https://www.andrew.cmu.edu/user/dl0p/laughlin/pdf/PMS.pdf

Amorphous and nanocrystalline materials for applications as soft magnets

Following their discovery1, carbon nanotubes have attracted interest not only for their unusual electrical and mechanical properties, but also because their hollow interior can serve as a nanometre-sized capillary2,3,4,5,6,7, mould8,9,10,11 or template12,13,14 in material fabrication. The ability to encapsulate a material in a nanotube also offers new possibilities for investigating dimensionally confined phase transitions15. Particularly intriguing is the conjecture16 that matter within the narrow confines of a carbon nanotube might exhibit a solid–liquid critical point17 beyond which the distinction between solid and liquid phases disappears. This unusual feature, which cannot occur in bulk material, would allow for the direct and continuous transformation of liquid matter into a solid. Here we report simulations of the behaviour of water encapsulated in carbon nanotubes that suggest the existence of a variety of new ice phases not seen in bulk ice, and of a solid–liquid critical point. Using carbon nanotubes with diameters ranging from 1.1 nm to 1.4 nm and applied axial pressures of 50 MPa to 500 MPa, we find that water can exhibit a first-order freezing transition to hexagonal and heptagonal ice nanotubes, and a continuous phase transformation into solid-like square or pentagonal ice nanotubes.

/pdf/formation-of-ordered-ice-nanotubes-inside-carbon-nanotubes-2b0c91xwu8.pdf

Formation of ordered ice nanotubes inside carbon nanotubes.

Synthesis of nanoparticles in the gas phase for electronic, optical and magnetic applications—a review

This review focuses on the synthesis, protection, functionalization, characterization and with some applications of magnetic nanoparticles (MNPs) The review begins with an overview on magnetic property and single domain particles The synthetic strategies developed for the generation of MNPs, with a focus on particle formation mechanism and recent modifications made on the synthesis of monodisperse samples of relatively large quantities are also discussed Then, different methodologies for the protection and functionalization of the synthesized MNPs, together with the characterization techniques are explained Finally, some of the recent industrial, biological, environmental and analyticals application of MNPs are briefly reviewed, and some future trends and perspectives in these research areas will be outlined

Magnetic nanoparticles: Synthesis, stabilization, functionalization, characterization, and applications

The synthesis of nickel nanoparticles by the reduction of nickel chloride with hydrazine in the cationic water-in-oil microemulsions of water/CTAB (cetyltrimethylammonium bromide)/n-hexanol at 73 °C has been studied. By the analyses of electron diffraction pattern and X-ray diffraction, the resultant particles were characterized to be the pure nickel crystalline with a face-centered cubic (fcc) structure. The investigation on the composition of microemulsion solution indicated that the average diameter of nickel nanoparticles was affected mainly by the ratio of CTAB to n-hexanol instead of the size of microemulsion droplets. Smaller particles could be obtained at higher ratios of CTAB to n-hexanol. At a constant nickel chloride concentration, the average diameter of nickel nanoparticles decreased with the increase of hydrazine concentration and then approached to a constant value when the concentration ratio of hydrazine to nickel chloride was above 10. At a sufficient high hydrazine concentration, the av...

/pdf/synthesis-of-nickel-nanoparticles-in-water-in-oil-4r45pl3m5f.pdf

Synthesis of Nickel Nanoparticles in Water-in-Oil Microemulsions

/pdf/controlled-size-nanocapsules-11053gjv18.pdf

Controlled-size nanocapsules

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 properties of graphitically encapsulated nickel 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 saturation magnetization, remanent magnetization, and coercive field of the particles on annealing temperature is reported. These data are described by transition of particles form single domain to multidomain as a result of particle growth due to annealing. We also present the particle size distribution measurements that show log-normal behavior, and indicate substantial particle size growth due to annealing.

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

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.

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

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.

J. J. Host

Papers

Controlled-size nanocapsules

Magnetic properties of graphitically encapsulated nickel nanocrystals

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

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

Graphite encapsulated nanocrystals produced using a low carbon: Metal ratio