US3372058A - Electrical device, method and material - Google Patents

Description

March 5, 1968 J YD ET AL ELECTRICAL DEVICE, METHOD AND MATERIAL Filed Dec. 18, 1963 WA w m w m o h F mi 4 v R\ Q W/ H/ mum G r m F .6 GRAMS 0F DISPERSING AGENT FIG. 3

INVENTORS JAMES R. BOYD CRYSTALLITE SIZE fi am; ANGSTROMS ATTORNEY United States Patent Office 3,372,058 Patented Mar. 5, 1968 3,372,058 ELECTRICAL DEVICE, METHOD AND MATERIAL James R. Boyd, Austin, Tex., and Arthur H. Mones, Poughkeepsie, and John C. Schottmiller, Penfield, N.Y., assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Dec. 18, 1963, Ser. No. 331,534 11 Claims. (Cl. 117-227) ABSTRACT OF THE DISCLOSURE Metal glaze resistive compositions suitable for application to a ceramic dielectric comprising 15 to 70% of a palladium or rhodium oxide having a crystallite size with a surface area of at least 0.75 m. gm. and up to 35% of a conductive metal dispersed in a glass matrix comprising 35 to 70% vitreous enamel frit.

This invention relates to metal glaze resistors, and, in particular, to metal glaze resistive compositions, to the method of fabricating metal glaze resistors, and to the metal glaze resistors produced therefrom.

The present trend toward microminiaturization has focused attention on metal glaze resistors as components of integrated electronic circuits. Made from mixtures of glass and noble metal powders, these metal glaze resistors offer Wide resistance ranges, flexibility in their processing, product versatility, economy and performance capabilities not available with other conventional resistors.

Of these metal glaze resistors, palladium-glass and palladium-silver-glass systems potentially offer rather desirable characteristics. These resistors are formed by combining glass, palladium and silver powders of small particle size and uniformly dispersing the mixture in an organic vehicle, the organic vehicle being chosen in some instances for its high boiling point and in others for the flow properties that it imparts to the dispersion. Through variation of the organic vehicle, metal glaze coatings are applied to almost any substrate configuration by methods such as screening, spraying, dipping or transfer wheel systems. Once applied to the substrate, the vehicle is volatilized at about 100 C. and the material is fired at a temperature which ranges between 750 C. to 850 C. to melt the glass and to suspend the metal' constituents in a fixed relationship, one to the other.

Although the palladium-glass and the palladium-silverglass metal glaze resistors offer advantages not available with other types of resistors, they are not altogether satisfactory in providing stability, uniformity and reproducibility of electrical characteristics at commercially acceptable yields. Drift and wide scattering in the resistance and temperature coefficient of resistivity (hereafter designated TCR) are experienced in preparing these metal glaze resistors. Particularly, when rigid end of life specifications are required for all circuit components to perform satisfactorily. The appearance of any of these aforementioned adverse phenomena upsets the electrical balance required for successful operation of the circuits. Accordingly, it has been an object of considerable research to find metal glaze resistors having stable electrical characteristics With a reproducibility factor at commercially acceptable yields.

It is an object of this invention to provide an improved metal glaze resistor having stable electrical resistance and temperature coefficient of resistivity.

It is another object of this invention to provide a metal glaze resistor composition having enhanced electrical characteristics and uniformity in performance.

It is yet another object of this invention to provide an improved method for producing metal-glaze resistors.

It is a further object of this invention to provide an economically feasible method for producing metal glaze resistors having product versatility and uniformity in performance characteristics.

Resistive metal glazes of this invention are formed by combining vitreous enamel frit with palladium or rhodium and thereafter firing the compact to produce a material having a microstructure which is characterized by a dispersion of metal oxide and metal in a glass matrix. To modify properties such as thermal stability, a conductive component such as silver is added to the vitreous enamelpalladium oxide reaction mixture. The silver serves to modify the oxidation reaction promoted during the firing process but the amount admissible to the reaction product is limited, since silver, when an electric field is applied, tends to migrate which results in variations in resistivity. Large amounts of silver also leads to poor reproducibility in resistivity because, presumably, of inconsistent silver linkages in parallel with the resistive phase of palladium oxide. What has been discovered is that stability and uniformity in electrical characteristics are achieved, when the palladium and rhodium, or their oxide, added to the reaction mixture, are maintained within selected crystallite sizes or surface areas. This is most surprising since it was previously thought that it was the size of the palladiu-m or oxide particle, a particle being defined as an agglomerate of crystallites, that exerts the influence on the resulting electrical characteristics; it was felt that it was only necessary to maintain palladium or its oxide particle to a size of less than 325 mesh, and, preferably to a size between 0.1 and 50 microns. It has been found that problems still exist with stability, reproducibility and uniformity in the desired electrical characteristics where selection of palladium or rhodium or their oxides are based solely on particle size. On the other hand, maintaining the crystallite size of the palladium to below 1500 Angstroms, and preferably to less than 1000 Angstroms, provides stability, uniformity and reproducibility in the resistive metal glazes. With extremely large crystallite sizes, that is, with those that exceed 1000 to 1500 Angstroms, oxidation becomes sluggish and it becomes difficult to assure the presence of a highly dispersed oxide phase in the resistor. Palladium crystallites within the selected values correspond (after sintering) to palladium oxide crystallites with surface areas of at least 0.75 m. gm. to 1.5 m. /gm., with it being preferable to use crystallites with a surface area of about 1 m. gm.

The significance of crystallite size is particularly evident in the preparation of loW resistivity material, that is, material having a sheet resistivity of about ohms per square. Low resistivity material may be prepared by: 1) increasing the silver concentration; (2) decreasing the glass concentration; (3) doping the resistor with, for example, the cation lithium. The last of these techniques is the subject of copending US. patent application Ser. No. 313,032, filed in behalf of Arthur H. Mones and Kenneth E. Neisser, Jr., and which is assigned to the assignee of the instant application. Increasing silver to levels Where the silver to palladium cation ratio is greater than about 1.5 results in poor reproducibility, extremely high positive TCRs and load instability which are due presumably to silver migration. Decreasing the glass concentration to levels much below 35 percent results in reduced moisture stability. Doping with lithium, for example, to produce resistivities of the order of 100 ohms per square usually results in high positive TCRs.

The effect of crystallite size (or surface area) on resistivity and TCR is brought out by Table I below. The first two columns in the table give the crystallite size as based on X-ray diffraction data along the crystal directions as specified. The third column presents surf-ace area measurements which are in meters square per gram of material. The fourth column in the table presents the particle size which is determined by the usual techniques of sedimentation and screening procedures. The last two columns in the table present resistivity in ohms per square and TCR in parts per million per degree (p.p.rn./ C.). What is observed from the table is that there is a relationship between crystallite size or surface area with resistivity and TCR, whereas a relationship is lacking between particle size and the electrical characteristics. The data is presented for fixed glass and silver concentration, the crystallite size of silver is also fixed and has a similar affect on the electrical characteristics.

Uniformity of resistivity is not achieved through particle size selection as was previously thought. This is further emphasized in Table II below where particle size is varied while crystallite size as based on, in this case, surface area of the palladium oxide is maintained constant at about 1 m. gr.

TABLE II Particle Size, Resistivity, ohms per TCR, p.p.rn./ 0.

microns square (ZS-100 C.)

It will be recognized by those skilled in the art that wherever silver is used as a conductive component it is replaceable wholly or partially with either gold or platinum or both with substantially similar results, and, where palladium or palladium oxide is contemplated, it is replaceable wholly or partially by rhodium or rhodium oxide.

Further beneficial effects are available with the addition of small amounts of dispersing agents. The addition of oxides such as silicon oxide and alumina into the initial reaction product enables the fabrication of resistors with still further improvements in reproducibility and enables reduction in TCR without substantially affecting resistivity.

The crystallites used in the initial reaction mixture are formed by treating palladium powder in a furnace under non-oxidizing conditions where the palladium power grows into crystallites of a desired size. They are then removed and oxidized at a temperature maintained between 750 C. to 800 C. for approximately 2 hours to convert the palladium to the oxide.

It is important that the crystallites incorporated into the initial reaction product undergo sintering prior to oxidation, sintering being defined as that process where the palladium grows into larger crystallites. If, for example, palladium black of arbitrary crystallite size is oxidized and the sintering step bypassed, the resistivity obtained in the end product lacks the reproducibility, uniformity and stability as that obtained with the sintered palladium crystallites of controlled size. The reasons for this are not really known but a working hypothesis has been formulated, namely, that electrical linkages composed of large crystallites as opposed to small crystallites for a fixed distance will have lower resistivity by virtue of having less grain boundary contacts. In the limiting case a single crystal will have minimum resistivity. It is important therefore to control the crystallite size for uniformity in resistivity and enhancement of other properties.

The reasons for conversion to oxide are: (1) finely di- 4. vided palladium tends to sinter in mixing operations whereas palladium oxide does not, and (2) finely divided palladium tends to catalytically affect the organic vehicle whereas palladium oxide does not. Uniformity and shelf life are enhanced by conversion to oxide.

The palladium crystallites are combined with silver, gold and platinum, dispersing agent, and organic vehicle, coated on a substrate and the reaction mixture fired in an oxidizing atmosphere to temperatures up to 750 C. Although it is usually not desirable to have the firing temperature exceed 790 C. for prolonged periods, since the palladium oxide tends to reconvert to palladium metal, a condition which has deleterious if not completely disastrous effects on the operation of the resistive metal glaze, still further improvement is achieved if the firing temperature is allowed to exceed 850 C. under particular conditions.

To take advantage of this high temperature firing, the palladium oxide, silver and vitreous enamel are mixed as previously. The compact is fired at 850 C. to decompose the palladium oxide to form a palladium-silver alloy. The reaction product is then refired at temperatures in the range between 450 C. to 750 C. to yield a product with a further degree of sensitivity than available from the low temperature firing. An additional advantage of the high temperature firing is that vitreous enamel frits that include glass that softens at temperatures above 800 C. are usable, thereby overcoming a limitation of the low temperature firing process. Additionally, with the palladium oxide and glass system firing the initial reaction product at temperatures exceeding 750 C. enhances the drift characteristics markedly over those observed with metal glaze resistive materials fired at temperatures of less than 750 C.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings.

In the drawings:

FIGURE 1 is a sectional view to a greatly enlarged scale of a portion of an electrical resistance element in accordance with the present invention;

FIGURE 2 is a graph of resistivity and TCR versus grams of dispersing agent for a palladium oxide silver glass composition;

FIGURE 3 is a plot of resistivity per square for a palladium oxide, silver and glass composition versus crystallite size.

A metal glaze resistor element 10 in accordance with the persent invention includes an electrically nonconducting base 11 of a suitable material such as a ceramic having fired thereon a thin layer 12 of a particular resistive metal glaze. The resistance composition includes a quantity of finely divided material from the group consisting of palladium oxide 13 and rhodium oxide. In addition the glaze includes divided glass 14 which may be lead borosilicate or the like and, in addition, the glass may also include a finely divided conductive material of noble metal, i.e., silver, gold, or platinum, which aids in adjusting the resistivity of the final resistor element.

Speaking more particularly now, resistive metal glaze compositions are formed for application as resistors with a microstructure that contains a dispersion of palladium oxide and silver-palladium or palladium phases in a matrix of glass. Wherever silver is used, it is replaceable wholly or partially with either gold or platinum or both with substantially similar effects in the microstructure and, where palladium or palladium oxide is contemplated, it is replaceable wholly or partially by rhodium or rhodium oxide, the replacement, of course, being based on equivalent amounts. The initial reaction mixture includes 35% to 70% by weight vitreous enamel frit and 15% to 70% by weight sintered palladium oxide, with a palladium crystallite size up to 1500 Angstroms, and preferably up to 1000 Angstroms, which corresponds to a palladium oxide crystallite with a surface area of at least 0.75 mF/gm. to 1.5 m. /gm. and preferably 1 m. gm. Where silver is a component of the initial reaction mixture, it may constitute up to 35% by weight of reaction mixture with the cation ratio of the silver to palladium maintained between 0.5 to 1.5. The preferred composition for a resistor includes 60% by weight vitreous enamel frit with the balance being silver and palladium, the silver being present up to 22% by weight and the palladium being present in the range between 18% to 60% by weight, with a silver to palladium cation ratio between about 0.9 to 1. The TCR of such a material is reducible to zero p.p.m./ C. (ZS-100 C.) and has optimum load and moisture stability characteristics. Typical formulations with the resulting electrical characteristics are illustrated in Table III.

The additions of inert oxides are eifective in altering TCR only when size is in colloidal range. Large crystallite sizes, for example, greater than 27 microns, appear 5 only to dilute the system which brings about an increase in resistivity.

TABLE III Yield 530% Percent R Av. 500 Percent R Example Ratio Ag/Pd Glass Cone, R/K-ohms TCR, p.p.m./ Resistors to hrs., w./sq. inch, Av. 1,000 hrs., Surface Area,

Percent 0. Nominal Value 60 0., in 90% RH same condition mJ/gm.

Circuit Bias 5 45 0. 9 70% 54 53 21 5 70 25 1, 000 70% 69 +1. 48 21 1. 0 57. 3 1. 5 175 53% 33 41 21 1. 0 57. 3 1.5 161 84% 28 36 21 1. 5 70 0.15 1, 000 60% -13. 8 26 21 0. 9 60 3 5:100 70% +0.17 +0.18 21 1. 1 60 0.12 +350 70% 0. 40 0. 41 1. 8

The table illustrates the significance of utilizing crystallite sizes or surface areas of selective values to achieve uniformity, reproducibility and stability in the electrical characteristics as opposed to the previous techniques of composition adjustments including those of particle size. In this manner compositional ranges are available yielding optimum characteristics, for example, in TCR drift and reproducibility. Increasing silver as illustrated in Example E to produce low resistivity material does not produce as a stable result in comparison to Example G where crystallite size or surface is varied within selected values to achieve a lower resistivity which is accompanied by additional desirable properties.

Returning now to Table III, a brief explanation may further enhance what is illustrated. The first two columns present the composition of the resistive metal glaze, the glass being given in Weight percent. Taking the weight percent of glass from 100 percent yields the amount of other constituents in the initial reaction mix. For example, where the glass comprises 45% by weight and the silver and palladium are present with a cation ratio of about 0.5, the silver constitutes about 18% and the palladium about 38% of the initial reaction mixture. The third and fourth columns present the resistance per square in kilo ohms and TCR in p.p.m./ C. The fifth column indicates the yield obtained with the techniques of the present invention. The data presented, for Example E, is for yields of 115%. The next to the last two columns present stability characteristics, that is, the change in resistance over periods of 500 and 1000 hours where tests are conducted with the application of 15 watts per square inch at 60 C. at 90% relative humidity with the circuit biased. The eifect of crystallite size on the electrical characteristics is graphically illustrated in FIG. 3 of the drawings.

Additional improvements with the electrical characteristics of the resistive metal glaze are achieved with the addition of up to about 5% of dispersing agent where the dispersing agent is selected from the group consisting of silicon or aluminum oxide or their equivalents. The effect of these dispersing agents on the electrical characteristics is graphically illustrated in FIG. 2 of the drawings. Further, the additional beneficial elfects of the dispersing agents are given in Table IV.

TABLE V Temperature, C. Soak Time, Minutes Best Value For Crystalte Size, Angstrom Units Room Temperature 189 285 .15 336 205 60 460 335 15 432 335 60 641 385 15 606 385 30 779 440 15 1, 056 440 60 1, 072 466 15 1, 090 466 60 1, 488 488 30 1, 635 514 15 1, 556 514 30 1, 815

To avoid oxygen contamination during the treatment of the palladium powder, the palladium powder is heated in a forming gas atmosphere. If the hydride is formed as a result of this treatment, the palladium is then treated in a vacuum for 2 hours at C. to decompose the hydride produced from the forming gas treatment. The palladium crystallites are then inserted in a furnace containing an oxidizing atmosphere maintained at a temperature of about 750 C. for about 2 hours where the palladium forms palladium oxide of the desired crystallite size. Any sintering temperature and time may be used provided it furnishes a substantially pure palladium oxide crystallite without a metal phase.

Although there is no lower limit for the crystallite size of the palladium save that which is producible from the process, the upper limit for the crystallite size is about 1500 Angstroms and preferably about 1000 Angstroms. This corresponds to palladium oxide crystallites with a surface area of at least 0.75 m. /gm., and, preferably at least 1 m. /gm. Where crystallites exceeding 1500 Angstroms are used, wide scatterings and poor scaling is achieved with the electrical characteristics. Accordingly, it is preferable to maintain the palladium crystallite size to less than 1500 Angstroms. After the crystallites of desired size are prepared they are combined with vitreous enamel frit, the frit used may be composed of glass frit such as borosilicate frit, lead borosilicate frit, or other borosilicate frit, the preparation of these being well known in the art. The vitreous enamel frit, the metal oxide powders, with or without silver being present, and the colloidal dispersing agent are milled for about 2 hours. The mixture is then combined with a vehicle which is in the form of a liquid or paste. Any inert liquid may be employed for the purpose, for example, water, organic solvent with or without thickening agents, stabilizing agents, or the like, all of which are well known in the art.

Preferably the dispersing agents such as silicon oxide or aluminum oxide added to the initial reaction mixture are maintained below a particle size of about 5 microns. The effect of the addition of dispersing agent is most pronouncedit acts to decrease the TCR. For example, in a resistive composition having initial resistivity of 2850 ohms per square and a TCR of +100 p.p.m./ C., the

addition of 1.6% to 4% by weight of dispersing agent brings about a 31.6% change in resistivity; it changes the resistivity to 3750 ohms per square while changing the TCR to about 100 p.p.m./ C.

After the initial reaction mixture is prepared, it is applied to a dielectric substrate capable of withstanding the firing temperature of the vitreous enamel palladium oxide metal composition. The substrate may be alumina, for example, glass, porcelain, refractory barium titanate or the like, preferably the ceramic substrate should have a smooth, substantially uniform surface. The initial re action mixture is applied in thickness to the order of about 0.001 inch. After application on the dielectric substrate, the substrate and mix coated thereon is inserted in an oxidizing atmosphere and fired at a temperature between 700 C. and 790 C. for periods of a few minutes to several hours and preferably for about 20 minutes. In this manner glaze resistors are formed having the desired electrical characteristics.

As previously brought out, the initial reaction mixture of the instant invention includes 35% to 70% by weight vitreous enamel frit with the balance being palladium or palladium oxide with a crystallite size up to 1500 Angstroms. Where silver is also incorporated into the reaction product the cation ratio is maintained between 0.5 to 1.5, and preferably between 0.9 to 1.1. The operable and preferred ranges for the initial reaction product are presented in Table VI in weight percent.

TABLE VI Ingredients Percent Operable Percent Preferred Example 1 Starting with palladium with a surface area of 2.1 m. /gm., the material is heated in forming gas at a temperature of 440 C. for 1 hour. The treated palladium has a surface area of 1.8 m. gm. This material is oxidized at 750 C. in oxygen for 2 hours. The resultant oxide has a surface area of 1 m. /grn.

21% by weight silver 19% by weight palladium oxide 60% by weight glass Ag/Pd: 1.1

To this mixture 'isadded finely divided colloidal silica such that the ratio is 1.5% colloidal silica to 98.5% of the above mixture. The solids are thoroughly mixed in'a high speed shaker for aboutZ hours. A vehicle, for example, beta terpineol, isadded to the solids and the solidsare thoroughly wet. Conventional milling equipment is used. Solid concentration is 80%. The mixture which is suitable for screening is deposited in 1 mil layers to a stencil on ceramic, typically 96% alumina. The deposited composition is dried at about 100 C. and the ceramic with deposit is fired at about 750 C. for about 20 minutes. Resistivities from 'such a mixture are about 100 ohms per square and the TCR about +300 parts per million per C.

Example 2 21% palladium oxide 19% silver 60% glass Ag/Pd=0.9

' Colloidal silica is added to represent 1.5% of the total solids. The compact is mixed and wetted as described in Example 1, similarly screen deposition and firing is performed as in Example 1. Resistivity is typically 3000 ohms per square accompanied by a TCR of about p.p.m./C.

Values for resistivity and TCR may be varied by various techniques and materials. It is known that changes in detail and purity of raw materials, wetting, mixing, dispersion, and firing techniques will alter resistivity and TCR. The methods and materials must be consistent and carefully controlled to realize good reproducibility any good mixing and wetting techniques are usable provided they are consistently followed.

To obtain the additional improvements available with the metal glaze resistors, when the mix constitutes palladium oxide and vitreous frit, the initial reaction mix is fired at temperatures that exceed 790 C. Drift characteristics are further enhanced in this manner. The procedure is illustrated in Example 3 below.

Example 3 A reaction mix constituting 45% by weight palladium oxide with the balance glass is prepared as in Example 1. After deposition on a ceramic surface, the mix is fired in an air atmosphere at about 850 C. for a selected period of time which may be as short as several minutes. The reaction mix is then quenched to room temperature in about 2% minutes to minimize reoxidation of the palladium. The mix exhibits a resistivity of about 4000 ohms per square and a TCR of about 220 p.p.m./C. and has drift characteristics which are maintained to within about 2% to 3% under accelerated stress conditions, that is, 250 C. for 16 hours. I

Where the reaction mix includes a conductive component, the high temperature firing requires an additional step. The reaction mixture is initially heated to a temperature of about 850 C. to decompose the palladium oxide to form a palladium-silver alloy. Following this, it is reoxidized at a temperature in the range between 450 C. to 700 C. to form selected amounts of palladium oxide. The procedure is illustrated in Example 4.

Example 4 A reaction mix is prepared containing 16.8% palladium, 23.2% silver, with the balance glass. The details of preparation are similar to those previously described. After screening on a ceramic substrate, the mix is fired at 850 C. for 20 minutes. X-ray diffraction analysis indicated that the palladium oxide decomposed and had a resultant resistivity of 1.5 ohms per square. The reaction product was then reoxidized at 650 C. for periods varying between 48 hours to 160 hours. Under these conditions the resistivities, as desired, are achieved between 223 to 3000 ohms per square.

What has been described are resistive metal glazes having outstanding electrical characteristics which are reproducible at commercially acceptable yields. These metal glaze resistors are characterized by a microstructure having a metal oxide and metal phase dispersed in a glass matrix. Through the regulation of the crystallite size, of the added constituents, wide range of electrical characteristics are-available with given compositional systems. In addition, these results are available through processing procedures that maintain the required compositional balance between metal and metal oxide phases in the dispersed glass matrix.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.

What is claimed is:

1. A metal glaze resistive composition adapted to be applied to and fired on a ceramic dielectric characterized by a microstructure having a dispersion of metal oxide and metal in a glass matrix where the initial reaction mix for said metal glaze resistive composition includes:

35% to 70% by weight of vitreous enamel frit;

15% to 70% by Weight of particles comprising agglomerates of crystallites of a metal oxide selected from the group consisting of palladium and rhodium oxide where the metal oxide has a crystallite size such that the surface area is at least 0.75 m. /gm.; and

up to 35% by weight of a conductive component selected from the group consisting of silver, gold, and platinum where the cationratio of said conductive component to said metal oxide component lies between 0.5 to 1.5.

2. A metal glaze resistive composition adapted to be applied to and fired on a ceramic dielectric characterized by a microstructure having a dispersion of metal oxide and metal in a glass matrix where the initial reaction mix for said metal glaze resistive composition includes:

40% to 60% by weight of vitreous enamel frit;

18% to 60% by weight of particles comprising agglomerates of crystallites of a metal oxide selected from the group consisting of palladium and rhodium oxide where the metal oxide has a crystallite size such that the surface area is at least 0.75 m. /gm.; and

up to 22% by weight of a conductive component selected from the group consisting of silver, gold, and platinum where the cation ratio of said conductive component to said metal oxide component lies between 0.9 to 1.1.

3. A metal glaze resistive composition adapted to be applied to and fired on a ceramic dielectric characterized by a microstructure having a dispersion of metal oxide and metal in a glass matrix where the initial reaction mix for said metal glaze resistive composition includes:

35 to 70% by weight of vitreous enamel frit;

up to 5% by Weight of a dispersing agent selected from the group consisting of the oxides of silicon and aluminum;

to 70% by weight of particles comprising agglomerates of crystallites of a metal oxide selected from the group consisting of palladium and rhodium oxide where said oxide constituent has a crystallite size such that the surface area is at least 0.75 m. /gm.; and

up to 35 by weight of a conductive component selected from the group consisting of silver, gold and platinum where the cation ratio of conductive component to said metal oxide component lies between 0.5 to 1.5.

4. A metal glaze resistive composition adapted to be 10 applied to and fired on a ceramic dielectric characterized by a microstructure having a dispersion of metal oxide and metal in a glass matrix where the initial reaction mix for said metal glaze resistive composition includes:

40% to 60% by weight of vitreous enamel frit;

1.6% to 4% by weight of a dispersing agent selected from the group consisting of the oxides of silicon and aluminum;

18% to 60% by weight of particles comprising agglomerates of crystallites of a metal oxide selected from the group consisting of palladium and rhodium oxide where said oxide constituent has a crystallite size such that the surface area is at least 0.75 m. /gm.; and,

up to 22% by weight of a conductive component selected from the group consisting of silver, gold and platinum where the cation ratio of'conductive component to said oxide component lies between 0.9 to 1.

5. An electrical resistor comprising a ceramic dielectric containing on the surface thereof a metal glaze resistive element characterized by a microstructure having a dispersion of metal oxide and metal in a glass matrix where the initial reaction mix for said metal glaze resistive composition includes:

35% to 70% by weight of vitreous enamel frit;

up to 5% by weight of a dispersing agent selected from the group consisting of the oxides of silicon and aluminum;

15% to 7 0% by weight of particles comprising agglomerates of crystallites of a metal oxide selected from the group consisting of palladium and rhodium oxide where said oxide constituent is formed by sintering a member selected from the group consisting of palladium and rhodium crystallites having a size up to 1500 Angstroms; and,

up to 35% by weight of a conductive component selected from the group consisting of silver, gold and platinum where the cation ratio of said conductive component to said oxide component lies between 0.5 to 1.5.

6. An electrical resistor comprising a ceramic containing on the surface thereof a metal glaze resistive element characterized by a microstructure having a dispersion of metal oxide and metal in a glass matrix where the initial reaction mix for said metal glaze resistive composition includes:

40% to 60% by weight of vitreous enamel frit;

1.6% to 4% by weight of a dispersing agent selected from the group consisting of the oxides of silicon and aluminum;

18% to 60% by weight of particles comprising agglomerates of crystallites of a metal oxide selected from the group consisting of palladium and rhodium oxide where said oxide constituent is formed by sintering in an oxidizing atmosphere a member selected from the group consisting of palladium and rhodium crystallites having a size up to 1500 Angstroms; and,

0 to 22% by weight of a conductive component selected from the group consisting of silver, gold and platinum where the cation ratio of conductive component to said oxide component lies between 0.9 to 1.

7. A method for making an electrical resistor comprising the steps of:

heating a metal selected from the group consisting of palladium and rhodium in a nonoxidizing atmosphere to form metal crystallites with size up to 1500 Angstroms;

sintering said metal crystallites in an oxidizing atmosphere to form substantially pure metal oxide particles comprising agglomerates of crystallites such that the surface area is at least 0.75 m. /gm.;

forming a reaction mixture with said metal oxide crystallites, said reaction mixture consisting essentially of 35% to 70% by weight of vitreous enamel frit, 15 to 70% by weight of said metal oxide and up to 35% by weight of a conductive component se- 11 lected from the group consisting of silver, gold and platinum; applying said reaction mixture to a ceramic surface;

and,

heating said reaction mixture to a temperature between about 750 C. to 790 C. to form a metal glaze resistive element on said ceramic surface characterized by a microstructure having a dispersion of metal oxide and metal in a glass matrix.

8. The method of claim 7 wherein said reaction mixture additionally contains a dispersing agent selected from the group consisting of the oxides of silicon and aluminum.

9. The method of claim 8 including the step of cooling said reaction mixture to room temperature to form an adherent metal-glaze resistive .elementon a ceramic surface.

v 10. The method of claim 9 wherein said reaction mixture consists essentially of 40% to 60% by weight of vitreous enamel frit, up to 5% by Weight of said dispersing agent, and 18% to 60% by weight of said metal oxide.

11. A method for making an electrical resistor comprising the steps of:

heating a metal selected from the groupconsisting of palladium and rhodium, in a nonoxidizing atmosphere to form metal crystallites with size up to 1500 Angstroms;

sintering said metal crystallites in an oxidizing atmosphere to form substantially pure metal oxide parti- 12 -cles comprising agglomerates of crystallites such that the surface area is at least 0.75 mF/gms, forming a reaction mixture with said metal oxide crystallites, said reaction mixture consisting essentially of to-% by weight of vitreous enamel frit, I 15% to 70% by weight of said metal oxide and up to 35% by weight of a conductive component selected from the group consisting of silver, gold and platinu-m; applying said reaction mixture to a ceramic surface; firing said reaction mixture on said ceramic surface to a temperature of about 850 C. in an oxidizing atmosphere; a thereafter reoxidizing said fired reaction mixture at a temperature in the range between 450 C. to 700 C.; and, I

cooling said mixture to form an adherent metal glaze resistive element on a ceramic surface.

References Cited UNITED STATES PATENTS 3,052,573 9/1942 Dumesnil 117-227 X 25 RALPH s. KENDALL, Primary Examiner.

ALFRED L. LEAVITT, Examiner.

E. B. LIPSCOMB III, Assistant Examiner.

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