Polyacrylonitrile fiber is a fiber spun from a copolymer of acrylonitrile and other second and third monomers. Polyacrylonitrile fiber has a fluffy texture, soft hand feel, low density, good warmth retention, high resilience, excellent light resistance, insect resistance, biodegradation resistance and good dyeability.
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High electrical conductivity materials tend to exhibit high thermal conductivity. However, scientists have also found many materials that have both low thermal conductivity and high electrical conductivity. For example, the porous material carbon aerogel has both ultra-high electrical conductivity and ultra-low thermal conductivity.
Materials with low thermal conductivity and high electrical conductivity are very important in fields such as energy conversion, miniaturized electronics, and high-temperature fuel cells. These application scenarios often require these materials to be easy to process, foldable, and non-flammable. Some researchers have reported an electrospun polyacrylonitrile material with extremely low thermal conductivity and high electrical conductivity. It uses carbon as the matrix, silicon-based ceramics as the nano-inclusion body, and has a sea-island nanostructure. Among them, the carbon phase modulates electron transport to achieve high electrical conductivity, while the ceramic phase induces phonon scattering through boundary scattering to achieve low thermal conductivity.
The preparation process of this composite nonwoven material is divided into three steps: first, polyacrylonitrile and different contents of oligosilazane are mixed, and the primary product is obtained through an electrospinning process. The average fiber diameter is 900±70 nm;
Subsequently, it is heated and annealed at 250 °C in an air atmosphere to achieve stabilization; finally, it is carbonized and ceramicized at 1000 °C in a nitrogen atmosphere. The fiber diameter is reduced to 470±50 nm, and a flexible and foldable nonwoven material is obtained (C/ SiCON).
Subsequently, the researchers discussed the impact of ceramics on the thermal and electrical conductivity of nonwoven materials. As the polysilazane (SiCON) content increases, the thermal diffusivity shows a clear downward trend, especially the cross-plane thermal diffusivity. When SiCON is 50% of the mass fraction of the polyacrylonitrile precursor, the nonwoven material has the lowest in-plane thermal conductivity (32±12 mW/m.K) and cross-sectional thermal conductivity (10±0.1 mW/m.K) and a cross-plane thermal conductivity (19.8±7.8 mW/m.K), which are approximate to the thermal conductivity of air (26 mW/m.K).
Of course, as the SiCON content increases, the conductivity decreases slightly. However, the ceramic phase does not prevent electrons from being transmitted through the carbon material phases in the fiber. Not only can the fiber light up a light-emitting diode, but after 5,000 cycles of bending and folding, the resistance of the material remains almost unchanged.
There is also no visual change in the brightness of the LEDs. In addition, the electrical conductivity of nonwoven materials only increased slightly from −50 °C to 300 °C, indicating that the material's operating temperature range is quite wide.
Compared with pure carbon fiber, the carbon material phase in C/SiCON fiber has a higher degree of disorder and lower graphite content, which makes the electron transmission path more complex and easily causes additional scattering, resulting in a decrease in electrical and thermal conductivity.
The characterization results show that the SiCON phase is evenly distributed in the carbon phase, and the carbon phase and SiCON phase are evenly distributed in the fiber, forming a sea-island nanostructure. This uniformly dispersed structure prevents the ceramic phase from significantly increasing the resistance, but effectively increases the boundary scattering of phonons, resulting in a reduction in the heat transfer efficiency of phonons. This explains that C/SiCON nonwoven materials have both high electrical conductivity and low thermal conductivity.
In addition, C/SiCON nonwoven materials also have very good thermal stability and flame retardancy. Pure carbon nonwoven materials are easily ignited, while C/SiCON will not burn or deform in a 100% O2 atmosphere. This may be attributed to the fact that after the surface carbon is burned out, the SiOCN ceramic phase forms a passivated silicon dioxide layer, which can effectively protect the fiber.
