Electromagnetic wave absorbing material and electromagnetic wave absorber

Abstract

To provide an electromagnetic wave shielding material which is lightweight yet has excellent electromagnetic wave absorbing performance.SOLUTION: An electromagnetic wave shielding material includes carbon nanotubes that have been surface-treated with a silane coupling agent. The bulk density of the electromagnetic wave shielding material is 1.23 kg / m3 or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an electromagnetic wave absorbing material and an electromagnetic wave absorber. Specifically, it relates to an electromagnetic wave absorbing material preferably applied to applications for absorbing electromagnetic waves, that is, suppressing reflection.

Background Art

[0002] Patent Document 1 discloses an electromagnetic wave absorber having a laminated structure in which a first layer made of a dielectric, a second layer having conductivity, a third layer made of a dielectric, and a fourth layer having conductivity are laminated in this order, wherein the sheet resistance of the second layer is 100 Ω / sq or more and 300 Ω / sq or less, and the fourth layer is a reflector of electromagnetic waves.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In information communication using electronic devices, weight reduction of carriers such as those mounted with electronic devices used in the aviation field, space field, and on the ground has become an essential issue from the perspective of fuel consumption reduction.

[0005]

[0006]

Means for Solving the Problems

[0007] The electromagnetic wave absorbing material according to one aspect of the present disclosure includes carbon nanotubes surface-treated with a silane coupling agent, and has a bulk density of 1.23 kg / m 3 as follows.

[0008] The electromagnetic wave absorber according to one aspect of the present disclosure includes a plurality of stacked layers, and the plurality of layers include a functional layer and at least one of a metal layer and a magnetic layer. The functional layer includes the electromagnetic wave absorbing material.

Advantages of the Invention

[0009] According to one aspect of the present disclosure, it is possible to provide an electromagnetic wave absorbing material and an electromagnetic wave absorber that are lightweight and have excellent electromagnetic wave absorption performance.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0011] 1. Embodiment The embodiment will be described. Note that the following embodiment is only a part of various embodiments of the present disclosure. Also, the following embodiment can be variously modified according to design and the like as long as the object of the present disclosure can be achieved. The drawings referred to below are schematic diagrams, and the dimensional ratios of the components in the drawings do not necessarily reflect the actual dimensional ratios.

[0012] The electromagnetic wave absorbing material according to the embodiment includes carbon nanotubes surface-treated with a silane coupling agent, and has a bulk density of 1.23 kg / m 3 as follows.

[0013] Therefore, the electromagnetic wave absorbing material is lightweight and has excellent electromagnetic wave absorption performance.

[0014] The electromagnetic wave absorber according to the embodiment includes a plurality of stacked layers. The plurality of layers includes a functional layer and at least one of a metal layer and a magnetic layer. The functional layer contains the above-described electromagnetic wave absorbing material.

[0015] Therefore, the electromagnetic wave absorber is lightweight and has excellent electromagnetic wave absorption performance.

[0016] First, the background leading to the present disclosure will be described. In order to realize wireless communication inside artificial satellites and absorb electromagnetic wave noise and unnecessary reflected waves caused by wireless communication, an ultra-lightweight electromagnetic wave absorbing material using carbon nanotubes (CNT) was fabricated. In the past, the increase in the addition amount has inhibited the absorption of electromagnetic waves due to reflection caused by the high conductivity of the material itself.

[0017] The realization of an ultra-lightweight and non-disruptive wireless communication system can contribute to greatly expanding the possibility of vehicle flight. In spacecraft such as flying cars, artificial satellites, and space probes, the change from wired communication, which requires complicated wiring, to wireless communication can be expected to further improve safety and security by reducing not only fuel consumption but also human errors such as wiring mistakes. If an electromagnetic wave absorption performance can be imparted to an ultra-lightweight material, it becomes possible to create an environment without communication obstacles due to electromagnetic wave interference even inside a flying object covered with a metal casing. Generally, since electromagnetic waves are reflected on the metal surface, electromagnetic wave interference occurs, which inhibits the transmission and reception of radio waves between communication devices inside the casing and makes smooth communication difficult. Therefore, an attempt is made to solve this problem by using an electromagnetic wave absorbing material.

[0018] In the present disclosure, the insulation treatment of the surface of CNT with a silane coupling agent was considered as a means for solving the problem. By performing insulation treatment on the surface of CNT, it is aimed to reduce the conductivity σ in Equation (1) and control the dielectric loss tangent δ shown in Equation (2) to 0.4 to 0.5.

[0019]

Equation

[0020]

Number

[0021] The silane coupling agent for the surface treatment of CNT preferably contains aminosilane. For example, as the silane coupling treatment, the surface of CNT is coated with aminopropyltrimethoxysilane (APMS) in which a hydrophilic amino group is chemically modified on a hydrophobic silane surface.

[0022] In addition, samples with an increased blending ratio of silane coupling-treated CNT are prototyped to improve the electromagnetic wave absorption performance. Furthermore, for the prototyped samples, the electromagnetic wave absorption performance and electromagnetic characteristics are evaluated, and the effect of the silane coupling treatment is examined.

[0023] 2. Experimental Method (1) Sample Preparation a. Silane Coupling Treatment For the insulation treatment of the surface of CNT, 3-Aminopropyltrimethoxysilane (T1255, manufactured by Tokyo Chemical Industry Co., Ltd.) was used.

[0024] First, CNT was coated with trimethoxysilane, but it was found that due to the hydrophobic group of SiO 2 being on the surface, it was difficult to achieve uniform dispersion in the CMC aqueous solution.

[0025] Therefore, it was necessary to chemically modify the hydrophilic group at the end. Since APMS has a hydrophilic amino group (-NH 2 ), it was selected because dispersion could be expected during mixing and it was suitable for the freeze-drying process.

[0026] As the CNT, a single-layer CNT manufactured by Nippon Zeon was used. For the case of high-temperature durability, a snap cup with CNT inside and APMS in the center was placed in the center of the case with CNT, and a two-layer structure coupling process was performed. The entire two-layer case was sealed, and the case was placed in an electric furnace at room temperature and heated to 200 °C over time for thermal decomposition.

[0027] b. Preparation of samples by freeze-drying A dispersion liquid for performing freeze-drying was prepared with the following formulation.

[0028] 0.05 g of CNT treated with silane coupling was added to the preliminarily mixed liquid, and this dispersion liquid was sonicated. Specifically, the dispersion liquid was sonicated for 3 minutes and then heat was removed for 3 minutes, and this was repeated 4 times as a set. The dispersion liquid was visually inspected, and if the dispersion was insufficient after 4 sets, additional sonication was performed.

[0029] Subsequently, the dispersion liquid was pre-cooled with cold water at 5 °C for 30 minutes or more. After pre-cooling, 45 g of the dispersion liquid was poured into a mold of 110 mm × 110 mm and leveled to a thickness of 3 mm. The leveled dispersion liquid was frozen for 2 hours. After freezing, it was placed in a freeze-dryer and freeze-dried. Table 1 shows the prepared samples.

[0030]

Table 1

[0031] For samples with a CNT blending ratio of 5 wt% (Experimental Examples 1 to 3), the amount of APMS shown in the table was coupling-treated with respect to 0.05 g of CNT. For the sample with a CNT blending ratio of 14 wt% (Experimental Example 4), 1000 μL of APMS was coupling-treated with respect to 0.05 g of CNT, and 0.15 g of the treated CNT was blended. The in-plane dimensions of the materials of all samples are 110 mm × 110 mm. The thickness of the samples is approximately 3.5 mm.

[0032] The coated CNT was taken at the ratio of the weight after coating.

[0033] (2) Measurement method of electromagnetic wave absorption performance Using a rectangular waveguide, the electromagnetic wave absorption performance of the fabricated samples was evaluated. The outline of the measurement system is shown in Fig. 1. As shown in Fig. 1, at the end of one waveguide 2, a sample 4 and a metal plate 5 were stacked via a sample holder 3. A power of 0 dBm was input into the waveguide 2, and S 11 was measured using a network analyzer 1 (N5224A, manufactured by Keysight Tech). The bandwidth was 10 kHz and the number of averaging times was 4. Also, as shown in Table 2, the dimensions of the waveguide were changed for each frequency band during the measurement.

[0034] [Table 2]

[0035] (3) Evaluation of electromagnetic wave absorption performance and electrical characteristics Fig. 2 shows a comparison of |S 11 | for each of the fabricated samples. Discontinuities occur in the measurement results at the switching frequencies of the waveguide. Also, maximum and minimum values, presumably due to wavelength resonance in the waveguide, were obtained approximately every 1 - 2 GHz. However, the high reproducibility of the measurement has been separately confirmed.

[0036] For all samples (Experimental Examples 1 - 3) with a CNT blending ratio of 5 wt%, a broad minimum value was obtained around 20 GHz. Based on the case where a sample with an AMPS amount of 0 μL was placed, |S 11 | increased for the sample with an AMPS amount of 500 μL. It is considered that the loss generated in the sample decreased due to insulation by silane coupling.

[0037] On the other hand, for the 2000 μL sample, |S 11 | decreased, and the frequency at which |S 11 | becomes minimum shifted to a lower band.

[0038] As a cause, it is conceivable that the sample thickness is about 10% thicker and the addition of a large amount of silane coupling agent may have increased the dielectric constant of the sample.

[0039] For the sample with a CNT blending ratio of 14% by weight, a minimum value of -46 dB was obtained at 12 GHz. However, it is necessary to consider the superposition of the effects of wavelength resonance.

[0040] (4) Electrical properties To estimate the electromagnetic properties of the sample, the sample was modeled as an equivalent electromagnetic field. First, the conductivity of the sample was measured using the probe method (RM3548, manufactured by Hioki E.E. Co., Ltd.). Next, to estimate the dielectric constant, the sample was modeled as an equivalent electromagnetic field as a flat plate with a uniform conductivity and complex dielectric constant. The measured value was input for the conductivity. For the dielectric constant, electromagnetic field simulation (Ansys HFSS, manufactured by Ansys Inc.) was performed and obtained by fitting with the experimental value. However, it was assumed that the dielectric constant does not depend on the frequency.

[0041] Table 2 shows the measured values of conductivity and the estimated values of dielectric constant. Compared with Experimental Example 1 without silane coupling treatment, the conductivity decreased significantly in Experimental Examples 2 and 3 with silane coupling treatment. This is considered to be the effect of surface insulation treatment. In the sample with an APMS amount of 500 μL (Experimental Example 2), the dielectric tangent also decreased, and it can be expected that the desired effect was obtained.

[0042] However, in the sample with an APMS amount of 2000 μL (Experimental Example 3), the real part of the relative dielectric constant and the dielectric tangent increased. This is considered to be the influence of the silane coupling agent itself.

[0043] For Experimental Example 4 with a CNT blending ratio of 14% by mass, the conductivity decreased significantly compared with Experimental Example 1. Also, a high relative dielectric constant was realized while maintaining the dielectric tangent. The possibility of controlling the dielectric constant of the sample by silane coupling treatment was shown, and an improvement in the degree of freedom of material design can be expected.

[0044]

Table 3

[0045] (5) Summary The surface was insulated with APMS, the CNT blending ratio was increased, the absorber in the absorbing material was densified, and they were mixed in the freeze-drying process to fabricate an ultra-lightweight electromagnetic wave absorbing material. The following results were obtained by evaluating the electromagnetic wave absorption performance and estimating the dielectric constant.

[0046] When the silane agent APMS chemically modified with an amino group is coupled to the CNT surface, the CNTs are uniformly dispersed in the aqueous solution.

[0047] By increasing the CNT blending ratio to three times the conventional ratio, the electromagnetic wave absorption performance could be improved to -46 dB at 12 GHz.

[0048] From this, it was confirmed that the surface insulation treatment by silane coupling has the potential as a means to improve the electromagnetic wave absorption performance.

[0049] 3. Embodiment The electromagnetic wave absorbing material according to the first embodiment includes carbon nanotubes subjected to surface treatment with a silane coupling agent, and has a bulk density of 1.23 kg / m 3 as follows.

[0050] According to this embodiment, an electromagnetic wave absorbing material that is lightweight and has excellent electromagnetic wave absorption performance can be obtained.

[0051] In the second embodiment, in the first embodiment, the silane coupling agent includes aminosilane.

[0052] The electromagnetic wave absorber according to the third embodiment includes a plurality of stacked layers. The plurality of layers include a functional layer and at least one of a metal layer and a magnetic layer. The functional layer includes the electromagnetic wave absorbing material of the first or second embodiment.

[0053] According to this aspect, an electromagnetic wave absorber that is lightweight and has excellent electromagnetic wave absorption performance can be obtained.

Claims

**Claim 1** An electromagnetic wave absorbing material comprising carbon nanotubes surface-treated with a silane coupling agent. The bulk density is 1.23 kg / m 3 or less. An electromagnetic wave absorbing material. **Claim 2** The electromagnetic wave absorbing material according to claim 1, wherein the silane coupling agent contains aminosilane. The electromagnetic wave absorbing material according to claim 1. **Claim 3** An electromagnetic wave absorber comprising a plurality of stacked layers, the plurality of layers including a functional layer and at least one of a metal layer and a magnetic layer, the functional layer including the electromagnetic wave absorbing material according to claim 1 or 2. An electromagnetic wave absorber.

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