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Metal and plastic are widely used materials in daily life. Now see if there are countless metal products around you who have money to spend and food to eat.

And countless plastic products?

After eating, you should learn more knowledge and think about why metals and plastics can be applied to all aspects of life on a large scale.

1. Are metal and plastic contradictory materials? usually, metal has a relatively high melting point and high mechanical strength. Human beings used metal materials a long time ago, and can be traced back to the Bronze Age recorded in human civilization.

Compared with earlier stone tools, metal copper and metal iron are more advanced materials. The reason why bronze and iron tools can replace stone tools is that they have superior mechanical properties and easy smelting and processing, so the first industrial revolution is also closely related to the large-scale use of iron and steel.

Compared with metal, plastic is only a material that began to rise at the beginning of the 20th century, and it has been used on a large scale because of its good deformability and processability.

Plastic is a kind of polymer amorphous material with irregular atomic arrangement, and its softening point is called glass transition temperature. Because the glass transition temperature of plastics is usually near room temperature, or even lower than room temperature, plastics can be processed near room temperature. With the characteristics of low deformation temperature and strong deformation ability, plastics have been widely used in daily life and industrial production since their invention.

Therefore, the factors that determine the prospect of materials require not only their own unique physical and chemical properties (such as metals), but also excellent processing and deformation properties (such as plastics).

Metal and plastic are two kinds of almost contradictory materials: metal is much stronger than plastic, but its processability is worse than plastic. So is it possible to develop a material with both the strength of metal and the excellent processability of plastic?

2. The answer to the discovery of metal and plastic is yes. In May 2005, the Institute of Physics of the Chinese Academy of Sciences reported a new metal material with the strength of metal and the excellent processability of plastic.

This material has the same strength as aluminum-magnesium alloy at room temperature, but when the temperature rises to the temperature of boiling water, it shows a variety of processing deformation behaviors, such as tension, compression, bending, imprinting and so on. It is precisely because of the excellent processability of general metal and plastic, this material is called "metal plastic".

Unlike the metals commonly used in life, metal plastics are not crystals, but amorphous structures, and are a kind of amorphous alloys. The arrangement of atoms, such as steel and aluminum alloy, which are often encountered at the microscopic level, is symmetrical, while the arrangement of non-crystal atoms does not have any periodic structure, so it is a complex disorder system.

The picture on the left shows the structure of amorphous copper-zirconium alloy simulated by computer, and the arrangement of particles is disorderly. On the right, the structure of crystalline copper-zirconium alloy is very regular.

In fact, there are many kinds of amorphous solid materials. We live in a world full of amorphous solids. The materials that people see every day: plastic, glass, asphalt, amber, rubber and so on are all amorphous solids.

As a disordered inorganic amorphous substance, glass also plays an important role in the development of human civilization, such as cups that are used every day:

For example, the development of modern science is the cornerstone of telescopes and optical lenses of microscopes:

For example, the source of architectural art and design:

For example, it is the optical fiber below (which uses a very pure glass medium to transmit light waves) that makes this push visible to more people.

Amorphous solids also have characteristics different from crystals:

1. Amorphous is metastable. When the amorphous material is heated up, the amorphous material will evolve to the equilibrium state, and its physical properties and atomic arrangement structure will also change with time. Therefore, when using amorphous materials, it is necessary to consider the stability factors in the process of use (whether the physical properties remain stable in a certain period of time).

The free energy comparison diagram of amorphous and crystal shows that the metastable amorphous state is in an energy unstable state 3.

two。 The isotropy of the physical properties of amorphous matter. The crystal has crystal axis orientation and anisotropy; the amorphous has no lattice orientation and is isotropic macroscopically, just like the broken amorphous glass has no regular shape.

3. Amorphous solids have no exact melting point. The crystal has a definite melting point at room temperature and pressure, and the amorphous has no exact melting point. Plastic and glass are gradually softened and melted at the same time. When the melt is cooled, if the cooling rate is higher than the cooling rate of crystal formation, the melt will form an amorphous solid near a certain temperature, which is called the glass transition temperature point.

The path of supercooled liquid, amorphous and crystal formed by liquid 3

The temperature range from the solidification point of the melt to the glass transition temperature is the supercooled liquid region. In this temperature range, the amorphous state is neither a hard solid nor a liquid without a specific shape, but shows a typical viscous fluid state, has a good superplastic forming ability, and can be permanently deformed by external force.

Molding of plastics in supercooled liquid region 4

After the amorphous alloy of the same kind as metal plastic is heated to the temperature of supercooled liquid zone, it can also be formed by blow molding to achieve extremely high deformation.

Zirconium-based amorphous alloy 5 formed by blow molding

So how is this metal plastic designed?

3. The design rules of metal plastics amorphous alloy is a kind of strange material by nature. On the one hand, at room temperature or below the glass transition temperature, it has higher mechanical strength than ordinary metal materials.

Zirconium-based bulk amorphous alloy mobile phone frame 6

On the other hand, after heating into the supercooled liquid range, it becomes as good deformable as a viscous fluid.

Micron structure formed by micro-nano forming of platinum-based amorphous alloy in supercooled liquid region 7

Therefore, in terms of the nature of amorphous materials, compared with amorphous alloys, polymer plastics is a kind of amorphous materials with low glass transition temperature, whose glass transition temperature is usually near room temperature, and some are even lower than room temperature. Compared with plastics, most amorphous alloys have high glass transition temperatures (usually in the range of 300 degrees Celsius to 600 degrees Celsius).

Therefore, for amorphous glass materials, the glass transition temperature Tg is one of the most important performance parameters and indicators, which directly determines the use temperature and deformation processing temperature of glass materials.

Within the range of strength and glass transition temperature, there is a separate zone between polymer plastics and amorphous alloys, which is the original design idea of metallic plastic materials, that is, the development of new amorphous alloy materials located in this unique range. In other words, this material has the same low glass transition temperature Tg as plastic, but also has the high strength of typical metal materials.

According to the design idea of metal-plastic material, combined with the relationship between glass transition temperature and elastic modulus of amorphous alloy (the ability of material to resist elastic deformation), and the relationship between elastic modulus of amorphous alloy and element modulus of amorphous alloy, firstly, metal-plastic system Ce70Al10Cu20 was found in cerium-based metal, and the glass transition temperature can be as low as 68 degrees Celsius.

Ce70Al10Cu20 metal plastics with a glass transition temperature of 68 degrees Celsius can be easily bent in boiling water.

Easily bend Ce70Al10Cu20 metal plastic in boiling water to get the "BMG" letter (bulk amorphous alloy) pattern 1

Cerium-based metal plastics can not only easily carry out various plastic deformations such as stretching, compression, bending and imprinting in boiling water, but also have a wide range of supercooled liquid phase, which can be processed in a wide temperature range.

In addition, a series of calcium-lithium matrix, strontium matrix and zinc matrix metal plastics have been developed, which can be deformed near the boiling water temperature.

However, as mentioned earlier, amorphous alloy is a metastable alloy that evolves to equilibrium over time. Metal plastic is a kind of amorphous alloy, so the stability of this material must be considered.

If the understanding of deviating from its equilibrium state is not deep enough, a physicist at the University of Queensland, Australia, in order to prove that "asphalt is a liquid, not a solid" (because asphalt has a high coefficient of viscosity near room temperature, it will be difficult for particles to change position to form a new configuration in a limited time), he heated the asphalt and poured it into a sealed glass funnel. When the asphalt is completely solidified, cut the lower end of the funnel and begin to record the time of a drop of bitumen. The experiment shows that the first drop of asphalt took 8 years. Therefore, sometimes it is not easy to observe the evolution of amorphous matter in a relatively short time scale, so it is often ignored.

Parnell Asphalt dripping Test device of the University of Queensland, Australia 9

4. To what extent can the stability of amorphous materials be verified by the super stability of metal plastics?

For example, amber is a typical amorphous substance (an amorphous sugar that can seal proteins for a long time), which is very stable. At the beginning of the formation of amber, animals and plants from ancient times were sealed in it. Due to the good preservation of amorphous materials, the creatures of millions of years ago and their scenes at that time could be preserved. This solidifies a space-time scene from millions of years ago and preserves it with unparalleled stability to this day.

On the other hand, the amorphous alloys with some components are easy to change to the equilibrium state when heated, that is, the amorphous alloys gradually become crystals.

Heating at a certain temperature makes the crystal in Zr46Cu46Al8 amorphous alloy gradually precipitate over time, and the crystal grows slowly (as shown by the blue arrow) 10

Therefore, some amorphous systems are easily transformed into crystals, while some systems are not easily transformed into crystals, which gives rise to a question: for a given disordered system, is there a unique lowest energy state? Like amorphous amber, its physical properties do not change significantly over tens of millions of years of evolution.

At present, the main characteristics of this kind of super-stable glass are 11:

The glass transition temperature increases. The higher the glass transition temperature is, the more energy the system needs to activate the molecular motion, that is, the higher the stability of the system, the less easy it is to change to crystal.

The density increases. The increase of density is a direct macroscopic reflection of the dense arrangement of ultra-stable glass particles.

Improved elasticity and strength. Due to the dense arrangement of ultra-stable glass particles, the ability to resist external force deformation is also improved.

Ultra-stable metallic glass can be used in flexible electronic devices 12 because of its high stability, physical properties that conventional crystals and amorphous crystals do not have, and wearable characteristics.

Because the metal plastic can be easily deformed in boiling water, placing it in the room temperature range can cause great changes in the microstructure evolution and energy state of the metal plastic. So does the metal plastic stored at room temperature for a long time become a more stable crystal or an ultra-stable substance like amber?

What will happen to the cerium-based amorphous plastics developed in 2005 since they have been kept at room temperature?

It is found that the cerium-based amorphous metal plastic sample aged at room temperature for 17.7 years still maintains a perfect amorphous state and does not transform into a crystal, showing a strong ability of crystallization resistance, which is a kind of amorphous alloy with high stability.

The transmission electron microscope images of cerium-based amorphous metal plastics aged at room temperature for 17.7 years show that the atoms are disordered and no ordered crystal structure is found.

Compared with other kinds of amorphous alloys, cerium-based metal plastics have a strong ability to resist crystal transformation, which is close to super-stable glass, that is, it can hinder the occurrence of nucleation and crystallization in the relaxation process. And the cerium-based amorphous metal plastics aged at room temperature in 17.7 years need to be heated to a higher temperature before they can be converted to liquid.

Comparing the crystal nucleation rate of Ce--based amorphous alloy with that of other alloy systems, it is found that cerium-based metallic plastics have very low nucleation rate and strong stability.

After aging at room temperature for 17.7 years, the glass transition point of cerium-based amorphous plastics increased by 27K, which indicated that the thermodynamic stability was significantly improved, and the long-term aging made it a kind of hyperstable amorphous alloy.

Why did cerium-based metal plastics become ultra-stable glass for a long time without being transformed into crystals?

To explain this problem, let's make an analogy here. The potential energy of the atomic configuration in the amorphous matter is just like the gravitational potential energy of the mountain in reality, the position of the peak corresponds to the configuration of the high energy state and the position of the valley corresponds to the configuration of the low energy state. The atomic configuration evolves to a low-energy configuration over time, just as water flows into the valley over time.

There is a large energy valley in the energy barrier diagram of cerium-based metal plastics, while the energy barrier of amorphous which is easy to transition to crystal is composed of several different high-energy valleys, which is easier to transition to the position of crystal in the process of evolution to equilibrium state. The microkinetics of cerium-based metal plastics is relatively uniform. This micro-configuration property enables cerium-based metal plastics to reach a lower energy state, that is, a super-stable glass state, through the transition between adjacent low-energy valleys continuously and rapidly during aging.

Cerium-based metal plastics belong to the strong glass system pictured above, which contributes to its continuous and rapid relaxation to ultrastable glass state.

Because metal-plastic materials have unique physical and chemical properties, they have application value and potential in different fields. Excellent processing and deformation ability is a common feature of metal plastic materials, which is very beneficial to microplastic forming.

The image of the gear object observed on the finger and the image of the gear observed under the scanning electron microscope 8

It has been more than ten years since the discovery of cerium-based metal plastics. At present, metal plastics as a new type of materials have been further developed, and metal plastic materials have been found in many alloy systems, which show great application potential in micro-nano processing and devices.

Finally, let's summarize the characteristics of metal plastics:

The same strength properties as metal.

The same molding ability as plastic.

A glass-like disordered structure.

Finally, imprint a logo with metal plastic.

reference

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Materials Science and Engineering R 100 (2016) 1-69

Progress in Physics, 2013, 33 (5): 177351.

Https://mp.weixin.qq.com/s/TnV0lhnTv9zdzWRHZrbgAQ

Journal of Nature, 2018. 40 (03): 157-168.

Materials, 2018, 11: 2338

Adv. Mater. 2011, 23,461-476

Acta Phys. Sin. Vol. 66, No. 17 (2017) 176411

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Acta Materialia 149 (2018) 108118

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Appl. Phys. Lett. 110 (2017) 031901

Editor: be careful

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

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