Structural Manipulation of Aminal-crosslinked Polybutadiene for Recyclable and Healable Elastomers

Recently,increasingly growing efforts have been devoted to incorporating dynamic covalent bonds into covalently crosslinked networks to address the persistent trade-offs between chemical crosslinking and malleability.Herein,a series of dynamic aminal bond crosslinked polybutadiene rubbers(PAPB)are prepared by crosslinking aldehyde group-terminated polybutadiene rubber(APB)with piperazine.By varying the molecular weight of APB,the crosslinking density of PAPB is changed,which offers the platform to regulate the mechanical characteristics and dynamic properties.Specially,with the decrease of APB molecular weight,i.e.with the increase of crosslinking density,the modulus of PAPB gradually increases while the elongation at break conversely decreases,and the activation energy for network rearrangement initially decreases and then increases.The resultant PAPB exhibit vitrimer-like behaviors that can alter the network topologies at elevated temperatures without the loss of structural integrity through dissociative aminal exchange reactions,while the protic source can accelerate aminal dissociation and result in network dissolution even at room temperature.Due to the aminal exchange,PAPB are thermally malleable and can almost restore the original mechanical characteristics after recycling;besides,they are capable of healing at a relatively low crosslinking density.

Close-packed layer spacing as a practical guideline for structure symmetry manipulation of IV-VI/I-V-VI_(2) thermoelectrics

The crystal-structure symmetry in real space can be inherited in the reciprocal space,making high-symmetry materials the top candidates for thermoelectrics due to their potential for significant electronic band degeneracy.A practical indicator that can quantitatively describe structural changes would help facilitate the advanced thermoelectric material design.In face-centered cubic structures,the spatial environment of the same crystallographic plane family is isotropic,such that the distances between the close-packed layers can be derived from the atomic distances within the layers.Inspired by this,the relationship between inter-and intra-layer geometric information can be used to compare crystal structures with their desired cubic symmetry.The close-packed layer spacing was found to be a practical guideline of crystal structure symmetry in IV-VI chalcogenides and I-V-VI_(2) ternary semiconductors,both of which are historically important thermoelectrics.The continuous structural evolution toward high symmetry can be described by the layer spacing when temperature or/and composition change,which is demonstrated by a series of pristine and alloyed thermoelectric materials in this work.The layerspacing-based guideline provides a quantitative pathway for manipulating crystal structures to improve the electrical and thermal properties of thermoelectric materials.

Broadband acoustic focusing by symmetric Airy beams with phased arrays comprised of different numbers of cavity structures

We realize broadband acoustic focusing effect by employing two symmetric Airy beams generated from phased arrays,in which the units of the phased arrays consist of different numbers of cavity structures, each of which is composed of a square cavity and two inclined channels in air. The exotic phenomenon arises from the energy overlapping of the two symmetric Airy beams. Besides, we demonstrate the focusing performance with high self-healing property, and discuss the effects of structure parameters on focusing performance, and present the characteristics of the cavity structure with straight channels. Compared with other acoustic lenses, the proposed acoustic lens has advantages of broad bandwidth（about 1.4 kHz）, high self-healing property of focusing performance, and free adjustment of focal length. Our finding should have great potential applications in ultrasound imaging and medical diagnosis.

Manipulating Backward Propagation of Acoustic Waves by a Periodical Structure

In the backward propagation of acoustic waves, the direction of phase velocity is anti-parallel to that of group velocity. We propose a scheme to manipulate the backward propagation using a periodicM structure. The dynamic backward propagation process is further experimentally observed. It is demonstrated that the oblique incident plane wave moves backward when it travels through the periodical structure and the backward shift can be controlled within a certain range.

Metasurfaces enabling structured light manipulation:advances and perspectives [Invited]

Metasurfaces and structured light have rapidly advanced over the past few years, from being paradigms to forming functional devices and tailoring special light beams for wide emerging applications. Here, we focus on harnessing metasurfaces for structured light manipulation. We review recent advances in shaping structured light by metasurfaces on different platforms（metal, silica, silicon, and fiber）. Structured light manipulation based on plasmonic metasurfaces, reflection-enhanced plasmonic metasurfaces, metasurfaces on fiber facets, dielectric metasurfaces, and sub-wavelength structures on silicon are presented, showing impressive performance.Future trends, challenges, perspectives, and opportunities are also discussed.

Tuning microwave absorption properties of Ti_(3)C_(2)T_(x) MXene-based materials:Component optimization and structure modulation

The current electromagnetic environment brings a growing demand for efficient microwave absorption(MA)materials.Ti_(3)C_(2)T_(x) MXene,one of the 2D transition-metal carbides,is considered to be a promising MA material owing to its superior dielectric properties and structural processability.In order to further improve the MA performance and environmental adaptability of Ti_(3)C_(2)T_(x) MXene,Ti_(3)C_(2)T_(x) MXene-based MA materials enhanced by composition and structure design have been extensively studied and the regu-lation ideas for its MA properties can be outlined as component optimization and structure manipulation strategies based on the microwave absorption mechanism.Herein,we briefly introduced the microwave absorption mechanism and focused on the design strategies of Ti_(3)C_(2)T_(x) MXene-based MA materials based on recent advances.In addition,the prospects of Ti_(3)C_(2)T_(x) MXene-based MA materials were also discussed.

Cobalt diselenide (001) surface with short-range Co-Co interaction triggering high-performance electrocatalytic oxygen evolution

Oxygen evolution reaction (OER) still suffers from the bottleneck in electrocatalytic water splitting. Herein, in virtue of volcano plots drawn by theoretical calculation, the (001) facet was screened as the superb facet of orthorhombic CoSe_(2) for OER. Afterwards, CoSe_(2)(001) nanosheets were synthesized and the exposure ratio of (001) facet is controllable with thermodynamics methods effectively. The single-facet CoSe2(001) delivered an overpotential as low as 240 mV at 10 mA·cm^(−2) in 1 M KOH, which outperformed the bulk (380 mV) as well as other CoSe_(2)-base OER catalysts reported before. Especially, a shorter Co-Co path was observed in CoSe_(2)(001) by X-ray absorption spectroscopy. Further density functional theory (DFT) studies revealed that the reversible compression on the shorter Co-Co path could regulate the electronic structure of active sites during the OER process, and thus the energy barrier of the rate-determining step was reduced by 0.15 eV. This work could inspire more insights on the modification of electronic structure for OER electrocatalysts.

Feedback control can make data structure layout randomization more cost-effective under zero-day attacks

In the wake of the research community gaining deep understanding about control-hijacking attacks,data-oriented attacks have emerged.Among data-oriented attacks,data structure manipulation attack(DSMA)is a major category.Pioneering research was conducted and shows that DSMA is able to circumvent the most effective defenses against control-hijacking attacks-DEP,ASLR and CFI.Up to this day,only two defense techniques have demonstrated their effectiveness:Data Flow Integrity(DFI)and Data Structure Layout Randomization(DSLR).However,DFI has high performance overhead,and dynamic DSLR has two main limitations.L-1:Randomizing a large set of data structures will significantly affect the performance.L-2:To be practical,only a fixed sub-set of data structures are randomized.In the case that the data structures targeted by an attack are not covered,dynamic DSLR is essentially noneffective.To address these two limitations,we propose a novel technique,feedback-control-based adaptive DSLR and build a system named SALADSPlus.SALADSPlus seeks to optimize the trade-off between security and cost through feedback control.Using a novel feedback-control-based adaptive algorithm extended from the Upper Confidence Bound(UCB)algorithm,the defender(controller)uses the feedbacks(cost-effectiveness)from previous randomization cycles to adaptively choose the set of data structures to randomize(the next action).Different from dynamic DSLR,the set of randomized data structures are adaptively changed based on the feedbacks.To obtain the feedbacks,SALADSPlus inserts canary in each data structure at the time of compilation.We have implemented SALADSPlus based on gcc-4.5.0.Experimental results show that the runtime overheads are 1.8%,3.7%,and 5.3% when the randomization cycles are selected as 10s,5s,and 1s respectively.