Laser-nanostructured metal oxide semiconductors for conductometric gas sensors.

Los materiales nanoestructurados presentan diferentes propiedades físicas si se comparan con su material “bulk” equivalente. La integración de este tipo de materiales en dispositivos convencionales, tales como sensores de gas, puede mejorar algunas de sus características como la sensibilidad, la selectividad y la respuesta, lo que es importante para el desarrollo de sensores de gas fiables. En particular, la nanoestructuración de óxidos metálicos semiconductores se ha investigado ampliamente para su aplicación en sensores de gas conductométricos. Los dos principales inconvenientes de la mayoría de las técnicas de nanoestructuración son la baja velocidad de los procesos, no escalable a la producción en la industria, y la necesidad de transferir las nanoestructuras al dispositivo (procesos ex-situ). Por este motivo, el presente trabajo estudia la detección de gases de semiconductores nanoestructurados mediante dos técnicas top-down que son rápidas, de bajo coste, de proceso in-situ y automatizables: patrón directo por interferencia láser (direct laser interference patterning (DLIP)) y nanoestructuración mediante láser de femtosegundos (femtosecond laser subwavelength patterning). DLIP es una técnica sin contacto, que utiliza los patrones de interferencia generados por dos o más haces láser coherentes para estructurar directamente los materiales. Por otro lado, la nanoestructuración mediante láser de femtosegundos genera estructuras periódicas inducidas por láser (LIPSS) cuando la radiación polarizada linealmente interactúa con un sólido. Este trabajo se centra en la detección de dióxido de nitrógeno (NO2), ya que es uno de los contaminantes más comunes, del que es necesario medir concentraciones muy bajas. De hecho, la recomendación del Comité Científico de Límites de Exposición Ocupacional para el Dióxido de Nitrógeno de La Comisión Europea establece 0.5 ppm como el TWA de 8 horas. En particular, esta tesis recoge el estudio de tres tipos diferentes sensores de gas nanoestructurados por láser para la detección de bajas concentraciones de NO2: sensores de ZnO procesados por DLIP, sensores de ZnO nanostructurado con LIPSS y sensores de WO3 procesados por DLIP. En todos los casos estudiados, se ha obtenido una mejora de la respuesta en los sensores nanoestructurados en comparación con dispositivos a los que se ha realizado un recocido, lo cual indica el potencial de las tecnologías láser. Además, se ha estudiado el efecto de las condiciones de detección (flujo y posición del sensor dentro de la cámara) en el rendimiento de los sensores comparando resultados experimentales con simulaciones de flujo de gas. Por último, se ha incluido la integración de los sensores fabricados en una plataforma inalámbrica.Nanostructured materials present different physical properties in comparison to their bulk counterparts and the integration of this type of materials in conventional devices, such as gas sensors, can improve some of their characteristics such as sensitivity, selectivity and response. These enhanced features are important for the development of reliable gas sensors. In particular, nanostructuration of semiconductor metal oxides has been widely researched to be applied in conductometric gas sensors. The two major drawbacks of most nanostructuring techniques are the low velocity of the process, not scalable for mass production and the need to transfer the nanostructures to the sensing device (ex-situ approaches). Hence, the present work studies the gas sensing performance of semiconductors nanostructured by two top-down techniques that are fast, inexpensive, in-situ process and automatable: direct laser interference patterning (DLIP) and femtosecond laser subwavelength patterning. The DLIP is a non-contact technique that uses the interference patterns generated by two or more coherent laser beams to directly structure materials. On the other hand, femtosecond laser subwavelength patterning generates laser-induced periodic structures (LIPSS) when linearly polarized radiation interacts with a solid. This work focuses on the detection of NO2, since it is one of the most common pollutants, and needs to be detected in very low concentrations. In fact, the recommendation from the Scientific Committee on Occupational Exposure Limits for Nitrogen Dioxide of the European Commission establishes 0.5 ppm as the 8-hour TWA . In particular, this thesis gathers the study of three different type of laser nanostru ctured semiconductor gas sensors for the detection of low concentration of NO2: ZnO based sensors processed by DLIP, ZnO based sensors nanostructured with LIPSS and WO3 based sensors processed by DLIP. In all the approaches, a response improvement has been obtained by the nanostructured sensors compared with classically annealed devices, pointing out the laser technologies potential. Furthermore, the study of the operating conditions influence (flow and position of the sensor inside the chamber) on the sensors performance is investigated comparing experimental results with gas flow simulations. Finally, the integration of the fabricated sensors into a wireless platform is included in this dissertation

Influence of the test-chamber shape on the performance of conductometric gas sensors

In this article, CFD simulations results are presented as a key tool to the comprehension of the target gas con- centration evolution in a test chamber, at different working conditions. The simulation results are compared with the experimental data, which shows a qualitative good correlation with the evolution of the concentration gradient detected. The experiments were carried out using an aluminum gas test chamber, where a WO3 based conductometric sensor is introduced. The results demonstrate how the response time is dependent on the sensor working conditions. Analyzing the CFD and experimental results, some assumptions for this behavior are proposed. The WO3 sensor needs a Pt heating element, which is heated up to 300 ◦C. As the response is highly temperature-dependent, the temperature distribution on the sensor surface was measured by an IR thermo- graphic camera. The simulation results show that the temperature distribution matches with those obtained experimentally. To validate the model, a mesh and time step convergence study was also implemente

Influence of the test-chamber shape on the performance of conductometric gas sensors

In this article, CFD simulations results are presented as a key tool to the comprehension of the target gas con- centration evolution in a test chamber, at different working conditions. The simulation results are compared with the experimental data, which shows a qualitative good correlation with the evolution of the concentration gradient detected. The experiments were carried out using an aluminum gas test chamber, where a WO3 based conductometric sensor is introduced. The results demonstrate how the response time is dependent on the sensor working conditions. Analyzing the CFD and experimental results, some assumptions for this behavior are proposed. The WO3 sensor needs a Pt heating element, which is heated up to 300 ◦C. As the response is highly temperature-dependent, the temperature distribution on the sensor surface was measured by an IR thermo- graphic camera. The simulation results show that the temperature distribution matches with those obtained experimentally. To validate the model, a mesh and time step convergence study was also implemente