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  • 1
    Language: English
    In: Carbon, 2011, Vol.49(4), pp.1058-1066
    Description: Dramatic increases in solid carbon deposition rates using palladium and cobalt are achievable at 550 °C while flowing ethylene and hydrogen by simple mixing of the powders of the two otherwise inactive metals. The synergistic mechanism is attributed to a favorable change in gas chemistry coupled with close proximity. ► Palladium and cobalt mixtures catalyze carbon deposition readily. ► When separate, neither metal is effective at carbon deposition. ► No extensive alloying was observed under normal reaction conditions. ► The synergism is attributed to favorable gaseous species being created. ► Close proximity is necessary for utilization of radicals by the other metal. The rate of catalytic carbon nanofiber formation from a mixture of ethylene and hydrogen at 550 °C was found to be dramatically faster over physical mixtures of palladium and cobalt micron scale particles than over either metal independently. The rate correlated with the metal fraction nearly identically for either Pd or Co rich mixtures. The highest rate increase over either pure metal was observed for a 1:1 mass ratio (∼150 times faster), although significant increases were found even at metal ratios of 11:1 (∼45 times faster). There was no direct evidence of extensive alloy formation from the mixed powders which suggests that a synergistic mechanism driven by proximity only may be responsible for the observed rate increases. It is thought a species (e.g. hydrogen atoms) formed at one metal (e.g. palladium) diffuses to the other where it accelerates carbon deposition by affecting the other catalyst material directly, or by generating favorable radical species. Kinetic synergism was also observed for Pd–Co alloys, although it was clearly less dramatic than that found for mixtures. Still, the fundamental similarity in behavior suggests that on the alloy surface two site types exist: one primarily Pd and one primarily Co.
    Keywords: Chemistry
    ISSN: 0008-6223
    E-ISSN: 1873-3891
    Source: ScienceDirect Journals (Elsevier)
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  • 2
    Language: English
    In: Carbon, June 2013, Vol.57, pp.363-370
    Description: A unique type of nonwoven carbon material has been developed which is flexible, resilient, and produced at modest temperature and near ambient pressure using catalytic deposition. This material is comprised entirely of nanoscale carbon fibers, which are extensively interlaced to create a coherent, bulk material. The structure and basic mechanical and electrical properties of this material were investigated through cyclic compression and resistance measurement. The material was highly elastic and capable of being repeatedly compressed without disintegration. The mechanical response varied with density, and the density was controlled by the amount of catalyst used. The material exhibited a high electrical resistivity, which varied nonlinearly with compression.
    Keywords: Chemistry
    ISSN: 0008-6223
    E-ISSN: 1873-3891
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  • 3
    Language: English
    In: Carbon, 2010, Vol.48(7), pp.1932-1938
    Description: Carbon nanofiber growth on palladium particles from ethylene–oxygen mixtures was investigated with respect to thermal history. Electron microscopy, combined with focused ion beam cross-sectioning show particles sinter quickly, but can be stabilized by the addition of a short carbon deposition step at a temperature below the general reaction temperature. This step generates a thin layer of carbon on the catalyst which reduces sintering once the temperature is raised to the optimal reaction temperature. For example, high temperature (e.g. 500 °C) catalyst pre-treatment leads to catalyst particle sintering, and subsequent fiber growth produces large diameter fibers. In contrast, small diameter fibers form on catalyst particles pretreated at low temperature (ca. 350 °C), even if the fibers are grown at a temperature at which deposition rates are faster (e.g. 550 °C). These results led to the development of unique multiple temperature fiber growth protocols that produce smaller diameter fibers while improving the deposition rate.
    Keywords: Chemistry
    ISSN: 0008-6223
    E-ISSN: 1873-3891
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  • 4
    Language: English
    In: Carbon, June 2018, Vol.132, pp.411-419
    Description: This study shows high stability Sn (10 wt %)/carbon Li-ion battery anodes can be made via the Reduction Expansion Synthesis (RES) process. Hybrid Sn/C anodes had an initial capacity of 425 mAh g-1 which stabilized to ∼340 mAh g-1 after less than 10 cycles. Unlike earlier Sn/C anodes, capacity remained virtually constant for more than 180 additional cycles. Neat carbon independently tested for Li capacity had a steady specific capacity of 280 mAh g-1. The difference detected between the pure carbon and Sn/C cases are consistent with Sn having the theoretical capacity of ∼1000 mAh g-1. The high stability of the RES derived anodes, relative to earlier Sn based electrodes, is postulated to exist because RES synthesis enables the formation of direct, strong bond between Sn and carbon substrate atoms, hence reducing the rate of Sn electrode disintegration and capacity fade due to expansion upon lithiation. X-ray diffraction and transmission electron microscopy are consistent with this postulate as both show an initial Sn particles size of only a few nanometers and minimal growth after cycling. Reduced interface resistance is also indicative of unique Sn-carbon bond.
    Keywords: Chemistry
    ISSN: 0008-6223
    E-ISSN: 1873-3891
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  • 5
    Language: English
    In: Carbon, 2009, Vol.47(13), pp.3071-3078
    Description: Employing a relatively new method, in which carbon structures are grown from fuel rich combustion mixtures using palladium particles as catalyst, multi-scale diameter nanometer – micrometer filament structures were grown from ethylene/oxygen mixtures at 550 °C on commercial PAN micrometer carbon fibers. The filaments formed had a diameter roughly equal to the palladium particle size. At sufficiently high metal loadings (〉0.05 wt.%) a bimodal catalyst size distribution formed, hence a bimodal filament size distribution was generated. Relative short, densely spaced nanofilaments (ca. 10 nm diameter), and a slightly less dense layer of larger (ca. 100 nm diameter) faster growing fibers (ca. 10 μm/h) were found to exist together to create a unique multi-scale structure. A protocol was developed such that only nano-scale fibers or a mixture of nano and sub-micron fibers could be produced. No large range order was evident in the filaments. This work demonstrates a unique ability to create a truly ’ carbon structure on the surface of carbon fibers. This fiber structure potentially can enhance composite material strength, ductility and energy absorption characteristics.
    Keywords: Chemistry
    ISSN: 0008-6223
    E-ISSN: 1873-3891
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  • 6
    Language: English
    In: Carbon, 2009, Vol.47(9), pp.2269-2280
    Description: The characteristics of carbonaceous materials deposited in fuel rich ethylene–oxygen mixtures on three types of palladium: foil, sputtered film, and nanopowder, are reported. It was found that the form of palladium has a dramatic influence on the morphology of the deposited carbon. In particular, on sputtered film and powder, tight ‘weaves’ of sub-micron filaments formed quickly. In contrast, on foils under identical conditions, the dominant morphology is carbon thin films with basal planes oriented parallel to the substrate surface. Temperature, gas flow rate, reactant flow ratio (C H :O ), and residence time (position) were found to influence both growth rate and type for all three forms of Pd. X-ray diffraction, high resolution transmission electron microscopy, temperature-programmed oxidation, and Raman spectroscopy were used to assess the crystallinity of the as-deposited carbon, and it was determined that transmission electron microscopy and X-ray diffraction were the most reliable methods for determining crystallinity. The dependence of growth on reactor position, and the fact that no growth was observed in the absence of oxygen support the postulate that the carbon deposition proceeds by combustion generated radical species.
    Keywords: Chemistry
    ISSN: 0008-6223
    E-ISSN: 1873-3891
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  • 7
    Language: English
    In: Carbon, 2003, Vol.41(13), pp.2555-2560
    Description: Analysis of the many studies of carbon nanotube formation in high-temperature ovens clearly indicates the key requirements of nanotube formation are an ‘atomic’ carbon source and a source of nanometal particles. We adapted this formulation to the high temperature (〉3000 K) environment found in a low-power (〈1000 W) atmospheric pressure, microwave plasma torch, by simultaneously feeding carbon monoxide (carbon source), and (presumably) iron carbonyl (source of metal catalyst particles) through an argon stabilized plasma flame. This technique led to the relatively rapid (25 mg/h) formation of carbon nanotubes of a unique form: macro-sized ‘woven’ threads. Scanning electron microscopy and high-resolution transmission electron microscopy studies revealed that the woven threads consist entirely of carbon nanotubes (primarily carbon single-wall nanotube) and associated nano-iron particles. The structures appear ‘fractal’ in that each woven thread appears to be constructed of smaller threads that in turn are formed of yet smaller woven threads. Simple mechanical tests show the threads can be bent without breaking, and the thread will spring to its original shape when the force holding it is released. Threads of the size produced can be woven together to form actual cloth or ropes and thus this result represents a step toward the ultimate application of carbon nanotubes for super strong/light structures.
    Keywords: A. Carbon Nanotubes ; B. Plasma Reactions ; C. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (Tem) ; Chemistry
    ISSN: 0008-6223
    E-ISSN: 1873-3891
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  • 8
    Language: English
    In: Carbon, 2000, Vol.38(5), pp.691-700
    Description: This study was designed to determine the value of microcalorimetry as a probe of acid/base sites on carbon surfaces. Calorimetric studies of ammonia adsorption on acid and ammonia pretreated activated carbon (BDH) samples demonstrated that the technique does titrate acid sites (at equilibrium) according to their relative strengths. However, only in conjunction with other techniques, including Boehm titration, point of zero charge (PZC) and temperature programmed desorption (TPD) is it possible to determine the probable identity of the acid sites present on a given carbon. Collective consideration of the data from all techniques suggests that ammonia pretreatments create a surface whose chemistry is completely different from that of the original carbon. Not only are new acidic sites created, but a high concentration of basic sites are introduced as well. Thus, the final surface is clearly amphoteric, to an extent which is largely dictated by the pretreatment temperature.
    Keywords: A. Activated Carbon ; B. Chemical Treatment ; D. Functional Groups, Surface Properties ; Chemistry
    ISSN: 0008-6223
    E-ISSN: 1873-3891
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  • 9
    Language: English
    In: Carbon, 01 January 2009
    Description: The characteristics of carbonaceous materials deposited in fuel rich ethylene-oxygen mixtures on three types of palladium: foil, sputtered film, and nanopowder, are reported. It was found that the form of palladium has a dramatic influence on the morphology of the deposited carbon. In particular, on sputtered film and powder, tight 'weaves' of sub-micron filaments formed quickly. In contrast, on foils under identical conditions, the dominant morphology is carbon thin films with basal planes oriented parallel to the substrate surface. Temperature, gas flow rate, reactant flow ratio (C2H4:02), and residence time (position) were found to influence both growth rate and type for all three forms of Pd. X-ray diffraction, high-resolution transmission electron microscopy, temperature-programmed oxidation, and Raman spectroscopy were used to assess the crystallinity of the as-deposited carbon, and it was determined that transmission electron microscopy and x-ray diffraction were the most reliable methods for determining crystallinity. The dependence of growth on reactor position, and the fact that no growth was observed in the absence of oxygen support the postulate that the carbon deposition proceeds by combustion generated radical species.
    Keywords: Carbon ; Carbonaceous Materials ; Catalysts ; Combustion ; Deposition ; Ethylene ; GAS Flow ; Mixtures ; Morphology ; Oxidation ; Oxygen ; Palladium ; Radicals ; Raman Spectroscopy ; Substrates ; Thin Films ; Transmission Electron Microscopy ; X-Ray Diffraction ; Chemistry
    ISSN: 0008-6223
    E-ISSN: 1873-3891
    Source: SciTech Connect (U.S. Dept. of Energy - Office of Scientific and Technical Information)
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