UNSATURATED HIGH-PRESSURE SODIUM LAMP

Abstract: The invention relates to an unsaturated high-pressure sodium lamp provided with a discharge vessel enclosing a discharge space, having a ceramic wall and closed at both ends by a leadthrough element to which an electrode is secured, while at least one electrode is provided with emitter material. The filling of the discharge vessel contains besides sodium at least mercury and a rare gas. According to the invention, the discharge space contains in metallic form one or more of the elements Mg, Ca, Sr and Ba up to at most 10% by weight of the mercury metered.

Description: The invention relates to an unsaturated high-pressure sodium lamp provided with a discharge vessel enclosing a discharge space, having a ceramic wall and closed at both ends by a leadthrough element, a respective electrode secured to each leadthrough element, at least one electrode provided with emitter material, the discharge vessel having a filling containing sodium and at least mercury and a rare gas. The invention further relates to a method of manufacturing such a lamp. The term "ceramic wall" is to be understood herein to mean a wall formed by translucent crystalline metal oxide which may be either monocrystalline (for example sapphire) or polycrystalline. Known polycrystalline metal oxides in this respect are aluminium oxide and yttrium aluminium garnet. In polycrystalline form the material is sintered to gas-tightness. Such a lamp is known from US-A-3,453,477. In the known lamp, it is achieved by a suitable choice of the temperature and the composition ratio of Na/Hg that formation of sodium aluminate is counteracted. However, experiments have shown that not only the metal oxide of the ceramic wall gives rise to disappearance of Na as filling component, but that also a number of oxygen sources give rise to disappearance of sodium. A very important source is found to be emitter material, for instance in the case in which this material contains alkaline earth metal oxide or for instance yttrium oxide. Thus, it has been found that the use of such an emitter material results in a substantial disappearance of sodium during the first hundred hours of the lamp life and in a small, but constant disappearance thereof during the further life of the lamp. Although a number of metals are known per se as oxygen getter in a discharge space, these metals do not lead in the presence of emitter material to an acceptable suppression of processes extracting sodium as filling component from the discharge space. Although it is conceivable in principle to compensate the loss of Na occurring at the beginning of the life by increasing the quantity of Na in the provided filling, it is thus hardly possible in practical conditions to manufacture lamps on an industrial scale in a reproducible manner. Moreover, also in this case the lamp will exhibit a deviating behaviour during the first hours of its life. US-A-3 558 963 discloses a high-pressure vapour arc lamp wherein sodium atoms are a principal radiating species and wherein a reactive metal preventing the formation of aluminate is provided in direct contact with the atmosphere within the arc tube. This document enumerates also Mg, Ca, Sr and Ba as possible reactive metals which latter metals, however, should not be placed inside the arc tube without placement in a hermetically sealed oxygen-permeable jacket, because, if placed within the arc tube, their vapour pressures or amalgam-forming characteristics will result in operational difficulties in the presence of mercury. EP-A-0 123 397 discloses an unsaturated high-pressure sodium lamp wherein a getter forming a metal oxide having a free energy of formation per mole of oxygen more negative than sodium oxide and more positive than aluminium oxide is located within the arc tube in direct contact with the fill gas and preferably adjacent one of the electrodes within the arc tube to reduce the loss of sodium. The getter is of a material which does not react with mercury. The invention has for its object to provide a measure by which it is possible to obtain an unsaturated high-pressure sodium lamp provided with electrodes comprising emitter material whilst maintaining satisfactory light-technical properties and a long life. For this purpose, according to the invention, an unsaturated high-pressure sodium lamp of the kind mentioned in the opening paragraph is a lamp wherein at least one of the elements of the group consisting of Mg, Ca, Sr and Ba is provided in open communication with the discharge space in metallic form in a quantity up to at most 10% by weight of the Hg present in the discharge space. A surprising advantage is that the disappearance of sodium is limited to a minimum and difficulties in operation due to, for example, the formation of amalgam do not occur. A possible explanation is that the said elements have a comparatively high vapour pressure. Due to the direct contact with the gas filling, also the vapour of the said metals will be distributed through the whole discharge space, which strongly promotes a rapid getter effect. This is in sharp contrast with the use of the said metals surrounded by a holder only pervious to oxygen. It moreover appears that no influencing of the spectrum of the light emitted by the lamp occurs with respect to the illumination source. If a larger quantity of metal of the above-mentioned elements than 10% by weight with respect to Hg is provided in the discharge space, the necessarily occurring formation of amalgam will at least delay the build-up of a desired mercury pressure and problems arise which are known from literature (US-A-3,558,963). Ba, Ca and Sr are known per se as filling components for influencing with respect to the illumination source the spectrum of the light emitted by the lamp. The quantities required to this end are such, however, that on the one hand excess filling can hardly be avoided in connection with amalgam formation, while on the other hand a substantial attack of the ceramic wall takes place due to reactions between the relevant metal vapour and the metal oxide of the ceramic wall. In order that a sufficient oxygen getter effect is ensured also during the life, in practice at least 0.5% by weight is supplied with respect to the metered quantity of Hg. A lamp according to the invention, in which at least one electrode is provided with emitter material, is preferably manufactured by a method comprising the following steps: securing in a gas-tight manner a first leadthrough element provided with an electrode in a first end of the discharge vessel, metering mercury and sodium into the discharge vessel, filling the discharge vessel with a rare gas up to a pressure corresponding to the desired pressure in the finished lamp, providing a quantity of metal of one or more of the elements Mg, Ca, Sr and Ba in open communication with the discharge space, and securing in a gas-tight manner a second leadthrough element provided with an electrode in a second end of the discharge vessel. In the preferred method, the leadthrough elements constitute a hermetic seal already when being secured in the discharge vessel. Temperature control of the discharge space can then be realized entirely within the space enclosed by the ceramic wall of the discharge vessel, which is generally advantageous. The construction of a discharge vessel provided with leadthrough elements obtained by this method is known per se . Especially in high-pressure sodium lamps whose filling is partly saturated during operation, this construction is frequently used. The use of the same construction in an unsaturated high-pressure sodium lamp therefore has the great advantage that it is possible to use the same production method and hence the same machines and tools for the manufacture of both lamp types. In a preferred method, the quantity of metal of one or more of the elements Mg, Ca, Sr and Ba is provided simultaneously with at least the step of metering mercury in the form of amalgam. This can be effected advantageously because due to the small quantity of metal, liquid amalgam having a comparatively low boiling trajectory will readily be obtained. An embodiment of a lamp according to the invention will be described more fully with reference to a drawing, in which: Figure 1 is a side elevation of the lamp according to the invention, and Figure 2 shows the variation of the quantity of sodium in the discharge as a function of the life of the lamp. In Figure 1, a discharge vessel 3 enclosing a discharge space 10 is arranged in a glass outer envelope 1 provided with a lamp cap 2 between current conductors 4, 5. The discharge vessel has a ceramic wall 3a provided on both sides with leadthrough elements 6, 7 secured in a gas-tight manner. The leadthrough elements 6, 7 are in the form of niobium sleeves. Within the discharge vessel 3, the leadthrough elements 6 and 7 are each provided with an electrode 11,12, between which electrodes the discharge extends in the operating condition of the lamp. The electrodes 11, 12 each contain emitter material. The current conductor 5 is passed with a certain amount of clearance into the niobium sleeve 6. A good electrical contact between these two is guaranteed by a metal wire 8, for example of nickel or niobium. The filling of the discharge vessel consists of 3.6 mg of mercury, 0.025 mg of Na, 100 µg of Mg and xenon having at 300 K a pressure of 13.3 kPa. The discharge vessel has an inner length of 82 mm and an inner diameter of 6.8 mm. The lamp described has a nominal power of 220 W and is suitable to be operated at a supply source of 220V, 50 Hz. The lamp described is manufactured by means of a method, in which the leadthrough element 7 is secured by means of melting glass in a gas-tight manner to the wall 3a of the discharge vessel. The melting glass used consists of 45.4% by weight of Al₂O₃, 5.6% by weight of MgO, 38.6% by weight of CaO, 8.7% by weight of BaO and 1.7% by weight of B₂O₃ of which 16 mg is provided. Subsequently, the discharge vessel is filled with Hg and Na in the quantities described. The discharge vessel is then arranged in a xenon atmosphere of 23.5 kPa at 267°C, which corresponds to 13.3 kPa at 300 K. Subsequently, a rod formed from Mg having a mass of 100 µg is arranged in the discharge vessel together with the leadthrough element 6, whereupon the leadthrough element 6 is secured correspondingly as done with the leadthrough element 7 in a gas-tight manner in the discharge vessel. Both electrodes 11, 12 are provided with tri-barium yttrium tungstate as emitter material. In Figure 2, the quantity of sodium in the discharge vessel is plotted on the ordinate (expressed in an arbitrary relative unit). The life of the lamp in hours is plotted on the abscissa. The curve I indicates the connection between the quantity of sodium and the life for the lamp described. The curve II indicates the same connection for a lamp of a corresponding type, but with omission of Mg from the filling of the discharge vessel. It is clearly visible that the quantity of sodium in the case of the lamp without Mg strongly decreases immediately at the beginning of the life and then remains at a low level.

Claims: An unsaturated high-pressure sodium lamp provided with a discharge vessel enclosing a discharge space, having a ceramic wall and closed at both ends by a leadthrough element, a respective electrode secured to each leadthrough element, at least one electrode provided with emitter material, the discharge vessel having a filling containing sodium and at least mercury and a rare gas, wherein at least one of the elements of the group consisting of Mg, Ca, Sr and Ba is provided in open communication with the discharge space in metallic form up to at most 10% by weight of the Hg present in the discharge space. A method of manufacturing an unsaturated high-pressure sodium lamp as claimed in Claim 1, in which at least one electrode is provided with emitter material, characterized in that the method comprises the following steps: securing in a gas-tight manner a first leadthrough element provided with an electrode in a first end of the discharge vessel, metering mercury and sodium into the discharge vessel, filling the discharge vessel with a rare gas up to a pressure corresponding to the desired pressure in the finished lamp, providing a quantity of metal of one or more of the elements Mg, Ca, Sr and Ba in open communication with the discharge space, and securing in a gas-tight manner a second leadthrough element provided with an electrode in a second end of the discharge vessel. A method as claimed in Claim 2, characterized in that the step of providing the quantity of metal of one or more of the elements Mg, Ca, Sr and Ba is carried out simultaneously with at least the step of metering mercury in the form of amalgam.

Inventors: MEEUWSSEN, MARINA MARIA JOSEPHA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1989
Main classification: H01J61
IPC codes: H01J61, H01J9