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Intégration et caractérisation de couches sensibles de ZnO : Ga et de cobaltites spinelles (CoxFe3-xO4 ; 1<=x<=3) sur véhicules de test pour la détection de dioxyde d'azote sub-ppm et d'éthanol

Abstract : Measuring air pollutants is a challenge. Most gaseous pollutants, such as nitrogen dioxide (NO2) or volatile organic compounds (VOCs), occur at parts per billion (ppb) levels in the air and are mixed with thousands of other compounds. Today, the public air quality monitoring network relies on fixed measuring stations, monitoring organisations use the most efficient but expensive detectors and the cost of a multi-detector station is high, resolution of the measurements is strongly reduced. In this thesis, we propose to find materials capable of detecting very low concentrations of polluting gases and to integrate them by a vapour deposition technique in the form of thin layers (a few tens of nanometres thick) on micro-platforms in order to manufacture low-cost resistive gas sensors. To this end, gallium-doped zinc oxide (ZnO:Ga) was first studied for the detection of NO2. This material was deposited by radio frequency sputtering on fused silica substrates for structural and microstructural characterisation. The resulting layer appears as a columnar and circular polycrystalline growth with a porous surface, ideal for gas adsorption. X-ray diffraction shows a wurtzite structure with the (002) plane parallel to the substrate. The electrical and thermoelectric characterisations show a low resistive n-type material (compared to bulk ZnO). These ZnO:Ga thin films were then deposited on characterisation platforms (developed and fabricated during this thesis) especially for these preliminary detection tests under controlled atmosphere, but also for the determination of the optimal detection parameters of the material. These tests proved that at 250°C the ZnO:Ga has a good sensitivity to 500 ppb NO2 in an atmosphere with 50% relative humidity. Then, the thin films were deposited on micro-platforms using a shadowmask (developed and fabricated for this process) to obtain well localised depositions in order to fabricate the sensor and characterise it under real operating conditions. Thus, the fabricated sensors are very sensitive to 500 ppb NO2 at 50% relative humidity. In parallel to the study of ZnO:Ga, an exploratory evaluation was conducted on iron cobalt powders for the detection of VOCs. Powders with different compositions were studied: Co1.16Fe1.84O4, Co1.5Fe1.5O4, Co1.7Fe1.3O4, Co2FeO4, Co2.7Fe0.3O4 and Co3O4. These powders were deposited on alumina substrates, heated to different temperatures and tested under 25 ppm ethanol in dry air. Of all the compositions, Co3O4 showed the best response at low temperatures. In conclusion, this thesis presents ZnO:Ga thin films as a promising material for NO2 detection at very low concentrations. However, it remains to determine a more correct way to exploit this material, depending on the applications, considering its assets (sensitivity, response time, temperature ...) and its shortcomings (aging etc.). Co3O4 seems to be interesting for ethanol detection, and it is now necessary to study its thin layers in order to detect lower concentrations and to integrate it into micro-platforms for the manufacture of functional sensors.
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Submitted on : Tuesday, March 1, 2022 - 4:08:28 PM
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  • HAL Id : tel-03592964, version 2

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Vignesh Gunasekaran. Intégration et caractérisation de couches sensibles de ZnO : Ga et de cobaltites spinelles (CoxFe3-xO4 ; 1<=x<=3) sur véhicules de test pour la détection de dioxyde d'azote sub-ppm et d'éthanol. Matériaux. Université Paul Sabatier - Toulouse III, 2021. Français. ⟨NNT : 2021TOU30105⟩. ⟨tel-03592964v2⟩

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