Means for suppressing reflection of electromagnetic radiation

Abstract

Composites are produced which are adapted to cover structures which are to be shielded from detection by electromagnetic radiation. The composites are in the form of a substrate, such as Mylar film or the like, having on at least one surface thereof a layer of ferrite having dispersed therein discrete particles of a ferromagnetic metal or ferrite, the composite having a high dielectric constant and a high magnetic permeability. The orientation of the particles is such that their planes are parallel to each other and to the plane of the first said ferrite. The particle thickness is substantially less than the electromagnetic skin depth of the particles and the length of the particles is substantially less than the wave length of a predetermined electromagnetic radiation.

Description

DESCRIPTION

1. Technical Field

The invention relates to means for suppressing the reflection of electromagnetic radiation from structures, comprising the formation of composites having high dielectric constants and high magnetic permeability, and the application of such composites to the structures.

2. Background Art

That suppression of reflection of electromagnetic radiation, and the like, as for example, that represented by radar waves, is important, particularly in its application to military aircraft, missiles, and the like, is self evident. Attempts to effect such suppression by coating or otherwise covering or enveloping the structure to be shielded have not been wholly successful for various reasons, stemming in part, at least from the unavailability of coatings which would be effective over a wide range of electromagnetic frequencies, which would be easily applicable to the surfaces of e.g., an aircraft, and which would be lightweight and thin enough so as not to unduly interfere with the aerodynamic characteristics of the aircraft, missile, or the like. Ceramic materials having a magnetic property have been used for such purpose by gluing specimens in the form of little tiles to the surface of the aircraft; however, these are extremely heavy, expensive, difficult and time-consuming to apply, and, when applied, the volume and the mass of the applied materials is such that it affects the aerodynamic characteristics of the aircraft.

Prior art discloses, for example, a composite of thin ferromagnetic metallic platelets, embedded in a matrix of insulating dielectric material such as SiO as set out for example, in U.S. Pat. No. 3,540,047 to Wasler, et al., issued Nov. 10, 1970. Composites as described in this patent are limited in utility for purposes of effectively suppressing reflection of electromagnetic radiation from a structure covered with such, because of the low values of both the dielectric constant and the magnetic permeability of the matrix material used in the composite. By way of further background, reference is made to R. W. Cohen, G. D. Cody, M. D. Coutts, and B. Abeles, Physical Review B, Vol. 8, #8, pages 3689-3701, 1973, and J. C. Maxwell-Garnett, Philosophical Transactions of the Royal Society of London, vol. 203, pages 385, 1904.

3. Disclosure of the Invention

My invention is directed to a method and means for producing unique composites of two or more materials, characterized in that the resulting composite has high values of dielectric constant (epsilon) and magnetic permeability (mu), and is penetrable to electromagnetic radiation in the milliliter to several meter wavelength range. The composites, in one preferred form, consist of thin platelets or the like, of one or more ferromagnetic metals, or ferrites, in a matrix of another ferrite. Geometrically, an illustrative composite may consist of a thin film of the matrix material, with the platelets dispersed therethrough, the orientation of the platelets being such that their planes are parallel to each other, as well as to the plane of the thin film matrix. Furthermore the thickness of the platelets (the small dimension) is preferably substantially less than electromagnetic skin depth of the platelet material, and the lengths in the other two dimensions of the platelets are substantially less than the wavelengths of the electromagnetic radiation which the composite is to shield against.

BRIEF DESCRIPTION OF THE DRAWINGS

My invention will be better understood by reference to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a plan view of one embodiment of a composite, showing its geometry, with the metal platelet components deposited with aid of a physical mask.

FIG. 2 is a side view of the composite of FIG. 1 taken along the line A--A, looking in the direction of the arrows.

FIG. 3 is a plan view of one of the metal platelets of FIG. 1.

FIG. 4 is a side view of the platelets of FIG. 3.

FIG. 5 is a plan view of another embodiment of my invention showing the geometry of a composite produced by a laser scribing a stack of alternating ferrite/magnetic metal layers.

FIG. 6 is a side view of the composite of FIG. 5 taken along the line B--B looking in the direction of the arrows.

FIG. 7 shows a side view of the ferrite platelets of FIG. 6.

FIG. 8 is a plan view of a metal platelet of FIG. 6.

FIG. 9 is a side view of one of the metal platelets of FIG. 6.

ILLUSTRATIVE EMBODIMENT OF THE INVENTION

Referring now to the drawings and in particular to FIGS. 1-4.

This form of my invention is prepared by depositing a pattern of platelets 11 of magnetic metal, for example permalloy, a nickel-iron alloy, onto a film 12 such as Mylar, which has had deposited on it a layer of ferrite 13, forming a matrix. While film made of Mylar--a tradename for a polyester of glycol and terephthalic acid--is presently preferred as the film of choice, it will be understood by those skilled in the art that other polyester films, as well as films of polyethylene, polyimide, and the like may be used. In this embodiment the ferrite layer 13 is deposited as a continuous layer onto film 12, using the method of deposition known as sputtering. Following this, a perforated mask (not shown) is placed between the magnetic metal target in the magnetron and the ferrite-coated film 12, and platelets 11 of the magnetic metal, are deposited on ferrite 13 of the matrix. Perforations in the mask dictate the size, shape, and number of platelets of magnetic metal which pass through and are deposited onto the ferrite-coated film forming the matrix. This procedure is repeated until the desired number of alternate layers of ferrite 13 and metal platelets 11 are obtained. As shown in the drawing, the thickness of the ferrite material separating the metal platelets should be as small as possible, as for example, about .about.100 A. The restriction on thickness being that the metal platelets remain electrically insulated from one another.

As shown in FIGS. 5-9, a composite in accordance with my invention can also be produced by depositing alternating layers (that are continuous) of the ferrite and magnetic metal onto a film 12a, such as Mylar, by sputtering. After deposition, this relatively large area of alternating layers is laser-scribed (i.e., cut into small stacks of platelets of alternating ferrite 14/magnetic metal 15) by a finely focused laser beam. This method leaves the planes of metal platelets 15 separated by relatively magnetic ferrite 14, while the edges are separated by a narrow air gap 16. This air gap volume can have a negligible effect on performance if the total gap volume is small relative to the total volume of the composite structure.

The materials used in my invention are selected from those which meet certain optimum characteristics of magnetic permeability and dielectric constants, as will be evident from the disclosures which follow.

Consider a layer of my composite material applied to a metal surface. If .epsilon. and .mu., for the composite, are not equal, there will be a reflection from the first surface (the air/composite interface). Further, any radiation incident on the second surface (the composite/metal interface) will be reflected back towards the first surface. In order that none of this second reflected wave escape back into space at the first interface, the absorption in the medium should be great enough to absorb all of the transmitted wave during its "round trip" through the thin film composites. Therefore, to reduce the initial reflection, the ratio of magnetic permeability to dielectric constant (.mu./.epsilon.) should be as close to 1 as possible. In order to reduce the intensity of the wave reflected from the second surface, the attenuation in the medium should be as high as possible. While metals have very high absorption, the ratio of (.mu./.epsilon.) is generally substantially less than 1, even for magnetic metals wherein .mu. is substantially greater than 1. For dielectric materials the .mu./.epsilon. ratio is much more favorable, but the absorption is generally very small.

By producing composites of magnetic metal particles dispersed in a dielectric (or magnetic dielectric-ferrite) the effective magnetic permeability may be maintained fairly high due to the magnetic materials, while the effective dielectric constant may be reduced relative to a metal but remain much higher than in a dielectric. Since radar waves are in the range of 2 cm to about 2 m, it is only necessary that the linear dimensions of the magnetic particles be substantially less than a couple of centimeters. Furthermore, for waves which are near normal incidence the particles need only be flat platelets with their normals parallel to the normal of the plane of incidence. This geometry also leads to very small depolarization (or demagnetization) values. 1/2 cm diameter circular platelets layered in a ferrite matrix can be fabricated readily by use of the sputtering techniques, previously referred to. For the greatest effect, ferrites having high magnetic permeability and high dielectric constants are used for the matrix material while metals having a high magnetic permeability, such as permalloy, are used for the inclusions, e.g. the platelets. Selection of appropriate ferrites and metals can be facilitated by use of the following equations. ##EQU1## where (.epsilon., .mu.) refer to the composite, while (.epsilon..sub.m, .mu..sub.m) and (.epsilon..sub.f, .mu..sub.f) refer to the magnetic metal and ferrite materials respectively. Also X.sub.v is the volume fraction of magnetic metal in the composite and L is the shape depolarization factor for the metal platelets.

Essentially, one would like high values for .epsilon..sub.f, .mu..sub.f, .epsilon..sub.m, and .mu..sub.m. In practice, the most difficult constraints are .epsilon..sub.f and .mu..sub.f. Ferrites with high .epsilon..sub.f and .mu..sub.f at frequencies of or greater than 100 MHz are rare, with the best condidates appearing to be the manganese-zinc ferrites and the nickel-zinc ferrites. The manganese-zinc ferrites have the advangtage of having a large absorption also.

It should also be noted that according to equations 1-A and 1-B, .epsilon. or .mu. may become extremely large if the denominators on the right hand side of the equations become close to or equal to zero. In optics this is known to happen to .epsilon. for certain values of .epsilon..sub.m and .epsilon..sub.f (i.e. when ##EQU2## and is known as the dielectric anomaly. The wavelength at which the anomaly occurs can be predicted if the wavelength dependences of .epsilon..sub.m and .epsilon..sub.f are known. An analogous magnetic anomaly should occur if ##EQU3## Such a magnetic amomaly would significantly enhance the radar reflection suppression properties of the composite over a range of wavelengths. This enhancement is, of course, over and above the already superior properties of the composite.

My novel composites in that form in which the components have been deposited on a film such as Mylar, may be applied to the structure, such as the skin of an aircraft, by means of adhesive, including adhesive layers applied to the uncoated surface of the Mylar film, or in any other suitable way as those skilled in the art will understand.

An important advantage of my invention is that the composites are substantially lighter in weight than magnetic dispersion paints or tiles which are presently used on aircraft for purposes of shielding the aircraft from radar detection. The advantages of using my composite over a composite made by dispersing ferromagnetic platelets in a dielectric are two-fold, namely, an overall increase in effective .epsilon. and .mu. due to the high .mu. and .epsilon. of the ferrite matrix, and manifestation of a magnetic resonance, similar to the well-known Maxwell-Garnett dielectric anomaly in optics, providing great enhancements in .mu. over a selected wavelength range.

In the foregoing specification I have described preferred forms of my invention in their application as suppressants of reflection from electromagnetic radiation, when applied to such structures as aircraft, missiles and the like. However those skilled in the art will appreciate that my invention may be modified in various ways, only some of which may have been specifically mentioned herein, and may be applied to different types of structures, all without departing from the spirit and scope of my invention. Therefore, it is to be understood that my invention is not limited to the specific forms described herein or in any manner other than by the scope of the appended claims when given the range of equivalents to which my invnetion may be entitled.

Claims

1. A composite for covering a structure whereby to suppress reflection of electromagnetic radiation from said structure comprising a sheet or film having deposited on at least one surface thereof, by sputtering, alternating layers of a ferrite and of a member selected from the group consisting of ferrites other than the first said ferrite and magnetic metals whereby to form a composite having a high dielectric constant with a high magnetic permeability, the layered product being in the form of stacks of platelets, the planes of metal platelets being separated by planes of relatively magnetic ferrite, with the stack edges being separated by a relatively narrow air gap, the said planes being parallel to each other, the thickness of the platelets being substantially less than the electromagnetic skin depth of the platelets, and the length in the other dimensions of the platelets being substantially less than the wavelength of a predetermined electromagnetic radiation.

2. The product of claim 1 wherein at least one of the said ferrites has relatively high.epsilon..sub.f and.mu..sub.f at frequencies greater than about 100 MHz.

3. The product of claim 1 wherein the first said ferrite is a manganese-zinc ferrite.

4. The product of claim 1 wherein the first said ferrite is a nickel zinc ferrite.

5. The product of claim 1 wherein the said magnetic metal is a nickel-iron alloy.

6. The product of claim 1 wherein the spaces between said stacks have been effected by laser scribing.

7. The product of claim 1 wherein the thickness of the ferrite layer separating the platelets is from about 10-100 A.

8. The product of claim 1 wherein the sheet or film is a polyester of glycol and terephthalic acid.

9. The product of claim 1 wherein the dielectric constant of the magnetic metal is ##EQU4## and the magnetic permeability of the metal is ##EQU5## where.epsilon..sub.f is the dielectric constant of the ferrite;

.mu..sub.f is the magnetic permeability of the ferrite;

L is the shape depolarization factor for the metal platelets; and

X.sub.v is the volume fraction of magnetic metal.