EP0136726A2

EP0136726A2 - Emissive material for high intensity sodium vapor discharge device

Info

Publication number: EP0136726A2
Application number: EP84111973A
Authority: EP
Inventors: Philip J. White
Original assignee: GTE Products Corp
Current assignee: Osram Sylvania Inc
Priority date: 1983-10-06
Filing date: 1984-10-05
Publication date: 1985-04-10
Also published as: EP0136726A3; CA1227521A

Abstract

A high intensity sodium vapor discharge device includes an elongated envelope containing an elongated arc tube having a fill of noble gases and an electrode with emissive material thereon or therein with a free energy of formation per mole of oxygen more negative than the free energy of formation of barium oxide.

Description

CROSS REFERENCE TO OTHER APPLICATIONS

U.S. Ser. No. 473,895, filed March 10, 1983, entitled "Unsaturated Vapor Pressure Type High Pressure Sodium Lamp" in the name of the Inventor and assigned to the Assignee of the present invention relates to high pressure sodium lamps.

TECHNICAL FIELD

This invention relates to high intensity sodium vapor discharge devices and more particularly to emissive materials for high intensity sodium vapor discharge devices.

BACKGROUND ART

High pressure sodium lamps having an elongated arc tube filled with large amounts of sodium and mercury and disposed within an evacuated glass envelope are well known in the art. Also well known is the fact that sodium loss has long been a problem in such lamps. Moreover, such undesirable features as increased voltage drop across the lamp and a reduction in the period of useful "life" have been traced to this undesired loss of sodium. Thus, it is a common practice to load or "saturate" high pressure sodium lamps with large amounts of sodium in order to compensate for the uncontrolled sodium losses during operational use of the discharge device.
One known attempt to reduce the sodium loss problem is an effort to reduce the level of oxygen impurity of the lamp which, in turn, reduces the sodium loss since there is a shortage of oxygen for combination with the sodium. Such an effort is set forth in U.S. Patent No. 4,075.530 by Furukubo et al wherein a decomposable material, NaN₃, is located in an adjacent exhaust tube, heated to decompose the NaN₃ and cooled to condense to a resultant material while the undesired nitrogen gas is withdrawn. Obviously, such a process is relatively cumbersome of apparatus and expensive of labor and materials.
Another known attempt to reduce sodium loss in high pressure sodium lamps is set forth in the above-mentioned filed application bearing U.S. Ser. No. 473,895. Therein, a getter is located within the arc tube of the discharge device and provides a metal oxide having a free energy of formation per mole of oxygen more negative than sodium oxide. However, the numerous advantages provided do not come without cost since the getter must be added to the structure.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to provide an enhanced high pressure sodium lamp. Another object of the invention is to improve the stability of a high pressure sodium lamp. Still another object of the invention is to reduce the sodium losses in a high pressure sodium lamp. A further object of the invention is to provide an improved unsaturated vapor high pressure sodium lamp.
These and other objects, advantages and capabilities are achieved in one aspect of the invention by a high pressure sodium lamp having an elongated ceramic tube filled with sodium and noble gases with leads extending therethrough and coupled to electrodes therein. The leads are connected to electrical conductors within an envelope, and at least one of the electrodes within the arc tube includes therein or has an emissive material thereon which has a free energy of formation per mole of oxygen more negative than that of barium oxide (BaO).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a high pressure sodium lamp of the invention; and
FIG. 2 is a comparison chart illustrating the sodium loss with time in a high pressure sodium lamp utilizing various electron emissive materials.

BEST MODE FOR CARRYING OUT THE INVENTION

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 conjunction with the accompanying drawings.
Referring to FIG. 1 of the drawings, a high pressure sodium lamp includes an elongated outer glass envelope 3 attached to a normal screw-type base 5. The lamp has a glass stem portion 7 hermetically sealed to the envelope 3, and a pair of electrical leads 9 and 11 are sealed into and pass through the glass stem portion 7 of the lamp. An electrically conductive support member 13 is affixed to one of the electrical leads 9 and extends along the longitudinal axis of the envelope 3. A pair of cross-members 15 and 17 are attached to opposite ends and extend normal to the longitudinal axis of the electrically conductive support member 13.
An elongated arc tube 19 of a light transmissive ceramic material, such as polycrystalline alumina for example, is supported within the outer glass envelope 3 by a pair of electrical conductors 21 and 23 sealed into opposite ends thereof. These electrical conductors 21 and 23 extend outwardly from the arc tube 19 and are attached to and supported by the cross members 15 and 17 respectively. Affixed to the other end of each of the electrical conductors 21 and 23 and spacedly arrayed within the arc tube 19 is a pair of electrodes which will be explained in detail hereinafter.
Also, heat insulating sleeves 29 and 31 are wrapped about the opposite ends of the arc tube 19 in the vicinity of the electrodes 25 and 27. The glass envelope 3 is evacuated and one or more getters 24, preferably barium, are positioned therein. Moreover, a lamp fill gas, preferably sodium and mercury, is disposed within the arc tube 19. This lamp fill gas may be of an amount sufficient to "saturate" or provide an excess amount of sodium therein, but preferably only sufficient fill gas is added to provide a lamp of the unsaturated vapor type. In other words, the "saturated" lamp includes a gas phase saturated with sodium and mercury and a pool of excess sodium and mercury while the "unsaturated" does not include a pool of excess sodium and mercury.
Additionally, it has been observed that high pressure sodium lamps employing the usual di-barium calcium tungstate emission material tend to exhibit an excessive loss of sodium during operation. Moreover, it is believed that decomposition of the emissive material liberates oxygen which, in turn, undesirably combines with the sodium and the alumina of the arc tube to form a non-volatile compound.
As to the electrodes 25 and 27, a material having a free energy of formation more negative than about -180 K cal/mole at 1200° or more negative than barium oxide (BaO) is a preferred electrode emissive material. For example, thorium oxide (Th0₂) coatings, as well as coatings selected from the following emissive material coatings, are appropriate: scandium oxide (Sc₂O₃): yttrium oxide (Y₂O3); beryllium oxide (BeO); lanthanum oxide (La₂O₃); cerium oxide (Ce 0 ) and hafnium oxide (HfO₂). Moreover, tungsten containing one or more of these emissive materials as inclusions in the tungsten, such as thoriated tungsten, is also an appropriate material for the electrodes 25 and 27.
As a specific example of the advantages of the above- described configuration, a comparison was made for sodium content of unsaturated high pressure sodium lamps. A series of 400-watt unsaturated vapor high pressure sodium lamps having a volume of about 5 cubic centimeters were filled with relatively low amounts of sodium, about 6 X 10^-5 gms, xenon in the range of about 15 to 300 Torr and about 1.8 mgs. of mercury. Moreover, one series of lamps employed tungsten electrodes with a coating of emissive materials commonly employed in sodium vapor lamps known as di-barium calcium tungstate. The other series of lamps employed a similar tungsten electrodes but utilized a coating of thorium oxide (ThO₂) as the emissive material.
A comparison test, FIG. 2, was conducted wherein neither of the test groups included any gettering within the group. A comparison can readily be seen in the chart of FIG. 2 wherein the test group (A) employed thorium oxide (ThO₂) as the emissive material and the control group (B) employed di-barium calcium tungstate as the emissive material. Operating under the same conditions, the test group (A), using Th0₂ as the emissive material, maintained a substantially constant level of sodium over an extended time period. On the other hand, the control group (B) using di-barium calcium tungstate as an emissive material indicated an almost complete loss of sodium content within a relatively short operating time, about 400 minutes.
Thus, test results have indicated that high pressure sodium lamps, saturated and preferably unsaturated, are enhanced by the utilization of an emissive material having a free energy of formation per mole of oxygen more negative than that of barium oxide (BaO). Also, observation of unsaturated high pressure sodium lamps made with both di-barium calcium tungstate and with thorium oxide emissive materials indicates a very definite reduction in end blackening when thorium oxide is used as the emissive material. Both tests are indicative of the reduced loss of sodium within lamps employing an emissive material with a free energy of formation per mole of oxygen more negative than about -180 K cal/mole at about 1200° C.
While there has been shown and described what is at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined by the appended claims.

Claims

1. A high intensity sodium vapor discharge device comprising:

an elongated envelope having a pair of electrical leads sealed therein and passing therethrough:

an elongated arc tube having a fill of noble gas and disposed within said envelope; and

a pair of electrodes sealed within said arc tube and connected to an electrical conductor passing through said arc tube and connected to one of said electrical leads with at least one of said pair of electrodes including thereon or therein emissive material having a free energy of formation per mole oxygen more negative than the free energy of formation of barium oxide (BaO).

2. The high intensity sodium vapor discharge device of Claim 1 wherein said discharge device is an unsaturated vapor type high pressure discharge device.

3. The high intensity sodium vapor discharge device of Claim 1 wherein said at least one of said pair of electrodes has a coating of thorium oxide.

4. The high intensity sodium vapor discharge device of Claim 1 wherein said at least one of said pair of electrodes is of a thoriated tungsten material.

5. The high intensity sodium vapor discharge device of Claim 1 wherein at least one of said pair of electrodes has a coating thereon of a material selected from the group consisting of scandium oxide (Sc₂O₃); yttrium oxide (Y₂O₃); beryllium oxide (BeO); lanthanum oxide (La₂O₃); cerium oxide (Ce 0 ) and hafnium oxide (HfO₂).

6. A high intensity sodium vapor discharge device comprising:

a light transmittable envelope having a pair of electrically conductive leads sealed therein and passing therethrough:

a ceramic arc tube disposed within said envelope and having a fill of sodium and noble gases: and

a pair of spaced electrodes within said ceramic arc tube. each of said pair of electrodes attached to an electrical conductor sealed into and passing through the end of said ceramic arc tube and connected to one of said pair of electrically conductive leads of said envelope, at least one of said electrodes including emissive material thereon or therein having a free energy of formation per mole oxygen more negative than about -180 K cal/mole at about 1200° C.

7. The high intensity sodium vapor discharge device of Claim 6 wherein said discharge device is of an unsaturated high intensity sodium vapor discharge device.

8. The high intensity sodium vapor discharge device of Claim 7 wherein at least one of said pair of electrodes is of a thoriated tungsten material.

9. The high intensity sodium vapor discharge device of Claim 6 wherein at least one of said pair of electrodes has a coating of thorium oxide thereon.

10. The high intensity sodium vapor discharge device of Claim 7 wherein at least one of said pair of electrodes has a coating thereon of a material selected from the group consisting of thorium oxide (ThO₂); scandium oxide (Sc₂0₃): yttrium oxide (Y₂O₃); beryllium oxide (BeO); lanthanum oxide (La₂0₃); cerium oxide (Ce₂0₃) and hafnium oxide (HfO₂).