We study the resonance behavior of the unusual Raman feature known as the coalescence-inducing mode ͑CIM͒, observed at ϳ1850 cm −1 , in samples of double-wall carbon nanotubes annealed at high temperatures. Resonance Raman spectra taken with different laser energies show that the intensity of the CIM band exhibits a maximum around 2.20 eV. By comparing the resonance Raman experimental results with first-principles calculations for the vibrational frequency and the energy gap, we propose that the CIM feature is associated with short carbon chains with an odd number of atoms, interconnecting the nanotube surfaces. DOI: 10.1103/PhysRevB.73.193408 PACS number͑s͒: 78.30.Na, 63.22.ϩm, 78.20.Bh, 78.66.Tr The Raman spectra of carbon nanotubes exhibit weak features in the spectral range between 1600 and 2000 cm −1 that are ascribed to a second-order Raman process involving the combination of two phonons.1,2 However, the observation of unusual and strong spectral features around 1850 cm −1 have been reported recently in the Raman spectra of carbon nanotube systems, [3][4][5] and have been ascribed to the vibration of one-dimensional ͑1D͒ chains of carbon atoms. In particular, a strong and sharp feature at ϳ1850 cm −1 is observed in the Raman spectra of double-wall nanotube ͑DWNT͒ samples heat treated at high temperatures and, since it appears at specific annealing temperatures ͑T htt ͒ that occur just below the T htt needed for full coalescence of DWNTs, it is named the coalescence-inducing mode ͑CIM͒.To characterize this unusual Raman feature at ϳ1850 cm −1 , we report a complete resonance Raman study of this phonon band, using many different laser excitation energies ͑E laser ͒. The intensity dependence of the CIM band on E laser is here determined, and is shown to exhibit a maximum at around 2.2 eV. The experimental results are compared with theoretical calculations, using either density functional theory ͑DFT͒ or Hartree-Fock ͑HF͒ approaches, for finite carbon chains containing a small number of carbon atoms for which the normal mode frequencies and energy gaps were obtained. According to the calculations presented here, the vibrational frequencies and the resonance behavior obtained experimentally are in close agreement with the formation of short linear carbon chains during the coalescence process, with the chains having an odd number of carbon atoms fixed to the carbon nanotube walls under different environments ͑deformed and twisted chains͒.Room-temperature Raman spectra were recorded in the backscattering configuration using a Dilor XY triple monochromator, equipped with a cooled charge-coupled device ͑CCD͒ and using several different laser line excitations from an Ar-Kr ion laser and a dye laser in the range 1.9-2.7 eV. A laser power Ͻ1 mW was focused on a ϳ2 m 2 spot during the measurements.The samples studied here consist of highly purified DWNT bundles synthesized by a catalytic chemical vapor deposition method. 6,7 The diameter distribution of the sample is 0.77Յ d t Յ 0.90 nm for the inner tubes and 1.4...

No abstract

The technical feasibility of the simultaneous desulfurization and dephosphorization process of hot metal with high basicity slags of the BaO‐BaF2 system is discussed from the thermodynamic point of view. The partition ratios as well as the degrees of removal of sulphur and phosphorus from the hot metal obtained after a single refining step with BaO‐saturated slags have been compared to those achieved when typical hot metal pretreatment slags of the system CaO‐CaF2‐SiO2 (double saturated with CaO and 3CaO•SiO2) were used. The experiments were carried out in graphite crucibles at 1573 K, under CO atmosphere. Considering that both the desulfurization degree and the dephosphorization degree of hot metal achieved with BaO‐based slags, around 95 %, were much higher than those obtained when CaO‐based slags were used, maximum 45 %, it was concluded that the simultaneous desulfurization and dephosphorization process of hot metal with high basicity BaO‐BaF2 slags is technically feasible. It has been verified also that the high basicity BaO‐based slags led to manganese recovery degrees as high as 86 % and prevented at the same time the undesirable oxidation of carbon from the metal phase.

No abstract

No abstract