JPH077875B2 - Radio wave absorber - Google Patents

Description

TECHNICAL FIELD The present invention relates to a radio wave absorber, and more specifically, it absorbs and attenuates radio waves such as VHF, UHF, microwaves, radar waves, and quasi-millimeter waves and millimeter waves over a wide band. The present invention relates to a high-performance electromagnetic wave absorber.

Conventional technology Conventionally, as electromagnetic wave absorbers, ferrite sintered bodies, ferrite powder, semi-conductive carbon (fiber, powder),
Metal fibers or the like are mixed or dispersed in rubber or resin, or expandable styrene or urethane, and further, those obtained by mixing or dispersing them in a non-woven fabric are used. Furthermore, recently, the surface of insulating potassium titanate short fibers is reduced to a semi-conductive material by a reduction treatment or a coating treatment (resistivity of a powder compact pressed at a pressing force of 100 kg / cm ² of 10 ⁻² to 10 ² Ω.
.Cm), and a radio wave absorber blended in a resin has been proposed (JP-A-62-123799). However, this does not always give sufficiently satisfactory performance in terms of electromagnetic wave absorption.

Further, a radio wave absorption electromagnetic shield material in which the surface of a ferromagnetic powder is coated with a conductive material having a conductivity of 10 ⁻² to 10 ² Ω · cm has been proposed (Japanese Patent Laid-Open No. 63-12198). This material is a material having a strong electromagnetic shielding property rather than an electromagnetic wave absorbing material, and has a low electromagnetic wave absorbing performance, so that it is unavoidable that the absorption frequency band is narrow.

Problems to be Solved by the Invention These conventional electromagnetic wave absorbers have many problems to be solved as follows.

That is, in the conventional electromagnetic wave absorber, the electromagnetic wave absorbency depends on the incident direction of the electromagnetic wave, the electromagnetic wave absorption is directional with respect to the polarized wave, and it is extremely difficult to obtain sufficient absorption performance. There is a problem that the frequency band exhibiting the radio wave absorption effect is relatively narrow. Furthermore, when applied to an actual electromagnetic wave absorber, it cannot be said that the weather resistance and the stability of the characteristics are sufficient in the design, manufacture, and actual use of the absorber. There is a problem in adhesiveness to (materials) and the flexibility of the electromagnetic wave absorber, and further, deformation such as warpage is likely to occur during actual use, which adversely affects the electromagnetic wave absorption performance. And a kneader or stirrer,
When manufacturing a radio wave absorber using a mixer, a molding machine, etc., there is a problem that the radio wave absorber accelerates wear of the machine.

Therefore, realization of an electromagnetic wave absorber in which many of these problems have been solved or improved has been strongly desired.

It is an object of the present invention to provide a radio wave absorber having a very high performance, which solves such a problem.

Means for Solving the Problems The present invention uses zinc oxide (ZnO) whiskers, which is a completely new material, as a radio wave absorber. And more preferably, the zinc oxide whiskers have a length from the base to the tip of 3 μm or more. Further, the zinc oxide whiskers have a core portion and needle-shaped crystal portions extending from the core portion in different axial directions. Further, the number of axes of the needle-shaped crystal portion extending in the plural axis directions of the zinc oxide whiskers is 4. That is, the present invention, in the radio wave absorbing or radio wave transmitting holding material, has a core portion and a needle-shaped crystal portion extending in four different directions from the core portion, from the base to the tip of the needle-shaped crystal portion. Hold zinc oxide whiskers with a length of 3 microns or more with a holding material, or
An electromagnetic wave absorbing material characterized by being mixed or dispersed in a holding material. Furthermore, the present invention has a radio wave absorbing or radio wave transmitting holding material having a core portion and needle-shaped crystal portions extending from the core portion in four different directions, and the length from the base to the tip of the needle-shaped crystal portion. A triaxial, biaxial, or uniaxial needle-shaped crystal part formed by breaking a part of a 4-axis needle-shaped crystal part extending in four directions of a zinc oxide whisker having a diameter of 3 μm or more. A radio wave absorber characterized in that the zinc oxide whiskers alone or in combination selected from the zinc oxide whiskers are mixed or dispersed in a holding material. Furthermore, the present invention provides a radio wave absorbing or radio wave transmitting holding material having a core portion and needle-shaped crystal portions extending from the core portion in four different directions. Zinc oxide whiskers having a length of 3 μm or more, or triaxial or biaxially formed by breaking a part of four-axis needle crystal parts extending in four directions of the zinc oxide whiskers, or The zinc oxide whiskers selected from zinc oxide whiskers having uniaxial needle-shaped crystal parts are composed of a member and a reflecting plate mixed or dispersed in a holding material. It is a radio wave absorber.

Furthermore, the present invention has a radio wave absorbing or radio wave transmitting holding material having a core portion and needle-shaped crystal portions extending from the core portion in four different directions, and the length from the base to the tip of the needle-shaped crystal portion. Saga 3
1% by weight or more of zinc oxide whiskers of micron or more, ferrite, carbon, conductive potassium titanate,
It is a radio wave absorber that holds or disperses silicon carbide and any one or a combination of metal particles and fibers.

Further, the present invention has a radio wave absorbing or radio wave transmitting holding material having a core portion and a needle-shaped crystal portion extending in four different directions composed of the core portion, from the base to the tip of the needle-shaped crystal portion. 1% by weight of zinc oxide whiskers with a length of 3 microns or more
Electromagnetic wave absorption composed of a member that holds or disperses any one or a combination of the above, ferrite, carbon, conductive potassium titanate, silicon carbide, and metal particles or fibers, and a reflector. It is a material.

Further, the present invention provides a surface of a zinc oxide whisker having a core portion and needle-shaped crystal portions extending from the core portion in four different directions, and the length from the base to the tip of the needle-shaped crystal portion is 3 microns or more. Is coated with a magnetic material, and the zinc oxide whiskers are held or dispersed in a radio wave absorbing or radio wave transmitting holding material.

The present invention also provides a zinc oxide whisker having a core portion and a needle-shaped crystal portion extending in four different directions from the core portion, and the length from the base to the tip of the needle-shaped crystal portion is 3 microns or more. A radio wave absorber having a surface coated with a magnetic material, the zinc oxide whiskers being held or dispersed in a radio wave absorbing or radio wave transmitting holding material and a reflector.

The holding material is resin, rubber, paint, ceramic, glass, concrete, mortar, inorganic or organic fiber, granular material, powder or flake, inorganic binder or organic binder, wax, gel-like semi-solid substance,
Alternatively, a foam is used.

Action The mechanism by which the electromagnetic wave absorbing material of the present invention has a significantly superior electromagnetic wave absorbing property as compared with the conventional product remains unsolved, but at present it is speculated as follows.

First, the radio wave absorber of the present invention has an effect of effectively guiding radio waves to the inside of the absorber. The main reasons for this are as follows. Since the zinc oxide whiskers themselves have conductivity as an appropriate semiconductor and a dielectric constant, it is considered that the radio wave reflection is small. Zinc oxide whiskers are completely different from conventional short fiber whiskers,
Has a three-dimensional structure with a core and needle-shaped crystal parts extending from the core to the different four directions at the same angle, and when they are gathered, a three-dimensional mesh structure with appropriate voids is formed. Easy to make. Furthermore, zinc oxide whiskers are completely different from the image of conventional whiskers,
All of them are neat single crystals, and there are few irregularities on the surface, and since they have a colorless and transparent glossy surface, there is little irregular reflection of radio waves.

The radio wave absorber of the present invention has a much more effective effect of absorbing radio waves than the conventional materials, and the reason is presumed to be as follows.

As described above, the radio waves are effectively guided to the inside of the absorber, and more radio wave absorbers have a chance of turning, so that extremely good absorption performance can be obtained. Zinc oxide whiskers have a three-dimensional structure with a core and needle-shaped crystal parts extending from the core in four different directions. There is no direction in the absorption of waves. Since zinc oxide whiskers have the above-mentioned structure, unlike ordinary short fiber whiskers, it is easy to make a three-dimensional mesh structure, and therefore when electromagnetic waves arrive, it acts like a kind of loop antenna in electromagnetic induction. And shows a highly efficient absorption performance.
Further, when the zinc oxide whiskers having the above structure are dispersed in the matrix, the zinc oxide whiskers are uniformly dispersed at appropriate intervals, so that a highly efficient radio wave absorber can be obtained. Since the tips of the needle-shaped crystals of zinc oxide whiskers are extremely sharp, the electric field at the tip portion becomes extremely high, and remarkable electromagnetic wave absorption is exhibited at that portion. The zinc oxide whiskers are a uniform single crystal body, and unlike the one in which the surface is made into a semiconductor by surface processing, the entire whisker is a semiconductor and a uniform radio wave absorber, so that it is an efficient radio wave absorber. Furthermore, since zinc oxide whiskers are semiconductors with a large aspect ratio, large polarization can be expected, and ε ′,
A large electromagnetic wave absorber is obtained for both ε ″. Further, unlike the conventional electromagnetic wave absorbers, the electromagnetic wave absorber of the present invention is a material exhibiting large photoconductivity and varistor characteristics. Is a material that exhibits diamagnetic properties with a magnetic susceptibility of −0.31 × 10 ⁻⁶ / 0 ° C. (CGS electromagnetic unit system), and has been used to mix ferrite with ferrite for its unique properties. It is quite possible that this magnetic effect contributes.

As described above, in addition to the rare multifunction of the zinc oxide material, the zinc oxide whiskers are unique in that they have a core part and needle-shaped crystal parts extending from the core part in four directions at equal angles. It has a shape, and it is considered that such a topological property, a crystalline, a semiconductor-like, or a magnetic property acts to reveal a high-performance electromagnetic wave absorber.

Next, in the radio wave absorber of the present invention, the zinc oxide whiskers have the unique shape and the sharp shape as described above, and further promote the improvement of the adhesiveness due to the inherent strength and reinforcing property of the whiskers. .

Further, when it is a composite material, the zinc oxide whiskers have an isotropic orientation due to the peculiarity of its shape, so that warp deformation as a structural material is substantially eliminated and dimensional stability is secured.

Furthermore, since zinc oxide is a metal oxide, it does not undergo oxidative deterioration, and since it has ultraviolet absorption and choking resistance, it has excellent weather resistance. And, as described above, since it is possible to uniformly disperse due to the characteristics of the shape, there is no possibility of deterioration due to oxidation, etc.
The characteristics are the same among the individual wave absorbers,
A stable product can be obtained over time. Further, the zinc oxide whiskers themselves have a hardness of 4 to 4.5, and have a cleavage plane perpendicular to the C-axis, and are therefore extremely supple as whiskers, which imparts a reinforcing effect and flexibility to the composite.

Further, unlike a radio wave absorber such as a ferrite-based or silicon carbide-based radio wave absorber, since it is a relatively soft radio wave absorber, it is possible to suppress wear of a molding machine or the like.

Another radio wave absorber of the present invention is a highly efficient radio wave absorber that organically combines magnetic loss and high conduction loss.

That is, zinc oxide whiskers are electrically conductive single crystals having an appropriate resistivity, and by coating the surface thereof with a magnetic material, highly efficient magnetic loss can be expected on the surface of the zinc oxide whiskers. Further, the radio waves transmitted through the magnetic film penetrate into the zinc oxide whiskers, but since the whiskers themselves are resistors having an appropriate resistivity, the electric field acts on the free electrons to cause an efficient conduction loss. As described above, the present invention is a radio wave absorber that organically and effectively combines magnetic loss and conductive loss.

Still another aspect of the present invention is a composite electromagnetic wave absorber that uses zinc oxide whiskers in combination with at least one of ferrite, carbon, conductive potassium titanate whiskers, metal particles and fibers. Zinc oxide whiskers have various advantages as described above, and by combining them with other materials, the performance of conventional electromagnetic wave absorbers can be greatly improved.

That is, the zinc oxide whiskers guide the radio waves to the inside of the absorber, and have many chances to combine with conventional radio wave absorbing materials such as ferrite and carbon, so that effective radio wave absorbing performance can be obtained.

In addition to the above, the compounding can improve the adhesiveness, the warp deformation prevention, the dimensional stability, and the weather resistance of the electromagnetic wave absorber.

Examples Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to the following examples.

In the present invention, a completely new zinc oxide whisker is used as the radio wave absorber. Among these zinc oxide whiskers, what has a particularly characteristic effect is a three-dimensional structure having a core and needle-shaped crystal parts extending from the core in four different directions. Such a zinc oxide whisker having a special shape can be produced by heat-treating a metallic zinc powder having an oxide film on its surface in an atmosphere containing oxygen. The obtained zinc oxide whiskers are
The apparent bulk specific gravity is 0.02-0.1, and the production yield is 70wt.
%, Which is an extremely high value, and can be obtained with excellent mass productivity.

1 and 2 are electron micrographs showing an example of the product. According to this, as described above, the shape characteristic and the dimensional characteristic including the core portion and the needle-shaped crystal portions extending from the core portion in the four different directions are clearly recognized.

By the way, this zinc oxide whisker has 3 needle-shaped crystal parts.
Axial or biaxial, or even monoaxial may be mixed. This is due to the fact that part of the four-axis crystal was originally broken. Also, when mixing this zinc oxide whisker into rubber, resin, ceramics, glass, etc., if you do not pay sufficient attention to it, the peculiar structure as described above will collapse during mixing and molding, and it will change to a simple needle-like whisker. Often.

When X-ray diffraction of zinc oxide whiskers is observed, all show zinc oxide peaks, and the electron diffraction results also show single crystallinity with few dislocations and lattice defects. The content of impurities was also small, and the result of atomic absorption analysis was 99.98% zinc oxide.

On the other hand, a simple acicular zinc oxide whisker can be produced, for example, metallic zinc powder can be fired at the same time as charcoal or the like to be produced on the wall surface of a crucible, but it is not mass-produced.

Also, from the standpoint of electromagnetic wave absorption performance, the length of the needle crystal part is 30μ.
It is desirable to use 3 wt% or more of zinc oxide whiskers of m or more. More preferably, 70 wt% or more of zinc oxide whiskers having a length of 50 μm or more is used. A system in which the zinc oxide whiskers having a needle-shaped crystal portion length of less than 30 μm account for a large proportion (for example, 99 wt% or more) is not preferable.

The average aspect ratio of zinc oxide whiskers is 3
The above is desirable. More preferably 10 on average
That is all. In addition, the value of the ratio of the diameter of the tip part of the needle-shaped crystal part to the diameter of its base part is 0.8 from the point of electromagnetic wave absorption characteristics.
The following is desirable. More preferably 0.5 or less,
More preferably, it is 0.1 or less.

The resistivity of the zinc oxide whiskers used in the present invention is 5k.
It is a value measured by applying a DC voltage of 50 V to a 0.2 mm thick green compact pressed at a pressure of g / cm ² , and it may be in the range of 10 to 10 ⁸ Ωcm, depending on the application. Therefore, zinc oxide whiskers having a resistivity suitable for that can be used properly. From the radio wave absorption performance, 6 × 10 ² to 8 × 10 ⁵ Ω · cm is preferable, and 5 × 10 ³ to 8 × 10 ⁴ Ω · cm is particularly effective in view of production cost. Zinc oxide whiskers (average leg length: 70 μm) having a core part and needle-shaped crystal parts extending in four directions from the core part and having a pressure of 350 kg / cm ² and a pressure of 5 mm
The resistivity of the thick green compact was 1.2 × 10 ⁷ Ω · cm (manufactured by HIOKI: measured with a digital tester).

The resistivity of the zinc oxide whiskers used in the present invention is selected by selecting firing conditions during the production of the whiskers, performing reduction firing treatment, or doping with other elements such as aluminum, lithium and copper by an appropriate method. By doing so, a desired value can be obtained.

There are various usage forms of the radio wave absorber of the present invention.

That is, the zinc oxide whiskers are held in a powder state, a deposit state, a sintered state, and various holding materials by an appropriate method and form.

Zinc oxide whiskers in powder form can be placed in a container or bag of woven cloth, non-woven cloth, ceramics, glass, resin, rubber, concrete, mortar, wax, gel-like semi-solid material, foam, etc., or sealed with these materials. It is also used.

What is the state of deposits of zinc oxide whiskers? Papermaking (papermaking)
Method whisker paper and wet filtration (vacuum filtration, etc.)
Zinc oxide whisker filtration deposits and the like. In this case, a suitable organic binder or inorganic binder can be used.

Furthermore, it is possible to use a sintered body obtained by sintering the zinc oxide whisker aggregate while pressing at a suitable pressure or after pressing at a suitable temperature, for example, 500 to 1600 ° C. In this case, it is effective to use a generally used sintering aid in an appropriate amount. Regarding the pressing pressure, the value is not particularly limited, but in the range of 1 to 2000 kg / cm ² ,
Particularly good results are obtained in the range of 10 to 400 kg / cm ² .

The zinc oxide whisker holding material may be radio wave absorptive or transparent. Various resins are used as the holding material. Specifically, both a thermosetting resin and a thermoplastic resin can be used.

As the thermosetting resin, for example, epoxy resin, unsaturated polyester resin, urethane resin, silicone resin, melamine-urea resin, phenol resin, etc. can be used. Examples of the thermoplastic resin include polyvinyl chloride, polyethylene, chlorinated polyethylene, polypropylene,
Polyethylene terephthalate, polybutylene terephthalate, polyamide, polysulfone, polyetherimide, polyether sulfone, polyphenylene sulfide, polyether ketone, polyether ether ketone, ABS resin, polystyrene, polybutadiene, methyl methacrylate, polyacrylonitrile, polyacetal, polycarbonate, Polyphenylene oxide, ethylene-vinyl acetate copolymer, polyvinyl acetate, ethylene-tetrafluoroethylene copolymer, aromatic polyester, polyvinyl fluoride, polyvinylidene fluoride, polyvinylidene chloride, polyethylene fluoride, etc. are used. It Needless to say, the resin used for the holding material is not limited to these exemplified ones.

In particular, from the viewpoint of radio wave absorption characteristics, zinc oxide whiskers and acicular zinc oxide whiskers having a core and needle-shaped crystal parts extending from the core in four different directions are used as a holding material without breaking as much as possible. Since it is desirable to be retained, various methods conventionally proposed or implemented for that purpose can be applied. That is, a mixing method, a kneading method, an extruding method, or a molding method, in which stress or shearing force is not generated as much as possible with respect to the zinc oxide whiskers, can be applied. Then, zinc oxide whiskers are mixed with a resin in a liquid phase state having a relatively low viscosity such as a thermosetting resin such as an epoxy resin, an unsaturated polyester resin, a urethane resin, or a silicone resin, followed by molding and curing. That method is practical. Alternatively, a method in which a resin is dissolved in a solvent, zinc oxide whiskers are mixed in a low-viscosity solution, and then the solvent is volatilized can be applied. Furthermore, a method of mixing fine powder of resin having a particle size of several μm to several tens of μm and zinc oxide whiskers, and then causing heat or a solvent to act to dissolve the resin component and hold the zinc oxide whiskers is also preferable.

As the rubber material used as the holding material, natural rubber or synthetic rubber is used. In particular, a rubber material that does not adversely affect zinc oxide and has excellent radio wave absorption characteristics is preferable. In that respect, polyurethane rubber is most preferable, and then acrylic rubber, silicone rubber, butadiene rubber, isobutylene rubber, polyether rubber,
Isobutylene-isoprene copolymer and isocyanate rubber are preferred. Depending on the application, nitrile rubber, chloroprene rubber, chlorosulfonated polyethylene, polysulfide rubber, and fluorine-based rubber are also used. Also, natural rubber dissolved in a solvent, polyethylene fine powder dispersed in water, polymer emulsion Etc. are also used. Needless to say, when a rubber material is used as the holding material, it is more preferable to apply a generally practiced method that minimizes breakage of the whiskers. It goes without saying that various commonly used additives, fillers, etc. are used at the same time.

Further, the zinc oxide whiskers can be dispersed in various paints, or the paint material can be used as a holding material. That is, as the paint, for example, epoxy-based, acrylic-based, urethane-based and other various paints can be used. Of course, although not limited to this, a coating material having particularly high heat resistance and weather resistance is more preferable.

A radio wave absorber can be constructed by dispersing zinc oxide whiskers using various inorganic solid materials (powder, fibrous, flake, granular, solid) as a holding material.

Specifically, it is dispersed in various ceramics, glass, enamel, etc., or constitutes a radio wave absorber using them as holding materials, or zinc oxide is added to clay powder, asbestos, mica, sand, glass fibers, etc. It is possible to construct an electromagnetic wave absorbing powder in which whiskers are dispersed, an electromagnetic wave absorbing fiber aggregate (woven cloth, non-woven cloth shape), or the like.

In addition, concrete or mortar can be used as the holding material.

Waxes such as paraffin wax, polyethylene wax, and microcrystalline wax can also be used as the holding material. Further, a gel-like semi-solid substance such as agar or gelatin, a substance having a high viscosity or adhesiveness such as glue, rubber glue, or high-viscosity polybutene can be used as the holding material.

Further, various foams are suitable as the holding material. In particular, urethane type is excellent in characteristics, followed by epoxy type, and then styrene type.

The holding material system as described above is not limited because it varies depending on the strength of radio waves, the size of zinc oxide whiskers, the degree of breakage and shape thereof, the type of matrix material and the holding material, and the holding form. But almost 5 wt%
By dispersing the above, the electromagnetic wave absorption characteristics can be confirmed. This becomes remarkable at 10 wt% or more, and a large absorption effect can be obtained even if an extremely small amount is used.

Also, if necessary, it may be used together with or mixed with other particles or fiber-shaped electromagnetic wave absorbing materials (carbon-based, silicon carbide-based, ferrite-based, metal-based, conductive potassium titanate-based, etc.). It doesn't matter. The effective amount of zinc oxide whiskers is 1 wt% or more, more preferably 5 wt% or more, based on the total amount of the compound.

As the magnetic material for coating the surface of the zinc oxide whiskers, soft magnetic material, hard magnetic material, metallic magnetic material (iron, cobalt, nickel etc. or alloy), oxide magnetic material (ferrite), paramagnetic material or strong magnetic material. A magnetic material, an antiferromagnetic material, or a ferrimagnetic material is selected according to the application. In particular, an oxide magnetic material having a large μ ″ at the used frequency is preferable.

Specifically, in particular, among the ceramic ferrimagnetic oxides represented by MO · Fe ₂ O ₃ , a polycrystalline composite ferrite having an inverse spinel structure and mixed with two or more divalent ions as M ²⁺ is used. desirable. That is, as the main composition of M ²⁺ ,
(Ni, Zn), (Ni, Cu, Zn), (Ni, Mg, Zn), (Ni, Zn, C
o), (Ni, Cu, Zn, Co), (Mn, Zn), (Cu, Zn), etc. give desirable results. In addition, a hexagonal crystal material, which is a type of ferrite and has a trade name of "ferroc sprayer", can also be used.

The coating thickness used is in the range of 50Å to 200 μm. Especially 100-500Å is effective. Regarding the coating,
It may be a full surface coating, a partial coating or a local coating, and produces a great effect depending on each application.

Various methods are used for coating the surface of the zinc oxide whiskers with the magnetic material. In particular, electrolytic plating method, electroless plating method, vapor deposition method such as vacuum deposition method, emulsion coating method (a solution in which fine particles are dispersed is applied on the surface of whiskers and dried), paint coating method (fine particles are dispersed in the paint) Then, it is applied to the surface of the whiskers and dried).

Also, in some cases, this type of electromagnetic wave absorber may be used in combination with other electromagnetic wave absorbing materials (zinc oxide whiskers, carbon type, silicon carbide type, ferrite type, metal type, conductive potassium titanate type, etc.), The characteristics can be further improved by mixing them.

The radio wave absorber of the present invention is not limited to applicable frequencies and intensities as long as the radio wave absorption effect is practically obtained and the radio wave frequency and radio wave intensity. In particular, VHF, UHF, microwave, radar It can be used for waves, quasi-millimeter waves, millimeter waves, etc. Frequency 100MHz-100GHz, more preferably 100M
It can be applied in the range of Hz to 20 GHz, more preferably 1 to 15 GHz, and most preferably 10 to 15 GHz, and it is a radio wave absorber exhibiting excellent absorption in a wide frequency bandwidth.

[Example 1] Metal zinc powder having an oxide film on its surface is heat-treated in an atmosphere containing oxygen to extend in four directions at the same angle from the core portion as shown in FIG. A zinc oxide whisker having a shape including a needle-shaped crystal part was produced. This zinc oxide whisker has an average length from base to tip
The average diameter of the base was 5 μm, and most of them had the structure shown in FIG. The resistivity was 2.5 × 10 ⁴ Ω · cm as a value measured by applying a DC voltage of 50 V to a 200 μm-thick green compact to which a pressing force of 5 kg / cm ² was applied.

Next, a two-component low-viscosity epoxy resin cured by epi-bis / acid anhydride is prepared, and the zinc oxide whiskers are added at 16% by weight.
The mixture was gently mixed at a ratio of 120 rpm, mixed for 2 minutes at 120 rpm, preheated at a temperature of 60 ° C. for 15 minutes, and further deaerated under reduced pressure for 15 minutes. Then, the epoxy resin composition was poured into a mold preheated at a temperature of 80 ° C., deaerated under reduced pressure for another 15 minutes, and then cured at a temperature of 90 ° C. for 5 hours.

The obtained sample was a thick plate with a size of 10 cm square and 4 mm.
When the cross section of this sample was observed with an electron microscope, most of the zinc oxide whiskers maintained a shape with a nucleus and needle-shaped crystal portions extending from the nucleus in four directions at the same angle, It was evenly distributed in the interior and had a uniform three-dimensional mesh structure. The resistivity of this sample is 2 x 10
It was ⁶ Ω · cm.

When the frequency-reflection reflection characteristics of this sample were measured, it was found that a large attenuation was obtained in a wide frequency band of 10 to 15 GHz as shown in Fig. 3, and the same attenuation characteristics were exhibited for polarized waves and oblique incidence. Was confirmed.

[Example 2] 25 parts by weight of the zinc oxide whiskers used in Example 1 were gradually added to 100 parts by weight of liquid nitrile rubber (Nipol 1312 Nippon Zeon Co., Ltd.), and a vulcanizing agent as an additive was added to 2.0 parts by weight.
By weight, 3.0 parts by weight of activator zinc oxide, 1.0 part by weight of accelerator and 1.0 part by weight of antioxidant were added, and the mixture was stirred gently and mixed sufficiently without applying a shearing force. The obtained paste-like mixture was passed through a constant-velocity three-roll roll subjected to a surface release treatment to form a sheet having a predetermined thickness. This was heated under pressure using a piston mold to obtain a vulcanized rubber sheet having a thickness of 2 mm.

When observing the cross section of this sample with an electron microscope, about 40% of the zinc oxide whiskers were in a broken state, but the majority of zinc oxide whiskers are the needles that extend in the four directions at the same angle from the core and this core. And a shape having a crystal-like portion was maintained.

When the frequency-reflection reflection characteristic of this sample was measured, a large attenuation characteristic of -7 dB or less was obtained in a wide frequency band of 10 to 15 GHz.

It was confirmed that this rubber has extremely high adhesiveness to various adhesives and double-sided adhesive tape.

[Example 3] 100 parts by weight of polyethylene fine powder having a particle size of 3 to 10 µm and 40 parts by weight of the same zinc oxide whiskers as those in Example 1 were gently mixed sufficiently, and then charged into a press mold,
It heated at the temperature of 160 degreeC for 5 minutes. And further pressure 1kg / c
A plate-like sample having a size of 4 mm and a 10 cm square was obtained by press molding at m ² .

Observation of the cross section of this sample with an electron microscope showed that most of the zinc oxide whiskers maintained a shape with a nucleus and needle-like crystal portions extending from the nucleus in the four directions at the same angle, The cross section was formed and the polyethylene served as the binder.

Also in this sample, extremely good electromagnetic wave absorption characteristics, polarization and oblique incidence characteristics were obtained.

Example 4 A shape having a core portion and an acicular crystal portion extending from the core portion in the four directions at the same angle to each other in an average length of 45 μm from the base portion to the tip and an average diameter of the base portion of 2 μm. A zinc oxide whisker was prepared, and 200 parts by weight of EPDM was mixed with 200 parts by weight, kneaded, heated, vulcanized, and molded. 2m in this way
A sample with a thickness of 10 m and a size of 10 cm square was obtained.

When the cross section of this sample was observed with an electron microscope, it was found that the shape of most of the zinc oxide whiskers collapsed to form needle-shaped or rod-shaped zinc oxide whiskers.
The average value of the aspect ratio (= length / large diameter) of this zinc oxide whisker was 11, and the ratio of the diameter of the base portion to the diameter of the tip portion was 0.65 on average.

Using this sample, we measured the frequency vs. return loss characteristics, and as shown in Fig. 4, in the frequency band of 10 to 15 GHz,
It was found that a high attenuation characteristic of 5 to -17 dB was obtained, and that the performance was extremely higher than that of rubber ferrite type electromagnetic wave absorbers that have been commercially available in the past.

The resistance value of this rubber was 10 ¹⁵ Ω or more.

[Example 5] The same zinc oxide whiskers as in Example 1 were prepared.
Gradually add it to polypropylene pellet melt so as to have a ratio of 1.5 wt%, knead it, and then injection-mold it.
A 10 cm square sample having a size of 3 mm was obtained.

When the cross section of this sample was observed with an electron microscope, most of the zinc oxide whiskers did not retain the original shape and were oriented in the resin flow direction. However, it was found that the kneading machine and the molding machine suffer less wear in the manufacturing process than other materials, and that the molded sample is soft and supple against bending etc. even though the compounding ratio is high. did. Further, the aspect ratio of zinc oxide whiskers observed in the cross section was 15 or more on average.

When the frequency vs. reflection attenuation characteristics were measured using this sample, it was found that high attenuation characteristics were obtained in the frequency band of 10 to 15 GHz as shown in FIG.

[Example 6] Zinc oxide whiskers, silicon carbide whiskers and potassium titanate whiskers subjected to a reduction conductive treatment (thickness 1 to 5 µm, aspect ratio 100 to 600, resistivity 1 Ω · c) used in Example 1.
m), non-conductive potassium titanate whiskers, zinc white,
When carbon black was placed in a polyethylene bag and molded into a thickness of 30 mm, and the frequency vs. reflection attenuation characteristics were measured, it was found that the attenuation effect in the frequency band of 10 to 15 GHz was zinc oxide whiskers >> Silicon carbide whiskers >
Conductive potassium titanate whiskers ≫ zinc white> carbon black ≈ non-conductive potassium titanate whiskers, and it was clearly confirmed that zinc oxide whiskers are extremely suitable materials for electromagnetic wave absorption.

[Example 7] When the same samples as in Examples 1 to 6 were evaluated for electromagnetic wave absorption in the frequency band of 100 MHz to 1 GHz, remarkable electromagnetic wave absorption characteristics were obtained in an extremely wide band.

In these examples, evaluation was made as a single-layer type electromagnetic wave absorber, but the electromagnetic wave absorber of the present invention is not limited to this, and is a multilayer type electromagnetic wave absorber in combination with a metamorphic layer or in combination with various electromagnetic wave absorbers. It goes without saying that can be configured.

[Example 8] Metal zinc powder having an oxide film on the surface was heat-treated in an atmosphere containing oxygen to produce zinc oxide whiskers.
The zinc oxide whiskers have an average length from the base to the tip of 100 μm, an average diameter of the base of 6 μm, and are mostly composed of a core and needle-like crystal parts extending from the core in four directions at the same angle to each other. It had a structure with. Its resistivity is
DC voltage of 50 is applied to a 200 μm thick green compact molded at a pressure of 5 kg / cm ^2.
The value measured by applying V was 1.5 × 10 ⁴ Ω · cm.

Next, an acrylic coating solution 1 diluted with toluene to a concentration of 10% was prepared, and 100 g of MO · Fe ₂ O ₃ ferrite powder (M: Ni, Zn) having a particle size of 2 to 5 μm and 100 g of the zinc oxide whiskers were prepared. And gently agitate the mixture at a temperature of 50 ° C.
To obtain a ferrite-coated zinc oxide whisker. Its sectional structure is shown in FIG.

This was mixed in an acrylic paint at a weight ratio of 30 wt% to obtain a ferrite-coated zinc oxide whisker-containing paint. The ferrite coating thickness was 2-100 μm on average.

This paint was uniformly applied to a 2 mm thick aluminum plate with a thickness of 3.5 mm, and was sufficiently dried to obtain a measurement sample.

When the return loss characteristics of this sample were measured, it became clear that as shown in FIG. 7, a remarkable electromagnetic wave absorption characteristic was exhibited in a wide band.

[Example 9] The zinc oxide whiskers used in Example 8 were nickel-plated by an electroless plating method to obtain a nickel metal-coated zinc oxide whiskers. The nickel coating thickness averaged 100Å.

500 parts by weight of this whisker was kneaded into EPDM rubber and press-molded to obtain a rubber plate having a thickness of 5 mm.

When an aluminum plate (2 mm thick) was applied to the back surface of this sample and the return loss at a frequency of 1 GHz was measured, it was -14 dB, and it was confirmed that the sample showed remarkable radio wave absorption.

In these examples, a single-layer type electromagnetic wave absorber was evaluated, but the electromagnetic wave absorbing material of the present invention is not limited to this, and may be combined with a metamorphic layer or various electromagnetic wave absorbing materials to form a multilayer type structure. It goes without saying that the characteristics can be further improved by doing so.

[Example 10] Fig. 8 is an explanatory view of a composite electromagnetic wave absorber in which powdery zinc oxide whiskers 1 and ferrite 2 are mixed.

As the ferrite 2, the same type as the ferrite powder used in Example 8 was used. The zinc oxide whisker 1 has a structure having a core portion and needle-like crystal portions extending from the core portion in four directions at the same angle, and the average length from the base to the tip is 100 μm, and the thickness of the base is large. Has an average diameter of 4 μm and diameter ratio
0.05 whiskers were used. This zinc oxide whisker 1 was mixed with ferrite 2 at a ratio of 6 wt%, and 200 parts by weight of this mixture was added to liquid silicone rubber and sufficiently kneaded to form a 3 mm thick sheet.

The return loss of the obtained sheet was measured and found to be -15 dB.
The following bandwidths are more than doubled compared to conventional ferrite only sheets.

Example 11 A structure having a core part and needle-shaped crystal parts extending from the core part in the four directions at the same angle to each other, the length from the base part to the tip is 120 μm on average, and the thickness of the base part is 6 μm on average. Diameter ratio average 0.0
A zinc oxide whisker of 1 was prepared. Its resistivity is 5kg
The value measured by applying a DC voltage of 50 V to a green compact having a thickness of 200 μm obtained by applying a pressure of / cm ² was 1 × 10 ⁴ Ω · cm. Liquid silicone rubber (Toshiba Silicon Co., product name
YE5822) in a ratio of 17 wt%, then pour into a mold and heat cure at 100 ° C for 1 hour,
A white rubber sheet (thickness 3.5 mm) was obtained.

Next, when the reflection attenuation characteristic of the rubber sheet was measured using an aluminum reflector, as shown in FIG.
It was found that the above rubber sheet is a radio wave absorber having a large attenuation over a wide frequency band.

Also, in order to investigate the degree of orientation of the whiskers, the rubber sheet was rotated 90 ° and the return loss characteristics were examined, but no change was observed, and the zinc oxide whiskers were randomly oriented in all directions. It was confirmed that radio waves are effectively absorbed regardless of polarization.

EFFECTS OF THE INVENTION According to the present invention, by using zinc oxide whiskers, a novel radio wave absorber capable of absorbing and attenuating radio waves very efficiently can be obtained. Recently, demands for EMI and EMC are rapidly increasing, such as problems related to the start of satellite broadcasting, microwave communication, satellite communication, radar, leaked radio waves from microwave ovens, and VHF and UHF VT radio wave ghost problems. In a sense, the present invention not only facilitates the design of those devices, but can significantly increase the reliability of the devices and systems.

[Brief description of drawings]

1 and 2 are electron micrographs showing the crystal structure of the zinc oxide whiskers used in the present invention, FIG. 3 and FIG.
FIGS. 4 and 5 are radio wave absorption characteristics of the radio wave absorber of the present invention, FIG. 6 is a schematic sectional view of the coated zinc oxide whiskers used in the present invention, and FIG. 7 is the radio wave absorption of the present invention. Fig. 8 is a radio wave absorption characteristic diagram of the material, Fig. 8 is a sectional view of the radio wave absorption material of the present invention, and Fig. 9 is a diagram showing the radio wave absorption characteristic of the radio wave absorption material of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kojiro Matsuo 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Hidenosuke Nakamura, 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. 56) References JP-A-58-82403 (JP, A) JP-A-63-47999 (JP, A)

Claims (7)

[Claims] 1. A holding material that absorbs or transmits radio waves,
A zinc oxide whisker having a core part and needle-shaped crystal parts extending from the core part in four different directions and having a length from the base part to the tip of the needle-shaped crystal part of 3 μm or more is held by a holding material. Or, a radio wave absorber characterized by being mixed or dispersed in a holding material. 2. A holding material that absorbs or transmits radio waves,
A zinc oxide whisker having a core portion and needle-like crystal portions extending from the core portion in four different directions, and having a length from the base to the tip of the needle-like crystal portion of 3 μm or more extends in the four directions. 3-axis formed by breaking a part of 4-axis needle crystal part,
Alternatively, a radio wave characterized in that the zinc oxide whiskers alone or in combination, selected from zinc oxide whiskers having biaxial or uniaxial needle-shaped crystal parts, are mixed or dispersed in a holding material. Absorbent material. 3. An electromagnetic wave absorbing or electromagnetic wave transmitting holding material,
A zinc oxide whisker having a core part and needle-shaped crystal parts extending from the core part in four different directions, and the length from the base part to the tip of the needle-shaped crystal part is 3 μm or more, or
The zinc oxide whiskers are selected from zinc oxide whiskers having triaxial, biaxial, or uniaxial acicular crystal parts formed by breaking a part of tetraaxial acicular crystal parts extending in four directions. A radio wave absorber, characterized in that the zinc oxide whiskers alone or in combination are composed of a member and a reflection plate mixed or dispersed in a holding material. 4. A holding material that absorbs or transmits radio waves,
1% by weight or more of zinc oxide whiskers having a core portion and needle-shaped crystal portions extending from the core portion in four different directions, and the length from the base portion to the tip of the needle-shaped crystal portion is 3 microns or more,
A radio wave absorber that holds or disperses any one or a combination of ferrite, carbon, conductive potassium titanate, silicon carbide, and metal particles or fibers. 5. An electromagnetic wave absorbing or electromagnetic wave transmitting holding material,
1% by weight or more of zinc oxide whiskers having a core portion and needle-shaped crystal portions extending from the core portion in four different directions, and the length from the base portion to the tip of the needle-shaped crystal portion is 3 microns or more,
An electromagnetic wave absorber composed of a member that holds or disperses any one or a combination of ferrite, carbon, conductive potassium titanate, silicon carbide, and metal particles or fibers, and a reflector. 6. A surface of a zinc oxide whisker having a core portion and needle-shaped crystal portions extending from the core portion in four different directions, and the length from the base portion to the tip of the needle-shaped crystal portion is 3 μm or more. An electromagnetic wave absorbing material in which the zinc oxide whiskers are covered with a magnetic material and held or dispersed in an electromagnetic wave absorbing or electromagnetic wave transmitting holding material. 7. A surface of a zinc oxide whisker having a core portion and needle-shaped crystal portions extending from the core portion in four different directions, and the length from the base portion to the tip of the needle-shaped crystal portion is 3 μm or more. An electromagnetic wave absorber comprising a member and a reflector in which the zinc oxide whiskers are covered with a magnetic material, and the zinc oxide whiskers are held or dispersed in an electromagnetic wave absorbing or electromagnetic wave transmitting holding material.