US3432712A

US3432712A - Cathode ray tube having a perforated electrode for releasing a selected gas sorbed therein

Info

Publication number: US3432712A
Application number: US595104A
Authority: US
Inventors: David Benda
Original assignee: Sylvania Electric Products Inc
Current assignee: GTE Sylvania Inc
Priority date: 1966-11-17
Filing date: 1966-11-17
Publication date: 1969-03-11
Anticipated expiration: 1986-03-11
Also published as: GB1194357A

Description

United States Patent 3 Claims ABSTRACT OF THE DISCLOSURE An improved color cathode ray tube employing an open metallic structure positioned relative to the screen, for example, a shadow mask color tube wherein the mask is a continuous selected gaseous giving mechanism during tube operation and a method of processing the tube for achieving the same. During tube processing the mask sorbs a selected introduced gas such as hydrogen. Electron beam impingement of the mask during subsequent tube operation effects gradual release of the occluded selected gas from the mask to provide a replenishable partial pressure of hydrogen which in relation to the total tube pressure is consistent for the promotion of enhanced emission and extended tube life.

This invention relates to cathode ray tubes and more particularly to a cathode ray tube employing a substantially open metallic member spaced relative to the screen whereof the tube has improved life performance and a method for processing the tube to achieve the desired performance.

The useful operational life of electron tubes, of which cathode ray tubes are an example, is dependent largely upon the level of electron emission available for utilization in the device. In color cathode ray tubes, for example, one or more cathodic sources are incorporated in the electron gun structures oriented within the conventionally evacuated envelope to provide a continuous supply of electrons to effect tube operation. These electrons substantially released by heat from the barium compounds of the cathodes are formed or shaped into beams, focussed, accelerated, and directed from the terminal end of the gun structure toward an electron responsive screen by appropriately associated gun elements. Means external of the tube are utilized to deflect the beams in a predetermined sweeping manner to provide discrete impingement of the beams on the screen thereby producing desired luminescent displays. Thus, the sustained generation of electrons of a predetermined level of supply is necessary to maintain prolonged tube operation of a desired degree.

In shadow mask cathode ray tubes of the type described, it is customary to position a getter structure adjacent the terminal end of the gun. This getter is formed to effuse a gas-adsorbing material, such as barium, during a specific sequence in tube processing to dispose a thin film of gettering material substantially on the walls of the envelope and on the surface of the shadow mask facing thereto. While a thin film of gas-adsorbing barium material, having a substantially uniform thickness, is desired for optimum gas clean-up, the directional dispersion of the gettering material in the reduced atmosphere tends to dispose a thicker and somewhat less efiicient film on predominantly the central surface area of the mask.

While degassing of the tube components is practical before and during tube processing,-additional occluded gases are released during tube operation from the various elemental tube structure and envelope into the substantially evacuated interior. These gases, as for example,

N 0 H C0, C0 and H 0 are for the most part effectively adsorbed by the getter film, but as tube life progresses getter clean-up efliciency decreases, and some of the heavier gaseous hydrocarbons, such as acetylene (C H are sometimes evidenced. These hydrocarbon ions, being attracted by the cathode, deleteriously bombard and impair the emissive surface thereof. As the supply level of electron generation decreases as a result of cathode deterioration, a change is evidenced in the tube operating characteristics. When these characteristics drop below a certain prescribed parameter, tube life is said to be afiected.

It is known that the electron emission of electron tubes may be benefited by the introduction thereinto of specific pressures of selected gases such as, for example, nitrogen or hydrogen, but there has been no readily feasible means for maintaining an optimum emission-promoting partial pressure of the desired gas within the operating tube.

Accordingly, it is an object of this invention to reduce the aforementioned disadvantages and to produce an improved cathode ray tube that, when processed, has a continuous selected gaseous giving mechanism during tube operation to enhance electron emission and promote extended tube performance.

A further object is to produce a color cathode ray tube that has improved gas adsorbing capabilities.

An additional object is to provide a method for processing a color cathode ray tube to effect means for maintaining therein an electron emission-promoting-partial pressure of a selected gas during tube operation to extend the life thereof.

The foregoing objects are achieved in one aspect of the invention by the provision of a method for processing a cathode ray tube for example a color tube employing an open metallic member, such as a shadow mask, wherein the tube is heated and substantially evacuated of occluded gases, the cathode materials converted to the electron emission state, the tube evacuation terminated, and a volume of a selected gas introduced into the substantially evacuated tube while the mask temperature is of a level to effect selected gas sorption therein. The selected gas, being chemically compatible with the screen and electron emission materials, is appreciably sorbed by the conditioned mask and released in a gradual manner therefrom by electron bombardment of the mask in said subsequently operating tube. Thus, there is provided therein a replenishable partial pressure of the selected gas in relation to a total tube pressure to promote enhanced electron emission and extended tube life. Subsequent to the initial introduction of the selected gas, a layer of gas sorbing getter material is diffused over the mask surface proximal to the electron gun; this getter material having a low sorption sensitivity for the selected gas.

For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following specification and appended claims in connection with the accompanying drawings in which:

FIGURE 1 is a sectional view of a shadow mask color cathode ray tube;

FIGURE 2 is a perspective view of one type of material eifusing structure; and

FIGURE 3 is a plan view showing one embodiment for introducing the selected atmosphere into the evacuated tube.

For simplicity and ease of understanding, while in no way limiting, the invention will be described with reference to a 25 inch rectangular shadow mask color cathode ray tube having substantially degree electron beam deflection as indicated by 0: in FIGURE 1. The shadow mask is but one type of open metallic member. Other structures intended to be within the scope of the invention include grids, perimetric frames and other structures that are oriented relative to the screen and can be impinged 'by the scanning electron beam.

With further reference to the figures there is shown a shadow mask color cathode ray tube 11 of the type above noted having an envelope 13 integrally comprising a neck portion 15, a funnel portion 17, and a panel portion 19. A patterned cathodoluminescent screen 21 of selected electron responsive phosphors is formed on the inner surface of the panel portion 19. Adjacent to the screen and spaced therefrom is the foraminous shadow mask structure 23 which comprises the supporting frame 25 and the peripherally attached apertured mask 27. The mask frame is spacedly oriented within the panel by suitable support means 29.

In greater detail, the formed apertured mask portion 27 is of low carbon steel material, such as SAE 1010 formulation or a similar material, of a thickness in the order of .006 inch. The supporting frame portion 25 is of similar low carbon steel material having a nominal thickness of .093 inch. The holes 26 in the apertured portion which are associated with the screen pattern therebeneath are substantially circular in shape and range in diametrical size from about .0110 inch at the center to about .0098 inch at the edge. While the apertured mask portion comprises a multitudinous number of these holes, the transmission of the mask is in the order of about 16 percent at the center diminishing to about 11 percent at the edge. Thus, the solid web of mask material 28 comprises about 84 to 89 percent of the mask area.

Extending from the mask frame is electrical connective means 31 which makes contact with the aquadag coating 33 disposed on the interior surface of the funnel 17 and extending partially into the neck portion 15.

Disposed within the neck of the tube is the electron gun mount structure 35 which for clarity is only partially detailed and illustrates only one electron source or cathode 37. The beam convergence means 39 terminally oriented on the mount has resilient support and connective means 41 making pressured electrical contact with the aquadag coating extending into the neck portion. Spaced from and supported relative thereto by a positioner 42 extending from the convergence means is one type of a material elfusing structure 43 which will be detailed later in this specification. The mount structure 35 is further positionally supported on electrically conductive pins (of which only four are shown) 45, 46, 47, and 48 which are hermetically sealed in the stem wafer closure portion 51 to extend interiorly and exteriorly therefrom.

An exhaust tubulation 53 is connected through appropriate valving 55 to a conventional vacuum or gas evacuation system. In the tubulation there is noted, by dotted lines, the region of hermetic tubulation seal 57 which is consummated by heat prior to removal of the tube from the valve.

In processing, the tube 11 is oriented in a manner to expedite connection of the exhaust tubulation 53 with the evacuation system. External heat is applied to the tube, by means not shown, to substantially degas the envelope 13, the screen 21, the shadow mask structure 23, the aquadag coating 33, and the gun mount structure 35 of gases occluded therein. During this out-gassing heating step the mask reaches a temperature in the order of 380 to 400 degrees centigrade. The internal ambient and released occluded gases within the tube are evacuated through the externally connected vacuum system by extended pumping during the heating sequences. It is conventional to supply extra heat to the electron gun mount structure 35, especially to the lower portion thereof, by induction heating means substantially localized relative the neck portion of the tube.

Additional heat is applied to the cathode 37 during at least part of the latter sequences of the evacuation period and during at least a portion of the lat er p rt f the tu e heating period to chemically convert the emission materials 38 to an electron emitting state. For example, the major constituent of the emission materials combination, barium carbonate (BaCO is converted during tube processing to barium which is the functional electron emitter during subsequent tube operation, the emissive action of which may be augmented by including strontium and calcium in the emissive coating. The aforementioned additional cathode heat is supplied thereto by electrically activating the cathode heater 40 which is insulatively positioned within the nickel alloy cathode sleeve 37. This heater activation is accomplished by connecting

heater pins

46 and 47 to an appropriate electrical supply source, not shown.

When degassing and evacuation have reached predetermined levels, the externally connected evacuation period is terminated in accordance with the way the selected gas is to be introduced into the tube. If the selected gas 56 is supplied by a subsequently activated giver oriented within the tube, the evacuation termination is consummated by elfecting a tip-off heat seal 57 in the exhaust tubulation. Alternatively, if the selected gas is to be supplied from an external pressurized supply as shown in FIGURE 3, the valve 55 is replaced by a two-way valving device 55 which terminates the evacuation, and when desired, can be adjusted to allow a predetermined pressure of the selected gas to enter the substantially evacuated tube envelope, after which the valve is closed and the tubulation seal 57 effected.

At the termination of externally connected evacuation period, the temperature of the shadow mask is approximately 200 degrees centigrade. It has been found desirable to introduce the selected gas while the mask temperature is at least degrees centigrade and preferably while it is in the range between 100 and degrees centigrade. While the mask is cooling through the aforementioned temperature range, the expansive surface of the foraminous mask structure of degassed porous low carbon steel sorbs or getters a large amount of the selected gas. Naturally other internal components of the tube sorb a certain amount of the selected gas, but the amount is far less than that sorbed by the mask.

The term selected gas is herein used with reference to a gaseous composition, of one or more gases, that is chemically compatible with the phosphors of the screen and the converted electron emission materials of the cathode, and one that is not appreciably sorbed by the subsequently applied getter material, such gases for example may be hydrogen or nitrogen or an inclusive mixture. By way of example in this instance, hydrogen (H will be described as the selected gas of the desired type. It has been discovered that maintenance of a predetermined partial pressure range of H in the subsequently operating cathode ray tube enhances electron emission and improves overall tube life performance.

As previously mentioned, the selected gas can be introduced in several ways. By way of example, one type of material effusing structure 43 will be described. This ring-like structure is of a metallic material such as nonmagnetic stainless-steel formed as an open trough or channel facing the mask and containing at least two types of efi'using materials 44. One of these is a hydrogen giver as for example a hydride of a metal such as zirconium or titanium, of an amount which when heated will release the desired volume of hydrogen; the other material is a gettering substance such as BaAl from which barium is released upon heating. Although not shown, it is in keeping with the invention to utilize a separate H effusing structure and a separate getter structure, if so desired.

When the mask is in the desired temperature range, the material effusing structure 43 is heated by localized induction means, not shown. As the ring reaches the 500 to 600 degree centigrade temperature range, dissociation of the hydride materializes and H is released into the substantially evacuated interior of the tube. As previously mentioned, a large quantity of the released H is sorbed by the conditioned mask while the remainder constitutes a partial pressure within the tube. Another gas evidenced as a partial pressure at this stage of processing is argon (Ar). This inert gas, which is conventionally utilized as an ambient medium during the storage of etfusing structures and sorbed to a limited degree thereby, is released by heat to contribute to the initial total tube pressure. During early tube life, this inert gas appears to be largely sorbed in a seemingly harmless manner by the tube components other than the Ba getter material. Increasing the ring temperature to approximately 1100 degrees centigrade volatilizes the Ba gettering material which is directively elfused into the partial pressures of hydrogen and argon by the open channel shaping of the ring. The barium molecules in contacting and colliding with the predominantly hydrogen molecules, sorb a limited amount of the hydrogen and are beneficially deflected and diffused to form a layer of gas sorbing getter material of an efiicient thickness over the surface of the mask proximal to the electron beam source .Some of the getter material is also difiused toward and deposited on the surface of the aquadaged funnel portion. The presence of the additional partial pressure of H provides a more uniform distribution of gettering material than is possible in a tube having a low total pressure. After flashing of the getter, the tube is high voltage conditioned and further electrically processed in substantially the conventional manner.

If it is desired to introduce the H from a pressurized external source as aforementioned and shown in 'FIGURE 3, the material etfusing structure would be a conventional channelized getter, formed similar to the structure already described but containing only getter material. After the H is introduced and the tube seal 57 accomplished, the conventional getter ring is inductively heated whereupon the Ba material is advantageously flashed or diffused as previously described.

As an aid to further description, the cathode ray tube shown in FIGURE 1 will be considered as a sealed and finished tube having a hydrogen atmosphere 56 therein and operating in a typical situation, the conditions of which are not shown. The electron beam 59 emanating from the electron gun is appropriately deflected through the a to sweep the screen, and in so doing, usually overscans the mask. Thus, the beam impinges substantially the whole of the gettered surface of the shadow mask structure 23. In a color cathode ray tube the electron guns operate at much higher cathode currents and anode voltages than do monochrome guns; the color gun conditions being in the order of 1 ma. cathode current and 25 kv. anode potential. The beam impingement on the aforedescribed expansive surface of the mask structure converts a large portion of the electrokinetic energy of the beam into heat. The impact of the beam appears to be at least two-fold, namely the high velocity electron impingement of the moving beam frees some of the loosely held sorbed H from the Ba getter layer, and the heat resultant from the sequential impacts promotes continued release of occluded H from the mask material proper. The mask temperature due to beam bombardment in a normally operating color tube will be substantially in the range of 55 to 60 degrees centigrade. This is substantially an equalized temperature resulting from the conduction and dissipation of the electron-mask impact heat within the material web of the mask effected by the rapidly scanning electron beam; whereof the momentary temperature rise at the point of impact is appreciably above 60 degrees centigrade. Thus, it has been found that the mask, which is processed to be literally impregnated with H becomes a continuing H giver under operational electron bombardment and the heat resultant therefrom to provide a replenishable partial pressure of hydrogen at a rate to promote enhanced electron emission and extended tube life.

The eflicient and substantially uniform thickness of the getter layer not only provides improved gettering but also 6 promotes uniform heating of the mask by the beam which augments constancy of H release.

Since the low carbon steel mask and frame material exhibits a great aflinity for hydrogen, a greater partial pressure of this selected gas is introduced into the tube during processing than is evidenced in the subsequently finished tube. The desired amount of hydrogen content has been determined by extensive experimentation and observation of total tube pressures and related partial pressures during tube life. Thus, by analytical observations of the desired results, desired initial gaseous content can be calculated.

Immediately following gettering, the total tube pressure is relatively high, and may be, for example, in the order of 10- torr. At this stage, the major partial pressures contributing to the total include H Ar, C0, C0 and N During tube aging, stabilization, and testing a semblance of gaseous equilibrium becomes evidence within the tube. For example, at a very early period in life, such as during the one to two hour period, the total gas pressure may drop to the vicinity of 10 torr, the major portion of which is a partial pressure of H not lower than substantially one magnitude (9X10" below the total tube pressure. In other words, the partial pressure of hydrogen comprises substantially ninety percent of the total tube pressure. This relationship is substantially maintained during ensuing tube life; for example, at 1000 hour life the total tube pressure may be 1 10- torr whereof the H partial pressure Would not be less than substantially 9 10- torr. Likewise, at the S000 hour level, the range between total tube pressure and the H partial pressure would not exceed substantially one magnitude. Too great a H pressure is not desired; for optimum benefits, it

should be substantially less than the total tube pressure but not more than substantially one magnitude therebelow.

During tube operation, there are minor partial pressures of gases evolved, most of which are of insignificant pressure gradients or of a type successfully gettered in accordance with the capabilities of the respective gettering materials utilized.

The reasons for the beneficial effects of the major partial H pressure in the operating tube are not fully understood. In most conventional color cathode ray tubes, a minor partial H pressure, evidenced during early life, disappears or drops to an insignificant level as life progresses. The enhanced electron emission and extended tube life are results evidenced from the conditioned presence of a major partial H pressure as furnished by the replenisher within the tube. Hydrogen content of the pressure values indicated appear to deter the formation of certain heavy hydrocarbons, such as ethane (C H and acetylene (C H which seem to be associated with slumping emission in conventional tubes. It is thought that the positive ions of these undesirable hydrocarbons deleteriously bombard the negatively charged emissive cathode coating. It is further thought that the presence of a major partial H pressure may effect a carbon combination in other than a gaseous form. Whatever the chemical and electrical mechanisms involved, marked life improvement is noted when a cathode ray tube is processed in a manner that the open metallic structure becomes a continuous selected gaseous giving mechanism during tube operation.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

I claim:

1. A cathode ray tube having within its enveloped enclosure a low total tube pressure, said tube comprising:

a luminescent screen having a display surface formed of light-emitting phosphor material;

at least one electron gun having an electron emitter 7 8 therein located to beam electron energy to said screen said screen is formed as a pattern of selected electron refor selective excitation of said phosphor material; sponsive color-emitting phosphors, and wherein said open a metallic member having a plurality of openings metallic member is formed of a thin foraminous material therein positioned intermediate said screen and said capable of occluding said selected gas from an atmosphere electron gun in spaced relationship with said screen r thereof during tube processing. in the path of the electron beam directed to said screen; and References Cited a partial pressure of a selected gas comprising a major UNITED STATES PATENTS portion of said total tube pressure to provide enhancement of electron emission and extension of tube 10 i g a1 operation performance, said gas being occluded sub- 2640952 6/1953 S em X stantially in said metallic member during tube proc- 2766397 10/19,.6 y g i 180 essing and released therefrom in a gradual manner by 2886730 95 fizi gg '3' X scanned electron beam bombardment during tube 3:138:734 6/1964 Lineweaver 313 178 X operation to replenish said partial pressure of said 15 selected gas. 2. A cathode ray tube according to claim 1 wherein ROBERT SEGAL Primary Exammer' said selected gas is hydrogen exhibiting a partial pressure V. LAFRANCHI, Assistant Examiner. of said total tube pressure during tube operation that is U S C1 X R substantially ninety percent of said total tube pressure.

3. A cathode ray tube according to claim 1 wherein 313-179, 31612