Fischer, John E

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Now showing 1 - 10 of 37
  • Publication
    EWGP--a work in progress....
    (2007-03-09) Fischer, John E
    The need for energy affects almost every aspect of modern society. Indeed, the advent of coal as a widely-used energy source is sometimes viewed as the spark that started the Industrial Revolution. However, we are about to enter a new era due to increased competition from developing nations for the world's dwindling energy supplies and to the growing recognition that our current energy usage is unsustainable and is affecting the world's climate. It is apparent that research on energy-related issues will become increasingly important in the coming years.
  • Publication
    Carbide-Derived Carbons: Effect of Pore Size on Hydrogen Uptake and Heat of Adsorption
    (2006-10-27) Yushin, Gleb; Dash, Ranjan; Jagiello, Jacek; Fischer, John E; Gogotsi, Yury
    Cryoadsorption is a promising method of enhancing gravimetric and volumetric onboard H2 storage capacity for future transportation needs. Inexpensive carbide-derived carbons (CDCs), produced by chlorination of metal carbides, have up to 80 % open-pore volume with tunable pore size and specific surface area (SSA). Tuning the carbon structure and pore size with high sensitivity by using different starting carbides and chlorination temperatures allows rational design of carbon materials with enhanced C-H2 interaction and thus increased H2 storage capacity. A systematic experimental investigation of a large number of CDCs with controlled pore size distributions and SSAs shows how smaller pores increase both the heat of adsorption and the total volume of adsorbed H2. It has been demonstrated that increasing the average heat of H2 adsorption above 6.6 kJ mol-1 substantially enhances H2 uptake at 1 atm (1 atm = 101 325 Pa) and -196 °C. The heats of adsorption up to 11 kJ mol-1 exceed values reported for metal-organic framework compounds and carbon nanotubes.
  • Publication
    Synthesis and Post-growth Doping of Silicon Nanowires
    (2005-11-01) Byon, Kumhyo; Tham, Douglas; Fischer, John E; Johnson, Alan T
    High quality silicon nanowires (SiNWs) were synthesized via a thermal evaporation method without the use of catalysts. Scanning electron microscopy and transmission electron microscopy showed that SiNWs were long and straight crystalline silicon with an oxide sheath. Field effect transistors (FETs) were fabricated to investigate the electrical transport properties. Devices on as-grown material were p-channel with channel mobilities 1 - 10 cm2 V-1 s-1. Post-growth vapor doping with bismuth converted these to n-channel behavior.
  • Publication
    Single-walled carbon nanotubes in superacid: X-ray and calorimetric evidence for partly ordered H2SO4
    (2005-07-01) Zhou, Wei; Fischer, John E; Heiney, P. A; Fan, H.; Davis, Virginia A; Pasquali, M.; Smalley, Richard E
    Liquid anhydrous sulfuric acid forms a partly ordered structure in the presence of single-walled carbon nanotubes (SWNTs). X-ray scattering from aligned fibers immersed in acid shows the formation of molecular shells wrapped around SWNTs. Differential scanning calorimetry of SWNT-acid suspensions exhibits concentration-dependent supercooling/melting behavior, confirming that the partly ordered molecules are a new phase. We propose that charge transfer between nanotube π electrons and highly oxidizing superacid is responsible for the unique partly ordered structure.
  • Publication
    From Fundamental Understanding to Predicting New Nanomaterials for High-Capacity Hydrogen Storage
    (2007-03-09) Zhou, Wei; Simmons, Jason; Fischer, John E; Yildirim, Taner
    • H2-Storage Materials: Current Impasse • Promising Hybrid Materials • Direct Adsorption Measurements • Neutron Scattering Capabilities • First Principles Calculations • Use a combination of experimental and theoretical studies to understand the detailed guest-host interactions in novel storage materials • Armed with this, develop advanced guest-host materials that can meet the DOE hydrogen storage challenge
  • Publication
    Self-branching in GaN Nanowires Induced by a Novel Vapor-Liquid-Solid Mechanism
    (2008-01-01) Nam, Chang-Yong; Tham, Douglas; Fischer, John E
    Nanowires have great potential as building blocks for nanoscale electrical and optoelectronic devices. The difficulty in achieving functional and hierarchical nanowire structures poses an obstacle to realization of practical applications. While post-growth techniques such as fluidic alignment might be one solution, self-assembled structures during growth such as branches are promising for functional nanowire junction formation. In this study, we report vapor-liquid-solid (VLS) self-branching of GaN nanowires during AuPd-catalyzed chemical vapor deposition (CVD). This is distinct from branches grown by sequential catalyst seeding or vapor-solid (VS) mode. We present evidence for a VLS growth mechanism of GaN nanowires different from the well-established VLS growth of elemental wires. Here, Ga solubility in AuPd catalyst is limitless as suggested by a hypothetical pseudo-binary phase diagram, and the direct reaction between NH3 vapor and Ga in the liquid catalyst induce the nucleation and growth. The self-branching can be explained in the context of the proposed VLS scheme and migration of Ga-enriched AuPd liquid on Ga-stabilized polar surface of mother nanowires. This work is supported by DOE Grant No. DE-FG02-98ER45701.
  • Publication
    An infiltration method for preparing single-wall nanotube/epoxy composites with improved thermal conductivity
    (2006-04-13) Du, Fangming; Guthy, Csaba; Fischer, John E; Kashiwagi, Takashi; Winey, Karen I
    Recent studies of SWNT/polymer nanocomposites identify the large interfacial thermal resistance at nanotube/nanotube junctions as a primary cause for the only modest increases in thermal conductivity relative to the polymer matrix. To reduce this interfacial thermal resistance, we prepared a freestanding nanotube framework by removing the polymer matrix from a 1 wt % SWNT/PMMA composite by nitrogen gasification and then infiltrated it with epoxy resin and cured. The SWNT/epoxy composite made by this infiltration method has a micron-scale, bicontinuous morphology and much improved thermal conductivity (220% relative to epoxy) due to the more effective heat transfer within the nanotube-rich phase. By applying a linear mixing rule to the bicontinuous composite, we conclude that even at high loadings the nanotube framework more effectively transports phonons than well-dispersed SWNT bundles. Contrary to the widely accepted approaches, these findings suggest that better thermal and electrical conductivities can be accomplished via heterogeneous distributions of SWNT in polymer matrices.
  • Publication
    Polarized spectroscopy of aligned single-wall carbon nanotubes
    (2000-11-15) Hwang, J.; Gommans, H. H.; Ugawa, A.; Tashiro, H.; Winey, Karen I; Haggenmueller, Reto; Fischer, John E; Tanner, D. B.; Rinzler, A. G.
    Polarized resonant Raman and optical spectroscopy of aligned single-wall carbon nanotubes show that the optical transitions are strongly polarized along the nanotubes axis. This behavior is consistent with recent electronic structure calculations.
  • Publication
    Nanoporous Carbide Derived Carbon with Tunable Pore Size
    (2003-09-01) Gogotsi, Yury; Nikitin, A.; Ye, H.; Zhou, Wei; Fischer, John E; Yi, B.; Foley, H. C; Barsoum, M. W
    During the past decades major efforts in the field of porous materials have been directed toward control of the size, shape and uniformity of the pores. Carbide-derived carbons (CDCs) represent a new class of nanoporous carbons with porosity that can be tuned with sub-Ångström accuracy in the range 0.5-2 nm. CDCs have a more narrow pore size distribution than single-wall carbon nanotubes or activated carbons; their pore size distribution is comparable with that of zeolites. CDCs are produced at temperatures from 200-1200oC as a powder, a coating, a membrane or parts with near-final shapes, with or without mesopores. They can find applications in molecular sieves, gas storage, catalysts, adsorbents, battery electrodes, supercapacitors, water/air filters and medical devices.
  • Publication
    Magnetically aligned single wall carbon nanotube films: preferred orientation and anisotropic transport properties
    (2003-02-15) Fischer, John E; Zhou, Wei; Vavro, Juraj; Llaguno, Mark C; Guthy, Csaba; Haggenmueller, Reto; Casavant, M. J; Walters, D. E; Smalley, Richard E
    Thick films of single wall carbon nanotubes (SWNT) exhibiting in-plane preferred orientation have been produced by filter deposition from suspension in strong magnetic fields. We characterize the field-induced alignment with x-ray fiber diagrams and polarized Raman scattering, using a model which includes a completely unaligned fraction. We correlate the texture parameters with resistivity and thermal conductivity measured parallel and perpendicular to the alignment direction. Results obtained with 7 and 26 Tesla fields are compared. We find no significant field dependence of the distribution width, while the aligned fraction is slightly greater at the higher field. Anisotropy in both transport properties is modest, with ratios in the range 5–9, consistent with the measured texture parameters assuming a simple model of rigid rod conductors. We suggest that further enhancements in anisotropic properties will require optimizing the filter deposition process rather than larger magnetic fields. We show that both x-ray and Raman data are required for a complete texture analysis of oriented SWNT materials.