US2950995A

US2950995A - Electrical resistance element

Info

Publication number: US2950995A
Application number: US646888A
Authority: US
Inventors: Sr Thomas M Place; Jr Thomas M Place
Original assignee: Beckman Instruments Inc
Current assignee: Beckman Coulter Inc
Priority date: 1957-03-18
Filing date: 1957-03-18
Publication date: 1960-08-30
Anticipated expiration: 1977-08-30

Description

T. M. PLACE, SR. ETAL 2,950,995 ELECTRICAL RESISTANCE ELEMENT 'Aug. 30, 1960 Filed March 18, 1957 lNl/iNTORS. THOMAS M. PLACE 5R. THOMAS M. PLACE, UR.

. BY THE/R HTTORNEYJ'.

HAQR/s, K/iCH, Fosrae 1 Hmm/s United States Patent 2,950,995 ELECTRICAL RESISTANCE ELEMENT Thomas M. Place, Sr., Newport Beach, and Thomas M. Place, .lr., Costa Mesa, Calif., assignors to Beckman Instruments, Inc, Fullerton, Calif., a corporation of California Filed Mar. 18, 1957, Ser. No. 646,888 11 Claims. (Cl. 117-227) This invention relates, to resistance elements for use in electrical circuits and, in particular, to resistance elements which are formed by applying a layer of particular resistance material to an electrically nonconducting hightemperature-resistant base.

This is a continuation-impart of our co-pending application entitled Electrical Resistor and Method of Making the Same, Serial No. 439,650, filedon June 28, 1954, now abandoned.

Resistance elements have been made by winding metal wire or ribbon on nonconducting cores or cards. Such wound resistors are characterized by good stability and close control of resistance tolerances. However, such metals are available with only relatively low ohmic resistances and wound resistors are limited in resistance range by this low ohmic resistance and by the minimum size of wire or ribbon that can be drawn and wound economically.

in many applications, wound resistors are bent or shaped to particular configurations for use in specific equipment, after being wound. Therefore, the coresof such resistors must be of ductile material and, while such core materials are satisfactory at normal temperatures, suchcores maybe seriously affected by. the higher temperatures encountered inpresent dayapplications which may result rapid deterioration of the electrical characteristics. of the component.

Resistors have also beenformed by applying thin metallic films to nonconducting base materials, such as by sputtering or evaporative techniques. Conducting metals with relatively low ohmic resistances, such as gold, platinum, palladium, nickel, etc., have been used in the manufacture of such resistors. These metals are deposited in exceedingly thin films. For higher resistance values, the film thickness is only slightly greater than the molecular size of the metal, with the resistance characteristic resulting from the small cross-sectional area of the metal. Thesethin films deposited on relatively thick base materials are subject to deterioration from electrical currents and from temperature variations as well as from rnechan ical stresses and abrasion.

Lowcost resistance elements are manufactured by depositing carbon and boro-carbon on nonconducting base materials, the ohmic resistances of various forms of carbon being somewhat higher than thatof the metals previously referred to. However, such deposited carbon resistance elements have variable and inferior temperature-and voltage coefiicicnts of resistivity. and have numerous surface irregularities which present difiicultieswhen used as variable resistance elements.

Therefore, it is an object of the invention to provide a resistance material for use in resistance element, which material is hard, durable, resistant to mechanical abrasion and substantially unaffected by moisture, high humid ity and fungus. A further object of the invention is to provide such aresistance material which may be produced in a wide range of resistance values and which is stable over a wide range of temperature variations and has a predictable temperature coeflicient of resistivity.

It is another object of the invention to provide a resistance material which may be applied to bases haw'ng any of a variety of desired shapes for making resistance elements, which material has a smooth surface finish suitable for use in otentiometers and variable resistors as well as in fixed resistors. A further object of the invention is to provide a resistance material which is relatively thick with the overall ohmic resistance resulting primarily from the composition or structure of the resistance material rather than from an extremely small cross section or thin film of resistance material and with the magnitude of the ohmic resistance being determined by controlling the composition or structure of the material rather than by controlling the thickness of the layer as in the metallic films resistors.

In general, the resistance element of the invention is a layer of resistance material comprising. a heterogenous mixture of nonconducting material and conducting metals fixed to a nonconducting base with. the overall resistance of the element dependent upon the relative proportion of nonconducting material and conducting metal and upon the particular metal or combination of metals utilized in the resistance material. The nonconducting material is ceramic in nature and the layer is formed by heating the mixture at least to the. melting point of the ceramic but not to the melting point of the metals to create a smooth,v glassy phase.

Accordingly, itis an object of the invention to provide: a resistance material comprising a ceramic-type binder, such as a glass, having finely divided, amorphous par-- ticles of metal dispersed throughout the binder withthe" binder being the predominant portion of the resistance material and the metal being a relatively small portion. Another object of the invention is to produce such a resistance material by the dispersion of colloidal or molecular sized particles of the metal in'th'e ceram c glass which is melted into a continuous phase at a temperature below the melting point of the metal. A further object of the invention is to provide'such a resistance material in which the metal comprisesnot more than sixteen percent by weight of the resistance material. A further object of the invention is to provide a resistance element in which such a-resistance material is formed in relatively thick layers, such layers being'inthe'ordcr of .0085 to .003 inch thick.

It is a further object of the invention to providesuch a resistance material which utilizes a-finely ground metal or combination of metals which do not react with the other components of the7mixturer and which do not oxidize nor melt during the process of forming the hard resistance layer. A further object of the invention is to provide such aresistance material having-trace amounts of bismuth oxide or other lowmelting temperature ce-" ,ramic flux and/or trace amounts ofopacifiers mixedwith the nonconductiug binder and the metal for improving the uniformity of the resultant metal-glass layer.

The invention also comprises novel details of construction and novel combinations and arrangements of parts, which will more fully appear'in the courseof the following description. The'drawing merely shows and the description merely describes preferred embodiments-of the present invention which are given by way of illustration or example.

In the drawing:

Fig. 1 is an isometric view of a preferred embodiment of the invention which is suitable for use in rotary poten'-' tiometers;

Fig. 2 is an isometricviewofanother embodiment ofl the invention which is suitable for use in linear potentiometers as well as for a fixed resistor;

Fig. 3 is an isometric view of another embodiment of the invention having flexible leads for connection into an electrical circuit; and

Fig. 4 is an isometric view of another embodiment of the invention used as a button-type resistor.

In the structure of Fig. 1, a layer of resistance material is fired to a base 11 with

electrodes

12, 13 being provided at each end of the layer 10 for connecting it into an electrical circuit. This resistance element may be used as a fixed resistor or may be combined with a rotating contact arm for use as a rotary potentiometer. The base 11 may be of any suitable electically nonconducting material which will withstand the elevated temperatures used in fin'ng the resistance material. Various ceramic materials are suitable for this use, those having a smooth, fine textured surface and being impervious to moisture and other liquids being preferred. Steatite, fosterite, sintered or fused aluminas and zircon porcelains are examples of preferred materials for forming the base 11.

The electrical

conductive electrodes

12, 13 are conventional and may be formed by applying any of the well known conducting silver or other metal pastes over the layer of resistance material and firing the unit to convert the paste to a layer of metal which is firmly attached to the layer of resistance material. Then leads may be connected to the electrodes by suitable means, such as by clamping or by soldering. Alternatively, leads in the form of wire or ribbon may be placed in grooves or openings in the base 11 prior to firing on the layer 10 so that the wire or ribbon will project into the layer 10 and be .adhered thereto during the firing process.

binder should not absorb moisture and should be resistant to high humidity and fungus and should fuse to a smooth surface, continuous glassy phase on heating to a temperature below the melting point of the metal or metals mixed therewith. It has been found that a ceramic glass is suitable for this purpose and lead borosilicate glasses are preferred for use in the invention. The particular composition of the glass utilized is not critical to the practice of the invention and various changes in the composition of the glasses can be made to alter the fusion temperature, coefficient of thermal expansion, fluidity, solubility, etc., by one familiar with the ceramic arts to provide a particular desired characteristic. The composition of two glasses which have been used in the practice of the invention are given below as illustrative, but are in no way intended to be restrictive on the composition used in the resistance material.

While the glass may be produced by any conventional.

process, it is preferred that it be as homogeneous as possible. One method of making a glass includes thoroughly mixing a batch of raw materials together while dry, melting the batch in ceramic crucibles to produce a clear fluid glass, quenching the molten glass by pouring into cold water, drying the resulting shattered glass and 4 crushing and then grinding it to a very fine powder with all particles less than about 325 mesh in size.

The metal or metals used in the mixture are nonreactive and nonoxidizable. The term nonreactive means that the metal will not react with the other components of the mixture either at room temperature or at the elevated temperatures required to produce the continuous, glassy finished resistance element. The term nonoxidizable means that the'metal does not oxidize in a normal atmosphere at such elevated temperatures. Such metals are commonly referred to as noble metals and for the purposes of this specification include gold, silver, palladium, platinum, rhodium and iridium. However, this is not intended as an exclusive listing since other metals are known to have similar properties and may be used in the practice of the invention and are intended to be included in the class of noble metals.

In one method of preparing the resistance material of the invention, the glass binder, the metal or metals and other components which may be used in certain applications, are mixed together in particular proportions, to be described in detail below, with a volatile or evaporable liquid carrier to form a uniform mixture. The glass and/ or the metal may be added to the mixture in finely divided form or may be introduced as large particles with the mixture being ground in a ball mill or similar appa-' ratus to produce a finely ground mixture. It is preferred that the solid particles in the mixture have a screen fineness of less than about 325 mesh. The particular liquid carrier utilized is not critical to the practice of the invention and examples of suitable volatile carriers are toluol, xylol, oil, clear lacquer mixtures, isopropyl alcohol and even water. The mixture is applied to the base 11 to form the layer 10 by any suitable means such as brushing, spraying, stenciling or silk screening. The quantity of liquid carrier used in the mixture is selected to give the mixture the proper viscosity for the particular method used in applying the mixture to the base.

After the layer has been applied to the base, the base and layer are preferably permitted to dry in circulating warm air for a short period. Then the base and layer are fired in a kiln which may be a conventional ceramic kiln, preferably one utilizing electrical h'eat since such kilns produce a cleaner atmosphere.

The purpose of the firing operation is to solidify the glass into a continuous glass phase with the metal particles uniformly dispersed throughout the glass phase without melting the metal particles and without producing bubbles or blisters in the surface of the layer. The temperature to which the layer and base are fired is critical in that firing too low results in failure to achieve the continuous glass phase with the hard smooth surface 1 and firing too high produces bubbles or blisters and causes the metal particles to agglomerate. The firing temperature depends upon the particular glass utilized and upon the particular kiln and cannot always be determined in advance, since the heating characteristics of individual kilns vary. The time and temperature cycle of the firing step is not otherwise critical and one skilled in the ceramic art can devise a number of suitable firing procedures.

The following is illustrative of a suitable firing procedure. The base with the layer of resistance material is placed in the kiln and the temperature is increased to 1000 F. at a rate of approximately 400 F. per hour. The temperature is then held at 1000 F. for about 30 minutes to insure the removal of all volatile and organic materials from the mixture and, also, to insure the uniform distribution of heat through the base and layer before the glass starts to fuse. Then the temperature of the kiln is raised to 1490- F. at a rate of about 200 F. per hour. The temperature is maintained at the 1490 point for 30 minutes to insure uniform heat distribution and at the end of this period, the kiln is allowed to cool to room temperature by normal radiation. This particular firing cycle is used with a glass mixture consisting of 20% glass No. l and 80% glass. No. 2 as described above. The firing cycle and temperature may be varied over a wide range as required for different glass composition and different kilns. 5

When the unit is cool, the layer of resistance material is firmly attached to the base and is in the form of a smooth black glossy layer retaining the exact configuration in which it had been applied to the base. Then electrodes and leads may be applied as previously described if desired, after which the component is ready for connection into an electrical circuit.

It has been found that the introduction of fractional percentages or trace amounts of one or more refractory metal oxides, commonly known as opacifiers, to the glassmetal mixture reduces the contact resistance between moving contacts and the surface of the solidified glassmetal mixture. When added in small percentages to ceramic glasses, opacifiers have the property of being used as solid metallic conductors. Alloys of platinum and rhodium and gold and rhodium produce a lower range of ohmic resistance values with positive temperature coeflicients of resistivity. Gold, platinum and rhodium alloys produce low range ohmic resistance values with lower positive temperature coefficients of resistivity. Gold, palladium and rhodium alloys produce a higher range of resistance values and these resistance values increase with increasing percentages of palladium. The temperature coefficients of resistivity range from low positive to high negative values with increasing percentages of palladium. The addition of silver to these alloys will usually change the temperature coeflicients of resistivity to more negative values.

While innumerable combinations of materials may be used in making the resistance element of the invention, the following are set out as being illustrative of the range of mixtures which it is intended that this invention cover.

' Mixture No 1 Chromium Oxide.

uniformly dispersed as colloidal particles or flocs when the glass is melted and subsequently cooled, thereby producing a more uniform dispersion of metal particles throughout the resistance material when utilized in the invention. Examples of such opacifiers are tin oxide, antimony oxide, zirconia, molybdenum oxide and chromium oxide. Accordingly, although not essential to the performance of the invention, it is preferred in the practice of the invention to include fractional percentages of one or more of the opacifiers in the mixture of glass and metal, particularly when the resistance element is to be used in a potentiometer.

It has been found that the inclusion of a fractional percentage of low melting temperature ceramic flux, such as bismuth oxide, molybdenum oxide or vanadium oxide in the glass-metal mixture produces adhesion of the metal particles to glass particles at temperatures below the softening point of the glass and tends to prevent agglomeration of the metal particles as the firing temperature increases. Accordingly, although not essential to the performance of the invention, it is preferred in the practice of the invention to include a fractional percentage or trace amount of a low melting temperature ceramic flux in the metal-glass mixture.

The glass-metal mixture which constitutes a resistance material of the invention is predominantly glass with a relatively small amount of metal. The particular proportions utilized in a specific resistance element will depend upon the desired value of resistance. However, the range of proportions in finished resistance elements will be glass binder about 8499 percent by weight and metal about 1-16 percent by weight. The preferred range within which most resistance elements of the invention fall is 91-98 percent by weight of glass and 2-9 percent by weight of metal. The resistance characteristics of a particular mixture are determined by manufacturing and testing a resistance element utilizing the mixture, after which changes in resistance may be made by changing the proportions used.

Various electrical characteristics of the resistance material may be controlled by using different noble metals and different mixtures of noble metal in the resistance material. The electrical characteristics of the metals dispersed throughout the solidified glass tend to follow the electrical characteristics of the same metals when The configuration of the layer of resistance material which is applied to the base will depend upon the resistance characteristic of the particular mixture employed and upon the desired over-all resistance of the finished resistance element. As an illustration of the size of the layer, the annular strip 16 of Fig. 1 may be made in the order of one-half to one and one-half inches in diameter. The layers of resistance material will range from a fraction to a few thousandths of an inch in thickness with the preferred range being about .005.003 inch The majority of the resistance elements being produced at the present time are in the order of one thousandth of an inch thick. Since the resistance layer has a substantial thickness as compared to the sputtered and evaporated metallic film resistors, thickness control is much less critical in the resistor of the invention.

In the finished resistance element, amorphous metal particles are uniformly dispersed throughout the solidified glass forming a semi-conductive path through the resistance material. The exact electrical phenomena existing within the resistance material is not yet fully known. However, it appears that the metal particles are spaced from each other or are slightly touching to produce a high resistance. it is known that an increase in percentage of metal in the mix produces a decrease in overall resistance which would be consistent with a reduction in spacing between the metal particles and an increase in the number of particles which may be contacting adjacent particles. It has been suggested that the resistance of the layer is due to the fact that the particles are spaced apart less than the wavelength of an electron and that when the spacing between a majority of the particles exceeds this value, the layer will become a nonconductor and when a majority of the metal particles are in contact with each other, the layer will become a conductor having substantially zero resistance.

In an alternative method of preparing the resistance material of the invention, the metal or metals may be mixed with the finely ground glass binder with the metals being in the form of soluble metal compounds which are decomposable by heat. The metal compound or compounds are dissolved in a suitable solvent, such as in one of the essential oils, and thoroughly mixed or milled with the powdered glass to produce a uniform mixture. The volatile liquid carrier described in conjunction with the perviously disclosed method is ordinarily not then required, since the solvent for the metal compounds serves to make the mixture fluid and suitable for applying to the base. An important characteristic of such metal compounds is that the metal is present in colloidal form so that when the base with the layer of resistance material applied thereto is fired to drive off the organic material present in the mixture, the metal compounds will be decomposed, leaving a residue of molecular size metal particles uniformly dispersed throughout the layer. It is preferred to use metal-organic compounds such as metal resinates or abictates as the soluble metal compounds discussed above.

The metal oxides, such as bismuth oxide, tin oxide and chromium oxide, which are sometimes used in the resistance material of the invention, may also be introduced into the mixture in the form of soluble metal compounds as described in the preceding paragraph. Upon decomposition of the metal compounds, the metals will be converted to oxides.

In another embodiment of the method of preparing the resistance material of the invention, the resistance material may be prepared in large batches and stored indefinitel'y to be used in making a desired number of resistance elements as required. In this method, the glass binder, metal or metals and metal oxides, if used, are mixed or milled together with the noble metals and the oxidizable' -metals being present in the form of soluble metal compounds. The mixing is carried out thoroughly so that each glass particle will be wet with the metal solutions. This mixture is gradually heated to approximately 700 F.

being constantly stirred, to remove the volatiles and organic materials from the mixture, to decompose the metal compounds and to oxidize the oxidizable metals. The resulting dry material is ground to a fine powder and calcined at about 850 F. The resulting calcine is ground to a fine powder, preferably with all particles less than about 325 mesh, producing a dry material consisting of very small glass particles coated with an extremely thin layer of metal and metal oxide particles. This mixture may be stored indefinitely without change or deterioration and may be used in small portions to produce limited numbers of resistance element.

When it is desired to manufacture resistance elements using the material produced according to the preceding paragraph, this dry powder is mixed with a suitable liquid carrier to form a fluid composition which can be applied to the base, as in the manner described previously. The base With the layer applied thereto is then fired to produce the continuous phase of solidified glass in the same manner as described above.

Fig. 2 illustrates another form. of the resistance element of the invention in which a layer 15 of resistance material is applied to a rectangular base 16- and electrodes l7, 18 are then added at the ends of the layer 15. This the embodiments disclosed may be subjected to variouschanges, modifications and substitutions without necessarily departingfrom the spirit of the invention.

We claim as our invention:

1. A resistance element comprising a high-temperatureresistant, electrically nonconductive base having fired thereto a layer of resistance material comprising about 84-99 percent by weight of solidified glass and about 1-16 percent by weight of at least one of the noble metals in finely divided form dispersed throughout the solidified glass in electrically conductive relationship.

2. A resistance element comprising a high-temperatureresistant, electrically nonconductive base having fired thereto a layer of resistance material comprising 91-98 percent by weight of solidified glass and 2-9 percent by Weight of at least one of the noble metals in finely divided form dispersed throughout the solidified glass in electrically conductive relationship.

3. A resistance element comprising a high-temperatureresistant, electrically nonconductive base having fired thereto a layer of resistance material with a thickness of a fraction of to a few thousandths of an inch, said layer comprising about 84-99 percent by w'eight of solidified glass and about 1-16 percent by weightof at least one of the noble metals in finely divided form dispersed throughout the solidified glass in electrically conductive relationship.

form of the invention is particularly suitable for use in to the electrodes 30, 3 1 respectively. The materials comprising the resistors of Figs. 2, 3 and 4 and the methods of making the resistors are the same as described in conjunction with Fig. 1, the various resistors differing only in the physical shape of the finished product.

Although exemplary embodiments of the invention have been disclosed and discussed, it will be understood that other applications of the invention are possible and that 4. A resistance element comprising a high-temperatureresistant, electrically nonconductive base having fired thereto a layer of resistance material comprising about 84-99 percent by Weight of solidified glass and about 1-16 percent by weight of at least one of the noble metals in finely divided form dispersed throughout the solidified glass in electrically conductive relationship, the thickness of said layer being about .0005-003 inch.

5. A resistance element comprising a high-temperatureresistant, electrically nonconductive base having fired thereto a layer of resistance material comprising about 84-99 percent by weight of solidified glass, about 1-16 percent by weight of at least one of the noble metals in finely divided form dispersed throughout the solidified glass in electrically conductive relationship, and less than 1 percent by weight of a low melting temperature ceramic flux dispersed throughout the solidified glass.

6. A resistance element comprising a high-temperatureresistant, electrically nonconductive base having fired thereto a layer of resistance material comprising about 84-99 percent by weight of solidified glass, about l-l6 percent by weight of at least one of the noble metals in finely divided form dispersed throughout the solidified glass in electrically conductive relationship, and less than A2 percent by Weight of opacifier dispersed throughout the solidified glass.

7. A resistance element comprising a high-temperatureresistant, electrically nonconductive base having fired thereto a layer of resistance material comprising about 84-99 percent by weight of solidified glass, about 1-16 percent by weight of at least one of the noble metals in finely divided form dispersed throughout the solidified glass in electrically conductive relationship, and less than 1 percent by weight of low melting temperature ceramic flux and less than /2 percent by weight of opacifier, said flux and opacifier being dispersed throughout the solidified glass.

8. A resistance element comprising a high-temperatureresistant, electrically nonconductive base having fired thereto a layer of resistance material comprising 91-98 percent by weight of solidified glass, 2-9 percent by Weight of at least one of the noble metals in finely divided form dispersed throughout the solidified glass in electrically conductive relationship, and about A1 percent of bismuth oxide and trace amounts of tin oxide and chromium oxide dispersed throughout the solidified glass.

9. A resistance element comprising a high-temperaturereSiSlaLLt, electrically nonconductive base having 'fired thereto a layer of resistance material comprising 91-98 percent by weight of solidified glass, 2-9 percent by Weight of at least one of the noble metals in finely divided form dispersed throughout the solidified glass in electrically conductive relationship, less than 1 percent by weight of low melting temperature ceramic flux and less than /2 percent by weight of opacifier, said flux and opacifier being dispersed throughout the solidified glass, said layer of resistance material having a thickness between .0005-.003 inch.

10. A resistance element comprising a layer of amorphous particles of noble metal dispersed throughout a continuous phase of solidified glass, the melting temperature of said glass being lower than the melting temperature of said metal, said layer being fired to a high-temperature-resistant, electrically nonconductive base, said metal comprising about 1-16 percent by weight of the total of said layer.

11. A resistance device comprising: an electrically nonconductive base carrying on one of its surfaces a hard layer of resistance material comprising at least a threecomponent fused mixture, the first of said components being about 84-99 percent by weight of glass, the second of said components being about 1-16 percent by weight of at least one of the noble metals, the third of said 10 components being no more than 1 percent by weight of bismuth oxide, said metals and oxide being uniformly dispersed in a continuous phase of said glass; and means for conductively connecting said layer into an electrical circuit.

References Cited in the file of this patent UNITED STATES PATENTS 1,927,185 Power et al. Sept. 19, 1933 2,001,297 Boyles May 14, 1935 2,274,830 Gould et al Mar. 3, 1942 2,461,878 Christensen Feb. 15, 1949 2,472,533 Heyman June 7, 1949 2,472,801 Barfield et a1 June 14, 1949 2,552,626 Fisher May 15, 1951 2,564,707 Mochel Aug. 21, 1951 2,588,920 Green Mar. 11, 1952 2,703,354 Wainer Mar. 1, 1955 FOREIGN PATENTS 314,173 Germany Sept. 1, 1919 OTHER REFERENCES Electronic Engineering, July 1952, pages 324-327, article by Heritage, titled Metal Film Resistors.

Claims

2. A RESISTANCE ELEMENT COMPRISING A HIGH-TEMPERATURERESISTANT, ELECTRICALLY NONCONDUCTIVE BASE HAVING FIRED THERETO A LAYER OF RESISTANCE MATERIAL COMPRISING 91-98 PERCENT BY WEIGHT OF SOLIDIFIED GLASS AND 2-9 PERCENT BY