Shulou(Shulou.com)11/24 Report--

The original title: "three-body animation unexpectedly with the" zither project "beginning, is there really such a strong nanowires? "

Smell the whistle of the water from a distance

Then a huge ship broke through the waves.

Its height is several centimeters.

Suddenly there was a taut sound of strings.

Then the mast collapsed and the hull shattered.

Another steel column broke and boomed loudly.

When it is

The crew shouted and cried

The crowing of birds and the impact of steel

The sound of the surging river

At the same time, it shocked the world!

Fig. 1 screenshot of the animation of "three-body" this is the unforgettable picture of the "zither project" in "three-body", which is lucky to be the beginning of the animation. This frightening song is played by a special nanowire developed by the character Wang Miao. Seeing this, the editor can't help thinking: what is nanofilament? What determines the uniqueness of nanowires? Can nanowires really achieve "indiscriminate cutting"?

Part 1: nanowires = very fine filaments? In recent years, the word "nanometer" is not only a regular in scientific research reports, but also a "high-tech pronoun" that is gradually familiar to everyone. Perhaps in the eyes of many people in unrelated fields, nano is simply equated with "very small". For example, nanoparticles are "very small particles", nanotubes are "very thin tubes", and nanopores are "very small pores". Yes, this understanding gives the most intuitive appearance, but ignores the essence that really interests scientists-the nano-effect.

Fig. 2 diagrams and micrographs of certain gold nanoparticles however, before we elaborate on what the nano-size effect is, we must try to take you to the micro world to see how the world is different from the world we are familiar with. In general, an "atomic" or "electronic" understanding is enough to help us understand many phenomena in the material world. In this understanding, the matter we see macroscopically is made up of various nuclei and their extra-nuclear electrons at the micro level. What must be recognized at the same time is that there are interactions between nuclei, electrons, and between nuclei and electrons, which we often call "forces" (in which electromagnetic force plays a major role. Gravity is generally small and negligible).

Fig. 3 although the conceptual map of the micro-system is essentially electromagnetic force, people abstract some different situations and give them new labels one by one. (1) sometimes, some electrons are likely to stay in the area between the two nuclei, which seems to be shared by the two nuclei, which is called covalent bond; (2) sometimes, some electrons obviously tend to "wait" around one nucleus and stay away from the other, which is called ionic bond. (3) sometimes, some electrons travel almost freely around thousands of nuclei, as if the nuclei are "immersed" in a sea of electrons, which is called a metal bond.

Fig. 4 the formation of covalent bond, ionic bond and metal bond indicates that the above three bonds are collectively called "chemical bonds", and their strength is strong, in other words, it is not easy to break this state. When we summarize all the two atoms connected by a chemical bond (usually a "covalent bond" or "ionic bond") into a collective, all the atoms contained in the collective are collectively referred to as a "molecule". But sometimes, each atom in the material is connected to the surrounding atoms by a chemical bond, so that all the atoms form a "molecule", so it is called a "giant molecule". For crystals, those that are not "macromolecules" are generally called "molecular crystals" (such as ice and dry ice). According to the bonding types of macromolecules, they are divided into "atomic crystals" (such as diamond), "ion crystals" (such as NaCl crystals) and "metal crystals" (such as copper and silver).

Fig. 5 NaCl and diamond cell illustration | the picture is from [6] [7] so the chemical bond can naturally be approximately thought of as "intramolecular interaction" (except in the special case of intramolecular hydrogen bonding). If there is intramolecular interaction, of course there will be intermolecular interaction, including van der Waals force and intermolecular hydrogen bonding. Without going into the details, we can think of the intermolecular interaction as a significantly weaker force than a chemical bond. Therefore, the matter composed of molecules will move as a collective with molecules, and the molecules themselves are not easy to disintegrate.

Fig. 6 with such an image of hydrogen bonds and van der Waals forces between water molecules and carbon dioxide molecules in ice and dry ice, we can show how small it is to be on a nanometer scale. By definition, a dimension of 1-100nm is generally called nanoscale, while the distance between the two atoms connected by chemical bonds is generally 0.1-0.2nm. A molecule containing a dozen or dozens of atoms is about the size of 1nm or several nm. In this way, nanometer size can be understood as "the size of one to several molecules", or "the length of several to dozens of atoms side by side". In later parts, we will gradually realize how important such a seemingly "nonsense" description is!

Fig. 7 the atomic structure of a nanowire | the picture is derived from [8]. Therefore, we often refer to materials of nanometer size in three, two and one dimensions as nanospheres, nanowires / wires (solid), nanotubes (tubes) or nanoribbons (ribbons), and nanowires, collectively referred to as nanomaterials. In carbon materials, we often talk about C60, carbon nanotubes and graphene are examples of nanospheres, nanotubes and nanosheets. These three materials are also called zero-dimensional materials, one-dimensional materials and two-dimensional materials in turn.

Fig. 8 different dimensions of carbon materials: graphene (top left), diamond (top right), C60 (bottom left) and carbon nanotubes (bottom right) | Image from [9]

At this point, some people will ask: so, aren't nanowires very fine filaments? Yes, but not necessarily.

Part 2: when talking about the nano-size effect, we have to make an account: what percentage of atoms are on the surface of a bulk material? It is generally believed that the range of the outermost layer of the material a few nanometers (assuming 5nm) belongs to the surface. For a small copper sphere with a 1cm diameter, the proportion of surface atoms is about 0.0001%, but for a copper nanoparticles with a diameter of 100nm or even 10nm, the proportion is 27.1% and 100%, respectively.

Fig. 9 what does it mean by dangling bonds on the surface of nanoparticles? We just mentioned that chemical bonds are very strong, which means that once an atom becomes a chemical bond in all directions, the atom seems to be "tied" and cannot move at will. But the atoms on the surface are partially exposed, and the number of naturally formed chemical bonds is less than that of the interior, so they are relatively "free" and lively. This means that the properties of surface atoms are different from those of internal atoms. Therefore, when the proportion of surface atoms is different, they have completely different properties. Through the calculation, it is not difficult to find that in the dimension with nanometer scale, the surface proportion can no longer be ignored, and with the further decrease of the size, the proportion will increase significantly or even reach 100%!

Figure 10 shows the proportion of atoms on the surface. The picture comes from [10]. The reason why it is necessary to emphasize the number of molecules and atoms corresponding to nano-size is that the nature of nano-size effect is related to bonding. This microscopic action reflects the things between neighboring atoms or molecules, which are bound to become important in the countable size of molecules and atoms. Of course, the nano-effect is not just a surface effect. The small size effect will occur when the size is smaller than the optical wavelength, de Broglie wavelength and the coherent length of the superconducting state; the quantum size effect will be caused because the energy levels are no longer continuous but discrete; in addition, there will be nano-tunneling effect in electronic components.

For nanowires, in addition to the length of the dimension is macro, the other two dimensions are micro, which inevitably leads to its surface proportion can not be ignored. So what effect does this have on its mechanical strength? Besides the influence of surface, what other factors determine the particularity of nanowires?

Part 3: how is the wire broken? Before explaining the particularity of nanowires, we first introduce a little background knowledge about material mechanics from this side. Generally speaking, there may be several stages before a wire is broken: elastic deformation stage, plastic deformation stage, and final fracture stage. Its micro-mechanism is different.

When a line is pulled, its adjacent atoms parallel to the direction of the line are subjected to forces in the opposite direction, which try to separate the two atoms. However, as mentioned earlier, there are chemical bonds between adjacent atoms, which are essentially electromagnetic forces. When the atom leaves the equilibrium position, the electromagnetic force will significantly resist the external pull, as if there is a spring between the two atoms. If the pulling force is removed at this time, the atom will return to its original equilibrium position under the action of the electromagnetic force, which macroscopically shows that the line has returned to its original length. Therefore, such recoverable deformation is called elastic deformation.

Fig. 11 unlike electromagnetic force and tension confrontation, plastic deformation is irrecoverable because its microscopic mechanism is no longer the stretching of simple chemical bonds. It involves a change in the relative position of the atom. Plastic deformation mainly includes slip and twins. Only relatively simple slippage is introduced here. As the name implies, some of the atoms of the crystal slide along a face as a whole. This phenomenon generally occurs only in metals, because metal bonds are nuclei "immersed" in the ocean of electrons, so when the nucleus as a whole slips away from its original position, the dispersed electrons can still provide enough electromagnetic force, just like glue. This property of plastic deformation without failure is called ductility. Metals are therefore called ductile materials.

Fig. 12 slippage and twins | the picture is derived from [4]

Fig. 13 ductility of metal nails for covalent crystals such as diamond, if the atoms slide as a whole, all the bonds will break as soon as they leave the original position, and the material as a whole will break. This kind of material which can hardly produce plastic deformation is called brittle material.

Fig. 14 the moment the diamond is crushed, of course, there is another material that can undergo a large degree of elastic deformation, such as rubber. They usually have long-chain molecules curled up inside. When stretched, the intermolecular interaction in the process of unfolding and straightening of the molecular segment will be used as a counterweight to the external tension. Although this kind of material has no obvious plastic deformation, the behavior of elastic deformation is very prominent, which is generally called elastic material.

Fig. 15 Elastic deformation of a rubber band in the process of drawing a slingshot for an ordinary (non-nanoscale) thread with little elasticity and malleability, if we want to break it, we have to directly destroy all the chemical bonds on the cross section. It looks very difficult. But in the actual measurement, it is found that the silk thread is hundreds of times more fragile than the theoretical prediction. The root cause is that in the macro-level silk thread, there are almost certainly defects, such as small cracks. Once such a cracked line is pulled, the pressure on the cross section will no longer be borne by all atoms, but will be largely concentrated at the edge of the crack.

Fig. 16 the stress concentration caused by defects indicates that the nanowires are basically defect-free, so their strength may be close to the predicted value of the theory, which is an important mechanical property of nanowires. In addition, it is mentioned earlier that the nanowires have a high proportion of surface atoms. The distance and action between the surface atoms are different from the interior, and in some cases the strength of the surface layer is higher than that of the internal atoms. This provides a new dimension for the improvement of the overall mechanical properties of nanowires. Of course, the real situation is more complicated because the surface of the nanowires may also adsorb other molecules, such as water molecules. These molecules may also affect the strength of the surface layer.

Part 4: can "Flying Blade" be realized? Admittedly, the "flying blade" described in the "three-body" cannot be realized in reality, so the answer to this question is still unknown. But this does not mean that we can not give some reasonable perspective to enhance our understanding of the implementation of similar technologies.

The screenshot of the "my three-body" video in figure 17 is first of all about the dimension of measuring the mechanical properties of materials. In the previous section, we only revolve around the dimension stretched along the silk thread. However, looking back on the scene in the animation, we find that the nanowires undergo a more complex test. First of all, the ship collided from the side of the nanowires, and the width of the ship is less than the length of the line. This means that the middle part of the thread is mainly subjected to a force perpendicular to the line, that is, bending.

Fig. 18 the bending force of nanowires in fact, in addition to tension and bending, the material may also be subjected to compression, shear, torsion and so on.

Fig. 19 indication of compression, cutting and torsion | the picture is derived from [4]. In addition, the silk thread is not subjected to a single action, but to the force acting on different parts many times. So the nanowires will be tensioned, relaxed, retensioned and relaxed alternately. This reflects the anti-fatigue properties of nanowires. In general, the performance of the material will decline after many times of stress, that is, fatigue occurs.

Fig. 20 fatigue fracture of metal after repeated bending as a supplement, the metal material of the hull has high hardness after all, and even if the wire can cut through the metal, the metal will wear on its surface. This wear and tear is likely to lead to fatal defects. Cutting such a large ship requires nanowires to withstand wear for a long time and still have good strength, which is a very stringent requirement.

Figure 21 Wear indication | the picture is from [11] another challenge actually lies in length, which is the level of material preparation technology. As mentioned earlier, the reason why the strength of the macro-size material is much lower than the theoretical value is due to defects, and it is basically impossible to ensure that there are no defects on the macro-scale. For nanowires, although it is nano-scale in cross section, it is also a challenge whether it can be achieved without any defects in 150m length.

Fig. 21 the bullet pierced the object as to whether the nanowires would cut through the material and heal again. The editor thinks that there is no need to worry. Because when the thread is cut, it is like a bullet piercing an object, accompanied by the release of energy. It includes the collision caused by kinetic energy and the heat caused by friction. The release of these energy will cause irreversible damage to the cross section. Especially the overall amputation at the macro level, it is basically impossible to heal. For example, although the size of radiation is very small but contains high energy, it can still destroy structures such as macromolecules such as DNA.

Fig. 22 screenshot of the tension moment of "three-body" animation "flying blade" in addition, the description in the book is that the nanofilament is "1/10 of the hair" thickness, which corresponds to the order of several microns, which is likely to be formed by bundles of multiple nanoscale threads. Its diameter is several orders of magnitude larger than the distance between atoms. Such a "blade" is actually a "blunt knife", it is difficult to leave a neat incision and then allow the structure on both sides to heal.

As for whether the technology of "flying blade" can be realized? Just let time give us the answer.

Reference:

[1] Wang S, Shan Z, Huang H. The Mechanical Properties of Nanowires [J]. Current Sustainable/Renewable Energy Reports, 2017 (4): 4

Guan Zhenduo, Zhang Zhongtai, Jiao Jinsheng. Physical properties of inorganic materials [M]. Tsinghua University Press, 2011.

[3] the course courseware of Mechanical Properties of Materials. Liu Junqing

[4] the courseware of material Physics. Wang Danhong

[5] Why is ice lighter than water?

[6] Diamond-Wikipedia, free encyclopedia

[7] the courseware of structural Chemistry. Sun Hongwei

[8] Fang F, Zhang N, Guo D, et al. Towards atomic and close-to-atomic scale manufacturing [J]. International Journal of Extreme Manufacturing, 2019, 1 (1): 012001.

[9] RISE Electron Mirror Raman Integration system: carbon Materials and two-dimensional material Analysis Solutions

[10] what are the basic effects of nanomaterials? -zhihu.com

[11] detailed explanation of wear properties of materials-zhihu.com

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Yun Kai Ye Luo

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