels were prepared from the hydrolysis and condensation of organotrimethoxysilanes, RSi(OEt){sub 3}, and organotriethoxy-silanes, RSi(OEt){sub 3}.
Date: May 9, 2000
Partner: UNT Libraries Government Documents Department

Dynamical Properties of Polymers: Computational Modeling

Description: The free volume distribution has been a qualitatively useful concept by which dynamical properties of polymers, such as the penetrant diffusion constant, viscosity, and glass transition temperature, could be correlated with static properties. In an effort to put this on a more quantitative footing, we define the free volume distribution as the probability of finding a spherical cavity of radius R in a polymer liquid. This is identical to the insertion probability in scaled particle theory, and is related to the chemical potential of hard spheres of radius R in a polymer in the Henry's law limit. We used the Polymer Reference Interaction Site Model (PRISM) theory to compute the free volume distribution of semiflexible polymer melts as a function of chain stiffness. Good agreement was found with the corresponding free volume distributions obtained from MD simulations. Surprisingly, the free volume distribution was insensitive to the chain stiffness, even though the single chain structure and the intermolecular pair correlation functions showed a strong dependence on chain stiffness. We also calculated the free volume distributions of polyisobutylene (PIB) and polyethylene (PE) at 298K and at elevated temperatures from PRISM theory. We found that PIB has more of its free volume distributed in smaller size cavities than for PE at the same temperature.
Date: January 1, 2001
Partner: UNT Libraries Government Documents Department

Effect of tetralin on the degradation of polymer in solution

Description: The effect of a hydrogen-donor solvent tetralin on thermal degradation of poly(styrene-allyl alcohol) in liquid solution was investigated in a steady-state tubular flow reactor at 1000 psig at various tetralin concentrations, polymer concentrations, and temperatures. The experimental data were interpreted with continuous- mixture kinetics, and rate coefficients determined for the specific and random degradation processes.
Date: December 31, 1995
Creator: Madras, G.; Smith, J.M. & McCoy, B.J.
Partner: UNT Libraries Government Documents Department

A novel carbon fiber based porous carbon monolith

Description: A novel porous carbon material based on carbon fibers has been developed. The material, when activated, develops a significant micro- or mesopore volume dependent upon the carbon fiber type utilized (isotropic pitch or polyacrylonitrile). The materials will find applications in the field of fluid separations or as a catalyst support. Here, the manufacture and characterization of our porous carbon monoliths are described.
Date: July 1, 1995
Creator: Burchell, T.D.; Klett, J.W. & Weaver, C.E.
Partner: UNT Libraries Government Documents Department

Expancel Foams: Fabrication and Characterization of a New Reduced Density Cellular Material for Structural Applications

Description: This study was initiated to produce a low-density centering medium for use in experiments investigating the response of materials to shock-loading. While the main drivers for material selection were homogeneity, dimensional stability, performance and cost, other secondary requirements included fine cell size, the ability to manufacture 5--10 cm-sized parts and an extremely compressed development time. The authors chose a non-traditional methodology using a hollow, expandable, polymeric microballoon material system called Expancel{reg_sign}. These microballoons are made from a copolymer of polyacrylonitrile (PAN) and polymethacrylonitrile (PMAN) and use iso-pentane as the blowing agent. The average diameter (by volume) of the unexpanded powder is approximately 13 {micro}m, while the average of the expanded powder is 35--55 {micro}m, with a few large microballoons approaching 150--200 p.m. A processing method was developed that established a pre-mixed combination of unexpanded and expanded Expancel at a ratio such that the tap (or vibration) density of the mixed powders was the same as that desired of the final part. Upon heating above the tack temperature of the polymer, this zero-rise approach allowed only expansion of the unexpanded powder to fill the interstices between the pre-expanded balloons. The mechanical action of the expanding powder combined with the elevated processing temperature yielded flee-standing and mechanically robust parts. Although mechanical properties of these foams were not a key performance requirement, the data allowed for the determination of the best temperature to heat the samples. Processing the foam at higher temperatures enhanced both modulus and strength. The maximum allowable temperature was limited by dimensional stability and shrinkback considerations. Tomographic analysis of foam billets revealed very flat density profiles. Parts of any density between the low density expanded powder (approximately 0.013 g/cm{sup 3}) and the higher density unexpanded powder (approximately 0.5 g/cm{sup 3}) can be produced using this technique. The extremely wide range ...
Date: August 1, 2000
Creator: Whinnery, L.; Goods, S. & Even, B.
Partner: UNT Libraries Government Documents Department


Description: This research evaluated organoclays, zeolites, phosphates, and a biopolymer as sequestering agents for inorganic and organic contaminants. Batch experiments were conducted to identify amendments and mixtures of amendments for metal and organic contaminants removal and retention. Contaminant removal was evaluated by calculating partitioning coefficients. Metal retention was evaluated by desorption studies in which residue from the removal studies was extracted with 1 M MgCl{sub 2} solution. The results indicated that phosphate amendments, some organoclays, and the biopolymer, chitosan, were very effective sequestering agents for metals in fresh and salt water. Organoclays were very effective sorbents for phenanthrene, pyrene, and benzo(a)pyrene. Partitioning coefficients for the organoclays were 3000-3500 ml g{sup -1} for benzo(a)pyrene, 400-450 ml g{sup -1} for pyrene, and 50-70 ml g{sup -1} for phenanthrene. Remediation of sites with a mixture of contaminants is more difficult than sites with a single contaminant because metals and organic contaminants have different fate and transport mechanisms in sediment and water. Mixtures of amendments (e.g., organoclay and rock phosphate) have high potential for remediating both organic and inorganic contaminants under a broad range of environmental conditions, and have promise as components in active caps for sediment remediation.
Date: May 10, 2007
Creator: Knox, A; Michael Paller, M; Danny D. Reible, D; Xingmao Ma, X & Ioana G. Petrisor, I
Partner: UNT Libraries Government Documents Department