Have you ever seen glass cups, but have you ever seen

Wood is one of the earliest materials used by humans. For a long period of history, wood has dominated the field of building materials. Although wood has many advantages such as low cost, safety, environmental protection, easy processing, and renewability, it is gradually being replaced by glass due to its lack of transparency. The modern glass manufacturing industry is a high energy consuming and high polluting industry. Glass is widely used in the construction industry, and due to its poor thermal insulation, about 30% of the cooling and heating energy is consumed through glass windows. In addition, glass has disadvantages such as high price, easy breakage, and difficulty in processing, and people have been struggling to find alternatives to glass. How to turn wood into a "transparent" material has been a magical dream of humanity for thousands of years.

Associate Professor Zheng Rongbo's team has made significant progress in the field of "transparent wood" research by using chemical modification methods to turn ordinary wood into transparent wood. Speaking of this magical process, Teacher Zheng introduced that transparent wood is a new material that combines aesthetics and practicality, and has a promising application prospect. After processing, the wood that becomes transparent is still wood in structure, and even stronger than wood, harder than wood. At the same time, transparent wood is more easily degradable than glass and plastic, and also has unique optical properties such as high haze and high anisotropy that are not commonly found in transparent materials.

Making ordinary wood transparent is not magic, but science

Why is ordinary wood opaque? The first is the presence of lignin, which absorbs light. Secondly, the porosity of wood is very high, with many micropores inside. The refractive index of wood is relatively high, about 0.5 more than that of air. For example, ordinary paper is opaque due to the presence of micro porous structures inside. If the paper is soaked in oil, it will become transparent.

Scientists at home and abroad have been tirelessly exploring how to make ordinary wood transparent. As early as 1992, German scientist Siegfried Fink first proposed the research plan for "transparent wood". They extracted the "lignin" from wood and filled it with a polymer that matches the refractive index of wood cellulose, suppressing light scattering and absorption, thereby improving the transparency of wood and used it for microstructure testing of wood.

In 2016, Professor Lars Berglund, Director of the Wallenberg Centre for Wood Science at the Royal Institute of Technology in Sweden, first manufactured structural "transparent wood" in the laboratory and conducted functional research. In the same year, a research team led by Chinese scientist Hu Liangbing from the University of Maryland in the United States achieved a thickness similar to glass for transparent wood. The transparency of the prepared transparent wood reached 80% to 90%, and its mechanical strength was much stronger than that of the original wood. Its thermal insulation was 3 to 4 times that of glass. After being reported by domestic media, there has been a research craze for transparent wood in China.

The bottleneck restricting the industrialization development of "transparent wood"

Although scientists at home and abroad have successfully produced "transparent wood", they are still only making laboratory samples of small size and millimeter thickness. At present, there are two main challenges that urgently need to be addressed in this research field: firstly, in the first step of the "delignification" process, boiling bleaching solutions containing chlorine and sulfur are commonly used. During this process, the wood cellulose skeleton is easily broken in boiling bleach, making it impossible to prepare large-sized, ultra thick transparent wood. Secondly, the bleaching solution used for delignification is prone to environmental pollution due to the presence of toxic and harmful elements such as chlorine and sulfur, making subsequent environmental treatment very challenging. Therefore, finding green processes for industrialized manufacturing of large-sized and ultra thick transparent wood is a bottleneck problem that urgently needs to be solved in this research field.

Inspired by Chinese Steamed dumplings, successfully solved world-class problems

When the research trend of "transparent wood" was just emerging abroad, Professor Zheng organized his research team members to closely track the literature and conduct research around the direction of "large-sized, ultra thick transparent wood". Firstly, the research team has developed a green, environmentally friendly, and pollution-free hydrogen peroxide bleaching solution through experimental screening, which solves the problem of bleaching solution pollution caused by chlorine and sulfur. Secondly, using boiling solution method to remove lignin can easily cause the wood fiber skeleton to break. To solve this problem, inspired by the processing of Steamed dumplings in China, Mr. Zheng separated the wood from boiling hydrogen peroxide solution through grids. With the super permeability of hydrogen peroxide vapor, the residual lignin in the wood was controlled below 0.8%, which was only half of the delignification by hot boiling with bleach solution, and still maintained the integrity of the wood cellulose skeleton. Due to the steam delignification process, rich micro nano porous structures are generated inside the wood while removing lignin. These micro - and nano scale pores are conducive to the infiltration and filling of polymer materials. By densely filling with high polymers, light absorption is effectively suppressed, interface gaps are eliminated, transparency and mechanical strength are improved, and finally transparent wooden bricks with a thickness of up to 5 centimeters are prepared.

The achievements have been highly discussed and appreciated by international peers

After Professor Zheng's article was published in the prestigious journal "Journal of Materials Research" in the field of materials science, it has been highly praised and discussed by international peers. The "News" column of the American Society for Materials Science's MRS Bulletin has written a special article introducing this achievement (Steaming technique makes wood transparent). The American Physical Society website "Inside Science" also reported on this in the form of news (New technique improvements transparent wood). According to the Altmetric scoring criteria, this achievement received a high score of 14, ranking 13th out of 1398 articles in the same journal and in the top 10% out of 12.7 million articles worldwide.

Lars Berglund, a professor at the Royal Institute of Technology in Sweden and director of the Wallenberg Centre for Wood Science (the first scientist to create structurally transparent wood), commented that "The work is truly novel in that steam based approach has not been used before and it may offer real advantages" (the steam evaporation method used to prepare transparent wood is reported for the first time and has obvious advantages), giving high praise to this achievement.

In the future, transparent wood will become the "new darling" of these fields

Transparent wood, as a new type of functional material, has a very broad application prospect. Compared with untreated wood, the mechanical strength of transparent wood has increased by 3 to 6 times; Compared with ordinary glass, it has higher insulation and allows more light to enter; Compared to ordinary plastics, it is green and renewable. In short, transparent wood has a series of advantages such as simple preparation process, easy availability of raw materials, high light transmittance, high haze, excellent toughness, light weight, and low thermal conductivity. If it can be mass-produced on a large scale, it can have a wide range of application prospects in energy-saving building materials, home materials, automotive glass, solar panels, and electronic devices. Teacher Zheng said that the new technology he developed for manufacturing large-sized, ultra thick transparent wood is green and harmless. Based on its thermal conductivity of only one-third of glass, it is in line with national energy conservation and emission reduction, and has a very broad industrial prospect. During the interview, Teacher Zheng showed us a simple house model built with transparent wood. The simple model house built with ordinary wood is very dark, while the model built with two centimeter thick transparent wood on all three walls has excellent lighting. Whether it is sunny or cloudy, as long as there is sunrise, the indoor brightness is two to three times that of ordinary household fluorescent tubes, which can achieve the effect of basically no indoor lighting during the day. The "transparent wooden brick" thickened to five centimeters can also achieve the expected brightness after testing.

Continue to work hard and bridge the 'last mile' of achievement transformation

China is a major consumer of wood, and the application prospects of transparent wood are very broad. At present, large-sized and transparent wood is still in the stage of laboratory achievements, and there is still a long way to go to achieve true industrialization. The next step for the research group is to collaborate with relevant enterprises and take the path of school enterprise cooperation to quickly industrialize this technological achievement, take root in enterprises, and pave the way for industrialization, so as to bring transparent wood, a new material, to the benefit of humanity as soon as possible.