Low pressure discharge lamp

Abstract: A low pressure discharge lamp having a pair of electrodes, each electrode consisting of a sintered mixture of 50%-90% by weight of W, 5-25% by weight of BaO or of a 1:1:1 by weight mixture of BaO, CaO and SrO, and 5-25% by weight of a metal oxide selected from the group consisting of the oxides of Y, Zr, Hf and the rare earths, each electrode having a porosity of less than about 10% and a resistance of greater than 1 ohm.

Description: The invention relates to a low pressure discharge lamp comprising a closed discharge vessel in which two electrodes are arranged between which a discharge is maintained during operation. In the known low pressure discharge lamps, the electron emissive electrodes that are employed have a coil structure in which the electron emissive material is provided as a coating on a coiled tungsten wire. A problem with such an electrode is that it is difficult to provide an adequate control of the amount of emissive material provided on the coiled tungsten wire. As a result, it is very difficult to control the life distribution of the lamps so as to manufacture lamps having a narrowly controlled life distribution. This is because the lamp life is very sensitive to the quantity of emissive material provided on the electrode. Since it is almost impossible to uniformly control amounts of emissive material provided on a coated tungsten wire electrode it is difficult to manufacture lamps having an adequately narrow life distribution. Another problem exists in that fact that due to the physical nature of the electrode employing a tungsten coil, it is impossible to fabricate the electrode into a particularly desired shape. Further, fabricating an electrode in which the emissive material is loaded on to a double helix electrode, such as the ones presently employed, is a rather difficult operation and requires expensive equipment. It is an object of the invention to provide an improved low pressure discharge lamp having improved electrodes. According to the invention a lamp of the kind mentioned in the opening paragraph is characterized in that each electrode consists of a sintered mixture of 50%-90% by weight of W, 5-25% by weight of BaO or of a 1:1:1 by weight mixture of BaO, CaO and SrO, and 5-25% by weight of a metal oxide selected from the group consisting of the oxides of Y, Zr, Hf and the rare earths, each electrode having a porosity of less than about 10% and a resistance of greater than 1 ohm. By use of the sintered electrodes, it has been found that it is possible to more closely control the life expectancy of the lamp. Further, because of the greater ease of fabrication, the cost of the manufacturing electrodes and, therefore, the cost of the lamp is greatly reduced as compared with the a lamp employing a coiled electrode. Additionally the electrodes of the invention have relatively high resistance (greater than 1 ohm) thus requiring use of a minimum cathode current. Further, the lamps of the invention exhibit a relatively stable discharge. While the use of sintered electrodes in discharge lamps is known, the lamps in which sintered electrodes have been applied have been high pressure discharge lamps. Such a lamp is shown for example in U.S. Patent 4,303,848. However, while the low pressure discharge lamps of the invention pass a heater current through the electrodes before arc formation (hot cathode operation), therefore requiring the resistance of the electrodes to be high, no heater current is passed through electrodes employed in the high pressure lamps of this patent. Therefore for these lamps it is not of importance that the electrodes have a high resistance. In fact, preferably the electrodes have a low resistance. U.S. Patent 4,808,883 shows a discharge lamp containing an electrode formed of a semiconductor ceramic material. The electrode in this lamp, unlike the lamp of the invention, does not contain tungsten as the major ingredient but only in an amount up to 0.8 mol.%. U.S. Patent 3,766,423, shows low pressure mercury vapor discharge lamps containing hot cathode electrodes formed by mixing tungsten with oxides of barium or with mixtures of oxides of barium, calcium and strontium. However, no yttrium oxide is present. In addition, pressing and sintering is not carried out so as to produce an electrode having a porosity of less than about 10% in this patent. But sintering is carried out in such a manner that the electrode produced has a density gradient containing 80% voids in the surface of electrodes extending down 10% voids in the central portion of the electrode. As a result it has been found that such electrodes are very fragile and difficult to degas. While any metal oxide of the group consisting of the oxides of yttrium, zirconium and hafnium may be employed, it is found that best results are achieved when the metal oxide is Y₂O₃. Preferably, each electrode is made from a mixture of 50 to 80% by weight of tungsten, 10 to 25% by weight of yttrium oxide and 10 to 25% of barium oxide, the particle sizes of these ingredients being 0.05 - 10 µm. While the electrodes may have any desired shape they are conveniently rod-shaped with a length of at least 5 mm with a length of up to about 30 mm and preferably up to about 15 or 20 mm. Preferably the thickness of the rod is 0.5 - 2 mm. The electrodes are manufactured by pressing and sintering mixtures of powders of tungsten and the oxides or the tungsten powder may be first coated with the oxides by a sol-gel technique and the coated powders are then pressed and sintered. Pressing is generally carried out by isostatic pressing at a pressure of about 55 - 262 MPa (8,000 - 38,000 psi). Sintering is carried out in a reducing atmosphere preferably in an atmosphere containing up to about 5% of hydrogen in an inert gas such as helium at a temperature of about 1600°C - 2200°C for 5 minutes to 1 hour. While the electrodes may be directly pressed and sintered into bars, the electrodes may be first formed as sintered pellets, which pellets are then cut into bars of desired size. The electrodes are directly connected to the current lead-in wires, for example by point welding. Preferably the lamp is a low pressure mercury vapor discharge lamp containing a small amount of mercury and a noble gas at a pressure of 133 - 1333 Pa (1-10 torr). Example 80 weight percent of tungsten of a particle size of 0.4 µm was coated with 10 percent by weight of yttrium oxide and 10 percent by weight of barium oxide. The tungsten powder was coated with the yttrium oxide and the barium oxide employing a sol-gel technique. In carrying out this technique the tungsten powder was dispersed in a mixture of yttrium isopropoxide and barium butoxide in organic solvents in concentrations so as to provide 10 percent by weight of yttrium oxide and 10 percent by weight of barium oxide. The mixture was then formed into a dispersion and the resultant dispersion was heated at a temperature of about 90°C to remove solvents. The resultant coated powder was then fired at a temperature of about 620°C for two hours in a nitrogen atmosphere containing about 2% of hydrogen. The powder was then formed into pellets (1.4 mm thick and 25 mm in diameter) by pressing at a pressure of about 131 MPa (19000 psi). The pellets were then sintered at 2000°C for about 1 hour in an atmosphere of 95% helium and 5% hydrogen. The resultant pellets were then cut into bars of dimensions of 0.9 x 1.0 x 18 mm. The resultant bars had porosities of less than 10% at a resistance of 2-4 ohms. A low pressure mercury vapour discharge lamp was manufactured comprising two electrodes, each of which consisted of a rod prepared by the abovementioned example. The rods were positioned so that their axes were perpendicular to the axis of the discharge vessel. The following tests were carried out with this lamp. Employing a DC power supply (600 V, 1 A) and employing a resistor as a ballast a lamp voltage and current were monitored for different heating currents while the lamp was in an arc mode and carrying the cathode current. The time between the measurements was about two minutes and the ambient temperature was about 22°C. The results are shown in the following table. The values shown clearly indicate that the discharge provided by this lamp was stable at a wide range of cathode current and lamp currents. The relationship between cathode current and cathode voltage is shown in the following table. This table shows that the cold resistance of the cathode was about 0.5 ohms and that the resistance of the cathode was about 1.31 ohms at 2.8 A. The lamp was again started and the lamp current I LA was about 400 mA and the cathode current was decreased from 2.2 to 0 A. The discharge was stable. The lamp current was reduced from 400 mA to 150 mA. At the latter current the discharge became unstable. The results are shown in the following table. The discharge was stable until the lamp current was reduced to 150 mA. Thus the discharge provided in the lamp was stable between a wide range of lamp currents. The sole figure of the drawings is a longitudinal-sectional view of a fluorescent low pressure mercury vapor discharge lamp of the invention employing sintered electrodes. The lamp has a closed glass discharge vessel 1 which comprises mercury and a noble gas, e.g. argon. Electrodes 2 and 3 are arranged in the vessel 1, between which electrodes a discharge is maintained during operation of the lamp. The electrodes are rod-shaped sintered electrodes according to the invention. The discharge vessel 1 is on its inner side provided with a luminescent layer 4. The luminescent layer 4 comprises at least one luminescent material (phosphor) which emits visible radiation upon excitation by mainly 254 nm radiation from the mercury discharge.

Claims: A low pressure discharge lamp comprising a closed discharge vessel (1) in which two electrodes (2,3) are arranged and between which a discharge is maintained during operation, characterized in that each electrode (2,3) consists of a sintered mixture of 50%-90% by weight of W, 5-25% by weight of BaO or of a 1:1:1 by weight mixture of BaO, CaO and SrO, and 5-25% by weight of a metal oxide selected from the group consisting of the oxides of Y, Zr, Hf and the rare earths, each electrode (2,3) having a porosity of less than about 10% and a resistance greater than 1 ohm. A low pressure discharge lamp as claimed in claim 1, characterized in that the metal oxide is Y₂O₃. A low pressure discharge lamp as claimed in claim 1 or 2, characterized in that each electrode (2,3) consists of a sintered mixture of 50-80% by weight of W, 10-25% by weight of Y₂O₃ and 10-25% by weight of BaO. A low pressure discharge lamp as claimed in claim 1, 2 or 3, characterized in that each electrode (2,3) is rod-shaped with a length of at least 5 mm. A low pressure discharge lamp as claimed in claim 1, 2, 3 or 4, characterized in that before sintering the particle size of W is 0.05-10 µm, the particle size of BaO is 0.05-10 µm and the particle size of Y₂O₃ is 0.05-10 µm.

Inventors: GOLDBURT EFIM, HELLEBREKERS WIM, GOLDBURT, EFIM, HELLEBREKERS, WIM
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1991
Main classification: H01J61
IPC codes: H01J61, H01J9, B22F5