Made available in DSpace on 2014-06-11T19:30:19Z (GMT). No. of bitstreams: 0 Previous issue date: 2012-06-11Bitstream added on 2014-06-13T20:40:12Z : No. of bitstreams: 1 ywata_rs_me_bauru.pdf: 1172780 bytes, checksum: 9144da0c9124443e6313263e76ce6fbd (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / O objetivo deste trabalho é a caracterização de polímeros isolantes elétricos para seu uso na fabricação de dispositivos eletrônicos orgânicos. Os polímeros estudados foram: a poli(amida-imida), PAI, poli(fenil-metilsilsesquioxano), PSQ, e o fotoresiste SU8. O processo de fabricação dos filmes poliméricos foi realizado pela técnica de spin-coating e eles foram submetidos a um processo de cura apropriado para o entrecruzamento das cadeias poliméricas. Os filmes foram caracterizados espectroscopia de impedância e microscopia de força atômica (AFM). Curvas de capacitância e tan'delta' também foram analisadas e valores menores de 10-2 foram encontradas para a tn'delta' indicando que os polímeros são bons isolantes elétricos. Curvas características de corrente versus diferença de potencial elétrico (ddp) em diferentes temperaturas foram obtidas para todos os filmes. A condução elétrica foi analisada e conclui-se que o processo de condução predominantes nos filmes poliméricos é por injeção de cargas por efeito Schottky. A altura da barreira de injeção polímero/metal calculada foi da ordem de 1,1 eV para o PSQ e 1,2 eV para o SU8 e PAI / This work aims the films characterization of electric insulating polymers for the fabrication of organic electronic devides: the poly(amide-imide), PAI, poly(phenylmethylsilsesquioxane), PSQ, and the SU8 photoresist. The manufacturing process of the polymer films was performed by spin-coating and they were submitted to the appropriated curing process in order to promote the cross-linking of polymeric chains. The thin films were characterized by using electric impedance technique and scanning atomic force microscopy (AFM). Curves of capacitance and tan'delta' were also analyzed and values smaller than 10-2 were found for tan'delta' indicating that the polymers are good electrical insulators. Current versus voltage characteristics curves at different temperatures were obtained for all films. The electrical conduction was analyzed and it was concluded that the conduction process on polymeric films are dominated by Schottky charge injection. The injection barrier height of polymer/metal was found to be of the order of 1,1 eV for PSQ and 1,2 eV for SU8 and PAI

I. CURRENT FLUCTUATIONS IN A SUPERCONDUCTING CIRCUIT CARRYING A CIRCULATING CURRENT - Persistent currents in superconducting lead are free from fluctuations to less than 1.1 x 10⁻⁹ of full shot noise at approximately 2.4 Mc/s. Superconducting currents are also unaffected by the surface condition of the metal to the same limit as stated above. II. A NEW ABSOLUTE NOISE THERMOMETER AT LOW TEMPERATURES - If three resistors, which are kept at different temperatures, are arranged in form of a π network and if two of the thermal noise voltages appearing across the if network are multiplied together and averaged with respect to time, then under certain conditions the correlation between those voltages can be made zero. This condition is used to calculate the temperature of one noise source provided all the resistance Values and the other temperatures are known. A noise thermometer of this kind was constructed which is capable of measuring temperatures below approximately 140°K. The boiling points of liquid oxygen and liquid nitrogen were determined absolutely within 0.2 percent using the ice-point as reference. Between 1.3°K and 4.2°K the thermometer had to be calibrated due to errors arising in the equipment and the measured temperatures were then accurate within ± 1 percent. III. QUASI-PERSISTENT CURRENTS IN RINGS COMPOSED OF SUPERCONDUCTING AND NON-SUPERCONDUCTING REGIONS - A number of rings composed of a superconductor (Pb, In) apart from a small insert of normal metal (Cu) perpendicular to the current flow have been investigated between 1.30°K and 4.33°K for Pb-Cu and between 1.30°K and 3.20°K for In-Cu. It was found that for samples with good electrical contact the decay of the magnetic field due to the current is exponential and that the effective resistance increased compared with the bulk resistance of Cu by approximately 2.1 for the Pb-Cu rings and by 18.5 for the In-Cu rings. Two different thicknesses of the Cu inserts (0.0125 cm and 0.0053 cm) were used and it was found that the resistivity of the thin Cu insert increased with respect to the thick foil by 16% for the Pb-Cu system and by 36% for the In-Cu system. Part of this relative increase can be explained as a size effect due to electron scattering in the Cu insert. The effective resistance of the Pb-Cu rings shows a maximum at approximately 3.4°K. The resistance of the In-Cu samples decreases by about 10% between 3.2°K and 1.3°K. The resistivity of the Cu foil when measured separately was constant for the above temperature range. For samples with "poor" electrical contact (probably due to some copper oxide on the insert) two definite relaxation times were observed. For these samples the effective resistance was current and temperature dependent and it was decreasing for decreasing currents and decreasing temperatures. This can be explained in terms of a rectification effect of the two oxide layers on the insert. The decay of the magnetic field of the ring is consistent with the decay of a current in an L-R circuit. IV. THE DESTRUCTION OF SUPERCONDUCTIVITY IN TANTALUM WIRES BY A CURRENT - The transition from the superconducting to the normal state of various pre-stretched tantalum wires carrying current was investigated. When the resistance of the wire jumps discontinuously from the superconducting to the normal or intermediate state as a current is passed through it, then this current is defined as the critical current I(c). For temperatures T < (T(c)-5 millidegrees K) the resistance of the wire jumps directly from zero resistance to its normal value at the critical current, such that the total cross section of the wire goes effectively into the normal state. Between (T(c)-5 millidegrees K) and T(c) the resistance of the wire jumps at I(c) to any fraction of the normal resistance between approximately zero and one. For constant temperatures the resistance-current plots show a large hysteresis effect. The transition temperature, T(c), of the various samples is strongly dependent upon their normal resistivity at helium temperatures. If the wires with a small constant current (4.2 ma) flowing in them are cooled from above the transition temperature, the resistance decreases above T(c) and approaches zero at approximately T(c) where T(c) is defined by the extrapolation of the I(c)-T curve to I(c) = 0. If the wires are heated from below T(c) the same resistance-temperature curves are reproduced. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The technique of ellipsometry was applied to the study of nonuniform anodic oxide films resulting from electrolyte incorporation into the oxide on growth. Ellipsometry results obtained in air and in situ on oxides formed on Ta in phosphoric acid are consistent with the results of tracer studies in which the oxide consists of two layers which grow simultaneously due to metal and oxygen ion transport during anodization, and further that electrolyte incorporation into the outer layer on growth modifies its properties with respect to the inner layer. The ellipsometry results were not consistent with a single homogeneous layer film or with a film possessing an index of refraction changing linearly with thickness. Thus ellipsometry provides a new, nondestructive method of determining ion transport numbers. The index of refraction and thickness of each layer were obtained by curve fitting ellipsometry results obtained as a function of increasing oxide thickness. Computer methods for solving the ellipsometry equation and curve fitting are given. Computed results are given and discussed for cases of one and two layer films growing on a metal. Finally, an error analysis of ellipsometry is made. Ellipsometry results were obtained and curve-fitted for oxides grown in various solutions, at different rates and for anodization in a sequence of electrolytes. The major findings of this part of the study are as follows: Electrolyte incorporation decreases ionic conductivity, dielectric constant and index of refraction. At constant current formation, the fraction of oxide made up by the outer layer increases with current density, and electrolyte concentration, and depends on previous formation of the oxide. The log J-E characteristics at constant voltage in dilute phosphoric acid are curved and occur at higher fields than those for dilute sulphuric acid. Analysis of the above results indicates that: a) the conduction process is bulk controlled b) ionic conduction and dielectric properties arise from the same process and c) that electrolyte incorporation is responsible for part if not all the curvature in the logJ-E characteristics of ionic conduction. Photo-stimulated growth at low electric fields was investigated by ellipsometry. The effect of radiation is to first modify the properties of the existing oxide after which photo-stimulated growth occurs accompanied by a build up of secondary current. The secondary photocurrent is ionic in nature and the radiation rather than the applied field is responsible for the generation of ions to sustain this current. The photo-grown oxide consists of two layer with the outer layer having a much lower index of refraction than normally grown oxide. The thermal recrystallization of stripped anodic oxide films was studied using transmission electron microscopy. Various diffraction patterns were obtained and analyzed. The major result in terms of ionic conduction is that electrolyte incorporation inhibits recrystallization, again, consistent with a decreased ionic mobility with incroporation. A critical test has been devised and applied to a recently proposed theory of ionic conduction, the dielectric polarization theory. This theory postulates that the autocatalytic build up of ionic current on applying a constant high field to the oxide is due to an internal field controlled process and that the rate of build up of polarization (P) towards its equilibrium value (P₀) is enhanced by the passage of current, J, given by dp/dt = AJ(P₀-P). It is shown that this theory predicts an increase in small signal capacitance during the passage of the transient. However, measurements indicate that the capacitance decreases. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

I present two algorithms which solve the one-dimensional magnetotelluric (MT) problem of finding the electrical conductivity σ(z) as a function of depth in the earth. Together, these algorithms restrict and explore the nonuniqueness of the nonlinear MT inverse problem. They accept constraints which limit the space of acceptable conductivity models and they construct diverse classes of σ(z) in order to explore this space. To avoid pitfalls during interpretation, it is essential to investigate the extent of the nonuniqueness permitted by the MT data. Algorithm 1 is a two-stage process based on the inverse scattering theory of Weidelt. The first stage uses the MT frequency-domain data to construct an impulse response analogous to a deconvolved seismogram. Since this is a linear problem (a Laplace transform), numerous impulse responses may be generated by linear inverse techniques which handle data errors robustly. I minimize four norms of the impulse response in order to construct varied classes of limited structure models. Two least-squares norms minimize the energy in the impulse response or the energy in its derivative with respect to depth. Two least absolute value norms minimize the magnitudes of the response or its derivative. It is possible to use other norms. The different classes sample the range of acceptable models and the minimum structure criterion is unlikely to allow models with spurious features. The second stage of Algorithm 1 constructs the conductivity model from the impulse response using any of four Fredholm integral equations of the second kind. I evaluate the performance of each of the four mappings and recommend the Burridge and Gopinath-Sondhi formulations. I also evaluate three approximations to the second-stage equations. One of these is equivalent to the Born approximation which assumes the impulse response has negligible multiple reflections. The approximation that includes first-order multiple reflections is the most accurate and gives conductivity models similar to those given by the integral equations. Algorithm 2 solves an integral form of a nonlinear Riccati equation relating the measured frequency-domain data to a function of the conductivity. The iterative solution scheme sacrifices the efficiency of a direct inversion process such as Algorithm 1 for the advantages of incorporating localized conductivity constraints. The linear programming formulation readily accepts a wide variety of equality and inequality constraints on σ(z). I use these constraints in two ways to combat the nonuniqueness of this nonlinear inverse problem. First, I impose physical constraints derived from external sources to restrict the nonuniqueness and construct σ(z) models that are closer to reality. Second, I impose constraints specifically designed to estimate the extent ofthe nonuniqueness and explore the range of acceptable σ(z) profiles. The first technique enhances the reliability of an interpretation and the second assesses the plausibility of particular conductivity features. The convergence of Algorithm 2 is good because Algorithm 1 provides varied initial σ(z) which already fit the data well. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

LD2668 .T4 1966 C34 / Master of Science

No description available.

Thesis. 1975. M.S.--Massachusetts Institute of Technology. Dept. of Chemical Engineering. / Bibliography: leaves 63-65. / M.S.

The last two decades have seen huge improvements in measuring the conductance of single molecules, especially with the establishment of the scanning tunneling microscope break-junction (STM-BJ) method. The availability of such a robust and reliable measurement technique allows for the study of more exotic molecules with built-in functionality. In this thesis, we employ creative chemical design to manipulate transport in a single molecule junction by tuning the conducting frontier orbitals. We investigate three classes of materials – thiophene dioxides, mixed-valence bis(triarylamines), and benzotriazinyl-based Blatter radicals. Within each system, we probe changes in conducting behavior or interfacial interactions that arise from modifying the molecular structure. First, we demonstrate that a family of thiophene pentamers, which typically conduct through their highest occupied molecular orbital (HOMO), can be induced to conduct through their lowest unoccupied molecular orbital (LUMO) instead. This is akin to switching between from hole to electron transport. The switching was achieved using chemical modifications that drastically lower the LUMO level toward the Fermi energy of gold: oxidation at the sulfur position to form thiophene dioxides combined with installing electron-withdrawing groups at the 3- and 4-positions of the thiophene moiety. The ability to tune HOMO versus LUMO transport is potentially useful for assembling molecular circuits with n- and p-type components. Next, we found that oxidation of bis(triarylamine) molecular wires into their mixed-valence state shifts their conducting orbitals close to the Fermi energy of gold, making these wires highly conducting. We measured the length dependent transport of three bis(triarylamine) molecules. In their neutral state, the conductance of these compounds decreases with increasing length, which is observed for many different systems. However, when they are chemically oxidized, the mixed-valence molecular wires show an increase in conductance with increasing length. Such wires that maintain good electrical transport over long distances are valuable for building efficient molecular devices. We then investigated the interaction of half-filled orbitals in organic radicals with gold substrates to explore the potential of these compounds for spintronic and magnetic applications. We found that a Blatter radical functionalized with gold-binding thiomethyl groups displays different charge transfer behavior depending on the environment. Under ultra-high vacuum, X-ray spectroscopy shows that the radical molecules in contact with the gold substrate gain a charge from gold and their singly unoccupied molecular orbitals get filled. Contrastingly, in solution-based single molecule measurements, the radical loses the electron from its singly occupied molecular orbital instead, and only the conductance of the oxidized species is detected. We further probed the nature of charge transfer between the Blatter radical and gold in ultra-high vacuum by comparing spectroscopic measurements from three different derivatives. The derivative that was functionalized with two thiomethyl groups in order for it to be measured in the STM-BJ was the only molecule to undergo charge transfer in ultra-high vacuum. Two other Blatter derivatives that had only one and no thiomethyl groups did not show the same charge transfer; these retained their radical character even when in contact with the gold substrate. Therefore, the results indicate that only one of the thiomethyl groups mediates charge transfer between radical and substrate. The body of work presented herein shows that chemical modifications to old and new systems can be used to modulate transport in junctions via the intrinsic character of the molecules rather than external engineering factors. Thiophene dioxides are a relatively nascent class of materials that already show versatility as molecular conductors, while organic mixed-valence and radical systems have been heavily researched in other fields but less so in molecular electronics. This thesis therefore seeks to encourage further research that takes advantage of the unique electronic structure of these materials systems to discover new transport phenomena.

No description available.

Thesis (Ph. D.)--University of Michigan, 1937. / Lithoprinted. Errata slip attached to front fly-leaf. Bibliography: leaves 106-107.