Unusual Freezing and Melting of Gallium Encapsulated in Carbon Nanotubes

The freezing and melting behavior of gallium (Ga) encapsulated in carbon nanotubes was investigated through in situ observation in a transmission electron microscope. It is shown that Ga remains liquid up to $-80°\mathrm{C}$ when encapsulated in carbon nanotubes. Results of detailed electron diffraction analysis show that the encapsulated Ga can crystallize in either $\beta$ phase or $\gamma$ phase rather than the common $\alpha$ phase upon freezing. Both $\beta$-Ga and $\gamma$-Ga melt at around $-20°\mathrm{C}$. While this is very close to the melting point of bulk $\beta$-Ga ($-16°\mathrm{C}$), it is considerably higher than that of bulk $\gamma$-Ga ($-35.6°\mathrm{C}$). It was observed that upon solidification, Ga has its unique crystallographic orientation relative to the host carbon nanotube.