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  • Luhrs, Claudia  (26)
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  • 1
    Language: English
    In: Metallurgical and Materials Transactions B, 2013, Vol.44(1), pp.115-122
    Description: Recently the reductive expansion synthesis (RES) method was introduced as a means to create nano- and sub-micron metal particles and alloys by rapid heating of physical mixtures of urea with a metal nitrate. In the present work the generality of the RES method was demonstrated by creating metal micron and sub-micron particles from oxide and hydroxide precursors, and outlining the impact of temperature, precursor ratio, and gas flow rate on the product. For example, precursor selection impacted the temperature required for complete reduction, the amount of carbon present, and the size of the metal particles. For complete NiO reduction to micron scale particles, high urea content and a high temperature [ ca. 1073 K (800 °C)] were required. In contrast, Ni(OH) 2 was reduced to metal at far lower temperatures. Moreover, the Ni particles formed from NiOH were sub-micron ( ca. 200 nm) in size and carbon encapsulated. Other parameter variations had a similarly significant impact. Indeed, the reciprocal relationship between inert gas flow rate and the extent of reduction supports the supposition that the primary mechanism of reduced metal particle formation is the reduction of metal oxide particles by gases produced by urea decomposition. Collectively these and other findings indicate the RES method can be manipulated to create a range of micron and sub-micron reduced metal particle architectures appropriate for different applications.
    Keywords: Hydroxides ; Alloys ; Architecture ; Urea;
    ISSN: 1073-5615
    E-ISSN: 1543-1916
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  • 2
    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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  • 3
    Language: English
    In: Materials, 01 May 2014, Vol.7(5), pp.3699-3714
    Description: Samples of carbon nano-fiber foam (CFF), essentially a 3D solid mat of intertwined nanofibers of pure carbon, were grown using the Constrained Formation of Fibrous Nanostructures (CoFFiN) process in a steel mold at 550 °C from a palladium particle catalysts exposed to fuel rich mixtures of ethylene and oxygen. The resulting material was studied using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDX), Surface area analysis (BET), and Thermogravimetric Analysis (TGA). Transient and dynamic mechanical tests clearly demonstrated that the material is viscoelastic. Concomitant mechanical and electrical testing of samples revealed the material to have electrical properties appropriate for application as the sensing element of a strain gauge. The sample resistance versus strain values stabilize after a few compression cycles to show a perfectly linear relationship. Study of microstructure, mechanical and electrical properties of the low density samples confirm the uniqueness of the material: It is formed entirely of independent fibers of diverse diameters that interlock forming a tridimensional body that can be grown into different shapes and sizes at moderate temperatures. It regains its shape after loads are removed, is light weight, presents viscoelastic behavior, thermal stability up to 550 °C, hydrophobicity, and is electrically conductive.
    Keywords: Carbon Nanofiber ; Viscoelastic ; Strain Gauge ; Low Weight ; Porous ; Electrically Conductive ; Hydrophobic ; Engineering
    E-ISSN: 1996-1944
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  • 4
    Language: English
    In: Journal of Materials Research, 03/14/2011, Vol.26(5), pp.672-681
    Description: Upon rapid heating to a high temperature ( similar to 800 degree C), mixtures of nitrate compounds and urea created nano and submicron metal particles. The process (reductive/expansion synthesis, RES) results in atomic scale mixing. The product formed from mixed-nitrate (Fe + Ni)...
    Keywords: Ureas ; Nitrates ; Combustion Synthesis ; Metal Oxides ; Nanoparticles ; Decomposition ; General and Nonclassified (MD) ; General and Nonclassified (Ep) ; General and Nonclassified (Ed) ; General and Nonclassified (EC);
    ISSN: 0884-2914
    E-ISSN: 2044-5326
    Source: CrossRef
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  • 5
    Language: English
    In: Materials, 01 May 2014, Vol.7(6), pp.4182-4195
    Description: Carbon nanofilament and nanotubes (CNTs) have shown promise for enhancing the mechanical properties of fiber-reinforced composites (FRPs) and imparting multi-functionalities to them. While direct mixing of carbon nanofilaments with the polymer matrix in FRPs has several drawbacks, a high volume of uniform nanofilaments can be directly grown on fiber surfaces prior to composite fabrication. This study demonstrates the ability to create carbon nanofilaments on the surface of carbon fibers employing a synthesis method, graphitic structures by design (GSD), in which carbon structures are grown from fuel mixtures using nickel particles as the catalyst. The synthesis technique is proven feasible to grow nanofilament structures—from ethylene mixtures at 550 °C—on commercial polyacrylonitrile (PAN)-based carbon fibers. Raman spectroscopy and electron microscopy were employed to characterize the surface-grown carbon species. For comparison purposes, a catalytic chemical vapor deposition (CCVD) technique was also utilized to grow multiwall CNTs (MWCNTs) on carbon fiber yarns. The mechanical characterization showed that composites using the GSD-grown carbon nanofilaments outperform those using the CCVD-grown CNTs in terms of stiffness and tensile strength. The results suggest that further optimization of the GSD growth time, patterning and thermal shield coating of the carbon fibers is required to fully materialize the potential benefits of the GSD technique.
    Keywords: Carbon Fiber ; Carbon Nanofilaments ; Fiber Reinforced Composites ; Mechanical Properties ; Engineering
    E-ISSN: 1996-1944
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  • 6
    Language: English
    In: Nanoscience and Nanotechnology Letters, 03/01/2013, Vol.4(3), pp.316-322
    Description: Using the Aerosol Through Plasma (A-T-P) method in conjunction with simple chemical techniques a variety of complex and novel nanoparticle architectures were created. A-T-P was used to make metal core/carbon shell nanoparticles (ca. 50 nm diameter) of Sn/Carbon, which have potential for application as high energy density battery anodes for hybrid and electric vehicles. The initial Sn/carbon particles formed in the plasma were found to reach high capacity values, however presented poor cycle life possibly due to Sn volume expansion during lithiation and consequent shell breakage. Nonetheless, A-T-P Sn-core/carbon-shell material treated in acidic solution to partially remove tin, hence create void space, Sn/void space/carbon, showed higher discharge capacity and greatly improved cycling performance.
    Keywords: Carbon ; Voids ; Electric Potential ; Tin ; Shells ; Nanoparticles ; Nanostructure ; Energy Density ; Miscellaneous Sciences (So);
    ISSN: 19414900
    E-ISSN: 19414919
    Source: CrossRef
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  • 7
    Language: English
    In: Journal of Advanced Oxidation Technologies, July 31, 2015, Vol.18(2), p.295(8)
    Description: Orange II decomposition was studied on a variety of iron/carbon supported catalysts and control studies of the supports alone (carbon), and iron/alumina (non-active support). Variables tested included the impact of UV radiation, inclusion of hydrogen peroxide, catalyst treatment methods (oven treated and plasma torch treated) and type of the support. Results obtained for Orange II degradation indicated that active sites on carbon are more active for the catalytic decomposition of Orange II molecules, than metal sites. Oven-treated iron catalysts showed higher OII removal than catalysts prepared by plasma torch due to the fact that iron blocks carbon catalytic sites. XRD experiment on the non-active support allowed concluding that the oxidation state of Fe on the catalyst is not the main factor in the photocatalytic degradation of Orange II.
    Keywords: Plasma Physics ; Hydrogen Peroxide ; Radiation (Physics)
    ISSN: 1203-8407
    E-ISSN: 23711175
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  • 8
    Language: English
    In: Fibers, 01 February 2016, Vol.4(1), p.9
    Description: This work aimed to identify and address the main challenges associated with fabricating large samples of carbon foams composed of interwoven networks of carbon nanofibers. Solutions to two difficulties related with the process of fabricating carbon foams, maximum foam size and catalyst cost, were developed. First, a simple physical method was invented to scale-up the constrained formation of fibrous nanostructures process (CoFFiN) to fabricate relatively large foams. Specifically, a gas deflector system capable of maintaining conditions supportive of carbon nanofiber foam growth throughout a relatively large mold was developed. ANSYS CFX models were used to simulate the gas flow paths with and without deflectors; the data generated proved to be a very useful tool for the deflector design. Second, a simple method for selectively leaching the Pd catalyst material trapped in the foam during growth was successfully tested. Multiple techniques, including scanning electron microscopy, surface area measurements, and mechanical testing, were employed to characterize the foams generated in this study. All results confirmed that the larger foam samples preserve the basic characteristics: their interwoven nanofiber microstructure forms a low-density tridimensional solid with viscoelastic behavior. Fiber growth mechanisms are also discussed. Larger samples of mechanically-robust carbon nanofiber foams will enable the use of these materials as strain sensors, shock absorbers, selective absorbents for environmental remediation and electrodes for energy storage devices, among other applications.
    Keywords: Carbon Nanofibers ; Carbon Foam ; Engineering
    E-ISSN: 2079-6439
    Source: Directory of Open Access Journals (DOAJ)
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  • 9
    Language: English
    In: Nanoscience and Nanotechnology Letters, June 2009, Vol.1(2), pp.122-127
    Description: Carbon nanofilaments (CNFs) were grown on the surface of microscale carbon-fibers and glass fibers at low temperature using palladium as a catalyst to create multiscale fiber reinforcing structures with potential applications in structural composites. Employing a relatively new method, in which carbon structures are grown from fuel rich combustion mixtures on certain catalytic metals, multiscale filament structures were grown from ethylene/oxygen mixtures at 550 °C on commercial pitch carbon fibers and fiberglass. The filaments grew in different size distributions and distinct morphologies (whiskers and spirals) depending on the base fiber. Submicron fibers (ca. 200 nm) and spirals (ca. 50 nm) were dominating the grown species over the fiberglass substrate. Relatively short, densely spaced nanofilaments (ca. 10 nm), and a slightly less dense layer of larger (ca. 300 nm diameter) faster growing fibers were found to exist together to create a unique multiscale carbon structures over the pitch carbon fiber substrate. Transmission electron microscopy indicated poor crystallinity for the nanoscale carbon filaments grown on both pitch carbon fibers and fiberglass.
    Keywords: Carbon Fibers ; Fiber Glass ; Carbon Nanofibers ; Graphitic Structures ; Nanoropes ; Nanoropes
    ISSN: 1941-4900
    E-ISSN: 19414919
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  • 10
    Language: English
    In: Langmuir : the ACS journal of surfaces and colloids, 19 June 2007, Vol.23(13), pp.7055-64
    Description: Ceria-alumina particles of a wide variety of structures, from micrometer-sized hollow spheres to nanoparticles, were produced from aerosols of different natures, but all derived from nitrate salts passed through a low power (〈1000 W) atmospheric pressure plasma torch. The amount of water present with the nitrate salts was found to significantly affect the morphology of the resulting material. A model was proposed that explains the mechanism in which water acts as a blowing agent to create hollow metal oxide spheres that then shatter to form metal oxide nanoparticles. Further examination of the nanoparticles revealed that they display a core/shell morphology in which the core material is crystalline CeO2 and the shell material is amorphous Al2O3. These unique core/shell materials are interesting candidates for catalyst support materials with high thermal durability. In addition, experiments have shown that the nanoparticles can be readily converted into CeAlO3 perovskite.
    Keywords: Aerosol ; Aluminiumoxid ; Amorpher Stoff ; Atmosphärendruck ; Ceraluminat ; Ceroxid ; Hohlkugel ; Katalysatorträger ; Kern-Mantel-Struktur ; Kristalliner Stoff ; Mechanismus ; Mikrometerbereich ; Morphologie ; Nanopartikel ; Nitrat ; Perowskit ; Plasmabrenner ; Teilchengröße ; Thermische Stabilität ; Wassergehalt ; Chemistry;
    ISSN: 0743-7463
    E-ISSN: 15205827
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