Study On Doping,composite,and Thermoelectric Properties Of BiCuSeO Materials Modulated By High Pressure

Thermoelectric material is a kind of functional material which can realize the direct conversion between heat energy and electric energy.The Seebeck effect and Peltier effect promote the application from theory to practice,and the thermoelectric devices gradually step into the daily life.At present,thermoelectric technology is mainly used in thermoelectric power generation and refrigeration.The energy conversion efficiency of thermoelectric devices depends on the thermoelectric properties.High thermoelectric performance usually results in higher conversion efficiency and power density.In addition,electrode materials and module sizes influence the conversion efficiency of devices.The normal commercial thermoelectric materials is Bi₂Te₃-based alloys,while other thermoelectric systems are still being developed for application.Compared with intermetallic thermoelectric materials,the oxygen-containing thermoelectric materials have the advantages of high chemical stability,high temperature resistance,stable performance and so on,which are highly concerned by the field of thermoelectric power generation in the mid-temperature region.Bi Cu Se O,as an oxygen-containing thermoelectric material,is considered as a potential high-performance thermoelectric material due to its low intrinsic thermal conductivity.In this work,a series of electrothermal transport properties of P-type Bi Cu Se O-based materials were studied by using high-pressure preparation technology to manipulate the microstructure and chemical components.The main contents are shown as follows:1.High-performance Pb and Yb co-doped BiCuSeO samples were prepared by mechanical alloying and high temperature and high pressure（HPHT）.The structure and thermoelectric properties of doped samples were studied systematically.Pb&Yb substituting for Bi sites can increase the carrier concentration,and at the same time,The substitution of Bi³⁺by Yb³⁺could optimize the electronic structure,which inhibits the reduction of carrier mobility caused by Pb doping.combining high pressure and Pb&Yb co-doping introduced abundant grain boundaries,dislocations,and point defects,resulting in strong scattering of wide-frequency phonons and a reduction in lattice thermal conductivity(κ_lat=0.23Wm^-1K^-1).Through the collaborative optimization of pressure and Pb&Yb doping on electrothermal transport performance,the optimal quality factor z T_max=1.2 was obtained at 850K from Bi0.88Pb0.06Yb0.06Cu Se O.2.Pb-doped BiCuSeO samples were prepared by solid-state reaction and HPHT.The crystal structure,electronic structure and defect structure of the materials were systematically studied.The results show that pressure can significantly improve the structure distortion caused by Pb doping.Tigh density grain boundaries are generated in the matrix when pressure is applied,and high concentration of dislocations(N_D～9.05×10¹⁶ m^-2)are introduced in and located at grain boundaries to realize a chain scattering of phonons,leading to a reduction in lattice thermal conductivity(κ_lat=0.13 Wm^-1K^-1).It is found that pressure can modulate band structure,band gap,effective mass and thus improving carrier mobility.By optimizing the crystal structure,electronic structure,and dislocation density under pressure,a largerμ_w/κ_l=144 was obtained The Bi_0.96Pb_0.04Cu Se O samples prepared at 2GPa achieved the highest z T_max=1.69 at 767K and the average z T=0.85 in the temperature range of 374-767K.The theoretical simulation of single-leg thermoelectric devices showed that the energy conversion efficiency reached 12%when the temperature differenceΔK=500K,which is comparable to the conversion efficiency of most intermetallic thermoelectric devices.3.Commercial single-walled carbon nanotube（SWCNT）liquid and Pb-doped Bi Cu Se O-based material were used for liquid phase mixing and physical dispersion to obtain the precursor powder.The Bi Cu Se O&SWCNT composites were obtained by high-pressure sintering.The results showed that Pb doping significantly increased carrier concentration and cooperated with highly conductive SWCNT effectively improving the carrier mobility,and realizing the optimization of electrical transport performance.By studying the microstructure,the pressure induced obvious grain boundary and dislocations.The phase interfaces formed by SWCNT improves the phonon scattering efficiency.Incorporation of pressure,composite and doping,we obtained multiscale defect structure,which inhibited the increase of thermal conductivity caused by carbon-based material composite,and realized a decrease in lattice thermal conductivity at low contents.The superior composite obtained was Bi_0.95Pb_0.05Cu Se O&SWCNT material with the content of 0.1wt.%.The optimal quality factor zT_max=0.84@820K.