Abel Rousset

Specific surface area of carbon nanotubes and bundles of carbon nanotubes

The theoretical external specific surface area of single- and multi-walled carbon nanotubes and of carbon nanotube bundles is calculated as a function of their characteristics (diameter, number of walls, number of nanotubes in a bundle). The results are reported in diagrams and tables useful to correlate the microscopic characteristics and the specific surface area of samples. The calculated values are in good agreement with the microscopic characteristics and the specific surface area measurements which have been previously reported in the literature. The specific surface area is a macroscopic parameter which can be helpful to adjust the synthesis conditions of carbon nanotubes.

CARBON NANOTUBE-METAL-OXIDE NANOCOMPOSITES: MICROSTRUCTURE, ELECTRICAL CONDUCTIVITY AND MECHANICAL PROPERTIES

Carbon nanotube–metal–oxide composites (metal=Fe, Co or Fe/Co alloy; oxide=Al2O3, MgO or MgAl2O4) have been prepared by hot-pressing the corresponding composite powders, in which the carbon nanotubes, mostly single or double-walled, are very homogeneously dispersed between the metal–oxide grains. For the sake of comparison, ceramic and metal–oxide nanocomposites have also been prepared. The microstructure of the specimens has been studied and discussed in relation to the nature of the matrix, the electrical conductivity, the fracture strength and the fracture toughness. The carbon nanotube–metal–oxide composites are electrical conductors owing to the percolation of the carbon nanotubes.

Carbon nanotubes in novel ceramic matrix nanocomposites

Novel carbon nanotubes-metal-ceramic nanocomposite powders and dense materials have been prepared and their micro-structure and mechanical properties have been investigated. After a brief review on the structure, synthesis and physical properties of carbon nanotubes, we describe an original catalytic method that produces ceramic-matrix composite powders that contain in situ grown nanotubes. The synthesis parameters that favour the obtention of very high quantities of nanotubes are discussed. The quality of the nanotubes is also addressed. The microstructure and mechanical properties of the materials prepared by hot-pressing of these powders are presented. The in¯uence of carbon nanotubes in such composites is discussed in view of potential applications.

Carbon nanotubes-Fe-alumina nanocomposites. Part II : Microstructure and mechanical properties of the hot-pressed composites

Carbon nanotubes-Fe-Al2O3 massive composites have been prepared by hot-pressing the corresponding composite powders, in which the carbon nanotubes are arranged in bundles smaller than 100 nm in diameter and several tens of micrometers long, forming a web-like network around the Fe-Al2O3 grains. In the powders, the quantity and the quality of the carbon nanotubes both depend on the Fe content (2, 5, 10, 15 and 20 wt%) and on the reduction temperature (900 or 1000°C) used for the preparation. Bundles of carbon nanotubes are present in the hot-pressed materials but with a decrease in quantity in comparison to the powders. This phenomenon appear to be less pronounced for the powders containing higher-quality carbon, i. e. a higher proportion of nanotubes with respect to the total carbon content. The presence of carbon as nanotubes and others species (Fe carbides, thick and short tubes, graphene layers) in the powders modifies the microstructure of the hot-pressed specimens in comparison to that of similar carbon-free nanocomposites : the densifications are lower, the matrix grains and the intergranular metal particles are smaller. The fracture strength of most carbon nanotubes-Fe-Al2O3 composites is only marginally higher than that of Al2O3 and are generally markedly lower than those of the carbon-free Fe-Al2O3 composites. The fracture toughness values are lower than or similar to that of Al2O3. However, SEM observations of composite fractures indicate that the nanotubes bundles, which are very flexible, could dissipate some fracture energy.

Carbon nanotubes grown in situ by a novel catalytic method

Carbon nanotubes can be produced by the catalytic decomposition of hydrocarbons on small metal particles. However, nanotubes are generally produced together with non-tubular filaments and tubes coated by pyrolytic carbon. We propose a novel catalyst method for the in situ production, in a composite powder, of a huge amount of single- and multiwalled carbon nanotubes, having a diameter between 1.5 and 15 nm and arranged in bundles up to 100 mm long. We anticipate that dense materials prepared from such composite powders could have interesting mechanical and physical properties.

Aligned carbon nanotubes in ceramic-matrix nanocomposites prepared by high-temperature extrusion

Carbon nanotube (CNT)–metal-oxide nanocomposites are extruded at high temperatures. The superplastic forming is made easier by the CNTs. It is possible to align the CNTs in ceramic-matrix nanocomposites, which are bulk materials rather than fibers or thin films. The CNTs withstand the extreme shear stresses occurring during the extrusion. In addition to electron microscopy revealing the alignment, the materials show an anisotropy of the electrical conductivity, which could be adjusted by controlling the amount of CNTs.

High specific surface area carbon nanotubes from catalytic chemical vapor deposition process

A carbon nanotube specimen with a carbon content of 83 wt.% (95 vol.%) and a specific surface area equal to 790 m2/g (corresponding to 948 m2/g of carbon) is prepared by a catalytic chemical vapor deposition method. The nanotubes, 90% of which are single- and double-walled, are individual rather than in bundles. High-resolution electron microscopy shows a diameter distribution in the range 0.8–5 nm and Raman spectroscopy shows a high proportion of tubular carbon. Both techniques reveal a maximum in the inner wall diameter distribution close to 1.2 nm.

Powder synthesis of nanocrystalline ZrO2–8%Y2O3 via a polymerization route

Abstract A Pechini process has been used for preparation of yttria-stabilized zirconia gels and powders. Thermal behavior of gel and relationships between processing parameters and structure and microstructure of the powders have been determine to use this route for the preparation of thin film of YSZ. The thermal behavior of the gel has been studied by both thermogravimetric (DTA/TGA) and mass spectrometry analyses. The decomposition progress is in several steps and is based on a thermally induced anionic redox reaction. The transformation from amorphous powder (the gel) into a crystallized homogeneous oxide phase corresponds to an endothermic and two exothermic peaks in the DTA curve; the first one at 168°C is related to the evaporation of excess of ethylene glycol, the second peak at 300°C corresponds to a superposition of two reactions : the decomposition of some organics and the crystallization of YSZ. The last exothermic peaks correspond to the pyrolysis of organics residuals. A series of yttria-stabilized zirconia powders with different agglomerate structures have been synthesized by altering the acid citric–ethylene glycol ratio (CA/EG). The gel with CA/EG = 2.4 yields nanostructured powders at 325°C that is less porous and agglomerate with an average of 10–20 nm primary particles. Its specific surface area is around 55 m 2 /g.

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

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.

Influence of crystallite size on the oxidation kinetics of magnetite

Abstract The oxidation of magnetite yields the lacunar phase γ-Fe 2 O 3 , for sizes less than 5000 A and the rhombohedral phase, α-Fe 2 O 3 , for sizes above 10 000 A. For intermediate sizes, oxidation kinetics and X-ray analysis have confirmed that the γ-Fe 2 O 3 phase forms at the beginning of the reaction, followed by phase α-Fe 2 O 3 forming from γ-Fe 2 O 3 and then directly from the still-unoxidized magnetite. Influence of size could be accounted for in terms of structure and stresses at the crystal lattice level.