They are less than one-tenth the thickness of a human hair, but scientists can tell their flaws with just one look

Produced by: Science Popularization China

Author: Zhang Hao (Assistant Researcher, Shanghai Advanced Research Institute, Chinese Academy of Sciences)

Producer: China Science Expo

In the story of Edison, we all know that he finally discovered a durable filament after thousands of experiments, which brought the first beam of "light" to modern industrial civilization. However, what is less known is that the carbon filaments burned from bamboo fibers in his experiments inadvertently opened the chapter of modern new materials - carbon fiber.

Edison and the light bulb

(Photo source: veer photo gallery)

What is carbon fiber?

Carbon fiber is an inorganic fiber with a carbon content of more than 90% obtained by high-temperature conversion of organic fiber. As early as 1879, Edison obtained a fiber with good conductivity that could meet the needs of light bulbs by carbonizing plant fibers under high temperature. However, due to its short life and limited strength, it was later replaced by metal filaments.

In 1959, Japanese and American scientists developed carbon fiber production technology, which greatly improved its performance. In 1982, carbon fiber was first used in the aviation field by Boeing and Airbus.

Today, more than a hundred years later, with the advancement of carbon fiber spinning technology, high-strength carbon fibers have been produced that are as thin as a hair (5-10 microns), as light as a feather (density is about one-fifth of steel), and as strong as steel (breaking strength is about four times that of steel).

Carbon Fiber

(Photo source: China Science Daily)

Carbon fiber is widely used in various fields

Today, carbon fiber has become an indispensable underlying material in the modern industrial system. From lightweight sports equipment such as fishing rods to wind turbine blades; from F1 racing cars on the track to aviation giants such as the Boeing 787, carbon fiber plays an important role with its light weight and high strength.

Take the Boeing 787 as an example, up to 50% of its fuselage materials are made of carbon fiber composite materials. The reason why this material is so important is due to the many excellent properties of carbon fiber.

First, its low density greatly reduces the weight of the aircraft, thereby increasing the passenger capacity and transportation capacity. Secondly, carbon fiber can significantly reduce fuel consumption, making the flight longer and more efficient. In addition, the extremely high strength and good mechanical properties of carbon fiber also provide a strong guarantee for the safety and durability of the aircraft fuselage. These characteristics make carbon fiber one of the core technologies that promote the progress of the aviation industry.

Boeing 787

(Photo source: The Paper)

The manufacturing of high-performance carbon fiber faces challenges

Although my country has become a major producer of carbon fiber, it is still in the catching-up stage in the field of high-end carbon fiber. Top-level carbon fiber represented by carbon fiber products above T1000 has not yet achieved domestic substitution, while Western international leading companies such as Japan's Toray Corporation dominate the production of high-end carbon fiber and have imposed embargoes on my country. For this reason, my country still faces a supply bottleneck for high-end carbon fiber products. Therefore, it is particularly urgent to accelerate independent research and development, break through the technical bottleneck of carbon fiber, and realize the localization of technology.

However, manufacturing high-performance carbon fibers is not easy. One of the core issues is how to characterize and control defects in carbon fibers.

Defect structure evolution during carbon fiber production

(Photo source: Shenzhen University Xu Jian and Zhu Caizhen research team)

These defects are like ghosts hiding in the shadows and are directly related to the ultimate performance of carbon fiber.

Firstly, most of the defects are contained inside the fibers and cannot be detected by filling methods (such as mercury porosimetry, nitrogen adsorption, etc.). On the other hand, their three-dimensional structure, length, width and internal deflection angle will have an important impact on the performance of the material. The traditional electron microscope slice observation method actually observes the projection of its three-dimensional structure. Just like the three-view drawing when we draw, the projection figures of different three-dimensional structures may be the same.

Three views when drawing

(Photo source: provided by the author)

For example, for an ellipsoidal defect structure, its cross section is observed to be circular. If the relevant parameters of the circle are used to measure the real defect structure, it will inevitably lead to a huge error. Therefore, we need a method to directly observe the three-dimensional structure of the defect.

Cross section of an ellipsoidal defect structure

(Photo source: provided by the author)

If these defects cannot be discovered and controlled in time, the safety performance of equipment or devices may be reduced or even cause accidents.

For example, one of the reasons for the recent Titan submarine accident abroad was the failure to detect defects in the submarine hull in time. These defects are difficult to detect with conventional testing methods, like an enemy hiding in the shadows.

So, how to solve this problem?

A clever trick: use light to "illuminate" shadows!

Scientists came up with a clever way: using synchrotron radiation X-rays as a light source to illuminate the shadows in carbon fibers.

The three-dimensional information of defects is recorded and characterized by the scattering method. This method is different from traditional photography technology, which uses the particle effect of light.

When a beam of X-rays hits the sample, the photons interact with the electrons of the carbon fiber, just like two small balls colliding. After the collision, the angle and other parameters change, which are captured by the detector and form a certain scattering pattern. Due to the difference in electron density between the defective part and the regular part, the three-dimensional information of the defect structure will be reflected in the scattering pattern captured by the detector.

By analyzing these scattering data, scientists can obtain detailed information about the internal structure of the sample, such as the defect size, shape and distribution of the carbon fiber. Since synchrotron radiation has the characteristics of high brightness (flux is 1010 times that of the sun) and high collimation, as well as the penetration characteristics of X-rays, it can be characterized quickly without destroying the sample. A single spectrum can be obtained within one second, so the evolution of the carbon fiber defect structure can be studied online. Therefore, it is also widely used in the study of soft matter, polymers, nanomaterials and biomacromolecules, and is an important tool for exploring the microscopic world of matter.

It is with the help of this method that Chinese scientists have conquered the production process of 1,000-ton T1000 carbon fiber and successfully passed the scientific and technological achievement appraisal. This indicates that my country's domestically produced carbon fiber has reached the world's advanced level. This achievement not only broke the foreign technology blockade, but also injected new vitality into the development of my country's industry.

Shenzhen Evening News reported

(Photo source: Shenzhen Evening News)

Conclusion

In the days ahead, we can expect carbon fiber to play an important role in more areas. Using advanced technologies such as synchrotron radiation to illuminate the shadows and discover the defects in carbon fiber will be the key to driving the continuous development of carbon fiber technology.

As Su Shi said, "Looking from the side, you see a mountain range; looking from the front, you see a peak." As long as we are good at observing and thinking from different angles, we will be able to discover the truth hidden in the shadows and make this magical material, carbon fiber, better serve human society.

References:

[1]Zhu C, Liu X, Yu X, et al. A small-angle X-ray scattering study and molecular dynamics simulation of microvoid evolution during the tensile deformation of carbon fibers[J]. Carbon, Elsevier Ltd, 2012, 50(1): 235–243.

[2]Lu J, Li W, Kang H, et al. Microstructure and properties of polyacrylonitrile based carbon fibers[J]. Polymer Testing, Elsevier Ltd, 2020, 81(10): 106267.

[3]Thünemann AF, Ruland W. Microvoids in polyacrylonitrile fibers: a small-angle X-ray scattering study[J]. Macromolecules, 2000, 33(5): 1848–1852.