This paper explores the possibility of using AlN/sapphire piezoelectric bilayer structures for high-temperature SAW applications. To determine the temperature stability of AlN, homemade AlN/sapphire samples are annealed in air atmosphere for 2 to 20 h at temperatures from 700 to 1000°C. Ex situ X-ray diffraction measurements reveal that the microstructure of the thin film is not affected by temperatures below 1000°C. Ellipsometry and secondary ion mass spectroscopy investigations attest that AlN/sapphire is reliable up to 700°C. Beyond this temperature, both methods indicate ongoing surface oxidation of AlN. Additionally, Pt/Ta and Al interdigital transducers are patterned on the surface of the AlN film. The resulting SAW devices are characterized up to 500°C and 300°C, respectively, showing reliable frequency response and a large, quasi-constant temperature sensitivity, with a first-order temperature coefficient of frequency around -75 ppm/°C. Between room temperature and 300°C, both electromechanical coupling coefficient K(2) and propagation losses increase, so the evolution of delay lines' insertion losses with temperature strongly depends on the length of the propagation path.

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In-liquid arc plasma jet and its application to phenol degradation.

International audienceSynthesis of aluminium-containing nanoparticles (NPs) by electrical discharges was performed in three dielectric liquids (heptane, liquid nitrogen and water) with aluminium electrodes. The nature of the liquid plays an essential role in the synthesis yield and in the structural properties of NPs. Time-resolved optical emission spectroscopy of selected emission lines emitted during the discharge and its time afterglow was used to observe the chemical changes occurring in the gas phase. It turns out that in heptane and liquid nitrogen, crystalline metallic NPs (from 5 to 10 nm in diameter) are synthesized and oxidized next into amorphous alumina when they are in contact with air, once the liquid is evaporated. In heptane, the transformation of the liquid itself into hydrogenated amorphous carbon creates a kind a matrix in which the aluminium NPs are embedded. Sometimes, a protective graphite shell grows around the NPs and protects them from any further oxidation. In water, these crystalline metallic NPs are synthesized during the first 800 ns of the discharge process, when oxidation is limited by the outward flux of the metallic vapour. They are oxidized next in water. A second type of alumina NPs (several 10 s of nm in diameter) are produced from 800 ns on. They are likely formed from AlO molecules and no longer from aluminium atoms. In every liquid, sub-micrometric particles are also found due to droplet emission from the liquid well created during impacts of spark discharges on electrodes