|
| Figure 1. The radial breathing mode (RBM) frequency depends on the tube diameter d as ~1/d, which allows determination of the diameter distribution from Raman spectra. |
Institute for Space Systems Operations * 2001 Annual Report * 38-41
| Abstract--Carbon nanotubes are among the promising constituents of carbon nanonscale composites (CNCs) because of their unique properties, such as a one-dimensional structure (small diameter of ~1 nm and long length of many microns), high thermal and chemical stability, very good heat conduction, and high mechanical strength (elastic modulus comparable to that of a diamond). Currently, a large number of materials allow the implementation of SWNTs for the improvement of their properties or the acquisition of new qualities. Crucial for a successful design of CNCs is the knowledge of interactions between SWNTs and the matrix (polymers, epoxy resins, inorganic material) in the structural material. The ability of SWNTs to couple to the matrix is determined by the type of functionalization procedure that includes chemical activation such as oxidation or fluorination. UH researchers have developed several advanced Raman scattering methodologies. Raman imaging in conjunction with atomic force microscopy (AFM) has been successfully employed for studying of SWNTs' length and diameter distributions, electronic type (metallic or superconducting), and whether nanotubes are separated or in bundle. Raman micro-probe technique combined with transmission electron microscopy (TEM) has been used to collect Raman signals from a single nanotube bundle. A specially designed Raman scattering test provides a unique opportunity to probe, on a nanoscale level, the introduced strains in a structural material by measuring the changes of the SWNTs vibrational spectrum. An important consequence of this experiment is that it allows determination of load transfer from the matrix to the SWNTs and stress distribution in composites. The latter is of crucial importance for the design of structural materials. Proposed characterization methodologies should lead to weighty improvement of current composite technologies. |
Human exploration of space in the very future depends on advanced technologies such as nanotechnology. Toward this goal, the Johnson Space Center (JSC) is focusing on the development of nanotechnology based on single-wall carbon nanotubes (SWNTs). The JSC Carbon Nanotube team has made significant progress in refining SWNTs production techniques. Currently, pulsed laser vaporization (PLV) and a high pressure carbon oxide process (HiPCO), in collaboration with Rice University technology, are tuned to produce large amounts of SWNT material with reproducible characteristics. During the course of this project advanced purification methods have been developed that deliver SWNTs with specified degree of purity and abundance. In addition, JSC team is working towards fabrication of SWNT composites, with predicted strength-to-weight ratios that far exceed any of today's materials. In pursuing these goals JSC is demanding advanced characterization techniques.
The goal of the project is development of effective methodologies based on Raman spectroscopy technique for examination and characterization of SWNTs and nanocomposites.
Experimental and technical approach
The methodologies developed within the project are based on Raman spectroscopy.
Raman scattering is inelastic scattering of light, in which a transfer of energy and
(quasi)momentum from light to medium, or vice versa, occur. In solids, the requirement for
momentum conservation leads to coupling of light to excitations with a wave-vector close
to the center of the Brillouin zone. Practically important excitations in carbon nanotubes
are the phonons (carbon vibration modes). Raman scattering from phonons is always mediated
by electrons via electron-phonon coupling. Remarkably, the frequency of the radial
breathing modes (RMB), wr, in SWNTs obeys
the same functional dependence on the nanotube diameter d as that of the
transition energy between 1D density of states singularities: wr,
Eii~1/d. This peculiar feature of vibrational and electronic
excitations makes optical spectroscopy, in particular the resonant Raman scattering, a
powerful analytical tool for studying and characterization of SWNTs.
In addition, stresses and changes in the charge due to oxidation or reduction of SWNTs result in shift in frequency of the tangential mode at ~1590 cm-1, an important property with high impact on applications.
Results
SWNTs diameter distribution in parametric-varied materials
A thorough study of SWNTs produced at different parametric conditions of the laser
oven process is important for optimization of SWNT growth process. In Fig. 1 we show the
diameter distribution of parametric-varied SWNTs as it has been obtained by means of Raman
spectroscopy.
|
| Figure 1. The radial breathing mode (RBM) frequency depends on the tube diameter d as ~1/d, which allows determination of the diameter distribution from Raman spectra. |
Degree of SWNTs' alignment in bulk materials
SWNTs are highly anisotropic, true one-dimensional, Raman scatters. Raman intensity
of SWNTs is maximal when exciting light polarization is along the nanotube's axis. Raman
spectra measured in parallel incident and scattered light polarizations mostly represent
Raman signal from those SWNTs that are aligned along the incident polarization. This
property of SWNTs can be used for determination of degree of alignment of SWNTs in any
transparent matrix. An illustration is given in Fig. 2.
|
| Figure 2. Samples of SWNTS in polypropylene (E. Barrera, Rice University; B. Mayaux, NASA-JSC) with different degree of nanotubes' alignment measured for three angles between the light polarization direction and fiber/tape long axis. |
Raman imaging of SWNTs
Optical properties of SWNTs are rich in features. That is why a development of
imaging methods using electromagnetic waves in the optical range is highly compelling. In
Fig. 3 we present Raman spectra of a few well separated SWNT bundles recorder
simultaneously by using the line-scan attachment (2×10 mm2
area) of a LabRam HR spectrometer (in collaboration with S. Jandl, University of
Sherbrooke, Quebec, Canada).
Figure 3. Laser spot of ~2 microns run over a line segment of ~10 microns. Line image was projected on the spectrometer slit, dispersed by the spectrograph, and recorded on a 2D-CCD. The spectral window was 1300-1700 cm-1 conveniently covering the SWNT vibrations at ~1340 cm-1 (D-band), 1520-1550 cm-1 (G-band of metallic tubes), and 1590 cm-1 (G-line of semiconducting tubes).
Publications
Arepalli, S., P. Nikolaev, W. Holmes, V. G. Hadjiev, and B. S. Files. "Production and
Measurements of Isolated Single-Wall Nanotubes," in Nanonetwork Materials:
Fullerenes, Nanotubes and Related Systems. Ed. S. Saito, et. al., AIPCP 590, 11
(2001).
Hadjiev, V. G., M. N. Iliev, S. Arepalli, P. Nikolaev, and B. S. Files. "Raman
Scattering Test of Single-Wall Carbon Nanotube Composites," Appl. Phys. Lett.
78 (2001): 3193.
Presentations
Arepalli, S., P. Nikolaev, W. Holmes, V. G. Hadjiev, and B. S. Files. "Production and
Measurements of Isolated Single-Wall Nanotubes," NANOSPACE 2001, Int'l Conf. on
Integrated Nano/Microtechnology for Space and Biomedical Applications, March 13-16, 2001,
pp. 207.
Arepalli, S., P. Nikolaev, W. Holmes, V. G. Hadjiev, B. S. Files, and C. D. Scott.
"Direct Measurements of As-Produced Isolated SWNT and Small Ropes of Carbon,"
APS Meeting, Seattle, WA, March 12-16, 2001.
Hadjiev, V. G., M. N. Iliev, S. Arepalli, P. Nikolaev, B. S. Files, and C. D. Scott.
"Raman Spectroscopy of Nanocomposites: Test of Load Transfer, Interfaces, Residual
Stresses," MRS Meeting, Boston, MA, Nov. 27-Dec. 1, 2000.
Hadjiev, V. G., M. N Iliev., S. Arepalli, P. Nikolaev, B. S. Files, and C. D. Scott.
"Raman Spectroscopy Test of Nanocomposites," NANOSPACE 2001, Int'l Conf. on
Integrated Nano/ Microtechnology for Space and Biomedical Applications, March 13-16, 2001,
pp. 213.
Mayaux, B., B. S. Files, S. Arepalli, P. Nikolaev, O. Gorelik, and V. G. Hadjiev.
"Application of Carbon Nanotube Composites for Thermal Management," 8th
Foresight Conf. on Molecular Nanotechnology, Bethesda, MD, Nov. 3-5, 2000.
Nikolaev, P., O. Gorelik, S. Arepalli, C. D. Scott, B. S. Files, and V. G. Hadjiev.
"Working Towards Nanotube Composites," 6th Applied Diamond Conf./2nd Frontier
Carbon Technology Joint Conf., Auburn, AL, Aug. 6-10, 2001.
Funding and proposals
"Load Transfer in Carbon Nanotube Composites." National Science Foundation, Dec.
2001, $100,000; pending.
"Nanomechanics of Carbon Nanotubes Composites." Department of Energy, Feb. 2002,
$ 370,000; pending.
| Investigative Team UH PI: Milko N. Iliev, Ph.D.,
Research Professor UH Co-PI: Alexander P. Litvinchuk, Ph.D., Research Associate Professor JSC PI: Carl D. Scott, Ph.D. JSC Co-PI: Pavel Nikolaev, Ph.D. UH PDAF: Victor G. Hadjiev, Ph.D. Sivaram Arepalli, Ph.D. |
PDF (610K)
Table of Contents
Institute for Space Systems Operations - 2001
Annual Report
Copyright © 2002
|
|
