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Films minces de Ta₂O₅ et (NaₓK₁₋ₓ)₂Ta₄O₁₁ (x=0.93) : structures et propriétés

Abstract : Ta₂O₅ and (NaₓK₁₋ₓ)₂Ta₄O₁₁ type oxides are known to exhibit properties of optic, high K dielectric, photocatalysis for H2 production from H2O and piezoelecticity. In this work, thin films of these compounds were prepared by a chemical process. From X-ray diffraction and transmission electron microscopy (TEM) results, a transformation mechanism of Ta₂O₅ into (NaₓK₁₋ₓ)₂Ta₄O₁₁ is proposed. This mechanism involves Ta5+ vacancies substitued by H+ cations.A new process confirming this mechanism is to increase the concentration of Ta5+ vacancies substitued by H+ cations, gathered in structural defects which could improved a protonic conductivity.H+ cations would be in the form of bridging hydroxyl groups Ta-OH-Ta, stable at high temperature. Hydrogen analysis by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) confirms also this mechanism.For the piezoelectric property, different changes of space-groups R3c to R3 and P1, linked to huge strains were observed by TEM on the (NaₓK₁₋ₓ)₂Ta₄O₁₁ phase. Such changes result of electrical fields opposed to the crossing of the electron beam, giving rise to electro-mechanical effects of piezoelectricity. Large strains effects were confirmed by Laser Doppler Vibrometry measurements on (NaₓK₁₋ₓ)₂Ta₄O₁₁ layers of different texture subsmitted to alternative electrical fields.
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Marie Le Gallic. Films minces de Ta₂O₅ et (NaₓK₁₋ₓ)₂Ta₄O₁₁ (x=0.93) : structures et propriétés. Optique / photonique. Université de Grenoble, 2014. Français. ⟨NNT : 2014GRENI099⟩. ⟨tel-03219874⟩

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