EP0574427A1

EP0574427A1 - Component for the absorption of electromagnetic waves, and the use of such a component

Info

Publication number: EP0574427A1
Application number: EP19920904849
Authority: EP
Inventors: Wolfgang Jaschinski; Axel Berchtold; Kilian Friederich; Dietmar Klotz; Günter NIMTZ; Peter Marquardt; Rolf Pelster; Achim Enders
Original assignee: Cerasiv GmbH Innovatives Keramik Engineering
Current assignee: Cerasiv GmbH Innovatives Keramik Engineering
Priority date: 1991-03-07
Filing date: 1992-02-18
Publication date: 1993-12-22
Also published as: WO1992016033A1; DE4201871A1

Abstract

Un composant pour l'absorption d'ondes électromagnétiques consiste en une matière solide électriquement non-conductrice, telle que du grès argilo-calcaire, du gypse ou des mélanges céramiques, dans laquelle sont réparties des particules d'une matière électriquement conductrice à l'état macroscopique et communiquant en formant un réseau et au moins partiellement entre elles, la part de volume des particules étant située entre 5 et 30 %. Le composant présente une conductivité électrique de champ alternatif de 0,1 à 20 s/m, et la taille des particules va de 1 nm à 1 mum. La matière des particules présente, à l'état macroscopique, une conductivité électrique de 105 à 107 s/m.A component for the absorption of electromagnetic waves consists of an electrically non-conductive solid material, such as clay-limestone, gypsum or ceramic mixtures, in which particles of an electrically conductive material are distributed. macroscopic state and communicating by forming a network and at least partially between them, the part of volume of the particles being located between 5 and 30%. The component has an alternating field electrical conductivity of 0.1 to 20 s / m, and the particle size ranges from 1 nm to 1 mum. The matter of the particles has, in the macroscopic state, an electrical conductivity of 105 to 107 s / m.

Description

Component for the absorption of electromagnetic waves and its use

The present invention relates to a component for the absorption of electromagnetic waves, which essentially consists of a first phase component, formed from an electrically non-conductive solid and a second phase component introduced into the first phase component, from particles of a material that is electrically conductive in the macroscopic state. The invention also relates to the use of the specified component.

GB-A-22 01 551 strikes as an absorber for

Radar waves use non-stoichiometric

Compounds, especially of oxides, such as. B.

Titanium, chrome, tungsten and a. before, preferably using an electrically conductive oxide. The absorber material can contain up to 50% by weight of a magnetic material, e.g. B. iron added; furthermore, the electrical conductivity of the

Powder particles through a z. B. existing silver coating can be improved. The known

Absorber can be in a solid ceramic form or in Powder form is present or can be embedded in a flat structure made of polymer or elastomeric material or in a paint. The absorber material can also contain carbon. If the absorber material is embedded in polymeric or elastomeric material, the disadvantage is that the material is easily flammable. If the material is applied as a coat of paint, naturally only small thicknesses can be achieved, so that a satisfactory absorption effect is only achieved in a small microwave frequency range. On the other hand, use as a powder requires additional arrangement aids. Finally, the provision of substoichiometric oxides is complex, or the coating of the particles with metals such as silver often causes unacceptable costs.

According to DE-GM 18 42 857, a device for shielding in particular high-frequency waves is known, metal or graphite particles having different density distributions being arranged within individual foam components. The effect of the known components is compared approximately with the effect of a sound absorbing plate made of porous material, which explains that despite the reflection that is still occurring, the weakened waves inside the components are reflected several times by the embedded particles, and so that the waves are largely absorbed is coming.

A very similar proposal can be found in DE-GM 17 60 260, in which a low-reflection absorber for short electromagnetic waves is described. The known absorber consists of layers of different electrical conductivity, which is adjusted by adding differently large amounts of conductive powder. Preferably, the powder consists of graphite, which is used to produce the absorber in foam body made of z. B. polystyrene is entered.

The most serious disadvantage of these known proposals lies in the flammability of the foam materials, their chemical instability and in the high temperature and frequency-dependent high-frequency conductivity. Another disadvantage is that for the absorption of relatively long waves of 100 MHz (3 m), the components must be made correspondingly thick-walled and require a large building volume solely due to their large wall thickness, especially when creating large, low-reflection and low-echo measurement rooms. This disadvantage is explained by the fact that, due to the high air content of the foam, there is no significant wave shortening due to a high dielectric constant in the absorber material when electromagnetic waves occur on the component, which, as already stated, requires the provision of extremely thick-walled components when it comes to absorption long waves.

From DE-OS 38 02 150 an absorber material for electromagnetic waves is known, in which in one

Solid, such as ceramic, plastic or in one

Liquid called mesoscopic particles from im Macroscopic compact state of electrically conductive materials, such as metal or semiconductors in the order of 1 nm up to 1,000 nm.

The conductive particles are insulated from one another and, especially if they consist of low-melting materials and are intended for introduction into a liquid, such as oil, can be agglomerated by increasing the temperature

(Particle agglomeration). The structure of the absorber that arises in this case thus relates to a matrix from which the mesoscopic particles have been removed in places and which, on the other hand, has particles that have grown due to aggregation. In particular, if low-melting metals which are designated as particularly suitable are used, the fine particles are firmly bonded as a result of melting or sintering.

As studies of this known material showed, there is a clear dependence of the absorption on the frequency, so that the electrical conductivity of the absorber can be matched to the frequency range of the waves to be absorbed by selecting the particle dimensions and the fill factor. The reason for this is that the absorption is linearly proportional to the frequency. It is therefore not suitable for a wide range of applications.

In DE-PS 10 12 657 is a low-reflection absorber layer to complicate the radar location specified, which consists of pure concrete in the form of individual components, the concrete has an additional graphite admixture, the degree of filling, the size of the graphite particles and the microstructure of the known component, as well as its electrical AC field conductivity.

EP-A-0 383 142 specifies a means for absorbing or damping electromagnetic waves in granular form or as a shaped body, which can be produced using gas or foam concrete technology. In addition to air pores and residual quartz, the agent contains calcium silicate hydrate phases containing water of crystallization, which have a chemically bound and / or

Have crystal water content of more than 10 K%. Since the strongly frequency-dependent damping properties are adversely affected by the crystal water content and this z. B. reduced by exposure to heat, the impregnation or sealing of the surface of the known absorber material is proposed as an auxiliary measure according to this document. As a further auxiliary measure, the incorporation of carbon, aluminum or ferrite is provided, the incorporation of ferrites leading to the frequency dependence of the absorption properties. A disadvantage of the known absorber material is inadequate operational reliability due to a reduced water content, which cannot be ruled out in every case even with the specified auxiliary measures. The decisive disadvantage of the known absorbent arises in spite of it sufficient absorption in the range from 6 to 18 GHz, which is caused by the absorption maximum of water, which is known to be 10 GHz, in that the absorption effect of water at smaller frequencies decreases in proportion to the wavelength, so that the known absorption medium is only unsatisfactory compared to low-frequency waves Has an effect that can only be compensated for by a thick-walled design of the components.

A layer consisting essentially of silicon carbide fibers and serving to absorb electromagnetic waves is specified in DE-OS 33 11 001. The layer is applied to a metal plate. The known absorber is used for the frequency range from 8 to 16 GHz. The fabrics made of silicon carbide fibers can be reinforced by synthetic resin binders or ceramic masses, whereby in the case of organic binders insufficient temperature independence and high flammability remain as disadvantages and overall there is also the disadvantage that the known absorbers only exist for a narrowly limited frequency range can be used.

The object of the present invention is now to provide a component which is suitable for the absorption of electromagnetic waves and which has a high absorption effect (coefficient) for both a wide range of high-frequency waves and also for low-frequency waves its high-frequency conductivity over a wide range is less temperature-dependent than the materials known from the prior art, is unproblematic with regard to its fire behavior and allows the use of components of smaller thickness when lining highly absorbent measuring rooms, so that the construction of smaller buildings with the same size can still be used Internal volume is possible.

It has now been found that the absorption effect depends not only on the size of the particles embedded in an electrically insulating material, but also to a considerable extent on their character, on the degree of distribution of the particles and thus on the type of microstructure formed. Based on these findings, the invention provides a component for the absorption of electromagnetic waves, which essentially consists of a first phase component made of an electrically non-conductive solid and a second phase component introduced into the first phase component made of particles of a material that is electrically conductive in the macroscopic state, which Combination of the following features:

a) the solid mainly consists of one or more of the substances specified below: lime-sandstone, gypsum, glass, concrete, light or gas concrete, lime / sand-containing mass (plaster) in the hardened or fired state, fired clay-containing or sintered ceramic Masses,

b) the particles consist of one or more of the compounds specified below: corrosion-resistant, high-melting, non-magnetic metals, carbides, nitrides, carbonitrides or borides of titanium,

Zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten,

c) the average particle size is in a range from 1 nm to 1 μ and the material of the

In the macroscopic state, particles have an electrical direct current conductivity of 10 5 to

10 ⁷ S / m on,

d) the particles forming the second phase constituent are present within the contiguous first phase constituent in an amount of 5 to 30 vol., are distributed like a network and, at least in part, form a penetrating structure in connection with one another,

e) the component has an electrical alternating field conductivity of 0.1 to 20 S / m.

On the term penetration structure used in patent claims and description in

Connection with the further information of a network-like

Distribution and the indication that the particles are at least partially connected to one another is to be understood that the component is continuously electrically conductive, that is to say the particles, too if in the supervision of a cross section through a plane of the component according to the invention they are sometimes not connected to one another - but are connected to one another in other planes, so that, in other words, the component is traversed by a network of electrically conductive paths. The formation of the penetration structure according to the invention requires the use of particles made of such metals or the other particle substances provided according to the invention, which do not melt or sinter together during the drying, firing or sintering processes required for producing the component according to the invention, so that after the firing has been completed or sintering the particles are still in the state of distribution and in the form, as was the case after the shaping of the component, but before the drying, firing or sintering processes were carried out. Agglomeration, possibly followed by crystallite formation, is excluded according to the invention. The first phase component is designed to be homogeneous in itself and forms the isolator for the network of embedded particles it contains.

The small particle size of the particles to be stored according to the invention enables the storage of a large number of these fine particles per unit volume.

The term "electrically conductive in the macroscopic state" means with reference to that in DE-OS 38 02 150 mentioned phenomenon that the electrical conductivity depends on the particle size, ie in the particle size range> 1 μm corresponds to the values as given in the tables in the relevant literature, but in the mesoscopic range, ie in the range below 1 μm depending on the present particle size is reduced.

By choosing mineral substances, e.g. B. gypsum or ceramic masses as the first phase component in the component according to the invention, the problem of flammability in the known components made of foamed plastic is eliminated.

By not using graphite, another disadvantage, namely a temperature-dependent conductivity, is eliminated. The arrangement of the particles within the solid according to the invention gives the component an electrical one

AC field conductivity, which is preferably even in

Range is 0.2 to 4 S / m. The electrical

Alternating field conductivity results from the imaginary part of the dielectric function ε via the product 2Tf vx εo x _ε 2_. Here v is the corresponding frequency and ε is the permittivity of free space. The component according to the invention has a frequency independence sufficient for practical use, so that it can be used excellently in the frequency range between 10 MHz and 20 GHz. In the temperature range between -100 ° C and +300 ° C there is no technically relevant temperature dependence of the high-frequency conductivity. The absorbent In the case of particles with partially electrically conductive connections, properties of the component are proportional to the square root of the frequency and thus significantly less frequency-dependent than with the known absorber materials. For frequencies below the GHz range, in order to achieve a particularly good absorption effect, it is preferable for the particles to be as electrically conductive as possible, influenced by the fill factor and particle size, ie the particles should form a network.

Preferably, the first phase portion consists exclusively of one or more mineral solids, such as. B. from one of the known Beton¬ or lightweight concrete types, but in a minor amount one or more organic components, such as. B. a

Synthetic resin binders are present, or auxiliaries and additives, as are common in the construction industry, can be present in the usual amounts. For example, the component can be produced from the mixture by admixing the particles provided according to the invention into a conventional synthetic resin cleaning compound and shaping the component. However, the mixture can also be applied to a substrate, for example the wall of a building. Even if, as stated above, a composition containing the solid components specified according to the invention for the formation of the first phase portion and the particles intended for incorporation therein is not independently deformed into a component, but is applied to a substrate, this does not depart from the idea of the present invention, and the component according to the present invention thus also includes the formation of a layer according to the features of the invention on a substrate.

Small amounts of aids, such as. B. dispersants and thickeners, which are necessary for the uniform distribution of the particles in the solid, can be present in the component according to the invention. In most cases, however, there are no residues of these aids after curing or firing; only in exceptional cases are residues of less than 1%.

The selection of the particle size enables a more precise adjustment to the frequency range to be absorbed. Since the use of coarser particles is associated with increased reflection at a frequency of 10 GHz due to the low penetration depth of the high-frequency waves, particle sizes in the nanometer range are preferably considered for cm waves in the GHz range. At lower frequencies in the MHz range, however, the particle size can be chosen to be coarser, here the average particle size can be up to 1 μm.

If the component is designed with a certain porosity, there is also a reduced reflection effect. To adjust the porosity can - if necessary - on the usual tools such as those used in the production of the known Components are used, can be used. The component can preferably be designed such that the particles in the parts of the component which face the waves to be absorbed are present with a lower degree of filling, whereas the parts facing the building - ie facing away from the waves to be absorbed - have a higher degree of filling , which practically leads to a better "immersion" of the waves to be absorbed and subsequently to an improved wave absorption. Components with a different degree of filling across their cross-section can be produced by applying layers of masses with different particle concentrations. This effect can be further increased by the fact that the component has the pyramid-shaped or conical shape known from the prior art, although with the advantage that the pyramids or cones have a substantially lower height.

The present invention is particularly suitable for the low-transmission or, at the same time, low-transmission and low-reflection cladding of a building, for example in partial areas of an airport building which are to be shielded from external radiation, but also for shielding any type of electronic devices, for. B. a computer system. However, the component according to the invention is particularly preferred in the lining of test chambers, such as those used for the examination of electrical devices and machines to an ever greater extent to determine the EMC and EMP (electromagnetic compatibility or electromagnetic pulse) are required.

The following figures show:

1 is a diagram showing the dependence of the electrical alternating field conductivity on the frequency for a component according to the invention in comparison to other absorber materials not according to the invention,

Fig. 2 is a diagram showing the absorption effect as a function of frequency.

1 mean:

A: metallic component,

B: Component according to the invention made of 18 vol.% TiC, embedded in a first phase component made of 1 ₂ 0 _3; C: Component according to the prior art made of foamed plastic with embedded therein

Graphite, D: a non-inventive component, consisting of

Gypsum and 18 vol.% TiC embedded therein, the embedded TiC particles being agglomerated - without forming a penetration structure, E: tungsten particles isolated from one another with a filling factor of 11 vol. T and with a

Diameters of approx. 400 nm are embedded in polystyrene.

2 mean:

A: a component according to the invention with the component B in FIG.

1 specified composition, B: a component corresponding to E in FIG. 1. The following examples serve to explain the invention in more detail.

example 1

An aqueous suspension of TiC with a Dg _Q particle size <1 μm is pre-ground and gypsum (see Table 1) is added to this suspension and mixed well. After setting a homogeneous mixing state of the mass formed, test specimens with a thickness of 40 mm are produced from the mass and the electromagnetic properties are characterized by measuring the dielectric properties.

Table 1

Weight (g)

Try TiC gypsum water TiC pores

Vol.% After setting

The measurement results are recorded in Table 2:

Table 2

Measuring frequency 1 MHz 8.5 GHz 11 GHz experiment

a 8.5 0 8.5 b 11.4 1.3 7.8 c not measured 9.0 d 18.9 4.9 13.0

Measurement of a commercial foam sheet

- filled with graphite - at 10 GHz gave an effective dielectric function: ^ε eff ~ ^ε 1eff ^{+ ε} 2eff of 1.3 + 0.2 i. Example 2

30% by weight of TiC powder, as stated in Example 1, is added to a commercial plaster composition suitable for producing a so-called rough plaster, but without any coarse granulation of the mineral constituents containing a synthetic resin binder. A test specimen, which has a usual rough plaster pattern on one side, is produced with the mass formed. The elevations within the rough plaster pattern were between 1 and 4 cm high, while the base layer of the test specimen was approx. 2 cm thick. The TiC particle distribution in the total volume (plaster pattern and surrounding air) shows a constant gradient in relation to the distance from the base layer due to the plaster pattern.

The following measurement result was determined at a measuring frequency of 10 GHz:

^ε eff ^{= 8} ' ^{7 +} °' ^{4 L} '

The building materials specified in Examples 1 and 2 are suitable for lining an EMC test chamber.

Claims

1. Component for the absorption of electromagnetic waves, consisting essentially of: a first phase component, formed from an electrically non-conductive solid and a second phase component, introduced into the first phase component, from particles of a material that is electrically conductive in the macroscopic state, characterized by the combination of the following features:

a) the solid mainly consists of one or more of the substances specified below:

Lime-sandstone, gypsum, glass, concrete, light or gas concrete, lime / sand-containing mass (plaster) in hardened or fired state, fired clay-containing or sintered ceramic masses,

b) the particles consist of one or more of the following compounds: corrosion-resistant, high-melting, non-magnetic metals, carbides, nitrides, carbonitrides or borides of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten,

c) the average particle size is in a range from 1 nm to 1 μm and the material of the particles is macroscopic

State an electrical direct current conductivity of 10 5 to 107 S / m,

d) the particles forming the second phase constituent are present within the contiguous first phase constituent in an amount of 5 to 30 vol., are distributed in a network and form at least partially connected to one another, a penetrating structure,

2. Component according to claim 1, characterized by an electrical alternating field conductivity of 0.2 to 4 S / m.

3. Component according to one of claims 1 and 2, characterized by a direct current conductivity of the solid of <10 ~ S / m.

4. Component according to one of claims 1 to 3, characterized by a pore volume between 1 and 60%.

5. Component according to one of claims 1 to, characterized by a pore volume between 15 and 55%.

6. Component according to one of claims 1 to 5, characterized in that the particles are distributed over the cross section of the component with different filler gradients.

7. Use of the component according to one of claims 1 to 6 for the low-transmission cladding of a building or device from the outside or from the inside.

8. Use of the component according to one of claims 1 to 6 for lining an EMC test chamber for electrical devices and machines.