US2942331A - Resistor and method of preparing same - Google Patents

Description

June 28, 1960 w. D. SMILEY REsIsToR AND METHOD oF PREPARING SAME Filed Nov. 29, 1957 INVENTOR. l/l/////0m D Sm//ey/ BY /7/5 af/omeys Hv-wa-n ma( 'fvwm United States Patent RESISTOR AND METHOD OF PREPARING SAME William D. Smiley, Palo Alto, Calif., assignor to Frenchtown Porcelain Company, Trenton, NJ., a corporation of New Jersey Filed Nov. 29, 1957, Ser. No. 699,581

, 8 Claims. (Cl. 29-155.7)

Thisinvention relates to electrical resistors and to a method of making same. More particularly, this invention relates to a method for providing accurate resistors which are durable, resistant to oxidation, and capable of operating at high temperatures for extended periods of time.

With the present-day increase in various electrical devices such -as electronic instruments, there has been an increasing demand for electrical resistor elements which have and retain definite fixed resistances. In high frequency circuits where impedance values are difficult to control, it is particularly important to have accurate resistance values of substantially fixed characteristics.

In general, resistors dissipate electrical energy in the form of heat energy and frequently become quite hot in operation. Considerable research has been done in an effort to provide resistor materials which have a low thermal coefficient of resistivity, and -a number of such materials are available. In addition, surface resistors are now being provided which have the advantage of reduced inductance.

However, resistors are not only subject to change with changes in temperatures, but they are also subject to change due to corrosion which is particularly likely at high temperatures. Also mechanical injury such as scratching or abrasion caused by dust particles in the air will injure ,the surface in an amount sufficient to change the resistance to an unwanted value. Although protective coatings have been provided to protect such resistors, these also break down because of thermal and mechanical attack. Although they serve to increase the life of a resisted surface, they are not completely satisfactory. This is particularly true where-the resistors become extremely hot and coatings are likely to break down chemically. in fact, thermal decomposition of the coating may even supply materials such as oxygen which promote corrosion. t

It has also been extremely difficult to provide coatings having accurately-fixed resistances when it is desired to achieve a definite value of resistance between the terminal electrodes, and even a nonconducting protective coating can influence the resistance.

It is therefore a primary object of this invention to provide a method for fabricating resistors in which a predetermined value of resistance may be accurately obtained between the terminal electrodes.

Another object of this invention is to provide a resistor capable of operating for extended periods at high temperatures, which has an improved useful life, and is sufficiently rugged in structure to survive rough handling, adverse weather conditions, and the like.

It has now been found that these and other objects are accomplished, andY an improved resistor is provided by adhering a metallic coating or film `on a ceramic surface, attaching a pair of electrodes in electrical contact with the metallic coating, and then treating the coating to adjust the resistance until the desired resistance is the elect-rode.

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obtained between electrodes. Various methods of treating thi-n metallic coatings are known and in its broad aspect this invention embraces any of these known methods. However, I have found especially accurate resistances may be achieved by measuring the resistance across the electrodes during the treatment and using the measurement in order to control the treatment. I have also found that especially good resistances are made when the treatment consists of an anodic dissolution.

In its preferred aspect, this invention' provides for placing a comparatively thick film or coating on the material so that the electrodes may be easily soldered in place. In many of the past known methods of providing electrically conducting films, the electrodes are first placed on the ceramic surface and the electrically conducted film applied afterward, with the resulting problems Iof high resistance at the interface between the film and However, in the present method the electrode is soldered to the film or coating after the film is in place and thereby problems arising from poor electrical contact between the electrodes and the film are prevented. I

In general, the film initially provided will have a lower elecrtical resistance than that which is desired, and the resistance is then increased by treating the film, which treatment is controlled by the measurement of the resistance between the electrodes during the treating step.

Excellent results a-re `achieved by using an anodic dissolution process for increasing the resistance of the film, which process removes metal from the surface thereby reducing the thickness of the film and increasing its electrical resistance.

-In one embodiment, the invention involves providing .an electrolytic cell with the electrically conducting film to be treated serving as the anode. When current passes through such an electrolytic cell, metal is dissolved from the anode in accordance with Faradays law, providing that protective oxide films are not formed. The amount of metal removed is proportional to the quantity of current passing through the cell.

Four factors inuence the nature of the attack on the metal surface: (l) current density, (2) composition of the electrolyte, (3) composition of the metal film, and

(4) the ow rate of the electrolyte over the metal surface. With variations of these four factors the attack achieved may be 1) etching (selective attack at the grain boundaries), (2) pitting (random production of small holes throughout the surface), or (3) polishing (preferential dissolution of the high points on the metal surface, thus producing a smooth surface). In order to prepare thin film resistors by the anodic dissolution method, polishing is desired, and is obtained primarily through control of current density.

In general, the electrical resistor of this invention ccmprises a hollow ceramic body having two open ends, a metal film having a thickness of less than about 0.1 mm. on the inner surface of the ceramic body, and a pair of metal electrodes at each end of the said body in contact with said film, 4the metal film being treated as mentioned above to provide a definite fixed resistance between Ithe electrodes. In addition, means are provided to protect the film so that it retains its fixed resistance even at tempera-tures of as high as 500 C. by providing air-tight closures at each end of the ceramic body, and a non-oxidizing atmosphere within the body.

In order to fabricate the article, a hollow ceramic body is provided which is preferably cylindrical in shape having a smooth or polished inside surface. The degree of smoothness required of the inside surface will depend on how high a controlled fixed resistance is required.

A coating is then applied by using any known method of applying a metal coating to a ceramic surface such as spraying, painting, or thermal evaporation. Broadly, any metallic coating can be used for the film, however, alloys of Cu-Mn-Ni, Cu-Mn and Ni-Cr appear to be the most valuable.

Preferably, it is desired to use a metal having a low thermal coefficient of resistivity such as the materials sold under the trade names Nichrome (80% nickel, 20% chromium), Manganin (82% copper, 15% manganese, 3% nickel), Constantan (60% copper, 40% nickel), and Rheotan (52% copper, 25% nickel, 18% zinc, 5% iron).

Especially good results are obtained by using molymanganese coatings such as the coating obtained by firing a mixture of about 80% molybdenum powder and 20% manganese powder in the presence of hydrogen. However, these materials do not provide as continuous a structure on the ceramic as do certain other metallic materials, and where higher resistances are desired and the thermal coefficient of resistivity is not critical, metals which are known to provide a more continuous structure may be used. In addition, more than one coating of different metallic materials may be applied if desired. For example, a thin film of metal which provides the desired continuous structure on the ceramic may be applied lirst, and a second metallic material which is adapted to the treating step applied thereover. Also, adhesive layers, such as a thin coating of ferrie oxide which may be appiled by thermal evaporation, may be used where high resistances are desired to provide a base for a thin continous film.

After the film is in place the electrodes are fastened to the iilm and preferably fused or soldered thereto to provide a permanent bond. With the electrodes in place, the film is treated until the desired resistance is achieved and then protected from corrosion by inclosing an inert atmosphere inside of the ceramic body. This may be accomplished by placing the body in an inert atmosphere such as helium, and soldering the caps in place within this atmosphere.

The invention will be further described with reference to the accompanying drawings illustrating the preferred form of this invention and in which like numerals are used to designate like parts throughout.

Fig. 1 is a sectional view of a typical resistor provided by this invention.

Fig. 2 is a sectional view of the resistor taken along 'the .line 2-2 of Fig. 1.

Fig. 3 is a diagrammatic view illustrating a setup which may be used for achieving the desired fixed resistance value in the resistor of this invention.

Fig. 4 is zin enlarged crosssectlonal View of a part of the apparatus illustrated in Fig. 3 illustrating in greater detail, a method of making resistors in accordance with this invention.

Referring now more particularly to the drawing, there is shown a resistor comprising a hollow ceramic body 11 which is open at both ends. Preferably the ceramic body 11 is cylindrical as shown in Figs. 1 and 2 and should have a smooth interior surface. Within the ceramic body 11 there is provided a metal coating 12 which is permanently secured to the body 11 by such processes as fusing and which serves as the resistance element for the desired controlled fixed resistance.

The coating 12 will generally have a thickness of less than about .l mm. in order to provide sufficiently high resistance values without requiring an unduly long ceramic body. The smoothness of the interior surface of the ceramic body 11 required is determined by the resistance desired, because with a smoother surface it is possible to provide thinner iilms 12 capable of retaining a fixed resistance value.

Fitting within the ceramic body and in electrical contact with the lm V12 are electrodes 13 and 14. As illustrated in Fig. 1, a preferred form of electrode is a cylindrical body having an inner section 15 slightly smaller than the inside diameter of the lilm 12 and an outwardlyextending flange 16 having an external diameter substantially equal to the external diameter of the ceramic body 11. The electrode is generally soldered to the metal film 12 and its flange is generally soldered to the ceramic body 11 to provide a gas-tight seal between the electrodes and the ceramic body.

In general, the electrodes may be composed of any electrical conducting material which may be fused to the ceramic body and electrically connected to the film 12. Two caps 17 are provided which are sealingly connected to the electrodes 13 and 14 so that the film 12 is entirely enclosed in a gas-tight chamber.

Obviously different arrangements may be provided which will achieve the desired gas-tight seal, for example the caps could be sealed to the ceramic body or one of the caps could be integral with an electrode and in such a situation the electrodes 13 and 14 would not have flanges. However, it is preferred that the electrodes be in place before the tube is sealed so that the film 12 may be treated to achieve the desired xed resistance, and this i treatment requires the electrodes to be in place at the time of treatment. Otherwise, the placing of the electrodes would usually affect the resistance of the film in an amount to render it diflicult to arrive at the desired fixed value.

Generally the caps 17 will be of a metallic material and may be the same metal as electrodes 13 and 14 although any material may be used which will provide the desired seal. 1f the caps 17 are metal, lead Wires 18 may be soldered thereto as illustrated in Fig. 1 or if the caps are of a nonconducting material, the lead wires may be soldered directly to the electrodes 13 and 14.

As disclosed more fully hereinafter, the ceramic body is sealed in such a way that a non-oxidizing atmosphere is provided therewitlhin. Suitable atmospheres may be an inert gas or a vacuum, and I prefer to use helium for this purpose.

As stated above, one of the important aspects of this invention is to provide a method of achieving a given fixed value for the resistance. Broadly speaking, this is achieved by applying the coating 12 and electrodes 13 and 14 and then treating the coatingin such' a way that the resistance is changed while measuring the resistance between the electrodes. The treatment is discontinued when the desired value is achieved.

I have found that very accurate control of the change of resistance may be achieved by anodic dissolution of the surface of the lni until the desired resistance is achieved. I have also found that by choosing the proper electrolyte and by reducing the current density as the desired resistance is approached, an extremely accurate control is achieved. In addition, it is still possible to measure the film resistance during anodic dissolution, contrary to what may be expected, by using electrolytes of comparatively low conductivity. For more accurate control, calibration is necessary to translate apparent resistance values to true resistance values. However, the proferred electrolytes are of such a low conductivity that fairly-accurate results may be achieved without calibration. f

A typical laboratory setup for anodic dissolution is illustrated in Fig. 3. However, it is apparent that such a procedure is easily adaptable to production methods.

In Fig. 3, there is shown a storage battery 19 which supplies energy to the electrolytic apparatus 20 in which the anodic dissolution takes place. The power circuit also contains a slide wire rheostat 21 and an ammeter'ZZ for regulating the current and controlling the current density in the electrolyte. Satisfactory results have been ob tained with either a 6 or 12 volt battery and a control During the dissolution, electrolyte is supplied from dropping funnel 23 and circulated through container 24 in which the electrode dissolution is conducted and overflows therefrom into container 25.

The apparatus in which the electrolysis takes place is shown in detail in Figure 4, in which there is shown a plastic container 24 having a chamber 26 in which the ceramic body is placed for the electrolytic treatment. There is also provided a channel 27 for the entry of circulating electrolyte and an electrode 28 which extends from the outside of the plastic body 24 to the interior of the chamber 26. Electrode 28 is insulated so that only its tip 29 acts as a cathode and supplies an electric current to electrolyte within the rchamber 26. The tip 29 may be a point or extended to provide a line source along the axis of the cylindrical chamber 26.

In order to prepare the ceramic body for the electrolytic treatment, the ceramic cylinder 11 is first coated with film 12 in an amount suicient to provide a surface having a lower resistance than the resistance desired. This coating may be applied by any method known to the art for coating ceramic materials such as by dipping the ceramic in a suspension of metal powders or painting the suspension of metal powders on the ceramic with subsequent heat treatment to effect a ceramic-metal bond. Coatings may also be applied by melting metal on the ceramic allowing it to ow and wet the ceramic, or by vapor deposition. No matter what technique is used, it is important to provide a good bond which is not injured by the high temperatures which the material will attain when in operation.

With the coating in place, electrodes 13 and 14- are sealed in position. Preferably this is achieved by coating the electrode with a thin layer of solder and heating the unit to provide the desired fusion. For example, when nickel sleeves are used as the electrodes, they are rst coated with a thin layer of silver-copper solder by heating a small layer of solder on the nickel to a temperature of 1040" C. for five minutes in an atmosphere of helium.

The coated sleeve is then fitted in place and fused to the film by heating the entire unit to a temperature of 1040 C. for tive minutes in an atmosphere of helium. The exact material for the electrode and solder will vary depending on the material used for the coating; however when moly-manganese coatings are used, it is preferred to use a solder of about 72% silver and about 28% copper.

As stated above, the ceramic body having the film and electrodes in place is then treated to provide the desired resistance value in the coating. In accordance with the method illustrating in Fig. 4, the ceramic body is inserted axially into the chamber 26 of the plastic container 20, and the electrode 28 is adjusted to center its tip 29. Power lead wires 30 and 31 are then connected with lead wire 30 being connected to one or both of electrodes 13 and 14 such as electrode 14 as allustrated in Fig. 4, and lead wire 31 is connected to electrode 28. Electrolyte is supplied through channel 27 into the interior of the ceramic cylinder and it flows out through the top of the ceramic cylinder to provide circulation of fresh material. Wires 32 and 33 are connected to electrodes 13 and 14 and connected to the resistance-measuring device 34 such as a Wheatstone bridge.

Preferably non-aqueous electrolytes are used because with aqueous electrolytes the electrolysis process tends to liberate oxygen at the anode or metal films thereby tending to oxidize the metal films. However, certain aqueous solutions are satisfactory with certain coatings. In general, solutions containing little or no water and preferably solutions containing a major part of alcohol are used. The alcohol may be monohydroxy or polyhydroxy and usually will be of a short carbon chain of say one to six carbon atoms. Examples of suitable electrolyte compositions are 6% H2O, 5% H3PO4, 89% ethyl alcohol; 8% H20, 30% H3PO4, 62% ethyl alcohol; 1.75%

6 HF, 98.25 ethylene glycol; 3.7% HF, 96.3% ethylene glycol; and 30% H3PO4I and 70% water.

As the anodic dissolution progresses, the resistance of the film is read on the device 34 and the current density is decreased through operation of rheostat 21 whereby the resistance in the main circuit is increased to lower the current supplied to the electrodes. resistance value is attained in the film, the current is shut off and the cylinder removed from the electrolytic-treatingdevice. After cleaning to remove electrolyte, the cylinder is placed in a helium atmosphere in contact with nickel caps 15 and 16 where the assembly is heated to a temperature of say 840 C. to braze the nickel caps and the solder-coated electrodes. This provides a gas-tight ceramic metal unit containing a helium atmosphere with an accurately-measured resistance film. As stated above, the final sealing may be conducted in a vacuum or in an atmosphere containing inert gases so that the final unit contains a film protected from corrosion.

In order to illustrate the anodic dissolution procedure of this invention, the following examples are given, and it is to be understood that they are given for the sake of illustration and are not intended to limit the scope of this invention.

Example I A 10-ohm resistor was prepared by applying a molymanganese coating to the inner surface of a ceramic cylinder 3/16 of an inch in diameter and 2%: of an inch long and brazing nickel sleeves at the ends of the tube by using eutectic silver-copper solder wire and firing it in helium at 1040 C. for 5 minutes. The moly-manganese film contained about molybdenum and about 20% manganese. The cylinder was then placed within the apparatus illustrated in Figs. 3 and 4 with an electrolyte containing 98.25% ethylene glycol and 1.75% hydrofluoric acid. The initial resistance of the film was 0.6 ohm, and the initial current was l0 milliamperes which provided a current density of about 2.5 milliamperes per square centimeter. After a time interval of between 5 and 6 minutes, the resistance had increased to about 0.9 ohm, and the current was reduced to about 4.5 milliamperes. This was continued until a time interval of about 15-16 minutes when the resistance of the film started increasing rapidly and was at a value of about 1.9 ohms. At this point the current was further reduced to about 1.1 milliamperes which provided a current density of about 0.35 milliampere per square centimeter. After a complete time interval of about 40 minutes the resistance was between 6 and 7 ohms, and the current reduced to 0.8 milliampere.

At the end of about 45 minutes the desired resistance of l0 ohms was achieved and the current was cut off. The resistor was then rinsed in distilled water and alcohol to remove the acid solution. Solder-coated nickel caps and the resistor were placed in a furnace which was evacuatedv and ushed with helium. The caps were then placed in position and the unit fired to 840 C. for 5 minutes in the helium atmosphere to provide a scaled resistor having an accurately fixed resistance of l0 ohms which is capable of operating for extended periods at a temperature of 500 C. With the setup used, resistances as high as 20 ohmslmay be accurately prepared.

Example Il A 25-ohm resistor was prepared using the procedure of Example I except that the electrolytic solution contained 30% phosphoric acid and 70% water. The initial resistance was 0.9 ohm and the film was first subjected to a cu-rrent of milliamperes. The current was decreased as before and after 45 minutes a resistance of 25 ohms was achieved.

What is claimed is:

l, A method of making thin metallic film resistors which comprises firing a metallic coating on the inner surface of a hollow ceramic body having two open ends, providing a pair of electrodes in spaced relation in elec- When the desired trical `contact with said coating, reducing the thickness of the metallic coating by anodic dissolution, and measuring the resistance of said metallic coating during the reduction o f the thickness thereof to control the anodic dissolution and provide an accurately xed resistance.

2. A method of making thin metallic film resistors having low thermal coeicients of resistivity and operative at temperatures up to 500 C., which comprises iiring a metallic coating on the interior surface of a hollow ceramic body having two open ends, fastening a pair of terminals within and at each end of said Vceramic body in electrical contact with said metallic coating, inserting the ceramic body in an electrolyte solution, supplying a cathode within the electrolyte solution, applying a 4direct current voltage between said cathode and one of said terminals, and attaching a resistance. measuring device across said terminals for determining the resistance of said metallic coating.

3. A method claimedin claim 2, in which the current density within the electrolyte is decreased as the desired resistance value is approached whereby a comparatively high rate of dissolution is provided at first and a comparatively slow rate of dissolution is provided at the end of the electrolysis.

4. The method claimed in claim 2, in which the electrolyte contains a major portion of a low molecular weight alcohol and an ionizable material.

5. The method claimed in claim 2, in which the ceramic body is sealed Within caps in the presence of aninert 8 atmosphere to provide protection for the metallic coating.

`6. A method for making a resistor having a predetermined res-istance value which comprises tiring a metallic coating on a ceramic body, the thickness of said coating being such that the resistance of the coating is less than said predetermined value, and then dissolving metal from said coating by electrolytic action, while continuously measuring the resistance of the coating, until the predetermined value has been reached.

7. The method claimed in claim 6 wherein the metallic coating is tired on the interior of a hollow ceramic body having two open ends.

8. The method claimed in claim 7 in which the ceramic body is filled with an inert non-conductive substance after the coating has reached its predetermined resistance.

References Cited in the tile of this patent UNITED STATES PATENTS 540,073 Reed May 28, 18,95 1,023,485 '[howless Apr. 16, 1912 1,291,106 Payne Ian. 14, 1919 1,693,899 Horle Dec. 4, 19728 1,832,466 Means Nov. 17, 1931 i,936,934 Clear Nov. 28, 1933 2,092,133 Newman Sept. 7, 1937 2,440,691 lira May 4, 1948 2,662,957 Eisler Dec. Y15, 1953

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