Study of mechanical and physical properties Of thermoelectric materials for energy conversion.

Boumezrag, Maria Nor Elyakin (2025) Study of mechanical and physical properties Of thermoelectric materials for energy conversion. Doctoral thesis, Université Mohamed Khider (Biskra - Algérie).

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Abstract

This study investigates the fabrication and characterization of CuO thin films on glass substrates using the spin coating technique. Key experimental parameters, including solvent type, annealing temperature, film thickness, and alkali metal (Li, Na, K) doping at concentrations of 3%, 6%, 9%, and 12%, were optimized to enhance the structural, optical, electrical, and thermoelectric properties of the films. X-ray diffraction (XRD) analysis confirmed the polycrystalline monoclinic structure of CuO, with variations in crystalline peaks and lattice structure influenced by different solvents. Isopropanol, 1-propanol, ethanol, and 2-methoxyethanol promoted growth along the c-axis (002 plane), whereas pentanol and methanol favored (111) plane growth. Isopropanol-based films exhibited the largest crystallite size and improved optical transmittance, while crystallinity improved with annealing up to 500°C. Optimized film deposition with nine layers resulted in an approximate thickness of 400 nm, leading to smaller crystallites with higher strain. Other hand, across all doping levels, the monoclinic phase was preserved. Sodium and potassium doping led to lattice compression, whereas lithium reduced lattice volume at higher concentrations. Optically, sodium and potassium doping enhanced transmittance, with potassium providing the best optical clarity at 6%, while lithium reduced transmittance and increased the band gap. Urbach energy increased in all doped films, indicating higher disorder, with potassium-doped films showing the least disorder at 6%. For the electrical properties, electrical conductivity was improved with sodium and potassium doping, peaking at 6%, with sodium achieving a conductivity of 5.93 × 10⁻² (Ω cm) ⁻¹ and potassium exhibiting the lowest resistivity. Lithium doping enhanced conductivity up to 6%, but further doping led to a decline. The best thermoelectric performance was observed in Li-doped CuO, which exhibited the highest power factor (9.776 × 10⁻¹⁰ W. m⁻¹.K⁻²). Additionally, the study of mechanical and elastic properties confirmed CuO's strong anisotropy and mechanical stability. These findings highlight the potential of doped CuO thin films for applications in optoelectronics, thermoelectric, and flexible electronics, demonstrating the ability to fine-tune their properties through controlled doping and processing techniques.

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