Serial Cracking in 2D Van der Waals Layered Electrodes Mediated by Electrochemical Reaction and Mechanical Deformation
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In battery cycling, mechanical effects introduced by electrochemical reactions are commonly observed. In return, the mechanical deformations also have a large impact on the electrochemical process. However, such a coupling effect of electrochemical reaction and mechanical deformation has a complicated interplay on the atomic scale and an explicit elucidation is still challenging. Herein, we used in situ transmission electron microscopy to directly visualize the coupling process during the lithiation of two-dimension Van der Waals MoS 2 layered electrodes. A self-sustained cracking mechanism was identified; the first crack was created by the accumulation of the linear defects originated from the strain in lithiation. The formed defects including dislocations and antiphase boundaries, in turn accelerated the Li-ion diffusion, promoting the electrochemical reaction and cooperatively gave rise to the formation of a second and following cracks that resembled the “avalanche effect”. Meanwhile, it is observed that a threshold crystal size exists, under which the lithiation stress is not sufficient to initiate the first crack, and thus the serial cracking process could be avoided. The present work provides an atomistic insight into a cooperation from the mechanical and electrochemical effects toward the formation of the arrayed cracks. It also sheds light on the enhancement of mechanical properties of layered electrode materials for rechargeable batteries.The accurate description of van der Waals forces within density functional theory is currently one of the most active areas of research in computational physics and chemistry. Here we report results on the structural and energetic properties of graphite and hexagonal boron nitride, two layered materials where interlayer binding is dominated by van der Waals forces. Results from several density functionals are reported, including the optimized Becke88 van der Waals (optB88-vdW) and the optimized PBE van der Waals (optPBE-vdW) (Klimeš et al 2010 J. Phys.: Condens. Matter 22 022201) functionals. Where comparison to experiment and higher-level theory is possible, the results obtained from the two new van der Waals density functionals are in good agreement. An analysis of the physical nature of the interlayer binding in both graphite and hexagonal boron nitride is also reported.
Hexagonal boron nitride
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Electrostatics
DLVO theory
Hamaker constant
Particle (ecology)
Force Field
Electrostatic force microscope
Surface force
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Van der Waals interactions, primarily attractive van der Waals interactions, have been studied over one and half centuries. However, repulsive van der Waals interactions are less widely studied than attractive van der Waals interactions. In this article, we focus on repulsive van der Waals interactions. Van der Waals interactions are dipole–dipole interactions. In this article, we study the van der Waals interactions between multiple dipoles. Specifically, we focus on two-dimensional six-body van der Waals interactions. This study has many potential applications. For example, the result may be applied to physics, chemistry, chemical engineering, and other fields of sciences and engineering, such as breaking molecules.
Hamaker constant
DLVO theory
Non-covalent interactions
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Electrostatics
DLVO theory
Particle (ecology)
Hamaker constant
Surface force
Electrostatic force microscope
Electrostatic interaction
Force Field
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A mathematical model for analyzing the van der Waals interaction between the internal aqueous droplets (W1) and the external aqueous phase (W2) of double emulsions has been established. The effects of Hamaker constants of the materials forming the system, especially those of the two different adsorbed surfactant layers with uniform density (A1 and A2), on the van der Waals interaction were investigated. The overall van der Waals interaction across the oil film is a combined result of four individual parts, that is, W1−W2, A1−A2, W1−A1, and A2−W2 van der Waals interaction, and it may be either attractive or repulsive depending on many factors. It was found that the overall van der Waals interaction is dominated by the W1−W2 interaction at large separation distances between the W1/O and O/W2 interfaces, while it is mostly determined by the A1−A2 interaction when the two interfaces are extremely close. Specifically, in the cases when the value of the Hamaker constant of the oil phase is intermediate between those of W1 and W2 and there is a thick oil film separating the two interfaces, a weak repulsive overall van der Waals interaction will prevail. If the Hamaker constant of the oil phase is intermediate between those of A1 and A2 and the two interfaces are very close, the overall van der Waals interaction will be dominated by the strong repulsive A1−A2 interaction. The repulsive van der Waals interaction at such cases helps stabilize the double emulsions.
Hamaker constant
DLVO theory
Aqueous two-phase system
Non-covalent interactions
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Hamaker constant
London dispersion force
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The van der Waals volume is a widely used descriptor in modeling physicochemical properties. However, the calculation of the van der Waals volume (V(vdW)) is rather time-consuming, from Bondi group contributions, for a large data set. A new method for calculating van der Waals volume has been developed, based on Bondi radii. The method, termed Atomic and Bond Contributions of van der Waals volume (VABC), is very simple and fast. The only information needed for calculating VABC is atomic contributions and the number of atoms, bonds, and rings. Then, the van der Waals volume (A(3)/molecule) can be calculated from the following formula: V(vdW) = summation operator all atom contributions - 5.92N(B) - 14.7R(A) - 3.8R(NR) (N(B) is the number of bonds, R(A) is the number of aromatic rings, and R(NA) is the number of nonaromatic rings). The number of bonds present (N(B)) can be simply calculated by N(B) = N - 1 + R(A) + R(NA) (where N is the total number of atoms). A simple Excel spread sheet has been made to calculate van der Waals volumes for a wide range of 677 organic compounds, including 237 drug compounds. The results show that the van der Waals volumes calculated from VABC are equivalent to the computer-calculated van der Waals volumes for organic compounds.
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Van der Waals interactions are always present in handling micro objects and will influence the whole manipulation process. This paper investigates the effects of van der Waals forces on the design and planning of micromanipulation. Origins of van der Waals interactions are shown first. Van der Waals forces between micro objects of several typical configurations in micromanipulation process are characterized. The related aspects of van der Waals forces are discussed based on the theoretical analysis. Methods of control adhesion induced by van der Waals forces in micromanipulation processes are presented.
Hamaker constant
DLVO theory
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Shrinkage
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It has been observed that the van der Waals interaction can cause an extruded core of a multiwalled carbon nanotube (MWNT) to retract into the outer shells. In a previous report [Q. Zheng and Q. Jiang, Phys. Rev. Lett. $88,$ 045503 (2002)], the authors pointed out that the restoring force resulted from the excess van der Waals interaction energy due to the core extrusion would drive the core to oscillate with respect to its fully retracted position because of the small intershell sliding resistance force and they predicted that the oscillation frequency could be in the gigahertz range. The present article gives detailed theoretical calculations of the excess van der Waals energy due to the extrusion and the corresponding restoring force. The authors have further derived an explicit expression of the oscillation frequency in terms of the physical and geometrical parameters of the MWNT, with the interatomic locking effect taken into account, and they have shown that the oscillation frequency can be significantly higher than one gigahertz.
Oscillation (cell signaling)
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