Postdoctoral Fellow, University of Cambridge
Understanding and Scale-Up of a Gas Phase Process for Continuous Tailored Carbon Nanotube Synthesis
Manipulating structures that are as small as a few atoms does not only affect their mutual interactions within this small scale ‘nanoworld’, but can also have a strong impact on phenomena occurring in the ‘macroworld’, the scale on which our daily life takes place. ‘Nano’ describes the order of magnitude in which, for example, the growth of fingernails can be measured – about 1 nanometre per second. Among other things, this nanoworld is home of some incredible structures called carbon nanotubes (CNTs). On the nanoscale, individual carbon nanotubes have been shown to possess significantly improved mechanical, thermal and electrical properties compared to existing materials. However, their applications and macroworld impact are thus far limited. Extrapolating the properties of individual carbon nanotubes into macro-scale CNT materials using a continuous and cost effective process offers enormous potential for a variety of applications in fields such as composite materials, coatings and films, microelectronics, biotechnology, energy storage and energy conversion. The floating catalyst chemical vapour deposition (FCCVD) method, subject to my research, bridges the gap between generating nano- and macro-scale CNT material and has already been adopted by industry for exploitation. It is at the interface of chemistry, physics, engineering and materials science, and uniquely transforms a constant flow of precursors into gas-phase synthesised bulk CNT material which can be spun continuously from a reactor. A deep understanding of the phenomena that occur within the FCCVD reactor is essential to producing a desired and tailored CNT product and successfully scaling up the FCCVD process. My work has led to multiple patents and scientific publications. More importantly, it has fundamentally changed the understanding of the FCCVD process and got awarded a £3M research grant for the ‘Advanced Nanotubes Applications and Manufacturing Initiative’, led by our research group (www.anam.eng.cam.ac.uk). To build onto the insights gained within my previous studies, I now explore further how individual CNTs bond and stick to one another. As part of my postdoctoral research I am presenting my work on the cluster formation, or agglomeration, in CNT aerosols, which breaks new ground in the examination of the fundamental dynamics of the FCCVD method, at the American Association for Aerosol Research Conference.
Abstract: Many routes have been developed for the synthesis of carbon nanotubes, but their assembly into continuous fibers has been achieved only through postprocessing methods. We spun fibers and ribbons of carbon nanotubes directly from the chemical vapor deposition (CVD) synthesis zone of a furnace using a liquid source of carbon and an iron nanocatalyst. This process was realized through the appropriate choice of reactants, control of the reaction conditions, and continuous withdrawal of the product with a rotating spindle used in various geometries. This direct spinning from a CVD reaction zone is extendable to other types of fiber and to the spin coating of rotating objects in general.
Pub.: 16 Mar '04, Pinned: 29 Jun '17
Abstract: Worldwide commercial interest in carbon nanotubes (CNTs) is reflected in a production capacity that presently exceeds several thousand tons per year. Currently, bulk CNT powders are incorporated in diverse commercial products ranging from rechargeable batteries, automotive parts, and sporting goods to boat hulls and water filters. Advances in CNT synthesis, purification, and chemical modification are enabling integration of CNTs in thin-film electronics and large-area coatings. Although not yet providing compelling mechanical strength or electrical or thermal conductivities for many applications, CNT yarns and sheets already have promising performance for applications including supercapacitors, actuators, and lightweight electromagnetic shields.
Pub.: 02 Feb '13, Pinned: 29 Jun '17
Abstract: The complex structure of the macroscopic assemblies of carbon nanotubes and variable intrinsic piezoresistivity of nanotubes themselves lead to highly interesting piezoresistive performance of this new type of conductive material. Here, we present an in-depth study of the piezoresistive effect in carbon nanotube fibers, i.e., yarnlike assemblies made purely of aligned carbon nanotubes, which are expected to find applications as electrical and electronic materials. The resistivity changes of carbon nanotube fibers were measured on initial loading, through the elastic/plastic transition, on cyclic loading and on stress relaxation. The various regimes of stress/strain behavior were modeled using a standard linear solid model, which was modified with an additional element in series to account for the observed creep behavior. On the basis of the experimental and modeling results, the origin of piezoresistivity is discussed. An additional effect on the resistivity was found as the fiber was held under load which led to observations of the effect of humidity and the associated water adsorption level on the resistivity. We show that the equilibrium uptake of moisture leads to the decrease in gauge factor of the fiber decrease, i.e., the reduction in the sensitivity of fiber resistivity to loading.
Pub.: 23 Oct '14, Pinned: 04 Jul '17
Abstract: With their impressive individual properties, carbon nanotubes should form high-performance fibers. We explored the roles of nanotube length and structure, fiber density, and nanotube orientation in achieving optimum mechanical properties. We found that carbon nanotube fiber, spun directly and continuously from gas phase as an aerogel, combines high strength and high stiffness (axial elastic modulus), with an energy to breakage (toughness) considerably greater than that of any commercial high-strength fiber. Different levels of carbon nanotube orientation, fiber density, and mechanical properties can be achieved by drawing the aerogel at various winding rates. The mechanical data obtained demonstrate the considerable potential of carbon nanotube assemblies in the quest for maximal mechanical performance. The statistical aspects of the mechanical data reveal the deleterious effect of defects and indicate strategies for future work.
Pub.: 17 Nov '07, Pinned: 04 Jul '17
Abstract: The CVD process for the spinning of carbon nanotube (CNT) fibres combines the nucleation, growth and aggregation of CNTs in the form of an aerogel with fibre spinning into a single process step. The optimisation of the process requires agility in multi-dimensional parameter space, so one tends to find parameter 'islands' where spinning is possible, while exploration tends to follow 'routes' through this space. Here, we follow two such routes, one of which drastically improves fibre purity, the other changes the nature of the nanotubes comprising the fibres from multiwall to single wall. In the first case there is only a modest enhancement of the mechanical properties, but in the second a very considerable improvement is seen. In terms of the conditions required to make fibres comprising predominately single wall CNTs, the key factor appears to be the rigorous control of the sulphur addition, in trace quantities, coupled with the availability of carbon atoms at the earliest stage after injection, typically in the range 400-500 °C. A model is presented for the role of sulphur in floating catalysts CNT synthesis.
Pub.: 24 Oct '14, Pinned: 04 Jul '17
Abstract: The floating catalyst chemical vapour deposition (FC-CVD) method is unique in providing the capability for continuous carbon nanotube (CNT) synthesis at an industrial scale from a one-step continuous gas-phase process. Controlling the formation of the iron-based catalyst nanoparticles is widely recognized as a primary parameter in optimizing both CNT product properties and production rate. Herein the combined influences of pyrolytic carbon species and catalytic nanoparticles are both shown to influence CNT aerogel formation. This work studies the source of carbon in the formed CNTs, the location of aerogel formation, the in-situ behaviour of catalyst nanoparticles and the correlated morphology of the resultant CNTs. Axial measurements using isotopically-labelled methane (CH4) demonstrate that carbon within all CNTs is primarily derived from CH4 rather than some of the early-forming CNTs being predominantly supplied with carbon via thermal decomposition of catalytic precursor components. Quantification of CNT production along the axis of the reactor definitively dispels the notion that injection parameters influence CNT formation and instead shows that bulk CNT formation occurs near the reactor exit regardless of the carbon source (CH4, toluene or ethanol). Supply of carbon to different reactor locations indicates that CNT aerogel formation will occur even when carbon is delivered near the exit of the reactor so long as the carbon source reaches a sufficient temperature (>1000 °C) to induce pyrolysis. These results give an indication of how future large-scale CNT reactors may be optimized and controlled by modifying downstream catalyst and carbon delivery.
Pub.: 29 Nov '16, Pinned: 29 Jun '17