Graphite encapsulated nanocrystals produced using a low carbon : metal ratio

Abstract

Graphite encapsulated nanocrystals produced by a low carbon tungsten arc were analyzed to determine their chemistry, crystallography, and nanostructural morphology. Metallic nanocrystals of Fe, Co, and Ni are in the face-centered cubic (fcc) phase, and no trace of the bulk equilibrium phases of body-centered cubic (Fe) and hexagonal close-packed (Co) were found. Various analytical techniques have revealed that the encased nanocrystals are pure metal (some carbide was found in the case of Fe), ferromagnetic, and generally spherical. The nanocrystals are protected by turbostratic graphite, regardless of the size of the nanocrystals. The turbostratic graphite coating is usually made up of between 2 and 10 layers. No trace of any unwanted elements (e.g., oxygen) was found. The low carbon: metal ratio arc technique is a relatively clean process for the production of graphite encapsulated nanocrystals.

Plasma-assisted synthesis of carbon encapsulated magnetic nanoparticles with controlled sizes correlated to smooth variation of magnetic properties

“…This analysis shows that the average size of the γ-Fe crystallites were smaller than the corresponding α-Fe phases, under all pressure conditions. For iron nanoparticles, size dependence of the crystalline phases has been reported by other authors [30,31]. Another recent experiment on synthesis of CEMN by laser pyrolysis had an almost similar observation, where γ-Fe was the preferred structure for smaller particles [18].…”

Using diamond as a metastable phase carbon source to facilitate the synthesis of graphite encapsulated metal (GEM) nanoparticles by an arc-discharge method

“…To date, only the two-step encapsulation model [4,5] can be used to explain most experimental results. The first step of "phase separation" explains why the turbostratic graphitic shells enclosed a spherical metal nanoparticle.…”

“…The model predicts that these metals will help amorphous carbon crystallizing into graphite when it is at a high enough temperature. For more detail, please refer to Host et al [4] and Elliott et al [5].…”

“…Graphite encapsulated metal (GEM) nanocrystals are composite materials with a distinctive spherical core-and-shell structure (5-50 nm in diameter) [1][2][3][4][5][6][7]. The core is metal nanocrystal and the shell comprises of several to over 10 layers of graphitic sheets.…”

“…Through the protection of the outer graphitic shell, the inner metal nanoparticles are stable in strong acid, even at high temperatures if in an oxygen-free environment. Among many types of GEM with various core materials, those with ferromagnetic metal cores are particularly interesting, not only because of their ferromagnetic properties, but also because the typical ferromagnetic metals Fe, Co, and Ni have a unique catalyzing ability to transform amorphous carbon into graphite [4,5].…”