Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
  • H01J9/02—Manufacture of electrodes or electrode systems
  • H01J9/04—Manufacture of electrodes or electrode systems of thermionic cathodes
  • H01J9/042—Manufacture, activation of the emissive part

Definitions

• the present invention relates to the art of electron emissive coating materials and methods of application, and particularly to an electron emissive coating material having a novel composition for advantageous application to a base by cataphoresis, and to an improved cataphoretic coating method.
• a suspension useful in a cataphoretic coating method, of electron emissive materials containing a binder having the advantages of (1) stability of the suspension, (2) preservation of the normal positive charge on the coating material particles during coating, (3) providing a coating having desirable adherence to withstand normal handling without coating loss, (4) providing a coating that is easily removed from surfaces to be free of the coating, (5) complete elimination of the binder before or during exhaust of an electron tube in which a cathode coated according to the invention is used, (6) and complete coating weight control from .2 to 10 mgs./cm.
• the resin according to the invention will not only tend to stabilize the suspension and maintain the normal positive charge of the particles during coating by cataphoresis, but will produce 1) the correct type of adherence necessary for all handling of the filament without preventing an easy removal of the coating from those surfaces which must be free of coating, (2) satisfactory tube characteristics due to the complete elimination of the resin before or during exhaust, and (3) complete coating weight control from .2 to mgs./cm. over a very wide voltage range.
• suspensions containing polymeric methyl methacrylate are used in a cataphoretic coating method, coating starts as soon as voltage is applied and increases in a uniform manner as the voltage is increased.
• a coating suspension is prepared as follows:
• SUSPENDING MEDIUM Measure out 3,000 milliliters of acetone and 105 milliliters of dibutyl phthalate and pour into a glass bottle of 1 gallon capacity.
• CARBONATES Weigh out 525 grams of barium, strontium, and calcium carbonates in the relative amounts by weight of 57% barium carbonate, 39% strontium carbonate, and 4% calcium carbonate. Instead of mixing separate carbonates, it is feasible to coprecipitate the carbonates to form a single carbonate containing the same relative amount of elements indicated above. Where the carbonates referred to are prepared by the interaction of barium, strontium and calcium nitrates and sodium carbonate, they usually contain a small amount of sodium nitrate which is a soluble ionizable salt.
• the carbonates are prepared in other ways, leaving no residue of soluble ionizable salts such as sodium nitrate, such salts should be added to the carbonates referred to in the amount of .05% by weight for the purposes of the instant example.
• the carbonates including the soluble ionizable salt or salts referred to are heat treated for two hours at from 130 to 150 C.
• PREPARATION OF COATING SUSPENSION Place the 525 grams of the alkaline earth carbonates and the sodium nitrate into a clean dry porcelain ball mill jar of 1.25 gals. capacity and containing 2500 grams of porcelain balls 4 in diameter. Add 1000 ml. of the suspending medium, close the jar and rotate at aspee'd of 60-70 R. P. M. for a period of hours.
• Example 3 Coating Coating Wt., Current, Mgs. per Coat ng Voltage Ma. 200 mm length
• Example 4 The negative influence on the efiiciency of the applied coating by the addition of approximately .2% by weight of calcium nitrate to the described suspension is shown 55 by the following data:
• One process of manufacture of the carbonate powder aforementioned comprises precipitating a barium, strontium, calcium, nitrate solution with a solution of sodium carbonate. This reaction produces an amount of sodium nitrate, which is a soluble ionizable salt, small amounts of which remain in the resultant carbonate powder after washing. The final amount of such sodium nitrate depends upon the efliciency of the washing procedure following the precipitation. Normally, the sodium nitrate content of a carefully made and thoroughly washed carbonate precipitate, will be around .05% by weight of the carbonate powder.
• the amount of soluble ionizable salt in the carbonate powder should be confined within the range of from .01% to 0.5% by weight of the powder;
• the purposes served by the soluble ionizable salt according to the invention are to control the dispersion of the solids, i. e., the carbonate particles, in the suspension; and to provide desired charges on the particles required for a cataphoretic application thereof.
• the aforementioned range of salt content is critical for accomplishing these purposes.
• the lower limit of the range according to the invention dispersion is at a maximum which may result in quality defects, such as bare spots or uneven coating. If the amount of soluble ionizable salt is above .5 it induces excessive agglomeration of the coating particles with substantial loss in coating efficiency.
• sodium nitrate remains'in the carbonate powder from the process of manufacture aforementioned, the maximum amount being about .05
• the final amount of sodium nitrate may be appreciably less that .05% and in fact below .01%, the lower limit of the acceptable range according to the invention as stated before herein.
• the methacrylate content of the suspension may be varied for filaments having different diameters or cross-sections, to provide the bonding requirements of a particular filament. However, it should not be included in amount greater than grams per 100 milliliters of the suspension.
• the lower limit of methacrylate content according to the invention is four grams per 100 milliliters of suspension.
• the methacrylate content is increased from 4 grams to 10 grams per 100 milliliters of suspension, the bonding effect of an applied coating is, of course, also increased. Applicant has found that when the methacrylate is present in an amount of 10 grams per 100 milliliters of suspension, a coating made cataphoretically from the suspension has the bonding properties necessary for any filament application.
• the methyl ester has a combination of properties superior to others in the group. For instance, the hardness of the polymeric methyl ester is greatest, and therefore, by the introduction of a compatible solvent of low vapor pressure such as dibutyl phthalate in an amount from 0 to 50% by weight of the methacrylate, this hardness may be controlled. Also, on depolymerization the methyl ester produces the monomeric ester having a relatively low vapor pressure, which, in turn, is removed from the tube or tube parts at a relatively low temperature during tube processing.
• a compatible solvent of low vapor pressure such as dibutyl phthalate
• the coating suspension was contained in a single coating tank.
• the filament to be coated was continuously passed through the suspension in the tank over suitable pulleys at a speed of five meters per minute. Suitable connection of the filament to a D. C. electrical source of negative polarity of 0300 volts was effected.
• An electrode was extended into the suspension serving as an anode. The voltages and currents indicated in the aforementioned examples were across the anode and filament referred to through the suspension.
• Example 3 a ribbon filament about .040 inch wide was coated under conditions differing from those of EX- ample 1, only with respect to applied voltage. It will be noted that with a voltage of 25 volts a coating Weight of 11.5 milligrams was applied per 200 millimeters of ribbon length. This length, of course, provided a much larger arear than the similar length aforementioned of the filament having a diameter of .001 inch. As the voltage was increased -to volts, the coating weight increased to a value of 32.0 milligrams per 200 millimeters of ribbon length.
• the coating Weight increases substantially linearly with voltage increase.
• Examples 2 and 4 show the effect of adding more soluble ionizable salts to the suspension used in Examples 1 and 3, which included about .05% of such salts by weight of the carbonate.
• the conditions in Example 2 are the same as in Example 1, except that .05% more salt is added to the suspension, so that the total amount of ionizable salts therein is .10%. While coating weight control by the applied voltage s still feasible, agglomeration of the particles in the suspension has proceeded to such a degree that the coating efficiency of the suspension has been affected and higher voltages are required for the same weight.
• Example 4 the only change from the conditions in Example 3 is the addition of a further .2% of ionizable salts. This makes the total amount of such salts in Example 4, 25% by weight of the carbonate in the suspension. Such increase in the salt content, it will be noted, requires an appreciable increase in the voltage to produce the same coating weight.
• the speed of the base to be coated through the coating suspension was meters per minute. This speed is preferred when ribbons such as were used in Examples 3 and 4 or larger filaments, are to be coated. A speed of meters per minute is preferred for fine filaments such as were coated in Examples 1 and 2.
• Feeding the base at a lower rate than 5 meters per minute produces no quality disadvantage but reduces manufacturing efiiciency.
• the maximum speed of coating will be that speed at which the required coating Weight and coating quality are obtained and will vary with filament of different size and shape.
• Adjacent the tank referred to was disposed a furnace heated, for example, by resistance elements, and having a passageway through which the coated filament after emerging from the suspension in the tank, was adapted to pass.
• the function of the furnace was to vaporize and drive off volatile components of the suspension, such as the acetone used therein.
• the coated filament was dry and permitted practical handling incidental to further processing, without loss of or harm to thecoating.
• incidental handling involved cutting the filament to lengths and the application of pressure as by impact means to portions of the lengths to remove'the coating cleanly for welding to tube elements such as leads.
• novel coating suspension of the invention also permits an accurate control of coating weight by the voltage applied, which was impossible to accomplish heretofore. This control is efficiently feasible because of the presence of a predetermined amount of a soluble ionizable salt in the suspenslon.
• Both the methacrylate and the ionizable salt used in the coating suspension contribute to an improved method of manufacture of electron emissive filamentary cathodes and provide cathodes characterized by superior operatlon in an electron tube.
• a coating suspension of electron emitting material adapted for application to a base having a negative polarity, said suspension comprising a polymeric ester of methacrylic acid, an ionizable salt, particles of said emissive material, and a polar solvent for said polymeric ester and said salt consisting of acetone, said polymeric ester in solution in said solvent contributing to the formation of positive charges on said particles for migration of said particles to said base during a coating operation, said polymeric ester converting to a monomeric phase with a vapor pressure approximately that of water at a first predetermined temperature used in the manufacture of electron tubes and a gaseous phase at a second predetermined temperature used in said manufacture, whereby said ester is adapted to be removed during the processing of a tube having an element thereof coated with said suspension and to preserve said tube from contamination.
• a coating suspension of electron emitting material adapted for advantageous use in a cataphoretic coating operation wherein the base to be coated is caused to have a negative polarity for reduced erosion of said base, said suspension comprising particles of emitting material, a binder consisting of a polymeric ester of methacrylic acid, an ionizable salt, and a polar solvent consisting of acetone in which said polymeric ester and said salt are in solution, said polymeric ester, salt and polar solvent contributing to the formation of a positive charge on said particles which persists when said suspension is disposed in an electric field produced between said base of negative polarity and an electrode of positive polarity during a coating operation, said particles being adapted to form a coating mass up to 10 milligrams per square centimeter, for providing a coating of improved bonding quality.
• a coating suspension of electron emissive material adapted to be applied by cataphoresis to provide a coating having a mass up to 10 milligrams per square centimeter and characterized by improved bonding to a base and involving: reduced erosion of the base, said suspension consisting of particles of said material, a polymeric ester of methacrylic acid, an ionizable salt, and a polar solvent in which said polymeric ester and said salt are dissolved, the amount of said polymeric ester being from four to ten grams in each milliliters of suspension, the amount of said salt being from 0.01 to 0.5% by weight of said particles, said particles being adapted to receive a positive charge persisting during a coating operation for producing said coating mass when a base having a negative polarity and an electrode having a positive polarity are disposed in said suspension.
• a coating suspension of electron emissive material adapted to be applied to a base by cataphoresis to provide a coating having a thickness to provide a mass thereof up to 10 milligrams per square centimeter, said suspension consisting of particles of said material, a polymeric ester of methacrylic acid, the amount of said polymeric ester in relation to the amount of said particles being by weight from to 350 grams of said ester, to 525 grams of said particles for providing improved bonding of said particles to said base, an ionizable salt, and a polar solvent in which said polymeric ester and said salt are dissolved, said particles being adapted to receive a positive charge persisting when an electric field is produced between the base having a negative polarity and an electrode having a positive polarity in said suspension, for causing said particles to migrate to said base with reduced erosion of said base to provide saidcoating mass.
• Method of coating a metal filament by cataphoresis withelectron emissive material to form a coating having a mass of from 0.2 to 10.0 milligrams per square centimeter comprising continuously passing successive portions of said filament through a suspension containing said emissive material, a polar solvent, a polymeric ester of methacrylic acidand an ionizable salt, and with said polymeric ester and said salt dissolved in said solvent, producing a voltage difference between said filament and an electrode in said suspension by causing said filament to have a negative polarity with respect to said electrode, whereby said particles have a positive charge and migrate to said filament to provide said coating mass, heating said portions after leaving said suspension to drive ofi said solvent, whereby said coating is characterized by improved adherence and is sufficiently brittle for mechanical removal from portions of said filament.
• Method of making a coated cathode having a coating mass of from 0.2 to 10 milligrams per square centimeter comprising passing the base stock of said cathode through a single bath containing particles of electron emissive material, a polymeric ester of methacrylic acid, an ionizable salt and a polar solvent in which said polymeric ester and said salt are dissolved and in which said particles are suspended, while impressing a negative polarity on said base stock and a positive polarity on an electrode in said bath, drying the coated base stock to drive 01f said solvent, severing said coated base stock to provide a coated cathode, mounting said cathode in an electron tube, and heating said cathode in said tube to a temperature to cause said polymeric ester to change first to a liquid phase, and then to a gaseous phase for ready removal from said tube of substantially all of said polymeric ester in the coating on said cathode.

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• Engineering & Computer Science (AREA)
• Manufacturing & Machinery (AREA)
• Paints Or Removers (AREA)

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Cited By (8)

Citations (5)

• 0
  • BE BE530668D patent/BE530668A/xx unknown
• 1953
  • 1953-07-27 US US370657A patent/US2800446A/en not_active Expired - Lifetime
• 1954
  • 1954-07-26 FR FR1110855D patent/FR1110855A/fr not_active Expired

Patent Citations (5)

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