• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1560
  • 574
  • 7
  • 5
  • 5
  • 4
  • 4
  • 2
  • 1
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 4254
  • 4254
  • 1995
  • 1611
  • 678
  • 395
  • 232
  • 225
  • 188
  • 185
  • 175
  • 163
  • 162
  • 150
  • 148
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

A Statistical Study of Log-Gamma Distribution

Chan, Shing Ping 09 1900 (has links)
<p>In this thesis, we consider the log-gamma distribution and discuss some of its properties. We then study the order statistics from this distribution. We derive the explicit expressions for the means, variances and covariances of order statistics from the distribution for various choices of the shape parameter.</p> <p>Next, we discuss the following methods of estimation of the unknown location and scale parameters (i) bext linear unbiased estimation based on Type II censored samples, (ii) best linear unbiased estimation based on optimal selected order statistics, (iii) maximum likelihood estimation based on Type II censored samples, (iv) approximate maximum likelihood estimation based on Type II censored samples. We illustrate these estimation procedures through a real-life data set by Lieblein and Zelen (1956).</p> <p>We also study the construction of the confidence intervals of these parameters. Both conditional and unconditional approaches are discussed and comparison between these two approaches is made. We also discuss the determination of the tolerance limits and confidence limits for the reliability.</p> <p>After discussing the estimation methods of the location and scale parameters of the log-gamma distribution, we study the estimation method of shape parameter under Type II censoring. We derive the log-likelihood function of the shape parameter and its derivative and hence Newton-Raphson algorithm can be used to obtain the maximum likelihood estimate of the parameter. Asymptotic Fisher information matrix of all three parameters is also derived and so are the asymptotic variances and covariances of the maximum likelihood estimators.</p> <p>Finally, we study the relation between the log-gamma distribution and record value theory. A bivariate log-gamma model is proposed because of this relation. We also study the record values which come from populations other than log-gamma distribution. Statistical inference based on the record values is also studied.</p> / Doctor of Philosophy (PhD)

Title Effect of Deuteron Structure on Hyperfine Splitting of Deuterium

Azam, Sikandar 05 1900 (has links)
<p>The hyperfine splitting (hfs) of deuterium shows a substantial deviation from the prediction based on Fermi's formula. This reflects the effect of the deuteron structure which was not taken account of in Fermi's formula. Following Bohr's original work on the deuteron structure effect, several calculations were done prior to the 1960's, using nucleon-nucleon (NN) potentials which are from today's standard, rather primitive. In this thesis we reexamine this problem by using several modern realistic NN potentials which reproduce the deuteron properties and the NN scattering data very well. In addition to the correction of the type which was examined by Bohr and Low, we examine a number of other corrections. The Bohr-Low correction, which is the most important one, turns out to be remarkably insensitive to the choice of the potential, and this correction significantly over-estimates the experimentally observed anomaly. However, the correction arising through the angular momentum dependent terms in the NN potential is sensitive to the choice of the potential. When this effect is included the theory and experiment can be reconciled for some of the potentials. In this sense the long standing anomaly of the deuterium hfs can be explained.</p>

Seasonal Changes in Photosynthesis in the Lichen Peltigera Rufescens

Brown, Gregory Douglas 10 1900 (has links)
<p>The seasonal response of net photosynthesis to temperature, light and moisture has been examined in two populations of the lichen Peltigera rufescens. The sub-arctic population had a temperature optimum of 35°C and no seasonal changed in the photosynthetic response were observed. In contrast, the gas exchange rate did change on a seasonal basis in the second population studied (from a temperate climatic zone). In this population, the temperature optimum for the net photosynthesis shifted from 25°C in the winder to 35°C in the summer. This seasonal change can be induced in the laboratory storing the lichen under appropriate temperature and photoperiod conditions. This change was shown, with the use of photosynthetic-illumination curves, to be restricted to alterations in the light saturated rates of net photosynthesis only, indicating that the acclimation process resulted from a change in the dark reactions of photosynthesis. This hypothesis was further investigated with studies on an important rate controlling enzyme of the Calvin cycle, fructose-1, 6-bisohosphatase. This enzyme was purified from summer and winter collections of the temperate population of P. rufescens and the total activity, thermal stability, substrate affinity and activation energy was compared in these isolates. It was shown that the temperature response of the Km for this enzyme closely paralleled the net photosynthetic response and that the activation energy was lower in the summer form of the enzyme. These kinetic differences largely explain the seasonal change in net photosynthetic capacity observed in the temperate population and support the enzymatic theory of acclimation suggested by the photosynthetic-illumination curves.</p> / Doctor of Philosophy (PhD)

Molecular Cloning, Characterization and Expression in E. Coli of a cDNA Encoding the Growth Hormone in Rainbow Trout

Agellon, Benn Luis January 1986 (has links)
<p>RNA blot analysis, cell-free translation and protein immunoblot analyses were used to characterize mRNA purified from pituitary glands of rapidly growing rainbow trout. Cell-free translation products of total pituitary RNA, analyzed by direct immunoprecipitation or protein immunoblot analysis with an antiserum to chum salmon GH, indicated that the rainbow trout pre-GH is a polypeptide of 25 kDa. RNA blot analysis competitor, revealed the presence of at least four size classes of pituitary-specific RNA sequences. The abundant class of mRNA had a size immunoblot analysis, it was found that the GH mRNA size class. Furthermore, the size of the GH mRNA of rainbow trout is similar to that of mammalian GH mRNA.</p> <p>The immunochemical reactivity of rainbow trout GH with antiserum to other GH was also determined. Results of this study indicate that antisera to mammalian GH crossreact poorly with rainbow trout GH. This observation correlates with the inability of any of the mammalian GH nucleic acid probe sequences to hybridize with a specific mRNA in the pituitary gland of rainbow trout. On the other hand, antiserum to chum salmon GH shows good crossreactivity with rainbow trout GH present in cell-free translation products programmed by rainbow trout pituitary RNA or in extracts of rainbow trout pituitary glands. Several attempts were made to construct a cDNA library using total pituitary RNA of rapidly growing rainbow trout. The early attempts were unsuccessful and reasons for the failures are discussed. Subsequently, a simplified method for the synthesis and cloning of cDNA was developed. A library containing approximately 30,000 recombinant clones was obtained following this procedure. A simple immuno-screening procedure using an antiserum to chum salmon GH as probe was also developed and serveral recombinant clones carrying the complimentary DNA sequence of rainbow trout GH mRNA were isolated. One clone, designated pAF51, was found to program the synthesis of an immunoreactive polypeptide of 24kDa and contains a cDNA insert of approximately 900bp. The cDNA sequence in pAF51 was determined by direct super-coiled plasmid sequencing. The cloned sequence encode, a hybrid polypeptide that includes the first 9 amino acid residues of the E. coli B-galactosidase a portion of the predicted signal peptide of the rainbow trout pre-GH and the entire sequence of the mature GH polypeptide. The primary structure of the mature GH polypeptide predicted from the rainbow trout GH cDNA sequence shows complete homology with the chum salmon GH and weak homology to the primary structures of the mammalian GH. The tertiary structure of the rainbow trout GH was also compared with other GH polypeptides. The pairwise comparisons of the hydropathy profiles of bovine, human, rat and rainbow trout GH polypeptides indicate that regions of similarity exist between the rainbow trout GH and the mammalian GH. In particular, there are two major regions of similarity found near the amino terminal and at the carboxyl terminal region. These regions correspond to hydrophilic domains of the GH molecules. The significance of these domains is discussed.</p> <p>The biological activity of the rainbow trout GH synthesized in bacteria was tested in vivo. The chimeric hormone was partially purified from extracts of E. coli harboring plasmid pAF51. The hormone was administered to intact fingerling and yearling rainbow trout by intra-peritoneal injection, and to fry by the dip methoid. The results from the in vivo test indicate that the chimeric GH is biologically active with respect to its growth promoting activity; a marked enhancement of growth was seen in GH-treated fish. A dose of hormone as low as 0.2 ug/g of body weight when administered by injection of 50 ug/l GH when administered by dipping is sufficient to promote enchanced growth. However, high dosages used in this study consistently exhibited the least induction of growth, althought significantly higher than the controls. Certain morphological and behavioral effects were also observed. Analysis of compositiion of muscle tissue obtained from GH-treated and untreated fish indicated no detectable difference in tissue quality.</p> / Doctor of Philosophy (PhD)

Chemically Induced Grain Boundary Migration in Alloy Systems

Tashiro, Kelvin M. 10 1900 (has links)
<p>This thesis describes the phenomenon of chemically induced grain boundary migration, a process in which a grain boundary migrates in response to a chemical driving force. The migration characteristics from various binary alloy systems are documented, especially in the aluminum-zinc system where extensive measurements have been made in both static and dynamic situations. The dynamics of chemically induced grain boundary migration are also studied in conjunction with the growth of a second phase, a situation referred to as discontinuous precipitation. It is suggested that the force which is responsible for moving the boundary reveals different facets of itself for different cases. The facets of itself for different cases. The facets are extremes in the chemical force levels; one is at a low level and is suggested to be derived from coherency strains when solute penetration into the bulk is significant. The other force is a high level force and which originates from large concentration differences which exist across the boundary at low volume penetration levels. It is uncertain whether the transition between the two chemical driving force levels is continuous or discontinuous since the transition ranges are small. Elastic anisotropy is suggested to be responsible for the initiation of migration due to an unbalanced coherent strain energy present across the boundary. The migration process has been modeled macroscopically and computer simulated. At this time, current models of grain boundary structure are suggested to be inadequate to account for the coupling of a chemical force to grain boundary structural components.</p> / Doctor of Philosophy (PhD)

The cleaning of InP substrates for growth by MBE

Hofstra, Peter 06 1900 (has links)
<p>A novel technique of using an in-situ ECR generated H-plasma to produce high quality surfaces on InP substrates, for growth of n- and p-type InP layers by GSMBE, has been investigated. The initial substrate surface quality determines the quality of the subsequently grown layers and therefore, the cleaning procedure is of critical importance. The standard approach entails a thermal desorption of a passivating oxide; however, this technique leaves carbon on the surface and, because oxides may vary in composition depending on the growth conditions, a consistent temperature for desorption may not be obtained. The desorption process is also dependent on the atmosphere in which it is carried out; i.e. whether an overpressure of P₂ or As₂ is used. Thermal desorption of oxides from InP requires the substrate to reach temperatures higher than normal GSMBE growth temperatures which can lead to substrate decomposition and, for regrowth applications, can alter dopant profiles and layer composition in ternary and quaternary layer growths. As an alternative, H-plasmas in separate vacuum chambers have been used to remove oxides from InP but this typically produces highly defective substrates due to a loss of phosphorus from the substrate. In this work the combination of an in-situ H-plasma with a stabilizing atmosphere of P₂ is used for the removal of oxides at temperatures equal to growth temperature and below.</p> <p>The mechanism involved in the thermal desorption of an oxide is first clarified and this procedure is compared with oxide removal by H-plasma etch in a phosphorus atmosphere. The ECR source can produce various plasma modes which have been thoroughly characterized. The effect of the different plasma conditions on clean lnP is determined. These modes have different properties which result in different oxide etch rates. Various modes are compared and the mechanism of oxide removal is documented. As an alternative to oxide growth S-passivation has recently received attention as a surface passivation technique. Application of this surface, with and without H-plasma cleaning, has been investigated. The above treatments and other wet chemical surface treatments are compared to determine the optimal surface cleaning technique.</p> <p>The results indicate that thermal desorptions are driven by reaction with phosphorus from the substrate and therefore require high temperatures to promote significant phosphorus evaporation from the substrate. A H-plasma etch of the oxide is driven by the presence of atomic H in the plasma and results in the formation of water. Oxide removal rates were determined at various temperatures from 250°C - 490°C. The plasma etch was found to remove carbon contamination from the surface whereas, thermal desorptions do not. Defect levels in the underlying lnP are sensitive to the plasma properties but with careful choice of plasma conditions defect states can be minimized. An optimal surface cleaning procedure has been developed which involves a UV-ozone treatment and H-plasma cleaning, resulting in interfaces free of electrically active defects in n-type material and a defect concentration of 8x10¹¹ cm⁻² in p-type material.</p> / Doctor of Philosophy (PhD)

Development of environmental water monitors based on hydrostatic and flourescence detection techniques

Scott, Robin M. January 2009 (has links)
The research for this thesis led to the development of two environmental detectors, whose function was to identify predetermined threshold levels of the analyte. The research centred onto two types of analyte; immiscible and dissolved, both occurring in water based locations. The first detector was developed to detect the accumulation of oil within an oil/water interceptor. Pollution from oil spillages is a major contaminate of water systems and the control of this potentially hazardous material has legal obligations. This liquid, which naturally separates from water, accumulates within the interceptor enclosure and can be removed once the quantity of oil reaches the desired level. However, the often unpredictable nature of oil leaks and spillages means that the accumulation of oil within an interceptor is an irregular occurrence. Interceptor detectors based upon electrical techniques already exist. This research specifically developed a detection system that operated without any electrical devices within the interceptor. The research explored several possible avenues, eventually pursuing a technique based upon pressure change, based on the density differential between water and oil. The final system was capable of identifying when the oil had reached a depth of 200 to 250 mm within the interceptor. The second detection system, a portable microfluidic fluorimeter, was intended for placement in locations for the direct analysis of water. Glutathione was chosen as a model analyte, associated with a sex pheromone and prior to the onset of spawning may be found in high concentrations. The system that has been developed is capable of selectively sensing glutathione to below 10 μM. However, the threshold concentration in the proximity of its release as a pheromone is greater than 100 μM and within this range the fluorimeter produced a linear response. The fluorimeter used an LED light source with a PMT detector. An analysis could be made every seven minutes, using 150 μL each of analyte and reagent for every cycle. Through assessment of a pre-made standard, the viability of the microfluidic system could be assured with regards to blockages or other malfunctions of the system.

Fabrication of novel functional anisotropic micro-particles for foam stabilisation and structuring in food formulations

Campbell, Andrew Lee January 2009 (has links)
This thesis is concerned with the fabrication of novel food-grade anisotropic solid particles for foam stabilisation and applications in food formulations. Two main techniques were employed for the fabrication of these particles. Acidic hydrolysis of Nata de Coco bacterial cellulose was used to form cellulose nano-rods which were applied as a foam scaffolding material. Characterisation and modification of Nata de Coco nano-rods was also carried out. It was found that cellulose nano-rods modified with a surface layer of ethyl cellulose exhibited good foamability.Secondly, a combination of in-shear-flow dispersion and solvent attrition techniques were employed to form micro-rods from the edible polymeric materials shellac, ethyl cellulose and zein. Characterisation of the properties of shellac, ethyl cellulose and zein micro-rods was carried out, together with assessments of their foam stabilisation ability. Stable aqueous foams were produced using shellac, ethyl cellulose and zein micro-rods as stabilisers, and ethyl cellulose micro-rods were also used to form water-in-oil emulsions.An extension of this technique was used in the fabrication of novel functional anisotropic food-grade micro-rods with micro-particle inclusions giving altered morphology. Enhanced foamability and drainage retardation was achieved using lumpy yeast-shellac micro-rods to stabilise aqueous foams. The method was extended to the production of aqueous dispersions of ballooned micro-rods, formed by two different protocols.This procedure also allowed the formation of novel, dual-function micro-rod capsules or ‘micro-ampules’ for stabilisation and encapsulation by inclusion of oils into the dispersed phase prior to micro-rod fabrication. The oils became trapped within the micro-rods resulting in micro-ampules with a range of morphologies which were characterised by fluorescence doping and microscopy.

Analysis of hairy-rod polyimides for photoalignment

Droge, Stefan January 2008 (has links)
The evaluation of novel polyimides for nematic liquid crystal alignment by UV irradiation was the aim of this thesis. Homeotropic liquid crystal (LC) alignment was observed for the majority of the in-house synthesised hairy-rod polyimides (PIs). Oblique unpolarised UV irradiation of branched PIs results in pretilted LCs of 2 deg to 88 deg depending on chemical composition, processing history and fluence. The initial pretilted alignment collapsed to homogeneous alignment at higher fluences, which was dependent on the main chain, side chain combination. Minimal changes in the UV/VIS and FTIR absorption spectra were found for fluences at which pretilted LC alignment occurred. For higher fluences photodegradation could be observed in the FTIR spectra. AFM measurements revealed an increased surface roughness after UV irradiation, but no surface anisotropy could be observed. X-ray reflectivity measurements showed film shrinkage after in-situ UV irradiation with identical trends as was observed for the collapse of the LC pretilt. Modeling revealed an increased electron density at the surface and a less diffuse interface following UV exposure. Bragg features in the reflectivity curve suggested a periodical internal film morphology, which was confirmed by grazing incidence X-ray diffraction (GIXRD). Models of chain orientations are proposed for three PIs and the UV induced morphology change analysed. GIXRD probes the surface or bulk of the thin film and an anisotropic increase in order was found for the in-plane surface scan at low fluences, followed by a general order decrease for higher fluences. The LC tilts in the direction of UV irradiation and not toward higher order. An entropy-related photodegradation model is suggested whereby the LC tilts toward maximum surface disorder to minimise free energy. Polarised UV irradiation of main chain only PIs results in homogeneous LC alignment. This was used to photoalign a LC semiconductor material, resulting in a polarised UV absorption ratio of 11 to 1.

A matrix isolation study of transition metal halides and their structure

Wilson, Antony January 2009 (has links)
The work within this thesis has concentrated on the formation and isolation of titanium, vanadium, palladium, and mercury halides, with emphasis on the fluorides.TiF, TiF2, TiF3, and TiF4 have all been isolated within an argon matrix and infrared spectra obtained. From the titanium isotope splitting pattern a bond angle has been determined for TiF2 for the first time of 165o, or effectively linear. This work is also the first time that TiF has been isolated within an argon matrix. Work has also been conducted with vanadium which has lead to the isolation of VF5, VF4, VF3, and VF2, with VF4 undergoing Fermi Resonance. This is the first time that VF4 and VF2, consistent with a linear structure, have been isolated within a matrix.Work conducted upon palladium led to the isolation of numerous palladium fluorides, identified by the palladium isotope patterns in their IR spectra. Due to the similarity of the calculated stretching frequencies of PdF2, PdF3, PdF4, and PdF6 the assignment was challenging and so identification of these bands was conducted based photolysis and annealing behaviour in conjunction with computational calculations. This has allowed for the assignment of the bands present to PdF6, PdF4, PdF3, PdF2, and PdF.The bond length of molecular HgF2 has also been determined for the first time at 1.94(2) Å using the Hg L3-edge with EXAFS. Although the initial aim of this work was to isolate HgF4, using IR, UV/Vis, and XANES, no evidence could be found for oxidation states of mercury higher that HgII. The work also developed a new clean way of making HgF2 in a matrix. The identification of a new Hg…X2 complex was also discovered which when photolysed forms the HgX2 compound, this has only been proven for HgF2. This was achieved by isolating mercury atoms in an argon matrix doped matrices, photolysis of this matrix the led to the formation of HgF2 in significant amounts.

Page generated in 0.3075 seconds