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Complex band structure under plane-wave nonlocal pseudopotential Hamiltonian of metallic wires and electrodes

Description: We present a practical approach to calculate the complex band structure of an electrode for quantum transport calculations. This method is designed for plane wave based Hamiltonian with nonlocal pseudopotentials and the auxiliary periodic boundary condition transport calculation approach. Currently there is no direct method to calculate all the evanescent states for a given energy for systems with nonlocal pseudopotentials. On the other hand, in the auxiliary periodic boundary condition transport calculation, there is no need for all the evanescent states at a given energy. The current method fills this niche. The method has been used to study copper and gold nanowires and bulk electrodes.
Date: July 17, 2009
Creator: Yang, Chao
Partner: UNT Libraries Government Documents Department

Characterization of SEI layers on LiMn2O4 cathodes with in-situ spectroscopic ellipsometry

Description: In situ spectroscopic ellipsometry was employed to study the initial stage of SEI layer formation on thin-film LiMn{sub 2}O{sub 4} electrodes. It was found that the SEI layer formed immediately upon exposure of the electrode to EC/DMC (1:1 by vol) 1.0 M LiPF{sub 6} electrolyte. The SEI layer thickness then increased in proportion to a logarithmic function of elapsed time. In comparison, the SEI layer thickness on a cycled electrode increased in proportion to a linear function of the number of cycles.
Date: August 30, 2004
Creator: Lei, Jinglei; Li, Lingjie; Kostecki, Robert; Muller, Rolf & McLarnon, Frank
Partner: UNT Libraries Government Documents Department

Sensor for cell signaling proteins.

Description: Thiolated cyclodextrins have been shown to be useful as modifiers of electrode surfaces for application in electrochemical sensing. The adsorption of three different thiolated {beta}-cyclodextrin ({beta}-CD) derivatives onto gold (Au) electrodes was studied by monitoring ferricyanide reduction and ferrocene carboxylic acid (FCA) oxidation at the electrode surface using cyclic voltammetry. Electrodes modified with the {beta}-CD MJF-69 derivative bound FCA within the CD cavity. The monolayer acted as a conducting layer with an increase in the oxidation current. On the other hand, the {beta}-CD layer inhibited the reduction of ferricyanide at the electrode surface since ferricyanide is larger than the cavity of the {beta}-CD derivative and thus unable to form an inclusion complex.
Date: December 1, 2005
Creator: Farrow, Matthew & Yelton, William Graham
Partner: UNT Libraries Government Documents Department

Ionic liquids for rechargeable lithium batteries

Description: We have investigated possible anticipated advantages of ionic-liquid electrolytes for use in lithium-ion batteries. Thermal stabilities and phase behavior were studied by thermal gravimetric analysis and differential scanning calorimetry. The ionic liquids studied include various imidazoliumTFSI systems, pyrrolidiniumTFSI, BMIMPF{sub 6}, BMIMBF{sub 4}, and BMIMTf. Thermal stabilities were measured for neat ionic liquids and for BMIMBF{sub 4}-LiBF{sub 4}, BMIMTf-LiTf, BMIMTFSI-LiTFSI mixtures. Conductivities have been measured for various ionic-liquid lithium-salt systems. We show the development of interfacial impedance in a Li|BMIMBF{sub 4} + LiBF{sub 4}|Li cell and we report results from cycling experiments for a Li|BMIMBF{sub 4} + 1 mol/kg LIBF{sub 4}|C cell. The interfacial resistance increases with time and the ionic liquid reacts with the lithium electrode. As expected, imidazolium-based ionic liquids react with lithium electrodes. We seek new ionic liquids that have better chemical stabilities.
Date: September 29, 2005
Creator: Salminen, Justin; Papaiconomou, Nicolas; Kerr, John; Prausnitz,John & Newman, John
Partner: UNT Libraries Government Documents Department

Nanocrystal Solar Cells

Description: This dissertation presents the results of a research agenda aimed at improving integration and stability in nanocrystal-based solar cells through advances in active materials and device architectures. The introduction of 3-dimensional nanocrystals illustrates the potential for improving transport and percolation in hybrid solar cells and enables novel fabrication methods for optimizing integration in these systems. Fabricating cells by sequential deposition allows for solution-based assembly of hybrid composites with controlled and well-characterized dispersion and electrode contact. Hyperbranched nanocrystals emerge as a nearly ideal building block for hybrid cells, allowing the controlled morphologies targeted by templated approaches to be achieved in an easily fabricated solution-cast device. In addition to offering practical benefits to device processing, these approaches offer fundamental insight into the operation of hybrid solar cells, shedding light on key phenomena such as the roles of electrode-contact and percolation behavior in these cells. Finally, all-inorganic nanocrystal solar cells are presented as a wholly new cell concept, illustrating that donor-acceptor charge transfer and directed carrier diffusion can be utilized in a system with no organic components, and that nanocrystals may act as building blocks for efficient, stable, and low-cost thin-film solar cells.
Date: December 15, 2006
Creator: Gur, Ilan
Partner: UNT Libraries Government Documents Department

Nanoparticle modifications of photodefined nanostructures for energy applications.

Description: The advancement of materials technology towards the development of novel 3D nanostructures for energy applications has been a long-standing challenge. The purpose of this project was to explore photolithographically defineable pyrolyzed photoresist carbon films for possible energy applications. The key attributes that we explored were as follows: (1) Photo-interferometric fabrication methods to produce highly porous (meso, micro, and nano) 3-D electrode structures, and (2) conducting polymer and nanoparticle-modification strategies on these structures to provide enhanced catalytic capabilities and increase conductivity. The resulting electrodes were then explored for specific applications towards possible use in battery and energy platforms.
Date: October 1, 2011
Creator: Polsky, Ronen; Xiao, Xiaoyin; Burckel, David Bruce; Brozik, Susan Marie; Washburn, Cody M. & Wheeler, David Roger
Partner: UNT Libraries Government Documents Department

Investigating the Stability and Accuracy of the Phase Response for NOx Sensing 5% Mg-modified LaCrO3 Electrodes

Description: Impedance spectroscopy measurements were carried out on LaCr{sub 0.95}Mg{sub 0.05}O{sub 3} (LCM) asymmetric interdigitated electrodes supported on fully stabilized 8-mol% Y{sub 2}O{sub 3}-stabilized ZrO{sub 2} (YSZ) electrolytes. Experiments were carried out using 0-50 ppm NO{sub x}, 5-15% O{sub 2} with N{sub 2} as the balance, over temperatures ranging from 600-700 C. AC measurements taken at a constant frequency between 1-100 Hz indicated the phase response of the sensor was less sensitive to fluctuations in the O{sub 2} concentration and the baseline drift was limited. Specific frequencies were observed where the sensor response was essentially temperature independent.
Date: June 14, 2007
Creator: Murray, E P; Novak, R F; Kubinski, D J; Soltis, R E; Visser, J H; Woo, L Y et al.
Partner: UNT Libraries Government Documents Department

Water at an electrochemical interface - a simulation study

Description: The results of molecular dynamics simulations of the properties of water in an aqueous ionic solution close to an interface with a model metallic electrode are described. In the simulations the electrode behaves as an ideally polarizable hydrophilic metal, supporting image charge interactions with charged species, and it is maintained at a constant electrical potential with respect to the solution so that the model is a textbook representation of an electrochemical interface through which no current is passing. We show how water is strongly attracted to and ordered at the electrode surface. This ordering is different to the structure that might be imagined from continuum models of electrode interfaces. Further, this ordering significantly affects the probability of ions reaching the surface. We describe the concomitant motion and configurations of the water and ions as functions of the electrode potential, and we analyze the length scales over which ionic atmospheres fluctuate. The statistics of these fluctuations depend upon surface structure and ionic strength. The fluctuations are large, sufficiently so that the mean ionic atmosphere is a poor descriptor of the aqueous environment near a metal surface. The importance of this finding for a description of electrochemical reactions is examined by calculating, directly from the simulation, Marcus free energy profiles for transfer of charge between the electrode and a redox species in the solution and comparing the results with the predictions of continuum theories. Significant departures from the electrochemical textbook descriptions of the phenomenon are found and their physical origins are characterized from the atomistic perspective of the simulations.
Date: August 22, 2008
Creator: Willard, Adam; Reed, Stewart; Madden, Paul & Chandler, David
Partner: UNT Libraries Government Documents Department

Effect of electrode material and design on sensitivity and selectivity for high temperature impedancemetric NOx sensors

Description: Solid-state electrochemical sensors using two different sensing electrode compositions, gold and strontium-doped lanthanum manganite (LSM), were evaluated for gas phase sensing of NO{sub x} (NO and NO{sub 2}) using an impedance-metric technique. An asymmetric cell design utilizing porous YSZ electrolyte exposed both electrodes to the test gas (i.e., no reference gas). Sensitivity to less than 5 ppm NO and response/recovery times (10-90%) less than 10 s were demonstrated. Using an LSM sensing electrode, virtual identical sensitivity towards NO and NO{sub 2} was obtained, indicating that the equilibrium gas concentration was measured by the sensing electrode. In contrast, for cells employing a gold sensing electrode the NO{sub x} sensitivity varied depending on the cell design: increasing the amount of porous YSZ electrolyte on the sensor surface produced higher NO{sub 2} sensitivity compared to NO. In order to achieve comparable sensitivity for both NO and NO{sub 2}, the cell with the LSM sensing electrode required operation at a lower temperature (575 C) than the cell with the gold sensing electrode (650 C). The role of surface reactions are proposed to explain the differences in NO and NO{sub 2} selectivity using the two different electrode materials.
Date: September 23, 2009
Creator: Woo, L Y; Glass, R S; Novak, R F & Visser, J H
Partner: UNT Libraries Government Documents Department

Hybrid magnetic tunnel junction/spin filter device

Description: Surfaces and interfaces of complex oxides materials provide a rich playground for the exploration of novel magnetic properties not found in the bulk but also the development of functional interfaces to be incorporated into applications. We have recently been able to demonstrate a new type of hybrid spin filter/ magnetic tunnel junction. Our hybrid spin-filter/magnetic-tunnel junction devices are epitaxial oxide junctions of La{sub 0.7}Sr{sub 0.3}MnO{sub 3} and Fe{sub 3}O{sub 4} electrodes with magnetic NiMn{sub 2}O{sub 4} barrier layers. Depending on whether the barrier is in a paramagnetic or ferromagnetic state, the junction exhibits magnetic tunnel junction behavior where the spin polarized conduction is dominated by the electrode-barrier interface or spin filter behavior where conduction is dominated by barrier layer magnetism.
Date: July 10, 2008
Creator: Suzuki, Y.; Nelson-Cheeseman, B.; Wong, F.; Chopdekar, R.; Arenholz, E.; Chi, Miaofang et al.
Partner: UNT Libraries Government Documents Department

INFLUENCE OF TEMPERATURE ON THE CORROSION POTENTIAL OF THE 241-AN-102 MULTI PROBE CORROSION MONITORING SYSTEM SECONDARY REFERENCE ELECTRODES

Description: A test program using 241-AN-102 waste simulants and metallic secondary reference electrodes similar to those used on the 241-AN-102 MPCMS was performed to characterize the relationship between temperature and secondary reference electrode open-circuit corrosion potential. This program showed that the secondary reference electrodes can be used to make tank and tank steel corrosion potential measurements, but that a correction factor of approximately 2 mV per degree Celsius of temperature difference must be applied, where temperature difference is defined as the difference between tank temperature at the time of measurement and 30 C, the average tank temperature during the first several months of 241-AN-102 MPCMS operation (when the corrosion potentials of the secondary reference electrodes were being recorded relative to the primary reference electrodes).
Date: September 30, 2008
Creator: GL, EDGEMON & TM, TAYLOR
Partner: UNT Libraries Government Documents Department

Continuity and Performance in Composite Electrodes

Description: It is shown that the rate performance of a lithium battery composite electrode may be compromised by poor internal connectivity due to defects and inhomogeneities introduced during electrode fabrication or subsequent handling. Application of a thin conductive coating to the top surface of the electrode or to the separator surface in contact with the electrode improves the performance by providing alternative current paths to partially isolated particles of electroactive material. Mechanistic implications are discussed and strategies for improvement in electrode design and fabrication are presented.
Date: December 23, 2009
Creator: Chen, Guoying & Richardson, Thomas J.
Partner: UNT Libraries Government Documents Department

Overcharge Protection for 4 V Lithium Batteries at High Rates and Low Temperature

Description: Overcharge protection for 4 V Li{sub 1.05}Mn{sub 1.95}O{sub 4}/lithium cells at charging rates in excess of 1 mA/cm{sup 2} (3C) and at temperatures as low as -20 C was achieved using a bilayer separator coated with two electroactive polymers. High rate and low temperature overcharge protection and discharge performance were improved by employing a design in which the polymer-coated portion of the separator is in parallel with the cell rather than between the electrodes. The effects of different membrane supports for the electroactive polymers are also examined.
Date: January 4, 2010
Creator: Chen, Guoying & Richardson, Thomas J.
Partner: UNT Libraries Government Documents Department

Reduction of sample volume and waste generation in acid/base titrations using microelectrodes

Description: The Analytical Development Section (ADS) has developed microelectrode methods for use with pH titrations and pH determinations. These microelectrode methods offer increased sensitivity and enable analyses to be done with smaller sample and buffer volumes than are used with standard size electrodes. This report establishes the technical validity of the methods and describes the application of these methods to decreased detection limits, decreased waste generation, and decreased radiation exposure.
Date: March 22, 1996
Creator: Ekechukwu, A.A.
Partner: UNT Libraries Government Documents Department

Multiple Input Electrode Gap Control During Vacuum Arc Remelting

Description: Accurate control of the electrode gap in a vacuum arc remelting (VAR) furnace has been a goal of melters for many years. The size of the electrode gap has a direct influence on ingot solidification structure. At the high melting currents (30 to 40 kA) typically used for VAR of segregation insensitive Ti and Zr alloys, process voltage is used as an indicator of electrode gap, whereas drip-short frequency (or period) is usually used at the lower currents (5 to 8 kA) employed during VAR of superalloys. Modem controllers adjust electrode position or drive velocity to maintain a voltage or drip-short frequency (or period) set-point. Because these responses are non-linear functions of electrode gap and melting current, these controllers have a limited range for which the feedback gains are valid. Models are available that relate process voltage and drip-short frequency to electrode gap. These relationships may be used to linearize the controller feedback signal. An estimate of electrode gap may then be obtained by forming a weighted sum of the independent gap estimates obtained from the voltage and drip-short signals. By using multiple independent measures to estimate the gap, a controller that is less susceptible to process disturbances can be developed. Such a controller was designed, built and tested. The tests were carried out at Allvac Corporation during VAR of 12Cr steel at intermediate current levels.
Date: January 14, 1999
Creator: Beaman, J.J.; Hysinger, C.L.; Melgaard, D.K. & Williamson, R.L.
Partner: UNT Libraries Government Documents Department