US3502597A - Method of improving the electrical conductivity of sintered tin oxide electrodes - Google Patents

Description

March 24, 1970 OHM-,CMS.

E. A. BUSH METHOD OF IMPROVING THE ELECTRICAL CONDUCTIVITY 0F SINTERED TIN OXIDE ELECTRODES Filed June 28, 1967 CURVE-Y TEMPERATURE c INVENTOR.

EDWARD A. BUSH wmnmx A T TORNE Y 3,502,597 METHOD OF IMPROVING THE ELECTRICAL CONDUCTIVITY OF SINTERED TIN OXIDE ELECTRODES Edward A. Bush, Painted Post, N.Y., assignor, by mesne assignments, to Corhart Refractories Company, a corporation of Delaware Filed June 28, 1967, Ser. No. 649,671 Int. Cl. H01b 1/08, 5/00; C01g 19/02 US. Cl. 252518 '15 Claims ABSTRACT OF THE DISCLOSURE The electrical volume resistivity of sintered tin oxide electrodes for glass melting applications is greatly reduced United States Patent by cooling the heated electrode from about 1200 C. to I about 900 C. in an inert, nonoxidizing, nonreducing atmosphere including vacuum. Reduction of resistivity from 10 ohm-cm. to 10- or 10- ohm-cm. is possible. Electrodes may be treated after manufacture or method may form a step in manufacturing process.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to sintered bodies consisting principally of tin oxide, which bodies are particularly useful as electrodes in glass melting tanks. It further relates to a method of manufacture whereby tin oxide electrodes of low electrical volume resistivity are produced.

Tin oxide, because of its extreme resistance to the corrosive action of molten glass and its high electrical conductivity at glass melting temperatures, is an excellent material from which to fabricate electrodes for the use in question. Furthermore, as is known, tin oxide imparts no color to the glass, which fact increases its value as an electrode raw material.

In the manufacture of electrodes, finely divided tin oxide is intimately blended with a small amount of mineralizers or fiuxing aids and with an additional small amount of a material effective to enhance the electrical conductivity of the ultimate product. Green bodies are formed from this mixture in known manner, as by dry pressing, and are fired at a temperature and for a time effective to give a fired body of desired density and adequate strength. After firing, which is usually carried out at a temperature of at least 1400 C., the fried bodies are allowed to cool in the furnace at the cooling rate of the furnace. For electrode use, the bodies produced are generally either square or rectangular in cross section and are finally machined to insure desired dimension and further to insure that opposing faces of the article are parallel to each other.

As above stated, the electrical conductance of such electrodes is desirably high at glass melting temperatures. However, the conductivity value is observed to drop significantly when it is measured at a temperature below about 800 C. That is, as the fact would more usually be stated, the resistivity of the electrode increases significantly as the temperature of the electrodedrops below the stated temperature.

It is immediately apparent that, in use, at least a portion of the electrode is below this temperature. Since the electrode extends through a wall of the glass melting tank there exists a temperature gradient in the electrode between about 1300 C., a representative glass-melting temperature, and a temperature greater than ambient, but less than about 500 C. The external portion of the electrode will be at a temperature higher than ambient because of heat conduction and because of the passage of electrical current through this portion of relatively high resistivity.

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It is obviously desirable from the point of view of eflicient use of electric power in glass melting, that the glass tank electrode exhibit as low a value of resistivity as possible.

The manner of installation of the electrode in the tank wall also dictates that it exhibit minimum resistivity. Commonly, electrodes are installed in clusters. Relatively long electrodes, referred to here for convenience as master electrodes, pass through the tank wall so as to project a short distance, about one half inch, beyond the face of the redundant refractory lining within the tank. A number of these master electrodes, representatively five or six, are installed in side-to-side contiguous relationship. The projecting outer ends of these electrodes are each connected with a source of electric power. The other, or auxiliary electrodes, are generally about one-half inch longer than the thickness of the glass-contacting refractory lining of the tank. These are installed so that each of them is in contiguous relationship with a master electrode from the glass-contacting end to the outer boundary of the refractory lining; ordinarily the auxiliary electrodes are in contact with the top face of the master electrodes. Itis, of course, evident that these auxiliary electrodes are also in a contiguous, side-to-side relationship with each other.

The area of the face of the installed electrode is thus the sum of the cross-sectional area of all of both the master and the auxiliary electrodes. It is evident that achieving the desired substantially uniform current density across the face of the installed electrode will, in part, depend on the electrode components displaying relatively low resistivity, since only half of the electrodes are directly connected to a power source.

While the prior art has, in some measure, brought about improvements in the electrical conductivity of tin oxide electrodes by means of the inclusion therein of minor amounts of various additives, significant further improvement is obviously desirable. Furthermore, production of these electrodes in the past has been characteriztd by extreme variability in resistivity. This has made it requisite carefully match electrodes with electrical characteristics as nearly similar as possible before installation. Finally, it has not heretofore been possible to improve the conductivity of a given electrode once it has been manufactured.

Description of the prior art United States Patent 2,244,777 describes glass-com tacting sintered refractory bodies of tin oxides. Various metallic compounds, such as oxides of manganese, iron, copper, nickel and cobalt are disclosed as mineralizers or densifying agents. The electrical properties of these bodies are not mentioned.

In United [States Patent 2,467,144 the electrical conductivity of the above refractory bodies is recognized, as is the use of such bodies, modified by the inclusion of a small amount of uranium oxide, as electrodes for glass melting applications.

United States Patent 2,256,033 relates only to a method of introducing mineralizing materials into a lightly sintered tin oxide body.

The incorporation of a small amount of an oxide of either antimony or bismuth in a sintered stannic oxide refractory containing a sintering aid and uranium oxide is shown, in United States Patent 2,490,826, to be effective in lowering the resistivity of such refractory bodies, particularly at a temperature of about 500 C.

United States Patent 2,490,825 discloses oxides of tantalum, bismuth, arsenic and antimony as resistivity-reducing agents in sintered stannic oxide bodies containing mineralizer but not uranium oxide.

United States Patent 2,585,341 relates to a method of producing certain of the refractory bodies of the patent mentioned immediately above.

Finally United States Patent 3,287,284 discloses the value of zinc oxide as a resistivity-lowering agent in sintered tin oxide bodies containingncopper an antimony oxides.

As evidenced by these patents, his usual to employ the mineralizing or densifying agent in an amount of from about 0.5 toabout 2.0 weight per-eent and the resistivitylowering agent in an amount of from about 0.5 to about 5.0 weight percent of the total of tin oxide and the said agents.

None of the prior art patents suggests the method of lowering electrical resistivities of sintered stannic oxide bodies to extremely low values by cooling them through a certain temperature range in an inert, nonoxidizing, nonreducing atmosphere. Where mentioned at all, atmosphere is discussed only in reference to firing of the green bodies? An oxidizing atmosphere is indicated; the atmosphere obtained during cooling is nowhere specifically mentioned. In each of these prior art patents however, it is apparent that the cooling of the electrode was carried out in air.

SILJMMARY OF THE INVENTION .The present invention provides a method for the production of tin oxide electrodes of markedly'reduced resistivity relative to prior art products. It further provides a means whereby an already formed electrode of high resistivity may easily and conveniently be transformed into one of greatly enhanced conductivity,Additionally, electrodes produced by the method of the present invention display a narrow range in the variability of electrical resistivity between individual electrodes.

It has been found that if, in the manufacture of sintered tin oxide electrodes as above described, the fired body is allowed to cool in an, inert atmosphere, it will have a resistivity smaller by several orders of magnitude than that of an equivalent body cooled in air. More specifically, this effect is noteckif the cooling to a temperature of about 900 C. is so conducted. Furthermore, it has been found that if an electrode of high resistivity manufactured in conventiopal manner be heated to about 1200 C. and cooled from that temperature to about 900 C. in an inert atmosphere, its. resistivity will be reduced by several orders of magnitude.

While the term inert atmosphere include an atmosphere of the so-cailed noble gases, such as helium, it is tain the thus established atmosphere 'until the electrode is removed from the furnace.

DESCRIPTION oF THEVIDRAWIVVNG The accompanying drawing indicates graphically the effect of treating sintered tin oxide electrodes according to the method of the present invention. Curve X represents the volume resistivity of a commercial tin oxide electrode versus the temperature at which the resistivity was measured. Curve Y represents the resistivity fversus temperature of the same electrode after treatment according tothe present method. The commercial electrode was heated in air to 1200 C. When this temperafiure was reached a nitrogen atmosphere'was provided. The electrode was maintained at the stated temperature for 2 hours. It was then allowed to cool at the furnace rate to room temperature, the nitrogen atmosphere behig main-. tained during cooling. As expected, the curves are substantially convergent at the upper end of the tempera- DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the invention is illustrated by the following nonlimiting examples.

Example 1 Samples were cut from commercial sintered tin oxide electrodes known to contain, per 100 parts of stannic oxide, 0.5 part of a minerali zer, CuO, and 1.0 part of an agent to lower the electrical resistivity, Sb O The samples were heated to 1200 C. in a furnace and maintained at that temperature for four hours, an inert atmosphere having been provided when the temperature of 1200 C. was reached. The furnace was then shut down and the atmosphere provided was maintained until the furnace and contained sample had cooled to ambient temperature. The resistivity of the samples was then determined; the results: are set out in Table I in which the various inert atmospheres are noted ROOM lEMPERATURE RESISTIVITIES OF ELECTRODE SAMPLES AFTER HEATING IN THE INDICATED ATMOSPHERES AT 1,200 0. FOR 4 HOURS, FOLLOWED BY COOLING IN THE SAME ATMOSEHERES [Electrical resistivity robin-0mg} Electrode Initial a Air Oxygen Nitrogen C 0 g Argon Vacuum 2. 8x10 9. 8X10 1 7x10 6X10 1. 3X10 2. 4 10- 5. 2X10 3. 8x10 1 9X10 5. 6X10 7. 1X10- 9. 3))(10- 8. 0X10- 2. 2X10- 1. 0X16 1. 4X10 1. 8 10- 3. 0X10- 1. 2X10- 1 3X10- not limited thereto. The atmosphere must, of course, be nonoxidizing if the desired lowering of resistivity is to be achieved. Furthermore, to avoid reaction with the tin oxide to form tin metal it must be nonreducing. A nonreducing atmosphere is also required for the reason that in a reducing atmosphere tin oxide sublimes quite readily at elevated temperature. Finally, it must be incapable of reacting with the tin oxide any other fashion.

Advantageously, the inert atmosphere may be a vacaum. However, it is generally more convenient to employ a gaseous atmosphere. The so-called inert or noble gases themselves may be employed. Nitrogen and carbon dioxide are especially suitable, particularly in view of the safety of their use and their low cost.

While, as noted above, the cooling fired body need only be in the inert atmosphere during the cooling range of from about 1200 C. toabout 900 C., it may be convenient to introduce the gas to be employed or to evacuate the furnace when the firing is complete-d and to main- It is to be noted that neither an air nor an oxygen atmosphere is effective to produce the results afforded by the employment of the inert atmospheres listed above. In fact, in the case 'of the sample of commercial electrode 3, which initially displayed a relatively low resistivity, cooling in air or oxygen had the effect of markedly raising the resistivity. However, cooling in the listed inert atmospheres improved the electrical resistivity of this sample by an order of magnitude. in the case of the electrode samples of initially very high resistivities, cooling in each of the inert atmospheres effected a reduction of resistivity values amounting to several orders of magnitude.

' Example 2 A test piece was cut from an electrode of the type described in Example 1. Its room temperature resistivity was determined to be 2.8 l0 ohm-cm. It was then heated in a furnace to 1200" C. and maintained at that temperature for four hours. The furnace was shut down and a nitrogen atmosphere was provided and maintained until the sample, cooled to ambient temperature, was removed. The resistivity was again determined; a value of 3.6 lohm-cm. was obtained.

The sample was then heated in air to 500 C., maintained at that temperature for four hours, cooled rapidly in air and its room temperature resistivity measured. The resistivity was substantially unchanged from the value noted after the above described cooling in a nitrogen atmosphere. This procedure was repeated at increasing increments of temperature of about 100 C. Essentially no change in resistivity from the low value of 3.6 10 ohm-cm. until the heating was carried out at above about 900 C. The sample, after being heated in air for four hours at this temperature and cooled to room temperature, had a resistivity somewhat higher than the initial value of 3.6 10- ohm-cm. When the heating was carried out at 1000 C., the room temperature resistivity was found to be 3.9 10 ohm-cm, an increase of five orders of magnitude.

Example 3 Several test pieces were cut from an electrode of the type described in the preceding examples. The resistivity of each was found to be about 2.8 10 ohm-cm.

A test piece was heated to 800 C. When this temperature was reached, a stream of nitrogen was introduced into the furnace. The attained temperature was maintained for four hours, after which the furnace was shut down and allowed to cool to room temperature at its own rate. When the temperature of test piece reached about 300 C. the flow of nitrogen through the furnace was stopped. Room temperature resistivity of the test piece was found to be only about one order of magnitude lower than th initial value.

This procedure was repeated using the remaining test pieces at temperatures progressively raised by increments of 100 C. The resistivities of the cooled test pieces decreased in an essentially linear relationship to increasing temperature. Thus a resistivity of about 1 ohm-cm. was achieved when the heating of the sample was conducted at 1000 C.; when the temperature was 1200 C. a value of 2.8 10- ohm-cm. was obtained.

From the above, viewed together with the results of Example 2, it is evident that, in cooling a tin oxide electrode from, for example, 1475 C. a conventional sintering temperature, to room temperature, it is necessary to provide an inert atmosphere only in the range of from about 1200 C. to about 900 C. Moreover, it is also apparent, from a consideration of the above mentioned linear relationship, that the resistivity of an electrode may be predetermined.

Example 4 Four commercial tin oxide electrodes measuring 2" x 4" x 12" were heated in air to 1200 C. at a heating rate of 45 C./hour in a furnace having a volume of about cu. ft. When the temperature of 1200 C. was reached, nitrogen was introduced to the furnace at the rate of 80 cu. ft. per hour. This temperature was maintained for four hours. The furnace was then shut down and allowed to cool at its own rate. Nitrogen input was stopped when the temperature of the electrodes reached about 650 C. The resistivity of each electrode, both before and after the above treatment, is compared in Table II below.

TABLE II.-ELECTRICAL RESISTIVITY (OHMCM.) OF ELECTRODES COOLED IN NITROGEN Electrode Initial After treatment 1.69X 6. 31x10- 3. 30X10 3. 02X 10' 5. 08 10 2. 24x10- 1. 86 10 9. 55 10- As is apparent, the method of the invention was effective to reduce the resistivities of the electrodes by 5 orders of magnitude to extremely low values.

Example 5 In the above examples, all operations were carried out on preformed electrodes or samples thereof. However, the method of the invention is most advantageously integrated with the process of electrode manufacture inasmuch as a cooling step is a necessary part of the manufacture of the sintered electrode. In other words, it is immaterial Whether a formed electrode is reheated in order to be cooled according to the present method or if the cooling step in manufacture is modified by the method of the invention. Thus, green bodies of stannic oxide are formed containing small amounts of mineralizer and resistivity modifier. These are fired in conventional fashion at a temperature in the range of from about 1400 C. to about 1500" C. for a time sufficient to provide a sintered electrode of desired high density and adequate strength. Heating is discontinued and the furnace allowed to cool to room temperature at its own rate. When the temperature of the cooling electrode body has reached about 1200 C., the furnace is evacuated to a pressure of about 5 microns of mercury. Alternatively, at this temperature the furnace may be filled with a desired inert gas such as carbon dioxide. Cooling is continued and the chosen atmosphere is maintained. In the case of an inert gas atmosphere, this is accomplished by continuously admitting gas to the furnace chamber. When the fired electrodes have cooled to about 900 C., the gas flow is halted and cooling continues to room temperature. In the case of a vacuum atmosphere, vacuum maintaining pumping is stopped and air allowed to enter the chamber. In either case, the electrodes thus produced uniformly exhibit extremely low resistivities.

It is, of course, possible to provide the inert atmosphere at the time the furnace is shut down, at which time the fired body is at a temperature of between 1400 and 1500 C. and to maintain the atmosphere provided until the fired body has cooled to at least 900 C. or, if desired, to ambient temperature.

I claim:

1. A method of treating an electrically conductive sintered tin oxide body whereby the electrical volume resistivity of the body is substantially reduced comprising cooling said body, from the temperature at which it was sintered, in an inert, nonoxidizing, nonreducing atmosphere.

2. The method according to claim 1 in which the cooling sintered tin oxide body is maintained in the said inert, nonoxidizing, nonreducing atmosphere until it has cooled to a temperature of about 900 C.

3. The method according to claim 1 in which the temperature at which the said tin oxide body was sintered is from about 1400 C. to about 1500 C.

4. A method according to claim 2 in which the temperature at which the said tin oxide body was sintered is from about 1400 C. to about 1500 C.

5. The method of claim 1 in which the inert, nonoxidizing nonreducing atmosphere is a member of the group consisting of nitrogen, carbon dioxide and a noble gas and vacuum.

6. The method of claim 2 in which the inert, nonoxidizing, nonreducing atmosphere is a member of the group consisting of nitrogen, carbon dioxide, a noble gas and vacuum.

7. The method of claim 3 in which the inert, nonoxidizing, nonreducing atmosphere is a member of the group consisting of nitrogen, carbon dioxide, a noble gas and vacuum.

8. The method of claim 4 in which the inert, nonoxidizing, nonreducing atmosphere is a member of the group consisting of nitrogen, carbon dioxide, a noble gas and vacuum.

9. A method of treating an electrically conductive tin oxide body whereby the electrical volume resistivity of the body is substantially reduced comprising heating said body to a temperature of at least about 1200 C. and

cooling the said body in an inert, nonoxidizing, nonreducing atmosphere to a temperature no greater than about 900 C.

10. The method of claim 9 in which the inert, nonoxidizing, nonreducing atmosphere is a member of the group consisting of nitrogen, carbon dioxide, a noble gas and vacuum.

11. The method of claim 10 in which the sintered tin oxide body contains a small but effective amount of a mineralizer and a small but effective amount of a resistivity-lowering agent.

12. The method of claim 11 in Which the mineralizer is copper oxide and the resistivity-lowering agent is antimony oxide.

13. The method of claim 12 in which the sintered tin oxide body contains, per 100 parts of stannic oxide, about 0.5 part of copper oxide and about 1.0 part of antimony oxide.

14. An electrically conductive body produced by the method of claim 12.

15. An electrically conductive body produced by the method of claim 13. 7

References Cited UNITED STATES PATENTS 6/1949 Baxter 23144 2,474,645 2,585,341 2/1952 Mochel 23144 10 2,815,267 12/1957 Platteeuny 23l44 DOUGLAS I. DRUMMOND, Primary Examiner US. 01. X.R.

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