Nanofluidics in cellular tubes under oscillatory extension

Membrane nanotubes or tethers extruded from cells exhibit dynamic features that are believed to exhibit viscoelastic rheological properties. We have performed typical microrheology experiments on tethers pulled from red blood cells by measuring the force response to small oscillatory extensions or compressions. Our data, supported by a simple theoretical model, show that the force response does not reflect any intrinsic viscoelastic properties of the tethers themselves, but instead is dominated by the drainage of the internal cellular fluid into and out of the oscillating nanoconduit over a frequency-dependent penetration depth. The simplicity of tether rheology suggests its usage as a probe for measuring the local viscosity of the cytosol near the plasma membrane

Vertically aligned carbon nanotubes growth on aluminum substrates at low temperature

International audienceEnergy storage is a major challenge for the next years, with the development of renewable energy, mobile devices (smartphones, tablets, computers,...) and electric vehicles. Supercapacitors have a great potential, with performance located between the capacitors and batteries. Their principle is based on rapid ion charge / discharge cycles, using the common activated carbon based electrodes. However, these systems have limitations in terms of energy density and cost which involves the development of new electrode materials. During the last five years, a strong interest has been devoted to electrodes based on vertically aligned carbon nanotubes (VACNT) thanks to their large specific surface area developed, their anisotropy and good electrical conductivity. In this context, our first results obtained in collaboration with the Universities of Cergy Pontoise and Tours have demonstrated a high increase of capacitance due to the implementation, in ionic liquid, of electrodes based on VACNT covered with conducting polymers and standing on Si substrates [1]. The development of ultra-capacitor by NawaTechnologies Company is based on this VACNT-based technology. One of the challenge to penetrate the market is to replace the silicon collector by a conductive and inexpensive substrate. In this context, the aim of this collaborative work is to develop the growth of VACNT on aluminum foil by aerosol assisted Chemical Vapor Deposition (CVD). This method is well controlled on substrates such as stainless steel or quartz for synthesis temperatures between 800 and 850°C [2, 3 and 4]. Taking into account the aluminum melting temperature of about 660°C, the synthesis of VACNT on aluminum requires a significant lowering of VACNT growth temperature. At low temperatures, the decomposition of catalytic and carbon precursors commonly used is insufficient. To overcome this problem, the nature of the gas phase has been changed in terms of both carbonaceous precursor and carrier gas. Indeed, the decomposition of the catalyst precursor at low temperatures and thus the synthesis yield are increased by the addition of hydrogen in the atmosphere [3]. Moreover, in order to limit the decrease in growth rate it is necessary to use precursors with a catalytic and thermal decomposition more favorable around 600 ° C, such as acetylene recently reported for CVD growth on aluminum [5]. Therefore, the approach in this work is first to identify the most relevant synthesis parameters to reach VACNT growth at such a low temperature by varying them and analyzing subsequently the products obtained with SEM, TEM, Raman, ATG, to have information on CNT length, density, diameter,… The first results obtained on pure aluminum, without any surface pretreatment show that growth in VACNT is strongly influenced by the flow of the reactive gas phase, composition and the synthesis temperature as represented on figure 1. Moreover, attention is paid on study of Al surface before growth or during the initial steps of VACNT growth, and of CNT / Al interface with various analysis technics: SEM, TEM, XPS, AFM… in order to understand VACNT formation mechanisms at lower temperatures which has a direct link towards the optimization of VACNT synthesis process

Growth of vertically aligned carbon nanotubes on aluminium substrate at low temperature through a one-step thermal CVD process

International audienceThis study addresses the vertically aligned carbon nanotubes (VACNT) growth on specific substrates by thermal aerosol assisted CCVD at low temperature (LT). This one-step continuous process is based on the simultaneous injection of catalytic and carbon precursors into a reactor to form in-situ catalytic particles leading to the VACNT growth. It has initially been developed at high temperature (800-850 °C) [Pinault et al 2005, Castro et al 2013] and is easily scalable. Recently it has been adapted to grow VACNT on Al foils to fabricate ultracapacitor electrodes, requesting a lower process temperature [Nassoy thesis 2018]. According to our previous work, hydrogen adjunction in the gas phase promotes the catalyst precursor decomposition at LT. Using acetylene as a carbon precursor is more favorable for decomposition at LT. Recent results enabled to obtain clean, long and dense VACNT at LT with growth rates at the best level of state of the art for multi-step assisted CVD. However, a decrease in growth rate and a catalytic particle poisoning are observed for long time synthesis, inducing a carpet height limitation. The main goal is to strengthen our understanding of VACNT growth at LT and to identify mechanisms involved, in order to have a better control of the growth process

Growth of vertically aligned carbon nanotubes on aluminum foils

International audienceForests of vertically aligned carbon nanotubes (VACNTs) are attractive nanomaterials because of their unique structural, electrical and thermal properties. However, many applications require their growth on metallic substrates. Catalytic chemical vapor deposition (CCVD) is the best method to grow them but the catalytic particles can diffuse rapidly into the metal subsurface and thus become inactive. In this communication, I will address this issue through the recent results obtained in our laboratory. I will show how it is possible to grow VACNT on carbon fibers, stainless steel and aluminum surfaces by a single-step process, namely the aerosol assisted CCVD, where the catalyst and carbon precursors are injected simultaneously. In the case of aluminum, due to its low melting temperature, the synthesis of VACNT requires a significant reduction in the growth temperature as compared to conventional substrates. Our results show that, with our single-step process, it is possible to obtain clean, long and dense VACNTs, with a growth rate at the best state of the art level for such a low temperature. A particular attention has been paid to the study of the CNT/Al interface. The results suggest the crucial role of the interface for an efficient and reproducible VACNT growth. Finally, I will show that the aerosol-assisted CCVD process can be scaled-up to enable the fabrication of innovative ultracapacitors based on VACNTs grown on aluminum foils

Croissance en une seule étape de nanotubes de carbone vericalement alignés sur des feuilles d'aluminium

International audienceLes tapis de nanotubes de carbone verticalement alignés (VACNT) sont des matériaux aux propriétés structurales, électriques et thermiques très intéressantes pour de nombreuses applications. La méthode de choix pour la synthèse de VACNT de haute qualité est le dépôt chimique en phase vapeur catalytique (CCVD). Cette étude porte sur la croissance de tapis de VACNT sur des substrats d'intérêt par CCVD d'aérosols à basse température. Cette méthode, développée au sein du laboratoire Edifices Nanométriques (LEDNA) consiste à injecter simultanément dans le réacteur un précurseur catalytique et un précurseur carboné de manière à générer in-situ la formation des particules catalytiques à l'origine de la croissance des VACNT. Cette méthode est un procédé de synthèse en continu, en une seule étape, simple, peu coûteux et transposable à grande échelle. Elle a été jusqu'alors développée surtout à haute température (800 à 850°C) [1-3] et récemment elle a été ajustée à la croissance sur aluminium, pour la fabrication d'électrodes de supercondensateurs, qui imposait des températures plus basses de l'ordre de 600°C [4,5]. En se basant sur des travaux antérieurs, notre procédé a été modifié par l'adjonction d'hydrogène en phase gazeuse pour favoriser la décomposition du précurseur catalytique (ferrocène) à basse température [2], et par le remplacement du précurseur carboné liquide (toluène) par l'acétylène, facile à décomposer à basse température [6]. Les résultats récents mettent en évidence une croissance de nanotubes alignés et denses à ces faibles températures avec des vitesses de croissance qui sont comparables à celles obtenues dans l'état de l'art pour des méthodes en deux étapes et assistées (plasma [7] ou filaments chauds [8]). Toutefois, une diminution de la vitesse de croissance en fonction de la durée de synthèse se traduisant par une limitation de la hauteur des tapis de VACNT a été observée [9,10]. Dans ce contexte, l'objectif principal est d'approfondir notre compréhension de la croissance des VACNT spécifiquement à basse température et d'identifier les mécanismes mis en jeu. L'enjeu étant un meilleur contrôle du procédé de croissance dans ces conditions où les phénomènes physico-chimiques à l'oeuvre peuvent être modifiés ou ralentis

Vertically Aligned Carbon Nanotube Growth on Aluminium Substrate at Low Temperature

International audienceThis communication addresses the growth of VACNT on aluminium by a single-step process (simultaneously injecting catalyser and carbon source precursors), namely the thermal aerosol assisted CCVD. The aim is to get a scalable process to fabricate ultracapacitor electrodes exhibiting a great potential thanks to VACNT specific surface area, anisotropy and good electrical conductivity. Considering the aluminium melting temperature (c.a. 660°C), the synthesis of VACNT on such substrates requires a significant reduction in the growth temperature as compared to conventional substrates [1-3]. According to our previous work, when hydrogen is added in the gas phase, the decomposition of the catalyst precursor will be more efficient at low temperature [2]. Moreover, the use of acetylene as carbon source is more favourable for a decomposition at low temperature [4]. Our approach is first to identify the most relevant synthesis parameters to reach VACNT growth at such a low temperature by analysing the VACNT properties such as CNT length, density, diameter, … This optimization study, involving no surface pre-treatment of aluminium substrate, shows clean, long and dense VACNTs (Fig.1 A,B,C), with a growth rate (ca. 5µm/min) on par with the state of art the state of the art. Secondly, the objective is to understand VACNT growth mechanisms on Al substrate occurring at low temperature in order to optimise the VACNT synthesis process. Indeed, attention is paid on the Al surface prior and past the initial step of VACNT growth, and on the CNT/Al interface, with various analysis technics: SEM, TEM, EDX, XPS, GDOES … The results of the chemical analysis of the interface by STEM/EDX (Fig.2 D) exhibit clearly identifiable catalytic particles located at the CNT base within a well-defined oxide interface layer suggesting the crucial role of the interface for an efficient and reproducible VACNT growth

Growth of vertically aligned carbon nanotubes on aluminium substrate through a one-step thermal CVD process

International audienceThe aim is to grow vertically aligned carbon nanotubes (VACNT) on aluminium by a single-step process, namely the thermal aerosol assisted CCVD, in order to get a scalable process to fabricate ultracapacitor electrodes. The one-step synthesis of VACNT on such substrates requires a significant reduction in the growth temperature as compared to conventional substrates . According to previous work, our process is based on the adjunction of hydrogen in the gas phase to promote the decomposition of the catalyst precursor at low temperature, and of acetylene, easy to decompose at low temperature . Our approach is first to identify the most relevant synthesis parameters to reach VACNT growth on aluminium substrates by subsequently analysing the product features. This optimization study enabled to obtain clean, long and dense VACNTs at 580°C or 620°C with a growth rate (ca. 5$\mu$m/min) at the best level of the state of the art. Attention is paid on the study of VACNT thickness variation versus synthesis duration showing a limitation of CNT length from a certain time of synthesis. In order to understand this phenomenon, the trend in CNT length is examined according to different models reported in the literature, which gives evidence of a catalyst deactivation phenomenon occurring during the one step-CVD process performed at low temperature due to the additional formation of disordered carbon. In addition, the CNT/Al interface was analysed, enabling to clearly identify catalytic particles located at the CNT base and on the surface of a well-defined oxide interface layer suggesting the availability of catalyst nanoparticles for VACNT growth

Giant Vesicles under Flows: Extrusion and Retraction of Tubes

International audienc