APPLIED PHYSICS LETTERS 86, 062112 共2005兲
Positive temperature coefficient effect in multiwalled carbon nanotube/
high-density polyethylene composites
X. J. He, J. H. Du, Z. Ying, and H. M. Chenga兲
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China
X. J. He
Shenyang Institute of Chemical Technology, 11 Street, Shenyang Economic Development Zone,
Shenyang 110142, People’s Republic of China
共Received 29 September 2004; accepted 14 December 2004; published online 3 February 2005兲
Multiwalled carbon nanotube 共MWNT兲-filled high-density polyethylene 共HDPE兲 composites were
prepared by a solution-precipitation process, and the temperature dependence of electrical
conductivity of the MWNT/HDPE composites was studied. An obvious positive temperature
coefficient 共PTC兲 effect was found in the MWNT/HDPE composites with a relatively low MWNT
concentration. Compared with that of carbon black 共CB兲/HDPE composite, the negative temperature
coefficient 共NTC兲 effect of the MWNT/HDPE composite at temperature above the melting
temperature of HDPE was much less obvious and could be further eliminated after 80 kGy ␥-ray
irradiation. The mechanism of the PTC and NTC effects in MWNT/HDPE composites was
discussed. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1863452兴
In recent years, the electrical conductive properties of
carbon nanotube 共CNT兲-filled polymer composites have attracted great attention. Due to their good electrical conductivity and high aspect ratios, CNTs could dramatically improve the electrical conductivity of polymers with a
relatively low CNT loading. The electrical properties of
CNT-filled various polymer matrix composites were
reported.1–4 However, the temperature dependence of electrical resistivity for CNT/polymer composites is rarely studied.
The temperature-resistivity characteristics of carbon
black 共CB兲 or other conductive particles filled polymer composites have been widely investigated.5–8 It was found that in
the vicinity of melting point or glass transition temperature
of the polymer matrix, some conductive particle filled polymer composites could exhibit a positive temperature coefficient 共PTC兲 effect, i.e., their resistivity sharply increased
with the increase of temperature. The CB-filled polymer PTC
composites have found important applications in selfcontrolled heaters, over-current protectors, sensors, etc.
However, to reach the percolation concentration 共the filler
concentration at which a pathway of the conductive filler is
formed through the matrix兲, the PTC composites have to
have a high loading of CB, which usually results in degradation of mechanical and processing properties of the polymer matrix. Using CNTs instead of CB as conductive filler, it
is possible that polymer-based PTC devices with low filler
concentration and good performance be fabricated. In this
work, multiwalled carbon nanotube 共MWNT兲-filled highdensity polyethylene 共HDPE兲 composites were prepared and
their temperature-resistivity behavior was studied.
HDPE with a melt index of 0.3 g / 10 min was used as
polymer matrix and the MWNTs used were produced by a
CVD method.9 The transmission electron microscope 共TEM兲
image of the as-produced MWNTs is shown in Fig. 1共a兲. The
average diameter and length of the MWNTs are ⬃50 nm and
5 – 50 ␮m, respectively, and metal catalyst impurity is less
a兲
Electronic mail: cheng@imr.ac.cn
than 2.2 vol %. TEM observations showed that most of the
metal catalyst particles in the as-produced MWNTs were
covered by carbon shells, embedded in the caps or the inner
tubes of the MWNTs, or adhered to the outside layer of the
MWNTs. Because almost all the purification processes can
cause damage to CNTs and may result in decrease in the
electrical conductivity of CNTs, the MWNTs were used
without further purification.
MWNT/HDPE composites were prepared by a solutionprecipitation process. The MWNTs were first sonicated in
FIG. 1. 共a兲 TEM image of the as-produced MWNTs and 共b兲 SEM image of
the 5.4 wt % MWNT/HDPE composite.
0003-6951/2005/86共6兲/062112/3/$22.50
86, 062112-1
© 2005 American Institute of Physics
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062112-2
He et al.
Appl. Phys. Lett. 86, 062112 共2005兲
FIG. 2. Electrical resistivity vs MWNT content for the MWNT/HDPE
composites.
ethanol for 30 min. Then xylene solution of HDPE was
added into MWNT-ethanol dispersion under both sonication
and agitation, and HDPE precipitated from the solution onto
MWNTs. The mixture was agitated for 24 h to allow the
mixture to turn homogenous. After filtration, the composite
was dried in a vacuum oven and molded at 150 ° C to form a
sample sheet of 0.2 mm in thickness. The MWNT content in
the composite was determined by thermogravimetric analysis, which was carried out from room temperature to
1000 ° C in argon atmosphere. It was found that under this
condition HDPE was completely pyrolyzed and MWNTs
remained.
It is well known that the as-produced MWNTs exist in
an entanglement state. When dry powder of MWNTs was
directly mixed with molten HDPE as in the melt-mixing process, aggregates of MWNTs could not be disentangled10 because the interaction between HDPE molecular segments and
MWNTs was weaker than that between MWNTs. When
MWNTs were sonicated in ethanol, the MWNT entanglements turned loose or even disentangled because of the penetration of ethanol molecules. To precipitate HDPE onto
MWNTs in such a state, the dispersion of MWNTs could be
better, which was confirmed by the scanning electron microscope 共SEM兲 observations on the MWNT/HDPE composite,
as shown in Fig. 1共b兲.
The volume electrical resistivity of the MWNT/HDPE
composites with different MWNT contents is shown in Fig.
2. It can be seen that the resistivity change of the composites
exhibits a percolation behavior. The percolation threshold is
about 2 wt % MWNTs. For CB-filled HDPE composites, the
threshold reported by Hindermann-Bischoff11 was 10 vol %
CB, equivalent to 18 wt % CB. The high conductivity of
MWNT/HDPE composites is not only due to the good conductivity and high aspect ratio of MWNTs, but also due to
the good dispersion of MWNTs achieved by this processing
method, as described above. Consequently, a conductive network of MWNTs can be formed at a lower concentration and
the percolation threshold of the composite becomes lower.
Figure 3共a兲 is a plot of the volume electrical resistivity as
a function of temperature for 5.4 wt % MWNT-filled HDPE
composite. It can be seen that when temperature is between
120 and 130 ° C, the electrical resistivity of the composite
increases sharply with the increase of temperature, showing a
strong PTC effect. The PTC intensity 共ratio of peak resistivity to room temperature resistivity兲 reached 1.1⫻ 104. Figure
3共c兲 is the differential scanning calorimetry 共DSC兲 curve of
the same composite. A comparison between Figs. 3共a兲 and
FIG. 3. Comparison of resistivity-temperature curve for 共a兲 5.4 wt %
MWNT/HDPE composite and 共b兲 16 wt % CB/HDPE composite; 共c兲 DSC
curve of the 5.4 wt % MWNT/HDPE composite.
3共c兲 shows that the melting of HDPE crystallites and the
sharp increase in resistivity occurs in a similar temperature
range. Therefore, similar to the PTC mechanism of CB/
HDPE composites proposed by Kohler,12 the mechanism of
this PTC effect in MWNT/HDPE composites could be attributed to the thermal expansion of the crystalline HDPE during
melting, which results in the breakdown of CNT conductive
network. Figure 3共b兲 shows the temperature-resistivity curve
of 16 wt % CB-filled HDPE composite prepared by melt
mixing 共a commonly used processing method for such composites兲. To obtain the PTC effect, the loading concentration
of CB is nearly three times as high as that of MWNTs. Moreover, when temperature is above the melting temperature
共Tm兲 of HDPE, the CB/HDPE composite exhibits obvious
negative temperature coefficient 共NTC兲 effect, i.e., the resistivity of the composite decreases rapidly with the increasing
of temperature. The NTC effect is unfavorable for applications of the materials as PTC electrical devices.13 Nevertheless, from Fig. 3共a兲 it can be seen that the NTC effect in
MWNT/HDPE composites is quite small. It is generally accepted that NTC effect is caused by the agglomeration of
conductive particles. As can be imagined, in the polymer
PTC composites, conductive particles are separated and surrounded by many polymer chains. The agglomeration of conductive particles would require polymer segments adjacent to
them to move correspondingly so the larger the conductive
FIG. 4. Resistivity-temperature curves of the 5.4 wt % MWNT/HDPE composite before and after irradiation.
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062112-3
Appl. Phys. Lett. 86, 062112 共2005兲
He et al.
particles, the more energy is needed for their agglomeration.
Though the diameter of CNTs is similar to the size of CB,
because of its high aspect ratio, the geometric size of CNTs
is much larger than that of CB particles. Thus the agglomeration of CNTs is more difficult and only a weak NTC effect
is manifested.
It has been reported that the PTC properties of CB-filled
polymer composites could be effectively modified by the
␥-ray irradiation treatment.14 To examine the influence of
␥-ray irradiation on the PTC and NTC effect of the MWNT/
HDPE composite, the samples were irradiated with Co60 ␥
ray at room temperature to a dose of 80 kGy. The resistivitytemperature curves of the MWNT/HDPE composites before
and after irradiation are compared in Fig. 4. It can be seen
that after irradiation the NTC effect of the composite at temperature above Tm of HDPE was eliminated and the PTC
effect was slightly increased. The main function of irradiation is to induce the crosslinking of HDPE molecular chains.
Thus after irradiation, the movement of the HDPE molecular
chains was restricted and the agglomeration of MWNTs became even more difficult so the NTC effect in the MWNT/
HDPE composites was depressed.
In summary, the MWNT/HDPE composites prepared by
a solution-precipitation process exhibit an obvious PTC effect in the vicinity of Tm of HDPE. The NTC effect of the
composite at temperature above the Tm of HDPE is relatively
small and can be eliminated after 80 kGy ␥-ray irradiation.
The PTC intensity for the MWNT/HDPE composite containing 5.4 wt % MWNTs can reach 1.1⫻ 104. The mechanism
of PTC/NTC effect in MWNT/HDPE composites is similar
to that of CB-filled polymer composites.
This work was supported by National Science Foundation of China 共No. 50025204兲 and the National Hi-Tech Research and Development Program of China 共No.
2002AA327110兲.
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