Study On The Synthesis And Thermoelectric Properties Of Chalcogenide Cu₂CdSnSe₄

It is great convenience to people,s life due to the modernization of the world economy, while growing energy crisis zand environmental pollution pressure is becoming more and more serious. It has spurred research into new clean energy and renewable technologies that could convert waste heat in industry into electricity. One such avenue lies in the area of thermoelectric materials, which could convert waste heat in industry into electricity. Recently, a class of quaternary chalcogenides compounds with diamond-like structure and wide band gap has been found to have reasonable thermoelectric performance as a p-type thermoelectric materials. The structure of this compound can also be derived from cubic zinc blende binary II-VI compounds through substituting II atoms with both I and III atoms, followed by further substituting two III atoms with one II and IV atoms. The lattice thermal conductivity of quaternary chalcogenides compounds is low due to the strong phonon scattering by the intrinsic complex bonding type and/or the highly distorted crystal structure in these quaternary chalcogenides. Besides, colloidal synthesis of Cu₂CdSnSe₄ nanocrystals followed by spark plasma sintering is an effective way to enhance their thermoelectric performance. Therefore, we report the colloidal synthesis of quaternary chalcogenide Cu₂CdSnSe₄ nanocrystals follewed by spark plasma sintering, meanwhile complete to synthesize ex-situ homogeneous nanoinclusions in this paper. The major research results are summarized as follows:1. Cu₂CdSnSe₄ nanocrystals has been synthesized by colloidal synthesis followed by spark plasma sintering, while we study on thermoelectric properties of wide-band-gap dense materials embedding the nanostructure.There are some copper dopants and selenium vacancies during colloidal synthesis, which electrical conductivity is enhanced and thermal conductivity is greatly reduced compared to solid state method. These two factors boost the ZT of Cu₂CdSnSe₄ up to 0.62 at 745 K, improving 73% compared to polycrystal bulk sample, which indicates quaternary chalcogenide Cu₂CdSnSe₄ nanocrystals have potential research significance and application value in the medium temperature thermoelectric.2. We report Ag doped quaternary chalcogenide compounds Cu2-xAgxCdSnSe4 via the colloidal synthesis method followed by spark plasma sintering. The as-prepared Cu2-xAgxCdSnSe4 nanocrystals present a narrow size distribution ranging from 30 to 50 nm and a precisely controlled composition. Additionally, it was observed that the disordered zinc blende phases was embedded in the normal zinc blende phases and amorphous nanoparticles distributed on the grain surface and between grain boundary in Cu2-xAgxCdSnSe4 bulk samples. Moreover, a large increment in Seebeck coefficient and a dramatically reduction in the lattice thermal conductivity was found in Ag doped Cu₂CdSnSe₄ compounds, and the dimensionless thermoelectric figure of merit ZT reaches a peak value of 0.8 at 688 K.3. Quaternary chalcogenide Cu2 CdSn Se4 distributed with ex-situ homogeneous nanoinclusions has been synthesized by ball milling followed by spark plasma sintering. It is found that nanocrystallite inclusions strongly enhance electrical conductivity, while preserving the Seebeck coefficient. In addition, these inclusions significantly reduce the lattice thermal conductivity through scattering phonons with all-scale length due to the polymorphous structure feature of Cu₂CdSnSe₄ composites. These concomitant effects result in an enhanced thermoelectric performance with the dimensionless figure of merit ZT reaching a peak value of 0.5 at 760 K, which is 65% improvement compared to that of the pure Cu₂CdSnSe₄ matrix.