[0001] The invention relates to a method of manufacturing a dispenser cathode comprising
a barium compound for dispensing barium to an emissive surface of a porous cathode
body substantially comprising a metal melting at a high temperature.
[0002] The invention also relates to a dispenser cathode manufactured by means of such a
method and to an electron tube provided with such a cathode.
[0003] A characteristic feature of dispenser cathodes is that there is a functional separation
between the electron emissive surface on the one hand and a store of the emitter material
for realizing a sufficiently low work function on the emissive surface on the other
hand. The emitter material is present in the pores of the porous metal cathode body.
[0004] A method of the type mentioned in the opening paragraph is described in United States
Patent No. 4,077,393. This patent describes how a cathode body with a porosity of
approximately 20% compressed from tungsten powder and subsequently sintered is impregnated
with a mixture which comprises calcium oxide, aluminium oxide and scandium oxide in
addition to barium oxide.
[0005] European Patent Specification No. 0,091,161 describes how such cathodes can be improved
on sensitivity to and recovery after ion bombardment by compressing the cathode body
(notably the top layer) from a mixture of tungsten powder and scandium oxide and by
subsequently sintering it. To obtain a cathode body with a thin top layer (approximately
0.1 mm) which is as homogeneous as possible the compressing operation is generally
performed in two steps. Firstly, the tungsten portion of the cathode body is slightly
precompressed. Subsequently, the top layer powder is evenly distributed over a surface
of the tungsten portion whereafter the definitive compressing operation is performed.
[0006] It is an object of the invention to provide a different method of manufacturing such
a dispenser cathode, which method is simpler and leads to similar results as regards
current density and lifetime.
[0007] To this end a method according to the invention is characterized in that the cathode
body is compressed from a quantity of metal powder which is mixed with scandium or
scandium hydride whereafter the body is sintered and the cathode is provided with
emitter material. The quantity of scandium or scandium hydride in the quantity of
metal powder is preferably 0.3-0.7% by weight.
[0008] From a manufacturing technical point of view such a method is more advantageous because
compressing is only to be performed in one operation and the distribution of the top-layer
powder is no longer necessary. After the introduction of the impregnant the cathode
bodies manufactured by means of such a method can undergo mechanical treatments such
as turning or other types of shaping without any detrimental effects.
[0009] In order to prevent as much as possible that scandium is lost during sintering which
is preferably performed in a hydrogen atmosphere, this sintering operation is preferably
performed at a temperature which is lower than the melting point of scandium (1539°C).
However, on the other hand the sintering temperature must be chosen to be as high
as possible in order to obtain a sufficiently robust cathode body.
[0010] A preferred embodiment of a method according to the invention is therefore characterized
in that the sintering temperature is between 1430°C and 1500°C.
[0011] The invention will now be described in greater detail by way of example with reference
to the accompanying drawing, in which:
Figure 1 is a longitudinal cross-section of a cathode according to the invention and
Figure 2 is an elevational view of a cylindrical cathode according to the invention.
[0012] Figure 1 is a longitudinal cross-section of a cathode according to the invention.
The cathode body 1 is compressed from a mixture of tungsten powder and approximately
0.5% by weight of scandium or scandium-hydride. After compressing at a pressure of
approximately 3.5 atmosphere and sintering in hydrogen for approximately one hour
at 1450°, the cathode body of scandium and tungsten has a porosity of approximately
20%. The cathode body 1 now has, for example, a thickness of 0.5 mm and a diameter
of approximately 1.8 mm.
[0013] Subsequently, the cathode body 1 is impregnated in a hydrogen atmosphere with barium
calcium aluminate (for example, 5BaO; 2Al₂O₃; 3CaO or 4BaO; 1Al₂O₃; 1CaO), compressed
in a holder 2 and welded onto the cathode shank 3. The cathode shank 3 accommodates
a coiled cathode filament 4 comprising a helically wound metal core 5 and an aluminium
oxide insulating layer 6. The emission of the emissive surface 7 of such a cathode
was approximately 100 A/cm² at 950°C obtained at a pulse load at 1000 V in a diode
with a cathode-anode distance of 0.3 mm. Such an emission is comparable to that of
a cathode with a top layer of tungsten and scandium oxide as described in European
Patent Application No. 0,178,716 (PHN 11,169) which is more difficult to manufacture.
The recovery after ion bombardment was comparable to that of the cathode described
in that Application with a cathode body sintered at approximately 1900°C (approximately
65%). In a cathode according to the invention, sintered at 1500°C this recovery was
poorer and was approximately 58%. For the significance of the recovery percentages
and the way in which they have been determined reference is made to the European Patent
Application No. 0,178,716 or to the magazine Article "Properties and manufacture of
top layer scandate cathodes" in Applied Surface Science 26 (1986), pages 173-195.
[0014] In the above-mentioned example the impregnant absorption was approximately 4.5%.
Upon raising the quantity of scandium (hydride) in the mixture to be compressed to
1 percent by weight this absorption decreased to approximately 2% which shortens the
life time of the cathode. For a quantity of 0.3-0.7% by weight of scandium (hydride)
the quantity of absorbed impregnant is sufficient; the recovery after ion bombardment
did not show any significant change in this range.
[0015] A cylinder 20 with an emissive surface 21 in which a heating element is provided
and which is shown in an elevational view in Figure 2 can also be turned from a tungsten
body compressed in accordance with the method as described hereinbefore.
[0016] The cathodes according to the invention may be used in electron tubes such as, for
example magnetrons, transmitter tubes, etc., but also in cathode-ray tubes for e.g.
television applications and electron microscopy.
1. A method of manufacturing a dispenser cathode comprising a barium compound for
dispensing barium to an emissive surface of a porous cathode body substantially comprising
a metal melting at a high temperature, characterized in that the cathode body is compressed
from a quantity of metal powder which is mixed with scandium or scandium hydride whereafter
the body is sintered and the cathode is provided with emitter material.
2. A method as claimed in Claim 1, characterized in that the quantity of scandium
or scandium hydride in the mixture of metal powder and scandium or scandium hydride
is approximately between 0.3 and 0.7% by weight.
3. A method as claimed in Claim 1 or 2, characterized in that the sintering temperature
is lower than the melting point of scandium.
4. A method as claimed in Claim 3, characterized in that the sintering temperature
is between 1430°C and 1500°C.
5. A method as claimed in any one of the preceding Claims, characterized in that the
cathode body is definitively shaped after it has been provided with emitter material.
6. A dispenser cathode manufactured by means of a method as claimed in any one of
Claims 1 to 5.
7. An electron tube provided with a cathode as claimed in Claim 6.