WO2003098989A1

WO2003098989A1 - Microwave absorbant material

Info

Publication number: WO2003098989A1
Application number: PCT/GB2003/002037
Authority: WO
Inventors: Jason Michael Fallon
Original assignee: Qinetiq Limited
Priority date: 2002-05-18
Filing date: 2003-05-13
Publication date: 2003-11-27
Also published as: GB0211443D0; AU2003230015A1

Abstract

An anisotropic microwave absorbent material comprises a matrix material containing aligned magnetic material flakes. The matrix may be of epoxy resin or other electrically insulating material; the flakes may of FeCoSi, FeZr, FeCoB etc. Typical flake dimensions are up to 10 µm (e.g. 1 µm) thick and 10 to 1000 µm maximum in plane length. The flakes are typically about 5 % by volume in a range of 1 to 50 %. The composite's permittivity, and permeability are strongly orientation and aspect ratio dependent and can be composition dependent and these may be changed to vary the wavelength of maximum microwave absorption.

Description

Microwave absorbent material

Technical field.

This invention relates to a microwave absorbent material, to a method of producing such a material, and to an article coated with such a material.

Mobile phones transmit and receive signals at very narrow frequency intervals between 400 MHz and 2 GHz. The radiation may have undesirable side effects on entering the body that may be intensified by the fact that the mobile phone is located very close to the body. Reducing the user to exposure from this radiation may prove to be advantageous. Other requirements for a lightweight V-UHF microwave absorber are from antennas that broadcast signals at frequencies < 3GHz which require extraneous reflections to be suppressed to improve the signal/noise ratio.

Background

There are several materials used to absorb microwave radiation at V-UHF frequencies. Many are magnetic composites that contain a magnetic component (mostly in the form of spherical magnetic particles) dispersed in an insulating matrix such as a paint or polymer. In order to obtain a composite with a sufficiently high microwave permeability μ' and magnetic loss μ" to yield high absorption values at low microwave frequencies, the amount of necessary magnetic component makes coatings or layers heavy. Typically required magnetic volume fraction can exceed 40% thus incurring an undesirable weight penalty. This is especially so for absorbers that are required to operate at frequencies below several GHz which are often required to be at least 15-20 mm thick leading to large area densities. Examples of known absorbers are described in: EP-0884739, EP-0380267, WO-01/41255 It is an object of this invention to produce an absorber that is not only of a lower density than existing absorbing materials but also is a material that has an narrow absorption band width that may be readily tailored and tuned to different frequency ranges.

It is yet another object of this invention to produce an absorber, which has a broad absorption band.

Summary of the invention

The present invention overcomes this weight problem by the use of aligned magnetic ^■ flakes dispersed in an electrically insulating matrix material forming an anisotropic composite material that is capable of displaying the Ferromagnetic Resonance FMR effect. Flakes are thin items with regular or irregular shapes having maximum dimensions much greater than their thickness, and are distinguished from fibres, which have a regular cross section (such as circular) much smaller than their length.

According to this invention a method of producing a lightweight microwave absorber includes the steps of:

providing numerous flakes of a magnetic material, mixing the flakes into a liquid matrix material, aligning the flakes with a magnetic field, until the flakes are aligned in a range between parallel to one another and up to about 40° away from parallel, solidifying the matrix,^" so that the absorber has numerous flakes aligned in one or more parallel planes electrically isolated from one another.

According to the invention a microwave absorbing anisotropic composite material comprises: an electrically insulating matrix material carrying numerous magnetic material flakes arranged in one or more parallel planes electrically insulated from one another, the flakes being aligned in a range between parallel to one another and up to about 40° away from parallel. According to the invention a microwave absorber comprises: a shaped bulk material formed by an electrically insulating matrix material carrying numerous magnetic material flakes arranged in one or more parallel planes electrically insulated from oneanother, the alignment of the flakes being in the range from substantially perpendicular to the direction of travel of the microwaves to be absorbed to 40^s from the perpendicular.

Preferably the magnetic field is a rotating magnetic field or fields.

Preferably the absorber has a metallic layer backing to reflect microwaves back through the bulk material, and the bulk material has an electrical thickness of one- quarter wavelength measured at the wavelength of maximum absorption.

The anisotropic composite may contain aligned flakes with planes parallel to the sheet surface so they have substantially higher in-plane permeability per unit weight than for composites containing metal spheres; high in-plane permeability//' /magnetic loss //" assist the absorption of electromagnetic waves that propagate through the composite in a direction normal to the composite's surface. Circular flakes are beneficial as they do not discriminate between incident microwaves with different polarisations. Anisotropic flake composites may also have higher//', //" than composites of the same density containing fibre particles having their lengths contained in planes parallel to sheet surface.

The shaped bulk anisotropic composite may be of uniform or varying thickness as required. The alignment of the flakes may be parallel to the contours of the shaped article or varying over longer distances.

The flakes may be planar or substantially planar and thin, e.g. less than 10 /m thick, typically around 1//m or so that the flake's thickness is below the microwave skin depth at the frequency required to be absorbed, of regular or irregular shape, typically having in-plane lengths between 10/ m and 1000//m. The shape and/or size may be uniform or variable. The flake material may be in crystalline, amorphous form, or granular form. The flakes may be of one or more magnetic materials such as those with high intrinsic permeability, for example: - crystalline or amorphous Fe, Co or Ni rich alloys, or the 'granular magnetic alloys' that contain high concentrations of nano-sized Fe, Co or Ni rich crystalline grains in an electrically insulating matrix e.g. AI₂O_3ι or the classes of 'ferrite' such as Fe₃O₄ and especially those materials that would, in flake form, have an easy magnetisation direction i.e. an 'easy axis' contained within the plane of the flake.

Aligned flakes preferably have aspect ratios (in-plane length/thickness) above 10 to reduce the flake's demagnetisation fields and maximise the composite's //', //".

The flake distribution within the polymer may be uniform or variable throughout the polymer e.g. aligned flakes may have a concentration gradient normal to the surface of the material.

The flakes may all be aligned with their planes parallel to the sheet surface or there may be distribution of alignment angles up to 40°.

The flake volume fraction (percentage of flake volume to matrix volume) is typically about 5-10% but it may be within a range of 2 to about 50%.

The matrix material may be a thermosetting or thermoplastic material or any one of the following: epoxy resin, polyester resin, phenol, rubber, Teflon (TM), polyurethane, thermoplastics, styrene/butyl acrylate co-polymer, acrylic.

The anisotropic composite may be formed into flat sheets, then shaped e.g. by pressing into the desired shape, and polymerised or heat set. In some circumstances pressing can lead to some primitive preferred flake alignment due to lateral polymer flow in directions normal to press direction. Rolling calendering and extrusion of a soft solid precursor also produce some primitive degree of alignment. The polymerisation may be by UV illumination, or by inclusion of other initiators. The absorber may be in the form of a sheet of rigid or flexible material. The absorber may be formed as a thick sheet of rigid material; and subsequently machined into a required shape. The anisotropic composite when being formed in its liquid state may be held within a relatively rotating magnetic field that has field lines contained in a single plane. The magnetic field is held whilst the matrix sets into a solid material, or held until alignment is complete followed by solidification of the matrix.

Other manufacturing processing may form the microwave absorber. For example, it may be produced by forming alternate thin layers of a matrix material and a discontinuous layer of flake material. The discontinuous layer may be produced by a silk screen process or a printing process.

The anisotropic composite may be formed into a sheet of a required thickness, typically below 15 mm on a conductive metal backing to produce a microwave absorber for use at frequencies in the range 45MHz to 3 GHz.

In order to minimise weight of the flake based absorbers and controlling its absorption bandwidth it is necessary to consider the following:

When 1//m thick flakes with in-plane lengths above 100 μm, or any magnetic flake with aspect ratio (in-plane length/thickness) greater than 100, are preferentially aligned in polymer such that the magnetising fields in incident electromagnetic radiation lie in their plane, it is the composite's Ferromagnetic Resonance (FMR) that overwhelms the dispersion of the V-UHF permeability ' and magnetic loss //". The FMR of the composite decides its microwave absorption characteristics.

The FMR frequency (or the 'resonance absorption frequency') of the above structured anisotropic composite is now conveniently and chiefly decided by the flake's internal magnetic structure (the magnetic anisotropy fields within the flake) because the contribution from the flake's demagnetisation field to FMR has been reduced. This means that the composite's FMR and hence absorption characteristics may be changed simply by changing the flake's composition or adjusting its internal microstructure. For other composites, containing randomly orientated flakes, especially those containing flakes with aspect ratios (in-plane length/thickness) <10 (i.e. when the in- plane demagnetisation factor N_D of the flake is appreciably below 1 ) flake composition can be less important in deciding the composite's FMR character. This is because the FMR can become dominated by the flake's demagnetisation field rather than the other sources of magnetic anisotropy field within the flake.

For the above anisotropic composites containing suitably aligned flakes with aspect ratios above -100, the composite's resonance absorption may be tuned to any selected frequency by change of the flake's composition whilst minimising the necessary weight of the composite.

The above suitably structured composites can offer advantages over current absorbers as follows:

• Reduced weight.

•Over 95% vol. of the composite is polymer giving the composite added environmental stability.

• Lower metal volume fractions improve the absorbers mechanical properties. "Absorption properties are easily tailored by changes in the flake's composition and the flake's orientation/aspect ratio. •A practical, inexpensive manufacture route is feasible.

Brief description of drawings.

The invention will now be described, by way of example only, with reference to the accompanying drawings of which: -

Figure 1 is a diagrammatic cross section of an absorber;

Figure 2 is a view of apparatus for making the absorber material;

Figure 3 is a view of an alternative to Figure 2;

Figure 4 is a view of another alternative to Figure 2;

Figure 5 is a micrograph showing a cross section of the absorber at a low magnification with well aligned flakes;

Figure 6 is a micrograph showing the cross section of Figure 4 at a higher magnification;

Figure 7 is a micrograph showing a cross section of the absorber aligned in a lower magnetic field than that of Figure 4 and hence a less well aligned arrangement of flakes;

Figure 8 is a micrograph showing a cross-section of the anisotropic composite with partial alignment of the flakes;

Figure 9 is a micrograph showing a cross-section of the anisotropic composite with a random, non-aligned, arrangement of flakes; Figure 10 is a graph of reflectivity against frequency for three different thickness of anisotropic composite with on a metal backing showing selective absorption;

Figures 11a, b show coaxial in-plane permeability /¹, and magnetic loss/ " measurements from four different composites containing aligned FeCoSi alloy flakes at the same concentration (5% volume). Flake aspect ratios in the composites are as follows: A- 300 to 500; B- 150 to 300; C- 63 to 75; D- 38-45; E- 20-38;

Figures 12a, b show in-plane permeability/ ', and magnetic loss//" dispersion curves from an anisotropic composite effective density of 1.49 gem^"3 containing aligned FeCoSi flakes with aspect ratios 20-100;

Figures 13a, b show in-plane permeability//', and magnetic loss//" dispersion from ^■ composites of the same density 1.2gcm^"3 that either contain 'Well aligned', 'Misaligned' (Partial 2,3,4) or 'Randomly aligned' FeCoSi flakes;

Figures 14a, b show optical micrographs of two milled flake powders containing flakes with in-plane lengths of about 150//m and about 5-20//m respectively;

Figure 15 is a micrograph showing a cross section of the absorber formed by 1//m thick flakes in a low viscosity polymer and only partially aligned by a magnetic field of about 100 Oe; and

Figure 16 is a graph of reflectivity against frequency for three different thickness of , anisotropic composite shown in Figure 15 with a metal backing showing selective absorption.

Detailed description of embodiments.

The anisotropic composite 1 shown in Figure 1 comprises an epoxy resin matrix 2 carrying numerous magnetic flakes 3 aligned in spaced layers 4. The flakes 3 are of FeCoSi alloy of 1//m thickness and aspect ratio (diameter/thickness) >100. Ideally each flake is electrically isolated but is not a prerequisite for flakes with ferrite or granular alloy compositions. The anisotropic composite material of matrix and flakes are mounted on a metal backing of 5, for use e.g. as a mobile phone absorbing pad. The spacing 4 may be increased by increasing the field strength but has no significant bearing on the anisotropic composite's microwave properties.

Sputtered material on polymer substrates is commercially available and is currently used for soft magnetic thin film anti-theft markers. Flakes may be produced from this source of sputtered film by dissolving its substrate and breaking it into flakes by milling or grinding until the required flake size distribution is obtained. It is necessary to impart as little energy into the flake to preserve its original microstructure otherwise its intrinsic anisotropy (and the composite's FMR) will differ from that intended.

Figure 2 shows apparatus for producing an anisotropic composite of Figure 1 . It comprises a vial 7 (internal diameter 3cm) holding a mixture of 8g of liquid epoxy resin, 2g catalyst, and 2g of FeCoSi flakes. The vial 7 is suspended by a shaft 8 of an electric motor 9, and located between two large area ferrite blocks with north and south (N and S) poles made to face each other 10, 11. Magnetic field strength is variable by varying the interblock separation. Heat may be applied from a hot air gun 12. The shaft 8 is made to spin the vial 7 so flakes experience directional changing fields contained in a single plane. Rate of rotation may be used to further alter the microstructure of the final anisotropic composite through the centrifugal effect.

Figure 3 shows an alternative arrangement for aligning the flakes. This arrangement comprises two large diameter plate shaped ferrite block magnets 14, 15 (magnetised in their planes) with their planes facing each other at about 5-25mm separation. In this way the stray fields in each magnet emanating from their N poles are made to overlap. A supporting frame (to counteract magnetic repulsion) is needed and should be made of Al rather than mild steel to prevent circulating flux closure. This "magnet" produces compressed parallel field lines (contained in a plane approximately parallel to the faces of the magnet) within the gap. The gap (-10 mm across) allows a large mould 16 containing the flake-polymer mixture to be spun to achieve the preferred orientation. The mould 16 is held on the end of a spindle 17 which passes through a hole 18 cut through the centre of one of the magnets and is rotated by an electric motor 19 so that the flake mixture continually passes through the magnet. The magnetisation and separation of the two magnets 14, 15 can either be low or high to alter the field strength and thus the desired alignment.

Figure 4 shows another example of apparatus for aligning the flakes. It comprises two pairs of magnets 21 , 22 arranged with one of each pair of magnets on either side of a mould 16 containing a matrix and flake mixture. The magnetic field of each pair of magnet lies substantially parallel to the faces of the magnets as in Figure 3. The magnetic field direction 23 of the magnets 21 is about 45° to the magnetic field direction 24 of the magnets 22, although other non-zero angles can be used, together with additional pairs of magnets. The mould is arranged to be rotated by a motor (not shown) as in Figure 3. Rotation of the mould through the two magnetic field directions 23, 24, provides good alignment of flakes in a single plane parallel to the surface of the composite. The rotation may be about the mould centre 25, or may oscillate about this centre 25 to assist in flake alignment at the centre. This arrangement allows large area anisotropic sheets to be manufactured cheaply.

As alternatives to ferrite block magnets, solenoids, electro-magnets, or Helmhotlz coils may be used. Example 1.

A vial of internal diameter 4cm was partly filled with mixture of 8g of liquid epoxy resin, 4g catalyst, and 2g of FeCoSi flakes. The vial was placed on the end of the shaft between the ferrite blocks and rotated at about 40 rpm in a magnetic field of about 350 Oersteds (Oe) for about 60 minutes whilst the epoxy resin set to a solid. The mixture was heated to about 60^eC. The result was an anisotropic cylindrical block of absorber of density 1.2gcm^"3, volume fraction of flake/matrix of 5%.

Heating the epoxy resin speeds the cure time, but can introduce porosity into the final result. Such porosity may be useful in that the material density is reduced below that of the matrix. Composites may have densities ranging from about 0.6 to 2.5 gem^"3. Using stronger fields e.g. above 800 Oe makes it possible to align flakes at higher concentrations to raise permeability and density to beyond 2.5 gem^"3.

Example 2.

The material was similar to example 1 , but the flakes were added gradually to the matrix material and catalyst whilst stirring with a high shear stirrer. After complete mixing, the vial was placed in a magnetic field until the flakes had aligned to the required condition and mixture cured.

Figures 5, 6 show the results of alignment in a field of 350 Oe. The cross section optical images are of preferentially orientated layers of FeCoSi in epoxy resin. These layers are about 20//m apart, as indicated by the scale marking in the Figures 5, 6. Due to their orientation the flake edges only are visible. The higher magnification of Figure 6 shows clearly flakes in layers with individual flakes mostly isolated electrically from oneanother.

Figure 7 shows the effect of aligning the flakes of the above composition in a reduced magnetic field of about 80 Oe (c.f. the 350 Oe of Figure 5). The flakes are roughly in layers but not well aligned. Reducing the aligning field still further to about 20 Oe results in the partial alignment shown in Figure 8. Only a small amount of flakes are aligned. By comparison, Figure 9 shows a random distribution of flakes when the composite material is made without any applied magnetic field. Clearly, the small amount of field used with Figure 8 has made significant changes to the random alignment of Figure 9, but it is well short of the Figure 5 example. These Figures 5, 7, 8, 9 shows that the degree of alignment can be controlled as required to give the desired permittivity, permeability that define the absorbing characteristics merely by varying the amount of applied magnetic field.

Absorbers with a metal backing may be used as microwave absorbers in the V-UHF frequencies below 3 GHz. In these new lightweight ordered structures (e.g. Figure 1 ) flakes may have any suitable composition such that the composite's FMR frequency occurs below 40GHz (the FMR frequency is the frequency where the composite's magnetic losses are at their maximum and should be equal to the frequency of the radiation desired to be effectively attenuated). Mobile phones transmit signals at , frequencies close to 1 , 1.5 or 2.0 GHz which means that flakes should have soft magnetic compositions since their FMR is below 3GHz.

Figure 10 shows one effect of varying absorber thickness for a sample of density

1.21 gem^"3, FeCoSi alloy flakes of aspect ratios 150-300. The absorber was mounted on a metal back plate and electromagnetic parameters measured. The reflectivity for this non-optimised composite is quite low at a frequency around 1 GHz (where FMR occurs) when its thickness is about 8 mm. For a sample with an FMR at 2GHz, optimum thickness is expected to be only about 4mm. The observed performance is quite exceptional given the fact that the density of the composite is only 1.21 gem^"3 (the flake volume fraction is only 5%). Reflectivity may be further reduced by including a greater volume fraction of active aligned flakes. Reflectivity minimum may be shifted to other important communication frequencies by increasing the intrinsic anisotropy field within the flake; flake compositions like FeAIO may be useful for this. Figures 11 a, b show coaxial //', //" measurements from four different composites containing aligned FeCoSi alloy flakes at the same concentration (5% volume) so resulting anisotropic composites have a low density of 1.2 gem^"3. The magnetising fields in the coaxial line are directed along the planes of the flake in the coaxial line. Flake aspect ratios in the composites are as follows: A- 300 to 500; B- 150 to 300; C- 63 to 75; D- 38-45; E- 20-38. The 'FMR frequency' is the frequency where μ is greatest i.e. where resonance absorption is strongest. As the flake aspect ratio reduces, demagnetisation fields increase, causing permeability //' and magnetic loss / " to fall near the FMR frequency. The fall in //' and shift of the FMR frequency to higher frequencies reflect the increasing contribution from the 'shape anisotropy' field on the ', μ" dispersion curves as aspect ratio reduces. The FMR frequency for the composite containing flakes with average aspect ratios in the range 300-500 is 1.1 GHz. This is very similar to that measured directly from a thin film of the FeCoSi material supported on a Si wafer substrate using a microstripline technique, found to be 0.9GHz. This therefore demonstrates the potential the anisotropic composite has in allowing the magnetic losses to be adjusted through flake composition changes. The FMR effect is maximised (i.e. weight is minimised) when anisotropic composite contains aligned flakes with aspect ratios that exceed about 100.

The flake's composition may be controlled by sputter deposition or chemical vapour deposition or electroplating to change the FMR frequency. Flakes with suitable compositions that allow the FMR below 3GHz include high intrinsic permeability soft- magnetic alloy compositions like NiFe or amorphous FeCoSi, FeZr, FeCoB etc all having different intrinsic magnetic anisotropy's and hence would form anisotropic composites with different FMR frequencies. Many magnetic alloys containing nanometer sized crystalline Fe, Co or Ni rich grains electrically insulated in an insulating matrix like C, Si are the ceramics, have much lower electrical conductivity, and these tend to have FMR at frequencies above 3GHz. Conversely, to produce an anisotropic composite with high //" over a greater range of frequencies, to form a 'broad-band' microwave absorber, the anisotropic composite must contain a larger distribution of flake aspect ratios within about 10-200, e.g. above 5% vol of flakes, typically 20 to 25% vol. This is so that the composite has a larger distribution in anisotropy fields to spread out the FMR losses over a range of frequencies. This is shown in Figures 12a, b, but for an aspect ratio of 20-200.

To maintain a high //" over a larger frequency range the volume fraction of flake must be increased which consequently raises the composite's density.

To further tailor the composite's //', //" dispersion, and its effective permittivity ε' and dielectric loss ε", the flake alignment may be relaxed or staggered. In fact planes of flakes perfectly aligned within the surface of the anisotropic composite are not a prerequisite for a material with a high //', μ" in its plane. This is because the effective magnetising field within the plane of the flake varies with angle as a cosine function (a slow changing function) up to around 20°.

This is shown in Figures 13a and 13b, //', //" dispersion from composites of the same density 1.2gcm^"3 that either contain 'Well aligned', 'Misaligned' (Partial 2,3,4) or 'Randomly aligned' FeCoSi flakes.

Flakes may therefore have any angle <20 ° to the surface and still preserve the anisotropic composite's high effective in plane //', //". Composites can be prepared that contain flakes which are slightly misaligned very easily using weak magnetic aligning fields. Misaligned flakes also help reduce the effective permitivity of the composite.

Figures 13a, b show how, on changing from a composite that contains randomly to well-aligned flakes, the //', //" dispersion curves change accordingly with the onset of the development of the characteristic signature of a FMR i.e. rapid fall in //' over a narrow frequency range. Use of a low viscosity polymer matrix material, e.g. viscosity less than about 150 mPoise, may enhance performance as shown in Figure 16. For example, to achieve a composite with a wider- band absorption performance near say ~1 GHz (close to many mobile communication frequencies) it is necessary to select flakes with an alloy composition that provides ferromagnetic resonance absorption near this frequency. Preferably, the flakes are irregular shaped, as in Figure 14, and have an aspect ratio distribution in the range 20-100. Both these features increase the distribution in local pinning demagnetisation fields in the composite that tends to broaden the//', //" dispersion curve. Also the composite's permittivity is as low as possible to reduce unwanted reflection of radiation. This is done by ensuring flake-flake contact is suppressed (reduces electrical percolation) and that the flakes are stagger aligned with respect to the composite's surface (reduces electric polarisability).

Flakes in this configuration are shown in Figure 15, formed by irregularly shaped 1//m thick FeCoMoSiB flakes in a low-aligned configuration electrically insulated at 20 vol. % in low viscosity polymer. Flake in-plane length distribution was 10 to 100 m. This structure was prepared by dispersing flakes in an acrylic polymer and aligning using a sufficiently weak alignment field (-100 Oe), density was 2gcm^"3. The ultra-low viscosity of the polymer assists flake electrical insulation and tends to reduce unwanted porosity. Examples of this material formed as 4mm, 6mm, and 8mm thick layers with a metallic layer backing, have a reflectivity performance as shown in Figure 16; for comparison purposes the material without a metallic layer backing is also shown; note the improved reflectivity from that in Figure 10.

Claims

Claims.

1. A method of producing a lightweight microwave absorber includes the steps of:

providing numerous flakes of a magnetic material, mixing the flakes into a liquid matrix material, aligning the flakes with a magnetic field until the flakes are aligned in a range between parallel to one another and up to about 40° away from parallel, solidifying the matrix, so that the absorber has numerous flakes aligned in one or more parallel planes substantially electrically insulated from one another.

2. The method of claim 1 wherein the magnetic field is a rotating magnetic field.

3. The method of claim 1 wherein the flakes are added gradually to the liquid matrix material whilst being stirred with a high shear to give separation of individual flakes and substantially no air bubbles.

4. A microwave absorbing material comprising: an electrically insulating matrix material carrying numerous magnetic material flakes arranged in one or more parallel planes electrically insulated from one another, the flakes being aligned in the range between parallel to one another and up to 40° away from parallel.

5. The material of claim 4 wherein the flakes are of one or more materials selected from the list containing: Fe, Co or Ni based alloys and the Ferrites.

6. The material of claim 4 wherein the matrix material is a material selected from the list containing: epoxy resin, polyester resin, phenol, rubber, Teflon, polyurethane, thermoplastics, stytene/butyl acrylate copolymer, acrylic.

7. The material of claim 4 wherein the flakes are of uniform or non uniform shape, a thickness within the range of up to 10/ m, and a maximum in plane dimension between 10 /m and 1000//m.

8. The material of claim 4 wherein the amount of flakes within the matrix is in the range 1 % to 50% volume of flakes.

9. The material of claim 4 wherein the aspect ratio (in-plane length/thickness) distribution is within the range 20 to 200.

10. The material of claim 4 containing a gradient of reducing flake volume concentration toward the surface of the material.

11. The material of claim 4 wherein the flakes are aligned in the range between ' parallel to one another and up to 20^e away from parallel.

12. A microwave absorber comprising: a shaped bulk material formed by an electrically insulating matrix material carrying numerous magnetic material flakes arranged in one or more parallel planes electrically insulated from oneanother, the alignment of the flakes being substantially perpendicular from the direction of microwaves to be absorbed.

13. The absorber of claim 12 wherein the flakes comprise at least one soft magnetic composition and have thickness below the skin depth at the frequency band of maximum absorption.

14. The absorber of claim 12 wherein the shaped bulk material is of uniform or varying thickness.

15. The absorber of claim 12 supported on a conductive surface, the shaped bulk material having a uniform electrical thickness of substantially one-quarter wavelength at the wavelength of maximum absorption.

16. The absorber of claim 12 wherein the aspect ratio of the flakes is below about 200 and the absorber is a broadband absorber.

17. The absorber of claim 12 wherein the aspect ratio of the flakes is below about 200, the volume percent of the flakes is up to 30%, and the absorber is a broadband absorber.

18. The absorber of claim 12 wherein the aspect ratio of the flakes is above about 200 and the absorber is a narrow band absorber.

19. The absorber of claim 12 wherein the flakes have a distribution of alignment angles from parallel up to about 40⁹ to its surface.