Concave surfaces focus sound while convex surfaces disperse sound. It is therefore interesting to know if it is possible to make use of these two opposite characteristics to enhance the band gap performance of periodi...Concave surfaces focus sound while convex surfaces disperse sound. It is therefore interesting to know if it is possible to make use of these two opposite characteristics to enhance the band gap performance of periodic arrays of solid cylinders in air. In this paper, the band gap characteristics of a 2-D square array of semi-hollow circular cylinders embedded in air are investigated, both experimentally and theoretically. In comparison with the types of inclusion studied by previous researchers, a semi-hollow circular cylinder is unique in the sense that it has concave inner surfaces and convex outer surfaces. The finite difference time domain (FDTD) method is employed to study the propagation behavior of sound across the new phononic crystal of finite extent, and the influences of sample size and inclusion orientation on band gap characteristics are quantified in order to obtain the maximum band gap. For reference, the band gap behaviors of solid circular cylinder/air and hollow circular cylinder/air systems are considered and compared with those of semi-hollow circular cylinder/air systems. In addition to semi-hollow circular cylinders, other inclusion topologies such as semi-hollow triangular and square cylinders are also investigated. To validate the theoretical predictions, experimental measurements on square arrays of hollow Al cylinders in air and semi-hollow Al cylinders in air are carried out. The results demonstrate that the semi-hollow circular cylinder/air system has the best overall band gap performance.展开更多
Lithium-and manganese-rich(LMR)layered cathode materials hold the great promise in designing the next-generation high energy density lithium ion batteries.However,due to the severe surface phase transformation and str...Lithium-and manganese-rich(LMR)layered cathode materials hold the great promise in designing the next-generation high energy density lithium ion batteries.However,due to the severe surface phase transformation and structure collapse,stabilizing LMR to suppress capacity fade has been a critical challenge.Here,a bifunctional strategy that integrates the advantages of surface modification and structural design is proposed to address the above issues.A model compound Li_(1.2)Mn_(0.54)Ni_(0.13)Co_(0.13)O_(2)(MNC)with semi-hollow microsphere structure is synthesized,of which the surface is modified by surface-treated layer and graphene/car-bon nanotube dual layers.The unique structure design enabled high tap density(2.1 g cm^(−3))and bidirectional ion diffusion pathways.The dual surface coatings covalent bonded with MNC via C-O-M linkage greatly improves charge transfer efficiency and mitigates electrode degradation.Owing to the synergistic effect,the obtained MNC cathode is highly conformal with durable structure integrity,exhibiting high volumetric energy density(2234 Wh L^(−1))and predominant capacitive behavior.The assembled full cell,with nanograph-ite as the anode,reveals an energy density of 526.5 Wh kg^(−1),good rate performance(70.3%retention at 20 C)and long cycle life(1000 cycles).The strategy presented in this work may shed light on designing other high-performance energy devices.展开更多
基金Supported by the National Basic Research Program of China ("973" Project) (Grant Nos.2006CB601202,2006CB601204)the National Natural Science Foundation of China (Grant Nos.50676075,10572111,10632060)+1 种基金the National 111 Project of China (Grant No.B06024)the National High Technology Research and Development Program of China ("863" Project) (Grant No.2006AA03Z519)
文摘Concave surfaces focus sound while convex surfaces disperse sound. It is therefore interesting to know if it is possible to make use of these two opposite characteristics to enhance the band gap performance of periodic arrays of solid cylinders in air. In this paper, the band gap characteristics of a 2-D square array of semi-hollow circular cylinders embedded in air are investigated, both experimentally and theoretically. In comparison with the types of inclusion studied by previous researchers, a semi-hollow circular cylinder is unique in the sense that it has concave inner surfaces and convex outer surfaces. The finite difference time domain (FDTD) method is employed to study the propagation behavior of sound across the new phononic crystal of finite extent, and the influences of sample size and inclusion orientation on band gap characteristics are quantified in order to obtain the maximum band gap. For reference, the band gap behaviors of solid circular cylinder/air and hollow circular cylinder/air systems are considered and compared with those of semi-hollow circular cylinder/air systems. In addition to semi-hollow circular cylinders, other inclusion topologies such as semi-hollow triangular and square cylinders are also investigated. To validate the theoretical predictions, experimental measurements on square arrays of hollow Al cylinders in air and semi-hollow Al cylinders in air are carried out. The results demonstrate that the semi-hollow circular cylinder/air system has the best overall band gap performance.
基金The authors greatly appreciate the financial support from the National Science Foundation of China(22075048,51173027,21875141)Beijing National Laboratory for Condensed Matter Physics,Shanghai International Collaboration Research Project(19520713900).
文摘Lithium-and manganese-rich(LMR)layered cathode materials hold the great promise in designing the next-generation high energy density lithium ion batteries.However,due to the severe surface phase transformation and structure collapse,stabilizing LMR to suppress capacity fade has been a critical challenge.Here,a bifunctional strategy that integrates the advantages of surface modification and structural design is proposed to address the above issues.A model compound Li_(1.2)Mn_(0.54)Ni_(0.13)Co_(0.13)O_(2)(MNC)with semi-hollow microsphere structure is synthesized,of which the surface is modified by surface-treated layer and graphene/car-bon nanotube dual layers.The unique structure design enabled high tap density(2.1 g cm^(−3))and bidirectional ion diffusion pathways.The dual surface coatings covalent bonded with MNC via C-O-M linkage greatly improves charge transfer efficiency and mitigates electrode degradation.Owing to the synergistic effect,the obtained MNC cathode is highly conformal with durable structure integrity,exhibiting high volumetric energy density(2234 Wh L^(−1))and predominant capacitive behavior.The assembled full cell,with nanograph-ite as the anode,reveals an energy density of 526.5 Wh kg^(−1),good rate performance(70.3%retention at 20 C)and long cycle life(1000 cycles).The strategy presented in this work may shed light on designing other high-performance energy devices.
