WO2002080201A1

WO2002080201A1 - Electromagnetic energy absorbing material

Info

Publication number: WO2002080201A1
Application number: PCT/GB2002/001174
Authority: WO
Inventors: Andzrj Cichy; Hubert Kolodziej; Andrzej Sowa; Stanislaw Strzelecki; Andrzej Vogt
Original assignee: Emcco Research Limited; University Of Wroclaw
Priority date: 2001-03-28
Filing date: 2002-03-26
Publication date: 2002-10-10
Also published as: GB0107785D0; GB2379331B; GB2379331A

Abstract

A composite lossy material is provided, comprising: (i) a ferrimagnetic material FF or an alkyl or aryl ester of an inorganic acid, or both; and (ii) a ferromagnetic material, wherein the ferromagnetic material is a ferrite material or a ferromagnetic metallic material, or both, optionally together with an elastomer, plasticiser, organic polymer or mineral foam. Also provided is a ferrimagnetic material FF. The composite material exhibits strong absorption of electromagnetic energy within radio frequencies and microwave frequencies, and exhibits improved magnetic and dielectric lossy properties over its constituent parts.

Description

ELECTROMAGNETIC ENERGY ABSORBING MATERIAL

This invention relates to lossy materials that can absorb electromagnetic energy, and to a process for preparing these materials.

Electromagnetic energy absorbing materials are known and useful in many applications where the presence of electromagnetic radiation is undesired. For example, electromagnetic energy absorbers may be used to cover surfaces that would otherwise reflect electromagnetic radiation. Particular applications for electromagnetic energy absorbers include linings for the walls of electromagnetic test chambers used to simulate conditions of free space or open area test sites; coverings for the external walls of buildings and steel constructions; limiting radar cross-section (RCS); and coatings for electromagnetic compatibility (EMC) cables for the purpose of breaking unwanted radio frequency (RF) currents flowing in the cable or its screen, to name but a few.

In the case of electromagnetic test chambers (anechoic chambers), known absorbers of low reflectivity can be divided roughly into two groups. Absorbers of the first group are made of predominantly polyurethane foam loaded with graphite, and are shaped into the form of pyramids or wedges in order to ensure low reflectivity even for incidence angles away from the normal. The effectiveness of the absorbers of this group increases in proportion to their thickness (the height of the pyramids). Consequently, for effective broadband absorption, over both high and low (i.e. about 30 Mhz) frequencies, the absorbers of this group are required to have a thickness typically of 2 to 3 metres.

The absorbers of the second group are made of solid ferrite, in the form of tiles or grids. This group of absorbers has the advantage of low thickness, typically 6 to 7 millimetres for tiles or about 20 millimetres for grids, but can provide only narrow band absorption with a sharp selective minimum reflectivity, especially when in the form of tiles. Hybrid absorbers having a lower layer of solid ferrite and an upper layer of pyramidal foam are also known. These hybrid absorbers can provide broadband absorption whilst having a thickness of typically 0.5 to 1 metre.

Also known are elastic absorbers made of silicone rubber, polyurethane rubber, elastomers or other elastic polymer bases and loaded with ferromagnetic powder, which absorbers can be formed as thin flat sheets, sleeves, coatings or other shapes and forms. However, these, absorbers provide poor absorption properties compared with solid sintered ferrites, for instance, even at high ferromagnetic powder loadings, for example more than 90 weight percent.

There remains a need, therefore, for further electromagnetic energy absorbing materials that exhibit good absorption properties as well as desirable physical properties, and for a process for preparing, the materials. In particular, there remains a need for improved lossy materials and processes for their preparation. By a lossy material is meant a material that dissipates electromagnetic energy.

Accordingly, in one aspect, the present invention provides a composite lossy material comprising: (i) a ferrimagnetic material FF or an alkyl or aryl ester of an inorganic acid, or both; and (ii) a ferromagnetic material, wherein the ferromagnetic material is a ferrite material or a ferromagnetic metallic material, or both.

In another aspect, the present invention provides a process for preparing a composite lossy material comprising mixing: (i) a ferrimagnetic material FF or an alkyl or aryl ester of an inorganic acid, or both; and (ii) a ferromagnetic material, wherein the ferromagnetic material is a ferrite material or a ferromagnetic metallic material, or both, and extruding the mixture. It would also be desirable to provide ferrimagnetic materials useful in the preparation of lossy materials such as the composite lossy materials according to the invention, as well as a process for preparing the ferrimagnetic materials.

Accordingly, in another aspect, the present invention provides a process for preparing a ferrimagnetic material FF comprising: forming an aqueous solution of one or more soluble iron salts with an unsaturated fatty acid, heating the resulting solution, and adding a strong base so as to form a precipitate, and filtering off and washing the precipitate, wherein the washed precipitate is dissolved in an extractant solvent to form a colloidal solution, and acetone is added to the colloidal solution in order to precipitate out a solid phase sediment.

In a preferred embodiment, the present invention provides a process for preparing a ferrimagnetic material FF1 comprising: forming an aqueous solution of FeCl₃.6H₂O and FeSO₄.7H₂O and adding oleic, heating the resulting solution to a temperature of about 95 °C, and adding aqueous ammonia so as to form a precipitate, and filtering off and washing the precipitate with distilled water at room temperature, wherein the washed precipitate is dissolved in extraction naphtha to form a colloidal solution, and acetone is added to the colloidal solution in order to precipitate out a solid phase sediment.

In further aspects, the present invention provides ferrimagnetic materials FF and FF1 as herein defined.

Advantageously, the composite material according to the invention exhibits strong broadband absorption of electromagnetic energy within radio frequencies (RF) and microwave frequencies. Moreover, the absorption exhibited by the composite material is significantly stronger than would be expected from its constituent parts. Other advantages are that the electrical and mechanical properties of the material may be individually tailored within a broad range by appropriate formulation of the constituent parts. The material can for example be prepared in the form of a solid, plastic, elastic, foam or liquid.

Component materials

The composite lossy material according to the invention contains a ferrimagnetic material (preferably material FF) and/or an alkyl or aryl ester, together with a ferromagnetic material which is a ferromagnetic metallic powder and/or a powdered ferrite:

Ferrimagnetic material

The ferrimagnetic material improves the magnetic lossy properties of final material. Suitable ferrimagnetic materials include material FF, further described below, and in particular material FF1 described in Example 1.

The ferrimagnetic material, if used, should be present in the lossy material in amounts of from 10 to 90 %, preferably from 15 to 80 %, more preferably from 20 to 70 % , by weight of the material.

Ester

The alkyl or aryl ester, as used herein, is an alkyl or aryl ester of an inorganic acid such as phosphoric acid.

Preferred esters include mono-, di- and tri-alkyl esters, particularly mono-, di- and tri-alkyl esters of phosphoric acid. Preferred are tri-alkyl esters of phosphoric acid, in particular the tri-d-₆-alkyl esters, preferably tri-C₃-₅-alkyl esters. Most preferred is the ester tributyl phosphate.

The ester, if used, should.be present, in the lossy material in amounts of from 0.1 to 25 %, preferably from 1 to 15 %, more preferably from 2 to 10 % , by weight of the material. The ester modifies the electromagnetic properties of the material during physico- chemical processing, whereby the processed product shows improved magnetic and dielectric lossy properties compared with the additive effects expected from the constituent parts of the material. In particular, the esters improve the dielectric properties, especially dielectric losses, of the material. In other words, the ester, when combined and processed with the other components of the lossy material, provides a synergistic effect in terms of the magnetic and dielectric properties of the processed material.

Ferromagnetic material

Suitable ferromagnetic metallic materials include powders of iron, carbonyl-iron, other ferromagnetic metals such as cobalt and nickel, and metallic alloys thereof such as superalloys and permalloys. The preferred ferromagnetic metallic material is iron powder.

The ferromagnetic metallic powder, if used, should be present in the lossy material in amounts of from 5 to 95 %, preferably from 10 to 90 %, more preferably from 20 to 85 %, by weight of the material.

Preferably, the powder has an average particle size in the range from 1 to 500 micrometers, more preferably 1 to 200 micrometers, most preferably 5 to 100 micrometers.

Suitable ferrites in powder form may be selected from iron ferrite, copper ferrite, manganese ferrite, zinc ferrite, nickel ferrite, and mixtures thereof. Preferred is zinc ferrite or manganese ferrite.

It will be understood that although some ferrites show ferrimagnetic order, this effect relates to a short range interaction (intramolecular level) only. The macroscopic effects in both ferri- and ferromagnetic order ferrites are indistinguishable, particularly when in the powdered state, and accordingly these ferrites are all herein referred to simply as ferromagnetic materials.

The ferrite powder, if used, should be present in the lossy material in amounts of from 10 to 90 %, preferably from 15 to 80 %, more preferably from 20 to 70 %, by weight of the material.

Material FF

As mentioned above, the preferred ferrimagnetic material is a material herein referred to as 'material FF'. Material FF may be synthesised as follows:

A soluble iron salt or mixture of soluble iron salts is dissolved in distilled water. After dissolving the iron salt(s), an unsaturated fatty acid is added and mixed. The resulting solution is heated, preferably to a temperature of about 95 °C, and then a strong base is added whilst continuously mixing the heated solution, so as to form a precipitate of the reaction product.

The precipitate is filtered off and washed, preferably with distilled water at room temperature, then an extractant solvent is added to dissolve the product completely and thereby form a colloidal solution.

Acetone is added to the colloidal solution in order to precipitate out a solid phase sediment. The sediment is separated out magnetically, by applying a magnetic field, and the diamagnetic liquid phase removed. The separation procedure may be repeated as necessary.

The final precipitate is preferably washed with acetone, and dried. A ferrimagnetic material FF may thus be obtained as a solid. Suitable soluble iron salts which can be used include iron (HI) chloride, iron (H) sulfate, Mohr salt, iron (II) nitrate, iron (HI) nitrate, and mixtures thereof. Preferably, a mixture of the iron salts FeCl₃.6H₂O and FeSO .7H₂O is used.

Suitable unsaturated fatty acids which can be used are oleic acid and arachidonic acid, preferably oleic acid.

Suitable strong bases which can be used are aqueous ammonia, sodium hydroxide and potassium hydroxide, preferably aqueous ammonia.

Suitable extractant solvents include extractant naphtha, mineral spirits, liquid aliphatic hydrocarbons, liquid aromatic hydrocarbons, straight chain, branched or cyclic hydrocarbons, liquid aliphatic derivatives of benzene and/or liquid hydrogenated derivatives of aromatic hydrocarbons. Preferably, extractant naphtha is used.

The ferrimagnetic material FF is an electrically non-conductive, solid composite of non-stoichiometric substance. The magnetic domain structure of this material is such as to allow the preparation of composite materials, according to the invention, that have improved magnetic lossy properties.

Optional ingredients:

The composite lossy material according to the invention may further comprise an elastomer. Suitable elastomers include styrene-butadiene copolymer, polychloroprene (neoprene), nitrile rubber, butyl rubber, polysulfide rubber, cis-l,4-polyisoprene, ethylene-propylene terpolymers (EPDM rubber), silicone rubber, polyurethane rubber. A preferred elastomer is.Kraton C^® (ex Shell), a styrene-butadiene elastomer.

Elastomers may be present in the lossy material in amounts of up to 50 %, preferably from 0.1 to 40 %, more preferably from 0.5 to 35 %, by weight of the material. The composite lossy material according to the invention may further comprise a mineral foam or inorganic polymer foam. Preferred polymer foams are polyurethane foams, for example Irpur E - 33 A^® (ex Alfa System) and Irpur E - 33 B^® (ex Alfa System), and polystyrene foams.

Polymer foams may be present in the lossy material in amounts of up to 99 %, preferably from 5 to 95 %, more preferably from 5 to 50 %, by weight of the material.

If necessary, the composite lossy material according to the invention may further comprise a plasticiser. Suitable plasticisers include aromatic and aliphatic liquid hydrocarbons, mineral and synthetic oils, polyols and esters. A preferred plasticiser comprises lower aromatic fraction hydrocarbons having a vapour point of from 150 to 250 °C.

Plasticisers may be present in the lossy material in amounts of up to 20 %, preferably from 0.05 to 20 %, more preferably from 0.5 to 10 %, by weight of the material. It will be appreciated that if an alkyl or aryl ester is present in the material as one of the essential components, this may already provide a sufficient plasticising effect for the purposes of processing or forming the material, thus eliminating the need for an additional plasticiser.

Processing

To prepare the composite material, the essential components selected from the ferrimagnetic material and/or ester, and ferromagnetic material, are mixed together with the optional components such as elastomer and/or plasticiser. After or during mixing, the material is processed by treatment for a period at a temperature and pressure sufficient to achieve a synergistic improvement in at least one of the magnetic lossy and dielectric properties of the material (compared to the constituent components).

Preferably, the mixing and processing are effected in an extruder, such as are commonly known and used in the art. Alternatively, the components may be mixed initially before being extruded, in which case the processing may be effected either during the mixing or the extrusion step.

Suitably, the material is processed by treatment at a temperature in the range from room temperature to 160 °C and a pressure in the range from 5 to 300 atmospheres, for a period of up to 10 minutes.

When the material comprises a ferrimagnetic material combined with powdered ferrite, the material is preferably processed by treatment at a temperature in the range from room temperature to 160 °C and a pressure in the range from 50 to 300 atmospheres, for a period of up to 10 minutes.

When the material comprises ferromagnetic metallic powder, the material is preferably processed by treatment at a temperature in the range from room temperature to 110 °C and a pressure in the range from 50 to 300 atmospheres, for a period of up to 10 minutes.

. Depending on the nature and quantities of the particular components used in accordance with the invention, the composite material may be formed in any desired state, for example as a solid, foamed solid, semi-solid, semi-liquid, gel or liquid. The material, when semi-solid, semi-liquid, gel or liquid, may be produced to any suitable consistency or viscosity, for example in the form of a paste or putty, according to the desired end use or application.

The invention allows the production of lossy materials exhibiting excellent magnetic and dielectric lossy properties. Moreover, the invention allows the properties of the final material to be varied by altering any one or more of the processing conditions, components, and relative amounts of the components, so as to enable the properties of the final material to be tailored according to the desired end use or application. Accordingly, composite materials according to the present invention can be prepared that exhibit strong broadband absorption of electromagnetic energy within radio frequencies (RF) and microwave frequencies.

The present invention will be further illustrated by reference to the following, non-limiting, examples:

EXAMPLES

Example 1

A ferrimagnetic material was synthesised as follows:

562g of FeCl₃.6H₂O and 318g FeSO₄.7H₂O were placed in a vessel, and dissolved in distilled water. After dissolving both salts, 210 cm³ oleic acid was added and mixed. The resulting solution was heated to a temperature of about 95 °C, and then 1290 cm of 12.5% aqueous ammonia was added whilst continuously mixing the heated solution, so as to form a precipitate.

The precipitate was filtered off and washed with 5 aliquots of 0.5 dm³ distilled water at room temperature. After pouring off the last aliquot, sufficient naphtha extractant was added to dissolve the product completely and thereby form a colloidal solution.

Acetone was added to the colloidal solution in order to precipitate out a solid phase sediment. The sediment was separated out magnetically, by applying a magnetic field, and the diamagnetic liquid phase removed. The separation procedure was repeated as necessary.

The precipitate was washed once again with 50 cm³ acetone, and dried. A ferrimagnetic material ("material FF1") was obtained as a solid.

Example 2 A composite material was synthesised as follows:

20 g of material FF1 was added to a mixer with 72.4 g of powdered ferrite, 5.33 g of Kraton C^® elastomer, 1.75 g of lower aromatic fraction hydrocarbon (vapour point ~218°C), and 0.48 cm³ of tributyl phosphate. After mixing, the material was extruded at a temperature of ~120°C and pressure of ~200 atmospheres. A solid composite lossy material ("material Al") was obtained.

Material Al demonstrated markedly higher magnetic permeability, magnetic and dielectric loss than its constituent parts, and exhibited effective broadband absorption of electromagnetic energy.

Example 3

A composite material was synthesised as follows:

23.13 g of material FFl was added to a mixer with 74.8g of powdered ferrite. 3.17g of aromatic hydrocarbon (toluene), and 2.22g of tributyl phosphate. After mixing, the material was extruded at room temperature and pressure of 200 atmospheres. The mixture was then dried in order to remove toluene.

A solid composite lossy material ("material A2") was obtained.

Material A2 demonstrated markedly higher magnetic permeability, magnetic and dielectric loss than its constituent parts, and exhibited effective broadband absoφtion of electromagnetic energy.

Example 4

A composite material was synthesised as follows:

72.5 g of Kraton C^® elastomer was mixed with 28.5 cm³ of tributyl phosphate. The mixture was heated to a temperature of 80 °C under continuous stirring. When the mixture became semi-solid, 1.70 kg of 25 μm (average particle diameter) iron powder and 1.2 kg of FFl was added and mixed in a nitrogen atmosphere. After mixing, the resultant mixture was extruded at 80°C under pressure ~50 atmospheres and allowed to cool.

A solid composite lossy material ("material A3") was obtained. Material A3 demonstrated markedly higher magnetic permeability, magnetic and dielectric loss than its constituent parts, and exhibited effective absoφtion of electromagnetic energy.

Example 5

A composite material was synthesised as follows:

9.13 g of material FFl was added to a mixer with 12.8 g of powdered ferrite (25μm) and 12.3 g of powdered ferrite (60μm), and 0.95 g of butyl phosphate added. After mixing, the material was extruded at room temperature and pressure of ~200 atmospheres. Then the mixture was added into 4.54 g of pur E-33B^® (Alfa System) and mixed together. 4.69 g of hpur E-33A^® (Alfa System) was added and stirred intensively and the mixture poured into a cast of the desired shape.

A foam solid composite lossy material ("material A4") was obtained.

Material A4 demonstrated markedly higher magnetic permeability and dielectric loss than its constituent parts, and exhibited effective broadband absoφtion of electromagnetic energy.

Example 6

A composite material was synthesised as follows:

165.20 g of powdered ferrite was added to a mixer with 7.20 g of Kraton C^® elastomer, 2.86 g of tributyl phosphate, and 20.00 g of toluene. After mixing, the material was slowly dried at room temperature until the smell of toluene has disappeared.

A solid composite lossy material ("material A5") was obtained. Material A5 demonstrated markedly higher magnetic permeability and dielectric loss than its constituent parts, and exhibited effective broadband absoφtion of electromagnetic energy.

Results:

Example A:

A comparative material K-250 was prepared from a mixture of 165.2 g of powdered ferrite with 7.2 g of Kraton C^® elastomer, extruded at a temperature of 140° C and pressure of ~200 atmospheres.

The magnetic permeabilities m\ m" of material A5 of Example 6 and material K-250 were measured over a frequency range of 1 MHz to 1 GHz only. The results are shown in Figure 1.

From these results it may be seen that the presence of the ester in material A5 in accordance with the invention leads to a substantial synergystic increase in the magnetic permeability.

Example B:

The magnetic permeabilities m', m" of materials Al and A2 of Examples 2 and 3 were measured over a frequency range of 1 MHz to 1 GHz only, and compared with those measured for a commercially available composite material fsm-250 (a mixture of FSM- 250 powdered ferrite with Kraton C^® elastomer). The results are shown in Figure 2.

From these results it may be seen that the presence of ferrimagnetic material FFl in the materials Al and A2 in accordance with the invention leads to a substantial synergystic increase in the magnetic permeabilities.

Claims

CLAIMS:

1. A composite lossy material comprising:

(i) a ferrimagnetic material FF or an alkyl or aryl ester of an inorganic acid, or both; and

(ii) a ferromagnetic material, wherein the ferromagnetic material is a ferrite material or a feπomagnetic metallic material, or both.

2. A composite lossy material according to claim 1 wherein the ferrimagnetic material FF is powdered material FF 1.

3. A composite lossy material according to claim 1, wherein the ester is an alkyl ester of phosphoric acid, preferably tributyl phosphate.

4. A composite lossy material according to claim 1, wherein the ferromagnetic ferrite material is powdered ferrite.

5. A composite lossy material according to claim 1, wherein the feπomagnetic metallic material is powdered iron.

6. A composite lossy material according to any preceding claim, wherein component (i) comprises ferrimagnetic material FF and an alkyl or aryl ester.

7. A composite lossy material according to any of claims 1 to 6, wherein component (ii) comprises feπomagnetic ferrite material.

8. A composite lossy material according to any of claims 1 to 6, wherein component (ii) comprises feπomagnetic metallic material.

9. A composite lossy material according to any preceding claim, further comprising an elastomer.

10. A composite lossy material according to any preceding claim, further comprising an organic polymer or mineral foam.

11. A composite lossy material according to any preceding claim, further comprising a plasticiser.

12. A process for preparing a composite lossy material comprising mixing:

(ii) a feπomagnetic material, wherein the feπomagnetic material is a ferrite material or a feπomagnetic metallic material, or both, and extruding the mixture.

13. A process according to claim 12, further comprising mixing an elastomer.

14. A process according to claim 12, further comprising mixing a plasticiser.

15. A process according to claim 12, wherein the mixture is treated, during mixing , extrusion, or both, at a temperature in the range from room temperature to 160 °C, at a pressure in the range from 5 to 300 atmospheres, for a period of up to 10 minutes.

16. A process for preparing a feπimagnetic material FF comprising: forming an aqueous solution of one or more soluble iron salts with an unsaturated fatty acid, heating the resulting solution, and adding a strong base so as to. form a precipitate, and filtering off and washing the precipitate, characterised in that the washed precipitate is dissolved in an extractant solvent to form a colloidal solution, and acetone is added to the colloidal solution in order to precipitate out a solid phase sediment.

17. A process according to claim 16, wherein the sediment is separated out magnetically, by applying a magnetic field, and the diamagnetic liquid phase is removed.

18. A process according to claim 17, wherein the separated sediment precipitate is washed with acetone.

19. A process according to claim 18, for preparing a ferrimagnetic material FF, comprising: forming an aqueous solution of FeCl₃.6H₂O and FeSO .7H₂O with oleic acid, heating the resulting solution to a temperature of about 95°C, and adding aqueous ammonia so as to form a precipitate, and filtering off and washing the precipitate with distilled water at room temperature, characterised in that the washed precipitate is dissolved in extraction naphtha to form a colloidal solution, and acetone is added to the colloidal solution in order to precipitate out a solid phase sediment.

20. A process according to claim 19, wherein the sediment is separated out magnetically, by applying a magnetic field, and the diamagnetic liquid phase is removed, and the separated sediment precipitate is washed with acetone.