Carbon Fibers and Nanotubes as a Product of Interaction between Silicon Carbide and Carbonate Melt

In quite a number of works, the corrosion of SiC with molten carbonates was studied, but the products of this interaction were not investigated [1, 2]. In this work, the interaction of silicon, TiC and SiC (F 14 and F 36) powders with Na₂CO₃-K₂CO₃ melt at 750⁰С in Ar atmosphere and in air was studied. After washing, the interaction products were analyzed on a SEM and by Raman spectroscopy. A thermodynamic evaluation of the reactions proceeding in carbonates showed that the Gibbs energy for reactions given below is negative at 1023 K; if follows that the reactions proceed from left to right:

I.    3Si + 2Na₂CO₃ = 2C + 4Na + 3SiO₂

                ΔG^o_{1023 K} = -473 kJ

II. 3SiC + 2Na₂CO₃ = 5C + 4Na + 3SiO₂

                ΔG^o_{1023 K} = -277 kJ                      

III. 3TiC + 2Na₂CO₃ = 5C + 4Na + 3TiO₂

            ΔG^o_{1023 K} = -54 kJ                       

Were silicon interacts with carbonate melts, a large amount of graphite and carbon black is formed, which complicates the process of separation of reaction products. Because titanium oxides are slightly soluble in carbonate melts, the surface of carbon fibers was coated with this reaction product. Silicon oxide are readily soluble in carbonate melts, therefore the most promising method for the fabrication of carbon fibers and nanotubes is interaction between SiC and carbonate melts. When SiC interaction with molten carbonates, both carbon fibers and nanotubes are formed (Fig. 1.). Raman spectroscopy corroborated the presence of carbon fibers in the products.

References

1.   R. E. Dial, G.E. Mangsen,  Corrosion, 17,  35t  (1961).

2. R.E. Tressler, M.D. Meiser, T. Yonushonis,  J. Am. Ceram. Soc., 59,  278 (1976).

Fig. 1. SEM micrograph of SiC (F14) powder after 2 hours interaction with carbonate melts at 750⁰C .