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.