Showing papers by "Yury Gogotsi published in 2006"

Anomalous Increase in Carbon Capacitance at Pore Sizes Less Than 1 Nanometer

Abstract: Carbon supercapacitors, which are energy storage devices that use ion adsorption on the surface of highly porous materials to store charge, have numerous advantages over other power-source technologies, but could realize further gains if their electrodes were properly optimized. Studying the effect of the pore size on capacitance could potentially improve performance by maximizing the electrode surface area accessible to electrolyte ions, but until recently, no studies had addressed the lower size limit of accessible pores. Using carbide-derived carbon, we generated pores with average sizes from 0.6 to 2.25 nanometer and studied double-layer capacitance in an organic electrolyte. The results challenge the long-held axiom that pores smaller than the size of solvated electrolyte ions are incapable of contributing to charge storage.

Control of sp2/sp3 carbon ratio and surface chemistry of nanodiamond powders by selective oxidation in air.

Abstract: The presence of large amounts of nondiamond carbon in detonation-synthesized nanodiamond (ND) severely limits applications of this exciting nanomaterial. We report on a simple and environmentally friendly route involving oxidation in air to selectively remove sp2-bonded carbon from ND. Thermogravimetric analysis and in situ Raman spectroscopy shows that sp2 and sp3 carbon species oxidize with different rates at 375−450 °C and reveals a narrow temperature range of 400−430 °C in which the oxidation of sp2-bonded carbon occurs with no or minimal loss of diamond. X-ray absorption near-edge structure spectroscopy detects an increase of up to 2 orders of magnitude in the sp3/sp2 ratio after oxidation. The content of up to 96% of sp3-bonded carbon in the oxidized samples is comparable to that found in microcrystalline diamond and is unprecedented for ND powders. Transmission electron microscopy and Fourier transform infrared spectroscopy studies show high purity 5-nm ND particles covered by oxygen-containing sur...

Carbide-Derived Carbons: Effect of Pore Size on Hydrogen Uptake and Heat of Adsorption**

Abstract: Cryoadsorption is a promising method of enhancing gravimetric and volumetric onboard H2 storage capacity for future trans- portation 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 en- hanced 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 kJmol -1 substantially enhances H2 uptake at 1 atm (1 atm=101325 Pa) and -196°C. The heats of adsorption up to 11 kJmol -1 exceed values reported for metal-organic framework compounds and carbon nanotubes.

Effect of Thermal Treatment on the Structure of Multi-walled Carbon Nanotubes

Abstract: The effects of vacuum annealing and oxidation in air on the structure of multi-walled carbon nanotubes (MWCNTs) produced by a large-scale catalytic chemical vapor deposition (CCVD) process are studied using Raman spectroscopy and transmission electron microscopy (TEM). A detailed Raman spectroscopic study of as-produced nanotubes has also been conducted. While oxidation in air up to 400°C removes disordered carbon, defects in tube walls are produced at higher temperatures. TEM reveals that MWCNTs annealed at 1,800°C and above become more ordered than as-received tubes, while the tubes annealed at 2,000°C exhibit polygonalization, mass transfer and over growth. The change in structure is observable by the separation of the Raman G and D′ peaks, a lower R-value (I D/I G ratio), and an increase in the intensity of the second order peaks. Using wavelengths from the deep ultraviolet (UV) range (5.08 eV) extending into the visible near infrared (IR) (1.59 eV), the Raman spectra of MWCNTs reveal a dependence of the D-band position proportional to the excitation energy of the incident laser energies.

Effect of Graphitization on the Wettability and Electrical Conductivity of CVD-Carbon Nanotubes and Films

Abstract: The use of carbon nanomaterials in various applications requires precise control of their surface and bulk properties. In this paper, we present a strategy for modifying the surface chemistry, wettability, and electrical conductivity of carbon tubes and films through annealing in a vacuum. Experiments were conducted with 60−300 nm nanotubes (nanopipes), produced by noncatalytic chemical vapor deposition (CVD) in a porous alumina template, and with thin films deposited by the same technique on a glassy carbon substrate having the same structure and chemistry of the CNTs. The surface of the as-produced CVD-carbon, treated with sodium hydroxide to remove the alumina template, is hydrophilic, and the bulk electrical conductivity is lower by a factor of 20 than that of fully graphitic multiwalled nanotubes (MWNT) or bulk graphite. The bulk electrical conductivity increases to the conductivity of graphite after annealing at 2000 °C in a high vacuum. The analysis of CNTs by transmission electron microscopy (TEM)...

In Situ Raman Spectroscopy Study of Oxidation of Double- and Single-Wall Carbon Nanotubes

Abstract: In situ Raman spectroscopy allows for a detailed and time-resolved investigation of the kinetics of complex physical or chemical processes. Oxidation has become a frequently used method for the removal of disordered carbon species from carbon nanotubes. Oxidation, however, can also induce damage to the tubes and destroy most of the sample. We conducted an in situ Raman spectroscopy study of the oxidation of double- and single-walled carbon nanotubes (DWCNT and SWCNT) under isothermal and nonisothermal conditions to identify the temperature range in which the oxidation of amorphous carbon occurs without any changes with respect to the tubes and their structure. In situ Raman spectroscopy analysis of the oxidation of DWCNTs showed a decrease in the intensity of the D band starting around 370 °C, followed by complete D band elimination at 440 °C. Oxidation studies of SWCNTs showed a similar decrease in the D band intensity, but the D band was not completely eliminated. Furthermore, in situ measurements allow...

Formation of Carbide-Derived Carbon on β-Silicon Carbide Whiskers

Abstract: Carbon was synthesized on β-SiC whiskers by extraction of Si atoms from SiC. In this study, three different elevated temperature extraction methods were used to remove Si atoms from SiC: treatments in either Cl2 or HCl and vacuum decomposition. In all chlorination experiments and vacuum treatment at 1700°C, carbon preserved the original shape of SiC whiskers. At higher temperatures (2000°C), vacuum decomposition led to a distortion in the shape of the whiskers. High-resolution transmission electron microscopy and Raman spectroscopy showed that the structure of carbide-derived carbon depends on the Si extraction method and the process parameters. Chlorination of SiC resulted in the formation of mostly amorphous nanoporous carbon. High-temperature treatment of SiC in HCl environment produced fullerene-like structures, while high-temperature vacuum decomposition resulted in the formation of graphite. Transmission electron microscopy studies of the carbon coating thickness produced in Cl2 at various chlorination times revealed linear reaction kinetics at 700°C. Raman studies showed that the carbon structure became more ordered with increasing chlorination temperature. The results obtained demonstrate that by using the silicon extraction technique, one can precisely control the thickness and morphology of the carbon coating.

Nanotubes and Nanofibers

Abstract: Preface Carbon Nanotubes: Structure and Properties John E. Fischer Chemistry of Carbon Nanotubes Eduard G. Rakov Graphite Whiskers, Cones, and Polyhedral Crystals Svetlana Dimovski and Yury Gogotsi Inorganic Nanotubes and Fullerene-Like Materials of Metal Dichalcogenide and Related Layered Compounds Reshef Tenne Boron Nitride Nanotubes:Synthesis and Structure Hongzhou Zhang and Ying Chen Nanotubes in Multifunctional Polymer Nanocomposites Fangming Du and Karen I. Winey One-Dimensional Semiconductor and Oxide Nanostructures Jonathan E. Spanier Nanofiber Technology Frank K. Ko Index

Carbide-derived carbons: a comparative study of porosity based on small-angle scattering and adsorption isotherms.

Abstract: Porous carbons have received much attention recently for potential applications in energy generation and storage, molecular sieving, and environmental remediation. Property optimization for specific applications rests largely on controlling the volume, size, and shape of the pores at the synthetic level. Direct atom-scale experiments which might accurately and reliably measure these quantities are problematic, so indirect methods such as gas sorption are generally employed. Here we apply a second indirect method, small-angle X-ray scattering (SAXS), to study porosity in carbide-derived carbons (CDC). The results qualitatively confirm and reinforce model-dependent conclusions drawn from gas sorption isotherms. In particular, both techniques indicate the onset of broad polydispersity under the same processing conditions for particular porous carbon materials.

Anisotropic etching of SiC whiskers

Abstract: We have demonstrated a method of producing nanoplatelets or complex well-ordered nanostructures from silicon carbide (SiC) whiskers. Preferential etching of SiC whiskers in a mixture of hydrofluoric and nitric acids (3:1 ratio) at 100 °C results in the selective removal of cubic SiC and the formation of complex structures resembling a pagoda architecture. Possible mechanisms governing selective etching are discussed. Reproducible results on SiC whiskers manufactured in different laboratories suggest that the self-patterning phenomena are common in SiC whiskers, and the same electroless etching procedure can be used to synthesize various complex nanostructures from more conventional nano- and microscale objects for use as building blocks in the fabrication of sensors, cellular probes, and electronic, optoelectronic, electromechanical, and other devices.

Wetting of CVD carbon films by polar and nonpolar liquids and implications for carbon nanopipes.

Abstract: The handling, dispersion, manipulation, and functionalization of carbon nanotubes and nanopipes often require the use of solvents. Therefore, a good understanding of the wetting properties of the carbon nanotubes is needed. Such knowledge is also essential for the design of nanotube-based nanofluidic devices, which hold the promise of revolutionizing chemical analysis, separation, drug delivery, filtration, and sensing. In this work, we investigated the wetting behavior of individual nanopipes produced by the chemical vapor deposition (CVD) of carbon in porous alumina templates and of thin carbon films produced by the same technique. The carbon pipes and films have the same chemistry and structure as determined by Raman and infrared spectroscopies and, when similarly treated, demonstrate the same qualitative wetting behavior, as determined by optical microscopy. Thus, measurements conducted on the carbon film surface are relevant to the nanopipes. In the case of the nanopipes, filling with various liquids...

Formation of Porous SiC Ceramics by Pyrolysis of Wood Impregnated with Silica

Abstract: Biomorphous β-SiC ceramics were produced at 1400°C from pine wood impregnated with silica This one-step carbothermal reduction process decreases the cost of manufacturing of SiC ceramics compared with siliconization of carbonized wood in silicon vapor The synthesized sample exhibits a 14 m 2 /g surface area and has a hybrid pore structure with large 5-20 μm tubular macropores and small (<50 nm) slit-shaped mesopores SiC whiskers of 20-400 nm in diameter and 5-20 μm in length formed within the tubular pores These whiskers are expected to improve the filtration by removing dust particles that could otherwise penetrate through large pores After ultrasonic milling, the powdered sample showed an average particle size of ∼30nm The SiC nanopowder produced in this process may be used for manufacturing SiC ceramics for structural, tribological, and other applications

Synthesis and Tribology of Carbide-Derived Carbon Films

Abstract: Carbide-derived carbon (CDC) films are produced at atmospheric pressure on the surfaces of carbide-based ceramic materials and coatings by a high-temperature chlorination process. These nanoporous carbon films contain carbon nano-onions and amorphous carbon, and may contain nanocrystalline diamond and graphite as well, depending on the synthesis conditions. The combination of such diverse carbon phases in one material or coating provides unique and potentially useful properties for a wide range of engineering applications. In this paper, we will present the results of a comprehensive study on the tribological behavior of these films. The friction coefficient of CDC in open air is comparable with that of graphite and is typically in the range of 0.15–0.25. However, the friction coefficients of CDC tend to decrease with decreasing humidity. In dry nitrogen, its friction coefficient is ∼0.1 or less. Such behavior is in contrast to that of crystalline graphite, which normally exhibits low friction at high humidity, but high friction at low humidity or in vacuum. The friction coefficient of CDC becomes increasingly lower under heavier loads; however, increasing sliding velocity does not seem to affect its frictional behavior significantly. Using a hydrogenation process that removes residual chlorine from the CDC film, the friction coefficients of CDC can be further lowered to values as low as 0.03. In an attempt to understand some of the underlying mechanisms, we carried out comprehensive chemical and structural studies of the sliding surfaces as well as bulk films and correlated these findings with the friction and wear behavior of CDC films.

Functional nanoparticle filled carbon nanotubes and methods of their production

Abstract: Carbon nanotubes filled with a suspension or colloidal solution of functional nanoparticles and methods for production of carbon nanotubes loaded with functional nanoparticles are provided.

Nanocellular high surface area material and methods for use and production thereof

Abstract: Nanocellular high surface area materials of a carbon material with high surface area that is controllable and which exhibits high conductivity, controllable structure and a precisely controllable pore size and methods for production and use of these materials are provided.

Process for producing nanoporous carbide-derived carbon with increased gas storage capability

Abstract: Methods for producing and using nanoporous carbide- derived carbon in applications involving gas storage are provided .

Process for producing nanoporous carbide derived carbon with large specific surface area

Abstract: Processes for the synthesis of high specific surface area nanoporous carbons by reacting select carbides with one or more halogens to produce compositions comprising carbon and halogens and contacting the reacted carbides with a species capable of removing the halogen are provided. Methods for removing halogen impurities from carbon compositions having pores and for modifying the surface terminations of carbon compositions having pores are also provided.

Detection of Indentation Induced FE‐to‐AFE Phase Transformation in Lead Zirconate Titanate

Abstract: Instrumented indentation was combined with microscopy and spectroscopy analysis to investigate the local mechanically induced ferroelectric to anti-ferroelectric phase transformation of niobium-modified lead zirconate titanate 95/5 Indentation experiments to a depth of 2 μm were performed using a Berkovich pyramidal three-sided diamond tip Subsequent Raman spectroscopy and piezoelectric force microscopy revealed that indentation locally induced the ferroelectric to antiferroelectric phase transformation Piezoelectric force microscopy demonstrated the ability to map the individual phases within and near indented regions on the niobium-modified lead zirconate titanate ceramics

Detection of Indentation Induced FE-to-AFE Phase Transformation in Lead Zirconate Titanate

Abstract: Instrumented indentation was combined with microscopy and spectroscopy analysis to investigate the local mechanically induced ferroelectric to anti-ferroelectric phase transformation of niobium-modified lead zirconate titanate 95/5. Indentation experiments to a depth of 2 {micro}m were performed using a Berkovich pyramidal three-sided diamond tip. Subsequent Raman spectroscopy and piezoelectric force microscopy revealed that indentation locally induced the ferroelectric to antiferroelectric phase transformation. Piezoelectric force microscopy demonstrated the ability to map the individual phases within and near indented regions on the niobium-modified lead zirconate titanate ceramics.

Imaging of liquid crystals confined in carbon nanopipes

Abstract: This letter investigates the unexplored phenomenon of the wetting of liquid crystals inside carbon nanopipes. Carbon nanopipes are similar to carbon nanotubes but are open ended with straight walls. Using environmental scanning electron microscopy, the authors have observed liquid crystal menisci inside the nanopipes. Raman spectroscopy was used to confirm the presence of liquid crystals confined in the nanopipes and verify the physical interaction between the two materials. In addition, observations from reflection polarized optical microscopy experiments suggest the use of liquid crystals inside carbon nanotubes to develop electro-optic switches.

Coelectrospinning of carbon nanotube reinforced nanocomposite fibrils

Abstract: In order to assess the potential of carbon nanotube (CNT) for structural composite applications, a method was proposed to convert CNT to continuous fibrils and fibrous assemblies using the co-electrospinning process. Preliminary experiments demonstrated the feasibility for the formation of composite CNT fibrils. These fibrils provide a convenient material form to carry the CNT and facilitate the formation of macro composite structures. The level and the nature of CNT and fiber alignments, and the inclusion of CNT in the nanofiber were elucidated through SEM, TEM observations and Raman spectra analysis. Mechanical testing of the nanofibril, nanofibril spunbonded mats and composite yarns were carried out to assess the effect of fibril alignment and verify the nancomposite fibril concept.

Parameterization of a Piezoelectric Nanomanipulation Device

Abstract: Through measurement of hysteresis and drift in a piezoelectrically driven multi-axis multi-positioner nanomanipulator, we have quantified error over ranges of three primary operating parameters: gain, duty cycle, and actuation time. A total of sixty-nine curves were plotted, and error and drift measured in a single operating plane. All data shown were collected on the Zyvex L-100 nanomanipulator operating in an open-loop programmed method, in both coarse (12mm range, 2–5 μm precision) and fine mode (100 μm range, 10–50 nm precision). In general, the normalized error was reduced at larger actuation distances and in coarse mode. These results may be used to define optimum operating conditions for piezoelectrically actuated nanomanipulators and micromanipulators whereby the goal is to find a balance between speed and accuracy. The results also are intended to allow for optimal control in closed-loop operation and for task-oriented and pick-and-place operations.© 2006 ASME

Simulations of Multi-atom Vacancies in Diamond

Abstract: Multi-atom vacancies and pores in diamond-structured carbon phases play an important role in carbon sieves and carbon based storage. The size and shape of pores have a profound effect on the energetics of adsorptive storage. We are modeling large numbers of vacancy configurations with a combination of ab initio density functional theory (DFT), tight binding DFT and simpler methods based on Brenner-potentials and modified Brenner potentials. The more accurate calculations serve as the basis of the parametrization which is then used in lattice Monte Carlo simulations on more complex vacancies. The results will be put in the context of SiC-derived porous carbon materials with the purpose to explore basic questions of energetics in porous carbons.