Note: Descriptions are shown in the official language in which they were submitted.
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LD 7580
ALUMINA, CALCIA, BARIA SEALING coMæosITIoN
AND ARTICLE OF MANUFACTU~E
The invention xelates to a glassy sealing composition
of particular value in bonding to alumina ceramic, that is, bond-
ing ceramic to ceramic or ceramic to metal in electric lamp manu-
facture.
BACKGROUND OF THE INVENTION
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Alumina ceramic, either translucent or clear, is parti-
cularly suitable as an envelope material for high inten ity alkali
metal vapor lamps such as high pre~sure sodium vapor lamps, be-
cause it withstands the attack-of the vapors of the alkali metals
10 even at high temperaturesIn m~king such lamps, end closures
which support the electrodes must be bonded and hermetically seal-
ed to a ceramic tube and a ssa~ing glass or composition is used .
for this purpose. me closures may take the ~orm of metal end
caps which provide direct electrical connection to the electrode~,
or of a ceramic plug in which case a metal conductor must be seal-
ed through the plug to provide the electrical connection. The
metal having a coefficient of expansion closest to that of alumina
ceramic is niobium and it is the one generally used either for
the end cap or ~or the inlead in the cas~ o~ a ceramic plug. ¦ .
Desir~ble propertie~ of a seali~g compo ition are
favorable liquidus temperature, wide sealing range, ability to
form glasses when m~lted and cooled rapidly, and stabili~y in
the presence of alkali metal vapors at elevated temperaturas.
The liquidus temperature must of course be above the highest to
be encountered during operation o~ the la~p, but preferably not l-
m~ch above such maximum in oxder to facilitate manufacture and
assUre longer furnace lie~ A composition meeting the foregoing
requirements is disclosed in W.S. pat 3,588"577 - McVey et al. It
is centered on 45 wt~ A1203, 36 wt% CaO, 14 wt% BaO, and 5 wt~
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MgO. This composition, referred to as G45, has been widely used
by applicants' assignee in the commercial manufacture of high
pressure sodium vapor lamps of the kind using niobium metal end
caps. As in most manufacturing operations, some defective prod-
ucts, cracked or leaky seals in this ca~e, are produced when
using the G45 sealing composition. Such defective products must
be rejected and the proportion thereof is commonly referrad to
as the shrinkage rate. When the G4~ composition is used to seal
ceramic end plugs, a somewhat higher shrinkage has been experi-
enced.
The object of the invention is to provide a new compo-
sition for sealing metal or ceramic to alumina cPramic and which
is superior to the G45 composition. In particular a composition
is desired providing a lower shrinkage rate when sealing alumina
ceramic parts together in electric lamp manufacture.
SUMMARY OF TH~ INVENTION
In accordance with the invention we provide a~ improvea
sealing glass or composition far use between alumina ceramic paxts
or between alumina ceramic and a refxactory metal. It is centere~
on 47 wt~ A1203, 37 wt% CaO and 16 wt% BaO; these proportion~ re-
sult ~rom reformulating the G45 composition after eliminating it~
5 wt% MgO component~ The new co~po~ition, known a~ G47, has the
following properties which indicate several advantages over the
G45 composition~
1. The liquidus temperature o-E G47 at 1325C is 95C
lower than that of G45 at 1420, making possible a lower sealing
tempexature.
2. A1203 is less soluble and dissolves more 510wly -
into G47 than into G45 (12.5% vs. 30~ by weight at 1550C~ -
he~ce fewer problems due to seal glass-alumina ceramic inter-
- actions.
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3~ G47, unlike G45, does not ~orm the thermally incom-
i,X,
patible phase~ CaO 7A1203 when molten G47 containing dissolved
A1203 is cooled - hence a stronger bond.
4. The thermal expansion coef~icient of G47 is less
aEfected by dissolved A1203 than is that of G45, so that a closer
thermal expansion match with alumina ceramic can more easily be
maintained during seal processing.
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DESCRIPTION OF DRAWINGS
FIG. 1 shows an alumina ceramic lamp in which the G47
composition of the invention is used to seal a metal end cap~
. FIG. 2 shows an alumina ceramic lamp in which the ~47
composition i~ used to seal a ceramic end plug and a metal lead .
through the plug. .
FIG. 3 compares the liquidus curve of G47 with that of
G45. .
FIG. 4 compares the instantaneous coefficient of the~mal
expansion of G47 with that of G45 at 850C as a function of dis- .
-solve A12~3 .
DESCRIPTION OF PRE~ERRED EMBO~IMENT
A typical application of the sealing composition of the
invention in sealing an el~ctric lamp is illu~trated in FIG. 1. An
arc tube 1 comprises an envelope 2 of ceramic tubing consisting of
sintered high density polycrystalline alumina. A central portion
of the tube has been cut out to shorten the figu~e, and the in-
ternal construction can be seen in the sectioned lower por~ion~ B~way o~ example, in a 400-watt size of a l~mp, the arc tube is about
110 mm long by 7.5 mm in diameter.
The ends of the tub2 are closed by thimble-like niobium
closures or end caps 3,3' which are hermetically sealed to the
ceramic by means of the G47 sealing composition o~ the invention..
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The sealing composition is indicated at 4 and is loca-ted within
the space between the expanded shoulder portion 5 of the end
cap and the side and end of the ceramic tube. A niobium tube
6 penetrates into the thimble and is used as an exhaust tube
during manufacture aEter which it is closed off. A ther-
mionic electrode 7 is mounted in each end of the arc tube
and is supported by the niobium tube 6. The filling of the
arc tube may consist of a sodium mercury amalgam dose and an
inert gas such as xenon, or a neon-argon mixture to facilitate
starting. The arc tube is not operated in air but is mounted
within an evacuated outer jacket (not shown) which prevents
oxidation of the metal end caps.
FIG. 2 shows another typical application of our sealing
composition in a similar lamp using a ceramic plug for the
end closure. One end only of the lamp is shown and the con~
struction illustrated in that of U.S. Patent No.4,065,691
issued December 27, 1977 and similarly assigned. The end of
the alumina tube 2 is closed by a shouldered alumina ceramic
plug 11 (sometimes referred to as a 'manhole' plug) having
a central aperture through which extends a thin-walled niobium
tube 12 which serves as an exhaust tube and as an inlead. The
tube extends but a short distance through the plug and is
hermetically sealed there-through by sealing composition in-
dicated at 13. The plug in turn has its neck portion penetrat-
ing into ceramic envelope 2 whose end butts against the s
shoulder portion of the plug. A hermeric seal is effected
between the two parts by sealing composition indicated at 14.
In this lamp construction the electrode 7 is supported by
crimping exhaust tube 12 at 15 so as to clamp the electrode
shank 16 over an appreciable length.
The G47 sealing compositions used in the above-described
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lamp seals, and G45 compositions made for comparison purpOses, were
prepared through a solid state reaction. Appropriately weighed re-
agent grade chemicals consisting of A1203, CaC03, BaC03, and MgO
where called for, were mixed and blended in acetone and prereacted
r;i~at 1200C overnight in platinum crucibles. The samples were then
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~e~ and milled to a fine powder and a quantity pressed into a
sealing washer of appropriate size.
For studies of A1203 solubility and recrystallization,
appropriate amounts of A1203 were mixed with G45 and with G47
compositions. Several portions of these samples were melted to a
clear liquid in platinum crucibles. The liquid was then quenched
to a clear glass by plunging the outside of the crucible into cold
water. To characterize the recrystallization phases, the glass
fragments thus produced were heat treated overnight in platinum
crucibles at temperatures between 1200 and 1450C. The phases
present in various compositions were determined by petrographic
microscopic examination as well as by powder x-ray di~raction
patterns.
The solubility of A1203 in the compositions as a function
of ~empera~ure was measured by melting the composition with various
amounts of A1203 in ~ strip furnace equipped with an optical py- -
rometer viewed through a telescope. Instantaneous coefficient o~
thermal expansion aT was measured by a dilatometer using platinum
as a standard.
Solubility of A1203 In Composition:
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While the initiation of melting or solidus point occur~
a~ about 1275C for both compositions, the liquidus temperature of
G47 is about 1325C, as against 1420C for G45. Samples o~ G47
and of G45, both with added A1203 were heated in between the
respective solidus and liquidus points. The liquidus temperature
increases with A1203 content as is seen in FIG. 3 wherein the
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~curves show the maximum solubility of Al ~3 as a function o~ tem-
perature assuming complete thermal equilibrium. Samples equili-
brated between solidus and liquidus points were examined by x-ray
diffraction analysis. In the case of G47, A1203 was not found as
S a crystalline phase, indicating its complete dissolution inko the
compositlon or its crystallization products. The process of dis-
solution, viewed through the pyrometer telescope, appeared slug-
gish. In addition, when samples whi~h had been quenched from
temperatures as high as 50 to 70C above their liquidus points
were viewed under a petrographic microscope, numerous entrapped
air bubbles were o~served: this indicates high viscosity of the
glass or sealant.
The lower liquidus temperature o~ the G47 composition
according to the invention permits ceramic to ceramic or ceramic
to me~al seals to be made at temperatures around 1450~C. By
contrast, the current practice when using G45 as the sealant has
been to use a temperature o~ around 1550C. The lowering of the
sealing process temperature has certain de~inite advantag~s.
Firstly, it reduce~ manu~acturing difficulties and economizes on
fuel. Secondly, it cause~ less thermal stres~ and mechanical
tension in the seal joint area upon cooling after the seallng op-
era~ion.
In making a seal, some A1203 from the ceramic goes into
solution in the sealant. The lower sealing temperature, tha great- !
~5 er viscosity of the G47 melt and the lower solubili~y of A1203
therein all combine to reduce the amount of A1203 from the ceramic ,'
which goes into solution in our sealant. The invention thereby
appreciably reduces problem~ caused by sealing compo~ition/alumina
ceramic interaction. ` I -
Cr~ll zation Behavior-
.
The G47 sealing composition, unlike G45, does not form
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the thermally incompatible phase 12CaO 7A1203 when molten composi-
tion which contains additional dissolved A1203 is cooled. In
samples of G47 and G45 compositions containing additional A1203
heated overnight at 850C, x-ray diEfraction patterns showed
moderately intense recogniæable peaks of the 12 CaO~7A1203 phase
only in the G45 composition. G47 composition containing addi-
tional A1203 to the extent of 15~ by weight or more showed several
peaks indicating presence of BaA1204: HQWeVer, in no case ~f re-
crystalliæed or cooled G47 samples was metastable crystallization
10 of 12CaO~7A1203 observed. We have concluded from this that the
absence of MgO from the G47 composition according to the invention
accomplishes its intended purpose: recrystallization of th~ un-
desirable 12CaO'7A1203 phase is success~ully inhibited.
Thermal Expansion of G47 with Added ~1203:
The curves in FIG. ~ represent the coe~ficient of in-
stantaneous thermal expansion, aT, as a ~unction of dissol~ed
A1~03 at a temperature of 850C. This temperature i5 approxi-
mately that o the seals during opexation of the lamp. The co
efficient of thermal expansion of polycxystalline alumina cer~mi~,
20 95xlO 7/oC is indicated in the ~i~ure by a light dash dot lineO
~t will be observed that the aT value of a G47 sealing composition
is less influenced by the dissolved additional A1203 content than
is the G45 sealing compo~ition. In particular over the range of
additional dissolved alumina from 5 to 15 percent which include~ ¦
25 the practical sealing range, the coefficient of instantaneous
thermal expansion ~T of the G47 composition varies only from
97.5xlO 7/oC to 93.5xlO 7/oC. This provides a close thermal match
straddling the expansion coeficient of alumina ceramic at
95~10-7/oC. . .
Advantages of G47 Composition:
In studies of seal failu:es in ceramlc/ceramic seals
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using the prior art G45 composition as a sealant, it was observed
that the failure mode is partly due to microcrack formation ori-
ginated by thermal-expansion mismatch between the arc tube and
the seal-area composite (sealant ~ ceramic), as well as between
S dif~erent parts of the seal itself. We concluded that the ex-
pansion mismatch is caused by dissolution of A1203 in the sealant
melt followed by recrystallization of the thermally incompa~ible
phase 12CaO 7A1203. The greater shrinkage in ceramic-to ceramic
seals would be due at least in part to the dissolution of more
A1203 into the sealant inasmuch as A1203 can move into the sealant
~rom both ceramic surfaces in contact with it. This would explain
the greater shrinkage rate ob erved with the ceramic plug lamp
design of FIG. 2 when using G45 sealing compoqition.
When the data for solubility, thermal expansion and rate-
of dissolution of additional A1203 are compared together, addi-
tional advantages of the G47 sealing composition emerge. In a
practical sealing operation, it is un1ikely that complete thermal
equilibrium and maximum possible solubllity o~ A1~03 in ~he molten
sealant aXe achieved. In a typical sealin~ schedule, ~or example
about $ minutes of soaki~g time at 1550C when u ing ~he prlor G45
sealant, a true equilibrium in the seal area is probably not a-
chieved. On account of the rapid dissolution of A1203 in the
~45 m~lt, it can be assumed t~at locally about 20 to 25 weight per- ¦
cent of A1203 from the ceramic tube is dissolved in the sealant~
Then on cooling the metastable phase of 12CaO 7A1203 appears,
causing a significant decrease f~om ~he minimum acceptable ~alue
o thermal expansion of the seal/ceramic composite. This i5 borne
out by x-ray diffraction analysis of seal/~eramic composite and
fxom photomicrographs of sectioned seals. On ~he other hand, under
simllar seal process conditions using the G47 composition as the
sealant, only about 10 to 12~ or less alumina--due to the sluggi h
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process of dissolution with G47 as the sealant--is dissolved. In
addition, because of the high,viscosity of the melt, little crys-
tallization takes place on cooling, making the resulting seal a
close match to the alumina ceramic in thermal expansion.
Thus by reformulating the prior art G45 sealing compo-
sition to eliminate the 5 wt% of MgO, the invention has achieved
an improved sealing composition which allows seals to be made at
a lower temperature and wherein the sealant has a closer thermal
expansion match with alumina ceramic. Our new G47 sealing composi-
tion is advantageous for use with both niobium end oap lamps as
shown in FIG. 1, and ceramic plug lamps as shown in FIG. 2. In
tests of seals using ceramic plugs, a substantial reduction in the
shrinkage rate was achieved relative to the prior ar~ G45 composi-
tion.
While the preferred proportions of the G47 sealing compo-
sition are 47 wt~ A1203, 37 wt% CaO and 16 wt% BaO, the benefits
of the in~ention will be substantially realized with proportions
by wei3ht within the following limits:
CaO.......... ~................ 3~ to 42~,
BaO.......... 0... .....~....... 13 to 19%~
MgO.............. ........... up to 1%,
A1203.~.......... ............. .he balance making 100%.
It will be appreciated that even when one starts with sealing
composition in the preferxed proportions those will not be the
propor~ions in the sealant after melting and congealing in place
as binder in a sealed lamp axc tube. The sealant will contain
additional A1203 which dissolved into it from khe alumina ceramic,
fQr instance up to 20 wt~ additional A1203. The precise amount
will depend of course upon the nature of the joint and the tem-
perature and-soaking time used in cementing or sealing the parts
together.
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The avoidance of the 12 CaO 7A1203 phase when the sealant
cools and congeals is most effect.ively achieved by completely eli-
minating MgO from the composition. However the benefits of the in-
vention will be substantially realized notwithstanding the presence
of MgO in the composition up to about 1 wt%. This tolerance is
fortunate because it permits the use of components which may con-
tain MgO as impurity.
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