Electric lamp and discharge devices – Electrode and shield structures – Indirectly heated cathodes
Reexamination Certificate
1998-11-05
2002-07-09
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Electrode and shield structures
Indirectly heated cathodes
C313S337000, C313S341000
Reexamination Certificate
active
06417607
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrode for gas discharges which comprises an electrically conductive material.
2. Description of the Related Art
Cold electrodes for gas discharges which exploit the hollow cathode effect have been known and in use for some time in industry, e.g. for electron tubes or lighting purposes. (U.S. Pat. No. 1,125,476; for hollow cathode effect, see literature, e.g. Manfred von Ardenne (editor); “
Effekte der Physik und ihre Anwendungen
”; Verlag Harri Deutsch; Thun, Frankfurt/Main, 1990).
Cold electrodes are usually provided on the inner surface with a coating comprising mixtures of alkaline earth metal oxides, hereinafter referred to as activation, to reduce the work function (principle of Wehnelt in 1907). Since the oxides are not stable under normal ambient conditions, the emission coatings are applied in the form of carbonates to the support material of the electrode and are converted into the corresponding oxides at low pressures and high temperatures, e.g. with ignition of the support material.
The electric losses of the above-described electrodes with the associated disadvantages are very dependent on the boundary conditions during conversion of the carbonates in the conditioning step and also on residual gases in the discharge chamber during operation, which reduce the emission capability (“poisoning of the activation”).
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an electrode which is insensitive to the boundary conditions during processing and has low electric losses, and thus low heat evolution, during the entire life of the gas discharge device.
This problem is solved by the photoelectric work function of the material of the emission coating (
3
) being lower that that of the support material (
1
) in the region of the operating temperature of the electrode below 570 K, preferably below 420 K.
The key aspect of the solution according to the invention is accordingly that the coating of the electrode which emits the electrons (“emission coating”) is selected in a particular way taking into account its photoelectric work function.
This work function should be less than that of the support material of the electrode over the operating temperature range of the electrode which is typically from 260 to 450 K. Regardless of the support material, the photoelectric work function should be less than 5.6×10
−19
joule/electron in the temperature range from 0 to 500 K. Specific coating materials which can be used are, according to claim
3
, yttrium, praseodymium or rubidium or mixtures thereof.
The photoelectric work function is defined as the photoelectric quantum energy which has to be expended per electron to release the latter from the electrode (measured in eV/electron or joule/electron).
According to the invention, surfaces having a low and high photoelectric work function are combined. The electron-emitting layer can comprise metallic or semiconductor materials having a photoelectric work function lower than that of the support material in place of the oxides having a high photoelectric work function at low temperatures, often simultaneously exploiting the hollow cathode effect which is known in principle.
An advantage of the invention is the avoidance of an undesired chemical reaction on the electrode surface. This makes the electrode virtually independent of the gas atmosphere during manufacture and conditioning; it is neither possible for the activation composition to be poisoned nor for incomplete reaction during conversion to allow the release of reaction products into the atmosphere of the gas discharge chamber at a later point in time.
The use of appropriately chemically inert materials having a low photoelectric work function (e.g. yttrium) makes the electrode of the present invention largely secure against incorrect treatment during production and conditioning by, for example, untrained personnel. The avoidance of industrially very complicated preparation process for carbonate mixtures which was previously necessary can also lead to considerable cost advantages.
In addition, measurements indicated a considerably lower evolution of heat when operating the electrode of the invention compared to electrodes which were of the same dimensions and construction type but had been activated using oxide mixtures.
Measurements of the photoelectric work function at various temperatures demonstrate the considerably lower photoelectric work function of the electrode of the invention at an operating temperature of T=300 K (see FIG.
3
).
Oxide mixtures have, when excited thermally, a low photoelectric work function. In the thermal emission of electrons from inhomogeneous, multicomponent, insulating solids whose electronic band structure has indirect transitions, lattice vibrations (phonons) participate in the excitation of the transitions at the minimum of the band gap (references: e.g. Joseph Eichmeier, “
Moderne Vakuumelektronik
”, Springer Verlag, Berlin 1981).
For gas discharges using cold electrodes, the photoelectric work function has been found to be the critical parameter in determining the losses; under certain circumstances, it is different from the thermally determined work function. Since the phonon energy in cold electrodes is considerably lower than in thermally emitting electrodes, no indirect band transitions can be excited in the case of cold electrodes.
Coating materials of the invention have only almost direct band transitions and a small band gap which make participation of high-energy phonons in the excitation process dispensable.
In one embodiment of the invention, the electrode is configured as a hollow body, in particular cup-shaped, and the emission coating (
3
) is located on the inner surface of the hollow body. In this way, the hollow cathode effect can be exploited in a positive way in addition to the advantages of the coating of the invention. The hollow body can, in particular, have the shape of a cup and the emission coating is located on the inner surface of the hollow body where the emission of electrons takes place.
In a further embodiment of the hollow body electrode, the emission coating (
3
) has a lower photoelectric work function than the remaining surface of the electrode, in particular the outer surface of the hollow body. Electron emission is thus concentrated at the emission coating.
According to another embodiment of the invention, the support material (
1
) is provided on the outside of the hollow body with a surface layer (
4
), preferably of nickel or platinum, which has a high photoelectric work function, preferably higher than 8.0×10
−19
joule/electron. This advantageously allows an increase in the life of the electrode in operation by reducing the degree to which the discharge spreads to the outside of the support body and thus destroys it.
Another embodiment of the invention according to which the support material (
1
) has a low photoelectric work function, preferably less than 6.4×10
−19
joule/electron, leads to the advantage that the special coating on the inner surface of the electrode chamber can be dispensed with since support material and coating material can be identical.
The support material preferably comprises a metal, in particular iron. It is particularly preferred that the support material consists of the metal.
The emission coating (
3
) can further comprise dopants for reducing the photoelectric work function compared to the pure material, preferably the dopants, for example, calcium, cesium or barium in concentrations of from 10
−5
at % to 1 at %. In this way, a further reduction in the work function and thus the losses can be achieved by decreasing the band gap in the electronic band structure compared to the use of pure materials.
Further preference is given to part of the surface of the support material (
1
) being provided with an electrically insulating surface layer (
4
) to suppress an electron or ion current. This has the
Kueffner Friedrich
Patel Vip
Williams Joseph
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