Thermoelectrics enable waste heat recovery, holding promises in relieving energy and environmental crisis. Lillianite materials have been long-term ignored due to low thermoelectric efficiency. Herein we report the discovery of superior thermoelectric performance in Pb7Bi4Se13 based lillianites, with a peak figure of merit, zT of 1.35 at 800 K and a high average zT of 0.92 (450–800 K). A unique quality factor is established to predict and evaluate thermoelectric performances. It considers both band nonparabolicity and band gaps, commonly negligible in conventional quality factors. Such appealing performance is attributed to the convergence of effectively nested conduction bands, providing a high number of valley degeneracy, and a low thermal conductivity, stemming from large lattice anharmonicity, low-frequency localized Einstein modes and the coexistence of high-density moiré fringes and nanoscale defects. This work rekindles the vision that Pb7Bi4Se13 based lillianites are promising candidates for highly efficient thermoelectric energy conversion.

A simple but effective technique is proposed to generate cylindrical converging shock waves. The shock dynamics is employed to design a curved wall profile of the test section in a shock tube. When a planar shock wave propagates forward along the curved wall, the disturbances produced by the curved wall would continuously propagate along the shock surface and bend the shock wave. As an example, the wall profile for an incident shock Mach number of M 0 = 1.2 and a converging angle of 15°is tested numerically and experimentally. Both numerical and experimental results show a perfect circular shock front, which validates our method.In some applications of shock tube such as inertial confinement fusion, 1 supernova explosion, 2 and shock wave lithotripsy, 3 converging shocks are needed in order to concentrate energy in a small volume. However, it is difficult to generate a cylindrical converging shock wave in an ordinary shock tube due to the initial shape imperfections and the nonlinear wave interactions. 4-6 Experiments of cylindrical converging shock waves interacting with cylindrical bubble have been conducted in an annual coaxial vertical diaphragmless shock tube. 5 A suggestion of gas lens to generate a cylindrical converging shock wave in a twodimensional wedge geometry was proposed by Dimotakis and Samtaney. 6 This consideration can produce a perfect cylindrical converging shock wave theoretically, although the related experimental results have not yet been found from the open publications. In this letter, a simple but effective technique is proposed in order to avoid difficulties in the formation of cylindrical converging shock wave. Specifically, we apply shock dynamics to design a wall profile with a special shape, which transfers the planar shock wave in a shock tube to a cylindrical one.The shock dynamics is a simple and useful theoretical tool to analyze the process of the propagation of shock waves in various phenomena such as diffraction, reflection, refraction, interaction, and so on. Particularly, the theory of disturbance propagating on shock surface presented by Whitham 7 provides us with such a possibility to analyze complicated phenomena of shock diffraction and interaction. The Chester-Chisnell-Whitham ͑CCW͒ relation is the basis of shock dynamics for the case of a uniform quiescent gas ahead of shock, which refers to Chester, 8 Chisnell, 9 and Whitham, 7,10,11 who obtained the same relation using different methods, independently. According to the shock dynamics, when a planar shock wave propagates forward along a continuously concave wall, the disturbances produced by the curved wall would continuously propagate along the shock front and bend the shock wave such that the contact edge of the wall and the shock wave can keep vertically propagating forward. In order to obtain a perfect cylindrical convergent shock wave, the curvature of the profile line should be calculated by shock dynamics. Only a very brief introduction of the shock dynamics is provided here. The method by applying shock dynam...

Flying insects and swimming fishes have high efficiency and high maneuverability in air and water, respectively. Their wings and fins have evolved for many ages to adapt to propelling in the complex environment. In the paper, an integrative biomimetic robotic fish is proposed and developed, which combines the advantages of insect wings and fish fins to achieve a high agility underwater. In the robotic fish, two caudal fins were equipped at the tail of the robotic fish in parallel as the main propulsion mechanism, the opposite flapping of the two caudal fins generates mutually opposing lateral forces during cruising, which leads to a stable and high-performance swimming. In addition, two pectoral fins that mimic the function of insect wings were equipped at two sides of the robotic fish, which enhances the robotic fish maneuverability in vertical plane. Moreover, a central pattern generator (CPG) model was designed to achieve the versatile maneuvering motions, motion switching, and autonomous swimming with an obstacle avoiding ability. The experiments have demonstrated that the robotic fish can swim more stably and efficiently with versatile maneuver motions by taking advantage of the integrative propulsion mechanism. The developed robotic fish have many potential applications for its agility, stable swimming, and low-cost structure.

The local symmetry, beyond the averaged crystallographic structure, tends to bring unusual performances. Negative thermal expansion is a peculiar physical property of solids. Here, we report the delicate design of the localized symmetry breaking to achieve controllable thermal expansion in ScF nanoscale frameworks. Intriguingly, an isotropic zero thermal expansion is concurrently engineered by localized symmetry breaking, with a remarkably low coefficient of thermal expansion of about +4.0 × 10/K up to 675 K. This mechanism is investigated by the joint analysis of atomic pair distribution function of synchrotron X-ray total scattering and extended X-ray absorption fine structure spectra. A localized rhombohedral distortion presumably plays a critical role in stiffening ScF nanoscale frameworks and concomitantly suppressing transverse thermal vibrations of fluorine atoms. This physical scenario is also theoretically corroborated by the extinction of phonon modes with negative Grüneisen parameters in rhombohedral ScF. The present work opens an untraditional chemical modification route to achieve controllable thermal expansion by breaking local symmetries in materials.

The presence of high crystallographic symmetry and nanoscale defects are favorable for thermoelectrics. With proper electronic structures, a highly symmetric crystal tends to possess multiple carrier channels and promote electrical conductivity without sacrificing Seebeck coefficient. In addition, nanoscale defects can effectively scatter acoustic phonons to suppress thermal conductivity. Here, it is reported that the triple doping of Cu2SnSe3 leads to a high ZT value of 1.6 at 823 K for Cu1.85Ag0.15(Sn0.88Ga0.1Na0.02)Se3, and a decent average ZT (ZTave) value of 0.7 is also achieved for Cu1.85Ag0.15(Sn0.93Mg0.06Na0.01)Se3 from 475 to 823 K. This study reveals: 1) Ag doping on Cu sites generates numerous point defects and greatly decreases lattice thermal conductivity. 2) Doping Mg or Ga converts the monoclinic Cu2SnSe3 into a cubic structure. This symmetry enhancing leads to an increase in the effective mass from 0.8 me to 2.6 me (me, free electron mass) and the power factor from 4.3 µW cm−1 K−2 for Cu2SnSe3 to 11.6 µW cm−1 K−2. 3) Na doping creates dense dislocation arrays and nanoprecipitates, which strengthens the phonon scattering. 4) Pair distribution function analysis shows localized symmetry breakdown in the cubic Cu1.85Ag0.15(Sn0.88Ga0.1Na0.02)Se3. This work provides a standpoint to design promising thermoelectric materials by synergistically manipulating crystal symmetry and nanoscale defects.