Thermal energy storage is a major contributor to bridge the gap between energy demand (consumption) and energy production (supply) by concentrating solar power. The utilization of high latent heat storage capability of phase change materials is one of the keys to an efficient way to store thermal energy. However, some of the limitations of the existing technology are the high volumetric expansion and low thermal conductivity of phase change materials (PCMs), low energy density, low operation temperatures and high cost. The present work deals with encapsulated PCM system, which operates at temperatures above 500°C and takes advantage of the heat transfer modes at such high temperatures to overcome the aforementioned limitations of PCMs. Encapsulation with sodium silicate coating on preformed PCM pellets were investigated. A low cost, high temperature metal, carbon steel has been used as a capsule for PCMs with a melting point above 500° C. Sodium silicate and high temperature paints were used for oxidation protection of steel at high temperatures. The emissivity of the coatings to enhance heat transfer was investigated.

This study presents results from non-destructive testing to evaluate the degradation of the CFRP-masonry bond using thermal imaging. The goal of the research was to identify locations where there was evidence of bond deterioration that could subsequently be verified through destructive pull-off testing. Four full-scale masonry walls were built outdoors at the University of South Florida in 1995 to evaluate the effectiveness of CFRP for repairing settlement damage. Two of the settlement-damaged walls were repaired using single layer, commercially available unidirectional CFRP systems that used Tonen (wall 3) and Henkel (wall 2) epoxies. These two walls were the subject of this investigation. Before non-destructive tests were initiated, historical site data on temperature, humidity and rainfall variation was compiled. Over seventeen years, the walls experienced ambient temperatures as high as 98°F and as low as 25°F. The average rainfall in Tampa is about 34 inches and the annual average high humidity is around 87%#37;. Because of the high temperature and humidity, the CFRP-masonry bond was exposed to a particularly aggressive environment. Three types of thermal evaluation were carried out: thermocouple monitoring and both passive (solar) and active (localized heating) infrared thermal imaging. Twenty-four thermocouples were used to observe the spatial variations in temperature on the wall. Data showed that the surface temperatures of the wall are uneven with one end being hotter than the other. Measurements indicated that the wall temperatures went as high as 103°F during the week of data collection in late March and early April of 2012. In contrast, the highest ambient temperature over the same period was 92°F. The high temperature experienced by the wall is below the glass transition temperature for the epoxies, which ranges from 140°F to 180°F. A FLIR Tau 320 thermal imaging camera was used to identify localized de-bonding. Solar radiation heated the walls and the goal of thermal imaging was to detect hot spots which are indicative of de-bonding. Although this technique is ideal for exterior applications, initial attempts were unsuccessful. Once de-bonds were located by sounding, the camera was capable of confirming two hot spots on wall 2. A thermal scanner built by the university from a series of ten Omega OS137 thermal sensors was used to obtain more complete thermal images of the walls. This scanner had a heating element which supplied heat and allowed for active thermography. The scanner detected 16 hot spots not seen with the thermal camera. Ten of the twelve spots on wall 2 are concentrated on a region of the wall which experienced the highest daily changes in temperature, which indicates that higher thermal and environmental cycling has caused greater de-bond. Based on the number of hot spots found using both active and passive thermography the Tonen epoxy is performing better than the Henkel epoxy. In general, the bond has endured; however, there are a few localized areas that have de-bonded. Pull-off tests are recommended on walls 2 and 3. Five locations in regions suspected to have poor bond and five locations in regions suspected to have good bond are identified for each wall.

Thermal barrier coatings (TBC) are used for reduction of component temperatures in gas turbines. The service temperature for turbines can be as high as 1100ºC and the components are exposed to thermal cycling and gases that will cause the component to oxidize and corrode. The coatings are designed to protect the substrate material from this, but eventually it will lead to failure of the TBC. It is important to have knowledge about when this failure is expected, since it is detrimental for the gas turbine. The scope of this thesis has been to see if an existing life model for thin TBC also is valid for thick TBC. In order to do so, a thermal cycling fatigue test, a tensile test and finite element calculation have been performed. The thermal cycling fatigue test and finite element calculation were done to find correlations between the damage due to thermal cycling, the number of thermal cycles and the energy release rate. The tensile test was preformed to find the amount accumulated strain until damage. The thermal cycling lead to failure of the TBC at the bond coat/top coat interface. The measurment of damage, porosity and thickness of thermally grown oxide were unsatisfying due to problems with the specimen preparation. However, a tendency for the damage development were seen. The finite element calculations gave values for the energy release rate the stress intensity factors in mode~I and mode~II that can be used in the life model. The tensile test showed that the failure mechanism is dependent of the coating thickness and it gave a rough value of the maximum strain acceptable.

The aim and objective of this study was to determine the bond strength of a twostep and a single-step dentin bonding agent and to determine the effect of repeated thermal-cycling from 50C and 550C on the bond strength values of these dentin bonding agents.

Les travaux réalisés traitent de la caractérisation en fatigue thermique et mécanique d’assemblages collés par colle conductrice isotrope. Issu du domaine des circuits électroniques radio fréquences à applications spatiales, l’empilement type considéré dans l’étude est constitué d’un carré d’alumine reporté de manière automatique sur un support en Kovar.L’adhésif est à base d’époxyde chargé de microparticules d’argent pour la conduction électrique et thermique du report.Les surfaces de collage de l’alumine sont polies-rectifiées et dorées pour favoriser les fonctions RF, au détriment de la tenue du collage. L’objectif principal est l’identification des mécanismes de fatigue de l’assemblage face aux sollicitations thermiques subies pendant des essais de qualification thermiques réalisés au sol, qui se veulent représentatives de celles subies en orbite pendant toute la durée de la mission. Deux axes de recherche sont privilégiés dans cette étude. D’une part, le comportement du matériau adhésif est caractérisé par des essais de fluage et des essais cyclés réalisés sur des éprouvettes de colle. Le modèle viscoplastique Two-Layers est retenu et implémenté dans un modèle éléments finis de l’assemblage. La tenue de la colle aux interfaces constitue une seconde approche de caractérisation. Une faible tenue en mode I est constatée à l’issue d’une campagne DCB. D’autre part, deux plans d’expériences portant sur la géométrie et les conditions de report des assemblages sont réalisés en parallèle d’une étude paramétrique numérique. Ces deux approches complémentaires permettent de proposer des hypothèses relatives aux mécanismes de fatigue prépondérants au sein de l’assemblage dans son environnement thermique. La tenue en fatigue des collages s’avère très dépendante du procédé automatique qui insère de nombreux défauts (porosités) à l’interface du composant. Une caractérisation en fatigue par essai mécanique est proposée comme alternative au cyclage thermique lent en étuve. Bien que l’équivalence ne soit pas complète, les résultats présentent de bonnes perspectives pour aider à la caractérisation en fatigue de l’interface entre des composants céramiques et une colle conductrice. / This subject deals with the fatigue life characterization under thermal and mechanical loads of bonded assemblies with anisotropic conductive adhesive. In the framework of radio frequency electronics for space applications, the stack of study iscomposed of an alumina square bonded onto a Kovar plate following an automatic process. The adhesive is made of anepoxy based matrix loaded with silver flakes for electrical and thermal conduction. Surfaces of adhesion are polished andcoated with gold to improve RF functions despite the reduction of adherence. The main objective is to identify the fatigue mechanisms taking place in the assembly under thermal cycling due to its orbital environment during the mission and some representative on-ground qualification tests. The study is divided into two parts. First of all, the mechanical behavior of the adhesive is completely characterized by creep and recovery tests and cycling tests performed on bulk specimens. The model of material behavior, called Two-Layers, is identified and implemented in a finite elements model of the whole assembly. The adhesive resistance at the bonding interfaces is also studied. A certain weakness in mode I is observed from a DCB campaign. Secondly, two designs of experiments on geometry and thermal conditions are performed in parallel of a numerical parametric study. These two complementary approaches allow proposing some hypothesis upon the fatigue mechanisms occurring in the assembly under thermal cycling. The fatigue resistance of bonded assemblies depends on the automatic bonding process that creates numerous defects at the component interface (as porosities). A fatigue life characterization by the help of a mechanical test is proposed as an alternative to slow thermal cycling in an oven. Although, the equivalence is not entirely proven, results are promising and can help further with the fatigue study of the interface between such ceramic components and a conductive adhesive.

Submitted by Viviane Lima da Cunha (viviane@biblioteca.ufpb.br) on 2016-07-27T14:58:46Z No. of bitstreams: 1 arquivototal.pdf: 2690635 bytes, checksum: 755347a9fe593fec9ee6de7af48d4375 (MD5) / Made available in DSpace on 2016-07-27T14:58:46Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 2690635 bytes, checksum: 755347a9fe593fec9ee6de7af48d4375 (MD5) Previous issue date: 2015-07-16 / Shape memory alloys are widely used to produce mechanical actuators which operate in a temperature range that are responsible for the shape recovery of the sensor element. Such applications are numerous engineering. In this case, the main aspect responsible for a good performance of the system is the stability of the characteristics temperature of the alloy, i.e., the start and finish temperatures of the austenitic and martensitic transformations. Cu-Al-Be alloys are very attractive from the commercial point of view and have low transformation temperatures, thus, attractive for various applications. However, the stability of the phases is still the subject of various investigations. In this work, a device for conducting thermal cycling experiments was constructed which is able to cycling the samples up to 10,000 times, enabling the investigation of the thermal stability of Cu-Al-Be alloy containing Cr, as grain refining element. Experimental results have shown that the thermal cycling over extended periods of times increases the temperatures of the martensitic phase reversion - stabilizing the martensite phase. On the other hand tests carried out during the aging heat treatments did not significantly alter these temperatures. It was concluded that the main mechanism for stabilization of the martensitic phase is related to the immobilization of the interface due to defects introduced during the thermocycling. / Ligas passíveis do efeito memória de forma são muito usadas para confecção de atuadores mecânicos os quais trabalham em uma faixa de temperatura as quais são responsáveis pela recuperação de forma do elemento sensor. Este tipo de aplicação é numeroso na engenharia. Neste caso, o principal aspecto responsável para um bom desempenho do sistema reside na estabilidade das temperaturas características da liga, ou seja, nas temperaturas de início e fim das transformações austenítica e martensítica. Ligas Cu-Al-Be são bastante atrativas do ponto de vista comercial e possuem temperaturas de transformação baixas, sendo assim, atrativas para diversas aplicações. Entretanto, a estabilidade das fases é ainda tema de diversas investigações. Nesse trabalho, foi construído um dispositivo para realização de ciclagem térmica por longos períodos, até 10.000 ciclos, de modo a possibilitar a investigação da estabilidade térmica de ligas Cu-Al-Be contendo Cr como elemento refinador de grão. Os resultados experimentais demonstraram que a ciclagem durante longos períodos aumenta as temperaturas de reversão da fase martensítica - estabilização da fase martensítica. Por outro lado, os testes realizados durante os tratamentos térmicos de envelhecimento não alteram de modo significativo estas temperaturas. Foi concluído que o principal mecanismo para estabilização da fase martensítica está relacionado com a imobilização da interfase devido à introdução de defeitos durante a ciclagem térmica.

Les assemblages PoP pour « Package on Package » permettent d’augmenter fortement la densité d’intégration des circuits et systèmes microélectroniques, par superposition de plusieurs éléments semi-conducteurs actifs. Les interconnexions internes de ces systèmes sont alors soumises à des contraintes jamais atteintes. Nous avons pu identifier, caractériser, modéliser et simuler les mécanismes de défaillance potentiels propres à ces assemblages, et leur évolution : • Les gauchissements dans la phase d’assemblage du « PoP » et ses contraintes thermomécaniques sont plus importants que ceux de chacun des composants individuels. Un modèle analytique original a été construit et mis en ligne afin d’évaluer a priori ce gauchissement. • Les comportements hygroscopiques et hygromécaniques sont simulés et mesurés par une approche originale. L’assemblage « PoP » absorbe plus d’humidité que la somme des deux composants individuels, mais son gauchissement hygromécanique et ses contraintes hygromécaniques sont moins élevées. • Deux types d’essais de vieillissement accéléré sont réalisés pour étudier la fiabilité du « PoP » assemblé sur circuit imprimé : des cycles thermiques et des tests sous fort courant et température élevée. Dans ces deux types d’essais, l’assemblage d’un composant « top » sur un autre composant « bottom » pour former un PoP augmente les risques de défaillances. • L’évolution de la microstructure selon le type de vieillissement est comparée par des analyses physiques et physico-chimiques. Les fissures sont toujours situées dans l’interface substrat/billes, qui correspond aux zones critiques prédites par les simulations. / The assemblies PoP (Package on Package) can greatly increase the integration density of microelectronic circuits and systems, by vertically combining discrete semiconductor elements. The interconnections of these systems suffer the stresses never reached before. We were able to identify, characterize, model and simulate the potential failure mechanisms of these assemblies and their evolution: • The warpage in the assembly phase and thermomechanical stress of "PoP" are more serious than the individual components. An original analytical model has been built and put online for pre-estimating this warpage. • The hygroscopic and hygromechanical behaviors are simulated and measured by an original method. The assembly "PoP" absorbs more moisture than the sum of the individual components, but its hygromechanical warpage and stress are smaller. • Two types of accelerated aging tests are performed to study the reliability of "PoP" at the board level: the thermal cycling and the testing under current and temperature. In both types of tests, assembly a component "top" on another component "bottom" to form a “PoP” increases the risk of failure. • The microstructure evolution depending on the type of aging is compared by the physical and physico-chemical analysis. The cracks are always located in the interface substrate/balls, which corresponds to the critical areas predicted by the simulations.

Magister Scientiae Dentium - MSc(Dent) / The aim and objective of this study was to determine the bond strength of a twostep and a single-step dentin bonding agent and to determine the effect of repeated thermal-cycling from 50C and 550C on the bond strength values of these dentin bonding agents. / South Africa

In order to reliably test thermoelectric modules, it is necessary to have a adequate measuring apparatus. The presented diploma thesis deals with the design of measuring apparatus capable of performing both long-term testing of thermoelectric modules at a constant high temperature and thermal cycling extension has been designed. Both types can be changed in a short time.

No description available.