Degradation and Failure of Field Emitting Carbon Nanotube Arrays

It has been observed experimentally that the collective field emission from an array of Carbon Nanotubes (CNTs) exhibits fluctuation and degradation, and produces thermal spikes, resulting in electro-mechanical fatigue and failure of CNTs. Based on a new coupled multiphysics model incorporating the electron-phonon transport and thermo-electrically activated breakdown, a novel method for estimating accurately the lifetime of CNT arrays has been developed in this paper. The main results are discussed for CNT arrays during the field emission process. It is shown that the time-to-failure of CNT arrays increases with the decrease in the angle of tip orientation. This observation has important ramifications for such areas as biomedical X-ray devices using patterned films of CNTs.

Discontinuous shear thickening in confined dilute carbon nanotube suspensions

A monotonic decrease in viscosity with increasing shear stress is a known rheological response to shear flow in complex fluids in general and for flocculated suspensions in particular. Here we demonstrate a discontinuous shear-thickening transition on varying shear stress where the viscosity jumps sharply by four to six orders of magnitude in flocculated suspensions of multiwalled carbon nanotubes (MWNT) at very low weight fractions (approximately 0.5%). Rheooptical observations reveal the shear-thickened state as a percolated structure of MWNT flocs spanning the system size. We present a dynamic phase diagram of the non-Brownian MWNT dispersions revealing a starting jammed state followed by shear-thinning and shear-thickened states. The present study further suggests that the shear-thickened state obtained as a function of shear stress is likely to be a generic feature of fractal clusters under flow, albeit under confinement. An understanding of the shear-thickening phenomena in confined geometries is pertinent for flow-controlled fabrication techniques in enhancing the mechanical strength and transport properties of thin films and wires of nanostructured composites as well as in lubrication issues.

Macro-economic analysis of forestry options on carbon sequestration in India

The forestry sector provides a number of climate change mitigation options. Apart from this ecological benefit, it has significant social and economic relevance. Implementation of forestry options requires large investments and sustained long-term planning. Thus there is a need for a detailed analysis of forestry options to understand their implications on stock and flow of carbon, required investments, value of forest wealth, contribution to GNP and livelihood, demand management, employment and foreign trade. There is a need to evaluate the additional spending on forestry by analysing the environmental (particularly carbon abatement), social and economic benefits. The biomass needs for India are expected to increase by two to three times by 2020. Depending upon the forest types, ownership patterns and land use patterns, feasible forestry options are identified. It is found among many supply options to be feasible to meet the 'demand based needs' with a mix of management options, species choices and organisational set up. A comparative static framework is used to analyze the macro-economic impacts. Forestry accounts for 1.84% of GNP in India. It is characterized by significant forward industrial linkages and least backward linkage. Forestry generates about 36 million person years of employment annually. India imports Rs. 15 billion worth of forest based materials annually. Implementation of the demand based forestry options can lead to a number of ecological, economic and institutional changes. The notable ones are: enhancement of C stock from 9578 to 17 094 Mt and a net annual C-sequestration from 73 to 149 Mt after accounting for all emissions; a trebling of the output of forestry sector from Rs. 49 billion to Rs. 146 billion annually; an increase in GDP contribution of forestry from Rs. 32 billion to Rs. 105 billion over a period of 35 years; an increase in annual employment level by 23 million person years, emergence of forestry as a net contributor of foreign exchange through trading of forestry products; and an increase in economic value of forest capital stock by Rs. 7260 billion with a cost benefit analysis showing forestry as a profitable option. Implementation of forestry options calls for an understanding of current forest policies and barriers which are analyzed and a number of policy options are suggested. (C) 1997 Elsevier Science B.V.

Metal-billed and hollow carbon nanotubes obtained by the decomposition of metal-containing free precursor molecules

Vapor-phase pyrolysis of Fe(CO)(5) in the presence of another carbon source such as CO or Ca He yields iron-filled or hollow nanotubes depending on the relative concentration of the carbon source. Essentially single-walled nanotubes are obtained when the C6H6/Fe(CO)(5) ratio is high. Pyrolysis of metallocenes yields metal-filled nanotubes and hollow nanotubes are obtained when metallocenes are pyrolyzed along with benzene. Metal-decorated nanotubes are also obtained by this method.

Nitrogen-containing carbon nanotubes

Carbon nanotubes containing small amounts of nitrogen are produced by the pyrolysis of aza-aromatics such as pyridine, methylpyrimidine and triazine over cobalt nanoparticles in an Ar atmosphere; good yields of such nanotubes are obtained by carrying out the pyrolysis of a mixture of pyridine and Fe(CO)(5) in flowing Ar + H-2.

Oxide nanotubes prepared using carbon nanotubes as templates

Hollow nanotubes of SiO2, Al2O3, V2O5, and MoO3 have been prepared using carbon nanotubes as templates. The procedure involves coating the carbon nanotubes with tetraethylorthosilicate, aluminum isopropoxide, or vanadium pentoxide gel, followed by calcination and heating at higher temperatures in air to oxidize the carbon. SiO2 nanotubes containing transition metal ions have been prepared by this procedure since such materials may be of use in catalysis.

Some unusual features in the Raman spectrum of amorphous-carbon films obtained by pyrolysis of maleic anhydride

Laser micro-Raman spectroscopic measurements were done on the amorphous conducting carbon films obtained from maleic anhydride by pyrolysis process. We have found a predominant broad peak around 1140 cm(-1), in addition to the normally observed peaks in amorphous carbons around 1350 and 1600 cm(-1), and peak of medium intensity around 800 cm(-1). Here we discuss the possibility of conjugated polymer like bond alternating structure which can give rise to these unusual Raman features. (C) 1997 American Institute of Physics.

Structure of highly conducting amorphous carbon

We report on neutron diffraction study of a new form of conducting amorphous carbon up to Q similar to 14.5 Angstrom(-1). The bond distances from first two peaks in g(r) are 1.45 and 2.49 Angstrom, very similar to those in sputtered truly amorphous carbon films (Li and Lannin, Phys. Rev. Lett. 65 (1990) 1905). The first coordination number is 3.1 (+/- 0.1) indicating predominantly sp(2) hybridisation (ideal no. = 3). However, S(Q) itself shows vestiges of (0 0 2), (1 0) and (1 1) peaks, typical of glassy carbon (Mildner, J. Non-Cryst. Solids 47 (1982) 391). (C) 1998 Elsevier Science B.V. All rights reserved.

Absolute Entropy and Energy of Carbon Dioxide Using the Two-Phase Thermodynamic Model

The two-phase thermodynamic (2PT) model is used to determine the absolute entropy and energy of carbon dioxide over a wide range of conditions from molecular dynamics trajectories. The 2PT method determines the thermodynamic properties by applying the proper statistical mechanical partition function to the normal modes of a fluid. The vibrational density of state (DoS), obtained from the Fourier transform of the velocity autocorrelation function, converges quickly, allowing the free energy, entropy, and other thermodynamic properties to be determined from short 20-ps MD trajectories. The anharmonic effects in the vibrations are accounted for by the broadening of the normal modes into bands from sampling the velocities over the trajectory. The low frequency diffusive modes, which lead to finite DoS at zero frequency, are accounted for by considering the DoS as a superposition of gas-phase and solid-phase components (two phases). The analytical decomposition of the DoS allows for an evaluation of properties contributed by different types of molecular motions. We show that this 2PT analysis leads to accurate predictions of entropy and energy of CO2 over a wide range of conditions (from the triple point to the critical point of both the vapor and the liquid phases along the saturation line). This allows the equation of state of CO2 to be determined, which is limited only by the accuracy of the force field. We also validated that the 2PT entropy agrees with that determined from thermodynamic integration, but 2PT requires only a fraction of the time. A complication for CO2 is that its equilibrium configuration is linear, which would have only two rotational modes, but during the dynamics it is never exactly linear, so that there is a third mode from rotational about the axis. In this work, we show how to treat such linear molecules in the 2PT framework.

Graphitic Carbon as Durable Cathode-Catalyst Support for PEFCs

Long-term deterioration in the performance of PEFCs is attributed largely to reduction in active area of the platinum catalyst at cathode, usually caused by carbon-support corrosion. It is found that the use of graphitic carbon as cathode-catalyst support enhances its long-term stability in relation to non-graphitic carbon. This is because graphitic-carbon-supported- Pt (Pt/GrC) cathodes exhibit higher resistance to carbon corrosion in-relation to non-graphitic-carbon-supported- Pt (Pt/Non-GrC) cathodes in PEFCs during accelerated stress test (AST) as evidenced by chronoamperometry and carbon dioxide studies. The corresponding change in electrochemical surface area (ESA), cell performance and charge-transfer resistance are monitored through cyclic voltammetry (CV), cell polarisation and impedance measurements, respectively. The degradation in performance of PEFC with Pt/GrC cathode is found to be around 10% after 70 h of AST as against 77% for Pt/Non-GrC cathode. It is noteworthy that Pt/GrC cathodes can withstand even up to 100 h of AST with nominal effect on their performance. Xray diffraction (XRD), Raman spectroscopy, transmission electron microscopy and cross-sectional field-emission scanning electron microscopy (FE-SEM) studies before and after AST suggest lesser deformation in catalyst layer and catalyst particles for Pt/GrC cathodes in relation to Pt/Non-GrC cathodes, reflecting that graphitic carbon-support resists carbon corrosion and helps mitigating aggregation of Pt-particles.

Micro and macroscopic features of impact failed defect induced carbon-epoxy composites

Laminated composite structures are susceptible to damage under impacts with attendant properly degradation. While studies on damage tolerance behaviour are emphasised and the findings reported, the citations correlating impacts with the fracture features are limited. In the present study, therefore, attempts have been made to depict how the transition of the fracture features take place depending on the type and extent of defect introduced onto the carbon-epoxy system. The test specimens were subjected to differing levels of low energy pendulum impacts with a view to have specimens with varying levels of intial impacts history. Into such specimens, additional defect in the form of slits of varying depths were introduced by a mechanical process. The test coupons were then allowed to fail by impact. The fracture surface was studied under scanning electron microscope. The fractographic features that appear, based on the induced/inserted defects, are presented in this paper. It was noticed that the energy absorbed for final fracture could be associated with the defect introduced into the system. It was also observed that the size of the mechanically inserted defect had a significant influence on the features of the fracture surface.

Microstructure and properties of squeeze cast Cu-carbon fibre metal matrix composite

There have been reported attempts of producing Cu based MMCs employing solid phase routes. In this work, copper was reinforced with short carbon fibres by pressure infiltration (squeeze casting) of molten metal through dry-separated carbon fibres. The resulting MMC's microstructure revealed uniform distribution of fibres with minimum amount of clustering. Hardness values are considerably higher than that for the unreinforced matrix. Addition of carbon fibres has brought in strain in the crystal lattice of the matrix, resulting in higher microhardness of MMCs and improved wear resistance. Tensile strength values of MMCs at elevated temperatures are considerably higher than that of the unreinforced matrix processed under identical conditions. (C) 1999 Kluwer Academic Publishers.

Rearrangement approaches to sesquiterpenes containing multiple contiguous quaternary carbon atoms. Total synthesis of (+/-)-myltayl-8(12)-ene and (+/-)-6-epijunicedranol

Details of the first total syntheses of the sesquiterpenes myltayl-8(12)-ene and 6-epijunicedran-8-ol are described. The aldehyde 13, obtained by Claisen rearrangement of cyclogeraniol, was transformed into the dienones 12 and 18. Boron trifluoride-diethyl ether mediated cyclization and rearrangement transformed the dienones 12 and 18 into the tricyclic ketones 16 and 17, efficiently creating three and four contiguous quaternary carbon atoms, respectively. Wittig methylenation of 16 furnished (+/-)-myltayl-8(12)-ene (11), whereas reduction of the ketone 17 furnished (+/-)-6-epijunicedranol (23).

Synthesis of single-walled carbon nanotubes using binary (Fe, Co, Ni) alloy nanoparticles prepared in situ by the reduction of oxide solid solutions

Passing a H-2-CH4 mixture over oxide spinels containing two transition elements as in Mg0.8MyMz'Al2O4 (M, M' = Fe, Co or Ni, y + z = 0.2) at 1070 degrees C produces small alloy nanoparticles which enable the formation of carbon nanotubes. Surface area measurements are found to be useful for assessing the yield and quality of the nanotubes. Good-quality single-walled nanotubes (SWNTs) have been obtained in high yields with the FeCo alloy nanoparticles, as evidenced by transmission electron microscope images and surface area measurements. The diameter of the SWNTs is in the 0.8-5 nm range, and the multiwalled nanotubes, found occasionally, possess very few graphite layers. (C) 1999 Elsevier Science B.V. All rights reserved.

Correlated conformation and charge transport in multiwall carbon nanotube-conducting polymer nanocomposites

The strikingly different charge transport behaviours in nanocomposites of multiwall carbon nanotubes (MWNTs) and conducting polymer polyethylenedioxythiophene-polystyrene-sulfonic-acid (PEDOT-PSS) at low temperatures are explained by probing their conformational properties using small-angle x-ray scattering (SAXS). The SAXS studies indicate the assembly of elongated PEDOT-PSS globules on the walls of nanotubes, coating them partially, thereby limiting the interaction between the nanotubes in the polymer matrix. This results in a charge transport governed mainly by small polarons in the conducting polymer despite the presence of metallic MWNTs. At T > 4 K, hopping of the charge carriers following one-dimensional variable range hopping is evident which also gives rise to a positive magnetoresistance (MR) with an enhanced localization length (similar to 5 nm) due to the presence of MWNTs. However, at T < 4 K, the observation of an unconventional positive temperature coefficient of resistivity is attributed to small polaron tunnelling. The exceptionally large negative MR observed in this temperature regime is conjectured to be due to the presence of quasi-1D MWNTs that can aid in lowering the tunnelling barrier across the nanotube-polymer boundary resulting in large delocalization.