Ultrafast High-Energy Electron Diffraction Study of Photoexcited Bismuth Nanoclusters by Femtosecond Laser Pulses

The advancement in ultrafast electron crystallography (UEC) over the past few decades facilitated the study of structural dynamics in all phases of matter induced by femtosecond laser pulses. This technique became very powerful when the spatial resolution was combined with the temporal resolution, and succeeded in studying chemical reactions by ultrafast electron diffraction, bulk crystal phonons and melting by X-ray diffraction. In this dissertation, I demonstrate the uniqueness of UEC and its potential in monitoring in real time the structural dynamics of bismuth (Bi) nanoclusters and islands induced by femtosecond laser pulses. Our approach to accomplish this task includes building a time-resolved high energy electron diffraction setup that is capable of delivering high energy and short electron pulses, less than 3 ps, which will facilitate the real time measurement of the Bragg diffraction ring intensity, shift in the peak position and the diffraction ring full width at half maximum (FWHM) at different delay times with respect to the femtosecond excitation. Additionally, the temperature evolution of the same parameters, intensity, position and FWHM of the diffraction peaks, was monitored by using conventional direct current heating stage. Another task was accomplished in which I utilized the pump-probe ultrafast electron diffraction setup that I built and tested with picoseconds laser pulses in PERI lab - Old Dominion University, transferred later to the Applied Research Center where a femtosecond laser system was used to characterize the transient effects induced in Bi nanoclusters due to femtosecond laser excitation. The sample under consideration is excited by a femtosecond laser pulses with moderate fluence just to induce an observable change in the diffraction pattern and far from sample damage. The femtosecond laser pulses induce changes in the charge carrier distribution function of Bi nanoclusters, which leads to a disturbance in the lattice potential and drives the solid-liquid phase transformation. The melting is detected as decrease in the integrated intensity of the Bragg peaks with time delay. Another interesting behavior is observed in these experiments in which a lattice contraction following femtosecond laser excitation and proceeding over a time period of ∼ 6ps precedes the lattice expansion in Bi (012) planes. Again, the electronic excitation, here, plays an important role in inducing a sudden change in the interatomic forces which leads to A1g phonon excitation. Due to the limited resolution of our system (2–3 ps) we were not able to detect the A1g oscillation frequency/wavelength, but its effects which appear as lattice contraction upon its decay can be seen from the temporal evolution of the Bragg peak position over the time period, 0 \u3c t \u3c 6 ps. The incident laser fluence was not high enough to induce full melting, but was enough to induce partial lattice melting. This was observed as a gradual increase in the FWHM of the Bragg peaks as a function of delay time, i.e., formation of thin liquid layer which increases in size with time when the lattice temperature increases through electron-phonon and/or phonon-phonon relaxation. Also, the time evolution of the relative Bragg peak intensity, Δd/d and FWHM were monitored for Bi islands. Bi islands were prepared by annealing the as-deposited Bi thin film (5 nm, average coverage) solely by either raising its temperature slowly up to ∼ 525 K or with ultrafast laser pulses of fluence 0.8 mJ/cm² - 2.4 mJ/cm²

A Theoretical And Experimental Investigation Of Coke Gasification In A Batch Reactor

A computer model has been developed to simulate the char-gasification process occurring in the gasification component of a crossflow coal gasifier. This process corresponds to the batch gasification process in a combustion pot. Temperature and concentration profiles along the gasifier were obtained by solving the material and energy balances. In order to obtain the data necessary for evaluating and improving the computer model, an experimental system was developed to obtain data for the char-gasification process in the combustion pot. The results show that the gasification rate strongly depends on the air flow rate and that the reactivity of the char has a strong effect on the output gas composition. The results from the computer model were compared with available literature data on the fixed-bed gasifier and the experimental data obtained from this study, and were found to be in good agreement. A sensitivity analysis was performed on the input parameters (heat transfer coefficient, void fraction, particle diameter, and reactivity factor coefficients) of the computer model. The results show the importance of the input parameters in predicting the desired gas composition and total process time

Anisotropic Response of Nanosized Bismuth Films Upon Femtosecond Laser Excitation Monitored by Ultrafast Electron Diffraction

The lattice response of 5 nm thick bismuth film to femtosecond laser excitation is probed by ultrafast electron diffraction. The transient decay time after laser excitation is greater for diffraction from (012) lattice planes compared to (110) planes and is reduced for both planes with the increased laser fluence. These results indicate that different energy coupling mechanisms to the lattice occur depending on the crystal direction. The behavior of the diffraction peak width indicates partial disorder of the film upon photoexcitation that increases together with the laser fluence. © 2011 American Institute of Physics. [doi:10.1063/1.3652919

Management of information technology in Swedish firms: An empirical study

This study examines the role and the involvement of management in information technology acquisition, planning, and development. The results indicate that top management plays a major role in setting the overall information technology policies. In addition, the Chief Information Officer (CIO) plays a crucial role in the merger of information strategy and corporate strategy. The result also indicates that user\u27s satisfaction is considered to be the most important r factor for justification of information technology acquisition

Transient Characterization of Type B Particles in a Transport Riser

Simple and rapid dynamic tests were used to evaluate fluid dynamic behavior of granular materials in the transport regime. Particles with densities ranging from 189 to 2,500 kg/m3 and Sauter mean size from 61 to 812 μm were tested in a 0.305 m diameter, 15.5 m height circulating fluidized bed (CFB) riser. The transient tests involved the abrupt stoppage of solids flow for each granular material over a wide range gas flow rates. The riser emptying time was linearly related to the Froude number in each of three different operating regimes. The flow structure along the height of the riser followed a distinct pattern as tracked through incremental pressures. These results are discussed to better understand the transformations that take place when operating over various regimes. During the transients the particle size distribution was measured. The effects of pressure, particle size, and density on test performance are also presented

Applications of tribology and fracture mechanics to determine wear and impact attrition of particulate solids in CFB systems

In recent years, much attention has been focused on the development of novel technologies for carbon capture and chemicals production that utilize a circulating fluidized bed configuration; examples include chemical looping combustion and circulation of temperature swing adsorbents in a CFB configuration for CO2 capture. A major uncertainty in determining the economic feasibility of these technologies is the required solids makeup rate, which, among other factors, is due to impact and wear attrition at various locations, including standpipes, cyclones, and the gas jets in fluid beds. While correlations have been developed that estimate the attrition rates at these areas, these correlations are dependent on constants that are an unknown function of the solid properties and system. Thus, it is difficult to determine the attrition rate a priori without performing extensive experiments on the materials or scaling up entirely. In this work, the authors apply knowledge of fundamental material properties from fields of tribology (the study of wear) and fracture mechanics to the knowledge of forces and sliding distances determined from hydrodynamic models to develop basic attrition models for novel CFB systems. The equations are derived for common equipment found in CFBs, and the equations are compared to experimental data of attrition in the literature

Flow Regime Study in a High Density Circulating Fluidized Bed Riser with an Abrupt Exit

Flow regime study was conducted in a 0.3 m diameter, 15.5 m height circulating fluidized bed (CFB) riser with an abrupt exit at the National Energy Technology Laboratory of the U. S. Department of Energy. Local particle velocities were measured at various radial positions and riser heights using an optical fiber probe. On-line measurement of solid circulating rate was continuously recorded by the Spiral. Glass beads of mean diameter 61 μm and particle density of 2,500 kg/m3 were used as bed material. The CFB riser was operated at various superficial gas velocities ranging from 3 to 7.6 m/s and solid mass flux from 20 to 550 kg/m2-s. At a constant riser gas velocity, transition from fast fluidization to dense suspension upflow (DSU) regime started at the bottom of the riser with increasing solid flux. Except at comparatively low riser gas velocity and solid flux, the apparent solid holdup at the top exit region was higher than the middle section of the riser. The solid fraction at this top region could be much higher than 7% under high riser gas velocity and solid mass flux. The local particle velocity showed downward flow near the wall at the top of the riser due to its abrupt exit. This abrupt geometry reflected the solids and, therefore, caused solid particles traveling downward along the wall. However, at location below, but near, the top of the riser the local particle velocities were observed flowing upward at the wall. Therefore, DSU was identified in the upper region of the riser with an abrupt exit while the fully developed region, lower in the riser, was still exhibiting core-annular flow structure. Our data were compared with the flow regime boundaries proposed by Kim et al. [1] for distinguishing the dilute pneumatic transport, fast fluidization, and DSU

First documented presence of Galeocerdo cuvier (Péron & Lesueur, 1822) (ELASMOBRANCHII, CARCHARHINIDAE) in the Mediterranean basin (Libyan waters)

One male and one female specimen of tiger shark, Galeocerdo cuvier (Peron & Lesueur, 1822), were accidentally caught by a drifting longline for swordfish in the south Mediterranean (Libyan waters). This finding confirms beyond any doubt that the tiger shark may be encountered in the waters of the Mediterranean Sea. Although records of this species has previously been reported, the information is partial or dubious, due to the lack of a description of the individuals found or the uncertain provenance of preserved material. Our finding confirms the record of this species in the southern part of the Mediterranean basin. Images, as well as morphometrics and information on stomach contents are given. Based on the size of the individuals, it is considered that the two specimens were born recently, presumably inside the Mediterranean Sea and likely close to the area where the individuals were found

Urban Form and Real Estate Value in Msheireb Downtown Doha, Qatar

In the late 20th century, Doha’s rapid urbanization and globalization led to the loss of housing and the compact, traditional urban fabric in the old city center. The Qatari government and Msheireb Properties developed Msheireb Downtown Doha to bring urban living back with a contemporary re-interpretation of the traditional urban fabric and modern life conveniences. Our study's primary objective is to investigate the relationship between urban form and the rental value of residential units, identifying factors that might influence rental asking prices. The paper examines morphological characteristics through field surveys and the real-estate variables such as location, floor area, number of bedrooms/bathrooms, and asking price collected from publicly available real estate websites. The findings indicate that the residential units' layout and adjacent streets' morphological characteristics clearly define specific targeted user groups. Larger residential units target Qatari families via more bedrooms/bathrooms and quieter urban settings, emphasizing Islamic cultural values. Smaller units target ex-pat workers (especially Westerners) using open-plan layouts in more lively urban environments of the development. The price per square meter also increases for residential units closer to the Doha Metro station. The study reiterates the success of compact living for improving urban living in other neighborhoods of old Doha