INDIRECTLY HEATED CATHODE FOR AN ELECTRON DISCHARGE TUBE WITH AN INSULATED HEATING ELEMENT
United States Patent 3553521
An indirectly heated cathode for electron discharge tube having a refractory metal support, e.g. molybdenum or tungsten, the inner surface of which is coated with a thin layer of a metal of the platinum group to minimize oxidation of the refractory metal. The cathode is insulated from a heating element by a layer of alumina which surrounds the heating element.
US Patent References:
Electron device and the like adapted for alternating current
Myers - July 1929 - 1719988
/2733378.html
Aisenstein et al. - January 1956 - 2733378
Electronic tube structure
Millis - March 1959 - 2878410
Composite wire
Hagadorn et al. - August 1966 - 3268305
Cold-cathode glow-discharge tube
Hall et al. - November 1966 - 3283195
Electron device and the like adapted for alternating current
Myers - July 1929 - 1719988
/2733378.html
Aisenstein et al. - January 1956 - 2733378
Electronic tube structure
Millis - March 1959 - 2878410
Composite wire
Hagadorn et al. - August 1966 - 3268305
Cold-cathode glow-discharge tube
Hall et al. - November 1966 - 3283195
Inventors:
Bakker, Marinus Antonius Maria (Emmasingel, Eindhoven, NL)
Duran, Johannes Christiaan (Emmasingel, Eindhoven, NL)
Duran, Johannes Christiaan (Emmasingel, Eindhoven, NL)
Application Number:
04/701204
Publication Date:
01/05/1971
Filing Date:
01/29/1968
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Export Citation:
Assignee:
U. S. Philips Corporation (New York, NY)
Primary Class:
Other Classes:
313/355, 313/356, 313/629
International Classes:
H01J1/24; H01J1/26; H01J1/20; H01J1/20; H01J19/14
Field of Search:
313/311,337,340,356,217,218,355
US Patent References:
| 3401297 | Thermionic cathodes for electron discharge devices with improved refractory metal heater wires | September 1968 | Feinleib |
Primary Examiner:
Huckert, John W.
Assistant Examiner:
James, Andrew J.
Claims:
We claim
1. An indirectly heated cathode for an electron discharge tube provided with a heating wire which is insulated by means of a metal oxide from a support for an emissive layer, at least the surface of this support facing the heating wire consisting of molybdenum, said surface having thereon a layer of at least one of the metals of the platinum group having a thickness of at least 3μ.
2. A cathode as claimed in claim 1, wherein the heating wire has a layer of a metal of the platinum group.
1. An indirectly heated cathode for an electron discharge tube provided with a heating wire which is insulated by means of a metal oxide from a support for an emissive layer, at least the surface of this support facing the heating wire consisting of molybdenum, said surface having thereon a layer of at least one of the metals of the platinum group having a thickness of at least 3μ.
2. A cathode as claimed in claim 1, wherein the heating wire has a layer of a metal of the platinum group.
Description:
The invention relates to an indirectly heated cathode provided with a heating wire which is insulated by alumina from a support for the emissive material, while at least the surface of this support facing the heating wire consists of molybdenum.
It is known that the tungsten or molybdenum of a heating wire for an indirectly heated cathode may be attacked by oxygen, in which event the WO 3 or MoO 3 formed evaporates and reacts with the insulating material consisting of alumina. The metal is then released and the insulation resistance of the insulating material gradually decreases. According to the known method, this is prevented by coating the tungsten or molybdenum heating wire with a suitable metal of the platinum group and by fusing this metal to the heating wire by a electrical current transient and strong heat treatment. It is emphatically stated that only molten layers of these metals are effective, whereas nonmolten layers of these metals applied, for example, by electroplating may exhibit the said disadvantage to an even greater extent. In order to prevent the resistance of the heating wire from becoming unduly low, the layers must be extremely thin (at the most 1 μ). Platinum itself cannot be used, since at the high temperature of the heating wire it is dissolved in the molybdenum during the melting process, while rhenium has an unduly high melting temperature so that it cannot be fused to the heating wire.
However, it also has been found that if the heating wire is coated with a molten layer of one of the suitable metals of the Pt group such as rhodium, iridium or ruthenium, the insulation resistance still decreases in cathodes in which at least the surface of the support for the emissive material facing the heating wire consists of molybdenum, especially if the support has a positive potential with respect to the heating wire.
Many investigations have shown that this decrease in resistance is due to oxidizing gases released in the tube, such as water vapor, which have an oxidizing effect on the molybdenum, in which event the molybdenum oxide may be reduced in the A1 2O 3 and the metal may be released, or an oxide of lower valency which is dissolved in the A1 2O 3 .
In this case, the known method described above cannot be used, because, owing to the greater mass of the support, the process of melting the suitable metals of the Pt group cannot be carried out so rapidly that a reaction with the molybdenum can be avoided.
However, it has been found that in this case a favourable effect may also be obtained by the application of a nonmolten layer of one of the said metals of the Pt group if the thickness of such a layer is 3 microns or more. A Pt layer having a thickness of 3 microns is found to have a sufficient density. It has been found that due to the lower temperature of the support, only such a small quantity of Pt is dissolved in the Mo of the support and conversely that this does not give rise to difficulties during the life of the cathode. In this case, the comparatively thick metal layer does not give rise to difficulties either with respect to the resistance of the support so that such comparatively thick layers can be used without any objection.
The invention is preferably carried out in conjunction with the known method described.
The invention will be described more fully with reference to the accompanying drawing, which is a sectional view of a cathode according to the invention.
In the drawing, reference numeral 1 denotes the support which in this case entirely consists of molybdenum. The heater body 2 consists of a heating wire 3 to which a layer 6 of rhodium, ruthenium or iridium having a thickness of less than 1 μ is fused by the known method. The heating wire 3 is coated with alumina.
The support 1 is provided with an emissive layer 4 which preferably consists of a mixture of nickel powder and alkaline earth metal oxides. The surface of the support 1 facing the heating wire 3 is coated with a platinum layer having a thickness of 5 μ. Instead of platinum, also one of the other metals of the platinum group may be used for the layer 5. The layer 5 is preferably applied by absorption of a platinum suspension or by electroplating.
A cathode according to the invention is particularly suitable for use in circuit arrangements in which a potential difference of more than 400 v. may occur between the heating wire and the cathode support, the support 1 being positive with respect to the heating wire 3.
Although only one embodiment of a cathode according to the invention has been described, the invention also applies to differently shaped cathodes, for example, dispenser cathodes. Furthermore, the emissive material may also be applied to a tubular support. Instead of A1 2O 3 , MgO may be used.
It is known that the tungsten or molybdenum of a heating wire for an indirectly heated cathode may be attacked by oxygen, in which event the WO 3 or MoO 3 formed evaporates and reacts with the insulating material consisting of alumina. The metal is then released and the insulation resistance of the insulating material gradually decreases. According to the known method, this is prevented by coating the tungsten or molybdenum heating wire with a suitable metal of the platinum group and by fusing this metal to the heating wire by a electrical current transient and strong heat treatment. It is emphatically stated that only molten layers of these metals are effective, whereas nonmolten layers of these metals applied, for example, by electroplating may exhibit the said disadvantage to an even greater extent. In order to prevent the resistance of the heating wire from becoming unduly low, the layers must be extremely thin (at the most 1 μ). Platinum itself cannot be used, since at the high temperature of the heating wire it is dissolved in the molybdenum during the melting process, while rhenium has an unduly high melting temperature so that it cannot be fused to the heating wire.
However, it also has been found that if the heating wire is coated with a molten layer of one of the suitable metals of the Pt group such as rhodium, iridium or ruthenium, the insulation resistance still decreases in cathodes in which at least the surface of the support for the emissive material facing the heating wire consists of molybdenum, especially if the support has a positive potential with respect to the heating wire.
Many investigations have shown that this decrease in resistance is due to oxidizing gases released in the tube, such as water vapor, which have an oxidizing effect on the molybdenum, in which event the molybdenum oxide may be reduced in the A1 2O 3 and the metal may be released, or an oxide of lower valency which is dissolved in the A1 2O 3 .
In this case, the known method described above cannot be used, because, owing to the greater mass of the support, the process of melting the suitable metals of the Pt group cannot be carried out so rapidly that a reaction with the molybdenum can be avoided.
However, it has been found that in this case a favourable effect may also be obtained by the application of a nonmolten layer of one of the said metals of the Pt group if the thickness of such a layer is 3 microns or more. A Pt layer having a thickness of 3 microns is found to have a sufficient density. It has been found that due to the lower temperature of the support, only such a small quantity of Pt is dissolved in the Mo of the support and conversely that this does not give rise to difficulties during the life of the cathode. In this case, the comparatively thick metal layer does not give rise to difficulties either with respect to the resistance of the support so that such comparatively thick layers can be used without any objection.
The invention is preferably carried out in conjunction with the known method described.
The invention will be described more fully with reference to the accompanying drawing, which is a sectional view of a cathode according to the invention.
In the drawing, reference numeral 1 denotes the support which in this case entirely consists of molybdenum. The heater body 2 consists of a heating wire 3 to which a layer 6 of rhodium, ruthenium or iridium having a thickness of less than 1 μ is fused by the known method. The heating wire 3 is coated with alumina.
The support 1 is provided with an emissive layer 4 which preferably consists of a mixture of nickel powder and alkaline earth metal oxides. The surface of the support 1 facing the heating wire 3 is coated with a platinum layer having a thickness of 5 μ. Instead of platinum, also one of the other metals of the platinum group may be used for the layer 5. The layer 5 is preferably applied by absorption of a platinum suspension or by electroplating.
A cathode according to the invention is particularly suitable for use in circuit arrangements in which a potential difference of more than 400 v. may occur between the heating wire and the cathode support, the support 1 being positive with respect to the heating wire 3.
Although only one embodiment of a cathode according to the invention has been described, the invention also applies to differently shaped cathodes, for example, dispenser cathodes. Furthermore, the emissive material may also be applied to a tubular support. Instead of A1 2O 3 , MgO may be used.