EP0123397B1

EP0123397B1 - Unsaturated vapor pressure type high pressure sodium lamp

Info

Publication number: EP0123397B1
Application number: EP84301612A
Authority: EP
Inventors: Philip J. White
Original assignee: GTE Products Corp
Current assignee: Osram Sylvania Inc
Priority date: 1983-03-10
Filing date: 1984-03-09
Publication date: 1988-12-28
Also published as: CA1222273A; EP0123397A1; AU2547884A; DE3475855D1; AU578603B2; JPS59169050A

Description

This invention relates to high pressure sodium lamps of the unsaturated vapor pressure type and more particularly to an unsaturated vapor pressure type high pressure sodium lamp having an arc tube containing a fill gas and a getter in contact with the fill gas.
High pressure sodium lamps, and particularly so-called saturated high pressure sodium lamps, are known in the art, see US-A-3,384,798. Such a lamp comprises an evacuated glass envelope having a plurality of electrically conductive support members therein and extending therethrough, an elongated arc tube having a pair of electrodes extending therethrough and affixed to said electrically conductive support members within said glass envelope and a gas fill including mercury and sodium within said elongated arc tube.
As is well-known, sodium loss has long been a problem causing undesired increased voltage drop across the lamp and an accompanying reduction in the useful period of 'life' of the discharge lamp. Thus, the large amount or 'saturated' sodium content of the arc tube is an attempt to compensate for the uncontrolled loss of sodium during the operational use of the discharge lamp.
It has tong been recognised that one of the principal causes for this undesired sodium loss is the presence of oxygen impurities in the gas fill of the arc tube. More specifically, it is known that the sodium fill gas tends to combine with oxygen and the aluminum oxide arc tube to provide sodium aluminate whereby undesired sodium loss is encountered.
It has been proposed to prevent such sodium loss, in saturated vapor pressure lamps, by the provision of an oxygen absorbing getter, see US-A-3,485,343. Such a getter must, however, be isolated from the gas fill of the arc tube by an oxygen permeable barrier in order to prove effective, and, moreover, although improving the electrical characteristics of the saturated vapor pressure lamp, the presence of the getter was not shown to avoid the necessity for dosing the lamp with excess sodium to maintain the 'saturated' condition of the lamp.
It has, however, also long been known in theory that a lamp wherein the amount of sodium and mercury employed is only that which would become totally vaporised in use, i.e. an unsaturated vapor type lamp, would be a highly desirable structure insofar as efficiency, cost of manufacture and enhanced lighting capability are concerned, although no practical embodiment of such a lamp was known until the disclosure of US-A 4,075,530 of Furukubo et al. Therein, a niobium exhaust pipe is coupled to an arc tube and a decomposable material, NaN3, is located within the exhaust pipe. The exhaust pipe is heated to decompose the NaN3 while the arc tube is cooled to effect condensation. Thus, the material within this exhaust pipe is heated to effect decomposition, transferred to the arc tube wherein materials, such as sodium and mercury, are condensed and whereat undesired gases, such as nitrogen, are withdrawn.
Unfortunately, locating the decomposable materials in a container external to the arc tube necessitates the application of heat thereto in order to effect the desired decomposition. Thereafter, the decomposed materials must be transferred to the arc tube. Also, the arc tube must be cooled in order to effect the condensation of desired residual materials while permitting the exhaust of other undesired materials. Obviously, such a process is cumbersome of apparatus and unrealistically expensive of labor and materials.
Accordingly an object of the invention is to provide a practically usable unsaturated high pressure sodium lamp that is capable of manufacture by more economic means.
The invention accordingly provides a lamp of the kind initially referred to which is characterised in that said mercury and sodium are provided only in an amount that will become completely vaporised in use and that said lamp further comprises a getter located within said arc tube in direct contact with said fill gas and providing a metal oxide having a free energy of formation per mole of oxygen more negative than the free energy of formation per mole of oxygen of sodium oxide.
The invention is illustrated by way of example in the accompanying drawings, in which:

Figure 1 is an elevational perspective view of a high pressure sodium lamp of the unsaturated vapor type of the invention;
and Figure 2 is a chart comparing relative sodium content with operational time for unsaturated high pressure sodium lamps with and without a getter therein.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended Claims in connection with the accompanying drawings.
Referring to the drawings, Fig. 1 illustrates an unsaturated vapor pressure high pressure sodium lamp of the invention. Herein, a glass envelope 3 is formed for insertion in a normal screw-type metal base 5. A stem portion 7 of glass is hermetically sealed to the glass envelope 3 and extends therein. The stem portion 7 has a plurality of electrical conductors 9 sealed therein and extending therethrough. An electrically conductive support member 11 is affixed to one of the electrical conductors 9 and to a metal crossmember 13 attached to an electrode 15 at one end of an elongated arc tube 17. Another electrode 19 is located at the opposite end of the arc tube 17 and attached to one of the electrical conductors 9 passing through the stem portion 7. Heat insulating sleeves 21 and 23 are wrapped about the opposite ends of the arc tube 17 in the vicinity of the electrodes 19 and 15 respectively. Also, the glass envelope is evacuated and at least one getter, preferably barium, 25 is Positioned adjacent the stem portion 7.
Further, a lamp fill including sodium and mercury is disposed within the arc tube 17 in an amount only sufficient to provide an unsaturated vapor type high pressure sodium lamp. Importantly, 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 aluminum oxide is located within the arc tube 17. The getter is in direct contact with the fill gas and preferably adjacent one of the electrodes 15 and 19 within the arc tube 17.
The getter is of a material which does not react with mercury or sodium but does react with oxygen at a rate greater than the rate of reaction of sodium with oxygen. Also, the getter has a melting temperature greater than the operational end temperature of the arc tube 17. Moreover, a preferred getter is a zirconium-aluminum alloy getter manufactured by SAES Getters S.P.A., Milan, Italy, and known by the trade name St 101 getter having an alloy containing about 84% zirconium and 16% aluminum.
As a specific example, a number of substantially identical 400-watt unsaturated vapor high pressure sodium lamps were fabricated. Each of the lamps included an arc tube 17 of a polycrystalline aluminum oxide material having an inner volume of about 4.3 cubic centimeters. Each of the arc tubes 17 contained a relatively low amount of sodium, about 6 X 10-⁵ g, and one of the arc tubes 17 included about 10 mg of the above-described zirconium-aluminum getter material.
All of the lamps were processed in a substantially similar manner and lamps with and without the above-mentioned getter located within the arc tube 17 were energized under substantially normal operating conditions. As can readily be seen from the comparison chart of FIG. 2, the lamp having no getter (Curve "A") within the arc tube 17 lost essentially all of the sodium therein through arc tube wall reaction within a period of less than five (5) hours. On the other hand, the lamp having an arc tube 17 containing a getter (Curve "B"), as previously described, indicates a good supply of sodium after an operating period greater than about 700 hours.
Thus, a high pressure sodium lamp employing an unsaturated vapor pressure has been provided wherein a getter is positioned within the arc tube of the lamp. The arc tube has a relatively low amount of sodium introduced therein, and the getter forms a metal oxide with a more negative free energy of formation than that of sodium oxide. As a result, the sodium level of the arc tube is maintained and the period of operational use of the lamp is extended as compared with prior known structures.

Claims

1. A high pressure sodium lamp comprising an evacuated glass envelope (3) having a plurality of electrically conductive support members (9,11,13) therein and extending therethrough, an elongated arc tube (17) having a pair of electrodes (15,19) extending therethrough and affixed to said electrically conductive support members (9,13) within said glass envelope and a gas fill including mercury and sodium within said elongated arc tube (17), characterised in that said mercury and sodium are provided only in an amount that will become completely vaporised in use so that an unsaturated vapor pressure type lamp is provided, and that said lamp further comprises a getter located within said arc tube (17) in direct contact with said fill gas and providing a metal oxide having a free energy of formation per mole of oxygen more negative than the free energy of formation per mole of oxygen of sodium oxide.

2. An unsaturated high pressure sodium lamp as claimed in Claim 1, characterised in that said getter is in the form of a zirconium-aluminium alloy getter.

3. An unsaturated high pressure sodium lamp as claimed in Claim 1 or 2, characterised in that said getter is non-reactive with mercury and sodium.

4. An unsaturated high pressure sodium lamp as claimed in any one of Claims 1 - 3, characterised in that said getter is in the form of an alloy including about 84% zirconium and 16% aluminium.

5. An unsaturated high pressure sodium lamp as claimed in any one of Claims 1 - 4, characterised in that said arc tube has a given operational end temperature and said getter has a melting point at a temperature higher than said given operational end temperature of said arc tube.

6. An unsaturated vapor high pressure sodium lamp as claimed in any one of Claims 1-5, characterised in that said getter material reacts with oxygen at a rate greater than the rate of reaction of sodium with oxygen.

7. An unsaturated high pressure sodium lamp as claimed in any one of Claims 1 - 7, characterised in that said arc tube has a volume of about 4.3 cubic centimeters and contains about 6 x 10-⁵g of sodium and about 10-mg of 84% zirconium 16% aluminium getter.