CATHODE RAY TUBE AND METHOD OF MANUFACTURING A CATHODE RAY TUBE

Abstract: The invention relates to a cathode ray tube having an envelope 1 comprising a screen 2 and an electron gun 3 which is situated inside the envelope 1 opposite to the screen and which has three electrodes 4, 5 and 6, which electrodes are provided coaxially on the inner wall of a glass tube 7 in adjoining places having diameters differing per electrode. In order to obtain, for example, smaller dimensions and a better alignment of the electron gun and a higher resolution, according to the invention a focusing lens associated with the electron gun 3 is a high-ohmic resistance layer 8 having a spiral shape 9.

Description: The invention relates to a cathode ray tube having an envelope comprising a screen and a neck portion, an electron gun situated inside the neck portion of the envelope opposite to the screen and comprising an electron beam forming part for forming a single electron beam. The invention furthermore relates to a method of manufacturing a cathode ray tube. A cathode ray tube of the kind mentioned in the opening paragraph is knwon from GB-A 2,026,469. The envelope described herein comprises first and second engaging surfaces extending substantially normal to the display tube is against which first engaging surfaces the screen bears and against which second engaging surfaces a colour selection electrode bears which ensures a satisfactory landing of an electron beam produced by the electron gun on the screen. The construction of the electron gun used in the display tube as described in GB-A 2,026,469 is not further specified therein. Conventionally, an electron gun, as for example is described in EP-A 0 157 445, is constructed from a beam-forming part consisting of a cathode and a few electrodes which respectively serve to generate and modulate and pre-focus an electron beam, and a focusing lens which ensures that the beam is focused on the screen of the tube in which the gun is present. The said focusing lens in a conventional gun also consists of a number of electrodes. All these electrodes are usually held together by means of a number of insulation rods. As a result of the present-day development towards electron guns of smaller dimensions and electron beams having greater brightness, the requirements as regards the tolerances in the spacings between the electrodes mutually and the alignment of the electrodes become more stringent. In addition, it is endeavoured to simplify the construction if the electrode system used. One of the objects of the invention is to meet the requirements described hereinbefore at least to a considerable extent. Therefore, the cathode ray tube according to the invention mentioned in the opening paragraph is characterized in that the electron gun has a focusing lens which comprises a high-ohmic resistance layer having a spiral shape, a number of electrodes forming the electron beam forming part and a glass tube the inner wall of which carries at one end thereof the electrodes of the electron beam forming part and at the other end the high-ohmic resistance layer having a spiral shape forming the focusing lens, said layer enclosing only the path of the single electron beam formed by the electron beam forming part, and in that the inner wall of the glass tube has spaced engaging surfaces extending substantially normal to the glass tube axis, the electrodes of the beam forming part bearing against said engaging surfaces and having central apertures which are coaxial with the glass tube axis, said electrodes bearing also against parts of the inner wall of the glass tube which are at least substantially parallel to the axis of the glass tube. The colour cathode ray tube described in EP-A 0 157 445 has at least two electron guns and one common high-ohmic resistance layer forming a convergence lens enclosing the paths of all the electron beams formed. In contrast to the electron gun of the inventive cathode ray tube the electrodes of this known tube are mounted on a number of insulation rods. Preferably, a beam-forming part associated with the electron gun is also a prefocusing part having a high-ohmic resistance layer with spiral shape carried by the glass tube. The cathode ray tube according to the invention permits of realizing small dimensions, narrow tolerances, good alignment and simple construction. The said resistance layer serves as a potentio-meter. By varying the pitch, the spacing between the turns, or the resistance, a voltage distribution can be obtained which is necessary for a focusing lens having few aberration errors, as they can so far be realized only in lenses having very large diameters. The focusing lens and the prefocusing part may be formed in the same high-ohmic resistance layer. The cathode ray tube according to the invention is obtained in a particularly simple manner when the glass tube in which the high-ohmic resistance layer and the other electrode are to be provided is obtained by softening a glass tubular section by heating and drawing it on a mandril. Places having increasing diameters are obtained by drawing the part of the tube on a mandril whose diameter changes a few times in the longitudinal direction. As a result of this, a number a wall parts of the inner wall of the glass tube in the longitudinal direction of the glass tube are situated at substantially right angles to the tube axis and a number of wall parts are situated parallel to the tube axis. In this manner, reference faces for the axial and radial positions of the electrodes with respect to the axis of the glass tube are obtained in one operation, as a result of which a very good reproducibility of the resulting alignment of the electrodes within the electron gun is achieved. Preferably each electrode is a short thin-walled sleeve closed at the end facing the focusing lens by a plate in the centre of which an aperture is present to pass the electron beam. Thin-walled electrodes easily adapt the glass wall during the thermal treatment without fracture of the glass tube occurring. The electrodes can be assembled in the profiled glass tube in a simple manner. In this construction, insulation rods are not necessary. According to the invention, the high-ohmic resistance layer is obtained by providing on the inner wall of the glass tube at one end a layer of a stable binder-free suspension of ruthenium hydroxide and glass particles, from which layer a ruthenium-oxide-containing electric resistance layer is formed by heating. A mixture of ammonia and isopropanol is used as a suspension agent. The high-ohmic resistance layer can be given a spiral shape mechanically both prior to and after a thermal treatment. For contacting the high-ohmic resistance layer it is of importance that before the layer is provided the inside of the tube has a smooth surface without projections or holes and that after forming the spiral said smoothness is not disturbed. The high-ohmic resistance layer is therefore contacted in that, prior to providing the said layer, conical holes are provided by sand-blasting in the wall of the glass tube through which holes wires are threaded which are fused in the holes by means of a crystallizing glass, after which the wires on the inside of the tube are cut off flush with the glass wall and the resistance layer is then provided. The high-ohmic resistance layer and the other electrodes can also be contacted simultaneously in that, prior to providing the layer and the other electrodes, conical holes are provided in the wall of the glass tube by sand-blasting, in which holes indium balls are laid, after which wires are threaded which are fused in the holes by means of a crystallizing glass, after which at the area of the resistance layer and the other electrodes to be provided the indium balls are cut off, the resistance layer is provided, the electrodes are assembled and the electron gun is subjected to a thermal treatment. The invention will now be described in greater detail with reference to the accompanying drawing and a specific embodiment. In the drawing: Figure 1 is a diagrammatic cross-sectional view of a cathode ray tube according to the invention, Figure 2 is a diagrammatic sectional view in greater detail of a cathode ray tube manufactured by means of the method according to the invention, and Figure 3 is a diagrammatic cross-sectional view of a cathode ray tube according to another embodiment of the invention. The invention relates, for example, to a cathode ray tube having a glass envelope 1 (see Figure 1) comprising a screen 2 and having inside a neck portion 10 of the envelope 1 and opposite to the screen 2 an electron gun 3 having three electrodes 4, 5, and 6 which in accordance with the invention are provided coaxially on the inner wall of a glass tube 7 in adjoining places having diameters differing per electrode. A focusing lens associated with the electron gun 3 is a highohmic resistance layer 8 having a spiral shape 9. Figure 2 shows the electron gun in greater detail. In this case the electrodes referenced 23, 24, 25 and 26 are provided on the glass tube 27, and 20 denotes the high-ohmic resistance layer and 28 denotes the spirallike focusing lens. As compared with the electron gun described in the prior art, the electron gun of the cathode ray tube according to the invention means an important improvement, for example, as regards the small dimensions, the brightness of the electron beam, the tolerances in the spacings between the electrodes and their mutual alignment. The diameter of an electron gun in a cathode ray tube according to the invention, for example, may be 1.2 cm. In a prior art electron gun with comparable properties said diameter would be 4 cm. Preferably, a pre-focusing part of a beam-forming part to which belong, for example, the electrodes 23, 24, 25 and 26 also has a spiral shape 29 in the high-ohmic resistance layer 20. The electron gun is obtained in a simple manner when the glass tube 27 in which the high-ohmic resistance layer 20 and the other electrodes 23, 24, 25 and 26 have to be provided, is obtained by softening a glass tube section by heating and drawing it on a mandril. The high-ohmic resistance layer 20 is provided on the inside of one end of the glass tube 27 and the other electrodes 23, 24, 25 and 26 are provided on the inside 31 of the other end of the glass tube 27 in places having diameters increasing towards the other end. These places of increasing diameter are obtained with great reproducibility and accuracy by drawing the tube section on a mandril whose diameter changes a few times in the longitudinal direction. A glass tube 27 as shown in Figure 2 is obtained by drawing on a bipartite mandril the parts of which after drawing are removed from the glass tube in opposite directions. The electrodes 23, 24, 25 and 26 can be assembled in a particularly accurate manner in that during drawing engaging surfaces 32 against which the electrodes 23, 24, 25 and 26 bear are formed in the glass tube. As electrodes 23, 24, 25 and 26 are preferably used short, drawn, thin-walled sleeves having upper plates 34, 35, 36, 37 and 40, respectively, in the centre of which apertures 38, 39 and 40, respectively, are present to pass the electron beams. Such sleeves are easy and accurate to make, so that the mutual distance of the upper plates and the coaxial alignment of the apertures after assembly is also determined accurately (approximately 5 µm). Moreover, such sleeves easily adapt to the tube surface in subsequent thermal treatments. The spiral-like focusing lenses 28 and 29 are obtained with good quality and high breakdown voltage by providing on the inner wall of one end of the glass tube 27 a layer of a stable binder-free suspension of ruthenium hydroxide and glass particles, from which the ruthenium-containing electric resistance layer 20 is formed by heating and is given the spiral shapes 28 and 29 mechanically. Alcoholic ammonia is preferably used as a suspension agent for the ruthenium hydroxide. The suspension used is obtained, for example, by mixing glass enamel powder with water in a beaker glass. Ruthenium chloride (RuCl₃) is dissolved in water and added to the mixture. Ruthenium hydroxide is precipitated in the mixture by the addition of ammonia. The mixture is then allowed to precipitate after which the water is siphoned off and the precipitate is dried. The dried precipitate is transferred to a ball mill and isopropanol and ammonia are added. The mixture is then ground to obtain fine particles and a good mixing. Such a suspension is suitable to obtain a uniform resistance powder layer. The spiral shapes are scratched in said powder layer. The pitch in the spiral shape is, for example, 300 µm and the interruption in the resistance layer 60 µm. After a firing treatment, said interruptions are highly voltage-resistant. The thickness of the said layer is, for example, 1.3 µm. The layer is heated at 500°C for approximately 20 minutes and thus obtains a resistance of 10⁷ ohm/square. The high-ohmic resistance layer 20 and the electrodes 24, 25 and 26 can be contacted simultaneously in that, prior to providing the layer 20 and the other electrodes 24, 25 and 26, conical holes 41 (⊘ 0.9 x ⊘ 0.4 mm) are formed in the wall of the glass tube by sand-blasting, in which holes indium balls 42 are laid and lead-out wires 43 (⊘ 0.6 mm) are assembled (in order to avoid complexity of the drawing, always only one of the holes, balls and lead-out wires are referenced) which are fused in the holes 41 by means of a conventional crystallizing glass 44. At the area of the resistance layer 20 and the other electrodes 24, 25 and 26 to be provided, the indium balls are cut off and the electrodes 24, 25 and 26 are assembled. The electron gun is finally subjected to a thermal treatment, the electrodes 24, 25 and 26 contacting the lead-out wires 43 via indium. When only a high-ohmic resistance layer 20 is to be contacted, indium balls may be omitted and it will suffice before providing the layer 20, to cut off the ends of the lead-out wires 43 projecting at the area of the said layers. When the electron gun is ready it is mounted in the neck portion of the envelope. In an other embodiment of a cathode ray tube according to the invention which is schematically shown in Figure 3, the glass tube forms the neck portion 52 of the envelope. The envelope 50 comprises a screen 51 and the neck portion 52 in which an electron gun 53 is provided. The electron gun 53 comprises a cathode 61 and electrodes referenced 54, 55, 56 and 57 and are provided on the inside of the neck portion 52. The high-ohmic resistance layer is denoted by 58 and the spiral-like focusing lens 59 is provided on the inner wall of the neck portion 52. The envelope is closed in an air-tight manner by means of a closing plate 60 through which the connecting wires of the cathode 61 are led. It will be obvious that the invention is not restricted to the examples described but that many variations are possible to those skilled in the art without departing from the scope of this invention. Instead of ruthenium oxide, the high-ohmic resistance layer may also comprise manganese, nickel oxide, thallium oxide and the like.

Claims: Cathode ray tube having an envelope (1, 50) comprising a screen (2, 51) and a neck portion (10, 52), an electron gun (3, 53) situated inside the neck portion (10, 52) of the envelope (1, 50) opposite to the screen (2, 51) and comprising an electron beam forming part for forming a single electron beam, characterized in that the electron gun (3, 53) has a focusing lens (9, 27, 59) which comprises a high-ohmic resistance layer (8, 20, 58) having a spiral shape, a number of electrodes (4, 6, 23, 24, 25, 26, 54, 55, 56, 57) forming the electron beam forming part and a glass tube (7, 27, 52) the inner wall of which carries at one end thereof the electrodes (4, 5, 6: 23, 24, 25, 26; 54, 55, 56, 57) of the electron beam forming part and at the other end the high-ohmic resistance layer ( 8, 20, 58) having a spiral shape forming the focusing lens, said layer (8, 20, 58) enclosing only the path of the single electron beam formed by the electron beam forming part, and in that the inner wall of the glass tube (7, 27, 52) has spaced engaging surfaces (32) extending substantially normal to the glass tube axis, the electrodes of the beam forming part bearing against said engaging surfaces (32) and having central apertures (38, 39, 40) which are coaxial with the glass tube axis, said electrodes bearing also against parts of the inner wall of the glass tube which are at least substantially parallel to the axis of the glass tube. Cathode ray tube as claimed in Claim 1, characterized in that each electrode of the beam forming part is a short thin-walled sleeve closed at the end facing the focusing lens (9, 28, 59) by a plate (34, 36, 37) in the centre of which an aperture (38, 39, 40) is present. A cathode ray tube as claimed in Claim 1 or 2, characterized in that a pre-focusing part (29) of a beam-forming part associated with the electron gun (3) is a high-ohmic resistance layer (20) having a spiral shape carried by the glass tube (27). A cathode ray tube as claimed in Claim 1, 2 or 3, characterized in that the glass tube (52) forms the neck portion (10,52). A method of manufacturing a cathode ray tube as claimed in any of the Claims 1 to 4, characterized in that the glass tube (7, 27, 52) in which the high-ohmic resistance layer (8, 20, 58) and the other electrodes are to be provided is obtained by softening a glass tubular section by heating and drawing it on a mandril. A method as claimed in Claim 5, characterized in that places of increasing diameters are obtained by drawing the tubular section on a mandril whose diameter changes a few times in the longitudinal direction. A method of manufacturing a cathode ray tube as claimed in Claim 5 or 6, characterized in that the high-ohmic resistance layer (8, 20, 58) is obtained by providing on the inner wall of the glass tube (7, 27, 52) at one end a layer of a stable binder-free suspension of ruthenium hydroxide and glass particles, from which layer a rutheniumoxide-containing electric resistance layer is formed by heating. A method of manufacturing a cathode ray tube as claimed in Claim 7, characterized in that a mixture of ammonia and isopropanol is used as a suspension agent. A method of manufacturing a cathode ray tube as claimed in any r of the Claims 5 to 8, characterized in that the high-ohmic resistance layer (8, 20, 58) is contacted in that, prior to providing said layer, conical holes (41) are made in the wall of the glass tube by sand-blasting, through which holes (41) wires (43) are threaded and are fused in the holes (41) by means of a crystallizing glass (44), after which the wires (43) are cut off flush with the inside of the tube, and the resistance layer is then provided. A method of manufacturing a cathode ray tube as claimed in any of the claims 5 to 8, characterized in that the high-ohmic resistance layer (8, 20, 58) and the other electrodes are contacted in that, prior to providing the layer and the other electrodes, conical holes (41) are made in the wall of the glass tube by sand-blasting, in which holes (41) indium balls (42) are laid, after which wires (43) are threaded which are fused in the holes (41) by means of a crystallizing glass (44), after which the indium balls (42) are cut off at the area of the resistance layer ant the other electrodes to be provided, the resistance layer is provided, the electrodes are assembled and the electron gun is subjected to a thermal treatment.

Inventors: SPANJER, TJERK GERRIT, VRIJSSEN, GERARDUS ARNOLDUS HERMAN MARIA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: H01J29
IPC codes: H01J29, H01J9