US 3170966 A
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Description (OCR text may contain errors)
Feb- 23, 1965 J. KEMENY ETAL 3,170,966
PRODUCTION OF VOLTAGE DEPENDENT RESISTANCES Original Filed March 27, 1961 /I/Gl 1C/G2.
United States Patent O 2 Claims. (Cl. 264-22) The invention relates to a method of making a voltage dependent nonlinear electric resistance in which grains of hexagonal silicon carbide are enclosed in a metallic container.
Voltage dependent electric resistance elements made from granular silicon carbide are known in which the particles are held together by a binding agent containing an additive of metallic oxides. It is also known that the voltage sensitivity of the resistance element can be set to definite values by suitably choosing the binding agent and the contact pressure. This relationship to the binding agent and the contact pressure is accounted for by the fact that the binding agent is pressed away from the contact points between the crystals subjected to the applied contact pressure, so that the micro structure of the resistance element is changed accordingly.
According to the present invention, resistances are proposed in which the grains are used without binding agents in known manner. A surprising discovery has been made that even without the use of a binding agent there is a relationship between the voltage characteristic and the contact pressure if the contact pressure is sui'liciently high. This fact is surprising to the expert in so far that the literature on the subject has hitherto expressely deniedl a relationship between the slope of the characteristic curve of the resistance and the applied pressure.
The current-voltage function of the VDR resistances in question can be represented by the approximate mathematical formula where I is the current and U is the voltage. A and k are considered to be characteristic constants of the material of the VDR resistance. Since the slope of the currentvoltage function is principally affected by the factor k, thel aim during production is to make VDR resistances having the largest possible k-factor. Since the relationships between the k-factor and the measurable values of the pro duction Variables are still for the most part unknown, the manufacture of VDR resistances is carried out subtantially empirically. This means, therefore, that resistances are produced in which the k-factor fluctuates between certain limits. Accordingly, by means of measurement one must select from amongst the produced resistances those in which the k-factor has a certain value. However, this involves a large amount of wastage which -makes the manufacture of VDR resistances expensive. The above mentioned use of the binding agent serves, inter alia, to improve the k-factor and to control it reliably. However, with binding means such as metal oxides it has only been possible to obtain k-factors of about 4 during manufacture.
By means of the present invention it is now possible to make VDR resistances without the use of a binding agent, in which the k-factor is higher and, more important, in which the k-factor can be set to an accurately defined value during manufacture.
In the resistances made according to the invention, the grains are used in known manner without binding agents. The container is metallically pre-stressed and, in view of its ice resilient returning force, exerts on the grains a constant pressure which determines the slope of the voltage curve.
Preferably, the pressure exerted on the grains lies between kilograms per square centimetre and 300 kilograms per square centimetre.
An apparatus for making non linear electric resistances, in which the hexagonal silicon carbide grains are enclosed in a metallic container comprises a press in which the container filled with the silicon carbide grains is compressed. The press is controlled by an electric control circuit in which the silicon carbide grains are part of the circuit so that the variation in the resistance value of the arrangement of silicon carbide grains as caused by the pressure inuence of the press is measured and the press is switched off when a certain resistance value has been obtained.
The accompanying drawings serve to explain the present invention fully in conjunction with the following decription.
In the drawings:
FIG. 1 is a diagrammatic cross section of a VDR resistance of the known type.
FIG. 2 shows the current-voltage curves of VDR resistances under various pressures.
FIGS. 3 to 6 are sections through preferred embodiments of VDR resistances made according to the invention.
FIG. 7 is a diagrammatic illustration of an apparatus for making the VDR resistances according to the invention.
FIG. 1 is a cross section through a resistance material which has been made in known manner, in which metal oxide-coated Carborundum crystals were intermixed with a material which hardens at high temperatures and cornpressed into pastille form. In this known embodiment the semi conductor crystals form irregularV chains within the binding material 2. The binding material contains gas filled cracks 3. With the known manufacturing processes, therefore, an irregular inner structure is obtained in the resistance material. Since, however, the conductivity of the material is determined by its micro structure, the irregularity ofthe micro structurecauses large deviations in the I-JU functions of the resistances so made. Even with VDR resistances made according to the latest methods a tolerance of 40% must be expected. Thus, if precision resistances are required, the desired values can be obt tained only by choosing the correctly made products.
This makes manufacture veryexpensive indeed.
'Ihe present invention facilitates the manufacture of non linear resistances having a uniform inner structure and the accurate setting of their electric properties as a function of a readily measurable parameter of fabrication. According to the invention, the semi conductor grains, preferably the usual carborundum crystals provided with a thin silicon oxide or aluminium oxide layer, are introduced without binding agent into a container, preferably a metallic capsule having an insulated electrode, and are compressed together with the container, the container being finally closed Whilst the grains are being subjected to pressure, preferably by means of the same pressing means. After removing the container from the press, the grains remain under a constant pressure exerted on them by the resilent forces of the container. It has long been known that the electric resistance of a material consisting of semi conductor grains is a function of the pressure exerted on the grains. A thorough examination of this phenomenon has shown, however, that not only the resistance value of the grains but also the slope of their I-U functions, i.e., their k-factor, depends on the pressure exerted on the grains. v
FIG. 2 -shows the I-U function of grey Carborundum grains as a function of the pressure P. From this figure pose of the VDR resistance and to set the desired capsule,
pressure, for example by means of a hydraulic press. This precision manufacture opens up possibilities which were not attainable with hitherto known fabrication methods for VDR resistances. For example, in the voltage region of 100 to 200 volts the invention has made it possible to obtain k values of 6 to 6.3 and an accuracy in a series of 100 resistances below 5%.
FIGS. 3, 4, and 6 illustrate various preferred embodiments of VDR resistances according to the invention.
According to FIG. 3, the semi conductor grains 1 are inserted between electrodes 2 and 3 in an insulated capsule 4, `for example of Bakelite, which, in turn, is inserted into a metallicutube 5. The two ends of the metal tube are rounded ott in a press. Ametallie sub layer 6 serves to preserve the Bakelite capsule.
After relieving the pressure, the resilient return force of the grains seeks to expand the tube 5. The latter becomes elastically elongated and the pressure exerted on the grains ybecomes correspondingly smaller. This elastic release is proportional to the length of the tube. In order to minimize the pressure decrease, the length of that part of the metal which facilitates pressure decrease is made shorter. Such examples are shown in FIGS. 4 to 6.
According to FIG. 4, the Bakelite capsule 4 is inserted in two preferably identical metal cans 5 and 6, which are compressed and held together only at their edges by means ofthe annularinterrnediate member 7. During and after compression, the cylindrical portions of the metal cans 5 and 6 are subjected to constant tension. Only the short piece 8 of the ring 7 participates in the resilient pressure release, such piece 8 being in compression during pressing and being subjected to tension after compression. Accordingly, the pressure release is smaller than in the arrangement of FIG. 3.
In FIG. 5, two different metal cans Sand 6 are provided. The edge 7 of the can 6 is pressed against the edge of the can 5. The can 5 remains unaltered during and after Compression. The elastic pressure release is governed by the length of the piece 8.
According to the embodiment of FIG. 6, a single metal can 5 is provided having the edges 7, 8 and 9. A conical annular plate spring 6 of which the outer edge is compressed by the edge 7 of the can 5 is placed on the electrode 3. The two pressing means act on the upper faces 8 and 7. The cylindrical part of the can 5 remains unaltered after the load is removed and the annular plate spring remains in compression. Only the piece 9 participates in the elastic counter effect. Since the spring 6 is more resilient than the edge 9 of the can 5, it is possible to set the pressure exerted on the grains much more finely.
To permit a finer setting of the pressure, such a spring can also be inserted between the lower plate 3 and the lower rounded end of the tube 5 in FIG. 5 and between the plate 3 and the oppositely lying cover of the can 6 inFIGS. 4 and 5.
FIGS. 3 to 6 only illustrate examples of VDR resistances according to the invention. Other examples using different mechanical elements can be very easily constructed within the scope of the invention provided that the semi conductor grains are enclosed in -a vessel which, owing to its resiliency, exerts a constant pressure on the grains.
If the VDR resistances are to be used for high voltages, the breakdown resistance can be increased by introducing an insulating material between the grains, such insulating material on no account constituting a binding agent. The insulating material should always be added in a liquid condition or in the yform of a very fine powder. Further, care must be taken that any excess during compression of the metal capsule is removed through small apertures having `a diameter which is smaller than the grain diameter. Only in this way can one ensure mutual contact of the grains, which, as explained above, is a characteristic of the invention. The liquid or the powder also acts as a lubricant between the grains, which, therefore, can be more easily moved towards one another during compression and assume a stable position. Stearates such as zinc stearate are particularly suitable as lubricating powder.
A liquid insulating material could also be introduced through small holes to between the grains after their compression.
The insulating material could be hardened by drying or by other means. This hardening must take place after completion of manufacture when it is certain that every grain has made contact with adjacent grains and that the pressure is exerted on these grains. Only in this way can one be sure that the resistance has a homogeneous inner structure and that the introduction of the insulating material does not detrimentally influence the provided prop erties of the resistance. In this way, the insulating material does not act as a binding agent because the capsule pressure, which is preferably between 50 kilograms per square centimetre and 300 kilograms per square centimetre, is larger than the adhesion of the insulating material to the grains.
According t'o the invention, it is further proposed to provide an apparatus for making voltage dependent resist-ances, which works according to the following principle: during compression of the metal can, the electrodes are subjected to an electric voltage which permits a current to ow through the grains. Upon increase in pressure the current becomes larger until a current value is reached which corresponds to the desired I-U function. At this current Value the press is stopped.
To make this process automatic, it is preferred to provide a control circuit in which the VDR resistance under the press is associated with a relay which actuates the stop mechanism of the press at a predetermined current value.
FIG. 7 illustrates a preferred example of this apparatus. Compression of the metal can take place under a hydraulic press the pressure member 1 of which is insu-v lated from the frame 2 of the machine by means of an insulating plate 3. The other pressure member 4 moves in a cylinder 5 under an oil pressure which is derived from an oil pump 6. The weight of the member 4 is balanced by a spring 7. Between the member 1 and the frame 2 of the machine which is conductively connected to the other member 4, there is included a circuit comprising a current source, preferably a D.C. source 8 a rheostat 9 an electro magnet relay 10 and the VDR resistance between the two pressure members. The oil pump 6 is driven -by the electric motor 11. The motor 11 is connected to the terminals of its current source 12 by a switch 13 which can be switched off by means of the relay 10. The relay 10 actuates the switch at the instant in which the current of the VDR resistance has reached it desired value.
Instead of an electro magnetic relay one could also use an electronic valve relay or any other kind of relay. Instead of actuating the switch of the electric motor, the relay could for example actuate the valve of an oil duct which prevents a further pressure increase behind the pres sure member. The relay could also actuate any other part of the machine.
Other Variations can be made within the scope of the invention provided that the pressure increase on the VDR resistance is controlled by the current flowing through that resistance.
1. A method of making a non-linear electric resistor disposed in a metallic casing with a predetermined resistance value comprising the steps of filling a metallic container with discrete silicon carbide grains Without the addition of a binding agent, placing the lled container in a press, electrically connecting a control device associated with the press so that control current -ows through the silicon grains in said container, operating the press to compress and deform said metallic container and grains therein until the current flowing through said grains reaches a predetermined value, thereafter terminating the compression on said container and grains therein.
2. The method of claim 1 wherein said container is compressed with a pressure between 50 kilograms and 300 kilograms per square centimeter.
References Cited in the tile of this patent UNITED STATES PATENTS