Electric lamp and discharge devices – With support and/or spacing structure for electrode and/or... – Electrode supported by envelope
Reexamination Certificate
2003-06-26
2004-08-10
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With support and/or spacing structure for electrode and/or...
Electrode supported by envelope
C313S289000, C313S623000, C313S574000, C140S071500, C140S071600
Reexamination Certificate
active
06774547
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to discharge lamps, in particular to metal-halide discharge lamps.
FIG. 1
shows a conventional metal-halide discharge lamp. The lamp includes a ceramic discharge arc tube
5
with a capillary tube
10
extending from one side of the discharge tube
5
. A feed-through
15
is inserted into the capillary tube
10
and sealed with a frit seal
20
. The feed-through
15
includes four rod-like components; a tungsten electrode tip
25
, a molybdenum coil
30
, a cermet (50% Mo, 50% Al
2
O
3
) rod
35
, and a niobium rod
40
.
The tungsten electrode tip
25
extends into the volume of the discharge tube
5
, to function as the discharge termination point. The molybdenum coil
30
is laser welded to the tungsten electrode
25
and extends into the capillary tube
10
. The molybdenum coil
30
includes a molybdenum wire wound around a retained molybdenum mandrel. The particular geometry of the molybdenum coil
30
hinders the migration of salts from the fill gas in the discharge tube
5
into the capillary tube without causing excessive heat transfer up the capillary tube
10
from the discharge source.
The electrically conductive cermet rod
35
is laser welded to the other end of the molybdenum coil
30
to provide a material that is both resistant to the fill gases and salts as well as having a coefficient of thermal expansion similar to that of the wall of the capillary tube
10
and the frit seal
20
.
Niobium rod
40
is laser welded to the other end of the cermet rod
35
and functions as a material interface between the interior and the exterior of the discharge tube
5
at the end of the capillary tube
10
. Part of the niobium rod
40
sticks out of the end of the capillary tube
10
.
The frit seal
20
is a solder glass material used to seal the niobium rod
40
to the capillary tube
10
so as to seal the interior of the discharge arc tube
5
from an outside atmosphere. The frit seal
20
extends from the end of the capillary tube
10
between the niobium rod
40
and the capillary tube and into the area where the cermet rod
35
is located. Since niobium is not resistant to the corrosive effects of the discharge tube fill, the frit seal
20
functions not only to seal the discharge tube from atmosphere but also to protect the niobium rod
40
from the discharge tube
5
fill. Niobium has the specific characteristic that its thermal expansion coefficient is very close to that of the alumina that forms the discharge tube
5
and the frit seal
20
to minimize seal cracks and leaks caused by the large temperature variations that can occur when sealing and operating the lamp. These materials each have a thermal coefficient of expansion of about 8×10
−6
K
−1
. The cermet rod
35
would not be appropriate in the seal location even though it has appropriate thermal expansion characteristics because, unlike niobium, it can develop fissures that can spread and cause leakage of the seal.
The feed-through shown in
FIG. 1
requires three laser welds with three different pairs of materials—W to Mo, Mo to cermet, and cermet to Nb. The laser welds must provide intimate contact between the materials and provide good conductivity through the feed-through. The materials must be welded together to provide a straight feed-through that is easily slid into a capillary tube of a discharge lamp. The laser welds must be uniform and smooth so as to avoid burrs and the like that can inhibit passage of the feed-through into the capillary tube. The laser welds must be strong to avoid feed-through breakage during handling and shipping prior to being sealed in the discharge tube. Laser welding equipment is expensive and there are safety issues with its use. The laser set-ups are complicated requiring expensive fixturing with an inert atmosphere at the weld. Although the laser welding process is feasible, it is costly and complicated and one would rather not use laser welding.
In addition to the complications of laser welding, the feed-through contains four different materials, which have to be managed and understood from a material processing, lamp fabrication and lamp operating perspective. The cermet is expensive and its integrity is problematic at high temperatures relative to solid homogeneous refractory materials due to its potential to segregate into its base materials as well as develop fissures. Niobium absorbs hydrogen at low temperatures (<100° C.), oxidizes in air (>200° C.) and readily absorbs hydrogen, oxygen and nitrogen at higher temperatures that causes it to be brittle and to change its thermal expansion characteristics. Further, the niobium is exposed outside the discharge tube, and thus the discharge tube must be used in an atmosphere with which niobium does not react. Niobium also restricts the atmosphere in which the feed-through can be cleaned before manufacturing the discharge tube. For example, high temperature (~1100° C.) wet and dry hydrogen surface cleaning of the feed-through to rid surfaces of carbide and oxides impurities would be possible if not for the presence of the niobium. Additionally, unlike tungsten and molybdenum, niobium is not resistant to the corrosive effects of the discharge tube fill and has to be protected. The niobium puts further constraints on the lamp arrangement because the frit seal must cover the niobium and extend beyond the niobium/cermet weld, which in turn exposes the frit seal to higher temperatures.
In attempts to overcome the problems of the conventional discharge lamp, other high-intensity discharge lamps have been offered. U.S. Pat. No. 4,531,074 to Nagy et al., describes a feed-through for a 250 W high-pressure discharge lamp in which thin strands of molybdenum wire having a diameter of 0.05 mm, and preferably not more than 0.01 mm, are bundled together. The patent teaches that the diameter of the bundle should not exceed 0.15 mm in the case of molybdenum. The bundle is threaded through a bore in an aluminum oxide plug and connected to a tungsten electrode. The bore is sealed with melted vitreous enamel. The bundle is flexible to compensate for the heat expansion of the discharge tube.
U.S. Patent Publication 2002/0084754 to Allen et al. describes a feed-through for a low wattage ceramic metal halide (CMH) lamp with a niobium outer lead welded to an intermediate component comprising a molybdenum overwind on a Mo mandrel. The intermediate component is welded to an electrode comprising a tungsten shank with a W coil wound around one end of the shank. Allen et al. use reduced diameter mandrels with an increased overwind or use multiple overwinds to alleviate thermal expansion stresses that occur between the intermediate component and the ceramic lamp.
FIGS. 2
a
and
2
b
show another conventional discharge lamp such as that taught by U.S. Pat. No. 5,455,480 to Bastian et al. Specifically,
FIGS. 2
a
and
2
b
describe a 100 W high-pressure discharge lamp
5
a
with a ceramic sealing element
21
, an electrical feed-through
22
and a discharge vessel having cylindrical ends
6
through which the feed-through
22
extends. The feed-through
22
is made of alumina with metal wires threaded therethrough. The feed-through
22
is formed with at least two thin wires
23
having a diameter of about 0.25 mm. The wires
23
that extend into the interior of the discharge vessel are twisted together to form an electrode tip
25
′. The wires in the cylindrical ends are either loosely bundled and surrounded by glass melt or individual wires
23
are fed through a plurality of bores in a ceramic plug and then surrounded by glass melt
29
. The number of wires determines the current rating of the lamp. Bastian et al. also teach a lead wire connection extending outside of a capillary tube. The lead wire connection end of Bastian et al. is complicated requiring a niobium closing portion
28
and a niobium wound portion
27
in addition to a glass melt seal
29
.
These lamps have problems specific to the particular design choice. The bores of both Nagy et al. and Bas
Avallon James
Browne Joanne
Zaslavsky Gregory
Clark Robert F.
Leurig Sharlene
Osram Sylvania Inc.
Patel Nimeshkumar D.
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