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Carbon nanotubes for devices, electrodes and composites

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Industrial scale-up of the electric arc discharge process was the main objective of this work. The aim was to provide the quantities of single-walled carbon nanotubes required for the research activities of each partner in this project. A second objective of equal importance was optimisation of the quality of SWNT s produced by the electric arc discharge reactor and post synthesis purification of SWNTs. Research was focused on finding a suitable purification procedure, which is suitable for easy scale-up to industrial quantities when required. Delivery of SWNTs was the key requirement from Nanoledge in this program. Nanoledge was the leader of this work and the milestones were achieved. Sufficient quantities were delivered to the partners upon request and Nanoledge accommodated the significantly higher quantities required for large-scale purification routes and SWNT/polymer composite preparation by micro-compounding methods. Initial work was based on setting up the reactor to ensure homogeneous and reproducible quality and quantity of SWNTs was obtained. Nanoledge then concentrated its work efforts on two main areas: - Decrease of the production cost - Improvement of purity of SWNTs Various purification methods were investigated that yielded purity exceeding 90 %. However, the main hindrance was quantities in the milligram scale were obtained. The limitation was that these methods were suitable for lab scale and not industrial quantities since a filtration step is involved. Nanoledge found an alternative procedure for obtaining purer SWNTs. That was achieved by careful selection of product from the reactor. It was observed that specific and macroscopically distinguishable areas of the reactor contain statistically more SWNTs than the rest. The material collected from these areas gives the raw product. A purer SWNT product can be delivered by properly selecting parts of the raw product. A purity of 60 to 70wt% can be achieved using this method; around 35 wt% of the raw product being used. For reduction of production cost work was focused on anode preparation since it was the most labour intensive step. Usually, anodes were prepared manually with an extremely time consuming process. A new method was employed which decreased the cost by a factor of five on a lab scale level.
Highly purified cobalt colloids have been employed as a catalyst to grow aligned carbon nanofibres at temperatures as low as 300 °C in DC plasma enhanced chemical vapour deposition systems over large areas. Carbon nanotubes (CNTs) and less crystalline carbon nanofibres (CNFs) are nanoscale building blocks for an increasing number of applications. For many applications, such as electron field emission, fuel cells and super capacitors, it is necessary to grow CNTs and CNFs directly onto an appropriate surface. This implies depositing a catalyst onto a surface prior to the CNT growth. Typically the catalyst (mostly Ni, Co or Fe) is deposited by either thermal evaporation or sputtering. An alternative is to deliver the catalyst from a colloidal solution, as this can be used to cover any surface, independent of size, shape or structure, easily and at low cost. This method is particularly useful for coating complex shapes such as foams, meshes or cloths. Co nanoparticles are synthesised following the inverse micelle method. The nanoparticles can then be dispersed onto a flat or three dimensionally shaped surface and carbon nantoubes can be grown in a DC-plasma enhanced chemical vapour deposition (PECVD) system. We believe that the combination of colloidal catalysts and PECVD will allow the growth of CNTs and CNFs on more complex, sensitive substrates needed for more unusual applications such as electrochemistry and sensors. This result is the first use of colloidal catalysts and PECVD for the low temperature growth of CNFs.
The direct growth of vertically aligned carbon nanotubes onto flexible plastic substrates using plasma-enhanced chemical vapour deposition is reported. Only a few methods allow controlled growth directly on a substrate, which is important for many applications, especially as the individual manipulation of CNTs is difficult and expensive due to their size. Selective, aligned growth of CNTs on silicon and glass substrates has been demonstrated by plasma enhanced chemical vapour deposition (PECVD). However, despite the high level of control, PECVD growth typically involves processing temperatures over 500 °C, which significantly limits the choice of possible substrate materials and integration processes. The controllable growth of nanotubes on plastic substrates would open up many applications such as in fuel cells or field emission devices. We showed that individual lines and dots of freestanding 20–50 nm diameter nanotubes could be grown onto chromium covered commercially available polyimide foil. The scalable deposition method allows large area coverage without degrading or bending the sensitive substrate material. Field emission measurements show a low turn-on field ~3.2 V/micronsand a low threshold field ~4.2 V/microns. The result establishes a method of flexible field emitter fabrication, which is well suited for display production and integration of nanotubes into plastic electronics.
Epoxy composites based on aligned CVD-grown multi-wall carbon nanotubes with weight fractions ranging from as low as 0.001 up to 1 wt% were produced. The resulting electrical properties were analysed by AC impedance spectroscopy. The composite conductivity follows a percolation scaling law of the form sigma = constant x (p-pc)^t with the critical mean concentration pc to form a conductive network of approximately 0.0025 wt% and an exponent, t, of 1.2. The experimental percolation threshold for the aligned nanotubes used in this study represents the lowest threshold observed for carbon-nanotube-based polymer composites yet reported. Aligned multi-wall carbon nanotubes from an injection CVD process were dispersed as conductive fillers in an epoxy matrix. The resulting electrical properties were investigated by AC impedance spectroscopy and it is shown that sufficient conductivity for anti-static applications can be achieved at an average nanotube loading of approximately 0.005 wt%. The resulting bulk conductivity properties of the nanotube-epoxy composites arise from the formation of macroscopic nanotube aggregates. The experimental conductivity data follows a percolation scaling law. The use of the aligned multi-wall carbon nanotubes leads to a uniquely low percolation threshold, which is an order of magnitude smaller than best results previously achieved with entangled multi-wall nanotubes. Compared to carbon black particles, the use of nanotubes as filler represents a significant advance for epoxy systems, allowing anti-static Fig. 6. Epoxy composite conductivity as a function of filler weight fraction for the aligned CVD-grown multi-wall carbon nanotubes compared to results previously achieved with entangled nanotubes and carbon black particles conductivities at loading fractions where the mechanical properties of the matrix are not degraded, as well as higher maximum conductivities. In addition, these ultra-low loading fraction anti-static materials may prove to have interesting, low EM-radiation profiles. Based on the achievement of initially well-dispersed nanotube-resin mixtures, further work is aimed at establishing the percolation threshold as a function of nanotube aspect ratio as well as determining the nature of the interactions between individual carbon nanotubes as a function of processing conditions.
Carbon nanotubes were successfully synthesised with a ± 29 % thickness uniformity over an 8” substrate using a rescaled ICP reactor. Deposition rates of the order of 1 A/s were obtained at a gas composition, RF power, process pressure, and substrate temperature and bias voltage of 33 % C2H2 in NH3, 300 W, 1 mbar, 500oC and –100 V respectively. The predicted plasma conditions necessary for the production of aligned carbon nanotubes also correlated well with those obtained experimentally. The large area nanotube deposition system is not yet at a “ready product” level. Indeed further work will be required to achieve this goal. However, significant advances with respect to both equipment and process scale up in the 10-2 to 1 mbar pressure regime have been made in the context of this project. This information has recently allowed CCR Technology to expand and develop its range of inductively coupled plasma sources. CCR currently has patents pending on its ICP source technology. However together with appropriate partners within the CARDECOM consortium CCR will seek to evaluate the potential of submitting process patents associated with its scalable ICP source technology within the area of large area carbon nanotube and nanofibre deposition. This will be necessary if CCR is to be able to license its technology in order to fully commercialise the results of the CARDECOM project. The work conducted in the context of the CARDECOM project will permit CCR, within the next 6 months, to market and sell a unique scalable source technology with direct application to carbon nanotube deposition. The level of reliability data, process data and engineering detail required to be an effective player in this market will have principally come from CCR’s participation in the CARDECOM project. We expect the following economic benefits following participation in this project: - new markets for sources & hence additional sales of 5 sources per year at 30 k€ per source (150 k€); - new markets for systems & hence additional sales of 2 small systems per year at 165 k€ per system (330 k€);increased visibility and business in nanotechnology and related fields; - increase in yearly turnover of approximately 480 k€ for the three years following the optimisation of the current nanotube deposition system. The nanotube reactor developed for the CARDECOM project will be used as a demonstration tool for the deposition of carbon nanotubes and nanofibres. Such demonstration facilities will be necessary in order to fully commercialise this technology. With respect to information dissemination, CCR Technology will prepare both journal papers and marketing documents regarding the scaling of both the technology and the process. Reference will be given to both the European Union for financial support and for other key partners involved directly in both the basic research and commercialisation aspects of this project. Further information will be posted on the CCR web site.
Literature data on the storage capacities of hydrogen in carbon nanostructures show a scatter over several orders of magnitude, which cannot be solely explained by the limited quantity or purity of these novel nanoscale materials. With this in mind, this article revisits important experiments. Thermal desorption spectroscopy, as a quantitative tool to measure the hydrogen storage capacity needs an appropriate calibration using a suitable hybride. Single walled carbon nanotubes that have been treated by ultra-sonication show hydrogen uptake at room temperature. However, this storage can be assigned to metal particles incorporated during the sonication treatment. Reactive high-energie ball milling of graphite leads to a high hydrogen loading, however the temperatures for hydrogen release are far too high for application. In view of today’s knowledge, which is mainley based on experiment with small quantities and poorly characterised samples, carbon nanostructures at room temperature cannot store the amount of hydrogen required for automotive applications.
All nanotube-based transistors were fabricated using carbon nanotubes networks as both the gate layer and the conducting channel. The fabrication process is simple, scalable and done at room temperature. The nanotube networks are transparent and flexible, and can be fabricated on various different substrates, providing a wide range of potential applications.
SWNT were synthesized by arc-discharge methods. The as produced material was treated by different procedures in order to get purified materials. As it was measured on buckypapers the treatment indeed led to higher conductivity values and therefore can be named as purification. The SWNT were incorporated into a polymer matrix of polycarbonate by melt mixing using a small-scale compunder at 280 Grad Celsius, 50 rpm, and 15 min. The as produced SWNT material led to electrical percolation between 2 and 3-wt%. Unexpectedly, the composites with the treated SWNT showed higher percolation concentrations. This finding is in contradiction to theoretical expectations and the values found for the buckypapers.In addition, an increase in percolation concentration was also found when using a similiar treatment on SWNT produced by HIPco method (CNI). These results imply that increasing the purity of SWNT does not lead automatically to lower percolation contents when dispersing SWNT in a molton polymer by shear mixing. There is another property, which can be named as "dispersability". During the purification the interactions between SWNT bundles can be increased, thus leading to a more agglomerated SWNT material, which is more difficult to disperge during the melt mixing procedure.
We have developed a simple and practical method to prepare one-sided conductive transparent plastics. High performance and flexible (or not) materials based on Single Wall Carbon Nanotubes (SWNTs) were produced. Potential application lies where flexibility is required (ITO cannot be used then). The interesting part of the method resides in a transfer of SWNTs onto the surface of a variety of polymers. Purity is not compulsory, so low cost can be achieved. Current status: Patent applied for.
Integration of Carbon Nanotube (CNT) Material in a thermoplastic polymer matrix to improve its mechanical, thermal and electrical properties. Combination of limited CNT availability, high cost and extremely small size qualified CNT thermoplastic composites for demanding microinjection-moulding applications. Main results available at CRIF and ready for immediate dissemination are: - requires adapted equipment and methodology. - methodology and specific equipment for the compounding, processing and characterisation of thermoplastic composites using a very small amount of material. Whole process leading to characterisation of composite requires less than 5 grammes CNT and 100 grammes polymer. - identification of interesting rheological properties of CNT thermoplastic composites to reduce die swell and other behaviour of molten polymers - Development of methodology and experience in specific characterisation techniques for mechanical and thermal characterisation of thermoplastic micro-parts (holographic interferometry techniques for micro-mechanical characterisation and thermal characterisation using Hot Disk technique).
Using nitrene reaction, two individual single-walled carbon nanotubes can be covalently connected inducing only few defects to the nanotube sidewall. With this functionalisation method nanotubes can be linked within a bundle as well as between bundles.
A highly purified and concentrated monodisperse cobalt colloid was produced to use as a catalyst for growth of carbon nanotubes. Nanocontact printing was employed to deposit the cobalt nanoparticles in regular patterns with feature sizes down to 100 nm onto silicon wafers at low cost over large areas. Vertically aligned carbon nanotubes were selectively grown by direct current plasma enhanced chemical vapour deposition at temperatures ranging from 300 °C to 550 °C.
Chemical modification by SOCl2 of an entangled network of purified SWCNT, also known as ‘bucky paper’, is reported to profoundly change the electrical and mechanical properties of this system. Four-probe measurements indicate a conductivity increase by up to a factor of 5 at room temperature and an even more pronounced increase at lower temperatures. This chemical modification also improves the mechanical properties of SWNT networks. Whereas the pristine sample shows an overall semi conducting character, the modified material behaves as a metal. The effect of SOCl2 is studied in terms of chemical doping of the nanotube network. We identified the microscopic origin of these changes using SEM, XPS, NEXAFS, EDX, and Raman spectroscopy measurements and ab initio calculations. We interpret the SOCl2-induced conductivity increase by p-type doping of these pristine material. This conclusion is reached by electronic structure calculation, which indicates a Fermi level shift into the valence band, and is consistent with the temperature dependence of the thermo power.

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