WO2007107889A1

WO2007107889A1 - High intensity discharge device having low work function metal in the discharge space

Info

Publication number: WO2007107889A1
Application number: PCT/IB2007/050572
Authority: WO
Inventors: Peter D. Milewski; Ray G. Gibson
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-03-23
Filing date: 2007-02-22
Publication date: 2007-09-27
Also published as: CN101410934A

Abstract

A dose of Ce metal or other low work function metal is added to the arc tube (12) of 'high intensity discharge lamp e.g.' an unsaturated HPS lamp (10) to substitute for the emission material in the electrodes (22, 24). During initial lamp start-up, the emission metal coats the electrodes (22, 24), thus reducing their effective work function and allowing the lamp (10) to start more easily.

Description

HIGH INTENSITY DISCHARGE DEVICE HAVING LOW WORK FUNCTION METAL IN THE

DISCHARGE SPACE

This invention relates to a high intensity discharge (HID) device, and more particularly relates to such a device having a low work function metal in the discharge space.

An HID device such as a high-pressure sodium vapor (HPS) lamp comprises an arc tube having a discharge sustaining fill and a pair of electrodes sealed through opposite ends of the arc tube and adapted to have an elongated arc discharge maintained therebetween. Means is provided to connect current to the electrodes. Conventionally, an emissive material such as yttrium oxide is disposed on or incorporated into each electrode, to aid in lamp starting.

In accordance with current practice, the emissive material is provided on the electrode of an HPS lamp by forming a composite. This composite is prepared in a multi-step process in which a slurry of ceramic materials is formed, and the slurry is coated onto and forced into the interstices between the windings of the electrodes and dried. After drying, excess coating material is removed from the surface of the electrodes and the coated electrodes are sintered at high temperatures under a hydrogen atmosphere to convert the ceramic precursor materials into the desired emissive material and bind it to the electrode.

The procedure is highly labor-intensive and requires many steps to complete. In addition, the coating consistency varies from electrode to electrode. Electrodes with insufficient emission material can result in reduced life of the lamp, while electrodes with excess emission material can result in blackening of the arc tube, resulting in reduced lumen output, as well as a reduction in lumen maintenance. Finally, emission material from the electrode is known to foul the electrode welding and assembly machines, requiring frequent downtime for cleaning.

Accordingly, an object of the invention is to eliminate the electrode/emissive material composite and its manufacturing process.

In accordance with the invention, this object is achieved by dosing a small amount of a low work function metal (e.g., Ce) directly into the discharge space. The low work function metal serves as an emissive material, and thus renders unnecessary the manufacture of an electrode/emissive material composite. The low work function metal is dosed into the discharge space along with a fill of vapor discharge-sustaining materials (e.g., sodium-mercury amalgam) during the arc tube sealing process, thus avoiding costly extra steps. At sufficiently high temperatures, the low work function metal will melt and slowly vaporize, coating the electrodes, reducing their effective work function and allowing the lamp to start more easily.

In one aspect of the invention, a high intensity discharge lamp is provided comprising: a discharge vessel and an outer envelope surrounding the discharge vessel; the discharge vessel defining a cavity containing a pair of opposing discharge electrodes and a discharge-sustaining fill; wherein the discharge vessel also contains a low work function metal.

In another aspect of the invention, a method for making a high intensity discharge lamp is provided, the lamp comprising a discharge vessel and an outer envelope surrounding the discharge vessel, the discharge vessel defining a cavity containing a pair of opposing discharge electrodes and a discharge-sustaining fill, the method comprising adding a discharge-sustaining fill to the discharge vessel and sealing the discharge vessel in a gas- tight manner, wherein a low work function metal is added to the discharge vessel. Preferably, the discharge vessel is at least substantially unsaturated, i.e., the reserve of discharge chemicals (i.e., salts, mercury and/or amalgam) normally found in the liquid state in the region of the cold spots behind the electrodes in a saturated discharge vessel is absent or is present in an amount which is insufficient to cause substantial blackening of the discharge vessel. As is known, such discharge vessels are characterized by a shorter electrode insertion depth and/or a lower dose of discharge chemicals, leading to higher cold spot temperatures, e.g., within the range of about 800°C to about 1000°C.

Preferably, the low work function metal is selected from the group consisting of Ba, Ca, Ce, Eu, Li and Rb, and is present in the amount of from about 0.1 mg to 5 mg, below which dosing is difficult, and above which the cost becomes prohibitive. Most preferably, the low work function metal is Ce, and is present in the amount of about 0.5 to about 2.5 mg.

These and other aspects of the invention will be further elucidated with reference to the Figures, in which:

Fig. 1 shows one embodiment of an HPS lamp of the invention; Fig. 2 is a graph showing the total ignition time and glow-to-arc time (GTA) both in seconds for 250W unsaturated HPS lamp samples with 0, 0.5 and 1.5 mg Ce, and excess Na compared to a control with yttrium oxide emitter on the electrode; Fig. 3 is a graph showing the PCA wall temperature at the tip of an electrode for the 250W unsaturated HPS lamp samples of Fig. 2.

The Figures are diagrammatic and not drawn to scale. The same reference numbers in different Figures refer to like parts. Fig. 1 shows one embodiment of an HPS lamp Jj) of the invention, including a poly crystalline alumina (PCA) arc tube 12 having a main body portion 14 and end caps 16 and 18 forming a gas-tight cavity 20 containing a fill of sodium, mercury and cerium metal. Opposing electrodes 22 and 24 pass through end caps 14 and 16 into the interior of the cavity 20. The arc tube 12 is surrounded by a gas-tight outer glass envelope 26 sealed at one end by a dimple 28 and at the other end by a press seal 30 and connected to a standard base 32.

Electrical leads 34 and 36 are connected to the base 32 and extend through the press seal 30 into to interior of the outer envelope 26 to provide current to electrodes 22 and 24 via supporting leads 38 and 40. Further support is provided by loop 42 of lead 38, which surrounds dimple 28. In the invention, an uncoated electrode (with no emitter) is used in the arc tube. However, a low work function metal is introduced into to arc tube. It is thought that the emitter metal melts and either evaporates and condenses on the electrode or is drawn into the electrode by surface tension forces and coats the tip of the electrode.

Preferably, the low work function metal is selected from the group consisting of Ba, Ca, Ce, Eu, Li and Rb. The work functions and melting temperatures are given in Table I for these materials, and for W, the electrode wire composition, for reference. TABLE I

The low work function material may be present in the amount of from about 0.1 mg to 5 mg, below which dosing is difficult, and above which the cost becomes prohibitive. Most preferably, the low work function metal is Ce, and is present in the amount of from about 0.5 to about 2.5 mg, below which the effect of the Ce has been seen to be minimal, and above which significant blackening of the ends of the arc tube and/or cooling of the ends of the arc tube can occur. The metal may be introduced in the form of a disk, pellet or small piece of wire of the emitter metal during the dosing of the arc tube, or by forming an amalgam of the metal with mercury. This enables introducing all of the fill components, e.g., sodium, mercury and Ce, in one single pellet, thus eliminating the need for a separate mercury-dosing step.

Example

The addition of 1.5 mg dose of Ce metal to a 250-watt unsaturated HPS arc tube has been shown to reduce the total ignition time and glow-to-arc time by 70% and 75%, respectively, compared to a lamp with no Ce. This result is comparable to that obtained in lamps using conventional yttrium oxide coated electrodes. The addition of Ce has also been shown to decrease the wall temperature near the tip of the electrode, which can reduce sodium loss due to diffusion and also can reduce PCA corrosion caused by reaction with sodium.

Several unsaturated 250-watt HPS arc tube samples were prepared using polycrystalline alumina (PCA) with nominal IOOW tungsten rod electrodes having a coiled coil welded to the distal end, and a short insertion depth determined by a scrape height of 13 mm. Scrape height is the distance from the tip of the electrode to the location where the electrode assembly (electrode and niobium tube) abuts the end cap of the PCA arc tube. The arc tubes were dosed with one Al getter pellet having a diameter of 420-710 micron. The xenon fill pressure was set nominally at 95 Torr.

Samples were prepared having five different fill conditions and divided into groups A-E. Group A had no emitter; groups B and C had two different doses of Ce metal wire, 0.5 mg and 1.5 mg; group D had the standard yttrium oxide coated electrodes; and group E had a doubling of the Na dose. The mercury (Hg) dose for each arc tube test set was adjusted to 100V on the arc tube voltage tester. All groups were composed of 5 lamps except group E, which had only 4 lamps. All lamps were fitted with a standard getter. The details of the different fills are summarized in Table II.

TABLE II

Average glow-to-arc (GTA), total ignition time and PCA wall temperature at the electrode tip were determined for each sample. The total ignition time is defined as the GTA time plus the breakdown time. The wall temperature was measured at 14 mm from each end of the arc tube, which with a 13 mm scrape would be about 0.4 mm off the end of the electrode.

Results are shown graphically in Fig. 2, which is a plot of the total ignition time and glow-to-arc time (GTA), both in seconds, and in Fig. 3, which is a plot of wall temperature in Kelvin, for each group of samples. The "2xNa" lamps (group E) showed the longest GTA and total ignition times and the second highest average wall temperatures, at 1468K; the "No Ce" lamps (group A) had the highest average wall temperatures, at 1532K. Typically the design rule for unsaturated lamps is to keep the maximum PCA temperature below 1400K. The results also indicated a decrease in all three parameters with increasing Ce content. In addition, the results indicate that the group C arc tubes with the "1.5 Ce" have similar properties as the yttrium oxide control (group D).

It is theorized that the Ce reduces the work function of the electrode, thus reducing the final operating temperature. Consequently, the electrode is able to provide more current faster, and reach operating temperature faster, thus reducing the GTA time. The lower operating temperature has an added benefit of reducing the PCA wall temperature near the electrode. With unsaturated lamps this temperature is particularly important since it is significantly higher than with saturated lamps and the potentials from the electrodes attracts the highest concentration of photoelectrons to the ends of the arc tube which can result in PCA corrosion and sodium loss. The results showed that the dosing of Ce metal to the arc tube in unsaturated

250-watt HPS lamps results in a reduction in the PCA wall temperature near the tip of the electrode. Ce dosing also results in shorter ignition and GTA times and reduced end blackening. The cooling of the PCA with the addition of Ce, results in an improvement in the sodium retention in the first 100 hours of operation compared to arc tubes with an uncoated electrode and no Ce. The results are consistent with the theory that the Ce acts to reduce the work function of the electrode allowing stabilization temperatures lower than that of the uncoated and comparable to yttrium oxide coated electrodes. The invention has necessarily been described in terms of a limited number of embodiments. From this description, other embodiments and variations of embodiments will become apparent to those skilled in the art, and are intended to be fully encompassed within the scope of the invention and the appended claims. For example, while the invention has mainly been described in terms of HPS lamps, it will be appreciated that the benefits of the addition of a low work function metal such as Ce into the discharge space instead of incorporating an emissive material into the discharge electrodes will be obtained in any high intensity discharge device which can benefit from increased emissivity of the discharge electrodes and has operating conditions (e.g., cold spot temperature, vapor pressure) conducive to deposition of the low work function metal on the electrodes, such as ceramic discharge metal halide and mercury vapor discharge lamps.

Claims

CLAIMS:

1. A high intensity discharge lamp (IQ) comprising: a discharge vessel (12) and an outer envelope (26) surrounding the discharge vessel (12); the discharge vessel (12) defining a cavity (20) containing a pair of opposing discharge electrodes (22, 24) and a discharge- sustaining fill; wherein the discharge vessel (12) also contains a low work function metal.

2. The high intensity discharge lamp (10) of claim 1 in which the discharge vessel is at least substantially unsaturated.

3. The high intensity discharge lamp (10) of claim 2 in which the cold spot temperature of the discharge vessel is in the range of about 800 C to 1000 C.

4. The high intensity discharge lamp (IQ) of claim 1 in which the low work function metal is selected from the group consisting of Ba, Ca, Ce, Eu, Li and Rb.

5. The high intensity discharge lamp (IQ) of claim 4 in which the low work function metal is present in the amount of from about 0.1 mg to about 5 mg.

6. The high intensity discharge lamp (IQ) of claim 4 in which the low work function metal is Ce.

7. The high intensity discharge lamp (IQ) of claim 6 in which the Ce is present in the amount ofabout 0.5 to 2.0 mg.

8. The high intensity discharge lamp (IQ) of claim 1 in which the discharge-sustaining fill comprises Na and Hg.

9. A method for making a high intensity discharge lamp (IQ) comprising a discharge vessel (12) and an outer envelope (26) surrounding the discharge vessel (12), the discharge vessel (12) defining a cavity (20) containing a pair of opposing discharge electrodes (22, 24), the method comprising adding a discharge-sustaining fill to the discharge vessel (12) and sealing the discharge vessel (12) in a gas-tight manner, wherein a low work function metal is added to the discharge vessel (12).

10. The method of claim 9 in which the discharge vessel is at least substantially unsaturated.

11. The method of claim 10 in which the cold spot temperature of the discharge vessel is in the range of about 800 C to 1000 C.

12. The method of claim 9 in which the low work function metal is selected from the group consisting of Ba, Ca, Ce, Eu, Li and Rb.

13. The method of claim 12 in which the low work function metal is present in the amount of from about 0.1 mg to about 5 mg.

14. The method of claim 12 in which the low work function metal is Ce.

15. The method of claim 14 in which the Ce is present in the amount of about 0.5 to 2.0 mg.

16. The method of claim 9 in which a discharge-sustaining fill comprises Na and Hg.

17. The method of claim 14 in which the Ce is added in the form of a wire.

18. The method of claim 14 in which the Ce is added in the form of an amalgam with Hg.