Catalyst nanoparticle growth dynamics and their influence on product morphology in a CVD process for continuous carbon nanotube synthesis

Extrapolating the properties of individual CNTs into macro-scale CNT materials using a continuous and cost effective process offers enormous potential for a variety of applications. The floating catalyst chemical vapor deposition (FCCVD) method discussed in this paper bridges the gap between generating nano- and macro-scale CNT material and has already been adopted by industry for exploitation. A deep understanding of the phenomena occurring within the FCCVD reactor is thereby key to producing the desired CNT product and successfully scaling up the process further. This paper correlates information on decomposition of reactants, axial catalyst nanoparticle dynamics and the morphology of the resultant CNTs and shows how these are strongly related to the temperature and chemical availability within the reactor. For the first time, in-situ measurements of catalyst particle size distributions coupled with reactant decomposition profiles and a detailed axial SEM study of formed CNT materials reveal specific domains that have important implications for scale-up. A novel observation is the formation, disappearance and reformation of catalyst nanoparticles along the reactor axis, caused by their evaporation and re-condensation and mapping of different CNT morphologies as a result of this process.The authors thank Qflo Ltd for providing funding towards this research, C. Hoecker additionally thanks Churchill College Cambridge for financial support, M. Bajada gratefully acknowledges financial support through the 'Master it! Scholarship Scheme'.This is the accepted manuscript. The final version is available at http://dx.doi.org/10.1016/j.carbon.2015.09.05

Hall Effect in a Quasi-One-Dimensional System

We consider the Hall effect in a system of weakly coupled one-dimensional chains with Luttinger interaction within each chain. We construct a perturbation theory in the inter-chain hopping term and find that there is a power law dependence of the Hall conductivity on the magnetic field with an exponent depending on the interaction constant. We show that this perturbation theory becomes valid if the magnetic field is sufficiently large.Comment: 20 page

Spin Gap and Superconductivity in Weakly Coupled Ladders: Interladder One-particle vs. Two-particle Crossover

Effects of the interladder one-particle hopping, $t_{\perp}$, on the low-energy asymptotics of a weakly coupled Hubbard ladder system have been studied, based on the perturbative renormalization-group approach. We found that for finite intraladder Hubbard repulsion, $U$, there exists a crossover value of the interladder one-particle hopping, $t_{\perp c}$. For $0<t_{\perp}<t_{\perp c}$, the spin gap metal (SGM) phase of the isolated ladder transits at a finite transition temperature, $T_{c}$, to the d-wave superconducting (SCd) phase via a two-particle crossover. In the temperature region, $T<T_{c}$, interladder coherent Josephson tunneling of the Cooper pairs occurs, while the interladder coherent one-particle process is strongly suppressed. For $t_{\perp c}<t_{\perp}$, around a crossover temperature, $T_{cross}$, the system crosses over to the two-dimensional (2D) phase via a one-particle crossover. In the temperature region, $T<T_{cross}$, the interladdercoherent band motion occurs.Comment: 4 pages, 5 eps figures, uses jpsj.st

The influence of carbon source and catalyst nanoparticles on CVD synthesis of CNT aerogel

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

Vanadium Dioxide Cathodes for High-Rate Photo-Rechargeable Zinc-Ion Batteries

Photovoltaics are an important source of renewable energy, but due the intermittent nature of insolation, solar cells usually need to be connected to rechargeable batteries, electrochemical capacitors or other energy storage devices, which adds to the complexity and cost of these systems. In this work, we report a cathode design for photo-rechargeable zinc-ion batteries (photo-ZIBs) that is inherently capable of harvesting sunlight to recharge without the need for external solar cells. The proposed photocathodes comprising a composite of vanadium dioxide nanorods and reduced graphene oxide, are engineered to provide the necessary charge separation and storage for photocharging under illumination. The photo-ZIBs achieve gravimetric capacities of ~ 282 mAh g-1 in the dark and ~ 315 mAh g-1 under illumination, at 200 mA g-1, demonstrating the use of light not only to charge the deceives, but additionally to enhance their capacity. The photo-ZIBs also demonstrate enhanced high-rate capabilities under illumination, as well as a capacity retention of ∼ 90% over 1000 cycles. The proposed photo-ZIBs demonstrate a promising new technology for addressing energy poverty, due to their high performance and inherent cost-efficiency and safety.Newton International Fellowship-Royal Society (UK) grant NIF∖R1∖181656 ERC Consolidator grant MIGHTY - 866005 EPSRC Graphene CDT EP/L016087/

Crossover from Luttinger- to Fermi-liquid behavior in strongly anisotropic systems in large dimensions

We consider the low-energy region of an array of Luttinger liquids coupled by a weak interchain hopping. The leading logarithmic divergences can be re-summed to all orders within a self-consistent perturbative expansion in the hopping, in the large-dimension limit. The anomalous exponent scales to zero below the one-particle crossover temperature. As a consequence, coherent quasiparticles with finite weight appear along the whole Fermi surface. Extending the expansion self-consistently to all orders turns out to be crucial in order to restore the correct Fermi-liquid behavior.Comment: Shortened version to appear in Physical Review Letter

Weakly Coupled Piezoelectric MEMS Resonators for Aerosol Sensing.

This paper successfully demonstrates the potential of weakly coupled piezoelectric MEMS (Micro-Electro-Mechanical Systems) gravimetric sensors for the detection of ultra-fine particulates. As a proof-of-principle, the detection of diesel soot particles of 100 nanometres or less is demonstrated. A practical monitoring context also exists for diesel soot particles originating from combustion engines, as they are of serious health concern. The MEMS sensors employed in this work operate on the principle of vibration mode-localisation employing an amplitude ratio shift output metric for readout. Notably, gains are observed while comparing parametric sensitivities and the input referred stability for amplitude ratio and resonant frequency variations, demonstrating that the amplitude ratio output metric is particularly suitable for long-term measurements. The soot particle mass directly estimated using coupled MEMS resonators can be correlated to the mass, indirectly estimated using the condensation particle counter used as the reference instrument

Impurity-induced stabilization of Luttinger liquid in quasi-one-dimensional conductors

It is shown theoretically that the Luttinger liquid phase in quasi-one-dimensional conductors can exist in the presence of impurities in a form of a collection of bounded Luttinger liquids. The conclusion is based upon the observation by Kane and Fisher that a local impurity potential in Luttinger liquid acts, at low energies, as an infinite barrier. This leads to a discrete spectrum of collective charge and spin density fluctuations, so that interchain hopping can be considered as a small parameter at temperatures below the minimum excitation energy of the collective modes. The results are compared with recent experimental observation of a Luttinger-liquid-like behavior in thin NbSe$_3$ and TaS$_3$ wires.Comment: 11 pages, revtex, final version published in JETP Lett