JP2024080837A - Radio wave absorber, housing, and power conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radio wave absorber which enhances heat conductivity while securing electromagnetic wave absorption capability.

SOLUTION: A radio wave absorber absorbing electromagnetic waves includes: a radio wave absorption layer having a soft magnetic material and a silicone resin; and a heat conduction layer which is arranged on one surface of the radio wave absorption layer, has ceramic particles and a silicone resin, and has heat conductivity higher than that of the radio wave absorption layer.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2024,JPO&INPIT

Description

This disclosure relates to a radio wave absorber, a housing, and a power conversion device.

Conventionally, electromagnetic wave absorbers that absorb electromagnetic waves have been used to suppress the effects of electromagnetic waves emitted from various electric circuits, electronic devices, etc. For example, Patent Document 1 discloses an electromagnetic wave absorber that is used in the vicinity of equipment that generates electromagnetic waves and can become hot, and that has an electromagnetic wave absorbing heat conductive layer in which soft magnetic metal powder and an electrically insulating heat conductive filler are dispersed in a base polymer. In such an electromagnetic wave absorbing heat conductive sheet, electromagnetic waves are absorbed by the soft magnetic metal powder dispersed in the electromagnetic wave absorbing heat conductive layer, and the heat conductive performance of the electromagnetic wave absorbing heat conductive sheet is improved by the heat conductive filler dispersed in the electromagnetic wave absorbing heat conductive layer.

JP 2004-200534 A

However, even if the electromagnetic wave absorbing heat conductive sheet as described above contains soft magnetic metal powder and electrically insulating heat conductive filler, the electromagnetic wave absorbing performance and thermal conductivity may be insufficient. When fillers such as soft magnetic metal powder and heat conductive filler are added to the resin (base polymer), there is a limit to the total amount of filler added from the viewpoint of ensuring the flexibility of the electromagnetic wave absorbing heat conductive sheet. For example, if the amount of soft magnetic metal powder added is ensured to ensure the electromagnetic wave absorbing performance, the amount of heat conductive filler added is suppressed. In particular, the heat conductive filler dispersed in the resin has a property that it is difficult to increase the thermal conductivity unless the amount of heat conductive filler added is secured to a certain extent or more and the distance between the heat conductive fillers is shortened, since heat is transferred by the heat being transmitted between adjacent heat conductive fillers. Therefore, when the amount of heat conductive filler added is suppressed as described above, the thermal conductivity of the entire radio wave absorber may be insufficient. Therefore, a technology for increasing the thermal conductivity while ensuring the electromagnetic wave absorbing ability in a radio wave absorber that absorbs electromagnetic waves has been desired.

The present disclosure can be realized in the following forms.
(1) According to one aspect of the present disclosure, there is provided a radio wave absorber for absorbing electromagnetic waves, the radio wave absorber comprising: a radio wave absorbing layer including a soft magnetic material and a silicone resin; and a thermally conductive layer disposed on one surface of the radio wave absorbing layer, the thermally conductive layer including ceramic particles and a silicone resin, and having a thermal conductivity higher than that of the radio wave absorbing layer.
According to this embodiment of the radio wave absorber, since the radio wave absorber includes the heat conductive layer as well as the radio wave absorbing layer, it is possible to increase the thermal conductivity of the radio wave absorber while ensuring the electromagnetic wave absorbing ability.
(2) In the radio wave absorber of the above embodiment, the thermal conductivity may be 1.0 W/(m·K) or more. With such a configuration, the thermal conductivity of the radio wave absorber can be more sufficiently ensured.
(3) In the radio wave absorber of the above embodiment, the real component of the complex relative permeability may be 3 or more in the range of 1 MHz to 100 MHz. With this configuration, the radio wave absorber can function as a radio wave absorber for electromagnetic waves of 1 MHz to 100 MHz.
(4) In the radio wave absorber of the above embodiment, the Young's modulus may be 10 MPa or less. With such a configuration, the flexibility and flexibility of the radio wave absorber can be increased, and therefore, by bending the radio wave absorber, the radio wave absorber can be easily disposed on the surface of a member having an arbitrary shape.
(5) In the radio wave absorber of the above embodiment, the Young's modulus after treatment in the atmosphere at 100° C. for 1000 hours may be 11 MPa or less. With such a configuration, even when the radio wave absorber is placed in a device that becomes relatively hot and used for a long time, the flexibility of the radio wave absorber can be ensured, and the heat resistance and durability of the radio wave absorber can be improved.
(6) In the radio wave absorber of the above embodiment, the ceramic particles may contain at least one of aluminum nitride and boron nitride. With such a configuration, the thermal conductivity of the heat conduction layer is increased, and as a result, the thermal conductivity of the entire radio wave absorber is easily increased.
(7) In the wave absorber of the above embodiment, when the average particle diameter of the ceramic particles having a particle diameter of 7 μm or more is r1 and the average particle diameter of the particles having a particle diameter of less than 7 μm is r2, the value of r2/r1 may be 0.10 or more and 0.45 or less. With such a configuration, it is possible to ensure that the ceramic particles are dense in the thermally conductive layer, and it is easy to increase the thermal conductivity of the thermally conductive layer and the entire wave absorber.
(8) In the radio wave absorber of the above embodiment, the elongation rate at room temperature may be 30% or more. With such a configuration, the flexibility and flexibility of the radio wave absorber can be increased. In addition, when the radio wave absorber is attached to, for example, an inner wall surface of a housing that houses a heat-generating device, it becomes easy to alleviate distortion caused by a difference in thermal expansion between the radio wave absorber and the housing.
(9) According to another aspect of the present disclosure, there is provided a housing for accommodating an electronic component module, the housing including the radio wave absorber according to any one of (1) to (8), and an attachment portion for attaching a cooling structure for cooling the electronic component module to the housing from the outside of the housing, the radio wave absorber being attached to the inner wall surface of the housing at a position overlapping with the attachment portion such that the radio wave absorbing layer faces an inner wall surface of the housing and the thermally conductive layer faces the electronic component module.
According to this embodiment of the housing, the radio wave absorber is attached to the inner wall surface of the housing, thereby suppressing the influence of electromagnetic waves on the electronic component module and ensuring thermal conductivity between the housing and the electronic component module. At this time, since the radio wave absorber is attached to the inner wall surface of the housing so that the thermal conductive layer faces the electronic component module, the heat generated by the electronic component module can be immediately removed by the thermal conductive layer.
(10) According to yet another aspect of the present disclosure, there is provided a power conversion device including a power module having a semiconductor, a housing for housing the power module, and a cooling structure for cooling the electronic component module, the cooling structure being attached to the housing from the outside of the housing at an attachment portion provided on the housing. In this power conversion device, the housing includes the radio wave absorber according to any one of claims 1 to 8, and the radio wave absorber is attached to the inner wall surface of the housing at a position overlapping with the attachment portion, with the radio wave absorbing layer facing the inner wall surface of the housing and the heat conduction layer facing the electronic component module.
According to the power converter of this embodiment, the electromagnetic wave absorber is attached to the inner wall surface of the housing, thereby suppressing the influence of electromagnetic waves on the power module and ensuring thermal conductivity between the housing and the power module. At this time, since the electromagnetic wave absorber is attached to the inner wall surface of the housing so that the thermal conductive layer faces the power module, the heat generated by the power module can be immediately removed by the thermal conductive layer.
The present disclosure may be realized in various forms other than those described above, for example, in the form of a radio wave absorbing sheet, a method for manufacturing a radio wave absorber, a method for preventing the emission or intrusion of electromagnetic waves in electronic devices, etc.

1 is a schematic cross-sectional view illustrating a schematic configuration of a radio wave absorber according to a first embodiment. FIG. 4 is an enlarged schematic cross-sectional view showing the arrangement of a radio wave absorber in the device. FIG. 2 is an explanatory diagram showing an example of a model in which ceramic particles are most dense. FIG. 11 is an explanatory diagram showing the results of determining the particle diameter of R5 according to the particle diameters of particles R1 to R4. FIG. 2 is an explanatory diagram illustrating a specific configuration of a radio wave absorber. FIG. 11 is an explanatory diagram showing all the measured values related to the performance of the radio wave absorber. FIG. 13 is an explanatory diagram showing evaluation results of radio wave absorbers.

A. First embodiment:
1 is a schematic cross-sectional view showing a schematic configuration of a radio wave absorber 10 according to a first embodiment of the present disclosure. The radio wave absorber 10 includes a radio wave absorbing layer 12 and a thermally conductive layer 14 disposed on one surface of the radio wave absorbing layer 12.

The electromagnetic wave absorbing layer 12 comprises a soft magnetic material and a silicone resin. In the electromagnetic wave absorbing layer 12, the soft magnetic material is dispersed in the silicone resin in the form of powder particles.

The soft magnetic material may be, for example, at least one of a soft magnetic alloy and a metal oxide. Examples of soft magnetic alloys include Fe-Si alloys, Fe-Si-Cr alloys, sendust (Fe-Si-Al alloy), and permalloy (Fe-Ni alloy). Examples of metal oxides include ferrites such as Ni-Zn ferrite (nickel zinc ferrite) and Mn-Zn ferrite (manganese zinc ferrite), and iron oxides such as magnetite and maghemite.

The soft magnetic material may be appropriately selected according to the frequency band of the electromagnetic waves to be absorbed using the radio wave absorber 10. Since the frequency band of electromagnetic waves emitted from general electronic devices is kHz to MHz, a soft magnetic material that functions in a wide frequency band from kHz to MHz is preferable. The above-mentioned Ni-Zn ferrite, Mn-Zn ferrite, magnetite, Fe-Si alloy, Fe-Si-Cr alloy, sendust, permalloy, etc. are useful for electromagnetic waves in such frequency bands. In particular, ferrites such as Mn-Zn ferrite and Ni-Zn ferrite are preferable because they have a relatively high magnetic permeability in the frequency band from kHz to MHz. In addition, when the radio wave absorber 10 is used in the vicinity of a high-voltage device, it is desirable for the soft magnetic material to be a material with a higher electrical resistance. From the viewpoint of high electrical resistance, ferrites such as Ni-Zn ferrite and Mn-Zn ferrite are preferable. The amount of soft magnetic material added to the radio wave absorbing layer 12 is preferably 40% to 60% by volume from the viewpoint of increasing the real component μ' of the complex relative permeability in the radio wave absorber 10.

The thermally conductive layer 14 is a layer for increasing the thermal conductivity of the entire radio wave absorber 10, and comprises ceramic particles and silicone resin. Hereinafter, the ceramic particles in the thermally conductive layer 14 are also referred to as "highly thermally conductive ceramic particles", and the ceramic that constitutes the ceramic particles is also referred to as "highly thermally conductive ceramic". The thermally conductive layer 14 is formed in a state in which highly thermally conductive ceramic particles are dispersed in silicone resin. The thermally conductive layer 14 of this embodiment has a higher thermal conductivity than the radio wave absorbing layer 12.

The highly thermally conductive ceramic particles of the thermally conductive layer 14 are preferably made of ceramics that exhibit a higher thermal conductivity than general resins including silicone resins. Examples of such highly thermally conductive ceramics include alumina (Al ₂ O ₃ ), zirconia (ZrO ₂ ), silicon nitride, aluminum nitride, boron nitride, and silicon carbide. In particular, from the viewpoint of increasing the thermal conductivity of the thermally conductive layer 14 to be higher than that of the electromagnetic wave absorbing layer 12 and increasing the thermal conductivity of the entire electromagnetic wave absorber 10, the highly thermally conductive ceramic is preferably a ceramic having a thermal conductivity of more than 100 W/(m·K). From the viewpoint of forming highly thermally conductive ceramic particles using highly thermally conductive ceramics having a thermal conductivity of more than 100 W/(m·K), the highly thermally conductive ceramic particles preferably contain at least one of aluminum nitride and boron nitride.

To achieve sufficient thermal conductivity in the thermally conductive layer 14, the highly thermally conductive ceramic particles in the thermally conductive layer 14 are required to be in a dense state in the silicone resin. In order for the highly thermally conductive ceramic particles to be in a dense state in this way, it is desirable for the particle diameters of the highly thermally conductive ceramic particles to have a relatively wide distribution. Even if the particle diameter of the highly thermally conductive ceramic particles is relatively narrow, when particles with different particle size distributions are mixed, the ceramic particles can easily form a densely packed structure. The amount of highly thermally conductive ceramic particles added in the thermally conductive layer 14 is preferably 40% to 60% by volume.

In the radio wave absorber 10, the thermal conductivity of the entire radio wave absorber 10, which includes the radio wave absorbing layer 12 and the thermally conductive layer 14, is preferably 1.0 W/(m·K) or more. In this way, the heat transfer performance of the radio wave absorber 10 can be ensured. It is preferable that the above-mentioned thermal conductivity in the radio wave absorber 10 is realized particularly in the stacking direction of the radio wave absorbing layer 12 and the thermally conductive layer 14 (the direction perpendicular to the surface direction of the radio wave absorbing layer 12 and the thermally conductive layer 14, hereinafter also simply referred to as the "stacking direction"). In this way, in the radio wave absorber 10, the thermal resistance between the surface on the thermally conductive layer 14 side and the surface on the radio wave absorbing layer 12 side can be reduced to ensure the heat transfer performance. In FIG. 2 described later, the stacking direction in which it is particularly desired to suppress the thermal resistance in the radio wave absorber 10 is indicated by a double-headed arrow. The thermal conductivity value of the entire wave absorber 10, 1.0 W/(m·K), is considered to be almost impossible to achieve when ferrite, which has a thermal conductivity of about 5 W/(m·K), is used as a filler together with the silicone resin, and is extremely difficult to achieve even when alumina, which has a thermal conductivity of 30 W/(m·K), is used as a filler.

The thermal conductivity of the entire radio wave absorber 10 can be adjusted by the thermal conductivity and thickness fraction of each of the radio wave absorbing layer 12 and the thermally conductive layer 14. Specifically, it is easy to increase the thermal conductivity of the entire radio wave absorber 10 by increasing the thickness of the thermally conductive layer 14 to a certain degree and increasing the thickness fraction, as well as increasing the thermal conductivity of the thermally conductive layer 14. However, from the viewpoint of being able to arrange the radio wave absorber 10 even in an extremely narrow space, it is desirable that the thickness of the entire radio wave absorber 10 be 1 mm or less. The thermal conductivity of each of the radio wave absorbing layer 12 and the thermally conductive layer 14 can be adjusted by the type, amount of filler (soft magnetic material contained in the radio wave absorbing layer 12 or highly thermally conductive ceramic particles contained in the thermally conductive layer 14) contained in each layer, and the packing state (how dense the filler is).

The real component μ' of the complex relative magnetic permeability in the radio wave absorber 10 is preferably 3 or more in the range of 1 MHz to 100 MHz. In this way, the radio wave absorber 10 can function well as a radio wave absorber for electromagnetic waves in the above frequency band. The real component μ' of the complex relative magnetic permeability in the radio wave absorber 10 can be adjusted, for example, by the type and amount of soft magnetic material contained in the radio wave absorbing layer 12. The greater the amount of soft magnetic material added, the greater the real component μ' of the complex relative magnetic permeability. In addition, the thickness fraction of the radio wave absorbing layer 12 in the radio wave absorber 10 in which the radio wave absorbing layer 12 and the thermally conductive layer 14 are laminated also affects the real component μ'. The greater the thickness fraction of the radio wave absorbing layer 12, the greater the real component μ' of the complex relative magnetic permeability in the radio wave absorber 10.

To make the real component μ' of the complex relative permeability 3 or more, it is desirable to use Mn-Zn ferrite as the soft magnetic material of the radio wave absorbing layer 12. Since Mn-Zn ferrite has a relatively high permeability in the frequency band from kHz to MHz, using Mn-Zn ferrite as the soft magnetic material makes it easy to make the real component μ' of the complex relative permeability 3 or more in the range from 1 MHz to 100 MHz.

The Young's modulus of the radio wave absorber 10 is preferably 15 MPa or less, and more preferably 10 MPa or less. This ensures the flexibility of the radio wave absorber 10. By ensuring the flexibility of the radio wave absorber 10, it becomes easier to attach the radio wave absorber 10 along a shape that is different from a flat surface. For example, when attaching the radio wave absorber 10 to at least a part of the inner wall surface of a housing that contains a device that emits electromagnetic waves or is affected by electromagnetic waves, the radio wave absorber 10 can be easily attached even if the attachment portion is a curved surface. The Young's modulus of the radio wave absorber 10 is preferably 0.5 MPa or more, and more preferably 1 MPa or more.

The Young's modulus of the radio wave absorber 10 can be adjusted, for example, by the molecular weight and crosslink density of the silicone resin constituting the radio wave absorbing layer 12 and the thermally conductive layer 14. For example, if the molecular weight of the silicone resin is small, the crosslink density is likely to be high and the Young's modulus is likely to be large. The Young's modulus of the radio wave absorber 10 can also be adjusted by the type and content of the soft magnetic material in the radio wave absorbing layer 12, or the type and content of the highly thermally conductive ceramic particles in the thermally conductive layer 14. The lower the content of the filler in each layer, the lower the Young's modulus of the radio wave absorber 10 can be. The molecular weight of the silicone resin in particular has a large effect on the Young's modulus of the radio wave absorber 10, and it is preferable that the molecular weight of the main agent is 10,000 or more.

Furthermore, the Young's modulus of the radio wave absorber 10 after treatment at 100°C for 1000 hours in the atmosphere is preferably 15 MPa or less, more preferably 11 MPa or less. This ensures the flexibility of the radio wave absorber 10 even when the radio wave absorber 10 is placed in a device that is relatively hot and used for a long time, and can improve the heat resistance and durability of the radio wave absorber. The Young's modulus of the radio wave absorber 10 after the above-mentioned heat treatment can be adjusted, for example, by the molecular weight and crosslink density of the silicone resin constituting the radio wave absorbing layer 12 and the thermal conductive layer 14, or the temperature at which the resin is cured. For example, assuming that the amount of functional groups is the same, the larger the molecular weight of the silicone resin, the lower the crosslink density and the smaller the Young's modulus after heat treatment. Therefore, the molecular weight of the main agent of the silicone resin is preferably 20,000 or more, more preferably 50,000 or more, and even more preferably 70,000 or more. The Young's modulus of the radio wave absorber 10 after the above-mentioned heat treatment is preferably 0.5 MPa or more, more preferably 1 MPa or more. Furthermore, if the silicone resin is cured at a relatively low temperature, the Young's modulus of the radio wave absorber 10 may increase and change significantly when it is subsequently placed in a high-temperature environment, so in order to suppress the Young's modulus after heat treatment, it is preferable to cure the silicone resin at a temperature of 100°C or higher.

The radio wave absorber 10 desirably has an elongation rate of 30% or more at room temperature. This ensures the flexibility of the radio wave absorber 10. When the radio wave absorber 10 exhibits the above elongation rate, for example, when the radio wave absorber 10 is attached to the inner wall surface of a housing that contains a device that emits electromagnetic waves or is affected by electromagnetic waves, the strain caused by the difference in thermal expansion between the radio wave absorber 10 and the housing can be alleviated. The elongation rate of the radio wave absorber 10 can be adjusted by the molecular weight and crosslinking density of the silicone resin that constitutes the radio wave absorbing layer 12 and the thermally conductive layer 14, as with the above-mentioned Young's modulus. From the viewpoint of ensuring the above-mentioned effect of alleviating strain, it is preferable that the main agent of the silicone resin has a molecular weight of 10,000 or more.

Figure 2 is an explanatory diagram showing the configuration of a device equipped with the radio wave absorber 10 of this embodiment. Specifically, Figure 2 is a schematic cross-sectional view showing an enlarged view of the location where the radio wave absorber 10 is arranged in a device 50 equipped with a device 40 that emits electromagnetic waves or is affected by electromagnetic waves, a housing 20 that houses the device 40, and a cooling structure 30 for cooling the device 40. Note that Figure 2 does not accurately show the dimensional ratios of each part.

The housing 20 includes an outer shell 22 of the housing and a radio wave absorber 10. In the device 50 shown in FIG. 2, the radio wave absorber 10 is attached to the inner wall surface of the housing 20 with the radio wave absorbing layer 12 facing the inner wall surface of the housing 20, i.e., the inner wall surface of the outer shell 22 of the housing, and the thermally conductive layer 14 facing the device 40.

The cooling structure 30 can employ various cooling methods such as air cooling, but FIG. 2 shows a structure (e.g., a water-cooled jacket) that includes a flow path piping 32 and has a refrigerant flow path 34 formed therein through which the refrigerant flows. The cooling structure 30 is attached to the housing 20 from the outside of the housing 20 at a mounting portion 24 provided on the housing. Various methods can be used for mounting at the mounting portion 24, such as welding or bolt fastening. FIG. 2 shows, as an example, a state in which a part of the outer shell portion 22 of the housing that constitutes the mounting portion and a part of the flow path piping 32 of the cooling structure 30 are integrated. The radio wave absorber 10 is attached to the inner wall surface of the housing 20 at a position that overlaps with the mounting portion 24 in a direction perpendicular to the surface direction of the outer shell portion 22.

The device 50 can be, for example, a power conversion device that serves as the power source for an electric vehicle, i.e., a power train. The device 40 can be, for example, a power module having a semiconductor, and can be a transformer (a step-up converter that steps up the battery voltage, or a step-down converter that steps down a high voltage) composed of Si-IGBT or the like. The radio wave absorber 10 of this embodiment can be used in a device equipped with an electronic component module that operates at a lower voltage than high-voltage devices such as the power module described above. However, by using it together with a device 40 that generates a large amount of heat, such as a power module, the effect of improving the cooling performance of the device 40 by providing a thermally conductive layer 14 in the radio wave absorber 10, which will be described later, can be significantly obtained.

In the radio wave absorber 10 of this embodiment, an adhesive layer may be provided on one side of the radio wave absorber 10 (a side different from the side in contact with the thermally conductive layer 14 in the radio wave absorbing layer 12, and the side shown as the adhesive surface 13 in FIG. 1). This adhesive layer is a structure for attaching the radio wave absorber 10 to a location where the radio wave absorber 10 is to be placed, such as the inner wall surface of the housing 20 that houses the device 40 shown in FIG. 2. The adhesive layer can be formed by applying an adhesive or pressure-sensitive adhesive such as a silicone resin, a polyamide resin, an acrylic resin, or an epoxy resin. Since the radio wave absorber 10 is used in close proximity to the device 40 that may become hot during use, it is desirable to use a silicone resin that has a relatively high heat resistance and a relatively high performance of relaxing distortion caused by the difference in thermal expansion between the radio wave absorber 10 and the outer shell part 22 of the housing as the adhesive or pressure-sensitive adhesive. Note that when the radio wave absorber 10 is placed by being mechanically fixed using, for example, a fastener, instead of being attached, the adhesive layer described above may not be provided.

In addition, when the radio wave absorber 10 is arranged by pasting, for example, an alkoxide such as a silane coupling agent may be added to the silicone resin constituting the radio wave absorbing layer 12, and the radio wave absorber 10 may be used as an adhesive sheet without providing an adhesive layer. In this case, the radio wave absorber 10 is prepared in a state in which a part of the silicone resin constituting the radio wave absorbing layer 12 is cured, and then the radio wave absorber 10 is pasted on a desired location (an adherend such as an inner wall surface of a housing) and thermally cured, thereby bonding the radio wave absorber 10 to the adherend. There is no particular restriction on the silane coupling agent to be added to the silicone resin, and it may be appropriately selected from conventionally known silane coupling agents, such as those having any of a vinyl group, an epoxy group, a methacryl group, an amino group, a mercapto group, and an isocyanate group as an organic reactive group.

The radio wave absorber 10 of this embodiment configured as described above includes a radio wave absorbing layer 12 including a soft magnetic material and a silicone resin, and a thermally conductive layer 14 arranged on one surface of the radio wave absorbing layer 12, including highly thermally conductive ceramic particles and a silicone resin, and having a higher thermal conductivity than the radio wave absorbing layer 12. In this way, since the radio wave absorber 10 includes the radio wave absorbing layer 12 and the thermally conductive layer 14, the thermal conductivity of the radio wave absorber 10 can be increased while maintaining the electromagnetic wave absorption capacity. As a result, when the radio wave absorber 10 is used together with a device 40 that generates a relatively large amount of heat, such as a power module, as shown in FIG. 2, the electromagnetic noise from the device 40, etc., can be attenuated while heat is dissipated well from the device 40.

In addition, devices that generate heat, such as power modules, generally have the property that their characteristics deteriorate as their temperature rises. Therefore, as shown in FIG. 2, when the radio wave absorber 10 is arranged with the thermally conductive layer 14 facing the device 40, it is desirable that the heat generated in the device 40 can be immediately removed by the thermally conductive layer 14. It is also possible to provide other layers, such as an insulating layer, between the thermally conductive layer 14 and the device 40, but in order to improve the heat dissipation performance from the device 40, it is desirable that the thermally conductive layer 14 and the device 40 are in contact with each other. It is also possible to provide other layers, such as an insulating layer, between the radio wave absorbing layer 12 and the outer shell 22 of the housing 20, but it is desirable that the configuration related to thermal resistance is simple, and the contact between the radio wave absorbing layer 12 and the outer shell 22 allows the heat generated in the device 40 to be quickly transferred to the cooling structure 30.

Furthermore, when the radio wave absorber 10 is arranged so that the thermally conductive layer 14 faces the device 40, the thermally conductive layer 14 can function as an electrical resistance layer for the radio wave absorbing layer 12, so that the radio wave absorbing layer 12 can be protected even when the voltage used by the device 40 is relatively high. Furthermore, by providing the thermally conductive layer 14 in addition to the radio wave absorbing layer 12, the insulating properties of the radio wave absorber 10 can be improved. In particular, in this embodiment, ceramic particles are used as a filler added to the thermally conductive layer 14 to increase the thermal conductivity of the entire radio wave absorber 10, so that it is easier to increase the electrical resistance of the entire radio wave absorber 10 compared to when metal powder such as copper or silver is used as a filler to increase the thermal conductivity. In particular, when the flow path piping 32 constituting the water-cooled jacket or the like is made of aluminum or the like having low electrical resistance, it is important to ensure insulation between the device 40 and the cooling structure 30 in order to prevent the device 40 inside the housing 20 from being electrically connected to the outside via the cooling structure 30.

In addition, since the radio wave absorber 10 of this embodiment includes a silicone resin as a base polymer for dispersing the filler in the radio wave absorbing layer 12 and the thermally conductive layer 14, the heat resistance and durability of the radio wave absorber 10 can be improved, and the radio wave absorber 10 can be used without any problems even when the temperature rises to, for example, about 100°C due to heat generation from the device 40. Furthermore, since the radio wave absorber 10 includes a silicone resin as the base polymer, the flexibility of the radio wave absorber 10 is increased, and as described above, the Young's modulus of the radio wave absorber 10 and the Young's modulus after heat treatment are reduced, and the elongation rate of the radio wave absorber 10 is easily ensured. In this way, by increasing the flexibility of the radio wave absorber 10 and ensuring the elongation rate (deformation amount), for example, when the device 50 is used as the power train of the electric vehicle described above, problems such as damage to the radio wave absorber 10 and noise generation caused by vibration during driving can be suppressed.

B. Second embodiment:
In order to increase the thermal conductivity in the thermally conductive layer 14 of the wave absorber 10, it is preferable that the ceramic particles contained as a filler in the thermally conductive layer 14 are a mixture of particles having a relatively large particle diameter and particles having a relatively small particle diameter. Hereinafter, as a second embodiment, the relationship between the preferred particle diameters of particles A and particles B in the thermally conductive layer 14 including ceramic particles in which particles A having a relatively large particle diameter and particles B having a relatively small particle diameter are mixed will be described. Note that the wave absorber 10 of the second embodiment has a common configuration with the wave absorber 10 of the first embodiment, except for the above-mentioned configuration related to the ceramic particles.

The thermally conductive layer 14 of the radio wave absorber 10 of the second embodiment includes, as ceramic particles, particles A having a particle diameter of 7 μm or more and particles B having a particle diameter of less than 7 μm. When the average particle diameter of particles A is r1 and the average particle diameter of particles B is r2, the value of "r2/r1" is set to 0.10 or more and 0.45 or less. In the second embodiment, the ceramic particles satisfy the above-mentioned numerical range for the particle diameter, thereby ensuring a state in which the ceramic particles are dense in the thermally conductive layer 14, and improving the thermal conductivity of the thermally conductive layer 14. The above-mentioned numerical range for defining that the ceramic particles are dense in the thermally conductive layer 14 can be understood based on the so-called percolation theory. The percolation theory is known as a theory regarding how, in a specific system, a target substance is connected by forming clusters in the system, and as a result, how the properties of the above system are caused by the properties of the above substance. In the following explanation, the state in which the ceramic particles are most dense is understood using a model that simplifies the distribution of particle diameters.

Figure 3 is an explanatory diagram showing an example of a model showing the case where ceramic particles are most dense when ceramic particles of different particle sizes are mixed. Figure 3 shows how particle B, particle R5, is placed in the space formed between four particles A, particles R1 to R4. In Figure 3, the model is simplified by assuming each particle is a perfect sphere. Also, in Figure 3, particles R4 and R5, which are hidden by particles R1 to R3, are shown by dashed lines.

Figure 4 is an explanatory diagram showing the results of calculating the particle diameter of particle R5 to realize the densest state when the particle diameters of particles R1 to R4 are set variously in the model shown in Figure 3, and determining "r2/r1". Here, the particle diameter of particle R1 is assumed to be the same as the average particle diameter r1 of particle A, and the particle diameters of particles R2 to R4 are changed to calculate the particle diameter of particle R5 to realize the densest state. Specifically, the maximum diameter of a true sphere that can exist in the space created by particles R1 to R4 is calculated as the particle diameter of particle R5. Since the above-mentioned average particle diameter ratio "r2/r1" is the subject of consideration, the particle diameter of particle R1 (average particle diameter r1) is set to 1 in the models shown in Figures 3 and 4. In addition, the particle diameters of particles R2 to R4 are assumed to be the same, and "r2/r1" is calculated assuming that the particle diameter of particle R5 to realize the densest state corresponds to the average particle diameter r2 of particle B.

As shown in FIG. 4, the particle diameter of particle R5 was calculated for the cases where the particle diameters of particles R2 to R4 are all 1, the same as the particle diameter of particle R1 (average particle diameter r1) (Case 1), where they are all 0.5 (Case 2), and where they are all 2 (Case 3), and the value of "r2/r1" was obtained. Case 1 is a model that shows how particles having a particle diameter close to the average particle diameter r1 of particle A gather together, Case 2 is a model that shows how relatively small particles gather around a particle having a particle diameter close to the average particle diameter r1 to form a space in which particle B is arranged, and Case 3 is a model that shows how relatively large particles gather around a particle having a particle diameter close to the average particle diameter r1 to form a space in which particle B is arranged. As shown in FIG. 4, even if the variation in the particle diameter of particle A relative to the average particle diameter r1 is relatively large, it can be understood that the degree to which the ceramic particles are densely arranged can be increased by setting the value of "r2/r1" to 0.10 or more and 0.45 or less. If the particle size variation of the ceramic particles is too large, it may be difficult to arrange the ceramic particles densely, so the value of "r2/r1" is preferably 0.10 or more, more preferably 0.15 or more, and even more preferably 0.2 or more. In addition, the value of "r2/r1" is preferably 0.45 or less, more preferably 0.35 or less, and even more preferably 0.25 or less.

The particle size of the ceramic particles, which include particles A with a relatively large particle size and particles B with a relatively small particle size, can be set in various ways. For example, the range of particle sizes that can be set as the particle size of the larger particle A and the desired particle size of particle B according to the particle size of such particle A can be set according to the thickness of the thermally conductive layer 14 to be provided. Here, as an example, based on the expected thickness of the thermally conductive layer 14, when the particle size of particle A is set to about 15 to 30 μm, the particle size value that is the boundary between particles A and B is set to 7 μm so that particles having a particle size desirable for particle B are classified as particles B.

With this configuration, the degree to which the ceramic particles are densely arranged in the thermally conductive layer 14 can be increased, and the thermal conductivity of the thermally conductive layer 14 and the entire wave absorber 10 can be further increased. In order to obtain the effect of increasing the thermal conductivity of the thermally conductive layer 14 and the entire wave absorber 10 by setting the value of "r2/r1" within the above range, the content ratio of the filler in the thermally conductive layer 14 is preferably 35 wt% or less, and more preferably 10 wt% or less, when the total of particles A and particles B is 100 wt%. In addition, since the thermal conductivity of the thermally conductive layer 14 can be improved as the content ratio of the ceramic particles is increased, the content ratio of the ceramic particles in the thermally conductive layer 14 is preferably 40 vol% or more, more preferably 50 vol% or more, and even more preferably 60 vol% or more. However, from the viewpoint of ensuring the flexibility of the thermally conductive layer 14, the content ratio of the ceramic particles in the thermally conductive layer 14 is preferably 80 vol% or less, and more preferably 75 vol% or less.

C. Other embodiments:
In the above-described embodiments, the radio wave absorber is formed in a sheet shape, but may be formed in a different shape, however, forming the radio wave absorber in a sheet shape makes it easier to arrange the radio wave absorber along the surface of an adherend of any shape.

The present disclosure is not limited to the above-mentioned embodiments, and can be realized in various configurations without departing from the spirit of the present disclosure. For example, the technical features in the embodiments corresponding to the technical features in each form described in the Summary of the Invention column can be replaced or combined as appropriate to solve some or all of the above-mentioned problems or to achieve some or all of the above-mentioned effects. Furthermore, if a technical feature is not described as essential in this specification, it can be deleted as appropriate.

Figure 5 is an explanatory diagram showing the specific configurations (constituent materials, additive amounts, thickness, etc.) of 12 types of radio wave absorbers from sample S1 to sample S12. Figure 6 is an explanatory diagram showing the measured values related to the performance of sample S1 to sample S12. Figure 7 is an explanatory diagram showing the evaluation results of the radio wave absorbers of sample S1 to sample S12. The configuration and manufacturing method of each sample, and the results of evaluating the performance are described below. Note that sample S10 does not have a "radio wave absorbing layer 12 comprising a soft magnetic material and a silicone resin" because the radio wave absorbing layer does not contain a filler. Also, sample S11 does not have a "thermal conductive layer 14 comprising high thermal conductive ceramic particles and a silicone resin" because the thermal conductive layer does not contain a filler. Also, sample S12 does not satisfy the requirement that "the thermal conductive layer 14 has a higher thermal conductivity than the radio wave absorbing layer 12". Therefore, these samples are comparative examples.

<Preparation of each sample>
Each sample was produced by forming layers corresponding to the electromagnetic wave absorbing layer and the thermally conductive layer using the materials shown in FIG. 5 and having the thicknesses shown in FIG. 5, and then integrating these two layers.

The electromagnetic wave absorbing layer was prepared as follows. The filler for the electromagnetic wave absorbing layer shown in FIG. 5 was added to a silicone resin (having the molecular weight shown in FIG. 5) having a vinyl group as a functional group, and mixed using a stirrer with a stirring blade and a three-roll mill. A silane coupling agent, a crosslinking agent, and a metal catalyst were added thereto, and thoroughly mixed to obtain a paste. The obtained paste was formed into a sheet by the doctor blade method to the thickness shown in FIG. 5, and thermally cured (semi-cured) at a temperature of 100°C or less to obtain an electromagnetic wave absorbing layer.

The thermally conductive layer was prepared as follows. The filler for the thermally conductive layer shown in FIG. 5 was added to a silicone resin (having the molecular weight shown in FIG. 5) having a vinyl group as a functional group, and mixed using a stirrer with a stirring blade and a three-roll mill. A silane coupling agent, a crosslinking agent, and a metal catalyst were added and thoroughly mixed to obtain a paste. The resulting paste was formed into a sheet by the doctor blade method to the thickness shown in FIG. 5, and thermally cured (semi-cured) at a temperature of 100°C or less to obtain a thermally conductive layer.

The radio wave absorbing layer and the electrical resistance layer prepared as described above were laminated by vacuum pressing or by pressing with a roller, and further heat curing was performed at 100°C or higher to obtain the radio wave absorber for each sample.

<Measurement of thermal conductivity>
The thermal conductivity of each sample as a whole as a radio wave absorber and the thermal conductivity of each of the radio wave absorbing layer and the thermal conductive layer constituting each sample were measured using a thermal conductivity measuring device TRIDENT (manufactured by Rigaku Corporation). Specifically, the thermal conductivity of the radio wave absorber was measured by obtaining a specimen cut into a 100 mm square from each sample prepared as described above, and arranging the thermal conductive layer on the probe surface side. In addition, the thermal conductivity of the radio wave absorbing layer and the thermal conductive layer was measured in the same manner as the thermal conductivity of the radio wave absorber, by preparing each of the radio wave absorbing layer and the thermal conductive layer independently.

<Derivation of complex relative permeability>
The real component μ' of the complex relative permeability was measured using an E4991B impedance analyzer (manufactured by Keysight Technologies). During the measurement, each sample was cut out to have a toroidal shape with an outer diameter of 20 mm and an inner diameter of 10 mm. The real component of the complex relative permeability was read from a region where the value of μ' was constant. In general, permeability tends to be constant until a specific frequency (resonance frequency) is reached, and to decrease as the frequency increases beyond that. Since the frequency at which the permeability starts to decrease for all of the samples to be measured is 100 MHz or higher, the real component μ' of the complex relative permeability in the range from 1 MHz to 100 MHz can be obtained by reading the value of the region where the value of μ' is constant as described above.

<Measurement of Young's modulus and elongation rate>
The Young's modulus and elongation were measured at room temperature using a tensile tester (Shimadzu Autograph (AG-IS)). Each sample was cut into a rectangular test piece with a width of 1 cm and a length of 7 cm, and a tensile test was performed at the middle part with a length of 3 cm while holding the test piece at a position 2 cm from both ends with a jig, and the Young's modulus and elongation were obtained from the S-S curve (stress-strain curve). Specifically, the change in sample length and load was measured over time while pulling each test piece until it broke. At this time, the tensile stress was calculated by dividing the measured load by the cross-sectional area of the test piece before the tensile test. The Young's modulus was calculated as the slope in the elastic deformation region in a graph (stress-strain curve) with the strain calculated by the following formula (1) as the horizontal axis and the tensile stress as the vertical axis. The elongation was calculated by pulling each test piece until it broke, subtracting the original sample length (the middle length of 3 cm in the above sample) from the sample length at the time of breakage, and then dividing by the original sample length. The formula for calculating the elongation percentage is shown below as formula (2).

Strain (%) = [sample length during tension − original sample length] / original sample length (1)
Elongation (%) = [sample length at break − original sample length] / original sample length (2)

<Heat resistance test>
Each sample was placed in a dryer at 100° C. in the atmosphere and subjected to a continuous heat resistance test for 1000 hours. After the heat resistance test, the Young's modulus of each sample was measured by the same method as in the "Method for measuring Young's modulus" described above.

<Average particle size>
In measuring the particle size distribution of the ceramic particles as the filler contained in the thermally conductive layer, the thermally conductive layer was cut out from each sample as the radio wave absorber using a cutter. The obtained thermally conductive layer was impregnated with a silicone solvent to dissolve the silicone resin. After separating the solution from the filler, the filler was washed several times using a silicone solvent to obtain a filler for measurement. The particle size distribution of the filler for measurement (ceramic particles) was analyzed using a particle size distribution measuring device Partica LA-960V2 (manufactured by Horiba, Ltd.). From the graph of the obtained volumetric particle size distribution, the distribution was recalculated for particles A with a particle size of 7 μm or more and particles B with a particle size of less than 7 μm, and the volumetric particle size distribution value of the particles whose particle sizes belong to each range was obtained. Then, the average particle size (volume average diameter MV) was obtained from the obtained volumetric particle size distribution value. That is, the average particle size r1 of particles A and the average particle size r2 of particles B were calculated.

<Evaluation method>
As shown in FIG. 7, each item was evaluated based on the evaluation criteria. The "relationship of thermal conductivity between layers" was marked as "○" when the thermal conductivity of the thermally conductive layer was higher than that of the electromagnetic wave absorbing layer in each sample, and marked as "×" when the thermal conductivity of the thermally conductive layer was equal to or lower than that of the electromagnetic wave absorbing layer. The "thermal conductivity" was marked as "○" when the thermal conductivity of each sample was 1.0 W/(m·K) or higher, and marked as "×" when it was less than 1.0 W/(m·K). The "μ'" was marked as "○" when the real component μ' of the complex relative permeability in the range of 1 MHz to 100 MHz of each sample was 3 or higher, and marked as "×" when it was less than 3. The "Young's modulus" was marked as "○" when the Young's modulus of each sample was 10 MPa or lower, and marked as "×" when it exceeded 10 MPa. The "Young's modulus after heat resistance" was marked as "○" when the Young's modulus of each sample after the heat resistance test was 11 MPa or lower, and marked as "×" when it exceeded 11 MPa. For the "high thermal conductive ceramic" category, if the thermal conductive layer of each sample contained aluminum nitride (AlN) or boron nitride (BN) with excellent thermal conductivity as a filler, it was marked as "○", and if the thermal conductive layer did not contain the above filler, it was marked as "×". For "r2/r1", if the value of "r2/r1" calculated for each sample was 0.10 or more and 0.45 or less, it was marked as "○", and if it was less than 0.1 or more than 0.45, it was marked as "×". For "elongation", if the elongation of each sample was 30% or more, it was marked as "○", and if it was less than 30%, it was marked as "×".

Figure 7 further shows the results of the overall evaluation. The "overall evaluation" was marked as "X" when at least one of the evaluation items corresponding to the basic performance related to the effect of the radio wave absorber of the present disclosure, i.e., "relationship of thermal conductivity between layers", "thermal conductivity" and "μ'", was marked as "X". In addition, when the overall evaluation was other than "X", and there were no remaining evaluation items marked as "X", the evaluation was marked as "◎", when there was one evaluation item marked as "X", the evaluation was marked as "○", and when there were two or more evaluation items marked as "X", the evaluation was marked as "△".

<Evaluation Results>
As shown in FIG. 7, it was confirmed that the thermal conductivity can be increased while ensuring the electromagnetic wave absorbing ability for electromagnetic waves in the range of 1 MHz to 100 MHz by providing "an electromagnetic wave absorbing layer including a soft magnetic material and a silicone resin" and "a thermally conductive layer disposed on one surface of the electromagnetic wave absorbing layer, including ceramic particles and a silicone resin, and having a thermal conductivity higher than that of the electromagnetic wave absorbing layer" (comparison between samples S1 to S9 and samples S10 to S12). In addition, it was confirmed that the Young's modulus and the Young's modulus after the heat resistance test are reduced by using a resin having a larger molecular weight as the silicone resin constituting the electromagnetic wave absorbing layer and the thermally conductive layer, and thus the flexibility and heat resistance of the electromagnetic wave absorber are increased (for example, comparison between samples S1 to S3 and S4 to S9). In addition, it was confirmed that the ceramic particles in the thermally conductive layer are easily densely packed in the silicone resin by satisfying the criteria related to "r2/r1", and therefore the thermal conductivity of the electromagnetic wave absorber is increased (for example, comparison between samples S4 and S5, and comparison between samples S6, S7 and S8, S9). It was also confirmed that the thermal conductivity of the radio wave absorber was increased by using AlN or BN, which has excellent thermal conductivity, as the filler for the thermally conductive layer (for example, comparison between samples S1 to S5 and S6 to S9).

The present disclosure can also be realized in the following forms.
[Application Example 1]
A radio wave absorber that absorbs electromagnetic waves,
an electromagnetic wave absorbing layer including a soft magnetic material and a silicone resin;
a thermally conductive layer disposed on one surface of the electromagnetic wave absorbing layer, the thermally conductive layer comprising ceramic particles and a silicone resin and having a thermal conductivity higher than that of the electromagnetic wave absorbing layer;
A radio wave absorber comprising:
[Application Example 2]
The radio wave absorber according to Application Example 1,
A radio wave absorber having a thermal conductivity of 1.0 W/(m·K) or more.
[Application Example 3]
The radio wave absorber according to Application Example 1 or 2,
A radio wave absorber, characterized in that a real component of the complex relative permeability is 3 or more in the range of 1 MHz to 100 MHz.
[Application Example 4]
The radio wave absorber according to any one of Application Examples 1 to 3,
A radio wave absorber having a Young's modulus of 10 MPa or less.
[Application Example 5]
The radio wave absorber according to any one of Application Examples 1 to 4,
A radio wave absorber having a Young's modulus of 11 MPa or less after treatment in air at 100° C. for 1000 hours.
[Application Example 6]
The radio wave absorber according to any one of Application Examples 1 to 5,
The electromagnetic wave absorber, wherein the ceramic particles contain at least one of aluminum nitride and boron nitride.
[Application Example 7]
The radio wave absorber according to any one of Application Examples 1 to 6,
a value of r2/r1 being 0.10 or more and 0.45 or less, wherein r1 is an average particle diameter of particles having a particle diameter of 7 μm or more and r2 is an average particle diameter of particles having a particle diameter of less than 7 μm, in the ceramic particles.
[Application Example 8]
The radio wave absorber according to any one of Application Examples 1 to 7,
A radio wave absorber having an elongation rate of 30% or more at room temperature.
[Application Example 9]
A housing for housing an electronic component module,
A radio wave absorber according to any one of Application Examples 1 to 8,
a mounting portion is provided for mounting a cooling structure for cooling the electronic component module to the housing from an outside of the housing,
The radio wave absorber is attached to the inner wall surface of the housing at a position overlapping with the mounting portion, with the radio wave absorbing layer facing the inner wall surface of the housing and the thermal conductive layer facing the electronic component module.
[Application Example 10]
1. A power conversion device comprising: a power module having a semiconductor; a housing that houses the power module; and a cooling structure for cooling the electronic component module, the cooling structure being attached to the housing from an outside of the housing at an attachment portion provided on the housing,
The housing includes the radio wave absorber according to any one of Application Examples 1 to 8.
the radio wave absorber is attached to the inner wall surface of the housing at a position overlapping with the mounting portion, with the radio wave absorbing layer facing the inner wall surface of the housing and the thermal conduction layer facing the electronic component module.

Reference Signs List 10: radio wave absorber 12: radio wave absorbing layer 13: adhesive surface 14: thermally conductive layer 20: housing 22: outer shell portion 24: mounting portion 30: cooling structure 32: flow path piping 34: refrigerant flow path 40: equipment 50: device

Claims (10)

A radio wave absorber that absorbs electromagnetic waves,
an electromagnetic wave absorbing layer including a soft magnetic material and a silicone resin;
a thermally conductive layer disposed on one surface of the electromagnetic wave absorbing layer, the thermally conductive layer comprising ceramic particles and a silicone resin and having a thermal conductivity higher than that of the electromagnetic wave absorbing layer;
A radio wave absorber comprising: 2. The radio wave absorber according to claim 1,
A radio wave absorber having a thermal conductivity of 1.0 W/(m·K) or more. 2. The radio wave absorber according to claim 1,
A radio wave absorber, characterized in that a real component of the complex relative permeability is 3 or more in the range of 1 MHz to 100 MHz. 2. The radio wave absorber according to claim 1,
A radio wave absorber having a Young's modulus of 10 MPa or less. 2. The radio wave absorber according to claim 1,
A radio wave absorber having a Young's modulus of 11 MPa or less after treatment in air at 100° C. for 1000 hours. 2. The radio wave absorber according to claim 1,
The electromagnetic wave absorber, wherein the ceramic particles contain at least one of aluminum nitride and boron nitride. 2. The radio wave absorber according to claim 1,
a value of r2/r1 being 0.10 or more and 0.45 or less, wherein r1 is an average particle diameter of particles having a particle diameter of 7 μm or more and r2 is an average particle diameter of particles having a particle diameter of less than 7 μm, in the ceramic particles. 2. The radio wave absorber according to claim 1,
A radio wave absorber having an elongation rate of 30% or more at room temperature. A housing for housing an electronic component module,
A radio wave absorber according to any one of claims 1 to 8,
a mounting portion is provided for mounting a cooling structure for cooling the electronic component module to the housing from an outside of the housing,
The radio wave absorber is attached to the inner wall surface of the housing at a position overlapping with the mounting portion, with the radio wave absorbing layer facing the inner wall surface of the housing and the thermal conductive layer facing the electronic component module. 1. A power conversion device comprising: a power module having a semiconductor; a housing that houses the power module; and a cooling structure for cooling the electronic component module, the cooling structure being attached to the housing from an outside of the housing at an attachment portion provided on the housing,
The housing includes the radio wave absorber according to any one of claims 1 to 8,
the radio wave absorber is attached to the inner wall surface of the housing at a position overlapping with the mounting portion, with the radio wave absorbing layer facing the inner wall surface of the housing and the thermal conduction layer facing the electronic component module.

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