US2853639A - Cathode ray tube - Google Patents

Description

Sept. 23, 1958 w. J. KNocHEL ETAL 2,853,539

cATHoDE RAY TUBE Filed Feb. 27, 195e Fig.\2.

United States Patent() CATHODE RAY TUBE William J. Knochel, Elmira, and Glenn L. Cox, Horseheads, N. Y., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Penn- Sylvania Application February 27, 1956, Serial No. 567,885

6 Claims. (Cl. 313-82) This invention relates to apparatus for producing an electron beam and more particularly to methods and structure for eliminating ions from the electron beam.

One of the important factorsv in determining the life of a non-aluminized cathode ray tube'is the deterioration of the fluorescent screen by ion bombardment. Both positive and negative ions have been shown to be involved in this deterioration of the fluorescent screen and various methods and structures have been utilized. in order to reducethe ion'blemish.

Negative ions are believed to be 'formed nearthe electron gun cathode and the first accelerating aperture and are controlled byy various type ion traps Within the electron gun' structure. The problem of negative ions and suppression thereof is more thoroughly discussed. in a copending applicationy entitled Cathode Ray Tube," SerialNo. 518,871, filed lune 29, 1955, by A. T. RaczynskiA and assigned to the same assignee.

The positive ions are believed formed within the cathode ray tube in the' bulb. space between therelectro'n gun and t-he face plate andv are` much more dii'cult to control. The exact origin of these positive ions' has been uncertain' in the past, and the only successful' remedy is tofplace a coating of. a material of sufii'cient thickness upon the screen to prevent the ions from reaching the phosphor. This positive ion deterioration, and more specifically the'X burn found in non-aluminized cathode rayv tubes is more thoroughly discussed in copending application entitled Method of Producing Secondary Emission from Bombarded Surface, Serial No. 548,990, fled'November 25, 1955, by E. I. Sternglass and W. I. Knochel and assigned to the sameassignee.

Thisinvention is directed to the elimination of negative ions generatedv within the electron gun structures and not removed by the conventional negative' ion traps. The ion burn on the screen that we are specifically trying to prevent results in an ion burn spotV positioned to one sidel of the center of the phosphor screen andresultsY in a spot deterioration of the screen. The effect of this ion `bombardment on the phosphor or fluorescentiscreen of a cathode'ray tube is to produce a thin film ofldeactivated material. on the surface thereof. It is this deactivated area of fluorescent material which is referred to as Van ion burn or blemish. This deactivated area absorbs a higli proportion of the electron beam, resultingin only a small light output from the area.

' The mass of the ion is greaterA than the' electron while the' charge is'essentially the same. Thene'gative ion will be focused and deflected essentially along the same path as the electron in an electrostatic'field, while a magnetic eld sufcient to apply a force to the electrons will have little effect on the negative ions. It is, therefore;v seen that the negative ions generated Within an electron gun structure would bombardlthevv entire screen with electrostat-ically deflected type cathode ray tubes and essentially only one spot withv electromagnetically d'eected type cathode ray tubes. Since most television receivers ICC 2` are the electromagnetic deilected type, cathode ray tubes, this explanation will be limited only to the generation of the small spot ion burn.

There are several designs and modifications of electronl gun structures utilized in the tube industry for elimination of negative ions. These type designs may essentially be broken down into three classes, one in which the electron source or cathode of the electron gun isv located off the axis of the tube neck and in which the initial direction of the electron and ion beams is tiltedk with respect to the neck axis. The electron beam is then bent and aligned with the tube neck by a transverse magnetic eld. The ion beam continues in its original directionv and is trapped by a disc in the anode whose aperture does not lie on the path of the ion beam. In the second, the electron source `or cathode is again lo. cated o' the main axis of the tube neck, and the electron and ion beams are first bent oli the gun axis by a transverse electrostatic field and then the electron beam is bent and aligned along the axis of the tube by a transverse magnetic field or fields.` These fields may be practically superposed as in the previously mentioned copending application Serial No. 518,871, so that the electron beam path does not deviate significantly from the axis of the tube neck. In both of these latter structures, the unwanted ions are trappedA by a diaphragm in the anode whose aperture does not lie on the path of the ion beam. This invention is applicable to all three types of structure.

The second and third ty es of structure described above utilize what is known as a slashed or tilted electrostatic eldlwithin the gun structure to supply a transverse field component to deflect4 both the electrons and the ions generated at or near the cathode of the axis of the electron gun structure in order to effect the ion traping within the gun. lt has beenfound that the electron gun structures described above do not remove all of the negative ions and that an objectionable deactivated area positioned to one. side of the center of the phosphor screen appears during the operating life of the tube.

It is, accordingly, an object of our invention to provide an improved electron gun structure capable of providing a1substantially ionfree electron beam.

It is another object to provide an improved electron gun in which the parts are positioned coaxial with respect tothe gun axis and. tilted within the tube neck and requiring only one ion trap magnet.

It. is another object to reduce electron emission from surfaces bombarded by positive ions within an electron gun.

These and otherobjects are eifected in our invention as willbeapparentfrom thefollowing description taken in accordance. with the accompanying drawing throughout which like reference charactersindicate like parts, and in which:

Figure l is a side elevational view showing the relationship of the cathode ray gun embodying the principles of our invention to the other structural elements of a cathode ray tube;

Fig. 2 is a sideA elevational view, partly in section, of acathode ray gun positionedwithin the neck of a cathode ray tube embodying the principles of. our invention; and

l Fig. 3 is` an enlarged. sectionaly view of.V an electrode element in the cathode ray gun embodying theprinciples of our invention.

Referring in detail to Fig. 1, there is shown a cathode ray tube embodying our invention. The tube is comprised of an envelope having a tubular neck portion 12, a flared bulb portion 14 and a face plate 16., The face plate 16 of the envelope 10 has a suitable fluorescent coating 18 on the inside surface thereof and the flared bulb portion14 has a conductive coating 20 of a material such as aqueous suspension of graphite. The neck portion 12 is a straight tubularY member, and the axis 13 is normal to the face plate 16 and substantially at the center thereof. Y

Y The electron gun 22 is comprised of an indirectly heated cathode 30, a control grid or electrode 40, a screen grid or electrode 50, a first anode 60, a focusing electrode 70 and a second anode 80. The graphite vcoating 20 on the interior of the ared bulb portion 14 serves as the third anode of the cathode ray tube and a suitable voltage is applied at a terminal 26. The structure shown and Referring in detail toV Fig. 2 which shows a preferred l embodiment of our invention, the control grid 40 is comprised of a tubular member or skirt 42 coaxial with the electron gun axis 13 with a diaphragm or stop 44 positioned therein in which a small aperture 46 is centrally located on the gun axis. In the specific embodiment shown, the diaphragm 44 is located at the end of the cylindrical or tubular member 42 nearest the screen 18. The plane of the diaphragm 44 is perpendicular to the axis of the 4electron gun 13. Positioned within the control grid structure 40 is the cathode 30 comprised of a tubular member 32 of smaller diameter than the control grid skirt 42 with its longitudinal axis aligned on the .gun axis. The end of the tubular member 32 nearest the diaphragm 44 of the control grid 40 is closed and a coating 34 of electron emissive material is placed on the exterior surface. The tubular member 32 is supported within the grid skirt 42 by means of a ceramic collar 36. A heater filament 38 is provided within the tubular member 32 for controlling the temperature of the electron emissive coating 34. I

The screen grid or electrode 5'0 is comprised of a tubular skirt or cylindrical member 52 spaced along the gun axis and adjacent to the diaphragm end of the control grid 40. The screen grid skirt 52 has a diaphragm member 54 positioned therein, and in the specific illustration closes the end of the cylinder adjacent to the control grid 40 which is perpendicular to the gun axis 13. In a similar manner to the control grid diaphragm 44, an aperture 56 is placed at the center of the diaphragm 54 with its center also on the gun axis. The unclosed end or rim 58 of the screen grid skirt 52 may be rolled outward as shown in Figs. 2 and 3 to eliminate possibilities of distortion of electrostatic fields due to sharp edges and burrs on the materials. The plane of the unclosed end or rim 58 of the cylinder 52 is slanted at an angle of 10 to 13 with respect to a plane perpendicular to the gun axis 13. The screen grid 50, as generally described above, is more completely described in a copending application, Serial No. 374,240, tiled August 14, 1953, now Patent No. 2,773,212 entitled Electron Gun, by I. A. Hall, and assigned to the same assignee.

In our invention, a coating 57 of inert conductive material is deposited on the surface of the diaphragm 54 facing the screen 18. The coating' 57 surrounds the aperture slanted open end of the screen grid 50. The first anode 60 is opened at the'end adjacent the screen grid 50 and the rim 68 is also slanted so that the plane of the rim 68 is parallel to the rim 58 of the screengrid 50 and inclined to a plane perpendicular to the axis by about 10 to 13. The other end of the first anode skirt 62 is closed by a diaphragm 64 perpendicular to the gun axis 13 and also having a centrally located aperture 66 therein. The second anode is comprised of a cylindrical or skirt portion 82 positioned along gun axis 13 at a distance greater than the other members of the electron gun from the first anode 60. The end of the skirt 82 adjacent the rst anode is closed by a diaphragm 84 perpendicular to the gun axis 13 and having a centrally vlocated aperture 86 located therein. The opposite end of the' second anode skirt 82 also has a ldiaphragm 83 therein which is perpendicular to the gun axis 13 with a large aperture 85 positioned therein. Integral with the diaphragm 83 is a flange 88 extending outwardly from the surface of the second anode to -which flexible spring members 90 are attached which position the flange 88 on the electron gun 22 approximately centrally within the neck 12 and 'also make electrical contact with the graphite coating 20 on the ared portion of the envelope 10. The aperture 85 is centrally located within the diaphragm 83 and is therefore approximately located on the tube axis. The rst and second anodes 60 and 80 are connected together electrically and are supplied with voltage from the graphite coating 20 by means of the flexible conducting spring members 90.

A sleeve or focusing electrode 70 surrounds the space between the first andsecond anodes 60 and 80 and is also coaxial with the gun axis. The tubular sleeve 70 is of a larger diameter than the first and second anode cylinders 62 and. 68.` The control grid 40, the screen grid 50, the irst anode 60, focusing electrode 70 and the second anode 80 may be supported by providing radially projecting .anchor .pins on thecylindrical surface thereof. The anchor pins are embedded within longitudinal glass support rods which may extend along substantially the entire length of the gun structure. This desirable feature and improved assembly accuracy can be incorporated in this preferred embodiment because all gun electrodes are centered on a single gun axis.

Suitable voltages are applied to the filament 38, control grid 40, screen grid 50 and focusing electrode 70 -by means of leads extending through a button stern provided in the end of the tube neck 12. A table is given below of typical dimensions of a specific embodiment shownin Figs. l Iand 2 with representative voltages applied to each of the electrodes. The relative dimensions of these electrodes are expressed as ratios to the inside diameter of the control grid cylinder 42.

Cylinder f Diameter Length Voltage (Outside) M 0.68 (Inside) l-- 0.87 30. (Inside) 1 0.55 (maximum length) +275 to 500 v. (Inside) 1.- 2.64 (maximum length) +10 to 20 kv. 1 1.0 +10 to 20 kv. 1.3 1.4 .I to +450.

Aperture: Inside diameter 46 .062 56 .072 76 .200 y s6 I .24o Gap between electrodes along axis 11:

Inl the operation of the electron gun structure 22, the electrons as well as negative'ions are emitted from the electron emissive surface 34 of the cathode 30.v The electrostatic eld of the control grid l40 and the screen ,5 grid 50 acceleratesthe electrons andions'andformsalens of short' focal length bringing the electrons and ions` to what is known as a cross-over point'between or near the diaphragms 44l andI 54 The control grid 40 controls essentially the number of electrons passing' through the aperture.46A inthediaphragm 44` and thereby controls the intensity of the electron beam. The electronA and ion beamthus formed passes through the-aperture 56 in the diaphragm 54 of the screen grid 50. The electrostatic field or. lens set up between the screen grid.50 and the iirstanode 60acts on the beam atV the time that the beam enters the aperture 56 in the diaphragm 54 of the screen grid 50 and: acts-.to accelerate and deflect the electron and ion beam enteringthe slashedtield. Since the adjacent ends ofthe screen grid 50 and the irst anode 60 lie inn parallel planes, tilted-with respect-toy a plane perpendicular'to' the^gun'axis-13,theefectis to'produce a transverse electrostatic-held component which bends the paths of the electrons'and'ionswithinthe beamgjust after crossover. Since'theelectrons-and'ionsare moving-slowly at this point, the electron .and ion beams are bent sharply downward. The magnetic iield due to the ion trap magnet 21 is also concentrated at this point just after crossover, and directed to produce a force directed upward on the electron and ion beam opposing the electrostatic field force. This field may be adjusted to compensate for the electrostatic field so that the electron beam will be bent and aligned with the axis of the tube neck 12. The magnetic field due to the ion trap magnet 21 will have little effect on the ions Within the beam and as a result the ion beam wil-l follow substantially the deiiected path impressed thereon by the transverse component of the electrostatic field set up between the screen grid 50 and the anode 60 and will strike the inner surface of the diaphragm 64 of the first anode 60 while the electron beam will pass through the aperture 66 of the iirst anode 60.

It was found without the utilization of the inert conductive material coating 57 on the diaphragm 54 of the screen grid 50 that .a secondary ion blemish appears during the life of the tube. An attempt to increase the transverse component of the ion trap slashed electrostatic field did not improve or remove this ion blemish. A suggested explanation of this effect is as follows: It is believed that positive ions formed within the tube bombard the surface of the diaphragm 54 of the screen grid 50 facing the screen 18. It is believed that the primary source of these positive ions is within the first anode and since this first anode operates at a potential over 10,000 volts while the screen grid operates at about 350 volts, that these positive ions are focused and -accelerated into impingernent on the surface of the diaphragm 54 surrounding the aperture 56. This is further borne out by the fact that inspection of the surface of the diaphragm 54 of a gun after being in operation shows a darkened area near the aperture 56 and primarily on one side of the aperture 56. It it also noted that the ion blemish on the iiuorescent screen has the same shape as the darkened area on the diaphragm 54 of the screen grid 50. Although we do not wish to be limited to this explanation, it is believed that the primary negative ions which originate at or near cathodes are deected into the inner surface of the first anode 60 and strike the inner surface of diaphragm 64 thereby generating secondary electrons which, in turn, form positive ions. It is a known phenomenon that slow secondary electrons form positive ions with very great efficiency where the energies range up to the order of 50 electron volts. The threshold for ionization lies typically near Volts and the maximum ionization cross-sections occur around 50 to 75 volts. The positive ions generated within the first anode 60 are accelerated by the high potential difference between the screen grid 50 and the first anode 60 into impingement with the surface of the diaphragm 54. This bombardment of the surface of the diaphragm 54 by the high energy positive ions results in the release of either electrons or negative ions from the 6 surface. These electrons -wliich'- are emitted2 by'the screen grid-54 may, in turn, form negative-ions by attachment to gas molecules while they are still moving slowly immediately after leavingthe Vsurface of the screen grid. These secondary negative-ions originating off the axis of the screen grid 54 are not' trappedfinv the iirst anode but pass through the gun to cause'thevobserved otf centerv ion burn.

It isfound that cleaning the surface of the diaphragm 54 of screen grid 50l and then placing the inert conductive material coatingi57'such-as gold, platinum, silver, etc., on -it has-resulted''inthe `elimination ofthe secondaryiion blemishspot-near thecenter of the face plate 18-of` the tube; After the normal cleaning. and hydrogen baking operation, the part was degreased, dippedin muriatic acid to remove'surface oxides, nickel strike applied (very thin plate) and-thenplated with coating-57. Itis believed that the coating 57 onthe diaphragm 54 providesan-inert nonoxidizable surface under conditions found in the tube'and dur-ing its'manufacture.4 The non-oxidizabl'e surface isy a poor secondary electron emitter and, therefore, the production of negative ions near the screen grid 50 is substantially reduced so as to prevent the ion blemish on the face plate 18. It is believed that the oxide coating formed on the screen grid 50 without the gold coating may be a primary cause of secondary emission. The material normally utilized in the electron gun structure is a stainless steel on which oxide coatings are formed fairly readily. It is, therefore, seen that byl providing this inert coating 57 on the diaphragm surface an economical way of eliminating secondary ion blemishes is obtained. If the inert material utilized on the diaphragm 54 is also of a high atomic number material, the coating may cover the edge of the aperture 56 in order to further reduce secondary emission due to the )bombardment of electrons at grazing angles on the edge of the aperture 56. Other specific materials which may be utilized in the coating of the diaphragm in order to further reduce the cost are platinum-silver, etc. This effect may also be 'obtained or enhanced by applying a similar inert conductive coating to the inside of anode 60, especially on diaphragm 64 to prevent secondary electron emission from this part.

While we have shown our invention in only one form, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various other changes and modiiicationswithout departing from the spirit and scope thereof. In particular, it may be embodied in electron guns which are coaxial with the tube neck, to those which are bent to align the upper part of the gun with the tube neck and to those in which a bent anode is essential to the primary on trapping action.

We claim as our invention:

l. A cathode ray tube comprising an evacuatedenvelope, said envelope having a straight neck portion, the axis of which is substantially normal to the face plate of said cathode ray tube, an electron gun positioned within said neck portion, said gun comprising a plurality of electrodes consisting of at least a cathode, control grid, screen grid and anode, said control grid, screen grid and anode having coaxial tubular portions, said screen grid having an apertured conducting disc mounted transversely therein, said disc having a conductive coating of a low secondary emissive material on the surface thereof facing said screen.

2. An electron gun structure comprising a cathode, a

control grid, a screen grid and a high potential accelerating anode in the order named, said screen grid consisting of at least an apertured diaphragm positioned transverse to the path of the electron beam, said diaphragm having a relatively non-oxidizafble conductive -coating on the surface thereof facing said high voltage accelerating electrode.

3. A cathode ray tube comprising an envelope and having therein a target member, a cathode, means for forming an electron beam of the electrons generated by said cathode and directing said electron beam along a path to said target, said means including a beam' limiting apertured diaphragm of a first conductive material positioned transverse to said electron beam and a second conductive material coating inert to oxidation deposited on a portion of the surface of said diaphragm. l

y 4. A cathode ray tube comprising an envelope and having therein a target member, a cathode, meansfor forming an electron beam of the electrons generated by said vcathode and directing said electron beam along .a path to said target, said means including an apertured diaphragm member of a conductive material positioned transverse to said electron beam and a coating of a relatively nonoxidizable material deposited on a portion of the surface of said diaphragm.

, 5. A cathode ray tube comprising an envelope and having therein a target member, a cathode, means for forming -an electron beam of the electrons generated by said cathode and directing `said electron beam along a path to said target, said means including a diaphragmniember having an aperture located therein and a coating. of conductive material relativelyl inert to oxidation covering at least the inside surface of.said aperture.

6. cathode ray tube comprising an envelopehaving therein a. target member, a cathode, means for forming a beam of electrons generated by said cathode and directing said electron beam along a path toV said target, said means including a diaphragm member having aperture located therein and a coat of electrical conductive material relatively inert to oxidation and of a high atomic number material covering at least the inside surface of said aperture.

References Cited in the le ofths patent UNITED STATES PATENTS 2,496,127 Kelar Jan. 31, 1950 2,544,934 Nase Mar. 13, 1951 2,564,737 Szegho Aug. 2l, 1951 2,732,511 Dichter Jan. 24, 1956 Hoagland Jan.V 3l, 1956

Images