No description available.

The development of thermal barrier coatings (TBCs) has been undoubtedly the most critical advancement in materials technology for modern gas turbine engines. TBCs are widely used in gas turbine engines for both power-generation and propulsion applications. Metallic oxidation-resistant coatings (ORCs) are also widely employed as a stand-alone protective coating or bond coat for TBCs in many high-temperature applications. Among the widely studied durability issues in these high-temperature protective coatings, one critical challenge that received greater attention in recent years is their resistance to high-temperature degradation due to corrosive deposits arising from fuel impurities and CMAS (calcium-magnesium-alumino-silicate) sand deposits from air ingestion. The presence of vanadium, sulfur, phosphorus, sodium and calcium impurities in alternative fuels warrants a clear understanding of high-temperature materials degradation for the development of fuel-flexible gas turbine engines. Degradation due to CMAS is a critical problem for gas turbine components operating in a dust-laden environment. In this study, high-temperature degradation due to aggressive deposits such as V2O5, P2O5, Na2SO4, NaVO3, CaSO4 and a laboratory-synthesized CMAS sand for free-standing air plasma sprayed (APS) yttria stabilized zirconia (YSZ), the topcoat of the TBC system, and APS CoNiCrAlY, the bond coat of the TBC system or a stand-alone ORC, is examined. Phase transformations and microstructural development were examined by using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. This study demonstrated that the V2O5 melt degrades the APS YSZ through the formation of ZrV2O7 and YVO4 at temperatures below 747°C and above 747°C, respectively. Formation of YVO4 leads to the depletion of the Y2O3 stabilizer and the deleterious transformation of the YSZ to the monoclinic ZrO2 phase. The investigation on the YSZ degradation by Na2SO4 and a Na2SO4 + V2O5 mixture (50-50 mol. %) demonstrated that Na2SO4 itself did not degrade the YSZ, however, in the presence of V2O5, Na2SO4 formed vanadates such as NaVO3 that degraded the YSZ through YVO4 formation at temperature as low as 700°C. The APS YSZ was found to react with the P2O5 melt by forming ZrP2O7 at all temperatures. This interaction led to the depletion of ZrO2 in the YSZ (i.e., enrichment of Y2O3 in t' -YSZ) and promoted the formation of the fluorite-cubic ZrO2 phase. Above 1250°C, CMAS deposits were observed to readily infiltrate and significantly dissolve the YSZ coating via thermochemical interactions. Upon cooling, zirconia reprecipitated with a spherical morphology and a composition that depended on the local melt chemistry. The molten CMAS attack destabilized the YSZ through the detrimental phase transformation (t - > t - > f + m). Free standing APS CoNiCrAlY was also prone to degradation by corrosive molten deposits. The V2O5 melt degraded the APS CoNiCrAlY through various reactions involving acidic dissolution of the protective oxide scale, which yielded substitutional-solid solution vanadates such as (Co,Ni)3(VO4)2 and (Cr,Al)VO4. The molten P2O5, on the other hand, was found to consume the bond coat constituents significantly via reactions that formed both Ni/Co rich phosphates and Cr/Al rich phosphates. Sulfate deposits such as Na2SO4, when tested in encapsulation, damaged the CoNiCrAlY by Type I acidic fluxing hot corrosion mechanisms at 1000°C that resulted in accelerated oxidation and sulfidation. The formation of a protective continuous Al2O3 oxide scale by preoxidation treatment significantly delayed the hot corrosion of CoNiCrAlY by sulfates. However, CoNiCrAlY in both as-sprayed and preoxidized condition suffered a significant damage by CaSO4 deposits via a basic fluxing mechanism that yielded CaCrO4 and CaAl2O4. The CMAS melt also dissolved the protective Al2O3 oxide scale developed on CoNiCrAlY by forming anorthite platelets and spinel oxides. Based on the detailed investigation on degradation of the APS YSZ and CoNiCrAlY by various corrosive deposits, an experimental attempt was carried out to mitigate the melt-induced deposit attack. Experimental results from this study demonstrate, for the first time, that an oxide overlay produced by electrophoretic deposition (EPD) can effectively perform as an environmental barrier overlay for APS TBCs. The EPD protective overlay has a uniform and easily-controllable thickness, uniformly distributed closed pores and tailored chemistry. The EPD Al2O3 and MgO overlays were successful in protecting the APS YSZ TBCs against CMAS attack and hot corrosion attack (e.g., sulfate and vanadate), respectively. Furnace thermal cyclic oxidation testing of overlay-modified TBCs on bond-coated superalloy also demonstrated the good adhesive durability of the EPD Al2O3 overlay.

<p>  </p> <p>The next generation of nuclear reactor designs promise to provide clean, safe, and efficient energy to address our current climate crisis. But with these new technologies, nuclear fuel materials must be carefully designed and understood to meet these demands. Candidate oxide and metallic nuclear fuel materials being considered for use in these new reactor technologies, despite their potential, still have significant remaining materials challenges in understanding their long-term performance and integrity under extreme reactor conditions. As such these candidate fuels require extensive materials characterization to understand their long-term performance under reactor conditions. The objective of this study is to evaluate the microstructural evolution of candidate fuels U-50wt%Zr and ThO2 under the following contexts: 1) Investigation of phase stability in candidate metallic fuel U-50wt%Zr under thermal and irradiation treatment; 2) Investigate localized thermal properties of candidate oxide fuel ThO2 under irradiation through a novel correlative microscopy approach. </p> <p>The influence of thermal and irradiation treatment on phase stability in δ-U50wt%Zr was investigated through conventional APT-TEM methodology. U-Zr is a candidate metallic fuel for advanced fast reactor applications. However, there is still work remaining to better understand how these materials evolve under extreme reactor conditions, especially for the δU-50wt%Zr composition. Metallic fuels are susceptible to significant chemical redistribution under extreme conditions resulting in potential degradation of fuel properties and performance. In these experiments, U-50wt%Zr was subjected to thermal annealing and proton irradiation respectively. These treatments produced very different modulated structures in U-50wt%Zr, and the implications of such on phase stability in U-50wt%Zr will be discussed.</p> <p>Additionally, long-term nuclear reactor operation hinges upon efficient thermal transport in nuclear fuels. There is a critical need to understand localized thermal transport in these materials to enable intelligent design of high-performance fuels. A novel correlative atom probe tomography (APT)-transmission electron microscopy (TEM) approach was developed to investigate the influence of irradiation defects on localized thermal diffusivity in ThO2 upon proton irradiation, and implications of such results will be discussed. </p>

No description available.

The scarcity of freshwater is becoming a global challenge worldwide due to limited resources availability and increasing demand both for manufacturing and household use. For this reason, there is an important need to develop efficient, economic and sustainable desalination technologies able to take advantage of unconventional sources of water (seawater, brackish groundwater and wastewater) in order to produce freshwater. Sun is considered as the most promising abundant renewable (and free) energy source that can be employed in steam and vapor generation processes, which has a great importance in many applications such as: water desalination, domestic water heating, and power generation. This doctoral dissertation presents a study on the efficiency of different carbon based systems - nanofluids and hybrid natural composites - for the improvement of direct-solar evaporation systems, for the production of freshwater. The two main goals of this work consist of: (i) the synthesis and characterization of stable carbon-based nanofluids in water and of re-usable, economical and ecological hybrid composite materials, and (ii) the comparison of such carbon-based systems applied to water evaporation, understanding mechanisms, advantages and limitations. Carbon based materials (carbon black, graphene and multi-walled carbon nanotubes) were chosen because of their high sunlight absorption ability, unique thermal properties, as well as low cost and abundant availability. However, the hydrophobic character of such materials makes necessary to find efficient strategies to overcome this problem when dealing with water. In this work, the suspension stability of graphene-based nanofluids in water - a key parameter for the application of nanofluids in any field - was effectively improved by combining physical (by RF Sputtering coating) or chemical (by NaClO-NaBr solution) graphene surface modification treatments, and the use of common additives (Triton X-114, SDBS and gum arabic) showing different stabilization mechanisms. The best strategy to obtain long-time graphene suspension stability in water (both deionized water and saline solution with 3.5 wt% NaCl) turned out to be the combination of the easy chemical treatment with the electro-steric stabilization effect of gum arabic. In addition to nanofluids, a re-usable devices based on gum arabic cross-linked gelatin hydrogel were synthesized and characterized. Hydrophobic carbon-based materials were easily and uniformly embedded into the porous hydrogel matrix, thanks to the amphiphilic character of both gelatin and gum arabic. The effect of carbon-nanoparticles nature, morphology and concentration on the measured effective thermal conductivity of the composite material was studied and the thermal conductivity of the nanoparticles was evaluated applying several models based on the effective medium approach. The values obtained for the nanoparticles were far from the tabulated thermal conductivity values because of the combination of the composite features (such as nanoparticles concentration, Kapitza resistance) and the particles characteristics (such as aspect ratio, crystalline structure). The performance of carbon-based nanofluids and hybrid hydrogels on direct-solar evaporation of water was tested and compared to that of carbon-wood bilayer composite (which presents both hydrophilic character and natural channels for water transportation) under solar simulator. The effect of surface temperature, light-to-heat conversion efficiency of carbon-based materials, heat losses, water transport through a porous medium and suspension stability (in the case of nanofluids) were investigated in order to understand the advantages and limitations of such systems. All the tested systems were able to improve water evaporation rate and evaporation efficiency up to 70% and 82% under 1 sun and 2 suns respectively using a small amount of nanoparticles: the same amount of particles dispersed in nanofluid (0.01 wt%) was embedded into hydrogels or deposited onto wood. The high sunlight absorption ability of carbon-based nanoparticles appeared as a dominant parameter for the improvement of water evaporation rate. In fact, enhanced light absorption was directly related to a high photothermal conversion efficiency, which caused an improvement in the surface temperature, leading to a consequent enhancement in evaporation rate. It has been found that an adequate supply of water to the evaporation surface represents a fundamental parameter as well considering floating systems.

Tesis por compendio / [EN] In the present Doctoral Thesis, antimicrobial active packaging materials, at lab and at semi-industrial scale, have been developed with the aim to reduce the natural flora of peeled and cut fruit and extend its shelf life. Packaging prototypes have been developed for their further application. Prior to developing the active materials, the most suitable active agents were selected. To that end, the antimicrobial properties of the volatile active agents citral, hexanal and linalool and mixtures thereof were evaluated against typical microorganisms related to fruit spoilage, molds and yeast, concluding that the effectiveness of the mixture is higher than the sum of the effectiveness of the individual agents. Likewise, non-volatile antimicrobial agents such as potassium sorbate and sodium benzoate were selected, which are widely used in the food industry due to their antifungal properties. With the selected active agents, monolayer polypropylene (PP) films with different concentration of the active mixture citral, hexanal and linalool, at lab scale by means of extrusion, and bilayer films at semi-industrial scale with different active layer thickness by means of coextrusion were prepared. Besides, active packaging trays were developed at semi-industrial scale by thermoforming active sheets obtained by coextrusion of PP and ethyl vinyl acetate (EVA) compounds containing potassium sorbate and sodium benzoate as active agents. Mechanical, barrier and thermal properties of the developed active packaging materials, as well as their sealability and transparency were evaluated. In general, the materials' properties were not affected in a significant manner. However, active trays decreased in transparency due to the incorporation of non-volatile active agents. The release kinetics of the volatile and non-volatile active agents were studied at different temperatures, defining their diffusion coefficients by the adjustment to mathematic models based on Second's Law Fick. Among the volatile active agents, hexanal showed a higher diffusion coefficient, followed by citral and linalool. On the other hand, very small differences were observed between potassium sorbate and sodium benzoate diffusion coefficients, being of the same order of magnitude. In vitro tests were also performed at different temperatures to evaluate the antimicrobial properties of the developed materials. In general, the active packaging materials showed high antimicrobial properties which were enhanced with the increment of temperature. Once the properties of the developed materials were evaluated, in vivo tests with peeled and cut orange and pineapple were performed by packing these fruits with the active film, active tray and their combination (active packaging system). In general, the active packaging system improved the microbiological preservation of the fruit for longer times, between 2 and 7 days for orange and pineapple, respectively, and maintained quality parameters of the fruit at stable levels for longer times. Lastly, the safety of the active packaging materials was evaluated according to the European food contact materials and food legislation, and it was concluded that these materials were not of any safety concern for the consumers. / [ES] En la presente Tesis Doctoral se han desarrollado materiales de envase activo antimicrobiano, a escala laboratorio y a escala semi-industrial, con el objetivo de reducir la proliferación de la flora natural de la fruta pelada y cortada y extender su vida útil. Se han desarrollo distintos prototipos para su posterior aplicación industrial Previo al desarrollo de los materiales de envase, se ha realizado una selección de agentes activos más idóneos. Para ello se han estudiado mediante ensayos in vitro las propiedades antimicrobianas de agentes activos volátiles, citral, hexanal y linalool y diferentes mezclas de los mismos, frente a distintos microorganismos típicos del deterioro de las frutas, mohos y levaduras, concluyendo que la efectividad de la mezcla de los tres es superior a la suma de la efectividad de los activos de forma individual. Así mismo, también se han seleccionado antimicrobianos no volátiles como el sorbato potásico y benzoato sódico, los cuáles son ampliamente empleados en la industria alimentaria debido principalmente a sus propiedades antifúngicas. Con los agentes activos seleccionados, se han desarrollado películas monocapa de polipropileno (PP) con distintas concentraciones de la mezcla activa, citral, hexanal y linalool, a escala laboratorio, mediante técnicas de extrusión, y películas bicapa a escala semi-industrial con distintos espesores de capa activa mediante coextrusión. Por otra parte, se desarrollaron bandejas activas a escala semi-industrial mediante termoconformado de láminas obtenidas por coextrusión de compuestos de PP y etilvinilaceteto (EVA) con sorbato potásico o benzoato sódico como agentes antimicrobianos. Se han evaluado las propiedades mecánicas, barrera y térmicas de los materiales activos desarrollados, así como su sellabilidad y transparencia. En general, las propiedades de los polímeros no se vieron afectadas de manera relevante. Sin embargo, las bandejas activas perdieron su carácter transparente debido a la incorporación de los agentes activos no volátiles. Se ha estudiado la cinética de liberación de los compuestos activos volátiles y no volátiles a distintas temperaturas, determinando los coeficientes de difusión de los agentes activos mediante el ajuste a modelos matemáticos de difusión basados en la Segunda Ley de Fick. Entre los agentes volátiles, el hexanal mostró un mayor coeficiente de difusión seguido de citral y linalool. Por otra parte, no hubo apenas diferencia en los coeficientes de difusión del sorbato potásico y benzoato sódico, siendo éstos del mismo orden de magnitud. Igualmente, se han realizado diferentes experimentos in vitro a distintas temperaturas para determinar las propiedades antimicrobianas de los materiales desarrollados. En general, los materiales activos presentan una elevada capacidad antimicrobiana que se ve potenciada al aumentar la temperatura de exposición. Una vez evaluadas las características de los materiales desarrollados, se han efectuado ensayos de envasado de naranja y piña pelada y cortada con las películas y las bandejas activas y con la combinación del sistema de envase bandeja activa termosellada con la película activa. En general, el sistema de envase activo mejoró la conservación de la fruta por un mayor tiempo, entre 2 y 7 días para la naranja y piña, respectivamente, presentando una gran capacidad antimicrobiana y manteniendo los parámetros de calidad de la fruta en niveles estables por un mayor tiempo. Por último, se ha estudiado la seguridad de estos materiales de acuerdo a la legislación de materiales en contacto con alimentos y la legislación alimentaria europea, concluyendo que los materiales activos desarrollados no presentan preocupación para la seguridad de los consumidores. / [CA] En la present Tesi Doctoral s'han desenvolupat materials d'envasament actiu antimicrobià, a escala de laboratori i a escala semi-industrial amb l'objectiu de reduir la proliferació de la flora natural de la fruita pelada i tallada i estendre la seua vida útil. S'han desenvolupament diferents prototips per a la seua posterior aplicació industrial. Previ al desenvolupament dels materials actius, s'han seleccionat els agents actius mes idonis estudiant mitjançant assajos in vitro les propietats antimicrobianes d'agents actius volàtils, citral, hexanal i linalool i diferents mescles dels mateixos, enfront de diferents microorganismes típics de la deterioració de les fruites -floridures i llevats- concloent que l'efectivitat de la mescla dels tres és superior a la suma de l'efectivitat dels actius de forma individual. Així mateix, s'han seleccionat antimicrobians no volàtils, sorbat potàssic i benzoat sòdic, els quals son àmpliament empleats a l'industria alimentaria per les seues propietats antifúngiques. Amb els agents actius seleccionats, s'han desenvolupat pel·lícules monocapa de polipropilè (PP) amb diferents concentracions de la mescla activa, citral, hexanal i linalool, a escala laboratori, mitjançant tècniques d'extrusió, i pel·lícules bicapa a escala semi-industrial amb diferents espessors de capa activa mitjançant coextrusió. D'altra banda, s'han desenvolupat safates actives a escala semi-industrial mitjançant termoconformació de làmines obtingudes per coextrusió de compostos de PP i etil vinil acetat (EVA) amb sorbat potàssic o benzoat sòdic com a agents antimicrobians. S'han avaluat les propietats mecàniques, barrera i tèrmiques dels materials actius desenvolupats, així com la seua sellabilidad i transparència. En general, les propietats dels polímers no es van veure afectades de manera rellevant. No obstant això, les safates actives van perdre el seu caràcter transparent a causa de la incorporació dels agents actius no volàtils. S'ha estudiat la cinètica d'alliberament dels compostos actius volàtils i no volàtils a diferents temperatures, determinant els coeficients de difusió dels agents actius mitjançant l'ajust a models matemàtics de difusió basats en la Segona Llei de Fick. Entre els agents volàtils, l' hexanal va mostrar un major coeficient de difusió seguit de citral i linalool. D'altra banda, no va haver-hi a penes diferències en els coeficients de difusió del sorbat potàssic i benzoat sòdic, sent aquests del mateix ordre de magnitud. Igualment, s'han realitzat diferents experiments in vitro a diferents temperatures per determinar les propietats antimicrobianes dels materials desenvolupats. En general, els materials actius presenten una elevada capacitat antimicrobiana que es veu potenciada en augmentar la temperatura d'exposició. Una vegada avaluades les característiques dels materials desenvolupats s'han efectuat assajos d'envasament de taronja i pinya pelada i tallada amb la safata, la pel·lícula activa i la seva combinació (sistema d'envàs actiu). En general, el sistema d'envàs actiu va millorar la conservació de la fruita per un major temps, entre 2 i 7 dies per a la taronja i pinya respectivament, presentant una gran capacitat antimicrobiana i mantenint els paràmetres de qualitat de la fruita en nivells estables per un major temps. Finalment, s'ha estudiat la seguretat d'aquests materials d'acord a la legislació de materials en contacte amb aliments i la legislació alimentària europea, concloent que els materials actius desenvolupats no presenten preocupació per a la seguretat dels consumidors. / Lara Lledó, MI. (2016). Antimicrobial packaging system for minimally processed fruit [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/61388 / Premios Extraordinarios de tesis doctorales / Compendio

Cette étude constitue une contribution dans un grand projet européen dénommé : 3DICE (3D Integration of Chips using Embedding technologies). La fiabilité mécanique et thermomécanique des composants 3D a été étudiée par des essais normalisés et des simulations numériques. L’essai de chute et le cyclage thermique ont été sélectionnés pour la présente étude. Des analyses de défaillance sont menées pour compléter les approches expérimentales. Les propriétés mécaniques des éléments constituant les composants ont fait l’objet d’une compagne de caractérisation complétée par des recherches bibliographiques. Les simulations numériques, dynamiques transitoires pour l’essai de chute et thermomécanique pour l’essai de cyclage thermique, ont été réalisées pour une estimation numérique de la tenue mécanique des composants. Les modèles numériques sont utilisés pour optimiser le design des composants et prédire les durées de vie en utilisant un modèle de fatigue. / This work establishes a contribution in an important European project mentioned 3DICE (3D Integration of Chips using Embedding technologies). The mechanical and thermomechanical reliability of 3D microelectronic components are studied by employing standardized tests and numerical modeling. The board level drop test and thermal cycling reliability tests are selected for this study. Failures analysis has been used to complete the experimental study. The mechanical properties of elements constituting the microelectronic components were characterized using DMA, tensile test and nanoindentation. Bibliographical researches have been done in order to complete the materials properties data. Numerical simulations using submodeling technique were carried out using a transient dynamic model to simulate the drop test and a thermomechanical model for the thermal cycling test. Numerical results were employing in the design optimization of 3D components and the life prediction using a fatigue model.

La présente thèse doctorale vise à analyser le comportement tribologique, en corrosion et en tribocorrosion des nouveaux aciers inoxydables martensitiques et leurs mécanismes de dégradation pour applications aéronautiques. Pour ce faire, techniques électrochimiques, tribo-électrochimiques et d’analyse de surface ex-situ ont été utilisés. L'usure est affectée par la dureté du matériau et le durcissement lors du glissement de contact. Les aciers inoxydables martensitiques ont montré une augmentation de résistance à la rayure, mais une plus grande perte de matière lors des essais tribologiques à sec par rapport à l'acier inoxydable austénitique. La résistance à la corrosion des aciers inoxydables martensitiques est favorisée par leur passivité, dont la cinétique du film passif peut être décrite par un modèle de type high-field. La vitesse de dissolution passive dépend de la composition chimique de la surface du matériau, ce qui diminue la teneur en Cr dans la couche passive. Les mécanismes de dégradation en tribocorrosion des aciers inoxydables martensitiques comprennent déformation plastique, shakedown et fatigue de de type oligo-cyclique. Les conséquences de ces mécanismes impliqués dépendent des conditions électrochimiques qui prévalent. / The present Doctoral Thesis aimed to analyze the tribological, corrosion and tribocorrosion behavior of new martensitic stainless steels and their degradation mechanisms for aeronautical applications. For this, electrochemical, tribo-electrochemical and ex-situ surface analysis techniques were used. Wear damage was found to be critically affected by the hardness of the material and its hardening during sliding. Martensitic stainless steels showed higher scratch wear resistance but higher wear material loss when compared to the austenitic stainless steel. Corrosion resistance of martensitic stainless steels is driven by their passivity, whose kinetics can be described through a high field conduction model. Passive dissolution rate depends on the surface chemistry of the material, thus decreasing with the Cr content in the passive film. Degradation mechanisms involved in tribocorrosion of martensitic stainless steels included plastic deformation, shakedown and low-cycle fatigue. The consequences of those involved mechanisms depended on the prevailing electrochemical conditions. / La presente Tesis Doctoral tiene como objetivo analizar el comportamiento tribológico, frente a la corrosión y a la tribocorrosión de nuevos aceros inoxidables martensíticos y sus mecanismos de degradación en aplicaciones aeronáuticas. Para ello, se han utilizado técnicas electroquímicas, tribo-electroquímicas y de análisis de superficie ex-situ. El desgaste depende de la dureza del material y de su endurecimiento durante el deslizamiento. Los aceros inoxidables martensíticos mostraron una mayor resistencia al rallado pero una mayor pérdida de material en el ensayo tribológico de desgaste si se compara con el acero inoxidable austenítico. La resistencia a la corrosión de los aceros inoxidables martensíticos es promovida por su pasividad, cuya cinética puede ser descrita a través de un modelo de tipo high field. La velocidad de disolución pasiva depende de la química de la superficie del material, disminuyendo por lo tanto con el contenido de Cr en la película pasiva. Los mecanismos de degradación en tribocorrosión de los aceros inoxidables martensíticos incluyen deformación plástica, shakedown y fatiga de bajo ciclo. Las consecuencias de esos mecanismos dependen de las condiciones electroquímicas del sistema. / La present Tesi Doctoral té com a objectiu analitzar el comportament tribològic, front a la corrosió i a la tribocorrosió de nous acers inoxidables martensítics i els seus mecanismes de degradació en aplicacions aeronàutiques. Per tot això, s'han utilitzat tècniques electroquímiques, tribo-electroquímiques i d'anàlisi de superfície ex-situ. El desgast depen de la duresa del material i del seu enduriment durant el lliscament. Els acers inoxidables martensítics van mostrar una major resistència al ratllat però una major pèrdua de material en l'assaig tribològic de desgast si es compara amb l'acer inoxidable austenític. La resistència a la corrosió dels acers inoxidables martensítics és promoguda per la seva passivitat, i la seua la cinètica pot ser descrita a través d'un model de tipus high field. La velocitat de dissolució passiva depen de la química de la superfície del material, disminuint per tant amb el contingut de Cr en la pel·lícula passiva. Els mecanismes de degradació en tribocorrosió dels acers inoxidables martensítics inclouen deformació plàstica, shakedown i fatiga de baix cicle. Les conseqüències d'aquests mecanismes depenen de les condicions electroquímiques del sistema.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho, desenvolveu-se um sistema de detecção fotoacústico para medidas simultâneas e independentes dos sinais fotoacústicos dianteiro e traseiro, utilizando dois microfones e um único feixe de excitação. Utiliza-se a diferença de fase entre estes sinais para a determinação da difusividade térmica de materiais, com base na abordagem teórica da técnica da Diferença de Fase dos Dois Feixes (T2F). Na metodologia apresentada não há a necessidade de se alternar o feixe de excitação entre as faces da amostra. Esta característica torna mais rápido o procedimento de medida e simplifica o monitoramento automatizado de processos dinâmicos que afetam a difusividade térmica do material, como a cura de resinas poliméricas. É apresentado o procedimento utilizado para determinar a diferença entre as fases intrínsecas dos microfones e o método empregado para compensar tal diferença e, assim, obter a defasagem entre os sinais fotoacústicos dianteiro e traseiro. O sistema de detecção desenvolvido é avaliado em medidas de difusividade térmica de amostras metálicas (aço inoxidável AISI 304 e aço SAE 1020) e poliméricas (polipropileno e polietileno de baixa densidade). Os resultados obtidos concordam de forma satisfatória com dados disponíveis na literatura. Finalmente, a aplicação do sistema proposto ao monitoramento de cura de amostras de resina epóxi indicou sua potencialidade de acompanhar, em tempo real, este tipo de processo dinâmico. / In this work, a photoacoustic detection system was developed for simultaneous and independent measurements of both front and rear photoacoustic signals, using two microphones and a single beam illumination mode. The phase-lag between these signals is used in the determination of thermal diffusivity of materials, based on the theoretical approach of the Two-Beam Phase-Lag technique. In the experimental setup presented in this work there is no need to alternate the light beam between the sample surfaces. This feature provides faster measurements and simplify the automated monitoring of dynamic processes that affect the material thermal diffusivity, as crosslinking processes. The procedure to determine the difference between the intrinsic phases of the microphones is presented, as well as the method to compensate this difference and to obtain the phase-lag between front and rear photoacoustic signals. The developed detection system is tested in thermal diffusivity measurements of metallic (AISI 304 stainless steel and SAE 1020 steel) and polymeric (polypropylene and low-density polyethylene) samples. The results are in good agreement with the available literature values. Finally, the system here proposed is applied in studies of epoxy resin curing, which shows its potentiality for real-time monitoring of dynamic process.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho, desenvolveu-se um sistema de detecção fotoacústico para medidas simultâneas e independentes dos sinais fotoacústicos dianteiro e traseiro, utilizando dois microfones e um único feixe de excitação. Utiliza-se a diferença de fase entre estes sinais para a determinação da difusividade térmica de materiais, com base na abordagem teórica da técnica da Diferença de Fase dos Dois Feixes (T2F). Na metodologia apresentada não há a necessidade de se alternar o feixe de excitação entre as faces da amostra. Esta característica torna mais rápido o procedimento de medida e simplifica o monitoramento automatizado de processos dinâmicos que afetam a difusividade térmica do material, como a cura de resinas poliméricas. É apresentado o procedimento utilizado para determinar a diferença entre as fases intrínsecas dos microfones e o método empregado para compensar tal diferença e, assim, obter a defasagem entre os sinais fotoacústicos dianteiro e traseiro. O sistema de detecção desenvolvido é avaliado em medidas de difusividade térmica de amostras metálicas (aço inoxidável AISI 304 e aço SAE 1020) e poliméricas (polipropileno e polietileno de baixa densidade). Os resultados obtidos concordam de forma satisfatória com dados disponíveis na literatura. Finalmente, a aplicação do sistema proposto ao monitoramento de cura de amostras de resina epóxi indicou sua potencialidade de acompanhar, em tempo real, este tipo de processo dinâmico. / In this work, a photoacoustic detection system was developed for simultaneous and independent measurements of both front and rear photoacoustic signals, using two microphones and a single beam illumination mode. The phase-lag between these signals is used in the determination of thermal diffusivity of materials, based on the theoretical approach of the Two-Beam Phase-Lag technique. In the experimental setup presented in this work there is no need to alternate the light beam between the sample surfaces. This feature provides faster measurements and simplify the automated monitoring of dynamic processes that affect the material thermal diffusivity, as crosslinking processes. The procedure to determine the difference between the intrinsic phases of the microphones is presented, as well as the method to compensate this difference and to obtain the phase-lag between front and rear photoacoustic signals. The developed detection system is tested in thermal diffusivity measurements of metallic (AISI 304 stainless steel and SAE 1020 steel) and polymeric (polypropylene and low-density polyethylene) samples. The results are in good agreement with the available literature values. Finally, the system here proposed is applied in studies of epoxy resin curing, which shows its potentiality for real-time monitoring of dynamic process.