Tunable metasurface for independent controlling radar stealth properties via geometric and propagation phase modulation.

Metasurfaces have been verified as an ideal way to control electromagnetic waves within an optically thin interface. In this paper, a design method of a tunable metasurface integrated with vanadium dioxide (VO2) is proposed to realize independent control of geometric and propagation phase modulation. The reversible conversion of VO2 between insulator phase and metal phase can be realized by controlling the ambient temperature, which enables the metasurface to be switched quickly between split-ring and double-ring structures. The phase characteristics of 2-bit coding units and the electromagnetic scattering characteristics of arrays composed of different arrangements are analyzed in detail, which confirms the independence of geometric and propagation phase modulation in the tunable metasurface. The experimental results demonstrate that the fabricated regular array and random array samples have different broadband low reflection frequency bands before and after the phase transition of VO2, and the 10 dB reflectivity reduction bands can be switched quickly between C/X and Ku bands, which are in good agreement with the numerical simulation. This method realizes the switching function of metasurface modulation mode by controlling the ambient temperature, which provides a flexible and feasible idea for the design and fabrication of stealth metasurfaces.

Rapid fabrication of large-area concave microlens arrays on silica glasses by femtosecond laser bursts.

An efficient and flexible method using femtosecond laser bursts assisted by wet etching is presented to fabricate large-area high-quality microlens arrays (MLAs) on a silica glass surface. In this method, femtosecond laser bursts can ablate micro craters on silica glass in a fast, single-step process by controlling the electron density and a high-speed scanning galvanometer, and the influence mechanism of the number of pulses within a burst on the accuracy and quality of micro craters is analyzed in detail. The experimental results show that the preparation efficiency of micro craters is significantly improved to approximately 32,700 per second. By subsequent acid etching, concave microlenses with controllable dimensions, shapes, and alignments are easily obtained. A large area close-packed hexagonal concave MLA is successfully fabricated by using this method and shows high surface quality and uniformity, which excellently demonstrates the feasibility and flexibility of rapidly fabricating MLAs in the burst regime.

Two-dimensional/one-dimensional molybdenum sulfide (MoS2) nanoflake/graphitic carbon nitride (g-C3N4) hollow nanotube photocatalyst for enhanced photocatalytic hydrogen production activity.

The graphitic carbon nitride (g-C3N4) hollow nanotubes synthesized via a simple freeze-drying method are used for constructing Two-dimensional (2D)-one-dimensional (1D) molybdenum sulfide (MoS2) nanoflake/g-C3N4 hollow nanotube (MoS2/g-C3N4 nanotube) photocatalysts. The MoS2/g-C3N4 nanotube composite with 15 wt% MoS2 shows the highest hydrogen (H2) production rate (1124 µmol·h-1·g-1), much higher than bulk g-C3N4 (64 µmol·h-1·g-1) and g-C3N4 nanotubes (189 µmol·h-1·g-1). The excellent photocatalytic activity of MoS2/g-C3N4 nanotube composites can be ascribed to more exposed active edges of 2D-1D structure, multiple light reflection/scattering channels of 2D nanoflake/1D hollow nanotube composite structure and better carrier transfer and separation by heterojunction interface.

TiO2 nanobelts with anatase/rutile heterophase junctions for highly efficient photocatalytic overall water splitting.

Herein, we report surface coarsened titanium dioxide (TiO2) nanobelts with anatase/rutile heterophase junctions via a facile hydrothermal/calcination method for simultaneous hydrogen (H2) and oxygen (O2) productions from pure water, with excellent production rates of 0.614 and 0.297 mmol h-1 g-1 with platinum (Pt)/cobalt phosphide (CoP) as cocatalysts, respectively. Besides, the TiO2 nanobelts-900 °C with anatase/rutile heterophase junctions show a notable improvement in photocatalytic H2 and O2 production than pure anatase TiO2 nanobelts (TiO2 nanobelts-600 °C, 700 °C and 800 °C) and pure rutile TiO2 nanobelts (TiO2 nanobelts-1000 °C). The anatase/rutile heterophase junctions could effectively stimulate the transfer of electrons from rutile to anatase and then to Pt, and H2 generation on the surface of Pt. In the meantime, the holes can be transferred from anatase to rutile and then to CoP, and water oxidation on CoP's surface.

Fabrication of molybdenum and tungsten oxide, sulfide, phosphide (MoxW1-xO2/MoxW1-xS2/MoxW1-xP) porous hollow nano-octahedrons from metal-organic frameworks templates as efficient hydrogen evolution reaction electrocatalysts.

A metal-organic frameworks-derived sulfuration and phosphorization method is developed for constructing a hydrogen evolution reaction (HER) electrocatalyst (MoWOSP@C), which is composed of MoxW1-xO2 nanoparticles, MoxW1-xS2 nanosheets, and MoxW1-xP nanoparticles coated with a porous hollow carbon matrix. The three-dimensional MoWOSP@C nano-octahedron shows excellent catalytic activity with a low overpotential of -118 mV at a current density of -10 mA cm-2 and a small Tafel slope of 74.1 mV dec-1, which are all higher than those of the single phase components. The outstanding performance of the MoWOSP@C is due to the highly exposed active sites and synergistic effects of the MoxW1-xS2 and MoxW1-xP, carbon conductive matrix and mesoporous hollow structures.

Coarse-grained molecular dynamics simulations of the tensile behavior of a thermosetting polymer.

Using a previously developed coarse-grained model, we conducted large-scale (∼ 85 × 85 × 85 nm(3)) molecular dynamics simulations of uniaxial-strain deformation to study the tensile behavior of an epoxy molding compound, epoxy phenol novolacs (EPN) bisphenol A (BPA). Under the uniaxial-strain deformation, the material is found to exhibit cavity nucleation and growth, followed by stretching of the ligaments separated by the cavities, until the ultimate failure through ligament scissions. The nucleation sites of cavities are rather random and the subsequent cavity growth accounts for much (87%) of the volumetric change during the uniaxial-strain deformation. Ultimate failure of the materials occurs when the cavity volume fraction reaches ∼ 60%. During the entire deformation process, polymer strands in the network are continuously extended to their linear states and broken in the postyielding strain hardening stage. When most of the strands are stretched to their taut configurations, rapid scission of a large number of strands occurs within a small strain increment, which eventually leads to fracture. Finally, through extensive numerical simulations of various loading conditions in addition to uniaxial strain, we find that yielding of the EPN-BPA can be described by the pressure-modified von Mises yield criterion.

A coarse-grained model for epoxy molding compound.

We present a coarse-grained model for molecular dynamics simulations of an epoxy system composed of epoxy phenol novolac as epoxy monomer and bisphenol-A as the cross-linking agent. The epoxy and hardener molecules are represented as short chains of connected beads, and cross-linking is accomplished by introducing bonds between reactive beads. The interbead potential, composed of Lennard-Jones, bond stretching, and angle bending terms, is parametrized through an optimization process based on a particle swarm optimization method to fit certain key thermomechanical properties of the material obtained from experiments and previous full atomistic simulations. The newly developed coarse-grained model is capable of predicting a number of thermomechanical properties of the epoxy system. The predictions are in very good agreement with available data in the literature. More importantly, our coarse-grained model is capable of predicting tensile failure of the epoxy system, a capability that no other conventional molecular dynamic simulation model has. Finally, our coarse-grained model can speed up the simulations by more than an order of magnitude when compared with traditional molecular dynamic simulations.