US2883502A - Electrical resistors and other bodies with negligible temperature coefficient of expansion - Google Patents

Description

Aprll 21, 1959 M. A. RUDNER 2,

ELECTRICAL RESISTORS AND OTHER BODIES WITH NEGLIGIBLE TEMPERATURE COEFFICIENT 0F EXPANSION Filed Jan. 28, 1955 ELECTRICAL RESISTORS AND OTHER BODIES WITH NEGLIGIBLE TEMPERATURE COEFFI- CIENT OF EXPANSION Application January 28, 1955, Serial No. 484,681

7 Claims. (Cl. 201-63) This invention relates to plastic resin products and has "for an object the improvement in the characteristics of said products by reduction to a low or negligible value the temperature coeflicieut of expansion thereof.

This application is a continuation-in-part of my application Serial No. 245,280, filed September 6, 1951, now abandoned, and entitled Molded Resistors, and also of my application Serial No. 373,433, filed August 10, 1953, now abandoned, for Plastic Body With Negligible Temperature Coefficient of Expansion. This application is also a continuation-in-part of my application, Serial No. 221,197, filed April 16, 1951, for Fluorocarbon Resin Mixtures and Metal to Plastic Bonding.

Among the many plastic resins to which the present invention is applicable are the fluorocarbon or fluoroethylene resins. The fluorocarbon resins have excellent characteristics for use as electrical insulators. Moreover, they will withstand operating temperatures considerably higher than the operating temperatures customary for electrical or electronic components. Those resins, are, therefore, well suited for application in electronic and electrical components. One characteristic which is at times disadvantageous, however, is the large positive temperature coefficient of expansion that those fluorocarbon resins have. For example polytetrafluoroethylene, presently available in the market under the trademark Teflon,

is ideally suited as an insulating medium and is capable of operating up to 500 F., but it has a high temperature coefiicient of expansion. I

In its commercial form, the polytetrafluoroethyleue resin comes as a powder. The general procedure employed in forming a body from this powder consists in subjecting a quantity of the raw resin powder to a high pressure, for example from one to ten tons per square inch, to reduce the mass of the raw powder to a self-sustaining body of about one-fourth the original volume of the powder. The compressed body thus formed is then sintered at a temperature of about 725 F. It is then cooled, and, during that operation, may be maintained under pressure, in order that it may be confined to the desired size and shape while its temperature comes back down to normal.

The product thus formed, like products formed of other resins, exhibits a change in physical dimension upon variation in temperature. Such change takes place due to the resin material having a temperature coefiicient of expansion other than zero. Of course the greater the temperature coeflicient of expansion, which is a constant for a given material, the larger will be the extent of change in physical dimensions thereof. This fact is worthy of consideration in those instances where, for example, the article is part of an electrical element and constant physical relationships are desired and, in fact, may be required.

Although the fluorocarbon resins are possessed of excellent insulating characteristics and are capable of use over a wide range of temperature, 100 F. to +575 F. for polytetrafiuoroethylene and 100 F. to +375 F. for trifluorochloroethylene, their temperature coeflicients of expansion limit their application where close tolerance or dimensional stability is required.

One object of this invention is to provide a method of modifying the thermal expansion characteristic of a fluorocarbon resin to such an extent as to substantially eliminate the temperature expansion coetficient or to reduce it to a substantially negligible value.

Another object of this invention is to provide a mixture including a fluorocarbon resin and a loading material which will counterbalance any thermal expansion coeflicient of the resin to such an extent as to establish a net temperature expansion coefficient that shall be negligible.

In order to compensate for the expansion of the resin with increase in temperature, the loading material should have an appropriate negative coeflicient of expansion.

- Moreover, it should have other characteristics which make it compatible with the resin, to enable a homogeneous mixture and physical structure to be formed.

A material which I have found proper for this purpose is heat-treated lithium alumina silicate. The silicate is first fired, and after cooling is ground to a fine powder. The silicate as thus treated by the firing operation is available commercially under the trademark Stupalith, made by the Stupakotf Ceramic and Manufacturing Company.

The grades of Stupalith preferred are those with the maximum negative characteristics. Stupalith grade A-24l0 has been used with success. Any of the material after having been thus fired is stabilized in retaining a negative temperature coeflicient. In the claims appended hereto, I refer to heat-treated alumina silicate as the aforesaid material which has been so fired and stabilized. In accordance with the present invention, a quantity of raw powdered fluorocarbon, such as polytetrafluoroethylene or trifiuorochloroethylene, is mixed with a powdered dimensional stabilizing material having a negative tem- .perature coefiicient of expansion. The amount of dimensional stabilizer added with the fluorocarbon resin in mixture materially reduces the effects of the positive temperature coeflicient of expansion of the fluorocarbon resin.

With lithium alumina silicate selected as a dimensional stabilizing material, I have found that when it is mixed 1 ring, and micropulverizing have proved successful in providing the desired homogeneous distribution.

The mixture thus formed is then subjected to the pressure and temperature operations regularly employed for the resin alone, so that the final product Will be available either as an unfinished product for further machining operations, or as a finished product whose size, shape and contours have been established by the pressure-forming operation. Where the unfinished product is made up as raw stock material, the usual machining operations may be readily performed on such stock material for forming finished products of any desired size and shape.

By way of example, to illustrate the eflect of the mixture herein disclosed, a sample plate of polytetrafluoroethylene was formed to the dimensions of a square two inches by two inches, with a thickness of one-quarter of an inch, by the usual procedure of pressing a quantity of powder down to that thickness, and then sintering the body thus formed to the usual temperature of about 725 F and then cooling the plate thus formed down to normal ambient temperatures. A similar plate was then formed to the same dimensions but containing 25% by weight of the treated lithium alumina silicate. The plate formed from this mixture was subjected to the same pressure and temperature conditions during the pressure-forming and sintering operations. A

As presently conceived, there is utilized in accordance with the present-invention mechanism which involves an additive having the capability of reducing its volume with a rising temperature, and which when mixed throughout a mass of the fluoroethylene resin and the whole exposed to rising temperatures leaves microscopic voids within the plastic body into which the fluoroethylene resin may expand without increasing the overall dimensions of the'plastic body. While the foregoing mechanism is taking place, the strength of the body is not reduced or otherwise adversely aifected. By the dimensional stabilization of the molded, formed, or sintered body, stabilization of the other useful characteristics can 'be obtained, such for example as uniformity in electrical resistance characteristics of molded or formed resistors.

Both plates were then raised from a normal ambient temperature of 70 F. to a top temperature of 690 F. and the changes in dimensions were then measured. The change in linear dimension of the plate of plain material amounted to 0.100 inch, whereas the change in the plate formed from the mixture was less than 0.003 inch, representing a decrease'in the temperature-expansion coefficient to less than 3% of the expansion coefficient of the resin alone.

Slight variations of the given percentage of the treated silicate will correspondingly aifect the relation between the resin and the silicate to establish any satisfactory net temperature coeflicient of expansion for the integrated my co-inventor A. J. Pearl on November 14, 1951, both by their mineral names and by their formulae. They arepetalite (Li O -Al O 8SiO spo'dumene andeucryptite (Li -Al O -2SiO my preference, however, being the lithium alumina silicates Which were available commercially under the trademark Stupalith from the, Stupakoff Company at the time of filing the parent.

applications and, which are still available under the same trade-name.

Any or the foregoing lithium alumina silicates having negative temperature coefficients of expansion can be used in mixture with other fluorocarbon resins such as the one available on the market under the trade-names of Kel-F, Fluorothene. and Trithene, this resin being polychlorotrifluoroethylene. I have found that the same "techniques may also be applied to resins generally if of the type which may be reduced to a powdered form prior to molding or shaping, one or more of the lithium alumina silicates being added thereto in powdered form as above described.

Typical resins contemplated by the present invention are as follows: the methacrylates, the phenol formaldehydes, the furans, the urea formaldehydes, the melamine formaldehydes, the aniline formaldehydes, epoxy resins, silicate molding'compounds, ethyl cellulose, cellulose acetates, the cellulose acetate butyrates, plastics known under the trade-name of ?nylon (polyamide), the vinyl resins,

4 polyethylene, and polystyrene preferably of the modified type such as sold under the trade-names of Cerex," Gering Psx, and Pholite.

For greater uniformity and stability of the end product, it will sometimes be desirable to extrude more than once the mixture of a resin and the lithium alumina silicate. After the first extrusion of the material, it is pelletized and ground a second time. may again be repeated if desired.

While I have mentioned that the lithium alumina silicates should be present in amount about 25 of the. mixture by weight, this percentage is not critical; and

while 25% is adequate for many applications, I have used the lithium alumina silicates in amount as high as 70% by. weight. The exact amount will be adjusted in accordance with the end result desired and in respect to the limitations of the resin or carrier plastic material.

As the 'niechanism is presently conceived by which there is a material reduction in the positive natural temperature coefiicient of, expansion in the above plastic materials, it is believed the lithium alumina silicate homogeneously distributed throughout the mass exhibits a reduction in volume within the mass, with rising temperature, thus leaving microscopic voids or spaces into which .the plastic material can expand, due to its positive temperature coefficient of expansion, without affecting the overall dimensions of the molded body to the same degree as in the absence of the lithium alumina silicate. Thus, while the materials having the positive and. negative temperaturecoetfficients of expansion perform as dictated by their respective temperature coeflicients of expansion, nevertheless a body formed from a mixture of them exhibits properties possessed by neither and yet it retains to large degree the characteristics of the plastic or resin used.

There. will now be explained the application of the invention to resistors, and particularly to resistors formed by molding powdered materials or mixtures of powdered materials.

Resistors constitute important components in electronic systems. Either alone, or in combination with other electronic components, the resistors control the characteristics of a circuit or the method of operation of a cooperating component in order to achieve some desired end result or operation, or characteristic condition in a circuit, that will determine the constants of that circuit.

Resistors are used throughout the field of electrical engineering. Where they are used in the power engineering field, slight variations of current, with attendant slight variations of voltage-drop across the'resistor, ordinarily may have very little significance in the operation of the system in which the resistor is employed. Particularly,- since resistors used in the power field are ordinarily not connected to any kind of a sound-reproducing system, there is no problem in connection with What is termed internal noise from the variations of current due to changes in the internal resistance condition of a resistor during use and operation.

Inthe field of electronics, however, the resistance variations that occur within a resistor, even though slight, and the resultant slight variations in the current that traverses-the resistor, and the circuit containing the resistor, are significant. Those variations of current are fre quently amplified in the end result of the system and are usually present in a signal reproduction of the current traversing that resistor. Such reproduction may be as a sound, by way of speech or music, or it may be as a picture in a television system. In either case, the variations that occur within the resistor ordinarily are amplified within the system before they reach the reproducer and those variations cause a corresponding distortion of the true desired signal current within the reproducer.

Because of thelar ge number of resistors that are employedin electronic systems, economy is a factor of primary importance in 'the production of those resistors.

This operation Nevertheless, since their function is to control a circuit correctly, freedom from distortion is desirable and, therefore, is also important. The problem of producing resistors in quantity must, therefore, be solved coextensively with the problem of establishing and maintaining freedom from distortion within the resistor.

The method of forming resistors by molding certain materials which will have a conducting or a semi-conducting characteristic, lends itself to quantity production, which may satisfy the requirement of economy. The problem of obtaining a resistor with freedom from noise still remains however, and the desire for a quiet resistor with no current distortion is usually sacrificed in deference to the important consideration of economy.

An object of the present invention as applied to resistors is to provide a molded resistor that shall be relatively free of noise by reason of various novel features of construction in a resistor of the molded type.

' Another object of this aspect of the invention is to provide a line of resistors of the molded type, available throughout a range of resistance values that are ordinarily required in an electronic system, all of which shall be characterized by their imposed impedance or resistance values, which shall be relatively uniform among individual resistors having the same ingredients, and similarly processed in manufacture, and all of which shall be characterized by a relatively low noise factor, which shall be assured by a uniform homogeneous distribution of ingredients that establishes a relatively minimum average current density at any point throughout the resistor, and by a construction that makes available an exceptionally good terminal contact of relatively large area with a low resistance at the point of contact to a terminal for an external circuit.

Another object of my invention is to provide a resistor of the molded type having a high degree of temperature stability and structural stability, and characterized by a minimum change in internal structure or dimensional form throughout the normal range of operation of the resistor in its intended service.

The principles of this invention are applicable generally to resistors, and to the methods of making such resistors, formed from compressed powdered materials and from mixtures of such materials.

The present description is directed, by way of example, specifically to resistors made from various mixtures comprising fluorocarbon or fluoroethylene resins as a base or vehicle, with which various other materials in powdered form are preliminarily mixed to establish the resistance characteristic desired in the ultimate resistor. Two fluorocarbon resins are presently commercially available that have the chemical, physical and electrical characteristics which make them suitable materials for many applications, and particularly for the application here considered. One of those materials, polytetrafluoroethylene, is made and sold under the trademark Teflon, and the other, polychlorotrifluoroethylene, also known as polymonochlorotrifluoroethylene and as a thermoplastic polymer of trifluorochloroethylene, is sold under the names Kel-F, Fluorothene," and Trithene.

For convenience, the term fluorocarbon resin will be employed to indicate the materials, with the understanding that all such materials are included in the reference unless the contrary be explicitly stated. It is also contemplated, as earlier set forth, that other synthetic resins or plastics may be used as a base material, and mixed with conducting materials, as taught herein, to achieve the end results sought in forming molded resistors with the desirable characteristics indicated herein.

One of the striking physical characteristics of a fluorocarbon resin is its ability to resist wetting and sticking. It is, therefore, impervious to water and moisture. The resistivity of such material is very high. Its power factor is low.

These characteristics combine to make it an excellent base material for general electronic applications, and particularly for use in the manufacture of resistors.

The fluorocarbon resins are provided in their raw state as powders, which may be formed and molded by pressure and heat to any simple shapes that may be desired. The raw resins or materials may, also be formed in bar or sheet stock and then machined to shape, where complex shapes are desired that may not be readily adapted to simple molding or extruding operations, for example.

Because of its insulating qualities, each fluorocarbon resin has been used as an insulator. In that case, of course, it has been used solely as a spacing element in mechanical structures, and external auxiliary structures have been utilized to support the member formed of the fluorocarbon resin for use as an insulator. In practically all cases where it has been so employed 'in the past for its insulating characteristic, such external structure means have been required to support the body as a spacing medium, so its insulation would be available in the region where such insulation was Wanted. Because of the nonsurface sticking characteristics of each fluorocarbon resin, it has been considered impossible, and, in fact, it has been impossible heretofore to provide a direct physical. bonding between the body of fluorocarbon resin and any external material or member, particularly metal, with the usual supporting structure.

Another object of this invention, therefore, is to provide a body of synthetic resin, particularly a fluorocarbon, and specifically either polytetrafluoroethylene or trifluorochloroethylene, with a metallic surface layer strongly bonded thereto, in order to provide a metallic surface to accept a soldered connection with an external metal element capable of providing relative support between the body and an external element, so that either the body or the element may support the other.

A major feature and object of this invention, therefore, is to provide a resistor and a process of making the resistor, to contain a quantity of fluorocarbon resin raw material, mixed with a quantity of other selected raw material or materials, to provide a resistor body having a resistance of a selected value, with separate metallic layers integrally bonded at two spaced surface areas of the finished resistor, that shall serve as terminal areas to which external conductors may be suitably connected, as by a solder connection, in order to connect the resistor into some desired external circuit, or to another component thereof.

Another object of the invention is to provide a molded body as a resistor, and a method of making it, to embody a metallic element of substantial area at the point where an external conductor may be connected to that terminal area, in order that the current density through the body of the resistor may be kept relatively small up to the actual plane of contact with the external conductor, thereby to limit the noise-producing tendencies within the body of the resistor.

Another object of this invention is to provide a resistor, and a method of making it, to embody a contact surface having the contour of a mathematical curve or having the contour of any other desired form, in order to permit a resistance value or quantity to be derived by establishment of two contacts across the two selected points of the resistor for connection to an external circuit.

Another object of this invention is to provide a resistor of the molded type, utilizing a body of polytetrafluoroethylene, or of a similar fluorocarbon resin including trifluorochloroethylene, that shall provide its own surfaceprotecting insulation for the 'body of the resistor when completed.

The construction and the features of the invention, and the various advantages thereof as applied to electrical components including resistors, are more fully described and pointed out in the accompanying description and drawings, in which: I

' Figure 1 is a schematic view of a quantity of a fluorocarbon resin material in its'pure state, with juxtaposed end layers of the resin and conducting-material mixes in powdered form, before the entire quantity is compressed and heat applied, as for sintering;

Figure 2 is a similar view showing the reduced section and dimensions of the quantity of material of Fig. l, as an integral body, after it is compressed and heated;

Figure 3 shows the structureof Fig. 2 after the deposition of a surface layer of metal at the two ends of the body;

Figure 4 shows a'schematic side view of resistor as in Fig. 3, to which a pair of end terminals have been ap plied; I

Figure 5 is a view similar to that of Fig. 4, except a body of solder is appliedat each end of the resistor, by dipping in a tin bath, instead of applying a terminal, and the cohering tin then serves as a terminal surface for easy connection of an external conductor to the resistor;

Figure 6 is a schematic view of a molded resistor hav ing a sine-wave. contour along one edge, and having both that contour edge and its straight reference edge on the opposite side metallized in order to permit a variable resistance to be measured between two applied contacts movable along the reference edge and the contour edge; and

Figure 7 is a sectional view taken along the lines 77 of the element shown in Fig. 6.

In accordance with the principles of this invention, in order to form a resistor body from a fluorocarbon resin, for example, a quantity of powdered resin is thoroughly mixed with a quantity of correspondingly powdered material having electrically conducting characteristics that will impart to the finished resistor body a resistance of desired Value. In order to achieve a thorough homogeneous distribution of the materials, many mixing methods may be used, among them being micropulverization. The volume of the finished resistor and its respective dimensions will be determined by the value of resistance desired, and the desired rating in watts or fractions thereof, i.e. the current which the resistor is to conduct. Thus, the volumes of the resistors for equal resistance values will be determined by the permissible temperature rise. A larger volume with a greater radiating surface provides greater dissipation of heat and thus limits the normal operating temperature;

The materials which may be employed in the nature of first additive powders with the powdered'fiuorocarbon resin to impart the resistance characteristic to the mixture may be drawn, of course, from the entire range of materials which have current-conducting characteristics, and which may be stable and unchanging in that characteristic throughout the temperature range up to about 700 F, at which the resistor bodies will be molded or sintered, preferably the latter for polytetrafluoroethylene and the former for trifluorochloroethylene whose molding temperatures range from about 450 F. to about 650 F.

' As examples of the various materials which may be employed to'impart the resistance characteristic to such a mixture, the following are suggested:

1) Carbon in its various forms, such as lampblack, coke flour, animal carbon or charcoal, graphite (powdered), graphite (colloidal);

(2) Boron carbide;

(3) Ferrites (available under the trade-name of Feramics, a term used by the General Ceramic and Steatite Corporation, of Keasby, New Jersey, to designate their line of ferrites);

.(4) Powdered iron, in various forms, such as carbonyl;

(5 Any of the conducting metals, such as copper, tin, lead, silver, zinc, or allow mixtures of the metals.

In orderto achieve homogeneity within each resistor as lithium alumina silicate, with a negative temperature unit, and uniformity between the characteristics of units,

. base material, ofthe fluorocarbon resin for example, and

the additive material should both be mixed when in their respective finely comminuted states. The materials should be mixed and regroundduring the mixing operationin order to establish a maximum of kneading and interfolding of the two materials. Where the percentage of the additive may be relatively small as Within 25% of the complete mixture, forexample, a dry method of mixing may be adequate. However, where a large percentage of the additive is desired in the ultimate mixture, a

. wet mixing operation will be found more efiicient and.

satisfactory, and either or both of the ingredients are mixed and added to the mix as suspensoids in suitable liquid vehicles, and the mixed materials are later suitably separated from the liquid by any suitable filtering operation, after a homogeneous mixture has been achieved.

The various classes of materials designated above have been found suitable to form resistors within the followin ranges, for example:

(1) Under 1,000 ohms, various forms of carbon as additives;

(2) In the neighborhood of 1,000 ohms, the various metals;

(3) In the range from 10,000 ohms to 100,000 ohms, boron carbide;

(4) About one megohm, the ferrites.

The materials and ranges set forth above are, of course, merely exemplary. It is to be understood that in the actual manufacture of resistors, the value there--. of may be caused to vary over a very wide range by change in its size and change in the percentage of the materials incorporated therein. Accordingly, although resistors having a nominal range under 1,000 ohms may be made by adding powdered carbon to the fluorocarbon resin, resistors having a value as high as 100,000 ohms may also be made by using a combination of carbon and fluorocarbon resin by merely changing the percentage of the materials. The fact that resistors may be made to have a given value by incorporation of a certain material in predetermined amounts is set forth below by examples of resistors which were made in accord ance with the present invention and which possess measured values which varied substantially from the ranges set forth above.

A resistor having a low value of resistance was formed by a mixture of aluminum and 30% polytetrafluoroethylene. The measured value was 60 ohms. On the other hand, a resistor of higher value was formed with a mixture of 10% lampblack and polytetrafluoroethylene. The measured resistance of this resistor was 110,000 ohms.

The fluorocarbon resins are characterized by an ability to withstand relatively high temperature rises above ambient during operation. Of these, polytetrafluoroethylene has the best temperature-tolerating characteristic. However, if materials have a tendency to change their dimensions at the elevated temperatures, such variations in dimensions would, of course, be attended by internal variations in pressure, and, consequently, in contact resistance between various body particles. In order to counterbalance and neutralize any such tendency for the internal structure to vary under elevated temperature conditions, an additional additive is in:

cluded in the mix. Such an additive, which has been found satisfactorily effective, has been a material, such coefiicient, which will serve as a dimensional stabilizer to reduce the tendency on the part of the resistor to vary its physical dimensions under the heat and elevated tem-,

perature developed during operation, A number of ex;

amples of suitable lithium alumina silicates have already I been set' forth.

In producing electrical resistors, the dimensional stabilizer (one or more of the lithium alumina silicates) should be present in amount at least equal to by weight of the mixture, and preferably from about 10% to about 40% by weight of the mixture. Actually the percentage ofthe dimensional stabilizer present in the mixture may exceed 40% by weight of the mixture. However, once the amount of stabilizer exceeds 40%, physical degradation of the plastic begins to set in. For example, one of the most noticeable characteristic changes which takes place in a resistor having more than 40% of dimensional stabilizer is in the loss of desirable mechanical strength. However, such resistors may be made and the mechanical strength supplemented by enclosing the resistor in a supporting structure. The supporting structure may be a polytetrafiuoroethylene or polytrifluorochloroethylene tube. Where the use of basic materials having an excessively high positive temperature coefficient of expansion is contemplated, the total amount of filler (dimensional stabilizer) may be increased to as high as 90% by weight of the mixture. Such mixtures including this high amount of dimensional stabilizer may be fabricated using normal techniques, including extrusion.

A number of resistors embodying the present invention were manufactured prior to the filing of the parent applications. Examples of these resistors follow:

1. 25% boron carbide 10% lithium alumina silicate (Stupalith) 65% polytetrafluoroethylene Measured resistance=3.0 10 ohms 2. magnesium dioxide 15% lithium alumina silicate (Stupalith) 70% polytctrafiuoroethylene Measured resistance=l.1 10 ohms 3. 10% boron carbide 10% lithium alumina silicate (Stupalith) 80% polytetrafiuoroethylene Measured resistance=3.0 10 ohms In all cases the resistor samples were approximately .628 inch in diameter and had a length of approximately of an inch.

The homogeneous mixture of the current-conducting material and the fluorocarbon resin provides a very large number of current-conducting paths when the mixture is used to form the body of a resistor. Each of the current-conducting paths may be likened to a chain comprised of contacting current-conducting elements. Each path is a potential conductor of current for the resistor body. However, like all conductors, there is a limit as to the magnitude of current it will safely pass without becoming unduly overheated. If each of the parallel paths within the resistor body conducts a proportionate share of the total current, that proportionate share being well within the capacity of the path, then relatively stable resistance characteristics will be established and maintained in the resistor body. Under such stabilized conditions, a minimum of resistance variation due to heating effects will occur in each of the con ducting chains, and noise effects will be kept at a minimum.

Such optimum operation of the individual currentconducting chains may be assured and achieved if the entire current-conducting duty is reasonably equally distributed between the various internal chains in the resistor unit.

Such an optimum condition may be established and maintained, and is done so in the present case with the resistors manufactured in accordance with the present invention, by permitting each individual chain to conduct its own minute current stream from one terminal of the resistor to the other terminal of the resistor.

The method of making the mixtures and arranging the mixtures for the resistors, and several modifications of finished resistors are shown in the accompanying draw mgs.

As shown in Fig. 1, a quantity of powered material 11, such as a fluorocarbon resin, or other resin which may be similarly utilized, is preferably employed in a powdered state to constitute the base material for the resistor 10. In order to impart a resistance characteristic to the body of base material 11, a suitable electrical conductive-controlling additive 12 is thoroughly mixed with the base powder to form a homogeneous mass throughout which both the base powder and the additive are thoroughly and homogeneously distributed. The additive 12 may be any material having a conducting or a semi-conducting characteristic that will impart to the finished resistor a resistance value as desired.

The resistor 10 shown in Fig. 1 is cylindrical in form. In order to maintain substantial uniformity of current distribution throughout the body of the resistor, from one end of that resistor body to the other end of that resistor body, the two ends 13 and 14 of the resistor body 10 are formed to constitute terminals having an area coextensive with the cross-sectional area of the resistor body. Under such physical conditions, the current distribution with respect to the cross section of the resistor body he considered to the substantially equally distributed, as a result of which the current density at any point in the conductor body is a minimum.

In order to form the end terminal sections 13 and 14 for the resistor 10, a quantity of powder mix is disposed at the end of the original body powder quantity. Such end mixture consists of a mixture of the body powder mix plus a large proportion of a third material 15 that shall have a very high conductivity, such as copper, for example. In the case of this end mixture, the copper content may range from 40% to with the distribution of the copper content arranged to place the maximum proportion of the copper (above 50%) at the outer surface of the mix, in order that the conductivity at that outer surface may be a maximum, and in order that Such outer surface be free to accept a deposit of suitable metal sprayed thereon, or a deposit of metal such as tin or silver as a continuous monolithic layer strongly bonded to the copper particles in the mixture.

After suitable quantities of the body mixture and of the end terminal mixtures have been placed in a mold, the entire quantity of the mixture is then compressed to a volume about one-fourth of its initial volume, into a. Self-sustaining unit that may be handled without the unit falling apart. The compressed unit then has the relative appearance shown in Fig. 2, in which the main body section 11 and the two metallized end portions 13 and 14 are integrally bonded. After the body is thus compressed to the shape shown in Fig. 2, it is molded and/or sintered at a selected temperature, say 700 F. for polytetrafiuoroethylene, for an interval of time sulficient to assure the desired molding or sintering action throughout the entire body.

After the foreging operations are completed, a heated unit is permitted to cool gradually to ambient temperature. The two metallized end portions 13 and 14 are then subjected to a metal deposition process to deposit a layer of plain metal 16, Fig. 3, on those two end sections 13 and 14, suitable to accept a solder connection by means of which the two ends of the resistor may be connected to external conductors of an associated circ-uit. A deposition suitable for this purpose may be made by metal spray, by electro-deposition, or by mechanical application, or by chemical action, or by any other process whereby metal may be deposited onto a metal surface.

After the layer of metal 16 has been deposited onto the two ends of the resistor body, a layer of solder 17, Fig. 5, may also then be added, or the ends of the reyailablefor a soldering oper-ation dering operation, which may be readily performed-during..-'

the manufacturing processes by the use of a solder pre-. form-22 .at the two ends of theresistor-and under the inner surface of the ferrulesor contact members. 21..

the ends Alternatively, the ferrule may be applied after have been tinned, as in the bath.

This method of forming a resistor, from-a fluorocarbon. resin-rbody, or'from other suitable :base materials, lends.v

itself readily to the formation of otentiometers or'resistance cards havinga predetermined orcharacteristic resistance curve thatmaybe utilized .for. control purposes.v

For example; as shown in.Fig. 6, a potentiometer .25

may be made .to have a resistance variationfollowing a sine curve relationship along. its normal operating dimension. Such variation in the resistance value of. the potentiometer resistor may .then be-utilized to change-the. circuit constants ofexternal circuits to-which the po tentiometerlis connected.

As shown in Figures 6 and 7, the main body may be. formedwfrom the initial mix containing the fluoroa carbon resin and the material that is to impart the resistance characteristic in the range desired,.andthe=sur-.-

face may be formed from a metallized fluorocarbon-mix to form the'desired end or edge layer 26, similar .to-the terminal portions. 13 and 14 of Fig. 1.

The metallized layer 26 is then covered with a metallic deposit 27 ofcontinuous metal to serve as a contact-engaging surface. Similarly, the upper surface of the potentiometer. body. is covered with a metallic mix to form a metallized base-l 28-that will be covered With'a deposit of a continuous layer 29 that will serve as a contacting surface.

33.. whichmay be moved along thepotentiometer-in ac-.

cordance with any predetermined scheduleto establish-- 'a varyingresistance between the two contacts '31 and=3 2,

Two contacts 31 and 32 are supported on a movable frameto:control the external circuit to which the contactsare connected through conductors 34'and. 35.

By the -methodforming the resistor bodies, .as disclosed herein, the body of aresistor' is-uniformly homogeneous, the conducting-path along the body is of uniform-cross-sectional area, from one terminal to the'other terminal, and the current-conducting lines-or chains are of substantially equal. length from-terminal to terminal; due tothe end surface disposition of each terminal area.

Moreover, the additional of thestablizingingredient to the mixture of thevehicle and of the conductingmaterial reduces the-change in resistance characteristic, inducedwby physical expansion produced by heating throughout the normal thermali'range of operation,-.

whether the heat is produced by internal generation or whether established by ambient temperature conditions.-

The foregoing advantages are presentirrespective of:

the particular disclosedbase material employed. An additional. feature: and particular advantagewhen one ofthe fluorocarbon resins is employed as .a base materialisthat the-surface .of the body provides a thin layer insula-..-

tion of. the fluorocarbonresin itself to-a thickness of about one mil, which is adquate in'many cases for: ordinary low-voltage circuits inwhich theresistor may be utilized.

By my reference to thefluorocarbon resins I 'mean to.

include. the homopolymersv of chlorotrifluoroethylene. sold.undervarioustrade-names such :as Kel-F, Fluorothene and.Trithene, and thelhomopolymers of tetra-- fluoroethylene sold under various trade'names such:-.a.s-

Teflonfi Fluroflex? and :Fluonf v 7 What is claimed. is: I

,1 An articleofmanufactu-re comprising a solid body? formed "by a mixture of powderedpolytetrafluoroethylene. and powdered heat-treated lithium alumina silicate having: .1

a negative temperature coeflicient' of: expansionand presentin quantity from at least 10% by weight to about :'90% by; weight -materially-to reducethe-effect on said;, body of the positive temperature coefiicient" ofexpansiom of said polytetrafiuoroethylene'.

2. An article -of"manufacture comprising a 1 solid body formed by a mixture of powdered polytetrafluoroethylene r and powdered heat-treated. lithium alumina silibate-having a negative temperature coefficient of expan sion and-present in 'quantity from at least 10% by weight tot-about byweight substantially entirely to comapensate-for thepositive temperature coefiicient of expansion of said polytetrafluoroethylene.

3.- A molded resistor comprising a main body Portion having two opposite-ends and said main body consisting of-{a homogeneous and intimate mix of a fluorocarbon resin selected'from the group consisting of polytetrafluoro- 'ethyleneaand polytrifluorochloroethylene, a dimensional stabilizer-consisting 'ofa heat-treated lithium alumina; sili- 1 cate mtamount from about, 10% by Weight to about 90% by weight, and 'apowder'of low conductivity materiaLg. and two terminal portions disposed at the respective ends of the mambody portion and each terminal portion-consisting of a mix similar to that of the main body and including an added ingredient having ahigh current conductivity to cause the terminalportions to be. of relatively low impedance.

ized by the addition of a surface layer of metal bonded to the terminahportionsand conditioned'to accept .a soldered connection.

5. A molded resistor consisting of a pressure molded body containing a fluorocarbon resin sele'cted'froml the group consisting -of polytetrafluoroethylene "and polytrifluorochloroethylene and an admixture of a powdered material homogeneously distributed'therethrough, as selected from the group consisting of graphite'in'powderediand" colloidal form, animal-charcoal, lampblack, coke flour, boron carbide, the ferrocerami'cs,iron,' copper, silver, tin, zinc, aluminum, magnesium, cadmium and lead, and a dimensional. stabilizer consisting of beat-treated lithium 4. A molded resistor as in claim 3, further characteralumina 'silicate having anegative temperaturecoefii f 'ClGIlt of expansion and present in amount from about 10%-i to about'90% by weight.

6a A moldedresistor consisting of 1a body-of a synthetic; 1

resinselected from the --group-consisting 10f: polytetra-t fluoroethylene and polytrifluorochloroethylene'andserving as. a-vehicle-andin the absence of the dimensional. stabi-a hzerheremafter specifiedlzhavingra positive temperature.

coefiicient of expansion; a quantitytof conductingmaterial and a dIHICIISlOIlfllvSlIfibiliZCI- consisting of heat-treated lithiumxalumina silicate having a negative temperature 1 coefficient of expansionpresentin amount from about 10% to about 90%" by weight distributed homogeneously throughout-the body to impart a resistance characteristic thereto, and a separate layer of metal cohesively bonded' to eachof two spaced surface areas 'of the body.

carbon'resin selected from the group consisting of polytetrafluoroethylene and polytrifluorochloro'ethylene,, a current-conducting material homogeneously distributed throughout vsaid body, nnd-a dimensional-stabilizing material homogeneously, distributed throughoutsaid body,

said last-named material being selected from a group con- '7. A molded resistor *consistingiof a body'of a fluoro-.

sisting of heat-treated spodumene, eucryptitennd petalite. and having a negative temperature coelficient of'expan-- sion tandpresent in amount from about 10% to about 90% by weight forcounteracting anydimensional. variations-in said resistor-bodyqdue :to temperature-changes;

thereinv 7 (References. onfollowing page) r References Cited in the file of this patent UNITED STATES PATENTS Power July 28, 1931 Hansell J an. 2 1945 Joyce Jan. 8, 1946 Brubake'r May 14, 1946 Harvey July 9, 1946 Buehler Apr. 8, 1947 Agens Sept. 7, 1948 10 Warrick Feb. 1, 1949 Haayman et al Feb. 7, 1950 Ferguson et al Mar. 21, 1950 Zabel et a1 Oct. 17, 1950 Bromberg et al. May 29, 1951 15 14 Murdick et a1. June 12, 1951 Rubin May 12, 1953 Morin July 14, 1953 Pearl et a1. Dec. 29, 1953 Smith et a1 May 25, 1954 Podolsky et al Jan. 10, 1956 FOREIGN PATENTS Great Britain Mar. 10, 1936 OTHER REFERENCES Modern Plastics, October 1948, pages 168, 170, 172. Ladoo: Non-Metallic Minerals, 1951, pages 290 to

Claims (1)

1. AN ARTICLE OF MANUFACTURE COMPRISING A SOLID BODY FORMED BY A MIXTURE OF POWDERED POLYTETRAFLUOROETHYLENE AND POWDERED HEAT-TREATED LITHIUM ALUMINA SILICATE HAVING A NEGATIVE TEMPERATURE COEFFICIENT OF EXPANSION AND PRESENT IN QUANTITY FROM AT LEAST 10% BY WEIGHT OT ABOUT 90% BY WEIGHT MATERIALLY TO REDUCE THE EFFECT ON SAID BODY OF THE POSITIVE TEMPERATURE COEFFICIENT OF EXPANSION OF SAID POLYTETRAFLUORETHYLENE.

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