Made available in DSpace on 2014-10-09T12:37:50Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:31Z (GMT). No. of bitstreams: 1 05380.pdf: 5707781 bytes, checksum: f7a7b65bad72a837f9123ca6deee3226 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Given a graph G and an initial colouring of its vertices with two colours, say black and white, an irreversible k-threshold conversion process on G is an iterative process in which a white vertex becomes permanently coloured black at time t if at least k of its neighbours are coloured black at time t-1. A set S of vertices is an irreversible k-threshold conversion set (k-conversion set) of G if the initial colouring in which the vertices of S are black and the others are white results in the whole vertex set becoming black eventually. In the case where G is (k+1)-regular, it can be shown that the k-conversion sets coincide with the so-called feedback vertex sets, or decycling sets. In this dissertation we study the size and structure of minimum k-conversion sets in several classes of graphs. We examine conditions that lead to equality and inequality in existing bounds on the minimum size of a k-conversion set of G, for k- and (k+1)-regular graphs G. Furthermore, we derive new sharp lower bounds on this number for regular graphs of degree ranging from k+1 to 2k-1 and for graphs of maximum degree k+1. We determine exact values of the minimum size of a k-conversion set for certain classes of trees. We show that every (k+1)-regular graph has a minimum k-conversion set that avoids certain structures in its induced subgraph. These results lead to new proofs of several known results on colourings and forest partitions of (k+1)-regular graphs and graphs of maximum degree k+1. / Graduate

The aim of the diploma thesis is to analyze the WDM and OTDM optical telecommunication networks and the possibility of conversion between these two multiplexing methods. In the first chapter were mentioned the basics of optical communication. The next chapter describes the optical fiber. Specifically, their types, linear and nonlinear optical characteristics whitch are limiting transmission e.g. attenuation, dispersion, refraction, FWM, SPM and XPM. Followed by a general description of the multiplexing systems used in optics and individual elements used in various multiplexing systems. The last chapter was devoted to the design of the OTDM - WDM conversion model.

Biomass is a natural resource that is both abundant and sustainable. Its efficient utilization has long been the focus of research and development efforts with the aim to substitute it for fossil-based feedstock. In addition to the production of biofuels (e.g., ethanol) from biomass, which has been to some degree successful, its conversion to high value-added chemicals is equally important. Among various biomass conversion pathways, catalytic conversion is usually preferred, as it provides a cost-effective and eco-benign route to the desired products with high selectivities. The research of this thesis is focused on the conversion of biomass to various chemicals of commercial interest by selective catalytic oxidation. Molecular oxygen is chosen as the oxidant considering its low cost and environment friendly features in comparison with commonly used hydrogen peroxide. However, the activation of molecular oxygen usually requires high reaction temperatures, leading to over oxidation and thus lower selectivities. Therefore, it is highly desirable to develop effective catalysts for such conversion systems. We use kegging-type heteropoly acids (HPAs) as a platform for catalysts design because of their high catalytic activities and ease of medication. Using HPA catalysts allows the conversion taking place at relatively low temperature, which is beneficial to saving production cost as well as to improving the reaction selectivity. The strong acidity of HPA promotes the hydrolysis of biomass of giant molecules (e.g. cellulose), which is the first as well as the most difficult step in the conversion process. Under certain circumstances, a HPA combines the merits of homogeneous and heterogeneous catalysts, acting as an efficient homogeneous catalyst during the reaction while being easily separated as a heterogeneous catalyst after the reaction. We have successfully applied HPAs in several biomass conversion systems. Specially, we prepared a HPA-based bi-functional catalyst (Au/Cs2HPW12O40) that enabled the selective conversion of cellobiose to gluconic acid with a very high yield of 96.4% (Chapter II); we realized a direct oxidative conversion of cellulose to glycolic acid (yield of 49.3 %) in a water medium for the first time, by using a phosphomolybdic acid catalyst (chapter III); we found that a vanadium-substituted phosphomolybdic acid catalyst (H4PVMo11O40) is capable of converting various biomass-derived substrates to formic acid and acetic acid with high selectivity, and under optimized reaction conditions, high yield of formic acid (67.8%) can be obtained from cellulose (chapter IV); we discovered that the vanadium-substituted phosphomolybdic acids can also selectively oxidize glycerol, a low-cost by-product of biodiesel, to formic acid, and interestingly this conversion can be performed in highly concentration aqueous solution (glycerol: water = 50: 50), giving rise to exceptionally high conversion efficiency (chapter V). These results reveal that HPAs are useful and suitable catalysts for selective oxidation of biomass, and that the reaction pathway is largely determined by the type of addenda atom in the HPA catalyst. The optimization of the reaction conditions and processes in these systems are also discussed in this thesis.

No description available.

<p>Virtually all satellite repeaters use a traveling-wave tube (TWT) as their main power amplifier. Because on-board power is a limited commodity, it is highly desirable that the TWT be operated as efficiently as possible, namely in or near saturation where it is highly nonlinear. These nonlinear effects manifest themselves as an amplitude compression (AM/AM conversion) effect and an amplitude dependent phase modulation (AM/PM conversion) effect. In this thesis a number of investigations have been made in relation to the TWT nonlinearities and their effect on the performance of communication systems.</p> <p>A novel quadrature model of the TWT has been developed. This model is most useful in that it is analytic and requires the choice of only four parameters to obtain an excellent fit to the TWT characteristics.</p> <p>An optimal bandpass nonlinear transfer characteristic that maximizes its output signal to interference power ratio has also been derived. By making use of this optimal transfer characteristic and the quadrature model of the tube, a computer-aided design procedure has been described for obtaining a predistortion compensation network for the TWT. This network consists of a simple arrangement of attenuators and power-law devices and has been shown, by computer simulation, to yield about 1 dB improvement in system performance for the case when only a single carrier is present in the TWT.</p> <p>In the case when a single sample detection and majority logic decision circuit is assumed at the receiver, it has been possible to derive analytical expression for the probability of error of M-ary CPSK signals transmitted through a piecewise-linear envelope limiting repeater. An infinite series expression for the bit error rate of binary CPSK transmission through an actual TWT channel has also been derived.</p> <p>A performance analysis of a correlation receiver with a linear integrate and dump circuit has been carried out for the case of binary CPSK transmission through a bandpass nonlinearity exhibiting AM/PM conversion.</p> <p>For the case of purely amplitude-limiting channels, an optimal (maximum-likelihood) receiver structure and its approximate performance has also been investigated.</p> / Thesis / Doctor of Philosophy (PhD)

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Power Electronic Building Block (PEBB) concept is to provide generic building blocks for power conversion, regulation and distribution with control intelligence and autonomy. A comprehensive modeling and analysis of a PEBB based DC distributed power system (DPS), comprising of a front end power factor correction (PFC) boost rectifier, a DC-DC converter and a three phase four leg inverter is performed. All the sub-systems of the DC DPS are modeled and analyzed for stability and good transient performance. A comprehensive stability analysis of a PEBB based DC DPS is performed. The effect of impedance overlap on the system and individual sub-systems is examined. Ability of a PEBB based converter to stabilize the integrated system by actively changing the system bandwidth is presented. The fault tolerance capability in a PEBB based rectifier is established by ensuring stable system operation, with one leg of the rectifier failed open-circuited. / Master of Science

The conversion of the convent of San Francisco into a school of architecture in Havana, comes out of my omnipresent desire to work with old structures, my faith in architectural education, and my love for he city in which I was born. My intention was to propose an alternative to conventional restoration and preservation. From the mutilated body of the convent, and the seed of education planted by the monks the new school sprung. The memory of the lost limbs, still present in the city, began to materialize slowly, letting the old structure breathe again. / Master of Architecture

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