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1
Zytek, Roman. "Essays on east European economies." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39872.
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Tadi, Mohsen. "An optimal control problem for a Timoshenko beam." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39868.
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Hu, Kangping Karen. "Study of photon production in e + e - collisions at Tristan." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39778.
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Han, Man-Cheol. "Improved approaches to the indirect force determination problems via experimental modal analysis." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39412.
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Solving the inverse problem, finding the applied forces knowing the system characteristics and the response, has been a difficult problem in structural dynamics. Insufficient accuracy in the system identification and uncorrelated content in the response exacerbate the ill-conditioned nature of the indirect-force-determination problem. Numerical techniques for performing the force determination are exploited and compared. The characteristics of the force determination problems are investigated through least squares solution procedures and numerical examples. The credibility of the estimated forces are studied in the numerical examples using the correlations of the matrix condition number and the mode contribution factor with the resulting error.
The focus of this research is the improved estimation of the applied forces. The two important factors in reducing the force determination error are accurate system identification and improved conditioning of the system matrix. A variety of techniques are examined to reduce the system identification error and control the response measurement uncertainty. The use of rotational or curvature degrees of freedom as an alternative to the translational degrees of freedom for the response measurements and for the structural dynamics model yields a quite differently conditioned system matrix. The choice of a particular degrees of freedom is shown to depend on the frequency contents of the applied forces.<br>Ph. D.
5
Ji, Wan. "Expression of 3-hydroxy-3-methylglutaryl coenzyme a reductase in maize as influenced by light and bleaching herbicides." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39911.
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The activity of the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR, EC 1.1.1.34) is highly expressed in 4-day-old etiolated seedlings of normal ('DeKalb XL72AA'), dwarf (<u>d₅</u>), and albino (<u>lw₃</u>) maize (<u>Zea mays</u> L.). HMGR activity of maize seedlings appeared to be exclusively associated with the microsomal rather than the plastidic fraction of maize cells. Maize tissues with high meristematic activity such as germinating seeds, leaf bases, root tips, and the site of origin of lateral roots contained high levels of HMGR activity. The activity of HMGR extracted from leaf tips of normal, dwarf, and albino maize seedlings was regulated by light. HMGR activity from leaf tips of 4-day-old maize seedlings was inhibited significantly following exposure to strong light (600 μol/m²/s) for more than 10 h. In contrast, HMGR activity from leaf bases and root tips of maize was not inhibited by exposure to strong light. These results suggest that HMGR may play an important role in cell division and that light may regulate HMGR activity indirectly by increasing cell differentiation. Under conditions of strong light pretreatment with the bleaching herbicides clomazone, norflurazon, fluridone and acifluorfen stimulated by 4-to 7-fold the activity of HMGR extracted from 'Dekalb XL72AA' maize seedlings.<br>Ph. D.
6
Ho, Henjen. "An evaluation of the Iosipescu specimen for composite materials shear property measurement." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39826.
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A detailed evaluation of the suitability of the Iosipescu specimen tested in the modified Wyoming fixture is presented. An experimental investigation using conventional strain gage instrumentation and moire interferometry is performed. A finite element analysis of the Iosipescu shear test for unidirectional and cross-ply composites is used to assess the uniformity of the shear stress field in the vicinity of the notch, and demonstrate the effect of the nonuniform stress field upon the strain gage measurements used for the determination of composite shear moduli.
From the test results for graphite-epoxy laminates, it is shown that the proximity of the load introduction point to the test section greatly influences the individual gage readings for certain fiber orientations but the effect upon shear modulus measurement is relatively unimportant. A numerical study of the load contact effect shows the sensitivity of some fiber configurations to the specimen/fixture contact mechanism and may account for the variations in the measured shear moduli. A comparison of the strain gage readings from one surface of a specimen with corresponding data from moire interferometry on the opposite face documented an extreme sensitivity of some fiber orientations to eccentric loading which induced twisting and yielded spurious shear stress-strain curves.
In the numerical analysis, it is shown that the Iosipescu specimens for different fiber orientations have to be modeled differently in order to closely approximate the true loading conditions. Correction factors are needed to allow for the non uniformity of the strain field and the use of the average shear stress in the shear modulus evaluation. The correction factors, which are determined for the region occupied by the strain gage rosette, are found to be dependent upon the material orthotropic ratio and the finite element models.
Based upon the experimental and numerical results, recommendations for improving the reliability and accuracy of the shear modulus values are made, and the implications for shear strength measurement discussed.
Further application of the Iosipescu shear test to woven fabric composites is presented. The limitations of the traditional strain gage instrumentation on the satin weave and high tow plain weave fabrics is discussed. Test result of a epoxy based aluminum particulate composite is also presented.
A modification of the Iosipescu specimen is proposed and investigated experimentally and numerically. It is shown that the proposed new specimen design provides a more uniform shear stress field in the test section and greatly reduces the normal and shear stress concentrations in the vicinity of the notches. While the fabrication and the material cost of the proposed specimen is tremendously reduced, it is shown the accuracy of the shear modulus measurement is not sacrificed.<br>Ph. D.
7
Frink, Neal T. "Three-dimensional upward scheme for solving the Euler equations on unstructured tetrahedral grids." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39423.
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A new upwind scheme is developed for solving the three-dimensional Euler equations on unstructured tetrahedral meshes. The method yields solution accuracy and efficiency comparable to that currently available from similar structured-grid codes. The key to achieving this result is a novel cell reconstruction process which is based on an analytical formulation for computing solution gradients within tetrahedral cells. Prior methodology requires the application of cumbersome numerical procedures to evaluate surface integrals around the cell volume. The result is that higher-order differences can now be constructed more efficiently to attain computational times per cell comparable to those of structured codes.
The underlying philosophy employed in constructing the basic flow solver is to draw on proven structured-grid technology whenever possible in order to reduce risk. Thus, spatial discretization is accomplished by a cell-centered finite-volume formulation using flux-difference splitting. Solutions are advanced in time by a 3- stage Runge-Kutta time-stepping scheme with convergence accelerated to steady state by local time stepping and implicit residual smoothing. The flow solver operates at a speed of 34 microseconds per cell per cycle on a CRAY-2S supercomputer and requires 64 words of memory per cell.
Transonic solutions are presented for a broad class of configurations to demonstrate the accuracy, speed, and robustness of the new scheme. Solutions are shown for the ONERA M6 wing, the Boeing 747-200 configuration, a low-wing transport configuration, a high-speed civil transport configuration, and the space shuttle ascent configuration. Computed surface pressure-coefficient distributions on the ONERA M6 wing are compared with structured-grid results as well as experimental data to quantify the accuracy. A further assessment of grid sensitivity and the effect of convergence acceleration parameters is also included for this configuration.
The more complex configurations serve to demonstrate the robustness and efficiency of the new method and its potential for performing routine aerodynamic analysis of full aircraft configurations. For example, the basic transonic flow features are well captured on the space shuttle ascent configuration with only 7 megawords of memory and 142 minutes of CRAY-YMP run time.<br>Ph. D.
8
Pillay, Michael. "DNA systematic studies in the genus Bromus L. (Poaceae)." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39764.
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Ooi, Phillip S. K. "Design methods for deep foundations." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/28503.
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Zhou, Dan. "Biosynthesis of Caldariellaquinone in Sulfolobus acidocaldarius." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39854.
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Wood, Paul A. "Synthesis and characterization of novel bismaleimides derived from arylene phosphine oxides and ether-ketones." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39889.
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Yeo, Incheol. "Reliability and failure analysis of composite beams and plates containing stress concentrations." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39967.
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Spinu, Ionel M. "Durability of sheet molding compound/metal adhesive bonds." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39832.
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Vengsarkar, Ashish Madhukar. "Localized wave solutions in optical fiber wavelengths." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39963.
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Rais-Rohani, Masoud. "Integral aerodynamic-structural-control wing design." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39867.
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Huang, Jiann-Shiuh. "One-to-one correspondance between maximal sets of antisymmetry and maximal projections of antisymmetry." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39824.
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Let <b>X</b> be a compact Hausdorff space and <b>A</b> a uniform algebra on <b>X</b>. Let if be an isometric unital representation that maps <b>A</b> into bounded linear operators on a Hilbert space. This research investigated that there is a one-to-one correspondence between the collection of maximal sets of antisymmetry for <b>A</b> and that of maximal projections of antisymmetry for π (<b>A</b>) under the extension of π if π satisfies a certain regularity property.<br>Ph. D.
17
Smith, Robert S. "The insulated gate field effect transistor (IGFET) as a microluminometer." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39839.
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Franke, Warren D. "Effects of α1-receptor blockade on the hemodynamic responses to exercise in young normotensives and hypertensives." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/38955.
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The purpose of this study was to determine if α1- adrenergic receptor blockade alters the hemodynamic response to exercise in young (<25 yr) male adult borderline hypertensives differently than in young normotensives. Five hypertensive (HTN, MAP>105 mmHg) and 7 normotensive (NTN, MAP<95 mmHg) college-age males underwent two 30 min bouts of cycle ergometry exercise at 50% V02Pk in a warm (25°C, 50% rh) environment; one bout occurred followed α1-receptor blockade with prazosin (HTN-α, NTN-α) and the other following placebo administration (HTN-p, NTN-p). At rest, HTN-p exhibited an elevated cardiac output (Q, p=.024) and MAP (p=.007). Resting Q was similar for HTN-α and NTN-α. Resting heart rate (HR) was elevated more in HTN-α than NTNα (p=.013) and not different for placebo. Resting and exercise forearm blood flows were similar between groups and altered similarly with prazosin. Exercise resulted in greater (p=.035) Q for HTN vs NTN (HTN-u > NTN-α; HTN-p = NTN-p). HR was higher (p=.043) with prazosin for both groups. Regardless of drug treatment, MAP was stable for NTN while it declined after 10 min of exercise in HTN. Rectal temperatures rose above baseline after 10 min. since Q was similar between groups with placebo but not with α1- blockade, and FBF, MAP, and HR were similarly altered between drug trials, it was concluded that young male hypertensives have an elevated blood pressure due to an elevated Q. In this group, α1-blockade may reduce Q by reducing central venous return.<br>Ph. D.
19
Opengart, Kenneth N. "Studies on the immunopathologic mechanisms of intestinal lesion formation in turkey poults infected with hemorrhagic enteritis virus." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/37756.
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Rodarte, Alma Isabel MarÃn. "Predispersed solvent extraction of copper from dilute aqueous solution." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/38846.
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Predispersed Solvent Extraction (POSE) was used to extract copper ions from dilute acidic aqueous solution. POSE is based on the principle that there is no need to comminute both phases. All that is necessary is to comminute the solvent phase prior to contacting it with the feed. This is done by converting the solvent into aphrons. which are micron-sized globules encapsulated in a soapy film. Since the aphrons are so small, it takes a long time for the solvent to rise to the surface under the influence of gravity alone. Therefore, the separation is expedited by piggy-back flotation of the aphrons on especially prepared gas bubbles, which are somewhat larger than aphrons and are called colloidal gas aphrons (CGA).
Polyaphrons of various types were studied extensively. The apparatus used to generate the polyaphrons was upgraded. The residence time distribution of a liquid in the polyaphron generator was determined. The particle size distribution of polyaphrons was determined using photo-microscopy and sedimentation among other methods.
Batch tests were done using both conventional and POSE. Results showed that POSE approaches equilibrium much faster than conventional extraction. Equilibrium isotherms were drawn and empirical equilibrium relationships were developed. The dynamics of the kinetics of the extraction was modeled using film theory.
Equipment for the POSE process was built. Experiments were carried out in continuous mode and the process was optimized. An empirical statistical equation was developed for the extraction process in continuous mode. Depending on the aqueous to solvent ration, more than 99% copper can be extracted.<br>Ph. D.
21
Muellerleile, Joan Thérèse Polesnak. "Crystallization behavior and morphological features of two LARC polyimides and local orientation studies of two semicrystalline model composites." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39082.
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Results are presented for two unrelated studies. The primary topic involved the crystallization behavior and morphological features of two polyimides, namely LARCCPI (Langley Research .Center Crystalline folyimide) and LARC-TPI (Thermoplastic folyimide). The LARC-CPI study first considered feature~ affecting crystallization behavior including inherent viscosity, crystallization temperature, and melt temperature and time. Data were analyzed using the A vrami analysis. Morphological features were investigated using techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and small angle x-ray scattering (SAXS). Permanganic etching combined with SEM successfully revealed morphological detail, further supported by TEM data. These data support the Avrami analysis results. SAXS data reflected the influence of several crystalliz.ation variables on the long spacing. SAXS results also revealed the presence of a broad second scattering peak for semicrystalline samples appearing in the same position regardless of crystalliz.ation temperature or inherent viscosity. Molecular modelling predicted a low-energy helical conformation with a near-periodic repeat distance corresponding to the second SAXS peak. This conformation is offered as a tentative explanation for that peak. Finally, the effe.ct of nucleating agent on the recrystalliz.ability of a higher inherent viscosity LARC-CPI was considered. One of several nucleators appeared effective in enhancing recrystallization.
The LARC-TPI study involved the development of a thermal cycle based upon crystalliz.ation kinetics results, allowing the successful recrystallization of LARC-TPI following exposure to elevated temperatures. This cycle was then used to modify an existing composite fabrication process to produce semicrystalline composites. Two carbon fiber types were utilized to make composites via both fabrication cycles. These composites were evaluated using several techniques.
The second unrelated project examined very localized orientation behavior in two types of semi crystalline superstructure using FTIR microspectroscopy. Results were obtained utilizing linear dichroism for orientation within spherulitic and transcrystalline superstructure in poly(glycolide co-lactide) and isotactic polypropylene. An increase in orientation on an absolute basis as a function of position was successfully measured. Orientation function values were higher on an absolute basis for the transcrystalline superstructure. However, the sign of the isotactic polypropylene spherulite orientation function values was the opposite of what was anticipated. These results were considered in light of spherulite W AXS data also obtained.<br>Ph. D.
22
Jones, Stephen H. "Parameter robust reduced-order control of flexible structures." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/29264.
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This thesis generalizes the concept of internal feedback loop modeling, due to Tahk and Speyer, to arrive at two new LQG-based methods of parameter robust control. One component of the robustness procedure, common to both methods, is the application of an auxiliary cost functional penalty to desensitize the system to variations in selected parameters of the state-space model. The other component consists of the formulation of a fictitious noise model to accommodate the effect of these parameter variations.
The "frequency-domain method" utilizes knowledge of the system dynamics to create a frequency-shaped noise model with a power spectrum that approximates the frequency content of unknown error signals in the system due to parameter uncertainties. This design method requires augmentation of additional dynamics to the plant, which results in higher-dimensional full-order controllers. However, the controller design computations are identical to those of a standard LQG problem.
The "time-domain method" emulates the same error signals by means of a multiplicative white noise model which reflects the time-domain behavior of those signals. The resulting robust controller is of the same order as the standard LQG controller, although the design involves a more complex computational algorithm. The application of multiplicative white noise to the system model requires the solution of a system of four coupled equations - two modified Riccati equations and two modified Lyapunov equations.
In addition, the optimal projection equations are applied to both robustness methods to reduce the controller order with minimal loss in performance.
Comparisons are drawn between these and related robust control methods, and it is shown that the relative effectiveness of such methods is problem dependent. Parameter sensitivity analysis is carried out on a simply supported plate model subject to external disturbances. The appropriate robust controller is selected, and it is found to stabilize the plate with little sacrifice in performance.<br>Ph. D.
23
Pérez, Guerra Gema. "Biosystematics of the family dactylopiidae (Homoptera: Coccinea) with emphasis on the life cycle of dactylopius coccus costa." Diss., Virginia Tech, 2005. http://hdl.handle.net/10919/29024.
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The cochineal insects include nine species assigned to the genus Dactylopius Costa, 1835, the only genus in the family Dactylopiidae. The present research is a comprehensive review of all the species in the family Dactylopiidae, with special emphasis on the life cycle of the type species Dactylopius coccus Costa. The adult females of the nine species have been redescribed and illustrated in detail, with a discussion on their morphological affinities and relationships. Their hosts, natural enemies, distribution, etymology, and role as biological control agents are discussed. For several species many new distribution and host records are given. Also, new types have been designated for some of the species. These included designation of one neotype and three paratypes for Dactylopius coccus Costa, one neotype for D. tomentosus (Lamarck), and designation of eight new topotypes for D. opuntiae (Cockerell).
Methods are given on collecting, preservation and slide mounting, also on measurements and on preparation of illustrations. Cuticular ultrastructure is shown in scanning electron micrographs. All developmental stages of the type species D. coccus are described. The life cycles under two temperatures and relative humidities, for both males and females are discussed. Aspects of reproduction in D. coccus, its dispersal methods, factors affecting development, and its economic importance are also included.
A separate chapter deals with the host-plants of Dactylopiidae. This includes data on host plant suitability and host plant resistance.
Three identification keys are presented: one to the suborders of Homoptera, the other to the superfamilies and families of Coccinea, and another one for the determination of the species of Dactylopius.
The phylogenetic relationships of the family Dactylopiidae with respect to all the Coccinea families is discussed. A phylogenetic tree for the Dactylopius species is proposed.<br>Ph. D.
24
Wood, Paul A. III. "Synthesis and characterization of novel bismaleimides derived from arylene phosphine oxides and ether-ketones." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39889.
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Drumright, Ray E. "Radical anion rearrangements. aryl cyclopropyl ketyl anions." Diss., This resource online, 1991. http://scholar.lib.vt.edu/theses/available/etd-07282008-134853/.
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Williams, Barry L. "Control of endometrial secretion in cattle and production of transgenic swine." Diss., This resource online, 1991. http://scholar.lib.vt.edu/theses/available/etd-07282008-134227/.
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Sukhadia, Ashish Mahendra. "The in situ generation of liquid crystalline polymer reinforcements in thermoplastics." Diss., This resource online, 1991. http://scholar.lib.vt.edu/theses/available/etd-02052007-081237/.
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Shyu, In-Ming Kevin. "Forced, nonlinear, planar and nonplanar oscillations of a cantilvered beam including static deflection." Diss., This resource online, 1991. http://scholar.lib.vt.edu/theses/available/etd-06062008-171257/.
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Gaspar-Rolle, Maria Nelma Pinto. "Attachment of bacteria to teflon and buna-n-rubber gasket materials." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39818.
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Surface analysis of buna-N-rubber and teflon was performed. Scanning electron microscopy was used to analyze the topography of both materials and x-ray microanalysis identified the elemental chemical composition of the polymers.
Teflon was primarily a smooth surface with random irregular spots, while buna-N-rubber had a very rough topography with "caverns" and crevices spread over the surface. The x-ray microanalysis showed that there are no impurities on the surface of teflon; however, calcium, silicone and sulfur were present on the surface of buna-N-rubber. Water contact angle measurements indicated that buna-N-rubber was a more hydrophobic surface than teflon.
Qualitative analysis of the attachment of Pseudomonas fragi A TCC 4973, Listeria monocytogenes Scott A and Bacillus cereus ATCC 11778 to buna-N-rubber and teflon was assessed by scanning electron microscopy. These bacteria readily attached to both surfaces. Pseudomonas fragi attached after 2 hours in the presence of this microoorganism and Bacillus cereus and Listeria monocytogenes attached at 12 and 24 hours, respectively.
Quantitative analysis of the attachment of Pseudomonas fragi to both surfaces as affected by various milk fat concentrations and temperature, and the availability of nutrients (different dilutions of skim milk, casein, casein and lactose, and whey and lactose) was conducted. Attachment was assessed by impedance microbiology. Milk fat content did not play a significant role in the process of attachment of this organism to either type of surfaces; however, significantly greater numbers attached to buna-N-rubber than to teflon. Overall bacteria attached in higher numbers to both surfaces when grown at 21°C, compared to bacteria grown at 4°C. For buna-N-rubber, bacteria attached in significantly higher numbers when the concentration of nutrients was minimal, while for teflon, the results were, in most cases, opposite to these.<br>Ph. D.
30
Abdaoui, Fatima El. "Allelopathic effects of ferulic, gallic, and vanillic acids on corn (Zea mays L.)." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39816.
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Studies on the activity of femlic, gallic, and vanillic acids on germination
and growth of corn (Zea mays L.), radish (Raphanus sativus L.), and peanut
(Arachis hypogaea L.) showed that the inhibitory effects of these acids were
concentration and growth variable dependent. Ten days after treatment,
significant reduction in percent germination of the three species occurred with
higher phenolic acid treatments, except that gallic acid did not significantly inhibit
peanut germination. Among the growth parameters investigated, root elongation
and dry weight were more affected than either germination or shoot length and
dry weight. Radish and corn were more sensitive than peanut.
In two-combination experiments, the interactive effects of phenolic acids on
corn germination and shoot growth were generally not significant, indicating an
additive effect. Femlic acid, generally, antagonized higher concentrations of
vanillic or gallic acids on corn root length and dry weight, suggesting a differential
uptake of phenolic acids by corn roots or a limited uptake of gallic and vanillic
acids in the presence of ferulic acid. In a soil system, higher and repeated
phenolic acid treatments were required to bring about inhibition of corn growth
than those which were effective in petri dishes.
All levels of the synthetic auxin, 2,4-D (2,4-dichlorophenoxyacetic acid) were
effective in reversing the inhibitory effects of 1 mM ferulic acid on corn root
length when these two acids were applied in combination. No 2,4-D treatment
counteracted 10 mM of ferulic acid. All levels of 2,4-D combined with 1 mM
ferulic acid and the mixture of 0.1 nM 2,4-D with 10 mM ferulic acid were
antagonistic for corn shoot length. No significant interactions were obtained on
corn germination or seedling growth when 2,4-D was combined with gallic acid.
Using manometric techniques, no inhibitory effects of ferulic or gallic acids
observed on 02 consumption of germinating corn seeds. Ferulic acid did not
interfere with water uptake of corn seeds during imbibition and germination.
These findings indicate that the phytotoxicity of these acids observed on corn
germination and seedling growth are not due to their interference with water
uptake and respiratory activity of germinating seeds.<br>Ph. D.
31
Hussein, Mohamad M. "Thermal analysis of power hybrid microelectronic packages." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39971.
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In this dissertation a simplified nondimensional approach for the thermal analysis of power hybrid circuits is presented. The new technique uses only the metallization and the substrate as layers and represents everything below the substrate by an external thermal resistance (expressed as an equivalent convective heat transfer coefficient, h). In this study, the impact on thermal management of thick film metallization and copper cladding on alumina, aluminum nitride, and beryllia ceramic substrates is compared. The thermal conductivity of the substrate material, the thickness of the copper layer, the thermal resistance of the heat sink system, the size of the device, and the spacing between two heat dissipating devices are considered. The model results show that increasing the thickness of the copper layer can significantly decrease the device temperatures on alumina but may increase temperatures on high thermal conductivity substrates. Moreover, the model results show that increasing the thickness of the copper layer requires that the devices be placed farther apart to prevent thermal interaction. The results also demonstrate that the external heat sink resistance can have a significant impact on the heat flow paths and temperatures in the substrate. As the external resistance increases, the spacing required to prevent thermal interaction also increases.
In addition to the above, a series of experiments were conducted on various hybrid circuits samples for a low and high heat sink external resistance, i.e., large and small convective heat transfer coefficients, respectively. These samples were constructed using thick film resistors as heat sources on alumina and beryllia substrates. The temperature rise was measured using infrared thermal imaging technique. These experimental results were compared to results predicted by the thermal model. In general, the model underpredicts or overpredicts the experimental temperature rise by 0-2 ·C and the agreement is within the experimental uncertainty of ±2°C.<br>Ph. D.
32
Carman, Greg P. "Micromechanics of finite length fibers in composite materials." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39869.
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Baik, Jai Wook. "X control charts in the presence of correlation." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39954.
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In traditional quality control charts, fixed sampling interval (FSI) schemes are used where the time between samples has fixed intervals. More efficient methods called variable sampling interval (VSI) schemes have been developed where one takes the next observation sooner than usual if there is an indication that the process is operating off the target value.
Another traditional assumption behind most statistical process control charts is that the sequential observations are independent. However, there are many situations where the sequential observations should not to be treated as independent. Rather, a time series model, in particular the first order autoregressive (AR (1)) model, is appropriate. A Markov chain representation is used to study the properties of the FSI and VSI Shewhart X control charts.
First, the results show that if the process variance is properly estimated and if traditional control limits are used in the FSI control charts, then the detection time is shorter when the consecutive observations are negatively correlated than when they are positively correlated. If they are
positively correlated, then the false alarm rate decreases as the correlation between consecutive observations increases. On the other hand, the detection time increases as the correlation increases.
In VSI control charts with traditional control limits, if the process mean is on or near the target, then the average time to signal (A TS) and average number of samples to signal (ANSS) tend to decrease as the correlation increases until the correlation becomes rather moderate. Then, for more highly correlated data, the A TS and ANSS tend to increase as the correlation increases.
Next, the results show that, even under the AR (1) process, the VSI chart is more efficient than the FSI chart in terms of ATS. In contrast, the VSI chart is less efficient than the FSI chart in terms of ANSS. The efficiency (inefficiency) of ATS (ANSS) tends to decrease (increase) as the correlation between the consecutive observations becomes stronger.
Steady state ATS (A TS·) and steady state ANSS (ANSSO) under the AR (1) process show the same trend as the 'regular' ATS and 'regular' ANSS except when the deviation is very large. If the deviation is very large, then the VSI control chart does not seem to be more efficient than the FSI control chart in terms of steady state ATS.
If we have an AR (2) process, then for any given value of tP2 a PSI control chart has a shorter detection time when tPl is negative than when tPl is positive. In a FSI control chart, the effect of positive </>2 in addition to positive tPl is that the false alarm rate decreases even further and the detection time is even longer.<br>Ph. D.
34
Lapido, Folami Tesileem. "Oxidative addition of N-H and O-H bonds to iridium: developing active catalysts for N-H and O-H additions to unsaturates." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39829.
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The oxidative addition of the N-H bond of both heterocyclic and acyclic amines and the O-H bond of phenols, water and carboxylic acids to iridium(I) trimethylphosphines complexes was studied and the reactivity of the resulting hydrido amido-, aryloxo- and carboxylato iridium(III) complexes was investigated. Oxidative addition of the N-H bond of pyrrole, indole, 3-methylindole, 7-azaindole, carbazole and aniline to [Ir(COD)(PMe₃)₃]CI (<b>1a</b>) (COD = 1,5-cyclooctadiene) produces merIr( NR₂)H(PMe₃)₃CI (<b>2a-f</b>) complexes. That these amines were bound to iridium through an Ir-N bond was established by ¹H, ³¹P and ¹³C NMR spectroscopy, IR spectroscopy, C-H analyses and single crystal x-ray diffraction. Similarly, oxidative addition of the O-H bond of phenol, p-cresol, 3,5-dimethylphenol, and water to [Ir(COD)(PMe₃)₃]CI (la) produces mer-Ir(OR)H(PMe₃)₃C1 (<b>5a-c and 6</b>) which were characterized by ¹H, ³¹P and ¹³C NMR spectroscopy, C H analyses and single crystal x-ray diffraction. A preliminary study of the reactivity of both the amido and aryloxo complexes suggests that because of the increased electron affinity of the heteroatoms involved (N or 0), there is high electron density at the heteroatoms in these complexes and therefore the M-N or M-O bond can heterolyze more easily than M-C bonds. This increased tendency of the M-N or M-O bond to heterolyze leads to decomposition reactions when attempts are made to open up a coordination site at the metal center by removing the chloride ligand.<br>Ph. D.
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Fontana, Eddy Alejandro. "The effect of early feed restriction on the performance, organ weights, carcass composition, and lipid and protein metabolism in broiler chickens." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/41291.
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Five experiments were conducted to investigate the effects of early feed restriction on body weights, feed conversion, organ weights, fat deposition, carcass and muscle composition, plasma lipids, lipogenesis, lipolysis, and muscle tissue DNA and RNA in broiler chickens. In Experiments 1 and 2, broiler chicks were reared in litter pens for 49 days, while Experiments 3,4, and 5 were conducted in battery cages for 28 days. Feed restriction in all experiments was induced by providing male broiler chicks with 40 kcal/bird/day for 7 (Experiments 1 and 2) or 6 (Experiments 3, 4, and 5) days, starting at 4 days of age. Feed restriction (40 kcal/bird/day) for broiler females in Experiment 1 was imposed from 4 to 9 days of age. Ad libitum feeding was resumed after the restriction periods and continued to the conclusion of each experiment. Broilers fed ad libitum for the entire experimental period were used as controls in each study.
Broilers under early feed restriction had significantly (P s .05) lower mean body weights than ad libitum fed controls, for all ages measured. However, feed to gain ratios for restricted birds were Significantly lower at 28 (Experiments 1 through 4) and 49 (Experiments 1 and 2) days of age than for birds fed ad libitum. Total pen body weights for restricted and ad libitum fed groups were similar at 49 days of age in Experiments 1 and 2 which, reflected a significant reduction in the rate of mortality observed in the early restricted groups.
Significantly higher levels of lipogenic activity, plasma triglycerides and lipoproteins (VLDL + LDL), and significantly larger abdominal fat pads were observed in restricted broilers than in ad libitum fed controls at 28 days of age (Experiment 4). No significant differences were found in organ weights, carcass composition, lipolysis, and muscle tissue DNA/RNA levels, and muscle composition between early restricted and unrestricted broilers.
Results from these studies indicate that restricted broilers were not able to attain body weights comparable to ad libitum fed birds at 49 days of age. In contrast, early feed restriction resulted in consistently better feed efficiency in restricted birds when compared to controls. Furthermore, it appears that early feed restriction altered lipid metabolism early in life; however, organ weights, fat deposition, muscle composition, and carcass composition were only minimally affected by this procedure.<br>Ph. D.
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Knorr, Joseph Robert. "Catalytic hydroboration: a study of model hydridoiridium and hydridorhodium boron complexes." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39883.
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The mechanism of catalytic hydroboration was studied through the use of iridium and rhodium model complexes. Oxidative addition of the B-H bond in (1,2-phenylenedioxy) borane (catecholborane) to (Me₃P)₃Ir(Cl)(H) (BO₂C₆H₄ (<B>II</B>) produces <i>mer</i>-(Me₃P}₃Ir(Cl)(H)(B0₂C₆H₄) (<B>II</B>), which was characterized by ¹H NMR spectroscopy and single crystal X-ray diffraction. Compound <B>II</B> reacted with alkynes to form vinyliridium complexes and will catalyze the hydroboration of alkynes with (1,2- phenylenedioxy)borane. The reaction of <B>II</B> with acetylenes was inhibited by the presence of free Lewis bases indicating that the reaction proceeds by a dissociative mechanism. Exchange of the chloride ligand in <B>II</B> occurred with other Lewis bases, indicating that chloride dissociation was responsible for opening up the vacant coordination site on the complex and thus providing for acetylene coordination. When the chloride ligand on <B>II</B> was replaced with other Lewis bases, the reactivity towards trimethylsiliylacetylene was qualitatively determined to be inversely proportional to the strength of the new ligand. The above experiments indicated that the mechanism of catalytic hydroboration of acetylenes with catecholborane involves: oxidative addition of the B-H bond to the iridium center, followed by chloride dissociation and acetylene coordination, migratory-insertion into the Ir-H bond to form the metallo-vinyl complex, and finally reductive elimination to produce trans-alkylvinylborole esters.
The stable metallo-vinyl complex, <b>IX</b>, produced in the reaction of <b>II</b> with dimethyl acetylene dicarboxylate produced twO isomers in solution, one of which showed fluxional behavior. Single crystal X-ray diffraction elucidated a single solid state structure, but the structures of the isomers in solution and the fluxional properties observed have not yet been explained.
The rhodium complex was synthesized by oxidative addition of the B-H bond in (1,2- phenylenedioxy) borane to (Me₃P)₃RhCl producing <i>mer</i>-(Me₃Rh(CI)(H)(BO₂C₆H-₄) (<b>XXIV</b>), which was characterized by ¹H NMR spectroscopy. This complex reacted with acetylenes, but more slowly than the iridium complex, <b>II</b>. The resulting vinyl products were also different than those produced in the iridium case. Phosphine dissociation in <b>XXIV</b> was observed, indicating the possibility of a different mechanism than proposed for the iridium complex.<br>Ph. D.
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Solie, Daniel J. "A search for slow, lightly ionizing particles in cosmic rays." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39451.
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Park, Young Wook. "Large deviation theory for queueing systems." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39858.
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Won, Younsang. "Higher order numerical schemes for propagation of wind wave spectra." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39822.
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Tarrago-Trani, Maria Teresa. "Structural analysis of glycolipid-derived oligosaccharides from metabolically radiolabelled colorectal carcinoma SW1116 cells." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39852.
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Waehamad, Wae-asae. "Synthesis and characterization of polyether-ester liquid crystalline polyesters and poly(arylene ether ketone)-lcp segmented copolymers." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/38781.
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Tan, Chia K. "Movements of footings and retaining walls." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39850.
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Reichard, Karl Martin. "Distributed-effect modal domain optical fiber senors for flexible structure control." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39420.
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Tjatra, I. Wayan. "Transonic aeroelastic analysis of systems with structural nonlinearities." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39912.
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Wood, Charles Wade. "Oscillating shock impingement on low-angle gas injection into a supersonic flow." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39856.
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Nostrandt, Amy Carol. "Development of a model cell culture system in which to study early effects of neuropathy-inducting organophosphates." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39422.
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Gitti, Rossitza K. "The 1H and 13C dynamic nuclear polarization (DNP) enhancement for novel silica phase immobilized nitroxide (SPIN) samples." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/38922.
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The solid/liquid intermolecular transfer (SLIT) flow dynamic nuclear polarization (DNP) experiment potentially provides new methodology for studying interfacial phenomena (e.g., weak hydrogen bonding). In addition, the high efficiency of the transfer also ensures dramatically enhanced NMR signals. These large DNP enhancements could alleviate sensitivity limitations in various flow NMR experiments. Previous studies have established that silica phase immobilized nitroxide (SPIN) radical system are advantageous in the SLIT experiment. In favorable cases (e.g. DCCI₃/SPIN system) a ¹³C DNP enhancement 60 times in excess of the high magnetic field (4.7 T) magnetization has been achieved.¹² However a number of factors still limit the SPIN system presently available. For example, low magnetogyric ratio nuclides, ¹³C, ¹⁵N, which are not dominated by scalar relaxation mechanism require high surface radical concentrations.
The focal point of the present study is the preparation and characterization of several new SPIN radical systems and can be divided into two parts:
1). Preparation, EPR, and DNP Characterization of Achiral SPIN Radicals: a number of SPIN samples were prepared in order to examine the dependence of the observed SLIT DNP enhancements as a function of the surface spin concentration and also isotope-substitution of the immobilized radicals. The SPIN samples were characterized by EPR and DNP. The results show that the increase in the spin concentration does not offer any advantage for ¹H DNP studies. In contrast, ¹³C SLIT DNP results in improved SPIN sample demonstrate the possibility of monitoring dipolar dominated ¹³C DNP enhancements as a result of better leakage factors and suppressed three-spin effects at higher radical concentration. The effect of substitution of deuterons for protons in the immobilized radical also suggest an appreciable contribution of a solid-state three-spin effect.
2). Preparation, EPR, and DNP Characterization of Chiral SPIN Samples: This part of the study provides a chiral SPIN radical suitable for monitoring enantioselective ¹³C DNP enhancements. The DNP results suggest that selective enantiomer/chiral SPIN interactions are feasible. Specifically, differences in the ¹³C DNP enhancements for a model system: (R)- and (S)- enantiomers of bromocamphor, and a (R) chiral SPIN sample were observed.<br>Ph. D.
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King, Peter Samuel. "A parametric study of the hydrodynamic stability theory of 3-D compressible free shear flows." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39703.
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In this study, a new and efficient numerical algorithm is developed to solve both the two-dimensional and three-dimensional compressible hydrodynamic stability problem. A parametric study of free shear flows with two or more supersonic streams is performed. Flows examined included shear layers, jets/wakes, and various geometrical combinations of these flows. The effect of Mach number on the stability characteristics of the flow is studied and found to confirm the work of other researchers who found that increasing the relative (or convective) Mach number increases the stability of the flow. For 2-D mean flows, the most amplified disturbance is shown to be axial for M<1.2 and fully three-dimensional for M> 1.2. Disturbances for three-dimensional mean flows are found here to be axial in the presence of side walls. The variation of the eigenfunctions and flow field disturbances as a function of Mach number and the flow geometry was also studied. Comparisons of the stability code results are also made to several turbulent mixing experiments. The stability code correctly predicts which parameters will accelerate mixing. New correlations of the effects of some important parameters on stability are developed.<br>Ph. D.
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Stoll, Frederick. "A method for the geometrically nonlinear analysis of compressively loaded prismatic composite structures." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39814.
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DeBrunner, Linda Sumners. "Modeling reconfiguration algorithms for regular architecture." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/29254.
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Three models are proposed to evaluate and design distributed reconfigurable systems for fault tolerant, highly reliable applications. These models serve as valuable tools for developing fault tolerant systems. In each model, cells work together in parallel to change the global structure through a series of separate actions. In the Local Supervisor Model (LSM), selected cells guide the reconfiguration process. In the Tessellation Automata Model (TAM), each cell determines its next state based on its state and its neighbors' states, and communicates its state information to its neighbors. In the Interconnected Finite State Machine Model (IFS:MM:), each cell determines its next state and outputs based on its state and its inputs.
The hierarchical nature of the TAM and IFSMM provides advantages in evaluating, comparing, and designing systems. The use of each of these models in describing systems is demonstrated. The IFSMM: is emphasized since it is the most versatile of the three models. The IFSMM: is used to identify algorithm weaknesses and improvements, compare existing algorithms, and develop a novel design for a reconfigurable hypercube.<br>Ph. D.