PREPARAÇÃO DE PONTOS DE CARBONO E SUA CARACTERIZAÇÃO ÓPTICA: UM EXPERIMENTO PARA INTRODUZIR NANOCIÊNCIA NA GRADUAÇÃO

Carbon dots (CDs) constitute a new class of carbon-based nanomaterials that measure less than 10 nm and display attractive physical and chemical features such as fluorescence. CDs have been considered the new “power” carbon nanomaterials since their accidental discovery in 2004. This study reports a simple, easy, and accessible experiment for undergraduate courses. The experiment involves the preparation of CDs by pyrolysis using commercial gelatin as a low cost precursor as well as CD purification and optical characterization. The optical properties of CDs such as absorption and emission properties make them a promising material for teaching the basic concepts and techniques used for characterization of nanomaterials. Also, the reactants and final product are suitable for undergraduate courses since they are non-toxic materials. The prepared CDs can be used in such applications as bioimaging, solar cells, and photocatalysis.

Ba-DOPED ZnO MATERIALS: A DFT SIMULATION TO INVESTIGATE THE DOPING EFFECT ON FERROELECTRICITY

ZnO is a semiconductor material largely employed in the development of several electronic and optical devices due to its unique electronic, optical, piezo-, ferroelectric and structural properties. This study evaluates the properties of Ba-doped wurtzite-ZnO using quantum mechanical simulations based on the Density Functional Theory (DFT) allied to hybrid functional B3LYP. The Ba-doping caused increase in lattice parameters and slight distortions at the unit cell angle in a wurtzite structure. In addition, the doping process presented decrease in the band-gap (Eg) at low percentages suggesting band-gap engineering. For low doping amounts, the wavelength characteristic was observed in the visible range; whereas, for middle and high doping amounts, the wavelength belongs to the Ultraviolet range. The Ba atoms also influence the ferroelectric property, which is improved linearly with the doping amount, except for doping at 100% or wurtzite-BaO. The ferroelectric results indicate the ZnO:Ba is an strong option to replace perovskite materials in ferroelectric and flash-type memory devices.

Intelin, Toshiban ja Samsungin patenttipolitiikka 1990-luvun alusta lähtien

Tämä kandidaatintyö käsittelee puolijohdeteollisuusyritysten Intelin, Toshiban ja Samsungin patenttipolitiikkaa 1990-luvun alusta lähtien. Työn tarkoituksena on antaa vastauksia siihen, miksi patentointiaktiivisuudet (patenttiaineiston määrät) vaihtelevat niin suuresti yrityksestä toiseen saman alan sisällä. Yrityksiä tarkastellaan erityisesti kotimaidensa suhteen. Patentointiaktiivisuuden analyysissä käytetään IPC-luokittain jaoteltua patenttiaineistoa sekä patentointiin liittyviä artikkeleita ja kirjallisuutta. Yritysten sijainti ja paikallinen yrityskulttuuri vaikuttavat merkittävästi yritysten patenttipolitiikkaan. Vertailuyrityksillä on omat arvot ja toimintatavat, joilla patentointiin liittyviä asioita hoidetaan. Puolijohdeteollisuudessa ja yleisesti informaatioteknologiateollisuudessa, joissa käytetään paljon patentointia, on tärkeää, että maan johto suhtautuu positiivisesti immateriaalioikeuksiin. Kyseisen alan merkitys maiden hyvinvoinnille kasvaa koko ajan. Empiria-aineiston mukaan Intelin patentit ovat eteen- ja taaksepäin viittausten perusteella laadukkaimpia. 1990-luvulla Etelä-Korea panosti suuresti immateriaalioikeuksien kehittämiseen, mikä näkyy Samsungin patenttimäärän nousuna. Samsungilla on vertailuyrityksistä eniten patentteja, mutta ne ovat heikkolaatuisimpia. Toshiban patentit eivät saavuta määrällisesti Intelin taakse- ja eteenpäinviittauksia. Laadullisesti Toshiban patentit ovat kuitenkin parempia kuin Samsungin patentit. Kaikkiaan patentointi on lisääntynyt 1990-luvulla lähtien muun muassa parantuneen patenttisuojan ja helpottuneiden hakuprosessien ansiosta. Aasiassa on yleisempää käyttää strategista patentointia, muun muassa portfolion maksimointia. Yhdysvalloissa suhtaudutaan patentointiin enemmän taloudelliselta kannalta, kun taas puolestaan Aasiassa t&k-toiminta on pitkäjänteisempää.

Time-Resolved Photoluminescence in Antimonide and Dilute Nitride Quantum Well Structures

This Master's thesis is devoted to semiconductor samples study using time-resolved photoluminescence. This method allows investigating recombination in semiconductor samples in order to develop quality of optoelectronic device. An additional goal was the method accommodation for low-energy-gap materials. The first chapter gives a brief intercourse into the basis of semiconductor physics. The key features of the investigated structures are noted. The usage area of the results covers saturable semiconductor absorber mirrors, disk lasers and vertical-external-cavity surface-emittinglasers. The experiment set-up is described in the second chapter. It is based on up-conversion procedure using a nonlinear crystal and involving the photoluminescent emission and the gate pulses. The limitation of the method was estimated. The first series of studied samples were grown at various temperatures and they suffered rapid thermal annealing. Further, a latticematched and metamorphically grown samples were compared. Time-resolved photoluminescence method was adapted for wavelengths up to 1.5 µm. The results allowed to specify the optimal substrate temperature for MBE process. It was found that the lattice-matched sample and the metamorphically grown sample had similar characteristics.

Determinants of credit default swap spread, the effect of the financial crisis in the us markets

The purpose of this study is to define what determinants affect the Credit spread. There are two theoretical frameworks to study this: structural models and reduced form models. Structural models indicate that the main determinants are company leverage, volatility and risk-free interest rate, and other market and firm-specific variables. The purpose is to determine which of these theoretical determinants can explain the CDS spread and also how these theoretical determinants are affected by the financial crisis in 2007. The data is collected from 30 companies in the US Markets, mainly S&P Large Cap. The sample time-frame is 31.1.2004 – 31.12.2009. Empirical studies indicate that structural models can explain the CDS spreads well. Also, there were significant differences between bear and bull markets. The main determinants explaining CDS spreads were leverage and volatility. The other determinants were significant, depending on the sample period. However, these other variables did not explain the spread consistently.

Avaliação da limpeza de CDtrodo construídos a partir de CD de ouro gravável/fita adesiva de galvanoplastia

Os eletrodos de ouro são largamente utilizados em estudos eletroquímicos e eletroanalíticos, devido à sua elevada pureza, ampla faixa de potencial de trabalho, bem como a possibilidade de controle e modificação da superfície eletródica. Neste trabalho são descritos procedimentos para construção e limpeza de eletrodos a partir de CDs de ouro graváveis, denominados CDtrodos. Inicialmente, os CDs foram submetidos à ação de HNO3 concentrado para retirada da camada polimérica protetora e exposição da camada metálica, posteriormente, para a construção dos CDtrodos, delimitou-se a área eletródica com fita de galvanoplastia. Diversos métodos para a limpeza da superfície do ouro foram empregados, após ataque do HNO3, tais como, aplicação de potencial fixo em solução de NaCl, ciclagens sucessivas em H2SO4 e aplicação de ultra-som, sendo que os melhores resultados foram obtidos por tratamento com H2SO4. A literatura tem registrado vários trabalhos empregando CDtrodo no entanto são focados na aplicação e pouco se descreve sobre a limpeza do eletrodo; nenhum trabalho foi registrado empregando a fita de galvanoplastia para delimitar a área do eletrodo. A utilização desse tipo alternativo de eletrodo é de suma importância, uma vez que o mesmo apresenta desempenho eletroquímico comparável aos eletrodos comerciais, além de grande versatilidade e baixo custo.

Switching components for medium voltage drives

Nowadays power drives are the essential part almost of all technological processes. Improvement of efficiency and reduction of losses require development of semiconductor switches. It has a particular meaning for the constantly growing market of renewable sources, especially for wind turbines, which demand more powerful semiconductor devices for control with growth of power. Also at present semiconductor switches are the key component in energy transmission, optimization of generation and network connection. The aim of this thesis is to make a survey of contemporary semiconductor components, showing difference in structures, advantages, disadvantages and most suitable applications. There is topical information about voltage, frequency and current limits of different switches. Study tries to compare dimensions and price of different components. Main manufacturers of semiconductor components are presented with the review of devices produced by them, and a conclusion about their availability was made. IGBT is selected as a main component in this study, because nowadays it is the most attractive component for usage in power drives, especially at the low levels of medium voltage. History of development of IGBT structure, static and dynamic characteristics are considered. Thesis tells about assemblies and connection of components and problems which can appear. One of key questions about semiconductor materials and their future development was considered. For the purpose of comparison strong and weak sides of different switches, calculation of losses of IGBT and its basic competitor – IGCT is presented. This master’s thesis makes an effort to answer the question if there are at present possibilities of accurate selection of switches for electrical drives of different rates of power and looks at future possible ways of development of semiconductor market.

Coeficiente de descarga em emissores de pivô central

Este trabalho foi desenvolvido com o objetivo de determinar, em condições laboratoriais, o coeficiente de descarga (Cd) dos emissores fixos tipo “spray” (Senninger, Nelson e Fabrimar) utilizados em equipamentos pivô central no Brasil. Foram avaliados 45 bocais Senninger, 42 bocais Nelson e 36 bocais Fabrimar, com três repetições, totalizando 369 bocais ensaiados. As pressões de funcionamento variaram entre 6 e 30 PSI (41,37 a 206,84 kPa), com intervalo de leitura a cada 2 PSI (13,79 kPa), totalizando 13 pressões analisadas por bocal. Foram ajustadas as curvas do coeficiente de descarga em função do diâmetro de bocal, para quatro pressões de operação: 10; 15; 20 e 30 PSI (68,95; 103,42; 137,90 e 206,84 kPa). Utilizou-se de dois manômetros de precisão (analógico e digital) previamente calibrados em um manômetro de peso morto. As vazões foram obtidas utilizando três medidores eletromagnético-indutivos, calibrados em recipiente volumétrico. O diâmetro dos bocais foi mensurado através de uma ampliação em projetor de perfil da marca Starrett, modelo HB 400. Os resultados evidenciaram que o coeficiente de descarga varia em função da pressão e do diâmetro do bocal. Nos ensaios conduzidos, obtiveram-se Cds compreendidos na faixa de 0,88 a 0,98 para a faixa de emissores analisados.

Loss Modelling of Three-Level Inverters controlled with Space Vector Modulation Technique

The objective of this master’s thesis is to investigate the loss behavior of three-level ANPC inverter and compare it with conventional NPC inverter. The both inverters are controlled with mature space vector modulation strategy. In order to provide the comparison both accurate and detailed enough NPC and ANPC simulation models should be obtained. The similar control model of SVM is utilized for both NPC and ANPC inverter models. The principles of control algorithms, the structure and description of models are clarified. The power loss calculation model is based on practical calculation approaches with certain assumptions. The comparison between NPC and ANPC topologies is presented based on results obtained for each semiconductor device, their switching and conduction losses and efficiency of the inverters. Alternative switching states of ANPC topology allow distributing losses among the switches more evenly, than in NPC inverter. Obviously, the losses of a switching device depend on its position in the topology. Losses distribution among the components in ANPC topology allows reducing the stress on certain switches, thus losses are equally distributed among the semiconductors, however the efficiency of the inverters is the same. As a new contribution to earlier studies, the obtained models of SVM control, NPC and ANPC inverters have been built. Thus, this thesis can be used in further more complicated modelling of full-power converters for modern multi-megawatt wind energy conversion systems.

Positron measurements in Mg-doped GaN and development of digital Doppler broadening spectroscopy

In this work parameters of Mg-doped GaN samples were studied using positron annihilation spectroscopy and analyzed. It is shown that gallium vacancies exist in an unintentionally doped sample. Next, the sample with higher concentration of Mg and low growth temperature contains vacancy clusters. In case of low concentration of Mg the growth temperature does not affect the formation of defects. Analog electronics can be replaced by a modern digital device. While promising a high quantity of benefits, the performance of these digitizers requires thorough adjustment. A 14-bit two channel digitizer has been tested in order to achieve better performance than the one of a traditional analog setup, and the adjustment process is described. It has been shown that the digital device is unable to achieve better energy resolution, but it is quite close to the corresponding attribute of the available analog system, which had been used for measurements in Mg-doped GaN.

Performance and scalability of isolated DC-DC converter topologies in low voltage, high current applications

Fuel cells are a promising alternative for clean and efficient energy production. A fuel cell is probably the most demanding of all distributed generation power sources. It resembles a solar cell in many ways, but sets strict limits to current ripple, common mode voltages and load variations. The typically low output voltage from the fuel cell stack needs to be boosted to a higher voltage level for grid interfacing. Due to the high electrical efficiency of the fuel cell, there is a need for high efficiency power converters, and in the case of low voltage, high current and galvanic isolation, the implementation of such converters is not a trivial task. This thesis presents galvanically isolated DC-DC converter topologies that have favorable characteristics for fuel cell usage and reviews the topologies from the viewpoint of electrical efficiency and cost efficiency. The focus is on evaluating the design issues when considering a single converter module having large current stresses. The dominating loss mechanism in low voltage, high current applications is conduction losses. In the case of MOSFETs, the conduction losses can be efficiently reduced by paralleling, but in the case of diodes, the effectiveness of paralleling depends strongly on the semiconductor material, diode parameters and output configuration. The transformer winding losses can be a major source of losses if the windings are not optimized according to the topology and the operating conditions. Transformer prototyping can be expensive and time consuming, and thus it is preferable to utilize various calculation methods during the design process in order to evaluate the performance of the transformer. This thesis reviews calculation methods for solid wire, litz wire and copper foil winding losses, and in order to evaluate the applicability of the methods, the calculations are compared against measurements and FEM simulations. By selecting a proper calculation method for each winding type, the winding losses can be predicted quite accurately before actually constructing the transformer. The transformer leakage inductance, the amount of which can also be calculated with reasonable accuracy, has a significant impact on the semiconductor switching losses. Therefore, the leakage inductance effects should also be taken into account when considering the overall efficiency of the converter. It is demonstrated in this thesis that although there are some distinctive differences in the loss distributions between the converter topologies, the differences in the overall efficiency can remain within a range of a few percentage points. However, the optimization effort required in order to achieve the high efficiencies is quite different in each topology. In the presence of practical constraints such as manufacturing complexity or cost, the question of topology selection can become crucial.

Power cycling lifetime estimation of IGBT power modules based on chip temperature modeling

In this doctoral thesis, methods to estimate the expected power cycling life of power semiconductor modules based on chip temperature modeling are developed. Frequency converters operate under dynamic loads in most electric drives. The varying loads cause thermal expansion and contraction, which stresses the internal boundaries between the material layers in the power module. Eventually, the stress wears out the semiconductor modules. The wear-out cannot be detected by traditional temperature or current measurements inside the frequency converter. Therefore, it is important to develop a method to predict the end of the converter lifetime. The thesis concentrates on power-cycling-related failures of insulated gate bipolar transistors. Two types of power modules are discussed: a direct bonded copper (DBC) sandwich structure with and without a baseplate. Most common failure mechanisms are reviewed, and methods to improve the power cycling lifetime of the power modules are presented. Power cycling curves are determined for a module with a lead-free solder by accelerated power cycling tests. A lifetime model is selected and the parameters are updated based on the power cycling test results. According to the measurements, the factor of improvement in the power cycling lifetime of modern IGBT power modules is greater than 10 during the last decade. Also, it is noticed that a 10 C increase in the chip temperature cycle amplitude decreases the lifetime by 40%. A thermal model for the chip temperature estimation is developed. The model is based on power loss estimation of the chip from the output current of the frequency converter. The model is verified with a purpose-built test equipment, which allows simultaneous measurement and simulation of the chip temperature with an arbitrary load waveform. The measurement system is shown to be convenient for studying the thermal behavior of the chip. It is found that the thermal model has a 5 C accuracy in the temperature estimation. The temperature cycles that the power semiconductor chip has experienced are counted by the rainflow algorithm. The counted cycles are compared with the experimentally verified power cycling curves to estimate the life consumption based on the mission profile of the drive. The methods are validated by the lifetime estimation of a power module in a direct-driven wind turbine. The estimated lifetime of the IGBT power module in a direct-driven wind turbine is 15 000 years, if the turbine is located in south-eastern Finland.

Galvanomagnetic effects in InGaAs nanostructure

Investigation of galvanomagnetic effects in nanostructure GaAs/Mn/GaAs/In0.15Ga0.85As/ GaAs is presented. This nanostructure is classified as diluted magnetic semiconductor (DMS). Temperature dependence of transverse magnetoresistivity of the sample was studied. The anomalous Hall effect was detected and subtracted from the total Hall component. Special attention was paid to the measurements of Shubnikov-de Haas oscillations, which exists only in the case of magnetic field aligned perpendicularly to the plane of the sample. This confirms two-dimensional character of the hole energy spectrum in the quantum well. Such important characteristics as cyclotron mass, the Fermi energy and the Dingle temperature were calculated, using experimental data of Shubnikov-de Haas oscillations. The hole concentration and hole mobility in the quantum well also were estimated for the sample. At 4.2 K spin splitting of the maxima of transverse resistivity was observed and g-factor was calculated for that case. The values of the Dingle temperatures were obtained by two different approaches. From the comparison of these values it was concluded that the broadening of Landau levels in the investigated structure is mainly defined by the scattering of charge carriers on the defects of the crystal lattice