ABSTRACT Microwave interaction with single-walled, carbon nanotubes (SWNTs) in vacuum has been shown to result in rather dramatic effects including highly ionized plasmas, high temperatures, and unique morphological changes. The mechanism for absorption of microwave energy in SWNTs is contested and centers on the role of metallic impurities. Such impurities, especially Fe, are introduced during the synthesis process as a catalyst to control the morphology of the SWNTs. In this work, the absorption of microwave energy was determined for different types of carbon nanotubes, including multi-walled and double-walled varieties. Particle Induced X-Ray Emission (PIXE) analysis was done to identify the impurities and their concentrations in the samples. Also, low-temperature gas desorption from the carbon nanotubes was studied. The results indicate that metallic impurities do not play an important role in the absorption of microwave energy by carbon nanotubes but rather structural properties dominate

F D Mcdaniel

F Naab

J L Duggan

J Roberts

M Dhoubhadel

O W Holland

CiteSeerX

 10th International Conference on 
 Particle Induced X-ray Emission and its Analytical Applications  
 PIXE 2004, Portorož, Slovenia, June 4-8, 2004 
 http://pixe2004.ijs.si/ 
 
601.1 
The role of metallic impurities in the interaction of carbon nanotubes with 
microwave radiation 
F. Naab, M. Dhoubhadel, O.W. Holland, J.L. Duggan, J. Roberts and F.D. McDaniel 
Department of Physics, P.O. Box 311427, University of North Texas, Denton, TX 76203-1427, USA 
ABSTRACT 
Microwave interaction with single-walled, carbon nanotubes (SWNTs) in vacuum has been shown to result in rather 
dramatic effects including highly ionized plasmas, high temperatures, and unique morphological changes.  The 
mechanism for absorption of microwave energy in SWNTs is contested and centers on the role of metallic impurities.  
Such impurities, especially Fe, are introduced during the synthesis process as a catalyst to control the morphology of the 
SWNTs.  In this work, the absorption of microwave energy was determined for different types of carbon nanotubes, 
including multi-walled and double-walled varieties.  Particle Induced X-Ray Emission (PIXE) analysis was done to 
identify the impurities and their concentrations in the samples.  Also, low-temperature gas desorption from the carbon 
nanotubes was studied.  The results indicate that metallic impurities do not play an important role in the absorption of 
microwave energy by carbon nanotubes but rather structural properties dominate. 
Keywords: thick samples; PIXE; carbon nanotubes; impurities; microwave radiation. 
1. INTRODUCTION 
 The interaction of microwave radiation with matter is of fundamental and technological 
importance.  Microwave technology is widely utilized in the food industry for heating, sterilization, 
drying, freeze-drying, etc., and in chemical processing [1] where it provides better control of 
temperature/pressure during synthesis.  Recently, microwave heating of graphite to produce carbon 
nanotubes (CNTs) has been demonstrated [2].  A nanotube is an allotrope of carbon with unique 
electronic and mechanical properties derived from its Fullerene structure.*  Microwaves offer advantages 
over standard thermal methods that can be integrated into many CNTs applications where thermal 
activation is required. 
 Recently, studies have been done to understand the microwave absorption by single-wall carbon 
nanotubes (SWNTs) with conflicting results.  The formation of SWNTs involves the use of metallic 
catalyst such as Fe, Ni, Co, etc. to promote the formation of SWNTs over other varieties of nanotubes, 
i.e. multi-walled (MWNTs) and double-walled nanotubes (DWNTs).  Thus, carbon nanotubes are 
inherently not pure and contain a host of metallic impurities as well as other forms of carbon such as 
polyhedral particles and amorphous carbon.  One study [3] compared the behavior of raw and purified 
SWNTs and concluded that microwave energy is predominantly absorbed as a result of a magnetic 
resonance within the ferromagnetic Fe catalyst particles.  Another study reported no differences in the 
behavior of raw and purified SWNTs in a microwave field [4].  However, both studies suffered from a 
common problem.  CNTs have a propensity to adsorb gases [5].  In both studies, gas desorption during 
microwave irradiation initiated an extremely hot plasma contiguous to the SWNTs sample.  Since the 
plasma strongly couples with the microwave radiation, it provides an independent mechanism for 
absorbing microwave energy.  While this coupling will depend upon the amount and rate of gas 
desorption from the sample, it will not be affected by metallic impurities.  Also, the hot plasma can 
conductively couple to the sample to cause a substantial rise in temperature. 
 The interaction of various types of CNTs (i.e. SWNTs, DWNTS, and MWNTs) with 2.45 GHz 
microwaves from a 600 W source in high vacuum (~10-6 torr) conditions is reported in this paper.  A 
variety of other materials including powdered graphite were investigated for comparison to the CNTs.  
Prior to irradiation, all the samples were degassed to suppress plasma formation during microwave 
exposure.  Particle-Induced X-ray Emission (PIXE) analysis was done to identify impurities and their 
                                                 
* http://en.wikipedia.org/wiki/Carbon_nanotube 
601.2 
Proceedings of the 10th International Conference on Particle Induced X-ray Emission and its Analytical 
Applications, Portorož, Slovenia, June 4-8, 2004 
 
concentration in the carbon samples.  A residual gas analyzer (RGA) was used to monitor the released 
gases, e.g. H2, during the degassing process.  While differences in the microwave absorption were 
observed between the various types of nanotubes, the differences could not be correlated with impurities 
in the samples.  Rather, a correlation was found between the microwave interaction and thermally 
activated release of H2 in the different types of CNTs. 
2. PIXE ANALYSIS  
 Samples were irradiated with 1.5 MeV protons using a 2.5 MV Van de Graaff accelerator at the 
University of North Texas.  The HPGe X-ray detector was positioned at a backscattering angle θ = 145° 
and the beam’s incident angle on the sample was α = 17° (see figure 1).  The samples were biased at 
+300 V for efficient collection of secondary electrons for current integration. 
 
 
 
 
 
 
 
 
  
 
 
 
 
FIGURE 1.  Sketch of the  
experimental setup. 
   The CNTs samples were in two forms: powder and small balls of ~1 mm in diameter.  The 
samples were mounted between two plastic foils attached to a supporting metal frame; an 8.5-µm thick 
Kapton foil on the backside and a 0.15-µm AP1 Moxtek film on the front.  The Moxtek film is thin 
enough to avoid corrections due to beam energy loss and X-ray attenuation.  The aluminum frame was 
covered with carbon tape to avoid X-ray production in the frame due to the scattered beam.  Secondary 
X-ray fluorescence in the aluminum frame was completely negligible.   
 The efficiency of the X-ray detector was determined for eighteen Kα transitions of different 
elements using thin targets [6].  A polynomial fit was done using lspline and interp functions of Mathcad 
8.0 to the natural logarithm of the experimental values.   
 The calculation of the mass concentration for each element was done using the following 
expression corresponding to a thick sample:  
∫
= 0
0 )(
)()(
E EMS
EZTEZ
pZZ
ZX
dENb
AN
ZC σε  (1) 
where NX is the number of counts of the Kα or Lα transition in the X-ray spectrum, AZ is the atomic 
mass of the element, εZ is the detector’s absolute efficiency (which includes detector’s intrinsic efficiency 
and solid angle) for the Kα or Lα transition, bZ is the intensity fraction of the Kα or Lα transition [7], Np 
is the number of particles that interact with the target during the spectrum acquisition, σZ is the elemental 
K-shell or L-shell X-ray production cross section, TZ is the transmission in the sample of the Kα or Lα 
X-ray in its way towards the detector from the point where they are produced and SM is the stopping 
power of the matrix of the sample.  The AXIL code [8] was used to fit the spectra.  We assumed the 
matrix of the sample to be 100 % carbon.  The mass attenuation coefficients were obtained from Ref. [9].  
Discrete values for σZ and SM were obtained from ISICS code [10] and SRIM code [11]; respectively.  A 
polynomial function was fitted to each of these sets of values to evaluate Eq. (1) using the Mathcad code. 
3. RESULTS   
 Table 1 lists the nanotube materials analyzed using PIXE and some of their characteristics.  Figure 
2a shows the PIXE for the SWNTs used in this study.  The results show that all the SWNTs (independent 
1.5 MeV
proton beam
X-ray
detector
Sample 
θ = 145° 
α = 17° 
Normal to 
sample surface
Normal to  
detector window   
601.3 
Proceedings of the 10th International Conference on Particle Induced X-ray Emission and its Analytical 
Applications, Portorož, Slovenia, June 4-8, 2004 
 
of vendor and average size) contain large amounts (~ 9 % or less) of metallic impurities.  Elemental 
variations among the various types of SWNTS are mainly due to the different catalyst material used by 
the vendors during growth.  Despite these variations, the total amount of metallic impurities, given in the 
rightmost portion of the figure, is nearly the same in all the SWNTs samples.  Figure 2b gives the 
impurity concentrations in the different types of nanotubes, as well as graphite.  Significant differences 
are observed in these samples.  It is clear that both SWNTs and DWNTs contain substantially more 
metallic impurities than either MWNTs or graphite. 
 
Material I.D. [nm] O.D. (Avg.) [nm] Length [µm] Temperature [K] 
Single Wall1 
Buckypearls (SW)2 
Double Wall1 
Multi Wall1 
Graphite 
P-type Silicon 
 
 
1.3-2.0 
5-10 
0.8-2 (1.1) 
1-1.5    
<5 
60-100 
0.5-100 
 
0.5-50 
0.5-50 
 
~3000 
 
~1900 
~3000 
~1700 
~1400 
TABLE 1.  List of nanotubes materials analyzed using PIXE and exposed to microwave radiation. 
1 Vendor: Nanostructured & Amorphous Materials, Inc.   
2 Carbon Nanotechnologies, Inc. 
Al Si P S Cl K Ca Ti V Cr Mn Fe Co Ni Cu Zn Mo Ba La Metals
100
101
102
103
104
105
(a)
Elements
 Single Wall (1.1 nm)
 Single Wall (1.4 nm)
 Single Wall* (1.1 nm)
 Buckypearls (1.0-1.5 nm)
Al Si P S Cl K Ca Ti V Cr Mn Fe Co Ni Cu Zn Mo Ba La Metals
100
101
102
103
104
(b)
M
as
s 
C
on
ce
nt
ra
tio
n 
[p
pm
]
 Single Wall*
 Double Wall 
 Multi Wall 
 Graphite
 
FIGURE 2.  Mass Concentration for those elements found in different carbon nanotube materials.  Also, 
impurities in graphite are shown for comparison. 
 
 An Ocean Optics Spectrometer with a charge coupled device detector sensitive over a wavelength 
range of 300-900 nm was used to acquire emission spectra from the CNTs during microwave exposure.  
The spectrometer was set to a continuous acquire mode so that the temperature profile of the samples 
during the entire exposure could be determined (from the blackbody radiation).  Samples other than 
CNTs spanning a wide range in electrical resistivity were used to establish an Ohmic response, i.e. 
heating caused by induced currents through electrical (Ohmic) resistance.  Figure 3 compares the 
resistivity of these samples to the maximum temperature achieved during microwave exposure.  It is 
clear that the results intersect a smooth response curve indicating a normal retrograde behavior, i.e. small 
net absorption at both ends due to transmission for insulators and high reflectance for highly conductive 
metals.  Net absorption is expected to peak for some intermediate value of resistivity as is observed.  The 
result for the DWNTs (1800 K) is consistent with this trend and is nearly identical to that from graphite.  
601.4 
Proceedings of the 10th International Conference on Particle Induced X-ray Emission and its Analytical 
Applications, Portorož, Slovenia, June 4-8, 2004 
 
However, the results (~3000 K) for both the SWNTs and MWNTS are anomalous and not easily 
interpreted in terms of only Ohmic heating.  When compared with the PIXE results, there is no 
correlation between total metallic impurities and microwave absorption.  Therefore, it is concluded that 
such impurities do not play a dominant role in this process. 
 
10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104
0
300
600
900
1200
1500
1800
2100
2400
2700
3000
P-type
Silicon
Graphite
Nichrome
Hyperpure
Silicon
Copper
Te
m
pe
ra
tu
re
 [K
]
Resistivity [Ω cm]
 
 
FIGURE 3.  Qualitative behavior of the 
maximum temperature reached by different 
materials as a function of their resistivity at 
room temperature when exposed to 
microwave radiation in a high vacuum system 
(~10-6 torr).  The optical spectrometer is 
sensitive to temperatures greater than ~1000 
K.  Approximate values were placed on the 
plot for Cu and hyperpure Si.  The dashed line 
at 3000 K suggests the temperatures reached 
by SWNT and MWNT samples, since their 
resistivities are unknown.  
 Desorption measurements provided some indication that structural differences may be 
responsible.  Of the three types of CNTs, only DWNTs exhibited an observable release of hydrogen 
during degassing to ~ 400 ºC.  This behavior mimics the trend seen in the microwave experiments, i.e. 
the DWNTs behave differently than either SWNTs or MWNTs. 
4. CONCLUSIONS  
 Differences were observed in CNTs when irradiated by microwaves.  While Ohmic behavior during 
exposure is sufficient to account for the temperature rise in the DWNTs, the results for both SWNTs and 
MWNTs are more complicated.  They suggest another heating mechanism is operative in these samples.  
While this analysis ignores any differences in emissivity, large variations are not expected between 
CNTs.  No correlation was established between the microwave effects and impurities in the samples 
indicating that impurities do not play a substantial role in the heating process.  Structural differences may 
account for both the gas desorption and microwave results. 
ACKNOWLEDGEMENTS 
 Work at UNT supported in part by NSF, ONR, Texas Advanced Technology Program, and the 
Robert A. Welch Foundation. 
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and Tour J.M., Chem. Matter. 15, 3969-1970 (2003).  
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The role of metallic impurities in the interaction of carbon nanotubes with microwave radiation

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The role of metallic impurities in the interaction of carbon nanotubes with microwave radiation

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