US2276864A - Resistance device - Google Patents

Description

March 17, 1942. e. 1.. PEARSON 2,276,864

RESISTANCE DEVICE Filed Dec. 30, 1939 2 Sheets-Sheet l INVENTOR By G.L. PEARSON ATTORNEY ,March 17, 1942. G. 1.. PEARSON 7 ,8 4

RES I STANCE DEVICE Filed Dec. 30, 1939 2 Sheets-Sheet 2 40- I u, k a i 30- E I c- 7 l q :TAr/c DYNAMIC I l 20 g 1 E l I D I a. I cun NT I I0 IAMPL UDE I i I I I 1 c 1 I12: I I: h 1. 1 1 l o 0.2 0.4 0.6 o.a m L2 1.4 L6 m CURRENT IN MILLMMPEREJ //v VENTOR G. L. PEARSON ATTORNEY Patented Mar. 17, 1942 RESISTANCE DEVICE Gerald L. Pearson, Towaco, N. 1., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application December 30, 1939, Serial No. 311,736

9 Claim.

This invention relates to resistors and more particularly to temperature sensitive resistors having low thermal inertia.

All materials that are ordinarily designated as conductive or resistive materials ar more or less sensitive to changes in temperature. The terms temperature sensitive, thermally sensitive" and like expressions are, however, usually applied only to those materials that exhibit a large change in resistance for a small change in temperature. Such materials have been, for convenience, called thermistor materials and devices made therefrom designated as thermistors. Those materials having conductivities lying between conductive values normally associated with conductors and with insulators, and generally designated as semiconductive materials, have been found usually to be thermally sensitive or thermistor materials. The semiconductive materials at present comprise a good source of material for making thermistors.

Many of the semiconductive materials that have been investigated up to the present have a high negative temperature coeflicient of resistance. For this reason, it is convenient to discuss thermistor" action, from this viewpoint. However, positive resistance temperature coef iicient thermistors may also be employed in a similar manner to the negative coeflicient devices.

Heretofore th'ermistors" have in general been rather sluggish in operation due to thermal lag. For example, in the case of a thermistor employing a negative resistance temperature coefficient material, when voltage is applied, an appreciable time elapses before the unit heats up and its resistance decreases. Likewise, there is a time delay after the voltage is removed, before the unit cools and again attains a high resistance. For some purposes this time delay is useful and for others it is of no particular consequence. However, a thermally sensitive resistor that could heat and cool with sufficient rapidity to closely follow applied voltage variations at audio or higher frequencies would find many useful applications.

One object of this invention, therefore, is a high speed thermally sensitive resistor or thermistor capable of closely following audio or high frequency variations of applied voltage.

A further object of this invention is a high thermistor suitable for use as an oscillator, modulator, amplifier and the like.

One feature of this invention resides in a th'erinistor" unit characterized by low heat capacity and good heat transfer characteristics with relation to the surrounding medium.

A further feature of this invention lies in a thermistor unit having an active resistance portion of very small volume.

Another feature of this invention resides in a high speed thermistor" having a negative resistance characteristic.

A further feature of the invention is found in thermistors" made in accordance therewith, that exhibit a circuit characteristic which is the equivalent of reactance and which may be called effective reactance.

In accordance with another feature of this invention, a high speed "thermistor" comprises two bodies capable of electrical conduction and in mutual contact over an area that is very small with respect to the superficial area of said bodies, at least one of the bodies being of a material having a high temperature coemcient of resistance, for example, a semiconductor.

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

Figs. 1, 2 and 3 are diagrammatic views of illustrative forms of the invention;

Fig. 4 is a view in elevation of one form of device illustrative of the invention;

Fig. 5 is a sectional view of another device illustrative of this invention;

Fig. 6 is a sectional view of still another device illustrative of the invention;

Fig. '1 is a fragmentary section of part of FigfG to better illustrate details thereof; and

Fig. 8 is a plot to show the electrical characteristics of a high speed thermistor made in accordance with this invention.

As previously indicated the thermistor devices made in accordance with this invention comprise two conducting bodies, at least one of which has a high temperature coefficient of resistance and is generally a semiconductor, the bodies being in mutual contact over a very small area. Contact areas of the order of 2 1o' to 1 10 square centimeters have been found satisfactory. As will be pointed out more fully in the following description of specific embodiments of the invention, the foregoing values are only indicative of the order of magnitude of the contact area and are not intended as limiting values.

The use of a small contact area is one way of confining the efiective resistance of the device to a small volume. The very small volume of resistance material can be rapidly heated when voltage is applied or increased. Since rapid cooling is also necessary upon a reduction or removal of the applied voltage, the heat transfer from the body must be good. This may be accomplished in one way by having a good heat conducting material in intimate contact with the small volume of resistance material. Illustrations of this may be found in the detailed description of the embodiments of this invention.

According to Figs. 1, 2 and 8 there are shown several diagrammatic forms of high speed varistors. The device of Fig. 1 comprises two semiconductive bodies I0. such as boron crystals in single contact. Conductive members II are attached to the bodies l forming circuit connections. Another form illustrated in Fig. 2 comprises a semiconductive body III, which may be a boron crystal, in contact with a metallic plate l2. A conductive means ll forms one circuit connecting element and another connection may be made to the plate l2. In the embodiment of the invention shown in Fig. 3, a conductor H, such as a wire of platinum, bears on the surface of a thin layer ll of semiconductive material secured to the surface of a metallic plate l2. The contacting surface of conductor II should be shaped to minimize any tendency to deform the surface of the semiconductive layer l3 and still provide contact over a very small area. One way of doing this is to make the contacting surface of said conductor generally convex or spherical. For example, where conductor I l is a metal wire, a bead Il may be formed at its end or the end may be bent back upon itself to present a convexly curved contacting surface.

Thermistors made in accordance with this invention have a non-linear static voltage current characteristic. If a negative resistance temperature coefllcient thermistor of this type is subjected to a direct current of increasing magnitude, the voltage drop across it is found to increase to a maximum and then decrease. In other words, the device has a declining voltagecurrent characteristic. The static voltage-current curve of a typical thermistor is shown in Fig. 8.

Dynamically, the alternating current resistance is negative in the region beyond the voltage maximum Em for sufficiently low frequencies. Fig. 8 also indicates the dynamic characteristics. If a direct current of value It greater than 10 (that current corresponding to Em) to be applied to the thermistor, a superposed alternating current of frequency approaching zero will trace out a curve aob approximating the static characteristic. If the superposed current has a very high frequency, the thermal lag of the thermistor will prevent any change in temperature, and hence in resistance, from taking place. The voltage current trace therefore will be along the ohmic resistance line cod. At intermediate frequencies the superimposed current will produce traces as shown at e, f and g in the order of increasing frequency. At low frequencies the effective alternating current resistance is negative, at high frequencies it is positive and at intermediate frequencies it may be either positive or negative; thus for some critical frequency it becomes equal to zero. This latter is the maximum frequency at which oscillation can occur. If the resistance temperature coeflicient oi the thermistor is positive a. current voltage curve similar to the voltage current curve of Fig. 8 may be employed to show its characteristics. This curve will have a current peak similar to the voltage peak of Fig. 8.

As has been previously indicated and as will be evident by reference to the structures disclosed as illustrative embodiments of the invention, the volume of semiconductive material carrying most oi the current is very small. This is due in part to the small area of one of the contacts thereto and also in devices of the type shown in Fig. 3 to the thinness of the semiconductive layer. The small volume subjected to the heating effect of the current rapidly attains a high temperature. When the applied current is reduced, rapid cooling occurs because of the good heat conduction of the mass of metal in contact with the semiconductive material. The low thermal inertia of the device renders it capable of varying its resistance with sumcient rapidity to substantially follow the high speed current variations present when voltage at audio or higher frequencies is applied thereto. Although the thermal inertia is sufliciently low to allow the high speed action previously described, it is not negligible and is responsible for another unique characteristic of the high speed thermistor, namely, effective reactance. The thermal lag, which is frequency dependent, affects the electrical circuit through thermal control of its resistance in a manner equivalent to the effect of inductance or capacitance thereon. In the case of a negative resistance temperature coefficient device the effective reactance is inductive in character and with a positive coefllcient unit it is capacitive.

As indicated in connection with the foregoing discussion of Fig. 8, oscillation, amplification and other like phenomena can occur in the region of negative resistance, provided the frequency is not higher than the critical frequency. A high speed thermistor" of the negative resistance temperature coefficient type may be represented by an equivalent electrical network containing a resistance in parallel with a series combination of negative resistance and inductance. In such a network the resistance represents the positive ohmic resistance and the inductance the effective inductance due to the thermal lag. At any given frequency below the critical frequency (that frequency at which the effective resistance is zero, namely, positive and negative resistance equal); the effective network of such a high speed thermistor comprises an efiective negative resistance and an effective inductance in series. Similar networks comprising resistance and capacitance may be employed to represent the positive resistance temperature coefilcient thermistor. Several practical embodiments of high speed thermistor" devices of the type shown diagram matically in Fig. 3 are illustrated in Figs. 4, 5, 6 and '7.

In Fig. 4 there is shown a device comprising a thin layer 2| of semiconductive material secured to a metallic backing 22, which is attached to a metal block or plate 23 secured to the base plate 24 of a holder generally designated by the reference character 20.

The holder 20 comprises the base plate 24 of metal such as brass and a pair of blocks 25 and 26 serving as support means for spring members 21 and 22. The blocks 25 and 26 may be secured to the base plate 24 and springs 21 and 28 to the blocks by fastening means such as bolts 29 and III and nuts 3| and 32. The lower block 25 may be insulated from the base plate 24 by means such as insulating sheet 33. Insulating bushings 34 and washers 25 may be employed to insulate the bolts 29 and 20 and nuts 3| and 32 from the base plate.

The springs 21 and 28 which may be of the cantilever type have portions secured respectively between blocks 25 and 2C and on top of block 26. The projecting ends of the springs 21 and 28 are in alignment and respectively support contacting member 38 and adjusting member 31. The member 28, which may be of steel and in the form of a screw secured to the spring 21 by m ns of a nut ll, bears on the surface of semiconducting layer 2'. The adjusting member 31 may also be a screw threaded into a tapped hole in spring 25 and secured in adjusted position by a lock nut 39. The point of member 31 may bear on the head of member 35.

Circuit connections may be made to semiconducting layer 2| and contact member 35 by any convenient means. The terminal members 49 and 4| secured respectively to base plate 24 and block 25 are suitable. Other means than the insulating sheet 3!, bushings 34 and washers 35 may be employed to avoid a short-circuiting shunt around the contact between member 35 and semiconductive layer 2|. For example, the conducting block 23 may be insulated from base plate 24 and circuit connections made from said block and at some convenient point on the holder 25.

The point of contact member 35. which is convexly curved or spherical, is applied to the surface of semiconductive layer 2| with sufficient pressure to insure mechanical stability. The pressure is adjusted so that the resistance is not affected by mechanical vibrations. When the thermistor" is to be employed as an oscillator, further adjustment is made, if necessary. until the device oscillates in an appropriate circuit. If the thickness of semiconductive layer 2! is of the order of 1 mil or 2.5x centimeters, which has been found satisfactory, the contact area giving the desired result will probably be in the neighborhood of 2 10-' square centimeters. In devices of this type the pressure applied by screw 31, when proper adjustment is attained, has been found to be as high as two pounds. It will thus be seen that the pressure over the small contact area is very high. With some of the materials, which have given good results, such high contact pressures cause some deformation of the semiconductive surface. Other things being equal, materials giving the least deformation at the contact are to be preferred.

In view of the high contact pressure employed the semiconductive layer must be hard and strong to resist appreciable deformation and consequent increase of the contact area.

One suitable semiconductive element comprises a thin layer of uranium oxide, about 1 mil thick, on a platinum foil backing. A suitable method of preparing such an element is as follows: Finely ground uranium oxide and floated silicon dioxide are mixed in the approximate proportion of 85 per cent and per cent by weight respectively, water is added to form a thin paste which is applied to thin sand blasted platinum foil so that th oxide does not exceed a thickness of 1 mil. After thorough air drying the coated foil is passed through a furnace regulated to 1420110" C. at a steady rate and in a total time of about three minutes to sinter the active surface.

The optimum rate of travel through the furnace is about 0.25 foot per minute. At slower rates the silica forms large crystals while at faster rates the rapid quench produces cracking and chipping of the surface and peeling of the oxide layer.

The completed foil-oxide assembly may then be cut into strips of suitable size, for example, V4 inch by inch (.635 by .3175 centimeter), which are individually subjected to a pressure of about 600,000 pounds per square inch in a press, to remove surface irregularities. These strips are then secured as by spot welding to blocks of metal such as brass and the assembly subjected to about 300,000 pounds per square inch pressure. The resulting assemblies are then placed in a suitable holder. See. for example, assembly 2|, :2, 23 in holder ll of Fig, 4.

Other semiconductive materials may also be employed for making th thin film on a metal backing. Some of these are: A heat treated mixture of nickel and manganese oxides such as disclosed in application Serial No. 274,114 by Richard O. Grisdale, filed May 17, 1939 now Patent 2,258,646, issued October 14, 1941; a heat treated mixture of oxides of nickel, manganese and cobalt as disclosed in application Serial No. 280,692, by Ernest I". Dearborn, filed June 23, 1939 now Patent 2,274,592, issued February 24, 1942, or boron deposited from the vapor stage on a metal such as tungsten. Thin films may also be applied to a metal backing by subjecting superposed layers of metallic and semiconductive powders to high pressure as disclosed in application Serial No. 274,101, by Earle E. Schumacher, filed May 17, 1939 now Patent 2,267,954, issued December 30, 1941.

A further embodiment of the invention of the type shown in Fig. 3 is disclosedin Fig. 5. A thin layer of semiconductive material 5| is applied to a metallic backing 52 in one of the ways heretofore described. This unit is placed in a recess 53 in a metal block 54 The recess should be of such depth that the semiconductor surface and the face of the block 54 are coplanar. A thin sheet of insulation as such as mica or other insulating material that may be slightly compressed by application of reasonable force thereto, is placed over the block 54. The sheet 55 is provided with a small orifice 55 of about the same diameter as its thickness. The orifice 55 is located to come over the semiconductive layer 5|. A metal bead or sphere 51 of a diameter to fit orifice 56 snugly is placed therein. Another block of metal 58, similar to block 54 but having no recess, is placed over the insulating sheet 55 and metal bead 51. Clamping means, such as U- shaped member 59 of spring metal, hold the assembly together. The clamping means, if of conductive material, may be insulated from one of the blocks 54 or 58 (in the device illustrated, 54) by means of insulating material 60. The clamping means is adjusted to exert sufilcient pressure to slightly compress the insulation 55 and press the bead 51 firmly against the semiconductive layer 5|. The contact area and the pressure should be comparable to those employed in the device illustrated in Fig. 4. Circuit connections may be made by any suitable means such as terminals GI and 52 attached to metal blocks 55 and 54, respectively.

In Fig. 6 is shown a form of high speed thermistor similar in some respects to that shown in Fig. 5. A thin layer of semiconductive material 10 may be provided with a platinum or other suitable metal backing 1|. The backing 1| may be secured to a block of metal 12. On the face of the semiconductive layer 10 is a sheet or body of insulating material 13 The insulating sheet 13 may be provided with an orifice 14 That part of the orifice 14 adjacent the semiconductor 10 should have an area comparable to the contact area of the previously described devices. The remainder of the orifice may be made larger to facilitate the application of a metal contact to the semiconductor through the small opening. With the body of insulation 13 in place, a small amount of evaporated metal is condensed on the semiconductor through the orifice 14 to form a contact 15. This may be backed up with sprayed metal 18 in sufficient amount to fill the orifice.

' thermal The enlarged fragmentary view of Fig. '1 shows the details of the contact and backing Another metal block 11, similar to block II, may be placed over insulating sheet I3 and in contact with the sprayed metal backing 16. A clamp, such as the U-shaped spring member I8, may be employed to hold the stack together. If the clamp II is of conducting material it should be insulated from at least one of the metal blocks I2 and 11. In the device as illustrated, block 11 is insulated by means of insulating material IS. A ball bearing or similar means may be placed between clamp 18 and block I2 to aid in equalizing the pressure on the stack. Circuit connections may be made in any convenient manner as by means of terminals BI and 82 attached respectively to metal blocks I2 and 17. In the modification of the device just described, the contact area is fixed by the size of the small end of the orifice H and the applied pressure is therefore not critical. Such a device would also be free from contact variations due to mechanical shock or vibrations.

Resistor units have been constructed in accordance with this invention, that will follow applied voltage variations at frequencies as high as about 1'7 kilocycies per second. Several units of this type connected in suitable circuits have oscillated continually over a period of several months, except for short intervals of testing and checking.

Although specific embodiments of this invention have been shown and described, it will be understood that various modifications may be made therein without departing from the spirit and scope of this invention defined in the appended claims.

What is claimed is:

1. A temperature-sensitive resistor having low thermal inertia and a declining voltage-current characteristic, comprising a sheet of metal, a thin, relatively dense film of high resistance-temperature coefficient semiconductive material on said metal sheet and presenting a plane exterior surface, a conductive body having a convex portion making contact with said film over a small area, and means for varying the contact pressure between said body and film to thereby adjust the area of contact.

2. A temperature-sensitive resistor having low inertia and comprising a sheet of platinum, a thin, relatively dense film of uranium oxide on said platinum sheet and presenting a plane exterior surface, a conductive body having a convex portion making contact with said film over an area of about square centimeters, and means for adjusting the contact area by adjusting the pressure of said body on the film.

3. A temperature sensitive resistor having low thermal inertia comprising a sheet of conductive material, a thin film of semiconductive material on said sheet, a body of conductive material supporting said filmed sheet, a sheet of insulating material overlaying said film with an orifice therein adjacent the film for the reception of a conductive member to position said member on said film, a second body of conductive material on the insulating sheet and making contact with said member, and clamping means for holding the assembly together.

4. A temperature sensitive resistor having low thermal inertia comprising a sheet of platinum, a thin film of uranium oxide on said platinum, a body of conductive material having a recess in one face for the reception of said uranium oxidefilmed platinum sheet, whereby the uranium oxide surface and said surface of the conductive body are coplanar, a sheet of insulating material overlaying said face and film with an orifice therein adjacent the film for the reception of a conductive member, to position said member on said film, a second body of conducting material on the insulatlngsheet and making contact with said member, and resilient clamping means for holding the assembly together.

5. A quick-acting thermally sensitive variable resistor comprising a conductive body having a thin hard film of high resistance-temperature coefilcient semiconductive material on a surface thereof, said film having a plane exterior surface, a second conductive body having a convex portion in contact with said film, and means for adjusting the area of said contact by varying the pressure between said bodies.

6. A thermally sensitive variable resistor comprising a sheet of platinum, a firm film of uranium oxide in the order of one mil thickness on said sheet, a conducting body in firm contact with said film over an area, in the order of one millionth or less of the area thereof, and means for making electrical connection respectively to said sheet and said body.

7. A rapidly variable, thermally sensitive resistor comprising a metal body filmed with a dense high resistance-temperature coefficient semiconductive material, said material presenting a plane surface to a metal member having a convex portion thereof in contact with the material, and a mass of material having high thermal conductivity, in intimate thermally conducting relation to said-semi-conductive material.

8. A resistor comprising a body of material having high thermal and electrical conductivities, a thin film of high resistance-temperature coefiicient semiconductive material on said body, said film having a plane surface, a support for said body, a member of electrically conductive material having a convex portion, and means including two parallel resilient elements cooperating with the support and member to maintain the convex portion of said member in firm contact with the surface of said film.

9. A resistor device comprising a base plate of c;nductive material, a sheet of metallic material mounted on said base plate, a thin film of high zesistance-temperature coefiicient semiconductive material attached to said sheet and presenting a plane exterior surface, electrically conductive support means secured to said base plate and insulated therefrom, an electrically conductive member having a convex portion, spring means secured to said support means and maintaining the convex portion of said member in firm contact with the surface of said film, adjustable means cooperating with the spring means for varying the area of contact between said member and film by adjusting the contact pressure, and means for making electrical connections respectively to said support and base plate.

GERALD L. PEARSON.

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