Kaya Wei

Synthesis and Characterization of Nanostructured Stannite Cu2ZnSnSe4 and Ag2ZnSnSe4 for Thermoelectric Applications

Cu2ZnSnSe4 and Ag2ZnSnSe4 nanocrystals were synthesized by a colloidal synthesis route and subsequently densified to form dense polycrystalline bulk specimens with nanoscale grains employing spark plasma sintering (SPS). Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron diffraction spectroscopy (EDS) were used to characterize the nanocrystals. The optical bandgap, thermal stability, and low temperature transport properties of the nanostructured polycrystalline stannites Cu2ZnSnSe4 and Ag2ZnSnSe4 were investigated. The transport properties of Ag2ZnSnSe4 are reported here for the first time and indicate polaronic-type conduction. Cu2ZnSnSe4 is p-type while Ag2ZnSnSe4 is n-type. The thermal transport in these materials is also investigated, the thermal conductivity of nanostructured Cu2ZnSnSe4 being greatly reduced compared with that of the bulk. Our results are presented in light of the interest in these materials for thermoelectric applications.

Bournonite PbCuSbS3 : Stereochemically Active Lone-Pair Electrons that Induce Low Thermal Conductivity.

An understanding of the structural features and bonding of a particular material, and the properties these features impart on its physical characteristics, is essential in the search for new systems that are of technological interest. For several relevant applications, the design or discovery of low thermal conductivity materials is of great importance. We report on the synthesis, crystal structure, thermal conductivity, and electronic-structure calculations of one such material, PbCuSbS3 . Our analysis is presented in terms of a comparative study with Sb2 S3 , from which PbCuSbS3 can be derived through cation substitution. The measured low thermal conductivity of PbCuSbS3 is explained by the distortive environment of the Pb and Sb atoms from the stereochemically active lone-pair s(2) electrons and their pronounced repulsive interaction. Our investigation suggests a general approach for the design of materials for phase-change-memory, thermal-barrier, thermal-rectification and thermoelectric applications, as well as other functions for which low thermal conductivity is purposefully sought.

Clathrates and beyond: Low-density allotropy in crystalline silicon

In its common, thermodynamically stable state, silicon adopts the same crystal structure as diamond. Although only a few alternative allotropic structures have been discovered and studied over the past six decades, advanced methods for structure prediction have recently suggested a remarkably rich low-density phase space that has only begun to be explored. The electronic properties of these low-density allotropes of silicon, predicted by first-principles calculations, indicate that these materials could offer a pathway to improving performance and reducing cost in a variety of electronic and energy-related applications. In this focus review, we provide an introduction and overview of recent theoretical and experimental results related to low-density allotropes of silicon, highlighting the significant potential these materials may have for technological applications, provided substantial challenges to their experimental preparation can be overcome.

A synthetic approach for enhanced thermoelectric properties of PEDOT:PSS bulk composites

The thermoelectric properties of PEDOT:PSS/Bi0.5Sb1.5Te3 polymer/inorganic bulk composites with different Bi0.5Sb1.5Te3 content were investigated. The composites were prepared at various concentrations of Bi0.5Sb1.5Te3 by a solution-phase process before grinding to fine powders in liquid N2 for hot pressing into bulk polymer composite materials. The measured transport properties are well described within a theoretical model for effective media involving a tunneling mechanism induced by thermal voltage fluctuations. Our results present a strategy for the preparation of bulk polymer composites and demonstrate an avenue for optimization of the thermoelectric properties of PEDOT:PSS/Bi0.5Sb1.5Te3 bulk composites.

Synthesis, transport properties, and electronic structure of Cu2CdSnTe4

A new stannite phase was synthesized and its temperature dependent transport properties were investigated. Cu2CdSnTe4 possesses strong p-type conduction, while the temperature dependence of the thermal conductivity exhibits typical dielectric behavior. Electronic structure calculations allowed for a description of the transport characteristics in terms the energy band structure, density of states, and Fermi surface. The potential for thermoelectric applications is also discussed.

A type-II clathrate with a Li-Ge framework

Abstract Na16Cs8LixGe136−x (x≈2.8, space group Fd3̅m) is the first intermetallic type-II clathrate with a lithium-substituted framework. The phase was obtained from the elements in sealed Ta ampoules by annealing at 650°C for 5 days. Samples were investigated by synchrotron X-ray powder diffraction, solid-state NMR, microstructure and chemical analysis. The substitution of Ge by Li atoms causes a marked shrinking of the lattice parameter. Studies by 7Li NMR confirmed the presence of Li in the clathrate phase and the 23Na and 133Cs NMR spectra consistently showed distinct changes as compared to the ternary Na16Cs8Ge136. The SEDOR technique revealed a distance between Li and Cs atoms in agreement with the result of crystal structure refinement, indicating Li substitution at site 96g. The distinct Knight shift of all NMR signals implies metallic behaviour of the clathrate phase, measurements of the magnetic susceptibility indicate diamagnetic behaviour.

Polaronic transport in Ag-based quaternary chalcogenides

Low temperature resistivity measurements on dense polycrystalline quaternary chalcogenides Ag2+xZn1-xSnSe4, with x = 0, 0.1, and 0.3, indicate polaronic type transport which we analyze employing a two-component Holstein model based on itinerant and localized polaron contributions. Electronic structure property calculations via density functional theory simulations on Ag2ZnSnSe4 for both energetically similar kesterite and stannite structure types were also performed in order to compare our results to those of the compositionally similar but well known Cu2ZnSnSe4. This theoretical comparison is crucial in understanding the bonding that results in polaronic type transport for Ag2ZnSnSe4, as well as the structural and electronic properties of both crystal structure types. In addition to possessing this unique electronic transport, the thermal conductivity of Ag2ZnSnSe4 is low and decreases with increasing silver content. This work reveals unique structure-property relationships in materials that continue to ...

Synthesis, Structure, Te Alloying, and Physical Properties of CuSbS2

Materials with very low thermal conductivities continue to be of interest for a variety of applications. We synthesized CuSbS2 employing a mechanical alloying technique in order to investigate its physical properties. The trigonal pyramid arrangement of the S atoms around the Sb atoms allows for lone-pair electron formation that results in very low thermal conductivity. In addition to thermal properties, the structural, electrical, and optical properties, as well as compositional stability measurements, are also discussed. CuSbS1.8Te0.2 was similarly synthesized and characterized in order to compare its structural and transport properties with that of CuSbS2, in addition to investigating the effect of Te alloying on these properties.

Structure and Transport Properties of Dense Polycrystalline Clathrate-II (K,Ba)16(Ga,Sn)136 Synthesized by a New Approach Employing SPS

Tin clathrate-II framework-substituted compositions are of current interest as potential thermoelectric materials for medium-temperature applications. A review of the literature reveals different compositions reported with varying physical properties, which depend strongly on the exact composition as well as the processing conditions. We therefore initiated an approach whereby single crystals of two different (K,Ba)16(Ga,Sn)136 compositions were first obtained, followed by grinding of the crystals into fine powder for low temperature spark plasma sintering consolidation into dense polycrystalline solids and subsequent high temperature transport measurements. Powder X-ray refinement results indicate that the hexakaidecahedra are empty, K and Ba occupying only the decahedra. Their electrical properties depend on composition and have very low thermal conductivities. The structural and transport properties of these materials are compared to that of other Sn clathrate-II compositions.

Porosity Effects on the Thermoelectric Properties of Nanostructured Bismuth

Bismuth nanocrystals were synthesized using a polyol process and densified using spark plasma sintering to form polycrystalline bulk materials with uniformly dispersed nanoscale grains. The Bi nanocrystals were consolidated at different pressures in order to investigate the magnitude and temperature dependence of transport properties as a function of porosity. We also compare these transport properties with those of single-crystal bismuth. This investigation expands our understanding of the role of porosity in the thermoelectric properties of nanostructured bismuth.