JP-WO2025028488-A5 -

Dates

Publication Date

20260507

Application Date

20240729

Description

This disclosure relates to an electromagnetic wave absorber, and more particularly to an electromagnetic wave absorber that suppresses fluctuations in the frequency of absorbed electromagnetic waves due to temperature changes, and that can effectively absorb electromagnetic waves of a desired frequency even in environments with temperature fluctuations. To avoid the effects of leaked electromagnetic waves emitted from electrical circuits and other sources, as well as unwanted reflected electromagnetic waves, electromagnetic wave-absorbing compositions are used. These compositions are molded into predetermined shapes as electromagnetic wave-absorbing materials, such as block-shaped electromagnetic wave absorbers and sheet-shaped electromagnetic wave-absorbing sheets. They are also known to be used as electromagnetic wave-absorbing coatings applied to desired locations. In recent years, research has progressed on technologies that utilize electromagnetic waves in the centimeter-wave to millimeter-wave bands, and even higher frequency bands beyond the millimeter-wave band, for mobile communications such as mobile phones, wireless LANs, and automated toll collection systems (ETC). In response to this technological trend of utilizing higher frequency electromagnetic waves, there is a growing demand for electromagnetic wave absorbers that can absorb electromagnetic waves in the millimeter-wave band (approximately 30 GHz to 300 GHz) or higher. As an electromagnetic wave absorber (sheet) that absorbs electromagnetic waves in high frequency bands above the millimeter wave range, a material containing epsilon iron oxide and hexagonal ferrite in a resin binder has been proposed to suppress changes in the frequency of absorbed electromagnetic waves when the ambient temperature changes (see Patent Document 1). Japanese Patent Publication No. 2019-145534 This is a schematic diagram illustrating the general configuration of the electromagnetic wave absorbing sheet according to this embodiment.This figure shows the relationship between the frequency of electromagnetic waves from the electromagnetic wave absorbing sheet of Example 2 and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber.This figure shows the relationship between the frequency of electromagnetic waves and the transmission attenuation per unit thickness of the electromagnetic wave absorbing sheet of Example 2.This figure shows the relationship between the frequency of electromagnetic waves in the electromagnetic wave absorbing sheet of Comparative Example 1 and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber.This figure shows the relationship between the frequency of electromagnetic waves and the transmission attenuation per unit thickness of the electromagnetic wave absorbing sheet in Comparative Example 1.This figure shows the relationship between the change in the imaginary part of the complex relative permeability and the increase in thickness required to ensure the necessary amount of electromagnetic wave absorption at 105°C. The electromagnetic wave absorber disclosed herein is an electromagnetic wave absorber containing magnetoplanbite-type hexagonal ferrite that magnetically resonates in the millimeter-wave frequency band within a binder, wherein a portion of the metal sites in the magnetoplanbite-type hexagonal ferrite are replaced with La, and in a graph showing the relationship between the frequency of the incident electromagnetic wave and the imaginary part of the complex relative permeability of the electromagnetic wave absorber, when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, the value of the frequency at which it reaches a positive peak shows a positive rate of change, and the ratio of the change in the frequency at which it reaches a positive peak when the temperature changes from 25°C to 105°C to the value of the frequency at which it reaches a positive peak at 25°C is 1.5% or less. Here, "the ratio of the change in the frequency of the positive peak when the temperature changes from 25°C to 105°C to the value of the frequency of the positive peak at 25°C" is given by the following equation (1), where f 25 is the frequency of the positive peak at 25°C, μ'' 25 is the value of the imaginary part of the complex relative permeability at that time, and f 105 is the frequency of the positive peak at 105°C, μ'' 105 is the value of the imaginary part of the complex relative permeability at that time. (f 105 - f 25 )/f 25 × 100 (1) It is a numerical value that can be expressed as follows. Furthermore, the above-mentioned metal site contains at least one of the following: barium (Ba), strontium (Sr), and calcium (Ca). In this manner, the electromagnetic wave absorber disclosed in this application exhibits minimal change in electromagnetic wave absorption characteristics due to changes in ambient temperat