The "mysterious" physics behind the glass

The following article comes from WeChat public account: China Physical Society Journal Network, author Pan Deng Jin Yuliang compiled

Pan Deng and Jin Yuliang of the Institute of theoretical Physics of the Chinese Academy of Sciences are compiled from Jon Cartwright.physics World, 2022, (6): 24: 24

This article is selected from "Physics", No. 7, 2022

In the British Museum in London, there is a small blue -green bottle, which is native to Egypt during the rule of Thutmose III. This opaque small bottle is almost made from glass. However, although it has a history of more than 3,400 years, it is not the earliest glass made by humans. Historians believe that as early as 4,500 years ago, the Mesopbamian civilization had mastered the technology of making glass.

At the beginning, glass does not seem complicated. Glass material has an amorphous (disorderly) structure, that is, there is no long program between atoms or molecules. Ordinary glass generally contains three components: silicon dioxide (sand) that constitutes the basic structure, which is used to reduce the alkaline metal oxides (generally soda), which is used to reduce the melting temperature, and calcium oxide (lime) used to reduce water -soluble. In fact, the formula can be easier, we now know that almost any material can become glass state. As long as it is cooled fast enough, the atoms or molecules in the liquid will be "frozen" before forming an orderly solid structure, thereby forming a glass state. However, this simple description covers the deep physics under the appearance -for more than a century, some problems related to glass have been plagued by physicists.

Mystery of liquidity

Walking in the medieval church, you will find that the scene outside the window is distorted after through colorful glass windows. This phenomenon is doubtful that as long as the time is long, the glass will flow like a very sticky liquid. However, can this guess be confirmed?

This problem is not as simple as it looks. In fact, no one can accurately divide liquid and glass. Physicists generally believe that when the relaxation time of the atom (the distance of the atomic movement is close to the time required for the atomic diameter), the liquid becomes glass. The relaxation rate of the glass is 1010 times slower than the honey, and it is 1014 times slower than water. But no matter what, the choice of this judgment is arbitrary, but it does not reflect the physical difference between liquid and glass.

Even so, the relaxation time of 100 s is eternal for humans. At this rate, a piece of ordinary glass needs to be slowly flowing through tens of millions of years, and it is transformed into a more stable crystal (ie, quartz). Therefore, if the color glass in the medieval church is distorted, it is more likely to be caused by the poor technology of the glass maker at the time (according to modern standards). On the other hand, obviously no one has done thousands of years of experiments to test these speculations.

Find "ideal" glass

From the perspective of the "phase change" of physicist Lev Lang, when the state of material changes, the inner "preface" will change suddenly. However, when the liquid becomes glass, there seems to be no obvious changes in the order. The difference between the two is that liquids can traverse different disorder structures, while glass is stuck on one or more disorderly structures. In the process of formation, why choose a specific state?

When the liquid cools, it is either forming glass or crystallization. However, the temperature of the liquid to glass is not fixed. Under the premise of avoiding crystallization, as the cooling rate becomes slower, the temperature of the liquid -glass transition will be reduced, and higher -density glass will be formed. At the end of the 1940s, American chemist Walter Cosmanian noticed this phenomenon, and predicted the temperature of the liquid's glass at the "balance" cooling (unlimited slow cooling). The "ideal glass" formed in this way looks like a paradox: Although it is disorderly, it has the same entropy as the crystal. In essence, the ideal glass is composed of the molecules in the most tight and random way (Figure 1).

Figure 1 The ideal glass state imagined by the artist (the picture comes from the University of Bristeo)

In 2014, physicists such as George Parry (the winner of the Nobel Prize in Physics in 2021) obtained the ideal glass phase map under the limit of infinite dimensions. Generally, density can be a sequential parameter that distinguishes different states, but for glass and liquid, the density of the two is not much different. Therefore, physicists have to use another sequential parameter, the so -called "overlap" function. This function describes the similarity of the molecular position of different disorderly configuration at the same temperature. They found that when Cobazman's temperature, the system would enter a high degree of overlap, namely (ideal) glass state.

How to make better glass?

As a non -fixed solid, glass can be in a variety of different states. This feature causes the design of glass materials to be very flexible. Regardless of the composition or processing method, subtle changes may lead to different glass performance.

If you want to change the performance of the glass, there are two most basic ways: change the composition of the glass, or change its manufacturing method. Examples of the former include soda and lime in ordinary glass with borosilicate, so that the glass made in this way will not be too concentrated when heating (which causes cracks), so it can be used to make baking vessels. The latter's example is to use the principle of cooling and faster the surface of the glass in the "ignition" to prepare a stronger glass than internal cooling. Corning's first Pyrex heat -resistant glass was prepared based on this principle. Another innovation of Corning is Gorilla Glass used on smartphones. This glass has the characteristics of sturdy and scratch, while its ingredients and processing methods are more complicated. It is essentially a kind of aluminum -aluminum silicate material, which is produced on the suspended tablet with a special fast quenching "melting" process, and then immersed in the melting salt solution to further chemically strengthen it.

Generally speaking, the stronger the density of the glass, the stronger the density. In recent years, the researchers have found that the physical gas sedimentation method (condensing the gasification material in the vacuum to the base) can create very dense glass. This process allows molecules to find the most effective accumulation every time during condensation -similar to the Russian square game.

Conquer metal glass

In 1960, Belgian physicist Pol Duwez (working at the California Institute of Technology at the time) discovered that the solidified metal became glass state after a pair of cooling rollers (called SPLAT quenching). Since then, metal glass has attracted the attention of material scientists: on the one hand, because this material is extremely difficult to prepare, and on the other hand, because they have extraordinary characteristics.

Because there is an inherent crystal boundary like ordinary crystal metal, metal glass is not easy to wear. Using this performance, NASA (NASA) used metal glass to create gear components without lubricants (Figure 2), and tested its use of space machinery and equipment. Metal glass mechanical moving energy consumption is very low -for example, the ball made of metal glass can continue to bounce for a long time. Metal glass also has excellent soft magnetic properties, which can be used for high -efficiency transformers. In addition, it can make various very complicated shapes like plastic.

Figure 2 Gear made of metal glass (picture from NASA NASA)

Many metals can only become glass state at a very fast cooling rate (number of 〸 00 million degrees per second). Researchers usually find alloys that are easier to transform into glass by repeated trials. If we can predict the transformation temperature of the glass and the characteristics of the formed metal glass, then the development of metal glass with commercial value becomes possible. In fact, Apple, the United States, has a patent for metal glass mobile phone cases very early, but it has not been used for actual products -it may be precisely because it has not found a metal glass with enough economic costs.

The future of phase change materials

Although the mechanical properties of glass and crystal materials may be very different, their optics and electrical properties are often closer. For example, for those who are not trained, it is almost impossible to distinguish ordinary silicon glass and quartz (crystal state corresponding to silicon glass). However, some materials, especially sulfur, are obviously different in optics and electrical characteristics during glass and crystalline. If they just have poor glass formation capacity (moderate heating will crystallize), then it can be used as a so -called phase -changing material.

In fact, many people have already exposed to phase -changing materials: the data storage medium of the disc is this material. Inserting the disc into the supporting driver, the laser can make any comparison of any comparison on the disc in the glass state and crystalline state, which indicates 0 or 1 of binary. Sulfide glass is sometimes used for photon integrated optical circuits. In addition, phase -changing materials also have new applications in data storage, such as the Optane memory of Intel Company in the United States. The access speed is fast and the data will not be lost during power off. The problem worth further exploring is the source of phase change characteristics and predictability.

Unexpected glass

People who have participated in the music festival will notice a phenomenon: When you try to leave a performance with thousands of people, suddenly, the whole crowd stopped, and you can't move. Just like after the molecules in the silicon melting material are cooled, your activity range is suddenly limited -you and other audiences have become a large "glass" (or a state of glass -like).

Other "glass" include ant colony, biological cells sandwiched between glass, and colloids (such as foam used when shaving). Especially the colloid, its particle size can reach micron level, so its dynamics can be observed by microscope, which makes the colloid a system that facilitates the theory of vitrification transformation. What is even more surprising is that glassization will also appear in some computer algorithms. For example, if there is a lot of variables in a certain problem, due to its complexity, the general algorithm will get stuck in a certain non -optimal solution before finding the optimal solution. Researchers now know how to improve such algorithms with the help of statistics developing in glass problems to find better solutions.

This article is authorized to reprint from WeChat public account: China Physical Society Journal Author: Pan Deng

Reprinted content only represents the author's point of view

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It does not represent the high energy office of the Chinese Academy of Sciences

Edit: Liu Yan

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