JP2010260766A - Magnetoplumbite-type hexagonal ferrite and radiowave absorber using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To develop and provide a magnetoplumbite-type hexagonal ferrite which exhibits a property in which, when it is used in a radiowave absorber, the matching frequency is substantially insensitive to the variation of the thickness of the radiowave absorber. <P>SOLUTION: The hexagonal magnetoplumbite-type ferrite is represented by the compositional formula: AFe<SB>(12-x)</SB>(B1<SB>0.5</SB>B2<SB>0.5</SB>)<SB>x</SB>O<SB>19</SB>, wherein A is Ba and/or Sr; B1 is Ti and/or Zr; and B2 is a divalent metallic element, provided that B2 includes two or more selected from among Co, Mn, Cu, Mg, Zn, and Ni. One at least containing Zn, particularly Co and Zn or Mn and Zn as the B2, is preferably used. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a magnetoplumbite type hexagonal ferrite suitable for a radio wave absorber for the GHz band, and a radio wave absorber using the same.

 In recent years, with the advancement of information communication technology, radio waves in the GHz band have been used for various purposes. For example, a mobile phone, wireless LAN, sanitary broadcasting, intelligent road traffic system, non-stop automatic toll collection system (ETC), automobile driving support system (AHS), and the like can be mentioned. Thus, when radio wave usage forms in a high frequency range are diversified, there is a concern about failure, malfunction, malfunction or the like due to interference between electronic components, and countermeasures are important. As one of them, a method of absorbing unnecessary radio waves using a radio wave absorber and preventing reflection and intrusion of radio waves is effective. Recently, the demand for radio wave absorbers for the GHz band is increasing.

Conventionally, oxide-based magnetic materials such as ferrite are often used for radio wave absorbers for high frequency bands. Among ferrites, spinel type is mainly used in the MHz band, but magnetoplumbite type hexagonal ferrite is promising as a material that exhibits excellent characteristics in a high frequency band of GHz or higher. For example, Fe ³⁺ A magnetoplumbite-type hexagonal ferrite, in which a part thereof is substituted with a tetravalent cation and a divalent cation, is known (Patent Document 1, Non-Patent Document 1).

JP 11-354972 A Journal of Japan Society of Applied Magnetics, 22, 297-300 (1998)

A radio wave absorber using ferrite is an “impedance matching type”, and when a material constant is determined, a matching frequency and a matching thickness are determined. The inventors have investigated various radio wave absorption characteristics of a radio wave absorber using a magnetoplumbite type hexagonal ferrite in which a part of Fe ³⁺ is substituted. As a result, in the case of this type of wave absorber, it became clear that the change of the matching frequency with the thickness is large. That is, when the sheet thickness varies, the matching frequency changes greatly, and a problem that radio waves in the target frequency region cannot be accurately absorbed due to a subtle difference in thickness tends to occur. That is, in order to obtain stable quality in industrial production, high dimensional accuracy is required for the thickness, which contributes to an increase in production cost.

In view of such problems, the present invention intends to develop and provide a magnetoplumbite type hexagonal ferrite that exhibits the property that the matching frequency hardly changes even when the thickness of the absorber is changed. Is.

The object is represented by the composition formula AFe _(12-X) (B1 _0.5 B2 _0.5 ) _X O ₁₉ , wherein A is one or two of Ba and Sr, and B1 is one of Ti and Zr. Two types, B2, are divalent metal elements, and are achieved by a magnetoplumbite type hexagonal ferrite containing two or more of Co, Mn, Cu, Mg, Zn and Ni. B2 contains at least Zn, and particularly those containing Co and Zn or Mn and Zn are suitable targets.

Specifically, what is shown to the following compositional formulas is mentioned, for example.
It is represented by a composition formula AFe _(12-X) (B1 _0.5 (Co _(1-y) Zn _y ) _0.5 ) _X O ₁₉ , wherein A is one or two of Ba and Sr, B1 is Ti, A magnetoplumbite type hexagonal ferrite, which is one or two of Zr, x is 0.1 to 2.0, and y is 0.2 to 0.8. Moreover, it is represented by a composition formula AFe _(12-X) (B1 _0.5 (Mn _(1-y) Zn _y ) _0.5 ) _X O ₁₉ , A is one or two of Ba and Sr, and B1 is It is a magnetoplumbite type hexagonal ferrite, which is one or two of Ti and Zr, x is 0.2 to 4.0, and y is 0.5 ± 0.1.

The present invention also provides a radio wave absorber using the above-described magnetoplumbite type hexagonal ferrite powder.

According to the present invention, it is possible to provide a magnetoplumbite type hexagonal ferrite that suppresses a change in matching frequency due to a variation in the thickness of a radio wave absorber. Using this improved magnetoplumbite-type hexagonal ferrite, stable radio wave absorption performance in a specified frequency range can be stabilized without requiring strict dimensional accuracy as in the conventional manufacturing of radio wave absorbers. Can be obtained. In addition, it becomes easy to match the matching frequency to a desired frequency region even in a paint or injection molded product whose thickness is likely to fluctuate. Therefore, the present invention contributes to the spread of radio wave absorbers using magnetoplumbite type hexagonal ferrite by reducing the industrial production cost, and thus contributes to prevention of radio wave interference in the GHz band.

In the present invention, among the magnetoplumbite-type hexagonal ferrites in which a part of Fe ³⁺ is substituted with other elements, those represented by the following composition formula are targeted.
AFe _(12-X) (B1 _0.5 B2 _0.5 ) _X O ₁₉
Here, A is one or two of Ba and Sr, B1 is one or two of Ti and Zr, and B2 is a divalent metal element.
However, in the present invention, two or more of Co, Mn, Cu, Mg, Zn, and Ni are contained as the B2 element in the composition formula. At this time, the change of the matching frequency due to the thickness variation of the radio wave absorber can be remarkably reduced. The reason for this is not clear at this point, but by adding multiple elements in combination, the crystal strain is reduced and the crystal structure is stabilized, so that frequency fluctuations caused by the strain can be suppressed to a smaller level. Conceivable.

As an example of its practical composition, for example, a composition formula BaFe _(12-X) (Ti _0.5 (Co _(1-y) Zn _y ) _0.5 ) _X O _{19 in} which Co and Zn are added together as B2 element. The thing represented by is mentioned. As a result of the examination, as for the molar ratio of Co and Zn, an effect that the change in the matching frequency due to the thickness is remarkably reduced is obtained at least in the range where y in the above formula is 0.2 to 0.8. y is preferably in the range of 0.3 to 0.8, and a particularly large effect is obtained in the range of 0.4 to 0.8. The absorption frequency region can be controlled by the substitution amount x. For example, x can be in the range of 0.1 to 2.0.

To illustrate another practical composition, for example, a composition formula BaFe _(12-X) (Ti _0.5 (Mn _(1-y) Zn _y ) _0.5 ) _X O in which Mn and Zn are added in combination as B2 elements. What is represented by ₁₉ is mentioned. As a result of the study, at least when the molar ratio of Mn to Zn is around 1: 1 (y in the above formula is 0.5 ± 0.1), the effect of significantly reducing the change in the matching frequency due to the thickness is obtained. In this case, x can be in the range of 0.2 to 4.0, for example.

The magnetoplumbite type hexagonal ferrite of the present invention can be manufactured according to a general soft ferrite manufacturing method. For example, oxide and carbonate powders are used as raw materials, and these are weighed so as to have a predetermined ferrite composition, mixed, granulated, and then fired to produce a magnetoplumbite type hexagonal ferrite of a predetermined composition. Is obtained. This may be pulverized into powder.

The obtained magnetoplumbite type hexagonal ferrite powder is kneaded with a polymer base material to obtain a radio wave absorber material (kneaded product). The blending amount of magnetoplumbite type hexagonal ferrite powder in the kneaded product is preferably 60% by mass or more. However, if it exceeds 95% by mass, kneading with the polymer substrate becomes difficult. The mixing ratio of the magnetoplumbite type hexagonal ferrite powder is more preferably 80 to 95% by mass, and still more preferably 85 to 95% by mass.

As the polymer substrate, various materials satisfying heat resistance, flame retardancy, durability, mechanical strength, and electrical characteristics can be used depending on the use environment. For example, an appropriate material may be selected from resin (nylon or the like), gel (silicone gel or the like), thermoplastic elastomer, rubber or the like. Two or more kinds of polymer compounds may be blended to form a substrate.

In order to improve the compatibility and dispersibility with the polymer substrate, the magnetoplumbite type hexagonal ferrite powder can be subjected to surface treatment with a silane coupling agent, a titanate coupling agent or the like in advance. In addition, various additives such as a plasticizer, a reinforcing agent, a heat resistance improver, a heat conductive filler, and an adhesive can be added when mixing the magnetoplumbite type hexagonal ferrite powder and the polymer compound.

A radio wave absorber can be obtained by forming the above radio wave absorber material (kneaded material) into a predetermined sheet thickness by rolling. Further, when the magnetoplumbite type hexagonal ferrite of the present invention is used, the allowable amount of thickness dimensional accuracy of the radio wave absorber is relaxed, and therefore, injection molding can be performed instead of rolling. The electromagnetic wave absorber as a coating film can also be formed by dispersing the magnetoplumbite type hexagonal ferrite powder directly in the paint and applying it to the surface of the substrate.

The present invention will be specifically described below.
First, a method and apparatus for measuring physical properties of a sample used in the examples will be described.

<Specific surface area>
The specific surface area (SSA) of the ferrite powder was measured using a monosorb (Model Number: MS-17) manufactured by Yuasa Ionics Co., Ltd. based on the BET method.

<Compression density>
The ferrite powder was compressed at a pressure of 1 ton / cm ³ after filling 10 g of ferrite powder into a cylindrical mold having an inner diameter of 2.54 cmφ. The density of the ferrite powder at this time was measured as a compression density.

<Particle size distribution>
The particle size distribution of the ferrite powder is determined by using a laser diffraction particle size distribution measuring device (HELOS & RODOS, manufactured by Nippon Laser Co., Ltd.) under the conditions of focal length = 20 mm, dispersion pressure 5.0 bar, suction pressure 130 mbar. The diameter was measured.
In the particle size distribution, D16, D50, and D84 are cumulative particle size distributions at a volume of 16%, 50%, and 84%, respectively, -0.3μ, −0.52μ, −1μ, + 5μ, + 8.6μ is the ratio of particles having a particle size of 0.3 μm under, particle size of 0.52 μm under, particle size of 1 μm under, particle size of 5 μm upper and particle size of 8 μm upper, respectively.

<Magnetic properties>
The magnetic properties of the ferrite powder were measured for σs (emu / g), σr (emu / g), Hc (Oe), and SQ using VSM (manufactured by Toei Industry Co., Ltd., VSM-P7).

<X-ray measurement>
The measurement conditions of the X-ray diffraction of the ferrite powder are as follows: tube: cobalt tube, Goniometer: Ultimate + horizontal goniometer type I, Attachment: ASC-43 (vertical type), Monochromator: fully automatic monochromator, Scanning Mode: 2θ / θ, ScanningType: CONTINUOUS, X-Ray: 40 kV / 30 mA, diverging slit: 1/2 deg. , Scattering slit: 1/2 deg. , Receiving slit: 0.15 mm, measurement range: 30 ° to 70 °.

[Example 1]
Α-Fe ₂ O ₃ , BaCO ₃ , TiO ₂ , Co ₃ O ₄ and ZnO were used as raw material powders, and these were weighed at a quantitative ratio corresponding to the following composition.
Composition: BaFe _(12-X) (Ti _0.5 (Co _0.5 Zn _0.5 ) _0.5 ) _X O ₁₉ , x = 0.4
The raw material powder after weighing was mixed with a high speed mixer, and further mixed and strengthened by a dry method using a vibration mill. The obtained mixed powder was granulated and formed into pellets, and the formed body was placed in a roller hearth type electric furnace and fired by holding at 1250 ° C. in the atmosphere for 2 hours. The obtained fired product was dispersed with a hammer mill, and further subjected to wet dispersion with an attritor (AT) for 5 minutes for degreasing, followed by dehydration and drying. The dehydrated and dried dried product was crushed with a hammer mill to obtain a magnetoplumbite type hexagonal ferrite powder.

  Table 1 shows the measurement results of the composition, specific surface area (SSA), compression density (CD), particle size distribution, peak particle size, and magnetic properties of the obtained magnetoplumbite type hexagonal ferrite powder.

  Next, the powder and the polymer substrate are kneaded so that the content of the pulverized magnetic powder (magnet plumbite-type hexagonal ferrite powder) becomes the ratio shown in Table 2 to absorb radio waves. A body material (kneaded material) was prepared. Synthetic rubber (JSR (Nippon Synthetic Rubber), N215SL) was used as the polymer substrate. The radio wave absorber material was rolled to a predetermined thickness by a rolling roll to obtain a radio wave absorber sheet.

The radio wave absorption characteristics of each radio wave absorber sheet were examined by the free space method. Using a free space microwave measurement system (HVS FreeSpace Microwave Measurement System) manufactured by HVS Technologies, Inc. (HVS Technologies, Inc.), K band (18.0 to 26.5 GHz) and Ka band (26.5 to 40) .0 GHz) radio wave was incident on the sample, and the return loss was measured.

[Example 2]
Α-Fe ₂ O ₃ , SrCO ₃ , TiO ₂ , Co ₃ O ₄ and ZnO were used as raw material powders, and these were weighed at a quantitative ratio corresponding to the following composition.
Composition: SrFe _(12-X) (Ti _0.5 (Co _0.5 Zn _0.5 ) _0.5 ) _X O ₁₉ , x = 0.5
Thereafter, a ferrite powder was obtained through the same process as in Example 1. As a result of X-ray diffraction, it was confirmed that the finely pulverized product was a magnetoplumbite type hexagonal ferrite. Using this magnetoplumbite type hexagonal ferrite powder, a sheet having a predetermined thickness was prepared in the same manner as in Example 1 and subjected to measurement of radio wave absorption characteristics described later.

Example 3
The same raw material powder as in Example 1 was used and weighed at a quantitative ratio corresponding to the following composition.
Composition: BaFe _(12-X) (Ti _0.5 (Co _0.5 Zn _0.5 ) _0.5 ) _X O ₁₉ , x = 1.8
Thereafter, a ferrite powder was obtained through the same process as in Example 1. As a result of X-ray diffraction, it was confirmed that the finely pulverized product was a magnetoplumbite type hexagonal ferrite. Using this magnetoplumbite type hexagonal ferrite powder, a sheet having a predetermined thickness was prepared in the same manner as in Example 1 and subjected to measurement of radio wave absorption characteristics described later.

Example 4
Α-Fe ₂ O ₃ , BaCO ₃ , TiO ₂ , MnO ₂ and ZnO were used as raw material powders, and these were weighed at a quantitative ratio corresponding to the following composition.
Composition: BaFe _(12-X) (Ti _0.5 (Mn _0.5 Zn _0.5 ) _0.5 ) _X O ₁₉ , x = 0.4
Thereafter, a ferrite powder was obtained through the same process as in Example 1. As a result of X-ray diffraction, it was confirmed that the finely pulverized product was a magnetoplumbite type hexagonal ferrite. Using this magnetoplumbite type hexagonal ferrite powder, a sheet having a predetermined thickness was prepared in the same manner as in Example 1 and subjected to measurement of radio wave absorption characteristics described later.

[Comparative Example 1]
Α-Fe ₂ O ₃ , BaCO ₃ , TiO ₂ and Co ₃ O ₄ were used as raw material powders, and these were weighed at a quantitative ratio corresponding to the following composition.
Composition: BaFe _(12-X) (Ti _0.5 Co _0.5 ) _X O ₁₉ , x = 0.4
Thereafter, a ferrite powder was obtained through the same process as in Example 1. As a result of X-ray diffraction, it was confirmed that the finely pulverized product was a magnetoplumbite type hexagonal ferrite. Using this magnetoplumbite type hexagonal ferrite powder, a sheet having a predetermined thickness was prepared in the same manner as in Example 1 and subjected to measurement of radio wave absorption characteristics described later.

[Comparative Example 2]
Α-Fe ₂ O ₃ , SrCO ₃ , TiO ₂ and ZnO were used as raw material powders, and these were weighed in a quantitative ratio corresponding to the following composition.
Composition: SrFe _(12-X) (Ti _0.5 Zn _0.5 ) _X O ₁₉ , x = 0.5
Thereafter, a ferrite powder was obtained through the same process as in Example 1. As a result of X-ray diffraction, it was confirmed that the finely pulverized product was a magnetoplumbite type hexagonal ferrite. Using this magnetoplumbite type hexagonal ferrite powder, a sheet having a predetermined thickness was prepared in the same manner as in Example 1 and subjected to measurement of radio wave absorption characteristics described later.

[Comparative Example 3]
Α-Fe ₂ O ₃ , BaCO ₃ , TiO ₂ and Co ₃ O ₄ were used as raw material powders, and these were weighed at a quantitative ratio corresponding to the following composition.
Composition: BaFe _(12-X) (Ti _0.5 Co _0.5 ) _X O ₁₉ , x = 1.8
Thereafter, a ferrite powder was obtained through the same process as in Example 1. As a result of X-ray diffraction, it was confirmed that the finely pulverized product was a magnetoplumbite type hexagonal ferrite. Using this magnetoplumbite type hexagonal ferrite powder, a sheet having a predetermined thickness was prepared in the same manner as in Example 1 and subjected to measurement of radio wave absorption characteristics described later.

[Comparative Example 4]
Α-Fe ₂ O ₃ , BaCO ₃ , TiO ₂ , MnO ₂ and ZnO were used as raw material powders, and these were weighed at a quantitative ratio corresponding to the following composition.
Composition: BaFe _(12-X) (Ti _0.5 Mn _0.5 ) _X O ₁₉ , x = 0.4
Thereafter, a ferrite powder was obtained through the same process as in Example 1. As a result of X-ray diffraction, it was confirmed that the finely pulverized product was a magnetoplumbite type hexagonal ferrite. Using this magnetoplumbite type hexagonal ferrite powder, a sheet having a predetermined thickness was prepared in the same manner as in Example 1 and subjected to measurement of radio wave absorption characteristics described later.

Table 2 illustrates the relationship (measurement result) between the sheet thickness and the frequency in Example 1. From the measurement results of each sheet with different thickness, the vertical axis indicates the sheet thickness, and the horizontal axis indicates the peak value of the frequency (hereinafter referred to as the matching frequency) at which an absorption amount of about −20 dB or less is obtained. To do. As a result, the relationship between the matching frequency and the sheet thickness is a substantially linear relationship. Therefore, the arrangement of each plot is approximated by a straight line by the method of least squares. As the absolute value of the slope of the approximate line increases, the amount of change in the matching frequency with respect to fluctuations in the thickness of the radio wave absorber is reduced, and the allowable range of thickness applicable to a predetermined frequency region is expanded.

For other examples and comparative examples, the degree of change in the matching frequency due to the thickness variation of the radio wave absorber was evaluated by the same method as described above. Table 2 shows the target frequency and the calculation result of the absolute value of the slope for each example and comparative example.

As can be seen from Example 1, Example 2, Comparative Example 1, and Comparative Example 2, the absolute values of the slopes of Comparative Example 1 and Comparative Example 2 are 0.102 and 0.137, respectively. The absolute values of the slopes of Example 2 and Example 2 were as large as 0.319 and 0.315, respectively. That is, the magnetoplumbite type hexagonal ferrite powders of Examples 1 and 2 in which Co and Zn are added together as a divalent metal element are compared with those in Comparative Example 1 in which Co is added alone or Comparative Example 2 in which Zn is added alone. It can be seen that the degree of change in the matching frequency due to the thickness variation of the radio wave absorber is significantly reduced.

As can be seen from Example 3 and Comparative Example 3, the absolute value of the slope of Comparative Example 3 was 0.249, whereas the absolute value of the slope of Example 3 was as large as 0.536. That is, the magnetoplumbite type hexagonal ferrite of Example 1 in which Co and Zn are added together as a divalent metal element has a matching frequency due to the thickness variation of the radio wave absorber compared to that of Comparative Example 1 in which only Co is added. It can be seen that the degree of change is significantly reduced.

As can be seen from Example 4 and Comparative Example 4, the absolute value of the slope of Comparative Example 4 was 0.173, whereas the absolute value of the slope of Example 3 was as large as 0.429. That is, the magnetoplumbite type hexagonal ferrite of Example 4 in which Mn and Zn are added together as a divalent metal element has a matching frequency due to the thickness variation of the radio wave absorber compared to that of Comparative Example 4 in which Mn alone is added. It can be seen that the degree of change is significantly reduced.

Claims (3)

AFe _(12-X) (B1 _0.5 (Co _(1-y) Zn _y ) _0.5 ) _X O ₁₉ is represented by a composition formula, A is one or two of Ba and Sr, and B1 is Ti , Zr, magnetoplumbite type hexagonal ferrite, wherein x is 0.1 to 2.0, and y is 0.2 to 0.8. AFe _(12-X) (B1 _0.5 (Mn _(1-y) Zn _y ) _0.5 ) _X O ₁₉ is represented by the composition formula, A is one or two of Ba and Sr, and B1 is Ti. Magnetoplumbite type hexagonal ferrite, which is one or two of Zr, x is 0.2 to 4.0, and y is 0.5 ± 0.1. A radio wave absorber using the magnetoplumbite-type hexagonal ferrite powder according to claim 1.