Improvement Of Thermoelectric Properties Of BaCu₂Se₂ And BiCuSeO By Doping And Texturation

BaCu₂Se₂ and Bi CuSe O, as two kinds of promising p-type copper-chalcogenide based thermoelectric materials, have low thermal conductivity and high Seebeck coefficient. But the low electrical conductivity results in low ZT value. Therefore, improvement of electrical conductivity is the key issue for their practical application. In this thesis, doping was carried out to increase electrical conductivity of BaCu₂Se₂ and Bi Cu SeO, and further improve their ZT value. Influence of doping on the microstructure, transport behavior and thermoelectric properties of BaCu₂Se₂ and Bi Cu SeO was investigated by XRD, SEM, TEM, Hall measurement, ZEM-3 and laser flash diffusivity apparatus. Meanwhile, the related mechanism was also investigated.In order to improve electrical conductivity and ZT value, Na doping and Ag doping were developed to increase carrier concentration and mobility of BaCu₂Se₂, respectively. As Na doping content increased, carrier concentration of Ba1-xNaxCu2Se2 increased obviously, electrical conductivity（σ） and Seebeck coefficient（S） was increased and decreased, respectively. The maximum power factor（PF） of 8.1μWcm-1K-2 at 773 K was obtained when x=0.075. Lattice thermal conductivity（κlat） decreased slightly as Na doping increasing, while electronic thermal conductivity（κele） increased due to the improved electrical conductivity. According to the equation ZT=（S2σ/κtot）T, ZT first increased and then decreased with the increasing Na doping amount. Maximum ZT of 1 at 773 K was obtained when x=0.075. Proper amount of Ag substitution in the BaCu₂Se₂ increased the carrier mobility. Therefore, the electrical conductivity and PF was improved. Room temerpature ZT was enhanced from 0.018 to 0.034.Ba doping was chosen in BiCuSeO to increase the carrier concentration, thus improve the electrical conductivity and ZT. As Ba doping concentration increased, carrier concentration of Bi1-xBaxCu SeO increased remarkably, leading to electrical conductivity and Seebeck coefficient increase and decrease, respectively. Therefore, the maximum PF of 6.33μWcm-1K-2 at 923 K was obtained when x=0.125. Electronic thermal conductivity was increased because of the improved electrical conductivity after Ba doping, while lattice thermal conductivity was decreased due to enhanced phonon scattering by the fluctuation of atomic mass and size between Ba and Bi. The increase of κele surpassed reduction of κlat, so κtot increased after Ba doping. As a concequence, ZT first increased and then decreased. The maximum ZT of 1.1 at 923 K was obtained when x=0.125. The thermoelectric properties of Bi1-xBaxCuSe O showed no change after annealing at 923 K for one week, displaying their good thermal stability.Taking the advantage of the anisotropic characteristics of BiCuSeO, hot forging method was applied on the Ba heavily doped BiCuSeO to induce texture and improve the carrier mobility and electrical conductivity. For the direction perpendicular to the pressure, with the increase of hot forging times, carrier mobility（μ⊥）, electrical conductivity（σ⊥） and thermal conductivity（κ⊥） increased. In contrast, transport behavior showed a reverse regulation in the direction parallal to the pressure. Seebeck coefficient and carrier concentration were still unchanged after hot forging. σ⊥/κ⊥ increased while σ∥/κ∥ decreased as increasing hot forging times, consequently, ZT⊥ was boosted to 1.4 at 923 K after three times hot forging.S doping was applied to decrease the cost and thermal conductivity of BiCuSeO. Nearly pure phase Bi Cu Se1-xSxO solid solution can be formed within x=0¹. Lattice parameters of Bi Cu Se1-xSxO decreased linearly as increasing S content, obeying the Vegard’s law. Ab initio calculation for Bi Cu Ch O（Ch=S, Se） indicated that Cu-Ch bonding had strong covalent characteristic, while Bi-Ch bonding that connected（Bi2O2）2+ layer with（Cu2Ch2）2- layer had strong inoic characteristic. When Se was partially substituted by S, atomic mass and size difference between Se and S induced lattice distortion and intensified phonon scattering, so their thermal conductivity was lower than those of Bi CuSe O and Bi Cu SO.