Optical Spectra of Carbon Nanotubes for Stress

Measurements of Advanced Aerospace Structures

Gregory Freihofer, Adonay Jimenez and Seetha Raghavan�

Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL32816

SDM 2010 student paper competition

The potential of carbon nanotubes being utilized for structural applications presentsa need to develop non-invasive measurement capabilities for tailoring of reinforcementsand to establish the structural integrity of such materials. This research is focused ondeveloping relationships between mechanical properties and the optical spectra of single-walled carbon nanotubes. Laser excitation wavelengths were varied and e�ects on the Gbands of the Raman spectra are presented here. Results of an in-situ thermal experimenton bulk carbon nanotube samples show the temperature dependency of these bands, whichare commonly used to quantify the elastic modulus for CNT composites.

I. Introduction

Since the discovery of carbon nanotubes (CNTs), research has shown their potential to create a newclass of advanced composites.1 These CNT composites will bene�t from the unique and exceptional

thermal,2{4 electrical,5,6 and mechanical7{9 properties of individual CNTs. The many potential applicationsfor these CNT composites in the aerospace industry signi�es the need for the development of experimentalmethods to non-invasively measure and characterize these new composites under varying environmentalconditions. Such information is vital to manufacturers to determine optimal arrangements and methods ofimplementing the CNTs into various matrix materials and to understand their behavior in both terrestrialand space environments.

Raman spectroscopy o�ers an e�ective way to characterize several of these properties. The raman bandsrepresent atomic bond energies of the CNTs to the surrounding matrix. There are 6 Raman active peakswithin the G-band due to symmetry breaking e�ects associated with nanotube curvature,9 however usuallythese 6 modes are not resolvable.10 The D band is normally made up of 1 peak, the intensity of which iscommonly compared to the G bands for sp2 and sp3 ratio derivations.11 By calibrating individual ramanpeaks with temperature and stress e�ects on the material, empirical relationships can be established. Peakshifts in the Raman Stokes G band (tangential vibrational modes) and D band (disorder induced modes)have been associated with mechanical and thermal stress.8,9, 12 The shifts caused by temperature can beattributed to the coe�cent of thermal expansion (CTE) mismatch between the matrix and reinforcement�bers.8 The observed shifts from the composite, compared to shifts seen in as-received carbon nanotubes(nanotubes prior to impregnation into matrix) can provide important information on the transfer of stressfrom the surrounding matrix to the CNT.

Mechanical and thermal stresses and a systematic calibration of their e�ects on the optical spectra ofcarbon nanotube materials are the focus of this research. The overall goal is to develop methods and stress-spectral relationships that can be used in the future for design, analysis and non-destructive structural stresstesting. In this paper, the relationship between G and D peak positions under a large temperature variationis developed. Spectral characteristics from a Pseudo-Voigt deconvolution10 were monitored in the G and Dbands and initial results of the e�ects of this temperature variation as well as laser excitation wavelengthon engineered carbon nanopaper are presented here. Fig 1 shows the bands and the material, which was

�Assistant Professor, MMAE Department, 4000 Central Florida Blvd, Orlando, FL32816, Senior Member.

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AIAA 2010-3131

Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

selected due to good dispersion of the single-walled nanotubes and its potential for use as aerospace structuralmaterials with possibilities of presenting signi�cant increases of elastic modulus.13

II. Experimental Procedure

The experiments were conducted at the Center for Nanoscale Materials at the Argonne National Labora-tory. The set up was designed to simulate the temperature variation the CNT material would encounter onthe equatorial lunar surface during a lunar day. For the study, the Raman G-band and D-band were selectedfor observation and analysis throughout the experiment. The temperature range of 300K was performed onthe sample while excited with a 514nm laser. Additional laser excitation was conducted with 325nm, 442nmand 633nm lasers at 288K. The simulation of lunar surface temperature ranges along the equatorial surfaceof the Moon are from 100K (�173�C) at night to 400K (127�C) at daylight. A Linkam R temperature stagewith a silver, liquid nitrogen-cooled sample holder was utilized to simulate the temperature range of 88Kto 388K at a rate of 4K/s with data acquisitions being collected at every 10K. The sample was excitedusing a 514nm green argon-ion laser through a Raman Invia-Microscope. A total of thirty-one datasets werecollected over a six and half hour period. The system would be allowed to stay at each degree measurementincrement for a total of �ve minutes to allow the sample to stabilize at each measurement increment. Thespectral region at which data was collected includes the radial breathing modes (RBMs), G-band, D band,and G’-band.

 

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Figure 1. 10 (a) An enhanced look at the G and D doublet, shown from the endpoints where deconvolutionlimits were set. Most commonly assigned D, G+ and G� bands are shown; these bands were from a pseudo-Voigt deconvolution using a peak assignment of 5. (b) An SEM image of the Single Walled Carbon NanotubeEngineered paper.13(c) Here the complete Raman Spectra is shown for the SWCNT engineered paper.

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III. Results & Discussion

A. Laser Excitation

The G and D bands were very dependent upon excitation wavelengths. The D band disperses with manycarbonaceous materials14,15 varying according to grain size and atomic structure which a�ects the scatteredelectron and phonon momentums.16 The data from varying laser excitations were utilized to associate suit-able excitation wavelengths for speci�c peaks to monitor. By inspection of Fig 2, the D band’s intensitygreatly increases as infrared excitation is approached. With this long wavelength, there is also a subtledispersion of the G-band; normally only thought to be dispersive with nanotube curvature.17 The actualmechanism behind this G-band dispersion is not evident, however the resolvability of these bands are in-creased and suggests future studies of similar CNT composites should be conducted in the near infrared withmore peaks assigned.10 The higher energy ultraviolet excitation appeared as if the spectrum was not CNT,but instead a highly orderd graphite sheet, producing a single peak at 1580cm�1 . Distinctions betweenG�(longitudinal) and G+(transverse) phonons cannot be readily made. Results show that the intensity ratioof D=G increased linearly with excitation energy; a result of resonance with D bands energy. This linearrelationship may be related to the microstructures ability to transform sp2 bonds into sp3 bonds, as similarresults from temperature experiments give such ratios.

 

 

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Figure 2. Laser excitation e�ects of CNT engineered paper.13 (a) Double resonant dispersion of D band,causing the disorderd induced D mode to shift with laser exciation.14 (b) Intensity Ratio of D=G changes ina near perfect linear fashion with respect to laser excitation.(c) The spectral range of deconvolution is shownfor all 4 laser excitations used.

B. Temperature dependency of G and D bands

Monitoring raman bands in a temperature experiment yields a large amount of data and Fig. 3 illustratesthe various peaks monitored for the in situ temperature experiment. Linear relationships for peak positionand other spectral characteristics are commonly made with temperature.8,9, 18 Broad peaks are normallynot used in stress relationships,19 however with advances in the accuracy of spectral characteristics im-proved by deconvolution,10 this may be further investigated. Resolvability of the peaks may have changed

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with improvements of spectral analysis techniques and some peaks become more and more resolved withtemperature. Figure 3 illustrates the improvement in consistent spectral characteristics for the broad peaks.

At a temperature of 298K, there is a signi�cant change in temperature relationships with position andwidth. Fig. 3 shows that at this critical temperature, the peaks change their relationship with temperaturein both sign and magnitude. The location of the critical temperature is emphasized with two linear �ts,one before and one after the critical point. This could be an indicator of slippage, or shifts of atomicbonds on the surface of the CNT. Such critical points were noticed in spectral characteristic relationshipsto mechanical strain in literature.9 Di�erent CNT-matrix composite combinations will have di�erent valuesfor these critical points and should be used in future studies to be more accurate indicators of mechanicale�ciency for CNT composites.

D=G ratios are important to consider for analyzing the e�ciency of the CNT composite by yielding a sp3

to sp2 ratio. Large changes in these ratios are considered to be an indication of deterioration such that thematerial is considered unacceptable for industrial applications.18 Usually intensity ratios are used, however,if the G band is deconvoluted as several peaks, an area ratio serves as a suitable replacement to intensityratios.10 At the critical temperature of 298K, the D=G area and intensity ratios change, and the slopeincreases, indicating an acceleration of phase change from sp2 to sp3. The increase of this D=G ratio forthis CNT Engineered paper is not as severe as increases for other multi-walled CNT composites at highertemperatures.18 Non-invasive methods for calculating this sp2 to sp3 transformability may be possible witha tunable laser experiments, and are the focus of future work. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Figure 3. Results from temperature experiment: a) The Raman G and D bands indicating a critical temper-ature at 298K. b-d) The peak position and width for D, G� and G+ respectfully. The critical temperature isindicated by the change in slope for each of the sub�gures, linear relations are plotted. The deconvolution waswith 5 peaks assigned for the range 1000-1800cm�1.

IV. Conclusion

The shifts in surface energies and compositional changes, caused by thermal expansion and compression,are indicators of the composite’s potential strength. Temperature experiments of CNT composites are carriedout to observe CTE-mismatch stresses that form at the surfaces of CNTs and matrix material.8 IntrinsicCNT Raman bands in a CNT composite should be upshifted compared to isolated CNT Raman bands. Agreater upshift is regarded as a better bonding between the CNT and surrounding matrix; and a betterbonding results in a greater transfer of energy from the matrix to the nanotube. In future studies, new CNT

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composites will be analyzed by Raman spectroscopy for quantifying bonding e�ciency and searching for theproper mixture of matrix material and CNTs. Raman spectroscopy of CNTs has shown promise as a methodof characterizing the mechanical properties. The method of deconvoluting the collected data from the CNTexperiments was successful in assisting the process of �nding and analyzing the peak characteristics of theG and D bands. Future work will analyze additional intrinsic CNT composite peaks.

V. Acknowledgements

Use of the Center for Nanoscale Materials was supported by the U. S. Departmentof Energy, O�ce ofScience, O�ce of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Dr. Gou Jihua (UCF)is acknowledged for providing the CNT Engineered paper samples for our experiments and the images of thesamples. The genetic algorithm codes used in our spectral analysis programs were courtesy of Dr WilliamCrossley (Purdue University).

References

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