Abstract
Neutron-irradiated graphite has attracted attention as a useful material for creating new carbon phases. Recently, neutron-irradiated highly oriented pyrolytic graphite (n-HOPG) has been shown to transform into transparent amorphous diamond fragments under multiple processes of shock compression and rapid quenching [1]. Also, it has been reported that the n-HOPG can be transformed into nano-polycrystalline diamond [2] and compressed graphite [3], by applying high-pressure and high-temperature treatments. Both the structures are stable at room temperature and ambient pressure. However, the structure and the formation mechanism, especially the compressed graphite, has not yet been clarified.
The purpose of this study is to clarify the effect of irradiation defects on structural changes in n-HOPG under high pressure and high temperature by utilizing in-situ XRD and TEM-EELS to elucidate the formation mechanism of stable compressed graphite.
From the in-situ XRD experiments performed at SPring-8, it was found that graphite G(002) peak is represented by two components for the irradiated samples, whereas only one component for the un-irradiated samples. Under in-situ experiment, a recovery of irradiation defects was also suggested at around 600℃. On the other hand, the compressed graphite was also synthesized by high-pressure and high-temperature treatments of n-HOPG and un-irradiated HOPG at 15 GPa and 1500℃.
The structural and electronic properties of the specimen were subsequently characterized by TEM-EELS. Fig. 1 suggests the formation of compressed graphite and the presence of a slight sp2-σ* component, unlike graphite.
Reference
[1] K. Niwase et al., Phys. Rev. Lett. 102, 116803 (2009).
[2] M. Terasawa et al., Diam. Relat. Mater. 82, 132 (2018).
[3] K. Niwase et al., J. Appl. Phys. 123, 161577 (2018).
