Recent graphene research news has continued. Scientists at the Randolph Institute for Theoretical Physics in Russia found that the Poisson's ratio of graphene can be controlled by changing the applied pull force. This discovery ended the controversy over the Poisson ratio of graphene. For example, Poisson is important because it is not just a performance index. Behind it, hiding many features will directly affect our definition of graphene, so this discovery is subversive.
As we all know, the miracle material graphene is a two-dimensional material composed of a single layer of carbon atoms. It is very controversial, because it shows in many ways a completely different behavior from traditional materials. One of them is its electrical properties and elasticity. The relationship between graphene and graphene has an extremely high charge mobility, but this value is not fixed and will be affected by the elasticity. The mobility value will change dramatically under different elastic forces. Physicists have been trying to find a full reflection. The cause of this anomalous behavior, they hope to find the physical characteristics that can explain this phenomenon and are generally applicable. Once solved, we will be able to use graphene more efficiently, and it is easier to create new materials needed. However, researchers No reasonable explanation has been found until recently.
For most materials, when they are stretched, they contract laterally, just like pulling a rubber band. However, about a hundred years ago, the German physicist Wald M. Voyt found that the pyrite crystals were pulling. However, the material that shows abnormal behavior when stretched is called auxetic material. In the late 1970s, the scientists first produced such materials. The secret of abnormal bulging material comes from that they do not In ordinary geometrical shapes, when the material is relaxed, their structural units fold into each other, but when subjected to tensile stress, the folded structure is pulled and expanded, and the size becomes larger instantaneously.
Fig. a shows the state of the normal material and the auxetic material when they are stretched. It can be seen that the folding unit of the auxetic material expands when stretched, and the lateral dimension increases; Fig. b shows the product folded according to the miura-ori rule when straightened Appearance of bulge. Image Source: Langdo Institute of Theoretical Physics
The bulging material has many unusual features that will help improve the existing technology and create new technologies. Traditional materials will expand when heated, which will produce a variety of mechanical stress and further disturb their original performance. However, auxetic material is just the opposite. When heated, they may shrink, so we can try to use auxetic materials and traditional materials to make a composite material with zero expansion ratio. In this way, with the increase of temperature, traditional materials The volume is enlarged, but the auxetic material can be well compensated to achieve the final volume stability.
We usually define the material's ability to contract or stretch laterally under tension as the Poisson's ratio. Poisson's ratio is generally positive for normal materials, but Poisson's ratio for auxetic materials is negative. Kachorovskii said: 'Scientists We have been interested in graphene Poisson's ratio for a long time and we generally consider it to be negative. However, some recent numerical calculations show that the Poisson ratio of graphene may be positive or negative. The results of various calculations are completely contradictory.
Poisson's ratio is difficult to measure, and it is more difficult for graphene. Because grown graphene is generally on the substrate, various substrates will prevent us from measuring the true Poisson's ratio of graphene. If we do not use a substrate, the monolithic graphene is so small that it is impossible to clamp it on a jig for controlled tensile testing. Doesn't Poisson's ratio need to be measured? No, research on carbon material technology There is a need for this and engineers, who need to know exactly whether graphene is bulging.
So the scientists at the Randolph Institute for Theoretical Physics have been working on this issue. They started by trying to 'reconcile' the previously conflicting calculations and find precise graphene Poisson's ratio parameters. However, as the research Advancing, they found that this number is not a fixed value, it will change with the tension applied. Researcher Kachorovskii added: 'Graphene is subjected to a large tensile stress, it will have a positive Poisson's ratio like ordinary materials. However, as the tensile stress decreases, graphene starts to exhibit the characteristics of the auxetic material, showing a negative Poisson value.
Subsequently, they explained the unusual link between Poisson's ratio and stretching. Although most people see graphene images as flat two-dimensional carbon atoms, this is not the case. They are actually. There are many bends and waves that run along this 'slice'. They tend to change graphene from a flat to a wrinkled state, so graphene is not simply flat but pleated, they 'fold' It is so appropriate that it behaves like a flat two-dimensional structure. Kachorovskii explains: 'The scientific understanding of membranes for a long time is that there would be no two-dimensional crystals such as graphene. They think they always strive to shrink ball.
'However, as we have seen, the discovery of graphene crushes this theory. The presence of graphene surfaces must be similar to the fluctuations in the tension and compression. They and the surfaces of the folds will have a non-linear effect, preventing the graphene from shrinking into balls. In fact, graphene is actually not a two-dimensional crystal. It should be an intermediate state between two-dimensional and three-dimensional.
Why does the Poisson's ratio change sign? This is because the tension and pressure fluctuations inherent in the surface of the graphene will have a competitive effect with the sliding action caused by the external stress under applied tension. When the external stress is high, the auxetic action is increased. Inhibition and Poisson's ratio appear positive. When the external stress decreases, the tension and pressure fluctuations caused by the surface wrinkling of graphene play a leading role and the Poisson's ratio becomes negative. This is the reason why the Poisson's ratio sign changes.
Kachorovskii said: 'The extra energy is stored in the transverse bending wave folds. This is why graphene shows unusual elasticity and other special properties. This explains why graphene heat shrinks longitudinally because of its transverse folds. Collapsing occurs, so it exhibits contraction behavior that is different from most materials. So we think that the universal feature that can explain graphene behavior is Poisson's ratio. As long as we understand Poisson's ratio well enough, we will be able to clearly Explain the abnormal behavior of graphene and further predict other properties.
What is more significant is that the current results also explain why the previous research on the Poisson's ratio of graphene would be contradictory. 'By calculating, we have obtained an analysis of the elastic equilibrium equations of intact graphene sheets. The results show that graphene films There are two modes of behavior: Normally, all properties of graphene are determined by standard values, and Poisson's ratio is calculated to be positive. At the same time, for length than the so-called Günzburg (for graphene, the length of Günzburg) The range is from 40 to 70 Angstroms. For large samples, the bulge behavior occurs, and the negative Poisson's ratio is calculated. 'Kachorovskii added, 'The actual sample size used must be greater, so we can see the most unusual pull Swelling behavior. '
The explanation of this phenomenon is also related to the different types of waves that interact in a very complex way. The Günzburg length characterizes the scale by which these interactions are no longer ignored. At this scale they begin to make the material behave in an abnormal manner. Such large-scale interactions, for example, hinder the contraction of two-dimensional crystals into spheres. Different materials have different Gilzberg lengths, and knowing their specific ranges is extremely important for the development of new materials.
Kachorovskii cautioned that people usually create new materials without calculating the length of Günzburg and then try to find specialities in their properties. This is completely wrong. If Günzburg is as large as 1 km in length, then Normal-sized samples do not show any special properties at all. So it's very important to know the length of the Günzburg.
The debate over the graphene Poisson ratio has come to an end and the graphene bulging abnormal behavior has been perfectly explained. Given that the properties of graphene are so easily affected by external forces, we can use it to build highly sensitive sound sensors because acoustic waves can Drawing graphene films, the resistance of graphene changes significantly under different degrees of stretching. The Institute of Theoretical Physics of Randolph has put this application on the agenda. They calculated the sensitivity of this detector. High. In addition, the speed of sound propagation in auxetic materials is much higher than that of normal materials. Therefore, when graphene is in the auxetic state, the sound propagates extremely fast, which helps us to build a sensor with ultra-fast response speed. Quickly detect the oscillation of the sound.