Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C24/00—Coating starting from inorganic powder
• • 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
  • H01J9/08—Manufacture of heaters for indirectly-heated cathodes

Definitions

• Such heaters comprise a core wire of a refractory metal, such as tungsten, a first wire covering layer of an insulating material, such as aluminum oxide, and an outer dark coating such as a particulate mixture of tungsten and aluminum oxide.
• a purpose of the first coating is to provide insulation between the heater wire and the cathode, and a purpose of the outer coating is to increase'the thermal emissivity of theheater, thereby lowering the temperature at which the heater need operate to heat the cathode to its operating temperature.
• a heater outer coating consisting entirely of a conductive material, e.g., tungsten, significantly reduces the arcing problem regardless-of the type of contact between the heater outer coating and the cathode. This follows because the conductive coating and the cathode are at the same potential, whereby no potential gradient exists therebetween. While-a potential difference exists between the heater outer coating and the heater core wire, the first layer of insulating material provides a generally uniform spacing between the two, thereby avoiding the creation of non-uniform and excessively high potential gradients.
• a conductive material e.g., tungsten
• dip coating One method generally known for applying various coatings to heater structures is known as dip coating. This method involves dipping the heater to be coated into and out of a bath containing a suspension of the particularly high potential gradients frequently exist particles to be coated on the heater, the particles adhering to the heater as the emerging heater breaks through the surface of the bath. If the heater structure is in the form of a continuous wire or the-like which is drawn through the bath, the process is known as a drag coating process. The thickness of the coating is a function of the specific gravity and viscosity of the bath.
• a solution to this problem of penetration is the use of a bath in which the viscosity at the surface of the bathis and'remains' high during a reasonably long time adequate to perform the dipping operation.
• a bath which is substantially rheologically stable, i.e., one in which the rate of settling of the particles of the bath is so slow as to retain a high viscosity at the surface thereof.
• all that is required is a bath which is stable only during thetirrie it takes to dip a heater into and out of it, as a practical matter, it is preferred that the bath be so stable as to allow repeated use thereof over a period of hours without constant restirring and reconstitution.
• tungsten matcrial While it is possible to purchase on the market various grades of tungsten powder, the cost of the tungsten matcrial generally rapidly increases with decreasing particle size and particle size range. Thus, for reasons 6f cost, it is desirable to use tungsten material of large particle size for the coating bath. Conversely, for obtaining good control over the thickness of the coating applied to the heater, and providing good continuity of the applied coating for thin coats of tungsten, small particle size is desired. A satisfactory compromise is to use commercially-available tungsten predominantly in the particle size range of 0.5 to 10 microns, with the average particle size being about 2 microns.
• tungsten powder generally comprises relatively large agglomerates or tightly packed clusters of individual particles of tungsten.
• One means used in the past for deagglomerating the tungsten powder, in the fabrication, for example, of dark heaters comprising outer coatings of aluminum oxide mixed with tungsten particles, comprises dry or wet rolling the tungsten powder in a mill or the like with no milling media other than the tungsten itself. I discovered that in order to obtain an adequately stable bath, substantially complete deagglomeration of the tungsten powder is necessary, and that the prior art technique of rolling is totally inadequate.
• a relatively high shearing force must be applied to the individual tungsten particles.
• external agitation of adequate force can be applied by some suitable mechanical means, e.g., the commercially available Kady Mill sold by Kinetic Dispersion Corp., Buffalo, N.Y.
• material and solvent loss occurs during dispersion.
• ultrasonic vibration of the bath can be used. This, however, requires a rather large and expensive system, and vehicle breakdown occurs.
• a milling process employing, for example, a high density alumina milling jar with tungsten carbide milling media.
• tungsten carbide milling media Although not critical, I find that preferred results are achieved using relatively large media, e.g., 3/16 inch long, round ended cylinders.
• the use of such a milling operation is especially desirable since it causes no breakdown of the liquid vehicle and no adverse contamination of the bath, the materials of both the mill and the media being quite compatible with the heater materials.
• the proper duration of the milling operation for a given bath to provide complete dispersion of the tungsten particles can be determined by several means.
• the apparent specific gravity of the bath is less than the true specific gravity of the entire bath, as determined by dividing the weight of the entire bath by the volume of the bath.
• the reason for this is that the incompletely deagglomerated clusters very rapidly settle to the bottom of the bath and do not provide buoyancy for the hydrometer.
• the apparent specific gravity very nearly approaches, and often just about equals, the true specific gravity.
• the stability of the resulting suspension can be conveniently defined in terms of the change in the apparent specific gravity with time.
• a bath comprising (by weight) 40 percent tungsten and percent liquid vehicle, the vehicle comprising 1.68 percent 1,000.
• second nitrocellulose wet 30 percent by weight by alcohol and 98.5 percent butyl acetate after ball milling for about 6 hours, the change in apparent specific gravity of the suspension, owing to settling of the tungsten, is only in the order of 2 percent over a period of 8 hours.
• a rheologically stable bath is defined as one in which the change in apparent specific gravity of the bath is less that5 percent in 8 hours.
• the use of such a bath is especially desirable in automated dipping operations because the bath is so stable as to require little attention, i.e., stirring, during its useful life.
• the life of the bath is generally determined by the rate of evaporation of the liquid phase, such evaporation changing the coating characteristics of the bath. With the'above-described bath, the life is about 8 hours. Penetration of tungsten into the underlying layer of aluminum oxide is substantially avoided as long as the dipping operation is performed while the apparent specific gravity of the bath is still high, e.g., within 5 percent of the true specific gravity.
• Another method of determining when complete dispersion is reached is by monitoring the viscosity of the bath during the milling operation.
• Maximum dispersion of the particles corresponds to maximum viscosity of the bath. This follows because maximum dispersion of the particles within the liquid phase gives rise to maximum particle-to-particle interaction, i.e., maximum internal friction.
• penetration of the tungsten into the underlying layer of aluminum oxide is substantially avoided as long as the dipping operation is performed while the viscosity of the bath near the surface thereof is high, e.g., within 5 percent of the maximum bath viscosity. By near the surface is meant within a few mils of the surface.
• the degree of dispersion can also be measured using a commercially available fineness of grind meter," e.g., the Nipri Type G-l Meter, sold by Precision Gauge and Tool Company, Dayton, Ohio. Maximum dispersion of the tungsten particles is reached when further milling does not result in a finer aggregate particle size.
• the thickness of the tungsten coating applied to the heater in the dipping operation is a function of, among other things, the viscosity and specific gravity of the bath. Depending on the particular heater being made, different coating thicknesses are desired, hence different baths having'different viscosities and specific gravities are used.
• (b is the ratio of the volume of the tungsten material in the dry powder state to the sum of the volume of the dry tungsten powder plus the volume of the liquid phase.
• the following Table provides data concerning different baths for use in providing different thickness coatings of tungsten ina dipping operation.
• the invention has been described in connection with outer coatings consisting entirely of tungsten, the invention also has utility in providing heaters having outer coatings comprising tungsten and some other insulating material, generally aluminum oxide.
• outer coatings comprising tungsten and some other insulating material, generally aluminum oxide.
• tungsten in any of the baths listed in the aluminum table, a portion of the tungsten can be replaced with aluminum oxide, the same liquid vehicle being used.
• the bath is ball milled. using a tungsten carbide milling media, until maxium conditions of apparent specific gravity and viscosity are achieved.
• a method of providing a coating on a heater structure comprising:
• said bath consists essentially of, by weight, tungsten, in the range of 30 to 60 percent, said tungsten being provided as a powder having a particle size in the approximate range of 0.5 to 10 microns.
• a method of coating an aluminum oxide coated heater with an outer layer consisting essentially of tungsten comprising:
• tungsten in the range of 30 to 60 percent, and a liquid vehicle, in the range of 40 to. percent, said tungsten being provided as a powder having a particle size in the.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacturing & Machinery (AREA)
• Solid Thermionic Cathode (AREA)
• Powder Metallurgy (AREA)
• Compositions Of Oxide Ceramics (AREA)
• Electrolytic Production Of Metals (AREA)

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• 1972
  • 1972-04-07 US US00242240A patent/US3852105A/en not_active Expired - Lifetime
• 1973
  • 1973-03-07 CA CA165,430A patent/CA1014805A/en not_active Expired
  • 1973-04-04 GB GB1602173A patent/GB1418196A/en not_active Expired
  • 1973-04-06 FR FR7312564A patent/FR2179251B1/fr not_active Expired
  • 1973-04-06 JP JP3939673A patent/JPS5331591B2/ja not_active Expired
  • 1973-04-06 NL NL7304824A patent/NL7304824A/xx not_active Application Discontinuation

Patent Citations (5)

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