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  • 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.
51

Transport de phonons dans le régime quantique / Phonon transport in the quantum regime

Tavakoli-Ghinani, Adib 14 December 2017 (has links)
Ce travail de thèse est consacré à la mesure de transport de chaleur par les phonons dans le régime quantique dans des systèmes confinés à très basse température.Le contexte de ce sujet est de soumettre ces systèmes à deux conditions extrêmes : basse température et faibles dimensions et de comprendre les propriétés thermiques fondamentales issues de ces limites.Les échantillons étudiés au cours de cette thèse sont des structures suspendues (membrane ou nanofil) ; elles sont élaborées à partir de nitrure de silicium amorphe (SiN).En abaissant la température, les longueurs caractéristiques des phonons comme le libre parcours moyen ou la longueur d'onde dominante des phonons augmentent. Lorsque ces longueurs caractéristiques dépassent les dimensions latérales du système, la diffusion sur les surfaces (boundary scattering) régira les propriétés thermiques. Dans cette limite de diffusion, le transport des phonons va de la diffusion aux surfaces (régime de Casimir) au régime balistique (limite quantique). Dans ce régime balistique, le courant de chaleur peut être exprimé en utilisant le modèle de Landauer. La conductance thermique est alors exprimée par: K=N_α q T où, N_α est le nombre de modes vibratoires peuplés, q=((π²k_B^2)T)⁄3h est la valeur universelle du quantum de conductance thermique et T est le coefficient de transmission.Dans ce travail, les mesures de conductance thermique de nanofils suspendus ont été effectuées jusqu'à très basse température. Une plate-forme de mesure ayant une sensibilité sans précédent a été développée pour mesurer la variation d'énergie inférieure à l'attojoule. Ces nouveaux capteurs permettent de mesurer les propriétés thermiques du guide d'onde de phonon 1D dans le régime quantique du transport de chaleur. Nous montrons que le coefficient de transmission est le facteur dominant qui définit la valeur de conductance thermique. Ce coefficent dépend de la dimension et de la forme des réservoirs ainsi que de la nature du matériau utilisé ce qui rend difficile la mesure du quantum de conductance thermique. Nous montrons que dans toutes les structures de SiN mesurées, le transport thermique pourrait être dominé par des excitations de faible énergie qui existent dans les solides amorphes (a-solides).Le deuxième ensemble important d'expériences concerne la chaleur spécifique. Nous avons étudié les propriétés thermiques de membranes suspendues de SiN très minces que l'on pense être des cavités de phonon 2D. Nous montrons que la dépendance en température de la chaleur spécifique s'écarte du comportement quadratique comme prévu à très basse température. Les modèles pertinents donnant une explication quantitative des résultats sont encore à l'étude. La présence de systèmes à deux niveaux dans les matériaux amorphes pourrait être une explication possible de la valeur absolue élevée de la chaleur spécifique observée. / This PhD entitles Phonon heat transport in the quantum regime is based on the analysis of the thermal properties of confined systems at very low temperature.The context of this subject is putting the systems in two extreme conditions (low temperature and low dimensions) and understand the fundamental thermal properties coming from these limits.The studied samples during this PhD that are suspended structures (membrane or nanowire) are elaborated from amorphous silicon nitride.By lowering the temperature, the phonon characteristic lengths like the mean free path or the phonon dominant wavelength increase. When these characteristic lengths exceed lateral dimensions of the system, the boundary scattering will govern the thermal properties. In the boundary scattering, phonon transport goes from boundary limited scattering (Casimir regime) to ballistics regime (quantum limit). In this ballistic regime, the heat current can be expressed using the Landauer model. The thermal conductance is then expressed as: K=N_α q T where N_α is the number of populated vibrational modes, q=((π²k_B^2)T)⁄3h is the universal value of quantum of thermal conductance, and T is the transmission coefficient.In this work, thermal conductance measurements of suspended nanowires have been performed down to very low temperature. A measurement platform having an unprecedented sensitivity have been developed that can measure a variation of energy smaller than the attojoule. These new sensors allow the measurement of thermal properties of 1D phonon waveguide in the quantum regime of heat transport. We show that the transmission coefficient is the dominant factor that set the thermal conductance value. It depends on the dimension and the shape of the reservoirs, and the nature of the material in use rendering difficult the measurement of the quantum of thermal conductance. We show that in all of the SiN structures, the thermal transport could be dominated by low energy excitations that exist in amorphous solids (a-solids).The second important set of experiments concerns the specific heat. We have studied suspended the thermal properties of very thin SiN membranes that are thought to be 2D phonon cavities. We show that the temperature dependence of the specific heat departs from the quadratic behavior as expected at very low temperature. The true models giving a quantitative explanation of the results is still under consideration. The presence of tunneling two-level systems in amorphous materials could be one possible explanation for the high absolute value of specific heat that has been measured.
52

[en] TRANSIENT HEAT TRANSFER MODELING OF THERMALLY INSULATED OIL OR GAS PIPELINES / [pt] MODELAGEM TRANSIENTE DA TRANSFERÊNCIA DE CALOR EM DUTOS DE PETRÓLEO OU GÁS, TERMICAMENTE ISOLADOS

JHOANY JHORDANN BARRERA ESCOBEDO 07 February 2006 (has links)
[pt] Linhas submarinas são utilizadas na produção e transporte de petróleo e seus derivados. Em ambas as situações, o controle da transferência de calor do fluido para o ambiente externo pode ser um fator determinante para o escoamento. No caso de produção em águas profundas, o fluido aquecido perde calor para a água do mar gelada. A perda de calor é controlada através do isolamento térmico, o qual é projetado para operações de escoamento em regime permanente. Durante eventuais paradas de operação, o fluido estagnado no interior da tubulação ao perder calor para o ambiente frio, pode atingir níveis críticos de temperatura, acarretando graves problemas, tais como formação de hidratos ou deposição de parafina nas paredes da tubulação, o que pode levar ao bloqueio da linha e interrupção de produção. No transporte de produtos, o reinício de bombeio de fluidos muitos viscosos também é um problema crítico, devido ao aumento significativo da viscosidade com a redução da temperatura. O presente trabalho apresenta uma análise da influência da capacidade térmica da parede do tubo e das camadas de revestimento no transiente térmico de linhas com muito isolamento. A perda de calor da linha para o ambiente é determinada resolvendo-se a equação transiente de condução de calor para as camadas de revestimento da tubulação, utilizando um modelo uni-dimensional na direção radial. O método de volumes finitos é empregado para resolver o escoamento transiente no interior da tubulação acoplado com o transiente térmico na parede da tubulação, a partir do instante em que uma válvula é fechada na extremidade da tubulação interrompendo o fluxo. Comparações com as previsões de softwares comerciais foram realizadas e suas limitações são discutidas. Resultados obtidos das simulações para o escoamento tanto de líquidos quanto de gases, considerando e desprezando a capacidade térmica, mostram que o efeito da mesma é relevante na determinação do tempo de resfriamento da linha e do fluido em seu interior. / [en] Subsea pipelines are employed not only for production but also for transportation. In both situations, warm oil loses heat to the cold sea water. The heat loss to the ambient is controlled by means of thermal insulation, which is designed for steady state operations. During shutdowns, the stagnant fluid in the pipeline loses heat to the cold surrounding, eventually reaching some critical temperature. As a result, several problems can occur, such as formation of hydrates or deposition of high molecular weight paraffins on the inner wall of the subsea line, which can lead to flow line blockage and production shutdown. Restart of very viscous fluid after shutdown is also critical, since viscosity increases significantly with the reduction of the temperature. This work presents an analysis of the influence of the pipe wall thermal capacitance on the transient behavior of heavily insulated lines. The heat loss from the pipeline is determined, by solving the transient heat conduction equation for the pipewall layers, utilizing a simple one-dimensional model in the radial direction. The finite volume method is employed to solve the transient flow inside the pipeline, from the time instant that a valve at the end point of the line is closed, coupled with the pipe wall thermal transient. Comparisons with the prediction of commercial softwares were performed and their limitations are addressed. Numerical results obtained for flows of both liquid and gases, considering and neglecting the thermal capacitance, revealed that accounting for the thermal capacity of the wall is relevant to the determination of cooldown times
53

Structure interne et propriétés thermiques macroscopiques, application aux matériaux de construction / Internal structure and macroscopic thermal properties, application to construction materials

Faye, Mactar 26 May 2016 (has links)
L'objectif de cette thèse est d'étudier l'impact de la structure interne des matériaux isotropes granulaires sur leurs propriétés thermiques macroscopiques. Nous avons développé un code de calcul pour résoudre les transferts thermiques au sein d'un matériau hétérogène tridimensionnel. Ce code est couplé avec un algorithme de génération de structures aléatoires. Après validation expérimentale, nous avons généré des géométries granulaires dont nous avons caractérisé la structure interne, puis nous avons étudié l'impact de cette structure sur la conductivité thermique. Nous avons également développé une nouvelle méthode de mesure expérimentale de la capacité thermique surfacique d'un élément de paroi à structure interne complexe. L'originalité de cette méthode est le couplage d'un modèle analytique de la capacité thermique, indépendant des propriétés thermiques des constituants, et d'une étude expérimentale. / The objective of this thesis is to study the impact of the internal structure of isotropic granular materials on the macroscopic thermal properties. We have developed a model to solve the heat transfer problem within a heterogeneous three-dimensional material. This code is coupled with an algorithm generating random structure. After an experimental validation, we first generated granular materials and we characterized their internal structure; then we studied the impact of this structure on the thermal conductivity. We also developed a new experimental method for measuring the heat capacity area of a wall element with complex internal structure. The originality of this method is the coupling of an analytical model of heat capacity area, which is independent of the thermal properties of the constituents, and an experimental study.
54

Nanoscale structuration effects on phonon transport at low temperatures / Transport quantique de phonons dans des nanostructures à très basse température

Blanc, Christophe 05 November 2013 (has links)
Cette thèse, intitulé « Effet de structuration à l'échelle du nanomètre sur le transport de phonon à basse température » c'est déroulé pendant trois ans au sein du groupe Thermodynamique et Biophysique des Petits Systèmes de l'Institut Néel.Il s'agit de comprendre et de contrôler le transport de chaleur au sein d'échantillons ayant des variations de l'ordre du nanomètre. Ces échantillons ont surtout été des nanofils suspendus en silicium. La fabrication a été réalisée au sein de l'Institut Néel. Lors de ces trois années, trois résultats importants ont été réalisés.Tout d'abord, il a fallu vérifier que le transport de chaleur ne soit pas dominé par un effet dû aux contacts entre le nanofil suspendu et le bain thermique. Cela a pu être mis en évidence grâce à la concordance entre les mesures et le modèle appelé Casimir-Ziman. Mais cela a surtout été vérifié avec des fils dont la jonction au bain thermique a été adaptée afin de permettre une transmission proche de l'unité. Ces fils profilés ayant la même conductance thermique que les fils avec une jonction abrupte au bain thermique, cela prouve que la transmission est toujours proche de 1.Ensuite des mesures sur des fils dont la section est ondulée ont permis de montrer une réduction de la conductance thermique. Cette réduction est expliquée par la présence de rétrodiffusion des phonons à la surface, ce qui entraîne une grande réduction de leur libre parcours moyen. Ainsi, les phonons dans un nanofil droit ont un libre parcours moyen jusqu'à 9 fois plus grand que dans ces nanofils à la section ondulée. Des simulations avec la méthode de Monte-Carlo ont permis de mettre en évidence cet effet.Si ces premiers résultats ont été réalisés pour des fils de silicium monocristallin, le dernier travail a porté sur l'étude d'échantillon en nitrure de silicium. Ce matériau est un matériau amorphe. La physique du transport de chaleur au sein des matériaux amorphes n'est pas encore complètement comprise. Cependant les mesures faites sur ces matériaux montrent un comportement similaire, tant qualitatif que quantitatif, pour presque tous les matériaux amorphes. Nous avons donc mesurés des échantillons de différentes sortes, afin de vérifier si ce comportement était toujours valable, lorsque la dimension de l'échantillon est réduite. Le résultat de nos mesures est que la dimension joue un rôle sur le transport. Tout comme dans les matériaux cristallins, la basse dimension de l'échantillon va limiter le transport de chaleur. Cependant le transport dans les échantillons de basses dimensions montre le même comportement qualitatif que les matériaux amorphes massifs. Ce travail peut permettre de donner des pistes pour la compréhension du transport de chaleur au sein des matériaux amorphes.En conclusion ce travail m'a permis de fabriquer puis de mesurer le transport de chaleur dans différents types d'échantillons. Les résultats obtenus permettent une meilleur connaissance du transport des phonons, et donc aident à ouvrir la voie vers un meilleur contrôle du transport de la chaleur. / This PhD entitled "Nanoscale structuration effect on the phonon transport at low temperature" take place for three years in the Thermodynamique et Biophysique des Petits Systèmes of the Institut Néel.The context of this PhD is to understand and control the heat transport in samples with variations at the nanoscale. These samples were mostly suspended silicon nanowires. The production was performed in the Néel Institute. During these three years, three important results have been demonstrated.First, we verify that heat transport is not dominated by an effect due to the contact between the suspended nanowire and the thermal bath. This has been demonstrated by the agreement between the measurements and the model called Casimir-Ziman. It was also mainly verified with wires whose junction to the thermal bath has been adapted to allow transmission close to unity. These profiles nanowires have the same thermal conductance as a nanowire with abrupt junction to the thermal bath. This proves that the transmission is always close to 1.Then measurements on nanowires whose section is corrugated have shown a reduction in thermal conductance. This reduction is explained by the presence of backscatter phonons at the surface, resulting in a large reduction of their mean free path. Thus, the phonons in a smooth nanowire have a mean free path up to 9 times greater than in these corrugated nanowires. Simulations with the Monte-Carlo method also demonstrate this effect.If these first results were achieved for monocrystalline silicon nanowires, my last work has focused on the study sample of silicon nitride. This material is an amorphous one. Physics of heat transport in amorphous materials is not yet fully understood. However, measurements on these materials show a similar behavior, both qualitatively and quantitatively, for almost all amorphous materials. We have measured samples of different kinds, to see if this behavior was still valid when the sample size is reduced. The result of our measurements is that the size plays a role in transport. As in crystalline materials, the small sample size will limit the heat transport. However transport in low-dimensional samples shows the same behavior qualitatively as in bulk amorphous materials. This can help provide clues for understanding the heat transport in amorphous materials.In conclusion, this work has allowed me to make and measure the heat transport in different types of samples. The results allow a better knowledge of the phonon transport, thus helping to pave the way towards a better control of heat transport.
55

Implantação das técnicas de fotoacústica e pc e aplicações em sistemas vítreos

Silva, Alexandre Pinheiro da 30 September 2011 (has links)
Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-07-24T12:08:43Z No. of bitstreams: 1 alexandrepinheirodasilva.pdf: 29097377 bytes, checksum: 94bcde5f459e88c78cffbdf774e348a0 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-08-09T13:30:24Z (GMT) No. of bitstreams: 1 alexandrepinheirodasilva.pdf: 29097377 bytes, checksum: 94bcde5f459e88c78cffbdf774e348a0 (MD5) / Made available in DSpace on 2017-08-09T13:30:24Z (GMT). No. of bitstreams: 1 alexandrepinheirodasilva.pdf: 29097377 bytes, checksum: 94bcde5f459e88c78cffbdf774e348a0 (MD5) Previous issue date: 2011-09-30 / Neste trabalho vamos apresentar a implantação de duas técnicas fototérmicas: a fotoacústica de câmara aberta que possibilita encontrarmos a difusividade térmica dos materiais, e a técnica pc que fornece o valor do produto da densidade pelo calor específico da substância, conhecido como capacidade térmica volumétrica. Utilizamos também a técnica de lente térmica para a medida da difusividade térmica e do parâmetro dS/dT (taxa de variação do caminho óptico S). As técnicas mencionadas fornecem os parâmetros termo-ópticos (Difusividade Térmica, Condutividade Térmica e dS/dT ) para materiais vítreos como os vidros ferro fosfato, telurito, borossilicato e simulado lunar JSC-1. Modelos teóricos são apresentados e testados. Mostramos que o modelo da flexão termoelástica é dominante na maioria das amostras estudadas. Exceção foi observado no caso de vidros semicondutores (ferro fosfato). Neste caso, correções foram necessárias para o modelo da flexão termoelástica, possibilitando obter: difusividade térmica, coeficiente de difusão de portadores, velocidade de recombinação dos portadores na superfície e tempo de recombinação na superfície. As técnicas foram testadas com amostras de alumínio com 99,99 % de pureza, que possuem as informações térmicas disponíveis na literatura. Nos resultados, identificamos, através da técnica fotoacústica, que o vidro ferro fosfato é um vidro semi-condutor. Vidros ferro fosfato são candidatos para aplicações de vitrificação de resíduos nucleares. Comparações foram feitas com vidros borossilicato que são atualmente usados na vitrificação dos resíduos nucleares. Obtivemos a difusividade térmica, a capacidade térmica volumétrica e a condutividade térmica dos vidros do simulado lunar JSC-1 e telurito. O primeiro se mostrou eficiente para aplicações em exploração espacial e o segundo tem potencial aplicação em dispositivos fotônicos. / In this work we present the implantation of two photothermal techniques : the open cell photoacoustic which allows one to find thermal diffusivity of materials, and pc technique which gives the value of the product of density by specific heat of substance, known as the volumetric heat capacity. We also used the technique of thermal lens for measurement of thermal diffusivity and the parameter dS/dT (the temperature coefficient of the optical path S). The mentioned techniques furnished the thermo-optical parameters (Thermal Diffusivity, Thermal Conductivity and dS/dT) of glassy materials such as iron phosphate, tellurite, borosilicate glasses and lunar simulate JSC-1. Theoretical Models are presented and tested. It was shown that the model of thermoelastic bending is dominant in most samples studied. Exception is observed in the case of semiconducting glasses (iron phosphate). In this case, corrections were necessary to the bending model, and allowed one to obtain: thermal diffusivity, carrier 's diffusion coefficient, carriers surface recombination velocity and surface recombination time. Techniques were tested with aluminum samples with 99.99 % purity, which have the thermal information available in the literature. As a result, we identified, through the photoacoustic technique, that iron phosphate glass is a semiconductor glass. Iron phosphate are candidates for nuclear waste vitrification applications. Comparison was made with borosilicate glasses that are currently used for nuclear waste vitrification. We also obtained the thermal diffusivity, the volumetric heat capacity and thermal conductivity of simulated lunar JSC-1 and tellurite glasses. The former proved efficient applications in space exploration and the latter has potential applications as photonic device.
56

Thermodynamische Untersuchungen in den Systemen Lithium-Silicium und Lithium-Zinn

Taubert, Franziska 12 October 2017 (has links) (PDF)
Lithium-Ionen-Batterien besitzen ein ausgezeichnetes Potential für die Energiespeicherung. Das derzeit dominierende Anodenmaterial in Lithium-Ionen-Batterien mit einer Energiespeicherkapazität von 339 mAh/g ist Graphit. Als Alternative hierfür bieten sich Lithiumsilicide und Lithiumstannide an. Diese Materialien zeichnen sich durch eine viel größere Speicherkapazität und geringere Selbstentladungspotentiale aus. Für die kommerzielle Anwendung dieser beiden Systeme in Lithium-Ionen-Batterien werden grundlegende und verlässliche thermodynamische Daten benötigt. Derzeit ist die Existenz von sieben Lithiumsiliciden sicher nachgewiesen. Dazu zählen die sechs stabilen Phasen Li17Si4, Li16.42Si4, Li13Si4, Li7Si3, Li12Si7, die Hochdruckphase LiSi und die metastabile Phase Li15Si4. Für die ersten fünf genannten Phasen wurden in der ersten Förderperiode des Schwerpunktprogrammes 1473 Wärmekapazitäten und Standardentropien bestimmt. Bei den Lithiumstanniden sind derzeit sieben Phasen gesichert belegt. Allerdings existiert für keine Phase der Lithiumstannide ein verlässlicher thermodynamischer Basisdatensatz. Aus diesem Grund wurden für die beiden zuletzt genannten Lithiumsilicide (Li15Si4 und LiSi), sowie für die Lithiumstannide Li17Sn4, Li7Sn2, Li13Sn5 und Li7Sn3 die fehlenden thermodynamischen Daten experimentell bestimmt. Die hergestellten Phasen wurden zunächst mittels Röntgenbeugung, thermischer und chemischer Analyse charakterisiert. Ein Schwerpunkt dieser Arbeit lag auf der experimentellen Bestimmung der Wärmekapazitäten in einem Temperaturbereich von 2 K bis zur jeweiligen Zersetzungstemperatur der untersuchten Verbindungen. Hierfür wurden zwei unterschiedliche Kalorimeter verwendet: ein Physical Property Measurement System (Quantum Design) von 2 K bis 300 K und eine DSC 111 (Setaram), beginnend ab 300 K. Die experimentellen Daten konnten mit Messunsicherheiten von 1 % bis 2 % über 20 K und bis zu 20 % unterhalb von 20 K angegeben werden. Die Messungen bei niedrigen Temperaturen erlauben zudem die Berechnung der Standardentropien, sowie die Bestimmung von elektronischen Beiträgen und Gitterschwingungsbeiträgen zur Wärmekapazität. Weiterhin ist Fokus dieser Arbeit die Bestimmung der Standardbildungsenthalpien der Lithiumsilicide und Lithiumstannide auf Basis von Wasserstoffsorptionsmessungen mittels einer Sieverts-Apparatur. Hierfür wurden erstmals Messungen an den Lithiumsiliciden ausgehend von Li17Si4, LiH:Si (Li:Si = 17:4), Li16.42Si4 und LiSi durchgeführt. Für die Lithiumstannide dienten als Ausgangsmaterial Li17Sn4, LiH:Sn (Li:Sn =17:4), sowie Li7Sn2 und LiH:Sn (Li:Sn = 7:2). Die Anwendung des van´t-Hoff-Plots resultierte in Messunsicherheiten von mindestens 10 %. Aus diesem Grund wurde eine alternative Auswertemethode gewählt, bei der die ermittelten Wärmekapazitäten und Standardentropien mit den Gleichgewichtsdrücken aus den Wasserstoffsorptionsmessungen miteinander verknüpft werden. Auf diese Weise konnten Standardbildungsenthalpien für die untersuchten Phasen mit Fehlern kleiner 1 % ermittelt werden. Aus den Ergebnissen dieser Arbeit resultierte ein vollständiger, gesicherter thermodynamischer Datensatz für das System Li-Si. Das berechnete Li-Si-Phasendiagramm ist im sehr guten Einklang mit experimentellen literaturbekannten Daten. Für die Lithiumstannide erfolgte eine Validierung der ermittelten thermodynamischen Werte. Die in dieser Arbeit erzielten Ergebnisse liefern einen wesentlichen Beitrag zur Verbesserung der Datenbasis für thermodynamische Berechnungen und für das Verständnis von Phasensequenzen und Gleichgewichten beim Einsatz von Lithiumsiliciden bzw. Lithiumstanniden als Anodenmaterialien in Lithium-Ionen-Batterien.
57

Optimalizace návrhu solárního kolektoru využívající latentní teplo fázové přeměny / Optimal design of solar air collector with latent heat thermal energy storage

Zálešák, Martin January 2018 (has links)
The thesis deals with the creation for a numerical model of a solar collector with a phase change material as a heat storage medium. The model was implemented in Python. Using the created model, design optimization of several problems was carried out with the use of selected methods of heuristic optimization. The results of the behaviour of the created model and of design optimization were then analysed and evaluated.
58

Vliv rozdílné tepelné kapacity DPS a součástek na podélný teplotní profil u pájení přetavením / Temperature Profile in Reflow Soldering and Influence of Different PCBś and ComponentsThermal Capacities

Procházka, Martin January 2011 (has links)
This thesis mainly deals with the prediction of temperature on the components and the PCB during reflow soldering. The theoretical part describes the particular solder reflow process, types of heat transfer and temperature profiles. The practical part is divided into forecasting temperatures if the conveyor is stopped and the temperature predictions when the conveyor is in motion. In both parts of the measured temperature is compared with the predicted temperatures, which show the success rate of prediction. The last part of this work is part of the simulation, which helps in proper understanding of the issues discussed.
59

Nuclear magnetic resonance and specific heat studies of half-metallic ferromagnetic Heusler compounds

Rodan, Steven 26 January 2016 (has links)
Half-metallic ferromagnets (HMFs), with fully spin-polarized conduction electrons, are prime candidates for optimizing spintronic devices. Many Heusler compounds (a class of ternary and quaternary intermetallics) are predicted to be HMFs, in particular Co$_{2}YZ$ (where $Y$ is usually another transition metal, and $Z$ is an s-p element). Crystal structure is controlled by thermodynamics to a large extent. Ideally, one should be able to control and optimize properties which are of interest by appropriately "tuning" the structure (e.g. annealing), but first one must understand the structure and its relation to observed physical properties. A local structural probe technique such as nuclear magnetic resonance (NMR) is an essential tool for identifying and quantifying the various atomic-scale orderings. Different Heusler structure types and antisite disorders affect the material's physical properties. In this thesis, order-disorder phenomena in both bulk and thin film samples of Co$_2$Mn$_{1-x}$Si$_x$ and Co$_2$Mn$_{1-x}$Fe$_x$Si have been systematically studied using NMR. Though it is the films which are directly implemented in actual devices, studying bulk samples as model systems provides invaluable information regarding the material properties. The evolution of local atomic structure in numerous thin films has been shown to depend greatly on preparation parameters, including post-deposition annealing temperature, and specific stoichiometry. For Co$_2$MnSi films, the ideal post-annealing temperature for promoting the $L2_1$ atomic structure was found; the threshold temperature above which structure continues to become higher-ordered in the bulk, but where too much interdiffusion at the buffer interface occurs, degrading the smooth interfaces necessary for high magnetoresistance ratios. NMR also adds evidence that Co$_2$Mn$_x$Si$_{0.88}$ ($x>$1) electrodes in magnetic tunnel junctions have highest tunneling magneto-resistance because the excess Mn suppresses the formation of detrimental Co$_{Mn}$ antisites. A systematic investigation of several thermal and magnetic properties, including Sommerfeld coefficients, Debye temperatures, saturation magnetic moments, spin-wave stiffness, and magnon specific heat coefficient, were measured for selected Co$_2$-based ternary and quaternary Heusler compounds. Obtained values were compared with theoretical ones calculated using electronic band structure methods. It has been systematically shown that adding a magnon term to the specific heat has a negligible effect on the electronic contribution in all cases.
60

Thermodynamische und kinetische Untersuchungen im System Lithium-Silicium

Thomas, Daniel 16 January 2015 (has links)
Die vorliegende Dissertation stellt die experimentelle Bestimmung von grundlegenden thermodynamischen und kinetischen Stoffdaten im System Lithium-Silicium vor. Ausgehend von der Synthese qualitativ hochwertiger Lithiumsilicide wurden Wärmekapazitäten über einen großen Temperaturbereich (2-873 K) bestimmt, die aufgrund der Ergebnisse bei tiefen Temperaturen die Ermittlung weiterer Parameter wie beispielsweise der Standardentropien bzw. der Bildungsentropien der Lithiumsilicide ermöglichte. Die Eigenschaft der Silicide, mit Wasserstoff Verbindungen einzugehen, führte zudem zur Ausdehnung der Untersuchungen auf das System Li-Si-H. Aus der Erweiterung resultierte neben der formalkinetischen Beschreibung ablaufender Gleichgewichtsreaktionen die Bestimmung von Bildungsenthalpien der Silicide. Auf Grundlage der experimentell bestimmten Stoffgrößen (Cp, S°, ∆BH°), die für theoretische und praxisrelevante Berechnungen sehr verlässliche Stoffdaten darstellen, wurden thermodynamische Modellierungen im stofflichen System durchgeführt.

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