Revealing the Pressure-Induced Softening/Weakening Mechanism in Representative Covalent Materials

Diamond, cubic boron nitride(c-BN), silicon(Si), and germanium(Ge), as examples of typical strong covalent materials, have been extensively investigated in recent decades, owing to their fundamental importance in material science and industry. However, an in-depth analysis of the character of these materials' mechanical behaviors under harsh service environments, such as high pressure, has yet to be conducted. Based on several mechanical criteria, the effect of pressure on the mechanical properties of these materials is comprehensively investigated.It is demonstrated that, with respect to their intrinsic brittleness/ductile nature, all these materials exhibit ubiquitous pressure-enhanced ductility. By analyzing the strength variation under uniform deformation, together with the corresponding electronic structures, we reveal for the first time that the pressure-induced mechanical softening/weakening exhibits distinct characteristics between diamond and c-BN, owing to the differences in their abnormal charge-depletion evolution under applied strain, whereas a monotonous weakening phenomenon is observed in Si and Ge. Further investigation into dislocation-mediated plastic resistance indicates that the pressure-induced shuffle-set plane softening in diamond(c-BN), and weakening in Si(Ge), can be attributed to the reduction of antibonding states below the Fermi level, and an enhanced metallization, corresponding to the weakening of the bonds around the slipped plane with increasing pressure, respectively. These findings not only reveal the physical mechanism of pressure-induced softening/weakening in covalent materials, but also highlights the necessity of exploring strain-tunable electronic structures to emphasize the mechanical response in such covalent materials.

Temperature-dependent hardness of zinc-blende structured covalent materials

Understanding the temperature-dependent hardness of covalent materials is of fundamental scientific interest and crucial technical importance.Here we propose a temperature-dependent hardness formula for zinc-blende structured covalent materials based on the dislocation theory.Our results indicate that at low temperatures,the Vickers hardness is primarily modulated by Poisson’s ratio and the shear modulus,with the latter playing a dominant role.With an increase in temperature,the governing mechanism for the plastic deformation switches from shuffle-set dislocation control to glide-set dislocation control,and the hardness decreases precipitously at elevated temperatures.Moreover,the intrinsic parameter a3G is revealed for zinc-blende structured covalent materials,which represents the resistance of a material to softening at high temperatures.This temperaturedependent hardness model agrees remarkably well with the experimental data of zinc-blende structured covalent materials.This work not only sheds light on the physical origin of hardness,but also provides a practical guide for the design of superhard materials.

Is hardness constant in covalent materials?

It has long been commonly believed that the hardness of covalent materials is related only to chemical bonds,leading to a constant covalent material hardness.Here,we systematically investigated the hard-ness of Cubic-diamond(3C-diamond)and Hexagonal-diamond(2H-diamond)structures using the ordered structure of functional units(OSFU)strategy.We found that although chemical bonds are the decisive factor in determining the hardness of covalent materials,the effects of crystal lattice,dislocation density,and grain size and orientation are also very important.These are all internal factors that determine the hardness of a material.In addition,external factors such as temperature and strain rate can also influ-ence the hardness of a material to some extent by affecting the critical resolved shear stresses(CRSSs)of dislocation motion.In this work,we argue that the hardness of covalent materials is determined by a combination of internal and external factors,where internal factors such as the chemical bonds,crystal lattice,defects,and grains intrinsically determine the hardness of a material;likewise,external factors such as temperature and strain extrinsically affect the hardness of a material.Therefore,the hardness of covalent materials is not constant.

Synthesis of covalent organic framework materials and their application in the field of sensing

Covalent organic frameworks(COFs)are an emerging type of porous crystalline polymers formed by combining strong covalent bonds with organic building blocks.Due to their large surface area,high intrinsic pore space,good crystallization properties,high stability,and designability of the resultant units,COFs are widely studied and used in the fields of gas adsorption,drug transport,energy storage,photoelectric catalysis,electrochemistry,and sensors.In recent years,the rapid development of the Internet of Things and people’s yearning for a better life have put forward higher and more requirements for sensors,which are the core components of the Internet of Things.Therefore,this paper reviews the recent progress of COFs in synthesis methods and sensing applications,especially in the last five years.This paper first introduces structure,properties,and synthesis methods of COFs and discusses advantages and disadvantages of different synthesis methods.Then,the research progress of COFs in different sensing fields,such as metal ion sensors,gas sensors,biomedical sensors,humidity sensors,and pH sensors,is introduced systematically.Conclusions and prospects are also presented in order to provide a reference for researchers concerned with COFs and sensors.

Applications of covalent organic frameworks(COFs)：From gas storage and separation to drug delivery

Covalent organic frameworks（COFs） are an emerging class of porous covalent organic structures whose backbones were composed of light elements（B,C,N,O,Si） and linked by robust covalent bonds to endow such material with desirable properties,i.e.,inherent porosity,well-defined pore aperture,ordered channel structure,large surface area,high stability,and multi-dimension.As expected,the abovementioned properties of COFs broaden the applications of this class of materials in various fields such as gas storage and separation,catalysis,optoelectronics,sensing,small molecules adsorption,and drug delivery.In this review,we outlined the synthesis of COFs and highlighted their applications ranging from the initial gas storage and separation to drug delivery.

Assemblies of covalent organic framework microcrystals:multiple-dimensional manipulation for enhanced applications

Covalent organic frameworks (COFs) are well known as the next generation of shape-persistent zeolite analogues, which have brought new impetus to the development of porous organic materials as well as two-dimensional polymers. Since the advent of COFs in 2005, many striking findings have definitely proven their great potentials expanding applications across energy,environment and healthcare fields. With thorough exploration over a decade, research interest has been drawn on the scientific challenges on chemistry, while making full play of COF values has remained far from satisfactory yet. Thus opening an avenue to modulating COF assemblies on the multi-scale is no longer just an option, but a necessity for matching the application requirements with enhanced performances. In this mini-review, we summarize the recent progress on design of nanoscale COFs with varying forms. Detailed description is concentrated on the synthetic strategies of COF assemblies such as spheres, fibers,tubes, coatings and films, thereby shedding light on the flexible manipulation over dimensions, compositions and morphologies.Meanwhile, the advanced applications of nanoscale COFs have been discussed here with comparison of their bulky counterparts.

Dynamic covalent gels assembled from small molecules：from discrete gelators to dynamic covalent polymers

Dynamic covalent chemistry has emerged recently to be a powerful tool to construct functional materials.This article reviews the progress in the research and development of dynamic covalent chemistry in gels assembled from small molecules.First dynamic covalent reactions used in gels are reviewed to understand the dynamic covalent bonding.Afterwards the catalogues of dynamic covalent gels are reviewed according to the nature of gelators and the interactions between gelators.Dynamic covalent bonding can be involved to form low molecular weight gelators.Low molecular weight molecules with multiple functional groups react to form dynamic covalent cross-linked polymers and act as gelators.Two catalogues of gels show different properties arising from their different structures.This review aims to illustrate the structure-property relationships of these dynamic covalent gels.