After being baked at 1200 degrees, you can still hold it with your hands! What kind of magic is this?

When getting hot water, have you ever thought about this question: Why is it so hot when using a glass cup to get hot water, but a thermos cup can keep the water warm without getting hot?

Not only that, there is also a magical material - thermal insulation tile, which can be directly held by hand after being heated to 1200 degrees. Why are these items so different?

Image source: a video website

01. Why is the glass cup hot but the thermos cup is not?

Glass is hot because glass is a good conductor of heat . The microstructure of glass is relatively tight. Although the internal silicon-oxygen tetrahedron and other structural units are not arranged regularly like crystals, they can form a continuous network structure, and heat can spread quickly along this network structure. So when hot water comes into contact with the cup, the heat will quickly conduct through the wall of the cup, and we will feel it is very hot when we touch the cup.

Image source: Photo Network

The thermos cup is neither too hot nor too cold but can keep things warm, mainly due to its unique structural design and material selection.

The vacuum flask is mainly composed of an inner liner, an outer shell and a vacuum layer in the middle. The vacuum insulation layer in the middle is the key to the thermos flask's heat preservation and non-scalding properties. Vacuum is a spatial state where no material particles (molecules, atoms, etc.) exist. Since there are no material particles, heat cannot be transferred in the vacuum layer by heat conduction.

Thermal radiation is another way of heat transfer. It does not require a medium and can be transmitted in the form of electromagnetic waves. The inner liner of a thermos bottle is usually made of metal materials such as stainless steel, and the outer shell is generally also made of metal. There will be some coating or coating on the surface of the inner liner and the outer shell. This special material surface treatment can further reduce the transfer of thermal radiation , so we don’t feel it is very hot when we touch the outer shell of the thermos bottle [1].

Image source: Reference [1]

02. Why can “thermal insulation tiles” still be held directly by hand after being baked at 1200 degrees?

Thermal insulation tiles are made of high-temperature resistant materials such as ceramics and fibers, used to block heat transfer . They are usually made of some special ceramic materials or composite materials. For example, ceramic-based composite materials use ceramics as the matrix, and then add some reinforcement phases to it. These reinforcement phases can be fibers, such as carbon fibers. Ceramics themselves have certain high-temperature resistance and heat insulation properties. After adding carbon fibers, the material properties can be further improved. Carbon fibers have high strength, low density and good thermal conductivity. When combined with ceramics, they can prevent heat conduction in the ceramics to a certain extent [2].

Image source: Tencent

From the perspective of heat conduction, there is an interface between the ceramic matrix and the reinforcement phase in the thermal insulation tile. This interface can scatter the phonons of heat propagation (a quantized form of heat propagation in a solid), so that the heat is continuously scattered when it is conducted inside the material, thereby reducing the thermal conductivity of the material. At the same time, due to the complex microstructure of the material, the heat propagation path in it becomes tortuous , not as direct as in ordinary materials, which also reduces the heat transfer efficiency.

Image source: Reference [3]

In terms of thermal radiation, the surface of the thermal insulation tile may be specially treated, such as being coated with some low-emissivity coating. This coating can reflect most of the thermal radiation and reduce the heat absorbed by the thermal insulation tile itself. In addition, the material structure inside the thermal insulation tile also has a certain absorption and scattering effect on thermal radiation, further reducing the heat transferred by thermal radiation [3-4].

Image source: a video website

03. What are the applications of “thermal insulation tiles”?

Thermal insulation tiles play an irreplaceable role in many fields with their magical thermal insulation properties. For example, in the field of aerospace, when a spacecraft returns to the Earth's atmosphere, due to high-speed friction, the surface of the spacecraft will generate extremely high temperatures (up to 1650℃ or even higher). The thermal insulation tiles on the surface of the spacecraft can isolate the high temperature from the inside of the spacecraft , ensuring that the internal temperature remains within a suitable range, protecting the instruments and equipment inside the spacecraft and the safety of astronauts.

Image source: NASA

In some high-temperature industrial equipment, such as metallurgical furnaces, glass melting furnaces, etc., thermal insulation tiles can be used for heat insulation of furnace walls . By installing thermal insulation tiles on the furnace wall, heat loss can be reduced, fuel consumption can be reduced, and energy utilization efficiency can be improved. At the same time, thermal insulation tiles can also reduce the impact of high temperature radiation on workers and surrounding equipment, and improve the working environment.

Image source: CITIC Heavy Industries

Thermal insulation tiles also have certain applications around high-temperature components such as automobile engines. Automobile engines generate a lot of heat when working. If this heat cannot be effectively controlled, it will affect the performance and life of the engine. Thermal insulation tiles can isolate the heat generated by the engine from other parts of the car, which not only helps to improve the thermal efficiency of the engine, but also prevents heat from being transferred to the car's cockpit, improving driving comfort.

With the continuous development of science and technology, the performance of thermal insulation tiles will continue to improve and its application areas will continue to expand.

References

[1] Song Shiqi. A thermos cup and its manufacturing method: CN201010545440.7[P]. 2011-06-22.

[2] Wang Kangtai, Feng Jian, Jiang Yonggang, et al. Research progress of ceramic fiber rigid insulation tiles[J]. Materials Review, 2011, 25(23): 35-39.

[3] Lan Zhidan, Ren Weimin, An Nan, et al. Research progress of ceramic fiber thermal insulation tiles and their high emissivity coatings[J]. Bulletin of the Chinese Ceramic Society, 2023, 42(12): 4465-4474.

[4] Li Yizhen, Zhu Shizhen, Wang Hui, et al. Preparation and properties of high-emission ablation-resistant and oxidation-resistant coating on the surface of carbon-based ceramic insulation tiles[J]. Rare Metal Materials and Engineering, 2020, 49(2): 533-539.

Author: Xu Yan, PhD candidate in Materials Science at Fudan University

Planning & Editing: Ding Dong

Acknowledgements: Professor Ma Ruguang from the School of Materials Science and Engineering at Suzhou University of Science and Technology provided scientific guidance for this article

This project was supported by the Shanghai Science and Technology Commission's Science Popularization Project (Project No.: 24DZ2304000).