US3808043A

US3808043A - Method of fabricating a dark heater

Info

Publication number: US3808043A
Application number: US00257625A
Authority: US
Inventors: J Hale; G Merritt
Original assignee: RCA Corp
Current assignee: RCA Licensing Corp
Priority date: 1972-05-30
Filing date: 1972-05-30
Publication date: 1974-04-30
Anticipated expiration: 1991-04-30
Also published as: NL7307463A; CA1017208A; JPS4951868A; FR2186802B1; JPS5331592B2; FR2186802A1; IT987598B; GB1431571A; DE2326202A1; DE2326202B2

Abstract

A refractory metal heater wire is first coated in conventional fashion with a first coating of insulating material, e.g., aluminum oxide. The coating is then dried in air at a temperature below which oxidation of the heater wire occurs for a time sufficient to completely remove all solvents from the first coating. Then, a second coating of a darkening material is applied over the first coating. The complete removal of solvents from the first layer prior to the second layer application step reduces penetration of the material of the second layer into the first layer.

Description

United States Patent [1 1 Hale et al.

[ METHOD OF FABRICATING A DARK HEATER [75] Inventors: John Richard Hale, Lancaster, Pa.;

George Irvin Merritt, Clifton, NJ.

[52] US. Cl 117/217, ll7/227, 313/345 [51] Int. Cl B44d l/l8 [58] Field of Search 117/217, 29, 31, 227

[56] References Cited UNITED STATES PATENTS 6/1967 Scheible 117/217 9/1968 Feinleib 117/217 [451 Apr. 30, 1974 Primary Examiner-Cameron K. Weiffenbach Attorney, Agent, or Firm-G. l-l. Bruestle; L. Greenspan [57] ABSTRACT A refractory metal heater wire is first coated in conventional fashion with a first coating of insulating material, e.g., aluminum oxide. The coating is then dried in air at a temperature below which oxidation of the heater wire occurs for a time sufiicient to completely remove all solvents from the first coating. Then, a second coating of a darkening material is applied over the first coating. The complete removal of solvents from the first layer prior to the second layer application step reduces penetration of the material of the second layer into the first layer.

3 Claims, N0 Drawings METHOD OF FABRICATING A DARK HEATER BACKGROUND OF THE INVENTION This invention relates to electron discharge tubes, and particularly to a method of fabricating dark insulated heaters for such tubes.

In various types of electron tubes having indirectly heated cathodes, it is the practice to use dark heaters to heat the cathode to electron emitting temperatures. 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 the heater, thereby lowering the temperature at which the heater need operate to heat the cathode to'its operating temperature. The greater the proportion of tungsten in the outer coating, the darker the heater.

While the use of insulated heaters having an outer coating consisting entirely of tungsten has been suggested in the past (see, for example, US. Pat. No. 3,195,004 issued to Hassett on July 13, 1965), the use of such heaters has generally not been practical. One reason for this is that, in the past, as the percentage of tungsten in the outer coating was increased, the amount of current leakage between the heater and the cathode increased. The cause of such leakage is that particles of tungsten from the outer coating penetrate into the aluminum oxide undercoating and provide leakage paths for current through the undercoating. This is generally undesirable, and the practice in the past has beento limit the ratio of tungsten to aluminum oxide in the outer coating to some upper limit, e.g., in the orderof 40 percent.

A technique has recently been suggested (as described in co-pending US. Pat. application Ser. No. 242,240 filed Apr. 7, 1972 for John I-Iale) for fabricating heaters having outer coatings with a large percentage, ashigh as 100 percent, of tungsten while avoiding high levels of. leakage through the heater. Briefly, the technique comprises using a coating bath for the outer coating which isrheologically stable, i.e., which has a low settling rate of the particles therein. This results in a high viscosity bath whereby penetration of the bath into the undercoating is prevented.

In addition, to further prevent such penetration, it has, in the past, been thought necessary to render the undercoating as impervious to the overcoating bath as possible by means of a high temperature sintering process prior to performing the overcoating process. Disadvantages of the sintering process, however, are that expensive apparatus is generally required, the process tends to be time consuming and expensive, and, owing to the high temperatures involved, various fixtures, such as clips in which the heaters are mounted, have a short life requiring frequent maintenance and replacement. Also, to prevent oxidation of the heater wire, the sintering is done in a protective atmosphere, which adds further cost and complexity to the process.

DETAILED DESCRIPTION OF THE INVENTION One methodgenerally 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 an organic solvent and a suspension of the 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.

As previously noted, a problem in the past using dark heaters having large percentages of tungsten in the outer coating is that the tubes using the heaters generally have unacceptably high levels of heater to cathode leakage. This occurs because of penetration of particles of tungsten from the outer coating into the aluminum oxide undercoating.

We have discovered that probably the principal cause of penetration of the tungsten particles into the undercoating is the use of outer coating processes in which the viscosity of the coating bath is allowed to become too low. In such case, as described in the abovecited co-pending application, the highly fluid bath, including tungsten particles conveyed therewith, penetrates into the undercoating.

Although it is generally desirable that the undercoating be rendered relatively hard and impervious prior to the overcoating process, by a high temperature firing operation, for minimizing penetration of the overcoating bath into the undercoating, we have discovered that such a high temperature process is not necessary, and that penetration can be avoided even if the undercoating is in a non-sintered, relatively soft and porous condition during the overcoating process.

What is necessary, we discovered, is that the undercoating be substantially completely dry and free of all solvents used both in the undercoating process and in the usual rinsing step performed thereafter. That is, we discovered that the prevention of penetration of the overcoating bath into the undercoating is not so much a function of the hardness and imperviousness of the undercoating, as previously thought, but primarily a function of the degree of removal of the solvents from the undercoating. While not known for sure, it appears that the presence of such solvents in the undercoating tends to dilute and reduce the viscosity of the overcoating bath which contacts the undercoating, thus promoting penetration.

The prior art process of sintering the undercoating prior to the overcoating process also results in a complete drying and removal of the solvent. The advantage of our discovery, that sintering of the undercoating is not necessary, is that a much lower temperature drying operation can be used, thereby allowing the use of simple apparatus, no reducing atmosphere, and a processing rate higher than was heretofore possible.

In a specific embodiment of the invention, a heater wire of usual configuration is first coated by a conventional means with a layer of aluminum oxide having a thickness in the order of 5 mils. The coating operation can comprise the known dipping, spraying, or cataphoretic processes. In such operations, as known, an organic solvent or vehicle for the aluminum oxide is used which tends to remain with the aluminum oxide coating on the heater wire. Also, the aluminum oxide layer is somewhat soft and porous.

While it is not possible to give a quantitative definition of what is meant by soft" and porous," the difference in softness and porosity of an aluminum oxide layer before and after a high temperature firing operation is well known to persons skilled in these arts. In general, the soft layer can be scraped off the heater wire in a paste-like form, whereas the sintered layer is quite brittle and crumbles upon being scraped or bent.

Thereafter, in accordance with usual techniques, the coated heater is rinsed in a suitable solvent, such as methanol or acetone, to remove loosely adhered particles.

Then, in accordance with the instant invention, the heater is air dried at a temperature as high as possible for rapidly drying the heater without causing significant oxidation of the heater wire. The temperature used, and the time required to thoroughly dry the heater, are functions of the configuration of the heater and the size and mass of the heater. In general, however, to avoid oxidation of the heater wire and thus avoid the need to use a protective atmosphere during the drying operation, temperatures below 400 C are preferred.

Although the drying operation is performed at a temperature well below that used in the prior art sintering process (generally above l,600 C), some degree of hardening of the insulation coating occurs. This is desirable for further reducing the possibility of bath penetration.

Various means for performing the drying process can be used. For example, heaters can be placed in a conventional air oven and heated. Alternatively, a jet of hot dry air can be blown over the heaters. Further still, a source of radiations, such as infrared radiations from a conventional source, can be directed onto the heaters. The latter heating means is preferred since it is simple, noiseless, and the radiations can be accurately focused onto the desired portions of the heaters, thereby avoiding heating of fixtures used to carry the heaters.

In one embodiment of the invention, in which is air dried a heater having an undercoating of aluminum oxide of 5 mils thickness and weight of 5 miligrams, and a base wire of tungsten of 54 mm length and 3.5 mils diameter an infrared source is used to heat the heater coating to a temperature of about 300 C for a time of l2 seconds.

After thorough drying, the coated heaters are provided with the outer coating of tungsten. Preferably, in order to avoid penetration of the tungsten into the undercoating, a coating bath is used in accordance with the process described in the above-described co pending application. For example, with a coating bath comprising (by weight) 40 percent tungsten and 60 percent liquid vehicle, the tungsten having a particle size of 0.5 to microns, with the average particle size being about 2 microns, the vehicle comprising 1.68 percent lQQQ second nitrocellulose wet 30 percent by weight by alcohol and 98.5 percent butyl acetate, the bath is ball milled for about 6 hours. This provides a rheologically stable bath suitable for coating the tungsten particles onto the heater without bath penetration into the undercoating.

Other overcoating techniques, such as spraying or cataphoretic processes, can be used.

After the overcoating process, the heater is again rinsed, in a suitable organic solvent, to remove loosely adherent particles, and the heater is fired at an elevated temperature, in the order of l,600 C to complete the heater.

While the invention has been described using specific materials, the invention has utility in the fabrication of heaters using other materials. For example, the heater wire can be any refractory wire, such as molybdenum, normally used in heaters.

The undercoating can be various known insulating refractory materials normally used in heaters, such as zirconium oxide, beryllium oxide, and the mixture of chrome oxide and titanium oxide. These materials, and others known to workers skilled in these arts, tend to be relatively soft and porous when initially applied, and are normally later fired at high temperatures. Thus, the above-described problems of the prior art processes exist with the use of such materials, and advantages are obtained using the inventive process described herein.

Also, the darkening outer coating can comprise various refractory, high thermal emissivity materials such as carbon, titanium, chromium, and molybdenum. Again, avoidance of penetration of these materials into the underlying coating is generally to be desired.

We claim:

1. A method of fabricating a dark heater comprising:

coating a tungsten wire with a relatively soft and porous first layer of aluminum oxide containing a solvent,

heating said coated wire in air at a temperature below 400 C for a time sufficient to remove said solvent from said first layer,

coating said coated wire with a second layer of tungsten metal particles while said first layer is still relatively soft and porous and firing said coated wire at an elevated temperature to complete the heater.

2. A method of fabricating a dark heater comprising:

coating a refractory metal heater wire with a relatively soft and porous first layer of electrically insulating inorganic material containing a solvent,

heating said coated wire in air at a temperature below 400 C and below which oxidation of said wire occurs for a time sufficient to remove said solvent from said first layer,

coating said coated heater wire while said first layer is still relatively soft and porous with a second layer containing refractory metal particles of high thermal emissivity, and

firing said coated heater wire to sinter said coatings.

3. The method of claim 2 wherein said second layer is coated by a process in which a solvent is caused to contact said first layer.

Claims

2. A method of fabricating a dark heater comprising: coating a refractory metal heater wire with a relatively soft and porous first layer of electrically insulating inorganic material containing a solvent, heating said coated wire in air at a temperature below 400* C and below which oxidation of said wire occurs for a time sufficient to remove said solvent from said first layer, coating said coated heater wire while said first layer is still relatively soft and porous with a second layer containing refractory metal particles of high thermal emissivity, and firing said coated heater wire to sinter said coatings.
3. The method of claim 2 wherein said second layer is coated by a process in which a solvent is caused to contact said first layer.