Use of a nanoindentation fatigue test to characterize the ductile-brittle transition

When considering grinding of minerals, scaling effect induces competition between plastic deformation and fracture in brittle solids. The competition can be sketched by a critical size of the material, which characterizes the ductile-brittle transition. A first approach using Vickers indentation gives a good approximation of the critical size through an extrapolation from the macroscopic to the microscopic scales. Nanoindentation tests confirm this experimental value. According to the grain size compared to the indent size, it can reasonably be said that the mode of damage is deformation-induced intragranular microfracture. This technique also enables to perform cyclic indentations to examine calcite fatigue resistance. Repeated loadings with a nanoindenter on CaCO3 polycrystalline samples produce cumulative mechanical damage. It is also shown that the transition between ductile and brittle behaviour depends on the number of indentation cycles. The ductile domain can be reduced when the material is exposed to a fatigue process.Comment: Journal of European Ceramic Society accept\'e pour publication (2008) sous-press

Adhesion forces due to nano-triboelectrification between similar materials

Contact electrification and triboelectrification are well-known in the case of dissimilar materials, however the case of charge exchange during friction between nominally identical insulating materials is less documented. We experimentally investigated the triboelectrification between two smooth monocrystalline α-Al 2O 3 (sapphire) antagonists by surface force measurements with a Surface Force Apparatus (SFA). The force between a sphere and a plane, both in sapphire, was measured as a function of the sphere-plane distance D, before and after nano-friction tests, under dry argon atmosphere. Respective contributions of van der Waals, water meniscus and electrostatic forces were determined. The estimated Hamaker constant was in good agreement with the Lifshitz theory, and the dominant meniscus attraction at low separation could be overcome with small radius sphere. We demonstrated that electrostatic forces were generated by the nano-friction test and we quantified the adhesion that results from this contact-electrification. In the first stage of the unloading process, the short range electrostatic force was found to vary both with time and distance D. Experimental results were correlated with surface densities of mobile charges on the two surfaces, and the time-dependence was related to classical surface transport phenomena on alumina surfaces

Applications of multi-walled carbon nanotube in electronic packaging

Thermal management of integrated circuit chip is an increasing important challenge faced today. Heat dissipation of the chip is generally achieved through the die attach material and solders. With the temperature gradients in these materials, high thermo-mechanical stress will be developed in them, and thus they must also be mechanically strong so as to provide a good mechanical support to the chip. The use of multi-walled carbon nanotube to enhance the thermal conductivity, and the mechanical strength of die attach epoxy and Pb-free solder is demonstrated in this work

Effets de la génération, de l'injection et du piégeage des charges électriques sur les propriétés des isolants

The results presented in this report are first mainly related to the study of the generation, the injection, the transport, and the trapping of electrical charges in insulators. The aim of this work is to understand the mechanisms of creation (or injection), propagation and storage of the electrical charges thanks to the "Mirror" (SEMM) and Induced Current (ICM) Methods performed in a Scanning Electron Microscope. Secondly the influence of these electrical charges on various properties such as the friction, the adhesion, or the breakdown strength of the insulating samples has been put in evidence. The methods of charging characterization have been coupled with other experimental set-ups such as Surface Force Apparatus, and the appropriate models and simulations have been developed to interpret the experimental results.Les travaux de recherche exposés dans ce mémoire concernent principalement l'étude des phénomènes de génération, de transport et de piégeage des charges électriques dans les isolants, ainsi que leur influence sur les propriétés (frottement, adhésion, tenue au claquage...) de ces matériaux. Le but est d'accéder à la compréhension des mécanismes d'apparition, de propagation, de stockage et de déstockage des charges dans des matériaux divers, par l'intermédiaire d'une méthode de caractérisation originale, "la méthode miroir", par son couplage avec d'autres appareillages (machine à force de surface...) et en développant les modèles ou outils de simulations approprié

Carbon nanotube-based composites and production and applications thereof

Title composite material is composed of 0.01-99 wt.% carbon nanotubes, 0-99.9 wt.% polymer (P1) selected from poly(alkyl acrylate), polystyrene, polyvinyl chloride, chlorinated polyvinyl chloride, polyvinylidene fluoride, PMMA, polycarbonates, polyamides, polyesters, polylactones, polyepoxides, polyimines, polyphosphazenes, polyolefins, polybutadienes, polyvinyl acetate, polyvinyl alc., polyketone, and polyurethanes, and a block copolymer having ≥1 block bearing ionic or ionizable functions and a block that is compatible with P1. The compatibilizer prepd. by controlled radical polymn. can control and optimize the interfacial interactions and thus to obtain a stable composite material that can be used in paints, coatings, antistatic materials, thermosets, and thermoplastics. Thus, carbon nanotubes and acrylic acid were dispersed in 1,4-dioxane and polymd. using alkoxyamine as initiator, followed by introducing a second monomer, Me acrylate, to provide a block copolymer

Electrical charges and tribology of insulating materials.

International audienceElectrical charges generation occurring during contact and friction of insulating materials has been identified for a long time. However the contribution of these electrical charges to the friction behaviour is usually neglected in the energetical balances. Based on published results and on our own experimental results on the ability of the dielectric materials to trap charges, we show in this study that the interaction energy during friction depends markedly on these trapped charges. Eventually, we propose the complementary use of the "mirror " method and of surface forces measurements to obtain a quantitative evaluation of this contribution. dielectric behaviour / friction / triboelectrification / interfacial interactions / surface free energy / space charge / surface force

Relation between metal electronic structure and morphology of metal compounds inside carbon nanotubes

International audienceSEVERAL attempts have been made to fill carbon nanotubes(1) with metals or metallic compounds to obtain nanocomposite materials with potentially interesting properties. Capillary action, predicted(2) to be a filling mechanism, has been used(3,4) to encapsulate lead and bismuth in open tubes. Compounds of yttrium(5), manganese(6) and gadolinium(7) have also been encapsulated by formation of the nanotubes in an are discharge with the metals present in situ. Very recently, Tsang et al.(8) showed that oxides of nickel, cobalt, iron and uranium can be encapsulated by opening the tubes and depositing the filling material using wet chemical techniques. Here we report a search for general principles relating to the nature and structure of the filling material, using the are-discharge method to fill tubes with fifteen metals and/or their compounds: Ti, Cr, Fe, Co, Ni, Cu, Zn, Mo, Pd, Sn, Ta, W, Gd, Dy and Yb. We find that the propensity for forming continuous 'nanowires' throughout the length of the tubes seems to be strongly correlated with the existence of an incomplete electronic shell in the most stable ionic state of the metal. We also find that the interplay between growth of the nanotube and growth of the filling results, in one case, in the formation of an unusual helical filling morphology

Alumina based ceramics for high-voltage insulation

International audienceDielectric breakdown constitutes an important limitation in the use of insulating materials under high-voltage since it can lead to the local fusion and sublimation of the insulator. The role of electrical charge transport and trapping in alumina ceramics on their resistance to this catastrophic phenomenon is studied in this work. In polycrystalline materials, the interfaces between the various phases play a main role because they constitute potential sites for the trapping of electrical charges. The density and the nature of these interfaces can be controlled by the way of the microstructure parameters. So, the aim of the present paper is to highlight the influence of average grain size and intergranular phase crystallization rate on the ability of polycrystalline alumina materials to resist to dielectric breakdown. Thus, it is shown that the control of the process conditions (sintering aids content, powder grain size and thermal cycle) makes it possible to change not only the density (by the average grain size) but also the nature (by the crystallization or not of anorthite) of the grain boundaries. On one hand, at room temperature a high density of interfaces, due to low grain size and highly crystallized intergranular phase, leads to a high dielectric strength. On the other hand, at higher temperature (250 degrees C), the presence of vitreous intergranular phase makes it possible to delay breakdown. That behaviour is explained thanks to charge transport and trapping characterizations