US2278072A - Electrical resistance device and method of manufacture thereof - Google Patents

Description

- w issrrfrri u (JHOGS nu LI\L-IJSL March 31, 1942. H. L. B. GOULD ETAL 2,278,072

ELECTRIbAL RESISTANCE DEVICE AND METHOD OF MANUFACTURE THEREOF Filed June 5, 1939 I II . H.L.8.GOULD BAKINGSBURY INVENTORS A TTORNEV Patented Mar. 31, 1942 ELECTRICAL RESISTANCE DEVICE AND METHOD OF MANUFACTURE THEREOF Harold L. B. Gould,

Towaco, and Burton A.

Kingsbury, East Orange, N. .L, assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 3, 1939, Serial No. 277,126

7 Claims.

This invention relates to electrical circuit elements the resistance of which varies markedly with temperature. More particularly, this invention relates to resistance devices having a high positive temperature coemcient of resistance, and to methods of manufacturing such de-v vices.

Resistance devices in which theqzes'istance is highly sensitive to temperature are used as regulating and control elements in electric circuits. The so-called semiconductors, because of their high resistance-temperature coefficient and intermediate specific resistance values, have been employed considerably in this field. The resistance-temperature coefiicient of semiconductive materials and resistors made therefrom have heretofore been found to be negative at ordinary temperatures.

The positive resistance-temperature coefficient characteristic of materials ordinarily classed as conductors is so small that such materials are inadequate for resistor units where a wide range of resistance variation is desired. For example, iron does not double in resistance for less than 70 C. rise in temperature at any point below 500 C.

One object of this invention is to produce resistance devices having a high positive temperature coemcient at relatively low temperatures, for example, temperatures between and 100 C.

Another object of this invention is to obtain firm, uniform low resistance contact between the body of a resistor unit and electrodes therefor.

A further object of this invention is to assure uniform spacing of the electrodes in formed resistance devices.

Still another object of this invention is to increase the density and ruggedness of formed electrical resistance devices.

In one illustrative embodiment of this invention, an electrical resistance device comprises a body of a material having a positive temperature coeflicient of resistance at relatively low temperatures. and electrodes attached to spaced portions of the body. I

In accordance with one feature of this invention. the body of the resistance device is formed of an oxide of chromium, more particularly chromic oxide.

In accordance with another feature of this invention, the electrodes are made of metallic sheets or gauze intimately adhered to the body of the resistance device.

In accordance with a further feature of this invention, insulating spacers, such as glass rods,

are incorporated in the body of the resistance device to assure uniform spacing of the electrodes.

In accordance with still another feature of this invention, the formed resistance device is o subjected to heat treatment to produce a dense rugged unit.

Other and further objects and features of the invention will be understood more fully and clearly from the following detailed description with reference to the accompanying drawing in which:

Fig. 1 is a plan view of an illustrative form of resistor unit embodying the invention, with parts broken away to show internal structure;

Fig. 2 is a section taken on line 2-2 of Fig. l; and

Figs. 3, 4, 5 and 6 are sectional views of other forms of resistor units illustrative of the invention.

Referring now to Figs. 1 and 2 of the drawing, I ll denotes a body of high positive resistancetemperature coeflicient material having elec-- trodes H attached thereto. Members of insulating material, such as glass rods l2, may be disposed at intervals throughout the body III to insure uniform spacing of the electrodes. Circuit connections may be made by conductors l3 which are secured to the electrodes II by welding or other suitable means. The body It comprises finely divided, pressed, chromic oxide or chromic oxid plus a binder material, such as sodium silicate The electrodes Il may comprise a wiregauze as illustrated, a preferred material being nickel or platinum. Sheet metal with or without perforations may also be employed for the electrodes. A reticulated or foraminated electrode is preferred as it adheres better to the body of oxide material It.

A preferred method of fabricating these units is as follows: An electrode member ll of suitable size is placed on a flat surface and a layer of oxide material applied thereto. The oxide shomely divided and may be applied as a powder or a paste. The paste has been found to be preferable. Small amounts of water may be mixed with the powdered oxide t'd'foriii the paste. It is preferable, however, to employ a binder, such as sggium sili cat e It has been found that a cg 1;s m a ig gf n insulating solid aid ig maintaining a uniform thickness of body T02 For this reason, fods lfof glassor like material may be inserted at intervals in the oxide. A second electrode II is then placed over the oxide layer and the assembly pressed sufficiently to bring the electrodes in contact with the spacers CROSS REFEREiCE it. In units of the type illustrated in Figs. 1 and 2 spacer rods of from 3 to 10 mils diameter have been employed. If desired, the electrodes ll may be flattened to remove major surface irregularities before using. in order to insure a more uniform thickness of the unit.

If the oxide has been applied as a paste, the units are dried after assembly. It is not necessary to heat treat the units to obtain a positive temperature coefilcient of resistance. However, heat treatment does aid in producing a more dense, rug ed unit. The temperature of heat treatment may be from 800 to 700 C. but should not go above 700 C. Conductive-leads, such as wires II of nickel, platinum or other suitable material, may be welded or otherwise secured to each electrode ll.

Units of the type shown in Figs. 1 and 2 and about 1 centimeter square with an electrode spacing of from 8 to 10 mils have a cold resistance of from 25,000 to 50,000 ohms. The half resistance interval or half temperature is from 3 to C. within the range O'to 100 C. The half resistance interval or half temperature may be defined as the temperature interval over which the resistance doubles or decreases to one-half.

By modification of the foregoing method of fabrication, other units, such as illustrated in Figs. 3, 4, 5 and 6, may be produced. The unit shown in Fig. 3 comprises a body ll of pressed semiconductive material having electrodes ll of pressed metallic powder. In the making of such a unit a layer of powdered chromic oxide material is placed between two layers of metallic powder, such as permalloy dust (over 30 per cent nickel, remainder iron) and subjected to sufficient pressure to form a self-sustaining body. The oxide and metal particles interengage to bond the electrode portions I l firmly to the body It. Conductive leads ll may be soldered or otherwise secured to the electrodes I I.

In Figs. 4 and 5 are shown units which are made by pressing finely divided oxidic material into a self-sustaining body II. For the type shown in Fig. 4, a metallic paste, such as silver paste, is applied to two portions of body II, and conductors I! are embedded therein. Sufficient heat is applied te convert the paste to metal electrodes ll firmly bonding the conductors it to the unit.

In the units illustrated in Fig. 5, the electrodes H are applied to the body III by evaporation or plating of metal, such as silver or gold. The conductors it may then be applied to the electrodes by solder or equivalent means.

In Figs. 3, 4 and 5, the thickness of the conducting layers or lectrodes II has been shown exaggerated in the interest of clarity of illustration. These electrodes, particularly in the forms shown in Figs. 3 and 5, are ordinarily made quite l- As indicated with respect'to the units illustrated in Figs. 1 and 2, heat treatment is not required to produce units having a relatively high positive resistance-temperature coemcient. However, where heat treatment is desirable from other considerations, such as mechanical strength, the temperature should be kept below 700C. A suitable range is 300 to 700 0.

As illustrated in Fig. 6, units may also be made in bead form. For this type of unit, the finely divided oxide material is made into a paste, preferably with a sodium silicate binder. The paste is formed into a bead 20 on uniformly spaced EXAMINER wires 2|. The wires 2| preferably are of platinum or other conductive material that will not be affected by heat treatment. The beads are dried and then preferably given a heat treatment in 5 air. The temperature range as for the cther units may be from 300 to 700 C. but should not go above 700' C.

As has been stated heretofore, the chromic oxide is preferably mixed with a bindermaterial, such as sodium silicate. It has been found that the binder insures a more rugged unit than is ordinarily possible without a binder. However, the binder is not necessary to obtain a positive resistance-temperature coemcient. Units made with no binder and with sodium silicate binder up to 25 per cent all have the positive resistancetemperature coefficient.

. By making suitable circuit changes, units having a high positive temperature coefiicient of resistance may be employed to perform the same function as negative resistance-temperature coefilcient devices. In general, the positive coefllcient unit is connected in series to perform the same function as a negative coefiicient device in parallel or vice versa. For some regulating purposes, a positive coeificient device may be used with a negative coefiicient device with good results. An advantage of the positive coefilcient material is its inherent ability to prevent the formation of (hot spots," which must be avoided in negative coefilcient units.

A "hot spot," as the name implies, is a portion of a resistor unit that in use attains a higher temperature than the surrounding portions. This may be, for example, due to non-uniformity of the resistance material. In the case of negative resistance-temperature coemcient material, the resistance is reduced at the "hot spot," due to v the high temperature. The resulting greater fiow of current further raises the temperature thereby further lowering the resistance. In the absence of exterior current limiting means the hot spot" condition may become so bad as to permanently change the character of the resistance material or even destroy it. In a positive resistance-temperature coemcient material, the hot spots" are self-limiting since the resistance goes up with increase in temperature.

Although specific embodiments of this inven- 50 tion have been shown and described, it will be understood that they are but illustrative and that various modifications may be made therein without departing from the scope and spirit of this invention as defined in the appended claims.

What is claimed is:

l. A high positive resistance-temperature coefilcient resistor unit comprising a body of finely divided, pressed chromic oxide heat treated in air at a temperature below 700' C., and electrodes attached to spaced portions of said body.

2. A resistor having a high positive temperature coefficient of resistance and comprising a thin body of untreated chromic oxide, two metal- -lic electrodes on opposite faces of said body, and insulating means embedded in said body for maintaining separation between said electrodes.

3. A resistor having a high positive temperature coemcientof resistance and comprising a body of pressed, finely divided, chromic oxide between two metallic electrodes, and a plurality of glass rods embedded in said body for maintaining separation between said electrodes.

4. A resistor having ahigh positive temperature coefiicient of resistance and comprising chromic oxide heat treated at a temperature between room temperature and 700 C.

5. The method 01' making a positive resistancetemperature coemcient resistor unit that comprises forming a body from finely divided chromic oxide, and heat treating said body in air at a temperature below 700 C.

6. The method of making a high positive resistance-temperature coefllcient resistor unit that comprises mixing finely divided chiomic oxide and sodium silicate to a paste, applying the paste to a reticulated metallic sheet, inserting a plurality of insulating spacer members into the paste and applying another reticulated metallic sheet thereto, pressing the assembly, drying, and heat

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