Note: Descriptions are shown in the official language in which they were submitted.
~37~
This in-~ention relates to electric inca~descent
lamps, and mor~ especially to incc~descent lamps operating
by a halogen Gycle.
I~ any incandescent twugsten fila~ent lamp co.~tainin~
a reactive fill such as halogen or hali.de, the choi.ce of
material for the internal components and ellvelope is usually
very restricted. ~or lalrlps having iodin2, bromine or chlorine
in the ~ill the envelope is preferably fused quartæ or a high
silica co~ten~ glass a~d the lead-in wire, filament supportst
internal re~lectors, shields and other internal componellts
are substantially composed of molybdenum or tungstenO If less
e~ensi~e~ common materials such as nickel, iron, copper,
alum ni~ and alloys containing these are used they react ~Jith
the halogens to form halides which can cau~e ~ilament em-
brittlementg and/or a halogen deficiency, both resulting inseverel~ reduced filament life~ Also, if soft glass~ such as
soda li~e silicate, is used for the e~elope, apart from the
obvious difficulties of the low softenin~ temperature and hlgh
water content~ the alkali metals can react wi.th the halogen
ox halides, again reducing filament li~e.
In ope~ation, tungsten-halogen la~ps normally contai.n
a non-reactive gas filling such as ~2~ ~rg Kr or X.e together
with iodine, bromine or chlorine vapour whiGh combin.es with
the evaporated tungste~. escaping from the inca~descent
fi7ament. A~ equilibr.ium concentration i.s attained by the
gaseous species within the lamp between the 1:emperattlre limits
defined by the i.ncandescent filament and col~est spot on the
.~
.
.
7~
lamp envelope. ~he cold spot temperature must be suffi~
ciently high to prevent an-g tun~sten hali(le from condensing t
and provided that this condition i5 met a continuous t~ngs~er
trc~sport cycle operates which keeps t-he e~velope ~xee from
tungsten~ The minimum envelope tempera-ture depends upon the
halo~en ox halogens taki.ng part in the cycleO However, the
maxi'num envelope temperature is usually well above the
acceptable limit for soft glass, and for this reason tun~sten-
halogen lamp envelopes are usually made from vitreous fus0d
silica or high silica content glasses.
The return o~ tungsten to the filament does not in
itself increase filament life since tungsten iodides, bromldes
and chlorides dissociate well below normal filament operatin~
. temperatures. Radio-chemical tracers have shown that
evaporated tungsten is redistribu.ted during the life of the
lamp so that the cooler parts of the filament collect -tungsten
at a ~reater rate than the hotter partsO ~ilament failure
usually occurs quite normally by the subseque.nt burn~out of a
'hot spot'. '~he improvement in life of tungsten-halo~en lamps
. 20 in comparison with conventional incandescent lamps is for
quite a differeLlt reason~ The absence of envelope blackening
coupled with the requiremen.t for a well-defined minimum
envelope temperature dictates tha-t the envelope must be
. su~stantially smaller than that of a conventional courlterpart.
In fact, tungsten-halo~en lamp envelopes are usually small and
mechanicall~r strong and in consequence can be safely gas-
. filled to several a-tmospheres pressure. ~'his increased.~as
: -3-
7~
filling pressure accounts for the gain in life.
If f;lament 'hot spots' could be healed or prevented
a further extension in filament life would be possible. This
is feasible with a tungsten-fluorine transport cycle because
in this case the most stable tungsten fluoride dissociates
at a temperature above 3000C, and tungsten is returned to
the incandescent filament surface. Again, this has been
substantiated by radiochemical tracer experiments, which show
that tungsten vapour returned from the region of the envelope
is evenly distributed along the incandescent part of the
filament. Technological difficulties have prevented the
further development of tungsten-fluorine lamps, the principal
problem being that free fluorine reacts rapidly with solid
tungsten below about 2000C, the cold parts of the filament,
the lead wires and the supports being rapidly eroded, and
that the fluorides formed (e.g. tungsten fluorides) react with
the silica contained in the envelope material to form SiF4,
depositing tungsten on the tube wall. This uses up the free
fluorine in a very short time. Various methods have been
proposed for protecting the envelope and tungsten components
but these have been unsuccessful because of the inability to
produce a continuous thin layer of protective material free
from pin-holes and minor defects.
In our Canadian Patents Nos. 986,980 of April 6, 1976
and 1,050,605 of March 13, 1979, both of which are assigned to
Thorn Lighting Limited, we describe the use of glassy coatings
for the internal surfaces of halogen-containing electric lamps,
and describe a process for
~ 9~
the form?~tion cf defect~free coatin~s by deposition of a
solutio~l of compounds of the metal and phosphorus or arsesiG,
followed by e~aporation of the solven-t and bakin~ of the
resultin~ layer~
5 United States Patent ~o, 3,067,356 describes a fluores-
cent lamp havin~ an inter:rlal barrier layer of, inter alia,
aluminilun oxide i.ntended to reduce darkenin~ of the lamp by
reaction of mercury in the gas fill with alkali in the glass
of the lam~ tube. ~ot only is the use of this coati~g
different from that required for this invention, but the layer
in the Patent re~erred to is ~ormed by applying the particu-
late oxide in a lacquer vehicle, follcwed bg- drying G~nd baking~
and such a layer does not provide effective protection against
a hi~hl~ reactive gas such as fluorine.
r~he presellt invention now provides an alternative or
improved. protective layer for the exposed internal surfaces oi'
incandescent lamps which tend to react with the f-ill in -the
lamp en-velope, particularly where -this includes a halogen~
and more especially fluorine or a fluorine-contai.nil~g compound,
2~ Xn accordance with this invention at least those
; portions of the i.nterna]. surface of -the en~el.ope o~ a halogen
cycle incandescen-t lamp, and preferably also the exposed
sur~aces of inter~al components which tend to react wi-th the
- fill in the envelope during operation of the lamp are coated.
with a continuous i~perforate coating substanti.ally consistirlg
of a me~al oxide resistant to halogen attack ~nd derived
from a compound of the said metal deposited on such surfaces
from solution~
5--
i7~
~ urther in accord.~nce with.-t~is in~ention there is
provided a ~ethod of making a halogen cycle incandescerlt lamp
which comprises coat-ing at least those portlons of the
i~-ternal surface of the envelope and tne exposed surfaces of
5 internal components which tend to react with halogen during
operation of the lamp with a solution of a metal compoun.d
capable of generating on being heated a halogen~-resistant
oxide o~ the metal, and heating the resulting coating -to form
on the surface a coating of the said oxide.
~he pre~erred metal o~ide ~or the formation of the
coating is aluminium oxide, but other halo~en-resistan-t oxides,
more especially of polyvalent or transition metals such as
cerium, thorium and yttrium, and others of the lanthanide
series, may be used. All~inium oxide has an especial adva~
tage in -that coatings formed thereby are both colourless and
~ranspalent. The coatings are preferably formed ~rom solutions
of halides of the metals in polar organic solvents such as
methanol~
Preferred solvents for the preparation of -the coating
solution a.re oxygen~containing organlc solvents such as
alcohols, estexs, ketones, aldehydes, nitro-compounds and
ethers, or miætures thereof. Particwlarly preferred are
nliphati~. alcohols, especially lower molecular weight
. alcohols containin~ 1 to 4 carbon atoms, for example methanol,
: ~5 ethanol, n- o:r iso-propanol or substituted alcohols such as
methoxy- or ethoxy-ethallol. A wide variet~ of compound.s of
the me-tals can be used, provi.aed they are solllble :in the
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99~
chosen solvent and generate on bei.ng heated the desired
oxide without substantial contamination by other elements
Simple inor~anic salts of the metals are preferred,
especially the halides, but org~nic salts such as acetates
~!ay also be used. ~he sa.lt may form in ~olution a complex
with molecules of the solvent.
~ 'he coated surfaces in the la~ps of this invention
may include the internal surface of the envelope, the
filament tails or lead-in wires or the filament supports~
depending on t~e nature of the fill gas employed and on the
materials from which the envelope and the internal components
are fabricated. Part or al.l of the filament or filaments may
be initially provided with a coating, for examAple ~here the
coating technique according to the invention ca~no-t conven-
iently avoid this, but the coating on the filament will be
remove~ when the fi.lament is heated to incandescence.
~ he protective coatin~s provided in accordance wi.ththis invention may be applied to conventional materials used
for the fabrication of lamp components, for exa~lple to
:20 protect them from hi.ghly reactive fill substances~ or they
- ~ay enable cheaper and more readily available mclteria].s to
be substituted for conventionally used materials wlthou-t
unacceptable loss in performance or life~
- ~he oxide coating should be continuous and free from
pin-holes or other de~ect or imperfection ~rhich might cause
it to bre~ down durin~ operation of the lamp. Suitable
coati.n~s are glass-like in appearallce but may have a micro~
7--
c.rystalline stl~cture. They preferably have a thickness in
the range 0~001 - 1 micron. Alt~rnatively, the coati.ng weigh-t
may be 0~ - 1000 microgram per cm2.
In one preferred method of makin~ lamps according to
this invention, the desired portions of the internal surface
of the envelope and the surfaces of in-ternal components which
are exposed in the finished lamp are coated either separately
or after assembly with an organic solveIlt solution of c~n
aluminium compound or complex capabl~ of ~enerating aluminium
oxide, and subsequently heated to evaporate the solvent and
cure the composition to form a defect-free alumi.nium oxide
coatingO It has been found valua~ie in the producti.on of
d.efect-free coatings to allow the applied liquid coating
composition to drain thoroughly and thereafter -to bake
initially a-t a relatively low temperature to remove the
solve~t and subsequently at a controlled higher temperature
to complete the formation of the protective coa-ting The
preferred baking temperatures vary with the particular com-
position en~ployed to generate the aluminium oxide, but c~m
be determined by experiment.
One example of this techni~ue wi1l now be described
with reference to the accompanying drawing which shows dia~
grammatical:Ly a tungsten-halogexl lamp assembly ln the course
of manufacture.
As shown i~ the drawin~, a 12V. 55W. tungsten-halogen
lamp, o~ the type commonly used i.~ projector and motor
~ehicle lighting applications, comprises a fused quartz
g~
envelope 1 in whiGh is sealed a tungsten filament 2
supported on filament tails o.r lead-in wires ~ and is
; provided with an exhaust tube 4. '~he lamp is -to be providecl
with an aluminil~-n oxide barxier layer covering the inside
surface of the envelope 1, the filament 2 and filament tails
A liquid CQating composition capable of generating
the aluminium oxide is dlspensed ~rom a h~podermic syringe
through the lamp exhaust -tube 4 by inserting the needle of the
syringe, discharging the liquid composition and then almost
immedlately drawing it back into the syringe, lecaving only a
thin layer adhering to the inside surfaces of the lamp struc~
turer At this stage the lamp is inverted to drain, and then
heated in a vacuum or sui-tably inert atmosphere, for example
at approx.imately 100C for an hour in the case of a
methanolic composition. ~he aluminium oxide coating is
finally formed by bakillg at a higher tempera-ture, for example
at 500C in a vacuum or suitably inert atmosphere ~or about
5 minutes~ The final b~e can be e~fectively incorporated in
20 subsequent lamp processing.
The initial heating cycle is chosen to substantially
remove the solvent and the time, temperature and atmo~phere
will depend upon the solvent selectedO '~he temperature of
the subse~uent bake depends on ths particula.r formulation
used, but will in general be below 1000C.
The lamp i.s then processed in the noI~lal ma~ner lor
tungsten--halogen lamps. Wherl the filament is first ener~.ised
_9~
~13~9~
the aluminium oxide layer on the incandescent filament surface
and part of the filament tail adjacent to the filament is
removed, leaving a protective barrier on the envelope surface
and cold parts of the filament tails or lead-on wires.
In accordance with one aspect of this invention it has
been found that when such lamps are provided with a fluorine-
containing fill they can be operate~ with less or even
substantially no attack on the filament tails, the filament or
the envelope surface by fluorine or fluorides. The fluorine
can be added as the element, or more conveniently as WF6
within the pressure range of 1 to 10 Torr, or as NF3 or SF6 or
a solid such as NF4SbF6, NF4AsF6, XeF4SbF6, XeF4AsF6, TeF4SbF5
or SeF4SbF5. Solids may also be added in solution in suitable
solvents as disclosed in the specification of our United States
Patent No. 3,898,500.
An alternative source of fluorine is described in our
Canadian Patent No. 1,069,576 of January 8, 1980, namely a
soluble fluorocarbon polymer, which can be metered into the
lamp envelope in solution in, for example, a fluorinated
organic solvent.
In accordance with another aspect of this invention r
cheaper or more easily obtainable or workable materials are
used for the envelope or internal components of halogen cycle
lamps by providing on the exposed surfaces of such parts of
the structure a coating of aluminium oxide as described above.
In certain established tungsten-halogen lamps (eAg.
-`10--
t~ri~l filament car lamps) a molybdemlm ~rame or wires is or
are used both as lead~in conductors and as a member to carr~-
a molybdenum ~or tungsten) shield~ There is some evidence
to suggest that there is a limited chemical reaction
b~tween -these components and the fill, ~nd in such a case it
is advantageous to coa-t them with a halogen or halide-
resistant la~ex of the aluminium oxide. However 5 as an
alternative, the refractory me-tal i~ these components can
be replaced by a less expensive and easier to work metal,
such as iron or nickel, coated with the aforementioned layer~
- A further possibility is to use a glass en~elope
coated with a halogen- or halide-resistant layer o~ alwninium
oxide in place of the fused quartz conventionally ernployed
for such envelopes. This may involve a direct xeplacement of
fused ~uartz ~y a hard glass, such as borosilicate or aluminc
silicate, or the use o~ inexpensive soda-lime silicate soft
glass. In the latter case the en~elope dimensions should be
carefully chosen so that the hottest part is below the glass
strain temperature and the coldest part i5 above the well
establ~ished minim~ for the par-ticular tungsten-halogen cycle
to function. ~his also would reduce material and mallu~actur~
in~ costs~ lt should be noted that aluminosilicate glass is
used for the envelope material of certain tungsten-halogen
lamps but Camlot be considered as a replacement for fused
~uartz. It will thus be apparent -that individual components
or all the internal surfaces wi-thin the l~p may be coa-ted~
The following ~re specifjc examples of the practical
. ' .
:
7~
application of ~he present irvention and the production of
tu~gsten-halogen lamps.
_ample 1
A liquid aluminium oxide coa-ting cornposition was
prepared by dissol~ing anhydro1ls aluminlu~ chloride (3O95 g.)
in methanol (396.05 g.). A tungsten filament lamp assembly
was coated internally with this composition by the technique
described above and the coated assembly thoroughly drained~
heated at 100C in vacuo for one hour, and baked at 500C for
5 minutes also i vacuo~ q'he lamp was subsequently filled
with ~ atm~ argon and 4 ~orr ~16 and finished in the usual
way.
~ he lamp was rated at 12V. 100W~ in operation and was
successfull~ run at a filamerl~ temperature of 3200C for
mo~e -than 25 hours without bre~down of the coa-ting. In
contrast, similar lamps withou-t the coating of this invention
showed extremel~ rapid loss of fluorine due to reaction with
the lamp components a~d had a useful li.fe of only a few minutes~
Ex~m~le 2
,
I,amps were made as described ln ~xample 1 excep-t tha-t
4 '~orr of S~6 was used instead of the ~6. The lamp was
e~ually successfu1.
La~ps were made as described in EY.ample 1, except that
the coating compositiorl consisted of ~5 ~. CeriU~l chloride
~CeC13) dissolved in ~6~5 gO methanol, any precipit~te
separatirlg out being filtered off befoxe use~ ~he larnps
- 12-
.
~. .. . .
- were b~ed at ~700C ~or 5 minutes ~n vacuo and showed
similar performance.
Examples_~ to 10
~he followi..ng are further examples of coating
compositions employing different metals, whi.ch may be u~ed
for the purposes o~ this invention:
E~ample 4: 6.2 gl Ce(N03)3~20 dissolved in 93~8 g~ m~th.~ol
(and filtered).
ExaDlple 5: 4.4 g~ SnClL~ ydrous, dissol-ved in 95~6 g~
methanolO
Example 6: 5.9 gO SnGl~,~5H20 dis~olvecl in 94~1 g. me-thanol.
Ex~lple 7: 7.0 gO ZrOCl2.~H20 dissolved in 93O0 gO methanolO
~xample 8: ~.9 g. Sn(N0~3 h~drate~ dissolved in 96,1 gO
methanvl~
Exa~ple 90 '~.4 g~ YCl3 hydrate, clissolved in 95.6 ~. methanol~
Example 10: 392 g~ ThC14 h~drate, dissolved in 9608 g-
methanol.
The salts used we:re of technical grade. Yn the case of
Fxample 4 some precipitate formed on s-tandin~ ~nd this wa5
filtered off~ In the other 'æx~nples no t`~ltration w~s
necessa, y~
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