http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

High-density nickel–dispersed-alumina (Al2O3/nickel) composites with superior mechanical properties were obtained by the hydrogen reduction and the hot pressing of alumina–nickel oxide (Al2O3/NiO) mixed powders. The mixtures were prepared by using NiO or nickel nitrate (Ni(NO3)2·nH2O) as a dispersion source of nickel metal. Microstructural investigations of the composite fabricated using nitrate powder revealed that fine nickel particles, } 100 nm in diameter, dispersed homogeneously at the matrix grain boundaries, forming the intergranular nanocomposite. High strength (.1 GPa) and high-temperature hardness were registered for the composite that contained a small amount of nickel dispersion. The ferromagnetic properties of nickel, such as high coercive force, were observed, because of the fine magnetic dispersions, which indicates a functional value of structural composites.

Reduction and Sintering of a Nickel–Dispersed‐Alumina Composite and Its Properties

The reduction in H2/CH4 atmosphere of aluminum-iron oxides produces metal particles small enough to catalyze the formation of single-walled carbon nanotubes. Several experiments have been made using the same temperature profile and changing only the maximum temperature (800-1070 °C). Characterizations of the catalyst materials are performed using notably 57Fe Mo¨ssbauer spectroscopy. Electron microscopy and a macroscopical method are used to characterize the nanotubes. The nature of the iron species (Fe3+, R-Fe, c-Fe-C, Fe3C) is correlated to their location in the material. The nature of the particles responsible for the high-temperature formation of the nanotubes is probably an Fe-C alloy which is, however, found as Fe3C by postreaction analysis. Increasing the reduction temperature increases the reduction yield and thus favors the formation of surface-metal particles, thus producing more nanotubes. The obtained carbon nanotubes are mostly single-walled and double-walled with an average diameter close to 2.5 nm. Several formation mechanisms are thought to be active. In particular, it is shown that the second wall can grow inside the first one but that subsequent ones are formed outside. It is also possible that under given experimental conditions, the smallest (<2 nm) catalyst particles preferentially produce double-walled rather than single-walled carbon nanotubes.

/pdf/a-study-of-the-formation-of-single-and-double-walled-carbon-2bmkdge70c.pdf

A study of the formation of single- and double-walled carbon nanotubes by a CVD method.

https://shell.cas.usf.edu/~phanm/ActaMaterialia-2013-published.pdf

Mechanical and magnetocaloric properties of Gd-based amorphous microwires fabricated by melt-extraction

Oxides based on a-alumina and containing various amounts of Fe (2, 5, 10, 15 and 20 cat.%) were prepared by decomposition and calcination of the corresponding mixed-oxalates. Selective reduction of the oxides in a H2-CH4 atmosphere produces nanometric Fe particles which are active for the in-situ nucleation and growth of carbon nanotubes. These form bundles smaller than 100 nm in diameter and several tens of micrometers long. However, the carbon nanotubes-Fe-Al2O3 nanocomposite powders may also contain Fe carbide nanoparticles as well as undesirable thick, short carbon tubes and thick graphene layers covering the Fe/Fe carbide nanoparticles. The influence of the Fe content and the reduction temperature on the composition and micro/nanostructure of the nanocomposite powders have been investigated with the aim of improving both the quantity of nanotubes and the quality of carbon, i. e. a smaller average tube diameter and/or more carbon in tubular form. A higher quantity of carbon nanotubes is obtained using a-Al1.8Fe0.2O3 as starting compound, i. e. the maximum Fe concentration (10 cat.%) allowing to retain the monophase solid solution. A further increase in Fe content provokes a phase partitioning and the formation of a Fe2O3-rich phase which upon reduction produces too large Fe particles. The best carbon quality is obtained with only 5 cat.% Fe (a-Al1.9Fe0.1O3), probably because the surface Fe nanoparticles formed upon reduction are a bit smaller than those formed from a-Al1.8Fe0.2O3, thereby allowing the formation of carbon nanotubes of a smaller diameter. For a given Fe content (≤ 10 cat.%), increasing the reduction temperature favours the quantity of nanotubes because of a higher CH4 sursaturation level in the gas atmosphere, but also provokes a decrease in carbon quality.

https://hal.archives-ouvertes.fr/hal-00959264/document

Carbon nanotubes–Fe–alumina nanocomposites. Part I: influence of the Fe content on the synthesis of powders

The influence of various parameters controlling the chemical vapour deposition of carbon-boron-nitrogen mixtures has been studied using the experimental designs methodology which provides a valuable phenomenological approach for the initial study of complicated chemical systems. The gaseous precursors used were acetylene (C2H2), boron trichloride (BCl3), and ammonia (NH3). An extended experimental field was investigated, which leads to a very wide composition range for the deposits. All the deposits may be described as hexagonal single phase, including the C-BN-“BC3” ternary domain. In this composition triangle, no thermodynamically stable boron carbides were found.

Parametric study of the chemical vapour deposition of carbon-boron-nitrogen compounds

Iron-alumina nanocomposite powders containing 10 wt % iron were prepared by selective reduction of alumina-haematite solid solutions Microstructural study showed three types of metal dispersion in the alumina matrix according to the elaboration process: iron grains that were >70 nm, most of the iron particles were <10 nm and directly epitaxied in the alumina matrix, and iron particles that were surrounded by an interfacial phase In agreement with transmission electron miscroscopy (TEM) observations, magnetic study confirmed a distribution of the iron particles size, showing the superposition of a ferromagnetic behaviour (larger particles) and a superantiferromagnetic behaviour (smaller particles) Furthermore, analysis of thermoremanent behaviour, coercive field and dissymmetry of hysteresis loops allowed the interfacial phase surrounding some iron particles to be identified as an antiferromagnetic phase, Fe1+xAl2−xO4 Nevertheless, at the interface of metallic iron epitaxied on the alumina matrix some atomic planes always existed where iron was ionic (even if no other phase was detected) As a consequence the mean magnetic moment of iron in these nanocrystals is larger than that in bulk metallic iron

Microstructural and magnetic characterization of alumina-iron nanocomposites

On a series of nanoparticles of iron epitaxed on Al2O3: a new field, temperature and concentration (of Fe) scaling plot of the magnetization curves

/pdf/high-field-magnetization-of-two-types-of-shortly-correlated-1u5vydpc2b.pdf

A. Marchand

Papers

Parametric study of the chemical vapour deposition of carbon-boron-nitrogen compounds

Microstructural and magnetic characterization of alumina-iron nanocomposites

On a series of nanoparticles of iron epitaxed on Al2O3: a new field, temperature and concentration (of Fe) scaling plot of the magnetization curves

High field magnetization of two types of shortly correlated systems : random anisotroy systems and nanoparticles