Ce travail a pour objectif de développer de nouveaux microcapteurs à onde acoustique de surface (SAW) capables de détecter le dioxyde de souffre (SO₂) à l’échelle de traces par l’intermédiaire d’une couche sensible à base d’un polymère fonctionnel. Une famille originale de cinq polyuréthaneimides (PUIs) à blocs contenant un nombre contrôlé de sites amines tertiaires de structures différentes a été synthétisée et caractérisée. Ces matériaux présentent des propriétés originales, en solution et à l’état solide, qui sont dues essentiellement à la présence des sites basiques et à la structure à blocs associant des blocs souples polyéthers et des blocs rigides en partie fluorés. Leur excellent caractère filmogène a permis leur application en tant que couche sensible au SO₂ sur des microcapteurs SAW. Deux structures de microcapteurs à onde de Love, bicouche et tri-couche, ont été développées en respectant les conditions de génération de l’onde. La structure bicouche comporte le Quartz-ST 90° comme substrat et l’un des PUIs joue le double rôle de couche guidante et sensible. Cette structure génère bien l’onde de Love mais elle présente une forte sensibilité à la température, inconvénient majeur pour les capteurs de gaz. L’ajout d’une couche guidante à base de ZnO, dans une structure tri-couche, a permis de rendre le dispositif peu sensible à la température et donc compatible avec l’application visée. Les résultats des tests de détection du SO₂ montrent que tous les PUIs étudiés contribuent à une amélioration de la sensibilité comparativement au dispositif sans couche polymère. La présence des sites amines tertiaires conduit à une amélioration importante de la sensibilité qui n’est pas seulement gouvernée par leur basicité mais également par leur encombrement stérique dont le rôle apparaît déterminant / This work aimed at developing new surface acoustic wave (SAW) microsensors capable of detecting traces of sulfur dioxide (SO₂) through a sensitive layer based on a functional polymer. An original family of five polyurethaneimide (PUI) block copolymers containing a controlled number of tertiary amine sites with different chemical structures was synthesized and characterized. These materials exhibited original properties in solution and solid state, which were mainly due to the presence of the basic sites and the block structure combining polyether soft blocks and partially fluorinated hard blocks. Their excellent film-forming character allowed their application as SO₂ sensitive layers on SAW microsensors. Two bi-layer and three-layer structures of Love wave microsensors were developed by respecting the conditions of the Love wave generation. The bi-layer structure included the Quartz ST-90° as the piezoelectric substrate and one of the PUIs acting as both guiding and sensitive layer. This structure generated the Love wave but its high sensitivity to temperature was a major drawback for gas sensors. The addition of a ZnO guiding layer in a three-layer structure led to a microsensor almost insensitive to temperature and thus compatible with the targeted application. The experimental results for SO₂ detection showed that all the PUIs contributed to improve the sensitivity compared to the device without polymer layer. The presence of tertiary amine sites led to a significant improvement in gas sensitivity which was not only governed by their basicity but also by their steric hindrance which played a determining role

In this thesis, gas sensors based on on-chip hydrothermally grown 1-D zinc oxide (ZnO) nanostructures are presented, to improve the sensitivity, selectivity, and stability of the gas sensors. Metal-oxide-semiconductor (MOS) gas sensors are well-established tools for the monitoring of air quality indoors and outdoors. In recent years, the use of 1-D metal oxide nanostructures for sensing toxic gases, such as nitrogen dioxide, ammonia, and hydrogen, has gained significant attention. However, low-dimensional nanorod (NR) gas sensors can be enhanced further. Most works synthesize the NRs first and then transfer them onto electrodes to produce gas sensors, thereby resulting in large batch-to-batch difference. Therefore, in this thesis six studies on 1-D ZnO NR gas sensors were carried out. First, ultrathin secondary ZnO nanowires (NWs) were successfully grown on a silicon substrate. Second, an on-chip hydrothermally grown ZnO NR gas sensor was developed on a glass substrate. Its performance with regard to sensing nitrogen dioxide and three reductive gases, namely, ethanol, hydrogen, and ammonia, was tested. Third, three 1-D ZnO nanostructures, namely, ZnO NRs, dense ZnO NWs, and sparse ZnO NWs, were synthesized and tested toward nitrogen dioxide. Fourth, hydrothermally grown ZnO NRs, chemical vapor deposited ZnO NWs, and thermal deposited ZnO nanoparticles (NPs) were tested toward ethanol. Fifth, the effect of annealing on the sensitivity and stability of ZnO NR gas sensors was examined. Sixth, ZnO NRs were decorated with palladium oxide NPs and tested toward hydrogen at high temperature. The following conclusions can be drawn from the work in this thesis: 1) ZnO NWs can be obtained by using a precursor at low concentration, temperature of 90 °C, and long reaction time. 2) ZnO NR gas sensors have better selectivity to nitrogen dioxide compared with ethanol, ammonia, and hydrogen. 3) Sparse ZnO NWs are highly sensitive to nitrogen dioxide compared with dense ZnO NWs and ZnO NRs. 4) ZnO NPs have the highest sensitivity to ethanol compared with dense ZnO NWs and ZnO NRs. The sensitivity of the NPs is due to their small grain sizes and large surface areas. 5) ZnO NRs annealed at 600 °C have lower sensitivity toward nitrogen dioxide but higher long-term stability compared with those annealed at 400 °C. 6) When decorated with palladium oxide, both materials form alloy at a temperature higher than 350 °C and decrease the amount of ZnO, which is the sensing material toward hydrogen. Thus, controlling the amount of palladium oxide on ZnO NRs is necessary.

Ces travaux ont donc visé le développement d’une plateforme complète de détection de gaz pour environnement sévère haute température. Cette plateforme intègre un dispositif à ondes acoustiques de surface sur substrat Langasite, une résistance chauffante, une couche sensible inorganique nanostructurée et est placée dans une enceinte hermétique. Des températures de l’ordre de 450°C ont été atteintes et des tests de cyclages ont démontré un fonctionnement en accord avec les modèles théoriques et une reproductibilité des mesures. Des tests de détection de composés organiques volatils (éthanol et toluène) ont mis en avant des seuils de détection de l'ordre de quelques ppm. / Measuring pollutants concentrations in gas and vapors emissions are important environmental issues. This work presents a stand-alone portable device for high temperature assessment. The system includes a Langasite (LGS) acoustic sensor, a ceramic heater and a platform with RF connections for remote in-situ measurements. The packaging consists in a hermetic stainless steel cell which enables safe gas detection. In situ temperature measurements have been achieved and the thermal behavior was successfully investigated in the temperature range 25-450°C. The designed cell highlights good agreement with theoretical models and reproducibility of the measures. Volatile organic compounds exposures have been investigated and promising ppm level detections have been obtained.

L'émergence des nouvelles applications dans le domaine de la micro et nanotechnologie requière de faibles coûts de fabrication et la caractérisation de dispositifs électroniques ayant des propriétés telles que la flexibilité, la portabilité, la légèreté, et des matériaux de faibles coûts. Les méthodes traditionnelles de fabrication impliquent de longues étapes de production, et des procédés de fabrication impliquant des étapes avec des produits chimiques. Le but de cette thèse est d'étudier la conception et la caractérisation de capteurs d'ammoniac et d'ozone sur support souple fabriqués par des processus de photolithographie et de gravure laser. Le support flexible est composé de Kapton avec des électrodes interdigitées de Ti/Pt pour la détection de gaz et un microchauffage. Les motifs du circuit ont été réalisés par photolithographie et gravure laser. L'utilisation de gravure laser sur support souple permet de réduire les coûts liés au temps de fabrication, aussi représente une excellente alternative aux processus chimiques. Des nanoparticles de ZnO déposées par gouttes ont été utilisées comme matériaux sensibles en raison de leurs excellentes propriétés dans la détection de gaz. Les conditions de détection de gaz ont été étudiées pour différentes concentrations d'ozone et d'ammoniac. Afin de tester une méthode de dépôt utilisée dans la production industrielle à grande échelle, un dépôt par spray ultrasonique a été effectué. Les capteurs réalisés montrent une large gamme de détection de 5 ppb à 500 ppb à 200 °C pour l'ozone et de 5 ppm à 100 ppm à 300 °C pour l'ammoniac avec une bonne reproductibilité, stabilité et de rapides temps de réponse et de retourn. / Nowadays the emerging of new applications in the micro and nanotechnology field required to reduce fabrication costand to improve electronic devices with properties such as flexibility, portability, lightweight, and low cost. Traditional methods involve expensive and long production steps, and chemical vapor deposition. The purpose of this work is to present the conception and characterization of flexible ammonia and ozone sensors fabricated by photolithography and laser ablation processes. The flexible platform is composed of Kapton substrate with interdigitated Ti/Pt electrodes for gas detection and a micro-heater device. The circuit patterns were realized by photolithography and laser ablation. Photolithography is a well-known and reliable patterning process used on rigid substrate. The application of laser ablation process not only reduces fabrication time, but also represents an excellent viable alternative instead of chemical processes. ZnO thin films deposited by drop coating have been used as sensitive materials due to their excellent properties in the gas detection. The gas sensing condition and the performances of the devices are investigated for ozone and ammonia at different gas concentrations and different thin film thicknesses. In order to test a deposit methodology used in large scale industrial production, an ultrasonic spray deposition was done. The sensor provides a wide range of detection from 5 ppb to 500 ppb for ozone and from 5 ppm to 100 ppm for ammonia. Their best sensibilities were obtained at 200°C for ozone and 300 °C for ammoniac with good repeatability, stability and fast response/recovery time.

O desenvolvimento de sensores em sistemas para controle ambiental tem-se mostrado uma área de elevado interesse científico e técnico. Os principais desafios nesta área estão relacionados ao desenvolvimento de sensores com capacidade de detecção de várias substâncias. Neste contexto, os capacitores MOS apresentam-se como dispositivos versáteis para a geração de imagens químicas com potencial de detecção e classificação de diferentes substâncias a partir de apenas um único sensor. No presente trabalho, foi proposto um sensor MOS com um perfil geométrico de porta em forma de cata-vento composta por Pd, Au e Pt. A resposta do sensor mostrou ter alta sensibilidade a moléculas ricas em átomos de H, como os gases H2 e NH3. As medidas de capacitância mostraram que o sensor tem uma resposta não linear para H2 e NH3 obedecendo à lei da isoterma de Langmuir. O sensor MOS mostrou-se eficiente na geração de imagens químicas através da técnica de escaneamento por luz pulsada. As imagens químicas correspondentes aos gases H2 e NH3 mostraram diferentes padrões quando o N2 foi utilizado como gás transportador. A diferença entre os padrões aconteceu principalmente devido ao perfil geométrico da porta metálica. A sensibilidade do sensor mostrou dependência com o potencial de polarização. Nas medidas de capacitância, a maior sensibilidade foi observada para potenciais próximos da tensão de banda plana. Já para as imagens químicas, a maior sensibilidade foi observada para potenciais inteiramente na região de depleção. A sensibilidade do sensor também se mostrou dependente do gás transporta- dor. O sensor mostrou ser mais sensível com N2 como gás transportador do que com ar seco. No entanto, o processo de dessorção dos íons H+ resultou ser mais eficiente em ar seco. Os resultados obtidos no presente trabalho sugerem a possibilidade de fabricação de um nariz optoeletrônico utilizando apenas um único sensor MOS. / The development of sensors and systems for environmental control has been shown to be an area of high scientific and technical interest. The main challenges in this area are related to the development of sensors capable of detecting many different substances. In this context, the MOS devices present themselves as versatile devices for chemical imaging with potential for detection and classification of different substances only using one single sensor. In the present work, was proposed a MOS sensor with a wing-vane geometric profile of its gate constituted of Pd, Au and Pt metals. The sensor\'s response showed to have high sensitivity to molecules rich on H atoms, such as H2 and NH3 gases. Capacitance measurements showed that the sensor has a nonlinear response for H2 and NH3 obeying the Langmuir isotherm law. The MOS sensor proved to be efficient in Chemical Imaging generation through the scanned light pulse technique. The chemical images of the H2 and NH3 gases showed different patterns when the N2 was used as carrier gas. The different patterns responses happened mainly due to geometric profile of the metallic gate. The sensor sensitivity showed dependence on the bias potential. In the capacitance measures, greater sensitivity was observed for potential near the flat-band voltage. In the chemical images, the greater sensitivity was observed for bias potential within depletion region. The sensor sensitivity was also dependent on the carrier gas. The sensor showed to be more sensitive with N2 as carrier gas than to dry air. However the desorption process of H+ have been more efficient in dry air. The results obtained in the present work suggest the possibility of manufacturing an optoelectronic nose using only a single MOS sensor.

O desenvolvimento de sensores em sistemas para controle ambiental tem-se mostrado uma área de elevado interesse científico e técnico. Os principais desafios nesta área estão relacionados ao desenvolvimento de sensores com capacidade de detecção de várias substâncias. Neste contexto, os capacitores MOS apresentam-se como dispositivos versáteis para a geração de imagens químicas com potencial de detecção e classificação de diferentes substâncias a partir de apenas um único sensor. No presente trabalho, foi proposto um sensor MOS com um perfil geométrico de porta em forma de cata-vento composta por Pd, Au e Pt. A resposta do sensor mostrou ter alta sensibilidade a moléculas ricas em átomos de H, como os gases H2 e NH3. As medidas de capacitância mostraram que o sensor tem uma resposta não linear para H2 e NH3 obedecendo à lei da isoterma de Langmuir. O sensor MOS mostrou-se eficiente na geração de imagens químicas através da técnica de escaneamento por luz pulsada. As imagens químicas correspondentes aos gases H2 e NH3 mostraram diferentes padrões quando o N2 foi utilizado como gás transportador. A diferença entre os padrões aconteceu principalmente devido ao perfil geométrico da porta metálica. A sensibilidade do sensor mostrou dependência com o potencial de polarização. Nas medidas de capacitância, a maior sensibilidade foi observada para potenciais próximos da tensão de banda plana. Já para as imagens químicas, a maior sensibilidade foi observada para potenciais inteiramente na região de depleção. A sensibilidade do sensor também se mostrou dependente do gás transporta- dor. O sensor mostrou ser mais sensível com N2 como gás transportador do que com ar seco. No entanto, o processo de dessorção dos íons H+ resultou ser mais eficiente em ar seco. Os resultados obtidos no presente trabalho sugerem a possibilidade de fabricação de um nariz optoeletrônico utilizando apenas um único sensor MOS. / The development of sensors and systems for environmental control has been shown to be an area of high scientific and technical interest. The main challenges in this area are related to the development of sensors capable of detecting many different substances. In this context, the MOS devices present themselves as versatile devices for chemical imaging with potential for detection and classification of different substances only using one single sensor. In the present work, was proposed a MOS sensor with a wing-vane geometric profile of its gate constituted of Pd, Au and Pt metals. The sensor\'s response showed to have high sensitivity to molecules rich on H atoms, such as H2 and NH3 gases. Capacitance measurements showed that the sensor has a nonlinear response for H2 and NH3 obeying the Langmuir isotherm law. The MOS sensor proved to be efficient in Chemical Imaging generation through the scanned light pulse technique. The chemical images of the H2 and NH3 gases showed different patterns when the N2 was used as carrier gas. The different patterns responses happened mainly due to geometric profile of the metallic gate. The sensor sensitivity showed dependence on the bias potential. In the capacitance measures, greater sensitivity was observed for potential near the flat-band voltage. In the chemical images, the greater sensitivity was observed for bias potential within depletion region. The sensor sensitivity was also dependent on the carrier gas. The sensor showed to be more sensitive with N2 as carrier gas than to dry air. However the desorption process of H+ have been more efficient in dry air. The results obtained in the present work suggest the possibility of manufacturing an optoelectronic nose using only a single MOS sensor.

Cette thèse est dédiée à l’élaboration de couches minces d'oxydes de tungstène par pulvérisation cathodique réactive. Afin de jouer sur la composition des films, le procédé de pulsation du gaz réactif (RGPP) est mis en œuvre pour changer les concentrations en oxygène et en tungstène. En parallèle, la technique de dépôt sous incidence oblique (GLAD) est développée pour produire différentes architectures, à savoir des colonnes inclinées, des zigzags ou encore des spirales, et augmenter le rapport surface-volume dans les films. La co-pulvérisation GLAD est également étudiée en utilisant deux cibles inclinées et séparées de W et WO3. Les relations entre la microstructure, la composition, les propriétés électroniques et optiques des films d'oxydes de tungstène sont systématiquement étudiées. Ils sont finalement appliqués comme couches actives pour des capteurs résistifs afin d'améliorer la détection de vapeur de dodécane et d'ozone gazeux. La microstructure poreuse élevée des colonnes inclinées produite par GLAD combinée à une composition ajustée par RGPP conduit à définir une gamme de films d'oxydes de tungstène attractifs pour améliorer les performances capteurs. / This thesis is focused on the reactive sputter deposition of W-O thin films. In order to play with their composition, the Reactive Gas Pulsing Process (RGPP) is implemented and allows tunable oxygen and tungsten concentrations. Similarly, the GLancing Angle Deposition (GLAD) technique is developed to produce various architectures, namely inclined columns, zigzags and spirals, and increases the surface-to-volume ratio of the films. The GLAD co-sputtering approach is also investigated by means of two inclined and separated W and WO3 targets. Relationships between microstructure, composition, electronic and optical properties of W-O films are systematically studied. They are finally applied as active layers for resistive sensors in order to improve detection of dodecane vapor and ozone gas. The high porous microstructure of inclined columns produced by GLAD combined to the suitable composition adjusted by RGPP leads to define a range of W-O films attractive for sensing performances.

Planar two-dimensional (2-D) nanostructured indium oxide (InOx) and one-dimensional (1-D) tin oxide (SnO2) semiconductor metal-oxide layers have been utilised for gas sensing applications. Novel layered Surface Acoustic Wave (SAW) based sensors were developed consisting of InOx/SiOxNy/36°YXLiTaO3, InOx/SiNx/SiO2/36°YXLiTaO3 and InOx/SiNx/36°YXLiTaO3 The 1 µm intermediate layers of silicon oxynitride (SiOxNy), silicon nitride (SiNx) and SiO2/SiNx matrix were deposited on lithium tantalate (36°YXLiTaO3) substrates by r.f. magnetron sputtering, electron-beam evaporation and plasma enhanced chemical vapour deposition (PECVD) techniques, respectively. As a gas sensitive layer, a 100 nm thin layer of InOx was deposited on the intermediate layers by r.f. magnetron sputtering. The targeted gases were ozone (O3) and hydrogen (H2). An intermediate layer has multiple functions: protective role for the interdigital transducers' electrodes as well as an isolating effect from InOx sensing layer, thereby improving the sensor performance. The developed SAW sensors' exhibited high response magnitudes with repeatable, reversible and stable responses towards O3 and H2. They are capable of sensing concentrations as low as 20 parts-per-billion for O3 and 600 parts-per-million for H2. Additionally a conductometric type novel sensing structure of SnO2/36°YX LiTaO3 was also developed by depositing a thin layer of SnO2 nanorods by PECVD. The gas sensing performance exhibited repeatable, reversible, stable responses towards NO2 and CO. The surface morphology, crystalline structure and preferred orientation of the deposited layers were investigated by Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). A polycrystalline, oxygen deficient non-stoichiometric InOx with grain sizes of 20-40 nm was revealed. The 1-D nanostructures were characterised by Transmission Electron Microscopy (TEM) showing nanorods with needle-like shape , diameters of 10-20 nm a t the top and 30-40 nm at the base as well as a preferential growth orientation of [ ] on the LiTaO3 substrate. The developed sensors are promising for O3, H2 and CO sensing.

Gas sensors are today used in many different application areas, and one growing future market is battery operated sensors. As many gas sensor components are heated, one major limit of the operation time is caused by the power dissipated as heat. AppliedSensor is a company that develops and produces gas sensor components, modules and solutions, among which battery operated gas sensors are one targeted market. The aim of the diploma work has been to simulate the heat transfer on a hydrogen gas sensor component and its closest surroundings consisting of a carrier mounted on a printed circuit board. The component is heated in order to improve the performance of the gas sensing element. Power dissipation occurs by all three modes of heat transfer; conduction from the component through bond wires and carrier to the printed circuit board as well as convection and radiation from all the surfaces. It is of interest to AppliedSensor to understand which factors influence the heat transfer. This knowledge will be used to improve different aspects of the gas sensor, such as the power consumption. Modeling and simulation have been performed in FEMLAB, a tool for solving partial differential equations by the finite element method. The sensor system has been defined by the geometry and the material properties of the objects. The system of partial differential equations, consisting of the heat equation describing conduction and boundary conditions specifying convection and radiation, was solved and the solution was validated against experimental data. The convection increases with the increase of hydrogen concentration. A great effort was made to finding a model for the convection. Two different approaches were taken, the first based on known theory from the area and the second on experimental data. When the first method was compared to experiments, it turned out that the theory was insufficient to describe this small system involving hydrogen, which was an unexpected but interesting result. The second method matched the experiments well. For the continuation of the project at the company, a better model of the convection would be a great improvement.

Un dels majors problemes experimentats pels sistemes de detecció de gasos basats en sensors d'òxids metàl·lics és la seva manca de reproduibilitat, estabilitat i selectivitat. A fi i a efecte d'intentar resoldre aquest problemes, diferents estratègies han estat desenvolupades en paral·lel. Algunes es relacionen a la millora dels materials i d'altres impliquen el condicionament o el pre-tractament de les mostres. Les més emprades han consistit en aprofitar que els sensors presenten sensibilitats solapades per construir matrius de sensors i emprar tècniques de processament del senyal o bé utilitzar característiques de la resposta dinàmica dels sensors. En els darrers anys, modular la temperatura de treball del sensors d'òxids metàl·lics s'ha convertit en un dels mètodes més utilitzats per incrementar-ne la selectivitat. Això s'esdevé així donat que la resposta del sensor varia amb la seva temperatura de treball. Per això, en determinats casos, mesurant la resposta d'un sensor a n temperatures de treball diferents pot ser equivalent a tenir una matriu de n sensors diferents. Això permet obtenir informació multivariant de cada sensor individualment i ajuda a mantenir baixa la dimensionalitat del sistema de mesura per resoldre una determinada aplicació. Malgrat que molts i bons resultats han estat publicats dins aquest àmbit, la tria de les freqüències emprades en la modulació de la temperatura de treball dels sensor ha consistit fins ara en un procés empíric que no garanteix la obtenció dels millors resultats per una determinada aplicació. En aquest context, el principal objectiu d'aquesta tesi doctoral ha consistit en desenvolupar un mètode sistemàtic que permeti determinar quines són les freqüències de modulació òptimes que caldria emprar per resoldre un determinat problema d'anàlisi de gasos. Aquest mètode, extret del camp d'identificació de sistemes, ha esta desenvolupat i implementat per primer cop dins l'àmbit dels sensors de gasos. Aquest consisteix en estudiar la resposta dels sensors en presència de gasos mentre la temperatura de treball dels sensors és modulada per un senyal pseudo-aleatori de longitud màxima. Aquest senyals comparteixen algunes propietats amb el soroll blanc, i per tant poden ajudar a estimar la resposta lineal d'un sistema amb no-linealitats (per exemple, la resposta impulsional d'un sistema sensor-gas). El procés d'optimització es duu a terme mitjançant la selecció entre els components espectrals de les estimacions de la resposta impulsional, d'aquells que millor ajuden bé a discriminar o a quantificar els gasos objectiu dins una aplicació d'anàlisi de gasos donada. Tenint en compte que els components espectrals estan directament relacionats amb les xvii Improving the performance of micro-machined metal oxide gas sensors: Optimization of the temperature modulation mode via pseudo-random sequences. freqüències de modulació, la tria d'uns pocs components espectrals resulta en la determinació de les freqüències òptimes de modulació. En els primer experiments, senyals binaris pseudo-aleatoris van ser emprats per modular la temperatura de treball de sensors de gasos basats en òxids metàl·lics micro-mecanitzats dins d'un rang comprès entre 0 i 112,5 Hz. La freqüència superior és lleugerament superior a la frequència de tall de les membranes dels sensors. El resultat principal derivat d'aques estudi va ser que les freqüències de modulació interessants es trobaven en un rang comprès entre 0 i 1 Hz. Això és comprensible donat que la cinètica de les reaccions i dels processos d'adsorció que es produeixen en la superfície dels sensors són lentes i si aquestes s'han de veure modificades per la modulació térmica, llavors caldran senyals de modulació de baixa freqüència. Això explica perquè s'han vingut emprant senyals moduladores de temperatura en el rang dels mHz, malgrat que les membranes d'un dispositiu micromecanitzat presenten respostes tèrmiques molts més ràpides (típicament de l'ordre de 100 Hz). En els experiments que continuaren els primers, un mètode evolucionat per determinar les freqüències de modulació tèrmica òptimes va ser implementat. Aquest es basa en l'ús de seqüències pseudo-aleatòries multi-nivell de longitud màxima. Els senyals de tipus multi-nivell van ser considerats en substitució dels senyals binaris ja que els primers permeten obtenir una millor estimació que els segons de la dinàmica lineal d'un sistema amb no linealitats. I és ben conegut que els sensors de gasos basats en òxids metàl·lics presenten no linealitat en la seva resposta. Aquests estudis sistemàtics van ser completament validats mitjançant la síntesi de senyals multi-sinusoïdals amb les freqüències prèviament identificades emprant sequències pseudo-aleatòries. Quan la temperatura de treball dels sensors va ser modulada amb un senyal, el contingut freqüencial del qual era l'òptim, els gasos i les mescles de gasos considerades van poder ser discriminades perfectament i es va mostrar la possibilitat d'obtenir models de calibració acurats per predir la concentració dels gasos. En alguns casos, aquest procés de validació es va portar a terme emprant sensors que no havien estat utilitzats durant el procés d'optimització (per exemple, una agrupació de sensors diferent però del mateix lot de fabricació). En resum, el nou mètode desenvolupat en aquesta tesi per seleccionar les freqüències de modulació òptimes s'ha mostrat consistent i efectiu. El mètode és d'aplicació general i podria ser emprat en qualsevol problema d'anàlisi de gasos o bé estès a altres tipus de sensors (per exemple sensors polimèrics). Les contribucions científiques d'aquesta tesi s'han recollit en quatre articles en revistes internacionals i 13 llibres d'actes de conferències. / Uno de los mayores problemas experimentados en los sistemas de detección de gases basados en dispositivos de óxidos metálicos es su falta de reproducibilidad, estabilidad y selectividad. Con el fin de intentar resolver estos problemas, diferentes estrategias han sido desarrolladas en paralelo. Algunas de ellas se relacionan con la mejora de los materiales y otras implican acondicionamiento o pre-tratamiento de las muestras. Otras estrategias ampliamente empleadas consisten en aprovechar que los sensores presentan sensibilidades solapadas para construir matrices de sensores y emplear técnicas de procesamiento de señal o bien utilizar características de la respuesta dinámica de los sensores.En los últimos años, modular la temperatura de trabajo de los sensores de óxidos metálicos se ha convertido en uno de los métodos más utilizados para incrementar su selectividad. Esto se debe a, dado que la respuesta del sensor varía con su propia temperatura de trabajo, entonces, en determinados casos, midiendo la respuesta de un sensor a n temperaturas de trabajo diferentes, es equivalente a tener una matriz de n sensores diferentes. Esto permite obtener información multivariante de cada sensor individualmente y ayuda a mantener baja la dimensionalidad del sistema de medida para resolver una determinada aplicación. A pesar de los buenos resultados que han sido publicados dentro de este ámbito, la selección de las frecuencias empleadas en la modulación de la temperatura de trabajo de los sensores ha consistido, hasta el momento, en un proceso empírico lo que no garantiza la obtención de los mejores resultados para una determinada aplicación.En este contexto, el principal objetivo de esta tesis doctoral ha consistido en desarrollar un método sistemático que permita determinar cuales son las frecuencias de modulación óptimas que podrían emplearse para resolver un determinado problema de análisis de gases. Este método, extraído del campo de identificación de sistemas, ha sido desarrollado e implementado por primera vez dentro del ámbito de los sensores de gases. Éste consiste en estudiar la respuesta de los sensores en presencia de gases mientras la temperatura de trabajo de los sensores es modulada mediante una señal pseudo-aleatoria de longitud máxima. Estas señales comparten algunas propiedades con el ruido blanco, y por tanto pueden ayudar a estimar la respuesta lineal de un sistema con no-linealidades (por ejemplo, la respuesta impulsional de un sistema sensor-gas).El proceso de optimización es llevado a cabo mediante la selección entre las componentes espectrales de las estimaciones de la respuesta impulsional, de aquellas que más ayudan ya sea a discriminar o a cuantificar los gases objetivo dentro de una aplicación de análisis de gases dada. Teniendo en cuenta que las componentes espectrales están directamente relacionadas con las frecuencias de modulación, la selección de unas pocas componentes espectrales resulta en la determinación de las frecuencias optimas de modulación.En los primeres experimentos, señales binarias pseudo-aleatorias fueron utilizadas para modular la temperatura de trabajo de los sensores de gases basados en óxidos metálicos micro-mecanizados en un rango comprendido entre 0 a 112.5 Hz. La frecuencia superior es ligeramente mayor a la frecuencia de corte de las membranas de los sensores. El resultado principal derivado de estos estudios fue que las frecuencias de modulación interesantes se encuentran en un rango comprendido entre 0 y 1 Hz. Esto es comprensible dado que la cinética de las reacciones y de los procesos de adsorción que se producen en la superficie del sensor son lentos y si estos se han de alterar mediante la modulación térmica, se habrá de elaborar señales de modulación a bajas frecuencias. Esto explica por que se han venido empleado señales moduladoras de temperatura en el rango de los mHz, a pesar que las membranas de un dispositivo micro-mecanizado presentan respuestas mucho más rápidas (típicamente en el orden de los 100 Hz).En los experimentos posteriores a los primeros, un método evolucionado para determinar las frecuencias de modulación óptimas de los sensores micro-mecanizados fue implementado, el cual se basa en el uso de secuencias pseudo-aleatorias multi-nivel de longitud máxima (MLPRS). Las señales de tipo multi-nivel fueron consideradas en lugar de las binarias ya que las primeras permiten obtener una mejor estimación que las segundas de la dinámica lineal de un sistema con no linealidades. Y es bien conocido que los sensores de gases basados en óxidos metálicos presentan no-linealidades en su respuesta.Estos estudios sistemáticos fueron completamente validados mediante la síntesis de señales multi-senoidales con las frecuencias previamente identificadas utilizando secuencias pseudo-aleatorias. Cuando la temperatura de trabajo de los sensores fue modulada por una señal, el contenido frecuencial de la cual es el óptimo, los gases y mezclas de gases considerados pudieron ser discriminados perfectamente y se verificó la posibilidad de obtener modelos de calibración precisos para predecir la concentración de los gases. En algunos casos, estos procesos de validación se llevaron a cabo con sensores que no habían sido utilizados durante el proceso de optimización (por ejemplo, una agrupación de sensores diferentes pero del mismo lote de fabricación).En resumen, El nuevo método desarrollado in esta tesis para seleccionar las frecuencias de modulación optimas se a mostrado consistente y efectivo. El método es de aplicación general y podría ser utilizado en cualquier problema de análisis de gases o bien extendido a otro tipo de sensores (por ejemplo sensores poliméricos).Las contribuciones científicas de esta tesis se han recogido en 4 artículos en revistas internacionales y trece actas de conferencias. / One of the major problems in gas sensing systems that use metal oxide devices is the lack of reproducibility, stability and selectivity. In order to tackle these troubles experienced with metal oxide gas sensors, different strategies have been developed in parallel. Some of these are related to the improvement of materials, or the use of sample conditioning and pre-treating methods. Other widely used techniques include taking benefit of the unavoidable partially overlapping sensitivities by using sensor arrays and pattern recognition techniques or the use of dynamic features from the gas sensor response.In the last years, modulating the working temperature of metal oxide gas sensors has been one of the most used methods to enhance sensor selectivity. This occurs because, since, the sensor response is different at different working temperatures, and therefore, measuring the sensor response at n different temperatures is, in some cases, similar to the use of an array comprising n different sensors. This allows for measuring multivariate information from every single sensor and helps in keeping low the dimensionality of the measurement system needed to solve a specific application. Although the good results reported, until now, the selection of the frequencies used to modulate the working temperature remained an empirical process and that is not an accurate method to ensure that the best results are reached for a given application.In view of this context, the principal objective of this doctoral thesis was to develop a systematic method to determine which are the optimal temperature modulation frequencies to solve a given gas analysis problem. This method, which is borrowed from the field of system identification, has been developed and introduced for the first time in the area of gas sensors. It consists of studying the sensor response to gases when the operating temperature is modulated via maximum-length pseudo-random sequences. Such signals share some properties with white noise and, therefore, can be of help to estimate the linear response of a system with non-linearity (e.g., the impulse response of a sensor-gas system).The optimization process is conducted by selecting among the spectral components of the impulse response estimates, the few that better help either discriminating or quantifying the target gases of a given gas analysis application. Since spectral components are directly related to modulating frequencies, the selection of spectral components results in the determination of the optimal temperature modulating frequencies.In the first experiments, pseudo-random binary signals (PRBS) were employed to modulate the working temperature of micro-machined metal oxide gas sensors in a frequency range from 0 up to 112.5 Hz. The upper frequency is slightly higher than the cutoff frequency of the sensor membranes. The outcome of this initial study was that the important modulating frequencies were in the range between 0 and 1 Hz. This is understandable, since the kinetics of reaction and adsorption processes taking place at the sensor surface (i.e., physisorption/chemisorption/ionosorption) are slow and if these are to be altered by the thermal modulation, low frequency modulating signals need to be devised. This explains why low-frequency temperature-modulating signals (i.e. in the mHz range) have been used with micro-hotplate gas sensors, even though the thermal response of their membranes is much faster (typically, near 100 Hz).In the experiments that followed the first ones, an evolved method to determine the optimal temperature modulating frequencies for micro-hotplate gas sensors was introduced, which was based on the use of maximum length multilevel pseudo-random sequences (MLPRS). Multilevel signals were considered instead of the binary ones because the former can provide a better estimate than the latter of the linear dynamics of a process with non-linearity. And it is well known that temperature-modulated metal oxide gas sensors present non-linearity in their response.These systematic studies were fully validated by synthesizing multi-sinusoidal signals at the optimal frequencies previously identified using pseudo-random sequences. When the sensors had their operating temperatures modulated by a signal with a frequency content that corresponded to the optimal, the gases and gas mixtures considered could be perfectly discriminated and the building of accurate calibration models to predict gas concentration was found to be possible. In some cases, the validation process was conducted on sensors that had not been used for optimization purposes (e.g. a different sensor array from the same fabrication batch).Summarizing, the new method developed in this thesis for selecting the optimal modulating frequencies is shown to be consistent and effective. The method applies generally and could be used in any gas analysis problem or extended to other type of sensors (e.g. conducting polymer sensors).The scientific contributions of this thesis are collected in four journal papers and thirteen conference proceedings.