COLOUR DISPLAY TUBE PROVIDED WITH A COLOUR SELECTION ELECTRODE

Abstract: In the manufacturing process of a colour display tube (1), the display window (3) is assembled with the funnel shaped part (4) in a furnace by subjecting them to a heat treatment. This heat treatment can lead to deformation of the colour selection electrode (12), because in colour display tubes (1) with an invar shadow mask (13), the diaphragm parts (15) are made from invar as well, while the corner sections (16) are made from iron in order to obtain enough stiffness. The difference in expansion coefficient between invar and iron leads to shearing of the welds (22) that connect the diaphragm parts (15) to the corner section (16). To overcome this problem the invention proposes to provide the diaphragm parts (15) with slits (23) or impressions (24), which are arranged so that no stress occurs between the spot welds (22) when the colour selection electrode (12) is heated.

Description: Colour display tube provided with a colour selection electrode The invention relates to a colour display tube comprising a colour selection electrode having a frame comprising corner sections and diaphragm parts which are coupled to the corner sections. The invention further relates to a colour selection electrode for use in such a colour display device and to a diaphragm part for use in said colour selection electrode. A colour display tube as described in the opening paragraph is disclosed in United States Patent US 5,003,218. The colour display tube according to this specification is provided with a colour selection electrode having a frame consisting of four diaphragm parts and four corner sections, suspended in the corners of a display window. The colour display tube described in US 5,003,218 is provided with a colour selection electrode to ensure that electron beams coming from three electron guns, mounted in a neck portion of the tube, only excite electroluminescent material of one colour on the inner side of the display window. This colour selection is achieved by applying, for instance, a shadow mask in the tube. This mask comprises a large number of apertures, which are customarily arranged in either a slotted pattern or a dotted pattern. If the colour selection electrode is not stably positioned in the colour display tube, small deviations of its position will lead to a deterioration of the picture quality. When, for instance, the colour selection electrode is slightly deformed, the shadow effect of the colour selection electrode changes and consequentially, the electron beams do not hit the appropriate electroluminescent material on the display window. These registration errors cause discolouration of the display tube that leads to a deterioration of the quality of the picture on the colour display tube. The colour display tube of the prior art has the disadvantage that the shadow mask, especially when this shadow mask is made from invar - which is a nickel-iron alloy - material, shows deformations during the manufacture of the colour display tube. In most high performance colour display tubes, the shadow mask as well the diaphragm parts are made of invar material, because the thermal expansion coefficient of invar is very low, resulting in a good doming performance. The corner sections of the colour selection electrode are normally made of iron because this offers an optimal stiffness of the colour selection electrode. After screen processing, i.e. the process wherein the black matrix and phosphor layers are applied to the inner side of the display window to form the screen, the display window is assembled with the funnel shaped part in a furnace. The high temperatures in this furnace cause permanent deformations in the frame of the colour selection electrode, due to the differences in expansion between the diaphragm parts and the comer sections. It is an object of the invention to provide a colour display tube having an improved colour selection electrode with respect to the type described in the opening paragraph, which strongly reduces the registration errors on the display window. According to the present invention, this object is achieved by means of a colour display tube which is characterized in that the diaphragm parts are provided with coupling means that increase the thermal stability of the frame. The invention is based on the insight that the registration errors are diminished when the positional stability of the colour selection electrode with respect to the display window is increased during the heat treatment in the furnace where the display window and the funnel shaped part are assembled. This can be realized by providing the frame with coupling means for interconnecting the diaphragm parts and the comer sections in such a way that the differences in thermal expansion between the invar diaphragm parts and the iron comer section do not lead to a deformation of the frame of the colour selection electrode. In this way, the thermal stability of the frame is improved. A preferred embodiment of the colour display tube according to the present invention is characterized in that the coupling means comprise an area for welding the diaphragm part to the comer section, which area further comprises slits. When the area of the diaphragm part at which it is welded to the comer section is provided with slits, the diaphragm parts will not deform when the comer sections show a much larger expansion, because the slits in between different welding points ensure that the welds cannot shear. In another embodiment, the coupling means comprise an area for welding the diaphragm part to the comer section, which area further comprises impressions. This embodiment also prevents shearing of the welds between the diaphragm parts and the comer section. This can be attributed to the fact that the impression is stretched when the corner section expands more than the diaphragm part. In a further embodiment, the diaphragm parts are welded to the comer sections by means of a plurality of spot welds. The use of spot welds further increases the dimensional stability of the frame of the colour selection electrode, because the welds are small enough to prevent that stress occurs between the diaphragm part and the comer section at the location of the weld. Yet another embodiment is characterized in that the diaphragm parts further comprise mathematical lines connecting the two spot welds of any pair, which mathematical lines are intersected by at least one slit or impression. This embodiment has the advantage that in between each pair of welds a slit or impression is present. As a result, the differences in thermal expansion can be compensated for between all the welds. As a result, the diaphragm part will not be deformed and the positional stability of the frame is improved. The invention also relates to the colour selection electrodes and the diaphragm parts used in a colour display tube according to the invention. These and other aspects of the invention will be apparent from and elucidated by way of non-limitative examples with reference to the drawings and the embodiments described hereinafter. In the drawings: Figure 1 is a sectional view of a colour display tube according to the invention; Figure 2 is an elevational view of a colour selection electrode to be mounted in the tube of Figure 1 ; Figure 3 is a perspective view of the comer section of the colour selection electrode; Figures 4A and 4B are, respectively, an elevational view and a cross-sectional view of the assembly of a comer section and diaphragm parts with slits according to the invention; Figures 5A and 5B are, respectively, an elevational view and a cross-sectionial view of the assembly of a comer section and diaphragm parts with impressions according to the invention. The colour display tube 1 shown in Figure 1 comprises an evacuated glass envelope 2 with a display window 3, a funnel shaped part 4 and a neck 5. On the inner side of the display window 3 a screen 6 having a pattern of for example lines or dots of phosphors luminescing in different colours (e.g. red, green and blue) may be arranged. The phosphor pattern is excited by the three electron beams 7, 8 and 9 that are generated by the electron gun 10. On their way to the screen, the electron beams 7, 8 and 9 are deflected by the deflection unit 11, ensuring that the electron beams 7, 8 and 9 systematically scan the screen 6. Before the electrons hit the screen 6, they pass through a colour selection electrode 12. This colour selection electrode 12 comprises a shadow mask 13, which is the real colour selection part: it intersects the electron beams so that the electrons only hit the phosphor of the appropriate colour. The shadow mask 13 may be a mask having circular or elongate apertures, or a wire mask. Further, the colour selection electrode 12 comprises the frame 14 for supporting the mask. Parts that can be distinguished in the frame 14 are, amongst others, the corner sections 16 and the diaphragm parts 15, interconnecting the comer sections 16. By means of the suspension elements 20 which are coupled to the comer sections 16, the colour selection electrode 12 is suspended from the display window 3 by using supporting elements 17, which are secured in the upright edge of the comer areas 18 of the display window 3. This way of suspending the colour selection electrode 12 in a colour display tube 1 will further be referred to as comer suspension. In Figure 2 an elevational view of a colour selection electrode 12 is given. The corner sections 16 in this Figure comprise two major portions, a rigid portion 19 for interconnecting the diaphragm parts 15 and a suspension element 20 for suspending the colour selection electrode 12 from the display window 3. The shadow mask 13 is coupled to the diaphragm parts 15. The section 21 of the mask as indicated in Figure 2 serves only as an example. Figure 3 gives a perspective view of the comer section 16, which comprises a rigid portion 19 and a suspension element 20. The rigid portion 19 is normally made from iron in order to obtain enough stiffness, which is necessary for giving the colour selection electrode 12 its mechanical stability. In colour display tubes 1 with an invar shadow mask 13, or with a shadow mask made of another low-expansion steel, the diaphragm parts 15 are made from the same material to improve the doming performance of the colour display tube. The overlapping areas 21 between the rigid portion 19 of the comer section 16 and the diaphragm parts 15 are used for interconnecting both parts. Figures 4 A and 4B give a part of the frame 14 comprising a comer section 16 and two - partly shown - diaphragm parts 15. The suspension means 20 are depicted only schematically. The areas 21 indicate where the diaphragm parts 15 and the rigid portion 19 of the comer section 16 overlap. In this example, the connection between diaphragm part 15 and comer section 16 is made by four spot welds 22. The diaphragm parts 15 are in this example provided with slits 23 which have been cut crosswise in the area 21. By virtue of this construction, a slit 23 is formed crosswise in between any pair of spot welds 22; this is indicated by the mathematical lines 25 connecting any two spot welds 22, and each mathematical line is intersected by a slit 23. A cross-section of this embodiment is given in Figure 4B. When the colour selection electrode is heated to a temperature of about 450 ° C - which is about the temperature in the furnace for assembling the display window 3 and the funnel shaped part 4 - the iron comer section 16 will expand more than the invar diaphragm part 15. In the absence of the slits 23, stress occurs between the welds 22 leading to shearing of these welds. Consequently, the diaphragm parts 15, which are thinner than the comer sections 16, are deformed. The slits which cross each interconnecting line between two spot welds prevent this shearing of the spot welds 22, and the diaphragm parts 15 are not deformed. An alternative embodiment is given in the Figures 5 A and 5B. Instead of slits 23, the diaphragm parts are provided with impressions 24. By virtue of these impressions 24, the diaphragm parts 15 contain, between any two spot welds 22, enough material that is stretched when the comer section 16 expands more than the diaphragm part 15, the effect being the same: preventing shearing of spot welds 22. It will be clear to a person skilled in the art that this invention is not limited to the examples given here. Alternatives may be considered, such as combinations of slits and impressions, rows of small round apertures, impressions with different profiles etc. Summarizing, in the manufacturing process of a colour display tube 1, the display window 3 is assembled with the funnel shaped part 4 in a furnace by subjecting them to a heat treatment. This heat treatment can lead to deformation of the colour selection electrode 12, because in colour display tubes 1 with an invar shadow mask 13, the diaphragm parts 15 are made from invar as well, while the comer sections 16 are made from iron in order to obtain enough stiffness. The difference in expansion coefficient between invar and iron leads to shearing of the welds 22 that connect the diaphragm parts 15 to the comer section 16. To overcome this problem the invention proposes to provide the diaphragm parts 15 with slits 23 or impressions 24, which are arranged so that no stress occurs between the spot welds 22 when the colour selection electrode 12 is heated.

Claims: CLAIMS: 1. A colour display tube (1) comprising a colour selection electrode (12) having a frame (14) comprising comer sections (16) and diaphragm parts (15), which are coupled to the comer sections (16), characterized in that the diaphragm parts (15) are provided with coupling means that increase the thermal stability of the frame (14). 2. A colour display tube (1) as claimed in claim 1, characterized in that the coupling means comprise an area (21) for welding the diaphragm part (15) to the comer section (16), which area further comprises slits (23). 3. A colour display tube (1 ) as claimed in claim 1 , characterized in that the coupling means comprise an area (21) for welding the diaphragm part (15) to the comer section (16), which area further comprises impressions (24). 4. A colour display tube (1) as claimed in claim 2 or 3, characterized in that the diaphragm parts (15) are welded to the comer sections (16) by means of a plurality of spot welds (22). 5. A colour display tube (1) as claimed in claim 4, characterized in that the diaphragm parts (15) further comprise mathematical lines (25) connecting the two spot welds (22) of any pair, which mathematical lines (25) are intersected by at least one slit (23) or impression (24). 6. A colour selection electrode (12) for use in the colour display tube (1) of one of the preceding claims. 7. A diaphragm part (15) for use in the colour selection electrode (12) of claim 6.

Inventors: HENS THEODOOR C A, VAN DE STEEN WILHELMUS M M, WINTERS JEAN, HENS, THEODOOR, C., A., VAN DE STEEN, WILHELMUS, M., M., WINTERS, JEAN
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H01J29
IPC codes: H01J29