P.C. Eklund

DISSOLUTION OF SINGLE-WALLED CARBON NANOTUBES

834 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/1007-0834

Lattice vibrations in graphite and intercalation compounds of graphite

17.50+.50/0 Adv. Mater. 1999, 11, No. 10 be detected. Furthermore, the mechanical properties of the complex are solid-like at temperatures below the observable glass transition, which would not be the case for a phase separated material containing rubbery microdomains. Instead, we advance an explanation for the extraordinary properties of this liquid crystal complex by invoking a high degree of coupling between mesogenic side groups and the ethylene oxide backbone which thereby inhibits the formation of helical conformations favored by afreeo poly(ethylene oxide) chains. Within such helical arrangements the lithium ions would be tightly coordinated below Tg and effectively trapped. By suppressing the formation of such helical structures, an open ethylene oxide structure is obtained within which lithium ions are free to move and where empty sites exist for the ions to occupy. It is remarkable that a similar (but weaker) effect is observed in the amorphous material. It seems that insertion of rigid isophthalate units also inhibits helical formation, sufficient to provide a measure of ionic decoupling, but for the liquid crystal complex the open structure is further stabilized via the interactions between the liquid crystal side groups. It is worth noting that this view is supported by the continuity of behavior from the melt into the glassy state in both the heat capacity and electrical conductivity, indicating that the structure of the melt is not strongly influenced by temperature. In addition, the dissolution of the ions in the backbone does not swell the smectic layer and thus, the open network must be relatively unchanged at least in the direction normal to the smectic layers. This suggests that by careful engineering of the types of liquid crystal phase present, it should be possible to tailor the conductivity mechanism to particular applications. It is also remarkable that the conductivity in the MeOC6G6 complex increases strongly (by several orders of magnitude) as the AO:Li ratio is decreased from 10:1 to 3:1. We should now be able to employ more concentrated polymer electrolytes than is presently possible with conventional materials. Transport number data for these electrolytes are not yet available, but it is tempting to speculate that the elimination of acation trappingo within the ethylene oxide helix will lead to substantial increases in cation mobility. For many years this has been one of the principal goals of polymer electrolyte research.

Applications for activated carbons from used tires: butane working capacity

Abstract The lattice modes of pure graphite can be divided into intraplanar displacements and interplanar displacements. Upon intercalation, the intraplanar vibration frequencies are only slightly shifted, though their Raman and infrared intensities can be greatly changed. These characteristics of the intraplanar vibrational modes are interpreted in terms of symmetry changes associated with intercalation. These symmetry changes give rise to both inplane and c-axis zone folding effects. Detailed results are reported for the Raman spectra of graphite-halogen acceptor compounds and graphite-alkali metal donor compounds. In the case of the graphite-halogen compounds, spectra showing both graphitic modes and intercalate modes are interpreted in terms of this symmetry model. In the case of the alkali metal compounds, the spectra for stage 1 compounds are considered separately from those for higher stage compounds. Particular attention is given both to the broad asymmetric Breit-Wigner high frequency structure and to the sharp doublet structure observed at intermediate frequencies. Infrared spectra of lattice modes in graphite intercalation compounds are reported.

Optical reflection studies of the electronic properties of stages 2 – 5 graphite-SbCl5

Abstract Activated carbons obtained from the pyrolysis (600–800°C) of used tires have been synthesized and studied for n-butane adsorption. To obtain optimal butane working capacity (BWC), higher burn-off was found necessary. It was also found that higher pyrolysis temperatures (∼800°C) allow mesoporosity to develop more readily and furthermore this porosity leads to higher BWC.

Synthesis of graphite-SbCl5 and optical tests of its environmental stability

Abstract We have measured the absolute reflectance spectra at near-normal incidence for well-characterized stages 2, 3, 4 and 5 graphite-SbCl 5 . The measurements were made with a prism monochromator in the range 0.1 – 3.0 eV. Because of the air stability of these compounds we have also been able to determine experimentally the values of the real and imaginary parts of the dielectric constant at 1.96 eV by measuring the absolute reflectance of the samples in air and CCl 4 using a HeNe laser. The reflectance spectra were analyzed in terms of a phenomenological dielectric constant model with contributions from the core, free carries, and interband transitions. Interband transitions are modeled as Lorentz oscillators. The spectra were fitted with the additional constraint that the calculated values of the real and imaginary dielectric constant at 1.96 eV matched those directly determined by experiment. The data are discussed in terms of the band model of Blinowski et al . and the more recent band model reported by Dresselhaus and Leung.

Charge-transfer-induced changes in the electronic and lattice vibrational properties of acceptor-type GICs

Abstract Details of the synthesis of stage 2 to stage 5 graphite-SbCl 5 are presented. The samples were prepared by the reaction of highly oriented pyrolytic graphite with SbCl 5 in a closed Pyrex tube by a two-temperature technique. We characterized our samples by X-ray diffractograms of the (001) reflections. The X-ray data were analyzed for the various c axis repeat distances and we found that I c ( n ) = 9.37 + ( n − 1)3.35 ± 0.02 A where n is the stage number of the compound. This results is in good agreement with previous work. Results of optical reflectance studies on samples measured in situ and on samples exposed to laboratory air for several months are presented. These studies indicate that SbCl 5 intercalates into graphite to form an environmentally stable synthetic metal.

X-ray study of the layer structure of graphite-antimony chloride intercalation compounds

Abstract We report on several charge-transfer-induced changes in the electronic and phonon properties of stage 1 and 2 graphite-H2SO4: c-axis LO and LA phonon dispersion, E2g (q = 0) intralayer graphitic phonon frequency, and resonant Raman scattering from the E2g modes. The data are discussed in terms of microscopic models.

CC bond distance and charge transfer in D2SO4 — Graphite compounds

Abstract We present the results of an X-ray diffraction study of stages 1, 2 and 3 graphite-antimony chloride intercalation compounds. Integrated intensity data on the (00l) diffraction peaks were analyzed in terms of a centro-symmetric structural model involving periodic stacking of atomic layers of carbon, antimony and chlorine. The results of our X-ray data analyses are found to be in good agreement with recent (121Sb) Mossbauer results which report large concentrations of both trivalent (Sb3+) and pentavalent (Sb5+) antimony in the intercalate layers. The X-ray derived stoichiometries were found to be C13.2SbCl4.8, C26.9SbCl5.0, and C47SbCl5.7 for the stage 1, 2, and 3 compounds, respectively. We are unable to distinguish separate contributions to a central Sb5+ layer from SbCl5 and SbCl6 molecules. We find the Sb3+ layers located at z = ± 0.07 A relative to an Sb5+ mirror plane (z = 0) in the center of the intercalate layer. These locations for the Sb3+ layers suggest that SbCl3 molecules are oriented in a configuration in which each SbCl3 molecule bridges the space between bounding carbon layers. The Cl− layers ( z = ± 1.40 A ) were found to contact the bounding carbon layers.

Experimental Studies of Phonon Dispersion and C-Axis Structure in Graphite Intercalation Compounds

Abstract The relationship between the carboncarbon bond distance and charge transfer in GICs has been studied experimentally, mainly by in situ neutron diffraction measurements on D 2 SO 4 — graphite during electrochemical intercalation. There is substantial, but not perfect, agreement with the theory of Pietronero and Strassler.

Optical and photochemical properties of C60 films

In this paper we present new results, obtained from a variety of experiments, which probe the structure and lattice dynamics of donor- and acceptor-type graphite intercalation compounds (GIC’s). Specifically, we wish to discuss conclusions deduced from x-ray diffraction, Mossbauer, inelastic neutron scattering and Raman scattering investigations of well-staged GIC’s. The paper is organized as follows: in section II we discuss the experimental details, in section III we collect results pertaining to the chemistry, layer structure and c-axis dispersion of low-frequency phonon branches in SbCl5:graphite; in section IV and V we present the results of Raman scattering studies; section IV — the stage-dependence of two-phonon scattering from high-frequency graphitic modes in SbCl5:graphite (acceptor system) and Rb:graphite (donor system); section V- the observation and study of the temperature dependence of low frequency shear modes (ω ≃ 15, 20 cm-1) in stage 2 Rb:graphite.