US 3370262 A
Description (OCR text may contain errors)
' Feb. 20, 1968 R. 's. MARTY ET AL ELECTRICAL RESISTOR Filed May 27, 1963 INVENTORS I Robert .Mar
John H Fabmcuw %Wg ATTORNEYS United StatesPatent O 3,370,262 ELECTRICAL RESISTOR Robert S. Marty, Nashua, N.H., and John H. Fabricius, Stamford, Vt., assignors to Sprague Electric Company, North Adams, Mass., a corporation of Massachusetts Filed May 27, 1963, Ser. No. 283,176 Claims. (Cl. 338309) This invention relates to a new and improved metal film resistor, and more particularly to a preclous metal film resistor having a thin resistive film of metal formed 'on a surface in a monolithic sandwich configuration.
Thin metallic films used as resistances may be produced by firing from metallo-organic formulations. The resistive into a structure which will permit it to be used in normal electronic and electrical usages. The thin resistive film must be supported on a base. It must be suitably protected against mechanical damage, and it must receive electrodes which complement the properties of the film. It is also advantageous to incorporate the thin film in a single composite structure which contains more than one individual electrical component. For example, it is desirable to mount the resistive film on a rigid base material carrying layers including as an active component a resistive metal film.
Although they are advantageous, the resistor constructions made up of layers on an inert base and carrying a metal resistive film suffer from several distinct disadvantages. The layers do not adhere satisfactorily to the inert base material. The plurality of coatings have a tendency to separate from the base and from each other.
It is an object of this invention to provide a resistor construction having a resistive film contained within layers on an inert base and having superior mechanical and electrical stability.
It is another object of this invention to produce a precious metal film resistor with high resistivity and good electrical properties.
Still other objects of this invention will become apparent when read in conjunction with the following description and accompanying drawings in which:
FIGURE 1 is a plan view of a resistor construction of this invention greatly enlarged;
FIGURE 2 is a sectional view of a build-up of the resistor construction taken on lines II-II of FIGURE 1 in the direction of the arrows, and
FIGURE 3 is a sectional view of a higher resistance buildup of alternate layers of vitreous glaze and resistive film greatly enlarged.
In general this invention involves a resistor structure made up of a thin resistive film deposited from a metalloorganic composition on a fused inorganic non-conducting layer. After the organic resinate is burned out of this composition the remaining metal resistive film becomes the electrically active portion of the resistor. Finally, over this resistive film is lain an overcoat of a glaze similar to the under layer and having substantially the same or a slightly lower coefiicient. The resistive film is provided with suitable electrodes. In a further general aspect, an inert base carries the fused inorganic non-conducting layer which has a lower coeificient of expansion under thermal change than the inert ceramic base.
Referring to the invention as illustrated in FIGURE 1,
3,370,262 Patented Feb. 20, 1968 a fiat ceramic substrate 10 such as barium titanate in a suitable form has applied to a surface thereof an undercoat 11 composed of a vitreous glaze. The glaze of undercoat 11 is made up of a suitable mixture of vitreous materials such as boro-silicates mixed with Pyrex glass to have a coefi'icient of linear expansion with temperature lower than the ceramic of the substrate 10 in the temperature range of from room temperature to 1100 C. Suitable electrodes 12 composed of a non-diffusing material such as platinum-gold alloy are spaced apart in narrow strips on the undercoat 11. A thin resistive film 13 composed of an alloy of precious metals and basemetal extends across the undercoat 11 from one electrode 12 to the other. An overcoat 15 of vitreous glaze covers the resistive film 13 and adjacent portions of electrodes 12. Suitable leads 14 are attached to the uncovered portions of electrodes 12. The vitreous glaze compositions of the undercoat 11 and the overcoat 15 may be of substantially the same composition with the composition of the overcoat 15 preferably having a slightly lower coefficient of expansion and a lower firing temperature.
FIGURE 2 shows the arrangement of the layers in section taken on line II-II of FIGURE 1. FIGURE 2 illustrates the relationship between the electrodes 12 which placed on the undercoat 11 are partially overlain by the resistive film 13 and also the overcoat 15. The overcoat 15 extends laterally across the resistive film 13 to contact the undercoat 11 adjacent the resistive film 13 as well as elsewhere on the substrate 10.
Before applying undercoat 11, resistive film 13, and overcoat 15, the surface of substrate 10 is suitably cleaned and prepared for the reception of the layers thereon. The undercoat 11 is preferably of a finely divided material such as a mixture of borosilicate with powdered Pyrex glass. Suitable other inorganic non-conducting materials may be used. These may be applied by mixing with a suitable carrier or binder; the combination being applied either across the entire substrate surface or in a desired pattern by any desired process such as printing, screening, painting or rolling. For the purpose of the description screening is preferred. The preferred material of this embodiment is a borosilicate of a general formula Na O, BaO, B 0 A1 0 SiO mixed with equal parts of Pyrex glass. A suitable carrier may consist of 5% ethyl cellulose and 95 pine oil. The overall mixture may be 25% carrier and finely divided inorganic material depending upon the consistency desired for the method of the application being employed. All elements of the carrier are evaporated or burned out during subsequent operations. The undercoat 11 should provide a continuous layer for the resistive film 13 and the adjacent portions of the electrodes 12.
The frit or mixture of carrier and inorganic material is applied to the surface of substrate 10, as by screening. The preferred frit fuses at temperatures of approximately 1800-1900 F. As described at greater length below, the frit is selected to provide an undercoat 11 having a lower temperature coeflicient of expansion than the substrate 10. The applied frit is fired on the substrate 10 at a temperature capable of forming a continuous film by fusion (1800-1900 F.).
One, two or three applications may be made to buildup the desired coating. An undercoat 11 for the resistive film 13 is thus produced on the ceramic base 10.
In the preferred embodiment of this invention, the substrate 10 is composed of a barium titanate base material having a suitable temperature ccefiicient of expansion. The following table sets forth the coeificients for four typical barium titanate ceramic base materials, but is not intended to be limitative.
3 TABLE I Range of expansion Temperature range coefficient in 10 in C.: cm./cm./ C.
(1) 27 to 1100 8.6 to 13.1 (2) 30 to 1100 6.6 to 12.7 (3) 25 to1100 9.1 to 11.6 (4) 25 to 1100 6.9 to 13.2
A suitable vitreous glaze for the undercoat 11 is composed of one half borosilicate and one half Pyrex glass and has an expansion coefi'icient of 5 x cm./cm./ C.; thus, an undercoat 11 having a lower coefficient of expansion is provided.
The electrodes 12 of platinum-gold alloy are applied by screen printing in a suitable manner. The electrodes are selected to minimize diffusion into the resistive film. Suitable platinum-gold formulations which fire in a temperature range of 1300l400 F. are next applied and fired in that temperature range to form the electrodes 12. Any suitable pattern capable of providing termination of resistance film 13 may be employed for electrodes 12. However, the dumbbell-like configuration shown in FIGURE 1 offers the advantage of economy of the costly electrode material.
Then a thin coating of an organic metallic compound is suitably applied to the undercoat 11 across the electrodes 12 for the production of the resistive film 13. The organic metallic compound is made up of an organic resinate of the alloy of the resistive film. Firing of the alloy provides a resistive film having desired electrical characteristics including preferably a zero temperature coefiicient of resistivity and is capable of providing a film in the order of 2 X 10* inches thick.
The organic resinate compound is applied to the undercoat 11 in an appropriate vehicle by dipping, brushing, spraying, or screening. The resinate in the vehicle is applied as thinly as is necessary to give to the deposited metal film the desired thickness and ohmic value of resistance. The vehicle functions as a medium for carrying the resinate so that the resinate is evenly distributed upon the undercoat 11. That is to say, the organic metallic resinate is evenly dispersed in the vehicle, and when fired to the undercoat gives a good thin metal film. The resinates may include as constituents natural occurring resinates, resins and synthetic preparations. The resinates are precious metal compounds with natural or synthetic resins or simple reactive organic compounds. The metal resinate is suitably prepared for use in the production of the resistive film 13 by known methods as for example by simple solution of resin in a base followed by addition of the precious metal salt.
After the coating comprising the metal resinate compound and its vehicle is applied to the glaze underlayer 11, the base 10 and underlayer 11, and the coating are given the first stage of heat treatment to decompose by pyrolysis the binder and organic portion of the metal compound. Deposition of the metal from the resinate starts at temperatures ranging anywhere from approximately 200 to 400 C. Crystal formation of the precipitated metal occurs at a temperature in this range and progresses with time until approximately 100% metallic deposit results. The time was found to vary anywhere from 15 to 30 minutes. Accompanying the precipitation of the metal is the deposition of some carbon and base metal oxide ash from the binder and flux or frit respectively.
The second stage heating is to completely oxidize the ash or residue and to ensure a thorough precipitation of the precious metal film. The second stage heating may range from 400 to 750 C. for about one hour. FIGURES 1 and 2 show the deposit thoroughly oxidized leaving the thin metal film 13 which may be characterized as the basic resistance. While metal film 13 is shown in FIGURES 1 and 2 as having a straight-line pattern between electrodes 12, other patterns, such as zig-zag paths, are utilized when increased resistance is required. The metallic film 13 possesses good bonding properties so that the underlayer 11 and the deposited metal are virtually integral.
Other well-known techniques for using metal resinates are similarly satisfactory. The metal resinate film is fired at about 1300 F. to produce a resistive film. The organic portion of the resinate and the vehicle are driven 011.
The metal composition is made up of a relatively large proportion of an alloy of precious metals and a relatively small proportion (up to 20% of the precious metals) of a base metal, such as bismuth oxide. This invention is described in connection with a series of precious metal combinations containing gold or iridium, platinum and rhodium, but it is understood that it includes their equivalents. Palladium may be included in the previous metal alloy to increase the resistivity of the alloy.
To illustrate the resistive films the following proportions are given for desirable organo-metallic formulations, but as exemplifications only. The percentages in these examples are by weight.
The precious metal component of Formulation I is composed of 76.8% gold, 19.2% platinum and 4% rhodium. A modification may be made in the gold content by replacement by an alloy of from 70-90% gold and 1030% palladium. About 5.1% of the total resinate composition is precious metal. Base metal oxide is present in a minor proportion that is about 19.6% of the precious metal.
The precious metal component of Formulation II may be 69% platinum, 29.8% iridium and 1.2% rhodium. The precious metal is about 2% of the total composition, and base metal oxide is present to the extent of about 10.5% of the precious metal.
The overcoat 15 of vitreous glaze is applied over the resistive film 13 and adjacent portions of electrodes 12 to come into contact with the undercoat 11. The overcoat glaze is selected to fire at a temperature of about 1100 F. In the final step the overcoat 15 is fired at about 1100 F. and the glazes of the undercoat 11 and overcoat 15 tend to react to produce the monolithic sandwich structure of this invention.
Among other features of this invention the vitreous glaze of: the undercoat remains relatively hard at the temperature of 1300-1400 C. during the firing of the resistive material. Further, the undercoat and overcoat are under compression and it has been found that this provides improved mechanical stability in the metal film resistor structure. The resistance has a temperature coetficient of resistivity which is close to zero.
FIGURE 3 illustrates a modification of this invention. In FIGURE 3, a layered structure is made up of glaze layers 15 and resistive films 13 on an underlayer 11. The resistive structure is made up by first applying the underlayer 11 to the substrate 10. The underlayer 11 is then fired on the base as described above. The layered substrate base is then removed from the oven and an electrode 12 (as in FIGURE 2) is applied adjacent the left side as seen in FIGURE 3. Electrode 12 is applied adjacent the right side of layer 11 and does not extend onto substrate 10, inasmuch as 12' is not a terminal electrode. The electrodes are suitably fired as described above. In the next step, the metal resinate is applied overlying the electrodes 12 and 12' and is fired to form resistive film 13. Overglaze 15 is then applied to cover film 13 and adjacent portions of electrodes 12 and 12'. Another electrode 12" is applied over a part of glaze 15 to provide another intermediate electrode permitting the continued build-up of the serpentine resistance path. The layers and films 15 and 15" and 13 and 13" are then applied and fired successively in the manner described above. Suitable electrodes 12" and 12" are also laid down and fired and receive the respective resistive films 13' and 13". The resistive films 13 and 13" are sandwiched between the overcoats 15, 15' and 15" and each resistive film extends from its respective electrode 12, 12 and 12" to the opposite side in a continuous zig-zag path. Conventional leads 14 are attached to the electrodes 12 and 12".
The invention has been described with particular reference to specific embodiments thereof including modification; however, it will be understood that it is susceptible to embodiment in a large number of forms still within the spirit of this invention. For example, the arrangement of the resistive films and their electrodes may be rearranged and the application of the resistive film need not be through evaporation of a solvent from the metal resinate, but may be produced by other techniques. Other ceramic substrates such as alumina may also be used. Accordingly, the scope of this invention is limited by the appended claims.
What is claimed is:
1. In an electrical resistor, an inert electrically nonconductive base having a coefficient of expansion of over 6 l cm./cm./ C. over a temperature range of from 27 C. to 1100 C., a fired-on continuous inorganic homogeneous electrical non-conducting first layer on a surface of said base having a lower temperature coeflicient of expansion than said base, a fired-on electrically resistive metal film on said first layer having a firing temperature not greater than said first layerwith spaced electrodes in electrical connection with said film, and a fired-on continuous inorganic homogeneous electrically non-conducting second layer overlying said film and adjacent portions of said electrodes and joined to said first layer in areas of contiguity, said second layer having a firing temperature not greater than said film and a temperature coefficient expansion not greater than first layer and said base.
2. An electrical resistor as claimed in claim 1 wherein said first layer and said second layer are comprised of borosilicate.
References Cited UNITED STATES PATENTS 2,357,473 9/1944 Jira 338-308 X 2,521,894 9/1950 Brown 338-314 X 2,808,351 10/1957 Colbert et al. 338-308 X 2,927,048 3/l960 Pritikin 117-215 3,134,689 5/1964 Pri'tikin et al. ll7-2l2 3,202,951 8/ 1965 Krinsky 338-2 3,217,281 11/1965 Greist et al. 338-309 3,244,559 4/1966 Sivertsen et al. 338-309 X 2,786,925 3/1957 Kahan 338-262 2,859,321 11/1958 Garaway 338-262 X 2,939,807 6/1960- Needham 117-212 2,950,995 8/1960 Place et al. 338-308 X 2,950,996 8/1960 Place et a1 338-308 X 3,114,868 12/ 1963 Feldman 117-217 X 3,248,680 4/1966 Ganci 338-266 FOREIGN PATENTS 8/ 1946 Great Britain.
RICHARD M. WOOD, Primary Examiner.
VOLODYMYR Y. MAYEWSKY, Examiner.