Figure 2. Transmission electron micrographs of the multi-walled carbon
nanotubes for the preparation of epoxy matrix composite materials.
The thermogravimetric analysis results and derivative thermogravimetric
analysis results of the multi-walled carbon nanotubes are presented in
Figure 3 for the preparation of epoxy matrix composite materials. In
thermogravimetric analysis of multi-walled carbon nanotubes which are
usually commercially available, there is one combustion peak at a
temperature of about 600 °C. This is because carbon impurities are
attached to carbon nanotubes. An apparent purity of carbon nanotubes
appears high, but this does not indicate the state of carbon nanotubes
having a high purity, an attention must be paid. Further, it is thought
that heat resistance of carbon nanotubes varies depending on the
diameter of carbon nanotubes and the quality of a graphene sheet
constituting the wall of carbon nanotubes, and in the prior technique,
it is difficult to realize high resistance with multi-walled carbon
nanotubes having a small outer diameter. At this time, carbon nanotubes
are preferably in the state where they are attached to a catalyst body.
Then, after a catalyst body is removed, thermogravimetric analysis is
performed, the resulting weight reduction curve is differentiated with a
time to obtain a differential thermogravimetric curve, and a heating
temperature is determined as follows, based on a graph obtained by
plotting, letting an x-axis to be a temperature and a y-axis to be the
differential thermogravimetric curve, and carbon nanotubes are heated.
That is, a sample is heated to an inflection point at which a peak is
changed from a low temperature combustion peak to a high temperature
combustion peak, of two combustion peaks which are on a high temperature
side and on a low temperature side in the differential thermogravimetric
curve under the oxygen atmosphere and, at the time point at which a
temperature reaches the temperature, heating is stopped, and oxygen is
replaced with a nitrogen gas.