Note: Descriptions are shown in the official language in which they were submitted.
LD 8915
Cl~RAMIC SEAL FOR HIGH PRESSURE SODIUM VAPOR LAMPS
BACKGROUND OF THE INVENTION
Various type ceramic seals are already known to
close each end o~ the tubular light-transmitting ceramic
envelope used ~or sodium vapor lamps such as described i~
United States patents 4,034,252 and 4,065,691, both
assigned to the assignee o~ the present in~ention. In
said earlier issued patent, the end of said arc tube is
closed with a ceramic plug that is inserted into the arc
tube end and which has a central aperture to permit
passage of an electrode inlead therethrough, with all of
said members being hermetically sealed together by seali~g
frit. The sealing ~rit is said to be comprised primarily
of aluminum o~ide and calcium oxide which i~ heated to
provide a molten glass during the sealing operation. A
metal cross piece is attached directly to said inlead
immediately next to the ceramic plug which e~tends
transversely across the end of the arc tube and physically
supports the therm~onic electrode j oined thereto. The
hermetic seal is formed with a liquid slurry of suspended
~rit particles that is drawn by capillary action with
assistance of the cross piece into the free space e~isting
between joined components. In the subsequently issued
aforementioned patent, the end closure is provided by a
ceramic plug inserted into the end of the arc tube ~hich
2S- again permits passage of an electrode i~lead ~ire
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therethrough, and with a hermetic seal being effected
between all of the joined members. A significant
difference does e~ist, however, with respec-t to the manner
i~ which a hermetic seal is achieved in this end closure
between two spaced apart sealing regions. Two preformed
rings or ~ashers of the ceramic frit mater~al are employed
further requiring that close tolerance dimensions be
maintained betwee~ all of the components being joined
together.
Difficulties can still be e~perienced with these
end closures in the ability to produce reliable hermetic
seals between the metal and ceramic lamp components being
joinedO For e~ample, sealing the ceramic plug to the
ceramic arc tube has prove~ di~ficult with success
generally requiring careful control of both plug and tube
dime~sions along with precise atte~tion being given to the
amount of seali~g frit used. Too little sealing frit
produces i~complete filling of the sur~aces being joined
and with leakage occurring at such locations. Too much
frit produces large sealing fil~ets that can cause
cracking of the ceramic tubing material due to any slight
mismatch in thermal e~pansion between the sealing frit and
the ceramic tubing. Accordingly, it would be desirable to
still further reduce the number o~ sealing ~rit rings or
~5 ~ashers now being used ~ithout decreasing the reliability
of seal formation. It would be further desirable to
reduce difficulties now being e~perienced in maintaining
close physical dimensions for the ceramic plug members
used in these closure structures. It would be still
further desirable to improve the overall end closure for
this type lamp without undue modi~ication or additional
cost in either the lamp manufacture or subsequent lamp
operation,
SUMMARY OF THE IN~ENTION
_
Novel means for arc tube e~d closures have no~
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been disco~ered which modifies one of the previously
described end closure structures. More particularly, the
end closure disclosed in the aforementioned 4,065,691
patent is modified for use at one end of the arc tube in
combination with the u~modi~ied end structure disclosed in
the aforementioned 4,034,252 patent being used at the
opposite end of said arc tube. Basically, the modified
end closure replaces the single ceramic plug member with a
composite assembly of a ceramic disc and ceramic plug that
are bonded together with a single frit washer. The
substituted ceramic disc has a larger diameter than the
inner diameter of the arc tube for butt-sealing engagement
with the end of said arc tube and the single ~rit washer
provides sufficient bonding material to hermetically seal
all joints in the assembled end closure. Both said
ceramic disc and ceramic plug have central openings
through which e~tend a~ inlead and e~haust tube member for
one thermionic electrode which is hermetically sealed at
the central opening of said ceramic plug with material
provided by the frit washer.
The improved high pressure sodium vapor lamp of
the present invention thereby includes a tubular
light-transmitting ceramic envelope with a pair of
thermionic electrodes being sealed at its ends by ceramic
closures, wherein the improvement comprises one ceramic
plug being inserted into one end of said light-
transmitting envelope and being hermetically sealed to the
inner wall of said light-transmitting envelope with
ceramic sealing frit, said ceramic plug having a centr~l
opening through which e~tends an inlead for one thermionic
electrode which is hermetically sealed at said opening
with ceramic sealing frit, and having a transverse metal
cross piece joined to said inlead at the e~terior surface
of said ceramic plug while the remaining ceramic closure
comprises a ceramic disc having a larger diameter than the
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inner diameter of said light-transmitting envelope which
is hermetically sealed with ceramic sealing frit to the
end of said light-transmitting envelope and the surface o~
a ceramic plug inserted into the Pnd o~ said light-
transmitting envelope, both said ceramic disc and ceramicplug having central openings through whîch e~tend an inner
lead and ~xhaust tube member ~or the remaining the~mionic
electrode ~hich is herme~ically sealed at the central
opening of said ceramic plug with ceramic sealing frit.
In a preferred lamp embodiment, both thermionic electrodes
comprise refractory metal ~ire coils wound around a
tungsten shank with the metal cross piece extending
transversely beyond the inner ~all of said light-
transmitting envelope so as to physically support one
thermionic electrode joined thereto~
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a high pressure sodium vapor lamp
employing the presently improved ceramic closure
structure;
FIG. 2 is an enlarged detailed vie~ of one end
closure employed in the FIG. 1 lamp embodiment; and
FIGo 3 is an enlarged detailed vie~ of the
remaining end closure employed in said FIG. 1 lamp
embodiment.
DETAILED DESCRIPTION OF T~E PREFERRED EMBODIMENTS
Referring to FIG. 1, a representative high
pressure sodium vapor lamp 1 corresponding to a 400-~att
size with ~ given lamp voltage drop of about 100 vol~s is
illustrated which comprises a vitreous outer envelope 2
having a standard mogul screw base 3 attached to one end
and comprising a reentrant stem press seal 4 through whlch
e~tend, in conventional fashion, a pair of relatively
heavy lead-in conductors 5 and 6 whose outer ends are
connected to the screw shell 7 and eyelet 8 of the base.
The inner envelope or arc tube 9 centrally located within
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said outer envelope comprise~ a length of light-
transmitting ceramic tubing, preferably polycrystalline
alumina ceramic which is translucent or which can be
single crystal alumina ~hich is clear and transparent.
The upper end of the arc tube is sealed by a closure
structure 10 in accordance with the forementioned
4,034,252 patent through which e~tends the niobium inlead
wire 11 hermetically sealed. The inlead suppor~s the
upper electrode which may be ge~erally similar to the
lower eleGtrode and with ~oth of said electrodes being
more specifically illustrated in the following FIG. 2-3
embodiments. The e~ternal portion of inlead 11 passes
through a loop 12 in transverse support wire 13 attached
to a side rod 14. This arrangement allows for a thermal
e~pansion of the arc tube during lamp operation when the
lower end seal is rigidly fixed in place, and a resilient
metal ribbon 15 assures continued ~ood electrical
connectio~. Side rod 14 is welded to lead-in con~uctor 6
and has its upper end braced by spring clamp 16 which
20 engages inverted nipple 17 in the dome end of the outer ~.
vitreous envelope. The lower end closure 20 for said lamp
and modified in accordance with the present inventio~ has
a central aperture through which e~tend a thin-walled
niobium tube 21 serving as an e~haust tube and as an
inlead. Said tube extends but a short distance through
the end closure and is hermetically sealed therethrough by
sealing frit as will be further e~plained in the
description given ~or FIG. 3 dealing with said end closure
embodime~t. The closed arc tube is supported in the outer
envelope by connector 22 which is ~elded across ~rom
tubular inlead 21 to a support rod 23 joined to lead-in
conductor 5.
Upper ceramic closure 10 ~or the above
illustrated lamp embodiment is depicted in greater detail
as shown in FIG. 2. Speci~ically, said ceramic closure 10
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comprises a ceramic plug 24 which is entirely inserted
into one end of the arc tube and having a metal cross
piece 26, which can be a short wire length, joined to
inlead 11 at the egterior surface 28 of said ceramic plug.
Said cross piece is bent or arched slightly about its
midpoint in a horizontal plane transverse to said inlead
in order to have its ends engage the edges of the arc tube
diameter and help physically support the thermionic
electrode joined thereto~ A hermetic seal 29 iæ ~ormed in
the annular recess between the assembled ceramic plug and
inner wall o~ the arc tube as hereinafter further
e~plained. A second hermetic seal 30 is formed at the
central opening 32 in ceramic plug 24 and which e~tends to
a ridge of sealing frit 34 on the surface of said plug
reaching up to the cross piece 26. All hermetic sealing
in the illustrated closure embodiment can be provided with
a measured ~uantity o~ sealing glass frit being placed on
the ceramic plug, ~uitably as a slurry and the closure
assembly then being heated to the melting temperature of
the glass frit. The molten frit is drawn by capillary
action into the annular free space between the arc tube
and ceramic plug as well as into the ~ree space formed
hetween the central opening in said plug with the
electrode inlead. Said inlead 11 is joined to a tungs~en
25 shank 36 of a conventional thermionic elec~rode 38 having
one or more refractory metal coils wound around which is
further described in the a~orementioned 4,034,252 patent.
The pri~cipal ~eature of the present inventior
resides in the modification of ceramic closure 20 which is
depicted in greater detail for the lower electrode
structure and electrode support assembly therefor as shown
in FIG. 3. Specifically, said improved ceramic enclosure
20 comprises assembly of a ceramic disc 40 having a larger
diameter than the inner diameter of said arc tube 9 which
is hermetically sealed with ceramic sealing ~rit at one
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end of said arc tube as well as the outermost surface 42
of a ceramic plug 44 which has been inserted into the end
of said arc tube. The butt-sealed interface between said
ceramic disc 40 and ceramic plug 44 is formed with a ring
or washer 46 of ceramic frit material which further
provides a hermetic seal ~8 ~o associated electrode
structure 50. Said conventional electrode 50 is shown to
include part of a metal e~haust tube and inlead member 21
that is inserted through a cen~ral opening 52 in both
ceramic disc and ceramic plug for hermetic sealing after
assembly of said component parts. As can be noted,
hermetic sealing 48 between the electrode and ceramic
components is generally limited to sealing engagement with
the plug member only.
It will be apparent from the foregoing
descriptio~ that improved seal means for high pressure
sodium vapor type lamps has been disclosed ~hich is
generally useful. It will be further apparent to those
skilled in the lamp art, however, that said improved
20 closure means can replace conventional end closures in ~-
this type lamp although remaining features in the lamp
other than above specifically disclosed are employed. For
example, lamp designs utiliæing additional gettering and
starting aids can use the present ceramic closure means.
~5 Accordingly, it is intended to limit the present invention
only by the scope of the following claims.
