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An empirical investigation of the salient dimensions of Baby Boomer and Generation Y consumers' health care decision choices.

Description: The purpose of this research is to empirically investigate consumers' health care decision choices in a dynamic market setting. The unprecedented demands on the U.S. health care system coupled with the mounting controversies surrounding health care reform suggest that consumers' health care decisions warrant empirical research attention. Toward this end, this dissertation empirically explored (1) the characteristics of consumers who possess a willingness to use non-conventional treatments over conventional treatments, (2) the characteristics of consumers who elect self-medication in lieu of health care practitioner-directed medication, and (3) the salient dimensions of consumers' channel choice for the procurement of health care products. Each of these decision choice factors were tested across two U.S. generational segments to assess whether differences existed across Baby Boomers' and Gen Yers' health care decision choices. The conceptual framework for empirical assessment is Bandura's (1986) social cognitive theory. From Bandura's social cognitive theory, a general model of healthcare decision choice is proposed to assess consumers' states of mind, states of being and states of action (decision choice). Results indicate that social cognitive factors (e.g., self-efficacy, objectivism) play an important role in each of the decision domains explored in this dissertation. Moreover, health value was found to be an important moderator between the social cognitive factors and health care decision choices. The predictors of the health care decision choices were found to vary across the Baby Boomers and Generation Yers on several dimensions, confirming the notion that generational differences may be a salient dimension of consumers' health care decision choice. The research offers several implications for practitioners, academicians and policy makers. Both descriptive and normative implications are gleaned from the research findings. Most notably, the results indicate that consumers' social cognitive factors and health value may be mechanisms for managing health care decisions.
Date: August 2006
Creator: Krishnankutty Nair Rajamma, Rajasree
Partner: UNT Libraries

Constraints on Adoption of Innovations: Internet Availability in the Developing World.

Description: In a world that is increasingly united in time and distance, I examine why the world is increasingly divided socially, economically, and digitally. Using data for 35 variables from 93 countries, I separate the countries into groups of 31 each by gross domestic product per capita. These groups of developed, lesser developed and least developed countries are used in comparative analysis. Through a review of relevant literature and tests of bivariate correlation, I select eight key variables that are significantly related to information communication technology development and to human development. For this research, adoption of the Internet in the developing world is the innovation of particular interest. Thus, for comparative purposes, I chose Internet Users per 1000 persons per country and the Human Development Index as the dependent variables upon which the independent variables are regressed. Although small in numbers among the least developed countries, I find Internet Users as the most powerful influence on human development for the poorest countries. The research focuses on key obstacles as well as variables of opportunity for Internet usage in developing countries. The greatest obstacles are in fact related to Internet availability and the cost/need ratio for infrastructure expansion. However, innovations for expanded Internet usage in developing countries are expected to show positive results for increased Internet usage, as well as for greater human development and human capital. In addition to the diffusion of innovations in terms of the Internet, the diffusion of cultures through migration is also discussed in terms of the effect on social capital and the drain on human capital from developing countries.
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Date: December 2006
Creator: Stedman, Joseph B.
Partner: UNT Libraries

Connective Technology Adoption in the Supply Chain: The Role of Organizational, Interorganizational and Technology-Related Factors.

Description: Supply chain management (SCM) is an area that offers organizations significant opportunities for both cost reductions and revenue enhancement. In their article, "Supply Chain Management: Implementation Issues and Research Opportunities," Lambert, Cooper and Pagh defined SCM as the "integration of key business processes from end user through original suppliers that provides products, services, and information that add value for customers and other stakeholders." Adopting and implementing appropriate technology has emerged as a source of competitive advantage for supply chain member firms through the integration of business processes with suppliers and customers. It is important to understand the factors influencing an organization's decision to acquire such technology. In the context of this study, connective technologies are defined as wireless communication devices and their accompanying infrastructure and software which may enhance coordination among supply chain partners. Building on previous literature in the areas of supply chain management, marketing strategy, and organizational innovation, a model was developed to test the relationships between organizational, interorganizational, and technology-related factors and the adoption of advanced connective technology, using radio frequency identification (RFID) as the test case, in the supply chain. A Web-based survey of supply chain professionals was conducted resulting in 224 usable responses. The overall model was statistically significant with four of the predictors significantly influencing the adoption of RFID in the supply chain. Size, centralization, new product advantage and time to achieve targeted ROI were significantly related to adoption of connective technology (RFID). Interorganizational related factors were not significant predictors of connective technology adoption. The study contributes to theory by testing scales from marketing and management in a supply chain context in order to better understand behavioral dimensions of supply chain management and logistics. The conceptualization and measurement of market orientation at the interfirm level advances the market orientation literature. Finally, the study contributes ...
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Date: May 2006
Creator: Neeley, Concha Kaye Ramsey
Partner: UNT Libraries

Correspondence Between Verbal Behavior About Reinforcers and Performance Under Schedules of Reinforcement.

Description: Important advancements have been made in the identification of reinforcers over the past decade. The use of preference assessments has become a systematic way to identify preferred events that may function as reinforcers for an individual's behavior. Typically, preference assessments require participants to select stimuli through verbal surveys or engagement with stimuli as preferred or non-preferred. Not all studies go on to directly test the effects of the preferred stimuli, and even fewer studies directly test for the effects of the non- preferred stimuli. The present study systematically identified preferred and non-preferred stimuli in adult human subjects by verbal report and then proceeded to test the effects of both verbally reported preferred and non preferred events on single and concurrent schedules of reinforcement. The results are discussed in terms of contemporary concerns regarding preference and reinforcer assessments.
Date: August 2006
Creator: Bekker-Pace, Ruthie
Partner: UNT Libraries

Current Practices in Working With Special Education Paraeducators.

Description: With so many paraeducators working in special education, it is important for teachers, administrators, and researchers to know how paraeducators are being utilized, supervised, and managed in order to create the most effective programs for students with special needs. Research is needed regarding current practices in supervising paraeducators. The purposes of this study were to (a) delineate the current practices being utilized by special education teachers of students with emotional and behavioral disorders (EBD) who supervise paraeducators that work with students with EBD in the general education classroom and (b) determine how effective the supervised paraeducators perceive those practices to be. Current practices were revealed by answering the following questions: (1) According to special education teachers and paraeducators, what procedures and practices are being utilized to supervise paraeducators who work in the general education environment with students with EBD? (2) In what ways do teachers and paraeducators see these supervision practices as being effective? (3) What is the relationship between actual supervision practices and accepted best practices? There were 60 participants in all, 30 professional teachers and 30 paraeducators. All 60 participants completed a survey; of these 60, 5 teachers and 5 paraeducators were individually interviewed Findings from the study indicate that actual supervision practices of teachers do not represent the best practices found in the literature. The study found that each of the seven executive functions of supervision (orientation, planning, scheduling, delegating, training/coaching, monitoring/feedback, and managing the workplace) need additional attention from school districts in order to maximize paraeducator effectiveness.
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Date: December 2006
Creator: Asel, Crystal S.
Partner: UNT Libraries

Differential Scoring Patterns on the Clock Drawing Test: a Comparison of Vascular Dementia and Alzheimer's Dementia.

Description: This study examined differences in scoring patterns among those diagnosed with Alzheimer's dementia and vascular dementia on the clock-drawing test. Archival clock drawing data was retrieved on 279 patients presenting at a county hospital-based memory clinic. Analysis of drawings was based on frequency of qualitative errors, as well as an overall quantitative score. Mean comparisons found those patients with Alzheimer's dementia to perform worse on both quantitative and qualitative scoring measures. However, Pearson's chi-squared test revealed a significantly higher rate of spacing errors among subjects with vascular dementia. Such lends support to my hypothesis that impaired executive functioning in vascular dementia patients would lead to poor qualitative performance. Logistic regression found significant predictive ability for the qualitative criteria in diagnosis (χ2 = 25.49, p < .001), particularly the rate of omission (z = 8.96, p = .003) and addition errors (z = 7.58, p = .006). Such findings hold important implications for the use of qualitative criteria in cognitive screening assessments.
Date: May 2006
Creator: Everitt, Alaina
Partner: UNT Libraries

The Effect of Constructivist Learning Environments on Student Learning in an Undergraduate Art Appreciation Course.

Description: The purpose of this study was to determine the effects of constructivist methods on student learning in an undergraduate art appreciation class. Three constructivist learning activities were designed and implemented in an undergraduate art appreciation course for non-art majors at Mississippi College. Through these constructivist learning activities, students were involved in their learning throughout the semester in realistic art roles in which they worked as curators, Web page designers, and artists. Six subjects were selected to participate in this case study. Subject data was collected through three methods: interviews with subjects at three points during the semester, student documents produced during the three activities, and a field journal of observations made during the activities. The multiple data sources were triangulated to reveal nine patterns of learning. The data evidence that constructivism results in a deeper understanding of art and art processes than in a typical art appreciation course in which learners are merely passive recipients of knowledge. This was not only indicated by the nine patterns of learning which emerged from the data, but also in the students' awareness and regulating of their cognitive processes. Although the research provided an in-depth understanding of this case and should not represent or be generalized to the entire population of art appreciation students, the results of this study suggest that art appreciation instructors have an opportunity to facilitate high levels of student thinking and encourage metacognitive skills through constructivist methods such as the ones used in this study.
Date: August 2006
Creator: Busbea, Stephanie Dickson
Partner: UNT Libraries

Effects of Training Accurate Component Strokes Using Response Constraint and Self-evaluation on Whole Letter Writing.

Description: This study analyzed the effects of a training package containing response constraint, self-evaluation, reinforcement, and a fading procedure on written letter components and whole letter writing in four elementary school participants. The effect on accuracy of written components was evaluated using a multiple-baseline-across components and a continuous probe design of components, as well as pre-test, baseline, and post-test measures. The results of this study show that response constraint and self-evaluation quickly improved students' performance in writing components. Fading of the intervention was achieved quickly and performance maintained. Results also show that improvement in component writing improved whole letter and full name writing and letter reversals in the presence of a model were corrected.
Date: December 2006
Creator: Cline, Tammy Lynn
Partner: UNT Libraries

The Effects of Tests and Praise on Children's Hear-write and See-say Responses.

Description: Four elementary school children were tested on 120 words containing the short e (e.g., ten, pen) and short a (e.g., tan, pan) sounds. Words were tested in the hear-write (H/W) and see-say (S/S) channels. No programmed consequences were scheduled during baseline (BL) tests 1-3. After BL, an error analysis categorized words based on channel error and topography of error. Praise was delivered during tests 4-6 for correct responses. Children's responses were variable within channel and across channels for a majority of words. By the end of the praise phase, there was a decrease in the number of words with errors, for all children in their error word group. Error topographies began to stabilize for some words during praise.
Date: December 2006
Creator: Edwards, Bobbie
Partner: UNT Libraries

The Effects of Using Children's Literature with Adolescents in the English As a Foreign Language Classroom.

Description: This study provides quantitative and qualitative data about the effects of using children's literature with adolescents in a language classroom and the role of children's literature in students' second/foreign language development, including listening, speaking, reading, and writing skills. The study presents qualitative data about the role of children's literature in developing more positive attitudes toward reading in the second/foreign language and toward reading in general. With literature being a model of a culture, presenting linguistic benefits for language learners, teaching communication, and being a motivator in language learning, this study presents empirical data that show that inclusion of children's literature in adolescents' second/foreign language classroom promotes appreciation and enjoyment of literature, enhances the development of language skills, stimulates more advanced learning, and promotes students' personal growth.
Date: December 2006
Creator: Belsky, Stella
Partner: UNT Libraries

The Effects of ARCS-based Confidence Strategies on Learner Confidence and Performance in Distance Education.

Description: The purpose of this research was to manipulate the component of confidence found in Keller's ARCS model to enhance the confidence and performance of undergraduate students enrolled in an online course at a Texas university using SAM 2003 software delivery. This study also tested whether the aforementioned confidence tactics had any unintentional effect on the remaining attention, relevance, and satisfaction subscales of the ARCS model as well as on learners' overall motivation for the class and the instructional materials. This study was conducted over a 5.5-week period with an initial sample of 81 total students. Two quantitative surveys were used to measure confidence and motivation: (a) the Course Interest Survey (CIS), and (b) the Instructional Materials Motivation Survey (IMMS). The results indicated that the treatment group showed statistically greater gains than the control group in terms of learner confidence on the CIS but not the IMMS. In terms of performance, the treatment group outperformed the control group on all of the individual posttest measures and on the overall aggregate mean performance score. The results showed no statistically significant difference on the attention subsection of the ARCS model. However, statistically significant differences were noted for the relevance and satisfaction subscales of the model. There was also a statistically significant difference in overall learner motivation as measured on both surveys. This research study suggests the feasibility of improving overall learner motivation and performance through external conditions such as systematically applied confidence tactics. The research further supports claims about the effectiveness of the ARCS model as a viable tool for enhancing online learner motivation and performance. What was unclear in this study was whether individual subsections of the ARCS model, such as confidence, can be independently manipulated.
Date: May 2006
Creator: Huett, Jason Bond
Partner: UNT Libraries

The Effects of an Inquiry-based American History Program on the Achievement of Middle School and High School Students.

Description: Implicit in the call for educational reform in the teaching of social studies has been the suggestion that pursuing inquiry-based principles will lead to improvement in student achievement. The purpose of this study was to compare the effectiveness of two types of pedagogy: traditional and inquiry-based upon student achievement as measured by a standards-based, state administered examination. Second, this study examined the relationship between the treatment teachers' level of implementation and student achievement. A nonequivalent control group posttest and experimental design was used in this study. Subjects involved in this study include 84 secondary American history teachers and their respective students from a large urban public school district in Texas. The sample consisted of two groups, one taught by traditional/didactic instruction (n=48) and the other taught by inquiry-based pedagogy (n=36). Data for this study were collected using a classroom observation protocol based upon the level of use rubric developed by the concerns-based adoption model. An analysis of variance (ANOVA) (p<.05) was used to measure the effects of inquiry-based instruction and traditional pedagogy on student achievement. Student achievement results were measured by the Texas Assessment of Knowledge and Skills (TAKS) for American history, grades 8 and 11. The study found that mean scores of the Grade 8 History Alive! group were significantly higher than the scores of the control group, but not for the Grade 11 History Alive! group. However, a comparison of mean scores by teachers' level-of-use suggested that the more faithful the teacher in designing standards-based lessons and delivering them through inquiry, the greater retention of American history student's knowledge about the subject.
Date: May 2006
Creator: Harmon, Larry G.
Partner: UNT Libraries

Accuracy Variations in Human Facial Identification Based on Time of Exposure.

Description: This study examined the relationship between time of exposure to the human face and accurate subsequent photo line-up identification. A volunteer group of 124 undergraduate students was divided into three approximately equal sized subgroups. The three groups were then exposed to a video or a portion of a video depicting a theft. Exposure times ranged from two minutes to 30 seconds. The subjects were then given a questionnaire and shown a photo line-up of the mock perpetrator and five foils. Subjects were asked to identify the perpetrator and mark that identification on the questionnaire. Results of the experiment indicated that the longer a subject was exposed the greater the possibility of an accurate identification.
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Date: December 2006
Creator: Cowle, Kenneth M.
Partner: UNT Libraries

Search for Resonances in the Photoproduction of Proton-Antiproton Pairs

Description: Results are reported on the reaction {gamma}p {yields} p{bar p}p with beam energy in the range 4.8-5.5 GeV. The data were collected at the Thomas Jefferson National Accelerator Facility in CLAS experiment E01-017(G6C). The focus of this study is an understanding of the mechanisms of photoproduction of proton-antiproton pairs, and to search for intermediate resonances, both narrow and broad, which decay to p{bar p}. The total measured cross section in the photon energy range 4.8-5.5 GeV is {sigma} = 33 {+-} 2 nb. Measurement of the cross section as a function of energy is provided. An upper limit on the production of a narrow resonance state previously observed with a mass of 2.02 GeV/c{sup 2} is placed at 0.35 nb. No intermediate resonance states were observed. Meson exchange production appears to dominate the production of the proton-antiproton pairs.
Date: June 30, 2006
Creator: Stokes, Burnham
Partner: UNT Libraries Government Documents Department

High Pressure Scanning Tunneling Microscopy and High PressureX-ray Photoemission Spectroscopy Studies of Adsorbate Structure,Composition and Mobility during Catalytic Reactions on A Model SingleCrystal

Description: Our research focuses on taking advantage of the ability of scanning tunneling microscopy (STM) to operate at high-temperatures and high-pressures while still providing real-time atomic resolution images. We also utilize high-pressure x-ray photoelectron spectroscopy (HPXPS) to monitor systems under identical conditions thus giving us chemical information to compare and contrast with the structural and dynamic data provided by STM.
Date: May 12, 2006
Creator: Montano, M.O.
Partner: UNT Libraries Government Documents Department

Structural Investigations of Surfaces and Orientation-SpecificPhenomena in Nanocrystals and Their Assemblies

Description: Studies of colloidal nanocrystals and their assemblies are presented. Two of these studies concern the atomic-level structural characterization of the surfaces, interfaces, and interiors present in II-VI semiconductor nanorods. The third study investigates the crystallographic arrangement of cobalt nanocrystals in self-assembled aggregates. Crystallographically-aligned assemblies of colloidal CdSe nanorods are examined with linearly-polarized Se-EXAFS spectroscopy, which probes bonding along different directions in the nanorod. This orientation-specific probe is used, because it is expected that the presence of specific surfaces in a nanorod might cause bond relaxations specific to different crystallographic directions. Se-Se distances are found to be contracted along the long axis of the nanorod, while Cd-Se distances display no angular dependence, which is different from the bulk. Ab-initio density functional theory calculations upon CdSe nanowires indicate that relaxations on the rod surfaces cause these changes. ZnS/CdS-CdSe core-shell nanorods are studied with Se, Zn, Cd, and S X-ray absorption spectroscopy (XAS). It is hypothesized that there are two major factors influencing the core and shell structures of the nanorods: the large surface area-to-volume ratio, and epitaxial strain. The presence of the surface may induce bond rearrangements or relaxations to minimize surface energy; epitaxial strain might cause the core and shell lattices to contract or expand to minimize strain energy. A marked contraction of Zn-S bonds is observed in the core-shell nanorods, indicating that surface relaxations may dominate the structure of the nanorod (strain might otherwise drive the Zn-S lattice to accommodate the larger CdS or CdSe lattices via bond expansion). EXAFS and X-ray diffraction (XRD) indicate that Cd-Se bond relaxations might be anisotropic, an expected phenomenon for a rod-shaped nanocrystal. Ordered self-assembled aggregates of cobalt nanocrystals are examined with transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). SAED patterns from multilayered assemblies show that the nanocrystals have preferred crystallographic orientations. ...
Date: June 17, 2006
Creator: Aruguete, Deborah Michiko
Partner: UNT Libraries Government Documents Department

Mechanisms for fatigue and wear of polysilicon structural thinfilms

Description: Fatigue and wear in micron-scale polysilicon structural films can severely impact the reliability of microelectromechanical systems (MEMS). Despite studies on fatigue and wear behavior of these films, there is still an on-going debate regarding the precise physical mechanisms for these two important failure modes. Although macro-scale silicon does not fatigue, this phenomenon is observed in micron-scale silicon. It is shown that for polysilicon devices fabricated in the MUMPs foundry and SUMMiT process stress-lifetime data exhibits similar trends in ambient air, shorter lifetimes in higher relative humidity environments and no fatigue failure at all in high vacuum. Transmission electron microscopy of the surface oxides of the samples show an approximate four-fold thickening of the oxide at stress concentrations after fatigue failure, but no thickening after fracture in air or after fatigue cycling in vacuo. It is found that such oxide thickening and fatigue failure (in air) occurs in devices with initial oxide thicknesses of {approx}4-20 nm. Such results are interpreted and explained by a reaction layer fatigue mechanism; specifically, moisture-assisted subcritical cracking within a cyclic stress-assisted thickened oxide layer occurs until the crack reaches a critical size to cause catastrophic failure. Polysilicon specimens from the SUMMiT process are used to study wear mechanisms in micron-scale silicon in ambient air. Worn parts are examined by analytical scanning and transmission electron microscopy, while temperature changes are monitored using infrared microscopy. These results are compared with the development of values of static coefficients of friction (COF) with number of wear cycles. Observations show amorphous debris particles ({approx}50-100 nm) created by fracture through the silicon grains ({approx}500 nm), which subsequently oxidize, agglomerate into clusters and create plowing tracks. A nano-crystalline layer ({approx}20-200 nm) forms at worn regions. No dislocations or extreme temperature increases are found, ruling out plasticity and temperature-assisted mechanisms. The COF reaches a ...
Date: December 1, 2006
Creator: Alsem, Daniel Henricus
Partner: UNT Libraries Government Documents Department

On the Mechanical Properties and Microstructure of Nitinol forBiomedical Stent Applications

Description: This dissertation was motivated by the alarming number of biomedical device failures reported in the literature, coupled with the growing trend towards the use of Nitinol for endovascular stents. The research is aimed at addressing two of the primary failure modes in Nitinol endovascular stents: fatigue-crack growth and overload fracture. The small dimensions of stents, coupled with their complex geometries and variability among manufacturers, make it virtually impossible to determine generic material constants associated with specific devices. Instead, the research utilizes a hybrid of standard test techniques (fracture mechanics and x-ray micro-diffraction) and custom-designed testing apparatus for the determination of the fracture properties of specimens that are suitable representations of self-expanding Nitinol stents. Specifically, the role of texture (crystallographic alignment of atoms) and the austenite-to-martensite phase transformation on the propagation of cracks in Nitinol was evaluated under simulated body conditions and over a multitude of stresses and strains. The results determined through this research were then used to create conservative safe operating and inspection criteria to be used by the biomedical community for the determination of specific device vulnerability to failure by fracture and/or fatigue.
Date: December 15, 2006
Creator: Robertson, Scott W.
Partner: UNT Libraries Government Documents Department

Nanoscale chemical and mechanical characterization of thin films:sum frequency generation (SFG) vibrational spectroscopy at buriedinterfaces

Description: Sum frequency generation (SFG) surface vibrational spectroscopy was used to characterize interfaces pertinent to current surface engineering applications, such as thin film polymers and novel catalysts. An array of advanced surface science techniques like scanning probe microscopy (SPM), x-ray photoelectron spectroscopy (XPS), gas chromatography (GC) and electron microscopy were used to obtain experimental measurements complementary to SFG data elucidating polymer and catalyst surface composition, surface structure, and surface mechanical behavior. Experiments reported in this dissertation concentrate on three fundamental questions: (1) How does the interfacial molecular structure differ from that of the bulk in real world applications? (2) How do differences in chemical environment affect interface composition or conformation? (3) How do these changes correlate to properties such as mechanical or catalytic performance? The density, surface energy and bonding at a solid interface dramatically alter the polymer configuration, physics and mechanical properties such as surface glass transition, adhesion and hardness. The enhanced sensitivity of SFG at the buried interface is applied to three systems: a series of acrylates under compression, the compositions and segregation behavior of binary polymer polyolefin blends, and the changes in surface structure of a hydrogel as a function of hydration. In addition, a catalytically active thin film of polymer coated nanoparticles is investigated to evaluate the efficacy of SFG to provide in situ information for catalytic reactions involving small mass adsorption and/or product development. Through the use of SFG, in situ total internal reflection (TIR) was used to increase the sensitivity of SFG and provide the necessary specificity to investigate interfaces of thin polymer films and nanostructures previously considered unfeasible. The dynamic nature of thin film surfaces is examined and it is found that the non-equilibrium states contribute to practical applications of acrylates, blends and hydrogels. Lastly, nanoparticle surfaces and the catalytic activity and selectivity of ...
Date: May 19, 2006
Creator: Kweskin, S.J.
Partner: UNT Libraries Government Documents Department

Synthesis and Manipulation of Semiconductor Nanocrystals inMicrofluidic Reactors

Description: Microfluidic reactors are investigated as a mechanism tocontrol the growth of semiconductor nanocrystals and characterize thestructural evolution of colloidal quantum dots. Due to their shortdiffusion lengths, low thermal masses, and predictable fluid dynamics,microfluidic devices can be used to quickly and reproducibly alterreaction conditions such as concentration, temperature, and reactiontime, while allowing for rapid reagent mixing and productcharacterization. These features are particularly useful for colloidalnanocrystal reactions, which scale poorly and are difficult to controland characterize in bulk fluids. To demonstrate the capabilities ofnanoparticle microreactors, a size series of spherical CdSe nanocrystalswas synthesized at high temperature in a continuous-flow, microfabricatedglass reactor. Nanocrystal diameters are reproducibly controlled bysystematically altering reaction parameters such as the temperature,concentration, and reaction time. Microreactors with finer control overtemperature and reagent mixing were designed to synthesize nanoparticlesof different shapes, such as rods, tetrapods, and hollow shells. The twomajor challenges observed with continuous flow reactors are thedeposition of particles on channel walls and the broad distribution ofresidence times that result from laminar flow. To alleviate theseproblems, I designed and fabricated liquid-liquid segmented flowmicroreactors in which the reaction precursors are encapsulated inflowing droplets suspended in an immiscible carrier fluid. The synthesisof CdSe nanocrystals in such microreactors exhibited reduced depositionand residence time distributions while enabling the rapid screening aseries of samples isolated in nL droplets. Microfluidic reactors werealso designed to modify the composition of existing nanocrystals andcharacterize the kinetics of such reactions. The millisecond kinetics ofthe CdSe-to-Ag2Se nanocrystal cation exchange reaction are measured insitu with micro-X-ray Absorption Spectroscopy in silicon microreactorsspecifically designed for rapid mixing and time-resolved X-rayspectroscopy. These results demonstrate that microreactors are valuablefor controlling and characterizing a wide range of reactions in nLvolumes even when nanoscale particles, high temperatures, causticreagents, and rapid time scales are involved. These experiments providethe foundation for future microfluidic investigations into the mechanismsof nanocrystal growth, crystal ...
Date: December 19, 2006
Creator: Chan, Emory Ming-Yue
Partner: UNT Libraries Government Documents Department

Potential Applications of Microtesla Magnetic Resonance ImagingDetected Using a Superconducting Quantum Interference Device

Description: This dissertation describes magnetic resonance imaging (MRI) of protons performed in a precession field of 132 {micro}T. In order to increase the signal-to-noise ratio (SNR), a pulsed 40-300 mT magnetic field prepolarizes the sample spins and an untuned second-order superconducting gradiometer coupled to a low transition temperature superconducting quantum interference device (SQUID) detects the subsequent 5.6-kHz spin precession. Imaging sequences including multiple echoes and partial Fourier reconstruction are developed. Calculating the SNR of prepolarized SQUID-detected MRI shows that three-dimensional Fourier imaging yields higher SNR than slice-selection imaging. An experimentally demonstrated field-cycling pulse sequence and post-processing algorithm mitigate image artifacts caused by concomitant gradients in low-field MRI. The magnetic field noise of SQUID untuned detection is compared to the noise of SQUID tuned detection, conventional Faraday detection, and the Nyquist noise generated by conducting biological samples. A second-generation microtesla MRI system employing a low-noise SQUID is constructed to increase SNR. A 2.4-m cubic, eddy-current shield with 6-mm thick aluminum walls encloses the experiment to attenuate external noise. The measured noise is 0.75 fT Hz{sup -1/2} referred to the bottom gradiometer loop. Solenoids wound from 30-strand braided wire to decrease Nyquist noise and cooled by either liquid nitrogen or water polarize the spins. Copper wire coils wound on wooden supports produce the imaging magnetic fields and field gradients. Water phantom images with 0.8 x 0.8 x 10 mm{sup 3} resolution have a SNR of 6. Three-dimensional 1.6 x 1.9 x 14 mm{sup 3} images of bell peppers and 3 x 3 x 26 mm{sup 3} in vivo images of the human arm are presented. Since contrast based on the transverse spin relaxation rate (T{sub 1}) is enhanced at low magnetic fields, microtesla MRI could potentially be used for tumor imaging. The measured T{sub 1} of ex vivo normal and cancerous prostate tissue ...
Date: May 18, 2006
Creator: Myers, Whittier R.
Partner: UNT Libraries Government Documents Department

Improvement of Photon Buildup Factors for Radiological Assessment

Description: Slant-path buildup factors for photons between 1 keV and 10 MeV for nine radiation shielding materials (air, aluminum, concrete, iron, lead, leaded glass, polyethylene, stainless steel, and water) are calculated with the most recent cross-section data available using Monte Carlo and discrete ordinates methods. Discrete ordinates calculations use a 244-group energy structure that is based on previous research at Los Alamos National Laboratory (LANL), but extended with the results of this thesis, and its focused studies on low-energy photon transport and the effects of group widths in multigroup calculations. Buildup factor calculations in discrete ordinates benefit from coupled photon/electron cross sections to account for secondary photon effects. Also, ambient dose equivalent (herein referred to as dose) buildup factors were analyzed at lower energies where corresponding response functions do not exist in literature. The results of these studies are directly applicable to radiation safety at LANL, where the dose modeling tool Pandemonium is used to estimate worker dose in plutonium handling facilities. Buildup factors determined in this thesis will be used to enhance the code's modeling capabilities, but should be of interest to the radiation shielding community.
Date: July 1, 2006
Creator: Schirmers, F.G.
Partner: UNT Libraries Government Documents Department