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Hoy en día, la necesidad de monitorizar y controlar el medio ambiente es a cada vezmás importante debido al creciente nivel de gases tóxicos que provienen de la expansiónde las actividades industriales, amenazando así el medio ambiente y la salud humana. Eldesarrollo de la nano-tecnología ha permitido fabricar sensores de gases portables,altamente sensibles, selectivos, de bajo coste y de bajo consumo de potencia.Los nanotubos de carbono (NTC) están ganando un interés a cada vez más considerablepor parte de la comunidad científica debido a su geometría y morfología únicas y susexcelentes propiedades electrónicas, mecánicas, térmicas i ópticas. Esto hace de ellosunos candidatos prometedores para un amplio rango de aplicaciones como por ejemplonuevos sensores de gases con propiedades mejoradas. En este contexto, mediante lapresente tesis, se ha realizado un profundo estudio para explorar las propiedades dediferentes sensores basados en nano-materiales híbridos constituidos por nanotubos decarbono junto a otros materiales con el fin de detectar gases tóxicos de manera eficiente.El trabajo realizado consistió en el diseño, la fabricación, la caracterización, y laoptimización de nanosensores híbridos.Esta tesis fue financiada en el marco del proyecto Europeo "Nano2hybrids", cuyoobjetivo era de diseñar la interfaz de las nano-partículas del metal con los nanotubos decarbono a través del control de los defectos estructurales y químicos producidos por ladescarga de un plasma de radiofrecuencia y aplicarlo a la detección de gases. Elbenceno fue elegido como gas principal, debido a sus graves efectos tóxicos a niveles depocas ppb y también debido a la no existencia en el mercado de un detector de bajocoste para benceno. De hecho, no hay en el estado de la técnica, un sensor de gas quepuede detectar de forma selectiva este gas a nivel operativo de ppb y trabajando atemperatura ambiente. Así, el reto de esta tesis era obtener un sensor altamente sensible,selectivo y estable, portátil y de bajo coste para la detección de benceno.En este sentido, se estudiaron exhaustivamente diferentes materiales basados ennanotubos de carbono funcionalizados, decorados con nanopartículas de metal o biendecorados o mezclados con óxidos metálicos, en términos de su adecuación para ladetección de gases (por ejemplo, sus sensibilidad, selectividad, estabilidad, y elmecanismo de detección, etc.). En particular se estudió la detección de diferentes gasescomo (benceno (C6H6 ), monóxido de carbono (CO), dióxido de nitrógeno (NO2), eletileno (C2H4), el sulfuro de hidrógeno (H2S), amoníaco (NH3) y agua (H2O)). Nuestrastareas consistieron en investigar experimentalmente y teóricamente el efecto de lascondiciones de preparación de los materiales (p.e. el tratamiento con plasma, lanaturaleza del precursor y tamaño de las nanoparticulas de metales), fabricación delsensor (p.e., técnica de deposición, el efecto del tipo de metal del los electrodos delsensor), y de las condiciones de caracterización del sensor (p.e., temperatura deoperación, flujo de gas,) sobre las propiedades sensoras de los mismos. Todo ello hapermitido adquirir conocimientos, explicar los mecanismos de funcionamiento en elsensado de gases de los diferentes materiales investigados y con ello desarrollar unsensor de gases adecuado para la detección de benceno.Hemos encontrado que los materiales híbridos que consisten en nanotubos tratados conplasma de oxígeno y decorados con diferentes nanopartículas de metal, muestran unamayor capacitad de detección a temperatura ambiente respecto a los nanotubos decarbono en bruto o los funcionalizados sólo con plasma. Las propiedades interfacialesde los materiales híbridos resultantes pueden ser adaptadas, lo que ofrece una enormeflexibilidad para el ajuste de sus propiedades sensoras. Cuando se combinaron en unamatriz de micro-sensores que opera a temperatura ambiente, nanotubos decorados condiferentes metales, de forma que unos resulten sensibles al benceno y otros insensibles,esto permitió por primera vez la realización de un prototipo de bajo coste capaz dedetectar selectivamente y a temperatura ambiente el benceno presente a nivel de trazas(por debajo de 50 ppbs) en una mezcla de gases. El prototipo realizado presenta unostiempos de respuesta y de recuperación de 60 s y 10 minutos respectivamente además deuna buena estabilidad y reproducibilidad. Este prototipo se encuentra protegido por unapatente que ha sido licenciada a una compañía que se encargará de la comercializaciónindustrial del producto.In the last few years, there has been a growing demand for monitoring the environment,especially with the increasing concern by the release of toxic gases emitted by manmadeactivities. The development of nanotechnology has created a huge potential for buildinghighly sensitive, selective, low cost, and portable gas sensors with low powerconsumption.Nowadays, carbon nanotubes are receiving an intense interest from the scientificcommunity, due to their unique geometry, morphology, electronic, mechanical, thermaland optical properties, which make them a promising candidate for many industrialapplications including new gas sensors for the detection of toxic species. In this context,in this thesis a deep study is devoted to explore the sensing properties of differenthybrid nanomaterials based on carbon nanotubes for an efficient detection of toxicgases. The design, fabrication, characterization, and optimization of gas sensors usinghybrid materials have been carried out.This thesis was financially supported by the European project "Nano2hybrids", whichexploits the interface design of metal nanocluster-carbon nanotube hybrids via controlof structural and chemical defects in a plasma discharge, for designing gas sensors withsuperior performance. Benzene was chosen as the principal target gas due to its serioustoxic effects at low ppb levels and the fact that there are no reliable, low cost andselective benzene detectors in the market. In fact, no gas sensor able to selectivelydetect this gas at ppb levels and operating at ambient temperature has been reported upto now in the literature. So, the challenge of the project was to fabricate sensitive,highly selective, stable, portable, and low cost benzene gas sensor employing hybridnanomaterials.Herein, functionalized MWCNTs, metal decorated MWCNTs, and metal oxidedecorated MWCNTs or metal oxide and MWCNT mixtures were deeply investigated interms of their gas sensing performances (e.g, sensitivity, selectivity, stability, detectionmechanism,. etc) towards the detection of different gases (benzene (C6H6), carbonmonoxide (CO), nitrogen dioxide (NO2), ethylene (C2H4), hydrogen sulfide (H2S),ammonia (NH3), and water (H2O)). Our tasks were to investigate experimentally andtheoretically the effects of material preparation conditions (e.g., plasma treatment,nanocluster precursor and size), sensor fabrication (e.g., deposition technique,electrodes sensor metal), and sensor characterization conditions (e.g., operatingtemperature, gas flow) on the gas sensing properties of our devices, and to acquireknowledge in order to develop a selective benzene detector. Based on experimental andtheoretical results, different mechanisms for the interaction between gases and thehybrid materials tested have been proposed.We found that hybrid materials consisting of oxygen plasma treated multiwalled carbonnanotubes decorated with different metal nanoparticles showed room temperaturesensing capability. Responsiveness to gases of these hybrid materials was higher thanthat of pristine or plasma functionalized carbon nanotubes. Metal decoated CNTs can betailored for the recognition of different gases and vapors with different reactivities,which offers enormous flexibility for tuning the interfacial properties of the resultinghybrid materials and thus, of their sensing properties. When combined in a microsensorarray operating at room temperature, the use of benzene-sensitive and benzeneinsensitivemetal-decorated multiwalled carbon nanotubes, allowed for the first time theimplementation of a low cost detector prototype, which can selectively detect benzenewhen present at trace levels (below 50 ppb) in a gas mixture. Sensors present responseand recovery times of 60 s and 10 min respectively, good stability and reproducibility.This type of sensors are protected by a patent, and licensed to a company for industrialcommercialization.

In this work, a new approach to optimize facility layout for toxic release, fire and explosion scenarios is presented. By integrating a risk analysis in the optimization formulation, safer assignments for facility layout and siting have been obtained. Accompanying with the economical concepts used in a plant layout, the new model considers the cost of willing to avoid a fatality, i.e. the potential injury cost due to accidents associated with toxic release near residential areas. For fire and explosion scenarios, the building or equipment damage cost replaces the potential injury cost. Two different approaches have been proposed to optimize the total cost related with layout. In the first phase using continuous-plane approach, the overall problem was initially modeled as a disjunctive program where the coordinates of each facility and cost-related variables are the main unknowns. Then, the convex hull approach was used to reformulate the problem as a Mixed Integer Non-Linear Program (MINLP) that identifies potential layouts by minimizing overall costs. This approach gives the coordinates of each facility in a continuous plane, and estimates for the total length of pipes, the land area, and the selection of safety devices. Finally, the 3D-computational fluid dynamics (CFD) was used to compare the difference between the initial layout and the final layout in order to see how obstacles and separation distances affect the dispersion or overpressures of affected facilities. One of the CFD programs, ANSYS CFX was employed for the dispersion study and Flame Acceleration Simulator (FLACS) for the fires and explosions. In the second phase for fire and explosion scenarios, the study is focused on finding an optimal placement for hazardous facilities and other process plant buildings using the optimization theory and mapping risks on the given land in order to calculate risk in financial terms. The given land is divided in a square grid of which the sides have a certain size and in which each square acquires a risk-score. These risk-scores such as the probability of structural damage are to be multiplied by prices of potential facilities which would be built on the grid. Finally this will give us the financial risk. Accompanying the suggested safety concepts, the new model takes into account construction and operational costs. The overall cost of locations is a function of piping cost, management cost, protection device cost, and financial risk. This approach gives the coordinates of the best location of each facility in a 2-D plane, and estimates the total piping length. Once the final layout is obtained, the CFD code, FLACS is used to simulate and consider obstacle effects in 3-D space. The outcome of this study will be useful in assisting the selection of location for process plant buildings and risk management.

O objetivo desta tese de doutorado foi estudar a influência da adição do íon estanho (Sn4+) à rede do composto SrTiO3 em substituição ao íon de titânio visando otimizar as propriedades elétricas desse composto e, como consequência, obter uma melhora de seu desempenho como um sensor de gás na forma de filmes finos. Para realizar a deposição destes filmes finos através dos métodos de Deposição por Feixe de Elétrons (EBD), alvos cerâmicos de composição SrTi1-xSnxO3 (STSO) com x = 0; 0,20; 0,40; 0,60; 0,80; 0,85; 0,90; 0,95; 1 foram obtidos através do método dos precursores poliméricos modificado. A sequência de formação de soluções sólidas foi determinada através do refinamento Rietveld das amostras STSO na forma de pó sinterizadas, obtidas através dos métodos dos precursores poliméricos e de reação de estado sólido, mostrando que a transição da fase cúbica Pm3̄m do composto SrTiO3 até a fase ortorrômbica Pnma do composto SrSnO3 não depende do método de síntese. As medidas de espectroscopia Raman e absorção de raios-X (XANES, na borda K do Ti) das amostras tanto na forma de pó, obtidas através do método dos precursores poliméricos e de reação de estado sólido, quanto na forma de filme fino obtidas por EBD revelaram a existência de uma desordem local na rede do composto SrTiO3 que diminui com o aumento da temperatura e com a diminuição da concentração de Sn. Os filmes finos STSO obtidos por EBD foram avaliados como sensores utilizando-se os gases O3 e NH3. Em medidas realizadas com o gás ozônio (O3), os resultados mostraram que os filmes finos de 100 nm de espessura apresentaram uma maior sensibilidade tendo a amostra com 60% de Sn com o melhor desempenho a 350°C para 0,15 ppm do gás. As análises de performance dos filmes STSO quanto a seletividade indicaram que não foram seletivos e que apresentaram uma maior resposta ao gás ozônio quando comparados ao gás NH3. / The objective of this work was to study the influence of the addition of tin ion (Sn4+) into the SrTiO3 compound lattice, to replace the titanium ion (Ti4+). The aim was to optimize the electrical properties of SrTiO3 compound and, as a consequence, to obtain an improvement of its performance as a gas sensor in the thin films samples. To perform the deposition of these thin films through Electron Beam Deposition (EBD), ceramic targets of composition SrTi1-xSnxO3 (STSO) with x = 0; 0.20; 0.40; 0.60; 0.80; 0.85; 0.90; 0.95; and 1 were obtained by the modified polymer precursor method. The solid solution formation sequence was determined by the Rietveld refinement of the STSO sintered powdered samples, obtained by both polymeric precursor and solid-state reaction methods, showing that the transition from the cubic Pm3̄m phase of the SrTiO3 compound to the orthorhombic Pnma phase of the SrSnO3 compound does not depend on the synthesis method. The measurements of Raman spectroscopy and absorption of X-rays (XANES, at Ti K-edge), of the powdered samples obtained by both synthesis methods and of the thin films obtained by EBD, revealed the existence of a local disorder in the SrTiO3 compound lattice which decreases with increasing of temperature and with decreasing of Sn concentration. The STSO thin films were evaluated as sensors using the O3 and NH3 gases. In measurements accomplished with the ozone gas (O3), the results showed that thin films of 100 nm thickness had a higher sensitivity. The sample having 60% of tin showed the best performance at 350°C for 0.15 ppm of ozone gas. The performance analysis related to the selectivity of the STSO films indicated they were not selective and that presented a higher response to the ozone gas when compared to the NH3 gas.

O objetivo desta tese de doutorado foi estudar a influência da adição do íon estanho (Sn4+) à rede do composto SrTiO3 em substituição ao íon de titânio visando otimizar as propriedades elétricas desse composto e, como consequência, obter uma melhora de seu desempenho como um sensor de gás na forma de filmes finos. Para realizar a deposição destes filmes finos através dos métodos de Deposição por Feixe de Elétrons (EBD), alvos cerâmicos de composição SrTi1-xSnxO3 (STSO) com x = 0; 0,20; 0,40; 0,60; 0,80; 0,85; 0,90; 0,95; 1 foram obtidos através do método dos precursores poliméricos modificado. A sequência de formação de soluções sólidas foi determinada através do refinamento Rietveld das amostras STSO na forma de pó sinterizadas, obtidas através dos métodos dos precursores poliméricos e de reação de estado sólido, mostrando que a transição da fase cúbica Pm3̄m do composto SrTiO3 até a fase ortorrômbica Pnma do composto SrSnO3 não depende do método de síntese. As medidas de espectroscopia Raman e absorção de raios-X (XANES, na borda K do Ti) das amostras tanto na forma de pó, obtidas através do método dos precursores poliméricos e de reação de estado sólido, quanto na forma de filme fino obtidas por EBD revelaram a existência de uma desordem local na rede do composto SrTiO3 que diminui com o aumento da temperatura e com a diminuição da concentração de Sn. Os filmes finos STSO obtidos por EBD foram avaliados como sensores utilizando-se os gases O3 e NH3. Em medidas realizadas com o gás ozônio (O3), os resultados mostraram que os filmes finos de 100 nm de espessura apresentaram uma maior sensibilidade tendo a amostra com 60% de Sn com o melhor desempenho a 350°C para 0,15 ppm do gás. As análises de performance dos filmes STSO quanto a seletividade indicaram que não foram seletivos e que apresentaram uma maior resposta ao gás ozônio quando comparados ao gás NH3. / The objective of this work was to study the influence of the addition of tin ion (Sn4+) into the SrTiO3 compound lattice, to replace the titanium ion (Ti4+). The aim was to optimize the electrical properties of SrTiO3 compound and, as a consequence, to obtain an improvement of its performance as a gas sensor in the thin films samples. To perform the deposition of these thin films through Electron Beam Deposition (EBD), ceramic targets of composition SrTi1-xSnxO3 (STSO) with x = 0; 0.20; 0.40; 0.60; 0.80; 0.85; 0.90; 0.95; and 1 were obtained by the modified polymer precursor method. The solid solution formation sequence was determined by the Rietveld refinement of the STSO sintered powdered samples, obtained by both polymeric precursor and solid-state reaction methods, showing that the transition from the cubic Pm3̄m phase of the SrTiO3 compound to the orthorhombic Pnma phase of the SrSnO3 compound does not depend on the synthesis method. The measurements of Raman spectroscopy and absorption of X-rays (XANES, at Ti K-edge), of the powdered samples obtained by both synthesis methods and of the thin films obtained by EBD, revealed the existence of a local disorder in the SrTiO3 compound lattice which decreases with increasing of temperature and with decreasing of Sn concentration. The STSO thin films were evaluated as sensors using the O3 and NH3 gases. In measurements accomplished with the ozone gas (O3), the results showed that thin films of 100 nm thickness had a higher sensitivity. The sample having 60% of tin showed the best performance at 350°C for 0.15 ppm of ozone gas. The performance analysis related to the selectivity of the STSO films indicated they were not selective and that presented a higher response to the ozone gas when compared to the NH3 gas.