Composite materials and shielding against electromagnetic radiation

Abstract

The present invention relates to a composite material for shielding electromagnetic radiation in the GHz range, comprising a matrix material and metal fibers, the metal fibers comprising at least one element selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, chromium, vanadium, titanium, aluminum, silicon, lithium, combinations thereof, and alloys comprising one or more of the foregoing. Furthermore, the present invention relates to a shield against electromagnetic radiation comprising the composite material, and to an electronic device comprising at least one component shielded against electromagnetic radiation using the shield.

Description

[Technical Field]

[0001] This invention relates to composite materials for shielding electromagnetic radiation and shielding materials for electromagnetic radiation. [Background technology]

[0002] Electrical and electronic equipment can be affected by interference from electromagnetic radiation, also known as electromagnetic waves, which can impact their performance. To ensure the reliable and long-lasting operation of such equipment, it is crucial to shield and protect them from such electromagnetic radiation. Electrical and electronic equipment can be protected from stray radiation by using shielding materials. Conversely, shielding materials can also be used to protect the environment from electromagnetic radiation generated by electrical and electronic equipment.

[0003] Furthermore, high-frequency electromagnetic radiation is being discussed due to its potential health risks. This is especially true for radiation in the higher GHz range, the so-called millimeter wave range. GHz radiation is widely used in radar (3 MHz to 110 GHz) and microwave ovens (2.455 GHz) and can cause thermal damage to biological materials. For this reason, the WHO, ICNIRP, and IEEE limit human exposure to 2-10 Wm in the 400 MHz to 300 GHz range. 2 Guidelines and regulations have been developed to limit the values ​​of these frequencies. These are also the frequency bands increasingly used in new technological developments such as radar-based distance sensors (24 GHz and 77 GHz) and in 5G, the fifth generation of mobile communications (3-30 GHz and 30-300 GHz in later stages). Therefore, there is a growing and widespread demand for electromagnetic shielding materials to avoid equipment malfunctions and to protect human and animal health in a world where the number of such devices is rapidly increasing.

[0004] Furthermore, without effective shielding, electromagnetic radiation becomes a source of interference, impairing the functionality of electronic equipment through inductive and capacitive coupling of energy fields. Examples include crosstalk, noise, reflection, and, above all, scrambling of digital signals. As a result, network failures and complete equipment failures occur, as seen in critical fields such as healthcare, transportation, security, and data processing.

[0005] Therefore, with the development of 5G technology, improved and highly effective shielding is required. Highly effective electromagnetic interference (EMI) shielding materials are necessary not only to reduce unwanted radiation but also to protect the components themselves from external stray signals. Particularly desirable is the possibility of frequency-selective surface structures (FSS) for shielding (i.e., certain wavelengths are effectively shielded while other wavelengths are not).

[0006] One possibility for EMI shielding is to reflect radiation using conductive materials. A further possibility for EMI shielding, which will become increasingly important in the future, is to utilize the absorption of electromagnetic (EM) radiation when electrical and / or magnetic dipoles interact with radiation. The combination of both effects is based on the fact that EM radiation is confined by scattering centers and interfaces or defect sites through multiple internal reflections and scattering, which lead to the absorption and dissipation of EM waves.

[0007] Simple metal sheaths that lead to reflection are widely used for low frequencies. In small devices and components (e.g., handheld devices), metal sheaths have drawbacks due to their weight, susceptibility to corrosion, and higher processing costs compared to plastics. Therefore, even for EM shielding in the kilohertz region, lightweight, inexpensive, and easily processed shielding materials are required. Polymer composite materials containing conductive fillers already meet these conditions for shielding in the kHz and MHz regions. In the MHz region, they also have special advantages due to their ability to absorb EM radiation. However, in many cases, the processability and corrosion resistance are limited by a metal filler amount exceeding 35 wt.%. Conductive carbon black as a filler is used in such composite materials but also has drawbacks due to black wear. Other carbon forms such as carbon nanotubes and graphene have been studied, and combinations with magnetic components have also been considered, but limitations in the GHz region have been shown, and there has not yet been a leap-forward progress in applications.

[0008] The shielding efficiency of a material is defined as the attenuation of electromagnetic radiation when passing through the material for a specific wavelength. Shielding is caused by both reflection and absorption. Heterogeneous materials such as metal foams and metal composite materials using dielectric materials can be particularly effective absorbers of electromagnetic radiation because they combine multiple reflections and absorption to increase the absorption path length. Absorption is due to the generation of local currents (displacement currents) that cause heating of the material, i.e., the thermal dissipation of electrical energy. Summary of the Invention Problems to be Solved by the Invention

[0009] Therefore, there is a need for improved shielding materials, especially for the 3 - 300 GHz broadband radio frequency band of 5G mobile wireless technology. Means for Solving the Problems

[0010] This object is solved by the composite material for shielding electromagnetic radiation according to claim 1 and a shield against electromagnetic radiation comprising such a composite material. The composite material according to the invention is a material for shielding electromagnetic radiation. It comprises a matrix material and metal fibers. The metal fibers are embedded in the matrix material. The metal fibers are selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, chromium, vanadium, titanium, aluminum, silicon, lithium, combinations of the above and alloys comprising at least one, preferably one element from the above.

[0011] The shield according to the invention can provide strong absorption losses of 20 to 60 dB in the frequency band of 3 to 300 GHz. Furthermore, it is possible to apply this material even to small electronic devices as a coating, paint, laminate or adhesive tape. Also, in addition to being an effective shielding material, the composite material according to the invention has little influence on the properties of the matrix material due to the small amount of metal fibers, so it is lightweight, easy to process, impact resistant, has good corrosion resistance because the metal fibers are embedded in the matrix material, is relatively inexpensive due to the small amount of metal fibers, and has a high degree of design freedom.

[0012] Without being bound by any theory, the mechanism of the GHz electromagnetic absorption characteristics is based on dielectricity rather than intrinsic conduction loss. According to Maxwell's equations, in a perfect conductor, when charge carriers are accelerated and decelerated in an electromagnetic field, a radiation field that cancels the incident wave is generated. In the composite material according to the invention, the following three mechanisms are combined to improve the shielding efficiency.

[0013] Absorption by antenna mechanism: Enhanced absorption is achieved by adjusting the length of the metal fiber to a resonant length antenna in the GHz frequency band, i.e., to one-quarter of the wavelength. It has been found that metal fiber lengths in the range of 1.0 mm to 25 mm efficiently absorb electromagnetic radiation in the frequency band of 3 to 300 GHz, particularly in the range of 3 to 30 GHz. Therefore, the length of the metal fiber is preferably in the range of 1.0 to 25 mm, more preferably in the range of 1.3 to 20 mm, even more preferably in the range of 1.5 to 18 mm, even more preferably in the range of 1.8 to 15 mm, and very preferably in the range of 1.9 to 10 mm. Furthermore, it is also possible to use metal fibers with a length exceeding 2.5 mm.

[0014] Impedance matching: The ideal condition for impedance matching is Z in This is achieved when =Z0=377Ω. Here, Z in is the impedance of the conductive dispersion component, and Z0 is the impedance of the dielectric matrix component. The above conditions apply to a specific matching thickness (t m ) and matching frequency (f m ) is satisfied. Ideally, the effective frequency band should be as wide as possible, which is t m This can be controlled by adjusting it to a multiple of 1 / 4 wavelength (nλ / 4). nλ / 4 is the length of dielectric material required to achieve zero reflection due to destructive interference between the incident and reflected waves. A -20dB attenuation is considered to be 99% microwave absorption and is considered sufficient shielding in most cases.

[0015] As an empirical rule for Faraday cages, the ratio of the diameter of the aperture gap to the wavelength of the electromagnetic wave being shielded should be 1 / 10 or less. Therefore, to prevent any transmission, it is preferable that the average distance between metal fibers is about λ / 10. This can be achieved using the composite material of the present invention when the metal fiber content is preferably in the range of 0.02 to 2.5 wt.% based on the total weight of the composite material in the 3 to 300 GHz range.

[0016] In the composite material according to the present invention, the matrix is ​​preferably an electrically insulating material. This electrically insulates the metal fibers from each other. As a result, a high level of shielding efficiency can be obtained. For example, it is particularly preferable that the fibers do not come into contact with each other, for example, by providing the fibers at a concentration of less than 20 wt.% or by adjusting the length of the fibers. A range of 0.02 to 2.5 wt.% is particularly preferred. By preventing the fibers from coming into contact with each other, it is possible to obtain the composite material of the present invention that does not have significant DC conductivity. In this regard, it is particularly preferable that the composite material of the present invention does not contain any further conductive additives such as carbon black or carbon nanotubes, separate from the metal fibers.

[0017] The matrix is ​​preferably a polymer material. This allows the metal fibers to be separated from each other, and polymer material molding processes such as extrusion molding using a twin-screw or single-screw extruder, injection molding, melt blowing, calendering, and lamination can be used. The polymer material is preferably a thermoplastic material, a rubber material, or a thermosetting resin material.

[0018] Preferred examples of thermoplastic materials include polyethylene, polypropylene, polystyrene and its copolymers, polymethacrylate and its copolymers, polyvinyl chloride, polyamide, aliphatic polyester, and aromatic polyester, polyimide, polyacetal, polysiloxane, polyphenylene sulfide, polycarbonate, polyphenylene ether, especially PPO, polyether ketone, especially PEK and PEEK, thermoplastic polyurethane, and thermoplastic derivatives of cellulose.

[0019] Preferred examples of rubber materials include thermoplastic elastomers such as polybutadiene rubber, polyisoprene rubber, nitrile rubber (NBR), EPDM rubber, polysiloxane rubber, thermoplastic olefins (TPO), styrene copolymers such as SBS and SEBS, thermoplastic urethane (TPU), thermoplastic amide polymers and copolymers (TPA), thermoplastic polyester copolymers (TPC), and ionomers, such as copolymers of ethylene and methacrylic acid. Ionomers are available, for example, under the brand name Surlyn.

[0020] Preferred examples of suitable thermosetting resins and prepolymers that can be used as matrix materials include phenolic resins such as novolac and bakelite, melamine resins, epoxy resins, and polyurethanes.

[0021] The use of hydrophobic polymers is even more preferable because they improve the weather resistance of the embedded metal fibers. Preferred examples of such hydrophobic polymers include polyethylene, polypropylene, polystyrene and its copolymers, polyvinyl chloride, polysiloxane, polybutadiene rubber, polyisoprene rubber, nitrile rubber (NBR), EPDM rubber, polysiloxane rubber, thermoplastic elastomers such as thermoplastic olefins (TPO), styrene copolymers such as SBS and SEBS, and phenolic resins.

[0022] The electromagnetic shielding composite material according to the present invention can be realized with a plurality of metal fibers. According to the present invention, the metal fibers include at least one element selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, chromium, vanadium, titanium, aluminum, silicon, lithium, combinations of the above, and alloys containing one or more of the above. Preferably, the metal fibers include at least one element selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, iron, vanadium, aluminum, silicon, lithium, combinations of the above, and alloys containing one or more of the above.

[0023] Particularly preferred metal fibers are those made of at least one material selected from the group consisting of Cu and its alloys with Si, Fe, and Mn, Al and its alloys with Si, Mg, Ti, Fe, and Mn, mu-metal also known as permalloy and supermalloy, and gold and silver, and their alloys. Alloys consisting of cobalt and the remainder of iron, molybdenum, boron, and / or silicon, such as Co 66 Fe4Mo2B 12 Si 16 and metal fibers of cobalt alloys such as are also preferred.

[0024] The metal fibers can be produced by melt spinning, for example, using the apparatus and method for producing metal strands by melt spinning as described in the examples disclosed in European Patent Application No. EP19175749.1, International Publication No. WO2016 / 020493A1, and International Publication No. WO2017 / 042155A1 (the contents of which are incorporated herein by reference with respect to the method of forming and obtaining the metal fibers). Thus, the metal fibers can be, for example, metal fibers made of Cu, Cu 99 Si1, Cu 96 Si4, Al, Al 99 Si1, Fe 40 Ni 40 B 20 , Au, Ag, Pb, Si, or alloys containing one or more of the above combinations and one or more of the above. Surprisingly, very good long-term corrosion stability was achieved in all cases where copper and aluminum were alloyed with Si, Fe, and Mn and compounded with a polymer matrix, particularly a hydrophobic polymer such as a polyolefin. The same is true for Co 66 Fe4Mo2B 12 Si 16The same was observed with respect to the metal fibers. In contrast to other metal fibers obtained by methods such as condensed drawing, the metal fibers obtained by melt spinning, as described in European Patent Application No. EP19175749.1, International Publication No. 2016 / 020493A1, and International Publication No. 2017 / 042155A1, exhibit lower brittleness due to their amorphous or nanocrystalline structure. Due to their low brittleness, the metal fibers can be extruded together with the matrix material without fracturing. Highly brittle fibers fracture during extrusion, resulting in shortening. Therefore, if the metal fibers are as brittle as those obtained from condensed drawing, it is difficult to produce composite materials containing metal fibers of distinct lengths. Consequently, metal fibers obtained by melt spinning, particularly those described above, are preferred in the composite materials of the present invention. Due to the flexibility of the metal fibers obtained from melt spinning, such metal fibers can be provided at the desired length before extrusion and maintain their length during extrusion, so that the composite materials of the present invention can be obtained by extrusion using metal fibers of distinct lengths.

[0025] When amorphous metal fibers are used, especially when they are embedded in a hydrophobic polymer, excellent corrosion stability can be achieved. Therefore, it is particularly preferable that the metal fibers in the composite material according to the present invention are amorphous metal fibers. It is even more preferable that these amorphous metal fibers are embedded in a hydrophobic polymer material. In contrast to crystalline metal fibers, amorphous metal fibers have a glassy structure or a nanocrystalline structure. A simple method for determining whether metal fibers are amorphous is DSC measurement. Amorphous metal fibers exhibit exothermic phenomena upon heating in DSC measurements that cannot be observed in crystalline metal fibers. Exothermic phenomena can be caused, for example, by crystallization, i.e., a transition from a glassy structure to a crystalline structure. Typically, stainless steel fibers have a crystalline structure.

[0026] The absorption effect can be further improved if the metal compound is a magnetic material that has soft magnetism for magnetic induction. Therefore, it is highly preferable that the metal fibers be made of mu-metals, more preferably mu-metals selected from the group consisting of mu-metals having a nickel content of 76-81 wt.% and the remainder being iron, copper, chromium, and / or molybdenum. These mu-metals exhibit almost immediate soft magnetization and can absorb electromagnetic energy by magnetic induction. As a result, particularly efficient shielding can be realized using metal fibers made of mu-metals. This is also true when the composite material containing mu-metals is manufactured using molding techniques such as those described above for polymer materials.

[0027] Other highly preferred metal fibers include copper or copper alloys, preferably Cu 99 Si1, Cu 98 Si2, Cu 96 Si4, Cu 88 Si 12 or Cu 92 Sn8, or aluminum or aluminum alloy, preferably Al 99 Made of Si1. Copper alloys and aluminum alloys can be easily manufactured using melt spinning technology compared to their pure metals, but they exhibit almost the same conductivity. Fibers of copper, copper alloys, aluminum, and aluminum alloys exhibit excellent shielding effects because their superior conductivity makes them prone to generating displacement currents.

[0028] It is preferable that the metal fibers are not made of stainless steel. In principle, it is possible to use metal fibers made of stainless steel, but better shielding results can be obtained by using other metal fibers such as those mentioned above. Although not bound by theory, it should be noted that shielding depends on the distance to the material at which the intensity of electromagnetic waves is attenuated to 1 / e, and this distance is also known as the skin thickness δ. The skin thickness is ν -1 / 2The shielding effect is proportional to the frequency of the electromagnetic wave and depends on the material, where ν is the frequency of the electromagnetic wave. In the frequency band of 1 to 100 GHz, the skin thickness is 2.0 to 0.2 μm for copper, 2.6 to 0.2 μm for aluminum, 13.2 to 1.3 μm for steel, and 59 μm to 5.9 μm for carbon. Due to the large skin thickness of steel and carbon materials, it is preferable that the composite material of the present invention essentially does not contain steel (especially stainless steel) and carbon materials (such as carbon black, carbon fibers, or carbon nanotubes). By using metal fibers of copper, copper alloys, aluminum, aluminum alloys, and / or mu-metal, the desired shielding effect can be achieved with relatively fine fibers. The use of finer fibers increases the surface area-to-volume ratio and further improves the shielding effect.

[0029] The amount of metal fibers in the composite material according to the present invention is preferably set so that the fibers do not come into contact with each other, i.e., below the penetration threshold. In order to keep the metal fibers below the penetration threshold, the concentration of metal fibers is preferably less than 20 wt.%. More preferably, the metal fiber concentration is in the range of 0.02 to 2.5 wt.%. From the viewpoint of processability and shielding efficiency based on Faraday's law, the upper limit of the amount of metal fibers in the composite material is preferably less than 1 wt.%, more preferably less than 0.5 wt.%, and even more preferably 0.25 wt.% or less, based on the total weight of the composite material. From the viewpoint of absorption by the antenna mechanism and the shielding mechanism based on impedance matching, the lower limit of the amount of metal fibers is preferably 0.04 wt.% or more, more preferably 0.05 wt.% or more, and even more preferably 0.06 wt.%, based on the total weight of the composite material. By adjusting the metal fiber concentration as described above, the distance between fibers can be adjusted. By adjusting the distance between fibers, electromagnetic waves of a specific wavelength can be selectively shielded, but the shielding effect is not as pronounced at other wavelengths.

[0030] The effect of the present invention is for 0.5~4μm -1 Preferably 0.8 to 3.5 μm -1 , more preferably 1.5 to 3.2 μm -1 More preferably 2.0 to 3 μm-1 This can be achieved particularly well by using metal fibers having a surface area-to-volume ratio. Such fibers have a particularly large surface area relative to their weight, which can increase the induction of displacement currents, resulting in a highly effective shielding material for electromagnetic waves while simultaneously saving weight and material.

[0031] In the composite material of the present invention, various shapes can be used for the cross-section of the metal fibers. When the cross-section of the metal fibers is not circular, a higher surface area-to-volume ratio can be achieved. Therefore, it is preferable that the cross-section has one edge that is less curved than the edges of other parts of the cross-section. That is, the radius of curvature of the less curved edge is greater than the radius of curvature of the other parts of the cross-section. Preferred examples of cross-sections in which one edge is less curved than the other include elliptical, crescent-shaped, and semicircular cross-sections. It is particularly preferable that the cross-section has one edge that is not curved, i.e., flat. More preferably, the metal fibers have a cross-section with two flat edges. Even more preferably, the cross-section of the metal fibers is rectangular.

[0032] In the present invention, it is preferable to use metal fibers having a rectangular cross-section and metal fibers having an elliptical cross-section. In the case of metal fibers having a rectangular cross-section, the width of the metal fiber is preferably 80 μm or less, more preferably 70 μm or less, even more preferably 40 μm or less, and even more preferably 5 μm or less, and the thickness is preferably 50 μm or less, more preferably 30 μm or less, even more preferably 10 μm or less, and even more preferably 5 μm or less. There is no particular lower limit to the width and thickness of the metal fiber. However, the metal fiber may have a width of 1 μm or more, preferably 3 μm or more, and a thickness of 1 μm or more.

[0033] In the case of metal fibers having an elliptical cross-section, the average diameter of the metal fibers is preferably 80 μm or less, more preferably 70 μm or less, even more preferably 40 μm or less, and even more preferably 5 μm or less. The average diameter of the elliptical cross-section refers to the average of the maximum and minimum diameters. There is no particular lower limit for the minimum diameter of metal fibers having an elliptical cross-section. However, the minimum diameter may be 1 μm or more, preferably 3 μm or more, and more preferably 5 μm or more.

[0034] The cross-sectional area of ​​the metal fiber is preferably 50 μm, regardless of whether its shape is rectangular, elliptical, crescent-shaped, or circular. 2 More preferably 30 μm 2 More preferably 25 μm 2 Below, most preferably 20 μm 2 The following applies: The lower limit of the cross-sectional area of ​​the metal fiber is preferably 0.5 μm. 2 More preferably 1.0 μm 2 That concludes the explanation. With the cross-sectional area described above, a particularly fine network of metal fibers is formed in the composite material, resulting in high shielding efficiency necessary for shielding in the GHz range. Although not bound by theory, when the cross-sectional area is as small as described above, it is thought that the increase in shielding efficiency is brought about by an increase in surface area. As will be explained in more detail below, as the surface area increases, the induced displacement current increases, and the shielding effect improves. At the same time, since shielding can be achieved with a smaller amount of metal fibers, the weight of the composite material can be reduced. Furthermore, such 0.5~10μm 2 For small cross-sectional areas, metal fibers are very flexible, which can improve processability, especially when using metal fibers obtained by melt spinning.

[0035] In the composite material according to the present invention, it is preferable that the metal fibers are not in electrical contact with each other. This allows for optimization of absorption by the antenna mechanism by adjusting the length of the fibers. Furthermore, because the fibers are not in electrical contact with each other, the DC conductivity of the composite material is reduced.

[0036] The composite material according to the present invention does not require additional conductive materials such as carbon black, carbon fibers, or carbon nanotubes. Therefore, it is preferable that the composite material according to the present invention does not contain carbon black, carbon fibers, or carbon nanotubes. By using a composite material that does not contain carbon black, carbon fibers, or carbon nanotubes, fouling due to abrasion can be prevented. Furthermore, it is possible to provide a transparent shield if the shield needs to be seen through, or to color the shield by adding pigments. It should be understood that the term "pigment" as used herein does not include carbon black, carbon fibers, or carbon nanotubes.

[0037] The composite material of the present invention is preferably an electromagnetic shield. Therefore, the use of the composite material of the present invention as an electromagnetic shield, particularly in the GHz range of 3 to 300 GHz, is also part of the present invention. Its use may also be in shields for frequencies in the range of 3 to 30 GHz, or 30 to 300 GHz, or both.

[0038] Preferably, an electromagnetic shield containing the composite material according to the present invention may have a single layer or may be manufactured from a multilayer laminate of two or more layers. However, it is preferable that the electromagnetic shield has a single layer to realize the electromagnetic shield. Such a single-layer shield can be easily manufactured and remains very effective in shielding such electromagnetic radiation due to the composite material of the present invention, which combines three mechanisms for absorbing electromagnetic radiation.

[0039] Shielding devices against electromagnetic radiation, including the composite materials described above and in the claims of the present invention, are also part of the present invention.

[0040] Preferably, the shield of the present invention is a housing for an electronic device or its components.

[0041] In connection therewith, electronic devices including at least one component shielded from electromagnetic radiation using the shielding material described above and in the claims, i.e., the composite material described above and in the claims, are also part of the present invention.

[0042] The composite material is prepared by cutting fibers to the appropriate length corresponding to the shielded EM spectrum (see below). The metal fibers are mixed with a fine powder of the matrix polymer before consolidation by melting, or are compounded directly with the polymer melt or monomer resin mixture.

[0043] Direct compounding using a twin-screw extruder is possible for short fibers suitable for high-frequency EM shielding, provided that the average fiber length is not significantly altered.

[0044] Next, the present invention will be described in more detail by various embodiments of the composite materials and shieldings of the present invention, with reference to the attached drawings and figures, as just one example. The drawings show the following: [Brief explanation of the drawing]

[0045] [Figure 1] This is a schematic diagram showing the incident, reflected, and transmitted power, as well as the electromagnetic field strength, when an EM wave collides with a 3D material. [Figure 2] The impedance measured for the CuSi4-TPS-SEBS composite material of Example 3 in the range of 1 Hz to 8 MHz is shown. [Figure 3] The impedance measured for the CuSi4-TPS-SEBS composite material of Example 4 in the range of 1 Hz to 8 MHz is shown. [Figure 4] The moisture absorption amounts of the composite materials in Examples 3 and 4 are shown. [Figure 5] The impedance measured in the range of 1Hz to 8MHz for the CuSi4-nylon composite material of Example 3 before and after underwater aging is shown. [Figure 6]The impedance measured in the range of 1Hz to 8MHz for the CuSi4-nylon composite material of Example 4 before and after underwater aging is shown. [Figure 7] These are photographs of the samples from Example 3, which contain CuSi4 fibers in concentrations of 0.5 wt.%, 2.5 wt.%, 10 wt.%, 20 wt.%, 30 wt.%, and 50 wt.%. [Modes for carrying out the invention]

[0046] Shielding efficiency depends on reflection and absorption. Shielding efficiency (AE) measures how much an EM wave of a specific frequency is attenuated by a material as it passes through it. Figure 1 shows possible interactions between EM waves and materials. When an EM wave reaches the surface of the material, the incident power (P) E A specific part of ) reflects (P R ) is absorbed, but another part is absorbed and dissipated as heat, and the remaining part is transmitted through the shielding material (P T Therefore, three different processes, namely reflection, absorption, and multiple internal reflections, contribute to the total attenuation, which corresponds to the shielding effect (Equation 1).

[0047]

number

[0048] The main mechanism of EMI shielding is reflection. Reflection loss (AE) 反射 ) is related to the relative impedance mismatch between the surface of the shielding material and the EM wave. The magnitude of the reflection loss is proportional to the ratio of the conductivity (σ) to the permeability (μ) of the material (i.e., AE 反射 ∝σ / μ).

[0049] Due to their excellent conductivity, metals can reflect or bypass electromagnetic radiation very well as closed layers or networks. Electrical equipment where high-frequency fields can accumulate static charge is typically protected by grounded metal shielding. This prevents high-frequency electromagnetic radiation from leaking out of the equipment while also providing shielding against stray radiation from the outside. However, simple metal shielding in the GHz range is not very practical for small components or when conductive seals that need to be compressed under high pressure are required.

[0050] Another mechanism of EMI shielding is absorption. The intensity of EM waves decreases exponentially as they pass through a conductive material. Absorption loss is caused by the heating of the material by the current induced in the medium. Absorption loss (AE) is measured in decibels (dB). 吸収 ) depends on the conductivity (σ), permeability (μ), and thickness of the sample (d) (i.e., AE 吸収 ∝σμd).

[0051] In thin films, absorption is facilitated by multiple reflections between the two outer interfaces, where EM waves are reflected from the second interface, return to the first interface, and are reflected from the first interface back to the second interface.

[0052] Here, the penetration depth δ is given by δ = (fπσμ) -1 / 2 It is given by and defined as the thickness below the outer surface at which the incident field is attenuated to 1 / e of its original value, where f is the frequency of the incident wave.

[0053] Also, multiple reflection AE 多重反射 The shielding efficiency based on this is dependent on the thickness d, and the penetration depth δ can be expressed by equation (2).

[0054]

number

[0055] Multiple reflections can also be achieved through the internal structure of materials, such as porous or composite materials where the permeability of the component varies greatly. Such heterogeneous microstructures result in large fluctuations in the local field. Various electromagnetic properties of the nano / microstructure act as polarization spaces, causing a delay in displacement current compared to conduction current. Under these conditions, permittivity and permeability can be replaced with effective permittivity (ε = ε' + iε) and permeability (μ = μ' + iμ), respectively. ε' and μ' refer to the storage of electrical and magnetic energy, respectively. ε'' and μ'' represent dielectric and resistive losses, respectively. These are complexly dependent on the shape, size, conductivity, and volume fraction of each component. When the impedance of the composite material matches that of free space, reflections of GHz waves from the surface of the composite material are completely attenuated (absorption is maximized). The ideal condition for impedance matching is Z in This is the case where =Z0=377Ω. Here, Z0 is the intrinsic impedance of free space, and Z in This is the input impedance of the absorber.

[0056] A -20dB attenuation is considered equivalent to 99% microwave absorption, which is generally considered sufficient shielding. Furthermore, in the technical realm, in addition to efficient shielding, lightweight, minimal thickness, corrosion resistance, chemical resistance, good flexibility, adjustable form, easy processing, and cost-effectiveness are also required.

[0057] The dependence of AE on μ and σ (see Equation 2) indicates that shielding of conductive magnetic metals is governed by absorption rather than reflection. This is due to the low conductivity, resulting in low reflectivity but high penetration depth. Here, thermoplastics and mu-metals or alloys consisting of cobalt and the remainder iron, molybdenum, boron and / or silicon, e.g., available under the trademark name Vitrovac®. 66 Fe4Mo2B 12 Si 16Microdispersed composite materials made from particularly fine fibers of soft magnetic metals such as are ideal for shielding by absorption. As mentioned above, such fine fibers of soft magnetic metals can be manufactured by melt spinning. The dimensions of the magnetic material also greatly affect the permeability. With larger particles, the loss of displacement current increases due to the increase in the induced displacement voltage (U 変位 (∝ region), leading to better AE results (shielding effect). Furthermore, the anisotropy of the metal fibers increases electromagnetic attenuation. [Examples]

[0058] Next, the present invention will be described with reference to non-limiting embodiments.

[0059] Several exemplary composite materials were prepared. Their compositions are summarized in Table 1. The following compounding procedures were applied to prepare the composite materials.

[0060] Formula: Compounding was carried out using extrusion. A conical screw and a co-rotating twin-screw microcompounder with a 15 mL capacity (DSM Xplore, Netherlands) was used for the exemplary composite materials. The compositions are shown in Table 1. Note that the composite materials of Examples 1 and 2 were prepared with metal fiber concentrations of 0 wt.%, 10 wt.%, 20 wt.%, 30 wt.%, 40 wt.%, and 50 wt.%. The composite materials of Examples 3 and 4 were prepared with metal fiber concentrations of 2.50 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, 30 wt.%, and 50 wt.%. The matrix material and metal fibers were supplied simultaneously to the microcompounder.

[0061] In Examples 1 and 2, mixing was carried out at 210°C for 10 minutes with a screw rotation speed of 40 rpm. The entire process was performed under a nitrogen atmosphere. Subsequently, the resulting composite material was processed into test specimens with a gauge length of 60 mm, a width of 20 mm, and a thickness of 2 mm using a 5.5 mL injection molding machine (DSM Xplore) under the following conditions: cylinder temperature 210°C, mold temperature 60°C, holding pressure 7 MPa, holding time 10 seconds, and cooling time 20 seconds.

[0062] In Example 3, the same procedure was used, but the temperature during mixing and the cylinder temperature during injection molding were 190°C. Photographs of the composite material from Example 3 for various concentrations of metal fibers are shown in Figure 7. The photographs show that the average distance between metal fibers decreases as the concentration of metal fibers increases. The penetration threshold of 30–20 wt.% observed by impedance measurements is consistent with the optical impression from the photographs.

[0063] The composite material of Example 4 is identical to that of Example 3, but instead of using injection molding as in Example 3, it was processed by hot pressing. By hot pressing at a pressure of 1.7 bar and a temperature of 180°C for 10 minutes (MeltPrep, Austria), pellets with a diameter of 25 mm and a thickness of 1.6 mm were obtained from the composite material of Example 4.

[0064] [Table 1]

[0065] SEBS is a hydrogenated thermoplastic styrene-containing elastomer obtained by hydrogenating a block copolymer of styrene and butadiene, possessing excellent mechanical properties, chemical resistance, and heat resistance. SEBS compounds exhibit superior elastomer properties and a rubber-like appearance, along with excellent weather resistance, UV resistance, and ozone resistance, making them a preferred choice for outdoor and long-life applications. The chemical structure of SEBS follows the formula below.

[0066] [ka] [In the formula, x, m, n, v, and y are integers.]

[0067] The impedance measurements of the composite material of Example 3 are shown in Figure 2, and the impedance measurements of the composite material of Example 4 are shown in Figure 3. The impedance measurements of the composite material of Example 2 are equivalent to those of Example 3. It was found that the penetration threshold of the composite material of Example 3, obtained by injection molding, was 30-20 wt.%, and the same can be said for the composite material of Example 2. A similar penetration threshold was observed for the composite material of Example 4, but the penetration limit does not appear to be as abrupt. This can be explained by the lower homogeneity of the samples prepared by hot pressing compared to those obtained by injection molding.

[0068] Coating preparation A solvent-based coating was prepared using a KPG stirrer according to the following procedure: 20 g of Laropal A81 (60% solids content in PMS) and 5 g of GARAMITE-7305 in 62 mL of 1-methoxy-2-propyl acetate were stirred at 1000 rpm for 5 minutes. Then, 18 g of CuSi4 fibers (approximately 1.5 mm in length) were added and stirred for a further 5 minutes at 500 rpm. The dispersion was applied as a coating to a polycarbonate plate using a drawdown rod (doctor blade) and then cured at room temperature for 24 hours.

[0069] Corrosion test To evaluate the corrosion resistance of the obtained composite materials, the samples were aged in distilled water at 50°C for 7 days. After treatment, no pitting, general corrosion, or oxidation of CuSi4 fibers was observed. The samples were weighed before and after aging in water, and the total moisture content was evaluated to be 0.22-0.5 wt.%, which indicates high resistance of the polymer composite material to hydration. The moisture content of the composite materials of Examples 3 and 4 is shown in Figure 4, where the upper bar represents the composite material of Example 4 and the lower bar represents the composite material of Example 3. Note that complete oxidation of copper to CuO would result in a 25% weight increase. In particular, in samples with high copper content, copper oxidation is suppressed to less than 2-3% of the metal. This indicates high corrosion resistance. The relatively high relative oxidation amount in samples with low fiber content can be explained by surface effects that can be avoided by changing the compounding conditions.

[0070] As can be seen from Figure 5 for the composite material of Example 3 and from Figure 6 for the composite material of Example 4, the impedance did not change significantly after the corrosion test. This confirms the high corrosion stability of the obtained composite materials.

[0071] shielding An EMI shielding composition was prepared from the composition of Example 2, which included a CuSi4 band (2 μm thick, 10 μm wide, and 2 mm long) and nylon 6 as the matrix material. The composite material was shredded into pellets, and the pellets were molded using an injection molding machine. Impedances in the range of 1 Hz to 8 MHz with a 20 mV amplitude (excluding low-impedance samples without a dielectric barrier with 5 mV applied) were measured using an electrochemical workstation IM6 (ZAHNER-elektrik GmbH&Co.KG). For two-electrode impedance measurements across the pellet thickness, a measurement cell with a D=18 mm stainless steel disk electrode was used. A 25 μm thick Kapton film was placed on one of the electrodes as a dielectric barrier (DB). This eliminates the situation where a single highly conductive path in the material short-circuits the electrodes. The cell was connected to the IM6 using a short cable from the IM6 set, and the IM6 was configured for four-electrode measurement to compensate for the cable impedance. The results are shown in Figure 2.

[0072] Based on these results, the reflection loss (RL) is

[0073]

number

[0074] This results in an attenuation of -3 to -6 dB, or 10 to 30% absorption of electromagnetic energy. At higher frequencies, the power loss due to charge displacement increases. At 8 GHz, the observed power loss of the electromagnetic wave is 10 6 Because it increased (P∝f 2 ), virtually complete shielding is achieved in the GHz range.

Claims

1. A composite material for shielding electromagnetic radiation that causes absorption loss of 20 to 60 dB in the frequency band of 3 to 300 GHz, comprising a matrix material and metal fibers, The above metal fibers include at least one element selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, chromium, vanadium, titanium, aluminum, silicon, lithium, combinations of the above, and alloys containing one or more of the above. The composite material is characterized in that the amount of metal fibers in the composite material is set to be less than the penetration threshold, the metal fibers are not electrically in contact with each other, and the average distance between adjacent fibers is λ / 10 or less (where λ is the wavelength of the electromagnetic wave to be shielded).

2. The above metal fibers Cu and its alloys with Si, Fe and Mn, Al and its alloys with Si, Mg, Ti, Fe and Mn, Mu-metal, and Gold and silver, and their alloys The composite material according to claim 1, comprising at least one material selected from the group consisting of the following.

3. The composite material according to any one of claims 1 to 2, wherein the metal fibers are copper metal fibers or copper alloy fibers, or aluminum metal fibers or aluminum alloy fibers.

4. The composite material according to any one of claims 1 to 3, wherein the above-mentioned metal fibers are not made of stainless steel.

5. The above metal fibers are 0.5 to 4 μm thick. －１ A composite material according to any one of claims 1 to 4, having a surface area to volume ratio.

6. The composite material according to any one of claims 1 to 5, wherein the metal fibers have a cross-section that is not circular.

7. The composite material according to any one of claims 1 to 6, wherein the metal fiber has a cross-section in which one edge is not curved more than the edge of another part of the cross-section.

8. The composite material according to any one of claims 1 to 7, wherein the metal fiber has a cross-section having one non-curved edge.

9. The composite material according to any one of claims 1 to 8, wherein the amount of metal fibers in the composite material is in the range of 0.02 to 2.5 wt.% based on the total weight of the composite material.

10. The composite material according to any one of claims 1 to 9, wherein the matrix is ​​an electrical insulating material.

11. The composite material according to any one of claims 1 to 10, wherein the metal fibers are not in electrical contact with each other.

12. A composite material according to any one of claims 1 to 11, which is an electromagnetic shielding material.

13. The composite material according to claim 12, wherein the electromagnetic shielding material has one layer.

14. A shield against electromagnetic radiation comprising the composite material described in any one of claims 1 to 13.

15. An electronic device comprising at least one component shielded from electromagnetic radiation using the shielding material described in claim 14.

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