Evolution of dispersion of carbon nanotubes in Polyamide 11 matrix composites as determined by DC conductivity

Double-walled Carbon NanoTubes (DWCNTs) have been dispersed in a Polyamide 11 (PA11) matrix by two routes: in the solvent way, Polyamide 11 was first dissolved in its solvent to ensure a liquid state dispersion of carbon nanotubes by ultrasonic way; in the melt mixing way, an optimization of the extrusion parameters, such as mixing time, mixing speed, mixing temperature and screw rotation direction allow to reach satisfactory dispersion. Dispersion and percolation threshold have been compared thanks to the evolution of DC conductivity with carbon nanotubes weight fraction in Polyamide 11. An electrical percolation threshold of 1% in weight was found by the solvent way while the melt mixing way offers one of the lower percolation thresholds for a semi-crystalline matrix (0.93% in weight). An interpretation of the limitation of the electrical percolation threshold value in a semi-crystalline matrix will be proposed

Dissipation mechanisms identification of soft hollow particle-dampers in honeycomb structures for micro-vibrations environment

Particle dampers are enclosures partially filled with metallic or glass small spheres, attached to the vibrating structure. This paper deals with replacing hard classical particles by soft hollow ones to maximize damping and mass ratio. Hence, one aspect of this damping method is obtained by mixing the kinetic energy conversion of the structure into heat(frictional losses and collisions) and the elastic energy conversion into heat (visco-elastic deformation). This study is oriented toward experimental and theoretical investigations in order to distinguish the dissipation phenomena. The experimental approach first relies on identification and, then, on validation applied on composite aluminum honeycomb plates. Indeed, equivalent viscous damping is identified on small honeycomb samples; then cantilever honeycomb beams are filled with particles and studied. Theoretically, beyond the nonlinear dissipation by impact and friction, these particles add a visco-elastic behavior. The shapes of the hysteretic loops highlight that this behavior is predominant. Hence, oscillators are added in the FE model and permit to consider the effect of the particles. These kinds of particle dampers are highly nonlinear as a function of excitation frequency and amplitudes. The aim of this study is to provide a structural damping solution for space applications which require high pointing stability to enhance mission performances. In this perspective, damping of micro-vibrations was thought as a possible application; nevertheless it is shown that best efficiency is achieved in high frequency range

Discontinuity of physical properties of carbon nanotube/polymer composites at the percolation threshold

Evolution of DC conductivity with double-walled carbon nanotube concentration in a polyamide 11 matrix displays the electrical percolation phenomenon. The percolation threshold is observed for this composite at 0.85 wt.%. Thermal, mechanical and dielectric manifestations of the glass transition have been investigated as a function of carbon nanotube content. Thermal and mechanical analyses have shown a depression of the glass transition temperature or its mechanical manifestation at the percolation threshold. Regarding the dielectric modulus, three distinct behaviors (below, at, and above percolation threshold) have been observed. A discussion about the interactions between the polymer matrix and the conductive fillers will be proposed to further the understanding of the discontinuity of physical properties at the percolation threshold

New hybrid polymer nanocomposites for passive vibration damping by incorporation of carbon nanotubes and lead zirconate titanate particles

A new hybrid nanocomposite for vibration damping has been elaborated. Ferroelectric lead zirconate titanate particles and carbon nanotubes are dispersed simultaneously in an engineering semi-crystalline thermoplastic matrix by an extrusion processing. Ferroelectric particles have been made piezoelectric once incorporated into the polymer matrix through a poling step. The dynamic response of nanocomposites has been characterized by dynamic mechanical analysis and vibration test. The shear mechanical modulus exhibits an increase of the conservative and dissipative components after the poling step of nanocomposites. By vibration test, the first bending mode of the frequency response function has been followed and a significant damping inherent to poling is also recorded. These evolutions are heightened by the use of two constrained elastic layers. For the first time, a synergy between poled piezoelectric particles responsible for the transduction phenomena and conductive particles allowing a local dissipation of electric charges has been revealed by two complementary techniques for the improvement of the polymer damping

Integrated piezoelectric function in a high thermostable thermoplastic PZT/PEEK composite

A piezoelectric structural material has been developed. Lead Zirconate Titanate (PZT) submicronic nanoparticles have been dispersed in a thermostable high performance thermoplastic polymer Poly(Ether Ether Ketone) i.e. PEEK to ensure piezoelectric properties. The inorganic particles with a mean diameter of 900 nmare polycrystalline as highlighted by HRTEM with a grain diameter estimated at 15 nm. XRD patterns have shown that the crystalline structure is rhombohedral i.e. ferroelectric. The PZT/PEEK composites have been elaborated by extrusion which allows reaching a satisfactory dispersion of particles even at high volume fraction (30% in volume). One of the challenges was to find poling conditions compatible with the thermal stability of the matrix. Indeed, this composite must be poled above the polymer glass transition temperature to improve matching of dielectric permittivity between inorganic and organic phases. The influence of the poling electric field on the final piezoelectric activity of the composite has also been studied to better understand the role of the polymer matrix. Finally, after a poling step, the PZT/PEEK composite exhibits a piezoelectric strain coefficient which can be exploited over a wide temperature range

Dynamique du déploiement autonome d\u27un hexapode à rubans pour applications spatiales

Les programmes spatiaux ont des exigences de résolution de plus en plus importantes que les télescopes monolithiques ne peuvent atteindre sans dépasser les limites de taille des lanceurs. Thalès Alénia Space a le projet de concevoir un télescope déployable dont la position du miroir secondaire serait corrigée selon six degrés de liberté. Le principe repose sur un hexapode utilisant successivement deux architectures complémentaires : une plate-forme de Gough-Stewart pour le déploiement et une structure "Poignet actif" pour la correction de l\u27assiette finale de la plate-forme supérieure supportant le miroir secondaire. L\u27innovation réside dans l\u27utilisation de six actionneurs originaux pour le déploiement des jambes de l\u27hexapode. Une lame métallique incurvée, appelée lame de Carpentier ou ruban, est enroulée autour d\u27une bobine. La libération de l\u27énergie de déformation provoque le déroulement des actionneurs à rubans et le déploiement autonome de la plate-forme. Un premier modèle de l\u27hexapode déployé a été élaboré afin d\u27évaluer sa raideur structurelle dans sa configuration opérationnelle. Validé par une analyse modale expérimentale, le modèle permet d\u27analyser les capacités de correction de la plate-forme supérieure grâce au concept de "Poignet actif". La caractérisation expérimentale de l\u27actionneur à ruban fait apparaître un comportement hystérétique qui peut être approché par un modèle de force de restitution. Celui-ci est utilisé dans les deux modèles de déploiement proposés confrontés aux mesures issues du déploiement en gravité compensée du prototype. La recherche effectuée et les outils développés aboutissent à des analyses et recommandations sur le déploiement

Dynamique du déploiement autonome d'un héxapode à rubans pour applications spatiales

Les programmes spatiaux ont des exigences de résolution de plus en plus importantes que les télescopes monolithiques ne peuvent atteindre sans dépasser les limites de taille des lanceurs. Thalès Alénia Space a le projet de concevoir un télescope déployable dont la position du miroir secondaire serait corrigée selon six degrés de liberté. Le principe repose sur un hexapode utilisant successivement deux architectures complémentaires : une plate-forme de Gough-Stewart pour le déploiement et une structure "Poignet actif" pour la correction de l'assiette finale de la plate-forme supérieure supportant le miroir secondaire. L'innovation réside dans l'utilisation de six actionneurs originaux pour le déploiement des jambes de l'hexapode. Une lame métallique incurvée, appelée lame de Carpentier ou ruban, est enroulée autour d'une bobine. La libération de l'énergie de déformation provoque le déroulement des actionneurs à rubans et le déploiement autonome de la plate-forme. Un premier modèle de l'hexapode déployé a été élaboré afin d'évaluer sa raideur structurelle dans sa configuration opérationnelle. Validé par une analyse modale expérimentale, le modèle permet d analyser les capacités de correction de la plate-forme supérieure grâce au concept de "Poignet actif". La caractérisation expérimentale de l'actionneur à ruban fait apparaître un comportement hystérétique qui peut être approché par un modèle de force de restitution. Celui-ci est utilisé dans les deux modèles de déploiement proposés confrontés aux mesures issues du déploiement en gravité compensée du prototype. La recherche effectuée et les outils développés aboutissent à des analyses et recommandations sur le déploiement.Large space-borne programs have resolution requirements more and more important that monolithic telescopes can t achieve in the size limits of the launchers. Thus, Thalès Alenia Space wants to design a deployable telescope whom secondary mirror position would be corrected along six degrees of freedom. The principle lies on a hexapod using successively two architectures : a Gough-Stewart platform to achieve the deployment and a "active wrist" concept to correct the final position of the platform supporting the secondary mirror. Second order errors will be corrected by using adaptive optics. Innovation lies in the fact that six tape-spring actuators are used to deploy the hexapod legs. They are based on the principle of carpenter s tape measures. A tape-spring actuator is composed of a thin curved strip coiled on a rotating drum and guided by spiral grooves. The release of the tape-spring strain energies allows the autonomous deployment of the platform. A model of the deployed hexapod has been first performed in order to investigate the structural stiffness in its working configuration. The tape-spring flexibility is modelled by using a condensed finite element model and take into account the deformed and pre-stressed geometry. Validated by an experimental modal analysis, the model is considered as linear and elastic to investigate the platform correction capabilities with the active wrist concept. The influence of the upper joint stiffness is highlighted. The deployment prediction needs to model the tape-spring actuators and to solve the direct dynamic model of a parallel structure. The experimental identification of the tapespring actuator highlights a hysteretic behaviour that can be described by a restoring force model. This model is useful for the elaborating two deployment models which are compared to the experimental results of a gravity compensated deployment. Developed tools allow several analyses. The influence of the micro-vibrations made by the satellite equipments in orbit is observed on the deployment. A sensitivity analysis of the deployment evaluates acceptable tolerances on the tape-spring actuators. The modal evolution of the hexapod during the deployment indicates the swept natural frequency range. The detection of possible singularities during the deployment is carried on with uncertain parameters.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF

Screening tests for enhanced shielding against hypervelocity particle impacts for future unmanned spacecraft

Protection of components of unmanned spacecraft against particle impacts is typically provided by the spacecraft’s structure together with the intrinsic protection capabilities of the components themselves. Thus to increase the survivability of future spacecraft, one option is to enhance the protection already provided using enhanced materials and additional shielding. As part of the EU funded FP7 research project ReVuS (“Reducing the Vulnerability of Space systems”), the configurations of equipment typically found on board unmanned spacecraft were identified. For each of those configurations, potential solutions have been identified which enhance the robustness against particle impacts. The solutions are broken down into a number of shielding components that include e.g. additional protective layers made from aluminum, Kevlar, Nextel, stainless steel mesh and ceramics. To evaluate the characteristics and performances of these shielding components, a number of screening hypervelocity impact tests were performed. During these tests, representative configurations have been subjected to impacts of aluminum spheres of 3 mm and 5 mm diameter at a nominal impact velocity of 7 km/s. This paper describes the targets and presents and compares the results