Matching Items (22)

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Strain induced fragility transition in metallic glass

Description

Relaxation dynamics are the central topic in glassy physics. Recently, there is an emerging view that mechanical strain plays a similar role as temperature in altering the relaxation dynamics. Here,

Relaxation dynamics are the central topic in glassy physics. Recently, there is an emerging view that mechanical strain plays a similar role as temperature in altering the relaxation dynamics. Here, we report that mechanical strain in a model metallic glass modulates the relaxation dynamics in unexpected ways. We find that a large strain amplitude makes a fragile liquid become stronger, reduces dynamical heterogeneity at the glass transition and broadens the loss spectra asymmetrically, in addition to speeding up the relaxation dynamics. These findings demonstrate the distinctive roles of strain compared with temperature on the relaxation dynamics and indicate that dynamical heterogeneity inherently relates to the fragility of glass-forming materials.

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  • 2015-05-18

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Field Induced Changes in the Ring/Chain Equilibrium of Hydrogen Bonded Structures: 5-Methyl-3-Heptanol

Description

Using non-linear dielectric techniques, we have measured the dynamics of 5-methyl-3-heptanol at a temperature at which the Kirkwood correlation factor gK indicates the coexistence of ring- and chain-like hydrogen-bonded structures.

Using non-linear dielectric techniques, we have measured the dynamics of 5-methyl-3-heptanol at a temperature at which the Kirkwood correlation factor gK indicates the coexistence of ring- and chain-like hydrogen-bonded structures. Steady state permittivity spectra recorded in the presence of a high dc bias electric field (17 MV/m) reveal that both the amplitude and the time constant are increased by about 10% relative to the low field limit. This change is attributed to the field driven conversion from ring-like to the more polar chain-like structures, and a direct observation of its time dependence shows that the ring/chain structural transition occurs on a time scale that closely matches that of the dielectric Debye peak. This lends strong support to the picture that places fluctuations of the end-to-end vector of hydrogen bonded structures at the origin of the Debye process, equivalent to fluctuations of the net dipole moment or gK. Recognizing that changes in the ring/chain equilibrium constant also impact the spectral separation between Debye and α-process may explain the difference in their temperature dependence whenever gK is sensitive to temperature, i.e., when the structural motifs of hydrogen bonding change considerably.

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Date Created
  • 2016-08-16

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Novel anhydrous superprotonic ionic liquids and membranes for application in mid-temperature fuel cells

Description

This thesis studies three different types of anhydrous proton conducting electrolytes for use in fuel cells. The proton energy level scheme is used to make the first electrolyte which is

This thesis studies three different types of anhydrous proton conducting electrolytes for use in fuel cells. The proton energy level scheme is used to make the first electrolyte which is a rubbery polymer in which the conductivity reaches values typical of activated Nafion, even though it is completely anhydrous. The protons are introduced into a cross-linked polyphospazene rubber by the superacid HOTf, which is absorbed by partial protonation of the backbone nitrogens. The decoupling of conductivity from segmental relaxation times assessed by comparison with conductivity relaxation times amounts to some 10 orders of magnitude, but it cannot be concluded whether it is purely protonic or due equally to a mobile OTf- or H(OTf)2-; component. The second electrolyte is built on the success of phosphoric acid as a fuel cell electrolyte, by designing a variant of the molecular acid that has increased temperature range without sacrifice of high temperature conductivity or open circuit voltage. The success is achieved by introduction of a hybrid component, based on silicon coordination of phosphate groups, which prevents decomposition or water loss to 250ºC, while enhancing free proton motion. Conductivity studies are reported to 285ºC and full H2/O2 cell polarization curves to 226ºC. The current efficiency reported here (current density per unit of fuel supplied per sec) is the highest on record. A power density of 184 (mW.cm-2) is achieved at 226ºC with hydrogen flow rate of 4.1 ml/minute. The third electrolyte is a novel type of ionic liquids which is made by addition of a super strong Brønsted acid to a super weak Brønsted base. Here it is shown that by allowing the proton of transient HAlCl4, to relocate on a very weak base that is also stable to superacids, we can create an anhydrous ionic liquid, itself a superacid, in which the proton is so loosely bound that at least 50% of the electrical conductivity is due to the motion of free protons. The protic ionic liquids (PILs) described, pentafluoropyridinium tetrachloroaluminate and 5-chloro-2,4,6-trifluoropyrimidinium tetrachloroaluminate, might be the forerunner of a class of materials in which the proton plasma state can be approached.

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Date Created
  • 2013

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Elastic properties of molecular glass thin films

Description

This dissertation provides a fundamental understanding of the impact of bulk polymer properties on the nanometer length scale modulus. The elastic modulus of amorphous organic thin films is examined using

This dissertation provides a fundamental understanding of the impact of bulk polymer properties on the nanometer length scale modulus. The elastic modulus of amorphous organic thin films is examined using a surface wrinkling technique. Potential correlations between thin film behavior and intrinsic properties such as flexibility and chain length are explored. Thermal properties, glass transition temperature (Tg) and the coefficient of thermal expansion, are examined along with the moduli of these thin films. It is found that the nanometer length scale behavior of flexible polymers correlates to its bulk Tg and not the polymers intrinsic size. It is also found that decreases in the modulus of ultrathin flexible films is not correlated with the observed Tg decrease in films of the same thickness. Techniques to circumvent reductions from bulk modulus were also demonstrated. However, as chain flexibility is reduced the modulus becomes thickness independent down to 10 nm. Similarly for this series minor reductions in Tg were obtained. To further understand the impact of the intrinsic size and processing conditions; this wrinkling instability was also utilized to determine the modulus of small organic electronic materials at various deposition conditions. Lastly, this wrinkling instability is exploited for development of poly furfuryl alcohol wrinkles. A two-step wrinkling process is developed via an acid catalyzed polymerization of a drop cast solution of furfuryl alcohol and photo acid generator. The ability to control the surface topology and tune the wrinkle wavelength with processing parameters such as substrate temperature and photo acid generator concentration is also demonstrated. Well-ordered linear, circular, and curvilinear patterns are also obtained by selective ultraviolet exposure and polymerization of the furfuryl alcohol film. As a carbon precursor a thorough understanding of this wrinkling instability can have applications in a wide variety of technologies.

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Date Created
  • 2011

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Electric potential and field calculation of HVDC composite insulators by charge simulation method

Description

High Voltage Direct Current (HVDC) technology is being considered for several long distance point-to-point overhead transmission lines, because of their lower losses and higher transmission capability, when compared to AC

High Voltage Direct Current (HVDC) technology is being considered for several long distance point-to-point overhead transmission lines, because of their lower losses and higher transmission capability, when compared to AC systems. Insulators are used to support and isolate the conductors mechanically and electrically. Composite insulators are gaining popularity for both AC and DC lines, for the reasons of light weight and good performance under contaminated conditions. This research illustrates the electric potential and field computation on HVDC composite insulators by using the charge simulation method. The electric field is calculated under both dry and wet conditions. Under dry conditions, the field distributions along the insulators whose voltage levels range from 500 kV to 1200 kV are calculated and compared. The results indicate that the HVDC insulator produces higher electric field, when compared to AC insulator. Under wet conditions, a 500 kV insulator is modeled with discrete water droplets on the surface. In this case, the field distribution is affected by surface resistivity and separations between droplets. The corona effects on insulators are analyzed for both dry and wet conditions. Corona discharge is created, when electric field strength exceeds the threshold value. Corona and grading rings are placed near the end-fittings of the insulators to reduce occurrence of corona. The dimensions of these rings, specifically their radius, tube thickness and projection from end fittings are optimized. This will help the utilities design proper corona and grading rings to reduce the corona phenomena.

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Date Created
  • 2013

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High resolution spectroscopy, normal mode analysis, and Franck-Condon factors calculation of transition metal-containing molecules

Description

The objective of the present investigations is to experimentally determine the fundamental molecular properties of the transient metal containing pieces. The transient molecules have been generated using laser ablation production

The objective of the present investigations is to experimentally determine the fundamental molecular properties of the transient metal containing pieces. The transient molecules have been generated using laser ablation production technique and detected by using laser induced fluorescence technique. Ultra-high resolution spectra of the diatomic molecules, 87SrF, 135&137BaF, YbF, HfF, and IrSi were recorded at a resolution of approximately 30 Mhz. The fine and hyperfine structure of these molecules were determined for the ground and the excited state. The optical Stark splittings of 180HfF and IrSi were recorded and analyzed to determine the permanent electric dipole moments of the ground and the excited state. An effective Hamiltonian operator, including the rotational, centrifugal distortion, spin-orbit, spin-spin, spin-rotation, Λ-doubling, magnetic hyperfine and quadrupole interactions, and Stark effect, was employed to model and analyze the recorded spectra. The electronic spectra of the triatomic molecules, TiO2 and ZrO2, were recorded using pulsed dye laser, LIF, spectrometer at a resolution of 300MHz. These molecules have C2v symmetry. The harmonic frequencies, lifetime measurements were determined. These spectra of ZrO2 and TiO2 were modeled using a normal coordinate analysis and Franck-Condon factor predictions. High resolution field-free and Stark effect spectra of ZrO2 were recorded and for future investigation.

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Created

Date Created
  • 2013

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Evaluation of room temperature vulcanized (RTV) silicone rubber coated porcelain post insulators under contaminated conditions

Description

This thesis concerns the flashover issue of the substation insulators operating in a polluted environment. The outdoor insulation equipment used in the power delivery infrastructure encounter different types of pollutants

This thesis concerns the flashover issue of the substation insulators operating in a polluted environment. The outdoor insulation equipment used in the power delivery infrastructure encounter different types of pollutants due to varied environmental conditions. Various methods have been developed by manufacturers and researchers to mitigate the flashover problem. The application of Room Temperature Vulcanized (RTV) silicone rubber is one such favorable method as it can be applied over the already installed units. Field experience has already showed that the RTV silicone rubber coated insulators have a lower flashover probability than the uncoated insulators. The scope of this research is to quantify the improvement in the flashover performance. Artificial contamination tests were carried on station post insulators for assessing their performance. A factorial experiment design was used to model the flashover performance. The formulation included the severity of contamination and leakage distance of the insulator samples. Regression analysis was used to develop a mathematical model from the data obtained from the experiments. The main conclusion drawn from the study is that the RTV coated insulators withstood much higher levels of contamination even when the coating had lost its hydrophobicity. This improvement in flashover performance was found to be in the range of 20-40%. A much better flashover performance was observed when the coating recovered its hydrophobicity. It was also seen that the adhesion of coating was excellent even after many tests which involved substantial discharge activity.

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Date Created
  • 2013

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Evaluation of epoxy nanocomposites for high voltage insulation

Description

Polymeric materials containing nanometer (nm) size particles are being introduced to provide compact shapes for low and medium voltage insulation equipment. The nanocomposites may provide superior electrical performance when compared

Polymeric materials containing nanometer (nm) size particles are being introduced to provide compact shapes for low and medium voltage insulation equipment. The nanocomposites may provide superior electrical performance when compared with those available currently, such as lower dielectric losses and increased dielectric strength, tracking and erosion resistance, and surface hydrophobicity. All of the above mentioned benefits can be achieved at a lower filler concentration (< 10%) than conventional microfillers (40-60%). Also, the uniform shapes of nanofillers provide a better electrical stress distribution as compared to irregular shaped microcomposites which can have high internal electric stress, which could be a problem for devices with active electrical parts. Improvement in electrical performance due to addition of nanofillers in an epoxy matrix has been evaluated in this work. Scanning Electron Microscopy (SEM) was done on the epoxy samples to confirm uniform dispersion of nano-sized fillers as good filler dispersion is essential to realize the above stated benefits. Dielectric spectroscopy experiments were conducted over a wide range of frequencies as a function of temperature to understand the role of space charge and interfaces in these materials. The experiment results demonstrate significant reduction in dielectric losses in samples containing nanofillers. High voltage experiments such as corona resistance tests were conducted over 500 hours to monitor degradation in the samples due to corona. These tests revealed improvements in partial discharge endurance of nanocomposite samples. These improvements could not be adequately explained using a macroscopic quantity such as thermal conductivity. Thermo gravimetric analysis (TGA) showed higher weight loss initiation temperatures for nanofilled samples which is in agreement with the corona resistance experimental results. Theoretical models have also been developed in this work to complement the results of the corona resistance experiment and the TGA analysis. Degradation model was developed to map the erosion path using Dijkstra's shortest path algorithm. A thermal model was developed to calculate the localized temperature distribution in the micro and nano-filled samples using the PDE toolbox in MATLAB. Both the models highlight the fact that improvement in nanocomposites is not limited to the filler concentrations that were tested experimentally.

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Date Created
  • 2012

Temperature and polarizability effects on electron transfer in biology and artificial photosynthesis

Description

This study aims to address the deficiencies of the Marcus model of electron transfer

(ET) and then provide modifications to the model. A confirmation of the inverted energy

gap law, which is

This study aims to address the deficiencies of the Marcus model of electron transfer

(ET) and then provide modifications to the model. A confirmation of the inverted energy

gap law, which is the cleanest verification so far, is presented for donor-acceptor complexes.

In addition to the macroscopic properties of the solvent, the physical properties of the solvent

are incorporated in the model via the microscopic solvation model. For the molecules

studied in this dissertation, the rate constant first increases with cooling, in contrast to the

prediction of the Arrhenius law, and then decreases at lower temperatures. Additionally,

the polarizability of solute, which was not considered in the original Marcus theory, is included

by the Q-model of ET. Through accounting for the polarizability of the reactants, the

Q-model offers an important design principle for achieving high performance solar energy

conversion materials. By means of the analytical Q-model of ET, it is shown that including

molecular polarizability of C60 affects the reorganization energy and the activation barrier

of ET reaction.

The theory and Electrochemistry of Ferredoxin and Cytochrome c are also investigated.

By providing a new formulation for reaction reorganization energy, a long-standing disconnect

between the results of atomistic simulations and cyclic voltametery experiments is

resolved. The significant role of polarizability of enzymes in reducing the activation energy

of ET is discussed. The binding/unbinding of waters to the active site of Ferredoxin leads

to non-Gaussian statistics of energy gap and result in a smaller activation energy of ET.

Furthermore, the dielectric constant of water at the interface of neutral and charged

C60 is studied. The dielectric constant is found to be in the range of 10 to 22 which is

remarkably smaller compared to bulk water( 80). Moreover, the interfacial structural

crossover and hydration thermodynamic of charged C60 in water is studied. Increasing the

charge of the C60 molecule result in a dramatic structural transition in the hydration shell,

which lead to increase in the population of dangling O-H bonds at the interface.

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Date Created
  • 2019

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Molecular polarizability as a descriptor for molecular conductance

Description

We studied the relationship between the polarizability and the molecular conductance

that arises in the response of a molecule to an external electric field. To illustrate

the plausibility of the idea, we

We studied the relationship between the polarizability and the molecular conductance

that arises in the response of a molecule to an external electric field. To illustrate

the plausibility of the idea, we used Simmons' tunneling model, which describes image

charge and dielectric effects on electron transport through a barrier. In such a

model, the barrier height depends on the dielectric constant of the electrode-molecule-electrode junction, which in turn can be approximately expressed in terms of the

molecular polarizability via the classical Clausius-Mossotti relation. In addition to

using the tunneling model, the validity of the relationships between the molecular

polarizability and the molecular conductance was tested by comparing calculated

and experimentally measured conductance of different chemical structures ranging

from covalent bonded to non-covalent bonded systems. We found that either using

the tunneling model or the first-principle calculated quantities or experimental data,

the conductance decreases as the molecular polarizability increases. In contrast to

this strong correlation, our results showed that in some cases there was a weaker or

none correlation between the conductance and other molecular electronic properties

including HOMO-LUMO gap, chemical geometries, and interactions energies. All

these results together suggest that using the molecular polarizability as a molecular

descriptor for conductance can offer some advantages compared to using other

molecular electronic properties and can give additional insight about the electronic

transport property of a junction.

These results also show the validity of the physically intuitive picture that to a first

approximation a molecule in a junction behaves as a dielectric that is polarized in the

opposite sense of the applied bias, thereby creating an interfacial barrier that hampers

tunneling. The use of the polarizability as a descriptor of molecular conductance offers

signicant conceptual and practical advantages over a picture based in molecular

orbitals. Despite the simplicity of our model, it sheds light on a hitherto neglected

connection between molecular polarizability and conductance and paves the way for

further conceptual and theoretical developments.

The results of this work was sent to two publications. One of them was accepted

in the International Journal of Nanotechnology (IJNT) and the other is still under

review in the Journal of Physical Chemistry C.

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Created

Date Created
  • 2014