Title:
CATHODE FOR AN ELECTRIC DISCHARGE TUBE
United States Patent 3823337
Abstract:
A cathode suitable for poor vacuum conditions and repeated exposures to air consists of a molybdenum support, compact or compressed powder, welded on a tungsten wire and having thereon a compressed mixture of rhenium with lanthanum hexaboride. Instead of rhenium, a tungsten boride such as W2 B or W2 B5 may be chosen.
US Patent References:
Boride cathodes
Lafferty - November 1953 - 2659685
Rhenium supported metallic boride cathode emitters
Lafferty et al. - April 1967 - 3312856
LANTHANUM HEXABORIDE CATHODE SYSTEM FOR AN ELECTRON BEAM GENERATOR
Schiller et al. - April 1969 - 3440475
HOLDER FOR HIGHLY REACTIVE CATHODES OF RARE-EARTH BORIDES SUCH AS LANTHANUM HEXABORIDE,THE HOLDER PROVIDED WITH A COOLING MEANS OPPOSITE TO THE EMISSIVE END OF THE CATHODE IN ORDER TO REDUCE TENDENCY OF HOLDER DETERIORATION
Broers - August 1969 - 3462635
PYROLYTIC GRAPHITE SUPPORT FOR LANTHANUM HEXABORIDE CATHODE EMITTER
Vogel - October 1970 - 3532923
Boride cathodes
Lafferty - November 1953 - 2659685
Rhenium supported metallic boride cathode emitters
Lafferty et al. - April 1967 - 3312856
LANTHANUM HEXABORIDE CATHODE SYSTEM FOR AN ELECTRON BEAM GENERATOR
Schiller et al. - April 1969 - 3440475
HOLDER FOR HIGHLY REACTIVE CATHODES OF RARE-EARTH BORIDES SUCH AS LANTHANUM HEXABORIDE,THE HOLDER PROVIDED WITH A COOLING MEANS OPPOSITE TO THE EMISSIVE END OF THE CATHODE IN ORDER TO REDUCE TENDENCY OF HOLDER DETERIORATION
Broers - August 1969 - 3462635
PYROLYTIC GRAPHITE SUPPORT FOR LANTHANUM HEXABORIDE CATHODE EMITTER
Vogel - October 1970 - 3532923
Inventors:
Van Stratum, Antonius Johannes Alberta (Emmasingel Eindhoven, NL)
Zalm, Pieter (Emmasingel Eindhoven, NL)
Mitra, Niranjan Kumar (Best, NL)
Zalm, Pieter (Emmasingel Eindhoven, NL)
Mitra, Niranjan Kumar (Best, NL)
Application Number:
05/363877
Publication Date:
07/09/1974
Filing Date:
05/25/1973
View Patent Images:
Export Citation:
Assignee:
U.S. Philips Corporation (New York, NY)
Primary Class:
Other Classes:
313/337
International Classes:
H01J1/148; H01J1/15; H01J1/13; H01J19/06; H01J1/14
Field of Search:
313/336,337,346
US Patent References:
| 3631291 | FIELD EMISSION CATHODE WITH METALLIC BORIDE COATING | December 1971 | Favreau |
Primary Examiner:
Lawrence, James W.
Assistant Examiner:
Chatmon Jr., Saxfield
Attorney, Agent or Firm:
Trifari, Frank Steinhauser Carl R. P.
Claims:
What is claimed is
1. A directly heated cathode for an electric discharge tube comprising a tungsten filament, a molybdenum plate welded to said tungsten filament, and a layer consisting of 30 percent by weight of a compressed mixture of lanthanum hexaboride and the remainder essentially rhenium or a tungsten boride on said molybdenum plate.
2. A cathode as claimed in claim 1, characterized in that the tungsten boride consists of W2 B or W2 B5.
1. A directly heated cathode for an electric discharge tube comprising a tungsten filament, a molybdenum plate welded to said tungsten filament, and a layer consisting of 30 percent by weight of a compressed mixture of lanthanum hexaboride and the remainder essentially rhenium or a tungsten boride on said molybdenum plate.
2. A cathode as claimed in claim 1, characterized in that the tungsten boride consists of W2 B or W2 B5.
Description:
The invention relates to a directly heated cathode of an electric discharge tube which is activated with lanthanum hexaboride and which emits an electron beam of a small cross-section.
The known cathodes of this type consist of a rhenium wire bent in the form of a hairpin and covered cataphoretically with a layer of lanthanum hexaboride, which layer is sintered on it (Rev. Sc. Inst. 856-7, 1965). The drawback of such a cathode as a punctiform electron source is that the curved end of the hairpin filament is not very suitable for a good concentration of the electron beam because the surface is convex and not plane or concave. Also, a very expensive rhenium filament is required because tungsten is attacked by lanthanum hexaboride without a special intermediate layer.
It is known from Radio Engineering and Electronic Physics 12, 798-802, 1967, in particular p. 800, line 11 from bottom, to add to compressed lanthanum boride a small quantity (optimum 10 percent by weight) of rhenium, tungsten, titanium or iridium. However, such a mixture is very difficult to compress. The said metals release lanthanum from the lanthanum boride which produces a low work function at the surface of the cathode.
It is the object of the invention to provide a cathode which is activated with lanthanum hexaboride, enables a good focusing and in addition has a long life.
According to the invention, a directly heated cathode for an electric discharge tube which is activated with lanthanum hexaboride to which a material is added which releases lanthanum consists of at least one tungsten filament on which a molybdenum plate is welded having thereon a compressed mixture of lanthanum hexaboride and rhenium or a tungsten boride, the lanthanum boride being at most 30 percent by weight of the whole. In its simplest form the filament is a wire bent in the form of a hairpin on the top of which the molybdenum plate is welded. In cathodes of a slightly larger diameter, a wire bent in a zig-zag manner or a spirally wound wire, or separate hair-pin wires may be used as a filament.
The molybdenum plate may be compressed simultaneously with the layer of powder compressed on it or it may consist of a compact metal. Tungsten boride preferably consists of W 2 B or W 2 B 5 . No excessive formation of free lanthanum is obtained. In so far as rhenium is used, this occurs only in powder form in the compressed layer which has a smaller influence on the price than a rhenium filament.
In contrast with the known compositions, the mixture according to the invention can be excellently compressed.
In the construction according to the invention the molybdenum forms a protective layer between the lanthanum boride and the tungsten filament. As a result of this, attack of the tungsten and hence shorting of the life are prevented. The plate may be circular in crosssection or be polygonal and be more or less concave instead of plane in connection with the focusing of the produced electron beam.
At a temperature of 1,400° C, good emission densities and long lives can be obtained with such cathodes.
Just like other lanthanum hexaboride-activated cathodes, those according to the invention are also suitable for discharge tubes having poor vacuum conditions and for tubes in which the cathodes are exposed to air several times, for example, in electron microscopes.
It is to be noted that a cathode is known from the U.S. Pat. No. 3,027,480 in which a mixture of tungsten, thorium tetraboride and rhenium as a mixture is sprayed on a tungsten cylinder or in an aperture in a cap. The mixture may also be shaped previously to form a hollow cylinder. The less agressive thorium tetraboride makes a protection between the mixture and the tungsten supporting body which in addition is indirectly heated superfluous. Moreover, the mixture is not compressed which in the cathode according to the invention is necessary for a good life.
The invention will be described in greater detail with reference to the drawing which shows a cathode according to the invention.
In the FIGURE, 1 denotes a hard glass tube bottom in which supply wires 2 and 3 of an iron-nickel-cobalt alloy are sealed. A tungsten wire 4 of 0.18 mm diameter is welded to the wires 2 and 3. Welded to the top of the hairpin wire 4 is a compressed cathode body consisting of a layer 5 of molybdenum powder with on top a layer 6 of a mixture of rhenium powder and lanthanum hexaboride in a weight ratio of 5:2.
The overall thickness of the cathode body is 0.3 mm and the diameter is 1 mm. The pressure of compression was 3,000 kg/cm 2 .
Prior to compression, the molybdenum powder is fired in hydrogen at 1,300° C for 30 minutes. The whole cathode body is heated in hydrogen at 1,750° C for 15 seconds. The cathode is degassed in a discharge tube at 1,600° C and activated at 1,500° C. The saturation emission at 1,400° C may amount to 25 A/cm 2 .
The known cathodes of this type consist of a rhenium wire bent in the form of a hairpin and covered cataphoretically with a layer of lanthanum hexaboride, which layer is sintered on it (Rev. Sc. Inst. 856-7, 1965). The drawback of such a cathode as a punctiform electron source is that the curved end of the hairpin filament is not very suitable for a good concentration of the electron beam because the surface is convex and not plane or concave. Also, a very expensive rhenium filament is required because tungsten is attacked by lanthanum hexaboride without a special intermediate layer.
It is known from Radio Engineering and Electronic Physics 12, 798-802, 1967, in particular p. 800, line 11 from bottom, to add to compressed lanthanum boride a small quantity (optimum 10 percent by weight) of rhenium, tungsten, titanium or iridium. However, such a mixture is very difficult to compress. The said metals release lanthanum from the lanthanum boride which produces a low work function at the surface of the cathode.
It is the object of the invention to provide a cathode which is activated with lanthanum hexaboride, enables a good focusing and in addition has a long life.
According to the invention, a directly heated cathode for an electric discharge tube which is activated with lanthanum hexaboride to which a material is added which releases lanthanum consists of at least one tungsten filament on which a molybdenum plate is welded having thereon a compressed mixture of lanthanum hexaboride and rhenium or a tungsten boride, the lanthanum boride being at most 30 percent by weight of the whole. In its simplest form the filament is a wire bent in the form of a hairpin on the top of which the molybdenum plate is welded. In cathodes of a slightly larger diameter, a wire bent in a zig-zag manner or a spirally wound wire, or separate hair-pin wires may be used as a filament.
The molybdenum plate may be compressed simultaneously with the layer of powder compressed on it or it may consist of a compact metal. Tungsten boride preferably consists of W 2 B or W 2 B 5 . No excessive formation of free lanthanum is obtained. In so far as rhenium is used, this occurs only in powder form in the compressed layer which has a smaller influence on the price than a rhenium filament.
In contrast with the known compositions, the mixture according to the invention can be excellently compressed.
In the construction according to the invention the molybdenum forms a protective layer between the lanthanum boride and the tungsten filament. As a result of this, attack of the tungsten and hence shorting of the life are prevented. The plate may be circular in crosssection or be polygonal and be more or less concave instead of plane in connection with the focusing of the produced electron beam.
At a temperature of 1,400° C, good emission densities and long lives can be obtained with such cathodes.
Just like other lanthanum hexaboride-activated cathodes, those according to the invention are also suitable for discharge tubes having poor vacuum conditions and for tubes in which the cathodes are exposed to air several times, for example, in electron microscopes.
It is to be noted that a cathode is known from the U.S. Pat. No. 3,027,480 in which a mixture of tungsten, thorium tetraboride and rhenium as a mixture is sprayed on a tungsten cylinder or in an aperture in a cap. The mixture may also be shaped previously to form a hollow cylinder. The less agressive thorium tetraboride makes a protection between the mixture and the tungsten supporting body which in addition is indirectly heated superfluous. Moreover, the mixture is not compressed which in the cathode according to the invention is necessary for a good life.
The invention will be described in greater detail with reference to the drawing which shows a cathode according to the invention.
In the FIGURE, 1 denotes a hard glass tube bottom in which supply wires 2 and 3 of an iron-nickel-cobalt alloy are sealed. A tungsten wire 4 of 0.18 mm diameter is welded to the wires 2 and 3. Welded to the top of the hairpin wire 4 is a compressed cathode body consisting of a layer 5 of molybdenum powder with on top a layer 6 of a mixture of rhenium powder and lanthanum hexaboride in a weight ratio of 5:2.
The overall thickness of the cathode body is 0.3 mm and the diameter is 1 mm. The pressure of compression was 3,000 kg/cm 2 .
Prior to compression, the molybdenum powder is fired in hydrogen at 1,300° C for 30 minutes. The whole cathode body is heated in hydrogen at 1,750° C for 15 seconds. The cathode is degassed in a discharge tube at 1,600° C and activated at 1,500° C. The saturation emission at 1,400° C may amount to 25 A/cm 2 .