Thèse sur: Synthèse contrôlée de nanoparticules d'(oxyhydr)oxydes de manganese électro-actifs vis-à-vis du lithium. Synthèse en milieu aqueux, voie "chimie douce". Etude des propriétés électrochimiques des matériaux en tant qu'électrodes positives pour accumulateurs au lithium.
This exhaustive article reviews the synthetic pathways explored up to now for nanoscaled metal borides and phosphides, and provides a comparative discussion of similarities and differences between metal borides and metal phosphides from the point of view of the crystal structure and of the nanomaterials properties. It also provides some trails which would be worthy of investigation in the future.
Nanostructured sub-stoichiometric oxides for energy harnessing
The first air stable thermoelectric nanostructured titanium sub-oxides (nanosized Magnelis’ phases) were fabricated. These compounds were known since the 1950’s as good electrical conductors. They are experiencing a complete renewal of interest due to surprising properties for new information storage devices. Extending this trend to the energy harnessing field in a multidisciplinary approach, the first synthesis of nanostructured Magnéli compounds has been developed by combining the sol-gel process - emerging from materials chemistry - and spark plasma sintering, typical of materials physics. These nanomaterials exhibit improved thermoelectric performances (conversion of temperature gradients into electricity and vice versa) compared to the bulk and are promising for the development of new systems for energy conversion.
Boron-based nano-alloys through innovative processes
The first synthesis of non-oxide functional nanomaterials based on boron has been developed through original methods. Via an innovative ionothermal process performed under relatively mild conditions (low temperature molten salts) with environmentally benign solvents, a versatile route was opened toward non pyrophoric nanocrystals of numerous metal hexaborides, tetraborides, diborides, and lower borides.
Boron nitride is isoelectronic to carbon. As such, it shows strong similarities with solid state chemistry and physics of carbon, but the polarity of the B-N bonds yields for instance specific electric insulation and chemical inertness. Intimately mixing boron nitride with carbon into the ternary B-C-N system could yield exquisite control of the transport properties, but full coverage of the composition map is unachievable at the moment, especially at the nanoscale where graphene-like “BCNs” could emerge. Overcoming this challenge and combining compositional control with nanostructuration could lead to innovative nanostructured “boron carbon nitrides” with unprecedented properties.
Precise control of the materials properties implies mastering the atomic-, nano-, meso- and macro-scales. In the case of nanoparticles, this means tunability of the crystal structure, the particle size and morphology, and the order between the numerous particles into the material. Reaching such a control, especially for compounds of complex compositions, calls for the development of alternative production methods. This research axis aims at exploring new synthesis routes focused on the use of molecular sources and low environmental footprint processes.
While chemical pathways to nanostructured oxides of metals with high oxidation state are the topic of intensive research since three decades, many other compounds families were only scarcely, if ever, reported at the nanoscale. These systems show a the bulk scale mechanical, catalytic, optical and electronic properties without equivalent among common oxides and metals. Nanostructures could lead to important changes or even enhancement of the properties and novel processing possibilities. This is the motivation of this research axis devoted to the synthesis of novel nanostructures of original materials.
Ordering nanoparticles of different compositions, sizes, shapes could yield a significant advance in for further increase in materials complexity. In order to reach this goal, fundamental understanding of the reaction pathways to the final materials is of prime importance. This research axis aims at investigating the nucleation-growth mechanisms in colloidal systems, from the solution to the solid state nanoparticles, and in a second step at exploring new pathways to complex materials with nanoscale heterogeneities.