US2951246A - Absorbent for electromagnetic waves - Google Patents

Description

ABSORBENT FOR ELECTROMAGNETIC WAVES Otto Haipern, Pacific ialisades, Calif., and Montgomery H. Johnson, Boston. Mass assignors, by mesne assignments. to the United States of America as represented by the Secretary of the Navy No Drawing. Fiied Jan. 30, 1946, Ser. No. 644,396

6 Claims. (Cl. 34318) This invention relates to a method and means for minimizing the reflection of radio microwave radiation of preselected wavelength incident upon surfaces and objects which normally reflect such microwaves. More specifically, the invention relates to a method for controlling the index of absorption of a resonant coating to satisfy the condition for minimum reflection of incident {ago microwave radiation impinging upon the coating.

It is known in the art how to protect good reflectors such as electrically conductive surfaces from incident radio microwaves of high frequencies, one of thesemethods being designated as the resonant method. This method consists of affixing o. high-loss dielectric material layer to the metallic surface to be protected. In this case the dielectric material must have a thickness which is an odd multiple of one-quarter of the wavelength of the incident radiation, measured inside the material, and a high-frequency loss of sufileient magnitude, which may be due either to surface treatment of the material or internal losses in it. One such material is achieved by providing a layer preferably comprising myriads of electrically conductive particles dispersed in an appropriate binder, the particles being quasi-insulated from each other. Electrically conductive particles thus arranged will impart to the whole layer a dielectric constant which will be greatly in excess of that of the binder alone. To obtain a high value of the dielectric constant, it is advisable to employ flake particles in dense concentration and so oriented in the binder that the plane of the large dimension of the flakes is parallel to the plane of the aggregate. The most generally idealized type of layer, one therefore having the highest dielectric constant, may be formed of a binder such as polystyrene containing copper or aluminum flake-like particles. Aluminum flakes generally have the most desirable characteristics for this purpose. However, the high-frequency losses with aluminum particles may be so small that the index of absorption of the layer may not be suflicicntly high to meet the condition hereinafter described. Such a material is fully disclosed and described in the copending application of Otto Halpern, Serial No. 581,179, filed March 5. 1945, now Patent No. 2,923,934.

This invention relates to a method of controlling the index of absorption of resonant layers to satisfy the conditions for minimum reflection of incident radio microwave radiation impinging upon an otherwise reflective surface. and more particularly to an interference arrangement for controlling the index of absorption of a resonant layer having a high dielectric constant.

An object of this invention is to provide a method for controlling the index of absorption of a resonant, flakepigmentcd coating of material in order to minimize rcfiection of incident radio microwave radiation from a normally reflective surface covered with such a coating.

Another object of this invention is to provide a novel method for increasing the reflection coefiicient of a resonant, flake-pigmented coating of the class described 2,951,246 Patented Aug. 30, 1960 which comprises adding ferro-magnetic particles to said flake-pigmented coating.

Still another object of this invention is to provide a novel method for decreasing the reflection coefficient of a resonant flake-pigmented coating which comprises forming said coating of alternate layers of high and low dielectric constant materials.

Other objects and advantages of this invention as Well as its construction, operation and arrangement will be apparent from the following description and claims.

Consider a plane metal plate covered by a partially conducting dielectric coating of thickness d as heretofore described. A plane electromagnetic wave propagating through this dielectric medium is described by the general expression in which E and H are the electric and magnetic fields of the wave, respectively, 1' is the imaginary quantity VI, A is the wave length of the wave, 5 is the base of the natural logarithms. and n and k denote respectively the index of refraction and absorption of the layer. To include possible eddy current effects, and other effects,

n and k are allowed to be functions of the frequency where (1 equals the thickness of the layer, n equals the index of refraction of the layer, A equals the wavelength of the incident microwave radiation, and k equals the index of absorption of the layer. The index of refraction and the index of absorption. that is, n and k, are respectively the real and the imaginary parts of the square root of the generally complex dielectric constant.

It will readily be apparent that the necessary thickness of the layer according to Equation 20 will be prohibitively large unless the layer has a large index of refraction. Since this index of refraction is approximately equal to the square root of the effective dielectric constant, it is necessary to construct a layer having a high dielectric constant. Such a material may be obtained by imbedding electrically conductive metallic flakes of a non-magnetic nature in a non-conducting dielectric. Looking at such a layer microscopically, it may be described as an arrangement of giant molecules which show a large electric movement due to the displacement of the charges in the flakes while the intervening binder plays the part of a vacuum. An arrangement of this kind will give a substance of very high dielectric constant of the order of l5 to 5600, although the mathe matical calculation of this problem is very difiicult if at all feasible. However, it is qualitatively evident that a molecule of very large dimension in the direction of the electric vector and small thickness with a correspondingly small depolarizing factor is most advantageous in the construction of such an artificial dielectric medium. As heretofore stated, such a layer has been found to have a large dielectric constant, up to 5000.

Utilizing a non-reflecting dielectric layer as heretofore described to cover a metal surface in order to reduce reflection of radio microwaves therefrom, it is apparent that it is always possible to apply the substance in sufficient thickness so as to satisfy Equation 2a. and thereby satisfy the resonance condition so far as the inneighborhood of one (1).

dex of refraction is concerned. In order to obtainthe correct index of absorption as given by the Relation 2b it is possible to proceed in two alternative ways.

If the index of absorption of a substance produced is smaller than that demanded by Equation 2b it may easily be increased by adding a small amount of a conducting material to the substance of high dielectric constant, or the losses may be increased by using a magnetic mixture. For example, ferromagnetic particles (flakes) may be added to the normally non-magnetic pigment heretofore described. If, on the other hand, the value of the index of absorption is higher than that required by Equation 2b a composite material as hereinafter described must be formed. Such a composite material preferably may be constructed in striated form with alternate films of high dielectric constant (flake-pigmented) film and low dielectric constant material such as paper disposed between the high dielectric layers. The periodic structure is composed of thin layers of substances of high dielectric constant followed by intervening thin layers of a more or less indifferent substance of a dielectric constant in the The thickness of these layers is limited so that the phase change as well as the absorption inside an individual layer is rather small. There will thus be a considerable amount of back and forth reflections which, on one hand will increase the phase change in the passage of the wave through the layers, and on the other hand will diminish the value of the index of absorption. It will readily be understood that in this manner. it is possible with a comparatively small increase in total thickness of the composite coating to diminish the coefficicnt of absorption in a sufficient amount as to cause it to approximate the value demanded by Equation 2b and thereby satisfy both resonant conditions.

As set forth in the above copending application, conductors like aluminum, copper, iron, steel, Permalloy and graphite flakes and mixtures thereof have been utilized in the fabrication of the coatings. The flakes average in thickness between 3 l and 2X10" centimeters. with the long dimension as high as seventy times the average thickness. The commercial processes for producing so-called bronze paint pigment" of various metals yield flakes which are satisfactory in size and shape for the purposes of this invention. The shapes vary from that of disks to that of threadlike particles.

The grease present on the surfaces of commercial flakes depends upon the production process and can be varied so as to permit ready and optimum orientation. The flakes may be dispersed and thoroughly mixed in a variety of binder substances and materials such as waxes, resins, polystyrene, Vistanex. Vinylite or synthetic rubbers. Xylene and toluene are examples of suitable volatile solvents. Practically useful concentrations vary between 20 and 80 percent of metal content of the mixture.

The composition or mixture of flakes, binder and solvent may be applied by spraying or working with a flat smoothing tool at room temperature or slightly higher. Applied in fluid form the composition readily dries and becomes fixed as a layer and coating, as the solvent evaporates.

It is well known that with a composition prepared and applied in the above-described manner, the flakes generally assume the preferred orientation, i.e., parallel to the surface covered. Working the coating by a smoothing or stroking motion aids in obtaining a more uniform leafing of the flakes with their faces generally parallel to the plane of the layer. Although the resulting layer may be obtained in a single co'atihgfii has been found that a composite layer constructed of a plurality of thin coatings is more eflicient. Multiple coat ing's as just described tend to yield an effective dielectric constant which is higher than that obtained with a single coating application, because a better leafing of the flakes is obtained when the coatings are thin and individually worked.

When employing the nonsymmetrical commercially obtained flake particles, the layer may be made to be polarized to a certain degree, by causing the smoothing stroke to be always in the same direction. That mode of application and treatment tends to align and orient the long dimension of the flakes as well as to press the faces of the flakes parallel to the plane of the layer. By altering the direction of spraying or the smoothing stroke ninety (90) degrees in the successively applied coatings, a layer may be made which is isotropic in the plane thereof.

A number of representative layers made with different binders. particles and concentrations and with varying methods of preparation, as set forth in the above copending application, will now be dsecribed to illustrate the general explanation given hereinabove. The frequencies used in the measurements referred to are in the neighborhood of 2,500 megacycles or higher values.

Example N0. J.Aluminum flake 76%, Vistanex and Estergum 24%, prepared by cross smoothing. Index of refraction 57.5. Reflected intensity at resonance 8%. No directional properties in plane of film.

Example N0. 2.Aluminum flake, polystyrene with plasticizer 15% and 15%) by cross smoothing. Index of refraction 45. Intensity reflected at resonance 1%. No directional properties in plane of film.

Example No. 3.Copper flake in polystyrene, 37 /2 and 62 /2 pressed. Index of refraction 6. Intensity reflected at resonance immeasurably small. No directional properties in plane of film.

Example N0. 4.-Aluminum flake in polystyrene, 37 /2 and'6l /2 pressed. Index of refraction 8.8. Intensity reflected at resonance immeasurably small. No directional properties in plane of film.

Example No. 5.Steel flake, plus clay, plus synthetic rubber binder known as GR-l. 40%, 30%, 30%. Calendered and pressed, one layer crossed on top. Index of refraction 7.9. Intensity reflected at resonance 1%. No directional properties in plane of film.

Example N0. 6.--Aluminum fluke in \"istanex and Estergum, 807 15%, 5%. Hand smoothed. Principal indices of refraction 64.5 and 7L5. Intensities reflected at resonance 22% and 16%.

Example N0. 7.Aluminum powder in synthetic rubber binder known as GR-3, and 25%. Hand mixed, hand smoothed. Principal indices of refraction 46 and 52. Intensities reflected at minimum 31% and 3%.

It will be appreciated that the forth in Examples 1, 2, 3. 4, 6 and flakes and aluminum flakes or powder correspond to the nonconducting dielectric matciral containing electrically conducting nonmagnetic flakes, whereas the coating set forth in Example 5, which uses steel flakes, corresponds to a nonconducting dielectric material containing flakes of ferromagnetic material.

While a particular embodiment of my invention has been disclosed and described, it will readily be understood that various changes and modifications may be made therein without departing from the spirit and scope thereof as set forth in the appended claims.

What is claimed is:

1. In a method for improving the electromagnetic energy absorption characteristics of a quarter wave length dielectric coating which is made up of electrically concoatings or layers set 7 which employ copper ducting nonmagnetic flakes dispersed through a non conducting dielectric material and which has an index of absorption less than 2/ 1r the step of adding ferro magnetic particles in flake form to said dielectric material until the index of absorption of said coating is increased 2. In a method for reducing the percentage of incident A d 4n where A is the wave length of the incident electromagnetic energy and n equals the index of refraction of said layer, the step of increasing the index of absorption of said layer k to equal 2/1r by adding to said nonconducting dielectric material flakes of magnetic material.

3. In combination, a surfacenormally reflective of electromagnetic energy incident thereon, a coating applied to said surface, said coating consisting of electrically conducting nonmagnetic flakes and ferromagnetic flakes dispersed throughout an electrically nonconducting dielectric binder, said electrically conducting nonmagnetic flakes giving said coating a high electromagnetic dielectric constant thereby reducing the thickness d of said coating necessary to satisfy the expression where A is the wave length of said electromagnetic energy and n equals the index of refraction of said coating and said ferromagnetic flakes increasing the index of absorption of said coating k to equal 2/1r whereby a minimum of said electromagnetic energy is reflected from said surface.

4. In combination, a surface normally reflective of electromagnetic energy incident thereon, a coating having a thickness where A is the wave length of said electromagnetic energy and n equals the index of refraction of said coating applied to said surface, said coating consisting of a plurality of interleaved high dielectric sheets formed by dispersing electrically conducting nonmagnetic flakes in a nonconducting dielectric binder and low dielectric paper 1"1 sheets, the thickness of each of said sheets being small compared to A and the index of absorption of said high dielectric sheets and said low dielectric paper sheets k being equal to 2/1r.

5. The method of decreasing the index of absorption k of a dielectric coating having a thickness where A is the wave length of the electromagnetic energy being propagated in said coating and n equals the index of refraction of said coating which comprises the step of forming said coating from a plurality of alternate layers of electrically conducting, nonmagnetic, flake pigmented dielectric films and paper sheets, the thickness of each layer being small compared to A so that the phase change of the electromagnetic energy and the absorption of the electromagnetic energy in each layer is relatively small.

6. A method of decreasing the electromagnetic energy index coeflicient k of a quarter wave length dielectric coating to equal 2/1r which involves the step of interleaving and bonding together a plurality of sheets of electrically conducting nonmagnetic flakes dispersed through an electrically nonconducting dielectric and low dielectric paper, whereby the electromagnetic energy propagated therein experiences a multiplicity of backand-forth intern-a1 reflections due-to the discontinuities produced by the contacting boundary surfaces of said dissimilar sheets and whereby an increased phase change is produced by the composite coating.

References Cited in the file of this patent UNITED STATES PATENTS 1,173,452 Meirowsky Feb. 29, 1916 1,354,147 Thomas Sept. 28, 1920 2,014,399 Sprague Sept. 17, 1935 2,206,720 Ducati Julq 2, 1940 2,413,085 Tiley Dec. 24, 1946 2,418,479 Pratt et a1 Apr. 8, 1947 2,464,006 Tiley Mar. 18, 1949 2,594,971 Moullin Apr. 29, 1952 2,599,944 Salisbury June 10, 1952