US2885310A

US2885310A - Method and apparatus for making film resistors

Info

Publication number: US2885310A
Application number: US455417A
Authority: US
Inventors: Earl R Olson; Robert F Vance
Original assignee: Ohmite Manufacturing Co
Current assignee: Ohmite Manufacturing Co
Priority date: 1954-09-13
Filing date: 1954-09-13
Publication date: 1959-05-05
Anticipated expiration: 1976-05-05

Description

y 5, 1959 E. R. OLSON ET AL 2,885,310

METHOD AND APPARATUS FOR MAKING FILM RESISTORS 2 Sheets-Sheet 1 Filed Sept. 15, 1954 wq I F6 I g V i 4 I /Hol 1% s I I I :5 J00 I I I v I Q? I w i I i I i L INVENTORS I I l a a rlf 0;;

l 5 -fifiem .zzce o 50 200 250 30 fi/SW gq TEMPERATURE C y 5, 1959 E. R. OLSON ETAL 2,885,310

METHOD AND APPARATUS FOR MAKING FILM liESISTORS Filed Sept. 13, 1954 2 Sheets-Sheet 2 United States Patent METHOD AND APPARATUS FOR MAKING FILM RESIST ORS Earl R. Olson and Robert F. Vance, Columbus, Ohio,

assignors, by mesne assignments, to Ohmite Manufacturing Company, Skokie, 111., a corporation Application September 13, 1954, Serial No. 455,417

11 Claims. (Cl. 117-227) This invention relates to film resistors, and to methods and apparatus for producing such resistors.

There are various types of resistors designed for electrical circuits, such as wire wound resistors, composition resistors and film resistors. This invention is directed to the latter type wherein a core of glass, ceramic, or other desired material is coated with the resistive film element.

Film type resistors as available commercially at the present time, are generally comprised of a core material having disposed thereon a deposit of carbon or a metallic oxide. These film resistors are of limited utility and accuracy, and are frequently marked by a short life or service span. The films generally are unstable, and their resistive values vary from age, and from encounters with high temperatures when applied to electrical circuits. Most known film type resistors, for instance, are subject to failure at temperatures approaching 200 C. to 250 C.

The present invention is directed to the making of film resistors from the natural occurring transition or refractory type metals falling within specified groups of the periodic table, as hereinafter set forth. The film is obtained through a process of vapor decomposition of a relatively low boiling compound of the metal, resulting in a metal film having a high melting point. Thus by relatively low processing temperatures, a high boiling, hard and well anchored resistive film is obtained.

Specifically, the transition or refractory type metals above referred to, are those naturally occurring in classes IVB, VB, V113 and VIIB of the periodic table. It has been found that these transition metals, produced from vapor decomposition processing, as hereinafter set forth, provide resistive films which are stable with age and use, which have good mechanical adhesion to the core, and hardness, and which can be produced in requisite thicknesses to provide films of practical resistive value having a low temperature coefficient (hereinafter sometimes referred to as T.C.). The temperature coefiicient (T.C.) of a resistive film denotes the resistive value change per degree of temperature, as heat is applied to the film by the transmission of electric current, or other heating means.

It is an object of the present invention to provide an improved film type resistor having a stable resistance over protracted periods, in use and not in use, which provides a hard surface and firm bond with the core; and which may be produced to provide a predetermined resistance value having a temperature coefficient within low accepted values.

A further object of the invention is to provide an improved film resistor which minimizes the difliculties heretofore encountered with film type resistors of conventional structure and design.

A further object of the invention is to provide a film resistor produced from the natural occurring transition metals in classes IVB, VB, VIB and VIIB of the periodic table.

A further object of the invention is to provide a film i atented May 5, 1959 resistor, produced as above, by a vapor decomposition process, whereby to provide a high boiling, hard metal film through the use of relatively lower vapor decomposition temperatures.

A still further object of the invention is to provide an improved film resistor, produced by vapor decomposition of the transition metals enumerated above and using the starting materials hereinafter more particularly set forth.

Other objects and advantages of the invention will become apparent from the following description, taken in connection With the accompanying drawings, wherein certain preferred embodiments are set forth for purposes of illustration.

Fig. l is a view of a film type resistor comprising the invention;

Fig. 2 is a view of a film resistor having grooves formed therein, in the film element;

Fig. 3 is a schematic side view of the apparatus employed in producing the film resistor in accordance with the present invention;

Fig. 4 is a schematic top sectional view of a portion of the apparatus shown in Fig. 3, more particularly illustrating the reaction chamber wherein the film deposition occurs;

Fig. 5 is a schematic view of the apparatus employed for forming the metallic chlorides, preferably used in the methods comprising the invention; and

Fig. 6 is a chart illustrating the vapor pressure curves for the preferred vapor decomposition starting compounds, utilized in accordance with the present invention.

State generally, the present invention contemplates the production of film type resistors by vapor decomposition of selected compounds containing the naturally occurring transition metals of classes IVB, VB, VIE and VIIB of the periodic series; the metal film being deposited onto a ceramic body in predetermined thickness to form the resistor element. The process is carried out in a furnace apparatus including heating means for vaporizing the metal compound, and heating means for heating the ceramic body to a relatively higher temperature onto which the metal film is deposited by decomposition of the compound vapor. Means is provided for scavenging the furnace with inert gas to control the reaction; and means is provided for controlling the position of the ceramic core, the vapor pressure of the compound, and

the time of operation, so as to provide a film of uniform and controlled thickness. The coated bodies thus constituted are then cut and suitably provided with end electrodes or terminals, to complete the resistor structure; and if desired the resistive element may be spiralled to increase the resistive value.

In Fig. 1 there is shown a resistor produced by the present invention. The insulating core of ceramic, glass, or other suitable material, but preferably of steatite, is shown at 10, having the transition metal resistive coating 12 deposited thereon. Terminals are formed at the opposite ends of the resistive film by coatings of silver paint as indicated at 14 and 16, painted onto the film; there being

metal band terminals

18 and 20 embracing the silver paint upon which the

terminal lugs

22 and 24 are formed. Preferably a coating of lacquer or enamel 26 is provided over the resistive film, for mechanical protective purposes.

In Fig. 2 a resistor structure is shown, essentially similar to the structure of Fig. 1, except that in this instance the resistive film as indicated at Hz: is provided with a spiral groove 28 separating the film into a spiral filament or band, whereby to increase the resistance value. The groove 28 may be formed in any suitable way, as for example by cutting a spiralled groove with a lathe, into the resistive film.

Before discussing the particular materials and methods of the present invention, it is desirable to discuss briefly the requisites of a satisfactory resistor, and particularly of the fihn type.

A requirement is that the resistor shall exhibit a stable or constant resistive value with age and use, including use wherein the resistor may be repeatedly reheated to relatively high temperatures. A further requirement is that the film shall have a relatively hard surface, resistive to mechanical abrasion, and that it shall adhere with tenacity to the core material so that it does not flake off in use, particularly as the resistor is repeatedly heated and cooled. A further requirement is that the material of the film shall have a suificiently high bulk resistivity so that an excessively thin film is not required to provide a resistive element of the necessary resistive value. Another requirement is that the resistor shall have, for most installations, a relatively low temperature coefiicient (T.C.), viz., change in resistive value with temperature. A still further requirement is that the resistive film may be produced by processing steps which can be achieved and controlled, and which are not excessively expensive. It has been found that the resistors produced by the materials of the present invention, and in accordance with the process steps employed, satisfy the foregoing requirements.

It has been found that the resistivity of the transition material film varies inversely as the film thickness, viz., the thinner the film the higher the resistive value. There is not a straight line relation, however, and in general as the film is made thinner, the resistivity increases at an increasing rate. If the bulk resistivity is inadequate so that the film must be made excessively thin, the resistivity becomes unpredictable, and the film becomes unstable.

It furthermore has been found, as to the transition metals of the present invention, that the temperature cefficient (T .C.) of the film varies with the film thickness, a film thickness of predetermined value providing a substantially zero T.C., films of less thickness providing a negative T.C. which increases as the film thickness decreases, and films of greater thickness providing a positive T.C. which increases as the film thickness increases. It is desirable that the cross-over point, viz., the film thickness providing a substantially zero T.C., shall be sufiiciently wide in range so that such desired film can be practicably produced. It is furthermore required that the resistivity of the film at such cross-over point thickness shall be sufliciently high to provide a practical resistor.

It has been found that the transition metals of the present invention provide film resistors satisfying the foregoing requirements as to film thickness and cross-over range. Further, in accordance with the present invention apparatus and methods are provided for controlling the production of the film, so that the film thickness can be controlled to provide a resistor of desired resistance value, and of substantially Zero T.C. In some installations it may be desirable that the resistor provide a changing resistance value with temperature change, viz., a positive or negative T.C.; but in most installations it is desirable to provide a resistor the resistive value of which remains substantially unchanged with changes of temperature, or which has a substantially zero T.C. Film resistors as heretofore produced have not compared favorably with wire wound resistors in this respect. On the other hand, wire wound resistors become excessively expensive in the high resistance value ranges. The resistor as provided by the present invention satisfies both requirements.

Referring more particularly to the transition elements in classes IVE, VB, VIB and VIIB of the periodic table, technetium is not a naturally occurring element. Hafnium and niobium (columbium) are not readily available commercially, and while operative, are therefore of lesser import. Of the remaining nine transition metals in classes IVB, VB, VIB and VIIB, tungsten (Wolfram), tantalum,

4 molybdenum, and vanadium constitute a preferred subgroup (as compared with the remaining elements titanium, zirconium, chromium, manganese and rhenium); and within the preferred subgroup tungsten, tantalum, molybdenum and vanadium are preferred in the order stated, because of their properties.

As stated, the films of the foregoing metals, in accordance with the present invention, are produced by the process of vapor decomposition, from the starting composition comprising the metal. The starting compounds may comprise the poly-halides, carbonyls, or hydrides, of the metals, or organic compounds containing the metals. Of these the poly-halides, and specifically the chlorides, are preferred because of their availability, volatility, and susceptibility to vapor decomposition. The vapor decomposition n.ay be effected by direct pyrolysis, but in the case of the halides preferably the decomposition is facilitated by reduction with hydrogen, as will be more specifically hereinafter discussed.

The apparatus by which the film deposition is effected is illustrated in Figs. 3 and 4. Referring to Fig. 3, there is illustrated a reaction chamber or furnace 3t and a drive mechanism for controlling the movements of the core material or substrate to be coated within the furnace, generally indicated by the reference numeral 32.

The drive mechanism more specifically comprises a carriage 34 actuated along a predetermined path of longitudinal travel, and at predetermined speed, by an elongated drive screw 36. The carriage 34- is provided with a half nut 38 selectively engageable with the drive screw by means of a control handle 40, the arrangement being such that when the half nut is in engagement with the screw, upon screw rotation the carriage 34 will be driven.

Screw 36 is journalled at its opposite ends in a pair of

support uprights

42 and 44, and is arranged to be driven through a reversible, variable speed motor 46 and a transmission 48. By means of the motor the screw may be driven at varying rates of speed, and selectively in either direction. The control circuit for the motor includes limit switches 54) and 52., for preventing over travel of the carriage 34 in either direction of movement. The carriage is guided by means of guide rods, as indicated at 54 and 56.

Carriage 34 carries a variable speed motor 58 arranged to drive rotatably, a core or substrate holder 60 arranged to project into the furnace 3t).

Referring to Fig. 4, wherein the furnace or reaction chamber is more particularly shown, it will be seen that the furnace comprises a chamber 62, within which is mounted a pair of Globars or

heating elements

64 and 66, the control circuit for one of which is shown at as. By means of these heating elements the substrate within the furnace may be heated to a desired deposition temperature.

A deposition chamber is provided within the furnace by a tube 72, into the opposite ends of which project elongated tubes 74 and 7 6, which latter tubes also project through the end walls of the furnace. Tube 74 is provided at its end with a removable cap 78 providing a bearing St) for the rotatable core holder 60, whereas tube 76, which projects through the opposite or discharge end of the furnace, is provided with a pair of large bore stopcocks 32 and 84. To provide an inert atmosphere within the tubes, tube 74 is provided with an inlet conduit 86 for the introduction of an inert scavenging gas such as helium, under control of a valve 88, and tube 76 is similarly provided with a helium inlet conduit 99 controlled by valve 92. The helium gas may pass through the open ends of the tubes 74 and 75, into the deposition tube 76, as indicated by the arrows, and tube 76 is provided also with an exhaust conduit or pipe 94 adjacent the stopcock 84, controlled by a valve 96.

Associated with the furnace 30 is a pair of lower temperature vaporizing chambers 109 and 102, provided with heating elements as indicated at 164, 1'36, 108

and 110, the control for one of which is shown at 112.

Chambers

100 and 102 are provided, respectively, with

trays

114 and 116 into which is placed the transition compound to be decomposed, these trays communicating with conduits 118 and 120 leading to the reaction tube 70, and being provided with inlet pipes or conduits 122 and 124, under control of valves 126 and 128, respectively, for the introduction of a reacting gas, specifically hydrogen as a reducing gas, in the embodiment set forth.

The reaction tube 72 is further provided with a pair of

exhaust conduits

130 and 132, under control of

valves

134 and 136, respectively.

In preparing a typical film deposit, the starting material containing the transition metal, for example tungsten hexachloride, is introduced into the

trays

114 and 116. The substrate or core rod, of steatite, Vycor, or the like, indicated by the reference numeral 140 in Fig. 4, is secured to the chuck 142, provided at the end of the rotatable holder 60.

Upon operation of the

drive motors

46 and 58, Fig. 3, the core rod 140 will be rotated and longitudinally advanced, at a predetermined rate, to the right as shown in Fig. 4. Upon application of heat within the vaporizing

chambers

100 and 102, the tungsten hexachloride will be vaporized, and propelled by its vapor pressure, along with hydrogen, into the deposition chamber 70 from the conduits 118 and 120.

As will be understood, upon the application of a higher degree of heat to the core 140, by the action of the

heating elements

64 and 66, as the tungsten hexachloride contacts the surface of the rod, a reducing action or vapor decomposition takes place, the tungsten being deposited as a metal film upon the surface of the core rod.

It will be noted that as the core approaches the deposition zone within the inlet tube 74, it is preheated while being maintained within the inert atmosphere of helium within the tube. Similarly, as the core leaves the deposition zone, it moves into the tube 76 wherein it is again subjected to an atmosphere of inert helium. This protection of the film as it leaves the reaction chamber is desirable to prevent contamination of the film with partially reduced metallic chloride. With proper balance of the volumes of hydrogen and helium, and the pressures thereof, a protective helium atmosphere may be maintained within the

tubes

74 and 76, without undue dilution of the reactive gases within the decomposition chamber.

It will furthermore be seen that by proper control and operation of the

drive motors

46 and 58, not only is a uniform film deposited on the core 140, as the core is rotated and longitudinally translated through the reaction Zone, but by controlling the speeds of movement, in relation to the vaporizing temperatures existing within the

chambers

100 and 102, the thickness of the metallic film which is produced may be accurately controlled.

After the coated substrate 140 has been projected into the cooling section 144 of the tube 76, the chuck 142 of the holder is released by manipulation of a control handle 146, Fig. 3, whereupon the holder may be retracted and the operation repeated. The coated core or substrate cools within the tube section 144, in a helium atmosphere, and when cooled may be withdrawn through the stopcock 84.

The exhaust gases from the

exhaust tubes

130 and 132 may be vented to a recovery chamber, for recovery of any unexpended metallic chloride.

It will be seen that the vapor decomposition process, as above described, comprises the vaporization of the transition element compound, by the application of willcient heat to effect vaporization, and the subsequent decomposition of the compound upon contact with the more highly heated substrate, whereby to provide the metallic film thereon. This process is in contrast with the direct vaporization of metals, at high temperature, and under vacuum, where temperatures must be employed equal to or greater than the vaporizing temperatures of the metal films.

Proper control of the temperature of the substrate is necessary to produce a properly adherent film. If the temperature of the substrate is excessive, the films deposited thereon will be colored and not reproducible, whereas if the temperature of the substrate is too low, the deposited films will be non-adherent. Proper temperature conditions are those resulting in a film which is silver-like in appearance, and strongly adherent to the core material. The flow rate of hydrogen, in the case of decomposition by hydrogen reduction, also reflects upon the thickness of the film produced, for any given core transmission speed.

Conditions are set forth in the following table, as to films which have been produced from chlorides of the transition metals indicated.

Conditions of deposition for transition metal films Temp. of Reaction Reaction H Flow Metal Chloride, Temp., Time, Rate,

0. 0. Min. cc./rnin Titanium 1-27 500-1, 000 10-15 2, 000-3, 800 Zirconium 275-365 950-1, 135 4-15 2, IOU-6,900 Vanadium Room 950-1, 000 l-5 3, 900-7, 900 Tantalum -250 750-1,000 5-15 2, 9008,600 Molybdenum- -140 850-900 0.5-5 9,000 Tungsten 775-800 6 to 20 6 900-8, 700 Ti-Zr Codeposi Rggm Z0131) 580-800 5-10 3, 100-5, 000

5 r Tl-Va Room 875-925 2-5 3, 900

As examples, excellent tungsten films were obtained when tungsten hexachloride at a temperature of 150 C. was introduced into the reaction chamber heated to 77 5- 800 C. for a time interval of six to twenty minutes, during which period a volume of 6900 to 8700 cc./min. of hydrogen was caused to flow.

As further illustrative, a tantalum film was obtained by subjecting a steatite core at temperatures ranging from 750-1000 C. to a vapor of tantalum pentachloride vaporized at temperatures of 100-250 C., for five to fifteen minutes in a hydrogen flow of 2900-8600 cc./min.

The reaction that is obtained may be described generally by the formula:

Where M refers to the transition metal under consideration, and X is the number of chloride atoms necessary to satisfy the highest valence of the metal. From the apparatus and process provided, the flow system involves an unrestricted volume, and the reaction normally will go to completion.

As previously pointed out, the resistance value of the metal film depends upon the bulk resistivity of the metal, and the film thickness. The resistance value of the film is expressed as resistance per square (R/sq.), which is a constant for a given metal and film thickness. As further previously pointed out, the temperature coefiicient (T.C.) of the film varies with the film thickness; and in accordance with the methods and apparatus heretofore set forth, the film thickness can be controlled to produce a substantially zero T.C., or a negative T.C. of given value, or a positive T.C. of given value, by controlling the thickness of the deposited film. The T.C. of any given film can be determined by actual test, as will be understood. In films which have been produced, a resistance of about 3000 ohms per square characterized tungsten films at a zero T.C.; whereas tantalum films at zero T.C. cover an approximate resistance range of 100- 500 ohms per square.

After a film thickness has been produced, to provide a desired temperature coefiicient, the coated core material may then be cut to proper length to provide the desired total resistor value for the unit, and if desired or necessary, the deposited film may be spiralled as indi cated in Fig. 2, to increase the resistance value. The bulk resistivity of the transition metals within classes IVB, VB, VIB and VIIB of the periodic table varies rather widely, so that a selection of resistive films is provided. The melting points are quite high, contributing to the usefulness of the resistive films produced. The films may be codeposited, as indicated in the chart heretofore set forth.

In certain instances the stability of the films has been increased by the preheating thereof, and it is within the contemplation of the present invention that the films, produced as above set forth, may be heated, and reheated, and maintained at elevated temperatures for a protracted period, prior to use, to increase the stability of the resistive films.

As previously stated, within the group classification set forth, tungsten films have exhibited exceptional qualities in respect to aging, stability under load, and stability of temperature coeflicient; along with films of tantalum, molybdenum and vanadium in the preference order stated.

The following table is representative of tungsten films of various thicknesses produced from tungsten hexachloride:

Molybdenum pentachloride was similarly prepared by direct synthesis from the elements. In a representative run a charge of 95 grams of molybdenum powder, 99.9% pure, was used in reaction with the chlorine. The reaction commenced at 300 C. and attained a satisfactory rate of 450 C. Approximately 164 grams of blue-black molybdenum pentachloride were obtained, representing a yield of 61% of theoretical.

Titanium tetrachloride may be obtained commercially. For use in the present invention commercial titanium tetrachloride was purified by a refluxing process for six hours in contact with by weight of copper gauze. After standing overnight, the tetrachloride was distilled through a Vigreux column, the fraction boiling at 131- 132 C. being retained.

For use in the present process, anhydrous chromic chloride (CrCl was prepared by heating hydrated chromic chloride in the presence of carbon tetrachloride. A quantity of the CrC1 -6H O was placed in a flask and then into a hot furnace, and when the temperature reached 150 C., carbon tetrachloride was introduced at a rate of two or three drops per second. At 300 C. phosgene evolution commenced. About 400 milliliters of CCL, were required to dehydrate 148 grams of Temp. of Vapor Reaction Reaction Hydrogen Helium Speed of Run Number W01 Pressure Temp., Time, Flow Flow Substrate Remarks C. of W01 C. min. Rate, Rate, Rotation,

mm. of ccJmin. cc./min. r.p.m.

150 25 800 6, 900 100 1 Silver appearance. 150 800 20 6, 900 100 1 Do. 150 25 800 15 6, 900 100 1 D0. 150 25 800 9. 5 8,700 110 6 Do. 150 25 800 9. 5 8, 700 110 6 Do. 150 25 800 9. 5 8, 700 110 6 Do. 150 25 775 9. 5 8, 700 110 6 D0. 150 25 775 8. 0 8, 700 110 6 D0. 150 25 775 6. 0 8, 700 110 6 Do.

As examples of starting materials, the preparation of the polychlorides will be particularly discussed.

In the preparation of zirconium tetrachloride, for instance, apparatus such as that schematically outlined in Fig. 5 may be and has been employed; as well as for the preparation of tantalum pentachloride, molybdenum pentachloride and tungsten hexachloride.

In preparing zirconium tetrachloride, zirconium of good purity may be placed into a reaction tube 150, Fig. 5, preferably fashioned from Vycor tubing, associated with a resistance wound tube furnace 152. A thermocouple 154 may be employed for controlling the temperature of the reaction. Argon gas is introduced into the reaction system through a conduit 156 as the furnace is brought to reaction temperature. When such temperature is reached chlorine gas is introduced through conduit 158, and passed at a moderate rate over the zirconium, the resultant zirconium tetrachloride being collected into a receiving flask 160, maintained at cool temperatures. While the reaction begins at 300 C., it is preferably conducted at a temperature of approximately 425 C. The rate of chloride formation at such temperature, in a representative run made, yielded 149 grams of zirconium tetrachloride in a period of ten hours, which was approximately a yield of 80% of theoretical.

Similar procedure is followed in the preparation of tungsten hexachloride by direct synthesis of its elements at 770 C. Tungsten chips greater than 99% purity are used, and a yield of 90% of theoretical is obtained.

Tantalum pentachloride is prepared by the direct combination of tantalum chips with dry chlorine. In a representative run, in two hours a charge of grams of tantalum chips yielded 20 grams of tantalum pentachloride run at a temperature of 250 C. This is 43% of the theoretical yield.

CrCl 6H O. The temperature was raised slowly throughout the reaction. Final temperature was 650 C. A yield of 77 grams of chromic chloride crystals was obtained, which is 88% of the theoretical yield.

Anhydrous manganous chloride (MnCl was prepared by a Vacuum dehydration of the tetrahydrate (MnCl 4H O) A quantity of the dehydrate of 200 grams was ground and placed in a flask heated by an electric mantle. The flask was evacuated, the water being collected in an ice trap as it evolved. The powder was heated for two hours at 240 C., and then thirteen hours at 350 C. The yield of anhydrous MnCl was 124 grams, which was 97% theoretical.

Vanadium tetrachloride (VC1 was prepared by direct combination of vanadium and chlorine. Chips of vana dium of 99.8% purity were placed in a flask and heated by an electric mantle. Dry chlorine was introduced directly over the vanadium via a long tube through a distillation head. The head was connected to a receiving flask by a long glass tube, and the receiving flask was cooled by a slurry of ice and water, and was open to the atmosphere through a drying tube containing calcium chloride. The reaction occurred at 350 C., the red VCl distilling into the receiving flask as it was formed, the vertical section of the still head being heated to prevent reflux of the VC14. A charge of 53 grams of vanadium yielded 200 grams of VCL; in six hours, which was 99% of theoretical.

In Fig. 6 the vapor pressure curves are shown for tungsten hexachloride, tantalum pentachloride, molybdenum pentachloride, and vanadium tetrachloride, which are the preferred starting materials for the preferred transition elements as hereinbefore set forth and as herein used.

Changes may be made in the form, construction and arrangement of the apparatus from that disclosed herein, and in the method steps outlined, without departing from the spirit of the invention. The invention is accordingly not to be limited to the specific embodiments set forth, but only as indicated in the following claims.

The invention is hereby claimed as follows:

1. The method of making a film resistor which comprises vaporizing a chloride selected from the group consisting of the chlorides of tantalum, molybdenum, tungsten and vanadium and mixing said vaporized chloride with hydrogen while maintaining said vaporized chloride at a temperature below that temperature at which said chloride will be decomposed to form a free metal; heating a dielectric core to a temperature at which said chloride will be reduced by said hydrogen; and contacting said vaporized chloride-hydrogen mixture with said heated core whereby said chloride is reduced at the surface of said core and said core is coated to form a resistive film thereon.

2. A method according to claim 1 in which said chloride is vaporized in the presence of hydrogen.

3. A method according to claim 1 in which said core is heated in an inert atmosphere prior to said contacting step.

4. A method according to claim 1 in which said core is cooled in an inert atmosphere subsequent to said contacting step.

5. A method according to claim 1 in which said core is subjected to a heat treatment for stabilizing purposes subsequently to the coating of said core.

6. A method according to claim 1 in which said chloride is vanadium chloride; in which said vaporized vanadium chloride is maintained at about room temperature; and in which said core is heated to about 950 C. to 1000 C.

7. A method according to claim 1 in which said chloride is tantalum chloride; in which said vaporized tantalum chloride is maintained at about 100 C. to 250 C.; and in which said core is heated to about 750 C. to 1000 C.

8. A method according to claim 1 in which said chloride is molybdenum chloride; in which said vaporized 10 molybdenum chloride is maintained at about C. to C.; and in which said core is heated to about 850 C. to 900 C.

9. A method according to claim 1 in which said chloride is tungsten chloride; in which said vaporized tungsten chloride is maintained at about C.; and in which said core is heated to about 775 C. to 800 C.

10. Apparatus for making film resistors which comprises means defining a closed treating chamber penneable to radiant heat and adapted to receive a core for a film resistor, a source of radiant heat outside said treating chamber for heating a core received in said treating chamber, means defining a closed vaporizing chamber permeable to radiant heat and adapted to receive a coating material to be vaporized, a source of radiant heat outside said vaporizing chamber for heating coating material received therein, means for conducting said vaporized coating compound from said vaporizing chamber to said treating chamber, and means communicating with one of said chambers for mixing hydrogen with said vaporized coating compound.

11. Apparatus according to claim 10 additionally comprising means defining preheating and cooling chambers adapted to receive said core, respectively, before and after said core is coated in said treating chamber, and means for supplying an inert gas to said preheating and cooling chambers.

References Cited in the file of this patent UNITED STATES PATENTS 1,399,722 Heany Dec. 6, 1921 1,497,417 Weber June 10, 1924 1,923,845 Rentschler Aug. 22, 1933 1,965,059 Seibt July 3, 1934 2,047,351 Alexander July 14, 1936 2,183,302 Brauer et a1. Dec. 12, 1939 2,357,473 Jira Sept. 5, 1944 2,382,432 McManus et a1. Aug. 14, 1945 2,418,804 Hood Apr. 8, 1947 2,656,284 Toulmin Oct. 20, 1953 2,667,432 Nolte Jan. 26, 1954 2,671,735 Grisdale et a1. Mar. 9, 1954 2,698,812 Schladitz Jan. 4, 1955

Claims

1. THE METHOD OF MAKING A FILM RESISTOR WHICH COMPRISES VAPORIZING A CHLORIDE SELECTED FROM THE GROUP CONSISTING OF THE CHLORIDES OF TANTALUM, MOLYBDENUM, TUNGSTEN AND VANADIUM AND MIXING SAID VAPORIZED CHLORIDE WITH HYDROGEN WHILE MAINTAINING SAID VAPORIZED CHLORIDE AT A TEMPERATURE BELOW THAT TEMPERATURE AT AWHICH SAID CHLORIDE WILL BE DECOMPOSED TO FORM A FREE METAL; HEATING A DIELECTIC CORE TO A TEMPERATURE AT WHICH SAID CHLORIDE WILL BE REDUCE BY SAID HYDROGEN; AND CONTACTING SAID VAPORIZED CHLORIDE-HYDROGEN MIXTURE WITH SAID HEATED CORE WHEREBY SAID CHLORIDE IS REDUCED AT THE SURFACE OF SAID CORE AND SAID IS COATED TO FORM A RESISTIVE FILM THEREON.