Jonathon J. Host

Magnetic Properties of Graphitically Encapsulated Nickel Nanocrystals

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.

Effect of annealing on the structure and magnetic properties of graphite encapsulated nickel and cobalt nanocrystals

We report the structure and magnetic studies of carbon coated nanocrystals of nickel and cobalt synthesized in a special low carbon to metal ratio arc chamber. Powder x-ray diffraction profiles show peaks associated with single phase of fcc nickel or cobalt and major peaks of graphite with no evidence of carbides or solid solutions of carbon in the metal. Measured lattice spacing of crystalline particles and that of graphite coating from high-resolution transmission electron microscope images also confirm such findings. Magnetization measurements as a function of temperature in the range 20–900 °C give a Curie temperature equal to that of bulk metal within the experimental error. Upon heating and recooling of the particles a larger magnetization as high as 57% of bulk Co and 53% of bulk Ni was measured. Also M–H hysteresis loop of the particles have been measured at room temperature after annealing in the temperature range 20–650 °C for Ni, and 20–900 °C for Co. The dependence of room temperature saturati...

A descriptive model linking possible formation mechanisms for graphite-encapsulated nanocrystals to processing parameters

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). LCMs select which mechanisms are active along each pathway within the reactor. The GCM places bounds upon and determines the weighting between different LCMs 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.

Graphite encapsulated nanocrystals produced using a low carbon : metal ratio

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.

NANOPHASE NI PARTICLES PRODUCED BY A BLOWN ARC METHOD

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.

SYSTEMATIC STUDY OF GRAPHITE ENCAPSULATED NICKEL NANOCRYSTAL SYNTHESIS WITH FORMATION MECHANISM IMPLICATIONS

By systematically varying the carbon content, chamber pressure, arc current, and blowing gas velocity in a tungsten-arc encapsulation setup, the effects of each of these variables on the encapsulation of nickel in graphite layers were observed. The data from these optimally designed experiments revealed that the properties of the arc translate into changes in the encapsulated product. Specifically, a larger, hotter arc results in more encapsulation in the final sample. These findings, along with evidence of graphite layers which have formed on precrystallized particles, indicate that the graphite layers may form by two sequential formation steps. The first step is the simple phase segregation of carbon from a cooling liquid particle, resulting in surface graphite. The second step is the growth of carbon on a crystallized nickel particle, regardless of the temperature at which this occurs. The proposed formation mechanism has significant implications for both a scientific understanding of the encapsulation phenomena, and possible commercial applications.

The magnetic properties of annealed graphite-coated Ni and Co nanocrystals

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/spl deg/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/sub s/, M/sub r/, and H/sub 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.