US2082102A - Resistance element - Google Patents

Description

June l, 1937. I J. R. FISHER 2,082,102

REsIsTANE ELEMEmn Filed April 25, 1955 /N VEA/TOR y J. RF/SHE'R A TTORNEV Patented June l, 1937 RESISTANCE ELMNT Joseph li.. Fisher, Brooklyn. N. Y., assigner to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application April 25, 1935, Serial No. 18,217

6 Claims.

This invention relates to electrical resistance units.

The object of the invention is to produce a resistance unit of silver sulphide and a method of 6 making the same.

' A further object of the invention is the produotion of a resistance tunit which will have a high negative temperature coeioient of resistance; that is, one whose resistance decreases with in- 10 crease in temperature, over the range of temperatures from room temperature to about 200 C. and whose resistance will be reproducible and reversible, that is, have the same value-at a given temperature regardless of the direction from l which this temperature is approached.

Another object of this invention is to provide a resistance having a high negative temperature coeflicient of resistance whose resistance will be independent of voltage and frequency. 20 A further object of this invention is the provision of `a resistance having a high negative temperature coemcient of resistance which can be easily formed into units having a range of resistance depending upon their size and shape.

Many materials are known to have a high negative temperature coeicient of resistance and this property is possessed in common by all semiconductors to some degree. However, most of these materials are unsatisfactory for use in elec- 30 trical control devices either because the change in resistance is not sucient, or because they are not stable and/or reversible, or because of their refractory nature they are not readily fabricated into a suitable form.

I have discovered that silver sulphide has a combination of properties which make it uniquely suitable for use in electrical devices either as a means of compensating for resistance changes in metallic structures, which have a positive tem- 40 'perature coeicient of resistance, or as a control device to operate when a change in resistance reaches a certain value.

I have discovered that by the following procedute, resistor umts may be prepared which are 45 suitable for use in electrical devices of the abovementioned nature. Chemically pure silver sulphide in the form of a powder, after sleving through a fine mesh screen, is placed in a steel die of suitable size and shapeand molded under 50 a pressure of approximately ten thousand pounds or more per square inch in a hydraulic press or ome similar device. By the use of a suitably .dnepowder no heat treatment or binder is necessary, the natural coherence of the particles being 55 sufliclent to produce a mechanically strong unit.

In the event that pure silver sulphide free from excess silver or sulphur is not available, ordinary silver sulphide may be used. However,

if ordinary silver sulphide powder is used ther sister has been formed and ls removed from the mold, it is given aheat treatment, that is, by

heating it rst in sulphur vapor at between 300 and 400 C. for approximately an hour and then subjecting it to an atmosphere of pure dry nitrogen gas at between 200 and 300 C. for' about an hour. Without this treatment the resistor may contain more or less sulphur than is required to exactly combine with the silver which will cause a resulting change in resistance with use.

Referring now to the drawing:

Fig. 1 illustrates, in cross-section, one form. of die which may be used for forming applicants resistance element, before force has been applied to the upper member;

Fig. 2 is a view, similar to Fig. l, except that Vthe upper member has been forced down and has compressed the powdered material located in the recessed portion of the lower member;

Fig'. 3 is the resistance element immediately following its removal from the die;

Figui is a view in cross-section illustrating the heat treating apparatus;

Fig. 5 illustrates one method of applying a conducting surface to the ends of the resistance element;

Fig. 6 illustrates the resistance element after it has been sprayed with molten metal in. accordance with the method shown in Fig. 5; and

Flg. 7 shows the completed resistance element with the terminal wires secured to the conducting surface of the resistor.

In accordance with the preferred form of the applicants invention. a quantity of chemically pure silver sulphide, in the form of a powder I0 is placed in the recess II of the lower die member I2, as shown in Fig. l. When this recess I I has the required amount of powder placed therein, lthe upper die member I3 is brought downward thus causing the raised portion I4 of the upper member I3 to compress the powder III and enter the recess I I, as shown in Fig. 2. Sullioient pressure is exerted on the upper member I3 to compress the powder I0 into a coherent mass I5, as shown in Fig. 3, no binder or other material being necessary.

Applicant has found that pressures somewhere in the neighborhood of ten thousand pounds per square inch have proved very satisfactory, the resulting resistances being very strong mechani-v cally. 0

After the resistanceunit I5 has been removed from the die block I2, it may in some instances be desirable to heat treat the unit'before securing the terminals thereto. This heat treatment is done by subjecting the unit I5 to a sulphur vapor which is heated to a temperature of between 300 and 400 C. for approximately one hour and then heating it in an atmosphere of pure dry nitrogen gas between 200 and 300 C. for about an hour.

Applicant may employ any suitable closed vessel which may be heated to the desired temperature for heat treating his resistances. However, he

has found the apparatus lshown inFig. '4 very' satisfactory.

As shown in- Fig. 4 the resistance unit I5 is suspended in the vessel I6, this vessel containing a s mail quantity of sulphur I1, the sulphur' IIl is caused to vaporize` by' means of the heating element I8 which raises the temperature in the vessel IS- to a temperature somewhere in 'the neighborhood of 300 or 400 C. This temperature *is held at approximately this level for about an hour, the heat in the interior of the vessel being registered on the. thermometer I9, the heatedsulphur vapor in the vessel thoroughly saturating the unit I5. After the .unit has been in this vapor a suilicient length .of time. the 'temperature islowered to approximately 200 or ,300 -C. and dry nitrogen gas is introduced through the inlet`20,'

the 'gas passing through the tube 20 down into the vessel' and escapingthrough the outlet 2l,

thus driving oi the sulphur vapor and allowing the resistance unit to be saturated with the corthe center portion covered by a tape or some other lead-in wires, 21 .and 20 are theny secured tothe tin coated portions ofthe resistance unit by fso similar material 22. The exposed portions 23 and 24 of the unit are now sprayedwith a molten metal 25, for 'example tin from the gun 26, which vby theway, is the welll-known Shoop process.

Thus, a coating of metal is applied to each end of the resistanceunit I5 as shown in Fig. 6. The

soldering or any other suitable method.

A resistance unit formed in accordance with the above method is simple and inexpensive to manu- -facture and the uniformity-of the nishedproduct It will be understood that I do not limit myself to the precise apparatus herein disclosed as the same may be changed, modified or altered without departing from the spirit of the invention and I am onlylimited by the scope of the appended claims. What is claimed is:

1.' A method of producing resistors having a high negative temperature coeflicient of restst ance, for use in electrical circuits, vwhich consists in placing a quantity, of silver sulphide powder between a pair of dies, applying suiiicient pressure 'to said dies to compress the silver sulphide powder into a coherent mass; removing the resistance 'from the lower' die, heating it in an atmosphere oi?l sulphur vapor for a period of approximately. one hour at 300 or 400 C. and-then heating it in a stream of pure drynitrogen-at 200er 300 C. for

approximately one hour.

2. A method of producing resistors having 'ahigh negative temperature coeii'icient ofresistance, for use vin electrical circuits, which .consists in placing-a quantityof silver sulphide powder between a pair of dies; applying a pressure of approximately ten thousand pounds per square inch to said powder by 'means ofl said dies to i'orm the silver sulphide powder into a coherent mass, vremoving the resistor' from the lower die, 'heating itA in an atmosphere of sulphur vapor fon a period of one hour at approximately 300 or 400 C. and

then heating it in a 'stream of pure dry nitrogen. at a temperature of approximately 200 or V300" C.

' for about one hour.

3. A method of producing resistors having a high negative temperature coeflcient of resist- '.ance, 'for use in electrical circuits', which consists in placing a quantity of silver sulphide powderbetween a pair of dies', applying sufficient pressurel to said dies' to compress the silver sulphide powder into a coherent mass, removing the resistor from vance, for use in electrical circuits whichgcohsists. in placing a quantity of silversulphidev 'powderbetween a pair of dies, applying apressure of approximately ten thousand poundspersquare inch to said powder by means of said dies to form the silver sulphide powder into a. coherent mass, removing the resistor from the lower die,heating it in an atmosphereof sulphur vapor for a period of approximately one hour at 300 or 400 C.,.and then heating it in a stream oipure dry nitrogen at a temperature between 200 and 300 C. for about an hour, masking out aportion oi! said resistance and spraying the exposed portion with molten metal to'provide contacts thereon.

5. A method .of producing resistors' having a high negative temperature coefllcint -of resistance, for` use in' electrical circuits, which consists in placing a quantity of silver sulphide powder between a'pair of dies, applying sufficient pressure tosaid dies to compress the silver sulphide powderv into a coherent mass, removing the resistor from f unit. from the heattreatingvessel, masking out a portion of said resistance, spraying the exposed portion with molten metal to provide-contacts thereon and then securing lead wires to said coated surfaces.

6. A method of producing resistorsfhaving a high` negative temperature coefficient of resistance.- for' use in electrical. circuits, which consists in placing a' quantity of silver sulphide powder between apair of'dies, applying'a pressure of ap proximately ten thousand pounds per square inch in an atmosphere of sulphur vapor for a period 'of approximately one hour at1300 or .400 C., and then heating itin a stream of puredry nitrogen at a temperature between 200 and 300 C. for

about an hour, masking out a portion of said resistance spraying the, exposed portion with' molten metal to provide contacts thereon and JOSEPH a.

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