US1821022A - Resistor block and process of making - Google Patents

Description

Patented Sept. 1, 1931 UNITED STATES STERN'E MORSE, OF CLEVELAND HEIGHTS, OHIO RESISTOR BLOCK. AND PROCESS OF MAKING No Drawing.

This invention relates to resistor blocks, and their method of manufacture and more particularly to blocks such as are used in heating elements such as are described in my Patent No. 1,663,810, issued March 27, 1928.

-An object of this invention is to produce graphite blocks of homogeneous composition, the blocks of the same size having substantially uniform resistances.

A further object of this invention is to produce such graphite resistor blocks of substantially the same size and with surfaces which do not need further finishing by grinding or machining.

A further object of this invention is to secure resistor blocks having surfaces such that the contact resistance is relatively sensitive to slight changes in pressure.

A further object of this invention is to produce such graphite resistor blocks of high density, and with considerable mechanical strength and heat conductivity.

Another object of this invention is to produce such blocks with a minimum. amount of 26 interstices between the individual particles of graphite to insure a minimum amount of oxidation when at high temperature.

A further objectof the invention is the process for making graphite resistor blocks 30 from artificial or natural pulverized graphite, or from such forms of amorphous carbon as-may be readily graphitized.

The artificial or natural graphite, which isusually powdered, is preferably mixed with v a suitable binder, such as coal tar pitch, dis

solved in benzol or some other suitable solvent Which may bereadily evaporated. Sufiicient a binder is preferably used to cause the individual particles of the mass to readily and quickly adhere to each other under moderately heavy ressures, thus avoiding the use of continued heavy pressures, such as are customarily necessary when no binder is used.

.The pulverized graphite is thoroughly mixed with the binder, preferably until the consistancy 'is' that of fairly plastic mortar, "and the solventof the binder is then evaporated in any suitable manner. After evapo- J 50. ration ofthe solvent, the'material is in the Application filed September 24, 1927. Serial No. 221,819.

state of a very soft friable mass whichbreaks up easilyinto a powder consisting of themdividual particles of graphite coated with a fine coating of the pitch binder. This material can be compressed in a slow acting press to form a firm mass, but cannot, under ordinary conditions, be advantageously pressed in a rapid, automatic press, such as a tablet pressing machine, since it cannot be made to feed properly. The material also contains so much air that the individual particles are held a art during the instant when pressure is app ied, so that it is extremely difficult, if not impossible, to form a firm mass by pressing' in a rapid, automatic press. For the above reasons the material is pressed by rolls, or in a suitable slow acting press, with sufiic'ient pressure to form fairly firm cakes or sheets. These cakes or sheets'are then again broken up and grained with very'low pressures in order to keep the size of the particles as large as possible so as to produce a coarse grained material. It is very desirable, in order to make the material feed properly in an automatic press of the tablet machine type, that it be brought to the physical condition Where it appears almost like sand, that is, the individual grains must be relatively compact sothat they contain little air, and be of approximately the same size. The channels between the individual grains being relatively large as a consequence of the graining process, there is time, even during the short instant when full pressure isapplied in a suitable tablet press, for the air to pass out between the grains and for full coalescence of the grains to be secured in the subsequent final pressing to the desired shape. In this graining grinding must be avoided to prevent the formation of powder, as the mass tends to break into fine powder very easily if it is ground, whereas coarse grains are desired.

This coarse-grained material is then classified in any suitable manner, such as by screening or pneumatic classifiers, into 'various grain sizes, so that the grains in one class will be of substantially the same size with a View of pressing the material of each range of sir-es separately. One good classification to profor example, a die, so that when pressure is applied tothis loose material resistor blocks of substantially the same weight and density will be formed. The resistor blocks, when so made from grains of substantially the same size, are quite uniform in character, so that they may be used interchangeably and form heating elements for use as set forth inmy copending application, Serial No. 86,413, filed February 26, 1926.

The material which passes the 40 mesh screen may be mixed with a suitable solvent to redissolve the binder and may be treated in the same manner as above to yield coarsegrained material, or it may be added to another batch of graphite and treated in the same manner as previously described.

The granules from each classification are then separately fed to a suitable press, such as, for example, a pellet or tablet forming machine, and pressed with a considerable pressure to form a block of the desired size and density.

The blocks are then mixed with an excess of powdered conductive material, such as graphite or carbon, and after the blocks and powdered material have been thoroughly mixed together so as to form a conductivemass, the mass is packed into a suitable c011- tainer through which an electric current may be passed.

The mass of tablets with the powdered conductive matter is made the resistor of a carbon resistance furnace with suitable graphite electrodes disposed at each end of the material. Preferably the furnace is one of the type in which an adjustable pressure can be applied to all sides of the resistor mass, thus insuring that the mass of graphite blocks and conductive material is compacted suificiently to readily conduct the .current therethrough and allow easy control of the amount of energy supplied to the furnace.

A low voltage current is applied to the electrodes, and the mass is heated very slowly until the carbonizing temperature of the binder is reached. The mass is held at this temperature until carbonization of the hinder is substantially completed, and then the temperature is raised to about 1800 F. and held there for several hours to insure complete carbonization of the binder. desired to have the blocks completely graphitized so that they will be more durable and suitable for service at higher temperatures,

the temperature of the furnace can be raised If it be terial may be removed from the furnace after i the carbonization of the binder is entirely completed.

If it be desired to have the resistor blocks .comp'osed completely of graphite, it may be preferable to use powdered petroleum coke as the original material from which to form the resistor blocks.- When this material is used, the process is carried on with the binder to form .the preliminary cake from which the coarse-grained particles are formed, and which are classified and pressed as hereinbefore described. The blocks are mixed with powdered conductive material and heated in a furnace similar to the process above described, but graphitization is carried to completion to graphitize both the grains of the block and the binder.

- When the carbonaceous material is derived from the aliphatic series, such as from cellulose or ordinary coal, a graphite will be obtained which is similar in crystallization to ordinary natural graphite, but if it is obtained from the benzol series, as from petroleum coke, graphite is obtained which differs from the graphite from the aliphatic series in its type of crystallization. The latter is somewhat preferable, as it yields a harder block of'higher resistance and also has the property of being more refractory.

In order to secure a homogeneous mixture between the powdered graphite and the binder, the coal tar pitch binder is preferably ground with benzol, or other suitable solvent, in a ball mill until a smooth, even-flowing mixture is secured. One good grade of coal tar pitch is that commonly known in the trade'as having a melting point of 175 F. It is important that the coal tar pitch used be relatively low in so-called' fixed carbon, as such fixed carbon is relatively insoluble in the ordinary solvents, and when present in appreciable quantities, seriously affects the adhesive quality of the binder.

If it is desired to graphitize the hinder, or to graphitize the carbonaceous material when such carbonaceous material is used to i form the blocks, it is preferable to incorporate a small percentage of a suitable catalyst,

such as iron oxide, alumina, silica, ferrous boroslllcate, or boron in any other form. The

presence of such catalyst materially assists 1 for this might cause the to be less stable.

It may be noted that by keeping the die and plunger of the press highly polished, the resistor blocks will have good finished surfaces for making electrical contact without the necessity of grinding or machining.

The density and strength of the resistor blocks may, of course, be increased with an increase of pressure in the tablet forming machine or other press that may be used. If the grained material to be pressed is heated and pressed hot, less pressure is necessaryto obtain blocks of the samecharacteristics, and the time of pressing may be reduced.

If it be desired to give the resistor blocks a higher resistance a non-carbonaceous binder such as clay may be used which may be mixed with any suitable liquid binder, such as a solution of water-glass or the like, if the characteristics of the clay are such that an additional liquid binder is desirable. When clay is used as a binder the process may be carried on as above described except that in the process of baking the resistor blocks no attention need be paid to the carbonlzation of the binder.

It may also be noted-that if the resistor electrical properties blocks are to be used at relatively low temperatures and relatively high resistances are desired as, for example, in the control of refrigerating devices, it may be unnecessary to even bake the blocks if they have been formed with relatively high pressure. Preferably, however, they should be heated suflicien tly to'insure removal of all of the volatile matter, and thereby prevent an adhesion of the resistorblocks to each other during use. This may also be prevented to some extent by the additional coating of these blocks with powdered graphite. It is usually preferable, however, when the resistor blocks are to be used with relatively large currents or above normal temperatures to give them a thorough baking and carbonize the binder.

Furthermore, it is to beunderstood that the particular compounds disclosed, and the procedure set forth, are presented for purposes of explanation and illustration, and that various equivalents can be used and modifications of said procedure can be made without departipg from my invention as defined in the appended claims.

What I claim is 1. The process of making carbmiaceous bodies which comprises mixing powdered carbonaceous material with a suitable binder, in solution, forming coarse solid granules of the carbonaceous material of predetermined size, pressing such granules with a relatively great pressure in a mold chamber having highly polished metal engaging surfaces, mixing the bodies so formed with carbonaceous conductive material and heating the bodies and carbonaceous conductive material in a furnace to substantially carbonize the binder.

2. The process of making carbonaceous bodies, which comprises mixing powdered carbonaceous material with a suitable binder in a volatile solvent, evaporating the solvent from the mixed mass, pressing the dry mass into cakes, breaking the cakes into solid granules of predetermined size, pressing said granules into blocks, mixing the bodies with conductive material, and heating said bodies and conductive material as the resistor in a resistor furnace to substantially carbonize said binder.

3. The process of making carbonaceous bodies having predetermined characteristics, which comprises mixing powdered carbonaceous material with a binder dissolved in a solvent to form a mass of plastic consistency, evaporating the solvent from the mass, pressing the residue into cakes, breaking the cakes into coarse solid granules, sorting the granules into classes according to predetermined sizes, pressing a predetermined quantity of said granules of a predetermined size under great pressure into a body, mixing said body with carbonaceous conductive material, and baking said body and conductive material in a furnace to completely earbonize said binder and produce a homogeneous body of predetermined characteristics.

4. The process of making carbonaceous bodies having predetermined characteristics, which comprises mixing finely divided carbonaceous material with a suitable binder, pressing the mixture into cakes, breaking the cakes into solid granules, sorting the granules into classes according to predetermined sizes, pressing equal predetermined quantities of the granules of one class into bodies, and carbonizing the binder by baking the bodies.

In testimony whereof I afiix my signature.

STERNE MORSE.