ELECTRIC LAMP PROVIDED WITH A GETTER

Abstract: The electric lamp has a getter capable of binding hydrogen, oxygen and stoichiometrically water at a com­ parativle low temperature. The getter comprises Pd as a first metal, chemically bound to a second metal from the group Zr and Y, the mol.% of the first metal in the getter lying between 0.4 and 15. The getter further comprises chemically bound oxygen, mole 0/mole second metal being 0.02 - 1.0. The particle size is ≦ 40µm.

Description: The invention relates to an electric lamp provided with a translucent lamp vessel sealed in a vacuum-tight manner; a light source arranged inside the lamp vessel; current supply conductors extending from the light source through the wall of the lamp vessel to the exterior; a getter inside the lamp vessel, which comprises an intermediate compound of a first metal with a second metal. Such a lamp is known from German Offenlegungsschrift DE-A-1,905,646. In the known lamp, the getter is an alloy of at least 5% by weight of at least one metal of the group III, IV, V and tungsten with at least one metal of the group VIII, aluminium and copper, which alloy has a melting point of at most 1250°C. This getter may be inter alia the zirconium/nickel alloy containing 5% by weight of Zr or Zr₂Ni, which latter alloy contains 75.7% by weight of Zr. The getter serves to bind oxygen in the lamp. In various types of lamps, however, water is a very harmful impurity. This substance may be present in a large quantity in lamps having a lamp vessel which is coated electrostatically with a powder. In order that a lamp vessel can be coated electrostatically, the resistivity of the powder to be applied is in fact of importance and this value is just strongly influenced by the moisture content of the powder. So, by coating a lamp vessel electrostatically moisture is introduced into the lamp vessel. In a lamp having incandescing tungsten parts, for example a filament, water can produce tungsten oxide and hydrogen. The oxide can evaporate and be deposited on the wall of the lamp vessel. Tungsten oxide may also react with the hydrogen formed to tungsten, which is deposited at colder areas, and water. Consequently, water is the carrier of a cycle process, in which tungsten is transported from the filament to colder area. This leads to a reduced transmission of light and to an accelerated disintegration of said filament and a short life of the lamp. Hydrogen, for example hydrogen obtained by decomposition of water, may lead to reduction of glass/metal connections, as a result of which a lamp vessel becomes leaky along current supply conductors and the lamp extinguishes prematurely. Hydrogen may further cause flashover, for example in evacuated lamp vessels, or may penetrate through a quartz glass wall into a discharge vessel and lead to an increase of the ingition voltage of the discharge arc. Oxygen in a lamp may lead to an undesired oxidation. Water may be such a harmful substance in lamps because its harmful effect is stronger than that of oxygen and hydrogen together. It is therefore of great importance that means are available by which water can be bound. Furthermore, it is of importance that, when binding water, no hydrogen or oxygen is formed which is not also bound. It is also of importance that means are available which are capable of binding molecular oxygen and hydrogen. The invention has for its object to provide a lamp of the kind described in the opening paragraph having a getter which is capable of binding not only hydrogen and oxygen, but also practically stoichiometrically water, especially also at comparatively low temperatures. According to the invention, this object is achieved in that the getter comprises Pd as a first metal, which metal is chemically bound to at least one second metal from the group of Zr and Y, the ratio "mole first metal × 100%/(mole first metal plus mole second metal)" lying in the range of 0.4-15%, and further chemically bound oxygen, the ration "mole 0/mole second metal" lying in the range of 0.02-1.0 and the getter having a particle size of mainly ≦ 40 µm. The getter according to the invention is capable of binding substantially stoichiometrically water also at comparatively low temperatures, for example temperatures in the range of 150-300°C and of further binding oxygen and hydrogen. The working rate of the getter and further its capacity are considerably higher than those of the related getters known from the said Offenlegungsschrift. It is easy to provide an electric lamp with the getter. The getter can be provided as a powder layer on a part of the lamp, for example on a current supply conductor or on a support wire or a mount. For the purpose, a dispersion of the getter in a solvent with or without a binder, may be used, for example a dispersion in a solution of nitrocellulose in butyl acetate. The getter may alternatively be present as powder in an envelope open to gas or may be present as a moulding, for example a pressed or sintered pill. The getter can be readily manipulated and stored at room temperature. It is also possible to subject the lamp to manufacturing steps in which lamp components are exposed to air at elevated temperature. In this case, if desired, to obtain the getter, material of the said composition of metals may be used which has an insufficient oxygen content. The initial oxygen content a material must have to have reached already immediately after the manufacture of the lamp the said ratio "mole 0/mole second metal" depends upon the conditions to which the material is subjected during the manufacture of the lamp. In a small series of experiments this initial oxygen content can be readily determined for a given lamp and a given manufacturing process. With ratios of the getter metals lying considerably above 15% not only the capacity for gas absorption is comparatively low, but also the hydrogen pressure at which hydrogen absorption takes place is comparatively high. With ratios lying considerably below 0.4%, the speed of gas absorption is low. In a favourable embodiment, the ratio of the metals in the getter lies in the range of 2-10% (mol/mol). The getter then not only has a high capacity and a low hydrogen residual pressure, but also a high gas absorption rate. It is further favourable that the content of the comparatively expensive metal Pd, is then low. It is favourable for the capacity of the getter if its oxygen content at the beginning of the life of the lamp lies lowely in the said range of 0.02-1.0 (mol 0/mol second metal), for example between 0.05 and 0.02. With ratios lying considerably below said broader range, hydrogen is absorbed only very slowly. If the particle size of the getter is considerably larger than the said value of 40 µm, the specific surface area of the getter is small and hence its absorption rate is low. If the particle size of the getter lies far below 0.1 µm, the getter has a very high rate of absorption, it is true, but the getter is then only slightly capable of withstanding the conditions of manufacturing of the lamp. An optimum getter effect is obtained with a particle size in the range of 0.1-40 µm. The lamp according to the invention may be an incandescent lamp, the light source is then a filament, or it may be a gas discharge lamp, for example a high-pressure discharge lamp. The light source may then be a pair of electrodes in an ionizable medium surrounded by an inner envelope. Alternatively, the lamp may be, for example, a low-pressure mercury discharge lamp. The light source may then be a pair of electrodes in a mercury-containing gas. An embodiment of the lamp according to the invention is shown in the drawings. The drawings also show results of experiments with the getter and with reference material. In the drawings: Figure 1 is a side elevation, partly broken away, of an incandescent lamp. Figures 2-7, 9 and 10 show a graph of the reaction of a number of materials with water vapour. Fig. 8 shows the reaction speed of two materials with hydrogen. In Fig. 1, the incandescent lamp has a translucent glass lamp vessel 1, which is sealed in a vacuum-tight manner and in which a light source 3, a filament, is arranged. Current supply conductors 4 extend from the light source 3 through the wall of the lamp vessel 1 to the exterior and are connected there to a lamp cap 5. The lamp vessel 1 is coated at its inner surface with an electrostatically applied powder layer 2. A getter 6 comprising particles of an intermetallic compound of a first metal with a second metal is arranged within the lamp vessel 1. The getter 6 comprises Pd as a first metal, chemically bound to at least a second metal from the group of Zr and Y, the ratio "mole first metal × 100%/(mole first metal plus mole second metal)" lying in the range of 0.4-15%; and chemically bound oxygen, the ratio "mole 0/mole second metal" lying in the range of 0.02-1.0 and the particle size of the getter being mainly ≦ 40 µm. In the Figure, the getter particles are pressed around a wire 7 to form a pill. On behalf of an experiment, lamps consuming at 225 V a power of 40 W were manufactured on a normal production machine. The lamps had an uncoated transparent lamp vessel having a diameter of 60 mm or had such a lamp vessel provided with a white electrostatically applied coating of about 57 mg of SiO₂ and about 6 mg of TiO₂. The filament was provided with 170 µg of red phosphorus. All lamps were evacuated because the failure of the getter for harmful gases, such as oxygen, hydrogen and especially water becomes manifest therein most strongly. The lamps were operated till the end of their lives, as the case may be in a "hot pot" (H.P), i.e. a substantially closed luminaire in which the temperature increases to a comparatively high value during operating. Lamps were manufactured with and without a getter according to the invention. The getter consisted of 8 mg of powder having a particle size of 0.1-40 µm of Pd chemically bound to Zr, where mole Pd × 100%/(mole Pd plus mole ZR)" = 8.7%, and chemically bound oxygen, where "mole 0/mole Zr" = 0.1. The powder was mixed with 16 mg of nickel powder and was pressed to a pill of 24 mg. As will be illustrated hereinafter, the nickel powder itself does not exhibit absorbing properties. The nickel powder serves to prevent that the pill after absorption of gases cracks and disintegrates and thus does not retain its position in the lamp. The temperature of the getter during operation of the lamp amounted to about 300°C. The results of the experiment are stated in Table 1: The lamps I and II are in accordance with the invention. The lamps III and IV are identical thereto, but no getter is present therein. The lamps V are reference lamps, which, just like the remaining lamps, are manufactured on a production machine, but in which the water-containing powder layer is not present. There were fifteen lamps per group I to V. A comparison of lamps I and V shows that in lamps I according to the invention the unfavourable effect of water from the powder layer is completely eliminated (see lamps III), while the getter further has neutralized the harmful effect of residual gases, which were present in the reference lamps V. The deviation of the life of the lamps I is of the same order as that of lamps V, but smaller. The getter has a very strong effect on lamps operated in a hot environment, which appears from comparison of the lamps II with the lamps IV. The deviation of the life is moreover considerably smaller. The getter is consequently very active in suppressing the harmful effect of residual gases, such as water, hydrogen and oxygen. The getter was manufactured as follows. Pd and Zr were mixed in a molar ratio of 8.7/91.3 in powder form and molten under argon in a discharge arc. After cooling, the melt was crumbled and hydrogenated. The reaction product was pulverized and sieved to obtain the particles having a size of 0.1-40 µm. The powder was x dehydrogenated by heating at 650°C in vacuo for one hour. The powder was passivated by exposing it at room temperature successively to oxygen at a pressure of 13.3, 133.3, 1333 and 13330 Pa. The resulting powder does not react in air at room temperature. The powder was examined with X-ray diffraction; it was then found that it contains Zr₂Pd as intermetallic compound. Said compound is present in a matrix of Zr, as appeared from interference microscopy. The powder was then oxidized in oxygen portions at a pressure of 133 Pa at 200-250°C to such an extent that the ratio 0/Zr, after incorporation in a lamp, was 0.1 (mol/mol). The powder was mixed with nickel powder and was pressed at a pressure of 1 Mpa around a molybdenum wire of 250 µm to a cylindrical pill having a diameter of 2 mm. In Fig. 2, the mass increase Δ M of a number of materials upon reaction with water vapour is plotted against the accumulation Q of hydrogen gas associated with said reaction. The dotted line A indicates (also in Figures 3 and 4) the accumulation of hydrogen gas in case a material solely binds oxygen from water. If a substance, after having initially bound hydrogen and oxygen, will bind solely oxygen at a given instant, the curve of this substance will extend from that instant parallel to the dotted line A. In the group of getters described in the aforementioned German Offenlegungsschrift DE-A-1,905,646 getters are included containing at least 5% by weight of Zr and another metal. Since no minimum quantity of the other metal is indicated, pure zirconium would be a material which falls just outside the described group of getters. However, the known getters have a melting point lower than 1250°C. This results in that the known Zr/Ni getters have an Ni content of at least 17 mol.%. Curve 21 indicates the reaction of Zr with water vapour at 300°C. Initially, with an increasing mass Δ M of the material, a small quantity of the hydrogen formed is absorbed, but soon the curve extends parallel to the dotted line. At the said temperature, zirconium is not a water getter. Also at 350°C (curve 22), from the beginning hydrogen is released if Zr binds oxygen from water vapour. Soon hydrogen is no longer bound at all. Zr₂Ni (curve 23) at 300°C initially binds solely oxygen from water (curve 23 coincides with the dotted line). Subsequently, the developed hydrogen is absorbed to a fairly low residual pressure. Finally, hydrogen is no longer absorbed, while oxygen continues to be bound. Zr₂Pd (curve 24) at 250°C substantially does not develop initially any hydrogen according to this graph and will lose its capability of absorbing hydrogen only at a larger Δ M than Zr₂Ni. Zr₂Pd is further more active (curve 24 at 250°C) than ZR₂Ni (curve 23 at 300°C). Zr₂Ni and Zr₂Pd are intermetallic compounds containing 33.3 mol.% of Ni and Pd, respectively. Fig. 3 shows that Zr now at 250°C binds (curve 31) only initially oxygen and a little hydrogen from water and subsequently binds solely oxygen. Curve 32 corresponds to curve 24 in Fig. 2 (Zr₂Pd at 250°C). Curve 33 shows that a getter having a metal composition according to the invention containing 8.7 mol.% of Pd, rest Zr, can absorb a considerably larger quantity of water vapour stoichiometrically without hydrogen being released than the intermetallic compound Zr₂Pd of curve 32. It appears on the other hand from curve 33 that the alloy containing 8.7 mol.% of Pd initially releases hydrogen when absorbing oxygen from water vapour. When the 0/Zr ratio (mol/mol) has become about 0.07 however, the hydrogen absorption arrears have been made up. With an 0/Zr ratio of about 0.03, a larger wuantity of hydrogen is already absorbed than that formed by oxygen absorption from water vapour (it should be noted that zirconium/palladium alloys having a palladium content of less than 19 mol.% have a melting point lying above 1250°C). Fig. 4 shows aimilar curves for alloys containing 8.7 (curve 41), 4.3 (curve 42) and 0.43 mol.% of Pd (curve 43), respectively. With an increasing Pd content, initially a larger quantity of hydrogen is released, but this quantity is then absorbed as yet. The oxygen content of the material with which the hydrogen is absorbed substantially entirely is slightly higher with a lower Pd content. It appears from Fig. 5 that Zr (curve 51) no longer absorbs substantially any hydrogen with an increasing content of oxygen originating from water and finally no longer contains any hydrogen at the point where O/Zr = 1 (mol/mol). The dotted line B indicates the variation of a material absorbing a number of hydrogen moles twice that of oxygen moles, that is to say that this material binds water stoichiometrically. Curve 52 shows that the intermetallic compound Zr₂Pd absorbs water stoichiometrically from water vapour. However, already at a low loading, the compound starts to release hydrogen if an additional amount of oxygen is bound. Curve 53 shows that with an alloy containing 8.7 and 4.3 mol.% of Pd, respectively, the stoichiometric water vapour absorption continues until the zirconium in the alloy is fully loaded. This is the case where the dotted line B intersects the dotted line C. The getter consequently has the theoretically maximum capacity. The dotted line C indicates the composition of zirconium material fully loaded with hydrogen (intersection point line C with ordinate, δ-ZrH 1.6 ) and fully charged with oxygen (intersection point with abscissa, ZrO₂) or with hydrogen and oxygen. When the curve 53 has reached the dotted line C, the material absorbs an additional quantity of oxygen whilst displacing hydrogen. The Figure shows that materials having the metal composition of the getter according to the invention have a higher getter capacity for water vapour than Zr and Zr₂Pd. The favourable difference between zirconium/palladium alloys in the getter according to the invention and Zr₂Pd is also advantageous in view of the comparatively high cost price of Pd. Fig. 6 shows the water vapour absorption behaviour of a getter pill. The pill consists of 8 mg of zirconium/palladium alloy with Pd = 8.7 mol.% and O/Zr = 0.1 (mol/mol) without (curve 61) or with 16 mg of Ni powder addition (curve 62). Fig. 7 shows the absorption rate of the two getter pills for water vapour. It appears from these Figures 6, 7 that Ni powder does not contribute to the getter effect. Due to the presence of Ni, however, mechanical stresses in the getter pill are neutralized, as a result of which the latter does not crack and crumble. Consequently, a pill can be readily held in place in the lamp., In Fig. 8, curve 81 indicates the hydrogen absorption by a sample of passicated zirconium/palladium alloy poor in oxygen (Pd = 8.7 mol.%) and curve 82 indicates that of a sample of zirconium/palladium getter according to the invention (Pd = 8.7 mol.%; O/Zr = 0.1 mol/mol) with a continuously increasing temperature. The considerably higher absorption rate of the getter according to the invention at temperatures up to 350°C is clear. The absorption rate of the getter sample according to the invention is lower above 350°C in the Figure shown because the sample is then already saturated to a great extent with hydrogen. In Fig. 9, of the aforementioned pill with Pd = 8.7 mol.%, O/Zr = 0.1 (mol/mol) and 16 mg of Ni for 8 mg of getter material the logarithm of the mass increase due to binding of oxygen (Δ M O ) upon reaction with water vapour in dependence upon the temperature is plotted against the logarithm of time. The comparatively high reaction speed at comparatively low temperatures appears therefrom. In Fig. 10, for various temperatures of the same pill as measured in Fig. 9 the hydrogen accumulation upon reaction with water vapour is plotted against the O/Zr (mol/mol) ratio in the getter. It has been found that at 350°C a very low residual pressure (less than 0.4 Pa) of hydrogen exists. At 250 and 350°C, the hydrogen residual pressure is less than 0.1 Pa.

Claims: 1. An electric lamp provided with a translucent lamp vessel sealed in a vacuum-tight manner; light source arranged within the lamp vessel; current supply conductors extending from the light source through the wall of the lamp vessel to the exterior; a getter within the lamp vessel comprising an inter-metallic compound of a first metal with a second metal, characterized in that the getter contains Pd as a first metal, which metal is chemically bound to at least one second metal from the group of Zr and Y, the ratio "mole first metal × 100%/(mole first metal plus mole second metal)" lying in the range of 0.4-15%, and further chemically bound oxygen, the ratio "mole 0/mole second metal" lying in the range of 0.02-1.0 and the getter having a particle size of mainly ≦ 40 µm. 2. An electric lamp as claimed in Claim 1, characterized in that the ratio "mole first metal × 100%/(mole first metal plus mole second metal)" is 2-10%. 3. An electric lamp as claimed in Claim 1 or 2, characterized in that the ratio "mole 0/mole second metal" is 0.05-0.2. 4. An electric lamp as claimed in Claim 1, 2 or 3, characterized in that the getter mixed with nickel powder is formed to a pill.

Inventors: MARTENS, WILHELMUS ADRIANUS ANTONIUS ALOYSIUS, STEINMANN, MAARTEN WALTER, WILLEMS, JOHANNES JOZEF GERARDUS STEPHANUS A.
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01J61
IPC codes: H01K1, H01J61