US2716190A - Dielectric material - Google Patents

Description

g- 23, 1955 E. B. BAKER 2,716,190

DIELECTRIC MATERIAL Filed Feb. 25, 1951 Microwave Lenses: magnesium meza/ parflc/es d/Lgoerseafn ,oaxs/yrene.

Fry 5 19 4 11 i5 5 15 ill 5 I" Half Wave coating of Magnesium same componenfs w/Z/z o /eleczrfc consianl ba/f INVENTOR' Edward 84 Baker zf/ml ofboa y BY ATTORNEYS parlic/es /'/2 polystyrene Ii United States atent DIELECTRIC MATERIAL Edward B. Baker, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application February 23, 1951, Serial N 0. 212,365

6 laims. (Cl. Mil-33.63)

This invention relates to an improved dielectric material, and, more specifically, to a dielectric material consisting essentially of small metal particles dispersed in a polymerized plastic matrix.

Polystyrene-metal compositions having the electric properties required for such diverse uses as special capacitors or resistors for high frequency transmission lines, and lenses used in microwave beam transmission work, have heretobefore been produced. For example, U. S. Patent 2,403,657 issued to Harvey on July 9, 1946, teaches a dielectric material consisting of copper particles embedded in polystyrene. Such a material is said to be distinguished from polystyrene in that the ratio of reactance to resistance is constant or increases slightly with increase of frequency in the range from to 400 megacycles. As a result, it is suitable for use in capacitors or resistors designed to operate over such a range. Another type of special dielectric material has been produced by placing metal strips in a predetermined pattern in slotted foamed polystyrene sheets. The sheets so fitted with metallic strips are then stacked to form a solid foamed polystyrene body having metal strips embedded therein in a desired pattern. Lenses used in microwave transmission have been produced in this way. A further discussion of this latter special dielectric material can be found in The Bell System Technical Journal, vol. XXV H, No. 2, page 210 (1948).

The present invention is based upon the discovery of a simple, inexpensive dielectric material having a high dielectric constant and a low power factor so that it is suitable for the manufacture of microwave lenses.

The principal object of the present invention is to provide an improved dielectric material.

A further object is to provide an improved microwave lens that can be produced at a fraction of the cost of previous lenses.

Still another object of the invention is to provide an improved microwave transmission system.

More specific objects and advantages are apparent from the description and drawings, which illustrate and disclose, but are not to be construed as limiting the inventron.

According to the invention an improved dielectric material is provided. This dielectric consists essentially of metal particles of size smaller than 100 mesh, U. S. Sieve Series, dispersed in a polymerized matrix of polyethylene, a silicone rubber, of a polymerized 2-aryl alkene. The metal particles are aluminum or magnesium, atomic numbers 12 and 13, and must have an electric-insulating surface coating. If the matrix is a polymerized 2-aryl alkene, the alkene has not more than three carbon atoms, and the aryl radical is mononuclear, contains not more than ten carbon atoms, has not more than two substituents, and has no substituent other than chlorine and alkyl. The matrix of dielectric materials of the invention can be either a solid matrix of the type produced by ordinary molding techniques, or can be an expanded matrix of the type produced according to the teachings of U. S.

Patent 2,450,436 to Mclntire. The term silicone rubher is used in its usual sense, namely to include vulcanized silicone resins, usually comprising methyl and phenyl silicones. All mesh sizes refer to the U. S. Sieve Series.

The magnesium or aluminum particles that are used to produce the compositions of the invention should be in the shape of rods, spheres or plates, the two first-named shapes being preferred, and in the order named. When the particles are rods it is ordinarily preferred that they have a maximum length not greater than about 0.1 inch and a diameter not greater than about 0.01 inch, and the smaller the diameter thereof the better are the electric characteristics of the composition; this latter is true so long as the rods are of sufiicient size that they are not deformed in processing. When the particles are spheres or plates it is usually preferred that substantially all of them be finer than mesh, and that at least a substantial portion thereof be finer than 200 mesh. Particles as fine as can be produced are effective, e. g., even particles smaller than 325 mesh.

When the particles are magnesium no particular treatment is required to form the electric-insulating coating. The action of air on magnesium particles, even at room temperature, is suflicient to produce an oxide film satisfactory as an insulation. The oxide coating that forms on aluminum exposed to air is not sufficient to effect the required insulation. Accordingly, when the particles are aluminum it is advantageous that the insulating surface coating be applied artificially, by some such method as anodizing the aluminum, or coating the particles with such an insulating material as a silicone. It will be readily seen from the foregoing remarks that the preferred electric-insulating coating is an oxide of the coated metal, and that magnesium is the preferred metal for use in producing compositions of the invention because special treatment thereof is not required.

It is usually preferred that the volume ratio of metal particles to total volume of dielectric material be from 0.52100 to 40:100. The exact proportion of metal particles used is determined by the characteristics required of the dielectric material. For example, when a non-expanded dielectric material is used to produce a microwave lens, it is desired that the material have a high dielectric constant. Such a characteristic is achieved by using a large ratio of metalparticles, e. g., from 30 to 40 volumes per 100 volumes of dielectric. When the dielectric material consists of metallic particles dispersed in a foamed matrix it is usually preferred that the volume ratio of metallic particles to dielectric material be from 0.5: 100 to 10:100. A larger proportion of metallic particles can be employed to produce a dielectric with a foamed matrix or a smaller proportion than 30 volumes per 100 volumes of dielectric with a non-expanded matrix, if desired.

Preferred dielectric materials of the invention have matrices of a polymerized 2-aryl alkene of the genus hereinbefore defined. Most desirably, the matrix is polystyrene, polyalphamethyl styrene, or a copolymer of styrene and alphamethyl styrene.

Improved dielectric materials of the invention are readily produced by conventional methods. The metal particles and the polyethylene, silicone rubber, or polymerized 2-aryl alkene are mixed. A dielectric having a non-expanded matrix can then be produced from the mixture by any of the conventional molding techniques, or by extrusion. A dielectric material having a foamed matrix is readily produced from such a mixture by adding a normally gaseous substance to a vessel containing the mixture and processing as described in U. S. Patent 2,450,436 to McIntire.

The invention may be more fully understood by reference to the drawings, in which Fig. l is a front view of a microwave lens of the invention;

Fig. 2 is a side view of the microwave lens of Fig. 1;

Fig. 3 is a front view of an alternative shape of a microwave lens of the invention;

Fig. 4 is a side view of the microwave lens of Fig. 3, in section along the line 44 in Fig. 3;

Fig. 5 is a schematic diagram showing a transmission system with which microwave lenses according to the invention are used; and

Fig. 6 is a cross-section, similar to Fig. 4, of an alternative form of lens according to the invention.

Figs. 1 and 2 illustrate a lens constituting a frustum of a cube. Such a lens is effective by virtue of variations in dielectric constant throughout the lens. These variations can be achieved by compression molding the lens from a composite preform made from various samples of polystyrene containing differing proportions of aluminum or magnesium, for example, a series of nesting annular disks in which the proportion of aluminum or magnesium increases with increasing ring diameter. This is practicable only for lenses having a non-expanded matrix.

Figs. 3 and 4 illustrate a double convex lens of the invention. Such a lens converges microwaves by virtue of variations in angle of incidence, and can be produced with an expanded or non-expanded matrix. In some instances when a foamed matrix is produced, it may be advantageous to mold the foamed composition to approximately its final form before it sets. This can readily be accomplished by forming a gel from a styrene-magnesium composition in the manner described by McIntire in a vessel that will serve as a mold of approximately the desired lens shape, and, after the desired cure, releasing the pressure therein. The lenses so produced affect microwaves in a manner analogous to that in which optical lenses affect light waves, dielectric constant being the counterpart of refractive index, so that microwave lens shapes are similar to optical lens shapes.

Fig. 5 illustrates schematically the transmission system with which microwave lenses of the invention are used. The system consists essentially of a transmitter 11, and a microwave receiver 12, with two microwave lenses 13 interposed between the transmitter and the receiver. The transmitter is essentially a point source of microwaves, and is located at the focal point of the first lens, which then converts the waves to a beam. The second lens converges the microwaves of the beam on the receiver, which is located at the focal point of the second lens. A horn, not shown, is frequently employed to concentrate the signals on the first lens. The transmitter, the lenses, and the receiver are usually mounted in towers. One lens is mounted close to the transmitter to form a beam; the other is mounted to converge the beam on the receiver, which may be as many as miles from the transmitter.

A particularly advantageous dielectric material of the invention is produced when aluminum or magnesium rods are mixed with the desired matrix material, and the dielectric is produced by compression molding or extrusion of the resulting composition. Either of these two methods of production results in the orientation of the rods in the matrix so that their long dimension is parallel to one surface of the dielectric material. This orientation causes the resulting dielectric material to be selective in that its dielectric constant is much higher when an applied E. M. F. has an electric vector parallel to the surface of the material than when an applied E. M. F. has no such vector. In practice, imcrowaves are so polarized before they pass through a lens that their principal electric vector lies in a plane perpendicular to the direction of wave transmission. Accordingly, dielectric materials of the invention having rods oriented in a plane parallel to the fiat surfaces of the material are particularly effective materials for the production of microwave lenses, and lenses produced from such a dielectric can be substantially thinner for a given focal length than can microwave lenses having a foamed or solid matrix containing unoriented aluminum or magnesium particles.

Because the dielectric constant of dielectric materials of the invention can be varied by changing the proportion of aluminum or magnesium incorporated therein, it is possible to minimize wave reflection on the surface of lenses of the invention by applying to the surface thereof a film of a dielectric material of the invention having a dielectric constant about one-half that of the main body of the lens. An especially advantageous technique in this connection involves the application of a film having a thickness of about one-half of the wave length of the microwave with which the lens is to be employed. In this way, reflection is minimized, and reflection from the surface of the coating tends to cancel reflection from the surface of the lens itself so that interference with wave transmission is minimized (see Fig. 6).

The following examples illustrate and disclose, but are not to be construted as limiting, the invention:

EXAMPLE 1 Different magnesium powders were incorporated in polystyrene to produce new dielectric materials. The samples of magnesium were produced as follows:

Sample No. ].Magnesium metal, atomized by the method described in U. S. Patent 1,351,865, to Nicol, issued September 7, 1920, screened to remove all material coarser than 100 mesh;

Sam le No. 2.Same material, screened to remove everything coarser than 200 mesh;

Sample No. 3.Fines not collected in the atomization process, but removed from the exhaust gases from this process in a cyclone separator, screened to remove everything coarser than 200 mesh;

Sample No. 4.Magnesium metal filings, mostly to mesh; and

Sample No. 5.Smaller than 200 mesh ground magnesium metal.

Dielectric materials were produced from each of the above magnesium samples by milling from one to three parts by weight of the appropriate magnesium metal sample with three parts by weight of polystyrene to disperse the magnesium in the polystyrene, and compression molding a disc from the resulting mixture. This resulted in a material containing 16.9 to 33.8 volume per cent of magnesium. The dielectric materials so produced had the properties set forth in Table 1 below.

Table I Fartl lstb fr Porer Factor erg 0 ercent Magnesium Sample No. F Metal to at 10,405

P 3 2 Cycles IO Cy- 10 Gy- Styrene eles cles If, for purposes of comparison, but not in accordance with the invention, copper powder is mixed with polystyrene, and small discs are molded from the resulting mixture, dielectric materials are produced which have higher dielectric constants than polystyrene itself, but are far inferior to dielectrics of the invention. For example, if a composition containing 20 weight per cent of copper powder is produced, it is found that the dielectric constant in the range from 10 to 10 cycles per second is only about 3, although the per cent power factor is a maximum of 0.2 at 10 cycles per second.

EXAMPLE 2 3. A microwave lens composed of a low-density dielectric material consisting essentially of substantially rod-shaped particles of metal of atomic number 12 to 13 having an electric-insulating oxide surface coating and having a length not greater than 0.1 inch and a diameter not greater than 0.01 inch dispersed in a foamed matrix of a polymerized 2-aryl alkene in which the alkene radical has not more than three carbon atoms and the aryl radical is mononuclear, contains not more than ten carbon atoms, has not more than two subl n01} number the gram.s i Polystyrene grams of mag stituents, and has no substitutent other than chlorine nesium rods, c. c. of liquid propylene, temperature and and alkyl i amp 3 i i i i thefdlelecltnc fi 4. A microwave lens composed of a dielectric ma- 1 eexpanthe ma f fil P g terial consisting essentially of substantially rod-shaped s gg e imwer ac or o 6 ma ena was 5 particles of metal of atomic number 12 to 13 having P 9911 an electrically insulating oxide surface coating and hav- T able 2 G C O f Curing Cycle Grams rams of .0 Run N0. of Poly- Magne- Propyafigggi styrene sium 1 lane O C Time, a C Time,

' days days September 16, 1949, by Leontis and Pashak.

Expanded polystyrene itself has a dielectric constant of 1.1 to 1.2.

EXAMPLE 3 Other samples of dielectric materials were produced by the procedure of Example 1, except that the magnesium was merely stirred into powdered polystyrene, without milling, using rod-like magnesium particles produced as described in Example 2. The weight per cent of magnesium used, and the electrical properties of the final materials are set forth in Table 3, below.

1 At 10 cycles.

Microscopic examination of a sample of the dielectric material showed the orientation of the magnesium rods in planes parallel to the surface of the dielectric. It is noted that dielectric constant was measured with the electric vector of the applied voltage at right angles to the surface of the dielectric material in the tests reported above. As is discussed herein the dielectric constant is at a maximum toward an applied voltage having an electric vector in a plane parallel to the surface of such a dielectric material.

I claim:

1. A microwave lens composed of a low-density dielectric material consisting essentially of substantially rod-shaped magnesium particles having a length not greater than 0.1 inch and a diameter not greater than 0.01 inch dispersed in a foamed polystyrene matrix.

2. A microwave lens composed of a low-density dielectric material consisting essentially of substantially rod-shaped magnesium particles having a length not greater than 0.1 inch and a diameter not greater than 0.01 inch dispersed in a foamed polyalphamethyl styrene matrix.

ing a length not greater than 0.1 inch and a diameter not greater than 0.01 inch dispersed in a matrix of a polymerized 2-aryl alkene in which the alkene radical has not more than three carbon atoms and the aryl radical is mononuclear, contains not more than ten carbon atoms, has not more than two substituents, and has no substituents other than chlorine and alkyl, the particles being oriented so that their long dimension is parallel to one surface of the dielectric material.

5. A microwave lens composed of a dielectric material consisting essentially of substantially rod-shaped particles of magnesium having a length not greater than 0.1 inch and a diameter not greater than 0.01 inch dispersed in a polystyrene matrix, the particles being oriented so that their long dimension is parallel to one surface of the dielectric material.

6. A microwave lens composed of a dielectric material consisting essentially of substantially rod-shaped particles of magnesium having a length not greater than 0.1 inch and a diameter not greater than 0.01 inch dispersed in a polystyrene matrix, the particles being oriented so that their long dimension is parallel to one surface of the dielectric material, said lens having a surface coating of dielectric material formed of the same components as aforesaid but having a dielectric constant about one-half that of the body of the lens, the thickness of the coating being about one-half the wavelength for which the lens is designed.

References Cited in the file of this patent UNITED STATES PATENTS 1,947,112 Ruben Feb. 13, 1934 2,054,454 Theis et al. Sept. 15, 1936 2,202,380 Hollmann May 28, 1940 2,343,531 Buchholz Mar. 7, 1944 2,379,790 Dimmick July 3, 1945 2,415,352 Iams Feb. 4, 1947 2,508,479 Wheeler May 23, 1950 2,547,416 Skellett Apr. 3, 1951 2,579,324 Kock Dec. 18, 1951 FOREIGN PATENTS 406,267 Great Britain Feb. 15, 1934 793,574 France Nov. 23, 1935

Claims (1)

1. A MICROWAVE LENS COMPOSED OF A LOW-DENSITY DIELECTRIC MATERIAL CONSISTING ESSENTIALLY OF SUBSTANTIALLY ROD-SHAPED MAGNESIUM PARTICLES HAVING A LENGTH NOT GREATER THAN 0.1 INCH AND A DIAMETER NOT GREATER THAN 0.01 INCH DISPERSED IN A FOAMED POLYSTYRENE MATRIX.

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