New Material: Silicon Dioxide Composite Fiber With Rabbit Hair Like Single Medullary Cavity Structure Is Heat Insulated And Durable

 

The research team found that the hollow structure in the shape of bamboo can keep a large amount of static air inside the fiber, which is conducive to heat insulation, after simple impregnation, the surface of the rabbit hair like silica nanofiber is attached with a thin layer of polyimide; The polyimide thin layer endows silica/polyimide composite nanofibers with good mechanical properties.

Fur is a secret weapon for mammals to adapt to environmental temperature. Natural fibers, such as rabbit hair, have excellent thermal insulation performance, elasticity and other mechanical properties. Their thermal insulation function mainly comes from their special hollow structure or porous structure.

It is learned from Zhejiang University of Science and Technology that Professor Fu Yaqin's team has prepared silica/polyimide composite nanofibers with rabbit hair like single pulp cavity structure through electrospinning and impregnation processes. Experiments show that the fiber membrane made of this composite fiber overcomes the shortcomings of low tensile strength and poor durability of pure silica fiber membrane, and is light in weight and fireproof, which has a broad application prospect. Relevant research papers were published online in the international journal Composite Materials B: Engineering.

Simulating the Slub Structure of Rabbit Hair with Silicon Dioxide

Observing the rabbit hair with an optical microscope, it can be seen that the inside of the rabbit hair is a regular, bamboo shaped hollow structure, and a large amount of static air is sealed inside the structure. The free movement of static air molecules is limited, making the rabbit hair has a very low thermal conductivity along the radial direction.

"The silica nanofibers obtained by electrospinning have good thermal insulation performance, good stability at high temperature, non-toxic, and cheap raw materials." Si Yinsong, the corresponding author of the paper and associate professor of the School of Materials Science and Engineering of Zhejiang University of Technology, "Based on the previous research, we thought of embedding hollow silica microspheres in hollow silica nanofibers to simulate the internal structure of rabbit hair and further improve the thermal insulation performance, which requires coaxial electrospinning technology."

He explained that the coaxial electrospinning technology is "fine in rough", which is like making a sandwich bar, injecting spinning solution of core layer and shell layer into two coaxial thin tubes with different inner diameters, and the two meet at the end of the nozzle. Under the effect of electric field force, the solvent volatilizes rapidly, and the solute is gradually drawn and stretched into nanofibers.

Polyvinyl alcohol (PVA) is a common raw material in spinning process, and its solution has good adhesion and film forming properties. In this study, the team used PVA solution and silica hollow microspheres as the core spinning solution, and PVA and silica small molecule sol as the shell spinning solution. After coaxial electrospinning, drying and calcination, they made rabbit hair like silica nanofibers with hollow bamboo structure inside.

Silica is one of the components of ceramic fiber. The rabbit hair like silica nanofibers obtained by the team belong to inorganic ceramic nanofibers. Inorganic ceramic nanofibers are considered to be one of the most promising thermal insulation materials in aerospace, construction, industrial pipelines, fire-resistant clothing and other fields due to their light weight, non combustible, fire resistance, corrosion resistance, and excellent thermal insulation properties. However, inorganic ceramic nanofibers are generally brittle, and the tensile strength of their films is usually low, which severely limits their wide application.

"When hollow structure or porous nanoparticles are introduced into inorganic ceramic nanofibers, the fiber membranes prepared by them may be more brittle." Si Yinsong said that how to improve the thermal insulation of inorganic ceramic nanofibers without losing flexibility is the key to this research, which is quite challenging.

Adding "versatile" materials to improve mechanical properties

Polyimide is known as the "versatile" of polymer materials, and has been used in aviation, microelectronics, nano, liquid crystal, separation membrane, laser and other fields. The film made of polyimide is called "golden film".

Si Yinsong said that polyimide has excellent thermal stability, chemical resistance and excellent mechanical properties. Theoretically, the synergetic effect of bamboo hollow structure and polyimide thin layer can overcome the brittleness of inorganic nanofibers without significantly reducing their overall thermal stability and thermal insulation performance.

The research team found that the hollow structure in the shape of bamboo can keep a large amount of static air inside the fiber, which is conducive to heat insulation, after simple impregnation, the surface of the rabbit hair like silica nanofiber is attached with a thin layer of polyimide; The polyimide thin layer endows silica/polyimide composite nanofibers with good mechanical properties.

According to the experimental data, the tensile strength of silica/polyimide composite nanofiber membrane can reach 19.7 MPa, about 10.1 times of that before impregnation; After 20000 dynamic bending cycles, there was no obvious damage; It can withstand at least 500 friction cycles, showing good durability.

"These outstanding characteristics are mainly attributed to the 3D network structure formed in the composite nanofiber film and the impregnation and reinforcement effect of a single silica nanofiber." Si Yinsong said that from a macro perspective, the strength of this composite nanofiber also depends on the interaction between fibers. The coating of polyimide makes the fiber bond better at the junction. When pulled, many fibers are stressed at the same time, which improves the overall strength.

In addition, the thermal conductivity of silica/polyimide composite nanofibers is far lower than that of natural rabbit hair, and the bending stiffness is significantly lower than that of ordinary A4 printing paper.

Si Yinsong said that high precision instruments, aerospace transportation, fire safety and other fields generally need thermal insulation materials with better comprehensive performance, such as heat insulation, light weight, and fire prevention. The silica/polyimide composite nanofibers obtained from this research, which mimic the single pith cavity structure of rabbit hair, have great advantages in these aspects. Next, the team will improve the preparation efficiency of this composite nanofiber, and gradually carry out achievements transformation in combination with specific needs such as reducing energy waste, preventing thermal damage, reducing equipment weight/occupied volume, and improving wear comfort.