Getter for use in the manufacture of an electric discharge tube

#The invention relates to a method of manufacturing an electric discharge tube in which a holder which contains a mixture, termed getter, which mainly consists of barium-aluminium (BaAl.sub.4) and a titanium-containing material, is placed in a location inside the glass envelope of the tube, after which at least two glass parts of the envelope of the tube are secured together at high temperature and barium, as the actual getter of the residual gases still present in the tube after evacuation, is then evaporated from the said mixture.

#The invention furthermore relates to a getter suitable for such a method, as well as to an electric discharge tube obtained by said method.

#In manufacturing an electric discharge tube, for example, a television display tube, it is usual that the getter is placed inside the envelope of the tube after securing the glass conical part of the tube to the glass window. In certain cases, however, it is necessary for the getter to be present already inside the cone before it is secured to the window. Such a case presents itself, for example, when a resistance layer which should restrict the detrimental results of high voltage breakdown, is provided on the inside of the envelope of the display tube at the area where the narrow neck changes into the conical part. It is usual that the getter holder is provided in the conical part of the tube close to the transition from neck to cone by means of a metal strip which is secured at one end to the system of guns. This method cannot be used because the said resistance layer would be short-circuited by the gettering material deposited thereon from the getter. In addition, discharges between the metal strip and the resistance layer are hard to avoid. Another method of placing the getter holder, however, cannot be realized at all or can be realized with difficulty only when the window and the cone are already secured together. For that reason, the holder with the getter provided therein is placed in its position inside the conical part of the tube before the window is secured to the cone. The result of this is that the getter is present during the whole process in which the cone and the window are secured together. This process is carried out in air in a furnace at a temperature of 450.degree. C. A usual composition of the getter, for example, a mixture of barium aluminium powder and nickel powder, can in that case not be used as such since in these circumstances the nickel of the getter is at least partly converted into nickel oxide. It has been found that the resulting variation of the composition produces a getter the constituents of which react so vehemently upon heating that several particles are flung away from it. In addition, too low a barium efficiency which, in particular, is not reproducible, is obtained. Moreover, said separate particles may produce spots on the display screen or result in electric breakdown in the tube.

#The invention will be described in detail with reference to the accompanying drawing, in which

#FIG. 1 is an axial sectional view of a getter holder having an annular channel and

#FIG. 2 is an axial sectional view of a colour television display tube obtained according to the invention.

#The holder in FIG. 1 comprises a chromium-nickel-steel channel 1 the depth h of which is 2 millimetres and the width b is 5 millimetres. The outward diameter of the annular channel 1 is 29.8 millimetres. The getter 2 is compressed in the channel 1 and consists of a mixture of 12 parts by weight of BaAl.sub.4, 3 parts by weight of NiTi.sub.2 and 1 part by weight of Ni.sub.3 Ti.

#The colour television display tube shown diagrammatically in FIG. 2 comprises a neck 10, a cone 11 and a glass window 12. A phosphor layer 13 consisting of a red, green and blue fluorescent phosphor is provided on the inside of the window. The tube furthermore comprises a shadow mask 15 which is secured to the mask ring 16 as is also a metal screening cap 17. A getter 21 according to the invention is present in a metal annular holder 20 which is secured to one end of a metal strip 19. The other end of the metal strip 19 is welded to the screening cap 17 at 22. At a temperature of 450.degree. C., the window 12 is then secured to the cone 11 at 18. The diagrammatic system of guns 14 is then placed in the neck 10 of the tube after which the tube is evacuated. After evacuation of the tube, the holder 20 and the getter 21 present therein is heated inductively for 30 seconds in such manner that approximately 11 seconds after the beginning thereof the first barium is released from the getter. The place and position of the holder 20 are so that no barium vapour is deposited on the resistance layer 23 which is provided near the transition from neck 10 to cone 11 and which changes at 24 into the usual readily conductive layer 25 provided on the inside of the cone 11. Said vapour is now deposited inside the space formed by the mask 15 and the screening cap 17.