Electric lamp and discharge devices – Electrode and shield structures – Cathodes containing and/or coated with electron emissive...
Reexamination Certificate
2002-07-30
2004-07-06
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Electrode and shield structures
Cathodes containing and/or coated with electron emissive...
C313S337000, C445S050000
Reexamination Certificate
active
06759799
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of electron tubes, and especially cathodes whose function in these tubes is to emit electrons and thus constitute the source of an electron current.
More particularly, the invention relates to so-called oxide cathodes. These cathodes, which are the most widely used, comprise a layer of oxides which are strong electron emitters on one face of a metal support. The support is connected to an electric potential which is negative relative to the surrounding potential, allowing an electron flux to be emitted from the oxide layer.
BACKGROUND OF THE INVENTION
FIG. 1
is a simplified sectional view showing a cross section through a conventional oxide cathode
2
. The support
1
consists of a thin nickel plate forming a pill which has a face
1
a
covered with an oxide layer
3
in the form of a washcoat. The washcoat is a coating consisting of an active compound filler and of a binder. The active compound is generally based on barium carbonate (BaCO
3
) and on carbonates of other elements, which are subsequently converted to barium oxide (BaO) and oxides of other elements.
The oxide layer normally has to be at a relatively high temperature to emit. In the conventional case of a so-called indirectly-heated cathode, a heat source, such as a filament, is provided near the support and connected to a low-voltage current source.
In operation, an electron current flows though the thickness of the oxide layer
3
(arrow I) due to the effect of the surrounding electric field. The electric field is created by establishing a potential difference between the support
1
and an electrode
5
located near the external surface
3
a
of the layer
3
. In the example, the support is referenced at an earth voltage while the electrode
5
is biased at a high positive voltage +V. The electron flux obtained by the cathode
2
is proportional to the intensity of this electron current I.
FIG. 2
shows the same cross section through the cathode
2
after it has changed over time. It may be seen that a resistive layer
6
, called an interface layer, has grown between the metal support
1
and the washcoat layer
3
.
In some applications, it is necessary to try to obtain as high an electron current in the cathode as possible. This is especially the case with cathode-ray tubes for “multimedia” and “high-resolution” display screens, as well as for video projectors and other types of electron tubes, such as those used in the microwave field.
It is known that the intensity of the electron current that can be obtained from an oxide cathode is limited because it does not have a high enough conductivity. This is essentially the conductivity through the thickness of the washcoat layer
3
and the interface layer
6
—that through the support
1
may be regarded as negligible. It should be noted that the conductivity of a layer is inversely proportional to its resistivity.
Moreover, it appears that oxide cathodes do not withstand a high current density well, particularly when the current is constant over time, on account of their insufficient electrical conductivity.
It is generally accepted that the insufficient electrical conductivity of oxide cathodes is due to two parameters: the fact that the emissive washcoat
3
is based on oxides which by nature are poor conductors and the fact that the resistive interface layer
6
, between the metal of the support
1
and the washcoat, grows.
FIG. 3
is an equivalent electrical circuit of the components R
1
and R
2
of the electrical resistivity of the oxide cathode, deriving from the emissive washcoat layer
3
and from the interface layer
6
, respectively. As these two layers are superposed, the components R
1
and R
2
combine as resistors in series.
The contribution of the washcoat layer
3
to the electrical resistivity changes over the lifetime of the cathode. This is because metallic barium is created in this layer by the reaction between the barium oxides BaO and the reducing elements which diffuse out of the nickel. This metallic barium, the primary role of which is to move to the surface of the washcoat in order to allow electron emission, gives the washcoat electrical conductivity. However, the amount of metallic barium decreases for two reasons:
the generation of metallic barium is gradually exhausted because of the fact that the reducing elements must come, by diffusion, from an increasing depth in the nickel; and
the interface layer
6
itself acts as a diffusion barrier with respect to these reducing elements.
The contribution of the interface layer
6
to the electrical conductivity changes during the lifetime because this interface grows. The growth of this interface is due to chemical reactions between the washcoat and the reducing elements contained in the nickel (such as Mg, Si, Al, Zr, W, etc.) which accumulate compounds in this interface. These compounds are rather poor conductors since they are, above all, oxides such as MgO, Al
2
O
3
, SiO
2
, Ba
2
SiO
4
, BaZrO
3
, Ba
3
WO
6
, etc.
The origin of the electrical resistivity of oxide cathodes and its changeover time have been studied in the prior art for the purpose of increasing the electron current density that can be sustained.
Certain known solutions aim to reduce the resistivity of the oxide layer
3
by generally incorporating a conductive filler into it. For example:
U.S. Pat. No. 4,369,392 proposes to incorporate nickel powder into the washcoat, which in this case is carried out by pressing and then sintering;
U.S. Pat. No. 4,797,593 provides a solution which comprises adding scandium oxide or yttrium oxide to the washcoat, one of the effects of which is to improve the electrical conductivity;
U.S. Pat. No. 5,592,043 proposes a washcoat in the form of a solid object containing metals (W, Ni, Mg, Re, Mo, Pt) and oxides (of Ba, Ca, Al, Sc, Sr, Th, La) which increase the electrical conductivity by a “percolation” effect; and
U.S. Pat. No. 5,925,976 proposes the addition of metals (Ti, Hf, Ni, Zr, V, Nb, Ta) to the washcoat.
Other known solutions aim to attenuate the effect of the interface layer
6
. For example:
U.S. Pat. No. 4,273,683 pertains to the case of an interface formed above all from Ba
3
WO
6
. A layer of nickel powder is deposited on the nickel support prior to washcoating and, in addition, a barium carbonate concentration gradient is produced in the thickness of the washcoat. The BaCO
3
concentration is less in the region touching the interface, so that less Ba
3
WO
6
compound is created;
U.S. Pat. No. 5,519,280 describes a solution in which indium tin oxide (a complex based on In
2
O
3
and SnO
2
) is incorporated into the washcoat and acts by providing conductivity and by limiting the growth of the interface;
U.S. Pat. No. 5,977,699 proposes the addition of a zirconium (Zr)-based layer between the nickel of the support and the washcoat, this layer decreasing the interface in terms of its reducing-agent property; and
in the minutes of the conferences “International Vacuum Electron Sources Conferences” IVESC98, which were held at Tsukuba (Japan) on Jul. 7-10, 1998, the publication entitled “An analysis of the surface of the NiW layer of tungsten film coating cathode” by Takuya Ohira et al. describes a solution in which a layer of tungsten powder is deposited on the nickel of the support prior to the washcoating and explains that this layer has an effect of dispersing the reducing elements (Si and Mg) so that the compounds (especially Ba
2
SiO
4
) resulting from the chemical reactions at the interface are less concentrated and that, consequently, the interface is less of a barrier.
It has also been proposed in U.S. Pat. No. 4,924,137 to ensure that the barium produced by reaction between the oxide layer and the support is absorbed in the washcoat rather than disappearing by evaporation. For this purpose, scandium oxide and an oxide of Al, Si, Ta, V, Cr, Fe, Zr, Nb, Hf, Mo, or W are incorporated into the washcoat.
Finally, solutions have also been proposed in the context of so-called directly heated cathodes. By w
Herrera Carlos M.
LaPeruta, Jr. Richard
Thomson Licensing S. A.
Tripoli Joseph S.
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