Pierre Gibot

The effects of moderate thermal treatments under air on LiFePO 4-based nano powders

The thermal behavior under air of LiFePO4-based powders was investigated through the combination of several techniques such as temperature-controlled X-ray diffraction, thermogravimetric analysis and Mossbauer and NMR spectroscopies. The reactivity with air at moderate temperatures depends on the particle size and leads to progressive displacement of Fe from the core structure yielding nano-size Fe2O3 and highly defective, oxidized LixFeyPO4 compositions whose unit-cell volume decreases dramatically when the temperature is raised between 400 and 600 K. The novel LiFePO4-like compositions display new electrochemical reactivity when used as positive electrodes in Li batteries. Several redox phenomena between 3.4 V and 2.7 V vs.Li were discovered and followed by in-situX-ray diffraction, which revealed two distinct solid solution domains associated with highly anisotropic variations of the unit-cell constants.

Study of the structural evolutions of mesoporous MCM-48 silica infiltrated with carbon by different techniques

Abstract The physico-chemical properties of ordered mesoporous carbons obtained by a negative replication process are strongly influenced by their preparations. In order to get a better understanding of the carbon formation, the structural evolution of the mesoporous MCM-48 silica templates before and after C infiltration by two different processes (and a subsequent oxidation), was studied using various techniques such as XRD, N 2 adsorption/desorption and TEM. It was shown that the use of a liquid carbon precursor such as a sucrose solution led to a strong alteration of the silica template (loss of long-range ordering, disappearance of the narrow mesopore size distribution). This was attributed to (i) the high temperature of the process (1173 K) and (ii) to the water vapor released during the carbonization that hydrolysed the poorly hydrothermally stable silica network. On the contrary, the pyrolytic decomposition of a gaseous carbon precursor such as propylene, performed at lower temperature (1023 K) and without water release, only led to minor modifications. These behaviors may influence the physico-chemical properties of the resulting carbon. For both impregnation processes, carbon acts as a highly stable porogene agent. Heat-treatment at a temperature as high as 1473 K in an inert atmosphere and a subsequent oxidation for the removal of the carbon succeeded in preserving the long-range ordering of the starting silica template and its surface area.

Modulation of the Reactivity of a WO3/Al Energetic Material with Graphitized Carbon Black as Additive

Although pyrotechnic performance is fundamental, the strong mechanical and electrostatic intrinsic sensitivities of nanothermite energetic composites represent an obstacle to their development. The addition of a ternary component to the classical binary energetic composite appears to be a promising idea to overcome the problem. A carbon black additive (V3G) was used on a WO3/Al nanothermite. The effect of the pristine and modified carbon particles on the mechanical and electrical sensitivities of the composites was measured together with the pyrotechnic properties. The results show a complete desensitization to friction with a ball-milled carbon when the combustion velocity is slightly reduced.

Nanostructuring of carbon materials by means of a calcium phosphate template

Nanostructured porous carbon powders were elaborated by means of a hard-template approach, using environmentally friendly materials such as sucrose and Ca3(PO4)2 tricalcium phosphate hydrate as carbon and template sources, respectively. While the naturally occurring carbohydrate is widely used for carbon materials synthesis, the tricalcium phosphate was never suggested as template despite its efficiency as shown in the present study. The resulting carbon materials were characterized by elemental analysis, Raman spectroscopy, nitrogen adsorption and electron microscopies. Porous carbon powders with disordered hierarchical porous structure, exhibiting tunable textural properties (specific surface areas, pore volume…), were thus synthesized. For instance, with a template/carbon precursor weight ratio of 1, the specific surface area and pore volume were significantly increased compared to the counterpart which was elaborated without a Ca3(PO4)2 template. The ease of implementation coupled to the low cost of different reagents make the present process potentially competitive for synthesizing porous carbon powders.

Safer and performing energetic materials based on polyaniline-doped nanocomposites

ABSTRACT Nanothermites, combining a fuel with an oxidizer at the nanoscale, represent a class of energetic material that has been attracting increasing attention over the past decade. This intensive interest is due to their tuneable pyrotechnic performance, making the materials promising candidates for ordnance applications. However, the extreme mechanical and electrostatic sensitivities of energetic composites make handling them hazardous. In this study, a realistic desensitization method is suggested via the addition of polyaniline while maintaining an interesting combustion velocity in contrast to the literature values. This investigation claims a major scientific breakthrough in the preparation of safer energetic nanocomposites.