electrical conductivity of microwave heated polyaniline ... - JMPEE
electrical conductivity of microwave heated polyaniline ... - JMPEE
electrical conductivity of microwave heated polyaniline ... - JMPEE
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Electrical Conductivity <strong>of</strong> Microwave<br />
Heated Polyaniline Nanotubes and Possible<br />
Mechanism <strong>of</strong> Microwave Absorption by<br />
Materials<br />
Takahiro Murai, Ryo Fukasawa, Tohru Muraoka, Hiroyuki Takauchi,<br />
Yasuo Gotoh, Tokihiro Takizawa and Takehiro Matsuse *<br />
Faculty <strong>of</strong> Textile Science and Technology, Shinshu University,<br />
Ueda, Nagano 386-8567, Japan<br />
*<br />
matsuse@shinshu-u.ac.jp<br />
In the course <strong>of</strong> experiments to perform deprotonation and carbonization <strong>of</strong> doped <strong>polyaniline</strong><br />
(PANI) nanotubes (NTs) by irradiating directly 2.45 GHz <strong>microwave</strong> (MW) in our <strong>microwave</strong><br />
heating system (MWHS), we have discovered that the PANI-NTs self heat by absorbing the MW, but<br />
the temperature <strong>of</strong> the PANI-NTs stops rising around 300 °C in spite <strong>of</strong> the heightened MW power.<br />
Furthermore, we have found that the MW irradiated PANI-NTs have transferred from <strong>electrical</strong><br />
conductor to insulator depending on the temperature <strong>of</strong> the PANI-NTs.<br />
By measuring electron spin resonance (ESR) spectra <strong>of</strong> the MW <strong>heated</strong> PANI-NTs, the<br />
existence <strong>of</strong> the unpaired electrons is shown to have a strong correlation between the degree <strong>of</strong><br />
MW absorption and the transition in the <strong>electrical</strong> conductivities.<br />
In order to deprotonate and carbonize further the PANI-NTs, we have performed heat treatment<br />
for the PANI-NTs up to a temperature (T HT<br />
) <strong>of</strong> about 1200 °C in the same MWHS using carbon<br />
fiber which self heats by absorbing MW.<br />
The chemical transformations in the PANI-NTs induced by the heat treatments are discussed<br />
by measuring the X-ray photoelectron spectroscopy (XPS) spectra. Finally, the temperature<br />
dependence <strong>of</strong> <strong>electrical</strong> conductivities <strong>of</strong> the PANI-NTs are measured in order to investigate the<br />
mechanism <strong>of</strong> <strong>electrical</strong> conduction <strong>of</strong> the heat treated PANI-NTs.<br />
Submission Date: 9 September 2008<br />
Acceptance Date: 13 February 2009<br />
Publication Date: 16 March 2009<br />
INTRODUCTION<br />
Carbon nanotubes (CNTs) [Iijima,1991]<br />
have been expected to be one <strong>of</strong> the most<br />
versatile materials in the field <strong>of</strong> materials<br />
science. However, the applications are limited<br />
because metallic elements used as catalysts for<br />
Keywords: PANI nano-tube, <strong>microwave</strong> absorption,<br />
deprotonation and carbonization, <strong>electrical</strong> <strong>conductivity</strong>,<br />
ESR-spectra, unpaired electron, XPS-spectra<br />
synthesizing CNTs remain inside the CNTs.<br />
To overcome the limitations, a synthesis <strong>of</strong> the<br />
CNTs in the metal free processes could be an<br />
option.<br />
It has been pointed out that the doped<br />
PANI-NTs are expected to be very promising<br />
process [Langer et al., 2007; Kyotani, et al.,<br />
2008] because the doped PANI has been found<br />
to form tubular shaped self assembly at room<br />
temperature [Wan et al., 2000; Zhang et al.,<br />
43-1-34 Journal <strong>of</strong> Microwave Power & Electromagnetic Energy ONLINE Vol. 43, No. 1, 2009
Figure 1. Morphologies <strong>of</strong> pristine-PANIs, (a) SEM and (b) TEM images.<br />
2005].<br />
In this report, MW heat treatments are<br />
applied to deprotonate and carbonize the PANI-<br />
NTs which are synthesized self assembly by<br />
the same method shown in Zhang et al. [2005],<br />
and the properties <strong>of</strong> the heat treated PANI-<br />
NTs are investigated by measuring <strong>electrical</strong><br />
conductivities, ESR, and XPS spectra.<br />
In the low T HT<br />
region (T HT<br />
< 300°C), a<br />
possible mechanism <strong>of</strong> MW absorption by the<br />
doped PANI-NTs is discussed by comparing<br />
the strength <strong>of</strong> measured ESR-spectra with<br />
the transitions <strong>of</strong> <strong>electrical</strong> conductivities<br />
which depend strongly on the temperature <strong>of</strong><br />
MW irradiated PANI-NTs. The conduction<br />
mechanism is discussed from the point <strong>of</strong> view<br />
<strong>of</strong> the variable range hopping (VRH) <strong>of</strong> unpaired<br />
electrons with localized wave function [Mott et<br />
al., 1979; Kobayashi et al., 1993].<br />
The measuring <strong>of</strong> the XPS spectra <strong>of</strong> the<br />
MW irradiated and heat treated PANI-NTs<br />
reveal the kind <strong>of</strong> chemical transformations<br />
that have been induced depending on the heat<br />
treated temperature <strong>of</strong> the PANI-NTs.<br />
PREPARATION <strong>of</strong> PANI-NTs<br />
The PANI-NTs were synthesized self assembly<br />
according to a method presented in the<br />
paper by Zhang et al. [2005] using (±)-10-<br />
camphorsulfonic acid (CSA) as a dopant. Firstly,<br />
aniline 0.12 mol and CSA 0.06 mol were mixed<br />
in 600 mL <strong>of</strong> distilled water. Before oxidative<br />
polymerization, the solution was cooled in an<br />
ice bath (0 – 5 °C). Next, an aqueous solution<br />
<strong>of</strong> ammonium per sulfate 0.12 mol pre-cooled<br />
in 300 mL <strong>of</strong> distilled water was added to the<br />
above-cooled mixture solution and reacted in<br />
the ice bath for 15 hours. The precipitate was<br />
filtered and washed with distilled water and<br />
methanol several times, and dried in vacuum<br />
at 40 °C for 24 hours. Finally, the raw PANI<br />
doped with CSA was obtained as a powder<br />
with a dark green color.<br />
Morphologies <strong>of</strong> the obtained PANI are<br />
shown in Figure 1(a) and (b) in the images <strong>of</strong><br />
SEM and TEM observations, respectively. We<br />
have then confirmed that the PANI synthesized<br />
self assembly forms NTs with an outer diameter<br />
<strong>of</strong> about 100 nm, inner diameter <strong>of</strong> about a few<br />
10 nm and a length <strong>of</strong> a few μm.<br />
MICROWAVE HEATING SYSTEM<br />
To deprotonate and carbonize the PANI doped<br />
with CSA (pristine PANI-NTs), we have tried<br />
to irradiate 2.45 GHz MW to the pristine PANI<br />
NTs using our <strong>microwave</strong> heating system<br />
(MWHS) shown schematically in Figure 2.<br />
The continuous power controllable MW is<br />
generated by the use <strong>of</strong> an MW oscillator. The<br />
power <strong>of</strong> the MW is controlled by monitoring<br />
the temperature <strong>of</strong> the MW <strong>heated</strong> sample,<br />
which is installed in a quartz test tube. The<br />
International Microwave Power Institute 43-1-35
Figure 2. Principal layout <strong>of</strong> MWHS in a side view <strong>of</strong> E-face <strong>of</strong> H 01<br />
-mode waveguide. (1)<br />
<strong>microwave</strong> oscillator, (2) circulator, (3) power monitor, (4) short circuit plunger, (5) quartz test<br />
tube as reaction vessel, (6) junction, (7) condenser and trap, (8) gas outlet, (9) gas inlet, (10)<br />
thermocouple, and (11) controller.<br />
temperature is measured using a thermocouple<br />
<strong>of</strong> alumel-chromel covered by thin quartz glass<br />
inserted in the quartz test tube parallel to the<br />
E-face. To control the ambient atmospheric<br />
condition <strong>of</strong> the sample, gas is injected through<br />
a thin quartz glass tube inserted in the bottom<br />
<strong>of</strong> the test tube.<br />
MW direct irradiation<br />
Firstly, we have irradiated directly MW to heat<br />
the pristine PANI-NTs in our MWHS under the<br />
circulation <strong>of</strong> the N 2<br />
gas with a flow rate around<br />
300mL/min. The applicator part <strong>of</strong> the MWHS<br />
is presented schematically in a cross-section<br />
<strong>of</strong> the H 01<br />
-wave guide shown in Figure 3(a).<br />
The MW irradiation time dependence <strong>of</strong> the<br />
measured temperature rise <strong>of</strong> the pristine PANI-<br />
NTs are shown in Figure 3(b). It is confirmed<br />
that the pristine PANI-NTs self-heat in the<br />
lower temperature region through absorption<br />
<strong>of</strong> irradiated MW. However, the temperature<br />
rises are recognized to stop around 300 °C in<br />
spite <strong>of</strong> the higher power <strong>of</strong> MW irradiation<br />
<strong>of</strong> 1kW. Furthermore, the temperature <strong>of</strong><br />
MW <strong>heated</strong> PANI-NTs revealed a tendency to<br />
descend to lower temperatures during longer<br />
MW irradiation with constant power.<br />
MWHS with an external heat source<br />
To heat up the pristine PANI-NTs to a<br />
temperature region higher than approximately<br />
300 °C, we have changed the heating method<br />
to carbon fiber (Besfight® produced by TOHO<br />
TENAX CO., LTD) which self heats rapidly<br />
by absorbing 2.45 GHz MW. A quartz test<br />
tube is wound around the carbon fiber (shown<br />
schematically in Figure 4(a)), then the samples<br />
installed in the quartz tube are treated thermally<br />
by the heat <strong>of</strong> the MW absorbed carbon fiber.<br />
To demonstrate the performance <strong>of</strong> MWHS<br />
as an external heating system, MW irradiation<br />
time dependence <strong>of</strong> the measured temperature<br />
<strong>of</strong> air inside the quartz test tube are shown in<br />
Figure 4(b) for several cases <strong>of</strong> the MW power<br />
under circulation <strong>of</strong> air with a flow rate <strong>of</strong> about<br />
1000mL/min. These demonstrations show that<br />
the MWHS is used as an external heating system<br />
by controlling the power <strong>of</strong> MW oscillator.<br />
Next, we have performed the pristine<br />
PANI-NTs’ heat treatment using our MWHS<br />
in the temperature region from T HT<br />
=300 °C<br />
to about 1200 °C under N gas circulation <strong>of</strong><br />
2<br />
flow rate about 300mL/min. Of course we<br />
have confirmed that the properties <strong>of</strong> the heat<br />
treated PANI-NTs <strong>of</strong> T HT<br />
=300 °C have the same<br />
43-1-36 Journal <strong>of</strong> Microwave Power & Electromagnetic Energy ONLINE Vol. 43, No. 1, 2009
Figure 3. (a) Schematic view <strong>of</strong> applicator parts <strong>of</strong> MWHS shown Figure 2 in cross section <strong>of</strong><br />
H 01<br />
mode wave guide, (b) MW power and irradiation time dependences <strong>of</strong> measured temperature<br />
rise <strong>of</strong> MW <strong>heated</strong> pristine PANI.<br />
Figure 4. (a) Schematic view <strong>of</strong> applicator part with reaction vessel winded by carbon fiber <strong>of</strong><br />
MWHS in cross section <strong>of</strong> H 01<br />
mode wave guide, (b) MW power and irradiation time dependences<br />
<strong>of</strong> measured temperature rise <strong>of</strong> the atmosphere <strong>of</strong> the reaction vessels under the circulation <strong>of</strong><br />
air with flow rate about 1000mL / min.<br />
International Microwave Power Institute 43-1-37
Figure 5. Morphologies <strong>of</strong> heat treated PANIs at T HT<br />
= 900 °C (a) SEM and (b) TEM images.<br />
properties <strong>of</strong> the MW directly irradiated PANI-<br />
NTs for T HT<br />
= 300 °C.<br />
Figure 5 shows images <strong>of</strong> SEM and TEM,<br />
indicating that the heat treated PANI-NTs are<br />
confirmed to have almost the same morphologies<br />
as the pristine PANI-NTs. However, the surface<br />
<strong>of</strong> the heat treated PANI-NTs has become<br />
smoother than that <strong>of</strong> the pristine PANI-NTs<br />
shown in Figure 1.<br />
RESULTS AND DISCUSSION<br />
The whole PANI-NTs were treated by keeping<br />
the T HT<br />
for 30 minutes by controlling the<br />
power <strong>of</strong> the <strong>microwave</strong> oscillator shown in<br />
Figure 2. The measured <strong>electrical</strong> resistivities<br />
at room temperature <strong>of</strong> the MW irradiated and<br />
heat treated PANI-NTs are shown in Figure 6,<br />
dependent upon the heat treatment temperature<br />
T HT<br />
up to 1200 °C. To measure the <strong>electrical</strong><br />
resistivities <strong>of</strong> the powdery PANI-NTs, the<br />
powder is screwed in a hole opened Teflon rod<br />
with two screws with electrode. The <strong>electrical</strong><br />
resistivities are then measured by the usual two<br />
terminal methods.<br />
What is shown in the present study on the<br />
resistivity measurements is similar globally<br />
to those mentioned in a paper by Langer et al.<br />
[2007]; here the mechanism <strong>of</strong> transition from<br />
conductor phase to insulator phase in <strong>electrical</strong><br />
<strong>conductivity</strong> is investigated from the view point<br />
<strong>of</strong> changes <strong>of</strong> electronic states and chemical<br />
transformations in the doped PANI-NTs by<br />
heat treatment.<br />
From the measured <strong>electrical</strong> resistivities,<br />
we have confirmed that the heat treated PANI-<br />
NTs <strong>of</strong> T HT<br />
from about 300 °C to about 600 °C<br />
transferred to the insulator. These temperature<br />
regions where the heat treated PANI-NTs<br />
transferred to the insulator are emphasized<br />
to coincide completely with the temperature<br />
region where the pristine PANI-NTs stopped to<br />
self heat by MW direct irradiation as shown in<br />
Figure 3.<br />
The doped PANI-NTs in the low T HT<br />
region<br />
<strong>of</strong> about 200 °C have <strong>electrical</strong> conductive<br />
properties with resistivity <strong>of</strong> about a few<br />
10Ω⋅cm, but the resistivity changed suddenly<br />
to much higher values in higher T HT<br />
than 300<br />
°C. In the T HT<br />
region higher than 600 °C, the<br />
heat treated PANI-NTs transfer gradually to<br />
conductive materials.<br />
To investigate the degree <strong>of</strong> deprotonation<br />
and carbonization in the doped PANI-NTs, we<br />
have measured XPS spectra <strong>of</strong> the heat treated<br />
PANI-NTs. The XPS measurements <strong>of</strong> the<br />
PANI-NTs were made on a system <strong>of</strong> KRATOS<br />
ANALYTICAL AXIS ULTRA DLD HAS SV2-<br />
003A with a MgK α<br />
X-ray source (1253.6eV<br />
with energy resolution about 0.7eV).<br />
From the XPS spectra shown in Figure 7,<br />
where the intensities are normalized to those <strong>of</strong><br />
C1s core levels, the main origin <strong>of</strong> deprotonation<br />
<strong>of</strong> the doped PANI-NTs is attributed to the<br />
evaporation <strong>of</strong> sulfur elements. These results<br />
on the deprotonation processes are almost the<br />
same as those mentioned in a paper by Langer<br />
et al. [2007]. Of course, the rapid evaporation<br />
43-1-38 Journal <strong>of</strong> Microwave Power & Electromagnetic Energy ONLINE Vol. 43, No. 1, 2009
Figure 6. T HT<br />
dependences <strong>of</strong> measured resistivity at room temperature <strong>of</strong> the heat treated PANI.<br />
Figure 7. T HT<br />
dependence <strong>of</strong> measured XPS spectra <strong>of</strong> the heat treated PANI in T HT<br />
region
Figure 8. T HT<br />
dependence <strong>of</strong> measured XPS<br />
spectra <strong>of</strong> S2p core level <strong>of</strong> the heat treated<br />
PANI in low T HT<br />
region < 500°C.<br />
<strong>of</strong> oxygen elements is recognized, but nitrogen<br />
elements remain up to the higher heat treatments<br />
<strong>of</strong> about 1000°C.<br />
For the XPS spectra <strong>of</strong> S2p core levels,<br />
the heat treatment dependences are shown in<br />
Figure 8 in the low T HT<br />
region. The intensities<br />
and pr<strong>of</strong>ile <strong>of</strong> S2p <strong>of</strong> PANI-NTs <strong>of</strong> T HT<br />
=100 °C<br />
did not change those <strong>of</strong> the pristine PANI-NTs.<br />
The XPS-spectra <strong>of</strong> the PANI-NTs <strong>of</strong> T HT<br />
=<br />
300 °C decreased the changing pr<strong>of</strong>ile, but the<br />
intensities remained about half <strong>of</strong> those <strong>of</strong> the<br />
pristine PANI-NTs. The intensities <strong>of</strong> S2p <strong>of</strong><br />
PANI-NTs became negligibly small at T HT<br />
=<br />
500 °C.<br />
From these T HT<br />
dependences <strong>of</strong> XPS spectra<br />
<strong>of</strong> S2p <strong>of</strong> PANI-NTs, the evaporation <strong>of</strong> sulfur<br />
does not seem enough to explain the change <strong>of</strong><br />
degree <strong>of</strong> MW absorption (Figure 3) and the<br />
transition <strong>of</strong> <strong>electrical</strong> conductivities (Figure<br />
6).<br />
For the XPS spectra <strong>of</strong> N1s core levels, the<br />
Figure 9. T HT<br />
dependence <strong>of</strong> measured XPS<br />
spectra <strong>of</strong> N1s core level. Dotted lines are the<br />
results <strong>of</strong> Gaussian deconvolution.<br />
heat treatment dependences up to T HT<br />
= 1200°C<br />
are shown in Figure 9. To identify the type <strong>of</strong><br />
chemical bonds that compose the spectra, the<br />
results obtained by the Gaussian deconvolution<br />
are also shown using dotted lines.<br />
From the heat treatment dependences <strong>of</strong><br />
the XPS spectra <strong>of</strong> N1s core levels, we can<br />
recognize that the chemical transformations<br />
induced by changes <strong>of</strong> chemical bonding in<br />
nitrogen coincide clearly with transitions <strong>of</strong><br />
<strong>electrical</strong> conductivities <strong>of</strong> the PANI-NTs<br />
shown in Figure 6.<br />
The assigned binding energies by Gaussian<br />
deconvolution for pristine PANI-NTs and<br />
the MW irradiated PANI-NTs <strong>of</strong> T HT<br />
= 100°C<br />
and T HT<br />
=200°C almost coincide with those <strong>of</strong><br />
well established conductive PANIs [Kang et<br />
al., 1998]. Accompanying the deprotonation<br />
induced by evaporation <strong>of</strong> sulfur, the N1s core<br />
levels <strong>of</strong> the heat treated PANI-NTs <strong>of</strong> T HT<br />
=<br />
300°C, 400°C and 500°C shifted to the lower<br />
43-1-40 Journal <strong>of</strong> Microwave Power & Electromagnetic Energy ONLINE Vol. 43, No. 1, 2009
Figure 10. T HT<br />
dependence <strong>of</strong> measured X-band (9.15 GHz) ESR spectra <strong>of</strong> the heat treated<br />
PANI-NTs in low T HT<br />
region < 300 °C.<br />
binding energy region by about 2.5eV. These<br />
energy shifts <strong>of</strong> approximately 2.5eV are<br />
large when compared to binding energy shifts<br />
induced by chemical reduction methods in<br />
standard PANIs [Kang et al., 1998].<br />
For the PANI-NTs <strong>of</strong> the heat treatment<br />
temperature higher than 600°C, the binding<br />
energies shift gradually depending on the heat<br />
treatment temperature. These tendencies <strong>of</strong><br />
measured XPS spectra <strong>of</strong> heat treated PANI-<br />
NTs seem to be globally similar to that <strong>of</strong> the<br />
measured IR absorption presented in Langer et<br />
al. [2007].<br />
We can infer the type <strong>of</strong> chemical bonding<br />
concerned with nitrogen by using the wellestablished<br />
binding energies <strong>of</strong> standard PANIs<br />
[Monkman et al., 1991; Kang et al., 1998], and<br />
we can also identify the character <strong>of</strong> the bonding<br />
by comparing the measured FTIR spectra with<br />
the well-established spectra <strong>of</strong> PANI [Kang et.<br />
al., 1998]. The detailed investigations on the<br />
change <strong>of</strong> chemical bonding will be presented<br />
elsewhere.<br />
To find out the fundamental mechanism<br />
<strong>of</strong> MW absorption by the pristine PANI-NTs,<br />
we have measured the ESR spectra <strong>of</strong> the heat<br />
treated PANI-NTs. The ESR measurements<br />
were made using a system <strong>of</strong> JEOL JES-<br />
RE200S. The measured ESR spectra are shown<br />
in Figure 10.<br />
The measured magnetic field H dependence<br />
<strong>of</strong> differential intensity dP/dH in ESR spectra<br />
is shown for the PANI-NTs <strong>heated</strong> by MW<br />
direct irradiation.<br />
Thus, the obtained ESR-spectra <strong>of</strong> heat<br />
treatment dependence for the doped PANI-<br />
NTs are very instructive to compare the ESRspectra<br />
for heat treated phenolformaldehyde<br />
resin [Tanaka, 1987] where the strength <strong>of</strong> dP/<br />
dH has been measured to become weaker with<br />
increasing the heat treatment temperature.<br />
The measured ESR-spectra for the pristine<br />
PANI-NTs are considered to be a pr<strong>of</strong>ile similar<br />
to that shown in PANI doped with 2-acrylamido-<br />
2-methyl-1-propane sulfonic acid [Sitaram<br />
et al., 2005]. Furthermore, the drastic change<br />
observed in the spectra from T HT<br />
= 200 °C to<br />
300 °C has been confirmed. It is then expected<br />
that the lower temperature heat treated PANI-<br />
NTs are composed <strong>of</strong> more localized wave<br />
functions occupied with unpaired electrons.<br />
For the higher temperature heat treated<br />
International Microwave Power Institute 43-1-41
Figure 11. Measured temperature T dependence <strong>of</strong> <strong>electrical</strong> conductivities <strong>of</strong> the heat treated<br />
PANI-NTs plotted in a scale T -1/2 .<br />
PANI-NTs, the strength <strong>of</strong> the ESR spectra<br />
becomes weak, and the changes suggest that<br />
spin density with unpaired electrons decreases<br />
suddenly at around T HT<br />
= 300 °C as the results<br />
<strong>of</strong> the sulfur elements evaporation shown in<br />
XPS spectra (Figures 7 and 8) and chemical<br />
transformations explored in XPS spectra <strong>of</strong><br />
N1s core levels (Figure 9).<br />
Therefore, it should be mentioned that<br />
the temperature rise stopping around 300°C<br />
in MW direct heating as shown in Figure<br />
3(b) is explained by the elimination <strong>of</strong> the<br />
unpaired electron in heat treated PANI-NTs<br />
and the <strong>electrical</strong> transitions from conductor to<br />
insulator.<br />
To identify the elimination <strong>of</strong> the unpaired<br />
electron depending on the heat treatment, we<br />
have measured temperature T dependence <strong>of</strong><br />
<strong>electrical</strong> conductivities <strong>of</strong> the heat treated<br />
PANI-NTs. The results are shown in Figure 11<br />
using a variable T -1/2 .<br />
From the temperature T dependence <strong>of</strong><br />
<strong>electrical</strong> conductivities σ <strong>of</strong> the heat treated<br />
PANI-NTs, the conductivities in low heat<br />
temperature treated PANI-NTs are understood<br />
by the variable range hopping (VRH) mechanism<br />
with localized unpaired wave function [Mott et<br />
al., 1979; Kobayashi et al., 1993]. In the higher<br />
temperature heat treated PANI-NTs <strong>of</strong> T HT<br />
>900<br />
°C, the <strong>electrical</strong> conductivities change to the<br />
characteristics <strong>of</strong> temperature dependences <strong>of</strong><br />
metallic conductor depending on the degrees <strong>of</strong><br />
the carbonization.<br />
CONCLUSION<br />
We have discovered a method for transferring<br />
the <strong>electrical</strong> conducting PANI-NTs to<br />
insulating ones by utilizing the characteristic<br />
response <strong>of</strong> the PANI-NTs to MW. Namely, the<br />
conducting PANI-NTs with unpaired electron<br />
absorb MW and self heat. The self heat<br />
drives the deprotonation <strong>of</strong> the PANI-NTs by<br />
accompanying evaporation <strong>of</strong> sulfur and the<br />
chemical transformation.<br />
Finally, the PANI-NTs that lost the unpaired<br />
electron transfer to the <strong>electrical</strong> insulator<br />
which does not absorb MW. Using our MWHS<br />
with N 2<br />
gas circulation, this method has been<br />
demonstrated by showing the relations between<br />
the ESR spectra and <strong>electrical</strong> conductivities.<br />
We can then summarize that the mechanism<br />
<strong>of</strong> the MW absorption by materials depends<br />
strongly on the existence <strong>of</strong> macroscopic<br />
43-1-42 Journal <strong>of</strong> Microwave Power & Electromagnetic Energy ONLINE Vol. 43, No. 1, 2009
<strong>electrical</strong> current with unpaired electrons in the<br />
PANI-NTs.<br />
To perform further the heat treatment <strong>of</strong><br />
the PANI-NTs transferred to insulator, we have<br />
utilized the carbon fiber which self heats by<br />
absorbing MW. We have also carried out heat<br />
treatment for the PANI-NTs up to about T HT<br />
=<br />
1200 °C in the same MWHS.<br />
By performing the MW assisted heat<br />
treatments, we have found that the <strong>electrical</strong><br />
insulating PANI-NTs generated by MW direct<br />
irradiation have been transformed finally to<br />
conductor depending on the degree <strong>of</strong> the<br />
chemical changes and carbonization in the heat<br />
treated PANI-NTs.<br />
What we have demonstrated in this report<br />
will be applied for fabricating the new materials<br />
by utilizing the characteristic response to MW.<br />
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ACKNOWLEDGMENTS<br />
This work was supported partly by Grant-in-<br />
Aid for “Global COE Program” by the Ministry<br />
<strong>of</strong> Education, Culture, Sports, Science, and<br />
Technology, Japan.<br />
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