US2894224A - Ferromagnetic microwave device - Google Patents

Description

July 7, 1959 EN 2,894,224

FERROMAGNETIC MICROWAVE DEVICE Original Filed May 19, 1954 I INVENTOR.

Arthur H. lversen,

' AGENT.

CalISGqCOHSiClBTablG heating of the compact.

United States Patent 2,894,224 FERROMAGNETIC MICROWAVE DEVICE Arthur H. Iversen, Santa Monica, Calif., 'assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Original application May 19, 1954, sen No. 430,341. Divided and this application December 12, 1'956, Serial No. 627,930

2 Claims. (Cl. 33334) This invention relates to hermetically-sealed ferromagnetic dielectric materials, and more particularly to vitreous-coated ferromagnetic dielectric compacts.

This is a divisional application of application Serial Number 430,841, filed May 19, 1954.

Ferromagnetic dielectric materials, known in the art simply as ferrites, are crystalline ceramics in which the major constituent is iron oxide. These ceramics are generally formed into a compact by a closely-controlled sintering process from a powder mixture which includes a plurality of bivalent-metal oxides. Ferrite compacts have a wide range of physical and electrical properties which arede'penden't on their composition and the sintering technique employed in their formation.

:The ferromagnetic dielectrics are particularly useful in microwave applications because at these frequencies they exhibit a characteristic known as the Faraday effect.

However, in use, the heating effect, power factor, and polarizationgrotating ability of the ferrite compacts are adversely aifected by increasing moisture content.

An example of such utilizations of the ferrite compacts is shown in copending application, Serial No. l 19,259 by Willard A. Hughes, filed on March 29 1954. In this example, the ferrite compact is utilized to rotate incident wave energy 45 degrees for transmission to a load. The reflected energy is rotated another 45 degrees in again traversing the ferrite compact so that it has a final pclarization at the source end of the ferrite which is normal to that of. the incident energy. This relationship between the planes of polarization of the incident and reflected energy makes it feasible to divert the reflected energy out of the working system and so prevent it from traveling 'ba'ck to the source.

In addition to the utilization of the ferrites in microwave transmission systems, they are also useful in certain types of electron discharge devices, such as, for example, the traveling-wave type. The susceptibility of the ierrites to release occluded gases over a long period of time is an impediment to this type of use because of the necessary high vacuum which must be maintained in the electron discharge device. it follows that this problem can be. obviated by hermetically sealing the ferrite cornpact in accordance with the present invention. Accordingly, it follows that if the ferrite compacts can be hermetically sealed. in an envelope or skin in a dry condition,,their usefulness will be very much enhanced. The

manner in which the ferrites are utilized, as Well as their inherent physical properties, interposes a number of difficulties in the way of achieving a satisfactory hermetic envelope.

The ferromagnetic dielectric compact or ferrite is utilized in high intensity microwave applications which it follows that. ascalingskin for the compact must be capable of withstanding a temperature of at least several hundreddegrecs centigradewithout deterioration, cracking or otherwise developing porosity. The substances capable of perice 2 forming a scaling function at these temperatures are the vitreous products, notably glass.

An obstacle in the way of utilizing a vitreous skin on ferrite compacts is the inherent susceptibility of the compact to deterioration, 'i.e., cracking, due to thermal shock. That is, the tensile and compressive strengths of the ferrite compact are insufficient to prevent disruption of the compact due to sudden heating to a high temperature of the outer surfaces thereof. Consequently, since a glass surface must be applied to the compact at the molten temperature of the glass, it is necessary to utilize a glass which has a temperature of melting which is not so high as to cause breakup or cracking of the compact. On the other hand, because, as previously mentioned, the ferrite compact can become quite hot in operation, it is necessary to utilize a glass which has a high enough melting temperature to preclude deterioration of the glass skin or envelope.

In addition to the above problems involved in providing a ferrite compact with a vitreous coat or skin, it is also necessary that the compact with its vitreous skin withstand variations in temperature over several hundred degrees Centigrade without deterioration of either the compact or seal. This requires that the vitreous substance utilized for sealing the compact have a coeflicient of thermal expansion sufliciently similar to that of the ferrite compact to limit the stresses in either the skin or the compact itself so as to avoid deterioration of either the skin or the compact.

It follows that a prime objective of this invention is to provide a sealing coat or skin onto a ferromagnetic dielectric compact. v i

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will become apparent from the following description considered in connection. with the acoom panying drawing, made a part of this specification.

In the drawing:

Figure 1 is a partial broken section of a hermetically sealed ferromagnetic ferrite device of the present invention; and V Fig. 2 is an enlarged cross-sectional view, section 2-2, of the device shown in Fig. 1.

Referring to the figures of the drawing, a typical embodiment of the present invention is shown comprising an elongated figure of revolution ferrite compact or slug 10 having a glass coating or skin 11 which hermetically seals the compact 11 from the surrounding medium. In the embodiment shown, the diameter of the middle portion of the compact is of the order of from 0.1 to 1.0 inch and its length is generally within the range of from one to four inches, the dimensions being determined by the particular application. Inasmuch as ferrites may be used in numerous other forms, such as, for example, hollow or solid cylinder, it is readily apparent that the scope of the present invention is not limited to the sizes mentioned or particular configuration shown.

In order to provide a suitable glass skin on the ferrite compacts, it is first necessary to determine the physical properties of the ferrite. Ferrites of many varied and diverse compositions are commercially available. Many different compositions are employed because of the unusually large number of combinations of different useno physical properties exhibited by all of them. Representative samples of a ferromagnetic ferrite may contain the, following constituents: MgO, MnO CuCO ZnO, Z (unstable), 1350, C210, Zr, ZrO The ferrite are normally chemically reactive with lead peroxides and bora'tes, but they are not reactive with oxides of aluminum or magnesium or the carbonates of sodium or calcium.

terial and stick thereto.

tivity and high permeability in comparison to powdered iron materials. Their specific gravity lies between 4 and 5 and their dielectric constant is about 9. These and other peculiar properties have led to a wide field of applications, many of which, as previously mentioned, require ferrites to be hermetically sealed.

The thermal coeflicient of expansion of ferrites is primarily limited to the range 71 to 93X inch per inch per C. They have a relatively high modulus of elasticity and their ultimate strength is usually about 10,000 p.s.i., i.e., pounds per square inch, in compression and 2000 p.s.i. in tension. are generally the most important when correlated with those of glass, the thermal expansion coefficient normally being the most important of the three.

To produce a hermetic seal that will withstand high vacuum and large temperature variations, glass probably has better physical and chemical properties than any other material.

In obtaining a glass seal for a material it is necessary for the glass to wet and to match the material. Since most all glasses wet metal oxides, there is obviously no problem presented with respect to this requirement in coating ferrites since they are almost totally constituted of the oxides of bivalent metals. What .is normally meant by matching a glass to a material is to seek out a glass having a thermal coeflicient of expansion near enough to that of the material to prevent a structural failure in either by stresses set up by differential expansion.

In the manufacture of a good seal, being able to match a glass and a material from the standpoint of thermal expansion, i.e., being able to keep the strains in the glass or the material below their breaking strength in case their thermal expansions donot match, is not sufiicient when accomplished only at room temperature because the glass and material must first of all be heated to a high temperature in order to make the seal. For this purpose, the glass is heated to its working temperature, the temperature above that at which it begins to soften or change its shape. In fact, the glass should be rendered plastic or fluid-like so that it will wet the ma- Such a temperature is in the neighborhood of 800 C. or higher, although, as it will be seen, one type of useful glass has a working temperature of 560 C.

The working temperature of a glass is to be distinguished from the softening temperature which may be as low as 440 C. and which is defined as being the temperature at which the glass becomes suliiciently yielding .so that strains are relieved at an extremely rapid rate or within a very short period, for example, less than one minute. The seal is then allowed to cool directly to room temperature or, as is the more usual case, the cooling process is temoprarily arrested at a predetermined elevated temperature, termed the annealing point or the lowest temperature below the softening temperature at which 90% of the internal stresses of the glass will be removed in about fifteen minutes.

In choosing a suitable glass in which to seal a ferrite, it is to be noted that a considerable choice as to which and how many raw materials, in addition to sand, are to be incorporated into a batch of a useful silicate glass. Silica glass or fused quartz and fused silica, made from a batch consisting of sand alone, appears to be the ideal in glass in nearly all its physical and chemical properties. However, if too high a percentage of sand is utilized, the glass formed in a batch involves melting temperatures of about 1700 C. which are too expensive and impractical even with the best commercial glass-furnace These three physical propertiesefiicient of expansion, being about 8 l0-" -two-component silicate glasses, which are unsuitablefor making glass articles because of water solubility or de- ;vitrification i.e.,crystallization, are classified in five chemical types: soda-lime glasses, sometimes called simply soda glasses and sometimes lime glasses; .lead glasses; and three new low-expansion and therefore heat-resistant type glasses, namely, borosilicate glass; and 96% silica glass, and pure silica glass or quartz. This listing is .more or less in order of increasing silica content, in-

creasing mechanical durability, improving electrical properties, increasing cost, increasing shaping difficulty, and decreasing thermal expansion coefficient.

The composition of these types, given in Table I, are the analyses of the finished glasses for elements, except oxygen, converted to oxide equivalent. a

Table l.-C0mpositions of commercial glasses Composition, Percent a Component Soda-lime Lead Borosilicate 96% Silica Silica glass SlOz- 70-76 (72) 53-68 (68) 73-82 96 19.8 N820 1218 (15) 510 0) 3-10 (4) K@O 0-1 1-10 (6 0-4-1 OaO- 5-14 (9) 0-6 (1) 01 PbO- 15-40 (15) 0-10 B103- 6-20 (14) 3 A1 0; 52.6 (1) 0 2 2-3 (2) MgO 0-4 (3) a The figures in parentheses give the approximate composition 0! a typical member.

In order to make a successful seal, the expansion of ferrite 10 and glass 11 must be substantially theflsa me over the temperature range within which the glass is elastic. A large difference in expansion between them produces stresses which may cause either or both to crack when cooled to room temperature. Some degree of differential expansion, however, is tolerable and sometimes desirable. The relaxation characteristics of the glass determine the upper temperature limit to which matched 'glasses and the lead glasses. The thermal coeflicientsof each of these types have the following respective ranges: 67 to 105 and 9 to 10-' inch per inch per C. Although soft glasses may go lower to possibly 67x10 inch per inch per C., few will go as high as 140x10 inch per inch per C. Fused silica or 100% silica glass has a coefiicient of expansion which is too low for the present purposes, being about 5.5)(10 inch per inch per C. and 96% silica glass likewise has too low a co- .inch per inch per C. The glasses called borosilicate glasses contain 5% or more of boric oxide. They are also usually too hard to seal a ferrite, the range of their thermal expansion coeificients being about 13 to 60Xl0' inch per inch per C.

Glass in general has an ultimate strength in compresp.s.i., except for very small glass fibers.

T It is, however, pertinent to note that the use of any glass with a ferrite QQQQQQQ demands that he thermal rnansionroeflicient .be wel matched to prevent a struc ural f ilure Particular y 111 the ferrite because of the low intimate istltength of the latter. e

Next in importance to therthermal expansion coeificient is the modulus ofelasticity of a glass. .is :tme .because the glass should also be. pliable at low temperatures to be able totake alarge deformation with very little stress, i.e., its modulus of .elasticity should be as low 1as possible. Youngs modulus does not appear to be especially relatedtothe fhardness of a glass or to any particular element of its composition. It is also true that there appears to be no correlation between Youngs modulus and the thermal expansion .coeflicient. This modulus of elasticity, however, ,does not normally vary appreciably for soft glasscstofcommon compositions and is, therefore, not nearly .so importantas the thermal expansion of glass. For example,.,all Youngs :moduli of glasses fall within the range 65 to 127 x p.s.i. Some s as w as 6 1 0 PB -i withta therma expans efficient of 796x10 inch per inch C. The moduli of hard glasses are scattered over an equally wide range, going as low as 68 l0--' p.s.i. for a glass having a thermal expansion coeflicient of 32x10 inchper inch per C. andgoing as high as 127 ,10 ;p.s.i. fortthermal expansion coeflicient of 42 l0' inch per inch per C.

Although the soft \glasses are generally satisfactory for coating a ferrite, there are a few glasses which may be used to particular advantage to reduce the risk of structural failures during the manufacture of the seal and especially during the handling of the component materials. For example, in the case of ferrites having a thermal expansion coefficient falling within the range 71 86 l0" inch per inch per C., they have an effective expansivity between 65 and l00 l0 inch per inch per C. A glass having characteristics approximating these is commonly known as lime glass. This glass is composed of 73.3% SiO 15.6% Na O; 5.4% CaO; 3.8% MgO; 1.4% R 0 0.5% K 0 and has an effective expansivity of 92i2 10-' inch per inch per C., and a Youngs modulus of 98 10"' p.s.i.

It has also been found that a seal between the soft glasses known to the trade as 0080 and 7570 manufactured by the Corning Glass Company may be used in hermetically sealing the ferrites of the above description. The Corning Glass Company designates clear sealing bulb glass and solder glass with the number 0080 and 7570, respectively. These glasses are kept as closely as possible to a standard chemical composition at all times.

In order to apply the glass coating 11 to the ferrite slug 10, a suitable glass of the above-described types is first powdered and then suspended in a liquid, such as, for example, water, methyl alcohol, or a solution of nitrocellulose in amyl acetate. This suspension of powdered glass is then brushed or sprayed uniformly over the ferrite slug 10 or alternatively the ferrite slug 10 may be immersed in a suspension of powdered glass in the solution of nitrocellulose in amyl acetate.

The ferrite slug 10 covered with the suspension of powdered glass is then heated in a controlled atmosphere to the working temperature of the glass and then slowly cooled. This process is repeated until the glass coating 11 has a glaze finish and is of a suitable thickness. The thickness of glass coating 11, depending on the uses to which the ferrite is to be put, is of the order of from 0.003 inch to more than 0.06 inch, the thinner coatings being used where permissible in that the electrical losses are less.

Referring again to Fig. 1, it is seen that glass coating 11 of the device has elongated pointed ends 12 where the glass is considerably thicker than the remaining portions of the coating. The device, as previously mentioned, is employed to rotate the plane of polarization of the electric field of a propagated wave. In order to accomplish this, it is necessary for the electromagnetic wave to propagate through the medium .lQf will? de ice." in that the dielectric constant of ferrite quite h gh. ther i ge erally .aimpedanee mismatch h 8 111s mediumeurrounding the device and the mcdinmrofrthe f it slug .10. Thus, if the :glass :has adielectricrconstant approximately equal tothe geometric mean of tthfi dielectric constants of the surrounding and ferrite me ms. t may e empl ye t substant al y improve the i d e match of the ferrite lug :10 to 111.653.1 3.001 ingrruedium. The thickness of glass points 12 for optimum matching as .measured along the longitudinal axis of the device is of the order of one-half guide wavelength.

A glass suitable for making the glass points 12-isknowu as Corning glass 0120. This glass hasathe particularly low dielectric constant of 6.6 which is necessary to approximate the aforementioned geometric mean of the dielectric constants of the surrounding medium and the ferrite. The Corning:0l20 glassis a-clear potash :soda lea glasshaving a thermal expansion of '89 910 tinch per finch per 2G,, and :a Youngs modulus of x10 p sgi. which suitable for .use in conjunction with the glass constituting'therglass coating 11.

it has "been found .inflatternpting to hermetically seal sorne ferrites that ,they are sufliciently porous so ,as .to continuously absorb the glass when heated to the fluid state. This effect is to be avoided in that the characteristics of the ferrite are deleteriously afiected. Accord ing to the present invention, ferrites of this type are sealed by first applying a coating of glass 13 having a high working temperature to the ferrite. This coating will generally not effectively seal the ferrite, but it will prevent subsequent coatings of glass having a lower working temperature from being absorbed into the ferrite.

More particularly, this process of hermetically sealing a porous ferrite is as follows:

(1) Suspend powdered Corning glass 0080 in water.

(2) Brush glass 0080 onto the ferrite.

(3) Heat glass 0080 in an oven in air to its sintering temperature, i.e., the temperature at which substantial fusion but not fluidity is indicated, e.g., 700-800 C.

(4) Allow the glass and the ferrite to cool.

(5) Brush a liquid suspension of powdered Corning glass 7570 over the sintered 0080 glass.

(6) Heat the ferrite and glasses to the working temperature of the 7570 glass, i.e., about 560 C.

(7) Allow both glasses and ferrite to cool.

Corning glass 7570 by itself has utility in the manu' facture of the ferrite seal of the present invention in that its working temperature is 560 0, whereas most other glasses have a working temperature above 800 C. The use of this glass reduces the risk of structural failure due to the low thermal shock resistance of fcrrites. Cornlng glasses 7570 and 0080 have thermal expansion coetficrents of 84 and 92 l0 inch per inch per C., respectively, and in this regard are useful in preventing dllferential stresses. Youngs modulus of Corning 7570 is w1th1n the usual soft glass range. Likewise, Corning 0080 has a Youngs modulus of 98x10 p.s.i.

A few other glasses may also be used to hermetically seal a ferrite material. Among these glasses are clear seallng glass," which is designated by the present Corn- Ingnumber code, by 8870. Its application is somewhat llmited, particularly because of its high dielectric con stant, viz., 9.5; however, its thermal expansion coefficient is 91x10 inch per inch per C. and a large advantage accompanying its employment is its modulus of elasticrty, 76 10 p.s.i., which is comparatively low.

What is claimed is:

1. A device for rotating the plane of polarization of an electromagnetic wave comprising: an elongated figure of revolution ferrite compact slug having a cylindiical mid-portion and tapered end portions, said compact slug being composed of a ferromagnetic ferrite material having predetermined moisture sensitive and heat sensitive physical characteristics; and a multi-layer protective glass coating disposed over the entire exposed surface of said slug for hermetically sealing said slug, said coating comprising a first relatively very thin layer of a glass having a predetermined working temperature which is lower than temperatures harmful to said ferrite material, said first layer substantially sealing said slug while not appreciably soaking into said slug, and a second layer of glass fused over said first layer and having a working temperature which is lower than said predetermined working temperature, said second layer totally sealing said first layer and thereby hermetically sealing said ferrite slug without appreciable soaking of said second layer of glass into said ferrite slug.

2. A device for rotating the plane of polarization of an electromagnetic wave comprising: an elongated figure of revolution ferrite compact slug having a cylindrical mid-portion and tapered end portions, said compact slug being composed of a ferromagnetic ferrite material having predetermined moisture sensitive and heat sensitive physical characteristics; and a multi-layer protective glass coating disposed over the entire exposed surface of said slug for hermetically sealing said slug, said coating comprising a first relatively very thin layer of a glass having a predetermined working temperature which is lower than temperatures harmful to said ferrite material, said first layer substantially sealing said slug while not appreciably soaking into said slug, and a second layer of glass fused over said first layer and having a working temperature which is lower than said predetermined working temperature, saidsecond layer totally sealing said first layer and thereby hermetically sealing said ferrite slug without appreciable soaking of said second layer of glass into said ferrite slug; and a tip having a taper less than that of said tapered end portions over at least one of said end portions of said slug for providing a gradual change of index of refraction for said electromagnetic waves as they traverse from an evacuated atmosphere to the mid-portions of said ferrite slug, said tip being composed of a glass having a dielectric constant substantially equally intermediate that of said fer rite material and said evacuated atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 2,568,881 Albers-Schoenberg Sept. 25, '1951 2,745,069 Hewitt May 8, 1956 2,748,353 Hogan May 29, 1956

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