US2589157A - Voltage-dependent resistance blocks - Google Patents

Description

Patented Mar. 11, 1952 VOLTAGE-DEPENDENT RESISTANCE BLOCKS Bcrtil Stalhane, Stockholm, Sweden, assignor to Elektriska Aktiebolaget, Vasteras, Sweden, a Swedish corporation No Drawing. Application April 29, 1949, Serial No. 90,539

Allmanna Svenska 1 Claim. 1

The present rinvention relates to voltage-dependent resistances and more particularly to such type of resistances in which, for a rising voltage, thecurrent rises more rapidly than the voltage. Such resistances are used, among other purposes, for protecting transmission systems against lightning and other voltage surges, and, on a smaller scale, for protecting contacts against arcs at an interruption of a current by a separation of the contacts. In both cases, the voltage-dependent resistance acts by absorbing, under a moderately increased voltage, a much larger current than under normal voltage.

In a frequently employed type of such voltagedependent risistances, the main constituent of the resistance is a semi-conductor such as silicon carbide. Such a semi-conductor cannot, however, be in the form of a solid homogenous body, but its action depends to a large extent on the fact that the semi-conductor appears in the shape of small grains, for large resistance blocks usually of the order of magnitude of one or some tenths of a millimeter in diameter, said grains being held together by a binder which is more or less an insulator. Such binders have been of many different types of composition, and a particularly appropriate one has been described in my U. 5. Patent specification No. 2,206,792. Its principal constituent may for instance be aluminium phosphate which is gradually heated to about 1000 C. for accomplishing the proper chemical binding by combination at a raised temperature.

It is the main purpose of the present invention to provide voltage-dependent resistance blocks which consist of granular semi-conductors held together by a binder chemically combining at a raised temperature, and which have improved mechanical properties as compared with blocks composed of only semi-conducting grains and binder.

When a block is formed of grains of an active material, such as a semi-conductor, it is important for obtaining constant electrical properties, that the size of the said grains is kept between rather narrow limits. On the other hand, it has been found desirable for mechanical reasons, that also smaller grains form part of the body, preferably in a certain proportion to the bigger grains. The plasticity of the mass during forming and pressing and its cohesion after forming and especially during baking is improved hereby, and the baked product shows less porosity than a mass formed only of grains of substan- 2 tially uniform size and is therefore also less hygroscopic and mechanically stronger.

The present invention has for its purpose to provide a resistance block of the type specified, which has good mechanical properties and still retains the electrical properties characteristic to the block composed only of semi-conducting grains and a binder chemically combining at a raised temperature.

According to the present invention, there is added to the mixture of semi-conducting grains and a binder chemically combining at a raised temperature, a certain quantity of electrically insulating grains which are essentially smaller than the semi-conducting grains and preferably vary more in size than the semi-conducting grains. For instance, if the size of the semiconducting grains varies from .1 to .3 millimeters, the size of the insulating grains may vary from .01 to .1 millimeters. The total quantity of such insulating grains will generally be less than the total quantity of semi-conducting grains, for instance 20-40 per cent by weight of the total mass for 50-75 per cent of semi-conducting grains.

The material of the insulating grains should be such as not to react with the binder nor with the semi-conducting grains at their binding temperature. Preferably, the material should be a simple chemical compound, such as an oxide, of, neither decidedly basic nor decidedly acid character, and of high melting point, such as aluminium oxide or zirconium oxide. Aluminum oxide in the form of artificial melted micro-crystalline corundum has been found particularly suitable in combination with the binder specified in the said U. S. specification No. 2,206,792, as it is not only chemically inert towards the other constituents of the resistance blocks and towards the binder also at high temperatures, but also can be easily formed to grains of any desirable size. Zirconium dioxide may be also be used and in some cases titanium dioxide or magnesium oxide, although the two latter oxides are comparatively inferior in quality to aluminum oxide for the purpose of the present invention.

As an example of a particularly suitable mixture, the following may serve:

50-75 per cent by weight of size of grains .l.3 millimeters;

20-40 per cent by weight of corundum, size of grains .01-.1 millimeters;

5-20 per cent of phosphatic binder, substansilicon carbide,

tiaily as described in specification No. 2,206,792.

I claim as my invention: REFERENCES CITED A Voltage-dependent resistance block 130m The following references are of record in the posed of from 50 to 75 per cent by weight of file of this patent; silicon carbide of grain sizes from .1 to .3 milli- 5 meter maximum dimension, from 20 to 40 per NITED STATES PATENTS cent by weight of micro-crystalline aluminum Number Name Date oxide of grain sizes from .01 to .1 maximum 1,293,421 Thomson Mar. 25, 1919 di sio and fIOm 5 20 20 per cent by weight 2 20 792 st July 2, 4 of a binder containing phosphoric oxide as a 10 2,276656 Johnson 17, 1942 characteristic constituent. l 2 27 732 Ludwig et 1 a 17, 1942 BERTIL STALHANE.