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Yes / Owing to their small size, good wettability, uniform dispersion ability and high thermal properties, the nickel-plated carbon nanotubes (Ni-CNTs) with different aspect ratios are used to reinforce reactive powder concrete (RPC) through modifying the nano/micro- structural units of concrete. Incorporating only 0.075 vol% of Ni-CNTs (0.03 vol% of CNTs) can significantly increase mechanical properties of RPC. The enhancement effect on compressive strength caused by the incorporation of Ni-CNTs with aspect ratio of 1000 reaches 26.8%/23.0 MPa, mainly benefiting from the high polymerization C-S-H gels, low porosity, and refined pore structure. The 33.5%/1.92 MPa increases of flexural strength can be attributed to the decrease of large pore, original cracks, molar ratio of CaO to SiO2, and gel water content when Ni-CNTs with aspect ratio of 125 are added. Ni-CNTs with aspect ratio of 1500 have the largest utilization rate of being pulled-out, resulting from the improvement of dispersibility and the pining effect of nickel coating and then leading to the increased toughness. Therefore, incorporating Ni-CNTs can fundamentally modify the nano/micro- scale structural nature of RPC, providing a bottom-up approach for controlling the properties of RPC. / Funding supported from the National Science Foundation of China (51908103 and 51978127) and the China Postdoctoral Science Foundation (2019M651116).

Polymer bonded explosives (PBXs) are composites comprised of energetic crystals with a very high energy density surrounded by a polymer binder. The formation of hotspots within polymer bonded explosives can lead to the thermal decomposition and initiation of the energetic material. A frictional heating model is applied at the mesoscale to assess the potential for the formation of hotspots under low velocity impact loadings. Monitoring of the formation and growth of damage at the mesoscale is considered through the inclusion of a piezoresistive carbon nanotube network within the energetic binder providing embedded strain and damage sensing. A coupled multiphysics thermo-electro-mechanical peridynamics framework is developed to perform computational simulations on an energetic material microstructure subject to these low velocity impact loads. With increase in impact energy, the model predicts larger amounts of sensing and damage thereby supporting the use of carbon nanotubes to assess damage growth and subsequent formation of hotspots. The framework is also applied to assess the combined effects of thermal loading due to prescribed hotspots with inertial effects due to low velocity impact loading. It has been found that the present model is able to detect the presence of hotspot dominated regions within the energetic material through the piezoresistive sensing mechanism. The influence of prescribed hotspots on the thermo-electro-mechanical response of the energetic material under a combination of thermal and inertial loading was observed to dominate the lower velocity impact response via thermal shock damage. In contrast, the higher velocity impact energies demonstrated an inertially dominated damage response. Quantifying the piezoresistive effect derived from embedding carbon nanotubes in polymers remains a challenge since these nanocomposites exhibit significant variation in their electro-mechanical properties depending upon factors such as CNT volume fraction, CNT dispersion, CNT alignment and properties of the polymer. Of interest is electrical percolation where the electrical conductivity of the CNT/polymer nanocomposite increases through orders of magnitude with increase in CNT volume fraction. Estimates and distributions for the electrical conductivity and piezoresistive coefficients of the CNT/polymer nanocomposite are obtained and analyzed with increasing CNT volume fraction and varying barrier potential, which is a parameter that controls the extent of electron tunneling. The effect of CNT alignment is analyzed by comparing the electro-mechanical properties in the alignment direction versus the transverse direction for different orientation conditions. Estimates of piezoresistive coefficients are converted into gage factors and compared with experimental sources in literature. The methodology for this work uses automated scripts which are used in conjunction with high performance computing to generate several 5 μm ×5 μm realizations for different CNT volume fractions. These realizations are then analyzed using finite elements to obtain volume averaged effective values, which are then subsequently used to generate measures of central tendency (estimated mean) and variability (standard deviation, coefficient of variation, skewness and kurtosis) in a descriptive statistical analysis. / Doctor of Philosophy / Carbon nanotubes or CNTs belong to a class of novel materials known as nanomaterials which are materials with length scales on the order of nanometers. CNTs have been widely studied due to their unique mechanical, electrical and thermal properties in comparison to traditional materials such as metals or plastics. Often times, research and applications concerning the use of CNTs involves embedding the CNTs as a filler within a larger composite material system. In the present work, CNTs are considered to be embedded within a polymer. It is known that the electrical properties of such a CNT/polymer composite change in response to the application of a mechanical force. This change in electrical properties is caused due to the presence of CNTs and is used as a means of sensing the mechanical state of the composite, i.e. real time structural monitoring. The extent of the change in electrical properties, also known as sensing, depends upon a number of different factors such as the amount of CNTs used per unit volume of the polymer, how well dispersed or clumped together the CNTs are within the polymer and the type of polymer material used, among other factors. A statistical analysis is performed with several case studies where these factors are varied and the resulting change in the sensing response is monitored. Several important conclusions were made from the statistical analysis with some of the results providing new insights into the sensing behavior of CNT/polymer composites. For example, it was found that a key parameter known as barrier potential, which directly influences the extent of sensing achieved through a mechanism known as electrical tunneling, needs to be several orders of magnitude lower than previously reported values to accurately capture the sensing effects. Key metrics quantifying the extent of sensing from the analysis were found to be in agreement with previously reported experimental results. The significance of such a statistical study lies in the fact that CNT embedded composites are increasingly being proposed and used for sensing applications. The use of CNT embedded polymers to encase explosive crystalline grains such as HMX or RDX is one such example. These explosive grains are used in a number of different civil and military applications such as fuel rocket propellants, industry explosives, military munitions etc. The grains possess extremely high energy densities and are susceptible to undergo violent chemical reaction if a trigger is provided through thermal or mechanical means. As such, the monitoring of the structural state of these explosives is crucial for their safe handling and processing. In this work, the sensing response of a composite material comprising of explosive grains surrounded by polymer material containing CNTs is studied in response to different types of mechanical loads, ranging from mild stimuli to impact. It was found that the sensing mechanism was capable of tracking mechanical damage as well as the resulting temperature increases interior to the composite. In addition to its application to safety and preventative measures, the use of CNTs in this context also provided insight into the mechanisms related to the sudden release of energy in these explosive grains which is of significant interest since this is an active area of research as well.

Conducting vertical nanopillar arrays can serve as three-dimensional nanostructured electrodes with improved performance for electrical recording and electrochemical sensing in bio-electronics applications. However, vertical nanopillar-array electrodes made of inorganic conducting materials by conventional nanofabrication approach still faces challenges in high manufacturing costs, poor scalability, and limited choice of carrier substrates. Here, we report a new type of conducting nanopillar arrays composed of multi-walled carbon nanotubes (MWCNTs) doped polymeric nanocomposites, which are manufactured over the wafer-scale on both rigid and flexible substrates by direct nanoimprinting of perfluoropolyether nanowell-array templates into uncured MWCNT/polymer mixtures. By controlling the MWCNT ratios and the annealing temperatures during the fabrication process, MWCNT/polymer nanopillar arrays can possess outstanding electrical properties with high DC conductivity (~4 S/m) and low AC electrochemical impedance (~104 Ω at 1000 Hz). Moreover, by electrochemical impedance spectroscopy (EIS) measurements and equivalent circuit modeling-analysis, we can decompose the overall impedance of MWCNT/polymer nanopillar arrays in the electrolyte into multiple bulk and interfacial circuit components, and thus can illustrate their different dependence on the MWCNT ratios and the annealing temperatures. In particular, we find that a proper annealing process can significantly reduce the anomalous ion diffusion impedance and improve the impedance properties of MWCNT/polymer nanopillars in the electrolyte. / Master of Science / Conducting vertical nanopillar arrays can serve as three-dimensional nanostructured electrodes with improved performance for electrical recording and electrochemical sensing in nano-bioelectronics applications. However, vertical nanopillar-array electrodes made of inorganic conducting materials by conventional nanofabrication approach still faces challenges in high manufacturing costs, poor scalability, and limited choice of carrier substrates. Compared to conventional nanofabrication approaches, nanoimprint lithography exhibits unique advantages for low-cost scalable manufacturing of nanostructures on both rigid and flexible substrates. Very few studies, however, have been conducted to achieve the scalable nanoimprinting fabrication of conducting nanopillar arrays made of MWCNT/polymer nanocomposites. Here, I'm reporting a new type of conducting nanopillar arrays composed of multi-walled carbon nanotubes (MWCNTs) doped polymeric nanocomposites, which can be manufactured over the wafer-scale on both rigid and flexible substrates by direct nanoimprinting of the perfluoropolyether nanowell-array template into uncured MWCNT/polymer mixtures. We find that the nanoimprinted conducting nanopillar arrays can possess appealing electrical properties with a high DC conductivity (~4 S/m) and a low AC electrochemical impedance (~104 Ω at 1000 Hz) in the physiologically relevant electrolyte solutions (1X PBS). Furthermore, I've conducted a systematic equivalent circuit modeling analysis of measured EIS results to understand the effects of the MWCNT ratios and the annealing temperatures on the impedance of different bulk and interfacial circuit components for MWCNT/polymer nanopillar arrays in the electrolyte.

Ion-containing segmented polyurethanes exhibit unique morphology and physical properties due to synergistic interactions of electrostatic, hydrogen bonding, and hydrophobic interactions. A fundamental investigation on a series of well-defined ion-containing polyurethanes elucidated the influence of charge placement, charge density, and soft segment structure on physical properties, hydrogen bonding, and morphologies. An unprecedented comparison of poly(ethylene oxide)(PEO)-based sulfonated polyurethanes containing sulfonate anions either in the soft segments or hard segments revealed that sulfonate charge placement dramatically influenced microphase separation and physical properties of segmented polyurethanes, due to altered hydrogen bonding and thermodynamic immiscibility between soft and hard segments. Moreover, studies on sulfonated polyurethanes with identical sulfonated hard segments but different soft segment structures indicated that soft segment structure tailored sulfonated polyurethanes for a wide range of mechanical properties. Sulfonated polyurethanes incorporated with ammonium-functionalized multi-walled carbon nanotubes (MWCNTs) generated novel polyurethane nanocomposites with significantly enhanced mechanical performance. Modification of MWCNTs followed a dendritic strategy, which doubled the functionality by incorporating two ammonium cations per acid site. Complementary characterization demonstrated successful covalent functionalization and formation of surface-bound ammonium salts. Upon comparison with pristine MWCNTs, ammonium-functionalized MWCNTs exhibited significantly enhanced dispersibility in both DMF and sulfonated polyurethane matrices due to good solvation of ammonium cations and intermolecular ionic interactions between anionic polyurethanes and cationic MWCNTs. Segmented polyurethanes containing sulfonated PEO-based soft segments and nonionic hard segments were incorporated with various contents of room temperature ionic liquid, 1-ethyl-3-methylimidazolium ethylsulfate (EMIm ES), to investigate the influence of ionic liquid on physical properties, morphologies, and ionic conductivity. Results indicated that EMIm ES preferentially located in the sulfonated PEO soft phase, leading to significantly enhanced ionic conductivity and well-maintained mechanical properties. These properties are highly desirable for electromechanical transducer applications. Electromechanical actuators fabricated with sulfonated polyurethane/IL composite membranes exhibited effective response under a low applied voltage (4 V). However, in the case of an imidazolium-containing segmented polyurethane with imidazolium ionic hard segments and hydrophobic poly(tetramethylene oxide) (PTMO) soft segments, EMIm ES selectively located into the imidazolium ionic hard domains, as evidenced with a constant PTMO soft segment glass transition temperature (Tg) and systematically reduced imidazolium hard segment Tg. Dielectric relaxation spectroscopy demonstrated that ionic conductivity of imidazolium-containing segmented polyurethanes increased by five orders of magnitude upon incorporation of 30 wt% EMIm ES. Imidazolium-containing sulfonated pentablock copolymers were also investigated to elucidate the influence of imidazolium counter cation structures on solution rheology, morphology, and thermal and mechanical properties. Combination of living anionic polymerization and post functionalization strategies provided well-defined sulfonated pentablock copolymers containing structured imidazolium cations in sulfonated polystyrene middle block. Varying alkyl substitute length on imidazolium cations tailored physical properties and morphologies of sulfonated pentablock copolymers. Results indicated that long alkyl substitutes (octyl and dodecyl) on imidazolium cations significantly influenced solution rheological behavior, morphology, and water uptake properties of sulfonated pentablock copolymers due to the altered characteristic of imidazolium cations. Imidazolium-containing sulfonated pentablock copolymers exhibited systematically tailored mechanical properties due to the plasticizing effect of alkyl substitutes. In addition, incorporation of ionic liquids into sulfonated pentablock copolymers further tailored their mechanical properties and ionic conductivity, which made these materials suitable for electromechanical transducer applications. All sulfonated pentablock copolymers were successfully fabricated into actuator devices, which exhibited effective actuation under a low applied voltage (4 V). / Ph. D.

A multiscale model is proposed to study the influence of interfacial interactions at the nanoscale in carbon nanotube(CNT)-polymer nanocomposites on the macroscale bulk elastic material properties. The efficiency of CNT reinforcement in terms of interfacial load transferring is assessed for the non-functionalized and functionalized interfaces between the CNTs and polymer matrix using force field based molecular dynamic simulations at the nanoscale. Polyethylene (PE) as a thermoplastic material is adopted and studied first because of its simplicity. Characterization of the nanoscale load transfer has been done through the identification of representative nanoscale interface elements for unfunctionalized CNT-PE interface models which are studied parametrically in terms of the length of the PE chains, the number of the PE chains and the "grip" position. Referring to the non-functionalized interface, CNTs interact with surrounding polymer only through weakly nonbonded van der Waals (vdW) forces in our study. Once appropriate values of these parameters are deemed to yield sufficiently converged results, the representative interface elements are subjected to normal and sliding mode simulations in order to obtain the force-separation responses at 100K and 300K for unfunctionalized CNT-PE interfaces. To study the functionalization effects, atomistic interface representative elements for functionalized CNT-PE interface are built based on non-functionalized interface models by grafting functional groups between the PE matrix and the graphene sheet. This introduces covalent bonding forces in addition to the non-bonded vdW forces. A modified consistent covalent force field (CVFF) and adaptive intermolecular reactive empirical bond order (AIREBO) potentials, both of which account for bond breaking, are applied to investigate the interfacial characteristic of functionalized CNT-PE interface in terms of the force-separation responses at 100K in both normal opening and sliding mode separations. In these studies, the focus has been on the influence of the functionalization density on the load transfer at the nanoscale interface. As an important engineering material, Epon 862/DETDA epoxy polymer,a thermoset plastic, has also been used as the polymer matrix material in order to see the difference in interfacial load transfer between a network structured polymer and the amorphous entangled structure of the PE matrix. As for thermoset epoxy polymer, emphasis has been put on investigating the effects of the crosslink density of the epoxy network on the interfacial load transfer ability for both non-functionalized and functionalized CNT-Epoxy interface at different temperatures(100K and 300K) and on the functionalization effect influenceing the interfacial interactions at the functionalized CNT-Epoxy interface. Cohesive zone traction-displacement laws are developed based on the force-separation responses obtained from the MD simulations for both non-functionalied and functionalized CNT-PE/epoxy interfaces. Using the cohesive zone laws, the influence of the interface on the effective elastic material properties of the nanocomposites are observed and determined in continuum level models using analytic and computational micromechanics approaches, allowing for the assessment of the improvement in reinforcement efficiency of CNTs due to the functionalization. It is found that the inclusion of the nanoscale interface in place of the perfectly bonded interface results in effective elastic properties which are dependent on the applied strain and temperature in accordance with the interface sensitivity to those effects, and which are significantly diminished from those obtained under the perfect interface assumption for non-functionalized nanocomposites. Better reinforcement efficiency of CNTs are also observed for the nanocomposites with the functionalized interface between CNTs and polymer matrix, which results in large increasing for the effective elastic material properties relative to the non-functionalized nanocomposites with pristine CNTs. Such observations indicates that trough controlling the degree of functionalization, i.e. the number and distribution of covalent bonds between the embedded CNTs and the enveloping polymer, one can tailor to some degree the interfacial load transfer and hence, the effective mechanical properties. The multiscale model developed in this study bridges the atomistic modeling and micromechanics approaches with cohesive zone models, which demonstrates to deepen the understanding of the nanoscale load transfer mechanism at the interface and its effects on the effective mechanical properties of the nanocomposites. It is anticipated that the results can offer insights about how to engineer the interface and improve the design of nanocomposites. / Ph. D.

Tesis por compendio / [EN] The current PhD thesis deals with the development and characterization of novel nanocomposites based on biodegradable poly(mannitol sebacate) (PMS) matrices with tailored properties and shape-memory capabilities for biomedical applications. Two types of fillers -cellulose nanocrystals (CNC) and electrospun poly(lactic acid) nanofibers (NF-PLA)- were used as reinforcement in order to induce and/or enhance the shape-memory properties of PMS matrices. Also, different crosslinking profiles and stoichiometric ratios between mannitol and sebacic acid (1:1 and 1:2) were studied and evaluated to obtain samples with low and high degrees of crosslinking. An appropriate combination of the crosslinking profile and the monomer ratio for PMS matrix, as well as the addition of low content of CNC, allowed the development of PMS/CNC nanocomposites with a wide range of mechanical properties and degradation profiles. On the other hand, highly oriented poly(lactic acid) (PLA) nanofiber mats obtained by electrospinning were embedded in the PMS matrices. An enhancement of up to 53-fold in the Young's modulus was observed for PMS/NF-PLA nanocomposites filled with 15 wt% of PLA nanofibers. The incorporation of fillers (CNC and NF-PLA) allowed the development of thermally active shape-memory nanocomposites with an enhancement of parameters such as recovery stress and shape fixity. The electrospun PLA-reinforced nanocomposites, offered the best balance of mechanical and thermal properties, as well as a greater control of the transition temperature for switching the change of shape, within a useful range of temperatures. Owing to that, these materials may be of interest as smart responsive systems in long-term biomedical applications. / [ES] La presente tesis doctoral, se centra en el desarrollo y caracterización de nuevos nanocompuestos biodegradables, a partir de matrices de poli(mannitol sebacato) (PMS) con propiedades a medida y capacidades de memoria de forma para aplicaciones biomédicas. Dos tipos de cargas -nanocristales de celulosa (CNC) y nanofibras de ácido poliláctico (NF-PLA) obtenidas mediante electrospinning- se han utilizado como refuerzo, con la finalidad de inducir y/o mejorar las propiedades de memoria de forma en matrices de PMS. Se han estudiado y evaluado diferentes tratamientos de curado y ratios de reacción entre el mannitol y ácido sebácico (1:1 y 1:2), con la finalidad de obtener muestras con bajo y alto grado de reticulación. Una combinación adecuada del tratamiento de curado y el ratio entre monómeros del PMS, así como la adición de bajos contenidos de CNC, permitió desarrollar nanocompuestos de PMS/CNC con un amplio rango de propiedades mecánicas y perfiles de degradación. Por otro lado, se han producido mats de nanofibras de ácido poliláctico (PLA) con alta orientación mediante la técnica de electrospinning, para embeberse en matrices de PMS, observándose una mejora de hasta 53 veces en el módulo de Young para nanocompuestos de PMS/NF-PLA con un 15% en peso de nanofibras. La incorporación de cargas (CNC y NF-PLA) permitió el desarrollo de nanocompuestos con memoria de forma activada térmicamente, con una mejora de parámetros tales como la fuerza de recuperación y la capacidad de fijación. Los nanocompuestos reforzados con NF-PLA obtenidas por electrospinning, ofrecieron el mejor balance de propiedades mecánicas y térmicas, así como un mayor control de la temperatura de transición para la activación del cambio de forma en un intervalo útil de temperaturas. Por todo ello, estos materiales pueden resultar de interés como sistemas activos en aplicaciones biomédicas de larga duración. / [CA] La present tesi doctoral se centra en el desenvolupament i caracterització de nous nanocompostos biodegradables a partir de matrius de poli(mannitol sebacato) (PMS) amb propietats a mesura i capacitats de memòria de forma per a aplicacions biomèdiques. Dos tipus de càrregues -nanocristals de cel·lulosa (CNC) i nanofibres d'àcid polilàctic (NF-PLA) obtingudes mitjançant electrospinning- s'han utilitzat com a reforç amb la finalitat d'induir i/o millorar les propietats de memòria de forma en matrius de PMS. S'han estudiat i avaluat diferents tractaments de curat i ràtios de reacció entre el mannitol i àcid sebàcic (1:1 i 1:2) amb la finalitat d'obtenir mostres amb baix i alt grau de reticulació. Una combinació adequada del tractament de curat i el ràtio entre monòmers del PMS, així com l'addició de baixos continguts de CNC, va permetre desenvolupar nanocompostos de PMS/CNC amb un ampli rang de propietats mecàniques i perfils de degradació. D'altra banda, s'han produït mats de nanofibres d'àcid polilàctic (PLA) amb alta orientació mitjançant la tècnica de electrospinning, per embeure's en matrius de PMS, observant-se una millora de fins a 53 vegades en el mòdul de Young per nanocompostos de PMS/NF-PLA amb un 15% en pes de nanofibres. La incorporació de càrregues (CNC i NF-PLA) va permetre el desenvolupament de nanocompostos amb memòria de forma activada tèrmicament, amb una millora de paràmetres tals com la força de recuperació i la capacitat de fixació. Els nanocompostos reforçats amb NF-PLA obtingudes per electrospinning, van oferir el millor balanç de propietats mecàniques i tèrmiques, així com un major control de la temperatura de transició per a l'activació del canvi de forma en un interval útil de temperatures. Per tot això, aquests materials poden resultar d'interés com a sistemes actius en aplicacions biomèdiques de llarga durada. / Sonseca Olalla, A. (2015). DEVELOPMENT OF SHAPE-MEMORY COMPOSITES BASED ON A BIODEGRADABLE POLYESTER ELASTOMER [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/54129 / Compendio

Alumina (Al2O3) matrix nanocomposites reinforced with a second nanometric phase have better mechanical properties, especially of hardness, fracture toughness and wear resistance compared to the monolithic alumina material. On the other hand, niobium carbide (NbC), as a second phase reinforcement, has properties that make it an ideal material for Al2O3 matrix ceramics, such as high melting temperature, high hardness, low chemical reactivity and a coeffi-cient of thermal expansion similar to the material of Al2O3, thus avoiding the appearance of cracks that diminish the resistance of the material. Currently, the largest reserves of niobium are in Brazil and the study on their use is a very important milestone for the country. Therefore, the aim of this thesis is to obtain and character-ize Al2O3 matrix nanocomposites with a second phase of 5% of vol-ume of NbC nanoparticles obtained by reactive high-energy milling, using conventional sintering, Spark Plasma Sintering (SPS) and Microwave. For this, the nanometric precursor powders of Al2O3-NbC were obtained reactive high-energy milling in SPEX mill, were sub-sequently deagglomerated, leached with hydrochloric acid, added to the Al2O3 matrix in the proportion of 5% of volume and dried under air flow. Powders of Al2O3-5vol.% NbC were sintered by different methods: conventional under an argon atmosphere, microwave and SPS using different temperatures. The precursor powders were characterized by X-ray diffraction (XRD), scanning electron micros-copy (SEM) and particle size measurement. Conventional and SPS sintered nanocomposites were characterized microstructurally, the bulk density and hardness was studied by nanoindentation. SPS sintered nanocomposites were characterized with respect to Young's modulus by nanoindentation, fracture toughness and flex-ural strength. On the other hand, conventionally sintered and SPS nanocomposites were characterized with respect to wear resistance by the pin-on-disc technique, using WC-6% Co spheres with loads 30 and 60 N and Al2O3 spheres with loads 15 and 30 N. The results show that reactive high-energy milling has been complete and effi-cient in obtaining nanometric powders with crystallite sizes of 9.1 and 9.66 nm for Al2O3 and NbC, respectively. In addition, the deag-glomeration, after the reactive high-energy milling process, was ef-fective in the dispersion of the NbC inclusions in the Al2O3 matrix. However, it has not been possible to obtain high density Al2O3-5vol.% NbC nanocomposites using conventional sintering and mi-crowave sintering processes. In the sintering process by SPS, the nanocomposites presented high densities, close to the theoretical and, consequently, better hardness and resistance to wear com-pared to the materials obtained in a conventional furnace. The re-sults corresponding to the wear resistance have confirmed that this property is influenced by several factors such as the sintering meth-od and temperature, the spheres used as counter-material and the loads applied during the test. The final results indicated that the Al2O3-5vol.% NbC nanocomposites obtained by SPS have a great potential for the different industrial applications, which require mate-rials of high mechanical and wear performance. / Los nanocomposites de matriz alúmina (Al2O3) reforzados con una segunda fase nanométrica presentan mejores propiedades mecáni-cas, especialmente de dureza, tenacidad a la fractura y resistencia al desgaste, en comparación con el material monolítico de alúmina. Por otra parte, el carburo de niobio (NbC), como refuerzo de segun-da fase, presenta propiedades que lo convierten en un material ideal para las cerámicas de matriz Al2O3, tales como alta temperatura de fusión, alta dureza, baja reactividad química y un coeficiente de expansión térmica similar al material de Al2O3, evitando así la apari-ción de grietas que disminuyen la resistencia del material. Actual-mente, las mayores reservas de niobio se encuentran en Brasil y el estudio sobre su uso es un hito muy importante para el país. Por lo tanto, el objetivo de esta tesis es obtener y caracterizar nanocompo-sites de matriz de Al2O3 con una segunda fase del 5% en volumen de nanopartículas de NbC obtenidos por molienda reactiva de alta energía, y utilizando la sinterización convencional, Spark Plasma Sintering (SPS) y microondas. Para ello, los nanométricos polvos precursores de Al2O3-NbC fueron obtenidos mediante molienda reactiva de alta energía en molino SPEX, desaglomerados, lixivia-dos con ácido clorhídrico, añadidos a la matriz de Al2O3 en la pro-porción de 5% en volumen y secado bajo flujo de aire. Los polvos de Al2O3-5vol.%NbC fueron sinterizados por diferentes métodos: con-vencional bajo una atmósfera de argón, microondas y SPS usando diferentes temperaturas. Los polvos precursores se caracterizaron por difracción de rayos X (XRD), microscopía electrónica de barrido (SEM) y la medición del tamaño de partícula. Los nanocomposites sinterizados convencionalmente y mediante SPS se caracterizaron microestructuralmente, se estudió la densidad aparente y la dureza por nanoindentación. Los nanocomposites sinterizados mediante SPS fueron caracterizados respecto el módulo de Young por nano-indentación, la tenacidad a la fractura y la resistencia a la flexión. Por otra parte, los nanocomposites sinterizados convencionalmente y mediante SPS fueron caracterizados respecto a resistencia al desgaste mediante la técnica de pin-on-disc, utilizando esferas de WC-6%Co con cargas 30 y 60 N y esferas de Al2O3 con cargas 15 y 30 N. Los resultados muestran que la molienda reactiva de alta energía ha sido completa y eficaz en la obtención de polvos nano-métricos con tamaños de cristalito de 9,1 y 9,66 nm para la Al2O3 y NbC, respectivamente. Además, la desaglomeración, después del proceso de molienda reactiva de alta energía, fue eficaz en la dis-persión de las inclusiones de NbC en la matriz de Al2O3. Sin embar-go, no ha sido posible obtener nanocomposites de Al2O3-5vol.%NbC con alta densidad usando procesos de sinterización convencional y microondas. En el proceso de sinterización mediante SPS, los nanocomposites presentaron altas densidades, cercanas a la teóri-ca y, en consecuencia, mejor dureza y resistencia al desgaste en comparación con los materiales obtenidos en un horno convencio-nal. Los resultados correspondientes a la resistencia al desgaste han confirmado que esta propiedad está influenciada por varios fac-tores tales como el método y temperatura de sinterización, las esfe-ras utilizadas como contramaterial y las cargas aplicadas durante el test. Los resultados finales indicaron que los nanocomposites de Al2O3-5vol.%NbC obtenidos mediante SPS tienen un gran potencial para las distintas aplicaciones industriales, las cuales requieren ma-teriales de alto rendimiento mecánico y al desgaste. / Els nanocomposites de matriu alúmina (Al2O3) reforçats amb una segona fase nanométrica presenten millors propietats mecàniques, especialment de duresa, tenacitat a la fractura i resistència al desgast, en comparació amb el material monolític d'alúmina. D'altra banda, el carbur de niobi (NbC), com a reforç de segona fase, presenta propietats que ho convertixen en un material ideal per a les ceràmiques de matriu Al2O3, com és l'alta temperatura de fusió, alta duresa, baixa reactivitat química i un coeficient d'expansió tèrmica semblant al material d'Al2O3, evitant així l'aparició de clavills que disminuïxen la resistència del material. Actualment, les majors reserves de niobi es troben a Brasil i l'estudi sobre el seu ús és una fita molt important per al país. Per tant, l'objectiu d'esta tesi és obtindre i caracteritzar nanocomposites de matriu d'Al2O3 amb una segona fase del 5% en volum de nanopartículas de NbC obtinguts per mòlta reactiva d'alta energia, i utilitzant la sinterització convencional, Spark Plasma Sintering (SPS) i microones. Per a això, les pols precursores d'Al2O3-NbC van ser obtinguts per mitjà de mòlta reactiva d'alta energia en molí SPEX, desaglomerats, lixiviats amb àcid clorhídric, afegits a la matriu d'Al2O3 en la proporció de 5% en volum i assecat baix flux d'aire. Les pols d'Al2O3-5vol.%NbC van ser sinteritzats per diferents mètodes: convencional davall una atmosfera d'argó, microones i SPS usant diferents temperatures. Les pols precursores es van caracteritzar per difracció de rajos X (XRD), microscòpia electrònica d'agranat (SEM) i el mesurament de la grandària de partícula. Els nanocomposites sinteritzats convencionalment i per mitjà de SPS es van caracteritzar microestructuralment, es va estudiar la densitat aparent, i la duresa es van estudiar per nanoindentació. Els nanocomposites sinteritzats per mitjà de SPS es van caracteritzar el mòdul de Young es van estudiar per nanoindentació, la tenacitat a la fractura i la resistència a la flexió. D'altra banda, els nanocomposites sinteritzats convencionalment i per mitjà de SPS van ser caracteritzats respecte a resistència al desgast per mitjà de la tècnica de pin-on-disc, utilitzant esferes de WC-6%Co amb càrregues 30 i 60 N i esferes d'Al2O3 amb càrregues 15 i 30 N. Els resultats mostren que la mòlta reactiva d'alta energia ha sigut completa i eficaç en l'obtenció de pols nanométrics amb grandàries de cristalit de 9,1 i 9,66 nm per a l'Al2O3 i NbC, respectivament. A més, la desaglomeració, després del procés de mòlta reactiva d'alta energia, va ser eficaç en la dispersió de les inclusions de NbC en la matriu d'Al2O3. No obstant això, no ha sigut possible obtindre nanocomposites d'Al2O3-5vol.%NbC amb alta densitat usant processos de sinterització convencionals i microones. En el procés de sinterització per mitjà de SPS, els nanocomposites van presentar altes densitats, pròximes a la teòrica i, en conseqüència, millor duresa i resistència al desgast en comparació amb els materials obtinguts en un forn convencional. Els resultats corresponents a la resistència al desgast han confirmat que esta propietat està influenciada per diversos factors com ara el mètode i temperatura de sinterització, les esferes utilitzades com contramaterial i les càrregues aplicades durant el test. Els resultats finals van indicar que els nanocomposites d'Al2O3-5vol.%NbC obtinguts per mitjà de SPS tenen un gran potencial per a les distintes aplicacions industrials, les quals requerixen materials d'alt rendiment mecànic i al desgast. / Ribeiro Rodrigues Alecrim, L. (2017). DESENVOLVIMENTO DE NANOCOMPÓSITOS DE ALUMINA-CARBETO DE NIÓBIO POR SINTERIZAÇÃO NÃO-CONVENCIONAL [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90395