JP7596959B2 - Carbon nanotube aggregates - Google Patents

Description

The present invention relates to an aggregate of carbon nanotubes (hereinafter referred to as CNTs).

CNT aggregates composed of multiple CNTs are expected to be applied to conductive films and windings, and in recent years, there has been a demand for low resistance due to the miniaturization of devices. For example, Non-Patent Document 1 proposes a method of reducing resistance by doping CNTs with _AuCl3 .

ACS Nano, vol. 5, P. 1236, 2011

In addition to lowering resistance, CNT aggregates are required to be able to withstand high-temperature environments such as vehicle motors. For example, they must show little change in resistance and maintain electrical resistance even when placed in an atmosphere at 200°C or higher for 100 hours or more. However, in the method described in Non-Patent Document 1, although the effect of reducing resistance is maintained at temperatures of around 100°C, the effect of reducing resistance decreases at temperatures of 200°C or higher. This is thought to be due to the detachment of additives or degradation due to reactions with oxygen and water in the atmosphere.

In addition, for n-type CNT materials for thermoelectric materials, a technology has been proposed in which alkali elements such as lithium and potassium or a complex of potassium ions and crown ether is added to the inside of the CNT, but stability above 200°C in air has not been confirmed. Also, there are few examples of adding p-type dopants, which are relatively stable in air compared to n-type, to the inside of CNT, and similarly, there are no examples of heat resistance for more than 100 hours in air at 200°C or higher.

In view of the above, the present invention aims to provide a highly heat-resistant CNT aggregate.

To achieve the above object, the invention described in claim 1 is a CNT aggregate comprising a plurality of CNTs (2), in which a metal compound (3) is added to at least one of the interior and exterior of the CNTs, and the surface is covered with an oxide film (4) composed of an oxide of the metal compound.

As a result, the metal compounds are isolated from the atmosphere by the oxide film that covers the surface of the CNT aggregate, suppressing the release of the metal compounds and their reactions with oxygen and water in the atmosphere, improving the heat resistance of the CNT aggregate.

The reference symbols in parentheses attached to each component indicate an example of the correspondence between the component and the specific components described in the embodiments described below.

FIG. 2 is an external view of a CNT aggregate. FIG. 2 is a diagram showing the internal structure of a CNT aggregate. FIG. 2 is a cross-sectional view of a CNT aggregate. 1 is a micrograph showing the molybdenum content. 1 is a micrograph showing the oxygen content. 1 is a micrograph showing the chlorine content. FIG. 13 is a diagram showing the relationship between heating time and electrical resistance. FIG. 1 is a graph showing the temperature dependence of electrical resistivity in samples C-1 to C-3. FIG. 1 is a diagram showing the intensity of the X-ray photoelectron spectroscopy (hereinafter referred to as XPS) spectrum versus the binding energy of the Cl2p region in sample C-3. FIG. 13 is a diagram showing the intensity of the XPS spectrum versus the binding energy of the Fe2p region in sample C-3. FIG. 1 shows the results of measuring the G band peak positions in the Raman spectra of samples C-1 to C-3. FIG. 1 shows the results of thermogravimetry and differential thermal analysis (hereinafter referred to as TG-DTA) performed on samples C-1 to C-3 and FeCl ₃ powder.

The following describes embodiments of the present invention with reference to the drawings. In the following embodiments, parts that are the same or equivalent to each other are denoted by the same reference numerals.

First Embodiment
A first embodiment will be described. As shown in FIG. 1 and FIG. 2, the CNT aggregate 1 of this embodiment is formed by assembling a plurality of CNTs 2 to form a rod-shaped or tape-shaped structure as shown in FIG. 1. Such a CNT aggregate is used, for example, in a CNT conductive film, a CNT winding for a device, a CNT winding for a motor, a CNT wire harness, etc. The length of one side of the CNT aggregate 1 is, for example, 10 μm or more. In addition, when the CNT aggregate 1 is cylindrical, the diameter of the CNT aggregate 1 is, for example, 10 μm or more. In addition, the density of the CNT aggregate 1 is, for example, 0.8 g/cm ³ or more. The average inner diameter of the plurality of CNTs 2 is, for example, 0.55 nm or more and 1000 nm or less.

As shown in Fig. 2, in the CNT aggregate 1, a metal compound 3 such as a metal chloride is added to the inside and outside of the CNT 2 in order to reduce resistance. Here, a case where molybdenum chloride (MoCl ₅ ) is added as the metal compound 3 will be described. Note that in Fig. 2, the CNT 2 is shown as a cylinder for simplicity. Also, the metal compound 3 added to the inside of the CNT 2 is shown by a dashed line, and the metal compound 3 added to the outside of the CNT 2 is shown by a solid line.

3, the surface of the CNT aggregate 1 is covered with an oxide film 4 composed of an oxide of the metal compound 3. The oxide film 4 is intended to isolate the metal compound 3 from the atmosphere and to suppress the release of the metal compound 3 and the reaction of the metal compound 3 with oxygen and water in the atmosphere. When MoCl ₅ is added as the metal compound 3, the oxide film 4 is composed of molybdenum oxide (MoO ₃ , MoO ₂ ) or molybdenum chloride partial oxide (MoO _x Cl _5-x ).

Such a CNT aggregate 1 is manufactured, for example, by the following procedure. First, a CNT film (manufactured by DexMat INC. (USA)) is placed in a vacuum and heated at 500°C or higher to remove moisture from the CNTs 2. Next, the CNT film is mixed with _MoCl5 gas in an Ar atmosphere with _O2 of 1 ppm to 20 ppm, and then the CNT film is sealed in a container to isolate it from the atmosphere and heated at 260°C to 350°C. Then, the _MoCl5 on the surface of the CNT film is removed by washing with ethanol.

In this way, MoCl ₅ is added as the metal compound 3 to the inside or outside of the CNT 2, and the CNT aggregate 1 is manufactured in which the surface is covered with an oxide film 4 such as MoO _2. In this manufacturing method, the surface of the CNT aggregate 1 is covered with the oxide film 4, and the opening of the CNT 2 is capped with the oxide of the metal compound 3 such as MoO ₂ .

Figures 4 to 6 are scanning electron microscope photographs of the cross section of the CNT aggregate 1 produced by the above method, showing the molybdenum, oxygen, and chlorine contents inside and on the surface of the CNT aggregate 1. In Figures 4 to 6, the white parts have higher molybdenum, oxygen, and chlorine contents than the black parts. As shown in Figures 4 to 6, the surface of the CNT aggregate 1 has higher molybdenum and oxygen contents and lower chlorine contents than the inside of the CNT aggregate 1. This shows that an oxide film 4 composed mainly of molybdenum and oxygen has been formed on the surface of the CNT aggregate 1.

Fig. 7 is a diagram showing the results of investigating the heat resistance of the CNT aggregate 1 manufactured by the above method. The horizontal axis of the graph in Fig. 7 is the heating time when a heating test was performed at 230 ° C in the atmosphere, and the vertical axis is the electrical resistivity measured by a four-terminal measurement method at room temperature after removing the CNT aggregate from the heating device. In Fig. 7, the white circles indicate the electrical resistivity of the CNT aggregate 1 having a molybdenum oxide film thicker than 1 nm and thinner than 10 nm formed on the surface, and the white squares indicate the electrical resistivity of the CNT aggregate 1 having a molybdenum oxide film with a thickness of 10 nm formed on the surface. In Fig. 7, the white triangles indicate the electrical resistivity of the CNT aggregate without an oxide film formed on the surface, and AuCl ₃ is added to this CNT aggregate.

As shown in Fig. 7, in a CNT aggregate without an oxide film formed on its surface, when it is heated at 230°C, the electrical resistance increases in a short time and returns to the electrical resistance before the addition of AuCl _3. In contrast, in a CNT aggregate 1 with an oxide film 4 formed on its surface, the electrical resistance does not increase significantly even when heated for a long time, and remains small.

As explained above, in this embodiment, the resistance of the CNT aggregate 1 is reduced by adding the metal compound 3 to the inside and outside of the CNT 2. Then, by covering the surface of the CNT aggregate 1 with an oxide film 4 made of an oxide of this metal compound 3, the metal compound 3 is shielded from the atmosphere, improving heat resistance compared to conventional methods and achieving heat resistance of 200°C or more.

In addition, as mentioned above, by capping the openings of the CNT2 with an oxide of the metal compound 3, the metal compound 3 inside the CNT2 is isolated from the atmosphere, further improving heat resistance.

This can also be confirmed by a test in which the CNT film was doped with FeCl _3. Here, Galvorn CNT Tape manufactured by DexMat INC. (USA) was used as the CNT film. The initial resistance of the CNT film was 33.8 ± 6.9 μΩcm. This CNT film was heat-treated in air at 350°C for 1 hour, and the initial resistance after the heat treatment was 61.2 ± 8.4 μΩcm. In addition, three types of samples, C-1 to C-3, were prepared by using the heat-treated CNT film and the heat-treated CNT film that was further doped. C-1 is a CNT film that was only heat-treated, C-2 is a CNT film that was doped with IBr after the heat treatment, and C-3 is a CNT film that was doped with FeCl ₃ after the heat treatment. For C-2, anhydrous _FeCl3 with a purity of 99.9% manufactured by Sigma Aldrich was used without purification and added as a dopant. For C-3, IBr with a purity of 98% manufactured by Sigma Aldrich was used without purification and added as a dopant. In the doping treatment, the CNT film and the dopant were allowed to coexist in a sealed container filled with an inert gas, and the treatment was performed at 270°C for 24 hours. After the doping treatment, the CNT film was washed with ethanol several times to remove excess dopant adsorbed on its surface.

Then, to investigate the temperature dependency of electrical resistivity in C-1 to C-3, the electrical resistivity was measured in an inert gas atmosphere. The electrical resistivity was measured at a rate of increasing the temperature by 25°C per minute. Figure 8 shows the measurement results. As shown in this figure, the electrical resistivity increased in all three types of samples when heated from room temperature. Among them, C-3 showed a lower rate of increase in resistance with increasing temperature than C-2, and the change was also small.

The thermal stability of the doped CNT films was also confirmed by analyzing their properties. The analysis included Raman spectroscopy (hereinafter referred to as Raman), XPS, and TG-DTA. For Raman, an excitation wavelength of 532 nm was used, and a confocal Raman microscope, InVia Qontor (UK), manufactured by Renishaw, was used. For XPS, a PHI 5000 VersaProveII manufactured by ULVAC-PHI, Inc. (JAPAN), was used to focus X-rays to a spot size of 200 μm in diameter on the sample, and the X-rays were performed under 50 W. The X-ray source used was Al-Kα radiation, hν = 1486.6 eV. For TG-DTA, which was performed to evaluate the thermal stability of each sample, a TG-DTA 6300 manufactured by Hitachi High-Tech Corporation (JAPAN) was used. The TG-DTA measurement temperature range was 40°C to 1000°C, and the scanning rate was +10.0°C temperature increase per minute. An alumina pan was used, and air was introduced at 100 mL per minute.

In the XPS measurement, in order to detect Fe and Cl inside the _FeCl3 -doped CNT, the peaks in the Fe2p region near 712 eV and the Cl2p region near 198 eV were focused on. Figures 9A and 9B show the relationship between the intensity (a.u.) of the XSP spectrum and the binding energy (eV) in the Cl2p region near 198 eV and the Fe2p region near 712 eV in C-3, respectively. Each figure shows, from the top, before heating, after heating for 50 hours, and after heating for 120 hours.

As can be seen from Figures 9A and 9B, in the XPS spectrum of C-3, no disappearance of the peak or change in peak shape was observed in the region of interest before and after heating. Therefore, it can be said that _FeCl3 is held in the CNT even at high temperatures of about 220°C, which is assumed when the CNT aggregate 1 is applied to a motor or the like.

The shift in the G-band peak position in the Raman spectrum reflects the state of p-type doping of the CNTs. Figure 10 shows the results of measuring the G-band peak position in the Raman spectrum for C-1 to C-3 using excitation light with an excitation wavelength of 532 nm. A high-temperature storage test was conducted for each sample, and the G-band peak position was measured before and after heat treatment in the high-temperature storage test. However, since C-1 is an undoped sample and is not expected to change before and after heat treatment, only the measurement result before heat treatment is shown.

The G-band peak positions of C-2 and C-3 before the heat treatment in the high-temperature storage test were 1601 cm ^-1 and 1609 cm ^-1 , respectively, and compared with the peak position of 1593 cm ^-1 in C-1, the peak shift was large in proportion to the change in electrical resistivity. In contrast, after the heat treatment in the high-temperature storage test, these peak shifts were 1596 cm ^-1 in C-3, while the value did not change in C-3. This suggests that the p-type doping state was maintained in C-3 before and after the heat treatment.

In TG-DTA, analysis was performed on C-1 to C-3 and FeCl ₃ powder. As a result, as shown in FIG. 11, FeCl ₃ showed different thermal stability depending on whether it was doped or not. That is, in the thermogravimetric measurement (TG) of only FeCl ₃ powder, a weight loss was confirmed from around 200°C, while in C-3, a rapid weight loss was confirmed from around 500°C and 700°C. The latter is a weight loss of CNT itself, since the slope is almost the same as the rapid weight loss in C-1 at temperatures near 500°C and 700°C, and the former is a weight loss of FeCl _3. From this, it is considered that FeCl ₃ and CNT encapsulated in CNT became thermally stable compared to when they were alone. This phenomenon was also confirmed in C-2, and it can be said that this supports the result that thermal stability is improved by encapsulating other substances.

Other Embodiments
The present invention is not limited to the above-described embodiment, and can be modified as appropriate within the scope of the claims.

For example, in the above embodiment, _MoCl5 is added to the CNT aggregate 1 as the metal compound 3, but other metal chlorides may be added, or metal compounds other than metal chlorides may be added. Also, the metal compound 3 may be added only to the inside of the CNT2, or may be added only to the outside of the CNT2.

2. CNTs
3 Metal compound 4 Oxide film

Claims (5)

A carbon nanotube aggregate comprising a plurality of carbon nanotubes (2),
A metal compound (3) is added to at least one of the inside and the outside of the carbon nanotube,
An aggregate of carbon nanotubes, the surface of which is covered with an oxide film (4) composed of an oxide of the metal compound. The carbon nanotube aggregate according to claim 1, wherein the metal compound is a metal chloride. the metal compound is molybdenum chloride;
3. The carbon nanotube aggregate according to claim 2, wherein the oxide is a molybdenum oxide or a molybdenum chloride partial oxide. A carbon nanotube aggregate according to any one of claims 1 to 3, the length of one side or the diameter of which is 10 μm or more. 5. The aggregate of carbon nanotubes according to claim 1, having a density of 0.8 g/cm ^<3> or more.

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