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Etude de couches minces à base de delafossite CuCr 1-x Fe x O 2 (0 ≤ x ≤ 1) dopées au Mg déposées par pulvérisation cathodique radiofréquence en vue d'optimiser leurs propriétés thermoélectriques

Abstract : The aim of this thesis was to study the thermoelectric properties of delafossite type oxides thin-films deposited by RF-magnetron sputtering. Several thicknesses of CuCrO2:3%Mg, CuFeO2:3%Mg and CuCr0,84Fe0,16O2:3%Mg oxides were deposited on fused silica then annealed under vacuum at different temperatures in order to obtain delafossite structure. The optimal annealing temperature which leads to acceptable thermoelectric properties is 550 °C for CuCrO2:Mg and CuCr0,84Fe0,16O2:Mg thin films and 700 °C for CuFeO2:Mg thin film. The optimal thickness is 100 nm for the delafossite with chrome and 300 nm for delafossite with iron. The electrical conductivity of the studied thin films increases with the temperature, while maintaining a positive and constant Seebeck coefficient for the three given compositions that implies a hopping mechanism. The power factor of CuCrO2:Mg, CuFeO2:Mg and CuCr0,84Fe0,16O2:Mg thin films for which the annealing temperature and the thickness were optimized, reached 59 µW.m-1K-2, 84 µW.m- 1K-2 and 36 µW.m-1K-2 respectively at 200 °C. The microstructural and structural analysis allowed to understand the variation of the power factor with the annealing temperatures and the thicknesses. In particular, they showed that the decrease in the electrical conductivity of the thin films annealed at high temperature is due to concomitant phenomena of film cracking and magnesium segregation. A thermal analysis using modeling with the finite element method has demonstrated that in the case of thin films, the thermal conductivity of the substrate can be substituted for the thermal conductivity of the film in the calculation of figure of merit. The validity of the modified figure of merit ((ZT)* = S²s/ksubstrate) was given as a function of the film thickness, emissivity and thermal conductivity. The thermal conductivity of CuFeO2:Mg was measured using the 3? method and it was 4.82 W.m-1k-1 at 25 °C which is within the range of validity established for the use of (ZT)*. Thermoelectric modules based on these delafossite have also been elaborated and studied in this thesis. The uni-leg module made with CuCrO2:Mg thin films showed a maximum power of 10.6 nW for an applied hot side temperature at 220 °C and uncontrolled cold side temperature. Thanks to their stability in temperature and in air, the delafossite oxides thin films are promising candidates for thermoelectric applications in the field of microelectronics as a thermogenerator and specifically in high accuracy temperature measurement devices.
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https://tel.archives-ouvertes.fr/tel-02093143
Contributor : Inthuga Sinnarasa <>
Submitted on : Monday, April 8, 2019 - 4:34:58 PM
Last modification on : Friday, January 10, 2020 - 9:08:26 PM

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Inthuga Sinnarasa. Etude de couches minces à base de delafossite CuCr 1-x Fe x O 2 (0 ≤ x ≤ 1) dopées au Mg déposées par pulvérisation cathodique radiofréquence en vue d'optimiser leurs propriétés thermoélectriques. Science des matériaux [cond-mat.mtrl-sci]. Université Toulouse 3 - Paul Sabatier, 2018. Français. ⟨tel-02093143⟩

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