US2376757A - Electrical resistor - Google Patents

Description

Fatented May 22, i

- were ELECTRICAL RESISTOR Adolph Chanosky, Fort Worth, Tex.

No Drawing. Application July 12, 1943, Serial No. 494,371

'4 Claims. (01. 201-76) This invention relates to improvements in electrical resistors and similar devices for regulating current flow in electrical circuits, and method of making the same.

The use of wires made of special alloys in resistors is well known, as are also the more recent developments in the use of materials such as carbon, graphite and silicon carbide in these fields, which materials have advantages for certain applications of electrical resistors/and one object of the present invention is to produce a resistor which embodies the simplicity and convenience which characterizes this latter class of resistors.

Another object is to provide a resistor of the type indicated which possesses the feature of high electrical resistance, combined with a temperature coeificient of electrical resistance such that it furnishes a given current for a definite and rather large range of voltages applied to its terminals (so-called ballast resistors as used in the radio industry).

Still another object is to produce an electrical resistance unit of metallic particles of normal electrical conductivity or resistivity, combined with a powdered insulating material, the composition being compressed with or without a binder into the shape desired for the unit, whereby the metallic particles of conductive material are substantially sheathed or coated with the insulating material, although the resistance of the resistor, as a unit, can be varied temporarily by simply varying the density thereof through the application of additional pressure.

In the preferred embodiment of the invention, the resistor or other electrical unit is made of a mixture whose principal constituents are titanium oxide and iron. In actual practice, I have made such o ts from raw material which, in its natural state, occurs in the form of a light bud-colored, sandy loam containing a black, shiny granular substance which can be separated from the bearing send by Deister or wellhnown concentrating machines, or flotation processes. This refined, black granular material, which appears to be somewhat similar to ilmenits, in powdered form, comprises substantially the following substances, in approximately the proportions indicated, namely:

Per cent Titanium oxide 46.95 Iron 41.422 Magnesium oxide; 4.1K Aiuzninum oxide 2.71 Silicon oxide 1.56 Manganese pride 1.52 Calcium odds 0.53 Zirconium oxide (3.88 Phosphatm 0.12

This analysis was made of the concentrate after it had been screened through a 100 mesh sieve and without any further crushing or grinding. Before compressing this granular concentrate into a resistor element or unit, it is sub- Jected to a reducing action wherein the titanium oxide is not aifected. For instance, it may be subjected to the reducing action of hydrogen gas passed over the concentrate in a tube furnace maintained at a temperature of approximately 1650 F. The resultant reduced product is then compressed into a body or unit of the desired shape and size in a press such as is used in making objects by the so-called powder metallurgical process, after which the compressed body is heat treated or sintered by heating it to approximately 2200 F. for one hour. Thesetemperature and time factors in this sintering step are not critical in that the temperatures ranging from 1900 F. to 2500" F. may be used for periods of time ranging from one-half to three hours, depending upon the properties desired in the finished product. These properties will also be depending upon the pressures used in compressing the reduced concentrate. Also, the pressures used will vary from 50 to tons, depending upon the size of the resistor unit to be produced. However, using concentrate reduced in a hydrogen furnace at 1650 F. and sintered at 2200 F. for one hour, and compressing the sintered material under an average pressure of '70 tons into a unit in the form of a bar one-half inch square and two and one-half inches long, such a unit has been found to have a normal resistance such that a six-volt battery connected thereto would give a current of ten amperes when the bar was red hot. On the other hand, when this bar was placed in a vise and subjected to further compression, its resistance was reduced approximately 50%. The following explanation is given for this special combination of properties exhibited by a resistor element thus produced from this ore. It is believed the reduction of the black granular material, which has been likened to ilmenite, results in the formation of metallic iron without reducing the titanium oxide, which, with the iron content of the concentrate, constitutes the two principal ingredients of the unit. The reduced iron, having a certain amount of plasticity, makes it possible to obtain a coherent bar by the application of pressure, while the unreduced titanium oxide acts as an insulating, non-conductive film or sheath on or between the granules of reduced iron and hinders or prevents complete contact between contiguous metallic particles. In other words, the compressed bar or unit consists of an intimate mixture of finely divided particles of metallic iron and titanium oxide and, as the metallic iron is a fair conductor of electricity and the titanium oxide is practically a complete insulator, the intimate mixture of the two results in a material which has a high electrical resistance, combined with the features characteristic of compressed powders. That is, in the case of 5011151 metallic conductors of electricity, one distinguishing feature is their positive temperature coefllcient whereby their resistance increases as their temperature is raised. On the other hand, electrical conductors made-from compressed powdered material are anomalous as to their temperature coefficient in that it may be positive at low temperatures but changes readily to become negative at some given higher temperature. The resistor unit of the present invention possesses this characteristic of conductors made from compressed powder, and by proper adjustment or regulation of the pressure used in compressing the mixture into the resistor,

it is possible to produce resistors possessing the valuable property of maintaining a constant current flow within narrow limits for a relatively wide range of applied voltages. This explanation of the present resistor structures is entirely consistent with their remarkable property of changing their resistivity when subjected to pressure.

As previously indicated, the raw material above described is substantially the equivalent of ilmenite, particularly so far as concerns the high percentages of iron and titanium oxide present in both materials and, hence, the present invention is not limited to the use of such raw material as the source for the conductive and non-conductive materials required for the present resistor. In fact, a prepared mixture of iron and titanium oxides can be used as may also other prepared combinations of a granulated metal and an inert insulating powder such as, for instance, a combination of red iron ore and magnesium oxide or magnetite and aluminum oxide. In the use of any of these mixtures, the reducing agent used in the reduction step should be such as to not efiect reduction of the titanium or other insulating ingredient. For instance, carbon may be used in lieu of the hydrogen gas. Also, as indicated, a hinder or sizing material may be used if desired, molasses, sugar, casein and other organic materials being capable of use for this purpose.

The electrical properties (resistivity, temperature coeiilcient and microphone sensitivity) of resistors of the type described herein may be varied or regulated and controlled not only by the pressures and sintering temperatures used, but also by the size of particle and screen analysis of the original materials. Hence, the size of screen used in screening the particles will depend upon the properties desired in the finished article.

Resistors made in accordance with the present invention may be used not only as substitutes for metals and alloys in rheostats and heating appliances, but are specially adapted for use in microphone buttons or carbon pile regulators. For instance, in the case of the pile-type of regulator resistance units, a multiplicity of carbon rings are used. This multiplicity of delicate rings is dispensed with by the use of the present invention by reason of the fact that a single, integrated ring made of the present materials, can be used in lieu of such a multi-ring unit.

What I claim is:

1. An electrical resistor composed essentially of a compressed mixture of reduced metallic iron in granular form and titanium oxide, said granules of reduced iron oxide being substantially insulated from one another by the. titanium oxide, said resistor having a flat-curve voltage-current characteristic.

2. An electrical resistor composed of a compressed mixture consisting essentially of a reduced metallic electrical conductor and unreduced titanium oxide, said metal being present in the form of chemically reduced granules substantially insulated from one another by said titanium oxide.

3. The method of making electrical resistors which comprises reducing a mixture of iron oxide granules in the presence of titanium oxide without eifecting reduction of said titanium oxide, and compressing the reduced material in the presence of a binder.

4. The method of making an electrical resistor which comprises reducing a. granular mixture consisting essentially of a metallic electrical conductor and titanium oxide in the presence of a reducing agent for said metallic conductive ingredient, compressing the reduced mixture, and sintering the same.

ADOLPH CHANOSKY.