Graphitically encapsulated ferromagnetic Ni nanocrystals have been synthesized via a modified tungsten arc-discharge method. By virtue of the protective graphitic coating, these nanocrystals are stable against environmental degradation, including extended exposure to strong acids. The magnetic properties of the encapsulated particles are characterized with regard to the nanoscale nature of the particles and the influence of the graphitic coating which is believed to be benign insofar as the intrinsic magnetic properties of the encapsulated nanocrystals are concerned. The Curie temperature of graphitically encapsulated Ni nanocrystals is the same as that of microcrystalline Ni. However, saturation magnetization, remanent magnetization, and coercivity of these particles are reduced, for a range of temperatures. The unique features are compared with those of unencapsulated nanocrystalline and coarse microcrystalline nickel particles.

Magnetic studies of graphite-coated nanocrystals of nickel and cobalt (5 - 50 nm) synthesized in a special fullerene are chamber are reported. Magnetization measurements as a function of temperature in the range 20 - 900 degrees C indicate the ferromagnetic nature of the nanocrystals, Upon heating and recooling of the nanocrystals a larger magnetization was measured. The dependence of room temperature M-s, M-r, and H-c of the nanocrystals on annealing temperature is reported. The data are consistent with increase in size of single domain particles, or transition of the nanocrystals from single domain to multi-domain due to particle size growth in the annealing process.

New and modified mechanisms are proposed to account for detailed observations of carbon encapsulation of Fe, Ni, and Co nanocrystals. The mechanisms are based on aerosol and gas phase chemistry and on the catalytic effects of transition metals. Two parameters are found to qualitatively dominate production: the local-path carbon-to-metal ratio (LCM) and the global carbon-to-metal ratio (GCM). LCM’s select which mechanisms are active along each pathway within the reactor. The GCM places bounds upon and determines the weighting between different LCM’s and thus determines the distribution of different nanoscale products within the collected, macroscopic product. A two part processing parameter → mechanism → product map links the components. The generality of the model is discussed throughout with reference to related processes and the encapsulation of other materials.

Small particles are of interest for magnetic study due to their range of magnetic properties with particle size and unusual magnetic properties. In this study we produced graphite encapsulated magnetic cobalt particles and measured their structural and magnetic properties both before and after annealing at 550°C for 6 hours. No large change in the structural properties (particle size, shape, and lattice parameter) was observed as a result of the annealing. There were, however, significant changes in the magnetic properties. The coercivity was found to decrease at all temperatures, especially at temperatures below 100° K. The possible reasons for this reduction in coercivity are examined, with the change in the shape of the particles appearing most likely.

Nanophase Ni particles (<10 nm in diameter) were produced by a blown arc method. A helium gas stream directed at the arc reduces the Ni vapor concentration and increases the quench rate. The helium gas velocity is the predominant factor influencing the size of the Ni particles. Gas velocities of 20 m/s and 56 m/s (at 26.6 kPa total helium pressure) resulted in Ni particle sizes of 13 nm and 7 nm, respectively.

Since it was discovered that crystalline materials could be encased in graphite shells, protection in this manner has been the focus of significant research. Previously, production of graphite encapsulated crystalline nanoparticles has been reported using an arc discharge between an anode stuffed with a metal oxide and a graphite cathode, resulting in a mixture of carbides, encapsulated carbides and carbonaceous debris. More recently, an arc discharge between metal electrodes has been used to produce large quantities of pure metal nanoparticles, with the addition of graphite to the anode resulting in the large scale production of graphite encapsulated metal nanoparticles (GEMN). In earlier studies, these materials were separated from the other products of the arc discharge only by a magnetic gradient, which does not remove the non-encapsulated crystalline particles. Therefore, the immersion of the product mixture in an acid bath was added as a subsequent processing step. The combined arc discharge/acid bath technique reduces both production and separation problems, producing a large quantity of Fe, Co, and Ni GEMN free from any non-encapsulated metal particles. Subsequent characterization of the resulting material has led to a better understanding of the GEMN produced.

Nanophase Ni particles (<10 nm in diameter) were produced by a blown arc method.A helium gas stream directed at the arc reduces the Ni vapor concentration and increases the quench rate.The helium gas velocity is the predominant factor influencing the size of the Ni particles.Gas velocities of 20 m/s and 56 m/s (at 26.6 kPa total helium pressure) resulted in Ni particle sizes of 13 nm and 7 nm, respectively.