Macroscopic fibers of carbon nanotubes (CNT) have emerged as an ideal architecture to exploit the exceptional properties of CNT building blocks in applications ranging from energy storage to ...reinforcement in structural composites. Controlled synthesis and scalability are amongst the most pressing challenges to further materialize the potential of CNT fibers. This work shows that under floating catalyst chemical vapor conditions in the direct spinning method, used both in research and industry, the ceramic reactor tube plays an unsuspected active role in CNT growth, leading for example to doubling of reaction yield when mullite (Al
Si
O
(x ≈ 0:4)) is used instead of alumina (Al
O
), but without affecting CNT morphology in terms of number of layers, purity or degree of graphitization. This behaviour is confirmed for different carbon sources and when growing either predominantly single-walled or multi-walled CNTs by adjusting promotor concentration. Analysis of large Si-based impurities occasionally found in CNT fiber fabric samples, attributed to reactor tube fragments that end up trapped in the porous fibers, indicate that the role of the reactor tube is in catalyzing the thermal decomposition of hydrocarbons, which subsequently react with floating Fe catalyst nanoparticles and produce extrusion of the CNTs and formation of an aerogel. Reactor gas analysis confirms that extensive thermal decomposition of the carbon source occurs in the absence of Fe catalyst particles, and that the concentration of different carbon species (e.g. carbon dioxide and ethylene) is sensitive to the reactor tube type. These finding open new avenues for controlled synthesis of CNT fibers by decoupling precursor decomposition from CNT extrusion at the catalyst particle.
Macroscopic fibres of carbon nanotubes are hierarchical structures combining long building blocks preferentially oriented along the fibre axis and a large porosity arising from the imperfect packing ...of bundles. Synchrotron small-angle X-ray scattering (SAXS) measurements show that such structure is a surface fractal with fractal dimension (Ds) of 2.5 for MWCNT fibres and 2.8 for SWCNT fibres. N2 adsorption measurements give similar values of 2.54 and 2.50, respectively. The fractal dimension and deviation from Porod's law are related to density fluctuations associated with the wide distribution of separations between CNTs. These fluctuations are also evident as diffuse wide-angle X-ray scattering (WAXS) from CNTs at distances above turbostratic separation. The structure of CNT fibres produced at different draw ratios is compared in terms of degree of orientation and characteristic lengths parallel and perpendicular to the fibre. Drawing not only increases alignment but also the fraction of graphitic planes forming coherent domains capable of taking part in stress transfer by shear; thus increasing both tensile modulus and strength. The invariant-normalized intensity of the (002) equatorial reflection thus takes the form of a degree of crystallinity closely related to tensile properties.
Macroscopic fiber assemblies of carbon nanotubes have a fractal network structure comprising a large mesoporosity coexisting with large oriented crystalline domains. Structure-properties are thus extracted from gas adsorption studies as well as WAXS/SAXS. Display omitted
This work evaluates the feasibility of several technologies for the removal of trihalomethanes (THM) precursors, usually humic substances found in real surface water, which are transformed into these ...potential carcinogens (THMs) during the further disinfection by chlorination. The aim was to compare, an Activated Carbon Bed (ACB), an Ozonation Reactor (OR) and a Submerged Membrane Photocatalytic Reactor (SMPR) with TiO2/UV in terms of reduction efficiency of these THM precursors after chlorination. Taguchi's parameter design methodology was selected to study the most relevant factors that might influence the removal of THM precursors with each technology keeping the number of experiments at minimum. According to the study, OR showed low removal percentages of THM precursors (of 40–50%) while SMPR and ACB were more efficient technologies. SMPR achieved 86% removal of THM precursors when operating at the optimal conditions of TiO2 catalyst concentration of 0.5gL−1 (type PC500® from Cristal Global). ACB attained the 87% removal of THM precursors when Filtracarb® CC60 8×30 (steam activated mineral coal) was used with a hydraulic retention time of 15min.
•Evaluation of several technologies for the reduction of THMs in drinking water•Ozonation shows low removal THM percentages of 40–50%,•Membrane photocatalysis with TiO2 and activated carbon provide about 86% removal of THM.•The addition of synthetic HA increases the ratio of bromo derivatives to chloroform.•Taguchi's parameter design and statics (ANOVA) was used to optimize the systems.
Macroscopic fibers of carbon nanotubes (CNT) have emerged as an ideal architecture to exploit the exceptional properties of CNT building blocks in applications ranging from energy storage to ...reinforcement in structural composites. Controlled synthesis and scalability are amongst the most pressing challenges to further materialize the potential of CNT fibers. This workshows that under floating catalyst chemical vapor conditions in the direct spinning method, used both in research and industry,the ceramic reactor tube plays an unsuspected active role in CNT growth, leading for example to doubling of reaction yield when mullite (Al4+2xSi2-2xO10-x(x = 0.4)) is used instead of alumina (Al2O3), but without affecting CNT morphology in terms of number of layers, purity or degree of graphitization. This behaviour has been confirmed for different carbon sources andwhen growing either predominantly single-walled or multi-walled CNTs by adjusting promotor concentration. Analysis of large Si-based impurities occasionally found in CNT fiber fabric samples, attributed to reactor tube fragments that end up trapped inthe porous fibers, indicate that the role of the reactor tube is in catalyzing the thermal decomposition of hydrocarbons, which subsequently react with floating Fe catalyst nanoparticles and produce extrusion of the CNTs and formation of an aerogel. Reactor gas analysis confirms that extensive thermal decomposition of the carbon source occurs in the absence of Fe catalystparticles, and that the concentration of different carbon species (e.g. carbon dioxide and ethylene) is sensitive to the reactor tube type. These finding open new avenues for controlled synthesis of CNT fibers by decoupling precursor decomposition from CNT extrusion at the catalyst particle.
Macroscopic fibres of carbon nanotubes are hierarchical structures combining long building blocks preferentially oriented along the fibre axis and a large porosity arising from the imperfect packing ...of bundles. Synchrotron small-angle X-ray scattering SAXS measurements show that such structure is a surface fractal with fractal dimension (Ds) of 2.5 for MWCNT fibres and 2.8 for SWCNT fibres. N2 adsorption measurements give similar values of 2.54 and 2.50, respectively. The fractal dimension and deviation from Porods law are related to density fluctuations associated with the wide distribution of separations between CNTs. These fluctuations are also evident as diffuse wide-angle X-ray scattering (WAXS) from CNTs at distances above turbostratic separation. The structure of CNT fibres produced at different draw ratios is compared in terms of degree of orientation and characteristic lengths parallel and perpendicular to the fibre. Drawing not only increases alignment but also the fraction of graphitic planes forming coherent domains capable of taking part in stress transfer by shear; thus increasing both tensile modulus and strength. The invariant-normalized intensity of the (002) equatorial reflection thus takes the form of a degree of crystallinity closely related to tensile properties.
•This paper discusses the use of frequency analysis in GPR assessment of pavements.•A section of a highway was studied in two different stages of its service life.•Changes in frequencies are ...associated to changes of the layers thicknesses and moisture.•The spectrum shows divisions that could be in this case associated to different layers.
Ground penetrating radar (GPR) is frequently used in pavement assessments, mainly using the evaluation of wave travel times. However, GPR data provide further information that could be used in order to determine the inner conditions and characteristics about materials. In this paper, the possible analysis of the frequency spectrum of GPR signals is analysed and discussed. Several tests were carried out in a portion of a highway in two different stages of its service life. Results highlight the relationship between the shape of the spectrum and the frequency signature with the structure and conditions of the pavement.
The loss of functional and accreting coral reefs reduces coastal protection and resilience for tropical coastlines. Coral restoration has potential for recovering healthy reefs that can mitigate ...risks from coastal hazards and increase sustainability. However, scaling up restoration to the large extent needed for coastal protection requires integrated application of principles from coastal engineering, hydrodynamics, and ecology across multiple spatial scales, as well as filling missing knowledge gaps across disciplines. This synthesis aims to identify how scientific understanding of multidisciplinary processes at interconnected scales can advance coral reef restoration. The work is placed within the context of a decision support framework to evaluate the design and effectiveness of coral restoration for coastal resilience. Successfully linking multidisciplinary science with restoration practice will ensure that future large‐scale coral reef restorations maximize protection for at‐risk coastal communities.
PDF
