Materialförluster och självuppvärmning är ett vanligt problem vid lagring av biomassa i stor skala. Biomassan som lagras bryts med tiden ned av mikroorganismer och kemiska processer vilket resulterar i en förlust av torrsubstans, ofta kring 1-4% per månad. Självuppvärmningen kan leda till mycket höga temperaturer i högarna av biomassa och kan i vissa fall leda till självantändning av biomassan. I detta examensarbete utvecklas en matematisk modell bestående av 21 kopplade partiella differentialekvationer som beskriver värme- och massflöden med målet att beskriva självuppvärmning och massförlust i systemet. Med syftet att undersöka hur värme- och massflöden påverkas av högarnas geometri, storlek, kompaktering och eventuella temperaturgradienter, skapades först en modell ii COMSOL. Resultatet från dessa simuleringar används sedan för att bygga en modell i MATLAB där flödena kan undersökas tillsammans med den komplexa reaktionskinetiken. Resultatet från denna två-dimensionella modell jämförs sedan med data från pågående forskning av SLU. Resultaten visar att de simulerade värdena liknar de experimentella data från SLU men modellen har svårt att beskriva hur materialet torkar, vilket är en vanlig observation vid lagring av biomassa. Modellen visar att fukthalten i det lagrade materialet, tillsammans med andelen lättnedbrytbart material är de två viktigaste parametrar som avgör magnituden av substansförlusten och dessutom potentialen för självuppvärmning. / Loss of material and self-heating is a common problem during large-scale storage of biomass. The material is degraded by microorganisms and chemical processes which commonly result in dry matter losses of 1-4% per month. The self-heating often leads to high temperatures in the piles, which in some cases can lead to fires. In this thesis, a mathematical model consisting of 21 coupled partial differential equations describing heat and mass transfer is presented with the aim to describe the variations in heat and dry matter loss within the pile. For describing mass and heat transfer inside the biomass pile, a COMSOL model is setup, where effects of compaction, temperature gradients, shape and size of the pile are studied. Results from the COMSOL simulations are then transferred to MATLAB to simulate the complex kinetics coupled with the heat and mass transfer. The simulated results are then compared to experimental values obtained from an ongoing research project conducted by SLU. Results show that simulations do resemble experimental data but is limited in terms of describing drying process observed during storage.  Important factors highlighted are the effect of moisture content in the starting material, as well as the amount of easily degradable content in the biomass as these factors greatly determine the magnitude of both dry matter loss and heat development.

Self-heating leads to temperature rise of laser diode and limits the output power, efficiency and modulation bandwidth due to increased loss and decreased differential gain. The main heat sources in laser diode during continuous wave operation are Joule heating and free carrier absorption loss. To control device self-heating, the epi structure needs to be designed with low electrical resistance and low absorption loss, while the heat flux must spread out of the device efficiently. This dissertation presents the control of self-heating of both edge emitting laser diodes and vertical cavity surface emitting lasers (VCSELs). For the 980nm high power edge emitting laser, asymmetric waveguide is used for low free carrier absorption loss. The waveguide and cladding materials are optimized for high injection efficiency. BeO heatsink is applied to spread the heat efficiently. Injection efficiency of 71% and internal loss of 0.3 cm-1 have been achieved. A total output power of 9.3 W is measured from 0.5cm long device at 14.5A injection current. To further reduce the internal loss, the development of 980nm quantum dot active region is studied. Threshold current density as low as 59A/cm2 is reached. For the VCSELs, oxide-free structure is used to solve the self-heating problem of oxide VCSELs. Removing the oxide layer and using AlAs in the DBRs leads to record low thermal resistance. Optimization of the DBRs leads to low resistance and low free carrier absorption. Power conversion efficiency higher than 50% is achieved. To further reduce device voltage and heat generation, the development of intracavity contacts devices is introduced.

<p>As the miniaturization of the transistors in integrated circuits approaches the atomic scale limit, novel materials with exceptional performance are desired. Moreover, to conduct enough current with an ultrathin and small-scale body, high drain current density is preferably required. Nevertheless, devices may suffer seriously from self-heating effect (SHE) with high drain bias and current if the generated heat cannot be dissipated efficiently. In this thesis, we introduce two material systems and several techniques to accomplish the demand without SHE. Tellurium, as a van der Waals material composed by atomic helical chains, is able to realize its one-dimensional structure. We illustrate that the cross-sectional current density of 150 MA/cm2 is achieved through boron nitride nanotube (BNNT) encapsulation without SHE due to the superior thermal conductivity of BN. With the nanotube encapsulation technique applied, one-dimensional tellurium nanowire transistors with diameter down to 2 nm are realized as well, and single tellurium atomic chain is isolated. Furthermore, atomic-layer-deposited indium oxide (In2O3) as thin-film transistors exhibit even better current carrying capacity. Through co-optimization of their electrical and thermal performance, drain current up to 4.3 mA/μm is achieved with a 1.9-nm-thick body without SHE. The alleviation of SHE is due to a) the high thermal conductivity of the substrate assisting on efficiently dissipating the generated thermal energy, b) SHE avoidance with short-pulse measurement, and c) interface engineering between the channel stack and the substrate. These two material systems may be the solid solution to the desire of high current density transistors in the post-Moore era.</p>

Darbo tikslas ‒ ištirti drėgnio įtaką susmulkinto gluosninio žilvičio savaiminiam kaitimui ir šilumos sklaidai pjaustinio sampile. Atlikti tyrimai ir literatūros analizė leidžia teigti, kad didelis drėgmės kiekis lemia pjaustinio biologinį aktyvumą, taip pat jo tankį, poringumą ir kitas savybes. Susidaro palankios sąlygos pjaustinio savaiminio kaitimo procesui, kuris siejamas su medienos audinių kvėpavimu, grybų ir bakterijų veikla, biodegradacijos proceso intensyvumu bei medienos pjaustinio savaiminio užsiliepsnojimo rizika. Atlikus tyrimus nustatyta, jog didžiausiu biologiniu aktyvumu (800 ÷ 340 W/(t•h)) pasižymi nuo 50 % iki 33 % drėgnio gluosninio žilvičio pjaustinys. Jam džiūstant toliau, išskiriamas šilumos srautas intensyviai mažėja ir išdžiūvus iki 12 % drėgnio, tampa visiškai biologiškai neaktyvus. Nustatyta, jog medienos pjaustinio temperatūrinis laidis nevienodas įvairuojant jo drėgniui. Temperatūrinio laidžio maksimali vertė 21,6 m2/s fiksuota, kai gluosnių pjaustinio drėgnis siekė 30 %. / The aim of this paper is to explore the influence of humidity to self‒heating and thermal dissipation pile chaff to cut Salicaceae osier. Performed research and analysis of literature suggests that high moisture content determines the biological activity of carving its density, porosity and other properties. It creates favourable conditions for self‒heating process of carving, which is associated with the wood tissue of breath, fungi and bacteria activity, biodegradation process, the intensity of wood carving and the risk of spontaneous ignition. The investigation revealed that the highest biological activity (800 ÷ 340 W/(t•h)) have between 50 % and 33 % of humidity Oyster osier chops. Further drying, heat flow decreases intensively and drying up to 12 % humidity, it is completely becoming biological inactive. It was found that the conductivity of wood carving temperature difference of unequal moisture. Thermal conductivity maximum value of 21,6 m2/s fixed, the willow carving moisture content was 30 %.

La détermination des propriétés en fatigue à grand nombre de cycles des aciers laminés destinés à l'industrie automobile est un processus coûteux en temps et en quantité de matière : 25 éprouvettes et presque un mois d'essais sont nécessaires à l'obtention d'une courbe de fatigue standard. Dans l'objectif de réduire ces temps de caractérisation, une méthode rapide, basée sur l'auto-échauffement de la matière sous sollicitation cyclique est mise en place sur un très grand nombre de nuances. Les mesures d'auto-échauffement mettent en évidence la présence de deux régimes dissipatifs distincts, un pour les plus faibles amplitudes de chargement cyclique et un pour les plus hautes. Un modèle probabiliste à deux échelles est ensuite développé, dont le but est de prévoir le comportement en fatigue à grand nombre de cycles à partir des mesures d'auto-échauffement. Il est composé d'une matrice au comportement élasto-plastique et d'une population d'inclusions possédant un second comportement élasto-plastique dont le seuil d'activation est aléatoire. Les deux régimes d'auto-échauffement peuvent ainsi être décrits fidèlement. En utilisant l'hypothèse du maillon le plus faible et un critère énergétique, une prévision du comportement en fatigue est réalisée. Trois jours seulement sont alors requis pour obtenir une courbe de fatigue complète. La pertinence de l'approche est validée en comparant avec des courbes de fatigue standards. Puis, des observations par microscopie optique, par microscopie à force atomique et par EBSD sont entreprises sur un acier micro-allié. L'objectif est double : en premier lieu, mieux comprendre les phénomènes qui se produisent sous sollicitation cyclique conduisant à l'obtention des deux régimes d'auto-échauffement ; en second lieu, justifier la pertinence des ingrédients introduits dans la modélisation. Dans une seconde grande partie, il est question de l'influence d'une pré-déformation plastique sur l'évolution des propriétés en fatigue. En effet, les composants automobiles obtenus à partir de tôles en acier laminé subissent diverses opérations de mise en forme, conduisant à déformer plastiquement la matière. Ces modifications de l'état de la matière engendrent des évolutions des propriétés en fatigue qui ne sont cependant pas prises en compte dans le dimensionnement actuel des pièces (non déterminées par la méthode standard car trop coûteux en temps). La rapidité de la méthode développée autorise à caractériser ces évolutions. A partir des modifications des propriétés à l'auto-échauffement après divers modes de pré-déformation plastique (traction, traction plane, cisaillement) et en étudiant diverses directions de sollicitation, il est possible de prévoir l'évolution des propriétés en fatigue associée pour de larges gammes de pré-déformations. La qualité des prévisions est validée en comparant avec des courbes de fatigue standards. / The determination of high cycle fatigue properties of high strength steel sheets for automotive industry is time and specimens consuming : 25 specimens and almost one month are required to obtain a traditional fatigue SN curve. In order to reduce the time dedicated to the fatigue characterization, a fast method, based on the self-heating of steels under cyclic loading is developed and applied to a wide range of grades. Self-heating measurements show the presence of two distinct dissipative regimes, a firstone for the low amplitudes of cyclic loading and a secondary one for the highest. A two scales probabilistic model is then developed in order to establish a dialogue between self-heating measurements and the fatigue properties. It is composed by a matrix having an elasto-plastic behaviorand a population of inclusions having a second elasto-plastic behavior with a random activation threshold. Both self-heating regimes can be perfectly described. By using the weakest link theory and an energetic criterion, a prediction of fatigue properties is made. Only three days are required to obtain acomplete fatigue curve. The pertinence of the approach is validated by a comparison with standard fatigue curves. Then, observations with optical microscopy, atomic force microscopy and EBSD are made on a high strength low alloyed steel grade. This study has two objectives: a better understanding of phenomenon occurring during cyclic loading leading to the two self-heating regimes; a justification of the ingredients introduced into the model. In a second important section, it deals with the influence of aplastic pre-strain on the fatigue properties evolution. Indeed, automotive components obtained from high strength steel sheets are subjected to primary forming operations, inducing plastic strain. These modifications lead to fatigue properties evolutions that are nevertheless not taken into account intraditional fatigue design of components (not determined with the standard method because ofprohibitive time). The speed of the proposed approach authorizes to characterize these evolutions. From the modifications of the self-heating properties after different modes of plastic pre-strain (tension, planetension, shearing) and by studying different directions of loading, it is possible to predict the fatigue properties evolutions for a wide range of plastic strain. The quality of the predictions is validated by acomparison with standard fatigue curves.

Este trabalho contempla um estudo sistemático das propriedades elétricas de óxidos cerâmicos a base de manganês. Ênfase foi dada a sistemas onde uma correlação forte entre os graus de liberdade de carga, spin e rede com ordenamento orbital resultam em um estado fundamental heterogêneo, devido a uma separação de fases. Com esse objetivo, foram preparadas amostras policristalinas e monocristalinas de Nd0.5Ca0.5Mn1-xCrxO3, 0.0 x 0.07. A caracterização destas amostras, via medidas de transporte elétrico (T) e de susceptibilidade magnética (T), revelou a ocorrência de uma temperatura de ordenamento de carga CO em TCO 250 K e que uma substituição pequena de Mn por Cr resulta na supressão desse estado CO, induzindo uma transição de fase do tipo metal-isolante (MI) no sistema. Concomitantemente a esta transição MI observa-se uma transição de fase do estado paramagnético PA isolante para um estado ferromagnético FM metálico em TMI ~ TC ~ 140 K. A análise combinada dos resultados experimentais de resistividade elétrica (T,H), magnetização (T) e de espectroscopia de impedância Z(,T) revelaram uma coexistência e competição entre fases na determinação do estado fundamental dessas manganitas. Tal competição foi observada ocorrer em uma larga faixa de temperatura, ou seja, abaixo da temperatura TCO 250 K até a mais baixa temperatura estudada de 1.4 K. Os dados também permitiram concluir que a natureza do estado fundamental desses materiais compreende de uma mistura de fases isolantes entre as temperaturas TCO 250 K e TMI ~ TC ~ 140 K. Por outro lado, e abaixo de TMI, o estado fundamental do sistema pode ser visualizado como sendo composto de uma fina mistura de duas fases: uma com ordenamento de carga e orbital (CO/OO) e de caráter isolante e uma outra ordenada ferromagneticamente FM e com características metálicas. A natureza deste estado fundamental heterogêneo foi confirmada através de medidas de relaxação da resistência elétrica (T,t) obtidas nas duas regiões de temperatura acima citadas. Os dados de (T,t) ainda permitiram concluir que o estado fundamental desses materiais além de heterogêneo é dinâmico, como esperado em um cenário de separação de fases. Uma outra característica desse estado heterogêneo, notadamente abaixo de TMI, é que o mesmo responde de forma não convencional a estímulos diversos, incluindo grandes excitações de corrente elétrica aplicada I. Nesse contexto, a natureza heterogênea do estado CO para T < TCO, bem como da coexistência de fases CO e FM em T < TMI foi provada via um estudo sistemático das propriedades de transporte e magnetização usando diferentes intensidades de corrente elétrica aplicada em medidas de (T,I), M(T,I) e através de curvas características V-I. A observação de fenômenos não lineares, principalmente em curvas características V-I, indicou que os mesmos são precursores de transições de fase abruptas, quando altas densidades de corrente são aplicadas nos materiais. Os dados também permitiram concluir que a corrente elétrica não é distribuída homogeneamente neste estado fundamental heterogêneo. Isto implica em uma localização de corrente e conseqüente efeito Joule dentro do material. A dissipação devido ao efeito Joule é responsável por um auto-aquecimento do material e pode ser suficiente para induzir transições de fase devido ao aumento de temperatura da amostra. A aplicação de um modelo simples de dissipação de calor aplicado aos dados experimentais indicam que o fenômeno de localização de corrente e efeito Joule são fundamentais para o entendimento de transições de fase induzidas por corrente elétrica nessas manganitas. / A systematic study of the electrical properties in doped manganese oxides is presented. Special attention was given to compositions where the strong correlation between charge, spin, and lattice degrees of freedom with orbital ordering resulting in a heterogeneous ground state leads to phase separation. To do this work, polycrystalline and monocrystalline Nd0,5Ca0,5Mn1-xCrxO3, 0,0 x 0,07 samples were prepared. The results obtained through electrical transport (T) and, magnetic susceptibility (T) have revealed the occurrence of charge ordering at TCO 250 K. A small partial substitution of Mn by Cr results in a suppression of the long range charge ordering state and induces both a magnetic from paramagnetic PA to ferromagnetic FM and a electronic from insulating to metallic phase transition at TMI ~ TC ~ 140 K. A combined analysis of the experimental results performed through (T,H), (T), and impedance spectroscopy Z(,T) revealed the coexistence of competing phases in the ground state of these manganites. Such a competition has been found in a large temperature range, from TCO 250 down to 1,4 K. In addition, it is suggested that the ground state comprises a delicate mixture of insulating phases between TCO 250 K e TMI ~ TC ~ 140 K. On the other hand, below TMI, the ground state can be visualized as comprised of two phases: (1) insulating charge orbital ordering (CO/OO) and (2) ferromagnetic metallic phases. The nature of this heterogeneous ground state was confirmed through relaxation measurements (T,t) performed in both temperature intervals cited above. The data indicated that besides to be heterogeneous this ground state is dynamical, as expected in the phase separation scenario. Moreover, this ground state responds in an unconventional fashion when the system is stimulated by electrical current, notably below TMI. Within this context, the heterogeneous nature of the CO state for T < TCO, and the coexistence of CO and FM phases for T < TMI, were studied through magnetic and electrical measurements using electrical current of different magnitude (T,I), M(T,I) and characteristic V-I curves. The non-linear phenomena are precursors of the very sharp transition when high electrical current density is applied. The data also allows to conclude that the electrical current is not homogeneously distributed throughout the sample in this ground state. Differently, the electrical current is localized in thin channels bringing about a large self-heating Joule effect. We argue that the dissipation due to Joule effect is responsible for the self-heating which in turn is large enough to induce phase transition due to the temperature raise. The application of a simple heat dissipation model to the experimental data reveals that both the electrical current localization phenomenon and the Joule effect are very important to the understanding of the current-induced phase transition in these manganites.

The ultra-short gate (LG < 20 nm) CMOS components (Complementary Metal-Oxide-Semiconductor) face thermal limitations due to significant local heating induced by phonon emission by hot carriers in active regions of reduced size. This phenomenon, called self-heating effect, is identified as one of the most critical for the continuous increase in the integration density of circuits. This is especially crucial in SOI technology (silicon on insulator), where the presence of the buried insulator hinders the dissipation of heat.At the nanoscale, the theoretical study of these heating phenomena, which cannot be led using the macroscopic models (heat diffusion coefficient), requires a detailed microscopic description of heat transfers that are locally non-equilibrium. It is therefore appropriate to model, not only the electron transport and the phonon generation, but also the phonon transport and the phonon-phonon and electron-phonon interactions. The formalism of the Boltzmann transport equation (BTE) is very suitable to study this problem. In fact, it is widely used for years to study the transport of charged particles in semiconductor components. This formalism is much less standard to study the transport of phonons. One of the problems of this work concerns the coupling of the phonon BTE with the electron transport.In this context, wse have developed an algorithm to calculate the transport of phonons by the direct solution of the phonon BTE. This algorithm of phonon transport was coupled with the electron transport simulated by the simulator "MONACO" based on a statistical (Monte Carlo) solution of the BTE. Finally, this new electro-thermal simulator was used to study the self-heating effects in nano-transistors. The main interest of this work is to provide an analysis of electro-thermal transport beyond a macroscopic approach (Fourier formalism for thermal transport and the drift-diffusion approach for electric current, respectively). Indeed, it provides access to the distributions of phonons in the device for each phonon mode. In particular, the simulator provides a better understanding of the hot electron effects at the hot spots and of the electron relaxation in the access.

The objective of the dissertation is to combine the recent Mixed-Mode reliability stress studies into a single text. The thesis starts with a review of silicon-germanium heterojunction bipolar transistor fundamentals, development trends, and the conventional reliability stress paths used in industry, after which the new stress path, Mixed-Mode stress, is introduced. Chapter 2 is devoted to an in-depth discussion of damage mechanisms that includes the impact ionization effct and the selfheating effect. Chapter 3 goes onto the impact ionization effect using two-dimensional calibrated MEDICI simulations. Chapter 4 assesses the reliability of SiGe HBTs in extreme temperature environments by way of comprehensive experiments and MEDICI simulations. A comparison of the device lifetimes for reverse-EB stress and mixed-mode stress indicates different damage mechanisms govern these phenomena. The thesis concludes with a summary of the project and suggestions for future research in chapter 5.

More and more electronic products, like battery chargers and power supplies, as well as applications in telecommunications and automotive electronics are based on System-on-Chip solutions, where signal processing and power devices are integrated on the same chip. The integration of different functional units offers many advantages in terms of reliability, reduced power consumption, weight and space reduction, leading to products with better performance at a hopefully lower price. This thesis focuses on the integration of vertical high-voltage double-diffused MOS transistors (DMOSFETs) on Silicon-on-Insulator (SOI) substrates. MOSFETs possess a number of features which makes them indispensable for Power Integrated Circuits (PICs): high switching speed, high efficiency, and simple drive circuits. SOI substrates combined with trench technology is superior to traditional Junction Isolation (JI) techniques in terms of cross-talk and leakage currents. Vertical DMOS transistors on SOI have been manufactured and characterized, and an analytical model for their on-resistance is presented. A description of self-heating and operation at elevated temperatures is included. Furthermore, the switching dynamics of these components is investigated by means of device simulations with the result that the dissipated power during unclamped inductive switching tests is reduced substantially compared to bulk vertical DMOSFETs. A large number of defects is created in the device layer if the trenches are exposed to high temperatures during processing. A new fabrication process with back-end trench formation is introduced in order to minimize defect generation. In addition, a model for the capacitive coupling between trench-isolated structures is developed.

Wood pellets have become a solid biomass fuel of choice because they are a standardized product with known quality properties. However, the self-heating and off-gassing tendencies during storage of wood pellets threatens the consistency of the pellets’ quality properties. The aim of this thesis was to increase the body of knowledge towards understanding of the off-gassing of wood pellets. The effects of total wood extractive content and types of extractives in the raw material on off-gassing of wood pellets were investigated through two separate studies. In the first study, the pellets were produced from fresh and pre-treated Scots pine sawdust. The pre-treatments involved storing, extraction and adding additive oils. The second study used synthetic pure cellulose that was pre-treated by adding different additive oils. The pellets were subjected to off-gassing tests under controlled conditions. The results from the first study showed that the total amount of extractives in the raw material has little effect on off-gassing. While gas emissions were reduced for stored and acetone extracted pine pellets, the coefficients of determination (R2) from the linear correlation analysis between off-gassing and the total extractive content of the raw materials were below 0.5 for all the three off-gasses indicating no correlation. The results of cellulose pellets with added additive oils in the second study showed that the off-gassing is highly dependent on the type of extractives in the raw material. The highest mean concentrations of the carbon oxides and methane were recorded from cellulose pellets with added linseed oil. Pellets with added linseed oil had higher off-gas emissions due to the high content of unsaturated fatty acids of 73.9% linolenic and 7.6% linoleic. Based on these results, it was concluded that one of the main causes of off-gassing during storage of wood pellets is the α-oxidation of unsaturated fatty acids. The other notable effect was that methane formation is dependent on anaerobic conditions, whereas formation of carbon oxides can occur both under aerobic and anaerobic conditions. / Wood pellets have over the years become a solid biomass fuel of choice for heat and power generation because they are a standardized product with known quality properties. However, the self-heating and off-gassing tendencies of wood pellets threatens the consistency of the quality properties. Self-heating and off-gassing can cause disintegration of the pellets resulting in dry matter losses and reduction in quality properties. Additionally, self-heating may lead to fires while off-gassing of toxic gasses such as carbon monoxide is a human health and environmental hazard. The aim of this thesis was to increase the body of knowledge towards the understanding of off-gassing of wood pellets. The results showed that total amount of extractives in the raw material has little effect on off-gassing. One of the main causes of off-gassing of carbon oxides by wood pellets is the α-oxidation of unsaturated fatty acids and other triglyceride oils that are chemically unstable. The other notable effect was that methane formation is dependent on anaerobic conditions, whereas formation of carbon oxides can occur both under aerobic and anaerobic conditions. / <p>This study was part of the project on secure and well-characterised raw materials and products through innovative adaption of pellets manufacturing processes (SVINPELS, project no 47997–1). This was a collaborative research project between Karlstad University and the Swedish University of Agricultural Sciences, SLU and was funded by the Swedish Energy Agency.</p>