DE1184424B - Indirectly heated cathode for an electrical discharge tube and process for its manufacture - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Boarding school Kl .: HOIj

German class: 21 g - 13/04

Number: 1184424

File number: N 23866 VIII c / 21 g

Filing date: October 11, 1963

Opening day: December 31, 1964

The invention relates to an indirectly heated cathode for an electrical discharge tube with a radiator, which consists of a covered with an insulating layer and with a low slope coiled tungsten wire and its insulating layer with one made of insulating material hollow helix is provided, the pitch of which exceeds that of the tungsten wire. Furthermore relates the invention to a method for producing such a cathode, preferably in a electric discharge tube is used, where large between the cathode and the filament Potential differences can occur.

It has been found that the risk of a breakdown between the filament and the Cathode shell always increases during the service life of an indirectly heated cathode. This is due to the fact that the insulation resistance between the filament and the cathode is always gets worse as metal ions from the filament gradually enter the insulating material of the radiator diffuse in.

It is therefore important that the insulating material has as few points of contact with the filament as possible or the cathode envelope. It has therefore already been proposed to use an insulated wire with a large Wrap the slope around the radiator, acting as a spacer between the radiator and the Cathode shell is used.

However, this type of construction has several disadvantages. Although the spacer wire is only proportionate has a small number of points of contact with the insulating material of the radiator, all of them Leakage currents flowing through these points are absorbed by the metal wire and throughout its whole Length, and the wire can get a relatively high potential ·. Because the outside the entire length of the wire with the insulating layer rests against the inside of the cathode sheath, In this case, the insulation resistance is much lower and the leakage current is relatively higher. In addition, after some time, as a result of diffusion of metal ions into the insulating material a breakdown occurs again.

The spacer wire also has a large heat capacity, so that the warm-up time of the cathode through this type of construction is considerably lengthened.

In another known design, these disadvantages have been avoided in that a with Insulating material coated spacer wire made of molybdenum is used, which is subsequently based on chemical Paths is removed, leaving a hollow helix

Indirectly heated cathode for an electrical one
Discharge tube and procedure for its
Manufacturing

Applicant:

N. V. Philips' Gloeilampenfabrieken,

Eindhoven (Netherlands)

Representative:

Dipl.-Ing. E. E. Walther, patent attorney,

Hamburg 1, Mönckebergstr. 7th

Named as inventor:

Pieter Geert van Zanten;

Friedrich Hermann Raymund Almer;

Adrianus Kuiper, Eindhoven (Netherlands)

Claimed priority:

Netherlands of October 15, 1962 (284 356)

of insulating material remains on the radiator.

Although in this case the insulation resistance is very good and no breakdown even after long use occurs, it turns out that the hollow coil is very brittle, so that by breaking the coil When inserting the. Radiator in the cathode shell of the committee is large. Next is the im strong spacer wire processed the amount of molybdenum relatively large, so that the chemical dissolving of the wire takes up a lot of time. In addition, molybdenum is relatively expensive.

The disadvantages of this known design can be reduced to a considerable extent, i. b one helix is more sufficient Strength established and the amount of molybdenum to be dissolved can be significantly reduced if According to the invention, the wall of the hollow helix is provided with local thickenings. This can thereby can be achieved that the massive spacer wire by a thin wire wound on a core is replaced. Even if the core is also made of molybdenum, the total amount of molybdenum is of the core and the coiled thin wire is considerably lower than that of a solid wire with same outside diameter. The core may optionally be made of a cheaper material, e.g. B. iron, exist. The metal of the core must, as well as

409 760/284

Molybdenum, to be soluble in a liquid that does not attack the tungsten of the filament, to the chemical Allow loosening after applying the insulating material.

The hollow coil can be placed around the coiled filament, but can also be attached to the radiator as such, which is created by folding the heating coil, which is coated with insulating material is obtained. According to the chemical route carried out Solution of the helically wound spacer wire on a core results in a hollow one Helix, the wall of which extends helically over its inner surface and possibly also over has its outer surface extending thickening. It turns out that the hollow helix has now become much stronger mechanically, so that no damage is caused when it is pushed into the cathode envelope occurs. In addition, it turns out that the heat transfer is significantly improved, what is probably due to the fact that the heat rays are reflected in the hollow coil due to the presence of the reinforcing ribs, it is better absorbed by the wall and directed towards it the cathode shell are radiated.

The invention is explained in more detail below with reference to the drawing in which FIGS. 1, 2 and 3 show an embodiment of a radiator according to the invention, and F i g. 4 and 5 another Illustrate embodiment.

In FIGS. 1, 2 and 3, 1 denotes the coiled filament made of tungsten, which is covered with insulating material 2 which is provided with a hollow, insulating coil 3. Like the top half of the Fig. 2 shows, the heating element is produced by winding the heating filament 1 with a low pitch on a molybdenum core 4, after which a structure consisting of a wire 5 wound on a core 6 is wound on this heating filament 1 with a high pitch. The wire 5 is made of molybdenum. The core 6 can also consist of molybdenum, but also of another metal which, like molybdenum, can be dissolved chemically by a liquid which does not attack tungsten. Then the whole thing, e.g. B. by cataphoretic means, coated with an insulating layer, which preferably consists of pure aluminum oxide (Al ₂ O ₃ ), after which it is sintered to 1600 ° C. This makes the insulating layer hard. Then the molybdenum is dissolved in the usual way in a bath containing nitric acid. A hollow heating coil consisting of tungsten wire is then created, which is embedded in insulating material 2 and surrounded by a coil 3 with a cavity 7. The cavity 7 has a surface with a likewise helical thickening 8. If the insulating layer 3 is not too thick, a thickening 9 is also present on the outside.

The thickenings 8 and 9 provide several surprising advantages.

First, it turns out that the wall of the helix 3 has become much stronger mechanically.

It is also found that the heat transfer is greatly improved while also maintaining a more uniform filament temperature. This is probably due to the fact that the infrared rays which are reflected in the cavity 7 are deflected by the thickenings 8 in the direction of the cathode shell. The Al ₂ O ₃ is in fact almost completely permeable to these rays, so that the infrared rays, when they reach the outer wall 3, are directed towards the inner surface of the cathode shell by refraction and reflection.

Furthermore, the outer surface of the hollow helix 3 is only with the narrow upper edge of the thickening 9 in contact with the cathode envelope. As a result, the insulation resistance is very high. In contrast to the known embodiment, no metal ions can get into the insulating material of the Diffuse in hollow coil 3.

According to the embodiment of FIG. 4 and 5 the radiator is initially in the usual way formed in that the tungsten wire 1 wound on a molybdenum core is folded in a zigzag shape and then coated with insulating material. After the insulating material has been sintered, the MolybdSakern removed chemically.

If necessary, a heating element of this type can then be wound onto a core afterwards Spacer wire 5, 6 are wound, and after attaching and sintering the insulating material can the wire parts 5 and 6 are removed chemically. On the first applied insulating layer 10 of the heating element is now located a second insulating layer 11 with a hollow helix 12, which on the inner surface and possibly also has thickenings in the form of ribs 13 and 14 on the outer surface.

The invention will be explained in more detail using an exemplary embodiment.

A tungsten filament with a thickness of 68 μ is wound with a pitch of 110 μ on a molybdenum core with a diameter of 280 μ.

A molybdenum wire of 60 μ is with a pitch of 180 μ around the molybdenum core of 100 μ wrapped.

This second core with the wound wire is wound around the first, wound with the tungsten wire core, preferably with a winding direction, which is opposite to that of the tungsten wire. Then the whole thing becomes cataphoretic covered with an insulating layer with an average thickness of about 100 μ and for 10 minutes heated to 1600 ° C. Then the radiator will dissolve the molybdenum cores during about a half Put in a mixture of about 2 parts of nitric acid and 1 part of sulfuric acid for an hour. After this he dried and possibly burned has been, the radiator is ready for insertion into a cathode can.

Instead of a hollow helix, two helices running in opposite directions can also be used be used.

Claims (3)

Patent claims: 1. Indirectly heated cathode for an electrical discharge tube with a heating element, which consists of a coated with an insulating layer and coiled with a slight slope Consists of tungsten wire and its insulation is provided with a hollow coil made of insulating material, the pitch of which is that of the Tungsten wire, thereby indicates that the wall of the hollow coil is provided with local thickenings. 2. A method of making a cathode according to claim 1, wherein a tungsten filament With a low pitch, helically wound on a molybdenum core, with insulating material coated and sintered and then after chemical removal of the molybdenum core in a cathode envelope is inserted, characterized in that prior to attaching the Insulating material a second core on which a molybdenum wire is wound, with a larger one Pitch than that of the tungsten wire is wound over this, after which the whole thing once or is bent several times in a zigzag. 3. The method for manufacturing a cathode according to claim 1, wherein a tungsten wire with ίο wound on a molybdenum core with a low pitch, the whole thing bent once or several times in a zigzag, covered with insulating material and is sintered, characterized in that a second core on which a molybdenum wire is wound, helically thereon insulated radiator attached and the whole thing covered with a second layer of insulating material and sintered, after which the molybdenum is chemically removed and the whole thing after drying and possibly burning, is inserted into a cathode envelope. Considered publications:
German Patent No. 1124158;
French patent specification No. 1236 862. 1 sheet of drawings 409 760/284 12.64 © Bundesdruckerei Berlin