M echanical properties of carbon nanotubes: a fiber digest for beginners

Abstract A condensed review of mechanical properties of carbon nanotubes is given. Theory as well as experiments is examined with a view to extracting the fundamental elements that should allow the reader to build his own perspective of the subject. . Introduction erties and for the clarity of analysis, it appears worthwhile to separate elastic properties from breaking strength. Carbon nanotubes (CNT), like whiskers, are acicular Indeed, establishing the elastic parameters of a solid from single crystals of high aspect ratio which contain only a its microscopic elements (atomic composition and bonds) few defects. It is chiefly this low density of defects that is nowadays a far less difficult task than predicting the way confers excellent mechanical properties to CNT. In addiit may break. The fracture of materials is a complex 2 tion, the intrinsic strength of the carbon-carbon sp bond phenomenon that requires a multiscale description involvis expected to give CNT not less than the highest strength ing microscopic, mesoscopic and macroscopic modeling. and modulus among all existing whiskers. This has been Most studies are based on a macroscopic classical conconfirmed recently by a series of acrobatic experiments tinuum picture that provides an appropriate modeling that are going to be discussed. Despite their early entry in except at the region of failure where a complete atomistic the Hall of Fame of material science, it remains that 10 description (involving bond breaking in real chemical years after their discovery nanotubes are still more in the species) is needed. Classical molecular dynamics simulalaboratories than on the stall. Does it reflect the natural tions using empirical potentials can be used to bridge the time lag between discoveries and their exploitation, or may mesoscopic and microscopic modeling which can help to this stem from more fundamental problems? This paper elucidate several relevant processes at the atomic level. To aims to describe recent developments around mechanical bridge micro-, meso-and macroscales it is desirable to find properties of CNTs, both from a fundamental point of view a hierarchy of models, both for ease of computation and and in the direction of applications, specifically those for conceptual understanding. When models bridging difinterested in reinforcements for composite materials. ferent scales are worked out we will be able to analyze and In the course of a discussion about mechanical propoptimize materials properties at different levels of approximation, eventually leading to the theoretical synthesis of novel materials. Nanotubes offer the possibility of addressing the validity of different macroscopic and microscopi

Résonance ferromagnétique et structure de nanoparticules bimétalliques à base de fer

Cette étude a pour objectif de montrer la pertinence de l utilisation de la résonance ferromagnétique dans l étude du magnétisme des nanoparticules. Les recherches en nanomagnétisme présentent des enjeux fondamentaux et appliqués fortement liés nécessitant une étude minutieuse des nouvelles propriétés qu offrent les nanomatériaux. C est le cas des alliages bimétalliques à base de fer lorsqu ils se présentent en dimension réduite. Parmi les moyens disponibles pour révéler les propriétés magnétiques de nanoparticules, la résonance ferromagnétique, lorsqu elle est mesurée en fonction de la température, présente l avantage d être sensible à la fois à la distribution en taille, à l anisotropie magnétique et à l aimantation des nanoparticules. Parallèlement, des études structurales, s appuyant sur l absorption et la diffraction des rayons X ainsi que la diffraction électronique, ont été menées afin de déterminer leurs compositions chimique et cristalline en fonction de la température. L utilisation conjuguée de ces techniques a permis de mettre clairement en évidence l apparition au sein des nanoparticules d un couplage d échange entre un cœur bimétallique à très forte anisotropie magnétocristalline, et une coquille formée d un oxyde de fer magnétique à température de Curie élevée.ORLEANS-BU Sciences (452342104) / SudocSudocFranceF

Multi-MHz micro-electro-mechanical sensors for atomic force microscopy

International audienceSilicon ring-shaped micro-electro-mechanical resonators have been fabricated and used as probes for dynamic atomic force microscopy (AFM) experiments. They offer resonance frequency above 10 MHz, which is notably greater than that of usual cantilevers and quartz-based AFM probes. On-chip electrical actuation and readout of the tip oscillation are obtained by means of built-in capacitive transducers. Displacement and force resolutions have been determined from noise analysis at 1.5 fm/√Hz and 0.4 pN/√Hz, respectively. Despite the high effective stiffness of the probes, the tip-surface interaction force is kept below 1 nN by using vibration amplitude significantly below 100 pm and setpoint close to the free vibration conditions. Imaging capabilities in amplitude- and frequency-modulation AFM modes have been demonstrated on block copolymer surfaces. Z-spectroscopy experiments revealed that the tip is vibrating in permanent contact with the viscoelastic material, with a pinned contact line. Results are compared to those obtained with commercial AFM cantilevers driven at large amplitudes (>10 nm)

Fermi level engineering for large permittivity in BaTiO3-based multilayers

Multilayered doped BaTiO3 thin films have been fabricated by physical vapor deposition (PVD) on low-cost polycrystalline substrates with the aim to improve dielectric properties by controlling point charge defects at the interfaces. We show that carefully designed interfaces lead to increasing the relative permittivity of the BaTiO3 thin films, in contradiction with the common belief that interfaces behave as dead layers. High relative permittivity up to 1030 and tanδ = 4% at 100 kHz and room temperature were obtained on BaTiO3 multilayered films deposited on Si/Pt substrates by PVD. The large permittivity is suspected to be an extrinsic contribution due to band bending at the interfaces, as inferred by in-situ X-ray photoelectron spectroscopy. A 20-nm depletion layer was found to be associated with an interdiffusion of dopants, as measured by depth profiling with time-of-flight secondary ion mass spectrometry. The films exhibit high permittivity and low dielectric losses stable between 200 and 400 K, which meet the requirement of electronic application

Fermi Level Engineering for Large Permittivity in BaTiO3-Based Multilayers

Multilayered doped BaTiO3 thin films have been fabricated by physical vapor deposition (PVD) on low-cost polycrystalline substrates with the aim to improve dielectric properties by controlling point charge defects at the interfaces. We show that carefully designed interfaces lead to increasing the relative permittivity of the BaTiO3 thin films, in contradiction with the common belief that interfaces behave as dead layers. High relative permittivity up to 1030 and tanδ = 4% at 100 kHz and room temperature were obtained on BaTiO3 multilayered films deposited on Si/Pt substrates by PVD. The large permittivity is suspected to be an extrinsic contribution due to band bending at the interfaces, as inferred by in-situ X-ray photoelectron spectroscopy. A 20-nm depletion layer was found to be associated with an interdiffusion of dopants, as measured by depth profiling with time-of-flight secondary ion mass spectrometry. The films exhibit high permittivity and low dielectric losses stable between 200 and 400 K, which meet the requirement of electronic applications