Supercapacitor is the best potential candidate of the energy storage system due to the superior charge or discharge efficiency, high power density (>10 kW kg-1), and long cycling life. Porous carbon materials as the promising electrode material have been widely used in supercapacitor. In fact, conventional porous carbon supercapacitor electrodes cannot fulfil the growing demand of high energy and power densities of supercapacitor. A large number of studies show that nitrogen doping can change the surface electronic structure of carbon materials, thus significantly improving the electrochemical properties. In addition to, the pore structure and morphology of carbon materials have great influence on the electrochemical performance. In this work, we firstly fabricated nitrogen-doped porous carbon nanotubes by using a simple mixed salts (NaCl/ZnCl2) activation strategy. The as-obtained porous carbon nanotubes exhibited excellent electrochemical performance in supercapacitor. Furthermore, two- dimension nitrogen-doping porous nanosheets were prepared by a salt template-assisted monomer deposition method. In this study, by optimizing the synthesis conditions, the as-obtained carbon nanosheets showed a high specific capacitance of 277 F g-1 at 1 A g-1 and excellent cycle stability retained 91 % after 10,000 cycles. / College of Engineering, Science and Technology / M. Tech.( Civil and Chemical Engineering

No description available.

The influence of uniaxial compression upon the Hall effect ad resistivity of cadmium-doped samples of InSb at 77 K, 64 K, and 12 K are reported. Unilaxial compressions as high as 6 kbar were applied to samples oriented in the {001} and {110} directions. The net hole concentration of the samples were about 5x10^13 cm^-3 at 77 K as determined from the Hall coefficient at 24 kilogauss. The net concentration of hole carriers decreases and then increases exponentially with stress at 77 k and 64 k, while at 12 k there is only a monotonic increase of carrier concentration with stress. Analysis of the hole concentration as a function of stress shows the presence of a deep acceptor level located about 90 meV above the valence band edge in additionb to the 10 meV vadmium acceptor level. The shallow acceptor level does not split with stress. The hole density data is represented very well by models which describe both the variation in the net density of states and motion of the acceptor levels as a function of stress.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1979. / Microfiche copy available in Archives and Science. / Bibliography: leaf 93. / by Carolyn Alexandria Morrow. / M.S.

In this thesis we investigate concepts associated with aggregation. The basic idea of aggregation is that there exists a reduced order model such that, for an appropriate initial condition, the trajectories of the reduced-order model are linear combinations of the trajectories of the ful 1-order model. We study systems which do not aggregate exactly, but which "nearly aggregate". It is shown that for "nearly aggregable" systems there exists a reduced-order model such that, for an appropriate initial condition, the trajectories of the reduced-order model are near a linear combination of the trajectories of the full-order model. Under certain conditions it has also been shown that near-aggregation is equivalent to near-unobservability (roughly, an invariant subspace close to the null space of C). Here we establish a relationship between near-unobservability and modal measures of observability as suggested by Selective Modal Analysis. With this result we then obtain an upper bound on the norm of the transfer function residue using near-unobservability measures. The Generalized Hessenberg Representation (GHR) and Dual GHR are examined throughout this analysis. It is finally shown that for SISO systems, the residue norm may be expressed in terms of certain parameters of the Dual GHR. / M.S.

Polypyrrole is an electrochemically synthesized conductive polymer that has physical properties which impede efforts to develop structure-properties relationships. The extent of conjugation, as limited by the presence of structural disorders in the polymer, is important in determining its inherent conductivity. The extent of conjugation in thin electrochemically generated films of polypyrrole and poly-N-methylpyrrole has been examined with resonance Raman spectroscopy. The Raman experiment was performed within the electrochemical cell and does not suffer from exposure to the contaminants encountered when transfer techniques are employed. Electrochemically reduced films of polypyrrole exhibited intense resonance Raman spectra of the carbon-carbon stretching frequencies. The position of these bands is a function of the number of double bonds in conjugation. The conjugation length within the polymer chain was found to be between 3 and 4 rings for PP and slightly less in PNMP (2-3 rings). This is the first reported determination of the conjugation length in PP and PNMP. This data confirms the idea that PNMP is less conductive than PP due to reduced planarity within the chain, thus less conjugation. Reduced films of PP and PNMP yielded intense luminescence that disappeared upon oxidation. The luminescence is a broad featureless band that consumes the weakly enhanced Raman of PNMP. The intensity of the luminescence increased as the reduction potential increased and the highest intensities occurred at potentials far cathodic of the E₀ for the film. The explanation for this is still obscure but may involve either further reduction of highly luminescent segments or a decrease in the amount of quenching by solvent or counter-ion interactions with the luminescer. / Ph. D.

Indiana University-Purdue University Indianapolis (IUPUI) / Preparation and characterization of multiferroic materials in which ferroelectricity and ferromagnetism coexist would be a milestone for functionalized materials and devices. First, electric properties of polyvinylidene (PVDF) films fabricated using the Langmuir-Schaefer method have been studied. Films of different thickness were deposited on silicon substrates and analyzed using several techniques. X-ray diffraction (XRD) data showed that PVDF films crystallize at an annealing temperature above 130 °C. Polarization versus electric field (PE) ferroelectric measurements were done for samples prepared with electrodes. PE measurements show that the coercivity of the films increases as the maximum applied electric field increases. The coercivity dependence on the frequency of the applied electric field can be fitted as . The results also show that the coercivity decreases with increasing the thickness of PVDF film due to the pinning effect. Next, we have demonstrated that those PVDF properties can be controlled by applying an external magnetic field. Samples were created in a layered heterostructure, starting with a Fe thin film, PVDF above that, and followed by another thin film of Fe. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to study the interface between PVDF polymer films and ferromagnetic iron thin films. Conventional EXAFS was applied to identify the structure of a Fe film sandwiched between two PVDF layers. An electric signal was then applied to the polymer to study the effects polarizing the polymer has on the Fe atoms at the interface. This shows that the Fe atoms diffuse into the PVDF layer at the interface between the two layers. Polarizing the film causes further diffusion of Fe atoms into the polymer. We also found that as the applied magnetic field is changed, the switching of electric polarization for the PVDF displayed a dependence on the external magnetic field. We also noticed that both the coercivity and polarization for the PVDF polymer display hysteretic features as the applied magnetic field is changed. We also found that the thickness of both the iron layers and the PVDF layer has an effect on the magnetoelectric coupling in our samples. The same strain applied to a thicker PVDF layer becomes tougher to flip the polarization compared to a thinner PVDF layer. As the iron film thickness increases, the strain also increases, and the polarization of the PVDF polymer is more easily flipped. We also found that the magnetoelectric sensitivity increases as both the PVDF and iron layers increase in thickness. We have shown that it is possible to control the ferroelectric properties of a PVDF film by tuning the magnetic field in a heterostructure. Our experiments show a coupling between the electric polarization and applied magnetic field in multiferroic heterostructures much larger than any previously reported values. Previous reports have used inorganic materials for the ferroelectric layer. Organic polymers have an electric dipole originating at the molecular level due to atoms with different electronegativity that are free to rotate. To flip the polarization, the chains must rotate and the position of the atoms must change. This increases the force felt locally by those chains. Using this polymer, we are able to increase the magnetoelectric coupling.

The electrochemistry of solid electrodes of charge transfer complexes of tetracyanoquinodimethane (TCNQ) and copper complexes with nitrogen containing chelates, such and dipyridylamine (dpaH), bipyridyl (bpy) and 1,10-dimethyl-2,9-phenanthroline (dmp), has been investigated with cyclic voltammetry. Pressed pellet electrodes of these complexes exhibit a broad electrochemically stable region. The oxidative and reductive breakdown reactions involve solid state reactions into the bulk electrode. These materials also act as electron mediators for glucose oxidation in glucose oxidase modified electrodes. The structure of the model compound, copper(I)(dpaH)₂Cl has been determined to have a distorted tetrahedral coordination sphere. The electrochemistry of solid electrodes of charge transfer complexes of tetrathiafulvalene (TTF) with copper chloride and copper bromide has been investigated with cyclic voltammetry. Pressed pellet electrodes do not exhibit a broad stable region, as do the TCNQ complexes. A preliminary structure of the organic part of tetramethyltetraselenafulvalene copper chloride has been determined from the solution of the Patterson function and exhibits a displacive modulation with a repeat unit of seven TMTSF molecules. (TTF)(SCN)₀ͺ₆₆ and (TTF)Cu(SCN)₂ have been investigated by infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. (TTF)(SCN)₀ͺ₆₆ crystallized in a tetragonal space group with a disordered column of thiocyanate anions. (TTF)Cu(SCN)₂ is an insulator with a two-dimensional network of Cu(SCN)₂⁻ ions. X-ray crystal structures of four compounds prepared in association with copper complex chemistry have been determined; (1) 5,5'-dibromo-2,2'-bithiophene, (2) 3,5,5'-tribromo-2,2'-bithiophene, (3) Cu(dmp)(CN)₂ ⁻ · Bu₄N⁺ and (4) the 1:2 adduct of dimercaptosuccinic acid and dimethylformamide.

Thesis (MSc)--University of Stellenbosch, 2007. / ENGLISH ABSTRACT: Silicone rubber, particularly poly(dimethylsiloxane) (PDMS), has been increasingly used in the manufacture of outdoor high voltage insulators in the recent years. PDMS offers several advantages that make it suitable for outdoor use, such as low weight, a hydrophobic surface, stability, and excellent performance in heavily polluted environments. PDMS surfaces can, however, become progressively hydrophilic due to surface oxidation caused by corona discharge, UV radiation and acid rain. In this study, PDMS samples of controlled formulations as well as six commercial insulator materials four PDMS based and two ethylene propylene diene monomer (EPDM) based were exposed to various accelerated weathering conditions for various periods of time in order to track changes in the material over time. The ageing regimes developed and used to simulate the potential surface degradation that may occur during in-service usage included needle corona and French corona ageing, thermal ageing, UV-B irradiation (up to 8000 hours) and acid rain (up to 200 days). Both the chemical and physical changes in the materials were monitored using a wide range of analytical techniques, including: static contact angle measurements (SCA), optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), gas chromatography (GC), gas chromatography/mass spectroscopy (GC/MS), size-exclusion chromatography (SEC), Fourier-transform infrared photoacoustic spectroscopy (FTIR-PAS) and slow positron beam techniques (PAS). A low molecular weight (LMW) uncrosslinked PDMS model compound was used to further study the chemical effects of corona exposure on PDMS materials. PDMS showed far better performance than EPDM, in terms of resistance to the various ageing regimes and “hydrophobicity recovery”. / AFRIKAANSE OPSOMMING: Silikoonrubber, spesifiek polidimetielsiloksaan (PDMS), is gedurende die afgelope paar jaar toenemend gebruik in die vervaardiging van buitelughoogspanningisolators. PDMS het baie voordele vir gebruik in elektriese isolators soos ‘n laer massa, ʼn hidrofobiese oppervlak, stabiliteit en uitstekende werking in hoogsbesoedelde omgewings. Die hidrofobiese oppervlakte kan egter gelydelik hidrofilies word weens oppervlakoksidasie as gevolg van korona-ontlading, UV-bestraling en suurreën. In hierdie studie is PDMS monsters van verskillende samestellings sowel as ses kommersiële isolators (vier PDMS en twee etileenpropileenrubber (EPDM)) blootgestel aan verskillende versnelde weersomstandighede vir verskillende periodes om die veranderinge in die materiale te monitor. Die verskillende materiale is gerangskik volgens hulle werking oor ‘n periode van tyd. Dit het ook ‘n geleentheid gebied om die eienskappe van die verskillende samestellings te bestudeer. Die tegnieke wat ontwikkel is om die moontlike oppervlakdegradasie te simuleer, het naald-korona, “French” korona, UVB-bestraling (tot 8000 uur) en suurreën (tot 200 dae) ingesluit. Beide die chemiese en die fisiese veranderinge in die materiale is gemonitor met behulp van verskeie tegnieke soos statiese kontakhoekbepaling, optiese mikroskopie, skandeerelektronmikroskopie, energieverspreidingsspektroskopie, gaschromatografie, grootte-uitsluitingschromatografie, foto-akoestiese Fouriertransforminfrarooi (PASFTIR) en stadige-positronspektroskopie (PAS). ʼn Lae molekulêre massa PDMS modelverbinding is gebruik om die chemiese effek van korona te bestudeer. Die PDMS materiale het baie beter vertoon teenoor die EPDM materiale in terme van hulle herstel van hidrofobisiteit.

We investigated the electronic structure of the two nematic materials Sr₃Ru₂O₇ and Ca(Fe₀.₉₇Co₀.₀₃As)₂ using spectroscopic – imaging scanning tunneling microscopy (SI-STM) and angle resolved photoemission spectroscopy (ARPES). – – – Sr₃Ru₂O₇ is an itinerant metamagnet that shows a putative quantum critical endpoint at 8 Tesla, submersed by the formation of a nematic electronic phase. Using ARPES, we identified at least 5 Fermi pockets in agreement with quantum oscillation measurements. Surprisingly, we found Fermi velocities up to an order of magnitude lower than in single layer Sr₂RuO₄ and up to 35 times lower than predicted by ab initio calculations. Many bands are confined in an energy range of only ∼10 meV below the Fermi level. This, as well as distinct peak-dip-hump shapes of the spectra with a characteristic energy of around ∼5 meV indicate strong correlations and a possible nontrivial mechanism that is absent in single layer Sr₂RuO₄ and connected to the nematicity. The quasiparticle interference of one of the bands was detected by SI-STM, which was also used to measure subatomic features with the symmetries of the relevant Ru d orbitals. – – – In the second mate- rial, the iron-based high-temperature superconductor Ca(Fe[subscript(1-x)]Co[subscript(x)]As)₂, we discovered electronic nematic nano-pattern in its under-doped ‘parent’ state. We spectroscopically imaged this state in real space over large areas and across domain boundaries that change the directionality of the nano-pattern by 90°. We propose that oriented, dimer-shaped electronic nematogens are responsible for this pattern, in striking contrast to what has been expected and observed in electronic nematic materials. The dimers consist of two Gaussian conductance peaks separated by about 8 a[subscript(FeFe)]. Unidirectionality also shows in the quasiparticle interference pattern of the delocalized electrons. The dispersion is in agreement with scattering from the α₂ band discovered by ARPES but has distinct C₂ symmetry, not inconsistent with a C₄-symmetric band scattered by the proposed dimers.