US2362428A - Dielectric materials and electrical apparatus containing them - Google Patents

Description

Nov. 7, 1944.

B. S. BIGGS ETAL DIELECTRIC MATERIAL AND ELECTRICAL APPARATUS CONTAINING IT Filed Dec. 6, 1958 FIG] CORRELA TION OF DIELECTRIC CONSTANT AND TEMPERATURE OF TE TRACHLORO ORTHO -XYLENE- PAPER IMPRE GNA TED WITH A MATERIAL SUCH AS A HEXA SUBSTITUTED BENZENE. THE SUBSTITUENTS CONS/STING OF METHYL AND POLAR GROUPS,ATLEAST ONE OF THE SUBSTITUENTS BEING A METHYL GROUP AND ATLEAST ONE A POLAR GROUP- FIGZ CORRELA TION OF DIELECTRIC CONSTANT AND TEMPERATURE OF 4, 5,6 TRICHLORO ORTHO-XYL ENE.

8 souo L/oum 0 20 4-0 60 80 I00 TBIPERATURE IN DEGREES CENTIGRADE CORRELATION 0F DIELECTRIC CONSTANT I TEMPERATURE OF A MIXTURE OF I M01. 0F TETRA H -0RTHO G LORO XYLENE I 7 MOLS 0F 4,5,6 TR/CHLORO-ORTHO-XYLENE urc s sxc j IOKG- L, 4 30kt 1 I00 KC 02 3 '3 Q 7 -l80 I20 -80 4O 0 TEMPERATURE IN DEGREES CENT/GRADE FIG.6

A MATERIAL SUCH AS A HEXA SUBSTITUTED BENZENE, THE SUBS TITUENTS CONS/STING OF METHYL AND POLAR GROUPS AT LEAST ONE OF THE SUBSTITUENTS BEING A METHYL GROUP AND AT LEAST ONE A POLAR GROUP- flWIIIIIIIII/IIIIIII FIG. 7

A MATERIAL SUCH AS A HEXA SUBSTITUTED BENZENE, THE SUBSTITUENTS CONS/S TING OF METHYL AND POLAR GROUP5,ATLEAST ONE OF THE SUBSTITUENTS BEING A ME THYL GROUP AND ATLEAST ONE A POLAR GROUP gs sz zzaa WENZSR AJiwH/TE A TTQR-ALEV Patented Nov. 7, 1944 UNITED STATES PATENT OFFICE DIELECTRIC MATERIALS AND ELECTRICAL APPARATUS CONTAINING THEM York Application December 8, 1938, Serial No. 244,171

28 Claims.

This invention relates to dielectric materials and more particularly to electrical apparatus embodying these materials.

In telephone exchanges, particularly in urban centers, space is at a premium and any substantial reduction in the size of apparatus is of paramount importance. Then too, in mobile bodies, such as airplanes, weight and space are factors which dictate the reduction of communication apparatus employed therein to the minimum in size necessary for successful operation. In both telephone and communication systems condensers are used in relatively great numbers and any reduction in their size without appreciable diminution in their functional characteristics results in material economies.

An object of this invention is to reduce substantially the size of apparatus employed in electrical systems which are subjected to wide variations in temperature.

Another object is to utilize in electrical apparatus materials having high dielectric constants and desirable physical and electrical properties. In accordance with this invention dielectric materials are employed which under conditions of use are solids and which either alone or in conjunction with other dielectrics, such as paper, provide high capacity in electrical apparatus and which at the same time have physical properties such as melting point, compressibility, viscosity and penetrabillty, which render themadmirably adapted for use in electrical apparatus. Further, these materials retain the high dielectric constant at high frequencies and over a wide range of temperatures, are non-hygroscopic, are

' chemically stable, do not undergo electrolytic changes and do not decompose or break down under conditions of use in electrical apparatus. The materials to which this invention relates include polar compounds selected from the group which consists oi hexasubstituted benzenes, the substituents consisting of methyl and polar groups, at least one of the substituents being a methyl group and at least one a polar group; pentasubstituted benzenes, the substituents consisting of methyl and polar groups, at least two of the substituents being methyl groups and at least one a polar group; pentasubstituted benzenes, the substituents consisting of methyl and polar groups, at least one of the substituents being a methyl group and at least two other sub stituents being two different polar groups; pentasubstituted benzenes, the substituents consisting of methyl and polar groups, at least one of the substituents being a methyl group and at least one of the polar substituents being a member of the class which consists of amino, bromo, fluoro, iodo, hydroxy, and nitro, all of the compounds having sufllcient geometric symmetry in one plane about the molecular center of gravity to permit molecular rotation..

When polar molecules, that is, those containing an electric dipole moment greater than zero, engage in rotational motion, materials containing these molecules exhibit a dielectric constant higher than the square of the optical refractive index by an amount indicated by the magnitude of the dipole moment. All liquids composed of such polar molecules as a consequence manifest correspondingly high dielectric constants since rotational motion of molecules in liquids is relatively free. It would be expected that when such liquids were solidified, the dielectric constant would fall to the square of the refractive index a value of between 2 and 3-because of the elimination of molecular rotation in the more rigid solid state.

Materials included in the class to which this ,iluorotoluene, dinitrophehnitene, dinitroisodurene, 2,4,5-trichloro-meta-xylene, 4,5,6-trichlo ro-ortho-xylene, 2,4,6-trlchloro-meta-fluorotol uene, and 5-nitro-3,4-dichloro-ortho-xylene.

It has been established heretofore that in an organic crystal, molecules cannot engage in translational motion to any great extent, their locations being fixed in a regular array in space according to the symmetry of the crystal. For a molecule thus fixed to engage in rotational motion to any great degree it is necessary that its energy of interaction with its neighbors should not change greatly as the, orientation of the molecule changes. As a consequential requirement for molecular rotation, the smallest distance between any part of the molecule in question and the neighboring molecules must not change greatly as the orientation of the molecule changes. Hence, the molecule must possess a high degree of geometrical symmetry about its center of gravity, which determines the axis of the rotational motion, for such motion to occur appreciably in the crystal. In theory, this symmetry may be either spherical or circular, molecular rotation occuring only about the axis perpendicular to the plane of the circle in the latter case. The effect on the dielectric constant of a specimen containing a large number of randomly oriented single crystals is the same in either case, provided the dipole lies in the plane of the circle when the symmetry is circular. The compounds of this invention have symmetry about one axis only and these compounds have sufficient geometric symmetry in one plane about their molecular center of gravity to permit molecular rotation. This symmetry may be ascertained by an examination of scale models and diagrams of the pentaand hexa-substituted benzene compounds to which this invention relates. The symmetry arises from the similar eifective volume of the methyl and polar groups and atoms as compared with the extremely small hydrogen atoms which they replace.

The use of a methyl group or groups suitably located with respect to other polar groups is necessary to obtain a large dipole moment since if all the hydrogens of benzene were substituted with the same groups or with groups or atoms of similar polarity, then polarities would tend to cancel each other due to their opposing directions. For example, the geometrical symmetry of the dichloroprehnitene molecule is highabe; cause of the similar volume of. the chlorine atoms and methyl groups, while the molecule is polar because of the location of these substituents about the benzene nucleus; thus the molecule is able to rotate in the crystal because ofits geometrical symmetry and the rotation in turn produces a high dielectric constant because of the dipole in the molecule. Geometrical symmetry about the molecular center of gravity is somewhat lower in dinitroprehnitene because of the adjacent nitro groups weight one side of the molecule so heavily that its center of gravity is considerably displaced from the center of the benzene ring. In the pentasubstituted benzenes, symmetry is reduced because of the small size of the one hydrogen atom as compared with the substituents. The effect of the lower symmetry of these compounds appears in the abrupt drop of their dielectric constant at a transition, in place of the of measurement, which characterizes the dielectric constant of the hexasubstituted benzenes that are most symmetrical about their centers of gravity. Among solids whose molecules have two or more unsubstituted positions, such as 2,3,5-trichlorotoluene and ortho-dichlorobenzene, no evidence of a high dielectric constant in the solid state has been obtained. This demonstrates that with two or more unsubstituted positions benzene derivatives become so unsymmetrical as to reduce greatly the possibility of molecular rotation in the crystal.

It has been established heretofore that in pure homogeneous non-ionizing polar liquids, the difference observed between the dielectric constant at radio frequencies and at optical frequencies (the latter being the square of the optical refractive index according to Maxwell's law) is due almost entirely to the rotational motion of molecules containing dipoles. Furthermore, the magascacae nitude of this diiference between the dielectric constant and the square of the refractive index is roughly determined by the mlsnitude of the molecular dipoles, the dielectric constant increasing with increasing dipole moment. Heretofore, it was believed that when a polar benzene derivative changed from the liquid to the solid state by freezing, rotational motion of the molecules ceased, the molecules being almost rigidly oriented in the crystal. This belief was supported by evidence including the invariable fall of dielectric constant upon freezing to a value about equal to the square of the refractive index, as well as x-ray studies which showed the orientation of the molecule in the crystal to be fixed. However. none of these studies involved the use of any oi the materials falling within the class to which this invention relates. These materials have high dielectric constants through considerable ranges.

of temperature in the crystal as well as in the liquid state. It is believed that this high dielectric constant in the solid state is due to a continuation in the solid of the rotational motion of the molecule, which is known to exist in the liquid and to account for its dielectric constant. Support of this belief is found in the absence of any other known mechanism whereby a pure nonionizing homogeneous organic solid can exhibit a dielectric constant appreciably greater than the square of the refractive index, and the relatively small change of dielectric constant which occurs at the melting points of the compounds within the class to which this invention relates. These facts indicate that the same mechanism is responsible for the observed values in the solid as the liquid state. Finally a rough correlation exists between the values of dielectric constant of the solids and molecular dipole moment, which demonstrates clearly that the value of dielectric constant, although influenced by other factors such as density and molecular interaction, is primarily dependent on the dipole moment of the molecule. Since it has been established that the dipole moment cannot possible influence the dielectric constant except through molecular rotation, the high dielectric constant in the solid state of the materials to which this invention relates is due to a continuation of rotational motion in the molecule of the solid compound.

A molecule is said topossess a permanent electric dipole when it is electrically asymmetrical, that is, when the center of gravity of the positive charges is separated from the center of gravity of the negative charges by a finite distance. A molecule containing a permanent electric dipole tends to orient itself in an electric field analogously to the well-known behavior of a permanent magnet orienting itself in a magnetic field. The compounds to which this invention relates possess a permanent electric dipole which is electrically asymmetrical and the center of gravity of the postive charges is separated from the center of gravity of the negative charges by a finite distance. The existence of an electric dipole may be ascertained by any of the well-known standard methods.

. In the second class of compounds when the tem-' swam Fig. 7 is an alternate form oi the invention relates may be divided into two classes. The

distinguishing behavior of the first class is characteriaed by' the manner in which, as the temperature of a polar compound is reduced the dielectric constant drops of! from a high value at a different temperature for each frequency of the applied electric field used for measurement.

peratiire of the compound is reduced the dielectrio constant drops sharply at all frequencies alike from a high to a low value. The dielectric behavior at various temperatures oi a member oi the first class is illustrated in Fig. 1 which shows the dielectric constant of tetrachloro-crtho-xylene plotted against temperature for a number of different frequencies of measurement. The dielec tric constant for one, three, ten. thirty and one hundred kilocycles of various temperatures is shown by means of labeled black lines. The melting point of the tetrachloro-ortho-xylene is indicated by the M. P. symbol. It is observed that as the frequency is increased the temperature at which the dielectric constant drops oil also increases. Such dependence of dielectric constant on frequency is commonly termed anomalous dispersion. The dielectric behavior oi 'members of the second class is typified by 4,5,6 trichlcro-ortho-xylene. The temperature variation of the dielectric constant for all frequencies between one and one hundred kilocycles is shown graphically in Fig. 2. When a compound of this latter class is heated from low temperatures, a temperature is reached where a sharp rise of dielectric constant to the original high value is observed. The black line shown in Fig. 2 shows the dielectric constant of the 4,5,8 trichloro-ortho-xylene at various temperatures, while the symbol M. P. indicates the melting point or that compound. The arrows represent the direction of the temperature change being effected. when the temperature is decreased from a high value to"below approximately 20 0., a slight increase in dielectric constant is discernible and this increase continues to approximately C. However, as indicated by the arrows, if the temperature oi the 4,5,6 trichloro-ortho-xylene is increased from about0 C. the dielectric constant remains .at a substantially low level until approximately C. is attained. At this point. the dielectric constant increases to that-corresponding to the dielectric constant obtained with decreasing temperature; Frequently, this hysteresis occurs in compounds oi the second class when subjectedto rapid temperature change.

Except at low temperatures, compounds oi both classes have dielectric constants which are greater than the squares of their reiractive indices, which in all cases are found to be no more than three by calculation according to well-known methods. At low temperatures the dielectric constans of both classes of material are approximately equal to the squares 0! their refractive indices. The dielectric behavior or both classes of compounds in the solid state resembles that of liquids and in all but a few cases, such as dinitroprehnitene. the dielectric constant is higher in the crystalline than in the liquid-state as shown in Figs. 1 and 2.

The dielectric constants at C. and at --195' C. together with the molecular dipole moments Dielectric Dieledrie Moleulhr constant constant dipole at I) 0 at -i0l' C. moment Pentsmothyidflmohonmne.-. 8.7 2.85 I l. Didiloroprebniteno.-...-.. I. i 2.75 3. ichlom-iso-durene 6.8 2.0 l. 8.5 2W 3. 5.3 2.0 l. 3.7 2.97 l. 0.0 2. 3. 5.2 3.0 l. Pontamothyliiuorobensene 3.5 2.8 l. 'letrnclilorometaiiuorotoiuene 5.8 2.01 l. Pentachloromethylbenscne-.. 5. l 2.8 l.

The dielectric constant was determined at '20 V C. and at -195 C. since at the former temperature rotation of the molecule occurs, while at the latter temperature it does not.

The transition temperature, dielectric constants above the transition, the dielectric constants below the transition and the molecular dipole moments of some of the compounds of the second class are as follows:

Transition Dielectric Dielectric tern xii-alum cortigiant cotigiant fl g g I 8 ill I! \6 0W temperature transition transition mmnem Dinitroprehnl- "C.

tone +160 18. 5 2. [l5 7. 60 2,4,li-trichloromcta-xvlcnc. 2i 4. 2 2. 02 i. l 4,5,6-trichioroortho-xylene. i7 7. 0 2. 82 2. 2 2,4,6-trichloromota-iluorotoluene --65 4. 4 2. 85 i. i 6-nitro-3,t-diohloro-orthoxylene 0 9. i 2. 9

All-of the compounds to which this invention oiaomeottheccmpcimdsoitheiirstclassare asiollows:

relates are polar materials selected from the class which consists of helm-substituted benzenes, the substituents consisting of methyl and polar groups, at least one of the substituents being a methyl group and at least one a polar group; penta-substituted benzenes, the substituents consisting of methyl and polar groups, at least two of the substituents being methyl groups and at least one a polar group; penta-substituted benzenes, the substituents consisting of methyl and polar groups, at least one of the substituents' being a methyl group and at least two other substituents being two different polar groups; pentasubstituted benzenes. the substituents consisting of methyl and polar groups at least one of the substituents being "a methyl group and at least one of the polar substituents being a member of the class which consists of amino, bromo, iiuoro. iodo, hydroxy, and nitro. The penta-substituted derivatives are those in which all but one of the hydrogen atoms are replaced with other atoms or atomic groups and the heirs-substituted derivatives are those in which all the hydrogen atoms are replaced with other atoms or atomic groups. The atoms and groups in each compound are so distributed among the substituted positions on the benzene ring that the molecule as a whole is polar, that is, that it s a known electrical dipole moment greater than zero. Accordingly, the compounds have a iinite dipole moment. Although the most desirable compounds are those in which the polar substituent groups are methyl, chlorine, fluorine or nitro other polar substituents such as, amino and hydroxyl may likewise be employed. Examples compounds of this invention are fluoro-nitro-prehnitene, difluoroprehnitene, polar isomers of dichloro-fluoroxylene, the polar isomers of trimethyl dichloro- The presence of one or more atoms of fluorine which are substituted for a chlorine atoms in a pentaor hexasubstituted benzene lowers the transition temperature and thus increases the range of temperatures in which the material has a high dielectric constant. For example, the transition temperature of 2,4.fi-trichloro-meta-iiuoro-toluene is -65 C., whereas that of 2,3,4,6-tetra-chlorotoluene is 22 C. In the hexa-substituted compounds the introduction of adiuorine atom or atoms has a similar eii'ect in lowering the temperature at which the dielectric constant falls off for any given frequency although no transition is involved; fo example, the fall of the dielectric constant of tetrachloro-meta-fluorotoluene is for one hundred kilocycles most rapid at 78 C. while the corresponding temperature in pentachloro-toluene is +10 C.

The dielectric materials in accordance with this invention may be employed in condensers as shown in Figs. 5, 6, and '7. In Fig. 5 sheets of a porous dielectric H such as paper are impregnated with a compound to which this invention relates. Interposed between the sheets of dielectric are metal foils [0. Both the dielectric and foils are wound in a well-known manner to form the condenser. Two metallic strips [2 are electrically connected to the electrodes Hi to form the terminals of the condenser. The compounds to which the invention relates are stable when aluminum foils are employed as electrodes l0. For example, the chlorine derivatives do not evolve any appreciable quantity of hydrochloric acid and accordingly condensers in which these derivatives are employed are not prone to break down. Alternately the polar pentaor hexa-substituted benzene may be a coating 14 on a metal foil i6 as shown in Figs. 6 and 7. The coated metal foils may be separated as shown in Fig. 6 by a strip of another dielectric l5 and the coated foils piled alternately in a well-known manner to form a condenser.

The polar pentaor hexasubstituted benzenes may also be employed in other electrical apparatus such as wave guides in accordance with the disclosure in U. S. Patent 2,129,711 granted toG. C. Southworth on September 13, 1938, where a relatively high capacitance is required.

Mixtures or solid solutions of the compounds to which this invention relates may be used to obtain combinations of dielectric and physical properties which greatly enhance their usefulness for the impregnation of paper condensers. The proportions of the constituents of these mixtures are critical for this particular use. For example, a mixture of approximately 1 mol of dichloroprehnitene with 7 mols of 4,5,6-trichloro-orthoxylene is found to have a transition reduced in magnitude and displaced to lower temperature than that found in pure 4,5,6-trich1oro-ortho-' xylene. The dielectric properties of this mixture are shown graphically in Fig. 3 in which the dielectric constant is correlated with the temperature of the mixture. The labeled black lines represent the dielectric constant for one, three, ten,

2,se2,42a

thirty and one hundred kilocycles when the temperature is increased from 140' C. The desirably high dielectric constant of 4,5,6-trichloroortho-iwlene is thus made available at temperatures below 10" C. while it may be undesirably low at about +15 C. in the pure material. At-

the same time the undesirable melting point of 191 C. found for dichloroprehnitene is reduced in the mixture to an initial crystallization temperature of 112 C. The characteristics 0! this mixture are desirable when paper condensers are impregnated since the dielectric must be liquefied. At temperatures above 130 C. paper tends to deteriorate and the vapor pressure of the compounds becomes so high as to render vacuum impregnation of paper units dimcult. To secure the desired combination of melting point and transition temperature of this mixture, it is necessary that the proportions be extremely close to 1 mol of dichloroprehnitene and 7 mols of 4,5,6-trichloro-ortho-xylene. Any considerable increase of the 4,5,6-trichloro-ortho-xylene results in a transition temperature in the operating range of the condenser, while any considerable increase of the proportion of dichloroprehnitene raises the melting point of the mixture beyond the temperature of practicable impregnation of paper condensers.

Another mixture which has been found extremeILy satisfactory for paper condensers consists of approximately 1 mol of tetrachloro-orthoxylene and 7 molsof 4,5,6-trichloro-ortho-xylene. The dielectric properties of this mixture at various temperatures are shown in Fig. 4. The isbeled black lines represent the dielectric constant for one, three, ten, thirty and one hundred kilocycles when the temperature is increased from l40 C. The temperature of the initial crystallization of this mixture is 124 0., an extremely great improvement over the melting point of tetrachloro-ortho-xylene which is 230 C. The proportions of the ingredients in this mixture are also very critical, depending on the proportions specified.

In addition to the example cited above, other mixtures of two or more polar compounds of the type to which this invention relates may be employed: For example, mixtures of 4,5,6-trichloro-ortho-xylene with tetrachloro-meta-fluore-toluene, of 4,5,6-trichloro-ortho-xylene with pentachlorO-toluene, of 4,5,6-trich1oro-ortho-xylene with tetrachloro-meta-xylene, of 4,5,6-trichloro-ortho-xylene with trichloro-pseudocumene, of 4,5,6-trichloro-ortho-xylene with trichloro-hemimellithene, of 4,5,6-trichloro-orthoxylene with penta-methyl chlorobenzene, of 4,5,6- trichloro-ortho-xylene with penta-methyl fluorobenzene, of 2,4,6-trichloro-meta-fluoro-toluene with tetrachloro-meta-fluorotoluene, of 2,4,6-tri- 2,862,428 lates, as dielectric materials in electrical devices are illustrated by various examples:

Example 1.A paper insulated capacitor such as that shown in Fig. designed for operation at low voltage (500 volts or less) and having 9. volume of 13.3 cubic centimeters has, when impregnated according to standard practice with halowax, a commercial dielectric comprising chlorinated naphthalenes, a capacity oi 1.0 microfarad. When such a paper insulated capacitor unit is similarly impregnated with a mixture of on mol of tetrachloro-ortho-xylene and. 7 mols oi 4,5,6-trich1oro-orthoxylene, its capacity is found to be 1.0 microfarad and it withstands severe treatment such as subjection to a temperature of 100 C. over a long period of time before showing evidence 01' decomposition.

Example 2.'A similar condenser impregnated with a mixture of one mol of dichloroprehnitene and 7 mols of 4,5,6-trichloro-ortho-xylene exhibited a capacity of 1.0 microfarad and is capable of withstanding severe heat treatment. Example 3.A paper insulated condenser impregnated with tetrachloro-meta-fluorotoluene by a continuous process whereby the temperature of about 150 C. necessary to maintain the liquid state does not affect the paper long enough to cause appreciable decomposition, may have a capacity equal to that obtainable with halowax.

Example 4.--A paper insulated condenser impregnated with difiuoroprehnitene may have a capacity equalv to that obtained with halowax combined with excellent chemical stability, long life and dielectric breakdown strength because of the intrinsic stability of the aromatic fluorine bond and the absence of any catalysis of decomposition by aluminum fluoride, which may be formed in small quantities due to the contact of the aluminum foil of the condenser with the difiucroprehnitene.

While preferred embodiments of this invention have been illustrated and described various modifications may be made therein without departing from the scope of the appended claims.

What is claimed is:

1. An electrical condenser comprising armatures separated by a solid dielectric comprising a solid polar substituted benzene having at least five substituents. the substituents consisting of methyl and polar groups, at least two of the substituents being methyl groups and at least one a polar group, said compound having a finite dipole moment and having suilicient geometric symmetry in one plane about the molecular center of gravity to permit molecular rotation in the solid state. whereby the compound in the solid state, throughout at least a limited temper-- ature range, possesses a dielectric constant which is substantially greater than the square of the optical refractive index of the substance.

2. An electrical condenser comprising armatures separated by a dielectric comprising a solid polar substituted benzene having at least five substituents, the substituents consisting of methyl and polar groups, at least one of the substituents being a methyl group and at least two other substituents being two different polar groups. said compound having a finite dipole moment and having sufiicient geometric symmetry in the one plane about the molecular center of gravity to permit molecular rotation in the solid state. whereby the compound in the solid state, throughout at least a limited temperature range, possesses a dielectric constant which is substantially greater than the square 01' the optical refractive index or the substance.

3. Electrical apparatus comprising two conducting elements separated by a dielectric element comprising a substance having a dielectric constant in the solid state substantially in excess of the square of its optical refractive index said substance comprising a solid polar substituted benzene having at least five substituents, the substituents consisting of at least one methyl group, at least one nitro group and the remainder, if any, halogens.

4. The device'described in claim 3 wherein the electrical apparatus is an electrical condenser and the conducting elements are condenser armatures.

5. An electrical condenser comprising armatures separated by a solid dielectric comprising a substance having a dielectric constant in the solid state substantially in excess of the square or its optical refractive index said substance consisting of a nitrcdichloroxylene substance.

6. An electrical condenser comprising armatures separated by a solid dielectric comprising 5-nitro, 3,4-dichloro-ortho-xylene.

7. Electrical apparatus comprising two conducting elements separated by a solid dielectric element comprising a polar substituted benzene having at least five substituents, the substituents consisting of methyl and halogen groups, at least two of said substituents being methyl groups and at least one of said substituents being a halogen.

8. The device described in claim 7 wherein the electrical apparatus is an electrical condenser and the conducting elements are condenser armatures.

9. The device described in claim 7 wherein at least one of the halogen substituents is fluorine.

10. Electrical apparatus comprising two conducting elements separated by a solid dielectric element comprising a polar substituted benzene containing at least five substituents, said substituents consisting of methyl and polar groups, at least one of said substituents being a methyl group and at least one of said substituents being fluorine, said substituted benzene having a dielectric constant in the solid state substantially in excess of the square of its optical refractive index.

11. The device described in claim 10 wherein all of the polar groups are halogens.

12. An electrical condenser comprising armatures separated by a solid dielectric comprising a penta-substituted benzene, the substituents consisting oi methyl and halogen groups, at least two of said substituents being methyl groups and at least one of said substituents being a halogen.

13. An electrical condenser comprising armatures separated by a solid dielectric comprising a penta-substituted benzene, the substituents consisting of methyl and chlorine groups, at least two of said substituents being methyl groups and at least one of said substituents being chlorine.

14. An electrical condenser comprising armatures separated by a solid dielectric comprising dichloroprehnitene.

15. An electrical condenser comprising arma-. tures separated by a solid dielectric comprising 4,5,6-trichloro-ortho-xylene.

16. An electrical condenser comprising armatures separated by a solid dielectric comprising tetrachloro-ortho-xylene.

17. An electrical condenser comprising arma-' tures separated by a solid dielectric element comprising 2,4,6-trichloro-meta-fiuoro-toluene.

18. An electrical condenser comprising armltures separated by a dielectric element comprising a plurality of polar compounds forming s solid solution, the dielectric constant of each of said compounds being substantially higher than the square of its respective optical refractive index, at least one of said polar compounds being a substituted benzene having at least five substituents, said substituents consisting of methyl and halogen groups. at least two of said substituents being methyl groups and at least one of said substituents being a halogen.

19. A device as described in claim 18 wherein each of said polar compounds is a substituted benzene having at least five substituents, said substituents consisting of methyl and polar groups.

20. A dielectric material comprising a solid solution of approximately one mol of dichloroprehnitene and seven mols or 4,5,6-trichloroortho-xylene.

21. An electrical condenser comprising armatures separated by a dielectric comprising a solid solution oi dichloro-prehnitene and 4,5,8-trichloro-ortho-xylene.

22. A dielectric element comprising a solid solution of dichloroprehnitene and a nitro-dichloroxylene.

23. A dielectric element comprising a solid solution of tetrachloro-ortho-xylene and 2,4,6-trichloro-meta-fluoro-toluene.

BURNARD S. B1008. STANLEY 0. MORGAN. ADDISON H. WHITE.

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