US2915730A - Electrical resistor and method - Google Patents

Description

' Dec. 1, 1959 J. K. DAVIS ELECTRICAL RESISTOR AND METHOD Filed Sept. 30, 1955 INVENTOR. JANA-5 AQAV/S BY My 4. 119.}.

Afro/ilvir United States Patent ELECTRICAL RESISTOR AND METHOD James K. Davis, Elmira, N.Y., assignor to Corning Glass Works, Corning, N.Y., a corporation of New York Application September 30, 1955, Serial No. 537,810

16 Claims. (Cl. 338-309) This invention relates to electrical resistors of the type comprising a ceramic body, such as a tube, rod or sheet of glass, porcelain, sillimanite, or the like, an adherent, electroconductive, metal oxide film on the surface of the body, and spaced, electroconductive terminals in electrical contact with the film.

In making this type of resistor, the ceramic body is heated to a temperature in the neighborhood of 500 700 C. The heated body is then contacted with the vapor or atomized solution of a selected hydrolyzable material to produce on the exposed ceramic surface a thin, strongly adherent, electroconductive film. Suitable materials and mixtures for producing such films include the chlorides, bromides, iodides, sulfates, nitrates, oxalates, and acetates of tin, indium, cadmium, tin and antimony, tin and indium, or tin and cadmium either with or without a similar hydrolyzable salt or other compound of a modifying metal such as zinc, iron, copper, or chromium. The film consists of the corresponding metal oxide or oxides. For a clearer understanding of such films, their formation and their characteristics, reference is made to United States Patents Number 2,564,706 and 2,564,707 issued in the name of John M. Mochel.

The thickness of the film increases with the length of time the heated body is contacted with the vapor or atomized solution, and its electrical resistance generally decreases as its thickness increases. Films having thicknesses from less than the first order of interference colors up to about the tenth order with electrical resistances of 1,000,000 or more down to 5 or less ohms per unit square can thus be produced. Resistances may also be adjusted upward by cutting through a film of a given resistance on a cylindrical ceramic body to shape the film into a spiral strip of predetermined width and length.

Resistors comprising electroconductive films of this type provide distinct advantages over other types of re sistors for many purposes. Prior to the present invention, however, it has not been feasible to produce commercially acceptable, metal oxide film resistors having aresistance over about 60 ohms/square. This undesirable situation has been due primarily to the general tendency of metal oxide films to have a high negative temperature coeflicient of resistance and to be quite unstable electrically, the latter being particularly-trouble some when the films are operated under, direct current load. This condition of instability manifests itself by a temporary or permanent change in resistance during operation of the resistor. While the tendency is ap parent at low temperatures, it becomes progressively aggravated as the temperature involved increases.

The temperature coefiicient of a film or other form of resistance element is measured over a given temperature range. The particular temperature range selected will, of course, vary with different types of resistors and resistor applications, but will ordinarily correspond ap proximately to the expected operating range of the resistance element. It is known that electroconductive metal oxide films tend to have a negative temperature coefficient which is usually of greater magnitude in higher resistance compositions. t

Patented Dec. 1, 1959 It has been found, however, that tin oxide films con taining up to about 6% antimony oxide are useful as resistor elements since their relatively small temperature coefficients are within the limits ordinarily specified for that purpose. As antimony oxide is initially added to a tin oxide film small positive temperature coefiicients are produced in contrast to the usual tendency. As the antimony oxide content reaches about 3% the coefiicient reverses in sign, becoming negative, and increases rapidly in magnitude, becoming too high to be practical for resistors at about 6% antimony oxide.

The standard resistance of films in this composition range, that is tin oxide films containing up to 6% antimony oxide, does not exceed about 60 ohms/square, however. By standard resistance is meant the resistance in ohms/square of a film exhibiting a third order red inter ference color. It has been readily apparent then that, unless some practical means could be found to provide higher resistances on the order of 1004000 ohms/ square, the metal oxide film type resistor would be of limited significance.

There are, of course, two readily available procedures for producing higher resistance films. The film composition may be varied as, for example, increasing the anti mony oxide content of the present tin oxide films beyond the present 6% limit. This is of no avail, however, be cause as previously indicated, excessively high temperature coefficients are invariably associated with higher resistances produced in this manner.

In the alternative method of increasing film resistance, film composition is maintained constant but thinner films are formed on the base material, film resistance increasing as the thickness is decreased. This method may be carried out either by shortening the time of exposure or by diluting the film forming material. In accordance with this method then, film compositions, previously observed to produce low resistance films having acceptable temperature coefficients, can be used to form thinner, higher resistance films without substantially changing the nature of the temperature coefficient.

However, when such compositions are deposited as very thin films, for example in thicknesses exhibiting only first order interference colors, to obtain resistance values on the order of 100 to 1000 ohms/square, such films are found to be highly unstable electrically, the degree of instability being such thatthe films are wholly un usable for resistor purposes. Thus, it would appear that the instability is a surface phenomenon which has its most pronounced effect on thin films.

It has been observed that moisture and other atmospheric gases and vapors can effect changes in the con ducting film and that exposure of the film to these atmospheric influences is a major factor in the electrical instability. This situation can be largely corrected, and a substantially improved resistor produced, by covering the exposed portion of the metal oxide film between the spaced electroconductive terminals with a fused layer of a ceramic glaze or enamel frit. The coating method and resultsing. product are described in detail in my co" pending application S.N. 434,150, filed June 3, 1954.

This ceramic coating was developed in conjunction with films having a third order or greater thickness and under such circumstances has proven highly satisfactory. Unfortunately, when it is applied and fused over thinner films a marked change in film resistance occurs which may be as detrimental to the film, from a resistance standpoint, as is the condition which the coating is designed to avoid. This effect may be a slight chemical interaction with, or physical disruption of, the metal oxide film during firing. With thicker films the effect is apparent but not especially serious. However, it produces such erratic effects on thinner films as to rule out its use completely.

Even where the ceramic coating method provides satisfactory protection. for the film, it is rather expensive and time consuming because of the additional coating and firing. steps involved. Thus, it is necessary to heat the ceramic base prior to film formation, then apply the terminals and reheat to fire them on, and thereafter spray on the frit or glaze and again reheat to fuse the frit or glaze. Further, both reheating operations must in many cases, be conducted in a neutral or reducing atmosphere to avoid damage to the film. It will be readily appreciated that, in an inexpensive product such as a resistor, the nature and number of these production steps are such as to render the ultimate cost completely prohibitive for many applications.

I have now discovered a greatly simplified method of producing protectively coated, film type resistors which provides adequate protection for the conducting film rcgardless of film thickness. This new method involves depositing, as a protective coating, an electroconductive metal oxide film similar to the primary conducting film but differing both in composition and resistance. It fur ther involves applying the primary and protective films successively and as part of a single coating operation, and thereafter applying terminals over the protective film. This method eliminates one of the customary reheating steps and in most cases, the need for a special firing atmosphere. The production operation is then considerably shorter and less expensive than heretofore. It has been further found that deposition of the second metal oxide film can be carried out with little or no disturbance of the lower or primary film properties even on very thin films. This means that it is now commercially feasible to produce film type resistors for a much wider range of applications both because of the greatly increased resistance range and the lower, more competitive production cost.

The improved electrical resistor resulting from such method and forming a part of this invention comprises a ceramic body, an adherent, electroconductive, metal oxide film on the surface of the body, a second electroconductive metal oxide film superimposed on the first film, and electrically conducting terminal members formed on the second film and in electrical contact with the first film.

The secondary or protective film may contain oxides differing from those in the primary film, or the two films may contain the same oxides in different proportions. In either event, the protecting film must have a sufiiciently higher resistance than the primary film so that a major proportion of longitudinal electrical current flow in the resistor preferentially occurs in the protected or primary film. On the other hand, the protecting coating must possess sufficient conductivity to permit establishment of electrical contact between the terminals and the primary conducting film by a transverse flow of current through the protective film.

The single figure in the accompanying drawing illustrates one embodiment of the present invention and shows partly in section an electrical resistor composed of a cylindrical ceramic body provided with two superimposed electroconductive metal oxide films and spaced terminal members.

In producing such a resistor and with reference to the drawing, ceramic body is heated to a temperature of about 450 C. or higher, but not above its softening or deformation point, and preferably to a temperature of about 600-650 C. Ceramic body 10, while shown as solid, may also be a hollow body and is preferably a length of glass tubing or cane. The heated body is then contacted by vapors from, or an atomized solution of, a selected metal salt or salts to produce electroconductive film 11. Subsequent to formation of this initial coating and preferably while the ceramic body is still within the elevated temperature range at which film formation occurs, it is contacted with a second film forming material to produce a protective film 12.

The materials used in producing each film may be anhydrous and fumed onto the body or may be dissolved in compatible organic solvents and applied in solution form. It is usually more convenient, however, to employ an aqueous solution of the salt or salts with sulficient acid in the solution to prevent separation of hydrolysis products. This aqueous solution may then be sprayed on the surface of the heated ceramic body to produce the desired film, or may be thermally converted into a hot vapor phase to which the ceramic body is exposed. Exposure of the ceramic body to the film forming material, in each instance, is continued until. a film having the desired thickness, and consequently the desired resistance, is formed from the contacting material.

In order to avoid any possible interaction between the films, it is preferable that the materials used in producing each film contain the same components, although necessarily in different proportions. correspondingly then the films will contain the same oxides although indifferent proportions to provide the required higher resistance in the protective cover film. Thus the primary or conducting film, which customarily contains up to about 6% antimony oxide, may be produced from a suitable mix ture of SnC1 -5H O and SbCl In practice, for example, a solution containing 99 parts of the tin chloride to 1 part of the antimony chloride, with water and concentrated or 37% hydrochloric acid in a ratio of 5/1 as solvent, has been found particularly suitable because of the positive temperature coefficient of resistance in the resulting film. It is necessary, however, that a film of this composition be very thin, e.g. about a first order film having reference to interfer ence colors as a measure of thickness, where higher resistances on the order of 500 ohms/square are desired.

The protective film must, of course, have a considerably higher content of antimony oxide to provide the desired higher resistance and preferably is formed from an acid solution containing about 30-60 parts antimony chloride and -40 parts tin chloride with the relative oxide contents in the film roughly corresponding to these ranges also.

Alternatively other film-forming materials or mixtures suited to forming high resistance films may be employed in producing the protective film. Thus, tin oxide films may be poisoned, that is given a much higher resistance, by incorporating minor amounts of such oxides as those of bismuth, iron, chromium and zinc, and rendered quite satisfactory for cover films. By way of illustration, examples of a number of suitable compositions for forming cover films are shown in the table below along with the resistance in ohms/ square of an oxide film formed from the composition and having a third order red thickness in terms of interference colors.

It should be noted that the recited compositions are used in solution form. The solution is made by dissolving one gram of the tin chloride in a 1 to 5 mixture of concentrated hydrochloric acid and H 0 to produce 1 ml. of solution; one gram of SbCl (when used) in a 1 to 1 mixture of concentrated hydrochloric acid and H 0 to make 1 ml. of solution; mixing these solutions in indicated proportions; adding other chlorides (in grams) and phenol (in mls.) as required.

The essential function of the top or cover film is to insulate the primary film from'atmospheric and other deleterious external influences. It must then be sufficiently thick to provide such insulation and in general cover films should be at least third order films.

Following the formation of film 12, terminals 13 are applied over film 12. They are desirably of a metallic nature and applied as a thin band at either end of the resistor. In forming such terminals any well-known metallizing procedure may be employed. For example, a thin coating of organo-metal material such as the noble metalresinates may be fired. on the filmed body. Alternatively, metallizing pastes containing a vitreous flux, such as commercially available silver pastes, may be used.

. Preferably terminals 13 take the form of thin, metallic bands encircling the end of the resistor, as shown, and an eighth to a quarter inch in width. This not only provides a large surface on which to fasten leads, terminal caps or the like, but also furnishes a larger contact area on the upper or cover film.

. In establishing electrical contact between the terminals and conducting film 11 it is necessary to obtain transverse current flow in film 12 while avoiding any appreciable contact resistance or impedance to current flow which could cause overheating in service. The contact area between terminals 13 and cover film 12 will ordinarily be extremely large in contrast to the film thickness which will be in the range of first to tenth order of interference colors or to 10- millimeters. Under these circumstances, it might be expected that contact resistance would be a negligible factor even though the resistance of the cover film might be on the order of billions of ohms per square. Experimental evidence indicates, however, that such is not the case and that apparently most of the transverse current flow through the film follows a relatively narrow path under the extreme inner edge of the contact.- Accordingly, the resistance of the cover films cannot be as great as might be thought.

If on-the other hand, the ratio of the resistance of the top film to that of the under or primary conducting film is too small, a substantial part of the longitudinal current flow will be shunted through it. In other words, with respect to longitudinal current flow the two films then function as parallel resistances. To the extent that the top fllrn carries any longitudinal flow of current, it functions as an exposed conducting film rather than a protecting film. In general, high resistance films of the type used for cover films have very. poor electrical stability as well as relatively high negative temperature coeflicients of resistance. In order that these unfavorable characteristics may not be imparted to any substantial extent in the composite resistor film, it is desirable that the resistance of the protecting film be sufficiently high so that the resistance of the primary conducting coating alone istsubstantially the same, that is within 1% or so, of the final composite multi-coat film.

Onthe other hand, I have found that under some circumstances, there are advantages in having up to about 10% .of the longitudinal current flow shunted through the higher resistance, protective film. For example, when aprimary conducting film has a positive temperature coefficient, the higher resistance protective film has a negative temperature coefficient, and the latter carries a small fraction of the current flow, the temperature coeffi c ients tend to compensate or cancel each other out.

Metal oxide films suitable for use as' the primary con-- ducting element in a resistor may vary in resistance from about 20 to 10,000 ohms/square. In order to satisfy the various conditions discussed above, a cover or protecting film should have a resistance at least ten times that of the conducting film in conjunction with which it is used. Hence cover films should have a resistance from at least 200 ohms up to about 10 megohms/ square.

In producing the improved dual-coated resistors it is convenient to pass a continuous length of heated glass or other ceramic tube or cane past a suitable fuming or spray apparatus. Glass is particularly convenient to use since it has been found that glass cane or tubing, as drawn from the melting chamber, may be maintained at a sufficiently high temperature to enable continuous film formation on the surface without reheating. Two substantially similar devices for projecting the film forming materials onto the glass surface may be set up in succession with the spray or fume pattern and length of exposure in each instance correlated with drawing speed of the glass so that a proper thickness of each film is achieved. Following that, terminals 13 may be applied in the usual manner over the cover film.

While any ceramic material capable of withstanding the film forming temperatures may be used for the base 10, maximum electrical stability is achieved with a smooth, non-porous surface. For this reason, as well as the ease of fabrication and adjustment of physical properties, glass is preferred.

Base 10 is also preferably substantially free of alkali metal ions for optimum electrical stability. It is known that alkali metal ions can migrate in vitreous or glassy media. As explained in my co-pending application filed of even date herewith, this condition appears to exert a serious and erratic influence on the electrical properties of electroconductive metal oxide films and to have been a major contributing factor in their electrical instability. By substantially free is meant free from any but trace impurities.

By way of further illustration the following specific example is referred to:

Cane, 0.260 inch in diameter, was drawn in conventional manner from an alkali-free glass having the following compositions: 58% SiO 15% A1 0 10% CaO, 7% MgO, 6% BaO and 4% B 0 During the drawing process and while the glass cane was still at an elevated temperature, it was successively passed through two adjacent coating chambers. In the first chamber the cane was exposed to the hot vapors of a hydrochloric acid solution of mixed chlorides containing 97.5 parts SnCl -5H O and 2.5 parts SbCl This produced on the cane a first order white, metal oxide film having a resistance of about 600 ohms/square. The resistance varied somewhat depending on the glass temperature and the rate of draw and hence the length of exposure. As a further means of control the solution concentration may be varied, a dilute solution producing a thinner film in a given time. In the second chamber the coated cane was exposed to the hot vapors of a hydrochloric acid solution of mixed chlorides containing 40 parts SnCl -5H O and 60 parts SbCl This produced a sixth order film having a resistance of 50,000 ohms/square.

Subsequent to coating the cane was cut up in short lengths and metal handed to produce resistor elements. These resistors were then subjected to various tests. It was observed that the temperature coefficient of the unit was negative in sign and, when measured between 37 and 97 C., was from 200400 parts per million/degree C. The variations were primarily due to changes in the unstable cover film during metallizing but well-within a specified limit of 500 ppm.

Twenty of the resistor units were placed on direct current electrical load test with ten operating at a maximum temperature of C. and ten at 200 C. The resistance of each unit was measured prior to placing it on test and resistance measurements were observed periodically throughout the 1000 hour test to ascertain, for each unit, the maximum change in resistance from the original value. The maximum change observed in any of the ten units operating at 140 C. was under 0.5% whereas specifications for this type of resistor operating under these conditions ordinarily allow for changes up to 1%. Among the ten units operating at 200 C. no change greater than 0.8% was observed while changes up to 2.0% are usually acceptable under these operating conditions. Thus, the resistors of the present invention provide the desired combination of high resistance values, low temperature coefficient, and satisfactory stability under electrical load.

What is claimed is:

1. An electrical resistor comprising a ceramic body, an adherent, electroconductive, metal oxide film on the surface of the ceramic body, a second electroconductive metal oxide film superimposed on the first film, the second film having a different composition from and a higher resistance than the first film, and spaced, electrically conducting terminal members in physical contact with said second film and spaced from said first film by said second film, but in electrical contact with said first film through said second film.

2. An electrical resistor in accordance with claim 1 in which the ceramic body is composed of glass.

3. An electrical resistor in accordance with claim 1 in which the ceramic body is substantially free from alkali metal ions.

4. An electrical resistor in accordance with claim 1 in which the resistance in ohms per square of the first metal oxide film is 20-10,000 ohms and that of the second film is from 200 ohms to megohms, and the resistance of the second film is at least ten times that of the first.

5. The electrical resistor of claim 4 in which the first film is composed of tin oxide and up to about 6% antimony oxide.

6. An electrical resistor comprising a ceramic body, an adherent, electroconductive, metal oxide film on the surface of the ceramic body, a second electroconductive metal oxide film superimposed on the first film and spaced, electrically conducting terminal members in physical contact with the second film and spaced from said first film by said second film, but in electrical contact with said first film through said second film, the electrical resistivity of the second film being such that under electrical load no significant impedance is offered to a transverse fiow of current through those portions of the second film between the terminals and the first film, but

being sufiiciently greater than the first film so that no substantial portion of the longitudinal flow of current between the terminals occurs in the second film.

7. A method of making an electrical resistor which comprises exposing a heated ceramic body to a film forming material to form an adherent, electroconductive metal oxide film on its surface, thereafter exposing the filmed ceramic body to a second film forming material to superimpose on the first film a second electroconductive metal oxide film which is of such composition and thickness relative to the first that no significant impedance is offered to a transverse fiow of electrical current while no substantial longitudinal current flow occurs in the second film, and thereafter applying metallic terminals over the second film in such manner that they are spaced from said first film by said second film, but in electrical contact with said first film through said second film.

8. A method of making an electrical resistor which comprises successively exposing a heated ceramic body to two different film-forming materials whereby two separate, electroconductive, metal oxide films are formed in superimposed relationship on the surface of the body with the second film having a higher resistance than the first film, and then applying electroconductive terminals over the second film and spaced from said first film by said second film, but in electrical contact with the first film through the second film.

9. A method in accordance with claim 8 in which the ceramic body is substantially free from alkali metal ions.

10. A method of making an electrical resistor which comprises heating a ceramic body to a temperature of at least 450 C., exposing the heated body to a material capable of forming on the surface of the body an electroconductive metal oxide film, continuing such exposure until the resistance of such film reaches a predetermined value within the range 2010.000 ohms per square, thereafter exposing the heated body to a second material capable of forming an electroconductive metal oxide film having a different composition and a higher resistance than the first film, continuing such exposure until a film having a selected resistance at least ten times that of the first film and within the range of 200 ohms to 10 megohms per square is produced, and then applying electroconductive terminals on said second film.

11. A method in accordance with claim 8 in which the first film forming material to which the ceramic body is exposed is capable of producing a tin oxide film containing up to 6% antimony oxide.

12. The method of making electrical resistors in which the current carrying element is an electroconductive metal oxide film deposited on the surface of a ceramic body which comprises drawing an elongated glass body from a molten glass reservoir, successively contacting the glass body while at a temperature in excess of 450 C. with two difierent film-forming materials whereby two separate, electroconductive, metal oxide films are formed in superimposed relationship on the surface of the glass with the second film having a higher resistance than the first film, separating the filmed glass body into appropriate lengths and applying spaced electroconductive terminals to the surface of the second film on each length in such manner that the terminals are physically spaced from the first film, but in electrical contact therewith through the second film.

13. A method in accordance with claim 12 wherein the composition of the first film-forming material with which the glass body is contacted is such as to produce a film containing tin oxide and antimony oxide, the latter being present in amounts up to 6%, and the contact time with this film-forming material is such that the film produced has a thickness less than third order red and a resistance of over 60 ohms per square.

14. An electrical resistor comprising a ceramic body, an adherent, electroconductive, metal oxide film on the surface of the ceramic body, such metal oxide film having a resistance of over 60 ohms per square, a thickness less than third order red and being composed of tin oxide and up to 6% antimony oxide, a second electroconductive metal oxide film superimposed on the first film, the second film having a different composition from and a higher resistance than the first film but physically separated therefrom by said second film, and spaced, electrically conducting terminal members formed on said second film and in electrical contact with said first film.

15. in an electrical resistor composed of a ceramic body, an electroconductive metal oxide film adherent to the surface of such body, a superimposed metal oxide film having a higher resistance than the first film and serving.

to protect that film against environmental influences, and terminals in electrical contact with the initial film, an improved construction wherein the superimposed film is sufl'iciently conductive to permit transverse flow of electrical current and the terminals are applied to the surface of the second film and thus physically spaced from the first film but in electrical contact therewith through said second film.

16. In the production of electrical resistors including a ceramic base and an electroconductive metal oxide film formed thereon, an improved method which comprises forming successively and in superimposed relationship tWo electroconductive metal oxide films and electroconductive terminals, the terminals being physically spaced from the initial film but in electrical contact therewith through the second film.

References Cited in the file of this patent UNITED STATES PATENTS 1,771,236 Schellenger July 22, 1930 10 Davis Aug. 21, 1951 Mochel Aug. 21, 1951 Mochel Aug. 21, 1951 Patai July 28, 1953 Lytle Aug. 11, 1953 Kohring July 2, 1957 BEST AVAILA COPY UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2,915,730 December 1, 1959 James K, Davis It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Columns 3 and in the table at the bottom of the page, in the heading +2 of column 3 thereof, for the numeral "1" read *2 column 5, inc 29, for "larger" read large column 8, line 20, after "film" and before the period, insert in such manner that they are spaced from said first film b said second film, but in electrical contact With said first film through saic second film lines 57 and 5-8, strike out "but physically separated therefrom bys'aid second film" and insert the same after first film" and before the period in line 60, same column 8,,

Signed and sealed this 17th day of May 1960.

(smmj Attes t:

KARL HQ AXLINE 1 ROBERT C. WATSON I Commissioner of Patents Attesting Officer

Claims (1)

1. AN ELECTRICAL RESISTOR COMPRISING A CERAMIC BODY, AN ADHERENT, ELECTROCONDUCTIVE, METAL OXIDE FILM ON THE SURFACE OF THE CERAMIC BODY, A SECOND ELECTROCONDUCTIVE METAL OXIDE FILM SUPERIMPOSED ON THE FIRST FILM, THE SECOND FILM HAVING A DIFFERENT COMPOSITION FROM AND A HIGHER RESISTANCE THAN THE FIRST FILM, AND SPACED, ELECTRICALLY CONDUCTING TERMINAL MEMBERS IN PHYSICAL CONTACT WITH SAID SECOND FILM AND SPACED FROM SAID FIRST FILM BY SAID SECOND FILM, BUT IN ELECTRICAL CONTACT WITH SAID FIRST FILM THROUGH SAID SECOND FILM.

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