◎ Intern reporter Li Zhao Yu Dou Peng
From towering skyscrapers to small and exquisite chronograph watches, glass can be seen everywhere in life.
Peng Shou, deputy to the National People’s Congress, academician of China Academy of Engineering, chief engineer of China Building Materials Group and president of China Building Materials Research Institute of Glass New Materials, said in an interview that it is necessary to vigorously promote energy conservation and carbon reduction in the glass industry, so that the glass industry in China can "go green".
In fact, glass should not only "go green", but also be expected to "rejuvenate" — — With the increase of service time, glass will age, which is usually accompanied by the deterioration of physical and mechanical properties. In recent years, more and more attention has been paid to how to "rejuvenate" aging glassy substances and restore their properties.
At the end of 2022, Jiang Minqiang, a researcher at the Institute of Mechanics, Chinese Academy of Sciences, revealed a new mechanism for the rejuvenation of severely aging metallic glass through research, which deepened the understanding of the rejuvenation of glass structure. Relevant research results were published in Fundamental Research, a multidisciplinary English journal headed and sponsored by the National Natural Science Foundation of China.
Glass aging: a slow transition from disorder to order
To make glass "rejuvenate", we must first understand how glass "ages".
Jiang Minqiang introduced that from a microscopic point of view, glass is an amorphous solid with irregular structure. He gave an example to the reporter: in crystalline solids such as steel, atoms are arranged in an orderly manner like students sitting quietly in a classroom, showing a regular shape. In glass, an amorphous solid with irregular structure, atoms are just like students after class, running around the campus, showing a disorderly arrangement.
The phenomenon of glass aging, in essence, is the transition of glass from the disordered state when it was initially formed to the orderly state. "Generally speaking, the total energy of matter is higher in disorder state, while in order state, the total energy of matter is lower. As time goes on, glass will gradually change from a high-energy state to a low-energy state. This process is generally called glass aging. " Jiang Minqiang explained that if the aging time is long enough, or the aging is accelerated by heating up, the glass can even be transformed into a solid crystal with a regular structure.
Glass aging will affect many properties of glass, such as toughness, optical properties and electrical conductivity, which is a phenomenon that people want to delay or even avoid. Therefore, as a process of reversing glass aging, glass rejuvenation has long been widely concerned by researchers.
"The rejuvenation of glass is the reverse process of glass aging, that is, the glass that makes the atoms relatively orderly after aging slowly returns to the relatively disordered state of atoms." Jiang Minqiang said. In the previous research on glass rejuvenation, the researchers found that the younger glass will release some enthalpy when it is heated to a certain temperature, and the higher the younger the glass is, the more enthalpy will be released during heating. Enthalpy is an important state parameter to characterize the energy of material system. Generally speaking, enthalpy is the energy released by young glass during heating.
"Our research believes that the above views do not apply to severely aging glass. With the increase of the degree of youthfulness, the enthalpy release of severely aged glass has not changed, or even has no enthalpy release at all. " The research of Jiang Minqiang’s team shows that the previous views on glass aging mechanism are not applicable to severely aging glass, which updates people’s understanding of the mechanism of glass structure rejuvenation.
"Unintentionally inserted willow": The research result is unexpected.
"In fact, the breakthrough of this research stems from our research team ‘ Unintentionally inserted willow ’ An attempt. " Jiang Minqiang said that the initial purpose of this experiment was only to prepare experimental samples. "Our team originally prepared samples for another experiment. In order to strengthen the scientific nature of the experiment, it is necessary to eliminate the thermal history of the samples and ensure that their structures are consistent. We anneal the glass samples at low temperature — — Slowly heat the metallic glass to a certain temperature and keep it for enough time, and then cool it to room temperature at a certain speed. " Subsequently, the research team rejuvenated these glasses in a severely aging state through mechanical deformation. The result was unexpected — — "We obviously input energy into the glass through mechanical work. Why don’t these glasses release enthalpy and become younger?" This is contrary to the previous mainstream view.
This result puzzled the research team. In order to solve the mystery, in addition to measuring enthalpy, the research team also measured the high temperature of glass samples (450K— 750K) and low temperature (1.9K— 100K), and then the atomic vibration information and topological structure information of glass are investigated. "In the course of the experiment, the research team found that although the parameter of enthalpy release before glass transition remains unchanged in some cases, the effective enthalpy change during glass transition and the atomic vibration Bose peak reflected by low temperature specific heat will change accordingly." Jiang Minqiang further explained, "This shows that enthalpy release is not the only physical quantity reflecting the rejuvenation of glass."
When talking about why the enthalpy release remains unchanged, Jiang Minqiang once again explained with vivid examples: "If we put a ball in ‘ Concave ’ The concave part of the zigzag plane, this ball will naturally remain in place. This stable state is like severely aged glass. And if we put this ‘ Concave ’ The zigzag plane is inclined at some angles. Although the height of the recess, that is, the energy level of the glassy substance, remains almost unchanged, the glass state represented by the ball will become unstable, and then the phenomenon of glass rejuvenation will appear. "
The results show that, in addition to the previous mainstream point of view, the rejuvenation of glass can be directly reflected in the release of enthalpy, that is, the improvement of energy level, and can also be reflected in the inclination of energy plane, that is, the free volume can be redistributed in space through local structural rearrangement. "This is also the new mechanism of rejuvenation of severely aging glassy substances that we have discovered." Jiang Minqiang said.
Expand the scene: provide a broad application space
This study also found that with the glass entering a steady flow state, the above three physical parameters representing the rejuvenation will tend to saturation values, so it was first determined experimentally that the upper limit of the glass structure rejuvenation was a "frozen" steady flow state.
If water is used as an analogy, glass that forms liquid at high temperature is like water, while glass that solidifies at low temperature is like ice. "The limit of the rejuvenation of the glass structure is that the high-temperature glass liquid is suddenly frozen by rapid cooling, thus forming a similar ‘ Frozen running water ’ The material state. " Jiang Minqiang explained, "In this case, the glass will maintain almost the same material structure as the liquid state under the solid appearance, and its fluidity will reach the limit of current understanding."
The new mechanism of rejuvenation of glassy materials revealed in this study not only allows us to better understand the related causes and processes of glass aging in physical essence, but also has great potential application space in promoting the batch rejuvenation of aging glass. "The research team is currently communicating with companies engaged in glass production or research and development, trying to find a good combination point to promote technology to the market."
In addition, Jiang Minqiang also found that the new mechanism revealed in this study is also expected to be applied to the preparation of advanced metal materials.
"Generally speaking, the strength and toughness of metal materials cannot be both. With the increase of strength, toughness will decrease, and vice versa. " Jiang Minqiang said, "How to overcome this inherent inversion relationship is a problem that must be faced in preparing advanced metal materials with both strength and toughness."
The total energy level of high-strength metal materials at the microscopic level is generally very low. If energy is input by heating and other methods, it is almost impossible to try to improve the toughness of metal materials by raising the total energy level, which often requires extremely high energy input.
"If we can use the new mechanism discovered in this study to adjust the energy plane angle of metal materials when the total energy level is low, we can improve the disorder of atoms and enhance the toughness of metal materials while maintaining the macro strength unchanged. Through this method, we can effectively avoid huge energy input and greatly reduce the cost of preparing high-toughness metal materials. " Jiang Minqiang said that his team is constantly trying to make a decisive breakthrough and provide new ideas for solving the irreconcilable contradiction between the strength and toughness of metal materials for a long time.
