Highlights
•The glass forming ability of high entropy metallic glasses (HE-MGs) and strategies to fabricate high entropy bulk metallic glasses (HE-BMGs) has been reviewed and summarized.
•The crystallization process and products of HE-MGs and HE-BMGs have been reviewed.
•The mechanical properties of HE-BMGs have been reviewed.
•The magnetic properties and related parameters have been summarized.
The team of Li Jianqiang, a researcher at the Institute of Process Engineering, Chinese Academy of Sciences, has developed a high-entropy glass. This glass sample has record-breaking hardness and modulus, and many indicators far exceed Corning's mainstream product-the sixth-generation Gorilla Glass (which was once known as the "strongest" mobile phone screen ever).
Record-breaking "hard" glass
"This kind of oxide high-entropy glass has record-breaking hardness and modulus, as well as excellent fracture toughness." Li Jianqiang told the "Chinese Journal of Science", "The glass sample far exceeds the sixth generation in hardness, modulus and fracture toughness. Gorilla Glass has important potential applications in the field of cover glass."
High entropy is the name of a new type of material system in the fields of alloys and ceramics in recent years. This concept originated from high-entropy alloys. Li Jianqiang introduced that as a brand-new material system, high-entropy materials have broken the design concepts of traditional materials and have received widespread attention in the fields of metals, ceramics, and intermetallic compounds. Compared with traditional materials, high-entropy materials show unique advantages in terms of mechanical, physical and chemical properties, and have become one of the important research hotspots in the international material academia.
Traditional alloy materials are mainly composed of one or two main components, and some other trace elements are added to enhance the characteristics. The main components of high-entropy alloys are usually more than four or five, and the content of various components is close to the same proportion.
"Entropy can be used to describe the complexity of a system, and high entropy can be understood as a high degree of chaos. Compared with conventional material systems, high-entropy materials have a higher degree of chaos. Existing research believes that this degree of chaos is at a certain level. These aspects can break the routine and produce some unexpected performance." Guo Yongchang, the first author of the paper and a doctoral student at the University of Chinese Academy of Sciences, told the China Science News, "In the past, the more metal types added to the alloy, the more It will be more brittle, but the high-entropy alloy is different from the previous alloys, and the addition of multiple metals will not be embrittled. It is a subversive new method of material design."
In addition, Li Jianqiang explained that modulus (referring to Young's modulus) is a physical quantity that describes the ability of a solid material to resist elastic deformation under an external force, and high modulus can reflect the ability of glass to resist scratches to a certain extent.
"The author reported three new types of multi-component glasses, prepared by a containerless melt quenching process, which can make the glass have high hardness and Young's modulus…" the reviewer of the paper commented, "These results lead to A lot of interest in the materials science community."
"Hardcore" technology makes it stronger
As electronic products such as mobile phones and tablet computers have become thinner and lighter, and screens have become larger and larger, the performance requirements of cover glass have become higher and higher. In the test of the drop resistance of the cover glass, the mainstream method is to drop the prototype freely from a certain height, and then count the breakage of the cover glass.
In 2018, Corning announced the successful development of the sixth-generation Gorilla Glass. According to the company's laboratory data, the sixth-generation Gorilla Glass dropped from a height of 1 meter onto a rough surface and can withstand 15 hits without being damaged.
According to market surveys, the average number of people dropping their mobile phones is about 7 times a year. Using the sixth-generation Gorilla Glass, it can still be used for two years without the need for a phone case and a tempered film. This kind of glass once became the "selling point" of many smart phones.
Nevertheless, "broken screen insurance" is still a standard configuration for many people when they buy mobile phones. Users still urgently need "hard core" technology to create a more "strong" mobile phone.
The hardness, modulus and fracture toughness of glass depend on the composition and microstructure of the material. Under normal circumstances, selecting oxide components with high dissociation energy (such as aluminum oxide), and optimizing the preparation process to increase the atomic packing density, will help improve the mechanical properties of the glass, but at the same time will lead to the glass forming ability Severe decline.
"For example, the sixth generation of Gorilla Glass, in order to improve the hardness, modulus and fracture toughness during the production process, usually requires chemical strengthening treatment. The strengthened glass due to the existence of the surface stress layer, the hardness, modulus and fracture toughness There is a certain improvement, but this post-processing method limits the later design of the glass shape, once it is chemically strengthened, it is difficult to reprocess." Li Jianqiang said, "Therefore, through innovative component design and preparation methods, the prepared Glass with hardness, high modulus and high fracture toughness is of great significance to the electronic glass industry."
The cover glass of the screens of existing mobile phones and tablet computers can be divided into high-alumina glass (represented by the sixth-generation Gorilla Glass) and soda-lime glass according to different components. In contrast, the former has better performance, but the two glasses have one thing in common, that is, the composition contains more network formers (such as SiO2, B2O3) and so on.
"Containing network formers will limit the development and performance improvement of new glass systems to a certain extent." Li Jianqiang said, "Our glass samples do not contain network formers. At the same time, we introduce high-entropy material design concepts and add titanium oxide and zirconium oxide to the glass. Such components that can change the microscopic coordination number together form the main body of the glass. The biggest difference between this new glass and the traditional glass in terms of microstructure is that it has a higher coordination number, which can bring excellent performance."
However, this kind of glass has a low forming ability, is easy to crystallize during cooling and has a high melting point, which is difficult to prepare by traditional melting and cooling methods. In order to solve this contradictory key scientific problem, researchers based on years of research and accumulation in the field of special glass, rationally selected multi-principal components, and adopted the method of "laser heating melting-containerless solidification" to melt zirconia and other super High melting point substances.
"Because the sample is in a suspended state during the preparation process, it can effectively inhibit the non-uniform nucleation formed by contacting the container wall, thereby inhibiting crystallization." Guo Yongchang said, "At the same time, laser heating has the advantages of rapid heating and cooling, which can make the melt reach Deep subcooling and rapid solidification have solved the problem of glass sample preparation."
When to bid farewell to broken screen insurance?
Using these technologies, researchers have successfully prepared high-entropy glass with high hardness and high modulus, which has record-breaking hardness (12.58 GPa) and modulus (177.9 GPa), and excellent fracture toughness (1.52 MPa·m0.5). ) And good visible light-near-mid-infrared transmission (maximum 86.8%), many indicators far exceed Corning's sixth-generation Gorilla Glass (hardness 6.78 GPa, modulus 77 GPa, fracture toughness 0.7 MPa·m0. 5).
Judging from several main data, this glass sample is about twice as high as the sixth-generation Gorilla Glass. But Li Jianqiang said frankly that he cannot simply say that "fall resistance can be doubled."
"The anti-drop degree of the cover glass comprehensively reflects the brittleness and fracture toughness parameters, and has a certain relationship with the hardness." Li Jianqiang said, "At present, our samples are still in the laboratory stage, and the glass's anti-drop resistance has not been tested. However, the fracture toughness (physical quantity describing the ability of the material to resist crack growth) of this glass sample is higher than that of the sixth-generation Gorilla Glass, which to a certain extent indicates that the sample will achieve better results in the later assembly test of the whole machine. Fall resistance."
Li Jianqiang said that a mature product going to the market not only needs some outstanding advantages, but also needs a better balance of performance in all aspects. For cover glass, in addition to modulus, the glass is often required to have good permeability, brittleness, fracture toughness, etc. The complexity and cost of the preparation process are also important factors affecting its application.
"The focus of our early research is mainly on mechanical properties, which are the most basic and most important parameters of cover glass. For its electrical properties, such as conductivity, sensitivity, and dielectric constant, we will focus on special The module is tested and analyzed.” Li Jianqiang said, “Next, we will try to prepare cover glass that meets the requirements of the installation, and gradually advance various performance tests.”