US2690489A - Conductor device having a variable electric resistance - Google Patents

Description

P 1954- J. M. B. JARRET ETAL 2,690,489

CONDUCTOR DEVICE HAVING A VARIABLE ELECTRIC RESISTANCE Filed Jan. 5, 1950 a s s Q 5; 5 F5 8 2 INVENTUR v 196 a/v-rs Patented Sept. 28, 1954 CONDUCTOR DEVICE HAVING A VARIABLE ELECTRIC RESISTANCE Jean Marie Baptiste Jarret and Jacques Henri J arret, Lyon, France Application January 5, 1950, Serial No. 136,932

Claims priority, application France January 8, 1949 10 Claims.

The present invention relates to conductor devices having a variable electric resistance, the words "electric resistance being meant to designate any phenomenon opposing the flow through these devices of a current, either of constant or of variable intensity. Our invention relates more particularly, but not exclusively, to such conductor devices the resistance of which is variable either as a function of the voltage across the terminals of the device or as a function of the pressure applied thereto.

It is known to produce semi-conductors by admixing particles of carbon to materials having a more or less elastic nature, for example rubber. With carbon microphones in mind as the starting point, several workers in the art conceived that it would be possible to incorporate carbon particles in a rubber-like material and to bring these carbon particles into contact with each other when the rubber-like material is deformed, thereby to constitute an electrical resistor the value of which would be a function of the pressure applied thereto.

It is also known to impart conductivity to rubber by including therein particles of carbon, prior to vulcanization of the rubber, the size and distribution of the carbon particles being such as to constitute chains within the rubber acting as minute conducting wires embedded in the insulating material. It was found that when a semiconductor of this nature is only slightly deformed, within limits which would not break the chains of carbon particles, as soon as the deformations reach a certain amplitude within these limits, the electrical resistance of the material increases in an irreversible manner.

It has been known for about 10 years prior to the filing of the present application, that when a test piece of conductive rubber is placed in contact with a metallic electrode, a substantial electrical resistance of contact may be established which constitutes a barrier to the passage of current between the two elements, and various means and devices have been conceived for reducing this resistance of contact which ischaracteristic of conductive rubbers.

The chief object of our invention is to provide devices of this kind which are better adapted to meet the various requirements of practice than those existing at the present time, and in particular which have a higher sensitivity without impairing the desirable fidelity.

Our invention consists chiefly in incorporating in the devices of the kind in question a piece of a conductor or semi-conductor material, such as a metallic piece or electrode, placed in contact with an element or pellet made of an elastomer or a material of rubber-like elasticity made which is rendered conductive by incorporating into the mass thereof extremely fine conductor particles, such for instance as graphitic carbon particles of a diameter smaller than five hundredths of a micron.

It should be well understood that the term conductor as applied to said particles and pellets is used in its usual meaning, as opposed to the term insulator, and that this term conductor includes in this case bodies which are, to be accurate, only semi-conductors, and on the other hand that the term pellet is not, in this case, limitative of for-ms or dimensions.

A second feature of our invention consists in choosing the elastomer or material of rubber-like elasticity, among bodies having the following characteristics:

Resistivity ranging from 0.1 ohm to 1 megohm per cm./cm.

Modulus of elasticity ranging from 1 kg. to 1000 kg. per cm? upon compression and maintained, without particular damage for the mechanical characteristics of the material, at temperatures at least equal to 80 0.;

Resistance of contact such that its value under a voltage of 1 volt, for l cm. of area of contact with a polished metallic electrode and under a pressure of 1 kg. per cm?, is higher than the value of the resistivity of said material, and such that this resistance of contact decreases when the voltage that is applied increases, which corresponds to a downwardly sloped characteristic curve of hyperbolic configuration.

The invention is more particularly concerned with certain applications, and in particular:

To electronic devices, such, for instance, as rectifier devices, amplifier devices and ear-piece devices;

To electro-acoustic devices such, for instance, as pick-ups, microphones and magnetostriction amplifiers;

And to metrology and adjustment devices such, for instance, as extensometers, vibration collectors and regulating rheostats.

A preferred embodiment of the present invention will be hereinafter described with reference to the accompanying drawings, given merely by way of example and in which:

Fig. 1 shows two curves representing respectively a phenomenon characterizing a device according to our invention, and the same phenomenon in the case of a device outside of the scope of our invention;

Fig. 2 is another characteristic curve of a unit according to our invention; and

Fig. 3 is a diagrammatical view of the construction of an apparatus made according to our invention, to wit: a vibration collector.

Several applications of conductor rubbers or the like are already known, and in particular one according to which a conductor rubber pellet is applied in contact with a piece of a conductor or semi-conductor material. In this known application of conductor rubbers, advantage is taken of the fact that the area of contact between the rubber-like pellet and the metallic piece varies as a function of the pressure with which these two pieces are applied against each other; and also of certain variations of the electric resistance of the rubber-like pellet, which seem to be due to variations in the distance between one another of the carbon grains in the pellet when it is compressed.

However, the effects thus obtained were insuilicient to permit important applications. We have shown at A on Fig. l a curve showing the increase, as a function of the crushing in microns, in the conductance of a conventional unit constituted by a metallic electrode in contact with a pellet of an ordinary conductor rubber, and more especially by incorporation in the rubber carbon particles of a mean diameter of five hundredths of a micron, this diameter being however already smaller than the diameter of carbon grains known up to a recent date.

We obtain effects of very difierent nature and scope by constituting the unit, according to our invention, by a piece of a conductor or semiconductor body, such as a metallic electrode, placed in contact with an element or pellet constituted by a material of rubber-like elasticity, preferably vulcanized, and made conductor by incorporation thereinto of extremely fine conductor particles, such for instance as particles of graphitic carbon of a mean diameter lower than five hundredths of a micron.

We have shown at B on Fig. 1 the relative conductance variation, expressed in percents, obtained by compressing the conductor rubber pellet of a special unit of general dimensions equal to the dimensions of the conventional unit. This special pellet was made by incorporating into the rubber mass, the same total proportion as in the case of the conventional unit, graphitic carbon particles of a mean diameter of two hundredths of a micron, that is to say particles of nature and dimensions coming within the scope of our invention.

When the conductor powder is carbon powder, and the rubber-like material is rubber (either natural or artificial), the proportion of said conductor powder should preferably not exceed one hundred parts of carbon for one hundred parts of rubber.

.We obtain results of a nature and of an order of magnitude very diiTerent from those obtained up to now by choosing, according to a second feature of our invention, for the material of rubber-like elasticity which constitutes an element or pellet placed in contact with an at least semi-conducting piece or electrode among, those having simultaneously the following characteristics:

Resistivity ranging from 0.1 ohm and l megohm per cm./cm.'-;

Modulus of elasticity ranging from 1 kg. to 1000 kg. per cm. under the effect of compression and maintained without particular damage for the mechanical characteristics of the material at temperatures at least equal to (3.;

Resistance of contact such that its value under a voltage of 1 volt, for an of contact of 1 cm. with a polished metallic electrode, and under a pressure of 1 leg/cur is higher than the value of the resistivity of said material, and such that this contact resistance decreases when the voltage that is applied increases, which corresponds to a characteristic curve of hyperbolic configuration having a downward slope.

Concerning the maintaining of the modulus of elasticity at temperatures at least equal to 80 C. it will be noted that this temperature is very frequently reached and exceeded at the contact between the electrode and the rubberlike pellet, and that it will he often advantageous, to have a rubber-like material capable of supporting without damage temperatures averaging C.

Concerning the downwardly sloping charac teristic curve of hyperbolic configuration which indicates the values of the current as a function of the voltage across the terminals of the device, this curve may have various aspects. By way of'exainple, Fig. 2 shows a curve wherein the resistance is plotted vertically and the voltagehorizontally in the case of a unit made ac cording to our invention, the rubber-like elasticity pellet of which was four millimeters thiclz, and of a diameter of also four millimeters; the proportion of carbon was 65% of the proportion of rubber; the mean diameter of the carbon grains was 2 hundredths of 9, micron: the constant pressure applied to this pellet was 20 grammes, and the temperature 19 C.

It seems that the phenomenons that were ob served, which can be given the name of superconductance, can be explained as follows. For

the sake of clarity, the conductor orsemi-conductor piece will be called electrode and the element made of a material of rubber-like elasticity rubber pellet"; besides, it will be supposed that the rubber pellet is made conducting by incorporation of carbon grains, and for instance of lampblack, carbon black, etc.

When the electrode and the rubber pellet are placed in contact, and apotential difference is provided between these two elements, a certain resistance of contact is produced, as usual clue to the very thin skin of pure rubber at the surface of the pellet. Consequently, and in view of the fact that the carbon particles on the one hand and the molecules of the electrode on the other hand are very close to one another, due

in particular to the fineness of the c ticles that are used, a high inten field (probably averaging from "boo par electric Y 19 volts/cm.) is established in the zone of contact. However, due to the rubber-like deformations of the surface of the pellet, there is no effective contact between the carbon and the electrode. It can be ascertained that this electric field then modifies, according to its intensity, the resistance opposed to the flow of current, the word resistance being used with the meaning above specified. Now, the intensity of this field can be modified either by varying the potential difference that is applied, or by varying the mean thickness of the very thin resistant layer or skin which remains interposed between the extremely fine carbon grains and the elec trade. It is therefore possible to act upon a unit made according to our invention to obtain results analogous to those indicated.

Among the rubber-like materials which may comply at least for a certain time with the preceding characteristics, we will cite in particular silicone rubbers, or silastics, i. e. organosilicic bodies having properties similar to those of rubber, which seem particularly advantageous for making devices according to our invention.

Concerning the shape to be given to the rubber-like pellets of devices according to our invention, this form may be varied, in particular according to the applications of the device.

The invention can be applied to various fields, and in particular to the radio-electric or electronic field, to the electro-acoustic field (transformation of sound vibrations into electric currents) and to the regulation or metrology field (electrical measurements of pressures capable of expressing various phenomenons). Only one example is given hereinafter, but the invention is clearly applicable to many other forms of apparatus.

The vibration collector illustrated in Fig. 3 is used for instance to detect and measure the am litude and frequency of the oscillations of a mechanical part with respect to a fixed frame, by making one of these two elements rigid with an electrode, and the other rigid with a rubberlike pellet according to our invention.

Let it be supposed, for instance, that it is desired to provide such an apparatus, mounted on a tool of a machine, in order to measure the vibrations of said tool. The rubber-like pellet 49 is placed inside a casing having a deformable cover, and this pellet is secured by vulcanizing to a first electrode 5|, which is connected with the positive terminal 52 of a direct current source. Electrode 5| is made rigid with the tool 52' to be studied, and this electrode is comiected to the mass together with the terminal 52 of the voltage source. On the other hand, rubber-like pellet 49 is placed in contact with a second electrode 55, connected both with the resistance 56 of the measurement apparatus 51, which may be an oscilloscope, and with the negative terminal 58 of the source of voltage. This electrode is secured on the support 59 of the tool, with the interposition of suitable insulating means 69. It is further advantageous to give this electrode a cup-like shape with cylindrical edges, or to fit it with a ring forming such edges,

in order to avoid crushing of the rubber-like pellet during operations.

In these conditions, during the testing of a tool, the variations of the contact resistance between pellet 49 and electrode 55 modulate the current and thus make it possible to measure the frequency and amplitude of the oscillations.

In a general manner, while we have, in the above description, disclosed what we deem to be a practical and emcient embodiment of our invention, it should be well understood that we do not wish to be limited thereto as there might be changes made in the arrangement, disposition form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.

What we claim is:

l. A variable resistance device comprising a first element of relatively good conducting material, a second element of substantially less conducting material, and means integral with said second element and in intimate contact With said first element defining a barrier impeding the flow of current therebetween; said second element comprising a mass of resilient dielectric material having discrete conducting particles dispersed therein, said dielectric material having a modulus of elasticity of from 1 kg. to 1,000 kgs. per cm. under compression and being capable of withstanding temperatures of up to at least C. without substantial deterioration of the mechanical properties thereof, and the internal resistivity of said second element ranging from 0.1 ohm to 1 megohm per cm./cm. and said means comprising a very thin skin of dielectric material substantially free of conducting particles having a resistance to the fiow of current therethrough when in contact with a polished metal surface which is greater than the internal resistivity of said second element when subjected to a difiference of potential of 1 volt per cm. of contact area and to a pressure of 1 kg. per cm. and which decreases when said difference of potential is increased.

2. A device according to claim 1 wherein the resistance of said means to the flow of current therethrough decreases at first very sharply when said difference of potential is increased and thereafter only gradually, thereby to define a curve of hyperbolic configuration asymptotic to a vertical coordinate of resistance and a horizontal coordinate of voltage applied.

3. A device according to claim 1 wherein said conducting particles comprise graphitic carbon particles.

4. A device according to claim 3 wherein said particles have a mean diameter of not more than 50 millimicrons.

5. A device according to claim 1 wherein said second element is cured in contact with said first element.

6. A device according to claim 1 wherein said second element comprises rubber and graphitic carbon particles in the ratio at least 1:1 by weight, said particles having a mean diameter of 20 millimicrons.

'7. A device according to claim 1 further comprising terminals connected respectively to said first and second elements, and a circuit including said terminals.

8. A device according to claim 7 further comprising a fixed support operatively connected to one of said elements, a movable member connected to the other of said elements, and means determining movement of said member relatively to said support as a function of pressure applied to said second element.

9. An instrument for measuring the vibrations of a mechanical part comprising, in combination: a support for the mechanical part; a variable resistance device comprising a first element of relatively good conducting material, a second element of substantially less conducting material, and means integral with the second element and in intimate contact with said first element defining a barrier impeding the fiow of current therebetween, said second element comprising a mass of resilient dielectric material with discrete conducting particles dispersed therein having a mean diameter of not more than 50 millimicrons thereby to determine the internal resistivity of said second element in a range from about 0.1 ohm to about 1 megohm per cm./cm. and said means comprising a very thin skin of dielectric material substantially free of conducting particles and having a resistance to the flow of current therethrough which is greater than the internal resistivity of said second element when subjected to a difference of potential or" 1 volt per cm. and a pressure of 1 kg. per 0111. and which decreases when said diiference of potential is increased; said device being interposed between said part and support; an electric circuit including termi- 112115 for applying a difierence of potential across said first and second elements; and a measurement apparatus in said circuit.

10. An instrument according to claim 9 further comprising a cup-shaped member enclosing said device for limiting the deformations thereof.

References Cited in the file of this patent UNITED STATES PATENTS Number 5 749,854 2,305,717 2,559,077

10 Number Name Date Fahrney Jan. 19, 1904 Bell Dec. 22, 1942 Johnson et al. July 3, 1951 FOREIGN PATENTS Country Date Great Britain July 28, 1943 France Feb. 16, 1931

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