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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 4+2x Si 2−2x O 10−x (x ≈ 0:4)) is used instead of alumina (Al 2 O 3 ), 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.

The kinetics of the thermal decomposition of methylal has been studied over the range 472-520" and 50-450 mm. in a static system, by studying the change of pressure and by infrared analyses. The system of steadystate equations is too complex to have a straightforward solution but by means of them the experimental order is explained, several stoicheiometric relations among the products become amenable to experimental test, and ratios of elementary rate constants and changes of some radical concentrations have been calculated.PART I of this series1 dealt with the thermal decomposition of ethylal, dimethyl acetal, and diethyl acetal in a static system. The present paper extends the investigation to the simplest acetal, namely, methylal. With it a more thorough study of the decomposition mechanism has been undertaken by applying the steady-state approximation. EXPERIMENTAL AND RESULTSThe experimental technique was as described in Part I. Methylal was a purum grade " Fluka A.G." chemical.Influence of the Surface.-Experiments were made in two vessels of the same size and shape, one empty and theother filled with silica powder, which varies the surface : volume ratio -500-fold.Plots of pressure increase against time for the first experiments in the filled vessel showed a shift towards greater values of Ap, compared with those obtained with the empty vessel. However, after several runs the shift decreased and finally the curves were superimposable. It seems that the thermal decomposition of methylal can be affected by the surface of the reaction vessel, but that this influence is probably relevant to a non-seasoned surface. These results suggest that in a seasoned vessel measurements refer to the true homogeneous decomposition. Other results (below) were obtained in seasoned vessels. TABLE 1. Rates of decomposition of methylal.

Composites of nanocarbon network structures are interesting materials, combining mechanical properties and electrical conductivity superior to those of granular systems. Hence, they are envisaged to have applications as electrodes for energy storage and transfer. Here, we show a new processing route using Joule heating for a nanostructured network composite of carbon nanotube (CNT) fabrics and an inorganic phase (namely, MoS 2 ), and then study the resulting structure and properties. To this end, first, a unidirectional fabric of conductive CNT bundles is electrochemically coated with MoS 2 . Afterward, the conformally coated inorganic phase is crystallized via heat generated by direct current passing through the CNT ensemble. The Joule heating process is rapid (maximum heating rate up to 31.7 °C/s), enables accurate temperature control, and takes only a few minutes. The resulting composite material combines a high electrical conductivity of up to 1.72 (±0.25) × 10 5 S/m, tensile modulus as high as 8.82 ± 5.5 GPa/SG, and an axial tensile strength up to 200 ± 58 MPa/SG. Both electrical and mechanical properties are orders of magnitude above those of wet-processed nanocomposites of similar composition. The extraordinary longitudinal properties stem from the network of interconnected and highly aligned CNT bundles. Conductivity and modulus follow approximately a rule of mixtures, similar to a continuous fiber composite, whereas strength scales almost quadratically with the mass fraction of the inorganic phase due to the inorganic constraining realignment of CNTs upon stretching. This processing route is applicable to a wide range of nanocarbon-based composites with inorganic phases, leading to composites with specific strength above steel and electrical conductivity beyond the threshold for electronic limitations in battery electrodes.

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