Multinary chalcogenide semiconductors in the Cu−Zn−Sn−S system have numerous potential applications in the fields of energy production, photocatalysis and nonlinear optics, but characterization and control of their microstructures remains a challenge because of the complexity resulting from the many mutually soluble metallic elements. Here, using state-of-the-art scanning transmission electron microscopy, energy dispersive spectroscopy, first-principles calculations and classical molecular dynamics simulations, we characterize the structures of promising thermoelectric materials Cu 2 (Zn,Sn)S 3 at different length scales to gain a better understanding of how the various components influence the thermoelectric behavior. We report the discovery of a mosaic-type domain nanostructure in the matrix grains comprising welldefined cation-disordered domains (the "tesserae") coherently bonded to a surrounding network phase with semiordered cations. The network phase is found to have composition Cu 4+x Zn x Sn 2 S 7 , a previously unknown phase in the Cu−Zn−Sn−S system, while the tesserae have compositions closer to that of the nominal composition. This nanostructure represents a new kind of phonon-glass electron-crystal, the cation-disordered tesserae and the abrupt domain walls damping the thermal conductivity while the cation-(semi)ordered network phase supports a high electronic conductivity. Optimization of the hierarchical architecture of these materials represents a new strategy for designing environmentally benign, low-cost thermoelectrics with high figures of merit.

Controlling thermal conductivity in nanocrystalline materials is of great interest in various fields such as thermoelectrics. However, its reduction mechanism has not been fully given due to the difficulty to assess local thermal conduction at grain boundaries (GBs) and grain interiors. Here, we calculated spatially decomposed thermal conductivities across and along MgO symmetric GBs using perturbed molecular dynamics, varying the GB separation from 2.1 to 20.0 nm. This reveals the different length scale of GB scattering for two directions: over hundreds of nanometers across GBs while within a few nanometers along GBs. Numerical analyses based on the spatially decomposed thermal conductivities demonstrate that the former is dominant upon suppressing thermal conductivity in polycrystalline materials, whereas the latter has a non-negligible impact in nanocrystalline materials because of a large reduction of intragrain thermal conductivity along GBs. These insights provide the exact mechanisms of heat transport in nanocrystalline materials toward more precise control of thermal conductivity.

No abstract

No abstract

Active Strobe Imager enables us to visualize the dynamic behavior of tissue, even under a high frequent vibration that cannot be followed by the naked eye. A pneumatic actuator imparts a vibration to the target object. By flashing light with a slightly different frequency from the object frequency, we can see the dynamics of object by the naked eye. The vibration we can observe depends on various parameters such as frequency and duty ratio. We formulate the parameter optimization problem by considering the observability of object under strobe condition. By applying the full search method for the problem, we discuss the optimum set of parameters. We also confirmed the effectiveness of the optimized parameters through experiment.