Covalent Crystals Research Articles

Investigation on the machining mechanism of silicon modified by Au ion irradiation in elliptical vibration diamond cutting

Single-crystal silicon (Si) has important applications in semiconductor, infrared optics, and photovoltaic industries. However, Si is difficult to be machined precisely due to its hard and brittle characteristics. Ion irradiation is proposed as an advanced technology to reduce the hardness and brittleness for a covalent crystal, which is beneficial for the ductile machining process. In the present research, the simulation and experimental investigation of Si with Au ion irradiation were carried out firstly. As following, the grooving experiment is carried out by elliptical vibration cutting (EVC). the machining performance is compared with ordinary cutting (OC) and the material removal mechanism is elaborated. The critical depth of cut for brittle-to-ductile transition is nearly 7 times higher by EVC compared to OC. Initial verification of material modification was conducted by Raman spectra and cutting microgrooves. Finally, the ion irradiation damage mechanism and amorphous Si (a-Si)/crystalline Si (c-Si) machining deformation mechanism were analyzed in detail by transmission electron microscopy. The irradiated sample contains an amorphous layer of 1050 nm, a transition layer containing dislocations and nanocrystals, and a fully crystalline layer. During machining a-Si/c-Si interface, the machining defects in the amorphous layer will first absorb the energy and ensure that the crystalline layer does not produce subsurface damage. In summary, ion-irradiated Si can be achieved in the amorphous region at any depth position and the substrate ductile machining without subsurface damage generation by EVC.

Closed-Loop Recyclable and Totally Renewable Liquid Crystal Networks with Room-Temperature Programmability and Reconfigurable Functionalities.

Dynamic covalent liquid crystal networks (DCv-LCNs) with straightforward (re)programmability, reprocessability, and recyclability facilitates the manufacture of sophisticated LCN actuators and intelligent robots. However, the DCv-LCNs are still limited to heat-assisted programming and polymer-to-polymer reprocessing/recycling, which inevitably lead to deterioration of the LCN structures and the actuation performances after repeated programming/processing treatments, owing to the thermal degradation of the polymer network and/or external agent interference. Here, a totally renewable azobenzene-based DCv-LCN with room-temperature programmability and polymer-to-monomers chemical recyclability is reported, which was synthesized by crosslinking the azobenzene-containing dibenzaldehyde monomer and the triamine monomer via the dynamic and dissociable imine bonds. Thanks to the water-activated dynamics of the imine bonds, the resultant DCv-LCN can be simply programmed, upon water-soaking at room temperature, to yield a UV/Vis light-driven actuator. Importantly, the reported DCv-LCN undergoes depolymerization in an acid-solvent medium at room temperature because of the acid-catalyzed hydrolysis of the imine bonds, giving rise to easy separation and recovery of both monomers in high purity, even with tolerance to additives. The recovered pure monomers can be used to regenerate totally new DCv-LCNs and actuators, and their functionalities can be reconfigured by removing old and introducing new additives, by implementing the closed-loop polymer-monomers-polymer recycling.

Closed‐Loop Recyclable and Totally Renewable Liquid Crystal Networks with Room‐Temperature Programmability and Reconfigurable Functionalities

AbstractDynamic covalent liquid crystal networks (DCv‐LCNs) with straightforward (re)programmability, reprocessability, and recyclability facilitates the manufacture of sophisticated LCN actuators and intelligent robots. However, the DCv‐LCNs are still limited to heat‐assisted programming and polymer‐to‐polymer reprocessing/recycling, which inevitably lead to deterioration of the LCN structures and the actuation performances after repeated programming/processing treatments, owing to the thermal degradation of the polymer network and/or external agent interference. Here, a totally renewable azobenzene‐based DCv‐LCN with room‐temperature programmability and polymer‐to‐monomers chemical recyclability is reported, which was synthesized by crosslinking the azobenzene‐containing dibenzaldehyde monomer and the triamine monomer via the dynamic and dissociable imine bonds. Thanks to the water‐activated dynamics of the imine bonds, the resultant DCv‐LCN can be simply programmed, upon water‐soaking at room temperature, to yield a UV/Vis light‐driven actuator. Importantly, the reported DCv‐LCN undergoes depolymerization in an acid‐solvent medium at room temperature because of the acid‐catalyzed hydrolysis of the imine bonds, giving rise to easy separation and recovery of both monomers in high purity, even with tolerance to additives. The recovered pure monomers can be used to regenerate totally new DCv‐LCNs and actuators, and their functionalities can be reconfigured by removing old and introducing new additives, by implementing the closed‐loop polymer‐monomers‐polymer recycling.

Spheroid models to elaborate the broken symmetry and equivalent volume of molecules in crystalline phase.

Dense packing of particles has provided powerful models to elaborate the important structural features of matter in various systems such as liquid, glassy, and crystalline phases. The simplest sphere packing models can represent and capture salient properties of the building blocks for covalent, metallic, and ionic crystals; it, however, becomes insufficient to reflect the broken symmetry of the commonly anisotropic molecules in molecular crystals. Here, we develop spheroid models with a minimal degree of anisotropy, which serve as a simple geometrical representation for a rich spectrum of molecules-including both isotropic and anisotropic, convex and concave ones-in crystalline phases. Our models are determined via an inverse packing approach: Given a molecular crystal, an optimal spheroid model is constructed using a contact diagram, which depicts the packing relationship between neighboring molecules within the crystal. The spheroid models are capable of accurately capturing the broken symmetry and characterizing the equivalent volume of molecules in the crystalline phases. Moreover, our model retrieves such molecular information from low-quality x-ray diffraction data with poorly resolved structures, and by using soft spheroids, it can also describe the packing behavior in cocrystals.

Causal emergent principles and relations for mechanical properties of covalent and ionic crystals

A knowledge and data-synergized intelligent computation architecture for materials was proposed within the data science paradigm. As a vital operation, two digital ensemble descriptors implying chemical composition and structural trend for crystals were created using the features contained in the Periodic Table of elements without a priori assumption, which affords causal emergence and regulation principles for the mechanical response of covalent and ionic solids. In addition to a linear correlation among structural state/mechanical response parameters, causal analytic relations in an exponential form between structural and thermodynamic state/mechanical response parameters and a digital ensemble descriptor were unveiled through least squares regression, in which the coefficients are classified in accordance with symmetry principles on the atom and lattice. Thereafter, the underlying physicochemical mechanisms of chemical pressure and chemical bonding are found responsible for the mechanical responses of bulk modulus and hardness of solids. At last, a physical prediction model was established for crystalline solids and demonstrated the feasibility of the predictive design of novel superhard materials. It is believed that by constructing suitable digital ensemble descriptors, this intelligent computation architecture and consequent physical prediction models on the basis of causal analytic relations are able to generalize by depicting crystalline solids with covalent and ionic bonds in other crystallographic structures.

Mechanical quenching phenomenon in diamond

The structure of dislocation cores, the fundamental knowledge on crystal plasticity, remains largely unexplored in covalent crystals. Here, we conducted atomically resolved characterizations of dislocation core structures in a plastically deformed diamond anvil cell tip that was unloaded from an exceptionally high pressure of 360 GPa. Our observations unveiled a series of nonequilibrium dislocation cores that deviate from the commonly accepted "five-seven-membered ring" dislocation core model found in FCC-structured covalent crystals. The nonequilibrium dislocation cores were generated through a process known as "mechanical quenching," analogous to the quenching process where a high-energy state is rapidly frozen. The density functional theory-based molecular dynamic simulations reveal that the phenomenon of mechanical quenching in diamond arises from the challenging relaxation of the nonequilibrium configuration, necessitating a large critical strain of 25% that is difficult to maintain. Further electronic-scale analysis suggested that such large critical strain is spent on the excitation of valance electrons for bond breaking and rebonding during relaxation. These findings establish a foundation for the plasticity theory of covalent materials and provide insights into the design of electrical and luminescent properties in diamond, which are intimately linked to the dislocation core structure.

Structure formation of ultradispersed diamond by detonation synthesis

In the present work, we report on structure formation of ultradispersed diamond through detonation synthesis. In particular, based on the present data, we can state that ultradispersed diamond of detonation (UDD) synthesis is characterized by a rail substructure formation, which is typical for diamonds of dynamic and static synthesis formed from graphite by the martensitic mechanism. The formation of UDD is characterized by a two-stage process: the first stage is the formation of graphite from detonation products, and the second stage is the transformation of graphite into diamond by the martensitic mechanism. A microlamellar substructure in the rails of diamonds formed from the original graphite is due to the presence of basic packing defects that is characteristic for covalent crystals with a wurtzite lattice deformed at high pressures and temperatures typical for lonsdaleite. The presence of lonsdaleite in UDD also indicates its formation in the process of synthesis of diamond from graphite through the martensitic transformation.

Visualizing dynamic competitive adsorption processes between iodine and methyl iodide within single covalent organic framework crystals

Covalent organic frameworks (COFs) are important porous adsorbents for volatile iodine (I2) and methyl iodide (CH3I). In situ monitoring of the dynamic adsorption process of single COF crystals toward I2 and CH3I is a critical and fundamental issue for understanding the reaction mechanism and improving the sorption performance. Here, we report operando real-time dark-field optical microscopy (DFM) imaging of visually studying the dynamic adsorption behavior of LZU-111 (LZU=Lanzhou University) COFs in the I2/CH3I binary gaseous mixture at the single-crystal level. Time-lapse imaging shows that the uptake of CH3I and I2 results in the R intensity increase and B intensity decrease of the DFM images. Employing the R and B intensities as two indicators, we find an unusual blinking of R/B intensities from single LZU-111 crystals, which is attributed to the intermittent sorption-desorption processes of CH3I and I2 within the LZU-111 framework. The visualization of the dynamic reaction process provides clear evidence that the competitive adsorption between CH3I and I2 goes through a multi-time and oscillatory reaction pathway instead of a successive procedure. Combined with theoretical calculations, the difference in the migration capability and initial pressure is identified for initiating the intermittent blinking events.

Electronegativity Force Field for Prediction of Elastic Moduli.

Macroscopic elastic moduli (i.e., bulk modulus and shear modulus) of covalent crystals are mainly determined by microscopic structures and stiffnesses. Herein, the microscopic bond and angle force constants of covalent crystals were parameterized from their atomic electronegativities, which is named the electronegativity force field (EFF). Based on this force field, the elastic moduli of covalent crystals can be directly obtained by molecular mechanics calculations. The calculated moduli for various covalent crystals are generally consistent with first-principles calculations, while the computational cost is reduced by several orders of magnitude, indicating the accuracy and efficiency of the EFF. Finally, we found 25 ultrahigh-modulus crystals with a bulk modulus greater than 350 GPa, which demonstrates that this force field can be used for screening of ultrahigh-modulus materials from numerous crystal candidates.

Dislocation-mediated brittle-ductile transition of diamond under high pressure

Recent confirmation of room temperature dislocation plasticity in single-crystal diamond and the activation of {001}<110> slip system in experiments has opened up a new era to understand the deformation mechanism of brittle covalent crystals. However, few theoretical research and simulation has been conducted on the dislocation gliding in {001}<110> slip system. In this work, a new potential function is derived from the original Tersoff potential to the Tersoff High Pressure potential with a novel bond order term with the nearest neighbors. This model has been validated with series of molecular dynamic simulations of both {001}<110> and {111}<110> dislocation slip systems in diamond, successfully describing the dislocation mediated brittle-ductile transition under high hydrostatic pressure. The gliding mechanism of dislocation on {001} plane presents a jumping character with a large burgers vector under high hydrostatic pressure. Whereas, the diamond exhibits a zipper-like Griffith cleavage upon shearing under zero hydrostatic pressure. Similar deformation processes are also found in polycrystalline diamond. The current model is expected to prove useful in related simulations focusing on the strengthening mechanism in diamond and related research.

Artificial neural network potentials for mechanics and fracture dynamics of two-dimensional crystals ** ** Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, world-wide license

Understanding the mechanics and failure of materials at the nanoscale is critical for their engineering and applications. The accurate atomistic modeling of brittle failure with crack propagation in covalent crystals requires a quantum mechanics-based description of individual bond-breaking events. Artificial neural network potentials (NNPs) have emerged to overcome the traditional, physics-based modeling tradeoff between accuracy and accessible time and length scales. Previous studies have shown successful applications of NNPs for describing the structure and dynamics of molecular systems and amorphous or liquid phases of materials. However, their application to deformation and failure processes in materials is still uncommon. In this study, we discuss the apparent limitations of NNPs for the description of deformation and fracture under loadings and propose a way to generate and select training data for their employment in simulations of deformation and fracture simulations of crystals. We applied the proposed approach to 2D crystalline graphene, utilizing the density-functional tight-binding method for more efficient and extensive data generation in place of density functional theory. Then, we explored how the data selection affects the accuracy of the developed artificial NNPs. It revealed that NNP’s reliability should not only be measured based on the total energy and atomic force comparisons for reference structures but also utilize comparisons for physical properties, e.g. stress–strain curves and geometric deformation. In sharp contrast to popular reactive bond order potentials, our optimized NNP predicts straight crack propagation in graphene along both armchair and zigzag (ZZ) lattice directions, as well as higher fracture toughness of ZZ edge direction. Our study provides significant insight into crack propagation mechanisms on atomic scales and highlights strategies for NNP developments of broader materials.

Hypercubane covalent crystals: Insight from density functional theory

A series of new covalent crystals inspired by the carbon tesseract or hypercubane has been introduced using the density functional theory (DFT) calculations. Three types of possible packing of hypercubane crystals were identified: simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc). Based on the structure of the materials, it follows that the hypercubane crystals cannot be attributed to superdense materials, since their density is inferior to that of the diamond. An analysis of the electronic characteristics showed that crystals with sc and bcc packing belong to the class of semiconductors, whose band gaps are 2.23 and 1.46 eV, respectively, while a crystal with fcc packing possesses a metallic nature. The calculated Raman and IR spectra for semiconductor hypercubane crystals made it possible to find characteristic peaks for their identification. Thus, the characteristic IR peaks for sc hypercubane turned out to be at frequencies of 1053 and 1205 cm–1, and for the bcc hypercubane, the characteristic peak turned out to be only one at the frequency of 851 cm–1. For the Raman spectrum, the characteristic frequency values were 961 and 1300 cm−1 for sc hypercubane crystal and 1051, 1142, and 1764 cm–1 for bcc covalent hypercubane. The calculated UV–visible spectra for both the sc and bcc crystals showed an absorption maximum of ∼100 nm, which corresponds to the far UV region.

Calculation of the energy of a two-dimensional hydrogen molecule

ABSTRACT The equilibrium distance and energies of singlet and triplet states of a two-dimensional (2D) hydrogen molecule are calculated. Variational calculations were carried out using a multiparameter system of Gaussian functions with correlation multipliers. The results are compared with the calculations performed by other authors. Currently, the most accurate values of the singlet and triplet states energy of the 2D-molecule in two-dimensional systems have been obtained: Es = –5.2843Eh , Et = –2.9277Eh ; the equilibrium state corresponds to the distance Rm = 0.36614ao , where Eh is the Hartree energy, and ao is the Bohr radius. In the mathematical sense, a 2D hydrogen molecule is a direct analog of a pair of shallow hydrogen-like paramagnetic centres in covalent two-dimensional crystals. The energies of the singlet and triplet states of 2DH2 are significantly lower than the corresponding values of the three-dimensional hydrogen molecule (3DH2), while the singlet–triplet splitting is much higher than that of 3DH2. In the equilibrium state, 2DH2 protons are much closer to each other, which leads to a significant increase in correlation effects associated with the direct dependence of the wave function on the interelectron distance compared to 3DH2.

Elastomers Grow into Actuators.

It is common knowledge that when an elastomer (rubber) is stretched, its length will bemaintained if its two ends are fixed. Here, it is serendipitously found that whenan elastomer isslowly elongated further to achieve buckling under such conditions, the final length is much longer than the pre-stretched length. This allows the designof untethered autonomous synthetic-material-based soft robots that do not need any other chemical or electrical energy sources or external stimuli after the pre-strain is installed. Once the growth starts, the elongation continues to proceed even when the applied force is removed. Moreover, the elastomer, after growing, eventually forms a robust soft actuator that can be reshaped for different purposes. Few synthetic materials can grow like this, so far. This investigation shows that the material has an uncommon liquid crystal phase. Contrary to normal liquid crystals, it becomes birefringent only at high temperatures. The formation and the reshaping of the resulting soft actuators relate to a usually unnoticed reversible reaction. The work is promising to promote further understanding of dynamic covalent chemistry and liquid crystal elastomers, as well as to foster new designs and high-impact applications of bioinspired sustainable soft actuators in areas other than soft robots.

A self-standing three-dimensional covalent organic framework film

Covalent crystals such as diamonds are a class of fascinating materials that are challenging to fabricate in the form of thin films. This is because spatial kinetic control of bond formation is required to create covalently bonded crystal films. Directional crystal growth is commonly achieved by chemical vapor deposition, an approach that is hampered by technical complexity and associated high cost. Here we report on a liquid-liquid interfacial approach based on physical-organic considerations to synthesize an ultrathin covalent crystal film. By distributing reactants into separate phases using hydrophobicity, the chemical reaction is confined to an interface that orients the crystal growth. A molecular-smooth interface combined with in-plane isotropic conditions enables the synthesis of films on a centimeter size scale with a uniform thickness of 13 nm. The film exhibits considerable mechanical robustness enabling a free-standing length of 37 µm, as well as a clearly anisotropic chemical structure and crystal lattice alignment.

A unified non-empirical strength model

Strength, as an important indicator of structural materials, has always been an important research topic in materials science. Theoretically, building a strength model is a rewarding method to understand the relationship between the mechanical properties and microstructure of materials. Although many strength models can reduplicate experimental values very well, they are empirical models, and their applicability is limited to materials for which empirical parameters have been obtained. Here, a non-empirical strength model is proposed based on the two-dimensional (2D) displacement potential of dislocation slipping, which can be applied to different chemically bonded crystals. Owing to the large electron localization function (ELF), covalent and ionic crystals have a high 2D displacement potential of dislocation slipping, and their dislocation slip mode prefers the kink-pair mode, further exhibiting a high critical resolved shear stress (CRSS). In contrast, metallic crystals with a small ELF have a low 2D displacement potential of dislocation slipping, and their dislocation slip mode is more inclined to the string mode, showing a low CRSS. This work provides new insights into dislocation-slipping configurations that will be useful for the development of new high-performance structural materials.

On the brittleness of elementary semiconductors

It is shown that the brittleness of a single-component covalent crystal (diamond, Si, Ge) is due to the &amp;quot;duplicity&amp;quot; of the paired potential of interatomic interaction for elastic (reversible) and for plastic (irreversible) deformation. This leads to the fact that the specific surface energy during plastic deformation of a covalent crystal is more than two times less than the specific surface energy during elastic deformation. Therefore, with a small deformation of a covalent crystal, it is energetically more advantageous to create a surface by irreversible breaking than by reversible elastic stretching. It is indicated that the brittle-ductile transition in a single-component covalent crystal is accompanied by metallization of covalent bonds on the surface. It is shown that the brittle-ductile transition temperature (TBDT) for single-component covalent crystals under static load has an upper limit: TBDT/Tm&amp;lt;0.45, where Tm --- is the melting temperature. Keywords: interatomic covalent bond, brittleness, ductility, elementary semiconductors, brittle-ductile transition.

О хрупкости элементарных полупроводников

It is shown that the brittleness of a single-component covalent crystal (diamond, Si, Ge) is due to the “duplicity” of the paired potential of interatomic interaction for elastic (reversible) and for plastic (irreversible) deformation. This leads to the fact that the specific surface energy during plastic deformation of a covalent crystal is more than two times less than the specific surface energy during elastic deformation. Therefore, with a small deformation of a covalent crystal, it is energetically more advantageous to create a surface by irreversible breaking than by reversible elastic stretching. It is indicated that the brittle-ductile transition in a single-component covalent crystal is accompanied by metallization of covalent bonds on the surface. It is shown that the brittle-ductile transition temperature (TBDT) for single-component covalent crystals under static load has an upper limit: TBDT/Tm &lt; 0.45, where Tm is the melting temperature.

Composite formation of covalent organic framework crystals and sugar alcohols for exploring a new class of heat-storage materials.

We impregnated the sugar alcohols (SAs) erythritol and mannitol (Man) in >0.1 mm single crystals of a covalent organic framework, COF-300, to propose new solid-state heat storing materials. The fusion-freezing cycles of the Man-COF composite occur in a narrow range of 130-155 °C without large or random supercooling, which has been a crucial problem of SAs, indicating the significance of this materials concept. The Man-COF composite displayed two endothermic and two exothermic features in the heating/cooling cycles. We measured the heat transmission properties in detail, based on which we discuss and suggest a postulate on the phase change behaviour of Man in the pores of the COF.

Study of the brittleness in covalent crystals

Based on the previously proposed model of paired covalent bond, the reasons for the brittleness for covalent crystals have been studied. It is shown that the brittleness of a covalent crystal is due to the “duplicity” of the paired interatomic interaction potential for elastic (reversible) and plastic (irreversible) deformation. This leads to the fact that the specific surface energy during plastic deformation of a covalent crystal is more than two times less than the specific surface energy during elastic deformation. Therefore, with a small deformation of a covalent crystal, it is energetically more advantageous to create a surface by breaking than by elastic stretching it. It is shown that the brittle-ductile transition temperature for single-component covalent crystals has an upper limit: TBDT/Tm < 0.45, where Tm is the melting temperature.