Cellulose-Based Carbon Fibers: Enhanced Orientability by Boric Acid During Carbonization

In the present paper, a scalable and continuous process with maximum treatment temperatures of 2000 °C for making cellulose-based carbon fibers (CFs) having Young's moduli of up to 230 GPa is presented. This unexpected high modulus was realized by using a boric acid (BA)-doped viscose precursor yarn. Such a precursor shows significantly improved orientability during carbonization, resulting in highly oriented CFs. For clarifying the underlying effect, a BA-doped and an undoped precursor (reference) were carbonized at different stretch levels, and the resulting CFs were systematically analyzed in terms of structural parameters characterizing the crystalline phase, i.e., crystallite dimensions (La, Lc), lattice plane spacing (d002), and crystallite orientation (⟨cos2ϕ⟩). Moreover, electrical resistivity and mechanical properties were determined. It was found that BA promotes the formation of graphite-like structures and their alignment with the fiber axis. Finally, both effects result in significantly improved CF properties, particularly electrical conductivity and Young's modulus, which are nearly three times and two times higher, respectively, than those of the reference. To explain the effectiveness of BA during thermal conversion, a microstructural mechanism is proposed based on results from a uniform stress model.