CA2062889A1 - Silicon nitride coatings in metal halide lamps to reduce sodium loss - Google Patents

Abstract

SILICON NITRIDE COATINGS IN METAL HALIDE LAMPS
TO REDUCE SODIUM LOSS
ABSTRACT

Metal halide lamps are provided which have better lumen maintenance and concomitant lower operating voltage rise over the life of the lamp when a coating comprising silicon nitride is deposited on one or more metal lamp components of a metal halide lamp or on such components and the arc tube's outer surface.
When deposited on the outer surface of the arc tube, silicon nitride acts as a barrier to sodium diffusion, thereby retarding or reducing the loss of sodium from within the are tube. When sodium nitride is deposited on metal lamp components, the coating reduces the emission of photoelectrons from such metal lamp components that are exposed to ultraviolet radiation from the arc tube. A method for reducing sodium loss in metal halide lamp is also provided.

Description

~` 2~2~

SILICON NITRIDE COATINGS IN METAL HALIDE LAMPS
TO REDUCE SODIUM LOSS
~çhnieal Pleld The pr-~ent invention r~late~ to high-eSfieiency gaseous eleetrie diseharg- lamps, and, mor~
particularly, to improv~d ~etal halide lamp~ having a eoating o~ ~ilieon nitride ov~r one or more o~ the ~etal component~ of the lamp or ovQr one or more of ~ueh componQnts and th~ arc diseharge tube Th~
pre~-nt invention al~o r~late~ to a mothod ~or r-dueinq sodium lo~ in m~tal halid~ lamps.

Convsntional metal halide lamps eontain an elQetrie light ~ource comprising an are discharge tube mad- o~ a vitreous ~aterial such as quartz or a high temperatur- gla~ which i~ g~ner~lly centrally dispo~d within a vitreous outer Qnvelope and ~upported by a metal fram Th~ outer onvelope generally ha~ a stem or neck shaped portion on at l~a~t on- ~nd thereof which t-rminat~ in a sub~t~ntially m~tal base portion. Th- ~rc di~charge tub~ or "are tu~e~ ecommodated in ~he envelop~ i~
eonneeted to th~ m~tal base by current supply eonduetors Th~ are tube contain~ an electrode dispos~d at each end and eontain~ a fill comprising mor~ury, a hallde o~ sodium and a halid~ o~ one or - ' . . . " ~
. ~

~2~

more metals such as scandium, cesium, calcium, cadmium, barium, mercury, gallium, indium, thallium, germanium, tin, thorium, selenium, tellurium, etc Generally the iodides of these metals are preferred, although the bromides and, in some casefi chlorides may also be used Usually, the arc tube also contains an inert gas such as argon Several types of arc tube support structures have b-en used in metal halide lamps One principle form employs a metal harnea~ wherein the arc tube is supported within a harnQss by metal straps around the pinched ends of the arc tube with metal rods present along the sidQ~ of the arc tubo ~o that a substantial ~mount of the m tal harno~ ~tructur- 1~ proximate to the arc tube This typ- of harne~ support is disclosed, for example, in U S Patent Nos 3,452,238 and 3,559,766 A second principle form o~ arc tube support involve~ a frame structure supporting the arc tube in which there are no structural ~upport members present along the side~ o~ the arc tub-, and which utilizes a thin wire known as a fly lead located away from the arc tube to carry the lamp current to one of the arc tube electrode~ This type of lamp construction is in wide use today and is shown, for example, in U S
Patent No~ 3,937,996 and 4,581,557 It ha~ long been recoqnized that the chemistry encountered in metal halide lamp~ is such that ultraviolet radiation from the arc tube ~trikes metal component~ within the lamp causing the emis~ion of photoelectrons Under certain conditions, these photoelectrons collect on the outer gurface of the arc tube and create a negative potential that attracts the po~itive sodium ions and accelerates their diffusion through the wall of the arc tube The production of such photoelectrons substantially accelerates the `20~8~

depletion of sodium within the arc tube and thus ~hortens the useful life of the lamp. The problems associated with sodium loss in metal halide lamps are described in: J. F. Waymouth, Elec~ric Dischar~e Lamps, pp. 266-277, (MIT Press 1971).
Prior to the present invention, different measures have been taken in order to reduce sodium loss in metal halide lamps and have been used with varying degree~ of succe~. For example, U.S. Patent No.

4,950,g38 (Ramaiah) deQcribe~ the use of zirconium oxide on the metal mounting frame for reducing the sodium diffusion level on the metal halide lamp.
Al~o, U.S. Patent No. 4,678,960 (Reiling) describes a metal halide lamp with an infrared reflective coating to reduce ultraviolet radiation and a~ a re~ult reduce o~lu~ tran~port.
There continue~ to be a n-ed for n-w materials that are effective for reducing or sUpprecsing both the emission of photoolectron~ under ultraviolet radi~tion expo~ure and the di~fu~ion of ~odium ion~
from within th- arc tube of metal halide lamps.

Sum~a~y of the Invention The pre~ent invention relates to the discovery that metal halide lamps have better lumen maintenance and concomitant lower operating voltage ri~e ov~r the life of the lamp when a coating compri~ing silicon nitr~de i~ d~posited on one or more metal lamp component~ of a metal halid~ lamp or on the metal lamp component~ and the arc tube's outQr surface. When silicon nitride i~ depo6ited on metal lamp components, the coating reduces the emission of photoelectrons from such metal lamp components that are exposed to ultraviolQt radiation from the arc tube. Such photoeleotron~, as discussed above, often enhance the rate of 80dium 10s5 due to neutralization of sodium 2~2g~3 ions on the arc tube's outer surface. When also deposited on the outer surface of the arc tube, silicon nitride acts as a barrier to sodium diffusion, thereby retarding or reducing the loss of sodium from within the arc tube.
As employed herein, the term "metal lamp components" is meant to include all of the metal components employed in positioning and/or supporting a metal halide arc tub~ within an outer vitrQous envelope of a metal halide lamp, irrespective as to whether or not they function as current supply conductors, aJ well as any current supply conductors within the outer vitreous envelope such as fly leads, starter probe~ and the like.

~Ll~C De9cri~iQ~ of the Drawing The ~ingle figure i~ a sch-matic illustration of an embodiment of a single-endQd metal halide lamp having a ~Qtal screw ba~e useful in the practice of the pre~ent invention.

Detaile~ ~Q~ç~ Qn As ~et forth above, the present invention relates to m-tal halide elQctric lamps which comprise an outer envelope of vitreous, light transmissive material such a~ gla~ having an electric light source accommodated within ~a~d envelope which is connected to an appropriate current supply source by means of current 3upply conductors, wherein said light source comprises a sodium-containinq metal halidQ arc discharge tube po~itioned ~nd ~upported within said env~lope by metal la~p component~. In accordance with the invention, a coating comprising silicon nitride (Si3N4) is depo~ited one or more of the metal lamp components of a metal halidQ lamp or on one or more of the metal lamp co~ponents as wQll as on the outer surface of the ~2 ~9 arc tube and/or by any suitable means such as atmospheric chemical vapor deposition (CVD) and the like prior to assembly of the lamp components into the lamp. In one embodiment, one or more of the metal S lamp components will be coated with at least about 500 angstroms of silicon nitride and up to about 20,000 angstroms of silicon nitride prior to assembly of lamp components into the outer vitreous envelope. When metal lamp components are coated, mo~t often all metal lamp components are coated. However, when less than all of the metal lamp components are coated it is preferred that at least the conductive msmbers such as the fly lead and starter probe are coated. In another embodiment, the arc discharge tube will be coated in addition to one or more of the metal lamp components with at least about 500 angstroms of silicon nitride and up to about 20,000 angstroms of silicon nitride prior to a88embly 0~ lamp co~pon-nts into the outer vitreous envelope.
In addition to sodium, the arc tube will also contain a fill comprising mercury, a halide of one or m-tals such a~ scandium, cesium, calcium, cadmium, barium, mercury, gallium, indium, thallium, dy~prosium, germanium, tin, thorium, selenium, tellurium, etc., and, usually, an inert gas such as arqon. Generally, the iodide~ of these metal~ are pr~erred, although the bromides and, in some cases, chloride~ may also be used. The arc tube may contain sodium in the ~orm o~ an amnlgam with mercury, as one or more sodium halides, or mixture thereof.
Generally, it is d~ired that all of the sodium in the arc tube is in the form of a ~odium halide, such as sodiu~ iodide, in order to minimize migration of the ~odium into the wall of the arc tube. The metal lamp components will generally comprise an iron alloy such a~ nickel plated steel. However, conductive members - 2~2~

are often constructed from other metals such as tungsten and molybdenum ~8 noted above, the surface of one or more metal lamp component~ is, in one embodiment, coated with at least about 500 angstroms of silicon nitride (Si3N4) prior to assembly of the lamp components into the lamp by any suitable means such as low pressure chemical vapor deposition (CVD) including the techniquQ described in Vossen &
Kern eds , Thin Film Processes, pp 298-299, (Academic Pre~s 1978) Similarly, when the surfac~ of the arc di-charge tub~ i8, in on- sm~odiment, coated along with the m tal lamp co~pon-nt~, it i~ coat-d with at l~a~t about 500 ang-trom~ o~ ~ilicon nitride mploying the conventional methods noted above Generally, an inert ga~ such ac nitrogen or argon i8 present in the space batween the arc tube and in th~ interior surface of the outer vitreous envelope, although lamp~ in which a vacuum exi~t~ in~ide the outer vitreou~ envelope can also b- employed in the practice of the pre~ent invention Ref~rring now to the ~ingle drawing figure, which i~ a ~chematic view of an illu~trative, but non-limiting embodiment of a metal halide lamp useful in the practice of the present invention, lamp 10 include~ an outer, light transmi~sive, vitreous env-lope 12 containing within light transmissive arc tube 14 made of, e g , quartz Metal base 16 i~
attached to one nd of envelop~ 12 ~or making an el~ctrical conn~ction to the arc tube Arc tube 14 ~ cured in envelop2 12 ~y m~tal ~r~me part~, e g , p~rt~ m~de of nickel plated ~tesl One end of arc tube 14 i~ secured by frame part~ comprising a ~pring clip metal band 18 currounding dimple 20 moldad in envelope 12 to which i~ attached support memb~r 22 which, in turn, ia attached to ~trap 2~?~

member 24 which is securely mechanically fastened about the pinch seal region 14a of arc tube 14.
The other end of the arc tube 14 is secured by a combination of strap member 26 which is securely S mechanically fastened about the other pinch seal region 14b of the arc tube 14 and which is attached to support member 28. The conductive members 30 and 32 are attached to support members 28 and 22, respectively, and to inleads 34 and 36 of the two arc tube electrodes ~not shown). Support member 28 is secured by attachment to conductive member 31 which passes through seal 38 of env-lope 12. Conductive member 40, which also passe~ through 3eal 38, i~ attached to a starter wire 42 made of tungsten, whereas conductive member 44 which al80 paBse~ throuqh seal 38, is attached to conductive fly lead 46 made of molybdenum which, in turn, is attached at its other ~nd to support 22 for supplying current to one of the arc tube electrodes. The lamp also contains a getter strip 48, coated with a m~tal alloy material to absorb hydrogen from inside the lamp envelope.
Except for inlead~ 34 and 36, starter wire 42 and fly lead 46, all of the lamp parts mentioned are generally made of an iron alloy such as nickel plated steel.
During lamp operation an electric discharqe is developed between the pair of arc tube electrodes (not ~hown). The di~charge dQvelops highly intense visible light which i5 transmitted ~rom the arc tube 14 and through the vi~reou~ envelope 12 for the purpose of illumination. Additionally, a strong flux in the ultraviolet region is emi~ted from the mercury vapor ionization within the arc tube 14. Ultraviolet photons which strike the metal lamp components noted above cause the emission of photoelectrons from the ~2~

metal. The free photoelectrons can accumulate on the outer surface of the arc discharge tube 14 and impart a negative charge to it. The negative charge will accelerate the diffusion of sodium ions through the wall of the discharge tube 14 resulting in the progressive depletion of the sodium concentration within it. This phenomena is deleterious to lumen maintenance. Also, as the sodium concentration within the arc tube 14 decreases the lamp voltage increases.
In order to reduce the production of photoelectron~ caused by the ultraviolet photons from the marcury vapor di~charge, a coating comprising silicon nitride is deposited on one or more of the 1~ metal lamp components noted above. Likewise, in order to reduce the diffusion of sodium ions from the inner surface of arc tube 14 to the outer surface of the arc tube, a coating comprising silicon nitride can also be deposited on the outer surface of the arc tube 14. In one embodiment, the surface of one or more o~ these m~tal lamp components including the fly lead 46 and the starter probe 42 will b~ coated with 3ilicon nitride, usually at least about 500 angstroms, prior to assembly of the lamp components into a f~nished lamp. In this embodiment, usually at least the fly lead and starter probe are coat~d. However, it is often advantageous to coat all the metal lamp components. In another embodiment, the arc discharge tub~ 14, in addition to one or more of the m~tal la~p components, is coated with silicon nitride, u~ually with ~ thic~ness of at least about 500 angatroms.
In another of its aspects, the present invention relate~ to a method for reducing sodium 10s8 in metal halide lamps which comprises coating one or more of th~ ~etal lamp componentc or the metal lamp components 2 3 ~ 2 ~) 8 ~

and the arc tube with silicon nitride in an amount sufficient to, on the one hand, reduce emission of photoelectrons from metal lamp components and, on the other hand, reduce diffusion of sodium ions from the inner to the outer surface of the arc tube.
CVD Techniques Numerous methods for low pressure or atmospheric chemical vapor deposition (CVD) are.known; suitable methods are described, for example, in Vossen and Kern, eds., Thin Film Processes, pp. 298-299, Academic PrQss, Orlando (197B); HQS~ and Jen~en, Microelectronics Processing: Chemical Enaineering Q~, ACS, WAshington, DC (1989); Pulker, Coatings on Gla~, El~evier, Am~terdam (1984): Bunshah, ed., Deposition Technoloaie~ For Films And Coatinas, Noxes PUb., (1982).
Two d$fferent silicon nitride atmospheric CVD
reaction pathways which have been found to be particularly useful in coating arc discharge tubes and metal lamp parts of the metal halide lamps according to th~ invention are:
3 SiC14 + 4 NH3 550-C >Si3N4 + 12 HCl (1) >550-C
R~action (2) i~ advantageous since reagent handling is more convenient in that SiH2C12 is a g~ at 25-C and atmospheric pressure. Therefore, it is po~sible to directly meter thi~ reagent into a reactor with a flow meter. In reaction (1), SiC14 ga4 is delivered to a reactor by bubbling nitrogen carrier ga~ through liquid SiC14 at 25-C. In bo~h r~actions (1) and (2), it i5 preferr~d that the Si-containing vapor be delivered to the reactor separately from the ammonia to mini~ize qas phase reactions which can result in particle formation.

2~-~2;3 The invention will be further understood by reference to the following examples.
Examples ,Exam~,,e~ 1 1. Atmospheric CVD Reactor The CVD reactor employed was a reaction tube furnace having a single hot zone 12 inches in length.
The reaction tube was fabricated from fused silica and was 24 inches in length with a 2 inch inside diameter. Reaction gases leave the reactor through a 4 foot long tube to reduce back diffusion of ambient air into the reactor.
2. Coating Arc Tubes and Metal Lamp Parts Prior to coating the arc tubes, the arc tubes were washed in a 10% solution (by volume) of ammonium bifluoride (NH4FHF) for one hour in a heated sonic bath. The arc tubes are rinsed in deionized water and oven dried. Prior to coating the metal lamp components, they were wa~hed in methanol for one hour in a heated sonic bath. The metal compon~nts were then rin~ed with deionized water and oven dried.
Arc tubes or metal lamp parts were loaded into the abov- CVD reactor at 25-C. The reactor was then sealed and purged with N2 for 1 hour to substantially remove ambient air. The reactor was subsequently heated to 625-C and held for 25 minutes prior to coating. Silicon nitride deposition was initiated by introducing the following reaction gases via a ~low meter (rotameter):
Table I
ÇQ~TING ARC TUBES

R~tion (1) G~s ,Flow Rate N2 diluent 1700 SiC14 + carrier (N2) 8 .. . ..
1 standard cubic cm/~in 2 ~

~able II
COATING METAL LAMP COMPONENTS
Reaction (2) Gas Flow Rate N2 diluent 1700 siH2cl2 5 _ 1 standard cubic cm/min A large exces~ of ammonia wa~ used to avoid depo~ition of non~toichiometric silicon nitride wh$ch could re~ult in the pre~ence of ~ilicon metal. It is pre~erred that the presQnce of silicon metal be avoided becau~e it is not transparent to visible light radiation and can result in less visible light output from the arc tube. The pre~ence of silicon metal on the m-tal components i~ also to be avoided, because silicon metal i9 slightly conductive to electricity.
Th- arc tube~ were coated for approximately 3 hours with 2000-3900 angstroms of Si3N4 film. The metal la~p components were costed for 2.S to 3 hours with 2800-6S00 angstroms o~ Si3N4 film. A~ter the reaction the reactor was cooled under a nitrogen purg~. Film thicknesses were obtained through optical interf~rence mea~urements on silicon wafers which were slide~ coated along with the arc tubes or metal lamp component~ for the purpose of film thickness m~a~ur~Qnts.
EXa~Ql~s 2-5 In these examples a number of commerc$ally availabl- lamps of the typQ illustrated in Fig. 1 were u~d. The~e lamp~ are available from General Electric (lamp d~scription: MXR175/BU - Ordering Code:
11417). The outer, oval-shaped envelope 12 is about 191 millimeter~ long, about 75 millimeter~ in diameter at its widest poin~ and contains an arc tube 14 2~ g~

about 29 millimeters long and about 14 millimeters in diameter which is made of quartz and filled with mercury, sodium iodide, scandium iodide, thorium iodide and argon as a starting gas. The outer envelope 12 is filled with nitrogen at a pressure of about 380 torr. The volume inside the outer glass envelope is about 325 cm3. The metal lamp components have a total surface area of about 15 cm2. Fifteen lamps were employed wherein none or all of the internal components were coated with silicon nitride prior to assembly. These lamps were designed for operation at 175 watts at a nominal voltag~ of about 135 volts. The light output in lumens after 100 hours of operation averages about 17000. After 8000 hour~ of lamp operation, the light output in lumens and operating voltage of the various lamps were compared. In the following table (A~) stAnds for arc tube and (MLC) ~tand~ for metal lamp componsnts.

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C
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~ o o o, _ _ ~U o ~ ~ ~ o ~ ~ ~ N ~ ~ qC O
o D ~ 'C ~ ' U
~114 g ~ U~ Ul ~
~~, c r~ ,~ ~ r7 E~o ~ _ E~ ~ u E ~ CD t'- O~
r r CL
'' ~n 'I
~t DJ `O
~ ~ O ~ O
C. ~ ~
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z ~:

e ~ c C"

C ~ ~ ~r U~
,~ ~-' .4 .a'' ~ U

U~ ~

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Although the invention has been described in connection with a particular metal halide discharge lamp, those skilled in the art will appreciate its application to any type of metal halide discharge lamp for reducing sodium loss and increasing lamp efficacy. Accordingly, the particular embodiment of the invention shown in the drawing is merely illustrative and the SCOpQ of the invention is d~termined by th~ following claims.

Claims (17)

1. A metal halide discharge lamp comprising:
an outer vitreous envelope;
an arc discharge tube disposed within said envelope:
a plurality of metal lamp components disposed within said envelope;
wherein one or more of said metal lamp components or one or more of said metal lamp components and said arc discharge tube contains thereon a coating comprising silicon nitride.

2. A lamp according to claim 1 wherein said coating of silicon nitride is at least about 500 angstroms.

3. A lamp according to claim 1 wherein one or more of said metal lamp components is coated with about 500 to about 20,000 angstroms of silicon nitride.

4. A lamp according to claim 1 wherein one or more of said metal lamp components and said arc discharge tube are coated with about 500 to about 20,000 angstroms of silicon nitride.

5. In combination with a metal halide discharge lamp comprising an outer vitreous envelope, a plurality of metal lamp components within said envelope, an arc discharge tube within said envelope, said discharge tube having an inner surface and an outer surface, an ionizable mixture within said arc discharge tube including a sodium halide ionizable to a sodium ion and a halogen ion, said sodium ion being capable of diffusing from said inner surface to said outer surface; and means for reducing diffusion of sodium ions from said inner surface of said discharge tube to said outer surface of said discharge tube comprising:
a coating of silicon nitride on at least one of said metal lamp components or on at least one of said metal lamp components and said arc discharge tube.

6. A metal halide discharge lamp comprising:
an outer vitreous envelope;
an arc discharge tube disposed within said envelope;
a plurality of metal lamp components disposed within said envelope;
wherein at least one of said metal lamp components has coating means of silicon nitride thereon for reducing the emission of photoelectrons from said metal lamp components.

7. A metal halide discharge lamp according to claim 6 wherein a plurality of said metal lamp components have said silicon nitride coating thereon.

8. A metal halide discharge lamp according to claim 6 wherein all of said metal lamp components have said silicon nitride coating thereon.

9. A metal halide discharge lamp according to claim 6 wherein said metal lamp components comprise metal support means for supporting said arc discharge tube.

10. A metal halide discharge lamp according to claim 6 wherein said metal lamp components comprise current conducting means.

11. A metal halide discharge lamp according to claim 10 wherein said metal lamp components comprise currently supply conductor means.

12. A metal halide discharge lamp comprising:
an outer vitreous envelope;
an arc discharge tube disposed within said envelope;
a plurality of metal lamp components disposed within said envelope;
wherein at least one of said metal lamp components and said arc discharge tube has coating means of silicon nitride thereon for reducing the diffusion of sodium ions from said arc discharge tube and for reducing the emission of photoelectrons from said metal lamp components.

13. A method of reducing sodium loss in a metal halide discharge lamp including an arc discharge tube and a plurality of metal lamp components disposed within an outer vitreous envelope which comprises:
providing coating means of silicon nitride on on or more of said metal lamp components or on one or more of said metal lamp components and said arc discharge tube, aid coating means being effective to reduce the emission of photoelectrons from one or more of said metal lamp components when coated and effective to reduce diffusion of sodium ions from within said discharge tube when coated.

14. A method according to claim 13 wherein said coating means of silicon nitride is at least about 500 angstroms.

15. A method according to claim 13 wherein one or more of said metal lamp components is coated with about 500 to about 20,000 angstroms of silicon nitride.

16. A method according to claim 13 wherein one or more of said metal lamp components and said arc discharge tube is coated with about 500 to about 20,000 angstroms of silicon nitride.

17. The invention as defined in any of the preceding claims including any further features of novelty disclosed.